Mono- versus bidentate coordination of the NONOate [Et₂N(N₂O₂)]^- to copper(II) complexes of tetradentate ligands

Copper(II) complexes of [Et₂N(N₂O₂)]^- with tetradentate, tripodal ligands composed of 1,4,7-triazacyclononanes with pyridine (L^Py), amide (L^HAmH) or phenolate (L^R2, R = Me or Bu^t phenolate substituents) groups as the fourth donor were prepared and characterized by spectroscopic and X-ray crystallographic methods. Bidentate O,O-coordination of [Et₂N(N₂O₂)]^- was observed in the complexes supported by L^Py and L^HAmH, but the stronger phenolate donor in L^R2 induced novel monodentate coordination via the terminal oxygen atom, defined by X-ray crystallography for R = Bu^t. Comparison of the bonding parameters of [Et₂N(N₂O₂)]^- when bound bidentate versus when bound monodentate revealed subtle but significant differences in the N-O distances. The amount and rate of nitric oxide (NO) release from the compounds was measured using a chemiluminescence detection method. Despite the differences in [Et₂N(N₂O₂)]^- structural parameters resulting from the divergent coordination modes, insignificant variation of the rates of NO release therefrom were found.