Gradual Release of Strongly-Bound Nitric Oxide from Fe 2 (NO) 2 (dobdc) (original) (raw)

2015, Journal of the American Chemical Society

An iron(II)-based metal−organic framework featuring coordinatively unsaturated redox-active metal cation sites, Fe 2 (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 Mossbauer 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, Fe 2 (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. M etal−organic frameworks, which have received a great deal of attention for gas storage and molecular separations, 1 have also recently shown promise for applications in biomedicine, typically for drug storage and delivery. 2 Although a number of structures have been synthesized from biologically active ligands, 3 bioactive molecules can also be incorporated into a metal−organic framework postsynthetically 4 or produced via the catalytic decomposition of precursor molecules such as S-nitrosothiols. 5 An important example of this is the adsorption and release of nitric oxide by frameworks featuring coordinatively unsaturated metal centers. The first investigation involved the widely studied metal−organic framework Cu 3 (btc) 2 (btc 3− = 1,3,5-benzenetricarboxylate, HKUST-1). 6,7 This compound adsorbs nearly 4.0 mmol/g of NO, a significant improvement over zeolites, which can adsorb NO with maximum capacities of 1 mmol/g under similar conditions. 8 Upon exposing HKUST-1 to humid air, a slow release of a fraction of the coordinated NO occurs over the course of an hour. Although the amount released is limited to just ∼2 μmol/g, it proved sufficient to inhibit platelet aggregation in biological experiments, which is necessary to prevent blood clotting. In addition to antithrombotic applications, porous materials that can store and deliver the critical biological signaling molecule NO may be useful for antibacterial and wound healing applications. 9 In order to improve upon the NO release properties of HKUST-1, storage and release by Ni 2 (dobdc) and Co 2 (dobdc) were subsequently studied. 10,11 These materials feature exceptionally high densities of coordinatively unsaturated metal cations and are unable to form the M I −NO + adducts likely responsible for the poor NO release displayed by HKUST-1. Indeed, the frameworks adsorbed close to 7 mmol/g NO at room temperature and released the entire quantity within 15 h of exposure to humid air. These frameworks, however, suffer from biocompatibility issues, as they are based on cobalt or nickel. The NO storage and release properties of a family of biocompatible MIL-88(Fe)-based metal−organic frameworks were recently reported. 12 However, these materials adsorb and release <0.35 mmol of NO per gram, significantly less than the M 2 (dobdc) frameworks.