Four-Center Oxidation State Combinations and Near-Infrared Absorption in [Ru(pap)(Q)2] n (Q=3,5-Di- tert-butyl- N-aryl-1,2-benzoquinonemonoimine, pap=2-Phenylazopyridine).

The complex series [Ru(pap)(Q)₂] ⁿ ([ 1] ⁿ-[ 4] ⁿ; n=+2, +1, 0, −1, −2) contains four redox non-innocent entities: one ruthenium ion, 2-phenylazopyridine (pap), and two o-iminoquinone moieties, Q=3,5-di- tert-butyl- N-aryl-1,2-benzoquinonemonoimine (aryl=C₆H₅ ( 1⁺); m-(Cl)₂C₆H₃ ( 2⁺); m-(OCH₃)₂C₆H₃ ( 3⁺); m-( tBu)₂C₆H₃ ( 4⁺)). A crystal structure determination of the representative compound, [ 1]ClO₄, established the crystallization of the ctt-isomeric form, that is, cis and trans with respect to the mutual orientations of O and N donors of two Q ligands, and the coordinating azo N atom trans to the O donor of Q. The sensitive CO (average: 1.299(3) Å), CN (average: 1.346(4) Å) and intra-ring CC ( meta; average: 1.373(4) Å) bond lengths of the coordinated iminoquinone moieties in corroboration with the NN length (1.292(3) Å) of pap in 1⁺ establish [Ru^III(pap⁰)(Q^.−)₂]⁺ as the most appropriate electronic structural form. The coupling of three spins from one low-spin ruthenium(III) (t_2g⁵) and two Q^.− radicals in 1⁺- 4⁺ gives a ground state with one unpaired electron on Q^.−, as evident from g=1.995 radical-type EPR signals for 1⁺- 4⁺. Accordingly, the DFT-calculated Mulliken spin densities of 1⁺ (1.152 for two Q, Ru: −0.179, pap: 0.031) confirm Q-based spin. Complex ions 1⁺- 4⁺ exhibit two near-IR absorption bands at about λ=2000 and 920 nm in addition to intense multiple transitions covering the visible to UV regions; compounds [ 1]ClO₄-[ 4]ClO₄ undergo one oxidation and three separate reduction processes within ±2.0 V versus SCE. The crystal structure of the neutral (one-electron reduced) state ( 2) was determined to show metal-based reduction and an EPR signal at g=1.996. The electronic transitions of the complexes 1 ⁿ- 4 ⁿ ( n=+2, +1, 0, −1, −2) in the UV, visible, and NIR regions, as determined by using spectroelectrochemistry, have been analyzed by TD-DFT calculations and reveal significant low-energy absorbance ( λ_max>1000 nm) for cations, anions, and neutral forms. The experimental studies in combination with DFT calculations suggest the dominant valence configurations of 1 ⁿ- 4 ⁿ in the accessible redox states to be [Ru^III(pap⁰)(Q^.−)(Q⁰)]²⁺ ( 1²⁺- 4²⁺)→[Ru^III(pap⁰)(Q^.−)₂]⁺ ( 1⁺- 4⁺)→[Ru^II(pap⁰)(Q^.−)₂] ( 1- 4)→[Ru^II(pap^.−)(Q^.−)₂]⁻ ( 1⁻- 4⁻)→[Ru^III(pap^.−)(Q²⁻)₂]²⁻ ( 1²⁻- 4²⁻). [ABSTRACT FROM AUTHOR]