The reaction of a strongly basic phosphazene (Schwesinger base) with water afforded the corresponding metastable hydroxide trihydrate [OH(OH2)3]− salt. This is the first hydroxide solvate that is not in contact with a cation and furthermore one of rare known water‐stabilized hydroxide anions. Thermolysis in vacuum results in the decomposition of the hydroxide salt and quantitative liberation of the free phosphazene base. This approach was used to synthesize the Schwesinger base from its hydrochloride salt after anion exchange in excellent yields of over 97 %. This deprotonation method can also be used for the phosphazene‐base‐catalyzed preparation of the Ruppert–Prakash reagent Me3SiCF3 using fluoroform (HCF3) as the trifluoromethyl building block and sodium hydroxide as the formal deprotonation agent.
We report on the first examples of isolated silanol–silanolate anions, obtained by utilizing weakly coordinating phosphazenium counterions. The silanolate anions were synthesized from the recently published phosphazenium hydroxide hydrate salt with siloxanes. The silanol–silanolate anions are postulated intermediates in the hydroxide‐mediated polymerization of aryl and alkyl siloxanes. The silanolate anions are strong nucleophiles because of the weakly coordinating character of the phosphazenium cation, which is perceptible in their activity in polysiloxane depolymerization.
The renaissance of Brønsted superbases is primarily based on their pronounced capacity for a large variety of chemical transformations under mild reaction conditions. Four major set screws are available for the selective tuning of the basicity: the nature of the basic center (N, P, …), the degree of electron donation by substituents to the central atom, the possibility of charge delocalization, and the energy gain by hydrogen bonding. Within the past decades, a plethora of neutral electron-rich phosphine and phosphazene bases have appeared in the literature. Their outstanding properties and advantages over inorganic or charged bases have now made them indispensable as auxiliary bases in deprotonation processes. Herein, an update of the chemistry of basic phosphines and phosphazenes is given. In addition, due to widespread interest, their use in catalysis or as ligands in coordination chemistry is highlighted.
The reaction of the strong monophosphazene base with the weakly acidic phenol leads to the formation of a phenol–phenolate anion with a moderately strong hydrogen bond. Application of the more powerful tetraphosphazene base (Schwesinger base) renders the isolation of the corresponding salt with a free phenolate anion possible. This compound represents the first species featuring the free phenolate anion [H5C6‐O]−. The deprotonation of phenol derivatives with tetraphosphazene bases represents a great way for the clean preparation of salts featuring free phenolate anions and in addition allows the selective syntheses of hydrogen bonded phenol‐phenolate salts. This work presents a phosphazenium phenolate salt with a redox potential of −0.72 V and its capability for the selective activation of the chemically inert greenhouse gas SF6. The performed two‐electron reduction of SF6 leads to phosphazenium pentafluorosulfanide ([SF5]−) and fluoride salts.
Die Reaktion eines stark basischen Phosphazens (Schwesinger‐Base) mit Wasser lieferte das entsprechende metastabile Hydroxid‐Trihydrat‐Anion [OH(OH2)3]−. Dies ist das erste Hydroxid‐Solvat, das nicht mit einem Kation in Kontakt steht. Darüber hinaus stellt es eines der seltenen bekannten wasserstabilisierten Hydroxid‐Anionen dar. Die Thermolyse im Vakuum führt zur Zersetzung des Hydroxidsalzes und zur quantitativen Freisetzung der freien Phosphazenbase. Dieses Verhalten wurde für die Synthese der Schwesinger‐Base aus ihrem Hydrochloridsalz mit einem Anionenaustauscherharz in exzellenten Ausbeuten von über 97 % genutzt. Diese Deprotonierungsmethode kann auch für die phosphazenbasenkatalysierte Synthese des Ruppert‐Prakash‐Reagenzes Me3SiCF3 unter Verwendung von Fluoroform (HCF3) als Trifluormethylbaustein und Natriumhydroxid als formalem Deprotonierungsmittel verwendet werden.
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