Iron and Manganese Pyridoxal-Based Complexes as Fluorescent Probes for Nitrite and Nitrate Anions in Aqueous Solution

Water-soluble iron and manganese pyridoxal-based complexes pyr(2)enFeCl (1) and pyr(2)enMnCl (2) (pyr(2)en = N,N'-ethylenebis(pyridoxylideneiminato) have been synthesized and characterized. Proof-of-principle results that the title complexes can be efficiently used as fluorescent probes for nitrite and nitrate detection are provided. The fluorescent complex pyr(2)enFeCl (1) selectively recognizes nitrate anions by fluorescent enhancement in an aqueous solution displaying high binding affinities. ESI-MS experiments clearly show the binding of these anions via the substitution of the chloride anion at the metal center. The pyridoxal-based manganese complexes of NO2- and NO3- have been isolated, and their structures have been established by single-crystal XRD. Density functional theory (DFT) calculations show that the substitution of the chloride anion at the metal center by the nitrite or nitrate anion is favored by the gain in the energies of hydration.