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 pyr2enFeCl (1) and
pyr2enMnCl (2) (pyr2en = 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
pyr2enFeCl (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.