Metal−Metal Cooperativity Effects in Promoting C−H Bond Cleavage of a Methyl Group by an Adjacent Metal Center

Abstract

The reaction of [Ir2(CO)3(dppm)2] (dppm = Ph2PCH2PPh2) with methyl triflate yields the methylene-bridged hydride [Ir2H(CO)3(μ-CH2)(dppm)2][CF3SO3] (2), in which the hydride and methylene hydrogens are rapidly scrambling at ambient temperature. Under CO this species yields the methyl and acyl products [Ir2(R)(CO)4(dppm)2][CF3SO3] (R = CH3, C(O)CH3). Removal of one carbonyl from 2 yields the fluxional methyl complex [Ir2(CH3)(CO)2(dppm)2][CF3SO3] (3) in which the methyl group readily migrates from metal to metal. Addition of CO, PR3, CNtBu or SO2 to 3 results in C−H bond cleavage of the methyl group yielding the methylene-bridged, hydride species, [Ir2H(CO)2L(μ-CH2)(dppm)2][CF3SO3] (L = CO, PR3, CNtBu) or [Ir2H(CO)2(μ-CH2)(μ-SO2)(dppm)2][CF3SO3] (11). Both the carbonyl and SO2 adducts are fluxional, having the hydrogens of the hydrido ligand and the methylene group exchanging rapidly at ambient temperature. The activation parameters for this reversible C−H bond-making and -breaking step have been determined (ΔH ‡ = 10.3 kcal/mol, ΔS ‡ = −11.2 cal/mol K (CO); ΔH ‡ = 6.1 kcal/mol, ΔS ‡ = −6.2 cal/mol K (SO2)). X-ray structure determinations of compound 3, [Ir2H(CO)2(PMe3)(μ-CH2)(dppm)2][CF3SO3] (6), and compound 11 have been determined to confirm the proposed structures. Density functional theory calculations have been carried out on cations related to 3 and 11 by substitution of the phenyl substituents on the dppm ligands by hydrogens, and on key isomers of these, to gain an understanding of the factors promoting C−H bond cleavage in this system. A proposal is presented rationalizing the facile C−H bond cleavage in 3 upon addition of the substrate molecules, in which the roles of the adjacent metals are described.

Facile Carbon–Fluorine Bond Activation and Subsequent Functionalisation of 1,1‐Difluoroethylene and Tetrafluoroethylene Promoted by Adjacent Metal Centres

“…. [22] In this case, the reaction proceeds much more slowly, requiring 24 h, but the final product is significantly cleaner as a result of compound 6 a always being contaminated with 6 b, and owing to the lability of the 1,1-difluoroethylene ligand, as discussed earlier. The 31 P{ 1 H} NMR spectrum of 10, which appears as a singlet that suggests the chemical equivalence of all four 31 P nuclei, Scheme 4.…”

Double Activation of the Geminal Carbon−Hydrogen Bonds in 1,3-Butadiene by a Diiridium Complex

“…The possibility of initial C-H activation of the methyl group, as previously observed or proposed in reactions of 1 with various substrates, 8,9 has been tentatively ruled out by experiments involving CD 3 -labeled 1, in which no deuterium incorporation into the butadiene ligand of 3 or into the hydride or vinylvinylidene ligands of 2 was observed. Support for our proposal, that precoordination of butadiene at both metals precedes the double C-H activation, comes from attempted reactions of 1 with monosubstituted butadienes.…”

“…8 In addition, reaction with unsaturated organic substrates yielded vinyl carbenes, and these have also been proposed to proceed via methylenebridged intermediates. 9 The present study was initiated as part of a continuation of the reactivity studies of 1, in which olefins were investigated as the substrates.…”

Methyl Complexes of the Transition Metals

“…This offers a striking scenario still to be exploited in the search for new homogeneous catalysts for hydrocarbon functionalization. Cowie has reported this bimetallic reaction in Rh/Os 22 and Ir/Ir (Scheme 42a) 250 complexes built around diphosphines, while in a related Ir/Ru system the agostic μ-CH3 complex prevails and no α-hydrogen elimination is observed. 251 The reaction has also been described for an Os trinuclear species.…”