Chelating N-Heterocyclic Silylenes as Steering Ligands in Catalysis

Abstract

N-Heterocyclic silylenes (NHSis) have been a key focus in many research groups in recent years and have emerged as a new class of ligands to stabilize transition metal centers. For the most part, the existing examples that have been reported exhibit monodentate NHSi-stabilized complexes that show promise as precursors for catalytic transformations and other small molecule bond-activation reactions. More recently, however, a new class of multidentate NHSi ligands have emerged unraveling new structural motifs and alternative possibilities to stabilize reactive metal centers and use the emerging complexes as precursors in catalytic transformations. Three subclasses of these ligands have emerged: type I [:Si]-X-[Si:], where two NHSi centers are connected via a linker (such as O or some other groups that are noncoordinating to the metal); type II [:Si]-[D:]-[Si:], where the linker [D:] between the two NHSi centers is also capable of acting as a σ-donor or forming a σ-bond to the metal center affording a tridentate or pincer-type ligand system; and finally ligands of type III [Si]-X-[C:], which are mixed NHSi–NHC (N-heterocyclic carbene) ligand systems bearing a nonchelating X linker as with type I. This article will focus on the very recent developments of these multidentate ligand types (I–III), with the first reports emerging in 2010, their associated complexes, and the use of these emerging complexes in catalytic transformations.