Designing Geometric Degrees of Freedom in ReO_3-Type Coordination Polymers

摘要

Engineering the interplay of structural degrees of freedom that couple to external stimuli such as temperature and pressure is a powerful approach for material design. New structural degrees of freedom expand the potential of the concept, and coordination polymers as a chemically versatile material platform offer fascinating possibilities to address this challenge. Here, we report a new class of perovskite-like AB_2X_6 coordination polymers based on a BX_3~- ReO_3-type host network (Mn(C_2N_3)_3~?), in which the spatial orientation of divalent A~(2+) cations (R_3N(CH_2)_nNR_3~(2+)) with separated charge centers that bridge adjacent ReO_3-cavities is introduced as a new geometric degree of freedom. Herringbone and head-to-tail order pattern of R_3N(CH_2)_nNR_3~(2+) cations are obtained by varying the separator length n and, together with distortions of the pseudocubic BX_3~? network, they determine the materials’ stimuli-responsive behavior such as counterintuitive large negative compressibility and uniaxial negative thermal expansion. This new family of coordination polymers highlights the chemists’ capabilities of designing matter on a molecular level to address macroscopic material functionality and underpins the opportunities of the design of structural degrees of freedom as a conceptual framework for rational material synthesis in the future.