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

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 [Fe^IIIMo^VI₆O₁₈{(OCH₂)₃CNH₂}₂]^3- ([TrisFe^IIIMo^VI₆O₁₈]^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 [TrisFe^IIIMo^VI₆O₁₈]^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 Fe^III in [TrisFe^IIIMo^VI₆O₁₈]^3- was disclosed. The d-orbital of Fe^III in [TrisFe^IIIMo^VI₆O₁₈]^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 Fe^III of [TrisFe^IIIMo^VI₆O₁₈]^3- with Co^III, Ru^III, or Mn^III is expected to improve the catalytic activity for several reasons: (i) the unoccupied molecular orbitals of POM-based compounds containing Co^III or Ru^III are low, which is beneficial for the ET of RDS; (ii) For N-H bond activation catalyzed by the Mn^III-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.