The Synthesis of [Sn₁₀(Si(SiMe₃)₃)₄]^2- Using a Metastable Sn(I) Halide Solution Synthesized via a Co-condensation Technique

The overall goal of the preparative co-condensation technique is to access novel reagents based on molecules that are unattainable from classical synthetic procedures, which opens the door to new fields in chemistry. The main advantage of this method is that highly reactive reagents are obtained and subsequent reactions can be done at low temperature, which is important for the synthesis of metastable metalloid clusters. This method can help to answer key questions in the field of nanotechnology, as structure-property relations might be established from structurally well-characterized metalloid cluster compounds.

Though the metastable tin-halide solution gives access to nanoscale cluster, it might also be used for coordination or adducts chemistry or for the synthesis of novel materials. Visual demonstration of this method is critical as the construction steps are difficult to learn due to the high complexity of the co-condensation apparatus. First, fill five reaction chambers with six grams of elemental tin.

Stack the reaction chambers over a terminal ring, each of them twisted by 30 degrees, and fix the stack with a 0.7 millimeter graphite rod. Insert the stack into a hollow graphite tube, so that the holes of the terminal ring fit with the holes in the graphite tube. Fix the setup with a second 0.7 millimeter graphite rod.

Then, weigh the whole setup with a scale and note the value to determine the amount of tin consumed during the reaction. Next, place the graphite reactor inside the copper induction coil of a co-condensation apparatus, and pass the cord's tube through the central hole of the top copper sheet, placed above the induction coil. Fix the cord's tube to a hydrogen chloride supply pipe with a gasket and a screw nut.

Check the position of the reactor inside the coil, to ensure that the reactor protrudes from the coil by five, plus or minus 0.5, millimeters. Connect the cooling shield to the water pins of the connector with screw nuts. After fixing the cooling shield at the other side with a screw, test the water tightness of the cooling cycle by opening the tap water for 30 seconds.

Following this, fix the fiber optics of a pyrometer to the holder at the copper shield with a screw. Fix the solvent vapor diffuser with a screw nut, and ensure that it is centered under the cavity of the copper shield. Re-check that the graphite reactor is still in the center of the induction coil and that there is no contact of the coil to the top copper sheet.

Connect a steel tube with a valve to the vapor diffuser. Now, mount the Schlenk vessel for solvent evaporation onto the valve. Attach a previously prepared solvent mixture flask to the Schlenk vessel.

Then, add a hemispherical heating mantle to the evaporating Schlenk vessel. Connect a long steel cannula to the apparatus with a screw nut, and close the outer tail with a small round-bottom flask. After placing a 10 centimeter magnetic stir bar inside a stainless steel vessel, place a gasket into the cavity of the plane flange, and connect the vessel to the co-condensation apparatus.

Next, connect the hydrogen chloride-containing glass vessel to a small steel tube, and then to a four point adapter. Close the rear exit of the four point adapter with a blind flange. Then, connect a differential pressure manometer to the front exit, and set up a valve at the top exit.

Now, connect a fine needle valve to the valve at the top exit of the four point adapter. Then, use a long steel tube to connect the gas supply part to the co-condensation apparatus. After complete assembly, evacuate the co-condensation apparatus overnight to a final pressure of around 10 to the minus five millibar.

The next day, fill the cooling trap of an oil diffusion pump with four liters of liquid nitrogen. Switch on the cooling water cycle of a high frequency generator, and open the tap water. After turning on the generator, slowly bake out the graphite reactor by increasing the output power in incremental steps of 0.1 to 0.5 kilowatts until the reaction temperature is around 1300 degrees Celsius.

Following bake-out, adjust the value of the generator to 1.0 kilowatt to cool down the reactor. After filling a steel dewar with 30 liters of liquid nitrogen, lift it with a lifting platform so that the reaction vessel is placed within the dewar. Then, add more liquid nitrogen to the dewar to reach the final level of liquid nitrogen.

Switch on the hemispherical heating mantle on the lowest level to maintain evaporation. At this point, evaporate the solvent drop-wise. Fix the drop rate so that all solvent is used during the reaction.

Close the fine needle and the gas valve, then open the hydrogen chloride-containing glass vessel. If the displayed voltage at the differential pressure manometer is not higher than 1600 millivolts, apply low pressure at the second connection of the manometer by connecting an evacuated vessel to the outer connection of the manometer. Record the displayed starting value.

Next, heat the graphite reactor to 1300 degrees Celsius by adjusting the value of the generator to 3.6 kilowatts. After opening the gas valve, slowly open the fine needle valve to let in the hydrogen chloride gas at a constant rate of eight millivolts per minute. Check the rate at least every 10 minutes, and record the measured values.

Add two liters of liquid nitrogen to the dewar, approximately every 10 minutes, so that the level of liquid nitrogen is always at the upper third of the stainless steel vessel. After the value of the differential manometer is decreased by 1600 millivolts, close the gas valve. Then introduce the remaining hydrogen chloride gas that is within the gas supply part by slowly opening the fine needle valve, and shut down the high frequency generator.

Disconnect the oil diffusion pump by closing the main valve, and flush the apparatus with dry nitrogen to a pressure around one atmosphere. Replace the liquid nitrogen-filled steel dewar with an insulated bucket placed on a magnetic stirrer. Then, add about five kilograms of fine powdered dry ice to the bucket, and turn on the magnetic stirrer.

After allowing the solution to stir for at least 45 minutes, replace the round-bottom flash at the press cannula with a double-valve Schlenk tube while still cooling with dry ice under constant flow of nitrogen. Fix the height of the press cannula so that it touches the bottom of the stainless steel vessel. Now, push out the tin chloride solution with slight over-pressure by opening the valves at the double-valve Schlenk tube.

To determine the solution quality, perform a halide titration by dissolving two milliliters of the solution in 50 milliliters of dilute nitric acid. Then, add one milliliter of 30 percent hydrogen peroxide to the mixture. After stirring for 10 minutes, perform a potentiometric titration of silver nitrate against the calomel electrode.

Following this, weigh the graphite tube to determine the amount of consumed tin, which should be between 4 and 4.8 grams. Using a steel or Teflon cannula, add 20 milliliters of the 0.2 molar tin chloride solution to a cooled Schlenk vessel, containing two grams of the ligon source. Turn on the magnetic stirrer, allowing the reaction to stir, and slowly warm to room temperature within three hours.

Once the reaction is complete, stop the magnetic stirrer and allow all insoluble precipitate inside the Schlenk tube to settle. Then, filter the dark brown solution into another Schlenk vessel. Finally, add 0.2 milliliters of TMEDA to the mixture, allow it to stand overnight to form black crystals.

After isolation, identify the crystals via proton NMR. The principle of the matrixed isolation technique, in conjugation with preparative co-condensation technique, is shown here. A schematic of the co-condensation apparatus setup is displayed here.

Here, the synthetic root to the metalloid cluster compound one is shown, applying the disproportionation reaction of the metastable sub-valent tin chloride. The proton, carbon, and silicon NMR spectra of the metalloid tin 10-cluster crystals are shown here. The molecular structure of compound one, as determined by x-ray crystallographic analysis, is displayed here.

Once mastered, the co-condensation can be performed within two days, if it is performed properly. The synthesis of the metalloid tin 10-cluster requires another week for the crystallization of the product. While attempting this procedure, it's important to remember that the settings during the co-condensation reaction must at constant to obtain a suitable metastable solution that can be used for the synthesis of metalloid clusters.

Following this procedure, subsequent reactions with the metalloid tin 10-cluster can be performed in order to answer additional questions like chemical reactivity of metalloid tin clusters. After its development, this technique paved the way for researches in the field of nanotechnology to explore properties on defined nano-scaled metalloid cluster compounds. After watching this video, you should have a good understanding of how preparative co-condensation is performed, leading to novel reagents, based on molecules that are unattainable via classical synthetic procedures, which opens the door to new fields in chemistry.

Don't forget that working with a co-condensation apparatus and the highly reactive metastable sub-halide solution can be extremely hazardous, and personal protective equipment such as safety glasses, gloves, and lab hood should be worn while performing the reactions.