The σ‐CAM Mechanism: σ Complexes as the Basis of σ‐Bond Metathesis at Late‐Transition‐Metal Centers

Abstract

Complexes in which a sigma-H--E bond (E=H, B, Si, C) acts as a two-electron donor to the metal center are called sigma complexes. Clues that it is possible to interconvert sigma ligands without a change in oxidation state derive from C--H activation reactions effecting isotope exchange and from dynamic rearrangements of sigma complexes (see Frontispiece). Through these pathways, metathesis of M--E bonds can occur at late transition metals. We call this process sigma-complex-assisted metathesis, or sigma-CAM, which is distinct from the familiar sigma-bond metathesis (typical for d(0) metals and requiring no intermediate) and from oxidative-reductive elimination mechanisms (inherently requiring intermediates with changed oxidation states and sometimes involving sigma complexes). There are examples of sigma-CAM mechanisms in catalysis, especially for alkane borylation and isotope exchange of alkanes. It may also occur in silylation and alkene hydrogenation.

Methyl Complexes of the Transition Metals

“…The hydrogenolysis mechanism of a pincer Ir(III)-CH3 complex was analyzed in detail by Brookhart and co-workers (Scheme 36a). Spectroscopic studies and DFT calculations revealed that reductive elimination of methane proceeds through a σ-metathesis or σ-CAM mechanism, 207 in which the oxidation state of iridium is unaltered over the course of the transformation and where a σ-methane complex 3.18b is an instrumental intermediate. 92b The metal-ligand cooperative hydrogenolysis of a Ni-CH3 bond in 3.19 has recently been disclosed and the role of the boryl ligand in assisting such transformation is supported by computational studies (Scheme 36b).…”

Mechanisms of C–H bond activation: rich synergy between computation and experiment

“…E is most commonly H, SiR 3 or BR 2 , but in principal this idea extends to C-H s-complexes. 37 Hall has used the atoms-in-molecules (AIM) approach 38 to characterise different transition states in terms of the varying patterns of bond critical points (BCP) and ring critical points (RCP) (see Fig. 6 Despite this elegant outcome, it seems likely that a continuum of transition state structures will be formed as more computational data become available on SBM processes.…”

Silane(silyl) and Bis(silyl)hydrido Manganese Complexes with Different Mn···H···Si Interaction: Observation of Gradual Si–H Bond Activation on the Metal Center

“…2 Thus, coordination of the hydrogen of a silane SiH bond to the coordinatively unsaturated metal center in a complex ML n can lead to the formation of A bearing an η 1 (H)-silane ligand, and then the coordination mode of the silane ligand in A can change to an η 2 -mode to give complex B that has stronger M£Si interaction compared with that in A. Finally, B can be converted either into a hydrido(silyl) complex C via complete oxidative addition or into other types of intermediate via ·-CAM mechanism, 3 etc. Therefore, to understand the detailed reaction mechanism of the SiH activation, it is indispensable to experimentally elucidate how bonding interaction between a metal and an SiH bond (M£H£Si interaction) can gradually change at some different stages of the process.…”