Charge Distribution in Chromium and Vanadium Catecholato Complexes: X-ray Absorption Spectroscopic and Computational Studies

Transition-metal complexes with redox-active catecholato ligands are of interest as models of bioinorganic systems
and as potential molecular materials. This work expands our recent X-ray absorption spectroscopic (XAS) studies
of Cr(V/IV/III) triscatecholato complexes (Levina, A.; Foran, G. J.; Pattison, D. I.; Lay, P. A. Angew. Chem., Int. Ed.
2004, 43, 462−465) to a Cr(III) monocatecholato complex, [Cr(tren)(cat)]+ (tren = tris(2-aminoethyl)amine, cat =
catecholato(2−)), and its oxidized analogue, as well as to a series of V(V/IV/III) triscatecholato complexes ([VL3]n-,
where L = cat, 3,5-di-tert-butylcatecholato(2−), or tetrachlorocatecholato(2−), and n = 1−3). Various oxidation
states of these complexes in solutions were generated by bulk electrolysis directly in the XAS cell. Increases in the
edge energies and pre-edge absorbance intensities in XANES spectra, as well as decreases in the average M−O
bond lengths (M = Cr or V) revealed by XAFS data analyses, are consistent with predominantly metal-based
oxidations in both the Cr(V/IV/III) and V(V/IV/III) triscatecholato series, but the degree of electron delocalization
between the metal ion and the ligands was higher in the case of Cr complexes. By contrast, oxidation of [CrIII(tren)(cat)]+ was mainly ligand-based and led to [CrIII(tren)(sq)]2+ (sq = semiquinonato(−)), as shown by the absence
of significant changes in the pre-edge and edge features and by an increase in the average Cr−O bond length.
The observed differences in electron-density distribution in various oxidation states of Cr and V mono- and
triscatecholato complexes have been discussed on the basis of the results of density functional calculations. A
crystal and molecular structure of (Et3NH)2[VIV(cat)3] has been determined at 25 K and the same complex with an
acetonitrile of crystallization at 150 K.