Structures and catalytic oxidative coupling reaction of four Co-MOFs modified with R-isophthalic acid (RH, OH and COOH) and trigonal ligands

Abstract

In this paper, we present four Co-MOFs using R-substituted isophthalic acid (R-H₂BDC, R = H, OH and COOH) along with trigonal n-TBT (n = 3 or 4) as organic ligands. These Co-MOFs are formulated as {[Co(1,3-BDC)(4-TBT)_2/3]·(H₂O)(DMF)_1.5}_n (Co-MOF-1), {[Co(HO-BDC)(4-TBT)]·(EtOH)₂(DMF)₂}_n (Co-MOF-2), {[Co(HO-BDC)(4-TBT)_2/3]·(H₂O)₃(MeOH)₃(DMA)}_n (Co-MOF-3) and {[Co₃(BTC)₂(3-TBT)₂(H₂O)₂]·(H₂O)₂(EtOH)₅(DMA)_1.5}_n (Co-MOF-4), where H₂BDC = isophthalic acid, OH–H₂BDC = 5-hydroxyisophthalic acid and H₃BTC = 1,3,5-tricarboxybenzene, 4-TBT = 1,3,5- tris(4-pyridyl)benzene and 3-TBT = 1,3,5-tris(3-pyridyl)benzene. Co-MOF-1, 2 and 3 are constructed from the dimeric Co₂(COO)₂ unit and exhibit 3D frameworks. In Co-MOF-4, the dimeric Co₂(COO)₂ unit and single nuclear Co^II center are connected by BTC³⁻ and 3-TBT ligands into a 3D network. The catalytic experiments revealed that four Co-MOFs can catalyze the oxidative coupling reaction of benzyl alcohols and aniline to imines with good to excellent conversions under solvent-free conditions and an air atmosphere. Co-MOF-4 exhibited the best catalytic performance and the catalyst could be reused for at least five cycles without losing its structural integrity and catalytic activity.