title: 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).
abstract: 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)(pap(0))(Q(·-))2 ](+) as the most appropriate electronic structural form. The coupling of three spins from one low-spin ruthenium(III) (t2g(5)) 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 (λmax >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)(pap(0))(Q(·-))(Q(0))](2+) (1(2+)-4(2+))→[Ru(III)(pap(0))(Q(·-))2](+) (1(+)-4(+))→[Ru(II)(pap(0))(Q(·-))2] (1-4)→[Ru(II)(pap(·-))(Q(·-))2](-) (1(-)-4(-))→[Ru(III)(pap(·-))(Q(2-))2](2-) (1(2-)-4(2-)).
authors.affiliation: Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India.
authors.forename: Dipanwita
authors.initials: D
authors.lastname: Das
authors.forename: Hemlata
authors.initials: H
authors.lastname: Agarwala
authors.forename: Abhishek Dutta
authors.initials: AD
authors.lastname: Chowdhury
authors.forename: Tuhin
authors.initials: T
authors.lastname: Patra
authors.forename: Shaikh M
authors.initials: SM
authors.lastname: Mobin
authors.forename: Biprajit
authors.initials: B
authors.lastname: Sarkar
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Kaim
authors.forename: Goutam Kumar
authors.initials: GK
authors.lastname: Lahiri
mesh_terms: D000042:Absorption; D056831:Coordination Complexes / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D004583:Electrons; D007536:Isomerism; D008024:Ligands; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D012428:Ruthenium / Q000737:chemistry*
chemical_list: D056831:Coordination Complexes; D008024:Ligands; D009942:Organometallic Compounds; D012428:Ruthenium