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Pure Appl. Chem., 2013, Vol. 85, No. 2, pp. 329-342

Published online 2012-12-18

Structural and electron paramagnetic resonance (EPR) characterization of novel vanadium(V/IV) complexes with hydroquinonate-iminodiacetate ligands exhibiting “noninnocent” activity

Chryssoula Drouza1, Marios Stylianou1, Petri Papaphilippou2 and Anastasios D. Keramidas2*

1 Department of Agricultural Sciences, Biotechnology and Food Science, Cyprus University of Technology, 3603 Limasol, Cyprus
2 Department of Chemistry, University of Cyprus, 1678 Nicosia, Cyprus

Abstract: Reaction of KVO3 with 2-[N,N'-(carboxymethyl)aminomethyl]-5-methylhydroquinone (H4mecah) in aqueous solution at pH 8.2 results in the isolation of mononuclear K2[VV(O)2{Hmecah(-3)}]·2H2O complex. On the other hand, reaction with the 2-[N,N'-(carboxymethyl)aminomethyl]-5-tert-butylhydroquinone (H4tbutcah) under the same conditions gives the tetranuclear mixed-valent complex K6[{VVO(μ-O)VIVO}{μ-tbutbicah(‑6)}]2·10.5H2O (H6tbutbicah, 2,2'-({2-[bis(carboxymethyl)amino]-3,6-dihydroxy-4-methylbenzyl}azanediyl)diacetic acid). The structures of both complexes were determined by single-crystal X-ray crystallography. The coordination environment of vanadium ions in both complexes is octahedral, with four out of the six positions to be occupied by the two cis carboxylate oxygens, one hydroquinonate oxygen, and one amine nitrogen atoms of the ligands’ tripod binding sites. The importance of the chelate ring strains in the stabilization of the p-semiquinone radical is also discussed. A protonation of the ligated to vanadium(IV) ion hydroquinonate oxygen at low pH was revealed by continuous wave (cw) X-band electron paramagnetic resonance (EPR) and UV–vis spectroscopies.