High nuclearity ruthenium carbonyl cluster chemistry VII. Synthesis, NMR studies, electrochemistry and X-ray crystal structure of [PPN] [Ru₈(μ₈-P)(CO)₂₂]

The reaction between [Ru₃(μ-H)(μ-NC₅H₄)(CO)₁₀] and chlorodiphenylphosphine in refluxing chlorobenzene, followed by metathesis with bis(triphenylphosphoranylidene)ammonium chloride ([PPN]Cl), affords [PPN][Ru₈(μ₈-P)(CO)₂₂] (1) in around 30% yield. ³¹P-NMR solution spectra are consistent with the presence of at least two isomers of the cluster anion, presumably due to differing carbonyl distributions; the chemical shifts for these configurations (600-800 ppm downfield of H₃PO₄) are consistent with a highly deshielded interstitial phosphorus atom. An X-ray structural study of one isomer of 1 reveals that the phosphorus atom occupies an interstitial square antiprismatic site defined by the eight ruthenium atoms, with two bridging carbonyl ligands on opposite faces spanning interplanar Ru-Ru bonds, and twenty terminal carbonyl ligands completing the ligand set. The solid state ³¹P-NMR spectrum of the crystallographically-identified isomer reveals a signal at 596.1 ppm assigned to the square antiprismatic interstitial phosphorus atom. Formal electron counting suggests that 1 has four electrons less than expected using Wade's rules. The reductive electrochemistry of 1 has been examined by cyclic voltammetry, and reveals the presence of two one-electron and one two-electron reduction waves, an uptake of four electrons in total, consistent with the cluster's theoretical electron deficiency in the resting state.