Cyclic phosphonium ionic liquids

Article abstract of DOI:10.3762/bjoc.10.22

Ionic liquids (ILs) incorporating cyclic phosphonium cations are a novel category of materials. We report here on the synthesis and characterization of four new cyclic phosphonium bis(trifluoromethylsulfonyl)amide ILs with aliphatic and aromatic pendant g

Full text of DOI:10.3762/bjoc.10.22

Cyclic phosphonium ionic liquids
Sharon I. Lall-Ramnarine*1, Joshua A. Mukhlall1,2, James F. Wishart*3,
Robert R. Engel*2, Alicia R. Romeo1, Masao Gohdo3,4, Sharon Ramati1,2,
Marc Berman5 and Sophia N. Suarez6
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 271–275.
1Chemistry Department, Queensborough Community College of the
City University of New York, 222-05 56th Avenue, Bayside, NY,
11364, USA, Fax: +1 718-281-5078, Tel: +1 718-281-5572,
2Department of Chemistry and Biochemistry, Queens College of the
City University of New York, 65-30 Kissena Boulevard, Flushing, NY,
11367, USA, Fax: +1 718-997-5531, Tel: +1 718-997-4106,
3Chemistry Department, Brookhaven National Laboratory, Upton, NY,
11973, USA, Fax: +1 631-344-5815, Tel: +1 631-344-4327, 4The
Institute of Scientific and Industrial Research (ISIR), Osaka University,
8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan, 5Department of
Physics and Astronomy, Hunter College of the City University of New
York, 695 Park Ave., New York, NY, 10065, USA and 6Physics
Department, Brooklyn College of the City University of New York,
2900 Bedford Avenue, Brooklyn, NY 11210, USA,
doi:10.3762/bjoc.10.22
Received: 06 August 2013
Accepted: 04 January 2014
Published: 24 January 2014
This article is part of the Thematic Series "Organophosphorus chemistry".
Guest Editor: P. R. Hanson
© 2014 Lall-Ramnarine et al; licensee Beilstein-Institut.
License and terms: see end of document.
Fax: +1 718-951-4407, Tel +1 718-951-5000, ext. 2869
Email:
Sharon I. Lall-Ramnarine* - slallramnarine@qcc.cuny.edu;
James F. Wishart* - wishart@bnl.gov; Robert R. Engel* -
Robert.engel@qc.cuny.edu
* Corresponding author
Keywords:
cyclic phosphonium; ionic liquid; organophosphorus; phosphinanium;
phospholanium; phosphonium
Abstract
Ionic liquids (ILs) incorporating cyclic phosphonium cations are a novel category of materials. We report here on the synthesis and
characterization of four new cyclic phosphonium bis(trifluoromethylsulfonyl)amide ILs with aliphatic and aromatic pendant groups.
In addition to the syntheses of these novel materials, we report on a comparison of their properties with their ammonium congeners.
These exemplars are slightly less conductive and have slightly smaller self-diffusion coefficients than their cyclic ammonium
congeners.
Introduction
The most widely investigated and commercially available ionic related quaternary ammonium) cations with a wide range of
liquids (ILs) are composed primarily of nitrogen-centered anions [1]. While a number of these ILs have been applied
(particularly imidazolium, pyridinium, pyrrolidinium, and successfully to commercial processes [1], efforts are continu-
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Beilstein J. Org. Chem. 2014, 10, 271–275.
ally underway to synthesize ILs with improved properties (such air-stable 1-n-butylphospholane–borane and 1-n-butylphosphi-
as viscosity, thermal stability, melting point, etc.) to expand nane–borane complexes that were purified by silica-gel chroma-
their applications into larger scale processes in diverse areas. tography. The pure compounds were then treated under nitrogen
Along that vein, phosphonium ILs have been observed to have with 1,4-diazabicyclo[2.2.2]octane (DABCO) at 80 °C to
favorable characteristics, including lower costs [2-6], lower remove the borane, and the resultant mixture was purified in a
viscosity (by ~50%) [7-11], greater thermal stability (by nitrogen-filled glove box on a short silica-gel column. The pure
~100 °C) [10-12], and wider electrochemical windows cyclic phosphines (1-n-butylphospholane and 1-n-butylphosphi-
compared to their ammonium congeners. The family of ILs nane) were allowed to react immediately with iodomethane at
bearing cyclic cations (pyrrolidinium, piperidinium, and azepa- room temperature to produce the 1-n-butyl-1-methylphosphola-
nium) [13,14] have been shown to have better transport and nium iodide and 1-n-butyl-1-methylphosphinanium iodide, res-
physical properties than most acyclic tetraalkylammonium salts. pectively. These materials were each mixed with an equivalent
While cyclic phosphonium salts have been noted in patent amount of lithium bis(trifluoromethylsulfonyl)amide (LiNTf2)
applications [15], their characteristics have not been reported in water at room temperature to produce the water-insoluble
therein nor in journals, even though the comparisons noted salts, 1-n-butyl-1-methylphospholanium bis(trifluoromethylsul-
above suggest those properties might be advantageous. To fonyl)amide and the corresponding 1-n-butyl-1-methylphosphi-
address such questions, we report here on the synthesis and nanium bis(trifluoromethylsulfonyl)amide. In a corresponding
physical characterization of some phosphonium analogues to manner were prepared the 1-phenylphospholane and the
pyrrolidinium and piperidinium ILs, i.e. phospholanium and 1-phenylphosphinane by the reaction of dichlorophenylphos-
phosphinanium ILs respectively.
phine with the appropriate bis-Grignard reagents,
BrMg(CH2)4MgBr and BrMg(CH2)5MgBr, respectively. The
The favorable stability of phosphonium ILs makes them good resulting five-membered cyclic 1-phenylphospholane was
choices for applications where the IL would be exposed to isolated in the same manner as was the 1-n-butylphospholane:
extreme conditions, such as environmental exposure to high i.e. by conversion with BH3·THF to give the air-stable
temperatures or exposure to extreme electromagnetic radiation 1-phenylphospholane–borane complex, followed by silica-gel
(as in a dye-sensitized solar cell or ionizing radiation as in the chromatography and then removal of the borane with DABCO,
recycling of spent nuclear fuel). ILs can play important roles in to give the pure 1-phenylphospholane. The six-membered cyclic
both of these latter areas [16]. While dialkylimidazolium ILs phosphine, which appears to be less air sensitive than the
can be efficient extraction media for nuclear separations [17- 1-butylphosphinane species, was purified in open air using a
19], the imidazolium cation is also a good acceptor of radiolyti- short column of silica gel. Both cyclic phosphines were each
cally produced excess electrons. The resultant radical partici- allowed to react with 1-bromobutane at reflux under nitrogen to
pates in radiation damage mechanisms that alter the IL's prop- give the 1-n-butyl-1-phenylphospholanium bromide (4c) and
erties [20]. On the other hand, phosphonium ILs, known for the 1-n-butyl-1-phenylphosphinanium bromide (4d), respective-
their durability and already made in large quantities, can be ly. Each of these salts was then mixed with aqueous LiNTf2 to
optimized for extraction of specific species from radioactive form the 1-n-butyl-1-phenylphospholanium bis(trifluoromethyl-
spent nuclear fuel. Inclusion of phenyl groups, as in two of the sulfonyl)amide (5c) and the corresponding 1-n-butyl-1-
new ILs reported here, can stabilize excess charges (electrons or phenylphosphinanium bis(trifluoromethylsulfonyl)amide (5d).
holes) leading to a more durable extraction system. The ques- The overall synthetic route is summarized in Scheme 1.
tion of whether cyclic phosphonium ILs would succeed in solar
photoconversion or spent fuel processing depends not only on Results and Discussion
whether their properties can be tuned to the application, but also The conductivities, viscosities, self-diffusion coefficients and
whether their reactivity under extreme conditions can be thermal properties of the phosphonium ILs 5a–d and their am-
managed.
monium congeners are given in Table 1.
A major challenge for this work has been the handling of the Comparison of the five-membered cyclic cation ILs C4mPphol
tertiary phosphine precursors, which are pyrophoric and NTf2 5a and C4mPyrr NTf2 (butylmethylpyrrolidinium NTf2)
extremely air sensitive. 1-n-Butylphospholane and 1-n- reveals that the viscosity of the phosphonium salt is one third
butylphosphinane were synthesized by the reaction of higher than that of the ammonium salt (101 cP vs 75 cP at
n-butyldichlorophosphine [21] with the appropriate bis-Grig- 25 °C). For the six-membered rings the phosphonium IL
nard reagents, BrMg(CH2)4MgBr and BrMg(CH2)5MgBr res- C4mPphin NTf2 5b is slightly less viscous than C4mPip NTf2
pectively. The resultant cyclic phosphines are air sensitive and (butylmethylpiperidinium NTf2), 171 cP vs 183 cP. Interest-
therefore were allowed to react with BH3·THF to provide the ingly, the cyclic cations do not follow the common trend among
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Beilstein J. Org. Chem. 2014, 10, 271–275.
Scheme 1: Reaction scheme for the preparation of cyclic phosphonium ionic liquids.
Table 1: Physical characteristics of cyclic phosphonium NTf2 ILs.a
Cationb
Viscosity
cP
Conductivity
mS/cm
D+ (1H)
10−7 cm2/s
D− (19F)
10−7 cm2/s
Tg
°C
Tm
°C
Td
°C
5a: C4mPphol+ 101
2.2 (23 °C)
2.8 [22]
0.92
1.27
1.01
−85
15
444
C4mPyrr+
75 [22]
2.2 [23] (30 °C)
0.65
1.8 [23] (30 °C)
0.68
−89
−8 [24]
6
431 [22]
386
5b: C4mPphin+ 171
−78
C4mPip+
183 [25]
1.2 [25]
–
0.98 [25]
–
0.85 [25]
–
−77 [26]
−66
−25c
29
400 [25]
422
5c: C4PhPphol+ 223
5d: C4PhPphin+ 48 (75 °C) 13.3 (76 °C)
1.56 (75 °C)
2.47 (75 °C)
−48
54
387
aAll values are for measurements at 25 °C except where noted. Viscosities are calculated from VFT fits. All glass-transition temperatures (Tg) and
melting points (Tm) are onset temperatures. Water contents of samples ranged from 76–155 ppm. Uncertainties of viscosities, conductivities, diffu-
sion coefficients (D+ and D−) are 1 cP, 7%, 5% and 1 °C respectively. Td is the thermal decomposition temperature. bAbbreviations and structures of
ammonium congeners: NTf2 - bis(trifluoromethylsulfonyl)amide anion; C4mPphol - butylmethylphospholanium; C4mPyrr - butylmethylpyrrolidinium;
C4mPphin - butylmethylphosphinanium; C4mPip - butylmethylpiperidinium; C4PhPphol - butylphenylphospholanium; C4PhPphin - butylphenylphosphi-
nanium.
cValue subject to dispute, either −25 °C [25] or just below 0 °C [27].
the quaternary tetra-n-alkylated cations wherein the room than that of the C4mPyrr (2.8 mS/cm) although not as much as
temperature viscosities of the phosphonium salts are approxi- expected from the viscosity difference. The self-diffusion coef-
mately one-half of those of their ammonium congeners [7-11]. ficients of the anion (D−) and the cation (D+) are somewhat
This difference draws attention to the particular role that the lower for the phosphonium salts than the ammonium ones.
cyclic alkyl functionality plays in IL fluid dynamics in contrast Interestingly, despite its slightly lower viscosity, C4mPphin IL
to two linear chains. This topic merits further exploration via has lower diffusion coefficients than C4mPip NTf2.
molecular dynamics simulations [28] and physical studies such
as NMR diffusion measurements, nuclear Overhauser effect For the P-versus-N pairs where direct comparisons can be
spectroscopies, and quasi-elastic neutron scattering. As seen for made, the glass-transition temperatures Tg are very close,
instance for the cyclic ammonium salts, the viscosities of the allowing for experimental differences. In contradiction to the
cyclic phosphonium salts increase with increasing ring size for trend for linear tetraalkyl cations, the melting temperatures (Tm)
two of the compounds synthesized. As suggested by the for the cyclic phosphonium salts are higher than those of their
viscosity results, the conductivities of the phosphonium ILs are cyclic ammonium congeners. C4mPphol NTf2 melts 23 °C
lower than those for their ammonium congeners, and that for higher than does C4mPyrr NTf2, and C4mPphin NTf2 melts
both families they decrease with increasing ring size. The 31 °C higher than does C4mPip NTf2. Henderson and Passerini
conductivity of C4mPphol NTf2 (2.2 mS/cm) is slightly smaller [29] showed that the often-cited Tm of −18 °C for C4mPyrr
273

Beilstein J. Org. Chem. 2014, 10, 271–275.
NTf2 was due to a metastable crystalline form by using an have slightly smaller self-diffusion coefficients than their cyclic
annealing process. The DSC scan of un-annealed C4mPphol ammonium congeners. Their viscosities are also lower than
NTf2 shows all the thermal features exhibited by un-annealed their ammonium congeners. In other instances, such as the
C4mPyrr NTf2 (glass transition, cold crystallization, solid–solid peculiarities in melting point and viscosity trends of the C-2-
transition, and melting). When the recommended annealing methylated imidazolium salts [33], ether-substituted ILs [34],
method [24] was applied to C4mPphol NTf2 the first three and bis(oxalato)borate salts [35], detailed investigation into
thermal features listed just above were eliminated, but the pos- those intriguing behaviors led to deeper insights into the
ition of the melting point did not change. The apparent thermal dynamical workings of ILs that have had important impacts on
decomposition temperature (Td) of the cyclic phosphonium the field. For this reason we will continue to explore cyclic
NTf2 ILs are not particularly different from those of the cyclic phosphonium ILs through synthesis of a wider variety of struc-
ammonium NTf2 ILs, which is similar to the situation of linear tural types and substituents, variation of anions, and more
alkyl quaternary cation ILs [11]. In some instances where the extensive characterization of their physical, structural, and
anion is nucleophilic, such as with dicyanamide [12], or the dynamical properties. In particular, this family of ILs, by com-
leaving group is resonance-stabilized, such as a benzyl radical parison with the more familiar cyclic ammonium ILs, should
[10], the phosphonium cations have greater thermal stability reveal the dynamical effects of cyclic vs linear alkyl chains on
than the ammonium ones. One trend observed for both cation the physical properties of ionic liquids.
types is that the ILs with five-membered rings have greater
thermal stability than those with six-membered rings. Further
Supporting Information
work will be required to determine if the observed TGA behav-
iors are due to actual decomposition or to volatility of the ILs
Detailed synthesis and characterization procedures are
[30].
provided for all compounds synthesized and characterized.
NMR spectra are provided for all compounds for which
While developing our synthetic approach to the aliphatic
NMR data is reported. DSC thermograms are provided for
C4mPphol+ and C4mPphin+ cations, which requires the use of
the bis(trifluoromethylsulfonyl)amide ionic liquids
highly air-sensitive reagents and intermediates as described
(5a–5d).
above, we refined our techniques using less reactive
Supporting Information File 1
NMR Spectra.
phenylphosphines. The resulting butylphenylphospholanium
(C4PhPphol NTf2) and butylphenylphosphinanium (C4PhPphin
NTf2) ionic liquids are examples of a rare class of ILs with
direct attachment of the aryl group to the cationic center. We
were unable to find any information on ILs containing analo-
gous nitrogen-centered cations for comparison, although ILs
with aryl groups attached directly to the nitrogen atoms of
imidazolium cations are reported [31]. The melting points of the
phenylphosphonium ILs are both above room temperature (see
Table 1), however C4PhPphol NTf2 is persistently metastable as
a supercooled liquid under ambient conditions. The most
significant effects observed for the replacement of a methyl
group with a phenyl ring in this family of cations are the
substantial increases of 19 °C and 30 °C in the glass-transition
temperatures for the 5- and 6-membered ring cations respective-
ly. Similar effects on Tg have been reported for other classes of
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-22-S1.pdf]
Supporting Information File 2
DSC Thermograms.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-22-S2.pdf]
Supporting Information File 3
Experimental.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-22-S3.pdf]
ILs bearing benzyl groups [32]. This family of cations may Acknowledgements
have the useful attributes for reducing radiation-induced The authors thank the office of Educational Programs at
damage of the IL owing to the stabilization of excess charges on Brookhaven National Laboratory and the NSF NYC Louis
the phenyl groups.
Stokes Alliance for Minority Participation program for student
internship support. This work was supported by the City
University of New York Energy Institute, the Professional Staff
Conclusion
We have successfully prepared a series of new cyclic phospho- Congress of the City University of New York (PSC-CUNY)
nium ILs that are representatives of a wide class of potentially Research Award Program - 43 and the U. S. Department of
useful ILs. These exemplars are slightly less conductive and Energy, Office of Basic Energy Sciences, Division of Chemical
274

Beilstein J. Org. Chem. 2014, 10, 271–275.
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