Chemistry
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Synthesis of a Thiol Building Block for the Crystallization of a Semiconducting Gyroidal Metal-sulfur Framework
Chapters
Summary April 9th, 2018
Here, we present a one-pot, transition-metal-free synthesis of thiols and thioesters from aromatic halides and sodium thiomethoxide, followed by the preparation of single crystals of a metal-dithiolene network using thiol species generated in situ from the more stable and tractable thioester.
Transcript
The overall goal of this protocol is to synthesize thiol linker molecules and to prepare single crystals of porous, metal-sulfur frameworks from these thiol species. This method can help answer key questions in the field of materials chemistry as to how to better prepare crystalline networks from metal ions and mercaptan molecules. The main advantage of this technique is provided by the convenient and reduced-time synthesis which utilize the relative stable thioester as the precursor for framework assembly.
Demonstrating the procedure will be Mr.He Yonghe, a graduate student in my laboratory. To begin the procedure, connect a 200 milliliter Schlenk flask, equipped with a stir bar, to a vacuum-gas manifold. Evacuate and backfill the flask with nitrogen gas three times.
From then on, keep the flask filled with flowing nitrogen gas at a slight positive pressure. Next, remove a block of metallic sodium from the mineral oil in which it was stored. Wipe away residual mineral oil with a paper towel and scrape off the oxide layer with a knife.
Quickly weigh out approximately 6.7 grams of metallic sodium, and note the weight. Cut the sodium into small pieces and transfer the pieces to the Schlenk flask under a counter-flow of nitrogen gas. Immediately seal the flask with a septum.
Add 80 milliliters of anhydrous air-free tetrahydrofuran to the flask via cannula transfer. Then, draw 14.0 milliliters of nitrogen-purged dimethyl disulphide into a syringe. Add the DMDS to the mixture drop wise under a nitrogen atmosphere.
Replace the septum with a ground-glass stopper. Then, stir the mixture at room temperature for 24 hours under a flow of nitrogen gas. Then, stir the mixture at 60 degrees celsius for three hours.
Ensure that the vacuum manifold is equipped with a cold trap in-line with the gas outlet. With the mixture stirring at 60 degrees celsius, evaporate the THF and excess DMDS under a steady stream of nitrogen gas until a solid has formed. Remove the residual THF and DMDS from the solid under vacuum for two hours to obtain sodium thiomethoxide as a light yellow solid.
Store the product under a nitrogen atmosphere in the absence of light. To begin the HVaTT synthesis, connect a 50 milliliter Schlenk flask, equipped with a stir bar, to a vacuum manifold and purge the flask. Quickly measure out approximately 0.664 grams of sodium thiomethoxide and note the weight.
Under a counter-flow of nitrogen gas, quickly load the sodium thiomethoxide into the reaction flask, and stopper the flask. Add 0.216 grams of HBT to the flask under a counter-flow of nitrogen gas and seal the flask with a septum. Add 10 milliliters of anhydrous air-free DMEU to the flask via cannula transfer.
Afterwards, replace the septum with a ground-glass stopper. Heat the mixture to 240 degrees celsius with a salt bath and stir the mixture under a nitrogen atmosphere for 48 hours. When ready to check the reaction progress, while under a flow of nitrogen gas, use a glass pipette to transfer about 0.1 milliliters of the reaction mixture to a plastic micro-centrifuge vial containing 0.1 milliliters of neat valeroyl chloride.
Quickly close the vial. Shake the mixture for one minute. Then, add to the turbid, gray-white mixture 0.4 milliliters of deionized water and 0.1 milliliters of ethyl acetate.
Shake the mixture for a few more seconds and allow the mixture to separate. If an insoluble white substance is between the layers, the reaction is likely incomplete. If nothing is observed between the water and ethyl acetate layers, the reaction is potentially complete and the sample may be further examined by thin-layer chromatography.
Spot the upper layer on a TLC plate. Develop the plate with a 1 to 4 mixture of ethyl acetate and petroleum ether as the eluent, and visualize the plate with UV light. Examine the plate to determine whether the reaction is complete.
Once the reaction is complete, remove the flask from the salt bath and allow the mixture to cool to room temperature. Then cool the mixture to zero degrees celsius with an ice bath. Replace the stopper with a rubber septum.
Draw 1.5 milliliters of valeroyl chloride into a syringe. Add the valeroyl chloride to the mixture drop wise over about two minutes while under a nitrogen atmosphere. Replace the septum with a ground-glass stopper and stir the mixture at zero degrees celsius for two hours.
Then dilute the mixture with 50 milliliters of ice water. Extract the product into 30 milliliter portions of ethyl acetate three times. Wash the combined organic layers with 60 milliliter portions of deionized water six times.
Dry the washed organic fractions over anhydrous magnesium sulfate. Filter off the magnesium sulfate and remove volatiles from the filtrate with a rotary evaporator. Isolate the oily crude product by column chromatography with a 1 to 10 mixture of ethyl acetate and petroleum ether as the eluent.
Remove volatiles with a rotary evaporator to obtain a light yellow oil. Combine the oil with five milliliters of methanol, sonicate the mixture for two minutes, and collect the resultant off-white solid product by vacuum filtration. To begin the HTT-Pb synthesis, dissolve 5.7 milligrams of lead two acetate trihydrate in 1.0 milliliters of ethylene diamine in a five milliliter vial.
Place 4.6 milligrams of HVaTT, 1.0 milliliters of a degassed 140 millimolar solution of sodium hydroxide in methanol, and 0.7 milliliters of ethylene diamine in a 10 milliliter Schlenk tube. Crystals of various sizes can be obtained by adjusting the solvents and the basicity of the reaction mixture. For example, by varying the ethylene diamine methanol volume ratio between one to two and one to one, and the HVaTT sodium hydroxide molar ratio between one to 14 and one to 28.
Sonicate the HVaTT mixture for five minutes. Then add the lead two acetate solution to the HVaTT mixture. Bubble nitrogen gas through the mixture for one minute without stirring.
Seal the Schlenk tube with a screw cap and heat the mixture in an oven at 90 degrees celsius for 48 hours. Then allow the mixture to cool to room temperature to grow yellow-orange octahedral single crystals, suitable for single-crystal x-ray diffraction. Use a pipette to remove the solution from the crystals and wash the crystals with methanol several times.
Store the washed single crystals in five milliliters of air-free methanol. Next, obtain an aqueous solution of 0.1%by weight paraquot diiodide. Transfer a few HTT-Pb single crystals from the methanol stock to a petri dish.
Use laboratory tissue or filter paper to soak up excess methanol in the dish. Carefully apply a few drops of the paraquot diiodide solution to the crystals. Observe the color change of the crystals under a microscope or with the naked eye.
The proton NMR spectrum of HVaTT showed a singlet from the aromatic hydrogens and multiplets from the aliphatic protons. Aliphatic C-H stretching was also observed in the HVaTT IR spectrum. The strong absorption at 1700 reciprocal centimeters was attributed to carbonyl stretching from the carbonyls protecting the thioester groups.
This carbonyl stretching was correspondingly absent in the IR spectrum of crystalline HTT-Pb. The HTT-Pb also lacked the aliphatic C-H stretching observed in HVaTT. Powder x-ray diffraction of crystalline HTT-Pb was consistent with the reported HTT-Pb single crystal structure.
Demonstrating the crystalline-phase purity of the product. The HTT-Pb crystals were highly responsive to paraquot diiodide in water, with the crystals changing color within one to two minutes of contact with an aqueous paraquot diiodide solution. This behavior was attributed to strong charge transfer interactions between the electron-rich HTT-Pb framework and the highly electron-deficient paraquot guest.
This technique paves the way for researchers of open framework materials to explore the construction of various metal-thiolate networks for studies of their electronic and catalytic properties. This procedure will facilitate deeper studies on these important metal-sulfur framework materials. Aromatic thioester can also be prepared for molecular chemistry and solution chemistry identification using this procedure.
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