Benchmarking the Reactivity of Caged Iron(IV)-Oxo Sites within Metal-Organic Frameworks

Three recently reported iron-containing metal-organic frameworks, FeZn₄(prv)₄(btdd)₃ (Fe-1), Fe_1.8Zn_3.2(prv)₄(btdd)₃ (Fe_1.8-1), and FeZn₄(moba)₄(btdd)₃ (Fe-2) (Hprv = pyruvic acid, Hmoba = 3,3-dimethyl-2-oxobutanoic acid, and H₂btdd = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin), that react with O₂ at low temperatures to generate high-spin (S = 2) ferryl species provide an unprecedented opportunity to explore the effects of spin state and site-isolation on hydrocarbon oxidation reaction mechanisms. These reagents oxidize the radical clock substrate norcarane, yielding radical lifetimes in the low nanosecond regime, consistent with a diffusion-limited rebound mechanism. Rebound products derived from ligands on the iron centers inside the framework are detected and recapitulate chemistry seen in the structurally related α-ketoglutarate-dependent dioxygenases such as TauD, which also generate high-spin (S = 2) ferryl intermediates. Framework models indicate that productive reactions occur when norcarane approaches the ferryl species from the larger of the two pores of the MFU-4l-type framework.