Redox and pH-induced switching of the coordination sites in the 3-hydroxypicolinate ruthenium(III)-edta complex

Ruthenium(III)-edta reacts with the 3-hydroxypicolinate ligand (Hhpic(-)) a t pH 5, yielding the practically colorless [Ru-III(edta)(kappaN, kappaO-Hhp ic)](2-) complex (H(2)hpic = 3-hydroxypicolinic acid; H(4)edta=ethylenedini trilotetraacetic acid). Above pH 9, deprotonation of the phenolic group pro motes an intramolecular linkage isomerization process, generating the faint red [Ru-III(edta) (kappaO, kappaO-hpic)](3-) complex. Both isomers can be electrochemically reduced, converting into a single deep red [Ru-II(edta)(k appaN, kappaO-Hhpic)](3-) complex strongly stabilized by ruthenium-to-pyrid inecarboxylate d(pi) -->p(pi)*, charge-transfer interactions. The observed distinct binding properties as a function of the oxidation states and pH ha ve been rationalized based on semiempirical theoretical calculations for th e complexes.