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Abstract: Porous materials with immobilized metal complexes of defined structure have wide applications in catalysis, gas storage, and sensor technology. Reported herein is the use of template copolymerization methods to design and synthesize reversible dioxygen binding sites in a porous organic host. The immobilized metal sites are formed using molecular precursors including a substitutionally inert Co(III) complex that ensures the desired square-pyramidal coordination geometry around the immobilized metal ions. Analysis of the resulting mesoporous polymer by EPR spectroscopy reveals an equilibrium of four- and five-coordinate sites which is similar to that observed for molecular analogues in solution. This equilibrium is sensitive to the solvent used to suspend the polymer, with the highest percentage of five-coordinate sites (60%) observed in CH3NO2, which is the same solvent used to synthesize the polymer. In the presence of dioxygen similar to 90% of the immobilized sites convert to Co-O-2 adducts. Dioxygen binding to the material is reversible-revision to the dioxygen-free form is obtained by purging with N-2. The large percentage of sites involved in dioxygen binding can only occur if the spatial arrangement of ligands within the immobilized sites is maintained throughout the copolymerization process. This level of molecular control within the porous organic hosts illustrates the effectiveness of this method for developing metal complexes in solid supports