Comparative photo-release of nitric oxide from isomers of substituted terpyridinenitrosyl-ruthenium(II) complexes: experimental and computational investigations

The 4'-(2-fluorenyl)-2,2':6',2"-terpyridine (FT) ligand and its cis(Cl, Cl)- and trans(Cl, Cl)-[Ru-II(FT)Cl-2(NO)]-(PF6) complexes have been synthesized. Both isomers were separated by HPLC and fully characterized by H-1 and C-13 NMR. The X-ray diffraction crystal structures were solved for FT (Pna2(1) space group, a = 34.960(4), b = 5.9306(7), c = 9.5911(10) angstrom), and trans(Cl, Cl)-[Ru-II(FT)Cl-2(NO)](PF6)center dot MeOH (P (1) over bar space group, a = 10.3340(5), b = 13.0961(6), c = 13.2279(6) angstrom, alpha = 72.680(2), beta = 70.488(2), gamma = 67.090(2)degrees). Photo-release of NO. radicals occurs under irradiation at 405 nm, with a quantum yield of 0.31 and 0.10 for cis(Cl, Cl)-[Ru-II(FT)Cl-2(NO)](PF6) and trans(Cl, Cl)-[Ru-II(FT)Cl-2(NO)](PF6), respectively. This significant difference is likely due to the trans effect of Cl-, which favors the photo-release. UV-visible spectroscopy and cyclic voltammetry indicate the formation of ruthenium(III) species as photoproducts. A density functional theory (DFT) analysis provides a rationale for the understanding of the photo-physical properties, and allows relating the weakening of the Ru-NO bond, and finally the photo-dissociation, to HOMO -> LUMO excitations.