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

The new redox-active complexes [RuH(CO)(EPh ) (μ-Q )RuH(CO)(EPh ) ], E=P (1) and E=As (2) with the bis-chelate bridging ligand Q =1,4-dioxido-9,10-anthraquinone were prepared and characterised. The related compound [RuCl(CO)(PPh ) (μ-Q )RuCl(CO)(PPh ) ] (4) with E=P and Q =5,8-dioxido-1,4-naphthoquinone 4 revealed trans-positioned PPh 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 or As hyperfine splitting of about 16 Gauss, small Ru coupling and small g-anisotropy in the frozen solution state. The P and 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 of the precursors 1 and 2 shift by about 25 cm 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 EPh is made responsible for the anthrasemiquinone formation instead of metal-centered oxidation.