Aqueous behavior and reactivity towards nitric oxide of NAMI-A type complexes bearing bulky N-heterocyclic ligands

Abstract

The comparative studies on stability and reactivity towards nitric oxide (NO) of two ruthenium complexes of general formula HL[trans-RuCl4(dmso)L], were L=Hind=1H-indazole or Isq=isoquinoline under physiological mimicking conditions (pH=7.4, T=37°C) are reported. Progression of hydrolysis and nitrosylation has been followed by spectroscopic (UV–Vis stopped-flow, 1HNMR, IR), chromatographic (HPLC) and electrochemical (DPV) techniques. Both ruthenium complexes undergo multi-stage hydrolysis leading to dissociation of not only Cl− and dmso ligands but also bulky N-heterocyclic ligand (79% of Hind and 32% of Isq after 1h at physiological conditions). The parent ruthenium complexes and their hydrolytic products have been evaluated with regard to their redox potentials. Comparison with NAMI-A ((H2Im)[trans-RuCl4(dmso)(HIm)], where HIm=imidazole) allowed to observe correlations: the lower the N-heterocyclic ligand basicity the higher the redox potential of the complex and the higher the degree of N-heterocyclic ligand loss. The reaction with NO is relatively slow and requires opening up a coordination site in the parent complexes via aquation. HPLC analysis revealed formation of several nitrosylation products depending on the progress of hydrolysis of the studied complexes. Diminished N-heterocyclic ligand loss has been observed during the reaction with NO when compared with similar behavior during hydrolysis. These findings point towards higher stability of nitrosylated complexes bearing coordinated indazole or isoquinoline ligands.