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 RuII complex having tris(2-pyridylmethl)amine (TPA) and 2,2‘-bipyridine (bpy), [Ru(TPA)(bpy)]X2 (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 SN2-like mechanism with
activation energy of 117 kJ/mol. In contrast, the complex showed
reversible photochemical dissociation and rebinding via an SN1-like mechanism by MLCT irradiation. The photochemical dissociation was accelerated ∼200-fold faster than the thermal process.
The dissociation process involves selective binding behavior toward
external ligands (solvents) with π-acceptor character, which is
indispensable, and no σ-donating molecules could bind to the RuII
center. The guest molecule can be released upon photoirradiation
after its thermal binding.