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Pure Appl. Chem., 2009, Vol. 81, No. 7, pp. 1217-1227

Published online 2009-05-29

Scorpionate complexes of vanadium(III or IV) as catalyst precursors for solvent-free cyclohexane oxidation with dioxygen

Gopal S. Mishra1, Telma F. S. Silva2,1, Luísa M. D. R. S. Martins1,3 and Armando J. L. Pombeiro1*

1 Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, TU Lisbon, Av. Rovisco Pais, 1049-001, Lisbon, Portugal
2 Área Científica de Física, ISEL, R. Conselheiro Emídio Navarro, 1959-007, Lisbon, Portugal
3 Departamento de Engenharia Química, ISEL, R. Conselheiro Emídio Navarro, 1959-007, Lisbon, Portugal

Abstract: 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.