Aminotroponiminates as Ligands for Potential Metal-Based Nitric Oxide Sensors

A family of new fluorescently labeled ligands, HRDATI, 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-PrDATI)2] (12), [Co(t-BuDATI)2] (14), [Co(BzDATI)2] (15)) and Zn2+ ([Zn(i-PrDATI)2] (13)) complexes
were prepared and characterized by X-ray crystallography. The bis(ATI) complex [Co(i-Pr2ATI)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 Cp2Fe/Cp2Fe+) in THF for the Co2+/Co+ and Co3+/Co2+ couples, respectively, of 11. 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(RDATI)]. 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 H2DATI-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.