Modulation of fluorescence emission of 1-(2-pyridyl) pyrazole derived Schiff base ligands by exploiting their metal ion sensitive binding modes

Abstract

Dioxomolybdenum(VI) complexes [MoO 2(B 1)H 2O] ( 1), [MoO 2(B 2)EtOH] ( 2), [MoO 2(B 3)EtOH] ( 3) and [MoO 2(B 4)EtOH] ( 4) were synthesized using the Schiff base ligands H 2B 1(previously reported), H 2B 2, H 2B 3 and H 2B 4, respectively. These ligands were prepared by condensation of 1-(2-pyridyl) 5-methyl 3-pyrazole carbohydrazide with salicylaldehyde, o-hydroxy acetophenone, 5-bromo salicylaldehyde and 5-nitro salicylaldehyde respectively. Due to the presence of a substituted 1-(2-pyridyl) pyrazole unit, ligands H 2B 1, H 2B 2 and H 2B 3 exhibit fluorescent emissions, and the most intense emission was obtained for H 2B 3. H 2B 4 is incapable of showing fluorescence emission. As the ligands are capable of using different binding modes, according to the demands of the guest metal ions, their emission properties also change accordingly. The dioxomolybdenum(VI) complex of the ligand H 2B 1, i.e. complex 1, shows quenched emission compared to H 2B 1. Again when Cu 2+, Co 2+ or Ni 2+ ions are added to a solution of 1, in each case a new complex of Cu 2+ Co 2+ or Ni 2+ is formed in solution and further quenching was observed. However, with Zn 2+ input to a solution of 1, fluorescence recovery was observed up to the level of the free ligand. The copper(II) complex of H 2B 1 (complex 5), produced by adding equivalent amount of Cu 2+ salt to a solution of 1, was isolated and characterized. One of the dioxomolybdenum(VI) complexes, 3, when subjected to an oxo-transfer reaction with PPh 3 produces complex [MoO(B 3)CH 3CN] ( 6). Complex 6 shows reduced fluorescence emissions compared to 3 in the solid phase. These observations open up the possibilities for these ligands to work as fluorescent signaling system with different metal ion inputs. All the complexes are characterized by elemental analyses, electronic spectra, IR, 1H NMR, magnetic measurements, EPR and by cyclic voltammetry. Complexes 1 and 5, as well as the ligands H 2B 2 and H 2B 3, have been crystallographically characterized.