A Facially Coordinating Tris‐Benzimidazole Ligand for Nonheme Iron Enzyme Models

Metal-α-Helix Peptide Frameworks

Metal-peptide frameworks (MPFs) are a growing class of metal-organic frameworks with promising applications in metalloprotein mimicry, chiral separations, and catalysis. There are limited examples of MPFs, especially those with both secondary structure and natural amino acid side chains that coordinate to metal nodes, which are important for accurately mimicking metalloprotein active sites. Here, we design a robust and modular strategy based on short α-helical peptides (nine amino acids long) to form frameworks with many types of biomimetic metal sites. Peptides were designed to have Glu and His metal-binding residues, hydrophobic residues, and noncanonical helix-enforcing residues. With Co(II), it was shown that mutagenesis of a single amino acid near the metal-binding residues generates a diverse library of frameworks with varying metal node coordination geometries and compositions. Structures for 16 out of 20 variants were characterized by single-crystal X-ray diffraction, revealing how noncovalent interactions impact the metal primary sphere. In one case, a point mutation turns on reversible ligand-triggered conformational changes, demonstrating that this platform allows for dynamic behavior like that observed in metalloproteins. Furthermore, we show that frameworks readily assemble with Mn(II), Fe(II), Cu(II), and Zn(II) ions, highlighting the generality of this approach. The ease-of-synthesis, modularity, and crystallinity of these materials make this a highly accessible platform for studying and engineering biomimetic metal centers in porous materials.

Direct Aerobic Generation of a Ferric Hydroperoxo Intermediate Via a Preorganized Secondary Coordination Sphere

Enzymes exert control over the reactivity of metal centers with precise tuning of the secondary coordination sphere of active sites. One particularly elegant illustration of this principle is in the controlled delivery of proton and electron equivalents in order to activate abundant but kinetically inert oxidants such as O2 for oxidative chemistry. Chemists have drawn inspiration from biology in designing molecular systems where the secondary coordination sphere can shuttle protons or electrons to substrates. However, a biomimetic activation of O2 requires the transfer of both protons and electrons, and molecular systems where ancillary ligands are designed to provide both of these equivalents are comparatively rare. Here, we report the use of a dihydrazonopyrrole (DHP) ligand complexed to Fe to perform exactly such a biomimetic activation of O2. In the presence of O2, this complex directly generates a high spin Fe(III)-hydroperoxo intermediate which features a DHP• ligand radical via ligand-based transfer of an H atom. This system displays oxidative reactivity and ultimately releases hydrogen peroxide, providing insight on how secondary coordination sphere interactions influence the evolution of oxidizing intermediates in Fe-mediated aerobic oxidations.