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

The chromium phenylcarbyne complexes (eta(5)-C5R5)(CO)(2)Cr=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)(2)Cr=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 alpha-phosphoniocarbene complex Cp(CO)(2)Cr=C(PMe3)Ph (3), whereas 2b undergoes with PMe3 a carbyne-carbonyl coupling reaction to afford the green eta(2) 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=0.39 V, E-1/2=0.20 V and E-1/2=-0.52 V 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. (C) 1997 Elsevier Science S.A.