Early cooperative reactivity

In the simplest case, heterodinuclear complexes will react as pairs of metal electrophiles and nucleophiles. Cooperative reactivity is particularly likely with polar substrates. In this case, the electropositive early transition metal center may react with the more Lewis-basic part of the substrate, while the nucleophilic late transition metal complex fragment will attack the more Lewis-acidic part of the substrate molecule. For metal- metal single bonds between the two metal centers, this interaction wiU go along with the cleavage of the metal-metal bond. In many cases, it is difficult to establish whether the scission of the metal-metal bond precedes the transformation of the substrate or whether it occurs at a later stage. 

The analysis of the regioselective reactivity of olefins in identical topochemical environments by three computational methods concludes that both steric factors (cavity and potential energy) and electronic factors (perturbation energy from orbital interactions) play important cooperative roles in determining which C—C double bond in a molecule reacts first in [2-1-2] photodimerization. The steric factor is considered to be effective in the movement of olefins at an early stage of the reaction, whereas the electronic factors are effective in the adduction of olefins at a later stage of the reaction. 

Abstract By combining an ever-increasing number of catalysts or catalytic functions, cooperative catalysis is a research area that grows fast. In the field, early-late heterobimetallic complexes are rather old objects but they still continue to fasdnate chemists because of their latent reactivity. After a brief and ccaicise overview of cooperative catalysis, this review focuses on early-late heterobimetallic complexes that were used in catalysis over the last decades. Examples of dual catalysis using early and late metal partners are also described. This chapter ends with an opening towards therapeutic applications of early-late heterobimetallic complexes. 

The combination of hard oxophilic early transition metals and soft nucleophilic late transition metals with opposite functionalities, provided they do not inhibit one another, is a priori ideal for promoting cooperative effect. A proof of concept can be found in the stoichiometric reactivity of early—late heterobimetallic complexes featuring a metal-metal bond [76]. It has been shown that such complexes are good candidates to realize the heterolytic cleavage of a bond in polar and apolar substrates. An illustrative example by Bergman et al. is the reaction of the Zr-lr complex 20 with carbon dioxide which leads to the rupture of the metal—metal bond (Scheme 18) [77]. The CO2 insertion occurs in the expected fashion with the CO2 bridging the two metals, the carbon atom coordinated to iridium, and the oxygen atom on the zirconium center. 

To resolve this reactivity problem, we explored the possibility of employing a Lewis acid to activate the iV-acyl hydrazone by lowering its LUMO energy (Scheme 22) [145-147]. If successful, such an achievement would expand the possibilities for the development of new NHC reactions. The most important challenge to this idea is the known ability of carbenes to act as ligands and form stable complexes, particularly with late transition metals such as palladium and copper [148-151]. We hypothesized that early metals would react reversibly with NHCs, allowing for the development of a cooperative catalytic system. After... 

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