Ruthenium-hydride-mediated stabilisation of the azo anion radical of azobis(benzothiazole) and its reversible electron-reservoir feature

The study addresses the question of stabilisation of the azo anion radical state of azobis(benzothiazole) (abbt, L) on a selective ruthenium platform without the involvement of direct intramolecular electron transfer (IET) or solvent oxidation, as has been demonstrated earlier, and its reversible electron-reservoir feature were investigated. Specifically, the ruthenium-hydride metal precursor (Ru-II(H)(Cl)(CO)(PPh3)(3))-mediated stabilisation of the azo anion radical of abbt derivatives in Ru-II(L center dot-)(Cl)(CO)(PPh3)2 (1: L1 = 2,2 '-azobis(benzothiazole), 2: L2 = 2,2 '-azobis(6-methylbenzothiazole), 3: L3 = 2,2 '-azobis(6-chlorobenzothiazole)) without the alteration of the metal oxidation state (Ru-II). The radical state of L center dot- in 1-3 (S = 1/2, mu(eff)/B.M. = 1: 1.83, 2: 1.78, 3: 1.86 in CDCl3 at 298 K, determined by Evans method) displayed free radical electron paramagnetic resonance (EPR) (g approximate to 2.0) with the partial resolution of nitrogen hyperfine splitting. The effective role of the Ru-H metal precursor towards the {L-0-Ru-II-H} -> {Ru-II-L center dot- + 1/2 H-2} transformation in 1-3 was corroborated by the detection of H-2 gas from the reaction mixture as well as by its calculated favourable thermodynamic parameters (Delta G: -76 to -84 kcal mol(-1)). Moreover, the low oxidation potential of L center dot- -> L-0 in 1-3 (0.14-0.31 V versus SCE) facilitated the isolation of the corresponding oxidised complexes [Ru-II(L-0)(Cl)(CO)(PPh3)(2)]ClO4 ([1]ClO4, [2]ClO4, [3]ClO4) by using AgClO4 as the chemical oxidant. The structural evaluation of [1]ClO4 to [3]ClO4 revealed: (i) a lengthening of the azo bond (N1-N4 > 1.3 angstrom) with respect to the corresponding free ligand (L approximate to 1.25 angstrom) due to d pi(Ru-II) to pi*(azo) back bonding and (ii) the planar/slightly non-planar L2, L1/L3 (torsional angle: 8.09(degrees), 9.07(degrees)) introduced two equivalent and inequivalent PPh3 co-ligands in [2]ClO4, [1]ClO4, and [3]ClO4, respectively. The L-0 state in 1(+)-3(+) could also be reduced to L center dot- in 1-3 by hydrazine hydrate, attributed to its reversible electron-reservoir feature. Also, 1(n)-3(n) (n = 1, 0, -1) exhibited mixed metal-ligand-based charge-transfer transitions in the UV-visible region as a function of L and n.