The systematic synthesis of Heptanuclear Osmium clusters

Heptanuclear osmium clusters, Os7(CO)20(L) (L=CNBut, PMe3, PEt3, P(OMe)3, P(OCH2)3CMe, PPh3), have been systematically synthesized by reacting Os6(CO)18 with Me3NO and Os(CO)4(L) at -78 °C; as the cone angle of the ligand increased, the yields decreased. Crystal structures of Os7(CO)20(L) (L=CNBut, PMe3, P(OCH2)3CMe) revealed a monocapped octahedral arrangement of metal atoms, with the ligand attached to the capping osmium group. As the ligand becomes a better ð–acceptor, the Os–Os bond trans to the ligand shortens, while an increase in the steric properties lengthened the two cis–Os–Os bonds. Carbonyl exchange in the Os7(CO)20(L) clusters were investigated by variable temperature 13C{1H} NMR spectroscopy. Room temperature spectra were consistent with rotation about the individual Os(CO)2(L) (L=CO, CNBut, PR3) units. At low temperatures, collapse of the resonances at different temperatures indicates slowed rotation of the Os(CO)3 groups of the octahedral core, while the capping osmium group continues to exchange rapidly on the NMR timescale.