Ultrathin Nanosheets Formed by Metal–Organic Cages Connected via Hydrogen Bonds

Ultrathin two-dimensional (2D) materials commonly feature repeat units interconnected through robust covalent or coordination bonds. In this work, we report a distinctive 2D material formed by assembling discrete metal-organic cages (MOCs) into extended nanosheets through directional hydrogen bonding. Coordination of a dicarboxylate-functionalized boron dipyrromethene (bodipy) ligand with bis(cyclopentadienyl)zirconium dichloride affords a layered crystal composed of bodipy-based tetrahedrons. The tetrahedra within the intralayer possess hydrogen bonds in close contact, while interactions across interlayers are supported by weak van der Waals interactions. This discrepancy enables a freeze-thaw method to exfoliate the layered crystal into nanosheets that are composed of one to three layers, approximately 2-6 nm thick, with aspect ratios up to 15,000:1. These nanosheets are solely stabilized by hydrogen bonds, and their structural integrity is confirmed by high-resolution transmission electron microscopy (HR-TEM), enabling direct observation of the arrangement of tetrahedrons within the nanosheet at high resolution while preserving the integrity of the original structure. As a visible light photocatalyst, the nanosheets exhibit significantly enhanced activity in C(sp²)-H/N-H cross-dehydrogenative coupling (CDC) amination under ambient air, outperforming both the layered crystals and the individual cages. This work introduces a new paradigm for the design of 2D materials and opens new avenues for the synthesis of 2D materials from other hydrogen bond-directed assemblies.