Mono- and bis-N-heterocyclic carbene complexes of tantalum and niobium with high oxidation states

Abstract

Reactions of two free N-heterocyclic carbenes (NHCs) 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and 1,3-dimesitylimidazol-2-yliden (IMes) with group VB transition metals MCl₅ at low temperature gave a mono-NHC complex [(IPr)TaCl₅] (1) and two bis-NHCs [(IMes)₂MCl₄][MCl₆] (M = Ta, 2; =Nb, 3). Crystallographic studies revealed that the substitution effect plays an important role in the construction of these two different types of NHC complexes. Although mono-NHC complex 1 has been theoretically analyzed by F. Marchetti, complexes 2 and 3 represented the first examples of the structurally characterized bis-NHCs Ta(V) and Nb(V) complexes. In addition, complexes 1–3 showed catalytic efficiency in the cycloaddition between CO₂ and propylene oxide to synthesize cyclic carbonates under mild conditions.