Observation of O₂ Molecules Inserting into Fe-H Bonds in a Ferrous Metal-Organic Framework

Exploring the interactions between oxygen molecules and metal sites has been a significant topic. Most previous studies concentrated on enzyme-mimicking metal sites interacting with O₂ to form M-OO species, leaving the development of new types of O₂-activating metal sites and novel adsorption mechanisms largely overlooked. In this study, we reported an Fe(II)-doped metal-organic framework (MOF) [Fe₃Zn₂H₄(bibtz)₃] (MAF-203, H₂bibtz = 1H,1'H-5,5'-bibenzo[d][1,2,3]triazole), featuring an unprecedented tetrahedral Fe(II)HN₃ site. This MOF exhibits selective adsorption behavior for O₂ from air, achieving an O₂/N₂ separation selectivity of up to 67.1. Breakthrough experiments confirmed that MAF-203 can effectively capture O₂ from the air even under a high relative humidity of 60%. X-ray absorption spectroscopy, in situ diffuse reflectance infrared Fourier transform spectra, and ab initio molecular dynamics simulations were utilized to monitor the O₂ loading process on the Fe(II)HN₃ site. Interestingly, O₂ molecules could insert into the Fe-H bonds of the tetrahedral Fe^IIHN₃ sites, forming Fe^III-OOH species (instead of the commonly observed Fe-OO species) with an ultrahigh adsorption enthalpy of -99.2 kJ mol^-1. Consequently, the O₂ capture behavior of MAF-203 enables efficient electrochemical 2e^- oxygen reduction for the production of H₂O₂ with air as the feedstock. Specifically, in a solid-state electrolyte electrolyzer without any liquid electrolyte, MAF-203 achieved selective O₂ capture and continuous production of medical-grade H₂O₂ (3.2 wt %) solution without salts for 70 h, with performance comparable to that under pure O₂ conditions. The O₂ adsorption and activation mechanisms inaugurate a fresh chapter in grasping the interaction between O₂ molecules and metal sites.