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
(H2mbpz), four new three-dimensional (3-D) homochiral metal–organic
frameworks (MOFs), Cu2(l-OBAla)­(H2mbpz)·2H2O (1), Cu2(l-OBVal)­(H2mbpz) (2), Cu2(l-OBPhe)­(H2mbpz) (3), and [Cu6(l-OBLeu)3(H2mbpz)3 (H2O)2]·7H2O (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 H2 over N2 as a result of the
limited window size.