J. Am. Chem. Soc. 1999, 121, 613-616
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Hybrid Redox Polyether Melts Based on Polyether-Tailed Counterions Enders Dickinson V, Mary Elizabeth Williams, Susan M. Hendrickson, Hitoshi Masui,† and Royce W. Murray* Contribution from the Kenan Laboratories of Chemistry, The UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290 ReceiVed September 8, 1998
Abstract: Interesting ionic materials can be transformed into room temperature molten salts by combining them with polyether-tailed counterions such as polyether-tailed 2-sulfobenzoate (MePEG-BzSO3-) and polyethertailed triethylammonium (MePEG-Et3N+). Melts containing ruthenium hexamine, metal trisbipyridines, metal trisphenanthrolines, and ionic forms of aluminum quinolate, anthraquinone, phthalocyanine, and porphyrins are described. These melts exhibit ionic conductivities in the 7 × 10-5 to 7 × 10-10 Ω-1 cm-1 range, which permit microelectrode voltammetry in the undiluted materials, examples of which are presented.
Scheme 1
Introduction We describe an apparently general synthetic route to room temperature melts of ionic substances that considerably enlarges the available library of electrochemically active, semisolid, polyether-based melts. Our previous investigations1 of mass and charge transport in rigid media have relied on amorphous, highly viscous, room temperature melts (termed hybrid redox polyether melts) synthesized by covalently attaching polyether chains to redox active molecules. This approach can be challenging and is not applicable to many redox moeities. The new route follows the very recent work of Ohno et al.2 and is based on using polyether-tailed counterions. This paper describes new molten salts containing either polyether-tailed 2-sulfobenzoate (MePEG-BzSO3-) or polyethertailed triethylammonium (MePEG-Et3N+) as counterions of ruthenium hexamine, metal trisbipyridines, metal trisphenanthrolines and ionic forms of aluminum quinolate, anthraquinone, phthalocyanine, and porphyrins. These generally quite viscous melts are prepared from readily available starting materials with neutralization and precipitation reactions (Scheme 1).3 The MePEG-BzSO3- anion was prepared by reacting 2-sulfobenzoic acid cyclic anhydride with MePEG, and the MePEG-Et3N+ cation by reacting triethylamine with tosylated MePEG. The melts are prepared by neutralizing (#1) the hydroxide of a redox cation with (MePEG-BzSO3-)(H+) [I] or (#2) the acid form of a redox anion with (MePEG-Et3N+)(OH-) [VI], by precipitating (#3,4) AgCl from mixtures of silver and chloride salts of the tailed counterions and redox species, or by using a “one pot” reaction (#5) of a metal salt, ligand, and I. Examples of each method are presented in the Experimental Section. Stoichio† Current address: Department of Chemistry, Kent State University, Kent, Ohio, 44242-0001. (1) (a) Williams, M. E.; Masui, H.; Long, J. W.; Malik, J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 1997. (b) Masui, H.; Murray, R. W. Inorg. Chem. 1997, 36, 5118. (c) Long, J. W.; Kim, I. K.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 11510. (d) Williams, M. E.; Crooker, J. C.; Pyati, R.; Lyons, L. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 10249. (2) Ito, K.; Ohno, H. Electrochem. Acta 1998, 43, 1247. (3) Abbreviations: AQSO3- (anthraquinone-2-sulfonate); phen(SO3)22(4,7-diphenyl-1,10-phenanthroline disulfonate); bpy (2,2′-dipyridine); BzSO3(2-sulfobenzoate); Et3N+ (triethylammonium); MePEG (poly(ethylene glycol) methyl ether, MW 350); PC(SO3)44- (phthalocyanine tetrasulfonate); quin-SO3- (5-sulfoquinolate); TMPP4+ (meso-tetra-(n-methyl-4-pyridyl)porphrin); TPP(SO3)44- (meso-tetra(4-sulfonatophenyl)porphyrin).
metric proportions were used insofar as possible to facilitate product isolation. The approach of Scheme 1 offers a convenient synthetic generality in that the melting, or disorganizing, role of the polyether tail is compartmentalized to the counterion and thus readily extended to other interesting ionic moieties. Experimental Section Synthesis of (MePEG-BzSO3-)(H+), I. A modified literature synthesis was used.5 To a 15 mL CHCl3 mixture of 2-sulfobenzoic acid cyclic anhydride (7.5 g; 41 mmol; Aldrich) was added, dropwise over a 45 min period, a 25 mL CHCl3 solution of MePEG (12.95 g; 37 mmol; Aldrich), which had been dried at ∼70 °C under vacuum for ∼12 h. The anhydride dissolved, yielding a clear, light yellow-brown solution. The solution was refluxed for ∼2 h, filtered, flash-evaporated, and dried at ∼70 °C under vaccum for several days, yielding a sweet smelling, pale yellow syrup. 1H NMR in CDCl3: δ 8.10 (m, 1H), 7.62 (m, 3H), 4.51 (t, 2H), 3.94 (t, 2H), 3.60 (m, 25H), 3.32 (s, 3H). Synthesis of (MePEG-Et3N+)(tosylate-), VI. MePEG was converted to its tosylate form according to a previously published (4) Some differences in calculated versus found elemental analysis are thought to arise either from water contaminant or from inaccuracy in the manufacturer’s stated average 350 MePEG molecular weight, or both. (5) Ito, K.; Ohno, H. Solid State Ionics 1995, 79, 300.
10.1021/ja983184e CCC: $18.00 © 1999 American Chemical Society Published on Web 01/15/1999
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Table 1. Glass Transition Temperatures and Ionic Conductivities of Hybrid Redox Polyethers with Polyether-Tailed Counterions melt no.
melt
TG (°C)
σION(25°C) (Ω-1 cm-1)
I II III IV V VI VII VIII IX X XI XII XIII
(MePEG-BzSO3-)(H+) [Ru(bpy)32+](MePEG-BzSO3-)2 [Co(bpy)32+](MePEG-BzSO3-)2 [Ru(NH3)63+](MePEG-BzSO3-)3 [TMPP4+](MePEG-BzSO3-)4 (MePEG-Et3N+)(tosylate-) (MePEG-Et3N+)4[TPP(SO3)44-] (MePEG-Et3N+)4[Fe(phen(SO3)2)34-]a (MePEG-Et3N+)4[Ru(phen(SO3)2)34-]b (MePEG-Et3N+)4[Co(phen(SO3)2)34-]c (MePEG-Et3N+)3[Al(quin-SO3)33-] (MePEG-Et3N+)[AQSO3-] (MePEG-Et3N+)4[PC(SO3)44-]
-47 -13 -10 -34 -63 -65 -43 -26 -3 -20 -40 -46 -49
5.3 × 10-6 1.1 × 10-8 1.5 × 10-8 4.4 × 10-7 7.1 × 10-8 7.6 × 10-5 6.8 × 10-8 6.9 × 10-9 6.1 × 10-10 7.4 × 10-10 2.6 × 10-7 4.7 × 10-6 d 4.6 × 10-7
a With ≈20% excess tailed counterion, relative to ideal 4:1 stoichiometry. b With
