Reversible CO Scavenging via Adsorbate-Dependent Spin State Transitions in an Iron(II) Triazolate Metal-Organic Framework

A new metal-organic framework, Fe-BTTri (Fe-3[(Fe4Cl)(3)(BTTri)(8)](2)center dot 18CH(3)OH, H(3)BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yObenzene)), is found to be highly selective in the adsorption of CO over a variety of other gas molecules, making it extremely effective, for example, in the removal of trace CO from mixtures with H-2, N-2, and CH4. This framework not only displays significant CO adsorption capacity at very low pressures (1.45 mmol/g at just 100 mu bar), but, importantly, also exhibits readily reversible CO binding. Fe-BTTri utilizes a unique spin state change mechanism to bind CO in which the coordinatively unsaturated, high-spin Fe-II centers of the framework convert to octahedral, low-spin Fe-II centers upon CO coordination. Desorption of CO converts the Fe-II sites back to a high-spin ground state; enabling the facile regeneration and recyclability of the material. This spin state change is supported by characterization via infrared spectroscopy, single crystal X-ray analysis, Mossbauer spectroscopy, and magnetic susceptibility measurements. Importantly, the spin state change is selective for CO and is not observed in the presence of other gases, such as H-2, N-2, CO2, CH4, or other hydrocarbons, resulting in unprecedentedly high selectivities for CO adsorption for use in CO/H-2, CO/N-2, and CO/CH4 separations and in preferential CO adsorption over typical strongly adsorbing gases like CO2 and ethylene. While adsorbate-induced spin state transitions are well-known in molecular chemistry, particularly for CO, to our knowledge this is the first time such behavior has been observed in a porous material suitable for use in a gas separation process. Potentially, this effect can be extended to selective separations involving other pi-acids.