Addition of some gold electrophiles to an octa-osmium carbonyl cluster: Thermodynamic vs. kinetic control

The reaction of the dianionic cluster [PPN]₂[Os₈(CO)₂₂] {[PPN]⁺=[N(PPh₃)₂]⁺} with Au₂Cl₂L {L=chelating phosphines bis-(diphenylphosphino)methane (dppm), 1,2-bis-(diphenylphosphino)ethane (dppe) or 1,4-bis-(diphenylphosphino)butane (dppb)} in the presence of an excess of the halide acceptor TlPF₆, affords the neutral digold-substituted clusters [Os₈(CO)₂₂ (Au₂L)] {(1) L = dppm; (2) L = dppe; (3) L = dppb} in almost quantitative yield. The ³¹ P{1H} NMR spectra of these compounds indicate that the gold atoms are in non-equivalent environments and therefore cannot have a metal geometry similar to that previously reported for the related cluster [Os₈(CO)₂₂(AuPPh₃)₂]1 (4). A reinvestigation of the reaction between [PPN]₂ [Os₈(CO)₂₂] and monogold fragment 'AuPPh₃⁺' has shown that, in addition to (4), a second isomer of Os₈(CO)₂₂(AuPPh₃)₂] (5) is formed, which on the basis of the spectroscopic data is thought to have the same metal geometry as compounds (1-3). A single crystal X-ray diffraction study of the compound [Os₈(CO)₂₂(Au₂dppb)] (3) has shown that the metal core consists of a bicapped octahedron of osmium atoms, with one of the gold atoms capping the osmium octahedron, and the other gold atom bridging an edge of the osmium core.