Reductive Addition of a Zn–Zn Bond to Main Group Carbene Analogues

Oxidative Addition involves the addition of a substrate to a metal centre. This reaction is fundamental across synthetic chemistry and underpins numerous catalytic methods. In the textbook description of oxidative addition reactions, charge flow predominately occurs from the metal centre to the substrate, leading to a net increase in the formal oxidation state of the metal; this occurs with simultaneous bond breaking at the substate. The majority of known reactions, however, involve substrates bearing relatively electronegative elements (e.g. H, C, N, O, halogen) and there has been little discussion of how addition processes may fundamentally change if substrates were constructed from more electropositive elements. Here we show that the zinc–zinc bonded complex, Cp*ZnZnCp*, which is isoelectronic with dihydrogen, undergoes facile addition to the metal (or semi-metal) centres of a series of main group carbene analogues based on aluminium, gallium, or silicon. Reactions proceed with complete breaking of the Zn–Zn bond and increase in the coordination number of the central metal from 2 to 4. Based of DFT calculations and reactivity, these addition processes are not oxidative. Rather data suggest that they are better described as a reductive addition, from the perspective of the main group centre. Our findings challenge our preconceptions regarding text-book definitions and hint that when involving electropositive substrates that oxidative addition processes may not always be oxidative.