Computational study of fluxional hydride bridged binuclear transition metal complexes: Effect of secondary bridging ligands

The binuclear complex [Ir2(CH3)(CO)2(dppm)2]+ (dppm = Ph2PCH2PPh2) coordinates the olefins of 1,3-butadiene and catalyzes double geminal C–H activation via a proposed fluxional hydride migration. Using DFT computational studies, we examine the fluxional behavior of three model hydride bridged bimetallic systems to elucidate the main factors in this transformation. Our results indicate that the bridging ligand opposite the μ-H controls the barrier to hydride fluxionality and that breaking this bridging interaction is the largest component of the transition state barrier. We found a low barrier, a medium barrier, and a high barrier in systems with no bridging ligand, a partial μ-CO, and μ-CH2, respectively. The respective systems with their corresponding barriers are: (1) [RhRe(μ-H)(CO)4(dhpm)2]+, 2.4 kcal/mol, (2) [IrRu(μ-H)(μ-CO)(CO)3(dhpm)2]2+, 8.5 kcal/mol, and (3) [RhOs(μ-H)(μ-CH2)(CO)3PH3(dhpm)2]2+, 26.4 kcal/mol (dhpm = PH2CH2PH2). The predicted fluxional hydride migration in the activation of 1,3-butadiene occurs easily and is consistent with these findings.