Dynamic Rotation of Bridging Aryl Ligands in Unsaturated Metal Carbonyl Cluster Complexes

Variable-temperature NMR studies of the compound Os-3(CO)(10)(mu-eta(1)-C6H5)(mu-AuPPh3) (1) have revealed a dynamic process of hindered rotation of the bridging phenyl ligand about the metal metal bond. The activation parameters for the process, Delta H-double dagger = 73.33(42) kJ/mol and Delta S-double dagger = -2.66(1.25) J/(K mol), were determined by analysis of variable-temperature H-1 NMR spectra. A density functional theory analysis has provided a mechanism that involves a shift of the ligand out of the bridging position with the formation of an agostic interaction of one of the ortho-positioned CH bonds of the phenyl ring to the neighboring metal atom. The related compound Os-3(CO)(10)(mu-eta(1)-Py)(mu-AuPPh3) (2; Py = 2-C15H9) was synthesized and was found to exhibit a similar rotation of the bridging pyrenyl ligand about the metal-metal bond: Delta H-double dagger = 70.93(61) kJ/mol and Delta S-double dagger = -6.98(1.83) J/(K mol).