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Synthesis and structures of alkaline earth metal salts of bis[(trifluoromethyl)sulfonyl]imide
Solid State Sciences 7 (2005) 311–318 www.elsevier.com/locate/ssscie
Synthesis and structures of alkaline earth metal salts of bis[(trifluoromethyl)sulfonyl]imide Lixin Xue, Darryl D. DesMarteau ∗ , William T. Pennington Department of Chemistry, Hunter Research Laboratories, Clemson University, Clemson, SC 29634-0973, USA Received 16 August 2004; received in revised form 30 September 2004; accepted 10 October 2004 Available online 28 January 2005
Abstract A series of alkaline earth metal salts of the bis[(trifluoromethyl)sulfonyl]imide anion have been prepared and structurally characterized. The magnesium cation is fully hydrated with no direct interaction with the anion, although there is extensive hydrogen bonding involving coordinated and lattice water molecules and the anion. The calcium cation is heavily hydrated, but also directly interacts with two anions. As with the magnesium salt, hydrogen bonding plays a major role in determining the crystal packing. The strontium salt is anhydrous, and the eight-coordinate cation interacts directly with several anions to form a two-dimensional layered structure. The barium salt is a monohydrate, with a nine-coordinate cation. As with strontium, it also forms a layered structure. Despite the differences, all of the structures exhibit extensive fluorine segregation which results in the formation of hydrophilic and hydrophobic domains. In all but the magnesium salt, the anion is chelated to the metal and has a cisoid conformation with the trifluoroalkyl groups lying to the same side of the S–N–S plane. This is in keeping with our previous observations that the cisoid conformation is preferred when the anion is coordinated to a metal ion, while the transoid is preferred for noncoordinating cations. 2005 Elsevier SAS. All rights reserved. Keywords: Fluorine segregation; Bis-trifluoromethylsulfonyl imide; Layered structures; Alkaline earth; Crystal engineering
1. Introduction Bis[(perfluoroalkyl)sulfonyl]imides, particularly the trifluoromethyl derivatives, are of increasing interest for their remarkable acidity [1,2], which is a result of the resonance stabilization of the conjugate base anions of the acids due to extensive delocalization of charge over the SO2 –N–SO2 framework. In addition, their interesting electrochemical properties, high chemical and thermal stabilities, and ease of preparation make them valuable for a wide variety of applications.
bis(triflouromethylsulfonyl)imide * Corresponding author.
E-mail address: [email protected] (D.D. DesMarteau). 1293-2558/$ – see front matter 2005 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.10.029
Salts of these anions and derivatives have been found to serve as solutes for polymer electrolytes, leading to dramatically improved performance in lithium batteries and fuel cells [3–5]. They have also proven useful as catalysts for Diels–Alder [6] and Friedel–Crafts acylation [7]. With noncoordinating cations, these anions have been found to give salts with good potential as ionic liquids [8,9]. On the other hand, with metal cations they serve as multidentate ligands that bond to multiple metal centers to maximize electrostatic interaction [10]. These interactions result in the formation of ionic hydrophilic and perfluoroalkyl hydrophobic regions, which typically associate into layers consisting of an ionic core with perfluoroalkyl surfaces [11]. This “fluorine segregation” effect [12,13] may provide a feasible driving force for the directed design of new solid state materials. To extend our earlier work on a series of alkali metal salts of An− [14,15], we have prepared and structurally characterized a series of alkaline earth metal salts. As with the group 1 cations, a variety of structures are formed, all of which share
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the common feature of fluorine segregation as a significant packing interaction.
3659 (m), 3584 (w), 3394 (m), 1615 (m), 1333 (s), 1194 (s), 1142 (s), 1054 (s), 863 (w), 800 (m), 771 (w), 749 (m), 647 (m), 599 (s), 575 (s), 516 (s). 19 F NMR (in CD3 CN) δ − 78.8 (s, –CF3 ).
2. Experimental section 2.1. Materials Bis[(trifluoromethyl)sulfonyl]imide in acid form (HAn) was prepared from the corresponding sulfonyl fluoride by previously reported procedures [16,17]. All other reagents were commercially available and were purified before use as appropriate. 2.2. Preparation of [Mg(H2 O)6 ](An)2 ·2H2 O Basic magnesium carbonate [(MgCO3 )4 ·Mg(OH)2 ·5H2 O] (0.858 g, 1.77 mmol) was combined with HAn (5.02 g, 17.9 mmol) in 20 mL of distilled water and stirred for 20 min at 25 ◦ C. The water was then removed by heating to 100 ◦ C in an oil bath for twelve hours. Yield 99% based on Mg. IR (KBr, cm−1 ): 1325 (s), 1213 (s), 1148 (s), 1056 (s), 863 (w), 809 (m), 753 (m), 657 (m), 608 (s), 517 (m). Repeated IR measurements indicated that the material is very hygroscopic, picking up water quickly. 19 F NMR (in CD3 CN) δ − 78.4 (s, –CF3 ). 2.3. Preparation of [Ca(H2 O)4 (An)2 ] Calcium hydroxide (0.600 g, 8.10 mmol) was combined with HAn (4.66 g, 16.6 mmol) in 30 mL of distilled water and stirred for 20 min at 25 ◦ C. After all the solid had dissolved, the water was removed by heating to 100 ◦ C in an oil bath for twelve hours. Yield 95% based on Ca(OH)2 . IR (KBr, cm−1 ): 1329 (s), 1207 (s), 1145 (s), 1061 (m), 863 (w), 804 (m), 752 (w), 655 (m), 600 (m), 576 (m), 512 (m). 19 F NMR (in CD CN) δ − 78.9 (s, –CF ). 3 3 2.4. Preparation of [Sr(An)2 ] Strontium carbonate (2.20 g, 14.9 mmol) was combined with HAn (8.65 g, 30.8 mmol) in 30 mL of distilled water and stirred for 30 min. The water was then removed by heating to 100 ◦ C in an oil bath for twelve hours. Yield 99% based on SrCO3 . IR (KBr, cm−1 ): 1321 (s), 1210 (s), 1134 (s), 1059 (s), 864 (w), 804 (m), 772 (w), 750 (m), 654 (m), 603 (s), 576 (m), 514 (m). 19 F NMR (in CD3 CN) δ − 78.9 (s, –CF3 ). 2.5. Preparation of [Ba(H2 O)(An)2 ] Barium hydroxide octahydrate (1.38 g, 4.38 mmol) was combined with HAn (2.69 g, 9.57 mmol) in 30 mL of distilled water and stirred for 30 min. The water was then removed by heating to 100 ◦ C in an oil bath for twelve hours. Yield 98% based on Ba(OH)2 ·8H2 O. IR (KBr, cm−1 ):
2.6. Structure determination Colorless crystals of each of the compounds, suitable for X-ray studies, were grown by slow evaporation of a nitromethane solution in air. Specific details of the crystallographic experiment and results for each compound are given in Table 1. The data were measured at either room or reduced temperature on a four-circle diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The data were corrected for Lorentz and polarization effects. The intensities of three reflections, remeasured periodically throughout data collection, varied by less than 2% for each of the compounds. An absorption correction, based on azimuthal scans of several intense reflections, was applied to the data for each compound. All structures were solved by direct methods and refined (on F 2 ) using full-matrix, least-squares techniques. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were refined isotropically for the magnesium salt, and were included at optimized difference Fourier positions (d O–H = 0.86 Å), but were not included for the calcium salt. Structure solution, refinement and calculation of derived results was performed with the SHELXTL [18] package of computer programs. Neutral atom scattering factors and the real and imaginary anomalous dispersion corrections were taken from International Tables for X-ray Crystallography, vol. IV [19].
3. Results and discussion 3.1. General structural features Bis(perfluoroalkylsulfonyl)imides can exist in two different conformations when in anionic form: a transoid form with the perfluoroalkyl groups lying on opposite sides of the S–N–S plane or a cisoid form with the perfluoroalkyl groups lying on the same side of the S–N–S plane, as shown in Fig. 1 [11,14]. This can also be characterized by the C–S· · ·S–C dihedral angle, with a value of ∼180◦ being observed for the transoid form and a value of ∼0◦ being observed for the cisoid form. Computational analysis indicates that the transoid form is slightly favored, and, in general, this is the conformation observed when there are only weak cation· · ·anion interactions. The cisoid form is often found when the anion is chelated to a metal center. Table 2 lists the conformations observed along with C–S· · ·S–C dihedral angles for all known salts of An− structurally characterized to date.
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Table 1 Crystal data Compound
[Mg(H2 O)6 ](An)2 ·2H2 O
[Ca(H2 O)4 (An)2 ]
[Sr(An)2 ]
[Ba(H2 O)(An)2 ]
Formula Mw (g mol−1 ) Crystal size (mm3 ) Crystal system Space group (No.) a (Å) b (Å) c (Å) β (deg) V (Å3 ) Z Diffractometer Temperature (K) θrange (deg) µ (mm−1 ) Trans. range Refls. meas. Refls. uniq. (Rmerg ) Refls. obs.a R1 b wR2 c
[Mg(H2 O)6 ][(CF3 SO2 )2 N]2 ·2H2 O 728.74 0.23 × 0.26 × 0.42 Monoclinic P 21 /n (No. 14) 6.4046(8) 14.3153(16) 14.4399(18) 97.455(10) 1312.7(3) 2 Siemens R3mV 295 ± 2 2.01–24.06 0.54 0.92–1.00 2159 2074 (0.041) 1615 0.0518 (0.0660) 0.1425 (0.1481)
[Ca(H2 O)4 ((CF3 SO2 )2 N)2 ] 672.44 0.17 × 0.26 × 0.41 Monoclinic C2/c (No. 15) 13.453(2) 7.7160(10) 22.458(3) 94.770(10) 2323.2(6) 4 Siemens R3mV 295 ± 2 1.82–22.53 0.78 0.95–1.00 1613 1535 (0.027) 983 0.0467 (0.0772) 0.1101 (0.1158)
[Sr((CF3 SO2 )2 N)2 ] 647.92 0.15 × 0.40 × 0.40 Monoclinic P 2/c (No. 13) 11.199(4) 6.546(2) 12.523(4) 103.72(3) 891.8(5) 16 Rigaku AFC7R-18 148 ± 2 3.11–25.04 3.65 0.76–1.00 1671 1586 (0.042) 1328 0.0370 (0.0496) 0.1038 (0.1106)
[Ba(H2 O)((CF3 SO2 )2 N)2 ] 715.66 0.21 × 0.23 × 0.31 Monoclinic P 21 /n (No. 14)d 8.660(3) 22.315(7) 9.807(3) 90.052(11) 1895.2(11) 4 Siemens R3mV 295 ± 2 1.83–22.58 2.70 0.76–1.00 3060 2496 (0.084) 1830 0.0548 (0.0731) 0.1389 (0.1460)
a b c d
(I > 2σ (I )). R1 = ||Fo | − |Fc ||/ |Fo | for observed data (I > 2σ (I )); number in parentheses is for all data. 2 2 wR2 = { [w(Fo − Fc )2 ]/ [w(Fo2 )2 ]}1/2 for observed data (I > 2σ (I )); number in parentheses is for all data. Although the metric symmetry of the Ba salt appears to indicate an orthorhombic crystal system, the Rmerg was significantly higher (0.24) than for the monoclinic system, and the systematic absences did not agree with any orthorhombic space group, and the solution does not appear to have higher symmetry.
(a)
(b)
Fig. 1. Observed conformations of the [CF3 SO2 NSO2 CF3 ]− anion. (a) Transoid (C–S· · ·S–C dihedral angle of ∼180◦ ); (b) cisoid (C–S· · ·S–C torsion angle of ∼0◦ ).
It should be noted that there are only two reported instances of the cisoid conformation for noncoordinated anions. For the 1,3-dimethylimidazolium salt, a bifurcated hydrogen bond provides a similar interaction to metal chelation [9], and in the diphenyliodonium salt a dimeric cation– anion structure formed through I· · ·O halogen bonds involving one SO2 group of An− provides steric constraints that favor the cisoid conformation [23]. Four examples of the transoid conformation in which the anion is coordinated to a metal ion have been reported, but the factors leading to these are not well understood. In addition to the structures included in Table 2, a Cu(I) salt of An− , Cu(CO)2 (An), has been reported in which the only coordination is through the nitrogen atom [27]. The anion in this compound has a transoid conformation, with a C–S· · ·S–C dihedral angle of 163.2◦ .
We have reported two alkali metal salts with a perfluorinated sulfonyl imide anion in which the trifluoromethyl groups are replaced by n-perfluorobutyl chains [28]. In the sodium salt, the anion possesses a transoid conformation (C–S· · ·S–C of 142.0(5)◦ ), while in the potassium salt both cisoid (C–S· · ·S–C of 10.6(9) and 25.3(9)◦ ) and transoid (C–S· · ·S–C of 176.1(10) and 158.4(11)◦ ) conformations are found. We have also prepared and characterized an iodonium zwitterionic salt, C6 H5 –I+ –C6 H4 –SO2 –N− –SO2 CF3 , which has a cisoid conformation (C–S· · ·S–C of 10.6(2)◦ ), but a dimethylsulfoxide solvate of this compound has a transoid conformation (C–S· · ·S–C of 145.2(3)◦ ) [29]. The distances and angles within the anion agree well with those of other salts An− , and the metal· · ·anion contacts are also typical. Selected distances and angles for the reported salts are given in Table 3.
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Table 2 An− conformations Cation Noncoordinating An− [Mg(H2 O)6 ]2+ [Co(H2 O)6 ]2+ [Yb(N,N -dimethylpropylurea)6 ]3+ 1-Bzl-2-Et-3-Me-imidazolium [Ph-I-CH2 CF3 ]+ N-Me-1-Me-pyrrolinium N,N-(Me)2 -pyrrolidinium [Pr4 N]+ [Me3 NH]+ 1,2,3-(Et)3 -imidazolium 1,3-(Me)2 -imidazolium [Ph-I-Ph]+ Coordinating An− Ca2+ ·4H2 O Sr2+ Ba2+ ·H2 O Li+ ·H2 O
C–S· · ·S–C (deg)
An− conformation
Reference
171.8(3) 170.9(5)
Transoid Transoid Transoid Transoid Transoid Transoid Transoid Transoid Transoid Transoid Cisoid
This paper, [20] [21] [22] [8] [23] [24] [24] [24] [24] [9] [9]
Cisoid
[23]
Cisoid Cisoid Cisoid Cisoid
This paper This paper This paper [14]
Cisoid Cisoid
[14] [14,25]
Cisoid Transoid Transoid Transoid Transoid
[14] [26] [14] [15] [7]
a
169.8 169.1(1) 174.6(2) 173.4(1) 166.3(1) 173.85(8) 175.67(16) 31.47(13), 37.88(13) 38.2(1) 10.2(4) 15.9(2) 30.9(6) 15.3(4), 16.5(3), 17.7(4), 18.4(3) 0.0 9.4(7), 13.2(5) 12.0(11) 171.7 172.6(3) 147.2(4) 133.3
Na+ ·H2 O·MeOH K+ Cs+ Li+ Rb+ ·H2 O Rb+ ·2dioxane Zn2+ a No details available.
Table 3 Selected bonding parameters Compound
[Mg(H2 O)6 ](An)2 ·2H2 O
[Ca(H2 O)4 (An)2 ]
[Sr(An)2 ]
[Ba(H2 O)(An)2 ]
S–O (Å)
1.412(3), 1.417(4) 1.396(5), 1.412(5)
1.424(4), 1.431(4) 1.422(4), 1.427(4)
1.426(3), 1.435(3) 1.434(3), 1.435(3)
1.428(7), 1.439(7), 1.435(8), 1.421(7),
S–N (Å)
1.582(4), 1.571(4)
1.568(5), 1.570(5)
1.572(3), 1.567(4)
1.568(8), 1.570(8) 1.571(8), 1.554(8)
S–N–S (deg)
125.1(2)
126.0(3)
C.N. (M)a
6
8
M· · ·O (H2 O)
2.046(3) 2.051(3) 2.060(3)
2.404(4) 2.416(5)
M· · ·O (An− )
a Coordination number about the metal ion.
2.466(4) 2.502(4)
125.6(2) 8
124.5(5) 125.5(5) 9 2.744(7)
2.542(3) 2.558(3) 2.561(3) 2.649(3)
2.735(7) 2.770(7) 2.786(6) 2.788(6) 2.800(7) 2.830(7) 2.853(7) 2.902(7)
1.429(7) 1.441(8) 1.450(7) 1.448(7)
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315
Fig. 2. Thermal ellipsoid plot (35% probability) of [Mg(H2 O)6 ](An)2 · 2H2 O.
3.1.1. Structure of [Mg(H2 O)6 ](An)2 ·2H2 O The structure of the [Mg(H2 O)6 ](An)2 ·2H2 O has been previously reported [25], but we include our own results here to facilitate comparison with the rest of the series. The formula unit (Fig. 2) contains a hexahydrated magnesium cation situated upon an inversion center (1/2, 1/2, 1/2), two anions and two lattice water molecules each occupying general positions within the unit cell. The crystal packing involves extensive hydrogen bonding involving the lattice water molecules with both the coordinated water molecules and the sulfonyl oxygen atoms, with O· · ·O contacts of 2.772(6)–3.074(5) Å. Cations related by translation along the a-axis are linked through lattice water molecules to form one-dimensional columns. The columns are coated with anions, and columns related by 21 screw and c-glide operations are connected by additional hydrogen bonds to extend the structure in the b- and c-directions (Fig. 3). The trifluoromethyl groups form the edges of the columns and fill the channels which result from the packing of the columns. 3.1.2. Structure of [Ca(H2 O)4 (An)2 ] The formula unit of [Ca(H2 O)4 (An)2 ], shown in Fig. 4, consists of a tetrahydrated calcium cation which also interacts with two anions through chelation involving two sulfonyl oxygen atoms. The eight-coordinate metal ion sits upon a crystallographic two-fold rotation axis (1/2, y, 0). Although the compound consists of discrete molecular ion pairs, these are connected in the b- and a-directions through hydrogen bonding between the coordinated water molecules and sulfonyl oxygen atoms with O· · ·O contacts of 2.822(7)–2.985(7) Å. The trifluoromethyl groups are oriented to the surfaces of the resulting layers, which stack along the c-axis, as shown in Fig. 5. 3.1.3. Structure of [Sr(An)2 ] The strontium cation in [Sr(An)2 ] is eight-coordinate and sits upon a crystallographic two-fold axis (1/2, y, 3/4). The cation interacts with two anions through chelated contacts and with four others through mono–hapto interactions, all involving sulfonyl oxygen atoms (Fig. 6). One pair of the latter links ion pairs along the b-axis. The resulting “chains”
Fig. 3. Crystal packing of [Mg(H2 O)6 ](An)2 ·2H2 O viewed down the a-axis. The origin is the upper, left, rear corner; +x is out, +y is down, and +z is to the right.
Fig. 4. Thermal ellipsoid plot (35% probability) of [Ca(H2 O)4 (An)2 ]. Upper case letters included in the atomic labels correspond to the following symmetry operators: (A) 1 − x, y, −z.
related by inversion (1/2, 1/2, 1/2) and a c-glide operation (x, 1/2, z) are linked through similar interactions to form a two-dimensional layer (Fig. 7). As compared to the potassium derivative, these layers possess small cavities, which appear to be vacant. Layers related by translation along the a-axis stack to complete the structure (Fig. 8). 3.1.4. Structure of [Ba(H2 O)(An)2 ] The barium cation is nine-coordinate and sits upon a general position in the unit cell. The cation interacts with one water molecule and with two anions through chelated contacts. It also interacts with four others through mono–hapto interactions, all involving sulfonyl oxygen atoms (Fig. 9). Ion-pair units, [Ba(H2 O)(An)2 ], related by n-glide opera-
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Fig. 7. View of one layer of [Sr(An)2 ].
Fig. 5. Crystal packing of [Ca(H2 O)4 (An)2 ] viewed down the b-axis showing the stacking of the layers along the c-axis. The origin is the lower, left, rear corner; +x is up, +y is out, and +z is to the right.
Fig. 8. Crystal packing of [Sr(An)2 ] viewed down the b-axis, showing the stacking of the layers along the a-axis. The origin is the lower, left, rear corner; +x is up, +y is out, and +z is to the right. Fig. 6. Thermal ellipsoid plot (50% probability) of [Sr(An)2 ]. Upper case letters included in the atomic labels correspond to the following symmetry operators: (A) 1 − x, y, 3/2 − z; (B) x, y + 1, z; (C) 1 − x, 1 + y, 3/2 − z; (D) 1 − x, 1 − y, 1 − z; (E) x, 1 − y, 1/2 + z; (F) x, y − 1, z.
tions (x, 3/4, z) are linked through the latter interactions to form a polar two-dimensional layer in the ac-plane (Fig. 10). The layers stack along the b-axis with adjacent layers related by inversion symmetry (0, 1/2, 0) to reverse the polarity of the layer (Fig. 11). As opposed to the strontium derivative, the vacancy in the layer as compared to the alkali metal salt is occupied, at least to some extent, by the coordinated water molecule. Strangely, there are no hydrogen bonding interactions involving this water molecule.
4. Conclusions Despite major differences in composition and connectivity, the alkaline earth salts of An− all exhibit fluorine segregation resulting in aggregation of the structure into hydrophilic and hydrophobic regions. The two lightest members, magnesium and calcium, are heavily hydrated and hydrogen bonding is the dominant packing interaction. For the heavier members, strontium and barium, strong cation· · ·anion interactions between the metal ions and sulfonyl oxygen atoms provide the dominant packing vector, the large barium salt forming as a monohydrate in which the water serves mainly to fill space. As previously observed, the conformation of the An− ion is cisoid for all of the salts
L. Xue et al. / Solid State Sciences 7 (2005) 311–318
Fig. 9. Thermal ellipsoid plot (30% probability) of [Ba(H2 O)(An)2 ]. Upper case letters included in the atomic labels correspond to the following symmetry operators: (A) −1/2 + x, 3/2 + y, −1/2 + z; (B) 1/2 + x, 3/2 + y, −1/2 + z; (C) −1/2 + x, 3/2 + y, 1/2 + z; (D) 1/2 + x, 3/2 + y, 1/2 + z.
317
Fig. 11. Crystal packing of [Ba(H2 O)(An)2 ] viewed down the a-axis, showing the stacking of the layers along the b-axis. The origin is the upper, left, rear corner; +x is out, +y is down, and +z is to the right.
(H2 O)6 ](An)2 ·2H2 O), CCDC 247560 ([Ca(H2 O)4 (An)2 ]), CCDC 247561 ([Sr(An)2 ]), CCDC 247562 ([Ba(H2 O)(An)2 ]). Copies may be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (http://www.ccdc.cam.ac.uk/conts/retrieving).
Acknowledgement The National Science Foundation is gratefully acknowledged for financial support of this project (CHE-0109377) and for assistance in the purchase of X-ray instrumentation (CHE-9207230).
References Fig. 10. View of one polar layer of [Ba(H2 O)(An)2 ].
in which it is coordinated to the metal, and is transoid only for the magnesium salt, in which the cation is fully hydrated. Continuing investigations are needed to understand the reasons for this behavior, and these will explore metal centers of various charges and of diverse degrees of hard/soft character.
Supplementary material Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre: CCDC 247559 ([Mg-
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[9] J.D. Holbrey, W.M. Reichert, R.D. Rogers, Dalton Trans. (2004) 2267. [10] D.D. DesMarteau, S. Zhu, W.T. Pennington, Y. Gotoh, M. Witz, S. Zuberi, S. Singh, J. Fluorine Chem. 45 (1989) 24. [11] L. Xue, D.D. DesMarteau, W.T. Pennington, Angew. Chem., Int. Ed. Engl. 36 (1997) 1333. [12] O.J. Dautel, M. Fourmigue, E. Faulques, Cryst. Eng. Commun. 4 (2002) 249. [13] J.J. Wolff, F. Gredel, T. Oeser, H. Irngartinger, H. Pritzkow, Chem. Eur. J. 5 (1999) 29. [14] L. Xue, C.W. Padgett, D.D. DesMarteau, W.T. Pennington, Solid State Sci. 4 (2002) 1535. [15] L. Xue, C.W. Padgett, D.D. DesMarteau, W.T. Pennington, Acta Crystallogr. C 60 (2004) m200. [16] J. Foropoulos Jr., D.D. DesMarteau, Inorg. Chem. 29 (1984) 3720. [17] M. Witz, D.D. DesMarteau, J. Fluorine Chem. 52 (1991) 7. [18] G.M. Sheldrick, SHELXTL, Version 6.10, Crystallographic computing system, Bruker AXS, Madison, WI, 2000. [19] International Tables for X-ray Crystallography, vol. IV, Kynoch Press, Birmingham, UK, 1974, Tables 2.2 and 2.3.1.
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Synthesis and structures of alkaline earth metal salts of bis[(trifluoromethyl)sulfonyl]imide Lixin Xue, Darryl D. DesMarteau ∗ , William T. Pennington Department of Chemistry, Hunter Research Laboratories, Clemson University, Clemson, SC 29634-0973, USA Received 16 August 2004; received in revised form 30 September 2004; accepted 10 October 2004 Available online 28 January 2005
Abstract A series of alkaline earth metal salts of the bis[(trifluoromethyl)sulfonyl]imide anion have been prepared and structurally characterized. The magnesium cation is fully hydrated with no direct interaction with the anion, although there is extensive hydrogen bonding involving coordinated and lattice water molecules and the anion. The calcium cation is heavily hydrated, but also directly interacts with two anions. As with the magnesium salt, hydrogen bonding plays a major role in determining the crystal packing. The strontium salt is anhydrous, and the eight-coordinate cation interacts directly with several anions to form a two-dimensional layered structure. The barium salt is a monohydrate, with a nine-coordinate cation. As with strontium, it also forms a layered structure. Despite the differences, all of the structures exhibit extensive fluorine segregation which results in the formation of hydrophilic and hydrophobic domains. In all but the magnesium salt, the anion is chelated to the metal and has a cisoid conformation with the trifluoroalkyl groups lying to the same side of the S–N–S plane. This is in keeping with our previous observations that the cisoid conformation is preferred when the anion is coordinated to a metal ion, while the transoid is preferred for noncoordinating cations. 2005 Elsevier SAS. All rights reserved. Keywords: Fluorine segregation; Bis-trifluoromethylsulfonyl imide; Layered structures; Alkaline earth; Crystal engineering
1. Introduction Bis[(perfluoroalkyl)sulfonyl]imides, particularly the trifluoromethyl derivatives, are of increasing interest for their remarkable acidity [1,2], which is a result of the resonance stabilization of the conjugate base anions of the acids due to extensive delocalization of charge over the SO2 –N–SO2 framework. In addition, their interesting electrochemical properties, high chemical and thermal stabilities, and ease of preparation make them valuable for a wide variety of applications.
bis(triflouromethylsulfonyl)imide * Corresponding author.
E-mail address: [email protected] (D.D. DesMarteau). 1293-2558/$ – see front matter 2005 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.10.029
Salts of these anions and derivatives have been found to serve as solutes for polymer electrolytes, leading to dramatically improved performance in lithium batteries and fuel cells [3–5]. They have also proven useful as catalysts for Diels–Alder [6] and Friedel–Crafts acylation [7]. With noncoordinating cations, these anions have been found to give salts with good potential as ionic liquids [8,9]. On the other hand, with metal cations they serve as multidentate ligands that bond to multiple metal centers to maximize electrostatic interaction [10]. These interactions result in the formation of ionic hydrophilic and perfluoroalkyl hydrophobic regions, which typically associate into layers consisting of an ionic core with perfluoroalkyl surfaces [11]. This “fluorine segregation” effect [12,13] may provide a feasible driving force for the directed design of new solid state materials. To extend our earlier work on a series of alkali metal salts of An− [14,15], we have prepared and structurally characterized a series of alkaline earth metal salts. As with the group 1 cations, a variety of structures are formed, all of which share
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the common feature of fluorine segregation as a significant packing interaction.
3659 (m), 3584 (w), 3394 (m), 1615 (m), 1333 (s), 1194 (s), 1142 (s), 1054 (s), 863 (w), 800 (m), 771 (w), 749 (m), 647 (m), 599 (s), 575 (s), 516 (s). 19 F NMR (in CD3 CN) δ − 78.8 (s, –CF3 ).
2. Experimental section 2.1. Materials Bis[(trifluoromethyl)sulfonyl]imide in acid form (HAn) was prepared from the corresponding sulfonyl fluoride by previously reported procedures [16,17]. All other reagents were commercially available and were purified before use as appropriate. 2.2. Preparation of [Mg(H2 O)6 ](An)2 ·2H2 O Basic magnesium carbonate [(MgCO3 )4 ·Mg(OH)2 ·5H2 O] (0.858 g, 1.77 mmol) was combined with HAn (5.02 g, 17.9 mmol) in 20 mL of distilled water and stirred for 20 min at 25 ◦ C. The water was then removed by heating to 100 ◦ C in an oil bath for twelve hours. Yield 99% based on Mg. IR (KBr, cm−1 ): 1325 (s), 1213 (s), 1148 (s), 1056 (s), 863 (w), 809 (m), 753 (m), 657 (m), 608 (s), 517 (m). Repeated IR measurements indicated that the material is very hygroscopic, picking up water quickly. 19 F NMR (in CD3 CN) δ − 78.4 (s, –CF3 ). 2.3. Preparation of [Ca(H2 O)4 (An)2 ] Calcium hydroxide (0.600 g, 8.10 mmol) was combined with HAn (4.66 g, 16.6 mmol) in 30 mL of distilled water and stirred for 20 min at 25 ◦ C. After all the solid had dissolved, the water was removed by heating to 100 ◦ C in an oil bath for twelve hours. Yield 95% based on Ca(OH)2 . IR (KBr, cm−1 ): 1329 (s), 1207 (s), 1145 (s), 1061 (m), 863 (w), 804 (m), 752 (w), 655 (m), 600 (m), 576 (m), 512 (m). 19 F NMR (in CD CN) δ − 78.9 (s, –CF ). 3 3 2.4. Preparation of [Sr(An)2 ] Strontium carbonate (2.20 g, 14.9 mmol) was combined with HAn (8.65 g, 30.8 mmol) in 30 mL of distilled water and stirred for 30 min. The water was then removed by heating to 100 ◦ C in an oil bath for twelve hours. Yield 99% based on SrCO3 . IR (KBr, cm−1 ): 1321 (s), 1210 (s), 1134 (s), 1059 (s), 864 (w), 804 (m), 772 (w), 750 (m), 654 (m), 603 (s), 576 (m), 514 (m). 19 F NMR (in CD3 CN) δ − 78.9 (s, –CF3 ). 2.5. Preparation of [Ba(H2 O)(An)2 ] Barium hydroxide octahydrate (1.38 g, 4.38 mmol) was combined with HAn (2.69 g, 9.57 mmol) in 30 mL of distilled water and stirred for 30 min. The water was then removed by heating to 100 ◦ C in an oil bath for twelve hours. Yield 98% based on Ba(OH)2 ·8H2 O. IR (KBr, cm−1 ):
2.6. Structure determination Colorless crystals of each of the compounds, suitable for X-ray studies, were grown by slow evaporation of a nitromethane solution in air. Specific details of the crystallographic experiment and results for each compound are given in Table 1. The data were measured at either room or reduced temperature on a four-circle diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The data were corrected for Lorentz and polarization effects. The intensities of three reflections, remeasured periodically throughout data collection, varied by less than 2% for each of the compounds. An absorption correction, based on azimuthal scans of several intense reflections, was applied to the data for each compound. All structures were solved by direct methods and refined (on F 2 ) using full-matrix, least-squares techniques. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were refined isotropically for the magnesium salt, and were included at optimized difference Fourier positions (d O–H = 0.86 Å), but were not included for the calcium salt. Structure solution, refinement and calculation of derived results was performed with the SHELXTL [18] package of computer programs. Neutral atom scattering factors and the real and imaginary anomalous dispersion corrections were taken from International Tables for X-ray Crystallography, vol. IV [19].
3. Results and discussion 3.1. General structural features Bis(perfluoroalkylsulfonyl)imides can exist in two different conformations when in anionic form: a transoid form with the perfluoroalkyl groups lying on opposite sides of the S–N–S plane or a cisoid form with the perfluoroalkyl groups lying on the same side of the S–N–S plane, as shown in Fig. 1 [11,14]. This can also be characterized by the C–S· · ·S–C dihedral angle, with a value of ∼180◦ being observed for the transoid form and a value of ∼0◦ being observed for the cisoid form. Computational analysis indicates that the transoid form is slightly favored, and, in general, this is the conformation observed when there are only weak cation· · ·anion interactions. The cisoid form is often found when the anion is chelated to a metal center. Table 2 lists the conformations observed along with C–S· · ·S–C dihedral angles for all known salts of An− structurally characterized to date.
L. Xue et al. / Solid State Sciences 7 (2005) 311–318
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Table 1 Crystal data Compound
[Mg(H2 O)6 ](An)2 ·2H2 O
[Ca(H2 O)4 (An)2 ]
[Sr(An)2 ]
[Ba(H2 O)(An)2 ]
Formula Mw (g mol−1 ) Crystal size (mm3 ) Crystal system Space group (No.) a (Å) b (Å) c (Å) β (deg) V (Å3 ) Z Diffractometer Temperature (K) θrange (deg) µ (mm−1 ) Trans. range Refls. meas. Refls. uniq. (Rmerg ) Refls. obs.a R1 b wR2 c
[Mg(H2 O)6 ][(CF3 SO2 )2 N]2 ·2H2 O 728.74 0.23 × 0.26 × 0.42 Monoclinic P 21 /n (No. 14) 6.4046(8) 14.3153(16) 14.4399(18) 97.455(10) 1312.7(3) 2 Siemens R3mV 295 ± 2 2.01–24.06 0.54 0.92–1.00 2159 2074 (0.041) 1615 0.0518 (0.0660) 0.1425 (0.1481)
[Ca(H2 O)4 ((CF3 SO2 )2 N)2 ] 672.44 0.17 × 0.26 × 0.41 Monoclinic C2/c (No. 15) 13.453(2) 7.7160(10) 22.458(3) 94.770(10) 2323.2(6) 4 Siemens R3mV 295 ± 2 1.82–22.53 0.78 0.95–1.00 1613 1535 (0.027) 983 0.0467 (0.0772) 0.1101 (0.1158)
[Sr((CF3 SO2 )2 N)2 ] 647.92 0.15 × 0.40 × 0.40 Monoclinic P 2/c (No. 13) 11.199(4) 6.546(2) 12.523(4) 103.72(3) 891.8(5) 16 Rigaku AFC7R-18 148 ± 2 3.11–25.04 3.65 0.76–1.00 1671 1586 (0.042) 1328 0.0370 (0.0496) 0.1038 (0.1106)
[Ba(H2 O)((CF3 SO2 )2 N)2 ] 715.66 0.21 × 0.23 × 0.31 Monoclinic P 21 /n (No. 14)d 8.660(3) 22.315(7) 9.807(3) 90.052(11) 1895.2(11) 4 Siemens R3mV 295 ± 2 1.83–22.58 2.70 0.76–1.00 3060 2496 (0.084) 1830 0.0548 (0.0731) 0.1389 (0.1460)
a b c d
(I > 2σ (I )). R1 = ||Fo | − |Fc ||/ |Fo | for observed data (I > 2σ (I )); number in parentheses is for all data. 2 2 wR2 = { [w(Fo − Fc )2 ]/ [w(Fo2 )2 ]}1/2 for observed data (I > 2σ (I )); number in parentheses is for all data. Although the metric symmetry of the Ba salt appears to indicate an orthorhombic crystal system, the Rmerg was significantly higher (0.24) than for the monoclinic system, and the systematic absences did not agree with any orthorhombic space group, and the solution does not appear to have higher symmetry.
(a)
(b)
Fig. 1. Observed conformations of the [CF3 SO2 NSO2 CF3 ]− anion. (a) Transoid (C–S· · ·S–C dihedral angle of ∼180◦ ); (b) cisoid (C–S· · ·S–C torsion angle of ∼0◦ ).
It should be noted that there are only two reported instances of the cisoid conformation for noncoordinated anions. For the 1,3-dimethylimidazolium salt, a bifurcated hydrogen bond provides a similar interaction to metal chelation [9], and in the diphenyliodonium salt a dimeric cation– anion structure formed through I· · ·O halogen bonds involving one SO2 group of An− provides steric constraints that favor the cisoid conformation [23]. Four examples of the transoid conformation in which the anion is coordinated to a metal ion have been reported, but the factors leading to these are not well understood. In addition to the structures included in Table 2, a Cu(I) salt of An− , Cu(CO)2 (An), has been reported in which the only coordination is through the nitrogen atom [27]. The anion in this compound has a transoid conformation, with a C–S· · ·S–C dihedral angle of 163.2◦ .
We have reported two alkali metal salts with a perfluorinated sulfonyl imide anion in which the trifluoromethyl groups are replaced by n-perfluorobutyl chains [28]. In the sodium salt, the anion possesses a transoid conformation (C–S· · ·S–C of 142.0(5)◦ ), while in the potassium salt both cisoid (C–S· · ·S–C of 10.6(9) and 25.3(9)◦ ) and transoid (C–S· · ·S–C of 176.1(10) and 158.4(11)◦ ) conformations are found. We have also prepared and characterized an iodonium zwitterionic salt, C6 H5 –I+ –C6 H4 –SO2 –N− –SO2 CF3 , which has a cisoid conformation (C–S· · ·S–C of 10.6(2)◦ ), but a dimethylsulfoxide solvate of this compound has a transoid conformation (C–S· · ·S–C of 145.2(3)◦ ) [29]. The distances and angles within the anion agree well with those of other salts An− , and the metal· · ·anion contacts are also typical. Selected distances and angles for the reported salts are given in Table 3.
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Table 2 An− conformations Cation Noncoordinating An− [Mg(H2 O)6 ]2+ [Co(H2 O)6 ]2+ [Yb(N,N -dimethylpropylurea)6 ]3+ 1-Bzl-2-Et-3-Me-imidazolium [Ph-I-CH2 CF3 ]+ N-Me-1-Me-pyrrolinium N,N-(Me)2 -pyrrolidinium [Pr4 N]+ [Me3 NH]+ 1,2,3-(Et)3 -imidazolium 1,3-(Me)2 -imidazolium [Ph-I-Ph]+ Coordinating An− Ca2+ ·4H2 O Sr2+ Ba2+ ·H2 O Li+ ·H2 O
C–S· · ·S–C (deg)
An− conformation
Reference
171.8(3) 170.9(5)
Transoid Transoid Transoid Transoid Transoid Transoid Transoid Transoid Transoid Transoid Cisoid
This paper, [20] [21] [22] [8] [23] [24] [24] [24] [24] [9] [9]
Cisoid
[23]
Cisoid Cisoid Cisoid Cisoid
This paper This paper This paper [14]
Cisoid Cisoid
[14] [14,25]
Cisoid Transoid Transoid Transoid Transoid
[14] [26] [14] [15] [7]
a
169.8 169.1(1) 174.6(2) 173.4(1) 166.3(1) 173.85(8) 175.67(16) 31.47(13), 37.88(13) 38.2(1) 10.2(4) 15.9(2) 30.9(6) 15.3(4), 16.5(3), 17.7(4), 18.4(3) 0.0 9.4(7), 13.2(5) 12.0(11) 171.7 172.6(3) 147.2(4) 133.3
Na+ ·H2 O·MeOH K+ Cs+ Li+ Rb+ ·H2 O Rb+ ·2dioxane Zn2+ a No details available.
Table 3 Selected bonding parameters Compound
[Mg(H2 O)6 ](An)2 ·2H2 O
[Ca(H2 O)4 (An)2 ]
[Sr(An)2 ]
[Ba(H2 O)(An)2 ]
S–O (Å)
1.412(3), 1.417(4) 1.396(5), 1.412(5)
1.424(4), 1.431(4) 1.422(4), 1.427(4)
1.426(3), 1.435(3) 1.434(3), 1.435(3)
1.428(7), 1.439(7), 1.435(8), 1.421(7),
S–N (Å)
1.582(4), 1.571(4)
1.568(5), 1.570(5)
1.572(3), 1.567(4)
1.568(8), 1.570(8) 1.571(8), 1.554(8)
S–N–S (deg)
125.1(2)
126.0(3)
C.N. (M)a
6
8
M· · ·O (H2 O)
2.046(3) 2.051(3) 2.060(3)
2.404(4) 2.416(5)
M· · ·O (An− )
a Coordination number about the metal ion.
2.466(4) 2.502(4)
125.6(2) 8
124.5(5) 125.5(5) 9 2.744(7)
2.542(3) 2.558(3) 2.561(3) 2.649(3)
2.735(7) 2.770(7) 2.786(6) 2.788(6) 2.800(7) 2.830(7) 2.853(7) 2.902(7)
1.429(7) 1.441(8) 1.450(7) 1.448(7)
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315
Fig. 2. Thermal ellipsoid plot (35% probability) of [Mg(H2 O)6 ](An)2 · 2H2 O.
3.1.1. Structure of [Mg(H2 O)6 ](An)2 ·2H2 O The structure of the [Mg(H2 O)6 ](An)2 ·2H2 O has been previously reported [25], but we include our own results here to facilitate comparison with the rest of the series. The formula unit (Fig. 2) contains a hexahydrated magnesium cation situated upon an inversion center (1/2, 1/2, 1/2), two anions and two lattice water molecules each occupying general positions within the unit cell. The crystal packing involves extensive hydrogen bonding involving the lattice water molecules with both the coordinated water molecules and the sulfonyl oxygen atoms, with O· · ·O contacts of 2.772(6)–3.074(5) Å. Cations related by translation along the a-axis are linked through lattice water molecules to form one-dimensional columns. The columns are coated with anions, and columns related by 21 screw and c-glide operations are connected by additional hydrogen bonds to extend the structure in the b- and c-directions (Fig. 3). The trifluoromethyl groups form the edges of the columns and fill the channels which result from the packing of the columns. 3.1.2. Structure of [Ca(H2 O)4 (An)2 ] The formula unit of [Ca(H2 O)4 (An)2 ], shown in Fig. 4, consists of a tetrahydrated calcium cation which also interacts with two anions through chelation involving two sulfonyl oxygen atoms. The eight-coordinate metal ion sits upon a crystallographic two-fold rotation axis (1/2, y, 0). Although the compound consists of discrete molecular ion pairs, these are connected in the b- and a-directions through hydrogen bonding between the coordinated water molecules and sulfonyl oxygen atoms with O· · ·O contacts of 2.822(7)–2.985(7) Å. The trifluoromethyl groups are oriented to the surfaces of the resulting layers, which stack along the c-axis, as shown in Fig. 5. 3.1.3. Structure of [Sr(An)2 ] The strontium cation in [Sr(An)2 ] is eight-coordinate and sits upon a crystallographic two-fold axis (1/2, y, 3/4). The cation interacts with two anions through chelated contacts and with four others through mono–hapto interactions, all involving sulfonyl oxygen atoms (Fig. 6). One pair of the latter links ion pairs along the b-axis. The resulting “chains”
Fig. 3. Crystal packing of [Mg(H2 O)6 ](An)2 ·2H2 O viewed down the a-axis. The origin is the upper, left, rear corner; +x is out, +y is down, and +z is to the right.
Fig. 4. Thermal ellipsoid plot (35% probability) of [Ca(H2 O)4 (An)2 ]. Upper case letters included in the atomic labels correspond to the following symmetry operators: (A) 1 − x, y, −z.
related by inversion (1/2, 1/2, 1/2) and a c-glide operation (x, 1/2, z) are linked through similar interactions to form a two-dimensional layer (Fig. 7). As compared to the potassium derivative, these layers possess small cavities, which appear to be vacant. Layers related by translation along the a-axis stack to complete the structure (Fig. 8). 3.1.4. Structure of [Ba(H2 O)(An)2 ] The barium cation is nine-coordinate and sits upon a general position in the unit cell. The cation interacts with one water molecule and with two anions through chelated contacts. It also interacts with four others through mono–hapto interactions, all involving sulfonyl oxygen atoms (Fig. 9). Ion-pair units, [Ba(H2 O)(An)2 ], related by n-glide opera-
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Fig. 7. View of one layer of [Sr(An)2 ].
Fig. 5. Crystal packing of [Ca(H2 O)4 (An)2 ] viewed down the b-axis showing the stacking of the layers along the c-axis. The origin is the lower, left, rear corner; +x is up, +y is out, and +z is to the right.
Fig. 8. Crystal packing of [Sr(An)2 ] viewed down the b-axis, showing the stacking of the layers along the a-axis. The origin is the lower, left, rear corner; +x is up, +y is out, and +z is to the right. Fig. 6. Thermal ellipsoid plot (50% probability) of [Sr(An)2 ]. Upper case letters included in the atomic labels correspond to the following symmetry operators: (A) 1 − x, y, 3/2 − z; (B) x, y + 1, z; (C) 1 − x, 1 + y, 3/2 − z; (D) 1 − x, 1 − y, 1 − z; (E) x, 1 − y, 1/2 + z; (F) x, y − 1, z.
tions (x, 3/4, z) are linked through the latter interactions to form a polar two-dimensional layer in the ac-plane (Fig. 10). The layers stack along the b-axis with adjacent layers related by inversion symmetry (0, 1/2, 0) to reverse the polarity of the layer (Fig. 11). As opposed to the strontium derivative, the vacancy in the layer as compared to the alkali metal salt is occupied, at least to some extent, by the coordinated water molecule. Strangely, there are no hydrogen bonding interactions involving this water molecule.
4. Conclusions Despite major differences in composition and connectivity, the alkaline earth salts of An− all exhibit fluorine segregation resulting in aggregation of the structure into hydrophilic and hydrophobic regions. The two lightest members, magnesium and calcium, are heavily hydrated and hydrogen bonding is the dominant packing interaction. For the heavier members, strontium and barium, strong cation· · ·anion interactions between the metal ions and sulfonyl oxygen atoms provide the dominant packing vector, the large barium salt forming as a monohydrate in which the water serves mainly to fill space. As previously observed, the conformation of the An− ion is cisoid for all of the salts
L. Xue et al. / Solid State Sciences 7 (2005) 311–318
Fig. 9. Thermal ellipsoid plot (30% probability) of [Ba(H2 O)(An)2 ]. Upper case letters included in the atomic labels correspond to the following symmetry operators: (A) −1/2 + x, 3/2 + y, −1/2 + z; (B) 1/2 + x, 3/2 + y, −1/2 + z; (C) −1/2 + x, 3/2 + y, 1/2 + z; (D) 1/2 + x, 3/2 + y, 1/2 + z.
317
Fig. 11. Crystal packing of [Ba(H2 O)(An)2 ] viewed down the a-axis, showing the stacking of the layers along the b-axis. The origin is the upper, left, rear corner; +x is out, +y is down, and +z is to the right.
(H2 O)6 ](An)2 ·2H2 O), CCDC 247560 ([Ca(H2 O)4 (An)2 ]), CCDC 247561 ([Sr(An)2 ]), CCDC 247562 ([Ba(H2 O)(An)2 ]). Copies may be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (http://www.ccdc.cam.ac.uk/conts/retrieving).
Acknowledgement The National Science Foundation is gratefully acknowledged for financial support of this project (CHE-0109377) and for assistance in the purchase of X-ray instrumentation (CHE-9207230).
References Fig. 10. View of one polar layer of [Ba(H2 O)(An)2 ].
in which it is coordinated to the metal, and is transoid only for the magnesium salt, in which the cation is fully hydrated. Continuing investigations are needed to understand the reasons for this behavior, and these will explore metal centers of various charges and of diverse degrees of hard/soft character.
Supplementary material Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre: CCDC 247559 ([Mg-
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