Sensitive Valence Structures of [(pap)₂Ru(Q)]ⁿ (n = +2, +1, 0, −1, −2) with Two Different Redox Noninnocent Ligands, Q = 3,5-Di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine and pap = 2-Phenylazopyridine

The complexes [(pap)₂Ru(Q)]ClO₄, [1]ClO₄−[4]ClO₄, with two different redox noninnocent ligands, Q = 3,5-di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine (-aryl = m-(Cl)₂C₆H₃ (1⁺), C₆H₅ (2⁺), m-(OCH₃)₂C₆H₃ (3⁺), and m-(^tBu)₂C₆H₃ (4⁺)) and pap = 2-phenylazopyridine, have been synthesized and characterized using various analytical techniques. The single-crystal X-ray structure of the representative [2]ClO₄·C₇H₈ exhibits multiple intermolecular C−H···O hydrogen bondings and C−H···π interactions. The C1−O1 = 1.287(4) (density functional theory, DFT, 1.311) and C6−N1 = 1.320(4) (DFT, 1.353) Å and intraring bond distances associated with the sensitive quinine (Q) moiety along with the azo(pap) bond distances, N3−N4 = 1.278(4) (DFT, 1.297) and N6−N7 = 1.271(4) (DFT, 1.289) Å, in 2⁺ justify the [(pap)₂Ru^II(Q^•−)]⁺ valence configuration at the native state of 1⁺−4⁺. Consequently, Mulliken spin densities on Q, pap, and Ru in 2⁺ are calculated to be 0.8636, 0.1040, and 0.0187, respectively, and 1⁺−4⁺ exhibit free radical sharp EPR spectra and one weak and broad transition around 1000 nm in CH₃CN due to interligand transition involving a singly occupied molecular orbital (SOMO) of Q^•− and the vacant π* orbital of pap. Compounds 1⁺−4⁺ undergo a quasi-reversible oxidation and three successive reductions. The valence structure of the electron paramagnetic resonance (EPR)-inactive oxidized state in 1²⁺−4²⁺ has been established as [(pap)₂Ru^II(Q°)]²⁺ instead of the alternate formalism of antiferromagnetically coupled [(pap)₂Ru^III(Q^•−)]²⁺ on the basis of the DFT calculations on the optimized 2⁺, which predict that the singly occupied molecular orbital is primarily composed of Q with 77% contribution. Accordingly, the optimized structure of 2²⁺ predicts shorter C1−O1 (1.264) and C6−N1 (1.317 Å) distances and longer Ru1−O1 (2.080) and Ru1−N1 (2.088 Å) distances. Compounds 1²⁺−4²⁺ exhibit the lowest energy transitions around 600 nm, corresponding to Ru(dπ)/Q(π) → pap(π*). The presence of two sets of strongly π-acceptor ligands, pap and Q, in 1²⁺−4²⁺ stabilizes the Ru(II) state to a large extent such that the further oxidation of {Ru^II−Q°} → {Ru^III−Q°} has not been detected within +2.0 V versus a saturated calomel electrode. The EPR-inactive reduced states 1−4 have been formulated as [(pap)₂Ru^II(Q²⁻)] over the antiferromagnetically coupled alternate configuration, [(pap)(pap^•−)Ru^II(Q^•−)]. The optimized structure of 2 predicts sensitive C1−O1 and C6−N1 bond distances of 1.337 and 1.390 Å, respectively, close to the doubly reduced Q²⁻ state, whereas the NN distances of pap, N3−N4 = 1.299 and N6−N7 = 1.306 Å, remain close to the neutral state. In corroboration with the doubly reduced Q²⁻ state, 1−4 exhibit a moderately strong interligand π(Q²⁻) → π*(pap) transition in the near-IR region near 1300 nm. The subsequent two reductions are naturally centered around the azo functions of the pap ligands.