Heme-Like Coordination Chemistry Within Nanoporous Molecular Crystals

Iron Exposure The macrocyclic heme motif coordinates iron ions in proteins and plays a widespread role in biochemical oxidative catalysis. Bezzu et al. (p. 1627) prepared crystals in which analogous iron-centered macrocycles were aligned in pairs. The outer faces of the pairs exposed the iron ions to vacant cavities, where ligand exchange could take place; the inner faces were bound together by rigid bridging ligands lending the crystals structural integrity. The stability and high porosity of these crystals lend themselves to potential catalytic applications. Metal-organic framework compounds expose iron atoms for reactions in a manner analogous to heme sites in proteins. Crystal engineering of nanoporous structures has not yet exploited the heme motif so widely found in proteins. Here, we report that a derivative of iron phthalocyanine, a close analog of heme, forms millimeter-scale molecular crystals that contain large interconnected voids (8 cubic nanometers), defined by a cubic assembly of six phthalocyanines. Rapid ligand exchange is achieved within these phthalocyanine nanoporous crystals by single-crystal–to–single-crystal (SCSC) transformations. Differentiation of the binding sites, similar to that which occurs in hemoproteins, is achieved so that monodentate ligands add preferentially to the axial binding site within the cubic assembly, whereas bidentate ligands selectively bind to the opposite axial site to link the cubic assemblies. These bidentate ligands act as molecular wall ties to prevent the collapse of the molecular crystal during the removal of solvent. The resulting crystals possess high surface areas (850 to 1000 square meters per gram) and bind N2 at the equivalent of the heme distal site through a SCSC process characterized by x-ray crystallography.