Pairing Heterocyclic Cations with closo-Icosahedral Borane and Carborane Anions, II: Benchtop Alternative Synthetic Methodologies for Binary Triazolium and Tetrazolium Salts with Significant Water Solubility

Abstract

Two efficient processes for the synthesis of 12 relatively water-soluble binary triazolium and the first tetrazolium borane [B₁₂H₁₂] and carborane [CB₁₁H₁₂] salts by a one-step, open-air metathesis reaction have been developed. First, a combination of exhaustive trituration of the two solid reactant salts with refluxing anhydrous acetonitrile followed by flash filtration through a plug of silica gel afforded excellent recovery for a broad series of otherwise water-soluble heterocyclium salts. Second, an alternative aqueous metathesis, driven to completion by precipitation of silver halides, followed by removal of water, redissolution in acetonitrile, and filtration through a silica-gel plug, also yielded such heterocyclium borane and carborane salts. Mixed 1:1 dication heterocyclium borane salts were first synthesized using this second procedure, and one example showed melting-point depression behavior.

GRAPHICAL ABSTRACT

Keywords:

• closo-Icosahedral borane dianions
• closo-icosahedral carborane anions
• exhaustive trituration
• heterocyclium halides
• mixed heterocyclium boranes
• silver-halide-mediated metathesis

ACKNOWLEDGMENT

The authors acknowledge the Air Force Research Laboratory (AFRL/RZSP, Edwards AFB, Calif.) for financial and administrative support.

Notes

^a Below detection limit.

*The term binary fits the carborane salts that stoichiometrically contain one heterocyclium cation paired with the carborane anion. For discussions in this report and others,^{[ 1 , 6 ]} the term binary also is extended to the borane salts that contain the same two heterocyclium cations paired with the borane dianion. The term ternary is reserved for the mixed borane salts where two different heterocyclium cations are stoichiometrically paired in a 1:1 ratio with the borane dianion. Similarly, salt 12 with a heterocyclium [Het]⁺ cation and potassium [K]⁺ cation stoichiometrically paired with the same borane dianion is considered to be a ternary salt.

†The synthetic method used, the significantly greater than 50% yields, and the proton NMR integrations are consistent with the formation of mixed salts 12–14. However, it is possible the same result would be observed if the isolated solids contained either an exact one-third mixture of the two pure salts and the mixed salt or of some mixture containing an equal percentage of the two pure salts with the mixed salt constituting the remaining 100% sample. In any case, two different heterocyclic mono-cations are in some arrangement of intimate contact with one borane dianion, which provided at least one interesting property, eutectic melting-point depression. Recrystallization of ternary salts 12–14 was not attempted because of a concern that a cation redistribution might result.