The great success of N-heterocyclic carbenes (NHCs) has inspired intensive efforts to explore carbon compounds of unusual valence. Of the compounds discovered, carbodicarbenes (CDCs), also referred to as bent allenes, are particularly distinct. This is because, unlike NHCs, they do not possess an empty p-orbital at their central C-atom. Instead, theoretical studies concluded that CDCs possess two pairs of largely non-bonding electrons. As such, they should be classified as carbones (B and C), rather than allenes (A). In principle, both pairs of the non-bonding electrons can interact with metal centres and upon σ-donation to a single metal centre, the remaining pair of electrons can potentially π-donate to the metal centre (forming a metal-carbon double bond) or, alternatively, bind to a second metal centre (geminal coordination). We recently reported a third option, in which the second pair of electrons render CDC ligands redox active. More specifically, we found that the bischelate, dicationic iron complex of the tridentate ligand L, [FeII(L)2]2+, could be reversibly oxidized in three single electron transfer steps to yield, ultimately, the corresponding pentacation. Subsequently, we have extended our studies to the analogous Co and Cr complexes, [M(L)2]2+, and found similar behaviour. Using a combination of spectroscopic measurements and DFT calculations it was shown that the tetracationic and pentacationic redox states are formed by ligand-centred oxidation and, therefore, these species are best described as being M3+ ions coordinated to ligand cation radicals. In addition to being the first examples of redox noninnocence in carbone ligands, this behaviour points at the untapped potential of carbones to stabilize high formal oxidation states and to support metal-ligand cooperative reactivity.

Carbodicarbene ligand redox noninnocence in highly oxidized first-row transition metal complexes

2019 Fall National Meeting

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