The binding properties of 1 with Cu2þ were further studied by
UVevis titration experiments. On the treatment with Cu2þ ion to
the solution of 1, the absorption band at 425 nm significantly decreased,
and a new band at 550 nm gradually reached a maximum
at 10 equiv of Cu2þ (Fig. 2).
The Job plot revealed a 1:1 stoichiometric ratio between the
Cu2þ ion and 1 (Fig. 3). In addition, the 1:1 stoichiometry of the
1eCu2þ complex was confirmed by ESI-mass spectrometry analysis.
The positive ion mass spectrum indicated the 1:1 binding model
between 1 and Cu2þ [m/z 420.1; calcd, 420.1] as shown in Fig. 4.
Based on Job plot and ESI-mass data analysis, we propose that
the two oxygen atoms and the imine nitrogen of 1 might bind to
Cu2þ and the remaining site of the 1eCu2þ complex may be occupied
by a nitrate anion (Scheme 3). The coordination of nitrate ions
was previously proposed when chemosensors were bound to Cu2þ
with a 1:1 stoichiometry in organic solvents