Half-sandwich scorpionate vanadium, iron and copper complexes: Synthesis and application in the catalytic peroxidative oxidation of cyclohexane under mild conditions

The new half-sandwich scorpionate complexes [hydridotris(I-pyrazolyl)methane]vanadium trichloride, ([VCl3{HC(pz)(3)}] 1 (pz=pyrazolyl), [tris(1-pyrazolyl)methanesulfonate]vanadium trichloride, [VCl3{SO3C(pz)(3)}] 2, [hydrido(1-pyrazolyl)methane]iron dichloride, [FeCl2{HC(pz)(3)}] 3, lithium [tris(1-pyrazolyl)methanesulfonate]dichloroferrate, Li[FeCl2{SO3C(pz)(3)}] 4, and [tris(1-pyrazolyl)methanesulfonate] copper chloride, [CUCl(SO3C(pz)(3)}] 5 were synthesized, characterized and shown to act, as well as the related (tripyrazolylmethane)copper dichloride, [CuCl2{HC(pz)(3)}] 6, as selective catalysts (or catalyst precursors) for the peroxidative oxidation (with hydrogen peroxide) of cyclohexane to cyclohexanol and cyclohexanone, under mild conditions (at room temperature and using an aqueous solution of H2O2). The iron complexes are the most active ones (reaching TON values up to ca. 690), the effects of a variety of factors are reported and the reaction is shown to proceed via both G and O-centred radical mechanisms, conceivably involving a metal-based oxidant.