Dense crystal packing as a limiting factor for bioactivity in Nd(iii) and Sm(iii) metal-organic frameworks: structural insights and biological evaluation.

The great rise in oxidative stress due to free radicals and multidrug-resistant (MDR) bacterial infections has led to a need to rationally develop multifunctional materials that combine antibacterial and antioxidant properties. The coordination of samarium(iii) and neodymium(iii) centers to 4,6-diamino-2-pyrimidinethiol in this work formed two lanthanide-based metal-organic frameworks (MOFs) designated as Sm-MOF and Nd-MOF through a solvothermal reaction method. The densely packed, highly crystalline, thermally stable frameworks with low surface accessibility and low porosity were confirmed by physicochemical characterization using XPS, PXRD, FTIR, FE-SEM/EDX, UV-vis spectroscopy, BET/BJH, TGA/DSC, and magnetic susceptibility measurements. Biological studies showed a weak antibacterial action towards Staphylococcus aureus ATCC 6538 and Escherichia coli ATCC 25922, and the MICs of both compounds were 5 mg mL-1. Nevertheless, the insignificant inhibition zones in the agar diffusion assays and the low bactericidal activity indicate poor overall biological efficacy, which additional testing confirmed. The DPPH radical-scavenging assay used to measure antioxidant activity yielded a maximum inhibition of only 22% for Sm-MOF and 24% for Nd-MOF, meaning IC50 values could not be determined. The main reasons contributing to this limited antibacterial and antioxidant activity include high framework packing, low surface area, blocked coordination sites, and restricted molecular diffusion within the rigid structures of the lanthanide frameworks. These findings highlight the impact of structural accessibility and porosity on the biological activity of Ln-MOFs and offer valuable insights for the rational design of next-generation, bioactive lanthanide-based MOFs.