Unveiling the Reactivity of Part Per Million Levels of Cobalt-Salen Complexes in Hydrosilylation of Ketones

A series of air-stable cobalt(III)salen complexes Co-1 to Co-4 have been synthesized and employed in the hydrosilylation of ketones. Notably, the most intricately tailored Co-3 pre-catalyst exhibited exceptional catalytic activity under mild reaction conditions. The developed catalytic hydrosilylation protocol proceeded with an unusual ppm level (5 ppm) catalyst loading of Co-3 and achieved a maximum turnover number (TON) of 200,000. A wide variety of aromatic, aliphatic, and heterocyclic ketones encompassing both electron-donating and electron-withdrawing substituents were successfully transformed into the desired silyl ethers or secondary alcohols in moderate to excellent yields.

Effects of silylene ligands on the performance of carbonyl hydrosilylation catalyzed by cobalt phosphine complexes

In order to study the effects of silylene ligands on the catalytic activity of carbonyl hydrosilylation catalyzed by cobalt phosphine complexes, readily available model catalysts are required. In this contribution, a comparative study of the hydrosilylation of aldehydes and ketones catalyzed by tris(trimethylphosphine) cobalt chloride, CoCl(PMe₃)₃ (1), and bis(silylene) cobalt chloride, Co(LSi:)₂(PMe₃)₂Cl (2, LSi: = {PhC(N^tBu)₂}SiCl), is presented. It was found that both complexes 1 and 2 are good catalysts for the hydrosilylation of aldehydes and ketones under mild conditions. This catalytic system has a broad substrate scope and selectivity for multi-functional substrates. Silylene complex 2 shows higher activity than complex 1, bearing phosphine ligands, for aldehydes, but conversely, for ketones, the activity of complex 1 is higher than that of complex 2. It is worth noting that in the process of mechanistic studies the intermediates (PMe₃)₃Co(H)(Cl)(PhH₂Si) (3) and (LSi:)₂(PMe₃)Co(H)(Cl)(PhH₂Si) (4) were isolated from the stoichiometric reactions of 1 and 2 with phenylsilane, respectively. Further experiments confirmed that complex 3 is a real intermediate. A possible catalytic mechanism for the hydrosilylation of carbonyl compounds catalyzed by 1 was proposed based on the experimental investigation and literature reports, and this mechanism was further supported by DFT studies. The bis(silylene) complex 4 showed complicated behavior in solution. A series of experiments were designed to study the catalytic mechanism for the hydrosilylation of carbonyl compounds catalyzed by complex 2. According to the experimental results, the hydrosilylation of aldehydes catalyzed by 1 proceeds via a different mechanism than that of the analogous reaction with complex 2 as the catalyst. In the case of ketones, complex 4 is a real intermediate, indicating that both 1 and 2 catalyze the reaction by the same mechanism. The molecular structures of 3 and 4 were determined by single crystal X-ray diffraction analysis.

Research on inorganic activators of dibromo Co-terpyridine complex precatalyst for hydrosilylation

The search for a stable, inexpensive, and easy-to-handle activator toward the catalyst precursor [Co(tpy)Br₂] in the hydrosilylation of olefins with hydrosilane revealed that K₂CO₃ is an effective activator. This inorganic salt is available on substrates with some functional groups and can be readily removed by simple filtration or centrifugation after the reaction. After examining and comparing the activator abilities of various salts, it was proposed that low MX lattice energy, high X-nucleophilicity, and a strong Si-X bond are necessary for an inorganic salt (MX) to be an excellent activator.

An Aryl Diimine Cobalt(I) Catalyst for Carbonyl Hydrosilylation

Through the application of a redox-innocent aryl diimine chelate, the discovery and utilization of a cobalt catalyst, (Ph2PPrADI)Co, that exhibits carbonyl hydrosilylation turnover frequencies of up to 330 s–1 is...

Extensive Redox Non-Innocence in Iron Bipyridine-Diimine Complexes: a Combined Spectroscopic and Computational Study

Metal-ligand cooperation is an important aspect in earth-abundant metal catalysis. Utilizing ligands as electron reservoirs to supplement the redox chemistry of the metal has resulted in many new exciting discoveries. Here, we demonstrate that iron bipyridine-diimine (BDI) complexes exhibit an extensive electron-transfer series that spans a total of five oxidation states, ranging from the trication [Fe(BDI)]3+ to the monoanion [Fe(BDI]-1. Structural characterization by X-ray crystallography revealed the multifaceted redox noninnocence of the BDI ligand, while spectroscopic (e.g., 57Fe Mössbauer and EPR spectroscopy) and computational studies were employed to elucidate the electronic structure of the isolated complexes, which are further discussed in this report.

Base Metal-terpyridine Complex Immobilized on Stationary Phase Aimed as Reusable Hydrosilylation Catalyst

The catalytic activity of a base metal-terpyridine complex immobilized on silica gel (M(tpy)X₂ @SiO₂ /H₂ O: M=Mn, Fe, Co, Ni, Cu; X=Cl, Br) for hydrosilylation was investigated. Co(tpy)Br₂ @SiO₂ /H₂ O in the presence of NaBHEt₃ exhibited the highest catalytic activity for hydrosilylation of 1-octene with diphenylsilane (Ph₂ SiH₂ ) to form the anti-Markovnikov-type hydrosilylation compound as the main product. The reusability of Co(tpy)Br₂ @SiO₂ /H₂ O activated by NaBHEt₃ was examined. It was found that the catalytic activity decreased with repeated use because of the peeling off of the Co complex anchor portion from the silica gel surface upon the attack of NaBHEt₃ . The introduction of Co(OAc)₂ instead of CoBr₂ to silica gel formed Co(tpy)(OAc)₂ - and Co(tpy)(OH)₂ -immobilized silica gel, which exhibited catalytic activity for the hydrosilylation in the absence of an activator such as NaBHEt₃ . The glassware in which Co(tpy)(OH)₂ was immobilized on the inner wall was prepared. It was found that the hydrosilylation catalytically occurred on the surface of a pretreated glassware and that the catalytic activity did not decrease even after 10 repeated uses.

Hydroboration of Nitriles, Esters, and Carbonates Catalyzed by Simple Earth-Abundant Metal Triflate Salts

During the past decade earth-abundant metals have become increasingly important in homogeneous catalysis. One of the reactions in which earth-abundant metals have found important applications is the hydroboration of unsaturated C-C and C-X bonds (X=O or N). Within these set of transformations, the hydroboration of challenging substrates such as nitriles, carbonates and esters still remain difficult and often relies on elaborate ligand designs and highly reactive catalysts (e. g., metal alkyls/hydrides). Here we report an effective methodology for the hydroboration of challenging C≡N and C=O bonds that is simple and applicable to a wide set of substrates. The methodology is based on using a manganese(II) triflate salt that, in combination with commercially available potassium tert-butoxide and pinacolborane, catalyzes the hydroboration of nitriles, carbonates, and esters at room temperature and with near quantitative yields in less than three hours. Additional studies demonstrated that other earth-abundant metal triflate salts can facilitate this reaction as well, which is further discussed in this report.

Carboxylate-Functionalized P, N-Ligated Cobalt Catalysts for Alkene Hydrosilylation

A series of N, P-ligands bearing carboxyl groups have been synthesized. These have been applied in conjunction with cobalt naphthenate in a facile, economic, and efficient method for the catalytic hydrosilylation of alkenes. In the presence of KO^tBu as an additive, the reaction time and activation energy are greatly reduced.

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.