Dois novos complexos [VO 2 (L)] e [VO 2 (HLox)]foram sintetizados e caracterizados por espectroscopias no IV, UV-Vis e RMN, voltametria cíclica, análise elementar e difração de raios X. A síntese do ligante inédito H 2 Lox também é descrita. Os complexos 1 e 2 foram obtidos pela reação de [VO(acac) 2 ] com os respectivos ligantes HL e H 2 Lox. Alternativamente, 2 foi preparado a partir da reação de HL com [VO(acac) 2 ] na presença de hidroxilamina, e através da reação de 1 com hidroxilamina. Dados cristalográficos mostram que 1 e 2 apresentam estruturas moleculares similares, onde o centro de vanádio(V) cis-dioxo encontra-se coordenado em um ambiente octaédrico distorcido formado pelos ligantes L -e HLox -, respectivamente. A atividade catalítica destes compostos foi avaliada na oxidação do cicloexano, utilizando H 2 O 2 e t-BuOOH como oxidantes. Ambos apresentam seletividade > 70% para formação de cicloexilidroperóxido. Cálculos B3LYP/6-31G(d) foram empregados na otimização da geometria e para auxiliar na atribuição do espectro eletrônico.Two novel complexes, [VO 2 (L)] (1) and [VO 2 (HLox)] (2), were synthesized and characterized by IV, UV-Vis and NMR spectroscopy, cyclic voltammetry, elemental analysis and X-ray diffraction. The synthesis of a new ligand, H 2 Lox, is also described. Complexes 1 and 2 were obtained by the reaction of [VO(acac) 2 ] with the ligands HL and H 2 Lox, respectively. Alternatively, 2 was also obtained by the reaction of HL with [VO(acac) 2 ] in the presence of hydroxylamine, and by the reaction of 1 with hydroxylamine. Crystallographic data show that complexes 1 and 2 have similar molecular structures, in which the cis-dioxovanadium(V) center is coordinated to L -or HLox -, respectively, in a distorted octahedral environment. The catalytic activity of these compounds towards cyclohexane oxidation was evaluated using H 2 O 2 and t-BuOOH as oxidants. Both complexes presented > 70% selectivity for cyclohexylhydroperoxide formation. B3LYP/6-31G(d) calculations were used to confirm the geometry and to help assign the electronic spectra.
Keywords: oxovanadium(V), bioinspired catalysts, peroxide activation
IntroductionVanadium coordination chemistry has received attention in recent years due to its presence in several biological systems and its applications in catalysis. The first example of vanadium-dependent enzyme are the vanadium haloperoxidases (VHPOs), isolated from the marine algae Ascophyllum nodosum.1,2 Later, these enzymes were isolated from different sources such as seaweed, lichens and other algae and characterized as iodoperoxidases, bromoperoxidases and chloroperoxidases, depending on the most electronegative halogen the enzyme is capable of oxidizing.3 These enzymes are responsible for the two-electron oxidation catalyses of chloride, bromide Silva et al. 661 Vol. 22, No. 4, 2011 and iodide by hydrogen peroxide and are involved in the halogenation of a large variety of organic substrates in vivo. 4 The discovery of its structure initiated the investigation of vanadium...
