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 Fe^IV-cyanide complexes supported by a bTAML ligand: [Fe^IV(CN)(bTAML)]^- and [Fe^IV(CN)₂(bTAML)]^2-. Despite possessing a 150 mV more positive Fe^IV/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 pK_a, 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.