Spin Interaction in Octahedral Zinc Complexes of Mono- and Diradical Schiff and Mannich Bases.

The four Schiff bases 2-tert-butyl-4-methoxy-6-[(pyridin-2-ylmethylimino)methyl]phenol, 2,4-di-tert-butyl-6-[(pyridin-2-ylmethylimino)methyl]phenol, 2-tert-butyl-4-methoxy-6-(quinolin-8-yliminomethyl)phenol, and 2,4-di-tert-butyl-6-(quinolin-8-yliminomethyl)phenol as well as one Mannich base, N,N',N,N'-bis[(2-hydroxy-3,5-di-tert-butylbenzyl)(2-pyridylmethyl)]ethylenediamine, and their Zn bis-phenolate complexes (1-5), resp., were prepd. The complexes 4 and 5 were characterized by x-ray diffraction crystallog., showing a Zn ion within an octahedral environment, with a cis orientation of the phenolate moieties. 1-5 Exhibit in their cyclic voltammetry curves two anodic reversible waves attributable to the successive oxidn. of the phenolates into phenoxyl radicals. Bulk electrolysis at ∼+0.1 V affords the Zn-coordinated monophenoxyl radical species (1·)+-(5·)+ characterized by UV-visible absorption bands at 400-440 nm. The more stable radicals are (3·)+ and (4·)+ (half-life {\textgreater} 90 min at 298 K), likely due to the increased charge delocalization within the quinoline moieties and exhibit a significant addnl. near-IR band ($ε$ {\textgreater} 1650 M-1 cm-1) attributed to a CT transition. In the two-electron-oxidized species (1··)2+-(5··)2+ the radical spins present a weak magnetic coupling. EPR reveals an antiferromagnetic exchange interaction for (1··)2+-(4··)2+, whereas an unusual ferromagnetic exchange coupling is operative in (5··)2+. The weak magnitude of exptl. |J| values (within the 1-5 cm-1 range) as well as their sign could be well reproduced by DFT calcns. at the B3LYP level. The small energy gap between the ground and the 1st excited spin states allows the authors to study the zero-field splitting (ZFS) of the triplet by EPR spectroscopy. This parameter is axial for all systems, with |D| values of 0.0163 cm-1 for (1··)2+, 0.0182 cm-1 for (2··)2+, 0.0144 cm-1 for (3··)2+, 0.0160 cm-1 for (4··)2+, and 0.0115 cm-1 for (5··)2+. The trend between exptl. ZFS is confirmed by DFT calcns., which give further insight regarding its sign (neg. for all the compds.). Lower ZFS values were obtained for (2··)2+ compared to (1··)2+ (and also for (4··)2+ compared to (3··)2+), which can be interpreted by an increased delocalization of the spin d. over the methoxy para substituent. Significant spin population on the quinoline also contributes to a lowering of the |D| value, as obsd. when (3··)2+ is compared to (1··)2+ (and also when (4··)2+ is compared to (2··)2+). [on SciFinder(R)]


Spin Interaction in Octahedral Zinc Complexes of Mono- and Diradical Schiff and Mannich Bases.
Type de publication
Article de revue
Année de publication
Inorg. Chem.
Soumis le 12 avril 2018