ИСТИНА

Crystalline solids exhibiting anomalous lattice dynamics–e.g., anisotropic or negative thermal expansion–represent significant interest as a versatile playground for the design of “smart” stimuli-responsive materials. In particular, metal propionates (Prop–) were reported to exhibit structural phase transitions, which are primarily driven by the thermally activated rotation of aliphatic groups. The present work reports an extensive crystallographic study of polynuclear and polymeric metal propionates with a primary focus on their thermally-driven lattice dynamics. Within the rare-earth (RE, Ln) propionate system, a family of 2D-layered coordination polymers [Ln2(H2O)2Prop6]∞, which consists of two distinct structural types (I – Ln=Ce, II – Ln=Ho–Lu, Y) has been revealed. As evidenced by powder- and single-crystal XRD, [Ce2(H2O)2Prop6] exists in the form of two polytypes (α and β), which differ from each other in the supramolecular arrangement of polymeric layers, the structure of these being identical for both structures. Given the pronounced crystallographic anisotropy of (α,β)-[Ce2(H2O)2Prop6], both polytypes exhibit a highly anisotropic thermal expansion, which additionally proved to be strongly packing-dependent. Indeed, α-[Ce2(H2O)2Prop6] demonstrate a non-monotonic temperature dependence of unit cell parameters with colossal positive (+899 MK–1) and negative (–427 MK–1) linear CTEs in the 190–210 K region. On the contrary, β-[Ce2(H2O)2Prop6] exhibits a moderate positively defined thermal expansion with no pronounced anomalies in the 100–300 K range. Experimental data on the thermal expansion of (α,β)-[Ce2(H2O)2Prop6] are discussed in terms of supramolecular interactions based on periodic DFT calculations. Similarly, [Ln2(H2O)2Prop6] of type II (Ln = Ho–Lu, Y) were also shown to form two distinct polytypes, which proved to be interconvertible upon thermal and mechanical treatment. Among heterometallic propionates, a 3D-coordination polymer [HKCu2Prop6] has been characterized. As revealed from the variable-temperature powder and single-crystal XRD experiments, [HKCu2Prop6] undergoes a martensitic phase transition, which, however, makes no impact on the magnetic exchange magnitude within discrete [Cu2Prop4] “paddlewheels” (2J = –333(4) cm–1). In summary, the present work reports a large family of structurally flexible crystalline solids that might shed light on the peculiar structure-property correlations among metal-organic systems.