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

Abstract

On the basis of amino acid-functionalized isophthalic\nacids, four\ninteresting homochiral metal–organic frameworks (HMOFs), namely,\n{[Pb₂(HL¹)₂]·11H₂O}_<i>n</i> (<b>1</b>), {[Pb₂(HL²)₂]·12H₂O}_<i>n</i> (<b>2</b>), {[Cd₆(L²)₄(H₂O)₁₁]­·6H₂O}_<i>n</i> (<b>3</b>), and [Pb­(HL³)]_<i>n</i> (<b>4</b>), were constructed\n(H₃L¹ = (<i>S</i>)-5-(((1-carboxyethyl)­amino)­methyl)­isophthalic\nacid, H₃L² = (<i>S</i>)-5-(((1-carboxy-3-methylbutyl)­amino)­methyl)­isophthalic\nacid, and H₃L³ = (<i>S</i>)-5-(((1-carboxy-2-phenylethyl)­amino)­methyl)­isophthalic\nacid). In <b>1</b>, <b>2</b>, and <b>4</b>, all\nPb^II ions have hemidirected coordination geometries despite\nthe difference in coordination number, and ligands H₃L¹–H₃L³ in negative bivalent zwitterionic\nforms adopt various coordination modes to bridge Pb^II centers\ninto two- and three-dimensional (2D–3D) coordination polymers,\nrespectively. Both <b>1</b> and <b>2</b> contain two independent\n6³-topological homochiral layers which further form a unique\nhydrogen-bonded double-layered HMOF with the stereochemically active\nlone pairs of all Pb^II ions pointing inward and the hydrophobic\nside arms of amino acid groups being located on both sides. For <b>4</b>, it is a 3D helicate of (4²·6·8³)­(4²·6³·8) topology, and the\nresulting one-dimensional (1D) channels are occupied by the hydrophobic\nbenzyls of amino acid groups in (HL³)^2– ligands. Coordination polymer <b>3</b> is an intriguing HMOF\nthat contains two independent 2D double-layered (6³)­(6⁵·8)-topological helicates with alternating distribution\nof 1D hydrophilic and hydrophobic helical channels. In <b>3</b>, all hydrophilic helical channels are occupied by water molecules,\nwhile all hydrophobic helical channels are filled by the isobutyls\nof amino acid groups. Remarkably, these chiral tricarboxylic ligands\nnot only have versatile coordination modes in the construction of\nHMOFs, but also subtly regulate the crystal structures through the\ndifferent hydrophobic side arms of amino acids implanted in these\nligands. Furthermore, the thermal stabilities and solid-state optical\nproperties, including CD spectra, and nonlinear optical and luminescent\nproperties of these complexes were also investigated. Interestingly\nenough, in our example, it was found that hemidirected, five-coordinate\nPb^II compounds perhaps have stronger emission than those\nhemidirected four- or six-coordinate Pb^II compounds.