Synthesis, Characterization and Catalytic Applications of N-Heterocyclic Carbene Based Vanadium Complexes.

Abstract

New tri- and tetradentate O-functionalized NHC oxo- and imido vanadium complexes have been synthesized and characterized. The complexes were preliminary tested for catalytic activity towards coupling of CO2 and epoxides in the presence of [PPN]Cl (PPN = bis(triphenylphosphoranylidene)ammonium) as co-catalyst. The tridentate oxo-vanadium complexes showed activity, although low, towards selective coupling of cyclohexene oxide (CHO) and CO2 to cyclohexene carbonate (CHC). The imido-vanadium complex had the same activity towards CHO/CO2 coupling as its corresponding oxo-vanadium complex owing to its reactivity towards [2+2] cycloaddition of CO2 to form the oxo-vanadium complex. Tetradentate NHC oxo-vanadium complexes were also tested in coupling of a wider variety of mono and di-substituted epoxides such as CHO, propylene oxide (PO), styrene oxide (SO), cyclopentene oxide (CPO) and 1,3-cyclohexadiene oxide (1,3-CHDO) with CO2 showing activity towards selective formation of cyclic carbonates.