Pentafluoroethylaluminates: A Combined Synthetic, Spectroscopic, and Structural Study.

Salts of the tetrakis(pentafluoroethyl)aluminate anion [Al(C2 F5 )4 ]- were obtained from AlCl3 and LiC2 F5 . They were isolated with different counter-cations and characterized by NMR and vibrational spectroscopy and mass spectrometry. Degradation of the [Al(C2 F5 )4 ]- ion was found to proceed via 1,2-fluorine shifts and stepwise loss of CF(CF3 ) under formation of [(C2 F5 )4-n AlFn ]- (n=1-4) as assessed by NMR spectroscopy and mass spectrometry and supported by results of DFT calculations. In addition, the [(C2 F5 )AlF3 ]- ion was structurally characterized.

Introduction

Perfluoroalkyl‐substituted borate anions are among the most weakly coordinating anions (WCAs), known to date.[ , ] Especially, the homoleptic tetrakis(trifluoromethyl)borate anion [B(CF3)4]− (Figure 1)[ , , ] and related mixed perfluoroalkylfluoroborate anions [RF BF4−]− (x=1–3; RF=perfluoroalkyl)[ , ] have found widespread applications, for example, for the stabilization of highly reactive cations,[ , ] in catalysis, ionic liquids (ILs), and battery applications.[ , ] In addition, perfluoroalkylboron compounds with functional groups were synthesized,[ , ] for example, RF 3BCO (RF=CF3, C2F5, C3F7 ), [(CF3)3BCPnic]− (Pnic=N, P, As), [(C2F5)BX 3]− (X=H, CN ), and the decomposition pathway of perfluoroalkylboranes, for example, (CF3)3B, was elucidated.

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]−, the gallane Ga(CF3)3, and some of its adducts (CF3)3Ga⋅L (L=PMe3, AsMe3) 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. In addition, first examples of related tris(pentafluoroethyl)gallane derivatives were described.[ , ] Tris(trifluoromethyl)indium In(CF3)3, some donor‐stabilized adducts (CF3)3In⋅L (L=PMe3, DMF), and few additional perfluoroalkylindium compounds, for example, (CF3)2InCl, were reported.[ , , ] Perfluoroalkylindium compounds and partially fluorinated indium organyls were applied in synthetic organic chemistry. Similar to (CF3)3 M (M=Ga, In), tris(trifluoromethyl)thallium (CF3)3Tl was obtained via metal vapor deposition synthesis. In addition, some adducts with donor molecules (CF3)3Tl⋅PMe3 and (CF3)3Tl⋅2 L (L=DMF, DMSO, Py) and a few further thallium derivatives with one or more perfluoroalkyl groups were reported,[ , , ] including a NMR report on the homoleptic thallate anion [Tl(CF3)4]−.

In case of perfluoroalkylaluminum, computational studies were performed, for example, on the Lewis acidity of Al(CF3)3, but to the best of our knowledge, neither AlRF 3 nor the corresponding anions [AlRF 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].[ , ] However, only limited spectroscopic data were presented. The formation of trifluoromethyl aluminum compounds via metal vapor deposition was claimed but without any spectroscopic evidence.[ , ]

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−AlF]− (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[ , , ] compared to the trifluoromethyl group, which tends to degrade via difluorocarbene elimination as shown for many trifluoromethylated compounds.[ , , , , , ] 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. 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)[Al(C2F5)4] was obtained by removal of all volatiles. However, Li(OEt2)[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)[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−AlF]− (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−AlF]− (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−AlF]− (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−AlF]− (n=1–4).[ , ] 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−AlF]− (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. [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]−. [h] 13C satellites of the 27Al{19F} NMR spectrum.

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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 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]−, [AlF4]−,[ , , ] 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. The calculated bonding parameters of the anions [(C2F5)4−AlF]− (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).

Table 2

Experimental and calculated bond distances of [(C2F5)4−AlF]− (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. [d] [PPh4][Ga(C2F5)4]; Symmetry: S 4. [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].

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Fluoroorganic compounds are formed as byproducts of the degradation of [(C2F5)4−AlF]− (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) 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 ‐7 and ‐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 ‐7 and ‐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. 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.[ , , ] 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. Silane (C2F5)SiF3 was employed as source for CF(CF3) and in carbene trapping reactions, for example, with PF3. The formation of CF(CF3) from (C2F5)SiF3 and (C2F5)PF4 was explained by intramolecular 1,2‐fluorine shifts (Cα−F activation) from CF2 to silicon and phosphorus, respectively.[ , , ] 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). Cα−F activation and formation of fluorocarbenes CF(CF2 R) is a general reaction of trifluorosilanes of the type (RCF2CF2)SiF3 (e.g. R=H). An X‐ray crystallographic study on pentafluoroethyl lithium provided evidence for a Li/F carbenoid, 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 (Cα−F activation) via CF(CF3) extrusion and 1,3‐fluorine shifts (Cβ−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, Cα−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−MF]− (n=0–3, M=Al, Ga) (top), and comparison of degradation of [Al(C2F5)4]− via Cα−F or Cβ−F activation (bottom) (B3LYP/6‐311+G(d)).

The successive degradation of the related gallate anion [Ga(C2F5)4]− (Figure 1) 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,[ , , , ] (C2F5)3SiF, and (C2F5)PF4[ , ] 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 (Cα−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 ( 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.

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Supplementary

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