Spectroscopic characterization of valence tautomeric behavior in a cobalt-dioxolene complex using an ancillary ligand containing quinoline groups

Co-dioxolene complex [Co(DBCat)(pbqa))(PF6) (DBCat = 3,5-di-tert-butylcatechol, pbqa = (2-pyridylmethyl)bis (2-quinolylmethyl)amine), was prepared from methanol solution. Single crystal X-ray structural analysis revealed that the valence of the cobalt ion to be + III, as DBCat is formally a catecholate dianion, whereas the acetonitrile solution exhibits an MLCT absorption near 575 nm, suggesting it is present as hs-Co-II (SQ). Of note, this MLCT band disappears with decreasing temperature. This behavior suggests that [Co(DBCat)(pbqa)1(PF6) undergoes a temperature-dependent change from hs-Co-II(SQ) to ls-Co-III(Cat). Magnetic susceptibility vs. temperature data for the compound displayed a gradual variation over 250 K, due to intramolecular electron transfer in the solid-state as well. (Co(DBCat)(pbqa)](PF6) demonstrates valence tautomerism due to a strong pi-pi interaction between the quinoline rings of the ancillary ligand. Comparison of this pbqa compound to similar Co compounds provides further evidence for these conclusions.