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

Abstract

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...p 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 p-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(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 Å, respectively, close to the doubly reduced Q^2- 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 Q^2- state, 1-4 exhibit a moderately strong interligand p(Q^2-) → 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.