From layered to porous: solvent-controlled structural diversity in Zn(II)-BTC coordination frameworks

Three Zn(II)-benzene-1,3,5-tricarboxylate (Zn-BTC) phases were obtained under distinct hydrothermal conditions and characterized by single-crystal X-ray diffraction. Although all compounds derive from the same Zn(II)/BTC chemical system, they exhibit markedly different crystallographic symmetries, framework dimensionalities, and inorganic building units. Compound 1, Zn(HBTC)(H₂O)₂, is a hexagonal two-dimensional layered framework (P6₃/m) in which the BTC linker is only partially deprotonated, and Zn(II) adopts a single distorted octahedral coordination geometry. Compound 2, Zn₉(BTC)₅(µ₃-OH)₃(H₂O)₉, is a three-dimensional trigonal framework (P3c1) built from trinuclear [Zn₃(µ₃-OH)] clusters in which Zn(II) occupies both octahedral and tetrahedral sites. Compound 3, C₇₂H50O₆₁Zn₁₂, is a three-dimensional orthorhombic porous framework (Cmc2₁) of higher structural complexity, incorporating twelve crystallographically independent Zn(II) centers and µ₃-hydroxo bridging units. Computational pore analysis reveals distinct accessibility regimes across the three phases, ranging from a nitrogen-inaccessible interlayer void space in the 2D framework 1 (pore limiting diameter 3.08 Å, below the N₂ probe diameter), through a narrow but accessible one-dimensional channel system in 2 (140.44 m² g⁻¹), to a genuinely open porous architecture in 3 (network-accessible surface area 640.68 m² g⁻¹, pore limiting diameter 6.98 Å). These results demonstrate that the Zn(II)/BTC system exhibits pronounced structural sensitivity to the reaction medium, generating phases with different dimensionalities, protonation states, and inorganic subunit nuclearity from relatively small variations in hydrothermal crystallization conditions.