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The ionothermal synthesis of metal organic frameworks, Ln(C9O6H3)((CH3NH)2CO)2, using deep eutectic solvents
Solid State Sciences 12 (2010) 418–421
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
The ionothermal synthesis of metal organic frameworks, Ln(C9O6H3)((CH3NH)2CO)2, using deep eutectic solvents Farida Himeur, Irene Stein, David S. Wragg, Alexandra M.Z. Slawin, Philip Lightfoot, Russell E. Morris* EaStChem School of Chemistry, University of St. Andrews, Purdie Building, St. Andrews KY16 9ST, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 January 2009 Received in revised form 4 May 2009 Accepted 21 May 2009 Available online 29 May 2009
Three new isostructural materials Ln(TMA)(DMU)2 (Ln(C9O6H3)((CH3NH)2CO)2; Ln: La 1, Nd 2, Eu 3; TMA: trimesate, DMU: dimethylurea) have been synthesised ionothermally using a choline chloride/dimethylurea deep eutectic mixture as the solvent. Normally in ionothermal synthesis the urea portion of the deep eutectic solvent is unstable, breaking down to release ammonium cations that act as templates. In the case of 1–3, however, the dimethylurea remains intact and is incorporated into the final structure. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Metal organic frameworks Ionothermal synthesis Deep eutectic solvents
1. Introduction Because of their attractive features, ionic liquids [1] now rank as a major research topic in fields as diverse as organic synthesis, electrochemistry, nanotechnology and more recently in materials science. In particular, over the past five years or so, much effort has been directed towards the use of these ionic solvents as water or volatile organic compounds (VOCs) replacements and in many cases structure-directing agents in materials synthesis, especially in the synthesis of zeolites [2], and transition metal organic frameworks [3]. Lanthanide compounds are widely studied due to their significance in biological and material chemistry. As luminescence materials, they have potential commercial applications in electroluminescent displays, fluorescent lighting, X-ray imaging, scintillators, fiber-optic amplifiers and solid state lasers as well as fluorescence tags of biological molecules [4]. Among these compounds the lanthanide-organic frameworks are of special interest owing to their unique topological structures and fascinating chemical/physical properties [5]. Due to the high coordination number and more variable nature of their coordination sphere, as well as their interesting magnetic and luminescent properties, they have been applied in fields such of porosity, luminescence, magnetism and catalytic activity [5–10]. Therefore, many
* Corresponding author. Tel.: þ44 1334 463818; fax: þ44 1334 463808. E-mail address: [email protected] (R.E. Morris). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.05.023
spectacular lanthanide-organic frameworks have been well documented recently. Although, they have been widely reported, there has not been – to the best of our knowledge – any preparation of such materials under ionothermal synthesis [11] where the ionic liquid acts as solvent and template. Indeed such technique presents many advantages over the traditional solvo/hydrothermal synthesis [11]. In addition, metal organic frameworks have many other uses, most famously in gas adsorption and storage applications [12]. In this paper we report the use of deep eutectic solvents (DESs) [13] as the reaction media for the synthesis of lanthanide-organic frameworks. DESs are a mixture of two compounds where there is a depression in the freezing point of the mixture compared with that of the separate compounds [13,14]. They can be formed between a variety of quaternary ammonium halide salts and a wide range of hydrogen bond donors such as amides, acids, amines and alcohols. In addition of having the exceptional features of ionic liquids, DESs exhibit some more significant advantages, such as relatively high polarity so they can dissolve many metal salts and metal oxides, their trivial preparation from easily available components and their relative unreactivity towards atmospheric moisture [14]. They can also be regarded as eco-friendly as they do not need any further purification, which is an important step in the preparation of most other types of ionic liquids. This step generally requires a large amount of organic solvents, raising the argument of how ‘‘green’’ these solvents really are. Previous reports on the synthesis of zeolitic materials [15], organophosphate compounds [16] and transition metal organic
F. Himeur et al. / Solid State Sciences 12 (2010) 418–421
frameworks [17] using choline chloride mixed with urea (and its derivatives) or carboxylic acids demonstrated that DESs can act as unusual reaction media, serving as template delivery agents in a controlled manner and can as well be used for the synthesis of materials that can not be prepared using other standard techniques. However, this invariably involves the breakdown of the urea or urea derivative in the DES under ionothermal conditions. Herein we report the ionothermal synthesis of three novel materials Ln(TMA)(DMU)2 (Ln: La 1, Nd 2, Eu 3) (TMA-H3 stands for trimesic acid, DMU for 1,3-dimethylurea) in a deep eutectic mixture of hydroxy-ethyltrimethylammonium (choline) chloride with 1,3dimethylurea. The important feature of the work is that for the first time we show that the 1,3-dimethylurea can not only act as a template delivery agent through its breakdown, but also it can be occluded into the final material as it is, opening up even more potential for use of this type of DES. 2. Experimental 2.1. Materials and methods All reagents were commercially available and used without further purification. The CHN analyses were performed on a Carlo Erba EA1110 CHNS Analyser. In addition, phase purity was confirmed by comparison of powder X-ray diffraction patterns simulated on the basis of the observed crystal structures to those measured experimentally on a Stoe STADI-P powder diffractometer, using X-rays Cu Ka radiation (1.5406 Å). 2.2. General synthesis The preparation of 1, 2 and 3 was conducted via ionothermal synthesis. The appropriate Ln(III) salt, TMA-H3, choline chloride and DMU were placed into a 23 ml Teflon lined autoclave in the molar ratios of (1:1:5:10). Exact amounts and the particular lanthanide salts used are given in Table 1. The reaction vessel was then sealed and placed in 110 C oven for six days for 1 and 2 and five days for 3. For 2 the addition of 0.05 ml of HF (48% in water) increases the crystallinity of the product. Upon cooling to room temperature, the products were filtered, washed with methanol and dried in the air for 24 h to give colourless prism-like crystals for 1 and 3 and light pink prism-like crystals for 2. Elemental analysis, together with comparison of the observed and simulated powder X-ray diffraction patterns, confirmed phase purity in each case (for 1: Calcd. C, 34.48; H, 3.64; N, 10.72%. Found: C, 34.38; H, 3.28; N, 10.21%; for 2: Calcd. C, 34.14; H, 3.60; N, 10.62%. Found: C, 33.76; H, 3.31; N, 10.12%; for 3: Calcd. C, 33.64; H, 3.55; N, 10.70%. Found: C, 33.80; H, 3.43; N, 10.70%). 2.3. Crystallography Compound 3 was the first material to be made with single crystals suitable for full X-ray structure determination. Compounds 1 and 2 were confirmed as isostructural with compound 3 using powder X-ray diffraction. Single crystal X-ray diffraction data for 3
Table 1 The lanthanide salts used and the exact amounts of other reagents added in the synthesis of compounds 1, 2 and 3.
1 2 3
Ln(III) salt used
Ln(III) salt g (mol)
Trimesic acid g (mol)
Cholinechloride g (mol)
La(NO3)3$6H2O NdCl3$6H2O Eu(NO3)3$5H2O
0.43 (1 103) 0.36 (1 103) 0.43 (1 103)
0.21 (1 103) 0.21 (1 103) 0.21 (1 103)
0.70 (5 103) 0.70 (5 103) 0.70 (5 103)
419
were collected using Mo Ka (0.7107 Å) radiation utilising a Rigaku rotating anode single-crystal X-ray diffractometer at the University of St. Andrews. The structure was solved with standard direct methods using SHELXS and refined with least-squares minimisation techniques against F2 using SHELXL under WinGX packages. All non-hydrogen atoms have been anisotropically refined. Hydrogen atoms for the methyl group and the N–H were fixed using, respectively, the SHELXL commands HFIX 137 and HFIX43. Full details of the structure determination of compound 3 can be found in Supplementary data. Crystallographic data and structural refinements are summarized in Table 2. 2.4. Structure description Single crystal and powder diffraction studies of the three materials reveal that compounds 1–3 are isomorphous and crystallise in the triclinic space group P1 (Z ¼ 2). Only the structure of compound 3 will be discussed in detail as an example. The asymmetric unit of compound 3 is composed of one Eu(III) ion, one TMA and two DMU molecules. The coordination environment around each central atom can be regarded as a distorted square antiprism, made up from two carboxylate groups from two different bidentate chelating TMA, two carboxylate oxygen atoms from two different bridging TMA and two DMU oxygens totally describing an eight coordination as illustrated in Fig. 1. The Eu–O distances range from 2.284(8) to 2.526(8) Å. In the structure, two of the COO groups of TMA are acting as bidentate ligands to chelate two different europium ions, the remaining COO group is acting as a bridging group between two further europium ions. Thus one TMA is connecting four different europium ions, the coordination behaviour of TMA is presented in Fig. 2. The structure consists of an infinite network spreading along the axis a, where the TMA acts as the linking agent between the neighbouring Eu atoms. Furthermore, one can observe that the basic constituting unit is the centrosymmetric dimer, consisting of two Eu atoms with two TMA acting as bridges between them (Fig. 3). The distance between two Eu atoms belonging to the same dimer is 5.295(2) Å. The infinite network is assembled by the coordination of the free carboxylate groups of the dimer to some next Eu atoms. DMU contains both an efficient coordination site and two hydrogen-bonding functionalities. Interestingly, in 3 the DMU is directly coordinated to the europium through the oxygen atom and also provides a hydrogen bonding donor site where all the NH
Table 2 Unit cell and space group data for compounds 1, 2 and 3, together with final refinement details for compound 3. Compound Formula FW Space group a/Å b/Å c/Å a/Å b/Å g/Å V/Å3 Z Crystal size/mm F(000) Reflections collected Rint GOOF on F2 R1, wR2 (I > 2s(I)) R1, wR2 (all data)
1 LaTMA(DMU)2 522.24 Pı 9.4558(4) 10.8193(7) 11.1193(6) 74.432(16) 65.817(14) 66.449(14) 953.14(82) 2
2 NdTMA(DMU)2 527.6 Pı 9.729(3) 11.0461(52) 11.3065(90) 78.88(13) 70.69(10) 64.705(91) 1034.92(10) 2
3 EuTMA(DMU)2 535.31 Pı 9.5970(40) 10.3970(40) 10.9840(40) 74.017(3) 65.569(3) 66.357(3) 906.18(18) 2 0.03 0.03 0.03 527.9 3307 0.0645 1.131 0.0657, 0.159 0.085, 0.173
420
F. Himeur et al. / Solid State Sciences 12 (2010) 418–421
Fig. 1. Coordination sphere of the metal ion in 3, for clarity oxygen atoms belonging to the DMU molecules are in black.
groups participate in H bonding. So a 3D supramolecular framework is achieved by the formation of hydrogen bridges between the DMU molecules and the nearest carboxylic groups of two separate dimers. An interesting feature of the present structure is the formation of distinct layers containing either the europium atoms or the DMU molecules, resulting from the presence of both the above-mentioned carboxylic bridges and hydrogen bonds (Fig. 4). In the vast majority of synthetic procedures using urea-containing DESs as the solvent in ionothermal synthesis, including the preparations of both zeolites and metal organic frameworks, the urea component of the DES has broken down to ammonium or alkyl ammonium cations (depending on the exact nature of the urea derivative used). These ammonium compounds are excellent structure directing agents and are almost always incorporated into the final materials. In the synthesis of 1–3, however, the occluded compound is the intact DMU molecule. Of course, this does not preclude the possibility of there being some breakdown of the urea component under the synthesis conditions, but it does indicate that the urea itself can also act as a ligand in such ionothermal preparations. This opens up new possibilities for the use of urea components of DESs in ionothermal synthesis that have not been seen before.
Fig. 2. Coordination behaviour of TMA in 3.
Fig. 3. Structure of 3 viewed along the a axis, DMU molecules have been omitted for clarity.
The three compounds are all prepared in the presence of a small amount of water, emanating from the lanthanide salts used and, in the case of 2, the aqueous HF. In ionothermal synthesis water can act as a very important reactant, increasing the rate of ionothermal reactions [11c] and improving the crystallinity of the products. Similarly, fluoride ions are extremely well-known mineralisers in both hydrothermal [18,19] and ionothermal reactions [11], again increasing the rate of reaction and improving crystallinity of the products. However, when using DESs as the solvent the addition of too much water or fluoride increases the likelihood of breakdown of the urea component [11]. Perhaps the most important feature of ionothermal synthesis is that the change in chemistry from molecular to ionic solvents very often results in a change in the product. For example, the use of ionic liquids as solvent reduces the tendency for water to be
Fig. 4. Structure of 3 viewed along the c axis showing the distinct layers of the europium atoms and the DMU (the hydrogen bonds in broken lines). The circle in the top right hand corner picks out the intact DMU that is coordinated to the europium.
F. Himeur et al. / Solid State Sciences 12 (2010) 418–421
accommodated into the resultant structure, even when there is a small amount of water present in the reaction mixture, as is the case in the synthesis of 1–3 reported here. The strong binding of isolated water molecules in ionic liquids such as DESs leads to another interesting effect that can be used in ionothermal synthesis – so called ‘water deactivation’. At low concentrations of water this strong hydrogen bonding leads to water being less reactive than similar amounts in other solvents. This effect is so strong that highly hydrolytically sensitive compounds such as PCl3 can be stored for relatively long periods, whereas they react quickly, and often violently, in other ‘wet’ solvents [20]. Such water deactivation is probably the reason why some of the materials prepared ionothermally can have unusual features. For instance SIZ-13, a cobalt aluminophosphate material has a layered structure that is closely related to a zeolite, but has Co–Cl bonds. Normally such bonds are hydrolytically unstable and under hydrothermal conditions it is unlikely that this material would be stable [2f]. In the synthesis of compounds 1–3 it may be that a similar effect leads to the urea component remaining intact (or at least not breaking down completely) so that it can be occluded in the final materials.
[3]
[4] [5] [6]
[7]
[8]
3. Conclusion The present work adds another dimension to the diversity of the ionothermal synthesis, as we have presented here the first lanthanide-organic frameworks synthesised ionothermally, this results arising from our studies in the aim of extending this method of synthesis. The reactions of the appropriate lanthanide salt and TMA-H3 in a deep eutectic mixture of hydroxy-ethyltrimethylammonium (choline) chloride with 1,3-dimethylurea have afforded single crystals of Ln(TMA)(DMU)2 (Ln: La 1, Nd 2, Eu 3). Of particular interest is the fact that the urea derivative used in the DES is incorporated into the material intact, unlike the vast majority of previous ionothermal syntheses where it breaks down to provide an ammonium-based cation. This opens new possibilities for ionothermal synthesis beyond what has been possible previously.
[9]
[10]
[11]
[12] [13] [14]
Appendix. Supplementary data Supplementary data associated with this article can be found in the online version, at doi:10.1016/j.solidstatesciences.2009.05.023. References [1] J.S. Wilkes, Green Chem. 4 (2002) 73. [2] (a) A. Taubert, Z. Li, Dalton Trans. (2007) 723; (b) E.R. Parnham, R.E. Morris, J. Mater. Chem. 16 (2006) 3682; (c) E.R. Parnham, R.E. Morris, Chem. Mater. 18 (2006) 4882;
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Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
The ionothermal synthesis of metal organic frameworks, Ln(C9O6H3)((CH3NH)2CO)2, using deep eutectic solvents Farida Himeur, Irene Stein, David S. Wragg, Alexandra M.Z. Slawin, Philip Lightfoot, Russell E. Morris* EaStChem School of Chemistry, University of St. Andrews, Purdie Building, St. Andrews KY16 9ST, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 January 2009 Received in revised form 4 May 2009 Accepted 21 May 2009 Available online 29 May 2009
Three new isostructural materials Ln(TMA)(DMU)2 (Ln(C9O6H3)((CH3NH)2CO)2; Ln: La 1, Nd 2, Eu 3; TMA: trimesate, DMU: dimethylurea) have been synthesised ionothermally using a choline chloride/dimethylurea deep eutectic mixture as the solvent. Normally in ionothermal synthesis the urea portion of the deep eutectic solvent is unstable, breaking down to release ammonium cations that act as templates. In the case of 1–3, however, the dimethylurea remains intact and is incorporated into the final structure. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Metal organic frameworks Ionothermal synthesis Deep eutectic solvents
1. Introduction Because of their attractive features, ionic liquids [1] now rank as a major research topic in fields as diverse as organic synthesis, electrochemistry, nanotechnology and more recently in materials science. In particular, over the past five years or so, much effort has been directed towards the use of these ionic solvents as water or volatile organic compounds (VOCs) replacements and in many cases structure-directing agents in materials synthesis, especially in the synthesis of zeolites [2], and transition metal organic frameworks [3]. Lanthanide compounds are widely studied due to their significance in biological and material chemistry. As luminescence materials, they have potential commercial applications in electroluminescent displays, fluorescent lighting, X-ray imaging, scintillators, fiber-optic amplifiers and solid state lasers as well as fluorescence tags of biological molecules [4]. Among these compounds the lanthanide-organic frameworks are of special interest owing to their unique topological structures and fascinating chemical/physical properties [5]. Due to the high coordination number and more variable nature of their coordination sphere, as well as their interesting magnetic and luminescent properties, they have been applied in fields such of porosity, luminescence, magnetism and catalytic activity [5–10]. Therefore, many
* Corresponding author. Tel.: þ44 1334 463818; fax: þ44 1334 463808. E-mail address: [email protected] (R.E. Morris). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.05.023
spectacular lanthanide-organic frameworks have been well documented recently. Although, they have been widely reported, there has not been – to the best of our knowledge – any preparation of such materials under ionothermal synthesis [11] where the ionic liquid acts as solvent and template. Indeed such technique presents many advantages over the traditional solvo/hydrothermal synthesis [11]. In addition, metal organic frameworks have many other uses, most famously in gas adsorption and storage applications [12]. In this paper we report the use of deep eutectic solvents (DESs) [13] as the reaction media for the synthesis of lanthanide-organic frameworks. DESs are a mixture of two compounds where there is a depression in the freezing point of the mixture compared with that of the separate compounds [13,14]. They can be formed between a variety of quaternary ammonium halide salts and a wide range of hydrogen bond donors such as amides, acids, amines and alcohols. In addition of having the exceptional features of ionic liquids, DESs exhibit some more significant advantages, such as relatively high polarity so they can dissolve many metal salts and metal oxides, their trivial preparation from easily available components and their relative unreactivity towards atmospheric moisture [14]. They can also be regarded as eco-friendly as they do not need any further purification, which is an important step in the preparation of most other types of ionic liquids. This step generally requires a large amount of organic solvents, raising the argument of how ‘‘green’’ these solvents really are. Previous reports on the synthesis of zeolitic materials [15], organophosphate compounds [16] and transition metal organic
F. Himeur et al. / Solid State Sciences 12 (2010) 418–421
frameworks [17] using choline chloride mixed with urea (and its derivatives) or carboxylic acids demonstrated that DESs can act as unusual reaction media, serving as template delivery agents in a controlled manner and can as well be used for the synthesis of materials that can not be prepared using other standard techniques. However, this invariably involves the breakdown of the urea or urea derivative in the DES under ionothermal conditions. Herein we report the ionothermal synthesis of three novel materials Ln(TMA)(DMU)2 (Ln: La 1, Nd 2, Eu 3) (TMA-H3 stands for trimesic acid, DMU for 1,3-dimethylurea) in a deep eutectic mixture of hydroxy-ethyltrimethylammonium (choline) chloride with 1,3dimethylurea. The important feature of the work is that for the first time we show that the 1,3-dimethylurea can not only act as a template delivery agent through its breakdown, but also it can be occluded into the final material as it is, opening up even more potential for use of this type of DES. 2. Experimental 2.1. Materials and methods All reagents were commercially available and used without further purification. The CHN analyses were performed on a Carlo Erba EA1110 CHNS Analyser. In addition, phase purity was confirmed by comparison of powder X-ray diffraction patterns simulated on the basis of the observed crystal structures to those measured experimentally on a Stoe STADI-P powder diffractometer, using X-rays Cu Ka radiation (1.5406 Å). 2.2. General synthesis The preparation of 1, 2 and 3 was conducted via ionothermal synthesis. The appropriate Ln(III) salt, TMA-H3, choline chloride and DMU were placed into a 23 ml Teflon lined autoclave in the molar ratios of (1:1:5:10). Exact amounts and the particular lanthanide salts used are given in Table 1. The reaction vessel was then sealed and placed in 110 C oven for six days for 1 and 2 and five days for 3. For 2 the addition of 0.05 ml of HF (48% in water) increases the crystallinity of the product. Upon cooling to room temperature, the products were filtered, washed with methanol and dried in the air for 24 h to give colourless prism-like crystals for 1 and 3 and light pink prism-like crystals for 2. Elemental analysis, together with comparison of the observed and simulated powder X-ray diffraction patterns, confirmed phase purity in each case (for 1: Calcd. C, 34.48; H, 3.64; N, 10.72%. Found: C, 34.38; H, 3.28; N, 10.21%; for 2: Calcd. C, 34.14; H, 3.60; N, 10.62%. Found: C, 33.76; H, 3.31; N, 10.12%; for 3: Calcd. C, 33.64; H, 3.55; N, 10.70%. Found: C, 33.80; H, 3.43; N, 10.70%). 2.3. Crystallography Compound 3 was the first material to be made with single crystals suitable for full X-ray structure determination. Compounds 1 and 2 were confirmed as isostructural with compound 3 using powder X-ray diffraction. Single crystal X-ray diffraction data for 3
Table 1 The lanthanide salts used and the exact amounts of other reagents added in the synthesis of compounds 1, 2 and 3.
1 2 3
Ln(III) salt used
Ln(III) salt g (mol)
Trimesic acid g (mol)
Cholinechloride g (mol)
La(NO3)3$6H2O NdCl3$6H2O Eu(NO3)3$5H2O
0.43 (1 103) 0.36 (1 103) 0.43 (1 103)
0.21 (1 103) 0.21 (1 103) 0.21 (1 103)
0.70 (5 103) 0.70 (5 103) 0.70 (5 103)
419
were collected using Mo Ka (0.7107 Å) radiation utilising a Rigaku rotating anode single-crystal X-ray diffractometer at the University of St. Andrews. The structure was solved with standard direct methods using SHELXS and refined with least-squares minimisation techniques against F2 using SHELXL under WinGX packages. All non-hydrogen atoms have been anisotropically refined. Hydrogen atoms for the methyl group and the N–H were fixed using, respectively, the SHELXL commands HFIX 137 and HFIX43. Full details of the structure determination of compound 3 can be found in Supplementary data. Crystallographic data and structural refinements are summarized in Table 2. 2.4. Structure description Single crystal and powder diffraction studies of the three materials reveal that compounds 1–3 are isomorphous and crystallise in the triclinic space group P1 (Z ¼ 2). Only the structure of compound 3 will be discussed in detail as an example. The asymmetric unit of compound 3 is composed of one Eu(III) ion, one TMA and two DMU molecules. The coordination environment around each central atom can be regarded as a distorted square antiprism, made up from two carboxylate groups from two different bidentate chelating TMA, two carboxylate oxygen atoms from two different bridging TMA and two DMU oxygens totally describing an eight coordination as illustrated in Fig. 1. The Eu–O distances range from 2.284(8) to 2.526(8) Å. In the structure, two of the COO groups of TMA are acting as bidentate ligands to chelate two different europium ions, the remaining COO group is acting as a bridging group between two further europium ions. Thus one TMA is connecting four different europium ions, the coordination behaviour of TMA is presented in Fig. 2. The structure consists of an infinite network spreading along the axis a, where the TMA acts as the linking agent between the neighbouring Eu atoms. Furthermore, one can observe that the basic constituting unit is the centrosymmetric dimer, consisting of two Eu atoms with two TMA acting as bridges between them (Fig. 3). The distance between two Eu atoms belonging to the same dimer is 5.295(2) Å. The infinite network is assembled by the coordination of the free carboxylate groups of the dimer to some next Eu atoms. DMU contains both an efficient coordination site and two hydrogen-bonding functionalities. Interestingly, in 3 the DMU is directly coordinated to the europium through the oxygen atom and also provides a hydrogen bonding donor site where all the NH
Table 2 Unit cell and space group data for compounds 1, 2 and 3, together with final refinement details for compound 3. Compound Formula FW Space group a/Å b/Å c/Å a/Å b/Å g/Å V/Å3 Z Crystal size/mm F(000) Reflections collected Rint GOOF on F2 R1, wR2 (I > 2s(I)) R1, wR2 (all data)
1 LaTMA(DMU)2 522.24 Pı 9.4558(4) 10.8193(7) 11.1193(6) 74.432(16) 65.817(14) 66.449(14) 953.14(82) 2
2 NdTMA(DMU)2 527.6 Pı 9.729(3) 11.0461(52) 11.3065(90) 78.88(13) 70.69(10) 64.705(91) 1034.92(10) 2
3 EuTMA(DMU)2 535.31 Pı 9.5970(40) 10.3970(40) 10.9840(40) 74.017(3) 65.569(3) 66.357(3) 906.18(18) 2 0.03 0.03 0.03 527.9 3307 0.0645 1.131 0.0657, 0.159 0.085, 0.173
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Fig. 1. Coordination sphere of the metal ion in 3, for clarity oxygen atoms belonging to the DMU molecules are in black.
groups participate in H bonding. So a 3D supramolecular framework is achieved by the formation of hydrogen bridges between the DMU molecules and the nearest carboxylic groups of two separate dimers. An interesting feature of the present structure is the formation of distinct layers containing either the europium atoms or the DMU molecules, resulting from the presence of both the above-mentioned carboxylic bridges and hydrogen bonds (Fig. 4). In the vast majority of synthetic procedures using urea-containing DESs as the solvent in ionothermal synthesis, including the preparations of both zeolites and metal organic frameworks, the urea component of the DES has broken down to ammonium or alkyl ammonium cations (depending on the exact nature of the urea derivative used). These ammonium compounds are excellent structure directing agents and are almost always incorporated into the final materials. In the synthesis of 1–3, however, the occluded compound is the intact DMU molecule. Of course, this does not preclude the possibility of there being some breakdown of the urea component under the synthesis conditions, but it does indicate that the urea itself can also act as a ligand in such ionothermal preparations. This opens up new possibilities for the use of urea components of DESs in ionothermal synthesis that have not been seen before.
Fig. 2. Coordination behaviour of TMA in 3.
Fig. 3. Structure of 3 viewed along the a axis, DMU molecules have been omitted for clarity.
The three compounds are all prepared in the presence of a small amount of water, emanating from the lanthanide salts used and, in the case of 2, the aqueous HF. In ionothermal synthesis water can act as a very important reactant, increasing the rate of ionothermal reactions [11c] and improving the crystallinity of the products. Similarly, fluoride ions are extremely well-known mineralisers in both hydrothermal [18,19] and ionothermal reactions [11], again increasing the rate of reaction and improving crystallinity of the products. However, when using DESs as the solvent the addition of too much water or fluoride increases the likelihood of breakdown of the urea component [11]. Perhaps the most important feature of ionothermal synthesis is that the change in chemistry from molecular to ionic solvents very often results in a change in the product. For example, the use of ionic liquids as solvent reduces the tendency for water to be
Fig. 4. Structure of 3 viewed along the c axis showing the distinct layers of the europium atoms and the DMU (the hydrogen bonds in broken lines). The circle in the top right hand corner picks out the intact DMU that is coordinated to the europium.
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accommodated into the resultant structure, even when there is a small amount of water present in the reaction mixture, as is the case in the synthesis of 1–3 reported here. The strong binding of isolated water molecules in ionic liquids such as DESs leads to another interesting effect that can be used in ionothermal synthesis – so called ‘water deactivation’. At low concentrations of water this strong hydrogen bonding leads to water being less reactive than similar amounts in other solvents. This effect is so strong that highly hydrolytically sensitive compounds such as PCl3 can be stored for relatively long periods, whereas they react quickly, and often violently, in other ‘wet’ solvents [20]. Such water deactivation is probably the reason why some of the materials prepared ionothermally can have unusual features. For instance SIZ-13, a cobalt aluminophosphate material has a layered structure that is closely related to a zeolite, but has Co–Cl bonds. Normally such bonds are hydrolytically unstable and under hydrothermal conditions it is unlikely that this material would be stable [2f]. In the synthesis of compounds 1–3 it may be that a similar effect leads to the urea component remaining intact (or at least not breaking down completely) so that it can be occluded in the final materials.
[3]
[4] [5] [6]
[7]
[8]
3. Conclusion The present work adds another dimension to the diversity of the ionothermal synthesis, as we have presented here the first lanthanide-organic frameworks synthesised ionothermally, this results arising from our studies in the aim of extending this method of synthesis. The reactions of the appropriate lanthanide salt and TMA-H3 in a deep eutectic mixture of hydroxy-ethyltrimethylammonium (choline) chloride with 1,3-dimethylurea have afforded single crystals of Ln(TMA)(DMU)2 (Ln: La 1, Nd 2, Eu 3). Of particular interest is the fact that the urea derivative used in the DES is incorporated into the material intact, unlike the vast majority of previous ionothermal syntheses where it breaks down to provide an ammonium-based cation. This opens new possibilities for ionothermal synthesis beyond what has been possible previously.
[9]
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