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 = L¹–L³) (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 ⁵¹V 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 ⁵¹V 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.