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)

Dipanwita Das, Hemlata Agarwala, Abhishek Dutta Chowdhury, Tuhin Patra, Shaikh M. Mobin, Biprajit Sarkar, Wolfgang Kaim, Goutam Kumar Lahiri

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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)2C 6H3 (2+); m-(OCH3)2C 6H3 (3+); m-(tBu)2C 6H3 (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 NN length (1.292(3) Å) of pap in 1+ establish [RuIII(pap0)(Q.-)2] + as the most appropriate electronic structural form. The coupling of three spins from one low-spin ruthenium(III) (t2g5) 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 1n-4n (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-4n in the accessible redox states to be [Ru III(pap0)(Q.-)(Q0)]2+ (12+-42+)→[RuIII(pap0)(Q .-)2]+ (1+-4+) →[RuII(pap0)(Q.-)2] (1-4)→[RuII(pap.-)(Q.-)2] - (1--4-)→[RuIII(pap .-)(Q2-)2]2- (12--4 2-). Proven not innocent! All four redox-active components of [Ru(pap)(Q)2]n behave non-innocently in not immediately predictable ways according to structural, spectroelectrochemical, and computational information for the accessible forms between n=2+ and n=2- (see figure). Cations, anions, and neutral forms are distinguished by significant near-infrared absorptions (λmax>1000 nm).

Original languageEnglish
Pages (from-to)7384-7394
Number of pages11
JournalChemistry - A European Journal
Volume19
Issue number23
DOIs
StatePublished - 3 Jun 2013
Externally publishedYes

Keywords

  • density functional calculations
  • electronic structure
  • non-innocent ligands
  • ruthenium
  • spectroelectrochemistry
  • structure elucidation

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