Not All 3 MC States Are the Same: The Role of 3 MC cis States in the Photochemical N ∧ N Ligand Release from [Ru(bpy) 2 (N ∧ N)] 2+ Complexes.

Ruthenium(II) complexes feature prominently in the development of agents for photoactivated chemotherapy; however, the excited-state mechanisms by which photochemical ligand release operates remain unclear. We report here a systematic experimental and computational study of a series of complexes [Ru(bpy) 2 (N ∧ N)] 2+ (bpy = 2,2'-bipyridyl; N ∧ N = bpy ( 1 ), 6-methyl-2,2'-bipyridyl ( 2 ), 6,6'-dimethyl-2,2'-bipyridyl ( 3 ), 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole ( 4 ), 1-benzyl-4-(6-methylpyrid-2-yl)-1,2,3-triazole ( 5 ), 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl ( 6 )), in which we probe the contribution to the promotion of photochemical N ∧ N ligand release of the introduction of sterically encumbering methyl substituents and the electronic effect of replacement of pyridine by 1,2,3-triazole donors in the N ∧ N ligand. Complexes 2 to 6 all release the ligand N ∧ N on irradiation in acetonitrile solution to yield cis- [Ru(bpy) 2 (NCMe) 2 ] 2+ , with resultant photorelease quantum yields that at first seem counter-intuitive and span a broad range. The data show that incorporation of a single sterically encumbering methyl substituent on the N ∧ N ligand ( 2 and 5 ) leads to a significantly enhanced rate of triplet metal-to-ligand charge-transfer ( 3 MLCT) state deactivation but with little promotion of photoreactivity, whereas replacement of pyridine by triazole donors ( 4 and 6 ) leads to a similar rate of 3 MLCT deactivation but with much greater photochemical reactivity. The data reported here, discussed in conjunction with previously reported data on related complexes, suggest that monomethylation in 2 and 5 sterically inhibits the formation of a 3 MC cis state but promotes the population of 3 MC trans states which rapidly deactivate 3 MLCT states and are prone to mediating ground-state recovery. On the other hand, increased photochemical reactivity in 4 and 6 seems to stem from the accessibility of 3 MC cis states. The data provide important insights into the excited-state mechanism of photochemical ligand release by Ru(II) tris-bidentate complexes.