Ligand as Buffer for Improving Chemical Stability of Coordination Polymers

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.