ChemComm
Published on 13 May 2014. Downloaded by Universidad de Vigo on 14/05/2014 05:36:25.
COMMUNICATION
Cite this: DOI: 10.1039/c4cc01023j Received 7th February 2014, Accepted 2nd May 2014
View Article Online View Journal
A cobalt(II) bis(salicylate)-based ionic liquid that shows thermoresponsive and selective water coordination† Yuki Kohno,a Matthew G. Cowan,a Miyuki Masuda,b Indrani Bhowmick,c Matthew P. Shores,c Douglas L. Gin*ad and Richard D. Noble*a
DOI: 10.1039/c4cc01023j www.rsc.org/chemcomm
A metal-containing ionic liquid (MCIL) has been prepared in which the [CoII(salicylate)2]2 anion is able to selectively coordinate two water molecules with a visible colour change, even in the presence of alcohols. Upon moderate heating or placement in vacuo, the hydrated MCIL undergoes reversible thermochromism by releasing the bound water molecules.
Ionic liquids (ILs) (i.e., neat salts with melting points below 100 1C) have attracted a great deal of interest as new functional materials in recent years due to their unique combination of physico-chemical properties (i.e., vanishingly low vapour pressure, broad liquid range, ionic conductivity).1 ILs having a metal complex as a component ion (i.e., metal-containing ILs (MCILs)) have been widely investigated in the literature.2 MCILs are considered a distinct subclass of ILs because the inclusion of a metal ion potentially allows for incorporation of metal-based optical, magnetic, catalytic, or molecular binding properties. A variety of transition-metal complexes have been used in the design of MCILs.2 Most of the studies on MCILs so far have been focused on characterising the fundamental coordination chemistry and properties of the MCILs in their neat states.2 Very little work has been performed on investigating their behaviour in, or interaction with, solvents. In the studies reported to date, the coordination number of the constituent metal ion has generally been unresponsive to external stimuli, rendering it difficult to promote isomerisation of the metal complexes via changes in external conditions. However, several recent studies on interactions of MCILs with molecular liquids a
Dept. of Chemical & Biological Engineering, University of Colorado, Boulder, CO 80309, USA. E-mail: [email protected], [email protected] b Dept. of Biotechnology and Life Science, Tokyo University of Agriculture and Technology, 2-24-16, Naka-cho, Koganei, Tokyo 184-8588, Japan c Dept. of Chemistry, Colorado State University, Fort Collins, CO 80523, USA d Dept. of Chemistry & Biochemistry, University of Colorado, Boulder, CO 80309, USA † Electronic supplementary information (ESI) available: Detailed synthesis procedures and characterisation data for [P4444]2[CoII(Sal)2] in both the neat state and mixed state with solvent molecules. See DOI: 10.1039/c4cc01023j
This journal is © The Royal Society of Chemistry 2014
have revealed properties previously unobserved for neat ILs.3 For example, Mochida and co-workers reported the reversible coordination of several organic vapours and light gases by Ni(II)- and Cu(II)-containing MCILs, affording a change between square-planar and octahedral geometries in the metal centers along with changes in colour and other physical properties.3b Such interactions and changes in the metal sites show the potential of MCILs for specific functional applications such as selective chemical binding or sensing. One particular area of MCIL–solvent research that has not received much attention is the interaction of MCILs with water. This is likely due to the chemical instability and/or low miscibility of many MCILs with water.3b,4 Only a handful of studies have reported on MCILs in which water is coordinated to the metal ion.2c,f However, to date, no studies have examined the reversibility or selectivity of these MCIL–water coordinative interactions. In general, the exclusion of water molecules from MCILs is considered desirable because some MCIL properties (e.g., their optical properties) can be altered compared to the anhydrous state.5 In contrast to traditional ‘protection’ from water, the recent work of Ohno and co-workers has revealed that mixtures of some non-metal-based ILs with a small amount of water can yield interesting properties such as enhanced dissolution of biopolymers,6 liquid-crystalline properties,7 and dynamic phase transition behaviour.8 This work hints at enticing possibilities for the discovery and development of interesting and useful phenomena in MCIL–water mixtures, achievable by careful design of the constituent ions. Herein, we report a new type of MCIL based on a Co(II) bis(salicylate) ([CoII(Sal)2]2 ) anion and two tetra-n-butylphosphonium ([P4444]+) cations that shows highly selective and thermally reversible coordination of two water molecules (Fig. 1). Upon exposure to liquid or gaseous water, the [CoII(Sal)2]2 anion in this MCIL is able to selectively coordinate two water molecules, even in the presence of alcohols, to form the [CoII(Sal)2(H2O)2]2 anion with accompanying visible colour and metal-center geometry changes. Upon heating to moderate temperatures or placement in vacuo at ambient temperature,
Chem. Commun.
View Article Online
Published on 13 May 2014. Downloaded by Universidad de Vigo on 14/05/2014 05:36:25.
Communication
Fig. 1 Structure of [P4444]2[CoII(Sal)2]; its reversible, highly selective coordination with water; and its accompanying thermochromism.
the resulting [P4444]2[CoII(Sal)2(H2O)2] MCIL undergoes reversible thermochromism via release of the metal-bound water molecules to re-form the original [P4444]2[CoII(Sal)2] complex. This type of highly selective and reversible thermochromic water coordination is unprecedented in MCILs and illustrates the potential of such systems for applications such as water sensing and water–alcohol sorptive separations. A small number of solid-state materials has been reported to be able to perform selective H2O–alcohol separations (e.g., poly(vinyl alcohol), zeolites)9,10 as well as colorimetric sensing and/or binding of H2O in the presence of alcohols (e.g., MOFs).11 However, to our knowledge, a liquid-phase material that is able to do both has not been reported, especially one that has the unique combination of physical properties inherent to ILs. [P4444]2[CoII(Sal)2] was prepared by adding aq. solutions of [P4444][HSal] and LiHSal to an aq. CoII(H2O)6Cl2 solution at a molar ratio of 0.5 CoII(H2O)6Cl2 : 1.0 [P4444][HSal] : 1.0 LiHSal. After workup and isolation, the resulting dark blue liquid was characterised as [P4444]2[CoII(Sal)2] by a combination of elemental analysis; solution magnetic moment measurements via 1H NMR spectroscopy; ATR-FTIR and UV-visible spectroscopies; and variable-temperature magnetic susceptibility studies on the neat material. Because the product was paramagnetic, it was not possible to perform traditional structural characterisation by 1H and 13C NMR spectroscopy (see the ESI† for full details and spectra). Characteristic changes in the FTIR spectra of the starting materials vs. that of the formed product indicated that the salicylate ligands in the product anion were coordinated in a bidentate fashion to the Co(II) center (see ESI†). In the solution state, [P4444]2[CoII(Sal)2] was established to be in the S = 1/2 lowspin state (m = 2.1 Bohr magnetons at 20 1C) by measuring its solution magnetic moment in dichloromethane using the Evans method (see ESI,† Table S1).12 The geometrical implication of this result is that the anionic Co(II) complex exists in solution as a low-spin square planar.13 UV-visible spectra of [P4444]2[CoII(Sal)2] in dichloromethane and in its neat liquid form both show absorption bands at 533 and 573 nm (see ESI†), suggesting that the magnetic and metal coordination environments of [P4444]2[CoII(Sal)2] in
Chem. Commun.
ChemComm
solution and in the neat state may be similar. However, magnetic susceptibility measurements taken of neat [P4444]2[CoII(Sal)2] suggested that the structure of the neat MCIL is more complex: it is not a magnetically independent Co(II) species in the neat state but rather a 1 : 1 mixture of high-spin Co(II) and low-spin Co(II) versions of the [CoII(Sal)2]2 anion, likely forming multinuclear Co(II) complexes or oligomers (see the ESI† for details and analysis). Work is in progress to more definitively characterise the geometry of [P4444]2[CoII(Sal)2] in both its solution and neat liquid states, and the effect of temperature on its magnetic and optical properties. Neat [P4444]2[CoII(Sal)2] was also found to have a broad liquid phase range (5 1C to 264 1C (decomposes)) and very good thermal stability by TGA and DSC analysis (see ESI†). In terms of interactions with molecular solvents, [P4444]2[CoII(Sal)2] was found to reversibly and selectively coordinate water to trigger a distinct colour change. It was observed that upon mixing with water under ambient conditions, [P4444]2[CoII(Sal)2] undergoes a colour change from deep blue to magenta (i.e., red-purple). The UV-visible spectra were then measured for [P4444]2[CoII(Sal)2] after mixing with different molar equivalents of water molecules per mole of MCIL (i.e., Mwater). As Mwater was increased, the intensity of the peak at 573 nm decreased compared to that at 533 nm. When the Mwater value in the mixture exceeded two, and the spectrum no longer underwent changes upon further addition; only the peak at 533 nm remained (see ESI,† Fig. S17). Similar colour changes have been observed when Co(II) complexes undergo a four-coordinate tetrahedral to a six-coordinate octahedral geometrical change.11b,14 The absorption intensity at 573 nm of the mixtures as a function of different Mwater values was then plotted to estimate the ratio of six-coordinate to four-coordinate species at different water loadings and the number of water molecules preferentially coordinated per MCIL. In this study, the absorption decreased rapidly with increasing Mwater and reached a plateau after two equivalents of added water molecules (Mwater = 2.0), which is consistent with the formation of [CoII(Sal)2(H2O)2]2 for the hydrated MCIL anion (see ESI,† Fig. S18). Also, FTIR analysis of the MCIL–water mixtures showed that the typical bands corresponding to coordinated water molecules were found upon increasing the Mwater value (see ESI,† Fig. S19). These results strongly suggest that the [CoII(Sal)2]2 anion in the original MCIL binds two water molecules as co-ligands and that the Co(II) center undergoes a four-coordinate to a six-coordinate geometry change. The colour change upon contact with water can be reversed by evaporating the coordinated water vapour in vacuo at room temperature over 30 min. In addition, the colour change can also be readily reversed by mild heating at ambient pressure. Fig. 2 shows photographs of the hydrated form of the MCIL, [P4444]2[CoII(Sal)2(H2O)2], upon varying the temperature between 25 and 55 1C. As can be seen in Fig. 2 (left), the hydrated sample is magenta at 25 1C, suggesting that the [CoII(Sal)2(H2O)2]2 anion exists as six-coordinate (presumably octahedral) complex. Upon heating to 55 1C at ambient pressure, the sample becomes deep blue (Fig. 2, right), consistent with release of the Co(II)-bound H2O molecules to re-form
This journal is © The Royal Society of Chemistry 2014
View Article Online
ChemComm
Communication
Published on 13 May 2014. Downloaded by Universidad de Vigo on 14/05/2014 05:36:25.
Fig. 2 Photographs of the thermoreversible colour change for neat [P4444]2[CoII(Sal)2(H2O)2] (left) interconverting to [P4444]2[CoII(Sal)2] + 2H2O (right) with moderate changes in temperature.
[P4444]2[CoII(Sal)2]. This colour change was observed reversibly and rapidly (within 10 s in a 55 1C water bath). In addition, there is no evidence of phase-separated water formation upon heating to the blue [P4444]2[CoII(Sal)2] state, suggesting that the released H2O molecules remain dissolved but uncoordinated in the MCIL. Variable-temperature UV-visible studies on the MCIL–water mixtures also indicated that the relative amount of four-coordinate [P4444]2[CoII(Sal)2] increases upon heating (see ESI,† Fig. S20 and S21). This reversible and rapid binding/release of water molecules under such mild conditions is distinct from previous reports on related Co(II) systems. Although the thermochromism of Co(II) complexes with water is well-known,14b,15 comparatively high temperatures (e.g., 100 1C) are generally required to reverse the water binding and associated colour change under ambient pressure. This phenomenon has not been previously observed for MCIL materials. To investigate the binding selectivity of [P4444]2[CoII(Sal)2], with water vs. other solvents, the neat MCIL was mixed with several common molecular liquids, and the UV-visible spectra of the systems before and after mixing were analysed. When two molar equivalents of methanol, ethanol, acetonitrile, and ethyl acetate were individually mixed with [P4444]2[CoII(Sal)2], no colour change was apparent. However, addition of excess amounts of methanol and ethanol were found to induce the aforementioned blue to magenta colour change, whereas excess amounts of the other organic solvents had no effect. The UV-visible spectra of [P4444]2[CoII(Sal)2] with increasing molar amounts (Msolv.) of methanol and separately ethanol revealed that the intensity of the 573 nm peak decreased gradually compared to the one at 533 nm. However, the latter peak was still evident even after addition of five molar equivalents of these two alcohols, unlike in the case of water (see ESI,† Fig. S22 and S23). The values of the ambient-temperature equilibrium constant (Keq) for binding of liquid water, methanol, and ethanol with [P4444]2[CoII(Sal)2] in its neat state and as an acetonitrile solution (Table 1) were then determined via UV-visible studies on the mixtures to estimate the ratio of six-coordinate to four-coordinate
Table 1 The ambient-temperature equilibrium constant (Keq) values for coordination of different solvents for [P4444]2[CoII(Sal)2] in the neat state and in acetonitrile solution. (For Keq of the neat MCIL, the value of Msolv. was set at 2.0.)
Solvent bound
Keq, neat MCIL (M 2)
Keq, MCIL as CH3CN soln (M 2)
Water Methanol Ethanol
205 7 0.4 0.1 0.15 0.02
14 3 1.6 0.1 0.74 0.02
This journal is © The Royal Society of Chemistry 2014
species present at different solvent loadings. A detailed description of these equilibrium binding studies is included in the ESI.† As can be seen in Table 1, a relatively high Keq value for water was obtained compared to methanol and ethanol, especially in the case of the neat MCIL. This result indicates that [P4444]2[CoII(Sal)2] has the potential to coordinate water molecules selectively over common small alcohols. ATR-FTIR measurements undertaken of neat [P4444]2[CoII(Sal)2] after mixing with water and these two alcohols also confirmed the competitive sorption of water over the alcohols. The O–H bending mode characteristic of coordinated water molecules16 was observed when two molar equivalents of water and alcohols were simultaneously added to neat [P4444]2[CoII(Sal)2] (see ESI,† Fig. S24). These data are consistent with the MCIL selectively coordinating water molecules even in the presence of these alcohols. This binding selectivity for water over alcohols was also found to apply for contact with vapours via preliminary studies in which the neat MCIL was exposed to a mixture of water + alcohol vapour (see ESI,† Fig. S25). In summary, a new functional MCIL ([P4444]2[CoII(Sal)2]) has been prepared that shows selective coordination of water with an associated visible colour change, which can be readily reversed under mild conditions. As a new liquid-phase material, this MCIL could find application for low-energy sensing and sorptive separation of water from other solvents such as alcohols. The continued development of this system towards these applications will be reported in the near future. Our ongoing work includes more detailed characterisation and quantification of the binding equilibria for [P4444]2[CoII(Sal)2] with several molecular solvents; determination of its sensitivity/detection limits for water vapour; determination of its sorptive separation efficiency of water from other miscible solvents under different water/solvent loading ratios; and calculation of the estimated heat/energy costs for its use in water–solvent separation processes. Financial support for this work was provided by the U.S. DOE ARPA-E program (grant DE-AR0000098) with matching funds from Total, S.A. (France), Colorado State University, and the NSF (CHE-1058889). The authors also thank Prof. H. Ohno, Prof. N. Nakamura, S. Saita, and T. Ando (Tokyo University of Agriculture and Technology) for valuable discussions, elemental analysis, and some spectroscopic studies.
Notes and references 1 (a) J. S. Wilkes and M. J. Zaworotko, J. Chem. Soc., Chem. Commun., 1992, 965; (b) T. Welton, Chem. Rev., 1999, 99, 2071; (c) N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37, 123; (d) P. Wasserscheid and T. Welton, Ionic Liquids in Synthesis, Wiley-VCH, Weinheim, 2nd edn, 2008. 2 (a) P. B. Hitchcock, K. R. Seddon and T. Welton, J. Chem. Soc., Dalton Trans., 1993, 2639; (b) S. Hayashi and H. Hamaguchi, Chem. Lett., 2004, 133, 1590; (c) P. Nockemann, B. Thijs, N. Postelmans, K. V. Hecke, L. V. Meervelt and K. Binnemans, J. Am. Chem. Soc., 2006, 128, 13658; (d) S. Tang, A. Babai and A.-V. Mudring, Angew. Chem., Int. Ed., 2008, 47, 7631; (e) B. Mallick, B. Balke, C. Felser and A.-V. Mudring, Angew. Chem., Int. Ed., 2008, 47, 7635; ( f ) T. Peppel, ¨ckerling, M. Geppert-Rybczyn ´ ska, R. V. Ralys, J. K. Lehmann, M. Ko S. P. Verevkin and A. Heintz, Angew. Chem., Int. Ed., 2010, 49, 7116; ( g) I. J. B. Lin and C. S. Vasam, J. Organomet. Chem., 2005, 690, 3498; (h) C. Chiappe and M. Malvaldi, Phys. Chem. Chem. Phys., 2010,
Chem. Commun.
View Article Online
Communication
3 4 5
Published on 13 May 2014. Downloaded by Universidad de Vigo on 14/05/2014 05:36:25.
6 7 8
12, 11191; (i) P. Zhang, Y. Gong, Y. Lv, Y. Guo, Y. Wang, C. Wang and H. Li, Chem. Commun., 2012, 48, 2334. (a) Z.-L. Xie and A. Taubert, ChemPhysChem, 2011, 12, 364; (b) Y. Funasako, T. Mochida, K. Takahashi, T. Sakurai and H. Ohta, Chem. – Eur. J., 2012, 18, 11929. J. Estager, J. D. Holbrey and M. Swadz´ba-Kwas´ny, Chem. Soc. Rev., 2014, 43, 847. S. Arenz, A. Babai, K. Binnemans, K. Driesen, R. Giernoth, A.-V. Mudring and P. Nockemann, Chem. Phys. Lett., 2005, 402, 75. (a) K. Fujita, D. R. MacFarlane, M. Forsyth, M. Yoshizawa-Fujita, K. Murata, N. Nakamura and H. Ohno, Biomacromolecules, 2007, 8, 2080; (b) K. Fujita and H. Ohno, Biopolymers, 2010, 93, 1093. T. Ichikawa, T. Kato and H. Ohno, J. Am. Chem. Soc., 2012, 134, 11354. (a) K. Fukumoto and H. Ohno, Angew. Chem., Int. Ed., 2007, 46, 1852; (b) Y. Kohno, H. Arai, S. Saita and H. Ohno, Aust. J. Chem., 2011, 64, 1560; (c) Y. Kohno and H. Ohno, Chem. Commun., 2012, 48, 7119.
Chem. Commun.
ChemComm 9 J.-W. Rhim, C.-K. Yeom and S.-W. Kim, J. Appl. Polym. Sci., 1998, 68, 1717. 10 Q. Liu, R. D. Noble, J. L. Falconer and H. H. Funke, J. Membr. Sci., 1996, 117, 163. 11 (a) K. Zhang, R. P. Lively, M. E. Dose, A. J. Brown, C. Zhang, J. Chung, S. Nair, W. J. Koros and R. R. Chance, Chem. Commun., 2013, 49, 3245; (b) Q. Chen, Z. Chang, W.-C. Song, H. Song, H.-B. Song, T.-L. Hu and X.-H. Bu, Angew. Chem., Int. Ed., 2013, 52, 11550. 12 (a) D. F. Evans, J. Chem. Soc., 1959, 2003; (b) S. K. Sur, J. Magn. Reson., 1989, 82, 169. 13 U. Beckmann and S. Brooker, Coord. Chem. Rev., 2003, 245, 17. 14 (a) S. Aizawa, S. Iida, K. Matsuda and S. Funahashi, Inorg. Chem., 1996, 35, 1338; (b) L. Wang, S. Lu, Y. Zhou, X. Guo, Y. Lu, J. He and D. G. Evans, Chem. Commun., 2011, 47, 11002. 15 J. H. Day, Chem. Rev., 1968, 68, 649. 16 M. H. Soliman and G. G. Mohamed, Spectrochim. Acta, Part A, 2013, 107, 8.
This journal is © The Royal Society of Chemistry 2014
Published on 13 May 2014. Downloaded by Universidad de Vigo on 14/05/2014 05:36:25.
COMMUNICATION
Cite this: DOI: 10.1039/c4cc01023j Received 7th February 2014, Accepted 2nd May 2014
View Article Online View Journal
A cobalt(II) bis(salicylate)-based ionic liquid that shows thermoresponsive and selective water coordination† Yuki Kohno,a Matthew G. Cowan,a Miyuki Masuda,b Indrani Bhowmick,c Matthew P. Shores,c Douglas L. Gin*ad and Richard D. Noble*a
DOI: 10.1039/c4cc01023j www.rsc.org/chemcomm
A metal-containing ionic liquid (MCIL) has been prepared in which the [CoII(salicylate)2]2 anion is able to selectively coordinate two water molecules with a visible colour change, even in the presence of alcohols. Upon moderate heating or placement in vacuo, the hydrated MCIL undergoes reversible thermochromism by releasing the bound water molecules.
Ionic liquids (ILs) (i.e., neat salts with melting points below 100 1C) have attracted a great deal of interest as new functional materials in recent years due to their unique combination of physico-chemical properties (i.e., vanishingly low vapour pressure, broad liquid range, ionic conductivity).1 ILs having a metal complex as a component ion (i.e., metal-containing ILs (MCILs)) have been widely investigated in the literature.2 MCILs are considered a distinct subclass of ILs because the inclusion of a metal ion potentially allows for incorporation of metal-based optical, magnetic, catalytic, or molecular binding properties. A variety of transition-metal complexes have been used in the design of MCILs.2 Most of the studies on MCILs so far have been focused on characterising the fundamental coordination chemistry and properties of the MCILs in their neat states.2 Very little work has been performed on investigating their behaviour in, or interaction with, solvents. In the studies reported to date, the coordination number of the constituent metal ion has generally been unresponsive to external stimuli, rendering it difficult to promote isomerisation of the metal complexes via changes in external conditions. However, several recent studies on interactions of MCILs with molecular liquids a
Dept. of Chemical & Biological Engineering, University of Colorado, Boulder, CO 80309, USA. E-mail: [email protected], [email protected] b Dept. of Biotechnology and Life Science, Tokyo University of Agriculture and Technology, 2-24-16, Naka-cho, Koganei, Tokyo 184-8588, Japan c Dept. of Chemistry, Colorado State University, Fort Collins, CO 80523, USA d Dept. of Chemistry & Biochemistry, University of Colorado, Boulder, CO 80309, USA † Electronic supplementary information (ESI) available: Detailed synthesis procedures and characterisation data for [P4444]2[CoII(Sal)2] in both the neat state and mixed state with solvent molecules. See DOI: 10.1039/c4cc01023j
This journal is © The Royal Society of Chemistry 2014
have revealed properties previously unobserved for neat ILs.3 For example, Mochida and co-workers reported the reversible coordination of several organic vapours and light gases by Ni(II)- and Cu(II)-containing MCILs, affording a change between square-planar and octahedral geometries in the metal centers along with changes in colour and other physical properties.3b Such interactions and changes in the metal sites show the potential of MCILs for specific functional applications such as selective chemical binding or sensing. One particular area of MCIL–solvent research that has not received much attention is the interaction of MCILs with water. This is likely due to the chemical instability and/or low miscibility of many MCILs with water.3b,4 Only a handful of studies have reported on MCILs in which water is coordinated to the metal ion.2c,f However, to date, no studies have examined the reversibility or selectivity of these MCIL–water coordinative interactions. In general, the exclusion of water molecules from MCILs is considered desirable because some MCIL properties (e.g., their optical properties) can be altered compared to the anhydrous state.5 In contrast to traditional ‘protection’ from water, the recent work of Ohno and co-workers has revealed that mixtures of some non-metal-based ILs with a small amount of water can yield interesting properties such as enhanced dissolution of biopolymers,6 liquid-crystalline properties,7 and dynamic phase transition behaviour.8 This work hints at enticing possibilities for the discovery and development of interesting and useful phenomena in MCIL–water mixtures, achievable by careful design of the constituent ions. Herein, we report a new type of MCIL based on a Co(II) bis(salicylate) ([CoII(Sal)2]2 ) anion and two tetra-n-butylphosphonium ([P4444]+) cations that shows highly selective and thermally reversible coordination of two water molecules (Fig. 1). Upon exposure to liquid or gaseous water, the [CoII(Sal)2]2 anion in this MCIL is able to selectively coordinate two water molecules, even in the presence of alcohols, to form the [CoII(Sal)2(H2O)2]2 anion with accompanying visible colour and metal-center geometry changes. Upon heating to moderate temperatures or placement in vacuo at ambient temperature,
Chem. Commun.
View Article Online
Published on 13 May 2014. Downloaded by Universidad de Vigo on 14/05/2014 05:36:25.
Communication
Fig. 1 Structure of [P4444]2[CoII(Sal)2]; its reversible, highly selective coordination with water; and its accompanying thermochromism.
the resulting [P4444]2[CoII(Sal)2(H2O)2] MCIL undergoes reversible thermochromism via release of the metal-bound water molecules to re-form the original [P4444]2[CoII(Sal)2] complex. This type of highly selective and reversible thermochromic water coordination is unprecedented in MCILs and illustrates the potential of such systems for applications such as water sensing and water–alcohol sorptive separations. A small number of solid-state materials has been reported to be able to perform selective H2O–alcohol separations (e.g., poly(vinyl alcohol), zeolites)9,10 as well as colorimetric sensing and/or binding of H2O in the presence of alcohols (e.g., MOFs).11 However, to our knowledge, a liquid-phase material that is able to do both has not been reported, especially one that has the unique combination of physical properties inherent to ILs. [P4444]2[CoII(Sal)2] was prepared by adding aq. solutions of [P4444][HSal] and LiHSal to an aq. CoII(H2O)6Cl2 solution at a molar ratio of 0.5 CoII(H2O)6Cl2 : 1.0 [P4444][HSal] : 1.0 LiHSal. After workup and isolation, the resulting dark blue liquid was characterised as [P4444]2[CoII(Sal)2] by a combination of elemental analysis; solution magnetic moment measurements via 1H NMR spectroscopy; ATR-FTIR and UV-visible spectroscopies; and variable-temperature magnetic susceptibility studies on the neat material. Because the product was paramagnetic, it was not possible to perform traditional structural characterisation by 1H and 13C NMR spectroscopy (see the ESI† for full details and spectra). Characteristic changes in the FTIR spectra of the starting materials vs. that of the formed product indicated that the salicylate ligands in the product anion were coordinated in a bidentate fashion to the Co(II) center (see ESI†). In the solution state, [P4444]2[CoII(Sal)2] was established to be in the S = 1/2 lowspin state (m = 2.1 Bohr magnetons at 20 1C) by measuring its solution magnetic moment in dichloromethane using the Evans method (see ESI,† Table S1).12 The geometrical implication of this result is that the anionic Co(II) complex exists in solution as a low-spin square planar.13 UV-visible spectra of [P4444]2[CoII(Sal)2] in dichloromethane and in its neat liquid form both show absorption bands at 533 and 573 nm (see ESI†), suggesting that the magnetic and metal coordination environments of [P4444]2[CoII(Sal)2] in
Chem. Commun.
ChemComm
solution and in the neat state may be similar. However, magnetic susceptibility measurements taken of neat [P4444]2[CoII(Sal)2] suggested that the structure of the neat MCIL is more complex: it is not a magnetically independent Co(II) species in the neat state but rather a 1 : 1 mixture of high-spin Co(II) and low-spin Co(II) versions of the [CoII(Sal)2]2 anion, likely forming multinuclear Co(II) complexes or oligomers (see the ESI† for details and analysis). Work is in progress to more definitively characterise the geometry of [P4444]2[CoII(Sal)2] in both its solution and neat liquid states, and the effect of temperature on its magnetic and optical properties. Neat [P4444]2[CoII(Sal)2] was also found to have a broad liquid phase range (5 1C to 264 1C (decomposes)) and very good thermal stability by TGA and DSC analysis (see ESI†). In terms of interactions with molecular solvents, [P4444]2[CoII(Sal)2] was found to reversibly and selectively coordinate water to trigger a distinct colour change. It was observed that upon mixing with water under ambient conditions, [P4444]2[CoII(Sal)2] undergoes a colour change from deep blue to magenta (i.e., red-purple). The UV-visible spectra were then measured for [P4444]2[CoII(Sal)2] after mixing with different molar equivalents of water molecules per mole of MCIL (i.e., Mwater). As Mwater was increased, the intensity of the peak at 573 nm decreased compared to that at 533 nm. When the Mwater value in the mixture exceeded two, and the spectrum no longer underwent changes upon further addition; only the peak at 533 nm remained (see ESI,† Fig. S17). Similar colour changes have been observed when Co(II) complexes undergo a four-coordinate tetrahedral to a six-coordinate octahedral geometrical change.11b,14 The absorption intensity at 573 nm of the mixtures as a function of different Mwater values was then plotted to estimate the ratio of six-coordinate to four-coordinate species at different water loadings and the number of water molecules preferentially coordinated per MCIL. In this study, the absorption decreased rapidly with increasing Mwater and reached a plateau after two equivalents of added water molecules (Mwater = 2.0), which is consistent with the formation of [CoII(Sal)2(H2O)2]2 for the hydrated MCIL anion (see ESI,† Fig. S18). Also, FTIR analysis of the MCIL–water mixtures showed that the typical bands corresponding to coordinated water molecules were found upon increasing the Mwater value (see ESI,† Fig. S19). These results strongly suggest that the [CoII(Sal)2]2 anion in the original MCIL binds two water molecules as co-ligands and that the Co(II) center undergoes a four-coordinate to a six-coordinate geometry change. The colour change upon contact with water can be reversed by evaporating the coordinated water vapour in vacuo at room temperature over 30 min. In addition, the colour change can also be readily reversed by mild heating at ambient pressure. Fig. 2 shows photographs of the hydrated form of the MCIL, [P4444]2[CoII(Sal)2(H2O)2], upon varying the temperature between 25 and 55 1C. As can be seen in Fig. 2 (left), the hydrated sample is magenta at 25 1C, suggesting that the [CoII(Sal)2(H2O)2]2 anion exists as six-coordinate (presumably octahedral) complex. Upon heating to 55 1C at ambient pressure, the sample becomes deep blue (Fig. 2, right), consistent with release of the Co(II)-bound H2O molecules to re-form
This journal is © The Royal Society of Chemistry 2014
View Article Online
ChemComm
Communication
Published on 13 May 2014. Downloaded by Universidad de Vigo on 14/05/2014 05:36:25.
Fig. 2 Photographs of the thermoreversible colour change for neat [P4444]2[CoII(Sal)2(H2O)2] (left) interconverting to [P4444]2[CoII(Sal)2] + 2H2O (right) with moderate changes in temperature.
[P4444]2[CoII(Sal)2]. This colour change was observed reversibly and rapidly (within 10 s in a 55 1C water bath). In addition, there is no evidence of phase-separated water formation upon heating to the blue [P4444]2[CoII(Sal)2] state, suggesting that the released H2O molecules remain dissolved but uncoordinated in the MCIL. Variable-temperature UV-visible studies on the MCIL–water mixtures also indicated that the relative amount of four-coordinate [P4444]2[CoII(Sal)2] increases upon heating (see ESI,† Fig. S20 and S21). This reversible and rapid binding/release of water molecules under such mild conditions is distinct from previous reports on related Co(II) systems. Although the thermochromism of Co(II) complexes with water is well-known,14b,15 comparatively high temperatures (e.g., 100 1C) are generally required to reverse the water binding and associated colour change under ambient pressure. This phenomenon has not been previously observed for MCIL materials. To investigate the binding selectivity of [P4444]2[CoII(Sal)2], with water vs. other solvents, the neat MCIL was mixed with several common molecular liquids, and the UV-visible spectra of the systems before and after mixing were analysed. When two molar equivalents of methanol, ethanol, acetonitrile, and ethyl acetate were individually mixed with [P4444]2[CoII(Sal)2], no colour change was apparent. However, addition of excess amounts of methanol and ethanol were found to induce the aforementioned blue to magenta colour change, whereas excess amounts of the other organic solvents had no effect. The UV-visible spectra of [P4444]2[CoII(Sal)2] with increasing molar amounts (Msolv.) of methanol and separately ethanol revealed that the intensity of the 573 nm peak decreased gradually compared to the one at 533 nm. However, the latter peak was still evident even after addition of five molar equivalents of these two alcohols, unlike in the case of water (see ESI,† Fig. S22 and S23). The values of the ambient-temperature equilibrium constant (Keq) for binding of liquid water, methanol, and ethanol with [P4444]2[CoII(Sal)2] in its neat state and as an acetonitrile solution (Table 1) were then determined via UV-visible studies on the mixtures to estimate the ratio of six-coordinate to four-coordinate
Table 1 The ambient-temperature equilibrium constant (Keq) values for coordination of different solvents for [P4444]2[CoII(Sal)2] in the neat state and in acetonitrile solution. (For Keq of the neat MCIL, the value of Msolv. was set at 2.0.)
Solvent bound
Keq, neat MCIL (M 2)
Keq, MCIL as CH3CN soln (M 2)
Water Methanol Ethanol
205 7 0.4 0.1 0.15 0.02
14 3 1.6 0.1 0.74 0.02
This journal is © The Royal Society of Chemistry 2014
species present at different solvent loadings. A detailed description of these equilibrium binding studies is included in the ESI.† As can be seen in Table 1, a relatively high Keq value for water was obtained compared to methanol and ethanol, especially in the case of the neat MCIL. This result indicates that [P4444]2[CoII(Sal)2] has the potential to coordinate water molecules selectively over common small alcohols. ATR-FTIR measurements undertaken of neat [P4444]2[CoII(Sal)2] after mixing with water and these two alcohols also confirmed the competitive sorption of water over the alcohols. The O–H bending mode characteristic of coordinated water molecules16 was observed when two molar equivalents of water and alcohols were simultaneously added to neat [P4444]2[CoII(Sal)2] (see ESI,† Fig. S24). These data are consistent with the MCIL selectively coordinating water molecules even in the presence of these alcohols. This binding selectivity for water over alcohols was also found to apply for contact with vapours via preliminary studies in which the neat MCIL was exposed to a mixture of water + alcohol vapour (see ESI,† Fig. S25). In summary, a new functional MCIL ([P4444]2[CoII(Sal)2]) has been prepared that shows selective coordination of water with an associated visible colour change, which can be readily reversed under mild conditions. As a new liquid-phase material, this MCIL could find application for low-energy sensing and sorptive separation of water from other solvents such as alcohols. The continued development of this system towards these applications will be reported in the near future. Our ongoing work includes more detailed characterisation and quantification of the binding equilibria for [P4444]2[CoII(Sal)2] with several molecular solvents; determination of its sensitivity/detection limits for water vapour; determination of its sorptive separation efficiency of water from other miscible solvents under different water/solvent loading ratios; and calculation of the estimated heat/energy costs for its use in water–solvent separation processes. Financial support for this work was provided by the U.S. DOE ARPA-E program (grant DE-AR0000098) with matching funds from Total, S.A. (France), Colorado State University, and the NSF (CHE-1058889). The authors also thank Prof. H. Ohno, Prof. N. Nakamura, S. Saita, and T. Ando (Tokyo University of Agriculture and Technology) for valuable discussions, elemental analysis, and some spectroscopic studies.
Notes and references 1 (a) J. S. Wilkes and M. J. Zaworotko, J. Chem. Soc., Chem. Commun., 1992, 965; (b) T. Welton, Chem. Rev., 1999, 99, 2071; (c) N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37, 123; (d) P. Wasserscheid and T. Welton, Ionic Liquids in Synthesis, Wiley-VCH, Weinheim, 2nd edn, 2008. 2 (a) P. B. Hitchcock, K. R. Seddon and T. Welton, J. Chem. Soc., Dalton Trans., 1993, 2639; (b) S. Hayashi and H. Hamaguchi, Chem. Lett., 2004, 133, 1590; (c) P. Nockemann, B. Thijs, N. Postelmans, K. V. Hecke, L. V. Meervelt and K. Binnemans, J. Am. Chem. Soc., 2006, 128, 13658; (d) S. Tang, A. Babai and A.-V. Mudring, Angew. Chem., Int. Ed., 2008, 47, 7631; (e) B. Mallick, B. Balke, C. Felser and A.-V. Mudring, Angew. Chem., Int. Ed., 2008, 47, 7635; ( f ) T. Peppel, ¨ckerling, M. Geppert-Rybczyn ´ ska, R. V. Ralys, J. K. Lehmann, M. Ko S. P. Verevkin and A. Heintz, Angew. Chem., Int. Ed., 2010, 49, 7116; ( g) I. J. B. Lin and C. S. Vasam, J. Organomet. Chem., 2005, 690, 3498; (h) C. Chiappe and M. Malvaldi, Phys. Chem. Chem. Phys., 2010,
Chem. Commun.
View Article Online
Communication
3 4 5
Published on 13 May 2014. Downloaded by Universidad de Vigo on 14/05/2014 05:36:25.
6 7 8
12, 11191; (i) P. Zhang, Y. Gong, Y. Lv, Y. Guo, Y. Wang, C. Wang and H. Li, Chem. Commun., 2012, 48, 2334. (a) Z.-L. Xie and A. Taubert, ChemPhysChem, 2011, 12, 364; (b) Y. Funasako, T. Mochida, K. Takahashi, T. Sakurai and H. Ohta, Chem. – Eur. J., 2012, 18, 11929. J. Estager, J. D. Holbrey and M. Swadz´ba-Kwas´ny, Chem. Soc. Rev., 2014, 43, 847. S. Arenz, A. Babai, K. Binnemans, K. Driesen, R. Giernoth, A.-V. Mudring and P. Nockemann, Chem. Phys. Lett., 2005, 402, 75. (a) K. Fujita, D. R. MacFarlane, M. Forsyth, M. Yoshizawa-Fujita, K. Murata, N. Nakamura and H. Ohno, Biomacromolecules, 2007, 8, 2080; (b) K. Fujita and H. Ohno, Biopolymers, 2010, 93, 1093. T. Ichikawa, T. Kato and H. Ohno, J. Am. Chem. Soc., 2012, 134, 11354. (a) K. Fukumoto and H. Ohno, Angew. Chem., Int. Ed., 2007, 46, 1852; (b) Y. Kohno, H. Arai, S. Saita and H. Ohno, Aust. J. Chem., 2011, 64, 1560; (c) Y. Kohno and H. Ohno, Chem. Commun., 2012, 48, 7119.
Chem. Commun.
ChemComm 9 J.-W. Rhim, C.-K. Yeom and S.-W. Kim, J. Appl. Polym. Sci., 1998, 68, 1717. 10 Q. Liu, R. D. Noble, J. L. Falconer and H. H. Funke, J. Membr. Sci., 1996, 117, 163. 11 (a) K. Zhang, R. P. Lively, M. E. Dose, A. J. Brown, C. Zhang, J. Chung, S. Nair, W. J. Koros and R. R. Chance, Chem. Commun., 2013, 49, 3245; (b) Q. Chen, Z. Chang, W.-C. Song, H. Song, H.-B. Song, T.-L. Hu and X.-H. Bu, Angew. Chem., Int. Ed., 2013, 52, 11550. 12 (a) D. F. Evans, J. Chem. Soc., 1959, 2003; (b) S. K. Sur, J. Magn. Reson., 1989, 82, 169. 13 U. Beckmann and S. Brooker, Coord. Chem. Rev., 2003, 245, 17. 14 (a) S. Aizawa, S. Iida, K. Matsuda and S. Funahashi, Inorg. Chem., 1996, 35, 1338; (b) L. Wang, S. Lu, Y. Zhou, X. Guo, Y. Lu, J. He and D. G. Evans, Chem. Commun., 2011, 47, 11002. 15 J. H. Day, Chem. Rev., 1968, 68, 649. 16 M. H. Soliman and G. G. Mohamed, Spectrochim. Acta, Part A, 2013, 107, 8.
This journal is © The Royal Society of Chemistry 2014
