CRYSTAL GROWTH & DESIGN
Supramolecular Organization in Lead Bromide Salts of Imidazolium-Based Ionic Liquids
2008 VOL. 8, NO. 5 1640–1644
A. Thirumurugan and C. N. R. Rao* Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for AdVanced Scientific Research, Jakkur P. O., Bangalore 560064, India ReceiVed NoVember 3, 2007; ReVised Manuscript ReceiVed December 6, 2007
ABSTRACT: By heating imidazolium bromide-based ionic liquids with lead(II) salts under ionothermal conditions, we have been able to obtain two imidazolium bromoplumbates, (EMIm)PbBr3, I, and (BMIm)2PbBr4, II, where EMIm and BMIm stand for 1-ethyl3-methyl and 1-butyl-3-methyl imidazolium cations respectively. Interestingly, both these compounds exhibit unusual supramolecular organization wherein dialkylimidazolium cations arrange themselves in a cylindrical fashion giving rise to channel structures. The bromoplumbate anions reside in the channels. The lead(II) cation exhibits either hemi- or holodirected coordination geometry. Introduction Ionic liquids (ILs) are salts with low melting points ( 2σ(I)] Rmerg goodness of fit R indexes [I > 2σ(I)] R indexes [all data]
C6H11N2PbBr3 558.07 orthorhombic P212121 (No. 19) 8.2131(7) 9.7717(8) 15.4345(13) 1238.71(18) 4 2.993 23.255 28200 2297 1995 0.0335 1.016 R1 ) 0.0282a wR2 ) 0.0563b R1 ) 0.0399a wR2 ) 0.0600b
C16H30N4PbBr4 805.24 trigonal R3j (No. 148) 16.6041(2) 16.6041(2) 24.1297(5) 5761.20(15) 3 2.089 12.838 15008 2391 1984 0.0233 1.021 R1 ) 0.0249a wR2 ) 0.0605b R1 ) 0.0348a wR2 ) 0.0642b
a R1 ) Σ|F0| - |Fc|/Σ|F0|. b wR2 ) {Σ[w(F02 - Fc2)2]/Σ[w(F02)2]}1/2. w ) 1/[σ2(F0)2 + (aP)2 + bP], P ) [max(F02,0) + 2(Fc)2]/3, where a ) 0.0247, b ) 4.1206 for I and a ) 0.0344, b ) 12.3057 for II.
Table 2. Selected Bond Distances in (EMIm)PbBr3, Ia bond
distance (Å)
Pb(1)-Br(1) Pb(1)-Br(2) Pb(1)-Br(2)#1 Pb(1)-Br(3) Pb(1)-Br(3)#2
2.789(1) 2.886(1) 3.219(1) 2.9748(9) 3.1177(9)
a Symmetry transformations used to generate equivalent atoms: #1 -0.5 + x, 0.5 - y, 1 - z, #2 0.5 + x, 0.5 - y, 1 - z.
Fine, India, 98.5%), methanol (Rankem, India, 99%), NaOH (Merck, India, 99%), 1-ethyl-3-methyl imidazolium bromide (EMImBr) (Aldrich, g98%), 1-butyl-3-methyl imidazolium bromide (BMImBr) (Aldrich, >97%), and of high purity double distilled water were used for the synthesis. The imidazolium bromoplumbates I and II were synthesized under ionothermal conditions by heating homogenized reaction mixtures in a 7 mL PTFE-lined bomb at the 180 °C temperature for 72 h under autogenous pressure. For the synthesis of I, the reaction mixture of the following composition was used. Pb(NO3)2 (0.3 mM), 1,3-H2CHDC (0.3 mM), NaOH (0.3 mM), methanol (2 mL), and 0.5 g of EMImBr. For the synthesis of II, the reaction mixture of the following composition was used. Pb(NO3)2 (0.3 mM), 1,3-H2BDC (0.15 mM), NaOH (0.3 mM), methanol (2 mL), and 0.5g of BMImBr. The change
10.1021/cg7010875 CCC: $40.75 2008 American Chemical Society Published on Web 04/08/2008
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Crystal Growth & Design, Vol. 8, No. 5, 2008 1641
Table 3. Hydrogen-Bonding Interactions in I and II compound
D-H · · · A
distance (Å) D-H
distance (Å) H · · · A
distance (Å) D · · · A
angle (°) D-H · · · A
I
C(4)-H(4b) · · · Br(2) C(4)-H(6a) · · · Br(1) C(4)-H(4c) · · · Br(2) C(2)-H(2) · · · Br(4)
0.9600 0.9600 0.9600 0.9600
2.896(2) 2.975(6) 2.942(5) 2.842(3)
3.781(4) 3.773(4) 3.813(5) 3.738(8)
153.7 141.9 151.4 161.6
II
of the starting compositions or omission of either carboxylic acid or NaOH did not yield any crystals but resulted in powders or gel-like products. The pH of the starting reaction mixture was 6-6.5. The pH after the reaction did not show appreciable change. The products of the ionothermal reaction were washed with methanol and ethanol without any residual carboxylic acid, vacuum-filtered, and dried under ambient conditions. Both compounds were obtained as single phase materials. The crystals were separated under a polarizing microscope and used for all the characterization. Elemental analyses of I and II were satisfactory. For I, (C6H11N2PbBr3) calcd: C, 12.90%; H, 1.97%; N, 5.02%. Found: C, 12.86%; H, 2.13%; N, 5.23%. For II, (C16H30N4PbBr4) calcd: C, 23.84%; H, 3.73%; N, 6.95%. Found: C, 24.16%; H, 4.02%; N, 6.72%. Powder X-ray diffraction (XRD) patterns of the products were recorded using Cu KR radiation (Rich-Seifert, 3000TT). The patterns agreed with those calculated for single crystal structure determination. Thermogravimetric analysis (TGA) was carried out (Metler-Toledo) in air (flow rate ) 50 mL/min) in the temperature range 25-800 °C (heating rate ) 5 °C/min). Infrared (IR) spectra of KBr pellets of the compounds were recorded in the mid-IR region (Bruker IFS-66v). The compounds show characteristic bands of the imidazole ring and the alkyl chains.9 Thermogravimetric analyses gave the following results. Both the compounds show weight loss at two temperature ranges. For I, the first weight loss of 26.42% occurred in the 280-380 °C range and the second weight loss of 65.12% occurred in the 380-700 °C range. For II, the first weight loss of 44.58% occurred in the 260-370 °C range and the second weight loss of 48.50% occurred in the 370-700 °C range. The first step in the weight loss is attributed to the loss of organic cations and the second to the decomposition of the bromoplumbate. Differential thermal analysis (DTA) of I shows an endothermic peak around 209 °C (melting point) and a small exothermic peak around 400 °C. II shows an endothermic peak around 138 °C (melting point) and an exothermic peak around 375 °C. A suitable single crystal of each compound was carefully selected under a polarizing microscope and glued to a thin glass fiber. Crystal structure determination by X-ray diffraction was performed on a BrukerNonius diffractometer with Kappa geometry attached with an APEXII-CCD detector and a graphite monochromator for the X-ray source (Mo KR radiation, λ ) 0.71073Å) operating at 50 kV and 30 mA. An empirical absorption correction based on symmetry equivalent reflections was applied using the SADABS program.10a The structure was solved and refined using SHELXTL-PLUS suite of program.10b For the final refinement the hydrogen atoms were placed geometrically and held in the riding mode. Final refinement included atomic positions for all the atoms, anisotropic thermal parameters for all the nonhydrogen atoms and isotropic thermal parameters for the hydrogen atoms. One of the carbon atoms (C6) in the butyl chain of the BMIm+ cation is disordered, so the C-C distances in the butyl chain were constrained to 1.50 Å. All the hydrogen atoms were included in the final refinement, and details of the structure solution and final refinements and selected bond distances of I and II are given in Tables 1, 2, and 3.
Pb atom is hemidirected and five-coordinated by bromine atoms (PbBr5) from three different types of Br- anions. The Pb-Br bond lengths are in the 2.789-3.219 Å range (Table 2), and all the bond distances are within the van der Waals contact limits. Four of the bromine anions have µ2 connections linking each Pb with two other Pb atoms. Thus, a PbBr5 polyhedron shares two of its edges with two different PbBr5 polyhedra forming an infinite 1-D Pb-Br-Pb chain (Figure 1a). EMIm+ cations exhibit weak C-H · · · Br interactions through the alkyl groups in the imidazolium ring, with three different Br- anions (Figure 1b, Table 3). These weak interactions lead to the formation of channels with the dimension ∼6.8 × 7.8 Å2, where the assembly of EMIm+ anions serves as walls. The channels are occupied with the infinite PbBr3 chains and surrounded by the EMIm+ cations (Figure 2). The structure is further stabilized by the alkyl-alkyl interactions between two neighboring EMIm+ cations. Bis(1-butyl-3-methyl imidazolium) tetrabromoplumbate, (BMIm)2PbBr4, II, has an asymmetric unit of 12.5 non-hydrogen atoms. The asymmetric unit contains two crystallographically distinct Pb2+ cations, one EMIm+ cation, and two distinct Branions. Pb(1) has a 1/6 occupancy and sits at the 3b position. Pb(2) has a 1/3 occupancy and sits at the 6c position. Pb(1) is holodirected and coordinated by six bromine atoms of the same type, Br(1), into a perfect octahedron PbBr6. The Pb-Br bond length is 3.021 Å. Pb(2) is also holodirected and coordinated to six bromine atoms of two types, three Br(1) and three Br(2), into a slightly distorted octahedron PbBr6. The Pb(2)-Br(1) bond length is 2.91, and the Pb(2)-Br(2) bond length is 3.227 Å, all within the van der Waals contact limit. All the six Br(2) anions have µ2 connections linking each Pb(1) with two Pb(2) atoms. Thus, a Pb(1)Br6 polyhedron shares its two faces with two different Pb(2)Br6 polyhedra forming a linear Pb3Br12 unit (Figure 3a). Each of the BMIm+ cation is hydrogen-bonded to Br(1) anions through H(2) in the imidazole ring (Figure 3b, Table 3). The BMIm+ anions interact with each other through methyl-methyl and butyl-butyl interactions (Figure 3b). These
Results and Discussion We have synthesized two compounds of imidazolium bromoplumbates, 1-ethyl-3-methyl imidazolium tribromoplumbate, (EMIm)PbBr3, I, and bis(1-butyl-3-methyl imidazolium) tetrabromoplumbate, (BMIm)2PbBr4, II. Both I and II possess structures with one-dimensional (1-D) channels made up of imidazolium cations and filled with the lead bromide anions. 1-Ethyl-3-methyl imidazolium tribromoplumbate, (EMIm)PbBr3, I, has an asymmetric unit of 12 non-hydrogen atoms. The asymmetric unit contains a crystallographically distinct Pb2+ cation, one EMIm+ cation, and three distinct Br- anions. The
Figure 1. (a) Infinite 1-D of Pb-Br-Pb chain in (EMIm)PbBr3, I. The dotted lines show H-bonding interactions between the cations and anions; (b) view of the C-H · · · Br interactions between the EMIm+ cations and Br- anions in I.
1642 Crystal Growth & Design, Vol. 8, No. 5, 2008
Figure 2. (a) Crystal structure packing and H-bonding interactions in (EMIm)PbBr3, I, viewed along the a-axis and (b) view of the infinite 1-D empty channels and ethyl-ethyl (cation-cation) interactions along the a-axis.
Figure 3. (a) The Pb3Br12 unit and its H-bonding interactions with the cations in (BMIm)2PbBr4, II and (b) view of the H-bonding interactions (highlighted in red, cation-anion interaction) between BMIm+ cation and Br- anion and alkyl-alkyl interactions (highlighted in blue, cation-cation interaction) between the BMIm+ cations.
interactions lead to the formation of a one-dimensional channel, where the BMIm+ assembly serves as a wall. The channel is
Thirumurugan and Rao
Figure 4. (a) View of a single capsule with the Pb3Br12 unit and its H-bonding interactions with the BMIm+ cations and (b) view of the crystal structure packing in (BMIm)2PbBr4, II, along the c-axis.
made up of capsules or cavities of the dimentions ∼8.5 × 8.0 Å2, and the Pb3Br12 units reside in the cavities, getting hydrogenbonded to the BMIm+ cations (Figure 4). The capsules are connected to form the infinite 1-D channel by a small window of the dimension ∼3.6 Å. This small window results due to the dangling butyl groups of the BMIm+ anions into the 1D channel (Figure 5). The structure is further stabilized by the alkyl-alkyl interactions between two neighboring BMIm+ cations. The difference between the channels in I and II arises due to the longer alkyl group in the latter. I possesses a whole simple rhombic 1-D channel, while II has a capsulized 1-D channel with small and large windows due to the dangling butyl groups. An important conformational feature of the BMIm+ cation is the torsional angle around C(5)-C(6) bond of the butyl chain. This angle is close to 60° for gauche and near 120° for the anti conformation. BMIm+ cations with butyl chains in both the gauche and the anti conformation are reported. In the gauche conformation,7e,11 the butyl chain is twisted towards the imidazolium ring, whereas in the anti conformation,12 it is projected away from the ring. These conformations are crucially affecting the hydrogen bonding and alkyl-alkyl interactions. In II, this angle is 57.88° with a gauche conformation of the twisted butyl chain. Another important feature is the angle between the imidazolium ring and the butyl chain. Butyl chains in most of the BMIm+ cations are directed perpendicular to the ring. Coplanar butyl chains are formed rarely.13 The butyl chain in II makes an interplanar angle of 88.67° with the ring. The melting points of I and II are 209° and 138 °C, respectively, as confirmed from DTA. Both the compounds
Supramolecular Organization in Lead Bromide Salts
Crystal Growth & Design, Vol. 8, No. 5, 2008 1643
with coordination number 5. The Pb2+cations in II exhibit holodirected geometry with coordination number 6. One of the Pb2+ in II (Pb2) with slightly long Pb-Br distances may be considered to be holodirected, with active lone-pairs in the discernible holes of the coordination spheres. Conclusions Ionothermal synthesis has enabled us to obtain two dialkylimidazolium bromoplumbates, (EMIm)PbBr3, I, and (BMIm)2PbBr4, II, derived from the corresponding ionic liquids. These compounds exhibit novel supramolecular architectures wherein the dialkylimidazolium cations organize themselves into channel structures with the bromoplumbate moieties residing in the channels. The difference between the channels of the two structures arises from the presence of the twisted, longer butyl chain with gauche conformation and the participation of ring hydrogen of II in hydrogen bonding interaction. The lead (II) cation in these compounds exhibits either hemi- or holodirected coordination geometry. Note Added after ASAP Publication. A version of this paper was posted ASAP to the web on April 8, 2008, with an incorrect reference 14. This reference has been updated in this new version posted April 22, 2008. Supporting Information Available: A crystallographic information file (CIF) for the compounds I and II is available free of charge via the Internet at http://pubs.acs.org. Figure 5. (a) View of the infinite 1-D empty channels and dangling butyl groups at the small window along the c-axis and (b) view of the schematic of the packing of the 1-D channels with capsules (Pb3Br12 unit in the capsule is highlighted in red), along the a-axis.
crystallize (polycrystalline powder) back to the original structures on cooling the melts. This observation suggests that both the compounds are formed under thermodynamic control rather than kinetic control. I has three H-bonding interactions per cation with three anions through alkyl groups. II has only one H-bonding interaction per cation with a Br- anion through a hydrogen atom from the imidazolium ring. Both I and II are stabilized by alkyl-alkyl interactions. The role of cooperative H-bonding interactions in directing and influencing the electronic properties of hybrid perovskites has been demonstrated in the recent literature.14 The stereochemical activity of lone-pair electrons in Pb2+ cation plays a role in the coordination geometry and thereby crystal engineering.15 The coordination geometry of the PbOn polyhedra in Pb(II) compounds is hemidirected for low coordination numbers (2-5) and holodirected for high coordination numbers (9,10). For intermediate coordination numbers (6-8) either type of stereochemistry is found.15 Ab initio molecular orbital studies of gas-phase Pb(II) complexes show that streochemically active hemidirected geometry occurs if the ligand coordination number is low, the ligands are hard, and there are attractive interactions between the ligands.16 In such cases, the lone pair orbital has a p character and fewer electrons are transferred from the ligands to the bonding orbitals of Pb(II), giving rise to more ionic bonds. Holodirected geometry occurs when the coordination number is high and the ligands are soft and bulky or show strong interligand repulsion. The lone pair orbital has negligible p character when the geometry is holodirected, and the bonds are more covalent than in the hemidirected structures. In the case of holodirected geometry, with weak donors and high Pb-L distances, the lone pair is still considered to be active with discernible holes in the coordination spheres.15a The Pb2+ cations in I exhibit hemidirected geometry
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