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The nuanced role of spin effects remains a critical gap in designing proficient open‐shell catalysts. This study elucidates an iron‐catalyzed allylic C(sp3)−H silylation/alkyne hydrosilylation reaction, in which the spin state of the open‐shell iron catalyst dictates the reaction kinetics and pathway. Specifically, spin crossover led to alkyne hydrosilylation, whereas spin conservation resulted in a novel allylic C(sp3)−H silylation reaction. This chemoselectivity, governed by the spin‐crossover efficiency, reveals an unexpected dimension in spin effects and a first in the realm of transition‐metal‐catalyzed in situ silylation of allylic C(sp3)−H bonds, which had been previously inhibited by the heightened reactivity of alkenes in hydrosilylation reactions. Furthermore, this spin crossover can either accelerate or hinder the reaction at different stages within a single catalytic reaction, a phenomenon scarcely documented. Moreover, we identify a substrate‐assisted C−H activation mechanism, a departure from known ligand‐assisted processes, offering a fresh perspective on C−H activation strategies. [ABSTRACT FROM AUTHOR]

The nuanced role of spin effects remains a critical gap in designing proficient open‐shell catalysts. This study elucidates an iron‐catalyzed allylic C(sp3)−H silylation/alkyne hydrosilylation reaction, in which the spin state of the open‐shell iron catalyst dictates the reaction kinetics and pathway. Specifically, spin crossover led to alkyne hydrosilylation, whereas spin conservation resulted in a novel allylic C(sp3)−H silylation reaction. This chemoselectivity, governed by the spin‐crossover efficiency, reveals an unexpected dimension in spin effects and a first in the realm of transition‐metal‐catalyzed in situ silylation of allylic C(sp3)−H bonds, which had been previously inhibited by the heightened reactivity of alkenes in hydrosilylation reactions. Furthermore, this spin crossover can either accelerate or hinder the reaction at different stages within a single catalytic reaction, a phenomenon scarcely documented. Moreover, we identify a substrate‐assisted C−H activation mechanism, a departure from known ligand‐assisted processes, offering a fresh perspective on C−H activation strategies. [ABSTRACT FROM AUTHOR]

Iron catalysts are ideal transition metal catalysts because of the Earths abundant, cheap, biocompatible features of iron salts. Iron catalysts often have unique open-shell structures that easily undergo spin crossover in chemical transformations, a feature rarely found in noble metal catalysts. Unfortunately, little is known currently about how the open-shell structure and spin crossover affect the reactivity and selectivity of iron catalysts, which makes the development of iron catalysts a low efficient trial-and-error program. In this paper, a combination of experiments and theoretical calculations revealed that the iron-catalyzed hydrosilylation of alkynes is typical spin-crossover catalysis. Deep insight into the electronic structures of a set of well-defined open-shell active formal Fe(0) catalysts revealed that the spin-delocalization between the iron center and the 1,10-phenanthroline ligand effectively regulates the iron center's spin and oxidation state to meet the opposite electrostatic requirements of oxidative addition and reductive elimination, respectively, and the spin crossover is essential for this electron transfer process. The triplet transition state was essential for achieving high regioselectivity through tuning the nonbonding interactions. These findings provide an important reference for understanding the effect of catalyst spin state on reaction. It is inspiring for the development of iron catalysts and other Earth-abundant metal catalysts, especially from the point of view of ligand development. [ABSTRACT FROM AUTHOR]