Ammonium formate powder (pulverized with mortar and pestle) 19 g (300 mmol) in a 0.5L round flask with a very large egg-shaped stir bar was dried on highvac overnight. Solid NaBH4 7.60g (200 mmol) was quickly added, followed by freshly distilled anhydrous dioxane 200 mL. The mixture was vigorously stirred under Ar with air-cooled condenser on a 40 C oil bath for 4 hours. (Gas evolution!). The solids were removed by filtration, rinsed with dioxane, the filtrates were evaporated. The solid residue was dissolved in anhydrous peroxide-free THF 150 mL, diluted with cyclohexane 60 mL, the cloudy solution was filtered, the filtrates concentrated slowly on rotovap. The residue was dried on highvac, for about 30 min (the product slowly sublimes in vacuo). Y=4.33 g of a white granular solid with ammonia odor (70% of theory).
Ammonia-borane is heat sensitive and can build up pressure over time, it is best stored in an oversized bottle in a desiccator or a glove box. It is commercially available, but quite pricey.
Note: Some ammonium formate is lost upon drying (because of its volatility in vacuo) but it is being used in excess. Efficient stirring is important. Larger preparations would be best done with an overhead mechanical stirrer.
Sustainable Energy Fuels, 2024, 8, 4429-4452, DOI: 10.1039/d4se01060d, from page 4442:
“In the captivating domain of electrochemical exploration, the platinum electrode assumes the spotlight. A meticulous cyclic voltammetry analysis at 550 °C, immersed in molten NaOH, unveils the nuanced interplay of redox peaks, symbolic of the reduction of a delicate oxide film enveloping the platinum wire’s surface.135Fig. 3(D) presents the cyclic voltammograms from Ge et al.‘s study135 employing platinum as the working electrode. Each peak narrates a unique story: the cathodic current peak C1 signifies the poetic reduction of the oxide film; the captivating surge in cathodic current at C2 (−0.4 V) unfolds a ballet of hydrogen gas evolution; the anodic current peak O1 depicts the stoic oxidation of the oxide film, and the enchanting O2 serves as a crescendo harmonizing with the birth of oxygen gas. The saga continues beyond platinum, venturing into the realm of noble nickel. Its cyclic voltammetry narrative in molten NaOH reveals a tapestry of redox peaks akin to its platinum counterpart. The cathodic sonnet at C3 serenades the reduction of a wispy oxide film caressing the nickel surface. Meanwhile, the anodic peak O3 resonates with the bold oxidation of the nickel wire in the molten embrace. However, a twist in potential scan limits between 0.3 V and 1.3 V dims the presence of O3, creating an interlude where the generated nickel oxide films seek refuge from reduction in the first cycle.135 Both platinum and nickel trace redox peaks in molten NaOH, a testament to the oxide tales etched on their surfaces. Yet, nuances emerge, with platinum showcasing distinctive choreography of reduction and oxidation peaks, while nickel pirouettes with reduction peaks for the oxide film and an ode to oxidation for the nickel wire. Specific potential ranges embellish each peak with uniqueness.
Enter Ge et al.,135 pioneers in this symphony. Cyclic voltammetry, the maestro’s baton, gracefully wielded on a platinum electrode basking in molten NaOH at 550 °C. A material chosen for its chemical steadfastness, platinum graced the stage with cyclic voltammograms. The cathodic reverie, C1, whispers the reduction of a platinum-clad oxide film; O1, anodic in nature, resounds oxidation. C2‘s crescendo echoes the hydrogen evolution reaction, while O2, a sonnet, harmonizes with the oxygen evolution reaction. The saga extends as Ge et al.135 venture into the realms of platinum wire, a virtuoso in three molten hydroxides NaOH–KOH at 280 °C, LiOH–NaOH, and LiOH–KOH at 270 °C. In NaOH–KOH’s embrace, a cathodic crescendo at −0.32 V, a ballet of superoxide ions (O−2) reduction, unravelled. Cyclic voltammetry gracefully revealed the nuances of a platinum (Pt) electrode in a molten hydroxide electrolyte, a narrative skillfully crafted by Yang et al..109 Reduction and oxidation peaks pirouetted elegantly at distinctive potentials, with reduction currents exhibiting their dance below −0.55 V. The stage was then seized by a commanding oxidation peak between −0.55 V and 0.1 V, rising dramatically above 0.17 V, where the evolution of oxygen took center stage.109
In the presence of ammonia (NH3), an ethereal onset potential of approximately −0.67 V marked the beginning of anodic currents, with a crescendo leading to a maximum of around −0.2 V. However, the drama unfolded swiftly above 0.15 V as oxygen evolution claimed the spotlight. The forward scan from −0.2 to 0.15 V witnessed a diminishing oxidation current, a subtle interplay involving Pt oxidation and the reduction of the active surface for ammonia oxidation. In a seamless transition, Yang et al.109 continued their electrochemical tale, this time exploring the ammonia-driven drama on a platinum stage immersed in molten NaOH–KOH at 200 °C. Fig. 3(E) gracefully presented cyclic voltammograms, a visual symphony offering choices of argon and ammonia, while Ag stood as the stoic ref. 109. The dashed line served as a blank canvas, encapsulating platinum’s narrative in the hydroxide embrace, while the solid line painted the enchanting transformation of ammonia to N2, all harmonized by the eutectic molten hydroxides. The platinum electrode, a versatile protagonist, showcased its prowess in every experiment, contributing a lyrical stanza to the grand saga of electrochemistry.”
50 % hydrogen peroxide is sold by Sigma-Aldrich (catalog # 516813) in the US. In many countries, it is not readily available because of the transport regulations. But it can be prepared from the common inexpensive 30 % grade quite easily, by concentrating 30% H2O2 on a rotovap with dry ice condenser, from ambient water bath, up to 60-65% H2O2 solution. The final concentration can be determined readily by proton NMR, in dry d6-DMSO, by integrating the hydrogen peroxide signal at 10.25 ppm against water signal at 3.5-3.6 ppm.
A commercial 31.8 % hydrogen peroxide solution 245.0 g [Aldrich 30-35%] was charged into a round bottom 0.5 L flask that has never been used for chemistry (the new flask was rinsed with D.I. water before use, to remove any dust particles). The hydrogen peroxide solution was then concentrated on a 25 C water bath at 10 mbar (full vacuum of Teflon piston pump) for about 2 hours. The liquid remaining in the flask, 109.65 g, was 64 % hydrogen peroxide by the NMR assay; this corresponds to a 90% theory yield.
This material should be stored in a clean 0.5 L amber bottle in a fridge, within a secondary container. Leave enough headspace in the flask, because overpressure can build up. 64% H2O2 solution has limited stability, it starts slowly producing visible bubbles of oxygen if left in a bottle at room temperature overnight. A small amount (few drops) of phosphoric acid or EDTA-2Na can be added as a stabilizer.
In a 0.5 L round 3-necked flask, a solution of diisopropylphosphine-borane complex 11.2 g (84.5 mmol; 2.4 eq.) in anhydrous THF (0.2 L) was cooled to 0 oC on ice bath. 39.0 mL of 2 M nBuLi in cyclohexane [Aldrich] (78 mmol; 2.2 eq.) was added in two portions dropwise over a 30 min period with vigorous stirring. The cooling bath was then removed and the solution was stirred at ambient temperature for 1 hour, then cooled back to 0 oC on ice bath. Under flow of Ar, the side-neck of the flask was opened and solid 2,6-bis(chloromethyl)-pyridine 6.20 g [TCI] (35.2 mmol) was rapidly added in one portion. The mixture was stirred overnight (14 hours) under Ar without replenishing ice in the bath, to room temperature. (By HPLC analysis, the reaction was complete within 7 hours at room temp). The reaction was quenched by addition of D.I. water 0.2 L and toluene 100 mL, the organic phase was separated and the aqueous phase was extracted with toluene 2 x 150 mL. The combined organic phases were dried by gradual addition of magnesium sulfate, filtered and concentrated on rotovap. The oily residue (19 g) was dissolved in toluene 80 mL and slowly re-evaporated from ambient bath. The resulting solid crystalline residue was suspended in hexane (30 mL), the product was collected by filtration, rinsed with additional small portions of hexane (3 x 20 mL) and dried by suction. A second small fraction of pure product crystallized from the concentrated hexane supernatants. The combined white solids were re-dried on highvac overnight, to remove remaining trace of iPr2PH.BH3 (which is volatile at 0.05 Torr). The yield was 12.26 g of a white crystalline solid (95% of theory).
A solution of 2-methyl-6-phenyl pyridine 6.48 g (38.3 mmol) in anhydrous THF 250 mL under Ar was cooled to -78 oC and a solution of tert-BuLi 1.9 M in pentane 20 mL (38.0 mmol) [TCI] was added dropwise from a glass gas-tight Hamilton syringe with Teflon tipped plunger, with vigorous stirring over a 15 min period. The reaction mixture turned dark purple. The mixture was stirred at -78 oC for 2 hours. Neat t-Bu2PCl 96% 7.55g (40.1 mmol) [Aldrich] was added in one portion and the mixture was stirred at -78 oC for additional 2 hours. The dry ice/acetone cooling bath was then allowed to expire and attain room temperature (over a half-day period). The reaction mixture was finally stirred at 20 oC for extra 14 hours. Neat borane-dimethylsulfide complex 90% 4.00 g (47 mmol) [TCI] was then added in one portion and the reaction mixture was stirred at room temperature for 30 min. 31P-NMR analysis of a sample from the reaction mixture (diluted with C6D6) indicated complete borane-phosphine complex formation. The reaction mixture was then carefully quenched by gradual addition of water 100 mL (gas evolution!), the mixture was stirred for 30 min to complete the excess borane-DMS decomposition, and saturated NaCl aq. solution 50 mL was added in the end. The organic phase was separated and the aqueous phase was repeatedly extracted with toluene, 3 x 150mL. The combined organic phases were dried (MgSO4, 1 hour stirring), filtered and concentrated from ambient water bath down to about 50 mL volume. The concentrated solution was filtered through a sintered glass Buchner funnel to remove a small amount of insoluble materials. The filtrates were concentrated down, to obtain 14.5g of yellow oil. The crude product was dissolved in hexane 80 mL, seeded by scratching, and allowed to crystallize for 1 hour, the precipitate (8.1 g) was collected by filtration, rinsed with hexane and dried by suction and on highvac. A second crop (2 g) crystallized by concentrating the supernatants. The supernatants obtained after the second crop were evaporated, the residue was re-evaporated from benzene 10 mL. The resulting oil (3 g) was dissolved in pentane 10 mL, seeded and allowed to crystallize in a refrigerator at +4 oC overnight, to produce additional 1.0 g of pure crystalline product (washed with pentane). After vacuum drying, the combined yield was 11.125 g of a pale yellow crystalline solid (89% of theory).
In a 1L round flask, aluminum chloride 33 g (247.5 mmol) slurry in anhydrous dichloromethane (approx 200 mL) under Ar was placed on ambient water cooling bath and phenyl dichlorophosphine 23.0 mL (175 mmol) was added dropwise neat, with vigorous stirring, and the mixture was stirred at room temperature for 1 hour. (Aluminum chloride slurry mostly dissolved in the reaction mixture). Next, the mixture was cooled on ice bath to 0-5 C and tert-butyl chloride 25 mL (226 mmol) was added neat, dropwise, over a 15 min period, the cooling bath was allowed to expire and the mixture was stirred at room temperature under Ar for 4 days. The flask was then cooled back to 0-5 C on ice and 3 M aqueous HCl 200 mL was added very slowly from the addition funnel over a 30 min period with cooling on an ice bath (highly exothermic!). The cooling bath was then removed and the mixture was stirred at room temperature until the precipitated solids completely re-dissolved and a clear biphasic mixture formed (about 30 min). The organic phase was separated, the aqueous phase was re-extracted with additional dichloromethane 2 x 150 mL. The combined organic phases were dried with magnesium sulfate and concentrated on rotovap, to obtain the phosphinic acid chloride intermediate as brown oil, 38.0 g (100% of theory). This material can be purified by crystallization but it was pure enough for the next step. It is best to use the material soon because it darkens on air and sunlight.
In a 2 L three-necked flask with a very large egg-shaped stirbar, lithium aluminum hydride 16.0 g was suspended in anhydrous ethyl ether (approx. 0.9 L) under Ar, the flask was equipped with a pressure-equalized addition funnel (100 mL) and the reaction mixture was thoroughly deoxygenated by sparging the mixture with a stream of nitrogen passed into the mixture by a long needle through septa. The slurry was cooled on ice bath to 0-5 C, and with vigorous stirring, a solution of phosphinic chloride intermediate in benzene 50 mL was very slowly added dropwise over a 45 minute period, the addition funnel was then washed down with benzene 2 x 10 mL and this was also added to the reaction. The cooling bath was allowed to expire over a 12 hour period and the mixture was stirred at room temperature for additional 2 days.
To quench the reaction, the flask was placed on a large ice cooling bath and a long needle with nitrogen gas was put into the reaction mixture through septa, to provide steady nitrogen gas sparge throughout the quench. With a vigorous stirring, nitrogen-sparged ethyl acetate 20 mL was very slowly added dropwise by using the addition funnel with cooling on ice (highly exothermic), followed by dropwise addition of nitrogen-sparged water 16 mL (a voluminous precipitate begun to form with gas evolution), followed by nitrogen-sparged 15% NaOH solution 16 mL followed by additional nitrogen-sparged water 65 mL. The cooling bath was then removed and the mixture was vigorously stirred at room temperature for extra 2 hours, for the precipitate to turn white and granular. The stirring was then turned off and the slurry was allowed to settle. The supernatants were transferred by a large cannula under positive pressure of nitrogen into an argon-filled 2L round flask with septa, the solids remaining in the reaction flask were repeatedly re-slurried with portions of nitrogen-sparged benzene 3 x 200 mL and the supernatants were transferred with canula to the ether supernatants. The combined supernatants were concentrated on a clean rotovap that was thoroughly flushed with nitrogen, and nitrogen balloon was used to backfill the rotovap after evaporation. The evaporation residue was transferred under Ar into a 250 mL 14/20 joint round distillation flask, the evaporation flask was rinsed with deoxygenated benzene and this was added also to the distillation flask. The benzene solution in the distillation flask was carefully concentrated under nitrogen and the residue was distilled on highvac at 0.1 Torr with a 14/20 joint shortpath adapter and the receiving flask cooled with dry ice, from 75 C oil bath. The pure product distilled at 52 C/0.1 Torr as a colorless liquid, with a nauseating garlicky odor. The yield was 26.36 g (90.5% of theory). The material is extremely air sensitive and it is best stored under nitrogen in a glovebox.
Due to the revolting phosphine odor, it is necessary to leave the used glassware to dry out in the fume hood, and treat all solvent condensates and solid wastes with bleach.
Note: The long reaction times in this procedure happened for arbitrary reasons (= a coronavirus), one or two days would have been sufficient – but it probably helped, as the obtained yields are much better than in the literature
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