Cobalt POCOP Pincer Complexes via Ligand C–H Bond Activation with Co2(CO)8: Catalytic Activity for Hydrosilylation of Aldehydes in an Open vs a Closed System

A series of cobalt POCOP pincer complexes with the formulas {2,6-(iPr2PO)2-4-R′-C6H2}­Co­(CO)2 (R′ = H (1a), NMe2 (1b), OMe (1c), CO2Me (1d)), {2,6-(Ph2PO)2C6H3}­Co­(CO)2 (1e), and {2,6-(tBu2PO)2C6H3}­Co­(CO) (2f) have been synthesized through C–H bond activation of the corresponding pincer ligands with Co2(CO)8. These complexes have been demonstrated to catalyze the hydrosilylation of PhCHO with (EtO)3SiH, which exhibits an induction period and the decreasing reactivity order 1b > 1c > 1a > 1d > 1e. The catalytic protocol can be applied to various aldehydes with turnover numbers of up to 300. The CO ligands in the dicarbonyl complexes have been shown to exchange with 13CO at room temperature and partially dissociate from cobalt at high temperatures. Substitution of CO by tert-butyl isocyanide has been accomplished with 1a at 50–80 °C, resulting in the formation of {2,6-(iPr2PO)2C6H3}­Co­(CNtBu)­(CO) (3a) and {2,6-(iPr2PO)2C6H3}­Co­(CNtBu)2 (4a). The catalytic reactions are more efficient when they are carried out in an open system or if the catalysts are preactivated by the aldehydes. The structures of 1a–e, 3a, and 4a have been studied by X-ray crystallography.