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

Three recently reported iron-containing metal–organic frameworks, FeZn 4 (prv) 4 (btdd) 3 (Fe- 1 ), Fe 1.8 Zn 3.2 (prv) 4 (btdd) 3 (Fe 1.8 - 1 ), and FeZn 4 (moba) 4 (btdd) 3 (Fe- 2 ) (Hprv = pyruvic acid, Hmoba = 3,3-dimethyl-2-oxobutanoic acid, and H 2 btdd = bis(1 H -1,2,3-triazolo[4,5- b ],[4′,5′- i ])dibenzo[1,4]dioxin), that react with O 2 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-4 l -type framework.