Sensitive valence structures of [(pap)(2)Ru(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)(2)Ru(Q)]ClO(4), [1]ClO(4)-[4]ClO(4), with two different redox noninnocent ligands, Q = 3,5-di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine (-aryl = m-(Cl)(2)C(6)H(3) (1(+)), C(6)H(5) (2(+)), m-(OCH(3))(2)C(6)H(3) (3(+)), and m-((t)Bu)(2)C(6)H(3) (4(+))) and pap = 2-phenylazopyridine, have been synthesized and characterized using various analytical techniques. The single-crystal X-ray structure of the representative [2]ClO(4).C(7)H(8) exhibits multiple intermolecular C-H...O hydrogen bondings and C-H...pi interactions. The C1-O1 = 1.287(4) (density functional theory, DFT, 1.311) and C6-N1 = 1.320(4) (DFT, 1.353) A 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) A, in 2(+) justify the (pap)(2)Ru(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 CH(3)CN due to interligand transition involving a singly occupied molecular orbital (SOMO) of Q(-) and the vacant pi 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 1(2+)-4(2+) has been established as (pap)(2)Ru(II)(Q degrees ) instead of the alternate formalism of antiferromagnetically coupled (pap)(2)Ru(III)(Q(*-)) 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 2(2+) predicts shorter C1-O1 (1.264) and C6-N1 (1.317 A) distances and longer Ru1-O1 (2.080) and Ru1-N1 (2.088 A) distances. Compounds 1(2+)-4(2+) exhibit the lowest energy transitions around 600 nm, corresponding to Ru(dpi)/Q(pi) --> pap(pi*). The presence of two sets of strongly pi-acceptor ligands, pap and Q, in 1(2+)-4(2+) stabilizes the Ru(II) state to a large extent such that the further oxidation of {Ru(II)-Q degrees } --> {Ru(III)-Q degrees } 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)(2)Ru(II)(Q(2-))] 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 A, respectively, close to the doubly reduced Q(2-) state, whereas the N horizontal lineN distances of pap, N3-N4 = 1.299 and N6-N7 = 1.306 A, remain close to the neutral state. In corroboration with the doubly reduced Q(2-) state, 1-4 exhibit a moderately strong interligand pi(Q(2-)) --> pi*(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.