Toward a photochemical and thermal molecular machine:: Reversible ligand dissociation and binding in a ruthenium(II)-2,2′-bipyridine complex with tris(2-pyridylmethyl)amine

A Ru-II complex having tris(2-pyridylmethl)amine (TPA) and 2,2'-bipyridine (bpy), [Ru(TPA)(bpy)]X-2 (X = ClO4, PF6), exhibited a severe distortion of the coordination of the axial pyridine moiety of TPA due to steric hindrance. The complex showed interesting dissociation-binding behavior of the axial pyridine arm to form a solvent adduct with TPA ligation in a unique meridional tridentate fashion. The complex undergoes thermal dissociation to form solvent-coordinated species via an S(N)2-like mechanism with activation energy of 117 kJ/mol. In contrast, the complex showed reversible photochemical dissociation and rebinding via an S(N)1-like mechanism by MLCT irradiation. The photochemical dissociation was accelerated similar to200-fold faster than the thermal process. The dissociation process involves selective binding behavior toward external ligands (solvents) with pi-acceptor character, which is indispensable, and no a-donating molecules could bind to the Ru-II center. The guest molecule can be released upon photoirradiation after its thermal binding.