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

Abstract

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.

Influence of Quinoidal Distortion on the Electronic Properties of Oxidized Divinylarylene-Bridged Diruthenium Complexes

“…Figure compares the experimental spectra of Ru 2 BTD •+ and Ru 2 v BTD 2 •+ to the simulated spectra obtained with the parameters summarized in Table ; those of the other complexes are displayed in Figures S35–S37. This renders radical cations Ru 2 v BTD 2 •+ , Ru 2 e BTD 2 •+ , and Ru 2 v p-phen 2 •+ further examples of mixed-valent systems that are delocalized on the slower EPR time scale despite exhibiting only partial valence delocalization on the faster IR time scale. ,,,,, Of further note is the even closer proximity of the g values of the present radical cations to the free electron value g e , which is usually considered as an indicator for an enhanced ligand character of such radicals. − This is particularly evident for Ru 2 v BTD 2 •+ and, to a somewhat lesser extent, its ethynyl-bridged analog Ru 2 e BTD 2 •+ . Computed spin density distributions, which are also displayed in Figures S35–S37, are in line with this kind of reasoning (Table ).…”

Materials with electronic transitions in the near-infrared

“…form may suggest the formation of a mixed-valent species such as the Creutz-Taube ion (Figure 4). However, both the EPR and IR results from corresponding spectroelectrochemical experiments 28,29 reveal very little metal contribution to the singly occupied MO, attributing the near IR transition to the p system of the anthrasemiquinone bridge. 9,10-Anthraquinones are known as useful dyes even in the non-radical state, and the formation of metal-containing radical intermediates with NIR absorption thus creates a further attractive pathway to remarkable chromophores.…”

Exciton Coupling in Redox‐Active Salen based Self‐Assembled Metallacycles

“…2.003 and exhibiting small g-anisotropy (Δg = 0.058-0.064). [90,91] Ru-based oxidation is expected to lead to much larger g-anisotropy (i. e., broader signals). [92] Furthermore, the solution spectrum of [2 b] + displays hyperfine coupling that was simulated in the fast-motion regime to four 1 H nuclear spins (I = 1 / 2 ); 99/101 Ru contribution was not included in the simulation as no satellite hyperfine coupling is observed in the spectrum ( 99/101 Ru: 12.7 % and 17.0 % abundance, respectively; I = 5 / 2 ).…”

“…A new band at 11,000 cm À 1 is assigned as an intra-ligand transition of the ligand radical, corroborated by a literature example of an organometallic bis-Ru complex bridged by anthrasemiquinone, resembling similar spectral features pre-and post-oxidation. [91] Furthermore, DFT calculations on [2 b] + reveal delocalization of the unpaired electron across the naphthazarin ligand scaffold, and TD-DFT calculations predict a number of transitions of high intensity in the energy range of the new band at 11,000 cm À 1 (Figure S37 The reversibility of the oxidation and the stability of the oxidized species were investigated through stepwise in situ reduction and re-oxidation (Figure S38). [3 a] 2* + and [(3 b) 2 ] 8* + were reduced with two and eight molar equivalents of decamethylferrocene (FeCp* 2 ; E 1/2 = À 0.48 V in dichloromethane [87] ), respectively, returning to the original spectra (Figure S38 Characterization of the energy, intensity, and shape of the NIR bands can inform the assignment of the degree of localization in the mixed-valence systems.…”