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

A new
metal–organic framework, Fe-BTTri (Fe3[(Fe4Cl)3(BTTri)8]2·18CH3OH, H3BTTri =1,3,5-tris­(1H-1,2,3-triazol-5-yl)­benzene)),
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 H2, N2, and CH4. This framework not only displays significant
CO adsorption capacity at very low pressures (1.45 mmol/g at just
100 μbar), 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 FeII centers of the framework convert to octahedral, low-spin
FeII centers upon CO coordination. Desorption of CO converts
the FeII 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, Mössbauer 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 H2, N2, CO2,
CH4, or other hydrocarbons, resulting in unprecedentedly
high selectivities for CO adsorption for use in CO/H2,
CO/N2, 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
π-acids.