Exploring the scope of redox-triggered chiroptical switches: Syntheses, X-ray structures, and circular dichroism of cobalt and nickel complexes of N,N-bis(arylmethyl)methionine derivatives

N,N-Bis(arylmethyl) methionine derivatives are chiral ligands whose complexes with metal ions may show molecular helicity that can be modulated by defined structural processes. It was shown previously that exciton-coupled circular dichroism (ECCD) spectral amplitude could be modulated by one-electron copper redox chemistry in copper complexes of these ligands. Here we describe the further development of novel systems that show conformational changes resulting in the inversion of exciton chirality. The phenomenon was probed in a N,N-bis(arylmethyl) methionine derivative containing quinoline/pyridine moieties and a methionine carboxylate moiety. The sign of the ECCD of the complex formed between this ligand and CoCl₂ is negative, which suggests that the deprotonated carboxylate oxygen coordinates to the metal, but the sulfur atom does not. The sign of the ECCD inverts to positive upon addition of ascorbic acid, which can be turned back to negative upon further treatment with persulfate. X-ray quality crystals of three cobalt complexes and one nickel complex were obtained. The ascorbate-treated cobalt complex of the ligand and the same ligand with nickel, however, vary from the behavior expected from their X-ray crystal structures. It is clear that the solution and crystallographic structures of these complexes differ in several cases.