The trans-labilization of nitric oxide in RuII complexes by C-bound imidazoles

The synthesis, characterization and reactivity of trans-[NO(L)(NH3)4Ru]Cl3 (L=imidazole, theophylline and caffeine) are presented. 1H NMR spectroscopy indicates that the imidazole ligands are coordinated to the RuII through a carbon atom (imκ2, 1,3Me2Xanκ8 and 1,3,7Me3Xanκ8). The nitrosyl stretching frequencies (νNO≅1913 cm−1) suggest the coordinated nitrosyl has substantial NO+ character. The complexes undergo a single-electron reduction (E°≅−0.50 V versus Ag/AgCl), which involves the coordinated nitrosyl. Dissociation of NO· in the reduced species is facilitated by the trans-imidazolylidene ligand. The lower than expected reduction potentials of these complexes may account for their inactivity in evoking neuronal firing in the hippocampus by releasing NO following reduction.