Copper(II), Zinc(II), and Cadmium(II) Formylbenzoate Complexes: Reactivity and Emission Properties

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.