Nitric oxide-induced fluorescence enhancement by displacement of dansylated ligands from cobalt

The cobalt complexes [Co(Ds-AMP)₂] (1) and [Co(Ds-AQ) ₂] (2), where Ds-AMP and Ds-AQ are the conjugate bases of dansyl aminomethylpyridine (Ds-HAMP) and dansyl aminoquinoline (Ds-HAQ), respectively, were synthesized in two steps as fluorescence-based nitric oxide (NO) sensors and characterized by X-ray crystallography. The fluorescence of the two complexes was significantly quenched in CH₃CN or CH₃OH compared to that of the free Ds-HAMP or Ds-HAQ ligands. Addition of NO to a CH₃CN solution of 1 or 2 enhanced the integrated fluorescence emission by factors of 2.1(+0.3) or 3.6(+0.4) within 35 or 20 min, respectively. Introduction of NO to methanolic solutions of the complexes similarly increased the fluorescence by 1.4(±0.1) for 1 or 6.5(±1.4) for 2 within 1 h. These studies demonstrate that 1 and 2 can monitor the presence of NO with turn-on emission and that their fluorescence responses are more rapid than those of previously reported cobalt systems in coordinating solvents such as CH ₄CN and CH₃OH. ¹H NMR and IR spectroscopic data revealed the formation of a {Co(NO)₂}¹⁰ cobalt-dinitrosyl adduct, with concomitant dissociation of one ligand from the cobalt center, as the metal-containing product of the NO reactions, a result indicating NO-induced ligand release to be the cause of the fluorescence increase.