Ligand control of the redox properties of dioxomolybdenum(VI) coordination complexes

Article abstract of DOI:10.1021/ic50225a024

Several new cis-dioxomolybdenum(VI) coordination complexes have been synthesized incorporating dibasic tridentate Schiff-base ligands derived from 5-substituted salicylaldehydes and o-aminophenol, 2-aminoethanol, or anthranilic acid. The synthesis and characterization of these complexes are described. Cyclic voltammetry was used to obtain cathodic reduction potentials (E_pc) for the irreversible reduction of the Mo(VI) complexes. The substituent on the salicylaldehyde portion of the ligand causes rather substantial differences in the E_pc's that are observed for the Mo(VI) complexes. Electron-withdrawing substituents cause an anodic shift in E_pc while the opposite effect is observed for electron-donating ligand substituents. Within a series of Mo complexes, a potential range of ～200 mV is spanned, and a linear relationship is observed between E_pc and the Hammett σ_p parameter for the ligand substituent. In addition to substituent effects within each series, there are structural effects between series that are important. Ligand delocalization and the substitution of a carboxylic acid group for the phenolic oxygen donor atom facilitate Mo complex reducibility. The trends in E_pc are also manifested in shifts observed in the low-energy ligand-to-metal charge-transfer transitions, and these are also discussed.