Scorpionate complexes of vanadium(III or IV) as catalyst precursors for solvent-free cyclohexane oxidation with dioxygen
The V-scorpionate vanadium complexes [VCl3{HC(pz)3}] (pz = pyrazolyl) and [VCl3{SO3C(pz)3}] catalyze cyclohexane oxidation with dioxygen, to cyclohexanol (the main product) and cyclohexanone, under solvent-free conditions. [VCl3{HC(pz)3}] provides the best activity (13 % conversion into the ketone and alcohol, with high selectivity, at the O2 pressure of 15 atm, at 140 ºC, 18 h reaction time). The reaction is further promoted (to 15 % conversion) by pyrazinecarboxylic acid (PCA). The use of C- or O-radical traps supports the involvement of a free-radical reaction mechanism. Several reaction parameters have been varied in a systematic study, directed toward optimization of the process.