Near-Stoichiometric O2 Binding on Metal Centers in Co(salen) Nanoparticles

Co(salen) [cobaltous bis(salicylaldehyde)ethylenediamine] complexes are well-known O2 carriers in solution. In the solid phase, these complexes exhibit some O2 binding but detailed studies have been complicated because few of the known polymorphs of Co(salen) actually bind O2. The O2 binding results for nanoparticulate Co(salen) are presented in this study. Rod-shaped Co(salen) nanoparticles, roughly 100 nm in diameter, were recrystallized by spraying a methylene chloride solution of commercially obtained Co(salen) into supercritical carbon dioxide. Temperature-programmed desorption, thermogravimetric analysis, and a Rubotherm magnetic suspension balance measurements reveal a reversible O2 uptake of ∼1.51 mmol/(g nanoparticles) at 25°C, consistent with a binding stoichiometry involving a bridging peroxo unit between two Co centers. In contrast, no measurable O2 uptake was observed with the commercial Co(salen). These results clearly show the potential for bottom-up design of nanoparticulate metal complexes for enhanced O2 storage and other applications. © 2009 American Institute of Chemical Engineers AIChE J, 2009