Pentafluoroethylaluminates: A Combined Synthetic, Spectroscopic, and Structural Study

Abstract

Salts of the tetrakis(pentafluoroethyl)aluminate anion [Al(C₂F₅)₄]⁻ were obtained from AlCl₃ and LiC₂F₅. They were isolated with different counter‐cations and characterized by NMR and vibrational spectroscopy and mass spectrometry. Degradation of the [Al(C₂F₅)₄]⁻ ion was found to proceed via 1,2‐fluorine shifts and stepwise loss of CF(CF₃) under formation of [(C₂F₅)_4−nAlF_n]⁻ (n=1–4) as assessed by NMR spectroscopy and mass spectrometry and supported by results of DFT calculations. In addition, the [(C₂F₅)AlF₃]⁻ ion was structurally characterized.

Tetrakis(pentafluoroethyl)aluminates: M[Al(C₂F₅)₄] with different cations (e.g. M=Rb, bis(triphenylphosphane)iminium (PNP)) have become available starting from readily available LiC₂F₅ and AlCl₃. The degradation of the [Al(C₂F₅)₄]⁻ anion resulting in the anions [(C₂F₅)_4−nAlF_n]⁻ (n=1–4) was elucidated providing a detailed insight into the general degradation of C₂F₅ element compounds.

Introduction

Perfluoroalkyl‐substituted borate anions are among the most weakly coordinating anions (WCAs), known to date.[ ¹ , ² ] Especially, the homoleptic tetrakis(trifluoromethyl)borate anion [B(CF₃)₄]⁻ (Figure 1)[ ³ , ⁴ , ⁵ ] and related mixed perfluoroalkylfluoroborate anions [R^F _xBF_4−x]⁻ (x=1–3; R^F=perfluoroalkyl)[ ⁶ , ⁷ ] have found widespread applications, for example, for the stabilization of highly reactive cations,[ ¹ , ⁶ ] in catalysis, ^[8] ionic liquids (ILs), ^[9] and battery applications.[ ⁴ , ¹⁰ ] In addition, perfluoroalkylboron compounds with functional groups were synthesized,[ ⁶ , ⁷ ] for example, R^F ₃BCO (R^F=CF₃, ^[11] C₂F₅, ^[12] C₃F₇ ^[12] ), [(CF₃)₃BCPnic]⁻ (Pnic=N, ^[13] P, As), ^[14] [(C₂F₅)BX ₃]⁻ (X=H, ^[15] CN ^[16] ), and the decomposition pathway of perfluoroalkylboranes, for example, (CF₃)₃B, was elucidated. ^[17]

Figure 1.

Homoleptic perfluoroalkylated anions of boron ([B(CF₃)₄]^−[3]), aluminum, and gallium ([Ga(C₂F₅)₄]^−[18]).

In contrast to perfluoroalkylboron derivatives, related perfluoroalkyl compounds of boron's higher homologues gallium, indium, and thallium have been rarely, studied. The tetrakis(trifluoromethyl)gallate anion [Ga(CF₃)₄]⁻, ^[19] the gallane Ga(CF₃)₃, ^[20] and some of its adducts (CF₃)₃Ga⋅L (L=PMe₃, AsMe₃) ^[20] have been synthesized and characterized by NMR spectroscopy. Recently, Hoge and co‐workers reported on salts of the tetrakis(pentafluoroethyl)gallate anion [Ga(C₂F₅)₄]⁻ (Figure 1) and its application in Li‐ion batteries. ^[18] In addition, first examples of related tris(pentafluoroethyl)gallane derivatives were described.[ ¹⁸ , ²¹ ] Tris(trifluoromethyl)indium In(CF₃)₃, ^[22] some donor‐stabilized adducts (CF₃)₃In⋅L (L=PMe₃, ^[22] DMF), and few additional perfluoroalkylindium compounds, for example, (CF₃)₂InCl, ^[19] were reported.[ ¹⁹ , ²³ , ²⁴ ] Perfluoroalkylindium compounds and partially fluorinated indium organyls were applied in synthetic organic chemistry. ^[25] Similar to (CF₃)₃ M (M=Ga, In), tris(trifluoromethyl)thallium (CF₃)₃Tl was obtained via metal vapor deposition synthesis. ^[26] In addition, some adducts with donor molecules (CF₃)₃Tl⋅PMe₃ ^[26] and (CF₃)₃Tl⋅2 L (L=DMF, DMSO, Py) ^[19] and a few further thallium derivatives with one or more perfluoroalkyl groups were reported,[ ²⁴ , ²⁶ , ²⁷ ] including a NMR report on the homoleptic thallate anion [Tl(CF₃)₄]⁻. ^[24]

In case of perfluoroalkylaluminum, computational studies were performed, for example, on the Lewis acidity of Al(CF₃)₃, ^[28] but to the best of our knowledge, neither AlR^F ₃ nor the corresponding anions [AlR^F ₄]⁻ are known, to date. The sole experimental information on perfluoroalkylaluminum compounds provided in the literature are on the reaction of Li[AlH₄] with perfluoropropyl iodide to give Li[Al(CF₂CF₂CF₃)H₂I].[ ²⁹ , ³⁰ ] However, only limited spectroscopic data were presented. ^[30] The formation of trifluoromethyl aluminum compounds via metal vapor deposition was claimed but without any spectroscopic evidence.[ ²⁶ , ³¹ ]

Here, we report on salts of the tetrakis(pentafluoroethyl)aluminate anion [Al(C₂F₅)₄]⁻ and present a detailed spectroscopic characterization. The degradation of the [Al(C₂F₅)₄]⁻ ion via the pentafluoroethylfluoroaluminate ions [(C₂F₅)_4−nAlF_n]⁻ (n=1–3) to give [AlF₄]⁻ is described and the discussion is aided by results of DFT calculations.

Results and Discussion

Synthetic aspects

Pentafluoroethyl lithium was reacted with aluminum trichloride in diethyl ether to give Li[Al(C₂F₅)₄] (Scheme 1). The pentafluoroethyl group was employed as substituent because of its high stability[ ¹⁶ , ¹⁷ , ³² ] compared to the trifluoromethyl group, which tends to degrade via difluorocarbene elimination as shown for many trifluoromethylated compounds.[ ¹⁷ , ³² , ³³ , ³⁴ , ³⁵ , ³⁶ ] The Hoge group has developed an alternative synthesis for Li[Al(C₂F₅)₄] in parallel to our study using the silane Si(C₂F₅)₃H and Li[AlH₄] as starting materials. ^[37] The crude product obtained from LiC₂F₅ and AlCl₃ in ether contained three aluminum‐containing side products, the anions [(C₂F₅)₃AlF]⁻, [(C₂F₅)₂AlF₂]⁻, and, most likely, [(C₂F₅)₃AlOEt]⁻ or less probable (C₂F₅)₃Al⋅OEt₂, as assessed by multinuclear NMR spectroscopy. The best result with respect to the [Al(C₂F₅)₄]⁻ content in the crude reaction mixture was achieved with a molar ratio of 1:3.0 to 1:3.5 for AlCl₃:LiC₂F₅. More than 3.5 equivalents of LiC₂F₅ resulted in the formation of side products of unknown composition. The lithium salt Li[Al(C₂F₅)₄] is stable in diethyl ether for several hours and such solutions were successfully employed in metatheses. Solid Li(OEt₂)_x[Al(C₂F₅)₄] was obtained by removal of all volatiles. However, Li(OEt₂)_x[Al(C₂F₅)₄] underwent fast decomposition in the solid state. Thus, immediate uptake of the solid in acetonitrile resulted in significant amounts of insoluble products and after three days of storage of the solid all [Al(C₂F₅)₄]⁻ had decomposed.

Scheme 1.

Synthesis of tetrakis(pentafluoroethyl)aluminates.

The reaction of aluminum trichloride with LiC₂F₅ in the presence of potassium or rubidium fluoride yielded M[Al(C₂F₅)₄] (M=K, Rb) as solids with small amounts of M[(C₂F₅)₃AlF] as side products (Scheme 1). Removal of Rb[(C₂F₅)₃AlF] was achieved by further work‐up to yield pure Rb[Al(C₂F₅)₄] that was characterized by IR and Raman spectroscopy (Figure S23 in the Supporting Information). These salts showed slow decomposition in the solid state under an inert atmosphere inside a glovebox. Decomposition of these solids is much slower than of Li(OEt₂)_x[Al(C₂F₅)₄], which is rationalized by the high fluoride ion affinity of the lithium cation. Full decomposition of M[Al(C₂F₅)₄] (M=K, Rb) was observed after approximately 2 months, only. According to elemental analysis of the solid remainder after 2 months, most of the fluoroorganic content had been lost. However, the NMR spectra of the solid remainder suspended in dichloromethane proved the formation of fluorinated substances during decomposition (vide infra).

Metatheses using ethereal solutions of Li[Al(C₂F₅)₄] and Rb[Al(C₂F₅)₄] with [PPh₄]Cl or PNPCl (PNP=bis(triphenylphosphane)iminium) afforded the corresponding tetrakis(pentafluoroethyl)aluminates (Scheme 1). The highest stability of all [Al(C₂F₅)₄]⁻ salts studied herein was found for PNP[Al(C₂F₅)₄] that was stable over months in the solid state and has shown no decomposition up to 80 °C (DSC). In contrast, salts with less bulky cations such as [PPh₄][Al(C₂F₅)₄] disclosed much faster degradation in the solid state.

Spectroscopic characterization of PNP[Al(C₂F₅)₄]

PNP[Al(C₂F₅)₄] was characterized, in detail, by multinuclear NMR spectroscopy, vibrational spectroscopy, mass spectrometry, and elemental analysis. The ²⁷Al NMR signal of the [Al(C₂F₅)₄]⁻ ion at 107.3 ppm is split into nine lines with ² J(²⁷Al,¹⁹F) of 32 Hz due to the coupling to four CF₂ groups (Figure 2). The ¹⁹F NMR spectrum shows two signals in the solid state as well as in solution (Figure 2). In CD₂Cl₂ solution, the signal corresponding to the CF₃ groups is located at −83.9 ppm and the one for the CF₂ groups at −128.4 ppm. The latter signal is split into a sextet as a result of ² J(²⁷Al,¹⁹F) coupling of 31 Hz. In the solid‐state ¹⁹F NMR spectrum two broad singlets that are shifted to slightly higher resonance frequencies compared to the NMR spectrum in solution are observed. In the ¹³C{¹⁹F} NMR spectrum the signals of the CF₂ and CF₃ groups at 124.8 and 121.2 ppm are split into sextets with ¹ J(²⁷Al,¹³C)=123 Hz and ² J(²⁷Al,¹³C) ≈9 Hz (Figure 2). In the solid state ¹³C{¹⁹F} CP/MAS NMR spectrum the signals are located at 125 and 122 ppm, respectively. Only the signal of the CF₂ groups shows the coupling to ²⁷Al with around 120 Hz.

Figure 2.

Selected NMR spectra of PNP[Al(C₂F₅)₄] in CD₂Cl₂.

Degradation of [Al(C₂F₅)₄]⁻ via [(C₂F₅)_4−nAlF_n]⁻ (n=1–4)

The degradation of tetraphenylphosphonium tetrakis(pentafluoroethyl)aluminate and of PNP[Al(C₂F₅)₄] in diethyl ether was investigated by NMR spectroscopy. The reaction proceeds via successive loss of the pentafluoroethyl groups under formation of the anions [(C₂F₅)₃AlF]⁻, [(C₂F₅)₂AlF₂]⁻, [(C₂F₅)AlF₃]⁻, and [AlF₄]⁻ (Figure 3). In addition to the NMR spectroscopic characterization, all anions were identified by mass spectrometry (Figure S24 and S25 in the Supporting Information). The assignment of the ²⁷Al and ¹⁹F NMR signals of the anions [(C₂F₅)_4−nAlF_n]⁻ (n=0–4) is aided by correlation spectra, selective decoupling experiments, and NMR data calculated at the B3LYP/6‐311++G(2d,p) level of theory (Table 1, Figure S26–S32 in the Supporting Information). The ¹⁹F NMR chemical shifts of the anions [(C₂F₅)_4−nAlF_n]⁻ (n=0–4) are in narrow ranges: CF₃ −83.6 to −84.6 ppm, CF₂ −127.7 to −133.1 ppm, and AlF −180.5 to −195.9 ppm. In contrast, δ(²⁷Al) reveals a distinct trend along the series with a reduction of δ(²⁷Al) of ca. 12–18 ppm per exchange of C₂F₅ against fluorine (Figure 3 and Table 1). An analogous behavior with larger differences was reported for [(CH₃)_4−nAlF_n]⁻ (n=1–4).[ ³⁸ , ³⁹ ] A further similar, univocal trend was found for ¹ J(²⁷Al,¹⁹F) that strongly decreases with decreasing number of pentafluoroethyl groups at aluminum. The coupling between ¹⁹F of the CF₂ units and ²⁷Al shows a parallel but less pronounced trend, whereas ¹ J(²⁷Al,¹³C) reveals opposite behavior (Table 1). The decomposition of [Al(C₂F₅)₄]⁻ in diethyl ether is accompanied by the formation of pentafluoroethylfluoroaluminium species with ethoxy groups and/or diethyl ether coordinated to aluminum as indicated by the NMR spectra depicted in Figure S28–S30 in the Supporting Information.

Figure 3.

Decomposition of [PPh₄][Al(C₂F₅)₄] in Et₂O monitored by ²⁷Al NMR spectroscopy (bottom) and the [(C₂F₅)AlF₃]⁻ ion that is disordered with [(C₂F₅)AlClF₂]⁻ in PNP[(C₂F₅)AlF₃]⋅Et₂O (top).

Table 1.

Experimental and calculated NMR parameters of [(C₂F₅)_4−nAlF_n]⁻ (n=0–4).^[a,b]

anion	[Al(C2F5)4]−	[(C2F5)3AlF]−	[(C2F5)2AlF2]−	[(C2F5)AlF3]−	[AlF4]−
δ(27Al)[c]	107.7 (108.0)	95.2 (95.1)	77.3 (76.5)	61.1 (61.0)	49.1[d,e] (50.3)
	115.4	102.6	81.1	64.3	51.8
δ(19F) Al‐F	–	−191.6	−180.5	−182.7	−195.9[d]
	–	−246.7	−229.2	−232.9	−250.5
δ(19F) CF2	−127.7	−131.8	−133.5	−133.1	–
	−151.3	−154.7	−156.2	−156.0	–
δ(19F) CF3	−83.6	−83.9	−84.3	−84.6	–
	−103.8	−104.1	−104.6	−105.0	–
δ(13C) CF2	124.8[d]	n.o.[f]	n.o.	n.o.	–
	135.6	135.2	135.5	136.8	–
δ(13C) CF3	121.2[d]	n.o.	n.o.	n.o.	–
	133.6	134.1	134.5	135.1	–
1 J(27Al,19F)	–	125	87	n.o.[g]	n.o. (38)[g]
2 J(27Al,19F)	32	32	31	29	–
1 J(27Al,13C)	123	135[h]	149[h]	n.o.	–

[a] Cation: [PPh₄]⁺; solvent: Et₂O with a (CD₃)₂CO capillary. [b] Calculated values in italics; B3LYP/6‐311++G(2d,p)//B3LYP/6‐311+G(d,p). [c] Solid state NMR spectroscopic data in brackets. [d] Cation: PNP ⁺; solvent: CD₂Cl₂. [e] Literature data for [AlF₄]⁻ in CD₃CN: δ(²⁷Al)=49.2 ppm, δ(¹⁹F)=−194.2 ppm, ¹ J(²⁷Al,¹⁹F)=37.8 Hz. ^[40] [f] n.o.=not observed. [g] The fluoroaluminate anions undergo fluorine exchange as proven by ¹⁹F‐¹⁹F EXSY experiments (Figure S31 and S32 in the Supporting Information). So, ¹ J(²⁷Al,¹⁹F) coupling was not observed for [(C₂F₅)AlF₃]⁻ and [AlF₄]⁻, rarely for [(C₂F₅)₂AlF₂]⁻, and in many spectra the signal of [(C₂F₅)₃AlF]^−‐ was broad without any resolved ¹ J(²⁷Al,¹⁹F) coupling. A similar, concentration‐dependent effect was reported for [AlF₄]⁻. ^[40] [h] ¹³C satellites of the ²⁷Al{¹⁹F} NMR spectrum.

Crystallization of a decomposition mixture of PNP[Al(C₂F₅)₄] afforded single crystals of PNP[(C₂F₅)AlF₃]⋅Et₂O and PNP[AlF₄] providing additional evidence for the successive replacement of C₂F₅ by fluorine during degradation of [Al(C₂F₅)₄]⁻. In both crystals studied, partial disorder of Al−F with Al−Cl was observed. The presence of chlorine is due to PNPCl that was employed in metatheses. PNP[(C₂F₅)AlF₃]⋅Et₂O crystallizes in the triclinic space group P $\bar{1}$ with Z=2 and PNP[AlF₄] in the monoclinic space group P2₁/c with Z=8. The bond parameters of the [(C₂F₅)AlF₃]⁻ anion (Figure 3) are in good agreement to values derived from DFT calculations (Table 2). A similar good agreement was achieved for experimental and calculated data of [Al(C₂F₅)₄]⁻, ^[37] [AlF₄]⁻,[ ³⁸ , ⁴⁰ , ⁴¹ ] and [Ga(C₂F₅)₄]^−[18] (Table 2). The crystal structure of [PPh₄][Al(C₂F₅)₄] is reported in the parallel contribution by Hoge et al. ^[37] The calculated bonding parameters of the anions [(C₂F₅)_4−nAlF_n]⁻ (n=0–4) are very close. However, trends have been found for d(M−C) and d(M−F) that are predicted to decrease and increase, respectively, with decreasing number of C₂F₅ groups (Table 2). The experimental and calculated bond distances of [M(C₂F₅)₄]⁻ (M=Al, Ga) are almost the same (Table 2), which nicely fits to the almost identical covalent radii of gallium (122 pm) and aluminum (121 pm). ^[42]

Table 2.

Experimental and calculated bond distances of [(C₂F₅)_4−nAlF_n]⁻ (n=0–4) and [Ga(C₂F₅)₄]⁻.^[a,b]

Anion	d(M−C)	d(C−C)	d(C−F2)	d(C−F3)	d(M−F)
[Al(C2F5)4]−[c]	204.0(2)	152.1(3)	138.4(3)	133.6(3)	–
	207.4	154.1	139.1	134.8	–
[Ga(C2F5)4]−[d]	204.14(11)	152.6(2)	137.72(13)	133.4(2)	–
	208.5	154.1	138.5	134.8	–
[(C2F5)3AlF]−	207.3	153.9	139.3	134.9	170.3
[(C2F5)2AlF2]−	206.9	153.8	139.5	135.0	170.5
[(C2F5)AlF3]−[e]	200.6(6)	149.9(8)	141.4(7)	134.0(10)	166.6(3)
	206.8	153.8	139.8	135.2	171.0
[AlF4]−	–	–	–	–	164.7(2)[f]
	–	–	–	–	171.7

[a] Calculated values in italics; d in pm; mean values where applicable. [b] B3LYP/6‐311+G(d). [c] [PPh₄][Al(C₂F₅)₄]; Symmetry: S ₄. ^[37] [d] [PPh₄][Ga(C₂F₅)₄]; Symmetry: S ₄. ^[18] [e] PNP[(C₂F₅)AlF₃]; the [(C₂F₅)AlF₃]⁻ anion is disordered with [(C₂F₅)AlClF₂]⁻ (d(Al‐Cl)=211.9(4) [exptl] and 217.2 pm [calcd]). [f] [PPh₄][AlF₄]. ^[41]

Fluoroorganic compounds are formed as byproducts of the degradation of [(C₂F₅)_4−nAlF_n]⁻ (n=0–3) in solution and the solid state. In Scheme 2 the fluoroorganic molecules are depicted that were identified by NMR spectroscopy and in the Supporting Information the spectra and experimental as well as calculated data are provided (Figures S33–S46, Tables S2 and S3). The formation of all fluoroorganic derivatives assigned is rationalized by initial elimination of fluoro(trifluoromethyl)carbene CF(CF₃) ^[43] from the pentafluoroethylaluminate anions: A) trans‐CF₃CF=CFCF₃ (1) is the dimer of CF(CF₃), B) 1,1,1,2‐tetrafluoroethane (3) and pentafluoroethane (4) are formed from the carbene and dichloromethane, C) ethyl trifluorovinyl ether (5) and fluoroethane (6) are derived from CF(CF₃) and diethyl ether, and D) the diasteromeric cyclopropanes cis ‐7 and trans ‐7 are the result of the addition of CF(CF₃) to 5 (Scheme 2).

Scheme 2.

Experimentally confirmed (data are given in the Supporting Information) products of reactions of fluoro(trifluoromethyl)carbene CF(CF₃), which was released from pentafluoroethylaluminate anions during degradation. For cis ‐7 and trans ‐7 only one of the two enantiomers are depicted, respectively.

Theoretical study on the degradation pathway of [Al(C₂F₅)₄]⁻

Only limited information on decomposition pathways of pentafluoroethyl derivatives can be found in the literature. Pentafluoroethyltetrafluorophosphorane (C₂F₅)PF₄ was reported to extrude CF(CF₃) on platinum at 240 °C. ^[44] Thermolysis of pentafluoroethyltrifluorosilane (C₂F₅)SiF₃ at 160 °C gives both dimers of CF(CF₃), trans‐CF₃CF=CFCF₃ (1) and cis‐CF₃CF=CFCF₃ in 92 % yield, together with SiF₄.[ ³⁵ , ⁴³ , ⁴⁵ ] Later, (C₂F₅)₃SiF was found to result in trans‐CF₃CF=CFCF₃ (1) and cis‐CF₃CF=CFCF₃, together with SiF₄, (C₂F₅)SiF₃, and further fluoroorganic compounds, upon thermolysis at 180 °C for 1 h. ^[36] Silane (C₂F₅)SiF₃ was employed as source for CF(CF₃) and in carbene trapping reactions, ^[43] for example, with PF₃. ^[44] The formation of CF(CF₃) from (C₂F₅)SiF₃ and (C₂F₅)PF₄ was explained by intramolecular 1,2‐fluorine shifts (C_α−F activation) from CF₂ to silicon and phosphorus, respectively.[ ³⁵ , ⁴³ , ⁴⁴ ] The release of CF(CF₃) from silane (C₂F₅)SiF₃ was predicted by DFT calculations to proceed barrierless and to be endergonic (141.8 kJ mol⁻¹). ^[46] C_α−F activation and formation of fluorocarbenes CF(CF₂ R) is a general reaction of trifluorosilanes of the type (RCF₂CF₂)SiF₃ (e.g. R=H). ^[43] An X‐ray crystallographic study on pentafluoroethyl lithium provided evidence for a Li/F carbenoid, ^[33] which can be regarded as a further indication for the favorable release of CF(CF₃) upon degradation of pentafluoroethyl compounds.

The degradation of the [Al(C₂F₅)₄]⁻ anion was modeled using DFT calculations (Figure 4). 1,2‐fluorine shifts (C_α−F activation) via CF(CF₃) extrusion and 1,3‐fluorine shifts (C_β−F activation) via release of tetrafluoroethylene (TFE) were considered. The 1,2‐fluorine shift of the degradation of the first C₂F₅ group of [Al(C₂F₅)₄]⁻ has a significantly lower free activation energy (ΔG ^≠) than the 1,3‐fluorine shift. Thus, C_α−F activation is predicted to be favorable, which is in perfect agreement to the experimental findings (vide supra) because there is proof solely for the formation of CF(CF₃) as an intermediate (Scheme 2) but no indication for the formation of TFE. Analogous results were obtained for the depletion of the remaining C₂F₅ groups at aluminum (Figure 4). In case of [(C₂F₅)AlF₃]⁻ the release of CF(CF₃) corresponds to the free activation energy since the transition state located is lower in energy than the separated species [(C₂F₅)AlF₃]⁻ and CF(CF₃). In summary, the extrusions of CF(CF₃) are all endergonic but the follow‐up reactions of the highly reactive carbene, for example, dimerization to trans‐CF₃CF=CFCF₃ (1), make the full reaction sequence strongly exergonic.

Figure 4.

Calculated free reaction energies (ΔG) and free activation energies (ΔG ^≠) for the consecutive degradation of pentafluoroethylaluminate and pentafluoroethylgallate ions [(C₂F₅)_4−nMF_n]⁻ (n=0–3, M=Al, Ga) (top), and comparison of degradation of [Al(C₂F₅)₄]⁻ via C_α−F or C_β−F activation (bottom) (B3LYP/6‐311+G(d)).

The successive degradation of the related gallate anion [Ga(C₂F₅)₄]⁻ (Figure 1) ^[18] that exhibits a much higher thermal and chemical stability in its salts than [Al(C₂F₅)₄]⁻ was studied by DFT calculations, as well (Figure 4). The data available show that the loss of CF(CF₃) via an 1,2‐fluorine shift is favored over TFE elimination via an 1,3‐fluorine shift. This behavior parallels the findings for the related aluminate anions. Together with earlier reports on (C₂F₅)SiF₃,[ ³⁵ , ⁴³ , ⁴⁵ , ⁴⁶ ] (C₂F₅)₃SiF, ^[36] and (C₂F₅)PF₄[ ⁴³ , ⁴⁴ ] these results point toward a general mechanism for the decomposition of pentafluoroethyl element compounds. However, the extrusion of CF(CF₃) requires significantly more energy for the pentafluoroethylgallate anions compared to the respective aluminate anions (>60 kJ mol⁻¹), which is the reason for the higher stability of [Ga(C₂F₅)₄]⁻ compared to [Al(C₂F₅)₄]⁻.

Conclusions

The reaction of pentafluoroethyl lithium with aluminum trichloride provides a convenient synthetic entry to salts of the tetrakis(pentafluoroethyl)aluminate [Al(C₂F₅)₄]⁻ ion with different cations. The combined experimental and theoretical study on the pentafluoroethylaluminate anions shows that the degradation proceeds via 1,2‐fluorine shifts (C_α−F activation) and loss of CF(CF₃). The much higher stability of the related gallate ions is rationalized by significantly higher barriers for the extrusion of the carbene CF(CF₃).

Experimental Section

Full experimental details and characterization data for all compounds and details of the DFT calculations are included in the Supporting Information. Deposition Number(s) 1979821 (PNP[(C₂F₅)AlF₃]⋅Et₂O) and 1979820 (PNP[AlF₄]) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supplementary

L. A. Bischoff, J. Riefer, R. Wirthensohn, T. Bischof, R. Bertermann, N. V. Ignat'ev, M. Finze, Chem. Eur. J. 2020, 26, 13615.