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-Bu2SQ)(trpy)](ClO4)2 (1s), [Cr(OH2)(3,5-Bu2Cat)(trpy)]ClO4 (1c), [Cr(OH2)(3,6-Bu2SQ)(trpy)](ClO4)2 (2), [Cr(OH2)(Cat)(trpy)]ClO4 (3), [Cr(OH2)(Cl4Cat)(trpy)]ClO4 (4), [Cr(OH2)(3,5-Bu2SQ)(Me3-tacn)](ClO4)2 (5), [Cr(OH2)(Cat)(Me3-tacn)]ClO4 (6), and [Cr(OH2)(Cl4Cat)(Me3-tacn)]ClO4 (7)
(Bu2SQ = di-tert-butyl-o-benzosemiquinonate anion, Bu2Cat = di-tert-butylcatecholate dianion, Cat = catecholate
dianion, Cl4Cat = tetrachlorocatecholate dianion, trpy = 2,2‘:6‘,2‘ ‘-terpyridine, and Me3-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 ≈ 1c > 7 > 5 ≈ 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, hydroxo−Cr(III)−semiquinone complexes were reduced to hydroxo−Cr(III)−catechol in H2O/THF at pH 11 under illumination
of visible light.