Metal-carbon multiple bonds: Half-sandwich phenylcarbyne complexes of chromium - Synthesis, structure, electrochemistry and reactions with PMe3

Article abstract of DOI:10.1016/S0022-328X(97)00084-3

The chromium phenylcarbyne complexes (η⁵-C₅R₅)(CO)₂Cr≡CPh (2a: R = H; 2b: R = Me) have been prepared and their structure, cyclic voltammetry and reactions with PMe₃ are reported. The compounds 2a and 2b are obtained as red and purple thermolabile solids respectively after treatment of cis-Br(CO)₂(pic)₂Cr≡CPh (pic = 4-methylpyridine) (1) with NaCp (Cp = C₅H₅) and KCp* (Cp* = C₅Me₅) respectively. Complex 2a adds PMe₃ at the carbyne-carbon atom to give the green α-phosphoniocarbene complex Cp(CO)₂Cr=C(PMe₃)Ph (3), whereas 2b undergoes with PMe₃ a carbyne-carbonyl coupling reaction to afford the green η² ketenyl complex Cp*(CO)(PMe₃)Cr[C(Ph)CO] (4). Cyclic voltammetry studies show the phenylcarbyne complexes 2a and 2b and the carbene complex 3 to undergo a reversible one-electron oxidation in CH₂Cl₂ at E_1/2 = 0.39V, E_1/2 = 0.20V and E_1/2 = -0.52V respectively (potentials vs. the ferrocene/ferrocenium redox couple). In comparison, the ketenyl complex 4 is electrochemically inactive in the potential range of - 1.5 to 1.0 V. The crystal structures of 2a and 3 are reported.