Turning on the flexibility of isoreticular porous coordination frameworks for drastically tunable framework breathing and thermal expansion

To study the potential flexibility of the (3,9)-connected xmz frameworks, a series of isoreticular metal carboxylate frameworks [M₃(μ₃-OH)(L)₃], namely MCF-18(L, M), were constructed by 3-connected, tripodal pyridyl-dicarboxylate ligands (H₂L1 = pyridine-3,5-dicarboxylic acid; H₂L2 = 4,4′-(pyridine-3,5-diyl)dibenzoic acid; H₂L3 = 2,6-di-p-carboxyphenyl-4,4′-bipyridine) and 9-connected, tricapped trigonal-prismatic M₃(μ₃-OH)(O₂CR)₆(py)₃(M = Fe, Co, Ni; py = pyridyl group) clusters. Powder and single-crystal X-ray diffraction studies showed that, while other isoreticular analogs do not show framework flexibility, the newly designed material MCF-18(L3, Ni) can drastically swell 70–105% in volume and 75–121% in length upon inclusion of different guests, the latter of which is the highest reported value to date. Comparison study showed that the nearly uniaxial framework breathing is generated by the special regulation effect of the xmz topology, but can only be activated by a ligand with a suitable shape. Moreover, the thermal expansion profile of MCF-18(L3, Ni) can be drastically tuned by guest, showing extremely large thermal expansion coefficients up to 430 × 10⁻⁶K⁻¹. The guest-included crystals also show water-like thermal expansion behaviors depending on the cooling rate, which is unanticipated for intrinsic crystalline materials.