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=C 6 H 5 ( 1 + ); m ‐(Cl) 2 C 6 H 3 ( 2 + ); m ‐(OCH 3 ) 2 C 6 H 3 ( 3 + ); m ‐( t Bu) 2 C 6 H 3 ( 4 + )). A crystal structure determination of the representative compound, [ 1 ]ClO 4 , 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 CO (average: 1.299(3) Å), CN (average: 1.346(4) Å) and intra‐ring CC ( 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 [Ru III (pap 0 )(Q .− ) 2 ] + as the most appropriate electronic structural form. The coupling of three spins from one low‐spin ruthenium(III) (t 2g 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 ]ClO 4 –[ 4 ]ClO 4 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 (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− ).