Synthesis and Characterization of Ruthenacarborane Complexes Incorporating ChelatingN-Donor Ligands: Unexpected Luminescence from the Complex [3-CO-3,3-{κ2-Me2N(CH2)2NMe2}-closo-3,1,2-RuC2B9H11]

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Abstract
A synthetic methodology using double carbonyl substitution of the starting tricarbonyl complex [3,3,3-(CO)3-closo-3,1,2-RuC2B9H11] (1) with 2 mol equiv of the reagent Me3NO has been employed to afford ruthenacarborane complexes with chelating N-donor ligands. Three of these complexes, [3-CO-3,3-{κ2-4,4‘-R2-2,2‘-(NC5H3)2}-closo-3,1,2-RuC2B9H11] (3a, R = H; 3b, R = (CH2)8Me; 3c, R = But), comprise 2,2‘-bipyridyl ligands with hydrogen, n-nonyl, or t-butyl groups in the 4,4‘-positions of the rings, respectively. Photophysical analysis revealed no substantial luminescent activity, but the complexes are electrochemically active, undergoing sequential (reversible and quasi-reversible) one-electron reductions, the second of which likely precipitating a ligand displacement. Cyclic voltammetry (CV) experiments revealed an irreversible one-electron oxidation (Epa ≈ 0.9 V) in MeCN, on the other hand, followed by rapid CO substitution by the solvent and reversible secondary reduction (E1/2 ≈ 0.1 V). The primary redox couple became quasi-reversible in CH2Cl2, and spectroelectrochemical analysis of complex 3c provided evidence of a closo → isocloso structural modification upon oxidation. An analogue of these complexes employing the TMEDA (N,N,N‘,N‘-tetramethylethylenediamine) ligand, [3-CO-3,3-{κ2-Me2N(CH2)2NMe2}-closo-3,1,2-RuC2B9H11] (4), was synthesized using the same methodology. Cyclic voltammetric measurements displayed a reversible metal-based one-electron oxidation whether in CH2Cl2 or MeCN, with no indication of subsequent CO substitution or a similar closo → isocloso adjustment. Complex 4 was unexpectedly weakly luminescent (λem = 360 nm) in THF (tetrahydrofuran) at ambient temperatures, demonstrating a more intense phosphorescent emission in MeTHF (2-methyltetrahydrofuran) glass at 77 K (λem = 450 nm, τ450 = 0.77 ms). The X-ray crystallographic structures of complexes 3a and 4 are reported along with spectroscopic IR, NMR (1H, 13C, 11B), UV−vis absorption, EPR, and CV data.

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