Correlation between active center structure and enhanced dioxygen binding in Co(salen) nanoparticles: Characterization by in situ infrared, Raman, and X-ray absorption spectroscopies

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 O-2. 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 O-2 binding at room temperature. In sharp contrast, the Co(salen) nanoparticles show near-stoichiometric O-2 adsorption, as demonstrated by microbalance gas binding experiments. The spectroscopy results indicate the presence of Co-II centers with distorted tetrahedral geometry in the Co(salen) nanoparticles with no evidence of metallic Co clusters, confirmed by the lack of Co-Co contributions at bonding distances in the EXAFS spectra and the presence of characteristic features of Co-II in the XANES spectra. The EXAFS data also indicate that there are on average two Co-N and two Co-O bonds with a distance of 1.81 +/- 0.02 and 1.90 +/- 0.02 angstrom, respectively, consistent with typical metal salen structures. Upon O-2 binding on the Co(salen) nanoparticles, the XANES results indicate oxidation of the Co-II to Co-III, consistent with the vibrational data showing new bands associated with oxygen species bonded to Co centers and the increase in the oxygen coordination number from 1.8 to 2.9 in the EXAFS data. The results indicate that the enhanced O-2 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.