The Schiff-base ligand (H2salhyhNH3)Cl (1) derived from salicylaldehyde and 6-aminohexanoic acid hydrazide hydrochloride reacts with ammonium metavanadate in methanol solution to yield the dioxidovanadium(V) complex [VO2(salhyhNH3)] (2). The utilized hydrazone ligand contains a flexible and protonated amino side chain. Crystallization from methanol affords complex 2 in the monoclinic space group P21/n, whereas crystallization from a methanol/water mixture 1:1 yields crystals, containing a water molecule of crystallization per two formula units (2⋅1/2H2O), in the orthorhombic space group Pbcn. In both cases the protonated amino group compensates the negative charge on the dioxidovanadium moiety and is involved in an extensive hydrogen bonding network particularly including the oxido groups from neighboring vanadium complexes. The reactivity of complex 2 toward protonation in aqueous solution has been investigated by spectrophotometric titrations and is characterized by two subsequent protonation steps at the hydrazide nitrogen atom of the ligand system and an oxido group leading to the formation of an oxidohydroxidovanadium(V) species with corresponding pKa values of 3.2 and 2.9, respectively. With larger excess of acid the oxidohydroxidovanadium(V) species starts to form the corresponding anhydride. The formation of the anhydride is strongly favored in the presence of methanol. The reaction of complex 2 with hydrogen peroxide in methanol solution leads to the formation of an oxidoperoxidovanadium(V) species, whereas in aqueous solution the addition of one equivalent of acid is required. Complex 2 catalyzes the oxidation of methylphenylsulfane to the corresponding sulfoxide in methanol/dichloromethane mixture using hydrogen peroxide as oxidant at room temperature.
Keywords: Hydrogen bonds; Protonation; Schiff-base ligands; Sulfoxidation; Vanadium.
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