Ruthenium(II) pyridylamine complexes with diimine ligands showing reversible photochemical and thermal structural change

Ruthenium(II)-TPA-diimine complexes, [Ru(TPA)(diimine)]²⁺ (TPA = tris(2-pyridylmethyl)amine ; diimine = 2,2′-bipyridine (bpy), 2,2′-bipyrimidine (bpm), 1,10-phenanthroline (phen)) were synthesized and characterized by spectroscopic and crystallographic methods. Their crystal structures demonstrate severe steric hindrance between the TPA and diimine ligands. They exhibit drastic structural changes on heating and photoirradiation at their MLCT bands, which involve partial dissociation of the tetradentate TPA ligand to exhibit a facially tridentate mode accompanied by structural change and solvent coordination to give [Ru(TPA)(diimine)(solvent)]²⁺ (solvent = acetonitrile, pyridine). The incoming solvent molecules are required to have π-acceptor character, since σ-donating solvent molecules do not coordinate. The thermal process is irreversible dissociation to give the solvent-bound complexes, which takes place by an interchange associative mechanism with large negative activation entropies. The photochemical process is a reversible reaction reaching a photostationary state, probably by a dissociative mechanism involving a five-coordinate intermediate to afford the same product as obtained in the thermal reaction. Quantum yields of the forward reactions to give dissociated products were lower than those of the backward reactions to recover the starting complexes. In the photochemical process, the conversions of the forward and backward reactions depend on the absorption coefficients of the starting materials and those of the products at certain wavelength, as well as the quantum yields of those reactions. The reversibility of the motions can be regulated by heating and by photoirradiation at certain wavelength for the recovery process. In the bpm system, we could achieve about 90% recovery in thermal/photochemical structural interconversion.