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).     

Hydrofluoroarylation of alkynes with fluoroarenes

A combination of Ni(cod)(2) and PCyp(3) is found to be an effective catalyst for chemoselective activation of the C-H bond of fluoroarenes over C-F bonds followed by insertion of alkynes to allow direct alkenylation of the electron-deficient arenes. The characteristics of the reactions are: a C-H bond ortho to a fluorine substituent is selectively activated; the reactivity of fluorobenzenes is roughly proportional to the number of fluorine atoms. The reaction conditions tolerate a broad range of both alkynes and fluoroarenes containing both electron-withdrawing and -donating groups, thus allowing efficient synthesis of a variety of substituted ethenes containing a fluoroaryl motif in high regio- and stereoselective manners. Mechanistic studies including both labeling experiments and stoichiometric reactions reveal that oxidative addition of C-H bonds in fluoroarenes to nickel(0) is kinetically highly facile whereas that of C-F bonds is thermodynamically favoured.     

Mechanisms of reductive eliminations in square planar Pd(II) complexes: nature of eliminated bonds and role of trans influence

The trans influence of various phosphine ligands (L) in direct as well as dissociative reductive elimination pathways yielding CH(3)CH(3) from Pd(CH(3))(2)L(2) and CH(3)Cl from Pd(CH(3))(Cl)L(2) has been quantified in terms of isodesmic reaction energy, E(trans), using the MPWB1K level of density functional theory. In the absence of a large steric effect, E(trans) correlated linearly with the activation barrier (E(act)) of both direct and dissociation pathways. The minimum of molecular electrostatic potential (V(min)) at the lone pair region of phosphine ligands has been used to assess their electron donating power. E(trans) increased linearly with an increase in the negative V(min) values. Further, the nature of bonds that are eliminated during reductive elimination have been analyzed in terms of AIM parameters, viz. electron density (ρ(r)), Laplacian of the electron density (?(2)ρ(r)), total electron energy density (H(r)), and ratio of potential and kinetic electron energy densities (k(r)). Interestingly, E(act) correlated inversely with the strength of the eliminated metal-ligand bonds measured in terms of the bond length or the ρ(r). Analysis of H(r) showed that elimination of the C-C/C-Cl bond becomes more facile when the covalent character of the Pd-C/Pd-Cl bond increases. Thus, AIM details clearly showed that the strength of the eliminated bond is not the deciding factor for the reductive elimination but the nature of the bond, covalent or ionic. Further, a unified picture showing the relationship between the nature of the eliminated chemical bond and the tendency of reductive elimination is obtained from the k(r) values: the E(act) of both direct and dissociative mechanisms for the elimination of CH(3)CH(3) and CH(3)Cl decreased linearly when the sum of k(r) at the cleaved bonds showed a more negative character. It means that the potential electron energy density dominates over the kinetic electron energy density when the bonds (Pd-C/Pd-Cl) become more covalent and the eliminated fragments attain more radical character leading to the easy formation of C-C/C-Cl bond.     

Mechanistic considerations for C-C bond reductive coupling at a cobalt(III) center

The diamagnetic cobalt(III) dimethyl complex, cis,mer-(PMe(3))(3)Co(CH(3))(2)I, was found to promote selective C-C bond formation, affording ethane and triplet (PMe(3))(3)CoI. The mechanism of reductive elimination has been investigated by a series of kinetic and isotopic-labeling experiments. Ethane formation proceeds with a rate constant of 3.1(5) × 10(-5) s(-1) (50 °C) and activation parameters of ΔH(double dagger) = 31.4(8) kcal/mol and ΔS(double dagger) = 17(3) eu. Addition of free trimethylphosphine or coordinating solvent strongly inhibits reductive elimination, indicating reversible phosphine dissociation prior to C-C bond-coupling. EXSY NMR analysis established a rate constant of 9(2) s(-1) for phosphine loss from cis,mer-(PMe(3))(3)Co(CH(3))(2)I. Radical trapping, crossover, and isotope effect experiments were consistent with a proposed mechanism for ethane extrusion where formation of an unobserved five-coordinate intermediate is followed by concerted C-C bond formation. An unusual intermolecular exchange of cobalt-methyl ligands was also observed by isotopic labeling.     

Mechanistic analysis of iridium(III) catalyzed direct C-H arylations: a DFT study

In a recent experimental work the Ir complex [Ir(cod)(py)(PCy(3))](PF(6)) (that is, Crabtree's catalyst) has been shown to catalyze the C-H arylation of electron-rich heteroarenes with iodoarenes using Ag(2)CO(3) as base. For this process, an electrophilic metalation mechanism, (S(E)Ar) has been proposed as operative mechanism rather than the concerted metalation-deprotonation (CMD) mechanism, widely implicated in Pd-catalyzed arylation reactions. Herein we have investigated the C-H activation step for several (hetero)arenes catalyzed by a Ir(III) catalyst and compared the data obtained with the results for the Pd(II)-catalyzed C-H bond activation. The calculations demonstrate that, similar to Pd(II)-catalyzed reactions, the Ir(III)-catalyzed direct C-H arylation occurs through the CMD pathway which accounts for the experimentally observed regioselectivity. The transition states for Ir(III)-catalyzed direct C-H arylation feature stronger metal-C((arene)) interactions than those for Pd(II)-catalyzed C-H arylation. The calculations also demonstrate that ligands with low trans effect may decrease the activation barrier of the C-H bond cleavage.     

Sequential C-F activation and borylation of fluoropyridines via intermediate Rh(I) fluoropyridyl complexes: a multinuclear NMR investigation

The C-F bond activation of fluoropyridines by [Rh(SiPh3)(PMe3)3] afforded Rh(I) fluoropyridyl complexes of the type [Rh(Ar(F))(PMe3)3] with concomitant formation of fluorotriphenylsilane; subsequent treatment with bis-catecholatodiboron yielded fac-[Rh(Bcat)3(PMe3)3] and the free fluoropyridyl boronate esters (Ar(F)Bcat).     

Copper-mediated chelation-assisted ortho nitration of (hetero)arenes

A novel copper-mediated chelation-assisted ortho C-H nitration of (hetero)arenes has been developed for the first time, which used dioxygen as terminal oxidant and 1,2,3-TCP as solvent, leading to the synthesis of nitroaromatics with excellent regioselectivity and in good yields. Mechanistic investigations indicate a mechanism involving a four-centered transition state, with simultaneous cleavage of an ortho C-H bond and a N-O bond of the nitrate anion on the 2-arylpyridine-coordinated copper(II) complex.     

Aryl-fluoride reductive elimination from Pd(II): feasibility assessment from theory and experiment

DFT methods were used to elucidate features of coordination environment of Pd(II) that could enable Ar-F reductive elimination as an elementary C-F bond-forming reaction potentially amenable to integration into catalytic cycles for synthesis of organofluorine compounds with benign stoichiometric sources of F(-). Three-coordinate T-shaped geometry of Pd(II)Ar(F)L (L = NHC, PR(3)) was shown to offer kinetics and thermodynamics of Ar-F elimination largely compatible with synthetic applications, whereas coordination of strong fourth ligands to Pd or association of hydrogen bond donors with F each caused pronounced stabilization of Pd(II) reactant and increased activation barrier beyond the practical range. Decreasing donor ability of L promotes elimination kinetics via increasing driving force and para-substituents on Ar exert a sizable SNAr-type TS effect. Synthesis and characterization of the novel [Pd(C(6)H(4)-4-NO(2))ArL(mu-F)](2) (L = P(o-Tolyl)(3), 17; P(t-Bu)(3), 18) revealed stability of the fluoride-bridged dimer forms of the requisite Pd(II)Ar(F)L as the key remaining obstacle to Ar-F reductive elimination in practice. Interligand steric repulsion with P(t-Bu)(3) served to destabilize dimer 18 by 20 kcal/mol, estimated with DFT relative to PMe(3) analog, yet was insufficient to enable formation of greater than trace quantities of Ar-F; C-H activation of P(t-Bu)(3) followed by isobutylene elimination was the major degradation pathway of 18 while Ar/F- scrambling and Ar-Ar reductive elimination dominated thermal decomposition of 17. However, use of Buchwald's L = P(C(6)H(4)-2-Trip)(t-Bu)(2) provided the additional steric pressure on the [PdArL(mu-F)](2) core needed to enable formation of aryl-fluoride net reductive elimination product in quantifiable yields (10%) in reactions with both 17 and 18 at 60 degrees over 22 h.     

A new class of transition metal pincer ligand: tantalum complexes that feature a [CCC] X3-donor array derived from a terphenyl ligand

A new class of [CCC] X(3)-donor pincer ligand for transition metals has been constructed via cyclometalation of a 2,6-di-p-tolylphenyl ([Ar(Tol(2))]) derivative. Specifically, addition of PMe(3) to [Ar(Tol(2))]TaMe(3)Cl induces elimination of methane and formation of the pincer complex, [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)MeCl (Tol' = C(6)H(3)Me), which may also be obtained by treatment of Ta(PMe(3))(2)Me(3)Cl(2) with [Ar(Tol(2))]Li. Solutions of [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)MeCl undergo ligand redistribution with the formation of [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)Me(2)and [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)Cl(2), which may also be synthesized by the reactions of [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)MeCl with MeMgBr and ZnCl(2), respectively. Reduction of [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)Cl(2) with KC(8) in benzene gives the benzene complex [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)(η(6)-C(6)H(6)) that is better described as a 1,4-cyclohexadienediyl derivative. Deuterium labeling employing Ta(PMe(3))(2)(CD(3))(3)Cl(2) demonstrates that the pincer ligand is created by a pair of Ar-H/Ta-Me sigma-bond metathesis transformations, rather than by a mechanism that involves α-H abstraction by a tantalum methyl ligand.     

Nickel-catalyzed alkenylation and alkylation of fluoroarenes via activation of C-H bond over C-F bond

Nickel/P(c-C(5)H(9))(3) (PCyp(3)) catalyst effects the addition reactions of fluoroarenes across alkynes, 1,3-dienes, and vinylarenes via the activation of C-H bonds over C-F bonds. The acidic C-H bonds located ortho to fluorine are exclusively activated to afford a range of alkenylated and alkylated fluoroarenes.     

Theoretical study of reaction pathways for the rhodium phosphine-catalysed borylation of C-H bonds with pinacolborane

The reaction mechanism of the rhodium-phosphine catalysed borylation of methyl-substituted arenes using pinacolborane (HBpin) has been investigated theoretically using DFT calculations at the B3PW91 level. Factors affecting selectivity for benzylic vs. aromatic C-H bond activation have been examined. It was found that [Rh(PR3)2(H)] is the active species which oxidatively adds the C-H bond leading to an eta3-benzyl complex which is the key to determining the unusual benzylic regioselectivity observed experimentally for this catalyst system. Subsequent reaction with HBpin leads to a [Rh(PR3)2(eta3-benzyl)(H)(Bpin)] complex from which B-C reductive elimination provides product and regenerates the catalyst. The electrophilic nature of the boryl ligand assists in the reductive elimination process. In contrast to Ir(L)2(boryl)3-based catalysts, for which Ir(III)-Ir(V) cycles have been proposed, the Rh(I)-Rh(III) cycle is operating with the system addressed herein.     

Theoretical study of the Cp2Zr-catalyzed hydrosilylation of ethylene. Reaction mechanism including new sigma-bond activation

The Cp(2)Zr-catalyzed hydrosilylation of ethylene was theoretically investigated with DFT and MP2-MP4(SDQ) methods, to clarify the reaction mechanism and the characteristic features of this reaction. Although ethylene insertion into the Zr-SiH(3) bond of Cp(2)Zr(H)(SiH(3)) needs a very large activation barrier of 41.0 (42.3) kcal/mol, ethylene is easily inserted into the Zr-H bond with a very small activation barrier of 2.1 (2.8) kcal/mol, where the activation barrier and the energy of reaction calculated with the DFT(B3LYP) method are given and in parentheses are those values which have been corrected for the zero-point energy, hereafter. Not only this ethylene insertion reaction but also the coupling reaction between Cp(2)Zr(C(2)H(4)) and SiH(4) easily takes place to afford Cp(2)Zr(H)(CH(2)CH(2)SiH(3)) and Cp(2)Zr(CH(2)CH(3))(SiH(3)) with activation barriers of 0.3 (0.7) and 5.0 (5.4) kcal/mol, respectively. This coupling reaction involves a new type of Si-H sigma-bond activation which is similar to metathesis. The important interaction in the coupling reaction is the bonding overlap between the d(pi)-pi bonding orbital of Cp(2)Zr(C(2)H(4)) and the Si-H sigma orbital. The final step is neither direct C-H nor Si-C reductive elimination, because both reductive eliminations occur with a very large activation barrier and significantly large endothermicity. This is because the d orbital of Cp(2)Zr is at a high energy. On the other hand, ethylene-assisted C-H reductive elimination easily occurs with a small activation barrier, 5.0 (7.5) kcal/mol, and considerably large exothermicity, -10.6 (-7.1) kcal/mol. Also, ethylene-assisted Si-C reductive elimination and metatheses of Cp(2)Zr(H)(CH(2)CH(2)SiH(3)) and Cp(2)Zr(CH(2)CH(3))(SiH(3)) with SiH(4) take place with moderate activation barriers, 26.5 (30.7), 18.4 (20.5), and 28.3 (31.5) kcal/mol, respectively. From these results, it is clearly concluded that the most favorable catalytic cycle of the Cp(2)Zr-catalyzed hydrosilylation of ethylene consists of the coupling reaction of Cp(2)Zr(C(2)H(4)) with SiH(4) followed by the ethylene-assisted C-H reductive elimination.     

Room-temperature regioselective C-H/olefin coupling of aromatic ketones using an activated ruthenium catalyst with a carbonyl ligand and structural elucidation of key intermediates

Mechanistic studies of the ruthenium-catalyzed reaction of aromatic ketones with olefins are presented. Treatment of the original catalyst, RuH(2)(CO)(PPh(3))(3), with trimethylvinylsilane at 90 °C for 1-1.5 h afforded an activated ruthenium catalyst, Ru(o-C(6)H(4)PPh(2))(H)(CO)(PPh(3))(2), as a mixture of four geometric isomers. The activated complex showed high catalytic activity for C-H/olefin coupling, and the reaction of 2'-methylacetophenone with trimethylvinylsilane at room temperature for 48 h gave the corresponding ortho-alkylation product in 99% isolated yield. The activated catalyst was thermally robust and showed excellent catalytic activity under refluxing toluene conditions. (1)H and (31)P NMR studies of the C-H/olefin coupling at room temperature suggested that an ortho-ruthenated complex, P,P'-cis-C,H-cis-Ru(2'-(6'-MeC(6)H(4)C(O)Me))(H)(CO)(PPh(3))(2), participated in the reaction as a key intermediate. Isotope labeling studies using acetophenone-d(5) indicated that the rate-limiting step was the C-C bond formation, not the C-H bond cleavage, and that each step prior to the reductive elimination was reversible. The rate of C-H/olefin coupling was found to exhibit pseudo first-order kinetics and to show first-order dependence on the ruthenium complex concentration.     

Synthesis, structure and DFT study of dinuclear iron, cobalt and nickel complexes with cyclopentadienyl-metal moieties

Reactions of 1,1'-bis(dipheny1phosphino)cobaltocene with Co(PMe(3))(4), Ni(PMe(3))(4), Fe(PMe(3))(4), Ni(COD)(2), FeMe(2)(PMe(3))(4) or NiMe(2)(PMe(3))(3) afford a series of novel dinuclear complexes [((Me(3)P)[lower bond 1 start]Co(η(5)-C(5)H(4)[upper bond 1 start]PPh(2)))((Me(3)P)M[upper bond 1 end](η(5)-C(5)H(4)P[lower bond 1 end]Ph(2)))] (M = Co(1), Ni(2) and Fe(3)) [Co(η(5)-C(5)H(4)[upper bond 1 start]PPh(2))(2)Ni[upper bond 1 end](COD)](4), [Co(η(5)-C(5)H(4)[upper bond 1 start]PPh(2))(2)Ni[upper bond 1 end](PMe(3))(2)] (5) and [((Me(3)P)[lower bond 1 start]Co(Me)(η(5)-C(5)H(4)[upper bond 1 start]PPh(2)))((Me(3)P)Fe[upper bond 1 end](Me)(η(5)-C(5)H(4)P[lower bond 1 end]Ph(2)))] (6). Reactions of 1,1'-bis(dipheny1phosphino)ferrocene with Ni(PMe(3))(4), NiMe(2)(PMe(3))(3), or Co(PMe(3))(4) gives rise to complexes [Fe(η(5)-C(5)H(4)[upper bond 1 start]PPh(2))(2)M[upper bond 1 end](PMe(3))(2)] (M = Ni (7), Co (8)). The complexes 1-8 were spectroscopically investigated and studied by X-ray single crystal diffraction. The possible reaction mechanisms and structural characteristics are discussed. Density functional theory (DFT) calculations strongly support the deductions.     

Carbon-halide oxidative addition and carbon-carbon reductive elimination at a (PNP)Rh center

An unsaturated (PNP)Rh fragment can be generated by means of C-C reductive elimination from (PNP)Rh(Me)(Ar) or (PNP)Rh(Ar)(Ar). This fragment undergoes carbon-halogen oxidative addition with aryl chlorides, bromides, and iodides at room temperature. The C-H oxidative addition products in reactions with haloarenes are not observed, and evidence is presented that carbon-halogen oxidative addition is thermodynamically preferred. C-C reductive elimination from (PNP)Rh(Me)(Ar) and (PNP)Rh(Ar)(Ar) proceeds near quantitatively as a clean, first-order reaction.     

C-H activation of terminal alkynes by tris-(3,5-dimethylpyrazolyl)boraterhodiumneopentylisocyanide: new metal-carbon bond strengths

C-H bond activation of terminal alkynes by [Tp'Rh(CNneopentyl)] (Tp' = hydridotris-(3,5-dimethylpyrazolyl)borate) resulted in the formation of terminal C-H bond activation products Tp'Rh(CNneopentyl)(C≡CR)H (R = t-Bu, SiMe(3), hexyl, CF(3), p-MeOC(6)H(4), Ph, and p-CF(3)C(6)H(4)). A combination of kinetic selectivity determined in competition reactions and activation energy for reductive elimination has allowed for the calculation of relative Rh-C(alkynyl) bond strengths. The bond strengths of Rh-C(alkynyl) products are noticeably higher than those of Rh-C(aryl) and Rh-C(alkyl) analogues. The relationship between M-C and C-H bond strengths showed a linear correlation (slope R(M-C/H-C) = 1.32), and follows energy correlations previously established for unsubstituted sp(2) and sp(3) C-H bonds in aliphatic and aromatic hydrocarbons.     

Rhodium-catalyzed nondecarbonylative addition reaction of ClCOCOOC2H5 to alkynes

Addition of ethoxalyl chloride (ClCOCOOEt) to terminal alkynes at 60 degrees C in the presence of a rhodium(I)-phosphine complex catalyst chosen from a wide range affords 4-chloro-2-oxo-3-alkenoates regio- and stereoselectively. Functional groups such as chloro, cyano, alkoxy, siloxy, and hydroxy are tolerated. The oxidative addition of ethoxalyl chloride to [RhCl(CO)(PR(3))(2)] proceeds readily at 60 degrees C or room temperature and gives [RhCl(2)(COCOOEt)(CO)(PR(3))(2)] (PR(3) = PPh(2)Me, PPhMe(2), PMe(3)) complexes in high yields. The structure of [RhCl(2)(COCOOEt)(CO)(PPh(2)Me)(2)] was confirmed by X-ray crystallography. Thermolysis of these ethoxalyl complexes has revealed that those ligated by more electron-donating phosphines are fairly stable against decarbonylation and reductive elimination. [RhCl(2)(COCOOEt)(CO)(PPh(2)Me)(2)] reacts with 1-octyne at 60 degrees C to form ethyl 4-chloro-2-oxo-3-decenoate. The catalysis is therefore proposed to proceed by oxidative addition of ethoxalyl chloride, insertion of an alkyne into the Cl--Rh bond of the resulting intermediate, and reductive elimination of alkenyl-COCOOEt.     

Reductive elimination/oxidative addition of carbon-hydrogen bonds at Pt(IV)/Pt(II) centers: mechanistic studies of the solution thermolyses of Tp(Me2)Pt(CH3)2H

Reductive elimination of methane occurs upon solution thermolysis of kappa(3)-Tp(Me)2Pt(IV)(CH(3))(2)H (1, Tp(Me)2 = hydridotris(3,5-dimethylpyrazolyl)borate). The platinum product of this reaction is determined by the solvent. C-D bond activation occurs after methane elimination in benzene-d(6), to yield kappa(3)-Tp(Me)2Pt(IV)(CH(3))(C(6)D(5))D (2-d(6)), which undergoes a second reductive elimination/oxidative addition reaction to yield isotopically labeled methane and kappa(3)-Tp(Me)2Pt(IV)(C(6)D(5))(2)D (3-d(11)). In contrast, kappa(2)-Tp(Me)2Pt(II)(CH(3))(NCCD(3)) (4) was obtained in the presence of acetonitrile-d(3), after elimination of methane from 1. Reductive elimination of methane from these Pt(IV) complexes follows first-order kinetics, and the observed reaction rates are nearly independent of solvent. Virtually identical activation parameters (DeltaH(++)(obs) = 35.0 +/- 1.1 kcal/mol, DeltaS(++)(obs) = 13 +/- 3 eu) were measured for the reductive elimination of methane from 1 in both benzene-d(6) and toluene-d(8). A lower energy process (DeltaH(++)(scr) = 26 +/- 1 kcal/mol, DeltaS(++)(scr) = 1 +/- 4 eu) scrambles hydrogen atoms of 1 between the methyl and hydride positions, as confirmed by monitoring the equilibration of kappa(3)-Tp(Me)()2Pt(IV)(CH(3))(2)D (1-d(1)()) with its scrambled isotopomer, kappa(3)-Tp(Me)2Pt(IV)(CH(3))(CH(2)D)H (1-d(1'). The sigma-methane complex kappa(2)-Tp(Me)2Pt(II)(CH(3))(CH(4)) is proposed as a common intermediate in both the scrambling and reductive elimination processes. Kinetic results are consistent with rate-determining dissociative loss of methane from this intermediate to produce the coordinatively unsaturated intermediate [Tp(Me)2Pt(II)(CH(3))], which reacts rapidly with solvent. The difference in activation enthalpies for the H/D scrambling and C-H reductive elimination provides a lower limit for the binding enthalpy of methane to [Tp(Me)2Pt(II)(CH(3))] of 9 +/- 2 kcal/mol.     

Roles of a tetrahydroborate ligand in a facile route to ruthenium(II) ethyl hydride complexes, and a kinetic study of ethane reductive elimination

The tetrahydroborate ligand in [Ru(eta(2)-BH(4))(CO)H(PMe(2)Ph)(2)], 1, allows conversion under very mild conditions to [Ru(CO)(Et)H(PMe(2)Ph)(3)], 7, by way of [Ru(eta(2)-BH(4))(CO)Et(PMe(2)Ph)(2)], 4. Deprotection of the hydride ligand in 7(by BH(3) abstraction) occurs only in the final step, thus preventing premature ethane elimination. A deviation from the route from 4 to 7 yields [Ru(eta(2)-BH(4))(COEt)(PMe(2)Ph)(3)], 6, but does not prevent ultimate conversion to 7. Modification of the treatment of 4 yields an isomer of 7, 10. Both isomers eliminate ethane at temperatures above 250 K: the immediate product of elimination, thought to be [Ru(CO)(PMe(2)Ph)(3)], 11, can be trapped as [Ru(CO)(PMe(2)Ph)(4)], 12, [Ru(CO)H(2)(PMe(2)Ph)(3)], 3a, or [Ru(CO)(C[triple bond]CCMe(3))H(PMe(2)Ph)(3)], 13. The elimination is a simple first-order process with negative DeltaS(++) and (for 7) a normal kinetic isotope effect (k(H)/k(D)= 2.5 at 287.9 K). These results, coupled with labelling studies, rule out a rapid equilibrium with a [sigma]-ethane intermediate prior to ethane loss.