A Mixed-Valence VIV/VV Alkoxo-polyoxovanadium Cluster Series [V6O8(OCH3)11]n+/−: Exploring the Influence of a μ-Oxo Ligand in a Spin Frustrated Structure
By Daniel, Charles & Hartl, Hans
Published in Journal of the American Chemical Society
2009
Abstract
The synthesis and structural characterization of the neutral mixed-valence methoxo-polyoxovanadium cluster [V6O8(OCH3)11] (1) and its single oxidation product in the hexachloroantimonate salt [V6O8(OCH3)11][SbCl6] (2) are presented here. The cluster comprises a hexauclear polyoxovanadate core of the Lindqvist structure, of which all but one of the μ-bridging oxo ligands are substituted by methoxo. As revealed by cyclic voltammetry, the cluster is highly redox active, displaying several further thermodynamically stable VIV/VV mixed-valence redox derivatives. Furthermore, valence sum calculations performed on the X-ray structural data as well as results from IR and UV-vis spectrometry characterize them as class II mixed-valence compounds. In the present article, we equally present results from cyclic voltammetry, UV-vis spectrometry, and magnetic measurements obtained for members of the previously reported [V6O7(OCH3)12] cluster series, which, as opposed to 1 and its derivatives, contain exclusively methoxo ligands as μ-bridging moieties. Magnetic measurements performed on the highly reduced cluster species [VIV5VV1O7(OCH3)12]- and [VIV6O7(OCH3)12]2- reveal net antiferromagnetic exchange interactions between the d-electrons, which at lower temperatures are in part suppressed for reasons attributed to geometric spin frustration. Among the present results, the comparison of the cyclic voltammograms of 1 and [V6O7(OCH3)12] has proven to be of considerable interest, showing an unexpectedly pronounced discrepancy in all but one of their corresponding redox potentials. In particular, a detailed analysis of the electrochemical conversions indicates that the observed shift is almost entirely the result of a different degree of d-electron spin-spin interactions in corresponding mixed valence species of the cluster series.