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Ruthenium Derivatives of in Situ Generated Redox-Active 1,2-Dinitrosobenzene and 2‑Nitrosoanilido. Diverse Structural and Electronic Forms
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posted on 2016-12-06, 19:19 authored by Prabir Ghosh, Soumyodip Banerjee, Goutam Kumar LahiriThe 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 NN (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.
