Light-induced geometrical changes in acyclic ruthenium(II) complexes and their ruthena-macrocyclic analogues

The two ligands 1 (4'-(3-anisylphenyl)-2,2';6',2' '-terpyridine) and 2 (2-mesityl-8-anisyl-1,10-phenanthroline) (Scheme 2) were synthesized and coordinated to ruthenium. The corresponding complexes Ru(1)(2)(L)n+, where L = Cl-, CH3CN, or C5H5N, have been fully characterized. Notably, the hindering mesityl group of the phenanthroline ligand was shown to lie opposite to the monodentate ligand L both in solution and in the solid state. Upon irradiation in acetonitrile or pyridine, quantitative isomerization of the complex occurred, which consisted of a 90 degrees rotation of the bidentate chelate. In the new isomers the mesityl group was shown to pi stack to the coordinated monodentate ligand with the anisyl group of the phen (1,10-phenanthroline) lying on the other side of the ruthenium atom. The back reaction was performed by heating the photochemical isomers of the complexes in DMSO and exchanging the DMSO with chloride anion, acetonitrile, or pyridine. The stability of the ruthenium(II)-pyridine bond was used in order to inscribe the Ru(terpy)(phen) motif in a molecular ring. Functionalization of the ligands and subsequent cyclization reaction on the complex were performed on the two isomers of Ru(1)(2)(C5H5N)2+. Four macrocyclic complexes including the Ru(terpy)(phen)(py)n+ moiety were obtained and characterized. A (CH2)18 alkane chain or polyethylene glycol chain formed the flexible part of the ruthena-macrocycles. Upon visible light irradiation a dramatic geometrical changeover of the cyclic complex took place, which could be reversed thermally.