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)center dot 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)mlapz)(3) (H2O)(2)]center dot 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 H2mbpz 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 H2mbpz 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.