A porous Anderson-type polyoxometalate-based metal–organic framework as a multifunctional platform for selective oxidative coupling with amines†

Abstract

Incorporating catalytic units into a crystalline porous matrix represents a facile way to build high-efficiency heterogeneous catalysts, and by rational design of the porous skeleton with appropriate building blocks the catalytic performance can be significantly enhanced for a series of organic transformations owing to the synergistic effect from the multicomponent and confined porous microenvironment around catalytically active sites. Herein, we demonstrate that the design and synthesis of a porous polyoxometalate-based metal–organic framework YL₂(H₂O)₂[CrMo₆O₁₈(PET)₂]·4H₂O (POMOF-1) constructed from Anderson-type [CrMo₆O₁₈(PET)₂] (PET = pentaerythritol), which can be employed as a multifunctional platform for synthesis of N-containing compounds via selective oxidative coupling with amines. POMOF-1 features microporous 1D channels defined by Y³⁺ and L, with [CrMo₆O₁₈(PET)₂] arranged orderly between adjacent Lvia electrostatic interactions. Upon using POMOF-1 as a catalyst and H₂O₂ as an oxidant, a variety of amines could be effectively converted to value-added amides, imines and azobenzenes via the oxidative cross-coupling with alcohols or homo-coupling. In particular, POMOF-1 showed dramatically improved activity for the N-formylation reaction owing to the synergistic and confinement effect, with the yield of amides up to 95% and 4 times higher than that of homogeneous [CrMo₆O₁₈(PET)₂]. Meanwhile, the oxidative homo-coupling of arylmethylamines and arylamines can be facilely tuned by adjustment of the amount of oxidant, solvent and additive, affording imines and azobenzenes in high selectivity and yield, respectively. POMOF-1 is robust and can be reused for 5 cycles with little loss of catalytic activity and structural integrity. The work demonstrates that the combination of catalytically active POMs with crystalline porous MOFs holds great potential to build robust and recyclable heterogeneous systems with enhanced activity and selectivity for multifunctional catalysis.