| Abstract |
The reaction of dibasic tripodal tetradentate ligands H2L1-4 (I-IV) (prepared by the reaction of 4-(2-amino-ethyl) morpholine and 2,4-disubstituted phenols) with [(VO)-O-IV(acac)(2)] or (VOSO4)-O-IV center dot 5H(2)O in a 1:1 molar ratio in ethanol gave the corresponding mononuclear oxidoethoxidovanadium(V) complexes [(VO)-O-V(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 [\(VO)-O-V(L-2)\(2)mu-O] (5) and [\(VO)-O-V(L-4)\(2)mu-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 V-51 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 V-51 NMR), mass spectrometry and DFT calculations. They exhibit distinct speciation in different solvents, and it was confirmed that in acetonitrile, they form mu-O oxido dinuclear species. The alkoxido ligands of complexes [(VO)-O-V(OR)(L)] in CDCl3 are exchanged upon addition of alcohols; namely the V-51 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. |
