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 = catecholato2-), 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-butylcatecholato2-, or tetrachlorocatecholato2-, 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 Cr(III)(tren)(cat)]+ was mainly ligand-based and led to Cr(III)(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)2V(IV)(cat)3] has been determined at 25 K and the same complex with an acetonitrile of crystallization at 150 K.