Green Methods for Processing and Utilizing Metal Complexes

The use of supercritical carbon dioxide (scCO(2)) and CO2-expanded liquids (CXLs) in metal-mediated processes is described. Two examples are examined: 1) heterogeneous oxidation catalysis and 2) processing nanoparticles of metal complexes. Oxidation processes with dioxygen as the terminal oxidant are often limited because of the low O-2 solubility in common organic solvents. The complete miscibility of dioxygen and scCO(2) alleviates this problem and provides improved mass transfer properties. These factors are exploited in the oxidation of substituted phenols with porous solids containing immobilized metal complexes that serve as catalytic sites. The development of nanoparticles composed entirely of metal complexes has lagged behind atom-based systems. Precipitation with compressed antisolvent, a method that uses scCO(2) as the precipitant, was employed to process nanoparticles of metal complexes. A correlation between molecular shape and particle morphology was discovered, in which planar compounds gave nanoparticles with rodlike structures. In contrast, three-dimensional molecular species produced spherical nanoparticles. Investigations into the binding of dioxygen and nitric oxide to the rod-like nanoparticles composed of cobalt complexes showed pronounced differences compared to their unprocessed analogs. Dioxygen reversibly binds to the nanoparticles, whereas nitric oxide is disproportionated into other NxOy species. This type of nitric oxide reactivity has not been observed for other cobalt complexes at room temperature and illustrates the promise of nanoparticles of metal complexes.