Ruthenium Derivatives of in Situ Generated Redox-Active 1,2-Dinitrosobenzene and 2‑Nitrosoanilido. Diverse Structural and Electronic Forms

The article describes one-pot synthesis and structural elucidation of <i>tc</i>-[Ru<sup>II</sup>(pap)<sub>2</sub>(L<sup>•–</sup>)]­ClO<sub>4</sub> [<b>1</b>]­ClO<sub>4</sub> and <i>tc</i>-[Ru<sup>II</sup>(pap)<sub>2</sub>(L′<sup>–</sup>)]­ClO<sub>4</sub> [<b>2</b>]­ClO<sub>4</sub>, which were obtained from <i>tc</i>-[Ru<sup>II</sup>(pap)<sub>2</sub>(EtOH)<sub>2</sub>]­(ClO<sub>4</sub>)<sub>2</sub> and benzofuroxan (L = 1,2-dinitrosobenzene, an intermediate tautomeric form of the biologically active benzofuroxan, L′<sup>–</sup> = 2-nitrosoanilido, pap = 2-phenylazopyridine, <i>tc</i> = <i>trans</i> and <i>cis</i> corresponding to pyridine and azo nitrogen donors of pap, respectively). The same reaction with the newly synthesized and structurally characterized metal precursor <i>cc</i>-Ru<sup>II</sup>(2,6-dichloropap)<sub>2</sub>Cl<sub>2</sub>, however, affords isomeric <i>ct</i>-[Ru<sup>II</sup>(2,6-dichloropap)<sub>2</sub>(L<sup>•–</sup>)]<sup>+</sup> (<b>3a</b><sup>+</sup>) and <i>tc</i>-[Ru<sup>II</sup>(2,6-dichloropap)<sub>2</sub>(L<sup>•–</sup>)]<sup><b>+</b></sup> (<b>3b</b><sup>+</sup>) (<i>cc</i>, <i>ct</i>, and <i>tc</i> with respect to pyridine and azo nitrogens of 2,6-dichloropap) with the structural authentication of elusive <i>ct</i>-isomeric form of {Ru­(pap)<sub>2</sub>} family. The impact of <i>trans</i> or <i>cis</i> orientation of the nitroso group of L/L′ with respect to the NN (azo) function of pap in the complexes was reflected in the relative lengthening or shortening of the latter distance, respectively. The redox-sensitive bond parameters of <b>1</b><sup>+</sup> and <b>3</b><sup>+</sup> reveal the intermediate radical form of L<sup>•–</sup>, while <b>2</b><sup>+</sup> involves in situ generated L′<sup>–</sup>. The multiple redox processes of the complexes in CH<sub>3</sub>CN are analyzed via experimental and density functional theory (DFT) and time-dependent DFT calculations. One-electron oxidation of the electron paramagnetic resonance-active radical species (<b>1</b><sup>+</sup> and <b>3</b><sup>+</sup>) leads to [Ru<sup>II</sup>(pap)<sub>2</sub>(L)]<sup>2+</sup> involving fully oxidized L<sup>0</sup> in <b>1</b><sup>2+</sup> and <b>3</b><sup>2+</sup>; the same in <b>2</b><sup>+</sup> results in a radical species [Ru<sup>II</sup>(pap)<sub>2</sub>(L′<sup>•</sup>)]<sup>2+</sup> (<b>2</b><sup>2+</sup>). Successive reductions in each case are either associated with pap or L/L′<sup>–</sup>-based orbitals, revealing a competitive scenario relating to their π-accepting features. The isolated or electrochemically generated radical species either by oxidation or reduction exhibits near-IR transitions in each case, attributing diverse electronic structures of the complexes in accessible redox states.