Rational Control of Conformational Distributions and Mixed-Valence Characteristics in Diruthenium Complexes

The electronic characteristics of mixed-valence complexes are often inferred from the shape of the inter-valence charge transfer (IVCT) band, which usually falls in the near infrared (NIR) region, and relationships derived from Marcus-Hush theory. These analyses typically assume one single, dominant molecular conformation. The NIR spectra of the prototypical delocalised (Class III Robin-Day mixed-valence) complexes [{Ru(pp)Cp'}₂ (μ-C≡C-C≡C)]⁺ ([1]⁺ : Cp'=Cp, pp=(PPh₃ )₂ ; [2]⁺ : Cp'=Cp, pp=dppe; [3]⁺ : Cp'=Cp*, pp=dppe) feature a 'two-band' pattern, which complicates band-shape analysis using these traditional methods. In the past, the appearance of sub-bands within or near the IVCT transition has been attributed to vibronic effects or localised d-d transitions. Quantum-chemical modelling of a series of rotational conformers of [1]⁺ -[3]⁺ reveals the two components that contribute to the NIR absorption band envelope to be a π-π* transition and an MLCT transition. The MLCT components only gain appreciable intensity when the orientation of the half-sandwich ruthenium ligand spheres deviates from idealised cis (Ω P-Ru-Ru-P=0°) or trans (Ω P-Ru-Ru-P=180°) conformations. The increased steric demand of the supporting ligands, together with some underlying inter-phosphine ligand T-shaped CH⋅⋅⋅π stacking interactions across the series [1]⁺ to [2]⁺ to [3]⁺ results in local minima biased towards such non-idealised conformations of the metal-ligand fragments (Ω P-Ru-Ru-P=33-153°). Experimentally, this is indicated by appearance of multiple bands within the IR ν˜ (C≡C) band envelopes and increasing intensity of the higher-energy MLCT transition(s) relative to the π-π* transition across the series, and the appearance of a pronounced 'two-band' pattern in the experimental NIR absorption envelopes. These conformational effects and the methods of analysis presented here, which combine analysis of IR and NIR spectra with quantum-chemical calculations on a range of energetically similar conformational minima, are expected to be quite general for mixed-valence systems.