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 [Fe III Mo VI 6 O 18 {(OCH 2 ) 3 CNH 2 } 2 ] 3- ([TrisFe III Mo VI 6 O 18 ] 3- , Tris = {(OCH 2 ) 3 CNH 2 } 2 ) for the N-H bond activation of hydrazine (PhHNNHPh) was investigated by density functional theory calculations. The results reveal that [TrisFe III Mo VI 6 O 18 ] 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. H 2 O 2 acts as an oxidant to regulate catalyst regeneration. Based on the proposed catalytic mechanism, the key role of the heteroatom Fe III in [TrisFe III Mo VI 6 O 18 ] 3- was disclosed. The d-orbital of Fe III in [TrisFe III Mo VI 6 O 18 ] 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 III Mo VI 6 O 18 ] 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.