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

© 2016 Wiley-VCH Verlag GmbH & Co.

KGaA, WeinheimThe 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 p-p* 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 (O P-Ru-Ru-P=0°) or trans (O P-Ru-Ru-P=180°) conformations. The increased steric demand of the supporting ligands, together with some underlying inter-phosphine ligand T-shaped CH···p 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 (O P-Ru-Ru-P=33–153°). Experimentally, this is indicated by appearance of multiple bands within the IR (Formula presented.) (C=C) band envelopes and increasing intensity of the higher-energy MLCT transition(s) relative to the p-p* 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 energe