Near-Infrared-Absorbing Organometallic Diruthenium Complex Intermediates: Evidence for Bridging Anthrasemiquinone Formation and against Mixed Valency

The new redox-active complexes [RuH(CO)-(EPh3)(2)(mu-Q(2-))RuH(CO)(EPh3)(2)], E = P (1) and E = As (2) with the bis-chelate bridging ligand Q(2-) = 1,4-dioxido-9,10-anthraquinone were prepared and characterised. The related compound [RuCl(CO)(PPh3)(2)(mu-Q(x)(2-))RuCl(CO)(PPh3)(2)] (4) with E = P and Q(x)(2-) = 5,8-dioxido-1,4-naphthoquinone 4 revealed trans-positioned PPh3 groups. The electrogenerated one-electron oxidised states 1(+) and 2(+) were examined using spectroelectrochemical techniques (EPR, IR and UV/Vis/NIR). In situ EPR studies gave spectra with P-31 or As-75 hyperfine splitting of about 16 Gauss, small Ru-99,Ru-101 coupling and small g-anisotropy in the frozen solution state. The P-31 and As-75 hyperfine values reflect axial positioning of the four Ru-E bonds relative to the plane of an anthrasemiquinone bridge. Single CO stretching bands around 1910 cm(-1) of the precursors 1 and 2 shift by about 25 cm(-1) to higher energies on oxidation. The direction, uniformity and the extent of the shifts confirm ligand bridge-based oxidation. Absorbance by the cations in the near infrared region is thus assigned to intra-ligand transitions of ruthenium(II)-bonded anthrasemiquinones and not to intervalence charge transfer of mixed-valent species. Ruthenium(II) stabilisation by CO and EPh3 is made responsible for the anthrasemiquinone formation instead of metal-centered oxidation.