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.

Six silyl cobalt(III) hydrides 1−6 with [PSiP] pincer ligands having different substituents at the P and Si atoms 5), and [(2-i Pr 2 PC 6 H 4 ) 2 PhSiCo(H)-(Cl)(PMe 3 )] ( 6)) were synthesized through the reactions of the ligands (L1−L6) with CoCl(PMe 3 ) 3 via Si−H bond cleavage. Compounds 1−6 have catalytic activity for alkene hydrosilylation, and among them, complex 3 is the best catalyst with excellent anti-Markovnikov regioselectivity. A silyl dihydrido cobalt(III) complex 7 from the reaction of 3 with Ph 2 SiH 2 was isolated, and its catalytic activity is equivalent to that of complex 3. Complex 7 and its derivatives 10−12 could also be obtained through the reactions of complexes 3, 1, 4, and 5 with NaBHEt 3 . The molecular structure of 7 was indirectly verified by the structures of 10−12. To our delight, the addition of pyridine N-oxide reversed the selectivity of the reaction, from anti-Markovnikov to Markovnikov addition. At the same time, the reaction temperature was reduced from 70 to 30 °C on the premise of high yield and excellent selectivity. However, this catalytic system is only applicable to aromatic alkenes. On the basis of the experimental information, two reaction mechanisms are proposed. The molecular structures of cobalt(III) complexes 3−6 and 10−12 were determined by single crystal X-ray diffraction analysis.

The reactions of unsymmetrical N-heterocyclic carbene (NHC) [CNC]-pincer preligands with CoMe­(PMe3)4 gave rise to NHC [CNC]-pincer cobalt­(III) hydrides, [(CcarbeneNaminoCnaphthyl)­Co­(H)­(PMe3)2] (3a) and (3b), via Csp2–H activation and the unexpected trans-bischelate [Ccarbene, Namino] cobalt­(II) complexes 4a and 4b via a disproportionation reaction, respectively. It was found that both 3a and 3b are efficient catalysts for hydrosilylation of alkenes. With aryl alkenes as substrates, 3a has high Markovnikov selectivity in excellent yields, while 3a is an efficient anti-Markovnikov catalyst in good yields with alkyl alkenes as substrates. The catalytic process could be promoted with pyridine N-oxide as an initiator. The catalytic mechanisms for the two different selectivities were proposed. Complexes 3a, 3b, 4a, and 4b were characterized by spectroscopic methods, and the molecular structures of 3b, 4a, and 4b were determined by single crystal X-ray diffraction.

[P,Si] chelate hydrido iron dinitrogen complex 3 obtained from reaction of preligand L3 with Fe(PMe3)4 reacted with dihydrogen to deliver iron dihydride 7. Complex 3 is an efficient catalyst for silylation of dinitrogen with KC8 as a reductant.

The hydrido silyl iron complex (o-PhPCHSiMe)Fe(PMe)H (2) was obtained via the activation of the Si-H bond of the bidentate silyl ligand o-PhP(CH)SiMeH (1) by Fe(PMe). 2 showed good to excellent catalytic activity in both the reduction of aldehydes/ketones and the dehydration of benzamide. In addition, with complex 2 as a catalyst, α,β-unsaturated carbonyls could be selectively reduced to the corresponding α,β-unsaturated alcohols. The mechanisms of the formation of 2 and the catalytic dehydration process are proposed and partly experimentally verified.