Cooperative bond activation reactions with carbene complexes

Abstract

Metal-ligand cooperation has emerged as a powerful concept in bond activation chemistry and catalysis. Here, both the metal and the ligand are directly involved in the bond breaking and bond formation processes, while in "classical" activation reactions the transformations solely take place at the metal centre. Several different types of cooperating ligand systems have been developed over the years and applied in catalysis. In this Feature Article, we will discuss the use of carbene ligands in cooperative bond activation reactions. We focus on two types of carbene complexes: mid and late transition metal carbene species (i) with PCcarbeneP pincer ligands and (ii) with strong anion-stabilizing α-substituents at the carbene carbon atom. These systems are active in a variety of bond activation reactions and have found first catalytic applications.

Carbon Dioxide Catalyzed Cyclometallation of a Carbene Complex: Synthesis and Mechanism

“…Recently, we and others reported on a series of nucleophilic, late transition metal carbene complexes which are able to activate a variety of E-H bonds of different polarity via [1,2] addition reactions across the metal carbon double bond. [10,11] Ruthenium complex 1 was found to be particularly active also giving previously reported cyclometallations at the sulfonyl unit. DFT calculations demonstrate that the ease and selectivity of the reaction can be explained by a unique mechanism in which CO 2 is coordinated by the carbene ligand.…”

“…Recently, we and others reported on a series of nucleophilic, late transition metal carbene complexes which are able to activate a variety of E–H bonds of different polarity via [1,2] addition reactions across the metal carbon double bond , . Ruthenium complex 1 was found to be particularly active also giving way to reversible processes.…”

Rhodium Complexes in P−H Bond Activation Reactions

“…[7] In addition, metals with formal d 0 electron count such as lanthanides, early transition metals and some actinides engage in s-bond metathesis as reported for complexes with alkyls, [8] silylamides, [9] amides, [10] and more recentlya lkoxy groups. [11] Moreover,P ÀHb ond activationt hrough metal-ligandc ooperationh as been recently reported for ruthenium and iridium complexes bearing carbene-type ligands, [12] whereas chelated assisted PÀHb ond cleavage has been described for diphosphane-phosphane oxides, [13] and diphosphane-phosphane compounds, [14] whichr esults in ap hosphanido functionality embedded within at ripodal ligand.…”

“…Indeed, such activations through the tautomer phosphinous acid have been previouslyp roposed for ruthenium complexes on the basis of DFT calculations. [12] Interestingly,t he reaction of OPHPh 2 with the bis(ethylene) complex [Rh(Tp)(C 2 H 4 ) 2 ]( 1)( in 1:1m olar ratio) did not give the mononuclear complex [Rh(Tp)(C 2 H 4 )(OPHPh 2 )] (analogous to 2), but the bis(h 1 Scheme9.Reactionof1 with OPHPh 2 .…”

C−N, C−S and S−S Bond Cleavage by Rhodium PCcarbeneP Pincer Complexes

“…Within this ligand class, PCP type pincers have proven versatile, especially amongst base metal systems, given their ability to partake in metal‐ligand cooperative bond activation . In particular, recent reports of PC carbene P pincers have demonstrated these ligands’ ability to activate a range of challenging single bonds including H−H, B−H, Si−H, N−H, O−H, C−H, Ge−H and S−S bonds metals via 1,2‐addition across the metal‐alkylidene bond or via [2+2] addition and subsequent β‐hydride elimination (Scheme , A) …”