Coordination Flexibility of Zn(II) in Trimesate-Based Metal–Organic Frameworks: Formation of Distinct Secondary Building Units

Two zinc(II)-trimesate metal–organic frameworks were synthesized under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction. Although both compounds originate from the same Zn(II)-benzene-1,3,5-tricarboxylate (BTC) chemical system, they crystallize in different space groups and exhibit distinct coordination environments and secondary building units (SBUs). One framework adopts a cubic structure and is built from a binuclear Zn paddlewheel type SBU, characterized by a short Zn–Zn internuclear distance and four μ2-bridging carboxylate groups. In contrast, the second framework crystallizes in a tetragonal lattice and features mixed Zn(II) coordination environments, with the coexistence of tetrahedral and octahedral metal centers assembled into a fundamentally different SBU. The comparison between these two structures highlights the coordination flexibility of Zn(II) and the sensitivity of Zn–BTC frameworks to crystallization conditions, such as solvent composition. These results underline the importance of detailed crystallographic analysis in revealing SBU diversity and provide insight into how variations in local coordination chemistry can lead to distinct framework architectures from identical chemical building blocks.