Fluorine in Cyclometalated Platinum Compounds - American

Review pubs.acs.org/Organometallics

Fluorine in Cyclometalated Platinum Compounds Margarita Crespo* Departament de Quimica Inorgànica, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain ABSTRACT: This article is focused on the chemistry of cyclometalated platinum compounds containing fluorine. Several types of platinum(II) or platinum(IV) compounds with [C,N], [C,N,N′], [N,C,N], [C,N,N,C], [C,P], or [P,C,P] cyclometalating ligands and fluorine as fluoro ligands or as fluoro substituents in either the cyclometalated or the ancillary ligands are included. The effects of the fluorine substituents upon the reactivity and properties of the cyclometalated platinum compounds are also presented.

nucleus is most useful in the NMR characterization of these compounds.8 On the other hand, the unique properties of fluorine, such as high electronegativity and relatively small size, render the presence of fluoro substituents an excellent choice to modify the electronic properties of a given compound. Because of the great strength and high stability of the C−F bond, fluorinated groups have been often used to stabilize organometallic complexes.9−11 For instance, metal−pentafluorophenyl complexes are more stable than their metal−phenyl counterparts, and an extensive chemistry based on M−C6F5 complexes has been developed.12 A study based on DFT calculations has demonstrated the very large increase in the M−C bond strength on ortho-fluorine substitution, whereas substitution at the meta and para positions has a minor influence.13 Moreover, fluorine can be used as a reporter atom since 19F NMR spectroscopy offers a number of advantages as compared to 1H NMR spectroscopy.14 In addition, the negative inductive effect (σ I = 0.51) and the positive mesomeric effect (σ R = −0.34) of fluorine are influential factors not only on the reactivity but also on intra- and intermolecular interactions.15 This article deals with cycloplatinated compounds containing fluorine substituents and the effect of these upon the reactivity and properties of the cyclometalated platinum compounds. As summarized in Figure 1, cyclometalated platinum(II) compounds containing fluoro or fluorinated ligands in the coordination sphere of platinum are considered, as well as platinum(IV) analogues, whereas ionic species in which the fluorine atom is exclusively present in a fluorinated anion, such as BF4− or PF6−, are not considered. It should be mentioned

1. INTRODUCTION Long-standing interest in cyclometalated compounds, in which a chelate ring contains a metal−carbon σ bond, derives from

Figure 1. Fluorine in cyclometalated platinum compounds. Color code for all the figures: Pt in blue and F or fluorinated groups in red for cycloplatinated compounds containing fluorine.

the fact that their properties can be easily tuned by modification of either the cyclometalated or the ancillary ligands. In particular, palladium and platinum cyclometalated compounds have been thoroughly investigated due to the wide range of their potential applications in many areas, such as organic and organometallic synthesis and the design of advanced materials and biologically active agents.1−7 An interesting feature of platinum derivatives is that, in addition to square-planar platinum(II) compounds, octahedral platinum(IV) compounds are readily accessible. Moreover, the magnetically active 195Pt © 2011 American Chemical Society

Special Issue: Fluorine in Organometallic Chemistry Received: September 6, 2011 Published: October 20, 2011 1216

dx.doi.org/10.1021/om200835g | Organometallics 2012, 31, 1216−1234

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Figure 2. Intramolecular C−F bond activation leading to [C,N,N′] cyclometalated platinum(IV) compounds.

that the synthesis of cycloplatinated compounds with fluoro substituents often involves intramolecular activation of C−H or C−F bonds. C−H bond activation at metal centers has been extensively studied,16−21 and analogous processes for the stronger C−F bond have attracted a great deal of interest over the past years.22−26 Studies aimed at comparing C−F and C−H bond activation have also been reported.27,28

2. SYNTHESES OF CYCLOPLATINATED COMPOUNDS CONTAINING FLUORINE 2.1. Cycloplatinated Compounds with Fluoro Ligands. Because the use of transition-metal complexes allows the cleavage of the robust C−F bonds under mild conditions, the presence of fluoro ligands in cycloplatinated compounds is often related to intramolecular C−F bond activation of fluorinated ligands at platinum. In these reactions, the driving force is formation of strong metal−fluorine and metal−aryl bonds. It has been reported that the presence of a strong metal−fluorine bond does not preclude the use of organometallic fluorides in catalysis.29 In addition, these compounds could function as molecular receptors for biologically relevant molecules due to the hydrogen-bonding ability of the fluoro ligand.25 The first chelate-assisted intramolecular activation of an aromatic C−F bond has been reported by Richmond and coworkers using [W(CO) 3 (PrCN) 3 ] as a substrate and adequately designed ligands containing a pentafluorophenyl group.30 The chelating nature and restricted conformation of the ligand, as well as the presence of nitrogen donor atoms, seem to be crucial to promote C−F bond activation. An

Figure 3. Intramolecular C−F bond activation leading to [C,N] cyclometalated platinum(IV) compounds.

analogous strategy has been used by Anderson and Puddephatt for the reaction of [Pt2Me4(μ-SMe2)2] with imine 2a (Figure 2).31 The reaction proceeds via substitution of the labile SMe2 ligands for the bidentate [N,N′] imine, followed by intramolecular oxidative addition of the C−F bond at the electronrich metal center. The obtained [C,N,N′] cyclometalated platinum(IV) compound 2b reacts with acetone solvent by cis addition of a C−H bond across the imine group, yielding 2c. According to the crystal structure determination, the resulting compound forms dimers associated by hydrogen bonding between the N−H and the Pt−F groups of molecules related by centers of symmetry. Further work carried out at Puddephatt’s 1217


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Figure 4. Intramolecular C−F versus C−X (X = Br, Cl, H) bond activation.

Figure 5. Intramolecular C−H versus C−F bond activation.

ortho-fluorine atoms, RCHNCH2CH2NMe2 (R = 2,4,6C6H2F3; 2,3,6-C6H2F3), shown in Figure 2, produce [C,N,N′] saturated cyclometalated platinum(IV) compounds via C−F bond activation and addition of acetone, as previously reported for the pentafluoro system, whereas analogous imines with aryl rings containing only one ortho-fluorine (R = 2,3,4- and 2,4,5C6F3H2) produce C−H bond activation exclusively.34 Intramolecular C−F bond oxidative addition also takes place for analogous ligands containing a single nitrogen donor, and two isomers of the corresponding cyclometalated [C,N] platinum(IV) compound are formed (Figure 3).32 Furthermore, activation of C−F bonds takes place in the presence of weaker C−X bonds (X = H, Cl, Br) for appropriately designed ligands (Figure 4).35,36 In this case, the selective C−F bond activation is related to the presence of a pentafluorophenyl group in the ligand and to the formation of a more stable endocycle (containing the CN group). However, the only cyclometalated platinum derivative that could be isolated from the reaction of cis-[PtCl2(dmso)2] with imine C6F5CH NCH2(4-ClC6H4) is an exo metallacycle arising from C−H bond activation (Figure 5),37 and this result indicates that a

Figure 6. Intramolecular C−F bond activation at platinum(0).

group, indicates that, when analogous imines with aryl rings containing only one ortho-fluorine, such as 2-C6H4F, 2,3C6H3F2, and 2,5-C6H3F2, are tested, activation of an ortho C− H bond always occurs in preference to C−F bond activation.32,33 The presence of only ortho-fluorine substituents is not a sufficient condition for C−F bond activation since no reaction is observed for the corresponding imine with the 2,6C6H3F2 aryl group.32 However, trifluorinated ligands with two

Figure 7. Synthesis of [C,P] cycloplatinated compounds with fluoro ligands. 1218


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Figure 8. Formation of [C,N,N′] cyclometalated platinum(II) and platinum(IV) compounds with fluorinated ligands.

Figure 9. Formation of [C,N] cyclometalated platinum(II) compounds with fluorinated ligands.

nucleophilic platinum substrate, such as [Pt2Me4(μ-SMe2)2], is required for C−F bond activation. The results obtained using [Pt2Me4(μ-SMe2)2] as a platinum substrate indicate that the reactions are favored for the more fluorinated systems and that the presence of a fluorine substituent in the position ortho to the C−F bond to be activated is decisive in the selectivity of the process. Kinetics studies carried out for intramolecular activation of C−X (X = H, F, Cl, Br) bonds of ligands ArCHNCH2Ph at platinum(II) confirm the existence of a common concerted mechanism,

with a highly ordered three-centered C−Pt−X transition state. Less negative activation volumes observed for C−F bond activation are taken as an indication of an earlier transition state for the oxidative addition of this bond.38 The formation of platinum(IV) derivatives arising from intramolecular C−F bond activation can be easily confirmed using NMR techniques. In particular, the disappearance of a fluorine atom in the aryl group and the presence of a fluoro ligand bound to platinum at higher field are easily observed in the 19F NMR spectra.32,34 In addition, the large range of 1219


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Figure 12. Cycloplatination at N-heterocyclic carbene complexes.

tone) has been reported for imines RCHNCH2CH2NMe2 (R = C6F5; 2,4,6- and 2,3,6-C6H2F3; 2,6-C6H3F2) containing two fluorine atoms in the ortho positions of the aryl ring.40 As previously observed for the reaction of 2,3,6-C6H2F3CH NCH2CH2NMe2 with [Pt2Me4(μ-SMe2)2],34 of the two nonequivalent ortho C−F bonds, only that having a fluorine atom in the adjacent position is selectively activated. However, in contrast to previous results for intramolecular processes leading to Pt(IV)−F bonds, no platinum(II) compounds with a fluoro ligand are isolated and the final products are obtained as bromo or chloro derivatives after reaction with the correspond-

Figure 10. Cycloplatination of 4-fluorophenylpyridine.

chemical shifts allows the use of 19F NMR spectroscopy as a sensitive technique for detecting diastereomers. For instance, a diastereomeric peak separation of ca. 4 ppm is observed for the F−Pt signals of the (C,S) and (A,S) isomers of platinum(IV) compound [PtMe2F(C6F4CHN-(S)-CHMePh)PPh3] arising from C−F bond activation at a fluorinated imine derived from (S)-α-methylbenzylamine.39 Intramolecular oxidative addition of C−F bonds at a reactive platinum(0) substrate [Pt(dba)2] (dba = dibenzylideneace-

Figure 11. Cyclometalated platinum(II) and platinum(IV) derivatives of 4-fluorophenylpyridine showing agostic interactions. 1220


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indicate a high barrier for C−F bond activation at platinum(0). Both this fact and the long Pt(II)−F bonds have been related to repulsion between the fluorine π orbitals and the occupied 5d orbitals of platinum.28 However, the intermolecular oxidative addition reaction of hexafluorobenzene has been reported by Hofmann using [Pt(dtbpm)] (dtbpm = bis(di-tertbutylphosphanyl)methane), a highly reactive substrate with a small bite angle chelating phosphine, and the reaction produces [Pt(C6F5)F(dtbpm)].46 Recent studies indicate distinct pathways for C−F bond activation of fluoropyridines at [Pt(PR3)2], either the now well-established C−F oxidative addition reaction or “phosphine-assisted” processes; the latter may produce Ptfluoride or Pt(alkyl)(fluorophosphine) products.28,47−49 Despite the plausible formation of platinum(II) compounds containing a fluoro ligand from intermolecular C−F bond activation, this type of compound could not be obtained from the intramolecular process at [Pt(dba)2], as stated above. Conversely, cyclometalated platinum compounds containing a Pt−F bond can be obtained through synthetic methods other than C−F bond activation. For instance, the fluoro complex cis[PtF(κ 2-C6H3-5-Me-2-PPh2)(PPh2-4-tol)] (7a in Figure 7) is obtained in good yield by treatment of a chloro precursor with AgF in the absence of light over a period of 3 days.50 The reaction of a cyclometalated platinum(II) compound with the electrophilic fluorination reagent XeF 2 gives a difluorocyclometalated platinum(IV) compound (7b in Figure 7).51 2.2. Cycloplatinated Compounds with a Fluorinated Cyclometalated Ligand. This section deals with platinum compounds containing a fluorinated cyclometalated ligand, but no fluorine atom directly bound to platinum. As indicated in

Figure 13. Cycloplatinated compounds with fluorinated oximes or ketimines.

ing lithium salts (Figure 6).40 In fact, fluoro complexes of palladium(II) and platinum(II) have been reported to be extremely labile, in particular in the absence of π-acceptor ligands trans to the fluoride. Some trans-[PtFAr(PR3)2] complexes have been described29,41−43 as well as platinum(II) and platinum(IV) difluorocomplexes, and an exceptional stability has been observed for the platinum(IV) compound.44,45 In contrast to nickel and palladium, examples of C−F bond intermolecular addition at platinum are scarce and DFT studies

Figure 14. Cycloplatinated compounds derived from fluorinated phenylpyridines (1). 1221


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Figure 15. Cycloplatinated compounds derived from fluorinated phenylpyridines (2).

the previous section, the reaction of platinum substrates with fluorinated imine ligands may produce cyclometalated compounds containing a Pt−F bond via intramolecular C−F bond activation. Nevertheless, the lability of this bond may prevent isolation of this type of compound, as in the reactions shown in Figure 6.40 In other cases, the platinum substrate is not well suited for oxidative addition and C−H bond activation takes place instead, as shown in Figure 5 for the reaction of ligand C6F5CHNCH2(4-ClC6H4) with cis-[PtCl2(dmso)2].37 Even when the nucleophilic compound [Pt2Me4(μ-SMe2)2] is used, competition between ortho C−F and ortho C−H bond activation favors the latter due to its relatively lower bond energy. In these reactions, activation of a C−H bond is followed by reductive elimination of methane, leading to platinum(II) cyclometalated compounds. Therefore, fluorinated ligands with either two31−34 or one nitrogen33,36,39,52−54 donor atom in which only one fluorine substituent is present at

the ortho positions lead to formation of [C,N,N′] or [C,N] cyclometalated platinum(II) compounds with a fluorinated cyclometalated ligand, as summarized in Figures 8 and 9. In these reactions, a high degree of chemo- and regioselectivity is observed and the presence of fluorine substituents in the ligands increases the reactivity of C−H bonds mainly via inductive effects. As an example of selectivity in these systems, we may note that, for ligand 2-C6H4FCHNCH2(2-ClC6H4), which could produce four distinct reactions, either C−F or C− H bond activation leading to endo-cycles, or C−Cl or C−H bond activation to give exo-metallacycles, the only observed process is C−H bond activation to produce an endo-cycle. On the other hand, for imine 3-C6H4FCHNCH2(2-ClC6H4), only the C−H bond having a fluorine atom in an adjacent position is exclusively activated. In contrast, for imine 3C6H4(CF3)CHNCH2C6H5, containing a bulkier CF3 substituent, activation of the C−H bond occurs selectively at the 1222


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Figure 16. Fluorinated [N,C,N], [C,N,N′], and [C,N,N,C] cycloplatinated compounds.

less-hindered of the two ortho positions of the aryl ring. Furthermore, as shown in Figure 8, the corresponding cyclometalated platinum(IV) compounds containing fluorinated ligands could be easily obtained by oxidative addition of methyl iodide to the platinum(II) compound (8a),34 or by an intramolecular oxidative addition of a C−Cl bond (8b).33 In related reactions, it was also observed that oxidative addition of methyl iodide is not inhibited by the presence of bulky, electronegative CF3 substituents.53 When imines derived from (S)-α-methylbenzylamine are used, the oxidative addition of methyl iodide gives a mixture of two diastereomers (C,S) and (A,S) of the octahedral platinum(IV) compounds, as described in the previous section for an analogous intramolecular process.39 Cyclometalated platinum compounds containing substituted 2-phenylpyridines have been prepared using K2PtCl4 as a

metalating agent. Although changes of the substituents have little effect on the electronic properties of the platinum center, the use of 4-fluorophenylpyridine revealed that both the 19F chemical shift and the coupling of this nucleus to 195Pt are very useful spectroscopic handles. As shown in Figure 10, the reaction of K2PtCl4 with 4-fluorophenylpyridine produces cyclometalated platinum(II) compound 10a containing a pendant phenylpyridine, from which derivatives 10b are obtained. In a novel process, the oxidation reaction of 10a results in formation of 10c, a doubly cyclometalated platinum(IV) compound via metalation at the pendant phenylpyridine group promoted by the more electrophilic platinum(IV) center.55,56 Further work carried out at Rourke’s group reveals that the reaction of K2PtCl4 with 2-tert -butyl-6-(4-fluorophenyl)pyridine gives a 14 e− species (11a in Figure 11) in which an 1223


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Figure 17. Cycloplatination of P-donor ligands with fluoro substituents.

well as for the loss of dimethylsulfoxide observed for compounds with R = Et or Pr.59 It is concluded that coordinatively unsaturated species can be stabilized by agostic interactions when a pendant alkyl chain is present. Recent studies on the chemistry of the agostic compound 11a disclose a unique reversible roll-over reaction leading to a [C,C]chelated ligand in polar solvents.60 As shown in Figure 12, cyclometalation at the 4-fluorophenyl group of N-heterocyclic carbene complexes leading to fivemembered rings has been achieved via silver complexes and using the electron-rich substrate [PtMe 2(dmso)2] as the starting material.61 On the other hand, orthometalated aryl oxime platinum(II) complexes, prepared from cis-[PtCl2(dmso)2], undergo deoxygenation of dimethylsulfoxide in methanol in the presence of HCl to afford the platinum(IV) dimethylsulfide complexes. In this process, compounds with electron-withdrawing substituents, such as fluorinated derivative 13a in Figure 13, react faster.62 Related compounds derived from fluorinated benzo-

agostic interaction from two of the hydrogen atoms on one of the methyl groups is present. In addition to the detection of agostic interactions, which are relevant in the activation of C− H bonds, this system is well suited to analyze the subtle balance between sp2 and sp3 C−H bond activation, which depends on the availability of additional ligating species. Thus, the reaction of 11a with dimethylsulfoxide gives a different cyclometalated product via C−H activation on the tBu group (11b), and upon addition of water, a doubly cyclometalated compound (11c) is formed, a result related to the fact that hydration and ionization of the released HCl acts as the driving force of the reaction. 57 Further work indicated that the agostic interaction involving an sp3 C−H bond detected for 11a is retained upon oxidation to the platinum(IV) species 11d. An agostic interaction from an sp2 C−H bond was also detected for platinum(IV) compound 11f arising from oxidation of 11b, followed by displacement of the dimethylsulfoxide ligand.58 Moreover, agostic interactions, suggested from NMR experiments and supported by DFT calculations, may account for the relatively fast isomerization of compounds 11h (S-bound dmso) to 11i (O-bound dmso) as 1224


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substituents on the supporting phosphine ligand have been used. Although this strategy failed, this study allows formation of oxo- and amido-platinacycles 17b and 17c, respectively. Although these compounds are not strictly cyclometalated platinum compounds, their formation, which involves C−F bond cleavage, is relevant in the context of this article. 78 The reaction of [PtCl2(NCPh)2] with a bisphosphonium salt containing fluoro substituents yields a platinum(IV) complex, [PtCl2(4-C6H4F){C6H3-5-F-2-P(p-C6H4F)2-CC(Me)CH2P(p-C6H4F)2}] (17d), in a process involving orthometalation at the phosphine unit, metalation at the olefinic fragment, and a P−C bond activation process.79 Isolation of this compound is related to the strong electron-withdrawing nature of the fluoro substituents, which inhibit dissociation of a ligand, which, in turn, prevents a C−C coupling from platinum(IV), as observed for unsubstituted analogues.79 A transmetalation reaction from 2-LiC6F4PPh2 allows access to the first series of homoleptic bis(orthometalated) complexes of the d8 elements, [M(2-C6F4PPh2)2] (M = Ni, Pd, Pt (17e)), and in this case, the fluorine substituents have a stabilizing effect on the four-membered rings.80 Cyclometalated platinum(II) compounds containing a perfluoroalkyl [P,C,P] ligand, such as 17f or 17g, have been prepared.81 Despite the poorly donating nature of these ligands, metalation proceeds under conditions similar to those reported for donor phosphine analogues. These [P,C,P] ligands impose a rigid mutually trans π-acceptor geometry, and several derivatives, some of them serving as catalysts for hydrogenation or hydrosylation processes, have also been reported.82 2.3. Cycloplatinated Compounds with a Fluorinated Ancillary Ligand. This section deals with cyclometalated platinum compounds with fluoro substituents in the anionic or neutral auxiliary ligands. In some cases, these compounds are closely related to those described in the previous sections. In a study involving the synthesis of monoaryl platinum complexes with a chelating (2-pyridyl)aryl ligand, compounds containing a fluorinated aryl group have been reported (18a and 18b in Figure 18). The mechanism of the process involves coordination of the (2-pyridyl)aryl ligand to a diarylplatinum(II), followed by intramolecular activation of a C−H bond, and arene reductive elimination. Along this work, in order to achieve isolation and X-ray characterization of the proposed coordination intermediate, the reaction with 2-phenyl-5(trifluoromethyl)pyridine has also been tested and gives, in addition to the proposed intermediate, a novel compound with a fluorinated cyclometalated ligand (18c).83 A number of cycloplatinated compounds with fluorinated ligands have been prepared with the aim of tuning their emission properties by introducing substituents with different steric and electronic properties on either the cyclometalating (see previous section) or the ancillary ligands. Some examples involving fluorinated σ-alkynyl (19a and 19b),75 or βdiketonates (19c),84,85 platinum(II) compounds are shown in Figure 19. Oxidative addition of I2 to the phenylpyridine and Nile Red cyclometalated platinum(II) compounds 19c produces the corresponding platinum(IV) derivatives 19d.86 In addition, heteroleptic bis-cyclometalated platinum(IV) compounds, such as 19e (analogous to those depicted in 15a), in which the fluorine substituents are present in the bipyridine ligand, should also be considered in this section.67 As shown in Figure 20, in an analogous process to that depicted in Figure 7, treatment of the bis(chelate) complex cis[Pt(κ 2-C6H3-5-Me-2-PPh2)2] with CF3CO2H produces com-

Figure 18. Aryl platinum complexes with cyclometalated (2-pyridyl)aryl ligands.

phenone oxime (13c) or diarylketimine (13d) have also been synthesized and their photophysical properties studied.63 Cycloplatinated complexes based on 2-phenylpyridines have been thoroughly studied due to their easy syntheses, high stabilities, and emission properties. Modification of the cyclometalating ligand allows for tuning of the emission color, and for this reason, several compounds with fluoro substituents have been described. As shown in Figure 14, these compounds can be prepared from the reaction of K2PtCl4 with the corresponding ligand64 and using ancillary ligands, such as β-diketonates.65,66 In the search of materials for luminescencebased applications, several types of related complexes, including heteroleptic bis-cyclometalated platinum(IV) (15a),67 ionic compounds with bulky counterions that suppress intermolecular interactions (15b),68 and complexes functionalized with triarylboron (15c),69 have been reported, and their syntheses are outlined in Figure 15. On the other hand, introduction of a cathecol ligand leads to a paramagnetic complex (15d) with the unpaired electron residing mainly on the cathecol ligand.70 In addition to bidentate [C,N] coordination, terdentate [N,C,N] or [C,N,N′] platinum compounds have also been investigated in relation to their photophysical properties, and several fluorinated derivatives, as those shown in Figure 16, have been prepared. The most general synthetic method consists of the reaction of K2PtCl4 with the corresponding ligand in refluxing acetic acid, followed by replacement or abstraction of the chloro ligand.71−75 In an attempt to improve the emission efficiency, more rigid tetradentate bis-cyclometalated platinum complexes, such as 16i, have been prepared, and in those systems, the electron-withdrawing effect of the fluorine substituents has also been studied.76 Compounds containing a fluorinated [C,P] ligand, such as 17a in Figure 17, have been obtained through platination of meta-fluorobenzylphosphines under drastic conditions.77 In an attempt to stabilize the amide bond to platinum(II), fluoro 1225


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Figure 19. Cyclometalated platinum compounds with fluorinated ancillary ligands.

pound 20a containing a fluorinated anionic ligand. Furthermore, the reaction of this compound with Li(C6F5) in diethylether produces cis-[Pt(C6F5)(κ 2-C6H3-5-Me-2-PPh2)(PPh2-4-C6H4Me)] (20b), a reaction that fails when the chloro derivative was used instead of the trifluoroacetate. A series of neutral and anionic derivatives have been prepared in these studies, and a trans-influence series of anionic and neutral ligands based on the J(P−Pt) values for the phosphorus atom of the four-membered ring has been established. It was confirmed that the trans influence of C6F5 is higher than that of Cl and smaller than that of C6H5.50 The first known [P,C,P] pincer complex of a group 10 metal with a fluoroalkyl ligand (20c) is also shown in Figure 20.87 Moreover, the first stable monometallic anionic [P,C,P] platinum(0) complex that is capable of intermolecular C−F

bond activation of hexafluorobenzene has been reported by Milstein’s group.88 In this process, a [P,C,P] platinum(II) compound with an ancillary C6F5 group is produced (20d). On the other hand, a phosphine containing perfluoroalkyl groups has been used as an ancillary ligand in order to increase the solubility of [C,N] cyclometalated platinum compounds derived from fullerenes (see 21a in Figure 21) and thus to facilitate the formation of macromolecular structures.89

3. REACTIVITY OF CYCLOPLATINATED COMPOUNDS CONTAINING FLUORINE 3.1. Oxidative Addition. The oxidative addition reaction is a fundamental process in transition-metal chemistry, and in particular, oxidative addition to platinum(II) compounds gives easy access to the corresponding platinum(IV) compounds. 1226


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Figure 20. [C,P] and [P,C,P] cyclometalated compounds with a fluorinated aryl or alkyl ligand.

Figure 22. Substitution reactions and metallacycle cleavage in cycloplatinated compounds containing fluorine.

metalated platinum(II)51 also produces the corresponding platinum(IV) derivatives (see Figures 7, 11, and 19). 3.2. Substitution Reactions. Substitution reactions of both anionic (see Figures 6,40 7,50 and 1465) or neutral (see Figure 10)56 ligands can be readily achieved at the platinum center and allow the synthesis of a large variety of compounds. It has been shown that the presence of a fluorine atom in the position adjacent to the Pt−Caryl bond is decisive in the reactivity of [C,N,N′] and [C,N] platinacycles with monodentate phosphines. As shown in Figure 22, dissociation of the dimethylamino group from platinum takes place readily upon reaction of the compounds [PtMe{Me2NCH2CH2NCHRf}] containing a fluorinated [C,N,N′] ligand with PPh3. However, the stronger Pt−Nimine bond is cleaved only if there is a fluorine atom (F5) adjacent to the Pt−Caryl bond.34 An analogous result

Figure 21. Cycloplatinated compounds containing perfluoroalkyl groups.

Some of these reactions are discussed in the previous section. For instance, oxidative addition of methyl iodide to the cyclometalated [C,N,N′] or [C,N] platinum(II) compounds containing fluorinated ligands produces the corresponding cyclometalated platinum(IV) compounds (Figure 8).34 Nitrogen donor ligands impart high nucleophilicity to the metal center, increasing its reactivity and allowing the process even in the presence of electron-withdrawing fluoro substituents and bulky groups, such as CF3.39,53 Oxidative addition of Cl2 (using PhICl2)58,59 or I286 to [C,N] cyclometalated platinum compounds or fluorination (using XeF2) of a [C,P] cyclo1227


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A distinct behavior in front of the diphosphine 1,2bis(diphenylphosphino)ethane (dppe) was observed for platinum(II) and platinum(IV) platinacycles (Figure 23). Cleavage of the metallacycle is produced for the platinum(II) compound, but the platinum(IV) analogue gives a dinuclear compound in which the diphosphine bridges two platinum(IV) moieties with the imine acting as a [C,N] bidentate ligand.54 The mechanisms of substitution reactions on cyclometalated octahedral platinum(IV) complexes have been studied for compounds of the general formula [PtMe 2 X(RCH NCH2Ph)L] in which X is F, Cl, or Br; R is a metalated aryl group; and L is the leaving ligand. In most cases, the reaction mechanism is dissociatively activated, including the formation of a pentacoordinated intermediate. Nevertheless, the presence of electron-withdrawing fluorine atoms as a fluoro ligand or as substituents in the aryl group allows the detection of an associatively activated path for relatively small entering and leaving ligands, as confirmed by ΔV‡ values.91,92 3.3. C−C and C−F Bond Formation. Although both biaryl linkages and fluorine substituents are entities frequently incorporated in biologically active pharmaceuticals, the crosscoupling of aryl fluorides has received less attention than that of other aryl halides, which can be associated with the challenge of activating the strong C−F bond. A strategy based on the reaction of cis-[Pt(C6F5)2(SEt2)2] with aryl imines produces C−C coupling between the pentafluorophenyl ligand and the

Figure 23. Substitution reactions at platinum(II) and platinum(IV) cyclometalated compounds containing fluorine.

has been obtained for ligands RfCHNCH2Ph52,54 and can be related to the unfavorable steric repulsion between the methyl group and F5 in chelated [C,N] compounds, which is relieved by cleavage of the metallacycles. This illustrates the fact that, although fluorine often replaces hydrogen in organic and organometallic compounds, the size and steric influences of the two atoms are quite different.90

Figure 24. Platinum-mediated C−C coupling in cyclometalated compounds with fluorinated ligands. 1228


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Figure 25. C−C and C−F bond formation at [C,P] cyclometalated platinum compounds.

derivative 25b in a process that is the first reported fluorination of a C−H bond assisted by a platinum complex.51 3.4. Catalytic Processes. Because of the presence of fluorine in bioactive molecules and pharmaceuticals,95 the development of routes for the generation of fluoro-organics by the transition-metal-mediated conversion of an aromatic C−F bond into a C−C bond is a research area of great interest. It is now clear that, although C−F bonds are quite strong, transition metals can be used to activate these bonds, and the feasibility of group 10-catalyzed cross-coupling of polyfluoroarenes has been established using nickel and palladium catalysts.96 Recently, Love and co-workers have reported the first example of a platinum-catalyzed cross-coupling of aryl fluorides, 97 thus showing that even considerably inert platinum compounds are able to activate and functionalize a strong bond, such as C− F. The ability of [Pt2Me4(μ-SMe2)2] to promote C−F bond activation of arylimines, as shown in Figures 2−4, is used to design a catalytic process involving conversion of a C sp2−F bond into a Csp2−Csp3 bond (see Figure 26). The process takes place in excellent yield and regioselectivity for several fluoro aryl imines. In addition, selectivity for ortho C−F bond activation is observed even in the presence of weaker C−Br bonds, in good agreement with the reported results for stoichiometric processes.35,36

aryl group of the incoming imine. Kinetico-mechanistic studies were carried out monitoring the process using UV−visible spectroscopy. In addition, 19F NMR spectroscopy proved to be an excellent handle for the establishment of the reaction sequence. The mechanism (see Figure 24) consists of intramolecular C−Br oxidative addition (step a), followed by Caryl−Caryl reductive elimination (step b), and intramolecular C−H activation of the phenyl ring with elimination of C6HF5 (step c). The final product 24a is a five-membered cyclometalated platinum(II) compound with an “exo Caryl−Caryl” bond.93 In a related process involving formation of a biaryl linkage, the fluorine atoms block the cyclometalation at the benzylidene ring and drive the reaction toward formation of an exoplatinacycle 24b.94 A related Caryl−Caryl bond formation, shown in Figure 25, was observed after removal of a fluoro ligand (using trimethylsilyl triflate) from the coordination sphere of platinum in a [C,P] cyclometalated platinum(IV) compound. The Caryl− Caryl coupling was followed by cyclometalation of the aromatic ring with elimination of HF, and the final product 25a was isolated as a PPh3 derivative. Interestingly, when an analogous reaction is tested using a mesityl group, a completely different product is formed. Quantitative fluorination of a C−H bond of the mesityl group was achieved, leading to fluorinated 1229


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Figure 26. Platinum-catalyzed cross-coupling of polyfluoroarylimines.

The proposed mechanism is shown in Figure 26.98 The presumed active species A is formed upon coordination of the imine ligand to platinum and dissociation of SMe2. Intramolecular C−F bond activation produces B (or an isomer), and transmetalation of the Pt−F species with dimethylzinc produces intermediate C (or an isomer). Reductive elimination and coordination of another equivalent of the imine gives the Csp2−Csp3 coupling product and regenerates A. The active catalyst [PtMe2(imine)] can be accessed from more user-friendly precatalysts, such as [PtCl2(SMe2)2]99 or [PtCl2(dmso)2].100 The greater lability of dimethylsulfoxide facilitates activation of the precatalyst. Along this work, a preference for Csp2−F versus Csp3−F bond activation was also found for imines containing both F and CF3 substituents at the ortho positions.99 A related work shows that, in addition to catalyzing the methylation of fluorinated aryl imines, compound [Pt2Me4(μSMe2)2] is able to catalyze Csp2−F to Csp2−O conversion in a process leading to fluorinated arylmethyl ethers. 101 As stated above, several cyclometalated platinum(II) compounds containing a perfluoroalkyl [P,C,P] ligand, such as 17f or 17g, have been tested as catalysts in several processes.82 On the other hand, fluorous palladacycles related to effective catalysts for Heck and Suzuki reactions102 have been prepared with the aim of facilitating catalyst recovery using a fluorous biphase system.103 Related [N,C,N]104 or [P,C,P]105 platinum compounds with a perfluoroalkyl group have also been prepared as model compounds (21b and 21c in Figure 21).

4. PROPERTIES AND APPLICATIONS OF CYCLOPLATINATED COMPOUNDS CONTAINING FLUORINE Although this article is mainly focused on the chemistry of cycloplatinated compounds containing fluorine, in this section, some interesting features of these compounds that are relevant to their applications, such as luminescence and mesogen properties, are presented. It is also interesting to point out that fluorine substituents can enhance the binding efficacy and selectivity in pharmaceuticals while increasing the resistance to metabolic degradation.15,95,106 On this basis, the incorporation of fluorine substituents on cycloplatinated compounds, some of which have shown high cytotoxicities,107−111 could also lead to interesting results in the field of bio-organometallic chemistry. 4.1. Luminescence. In the last several years, significant research effort has focused on the photophysical properties of luminescent square-planar platinum complexes. The aim of this research is to provide an understanding of the factors that govern the luminescence efficiencies of platinum(II) complexes as well as to apply these compounds in organic light-emitting diodes (OLEDs). Strong spin−orbit coupling of the platinum atom [ξ = 4481 cm−1] allows for the formally forbidden mixing of the singlet and triplet excited states and promotes emission from triplet states. To promote emission in solution at ambient temperature, it is necessary to ensure that the lowest-lying excited state is not a deactivating metal-centered state and that there is a large energy gap between the emissive and the metal-centered states.112 Many cyclometalated platinum complexes have proved to be luminescent in solution at ambient temperature because cyclometalating [C,N] ligands increase the energies of metal-centered excited states compared to analogous [N,N] ligands. In addition, rigidity generally favors luminescence over 1230


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Figure 27. Some further examples of compounds designed for their luminescence or mesogen properties.

nonradiative decay pathways, and, therefore, tridentate [C,N,N′] or [N,C,N] ligands may offer an advantage over bidentate [C,N] ligands. The ancillary ligands are also crucial to induce room temperature emission. For instance, ligands such as cyanide or acetylide push up metal-centered states and ligands such as acetylacetonate (bidentate and monoanionic) allow formation of neutral complexes that are better suited for OLED applications than ionic counterparts. From the combination of cyclometalating with strong-field ancillary ligands (see examples in Figures 14−16), high-efficiency luminescence can be anticipated. In addition, the emitting properties can be tuned according to the effect of electrondonating and electron-withdrawing substituents on the energies of the frontier orbitals. The emission properties have been studied for several cycloplatinated compounds with either fluorinated cyclometalating64−76 or fluorinated ancillary ligands,84−86 and some examples are shown in Figures 13−16 and 19,

respectively. A study based on [N,C,N] cyclometalated platinum complexes with different substituents, including fluorine (16a−16d in Figure 16), shows that most of these compounds are highly emissive and stable to sublimation, making them suitable as phosphorescent emitters for OLEDs.73,74 Complexes exhibiting an efficient blue emission are rare compared to those displaying green or red emission. A systematic study carried out for cycloplatinated complexes based on 2-phenylpyridines (shown in Figure 14) suggests that both platinum and ligand orbitals contribute to the HOMO level while the LUMO is largely localized on the cyclometalated ligand. For these compounds, electron-withdrawing F or CF3 substituents in the phenyl ring induce a blue shift in the emission due to stabilization of the HOMO, while donor substituents, such as NMe2 in the pyridyl, raise the LUMO and also induce a blue shift. The effects in the two rings are qualitatively additive, allowing quite substantial shifts toward the blue.65 The results for difluorosubstitution indicate that the 1231


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transition energy is sensitive to the substitution position on the phenyl ring as for related tris-cyclometalated iridium complexes. In a recent study carried out for [N,C,N] cyclometalated platinum compounds, a fluoro substituent para or meta to the platinum atom produces, respectively, a red or a blue shift of the emission, and this contrasting behavior is assigned to the mesomeric electron-donating effect operating for a fluoro substituent in a para position, which counterbalances the inductive effect.113 In another study, a blue shift of the luminescence of bis-cyclometalated platinum(IV) complexes is assigned to the larger ligand field splitting of platinum(IV).67 On the other hand, the square-planar geometry of platinum(II) compounds facilitates interactions of adjacent monomers via Pt···Pt or π−π stacking, and solid-state samples often show photophysical properties that are different from those of dilute fluid solutions. To reduce self-quenching at elevated concentrations, some times accompanied by formation of excimers,114 bulky substituents have been used to increase intermolecular distances as for 15b.68 Another approach shown for 27b and 27c (Figure 27) is to coordinate chelating phosphino-alcohols that inhibit stacking interactions due to their puckered nature in contrast to the planar 27a.115 A series of substituted pyrazolatebridged cyclometalated platinum(II) compounds (27d) have been designed in order to control the degree of Pt···Pt interaction and, thus, the photophysical properties.116 Furthermore, the propensity to form aggregates or excimers at elevated concentrations might provide a simple strategy for obtaining near-IR-emitting devices since excimers emit at longer wavelength than the monomers. In addition, a single dopant emitting simultaneously from monomer and excimer states could lead to efficient production of white light, as shown for dopant molecules based on a series of platinum(II)-[2-(4,6difluorophenyl)pyridinato]-β-diketonates, such as 27e.117,118 Formation of supramolecular nanosheets displaying nearinfrared phosphorescence and light-modulated conductivity 71 has been achieved from compounds, such as 16g (shown in Figure 16), through intermolecular Pt···Pt and C−H···π(C C) interactions in an orthogonal configuration. Another study reports on cyclometalated complexes with a [N,C,N] ligand in either a η 3-tridentate (27f) or a η 2-bidentate (27g) coordination mode aimed at developing a solid-state luminescent sensor for acidic vapors.119 On the other hand, cyclometalated platinum(II) compounds with a BMes 2 substituent at different positions of the phenylpyridine ligand have been designed as phosphorescent F − sensors, and formation of fluoride adducts, such as 27h, has been confirmed by 19F NMR.120 4.2. Metallomesogens. Palladium and platinum organometallic complexes with nitrogen-donor ligands represent an interesting class of metallomesogens (metal-containing compounds displaying liquid crystalline behavior). Luminescent metallomesogens are attractive due to their potential applications in optoelectronic devices. In a recent study, compounds based on cycloplatinated 2-phenylpyridines, such as 27i in Figure 27, were studied. Some of these compounds exhibit a lamellar smectic-type mesophase and different emission spectra in different phase states. Highly polarized phosphorescence is achieved for these compounds. 121 On the other hand, self-organization through extended Pt···Pt and hydrophobic interactions of phosphorescent cationic organoplatinum(II) complexes, such as 16e or 16f (Figure 16) in water, produced supramolecular polymers displaying lyotropic liquid crystallinity.72

5. CONCLUSIONS Cyclometalated platinum compounds are generally stable and easy to prepare, and their properties may be easily tuned by changes in the nature of the substituents. Incorporation of fluorine as fluoro ligands or into the cyclometalated or the ancillary ligands in [C,N], [C,N,N′], [N,C,N], [C,N,N,C], [C,P], or [P,C,P] platinum(II) or platinum(IV) cyclometalated compounds opens access to important achievements in distinct research areas. Cycloplatinated compounds with fluoro substituents are involved in fundamental processes of organometallic chemistry, such as intramolecular activation of C−H or C−F bonds, P−C bond activation, and stoichiometric C−C or C−F bond formation. A remarkable contribution is the first example of a platinum-catalyzed cross-coupling reaction of aryl fluorides. Moreover, an associatively activated path has been detected for substitution reactions at fluorinated cyclometalated platinum(IV) compounds, and novel agostic cyclometalated complexes based on a fluorinated ligand have been isolated. Fluorine substituents are used in either the cyclometalating or the ancillary ligands of many examples of cycloplatinated compounds aimed at the design of efficient emitting devices. As a whole, fluorine appears as a “little atom” with great opportunities to disclose novel findings in the chemistry of cycloplatinated complexes as well as to improve and expand their applications.

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AUTHOR INFORMATION Corresponding Author *Phone: +34934039132. Fax: +34934907725. E-mail: [email protected].

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ACKNOWLEDGMENTS

This work was supported by the Ministerio de Ciencia y Tecnologiá (project CTQ2009-11501).

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Fluorine in Cyclometalated Platinum Compounds - American

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