Reactions of Hydrosilanes with Transition Metal ... - ACS Publications

Review pubs.acs.org/CR

Reactions of Hydrosilanes with Transition Metal Complexes Joyce Y. Corey* Department of Chemistry and Biochemistry, University of MissouriSt. Louis, One University Drive, St. Louis, Missouri 63121, United States S Supporting Information *

ABSTRACT: This third review in a series involving reactions of hydrosilanes and transition metal complexes and characterization of the products covers the period 2009 thru 2013. After a brief discussion of other synthetic methods used for the formation of Si-TM complexes, Section 3 provides an extended discussion of the types of ligands and metal complexes used as reactants with hydrosilanes. The increase in use of pincer ligands in forming stable, isolable complexes is featured. Three tables list the complexes reported for primary, secondary, and tertiary silanes. Many reactions leading to SiTM complexes are initiated by loss of a ligand prior to oxidative addition of the hydrosilane or by a metathesis reaction. Structural data are tabulated for the isolated complexes and provide the Si-TM bond ranges for the elements in the “d” block. A major section on nonclassical (σ or agostic) complexes includes two general groupings with Si−H···TM and M−H···Si interactions. A section on solution processes identified by NMR spectroscopy is dominated by hydride exchange examples. A section on bonding outlines a unifying bonding description that has been proposed as well as calculations reported for Si-TM complexes, both real and hypothetical examples.

CONTENTS 1. Introduction 2. Background and Alternate Synthetic Methods for Si−TM Bond Formation 3. Reactions of Hydrosilanes with Transition Metal Complexes 3.1. Types of Ligands at the Metal and Substituents at Silicon 3.2. Reactions of Silanes Initiated by Loss of a Neutral Ligand 3.3. Exchange of Anionic Ligands: Preparation from TM-R, TM-Cl, TM-H, and TM-Si 3.4. Addition of Silanes to M(0) Complexes 3.5. Reactions of Silanes with Polynuclear Complexes and with Mononuclear Complexes That Form Polynuclear Complexes 3.6. Miscellaneous Methods 3.6.1. Formation of Silylene Complexes from Si(IV) Precursors and Reactions of Hydrosilanes with [TM] = X 3.6.2. Reactions of Silanes with a Ligand Coordinated to the Metal Precursor Complex 3.6.3. Reactions at Coordinated Silyl Ligands 3.7. Transfer of Hydride from Si to the TM 3.8. Summary 4. Solid State Structures 5. Nonclassical Si−H Interactions 5.1. General Comments 5.2. Examples of Data Interpretation for Nonclassical Complexes

© XXXX American Chemical Society

5.2.1. Changing Role of Si−H−M in Three Manganese Complexes, 5-35, 5-36, and 5-37, and the Relationship of 5-35 to Other η1-HSi Complexes 5.2.2. Chelating Silanes and Distinguishing the Role of Two Si−H Interactions in the Same Complex. 5.2.3. Products of the Reaction of Mo(PMe3)6 with PhxSiH4−x (x = 0, 1, 2, 3): How They Vary. 5.3. Complexes with Si−H···TM Interactions 5.3.1. η 1 and η 2 σ-Complexes (Including Mononuclear and Polynuclear Complexes) 5.3.2. σ, β, and Other Long-Range Agostic Interactions 5.3.3. More than One Agostic Interaction per Si Center: ηx-HxSi or ηx, ηx-HxSi Interactions 5.4. M−H···Si Interactions 5.4.1. SISHA Interactions 5.4.2. Hydrides That Span Metal−Silicon Multiple Bonds 5.4.3. Unclassified Examples: Complexes That Do Not Fit the Previously Described SiH Interactions 5.5. Summary 6. Solution Processes Determined by NMR Spectroscopy

B C Q AA BF BG BI

BJ BL

BL

BM BN BN BN BN CE CE CS

CS

CT

CU CU

CU CW

CY DB DB DB

DC DE DF

Received: September 21, 2015

A

DOI: 10.1021/acs.chemrev.5b00559 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 6.1. General Comments 6.2. Hydride Exchange Processes 6.3. Exchange Processes That Involve Other Groups and Additional Isomerization Processes 6.3.1. Exchange of Groups Other than Hydride 6.3.2. Isomerization Processes 7. Bonding and Calculations 7.1. A Possible Unifying Bonding Description for Metal Hydrosilane Complexes 7.2. Calculations for Specific Compounds 7.2.1. Titanium 7.2.2. Cr, Mo, W 7.2.3. Fe, Ru, Os 7.2.4. Co, Rh 8. Oxidative Addition Reactions of Lanthanides/ Actinides and of Other Si−X Bonds 8.1. Lanthanides/Actinides 8.1.1. Ln Complexes with β-SiH Interactions 8.1.2. Theoretical Calculations for Lanthanides 8.2. Oxidative Additions of Other Si−El Bonds 8.2.1. Si−Si Bonds 8.2.2. Si−C Bonds 9. Concluding Comments Associated Content Supporting Information Author Information Corresponding Author Notes Biography Acknowledgments References

Review

second reaction pathway, σ-bond metathesis, may provide a low energy route to [TM]-SiR3 (see Scheme 2 for a version featuring

DF DF

Scheme 2. Simplified Comparison of an Oxidative Addition Reaction Pathway and That of a σ-Bond Metathesis Pathway

DJ DJ DK DN DN DP DP DP DR DR

LnM-H and a HSiR3). A retrospective of σ-bond metathesis that focuses on the identification of features and reactivity characteristics of the mechanistic step for the σ-bond metathesis step with a focus on higher oxidation state metals has been published.3 The emphasis of the current review will still be on the formation of isolated (and characterized) products from the interaction of a hydrosilane with a transition metal complex although some systems are either too unstable to be isolated or can only be identified in a mixture. If such cases are adequately characterized spectroscopically, they will be included in the tables of compounds or in the related footnotes. In some instances the lowest energy reaction pathway involves the addition of the hydrosilane to a ligand bound to the reacting metal complex, and no [TM]-SiR3 is formed, although the silyl group was still observed in the isolated product. These instances are also included in the early tables. The tables (Tables 3−5) are organized by the type of silane reactant: primary silanes, secondary silanes, and tertiary silanes. The tables include both 2c/2e bonds (Si-TM and H-TM) as well as 3c/2e bonds with Si−H interactions to the metal center. Another reaction variation is one where the hydride of the silane actually functions as a reducing agent and hydride is transferred to the metal center but the silyl group is not present in the product. Silanes as “reducing agents” will be summarized in a separate table (Section 3.7). Hydrosilanes that have been reacted with transition metal complexes to provide variations in the complexes produced and illustrated in the tables are summarized in Figure 1. Other synthetic routes that are not as versatile have also been utilized for the formation of [TM]−Si complexes. Included in Section 2 (Background) are two quite different synthetic strategies. In the first, selected examples produced during this review period from salt metathesis are described. The examples shown would be very unlikely to be formed from a reaction of any transition metal precursor with a hydrosilane. The second route involves an emerging area that involves silylenes (SiII) as reactants with a transition metal complex. Two types of silylenes have been commonly used for this purpose: the N-heterocylic silylenes, NHSis (Table 1), and the N-heterocyclic carbenes, NHCs (Table 2) that are stabilized Si(II) systems (tricoordinated Si centers). Much less common routes involve a disilene, R2SiSiR2 or R2SiCR′2, that is coordinated to a metal, and the few cases that were reported during this review period also are described in this section. Section 3 represents the core of the reactions of [TM] with hydrosilanes, and the silyl−metal complexes that are formed are found in Tables 3 (primary silanes), 4 (secondary silanes), and 5 (tertiary silanes). The tables include the reactants and products with characterization data for representative

DT DT DU DU DW DW DZ EB ED ED ED ED ED ED ED ED

1. INTRODUCTION The reactions of hydrosilanes with transition metal complexes have provided fertile ground for new classes of compounds, and insight into catalytic cycles for a variety of reactions, not to mention new bonding modes that could not have been imagined 30 years ago. The interaction of a hydrosilane with a suitable transition metal complex can vary from a weak σ-interaction (usually a three-centered bond between the H, Si, and TM) to complete oxidative addition to the metal to form a 2-electron, 2-centered bond between H and the TM and Si and the TM (see Scheme 1). The most versatile approach to the Scheme 1. Idealized Sequence for Addition of a Si−H Bond to a LnM Complex

formation of a TM−Si bond is thus through a reaction with a hydrosilane. The current review follows two other previously published summaries that cover the time periods from 1981 through 19971 and 1998 through 2008,2 and this review covers publications from 2009 through 2013 with an additional few from 2014 on occasion. For an oxidative addition reaction to occur, a coordinatively unsaturated metal precursor is required. Even with such a metal center, a reductive elimination process of a different small molecule such as RH, H2, or HX may occur on the path to [TM]-SiR3 formation. When oxidative addition is not possible (especially for the early transition metals) or not favored, a B


Chemical Reviews

Review

Figure 1. Transition metal complexes that react with hydrosilanes to form isolable silyl-metal compounds as reported during the current review period.

2. BACKGROUND AND ALTERNATE SYNTHETIC METHODS FOR SI−TM BOND FORMATION With the exception of Tc, Ag, and Au, the remaining metals in Figure 1 (Section 1) have formed M−Si bonds from hydrosilanes as described in the previous reviews1,2 or as will be described in the current one. There are published examples of AgSi and AuSi complexes, but these have been formed with other silicon precursors. Various terms have been used to describe the products formed from hydrosilanes and transition metal complexes, especially those that fall under the category of “arrested SiH addition” or “nonclassical interactions”. Terms include σ-complex and agostic interaction (Section 5). In the nonclassical interaction, the Si−H σ-bonding orbital and an empty “d”-orbital on the metal interact and since the Si−H bonding orbital is more basic, it is a better donor than either H−H or C−H. In addition, the Si−H σ* orbital can overlap with a donor “d” orbital of π-symmetry and is a better π-type acceptor than either C−H or H−H. The term σ-complex seems to be employed for a variety of interactions, as will be described later in Section 5. Bond energy data are still lacking, and thus, examination of how changes in substituents (at the silicon center or the metal center), or as the metal is varied across the transition metal series, affect comparison of complexes of 3d vs 4d vs 5d metals is still not possible, and thus such discussions, unfortunately, will not be found in subsequent sections. In order to appreciate the role that hydrosilanes have in the synthesis of Si−TM complexes, a brief review of other approaches utilizing examples from the current review years will be included in this section. The first reaction equations occur in this section, and the reference to the work here, and for equations in the other sections, will be found in brackets underneath the reaction arrow. Methods that require silicon precursors other than the hydrosilanes include: anions (silyllithium and silylmagnesium reagents), silylenes and silenes. The transition metal complex that couples with the silylmetallic reagent would most likely be a metal halide or, more rarely, a M(0) complex. The counterpoint would be a TM-anion and a chlorosilane. The net result is a salt metathesis reaction. The number of either stable silyl anions or TM-anions is limited, although this would not be the case for either silyl halides or TM halides. The metathesis examples reported during this review period included formation of Si−Ti,Zr,Hf (eq 1)4 and Si−Zn (eq 2)5 complexes utilizing potassium salts or utilizing a SilylGrignard

complexes from a given publication (any additional examples of the same type will be indicated in a footnote). The entries are arranged according to the metal triad, starting with the early transition metals and ending with the late metals. For each listed compound, the 1H NMR data for TM−H and Si−H as well as 29Si NMR data and additional available characterization data (mp, 13C, 31P, IR, UV−vis, MS, X-ray, and calculations) are indicated if authors have supplied such information. Structures for selected examples will be found at the end of each table. The individual complex selected will be marked in the body of the table with an asterisk. The tables include silyl complexes with 2c/2e bonds as well as 3c/2e bonds. It should be noted that representations of the weaker interactions (3c/2e), especially those that contain a more remote HSi···TM (β- through ε-interactions) have not been standardized and are generally shown in the review as depicted by the original investigator. Table 6 summarizes the reactions where the silane has reacted as a reducing agent and a hydride from SiH was transferred to the metal center. In this case, the reacting TM complex and the silane are indicated as well as the product transition metal complex. The solid state structural data are described in Section 4 and summarized in Table 7, where the key interatomic distances, [TM]−H, Si−H, and [TM]−Si, are listed for the crystallographically characterized complexes that are reported in Tables 3, 4, and 5 and the examples found in the footnotes to these tables. Also included in the footnotes to Table 7 are the Si−TM distances for complexes that have been prepared by other routes and are reported in the Cambridge Structural Database. The ranges of Si−TM distances for each of the transition metals (observed during this review period) are summarized in Table 8a. Table S1 (Supporting Information) shows interatomic distance values for compounds subdivided into the categories: those given in Table 8a, those from the footnotes to Table 7, and finally those from Tables 1 and 2. Section 5 is devoted to nonclassical interactions. In this case, the coupling constants derived from NMR data become a key factor for these interactions in solution in addition to solid state structures and IR absorption data. The data are included in Table 9. A discussion of fluxional processes in solution as determined by NMR methods appears in Section 6. Bonding and calculations appear in Section 7. Current efforts on the oxidative addition reactions of other Si−X bonds are reported in Section 8 and include material on the silyl complexes of lanthanides and actinides. There are approximately 400 references cited in the current review. C


Chemical Reviews

Review

Figure 2. Examples of stable NHSi silylenes that have been coordinated to transition metals (see Table 1).14

reagent (eq 3)6 or, more rarely, a SilylCalcium reagent (eq 4).7 Related to these is the reaction that involves a KSiR3 reagent that displaces a Cp− from Cp2Ni to form a new complex with a Ni−Si bond (eq 5).8 Neutral ligands may be displaced by a silyl anion, as was the case when (PzRMe2Si)3SiK(18-c-6) (pz = pyrazole; R = tBu, iPr) was reacted with W(CO)6 to give (PzRMe2Si)3Si−W(CO)5K(18-c-6).9

Cluster anions of general composition Sinx−, have also been reacted with metal complexes. The first transition metal complex of a silicide anion was isolated from the reaction of K6Rb6Si17 and [Ni(CO)6(PPh3)2] in liquid ammonia to give [Rb([18]c-6]2[K[18]c-6]2Rb4][{Ni(CO)2}2](μ-Si9)2]·22NH3 (eq 7),11 where a structural representation of the anionic portion is shown. In another report11 which also utilized a phase of similar composition, K6Rb6Si17, in reaction with R3PCuCl and [18]crown-6, led to formation of a salt of a different silicide cluster anion, [(MesCu)2(η3-Si4)]4− (eq 8).12 In this case, the starting K6Rb6Si17 was shown to contain [Si4]4− units.

Normally Si(II) species such as SiCl2 and SiH2 are considered reactive intermediates. Base-stabilized Si(II) species, however, can be isolated and “stored in a bottle”. Various stable silylenes have been synthesized since the first report in 1994.13 The more common grouping contains the N-heterocyclic silylenes, which recently have been the focus of a review by Driess and co-workers,14 and selected examples of these silylenes are shown in Figure 2. The examples shown are basically cyclic systems that contain an N−Si−N sequence and are arranged in Figure 2 by increasing ring size (first line) but also include more recent examples that link two silylene units in the expectation that such systems might act as bis-silylene chelates. The NHSi systems that are included in Figure 2 have been reported to coordinate to metal centers as illustrated in Table 1.15−47 The advantage of the 4-membered NHSi is the

When 2 equiv of KSi(SiMe3)3 were reacted with freshly prepared AuI in liquid ammonia, the aurate complex, K[Au{Si(SiMe3)3}2] was the only product detected in solution and isolated in good yields as a toluene solvate.10 If solutions of the initially formed complex were left, further reactions were observed, resulting in the formation of other aurate complexes starting with [(toluene)K][Au2{Si(SiMe3)3}]. If the reaction was conducted at room temperature, additional aurate complexes of varying composition were formed (eq 6),10 and conducting the reaction in CH3CN led to still another product, KAu5[Si(SiMe3)2]6, that is also included in eq 6, where Si−Si bond metathesis of the -Si(SiMe3)3 fragment occurred, producing neutral (Me3Si)3Au as a possible intermediate. D


Chemical Reviews

Review

Table 1. Selected Parameters in Complexes Prepared from Stable NHSis and a TM Complex

E


Chemical Reviews

Review

Table 1. continued

F


Chemical Reviews

Review

Table 1. continued

G


Chemical Reviews

Review

Table 1. continued

H


Chemical Reviews

Review

Table 1. continued

I


Chemical Reviews

Review

Table 1. continued

J


Chemical Reviews

Review

Table 1. continued

K


Chemical Reviews

Review

Table 1. continued

L


Chemical Reviews

Review

Table 1. continued

1 Unless specified otherwise, the 29Si signal of the complex is observed downfield of the free silylene. 2Computed from molecular single-bond and double-bond covalent radii from Chem. Eur. J. 2009, 15, 186−197, and Chem. Eur. J. 2009, 15, 12770−12779.335,355 3New complexes were reported from reaction of Si−Cl with MeLi and with LiBHEt3 (forming the SiH analog).15 4Data taken from Schmedake, T. A.; Haaf, M.; Apeloig, Y.; Müller, T.; Bukalov, S.; West, R. J. Am. Chem. Soc. 1998, 121, 9479.17 5Formed from the SiCl complex, LSi(Cl)·M(CO)5 (L = PhC(NtBu)2) by treatment with Me3SnF.19 6Data taken from So, C−W.; Roesky, H. W.; Gurulasavaraj, P. M.; Oswald, R. B.; Gamer, M. T.; Jones, P. G.; Blaurock, S. J. Am. Chem. Soc. 2007, 129, 12049−12034.25 7The bonding in 1-21b is best described as a resonance hyrid between the ferriosilicon(II) species Cp*(CO)2Fe−SiCp* and the donor−acceptor complex, [Cp*(OC)2Fe]− → [SiCp*]+. The Wiberg bond order for Fe−Si was calculated to be 0.541.26 8Connectivity was established in the crystal structure, but rotational disorder precluded assigning metrical parameters.29 9The free silylene was not isolated.30 10Data obtained from Gehrhus, B.; Lappert, M. F.; Heinicke, J.; Boesse, R.; Bläser, D. J. Chem. Soc., Chem. Commun. 1995, 1931− 1932.30 11The related complex, [N3]Ru(Cl)2{Si(NN)} (X-ray, Si−Ru = 2.3356(9) was formed from [N3]Ru(Cl)2(C2H4)] + (Si(NN) as reported in Gehrus, B.; Lappert, M. F. J. Organomet. Chem. 2001, 617, 209.30 12Complex 1-39 was isolated from reaction at low temperature.34 13Complex 1-40 was isolated at room temperature.34 14Isolated from the complex 1-39 generated at low temperature followed by addition of B(C6F5)3.34 15Prepared by reaction of LSi-Ni(CO)3, 1-43, (L = [HC(CMeNAr)(C(CH2)NAr)]) with H2O (Meltzer, A.; Präsang, C.; Driess, M. J. Am. Chem. Soc. 2009, 131, 7232−7233).37 161-45a was prepared by the reaction of LSi-Ni(CO)3 (L = [HC(CMeNAr)(C(CH2)NAr)]) with H(OEt)2B(C6F5)4.37 17145c was prepared by the reaction of LSi-Ni(CO)3 (L = [HC(CMeNAr)(C(CH2)NAr)]) with H(OEt)2B(C6F5)4.37 18Prepared by addition of HCl to 1-43, [LSi-Ni(CO)3]. Additional example prepared by reduction of the chloride with Li[BHEt3] (29Si = 45.1, 1JSiH = 154 hz; Si-N = 2.2524(8).38 19 Prepared by reduction with Li[BEt3H].38 20Additional complex prepared from silylene where Ar = 2,4,6-Me3C6H2, 29Si = 77.8.39 21Complex 1-53 was too insoluble to obtain a 29Si NMR spectrum.44 22Complex 1-57 appears to undergo reductive elimination to provide Cl2SiNN (identified in the 29 Si NMR spectrum) and additional signals in the NNSi-metal region which were not assigned.40 23Complex 1-58 undergoes fluxional processes which were studied by variable temperature multinuclear NMR.40 24The 29Si NMR spectrum in C6D6 at ambient temperature exhibited no signal and thus a fast dissociative equilibrium

presence of an additional group at the Si(II) center that can be elaborated through further reaction chemistry. A second grouping of stable silylenes are those where an NHC ligand is coordinated to the silicon center to give (NHC)SiX2, a three-coordinate Si(II) species. The chemistry of (NHC)SiCl2 has recently been reviewed by Ghadwal, Azhakar, and Roesky.48 Table 2 contains examples of metal complexes produced from NHC-stabilized silylenes as well as those formed from a cyclic silylene stabilized by bulky groups alpha to the silicon center (referred to as dialkylsilylenes). The silicon center in NHC → SiCl2 can function as a donor toward BR3

(among other Lewis acids), but in the complex formed, NHC → SiCl2 → BH3, the SiCl bonds were successfully reduced by LiAlH4 to give NHC → SiH2 → BH3. Since the BH3 is labile, it could be replaced by a transition metal to form, as an example, the complex NHC → SiH2 → W(CO)5 (2−5, Table 2).52 In another example, both the Si−Cl and the GeCl bonds in the complex [Cl2(IPr → )SiGeCl2 → W(CO)5] were reduced by LiAlH4 to give a stabilized form of the “inorganic ethylene” H2SiGeH2, as [H2(IPr → )Si-GeH2 → W(CO)5].58 Thus, stabilized SiCl2 or SiH2 systems can be attractive reagents for substitution at the silicon center. Another novel M


Chemical Reviews

Review

Table 2. Selected Parameters in Complexes Prepared from Carbene-Stabilized Silylenes and Dialkylsilylenes

N


Chemical Reviews

Review

Table 2. continued

1 Unless specified otherwise, the 29Si signal of the complex is generally observed downfield of that of the free silylene. Thus, in complexes 2-2b, 2-5, 2-10, 2-11, 2-12, 2-13, 2-14, 2-15, and 2-17 the 29Si shift is upfield. 2Computed from molecular single-bond and double-bond covalent radii from ref 349. or375. 3IMe2iPr2 = 1,3-dihydro-4,5-dimethyl-1,3-bis(isopropyl)-2H-imidazol-2-ylidene.50 4The authors describe the Cr−Si bond in the complex, 2-2, as a polar donor−acceptor double bond with the σ component polarized toward the Si atom and a π-component polarized toward the Cr center.50 5L2 = IMe2iPr2 = 1,3-dihydro-4,5-dimethyl-1,3-bis(isopropyl)-2H-imidazol-2-ylidene.50 6The Si−Br bond is considered longer than those normally observed in metal bromosilyl complexes, suggesting the presence of M(dπ)-σ*(SiBr) hyperconjugation, giving a polarized Si−Br bond.50 7[SiCl2(SIdipp)] was generated from HSiCl3 and 2 equiv of (SIdippH) but could not be purified due to its instability.50 8[SiCl2(SIdipp)]

O


Chemical Reviews

Review

Table 2. continued was too unstable to obtain 29Si NMR data.50 9Prepared by salt metathesis of the silylene with Li[CpMo(CO)3].51 10Data taken from Ghadwal, R. S.; Roesky, H. W.; Merkel, S.; Henn, J.; Stalke, D. Angew. Chem. Int. Ed. 2009, 48, 5683−5686.49 11NBO analysis gives a single weak covalent Co−Si bond with occupation 1.82226 from one of the two Si centers, and the second involves a strongly donating Si lone pair (occupation 1.05441; the authors considered this as no covalent bond). However, the two Si−Co bonds are chemically equivalent, thus requiring an average of two or more contributing natural Lewis structures. The authors conclude that the silylene donor (in 2-7) is a strong σ-donor and a weak π-acceptor.53 12 Additional example reported with coordinated benzene (29Si = 357.2 (Si), −3.4 (SiMe)).54 13Complexes 2-10 and 2-11 exihibit significant backdonation from the metal to the silylene ligand.54 14DFT calculations for the model complex Me2SiPdSiMe2 indicate a significantly bent Si−Pd−Si in the optimized geometry, due to π-back-donation. In the actual complex, 2-12, the Si−Pd−Si angle is linear and the authors suggested that the steric effects of the bulkier silylene ligands overwhelm the secondary-electronic effects.55 15Identified in solution only.56 16Additional Pd derivative was reported, [LSi-Pd(PCy3) (PMe3)] mixed with Cy3P (29Si, 424.7, −2.04).54 17Additional Pt derivative was reported, [LSi-Pt(PCy3) (PMe3)] (29Si = 392.0 (dd), −2.12(SiMe), −1.23 (SiMe)).54 18Other 29Si NMR data: −1.3 (SiMe3), −1.2 (SiMe3), −1.0 (SiMe3), −0.4 (SiMe3), 10.3 (iPrSi), and 15.5 (iPrSi). Dsi = CH(SiMe3)2.57 19The observed cis-geometry may indicate an attractive interaction between the NHC ligand and the zinc center.57

silicon and a metal center. In the earlier report, ClRSi(Im-Me4) (R-C6H3-2,6-Trip, Im-Me4 = tetramethylimidazol-2-ylidene) was converted through a salt-metathesis reaction with Li[CpMo(CO)2] to give the silylidene complex, 2-4.51 The thermolysis of 2-4 provided a silylidyne complex, Cp(OC)2MoSiAr [MoSi, 2.2241(7); (MoSi = 2.345); Σcovr(triple) = 2.15].51 In a second paper by Filippou and co-workers, a similar reaction sequence to give a (NHC)Cr analog, 2-2a (and the Si−Cl analog, 2-3), was reported.50 However, the related chromium silylidyne complex was experimentally not reported although calculations for [Cp(OC)2CrSi-X] (X = Br, Cl) were presented and the CrSi (calculated) distances were ∼0.007 Å shorter than the CrSi distances.50 Although [Cp(OC)2CrSi-X] was not isolated, the cationic silylidyne complex and X-ray structure of [Cp*(OC)2CrSi-SIdipp]+ (SIdipp = 1,3-bis[2,6-bis(isopropyl)phenyl]imidazolidin-2-ylidene) gave a Cr−Si distance of 2.1219(9) Å. The reactions of Cp(OC)2MoSi−R (R = C6H3-2,6-Trip2) with nucleophiles and redox reagents have also been reported.61 There are MSi complexes that are prepared by other routes that primarily involve a reaction of a Si(IV) reagent, and these will be covered in Section 3 of the review. In the last entry in Table 2, the reaction of an NHC (1,3,4,5tetramethylimidazol-2-ylidene) with the disilyne, RSi-SiSi-SiR, produced the silylene RLSiSiR: (R = SiiPr[CH(SiMe3)2]2, L = NHC). The novel silylene reacted with ZnCl2 to form an adduct, 2-17, with a Si−Zn distance of 2.3954(10), which is slightly longer that the sum of the covalent radii (2.34 Å).55 A seemingly attractive starting point for the formation of a metal complex could be the reaction with a disilene, R2Si SiR2, the silicon analog of an olefin. In order to stabilize the disilene to polymerization, the groups on silicon are usually quite bulky, which tends to limit the coordination of the disilene to a metal center. However, there were three reports during the review period, each with a unique disilene. Reaction of a stable 1,2-disilacyclohexene with the Pd(0) complex, Pd(PCy3)2, produced the disilene adduct (B-1) as shown in eq 10, where theoretical calculations for the model (Me3P)Pd(R2SiSiR2) (R = SiH3) indicated that the π-complex character is enhanced when the complex has a symmetric Y-shaped structure.63 The complex shown in eq 10 appears to have the strongest π-complex character of the disilene complexes known at the time, as it exhibited a symmetrical Y-shape. The analog, (Cy3P)Pd(R2SiSiR2) (R = SiMe2tBu, B-2), is T-shaped.413 Other unique disilenes, Tip2SiSi(Tip)PR2 (Tip =2 ,4,6-iPr3C6H2; R = Ph, iPr, Cy, tBu), have also been prepared, and a complex prepared from Pd(PCy3)2 is shown in eq 11.64 The bent-back angles of the substituents in B-1 and

NHC-stabilized system was produced from 1,3,4,5-tetramethylimidazol-2-ylidene, which, when reacted with the disilyne, i PrDsi2Si−SiSi−SiDsi2iPr, followed by ZnCl2 produced the complex 2-17.57 Acyclic, room-temperature-stable, two-coordinate silylenes were essentially unknown until recently, when two examples were published simultaneously. A novel, stable 2-coordinate silylene, Si{B(NDippCH)2}{N(SiMe3)Dipp}, stabilized by bulky groups (Dipp = 2,6-iPr2C6H3), has been reported.59 This remarkable silylene exhibited a low singlet−triplet gap (calculated, 103.9 kJ/mol) and activated H2 at room temperature. The second 2-coordinate silylene was Si(SArMe6)2 [ArMe6 = C6H3-2,6(C6H2-2,4,6-Me3)2], a V-shaped silylene with a S−Si−S angle of ∼90°. However, this silylene does not activate H2 (the computed energy separation between the silicon lone pair and the 3p orbitals was 4.26 eV).60 As yet, neither of these new silylenes have been incorporated as ligands to transition metal centers. The silylene complexes undergo some interesting reactions, of which a selected few will be included in this section. The silylene complex, 1-43, undergoes 1,4-addition in the presence of protic reagents (illustrated with H2S in eq 9 in the formation

of 1-45b; other protic reagents studied included H2O, HOTf, NH3, iPrNH2, and H2NNPhH.35 With a series of 5 complexes, the authors compared the carbonyl stretching frequencies of the Ni(CO)3 unit with analogous phosphine complexes, and the NHSi systems were shifted to lower wavenumbers in comparison to those of the phosphines. Thus, the β-diketiminate silicon(II) amide ligands were considered the strongest donors of the N-heterocyclic silylenes.38 Of further utility was the reaction of 1-43 with HCl, which placed a chloride on the Si center of the silylene to give the complex 1-46a and provided a functional group at silicon that could be reacted further.38 In the case of 1-46a, reduction with LiBEt3H provided [LSiHNi(CO)3], 1-46e, and the SiH in the new complex was shown to hydrosilylate alkynes (3 examples).38 Another reaction observed with the same LSi: reagent was insertion into a Zn−Me bond, giving complex 1-62.47 Reaction of Ar(Cl)Si:, stabilized by an NHC base, illustrated the use of salt metathesis in forming multiple bonds between P


Chemical Reviews

Review

This area has been quite active over the past few years (Tables 1 and 2). Several cases of the coordination of SiSi bonds to metals have also appeared but are not expected to develop as a major synthetic route to Si-TM complexes due to their more difficult preparation and the necessity of the presence of bulky groups necessary to stabilize the SiSi multiple bond. In the next section, the versatility of the reaction SiH + TM for Si−TM bond formation will be amply illustrated.

B-2 are less than 10° whereas those in B-3 are 16° and 17° and indicate that complex B-3 has the greater metallacyclopropane character, as suggested in the structural representation. An interesting feature of B-3 is that the disilene coordination appears to compete with the coordination of the phosphine, which might be expected with extensive π-back-donation from Pd to the SiSi. Another uniquely substituted disilene that was reported was Bbt(Br)SiSi(Br)BBt (Bbt = 2,6-[CH(SiMe3)2]2-4-[CH(SiMe3)2]-C6H2). In this case the reaction of the disilene with Pd(PCy3)2 gave an intractable mixture with Pt black. However, the reaction with Pt(PCy3)2 took an entirely different path, as shown in eq 12. The SiSi was “cleaved” and a silylene−platinum complex was formed.65 Calculations showed a high s-character of the Si−Pt bond that was supported by increased 1JSiPt and 2JSiP coupling constants. From the calculations for a model compound (Bbt = 2,6-[CH(SiMe 3) 2 ]2 C6 H 3), the main Si−Pt bonding interaction originates from the s(Si) → 6s*(Pt) σ-donation with a Wiberg bond index (model) of 0.9924. A disilene complex of TiCp2 has also been reported from the reaction of a 1,2-dipotassiodisilane with Cp2TiCl2 (eq 13).66 The first complexes of a disilyne, in this case, Rs-SiSi-Rs (Rs = 1,1-bis(trimethylsilyl)-3,3-dimethylbutyl), with [(Cy3P)2Pd] and also with [(Cy3P)2Pt] have been reported. The Pd complex is shown in eq 14. Calculations seemed to favor significant metallacyclic character for the complexes.67 More recently, a stable silene has been shown to interact with both Ni and Pt, as shown in eqs 15 and 16.68 Clearly there are other methods by which Si-TM bonds are formed, in addition to the usual metathesis reactions of active organometals with chlorosilanes or silyllithium reagents with TM-Cl complexes, in addition to the subject of the current review: reactions of hydrosilanes and transition metal complexes. A relatively new entry into the scene is the use of Si(II) compounds that have been used to coordinate to metal centers.

3. REACTIONS OF HYDROSILANES WITH TRANSITION METAL COMPLEXES The products that have been obtained from the reactions of hydrosilanes with transition metal complexes are summarized in Tables 3 (primary silanes),69−117 4 (secondary silanes),71,118−149,418,415 and 5 (tertiary silanes)primarily for products that contain a Si−TM bond. Some researchers study reactions of primary and/or secondary and/or tertiary silanes. The number given in the earliest table is retained in the other two tables. A few reactions do not generate a Si−TM bond as the silane reacted with a ligand of the complex, but these cases are included in the tables. Silanes may also function as reducing agents, transferring a hydride to the metal center, and examples of this reactivity will be found in Table 6 (Section 3.7).215−238 Selected structural representations of products (indicated by an asterisk with the compound number in the body of the table) will be found at the end of the tables. For Tables 3−5, the product will be listed in the left-most column along with a number that designates that complex for the rest of the review. The second column contains both the metal complex and the hydrosilane reactant. The yield, color of the complex, and melting point (if supplied) follow in the third column. The key NMR data, solvent, and 1H NMR shifts for M−H, Si−H, as well as 29Si NMR data are supplied in the next columns. Additional characterization methods and the references are provided in the last 2 columns. The intent of Section 3 is to describe the various types of ligands (particularly chelates) that have been incorporated into the reacting metal complexes as well as silanes themselves that Q


Chemical Reviews

Review

Table 3. Complexes Formed from Reaction of RSiH3 with Transition Metal Complexes

R


Chemical Reviews

Review

Table 3. continued

S


Chemical Reviews

Review

Table 3. continued

T


Chemical Reviews

Review

Table 3. continued

U


Chemical Reviews

Review

Table 3. continued

V


Chemical Reviews

Review

Table 3. continued

W


Chemical Reviews

Review

Table 3. continued

X


Chemical Reviews

Review

Table 3. continued

Y


Chemical Reviews

Review

Table 3. continued

129

Si may have been determined by direct observe, INEPT or DEPT, or 2D 29Si−1H correlation. These methods are not distinguished in the table. Deuterated solvent key: toluene, C7D8; methylene chloride, CD2Cl2; benzene, C6D6; tetrahydrofuran, C4D8O. Ambient temperature unless otherwise noted. Temperatures in degrees Celsius. Solvent Key: C7D8, toluene; CD2Cl2, methylene chloride; C6D6, benzene; C6H5F, fluorobenzene; C4D8O, tetrahydrofuran. 3In ppm. Coupling constants for MH (or MSi) or SiH are in Hz. Assignments: s, singlet; d, doublet; t, triplet; q, quartet; quin, quintet; m, multiplet; br, broad; vt = virtual triplet, sat = satellite. Multiplicities are given directly as found in the experimental portions, including the Supporting Information. 4Will contain other characterization methods, including spectroscopic, X-ray, as well as calculations, if reported. If elemental analyses were reported, this will be indicated by EA. If analysis is outside ±0.5% of the calculated percentage value for carbon, this will be indicated by the symbolism [EA]. The 13C NMR data are proton decoupled unless indicated otherwise. 5The equilibrium constant for the reaction at room temperature was 3.04 for [1-1b][DMAP]/[LScNDIPP(DMAP)][PhSiH3].69 6Signals for SiH2 and ArCHMe2 overlap. Si NMR data from 29Si (DEPT 45) measurement.70 7Additional derivative reported from reaction of R = H3SiArF (ArF = 3,5-(CF3)2C6H3), 76%, X-ray; H3SiArOMe (Ar0Me = 4-MeOC6H4), 76%, X-ray; R = Bu, 22%) as well as PhSiD3, which demonstrated that D was incorporated in both the Ti−H and Si−H positions.71 8For the equilibrium between PhSiH3 and Cp*Ti{MeC(NiPr)2}(NNMe2) with 3-3 (R = Ph) measured from 293 to 349 K.71 9 Obtained for the models, CpTi{MeC(NMe2)} and MeSiH3.71 10Data given are for the C1 isomer. Data provided for the CS isomer: 1H, 13C, 15N, 29 72 11 Si. H3SiPh is the source of the SiH4 in the product.73 12Cartesian coordinates for geometry optimized structures are given in Table S2 of the publication. Calculations used DFT as implemented in the Jaguar 7.7 suite of ab initio quantum chemistry programs. “The geometry optimizations 2

Z


Chemical Reviews

Review

Table 3. continued were performed with the B3LYP density functional using the 6-31G** (C, H, N, and P) and LACVP** (Mo) basis sets. However, the results of these calculations were not discussed by the authors.73 13Ar = 2,6-diisopropylphenyl. Reaction also reported with PhSiD3.74 14Product was contaminated with Ph2SiH2 (∼5%) and (Cp)(ArN)Mo(SiH2Ph)(PMe3) (7−10%).74 15Ar = 2,6-diisopropylphenyl and Ar′ = 2,6dimethylphenyl.75,76 3-8 was isolated as a mixture of diastereomers (>95/5) and is fluxional at rt. 1H and 31P NMR reported in C6D6 and in C7D8 at rt.75 16Reactions of 3-9 with PhSiD3 resulted in incorporation of Si-D (∼55/45) into all SiH positions). Reaction also conducted with (m-Tol)SiH3.75 17Agostic Si−H was not observed.75 18When the reaction was monitored by NMR, an intermediate, (Ar′N)Mo(SiH2Ph) (PMe3)2(NAr′{SiH2Ph}), was observed.75 19When 3-10A and 3-10B were prepared in the presence of BPh3, the yields were 75% and 14%, respectively.75 20Complex 3-11 decomposed on attempted isolation. Proton NMR data also recorded for room temperature and −53 °C.75 21An additional derivative of complex 3-12 was prepared from [(Ar′N)Mo(η2-BH4)2(PMe3)2] (Ar′ = 2,6-Me2C6H3).76 22Reaction with PhSiD3 was also reported on an NMR scale. Scrambling of deuterium between silane, silyl, and hydride positions was observed with 59% H/D exchange for the hydride position and 65% H/D exchange in the silyl position.76 Additional example prepared from [(Ar′N)Mo(BH4)2(PMe3)2] (Ar′ = 2,6-Me2C6H3).76 23Additional derivatives prepared from (p-Tol)SiH3, (3,5-Xylyl)SiH3, and C6F5SiH3. The bulkier silanes MesSiH3, TripSiH3, and DMPSiH3 did not react with the starting material. Reaction of the secondary silane PhClSiH2 is found in Table 2.78 243-15a was originally reported in Watanbe, T.; Hashimoto, H.; Tobita, H. Angew. Chem. Int. Ed. 2004, 43, 218−221 from photolyis of Cp′(Me)W(CO)3 with H3SiC(SiMe3)3. NMR data are taken from this reference. Labeled analogs, [D2]3-15a and [D2]3−15b were also reported in ref 77 (2H and 29Si NMR data were identical to those of the protio analog except for the absence of signals for the W−H and Si−H groups). In addition, the product from reaction of the W complex with H2SiMes2 also was reported (Table 2). A neutron diffraction study of [D2]3-15b was also reported.79 25Additional derivative L2 = μ-dppm (oil), fac isomer produced from H3SiPh.81 26The (C−N−C) ligand is (2,6-bis(2,6-diisopropylphenyl)imidazole-2-ylidene)pyridine, and (C−NMe-C) = 2,6-bis(arylimidazol-2-ylidene)-3,5-dimethylpyridine (aryl =2,6-iPr2C6H3. Additional derivative prepared similarly: (2,6-bis(2,6diisopropylphenyl)imidazole-2-ylidene)-3,5-dimethylpyridine iron(II) bis(o-tolylsilyl)(η2-o-tolylsilane) (X-ray).82 27Solvent for 29Si NMR data.82 28 All attempts to obtain 1H NMR data were hampered by signal broadening.82 29LiPr = CH2CH(CH3)(3-isopropyl-4,5-dimethylimidazol-2-ylidene1-yl. See also Table 5 for product from Ph3SiH.84 30Related complexes were prepared from reaction with HSiCl3, HSiCl2Me (X-ray), HSiMe2Cl (X-ray), and HMe2Ph.87 31Additional derivative will be found in Table 4.88 32Additional derivatives prepared from reaction with H3Sidmp (dmp = 2,6-Mes2-C6H2; 79%) and H3SiMesF (2,4,6-(CF3)3-C6H2, 81%, X-ray). The reactions of H3SiPh and H3SiMes gave intractable product mixtures.89 33 Cp*(iPr2MeP)Ru(η3-CH2Ph) was generated in situ from Cp*(iPr2MeP)RuCl and Mg(CH2Ph)2(THF)2.89 34Attempts to obtain 29Si NMR data with direct detection and 2-D (HMBC) experiments (RT and variable temperature) were unsuccessful.89 35See Tables 4 and 5 for additional examples.90 36Additional complexes prepared from reaction of H3SiMes (90%), H3SiC6F5 (22%), H3SiSiPh3 (40%), and H3SiSi(SiMe3)3 (22%), as well as from secondary silanes (see Table 4).91 37Two additional derivatives of 3-33 were reported from the reaction of H3SiMes (X-ray) and H3SiPh.94 38X-ray data was of poor quality, and twinning problems did not allow the refinement of the structure to an acceptable value.95 39 [(CSi)Co(IMes′)] (CSi = bidentate silyl donor containing N-heterocyclic carbene ligand, IMes′ = cyclometalated IMes ligand, IMes = 1,2-dimesitylimidazol-2-ylidene). 1H NMR data are broad and were unassigned.96 Additional examples reported from reaction of PhMeSiH2 (4-26a) and Ph2SiH2 (Table 4, footnote 29).96 40By SST NMR (1H) there is no exchange between Si−H and Ir−H at −78 °C.98 41Additional complex prepared in the same way from H3SiMes (X-ray).98 42Reaction of 3-38 with D3SiPh resulted in deuterium scrambling in both silylene groups.98 43 Can also be prepared from 3-40 and PhSiH3.98 443-41 was also formed by addition of H3SiPh to 3-40. A similar experiment was performed using D3SiPh with deuterium scrambled over both silylene groups.98 45When 3-40 was reacted with H3Si(3,5-F2C6H3) (2 equiv), a mixture of 3-43(33%), 3-40 (50%), and 3-42 (17%) was obtained.98 46Related derivative prepared from H3SiMes.99 47Due to disorder, the structure could not be refined.99 48 Complex 3-48 could not be isolated from the product mixture.100 49Additional complexes prepared from reaction of (PNP)Ir(H)2 and H3SiCy (85%), H3SiC6F5 (76%) and H3SiTrip (trip =2,4,6-iPr3C6H3; 73%).101 50Additional complex formed from reaction of (PNP)Ir(H)2 and H3SiMes (two diastereomers; 56%).101 51Additional complex formed from reaction of H3Si(Xyl) (48%).102 52NMR data were also recorded at rt.109 53 The analog, [{1,2-C6H4(SiMe2)(SiH2)}Pd(dmpe)], was previously reported: Shimada, S.; Li, Y. H.; Choe, Y. K.; Tanaka, M.; Bao, M.; Uchimaru, T. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 7758.110 54INEPT measurement set to the SiMe2 unit.110 55DEPT measurement set to the SiH unit.110 56 [(μ-dcpe)Pd]2 was generated in situ by reduction of [PdCl2(dcpe)] with excess LiBEt3H. The 1JSiH value was determined by 31P-decoupled 1 H NMR at −30 °C.111 57Based on the reported integrated intensity at −20 °C.111 583-66-d4 was prepared analogously. 2H{1H} in toluene, −50 °C: 7.08 (br, 1D, SiD, 1.00 (br, 1D, SiDPd.112 59The corresponding complex [Cy-PSiP]PtSiH2Ph was also reported in 96% yd.114 60 Spectroscopic data in the table are those reported at 27 °C. Additional parameters were given for −80 and 90 °C.114 61Six isomers are possible, but only two were observed.115 62(depe)Pt(PEt3)2 was formed in situ by the reaction of Pt(PEt3)4 with depe.115 63Corresponding derivative [Pt3(dppe)3(μ3-SiH)2] (X-ray) was prepared by reaction of (dppe)Pt(CH2CH2) with H3SiSiMe2tBu (17%).116

that is at least two oxidation states lower than the maximum for that metal, or by a sigma-bond metathesis route which requires an empty frontier orbital on the metal. The latter is more often invoked for the early transition metals and rarely for the electron rich metals. If a reacting metal complex is coordinatively saturated, a ligand must be lost prior to reaction with the silane. The common precursor complexes in that case will contain a neutral ligand such as CO, PR3, olefin, alkyne, N2, or solvent, whose loss can often be promoted by photolysis or thermolysis to provide an open coordination site for a reaction. The number of commercially available complexes with easily displaceable ligands is relatively small, and a feature that distinguishes many of the ligand systems reported during this review period is the larger number of ligands that have been incorporated into a precursor complex that had to be synthesized prior to any reaction with a hydrosilane. Thus, there are synthetic steps before incorporation of the silane, but these

may also react as a type of chelate (Section 3.1). The next focus is on the ligand that is eliminated upon reaction with the silane and includes loss of neutral ligands (Section 3.2; PR3, CO, alkenes, alkynes, solvent, and H2); loss of “anionic” ligands, i.e. TM-R (Section 3.3, R = organic substituent, TM-H, or TM-SiR3); oxidative addition to a low valent metal without ligand loss (Section 3.4); reactions of silanes and/or complexes with 2 or more metal centers (Section 3.5); reactions of silanes that are not easily classified (Section 3.6); and finally, those reactions that lead to TM−H bonds but not to TM−Si bonds (Section 3.7). 3.1. Types of Ligands at the Metal and Substituents at Silicon

The most common reaction pathways when a hydrosilane interacts with a transition metal complex occur either by an oxidative addition to a coordinatively unsaturated metal center AA


Chemical Reviews

Review

Table 4. Complexes Formed from Reaction of R2SiH2 with Transition Metal Complexes

AB


Chemical Reviews

Review

Table 4. continued

AC


Chemical Reviews

Review

Table 4. continued

AD


Chemical Reviews

Review

Table 4. continued

AE


Chemical Reviews

Review

Table 4. continued

AF


Chemical Reviews

Review

Table 4. continued

AG


Chemical Reviews

Review

Table 4. continued

AH


Chemical Reviews

Review

Table 4. continued

129

Si may have been determined by direct observe, INEPT or DEPT, or 2D 29Si−1H correlation. These methods are not distinguished in the table. Deuterated solvent key: toluene, C7D8; methylene chloride, CD2Cl2; benzene, C6D6; tetrahydrofuran, C4D8O. Ambient temperature unless otherwise noted. Temperatures in degrees Celcius. Solvent Key: C7D8, toluene; CD2Cl2, methylene chloride; C6D6, benzene; C6H5F, fluorobenzene; C4D8O, tetrahydrofuran. 3In ppm. Coupling constants for MH (or MSi) and SiH are in Hz. Assignments: s, singlet; d, doublet; t, triplet; q, quartet; quin, quintet; m, multiplet; br, broad; vt = virtual triplet; sat, satellite. Multiplicities are given directly as found in the experimental portions (including the Supporting Information). 4Will contain other characterization methods, including spectroscopic, X-ray, as well as calculations, if reported. If elemental analyses were reported, this will be indicated by EA. If analysis is outside ±0.5% of the calculated percentage value for carbon, this will be indicated by the symbolism [EA]. The 13C NMR data are proton decoupled unless indicated otherwise. 5Reaction of Cp*Ti{MeC(NiPr)2}(NNPh2) with PhClSiH2 produced CpTi{N(NPh2)SiH2Ph}Cl2, and Cp*Ti{MeC(NiPr)2}(N-Tol) produced Ti{MeC(NiPr)2}(Cl){N(Tol)SiH2Ph}. Reaction of Cp*Ti{MeC(NiPr)2}(NNMe2) and PhSiH2Br gave Cp*Ti{MeC(NiPr)2}(Br){N(NMe2)SiH2Ph}, which decomposes above −53 °C.418 64-1b was previously reported in ref 337. A related derivative, [(CpMe)2Ti(PMe3)(HSiHPh2)] was also reported, as was the decomposition product, [(CpMe)2Ti(PMe3)(HSiHPh2)] (X-ray).118 7Also prepared from addition of the alkyne to Cp2Hf(nBu)2 in 84% yd.119 8Cartesian coordinates for geometry optimized structures are given in Table S2. Calculations used DFT as implemented in the Jaguar 7.7 suite of ab initio quantum chemistry programs. “The geometry optimizations were performed with the B3LYP density functional using the 6-31G** (C, H, N, and P) and LACVP** (Mo) basis sets. However, the results of these calculations are not discussed by the authors.73 9Reaction conducted at room temperature only provided cis-[MoH2X2(dppe)2]. Additional derivatives formed at 110 °C: R = c-C6H11, X = Cl (74%), R = Ph, X = Br (80%).121 10R = tBu-2,6-[P(O)(OiPr)2]2C6H2-.123 11Other secondary silanes, Ph2SiH2, Et2SiH2, and MesClSiH2 did not react. Reactions with primary silanes are found in Table 3.78 12Authors prepared the sample by literature methods.124 13Additional derivative L2 = μ-dppm (79%).81 14 (C−N−C) = 2,6-bis(arylimidazol-2-ylidene)pyridine. (C−Nme−C) = 2,6-bis(arylimidazol-2-ylidene)-3,5-dimethylpyridine (aryl =2,6-iPr2C6H3). Attempts to prepare the (C−NMe−C) analog of 4-13a failed.82 15[(CSi)Fe(IMes′)] (CSi = bidentate silyl donor containing N-heterocyclic carbene ligand, IMes′ = cyclometalated IMes ligand, IMes = 1,2-dimesitylimidazol-2-ylidene). Additional derivative from reaction of H2SiPh2 (X-ray). 1 H NMR data are unassigned.125 16An additional complex was prepared from the secondary silane, H2SiPhCl (2 diastereomers, 80:20; X-ray of one of the diastereomers). Also prepared was the deuterium analog of 4-15.86 17The 31P(1H) NMR spectrum at −80 °C shows a single resonance indicating fluxional behavior and potential scrambling of the hydride and silane hydrogens. Deuterium labeling studies (4-17 generated from D2SiPh2) confirm scrambling between the three hydrogen atoms of 4-17). The reaction of (d30-SiPPh3)Ru(H)(N2) with D2SiPh2 was also reported.127 18Additional derivative prepared from reaction of H2SiPhMe.127 19[SiPph2][P′Ph]Ru was prepared from [SiPPh3]RuCl by addition of MeLi at low temperature followed by warming to room temperature and elimination of MeH.127 20Trip = (trip = 2,4,6-iPr3-C6H2. Additional derivatives prepared from reaction with H2Si(dmp)Cl (59%) and H2Si(trip)Cl (62%; X-ray).89 21Cp*(iPr2MeP)Ru(η3-CH2Ph) was generated in situ from Cp*(iPr2MeP)RuCl and Mg(CH2Ph)2(THF)2.89 22D2O in a sealed capillary was used as an internal standard.90 23Additional complexes prepared from the reaction of H2SiPhMe (81%), H2SiEt2 (56%), H2SiiPr2 (53%), H2SiFlu (Flu = C12H8, 42%, X-ray).91 24The related complex, [PhBPPh3]Ru(H)(CN(Xyl)(η3-HSiHPhMe)) was also reported.128 25Additional complex prepared from reaction of H2SiPh2 (95%).92 26Λ-[cisRu(R,R)-Me-(BPE)2(PhSiH2)(H)] (X-ray) formed from PhSiH3 and Λ-[cis-Ru(R,R)-Me-(BPE)2(H)2].93 27The analogous PhSiH2- system as well as Δ-R,R-Ru2(Et2SiH)(H) (X-ray) were prepared in a like manner.93 28IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene (OTf = trifluoromethanesulfonate).130 29[(CSi)Co(IMes′)] (CSi = bidentate silyl donor containing N-heterocyclic carbene ligand, IMes′ = cyclometalated IMes ligand, IMes = 1,2-dimesitylimidazol-2-ylidene). Remaining substituents on Si are Me, Ph. Additional derivative from reaction of H2SiPh2 (X-ray). 1H NMR data are unassigned.96 30Analog of 4-26b was prepared from (dtbpe)Rh(CH2Ph) + H2SiEt2.105 31An additional derivative was prepared by the reaction of RhCl{xant(PiPr2)2} and HSiEt3.132 Decomposition at 50 °C for 5 days gave Rh(SiClPh2){xant(PiPr2)2} (X-ray).132 32 No attempt was made to isolate 4-31. A 13C enriched analog was also reported.98 3313C-enriched sample also prepared.98 34Both complexes decompose in solution at room temperature over 12 h.98 35Related derivatives prepared from reaction of PhClSiH2 (X-ray), PhMeSiH2(X-ray), and 2

AI


Chemical Reviews

Review

Table 4. continued from [RhIr(H)2(CO)2(μ-SiHC6H2F3)(dppm)2] and PhMeSiH2 (X-ray) and (2,4,6-C6H2Me3)SiH3.100 36Related derivative prepared from reaction of PhMeSiH2 (X-ray).100 37When 4-41b was heated at 50 °C for 48 h, IrH2(SiClPh2){xant(PiPr2)2} (X-ray) was isolated.132 384-45 was also prepared by reaction Cp2Fe+B(ArF)4 with (dtbpe)Ni-SiHMes2 in 85% yd.135 39At −60 C in C7D8, the SiH resonance is observed at 6.25 ppm with 1 JSiH = 167.9 Hz).137,138 404-47 was also prepared from 4-49 and 2 equiv of dmpe.139 4129Si was not observed due to decomposition of the complex during the measurement.140 424-50 was also prepared from Ni(COD)2 and Ph2SiH2 in the presence of (iPr3P).108 43Two related complexes were reported from RSiH3 (see Table 3). The SiH in 4-54 was not assigned due to overlap with Cy resonances, and the complex decomposed during the attempt to obtain the 29Si data.112 44The PEt3 ligands at cis-4-55b, trans-4-55b, and a Pt dimer overlap. Additional 1H, 29Si HMBC data: 8.04 (ortho Ar); 7.3/6 (meta Ar). Spectroscopic data for the Pt2 complex were also included.415 45Similar derivative prepared from the reaction of (Cl-nacnac)Pt(H)(1-pentene) with D2SiPh2, 4-55-d2, which demonstrated no H−D coupling in the 1H NMR and that the complex is a classical dihydride.145 No exchange between bound and free silane was observed.145 46When a solution of 4-57 was warmed to 0 °C for 2 days, a 1:1 mixture of 4-57 and 4-58 was obtained.146 47For additional derivatives, see Table 5.147

and the R groups contain pendant arms that terminate in a coordinating base (most often a phosphine), thus forming an E(∩LB)2 (LB = Lewis Base) sequence (shown are PNP in eq 17 and PSiP in eq 19). Thus, three positions in the coordination sphere of the metal are occupied by the pincer ligand. A modification of this approach involves a ligand of the general formula, E(∩LB)3 (E is usually Si or B) that then occupies four coordination sites at the metal. Examples of selected chelates (bidentates through tetradentates) utilized in metal precursors during this review period are shown in Figure 3. Only one example is shown for a specific chelate, and additional examples will be found in the tables. Generally, the chelates have a descriptive abbreviation that identifies the chelate, and these abbreviations (when supplied by the authors) are also listed in the tables. As can be seen in Figure 3, the chelates with a Lewis base portion in the pendant arms can contain P, N, or S. If silicon is the central element in the chelate, it is designated as an anionic center and is derived from a hydrosilane precursor. Certain silane systems have been used in the same sense as a chelate, that is, to tie together adjacent sites at the metal center. Examples include 1,2-(HMe2Si)2C6H4170,220 and xantsilH2 [xantsil = (9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl)]154 (Figure 4). Representative reactions are shown in eqs 20−22.

must obviously start with a commercial source of the metal. For the purpose of incorporating an anionic ligand at the metal, a metal halide (particularly a chloride) is often used. This includes the simplest metal compounds, such as MCl2, such as reported by Peters and co-workers,164 and other metal chlorides that also contain replaceable olefins and phosphines (partial listing of examples): (COD)RuCl2,187 [(COE)2MCl]2 (M = Rh,102 Ir131), [(COD)MCl]2 (M = Rh,196 Ir192), and [(COD)PdMeCl].212 Other commercial sources contain M(0) coordinated to phosphines and/or olefins, for example: [Ni(COD) 2 ], 139,140 Ni(PPh 3 ) 4 , 210 Pd(*PEt 3 ) 4 , 110 and Pt(PPh3)4.212 Selected examples of the use of metal halides in the preparation of complexes that were then used to react with a hydrosilane are shown in eqs 17 and 19 and similarly from

a commercial reagent in eq 18. One of the functions of the M−halide bond is to be a coreactant in a salt-metathesis process to build a more decorated complex, and an example is shown in eq 17, for the preparation of a PNP-pincer ligand, and in eq 18, for the formation of a novel unsymmetrical N-chelate. Another tactic (elimination of HX) is shown in eq 19 in the reaction of a synthesized hydrosilane with PdBr2. Although the reacting metal compound in eq 18 is commercially available, the majority of the reacting complexes listed in Tables 3−5 are prepared from precursors reported in the literature, thus adding to the steps required to incorporate a desired ligand into a targeted [TM]Si complex. Illustrated in eqs 17 and 19 are examples of the construction of pincer ligands, a feature of which includes an R2E¯ unit where the central element (N in eq 17 and Si in eq 19) becomes bound to the metal center

A disilylbenzene with two different silyl groups 1-(dimethylsilyl)2-silylbenzene has also been reported in the reaction with (depe)Pt(PEt3)2 to give two isomers of 3-72A and 3-72B {1,2C6H4-(SiMe2H) (SiH2)}-{1,2-C6H4(SiMe2)(SiH2)}(H)PtIV115 AJ


Chemical Reviews

Review

Table 5. Complexes Formed from Reaction of R3SiH with Transition Metal Complexes

AK


Chemical Reviews

Review

Table 5. continued

AL


Chemical Reviews

Review

Table 5. continued

AM


Chemical Reviews

Review

Table 5. continued

AN


Chemical Reviews

Review

Table 5. continued

AO


Chemical Reviews

Review

Table 5. continued

AP


Chemical Reviews

Review

Table 5. continued

AQ


Chemical Reviews

Review

Table 5. continued

AR


Chemical Reviews

Review

Table 5. continued

AS


Chemical Reviews

Review

Table 5. continued

AT


Chemical Reviews

Review

Table 5. continued

AU


Chemical Reviews

Review

Table 5. continued

AV


Chemical Reviews

Review

Table 5. continued

AW


Chemical Reviews

Review

Table 5. continued

AX


Chemical Reviews

Review

Table 5. continued

AY


Chemical Reviews

Review

Table 5. continued

AZ


Chemical Reviews

Review

Table 5. continued

BA


Chemical Reviews

Review

Table 5. continued

BB


Chemical Reviews

Review

Table 5. continued

BC


Chemical Reviews

Review

Table 5. continued

129 Si may have been determined by direct observation, or INEPT or DEPT or 2D 29Si−1H correlation. These methods are not distinguished in the table. 2Deuterated solvent key: toluene, C7D8; methylene chloride, CD2Cl2; benzene, C6D6; tetrahydrofuran, C4D8O. Ambient temperature unless otherwise noted. Temperatures in °C. Solvent Key: C7D8, toluene; CD2Cl2, methylene chloride; C6D6, benzene; C6H5F, fluorobenzene; C4D8O, tetrahydrofuran. 3In ppm. Coupling constants in Hz. Assignments: s, singlet; d, doublet; t, triplet; q, quartet; quin, quintet; m, multiplet; br, broad; vt = virtual triplet. 4Will contain other characterization methods, including spectroscopic, X-ray, as well as calculations if reported. If elemental analyses were reported, this will be indicated by EA. If analysis is outside ±0.5% of the calculated percentage value for carbon, this will be indicated by the symbolism, [E.A.]. The 13C NMR data are proton decoupled unless indicated otherwise. 5Previously reported: Varga, V.; Mach, K.; Polásě k, M.; Sedmera, P.; Hiller, J.; Thewalt, U.; Troyanov, S. I. J. Organomet. Chem. 1996, 506, 241.119 6Previously reported: Peulecke, N.; Ohff, A.; Kosse, P.; Tillack, A.; Spannenberg, A.; Kempe, R.; Baumann, W.; Barlakov, V. V.; Rosenthal, U. Chem. Eur. J. 1998, 4, 1852.119 The NMR characteristics of the product in the current study matched those earlier report.119 7Reaction of Cp*Ti{MeC(NiPr)2}(NNMe2) with HSiMe2Cl provided Cp*Ti{MeC(NiPr)2}(Cl){N(NMe2)SiHMe2}.418 85-5b was reported previously in ref 338. The X-ray structure was redetermined in the current study.118 9A mixture of zirconocene species was obtained, but two distinguishable, related sets of signals were assigned to isomers of 5-6. The signals were correlated with the aid of 1D (1H, 13C{1H}, 13C DEPT, 29Si{1H}, 29Si 1H-coupled INEPT) and 2D NMR measurements (HMQC, HMBC, 1 H-29Si HMQC).119 10When the mixture was crystallized from n-hexane at −78 oC, 5-9A and 5-9B were obtained in a 4:1 ratio.119 11Cp2HfCl2 + Mg + PhC2SiMe2H/THF (60 oC/5 days) gave 5-9B and 5-9C in ∼1:4.5 ratio.119 123-10 was formed in 95% purity.119 13Tbt = 2,4,6tris[bis((trimethylsilyl)methyl)]phenyl-. The 1H resonances were also reported at 50 °C in C7D8 and occurred at −7.38 ppm with 1JSiH = 91.2 Hz.150 14 Additional derivative also prepared, Cp*(CO)2Mo(η3-Ph2Si(CCiPr)), X-ray.151 1513C data recorded at −70 °C.151 16Additional derivatives: R′2 = MePh, Ph2, MeCl. NMR tube reactions were reported for R = 2,6-diisopropylphenyl, R′2 = Me2, MePh, MeCl and for R = 2,6-dimethylphenyl, R′2 = Me2(VT) and MeCl(VT).153 17Additional derivatives prepared from HSiPhMe2 (67%), HSiFcMe2 (59%).156 18Ru-analog of 5-22a also reported (42% yd; X-ray).156 19Additional derivatives prepared from HSiMe2Ph (86%), HSiMe(p-Tol)2 (71%, X-ray), HSi(p-Tol)3 (66%). Shown is the kinetic product. Heating gives the thermodynamic product which results from rearrangement to Cp*(OC)2W(DMAP)(SiMe2Ph) for 5-23a (analogs of the others also prepared; X-ray of product from HSi(p-Tol)3 has been previously reported). Complexes were also prepared from the photoreaction of a mixture of Cp*(OC)3WMe and HSiR2Ar.158 Photolysis of the thermodynamic product produces the silylene complex.158 20Ar′ = 2,6-dimethylphenyl; Ar = 2,6-iPr2C6H3. The addition of PMe3 to the mother liquor from 5-24 gave an isomer (ArN)(ArNSiMeHCl...)W(PMe3)2Cl.153 21The sign on the

BD


Chemical Reviews

Review

Table 5. continued JSiH was determined and is negative and thus indicates direct Si−H bonding.153 22R = tBu-2,6-[P(O)(OiPr)2]2C6H2-.123 23Additional derivative, R = Me.159 24cis-5-29 converts to the trans-form in solution over 3 days.159 25A sample of 5-23 was isolated on a preparative scale run in ∼95% purity.155 26 An analog was prepared from (CpMe)W(CO)3Me and HSiEt2SiMe2OMe.161 27Doublet of octets attributed to 1-bond coupling (JSiH = 46 Hz) with a H of the 5-35 Mn−H−Si bond, to 2-bond coupling (JSiH = 6 Hz) to the SiMe2, and to 3-bond coupling (JSiH = 6 Hz) with the ortho-aromatic H of the silicon atom in the xanthene backbone.162 28The IR absorption band for Mn−H was not observed in the region of 1700−2200 cm-1.162 29 The reaction of [ReBr2(MeCN(NO)(P∩P))], (P∩P) = (η5C5H4PPh2)2Fe(dppfc), [(η5-RC5H4)2Fe, R = PiPr2(diprpfc)], 2,2′-bis(diphenylphosphino)diphenylether (dpephos), 1,11-dihydro-4,5-bis(diphenylphosphino)dibenzo[b,f]oxepine (homoxantphos) with HSiEt3 yielded the analogs of 5-38 but were characterized only in solution.163 30An analog of 5-40 was prepared from H[SiPPh3] and FeCl2, in 53% yd.164 31The intermediate complex [SiPiPr3]FeCl2 could be isolated and was characterized by X-ray diffraction.164 32The intermediate complex [SiPiPr2SAd]FeCl was isolated from the reaction of HCl with 5-41 (X-ray). [SiPiPrSAd2]FeMe was also prepared.419 33The related derivative Cp(OC)FeH(SiEt3)(SnEt3) was similarly prepared in 58% yd.168 34A similar complex was prepared from (PMe2Ph)Fe(CO)4 and HSiPh3 to give (PMe2Ph)Fe(H)(SiPh3)(CO)3 (85%, X-ray).167 35LiPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene.84 36When the reaction was run in toluene, the analog of 5-50 with toluene replacing benzene was formed (28%, X-ray).170 37An analog where R = iPr was also reported (90%, X-ray).172 381H NMR data supplied but not assigned.172 39Additional derivatives formed from reaction with H2SiPh2, H2SiClPh (Table 2), H3SiPh (Table 1). The 2JSiH was not detected by J-HMBC nor 1H-coupled 1H−29Si HMQC.86 40Additional derivatives formed from reaction with HSiCl3 (90%), HSiMe2Cl (83%, X-ray), HSiMe2Ph (thermally unstable), and H3SiPh (Table 1).87 412-Pyridinetetramethyldisilazane.175 4213C NMR data reported at rt and the 31P data at −50 °C.175 43Solvent was removed from the supernatant, and the residue dissolved in CD2Cl2 and the 1H NMR spectrum obtained immediately, which showed the presence of 5-66A and 5-66B in a 1:1 ratio.179 44Data for cis-5-66B (compound not isolated): 1 H, 13C, 29Si, HRMS.179 455-69 is mixed with the starting material. Additional derivatives prepared from HSiMeCl2 (85%), HSiMe2Cl (80%), and HMe2Ph and also complexes prepared from the reaction [CpRu(PPh3)(CH3CN)2][BAF]− and HSiCl3, HSiMeCl2, and HSiMe2Cl (only solution data for each).87 46Additional example from reaction of RuH2(η2-H2)2(PCy3)2 and HSiMeCl2 (1:1).182 47All hydrides exhibited fast exchange from 193 K to 293 K.182 48When the reaction between RuH2(η2-H2)2(PCy3)2 and HSiMeCl2 was run with a 1:2 ratio, a product analogous to 5-72 was produced.182 49Reported earlier: Ng, S. M.; Lau, C. P.; Fan, F.; Lin, Z. Organometallics 1999, 18, 2484.183 50In equilibrium with starting complex. K ∼ 0.11 at 90 °C.183 511H−1H COSY, 1H−13C HMQC, 13C DEPT-135, and 1H−29Si HMBC NMR spectra.178 52No bond distance data were provided either in the paper or the Supporting Information.186 53Additional example prepared from reaction of HSiEt3, [OsH2(SiEt3)(η6-pcymene)(IPr)]OTf (82%), also from H2SiPh2 (Table 2).130 54IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene, OTf = trifluoromethanesulfonate.130 55Additional example was prepared where R = Ph. When R = Me, the 1H resonance of the diastereotopic Me signals appeared at 0.62 and −0.29 ppm. A 2D COSY showed a weak correlation between H4 and H1, and the 2 Me signals were reduced to singlets. At −50 °C the 29SiH satellites showed a J = 65 Hz, which is consistent with a Co η2-silane complex, as shown in the structure representation for 5-86 in Table 3.188 56 Addition complex prepared from the reaction of HSi(OMe)3. When 1.2 equiv of HSi(OEt)3 was used, a mixture of 5-92a (4 pts) and [Rh(H)((SiOEt)3)3((PEt3)3] (1 pt) formed.190 575-92b was identified by comparison to literature data. Other tertiary silanes, HSiR3 (R3 = MePh2, (OEt)2Me, and Et3) gave similar results.190 58Complex 5-93 was also formed from the reaction of HSi(OEt)3 with [Rh(CH3)(PEt3)3]. Similarly, [Rh(H)2{Si(OMe)3}2(PEt3)3] was prepared from HSi(OMe)3 with [Rh(CH3)(PEt3)3].190 59fac-[Rh(H)(CH3Si(OMe)3)] was similarly prepared.190 60The deuterium analog of 5-95 was also reported. Similarly, [Rh(H)(η2-HSiiPr3)(dippp)] was prepared from [Rh(η-F)(dippp)]2 and HSiiPr3.191 61Additional derivatives prepared from HSiEt3 (not isolated).192 62An additional derivative was prepared in a similar way from HSiEt3 to give (PyPyr)Rh(H)2(SiEt3)2.131 63Additional rhodium complexes formed from reaction of C5Me4tBu (73%), C5Me5(3,5-tBu2C6H3) (64%), C5Me4SiMe3 (58%).193 64Additional complex formed from reaction of HSiEt3 (2JSiH = 33.5).102 655-101 was also prepared from (PNPph)RhH(Cl)(THF) and H2SiPh2 (no details were provided).102 66In a related experiment, RhH2(SiPh3){xant(PiPr2)2} was observed in solution from RhH{xant(PiPr2)2} and HSiPh3 (258 K).132 67The germanium and tin analogs were also reported.194 68The reaction of other diaryldiimidates in solution was also reported: 2,6-(OMe)2 (stable complex), 2-Me-6-iPr (rearranges to 2 isomers of analog of 5-107, 2,6-(iPr)2 (no reaction), and 2-Me-6-tBu (no reaction). The reactions of other silanes with 2,6-Me2C6H3 were also studied: HSi(nC8H17)3 (rearrangement), HSi(nC8H17)Me2, and HSi(cy-C6H11)Me2 (both with rearrangement to two isomers, HSiPhMe2 (complex decomposes).107 69L = β-diimidate (see structure for 5-106).107 70When 20 equiv of HSi(OEt)3 was added to IrCl(CO)(PPH3)2, IrCl(H)2(CO)(PPh3)2 formed in 82% yield (X-ray) plus a mixture of Si(OEt)4, (EtO)2SiOSi(OEt)3 + (EtO)2HSiOSi(OEt)3. Also formed was the SiEt3 analog of 5-110.197 71Additional complexes prepared from reaction of (PNP)Ir(H)2 and primary silanes (Table 1) and a secondary silane (Table 2).101 72(PyPyr)− = 3,5-diphenyl-2-(2pyridyl)pyrrolide.131 73Xantsil = (9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl).199 74The T1 measurements for 5-115 at 183 K gave 2.3 s for the peak at −13.98 and 1.8 s for the peak at −4.69 (typical of dihydrido complexes).199 75The analog [IrBrH(bipSi)] was prepared treating 5-118 with NaBr in 75% yield, anti:syn = 92:8 (C6D6), 82:18 (CD2Cl2) (X-ray structures of both). The iodo analog, [IrIH(bipSi)], was prepared in the same way but with NaI in 74% yield, anti:syn = 45:55 (C6D6), ∼22:78 (CD2Cl2) (X-ray).200 76Additional complex prepared from reaction with HSiEt3 (83%).102 77Two additional complexes were reported from the reaction of (PNPph)Ir(I)(Me)(THF) with H3SiMes and with H3SiXyl (Table 3).102 78

Derivative related to 5-123 was prepared from H2SiPh2 with IrHC{xantPjPr2[jPrPCH(Me)CH2]} (see 4-41b).132 79The coupling constant was obtained from 1H−29Si DQF data.202 80(XyC∧C: = Cyclometalated Perimidine-Based Carbene, (C∧N) = Cyclometalated 2-Phenylpyridine). An analog of 5-126 was prepared from HSiMe2Bn (X-ray).203 81An analog of 5-129 was prepared in quantitative yield (NMR) from Et2PhSiH (2.5 equiv).206 82The structure of the dimer is the same as that for the Rh analog, 5-96. An analog of 5-130c was prepared with reaction of HSiEt3.192 83The other product in addition to 5-132 formed in the reaction was (dtbpe)NiOTf. Better yields were obtained from the condensation of [(dtbpe)Ni(μ-Cl)]2 with Mes2Si(H)K (86%). Although 1H NMR data are reported, peak assignments were not made.135 84Sample contains ∼2% [SiPiPr3]Ni-Cl impurity.208 855-136 was also prepared from reduction of [PSiP]Ni(PMe3)Cl with NaBH4 in 46% yd.173 865-137a was also prepared from NiCl2 + PMe3 (58%) and NiMeCl(PMe3)2 (69%) or from reaction of H[MeSi(2-Ph2PC6H4)2] and Me3SiCl or MeHSiCl2.173 87Additional derivative prepared from reaction of HSiPh3.137 885-139 was also prepared from (dippe)Ni(Ph)(Cl) in 61% yield.209 89Technically this does not involve a silane reacting with the Ni complex. However, it is likely that some interaction of Ni-H (formed by reduction with BH) with the Si of the ligand occurs and it leads to the product.141 90The silicon product may have been formed by chelate assisted Si−H bond OA of the starting silane followed by Si−C RE.207 91Characterization data are given for the silicon-containing product.207 92Same structure as the nickel complex, 5-145. Complex was also prepared from CpPd(C3H5) in 86% yield and from ClPd(PSiP) in 90% yield.210,211 93Additional example with [HBPh3]− also reported.213 94See also 5-142b. Elimination of H2 was observed as a byproduct by 1H NMR spectroscopy.141 955-149 was also prepared in 82% yield from (dmpe)Pd(SiPh2H)2 and Pd(PCy3)2.109 96A similar reaction of [Pt(PtBu3)2]with HSi(mtMe)3 did not lead to a Pt3 containing product but formed [PtH(PtBu3){Si(mtMe)3}, which could not be isolated.281 97Similar derivative prepared from the reaction of HSiEt3.145 985-153 was also BE


Chemical Reviews

Review

Table 5. continued prepared from [Pt(IPr)(dvtms)] [dvtms = (Vin)Me2SiOSiMe2(Vin)] in 78% yield.215 99The Me3Si-analog of 5-154 was reported in 94% yield from reaction of (PyPyr)Pt(C2H4)(Cl) and HSiMe3 as well as quantitatively from treatment of 5-153 with 4 equiv of HSiMe3.216 100The X-ray structure shows two pairs of associated monomers within the asymmetric unit.216 101The corresponding complex where R = Cy, [Cy-PSiP]PtCl, was prepared in 86% yield in a similar reaction.114 102Additional derivatives prepared from HPh2SiSiMe3 (X-ray) and H2PhSiSiMe3 (X-ray, 4-59).147 103 Additional analog prepared at room temperature from HPh2SiSiMe3 (80%, X-ray).147 104The complex [(dcpe)Pt{Si(OEt)3}] was similarly prepared from [(dcpe)PtH2] and HSi(OEt)3 (quantitative). Both complexes were also prepared by addition of (RO)3SiSi(OR)3 to the starting Pt complex; [(dcpe)PtH2] was generated in situ prior to addition of either HSi(OR)3 or (RO)3SiSi(OR)3.219 105Related derivatives with R = iPr, [SiP3R]Pt-Cl (from [SiP3R]H and (COD)PtCl2), and [SiP3R]-H (X-ray) (from [SiP3R]H and Pt(PPh3)4) were also prepared.212 106Also reported were cis-[Pt(H)(SiPh3)(PEt3)2] (98%) and cis-[Pt(H)(SiPh2Me)(PEt3)2] (93%).221 107Additional derivative, (Me3SiMe2Si)2Zn, prepared from Me3SiMe2SiH with tBu2Zn, tBu2Hg (0.05 equiv) at 90 oC in 95% yd. Previously prepared by Arnold, J.; Tilley, T. D.; Rheingold, S. J. Inorg. Chem. 1987, 26, 2106−2109.223 108Other conditions for the preparation of 5-179 included reaction of (Me3Si)3SiH with Et2Zn at 60 oC to give 30% yield of 5-179, and with (tBu)2Zn at 60 oC to give 70% yield of 5-179 and at RT (24 h) to give 50% yield.223 109With 1 equiv of HSiEt3, the product was U(OSiEt3)(OB(C6F5)3) (Ar = 3,5-tBu2C6H3), (18%).224

An example of the loss of each of the neutral ligands will be described in this order: phosphine (eq 23), carbon monoxide

and in the reaction with (depe)Pd(PEt3)2 to give the fourcoordinate complex [{1,2-C6H4(SiMe2)(SiH2)}Pd(depe)].110 An additional example involves 2,6-bis(dimethylsilyl)methylpyridine that formed a six coordinate complex in which the N−Si−N portion coordinated in a meridonal arrangement, 5-65.178 Although not very common, silicon “chelates” have been utilized as reactants, and examples were shown in Figure 4 with the products listed in Tables 3−5 identified. Silicon centers also play a role in pincer ligands, and these examples were listed previously in Figure 3. 3.2. Reactions of Silanes Initiated by Loss of a Neutral Ligand

Promoting a reaction between a metal complex and a hydrosilane involves the appropriate choice of a metal precursor. Since commercial sources for metal precursors are limited, synthetic work is often required to build the metal complex. Systematic studies such as reacting the same complex with the 3 types of hydrosilanes or the same hydrosilane with the same metal center but with different substituents, or with metals that are located in a single triad, are still rare. In order to initiate a reaction of a hydrosilane with a metal complex, two features for the reacting complex are required: (1) the metal should be in an oxidation state at least two less than the maximum for the metal; and (2) the metal should have an open coordination site. If the reacting complex is coordinatively saturated, a ligand must be “removed” from the metal center. The most common strategy involves loss of a neutral ligand promoted either by photolysis or by heating. Tables 3−5 contain many examples of the loss of the neutral ligands, including phosphines, carbon monoxide, olefins, coordinated solvents, or a dinitrogen ligand (Chart 1). If the complex contains an “anionic” ligand, particularly M-H, M-X (X = halogen), or M-C, then addition of HSiR3 may involve loss of H2, HX, or an HC unit. Inevitably, a more complex sequence of events may take place, but in the subsequent paragraphs only the above two types of processes will be described. The examples in Chart 1 involve primarily metal complexes from the iron, cobalt, and nickel triads. Overall, within these three triads, olefins (cyclic and acyclic) seem to dominate in the starting complexes that were employed during the current review period (relative to the previous reviews1,2). The loss of an olefin plays a more important role than it did previously1,2 and exceeds the use of phosphine in the same three triads. Another change in the current review period involved the use of a coordinated N2 molecule in the reacting complex for which there were only two examples in ref 2.

(Scheme 3), olefin (eq 24), solvent (eq 25), and dinitrogen (Scheme 4). Scheme 3 demonstrates the effect of photolysis conditions on the loss of 3 successive CO substituents and the changing role of the Si−H bond in the new complexes, 5-35 to 5-37, that were produced.162 An Fe(0) “pincer” ligand with terminal N2 substituents was utilized in related reactions where both primary and secondary silane reactants were employed as well as a third example where a minor perturbation with a substituent on a pyridine group of the “pincer” ligand was present (Scheme 4). When a silane is reacted with the iron precursor containing two terminally coordinated N2 groups, two silyl groups are incorporated and an additional substituent is found between the two silyl groups (3-17 to 3-19 and 4-13a, Scheme 4), each of which are different. With PhSiH3, the substituent produced was a σ-H-SiH2Ph, but when the size of the organic substituent of the reacting primary silane is increased from a Ph to a Mes group, only one of the terminal N2 ligands was lost, leaving one terminal N2 group in 3-19 still coordinated to Fe. Even a small change of the substituents on the pyridine group from H- to CH3- resulted in formation of a dihydride (3-18) instead of the σ-H-SiH2Ph product observed in 3-17. With the secondary silane, H2SiPh2, another variation was observed that involved formation of a σ-H-H in the same position of σ-HSiH2Ph in 3-17. Scheme 3 demonstrates the effect of photolysis conditions on the loss of 3 successive CO substituents and the BF


Chemical Reviews

Review

Figure 3. Selected chelates (bidentates through polydentates) utilized in metal complexes that were reacted with hydrosilanes or where the hydrosilane was the reactant that formed the chelate portion of the target complex.

may then be eliminated as HX. Although more complicated versions occur, the simplest sequence may involve elimination of a coordinated ligand followed by oxidative addition of a hydrosilane (similar to the cases discussed previously). In the cases outlined in Chart 2 described above, a coordinated anionic ligand (X−) is eliminated as HX with examples that include HX = H2, HCl (HF, HBr), and HR (R = an organic group). A probable, general sequence for ligand loss and elimination of HX (X = H, Cl(F, Br), R) is shown in eq 26. The loss of L opens a vacant site for reaction with the hydrosilane. Loss of HX provides a new metal complex with a TM−Si bond and is presented in the order HCl (eqs 27 and 28), solvent (eq 29), H2 (eq 30a), and alkane (eqs 27, 28, and 30b). During this review period the loss of an anionic ligand with the formation

changing role of the Si−H bond in the new complexes, 5-35 to 5-37, that were produced.162 Not unexpectedly, conditions are also important in determining the nature of the product that is produced. 3.3. Exchange of Anionic Ligands: Preparation from TM-R, TM-Cl, TM-H, and TM-Si

Elimination of a neutral ligand prior to the reaction of a hydrosilane with a metal complex cannot account for all the products shown in Tables 3−5, as is obvious from the listings. Some complexes are electron deficient (2.6 Å (therefore, nonbonding). DFT calculations were in agreement with the X-ray data, and the authors concluded that 5-67c is a formally 16-electron Ru(II) species. One of the more interesting features of 5-67c, however, was a broad signal at rt in the hydride region at δ = −4.34, and decoalescence occurred at 273 K to give two signals with a 1:1 integration. This suggested that the Ru−H and the Ru-η2-Si-H were in trans positions relative to each other; thus, exchange was blocked and characterized by a ΔG‡ = 48.5 kJ/mol. A 29Si{31P}{1H} spectrum exhibited a broad singlet (δ = 33) at 293 K, but at 324 K the broadening is reduced and the HMQC 1 H−29Si{31P} spectrum indicated a correlation between the 29Si signal at δ = 33 and the 1H signal at δ −4.4. At 213 K the 29Si signal resolves into two resonances at δ 54 and δ 16. An HMQC 1H−29Si{31P} experiment at 213 K showed a correlation of both Si resonances with only the high field 1 H resonance (δ −5.47), and the apparent JSiH for both was 37 Hz. Signals for 29Si could not be measured from 293 to 213 K, due to small T2 values, which prevented a determination of the 29 Si coalescence temperature. However, a DFT calculation gave a free energy of ΔG‡ = +16.9 kJ/mol (298 K) for 5-67b → 5-67c + H2, suggesting a facile transformation between these two complexes under the experimental conditions employed.180 5.3.3. More than One Agostic Interaction per Si Center: ηx-HxSi or ηx, ηx-HxSi Interactions. This was not a separate category in the previous review, but there are more than a dozen examples (in 7 publications) that will be outlined in this section. The two complexes, 5-76183 and 4-22b (R = Me)92 (Figure 13), that will be compared have structurally related ligands, HB(C3N2H3)3− for the former and PhB(CH2PPh2)3− for the latter, although the tridentate ligands are not electronically the same as the link between the boron and the metal center in 5-76, is B−N−N-Ru, and that in 4-22b, is

(eq 78) was observed upfield of TMS at −2.90 ppm with a 1 JSiH = 89 Hz at 300 K and 77 Hz at 193 K. The Si−H distance was 1.62(3) Å, but the Ni−H distance was shorter at 1.44(2) Å. The same parameters for the Pd complex, 5-145 (eq 78), were at a lower field for the 1H resonance and observed at 0.25 ppm with larger coupling constants of 1JSiH = 110 Hz (300 K) and 97 Hz (193 K). In this case the Si−H and Pd−H distances were close in value at 1.60(3) and 1.67(3) Å, respectively. For 5-145, the η2-(SiH)Pd(0) structure was the major component in solution between 193 and 200 K, and the values of the 1JSiH coupling constants for 5-141 also supported an η2-(SiH)Ni(0) structure in solution. The reaction of Pt(PPh3)4 with Ar2Si(H)Me took a different course, and two different complexes were isolated, depending on the solvent and temperature. In benzene at room temperature, the SiH of Ar2Si(H)Me oxidatively added to the Pt center to give 5-171 (eq 78) with a trigonal bipyramidal Pt center. However, in THF at −20 °C, the complex 5-172 (eq 78), with a square pyramidal Pt center, was isolated. DFT calculations for 5-171 and 5-172 were performed, and that for tbp-5-171DFT reproduced the parameters of the X-ray structure. The spy-5-172DFT calculations supported the square-pyramidal geometry with a PPh2 (from the pincer ligand) in the apical position. The kinetic experiments for the isomerization of spy-5-172 to tbp-5-171 suggested an intramolecular rearrangement process without dissociation of the PPh3 ligand, and the authors tentatively suggested a turnstile mechanism.210 Another Pd(0) complex, 9-34, had the same type of structure as 5-145, except that the monodentate ligand at Pd in 9-34 was Me3P instead of PPh3.291 The η2-(SiH)Pd(0) complex exhibited a 1H resonance for the SiH at −0.90 (tdq) and a J = 93 Hz (1H-decoupled 29Si). The Pd−H distance was 1.64(5) Å (the SiH distance was not provided). There are two related complexes in the last agostic category that involve coordination of an ε-SiH unit (eq 79; see also these CY


Chemical Reviews

Review

Figure 13. Complexes with 2 and 3 hydride ligands bridging Ru···Si.

The Ru···Si distances in the five complexes (Figure 13, 2.24, 2.26, 2.29, 2.38, and 2.40 Å) are on the low end of the reported Ru−Si distances and are actually similar to those reported for the NHSi complexes of Ru (Table 1) as well as for ruthenium silylene complexes (Table 2), 3-27 (plus 2 additional examples in footnote 33 of Table 3), and 5-63.177 A borderline case in this category could be complex 4-16,88 a ruthenium dihydride complex (see structure in Table 4) where there are relatively long Ru−H···SiPh2 contacts of 2.12 and 2.14 Å and 2JSiH = 9.8 Hz that the authors thought supported an assignment as a dihydridosilyl.88 Later, these distances were included in weak Si−H to M interactions (see Scheme 7). Tilley and co-workers suggested a bonding relationship for the frontier molecular orbitals in L nRu(η3-H2SiR2) and LnRuH2SiR2, as depicted in Figure 16.92 Later, a cationic ruthenium complex, [Cp*(iPr3P)RuH2SiHMes][CB11H6Br6], 9-41a (Figure 17), with a different set of ligands

B−C−P-Ru. Both complexes are formed with two Ru−H−Si linkages, although in the case of 5-76 this results from a ligand precursor that had a Ru−H in the starting ligand and in 4-22b both hydrogens arise from a reaction with an excess of a secondary silane. Nonetheless, both are depicted with a η3-H2Si interaction (Figure 13). Figure 13 illustrates how the reaction of a secondary silane can lead to either one or two of the SiH bonds coordinated to the metal in a σ-complex and can lead to two differrent Si−H bonding modes (Figure 14). The metal

Figure 14. Relationship between η2-H-Si and η3-H2Si interactions in σ-complex formation (adapted from ref 92).

centers in 4-22b and the related H2SiPh2 adduct (Table 4, footnote 25) are electrophilic and react with neutral bases such as THF and DMAP to give 6-coordinate complexes such as 9-38 (from the -SiPh2 analog and DMAP;92 shown in Figure 15), 9-39 from 4-22b, as well as a third complex formed as the THF adduct of 4-22b but studied only in solution (Table 9, footnote 57).92 Another 6-coordinate complex, 3-30, whose parameters are also shown in Figure 15, was prepared by a separate route that involved reaction of [PhBPPh3]RuCl(PMe3)3 and H3SiPh (3 equiv). Although the four-membered rings with 2H, a Ru, and a Si are represented as planar in Figure 15, they are all folded on an imaginary line connecting the two bridging hydrides, as in 5-77183 (Figure 15) and the closely related complexes 5-76 and 5-78 and as in 4-22b, or connecting H1,H3 as in 9-38, or connecting H2,H1 as in 3-30, but obviously, all the sides are not equal. In 5-77, for RuHbr, the distances (Å) are 1.49, 1.57 Å, and for SiHbr, they are 1.90, 1.96 Å. For 4-22b, the RuHbr distances (Å) are 1.73 and 1.76 Å (note that the RuHterm is 1.56 Å, typical of an RuHterm distance) and the SiH distances are smaller at 1.61, 1.66. For the approximately octahedral complexes, 9-38 and 3-30, the two Ru−Hbr distances are less than the two Si−Hbr distances. For 9-38 the RuHbr distances (Å) were 1.56 and 1.54 Å, and the SiH distances were 1.82 and 1.98 Å. In 3-30, the RuHbr distances were 1.59, 1.60, and 1.64 Å and shorter than the SiH distances (Å) of 1.81, 1.84, and 1.91 Å.

Figure 16. Comparison of the frontier molecular orbitals for LnRu(η3H2SiR2) and LnRuH2SiR2.92

Figure 17. H-bridging parameters for 9-41a.

also was published by Tilley and co-workers.241 This complex will be presented in the M−H···Si interaction section in the category, H−M(multiple)Si, as the structural features of this complex have been determined (details are shown in Figure 17241).

Figure 15. Bridging distances in 5-77, 4-22b, 4-22c, 9-37, 9-38, and 3-30 (in Å; reduced to 3 significant figures).183,92,129,95,92 CZ


Chemical Reviews

Review

an η4,η4-[H6Si]2− ligand and is the first species of this formulation to be isolated and characterized.94 The six Si−H distances spanned from 1.69 to 1.79 Å and the Ru−H distances from 1.62 to 1.73 Å, but the Ru to Si interatomic distances are shorter at 2.17 Å in 3-34, than in the η3,η3-H4Si (9-35) example.94 The last example that may be considered in the ηx-HxSi category comes from the Co-triad. The two examples (although not unequivocal assignments) involve Rh complexes from reaction with secondary silanes.105 When (dtbpe)Rh(CH2Ph) was reacted with Ph2SiH2, the product isolated was 4-26b, (dtbpe)Rh(H)2(SiBnPh2), which was depicted with two resonance structures as shown in eq 80. The two hydrides

The position of H2 in 9-41a was not stable under refinement and was fixed, but the DFT calculated H2−Si distances were 1.683 and 1.676 Å, which suggested that 9-41a could be described as an η3-H2SiHMes ligand.241 However, NMR data for 9-41a, which exhibited a 29Si resonance of 229 ppm and a JSiH = 62 Hz, supported a higher degree of silylene character accompanied by weaker Si−H interactions. Thus, the conclusion that there could be a continuum between Ru(IV) silylene dihydride and Ru(II) η3-silane structures could include the resonance structures shown in Figure 18. Cleavage of the two Si−H

Figure 18. Contributing resonance structures for Ru(IV) silylene and Ru(II) η3-H2Si complex.241

are equivalent and appear as a multiplet at −5.35 ppm with a JSiH = 44.4 Hz. The X-ray structure exhibited 2 Ru−H ligands with distances of 1.67(4) and 1.49(4) Å (the authors suggested that the difference observed may not be statistically meaningful). The Si−H bonds were elongated to 1.84(4) and 1.88(4), and the Rh−Si interatomic distance of 2.3536(19) is typical for Rh−Si single bonds. 4-26b could be viewed as an [η3-H2SiR3−] hydrosilicate complex, but since the Rh−H distances were short and the Si−H distances long, an alternative description may be that of a “silyl dihydride complex with strong intramolecular interactions between the hydride ligands and the silicon atom”. Therefore, Rh(I) and Rh(III) resonance forms are considered as shown in eq 80. A second complex, 4-27, was formed from TripPhSiH2 (not stable at room temperature in solution); thus, NMR data were collected at 0 °C. No Si−H group was observed, and the two hydrides were a complex multiplet at −7.04 ppm with JSiH = 51.8 Hz. The 29Si NMR resonance at 0.89 ppm is typical of a silyl ligand. The authors proposed that the best representation would be as a (dtbpm)RhI complex with an [η3-H2SiR3]− ligand.105 As discussed in this section, when there are two hydrogens that may be involved in bridging from the silicon to the metal, variations in the interaction have been observed from both Si−H bonds interacting with the M center (generally with similar interatomic distances, see Figure 15) to one of the two Si−H bonds being noninteractive as is the case for 4-3c73 (Figure 9) and a case where more than one interpretation was possible (4-26b,105 eq 80, and 4-1688). The least ambiguous cases all involved Ru as the metal in the complex.

bonds would give the dihydride structure (left in Figure 18), but incomplete activation would lead to a η3-silane on the right of the indicated contributing structures. Could an individual silicon center participate in two η3H2SiR2 or two (μ-H)3Si interactions? The answer is yes but not in an expected way. Tilley and co-workers described the reaction of [{(PhBPPh3)Ru(μ-Cl)}2] with a secondary silane to form 4-22b, an η3-H2SiRR′ adduct of Ru.92 One of the earliest of this structure type on record was communicated in 2000331 (full paper in 2003) from the reaction of PhMeSiH2 with RuH2(H2)2(PCy3)2 to produce (PR3)2H2Ru(SiH4)RuH2(PR3)2 (R = Cy, iPr) by Sabo-Etienne and co-workers, and the product resulted from the disproportionation of PhMeSiH2 to give SiH4 as one of the products which then was “trapped” by the Ru complex. In the current case, a Ru dimer was used by Tilley and co-workers as the starting point, and reaction with the primary silane, ArSiH3 (Ar = (o-MeO)C6H4, Mes, and Ph) gave hydridosilicates, 3-33,94 9-35,94 and 9-36,94 {[PhBPPh3]Ru}2(μCl)[μ-η3,η3-H4Si(Ar)], and related complexes.94 A structural representation of 9-35 is shown in Figure 19, including an expanded view of the central core with interatomic distances. The four Si−H distances in 9-35 range from 1.62 to 1.68 Å, and the Ru−H distances from 1.58 to 1.72 Å. The two Ru−Si interatomic distances are essentially equal (2.37 and 2.36 Å). When 4-22b (or the related 9-37) was reacted with (m-xylyl)SiH3, displacement of PhMeSiH2 occurred, giving the complex 3-34, [{(PhBPPh3)Ru}2(μ-η4,η4-H6Si)] (Figure 19), and silane disproportionation products such as Ph2SiH2, Me2SiH2, and MeSiH3. The complex is viewed as containing

Figure 19. Hydridosilicate structures and RuH and SiH parameters.94 DA


Chemical Reviews

Review

There are bound to be differences in distances between M−H and Si−H as measured by X-ray versus those obtained from Neutron Diffraction but especially for the M−H interactions. It should be noted that the Ru−H distances in Table 11 measured by ND are longer than those determined by X-ray. This observation is not always the case for Si−H distances (Si is not generally considered a “heavy” atom), which are generally longer when determined by ND than by X-ray. However, of the 11 Si−H distances reported in Table 11, there are 6 SiH distances that are actually shorter when determined by X-ray. For both 5-59 and 9-10 the DFT calculations more closely resemble the ND data. This suggests that in cases where X-ray data only are provided (which is the case for the vast majority of the structures), the DFT calculations are useful but will not match those of the X-ray data as closely as has been found for ND studies. Unfortunately, the number of complexes for which ND data are available is still rather small. Strong secondary interactions were also found for the HRuHterm dimer 5-83b shown in Figure 21. The RuHbr and RuHterm distances were 1.80(2) Å and 1.51 Å, respectively. The RuH···Si distance to the adjacent Si (cis relative to the Ru−H position) exhibited a distance of 1.96(2) Å, which supports the SISHA assignment.176 M−H distances for Si···HM between 1.9 and 2.4 Å where the hydride has been located in the solid state structure for metal centers in addition to ruthenium can be found from calculations or by using the cif files, but such has not often been done. When a MH or MSi are in the equivalent of cis-positions relative to each other, a weak interaction should be expected, as suggested by Lin332 over 10 years ago. There are examples in Table 7 where Si−H···M distances (X-ray or calculated) are reported in the 1.9 to 2.4 Å range, including 3-32 (and footnote 27, Table 4),93 4-22a (calc),128 5-62,176 footnote 41, Table 5 (ascribed to IHI),87 5-59 (eq 72),175 5-76,183 5-77,183 5-78,183 and 5-138137 (the last was attributed by the authors to the trans-effect of Si).137 5.4.2. Hydrides That Span Metal−Silicon Multiple Bonds. During this review period there were only a few examples of complexes where a hydride bridges MSi. Two contain WSi,79,80 and three examples contain RuSi.91,241 One of the tungsten examples, 3-15b,79 was formed from the reaction of [(η5-C5Me4Et)(OC)2(NCMe)WMe] and D3SiC(SiMe3)3 (the protio analog was reported previously333). With the earlier report 10 years ago, which featured an X-ray structure with DFT calculations, and the present ND study of the deutero analog, a comparison can now be made of the parameters determined by each method, as was presented for Ru complexes in Table 11. For the combination of the 3 characterizations, the ND distance for 3-15b, the X-ray distance, and DFT distances (both of the latter were for protio examples), the pertinent bond distances were: Si−Dbr, 1.720(10)/Si−Hbr, 1.71(6)/DFT, 1.71); Si−Dterm, 1.488(10)/SiHterm, 1.54(7)/DFT, 1.50; W−Dbr, 1.831(9), W−Hbr, 1.82(7)/DFT, 1.85.79,80 The second example in ref 79 involved the reaction of [η5-C5Me4Et(OC)3WMe] with the secondary silane H2SiMes2 to give [η5-C5Me4Et(OC)2(H)WSiMes2], 4-8. Data obtained from NMR spectroscopy indicated that the interligand interaction of 4-8 was stronger than that in 3−15b, but unfortunately, the hydride bound to tungsten could not be located in the crystal structure.79 Complexes with a RuSi unit have been reported by Tilley and co-workers. The cationic complexes 9-41b, [Cp*(PiPr3)Ru(H)2(SiHMes)][B(C6F5)4], and 9-42, [Cp*(PiPr3)Ru(H)2(SiHSi(SiMe3)3)][B(C6F5)4], are two of five examples

A smaller category of complexes involves the interaction of M−H with a Si center, and these will be covered in the next section. 5.4. M−H···Si Interactions

In this section, a M−H interacting with an electrophilic silicon center will be described. These are, in general, considered “weak” interactions. The classifications include Section 5.4.1, SISHA (Secondary Interaction between a Silicon and a Hydrogen Atom; M−H···Si where interatomic distances range from 1.90 to 2.40 Å), and Section 5.4.2, where an M−H bond interacts with Si in a (H)MSi complex. In the last grouping are complexes that do not fit particularly well into any category. 5.4.1. SISHA Interactions. Although developed from an analysis of Ru complexes, the following recommendations have been made for the determination of a SISHA interaction: Si−H distances (determined by X-ray diffraction or by DFT calculations) of 1.7 to 1.8 Å should suggest the formation of a σ-complex, but Si−H distances in the range of 1.9 to 2.4 Å imply the presence of secondary interactions (SISHA).313 No single parameter, JSiH, IR band assignment, or distances for Si−H (experimental or calculated) should be used independently to determine the extent of SiH activation, but it is necessary to use all these measurements to support an assignment of a σ-interaction. The examples in Table 9 under SISHA/Secondary Interactions are those for which X-ray data have been reported, and in some cases the data have been supported by DFT calculations. The Mn complex 5-37162 was one of three Mn complexes in the same study where the Si−H activation in the series ranged from an η1-SiH (5-35162), to η 2-SiH (5-36162), to Mn−Hterm (5-37162) (see Section 5.2.1). It is the complex 5-37 that exhibited Si−H···M distances of 1.88 (borderline) and 1.97 Å SISHA interactions.162 The majority of the remaining examples contained iron84 or ruthenium.175,176 Three related structures containing iron are shown in Figure 20.

Figure 20. Iron complexes with SISHA interactions.

The structures of 9-40 and 5-48 were determined, and the former has one long FeHbr···Si bond (1.90 vs 1.59 Å) and the latter has 2 relatively long FeHbr···Si bonds (1.80 and 1.87 Å), both of which would be considered borderline secondary interactions. No X-ray structure was reported for the tolyl complex, 9-41.84 The JH,Si values were 35 Hz (9-40) and 30 Hz (9-41), both of which have values further reduced from the previous σ-complexes as the H···Si distance has increased. The ruthenium complexes 5-59,175 5-83b,176 5-62,176 9-10,176 5-71,182 and 5-72182 were all reported by SaboEtienne and co-workers. A Neutron Diffraction study was published for 5-59 and also for 9-10 (included under η2-complexes) which allows comparison to the parameters found in the X-ray structure and also to DFT calculations (in 5-59, 9-10, and 5-62). The data are summarized in Table 11. DB


Chemical Reviews

Review

Table 11. Comparison of X-ray, ND, and DFT Calculations for Si−H and Ru−H Parameters in 5-59, 9-10, and 5-62175,176

5-59

9-10

5-62

El−H

X-ray

ND

DFTa

El−H

X-ray

ND

DFTb

El−H

X-ray

DFTc

Ru−H Sia−Ha Sib−Hb

1.41(5) 2.120(9) 1.45(3)

1.559(7) 2.154(8) 1.481(5)

1.557 2.162 1.491

Ru−Ha Ru−Hb Sia−Ha Sib−Ha Sic−Ha Sia−Hb Sib−Hb Sic−Hb

1.55(5) 1.47(4) 1.62(4) 3.20(5) 2.37(5) 3.07(4) 2.03(4) 1.89(4)

1.600(8) 1.587(7) 1.74(1) 3.28(1) 2.29(1) 3.057(9) 1.95(1) 2.00(1)

1.596 1.579 1.750 3.300 2.282 3.103 1.921 2.060

Ru−Ha Sia−Ha Sib−Ha Sic−Ha

1.42(3) 1.97(3) 2.00(3) 2.33(3)

1.586 1.891 2.042 2.929

a Calculations were performed (DFT/B3PW91) without any simplification of the model used. bFull optimizations of geometry without any constraint were performed. Further details can be found in the Experimental Section of ref167. cTwo structures (symmetric and a less regular one) were calculated for 5-62 and are degenerate. The geometry of the solid state structure is intermediate between the two lowest energy calculated geometries of 5-62.

H2Si(C12H8)) were also studied, but in these cases, curiously, no silicon satellites were detected for the RuH resonance and the absence of a significant interaction between the hydride ligand and the silicon center was assumed and attributed to an unfavorable H−Ru−Si−C torsion angle.241 The complex 9-41a, with a different counterion, is an effective catalyst for the hydrosilylation of olefins, although 9-41b is not, which implies that the counteranion may provide a crucial role in defining the catalytic chemistry, but that role is not understood as yet. 5.4.3. Unclassified Examples: Complexes That Do Not Fit the Previously Described SiH Interactions. The complexes featured in this last section are shown in Figure 22. These will be discussed briefly in the order of the triad (left to right), as has been the case in previous discussions and tables, but with the exception that the yttrium complex will be presented last. The complex 5-11 was isolated from the reaction of a silacyclohexadiene (Tbt and H the remaining substituents on Si) to provide the unprecedented silacyclohexadienyl complex that contained a 3-centered bond for the Si, H, and Cr atoms. The X-ray structure verified the placement of the hydride, and the overall structure exhibited the parameters: Si−Cr = 2.5480(12), Si−H = 1.52(3), Cr−H = 1.76(3). The reported Si−H bond is very close to that of a terminal Si−H distance, but the 1JSiH = 92.1 is similar to that of an η 2-complex. 5-11 was the first example of a transition metal complex bearing a 1-silacyclohexa-1,3-dienyl ligand.150 Complex 4-3b represents another “first”, as it is a structurally characterized metal disilane with Mo−Si distances, 2.5322(8) and 2.7140(8). The hydrides were all found in the X-ray structure Mo−Hterm = 1.679 and 1.689 Å (calculated from the cif file) and Mo−Hbr = 1.587 and 1.776 Å (the average Mobr and Moterm interatomic distances are essentially the same).73 The complex 9-43 was formed from the reaction of Cp*(OC)2(H)WSi(H){C(SiMe3)3} with MeOH.311 The DFT-optimized geometry with the model complex [Cp(CO)2W(H)2][SiH(OMe){C(SiH3)3}] (9-43DFT) indicated the presence of a W-silyl and two W−H bonds in addition to two weak Si···H interactions. Although this suggested a d2

Figure 21. Complex 5-83b176 with secondary Ru−H···Si interactions.

that were reported in ref 91. The reaction leading to these cationic complexes is shown in eq 81. The X-ray structure

was not reported for either of the complexes that were derived in two steps from the reaction of a primary silane with Cp*Ru(iPr3P)(OTf) followed by abstraction of the triflate anion.91 The 1H NMR spectrum of these two complexes exhibited a SiH resonance for 9-41b at 7.99 (1J = 226.5 Hz) and for 9-42 at 7.42 (1J = 214.9 Hz) ppm. The RuH resonances for 9-41b were observed at −11.47 (2JSiH 58.2) ppm, and those for 9-42, at −7.08 (2JSiH, 37.1 Hz). For the cation, 9-41b, the NMR data were consistent with a hydrogen-substituted ruthenium silylene complex with the Si−H resonance shifted to 7.99 ppm (1J = 58.2 Hz) in a region that has been identified previously for a H-substituted silylene complex. A Ru−H resonance at −11.47 with a 2J = 58 indicated a relatively strong H···Si interaction. Similar data were reported for 9-42, with a SiH resonance at 7.42 (1J = 215 Hz) ppm and a RuH resonance at −7.08 (2J = 37). Calculations for complex 9-41b were also presented and indicated that there was no direct σ-bond between Ru and Si and that this complex also appeared to exhibit η3-H2SiRR’ character with an electrophilic silicon center. Reactions of three secondary silanes (H2SiPh2, H2SiPhMe, DC


Chemical Reviews

Review

Figure 22. Unclassified Examples.

W(+IV) oxidation state, the MOs and population analyses indicated that the W center takes a d4 configuration (i.e., +II oxidation state) and that the (H)2[SiH(OMe){C(SiH3)3}] unit has a distorted, silicate-like electronic structure with a −1 charge. Although the two Si−H distances were longer than in a silicate anion, the Wiberg bond index indicated the presence of nonclassical bonding interactions in 9-43DFT. In addition to the Si−W bond, there are nonclassical interactions for both the Si−H bonds and the W−H bonds. The theoretical calculations also indicated that there are two processes that lead to fluxional behavior (see Section 6).311 When an equivalent amount of XylNC was added to [PhBPPh3]RuH(η3-H2SiRR′), the complexes formed were [PhBPPh3]Ru(H)[C(H)(NXyl)(η2-HSiRR′)], RR′ = PhMe, 9-32; PhPh, 9-33.128 The Ru−H ligands were characterized by moderate values of JSiH (9-32, JSiH = 46 Hz; 9-33, JSiH = 49 Hz), which supported a Ru−H−Si 3c/2e bonding interaction (the RuH resonance was located at −6.64 Hz for 9-32 and −6.32 Hz for 9-33). The values of JSiH were consistent with at least one Ru−H−Si 3c/2e bond. At low temperatures (−70 °C), a resonance for each RuH of 9-32 was observed at −6.52 ppm (JSiH = 36 Hz) and −6.59 ppm (JSiH = 56 Hz), suggesting chemically inequivalent Ru−H−Si interactions. This was verified by the X-ray structure, where both hydride ligands were refined and demonstrated inequivalent Si−H bond distances of 1.71(3) and 1.95(5) Å.128 When 3−6 equiv of a secondary silane are added to [PhBPPh3]Ru(OtBu)(CNXyl), the complexes formed were [PhBP Ph 3 ]Ru(H)(CNXyl)(η 3 -HSiHRR′) (RR′ = Ph 2 , 4-22a;128 PhMe, Table 4, footnote 24). The Ru−H ligands were characterized by moderate JSiH values in the low 30s (4-22a, JSiH = 34 Hz; PhMe analog, JSiH = 32 Hz), which supported a Ru−H−Si 3c/2e bonding interaction (RuH resonance at −6.73 Hz (2H)) for the PhMe analog, and −6.31 Hz for 4−22a; SiH resonances were observed at 6.10 ppm (1J = 213 Hz) for 4-22a and 6.10 (1J = 213 Hz) for the PhMe analog. Unfortunately, the hydride position in the X-ray structure of 4-22a could not be located. However, the structure was utilized as the starting point for the DFT calculation, which gave Si−H distances of 1.885 and 2.152 Å for 4-22a and 1.840 and 2.174 Å for the PhMe analog. These values were consistent with a η2-H-SiHRR′ assignment and an additional weak Ru−H → Si interaction. Note that the values for Si−H also fit the region for SISHA assignments.128

A pincer ligand has been used to form 5-coordinate complexes of both Rh and Ir, thus allowing a comparison of 4d vs 5d complexes with the same ligand set, although the comparison is somewhat limited.102 The X-ray structure of (PNPPh)IrH(SiPh3), 5-121, was obtained, but the Iridium hydride could not be located in the Fourier map, although this was not the case for the Rh analog, (PNPPh)RhH(SiPh3), 5-100. The key NMR resonance for Ir−H was found at −19.25 ppm, and that for the Rh analog, at −13.91 ppm. The 2 JSiH for Ir was 4 Hz and that for Rh, 35.9 Hz; thus, the latter exhibited a “weak” bonding interaction associated with the silyl and hydride ligands. The IR spectrum of 5-100 exhibited a peak at 1996 cm−1 and that for 5-121 at 1940 cm−1. The peak for 5-121 is in the region expected for a Rh−Hterm. Thus, the IR data did not distinguish differences between 5-121 and 5-100, but the NMR data illustrated a weak Rh−H···Si interaction between the silyl group and the rhodium hydride ligand (short Si−H distance, Et3Ge−H ≫ Et3SnH. The Si−Fe dissociation energy is 35.5 kcal/mol and is stronger than the Fe−Ge dissociation energy of 31.6 kcal/mol. However, the Si−H binding energy in the product is calculated to be −95.5 kcal/mol compared to that of Ge−H, which is −88.0 kcal/mol. The difference in binding energy (7.5 kcal/mol) indicates that reductive elimination of Et3SiH from the intermediate Cp(OC)Fe(SiEt3)GeEt3 is more favorable than that of Et3GeH. The RE of Et3Si−H from Cp(OC)(H)Fe(SiEt3)(GeEt3) is almost barrierless and gives the 16e species Cp(OC)Fe(GeEt3) with a free energy of 0.4 kcal/mol relative to that for RE of Et3GeH at 5.4 kcal/mol.355 7.2.4. Co, Rh. Three papers are summarized in this section: the first is on hydrosilylation of alkenes that involve CpCo complexes,357 the next is on Si···H interligand interactions in Co(V) and Ir(V) complexes,356 and the last is on a CpRh complex that contains a Si···H···Si unit.363 In the study of [CpCo(CO)], the aims were to determine how a high spin [CpCo(C2H4)] species could activate hydrosilanes to generate silylcobalt hydrides and how the resulting complex can react in the presence of alkenes to ultimately produce hydrosilylation products. The possible structures of silyl hydride CoIII versus CoI η2-silane σ-complexes were also targeted as well as the mechanism of the stereomutation. The C5Me5 ligand was replaced by C5H5 and Ph2SiH2 by Me2SiH2 in the calculations which were performed by DFT using the B3LYP or the BP86 functional as implemented in Gaussian and using the LACVP(d,p) basis set. The dissociation energy for the process, CpCo(CO)2 → 3 [CpCo(CO)] + CO, for which the experimental value of 44 ± 1 kcal/mol has been reported,358 could be replicated with BP86 methodology with a value of 44.2 kcal/mol (with zero point

smaller values of 0.37 (M = Cr), 0.54 (M = Mo), and 0.55 (M = W). The corresponding NBO analysis of the delocalized Kohn−Sham orbitals for σ-bond occupancy for MSi complexes was as follows: 1.892 (M = Cr), 1.888 (M = Mo), and 1.907 (M = W) compared to the M−Si for metallo-ylidene, M−Si−C complexes, which were 1.658 (M = Cr), 1.634 (M = Mo), and 1.602 (M = W). In the MSiMe bond, the M-Si σ-bonding orbitals are polarized toward the Si, and the polarization increases from Cr to W. However, for the M−SiMe (ylidene) bond, the M−Si σ-bonding orbitals are polarized toward the metal atom. Hybridization of the metal in ME (E = Si, Ge, Sn, Pb) indicates d character in the range 60.6− 68.8% while the M−E bonds (metallo-ylidenes) have a d character that is always >86% of the total atomic orbital contribution. Contributions of electrostatic interactions and covalent bonding have nearly the same values for the silylidyne. The silylidene, M−ER bonds have a lower degree of covalent bonding (34.9−44.9%). However, the σ-bonding contributions in M−SiMe are stronger than those in MSiR. The calculated NBO partial charges for the metal-ylidynes were as follows: for Cr (−1.09) and Si (1.18), for Mo (−0.69) and Si (1.10), and for W (−0.45) and Si (1.02). In the metallo-ylidenes the values were lower with Cr (−1.0) and Si (0.88), for Mo (−0.49) and Si (0.82), and for W (−0.34) and Si (0.82).352 A DFT calculation addressed the electronic nature of [1],[1]disilamolybdenocenophane and concluded that the complex was formulated consistent with the calculation results.404 7.2.3. Fe, Ru, Os. Two publications address the nature of the bonding in M−ER bonds for M = Fe, Ru, Os and E = Si, Ge, Sn, Pb,353 and one for donor−acceptor bonding in Fe−ER (E = Si, Ge, Sn, Pb),354 and another publication addresses the mechanism of ligand exchange reactions for Fe(IV) complexes.355 The nature of the M−E bond (E = Si, Ge, Sn, Pb) in [(η5C5H5)M(Me3P)(H)2(EPh)] (M = Fe, Ru, Os; E = Si, Ge, Sn, Pb) was investigated at the DFT/BP86/TZ2P/ZORA level of theory. Only the results for the hypothetical silicon complex will be included.353 The aims of the study were as follows: (a) to understand the nature of the TM−Si bond in the metallosilylene, (b) to sort out the role of the TM atom and the Si atom in the stability of the TM−SiPh, M−H, and M−P bonds, and (c) to determine the partitioning of the σ-bonding and π-bonding in the total MSiR and M−SiR bond energies. The optimized TM−Si bond distances in the silicon complexes were slightly longer than those expected from the sums of the covalent radii, which are 2.32(Fe−Si), 2.41(Ru−Si), and 2.45(Os−Si) compared to the calculated values of 2.356 (Fe−Si), 2.428 (Ru−Si), and 2.402 (Os−Si), indicating that the TM−Si bonds are single. The angles in the bent coordination geometry at silicon (M−E−C) were 101.7 (Fe), 101.2 (Ru), and 103.7 (Os) and consistent with a divalent Si(II) center, singly bonded to a M and the C of the phenyl group. The optimized Si−C bond distances, 1.933 (Fe), 1.935 (Ru), and 1.943 (Os), are similar to those for a single bond (Si−C = 1.91 Å). The Wiberg bond indices for TM−Si are 0.36 (Si−Fe), 0.43 (Si−Ru), and 0.63 (Si−Os), and those for Si−C are 0.78 (FeSi-C), 0.77 (RuSi-C), and 0.74 (OsSi-C). A possible bonding interaction between TM−Si···HTM is suggested by the WBI values for Si···H of 0.52 (Fe), 0.37 (Ru), and 0.21 (Os). The calculated NPA charge distributions are again negative for the metal center and positive for the silicon center, DR


Chemical Reviews

Review

Scheme 24. Adduct (Co···H−Si), Optimization of Which in the Singlet State Formed a through the Crossover Point, CP1 To Give Adducts B and C, the Most Stable of Which was −14 kcal/mol (overall transformation was exothermic by 14.5 kcal/mol)a.357

a

Energies in kcal are listed below A, B, and C as well as CP1.

The reactivity of “B” (Scheme 24) with ethene was also calculated. The calculations indicated that the ethene inserted into the Co−H bond (TS = 6.0 kcal/mol) rather than the Co−Si bond (TS = 22.5 kcal/mol). The author’s calculations provided a mechanistic proposal that requires revision of previous suggestions and eliminates the intermediacy of cobalt silenes during the formation of dihydro disilylcobalt complexes. Their simplified mechanism (Scheme 26) illustrates a two-state

correction). The triplet species resulted from the interaction of Me2SiH2 [Cp(CH2CH2)Co···HSiHMe2] with a long Co···H distance of 2.24 Å (normal Co−H bonds are usually in the range 1.46 to 1.51 Å), although the Si−H bond of the Co···Si−H fragment was only slightly elongated relative to that in Me2SiH2. The energy of the adduct was only 0.9 kcal/mol above that of the reactants. A crossover point after formation of the first adduct gave, exothermically, several rotamers around the Co−Si bond, of which 2 are shown in Scheme 24. The energies of the various species are given in kcal/mol relative to [CpCo(C2H4)]. The lowest energy rotamer is in agreement with the reported NOE effects in the related species [CpCo(SiHEt2)(H)(C2H4)].359 The mechanism for inversion of configuration was also studied in this report.357 An attempt to obtain an η2-silane transition state failed. The photolysis of CpCo(C2H4)2 led to the triplet 3[CpCo(C2H4)] species, and optimization of the triplet species in reaction with H2SiMe2 gave the adduct “A” (Scheme 24) with a long Co···H distance of 2.24 Å resembling the η1-silanes described previously, although the Si−H bond of the Co···Si−H fragment was only slightly elongated. A crossover to the singlet through CP1 gave several rotamers, two of which are given in Scheme 24 as “B” and “C”, the more stable of which had the H of the -SiMe2H ligand pointed away from the Cp center. An inversion at the metal proceeds in the singlet state to a silyl-Co complex via the bridging hydride. The authors also calculated the inversion of configuration in the related complexes, “D” and “E”, shown in Scheme 25. All

Scheme 26. Simplified Overview of Catalytic Cycles for the Hydrosilylation of Ethene by Mono- and Dihdridosilane and of the Path to Disilyl Dihydride CpCo Complexes357

mechanism that connects high- and low-spin species with a key step of insertion of ethene into the Co−H bond. A simplified view of the hydrosilylation of ethene resulting from their calculations is shown in Scheme 26.357 The Si···H interligand interactions in Co(V) and Ir(V) bis(silyl)bis(hydride) complexes were the subject of calculations by Horbatenko and Vyboishchikov utilizing the various functionals: B3LYP, BP86, MO6, Mo6L, PBEPBE, and TPSSh.363 The TPSSh function appeared to perform better than the remaining 5 functionals. The systems for which the calculations have been performed are shown in Figure 27.

Scheme 25. Inversion of Configuration at Co Showing Crossover Points CP2 and CP3357

the attempts to find an η2-silane transition state (TSA) failed; however, a solvate-like TSB such as is depicted in Scheme 25 was located where the silyl ligand interacts via a Co···H interaction that provides an achiral Co environment and leads to the other diastereomer. The free energy for this transformation (ΔG‡) was calculated to be 17.6 kcal/mol at 298 K (experimental value, at 303 K, was ΔE‡ = 14.2 kcal/mol).359 It is the 3Co state that activates Si−H bonds to ultimately generate singlet silylcobalt hydrides without the intervention of σ-silanes.

Figure 27. Complexes for which interligand interactions were calculated in Cp*M(SiR3)2(H)2.356 DS


Chemical Reviews

Review

The relevant complexes that have been reported are [Cp*Co(H)2(SiPh2H)2] (Co1, X-ray)360 and [Cp*Ir(H)2(SiEt3)2] (X-ray)365 as well as earlier in the 1980s by Maitlis and coworkers.364 The main question addressed in this study was whether any type of Si···H interaction occurs in complexes of the composition [Cp*M(H)2(SiR3)2], since neither exchange between Co−H and Si−H groups nor silicon satellites for the hydride signal were observed in the NMR spectrum,360 leading the authors to suggest that no interactions were present. In complexes with two silyl ligands and an intervening Si···H, the two silicon atoms could compete for the hydride, which would designate it as a silyl−silane complex, and interconversion could, in principle, occur between the two forms: [LnM(SiR3)(η2-H-SiR′3)] = [LnM(SiR′3)(η2-H-SiR3], or the hydrogen can interact with both silicon atoms simultaneously, with the entire R3Si···H···SiR′3 unit behaving as a single ligand, as was the case for [CpFe(CO)(SiMenCl3−n)2(H)(X)] (X = SiMenCl3−n, H, Me (n = 0−3)).362 All the complexes have a piano-stool geometry with the two hydrides in trans positions and two silyls also in trans positions. In the complexes Co2, Co3, Co4, and Co5, the Si1−H1 and Si2−H1 distances are the same, and the Si1−H2 and Si2−H2 distances are the same (but different from those to H1). The Si1−H1 and Si2−H1 bonds are shorter and fall between 1.89 and 2.47 Å, whereas the somewhat longer Si1−H2 and Si2−H2 bonds are between 1.90 and 2.50 Å. The parameters calculated (TPSSh method, in Å) for Co1 (with experimental values in parentheses) were Co−Si1, 2.258 (2.256−2.261 Å) and Co−Si2, 2.260 (2.243−2.247 Å), Si1−H1,2 = 2.210 and 2.246 Å (Si1−H1,2 were not located in the structure), Si2−H1,2 = 2.080 and 2.106 Å (Si2−H1,2 could not be located in the structure). Wiberg bond indices were computed for Co1 to Co7, and the values were between 0.10 and 0.21 for Si1,2−H1 distances and 0.06 and 0.12 for Si1,2−H2 distances (a WBI that is >0.05 is indicative of an Si···H interaction). There appear to be 3 types of interactions: (a) for Co2−Co5, the presence of four Si−H interactions is clear with Si1···H1···Si2 (slightly stronger) and Si1···H2···Si2 (slightly weaker); (b) Co1 is best described in terms of Si1···H1···Si2 and H1···Si2···H2 interactions; (c) Co6 and Co7 exhibit large Si1···H1 and Si1···H2 WBIs with a strong H1···Si1···H2, in accordance with the short Si1···H1 and Si1···H2 distances, but for the Si2···H1,2 pairs, the WBI almost vanishes. Although the values of the WBI are rather low, they correlate with Si−H distances within each of complexes. At least two residual Si···H interactions are present in Co1, Co2, Co5, Co6, and Co7, and all four Si···H in Co3 and Co4 were present. In contrast to the cobalt complexes, Ir1 through Ir7 are classical Ir(V) bis(silyl)bis(hydride) that are “symmetrical” complexes where the Ir−Si1 and Ir−Si2 bonds are equal with perhaps rudimentary Si···H interactions. The exception is Ir4, [Cp*Ir(H)2(SiF3)2], with weak Si···H interactions. The Ir−Si distances vary in the seven Ir complexes from 2.322 to 2.426, compared to the various Co−Si distances that range from 2.183 to 2.304. Therefore, the authors proposed that the larger atomic radius of iridium prevents the silyl and hydride ligands from a close interaction with each other, thus impeding a Si···H interaction.356 In a companion paper, the same authors reported the calculations for the corresponding Rh series shown in Figure 28.363 These may also be viewed as “piano-stool” complexes, and the following examples have been synthesized and characterized spectroscopically and in some cases by X-ray or neutron diffraction: [CpRh(SiMe3)2(H)2] (X-ray),366 [Cp*Rh(H)2(SiEt3)2],422,424

Figure 28. Complexes studied computationally for hydrogen behavior in an Si−H−Si sequence.363

[CpRh(SiMe3)2(PMe3)H]+,420 [Cp*Rh(SiMe3)2(H)(PMe3)]+421 In this series of 13 compounds, the hydride ligand interacts with at least one of the silyls and in some cases with both. All of the Cp complexes are asymmetric with one Si−H distance that is shorter (1.900−2.140 Å) than the other (1.981−2.196 Å). The TPSSh structures of the three Tp complexes are asymmetric and exhibit a η1-silyl−η2-silane coordination. Overall, the lowest CCSD energy was provided by TPSSh for Cp1-C-3, Tp1, and Tp2, and by M06L for both Tp3 and Cp4− Cp10. The Rh−Si distances in halogen-containing complexes Cp2− Cp4 (compared to Cp1) and Cp6−Cp8 (compared to Cp5) are shorter, and this shortening is attributed by the authors to IHI, as there is a halogen atom in the position trans to the hydride. The Si1−H distances in the Cp complexes vary from 1.900 to 2.006 Å, with WBI values ranging from 0.201 to 0.190, and the Si2−H distances from 1.981 to 2.110 Å, with WBI values of 0.142 to 0.121.363 In all cases the Si2−H distance was longer than the Si1−H distance. For the more limited Tp series, the Si1−H range was from 1.672 to 1.827 Å (WBI = 0.362 to 0.247) and the Si2−H range was 2.117 to 2.414 Å (WBI = 0.102 to 0.021). For the entire set of Cp and Tp complexes, the barrier to hydrogen transfer does not appear to correlate with the geometry parameters. From an analysis of ground-state vibrational wave functions, three classes of complexes were identified. A single-maximum vibrational wave function corresponded to hydrogen delocalization between both silyls for complexes with a single-well potential or with a very low barrier (example [TpRh(SiH3)2(SiMe3)(H)]). If both the minima and the TS are located close to each other on the potential energy surface (PES), the hydride behaves as a classical particle with 2 well-separated maxima near the PES minima. In this case the hydrogen transfer is coupled to the Si−Si motion. In the intermediate case between localized and delocalized hydride behavior, where the vibrational wave function has two weakly pronounced maxima, tunneling accounts for a large part of the hydrogen transfer (example, [TpRh(SiF3)2(PMe3)(H)]+).363

8. OXIDATIVE ADDITION REACTIONS OF LANTHANIDES/ACTINIDES AND OF OTHER SI−X BONDS 8.1. Lanthanides/Actinides

Only 7 publications concern reactions of, or calculations for, lanthanides and 1 for an actinide. There were no OA reactions DT


Chemical Reviews

Review

3,5-tBu2C6H3, in the presence of B(C6F5)3, as shown in eq 88. In this reaction U(VI) was converted to U(V) and no U−Si bond was formed in 5-180.224 When 1 equiv of Cp2Co was added to 5-180, the U(V) center was reduced to U(IV), forming the complex U(OSiEt3)2(Aracnac)2.2 Recently, it has been shown that NHC are reactive ligands to access redox inactive metals. Addition across the dative bond in d0 in Group 3 and f-block metal NHC complexes (Y, Ce, U) allows incorporation of units such as silanes and boranes to be incorporated resulting in complexes that are active for σ-bpmd metathesis chemistry.387 8.1.2. Theoretical Calculations for Lanthanides. The bond activations of PhSiH3 by Cp2SmH were studied by the DFT method by Eisenstein, Tilley, and co-workers.367 The experimental results were reported originally in 1996 for the reaction of Cp*2Sm[CH(SiMe3)2] with Ph2SiH2 as well as with PhSiH3.368 The reaction of the samarium complex with PhSiH3 provided a variation of products, including CH2(SiMe3)2 (quantitative), the cluster [Cp*2SmSiH3]3, PhSiH2-SiH2Ph, PhH, as well as the redistribution product SiH4. Reaction with PhSiD3 gave quantitative transfer of deuterium to give CHD(SiMe3)2, which suggested that the first step in the reaction could be formation of Cp*2SmSiH2Ph, although it

of a hydrosilane that led to the formation of a direct Si−Ln bond reported during this review period. However, there were examples of the formation of σ-agostic, β-SiH interactions that were included in Table 9 in examples 3-75,117 3-76,117 929,304,305 and 9-30.304 8.1.1. Ln Complexes with β-SiH Interactions. The first examples of Yb complexes with β-SiH interactions were generated from salt metathesis reactions between YbI2 and K[C(SiHMe2)3], as shown earlier in eq 74 (Section 5), and gave the complex [(HMe2Si)3C]2Yb·2THF (9-29).304,305 The parameters that supported the β-SiH interactions were lowenergy ν SiH bands in the IR spectrum, short Yb−Si distances, and small M−C−Si angles in the crystal structure.304,305 The Si−H distances in 9-29 varied from 1.36(3) to 1.48(4) Å and the Yb−H values from 2.50(3) to 3.59(4) Å. In the solid state, however, only 2 of the 6 Si−H bonds interacted with the Yb center. The related complex, Yb{C(SiHMe2)3}2·TMEDA (9-31)305 was prepared analogously to that of 9-29 but with excess TMEDA replacing THF to provide the TMEDA adduct, 9-31, that also exhibited parameters that supported β-SiH interactions.305 In this case, however, all 6 of the SiHMe2 groups were directed into the interior of the molecule and the methyl groups were pointing outward, but the combination of short Yb−Si distances, acute Yb−C−Si angles, small Yb−C−Si-H torsion angles, and short Yb−H distances provided structural support for two mono agostic C(SiHMe2)3 ligands bonded to the ytterbium center. The NMR spectrum suggested fluxionality on the NMR time scale in solution.305 In the reaction of Ar(H)NSiH3 with [Yb(NR2)2(thf)2] (R = SiMe3; Ar = 2,6-(3,5-Me2C6H3)2C6H3)), the product obtained was a function of the stoichiometry and the temperature.117 With 1 equiv of Yb in toluene at 70 °C, the complex isolated was {ArNSiH[N(SiMe3)2]}2Yb2 (3-75), which has a sixelement core, Yb2N2Si2, that approximates an octahedron, and with N centers that are (trans) to each other, and 2Yb centers and 2Si centers that are also trans to each other (but alternating giving an almost centrosymmetric core). A hydride bridges the Si−Yb bond. The complex was characterized by α-Si−H−Yb agostic bonding as described in Section 5. The171Yb NMR spectrum exhibited a sharp resonance indicating that the two Yb atoms were equivalent on the NMR time scale. If, however, the reaction was conducted at −78 °C with 2 equiv of Yb, a monomeric complex, {ArN[SiH2][N(SiMe3)2]}2Yb (3-76), formed.117 In this case the 1H NMR spectrum exhibited only one broad singlet for the H2Si groups at room temperature, suggesting fluxional behavior. When the temperature was lowered to 193 K, two doublets for the SiH2 unit were observed in the 1H NMR spectrum. The doublet at δ = 4.17 and 4.30 ppm (1JSiH = 257 Hz) was assigned to the Si−Hterm, and the other doublet at 4.19 and 4.27 (1JSiH = 104 Hz) was assigned to the β-agostic Yb···(H−Si) on the basis of the value of the coupling constant. The IR spectrum of 3-76 also had red-shifted bands at 1782 and 1749 cm−1, consistent with the agostic interaction, as compared to 2159 cm−1 for the Si−Hterm. The reaction of a primary aminosilane certainly provided unexpected products that contained both α- and β-agostic products, 3-76 and 3-75.117 Although the reactions of KC(SiHMe2)3 and YbI2 are salt metathesis reactions, the SiH of the ligand plays an important role in the nature of the β-agostic products that formed.304,305 Only one actinide publication involved reaction of a hydrosilane, and that was with the uranium complex, UO2(Aracnac)2 (Aracnac = ArNC(Ph)CHC(Ph)O, Ar =

could not be identified as an intermediate.367 The formation of redistribution products implies that Si−C bond formation could be occurring through a σ-bond metathesis transition state that places the Si center in a position β to the metal. Also, in an earlier publication, Cp*2SmPh was demonstrated to be the key intermediate in phenyl-transfer reactions.369 In the current report, six σ-bond metathesis possibilities were evaluated.367 It is possible that there could be two key intermediates for the reaction: Cp*2SmPh and Cp*2SmCH2SiPh. The silylation pathway by Si−C activation (removal of Ph from PhSiH3), leading to Sm-SiH3, does not occur by a direct, one-step process, as the energy required to reach the TS‡ is too high; therefore, at least two steps would be required.367 In a proposed mechanism, the first step was PhSiH3 coordinating to the metal center with a η6-PhSiH3 mode which then proceeded to a sigma-bond metathesis transition state with SiH3 in the β-position and an activation barrier of 19.2 kcal/mol, which would be a kinetically accessible reaction. This transition state gives a σ-adduct of H-SiH3. A Ph-Sm complex was not observed and probably reacted with molecules in the reaction mixture to produce Cp2Sm-SiH3. The σ-adduct of H-SiH3 could transform into another σ bond metathesis transition state with H in the β-position that would decay to [Sm]−SiH3 and Ph−H. The calculated activation barrier was 20.8 kcal/mol and sufficiently low to give an overall kinetically accessible reaction. The twostep pathway involved Siβ−Cα followed by a Siα−Hβ activation and was kinetically and thermodynamically favorable. The authors also calculated a reaction in which the first step would be reaction of PhSiH3 via Si−H (not Si−C) activation. A second Si−H activation of SiH4 would give the reaction DU


Chemical Reviews

Review

associated with the 1,2-insertion of propene into [Sm]−H to give Cp2SmCH2CH2CH3 (ΔG* = 4.8 kcal/mol), and the 2,1-insertion was higher (ΔG* = 7.7 kcal/mol). After the 1,2-insertion, a σ-bond metathesis reaction could occur with Cp2SmH or Cp2SmPr and one of the 3 sigma bonds of the silane, Me4−nSiNn (Si−H, Si−C, or C−H; n = 1−4). Activation of the Si−H bond with the silicon at the α-position gave the silyl complex, Cp2SmSiHn−1Me4−n plus H2. Activation of a Si−C of the silane would lead to Cp2SmSiMen−1H4−n and CH4 or to Cp2SmCH3 and Me4−n−1SiHn+1 (i.e., redistribution of silanes), depending on the orientation of the Si−C bond in the σ-bond metathesis transition state. Lastly, the activation of a C−H bond of a silyl-methyl group would lead to Cp2SmCH2SiMen−1H4−n and H2. Since the reactions of Si−H, Si−C, or C−H with Cp2SmH were higher in activation energy than the 1,-2-insertion of propene, the major product would be Cp2SmCH2CH2CH3. The rate-determining step for propene hydrosilylation by alkylsilanes was the silyl complex, Cp2SmSiHn−1Me4−n that would be formed from Cp2SmPr by Si(α)−H(β) activation. The substitution of SiH by SiMe reduced the reactivity, which could be explained by electronic factors. The calculations indicated the effects were sufficiently large as to disfavor hydrosilylation by R2SiH2 or R3SiH. Substitution on the olefin increased the energy for allylic activation and deactivates the catalyst.371 The last example from the lanthanide group does not contain either a M−H or Si−H but involves reactions that also take place by σ-bond metathesis and [1,2]-shifts, as reported by Eisenstein and co-workers.373 The study was prompted by observed chemistry where Cp′2CeH (Cp′ = 1,2,4(Me3C)3C5H2) and MeOSiMe3 gave the products Cp′CeOMe and HSiMe 3 whereas the related metallacycle, Cp′[(Me 3 C) 2 (C 5 H 2 CMe 2 ) 2 CH 2 ]Ce and MeOSiMe 3 gave Cp′2CeOCH2SiMe3 (presumably involving the hypothetical Cp′2CeCH2OSiMe3 by a [1,2]-shift). The solid state structure of the Cp′2CeOCH2SiMe3 complex unequivocally indicated the CeOCH2 sequence rather than a CeCH2OSiMe3 sequence. DFT(B3PW91) calculations, utilizing the models Cp2CeH and MeOSiMe3, gave the lowest energy pathway that was a H for OMe exchange at Ce through a σ-bond metathesis transition state with SiMe3 exchanging partners. In the formation of Cp2CeOCH2SiMe3, there was a low activation barrier to a [1,2]-shift (silyl-Wittig rearrangement) in Cp2CeCH2OSiMe3. The calculated potential energy surface for the reaction of

product, and this route was also thermodynamically plausible. Mechanistic pathways to dehydrocoupling products (such as PhH2 Si−SiH2Ph) and redistribution products (such as Ph2SiH2) were also calculated. Two other papers authored by Eisenstein and co-workers concerned hydrosilylation and are related. Part 1 concerned the reaction of propene with SiH4,370 and Part II probed the influence of a substituent on the olefin or the silane371 in the hydrosilylation of olefins. Both papers involved the catalyst, Cp2SmH. An earlier mechanistic proposal372 invoked the in situ formation of [Sm]−H from [Sm]−X (X = Cl, CH(TMS)2 or alkyl) followed by insertion of the olefin into the Sm−H bond and then an insertion/activation mechanism. The model used for the hydrosilylation in Part 1 of the Eisenstein papers was the addition of SiH4 to CH2CH−CH3 catalyzed by Cp2SmH, and the calculations were at the DFT level with the hybrid functional B3PW91. By their calculations, the authors demonstrated that Cp2SmH was not the active species for the hydrosilylation of propene with SiH4, as the insertion of propene into Cp2SmH would be required, followed by silane activation with Si at the α-position of a 4-centered transition state, which then would lead to the in situ formation of Cp2SmSiH3 and catalyzes the hydrosilylation of propene. Thus, the hydride complex is a “precatalyst” for this route. The course of the reaction summarized from calculations is shown in Scheme 27. In the second paper (DFT level, with hybrid functional B3PW91 with the Gaussian 03 suite of programs), Cp2SmH was considered a precatalyst, and the silyl complex Cp2SmSiH3 formed in situ from Cp2SmPr was preferred as the catalyst species. The catalytic process would begin with the insertion of propene into the Sm−Si bond of the catalyst.371 The aim of this second paper was to extend the theoretical work to more “real” systems by examining the influence that substituents on the silane and on the olefin had on the course of the reaction. The series of silanes included MeSiH3, Me2SiH2, and Me3SiH and was in line with the actual experimental trend that showed a decrease in hydrosilylation yield with increasing substitution of the silanes. The hydrosilylation of 1-hexene and isobuylene with SiH4 was also investigated.371 The reactions with the lowest activation barriers were 1,2- and 2,1-insertions of propene into Cp2SmH, and the activation of an allylic C−H bond (allylic activation) and all of these were exergonic. The lowest energy of activation was

Scheme 27. Calculated Sequence for Hydrosilylation of Propene Catalyzed by Cp2SmH370

DV


Chemical Reviews

Review

Cp′2CeH and MeOSiMe3 proceeded through a SiMe3+ cation in the σ-bond metathesis transition state. For the reaction of the metallacycle with MeOSiMe3, the calculations indicated that the favored path was CH bond activation over SiMe3 migration. The transition states for both reactions involved a silicon that was five coordinate. From the interaction diagrams for each of the reactions, it could be shown that the σ-bond metathesis and the silyl-Wittig rearrangement were isolobal processes when there was a migrating silyl group present.373

which the more realistic model, Pt[NHC(Dip)2](SiMe3)2 was used. All the geometries were optimized by the DFT method with the B3PW91 functional.377 In agreement with the previous bonding description, the Y-shaped structure became stable when the three ligands bound to the metal were strongly σdonating, as was the case for both a carbene complex and silyl groups. Where these authors differ from those of Markó and co-workers is based, in part, on the 195Pt NMR chemical shift which was quite different from those of Pt(II) complexes (the 195 Pt NMR shifts of the T-shaped complexes were similar to those of Pt(II)). The calculations indicated that the Si···Si bonding interaction was ∼50% of the usual Si−Si single bond energy. The interaction was still viewed in terms of Si−Si σ-donation and back-donation of Pt into the Si−Si σ*antibonding MOs. However, the view of Sakaki and co-worker was that the oxidation state of Pt is likely to be 0 for the σ-disilane complex and thus a snapshot after charge transfer from the disilane to the Pt center had taken place. The authors also examined σ-digermane, σ-diborane, and σ-silylborane complexes of Pt(0) as well as of Pd(0) and Ir(I) and concurred with the previous description that the complexes were good models of snapshots for the RE of not only disilane but also digermane, diborane, and borylsilane.377 In the following examples the reactions of a disilane were utilized for synthetic purposes. Some authors refer to the onestep formation of a Si−M−Si sequence from a disilane and a metal complex as insertion of a substrate into the Si−Si bond, and other authors refer to it as oxidative addition of a Si−Si bond to the metal center, but the net result is the same. In the following examples, the synthetic aspects of disilanes are highlighted. The Pd-catalyzed reaction of cis- and trans-3,4benzo-1,2-di(tert-butyl)-1,2-dimethyl-1,2-disilacyclobut-3-ene with diphenylacetylene was published earlier.378 Depicted in Scheme 30 is the reaction of the cis-isomer with PhCCPh, and reaction of the corresponding trans-isomer gave only transproduct.379 The reaction of RCCH (R = nBu, tBu, Ph, and Me3Si) was also reported and was presumed to go by the same mechanism.379 The insertion of Pd(PPh3)2 is shown in the reaction sequence sequence in Scheme 30. The reaction of alkenes (CH2CHPh and CH2CHnBu) required a higher temperature (110 °C for 24 h) and gave isomeric products. Reaction of the cis- and of the trans-isomer with styrene gave two isomers from each of the starting materials as shown in Figure 30. The reaction of the cis-isomer with H2CCH(nBu) gave two isomers related to those shown in Figure 30 from the cis-starting complex, but only a single isomer was formed in the reaction of the trans-starting complex. The authors concluded that the unfavorable stereoisomers were formed as the major products but offered no explanation for this observation.379 Later, the same starting disilane (eq 90 and Scheme 31) was reacted with CH2CH2 and demonstrated another reaction pathway. The products from the cis-form of the disilane are shown in eq 90, and that from the trans-form are shown in eq 91.380 The mechanism for the reaction with ethylene was proposed to be similar to that in Scheme 30, accept that after insertion of Pd into the Si−Si bond (last structure in brackets in Scheme 30), a second molecule of the starting cis-disilane oxidatively adds to the Pd center to form a spiro complex that involved 2 molecules of the starting disilane and one ethylene molecule followed by isomerization of the spiro complex as shown in Scheme 31, to give the product (58% yd) in eq 90.

8.2. Oxidative Additions of Other Si−El Bonds

This section introduces a sampling of other oxidative additions or metathesis reactions that have been reported for formation of Si−El linkages other than those produced from Si−H. The examples are summarized in Table 12, and for complexes that have been crystallographically characterized, the pertinent metal−silicon bond information will be found in the footnotes to Table 7. 8.2.1. Si−Si Bonds. Do Si−X bonds other than Si−H form σ-complexes with transition metals? One of the rare examples involves coordination of a cyclic disilane to Cu(I) as shown in Scheme 28.374 The formation of a Si−Si to Cu(1) was supported by both the X-ray structure and calculations. The observed Cu−Si distances are actually just within the sum of the covalent radii (2.28 Å)375 whereas the Si−Si bond has lengthened to 2.3581(11) from that in the free ligand, 2.24505(19) Å, but is only slightly longer than 2× the single bond radii (2.32).374 In contrast to the copper complex, in the silver analog (Scheme 28), the Si−Si interaction to the metal appeared to be absent, as indicated by the Si···Ag distances, which are unequal, 3.3896(11) and 3.4766(11). The sum of the single bond covalent radii for Si−Ag is 2.44 Å.375 DFT calculations and NBO analyses indicated a donor−acceptor interaction between the Si−Si σ bond and the Cu(I) center but not in the case of Ag(I). This was the first structural evidence for this type of bonding interaction for a coinage metal.374 A less compelling case was published for a bis(silyl)platinum(II)−NHC complex that is shown in eq 89.215 Complex 5-153

exhibited a unique trigonal-planar Y-shaped geometry with a relatively short Si···Si distance of 2.980(5) Å compared to that in [Pt(SiMe2Ph)2(PPhMe2)2], which was 3.233(1) Å.215 The DFT calculation for the model complex [Pt(SiMe3)2(Im)] (Im = imidazo-2-ylidene) also exhibited a Y-shaped geometry as one of the preferred geometries. If the Im ligand was replaced by PPh3 in the calculation, the Y shape was no longer a stable geometry. The Y-shaped geometry seemed to impose a degree of σ-bonding between the two silicon centers. Thus, 5-153 was viewed as an intermediate in the continuum of reductive elimination to the disilane, as shown in Scheme 29. Treatment of 5-153 with 11 equiv of diallyl ether at 60 °C gave a quantitative yield of the complex where diallyl ether was coordinated to the Pt displacing the two SiMe2Ph groups as the disilane (Me2PhSi)2.215 However, a later theoretical study by Sakaki and co-workers supported another view of the bonding in complex 5-153 in DW


Chemical Reviews

Review

Table 12. Reactions of Si−Si, Si−El, Si−C, and Si−X Bonds with Transition Metal Complexes (selected structures are shown at the end of the table for compounds marked with an asterisk)

1 Ambient temperature unless otherwise noted. Temperatures in °C. 2In ppm. Coupling constants in Hz. Assignments: s, singlet; d, doublet; t, triplet; q, quartet; quin, quintet; sept, septet; m, multiple; br, broad; vt, virtual triplet, sat, satellites. 329Si may have been determined by direct observation, INEPT or DEPT, or 2D 29Si−1H correlation. These methods are not distinguished in the table. 4Will contain other characterization methods including spectroscopic, X-ray, and calculations if reported. If elemental analyses were reported, this will be indicated by EA. If analysis is outside ±0.5% of calculated percentage value for carbon, this will be indicated by the symbolism, [EA] 51H, 29Si HMBC spectrum.415 612-4 decomposes above −60 °C.417 712-5a was also prepared by the reaction [Pd(ItBu)2] and Me3SiI.289 8(ItBu = 1,3-di-tert-butyl-imidazol-2-ylidene).289 9 The corresponding complexes with R = Me (91%, X-ray) and Et (93%, X-ray) were also prepared (characterization data in Supporting Information). A series of complexes were also prepared from {(Ph2P)C6H4}2SiMeR (R = H, Ph, Et, p-tolyl, p- methoxyphenyl, p-dimethylaminophenyl, and p-fluorophenyl (characterization data in Supporting Information).277 10The corresponding complexes with R = Me(81%, X-ray) and Et(95%, X-ray) were also prepared (characterization data in Supporting Information).277 11Prepared in situ from Pd(PCy3)2 and PMe3 (4 equiv).292

DX


Chemical Reviews

Review

Scheme 28. Coordination of Si−Si to Cu(I) but not Ag(I)374

Scheme 29. Proposed Reductive Elimination Continuum for the Formation of Disilane215

Figure 30. Reaction of PhHCH2 with benzodisilacyclobutenes to give isomeric insertion products.379

Scheme 31. Additional Intermediates in the Reaction of Heterocyclic Disilane in eq 90 with CH2CH2 To Account for the 2:1 Product Formed in 58%380

The Si−Si bond has also been activated by Cu(I) for the conjugate silylation of α,β-unsaturated compounds. The optimized reaction conditions involved Cu(OTf) 2/C 6H 6 (5 mol %), phenanthroline (7 mol %), DMPU, 100 °C, overnight with the disilane, PhMe2Si-SiMe2Ph (eq 92). The

Equation 93 shows the results for one allylic carbonate and the conditions for the reaction. The mechanism of the reaction is shown in Scheme 32 and involves the formation of a Si−Cu intermediate.

substrate, EWG(H)CCH2 (EWG = PhO2S, NC, Me2N(O)) provided alkylsilanes, (EWG)CH2CH2SiMe2Ph, in yields from 38% to 80% (3 examples). Alkynes, EWG--R (8 examples), and the same disilane gave vinylsilanes in yields varying from 27 to 74%. Two terminal acetylenes gave vinylsilanes as (E/Z) isomers (EWG = EtO2C-, 41% (87:13); MeO2C-, 51% (90:10). Six internal acetylenes gave vinylsilanes in yields from 27 to 74% with isomeric ratios varying from 100:0 to 66:34.381 In a separate study, the Cu(I) catalyzed reaction of PhMe2Si− SiMe2Ph with allylic carbonates gave allylsilane products under more mild conditions with the following amounts: substrate (# mol), PhMe2Si-SiMe2Ph (0.25 mol), CuCl (0.0125 mol), ligand (0−0.125), K(OtBu) (0.25 mol), THF (0.5 mL).

Gold, another coinage metal, in the form of gold particles supported on Au/TiO2, also has been reported to activate 1,2-disilanes in a reaction with water.383 As an example, Me6Si2 reacted with water in the presence of Au/TiO2 (1%) at room temperature to provide Me3SiOSiMe3 in >99% yd within 1.5 h. The reaction may also be utilized with a variety of disilanes to provide the silyl ethers of primary and secondary silanes (11 examples with yields ranging from 85% to 98%). Tertiary alcohols also react with the same catalyst system (Me6Si2/ R1R2R3COH = 5:1, at 55 °C, 1 to 24 h) to give two products, R1R2R3CH and R1R2R3COSiMe3, with the alkane as the principle product. In two cases of the reaction of tertiary alcohols, dehydration competed with the formation of the silyl

Scheme 30. Reaction Sequence Proposed for Insertion of “Pd(PPh3)2” into a Symmetrical Alkyne379

DY


Chemical Reviews

Review

with both Cl2MeSi-SiMeCl2 and ClMe2Si-SiMe2Cl also gave the cis platinum complex, the former in 93% yd after a 10 min reaction.214 The reaction of (EtO)3Si−Si(OEt)3 with [Pt(PEt)3] in the presence of H2 also provided both cis- and trans{(Et3P)2}PtSi(OEt)3(H) plus HSi(OEt)3. The ratio of cis:trans isomers was 10:1, which changed to 5:1 on irradiation for 5 h, but storage of the 5:1 mixture resulted in formation of the original 10:1 distribution. A mechanism for the reaction of the disilane was not proposed.221 The synthesis of organosilanes through Pd-catalyzed allyl C−OH functionalization is shown in eq 94 as one of the 9

Scheme 32. Proposed Reaction Mechanism for the Cu(I)Catalyzed Allylic Substitution Utilizing a Disilane Substrate382

ether, although the yield of the silyl ether was still >60%. The mechanism proposed for the reaction involved insertion of Au into the Si−Si bond in the preliminary step. The mechanism was suggested by the observation that when PhMe2SiSiMe2Ph or Ph2MeSiSiMePh2 was utilized for the alcoholysis, PhMe2SiH and Ph2MeSiH were observed in varying quantities during the course of the reaction. However, at the end of the reaction, these silanes had disappeared, presumably acting as silylating agents themselves, or were hydrolyzed.383 A mechanism for the alcoholysis has been provided based on this observation (Scheme 33). It should be noted that supported gold

examples from Me3SiSiMe3 (yields from 52 to 84%) and 3 examples with Me2PhSi−SiPhMe2 (yields, 78−79%) and also represented a new method for allylic silylation from allylic alcohols and disilanes.388 A simplified mechanism for the reaction is shown in Figure 31. In a later publication,389

Scheme 33. Proposed Mechanism for Reaction of Gold Nanoparticles with Alcohols383

Figure 31. Simplified mechanism for the activation of a disilane in silylation of allylic alcohols.388

nanoparticles have been used to oxidize silanes to silanols in water by three different groups, although the authors did not speculate as to how the reaction took place.384−386 Gold(I) also mediated a Si−Si bond metathesis reaction at room temperature.10 The products were summarized earlier in Section 2, eq 6. A DFT calculation on model systems indicated that the neutral, monomeric complex R3SiAu could insert into Si−Si bonds through a low-lying transition state followed by the sequence shown in Scheme 34. Activation of Si−Si bonds by the Pt(0) complex [Pt(PEt3)3] has been reported. In an earlier publication, Braun and coworkers had demonstrated the reaction of [Pt(PEt3)3] with Me3SiSiMe3 to give only cis-[(Et3P)3]2Pt(SiMe3)2 in about 50% conversion after 3 weeks214 whereas HPh2Si-SiPh2H and [Pt(PEt3)3] provided a mixture of cis- and trans-{(Et3P)2}Pt(SiPh2H)2 in a ratio of 1:0.27 after 10 min (isolated as an oil in 97% yd).415 The hydrogenolysis of HPh2Si-SiPh2H in the presence of cis- and trans-{(Et3P)2}Pt(SiPh2H)2 (4 mol %) provided a 72% yd of H2SiPh2.415 The reaction of [Pt(PEt3)3]

an expanded version of the mechanism was demonstrated with 2 consecutive transmetalations starting with 2 equiv of Me3SiSiMe3, where Me3Si−F and BF3 are formed and a RE regenerated 1 equiv of Me3Si−SiMe3. The Rh-catalyzed decyanative silylation of nitriles and also alkenylsilation of nitriles by Chatani and coworkers involved the use of Me3SiSiMe3.390,391 Activation of R3Si-B(epin) by Rh provided transmetallation of R3Si- from R3Si-B(epin) in conjugate addition of a α,β-unsaturated acceptors.271 Since then, several examples of Si-B activation for use in new synthetic methods have been reported by other metals including: Cu,406−411 Pd.287,289,412 8.2.2. Si−C Bonds. The activation of Si−C bonds by transition metals does not fit into one category. The closest to the overall theme of this review would involve reactions that lead to the formation of a Si−TM bond. However, other reactions of a TM complex with a silicon substrate result in transfer of an R group (mainly Me) from Si to the metal. Also, a

Scheme 34. DFT Calculated Reaction Path for Si−Si Bond Metahesis in Silyl Complexes of Gold.10

DZ


Chemical Reviews

Review

could not be separated and the two cocrystallized. However, the Si−Pd dimer was independently synthesized from the same Pd precursor through oxidative addition of Me3SiI. A mechanism for the reaction was proposed that involved OA of Me3SiCH2I to Pd, to form (ItBu)Pd(CH2SiMe3)I (+ ItBu), and the ItBu carbene attacks the silicon center, ejecting iodide and initiating a sequence leading to the products. Insertion of [Pt(PEt3)2] into strained Si-C bonds has been speculated.295 In several reactions, a Me-substituent on Si was transferred to a metal center.394,395 One example is shown in eq 98 (several

TM complex may be a catalyst for the formation of new silicon compounds, particularly silicon heterocycles. Examples of each of these will be presented. Two reviews have been published that deal with catalyzed reactions leading to silacycles. A review by Hartwig in Accounts of Chemical Research contains a section on Ir-catalyzed silylation of arenes in which benzoxasiloles are formed.392 The formation of silacycles through metal-mediated or catalyzed Si−C bond cleavage contains, in part, reactions that require TM catalysis.393 A well-documented reaction conducted by Nakazawa and coworkers, was the reaction of HM(PPh3)3(CO) (M = Rh, Ir) with {o-(Ph2P)C6H4}2SiR2, which resulted in the cleavage of an R group from the silicon center (elimination of RH) (eq 95).277

other cases have been published). From mechanistic studies, the methyl transfer from Si to Pt probably occurred after oxidation to Pt(IV) and was initiated by OH− attack at Si.395 In a somewhat similar reaction sequence, summarized in Scheme 35, a “PSiP” pincer formed and the Si center became Scheme 35. Reactions of Pd(0), and Pt(II) Precursors, with (2-Cy2PC6H4)2SiMe2396 The competitive cleavage of an R group in the silicon precursors with two different R (or Ar) groups was also examined and is summarized in eq 96. The % yd of the Si−R activation product was, R = H (>99%), R = Ph (61%), R = Et (11%), indicating a preference for Si−CMe bond cleavage over Si−CEt cleavage. The factors that influenced the Si−C bond cleavage were proposed to involve the extent of increase of the d(Rh) → σ*(Si−C) interaction at the Si−C oxidative addition stage and steric hindrance of the substituent. Relative rates (60 °C) of Si−C activation of the precursors in eq 95 where R is a p-substituted aromatic ring gave a linear plot of log(kSi‑C6H4X/kSi‑CMe) vs σp (Hammett constants) for the substituent X. Lesser electron density in the aromatic ring resulted in an increased rate of Si−Carene activation. Presumably an electron-withdrawing group decreased the energy level of the antibonding orbital of the Si−C bond, resulting in a stronger d(Rh)−σ*(SiC(sp2)) interaction at the OA stage. Additional experiments led to the conclusion that the entropy of activation is the dominant factor determining the reaction rate and that a phosphine exchange process is the ratedetermining step. The rate of Si−CR bond cleavage at 60 °C followed the order Si−CEt, Si−CMe < Si−CPh. The replacement of Rh with Ir increased the extent of the Si−CMe activation.277 Another Si−C bond activation led to a product mixture that contained a M−Si bond that involved the metal precursor [Pd(ItBu)2] and the silane, Me3SiCH2I, in the formation of a (M−Siterm)2 dimer as one of the products (b) in a mixture, as shown in eq 97.289 Complexes “a” and “b” (shown in eq 97)

coordinated to Pt(0 or II) or Pd(0).396 The Si-C(sp) bond in alkynylsilanes was activated by the first generation Grubb’s catalyst and provided a route to ruthenium styrylcarbene complexes.402 A Rh-porphyrin complex converted an unactivated Si-C(sp3) bond in a SiMe3 group to an SiOH.403 There are also cases where metal catalyzed Si−C cleavage has been targeted in the preparation of silacycles. Wang and Duan have reviewed recent work in this area.393 One of the examples from this review is shown in eq 99.397 This approach has been

most successfully applied to the formation of siloles, silaindenes, and silafluorenes.398,−401 Metals can insert into strained Si-C bonds just as into strained Si-Si bonds. Insertion of Pd into a strained silacyclobutane was involved in a σ-bond exchange between C-C and SiC substrates.405 There are few studies of the comparison of catalytic behavior of Group 14 Pincer ligands such as PMP (M = Si, Ge, Sn) but a report has shown that the “PMP” ligand when coordinated to Pd, EA


Chemical Reviews

Review

silylmetal complexes reported during the review period, and the data are listed in Table 7 by triad (3d metals, then 4d, then 5d; from the scandium triad to the zinc triad). The table contains values for the complexes from Tables 3−5 but also contains the examples in the footnotes to these tables. In addition, complexes prepared from other silicon reactants (other than the hydrosilane) are included as well as those reported in Tables 1 and 2 and in the footnotes to the tables. The temperature of the data collection for the silylmetal complex is also provided if it was given in the Supporting Information to the papers. There are still a few examples of data collection at or near room temperature, but most structures are now determined at low temperatures, where the data are presumably more reliable. The Si−TM bond distance ranges for each transition metal element, which involve 2c/2e, multicenter and silylene complexes, are summarized in Table 8a. These ranges vary from element to element and for those elements for which there are more than 10 complexes that have been reported (covering the three review periods) are found between 0.22 Å (Re, 36 values) and 0.79 (Fe, 453 values). The breakdown for the data collected during the current review period will be found in Table S1 into (a) values for compounds in Tables 3−5; (b) values from the footnotes to Table 7; and (c) values from Tables 1 and 2. Difficulties that still remain are establishing a reliable hydride distance for H in the vicinity of a heavy atom as well as just locating hydrides in the structure, or there is a problem with disorder. Only about 75% of the complexes reported in Table 7 during this review period have been relatively free of these problems. When a silane approaches a metal center, the Si−H bond will begin to stretch and become longer than 1.50 Å, but describing the interaction of the hydride (or the hydride and the silicon center) can be difficult. The variations of interaction are summarized in Section 5, Scheme 7, with the approximate distances associated with the interactions. However, some other designations have been introduced during this review period, including ASOAP(asymmetric) and SOAP(symmetric) oxidative addition products (Section 5, Figure 6), the complexes where a “nonclassical” (usually meaning multicentered bonding) or “agostic” interaction or “σ-complex” is present. The examples are found tabulated in Table 9 with characterization data. The nonclassical interaction of a hydrosilane and a metal may be divided into two approximate groups, one where the silicon hydride interacts with the metal (Si−H···M; the major grouping; Section 5.3) and the second one where a M−H interacts with the silicon center (M−H···Si; Section 5.4). Interactions of a silane with the metal center can involve a head-on approach of the hydride to the metal center (η1 sigma complex) and where no bond between Si and the TM is formed and with a Si−H−TM angle that is generally larger than ∼120° to η2-sigma complexes where the H and Si both are interacting with the TM center. In the agostic complex the interaction of a Si−H occurs when the SiH unit is tethered to the metal by intervening atoms in the α,β,γ,δ,ε-positions. The M−H···Si distances longer than 2.0 Å have been described as IHI and/or SISHA interactions previously, and another designation of “remenant interactions” has been introduced during this review period. Distances >2.5 Å for H···Si are considered nonbonding. Representative cases indicating how collected characterization data point the way to a structural representation are given in Section 5.2. There is no uniformly adopted convention for describing the various bonding possibilities for nonclassical complexes, although a possible view is described later in

promotes reductive aldol type reactions and that when M is Si, the strongest trans effect was observed.288 Although there are a few examples where a silyl group can be transferred from carbon or from a Si−Si bond, the examples are somewhat rare and would not be expected to displace the use of SiH bonds for the purpose of adding a silyl group to a metal center.

9. CONCLUDING COMMENTS The reactions of hydrosilanes with transition metal complexes continue to be an active research area. Tertiary silanes, HSiR3, continue to be the most studied silane reactants, as the number of examples in Table 5 (tertiary silanes) is greater than the sum of those in Tables 3 and 4 (primary silanes and secondary silanes, respectively). This may be due, in part, to the greater commercial availability of tertiary silane precursors. However, it is the reactions of R2SiH2 and RSiH3 that provide what may be considered the more interesting products that have nonclassical interactions (multicentered bonding) with novel bonding patterns. To realize the impact of the use of hydrosilanes in forming complexes with Si−TM bonds, other synthetic methods are briefly surveyed in Section 2. Since the previous review,2 the exploration of silylene, silicon(II) chemistry has expanded and is described in Section 2 well as a few salt metathesis reactions, and their limitations. Also described are a few complexes formed by transition metals with silenes (>SiSi

Reactions of Hydrosilanes with Transition Metal ... - ACS Publications

Recommend Documents