Pure and Applied Chemistry

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[V^V(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[{V^VO(μ-O)V^IVO}{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.