Combining Solid-state and Solution-based Techniques: Synthesis and Reactivity of Chalcogenidoplumbates(II or IV)

The synthesis of chalcogenidoplumbates(II,IV) via the in situ reduction of nominal "PbCh₂" (Ch = Chalcogen) and via a solid-state reaction and subsequent solvothermal reactions is presented. Additionally, reactivities of plumbate(II) solutions are portrayed, which yield the heaviest-known CO homolog known to date: the µ-PbSe ligand.

Transcript

The overall goal of this combinational approach of solid-state and solution-based chemistry is to synthesize novel materials with intrinsic semi-conducting properties. This method can help answer key questions in the inorganic and materials chemistry field, such as how to synthesize compounds not accessible via traditional synthetic methods or access compounds with uncommon oxidation states, coordination modes, or ligands. The main advantage of this technique is that it is high-yielding and provides access to reactive, pure metallate and iron solutions to be used as precursors or for further reactivity studies.

Though this method can provide insight into the chemistry of metalloid compounds of the heaviest metal atoms, it can also be applied to other metallates. Generally, individuals new to this method will struggle because all manipulations have to be performed under strict exclusion of air, moisture and, in some cases, light. To prepare the dry solvents for synthesis, first add one liter of freshly-purchased one-two diaminoethane to 25 grams of calcium hydride.

Heat the mixture under reflux until hydrogen is no longer generated, than distill the mixture under ambient pressure. Add one liter of tetrahydrofuran, THF, to 10 grams of sodium-potassium alloy. Heat the mixture under reflux for at least twelve hours, then distill the mixture at ambient pressure.

Make a saturated solution of Wilkinson's catalyst by adding 20 milliliters of THF to 300 milligrams of the catalyst. After stirring overnight at room temperature, filter the mixture with an inert gas filter frit of low porosity. Place 3.81 grams of elemental selenium in a silica ampule and add five grams of elemental lead.

Heat the solids with an oxygen-methane burner until optical homogeneity of the melt is achieved, which is approximately ten minutes. Knock the ampule gently with a cork ring throughout the synthesis to detach sublimed selenium from the ampule wall, which will then drop back into the reaction mixture. After allowing the reaction mixture to cool to room temperature, break the ampule with a pestle and a mortar.

Manually remove all remaining splinters of the ampule, then pestle the crude lead selenide thoroughly. Next, place 0.95 grams of elemental potassium and five grams of elemental lead in a thick-walled silica ampule. Slowly heat the solids with an oxygen-methane burner until optical homogeneity of the melt is achieved, which is approximately 20 minutes.

Carefully add 1.9 grams of elemental selenium pellets to the molten alloy. Upon complete addition, increase the temperature until the reaction mixture emits bright yellow-white radiation, and hold the temperature for 10 minutes. Decrease the reaction temperature slightly if the radiation color turns to pure, bright white.

After allowing the reaction mixture to cool to room temperature, break the ampule with a pestle and a mortar. Manually remove all remaining splinters of the ampule and the regulus of elemental lead, then pestle the crude potassium lead selenide thoroughly. Place one gram of lead selenide, 1.55 grams of 18 crown 6 and a large stir bar in a round-bottom nitrogen flask.

Transfer the flask onto a stir-plate and add 125 milliliters of one two diaminoethane. Stir vigorously at room temperature, and then add 0.23 grams of elemental potassium. The freshly-cut potassium is very sticky.

Here it was covered in the lead selenide powder prior to its addition. Zero moisture must absolutely not enter the flask during the addition of the diverse chemicals. We apply counter-flow technique with the inert gas blowing out of the flasks such that air cannot go in.

After stirring overnight at room temperature, fill the solution with an inert gas filter frit of low porosity. Now, place 0.5 grams of potassium lead selenide and two milliliters of one two diaminoethane in a five milliliter glass vial. Place the glass vial in a 15 milliliter PTFE vial and place the PTFE vial in a standard stainless steel autoclave, Close the autoclave tightly, and heat in a oven at 150 degrees Celsius for five days.

After five days, turn off the oven and allow it to slowly cool to room temperature for one day, then transfer the reaction mixture into paratone oil and manually select crystals of the product under a standard-light microscope at 15 to 40 fold magnification. Place ten milliliters of the crown ether solution in a 50 milliliter flask. Add ten milliliters of the saturated Wilkinson's catalyst solution and stir overnight.

Once the reaction is complete, filter the solution with an inert gas filter frit of low porosity and remove the solvent under dynamic vacuum slowly over 24 hours. After transferring the crude reaction product into paratone oil, manually select crystals of the Chevrel type cluster under a standard-light microscope at 15 to 40 fold magnification. Following this, place ten milliliters of the crown ether solution in a Schlenk tube and carefully layer it with ten milliliters of the saturated Wilkinson's catalyst solution.

Cover the Schlenk tube with aluminum foil and leave it undisturbed for four weeks. After four weeks, transfer the resulting solid into paratone oil and select single crystals under a light microscope. The existence of an ortho-selenidoplumbate anion was confirmed by single crystal diffraction experiments, elemental analysis, and quantum chemical calculations.

The crystal structure refinement confirms the almost-perfect tetrahedral coordination geometry, whereas DFT calculations rationalize the energetically stabilized A-one representation contributing to the overall stability of the anion. Other metallate materials can be obtained by this protocol, such as potassium two mercury two selenium three, which is a semi-and photo-conductor material with a polyanionic substructure. As it exhibits a too-large band gap, the band gap can be decreased by synthesizing the heavier tellurium homologue, which increases the photo-conductivity.

High-yield impurity metallate anions can be utilized for further reactivity studies yielding molecular Chevrel-type compounds. The phosphine-saturated species include mixed-valence rhodium ions, and as the charge is highly de-localized, the structure determined by single crystal diffraction does not allow for formal oxidation state assignment. The telluridopalladate cluster is electron-precise, and palladium ions adopt a distorted square planar geometry.

Similar synthetic procedures afford compounds with trirhodium disalinide units, adopting a trigonal, biparametal shape with selenium at the apical positions and rhodium in the basal plane. These units represent the core of different anionic cluster complexes. They can be isolated by the addition of counter-ion sequestering agents.

Once mastered, this technique can be done in a day to obtain high-purity metallates in high yield. While attempting this procedure, it's important to remember that the compounds are highly sensitive to air and moisture, and in the case of heavy-element metallates, like telluridoplumbates, also to light. Following this procedure, other metallate materials that contain, for instance, light or early transition metal atoms, should accessible, too.

This extends the library of known compounds and enables fine-tuning of desired properties. After watching this video, you should have a good understanding of how to synthesize novel metallate materials with a combination of solid-state and solution-based techniques. Don't forget that working with heavy elements and their compounds can be extremely hazardous, and precautions, such as wearing appropriate lab attire and having sand readily available for fire-extinguishing purpose, should always be taken while performing this procedure.

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Combining Solid-state and Solution-based Techniques: Synthesis and Reactivity of Chalcogenidoplumbates(II or IV)

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