Coordination Modulation Method to Prepare New Metal-Organic Framework-Based CO-Releasing Materials

Aluminum-based metal-organic frameworks (MOFs), [Al(OH)(SDC)]_n, (H₂SDC: 4,4′-stilbenedicarboxylic acid), also known as CYCU-3, were prepared by means of the coordination modulation method to produce materials with different crystal size and morphology. In particular, we screened several reagent concentrations (20-60 mM) and modulator/ligand ratios (0-50), leading to 20 CYCU x-y materials (x: reagent concentration, y = modulator/ligand ratio) with different particle size and morphology. Noteworthy, the use of high modulator/ligand ratio gives rise to a new phase of CYCU-3 (CYCU-3′ x-50 series), which was structurally analyzed. Afterward, to test the potential of these materials as CO-prodrug carriers, we selected three of them to adsorb the photo- and bioactive CO-releasing molecule (CORM) ALF794 [Mo(CNCMe₂CO₂H)₃(CO)₃] (CNCMe₂CO₂H = 2-isocyano-2-methyl propionic acid): (i) CYCU-3 20-0, particles in the nanometric range; (ii) CYCU-3 50-5, bar-type particles with heterogeneous size, and (iii) CYCU-3′ 50-50, a new phase analogous to pristine CYCU-3. The corresponding hybrid materials were fully characterized, revealing that CYCU-3 20-0 with the smallest particle size was not stable under the drug loading conditions. Regarding the other two materials, similar ALF794 loadings were found (0.20 and 0.19 CORM/MOF molar ratios for ALF794@CYCU-3 50-5 and ALF794@CYCU-3′ 50-50, respectively). In addition, these hybrid systems behave as CO-releasing materials (CORMAs), retaining the photoactive properties of the pristine CORM in both phosphate saline solution and solid state. Finally, the metal leaching studies in solution confirmed that ALF794@CYCU-3′ 50-50 shows a good retention capacity toward the potentially toxic molybdenum fragments (75% of retention after 72 h), which is the lowest value reported for a MOF-based CORMA to date.