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

Abstract: The new redox-active complexes [RuH(CO)(EPh3)2(-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(-Qx2-)RuCl(CO)(PPh3)2] (4) with E = P and Qx2- = 5,8-dioxido-1,4-naphthoquinone 4 revealed trans-positioned PPh¬3 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 31P or 75As hyperfine splitting of about 16 Gauss, small 99,101Ru coupling and small g anisotropy in the frozen solution state. The 31P and 75As 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 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.