Photochemistry of Heteroleptic 1,4,5,8-Tetraazaphenanthrene- and Bi-1,2,3-triazolyl-Containing Ruthenium(II) Complexes

Diimine metal complexes have significant relevance in the development of photodynamic therapy (PDT) and photoactivated chemotherapy (PACT) applications. In particular, complexes of the TAP ligand (1,4,5,8-tetraazaphenanthrene) are known to lead to photoinduced oxidation of DNA, while TAP- and triazole-based complexes are also known to undergo photochemical ligand release processes relevant to PACT. The photophysical and photochemical properties of heteroleptic complexes [Ru(TAP)n(btz)3-n]2+ (btz = 1,1′-dibenzyl-4,4′-bi-1,2,3-triazolyl, n = 1 (1), 2 (2)) have been explored. Upon irradiation in acetonitrile, 1 displays analogous photochemistry to that previously observed for [Ru(bpy)(btz)2]2+ (bpy = 2,2′-bipyridyl) and generates trans-[Ru(TAP)(btz)(NCMe)2]2+ (5), which has been crystallographically characterized, with the observation of the ligand-loss intermediate trans-[Ru(TAP)(κ2-btz)(κ1-btz)(NCMe)]2+ (4). Complex 2 displays more complicated photochemical behavior with not only preferential photorelease of btz to form cis-[Ru(TAP)2(NCMe)2]2+ (6) but also competitive photorelease of TAP to form 5. Free TAP is then taken up by 6 to form [Ru(TAP)3]2+ (3) with the proportion of 5 and 3 observed to progressively increase during prolonged photolysis. Data suggest a complex set of reversible photochemical ligand scrambling processes in which 2 and 3 are interconverted. Computational DFT calculations have enabled optimization of geometries of the pro-trans 3MCcis states with repelled btz or TAP ligands crucial for the formation of 5 from 1 and 2, respectively, lending weight to recent evidence that such 3MCcis states play an important mechanistic role in the rich photoreactivity of Ru(II) diimine complexes.