SPECTROSCOPIC CHARACTERIZATION OF SOME UNSTABLE ORTHO-SEMIQUINONE AND ORTHO-QUINONE COMPLEXES OF MN(I) BY VARIABLE-TEMPERATURE THIN-LAYER SPECTROELECTROCHEMISTRY AT OPTICALLY TRANSPARENT ELECTRODES

摘要

Four carbonyl complexes of Mn-1 containing 3,5-di-tert.butyl-1,2-semiquinone (DBSQ) and 3,5-di-tert.butyl-1,2-benzoquinone (DBQ) ligands, the radicals [Mn(CO)(3)(L)(DBSQ)] and the cationic complexes [Mn(CO)(3)(L)(DBQ)](+) (L = H2O, PPh(3)), have been characterized for the first time by. UV-Vis spectroscopy. These compounds possess limited stability at room temperature with the exception of [Mn(CO)(3)(PPh(3))(DBSQ)] which thermally decomposes only in the presence of an excess of PPh(3). The o-(semi)quinone complexes under study were electrogenerated inside a recently developed low-temperature optically transparent thin-layer electrochemical (LT OTTLE) cell. The cell is ide ally suited far UV-Vis and IR transmission spectroelectrochemical experiments at variable temperatures and allows the study of secondary reactions of the redox products. At T = 223 K, the CO disproportionation reaction of the complexes containing the H2O ligand, decomposition of the tricarbonyl DBQ complexes, and even substitution of the axial CO ligand in [Mn(CO)(3)(PPh(3))(DBSQ)] by PPh(3) were fully inhibited. The UV-Vis spectra indicate that the observed thermal lability of the o-quinone complexes [Mn(CO)(4-n)(L)(n)(DBQ)](+) (n = 0, 1; L = H2O, PPh(3)) most likely originates from a considerably weaker pi-acceptor character of the DBQ ligand in these species than imposed in the stable, delocalized complex [Mn(CO)(2)(PPh(3))(2)(DBQ)](+). [References: 29]