Nonoxido Vanadium(IV) Compounds Involving Dithiocarbazate-Based Tridentate ONS Ligands: Synthesis, Electronic and Molecular Structure, Spectroscopic and Redox Properties

A new series of nonoxido vanadium(IV) compounds [VL2] (L = L-1-L-3) (1-3) have been synthesized using dithiocarbazate-based tridentate Schiff-base ligands H2L1-H2L3, containing an appended phenol ring with a tert-butyl substitution at the 2-position. The compounds are characterized by X-ray diffraction analysis (1, 3), IR, UV-vis, EPR spectroscopy, and electrochemical methods. These are nonoxido V-IV complexes that reveal a rare distorted trigonal prismatic arrangement around the bare vanadium centers. Concerning the ligand isomerism, the structure of 1 and 3 can be described as intermediate between mer and sym-fac isomers. DFT methods were used to predict the geometry, g and V-51 A tensors, electronic structure, and electronic absorption spectrum of compounds 1-3. Hyperfine coupling constants measured in the EPR spectra can be reproduced satisfactorily at the level of theory PBE0/VTZ, whereas the wavelength and intensity of the absorptions in the UV-vis spectra at the level CAM-B3LYP/gen, where gen is a general basis set obtained using 6-31+g(d) for S and 6-31g for all the other elements. The results suggest that the electronic structure of 1-3 can be described in terms of a mixing among V-d(xy), V-d(xz), and V-d(yz) orbitals in the singly occupied molecular orbital (SOMO), which causes a significant lowering of the absolute value of the V-51 hyperfine coupling constant along the x-axis. The cyclic voltammograms of these compounds in dichloroethane solution display three one-electron processes, two in the cathodic and one in the anodic potential range. Process A (E-1/2 = +1.06 V) is due to the quasi-reversible V(IV/V) oxidation while process B at E-1/2 = -0.085 V is due to the quasi-reversible V(IV/III) reduction, and the third one (process C) at a more negative potential E-1/2 = -1.66 V is due to a ligand centered reduction, all potentials being measured vs Ag/AgCl reference.