Synthesis,
characterization, reactivity, and sensing properties
of 4-formylbenzoate complexes of copper(II), zinc(II), and cadmium(II)
possessing the 1,10-phenanthroline ancillary ligand are studied. The
crystal structures of the (1,10-phenanthroline)bis(4-formylbenzoate)(aqua)copper(II)
and (1,10-phenanthroline)bis(4-formylbenzo-ate)zinc(II) and a novel
molecular complex comprising an assembly of mononuclear and dinuclear
species of (1,10-phenanthroline)bis(4-formylbenzoate)cadmium(II) are
reported. These zinc and cadmium complexes are fluorescent; they show
differentiable sensitivity to detect three positional isomers of nitroaniline.
The mechanism of sensing of nitroanilines by 1,10-phenanthroline and
the complexes are studied by fluorescence titrations, photoluminescence
decay, and dynamic light scattering. A plausible mechanism showing
that 1,10-phenanthroline ligand-based emission quenched by electron
transfer from the excited state of 1,10-phenanthroline to nitroaniline
is supported by density functional theory calculations. In an anticipation
to generate a fluorescent d10-copper(I) formylbenzoate
complex by a mild reducing agent such as hydroxylamine hydrochloride
for similar sensing of nitroaromatics as that of the d10-zinc and cadmium 4-formylbenzoate complexes, reactivity of d9-copper(II) with hydroxylamine hydrochloride in the presence
of 4-formylbenzoic acid and 1,10-phenanthroline is studied. It did
not provide the expected copper(I) complex but resulted in stoichiometry-dependent
reactions of 4-formylbenzoic acid with hydroxylamine hydrochloride
in the presence of copper(II) acetate and 1,10-phenanthroline. Depending
on the stoichiometry of reactants, an inclusion complex of bis(1,10-phenanthroline)(chloro)copper(II)
chloride with in situ-formed 4-((hydroxyimino)methyl)benzoic acid
or copper(II) 4-(hydroxycarbamoyl)benzoate complex was formed. The
self-assembly of the inclusion complex has the bis(1,10-phenanthroline)(chloro)copper(II)
cation encapsulated in hydrogen-bonded chloride-hydrate assembly with
4-((hydroxyimino)methyl)benzoic acid.