Solution Behaviour and Catalytic Potential towards Oxidation of Dopamine by Oxidovanadium(V) Complexes of Tripodal Tetradentate Ligands

Abstract

The reaction of dibasic tripodal tetradentate ligands H2L1–4 (I–IV) (prepared by the reaction of 4-(2-aminoethyl)morpholine and 2,4-disubstituted phenols) with [VIVO(acac)2] or VIVOSO4·5H2O in a 1:1 molar ratio in ethanol gave the corresponding mononuclear oxidoethoxidovanadium(V) complexes [VVO(OEt)(L1–4)] (1–4) in good yields. Similar reactions with H2L2 (II) and H2L4 (IV) in acetonitrile resulted in the formation of the corresponding dinuclear complexes [{VVO(L2)}2µ-O] (5) and [{VVO(L4)}2µ-O] (6). Single-crystal X-ray diffraction analysis of 1, 2 and 4 confirmed the formulation of the mononuclear compounds as VVO complexes, with the ligands behaving in a tetradentate manner, overall yielding a distorted octahedral geometry. The molecular structures of 1, 2 and 5 were calculated by DFT, as well as the corresponding 51V NMR chemical shifts (in methanol and chloroform) and IR frequencies. The V compounds depict time-dependent solution behaviour, which was studied and rationalised through the combined interpretation of data from spectroscopic techniques (mainly UV/Vis and 51V NMR), mass spectrometry and DFT calculations. They exhibit distinct speciation in different solvents, and it was confirmed that in acetonitrile, they form µ-O oxido dinuclear species. The alkoxido ligands of complexes [VVO(OR)(L)] in CDCl3 are exchanged upon addition of alcohols; namely the 51V NMR chemical-shift values of [VVO(OR)(L2)] depict an increasing shielding effect of the OR moiety in the order: OMe < OEt < OiPr < OtBu. Compounds 1–4 were employed as catalyst precursors in the oxidation of dopamine to aminochrome in MeOH using dioxygen (air) as the primary oxidant. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim