NO Photorelease from a Ruthenium Complex Assisted by Formation of a Supramolecular Dimer

This work presents the NO release from compound [Ru(biq)2(H2O)(NO)](PF6)3 (biq = 2,2’-biquinoline) with visible light irradiation (λirrad = 660 nm), assisted by the low-absorbing photosensitizer [Ru(biq)2Cl2]. The structure of both compounds were characterized by means of ESI-MS (electrospray ionization mass spectrometry). The NO+ stretching, ν(NO) = 1995 cm-1, is atypically shifted to higher energy. This observation, along with the E1/2 = 0.49 V (vs. Ag/AgCl) assigned to the Ru2+/3+ redox pair observed for compound [Ru(biq)2Cl2] and its photoreactivity in solution suggest that the RuII ion, when coordinated to two biquinolines, behaves as a hard Pearson acid. Molecular modelling results confirmed the typical geometry distortion of ruthenium-polypyridine complexes bearing sterically hindered ligands. They also suggest the formation of a supramolecular dimer, assembled by weak interaction between biquinoline ligands from each compound, that is claimed to be responsible for the high efficiency of the NO photorelease bimolecular sensitization.