The Radical Chemistry of 2,3-Diguanidinopyridines: Coordination and Visible-Light Photochemistry

Guanidino-functionalized aromatics (GFAs) comprise an important class of redox-active organic molecules and ligands with a variety of applications. In this work, we report the synthesis and chemistry of novel GFAs with a pyridine core functionalized in 2 and 3 positions with two guanidino groups. The two guanidino groups turn the pyridine derivative into an electron donor that can be oxidized in two potentially separated one-electron redox steps to the dicationic redox state. The redox properties and the stability of the radical monocationic redox state are tuned by the addition of further substituents at the pyridine core. The new redox-active molecules are applied as ligands in mononuclear metal complexes, leading to significant anodic shifts of the ligand redox potentials. Depending on the substitution pattern, reaction with benzylbromide leads to benzylation at the pyridine N atom or the guanidino N atom. The pyridine N-benzylated molecules are photoactive and undergo chemical reactions, initiated by visible-light induced homolytic cleavage of the N─C bond with formation of radical monocationic 2,3-diguanidinopyridine and benzyl radicals, leading to photoconversion to the higher-energetic, photostable guanidino-benzylated molecules.