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Abstract

In the past, the search for soluble aluminium trifluoride (AlF3) led to the discovery of organoaluminium compounds (AlR3) and organoaluminium fluorides. However, suitable methods for the introduction of fluorine into AlR3 are still scarce, making the discrete AlF3 and organoaluminium fluoride compounds rare. Our investigation of the reactivity of cyclic (alkyl)(amino)carbene (CAAC) stabilised AlMe3 has shown that when a suitable neutral fluorinating reagent is used, one, two or three methyl groups on the aluminium centre can be substituted by fluorine, leading to discrete dimethylaluminium fluoride, methylaluminium difluoride and AlF3 adduct, respectively. Further fluorination led to the formation of salts with discrete fluoroaluminate anions.
AlR3 compounds have a Lewis acidic aluminium centre that tends to react with fluoride anions to form various organofluoroaluminates. However, discrete anions are still rare as fluorine atoms tend to engage in the formation of oligomeric or polymeric species. Our study on the reactivity of AlR3 compounds with fluorinating reagents based on imidazolium salts has shown that by using a suitable fluorinating reagent with the appropriate HF content, compounds with discrete [R3AlF]−, [R2AlF2]− and [RAlF3]− anions can be selectively prepared. Compounds with [AlF4]− anions were prepared when AlR3 reacted with an excess of HF-based reagents. The formation of salts was accompanied by the release of RH. We have developed selective synthesis procedures for the synthesis of various organofluoroaluminates.
Aluminium hydrides (AlH3) are important reagents in materials science and synthetic chemistry. However, relatively little attention has been paid to their partially substituted derivatives. Our study on the reactivity of N-heterocyclic carbene (NHC) stabilised AlH3 showed that even unusual reagents such as aldiminium-based triflate and chloride salts can be useful reagents for hydride/triflate and hydride/chloride substitution reactions, respectively. We have successfully prepared the first NHC-stabilised aluminium dihydride triflate adduct and a new NHC-stabilised aluminium hydride chloride adduct.
Fluorinating reagents based on imidazolium salts have recently been shown to be useful for the fluorination of organic and inorganic compounds. However, since the number of reliable methods for the introduction of fluorine into aluminium compounds is still limited and reagents based on imidazolium salts resulted in anionic species, we decided to prepare a series of new fluorinating reagents based on aldiminium salts. In our work, we prepared a series of poly(hydrogen fluoride) compounds starting from aldiminium-base chloride salt and anhydrous HF. By gradually removing HF, we also obtained an aldiminium-based fluoride compound.
Finally, we tested the reactivity of the newly prepared aldiminium-based fluorinating reagents on AlR3 compounds. The poly(hydrogen fluoride) reagents converted the AlMe3 into salts with discrete [RAlMe3]− and [AlF4]− anions, while the aldiminium-based fluoride compound successfully fluorinated the CAAC-stabilised AlMe3 to form the corresponding diorganoaluminium fluoride. These preliminary results show the potential of the newly prepared fluorinating reagents. In the future, their reactivity still needs to be exploited, not only in aluminium chemistry but also in other research areas.