J. Phys. Chem. B 2007, 111, 4685-4692
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Tetraalkylphosphonium Polyoxometalate Ionic Liquids: Novel, Organic-Inorganic Hybrid Materials† Paul G. Rickert,‡ Mark R. Antonio,‡ Millicent A. Firestone,§ Karrie-Ann Kubatko,| Tomasz Szreder,⊥ James F. Wishart,⊥ and Mark L. Dietz*,‡ Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439, Materials Science DiVision, Argonne National Laboratory, Argonne, Illinois 60439, Department of Geological Sciences, UniVersity of Miami, Coral Gables, Florida 33146, and Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: October 31, 2006; In Final Form: March 19, 2007
Pairing of a Keggin or Lindqvist polyoxometalate (POM) anion with an appropriate tetraalkylphosphonium cation is shown to yield the first members of a new family of ionic liquids (ILs). Detailed characterization of one of them, an ambient-temperature “liquid POM” comprising the Lindqvist salt of the trihexyl(tetradecyl) phosphonium cation, by voltammetry, viscometry, conductimetry, and thermal analysis indicates that it exhibits conductivity and viscosity comparable to those of the one previously described inorganic-organic POM-IL hybrid but with substantially improved thermal stability.
Introduction The development of hybrid organic-inorganic nanocomposites has emerged as one of the most potentially significant fields of investigation in contemporary materials chemistry.1 Such hybrid materials are widely regarded as offering the key to overcoming challenges in areas ranging from energy storage and production to catalysis and optoelectronics.1-3 Among the variety of routes to the preparation of these materials that have been described,1,4-10 the assembly of well-defined nanoscale “building blocks”1,10 has been of particular recent interest, a result of the range of materials properties accessible via variations in the nature, structure, functionality, and organization of the constituent building blocks.1 Of the numerous classes of nanoscale components that have been employed in nanocomposite fabrication, few have attracted more interest than polyoxometalates (POMs), a family of anionic, inorganic metal oxide clusters exhibiting a wide range of topologies and physicochemical properties.11 In the past several years, polyoxometalate chemistry has grown enormously as the potential of POMs, not just as inorganic components for novel materials, but also as “green” industrial catalysts12 and anti-tumor/anti-viral agents,13 has been increasingly recognized. Our own work has concerned, among other issues, the effects of various solutes, including countercations and supporting electrolytes, on the redox behavior of POM anions.14 Counter-ions, while often overlooked, are now known to determine critical aspects of the catalytic and aggregation behavior of POMs.15,16 In conjunction with these investigations, we have been evaluating the potential of ionic liquids (ILs), low-melting organic analogs of classical molten salts that have recently attracted intense interest as possible environmentally benign alternatives to conventional organic †
Part of the special issue “Physical Chemistry of Ionic Liquids”. * Corresponding author: E-mail address:
[email protected]. Chemistry Division, Argonne National Laboratory. § Materials Science Division, Argonne National Laboratory. | University of Miami. ⊥ Brookhaven National Laboratory. ‡
solvents in a wide range of synthetic,17-19 catalytic,20 electrochemical,21-23 and analytical24-27 applications, as media for electrochemical studies of POMs. Research in our laboratory14 and elsewhere28 has shown that the dissolution of various POM salts, including [(n-C4H9)4N]2M6O19, [Cnmim]3PW12O40, and [(n-C4H9)4N]4S2M18O62 (for M ≡ Mo, W; n ) 2, 5), in 1-nalkyl-3-methylimidazolium-based ILs (e.g., C5mimBF4 and C4mimPF6) influences their redox behavior and reversible reduction potentials. This result prompted us to extend our investigations to other families of ionic liquids. In the course of this work, we have found that the pairing of certain wellknown IL cations with an appropriate POM anion yields lowmelting, organic-inorganic hybrids. Despite the demonstrated utility of ionic liquids in the preparation of novel materials,29-38 the generation of nanocomposites combining IL-forming cations with inorganic species has received little attention to date. In fact, only a single report of a POM-based ionic liquid, prepared by partial replacement of the protons in 12-tungstophosphoric acid, H3PW12O40, with a PEG-containing quaternary ammonium cation,39 has appeared prior to the present work. In a preliminary study,40 we described our initial efforts to prepare and characterize another POM-based IL, an ambient temperature “liquid POM” comprising the trihexyl(tetradecyl) phosphonium salt of the Lindqvist isopolyoxoanion, [W6O19]2-. In this report, we describe the results of a more detailed examination of this “inorganic liquid”,41 extend our investigations to the preparation and preliminary characterization of other members of this family of aprotic, POM-based ILs, and consider their potential as organic-inorganic hybrid materials. Experimental Section Materials. Tetra-n-butylammonium hexafluorophosphate ([(nC4H9)4N]PF6), abbreviated TBAPF6, was used as received from Alfa Aesar (Ward Hill, MA). Acetonitrile, abbreviated MeCN, was obtained from Aldrich Chemical Co. (Milwaukee, WI) and distilled under N2 over activated Linde 13X sieves, according to standard purification methods,42 immediately prior to use. Ferrocene (Aldrich) was used without further purification as
10.1021/jp0671751 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/03/2007
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TABLE 1: Melting Points/Glass Transition Temperatures of Ionic Salts Incorporating Polyoxometalate Anions salt
melting point (°C)
tris(1-ethyl-3-methylimidazolium) Keggin14 tris(1-pentyl-3-methylimidazolium) Keggin14 tris(tetra-n-butylammonium) Keggind tris(tetra-n-butylphosphonium) Keggine tris[tributyl(tetradecyl)phosphonium] Kegginf bis[tributyl(tetradecyl)phosphonium] Lindqvistg tris[trihexyl(tetradecyl)phosphonium]Kegginh bis[trihexyl(tetradecyl)phosphonium] Lindqvisti
>215a >215a >215a >215a 129a 55a 65b -48b,c
a As determined by capillary melting point apparatus. b As determined by DSC. c Glass transition d Tris(tetra-n-butylammonium) Keggin: HR-FAB-MS, positive: 242.2859. Calcd. for C16H36N: 242.2848. Elemental analysis found: C, 15.65; H, 3.00. Calcd. for C48H108N3O40PW12: C, 15.99; H, 3.02. [Br]: 239 ppm. 31P NMR (vs 85% D3PO4 capillary; DMF): -14.411 (s). e Tris(tetra-n-butylphosphonium) Keggin: HR-FAB-MS positive: 259.2564. Calcd. for C16H36P: 259.2555. Elemental analysis found: C, 15.41; H, 2.94. Calcd. for C48H108O40P4W12: C: 15.77; H 2.98. [Br]: < 48 ppm. 31P NMR (vs 85% D3PO4 capillary; DMF): -14.421 (s), 34.701 (s). f Tris[tributyl(tetradecyl)phosphonium] Keggin: HR-FAB-MS, positive: 399.4104. Calcd. for C26H56P: 399.412. Elemental analysis found: C, 23.12; H, 4.23. Calcd. for C78H168O40P4W12: C, 22.98; H, 4.15. [Cl]: 35 ppm. 31P NMR (vs 85% D3PO4 capillary; DMF): -14.431 (s), 34.651 (s). g Bis[tributyl(tetradecyl)phosphonium] Lindqvist: HR-FAB-MS, positive: 399.4101. Calcd. for C26H56P: 399.412. Elemental analysis found: C, 27.94; H, 5.10. Calcd. for C52H112O19P2W6: C, 28.31; H, 5.12. [Cl]: < 74 ppm. 31 P NMR (vs 85% D3PO4 capillary; DMF): 34.586 (s). h Tris[trihexyl(tetradecyl)phosphonium] Keggin: HR-FAB-MS, positive: 483.5073. Calcd. for C32H68P: 483.5059. Elemental analysis found: C, 26.52; H, 4.80. Calcd. for C96H204O40P4W12: C, 26.64; H, 4.75. [Br]: 41 ppm. 31 P NMR (vs 85% D3PO4 capillary; DMF): -14.421 (s), 34.576 (s). i Bis[trihexyl(tetradecyl)phosphonium] Lindqvist: HR-FAB-MS, positive: 483.5050. Calcd. for C32H68P: 483.85. Elemental analysis found: C, 32.53; H, 5.95. Calcd. for C64H136O19P2W6: C, 32.37; H, 5.77. [Br]: 127 ppm. 31P NMR (vs 85% H3PO4; CDCl3): 33.400 (s).
an internal, reference-voltage standard. Tetraalkylphosphonium halides were obtained from Cytec Canada, Inc. (Niagra Falls, Ontario) or from Fluka Chemie GmbH (Buchs, Federal Republic of Germany) and used as received. Unless otherwise noted, all other materials were reagent grade and were used without further treatment. Tetra-n-butylammonium hexatungstate ([(n-C4H9)4N]2W6O19), abbreviated (TBA)2W6O19, was prepared according to published procedures.43 Isopolytungstate ILs were prepared from sodium tungstate by the method of Klemperer,44 modified by substitution of a tetraalkylphosphonium bromide for the analogous tetraalkylammonium salt. For synthesis of an IL comprising the Lindqvist hexatungstate anion, a stoichiometric reaction is formulated below:
6 Na2WO4 + 10 HCl + 2 R3R′PBr f (R3R′P)2W6O19 + 10 NaCl + 2 NaBr + 5 H2O (I) The corresponding Keggin heteropolytungstate, [PW12O40]3-, salts were prepared by reaction of 12-tungstophosphoric acid with a tetraalkylphosphonium bromide under the same conditions. In each case, addition of sufficient methanol to the reaction mixture (in acetic anhydride-hydrochloric acid-dimethylformamide) led to precipitation of the product, which was thoroughly washed with methanol and diethyl ether, dried at 80 °C for 24 h, and finally, characterized via NMR, elemental analysis, and/ or mass spectrometry (Table 1). Methods. Small-Angle X-ray Scattering (SAXS). SAXS data were collected at beam line 12-BM-B of the Advanced Photon Source at Argonne National Laboratory.45 An incident photon
TABLE 2: Average Interatomic Distances, r, about the W Atoms for Groups of Symmetry- and Distance-Related W-O and W-W Interactions within the Lindqvist Hexatungstate Framework in the Crystal Structure of [(n-C4H9)4N]2W6O1940 and from the W EXAFS for [(C14H29)P(C6H13)3]2W6O19a Cryst40
EXAFSc
elementb
CNb
r, Å
r, Å
σ 2, Å 2
O O O W O O W O O
1 4 1 4 4 4 1 4 1
1.712(7) 1.925(10) 2.329(6) 3.294(5) 3.523(17) 4.596(8) 4.659(11) 4.664(10) 6.370(11)
1.71(1) 1.92(1) 2.30(2) 3.32(4)
0.0003(4) 0.0040(10) 0.0006(17) 0.0025(2)
4.74(2)
0.0023(15)
a esds are in parentheses. b Target element (element) about W, i.e., W-Element interaction, and the element coordination number (CN). c No W-O interactions beyond the W-O6 coordination were fit because the W-W backscattering predominates the response at high k, Å-1. A single value of ∆E0 was refined, 7.7(9) eV, in the 5-shell fit of the k3χ(k) EXAFS, ∆k ) 16 Å-1, with 11 variables, including the 5 interatomic distances, r, and their Debye-Waller factors, σ2.
energy of 16.5 keV was chosen to provide good X-ray transmittance through the cell and, at the same time, to be as high above the X-ray fluorescence from the lower-energy W L-levels as the beam line optics would permit. (The 1/e attenuation length of 16.5 keV photons in acetonitrile is 20.5 mm, which is 12.1× longer than the optical path length of the cell.) Acetonitrile solutions of the Lindqvist-based phosphonium IL and the TBA salt of [W6O19]2- (5-30 mg per mL) were contained in thin-layer (1.7 mm path length), small volume (ca. 20 µL), parallel-plate cells fabricated of PCTFE with optically transparent PCTFE film windows of 0.010′′ thickness. The 2-D scattering profiles were recorded with a MAR-CCD-165 detector (Mar USA, Evanston, IL), which features a circular, 165 mm diameter active area and 2048 × 2048 pixel resolution. The sample-to-detector distance was such as to provide a detecting range for momentum transfer of 0.0025 < Q < 0.8 Å-1. The scattering vector, Q, was calibrated using a silver behenate standard.46 After correction for spatial distortion and detector sensitivity, the 2-D scattering images were radially averaged to produce plots of scattered intensity, I(Q) vs Q, where Q ) 4π sin θ/λ (Å-1), in which 2θ is the scattering angle and λ is the wavelength of the X-rays, following standard procedures.47 Guinier analysis of the I(Q) data in the low Q region provided the radius of gyration, Rg, of the molecular anions by use of traditional methods.48-50 The program CRYSOL51 was used to calculate the Rg value from the fractional atomic coordinates for the molecular structure of TBA2W6O19.40 Extended X-ray Absorption Fine Structure (EXAFS). A 0.01′′ thick film of the neat, Lindqvist-based phosphonium IL was used to acquire the W L3-edge transmission XAFS data, which were analyzed in the usual manner with EXAFSPAK,52 at APS beam line 12-BM-B.53 A five-shell fit with a fixed scale factor (1) as well as fixed O and W coordination numbers (pursuant to the crystal structure data of Table 2) was obtained using theoretical phase and amplitude functions calculated with FEFF8.054 from the atomic coordinates for [W6O19]2-.40 Voltammetry. An Epsilon potentiostat was used for the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments (ν ) 25 and 20 mV s-1, respectively) with 3.0 mm diameter glassy carbon (GC) working electrodes (BASi MF2012) and Pt wire auxiliary electrodes (BASi MW-1032). All sample manipulations and CV data acquisition for dilute (ca. 1
Tetraalkylphosphonium Polyoxometalate Ionic Liquids mM) solutions of the [(n-C4H9)4N]+ and [(C6H13)3P(C14H29)]+ complexes of [W6O19]2- in purified MeCN with a 0.1 M TBAPF6 electrolyte were conducted exactly as described previously40 under an N2 atmosphere with a Ag/Ag+ reference electrode (BASi MF-2062) filled with 0.1 M n-Bu4NPF6 and 0.01 M AgNO3, also in purified MeCN. With this electrode, the Fc/Fc+ half-wave potential was observed at +0.090(5) V. An approximately 4 µm thick film of the neat Lindqvist-based phosphonium IL, which is immiscible with water, was cast onto a freshly polished surface of a GC electrode for CV and DPV measurements,40 which were obtained under ambient conditions in an aqueous electrolyte of 0.5 M n-Bu4NBr (Aldrich 19,3119) with a Ag/AgCl reference electrode (BASi MF-2052). Viscosity Measurements. Viscosities were measured using a Cambridge Applied Systems (now Cambridge Viscosity) ViscoLab 4000 reciprocating piston viscometer equipped with a flow-through jacket for temperature control. The flow-through jacket was connected to a Lauda Proline RP-845 refrigerated circulating bath. Collection of the viscosity data as a function of temperature was automated using a program operating under LabVIEW (National Instruments) to control the bath and take temperature and viscosity readings from the viscometer. To avoid absorption of atmospheric moisture by the samples, the viscometer head was located in a nitrogen-purged acrylic box with hand access ports. In addition to the gas purge, two small Dewars of liquid nitrogen were placed in the box to further reduce the humidity through nitrogen boil-off and water vapor condensation. ConductiVity Measurements. Conductivites were determined by AC impedance at variable temperatures using a HewlettPackard 4129A Impedance Analyzer in the frequency range from 5 Hz to 10 MHz. A cell with Pt electrodes was used for the measurements. The cell was placed in a thermostated brass block, and the temperature was measured with a Teflon-coated type J thermocouple. The cell constant (1.97 and 2.05 cm-1 for the tetraalkylphosphonium Lindqvist and bromide salts, respectively) was calibrated prior to each set of measurements using an aqueous 0.053% KCl standard solution (YSI 3167, YSI Incorporated) at 25 °C. Thermal Analysis. Differential scanning calorimetry (DSC) was performed on a TA Instruments model Q100 interfaced with a refrigerated cooling system. Weighed amounts (5-10 mg) of the sample were sealed in aluminum pans and equilibrated at -75 °C for 5 min prior to starting the heating cycle, and data were collected at 2 °C/min. Instrument calibration was performed using an indium standard. Thermogravimetric analysis (TGA) was carried out on a TA Instruments Q50 by heating a known amount of sample (2-5 mg) in a platinum pan from 25 °C to a final temperature of 600 °C at a rate of 10 °C/min under N2 flow. The onset of decomposition (Tonset) was taken as the temperature corresponding to the intersection of the extrapolation of the predegradation portion of the mass curve and the tangent to the steepest portion of the curve during degradation.
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4687 ever, invariably failed to yield low-melting materials (Table 1). Such results imply that the inclusion of polyoxyethylene units in the cation, which represents an established strategy for reducing the melting point of organic solids,41,55 constitutes an essential feature of a POM-based ionic liquid. Although the precise relationship between the melting point of an ionic liquid and its chemical structure remains incompletely understood,56 it is known that an important factor governing the melting point is the magnitude of the Coulombic attraction between the constituent ions, described by eq 1
Ec ) MZ+Z-/4π0r
(1)
where Z+ and Z- are the ion charges and r is the interionic separation. Thus, the formation of low-melting salts will be favored when the charges on the ions are low (preferably (1) and when the ions are large (thus ensuring that the inter-ionic separation is large). Large ions also allow for charge delocalization, thereby reducing the overall charge density. We therefore reasoned that, among ionic salts pairing an organic cation with a polyoxometalate anion, low-melting materials were more likely to be obtained by employing a POM bearing a lower charge than that of the Keggin anion (PW12O403-), such as the Lindqvist (W6O192-) anion. Although the reduction in anion size that accompanies the change from the Keggin (r ) 5.5 Å) to the Lindqvist anion (r ) 4.3 Å) might be expected to lead to an increase in the melting point of the resultant salt, the results shown in Table 1, which compares the melting points/glass transition temperatures for a series of ionic salts comprising any one of several organic cations and either the Keggin or Lindqvist anion, indicate that this effect is outweighed by the impact of the lower charge of the Lindqvist anion. Thus, although the Keggin salt of the trihexyl(tetradecyl)phosphonium cation, for example, is a crystalline solid at room temperature (melting at ca. 65 °C), the analogous Lindqvist salt is a viscous liquid at ambient temperature. Confirmation of LindqVist POM Integrity. Because of its obvious interest as an ambient-temperature “liquid POM”41 and because, despite our best efforts to control the reaction conditions so as to produce the Lindqvist hexatungstate isopolyanion, modification of the synthetic procedure with the bulky trihexyl(tetradecyl)phosphonium countercation may have led to unanticipated and ancillary reactions, we first sought evidence to confirm the presence of the plenary hexatungstate anion in the (ostensible) Lindqvist-based room-temperature IL. In a prior report,40 we presented indirect evidence consistent with the presence of intact Lindqvist hexatungstate anion in the ionic liquid. That is, we noted that during efforts to acquire voltammetric data for the IL (in MeCN with a 0.1 M TBAPF6 electrolyte), crystals formed in the electrochemical cell that subsequent single-crystal X-ray diffraction studies showed to be the tetra-n-butylammonium salt of the Lindqvist hexatungstate dianion, formed via the following metathesis reaction:
Results and Discussion
[(C6H13)3P(C14H29)]2W6O19 + xs[(n-C4H9)4N]PF6 f [(n-C4H9)4N]2W6O19V + [(C6H13)3P(C14H29)]PF6
In an earlier report, Gianellis et al.39 noted that partial replacement of the protons of the acid form of the Keggin anion, H3PW12O40, with a PEGylated quaternary ammonium cation, (CH3)(C18H37)N+[(CH2CH2O)nH] [(CH2CH2O)mH] (m + n ) 15) yields a POM-IL hybrid that is liquid at ambient temperature. Our subsequent efforts to extend these results to systems comprising the Keggin anion and other, more “conventional” ionic liquid cations (e.g., 1-alkyl-3-methylimidazolium), how-
Along these same lines, we have found that when a solution of the IL in acetonitrile is subjected to a sufficiently negative potential the initially colorless solution turns blue, consistent with the reduction of the Lindqvist hexatungstate.57,58 Nevertheless, because a myriad of isopolytungstates turn blue upon reduction, this observation also does not constitute conclusive identification of the anion in the IL. In an effort to obtain direct molecular evidence for the presence of intact Lindqvist hexa-
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Figure 2. W L3-edge k3χ(k) EXAFS for [(C14H29)P(C6H13)3]2[W6O19] and (right) the corresponding Fourier transform showing the 3 shells of O (terminal, bridging, and central) bonds in the W-O6 octahedra as well as the two, distant W-W interactions.
Figure 1. Upper panel: log-log plot of the experimental SAXS data (circles) for the MeCN solution of the Lindqvist-based IL and the response calculated (solid line) from the hexatungstate structure of reference 40. Lower panel: Guinier plot (circles) of the measured SAXS data and the fit (solid line) to the data (Qmax*Rg ) 1.0) providing Rg ) 3.3 ( 0.1 Å.
tungstate anion in the IL, we turned to two other X-ray techniques: SAXS and EXAFS. SAXS. The experimental data obtained for a solution of the IL in MeCN are shown in Figure 1 (upper panel) as open circles. Also shown as the solid line is the I(Q) response calculated through use of CRYSOL51 and the X-ray crystallography data of [W6O19]2-.40 The close correspondence between the experiment and calculation provides direct evidence that the anion component of the RTIL has the architecture of hexatungstate, [W6O19]2-. A quantitative measure of the cluster size, in terms of the radius of gyration (Rg), is obtained by analysis of the low Q data using the Guinier approximation. The Guinier plot (Figure 1, lower panel) provides an Rg value of 3.3 ( 0.1 Å, which was extracted from the slope of the fit (line) to the experimental data (open circles). Within the experimental uncertainty ((0.1 Å), this Rg is in agreement with those calculated from the atomic coordinates of other Lindqvist hexatungstates, in both oxidized and 1e--reduced forms, values which range from 3.2-3.4 Å.59 Given its nearly spherical morphology, the Rg for the Lindqvist hexatungstate dianion can be related to its crystallographic radius, R, by R ) (5/3)1/2Rg,60 which amounts to 4.3 ( 0.1 Å, and is consistent with one-half of the average terminal O-O distance (4.04 Å) in the crystallographic structure of [W6O19]2-.40 EXAFS. The W L3-edge transmission spectrum of the neat [(C14H29)P(C6H13)3]+ RTIL is shown in Figure 2 (left; open circles) as k3χ(k) vs k, Å-1. The corresponding Fourier transform data (without phase shift correction) of Figure 2 (right) are typical of hexametalate Lindqvist structures with Zr4+, Mo6+, W6+, and Nb5+.61-65 The curve-fitting analysis was performed in accordance with the known coordination about W in the hexatungstate dianion. The fit to the experimental EXAFS data
Figure 3. Cyclic voltammograms showing two redox couples with E1/2 ) -1.25 and -2.21 V vs Ag/Ag+ for acetonitrile dilutions of the (a) Lindqvist-based IL and the (b) n-Bu4N+ salt of the Lindqvist hexatungstate. (c) Cyclic voltammogram of the neat IL obtained as a thin-film on the surface of a polished GC electrode in contact with an aqueous electrolyte of n-Bu4NBr, showing a strong response at E1/2 ) -0.72 V and a weaker one near -1.5 V vs Ag/AgCl. (d) Differential pulse voltammograms of the polished GC electrode without the RTIL film (gray lines), showing a featureless background response, and with the film (black lines), showing two redox couples with E1/2 ) -0.74 and -1.51 V vs Ag/AgCl. In each Figure panel, the electrode potential window is 1.70 V; -0.80 to -2.50 V vs Ag/Ag+ for (a, b); 0.00 to -1.70 V vs Ag/AgCl for (c, d).
using the molecular structure of the solid salt, (TBA)2W6O19,40 is persuasive. The W-O and W-W connectivities in the RTIL are diagnostic of the hexatungstate species. The average W-O and W-W distances of Table 2 indicate that the [W6O19]2structure is essentially independent of countercation, [(nC4H9)4N]+ and [(C14H29)P(C6H13)3]+, and the physical form, solid salt and ionic liquid, of the material. These results, taken together with those obtained through SAXS (above) and X-ray crystallography40 conclusively demonstrate the preparation of a room-temperature ionic liquid with the Lindqvist hexatungstate as its anionic component.
Tetraalkylphosphonium Polyoxometalate Ionic Liquids
Figure 4. Temperature-dependence of viscosity for trihexyl(tetradecyl) phosphonium bromide and the corresponding Lindqvist-based IL.
Characterization of POM-Based ILs. The utility of any inorganic-organic hybrid material is determined in large measure by the extent to which the desirable properties of its constituents are preserved or enhanced in the hybrid.1-4 Of obvious interest in the case of a POM-based IL is its electroactivity, which along with its viscosity, conductivity, and thermal properties will dictate its practical utility. ElectroactiVity. Figure 3 depicts the cyclic voltammogram obtained for the Lindqvist-based RTIL (a), along with (for purposes of comparison) that obtained for (TBA)2W6O19 (b) under identical conditions in acetonitrile solution. As can be seen, each compound exhibits two redox couples separated by 0.96 V, which is consistent with previous reports for [W6O19]2-.28,43,66,67 The CV data for the IL thin film (c) on the GC electrode immersed in a 0.5 M TBABr electrolyte also reveal two processes. The second one is weak and incompletely resolved because of its proximity to the low-potential polarization limit of the aqueous electrolyte. The corresponding DPV data, Figure 3d, shows the background response of the freshly polished GC electrode without the IL film (gray lines) and with it (black lines). The response for the film exhibits two redox couples with a peak separation of 0.77 V, a value consistent with the 0.69 V separation for [(n-C4H9)4N]2W6O19 adhered to a GC electrode in contact with C4mimPF6.28 In addition to providing yet another indication of the presence of the hexatungstate anion in the ionic liquid, these results clearly demonstrate that the redox activity of the anion is indeed preserved in the POM-IL hybrid. Viscosity. Viscosity is among the most important physicochemical properties of any liquid for which practical applications are envisioned,68 determining the rate of diffusion of reactants, the power requirements for mixing and pumping, and both the rate of mass transfer and ease of phase separation in two-phase
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4689 systems.68,69 Moreover, since the viscosity of an ionic liquid is dictated by a combination of electrostatics, van der Waals interactions, hydrogen bonding, and ion size and polarizability, studies of viscosity can provide information on its fundamental chemical characteristics.70 At ambient temperature, the viscosities of ionic liquids are often relatively high, with values of 10-500 mPa‚s (2-3 orders of magnitude greater than the viscosities of traditional organic solvents) considered to be typical.68 This viscosity frequently exhibits a pronounced temperature dependence, however, with values falling significantly upon modest increases in temperature.56 Figure 4 depicts the temperature dependence of viscosity for the Lindqvist-based RTIL, along with that of trihexyl(tetradecyl)phosphonium bromide, shown for comparison purposes. Several things are readily evident from these data. First, the viscosity of the former is quite high, exceeding the working range of the instrument (e20 000 mPa‚s), in fact, at temperatures below ca. 40 °C. This high viscosity is consistent with that previously reported for the Keggin-PEGylated quaternary ammonium POM-IL hybrid (75 000 mPa‚s at room temperature).39 Next, at any given temperature, the viscosity of the trihexyl(tetradecyl)phosphonium bromide is significantly (1 order of magnitude or more) lower than that of the Lindqvist-based IL, as expected from both the reported viscosity of trihexyl(tetradecyl)phosphonium chloride71 and the known relationship between viscosity and ionic radius for ionic liquids.72 Finally, as anticipated, the viscosity of each ionic liquid falls steeply with temperature. In fact, by ca. 90 °C, a temperature within the range of typical industrial reactions,71 the viscosity of the POM-IL declines to less than 1000 mPa‚s The dependence of ionic liquid viscosity on temperature is most commonly described by an Arrhenius-like equation of the following form:73
ln η ) ln η∞ + Eη/RT
(2)
where η∞ is the viscosity at infinite temperature and Eη is the activation energy for viscous flow (i.e., the energy barrier that must be overcome for ions in the ionic liquid to move past one another).73 The viscosity data for trihexyl(tetradecyl)phosphonium bromide and the Lindqvist-based IL are well fit (R g 0.999) by this equation, a result consistent with prior reports indicating that the Arrhenius-like treatment is especially wellsuited for ILs in which the cations are large (MW > 100) and unfunctionalized or the anions are symmetrical.73 From the slope of the best fit line for a plot of ln η vs 1/T, the activation energy for viscous flow, Eη, can be calculated as 52.1 and 59.3 kJ/mol for the bromide and Lindqvist RTILs, respectively, values consistent with that extracted from published data describing the temperature-dependence of viscosity for trihexyl(tetradecyl) phosphonium chloride (47.8 kJ/mol).71 Interestingly, extrapolation of the Arrhenius plot indicates that the viscosity of the Lindqvist-based IL (estimated as 1.1 × 105 mPa‚s at 20 °C) will fall to 1 mPa‚s, the viscosity of water at 20 °C,74 at ca. 288 °C, a temperature at which (as will be shown below) the ionic liquid still exhibits good thermal stability. ConductiVity. In any prospective application of ILs in electrochemical devices (e.g., capacitors, batteries, or fuel cells), knowledge of the ionic conductivity is critically important.75 As was the case for viscosity, the temperature-dependence of the ionic conductivity of the Lindqvist-based IL, as measured by impedance spectroscopy, is well-described by an Arrheniustype equation over the range of temperatures considered (Figure 5), with the slope of the best fit line corresponding to an activation energy for conductivity of 55 ( 1 kJ/mol. At any given temperature, the conductivity (estimated as 4.5 × 10-4
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Figure 6. Differential scanning calorimetry heating scans (scan rate: 2 °C/min) for trihexyl(tetradecyl) phosphonium bromide (A) and the corresponding Keggin (B) and Lindqvist (C) ILs.
Figure 5. Arrhenius treatment of the temperature dependence of conductivity for the trihexyl(tetradecyl) phosphonium Lindqvist IL.
mS/cm at 20 °C) is a factor of 2-4 lower than that reported previously39 for the Keggin-PEGylated quaternary ammonium POM-IL hybrid, a not unexpected result given the presence in this earlier hybrid of smaller, more mobile (vs bulky Lindqvist anions and tetraalkylphosphonium counterions) protons. Nonetheless, this conductivity is significantly greater than that reported for typical anhydrous solid POMs (e.g., 12-tungstophosphoric acid partially exchanged with a single K+ ion39). Thermophysical Properties. Differential Scanning Calorimetry. On the basis of results obtained for a number of imidazolium-based salts, Fredlake et al.76 have noted that three distinct types of behavior are commonly observed in DSC studies of ILs. In the first case, the ionic liquid exhibits definite freezing and/or melting points upon cooling and heating, respectively. The second type of behavior is characterized by an absence of true phase transitions. Instead, an amorphous glass is formed upon cooling and a liquid re-formed on heating. (Such ILs have no melting or freezing points, but rather glass transition temperatures.) A third group of ILs exhibits behavior analogous to that observed for many polymers and other amorphous materials. That is, upon cooling, the IL passes from a liquid to a glassy state, as is the case for the second type. Upon heating, however, the IL passes from a glass to a sub-cooled liquid, which then crystallizes. Further warming results in the melting of the crystals. As shown in Figure 6, which depicts the results of DSC heating scans carried out over the range -70 to +80/ 100 °C for the Lindqvist-based IL (Figure 6C) and the corresponding Keggin (Figure 6B) and bromide (Figure 6A) salts, each of the three types of behavior is represented by these ILs. That is, the trihexyl(tetradecyl) phosphonium bromide (A) exhibits multiple endo- and exothermic transitions. Specifically, a small endothermic peak at -58 °C associated with a glass to sub-cooled liquid transition, along with a large exothermic peak at -45 °C corresponding to cold crystallization, is observed. In addition, a large endothermic transition corresponding to
melting of the crystals is observed at -15 °C. Such a melting point is consistent with both the known effect of incorporation of large, asymmetric cations into ILs on their melting points77 and the observed liquid-like consistency of the sample at room temperature. The heating scan for the Keggin IL (B) is considerably simpler than that of the bromide salt, showing only a single endothermic transition, corresponding to the melting point, at 65 °C. (This transition, it should be noted, is irreversible, a common observation for ILs that has been attributed to very slow crystallization on cooling from the melt.77) In contrast, the heating profile for the Lindquist-based IL (C) is devoid of any primary transitions over the entire range studied (up to 100 °C) and displays only a glass transition at -48 °C. This material thus has no true melting or freezing points. Thermophysical Properties. ThermograVimetric Analysis. Superior resistance to thermal degradation is among the most widely touted advantages of ILs over conventional molecular solvents.69,78,79 Among ILs, phosphonium salts are not infrequently described as exhibiting particularly good thermal stability.71 Prior studies concerning the relationship between IL structure and physicochemical properties have noted that for a given cation, there exists a rough inverse relationship between anion nucleophilicity and the IL decomposition temperature.56 This suggests that POM-IL hybrids comprising phosphonium cations and bulky, charge-diffuse POM anions may exhibit exceptional high-temperature stability. Figure 7 depicts the results of fast-scan (10 °C/min) TGA measurements carried out on the Lindqvist-based IL (Figure 7C) and the corresponding Keggin salt (Figure 7B). Also shown for purposes of comparison are results for the trihexyl(tetradecyl)phosphonium bromide (Figure 7A). For the bromide (A), weight loss is seen to occur in two stages. The first, corresponding to only ∼2.5% of the sample weight, occurs at 100 °C and likely reflects the loss of water from the somewhat hygroscopic sample. The second, more significant loss (ca. 94%) occurs at 353.4 °C (onset; midpoint ) 375.4 °C), a temperature consistent with literature reports indicating dynamic thermal stabilities exceeding 300 °C for many phosphonium-based ILs.56 Decomposition to volatile products is essentially complete, with 100 °C) increase in the decomposition temperature. The onset of decomposition for the Keggin salt occurs at 455.3 °C (midpoint ) 472.8 °C), whereas that of the Lindqvist derivative falls at 482.5 °C (midpoint ) 490.5 °C). These onset temperatures, it must be noted, compare favorably with those reported for other phosphonium-based ILs
Tetraalkylphosphonium Polyoxometalate Ionic Liquids
Figure 7. Degradation of trihexyl(tetradecyl)phosphonium bromide (A), Keggin (B), and Lindqvist (C) ILs measured by temperatureramped TGA (10 °C/min; N2 flow; Pt pans).
Figure 8. Time-dependent degradation of trihexyl(tetradecyl)phosphonium bromide at 300 °C (A), the corresponding Keggin IL at 400 °C (B) and 360 °C (C), and the Lindqvist IL at 400 °C (D) and 360 °C (E), measured by isothermal TGA (N2 flow; Pt pans).
(Td for the Tf2N- and C(CN)3- salts of (C6H13)3(C14H29)P+, for example, falls at 400 and 415 °C, respectively80) and far exceed the decomposition temperature observed for the one previously reported IL-POM hybrid, 160 °C.39 No weight decrease associated with loss of water is observed for either sample, as expected from their greater hydrophobicity and the care taken to ensure that the samples remained free of moisture. The observed weight loss thus occurs in a single step, and given that the mass of the residue observed for the two ILs agrees well with that expected in the absence of any decomposition of the POMs, corresponds to the destruction of the organic (i.e., phosphonium) portion of the ILs. Numerous investigators have noted that conventional, fast TGA scans may not yield a realistic assessment of the thermal stability of an IL.69,78,79,81,82 (For example, at fast scan rates, slow decomposition reactions might not be adequately captured. In the alternative, inefficient heat transfer in the IL may mean that the IL temperature lags behind the measured temperature.82) As a result, isothermal gravimetric measurements were also carried out on the samples, with the objective of obtaining a more accurate description of the long-term stability of the POMILs at elevated temperatures. The results of these measurements are summarized in Figure 8. For all samples, rapid decomposition is observed when the temperature is maintained at the onset temperature determined in the fast scan TGA runs (results not shown). When held at a temperature 50 °C below the onset temperature (i.e., at 300 °C), nearly 50% decomposition of the
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4691 bromide IL is observed over a 2-h period (A). For the Kegginbased IL, substantially less decomposition (ca. 20-25%) is observed over the same period at a temperature corresponding to 50 °C below its onset temperature (400 °C), decreasing to 15-20% in the 360-380 °C temperature range (B and C). Although the Lindqvist-based IL does exhibit a somewhat lesser extent of decomposition at 400 °C ( Keggin > bromide) is the same as that determined from the rapid scan TGA measurements. It is interesting to note that the three ILs differ not only in their extent of decomposition under a particular set of conditions but also in the shape(s) of the time-dependence of the isothermal decomposition, suggesting a difference in the decomposition mechanism. Additional investigation, while beyond the scope of the present investigation, is clearly warranted. Taken together with the results of the DSC studies, the TGA results indicate that the useful temperature range of the POMbased ILs is quite wide. That is, if the working range of an IL is defined as the region between its melting point (or glass transition) and the onset of decomposition,69 then the useful range of the Keggin-based IL approaches 400 °C, whereas that of the Lindqvist-based IL exceeds 500 °C. Conclusions The results presented here provide a clear illustration of the possibilities afforded by inorganic-organic hybrids (i.e., POMbased ILs) comprising tetraalkylphosphonium cations and polyoxometalate anions. If results obtained for the Lindqvist and Keggin salts of the trihexyl(tetradecyl)phosphonium cation can be regarded as representative, these POM-based ILs maintain the advantageous properties of their constituents, exhibiting electroactivity, relatively high conductivity, and excellent thermal stability. Although the viscosity of these liquid POMs is quite high at modest temperatures, the significant decline in their viscosity that accompanies increasing temperature, together with their apparent resistance to thermal degradation, suggests that this problem is not insurmountable. Given the enormous structural variety possible in both the tetraalkylphosphonium cation and the POM anion, it seems certain that many more POM-based ILs remain to be identified. Through the use of these ILs, many of the distinctive characteristics of POMs (i.e., their nanomolecular morphology, electronic, and photochemical properties) might be more broadly exploited. In particular, the unique combination of properties exhibited by these materials suggests numerous potential applications in areas ranging from battery and fuel cell fabrication to high-temperature electrocatalysis. Efforts to explore these possibilities are now underway in this laboratory. Acknowledgment. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under contract numbers DE-AC02-06CH11357 (Argonne National Laboratory and the Advanced Photon Source) and DEAC02-98CH10886 (Brookhaven National Laboratory). Mass spectrometry was provided by the Washington University Mass Spectrometry Resource, an NIH Research Resource (Grant No. P41RR0954). References and Notes (1) Sanchez, C.; Soler-Illia, G. J. de A. A.; Ribot, F.; Lalot, T.; Mayer, C. R.; Cabuil, V. Chem. Mater. 2001, 13, 3061.
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