Received: October 2021

DOI 10.17677/fn20714807.2021.06.01

Fluorine Notes, 2021, 139, 1-2

SYNTHESIS OF LITHIUM (PERFLUOROALKYL)TRIHYDROALUMINATES Li[R_FAlH₃]

A.A. Tyutyunov^a, А.S. Peregudov^a, S.М. Igumnov^a,b

^aA.N. Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences,
28 Vavilova St., 119991 Moscow, Russian Federation

^bP&M-Invest Scientific Production Association,
28 Vavilova St., 119991 Moscow, Russian Federation

e-mail: tuytuynov@rambler.ru

Abstract: (Perfluoroalkyl)trimethylsilanes were shown to react with LiAlH₄ in ethereal solvents to give lithium (perfluoroalkyl)trihydroaluminates Li[R_FAlH₃], which, when R_F = C₂F₅ and n-C₃F₇, can be isolated as stable complexes solvated with ethereal solvents, for example Li[C₂F₅AlH₃]2DME, Li[ⁿC₃F₇AlH₃]2DME.

Key words: lithium aluminum hydride, (perfluoroalkyl)trimethyl silanes, lithium (perfluoroalkyl)trihydroaluminates

We have previously shown that the reaction of NaBH₄ with CF₃SiMe₃ in diglyme does not produce Na[CF₃BH₃], and instead, gives HCF₂SiMe₃ as a major product [1]. We also failed to prepare (trifluoromethyl)borohydrides by the action of NaBH₄ or BH₃ (obtained in situ by the reaction of NaBH₄ with ClSiMe₃) in a diglyme solution on CF₃B(OMe)₂ or K[CF₃B(OMe)₃]. The resulting products proved to be unstable and decomposed during isolation, in some cases with strong explosions. The recent report on the synthesis of (perfluoroalkyl)borohydrides indirectly supports our results, and it shows that more stable potassium and cesium (pentafluoroethyl)borohydrides, obtained in pure forms, explode upon friction [2].

A natural question that arose when studying the reaction of R_FSiMe₃ with NaBH₄, is how a similar reaction will proceed with LiAlH₄? We have shown that R_FSiMe₃ reacts with LiAlH₄ in ethereal solvents to furnish Li[R_FAlH₃] and HSiMe₃ [3]. In the present paper, we present the results of these studies.

It turned out, that following the addition of CF₃SiMe₃ (1.1 equiv.) to a solution of LiAlH₄ (1 equiv.) in diglyme at room temperature, a highly exothermic reaction is observed after a few minutes. When carrying out this reaction while controlling the temperature of the reaction mixture at ~20°C for 5.5 hours, analysis of the reaction mixture by ¹H NMR confirmed the formation of HSiMe₃ (¹H NMR, δ: 0.56 (d, 9H, ³J_HH = 5 Hz, SiMe₃), 4.44 (dec, 1H, HSi); ²⁹Si {¹H} NMR,δ: -16 (s)), the conversion of CF₃SiMe₃ is 58% (in 24 h, the conversion of CF₃SiMe₃ is 79%). The ¹⁹F NMR spectrum of the reaction mixture contains a broad peak with a maximum at -175 ppm, indicating the formation of aluminum fluoride complexes [4] (e.g., in [ⁿBu₄N][Me₂AlF₂] and Li[(Me₃Si)₃CAlF₃]THF, the fluorine atoms resonate at -154 [5] and -169 ppm [6], respectively), and also a residual signal of unreacted CF₃SiMe₃ at -67 ppm. This is consistent with the data of the ²⁹Si{¹H} NMR spectrum, which shows the peak corresponding to CF₃SiMe₃ at δ=5 (q, ²J_SiF = 37 Hz, CF₃Si). Taking into account that the ¹H NMR spectrum of the reaction mixture contains several broad upfield signals (δ: -0.7 and -0.75 ppm), it can be assumed that in this reaction, in addition to HSiMe₃, lithium aluminum hydrofluoride complexes are formed. No characteristic peaks appear in the ⁷Li and ²⁷Al NMR spectra. The reaction in THF provides a similar result. The degree of conversion of CF₃SiMe₃ to HSiMe₃ is 36% (8 h), 55% (32 h), and 63% (56 h); also, lithium aluminum hydrofluoride complexes are formed having similar spectral characteristics. The removal of THF in vacuo, together with the volatile components of the mixture (HSiMe₃ and unreacted CF₃SiMe₃), provides a white solid, which actively reacts with water and methanol. The mass of the residue, as well as the low fluorine content (5%), established by elemental analysis, suggests that this product is Li[AlFH₃], which contains unreacted LiAlH₄ and solvating THF, probably resulting from the decomposition of the initially formed Li[CF₃AlH₃]. Purification and identification of lithium aluminum hydrofluoride complexes formed in this reaction was not performed.

In contrast, C₂F₅SiMe₃ reacts with LiAlH₄ in diglyme virtually without heat evolution. According to ¹H and ²⁹Si{¹H} NMR data, the reaction provides HSiMe₃, and in the ¹⁹F NMR spectrum, in addition to peaks for the starting C₂F₅SiMe₃ (δ: -132 (s, 2F, CF₂), -82 (s, 3F, CF₃); ²⁹Si {¹H} NMR δ: 7.8 (t, ²J_SiF = 27 Hz, CF₂Si)), signals for the product bearing C₂F₅ moiety (δ: -126 (br. s, 2F, CF₂Al), -84 (s, 3F, CF₃)) are observed, where this product is, as we hypothesize, an ate complex of Li[C₂F₅AlH₃], which is consistent with the data of the ²⁷Al NMR spectra, showing the peak at 116 ppm (br.d, CF₂Al). However, based on the data of ¹H, ¹⁹F, ²⁷Al, and ²⁹Si NMR spectra, it turned out that C₂F₅SiMe₃ (1.1 equiv.) reacts with c LiAlH₄ (1 equiv.) in diglyme at ~20°C within several days with incomplete conversion of ~70%. This circumstance significantly complicates the isolation of the Li[C₂F₅AlH₃] complex in pure form, due to the presence of unreacted LiAlH₄ in the solution. This is confirmed by the ²⁷Al NMR spectrum containing a peak at 102 ppm, br.s (the signal, which should represent a quintet, was not resolved, probably due to the solvent effect [7-8]). Therefore, we investigated the effect of the nature of the solvent, the reagent ratio, and the reaction time on the degree of conversion of the starting compounds (see Table).

The above data show that the reaction proceeds equally well in strongly solvating donor ethereal solvents such as diglyme, monoglyme, and THF (runs 1-5). At the same time, the conversion of (pentafluoroethyl)trimethyl silane in diethyl ether for a similar time is significantly lower (run 6). In turn, C₂F₅SiMe₃ does not react with LiAlH₄ in 1,4-dioxane, most likely, due to the very low solubility of LiAlH₄ (solubility of LiAlH₄ in 1,4-dioxane at 25°C is 0.1 g in 100 g [9]). No reaction was also observed using hexamethyldisiloxane, triethylamine, and TMEDA as a solvent, probably, for similar reasons (runs 8-10). In terms of the convenience of isolating complex Li[C₂F₅AlH₃], relatively low boiling solvents such as THF and monoglyme are the most suitable. Thus, we have shown that when carrying out the reaction in monoglyme, to achieve almost complete conversion of LiAlH₄, a large (3-5 equiv.) excess of C₂F₅SiMe₃ and an increase in the reaction time to 5 days are required (runs 3-4). A further increase in the reaction time to 9 days leads to the accumulation of side-formed fluorine-containing impurities, the content of which in the isolated product increases from ~ 2% (if the reaction is carried out for 5 days) to 12% (during the reaction time of 9 days). Being carried out under optimal conditions, with the subsequent removal of volatiles in vacuo and toluene purification of the product, the reaction gives a solvated Li[C₂F₅AlH₃]2DME complex. Similarly, Li[C₂F₅AlH₃]2ТHF complex was obtained.

It should be noted that we failed to carry out this reaction using a slight excess of (pentafluoroethyl)trimethyl silane up to complete conversion of LiAlH₄ by shifting the equilibrium towards the formation of products through distilling off low-boiling HSiMe₃ (bp 7°C). The process almost terminates at 70-80% conversion, and only using a large excess of C₂F₅SiMe₃ (>3 equiv.) is it possible to achieve a high degree of LiAlH₄ conversion.

Ethereal solvents (Et₂O, THF) can be completely removed from their complexes with LiAlH₄ by heating in a dynamic vacuum (~0.5 Torr) at a temperature of 60-70°C [9-10]. However, an attempt to remove a solvent from Li[C₂F₅AlH₃]2DME by performing such a procedure at 55‑60^оС leads to decomposition of this complex hydride accompanied by a strong explosion (decomposition point of LiAlH₄ >100-150°C [9-10]).

Thus synthesized Li[C₂F₅AlH₃]2DME is a colorless liquid with the following spectral characteristics. Hydride protons appear in the ¹H NMR spectra as a broad singlet at 3.5 ppm (for the THF complex, the signal is sharper, with δ=3.2 ppm), which is quite close to the value of a chemical shift of protons in LiAlH₄ (δ=2.9 ppm (in DME)) [11]. The ¹⁹F NMR spectrum contains two pairs of signals corresponding to the C₂F₅ moiety (δ: -126 (br.s, 2F, CF₂Al), -83.8 (s, 3F, CF₃) and δ: -128 (br.s, 2F, CF₂Al), -84 (s, 3F, CF₃), in a ratio of 2.5:1. Accordingly, there are two signals for CF₃ moeties in the ¹³С{¹H} NMR spectrum appearing as a quartet of triplets at ~123 ppm (¹J_CF = 283 Hz, ²J_CF = 30 Hz) and a highly broadened triplet at 132 ppm corresponding to the aluminum-bonded CF₂ moiety. The ⁷Li and ²⁷Al NMR spectra show a singlet of -0.7 ppm and a highly broadened singlet at 117 ppm respectively. When recording the ¹⁹F and ²⁷Al NMR spectra in a dilute solution in monoglyme, the form of the spectra becomes similar to that of the reaction mixture. Namely, in the fluorine NMR spectrum, signals for nearly one C₂F₅ moiety are observed (δ: -126 (br.s, 2F, CF₂Al), -84.3 (s, 3F, CF₃)) (the content of the second pairs of signals for the C₂F₅ moiety decreases from 25% to 6%), while the ²⁷Al NMR spectrum contains a highly broadened quartet at 117 ppm (¹J_AlH = ~170 Hz) (for comparison, the ²⁷Al-¹H coupling constant in the AlH₄^- anion is 170-175 Hz [10]).

When stored in a sealed ampoule for several weeks, the liquid Li[C₂F₅AlH₃]2DME complex transforms into a crystalline-like but in fact a gel-like product. Raman spectra of the liquid complex and the gel-like compound formed 2 weeks later are identical, which suggests that the change in the state of aggregation does not occur through the decomposition of the complex. When refrigerated (~ -25°C), the liquid complex transforms into a white crystalline substance that can be stored for a long time without visible decomposition.

In the presence of chelating agents such as TMEDA or DABCO in ethereal solvents, Li[C₂F₅AlH₃] decomposes, so we failed to obtain complexes of the type LiAlH₄-TMEN [11] (TMEN = TMEDA), although triethylamine does not react with Li[C₂F₅AlH₃] under these conditions.

(n-Heptafluoropropyl)trimethyl silane reacts with LiAlH₄ similarly. At the same time, when carrying out the reaction of ⁿC₃F₇SiMe₃ with LiAlH₄, under optimal conditions (DME, 5 equiv. of the silane, 5 days), a noticeable decrease (to 77%) in the yield of Li[ⁿC₃F₇AlH₃]^.2DME along with an increase in the yield of fluorine-containing by-products up to 7-12% is observed. The resulting n-heptafluoropropyl complex Li[ⁿC₃F₇AlH₃]2DME is also a liquid and its spectral characteristics are identical to those for the complex bearing C₂F₅ moiety.

With a further elongation of the perfluoroalkyl substituent, reactions begin to dominate affording fluorine-containing by-products. Thus, ⁿC₆F₁₃SiMe₃ reacts with LiAlH₄ in THF for 2 days to give, according to ¹H, ¹⁹F, and ²⁷Al NMR spectra, the corresponding (n‑perfluorohexyl)trihydroaluminate complex in ~30% yield. However, as the reaction proceeds further, significant amounts of by-products begin to appear in the reaction mixture, which makes it impossible to isolate the Li[ⁿC₆F₁₃AlH₃]2THF complex in pure form.

Similar changes in the reactivity of (perfluoroalkyl)trimethyl silanes associated with the length of the perfluoroalkyl radical were also observed earlier. For example, it is known that CF₃SiMe₃ and C₂F₅SiMe₃ perfluoroalkylate B(OMe)₃ almost quantitatively in the presence of KF in THF or diglyme solution to afford corresponding borate salts. At the same time, ⁿC₃F₇SiMe₃ reacts with B(OMe)₃ under similar conditions to give K[ⁿC₃F₇B(OMe)₃] in a very low yield [12].

It is known from the literature, that Na[EtAlH₃] is unstable in solution and exists as an equilibrium mixture with the products of its disproportionation, namely, NaAlH₄ and Na[Et₂AlH₂], which was established from the ²⁷Al NMR spectrum containing three signals corresponding to these compounds [13]. Unlike its ethyl analog, Na[ⁱBuAlH₃] is stable and can be obtained via the reaction between NaAlH₄ and NaAlⁱBu₄ or AlⁱBu₃ [13]. Other examples of lithium alkyl- and aryltrihydroaluminates have also been reported, the stability of which is determined by the steric volume of an aliphatic or aromatic substituent on the aluminum atom, e.g., Li[(Me₃Si)₃CAlH₃] and Li[(Me₂PhSi)₃CAlH₃] [14], and also Li[(2,4,6-ⁱPr₃C₆H₂)AlH₃] and Li[(2,4,6-^tBuC₆H₂)AlH₃] [15].

We found that Li[C₂F₅AlH₃] and Li[ⁿC₃F₇AlH₃] solvated with DME or THF are fairly stable compounds and can be obtained in pure form from the corresponding (perfluoroalkyl)trimethyl silanes and LiAlH₄ in good yields. In this reaction, R_FSiMe₃ react with LiAlH₄ like aliphatic derivatives of alkali, alkaline-earth metals, Zn, Cd, B, and Al [9]. However, we failed to obtain pure forms of lithium (perfluoroalkyl)trihydroaluminates bearing CF₃ and ⁿC₆F₁₃ moieties, probably due to their instability under reaction conditions. It is interesting to note that recently, two papers have been simultaneously published on the synthesis of the tetrakis(pentafluoroethyl)aluminate anion, [Al(C₂F₅)₄]^- [16, 17], which, according to the authors, is the first characterized perfluoroalkyl aluminum derivative containing an R_F-Al bond. In this case, one of the synthetic procedures to obtain Li[(C₂F₅)₄Al], a salt which is unstable in ether solution or solid state and solvated with Et₂O, comprised the reaction of (C₂F₅)₃SiMe or (C₂F₅)₃SiH with LiAlH₄ to produce exclusively a tetrakis-substituted product and corresponding hydrosilanes [16]. In the reaction of C₂F₅SiMe₃ with LiAlH₄ discovered by us, the formation of poly-C₂F₅-substituted aluminates was not observed.

To conclude, a general possibility of synthesizing stable perfluoroalkyl aluminum derivatives by replacing the substituent in the coordination sphere of the tetrahedral aluminum anion with a perfluoroalkyl group has been demonstrated for the first time, which opens up prospects for further study of the properties of this, as yet exotic, class of compounds. Even though the first, the most natural idea of using Li[C₂F₅AlH₃]2DME for the synthesis of lithium (pentafluoroethyl) alkoxyaluminates has not been fully realized by us, since reactions with alcohols lead to partial substitution of the C₂F₅ group furnishing the difficultly separable mixture of products, one can expect that the replacement the Li cation with bulky organic cations such as PNP or EtP₄H (as in [16]) can be a possible approach to obtain [C₂F₅Al(OR)₃]^- anions.

Experimental

¹H, ¹³C, ¹⁹F, ²⁷Al, ²⁹Si, and ⁷Li NMR spectra were recorded on a Bruker AM-300, Bruker AVANCE-400, Bruker AVANCE-500, or Bruker AVANCE-600 spectrometers with 300.13, 400.13, 500.13, and 600.22 MHz working frequencies for ¹H, respectively. The frequencies of other nuclei, depending on the instrument used, are given in the Experimental Section below when referring to the NMR spectra of the corresponding compounds. Chemical shifts were referenced to the residual proton chloroform peak (7.26 ppm in CDCl₃) and reported in ppm units relative to TMS. Chemical shifts in ¹³C NMR spectra were referenced to the ¹³С nucleus peak (77.0 ppm in CDCl₃) and reported in ppm units relative to TMS. Chemical shifts in ¹⁹F NMR spectra were reported in ppm units relative to an external CFCl₃ reference. Chemical shifts in ²⁷Al NMR spectra were reported in ppm units relative to an external Al(NO₃)₃ reference. Chemical shifts in ²⁹Si NMR spectra were reported in ppm units relative to an external TMS reference. Chemical shifts in ⁷Li NMR spectra were reported in ppm units relative to an external LiCl reference. Positive values of chemical shift correspond to the downfield shift of the indicator nucleus signal. Raman spectra were recorded on a Jobin Yvon LabRam spectrometer. Elemental analysis was performed in the Laboratory of Microanalysis of INEOS RAS.

All experiments were carried out in an inert atmosphere using anhydrous solvents and standard working procedures. Before use, commercially available LiAlH₄ was purified by the standard method [9]: it was dissolved in Et₂O, the insoluble grey residue was allowed to settle down, the clear solution was separated, evaporated in vacuo to dryness or to a concentrated solution from which the hydride was precipitated by adding toluene and separated by filtration. The resulting white powdered LiAlH₄ was dried at 60-65°C (<0.5 Torr). (Perfluoroalkyl)trimethylsilanes: CF₃SiMe₃, C₂F₅SiMe₃, ⁿC₃F₇SiMe₃, and ⁿC₆F₁₃SiMe₃ are prepared by the procedure developed by us earlier [18-19].

To analyze the reaction mixtures, ~0.3 mL of the reaction solution was sealed in a glass ampoule (~3 mm diameter and 110-130 mm length). This ampoule was then placed in a standard 5 mm NMR ampoule, CDCl₃ was added as an external standard, and the reaction mixture was analyzed by NMR.

Synthesis of Li[C₂F₅AlH₃]2DME

To a stirred suspension of LiAlH₄ (1 g, 0.026 mol) in DME (50 mL), C₂F₅SiMe₃ (25 g, 0.13 mol) is added. The resulting mixture is stirred for several hours until homogeneous, and then kept for 5 days, periodically stirring the solution. Volatiles are then removed in vacuo (<0.5 Torr) and collected in a trap to provide a white solid, which turns into a liquid upon prolonged drying in vacuo (<0.5 Torr).

To the residue, toluene (20 mL) is added and the resulting mixture is stirred until dissolution and the formation of a liquid two-phase mixture, from which the solvent is distilled off in vacuo (<0.5 Torr). The liquid residue is dried for several hours in vacuo (<0.5 Torr) to give a colorless liquid (7.8 g, yield 90%).

Found, %: C, 35.52; H, 6.73. C₁₀H₂₃AlF₅LiO₄. Calculated, %: C, 35.73; H, 6.90.

¹H NMR (500.13 MHz) δ: 3.45-3.65 (br.s, 3H, AlH₃), 3.95 (s, 12H, OCH₃), 4.14 (s, 8H, CH₂CH₂);

¹⁹F NMR (470.59 MHz) δ: two sets of signals at -128 (s, 2F, CF₂), -84 (s, 3F, CF₃), and -126 (br.s, 2F, CF₂), -83.8 (s, 3F, CF₃) in a ratio of 1:2.5;

¹³С{¹H} NMR (125.75 MHz) δ: 59 (s, CH₃O), 70 (s, CH₂O), 123 (qt, ¹J_CF = 283 Hz, ²J_CF = 30 Hz, CF₃), 132 (br.t, CF₂Al);

²⁷Al NMR (104.26 MHz) δ: 117 (br.s);

⁷Li NMR (155.5 MHz) δ: -0.66 (s).

Raman spectrum of Li[ⁿC₃F₇AlH₃]2DME

Synthesis of Li[ⁿC₃F₇AlH₃]2DME.

Synthesized similarly to Li[ⁿC₃F₇AlH₃]2DME. Colorless viscous liquid, which is partially solidified at -25°C.

Found, %: C, 34.21; H, 6.26. C₁₁H₂₃AlF₇LiO₄. Calculated, %: C, 34.21; H, 6.00.

¹H NMR (500.13 MHz) δ: 3.45-3.65 (br.s, 3H, AlH₃), 3.9 (s, 12H, OCH₃), 4.1 (s, 8H, CH₂CH₂);

¹⁹F NMR (282.4 MHz) δ: two pairs of signals in the CF₂ region -128 (s, 2F, CF₂CF₂Al), -126.3 (s, 2F, CF₂CF₂Al), and -126.6 (s, 2F, CF₂CF₂Al), -126.1 (s, 2F, CF₂CF₂Al) in a ratio of 1:4, -81.3 (s, 3F, CF₃);

¹³С{¹H} NMR (125.75 MHz) δ: 59 (s, CH₃O), 70 (s, CH₂O), 112 (tq, ¹J_CF = 251 Hz, ²J_CF = 28 Hz, CF₂), 119 (qt, ¹J_CF = 288 Hz, ²J_CF = 38 Hz, CF₃), 135 (br.t, CF₂Al);

²⁷Al NMR (104.26 MHz) δ: -117 (br.s);

⁷Li NMR (155.5 MHz) δ: -0.64 (s).

Raman spectrum of Li[ⁿC₃F₇AlH₃]2DME

Acknowledgements

This study was financially supported by the Ministry of Science and Higher Education of the Russian Federation. The contribution of the Center for Molecule Composition Studies of INEOS RAS is gratefully acknowledged.

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ARTICLE INFO
Received 22 October 2021
Accepted 29 October 2021
Available online December 2021

Recommended for publication by PhD M. A. Manaenkova

Fluorine Notes, 2021, 139, 1-2