Reactivity of 17-, 18-, and 19-Electron Cationic Complexes Generated by the Electrochemical Oxidation of Tricarbonyl(mesitylene)tungsten

Article abstract of DOI:10.1021/ja00011a002

The electrochemical oxidation of (mesitylene)W(CO)₃ (W) in MeCN produces the 17-electron complex W⁺, which reacts very rapidly with solvent (S) or tri-n-butyl phosphite (P) to give 19-electron species (WS⁺, WP⁺) that undergo spontaneous further oxidation to the 18-electron analogues (WS²⁺, WP²⁺). The identities of WS²⁺ and WP²⁺ were established by voltammetric, IR spectroelectrochemical, and NMR experiments. Although the 17-electron ? 19-electron transformation is not directly observable, digital simulation techniques allowed selection of a probable mechanism and semiquantitative determination of the rate and equilibrium parameters describing the interconversion of the 17-, 18-, and 19-electron species: W⁺ + S ? WS⁺, k ? 10⁵ M^-1 s^-1, K_eq ? 10^-1 M^-1; W⁺ + P ? WP⁺, k ? 10⁷ M^-1 s^-1, K_eq ? 3 × 10³ M^-1 at 298 K. The related 18-electron complex WS²⁺ is quite reactive, but orders of magnitude less so than W⁺ and WS⁺. Experiments with (mesitylene)Cr(CO)₃ (Cr) suggest that associative attack by MeCN at the 17-electron Cr⁺ is 10⁴ times slower than attack at the W⁺ analogue. This study illustrates the power of digital simulation techniques for interpreting complex mechanistic schemes and characterizing important but unobservable reaction intermediates. Electrochemical oxidation of (arene)W(CO)₃ occurs without loss of arene or CO ligands, suggesting that the electroactivation of these complexes may have useful synthetic applications; this contrasts sharply with (arene)Cr(CO)₃ analogues, which decompose with loss of arene and CO ligands upon oxidation in MeCN.