Control of the Crystallization Process and Structure Dimensionality of Mg–Benzene–1,3,5-Tricarboxylates by Tuning Solvent Composition

Four new magnesium 1,3,5-benzenetricarboxylate metal–organic framework materials (NICS-n; n = 3–6) were synthesized solvothermally in the presence of solvents with different EtOH/H₂O ratios. We showed that the crystallization process of the Mg–1,3,5-benzentricarboxylate system strongly depends on the solvent composition, and that dimensionality of their structures can be tuned by changing the EtOH/water ratios in the reaction mixture. The presence of only water as a solvent yields the zero-dimensional molecular structure of Mg­(H₂BTC)₂(H₂O)₄ (NICS-3). One-dimensional (1D) chainlike Mg₃(BTC)₂(H₂O)₁₂ (NICS-4) and two-dimensional (2D) layered Mg₂(BTC)­(OH)­(H₂O)₄·2H₂O (NICS-5) structures were crystallized from EtOH/H₂O mixtures with molar ratios of 0.3 and 0.4–0.7, respectively. The crystallization in pure ethanol yields Mg₃(BTC)₂ material (NICS-6) with three-dimensional structure. Nuclear magnetic resonance investigations indicated that bulkier clusters of Mg species are formed in ethanol-rich solutions, even in the absence of the BTC ligand, and that the starting precursors formed with the reaction of Mg species and the BTC ligand at room temperature does not represent the final structures obtained by solvothermal reactions. NICS-4 and NICS-5 are formed from similar starting precursors but slightly different EtOH/H₂O ratios causing the crystallization to go in two different directions. Systematic investigation of phase formation using different EtOH/H₂O ratios, times, and temperatures of the synthesis along with the computational DFT studies confirmed that the 2D NICS-5 structure represents a thermodynamically more stable phase than 1D chainlike NICS-4. We showed that solvothermal reaction between Mg-precursors and the BTC ligand in EtOH/water mixture represents a complex and sensitive thermodynamic process.