Rational Control of Conformational Distributions and Mixed‐Valence Characteristics in Diruthenium Complexes |
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| Authors: | Dr Josef B G Gluyas Simon Gückel Prof Dr Martin Kaupp Prof Dr Paul J Low |
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| Affiliation: | 1. School of Chemistry and Biochemistry, University of Western Australia, Crawley, Australia;2. Institut für Chemie, Technische Universit?t Berlin, Sekr. C7, Berlin, Germany |
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| 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  (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. |
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| Keywords: | ab initio calculations conformation analysis electronic structure IR spectroscopy mixed-valent compounds |
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