Ionic Liquids as Crystallization Media: Weakly-Coordinating Anions

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Ionic Liquids as Crystallization Media: Weakly-Coordinating Anions Do Coordinate in 1¥[Eu(OTf)3(CH3CN)3] Sifu Tang†,‡ and Anja-Verena Mudring*,† † ‡

Anorganische Chemie I—Festk€orperchemie und Materialien, Ruhr-Universit€at Bochum, D-44780 Bochum, Germany Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, 266101 Qingdao, China

bS Supporting Information ABSTRACT: Tris(acetonitrile)tris(trifluoromethanesulfonato)-europium(III), 1¥[Eu(OTf)3(CH3CN)3], is the first structurally characterized Ln(OTf)3-solvate where triflate is still in the coordination sphere of the Ln(III) cation and not substituted by solvate molecules. This fact is attributed to the unique crystallization conditions that an ionic liquid provides: 1¥[Eu(OTf)3(CH3CN)3] was obtained from a solution of europium(III) triflate, Eu(OTf)3, with acetonitrile in the ionic liquid [C4py][OTf] (1-butylpyridinium triflate). The crystal structure (triclinic, P1 (no. 2), a = 5.734(1) Å, b = 10.500(2) Å, c = 19.088(4) Å, R = 98.09(3), β = 93.79(3), γ = 91.46(3), V = 1134.5(4) Å3, Z = 2, R1 for 4388 reflections with Io > 2σ(Io): 0.0335) contains 9-fold coordinated Eu(III) atoms: Six trans coordinating triflate anions and three equatorial acetonitrile molecules form a tricapped trigonal prism around the central Eu(III) cation. These polyhedra are linked to 1D chains by bridging triflate ligands and are further connected into a 3D supramolecular structure via C
H 3 3 3 O and C
H 3 3 3 F bonds. Eu(OTf)3 3 3CH3CN loses acetonitrile already at room temperature. Thermogravimetry shows that upon heating to 165 C all acetonitrile is lost and a complete decomposition occurs at 452 C.

’ INTRODUCTION It has been realized that ionic liquids and ionic liquid
cosolvent mixtures allow crystallizing compounds which are otherwise not accessible or only with great difficulty.1 These compounds often exhibit unusual coordination numbers, modes, or geometries in the solid state. The reaction of hydrated lanthanide(III) chlorides with 1-ethyl-3-methylimidazolium chloride under different conditions (with or without the addition of HCl (aq) in an open or sealed vial) yields two different compounds: LnCl3(OH2)4 3 2([C2mim]Cl) (Ln = Gd and Er, C2mim = 1-ethyl-3methylimidzolium)2 and [C2mim]3[LnCl6] (Ln = La, Pr, Nd, Sm, Eu, Gd, and Er).3 The isolation of the mixed valent neptunium compound [C4mim]5[Np(NpO2)3(H2O)6Cl12] (C4mim = 1-butyl-3-methylimidazolium) from wet [C4mim]2 [NpCl6] in the presence of oxygen is yet another proof how useful ionic liquids can be as crystallization media.4 Heptaaqua(p-toluenesulfonato-O)europium(III) bis(p-toluenesulfonate) dihydrate, [Eu(p-Tos)(H2O)7][p-Tos]2(H2O)2 could be crystallized from a solution of Eu(p-Tos)3 in 1-ethyl-3-methylimidazolium tosylate, (C2mim)(p-Tos).5 The compound is the structural link between the long-known lanthanide tosylate hydrates [Ln(H2O)9](p-Tos)3(H2O) with noncoordinating tosylate and [Ln(H2O)6(p-Tos)2](p-Tos)2(H2O)3, where two tosylate groups coordinate to the lanthanide cation. Similarly, the unprecedented octanuclear europium cluster [C4mpyr]6[Eu8(μ4O)(μ3-OH)12(μ2-OTf)14(OTf)2](HOTf)1.5 (C4mpyr = N-butylN-methylpyrrolidinium) could be obtained from wet [C4mpyr] [OTf].6 Under strictly anhydrous crystallization conditions, the first r 2011 American Chemical Society

structurally characterized homoleptic metal-triflate complex compound, [C4mpyr]3[Yb(OTf)6], was obtained.7 Lanthanide triflates have been extensively studied, mainly for their coordination chemistry and for their application in catalysis. Lanthanide triflates obtained from water are usually nonahydrated, with the general formula [Ln(H2O)9][OTf]3.8 In 1987 the use of lanthanide triflates as catalysts in the reactions of amines with nitriles under strictly anhydrous conditions was reported,9 followed by a large amount of pioneering work, mainly carried out by Kobayashi and co-workers.10 Still today, rare-earth triflates are still extensively studied and used in organic reactions for the formation of C
C, C
N, C
S, and C
O bonds. They are considered to be environmentally benign Lewis acids.11 It is generally accepted that this kind of reactions have to be performed under anhydrous conditions (although some reactions have been shown to be water-compatible).10a In that context, ionic liquids have received considerable interest as reaction media for lanthanide triflate catalyzed Diels
Alder, Friedel
Crafts, Kabachnik
Fields, and many other reactions.12 Though the structural identity of lanthanide triflates would be of great interest to tune and improve the catalytic properties, virtually no crystal structure information is available to date. To our knowledge, an astonishingly low number of compounds where triflate coordinates to Ln(III) could be structurally Received: January 17, 2011 Revised: April 1, 2011 Published: April 05, 2011 1437

dx.doi.org/10.1021/cg200064n | Cryst. Growth Des. 2011, 11, 1437–1440

Crystal Growth & Design characterized, often suffering from bad crystal quality and structural disorder.13 The structures of the simple anhydrous Ln(OTf)3 compounds are not known. Attempts to crystallize Ln(OTf)3 with neutral coligands typically lead to the partial or complete replacement of OTf in the coordination sphere of the lanthanide ion. For example, the structure of Ln(OTf)3 3 4Ph3PO is better represented by writing the formula as [Ln(OTf)2(Ph3PO)4][OTf], Ln = La, Ce, Nd, Er, Lu.14 Examples such as Pr(OTf)3(1,3,5trimethyl-1,3,5-triazacyclohexane)215 or [Ln(OTf)3(terpy)2](CH3CN)16 (terpy = 2,20 :60 ,20 -terpyridine, Ln = Ce, Nd), where all triflate anions coordinate to the metal cation in the presence of a suitable neutral coligand, are the exception. Only recently we were able to structurally characterize the first homoleptic Yb-triflate complex, [C4mpyr]3[Yb(OTf6)], crystallized from the ionic liquid [C4mpyr] [OTf].7 Together with the finding that [C4mpyr]6[Eu8(μ4-O)(μ3OH)12(μ2-OTf)14(OTf)2](HOTf)1.5 could be obtained from [C4mpyr][OTf] with small amounts of water6 and previous observations made by Rogers and co-workers,2,3 we believed that it might be possible to structurally trap a lanthanide triflate solvate with allcoordinating triflate ligands in an ionic liquid as the crystallization medium. Indeed, we succeeded in synthesizing and structurally characterizing Eu(OTf)3 3 3CH3CN (1) from Eu(OTf)3 with CH3CN in the ionic liquid (1-butylpyridinium triflate), [C4py][OTf].

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Figure 1. ORTEP plot (50% probability ellipsoids) of compound 1 showing hydrogen bonds (dashed lines). Symmetry codes: (A) x
1, y þ 1, z; (B) x, y þ 1, z; (C)
x þ 1,
y,
z þ 1; (D)
x þ 2,
y,
z þ 1; (E) x
1, y, z; (F)
x þ 1,
y,
z; (G) x þ 1, y, z.

Table 1. Selected Bond Lengths (in Å) in 1a

’ EXPERIMENTAL SECTION All operations were carried out under dry argon using standard Schlenk-line and glovebox techniques. Synthesis of Eu(OTf)3 3 3CH3CN (1). 0.3156 g of 1-butylpyridinium triflate ([C4py][OTf], 99%, iolitec) and 0.2209 g of Eu(OTf)3 (98%, Sigma-Aldrich) (IL/Eu = 3:1) were placed in a small Schlenk tube and stirred at 393 K under vacuum for 2 h. 2 mL of dry acetonitrile (Acros, AcroSeal, water < 10 ppm) was added after the reaction mixture was cooled to room temperature. Stirring was continued until a colorless clear solution was obtained. Needle-shaped crystals of sufficient quality for structure determination were obtained after two days in almost quantitative yield. Crystal Structure Determination of 1. Intensity data were collected on a Stoe IPDS-I single-crystal X-ray diffractometer with graphite monochromated Mo KR radiation (λ = 0.71073 Å) at 100(2) K. Crystal structure solution by direct methods using SHELXS-9717 yielded the heavy atom positions. Subsequent difference Fourier analyses and least-squares refinement with SHELXL-9718 allowed the location of the remaining atomic positions. The hydrogen atoms of the acetonitrile molecules were located from the difference Fourier maps, and their isotropic displacement factor was chosen as 1.5 times that of the preceding carbon atom. Data reduction was carried out with the program package X-red19 and a numerical absorption correction with the program X-Shape.20 Crystal Data for 1. C9H9EuF9N3O9S3, Mr = 722.33 g mol
1, triclinic, P1, a = 5.734 (1) Å, b = 10.500(2) Å, c = 19.088(4) Å, R = 98.09(3), β = 93.79(3), γ = 91.46(3), V = 1134.5(4) Å3, Z = 2, 2θmax = 55.42, λ = 0.71073 Å, T = 100(2) K, F = 2.115 g cm
3, μ = 3.159 mm
1, F(000) = 696. 9235 reflections were collected, of which 4388 were unique (Rint = 0.0518). GOF = 1.013. R1/R2 = 0.0335, 0.0765 (I > 2σ(I)). CCDC 769611 contains supplementary crystallographic data. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/datarequest/cif.

Eu(1)
O(2)#1

2.388(3)

Eu(1)
O(4)

2.411(3)

Eu(1)
O(5)#1

2.404(3)

Eu(1)
O(7)

2.417(4)

Eu(1)
O(8)#2

2.410(3)

Eu(1)
O(1)

2.420(3)

Eu(1)
N(3)

2.568(4)

Eu(1)
N(2)

2.599(4)

Eu(1)
N(1)

2.598(4)

Operators for generating equivalent atoms are as follows: ($1) x
1, y þ 1, z; ($2) x, y þ 1, z; ($3)
x þ 1,
y,
z þ 1; ($4)
x þ 2,
y,
z þ 1; ($5)
x þ 1,
y,
z. a

’ RESULTS AND DISCUSSION To obtain 1¥[Eu(OTf)3(CH3CN)3] (1), Eu(OTf)3 was first dissolved at elevated temperature in the ionic liquid [C4py][OTf]; then a small amount of dry acetonitrile was added. After the reaction mixture was cooled to room temperature, needle-shaped, transparent crystals of sufficient quality for structure determination grew. The result of the single crystal X-ray structure analysis could be confirmed by powder X-ray diffraction measurements (see Supporting Information). 1¥[Eu(OTf)3(CH3CN)3] crystallizes in the triclinic P1 (no. 2) space group with two formula units in the unit cell. The asymmetric unit contains one europium(III) cation, three triflate (OTf)
anions, and three acetonitrile molecules. Eu(III) is nine-coordinate by six oxygen atoms of six triflate anions and three nitrogen atoms from three acetonitrile molecules (see Figure 1). The Eu
O distances are in the range 2.388(3)
2.420(3) Å and are comparable to those found in other Eu(III)
OTf compounds.21 In comparison, the Eu
N interatomic distances are a little larger, falling in the range 2.568(4)
2.599(4) Å (see Table 1). However, they are a bit shorter than values found for other europium(III) nitrile compounds.22 The coordination geometry around the Eu(III) center can best be viewed as a tricapped trigonal prism with the three nitrogen atoms forming the caps. All the triflate ligands adopt a bridging coordination mode using their two sulfonyl oxygen atoms. With the linkage of triflate ligands, europium(III) ions are connected into infinite 1D chains (see Figure 2, top). Topologically, the 1438

dx.doi.org/10.1021/cg200064n |Cryst. Growth Des. 2011, 11, 1437–1440

Crystal Growth & Design

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mother liquor, it loses acetonitrile. This also prevents us from performing other analytical analyses, that require handling the sample at ambient temperature, such as elemental analysis. Thermogravimetry analysis (Supporting Information) shows that, upon heating with 10 C/min, all CH3CN is lost at 165 C. Total decomposition is observed at 342 C. Similarly, when stored at room temperature under argon, all acetonitrile is lost within a few days and the sample converts to amorphous Eu(OTf)3.

Figure 2. 1D chain of 1 along the crystallographic a-axis (top); the projection of the crystal structure along the a-axis (bottom).

Table 2. Hydrogen Bonds in 1a

a

D
H

H3 3 3A

D3 3 3A

— (DHA)

0.96

2.89

3.337(7)

109.4

0.96 0.96

2.49 2.63

3.414(8) 3.341(7)

160.6 136.1

0.96

2.59

3.334(7)

135.6

0.96

2.65

3.334(7)

129.0

0.96

2.85

3.262(6)

106.8

0.96

2.84

3.362(6)

115.1

C6
H6C 3 3 3 O6 C4
H4A 3 3 3 O9_$1 C4
H4B 3 3 3 O9_$2 C6
H6C 3 3 3 O6_$3 C6
H6A 3 3 3 O6_$4 C6
H6B 3 3 3 F5_$3 C8
H8B 3 3 3 F2_$5

The units of bond lengths and angles are Å and deg, respectively. Operators for generating equivalent atoms are as follows: ($1) x
1, y þ 1, z; ($2) x, y þ 1, z; ($3)
x þ 1,
y,
z þ 1; ($4)
x þ 2,
y,
z þ 1; ($5)
x þ 1,
y,
z.

arrangement of these 1D chains represents an intermediate between the spacewise most efficient, hexagonal, and the less dense tetragonal packing of parallel rods/cylinders.23 The topology of the rodpacking is determined by supramolecular H-bonds. Hydrogen bonds, with interaction parameters in the expected range,24 link the rods to a three-dimensional supramolecular structure (see Figure 2, bottom). The acetonitrile molecules in the equatorial positions exhibit interactions with the uncoordinated oxygen atoms and fluorine atoms of the triflate anions in the same or neighboring chains, forming an intra- and intermolecular C
H 3 3 3 O and C
H 3 3 3 F bond network (see Table 2). 1 ¥[Eu(OTf)3(CH3CN)3] could be obtained as phase-pure material (for the PXRD pattern, see the Supporting Information). However, it has a low vapor pressure, and once separated from the

’ CONCLUSIONS Triflate generally is considered as a weakly coordinating anion.25 In metal triflates it typically avoids strong interactions with the metal center. In addition, structural disorder and the many coordination modes of triflate seem to hamper the growth of crystals of sufficient quality for single crystal X-ray structure analysis. Indeed, to date, no crystal structures of anhydrous lanthanide triflates or simple solvates are known; albeit, these compounds are frequently used as transition metal catalysts. By adding an appropriate amount of acetonitrile to the solution of europium triflate in the triflate based ionic liquid, [C4mp][OTf], we successfully isolated well-grown crystals of the acetonitrile solvate of lanthanide triflate, Eu(OTf)3 3 3CH3CN, in quantitative yield. In Eu(OTf)3 3 3CH3CN, all triflate anions coordinate to Eu(III). It seems that the ionic liquid [C4py][OTf] is an uncommonly suitable crystallization medium. It not only acts as a low melting ionic flux but as both the IL cation and anion are weakly coordinating, it provides the suitable environment of the formation of the acetonitrile solvate. This is yet another of the still few scarce examples of how useful ionic liquids are as media for crystallization. However, our observations also have another practical implication: lanthanide triflate can be easily isolated from ILs by simply adding an appropriate amount of acetonitrile to the system. This is an easy way to recover the precious lanthanide triflate catalyst from an IL after the completion of the catalytic reaction. ’ ASSOCIATED CONTENT

bS

Supporting Information. Details of crystallographic data in CIF format, powder XRD patterns, and TGA (PDF). This material is available free of charge via the Internet at http://pubs. acs.org.

’ ACKNOWLEDGMENT Support by the DFG (Deutsche Forschungsgemeinschaft) priority program SPP 1166 “Lanthanoid specific functionalities” and the Fonds der Chemischen Industrie through a Dozentenstipendium for A.V.M. is gratefully acknowledged. ’ REFERENCES (1) Mudring, A.-V.; Tang, S. F. Eur. J. Inorg. Chem. 2010, 2569. (2) Hines, C. C.; Cocalia, V. A.; Rogers, R. D. Chem. Commun. 2008, 226. (3) Hines, C. C.; Cordes, D. B.; Griffin, S. T.; Watts, S. I.; Cocalia, V. A.; Rogers, R. D. New J. Chem. 2008, 32, 872. (4) Charushnikova, I.; Bosse, E.; Guillaumont, D.; Moisy, P. Inorg. Chem. 2010, 49, 2077. (5) Tang, S.-F.; Mudring, A.-V. Cryst. Growth Des. 2009, 9, 2549. (6) Babai, A.; Mudring, A.-V. Z. Anorg. Allg. Chem. 2006, 632, 1956. (7) Babai, A.; Pitula, S.; Mudring, A.-V. Eur. J. Inorg. Chem. 2010, 4933. 1439

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