Nonoxido Vanadium(IV) Compounds Involving Dithiocarbazate-Based
Tridentate ONS Ligands: Synthesis, Electronic and Molecular Structure,
Spectroscopic and Redox Properties
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Abstract
A new series of nonoxido vanadium(IV)
compounds [VL<sub>2</sub>] (L = L<sup>1</sup>–L<sup>3</sup>) (<b>1</b>–<b>3</b>) have been synthesized using
dithiocarbazate-based tridentate
Schiff-base ligands H<sub>2</sub>L<sup>1</sup>–H<sub>2</sub>L<sup>3</sup>, containing an appended phenol ring with a <i>tert</i>-butyl substitution at the 2-position. The compounds
are characterized by X-ray diffraction analysis (<b>1</b>, <b>3</b>), IR, UV-vis, EPR spectroscopy, and electrochemical methods.
These are nonoxido V<sup>IV</sup> complexes that reveal a rare distorted
trigonal prismatic arrangement around the “bare” vanadium
centers. Concerning the ligand isomerism, the structure of <b>1</b> and <b>3</b> can be described as intermediate between <i>mer</i> and <i>sym-fac</i> isomers. DFT methods were
used to predict the geometry, <b>g</b> and <sup>51</sup>V <b>A</b> tensors, electronic structure, and electronic absorption
spectrum of compounds <b>1</b>–<b>3</b>. 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 <b>1</b>–<b>3</b> can
be described in terms of a mixing among V-<i>d</i><sub><i>xy</i></sub>, V-<i>d</i><sub><i>xz</i></sub>, and V-<i>d</i><sub><i>yz</i></sub> orbitals
in the singly occupied molecular orbital (SOMO), which causes a significant
lowering of the absolute value of the <sup>51</sup>V hyperfine coupling
constant along the <i>x</i>-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 (<i>E</i><sub>1/2</sub> = +1.06 V) is due to the
quasi-reversible V(IV/V) oxidation while process B at <i>E</i><sub>1/2</sub> = −0.085 V is due to the quasi-reversible V(IV/III)
reduction, and the third one (process C) at a more negative potential <i>E</i><sub>1/2</sub> = −1.66 V is due to a ligand centered
reduction, all potentials being measured vs Ag/AgCl reference
