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( $\widehat{NN}$ )(btz) 2 ] 2+ ( 1 a – d : btz=1,1′‐dibenzyl‐4,4′‐bi‐1,2,3‐triazolyl; $\widehat{NN}$ =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( $\widehat{NN}$ )(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 $\widehat{NN}$ 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.