Co(ii)-cluster-based metal–organic frameworks as efficient heterogeneous catalysts for selective oxidation of arylalkanes

Abstract

To explore metal–organic frameworks (MOFs) based on Co-clusters as heterogeneous catalysts to selectively catalyze the reaction of C–H bond oxidation of aromatic alkanes to their corresponding ketones, three MOFs {[Co₅(pmbcd)₂(μ₃-OH)₂(H₂O)₄(DMF)₂]·4DMF}_n (MOF 1), {[Co₂(pmbcd)(bpea)₂]·2H₂O·2DMF}_n (MOF 2), and {[Co₂(pmbcd)(dpp)₂]·3H₂O·2DMF}_n (MOF 3) (H₄pmbcd = 9,9′-(1,4-phenylenebis(methylene))bis(9H-carbazole-3,6-dicarboxylic acid), bpea = 1,2-bis(4-pyridyl)ethane, dpp = 1,3-di(4-pyridyl)propane) were successfully synthesized and structurally characterized. MOF 1 was constructed from a pentanuclear Co(II) cluster and exhibited a porous framework with channels of 8 × 10 Ų along the b axis. MOF 2 was constructed from [Co₂(CO₂)₄] units and presented a porous three-dimensional (3D) framework with channels of 11 × 13 Ų along the b axis and of 10 × 12 Ų along the c axis. MOF 3 was a flat two-dimensional (2D) layer based on binuclear Co(II) units when dpp as an auxiliary ligand was introduced. The Co₅-cluster-based MOF 1 exhibited excellent catalytic activity for the direct C–H bond activation of arylalkanes to ketones in H₂O under room temperature because of its high density of Lewis acidic sites within the frameworks and suitable channel size to access the catalytic sites. It also presented the spatial confinement effect and catalyzed the reaction with high regioselectivity, forming mono-ketones as the sole products. Easy product separation, simple reaction procedures, and recyclability of these catalysts make the catalytic system attractive. Our work highlights the superiority of the MOF-based materials as heterogeneous catalysts.