Visible-Light-Assisted Photocatalytic Reduction of Nitroaromatics by Recyclable Ni(II)-Porphyrin Metal–Organic Framework (MOF) at RT

A microporous Ni­(II)-porphyrin
metal–organic framework (MOF), [Ni3(Ni-HTCPP)2(μ2-H2O)2(H2O)4(DMF)2]·2DMF, (MOF1) (where, Ni-HTCPP = 5,10,15,20-tetrakis­(4-benzoate) porphyrinato-Ni­(II))
has been synthesized by the solvothermal route. Single-crystal X-ray
diffraction study of 1 reveals a 2D network structure
constituted by Ni3 cluster and [Ni-HTCPP]3– metalloligand having (3, 6)-connected binodal net with {43}­2­{46·66·83}-kgd net topology.
The 2D layers are further stacked together through π–π
interactions between the porphyrin linkers to generate a 3D supramolecular
framework which houses 1D channels with dimension of ∼5.0 ×
9.0 Å2 running along the crystallographic a-axis. Visible-light-assisted photocatalytic investigation
of MOF1 for heterogeneous reduction of various nitroaromatics
at room temperature resulted in the corresponding amines with high
yield and selectivity. On the contrary, the Ni­(II)-centered porphyrin
tetracarboxylic acid [Ni–H4TCPP] metalloligand does
not show the photocatalytic activity under similar conditions. The
remarkably high catalytic performance of MOF1 over [Ni–H4TCPP] metalloligand has been attributed due to cooperative
catalysis involving the Ni-centered porphyrin secendary building units
(SBUs) and the Ni3-oxo node. Further, the MOF1 was recycled and reused up to three cycles without any significant
loss of catalytic activity as well as structural rigidity. To the
best of our knowledge, MOF1 represents the first example
of MOF based on 3d metal ion exhibiting visible-light-assisted reduction
of nitroaromatics under mild conditions without the assistance of
noble metal cocatalysts.