Spin State‐Driven Modulation of Catalytic Activity of an Iron Spin‐Crossover Complex: Toward Switchable Catalysis

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Abstract This paper investigates the effect of iron(II) spin state on the catalytic activity of a spin‐crossover (SCO) complex, specifically [Fe(NH 2 trz) 3 ](NO 3 ) 2 , in an acetalization reaction. Thanks to its bi‐stability, this complex enables the evaluation of its catalytic activity in both the high‐spin (HS) and low‐spin (LS) states under identical experimental conditions, thereby clearly distinguishing the effects of the spin state from other variables. A notable increase in catalytic efficiency is observed from the low‐spin to the high‐spin state, with the high‐spin complex promoting up to 1.7 times more acetal formation. Extensive experiments across multiple batches of the complex confirm that these findings are both reproducible and representative. To elucidate the underlying mechanism, we assessed the activation energy of the process. The activation energies for both spin states were found to be comparable, suggesting that the observed difference in catalytic activity is not due to electronic factors that stabilize the transition state, but rather to easier access to the catalytic active sites in the high‐spin state. This work represents a significant advancement toward developing a switchable catalytic system based on spin‐crossover phenomena, highlighting that the catalytic activity of SCO complexes can be effectively tuned by the spin state of the metal.

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