Chemistry of Monomeric and Dinuclear Non-Oxido Vanadium(IV) and Oxidovanadium(V) Aroylazine Complexes: Exploring Solution Behavior

A series of mononuclear non-oxido vanadium-(IV) [V-IV(L1-4)(2)] (1-4), oxidoethoxido vanadium(V) [(VO)-O-V-(L1-4)(OEt)] (5-8) , and dinuclear mu-oxidodioxidodivanadium(V) [(V2O3)-O-V(L-1)(2)] (9) complexes with tridentate aroylazine ligands are reported [H2L1 = 2-furoylazine of 2-hydroxy-1-acetonaphthone, H2L2 = 2-thiophenoylazine of 2-hydroxy-1-acetonaphthone, H2L3 = 1-naphthoylazine of 2-hydroxy-1-acetonaphthone, H2L4 = 3hydroxy-2-naphthoylazine of 2-hydroxy-1-acetonaphthone]. The complexes are characterized by elemental analysis, by various spectroscopic techniques, and by single-crystal X-ray diffraction (for 2, 3, 5, 6, 8, and 9). The non-oxido V-IV complexes (1-4) are quite stable in open air as well as in solution, and DFT calculations allow predicting EPR and UV-vis spectra and the electronic structure. The solution behavior of the [(VO)-O-V(L1-4)(OEt)] compounds (5-8) is studied confirming the formation of at least two different types of V-V species in solution, monomeric corresponding to 5-8, and mu-oxidodioxidodivanadium [(V2O3)-O-V(L1-4)(2)] compounds. The mu-oxidodioxidodivanadium compound [(V2O3)-O-V(L-1)(2)] (9), generated from the corresponding mononuclear complex [(VO)-O-V(L-1)]-(OEt)] (5), is characterized in solution and in the solid state. The single-crystal X-ray diffraction analyses of the non-oxido vanadium(IV) compounds (2 and 3) show a N2O4 binding set and a trigonal prismatic geometry, and those of the VVO complexes 5, 6, and 8 and the mu-oxidodioxidodivanadium(V) (9) reveal that the metal center is in a distorted square pyramidal geometry with O4N binding sets. For the mu-oxidodioxidodivanadium species in equilibrium with 5-8 in CH2Cl2, no mixed valence complexes are detected by chronocoulometric and EPR studies. However, upon progressive transfer of two electrons, two distinct monomeric (VO)-O-IV species are detected and characterized by EPR spectroscopy and DFT calculations.