Reactivity of Rhodium-Triflate Complexes with Diphenylsilane: Evidence for Silylene Intermediacy in Stoichiometric and Catalytic Reactions

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.

A two-coordinate Ni(i) silyl complex: CO2 insertion and oxidatively-induced silyl migrations

Synthesis of the first open-shell two-coordinate silyl complex and its oxidatively-induced silyl rearrangements.

Base-Free Iron Hydrosilylene Complexes via an α-Hydride Migration that Induces Spin Pairing

Two new base-free hydrosilylene complexes of iron were synthesized using the novel starting material Cp*( ⁱPr₂MeP)FeMes. These Cp*( ⁱPr₂MeP)Fe(H)SiHR (R = DMP, Trip) complexes are in equilibrium with the corresponding iron silyl complexes, Cp*( ⁱPr₂MeP)FeSiH₂R, which can be trapped and characterized for R = Trip. Unlike the Ru analogues, the Fe silylene complex with R = DMP is observed to undergo an intramolecular C-H activation involving formal addition of a benzylic C-H bond across the Fe-Si bond. This increased activity for bond activations is also observed for reactions with hydrogen, where Fe reacts faster than a Ru analog to form the hydrogenation product, Cp*( ⁱPr₂MeP)H₂FeSiH₂DMP.

Size-independent organosilane functionalization of silicon nanocrystals using Wilkinson’s catalyst

A low-temperature size-independent method for modifying the surface chemistry of the silicon nanocrystal (SiNC) surface is reported. Wilkinson’s catalyst has been applied to accelerate the dehydrogenative coupling reaction between organosilane molecules and hydride-terminated SiNCs. Two strategies and multiple organosilanes were evaluated to study surface modification efficiency. During the investigations, it was determined that surface functionalization efficiency showed some dependence upon the organic modifier in question. The comparatively low reactivity of octadecyldimethyl silanes may result from the formation of “Si–Rh–Si” intermediates hindered by the steric bulk of the silanes when reacted with activated SiNCs. Quenching of the SiNC-based photoluminescence is observed and the origin of this phenomenon has been attributed to the present of trace rhodium on SiNCs detected using X-ray photoelectron spectroscopy.

N-heterocyclic silylene complexes in catalysis: new frontiers in an emerging field

A review of all existing N-heterocyclic silylene (NHSi) complexes involved in catalysis. Remarkably few examples exist, but already show promise as an emerging new generation of catalysts with the potential to tune catalyst activities and selectivities.

Multiple reaction pathways in rhodium-catalyzed hydrosilylations of ketones

A detailed density functional theory (DFT) computational study (using the BP86/SV(P) and B3LYP/TZVP//BP86/SV(P) level of theory) of the rhodium-catalyzed hydrosilylation of ketones has shown three mechanistic pathways to be viable. They all involve the generation of a cationic complex [L(n)Rh(I)]+ stabilized by the coordination of two ketone molecules and the subsequent oxidative addition of the silane, which results in the Rh-silyl intermediates [L(n)Rh(III)(H)SiHMe2]+. However, they differ in the following reaction steps: in two of them, insertion of the ketone into the Rh-Si bond occurs, as previously proposed by Ojima et al., or into the Si-H bond, as proposed by Chan et al. for dihydrosilanes. The latter in particular is characterized by a very high activation barrier associated with the insertion of the ketone into the Si-H bond, thereby making a new, third mechanistic pathway that involves the formation of a silylene intermediate more likely. This "silylene mechanism" was found to have the lowest activation barrier for the rate-determining step, the migration of a rhodium-bonded hydride to the ketone that is coordinated to the silylene ligand. This explains the previously reported rate enhancement for R2SiH2 compared to R3SiH as well as the inverse kinetic isotope effect (KIE) observed experimentally for the overall catalytic cycle because deuterium prefers to be located in the stronger bond, that is, C-D versus M-D.

Hydrodefluorination of pentafluoropyridine at rhodium using dihydrogen: detection of unusual rhodium hydrido complexes

The pentafluoropyridyl complex [Rh(4-C5NF4)(PEt3)3] (3) reacts with H2 to give initially the dihydrido complex cis-mer-[Rh(H)2(4-C5NF4)(PEt3)3] (6). Within a few hours 2,3,5,6-tetrafluoropyridine as well as two rhodium(III) complexes mer-[Rh(H)3(PEt3)3] (mer-) and fac-[Rh(H)3(PEt3)3] (fac-) are formed. A catalytic C-F activation process for the formation of 2,3,5,6-tetrafluoropyridine starting from pentafluoropyridine and dihydrogen using 3 as a catalyst has been developed. Reaction of [RhH(PEt3)3] (1) with hydrogen affords fac-[Rh(H)3(PEt3)3] (fac-7) and mer-[Rh(H)3(PEt3)3] (mer-7) in a ratio of 1 : 7.25 at 193 K. The latter complex represents the first mononuclear rhodium compound bearing trans-hydrides.

From inconsistent results to high speed hydrosilylation

After noting that the presence of dihydrogen, generated in situ from the partial hydrolysis of a silane with residual water, significantly enhances the rate of the rhodium-catalysed hydrosilylation of acetophenone, we developed a high speed hydrosilylation reaction under dihydrogen pressure.

Mechanistic Insights into an Unprecedented C−C Bond Activation on a Rh/Ga Bimetallic Complex: A Combined Experimental/Computational Approach

The unusual rearrangement of [RhCp(GaCp)(CH(3))(2)] (1c) to [RhCp(C(5)Me(4)Ga(CH(3))(3))] (2) is presented and its mechanism is discussed in detail. (13)C MAS NMR spectroscopy revealed that the title reaction proceeds cleanly not only in solution but also in solid state, which supports a unimolecular reaction pathway. On the basis of (1)H, (13)C, and ROESY NMR spectroscopy as well as isolation and structural elucidation of the hydrolysis product, the compound [RhCp(endo-eta(4)-C(5)Me(5)GaMe(2))] (3a) was identified as a crucial reaction intermediate. DFT calculations on the B3LYP level of theory support this assignment and suggest a concerted C-C bond activation mechanism that topologically takes place at the gallium center. Furthermore, two fluxional processes of the reaction intermediate 3a were studied experimentally as well as by computational methods. First, a mechanism takes place similar to a ring-slipping process that exchanges a GaMe(2) group between adjacent ring carbon atoms within the same Cp ring. This process proceeds at a rate comparable to the NMR time scale and indeed is calculated to be energetically very favorable. Second, a unimolecular exchange process of the GaMe(2) group between the two Cp rings of 3a could be experimentally proven by the introduction of phenyl substituents as a label into the Cp ligands at both sites, the rhodium as well as the gallium center. A series of experiments including deuteration studies and competition reactions was performed to substantiate the suggested mechanism being in accordance with DFT calculations on possible transition states.