Molecular Scalpel to Chemically Cleave Metal-Organic Frameworks for Induced Phase Transition.

A bottom-up chemical synthesis of metal-organic frameworks (MOFs) permits significant structural diversity because of various combinations of metal centers and different organic linkers. However, fabrication generally complies with the classic hard and soft acids and bases (HSAB) theory. This restricts direct synthesis of desired MOFs with converse Lewis type of metal ions and ligands. Here we present a top-down strategy to break this limitation via the structural cleavage of MOFs to trigger a phase transition using a novel "molecular scalpel". A conventional CuBDC MOF (BDC = 1,4-benzenedicarboxylate) prepared from a hard acid (Cu2+) metal and a hard base ligand was chemically cleaved by l-ascorbic acid acting as chemical scalpel to fabricate a new Cu2BDC structure composed of a soft acid (Cu1+) and a hard base (BDC). Controlled phase transition was achieved by a series of redox steps to regulate the chemical state and coordination number of Cu ions, resulting in a significant change in chemical composition and catalytic activity. Mechanistic insights into structural cleavage and rearrangement are elaborated in detail. We show this novel strategy can be extended to general Cu-based MOFs and supramolecules for nanoscopic casting of unique architectures from existing ones.