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

• Single crystal X-ray structural analysis revealed that the valence of Co ion to be +III and DBCat is formally a catecholate dianion, also the electronic state is ls -CoIII(Cat). • However, its acetonitrile solution exhibits MLCT band at 575 nm, this band disappears with decreasing temperature. • This behavior suggests that [Co(DBCat)(pbqa)](PF 6) undergose a temperature-dependent change from hs -CoII(SQ) to ls -CoII(Cat). • [Co(DBCat)(pbqa)](PF 6) demostrates valence tautomerism due to a strong π-π interaction between the quinolyl rings of the ancillary ligand like pbqa. Co-dioxolene complex [Co(DBCat)(pbqa)](PF 6) (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 -CoII(SQ). Of note, this MLCT band disappears with decreasing temperature. This behavior suggests that [Co(DBCat)(pbqa)](PF 6) undergoes a temperature-dependent change from hs -CoII(SQ) to ls -CoIII(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)](PF 6) demonstrates valence tautomerism due to a strong π-π interaction between the quinoline rings of the ancillary ligand. Comparison of this pbqa compound to similar Co compounds provides further evidence for these conclusions. [ABSTRACT FROM AUTHOR]