Ligand as Buffer for Improving Chemical Stability of Coordination Polymers

Abstract

Chemical stability is one of the key concerns in coordination polymers (CPs). However, technologies to protect CPs against acidic or alkaline aqueous environments have yet to be implemented. Herein we demonstrate an approach for improving the pH stability by utilizing the ligand salt as buffering site to modify the unsaturated coordination sites of CPs. For the selective one-dimensional CP Eu–d-DBTA (d-H2DBTA = d-O,O′-dibenzoyltartaric acid) with a pH stability range of 6–8, the introduction of the ligand salt Na–d-DBTA extends the pH stability interval from 3 to 11. Crystallographic structure data reveal the formation of a Eu/Na–d-DBTA dynamic structure with Na–d-DBTA buffer sites on the Eu–O cluster of the Eu–d-DBTA skeleton. Benefiting from the dynamic single-crystal-to-single-crystal transformation, the buffer sites protect the skeleton from the impact of the acidic or alkaline aqueous environment. In addition, Eu/Na–d-DBTA produces stable photoluminescence properties and selective responses toward l-tryptophan (l-Trp) and further toward l-lysine (l-Lys) over the whole buffer capacity range of 3–11. Noticeably, other Ln/Na–d-DBTA CPs and star metal–organic frameworks also exhibit pH stability improvement when the ligand-as-buffer technology is used, which is significant for developing advanced inorganic–organic hybrid materials with superior functionality.