High-pressure study of a 3d–4f heterometallic CuEu–organic skeleton

We prepared a 3d–4f heterobimetallic CuEu–organic framework NBU-8 with a density of 1921 kg m⁻³ belonging to the family of dense packing materials (dense metal–organic frameworks or MOFs). This MOF material was prepared from 4-(pyrimidin-5-yl)benzoic acid (HPBA) with a bifunctional ligand site as a tripodal ligand and Cu²⁺ and Eu³⁺ as the metal centres; the molecular formula is Cu₃Eu₂(PBA)₆(NO₃)₆·H₂O. This material is a very promising dimethylformamide (DMF) molecular chemical sensor. Systematic high-pressure studies of NBU-8 were carried out by powder X-ray diffraction, high-pressure X-ray diffraction and molecular dynamics simulation. The high-pressure experiment shows that the (006) diffraction peak of the crystal structure moves toward a low angle with increasing pressure, accompanied by the phenomenon that the d-spacing increases, and as the pressure increases, the (10) diffraction peak moves to a higher angle, the amplitude of the d-spacing is significantly reduced and finally merges with the (006) diffraction peak into one peak. The amplitude of the d-spacing is significantly reduced, indicating that NBU-8 compresses and deforms along the a-axis direction when subjected to uniform pressure. This is caused by tilting of the ligands to become more vertical along the c direction, leading to its expansion. This allows greater contraction along the a direction. We also carried out a Rietveld structure refinement and a Birch–Murnaghan solid-state equation fitting for the high-pressure experimental results. We calculated the bulk modulus of the material to be 45.68 GPa, which is consistent with the calculated results. The framework is among the most rigid MOFs reported to date, exceeding that of Cu–BTC. Molecular dynamics simulations estimated that the mechanical energy absorbed by the system when pressurized to 5.128 GPa was 249.261 kcal mol⁻¹. The present work will provide fresh ideas for the study of mechanical energy in other materials.