V(IV), Fe(II), Ni(II) and Cu(II) complexes bearing 2,2,2-tris(pyrazol-1-yl)ethyl methanesulfonate: application as catalysts for the cyclooctane oxidation

Water-soluble compounds [VOCl2{CH3SO2OCH2C(pz)3}] (pz = pyrazol-1-yl) 1, [FeCl2{CH3SO2OCH2C(pz)3}] 2, [NiCl2{CH3SO2OCH2C(pz)3}] 3 and [Cu{CH3SO2OCH2C(pz)3}2](OTf)24 were obtained by reactions between the corresponding metal salts and 2,2,2-tris(pyrazol-1-yl)ethyl methanesulfonate, CH3SO2OCH2C(pz)3. They were isolated as air-stable solids and fully characterized by IR, FTIR, NMR (for 2), EPR (for 1), ESI-MS(+/−), elemental analysis and (for 4) single-crystal X-ray diffraction. In all, half- (1–3) or full-sandwich (4), compounds the C-scorpionate ligand shows the N,N,N-coordination mode. 3 and 4 appear to provide the first examples of a Ni(II) and a full-sandwich Cu(II) compound respectively, bearing that scorpionate ligand. Compound 3 is the first Ni(II) tris(pyrazol-1-yl)methane type complex to be applied as catalyst for the oxidation of alkanes. Compounds 1–4 exhibit catalytic activity for the peroxidative (with aq. H2O2) oxidation, in water/acetonitrile medium and under mild homogeneous conditions, of cyclooctane to the corresponding alcohol and ketone (yields up to ca. 27%). The effect of the presence of additives, such as nitric acid or pyridine, was studied.