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Two new bisphosphine [PCP] pincer cobalt(III) hydrides, [(L1)Co(PMe3)(H)(Cl)] (L11, L1= 2,6-((Ph2P)(Et)N)2C6H3) and [(L2)Co(PMe3)(H)(Cl)] (L21, L2= 2,6-((iPr2P)(Et)N)2C6H3), as well as one new bissilylene [SiCSi] pincer cobalt(III) hydride, [(L3)Co(PMe3)(H)(Cl)] (L31, L3= 1,3-((PhC(tBuN)2Si)(Et)N)2C6H3), were synthesized by reaction of the corresponding protic [PCP] or [SiCSi] pincer ligands L1H, L2H,and L3Hwith CoCl(PMe3)3. Despite the similarities in the ligand scaffolds, the three cobalt(III) hydrides show remarkably different performance as catalysts in alkene hydrosilylation. Among the PCP pincer complexes, L11has higher catalytic activity than complex L21, and both catalysts afford anti-Markovnikov selectivity for both aliphatic and aromatic alkenes. In contrast, the catalytic activity for alkene hydrosilylation of silylene complex L31is comparable to phosphine complex L11, but a dependence of regioselectivity on the substrates was observed: While aliphatic alkenes are converted in an anti-Markovnikov fashion, the hydrosilylation of aromatic alkenes affords Markovnikov products. The substrate scope was explored with 28 examples. Additional experiments were conducted to elucidate these mechanisms of hydrosilylation. The synthesis of cobalt(I) complex (L1)Co(PMe3)2(L17) and its catalytic properties for alkene hydrosilylation allowed for the proposal of the mechanistic variations that occur in dependence of reaction conditions and substrates.

Bidentate ligand 2-diphenylphosphinobenzaldehyde or 2-diisopropylphosphinobenzaldehyde reacted with CoCl(PMe3)3to give [P,C]-chelate cobalt(III) hydrides [mer-(Me3P)3Co(H)(Cl)(o-Ph2P–C6H4–C═O)] (1) or [mer-(Me3P)3Co(H)(Cl)(o-iPr2P–C6H4–C═O)] (2), respectively. Complex 2was new and characterized by spectroscopic methods and single-crystal X-ray diffraction analysis. It was found that both complex 1and 2are active catalysts for hydrosilylation of alkenes. Although the catalytic activity of 1is slightly higher, catalyst 1and 2have the same selectivity. The selectivity for aromatic alkenes is mainly of the Markovnikov type, while the selectivity for aliphatic alkenes is almost 100% anti-Markovnikov type. In the study of the reaction mechanism, a silyl cobalt dihydride [(Ph2ClSi)Co(H)2(PMe3)3] was isolated from the stoichiometric reaction of hydride 1with Ph2SiH2. The catalytic mechanism for alkene hydrosilylation with [HCo(PMe3)3] as a real catalyst is proposed and discussed with the experimental results.

Six silyl cobalt(III) hydrides 1–6with [PSiP] pincer ligands having different substituents at the P and Si atoms ([(2-Ph2PC6H4)2MeSiCo(H)(Cl)(PMe3)] (1), [(2-Ph2PC6H4)2HSiCo(H)(Cl)(PMe3)] (2), [(2-Ph2PC6H4)2PhSiCo(H)(Cl)(PMe3)] (3), [(2-iPr2PC6H4)2HSiCo(H)(Cl)(PMe3)] (4), [(2-iPr2PC6H4)2MeSiCo(H)(Cl)(PMe3)] (5), and [(2-iPr2PC6H4)2PhSiCo(H)(Cl)(PMe3)] (6)) were synthesized through the reactions of the ligands (L1–L6) with CoCl(PMe3)3via Si–H bond cleavage. Compounds 1–6have catalytic activity for alkene hydrosilylation, and among them, complex 3is the best catalyst with excellent anti-Markovnikov regioselectivity. A silyl dihydrido cobalt(III) complex 7from the reaction of 3with Ph2SiH2was isolated, and its catalytic activity is equivalent to that of complex 3. Complex 7and its derivatives 10–12could also be obtained through the reactions of complexes 3, 1, 4, and 5with NaBHEt3. The molecular structure of 7was 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 3–6and 10–12were determined by single crystal X-ray diffraction analysis.

A solvent-free cobalt-catalyzed highly selective hydrosilylation of alkenes has been developed. It was found that both Co(PMe3)4and CoCl(PMe3)3are highly active catalysts for hydrosilylation of alkenes. The former promoted Markovnikov-type hydrosilylation of the aryl alkenes, while the latter catalyzed anti-Markovnikov-type hydrosilylation of the alkyl alkenes. These two catalytic systems tolerate a variety of functional groups and provide high selectivity and medium to high yield. In the exploration of the reaction mechanism, a dinuclear silyl cobalt(I) complex [(PMe3)2Co(μ–η2-HSiPh2)2Co(PMe3)2] (4) from the Co(PMe3)4system and a silyl cobalt dihydride [(PMe3)3Co(H)2SiClPh2] (5) from the CoCl(PMe3)3system were obtained. It is proposed that the silyl cobalt(I) intermediate, [Co(PMe3)3(SiHPh2)], is the real catalyst for the Co(PMe3)4system, while the hydrido cobalt(I) intermediate, [HCo(PMe3)3], is the real catalyst for the CoCl(PMe3)3system. Complexes 4and 5were characterized by spectroscopic methods and single-crystal X-ray diffraction.

The reactions of unsymmetrical N-heterocyclic carbene (NHC) [CNC]-pincer preligands with CoMe(PMe3)4gave 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 4aand 4bvia a disproportionation reaction, respectively. It was found that both 3aand 3bare efficient catalysts for hydrosilylation of alkenes. With aryl alkenes as substrates, 3ahas high Markovnikov selectivity in excellent yields, while 3ais 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 4bwere characterized by spectroscopic methods, and the molecular structures of 3b, 4a, and 4bwere determined by single crystal X-ray diffraction.

An air-stable N-heterocyclic PSiP pincer iron hydride FeH(PMe3)2(SiPh(NCH2PPh2)2C6H4) (4) was synthesized by Si–H activation of a Ph-substituted [PSiP] pincer ligand. The analogous strong electron-donating iPr-substituted [PSiP] pincer ligand was prepared and introduced into iron complex to give an iron nitrogen complex FeH(N2)(PMe3)(SiPh(NCH2PiPr2)2C6H4) (6). Both 4and 6showed similar high efficiency for catalytic dehydration of primary amides to nitriles. Air-stable iron hydride 4was the best catalyst for its stabilization and convenient preparation. A diverse range of cyano compounds including aromatic and aliphatic species was obtained in moderate to excellent yields. A plausible catalytic reaction mechanism was proposed.

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)4via 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 L1and L2with 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 L3with 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 4is an effective catalyst for catalytic transformation of dinitrogen into silylamine. Among these four silyl cobalt(I) complexes, complex 1is the best catalyst for hydrosilylation of alkenes with excellent regioselectivity. For aromatic alkenes, catalyst 1provided 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 1–4were confirmed by single-crystal X-ray diffraction.

The two nickel chlorides 1and 2with [P,S] and [P,Se] bidentate ligands, respectively, were synthesized and used as catalysts for Sonogashira coupling reaction. Both 1and 2are efficient catalysts for Sonogashira C(sp3)–C(sp) coupling reactions. Complex 1has better catalytic activity than complex 2. In a combination of 1 mol % catalyst loading of 1and CuI/Cs2CO3/DMSO at 25 °C for the coupling reactions of alkyl iodides with terminal alkynes, the corresponding coupling products were obtained in moderate to excellent yields. This catalytic system is also suitable for alkyl bromides at 40 °C. It must be pointed out that with NaI as an additive complex 1could effectively catalyze the C(sp3)–C(sp) coupling of nonactivated alkyl chlorides with alkynes under mild conditions (50 °C) with low catalyst loading (1 mol %). Complex 1is easy to synthesize and has efficient catalytic activity, especially for alkyl chlorides, under mild conditions.