Homochiral Metal–Organic Frameworks of Lead(II) and Cadmium(II) Constructed by Amino Acid-Functionalized Isophthalic Acids: Synthesis, Structure Diversity, and Optical Properties

On the basis of amino acid-functionalized isophthalic acids, four interesting homochiral metal–organic frameworks (HMOFs), namely, {[Pb₂(HL¹)₂]·11H₂O}_n (1), {[Pb₂(HL²)₂]·12H₂O}_n (2), {[Cd₆(L²)₄(H₂O)₁₁]­·6H₂O}_n (3), and [Pb­(HL³)]_n (4), were constructed (H₃L¹ = (S)-5-(((1-carboxyethyl)­amino)­methyl)­isophthalic acid, H₃L² = (S)-5-(((1-carboxy-3-methylbutyl)­amino)­methyl)­isophthalic acid, and H₃L³ = (S)-5-(((1-carboxy-2-phenylethyl)­amino)­methyl)­isophthalic acid). In 1, 2, and 4, all Pb^II ions have hemidirected coordination geometries despite the difference in coordination number, and ligands H₃L¹–H₃L³ in negative bivalent zwitterionic forms adopt various coordination modes to bridge Pb^II centers into two- and three-dimensional (2D–3D) coordination polymers, respectively. Both 1 and 2 contain two independent 6³-topological homochiral layers which further form a unique hydrogen-bonded double-layered HMOF with the stereochemically active lone pairs of all Pb^II ions pointing inward and the hydrophobic side arms of amino acid groups being located on both sides. For 4, it is a 3D helicate of (4²·6·8³)­(4²·6³·8) topology, and the resulting one-dimensional (1D) channels are occupied by the hydrophobic benzyls of amino acid groups in (HL³)^2– ligands. Coordination polymer 3 is an intriguing HMOF that contains two independent 2D double-layered (6³)­(6⁵·8)-topological helicates with alternating distribution of 1D hydrophilic and hydrophobic helical channels. In 3, all hydrophilic helical channels are occupied by water molecules, while all hydrophobic helical channels are filled by the isobutyls of amino acid groups. Remarkably, these chiral tricarboxylic ligands not only have versatile coordination modes in the construction of HMOFs, but also subtly regulate the crystal structures through the different hydrophobic side arms of amino acids implanted in these ligands. Furthermore, the thermal stabilities and solid-state optical properties, including CD spectra, and nonlinear optical and luminescent properties of these complexes were also investigated. Interestingly enough, in our example, it was found that hemidirected, five-coordinate Pb^II compounds perhaps have stronger emission than those hemidirected four- or six-coordinate Pb^II compounds.