Aminotroponiminates as ligands for potential metal-based nitric oxide sensors

A family of new fluorescently labeled ligands, H(R)DATI, was prepared to develop transition-metal-based NO sensing strategies. The ligands are composed of aminotroponiminates (ATIs) with a dansyl fluorophore on one of the imine nitrogen atoms and an alkyl substituent, either i-Pr (8), t-Bu (9), or Bz (10), on the other. Bis(chelate) Co2+ ([CO((i-Pr)DATI)(2)] (12), [Co((t-Bu)DATI)(2)] (14), [Co((Bz)DATI)(2)] (15)) and Zn2+ ([Zn((i-Pr)DATI)(2)] (13)) complexes were prepared and characterized by X-ray crystallography. The bis(ATI) complex [Co(i-Pr(2)ATI)(2)] (11) was also prepared and its X-ray crystal structure determined. Cyclic voltammetry reveals reversible redox waves at -2.57 and -0.045 V (vs CprFe/Cp2Fe+) in THF for the Co2+/Co+ and Co3+/Co2+ couples, respectively, of ii. Only a Co2+/Co+ wave at -2.09 V is observed for 12. When excited at 350 nm, the HRDATI Ligands and the diamagnetic Zn2+ complex 13 fluoresce around 500 nm, whereas the paramagnetic Co2+ complexes quench the fluorescence. These air-stable cobalt compounds react with nitric oxide to dissociate a DATI ligand and form neutral dinitrosyl complexes, [Co(NO)(2)((R)DATI)]. The release of the fluorophore-containing ligand is accompanied by an increase in fluorescence intensity, thus providing a strategy for fluorescent NO sensing. Linking two DATI moieties via a tetramethylene chain affords the ligand H(2)DATI-4 (18). The Co2+ complex [Co(DATI-4)] (19) reacts more readily with NO than the bis(DATI) compounds and also displays an increase in fluorescence intensity upon NO binding.