Ligand-controlled synthesis of vanadium(i) β-diketiminates and their catalysis in cyclotrimerization of alkynes

Abstract

Three dimeric vanadium(I) β-diketiminates [V{μ-(η⁶-ArN)C(Me)CHC(Me)C(N–Ar)}]₂ (Ar = 2,6-Me₂C₆H₃ (2), 2,6-Et₂C₆H₃ (3), 9-anthracenyl (4)) were prepared and isolated upon reduction of their corresponding dichloro precursors VCl₂(Nacnac). Compounds 2–4 all show a structure with each vanadium atom being η² bonded to the β-diketiminate framework and η⁶ bonded to a flanking ring of a β-diketiminato ligand, attached to the other vanadium centre within the dimer. No metal–metal bonding interactions are observed in these dimers due to long vanadium–vanadium separations. Compounds 2–4 display an antiferromagnetic exchange between the two vanadium centres. An imido azabutadienyl complex (η²-PhCC(H)C(Ph)NC₆H₃-2,6-ⁱPr₂)VN(C₆H₃-2,6-ⁱPr₂)(OEt₂) (5) was isolated from the reduction of VCl₂(HC(C(Ph)NC₆H₃-2,6-ⁱPr₂)₂) by KC₈. Compounds 2–4 and the inverted-sandwich divanadium complex (μ-η⁶:η⁶-C₆H₅Me)[V(HC(C(Me)NC₆H₃-2,6-ⁱPr₂)₂)]₂ (1) reduce Ph₂S₂ to give two vanadium dithiolates V(SPh)₂[(HC(C(Me)NC₆H₃-2,6-R₂)₂)] (R = Et (6), ⁱPr (7)) through an oxidative addition. Most notably, 1 and 3 catalyze the cyclotrimerization of alkynes, giving tri-substituted benzenes in good yields and a 1,3,5-triphenylbenzene coordinated intermediate 8 was isolated and characterized.