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

Abstract

The article describes one-pot synthesis and structural elucidation of tc-[RuII(pap)2(L•-)]ClO4 [1]ClO4 and tc-[RuII(pap)2(L'-)]ClO4 [2]ClO4, which were obtained from tc-[RuII(pap)2(EtOH)2](ClO4)2 and benzofuroxan (L = 1,2-dinitrosobenzene, an intermediate tautomeric form of the biologically active benzofuroxan, L'- = 2-nitrosoanilido, pap = 2-phenylazopyridine, tc = trans and cis corresponding to pyridine and azo nitrogen donors of pap, respectively). The same reaction with the newly synthesized and structurally characterized metal precursor cc-RuII(2,6-dichloropap)2Cl2, however, affords isomeric ct-[RuII(2,6-dichloropap)2(L•-)]+ (3a+) and tc-[RuII(2,6-dichloropap)2(L•-)]+ (3b+) (cc, ct, and tc with respect to pyridine and azo nitrogens of 2,6-dichloropap) with the structural authentication of elusive ct-isomeric form of {Ru(pap)2} family. The impact of trans or cis 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 1+ and 3+ reveal the intermediate radical form of L•-, while 2+ involves in situ generated L'-. The multiple redox processes of the complexes in CH3CN 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 (1+ and 3+) leads to [RuII(pap)2(L)]2+ involving fully oxidized L0 in 12+ and 32+; the same in 2+ results in a radical species [RuII(pap)2(L'•)]2+ (22+). Successive reductions in each case are either associated with pap or L/L'--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.