Silane Activation with Cobalt on the POCOP Pincer Ligand Platform
Yingze Li, Jeanette A. Krause, Hairong Guan
Abstract
Cobalt POCOP pincer complexes {κP,κC,κP-2,6-(iPr2PO)2C6H3}Co(PMe3)2 (1) and {κP,κC,κP-2,6-(iPr2PO)2-4-NMe2-C6H2}Co(PMe3)2 (2) have been synthesized via C–H bond activation of the pincer ligands with HCo(PMe3)4. Silanes such as PhSiH3, Ph2SiH2, and (EtO)3SiH can undergo Si–H oxidative addition with these Co(I) complexes, though reversibly. One of the silane activation products, namely, {κP,κC,κP-2,6-(iPr2PO)2C6H3}Co(H)(SiH2Ph)(PMe3) (3), has been isolated and shown to eliminate PhSiH3 upon evaporation to form {κP,κC,κP-2,6-(iPr2PO)2C6H3}Co(PMe3) (4). Under heating, redistribution of PhSiH3 in 3 can take place, resulting in {κP,κC,κP-2,6-(iPr2PO)2C6H3}Co(H)(SiH3)(PMe3) (5) and Ph2SiH2. Complexes 1–3 have been established as catalysts for the hydrosilylation of aldehydes bearing various functional groups. According to the mechanistic studies, the silyl hydride species exists in the catalytic cycle, whereas the bis(trimethylphosphine) species sits outside the catalytic cycle. Dissociation of PMe3 is required prior to aldehyde insertion into the silyl hydride species, which is the turnover-limiting step of the catalytic cycle. Consequently, 3 outperforms 1 in catalyzing the hydrosilylation reaction due to the presence of only one PMe3 ligand. The structures of 1–4 have been studied by X-ray crystallography.