(668e) Correlation Between Structure and Gas Binding In Co(salen) Nanoparticles

Nanomaterials with novel size-dependent properties and potential applications for catalysis, gas storage, and controlled release formulations are increasingly emerging from rational design-based understanding of relationships between structure at the atomic/molecular level and performance. Precise structural characterization of these new nanomaterials, particularly in situ characterization under working conditions, is essential for establishing such structure-activity relationships and for guiding molecular designs of new nanomaterials. In this work, the structure and ligand environment of Co(salen) nanoparticles and unprocessed Co(salen) have been determined by the combined application of infrared, Raman, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies, and X-ray diffraction (XRD) experiments before and during interaction with O₂. The Co(salen) nanoparticles were prepared by the precipitation with compressed antisolvent (PCA) technique using commercially obtained Co(salen) [denoted as unprocessed Co(salen)] as the parent compound. The unprocessed Co(salen) particles exist as dimer species with a square-pyramidal coordination geometry that display no measurable O₂ binding at room temperature. In sharp contrast, the Co(salen) nanoparticles show near-stoichiometric O₂ adsorption, as demonstrated by microbalance gas binding experiments. The spectroscopy results indicate the presence of CoII centers with distorted tetrahedral geometry in the Co(salen) nanoparticles with no evidence of metallic Co clusters. The results indicate that the enhanced O₂ binding properties of Co(salen) nanoparticles are related to the unique distorted tetrahedral geometry, which is not observed in the unprocessed samples that contain mainly dimers with square planar geometry. The results presented here provide a fundamental relationship between active center structure and properties of novel molecule-based nanomaterials.