High nuclearity ruthenium carbonyl cluster chemistry -: VII.: Synthesis, NMR studies, electrochemistry and X-ray crystal structure of [PPN] [Ru8(μ8-P)(CO)22]2

The reaction between [Ru-3(mu-H)(mu-NC5H4)(CO)(10)] and chloridiphenylphosphine in refluxing chlorobenzene, followed by metathesis with bis(triphenylphosphoranylidene)ammonium chloride ([PPN]Cl), affords [PPN][Ru-8(mu(8)-P)(CO)(22)] (1) in around 30% yield. P-31-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 P-31-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. (C) 1998 Elsevier Science S.A. All rights reserved.