Tuning Nitric Oxide Adsorption in Cobalt–Triazolate Frameworks

Nitric oxide (NO) is an important signaling molecule in biological systems, and as such the ability of certain porous materials to reversibly adsorb NO is of interest for medical applications. Metal–organic frameworks have been explored for their ability to reversibly bind NO at coordinatively-unsaturated metal sites, however the influence of metal coordination environment on NO adsorption has yet to be studied in detail. Here, we examine NO adsorption in the frameworks Co₂Cl₂(bbta) and Co₂(OH)₂(bbta) (H₂bbta = 1H,5H-benzo(1,2-d:4,5-d′)bistriazole) via gas adsorption, infrared spectroscopy, powder X-ray diffaction, and magnetometry measurements. While NO adsorbs reversibly in Co₂Cl₂(bbta) without electron-transfer, adsorption of low pressures of NO in Co₂(OH)₂(bbta) is accompanied by charge transfer from the cobalt(II) centers to form a cobalt(III)–NO⁻ adduct, as supported by diffraction and infrared spectroscopy data. At higher pressures of NO, characterization data support additional uptake of the gas and disproportionation of the bound NO to form a cobalt(III)–nitro (NO₂⁻) species and N₂O gas, a transformation that appears to be facilitated in part by stabilizing hydrogen bonding interactions between the bound NO₂⁻ and framework hydroxo groups. This reactivity represents a rare example of reductive NO-binding in a metal–organic framework and demonstrates that NO binding can be tuned by changing the coordination environment of the framework metal centers.