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- i Pr2PC6H4)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- i Pr2PC6H4)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- i Pr2PC6H4)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.

Selectivity Reverse of Hydrosilylation of Aryl Alkenes Realized by Pyridine N-Oxide with [PSiP] Pincer Cobalt(III) Hydride as Catalyst

“…24 The selectivity of this catalytic system is opposite to that of our early catalytic system. 20 We speculate that the main reason may be the existence of trimethylphosphine dissociated from the catalyst in the previous catalytic system. Free trimethylphosphine may play a role for the selectivity.…”

“…A is the real catalyst for this catalytic system, and this is similar as our early work. 20 Although the experiment to isolate A failed because of the instability of the tetra-coordinate Co(I) species, its existence has been proved indirectly by the isolation of the penta-coordinate Co(I) complex 13 from the stoichiometric reaction between complex 3 and Ph 2 SiH 2 in the presence of 1 equiv of trimethylphosphine as a supporting ligand (Scheme 6). Complex 13 was reported in our recent work.…”

“…Complex 13 was reported in our recent work. 20 A interacts with diphenylsilane to form B having an η 2 -(Si−H) bond (semi-activated). 21 The π-coordination of the olefin to B gives rise to C. After 1,2-insertion, C is converted to disilyl alkyl complex D. D undergoes reductive elimination to deliver anti-Markovnikov silane with the regeneration of A.…”

“…17 We found that [PSiP] pincer metal complexes could catalyze hydrosilylation of carbonyl compounds, 18 dehydration of amides to nitriles, 19 and alkene hydrosilylation as well as nitrogen silylation. 20 As a continuation of our work in the direction of cobaltcatalyzed selective hydrosilylation of alkenes, in this paper, six [PSiP] pincer cobalt(III) hydrides with different substituents at the Si or P atom were prepared (Scheme 1). It was confirmed that compound 3 is the best catalyst for alkene hydrosilylation having anti-Markovnikov selectivity.…”

“…Turculet carried out the study on [PSiP] pincer metals complexes in 2007 . We found that [PSiP] pincer metal complexes could catalyze hydrosilylation of carbonyl compounds, dehydration of amides to nitriles, and alkene hydrosilylation as well as nitrogen silylation …”