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.