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

Variable-temperature NMR studies
of the compound Os3(CO)10(μ-η1-C6H5)­(μ-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, ΔH⧧ = 73.33(42) kJ/mol and ΔS⧧ = −2.66(1.25) J/(K mol), were determined
by analysis of variable-temperature 1H 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
Os3(CO)10(μ-η1-Py)­(μ-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: ΔH⧧ = 70.93(61) kJ/mol and ΔS⧧ = −6.98(1.83) J/(K mol).