An Mg Solid Solution Affords a Less Toxic and More Stable ZIF‑8

Porous coordination polymers (PCPs) and metal-organic frameworks (MOFs) are promising vehicles for drug delivery, provided that their metallic and organic constituents are biologically safe and their structure is stable under physiological conditions. ZIF-8 is the archetypical PCP for such applications; however, high Zn content causes concerns about its long-term use, while poor stability at acidic pH might limit the controlled delivery of active ingredients to target tissues. The combination of Zn²⁺ and Mg²⁺ affords a mixed-metal ZIF-8 analogue with up to 20% of the alkali heart. Besides reducing the dose of zinc, the solid solution shows increased water stability under mildly acidic conditions. DFT calculations point to a reduction of the MOF's polarizability as a cause for the observed behavior. Ideally, the changes in the Zn/Mg ratio would correspond to a modulation of such stability; however, equal stability improvement was measured over a range of compositions.