Structural diversity and catalytic properties of five Co2(COO)4 cluster-based coordination polymers modified with R-isophthalic acid (R = H, NO2, CH3, OH and tBu)

Abstract

The self-assembly of 5-substituted isophthalic acid (R-H₂BDC, R = H, –NO₂, –CH₃, –OH and –^tBu), Co^II ions and 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT) under solvothermal conditions yielded five Co-based coordination polymers (CPs): {[Co(BDC)(TPT)]·0.5(H₂O)}_n (CP1), {[Co(NO₂-BDC)(TPT)]·H₂O}_n (CP2), {[Co(CH₃-BDC)(TPT)]·0.5H₂O}_n (CP3), {[Co(HO-BDC)(TPT)]·0.5H₂O}_n (CP4), and {[Co(^tBu-BDC)(TPT)_2/3]·(CH₃CH₂OH)(H₂O)}_n (CP5) (H₂BDC = isophthalic acid, NO₂-BDC = 5-nitroisophthalic acid, CH₃-BDC = 5-methylisophthalic acid, OH-BDC = 5-hydroxyisophthalic acid and ^tBu-BDC = 5-tert-butylisophthalic acid). All compounds comprise Co₂(COO)₄ binuclear units, which are connected via a combination of R-H₂BDC and TPT ligands. In CP1, the binuclear Co₂(COO)₄ units are bridged into 1D chains extending along the [1 1 0] and [−1 1 0] directions, which are further interlinked by TPT ligands into a 3D network with CdS net topology. CP2, CP3 and CP4 are iso-structural and all exhibit 3D frameworks with pcu (α-Po) topology. In CP5, the Co₂(COO)₄ units are firstly linked into a 3D NbO net via^tBu-BDC ligands, which are further accommodated by size-matching trigonal TPT ligands into a novel (3,8)-connected 3D net. Catalytic measurements show that CP2 and CP3 are good catalysts for the Knoevenagel condensation reaction of benzaldehyde with ethyl cyanoacetate, exhibiting conversions of 91.3% and 96.6% in 120 min, respectively, compared with the other present compounds. In addition, CP2 and CP5 exhibit highly efficient catalytic properties for the selective epoxidation of styrene.