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

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 alpha-dine sections of the compounds exhibit intrachelate ring bond lengths 1.304 angstrom < d(cN) < 1.334 and 1.425 angstrom < d(cc) < 1.449 angstrom, which indicate considerable metal-to-ligand charge transfer in the ground state, approaching a Ru-III(BIAN(center dot-)) oxidation state formulation. The particular structural sensitivity of the strained pen-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 Ru-III 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 {(mu-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 (RuRuII)-Ru-III mixed-valent species but reduction to a tppz-centered radical state. The effect of the pi electron-accepting BIAN ancillary ligands is to diminish the metal-metal interaction due to competition with the acceptor bridge tppz