title: Rational Control of Conformational Distributions and Mixed-Valence Characteristics in Diruthenium Complexes.
abstract: 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.
authors.affiliation: School of Chemistry and Biochemistry, University of Western Australia, 35 Stirling Highway, Crawley, 6009, Australia.
authors.forename: Josef B G
authors.identifier: http://orcid.org/0000-0001-8577-3843
authors.initials: JB
authors.lastname: Gluyas
authors.affiliation: Institut für Chemie, Technische Universität Berlin, Sekr. C7, Strasse des 17. Juni 135, 10623, Berlin, Germany.
authors.forename: Simon
authors.initials: S
authors.lastname: Gückel
authors.affiliation: Institut für Chemie, Technische Universität Berlin, Sekr. C7, Strasse des 17. Juni 135, 10623, Berlin, Germany. martin.kaupp@tu-berlin.de.
authors.forename: Martin
authors.initials: M
authors.lastname: Kaupp
authors.affiliation: School of Chemistry and Biochemistry, University of Western Australia, 35 Stirling Highway, Crawley, 6009, Australia. paul.low@uwa.edu.au.
authors.forename: Paul J
authors.identifier: http://orcid.org/0000-0003-1136-2296
authors.initials: PJ
authors.lastname: Low
keywords: IR spectroscopy; ab initio calculations; conformation analysis; electronic structure; mixed-valent compounds