The Effect of Substituents on the Formation of Silyl [PSiP] Pincer Cobalt(I) Complexes and Catalytic Application in Both Nitrogen Silylation and Alkene Hydrosilylation.

Abstract

Four different [PSiP]-pincer ligands L1-L4 ((2-Ph2PC6H4)2SiHR (R = H (L1) and Ph (L2)) and (2-iPr2PC6H4)2SiHR' (R' = Ph (L3) and H (L4)) were used to investigate the effect of substituents at P and/or Si atom of the [PSiP] pincer ligands on the formation of silyl cobalt(I) complexes by the reactions with CoMe(PMe3)4 via Si-H cleavage. Two penta-coordinated silyl cobalt(I) complexes, (2-Ph2PC6H4)2HSiCo(PMe3)2 (1) and (2-Ph2PC6H4)2PhSiCo(PMe3)2 (2), were obtained from the reactions of L1 and L2 with CoMe(PMe3)4, respectively. Under similar reaction conditions, a tetra-coordinated cobalt(I) complex (2-iPr2PC6H4)2PhSiCo(PMe3) (3) was isolated from the interaction of L3 with CoMe(PMe3)4. It was found that, only in the case of ligand L4, silyl dinitrogen cobalt(I) complex 4, [(2-iPr2PC6H4)2HSiCo(N2)(PMe3)], was formed. Our results indicate that the increasing of electron cloud density at the Co center is beneficial for the formation of a dinitrogen cobalt complex because the large electron density at Co center leads to the enhancement of the π-backbonding from cobalt to the coordinated N2. It was found that silyl dinitrogen cobalt(I) complex 4 is an effective catalyst for catalytic transformation of dinitrogen into silylamine. Among these four silyl cobalt(I) complexes, complex 1 is the best catalyst for hydrosilylation of alkenes with excellent regioselectivity. For aromatic alkenes, catalyst 1 provided Markovnikov products, while for aliphatic alkenes, anti-Markovnikov products could be obtained. Both catalytic reaction mechanisms were proposed and discussed. The molecular structures of complexes 1-4 were confirmed by single-crystal X-ray diffraction.