Stereochemically Distinct Cyclotetrasiloxanes Containing 3‑Pyridyl Moieties and Their Functional Coordination Polymers

Synthesis of new cyclotetrasiloxane scaffolds containing peripherally functionalized 3-pyridyl moieties, [MeSiO­(CHCH³Py)]₄ (L¹) and [MeSiO­(CH₂CH₂³Py)]₄ (L²), and their reactivity studies with certain d¹⁰ metal ions are reported. The ligand L¹ is obtained by the Heck-coupling reaction of tetramethyl tetravinyl tetrasiloxane (D₄^vi) and 3-bromopyridine in the presence of the Pd(0) catalysts. The as-synthesized ligand L¹ shows the presence of three stereoisomers, <i>cis–trans–cis</i> (L^1A), <i>cis–cis–trans</i> (L^1B), and all-<i>trans</i> (L^1C), which are quantitatively separated by column chromatography. Subsequent reduction of L^1A, L^1B, and L^1C with triethylsilane in the presence of catalytic amounts of Pd/C leads to the formation of the ligands L^2A, L^2B, and L^2C with retention of stereochemistry due to the precursor moieties. Treatment of ZnI₂ with L^1A gives a one-dimensional coordination framework [(L^1A)₄(ZnI₂)₂]_∞, <b>1</b>. These 1D-chains are further connected by π–π stacking interactions between the pyridyl groups of the adjacent chains leading to the formation of a three-dimensional network with the topology of a PtS net. The reaction of silver nitrate with ligand L^1B gives a chain like one-dimensional cationic coordination polymer {[(L^1B)₄Ag₂]·2NO₃·H₂O·CH₃OH }_∞, <b>2</b>, consisting of two different kinds of 32-membered macrocycles. Treatment of the all-<i>trans</i> ligand L^2C with copper­(I) iodide salt results in the formation of a cubane-type Cu₄I₄ cluster MOF [(L^2C)₄Cu₄I₄]_∞, <b>3</b>, in a two-dimensional 4-connected uninodal sql/Shubnikov tetragonal plane net topology represented by the Schläfli symbol {4⁴.6²}. This MOF displays a thermochromic luminescence behavior due to Cu₄I₄ clusters showing an orange emission at 298 K and a blue emission at 77 K

Heteroatom-directed supramolecular helical-rich architectures in N-terminal protected pyridyl aromatic amino acids

Supramolecular helical structures formed by the assembly of biological and bio-inspired building blocks (typically amino acids, peptides and proteins) are an intriguing class of materials with prospective applications in sustainable biomedical technologies and electronics. Specifically, short peptide or single amino acid building blocks can give rise to ideal materials candidates in terms of low cost, adjustability, and compatibility. Yet, to date, reliable helical topologies with specific handedness have been highly challenging to obtain. Herein, we present simple N-terminal protected aromatic pyridyl amino acids that display helicity at the molecular level confirmed by single-crystal X-ray diffraction analysis. The helical structure is stabilized by strong intermolecular hydrogen bonding between the pyridyl nitrogen and carboxylic acid groups. By comparing the specific L and D isomers with the DL racemic mixture, we explicitly demonstrate the influence of amino acid chirality on supramolecular crystal packing, self-assembly, and electromechanical properties. Atomic force microscopy (AFM) nanoindentation analysis confirms the strong rigidity of the DL assembly with very high Young's modulus (31.8 ± 11.9 GPa) attributed to the stacked face-to-face dimers with macrocyclic architectures. The present study provides an effective strategy for precisely formulating supramolecular helical structures, which could pave the way for the development of new bio-electronic applications of smart chiroptical materials from functionalised amino acids.</p