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

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posted on 09.03.2016, 18:23 by Mahesh S. Deshmukh, Thangavel Vijayakanth, Ramamoorthy Boomishankar
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 (D4vi) 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 (L1A), cis–cis–trans (L1B), and all-trans (L1C), which are quantitatively separated by column chromatography. Subsequent reduction of L1A, L1B, and L1C with triethylsilane in the presence of catalytic amounts of Pd/C leads to the formation of the ligands L2A, L2B, and L2C with retention of stereochemistry due to the precursor moieties. Treatment of ZnI2 with L1A gives a one-dimensional coordination framework [(L1A)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 L1B gives a chain like one-dimensional cationic coordination polymer {[(L1B)4Ag2]·2NO3·H2O·CH3OH }, 2, consisting of two different kinds of 32-membered macrocycles. Treatment of the all-trans ligand L2C with copper­(I) iodide salt results in the formation of a cubane-type Cu4I4 cluster MOF [(L2C)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.