A Series of Chiral Metal–Organic Frameworks Based on Oxalyl Retro-Peptides: Synthesis, Characterization, Dichroism Spectra, and Gas Adsorption

By synergistic involvement of enantiopure oxalyl retro-peptide ligands and the conformationally flexible ligand 4,4′-bi-3,5-dimethylpyrazole (H 2 mbpz), four new three-dimensional (3-D) homochiral metal–organic frameworks (MOFs), Cu 2 ( l -OBAla)(H 2 mbpz)·2H 2 O ( 1 ), Cu 2 ( l -OBVal)(H 2 mbpz) ( 2 ), Cu 2 ( l -OBPhe)(H 2 mbpz) ( 3 ), and [Cu 6 ( l -OBLeu) 3 (H 2 mbpz) 3 (H 2 O) 2 ]·7H 2 O ( 4 ) ( l -OBAla = N, N ′-oxalylbis( l -alanine); l -OBVal = N, N ′-oxalylbis( l -valine); l -OBPhe = N, N ′-oxalylbis( l -phenylalanine); l -OBLeu = N, N ′-oxalylbis( l -leucine)), have been prepared under solothermal conditions. All of these structures contain binuclear subunits in which two copper atoms are coordinated by a single retro-peptide to produce a unique tetracyclic planar rigid moiety. Compounds 1 – 3 are isostructural. The self-assembly of the binuclear subunits generates two-dimensional (2-D) networks which are further pillared by H 2 mbpz ligands to give three-dimensional (3-D) frameworks with one-dimensional (1-D) channels. In compound 4, the interaction of the binuclear copper subunits forms an unexpected U-shaped secondary building unit containing six copper atoms, and further linked by H 2 mbpz ligands to furnish a 3-D architecture. The chiral nature of all compounds was confirmed by solid-state circular dichroism spectroscopy. Gas adsorption experiments indicate the microporous property of compounds 1 and 2 . Compound 1 exhibits the highest capability of gas adsorption among the four compounds due to its largest pore volume. Compound 2 reveals selectively adsorption of H 2 over N 2 as a result of the limited window size.