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)

Das, Dipanwita, Agarwala, Hemlata, Chowdhury, Abhishek Dutta, Patra, Tuhin, Mobin, Shaikh M., Sarkar, Biprajit, Kaim, Wolfgang and Lahiri, Goutam Kumar 2013. 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). Chemistry - A European Journal 19 (23) , pp. 7384-7394. 10.1002/chem.201204620

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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)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 [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 1n–4n in the accessible redox states to be [RuIII(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−–42−).

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Chemistry
Publisher:	Wiley
ISSN:	0947-6539
Last Modified:	06 May 2023 01:58
URI:	https://orca.cardiff.ac.uk/id/eprint/138418

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