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 FeIIHN3 sites, forming FeIII-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.