Iron(IV)–Cyanide Complexes Capable of Asynchronous Basicity-Driven PCET in C–H Activation upon Linkage Isomerization

High-valent iron–oxo complexes dominate C–H bond activation chemistry, whereas analogous reactivity of nonoxo iron oxidants remains largely unexplored. Here, we report two structurally characterized FeIV–cyanide complexes supported by a bTAML ligand: [FeIV(CN)(bTAML)]− and [FeIV(CN)2(bTAML)]2–. Despite possessing a 150 mV more positive FeIV/III potential, the five-coordinated complex is ∼550-fold less reactive and incapable of C–H bond activation, whereas the six-coordinated analogue oxidizes both O–H and C–H bonds. Kinetic studies reveal no correlation with C–H bond dissociation energies but a strong dependence on substrate pKa, indicating asynchronous (basicity-controlled) H atom abstraction. Large KIEs, Hammett analysis, and asynchronicity parameter of the HAA reactions support a proton-transfer-dominated asynchronous PCET mechanism, which is corroborated by quantum-chemical calculations that reproduce the reactivity trends and identify the electronic origin of the lower barrier in the dicyano complex, including revealing the Fe–C≡N → Fe–N≡C isomerization, which enables the PCET event. These results demonstrate that coordination-induced modulation of basicity and linkage isomerization can govern PCET asynchronicity in nonoxo iron complexes, establishing Fe–cyanide species as a distinct class of hydrogen-atom abstractors and providing a design principle for tuning C–H activation reactivity beyond conventional redox paradigms.