Solution stability of Cu(II) metal–organic polyhedra

In this report the structural stability of self assembled Cu(II) hydroxylated nanoballs (CuOH NB) [Cu2+2(5-OH-bdc)2L2]12 ((5-OH-bdc)2− = 5-hydroxybenzene-1,3-dicarboxylate; L = dimethyl sulfoxide, methanol or water ligand) are investigated using optical absorption and emission spectroscopies. Specifically, the effects of temperature, hydrostatic pressure, and solvent conditions on the CuOH NB stability were determined by monitoring both the ligand emission intensity and the metal to ligand charge transfer (MLCT) absorption band of the complex under different solution conditions. The results of temperature and pressure variation suggest that the CuOH NB is stable in solution with minimal perturbations between 10 and 60 °C and from ambient pressure up to 3.5 kbar. Degradation of the CuOH NB occurs with increasing concentrations of water with complete dissociation in methanol solutions containing ⩾0.7 mole fraction water. In methanolic solutions containing <0.7 mole fraction water the CuOH NB is stable over a pH range from 5 to 10 while at low pH (i.e., <4.5) the CuOH NB dissociates presumably due to protonation of the carboxylic groups associated with the ligand. The corresponding degradation at high pH (i.e., >10.5) is likely due to coordination of hydroxide ions to the Cu2+ ions resulting in disruption of the nanoball structure. Finally addition of imidazole disrupts the CuOH NB due to competition between the OH-bdc and imidazole for coordination to the Cu2+ ions. Overall these results provide important insights into the range of conditions required for the general solution stability of metal–organic polyhedra.

Optical spectroscopy (including steady state absorption and fluorescence) has been used to probe the stability of Cu(II) metal organic polyhedra (MOPs) in solution. The results demonstrate a stability towards water up to ∼0.7 mole fraction, stability at pH’s >4 and <10, and progressive degradation in the presence of bases such as imidazole. These studies ultimately provide a guide for the design of future MOPs with enhanced functionality and stability.Figure optionsDownload as PowerPoint slideHighlights
► Cu(II) metal–organic polyhedra exhibit solution stability up to 3.5 kbar hydrostatic pressure.
► Degradation is observed with water (>0.7 mole fraction), pH >10.5 and <4.5, and imidazole.
► Cu(II) metal–organic polyhedra exhibit stability up to 60 °C.
► A general mechanism is proposed for MOP degradation involving ligand competition.