Acid-base equilibrium of aqua-chromium-dioxolene complexes aimed at formation of oxo-chromium complexes

A series of aqua-Cr(III)-dioxolene complexes, [Cr(OH2)(3,5-Bu(2)SQ)(trpy)](ClO4)(2) (1s), [Cr(OH2)(3,5-Bu(2)Cat)(trpy)]ClO4 (1C), [Cr(OH2)(3,6-Bu(2)SQ)(trpy)](ClO4)(2) (2), [Cr(OH2)(Cat)(trpy)]ClO4 (3), [Cr(OH2)(Cl(4)Cat)(trpy)]ClO4 (4), [Cr(OH2)(3,5-Bu(2)SQ)(Me-3-tacn)](ClO4)(2) (5), [Cr(OH2)(Cat)(Me-3-tacn)]ClO4 (6), and [Cr(OH2)(Cl(4)Cat)(Me-3-tacn)]ClO4 (7) (Bu(2)SQ = di-tert-butyl-o-benzosemiquinonate anion, Bu(2)Cat = di-tert-butylcatecholate dianion, Cat = catecholate dianion, Cl(4)Cat = tetrachlorocatecholate dianion, trpy = 2,2':6',2-terpyridine, and Me-3-tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), were prepared. On the basis of the crystal structures, redox behavior, and elemental analyses of these complexes, dioxolene in 1c, 3, 4, 6, and 7 coordinated to Cr(III) as the catechol form, and the ligand in 1s, 2, and 5 was linked to Cr(III) with the semiquinone form. All the aqua-Cr(III) complexes reversibly changed to the hydroxo-Cr(III) ones upon dissociation of the aqua proton, and the pKa value of the aqua-Cr(III) complexes increased in the order 6 > 3 approximate to 1c > 7 > 5 approximate to 4 > 1s. Hydroxo-Cr(III)-catechol complexes derived from 1c, 3, 4, 6, and 7 did not show any signs of dissociation of their hydroxy proton. On the other hand, hydroxoCr(III)-semiquinone complexes were reduced to hydroxo-Cr(III)-catechol in H2O/THF at pH 11 under illumination of visible light.