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) (dobdc4– = 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 FeII sites to form FeIII–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.