While N-heterocyclic carbene (NHC) ligands are used abundantly with late-transition metals, they appear far less often in early transition metal chemistry and are particularly scarce for group 5. To explore the chemistry of NHC complexes of niobium, we use a monoanionic bis(carbene) ligand containing a dihydroborate linking the two rings. We have prepared a variety of low-valent niobium complexes using this supporting ligand and are currently exploring their reactivity. Unexpectedly, it was found that one of these complexes reacts with acetone at the formally anionic boron of the NHC ligand to form a boron alkoxide. This reactivity can be extended toward a variety of carbonyl-containing substrates resulting in reduction of C=O and in some cases C=N bonds, forming both mixed alkoxide hydride and bis(alkoxide) boron complexes. We have found that the nature of the substituents on the niobium center greatly affects the rate of this B-H insertion, indicating that there is significant electronic communication between the metal and the NHC ligand.

Hydroboration reactivity of niobium bis(NHC)borate complexes

253rd National Meeting (2017)

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