Bonding Situation of σ-E-H Complexes in Transition Metal and Main Group Compounds

Cross-Dehydrogenative Coupling of Secondary Amines with Silanes Catalyzed by Agostic Iridium-NSi Species

An active catalytic system for the cross-dehydrogenative coupling (CDC) of a wide range of secondary amines with silanes is reported. The iridium(III) derivatives [Ir(H)(X)(κ2-NSiDMQ)(L)] (NSiDMQ = {4,8-dimethylquinoline-2-yloxy}dimethylsilyl; L = coe, X = Cl, 2; L = coe, X = OTf, 3; L = PCy3, X = Cl, 4; L = PCy3, X = OTf, 5), which are stabilized by a weak yet noticeable Ir···H-C agostic interaction between the iridium and one of the C-H bonds of the 8-Me substituent of the NSiDMQ ligand, have been prepared and fully characterized. These species have proven to be effective catalysts for the CDC of secondary amines with hydrosilanes. The best catalytic performance (TOF1/2 = 79,300 h-1) was obtained using 5 (0.25 mol %), N-methylaniline, and HSiMe2Ph. The catalytic activity of the species [Ir(H)(OTf)(κ2-NSiQ)(PCy3)] (10, NSiQ = {quinoline-2-yloxy}dimethylsilyl) and [Ir(H)(OTf)(κ2-NSiMQ)(PCy3)] (11, NSiMQ = {4-methylquinoline-2-yloxy}dimethylsilyl), related to 5 but lacking the 8-Me substituent, is markedly lower than that found for 5. This fact highlights the crucial role of the 8-Me substituent of the NSiDMQ ligand in enhancing the catalytic performance of these iridium complexes.

Electrophilic Hydrosilylation of Electron-Rich Alkenes Derived from Enamines

The low-electron count, air-stable, platinum complexes [Pt(ItBu')(ItBu)][BArF] (C1) (ItBu=1,3-di-tert-butylimidazol-2-ylidene), [Pt(SiPh)3(ItBuiPr)2][BArF] (C2) (ItBuiPr=1-tert-butyl-3-iso-propylimidazol-2-ylidene), [Pt(SiPh)3(ItBuMe)2][BArF] (C3), [Pt(GePh3)(ItBuiPr)2][BArF] (C4), [Pt(GePh)3(ItBuMe)2][BArF] (C5) and [Pt(GeEt)3(ItBuMe)2][BArF] (C6) (ItBuMe=1-tert-butyl-3-methylimidazol-2-ylidene) are efficient catalysts (particularly the germyl derivatives) in both the silylative dehydrocoupling and hydrosilylation of electron rich alkenes derived from enamines. The steric hindrance exerted by the NHC ligand plays an important role in the selectivity of the reaction. Thus, bulky ligands are selective towards the silylative dehydrocoupling process whereas less sterically hindered promote the selective hydrosilylation reaction. The latter is, in addition, regioselective towards the β-carbon atom of both internal and terminal enamines, leading to β-aminosilanes. Moreover, the syn stereochemistry of the amino and silyl groups implies an anti Si-H bond addition across the double bond. All these facts point to a mechanistic picture that, according to experimental and computational studies, involves a non-classical hydrosilylation process through an outer-sphere mechanism in which a formal nucleophilic addition of the enamine to the silicon atom of a platinum σ-SiH complex is the key step. This is in sharp contrast with the classical Chalk-Harrod mechanism prevalent in platinum chemistry.

X-type silyl ligands for transition-metal catalysis

Given the critical importance of novel ligand development for transition-metal (TM) catalysis, as well as the resurgence of the field of organosilicon chemistry and silyl ligands, to summarize the topic of X-type silyl ligands for TM catalysis is highly attractive and timely. This review particularly emphasizes the unique σ-donating characteristics and trans-effects of silyl ligands, highlighting their crucial roles in enhancing the reactivity and selectivity of various catalytic reactions, including small molecule activation, Kumada cross-coupling, hydrofunctionalization, C-H functionalization, and dehydrogenative Si-O coupling reactions. Additionally, future developments in this field are also provided, which would inspire new insights and applications in catalytic synthetic chemistry.

σ-GeH and Germyl Cationic Pt(II) Complexes

The low electron count Pt(II) complexes [Pt(NHC')(NHC)][BAr^F] (where NHC is a N-heterocyclic carbene ligand and NHC' its metalated form) react with tertiary hydrogermanes HGeR₃ at room temperature to generate the 14-electron platinum(II) germyl derivatives [Pt(GeR₃)(NHC)₂][BAr^F]. Low-temperature NMR studies allowed us to detect and characterize spectroscopically some of the σ-GeH intermediates [Pt(η²-HGeR₃)(NHC')(NHC)][BAr^F] that evolve into the platinum-germyl species. One of these compounds has been characterized by X-ray diffraction studies, and the interaction of the H-Ge bond with the platinum center has been analyzed in detail by computational methods, which suggest that the main contribution is the donation of the H-Ge to a σ*(Pt-C) orbital, but backdonation from the platinum to the σ*(Ge-H) orbital is significant. Primary and secondary hydrogermanes also produce the corresponding platinum-germyl complexes, a result that contrasts with the reactivity observed with primary silanes, in which carbon-silicon bond-forming reactions have been reported. According to density functional theory calculations, the formation of Pt-Ge/C-H bonds is both kinetically and thermodynamically preferred over the competitive reaction pathway leading to Pt-H/C-Ge bonds.