Stabilization of CO2 as zwitterionic carbamate within a coordination polymer (CP): synthesis, structure and anion sensing behaviour of a Tb-CP composite

Abstract

A new gadolinium(III)-based coordination polymer (CP), [Gd(3,5-pydc)_1.5(CO₂)_0.5(H₂O)₄]·3H₂O (1, where 3,5-pydc = 3,5-pyridinedicarboxylate), has been successfully synthesized using a slow diffusion method at room temperature. Single-crystal X-ray diffraction study of 1 confirmed its one-dimensional ladder-like structure. The Gd³⁺ ions are in a distorted tricapped trigonal prismatic geometry. Two types of 3,5-pydc ligands are present in the structures: in one case, the pyridine nitrogen remains vacant, whereas in the other case, the pyridine nitrogen is coordinated with CO₂ to form zwitterionic carbamate. The phase purity of 1 has been confirmed by powder X-ray diffraction, thermogravimetric analysis and IR spectroscopic studies. The aqueous dispersion of compound 1 exhibits weak luminescence emission at 385 nm upon excitation at 270 nm. To explore the ligand-sensitized metal-centred luminescence, we have prepared a composite by mixing the CP with Tb³⁺ ions (Tb@1). The formation of the composite has been confirmed by IR spectroscopic studies in the solid state. The composite showed excellent ligand-sensitized metal-centred luminescence. We have utilized this luminescence property for the sensing of anions in aqueous media. The composite shows luminescence quenching behaviour in the presence of Cr₂O₇²⁻, CrO₄²⁻, and NO₂⁻, with high sensitivity, and their limits of detection are 147 ppb, 142 ppb and 20 ppb, respectively. Selectivity experiments show that the detection of these three anions is highly selective in the presence of other common anions. Detailed experimental studies suggest that the preferable interaction of the three anions with the Tb³⁺ ions inside the composite and the reduction of the excitation energy are probably responsible for the quenching behaviour.