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.

The binuclear complex [Ir2(CH3)(CO)(μ-CO)(dppm)2][CF3SO3] (1; dppm = μ-Ph2PCH2PPh2) reacts with allene and methylallene to ultimately yield the vinylcarbene products [Ir2H(CO)2(μ-η1:η3-HCC(CH3)C(H)R)(dppm)2][CF3SO3] (R = H (6), CH3 (7)). Monitoring the reactions by NMR spectroscopy (1H, 13C, 31P) between −78 °C and ambient temperature allows the observation of several intermediates in each of these transformations in which the allene moves from an η2 binding site on one metal, through an η1:η1-bridging geometry in which the cumulene is coordinated through the “H2CC” moiety, to an η1:η3-bridging geometry in which the central carbon of the cumulene is σ-bound to one metal, adjacent to the methyl ligand, while the three cumulene carbons are η3-bound to the adjacent metal. We propose that formation of the respective vinyl carbene products results from migration of the methyl ligand to the central cumulene carbon followed by activation of a cumulene C−H bond. 1,1-Dimethylallene reacts with 1 at −78 °C to yield a methylene hydride product containing an η2-bound cumulene on one metal, much as observed for the first products in the allene and methylallene reactions. Upon warming, this intermediate isomerizes to the final product containing a methyl ligand on one metal and an η2-bound cumulene on the other. No cumulene-bridged products are observed with this disubstituted allene. 1,1-Difluoroallene also yields a methylene hydride product at −78 °C, which is analogous to the first species observed in all cases noted above. In this case, warming results in movement of the cumulene to an η1:η1-bridging position in which this group binds to the metals via the “H2CC” moiety. Unlike the transformations observed with allene and methyl allene, difluoroallene undergoes no additional transformations as the temperature is raised. A rationalization of these transformations is presented together with a perspective on how the cumulene ligand moves over the dimetallic framework leading to the final products.

The reactions of the diiridium methyl complex [Ir 2 (CH 3 )(CO)(µ-CO)(dppm) 2 ][CF 3 SO 3 ] (1) with ethylene, fluoroethylene, Z-1,2-difluoroethylene, 1,1-difluoroethylene, trifluoroethylene, and tetrafluoroethylene have been investigated. Reaction of 1 with ethylene at -78 °C yields [Ir 2 H(η 2 -C 2 H 4 )(CO) 2 (µ-CH 2 )(dppm) 2 ][CF 3 SO 3 ] (2a), resulting from C-H activation of the methyl group induced by ethylene coordination, whereas reaction at higher temperatures yields the simple ethylene adduct [Ir 2 (CH 3 Reactions of 1 with fluoroethylene and Z-1,2-difluoroethylene yield only the olefin adducts analogous to 2b. At -78 °C reaction with 1,1-difluoroethylene yields the methylene-bridged hydride product [Ir 2 H(η 2 -C 2 F 2 H 2 )(CO) 2 (µ-CH 2 )(dppm) 2 ][CF 3 SO 3 ] (5a), which upon warming, yields first the olefin adduct [Ir 2 (CH 3 ). Trifluoro-and tetrafluoroethylene yield only the olefin-bridged products [Ir 2 7)). The structure of the tetrafluoroethylene-bridged, tricarbonyl species [Ir 2 (CH 3 )(CO) 3 (µ-C 2 F 4 )(dppm) 2 ][CF 3 SO 3 ] (8), determined by X-ray techniques, is reported.

No abstract