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 (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.