An Aliphatic Solvent-Soluble Lithium Salt of the Perhalogenated

Jan 19, 2016 - The lithium salt of perhalogenated alkoxy aluminate 4 is easily accessible in high yield (85%) through the reaction of purified LiAlH4(...
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An Aliphatic Solvent-Soluble Lithium Salt of the Perhalogenated Weakly Coordinating Anion [Al(OC(CCl3)(CF3)2)4]− Xin Zheng,† Zaichao Zhang,‡ Gengwen Tan,*,† and Xinping Wang*,† †

State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China ‡ School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huai’an 223300, P. R. China S Supporting Information *

Chart 1. Examples of Lithium Salts of Aluminum-Based Polyhalogenated Alkoxy Aluminate Anionsa

ABSTRACT: The facile synthesis of a new highly aliphatic solvent-soluble Li+ salt of the perhalogenated weakly coordinating anion [Al(OC(CCl3)(CF3)2)4]− and its application in stabilizing the Ph3C+ cation were investigated. The lithium salt Li[Al(OC(CCl3)(CF3)2)4] (4) was prepared by the treatment of 4 mol equiv of HOC(CCl3)(CF3)2 with purified LiAlH4 in n-hexane from −20 °C to room temperature. Compound 4 is highly soluble in both polar and nonpolar solvents, and it bears both CCl3 and CF3 groups, resulting in a lower symmetry around the Al center compared to that of Li[Al(OC(CF3)3)4] (1). Treatment of 4 with Ph3CCl afforded the ionic compound [Ph3C][Al(OC(CCl3)(CF3)2)4] (5) bearing the Ph3C+ cation with concomitant elimination of LiCl, suggesting the potential application of [Al(OC(CCl3)(CF3)2)4]− in stabilizing reactive cationic species. Compounds 4 and 5 were fully characterized by spectroscopic and structural methods.

a

are replaced by other alkyl groups, the solubility of lithium carboranes in cyclohexane was dramatically increased.12 However, Li[Al(OC(CF3)3)4] (1), a commonly used reagent to introduce the [Al(OC(CF3)3)4]− anion, has extremely low solubility in weakly basic solvents such as CH2Cl2, n-hexane, and toluene at room temperature, limiting its applications in some aspects. Also, some generated, highly electrophilic, and oxidizing cationic species with WCAs as the counterions could react with halogenated solvents; thus, the reactions need to be carried out in aliphatic solvents.7b,13 Moreover, the high symmetry at the Al center of 1 sometimes causes serious problems for crystal structure refinement.7 It would be desirable to synthesize aliphatic solvent-soluble lithium salts of WCAs in order to widen their applications. Until now, only two aluminum-based Li[Al(OCR(CF3)2)4] [R = Ph (2)6a and CH2SiMe3 (3);14 Chart 1] and several lithium carboranes12 that are hydrocarbon-solvent-soluble have been reported. Both Li[Al(OCR(CF3)2)4] bear extra hydrocarbon groups Ph and CH2SiMe3, in contrast to 1, which only contains the CF3 group. In addition, the electronic properties of the Ph and CH2SiMe3 groups are distinctively different from those of the CF3 group in 1, leading to different chemical properties of 2 and 3 compared with 1. For instance, the Brϕnsted acid [H(Et2O)2][Al(OC(CF3)3)4] can be generated and isolated in high yield, 1c,d whereas [H(Et 2 O) 2 ][Al(OC(CH 2 SiMe 3 )(CF3)2)4] is unstable, leading to anion decomposition with Al−O bond cleavage.14

W

eakly coordinating anions (WCAs) have attracted tremendous attention during the last decades owing to their applications in fundamental and applied chemistry.1 They have been widely utilized in homogeneous catalysis,2 electrochemistry,3 and organic as well as inorganic chemistry.4 In addition, WCAs have been applied as counterions in the synthesis of superacids.5 Up to now, a variety of WCAs have been synthesized.1 Among them, the polyfluorinated alkoxy aluminate anions [Al(ORF)4]− (ORF = per- or polyfluorinated alkoxy substituent) are one class of the most useful WCAs, which were first reported by Strauss,6 extensively developed by Krossing,4,7 and widely applied by many other research groups. We have recently isolated several nitrogen-,8 phosphorus-,9 sulfur-, selenium-,10 and transition-metal11-based radical cations with [Al(ORF)4]−. The [Al(OC(CF3)3)4]− anion, one of the most frequently utilized polyfluorinated alkoxy aluminate anions, exhibits quite high stability toward hydrolysis and electrophilic attack by Lewis acids because of the negative charge dispersed over the 36 F atoms around the anion surface and the steric protection of the Al−O bonds by the bulky OC(CF3)3 groups (Chart 1).7 Michl and co-workers have demonstrated that the solubility of lithium carboranes is largely dependent on the substituents at the anions. When one or two of the methyl groups of [CB11Me12]− © 2016 American Chemical Society

Li+ is omitted for clarity.

Received: November 23, 2015 Published: January 19, 2016 1008

DOI: 10.1021/acs.inorgchem.5b02674 Inorg. Chem. 2016, 55, 1008−1010

Communication

Inorganic Chemistry Because the Cl atom has electronic properties similar to those of the F atom, we reasoned that the partially chlorinated Li[Al(OC(CCl3)(CF3)2)4] (4; Chart 1) should have chemical properties similar to those of 1. The more bulky CCl3 may increase the volume of the [Al(OC(CCl3)(CF3)2)4]− anion, which should change the entropy of the anion in a favorable way, resulting in an improvement of the solubility in hydrocarbon solvents. Besides, when some of the F atoms are replaced by Cl atoms, a lower symmetry about the Al center in comparison to that of 1 results. This might be helpful to avoid the crystallographic disorder problem of the perfluorated anion. Herein, we report the synthesis of an aliphatic solvent-soluble perhalogenated alkoxy aluminate, 4, and its application in stabilizing the Ph3C+ cation, affording the salt [Ph3C][Al(OC(CCl3)(CF3)2)4] (5). The lithium salt of perhalogenated alkoxy aluminate 4 is easily accessible in high yield (85%) through the reaction of purified LiAlH415 with 4 mol equiv of HOC(CCl3)(CF3)2 in n-hexane from −20 °C to room temperature (eq 1). When the reaction temperature was above 40 °C, a color change to brown was observed and the yield of 4 decreased significantly. Compound 4 is highly soluble in n-hexane, toluene, acetonitrile, and CH2Cl2. As well as the high solubility, 4 also exhibits good thermal and chemical stability, which is indicated by the relatively high decomposition temperature (158−160 °C)16 and the inertness to HCl gas in a CH2Cl2 solution, respectively.17 The latter is in contrast to that of 1 and 3, which results in the formation of [H(Et2O)2]Al[OC(CF3)4]41c and anion decomposition with cleavage of the Al−O bond,14 respectively, when treated with HCl in an Et2O solution.

Figure 1. Molecular structure of 4 in the solid state. Disorders are omitted for clarity.

Figure 2. Part of the polymeric crystal structure of 1 in the solid state. Disorders are omitted for clarity.

Therefore, the monomeric structure of 4 and coordination of the Li center to the anion might be mainly attributed to its high solubility in organic solvents. Intrigued by the high stability of 4, we further investigated the capacity of the [Al(OC(CCl3)(CF3)2)4]− anion in stabilizing the [Ph3C]+ cation, which may act as a useful starting material to introduce the [Al(OC(CCl3)(CF3)2)4]− anion via hydride or alkyl group abstraction from the substrates.20 Compound 5 was readily prepared in high yield (80%) through the salt metathesis reaction of 4 and 1 mol equiv of Ph3CCl in a CH2Cl2 solution at ambient temperature, as shown in eq 2. The reaction initiated rapidly with an immediate color change from colorless to yellow once the two reagents were mixed together in a CH2Cl2 solution. The reaction also proceeds in n-hexane. Because the low solubility of the product in n-hexane, we carried out the synthesis in CH2Cl2. Compound 5 was isolated as a yellow crystalline solid. It is highly soluble in aliphatic solvents. The 1H resonance signals for the phenyl groups at δ 7.72 (6 H), 7.95 (6 H) and 8.34 (3 H) along with the low-field-shifted 13C signal (δ 210.8) indicate the presence of Ph3C+ in the molecule. A slightly upfield-shifted 27Al resonance (δ 25.4) is observed for 5 in comparison with that of 4 (δ 27.2) in the 27Al{1H,13C} NMR spectrum in a CD2Cl2 solution. The thermodynamic stability study of compound 5 shows that it is at least stable at room temperature for several months under an inert atmosphere.

LiAlH4 + 4HOC(CCl3)(CF3)2 n ‐ hexane

⎯⎯⎯⎯⎯⎯⎯⎯→ Li[Al(OC(CCl3)(CF3)2 )4 ] + 4H 2

(1)

Compound 4 was fully characterized by multinuclear {7Li, 13C, F, 27Al} NMR, elemental analysis, and single-crystal X-ray diffraction analysis. No 1H NMR signal for 4 is observed in the 1 H NMR spectrum in a CD2Cl2 solution, indicating full deprotonation of the polyhalogenated alcohol HOC(CCl3)(CF3)2 in compound 4. The 7Li nucleus resonates at δ 0.17 in the 7 Li{1H,13C} NMR spectrum, which is slightly low-field-shifted compared with that of 1 (δ −0.9).15 The 19F resonance signal is revealed as a single signal at δ −68.7, suggesting free rotation of the C(CCl3)(CF3)2 groups in a CD2Cl2 solution, and the 27Al NMR spectrum shows a broad signal at δ 27.2. Crystals of 4 suitable for single-crystal X-ray diffraction analysis were obtained by cooling the n-hexane solution to −20 °C for 12 h. It crystallizes in the monoclinic space group P21 and exhibits a monomeric structure (Figure 1).18 The Li center is attached by two F, two O, and two Cl atoms featuring a distorted octahedral geometry. Owing to coordination of the O atoms to the Li center, the Al1−O1 [1.808(8) Å] and Al1−O2 [1.793(7) Å] bond lengths are larger than those of Al1−O3 [1.713(7) Å] and Al1−O4 [1.704(7) Å]. Because of the presence of the CCl3 group, the Al center in 4 has a lower symmetry in comparison to that in 1, which might be beneficial for crystal structure refinement. In addition, our obtained crystal structure of 1,19 hitherto unknown, reveals that it features a 1D chainlike polymeric structure through coordination of the Li atom to one F atom of a second anion moiety (Figure 2). This is strikingly different from the solid-state structure of 4, in which the Li atom is intramolecularly entrapped by the anion. 19

Li[Al(OC(CCl3)(CF3)2 )4 ] + Ph3CCl CH 2Cl 2

⎯⎯⎯⎯⎯⎯⎯→ [Ph3C][Al(OC(CCl3)(CF3)2 )4 ] −LiCl

(2)

The crystal structure of 5 reveals a trigonal-planar geometry of the central C atom of the Ph3C+ cation, and the three phenyl groups feature a propeller-like arrangement (Figure 3), which is similar to that of the previously reported Ph3C+ cation.21 Owing to the absence of coordination of the Li center to the O atoms, the Al−O distances of the anion in 5 [av. 1.735(3) Å] are much shorter than those with lithium coordination in complex 4 [1.808(8) and 1.793(7) Å]. The facile isolation of 5 indicates that 1009

DOI: 10.1021/acs.inorgchem.5b02674 Inorg. Chem. 2016, 55, 1008−1010

Communication

Inorganic Chemistry

(5) (a) Reed, C. A. Chem. N. Z. 2011, 174−179. (b) Cummings, S.; Hratchian, H. P.; Reed, C. A. Angew. Chem., Int. Ed. 2015, DOI: 10.1002/anie.201509425. (6) (a) Barbarich, T. J.; Handy, S. T.; Miller, S. M.; Anderson, O. P.; Grieco, P. A.; Strauss, S. H. Organometallics 1996, 15, 3776−3778. (b) Ivanova, S. M.; Nolan, B. G.; Kobayashi, Y.; Miller, S. M.; Anderson, O. P.; Strauss, S. H. Chem. - Eur. J. 2001, 7, 503−510. (7) Selected examples: (a) Malinowski, P. J.; Krossing, I. Angew. Chem., Int. Ed. 2014, 53, 13460−13462. (b) Slattery, J. M.; Higelin, A.; Bayer, T.; Krossing, I. Angew. Chem., Int. Ed. 2010, 49, 3228−3231. (c) SantisoQuiñones, G.; Higelin, A.; Schaefer, J.; Brückner, R.; Knapp, C.; Krossing, I. Chem. - Eur. J. 2009, 15, 6663−6677. (8) (a) Su, Y.; Wang, X.; Zheng, X.; Zhang, Z.; Song, Y.; Sui, Y.; Li, Y.; Wang, X. Angew. Chem., Int. Ed. 2014, 53, 2857−2861. (b) Zheng, X.; Wang, X.; Qiu, Y.; Li, Y.; Zhou, C.; Sui, Y.; Li, Y.; Ma, J.; Wang, X. J. Am. Chem. Soc. 2013, 135, 14912−14915. (c) Wang, X.; Zhang, Z.; Song, Y.; Su, Y.; Wang, X. Chem. Commun. 2015, 51, 11822−11825. (9) (a) Su, Y.; Zheng, X.; Wang, X.; Zhang, X.; Sui, Y.; Wang, X. J. Am. Chem. Soc. 2014, 136, 6251−6254. (b) Pan, X.; Wang, X.; Zhang, Z.; Wang, X. Dalton Trans. 2015, 44, 15099−15102. (c) Pan, X.; Su, Y.; Chen, X.; Zhao, Y.; Li, Y.; Zuo, J.; Wang, X. J. Am. Chem. Soc. 2013, 135, 5561−5564. (10) (a) Zhang, S.; Wang, X.; Su, Y.; Qiu, Y.; Zhang, Z.; Wang, X. Nat. Commun. 2014, 5, 4127. (b) Zhang, S.; Wang, X.; Sui, Y.; Wang, X. J. Am. Chem. Soc. 2014, 136, 14666−14669. (11) (a) Wang, W.; Wang, X.; Zhang, Z.; Yuan, N.; Wang, X. Chem. Commun. 2015, 51, 8410−8413. (b) Zheng, X.; Wang, X.; Zhang, Z.; Sui, Y.; Wang, X.; Power, P. P. Angew. Chem., Int. Ed. 2015, 54, 9084− 9087. (c) Li, S.; Wang, X.; Zhang, Z.; Zhao, Y.; Wang, X. Dalton Trans. 2015, 44, 19754−19757. (12) (a) Körbe, S.; Schreiber, P. J.; Michl, J. Chem. Rev. 2006, 106, 5208−5249. (b) Valásě k, M.; Štursa, J.; Pohl, R.; Michl, J. Inorg. Chem. 2010, 49, 10255−10263. (13) Cameron, T. S.; Dionne, I.; Jenkins, H. D. B.; Parsons, S.; Passmore, J.; Roobottom, H. K. Inorg. Chem. 2000, 39, 2042−2052. (14) Müller, L. O.; Krossing, I. Z. Anorg. Allg. Chem. 2008, 634, 962− 966. (15) Krossing, I. Chem. - Eur. J. 2001, 7, 490−502. (16) We also carried out thermogravimetric analysis of 4; the result is shown in Figure 8s in the Supporting Information. It shows a fast weight loss at ca. 150 °C. (17) When we carried out the reaction in CH2Cl2, no precipitate of LiCl was observed, and ca. 70% of 4 could be recovered by recrystallization. (18) Sheldrick, G. M. Program for Crystal Structure Refinement; University of Göttingen: Göttingen, Germany, 1997. (19) The crystals were obtained when we carried out the synthesis of NO[Al(OC(CF3)3)4] through the reaction of NO(SbF6), 1, and 18crown-6 in CH2Cl2 at room temperature. We were also able to isolate the single crystals by slowly cooling the warm (ca. 40 °C) saturated solution of 1 to room temperature. Key crystal parameters of 1 can be found in the Supporting Information. (20) (a) Korolev, A. V.; Ihara, E.; Guzei, I. A.; Young, V. G.; Jordan, R. F. J. Am. Chem. Soc. 2001, 123, 8291−8309. (b) Korolev, A. V.; Delpech, F.; Dagorne, S.; Guzei, I. A.; Jordan, R. F. Organometallics 2001, 20, 3367−3369. (21) (a) Ivanov, S. V.; Davis, J. A.; Miller, S. M.; Anderson, O. P.; Strauss, S. H. Inorg. Chem. 2003, 42, 4489−4491. (b) Finze, M.; Bernhardt, E.; Berkei, M.; Willner, H.; Hung, J.; Waymouth, R. M. Organometallics 2005, 24, 5103−5109.

Figure 3. Molecular structure of 5 in the solid state. H atoms and disorders are omitted for clarity.

the [Al(OC(CCl3)(CF3)2)4]− anion is capable of stabilizing the Ph3C+ cation. In conclusion, we report the facile synthesis of a novel aliphatic solvent-soluble lithium salt of perhalogenated alkoxyaluminate 4, which represents a new WCA. The ability of the anion [AlOC(CCl3)(CF3)2]− in stabilizing cationic species was demonstrated, and the Ph3C+ cation was successfully stabilized in the form of 5, highlighting the potential of the [AlOC(CCl3)(CF3)2]− anion. The applications of [AlOC(CCl3)(CF3)2]− in generating cationic radicals are currently under investigation in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02674. Experimental procedures, NMR spectra, crystallographic details, and selected bond lengths and angles for 1, 4, and 5 (PDF) X-ray crystallographic files in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grants 21171087 and 21525102), the Natural Science Foundation of Jiangsu Province (Grant BK20140014) and the Doctoral Program of Higher Education (20110091120009) for financial support. Tan, G. would like to thank the National Postdoc Program of China for a scholarship.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02674 Inorg. Chem. 2016, 55, 1008−1010