New Routes to Transition Metal-Carbido Species: Synthesis and Characterization of the Carbon-Centered Trigonal Prismatic Clusters [W₆CCl₁₈]<i>ⁿ</i>^- (<i>n</i> = 1, 2, 3)

Abstract

Simultaneous reduction of WCl6 and CCl4 with bismuth metal at 400 degrees C affords a black solid, from which the new cluster [W6CCl18]2- is extracted into aqueous HCl. The cluster is isolated in 34% yield as (Bu4N)2[W6CCl18] and features a metal-metal bonded W6 trigonal prism centered by a carbon atom and surrounded by 12 edge-bridging and 6 terminal chloride ligands. A cyclic voltammogram of [W6CCl18]2- in DMF shows the cluster undergoes two reduction and two oxidation processes, suggesting five chemically accessible redox states. Consistent with this extensive electrochemistry, DFT calculations on the diamagnetic [W6CCl18]2- species reveal closely spaced frontier orbitals, with an a2' ' HOMO situated 0.61 and 0.71 eV below unoccupied e' ' and e' orbitals, respectively. Oxidation of the cluster by a single equivalent of NO+ gives [W6CCl18]1-, which, as expected on the basis of the [W6CCl18]2- HOMO character, possesses a less elongated W6 trigonal prism. Reduction of [W6CCl18]2- with a single equivalent of cobaltocene affords [W6CCl18]3-, wherein population of a low-lying e' orbital leads to a significant Jahn-Teller distortion.