Gradual Release of Strongly Bound Nitric Oxide from Fe₂(NO)₂(dobdc)

An iron(II)-based metal-organic framework featuring coordinatively unsaturated redox-active metal cation sites, Fe2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate), is shown to strongly bind nitric oxide at 298 K. Adsorption isotherms indicate an adsorption capacity greater than 16 wt %, corresponding to the adsorption of one NO molecule per iron center. Infrared, UV-vis, and Mössbauer spectroscopies, together with magnetic susceptibility data, confirm the strong binding is a result of electron transfer from the Fe(II) sites to form Fe(III)-NO(-) adducts. Consistent with these results, powder neutron diffraction experiments indicate that NO is bound to the iron centers of the framework with an Fe-NO separation of 1.77(1) Å and an Fe-N-O angle of 150.9(5)°. The nitric oxide-containing material, Fe2(NO)2(dobdc), steadily releases bound NO under humid conditions over the course of more than 10 days, suggesting it, and potential future iron(II)-based metal-organic frameworks, are good candidates for certain biomedical applications.