Metal-metal cooperativity effects in promoting C-H bond cleavage of a methyl group by an adjacent metal center

The reaction of [Ir-2(CO)(3)(dppm)(2)] (dppm = Ph2PCH2PPh2) with methyl tri flate yields the methylene-bridged hydride [Ir2H(CO)(3)(mu-CH2)(dppm)(2)] [ CF3SO3] (2), in which the hydride and methylene hydrogens are rapidly scram bling at ambient temperature. Under CO this species yields the methyl and a cyl products [Ir-2(R)(CO)(4)(dppm)(2)] [CF3SO3] (R = CH3, C(O)CH3). Removal of one carbonyl from 2 yields the fluxional methyl complex [Ir-2(CH3)(CO)( 2)(dppm)(2)] [CF3SO3] (3) in which the methyl group readily migrates from m etal to metal. Addition of CO, PR3, (CNBu)-Bu-t or SO2 to 3 results in C-H bond cleavage of the methyl group yielding the methylene-bridged, hydride s pecies, [Ir2H(CO)(2)L(mu-CH2)(dppm)(2)] [CF3SO3] (L = CO, PR3, (CNBu)-Bu-t) or [Ir2H(CO)(2)(mu-CH2)(mu-SO2)(dppm)(2)] [CF3SO3] (11). Both the carbonyl and SO2 adducts are fluxional, having the hydrogens of the hydride ligand and the methylene group exchanging rapidly at ambient temperature. The acti vation parameters for this reversible C-H bond-making and -breaking step ha ve been determined (Delta H double dagger = 10.3 kcal/mol? Delta S double d agger = -11.2 cal/mol K (CO); Delta H double dagger = 6.1 kcal/mol, Delta S double dagger: = -6.2 cal/mol K (SO2)). X-ray structure determinations of compound 3, [Ir2H(CO)(2)(PMe3)(mu-CH2)(dppm)(2)][CF3SO3] (6), and compound 11 have been determined to confirm the proposed structures. Density functio nal theory calculations have been carried out on cations related to 3 and 1 1 by substitution of the phenyl substituents on the dppm ligands by hydroge ns, and on key isomers of these, to gain an understanding of the factors pr omoting C-H bond cleavage in this system. A proposal is presented rationali zing the facile C-H bond cleavage in 3 upon addition of the substrate molec ules, in which the roles of the adjacent metals are described.