Cobalt(II) and Cobalt(III) Dipicolinate Complexes: Solid State, Solution, and in Vivo Insulin-like Properties

The synthesis and characterization of Co(II) and Co(III) 2,6-pyridinedicarboxylate (dipic2-) complexes are reported.
Solid-state X-ray characterizations were performed on [Co(H2dipic)(dipic)]·3H2O and [Co(dipic)(μ-dipic)Co(H2O)5]·2H2O. Two coordination modes not previously observed in dipicolinate transition metal complexes were observed
in these complexes; one involves metal coordination to the short C−O (CO) bond, and the other involves metal
coordination to a protonated oxygen atom. Solution studies, including paramagnetic NMR and UV−vis spectroscopy,
were done showing the high stability and low lability of the Co(III) complex, whereas the Co(II) complexes exhibited
ligand exchange in the presence of excess ligand. The [Co(dipic)2]2- complex has pH dependent lability and in this
regard is most similar to the [VO2dipic]- complex. The [Co(dipic)2]2- was found to be effective in reducing the
hyperlipidemia of diabetes using oral administration in drinking water in rats with STZ-induced diabetes. Oral
administration of VOSO4 was used as a positive control for metal efficacy against diabetes. In addition to providing
a framework to evaluate structure−function relationships of various transition metal complexes in alleviating the
symptoms of diabetes, this work describes novel aspects of structural and solution cobalt chemistry.