N–H Bond Activation Catalyzed by an Anderson-Type Polyoxometalate-Based Compound: Key Role of Transition-Metal Heteroatom

Polyoxometalates (POMs) have a broad array of applied platforms with well-characterized catalysis to achieve N–H bond activation. Herein, the mechanism of the Anderson-type POM-based catalyst [FeIIIMoVI6O18{(OCH2)3CNH2}2]3– ([TrisFeIIIMoVI6O18]3–, Tris = {(OCH2)3CNH2}2) for the N–H bond activation of hydrazine (PhHNNHPh) was investigated by density functional theory calculations. The results reveal that [TrisFeIIIMoVI6O18]3– as the active species is responsible for the continuous abstraction of two electrons and two protons of PhHNNHPh via a proton-coupled electron transfer pathway, resulting in the activation of two N–H bonds in PhHNNHPh and thus the product PhNNPh. H2O2 acts as an oxidant to regulate catalyst regeneration. Based on the proposed catalytic mechanism, the key role of the heteroatom FeIII in [TrisFeIIIMoVI6O18]3– was disclosed. The d-orbital of FeIII in [TrisFeIIIMoVI6O18]3– acts as an electron receptor to promote the electron transfer (ET) in the rate-determining step (RDS) of the catalytic cycle. The substitution of the heteroatom FeIII of [TrisFeIIIMoVI6O18]3– with CoIII, RuIII, or MnIII is expected to improve the catalytic activity for several reasons: (i) the unoccupied molecular orbitals of POM-based compounds containing CoIII or RuIII are low, which is beneficial for the ET of RDS; (ii) For N–H bond activation catalyzed by the MnIII-containing POM-based compound, the transition state of RDS is stable because the d-orbital of its active site is half-filled, which results in a low free-energy barrier.