Carbon-fluorine bond activation coupled with carbon-carbon bond formation at iridium. Confirmation of complete kinetic diastereoselectivity at the new carbon stereocenter by intramolecular trapping using vinyl as the migrating group

The iridium(perfluoropropyl)(vinyl) complex CpIr(PMe(3))(n-C(3)F(7))(CH=CH(2)) (5) has been prepared. It has been characterized by X-ray crystallography, and its ground state conformation in solution has been determined by (19)F{(1)H} HOESY NMR studies. It reacts with the weak acid lutidinium iodide to afford the eta(1)-allylic complex CpIr(PMe(3))((Z)-CH(2)CH=CFC(2)F(5))I (6), which has also been characterized crystallographically. The mechanism of C-F bond activation and C-C bond formation leading to 6 has been elucidated in detail by studying the reaction of 5 with lutidinium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate [LutH(+)B(ArF)(4)(-)], containing a weakly coordinating counteranion. The main kinetic product of this reaction, determined by (19)F{(1)H} HOESY studies at -50 degrees C, is the endo-CpIr(PMe(3))(anti-eta(3)-CH(2)CHCFCF(2)CF(3))[B(ArF)(4)] diastereomer 9, along with a small amount of the exo-syn-isomer 8. Isomer 9 rearranges at -20 degrees C to its exo-anti isomer 7, and subsequently to the thermodynamically favored exo-syn-isomer 8, which has been isolated and crystallographically characterized. Complex 8 reacts with iodide to afford complex6. On the basis of the unambiguously defined kinetically controlled stereochemistry of 9 and 8, a detailed mechanism for the C-F activation/C-C coupling reaction is proposed, the principal conclusion of which is that C-F activation is completely diastereoselective.     

Conformational analysis and assignments of relative stereocenter configurations in fluoroalkyl-iridium complexes using 19F[1H] HOESY experiments. Comparison with solid-state X-ray structural results

Solution conformations about the metal-carbon bond of the secondary fluoroalkyl ligands in iridium complexes [IrCp(PMe(3))(R(F))X] [Cp* = C(5)Me(5); R(F) = CF(CF(3))(2), X = I (1), CH(3) (2); R(F) = CF(CF(3))(CF(2)CF(3)), X = I (4), CH(3) (5)] have been determined using (19)F[(1)H] HOESY techniques. The molecules adopt the staggered conformation with the tertiary fluorine in the more hindered sector between the PMe(3) and X ligands, with CF(3) (and CF(2)CF(3)) substituents lying in the less hindered regions between PMe(3) and Cp or X and Cp. In molecules containing the CF(CF(3))(2) ligand, these conformations are identical to those adopted in the solid state. For compound 4, containing the CF(CF(3))(CF(2)CF(3)) ligand, two diastereomers are observed in solution. Solution conformations and relative stereocenter configuration assignments have been obtained using (19)F[(1)H] HOESY and correlated with the X-ray structure for the major diastereomer of 4, which has the (S(Ir), S(C)) or (R(Ir), R(C)) configuration. Relative stereocenter configurations of analogue 5, for which no suitable crystals could be obtained, were assigned using (19)F[(1)H] HOESY and proved to be different from 4, with 5 preferring the (S(Ir), R(C)) or (R(Ir), S(C)) configuration.     

Dissociation of carbanions from acyl iridium compounds: an experimental and computational investigation

Instead of reductive elimination of aldehyde, or decarbonylation to give a trifluoroalkyl hydride, heating Cp(PMe(3))Ir(H)[C(O)CF(3)] (1) leads to the quantitative formation of Cp(PMe(3))Ir(CO) (2) and CF(3)H. Kinetic experiments, isotope labeling studies, solvent effect studies, and solvent-inclusive DFT calculations support a mechanism that involves initial dissociation of trifluoromethyl anion to give the transient ion-pair intermediate [Cp(PMe(3))Ir(H)(CO)](+)[CF(3)](-). Further evidence for the ability of CF(3)(-) to act as a leaving group came from the investigation of the analogous methyl and chloride derivatives Cp(PMe(3))Ir(Me)[C(O)CF(3)] and Cp(PMe(3))Ir(Cl)[C(O)CF(3)]. Both of these compounds undergo a similar loss of trifluoromethyl anion, generating an iridium carbonyl cation and CF(3)D in CD(3)OD. Three additional acyl hydrides, Cp(PMe(3))Ir(H)[C(O)R(F)] (where R(F) = CF(2)CF(3), CF(2)CF(2)CF(3), or CF(2)(CF(2))(6)CF(3)) undergo R(F)-H elimination to give 2 at a faster rate than CF(3)H elimination from 1. Stereochemical studies using a chiral acyl hydride with a stereocenter at the beta-position reveal that ionization of the carbanion occurs to form a tight ion-pair with high retention of configuration and enantiomeric purity upon proton transfer from iridium.     

Imine-assisted C-F bond activation by electron-rich iron complexes supported by trimethylphosphine

C-F bond activation of ortho-fluorinated benzalimines 2,6-F(2)C(6)R1R2R3-CH=N-R (1-3) using the electron-rich complex Fe(PMe(3))(4) is reported. With the assistance of the imine group as the anchoring group, bis-chelated iron(II) complexes (C(6)FR1R2R3-CH=N-R)(2)Fe(PMe(3))(2) (4-6) were formed. The reaction of 2,6-difluorobenzylidenenaphthalen-1-amine 2,6-F(2)C(6)H(3)-CH=N-C(10)H(7) (9) with Fe(PMe(3))(4) affords [CNC]-pincer iron(II) complex (C(6)H(3)F-CH=N-C(10)H(6))Fe(PMe(3))(3) (10) through both C-F and C-H bond activation and π-(C=N) coordinate iron(0) complex (C(6)H(3)F-CH=N-C(10)H(7))(2)Fe(PMe(3))(2) (11) with C,C-coupling, while a similar reaction with perfluorobenzylidenenaphthalen-1-amine C(6)F(5)-CH=N-C(10)H(7) (14) gave rise to only [CNC]-pincer iron(II) complex (C(6)F(4)-CH=N-C(10)H(6))Fe(PMe(3))(3) (15). The proposed formation mechanisms of these complexes are discussed. The structures of complexes 5, 6, 10 and 11 were confirmed by X-ray single crystal diffraction.     

Electronic and medium effects on the rate of arene C [bond] H activation by cationic Ir(III) complexes

A detailed mechanistic study of arene C [bond] H activation in CH(2)Cl(2) solution by Cp(L)IrMe(X) [L = PMe(3), P(OMe)(3); X = OTf, (CH(2)Cl(2))BAr(f); (BAr(f) = B[3,5-C(6)H(3)(CF(3))(2)](4))(-)] is presented. It was determined that triflate dissociation in Cp(L)IrMe(OTf), to generate tight and/or solvent-separated ion pairs containing a cationic iridium complex, precedes C [bond] H activation. Consistent with the ion-pair hypothesis, the rate of arene activation by Cp(L)IrMe(OTf) is unaffected by added external triflate salts, but the rate is strongly dependent upon the medium. Thus the reactivity of Cp(PMe(3))IrMe(OTf) can be increased by almost 3 orders of magnitude by addition of (n-Hex)(4)NBAr(f), presumably because the added BAr(f) anion exchanges with the OTf anion in the initially formed ion pair, transiently forming a cation/borate ion pair in solution (special salt effect). In contrast, addition of (n-Hex)(4)NBAr(f) to [CpPMe(3)Ir(Me)CH(2)Cl(2)][BAr(f)] does not affect the rate of benzene activation; here there is no initial covalent/ionic pre-equilibrium that can be perturbed with added (n-Hex)(4)NBAr(f). An analysis of the reaction between Cp(PMe(3))IrMe(OTf) and various substituted arenes demonstrated that electron-donating substituents on the arene increase the rate of the C [bond] H activation reaction. The rate of C(6)H(6) activation by [Cp(PMe(3))Ir(Me)CH(2)Cl(2)][BAr(f)] is substantially faster than [Cp(P(OMe)(3))Ir(Me)CH(2)Cl(2)][BAr(f)]. Density functional theory computations suggest that this is due to a less favorable pre-equilibrium for dissociation of the dichloromethane ligand in the trimethyl phosphite complex, rather than to a large electronic effect on the C [bond] H oxidative addition transition state. Because of these combined effects, the overall rate of arene activation is increased by electron-donating substituents on both the substrate and the iridium complex.     

Effects of Cyclopentadienyl and Phosphine Ligands on the Basicities and Nucleophilicities of Cp'Ir(CO)(PR(3)) Complexes

Basicities of the series of complexes CpIr(CO)(PR(3)) [PR(3) = P(p-C(6)H(4)CF(3))(3), P(p-C(6)H(4)F)(3), P(p-C(6)H(4)Cl)(3), PPh(3), P(p-C(6)H(4)CH(3))(3), P(p-C(6)H(4)OCH(3))(3), PPh(2)Me, PPhMe(2), PMe(3), PEt(3), PCy(3)] have been measured by the heat evolved (DeltaH(HM)) when the complex is protonated by CF(3)SO(3)H in 1,2-dichloroethane (DCE) at 25.0 degrees C. The -DeltaH(HM) values range from 28.0 kcal/mol for CpIr(CO)[P(p-C(6)H(4)CF(3))(3)] to 33.2 kcal/mol for CpIr(CO)(PMe(3)) and are directly related to the basicities of the PR(3) ligands in the complexes. For the more basic pentamethylcyclopentadienyl analogs, the -DeltaH(HM) values range from 33.8 kcal/mol for the weakest base CpIr(CO)[P(p-C(6)H(4)CF(3))(3)] to 38.0 kcal/mol for the strongest CpIr(CO)(PMe(3)). The nucleophilicities of the Cp'Ir(CO)(PR(3)) complexes were established from second-order rate constants (k) for their reactions with CH(3)I to give [Cp'Ir(CO)(PR(3))(CH(3))](+)I(-) in CD(2)Cl(2) at 25.0 degrees C. There is an excellent linear correlation between the basicities (DeltaH(HM)) and nucleophilicities (log k) of the CpIr(CO)(PR(3)) complexes. Only the complex CpIr(CO)(PCy(3)) with the bulky tricyclohexylphosphine ligand deviates dramatically from the trend. In general, the pentamethylcyclopentadienyl complexes react 40 times faster than the cyclopentadienyl analogs. However, they do not react as fast as predicted from electronic properties of the complexes, which suggests that the steric size of the Cp ligand reduces the nucleophilicities of the CpIr(CO)(PR(3)) complexes. In addition, heats of protonation (DeltaH(HP)) of tris(2-methoxyphenyl)phosphine, tris(2,6-dimethoxyphenyl)phosphine, and tris(2,4,6-trimethylphenyl)phosphine were measured and used to estimate pK(a) values for these highly basic phosphines.     

A computational study of ethylene C-H bond activation by

It has previously been demonstrated that both [(C5Me5)Ir(PMe3)(CH=CH2)H] and [(C5Me5)Ir(PMe3)(H2C=CH2)] are formed when [(C5Me5)Ir(PMe3)] is thermolytically generated in the presence of ethylene. At higher temperatures, the vinyl hydride is converted to the eta2-ethylene adduct. Density functional theory has now been used to investigate this reaction, using the B3LYP functional, two types of basis sets (LanL2DZ and TZV*), and two models of the [(C5R5)Ir(PR3)] species (R=H and CH3). The study consists of full optimizations of local minima, first-order saddle points, and minimum energy crossing points (MECP). The experimental results are best accounted for by considering both singlet and triplet spin surfaces. The relative energies of singlet [(C5R5)Ir(PR3)(CH3)H], [(C5R5)Ir(PR3)(CH=CH2)H], and [(C5R5)Ir(PR3)(H2C=CH2)] are in good agreement with experiment, as is the calculated barrier for the conversion from the vinyl hydride to the eta2-alkene complex. However, the singlet surface alone fails to explain the experimentally observed product ratio, or the intermediate inferred from experimental isotope effect studies. Locating the MECP between singlet and triplet surfaces indicates that the thermolysis of the singlet alkyl hydride precursor directly forms triplet [(C5R5)Ir(PR3)]. The weak vanderWaals adduct of triplet [(C5R5)Ir(PR3)] and ethylene is proposed to be the key intermediate in the overall reaction. The interchanging of the available ethylene C-H bonds in this triplet sigma complex accounts for the observed kinetic isotope effects, and partitioning between alkene pi-complexation and C-H bond activation may also occur from this common intermediate. The possible role of steric factors and molecular dynamics are also discussed.     

Novel intramolecular C-H bond activation in an iridium dppm complex

Reaction of TpIr(C(2)H(4))(2) (Tp = tris-pyrazolylborate) with various chelating phosphine ligands has been explored. Reaction with bis-diphenylphosphinoethane leads to complete displacement of the Tp ligand. With bis-diphenylphosphinomethane, an intramolecular proton transfer from the methylene bridge to the iridium center occurs to give an iridium hydride complex formally resulting from oxidative C-H bond activation. Reaction with 2,2-bis(diphenylphosphino)propane (dppip) affords an Ir(I) complex formulated as kappa(2)-TpIr(dppip). Protonation of this Ir(I) complex gives a six coordinate Ir(III) hydride species.     

Facile carbon-fluorine bond activation and subsequent functionalisation of 1,1-difluoroethylene and tetrafluoroethylene promoted by adjacent metal centres

The bridging fluoroolefin ligands in the complexes [Ir(2)(CH(3))(CO)(2)(μ-olefin)(dppm)(2)][OTf] (olefin = tetrafluoroethylene, 1,1-difluoroethylene; dppm = μ-Ph(2)PCH(2)PPh(2); OTf(-) = CF(3)SO(3)(-)) are susceptible to facile fluoride ion abstraction. Both fluoroolefin complexes react with trimethylsilyltriflate (Me(3)SiOTf) to give the corresponding fluorovinyl products by abstraction of a single fluoride ion. Although the trifluorovinyl ligand is bound to one metal, the monofluorovinyl group is bridging, bound to one metal through carbon and to the other metal through a dative bond from fluorine. Addition of two equivalents of Me(3)SiOTf to the tetrafluoroethylene-bridged species gives the difluorovinylidene-bridged product [Ir(2)(CH(3))(OTf)(CO)(2)(μ-OTf)(μ-C=CF(2))(dppm)(2)][OTf]. The 1,1-difluoroethylene species is exceedingly reactive, reacting with water to give 2-fluoropropene and [Ir(2)(CO)(2)(μ-OH)(dppm)(2)][OTf] and with carbon monoxide to give [Ir(2)(CO)(3)(μ-κ(1):η(2)-C≡CCH(3))(dppm)(2)][OTf] together with two equivalents of HF. The trifluorovinyl product [Ir(2)(κ(1)-C(2)F(3))(OTf)(CO)(2)(μ-H)(μ-CH(2))(dppm)(2)][OTf], obtained through single C-F bond activation of the tetrafluoroethylene-bridged complex, reacts with H(2) to form trifluoroethylene, allowing the facile replacement of one fluorine in C(2)F(4) with hydrogen.     

Elimination of R-H from iridium acyl hydrides without loss of CO: evidence for the intervention of metal cation/carbanion pairs

Instead of reductive elimination of aldehyde, or decarbonylation to give a trifluoroalkyl hydride, heating Cp*(PMe3)Ir(H)[C(O)CF3] leads to the quantitative formation of Cp*(PMe3)Ir(CO) and CF3H. Kinetic experiments, isotope-labeling studies, solvent effect studies, and DFT calculations support a mechanism which involves dissociation of trifluoromethyl anion to give the transient ion-pair intermediate [Cp*(PMe3)Ir(H)(CO)]+[CF3]-. Further evidence for the ability of CF3 to act as a leaving group came from investigation of the analogous methyl and chloride derivatives Cp*(PMe3)Ir(Me)[C(O)CF3] and Cp*(PMe3)Ir(Cl)[C(O)CF3]. Both of these compounds undergo a similar loss of trifluoromethyl anion, generating an iridium carbonyl cation and CF3D in CD3OD.     

Synthesis, structure, and reductive elimination in the series Tp'Rh(PR3)(Ar(F))H; determination of rhodium-carbon bond energies of fluoroaryl substituents

A series of complexes of the type Tp'Rh(PR(3))(Ar(F))H, where PR(3) = PMe(3) (3) and PMe(2)Ph (9), Ar(F) = C(6)F(5) (a), 2,3,4,5-C(6)F(4)H (b), 2,3,5,6-C(6)F(4)H (c), 2,4,6-C(6)F(3)H(2) (d), 2,3-C(6)F(2)H(3) (e), 2,5-C(6)F(2)H(3) (g), and 2-C(6)FH(4) (h) and Tp' = tris(3,5-dimethylpyrazolyl)borate, has been synthesized as stable crystalline compounds by the reactions of the [Tp'Rh(PR(3))] fragment with the corresponding fluorinated aromatic hydrocarbons, and their structures were characterized by NMR spectroscopy and elemental analysis together with X-ray crystallography. The kinetics of the reductive eliminations of fluoroarenes from complexes 3a-h in benzene-d(6) solutions at 140 °C were investigated, but were complicated by the formation of the rhodium(I) bisphosphine complex, Tp'Rh(PMe(3))(2) (4). On the other hand, thermal reactions of (9) in THF-d(8) solutions at 120 °C resulted in the formation of an intramolecular C-H bond activated complex of the phenyl group on the phosphorus atom, Tp'Rh(κ(2)-C(6)H(4)-2-PMe(2))H (7), which prevents the formation of the corresponding bisphosphine complex. The experimentally determined rates of the reductive eliminations of fluoroarenes from the complexes 9a-h and their kinetic selectivities for formation in competition with the metallacycle have been used to determine relative Rh-CAr(F) bond energies. The Rh-CAr(F) bond energy is found to be dependent on the number of ortho fluorines. A plot of Rh-CAr(F) vs. C-H bond strengths resulted in a line with a slope R(M-C/C-H) of 2.15 that closely matches the DFT calculated value (slope = 2.05).     

Hydride transfer reactivity of tetrakis(trimethylphosphine)(hydrido)(nitrosyl)molybdenum(0)

The tetrakis(trimethylphosphine) molybdenum nitrosyl hydrido complex trans-Mo(PMe(3))(4)(H)(NO) (2) and the related deuteride complex trans-Mo(PMe(3))(4)(D)(NO) (2a) were prepared from trans-Mo(PMe(3))(4)(Cl)(NO) (1). From (2)H T(1 min) measurements and solid-state (2)H NMR the bond ionicities of 2a could be determined and were found to be 80.0% and 75.3%, respectively, indicating a very polar Mo--D bond. The enhanced hydridicity of 2 is reflected in its very high propensity to undergo hydride transfer reactions. 2 was thus reacted with acetone, acetophenone, and benzophenone to afford the corresponding alkoxide complexes trans-Mo(NO)(PMe(3))(4)(OCHR'R') (R' = R' = Me (3); R' = Me, R' = Ph (4); R' = R' = Ph (5)). The reaction of 2 with CO(2) led to the formation of the formato-O-complex Mo(NO)(OCHO)(PMe(3))(4) (6). The reaction of with HOSO(2)CF(3) produced the anion coordinated complex Mo(NO)(PMe(3))(4)(OSO(2)CF(3)) (7), and the reaction with [H(Et(2)O)(2)][BAr(F)(4)] with an excess of PMe(3) produced the pentakis(trimethylphosphine) coordinated compound [Mo(NO)(PMe(3))(5)][BAr(F)(4)] (8). Imine insertions into the Mo-H bond of 2 were also accomplished. PhCH[double bond, length as m-dash]NPh (N-benzylideneaniline) and C(10)H(7)CH=NPh (N-1-naphthylideneaniline) afforded the amido compounds Mo(NO)(PMe(3))(4)[NR'(CH(2)R')] (R' = R' = Ph (9), R' = Ph, R' = naphthyl (11)). 9 could not be obtained in pure form, however, its structure was assigned by spectroscopic means. At room temperature 11 reacted further to lose one PMe(3) forming 12 (Mo(NO)PMe(3))(3)[N(Ph)CH(2)C(10)H(6))]) with agostic stabilization. In a subsequent step oxidative addition of the agostic naphthyl C-H bond to the molybdenum centre occurred. Then hydrogen migration took place giving the chelate amine complex Mo(NO)(PMe(3))(3)[NH(Ph)(CH(2)C(10)H(6))] (15). The insertion reaction of 2 with C(10)H(7)N=CHPh led to formation of the agostic compound Mo(NO)(PMe(3))(3)[N(CH(2)Ph)(C(10)H(7))] (10). Based on the knowledge of facile formation of agostic compounds the catalytic hydrogenation of C(10)H(7)N=CHPh and PhN=CHC(10)H(7) with 2 (5 mol%) was tested. The best conversion rates were obtained in the presence of an excess of PMe(3), which were 18.4% and 100% for C(10)H(7)N=CHPh and PhN=CHC(10)H(7), respectively.     

C-H activation and C=C double bond formation reactions in iridium ortho-methyl arylphosphane complexes

The Vaska-type iridium(I) complex [IrCl(CO){PPh(2)(2-MeC(6)H(4))}(2)] (1), characterized by an X-ray diffraction study, was obtained from iridium(III) chloride hydrate and PPh(2)(2,6-MeRC(6)H(3)) with R=H in DMF, whereas for R=Me, activation of two ortho-methyl groups resulted in the biscyclometalated iridium(III) compound [IrCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)] (2). Conversely, for R=Me the iridium(I) compound [IrCl(CO){PPh(2)(2,6-Me(2)C(6)H(3))}(2)] (3) can be obtained by treatment of [IrCl(COE)(2)](2) (COE=cyclooctene) with carbon monoxide and the phosphane in acetonitrile. Compound 3 in CH(2)Cl(2) undergoes intramolecular C-H oxidative addition, affording the cyclometalated hydride iridium(III) species [IrHCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}] (4). Treatment of 2 with Na[BAr(f) (4)] (Ar(f)=3,5-C(6)H(3)(CF(3))(2)) gives the fluxional cationic 16-electron complex [Ir(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)][BAr(f) (4)] (5), which reversibly reacts with dihydrogen to afford the delta-agostic complex [IrH(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}][BAr(f)(4)] (6), through cleavage of an Ir-C bond. This species can also be formed by treatment of 4 with Na[BAr(f)(4)] or of 2 with Na[BAr(f)(4)] through C-H oxidative addition of one ortho-methyl group, via a transient 14-electron iridium(I) complex. Heating of the coordinatively unsaturated biscyclometalated species 5 in toluene gives the trans-dihydride iridium(III) complex [IrH(2)(CO){PPh(2)(2,6-MeC(6)H(3)CH=CHC(6)H(3)Me-2,6)PPh(2)}][BAr(f) (4)] (7), containing a trans-stilbene-type terdentate ligand, as result of a dehydrogenative carbon-carbon double bond coupling reaction, possibly through an iridium carbene species.     

Sulfur-bridged early-late heterobimetallics synthesized by incorporation of titanium,vanadium, and molybdenum into bis(hydrosulfido) templates of group 9 metals

Reactions of the bis(hydrosulfido) complexes [Cp*Rh(SH)(2)(PMe(3))] (1a; Cp* = eta(5)-C(5)Me(5)) with [CpTiCl(3)] (Cp = eta(5)-C(5)H(5)) and [TiCl(4)(thf)(2)] in the presence of triethylamine led to the formation of the sulfido-bridged titanium-rhodium complexes [Cp*Rh(PMe(3))(micro(2)-S)(2)TiClCp] (2a) and [Cp*Rh(PMe(3))(micro2-S)(2)TiCl(2)] (3a), respectively. Complex 3a and its iridium analogue 3b were further converted into the bis(acetylacetonato) complexes [Cp*M(PMe(3))(micro(2)-S)(2)Ti(acac)(2)] (4a, M = Rh; 4b, M = Ir) upon treatment with acetylacetone. The hydrosulfido complexes 1a and [Cp*Ir(SH)(2)(PMe(3))] (1b) also reacted with [VCl(3)(thf)(3)] and [Mo(CO)(4)(nbd)] (nbd = 2,5-norbornadiene) to afford the cationic sulfido-bridged VM2 complexes [(Cp*M(PMe(3))(micro2-S)(2))2V](+) (5a(+), M = Rh; 5b(+), M = Ir) and the hydrosulfido-bridged MoM complexes [Cp*M(PMe(3))(micro2-SH)(2)Mo(CO)(4)] (6a, M = Rh; 6b, M = Ir), respectively.     

A mechanistic investigation of the carbon-carbon bond cleavage of aryl and alkyl cyanides using a cationic RhIII silyl complex

Cationic Rh(III) complex [Cp(PMe(3))Rh(SiPh(3))(CH(2)Cl(2))]BAr(4)' (1) activates the carbon-carbon bond of aryl and alkyl cyanides (R-CN, where R = Ph, (4-(CF(3))C(6)H(4)), (4-(OMe)C(6)H(4)), Me, (i)Pr, (t)Bu) to produce complexes of the general formula [Cp*(PMe(3))Rh(R)(CNSiPh(3))]BAr(4)'. With the exception of the (t)BuCN case, every reaction proceeds at room temperature (t(1/2) < 1 h for aryl cyanides, t(1/2) < 14 h for alkyl cyanides). A general mechanism is presented on the basis of (1) an X-ray crystal structure determination of an intermediate isolated from the reaction involving 4-methoxybenzonitrile and (2) kinetic studies performed on the C-C bond cleavage of para-substituted aryl cyanides. Initial formation of an eta(1)-nitrile species is observed, followed by conversion to an eta(2)-iminoacyl intermediate, which was observed to undergo migration of R (aryl or alkyl) to rhodium to form the product [Cp*(PMe(3))Rh(R)(CNSiPh(3))]BAr(4)'.     

Synthesis, characterization, and application of two Al(OR(F))3 Lewis superacids

We report herein the synthesis and full characterization of the donor-free Lewis superacids Al(OR(F))(3) with OR(F) = OC(CF(3))(3) (1) and OC(C(5)F(10))C(6)F(5) (2), the stabilization of 1 as adducts with the very weak Lewis bases PhF, 1,2-F(2)C(6)H(4), and SO(2), as well as the internal C-F activation pathway of 1 leading to Al(2)(F)(OR(F))(5) (4) and trimeric [FAl(OR(F))(2)](3) (5, OR(F) = OC(CF(3))(3)). Insights have been gained from NMR studies, single-crystal structure determinations, and DFT calculations. The usefulness of these Lewis acids for halide abstractions has been demonstrated by reactions with trityl chloride (NMR; crystal structures). The trityl salts allow the introduction of new, heteroleptic weakly coordinating [Cl-Al(OR(F))(3)](-) anions, for example, by hydride or alkyl abstraction reactions.     

Octahedral Ru(II) amido complexes TpRu(L)(L')(NHR) (Tp = hydridotris(pyrazolyl)borate; L = L' = P(OMe)3 or PMe3 or L = CO and L' = PPh3; R = H,Ph, or tBu): synthesis,characterization, and reactions with weakly acidic C-H bonds

The octahedral Ru(II) amine complexes [TpRu(L)(L')(NH(2)R)][OTf] (L = L' = PMe(3), P(OMe)(3) or L = CO and L' = PPh(3); R = H or (t)Bu) have been synthesized and characterized. Deprotonation of the amine complexes [TpRu(L)(L')(NH(3))][OTf] or [TpRu(PMe(3))(2)(NH(2)(t)Bu)][OTf] yields the Ru(II) amido complexes TpRu(L)(L')(NH(2)) and TpRu(PMe(3))(2)(NH(t)Bu). Reactions of the parent amido complexes or TpRu(PMe(3))(2)(NH(t)Bu) with phenylacetylene at room temperature result in immediate deprotonation to form ruthenium-amine/phenylacetylide ion pairs, and heating a benzene solution of the [TpRu(PMe(3))(2)(NH(2)(t)Bu)][PhC(2)] ion pair results in the formation of the Ru(II) phenylacetylide complex TpRu(PMe(3))(2)(C[triple bond]CPh) in >90% yield. The observation that [TpRu(PMe(3))(2)(NH(2)(t)Bu)][PhC(2)] converts to the Ru(II) acetylide with good yield while heating the ion pairs [TpRu(L)(L')(NH(3))][PhC(2)] yields multiple products is attributed to reluctant dissociation of ammonia compared with the (t)butylamine ligand (i.e., different rates for acetylide/amine exchange). These results are consistent with ligand exchange reactions of Ru(II) amine complexes [TpRu(PMe(3))(2)(NH(2)R)][OTf] (R = H or (t)Bu) with acetonitrile. The previously reported phenyl amido complexes TpRuL(2)(NHPh) [L = PMe(3) or P(OMe)(3)] react with 10 equiv of phenylacetylene at elevated temperature to produce Ru(II) acetylide complexes TpRuL(2)(C[triple bond]CPh) in quantitative yields. Kinetic studies indicate that the reaction of TpRu(PMe(3))(2)(NHPh) with phenylacetylene occurs via a pathway that involves TpRu(PMe(3))(2)(OTf) or [TpRu(PMe(3))(2)(NH(2)Ph)][OTf] as catalyst. Reactions of 1,4-cyclohexadiene with the Ru(II) amido complexes TpRu(L)(L')(NH(2)) (L = L' = PMe(3) or L = CO and L' = PPh(3)) or TpRu(PMe(3))(2)(NH(t)Bu) at elevated temperatures result in the formation of benzene and Ru hydride complexes. TpRu(PMe(3))(2)(H), [Tp(PMe(3))(2)Ru[double bond]C[double bond]C(H)Ph][OTf], [Tp(PMe(3))(2)Ru=C(CH(2)Ph)[N(H)Ph]][OTf], and [TpRu(PMe(3))(3)][OTf] have been independently prepared and characterized. Results from solid-state X-ray diffraction studies of the complexes [TpRu(CO)(PPh(3))(NH(3))][OTf], [TpRu(PMe(3))(2)(NH(3))][OTf], and TpRu(CO)(PPh(3))(C[triple bond]CPh) are reported.     

Synthesis and reactivity of silyl ruthenium complexes: the importance of trans effects in C-H activation,Si-C bond formation,and dehydrogenative coupling of silanes

A series of octahedral ruthenium silyl hydride complexes, cis-(PMe(3))(4)Ru(SiR(3))H (SiR(3) = SiMe(3), 1a; SiMe(2)CH(2)SiMe(3), 1b; SiEt(3), 1c; SiMe(2)H, 1d), has been synthesized by the reaction of hydrosilanes with (PMe(3))(3)Ru(eta(2)-CH(2)PMe(2))H (5), cis-(PMe(3))(4)RuMe(2) (6), or (PMe(3))(4)RuH(2) (9). Reaction with 6 proceeds via an intermediate product, cis-(PMe(3))(4)Ru(SiR(3))Me (SiR(3) = SiMe(3), 7a; SiMe(2)CH(2)SiMe(3), 7b). Alternatively, 1 and 7 have been synthesized via a fast hydrosilane exchange with another cis-(PMe(3))(4)Ru(SiR(3))H or cis-(PMe(3))(4)Ru(SiR(3))Me, which occurs at a rate approaching the NMR time scale. Compounds 1a, 1b, 1d, and 7a adopt octahedral geometries in solution and the solid state with mutually cis silyl and hydride (or silyl and methyl) ligands. The longest Ru-P distance within a complex is always trans to Si, reflecting the strong trans influence of silicon. The aptitude of phosphine dissociation in these complexes has been probed in reactions of 1a, 1c, and 7a with PMe(3)-d(9) and CO. The dissociation is regioselective in the position trans to a silyl ligand (trans effect of Si), and the rate approaches the NMR time scale. A slower secondary process introduces PMe(3)-d(9) and CO in the other octahedral positions, most likely via nondissociative isomerization. The trans effect and trans influence in 7a are so strong that an equilibrium concentration of dissociated phosphine is detectable (approximately 5%) in solution of pure 7a. Compounds 1a-c also react with dihydrogen via regioselective dissociation of phosphine from the site trans to Si, but the final product, fac-(PMe(3))(3)Ru(SiR(3))H(3) (SiR(3) = SiMe(3), 4a; SiMe(2)CH(2)SiMe(3), 4b; SiEt(3), 4c), features hydrides cis to Si. Alternatively, 4a-c have been synthesized by photolysis of (PMe(3))(4)RuH(2) in the presence of a hydrosilane or by exchange of fac-(PMe(3))(3)Ru(SiR(3))H(3) with another HSiR(3). The reverse manifold - HH elimination from 4a and trapping with PMe(3) or PMe(3)-d(9) - is also regioselective (1a-d(9)() is predominantly produced with PMe(3)-d(9) trans to Si), but is very unfavorable. At 70 degrees C, a slower but irreversible SiH elimination also occurs and furnishes (PMe(3))(4)RuH(2). The structure of 4a exhibits a tetrahedral P(3)Si environment around the metal with the three hydrides adjacent to silicon and capping the P(2)Si faces. Although strong Si...HRu interactions are not indicated in the structure or by IR, the HSi distances (2.13-2.23(5) A) suggest some degree of nonclassical SiH bonding in the H(3)SiR(3) fragment. Thermolysis of 1a in C(6)D(6) at 45-55 degrees C leads to an intermolecular CD activation of C(6)D(6). Extensive H/D exchange into the hydride, SiMe(3), and PMe(3) ligands is observed, followed by much slower formation of cis-(PMe(3))(4)Ru(D)(Ph-d(5)). In an even slower intramolecular CH activation process, (PMe(3))(3)Ru(eta(2)-CH(2)PMe(2))H (5) is also produced. The structure of intermediates, mechanisms, and aptitudes for PMe(3) dissociation and addition/elimination of H-H, Si-H, C-Si, and C-H bonds in these systems are discussed with a special emphasis on the trans effect and trans influence of silicon and ramifications for SiC coupling catalysis.     

Computational study of the reaction of C6F6 with [IrMe(PEt3)3]: identification of a phosphine-assisted C-F activation pathway via a metallophosphorane intermediate

Density functional theory calculations have been used to model the reaction of C6F6 with [IrMe(PEt3)3], which proceeds with both C-F and P-C bond activation to yield trans-[Ir(C6F5)(PEt3)2(PEt2F)], C2H4, and CH4 (Blum, O.; Frolow, F.; Milstein, D. J. Chem. Soc., Chem. Commun. 1991, 258). Using a model species, trans-[IrMe(PH3)2(PH2Et)], a low-energy mechanism involving nucleophilic attack of the electron-rich Ir metal center at C6F6 with displacement of fluoride has been identified. A novel feature of this process is the capture of fluoride by a phosphine ligand to generate a metallophosphorane intermediate [Ir(C6F5)(Me)(PH3)2(PH2EtF)]. These events occur in a single step via a 4-centered transition state, in a process that we have termed "phosphine-assisted C-F activation". Alternative mechanisms based on C-F activation via concerted oxidative addition or electron-transfer processes proved less favorable. From the metallophosphorane intermediate the formation of the final products can be accounted for by facile ethyl group transfer from phosphorus to iridium followed by beta-H elimination of ethene and reductive elimination of methane. The interpretation of phosphine-assisted C-F activation in terms of nucleophilic attack is supported by the reduced activation barriers computed with the more electron-rich model reactant trans-[IrMe(PMe3)2(PMe2Et)] and the higher barriers found with lesser fluorinated arenes. Reactivity patterns for a range of fluoroarenes indicate the dominance of the presence of ortho-F substituents in promoting phosphine-assisted C-F activation, and an analysis of the charge distribution and transition state geometries indicates that this process is controlled by the strength of the Ir-aryl bond that is being formed.     

C-Cl bond activation of ortho-chlorinated benzamides by nickel and cobalt compounds supported with phosphine ligands

The C-Cl bonds of ortho-chlorinated benzamides Cl-ortho-C(6)H(4)C(=O)NHR (R = Me (1), nBu (2), Ph (3), (4-Me)Ph (4) and (4-Cl)Ph (5)) were successfully activated by tetrakis(trimethylphosphine)nickel(0) and tetrakis(trimethylphosphine)cobalt(0). The four-coordinate nickel(II) chloride complexes trans-[(C(6)H(4)C([double bond, length as m-dash]O)NHR)Ni(PMe(3))(2)Cl] (R = Me (6), nBu (7), Ph (8) and (4-Me)Ph (9)) as C-Cl bond activation products were obtained without coordination of the amide groups. In the case of 2, the ionic penta-coordinate cobalt(II) chloride [(C(6)H(4)C(=O)NHnBu)Co(PMe(3))(3)]Cl (10) with the [C(phenyl), O(amide)]-chelate coordination as the C-Cl bond activation product was isolated. Under similar reaction conditions, for the benzamides 3-5, hexa-coordinate bis-chelate cobalt(III) complexes (C(6)H(4)C(=O)NHR)Co(Cl-ortho-C(6)H(4)C(=O)NR)(PMe(3))(2) (11-13) were obtained via the reaction with [Co(PMe(3))(4)]. Complexes 11-13 have both a five-membered [C,N]-coordinate chelate ring and a four-membered [N,O]-coordinate chelate ring with two trimethyphosphine ligands in the axial positions. Phosphonium salts [Me(3)P(+)-ortho-C(6)H(4)C(=O)NHR]Cl(-) (R = Ph (14) and (4-Me)Ph (15)) were isolated by reaction of complexes 8 and 9 as a starting material under 1 bar of CO at room temperature. The crystal and molecular structures of complexes 6, 7 and 9-12 were determined by single-crystal X-ray diffraction.