Photochemistry of Ru^II 4,4′Bi-1,2,3-triazolyl (btz) complexes: Crystallographic characterization of the photoreactive ligand-loss intermediate trans-[Ru(bpy)(κ²-btz)(κ¹-btz) (NCMe)]²⁺

We report the unprecedented observation and unequivocal crystallographic characterization of the meta-stable ligand loss intermediate solvento complex trans-[Ru(bpy)(κ²-btz)(κ¹-btz)(NCMe)] ²⁺ (1a) 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(NŇ)(btz)₂]²⁺ (1a-d: btz=1,1′dibenzyl-4,4′bi-1,2,3-triazolyl; NŇ=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, 1a-d eventually convert to the bis-solvento complexes trans-[Ru(NŇ)(btz)(NCMe)₂]²⁺ (3a-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 3a and 3d confirmed the co-planar arrangement of the NŇ and btz ligands and the trans coordination of two solvent molecules. These conversions proceed via the observed intermediate complexes 2a-d, which are formed quantitatively from 1a-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 40C) allowed the isolation of 2a in the solid state, and the complex to be crystallographically characterized. Similarly to the structures adopted by complexes 3a and d, the bpy and κ²-btz ligands in 2a coordinate in a square-planar fashion with the second monodentate btz ligand coordinated trans to an acetonitrile ligand.