An unusual oxidation of the imidazolyl ring in a cobalt terpyridyl complex: crystal structure and photonuclease activity of the transformed cobalt terpyridyl complex

Abstract

This work describes a serendipitous oxidation of an imidazolyl ring to a carboxylic acid/ester group in a terpyridyl cobalt complex. The ligand transformation depends on both the oxidant and the solvent employed in the reaction and occurs during the oxidation of Co(II) to Co(III). While attempting to oxidize the parent [Co(Itpy)₂]Cl₂ (Itpy = imidazole terpyridine) complex using chlorine gas, it was found that both the metal and ligand had undergone oxidation, as was confirmed by single crystal XRD and ESI-MS. Studies using hydrogen peroxide as the oxidant revealed the formation of the expected cobalt(III) Itpy complex with an intact imidazole ring. The crucial role played by the solvent in the ligand transformation is established from the fact that while a methanolic medium transforms the imidazole ligand to a carboxymethyl ester, acetonitrile yields carboxylic acid functionality. It is likely that the chlorine radical generated during the oxidation of Co(II) to Co(III) is responsible for the unusual oxidation of the imidazolyl ring in the terpyridyl cobalt complex. The oxidized complex, [Co(Etpy)₂](ClO₄)₃, 1, (Etpy = carboxymethylester terpyridine) has been characterized spectroscopically and electrochemically, and DNA binding studies were carried out on it. From the absorption spectral studies, it was established that complex 1 binds to DNA in grooves and its intrinsic binding strength has been found to be (1.51 ± 0.18) × 10⁴ M^-1Since the imidazolyl ring was oxidized to a carboxymethyl ester, complex 1 lacks peripheral planarity and therefore binds in the grooves of the DNA. Further, molecular docking studies also confirm that the complex 1 is a major groove binder and is stabilized through hydrogen bonding interactions. The DNA cleavage experiments using pBR 322 show that the complex 1 exhibit efficient photonuclease activity in the presence of molecular oxygen.