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Sensitivity of a Strained C–C Single Bond to Charge Transfer: Redox Activity in Mononuclear and Dinuclear Ruthenium Complexes of Bis(arylimino)acenaphthene (BIAN) Ligands

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posted on 21.07.2014 by Prasenjit Mondal, Hemlata Agarwala, Rahul Dev Jana, Sebastian Plebst, Anita Grupp, Fabian Ehret, Shaikh M. Mobin, Wolfgang Kaim, Goutam Kumar Lahiri
The new compounds [Ru­(acac)2(BIAN)], BIAN = bis­(arylimino)­acenaphthene (aryl = Ph (1a), 4-MeC6H4 (2a), 4-OMeC6H4 (3a), 4-ClC6H4 (4a), 4-NO2C6H4 (5a)), were synthesized and structurally, electrochemically, spectroscopically, and computationally characterized. The α-diimine sections of the compounds exhibit intrachelate ring bond lengths 1.304 Å < d­(CN) < 1.334 and 1.425 Å < d­(CC) < 1.449 Å, which indicate considerable metal-to-ligand charge transfer in the ground state, approaching a RuIII(BIAN•–) oxidation state formulation. The particular structural sensitivity of the strained peri-connecting C–C bond in the BIAN ligands toward metal-to-ligand charge transfer is discussed. Oxidation of [Ru­(acac)2(BIAN)] produces electron paramagnetic resonance (EPR) and UV–vis–NIR (NIR = near infrared) spectroelectrochemically detectable RuIII species, while the reduction yields predominantly BIAN-based spin, in agreement with density functional theory (DFT) spin-density calculations. Variation of the substituents from CH3 to NO2 has little effect on the spin distribution but affects the absorption spectra. The dinuclear compounds {(μ-tppz)­[Ru­(Cl)­(BIAN)]2}­(ClO4)2, tppz = 2,3,5,6-tetrakis­(2-pyridyl)­pyrazine; aryl (BIAN) = Ph ([1b]­(ClO4)2), 4-MeC6H4 ([2b]­(ClO4)2), 4-OMeC6H4 ([3b]­(ClO4)2), 4-ClC6H4 ([4b]­(ClO4)2), were also obtained and investigated. The structure determination of [2b]­(ClO4)2 and [3b]­(ClO4)2 reveals trans configuration of the chloride ligands and unreduced BIAN ligands. The DFT and spectroelectrochemical results (UV–vis–NIR, EPR) indicate oxidation to a weakly coupled RuIIIRuII mixed-valent species but reduction to a tppz-centered radical state. The effect of the π electron-accepting BIAN ancillary ligands is to diminish the metal–metal interaction due to competition with the acceptor bridge tppz.