Pillared MOFs: structure and ring opening polymerization of cyclic esters

Abstract

The solvothermal reaction of Zn(NO₃)₂·6H₂O with 5-aminoisophthalic acid and 4,4′-bipyridyl (4,4′-bipy) led to the self-assembly of the known 3-D hybrid H-bonded/covalent structure {[Zn(5-AIP)(4,4′-bipy)0.5]·DMF}_n (1·DMF), but with DMF here (rather than H₂O as previously): an analogous reaction using the related 4,4′-azopyridine (4,4′-azopy) in place of 4,4′-bipyridyl afforded the structurally related framework {[Zn(5-AIP)(4,4′-azopy)0.5]·0.75DMF}_n (2·0.75DMF). Similar solvothermal reactions of Co(NO₃)·6H₂O, Mn(NO₃)·4H₂O and Cd(NO₃)·4H₂O with 5-aminoisophthalate and the potential linkers 4,4′-bipy, 2-di(4-pyridyl)ethylene (DPE), and 4,4′-azopy afforded the porous 3-D structures {[Co₂(NO₃)₂(5-AIP)(4,4′-bipy)₂]·2EtOH}_n (3·2EtOH), {[Co(5-AIP)(DPE)]·2DMF}_n (4·2DMF), {[Co(5-AIP)(4,4′-azopy)]·2DMA}_n (5·2DMA), {[Mn(5-AIP)(4,4′-bipy)]·2DMA}_n (6·2DMA), {[Mn(5-AIP)(DPE)]·6DMF}_n (7·6DMF), {[Mn(5-AIP)(4,4′-azopy)]·2.5DMF}_n (8·2.5DMF), the previously reported {[Cd(5-AIP)(4,4′-bipy)]·3DMF}_n (9·3DMF), {[Cd(5-AIP)(DPE)]·DMF}_n (10), and {Cd(5-AIP)(4,4′-azopy)(DMF)}_n (11), with structures 4–10 bearing the same network topologies with metal atoms and 5-AIP ligands in sheets, bipy ligands acting as pillars, and solvent molecules of crystallisation located around the bipy ligands. The activated MOFs were employed as catalysts for the ring opening polymerization (ROP) of ε-caprolactone and δ-valerolactone. ROPs were conducted as melts, and under N₂ only 1 with δ-VL (∼93% conversion) was active. In the case of ε-CL under air, all the systems were active with 1, 2, and 11 affording >90% conversion. Molecular weights (M_n) were in the range 3760–17 940 Da and the products formed were identified as both cyclic and linear PCL. For δ-VL, the catalysts performed somewhat better, with all systems (except 8) affording ∼90% conversion or more under air. Molecular weights (M_n) were in the range 2180–7940 and as for PCL, the products formed were identified as both cyclic and linear PCL.