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, cis–trans–cis (L^1A), cis–cis–trans (L^1B), and all-trans (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]_∞, 1. 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 }_∞, 2, consisting of two different kinds of 32-membered macrocycles. Treatment of the all-trans ligand L^2C with copper­(I) iodide salt results in the formation of a cubane-type Cu₄I₄ cluster MOF [(L^2C)₄Cu₄I₄]_∞, 3, 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.