Control over Selectivity in Alkene Hydrosilylation Catalyzed by Cobalt(III) Hydride Complexes.

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 L3H with 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, L11 has 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 L31 is 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.

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

Six silyl cobalt(III) hydrides 1-6 with [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)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 Ph2SiH2 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 NaBHEt3. 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 Effect of Substituents on the Formation of Silyl [PSiP] Pincer Cobalt(I) Complexes and Catalytic Application in Both Nitrogen Silylation and Alkene Hydrosilylation.

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.

Synthesis of silyl iron dinitrogen complexes for activation of dihydrogen and catalytic silylation of dinitrogen.

Three novel iron dinitrogen hydrides, [FeH(iPr-PSiMeP)(N2)(PMe3)] (1), [FeH(iPr-PSiPhP)(N2)(PMe3)] (2), and [FeH(iPr-PSiPh)(N2)(PMe3)] (3), supported by a silyl ligand are synthesized for the first time by changing the electronic effect and steric hindrance of the ligands through the reaction of ligands L1-L3 with Fe(PMe3)4 in a nitrogen atmosphere. The ligands containing an electron-donating group with large steric hindrance on the phosphorus atom are beneficial for the formation of dinitrogen complexes. A penta-coordinate iron hydride [FeH(iPr-PSiPh)(PMe3)2] (4) was formed through the reaction of ligand L3 with Fe(PMe3)4 in an argon atmosphere under the same conditions. The reactions between complexes 1-3 with an atmospheric pressure of dihydrogen gas resulted in Fe(II) dihydrides, [(iPr-PSiMe(µ-H)P)Fe(H)2(PMe3)] (5), [(iPr-PSiPh(µ-H)P)Fe(H)2(PMe3)] (6) and [(iPr-PSiPh(µ-H))Fe(H)2(PMe3)2] (7), with an η2-(Si-H) coordination. The isolation of dihydrides 5-7 demonstrates the ability of the dinitrogen complexes 1-3 to realize the activation of dihydrogen under ambient temperature and pressure. The molecular structures of complexes 1-7 were elucidated by single crystal X-ray diffraction analysis. The iron dinitrogen hydrides 1-3 are effective catalysts for the silylation of dinitrogen under ambient conditions and among them 3 is the best catalyst.

Correction: Pyridine N-oxide promoted hydrosilylation of carbonyl compounds catalyzed by [PSiP]-pincer iron hydrides.

Correction for 'Pyridine N-oxide promoted hydrosilylation of carbonyl compounds catalyzed by [PSiP]-pincer iron hydrides' by Guoliang Chang et al., Dalton Trans., 2020, 49, 9349-9354, DOI: 10.1039/D0DT00392A.

Pyridine N-oxide promoted hydrosilylation of carbonyl compounds catalyzed by [PSiP]-pincer iron hydrides.

Five [PSiP]-pincer iron hydrides 1-5, [(2-Ph2PC6H4)2HSiFe(H)(PMe3)2 (1), (2-Ph2PC6H4)2MeSiFe(H)(PMe3)2 (2), (2-Ph2PC6H4)2PhSiFe(H)(PMe3)2 (3), (2-(iPr)2PC6H4)2HSiFe(H)(PMe3) (4), and (2-(iPr)2PC6H4)2MeSiFe(H)(PMe3)2 (5)], were used as catalysts to study the effects of pyridine N-oxide and the electronic properties of [PSiP]-ligands on the catalytic hydrosilylation of carbonyl compounds. It was proved for the first time that this catalytic process could be promoted with pyridine N-oxide as the initiator at 30 °C because the addition of pyridine N-oxide is beneficial for the formation of an unsaturated hydrido iron complex, which is the key intermediate in the catalytic mechanism. Complex 4 as the best catalyst shows excellent catalytic performance. Among the five complexes, complex 3 was new and the molecular structure of complex 3 was determined by single crystal X-ray diffraction. A proposed mechanism was discussed.

Synthesis of silyl iron hydride via Si-H activation and its dual catalytic application in the hydrosilylation of carbonyl compounds and dehydration of benzamides.

The hydrido silyl iron complex (o-Ph2PC6H4SiMe2)Fe(PMe3)3H (2) was obtained via the activation of the Si-H bond of the bidentate silyl ligand o-Ph2P(C6H4)SiMe2H (1) by Fe(PMe3)4. 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,ß-unsaturated carbonyls could be selectively reduced to the corresponding α,ß-unsaturated alcohols. The mechanisms of the formation of 2 and the catalytic dehydration process are proposed and partly experimentally verified.