Designing Geometric Degrees of Freedom in ReO3-Type Coordination Polymers

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 adress this challenge. Here we introduce a new class of hierarchically organized, perovskite-like AB2X6 coordination polymers based on a [BX3]- ReO3-type host network ([Mn(C2N3)3]-), in which the spatial orientation of divalent A2+ cations with separated charge centers that bridge adjacent ReO3-cavities ([R3N(CH2)nNR3]2+) is introduced as a new geometric degree of freedom. Herringbone and head-to-tail order pattern of [R3N(CH2)nNR3]2+ cations are obtained by varying the separator length n and, together with distortions of the pseudocubic [BX3]- 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.