EPR characteristics of radical complexes with coordinated ammineruthenium(II) fragments. Evidence for the metal-to-ligand charge transfer (MLCT) nature of the low-lying excited states in precursor complexes

Abstract

The first EPR study of one-electron reduced ammineruthenium(II) complexes is reported, based on the reversible reduction of the following precursors: [(mpz)Ru(NH₃)₅]³⁺(mpz =N-methylpyrazinium), [(bpym)Ru(NH₃)₄]²⁺(bpym = 2,2′-bipyrimidine), [(bptz)Ru(NH₃)₄]²⁺ and {(µ-bptz)[Ru(NH₃)₄]₂}⁴⁺[bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine]. The partially resolved EPR spectra of [(mpz·)-Ru(NH₃)₅]²⁺ and [(bptz)Ru(NH₃)₄]⁺ confirm unambiguously that the unpaired electron resides mainly in the π system of the unsaturated ligand, however, the spin distribution and the ^99,101Ru isotope coupling reveal non-negligible contributions from the metal 4d orbitals to the singly occupied MO. The two other one-electron reduced compounds exhibit less well-resolved EPR spectra but are also Ru^II complexes of radical ligands as judged by their g anisotropy in comparison to the Ru^IIIcontaining oxidized forms. Distinctly different EPR characteristics of the oxidized and reduced forms support the MLCT formulation of low-lying excited states in the precursor compounds. The effect of the Ru^II ammine complex fragments on the EPR characteristics of the organic radical ligands is compared to that of other low spin d⁶ systems.