Synthesis and post-synthetic modifications of novel carboxylated triphenylphosphine-based coordination polymers
Over the last few decades, Porous Coordination Polymers (PCPs, a.k.a. MOFs) have been extensively studied for a number of applications due to their functional and structural versatility. Nonetheless, systematic studies that explore the structure-function relationship of frameworks decorated with different functional groups remain limited. In the present work, two families of coordination polymers that contain organophosphine building blocks have been synthesized and characterized. These materials contain a periodic array of P(III) Lewis base sites that are available for post-synthetic functionalization. PCM-10 is a porous coordination polymer based on Ca(II) and tris(p-carboxylated) triphenylphosphine. The material displayed a 3-dimensional surface, ideal for a series of post-synthetic modifications. The versatility of P(III)→P(V) chemistry allowed for decoration of the R₃P: sites of PCM-10 with different functional groups. Solid-state treatment of the framework with a variety of precursors, gave rise to a family of isostructural materials (ClAu-PCM-10, O=PCM-10 and (Me‒PCM-10)X; X = I⁻, F⁻, BF₄⁻ and PF₆⁻). These post-synthetic modifications directly affected the host-guest interactions, as demonstrated by comparative adsorption studied of CO₂, H₂ and other small molecules. For example, the phosphonium (Me-PCM-10)X composites displayed broad tunability of isosteric heats of CO₂ adsorption, depending on the counter-ion. Similarly, the ClAu-PCM-10 derivative resulted in selective adsorption of 1-hexene vs. n-hexane and displayed significantly increased H₂ uptake capacity, when compared to PCM-10. Ln-PCM-22 is a family of isostructural PCPs based on tris(p-carboxylated) triphenylphosphine and Ln(III) (Ln = Sm-Yb). Photoluminescence studies of Eu-PCM-22 and Tb-PCM-22 were used to elucidate certain aspects of the solid-state emissive properties of phosphine-based materials. In addition, a single-crystal-to-single-crystal post-synthetic oxidation of Tb-PCM-22 was achieved to yield the composite material Tb-O=PCM-22. This modification resulted in enhancement of the quantum efficiency, compared to that of the parent framework. These results will help further the understanding of PCMs and direct the upcoming synthesis and application of these materials.