Metal–Organic Derivatives with Fluorinated ... - ACS Publications

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Metal−Organic Derivatives with Fluorinated Ligands as Precursors for Inorganic Nanomaterials Shashank Mishra* and Stéphane Daniele Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON), UMR 5256, Université Claude Bernard Lyon1, 2 avenue Albert Einstein, 69626 Villeurbanne, France S Supporting Information *

4.1.1. Metallic Thin Films 4.1.2. Metallic Nanoparticles and Nanowires 4.2. Metal Oxide Thin Films and Composites 4.2.1. General Overview of Precursors for Metal Oxide Thin Films by MOCVD 4.2.2. Monometallic Oxide 4.2.3. Heterometallic Oxides and Composites 4.3. Fluoride-Doped Metal Oxide Thin Films and Nanoparticles 4.3.1. Fluoride-Doped Metal Oxides Thin Films 4.3.2. Fluoride-Doped Metal Oxides Nanoparticles 4.4. Metal Fluoride Thin Films and Nanoparticles 4.4.1. Metal Fluoride Thin Films 4.4.2. Metal Fluoride Nanoparticles 4.5. Miscellaneous 5. Conclusions and Looking Ahead Associated Content Supporting Information Author Information Corresponding Author Notes Biographies Abbreviations References

CONTENTS 1. Introduction 1.1. Scope and Focus of This Review 2. Synthetic Methods 2.1. Homometallic Derivatives 2.1.1. From Metal 2.1.2. From Metal Hydrides or Alkyls 2.1.3. From Metal Dialkylamides 2.1.4. From Metal Halides/Nitrates (Metathesis Reactions) 2.1.5. From Metal Oxides/Hydroxides 2.1.6. From Metal Alkoxides/Carboxylates (Ligand Exchange Reactions) 2.1.7. Miscellaneous Reactions 2.2. Heterometallic Derivatives 2.2.1. Lewis Acid−Base Reaction between Two Homometallic Complexes 2.2.2. Miscellaneous Reactions 3. Structures 3.1. Metal Complexes with Fluorinated Alcohols 3.1.1. Homometallic Derivatives 3.1.2. Heterometallic Derivatives 3.2. Metal Complexes with Fluorinated Carboxylic Acids 3.2.1. Homometallic Derivatives 3.2.2. Heterometallic Derivatives 3.3. Metal Complexes with Fluorinated β-Diketones 3.3.1. Homometallic Derivatives 3.3.2. Heterometallic Derivatives 4. Applications as Precursors in Materials Science 4.1. Metallic Thin Films and Nanoparticles

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1. INTRODUCTION Inorganic nanomaterials cover a wide range of applications in electronics, optics, magnetism, catalysis, biomedical, and environmental issues.1−3 The properties of these nanomaterials are determined by their composition and microstructure, and, therefore, the factors that control these aspects during their preparation become of utmost importance.1−6 Chemical vapor deposition (CVD) in the vapor phase and metal organic deposition (MOD) as well as the hydrolytic and nonhydrolytic sol−gel processing in solution phase represent the bottom-up (building-up) approach of the synthesis of nanomaterials. These chemical routes usually require metallic precursors with certain properties such as high purity, easy handling, facile storage, nontoxicity, clean and low temperature decomposition, etc.7−20 Depending upon the synthetic route chosen to transform these precursors into materials, some additional properties such as better stability, solubility, or volatility are also

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Scheme 1. Common Fluorinated Alcohols, -Carboxylic Acid, and -β-Diketones Described as Ligands in This Review

properties of metal−organic precursors for materials processing.36−43 A fluorinated ligand not only modifies the physicchemical properties, as mentioned above, but also alters the reactivity of the resulting metallic derivatives. For example, metal trifluoroacetate and hexafluoroacetylacetonate derivatives are the most commonly used precursors for the elaboration of YBa2Cu3O7−x (YBCO) superconductor coatings by TFA-MOD and MOCVD processes, respectively. In these cases, the thermal decomposition of the barium complexes proceeds via the formation of barium fluoride, thus avoiding the formation of the more stable barium carbonate.44−48 However, use of a fluorinated ligand in the design of metal precursors is not without some potential drawbacks. While both stabilities and vapor pressures are favored by these fluorinated ligands, the fluorine content might be sometimes incorporated in the deposited films either as undesired contaminants or as fluoride phases. This might pose some problem especially when extra pure metal oxide thin films are needed, for example, in microelectronics. Despite this limitation, the use of precursors with fluorinated ligands has surged ahead in recent years, mainly because this drawback can be overcome in the majority of the MOCVD process by using water saturated O2 stream as reaction gas, which removes the fluorine contamination from the growing films as the volatile HF.25,26 In some other cases, either the formed fluoride phase is a volatile species, which can easily be pumped out, or the metal oxide phases are thermodynamically more stable than the related metal fluorides or oxy-fluorides [e.g., CeO2, LnAlO3 (Ln = Y, La), and YBa2Cu3O7−δ]. On the other hand, this tendency of fluoride ions being retained during decomposition makes these fluorinated ligands an excellent in situ source of fluorides for nanostructured and functionalized inorganic fluorides, oxidefluorides, and fluorinated oxides. These nanomaterials, which range from powders or glass-ceramics to thin layers and

required. Bi- and multimetallic nanomaterials can be obtained by using either a mixture of precursors or a single source precursor (SSP) containing all of the required elements in a single molecule, the latter providing better homogeneity at the molecular level.7−20 However, a correct stoichiometry that matches with the final materials is then required. Three classes of metal compounds, metal alkoxides M(OR)n, carboxylates M(O2CR)n, and β-diketonates M(β-dik)n, are the most commonly used precursors for the synthesis of metal oxide nanomaterials.7−32 Modifications are often needed in these complexes for tailoring properties at a molecular level. The strategy to use fluorinated ligands helps in achieving many of the above-mentioned properties of the precursors.25−32 The hydrophobic nature of the fluoro groups can be exploited to control the hydrolysis rates of many moisture-sensitive complexes, for example, alkoxides derivatives, and thereby design more hydrolytically stable and hence easy to handle precursors.33−35 Also, the enhanced strength of the carbon− fluorine bond over the carbon−hydrogen bond leads to greater thermal stability. The presence of strongly electron-withdrawing CF3 groups (or perfluorinated alkyl chains) in alpha positions generates a less basic O donor site, making these ligands far less π donating than conventional ligands. As a result, mono- or low-nuclear complexes are generally formed, which are more soluble and volatile. Additionally, these fluorinated ligands have a tendency to form secondary M···F interactions with oxophilic metals such as alkaline earth metals and lanthanides, which helps increase the metal coordination number, leading to less aggregated and more soluble compounds.33,35 In this context, fluorinated ligands, even if they do not have an additional donor site, behave as functional ligands via the formation of short, secondary M···F bonds and thus increase the stability of the precursors. The use of fluorinated functional ligands also favorably modifies the B

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Scheme 2. Common Ancillary Coligands Described in This Review To Modify the Properties of Metal Precursors with Fluorinated Ligands

coatings, find many important applications as luminescent materials, integrated lasers and optical amplifiers, UV absorbers, planar optical waveguides, antireflective coatings and high Tc superconductors, catalysts, etc.49 While several reports are available on metal complexes with fluorinated ligands for synthesizing homometallic fluoride nanomaterials,50−54 precursor chemistry for the heterometallic fluoride nanomaterials, many of which are excellent materials as host matrixes for luminescent ions or prospective oxygen-free cathode materials,55−63 remains almost unexplored, and only very recently first single source precursors for such materials were reported.64−69

and application in materials science of selected homo- and heterometallic complexes that have been used (or at least hold potential) as molecular precursors to get different forms of the inorganic nanomaterials. Therefore, discussions on the derivatives that are unlikely to be good precursors (e.g., ionic compounds (metallates) or derivatives with water molecules or too many aryl groups) are omitted here, unless they present a direct comparison with their neutral or anhydrous counterparts. Wherever available, earlier review articles25−31 on certain complexes are cited, and these should be sought out for a more complete description because this Review includes only those compounds that have appeared after the publication of the above-mentioned review articles. It should further be noted that metal complexes with the above fluorinated ligands have also been studied for well-defined, single-site ring-opening polymerization catalysis,70 magnetism,71−74 and photoluminescence,75−78 topics that are beyond the scope of this Review. As stressed earlier, this Review focuses mainly on precursor’s chemistry for the bottom-up synthesis of nanomaterials. After an overview of synthetic strategies, this Review discuses extensively the structural diversity and applications of these compounds as precursors in materials science. Finally, it identifies and signifies the areas for future research in the looking ahead section. It is hoped that this Review will bridge the areas of precursor’s chemistry and materials science by providing a reference text for researchers working either in or at the interface of these two areas.

1.1. Scope and Focus of This Review

Surprisingly, despite the great utility of these fluorinated ligandcontaining metallic precursors in materials sciences, as reflected from the rapidly growing literature, no comprehensive effort has been made so far to review the subject matter. In this Review, we discuss well-defined and unambiguously characterized metal complexes containing mono- or multidentate fluorinated alkoxide, -carboxylate, and -β-diketonate ligands (Scheme 1). More often than not, the properties of these metal complexes are improved by using ancillary coligands such as those shown in Scheme 2. It is not the aim of this Review to present a complete list of all of the structurally characterized metal complexes containing these three families of fluorinated ligands (it is simply too vast to be adequately addressed here). Rather, the emphasis is put on providing more detailed information on general synthetic strategies, structural trends, C

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2. SYNTHETIC METHODS Several synthetic routes to metal−organic derivatives with fluorinated ligands have been reported using zerovalent metals, metal alkyls, halides, nitrates, oxides, hydroxides, or carboxylates as starting reactants. These are summarized in the following pages.

Andrews et al. studied the reaction of BiR 3 with hexafluoroisopropanol (H-HFIP) under different reaction conditions, which produced bismuth alkoxide complexes with varying degrees of substitution, cluster formation, and aggregation (Scheme 6).34 The volatile unsolvated bismuth fluoroalkoxide [Bi(ORf)3], isolated from the direct reaction of H-HFIP with BiR3 under solvent-free condition, gave, on extraction with toluene and Lewis base solvents, a variety of solvated mononuclear complexes and oxo species containing predominantly Bi2O cores. The BiPh3 reacted with hexafluoroisopropanol or hexafluoroacetylacetone to afford only the bissubstituted complexes “[PhBiL2]” as the main product.34,89 It was also possible to prepare a mixed hfac-TFA ligand complex. 2.1.3. From Metal Dialkylamides. This method is particularly useful for those metals that have greater affinity for oxygen than for nitrogen (Scheme 7),35,90−96 the advantage being the generally higher volatility of the liberated dialkylamines. Hoffman et al. reported reactions of metal amides [M(NR2)x] [M = Al, Ga, In (x = 3), Sn (x = 4), Pb (x = 2); R = Me, Et, But, and/or SiMe3] with fluorinated alcohols of different acidic character to give ligated or nonligated neutral complexes such as [M(ORf)3(L)] [M = Al, Ga; Rf = CH(CF3)2, CMe2(CF3), or CMe2(CF3)2; L = HNMe2, 4-Me2Npy],97 [In2{OC(CF3)Me2}6], [In{OCR(CF3)2}3(L)x] [R = H, Me; L = H 2 NBu t (x = 1, 3), py (x = 3)], 98 [Sn{OCH(CF 3 ) 2 } 4 (H NMe 2 ) 2 ], 9 9 or ion-pairs of th e t ype [H 2 NMe 2 ] + [Pb{OCH(CF 3 ) 2 } 3 ] − 90 and [LH] + [In{OCR(CF3)2}4(L)]− (R = H, Me; L = HNEt2, H2NBut, 2,2,6,6tetramethylpiperidine).98 2.1.4. From Metal Halides/Nitrates (Metathesis Reactions). The metathesis reaction between an alkali metal salt of the fluorinated ligand and a metal salt (usually a metal halide or nitrate) has been exploited for the synthesis of fluorinated metal alkoxides/carboxylates/β-diketonates. Classical alkoxides of 3d transition metals of the type [M(OR)2]∞ (M = Mn, Fe, Co, Ni, Cu, Zr; R = Me, Et, Pri) are usually insoluble in common organic solvents,21,22 making metathesis reactions unsuitable due to the problem of isolation of the alkali metal halides. The enhanced solubility of fluorinated alkoxides, especially in the presence of a donor ligand, removes this limitation. For example, the [Cu(ORf)2(py)2] (ORf = HFIP, HFTB), prepared from the reaction of CuBr2 and NaORf in the presence of pyridine, is soluble in common organic solvents and, hence, can easily be separated out from the NaBr.100 Chi et al. synthesized the heteroleptic fluorinated aminoalkoxide of ruthenium, [Ru(COD){OC(CF3)2CH2NHR}2] (R = H and Et), starting from [Ru(COD)Cl2] and sodium salt of the ligands.39 They also used the metathesis reaction of the sodium salt of fluorinated aminoalcohol [HOC(CF3)2CH2NMe2 (HamakF)] and iminoalcohol [HOC(CF3)2CH(Me)NMe (HimakF)] with GaCl3 in diethyl ether to prepare the bis complexes [Ga(amakF)2Cl] and [Ga(imakF)2Cl].42 In contrast, the reactions of same ligands with trimethylgallium afforded only the monosubstituted complexes [GaMe2(amakF)] and [GaMe2(imakF)].42 On the other hand, complete halide removal was possible in the reactions of MX4 [M = Ti (X = F), Zr (X = Cl)] with Li/Na salts of hexafluorophenyltertiarybutanol or a tetradentate β-diketoimine,101,102 which afforded the homoleptic complexes [Ti(HFPB)4]103 and [Zr{(CF3)C(O)CHC(Me)N(CH2)nNC(Me)CH(O)C(CF3)}2],104 respectively. Metal derivatives of fluorinated carboxylates and βdiketonates stabilized by diammine or glyme ligands have also

2.1. Homometallic Derivatives

2.1.1. From Metal. This route has been exploited for electropositive metals such as alkali, alkaline, and rare-earth metals, where the acidic character of the fluorinated ligands facilitates the reaction.35,79,80 In certain cases, the reactions were activated by the use of liquid ammonia, a method that makes use of the intermediate metal amide and proceeds via ligand exchange (Scheme 3). The THF ligand in the obtained [M(PFTB)2(THF)4] can be replaced by glyme ligands to afford [M(PFTB)2(glyme)x] (glyme = DME, diglyme; x = 1−3).35 Scheme 3. Synthesis of Alkaline Earth Metal Fluorinated Alkoxides and -Carboxylates Starting from a Zero-Valent Metal

Trifluoroacetate and hexafluoroacetylacetonate complexes of divalent metals could be prepared in quantitative yield by comproportionation reactions between trivalent metal complexes and the corresponding metal powders (Scheme 4).81,82 Scheme 4. Synthesis of Metal Fluorinated Carboxylates and -Acetylacetonates by a Comproportionation Reaction

Alternatively, the use of silver(I), copper(II), or mercury(II) complexes can also oxidize metal to give moderate yield of divalent metal derivatives [ML2] (M = Bi, Mn, Fe, Co, Ni; L = TFA, hfac). 2.1.2. From Metal Hydrides or Alkyls. The metal− hydrogen and metal−carbon bond cleavage reactions have also been used for the synthesis of several metal−organic derivatives with fluorinated ligands (Scheme 5).35,40,42,83−88 Sometimes, these reactions either lead to formation of adducts (e.g., [M(hfac)3(NMe3)] starting from MH3·NMe3, where M = Al, Ga)87 or completely substitute products [Al(β-dikF)3] from 1:2 reactions of AlMe3 with hfacH or tfhdH whatever the conditions used (temperature, solvent, addition order of reagents)].88 D

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Scheme 5. Synthesis of Metal Fluorinated Alkoxides and -Carboxylates Starting from Metal Hydrides or Alkyls

Scheme 6. Synthesis of Bismuth Fluorinated Alkoxides and -Acetylacetonates Starting from Bismuth Alkyls

Scheme 7. Synthesis of Metal Fluorinated Alkoxides Starting from Metal Dialkylamides

H-bonding in these fluorinated complexes.116 Tyrra et al. reported the synthesis of anhydrous trifluoroacetate complexes of the group 12 elements from the reactions of corresponding oxides and trifluoroacetic acid in the presence of trifluoroacetic acid anhydride to remove water molecules produced in the course of the reaction. 117 Solvated complexes [M(TFA)2(DMAP)2] (M = Zn, Cd, Hg) were formed after unligated [M(TFA)2] reacted with 2 equiv of DMAP ligand in the boiling methanol. On the other hand, anhydrous copper(I) trifluoroacetate could be prepared directly from the reaction of Cu2O with trifluoroacetic anhydride118 or with trifluoroacetic acid.119

been prepared starting from either metal halides or nitrates (Scheme 8).105−115 2.1.5. From Metal Oxides/Hydroxides. Direct reactions of oxides or hydroxides of the electropositive alkaline earth metals and the lanthanides with fluorinated ligands in refluxing solvent afford a cost-efficient synthesis of metal complexes. However, the reactions proceeds with the formation of water as a byproduct to give sometimes hydrated metal complexes. These hydrated complexes are, however, unattractive for the materials processing due to the uncontrolled reactivity, which water molecules may introduce in the precursor solution. Removal of the water molecules at high temperature and under vacuum often proves difficult, due to the presence of extensive E

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Scheme 8. Synthesis of Metal Fluorinated Carboxylates and -Acetylacetonates by a Metathesis Reaction

Scheme 9. Synthesis of Metal Fluorinated Alkoxides and -β-Diketonates by a Ligand Exchange Reaction

coordinating these large metals.26 A recent publication, however, reported the syntheses of anhydrous [Ln(hfac)3(DME)] (Ln = La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, Er, Tm) and thus contradicted the above assumption.132 A similar one-pot reaction is possible with metal hydroxides as well, although the products isolated often depend on the nature of metals as well as the reaction conditions. Thus, the reaction of hydroxides of strontium(II), cobalt(II), zinc(II), or yttrium(III) with hexafluoroacetylacetone in the presence of various glyme ligands afforded different complexes, the anhydrous [M(hfac)x(L)] [M = Sr (x = 2, L = tetraglyme),53 Co (x = 2, L = DME),133 Y (x = 3, L = DME, diglyme)],134 hydrated [M(hfac)x(H2O)2)]·L [M = Co (x = 2, L = di-, tri-, and tetraglyme),135,136 Zn (x = 2, L = tetraglyme),137 Y (x = 3, L = triglyme)134], and an ion-pair [Y(hfac)2(tetraglyme)]+[Y(hfac)4]−.134 2.1.6. From Metal Alkoxides/Carboxylates (Ligand Exchange Reactions). One of the characteristic features of

Another strategy to get anhydrous complexes is to use ancillary coligands in the reaction medium. In most cases, this affords anhydrous complexes [M(hfac)2(L)] (M = Pb,120,121 Cd,122 L = di-, tri-, and tetraglyme], [Ag(β-dikF)(L)] (β-dikF = hfac, fod; L = PMe3, PEt3),123−126 and [Ag(hfac)(L)x] [L = di-, tri-, and tetraglyme (x = 1),127,128 Me 3SiCCSiMe 3 , Me3SiHCCH2 (x = 1), THF, toluene (x = 0.5)],129 although the precursors may retain one or more water molecules sometimes, depending upon the reaction conditions and the ligands chosen. For example, anhydrous polyether adducts of lanthanide hexafluoroacetylacetonates, [La(hfac)3(L)] (L = diglyme, triglyme, tetraglyme), can easily be prepared through one-pot reactions of rare earth oxides, H-hfac, and polyether ligands in suitable solvents (benzene or dichloromethane).26 For the early lanthanides, however, monohydrated [Ln(hfac)3(DME)(H2O)] (Ln = La,130 Nd131) were obtained with the DME ligand, and it was assumed that the bidentate DME was not bulky enough to prevent a water molecule F

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Scheme 10. Synthesis of Heterometallic Hexafluoroacetylacetonates by a Lewis Acid−Base Reaction

[Bi2(TFA)4] or solvated [Bi(TFA)3(TFAH)] with M(II) carboxylates gave homo- and heteroleptic Bi(II)−M(II)/ Bi(III)−M(II) trifluoroacetate complexes [BiM(TFA)4] (M = R u, R h ) , 1 5 4 ci s - [ B i Rh ( T FA ) 2 ( O 2 C B u t ) 2 ] , [ B i R h (TFA) 3 (O 2 CMe)], 155 [Bi 2 Pd 2 (TFA) 10 (TFAH) 2 ], 156 and [BiRh(O2CRf)4] (Rf = CF3, C2F5).157 An acetone-bridged heterometallic trifluoroacetate complex {[Rh2(TFA)4]·(μ2OCMe2)·[Cu4(TFA)4]}2∞ was also prepared by codeposition of the volatile mono(acetone) [Rh2(TFA)4]·η1-OCMe2]2 and [Cu4(TFA)4].158 Similarly, the solid-state reaction of unsolvated homometallic β-diketonates gave quantitative yield of the heterometallics [NaM(hfac)3] (M = Mn, Fe, Co, Ni), [PbM(βdikF)4] (M = Mn, Fe, Ni; β-dikF = tfac, hfac), and [Pb2Fe(hfac)6] or heteroleptic [Pb2Fe2(hfac)6(acac)2].67,69,159 They have also utilized solid-state redox reaction between M(hfac)3 (M = Bi, Mn) and either transition metals, M′, or their complex, M′(hfac)2, to give homoleptic heterometallics of different stoichiometries160,161 (Scheme 10). The presence of multidentate ligands such as aminoalcohols and glymes not only helps in the formation of heterometallics but also permits one to fine-tune the properties such as thermal stability, moisture- and air-sensitivity, solubility, and volatility as well as Lewis acidity and steric hindrance about the metal centers in the obtained heterobimetallic molecules. Using this strategy, Wang et al. reported several Cu(I)−Ln(III)/Y(III) and Cu(I)−M(II) (M = an alkaline earth metal) heterometallic trifluoroacetate precursors for high Tc superconducting materials,162 and it continues to be used to synthesize interesting heterobimetallics with novel structural features and enhanced properties for materials process.64−66,163−173 2.2.2. Miscellaneous Reactions. The heterometallic [LiAl(ORf)4] [Rf = CH(CF3)2, OC(Ph)(CF3)2] was prepared from the reaction of LiAlH4 with a solution of HORf in 1,1,2C2Cl3F3 or toluene.174 Shen et al. reported the synthesis of polynuclear Na−Ln heterometallic clusters [Na8Ln2(TFE)14(THF)6] (Ln = Y, Sm, Yb) by the reaction of anhydrous LnCl3 with 7 equiv of NaOCH2CF3.175 Similarly, the reaction between anhydrous yttrium trichloride and barium hexaisopropoxide gave heterometallic [YBa2(HFIP)7(THF)], which on further reaction with Y(thd)3 yielded the mixedligand heterometallic [BaY2(HFIP)4(thd)4] as the major product.176,177 A 1:2 reaction of TiF4 and LiOC(CF3)2Ph in tetrahydrofuran afforded the heterometallic [Li(THF)2TiF3{OC(CF3)2Ph}2]2.103 Reaction of Cp2MoH2 and Cp2ReH with Bi2(HFIP)6(THF)2 in 1:2 and 1:1 molar ratios afforded [Cp2Mo{[Bi(HFIP)2}2]178 and [Cp2ReBi(HFIP)2],179 respectively, where direct M−Bi bond ensures the formation of heterometallics. The Ln−A1 trifluoroacetate complexes [(TFA)2Ln(μ-TFA)AlR2(THF)2]2 [Ln = Y, Nd (R = Bui), Eu (R = Et)] were obtained by the reaction between Ln(TFA)3 and aluminum alkyls, HAl(Bui)2 or AlEt3.180 Reaction of the in situ synthesized [Ba(ORf)2] (ORf = TFTB, HFTB) with

metal(loid) alkoxides and carboxylates is their ability to exchange OR−/RCO2− groups with deprotonated ligands, and this has been exploited for the synthesis of new fluorinated metal alkoxides, carboxylates, and β-diketonates. Factors such as steric demand of leaving and coming groups and relative strengths of M−O and O−H bonds of the reactants and products determine the extent of substitution (Scheme 9).88,101,138−144 Often, reactions with metal alkoxides are more controllable. For example, the reactions of Al(OPri)3 with 2 equiv of hfacH at room temperature afforded the heteroleptic complex [Al(hfac)2(OPri)],88 which is in contrast with the similar reactions with AlMe3 or AlMe2Cl that facilitated formation of the homoleptic tris-product (vide supra, section 2.1.2). Interestingly, same reaction of Al(OPri)3 with 2 equiv of hfacH at a higher temperature (55 °C) for 48 h afforded a moderate yield of a quite different compound, [(4Hhfac)(hfac)Al(THF)]2, as a result of an in situ Meerwein− Pondorf−Verley (MVP) reduction of an hfac− ligand by an active Al−OPri moiety.88 2.1.7. Miscellaneous Reactions. The cesium complexes [Cs(β-dikF)(H2O)x] (β-dikF = hfac, tfac, ptac, btac; x = 0 or 1) were prepared from the reaction of Cs2CO3 and an appropriate fluorinated β-diketone in diethyl ether.145 The reactions of Tl2CO3 and (MgCO3)4·Mg(OH)2·5H2O with H-hfac in the presence of diglyme afforded [Tl(hfac)(diglyme)]146 and [Mg(hfac)2(H2O)2]·2diglyme,51 respectively, the diglyme ligands not being directly bonded to magnesium center in the latter. Starting with Ru3(CO)12, Chi et al. synthesized mixedligand complexes [Ru(CO)2(L)2] (L = tfac, hfac, iminoalkoxo).39,147,148 2.2. Heterometallic Derivatives

2.2.1. Lewis Acid−Base Reaction between Two Homometallic Complexes. The Lewis acid−base reaction between two component metal alkoxides, carboxylates, or βdiketonates is one of the most used methods for the preparation of heterobimetallics. Thus, the reactions between alkali metal alkoxides (strong bases) with fluorinated alkoxides of a variety of metals and metalloids (Lewis acids) have yielded a number of heterobimetallics containing alkali metals: [MM′(PFTB)3(THF)] (M = Na, K; M′ = Mg, Sr, Ba, Eu),33 [Pb2Li2(HFIP)6],149 [Tl2Zr(HFIP)6],150 [Na2Y(HFIP) 5 (THF) 3 ], 151 [Na 2 Zr(HFIP) 6 ]·C 6 H 6 , 150 [Na 2 Y(HFIP) 5 (THF) 3 ], 151 [Na 2 Cu(HFIP) 4 ], 80 and [Na 3 Y(HFIP)6(THF)3].151,152 This strategy has also been used to prepare heteroleptic complexes as well, for example, [NaCu(HFIP)2(thd)]n,152 [Bi2SnO(HFIP)5(OBut)3]·THF,153 and [BiSnO(HFIP)3(OBut)2].153 Utilizing the reaction between two metal carboxylates or βdiketonates, mostly unsolvated and with at least one of them being coordinatively unsaturated, Dikarev et al. reported the synthesis of unligated heterometallic derivatives of the main group and the transition metals.154−161 Thus, the reactions of G

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alkyl chains. Some examples are (CF3)2CHOH (H-HFIP), (CF3)Me2COH (H-TFTB), (CF 3)2MeCOH (H-HFTB), (CF3)2PhCOH (H-HFPB), and (CF3)3COH (H-PFTB). Because of the presence of strongly electron-withdrawing fluorine atoms, these ligands are far less π-donating than conventional alkoxides, and, as a result, a much lower bridging tendency is observed. Even though the use of fluorinated alkoxide generally reduces oligomerization, this strategy may not be effective enough to prevent the formation of polynuclear clusters (sometimes templated by oxo- and/or hydroxo ligands) with oxophilic alkali and alkaline earth metals. Thus, the alkali metal derivatives with fluorinated alkoxide ligands [M4(ORf)4] (M = Li, Na, K; ORf = HFIP, TFTB, and PFTB) are tetrameric with a cubane structure.150,182−185 In addition to three M−O bonds [1.938(11)−1.991(11) Å for Li, 2.235(2)−2.342(2) Å for Na, and 2.578(2)−2.750(2) Å for K], each metal also forms several short M···F contacts to achieve a higher coordination number of six for lithium and eight or nine for sodium and potassium. In the case of [Na4(HFIP)4]150 and [Na4(TFTB)4],184 these Na···F interactions are both intracubic [2.635(2)−3.506(2) Å] and intercubic [2.332(5)−3.746(5) Å] in nature, the latter being even shorter sometimes and thus creating a polymeric structure. In contrast, [M4(PFTB)4] (M = Li, Na) has only intramolecular Li···F [2.213(1)−2.694(1) Å] or Na···F contacts [2.497(4)−3.747(4) Å]. The authors attributed this absence of intermolecular metal−fluorine contacts to the large steric bulk of the perfluoroalkoxide group, which shields the metal center.182,184 In a rare structure, the tetrameric lithium complex [Li4{OC(CF3)2(Mes)}4] has an eight-membered ring consisting of an alternating arrangement of Li and O atoms.186 Two of the four six-coordinated Li atoms form 4 Li···F interactions [2.123(9)−2.787(1) Å], whereas the two other Li atoms have weak Li···C interactions with Mes group [2.597(9)−2.791(10) Å]. A heteroleptic complex [Li4{OC(CF3)2(Mes)}4][LiF]2 with a double heterocubane structure is also known.186 These solvent-free homoleptic complexes are Lewis acids and, in the presence of a donor solvent, form adducts with lower nuclearity such as [Li2(μ-PFTB)2(THF)4]182 and [Li3(μ3-PFTB)(μPFTB)2(μ-acetone)(acetone)2].187 In the presence of moist air, the [Li4(PFTB)4] undergoes a stepwise hydrolysis forming first a water-bridged dimer [(PFTB)Li(H2O)2]2(μ-H2O), then an ion-pair [{(PFTB)Li(H2O)2(μ-H2O)Li(H2O)3}(PFTB)], and finally another ion-pair [{Li(H2O)4}(PFTB)] where the metal is no longer bonded to the alkoxide ion.188 The structure of pentanuclear [Ba5(μ5-OH)(μ3-HFIP)4(μHFIP)4(HFIP)2(THF)4(H2O)]·THF is based on a distorted square pyramid of barium atoms encapsulating an μ5-OH group (Figure 1).189 The basal barium atoms are connected to each other by doubly bridging fluoroisopropoxide groups, while triply bridging HFIP ligands capping the faces of the pyramid ensure bonding with the Ba metal in the apical position, which also bears a terminal alkoxide ligand. Short metal−fluorine interactions [2.99(2)−3.31(2) Å], three for the basal metals and four for the apical one, lead to the formal coordination number of barium centers between 9 and 11. With more bulky perfluoro tert-butoxide ligand, monomeric structures of the general formula [M(PFTB)2(L)x] (M = Mg, Ca, Sr, Ba; L = THF, DME, diglyme; x = 1−4)35 were obtained for the alkaline earth metals. These complexes display octahedral or pseudooctahedral (i.e., pentagonal or hexagonal bipyramidal) geometries with coordination numbers varying between six and eight. Depending on the nature of coordinating

copper mesityl, under different reaction conditions, gave [(THF)2Ba2Cu2(HFTB)6] or [(Me3NO)2Ba2Cu4(TFTB)8].181

3. STRUCTURES 3.1. Metal Complexes with Fluorinated Alcohols

An alkoxide ion RO− is a versatile ligand that coordinates to a metal center either in terminal or in bridging manner (μ or μ3 due to the presence of 3 pairs of electrons available on the oxygen atom, although there is also a possibility of the bridging mode involving a higher number of metal centers, e.g., μ4-, etc., due to delocalized bonding). The fluorinated alkoxide RfO− shows an additional coordination mode due to the secondary M···F bonding (Scheme 11). In the nonbridging mode, the Scheme 11. Various Bonding Modes Observed for the Fluorinated Alkoxide Ligands

angle R−O−M reflects the degree of π-donation in the bonding, which depends on the availability of vacant π-type orbitals on the metal ions.21 Supporting Information Table S1 contains some selected X-ray crystallographically characterized metal complexes with fluorinated alkoxide ligands, which have been used (or hold potential) as precursors to get different forms of the inorganic nanomaterials. 3.1.1. Homometallic Derivatives. 3.1.1.1. With Nonfunctional Alkoxides. 3.1.1.1.1. Main Group Metals. Because of its basic nature, the classical alkoxo group OR− has a general tendency to act as a bridging ligand, which often results in, especially with large, oxophilic metal centers, highly aggregated structures and complex compositions of the compounds. The use of highly stable fluorinated alkoxide ligands affords an efficient strategy to overcome this difficulty to a certain extent. As a result, most of the studies with fluorinated alcohols are with simple alcohols containing CF3 groups or perfluorinated H

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The coordination number of In(III) in derivatives with fluorinated alkoxides varies from 4 to 6. The indium center is tetra-coordinated in the unligated, alkoxo-bridged dimer [In 2 (μ-TFTB) 2 (TFTB) 4 ], the monomeric adduct [In(HFIP)3(H-TMP)] (where H-TMP = 2,2,6,6-tetramethylpiperidine), and the ion-pair [H2-TMP][In(HFTB)4], whereas the solvated [In(HFTB)3(py)3] possesses an octahedral indium center with meridional arrangement of the ligands.98 The ionpair [HL][In(HFIP)4(L)] (L = HNEt2, H2NBut), on the other hand, has a 5-coordinated indium center with square pyramidal geometry. 9 8 In contrast to the complexes [Ge(HFIP)2(H2NPh)] and [Sn(HFIP)2(HNMe2)], which have monomeric structures in the solid state, the lead compounds [Pb(μ-HFIP)(HFIP)(p-pyNMe2)]2 and [{Me2NH2}{Pb(μHFIP)(HFIP)2}2] are dimers with asymmetrically bonded alkoxide bridges.90 The lone pair on Pb does not appear to be stereochemically “active” in either structure. The metal centers in all four compounds have similar trigonal pyramidal geometry. As expected, the M−O and M−N distances increase significantly going down the column from Ge to Sn to Pb. Andrews et al. recently reported a variety of fluorinated alkoxides of bismuth(III) by extracting the unsolvated “Bi(HFIP)3” with toluene and Lewis base solvents.34 The mononuclear [Bi(HFIP)3(py)2] and mer-[Bi(HFIP)3(THF)3] are simple adducts with 5- and 6-coordination numbers for the metal center, respectively, the latter being different from the previously known [Bi(HFIP)2(μ-HFIP)(THF)]2199,200 where each bismuth center is 5-coordinated and is connected to only o ne TH F m olecule. Another dinuclear complex [ B i 2 ( H F I P ) 6 (DA B CO ) 3 ] , w h e r e D AB C O i s 1 ,4 diazabicyclo[2.2.2]octane, has two Bi(HFIP)3(DABCO) units bridged by an N,N-bound DABCO ligand. The bridging DABCO ligand is not symmetrically bound to both of the Bi atoms [Bi1−N4 = 2.988(8) Å and Bi2−N3 = 2.628(8) Å]. The Bi−N bonds for terminally bound DABCO ligands too vary a lot, from 2.580(8) to 2.851(8) Å. In contrast, the compound [PhBi(HFIP)(μ-HFIP)2)]n is a polymer with four coordinated bismuth centers bonded to a phenyl group and three alkoxy groups. Two of the alkoxy groups bridge successive bismuth atoms generating the linear chain polymer, while the third one is bonded in a terminal fashion. The other reported complexes, either dinuclear [Bi2(μ-O)(HFIP)4(TMEDA)2] or tetranuclear [Bi2(μ3-O)(HFIP)2(μ-HFIP)2(L)]2 (L = toluene, THF, Et2O), are oxo species containing a Bi2O core. In the tetranuclear complex with solvate toluene, one Bi center interacts with the toluene molecule, and the Bi···C distances (3.481, 3.555 Å) indicate that the toluene is bound in an η2-arene fashion. One more oxo complex [Bi9(μ3-O)7(HFIP)4(μ-HFIP)9] with a Bi9O7 core was isolated. Alkoxo ligands shroud the core where bismuth atoms are arranged in an octahedral fashion with three Bi atoms emanating from the oxo ligands that cap faces of the octahedral core (Figure 2).34 In the structures of [M4(HFPT)2] (M = Sn, Pb; HFPT4− = anion of 1,1,1,5,5,5-hexafluoropentane-2,2,4,4-tetraol),201 four metal atoms, bridged by two tetradentate tetraolate ligands, form a distorted tetrahedron. As expected for the lone-pair Sn2+ and Pb2+ ions, these metal centers exhibit a pyramidal coordination of four ligand oxygen atoms. Such M4(HFPT)2 unit was also observed in the structure of [Bi4(HFPT)2(hfac)4(THF)5],202 where additional hfac and THF ligands account for higher coordination number of the bismuth(III) centers (Figure 3). The coordination behavior of the tetraolate ligand in these structures is remarkable in the

Figure 1. Perspective view of [Ba5(μ5-OH)(μ3-HFIP)4(μ-HFIP)4(HFIP)2(THF)4(H2O)]. H atoms and Ba···F interactions are not shown for clarity. Adapted with permission from ref 189. Copyright 1994 Elsevier.

coligands, the bulky PFTB ligand occupies either the cis or the trans position. Thus, the THF and diglyme adducts adopt only the cis and trans conformations, respectively, whereas the DME adducts display both cis and trans arrangements. It is noteworthy that the volatility of these ligated complexes [M(PFTB)2(L)x] is significantly increased as compared to those of unligated complexes [M(PFTB)2].35 Homo- and heterometal alkoxides containing group 13 and 14 metals have attracted considerable attention because of their use in getting (mixed)metal oxide nanomaterials by MOCVD or sol−gel process.190−194 Hoffman et al. reported several metal derivatives of these groups with fluorinated alcohols of different Lewis acidity as potential precursors to undoped or fluorinedoped metal oxide.97−99 The homoleptic donor-free complexes [M(ORf)3] (M = B, Al, Ga) are Lewis superacids, which form adducts even with the weak donor ligands.195−197 The complexes [B(HFIP)3(MeCN)], [Al(ORf)3(L)] (ORf = HFTB, PFTB; L = Me2Npy, SO2, C6H5F, and 1,2-F2C6H4), and [Ga(ORf)3(Me2Npy)] (ORf = HFIP, TFTB) have distorted tetrahedral geometry around the metal center with O−M−O and O−M−N angles ranging from 102.4° to 117.0°.97,195,196 The structures of the two heteroleptic complexes [Al 2 (μ-F)(μ-PFTB)(PFTB) 4 ] and [Al 3 (μF)3(PFTB)6], obtained as decomposed products of the homoleptic Al(PFTB)3, are based on plannar four-membered Al2(μ-F)(μ-ORf) ring and six-membered Al3(μ-F)3 ring, respectively.195 Two terminal fluoroalkoxy ligands at each aluminum atom complete tetra-coordination for the metal centers in these complexes. In an unusual structure of [Al4(μ4OH)(μ-OCH2CF3)5(OCH2CF3)6], the four aluminum atoms are bridged by the μ-OCH2CF3 groups to form an eightmembered Al4O4 ring. All four Al atoms are further connected by an apical μ4-OH. An additional μ-OCH2CF3 group further bridges cross-ring Al centers, each of which also has one additional terminal alkoxy group. Two other Al atoms each have two terminal alkoxy ligands.198 I

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fluorine with titanium [Ti···F = 2.704(5) Å]. The complete replacement of the alkoxide ligands with fluorinated alkoxides resulted in an increased Lewis acidity of the metal ion, producing 6-coordinated solvated mononuclear complex [Ti(HFIP)4(MeCN)2]. In contrast, the homoleptic Ti(IV) and Zr(IV) complexes with bulkier hexafluorophenylbutoxide (HFPB) and hexafluorotertiarybutoxide (HFTB) ligands, [Ti(HFPB)4] and [Zr(HFTB)4], reveal an essentially tetrahedral configuration about the metal center.103,150 There is a vast amount of literature available on the fluoroalkoxo-supported complexes of groups 5 and 6 metals, especially on the related multiple-bonded dinuclear complexes.203−205 However, except for a few examples described below, the majority of these complexes have not been employed as precursors in materials science, and hence their discussion is omitted here. While the partially fluorinated oxoalkoxides of vanadium [VO(μ-TFE)(TFE)(C6F5)]2 and [VO(μ-TFE)(TFE)2]2 are centrosymmetric dimers where vanadium centers have trigonal bipyramidal geometry,206 the related tungsten(VI) complexes [WO(ORf)4] (ORf = TFTB, HFTB) adopt a monomeric structure.207 The 5-coordinated tungsten center in the TFTB complex has square pyramidal geometry where the oxo ligand occupies the apical position and the four alkoxy ligands coordinate almost equatorially. The complex with HFTB ligand, on the other hand, has an additional DMSO ligand coordinated trans to the oxo ligand, which makes the geometry of the tungsten center pseudooctahedral. As compared to this, the nitride analogues [NW(HFTB)3(DME)]208 and [NW(TFTB)3]3209 are monomeric and trimeric with 6- and 5-coordinated W centers, respectively, the former having a W−O bond trans to the nitride ligand substantially longer (0.305 Å) than that trans to alkoxide (due to the large trans influence of the triply bonded nitride ligand). In contrast, the N-bridged trimeric structure of the [NW(TFTB)3]3 is based on a hexanuclear plannar W3N3 ring.209 While the copper(II) complexes [Cu(ORf)2(TMEDA)] (ORf = HFIP, HFTB) are monomers with a 4-coordinated metal center,100 the [Ag(PFTB)]∞ forms a remarkable onedimensional polymeric structure built up from distorted eightmembered rings that are connected through rectangular fourmembered rings (Figure 4).210 One of the Ag atoms is almost linearly coordinated by two O atoms [∠O−Ag−O = 172.7(2)°], while the other Ag atom is coordinated by three O atoms in between a T-shape and a trigonal planar arrangement [av Ag−O = 2.268 Å (within ring), 2.494 Å (ring connecting); ∠O−Ag−O = 81.603(9)−152.302(6)°]. Bradley et al. reported reactions of Ln{N(SiMe3)2}3 (Ln = Sc, Y, La, and lanthanides) with fluorinated alcohols of different acidic character to give ligated or nonligated neutral complexes. 9 1 − 9 4 The trinuclear [Pr 3 (μ 3 -TFTB) 2 (μTFTB)3(TFTB)4] has three metals in a triangle capped above and below by two μ3-TFTB ligands.93 The μ-ligands span the edges of the triangle, and one Pr atom, which has two terminal alkoxo groups, is in a distorted octahedral environment, while the other two, with one terminal ligand, are fivecoordinated. However, 5-coordinated metals are close enough to F atoms of CF3 group (Pr···F = 2.756−2.774 Å) to suggest six- and seven-coordination for them. With the even more acidic HFTB ligand, the ammonia produced in a side reaction was captured by the praseodymium atom to give dimeric amine [Pr2(μ-HFTB)2(HFTB)4(NH3)4].91−93 The centrosymmetric molecule has the typical edge-shared octahedral structure but

Figure 2. Perspective view of [Bi9(μ3-O)7(HFIP)4(μ-HFIP)9]. Adapted with permission from ref 34. Copyright 2008 Royal Society of Chemistry.

Figure 3. Perspective view of [Bi4(HFPT)2(hfac)4(THF)5]. Adapted with permission from ref 202. Copyright 2010 Royal Society of Chemistry.

sense that it holds four metal atoms together, forming a discrete molecule rather than a coordination polymer. 3.1.1.1.2. Transition Metals and Lanthanides. Structures of a series of heteroleptic dinuclear titanium complexes [Ti(OR)4−x(ORf)x(HOR)] [Rf = HC(CF3)2, C6H3F2, C6F5; R = Et, Pri)] were reported by Van Der Sluys et al.101,102 While the Ti(IV) centers in two alcoholate derivatives [Ti(HFIP)2(OEt)2(EtOH)] and [Ti(OC6F5)3(OPri)(HOPri)] have regular octahedral geometry, the 5-coordinate metal centers in two unsolvated compounds [Ti(HFIP)2(OPri)2] and [Ti(OC6H3F2)3(OPri)] are in the trigonal bipyramidal environment. In the derivative [Ti(OC6H3F2)3(OPri)], the unique orientation of the phenyl group ensures a close interaction of a J

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Figure 4. Eight-membered ring structure of [Ag(PFTB)]∞ (a) and polymeric ring structure where CF3 groups have been omitted for clarity (b). Adapted with permission from ref 210. Copyright 2006 Wiley-VCH.

complexes,70 only those complexes that have appeared after the publication of this review article will be discussed here. Thus, [K{OC(CF3)2CH2(1-aza-12-crown-4)}]2 is a dimer bridged through the Oalkoxide atoms, where an 8-coordination sphere on each potassium is completed by the four heteroelements from the macrocyclic ether and two fluorine atoms situated at a short distance from the metals (average K···F = 2.98 Å).211 On the other hand, the tetrameric derivatives [K{OC(CF3)2CH2N(MeOCH2CH2)R}]4 (R = Me or MeOCH2CH2)211 adopt a slightly distorted cubic arrangement, where each metal atom in the central K4O4 core is adjacent to three Oalkoxide atoms. Each metal atom engages in two to four nonequivalent K···F interactions with neighboring CF3 groups [K···F = 2.862(1)− 3.786(2) Å] to increase the coordination numbers to 7, 8, or 9. Although all dimeric, the structures of the heteroleptic calcium complexes [(R2N)Ca{OC(CF3)2CH2N(C2H4OMe)(R′)}]2 (R = SiMe2H, SiMe3; R′ = C2H4OMe, Me)211 present some peculiar differences in bonding arrangements. Thus, the molecular structure of [{(SiMe2H)2N}Ca{OC(CF3)2CH2N(C2 H4OMe)2}]2 shows a potential Ca···H−Si agnostic interaction to afford 7-coordinated metal centers. This kind of Ca···H−Si interaction is replaced by a secondary, albeit weaker, Ca···F contact in [{(SiMe3)2N}Ca{OC(CF3)2CH2N(C2H4OMe)2}]2 and [{(SiMe2H)2N}Ca{OC(CF3)2CH2N(C2H4OMe)2(Me)}]2 to afford the same coordination number for the metal centers. The arrangement is different in [{(SiMe 3 ) 2 N}Ca{OC(CF 3 ) 2 CH 2 N(C 2 H 4 OMe)(Me)}] 2 , where the two calcium atoms are not equivalent. The atom Ca1 is 6-coordinated, having only one Ca···F interaction (3.08 Å); the atom Ca2 is 7-coordinated with two Ca···F contacts of 2.71 and 3.06 Å. The Ca2O2 central core is not symmetrical, and the single Ca1···F interaction induces slight deviations from the symmetry.211 The homoleptic fluorinated amino-alkoxides of the alkaline-earth metals, [M{OC(CF3)2CH2N(C2H4OMe)2}2] (M = Sr and Ba), are monomeric and show interesting structural contrast.36 While the strontium complex possesses an 8-coordinate distorted bicapped octahedral geometry with all O and N atoms coordinated to the central Sr cation, the barium complex adopts a rare 10-coordinate bicapped square antiprismatic structure, two Ba···F interactions (3.133−3.213 Å) accounting for the increased coordination number (Figure

with one HFTB and one NH3 in the Pr2O2 plane and the other HFTB at right angles to the Pr2O2 plane and trans to the second NH3 on each Pr. This is different arrangement from that found in [Sc2(μ-HFIP)2(HFIP)4(NH3)4].94 The bridging propensity of the alkoxo ligand can be reduced both sterically by using bulky alkyl groups, and electronically by using electronegative alkyl groups, which lower the electron density on the alkoxo oxygen donor atoms. Both factors are operating in the fluorinated tert-alkoxo groups, which afford mononuclear, octahedral complexes [Ln(HFTB)3(THF)3] (Ln = Y, La) and [Y(HFTB)3(diglyme)] with facial configuration.91−93 Very recently, dimeric [Ln2(μ-HFIP)2(HFIP)4(DME)2] (Ln = Ce, Tm) and mixed-valent polymeric compounds [Ln2II(μHFIP)3(μ3-HFIP)2LnIII(HFIP)2(L)2] (Ln = Eu, Yb; L = DME or THF/Et2O) have been described, where short Ln··· F interactions in the range 2.522(3)−2.920(4) Å help lanthanide atoms to achieve their common coordination numbers of 6−8.96 The cerium atoms in the crystal structures of [Ce(HFTB)4(TMEDA)] and [Ce(HFTB)4(diglyme)] are hexaand heptacoordinated, respectively.138 The Ce−O(HFIP) bond distances are quite short [2.115(5)−2.152(6) Å] and are associated with large Ce−O(HFIP) angles (>156°). Both complexes display a range of short F···C contacts, the shortest ones having values of 3.754 and 4.060 Å for complexes with TMEDA and diglyme ligand, respectively. The structure of [HPMDIEN]2[Ce(HFIP)6] is based on a centrosymmetrical [Ce(HFIP)6]2− anion associated with two [H-PMDIEN]+ cations.139 The surrounding of the six-coordinated cerium atom is quite regular [Ce−O distances 2.183(5)−2.208(5) Å] with O−Ce−O angles close to 90°, and the hexafluoroisopropoxide ligands form nearly a crown around the metal. 3.1.1.2. With Functional Alkoxides. Fluorinated amino- and imino-alkoxides have been used quite recently, primarily to design low-nuclear metal organic precursors for chemical vapor deposition (MOCVD) of metal, metal−oxide, or −fluoride thin films.36−43 Carpentier et al. have used main group and transition metal complexes with fluorinated amino- and imino-alkoxides as well as related diimino-dialkoxide salen-like ligands for well-defined, single-site ring-opening polymerization catalysis.70 As a recent review article covers most of these K

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5). Variable-temperature 19F NMR studies showed the existence of dynamic processes in these complexes, which

Figure 6. Perspective view of [InMe2{OC(CF3)2CH2NHCH2CH2OMe}]2. Adapted with permission from ref 40. Copyright 2003 American Chemical Society.

A number of other metal complexes (Ti,70 Zr,70 Hf,70 Ru,39 Pd,38 Ir,215 and Cu37,43) based on fluorinated amino- and imino-alkoxide ligands have also been reported. X-ray diffraction studies revealed a monomeric structure in all cases, with the anticipated coordination according to the type of ligand framework. For example, the Cu(II) complex [Cu{OC(CF3)2CH2C(Me)NMe}2] shows the usual N2O2 squareplanar geometry with the fluorinated iminoalkoxide ligand arranged in the all-trans orientation. In contrast, the complex [Cu{OC(CF3)2CH2CHMeNHMe}2], which bears a more flexible and sterically congested fluorinated aminoalkoxide ligand, revealed a highly distorted N2O2 geometry with a dihedral angle of 33°. As expected, most of these compounds sublimed intact at relatively low temperatures and, in turn, proved to be valuable precursors for the deposition of thin films made of quite pure Cu, Pd, and Ru metals with a low level of carbon and oxygen impurities, as well as polycrystalline IrO2 thin films, or even patterned IrO2 nanowires. 3.1.2. Heterometallic Derivatives. 3.1.2.1. With Two Main Group Metals. Recently, Buchanan and Ruhlandt-Senge described several heterobimetallic complexes containing alkali and alkaline-earth metals utilizing the perfluoro-tert-butoxide (PFTB) ligand.33 The isostructural [MM′(PFTB)3(THF)4] [M = Na (M′ = Ba), M = K (M′ = Sr, Ba)] contains an alkalineearth metal and an alkali metal bridged by all three PFTB ligands. The divalent metals (Sr, Ba) display a distorted octahedral geometry stemming from their coordination with three additional THF molecules. The alkali metal coordinates with one additional THF to have distorted tetrahedral environments. These complexes have several intramolecular M···F interactions, mainly with the alkali metals as the divalent metals are, by and large, coordinatively saturated by three THF ligands. While sodium has two short Na···F interactions [2.555(3)−2.753(3) Å], the larger potassium forms as many as six such interactions [2.848(2)−3.205(4) Å]. On the other hand, the structures of [NaMg(PFTB)3(THF)3] and [KMg(PFTB)3(THF)(toluene)]∞ have two of the metal centers bridged by the two PFTB ligands, the magnesium center additionally coordinated to a terminal PFTB and a THF ligand. In [NaMg(PFTB)3(THF)3], the sodium center is coordinated to two THF ligands and forms three short Na···F interactions (av 2.7 Å) to complete its coordination number. On the contrary, the alkali metal K in [KMg(PFTB) 3 (THF)(toluene)]∞ has the unusual formal coordination number of two in the absence of additional THF molecules. A very narrow O−K−O angle of 62.32(6)° exposes one side of the metal, prompting it to be engaged in a series of K···Cπ [3.297(3)− 3.489(3) Å] and K···F [3.014(2)−3.163(2) Å] interactions to achieve steric saturation, the intermolecular nature of the later

Figure 5. Perspective view of [Ba{OC(CF3)2CH2N(C2H4OMe)2}2]. Adapted with permission from ref 36. Copyright 2001 Royal Society of Chemistry.

were attributed to the intramolecular cis−trans isomerization. On the other hand, Sr and Ba metal centers show almost similar coordination chemistry with polyether substituted fluoroalcohols.212 For complexes [M{OC(CF3)2CH2(OCH2CH2)2OMe}2] (M = Sr, Ba), all four oxygen atoms of the ligand are coordinated to the metal cation, giving a distorted dodecahedral arrangement. In contrast, the complexes [M{OC(CF3)2CH2OCH2CH2N(CH2CH2OMe)2}2] (M = Sr, Ba) exist as centrosymmetric dimers with two alkoxy oxygen atoms linking between two nonbonded strontium or barium metal atoms. Each metal center is 9-coordinated and has a distorted tricapped trigonal prismatic geometry. Chi et al. reported several Ga(III) and In(III) derivatives with fluorinated amino- and imino-alcohols for the elaboration of transparent M2O3 thin films by MOCVD.40,42 The 5coordinated mononuclear, heteroleptic derivatives [Me2In{OC(CF 3 )CHC(CF 3 )NCH 2 CH 2 NMe 2 }], 4 0 [Ga{OC(CF3)2CH2NMe2}2Cl],42 and [Ga{OC(CF3)2CH2C(Me) NMe}2Cl]42 have a trigonal-bipyramidal geometry with nitrogen donors located at the axial positions. The tetracoordinated complex [Me2Ga(OC(CF3)2CH2NMe2)], on the other hand, shows a distorted tetrahedral framework.42 Variable-temperature 1H NMR studies of these compounds suggested rapid N− Ga bond scission, followed by recoordination with possible change of the absolute configuration. On the other hand, the dimeric compound [Me 2 In{OC(CF 3 ) 2 CH 2 NHCH 2 CH 2 OMe}]2 contains a centrosymmetric, four-membered In2O2 ring, where each indium metal atom adopts a distorted trigonal bipyramidal geometry with two methyl groups being in equatorial positions (Figure 6).40 The bridging alkoxide groups are located in both axial and equatorial positions, while the nitrogen atom of the aminoalkoxide group is in the axial position with the N−In−O bond angle to the opposite, axial alkoxide group being 147.58(3)°. Using fluorinated iminoalkoxides (L1) and diimino-dialkoxides (L2) ligands, Carpentier et al. have described several monomeric G13 metal complexes such as Me2Al(L1), MeAl(L1)2, (OPri)Al(L1)2 [L1 = (CF3)2C(OH)CH2C(R1)N−R2 where R1 = Me or Ph, and R2 = Ph or CH2Ph], and RIn(L2) [R = Me, Cl, CH2SiMe3; L2 = (CF3)2(OH)CCH2C(Me)N−R′−NC(Me)CH2C(OH)(CF3)2 where R′ is either CH2−CH2 or 1,2-cyclohexylene].213,214 L

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Figure 7. Perspective view of [Bi2SnO(HFIP)5(OBut)3(THF)] (a) and [BiSnO(HFIP)3(OBut)2]2 (b). Adapted with permission from ref 153. Copyright 2012 American Chemical Society.

ometallics, the molecular compositions of which depended on the reaction conditions.153 Thus, the reaction of two reactants under solvent-free conditions at 60 °C for 30 min, followed by an extraction from THF and toluene, afforded [Bi2SnO(HFIP)5(OBut)3(THF)], whereas a similar reaction performed in THF at 60 °C for 30 min and extracted from hexane and toluene produced [BiSnO(HFIP)3(OBut)2]2. The asymmetric unit of [Bi2SnO(HFIP)5(OBut)3(THF)] consists of a μ3-O bridged trinuclear metal core incorporating a hexa-coordinate Sn atom, and penta- and tetracoordinate Bi atoms, Bi(1) and Bi(2), respectively (Figure 7a).153 These three metal atoms are further bridged around the perimeter by three alkoxy groups: one HFIP group that bridges Sn and Bi atoms and two OBut groups that bridge Sn and Bi and the two Bi atoms. In addition to the bridging alkoxy ligands and the bond with the central O atom, the three metal centers have terminal ligands to complete their coordination number: three THF for Sn, one HFIP and one THF for Bi1, and one But for Bi2. In [BiSnO(HFIP)3(OBut)2]2, the central planar Bi2O2 parallelogram in each molecule resides on an inversion center (Figure 7b).153 The Sn atoms then adopt a trans orientation relative to the ring, with each emanating from separate ring O atoms at an angle of 119.6(3)° and bonding at a distance of Sn1−O1 = 2.100(7) Å. One bridging and two terminal HFIP groups, and two bridging OBut groups, contribute to a distorted octahedral coordination environment around the Sn atom. If the stereochemically active lone pair on Bi(1) is included in the coordination environment, then the Bi center adopts a highly distorted octahedral arrangement. 3.1.2.2. With a Main Group Metal and a Transition Metal. The fluoride-bridged dimeric structure of the heterometallic [Li(THF)2TiF3(HFPB)2]2 contains the Li(μ-F)2Ti(μ-F)2Ti(μF)2Li cage.103 Each titanium atom is in a distorted octahedral environment constructed from four fluorine atoms and two oxygen atoms, whereas two fluorine and two oxygen atoms form a tetrahedral environment around the lithium centers. The zigzag polymeric structure of [K2Fe(HFIP)4]∞ contains two K and one Fe atoms connected by all four HFIP ligands. While the Fe(II) atom has a distorted tetrahedral environment, the coordination environments of two potassium are different due to the varying number of K−O bonds (2 or 4) and K···F interactions (7 or 8).228

resulting in the overall molecule to be a 1D coordination polymer. In the crystal structures of [NaB(HFIP)4(sol)x] [sol = THF (x = 2), DME (x = 1)], the sodium atom besides being coordinated to the solvent molecule(s) [Na−O = 2.259(2)− 2.349(1) Å] is connected to the B(HFIP)4 moiety via O atoms of the two bridging alkoxides [Na−O = 2.344(1)−2.528(2) Å] as well as 2 F atoms of the CF3 groups [Na···F = 2.461(1)− 2.610(1) Å] to achieve 6-coordination number.216 Strauss et al. have reported several Li−Al and Tl−Al heterometallics with fluorinated alkoxides.174,217,218 In the monomeric [LiAl{OC(Ph)(CF3)2}4], the Li and Al centers are connected through 2 bridging alkoxide oxygens. The Li also forms 4 strong Li···F interactions [1.984(9)−2.354(10) Å] to have a rare trigonal prismatic coordination sphere. The [Al(ORf)4]− anion also acts in an hexadentate O2F4 manner with respect to the Tl centers in [TlAl(HFTB)4] and [TlAl(HFIP)4]. However, unlike the LiO2F4 coordination unit, the Tl centers in [TlAl(HFTB)4] and [TlAl(HFIP)4] are additionally connected to a second [Al(ORf)4]− moiety via a relatively long Tl···F [3.045(6)−3.471(10) Å] to have TlO2F9 and TlO3F8 coordination spheres, respectively. In contrast, the dimeric structure of [LiAl(HFIP)4]2 is composed of two Li+ and two tetrahedral [Al(HFIP)4]− ions.174 Each [Al(HFIP)4]− ion donates three of its four oxygen atoms to the Li+ cations, one each to the two Li+ cations, and a third that bridges the two Li+ cations forming a planar Li2O2 core [Li−O = 2.016(5)− 2.533(6) Å]. Each Li+ cation is also bonded to six fluorine atoms from four different CF3 groups [Li···F = 2.366(6)− 3.327(6) Å].174 In fact, fluorinated alkoxyaluminates, [Al(ORf)4]−, and related fluorinated alkoxymetallates have been extensively used as weakly coordinating anions to stabilize a plethora of cations to afford interesting compounds for different applications (e.g., as ionic liquids, conducting salts, or catalysts). Because these are beyond the scope of this Review, interested readers are referred to a review and few other recent articles.219−227 The structure of [Li2Pb2(HFIP)6(C6H6)] can be viewed as two Pb(HFIP)2 units bridged by a four-membered Li2(μ3HFIP)2 ring.149 The Pb centers have face-on interactions with benzene rings, which “cap” each end of a Li2Pb2(HFIP)6 unit to form a 1D linear polymer alternating metal alkoxide and benzene units. The reaction of Sn(OBu t ) 4 with Bi2(HFIP)6(THF)2 yielded oxo-bridged Bi(III)−Sn(IV) heterM

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In the crystal structures of [MM′(HFIP)6(sol)] (M = Li, Ag; M′ = Nb, Ta; Sol = 1,2-C6H4F2, C6H5F, H2O), the monovalent metal center M (Li or Ag) is connected to the M′(HFIP)6 moiety via the O atom of the two bridging alkoxides as well as F atoms of the CF3 groups [M···F = 2.136(5)−2.856(1) Å].229 Additionally, M is also connected with fluorinated solvent molecules 1,2-C6H4F2 and C6H5F via F and C atoms, respectively (M···F = 2.049(3) Å; M···C = 2.316(6)− 2.972(6) Å), which not only help M to achieve higher coordination number but also connect different heterometallic entities to obtain supramolecular structures. Depending on the nature of the fluorinated alkoxides, two different structures are obtained for [K{Cu(ORf)2}] (ORf = PFTB, HFPB).185 While [K{Cu(PFTB)2}] contains a plannar eight-membered ring with a [−K−O−Cu−O−]2 arrangement, where Cu(I) and K(I) centers have O2 [Cu−O = 1.832(2)−1.852(2) Å] and O3F7 coordination environments (av K−O = 2.76 Å; av K···F = 2.96 Å), respectively, the polymeric structure of [K{Cu(HFPB)2}] contains bridging K···F/O interactions to have an O2F6 coordination environment for K and has strong cuprophilic interactions (av Cu···Cu = 2.55 Å). In the solvated [K(18C6)][Cu(ORf)2] (ORf = PFTB, HFTB)185 and [K(18C6)][M(ORf)3] (M = Fe, Co, Zn; Rf = C4F9),230 the K+ is encapsulated by 18C6 and hence the number of K···F/O alkoxide contacts is decreased. The Cu(I)−Al(III) heterometallics with fluorinated alkoxides present an interesting case.231 The unsolvated [CuAl(HFIP)4]2 is a centrosymmetric dimer that contains a plannar eightmembered ring with a [−Al−O−Cu−O−]2 arrangement. In addition to two Cu−O bonds (av 1.907 Å), each Cu(I) atom shows three additional interactions with fluorine atoms of the HFIP ligand [Cu···F = 2.789−2.834 Å]. The Cu atoms are further held together by a weak cuperophillic interaction [3.040(1) Å]. The solvated complex [Cu(1,2-F2C6H4)2][Al(HFIP)4], on the other hand, is an ion-pair in which the copper atom coordinates to two parallel 1,2-F2C6H4 rings [Cu−C = 2.085(8)− 2.869(7) Å] to afford [Cu(1,2-F2C6H4)2]+ cation. In the neutral complex [Cu(CH2Cl2)Al(HFIP)4], the solvated dichlorometahne molecule is coordinated to the copper atom via one of its chloride atom [Cu−Cl = 218.8(7) Å]. This (CH2Cl2)Cu moiety is further connected to the Al(HFIP)4 moiety via the O atom of the two bridging alkoxides as well as 4 F atoms of the CF3 groups (av Cu···F = 2.910 Å).231 Quite a few Na−Y(Ln) heterometallic complexes such as [Na 2 Y(HFTB) 5 (THF) 3 ], [Y[Na(μ-HFIP) 2 (THF)] 3 ], [Na2Y6(μ6-O)(TFE)18(THF)12], and [Na8Ln2(TFE)14(THF)6] (Ln = Sm, Y, Yb) are known.151,175,232 In compound [Na2Y(HFTB)5(THF)3], the yttrium cation is surrounded by two [Na(μ-HFTB)2(THF)]−, one HFTB ligand, and one THF ligands. On the other hand, the tetranuclear derivative [Y{Na(μ-HFIP)2(THF)}3] adopts a star-shaped structure, which can be described as a central Y3+ cation being coordinated with three peripheral [Na(μ-HFIP)2(THF)]− units bridged through two HFIP ligands (Figure 8).151 The common feature of these two structures is the distorted octahedral surrounding of the yttrium center, the sodium atoms being connected as Na(ORf)2 units. The coordination sphere of the sodium atoms is complemented by neutral ligands such as THF but also by secondary Na···F bonds. These interactions allow sodium to attain high coordination numbers in small aggregates (but without polymerization), ensuring volatility as well as stability in the vapor phase. The crystal structure of the compound [Na2Y6(μ6-O)(TFE)18(THF)12] (TFE = 2,2,2-

Figure 8. Perspective view of [Y{Na(μ-HFIP)2(THF)}3]. Adapted with permission from ref 151. Copyright 1995 American Chemical Society.

trifluoroethoxy) is based on hexanuclear [Y 6 (μ6 -O)(μTFE)12(TFE)6]2−, with two [Na(THF)6]+ cations ensuring the neutrality of the crystalline framework. The hexanuclear anion may be described as an octahedron of six yttrium(III) ions, with an μ6-oxo ion occupying the central position and each of the 12 edges being replaced by a μ-alkoxo group. Each yttrium ion is 6-coordinated by one central oxo ligand, four μalkoxo, and one terminal alkoxo groups, the first two ligands also ensuring the connections between the yttrium ions.232 On the other hand, the isostructural [Na8Ln2(TFE)14(THF)6] (Ln = Sm, Y, Yb) are composed of two cubanes and a double open cubane, with one face of an Ln1Na2O4 open cubane capped by an additional Ln1O2 layer.175 Caulton et al. described heterometallic zirconium-hexafluoroisopropoxide complexes of the type [M2Zr(HFIP)6(C6H6)x] (M = Na, Tl; x = 0−2).150 The solid-state structure of [Na2Zr(HFIP)6(C6H6)2] reveals a Na(μ-ORf)2Zr(ORf)2(μORf)2 structural unit centered around a ZrO6 octahedron with two terminal, linear [Zr−O−C = 179.7(4)°] alkoxides and four bridging, bent [Zr−O−C = 135.7(3)°] alkoxides. Sodium is equidistant from the two oxygen donors [2.413(4) Å]. This bent two-coordinate environment of sodium is supplemented by bonding to four fluorines at Na···F distances of 2.673(3)− 2.810(3) Å as well as by the π-cloud of a benzene ring. In contrast, the monosolvate [Na2Zr(HFIP)6(C6H6)] adopts a solid-state structure with an infinite chain of alternating benzene and Na2Zr(HFIP)6 links. All of the alkoxide groups are equivalent and bent [Zr−O−C = 143.4(2)°], and every Na forms three bonds to oxygen (av Na−O = 2.50 Å) and three Na···F bonds (av 2.72 Å). Benzene is bound equally to two centrosymmetrically related sodiums with an average Na−C distance of 3.13 Å. Unlike the sodium system, the thallium species is solvent-free. The zirconium center is surrounded by six fluoroalkoxides in a slightly distorted octahedron, capped on trans faces by thallium ions giving the overall molecular unit Tl(μ2-ORf)3Zr(μ2-ORf)3Tl. In addition to three regular Tl−O bonds, each thallium forms six Tl···F secondary contacts with distances ranging from 3.068(8) to 3.287(11) Å. These secondary Tl···F interactions with neighboring Tl2Zr(ORf)6 molecules lead to a two-dimensional lattice in the solid state.150 N

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Quite a few Ba−Cu and Ba−Y heterometallics containing a fluorinated alkoxo ligand have been reported in relation with their use as precursors for high Tc YBa2Cu3O7−x superconductors.176,181,233 The X-ray structure of [BaCu2(HFTB)6] shows the presence of a monomeric molecule containing trigonal planar three-coordinated Cu(II) and with the Ba atom closely coordinated to four alkoxide oxygens (2.636−2.644 Å) and more weakly by intramolecular interactions with 8 fluorides (2.94−3.14 Å). The three-coordinated CuO3 core is Y-shaped and planar. The Cu−O distances to the HFTB ligands that are also coordinated to Ba are longer [1.889(6) and 1.878(7) Å] than those found for the unshared HFTB ligands [Cu−O = 1.781(7) Å].233 The structure of [Ba2Cu2(HFTB)6(THF)2] consists of a square planar center containing all four metal atoms (two barium and two copper atoms) and four oxygen atoms. Each of the four oxygen atoms is part of an alkoxide ligand and bridges between one barium and one copper atom. Copper is in its preferred linear coordination environment and has only alkoxide ligands. Two bridging alkoxides between the two barium centers and one terminal THF for each barium ensure a trigonal bipyramidal environment for this metal. Six short Ba···F contacts (2.91−3.75 Å) increase the coordination number of the barium centers.181 The structure of the Ba−Cu heterometallic complex with TFTB ligand, [(Me3NO)2Ba2Cu4(TFTB)8], is different in the sense that it has two more O−Cu−O units inserted into the Ba−(O−Cu− O)2−Ba unit. This gives the Ba2Cu4 core structure an octahedral shape and the barium atom a square-pyramidal coordination environment. The overall structure is that of a paddlewheel (Figure 9). Each barium center has a square

Figure 10. Perspective view of [BaY2(μ-HFIP)4(thd)4]. Adapted with permission from ref 176. Copyright 1996 Elsevier.

atoms have a quite regular hexacoordinate surrounding, the angles O1−Y1−O3 and O2−Y2−O4 of the bridges (av 74.8°) being comparable with the bite of the β-diketonate moiety (av 74.7°), ensuring a trigonal prismatic geometry. The coordination polyhedron of the central barium atom formed by the four alkoxide type oxygen atoms is supplemented by interactions with eight fluorine atoms (Ba···F 2.9−3.16 Å), which act as secondary bonds leading finally to a 12-coordinated metal.176 3.2. Metal Complexes with Fluorinated Carboxylic Acids

Structurally, metal carboxylates are a very diverse group of coordination compounds due to the various coordination modes of the carboxylate ligands. These ligands can act not only in a monodentate, chelating, bridging, or bridgingchelating manner but also as a counterion. Fluorinated carboxylates show additional bonding modes due to possible M···F interactions (Scheme 12). Some selected X-ray crystalloScheme 12. Various Bonding Modes Observed for the Fluorinated Carboxylate Ligands

Figure 9. Perspective view of [(Me3NO)2Ba2Cu4(TFTB)8]. Adapted with permission from ref 181. Copyright 1997 American Chemical Society.

pyramidal environment, with the Me3NO ligand in the axial position and the O−Cu−O paddles making up the base of the pyramid. The penta-coordination of the barium center is supplemented by the three Ba···F contacts [3.12(1)−3.91(1) Å].181 The trinuclear heterometallic species [BaY2(μ-HFIP)4(thd)4] presents an open, bent structure (∠Y−Ba−Y 129°) with alternate Y and Ba atoms (Figure 10).176 The different metals are linked by fluoroisopropoxides bridges, while all of the βdiketonate ligands are borne by the yttrium atoms. The yttrium O

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graphically characterized metal complexes with fluorinated carboxylate ligands, which have been used (or hold potential) as precursors to get different forms of the inorganic nanomaterials, are summarized in Supporting Information Table S2. 3.2.1. Homometallic Derivatives. 3.2.1.1. Homoleptic Derivatives. The interest in the homoleptic metal fluorinated carboxylates, especially trifluoroacetate, stems from their potential use as starting materials for the preparation of heterometallic single-source precursors for the mixed-metal oxide and fluoride materials. As expected, such complexes are coordinatively unsaturated (and hence Lewis acids in nature), which promptly form adducts even with weak donor ligands. As a result, very few crystal structures of these unligated derivatives are known, the majority of which consisted of paddlewheel M2(O2CRf)4 units. In general, the homoleptic dimetal tetracarboxylates molecules exhibit polymeric chain structures where each paddlewheel M2(O2CRf)4 unit has axial interdimer M···O interactions of metal centers with the carboxylate oxygen atoms of neighboring units. Within this common motif, two major packing patterns of dimetal units are usually observed: (i) flat ribbon, as found for [M2(TFA)4] [M = Rh(II),234 Mo(II),235 Ru(II)236] (Scheme 13a), and (ii) zigzag ribbon, as

one-dimensional coordination polymer through Bi···arene coordination. 3.2.1.2. Heteroleptic Derivatives. 3.2.1.2.1. Main Group Metals. As stated above, the unligated metal carboxylate complexes are coordinatively unsaturated in nature and hence form adducts even with weak donor ligands. Alkali metal trifluoroacetate adducts such as [Li 5 (μ-TFA) 5 (μ-tetraglyme)2]240 and [Na(μ-TFA)(μ-TFAH)2]∞241 are known. In the case of large alkaline earth metals, this tendency is particularly observed, and several alkaline earth metal derivatives with trifluoroacetate ligands have been reported as possible precursors for the trifluoroacetate-MOD route to high Tc superconductors. Boyle et al. reported the synthesis of anhydrous TFA derivatives of alkaline earth metals by dissolution of the alkaline earth metal in trifluoroacetic acid, followed by crystallization from Lewis basic solvents, pyridine or tetrahydrofuran.80 In the monomeric [Mg(TFA)2(py)4], the magnesium center adopts an octahedral geometry by binding four equatorial pyidine solvent molecules and two axial, terminal TFA ligands. In the polymeric [Ca(TFA)2(THF)2]∞, the repeating unit is [Ca2(μ-TFA)3(THF)4] dimer, where two calcium centers are bridged by three TFA ligands. Two THF molecules bonded in a cis equatorial arrangement to each Ca complete a slightly distorted octahedral geometry around metal. These dimers are then linked together by a bridging-TFA ligand to afford a 1D [{Ca2(μ-TFA)3(THF)4}(μ-TFA)]∞. The polymeric complexes [M(TFA)3(py)]∞(H-py) (M = Sr, Ba) have a pyridinium salt to balance the charge. In the strontium complex, each metal center possesses two μ:η1,η1-TFA and one μ:η2,η1-TFA ligand and completes its irregular 7-coordination by binding to each of the TFA ligands from the next metal center. The μ:η1,η1- and μ:η2,η1-TFA ligands are arranged in a trans arrangement down the polymer chain. In contrast, the barium complex has only μ:η2,η1-TFA. Each Ba adopts an irregular 10-coordinated geometry using the six oxygen atoms of three μ:η2,η1-TFA ligands and three oxygen atoms from the neighboring μ:η2,η1-TFA. The coordination sphere is completed through the binding of a py solvent molecule. Certain structural trends are observed in the Ba−TFA complexes with polyether ligands, as reported by HampdenSmith.84,85 The dimeric structures of [Ba2(TFA)4(12-crown4)2], [Ba2(TFA)4(15-crown-5)2], and [Ba2(TFA)4(triglyme)2] are similar, with four bridging trifluoroacetate ligands and sideon bonding of the polyether ligands. Clearly, the polyethers used here are too small to encapsulate the barium atom at its mid point and prefer to sit to one side of the metal center, which opens the opposite side of the metal atom to the bridging trifluoroacetate ligands. The addition of the hydroxymethylene group to the 15-crown-5 ring results in a partial structural change in the resulting complex [Ba2(TFA)4(15crown-5CH2OH)2]. The dinuclear structure is retained with two trifluoroacetate ligands bridging the barium centers, but two other trifluoroacetate ligands switching from bridging to chelating. This structural change allows the hydroxy group of the 15-crown-5CH2OH ligand to bond to the metal center. In contrast, the [Ba(TFA)2(cryptand-222)] is monomeric with 10-coordinated barium center. This complex has two dangling trifluoroacetate ligands and a cryptand ligand bonded to the barium atom through all six ether oxygen atoms and both nitrogen atoms. Clearly, the steric effects of the ether chains of the cryptand preclude the TFA ligands from chelating the barium center, which is roughly centered in the cryptand cavity. The barium centers in the complexes [Ba(TFA)2(18-crown-

Scheme 13. Common Packing Patterns of Paddlewheel M2(O2CRf)4 Unitsa

a

Reprinted with permission from ref 236. Copyright 2006 American Chemical Society.

found for [Cu2(O2CRf)4] (Rf = CF3,118 C2F5237) (Scheme 13b). It is interesting to compare these homoleptic complexes with the mixed-carboxylate complexes [Cu 2 (TFA)(O2CC2H5)3]237 and [Ru2(TFA)3(O2CC2H5)],236 where the axial interdimer interaction of the dimetal units is based on the contacts of the M(II) centers with the O atoms of the propionate only. This is consistent with the greater electrondonating properties of the propionate groups in comparison to the trifluoroacetate ligand. Such axial binding results in an unprecedented 2D network arrangement for the former237 and a unique 1D chain motif (Scheme 13c) for the latter.236 An unusual compound [Ru2(TFA)5] that has a Ru25+ core with one less electron in the metal−metal antibonding orbital has also been reported.238 Noteworthy, bismuth atoms in [Bi2(TFA)4] do not form any intermolecular bismuth oxygen coordination. This compound, first reported by Frank et al.239 and then by Dikarev et al.,81 forms an adduct with C6Me6 and exists as a P

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[SnII4SnIV(μ3-O)2(TFA)8] contains one SnII and two SnIV centers bridged by a μ3-O and four μ-TFA groups, two between 2 SnIV centers and one each between SnIV and SnII centers.243 The related [SnII2SnIV2(μ3-O)2(μ-TFA)8]·C6H6 consists of a SnIV2O2 ring in which two μ3-oxygen atoms form a bridge to two symmetry-related tin(IV) atoms.244 The remaining coordination site of the three-coordinate oxygen atom is occupied by a Sn(II) atom of the cluster to yield an almost planar rhombus, where each of the Sn(IV) and Sn(II) centers are connected with 2 and 1 μ3-oxygen, respectively. The Sn(IV) and Sn(II) centers are further bridged by 8 μtrifluoroacetate groups to complete 6- and 5-coordination spheres, respectively, around them. The structure of a heteroleptic mixed-valent complex [SnII2SnIV2F4(TFA)4]· 2TFAH consists of an eight-membered ring with a −SnII−F− SnIV−F− arrangement, with adjacent SnII and SnIV atoms also being bridged by one or two TFA groups.245 The remaining two trifluoroacetate groups are dangling in nature. 3.2.1.2.2. Transition Metals and Lanthanides. The trivalent or mixed-valent metal complexes of the types [M3(μ3-O)(μO2CRf)7(L)2] and [M3(μ3-O)(μ-O2CRf)6(L)3] (M = V, Cr; Rf = CF3, CHF2; L = TFAH, THF, py, Et2O, toluene) have classical triangular structure.246−249 The tetranuclear structure of [V4(μ3-Cl)2(μ-Cl)2(μ-TFA)4(THF)6]250 has four V(II) centers bridged by two μ3-Cl, two μ-Cl, and four μ-TFA ligands to give an almost planar rhombus. Although all four V(II) centers are hexacoordinated, they are in two distinct coordination environments: one that is connected to both μ3Cl groups and has only one THF molecule coordinated, and the other that is bonded with only one μ3-Cl and has two THF molecules attached with it. On the other hand, the hydrated tetranuclear cation species of [V4(μ-OH)4(μ-TFA)4(H2O)8]Cl4 contains hydroxo and trifluoroacetato ligands bridging the four coplanar V(III) centers above and below the plane in an alternate arrangement around the ring. The distorted octahedral coordination environment of each V(III) center is completed by two water ligands.251 Unligated [M2(TFA)4] (M = Cu, Mo, Ru, Rh) readily forms adducts with neutral donor ligands. A significant amount of literature is available on derivatives of the type [M2(TFA)4(L)2], where L is a small donor ligand such as THF, diethyl ether, acetone, DMSO, etc.236,252−260 Generally, the metal−metal distance in these adducts is slightly longer than those in the unligated [M2(TFA)4]. The strong Lewis acidity of these unligated [M2(TFA)4] complexes is further confirmed by the isolation of their thermally stable adducts with

6)(py)] and [Ba(TFA)2(18-crown-6CH2OH)] too are 10coordinated but have both chelating and dangling trifluoroacetate ligands. These two complexes are structurally quite similar, except the fact that the tenth coordination site in the latter is filled by a hydroxyl group rather than a pyridine molecule in the former case. The reactions of [Ba(TFA)2]∞ with MDEA-H2 ligand show a remarkable versatility, resulting in three different complexes under different conditions.141,163,164 The neutral [Ba(η1TFA)2(η3-MDEA-H2)2] or the ionic [{Ba(η3-MDEA-H2)3}(TFA)2] are monomeric. On the other hand, the zigzag polymeric [Ba3(TFA)6(MDEA-H2)4]1∞ has two types of barium atoms, Ba1 and Ba2 present in a 2:1 ratio (Figure 11).163 The formula [Ba3(η3-MDEA-H2)2(μ-η3:η1-MDEA-

Figure 11. Perspective view of [Ba3(TFA)6(MDEA-H2)4]∞.163 Reprinted with permission from ref 163. Copyright 2007 Wiley-VCH.

H2)2(η1-TFA)2(η2-TFA)2(μ3-η2:η2:η1-TFA)2]1∞ reflects the complex nature of its structure, which can formally be seen as the association between [Ba2(η3-MDEA-H2)2(μ-η3:η1MDEA-H2)2(μ,η2:η1-TFA)2]2+ and [Ba(η2-TFA)2(η1-TFA)2]2− units via one oxygen and one fluorine atom [Ba2···F = 3.010(3) Å] of μ,η2:η1-TFA, acting overall as a unique triply bridging ligand bonded in two chelate η2(O,O) and η2(O,F) manners. Partial oxidation of Sn(TFA)2 leads to a variety of oxocontaining and/or mixed-valent complexes.242−245 The structure of [Sn6(μ3-O)2(TFA)8]·TFAH has two groups of Sn3(μ3O) units, which are linked by TFA group to form an extended structure.242 On the other hand, the asymmetric unit of the

Figure 12. Perspective view of [Ru2(TFA)4·(μ-C16H16)]∞. Adapted with permission from ref 236. Copyright 2006 American Chemical Society. Q

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Figure 13. Perspective view of [Et2Zn9(O)2(O2CC2F5)12] (a) and [Me4Zn10(OMe)4(O2CC2F5)12] (b). Adapted with permission from ref 86. Copyright 2013 American Chemical Society.

[Ag2(O2CC2F5)2(bipy)] and [Ag6(O2CC3F7)6(bipy)4] are polymeric structures with bridging bipy ligand, the [Ag(TFA)(bipy)]2 is a dimer with monodentately linked carboxylate. Molloy and co-workers recently reported several fluorocarboxylates of zinc(II) as potential precursors for ZnO and ZnF2 nanomaterials.86 In the polymeric chain structures of [EtZn(O2CRf)] (Rf = C2F5, C3F7), each carboxylate ligand bridges three zinc atoms in a μ3,η1(O):η1(O′):η1(O′) manner. The overall structure can be seen as the [EtZn(μ,η1(O):η1(O′)O2CRf]2 dimers being joined via four-membered Zn2O2 rings. In contrast, the dimeric [Zn(μ,η1(O):η1(O′)-O2CC2F5)2(η2TMEDA)]2 has four carboxylate ligands bridging between the two zinc atoms. With one chelating TMEDA ligand, each zinc center adopts a distorted octahedral ZnO4N2 coordination. While the overall dimeric nature and ZnO4N2 coordination is retained in the related [Zn(μ,η1(O):η1(O′)-O2CC3F7)(η1O2CC3F7)(η2-TMEDA)]2·(H2O), the presence of a water molecule, which bridges in a μ-manner between metals, causes two of the carboxylate groups to become monodentate. In turn, these are then engaged in intramolecular hydrogen bonding with the water molecule. The heteroleptic derivatives RZnO2CRf were found to be sensitive toward O2 and/or H2O, and yielded the Zn8, Zn9, and Zn10 aggregated products. The asymmetric unit of the cluster [Et4Zn8(μ4-O)(OEt)4(O2CC3F7)6] consists of two central fivecoordinate ZnO5 centers in a distorted trigonal bipyramidal geometry [Zn(1), Zn(2)] and two peripheral four-coordinate ZnCO3 units in a distorted tetrahedral coordination. In addition to the μ4-O, there are two μ3-OEt groups, two μ3,η1(O):η1(O′):η1(O′) carboxylates, one μ,η1(O):η1(O) carboxylate, and one μ,η1(O):η1(O′) carboxylate group. The octanuclear complex is generated by a 2-fold axis through the μ4-O at the center of the cluster. The nonanuclear zinc(II) cluster [Et2Zn9(O)2(O2CC2F5)12] could conveniently be described in terms of its asymmetric [EtZn4(O)(O2CC2F5)6] unit, which contains one octahedral ZnO6 moiety, two tetrahedral ZnO4 species, and one ZnCO3 moiety containing the ethyl group (Figure 13a). Four out of six carboxylate groups are in μ3,η1(O):η1(O′):η1(O′) bridging mode, while the other two act in the μ,η1(O):η1(O′) fashion. The oxo ligand O13 acts in a μ4-manner linking Zn1−Zn4 inclusive. The nonanuclear cluster is generated by a 2-fold axis through one octahedral Zn at the center of the cluster. The structure of

molecular elements such as I2 and S8, which act as a neutral bior polydentate ligand to afford either one-dimensional monoadducts [{Rh2(TFA)4}(η2-I2)]·I2 and [{Rh2(TFA)4}(η2S8)] or a pseudo-two-dimensional bis-adduct [{Rh2(TFA)4}3(η3-S8)2].261,262 Utilizing the ability of [M2(TFA)4] (M = Rh, Ru) to interact with the π-electrons of aromatic rings, Cotton et al. and others have described several new supramolecular complexes of the general formula [M2(TFA)4(L)x] (L = C6Me6, Ph2C2, C2H4, benzene, p-xylene, naphthalene; [2.2]and [2.2.2]paracyclophanes; x = 1 or 2) and [Rh2(TFA)4· (OCMe2 )]2·(C 4I 2).236,263−271 Supramolecular complexes, [{Rh2(TFA)4}x(L)] [x = 2 or 3, L = CH3Si(C5H4N)3, (C6H5)2Si(C5H4N)2, and (HO)C(C5H4N)3], have also been reported by combining the Lewis acid [Rh2(TFA)4] with angular di- or trifunctional N-donor ligands.265 Among these, the structure with a weak aromatic donor ligand [2.2] paracyclophane, [Ru2(TFA)4·(μ-C16H16)]∞, is presented briefly here.236 The 1D polymeric chain consists of the alternating diruthenium(II,II) units and [2.2] paracyclophane ligands (Figure 12). The organometallic network is built on interactions between the Ru(II) centers and the carbon atoms of aromatic rings. Each ruthenium atom has the two closest Ru−C contacts with [2.2] paracyclophane at 2.682(4) and 2.764(4) Å, resulting in an η2-type coordination of C16H16 with respect to each metal center. Interestingly, the internal (bridgehead) carbon atoms are involved in metal binding. This complex represents a rare example of the transition-metal complex having both aromatic rings of [2.2] paracyclophane involved in the coordination. Structures of fluorinated carboxylates of copper, silver, and gold (group 11 metals) and their use as precursors to get metallic thin films by CVD were reviewed in 2005.27 Since then, new copper(II) fluorinated carboxylate complexes with functional alcohols,141 quinoline,272 or N-heterocyclic carbine273 have been described. The copper TFA or PFB adducts [Cu2(TFA)4(iPrOC2H4OH)2] and [Cu2(PFB)4(THF)2] are discrete dinuclear species in the solid state displaying the typical paddle-wheel structures found for other carboxylate adducts with four bridging carboxylates having syn−syn conformation.141 The Lewis bases occupy the axial position of a distorted tetragonal pyramid of the five-coordinate metal centers. Silver(I) perfluorinated carboxylate complexes with bipy ligand were reported recently. 114 Whereas the R

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Figure 14. Perspective view of [Ce(TFA)3(DMSO)2]∞ (a) and [Ce2(TFA)6(DMSO)6] (b). Adapted with permission from ref 282. Copyright 2013 Royal Society of Chemistry.

DMF, and/or DMSO (x = 2 or 3), diglyme (x = 1)282] are known. The structure of the dinuclear complexes [Y(TFA) 3 (L) x ] 2 [L = OHC 2 H 4 O i Pr (x = 2), OHC2H4OC2H4OMe (x = 1)] is based on a Y2(μ,η2-TFA)4(η1TFA)2 core, the coordination sphere being completed by neutral ligands to achieve eight-coordination for the metals. As a result, the complex with isopropoxyethanol shows two different coordination modes, monodentate η1- and bidentate η2- for the alcohol, whereas 2-propoxyethoxyethanol (OHC2H4OC2H4OMe) is tridentate. The Ln−TFA complexes with TMSO ligand show diverse structures. For example, the [Ln(TFA)3(TMSO)2] derivatives form infinite chains of dimers for Ln = La but discrete dimers for Ln = Nd.281 Whereas all of the trifluoroacetate groups act as bridges between neighbor lanthanum ions in the former, only two-thirds of TFA act as bridging ligands in the case of Nd, the remaining one-third acting in chelating manner. The 8-coordinated metal centers have a distorted trigonal dodecahedral and antiprismatic geometry for La and Nd, respectively. With HMPA as neutral ligand in the previously quoted Er derivative having different coordination geometries, the TFA ligands are monodentate. Similarly, ionic compounds for the Ln(TFA)3(HMPA)3 adducts (Ln = La, Nd, Er), which are not isomorphous, are based on [Er(η1-TFA)2(HMPA)4]+, [Er(η1TFA)2(HMPA)2(H2O)3]+, or [Nd(η2-TFA)2(HMPA)4]+ cations stabilized by a TFA counteranion.279 Recently, a series of anhydrous cerium(III) trifluoroacetate complexes with neutral O-donor ligands, [Ce 2 (OAc)(TFA)5(DMF)3], [Ce(TFA)3(L)x] [L = THF, DMF, DMSO (x = 2), diglyme (x = 1)], and [Ce2(TFA)6(DMSO)x(DMF)y] (x = 6, y = 0; x = 4, y = 2), were reported as precursors for the synthesis of scintillating CeF3 nanocrystals. Depending on the reaction/crystallization conditions, the Ce−TFA adducts with DMSO ligand show two different structural types, either a 1D polymeric or a dinuclear discrete molecular form (Figure 14).282 In the straight chain polymer [Ce(TFA)3(DMSO)2]∞, all of the Ce atoms are symmetrically bridged by three TFA ligands, all bonded only in one fashion, that is, μ,η1:η1. Two DMSO molecules on each metal center complete an 8coordinated environment with geometry being close to dodecahedral. In contrast, the Ce2(μ,η1,η1-TFA)4(η2-TFA)2 core in the dinuclear derivative [Ce2(TFA)6(DMSO)6] has two different coordination modes for the TFA ligand, chelating and bridging bidentate, the coordination sphere being completed by the DMSO ligands to achieve nine-coordination

[Me4Zn10(OMe)4(O2CRf)12] (Rf = C2F5, C3F7) consists of a cage of five zinc atoms, out of which Zn1 is in the center of the cage and is six-coordinated in a ZnO6 environment and the Zn3 is five-coordinate ZnO5 (Figure 13b). The remaining three zinc centers are all four-coordinate; while Zn2 has ZnO 4 coordination, the Zn4 and Zn5 bear methyl groups and are ZnCO 3 in ligation. This Zn 5 cage is built by three μ3,η1(O):η1(O′):η1(O′) carboxylates, two μ,η1(O):η1(O′) carboxylates, two OMe groups, and two Me groups. The sixth carboxylate ligand then bridges to a symmetry-related second Zn5 cage to yield an overall Zn10 aggregate. A large number of lanthanide trifluoroacetate complexes (including those of scandium and yttrium) have been reported, often for their luminescence properties, which display a large structural diversity. A survey of the structures of the “maximally” hydrated rare earth(III) trifluoroacetates Ln(TFA)3·xH2O274 showed that different polymorphs can be obtained. Whereas the lanthanum and cerium adducts are twodimensional polymers, the praseodymium and lutetium derivatives are dinuclear with two metal centers bridged by four trifluoroacetate ligands. Several partially hydrated mixedligands such as [Ln(TFA)3(L)2(H2O)x] [L = pyzNO (pyrazine N-oxide) 2 7 5 or aza (2-azacyclononane) 2 7 6 ], [{M(TFA)2(H2O)6}(TFA)]·(18-crown-6) (M = Y, Eu),277 and [Gd(η2-TFA)(η1-TFA)2(η2-phen)2(H2O)]278 are known, the latter possessing a monomeric structure with 9-coordinated gadolinium center having a distorted tricapped trigonal prismatic geometry. On the other hand, the structure of [Sm2(μ,η2-TFA)4(η1-TFA)2(aza)2(H2O)2]·2aza276 consists of a centrosymmetric dimer built by bridging and monodentate coordination mode of the TFA ligands: four connecting the two metal ions, while two, one for each Sm, are monodentate. The coordination spheres around metal centers are completed by two oxygens from the aza ligands and a water molecule. An ion-pair, where the [Er(η1-TFA)2(HMPA)2(H2O)3]+ cation is stabilized by a TFA counteranion, is also known.279 In [H3O][Y3(μ2-O)3(μ3-OH)(TFA)3(en)3(H2O)6], the anion has both μ3-OH and H3O+ ions located on the C3 axis, with the former capping the trinuclear [Y3(μ-O)3(μ3-OH)]+2 unit. The Y−η1-O(TFA) bond lengths of 2.44(2) Å (av) are much longer than those found for other 8-coordinated Y-TFA derivatives.280 Relatively few anhydrous species such as [Ln(TFA)3(L)x] [Ln = Y, L = OHC2H4OiPr (x = 2), OHC2H4OC2H4OMe (x = 1);280 Ln = La, Nd, L = TMSO (x = 2);281 Ln = Ce, L = THF, S

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different at either end of the molecule, with η2-environment at the Rh end and η6-coordination at the Bi end, the six Bi···C contacts being in the range 3.359(3)−3.607(3) Å. This varying coordination results in alternating orientations of the Rh−Bi compound along the chain, with each pyrene coordinated exclusively to two bismuth centers or two rhodium centers. The discrete tetranuclear structure of [Bi2Pd2(TFA)10(L)2] (L = TFAH or H2O) can be viewed as built of two paddlewheel [BiPd(TFA)4] units connected by two μ-O,O′ trifluoroacetate ligands through bismuth ends (Figure 15).156 On the periphery,

for the metals. The Ce···Ce distances have an average value of 5.042 Å. 3.2.2. Heterometallic Derivatives. 3.2.2.1. Without Ancillary Coligands. The Lewis acidity of the unligated homometallic complexes has been exploited for the synthesis of homoleptic heterometallic derivatives (vide supra, section 2.2). Thus, the reactions of two homoleptic derivatives in the absence of any coordinating ancillary ligand proceed to give homoleptic heterometallic derivatives, which may or may not be solvated. Dikarev et al. have reported two different types of assembly of these heterometallic paddlewheel carboxylates in the solid state (Scheme 14): (i) through the formation of a Scheme 14. Different Assemblies of Heterometallic Paddlewheel Carboxylates in Solid State

Figure 15. Perspective view of [Bi2Pd2(TFA)10(TFAH)2]. Adapted with permission from ref 156. Copyright 2009 American Chemical Society.

“true” heterobimetallic unit bridged by carboxylate ligands and further supported by direct metal−metal bonding (Scheme 14a), providing homogenization at the molecular level, and (ii) through the Lewis acid−base adduct formation giving an ordered or statistical arrangement of homonuclear units in infinite chains (Scheme 14b). The first type is represented by Bi(II)−Rh(II)/Ru(II) trifluoroacetates where Bi(TFA)2 acts as a metalloligand toward transition metal (M = Rh, Ru) fragments.154 The [BiRh(TFA)4], obtained either from the gas phase154 or from the solution phase,157 forms an extended chain structure through axial coordination of the transition metal center to one of the Bi bound carboxylate oxygen atoms from the neighboring unit. Interestingly, this compound crystallized in its unsolvated form from the reaction mixture (toluene/Ph2O), even though the homometallic components [Bi2(TFA)4] and [Rh2(TFA)4] show high affinity toward Ph2O or toluene to form solvated homometallics.283 However, the packing of the extended 1D chains is different for the crystals obtained from the gas phase or solution phase. The packing of 1D polymeric chains is more loose for the polymorph grown from solution at room temperature, as the Bi−Rh and Rh−O axial bond lengths are all longer than those in the polymorph crystallized from the gas phase at elevated temperature [2.5571(6) vs 2.5493(3) Å and 2.534(5) vs 2.413(3) Å, respectively]. The heteroleptic products cis-[BiRh(TFA) 2 (O 2 C t Bu) 2 ] and [BiRh( T F A) 3 ( O 2 C M e )] , o b t a i n e d b y t h e r e a c t i o n o f [Rh2(O2CR)4] (R = Me, tBu) with the Bi2(TFA)4, also form infinite chains by coordination of bismuth-bound oxygen to the rhodium center of the neighboring molecule.155 Both complexes were shown to act as Lewis acids toward basic substrates. However, this behavior was only observed at the Rh terminals of the molecules. Similar to the homoleptic homometallic derivatives, these heterobimetallic paddlewheel complexes also form infinite structures when cosublimed with the aromatic pyrene moiety, whereby aromatic units coordinate at both ends of the molecule.284 Pyrene coordination is

each palladium atom maintains a square planar coordination of four oxygen atoms, whereas each of the two central bismuth atoms is 8-coordinated by seven carboxylate oxygens and one O atom coming from either a dangling η1-trifluoroacetic acid or a water molecule. There are no metal−metal bonds in this tetramer. In the structure of [BiPd(TFA)5(THF)3], bismuth and palladium atoms are bridged by four trifluoroacetate groups. There are additional chelating trifluoroacetate groups and two TFA molecules residing on the bismuth(III) center, raising its coordination number to eight. In addition, one THF molecule is loosely attached [2.883(6) Å] to the axial position of the palladium atom. The structure of [MoRh(TFA)4]285 conforms to the type b with statistical distribution of homodinuclear units in the solidstate structure (Scheme 14b) and can be considered as a result of cocrystallization of isomorphous paddlewheel molecules. In fact, the single-crystal X-ray investigation does not provide unambiguous evidence of whether the product consists of the initial homometallic or newly formed heterometallic paddlewheel units. Both metal atoms occupy the same crystallographic position, while the M−M and M−O distances are averaged with respect to the parent homometallic compounds. Nevertheless, the results of mass spectrometric and magnetic measurements clearly indicate that the title bimetallic carboxylate contains a statistical mixture of homometallic dimolybdenum and dirhodium units. Although closely related to the type (b), the structure of the heterometallic assembly [{Rh2(TFA)4}·(μ2-OCMe2)·{Cu4(TFA)4}] is interesting.158 In this complex, two different Lewis acids interact concomitantly with the oxygen atom of a weakly basic carbonyl function of acetone (Figure 16). The bridging μ2-O coordination of the acetone molecules within the [{Rh2(TFA)4}·(μ2-OCMe2)· {Cu4(TFA)4}] unit is weak and asymmetric with the Rh−O T

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(TFA) 4 (diglyme)] (Ln = Y, Gd), [Na(triglyme) 2 ][Y2 (TFA) 7(THF)2], and [Na 2Y(TFA)5 (tetraglyme)] are among the first heterometal−organic single source precursors for heterometallic fluoride nanomaterials. The [Ln(μ-η1:η1TFA)2(μ3-η2(O,F):η1:η1-TFA)2Na(η3-diglyme)]∞ is a 1D polymeric Ln-TFA chain containing doubly and triply bridging TFA ligands, the latter also connecting peripheral [Na(η3diglyme)] units via one O and one F centers (Figure 17a). In comparison to the eight-coordinated lanthanide centers, the sodium atoms are 7-coordinate, each being connected with three oxygen atoms of a diglyme as well as one oxygen and one fluorine atoms each of two triply bridging TFA moieties. The ionic [Na(η4-triglyme)2][Y2(μ-η1:η1-TFA)7(THF)2] consists of 1D-polymeric chain of the composition [Y 2 (μ-η 1 :η 1 TFA)7(THF)2]−, the charge for this unit being balanced by a discrete [Na(η4-triglyme)2]+ cation. The yttrium centers are bridged by four and three η1:η1-trifluoroacetate ligands in an alternate manner, a tetrahydrofuran molecule on each yttrium center ensuring 8-coordination number and a bicapped trigonal prismatic geometry for the metal. The sodium ion in the [Na(η4-triglyme)2]+ cation is coordinated by eight oxygen atoms of the two perpendicular triglyme ligands, resulting in a dodecahedral geometry for it. The structure of [Na2Y(TFA)5(tetraglyme)] is composed of the repetitive [Y2Na4(μη 1 :η 1 -TFA) 4 (μ 3 -η 1 :η 2 (O,O):η 1 -TFA) 2 (μ 3 -η 2 (F,O):η 1 :η 1 TFA)2(μ3-η1:η1:η1-TFA)2Na(μ-η5:η1-tetraglyme)2] units, each unit being associated with other such unit via three μ-η1:η1-TFA groups, thus giving rise to a 1D polymeric chain (Figure 17b). Each yttrium center is 8-coordinated, whereas the sodium metal centers have two different coordination environments. The most peripheral 7-coordinate sodium atoms are bound by five oxygen atoms of a tetraglyme ligand and one oxygen each from the triply bridging TFA ligands. In contrast, the two sodium centers closer to Y-TFA chain are octahedral. A rare secondary F···M bond in [NaY(TFA)4(diglyme)] [F23···Na1 2.695(6) Å, F16···Na2 2.655(6) Å] and [Na2Y(TFA)5(tetraglyme)] [F21··· Y1 2.955(5) Å, F1···Y2 3.012(6) Å] is a rather unique feature for the metal trifluoroacetate complexes. Hubert-Pfalzgraf et al. investigated reactions between trifluoroacetate complexes of yttrium, barium, and copper in the presence of different aminoalkoxides to obtain hetero-

Figure 16. Perspective view of [{Rh 2 (TFA) 4 }·(μ 2 -OCMe 2 )· {Cu4(TFA)4}]. Adapted with permission from ref 158. Copyright 2004 American Chemical Society.

and Cu−O distances being 2.217(1) and 2.720(2) Å, respectively. 3.2.2.2. With Ancillary Coligands. Many researchers have investigated reactions between metal trifluoroacetate complexes in the presence of anciliary ligands with different denticity as a mean to afford heterometallic precursors for high tech materials. Using 2-hydroxylpyridine and amino alcohols such as 2,6-bis[(dimethylamino)methyl]4-methylphenol (bdmmpH) and 1,3-bis(dimethylamino)propan-2-ol (bdmapH) as ancillary ligands, Wang et al. have reported several Cu(I)−Ln(III)/ Y(III) and Cu(I)−M(II) (M = an alkaline earth metal) trifluoroacetate heterometallics.286−293 The structures and compositions of these mixed metal complexes were highly dependent on the stoichiometry of the starting materials, the solvents, and the adventitious water molecules in the system. In view of the availability of a review article on the structures of these complexes,162 further discussion is omitted here. 3.2.2.2.1. A Main Group Metal and a Transition Metal. We recently investigated reactions between yttrium(lanthanide) trifluoroacetate and sodium trifluoroacetate in the presence of glyme ligands to synthesize Na−Y(Ln) heterometallic precursors for lanthanide-doped NaYF4 up-converting nanomaterials.64−66 The isolated complexes [NaLn-

Figure 17. Perspective view of [NaY(TFA)4(diglyme)] (a) and [Na2Y(TFA)5(tetraglyme)] (b).65 Copyright 2012 Royal Society of Chemistry. U

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Figure 18. Perspective view of [Ba3Cu2(TFA)6(DMEA)4(MeOH)2·2MeOH]1∞. Adapted with permission from ref 164. Copyright 2004 Elsevier.

metallic precursors for high T c superconductors. 163,164 [Ba3Cu2(μ,η2-TFA)6(μ,η2-DMEA)4(MeOH)2·2MeOH]∞ is a 1D coordination polymer.164 Its structure can be considered as an assembly between a trinuclear Ba3(μ,η2-TFA)6(MeOH) unit and two peripheral Cu(η2-DMEA)2 molecules, the latter being connected to the Ba2 and Ba2′ centers via bridgingchelating dimethylaminoalkoxide ligands (Figure 18). The various Ba atoms are connected by triple bridges of μ,η2trifluoroacetate ligands. Whereas Ba1 is octa-coordinated, the presence of methanol as an additional ligand allows Ba2 and Ba2′ to reach 10-fold coordination. Copper displays a distorted square planar geometry. The structure of [Ba(DMAP)4Cu4(TFA)6]·THF can best be described as the barium ion being sandwiched between two dicopper [Cu2(μ-DMAP)2(μ, η1:η1-TFA)3] units, and the overall geometry can be seen as two Cu2Ba triangles sharing a common Ba corner.165 All of the metal centers are connected through bridging TFA and DMAP ligands. The oxygen atoms of DMAP ligands function as μ3-bridges between each two copper and one barium atom. Four out of the six TFA ligands are bridged between copper and barium atoms, while the remaining two are bridged between the two copper centers in the dicopper units. The O4N around each copper atom is made up from two bridging oxygen atoms of TFA ligand, two triply bridged oxygen atoms of the DMAP ligands, and a coordinating nitrogen atom of the DMAP ligand. A short Cu−Cu interaction of av 2.93 Å in each dicopper unit then completes a distorted octahedral environment around copper center. The 8coordinated environment around the barium atom is made up by four TFA ligands and four triply bridging oxygen atoms of the DMAP ligands, the geometry being a distorted square antiprismatic. The unprecedented structure of [Pb2Ti2(μ3O)2(μ2-TFA)8(THF)6] is based on the Pb2O2Ti2 core with each of the four M···M′ edges bridged by two carboxylate ligands. Additional O donors, two THF molecules for Pb and one THF for Ti atoms, complete the coordination numbers for the metal centers. The geometries of 8-coordinate Pb and 6coordinates Ti atoms are square-prismatic and octahedral, respectively (Figure 19).294 3.2.2.2.2. Two Transition Metals. The Y(III)−Cu(I) heterometallic [{CuY 3 (μ 3 -OH)(μ-η 3 :η 1 -MDEA-H) 3 (μ 3 η3:η1:η1-MDEA-H)2(η2-TFA)(μ,η2-TFA)2}(TFA)2(H2O)] is

Figure 19. Perspective view of [PbTi(μ3-O)(μ2-TFA)4(THF)3]2. Adapted with permission from ref 294. Copyright 2013 American Chemical Society.

an ionic complex, where Y1 and Y2 atoms of the triangular Y3 unit are connected to the copper atom via a μ3-OH group as well as by oxygen atoms of two μ,η2-trifluoroacetates and one deprotonated oxygen of a N-methylethanolaminate ligand (Figure 20).163 The third trifluoroacetate ligand is linked in a chelating manner to Y2, whereas all five aminoalcohols are only partially deprotonated affording MDEA-H ligands acting either in μ or in μ3 manner. The yttrium atoms are either 8- (Y1 and Y3) or 9-coordinated (Y2), whereas the copper atom has a 6coordinate environment (O5N). Mazhar et al. have reported several transition heterometallic trifluoroacetate single source precursors for mixed-metal oxide thin films and composites, either without167 or with assembling aminoalcohol ligands such as N,N-dimethylethanolamine ( DME A -H ) o r N, N-dimethylamino-1-propanolato (DMAP).165,166,169−173 The crystal structure of [Fe2Ti4(μO)6(TFA)8(THF)6]167 and [Mn2Ti4(μ-O)2(μOH)4(TFA)6(THF)6]168 can best be discussed as two [MTi2(μ3-O)2(TFA)4(THF)3] units connected via four μ-O between Ti(IV) centers (Figure 21). Each asymmetric part is based on a triangular FeTi2 fragment with a μ3-O at the center of the triangle. Each Ti center is further connected with an Fe atom via two μ2-O,O′ TFA ligands. All of the TFA groups act as bridging ligands. Two THF molecules on Fe and one THF V

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atoms of a chelating acac, two O from two OH groups, and one oxygen atom each from TFA and DMEA ligands, the NO5 environment around outer metal centers is achieved by one chelating DMEA-H ligand, two bridging, and one terminal TFA and a OH group. Each copper center has square pyramidal geometry with one bridging each TFA, one DMEA, one acac, and one OH ligand. In the octanuclear structure of [Cd4Ti4(μ3O) 6 (μ-DMEA) 4 (μ,η 1 :η 1 -TFA) 4 (η 1 -TFA) 4 (μ,η 2 :η 1 -OAc) 4 ], metal centers are connected by triply bridging oxo ligand, doubly bridging DMEA, TFA and OAc ligands, as well as metal···metal interactions (Figure 22b).173 In the asymmetric Ti2Cd2 unit, the first Cd is bonded to one chelating acetate oxygen atom, two chelating DMEA oxygens, and two monodentate TFA oxygens, while the second Cd center is bonded to one chelating acetate oxygen, two chelating TFA oxygens, and two triply bridging oxo ligands. There are also direct bonds present between the metal centers [Cd1···Ti4 = 3.313(1) Å, Ti4···Ti3i = 2.834(1) Å]. The coordination environment of cadmium atom is distorted square pyramidal, while those of titanium centers have a coordination number of six with an octahedral geometry. In the complexes [ZnCu3(μ3-OH)(TFA)3(L)3X]·THF (L = DMEA, DMAP; X = Cl and/or Br),170,171,295 the zinc atom forms the apex and the three copper ions constitute the base of the tetrahedral ZnCu3 core. Out of the four triangular faces of this ZnCu3 tetrahedron, three are centrally occupied by the oxygen atoms of the DMEA anions, acting as a μ3-bridging ligand between the zinc atom and two copper atoms. The center of the fourth face consisted of a μ3-OH group connected to all three copper ions. The zinc cation is then bonded to the halide ion to complete the preferred tetrahedral coordination geometry. On the other hand, the copper ions have a tetragonally elongated octahedral geometry. In addition to two μ3-O (DMEA) atoms and a μ3-OH anion, each of the copper centers is also bonded to one nitrogen atom (DMEA) and the two oxygen atoms of two TFA ligands, the latter bridging the copper ions and forming a calixarene like cavity that is occupied by the solvate THF molecule. The related complex [Zn(TFA)4Cu3(DMEA)4],171 where the OH group is replaced by a TFA ligand, shows a cubane-like central core M4O4 with metal and oxygen atoms occupying alternate corners of the cube. All of the TFA and DMEA ligands are coordinated in a doubly bridging manner.

Figure 20. Perspective view of [{CuY3(μ3-OH)(MDEA-H)5(TFA)3}(TFA)2].163 Reprinted with permission from ref 163. Copyright 2007 Wiley-VCH.

on each Ti complete the 6-coordinate environment around each metal. For simplicity, the complex [Co4Cu2(μ-OH)2(TFA)8(DMEA)2(THF)4]·0.5C7H8172 can be described as the central zigzag tetramer [Co4(μ-OH)2(THF)4(TFA)8]2− being coordinated to two [Cu(DMEA)]+ peripherally (Figure 22a). In Co4 tetramer, the terminal Co have from three TFA ligands, one μ3OH, and two terminally coordinated THF molecules, to complete octahedral geometry. On the other hand, 6coordinate environment around the central Co centers is achieved by three bridging TFA, two bridging DMEA, and one μ3-OH ligand. Each copper center has square pyramidal geometry with two bridging TFA groups, one bridging DMEA, and one μ3-OH ligand. All of the TFA anions act in a μ-O,O′ coordination mode between two metal atoms (Cu− Cu or Cu−Co). The structure of [Cu2M4(OH)(TFA)6(acac)2(DMEA)2(DMEA-H)2] (M = Co, Ni)166 is related to above structure of Co4Cu2 heterometallic and can be viewed as the central zigzag tetramer [M4(μ-acac)2(DMEA-H)2(η1-TFA)2(μTFA)4(μ3-OH)2]2− being coordinated to two [Cu(DMEA)]+ peripherally via OH, 1 TFA, and DMEA ligands. The metal centers in M4 (M = Co, Ni) are 6-coordinated. Whereas the central metals have O6 environment provided by two oxygen

Figure 21. Perspective view of [Fe2Ti4(μ-O)6(TFA)8(THF)6]. Adapted with permission from ref 167. Copyright 2011 Elsevier. W

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Figure 22. Perspective view of [Co4Cu2(μ-OH)2(TFA)8(DMEA)2(THF)4] (a) and [Cd4Ti4(O)6(DMEA)4(TFA)8(OAc)4] (b). Adapted with permission from refs 172 and 173, respectively. Copyright 2008 Elsevier and 2012 Royal Society of Chemistry, respectively.

3.3. Metal Complexes with Fluorinated β-Diketones

of the positively charged metal ions from the intermolecular interactions by surrounding hydro- or fluorocarbon shells. The degree of oligomerization depends on many factors, including the nature of the metal ion and the electronic and steric effects of the ligands. Some selected X-ray crystallographically characterized metal complexes with fluorinated β-diketonate ligands, which have been used (or hold potential) as precursors to get different forms of the inorganic nanomaterials, are listed in Supporting Information Table S3. 3.3.1. Homometallic Derivatives. 3.3.1.1. Without Ancillary Coligands. 3.3.1.1.1. Main Group Metals. Because of their volatility and stability, early work concentrated on socalled “first generation” homoleptic metal fluorinated βdiketonates [M(β-dikF)x]. In the absence of any ancillary coligand, the large and electropositive alkali metals often form polymeric complexes such as [Li(ptac)]∞296 and [Cs(β-dikF)]∞ (β-dikF = hfac, tfac). 145 These homoleptic fluorinated complexes also have a strong tendency to be coordinated by water and donor solvents, often leading to the formation of hydrated complexes such as [Cs(β-dikF)(H2O)]∞ [β-dikF = ptac, btac (benzoyltrifluoroacetonate)].145 Even use of functional β-diketonates often fails to produce monomeric homoleptic derivatives. For example, the fluorinated βketoiminate complexes [Li{(CF 3 )C(O)CHC(CF 3 )NHCH2CH2NEt2}]2297 and [Na{(CF3)C(O)−CHC(CF3)NHR}]4 (R = CH2CH2OMe, CH2CHCH2)298 are dimeric and tetrameric, respectively, and consist of the typical Li2O2 rhombohedron and Na4O4 cubane core arrangement. For the sodium complexes, there are also one or two weak Na···F interactions. Uncommonly, there is also a strong allyl to sodium π interaction when R is CH2CHCH2 (Figure 23), resulting in an enhanced volatility of this complex than the one with the CH2CH2OMe group. Group 2 metal β-diketonates are among the most studied precursors for MOCVD process, due to their generally higher volatilities, improved chemical stabilities, and better masstransport properties. The sterically crowded nonfluorinated βdiketone 2,2,6,6-tetramethylheptanedione (H-thd) was the first choice to prepare volatile group 2 CVD precursors.25,26,28,30 The large size and the strong polarity of the alkaline earth metals, however, lead to the formation of oligomeric complexes [M(thd)2]n (M = Ca, Sr, n = 3; Ba, n = 4),299,300 which are usually unstable at high temperatures for prolonged periods of

As compared to alkoxide and carboxylate ligands, a βdiketonate ligand mostly acts in a chelating manner to form stable metal complexes. However, with large metal ions of pronounced electron deficiency, it may function in a bridging fashion as well (Scheme 15), although this tendency diminishes significantly in the presence of Lewis bases. High volatility of the metal β-diketonates is associated with an efficient shielding Scheme 15. Various Bonding Modes Observed for the Fluorinated β-Diketonate Ligands

X

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Figure 24. Perspective view of [(μ,η1:η1-hfac-4H)(η2-hfac)Al(THF)]2. Adapted with permission from ref 88. Copyright 2010 American Chemical Society.

Figure 23. Perspective view of [Na{(CF3)C(O)−CHC(CF3)NHCH2CHCH2}]4. Adapted with permission from ref 298. Copyright 2000 Royal Society of Chemistry.

ketoiminate as CVD precursors.40 The heteroleptic indium(III) complex with this functionalized β-ketoiminate ligand, [Me2In{OC(CF3)CH2C(CF3)NCH2CH2NMe2}], exists as a monomer, where the 5-coordinate indium center has highly distorted trigonal bipyramidal geometry. There is a marked difference between the two In−N bond lengths, with the In−N dative bond 2.428(2) Å longer than the ketoiminate In−N bond of 2.321(2) Å. In the solid state, [Pb(ttac)2]∞304,305 and [Pb(fod)2]2306,307 are 1D zigzag polymer and dimer, respectively, via bridging βdiketonate ligands. Additional Pb···F interactions (3.219−3.451 Å) complete the preferred 8-coordination environment around the lead(II) center, which have “hemidirected” geometry due to the presence of a stereoactive lone pair of electrons on lead(II). In the homoleptic derivative [Sb(fod)3], the antimony adopts a pseudo-seven-coordinate geometry with three bidentate ligands in addition to the lone pair inherent on the metal.308 Each ligand binds to the antimony in an anisobidentate manner, with one short [2.064(2) Å] and one long Sb−O bond [2.441(3) Å]. Each short Sb−O bond is thus trans to a long bond of the same type, the C3F7 groups all lie on the same octahedral face and are associated with the short Sb−O bonds, while the tBu groups are on the more open octahedral face. Similar attempts to prepare homoleptic derivative with symmetric hfacH ligand generated a partially substituted derivative of the formula [Sb(EtO)(hfac)2]. Interestingly, the X-ray structure of this ethoxo-bridged dimer shows an unprecedented coupling of the two β-diketonate ligands on the Sb(III) center to form a functionalized 3,4-dihydro-2H-pyran ring.308 The crystal structures of the 6-coordinated heteroleptic phenylbismuth(III) bis(hexafluoroacetylacetonate) adducts with neutral O- and N-donor ligands, [BiPh(hfac)2(L)] (L = H2 O, Me2 CO, THF, DMA, DMSO, PhCN), reveal a pentagonal pyramidal geometry around the metal center (hemidirected geometry due to the presence of stereochemically active lone electron pair).89 The phenyl group occupies the apical position of the bismuth coordination sphere, while the equatorial plane is formed by two chelating hfac ligands and a coordinated Lewis base molecule. The mixed-ligand complex [BiPh(hfac)(μ,η2:η1-TFA)]2 is a dimer formed by bridging trifluoroacetate groups.89 The pentagonal pyramidal geometry around each bismuth centers contains a void, presumably occupied by a stereochemically active lone electron pair. The overall molecule could be seen as two fused pentagonal pyramids sharing one common edge. Two remarkable

time. Several efforts were, therefore, made to reduce the oligomeric character of these β-diketonates, including use of fluorinated ligands. However, such a strategy does not always work, especially for larger barium atom, as indicated from the tetrameric structure of [Ba(ptac)2]4.301 Therefore, a combined strategy of using fluorinated β-diketonate and a neutral ancillary ligand was employed resulting in the monomeric complexes with excellent thermal stability and mass transport properties (vide infra, section 3.3.1.2). In view of the availability of review articles on the structural aspects of group 2 metal βdiketonates,28 further discussion is abandoned here. The heteroleptic complexes [Al(hfac)2(OR)]2 (R = H, Pri) have dimeric structures where aluminum centers are bridged by two hydroxyl or isopropoxide ligands. 88 Whereas the [Al2(hfac)4(μ-OPri)2] crystallizes selectively from CH2Cl2 at −30 °C in the D, L form, the hydroxo-bridged crystallizes selectively as the meso isomer. This difference in the coordination mode, simply changing the bridging group, likely reflects the significant flexibility of the (hfac)− ligand. On the other hand, the molecule [Al3(μ-OPri)4(η2-hfac)5] has a trinuclear structure, which can formally be described as the central [Al(hfac)(μ-OPri)4]2− moiety connecting the two terminal [Al(hfac)2]+ cations via μ-bridging isopropoxide groups. While octahedral [Al(hfac)3] is monomeric with three hfac being coordinated to the aluminum atom in a bidentate fashion,302 the centrosymmetric dimeric structure of the unexpected molecule [(μ,η1:η1-hfac-4H)(η2-hfac)Al(THF)]2 contains one THF molecule, one chelating hfac ligand, and one reduced ligand (hfac-4H)2− bridged in a μ,η1:η1- manner via the oxygen atom (012) at the reduced carbon atom (C4) (Figure 24).88 The central cycle Al1i−O12−O12i−Al1 is planar, with a spatial distance between the two Al of 2.990 Å, even larger than that observed in the Al2(hfac)4(μ-OPri)2 (2.860 Å), which can be attributed to the larger steric hindrance induced by the neighboring groups at the quaternary carbon atom C4. The gallium centers in the compounds, [Mes2Ga(hfac)] and [Mes2Ga(hfac)(py)] (Mes = mesityl), are in a distorted fourcoordinate tetrahedral environment. The latter has an unusual bonding mode in the sense that it has one Ga−O bond consistent with literature values [1.971(8) Å] and one much longer Ga−O bond [2.590(8) Å], which is consistent with a dative interaction.303 Chi and co-workers used a β-ketoimine, synthesized by condensation of N,N-dimethylethylene diamine with Hhfac, to form the donor-functionalized metal βY

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homometallic bismuth oxo clusters with hfac− ligand were reported by Dikarev et al.309 The cluster [Bi9O7(hfac)13] contains bismuth atoms in two distinctly different chemical and coordination environments. Six bismuth atoms form a slightly distorted octahedron, seven faces of which are centered by oxo groups. The eighth face is unsymmetrically capped with an oxygen atom from one of the β-diketonate ligands. Three of the seven oxo groups centering the octahedron act as μ4-bridges to which Bi(hfac)x (x = 3 or 4) arms are attached. The second nearly spherical cluster [Bi38O45(hfac)24], which is among the largest known homometallic bismuth oxido clusters, has a fascinating giant [Bi38O45]24+ core with 24 β-diketonate groups located on its surface (Figure 25). Within the bismuth oxido

groups. The central unit consists of an octahedron of bismuth atoms, each face of which is centered by an oxo ligand. In addition, a μ6-O atom, which is unique for homometallic bismuth oxido clusters, resides at the center of the octahedron, on an inversion center. 3.3.1.1.2. Transition Metals. The real interest in the homoleptic transition metal β-diketonates, especially those of divalent transition metals, stems from their potential use as starting materials for the preparation of heterometallic singlesource precursors for the mixed-metal oxide and fluoride materials. This involves reaction between two unsolvated βdiketonates, with at least one of them being coordinatively unsaturated. The structures of these heterometallics are built on the Lewis acid−base interactions that cannot withstand the presence of donor solvent molecules. However, the unsolvated divalent transition metal derivatives with fluorinated βdiketonates are considerably more difficult to isolate because these fluorinated ligands increase the Lewis acidity of the metal center, thus making them more susceptible toward water and donor solvents. Unligated β-diketonates of many transition metals, mostly those that do not essentially require a regular octahedral environment, have been obtained and characterized structurally.160,252−255,310−313 The solid-state structure of [Mn(hfac)2] contains trinuclear molecules, in which one manganese center is sandwiched between the two [Mn(hfac)3] fragments.82 The geometry of the metal center in the [Mn(hfac)3] units is a distorted octahedral with three of the Mn−O bond distances (av 2.22 Å) being essentially longer than the others (av 2.11 Å) because of the bridging coordination of oxygen atoms to the central Mn in the former. The trinuclear molecule is held together by six bridging Mn···O contacts between the central Mn and βdiketonate oxygens at 2.20 Å. The dinuclear structure of [Fe(hfac)2] is formed by four Fe···O bridging contacts between two Fe(hfac)2 units at 2.87 and 3.02 Å, and with inclusion of these interactions the coordination environment of iron can be considered as a very distorted octahedral (Figure 26a). No strong interactions were observed between the dinuclear complexes, with the shortest Fe···O intermolecular contact measured at 3.59 Å. The solid-state structures of Co(hfac)2 and Ni(hfac)2 consist of trinuclear molecules, in which three M(hfac)2 units are held together by six M···O interactions (M = Co, Ni). Unlike previous trinuclear molecule [Mn(hfac)2], each

Figure 25. Perspective view of [Bi38O45(hfac)24]. Adapted with permission from ref 309. Copyright 2006 Wiley-VCH.

core, a central Bi6 unit can be identified whose metal atoms are connected only to oxo centers. All other 32 bismuth atoms are coordinated to both oxo and hfac− ligands, although eight of those that are directly attached to the μ4-O atoms of the central core make relatively long (2.69−3.08 Å) contacts with capping

Figure 26. Perspective view of [Fe(hfac)2] (a) and [Co(hfac)2] (b). Adapted with permission from ref 82. Copyright 2008 American Chemical Society. Z

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cobalt/nickel atom here has two chelating β-diketonate ligands and fulfills its distorted octahedral coordination environment with two Lewis acid−base contacts (2.09−2.42 Å) to the oxygens from the neighboring M(hfac)2 unit/units (Figure 26b). In contrast to above structures, the analogues copper(II) and zinc(II) complexes with hfac ligand are monomeric in nature. The [Cu(hfac)2],310 [Cu(ptac)2],311 [Cu(F6-thd)2],311 and [Cu(β-dikFSi)2]312 exhibit a square-planar geometry with long Cu···F intermolecular contacts for the first two complexes. Such interactions were absent in the [Cu(F6-thd)2].311 The unsymmetrical substitution of the ptac and F6-thd ligands allows for the formation of geometrical isomers. Only one geometrical isomer, cis in the crystals of [Cu(ptac)2] and trans in the [Cu(F6-thd)2],311 was observed. Soldatov et al. have previously isolated crystals of [Cu{F3CC(O)CHC(O)CMe2(OMe)}2] that contained only trans-isomer and a mixture of cis- and trans-isomers.313 Contrary to the square-planar geometry found in the above Cu(II) β-diketonates, the [Zn(hfac)2]160 has tetrahedrally coordinated zinc atom. Similarly, homoleptic hexafluoroacetylacetonates of tri- and higher-valent transition metals are also monomeric with the β-diketonate ligand being coordinated to the metal centers in a bidentate manner. As expected, the trivalent complexes [M(β-dikF)3] (M = V,314 Cr,315 Mn,316 Fe,317 Ir;318 β-dikF = hfac or tfac) are quasioctahedral in geometry, although Jahn−Teller distortion was observed in the manganese complex [Mn(hfac)3], which is tetragonally elongated due to the presence of long [2.141(3)− 2.147(3) Å] and short [1.906(2) and 1.937(3) Å] Mn−O distances.316 The tetravalent metal complexes [M(β-dikF)4] (M = Zr, Hf, Nb, Ce; β-dikF = ptac (pivaloyltrifluoroacetylacetonate), tfac, hfac), on the other hand, have a regular or distorted square antiprismatic geometry.319−323 These homoleptic [Hf(hfac)4]319,320,324 and heteroleptic [TiCl2(hfac)2]325 complexes undergo partial hydrolysis to afford either hydroxo-bridged dimeric complex, [Hf2(μ-OH)2(hfac)6], where 8-coordinated Hf centers have slightly distorted square antiprismatic geometry, or oxo-bridged dimeric [Ti2(μ-O)(hfac)4Cl2] and tetrameric cyclic [Ti4(μ-O)4(hfac)8] complexes in which titanium centers are in a distorted octahedral environment. 3.3.1.2. With Ancillary Coligands. 3.3.1.2.1. Main Group Metals. In contrast to the discrete monomeric [(15-crown5)Na(η2-hfac)],326 where sodium atom is 7-coordinated in a distorted pentagonal bipyramidal geometry, the potassium complex [(18-crown-6)K(η2-hfac)]n exists in the solid state as a linear polymeric chain via F···K interactions [2.934(7)− 3.081(6) Å].121 The use of intermolecular Lewis bases such as cyclic and acyclic ethers produces thermally unstable compounds for nonfluorinated β-diketones due to the weak interaction between the ether and the metal center. Tuning the Lewis acidity of the metal center via the use of fluorinated βdiketones affords a more stable interaction between the ether and the metal. Thus, [Ba(hfac)2(tetraglyme)] sublimes intact, which is in sharp contrast to the nonfluorinated analogue [Ba(thd)2(tetraglyme)].327 The thermal stability of the complex is believed to arise from the weakened interaction between the β-diketonate oxygen and the metal itself, which stems from the fact that the fluorine atoms pull the electron density away from the oxygen atoms. This leaves the metal in an extremely high state of electron deficiency, hence allowing for a stronger interaction between the tetraglyme oxygen and the metal. This theory has been supported by X-ray crystallographic data, which clearly indicate the formation of longer β-diketonate

oxygen−metal interatomic distances and a strong tetraglyme oxygen−metal interaction.28 Structural aspects of group 2 metal β-diketonates were reviewed by Otway and Rees in the year 2000.28 Since then, a number of fluorinated β-diketonate complexes of group 2 metals with neutral ancillary ligands have appeared in the literature. 5 1 , 1 0 8 , 1 0 9 , 3 2 8 − 3 3 4 The structure of [Ba(dfhd)2(tetraglyme)] (Hdfhd = 1,1,1,2,2,6,6,7,7,7-decafluoroheptane-3,5-dione) 3 2 8 is similar to that of [Ba(hfac)2(tetraglyme)],28 where 9-coordinated barium centers have distorted antiprismatic geometry comprised of five tetraglyme oxygen atoms in equatorial positions and four βdiketonate oxygen atoms arranged in nearly orthogonal fashions. The hydrated complex [Mg(hfac)2(H2O)2]·2diglyme has two water molecules bonded in the trans position.51 The diglyme molecule does not coordinate to the metal cation but interacts via hydrogen bonds with the coordinating water molecules. In the second type of hydrated complexes in [M(η2hfac)2(η3-diglyme)(H2O)] (M = Ca, Sr), the 8-coordinated metal centers have a distorted antiprismatic geometry.329 There are two crystallographically nonequivalent molecules in the asymmetric unit, which are combined into dimers by means of four hydrogen bonds between the oxygen atoms of the hfac− anions and the hydrogen atoms of H2O molecules. The crystal structure of anhydrous [Ba(η2-hfac)2(η3-diglyme)2] consists of monomeric molecules containing 10-coordinated barium centers.329 The diglyme ligands are coordinated on one side of the Ba2+ cation, while the hfac ligands are on the other side. Such a “cis”-arrangement of ancillary ligands is typical for the mixed ligand complexes with the general formula [M(hfac)2(L)2] (L = phen, bipy, etc.) with the coordination number of the alkaline earth cations being eight or more.28 Interestingly, the anhydrous derivative with higher glyme, that is, [Ba2(η2-hfac)4(η4-triglyme)2(μ:η2:η2-triglyme)], is a dimer where two [Ba(hfac)2(triglyme)] moieties are connected through a bridging triglyme molecule, which acts in a bidentate manner with respect to each of the barium ions, thus maintaining 10-coordinate environment around the metal center.330 In the structures of [Sr(η2-hfac)2(η6-18-crown-6)] and [Ba(η2-ptac)2(η6-18-crown-6)(THF)], the M2+ ion resides in the center position of the 18-crown-6 cavity and is coordinated by 10 oxygens, including four from the two βdiketonate ligands.108,331 The β-diketonate ligands are in a trans arrangement with respect to the nearly planar macrocycle. In contrast, the metal center in [M(η2-ptac)2(η6-15-crown-5)] (M = Ca, Ba) is outside the crown-ether cavity and coordinated by five O atoms of 15-crown-5 and four O atoms of two chelating fluorinated β-dikenonate ligands arranged in cis position.332 The 9-coordinated metal center has a distorted tricapped trigonal prismatic geometry. The 10-coordinated Ba center in [Ba(η2-ptac)2(η6-18-dibenzocrown-6)], which crystallizes with either dichloromethane or toluene as a solvated molecule, also has two β-diketonate ligands in the cis position relative to the plane of the macrocycle.331,333 Contrary to many alkaline earth metal complexes with hfac and diglyme ligands, 51,329 magnesium derivatives with various ancillary diamine ligands are anhydrous despite their synthesis in the EtOH/H2O media.109 In the isostructural derivatives [Mg(hfac)2(L)] (L = TMEDA, N,N′-DMEDA, N′,N′-DMEDA), the Mg2+ ion is surrounded by two β-diketonate ligands and one diamine ligand in a distorted octohedral 6-coordinate geometry. The symmetry of diamine ligands governs the Mg−N bond lengths in these complexes, which varies from being identical in the symmetric AA

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methyl-1-hexen-3-yne), the triple bond of the 2-methyl-1hexen-3-yne ligand is η2-coordinated to the copper atom, while the double bond stays free to afford square planar structures.357,358 The dimeric structure of [Ag(μ-η2,η2-hfac)(η3-diglyme)]2 contains two silver atoms bridged by four oxygens from the two hfac ligands and a η3-chelating diglyme molecule on each silver atom to achieve 7-coordination number for the metal.127 On the other hand, the tetraglyme and silylated alkyne ligands in the mononuclear complexes [Ag(hfac)(η3-tetraglyme)]127,128 and [Ag(hfac)(Me3SiC CSiMe3)]129 are coordinated through three of the five oxygen atoms and two carbon atoms, respectively, to afford 5- and 4coordinated silver atoms. The tri- and tetrametallic subunits in the [Ag3(hfac)3(VTMS)]∞ and [Ag4(hfac)4(THF)2]∞ are assembled via oxygen and carbon atoms of the bridging βdiketonate moiety to afford mixed 5/6- and 4/5-coordinated silver centers, respectively. In contrast, the [Ag4(hfac)4(toluene)2]∞ contains tetrametallic subunits assembled via toluene molecules to yield exclusively 6coordinated silver metal centers (Figure 27).129 The four-

TMEDA [2.227(2) Å] to unequal in the highly asymmetric N′,N′-DMEDA [2.281(3) and 2.219(3) Å]. In contrast, the diamine TEEDA leads to isolation of an ion-pair [HTEEDA]+[Mg(hfac)3]−.109 The corresponding barium derivatives [Ba(hfac)2(bpm)2] (bpm = 2,2′-bipyrimidine) and [Ba(hfac)2(Me4bpm)] (Me4bpm = 4,4′,6,6′-tetramethyl-2,2′bipyrimidine) are zigzag polymers with 10- and 8-coordinated barium atoms, respectively.334 The complex [Pb(hfac)2(μ-diglyme)]2 is a dimer bridged through the diglyme ligand. Each lead cation is octacoordinated by four oxygen atoms of the two hfac anion, three oxygens of a diglyme molecule, and an oxygen atom of the bridging diglyme molecule. The arrangement of the donor atoms around the Pb(II) cation is rather asymmetric. On the basis of the gap of coordination in a certain region and shortening of the Pb−O bonds lying in the opposite side of metal center, the authors suggested the presence of a stereochemically active lone pair on the Pb(II) cation.120,121 In the structure of [Pb(η2-hfac)2(η6-18-crown-6)], the Pb2+ ion resides in the center position of the nearly planar 18-crown-6 cavity. Further coordination with the two hfac ligands, arranged in a trans position with respect to the macrocycle, completes 10-coordination environment for the metal center.335 The Pb− O(18-crown-6) and Pb−O(hfac) distances are in the range 2.668(2)−2.772(2) Å and 2.629(2)−2.700(2) Å, respectively. In contrast, the centrosymmetric complex [Pb2(η2-tfac)4(η6-18crown-6)] has one polyether ligand 18-crown-6 against two Pb(tfac)2 fragments.336 The 7-coordinate Pb2+ coordination polyhedron includes three O atoms of 18-crown-6 and four O atoms of tfac. As compared to Pb−O bond lengths in [Pb(η2hfac)2(η6-18-crown-6)],335 the Pb−O(18-crown-6) distances here are much longer [2.919(4)−3.002(4) Å], but the distances Pb−O(tfac) are noticeably shorter [2.423(3)−2.489(3) Å]. The fluorinated β-diketonate complexes of lead(II) with Ndonor ligands [Pb2(β-dikF)4(L)2] (β-dikF = ttac, ftac; L = 2,9dimethyl-1,10-phenanthroline, bipy, phen, 4,4′-dimethoxy-2,2′bipyridine) are dimers through bridging β-diketonate ligand, where the 7- or 8-coordinated lead(II) atom has stereochemically active electron lone pairs.337−339 On the other hand, the 8coordinated lead(II) atom in [Pb(η2-hfac)(μ3,η1:η2:η1-OAc)(μ4,4′-bipy)]∞ has stereochemically inactive electron lone pairs.340 This compound with three different ligands shows a two-dimensional framework structure formed due to the doubly and triply bridging coordination modes of bipy and OAc ligands, respectively, as well as the C−H···F−C interactions. 3.3.1.2.2. Transition Metals and Lanthanides. 3.3.1.2.2.1. Monovalent Metals. In view of their potential application as precursors for CVD, several monovalent copper and silver complexes with fluorinated β-diketonates such as [M(β-dikF)(L)] (M = Cu, Ag; β-dikF = tfac, hfac; L = trimethylvinylsilane, trimethylphosphine, tricyclohexylphosphine, COD) have been isolated and characterized.341−356 Their structural aspects and application in materials science were reviewed by Hampden-Smith31 and Doppelt29 in 1995 and 1998, respectively. Since then, many new complexes such as [Cu(β-dikF)(L)] (β-dikF = hfac, tfac, pfac; L = 2-methyl-1hexen-3-yne357,358), [{(hfac)Cu}2(L)] (L = 2-methyl-1-hexen3-yne, hex-3-yn-1-ene),359 [Ag(hfac)(L)] (L = di- and tetraglyme,127,128 TMEDA, bipy113), [Ag(hfac)(Me3SiC CSiMe3)], [Ag4(hfac)4(L)2]∞ (L = THF, toluene), and [Ag3(hfac)3(VTMS)]∞129 have appeared in the literature, mainly for their application in MOCVD process to get metallic films. In [Cu(β-dikF)(L)] (β-dikF = hfac, tfac, pfac; L = 2-

Figure 27. Perspective view of a tetrametallic subunit of [Ag4(hfac)4(toluene)2]∞. Adapted with permission from ref 129. Copyright 2003 American Chemical Society.

coordinate [Rh(tfac)(L)] (L = PMe3, COD) with a square planar geometry around the metal is another example of a solvated monovalent metal complex with fluorinated βdiketonate.360 3.3.1.2.2.2. Divalent Metals. In the presence of coordinating donor molecules, the homoleptic [M(β-dikF)2] compounds instantly coordinate them to afford higher coordinated solvated species such as [M(hfac)2(L)2] [M = Cr,315 Mn,83,316 Co,361 Cd 3 6 2 (L = THF, py, OCMe 2 , or H 2 O)], [Cu(ptac)2(EtOH)]311 and [Cu(hfac)2(L)] (L = ButNH2, 2ethynylpyridine),363,364 [Cu(β-dikF)2(L)2] (L = PPh3, imidazole, H2O),364,365 [M(hfac)2(M3PO)]2 (M = Mn, Co, Ni; M3PO = 2,5,5-trimethyl-1-pyrroline-N-oxide),366 [M(hfac)(2PyBN)2][M(hfac)3] (M = Mn, Co, Ni, Fe; 2-PyBN = N-tertbutyl-R-(2-pyridyl)nitrone),366 [Cu(hfac)2(2-pyBN)],366 and [{Cu(hfac)2(H2O)}2(μ-tetracyanoethylene)].133 While the majority of these complexes are expectedly octahedral, a few of them are stabilized even in lower coordination number and have interesting structure; for example, the square planar structure of [Pd(hfac-C)(hfac-O,O)(L)] (L = SEt2, HNMe2, 2,6-Me2py, phenoxathiin, phenazine) has one carbon-bonded hfac ligands.367−369 A number of transition metal-hfac AB

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Figure 28. Perspective view of [Ce(η2-hfac)3(η2-DME)(H2O)] (a) and [{Ce(η2-hfac)3(η2-DME)}2(μ-DME)] (b). Adapted with permission from refs 132 and 379, respectively. Copyright 2012 Royal Society of Chemistry and 2012 Wiley-Blackwell, respectively.

Figure 29. Perspective view of [La(η2-hfac)3(η4-tetraglyme)] (a) and [Ce2(etbd)6(μ-tetraglyme)]2 (b). Adapted with permission from refs 385 and 386, respectively. Copyright 1998 Royal Society of Chemistry and 1998 Elsevier, respectively.

metals prefer to change the structure to retain the preferred 6coordinate geometry. Thus, anhydrous [Co(hfac)2(DME)]133 and dihydrated [M(hfac)2(H2O)2]·tetraglyme (M = Co,135 Zn137) complexes are formed with different glyme ligands. Clearly, the larger tetraglyme ligand is poorly suited for smaller ions such as Co2+ and Zn2+ and hence does not coordinate directly. 3.3.1.2.2.3. Trivalent Metals (Lanthanides). Because βdiketonates of lanthanides have been reviewed previously,376 only some important aspects of their structural diversity are discussed here. For Ln(hfac)3 complexes with monoglyme (DME) ligand, two types of structures are generally observed. The bidentate DME is usually not sufficient to complete the coordination sphere. Mononuclear hydrated complex of the type [Ln(η2-hfac)3(η2-DME)(H2O)] (Ln = La,130 Nd,131 Ce132) was obtained for the early lanthanides (Figure 28a). The relatively large ionic radius afforded a nine-coordinate lanthanide center with square antiprismatic geometry. In contrast, the late lanthanides form anhydrous complex [Ln(η2-hfac)3(η2-DME)] (Ln = Y,134 Ho377) where lanthanide centers were sufficiently sterically crowded even with 8coordination number to prevent any H2O ligation. It was assumed that the bidentate DME cannot efficiently act as a partitioning agent in the case of these large lanthanide metals, thus producing monohydrated adducts.378 A recent publication, however, reported syntheses of anhydrous [Ln(η2-hfac)3(η2-

complexes with chelating diamine ligands such as [M(hfac) 2 (TMEDA)] (M = Fe,106 Mn, 370 Co,371 Cu,105 Zn,111,372 Cd373), [M(ttac)2(TMEDA)] (M = Zn,142 Ni,143 Cd144), [Cd(ttac)2(phen)],374 [Zn(hfac)2(DEA)],111 and [Ni(hfac)2(pda)]110 have been isolated and employed as excellent CVD precursors. In these cases, the enhanced Lewis acidity of the metal center due to the presence of fluorinated groups enables it to bind to the ancillary diamine ligand, yielding stable six-coordinated M(II) complexes with MO4N2 environment around the metals. One general structural feature of these complexes is that the M−O bonds trans to the nitrogen atoms are slightly longer than the other ones. In the crystal lattice of these complexes, no intermolecular hydrogen bonds are present, thus enhancing significantly the volatility for the adducts and making them suitable for the CVD/ALD application. Following the great success of alkaline earth metal-hfac precursors with glyme ligands for MOCVD of metal-containing thin films, such studies were also extended to transition metals. Depending upon the reaction conditions, both anhydrous and hydrated TM-hfac complexes with glyme ligands have been isolated and characterized crystallographically. While metal centers such as cadmium(II) appear to be flexible enough to increase its coordination number from 6 to 8 to afford monomeric anhydrous complex of the type [Cd(hfac)2(glyme)] (glyme = DME,375 di- or triglyme122), other AC

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Figure 30. Perspective view of [InMn(hfac)3]. Adapted with permission from ref 161. Copyright 2008 Springer.

Figure 31. Perspective view of [PbMn(hfac)4]. Adapted with permission from ref 69. Copyright 2011 American Chemical Society.

coordinate, respectively, with distorted dodecahedral and monocapped square antiprismatic geometries.386 For smaller lanthanides, where such high coordination arrangement is unlikely, two types of structures have been reported: the hydrated [Ln(η2-hfac)3(H2O)2]·triglyme (Ln = Y,387 Ho388), where triglyme ligand is not directly connected with the metal centers, and ion-pair [Y(η2-hfac)2(glyme)]+[Y(η2-hfac)4]− (Ln = Y,387 Ho;389 glyme = tri- or tetraglyme). In both cases, the metal centers prefer a lower coordination number, eight in [Y(hfac)2(η4-triglyme)]+ and [Y(η2-hfac)4]− or nine in [Y(η2hfac)2(η5-tetraglyme)]+.387 3.3.2. Heterometallic Derivatives. In comparison to the homometallic β-diketonates, for which a plethora of structures have been reported, only a few heterometallic derivatives containing fluorinated β-diketonate moieties are known,67−69,159−161,390−395 the chelating character of the ligand being the main reason behind this. The infinite chain structures of [CsM(hfac) 4 ] (M = Y,390 Dy, Er,391 Eu, Am 392), [KM(hfac)4] (M = Dy, Er391), [NaM(hfac)3] (M = Mn, Fe, Co, Ni),67−69 and [M′Mn(hfac)3] (M′ = K,393 In161) consist of an alternating arrangement of M(hfac)4−/M(hfac)3− and Cs+/ K+/Na+/In+ fragments with all β-diketonate groups chelating to transition metal/lanthanide centers and acting in a chelatingbridging mode (Figure 30). Each of the alkali metal or In+ ion is rather loosely bonded to oxygen and fluorine atoms of the βdiketonate ligand. In the presence of weakly coordinating

DME)] for almost the whole series of the lanthanides (Ln = La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, Er, Tm) and thus contradicts the general assumption that DME is too small a polyether to act as a partitioning agent displacing coordinated water on the larger lanthanide(III) ions.132Interestingly, the cerium(III) also forms a DME-bridged dimer, [{Ce(η2-hfac)3(η2-DME)}2(μ-DME)] (Figure 28b).379 The formation of this complex can be justified in terms of higher coordination numbers required to satisfy large Ce(III) ion. Such a distinction between the large and the smaller lanthanides was found to be absent in the Ln(hfac)3 complexes with diglyme ligand, and only one type of the structure [Ln(η2hfac)3(η3-diglyme)] (Ln = Ce,380 La,130,378 Nd,131 Pr,381 Eu,382,383 Sm,383 Gd,384 Tb,383 Y134) has been observed along the lanthanide series. As expected, both the M−O(hfac) and the M−O(diglyme) distances decreased upon contraction of the metal ion. The large-sized lanthanum forms 10-coordinated complexes [La(η2-hfac)3(η4-triglyme)]378 and [La(η2-hfac)3(η4tetraglyme)]385 with longer tri- and tetraglyme ligands, respectively, the latter being connected with the metal center only by four out of five oxygen atoms (Figure 29a). The geometry was found to be a distorted bicapped antiprismatic. In contrast, the dimeric structure of [Ce2(etbd)6(μ-tetraglyme)]2 (etbd = 1-ethoxy-4,4,4-trifluorobutane-1,3-dionate) consists of two Ce(etbd)3 moieties bridged by a tetraglyme ligand (Figure 29b). The two Ce atoms in the molecule are 8- and 9AD

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solvents, however, this polymeric structure is converted to oligomeric adducts such as [NaMn(hfac)3]2·2Me2CO, where solvent molecules are coordinated to the alkali metal center.67−69 The heterometallic [PbM(β-dikF)4] (M = a transition metal; β-dikF = tfac, hfac) too was found to have a polymeric structure.67−69,159,394 Unlike InMn(hfac)3, however, the structures here consist of infinite zigzag chains of alternating M(hfac)3/Pb(hfac) (M = Co, Ni) or M(hfac)2/Pb(hfac)2 (M = Mn, Fe, Zn, Cu). For Co complex, the Co−O distances in [Co(hfac)3] units (2.04−2.07 Å) are similar to those in the [CoII(hfac)3]− anion (2.04−2.07 Å) but are significantly longer than the corresponding distances in CoIII(hfac)3 (1.88 Å), confirming the formulation {[PbII(hfac)]+[CoII(hfac)3]−}. For complexes where the transition metal is Mn, Fe, or Zn, the zigzag chain structure is composed of alternating [M(hfac)2] and [Pb(hfac)2] units. Each metal center has two chelating βdiketonate ligands, and the oxidation state of each metal atom is thus +2 (as also confirmed by magnetic measurements by the authors). The M(hfac)2 (M = Mn/Fe/Zn, Pb) fragments are connected through Lewis acid−base M−O interactions to complete the 6-coordinate environment around each metal center (Figure 31). The solid-state structure of [MMn2(hfac)6] (M = Cd, Pb) contains trinuclear molecules in which cadmium(II) or lead(II) ion is sandwiched between two Mn(hfac)3 groups (Figure 32).

Figure 33. Perspective view of [Hg2Mn2(hfac)6]. Adapted with permission from ref 161. Copyright 2008 Springer.

maintain a distorted octahedral coordination with three of the six Mn−O bonds being relatively longer (2.22 vs 2.13 Å) due to the bridging interactions of these oxygen atoms to dimercury unit. The isostructural heterometallics [Bi2M(hfac)8] (M = Mn, Fe, Co, Ni, Cu, and Zn) are, in fact, trinuclear molecules in the solid state, where a planar M(hfac)2 unit is sandwiched between the two Bi(hfac)3 groups. The heterometallic molecule is held together by two M−O contacts between the central transitionmetal center and one of the oxygens on a bismuth-chelating βdiketonate ligand. The coordination of the M(II) atom is thus octahedral, while the bismuth atom maintains a distorted pentagonal pyramidal coordination with two β-diketonate ligands approximately located in the basal plane and the third ligand being in a vertical mirror plane.160 The trinuclear structure of Pb2Fe(hfac)6 resembles the above structure, except that the former exhibits strong interactions between the trimetallic fragments to afford infinite zigzag polymeric chains.67 The mixed-ligand complex [Pb2Fe2(hfac)6(acac)2] is a discrete cyclic tetramer consisting of two [Fe(hfac)2] and two [Pb(acac)(hfac)] fragments (Figure 34). Each Fe atom features two additional bridging interactions to the oxygens of the acac ligands on both lead atoms, thus achieving an octahedral

Figure 32. Perspective view of [CdMn2(hfac)6]. Adapted with permission from ref 161. Copyright 2008 Springer.

As in In−Mn heterometallic, all of the β-diketonate ligands are chelating to the Mn centers in a distorted octahedral fashion, and the heterometallic molecule is held together by six M−O (M = Cd, Pb) contacts of average distance 2.34 Å. The Mn−O bond distances can be divided into two groups with three of those participating in the bridging coordination to cadmium being relatively longer (2.24 Å) than the others (2.12 Å). This is in contrast with the heterometallic In−Mn compound, in which all of the β-diketonates act in a chelating-bridging mode and thus exhibit similar Mn−O contacts. The structure of [Hg2Mn2(hfac)6] contains a tetranuclear molecule in which a dimercury unit is squeezed between two Mn(hfac)3 groups that are related by an inversion center at the midpoint of the Hg−Hg vector (Figure 33). The Hg−Hg bonds of 2.482 and 2.493 Å are on a short end of the range of known Hg22+ dumbbells. Each mercury atom has contacts with three β-diketonate oxygens from the [Mn(hfac)3]− anion at 2.23−2.63 Å and thus exhibits a distorted tetrahedral environment. Like previous molecules, manganese atoms here

Figure 34. Perspective view of [Pb2Fe2(hfac)6(acac)2]. Adapted with permission from ref 67. Copyright 2014 American Chemical Society. AE

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alternating neutral [Pb(hfac)2] and [Cu(β-dik)2] units, the lead atom showing asymmetric coordination due to the unshared electron pair effect.394,396,397 While the structures of the trans isomer of the heteroleptic heterometallics [PdPb(L)2(hfac)2] (L = 2-iminopentan-4-onate or 2,2,6,6-tetramethyl-3-iminoheptan-5-onate) are composed of chains of coordination polymers containing alternating molecules of the metal complexes, the cis isomer is actually a tetranuclear species.398

coordination. Each Pb atom also has two additional contacts with oxygens of the neighboring [Fe(hfac)2] units.67 The structure of [LnF(hfac)3K(diglyme)]2 (Ln = Nd, Eu, Sm) is comprised of two Ln2K triangles, which share the common Ln···Ln edge (Figure 35).383 The three metals in each

4. APPLICATIONS AS PRECURSORS IN MATERIALS SCIENCE As mentioned in the Introduction, using a fluorinated ligand is a commonly adopted strategy to modify the properties of various metal−organic precursors for materials processing. Metal−organic precursors with a fluorinated ligand have several advantages such as enhanced thermal and hydrolytic stability, better solubility, low melting and boiling points, and improved mass-transport properties, over nonfluorinated analogues. Researchers have exploited the strong electron-withdrawing property of a fluoro group as a means to enhance the volatility of metal−organic precursors.25−35 The presence of electronwithdrawing fluoro-groups increases intra- and intermolecular repulsions and generates a less basic O-donor atom of an alkoxide, carboxylate, or β-diketonate ligand. This results in a much lower bridging tendency of these ligands, leading very often to the selective formation of mono- (or low) nuclear complexes. As a consequence, the isolated products not only have higher volatility but also better solubility in organic solvents. An ancillary coordinating ligand such as a polyether or a polyamine is also often used in combination to fine-tune further the properties of these fluorinated metal−organic precursors. The coordination of such neutral Lewis bases saturates the coordination numbers and results in a better spatial shield of the metal center. Fluorinated β-diketonate complexes of large and electropositive alkaline- and rare-earth metals present striking examples of such a strategy.25,26 The complexes [M(hfac)2] (M = Ca, Sr, Ba) and [Ln(hfac)3] (Ln = a lanthanide), which have improved volatilities as compared to their nonfluorinated analogues, after getting coordinated with Lewis bases such as glymes or diamines, show even better properties in thermal stability upon sublimation and mass transport (vide supra, section 3.3.1.2). Thus, the sublimation of [M(hfac)2(L)] occurs in a single step against [M(thd)2(L)] (M = Sr, Ba; L = tetraglyme, pmdta), which loses the ancillary ligand upon vaporization.327 Whereas the complexes without Lewis bases leave a significant residue upon sublimation, these [M(β-dikF)2(L)] show a clean evaporation without any side decomposition. Importantly, many of these precursors [M(βdikF)x(L)] (L = polyether or diamine) have low melting points and thus are in the liquid state under usual CVD conditions. This allows a high and constant evaporation rate for longer deposition times. These low-melting precursors also act as a solvent for other precursors in a multielement system, forming a multimetal liquid mixture that evaporates with constant masstransport rates and stoichiometric ratios (vide infra, section 4.2.3.2). Because of these advantages, metal complexes with fluorinated ligands have been extensively used to get a variety of nanomaterials (mono- or heterometallic oxides, -oxofluorides, or -fluorides) in versatile form such as nanoparticles, nanotubes, thin films, composites, transparent gels, etc. These are described in the next section.

Figure 35. Perspective view of [EuF(hfac)3K(diglyme)]2. Adapted with permission from ref 383. Copyright 2002 Royal Society of Chemistry.

triangle are connected by (i) one triply bridging fluoride ligand, (ii) oxygen atoms from the two bridging hfac ligands on one edge, and (iii) an oxygen from the another bridging hfac ligand on the other edge. Each potassium center is coordinated by a bridging fluoride, the three bridging hfac oxygen atoms, and three oxygens from a terminal η3-diglyme ligand. In addition, there are two fluorides from hfac ligands, which are oriented toward the potassium. The 8-coordinated lanthanide is coordinated to both bridging fluorides, three bridging hfac oxygens, and three additional nonbridging hfac oxygen atoms. Overall, the four metal centers in the above structure have a planar rectangular geometry. The complex [{Zr2(η4-OiPr)9}Eu(η2-hfac)2] contains an eight-coordinate Eu(III) ion ligated by four oxygens of the {Zr2(η4-OiPr)9} ligand and four oxygens of two hfac ligands, defining a square antiprismatic geometry. In the heteroleptic heterometallics [PbCu(L)2(hfac)2]2, the lead dimer Pb2(hfac)4 is sandwiched between two cis-Cu(L)2 (L = 2-iminopentan-4-onate) moieties to give an overall tetrameric complex.395 The square planner geometry around copper center is achieved by the two oxygen (av Cu−O 1.944 Å) and the two nitrogen atoms (av Cu−N distances 1.924 Å) of the two ketoiminate ligands. Each lead atom in the dimer Pb2(hfac)4 is coordinated to four oxygen atoms of the two chelate hfac ligands and two oxygen atoms of the bridgingchelating ketoiminate ligands on copper. Two weak interactions with fluorine of the other Pb(hfac)2 moiety [Pb···F 3.263(4) Å] then complete the 8-coordination environment around each of the lead centers. On the other hand, the mixed-β-diketonate complexes [PbCu(β-dik)2(hfac)2]∞ (β-dik = acetylacetonate, trifluoroacetylacetonate, or 2-methoxy-2,6,6-trimethylheptan3,5-dionate) form polymeric chain structure composed of AF

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AG

MOCVD

MOCVD MOCVD MOCVD

MOCVD

MOCVD

MOCVD

MOCVD

sono-assisted self-reduction MOCVD MOCVD

spin coating

LP-MOCVD

MOCVD LP-MOCVD

MOCVD

AACVD

MOCVD

MOCVD MOCVD

MOCVD MOCVD

technique

[Ru(COD){OC(CF3)2CH2NHEt}2] [Ru(CO)2{OC(CF3)2CH2NHEt}2] [Ru{OC(CF3)CHCRNMe}3] (R = Me, CF3) [CpRu(η5-C5H3CH2NMe2)Pt(hfac)]

[Pd{CF3C(O)CHC(CF3)NR}(η3-allyl)] (R = Me, CH2CH2OMe) [Ru(hfac)2(CO)2]

[Pd{CF3C(O)CH2C(CF3)N(CH2)2OMe}2] and [Pd{OC(CF3)2CH2C−(Me) N(CH2)2OMe}2] [Pd(β-dikF)(η3-allyl)] (β-dikF = tfac, hfac, fod)

[Ni(hfac)2(PDA)] [Pd(hfac)2]

[Ag(hfac)(tetraglyme)]

[Ag(hfac)(tetraglyme)] in CHCl3

[Ag(fod)(PR3)] (R = Me, Et)

[Cu(HFIP)2(TMEDA)] [Cu{OC(CF3)2CH2NHBui}2], [Cu{OC(CF3)2CH2NHBut}2], and [Cu{OCMe(CF3)CH2NHBut}2] [Cu{OCMe(CF3)CH2NMe2}2] [Cu{OC(CF3)2CH2C(Me)NMe}2] and [Cu{OC(CF3)2CH2CH(Me)NHMe}2] [Cu(L)(μ-O2CRf)]n (R = CF3, C2F5; n = 2 or 3; L = vinylsilanes) [Cu(TFA)(deae)]4 (deaeH = N,N-diethylaminoethanol) [Cu2(μ-O2CRf)4(tBuNH2)2] [Rf = CnF2n+1 (n = 1−6)] [Cu(hfac)2(NEt3)] [Ag(hfac)(PMe3)]

metal precursor(s)

glass or Si, 300

Si, 325 Si, 375 Si, 325−425

Si, 275−450

Si, SiO2, 200−300

Si, SiO2, glass, 170−315

Si, SiO2, glass, 350

Ta/Si, 220−300 Si(100), 80−200

pure Ru metal films by using H2 or Ar+2% O2 carrier gas, small amounts of C and O impurities present Ru thin film with dense and smooth surface morphology Ru amorphous thin films consisting of granular crystallites no deposition of Ru with H2 until 425 °C, but Ar+2% O2 gas promoted Ru metal deposition at 325 °C homogeneous and well-adherent Pt−Ru thin films by using O2 gas, phase separation between Pt and Ru at 400 °C

pure Pd films at lower temp (140−230 °C) by H2, although using simple equipment was possible with O2 even if it required higher temp (330− 370 °C) high-purity palladium films by using either H2 or O2 as a carrier gas

monophasic cubic Ni films consisting of crystallite of 45−115 nm pure Pd films with rough surface, morphology improved with increase in deposition temp pure Pd thin films with the preferred (111) orientation

well-ordered arrays of crystalline Ag nanowires of about 200 nm diameters

1:1 solution of EtOH/H2O, 45

Si, 230−300

more stable than [Ag(hfac)(PMe3)] toward H2, thus pure Ag films were obtained nanostructured film consisting of Ag grains of 70 nm size

Ag(111), Pt(111), 230−300

Si wafer coated with TiN, 100−200 glass, Si, Cu, 300−350

Si(111), Si/SiO2 400−450

glass, 290

uniform films consist of small grains of average 20 nm precursor unstable toward H2, so films contain C impurities

reductive H2 required to induce metal deposition at lower temp low concentrations of O2 (2−8%) promotes partial ligand oxidation and releases the Cu0 film uniformity, crystallite size, and adherence depend on the nature of the substrates uniform and crystalline copper films in the absence of H2 consisting of well-defined grains of 0.3−0.6 μm size Cu films, obtained in the absence of H2,consisting of irregular grains

Si, 325 Si, 275−300 stainless steel, ITO, Si(111), quartz, 300

amorphous films consisting of round granules of 100−400 nm Cu films under inert atmosphere (Ar)

comments

Si, 300 Si, 250−300

substrate, T (°C)

XRD, SEM, EDX, XPS, cyclovoltametry

XRD, SEM, XPS XRD, SEM, XPS XRD, SEM, XPS

XRD, SEM, XPS

SEM, AES, XPS

SEM, XPS, AES

XRD, SEM, XPS

XRD, SEM XRD, SEM, AES

XRD, SEM, XPS XRD, SEM, EDX, XPS XRD, SEM, EDX, AFM, XPS XRD, SEM, TEM, EDX XRD, EDX, SEM

XRD, SEM, EDX

XRD, SEM, EDX

XRD, SEM, XPS

XRD, SEM, XPS XRD, SEM, XPS

XRD, XPS, SEM XRD, SEM, XPS

characterization techniques

ref

429

39 39 41

147, 148

423

420−422

38

110 417

430

127, 128

123−126

408 125

404

403

402

37, 399 43

100 37, 399

The CVD-generated Cu, Ag, or Au metallic films using fluorinated carboxylates as precursors were reviewed by Grodzicki et al. in 2005.27 bCopper metallic films via MOCVD using fluorinated βdiketonate complexes were reviewed separately by P. Doppelt and Hampden-Smith et al.29,31

a

Ru−Pt

Ru Ru Ru

19 20 21

22

Ru

18

Ag

12

Pd

Ag

11

17

Ag

10

Pd

Cu Ag

8 9

16

Cu

7

Pd

Cu

6

15

Cu

5

Ni Pd

Cu Cu

3 4

13 14

Cu Cu

thin film

1 2

no.

Table 1. Synthesis and Characterization of Metallic Thin Films Obtained from the Well-Defined Metal Precursors with Fluorinated Liganda,b

Chemical Reviews Review

DOI: 10.1021/cr400637c Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

requires relatively high deposition temperature (250−400 °C), could be modified by combining it with an appropriate coligand. Thus, the precursor [Cu(hfac)2(NEt3)] is liquid at room temperature, which allows deposition of Cu thin films at deposition temperature as low as 100 °C.408

4.1. Metallic Thin Films and Nanoparticles

4.1.1. Metallic Thin Films. Metallic thin films find several applications in different areas of science. Because of low resistivity and other related properties, thin films of copper and silver are used extensively in microelectronic industries. There are number of copper and silver complexes with fluorinated ligands that have been utilized for the elaboration of monometallic thin films. Several excellent review articles are available in this area. For example, the CVD-generated Cu, Ag, or Au metallic films using fluorinated carboxylates as precursors were reviewed by Grodzicki et al. in 2005.27 Similarly, the use of fluorinated β-diketonate complexes to deposit thin films of copper metal via MOCVD has been reviewed separately by P. Doppelt and Hampden-Smith et al.29,31 The discussion in this section, therefore, covers only those works that have appeared after the publications of these review articles (Table 1). Quite a few Cu(II) fluorinated alkoxide complexes, either with nonfunctional alkoxides such as [Cu(HFIP)2(TMEDA)]100 or with functional amino- and iminoalkoxides,37,43,399 have been used for the elaboration of Cu thin films. While low-melting and volatile copper(II) F-aminoalkoxides [Cu{OC(CF3)2CH2NHBui}2], [Cu{OC(CF3)2CH2NHBut}2], and [Cu{OCMe(CF3)CH2NHBui}2] having a secondary amino group were found capable of depositing copper metal at 250−300 °C under inert Ar carrier gas, the derivative with a tertiary amine group [Cu{OCMe(CF3)CH2NMe2}2] required the use of reductive H2 carrier gas to induce metal deposition at lower temperatures.37,399 On the other hand, the MOCVD of the fluorinated iminoalkoxides [Cu{OC(CF 3 ) 2 CH 2 C(Me) NMe}2] and [Cu{OC(CF3)2CH2CH(Me)NHMe}2] required a low concentration (2−8%) of O2 mixed with an inert argon carrier gas to produce high-quality copper films at 275−300 °C.43 No reducing carrier gas (H2) was required. The authors concluded that the low concentration of O2 promoted partial ligand oxidation, thus releasing the reduced copper on the substrate and affording the high-purity copper deposit. Since the publication of a review article in 2005,27 there have been few more reports on the application of fluorinated carboxylate derivatives for generating Cu metallic films.111,381,382,400,401 Using perfluorinated copper(I) carboxylates with vinylsilanes (L) of the general formula [Cu(L)(μO2CRf)]n (R = CF3, C2F5; n = 2 or 3), good quality copper films were deposited at 300 °C. A decomposition pathway based on the adsorption of the precursor on the substrate and disproportionation to Cu0 and [Cu(O2CC2F5)2]2, followed by desorption of the Cu(II) compound, was suggested by the authors.402 Using a heteroleptic complex [Cu(TFA)(deae)]4 (where deaeH = N,N-diethylaminoethanol), uniform and crystalline copper films consisting of well-defined grain boundaries with particles of 0.3−0.6 μm size were also deposited by AACVD in the absence of hydrogen.403 Recently, copper(II) perfluorinated carboxylates [Cu 2(μ-O 2 CRf) 4(tBuNH2)2] [Rf = CnF2n+1 (n = 1−6)] were used, in the absence of any reducing gas, for the CVD preparation of copper thin films.404 Because of increased volatility and a relatively clean decomposition pattern, fluorinated β-diketonate complexes have conventionally been used to deposit thin films of copper metal via MOCVD.29,31 The homoleptic copper(II) complex [Cu(hfac)2] was among the first precursors to be utilized for obtaining high-purity copper metal in the presence of an external reducing agent H2 (eq 1).405−407 The properties of this precursor, which is solid at room temperature (mp 88 °C) and

Cu(hfac)2 + H 2 → Cu 0 + 2hfac‐H

(1)

However, upon removal of the external reducing carrier gas H2, Cu(II) β-diketonate source reagents leave an excess of carbon and other contaminants on the thin film because of the unwanted heat-induced ligand fragmentation.409,410 On the other hand, CuI complexes such as [(hfac)CuL] (L = trimethylvinylsilane,340 2-methyl-1-hexen-3-yne,341−356 polymethylvinylsiloxane, and polydivinylsiloxane411) can be used to deposit high-purity copper thin films even in the absence of H2 gas, through a well-established process involving thermally induced disproportionation (eq 2). One drawback of these CuI complexes, though, is that they are thermally unstable and need to be stored at low temperature or added excess of parent ligands during CVD to stabilize them.412,413 2Cu(hfac)L → Cu 0 + Cu(hfac)2 + 2L

(2)

The strategy to combine ancillary coligand to the fluorinated β-diketonate architecture overcomes the problem of oligomeric and/or hygroscopic nature of the parent unadducted βdiketonates and leads to the formation of monomeric complexes. The obtained precursors [Ag(β-dikF)(L)] [β-dikF = hfac, fod (1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione); L = PMe3, PEt3, tetraglyme] show a clean, single-step decomposition pathway.123−128 Among these precursors, the [Ag(hfac)(PMe3)] was found least efficient as the silver films obtained at 310 °C in the absence of a reducing gas in the low pressure CVD experiments had considerable carbon impurities. The use of hydrogen as carrier gas to reduce the carbon impurity was unsuccessful as it caused premature decomposition in the precursor reservoir. The higher stability of [Ag(fod)(PR3)] (R = Me, Et), on the other hand, allowed the use of a hydrogen carrier gas, and pure silver films were obtained at 230−300 °C. The main volatile products of CVD were hfacH/fodH and PMe3/PEt3 (as determined by GC or NMR), either in the presence or in the absence of H2 carrier gas. No evidence of disproportionation, as is observed for copper analogous, was obtained. The precursor [Ag(hfac)(tetraglyme)], which is highly soluble in common organic solvents, allowed the deposition of silver or silver-containing nanostructured films by spin coating on a silicon substrate.127,128 Group 10 metal nanomaterials find extensive applications in catalysis. A two-phase system consisting of cubic Ni and Ni/ Ni3C solid solution was obtained from [NiL2(diamine)] (L = hfac, diamine = PDA;110 L = TFA, diamine = en, TMEDA, TEEDA).414,415 The pure, cubic monophasic Ni films could be obtained from both of the precursors at a substrate temperature of 300 °C. The crystallite sizes of the samples obtained from the [Ni(hfac)2(PDA)] were found to be 45−115 nm. Studies on the chemical routes (especially CVD) to palladium nanomaterials are rather restricted because of the lack of suitable precursors, as most Pd complexes have either low volatility or poor stability in moist air.416 Complexes containing fluorinated ligands overcome these problems mostly, and, therefore, it is not surprising that the complex [Pd(hfac)2]417 was first among the precursors employed for the Pd CVD. The AH

DOI: 10.1021/cr400637c Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

film growth was more favorable due to lower surface free energy, thus leading to the (001) preferential orientation. The CVD of an air-stable precursor [CpRu(η 5 C5H3CH2NMe2)Pt(hfac)], in the presence of O2 as the reactive gas, led to the formation of a homogeneous and well-adherent Pt−Ru thin films on glass or Si substrate at 300 °C.429 Upon raising the temperature to 400 °C, phase separation between Pt and Ru occurred, which then induced the growth of RuO2 grains at the substrate surface and caused depletion of the alloy in ruthenium. 4.1.2. Metallic Nanoparticles and Nanowires. The precursor [Ag(hfac)(tetraglyme)], which shows a clear single stage weight loss in the range 120−265 °C in the TGA to give a final residue of 21.3% at 450 °C (calcd value for Ag2O 21.4%), was used to prepare crystalline metallic silver nanowires by a sono-assisted self-reduction template process.430 Anodic aluminum oxide (AAO) membranes of ordered hole arrays (200 nm × 60 mm) were used as noninteracting templates. Sonochemical activation dramatically reduced the thermal input required for the self-reduction process from 220 to 45 °C. Not only did it allow the formation of crystalline Ag nanowires directly, it also did not require any reducing agent. These nanowires, obtained after the etching process, were characterized by XRD, EDX, and SEM analyses, the latter study showing well-ordered arrays of nanowires of about 200 nm diameter (Figure 36).

comproportionation reaction of [Pd(hfac)2] with metallic copper also yielded high-quality Pd materials (eq 3).418,419 Pd(hfac)2 + Cu → Pd + Cu(hfac)2

(3)

Because of the effective removal of intact ligands that are less susceptible to exhaustive oxidation, the fluorinated homoleptic Pd(II) β-ketoiminate and iminoalkoxide complexes were found suitable for the preparation of pure Pd thin films with the preferred (111) orientation on a Si substrate or Pd-containing alloys.38 Deposition of a Pd thin film was also possible with these precursors using a reactive O2 carrier gas. The composition and texture of the thin films could be engineered by varying the CVD conditions. Later, Puddephatt et al. and Chi et al. synthesized several heteroleptic allyl−β-diketonate and allyl−ketoiminate complexes of palladium, respectively, with required volatility and thermal stability for CVD applications.420−423 Many of these complexes contained one or more ligand with fluorine content, for example, [Pd(βdikF)(η3-allyl)] (β-dikF = tfac, hfac, fod) and [Pd{CF3C(O)CHC(CF3)NR}(η3-allyl)] (R = Me, CH2CH2OMe), and were either solids with low melting point or liquid at room temperature. Chi et al. synthesized a series of heteroleptic ruthenium complexes consisting of either carbonyl or 1,4-cyclooctadiene (COD) ligand and a fluorinated acetylacetonate/aminoalkoxide ligand.39,147,424 As compared to the organometallic ruthenium CVD precursors Ru3(CO)12425−427 and [Ru(CO)4(hexafluoro2-butyne)],428 the isolated mixed-ligand complexes [Ru(CO)2(hfac)2],147,407 [Ru(CO)2(amakF)2], and [Ru(COD)(amakF)2]39 had better stability and could be volatilized at temperatures below 250 °C. Deposition of the Ru thin film was achieved using a H2 carrier gas or a mixture of 2% oxygen in argon. For carbonyl complex [Ru(CO)2(amakF)2], deposition of amorphous thin films that consisted of granular crystallites was obtained at 375 °C. Both the fluorine and the nitrogen residues were below the detection limit under all conditions, and, as compared to the mixed carrier gas involving 2% O2, the H2 carrier gas was less effective in terms of the removal of carbon. On the other hand, the precursor Ru(COD)(amakF)2 afforded ruthenium thin film with dense and smooth surface morphology at temperature as low as 325 °C.39 In comparison, no deposition of Ru metal was observed even at deposition temperature as high as 425 °C with the homoleptic Ru(III) complexes [Ru{OC(CF3)CHCRNMe}3] (R = Me, CF3) and H2 as a carrier gas.41 The authors attributed this extra stability to higher oxidation number of ruthenium [+3 in Ru(OC(CF 3 )CHCRNMe) 3 as against +2 in Ru(L)2(amakF)2 (L = CO, COD) and zero in [Ru3(CO)12] and [Ru(CO)4(hexafluoro-2-butyne)], which required higher activation barrier to induce in situ metal reduction and deposition.41 They also suggested the chelating ketoiminate ligand having greater stability as another explanation for this unusual behavior. However, use of a mixture of 2% O2 in argon as the carrier gas promoted Ru metal deposition in Si wafer substrate at 325 °C. Upon increasing the temperature to 375 °C, the film thickness increased from 680 to 1000 Å. Furthermore, in contrast to the amorphous thin films deposited at 325 °C, the XRD pattern showed an enhanced (002) signal intensity at higher temperature, with respect to the XRD signal of hexagonal Ru metal standards, indicating the presence of caxis orientated Ru thin films. The authors attributed the formation of such structure to the smaller interplanar spacing of the (001) crystallographic planes, on which unidirectional thin

Figure 36. High-resolution SEM image of free-standing Ag nanowires. Reprinted with permission from ref 430. Copyright 2004 Royal Society of Chemistry.

More recently, [Pd(hfac)2] has been adapted to atomic layer deposition (ALD) techniques to yield well-controlled Pd nanoparticles on metal oxides such as CeO2, ZnO, TiO2, and Al2O3 under mild synthetic conditions.431−435 These nanostructured Pd/MxOy were found to be highly active catalysts for the alkane dehydrogenation, methanol decomposition, and alcohol oxidation reaction. Palladium nanoparticles with low polydispersity were also fabricated by thermally induced reduction of [Pd(fod)2] in o-xylene in the presence of tetraalkylammonium salts as the surfactants.436 The particle size, ranging from 6.2 to 18.5 nm, could be controlled by variation of the surfactant, and the concentration of precursor and surfactant. AI

DOI: 10.1021/cr400637c Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 37. TEM image of CoPt3 (a) and the core−shell Co@Pt NPs, with the high-resolution images shown in the insets. Reprinted with permission from ref 438. Copyright 2001 American Chemical Society.

Chemical vapor deposition of the bimetallic Ag−Cu films consisting of separate silver and copper grains were recently reported employing the precursors [Ag(μ-O2CC2F5)]n and [Cu(μ-O2CC2F5)(vtms)]n in the temperature range of 360− 440 °C.437 Using mass spectrometry and variable-temperature IR spectroscopy, the authors suggested the formation of mixedmetal species, [AgCu(μ-O2CC2F5)2], in the gas phase. The morphological studies (SEM and AFM) showed that the Ag(I)/Cu(I) precursor ratio, stability, and vaporization temperature of the metalated species transported in the gas phase were the main factors that influenced the structure, size, and packing density of Ag/Cu nanomaterials. The heterometallic complex Bi2Pd2(TFA)10(TFA-H)2 was used as precursor for the preparation of a bimetallic Pd−Bi carbon-supported catalyst.156 The SEM images of the obtained Bi−Pd/C samples showed small, homogeneously dispersed metallic particles. These Bi− Pd/C catalysts obtained from the above heterometallic were found to be more active than the reference materials prepared from multisource homometallic Pd and Bi precursors for the oxidation of D-glucose. The reaction between [Co2(CO)8] and [Pt(hfac)2] under reflux in a toluene solution containing oleic acid as a stabilizing agent resulted in the formation of bimetallic CoPt3 and CoPt nanoparticles.438 In contrast, when presynthesized Co nanoparticles were reacted with [Pt(hfac)2], the transmetalation redox reaction led to core−shell structured nanoparticles of Co@Pt. The resulting nanoparticles were coated with dodecyl isocyanide capping ligands and exhibited diameters of 6.27 nm (Figure 37). These nanoparticles were air-stable and dispersible in nonpolar solvents.

thermal decomposition, clean decomposition, and a good compatibility between coprecursors during the growth of complex oxides (in case of multi source process). It is, however, highly unlikely that these entire properties will be present in a single precursor, and, therefore, some sort of compromise is usually made during the selection and design of the precursors. Even though a number of MOCVD precursors (mostly with functional alkoxide and β-diketonate ligands) have been reported for the metal oxide thin film deposition over the past two decades, the majority of these precursors have insufficient thermal stability to withstand heating for long periods and decompose in the bubbler or in the inlet pipe-work leading to poor oxide layer uniformity and reactor blockages. These are, therefore, not very suitable for conventional MOCVD. It is, therefore, necessary to tailor the physical properties of the precursors as well as to modify the MOCVD techniques. One strategy to enhance the volatility of the precursor and ambient stability is to increase the steric hindrance of the ligand set or incorporate bi- or multidentate donor functionalized ligands, which inhibit oligomerization in the metal precursors.439−441 The other strategy is to use fluorinated ligands, as described in the beginning of section 4. Despite the possibility of the F-contamination in the final materials, the fluorinated precursors have been used extensively to elaborate metal oxide thin films because of their certain advantages over nonfluorinated precursors (e.g., fluorinated carboxylates lead to lower processing temperatures due to the avoidance of the formation of metal carbonates, or fluorinated β-diketonates tend to make the metal−ancillary ligand interaction robust, thus enhancing stability and mass transport properties of the precursors). Besides modifying the ligand set to optimize the precursor’s properties, the problems of premature thermal decomposition associated with low volatility of the precursors can also be overcome by employing the promising and increasingly used solution-based CVD techniques such as liquid injection MOCVD (LI-MOCVD) and aerosol-assisted CVD (AA-CVD), which allow excellent control of the feeding flux. Unlike in conventional MOCVD where low volatility may give upstream condensation, it poses no real problem in LI- and AA-MOCVD as the (aerosol) precursor solution is transported into the reaction chamber by use of a carrier gas, whereby the solvent is evaporated and precursor molecules decompose onto the substrate. Readers are referred to some excellent review and research articles, which are available on the use of IL-MOCVD and AA-CVD for the deposition of thin films.442−446 The following section describes results obtained for homo- and heterometallic oxides by using

4.2. Metal Oxide Thin Films and Composites

4.2.1. General Overview of Precursors for Metal Oxide Thin Films by MOCVD. Metal oxide thin films find an ever increasing application in advanced materials technology. Among several techniques used for the deposition of highpurity metal oxide films, MOCVD is the most versatile and promising technique, which offers the large area growth deposition with excellent composition control, high film uniformity, and good coverage on nonplanar irregular substrates. However, the MOCVD process requires suitable precursors. The inherent advantages of the precursors such as preformed metal−oxygen bond as well as the desired stoichiometry can easily be transferred to the materials by MOCVD, provided these precursors have appropriate properties and deposition characteristics such as adequate volatility, a sufficiently large temperature window between evaporation and AJ

DOI: 10.1021/cr400637c Chem. Rev. XXXX, XXX, XXX−XXX

films

AK

CeO2

21

CeO2

18

CeO2

CeO2

17

20

CdO

16

CeO2

CdO

15

19

CdO

ZnO

10

14

ZnO

9

CdO

Cu2O and/or CuO

8

13

Co3O4

7

ZnO

CoO

6

12

CoO/Co3O4

5

ZnO

CoO/Co3O4

4

11

[Fe(hfac)2(TMEDA)]

β-Fe2O3

3

[Ce(hfac)3(diglyme)]

[Ce(hfac)3(glyme)] (glyme = diglyme, diethyldiglyme, dibutyldiglyme) [Ce(hfac)3(diglyme)]

[Ce(hfac)3(glyme)] (glyme = mono-, di-, or triglyme)

[Ce(fod)4]

[Cd(ttac)2(TMEDA)]

TiO2(001), 450−1050

Hastelloy C276, 250−1050

MOCVD MOCVD

YSZ (001), 540

Si, Pt, TiN, 450

Si(100), 225−400

glass, Si, 300−700 (calcn temp)

glass, 350−380

SiO2, 400

(100) oriented CeO2 films at 350−550 °C but random films at higher temp (650−850 °C) epitaxially grown films in the range 450−750 °C, (111) oriented films above 750 °C

solid-phase precursor has poor reproducibility of sublimation rates, F- impurities present in the films mixed Ce(III)/Ce(IV) oxides with fluoride impurity, Ce4O7 being the main crystalline component; F-impurity absent if moist oxygen used as carrier gas or if films were annealed in oxygen (100) oriented CeO2 films

depending upon experimental conditions, formation of of pure CdF, CdS, or CdO films

phase-pure cubic, highly textured, and optically transparent films (85% transmittance)

cubic (100)-oriented thin films with about 90% transmittance in the visible range

(200) oriented CdO films with about 70% transmittance in the visible range

highly c-axis oriented hexagonal ZnO films of about 100 nm thickness

cubic Si(001), quartz, αAl2O3, 400−600 SiO2, 320−400

transparent and uniform ZnO films consisting of coalesced grains of about 100 nm

varying the temperature from 250 to 550 °C upon both dry O2 and O2 + H2O atmospheres led to a progressive transformation from Cu2O to Cu2O + CuO, to CuO nanosystems substrate-free, self-standing ZnO thin films by by rapid quenching from 500 °C to rt thin films consisting of nanoplatelates of different size

phase-pure, uniform, and crack-free cubic Co3O4 with no preferential orientations

depending on the CVD conditions employed, cubic and (100) oriented CoO films or (311) oriented Co3O4 thin films depending on the CVD conditions employed, CoO (200) or Co3O4 (311) oriented smooth films CoO films consisting of (200) oriented, cubic crystals of 20−200 nm size

high purity, nanostructured films consisting of plate-like grains of about 600 nm wide and 60 nm thick high purity, single-phase, and homogeneous β-Fe2O3 films

phase-pure, highly crystalline, and smooth films on glass; highly biaxially textured films on SrTiO3

comments

Si, quartz, 650

Si(100), 250−500

Si, 500

Si(100), 250−550

Si(100), 400

SiO2, 350

quartz, 400

Si(100), Herasil silica, 400− 500 SiO2, 350−500

Corning 1737F glass, Si, SrTiO3(100), SrTiO3 (110), 550−675 °C Si(100), 450−600

substrate, temp (°C)

LP-MOCVD

LP-MOCVD

spin-coating under wet O2 atmosphere MOCVD

MOCVD

LP-MOCVD

LP-MOCVD

[Cd(hfac)2(H2O)2]

[Cd(hfac)2(glyme)] (glyme = mono-, di-, or triglyme) [Cd(hfac)2(TMEDA)]

PA-MOCVD

MOCVD

[Zn(ttac)2(TMEDA)]

[Zn(ttac)2(TMEDA)]

LP-MOCVD

[Zn(hfac)2(H2O)2]·glyme (glyme = di-, tri-, and tetraglyme) [Zn(hfac)2(TMEDA)] MOCVD

MOCVD

LP-MOCVD

LP-MOCVD

LP-MOCVD

LP-MOCVD

LP-CVD

LP-CVD

LP-MOCVD

deposition technique

[Cu(hfac)2(TMEDA)]

[Co(hfac)2(TMEDA)]

[Co(hfac)2(H2O)2]·glyme (glyme = di-, tri-, and tetraglyme) [Co(hfac)2(DME)]

[Co(hfac)2(H2O)2]

[Mn(hfac)2(TMEDA)]

Mn3O4

[Mg(hfac)2(TMEDA)]

starting reactant(s)

2

Monometallic Oxides 1 MgO

no.

Table 2. Metal Oxide Thin Films Deposited Using Well-Defined Precursors with Fluorinated β-Diketonate Ligandsa

XRD, SEM, EDX

XRD, SEM, EDX

XRD

XRD, XPS, SEM

XRD, XPS, XRF, SEM

UV−vis, XRD, XPS, SEM, HR-TEM FE-SEM, AFM, GIXRD, XPS, EDX, UV−vis, XRD, SEM, EDX XRD, AFM, optical, and electrical properties UV−vis, XRD, XPS, EDX UV−vis, XRD, SEM, XPS, EDX XRD, EDX, SEM, EELS, optical measurements FT-IR, XRD, EDX, SEM

UV−vis, XRD, SEM, XPS UV−vis, XRD, SEM, XPS XRD, XPS, FE-SEM, EDX FT-IR, XPS, AES, GIXRD, FE-SEM, TEM

UV−vis, FT-IR, XRD, EDX, AFM UV−vis, XRD, XPS

XRD, EDX, FE-SEM

UV−vis, XRD, XPS, AFM

characterization techniques

454

453, 455

380

112

452

144

373

122, 375

362

460, 461

142, 461

372

137, 451

459

371

133

135, 136

361

106, 107

370

109

ref

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DOI: 10.1021/cr400637c Chem. Rev. XXXX, XXX, XXX−XXX

films

starting reactant(s)

AL

β-Fe2O3−CuO

ZnO−TiO2

ZnIn2Sn1.5Ox

36

37

38

[Zn(hfac)2(TMEDA)], [Ti (thd)2(OPri)2] [Zn(hfac)2(N,N′-DEA)], [In (dpm)3], and [Sn(acac)2]

[Ca(hfac)2(tetraglyme)], [Ba (hfac)2(tetraglyme)], [Cu(thd)2], and vapor of Tl2O [Fe(hfac)2(TMEDA)]

XRD, SEM, EDX

XRD, SEM, EDX, WDX, AFM UV−vis, XRD, SEM, EDX, EF-TEM, AFM XRD, SEM, EDX, AES, electrical transport properties XRD, FE-SEM, EDX, XPS, TXRF, AS GI-XRD, FE-SEM, EDX, XPS, XE-AES UV−vis, XRD, AFM, TEM, XPS

films having a columnar grain structure with crystallite sizes in the range 100 ± 200 nm epitaxially grown films with porous characteristics and consisting of grains size of 150−200 nm porous films consisting of well-distinct grains of 100−200 nm

monophasic, (011) oriented films

monophasic, strongly c-axis oriented films consisting of grains of 400 nm size

no pure CCTO films on SrTiO3(100), epitaxial films on LaAlO3(100), and polycrystalline films on Pt, TiO2, SiO2, and Si(001) a-axis oriented homogeneous and monophasic films

SrTiO3 (100), 900−1050 Si(100), 450 SiO2, TiN(100), 300−450 MgO (100), 700−800 SrTiO3, 650−850 yttria-stabilized zirconia, 880−980 LaAlO3 (100), 800 LaAlO3 (100), 650−850 SrTiO3(100), LaAlO3(100), Pt, TiO2, SiO2, Si(001), 600 LaAlO3 (100), 500

LP-MOCVD LP-MOCVD LP-MOCVD LP-MOCVD MOCVD MOCVD

MOCVD

MOCVD

MOCVD

MOCVD

PA- CVD + CuO RF sputtering CVD Corning 1737F glass, 500

Si(100), Al2O3, 350−400

ITO, 400

XRD, AFM

(111) oriented films of the mixed Ce(IV)/Y(III) oxide

SrTiO3 (100), 850−1050

LP-MOCVD

MOCVD

XRD, XPS, SEM

amorphous films needed to be annealed at 800 °C for crystallization

Si(100), 750−1000

characteristics of nanocomposite influenced by the host matrix porosity and the TiO2 deposition time transparent films made of discrete grains with sizes ranging from 100 to 200 nm

uniform distribution of CuO NPs in the porous β-Fe2O3 nanoplatelet arrays

epitaxially grown films have high-quality smooth surface

highly smooth films consisting of about 80 nm grains uniformly distributed

good quality films by one-step or two-step processes

XRD, FE-SEM, EDX, WDX, AFM

XRD, HR-SEM, EDX

XRD, SEM, EDX, AFM, XPS XRD, TEM, EDX, XPS

XRD, SEM, EDX

XRD, FE-SEM, EDX, luminescence XRD, SEM, EDX

perovskite, c-axis oriented films consisting of grains of about 100−150 nm

MOCVD

XRD, SEM, EDX, TEM

epitaxial, homogeneous films consisting of 100−150 nm square-shaped grains

SrTiO3(110), 1050

characterization techniques

MOCVD

comments

SrTiO3 (100), 900−1050

substrate, temp (°C)

MOCVD

deposition technique

111

457

456

475, 476

472−474

471

134

470

328

477

112

469

468

467

466

465

464

ref

Certain aspects of the second generation precursors [M(hfac)x(glyme)y] (M = alkaline earth metals, lanthanides, etc.; x = 2 or 3; glyme = mono-, di-, tri-, and tetraglyme; y = 1 or 2) were reviewed by Fragala et al.25,26 Abbreviations: FE-SEM = field effect scanning electron microscopy; GI-XRD = glancing incidence X-ray diffraction; PA-MOCVD = plasma-assisted MOCVD; XE-AES = X-ray excitedauger electron spectroscopy; RF sputtering = radio frequency sputtering; TXRF = total reflection X-ray; AS = absorption spectroscopy; WDX = wavelength dispersive X-ray; XRF = X-ray fluorescence.

a

Tl2Ba2CaCu2Ox

35

Heterometallic Oxides and Composites [Y(hfac)3(diglyme)] and [Al 22 YAlO3 (acac)3] 23 YAlO3:Er3+ [Y(hfac)3(diglyme)], [Er (hfac)3(DME)], and [Al(acac)3] 24 LaAlO3 [La(hfac)3(diglyme)] and [Al (acac)3] 25 LaAlO3 [La(hfac)3(diglyme)] and [Al (acac)3] 26 LaAlO3 [La(hfac)3(diglyme)] and [Al (acac)3] 27 PrAlO3 [Pr(hfac)3(diglyme)] and [Al (acac)3] 28 Ce2Y2O7 [Ce(hfac)3(diglyme)] and [Y (hfac)3(diglyme)] [Ba(hfac)2(pentaethylene glycole29 BaTiO3 thylbutylether)] and Ti(OPri)4 30 BaxSr1−xTiO3 [M(dfhd)2(tetraglyme)] (M = Sr, Ba) and Ti(OPri)4 31 La0.8Sr0.2MnO3 [La(hfac)3(diglyme)], [Sr (hfac)2(tetraglyme)], and [Mn (thd)3] [Y(hfac)3(DME)], [Ba 32 Y2−xBaxCuO4+δ (hfac)2(tetraglyme)], and [Cu (thd)2] [La(hfac)3(diglyme)], [Ba 33 La2−xBaxCuO4+δ (hfac)2(tetraglyme)], and [Cu (thd)2] 34 CaCu3Ti4O12 [Ca(hfac)2(tetraglyme)], [Cu (thd)2], and [Ti(thd)2(OPri)2]

no.

Table 2. continued

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a

Cu−ZnO

Cu−ZnO

4

5

[Co2Ti(μ3-O)(TFA)6(THF)3]

Cu−ZnO

Fe2TiO5−TiO2

CoTiO3−CoO

Cu2O−CoO

Cu0.75Co2.25O4− CuO

CuO−NiO

6

7

8

9

10

11

AM

AA-CVD

AA-CVD

AA-CVD

AA-CVD

AA-CVD

AA-CVD

AA-CVD

AA-CVD

glass, 350−500

glass, 390

glass, 350−500

glass, 500

glass, 500

glass, 450

glass, 250−500

glass, 250−475

FTO-coated glass, 550 glass, 400

FTO-coated glass, 450

substrate, temp (°C)

characterization techniques

composite thin films containing monoclinic CuO and cubic NiO

compact composite films containing Cu0.75Co2.25O4 and CuO

XRD, SEM, EDX

XRD, SEM

UV−vis, XRD, SEM, EDX, RBS transparent, crystalline, and fine-grained films consisting of grains of 0.3−1 μm size UV−vis, XRD, FE-SEM, EDX smooth, compact film consisting of well-defined grains with clear boundaries of 110−205 nm XRD, SEM, size EDX thin films with uniform distribution of cubic metallic Cu and hexagonal ZnO phases XRD, SEM, EDX uniform CuO−ZnO composite film consisting of av crystallite sizes of 40−80 nm XRD, SEM, EDX compact CuO−ZnO composite thin film consisting of spherical particles of 0.2−0.5 μm size XRD, SEM, EDX, RBS Fe2TiO5−TiO2 composite films with an average grain size of 0.10−0.12 μm XRD, SEM, EDX flower petal-like morphology with well-defined grain boundaries XRD, SEM, EDX XRD, SEM, dense composite films containing cubic Cu2O and CoO phases EDX

highly crystalline films consisting of grains of 30−50 nm size

comments

Abbreviation: FE-SEM, field effect scanning electron microscopy; GI-XRD, glancing incidence X-ray diffraction; PA-MOCVD, plasma-assisted MOCVD.

[Cu2Co4(OH) (TFA)6(acac)2(DMEA)2(DMEA− H)2] [Cu2Co4(μOH)2(TFA)8(DMEA)2(THF)4]· 0.5C7H8 [Cu2Ni4(OH) (TFA)6(acac)2(DMEA)2(DMEA− H)2]

[ZnCu3(μ-OH)(TFA)3(DMEA)3X]· THF (X = Cl and/or Br) [ZnCu3(μ-OH)(TFA)3(DMAP)3Cl]· THF [Fe2Ti4(μ-O)6(TFA)8(THF)6]

[ZnCu3(TFA)4(DMEA)4]

[BaCu4(TFA)6(DMAP)4(THF)]

BaCuO2−CuO

3 AA-CVD

AA-CVD

[Pb2Ti2(μ3-O)2(TFA)4(THF)3]

PbTiO3

2

deposition technique AA-CVD

starting reactant(s)

[Cd4Ti4O6(TFA)8(OAc)4(DMEA)4]

CdTiO3

films

1

no.

Table 3. Heterometallic Oxides and Composites Films Deposited Using Well-Defined Single Source Precursors with Fluorinated Ligandsa

166

172

166

169

167

170, 171 295

171

165

294

173

ref

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fluorinated ligand as a mean to enhance the volatility. Tables 2 and 3 summerize some important results of this section. 4.2.2. Monometallic Oxide. Metal oxide nanoparticles and thin films have received much attention in recent years for their wide range of applications spaning from heterogeneous catalysis to photoactivated energy production and pollutant removal, from chemical sensing to innovative batteries, to piezoelectric nanogenerators, information storage, light-emitting diodes, and photovoltaics. Except for few metal precursors with fluorinated alkoxides, most of the fluorinated precursors used for the formation of homometallic oxides are with β-diketonates. (Chi et al. have used many Ga(III) and In(III) fluorinated functional amino- and iminoalkoxides for the elaboration of M2O3 thin films by low pressure CVD.40,42 These low-melting and volatile complexes [Ga{OC(CF 3 ) 2 CH 2 NMe 2 } 2 Cl], [Me 2 Ga{OC(CF 3 ) 2 CH 2 NHBu t }], [Me 2 In{OC(CF 3 )CHC(CF 3 ) NCH2CH2NMe2}], and [Me2In{OC(CF3)2CH2NHCH2CH2OMe}]2 proved to be good precursors for the deposition of Ga2O3 and In2O3 on silicon and quartz at temperatures between 400 and 500 °C using O2 as a carrier gas. The monomeric tungsten oxo-fluoroalkoxides [W(O)(ORf)4] (ORf = TFE, TFTB, HFTB) have been used in atmospheric pressure or aerosol-assisted chemical vapor deposition experiments to deposit on the glass/ITO substrates either nonstoichiometric WOx (in nitrogen at low deposition temperature 100−250 °C) or stoichiometric WO3 (when O2 used as a coreagent).447,448 At growth temperatures above 300 °C, the W18O49 monoclinic crystalline phase was observed.448) Because of their volatility and stability, early work concentrated on so-called “first generation” homoleptic metal fluorinated β-diketonates. For example, Porchia et al.449 and Battiston et al.450 used [Ga(hfac)3] in a low pressure CVD process to form films of Ga2O3 on alumina and titania substrates at 450−470 °C using a nitrogen carrier gas. The asdeposited films were black and amorphous, but annealing at 700−1100 °C resulted in the formation of a white, crystalline film of monoclinic Ga2O3. Films deposited on alumina exhibited some interdiffusion of the Al2O3 and Ga2O3 layers during the annealing process, probably due to the similarity in ionic radius of Al3+ and Ga3+. However, no interdiffusion was observed with the films grown on TiO2.450 The XPS data of the as-deposited films indicated that stoichiometric Ga2O3 was formed with a low amount of carbon and fluorine contamination ( [Ce(hfac) 3 (triglyme)] (ca. 210°) > [Ce(hfac)3(diglyme)] (ca. 230°) > [Ce(hfac)3(tetraglyme)]2 (ca. 280°). The least volatile of the [Ce(hfac)3(tetraglyme)]2, even though it is a liquid precursor, can be attributed to its dinuclear structure and hence high molecular weight. The increase in the length of alkyl chain of the glyme ligands lowers the melting temperatures without significantly affecting volatilities, resulting in highly desirable low-melting precursors for MOCVD. In particular, the precursor [Ce(hfac)3(diglyme)], in view of its low melting point (75 °C), which allows high evaporation rates from a liquid phase, produced good quality CeO2 thin films at a significantly low temperature on a wide variety of substrates such as Si, Pt, TiN, random Hastelloy C276, YSZ (001), and TiO2(001) substrates.112,380,453−455 The X-ray patterns of all samples grown in the 350−550 °C temperature range pointed to the formation of (100) oriented CeO2 films, while at higher deposition temperatures (650−850 °C) random CeO2 films were formed. As expected, the surface morphology depended on the deposition temperature and changed from star-like grains to round-shaped islands upon increasing the deposition temperature for a constant deposition time of 1 h.455 The SEM images of CeO2 samples, deposited in the 350−550 °C range, showed uniform, adherent films with well-linked grains (Figure 38). The analogous [Y(hfac)3(diglyme)] and [La(hfac)3(diglyme)] have also been successfully used for the fabrication of multimetallic oxide systems such as LnAlO3. However, some other lanthanide precursors [Ln(hfac)3(diglyme)] (Ln = Pr, Gd, and Eu) were unsuccessful for producing pure Ln2O3 films. Fluorinated phases were always found in the deposited films, irrespective of deposition conditions (dry or water-vapor-saturated oxygen at various deposition temperatures). These complexes are, however, good precursors for metal fluoride and oxofluoride materials (vide infra, section 4.4.1). 4.2.2.2. Metal Oxides from M(β-dikF)x Complexes with Diamine Ligand. The favorable properties of [M(hfac)2(TMEDA)] (M = Mg,109 Mn,370 Fe,106,107,456 Co,371 Cu,105 Zn,111,372,451 Cd373) in terms of improved long-term stability and volatility with respect to conventional βdiketonates have been utilized for the chemical vapor deposion of metal oxide films. Additionally, many of these precursors are in molten form under normal CVD conditions, thus providing constant evaporation rates even for long deposition times. Often, the carrier/oxidizing gas was saturated with H2O vapor to minimize the fluoride contamination in the produced films

and to influence the precursor decomposition. The thermal behavior of the molecular precursors [Mg(hfac)2(L)] (L = TMEDA, N,N′-DMEDA, N′,N′-DMEDA) was reported by Wang et al.109 These derivatives exhibit single-step TGA curves with no significant residue or discontinuities, indicating clean single volatilization processes with negligible ligand decomposition and ensuring constant precursor transport properties in MOCVD growth processes. These derivatives evaporate completely at far lower temperatures (95% over the 300−3300 nm wavelength range for films with thicknesses of ∼170 nm on glass.

Figure 39. AFM image of MOCVD-derived MgO thin film grown at 600 °C on SrTiO3(110). Reprinted with permission from ref 109. Copyright 2005 American Chemical Society. AO

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Using [M(hfac)2(TMEDA)] (M = Mn,370 Fe106,107) in reduced pressure CVD, growth of single-phase and homogeneous Mn3O4 or β-Fe2O3 with high purity and controlled morphology was very recently reported on the Si(100) substrate. Interestingly, the manganese(II) precursor was also employed to get atmospheric pressure CVD-derived MnF2 nanostructures.458 The authors reasoned that, while the fluorine-containing byproducts produced during decomposition of the precursor are pumped out immediately in the reduced pressure CVD to afford metal oxide films, in AP-CVD, these species find enough time to react with the Mn(II) ion to yield the MnF2 nanostructures.370 As expected, MOCVD depositions of the [Co(hfac)2(TMEDA)] in O2 atmospheres on Si(100) substrate yielded phase-pure, uniform, and crack-free samples, bluish-gray cubic Co3O4 with no appreciable preferential orientations.371 The absence of contamination peaks pertaining to carbon or fluorine confirmed the clean decomposition pathway of the precursor. The FE-SEM micrograph revealed a compact and very uniform morphology, characterized by wellinterconnected triangular lamellar features with mean size ranging from 20 to 50 nm (Figure 40). Cross-sectional analyses allowed the estimation of an average film thickness of 200 nm.

Figure 41. Plane-view (left) and cross-sectional (right) FE-SEM images of two ZnO specimens synthesized at 400 °C: (a,b) under a dry oxygen atmosphere; (c,d) under a nitrogen + wet oxygen atmosphere. Reprinted with permission from ref 372. Copyright 2007 Wiley-VCH.

two samples obtained at 400 °C with and without the introduction of H2O into the reaction ambient. As can be observed, the use of dry oxygen led to uniform coatings (Figure 41a), characterized by interconnected rounded flakes (60 nm) formed by several nanoparticles (12 nm). Such observations were confirmed by the cross-sectional image in Figure 41b, displaying the compact morphology of the film resulting from an isotropic growth, with an average thickness of 63 nm. No significant variations in the deposit features with the substrate temperature were observed for samples deposited under dry O2. In a different way, high density, intertwined nanoplatelets uniformly distributed over the substrate surface (Figure 41c,d) were obtained in the presence of water vapor. The crosssectional image (Figure 41d) revealed that the obtained morphology resulted from the alignment of pseudocolumnar structures almost perpendicular to the substrate surface. Phase-pure, highly textured, and conductive as well as optically transparent thin films of CdO were grown by MOCVD using [Cd(hfac)2(TMEDA)] as a precursor.373 The carrier gas was argon flowing at 45−80 sccm, and the O2 reactant gas (flowing at 300 sccm) was saturated with H2O vapor to inhibit the formation of fluoride-containing defect phases. XRD analysis of the as-deposited films indicated a high degree of texturing as revealed by the intense (200) reflection of cubic CdO. Furthermore, energy dispersive X-ray (EDX) spectroscopy and electron energy loss spectroscopy (EELS) indicated negligible incorporation of the fluoride or carbon impurities from the ligand. The pale yellow CdO films exhibit high optical transmittance (∼85%) in most of the visible and near-infrared regions. The relatively small intrinsic bandgap estimated from the optical transmission spectrum, ∼2.6 eV, results in significant optical absorption at shorter wavelengths. The [M(ttac)2(TMEDA)] (H-ttac = 2-thenoyltrifluoroacetone; M = Zn,142,460,461 Ni,143,462 Cd144) was synthesized as potential multipurpose single source precursors for metal oxide nanomaterials. Like [M(hfac)2(TMEDA)], these precursors have clean, single step thermal decomposition and high solubility in organic solvents, and, therefore, could be used effectively in both chemical solution deposition (CSD) and chemical vapor deposition (CVD) processes. For example, the precursor [Ni(ttac)2(TMEDA)] evaporates quantitatively in

Figure 40. FE-SEM image of cubic Co3O4 thin film grown on the Si(100) substrate. Reprinted with permission from ref 371. Copyright 2009 American Chemical Society.

[Cu(hfac)2(TMEDA)] was used for the chemical vapor deposition (CVD) of copper oxide nanosystems.459 The syntheses were carried out under both O2 and O2 + H2O reaction atmospheres on Si(100) substrate, at temperatures ranging between 250 and 550 °C. Subsequently, the interrelations between the preparative conditions and the system composition, nanostructure, and morphology were elucidated. Variations of the substrate temperature from 250 to 550 °C upon both dry O2 and O2 + H2O atmospheres led to a progressive transformation from Cu2O to Cu2O + CuO and to CuO nanosystems. Water vapor introduction in the deposition environment improved the lateral deposit homogeneity and exerted an activation effect on the precursor decomposition, enabling one to lower the deposition temperature and resulting in a concomitant increase of the branch density due to the formation of a higher number of nucleation centers. Starting from [Zn(hfac)2(TMEDA)], zinc oxide nanoplatelets of as thin as 5 nm were deposited on Si(100) by CVD under a nitrogen and wet oxygen atmosphere.372 Figure 41 shows a comparison of representative SEM images for the AP

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could be deposited on all three substrates, hexagonal α-Al2O3 (0001), cubic Si(100), and quartz by plasma-assisted MOCVD. On the other hand, the spin coating of the dichloromethane solution of [Cd(ttac)2(TMEDA)] on glass or Si substrates in air, followed by post-thermal treatment in the 300−700 °C temperature range, resulted in different phases, CdO, CdS, or CdF2, depending on the conditions of thermal treatment.144 As expected, the cubic CdO phase was observed when the thermal treatments were carried out under O2 atmosphere. Thermal treatments under nitrogen (100 sccm) resulted in two different phases, depending on the reaction temperature. The XRD patterns of the films heated at 350−450 °C corresponded to the randomly oriented CdF2 cubic lattice, whereas it indexed well with CdS phase for those heated above 550 °C. In the temperature range 450−550 °C, both phases were present in the films. The authors suggested that at lower temperatures under N2, the precursor decomposition yields the CdF2 phase and some amorphous S and/or CdS that is not detectable using XRD (as confirmed by EDX analysis, which showed about 15− 20% S). Upon increasing the temperature, the amorphous S reacts with CdF2, yielding CdS and volatile species containing F. 4.2.3. Heterometallic Oxides and Composites. 4.2.3.1. From Single Source Precursors. Heterometallic oxides are used for many applications such as integrated-optics (LiNbO3, LiTaO3, BaTiO3), ferroelectrics in memory devices (BaTiO3, SrBi2Ta2O9), buffer layers (MgAl2O4), high Tc s u p e r c o n duc t or s (L a 2 − x Ba x CuO 4 + δ , Y B a 2 Cu 3 O 7 − δ , Bi2Sr2Can−1CunOx), or as corrosion and wear-resistant coatings (aluminum silicates). The fabrication of these complex oxides is not easy and requires well-optimized chemical routes. To a large extent, the success of theses routes depends on the properties of molecular precursors. The single source precursors containing all of the necessary elements in appropriate ratio of the desired material represent a bottomup approach for the preparation of mixed-metal oxide nanomaterials. Several well-defined SSPs with fluorinated ligands have been used for the preparation of heterometallic oxides (Table 3). These precursors allowed a strict control over chemical composition and ensured a high homogeneity among the constituent elements. Using the aerosol-assisted chemical vapor deposition technique (AA-CVD), Mazhar et al. have reported mixedmetal oxide thin films and composites from heterometallic trifluoroacetate single source precursors, mostly assembled by aminoalcohol ligands.165−173 Mesoporous, crack-free, and crystalline CdTiO3 thin films were deposited from [Cd4Ti4O6(TFA)8(OAc)4(DMEA)4] on a fluorine doped SnO2 coated conducting glass substrate at 450 °C.173 The XRD studies showed that the as-prepared films were highly crystalline with no preferred orientation. The SEM images showed that the films consisted of homogeneously dispersed grains of 30−50 nm size, the EDX confirming the absence of the fluorine impurity and a 1:1 atomic ratio of the Cd and Ti atoms. Employing [Pb2Ti2(μ3-O)2(TFA)4(THF)3] in AACVD under argon gas flow, uniform and transparent films of crystalline PbTiO3 were deposited on FTO-conducting glass substrate at 550 °C.294 Crystalline thin films of BaCuO2−CuO composites were deposited at 400 °C by AA-CVD using [BaCu4(TFA)6(DMAP)4(THF)] precursor.165 The X-ray diffractograms of the thin films indicated the formation of composites of two different types of highly crystalline oxide phases BaCuO2 and

the 120−250 °C range, with a residue left at 350 °C of lower than 2%.143 Its DTA curve consists of a single peak, indicating a single-step vaporization. A linear behavior was observed in the isothermal thermogravimetry experiments of this compound at a constant temperature (selected in the range 140−170 °C) and under reduced pressure (20 Torr) in N2 atmosphere, which also pointed toward a constant vaporization rate during the experimental time. 1 43 Thus, the MOCVD of [Ni(ttac)2(TMEDA)] on porous alumina template at deposition temperature of 500 °C for 1 h under reduced pressure using oxygen as reaction gas and argon as carrier gas yielded ordered homogeneous arrays of cubic NiO nanotubes.462 The field emission SEM images of the NiO nanotubes, after removing the template using an alkaline solution, indicated the formation of well-ordered nanotube arrays. The diameter of nanotubes was controlled by the characteristic pore dimension of the template membrane, whereas their length depended on the process conditions. The high magnification SEM image showed that nanotubes, which were about 1 μm in length, had an outer diameter of about 200 nm, the thickness of the nanotube walls being about 20 nm. TEM images showed that nanotubes were open on both sides (Figure 42).

Figure 42. HR-TEM micrographs of a free-standing NiO nanotube sheet. Reprinted with permission from ref 462. Copyright 2007 American Chemical Society.

Similarly, MOCVD of [Zn(ttac)2(TMEDA)] on Si (100) and quartz substrates at 650 °C yielded high-quality ZnO films in terms of phase purity, morphology, and transparency.142 During CVD, the precursor temperature was maintained at 170 °C. At this temperature, the precursor is molten, thus representing a liquid precursor. Thin films of ZnO were also grown by plasma-assisted MOCVD.460 The effects of growth parameters such as the plasma activation, the substrate, the surface temperature, and the ratio of fluxes of precursors on the structure, morphology, and optical and electrical properties of ZnO thin films were studied. Under a very low plasma power of 20 W, c-axis oriented hexagonal ZnO thin films were grown on hexagonal sapphire (0001), cubic Si(001), and amorphous quartz substrates. The substrate temperature mainly controlled the grain size. The MOCVD approach results in ZnO films with a (0001) preferential orientation only when grown on the hexagonal α-Al2O3 (0001).461 For other substrates, it resulted in either nonpolar ZnO orientation [Si(100)] or polycrystalline films (quartz). However, highly (0001) oriented ZnO thin films AQ

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CuO with mass ratios of 49% and 51%, respectively. The SEM images showed smooth and compact film consisting of homogeneously dispersed and well-defined grains with clear boundaries, the mean average particle sizes being 110−205 nm (Figure 43).

Figure 44. SEM micrographs of films deposited from [ZnCu3(μOH)(TFA)3(DMEA)3Cl]·THF at (a) 350 °C, (b) 400 °C, (c) 450 °C, and (d) 500 °C. Reprinted with permission from ref 170. Copyright 2011 Royal Society of Chemistry.

Figure 43. SEM image of BaCuO2 and CuO composites obtained from [BaCu4(TFA)6(DMAP)4(THF)]. Reprinted with permission from ref 165. Copyright 2009 Royal Society of Chemistry.

The Cu−Zn heterometallic precursors [ZnCu3(TFA)4(DMEA)4]171 and [ZnCu3(μ-OH)(TFA)3(L)3X]·THF (L = DMEA, DMAP; X = Cl and/or Br)170,171,295 were used to deposit thin films of crystalline Cu/ZnO composites at different temperatures. The XRD diffractograms of these films indicated the presence of cubic metallic copper and hexagonal zinc oxide in its wurtzite form. The microstructure and surface morphology of the films including the shape and size of the crystallites changed with variation of the substrate temperature. Thus, the SEM micrograph of the film deposited from [ZnCu3(μ-OH)(TFA)3(DMEA)3Cl]·THF at 350 °C showed a dense agglomeration of particles without any distinguishable features, which on increasing the deposition temperature, changed to a well-developed structure of intergrown platelets and flakes at 400−450 °C, and finally to large and thick plates, aligned almost perpendicular to the substrate at 500 °C (Figure 44a−d).170 The heterotransition metal precursors [Fe 2 Ti 4 (μO)6(TFA)8(THF)6]167 and [Co2Ti(μ3-O)(TFA)6(THF)3]169 were utilized for the deposition of Fe2TiO5−TiO2 and CoTiO3−CoO composite thin films, respectively, on glass substrates at 500 °C (Figure 45). The X-ray diffractogram of the as-obtained films indicated the formation of a composite of two different types of crystalline oxide phases, pseudobrookite Fe2TiO5 and rutile TiO2 in the first case, and hexagonal CoTiO3 and cubic CoO in the second case. The highresolution SEM image showed a compact morphology of the film consisting of well-defined rectangular and pyramidal particles with clear grain boundaries tightly attached to each other (Figure 45a). These particles showed a large particle size distribution, 0.2−0.8 μm for the length and 0.05−0.150 μm for the thickness. The CoTiO3−CoO composite thin film, on the other hand, showed flower petal-like morphology with welldefined grain boundaries (Figure 45b). The heterometallic complexes [Cu2M4(OH)(TFA)6(acac)2(DMEA)2(DMEA-H)2] (M = Co, Ni) were utilized for the deposition of thin films of crystalline Cu2O−CoO and CuO−

Figure 45. High-resolution SEM image of the Fe2TiO5−TiO2 (a) and CoTiO3−CoO (b) composite thin films deposited from [Fe2Ti4(μO)6(TFA)8(THF)6] and [Co2Ti(μ3-O)(TFA)6(THF)3], respectively, at 500 °C. Reprinted with permission from refs 167 and 169, respectively. Copyright 2011 Elsevier and 2012 John Wiley & Sons, Ltd., respectively.

NiO composites, respectively.166 The texture, morphology, and orientation of composite thin films were strongly dependent on the AACVD conditions. As expected, the surface morphology critically depended on the deposition temperature. Thus, the surface morphology of the CuO−NiO composite film changed from the dense nanostructured particles without any distinguishable features to a fine network of thin wires of varying lengths on increasing the substrate temperature from 350 to 500 °C. The related complex [Cu2Co4(μ-OH)2(TFA)8(DMEA)2(THF)4]·0.5C7H8 was also shown to be a promising precursor for the Cu0.75Co2.25O4/CuO composite by AACVD.172 4.2.3.2. From Multiprecursors. A disadvantage with the single-source approach is that a fixed ratio of the two metals in the heterometallic precursor makes it difficult to alter the concentration ratio of the heterometals in the film, which limits the ability to fine-tune the film properties. A more flexible approach employs two or more separate metallic precursors, which allows a more effective control of the metallic ratio. Thin films of Zn1−xCdxO solid solution were deposited on FTO coated glass substrate by aerosol-assisted CVD technique using a common solution of [Cd3(TFA)4(OAc)2(THF)4] and Zn(OAc)2.463 The authors studied various parameters to optimize the conditions for fabrication of stable, robust, and efficient photoelectrode thin films. The morphology of the AR

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grown thin films strongly depended upon the physical characteristics of the solvent. Photoelectrodes grown out of tetrahydrofuran solution had a higher concentration of Cd and showed better photocatalytic activity than the photoelectrodes grown from the methanol and the ethanol and having a lower Cd concentration. Fluorinated carboxylates, especially fluoroacetates, are also the most common precursors for the elaboration of high Tc superconductor coatings by metal organic deposition (TFAMOD). Large-area, uniform, high critical current density (Jc) YBa2Cu3O7−x (YBCO) superconductor films are now routinely obtained by TFA-MOD, a method that utilizes inexpensive and commercially available metal trifluoroacetates and does not require any expensive vacuum apparatus during the process. The preferential decomposition of barium trifluoroacetate into BaF2 represents a means to avoid the formation of highly stable barium carbonate, which is detrimental to the superconducting properties. The formed metal fluorides not only avoid fatal deterioration in Jc caused due to BaCO3 but also lead to perfectly c-axis-oriented epitaxial crystal growth. In conventional MOD, nucleation in the precursor film causes random orientation in the resulting film, but in TFA-MOD, nanocrystallites in the precursor film never cause such disorder. Furthermore, during the calcination process, water and HF gas diffuse quickly between the film surface and growth front of the YBCO layer, which never limits the growth rate of YBCO. Several excellent review articles44−48 are available on the different aspects of TFA-MOD, and, therefore, further discussion on it is omitted here. E x p l o i t i n g t h e c a p a b i li t y o f lo w - m e lt i n g [ M (hfac)x(polyether)] [M = alkaline earth metal (x = 2), Ln (x = 3)] precursors to act as a solvent for other species, Fragala et al. reported the fabrication of several perovskite-based materials such as MTiO3 (M = alkaline earth metal), LnAlO3 (Ln = Y, La), the high Tc superconductor La2−xBaxCuO4+δ (LBCO), and the giant k CaCu3Ti4O12 (CCTO) materials. The authors described these multimetal liquid mixtures as “single sources” because it could be easily and cleanly evaporated with constant mass-transport rates and stoichiometric ratios. Thus, homogeneous and smooth films of LnAlO3 (Ln = Y,464,465 La,466−468 Pr469) were deposited on SrTiO3(100)/(110) or Si(100) substrates at temperature ranging from 900 to 1050 °C from a molten multicomponent source containing the [Ln(hfac)3(diglyme)] (Ln = Y, La, Pr) and Al(acac)3 precursors. Films as thick as 600 nm could be fabricated upon tuning the deposition time. A water-saturated oxygen flow was required as reaction gas to preclude fluorine contamination in the growing phase. The XRD studies of these film deposited at 1050 °C showed perovskite LnAlO3 (Ln = Y, La, Pr) structure with a caxis preferred orientation. SEM images showed high-quality films with highly homogeneous surfaces consisting of uniformly distributed square- or plate-shaped grains of about 100−150 or 50−80 nm for yttrium aluminum garnate (YAG) and lanthanum aluminum garnate (LAG), respectively. The EDX analyses, while indicating negligible fluorine incorporation (lower than 0.2%), confirmed the expected 1:1 stoichiometry for Ln:Al in the films. Similarly, codeposition of [Ln(hfac)3(diglyme)] (Ln = Y, Ce) gave films of the mixed Ce(IV)−Y(III) oxide Ce2Y2O7.112 It also proved to be a facile route to prepare multimetallic oxides with much desired compositional control. Thus, using the low-melting [M(dfhd)2(tetraglyme)] (M = Sr, Ba) as liquid delivery precursors, and Ti(OPri)4 as the source for the

titanium, the BaxSr1−xTiO3 (BST) films of 0.2−1 μm thickness were grown on the SrTiO3 substrate.328 The substrate temperature had a drastic effect on the crystalline quality of the grown films, and the best crystalline quality was obtained at 740 °C. Similarly, porous La0.8Sr0.2MnO3 (LSMO) films for solid oxide fuel cell (SOFC) applications were deposited on yttria-stabilized zirconia (YSZ) using a molten mixture consisting of the [La(hfac)3(diglyme)], [Sr(hfac)2(tetraglyme)], and [Mn(thd)3] precursors by MOCVD technique.470 The La2−xBaxCuO4+δ (LBCO) and Y2−xBaxCuO4+δ (YBCO) are among the most studied high temperature superconducting materials. The parent compound Ln2CuO4+δ (Ln = Y, La) is an antiferromagnetic insulator (δ = 0), while superconductivity can be induced by partial modification of the formal copper oxidation state from +2 to +3 upon substitution of a divalent cation, such as Sr2+ or Ba2+, for the trivalent La3+. The Ln2−xBaxCuO4+δ (Ln = Y, La) films were efficiently grown on LaAlO3 (100) and MgO (100) substrates through an in situ MOCVD process from a suitable multicomponent mixture consisting of the [La(hfac) 3 (diglyme)] 4 7 1 or [Y(hfac)3(DME)],134 [Ba(hfac)2(tetraglyme)], and [Cu(thd)2] precursors in the appropriate molar ratio. The Y/La complex acts as the solvent for the remaining precursors. The XRD of the deposited films on LaAlO3 substrate provided evidence of a monophasic, strongly c-axis oriented YBCO/LBCO films. AFM images of the LBCO films showed a fine-grained structure with grains of about 400 nm. The giant K CaCu3Ti4O12 (CCTO) films were grown on Pt/TiO2/SiO2/Si(001) substrates at 750 °C using a multimetal source consisting of [Ca(hfac)2(tetraglyme)], [Cu(thd)2], and [Ti(thd)2(OPri)2] precursors.472−474 The XRD pattern indicated the formation of pure and polycrystalline CCTO films. The films showed a homogeneous surface, although some rounded grains were also present as outgrowths with average dimensions ranging from 200 to 400 nm. The cross section indicated a thickness of about 300 nm for the sample deposited for 3 h. Similar behavior was observed for other multicomponent mixtures, such as [Ca(hfac)2(tetraglyme)], [Ba(hfac)2(tetraglyme)], and [Cu(thd)2] used for the deposition of BaCaCuO(F) layers, precursor matrixes of the Tl2Ba2CaCu2Ox films.475,476 The source of a group 2 metal in the above nanomaterials was the precursor [M(hfac)2(tetraglyme)]. Even though these precursors exhibit good volatility, they remain solid under most of the CVD conditions due to a slightly high melting point (e.g., 151 °C for [Ba(hfac)2(tetraglyme)]). This leads to a slightly variable composition of the resulting films as the stable feeding rate of the vaporized precursor cannot be maintained for a long period. This problem could be solved by using a larger polyether than the tetraglyme as a coligand. For example, the compound [Ba(hfac)2(pentaethylene glycolethylbutylether)] is a low-melting precursor (mp 71 °C), which becomes liquid under normal CVD conditions and hence provides a stable vapor pressure.477 Using this precursor, the epitaxial films of BaTiO3 were deposited on (100) MgO, which provided better compositional control in comparison to the solid precursor [Ba(hfac)2(tetraglyme)]. Preparation of β-Fe2O3/CuO nanosystems was recently reported on indium tin oxide (ITO) substrate using a twostep strategy involving initial deposition of β-Fe2O3 nanostructure by plasma-assisted-chemical vapor deposition (PA-CVD) of [Fe(hfac)2(TMEDA)], followed by deposition of CuO nanoparticles by means of radio frequency (RF) sputtering.456 AS

DOI: 10.1021/cr400637c Chem. Rev. XXXX, XXX, XXX−XXX

AT

ZrF4

Na3ZrF7

KMnF3

nanoparticles

MF2, MF2: Tb3+

SrF2

12

13

14

no.

15

16

LaF3

GdF3

11

19

PrF3

10

CoF2

CeF3

9

18

LaF3

8

FeF2

MnF2

7

17

SrF2

6

[La(TFA)3(H2O)x]

[Co(hfac)2]·2H2O

[Fe(ttac)3]

[Sr(TFA)2]

solvothermal

oleic acid/1-octadecene

toluene

toluene

benzylamine

oleylamine

homogeneous films consist of grains of about 500 nm, which themselves are of aggregation of smaller particles MOCVD under Ar flow favored the formation of PrF3, but under water-saturated O2, it gave PrOF depending upon MOCVD conditions, different preferential orientations of the crystallites were obtained randomly oriented films with oxygen and carbon contamination

MOCVD at low temp (550 °C) under O2 flow allowed the formation of LaF3, but at higher temperature (750 °C) and under humid O2 gave LaOF

thin film consisting of MnF2 nanorods assembled in a pompon-like structure

homogeneous film at 450 °C consisting of well-defined grains of 200−300 nm

uniform films with (111) preferred orientation

homogeneous, oriented films with smooth surface at 400−450 °C

porous MgF2 films by spin coating, av grain size 15 nm

fine-grained films of different thickness

randomly oriented transparent film, which adheres very poorly to the substrates

comments

280, 1 h

400, 1 h

400, 1 h

240, 24 h

280, 1 h

T (°C), duration

triangular nanoplates, latter transformed in nanoarrays by slow evaporation of toluene/hexane solution

nanorods assembled into ribbon-like structures (100 nm to 2 μm) spheres consisting of dendritic-like structures (20−30 nm)

flowerlike superstructures composed of numerous aggregated nanoplates

monodisperse, highly crystalline, dispersible in nonpolar solvents

comments

randomly oriented transparent film with poor adhesion characteristics and carbon contamination perovskite (111) oriented KMnF3 at 700 °C Metal Fluoride Nanoparticles

solvent(s)/surfactatnt

MgO, 250 °C

glass, 255

Si wafer, SiO2, MgF2, glass, 375−460

glass, Si(111), 400−600

Si(111), 400−600

Si(111), 450

glass, Si(111), 400−600

Si, quartz, 400−600

Pt, TiN, SiO2, Si, 300−500

Si wafer, 300

glass, 350−650

glass, quartz, 450 (calc temp)

Si wafer, 500−600

[M(TFA)2] (M = Ca, Sr, Ba)

supercritical fluid method supercritical fluid method MOD

substrate, T (°C) Si, SiO2, MgF2, glass, 255−400

MOD

starting reactant(s)

[KMn(hfac)3]

[Gd(hfac)3(glyme)] (glyme = mono- or diglyme) [Zr(HFIP)4], Zr(TFTB)4], [Zr(HFTB)4], [Zr (PFTB)4] [Na2Zr(HFIP)6]

[Pr(hfac)3(diglyme)]

[Ce(hfac)3(diglyme)]

[La(hfac)3(diglyme)]

[Mn(hfac)2(TMEDA)]

[Sr(hfac)2(tetraglyme)]

technique

AP-CVD/ LP-CVD MOCVD

LPMOCVD LPMOCVD AP-CVD/ LP-CVD

CVD

APMOCVD APMOCVD

MOCVD

[Ca(hfac)2] + O3

CaF2

5

ALD

MgF2

4

[Mg (hfac)2(H2O)2]·2diglyme

MgF2

3

[Na(HFIP)], [Na(TFTB)], [Na(hfac)], [Na(fod)] [M(hfac)2] or [M(tfac)2] (M = Ca, Sr, Ba) [Mg(TFA)2] (soln in PVA)

metal precursor(s)

CVD

MF2

2

AP-CVD/ LP-CVD AP-CVD/ LP-CVD sol−gel

technique

NaF

thin film

1

no.

Metal Fluoride Thin Films

Table 4. Metal Fluoride Thin Films and Nanoparticles Obtained from the Well-Defined Metal Precursors with Fluorinated Liganda

ref

ref

509

507

507

506

52

characterization techniques FT-IR, XRD, TEM, HRTEM, EDX FT-IR, XRD, SEM, TEM, HR-TEM, EDX XRD, SEM, TEM, Raman XRD, SEM, TEM, Raman XRD, EDX, TEM, HRTEM

393

495

495

384

381

378, 385, 503 504

458

54, 501

500

51, 499

498

495, 496 497

XRD

XRD, SEM, XPS

RBS, AES, XRD, XPS, SEM

XRD, WDX

UV−vis, XRD, WDX, SEM XRD, WDX, SEM

UV−vis, XRD, SEM, AFM UV−vis, XRD, SEM, EDX, AFM, XPS UV−vis, XRD, AFM, TEM, RBS FT-IR, XRD, XPS, SEM, EDX XRD, FE-SEM, EDX XRD, XPS, TEM

RBS, AES, XRD, SEM, XPS XRD, SEM, AES

characterization techniques

Chemical Reviews Review

DOI: 10.1021/cr400637c Chem. Rev. XXXX, XXX, XXX−XXX

a

MOD

MYF5: Yb, Er/Tm/Eu (M = Ca, Ba) NaY(Gd)F4: Yb, Er(Tm) NaY(Gd)F4: Yb, Er(Tm) NaMF3

24

AU

flash vacuum pyrolysis flash vacuum pyrolysis

MOD

MOD

[NaM(hfac)3] (M = Mn, Fe, Co, Ni) [PbM(hfac)4] (M = Mn, Fe, Co, Ni, Zn)

[NaLn(TFA)4(diglyme)] (Ln = Y, Gd, Tm, Er, Yb) [Na2Ln(TFA)5(tetraglyme)]

[Na(TFA)] and [Ln (TFA)3(H2O)x] (Ln = Y, Er, Tm, Yb) [Ln(TFA)3(H2O)x] (Ln = Y, Yb, Er, Tm, Eu) and [M (acac)2] (M = Ca or Ba)

[Yb(TFA)2(OAc) (H2O)]·TFAH

1-octadecene

1-octadecene

1-octadecene, oleic acid, oleyl amine, trioctylphosphine, trioctylphosphine oxide 1-octadecene/oleic acid

methanol + HF

1-octadecene

oleic acid/1-octadecene

solvent(s)/surfactatnt

600

310

285−315

285−315

300−320, 1 h

300−330, 1−2 h

RT, 1 d

315, 1 h

290, 4 h

T (°C), duration

Metal Fluoride Nanoparticles

phase-pure Aurivillius-type structure for Pb2NiF6 and Pb2CoF6, but other Pb2MF6 (M = Mn, Fe, and Zn) contain different amounts of MF2 and metallic lead

XRD, ED, SEM

XRD, TEM, EDX XRD, TEM, EDX XRD, ED, SEM

β- NaGdF4 at 285 °C but a mixture of α + β NaYF4 α- NaYF4 and NaF NPs, the latter could be gotten rid of by washing with water phase-pure perovskite structure

FT-IR, XRD, TEM, HRTEM

XRD, SEM, TEM, HRTEM, SAXS FT-IR, XRD, TEM, EDX FT-IR, XRD, TEM, HRTEM, DLS XRD, TEM, EDX

characterization techniques

well-crystalline NPs of 12−15 nm size

monodisperse and size-tunable colloidal NCs

monodisperse sub-15 nm CeF3, highly crystalline and dispersible in nonpolar solvents transparent sol containing YbF3 NPs of about 5 nm

nanoplates, latter transformed in columnar assemblies

comments

Abbreviations: DLS, dynamic light scattering; SAXS, small-angle X-ray scattering; ED, electron diffraction; WDX, wavelength dispersion X-ray.

28

27

26

Pb2MF6

MOD

NaYF4:Yb, Er (Tm)

23

25

fluorolytic sol−gel

YbF3

22

[Ce(TFA)3(diglyme)]

MOD

CeF3

21

[Gd(TFA)3(H2O)x] in the presence of LiF

starting reactant(s)

MOD

GdF3, GdF3: Eu3+

20

technique

nanoparticles

no.

Table 4. continued

ref

69

68

65, 66

64−66

526, 527

518−525

511

282

510

Chemical Reviews Review

DOI: 10.1021/cr400637c Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

markedly after annealing to values as low as 1.43 × 10−3 Ω cm. Similar transparent conductive F:In2O3 thin films were also deposited by atmospheric-pressure CVD method using a mixed-carboxylate precursor [(C7H15CO2)2In{O2C(CF2)4H}] at 350−450 °C.480 For the 320 nm thick film deposited at 380 °C, the resistivity was 4.4 × 10−4 Ω cm. Low-pressure CVD of Sn(IV) and Sn(II) alkoxide precursors, [Sn(HFIP)4(HNMe2)2] and [Sn(HFIP)2(HNMe2)], afforded fluorine-doped tin oxide films, which were very different in nature.99 While CVD using [Sn(HFIP)4(HNMe2)2] and air at substrate temperatures of 180−450 °C gave highly transparent (>85% in the visible region) and electrically conductive films (O/Sn = 1.8−2.4; F/ Sn = 0.005−0.026), the films deposited from [Sn(hfac)2(HNMe2)] and air or water vapor at 200−250 °C were nonconductive. The film stoichiometries, SnO0.9−1.3F0.1−0.4, and properties suggested that the tin(II) precursor was not oxidized during the deposition and that hydrolysis was the primary film forming reaction. The F-doped SnO2 films were also produced from the single sourse alkoxide precursors [Bu3Sn(ORf)] [Rf = CH(CF3)2, CH2CF3, CH2C2F5, CH2(CF2)3CF2H, CH2CH2F], although only very little fluorine content could be incorporated.481 Recently, it was observed that, as a precursor class, the tin fluorocarboxylates perform better than tin fluoroalkoxides. Thus, films deposited from organotin fluorocarboxylates [Bu2Sn(TFA)2],482 [Me2Sn(TFA)2],483 and [Bu3SnO2C2F5]483 revealed a uniform film with a smooth surface and homogeneous substrate coverage (Figure 46). Different F-contents, 1% in Bu3SnO2C2F5 and

The ZnO−TiO2 nanocomposites were prepared on Si(100) and Al2O3 substrates by a CVD route that consisted of the initial deposition of porous ZnO matrix from the [Zn(hfac)2(TMEDA)], followed by the dispersion of TiO2 NPs on the above matrix from the [Ti(thd)2(OPri)2] in a subsequent step. The use of a processing temperature lower than 500 °C avoided ex situ thermal treatment and thus preserved the chemical identity of the host and guest phases. The gas sensing performances of these nanocomposites in the detection of volatile organic compounds (MeCOMe, EtOH, and CO) were found to be directly dependent on their composition and morphology, which in turn were dependent on the TiO2 content and the ZnO matrix porosity.457 Using [Zn(hfac)2(N,N′-DEA)], [In(dpm)3] (where dpm is dipivaloylmethanato ligand), and [Sn(acac)2] as precursors, thin films of ZITO with indium contents ranging from 40 to 70 cation %, were deposited on glass substrate at 500 °C.111 These films all exhibited polycrystalline microstructures as evidenced by XRD and electron diffraction patterns. The TEM image of a structurally and electrically representative film revealed discrete grains with sizes ranging from 100 to 200 nm, in good agreement with AFM and SEM data. The electron diffraction pattern was consistent with a randomly oriented In2O3 crystal structure. The electrical properties of the MOCVD-derived ZITO films were closely related to their chemical compositions. These ZITO films were highly transparent, all films exhibiting 80% or greater transmittance between 400 and 1500 nm. The transparency of these ZITO films was either comparable to or greater than that of commercial ITO (as indicated by absorption coefficient studies). 4.3. Fluoride-Doped Metal Oxide Thin Films and Nanoparticles

Because of the similarity in ionic radii of F− and O2−, doping of metal oxide by fluoride ion has attracted a lot of attention from researchers to modify the band gap and thus the optical properties of metal oxide nanomaterials. F-doped SnO2 and In2O3 are of interest as important transparent conducting materials, which have been commercially exploited in many areas such as liquid-crystal displays, electrochromic devices, and solar control coatings on glass.478 Some important metal fluoride thin films and nanoparticles obtained from the welldefined metal precursors with fluorinated ligands are summerized in the following subsections and in Table 4. 4.3.1. Fluoride-Doped Metal Oxides Thin Films. Unlike metal precursors with a direct M−F bond, which are usually insufficiently volatile as a result of generation of bridged oligomers/polymers through F → Sn interactions, metal complexes with fluorinated ligands have potential to be used as single-source precursors for fluoride-doped metal oxides materials, as they deliver the halogen to metal as part of the deposition process.99,479−484 Fluorine-doped indium oxide films were deposited at 400−550 °C in a low-pressure chemical vapor deposition process from [In(HFTB)3(H2NBut)] and O2 precursors.479 The films deposited on quartz at ≤500 °C contained 2−3 atom % fluorine, showed >85% transmittance in the 400−800 nm region, and had band gaps of 3.65−3.75 eV. Resistivities of 1.25 × 10−2−9.96 × 10−3 Ω cm were measured for the as-deposited films. Interestingly, the loss of fluorine content was quite facile and temperature sensitive. Thus, the In2O3 films deposited at 550 °C showed no detectable fluorine incorporation (by X-ray photoelectron spectroscopy with sputtering). The resistivities of films grown on silicon decreased

Figure 46. SEM image of the F:SnO2 film obtained from [Bu3SnO2CC2F5]. Reprinted with permission from ref 483. Copyright 2005 John Wiley & Sons, Ltd.

4.8% in [Me2Sn(TFA)2], suggested that the decomposition pathways were different for the two complexes. Similarly, using [Sn(TFA)2] as CVD precursor, the films deposited at 350 and 450 °C in air had F/Sn ratios of 0.026 and 0.028, respectively.484 The film grown at 400 °C showed a resistivity of 6 × 10−4 Ω cm. A plasma-enhanced CVD was recently used to yield F-doped β-Fe 2 O 3 on ITO starting from the precursor [Fe(hfac)2(TMEDA)].485,486 The fluorinated molecular precursor allowed homogeneous doping and in-depth distribution of the F-content in the metal oxide films. The fluorine content in the deposits could be adjusted between 4.3% and 11.2% simply by the variation of the deposition temperature (200−400 °C). The optical band gap showed a progressive blue-shift upon increasing F-content. It was revealed that increased F-content AV

DOI: 10.1021/cr400637c Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

fluorine contacts in the precursor [presence of intermolecular Na···F contacts in the solid-state structures of the [Na(HFIP)] and [Na(TFTB)], but absent in [Na(PFTB)]].495 The precursor [Na(HFIP)] gave good results, and NaF films of 3.6 μm were deposited on the silicon substrate during a 75 min sublimation of 200 mg of the precursor at ambient pressure and 300 °C. Thin film deposition of the alkaline earth metal fluoride MF2 (M = Ca, Sr, Ba) was first described by Purdy et al. using the precursors M(hfac)2 and M(tfac)2 in atmospheric or lowpressure CVD.497 More recently, a similar first generation Mg(TFA)2 was also used as a sol−gel precursor in polyvinyl acetate to deposite highly uniform and porous MgF 2 antireflective films on large-area glass substrates by spincoating.498 Using magnesium and strontium hexafluoroacetylacetonate precursors with glyme anciliary ligands, good quality thin films of MF2 were deposited by MOCVD.51 Even though the diglyme ligand is not coordinated directly with the metal center in [Mg(hfac)2(H2O)2]·2diglyme,51,499 its thermogravimetric analysis and the vaporization rate experiments showed high volatility and better thermal stability (less than 1% residue left) than the parent [Mg(hfac)2(H2O)2], which partially decomposes upon sublimation at significantly higher temperature. The very low melting point of this complex allowed its use as a thermally stable precursor in the liquid phase (and hence under constant vaporization and mass transport rates). The compositional purity of the deposited MgF2 films was confirmed by EDX and XPS studies, which showed no traces of oxygen or carbon impurities. The film deposited at 350 °C consisted of MgF2 grain of about 100 nm and was poorly crystalline. The SEM images of the films obtained at 450 °C showed rather homogeneous smooth surfaces with grains about 200 nm wide. At 650 °C, the surface became rougher and occasional cracks were observed. The atomic layer deposition of calcium fluoride using Ca(hfac)2 precursor with ozone was recently reported on Si wafer substrate at 300 °C.500 Ozone proved to be essential for the activation of the hexafluoroacetylacetonate adsorbed to the surface. From Rutherford backscattering spectroscopy (RBS) measurements, the film stoichiometry was determined to be CaF2.17 with less than 5 atomic % of oxygen. An alternate approach of first using nonfluorinated β-diketonate complex Ca(thd)2 and O3 subcycle, followed by applying a Hhfac + O3 cycle for fluorination, increased the purity of the CaF2 films. The deposited films were highly uniform and polycrystalline with (111) as the preferred orientation and had oxygen content below the detection limit of the RBS. Depending on the operational conditions, the MOCVD process for SrF 2 film growth from [Sr(hfac)2(tetraglyme)] precursor involved two different deposition regimes: reaction rate-limited and mass-transport ratelimited regimes.54,501 The study of the kinetics of film deposition, combined with “in situ” FTIR and “ex situ” XPS analyses of both the gas phase and the SrF2 films, provided crucial information on reaction pathways involved in the decomposition of the precursor. It was suggested that at temperature below 350 °C, a heterogeneous mechanism was based on the precursor adsorption, followed by the demolition of the β-diketonate framework operated. On the other hand, at higher temperatures (>375 °C), a homogeneous decomposition pathway leading to fluoroacylketenes and CF 2 O accompanied SrF2 film growth. Even though the oxygen was not involved in the rate-determining steps, it played an important role in preventing carbon contaminations.

resulted in a transition from an n-type to a p-type conduction regime. These F-doped β-Fe2O3 films were found to be efficient catalysts for water oxidation reaction. 4.3.2. Fluoride-Doped Metal Oxides Nanoparticles. Fdoped SnO2 nanoparticles have been prepared by the sol−gel method employing single-source precursors that contained a direct Sn−F bond. Unlike MOCVD, the lack of volatility of these precursors due to any possible intermolecular F: → Sn interactions is not an issue here. Under hydrolytic conditions, the robust Sn−F bond is maintained, thus introducing fluorine to the final material. Thus, the hydrolysis of mixed-valent fluorotin alkoxides [SnIIF2(μ-OR)3SnIV(OR)] (R = But, Amt), synthesized from the solvent-free reaction of SnF2 and Sn(OR)4, maintained the Sn:F ratio but removed most of the alkoxy groups. The optimum resistivity of 0.7 W cm was reached following calcination of the obtained nanopowder at 550 °C, which removed most of the residual carbon and some fluorine (14% remaining).487 Some other precursors containing a direct Sn−F bond such as [F2Sn(acac)2]488 and [FSn(βdik)2(OR)] (β-dik = acac, thd; R = Et, Pri, Amt)489−491 have been investigated. Their hydrolysis led initially to soluble oxo-tin oligomers or polymers, which retained the Sn−F and Sn−β-diketonate moieties, the latter being removed during pyrolysis. These precursors generated cassiterite powders, which incorporated typically 4−8% fluorine and showed enhanced conductivities over undoped analogues.490 Their resistivities were, however, generally higher than those of typical MOCVD films.490 The residual carbon and excess fluorine were suggested as possible causes for this enhanced resistivity.492 On the other hand, employing a precursor with a fluorinated ligand, [(tfac)2Sn(OAmt)2],490 resulted in low incorporation of F-content, most probably due to loss of the fluorinated ligand through Sn− O2CRf bond cleavage. It should be noted here that, although the optimum F-doping level in SnO2 is usually about 3% generating films of resistivity ca. 2 × 10−4 W cm, the electronic properties of the final material also depend on the manner of fluoride incorporation into the tin oxide lattice.493 A pure inorganic precursor KSnF3 has also been employed for getting F-dopped SnO2.494 Even though a high % of F could be incorporated in the final materials (up to 21%), the nonvolatility and nonsolubility of this precursor in organic solvents make it nonsuitable for MOCVD and sol−gel processing. 4.4. Metal Fluoride Thin Films and Nanoparticles

4.4.1. Metal Fluoride Thin Films. Thin films of sodium fluoride were deposited by chemical vapor deposition of the fluorinated metal−organic precursors such as [Na(HFIP)], [Na(TFTB)], [Na(HFTB)], [Na(PFTB)], [Na(hfac)], and [Na(fod)].495,496 Irrespective of the type of ligand (alkoxide or β-diketonate) and their degree of fluorination (partially or perfluorinated), all of the precursors were able to deposit the NaF films. The authors suggested several factors to explain the trends in deposition temperature of the fluorinated precursors, which followed [Na(HFIP)] < [Na(TFTB)] < [Na(HFTB)] < [Na(PFTB)]. These were (i) the steric bulk of the precursor ligands (with increasing size of the ligands, there was a corresponding increase in the temperature needed to initiate deposition), (ii) the easy transfer of F from carbon to metal via 1,2-migration of a group in the alkoxide to the carbon losing the fluorine (such migration being increasingly facile in the order CF3 ≪ CH3 < H), and (iii) the presence of metal− AW

DOI: 10.1021/cr400637c Chem. Rev. XXXX, XXX, XXX−XXX

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of FeF2 under standard conditions (972 and 663 kJ mol−1, respectively), the favorable SCF conditions led to the formation of latter. However, they did not rule out the possibility of FeF3 phase existing in the amorphous state. The SEM images of FeF2 revealed the presence of a variety of different structures consisting of small scale nanorods, which widely assembled into urchin-like features, to larger, more-elongated ribbon-like structures. The TEM analysis revealed an amorphous layer (5 nm), surrounding the FeF2 domains, which the authors attributed to the deposition of carbonaceous species following the synthetic treatment under SCF conditions. The SEM analysis of the CoF2 NPs, on the other hand, indicated the formation of near-uniform spherical morphologies, ranging in diameter from 0.7−1.8 mm. Higher-resolution TEM images confirmed further that the larger CoF2 spheres were formed from a series of dendritic-like structures, whose feature sizes ranged between 20 and 30 nm in diameter. The requirement for narrow emission profiles for several applications including those related to up-conversion or downconversion has led to an increase in the reports on rare-earth fluoride-based nanomaterials.508 Single-crystalline and monodisperse LaF3 triangular nanoplates in trigonal structure were synthesized via thermolysis of [La(TFA)3(H2O)x] in a hot oleic acid/1-octadecene solution.509 The high uniformity of these nanoplates allowed the formation of nanoarrays arranged with long-range translational and orientational order in several microns. After slow evaporation of a concentrated LaF3 nanoplate solution in toluene/hexane at the volume ratio (R) of 1:3, and by varying the solvent/substrate combination, a hexagonally close-packed (hcp) superlattice formed on the TEM grid, where the nanoplates either (i) lay flat on the face and self-assembled into nanoarrays via edge-to-edge formation, or (ii) stood on the edge and self-assembled into ribbonlike nanoarrays via face-to-face formation (Figure 47). The SAED pattern showed two sets of periodical diffraction spots, thus indicating the highly ordered arrangement of these nanoplates.

In addition to their widely adopted use as precursors for the deposition of mono- or multimetallic oxide films, the transition metal hexafluoroacetylacetonate derivatives with polyether or diamine ligands have also been used for the preparation of metal fluorides and oxo-fluorides. Generally speaking, the formation of competing fluoride phases is intimately associated with both thermodynamic stabilities and formation kinetics of the potential growing phases. Thus, MFx [M = Mn (x = 2); La, Pr, Ce (x = 3)] films could be deposited at atmospheric pressure and low temperature (up to 600 °C) from the [Mn(hfac)2(TMEDA)]458 or [Ln(hfac)3(diglyme)] (Ln = La,378,385,502,503 Pr,381 Gd,384 Ce504) adducts. For lanthanidecontaining films, the LnO1−xF1+2x (0 ≤ x ≤ 1) phase became more stable either upon increasing the temperature in the range 600−850 °C or at higher O2 flow. Similarly, the introduction of water vapor into the oxygen stream produced polycrystalline LnOF films with a (012) preferential texturing (Ln = Pr, Gd, and Eu).381,505 Using in situ synthesized unligated [Ln(hfac)3], good quality thin films of LnF3 (Ln = La, Y, and/or Er) were grown by MOCVD.53 FT-IR monitoring showed that the film deposition involved ligand dissociation on the substrate surface under Ar/O2 environments. This behavior is slightly different with respect to glyme-coordinated β-diketonate precursors, where such ligand dissociation was not observed. Clearly, the absence of ancillary glyme ligand favored hfacH dissociation (and hence the deposition process) even below 300 °C. Zirconium(IV) fluorinated alkoxide precursors, [Zr(HFIP)4], [Zr(TFTB)4], [Zr(HFTB)4], and [Zr(PFTB)4], have also been studied for the deposition of ZrF4 films.495 Interestingly, deposition of these metal fluoride films occurred at significantly higher temperatures than the deposition of NaF films from the sodium analogues. The authors attributed this increase in deposition temperature to the absence of intramolecular metal−fluorine contacts in Zr(ORf)4 complexes. Except for a few odd examples such as those of randomly oriented Na3ZrF7 films from the CVD of [Na2Zr(HFIP)6],495 and (111) oriented perovskite KMnF3 films from the flash evaporation MOCVD of [KMn(hfac)3],393 the chemical routes to heterobimetallic fluorides have rarely been explored. 4.4.2. Metal Fluoride Nanoparticles. Alkaline earth metal fluoride (MF2, M = Ca, Sr, Ba) nanocrystals were prepared by the thermal decomposition of the corresponding trifluoroacetate precursors in hot oleylamine or benzylamine.52,506 It was shown that the use of single-source precursors plays an important role in the formation of high-quality MF2 nanocrystals or flowerlike superstructures composed of numerous aggregated nanoplates, and the formation process was demonstrated in detail. The obtained MF2 nanocrystals were nearly monodisperse in size and highly crystalline, and could be dispersed in nonpolar solvents to form stable and clear colloidal solutions. The authors discussed the role of the solvent in providing size and shape control to form nanoparticle/ nanoplates and their self-assembling behavior to build into superstructures. The feasibility of introducing Tb3+ ions into the CaF2 host via this method was also demonstrated, the resulting material showing strong green emission corresponding to the characteristic emissions of the Tb3+ ions. Recently, highly crystalline nanoparticles of FeF2 and CoF2 were synthesized from the precursors [Fe(ttac)3] and [Co(hfac)2]·2H2O, respectively, using a novel supercritical fluid (SCF) method in toluene at 400 °C.507 The authors contended that, despite the availability of abundant F in the precursor and a relatively lower Gibbs energy of formation of FeF3 than that

Figure 47. TEM images of the edge-to-edge (a) and face-to-face (b) superlattices of LaF3 nanoplates. Reprinted with permission from ref 509. Copyright 2005 American Chemical Society.

Extending the above work, Murray et al. synthesized GdF3 nanoplates by the decomposition of [Gd(TFA)3(H2O)x] in a mixture of 1-octadecene and oleic acid in the presence of LiF and demonstrated that columnar and lamellar liquid crystalline superlattices of these nanoplates could be achieved by liquid interfacial assembly with long-range orientational and positional order.510 The choice of subphase was found to be an important factor in directing the orientation of the superlattices. While a more polar subphase favored lamellar liquid crystalline structure, a less polar subphase favored columnar liquid AX

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of NCs with high up-conversion luminescence.518−525 The selection of a proper coordinating ligand such as oleic acid (OA), oleyl amine (OM), trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), or ligand combinations such as OATOP controls the NaYF4 synthesis by dictating the particle nucleation and growth and is one of the key factors for achieving monodisperse and size-tunable colloidal NCs. Other lanthanide-doped host materials such as MYF5 (M = Ca, Ba) have also been prepared using [Ln(TFA)3(H2O)x] (Ln = Y, Yb, Er, Tm, Eu) and M(acac)2 (M = Ca or Ba) in oleic acid and 1octadecene.526,527 Mixed-metal species are a means to provide homogeneity at a molecular level to modify the reactivity pattern and solubility. Novel trifluoroacetate heterometallic precursors [NaLn(TFA)4(diglyme)] and [Na2Ln(TFA)5(tetraglyme)] (Ln = Y, Gd, Er, Tm, Yb) were recently reported, which contained all of the required elements (Na,Y/Ln, F) in a single molecule, for the one-step synthesis of upconverting NaY(Ln)F4 materials.64−66 These precursors had multiple advantages (e.g., formation of anhydrous species, lowering of decomposition temperature, no requirement of capping-reagents or surfactants for size control of nanocrystals, and render them monodisperse in organic media, etc.) over the extensively used homometallic Ln(TFA)3(H2O)3 for upconverting NaYF4:Ln3+ nanomaterials. Among these, formation of anhydrous heterometallic derivatives was particularly significant as the presence of water molecules is detrimental for the up-converting properties of NaY(Ln)F4 NCs. The thermal behavior of these heterometallic Na−Ln derivatives under argon atmosphere shows that their final decomposition temperature is considerably lower (265−285 °C) than that for the homometallic derivative Y(TFA)3(H2O)3 (310 °C), the precise order of the decomposition temperature being [Na2Y(TFA)5(tetraglyme)] < [NaLn(TFA)4(diglyme)] < [Y(TFA)3(H2O)3]. Thermogravimetric (TG) curves of these heterometallics show a two-step decomposition that lasts up to 300 °C (Figure 49a). These precursors decompose continuously in the first step (130−210 °C) that corresponds to partial loss of the glyme ligand. The second step, which is also the major step, corresponds mainly to thermal decomposition of the TFA ligands, as indicated by a prominent exothermic peak at 280.5, 282, and 261.7 °C in the differential thermal (DT) curves of Na−Y, Na−Gd, and Na2Y heterometallic, respectively, along with the loss of remaining glyme ligand. Whereas the remaining weight of the residues 33.5% in the case of Na− Gd heterometallic is consistent with the formation of NaGdF4 (33.4%) as the end product, the remaining residues 29.9% and 27.4% for Na−Y and Na2Y heterometallic, respectively, account for a little more than the expected yield of NaYF4 materials in [NaY(TFA)4(diglyme)] (26.9%) and NaYF4+NaF in [Na2Y(TFA)5(tetraglyme)] (24.0%), respectively. The decomposition of these heterometallics in 1-octadecene (bp 315 °C) gives NaY(Gd)F4 nanocrystals with controlled size and desirable dispersibility in organic media. Interestingly, the isostructural [NaY(TFA) 4 (diglyme)] and [NaGd(TFA)4(diglyme)] show different decomposition patterns. While [NaGd(TFA)4(diglyme)] decomposes at 285 °C to give directly the hexagonal NaGdF4 NCs, a phase many folds more efficient than the cubic phase for the upconversion process,64 the yttrium analogue [NaY(TFA)4(diglyme)] afforded only a mixture of the cubic and hexagonal phases of NaYF4, in a relative ratio that depended on the reaction temperature (Figure 49 b).65 These mixed-phase NaYF4 NCs

crystalline assemblies of GdF3 nanoplates. Several anhydrous cerium(III) trifluoroacetate complexes with neutral O-donor ligands, [Ce(TFA)3(L)x] (x = 1−3, L = THF, DMF, DMSO, diglyme) were recently reported, which on decomposition in 1octadecene in inert atmosphere gave CeF3 nanoparticles of sub15 nm size.282 The absence of water molecule in these complexes not only avoided the risk of the formation of oxofluoride phases, but also enhanced the luminescence by eliminating the possibility of the well-known quenching effect of water molecules retained on the surface of the resulted NPs.64−66 Among these complexes, the [Ce(TFA)3(diglyme)] proved to be the best precursor as the diglyme ligand coordinated strongly to the surface of CeF3 NPs and thereby led to the stabilization of the nanoparticles by preventing agglomeration and rendering them monodisperse in organic solvents (Figure 48). The radioluminosity of the obtained CeF3

Figure 48. (a) TGA curves of a series of anhydrous cerium(III) trifluoroacetate complexes with neutral O-donor ligands, [Ce(TFA)3(L)x] (L = THF, DMF, DMSO, diglyme), (b) HR-TEM image of the as-prepared CeF3 NPs obtained from the decomposition of [Ce(TFA)3(diglyme)], and (c) well-dispersed NPs in toluene.282 Reprinted with permission from ref 282. Copyright 2013 Royal Society of Chemistry.

nanoparticles was found to be in the order of the magnitude of CeF3 single crystal. The formation of a transparent sol containing YbF3 NPs of about 5 nm size was recently reported by the fluorolytic sol−gel of a mixed-ligand complex [Yb(TFA)2(OAc)(H2O)]·TFAH.511 Because of the unavailability of suitable metallic precursors, chemical routes to heterobimetallic fluorides remain least explored despite these mixed metal fluorides exhibiting many important applications in the field of magnetism,512,513 electrical conductivity,514 and nonlinear optical515 properties. Moreover, some rare-earth doped complex fluorides show photoluminescence and find applications in the fields of solidstate lasers, low-intensity IR imaging, NIR quantum counting devices, 3D flat-panel displays, and biological labels.516,517 For example, sodium yttrium tetrafluoride (NaYF4) as a very efficient host matrix for NIR-to-visible light upconversion has been the subject of special consideration. High-quality α- and/ or β-phase NaYF4:Yb,Er(Tm) nanocrystals (NCs) were prepared using Na-TFA and [Ln(TFA)3(H2O)x] in the presence of various coordinating ligands to control the sizes AY

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Figure 49. (a) TGA curves of different heterometallic Na−Ln trifluoroacetate derivatives under argon atmosphere, (b) XRD of the as-prepared NaYF4 and NaGdF4 NCs obtained from decomposition of the precursors [NaLn(TFA)4(diglyme)] (Ln = Y, Gd),( c) TEM image of as-prepared NaYF4:Yb3+, Er3+/Tm3+ NCs, and (d) the total upconversion luminescence of these NCs taken as 1 wt % solutions in CH2Cl2.66 Reprinted with permission from ref 65 and ref 66. Copyright 2012, 2010 Royal Society of Chemistry.

were required to be calcined at 400 °C to obtain the hexagonal phase. This difference in the decomposition pattern of the isostructural [NaLn(TFA)4(diglyme)] (Ln = Y and Gd) can be attributed to the different ionic radii of the two metal cations Y3+ (r = 0.89 Å) and Gd3+ (r = 0.94 Å), which would influence the free energy of the NaLnF4 system.524,528 In general, the hexagonal phase NaLnF4 is thermodynamically more stable than the cubic form, and the cubic-to-hexagonal phase transition requires sufficient free energy to overcome the activation barrier. While high temperature is often used to overcome the energy barrier between the two phases of NaLnF4,524,529,530 there have been few reports on the ionic radius dependence of the energy barriers for such system. For example, Yan et al. found a higher energy barrier for the lanthanides with small ionic radii as compared to those with larger ionic radius.524 Indeed, Liu et al. have reported the cubicto-hexagonal phase transition simply by doping NaYF4 with a larger Gd3+ cation.531 The precursor [Na2Y(TFA)5(tetraglyme)] decomposes at lower temperature (260 °C) to give mainly cubic NaYF4 nanocrystals along with an additional phase of NaF; the latter could easily be gotten rid of by washing the NCs with water. To obtain NaY(Gd)F4 NCs codoped with 20% Yb3+ and 2% Tm3+ cations, the derivatives [NaLn(TFA)4(diglyme)] (Ln = Y, Gd, Yb, Tm) taken in appropriate amounts were decomposed simultaneously in 1-octadecene. The transmission electron microscopy (TEM) images showed crystalline particles of sub15 nm size (Figure 49c). The NaYF4:Yb3+,Er3+/Tm3+ NCs thus obtained were well-dispersed in the organic solvents, which showed a significant upconversion of near-infrared light to blue/green/red light (Figure 49d).64−66 Very recently, Dikarev et al. reported fluorinated heterometallic β-diketonates [NaM(hfac)3] (M = Mn, Fe, Co, Ni)

and [PbM(hfac)4] (M = Mn, Fe, Co, Ni, Zn) as highly volatile single-source precursors for the preparation of mixed-metal fluorides.67−69 Complex fluorides of the compositions NaMF3 (M = Mn, Fe, Co, Ni) and Pb2MF6 (M = Mn, Fe, Co, Ni, Zn) were obtained by the flash vacuum pyrolysis of above precursors in a two-zone furnace under low-pressure nitrogen flow. XRD studies of these heterometal fluorides confirmed a phase pure perovskite structure for the sodium-transition metal fluorides and an Aurivillius-type structure for the lead-transition metal fluorides with layers of corner-sharing [MF6] octahedra separated by Pb2F2 blocks. For the latter, while Pb2NiF6 and Pb2CoF6 were phase-pure microcrystalline materials, those of Pb2MF6 (M = Mn, Fe, and Zn) contained different amounts of divalent metal fluorides and metallic lead. 4.5. Miscellaneous

Thin films of tin(II) sulfide were deposited on glass from the atmospheric pressure chemical vapor deposition using [Bun3Sn(TFA)] and H2S at 350−600 °C under nitrogen.532 The complex [Cd(ttac)2(TMEDA)] was used as a multifunctional single source precursor, which reproducibly and selectively yielded CdO or CdS films, depending on the processing parameters.144 The SEM images showed a nanostructured surface with grain dimensions less than 100 nm in diameter for CdS films, which was completely different from the larger rounded grains morphologies observed for the CdO films. The phase pure copper(I) nitride Cu3N was deposited in the temperature range 250−400 °C by CVD using Cu(hfac)2, NH3, and water; the latter reagent, that is, H2O, was instrumental in increasing the film growth rate on the SiO2 substrate.533 AZ

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5. CONCLUSIONS AND LOOKING AHEAD This Review highlights different aspects such as synthetic strategies, structural diversity, and applications in materials science of fluorinated homo- and heterometallic alkoxides, carboxylates, and β-diketonates. Their properties such as high solubility in common organic solvents, enhanced volatility and air-stability, mostly clean decomposition pathway, and rather easy preparative routes render these fluorinated precursors particularly suited for CSD and CVD routes to various nanomaterials. These metal−organic derivatives are versatile precursors, which have been used to obtain several forms of nanomaterials (nanoparticles, nanotubes, nanowires, thin films, composites, transparent gels...) of metals, metal oxides, fluorides, and oxo-fluorides. Recent advances show new research directions for unligated homoleptic metal complexes with TFA and hfac ligands, as synthons for the assembly of heterometallic complexes and polynuclear oxo-clusters. Exploiting structurally characterized precursors based on different metals for the formation of nanomaterials gives insight into relationships between the structural features of the precursors and the morphology of the materials. In this context, an integrated approach involving synthetic chemistry, physical chemistry, and chemical engineering has proved a boon for the materials science. Despite advancements in the chemistry and applications of these fascinating molecular precursors, some areas still remain to be explored further. The development of heterometallic single source precursors for bimetallic fluoride nanomaterials is one of those areas that needs to be explored further due to their potential applications. Some recent articles highlight the advantages of these SSPs over homometallic precursors.64−69 Metal precursors with fluorinated ligands also have potential to be used in the polymer matrix to afford transparent nanocomposite films for various applications (e.g., scratchresistant lenses, laser optics, and waveguide amplifiers). Metal fluorides exhibit the lowest refractive indices of all inorganic solids, and, therefore, by embedding different amounts of nano metal fluoride, a decrease and adjustment of the refractive index of polymeric materials is possible, without compromising on transparency. Indeed, it has been shown recently that using the fluorolytic sol−gel synthetic route, the nanometric metal fluoride content in the polymer materials can easily be varied without loss of transparency, and their refractive index properties can be fine-tuned toward application in lens systems or fiber optics.534−537 Further research for a better knowledge of reaction conditions helping to fine-tune the refractive index of polymers should prove rewarding in this area. Another area is the use of these fluorinated complexes as building blocks for the construction of metal−organic frameworks (MOFs), where the secondary M···F interaction may play a role in sustaining the framework structures. A recent publication shows that unique extended frameworks can be obtained from M2(O2CRf)4 paddlewheel units devoid of any exogenous ligands.538

AUTHOR INFORMATION Corresponding Author

*Tel.: +33 472445322. Fax: +33 472445399. E-mail: shashank. [email protected]. Notes

For the sake of homogeneity, the structures in Figures 1−35 were redrawn using the software program Diamond 3.2. The authors declare no competing financial interest. Biographies

Shashank Mishra is Assistant Professor of Chemistry at the Claude Bernard University of Lyon1 (UCBL), France. He obtained his Ph.D. degree (2002) under the supervision of Prof. Anirudh Singh, at the University of Rajasthan, Jaipur (India), and, subsequently, served as a lecturer (2002−2004) at Bundelkhand University (India). During this period, he received young scientist and best paper awards from the Indian Chemical Society (2003) and Indian Science Congress Association (2002). In 2004, he was selected for the French CNRS postdoctoral fellowship to work on the molecular precursors for hightech materials with (the late) Prof. Liliane G. Hubert-Pfalzgraf at the Institut de recherches sur la catalyse (now IRCELYON). Later, he was associated with Prof. Stéphane Daniele (2007−2010) and Prof. Christophe Dujardin (2010−2011) at the IRCELYON and Laboratoire de Physico-Chimie des Matériaux Luminescents (now Institut Lumière Matière), respectively, before being appointed as a faculty at UCBL in September of 2011. His present research interests are concerned with the design of molecular precursors of oxide and nonoxide materials and understanding their transformations via solutions or vapor phase routes for different applications (optics, energy, catalysis, ...). His other interest relates to the inorganic−

ASSOCIATED CONTENT

organic hybrid materials designed for luminescent properties, an area

* Supporting Information

where he has collaborated with Dr. Gilles Ledoux at ILM for the last 8

Tables S1−S3 listing selected X-ray crystallographically characterized metal complexes with fluorinated alkoxide, carboxylate, and β-diketonate ligands. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/cr400637c.

years. Dr. Mishra has published independently since 2007 and has, to

S

his credit, over 50 research papers in peer-reviewed international journals (total impact factor over 200), as well as 3 well-received review articles and a couple of book chapters. BA

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H-btac H-DMAP H-DMEA H-etbd H-fod H-ftac H-hfac H-HFIP H-HFPB H-HFTB H-imakF H-keimF H-ME H-pfac H-PFE H-PFP H-PFTB H-ptac

Stéphane Daniele was born in Cannes, France. He did his undergraduate education in chemistry at University of Nice (France) where he obtained his Master’s degree in Molecular Chemistry in 1991. In 1995, he obtained a Ph.D. in Inorganic Chemistry under the supervision of Prof. Liliane G. Hubert-Pfalzgraf, at the University of Nice (France), on the synthesis and characterization of molecular precursor for oxide ceramics by soft chemistry processes (sol−gel and MOCVD processes). In 1996, he was a JSPS research fellow at the Institute of Molecular Sciences in Japan (Okazaki) in the group of Prof. Koji Tanaka, and in 1996−1998 he obtained a European Marie Curie fellowship at the University of Sussex in England in the group of (the late) Prof. Michael F. Lappert. During these postdoctoral works, he developed new molecular transition metal-based catalysts for C−H bond activation or olefin polymerization. In 1998, he moved to Lyon (France) as a Lecturer in Inorganic Chemistry at the University of Lyon1 and became full Professor in 2008 at the “Institut de Recherches sur la Catalyse et l’Environement de Lyon” while having, in 2007, the direction of a 15 persons research group “Functional and Nanostructured Materials”. His main field of research concerns various aspects of coordination chemistry and the synthesis of functional (hybrid) nanoparticles with applications in the domains of catalysis, optics, microelectronics, cosmetics, environment, etc. In 2011, he started a company named “Lotus Synthesis” (www.lotus-synthesis.fr), which is adapted to the clean-technology elaboration of stable metal oxide (hybrid)nanoparticles dispersed in aqueous or organic (commercial) media or customers’ matrix (vanishes, paints, etc.) without any manipulation of dry nanopowder. He is the author/ coauthor of over 90 papers, 7 patents, and 63 international communications.

H-TFA H-tfac H-TFE H-TFTB H-thd H-TMP H-ttac MOCVD PDA phen PMDTA py RfOH RfCO2H sol TEEDA TGA THF TMEDA TMSO VTMS

ABBREVIATIONS ALD atomic layer deposition bipy bipyridine COD 1,4-cyclooctadiene DABCO 1,4-diazabicyclo[2.2.2]octane DAP 1,3-diaminepropane DEA N,N′-diethylethylenediamine β-dikF fluorinated β-diketonate DMA N,N-dimethylacetamide DME 1,2-dimethoxyethane DMEDA dimethylethylenediamine DMF dimethylformamide DMSO dimethyl sulfoxide H2-MDEA N-methyldiethanolamine H-acac 2,4-pentanedione (acetylacetone) H-amakF fluorinated aminoalcohol H-bdmap 1,3-bis(dimethylamino)propan-2-ol H-dfhd 1,1,1,2,2,3,3,7,7,7-decafluoro-4,6-heptanedione

benzoyltrifluoroacetonate N,N-dimethylaminopropanol N,N-dimethylethanolamine 1-ethoxy-4,4,4-trifluorobutane-1,3-dione 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione furoyltrifluoroacetone 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hexafluoroacetlyacetone) hexafluoroisopropanol hexafluorophenyltertiarybutoxide hexafluorotertiarybutanol fluorinated iminoalcohol fluorinated ketoimine 2-methoxyethanol perfluoroacetylacetone 1,1,2,2,2-pentafluoroethanol 2,2,3,3,3-pentafluoropropanoic acid perfluorotertiarybutanol pivaloyltrifluoroacetylacetone (1,1,1-trifluoro-5,5dimethyl-2,4-hexanedione) trifluoroacetic acid 1,1,1-trifluoro-2,4-pentanedione (trifluoroacetlyacetone) 2,2,2-trifluoroethanol trifluorotertiarybutanol 2,2,6,6-tetramethyl-3,5-heptanedione 2,2,6,6-tetramethylpiperidine 2-thenoyltrifluoroacetone metallorganic chemical vapor deposition propylenediamine phenanthroline pentamethyldiethylenetriamine pyridine fluorinated alcohol fluorinated carboxylic acid solvent N,N,N′,N′-tetramethylethylenediamine thermogravimetric analysis tetrahydrofuran tetramethlyethylenediamine tetramethylene sulfoxide vinyltrimethylsilane

REFERENCES (1) Goesmann, H.; Feldmann, C. Nanoparticulate Functional Materials. Angew. Chem., Int. Ed. 2010, 49, 1362−1395. (2) Schatz, A.; Reiser, O.; Stark, W. J. Nanoparticles as SemiHeterogeneous Catalyst Supports. Chem.Eur. J. 2010, 16, 8950− 8967. (3) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chemistry and Properties of Nanocrystals of Different Shapes. Chem. Rev. 2005, 105, 1025−1102. (4) Zhuang, Z.; Peng, Q.; Li, Y. Controlled synthesis of semiconductor nanostructures in the liquid phase. Chem. Soc. Rev. 2011, 40, 5492−5513. (5) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Synthesis of Monodisperse Spherical Nanocrystals. Angew. Chem., Int. Ed. 2007, 46, 4630−4660. (6) Hubert-Pfalzgraf, L. G. To What Extent Can Design of Molecular Precursors Control the Preparation of High Tech Oxides? J. Mater. Chem. 2004, 14, 3113−3123. BB

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DOI: 10.1021/cr400637c Chem. Rev. XXXX, XXX, XXX−XXX