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 (μ-Q2- )RuH(CO)(EPh3 )2 ], E=P (1) and E=As (2) with the bis-chelate bridging ligand Q2- =1,4-dioxido-9,10-anthraquinone were prepared and characterised. The related compound [RuCl(CO)(PPh3 )2 (μ-Qx 2- )RuCl(CO)(PPh3 )2 ] (4) with E=P and Qx 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 31 P or 75 As hyperfine splitting of about 16 Gauss, small 99, 101 Ru coupling and small g-anisotropy in the frozen solution state. The 31 P and 75 As 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.