Photochemistry of Ru(II) 4,4′-Bi-1,2,3-triazolyl (btz) Complexes: Crystallographic Characterization of the Photoreactive Ligand-Loss Intermediate trans-[Ru(bpy)(κ(2)-btz)(κ(1)-btz)(NCMe)](2+)

We report the unprecedented observation and unequivocal crystallographic characterization of the meta-stable ligand loss intermediate solvento complex trans-[Ru(bpy)(κ(2)-btz)(κ(1)-btz)(NCMe)](2+) (1 a) that contains a monodentate chelate ligand. This and analogous complexes can be observed during the photolysis reactions of a family of complexes of the form [Ru([Image: see text])(btz)(2)](2+) (1 a–d: btz=1,1′-dibenzyl-4,4′-bi-1,2,3-triazolyl; [Image: see text]=a) 2,2′-bipyridyl (bpy), b) 4,4′-dimethyl-2,2′-bipyridyl (dmbpy), c) 4,4′-dimethoxy-2,2′-bipyridyl (dmeobpy), d) 1,10-phenanthroline (phen)). In acetonitrile solutions, 1 a–d eventually convert to the bis-solvento complexes trans-[Ru([Image: see text])(btz)(NCMe)(2)](2+) (3 a–d) along with one equivalent of free btz, in a process in which the remaining coordinated bidentate ligands undergo a new rearrangement such that they become coplanar. X-ray crystal structure of 3 a and 3 d confirmed the co-planar arrangement of the [Image: see text] and btz ligands and the trans coordination of two solvent molecules. These conversions proceed via the observed intermediate complexes 2 a–d, which are formed quantitatively from 1 a–d in a matter of minutes and to which they slowly revert back on being left to stand in the dark over several days. The remarkably long lifetime of the intermediate complexes (>12 h at 40 °C) allowed the isolation of 2 a in the solid state, and the complex to be crystallographically characterized. Similarly to the structures adopted by complexes 3 a and d, the bpy and κ(2)-btz ligands in 2 a coordinate in a square-planar fashion with the second monodentate btz ligand coordinated trans to an acetonitrile ligand.