Co(salen) [cobaltous bis(salicylaldehyde)ethylenediamine] complexes are well known O2 carriers in liquid solution. In the solid phase, these complexes or their polymorphs are not known to exhibit any measurable dioxygen binding. However, when recrystallized as nanoparticles using rapid precipitation with supercritical CO2 as antisolvent, the rod-shaped nanoparticles (roughly 100 nm in diameter or less) are obtained that show reversible and near-stoichiometric dioxygen binding of 1.51 mmol/g at 25°C.
The combined application of infrared, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies in conjunction with X-ray diffraction (XRD) experiments reveal that the enhanced O2 binding properties of Co(salen) nanoparticles are related to the unique distorted tetrahedral geometry, which is not observed in the parent Co(salen) compound that contains mainly dimers with square planar geometry. These results provide a fundamental relationship between active center structure and properties of novel molecule-based nanomaterials, and clearly show the potential for bottom-up design of nanoparticulate metal complexes for enhanced O2 storage and other applications.
Commercially available Co(salen) (left) exhibits negligible oxygen adsorption capacity due to the formation of dimers with square-pyramidal geometry (center). Formation of Co(salen) nanoparticles by PCA (right) alters the geometry around the cobalt centers, and results in dramatically enhanced adsorption capacity for oxygen and nitric oxide.
For more, please see Johnson, C., Long, B., Nguyen, J. G., Day, V. W., Borovik, A.S., Subramaniam, B., and Guzman, J. "Correlation between Acitve Center Structure and Enhanced Dioxygen Binding in Co(salen) Nanoparticles: Characterization by In Situ Infrared, Raman, and X-ray Absorption Spectroscopies," J. Phys. Chem. C 2008 112 12272-12281. DOI: 10.1021/jp803985b. Additionally, NO disproportionation activity has also been recently demonstrated on these catalysts-- see Nguyen, J.; Johnson, C.A.; Subramaniam, B.; and Borovik, A.S. "Nitric Oxide Disproportionation at Mild Temperatures by a Nanoparticulate Cobalt(II) Complex," Chemistry of Materials 2008 20(19) 5939-5941. DOI: 10.1021/cm801406y.