Ambient temperature imidazolium-based ionic liquids with tetrachloronickelate(II) anions

Article abstract of DOI:10.1016/j.poly.2009.04.037

Deep blue tetrachloronickelate(II) salts of 1-alkyl-3-methylimidazolium cations [C_nmim]⁺ with intermediate chain lengths (n = 5, 7, 8 and an n = 4/6 mixture) have been prepared from imidazolium chlorides and NiCl₂. They are liquid at ambient temperatures (ca. 20 °C) and above, and display viscosities and thermal stability comparable to related [BF₄]^- compounds. The similarities may reflect the compensating effects of the dinegative charge of the [NiCl₄]^2- ion and its larger size.

Full text of DOI:10.1016/j.poly.2009.04.037

Polyhedron 28 (2009) 2355–2358
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Ambient temperature imidazolium-based ionic liquids with
tetrachloronickelate(II) anions
*
M. Brett Meredith, C. Heather McMillen, Jonathan T. Goodman, Timothy P. Hanusa
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, United States
a r t i c l e i n f o
a b s t r a c t
Deep blue tetrachloronickelate(II) salts of 1-alkyl-3-methylimidazolium cations [Cnmim]+ with interme-
diate chain lengths (n = 5, 7, 8 and an n = 4/6 mixture) have been prepared from imidazolium chlorides
and NiCl2. They are liquid at ambient temperatures (ca. 20 °C) and above, and display viscosities and ther-
mal stability comparable to related [BF4]ꢀ compounds. The similarities may reflect the compensating
effects of the dinegative charge of the [NiCl4]2ꢀ ion and its larger size.
Article history:
Received 21 January 2009
Accepted 28 April 2009
Available online 6 May 2009
Keywords:
Ionic liquid
Nickel
Ó 2009 Elsevier Ltd. All rights reserved.
Viscosity
Imidazolium
Thermal stability
1. Introduction
crystal structures for the compounds with n = 2 and 4 demon-
strated the existence of extensive cation-anion hydrogen-bonding.
In the past several decades, the chemistry of room temperature
ionic liquids (RTILs) has expanded markedly as these compounds
have become better understood and more broadly applied in aca-
demic and commercial settings [1]. Their unique properties,
including large liquid temperature ranges, negligible vapor pres-
sure, and adjustable viscosity and basicity, have stimulated uses
as ‘designer solvents’ [2–4], in electrochemistry [5–7], and in catal-
ysis [8]. A substantial part of their attractiveness stems from the
discovery of air- and moisture-stable RTILs with main group anions
such as [PF6]ꢀ [9], [BF4]ꢀ [10], [(CF3SO2)2N]ꢀ [11], and [CF3SO2]ꢀ
[11]. Metal-containing anions from across the periodic table have
also been incorporated into ionic liquids, and they also can possess
air- and moisture-stability; among these are zinc, tin, and iron-
containing [MClx]ꢀ anions associated with various choline-derived
cations [12]. In comparison with commonly used main group
monoanions, metal-containing anions offer the possibility of more
highly charged species (e.g., [LnX6]3ꢀ) [13] and additional varia-
tions in geometries (e.g., tetrahedral and square planar [MX4]2ꢀ
species) [14–16].
Pyridinium-based Ni-containing salts [Cnpy]2[NiCl4] (n = 12, 14, 16,
18) were investigated by Seddon and coworkers, who found all of
them to be mesogenic solids with melting points ranging from 80–
88 °C [19]. Abbott found that a choline-derived nickel chloride salt
had a melting point above 100 °C [12]; it thus falls outside the con-
ventional definition of RTILs (mp < 100 °C) [20].
In a search for nickel-containing ionic liquids that would be
fluid at ambient temperatures (ca. P 20 °C), and which might be
useful in film formation or electrochemical applications [5], we
have investigated the use of imidizolium cations with pendant al-
kyl chains of intermediate length, roughly C5–C10. This region has
been previously identified as one in which compounds with low
melting points are often found, as the hydrogen bonding that con-
tributes to the high melting points of [C(n 6 4)mim]-based RTILs is
disrupted, but the dispersion forces between longer chains that
cause [C(n P 10)mim]-based compounds to exhibit mesogenic char-
acteristics have yet to become dominant [9,10].
In this work we outline the synthesis of several imidazolium
RTILs that incorporate tetrachloronickelate anions and are liquid
at 25 °C. The composition, viscosity and thermal behavior of these
compounds was investigated.
Nickel-based anions have a long history of use in ionic liquids.
Osteryoung investigated the solubility of NiCl2 in chloroaluminate
melts, and determined that the predominant nickel containing
species was [NiCl4]2ꢀ, although several additional ions were also
detected [17]. Imidazolium-based Ni-containing salts [Cnmim]2-
[NiCl4] (n = 2 (mp = 92–93 °C) [18]; 4 (mp = 55–58 °C) [15]; 12–
18 (mp = 50–75 °C) [19]) were later prepared by other workers;
2. Results and discussion
2.1. Synthesis of compounds
The mixture of the alkyl imidazolium chlorides with hydrated
nickel(II) chloride results in dark blue liquids containing the
[NiCl4]2ꢀ anion (Eq. (1)).
* Corresponding author. Tel.: +1 615 322 4667; fax: +1 615 343 1234.
E-mail address: t.hanusa@vanderbilt.edu (T.P. Hanusa).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.04.037

2356
M.B. Meredith et al. / Polyhedron 28 (2009) 2355–2358
2½CnmimꢁCl þ NiCl2ðH2OÞ6 ! 2½Cnmimꢁ2½NiCl4ꢁ þ 6H2O
ð1Þ
than do those with shorter chains; e.g., the viscosity of
the [C8mim]+ salt is between 40% and 60% greater than for the
[C5mim]+ analogue. Below 30 °C, the viscosity of the mixed
[C4mim][C6mim][NiCl4] species (which could be considered to
have an ‘‘average” chain length of C5) tracks closely with that of
the single-chained [C5mim]2[NiCl4] salt. Above that temperature,
the viscosities start to diverge, and values for the mixed imidazoli-
um salt stay roughly 15% below that of the single. The effect of
mixed cations on the viscosities of ionic liquids is not well studied
(cf. the relative rarity of viscosity measurements in systems with
mixed anions [12,22,23]), so that it is not clear how typical the
behavior of [C4mim][C6mim][NiCl4] is.
The water can be removed by heating under vacuum. When n = 5,
the originally mobile liquid forms an opaque aqua-colored gel after
several days at room temperature. When n = 7 or 8, or for the mixed
cation species with n = 4, n = 6, the compounds remain transparent
blue liquids indefinitely. The previously described [C2mim]+ and
[C4mim]+ salts of [NiCl4]2ꢀ, in contrast, have melting points of 92–
93 °C [18] and 56 °C [15], respectively [21]. The salts reported here
are hygroscopic, and deliberate addition of water causes them to
turn green, but the blue color will be regenerated on heating under
vacuum.
Although the data are still limited, at similar temperatures the
[NiCl4]2ꢀ salts appear to have viscosities considerably less than
that observed for [PF6]ꢀ salts, and similar to or slightly greater than
[BF4]ꢀ salts [24,25]. The last point is somewhat surprising, consid-
ering that the dianionic charge on the [NiCl4]2ꢀ ion might be ex-
pected to strengthen coulombic attractions and hence to increase
viscosity. On the other hand, the larger size of [NiCl4]2ꢀ compared
to [BF4]ꢀ (with effective radii of ca. 2.9 Å versus 2.1 Å, respectively)
[26] should weaken such interactions, and serve as a compensating
influence. It is worth noting that the viscosity of the mixed
[C4mim][C6mim][NiCl4] salt is comparable to that of [C4mim]2-
[BF4], underscoring the potential viscosity lowering ability of
mixed cations.
2.2. Viscosity measurements
The viscosities of the nickelate salts were measured from 23–
60 °C (the exact range varied with the cation), and the available
viscosities closest to 25, 30 and 40 °C are given in Table 1. Not sur-
prisingly, the two longer-chained salts with [C7mim]+ and
[C8mim]+ cations have higher viscosities at a given temperature
Table 1
Representative values for substituted imidazolium [NiCl4]2ꢀ, [PF6]ꢀ, [BF4]ꢀ salts.
Ionic liquid
T (°C)
g
(cP)
Ref.
Plots of the complete data sets (Fig. 1) indicate slightly non-
Arrhenius behavior for the ionic liquids at higher temperatures.
Nevertheless, a fit of the data to an Arrhenius-like equation that ex-
presses the energy of activation for viscous flow (Eq. (2)),
[C5mim]2[NiCl4]
[C5mim]2[NiCl4]
[C5mim]2[NiCl4]
[C7mim]2[NiCl4]
[C7mim]2[NiCl4]
[C7mim]2[NiCl4]
[C8mim]2[NiCl4]
[C8mim]2[NiCl4]
[C8mim]2[NiCl4]
[C8mim][PF6]
24.9
30.2
40.2
26.3
30.1
40.0
27.7
30.2
40.0
30
125
76
33
178
116
49
144
109
50
452
82
122
70
30
204
This work
This work
This work
This work
This work
This work
This work
This work
This work
[24]
=RT
g
g
¼
g
0eE
ð2Þ
for which E is the energy of action for viscous flow, and
g0 is a con-
g
stant, is very good for all four liquids (r2 > 0.99). The E values (Table
g
2) cluster around 65–69 kJ molꢀ1, without an obvious correlation
with the chain length. These are near the value of 55.7 kJ molꢀ1
determined for the RTIL [choline][Zn2Cl5] [12], but are roughly four
times higher than the average of 17 3 kJ molꢀ1 found for various
RTILs with more common anions ([BF4]ꢀ, [PF6]ꢀ, [Tf2N]ꢀ, [Tf2C]ꢀ),
even when the cations are hydroxy-substituted, and thus capable
of engaging in hydrogen bonding [27].
[C8mim][BF4]
30
[25]
[C4mim][C6mim][NiCl4]
[C4mim][C6mim][NiCl4]
[C4mim][C6mim][NiCl4]
[C4mim][PF6]
[C4mim][BF4]
[C6mim][PF6]
25.1
30.0
40.0
30
30
30
This work
This work
This work
[24]
[24]
[24]
91.4
363
177
[C6mim][BF4]
30
[24]
Fig. 1. Viscosity of imidazolium [NiCl4]2ꢀ salts as a function of temperature.

M.B. Meredith et al. / Polyhedron 28 (2009) 2355–2358
2357
Table 2
charge, the viscosity and thermal properties of the resulting liquids
are similar to related [BF4]ꢀ-based species. The nickelate-based liq-
uids could provide useful substrates for electrochemical studies
and metal deposition reactions.
Energy of activation for viscous flow of tetrachloronickelate salts.
Compound
E
g
(kJ molꢀ1
)
r2
[C5mim]2[NiCl4]
[C7mim]2[NiCl4]
[C8mim]2[NiCl4]
[C4mim][C6mim][NiCl4]
65.6
67.8
64.6
68.7
0.997
0.998
0.995
0.996
3. Experimental
3.1. General considerations
Organic and inorganic starting materials were reagent grade
from Acros or Strem Chemical and used as received. The prepara-
tion of 1-butyl-3-methylimidazolium chloride ([C4mim]Cl), 1-pen-
tyl-3-methylimidazolium chloride ([C5mim]Cl), 1-hexyl-3-
methylimidazolium chloride ([C6mim]Cl), 1-heptyl-3-methylimi-
dazolium chloride ([C7mim]Cl), and 1-octyl-3-methylimidazolium
chloride ([C8mim]Cl) followed literature procedures [32].
Viscosity data were obtained on a Cambridge Applied Systems
Viscolab 400 Viscometer fitted with heating tape. All thermal mea-
surements were carried out on an Instrument Specialist TGA-1000
instrument. Elemental analyses were performed by the University
of Illinois Microanalytical Laboratory, Desert Analytics (Tucson,
AZ), or at the Micro-Mass Facility, University of California, Berkeley
(Berkeley, CA).
2.3. Thermal behavior
The thermal stabilities of imidazolium salts have been studied
under various conditions, and have been found to have more
dependence on anion type than cation size [24]. The three single-
chain [Cnmim]2[NiCl4] salts were investigated with scanning TGA,
and all were found to decompose in a similar, multistep manner.
A TGA scan for [C7mim]2[NiCl4] is presented in Fig. 2. Onset of
decomposition (2% weight loss) under N2 occurs just above
300 °C, and approximately two-thirds of the mass is gone by
430 °C, which corresponds to the loss of both imidazolium cations.
Such volatilization temperatures are typical for imidazolium salts
[28], but determination of the exact temperature and rate at which
decomposition occurs is a non-trivial matter [29]. With imidazoli-
um salts, whether decomposition occurs in a single or in multiple
steps varies with the anion; [C1mim][PF6] decomposes in a single
step, for example, but [C1mim][BF4] (like the tetrachloronickelate
liquids) does so in several steps [30].
3.2. Preparation of bis(1-pentyl-3-methylimidazolium)
tetrachloronickelate(II), [C5mim]2[NiCl4]
1-Pentyl-3-methylimidazolium chloride (5.26 g, 27.9 mmol)
and NiCl2(H2O)6 (3.31 g, 13.9 mmol) were placed in a 50 mL round
bottom flask equipped with a magnetic stirring bar. The flask was
flushed with N2 and the reaction was allowed to stir at 80 °C for
18 h. The reaction was removed from heat and allowed to cool to
room temperature. The solution was dried under vacuum at
95 °C to afford a dark blue liquid (6.24 g, 88%). Anal. Calc. for
C18H34Cl4N4Ni: C, 42.64; H, 6.76; N, 11.05. Found: C, 39.31; H,
6.38; N, 10.19%. Although individual percentage values are low,
the C:H:N values are in nearly exact molar ratios (18.0:34.8:4.0),
suggesting that combustion was incomplete.
Continued heating of [C7mim]2[NiCl4] eventually leads to a con-
stant weight near 690 °C, with a residual mass of 14%. This value
does not correspond to the presence of completely reduced nickel
or nickel chloride alone. To investigate this point further, a sample
of [C7mim]2[NiCl4] was placed in a tube furnace at 700 °C for 1 h
under N2. After the sample cooled, powder XRD analysis on the
resulting metallic-appearing film indicated the presence of both
elemental nickel and NiCl2; given this information, the residual
weight in the TGA experiment corresponds to a Ni:NiCl2 ratio of
3:1. Other modifications to the anion could make the decomposi-
tion to nickel alone more likely [31].
3.3. Preparation of bis(1-heptyl-3-methylimidazolium)
tetrachloronickelate(II), [C7mim]2[NiCl4]
2.4. Conclusions
The strategy of using imidiazolium cations with intermediate
chain lengths to produce ambient temperature ionic liquids is suc-
cessful with [NiCl4]2ꢀ anions. Despite the latters’ dinegative
1-Heptyl-3-methylimidazolium chloride (4.29 g, 19.8 mmol)
and NiCl2(H2O)6 (2.35 g, 9.89 mmol) were placed in a 100 mL
Fig. 2. Thermal behavior of [C7mim]2[NiCl4] under N2 (20° minꢀ1). The arrow at 305 °C marks the decomposition onset.

2358
M.B. Meredith et al. / Polyhedron 28 (2009) 2355–2358
[2] T. Welton, Chem. Rev. 99 (1999) 2071.
round bottom flask equipped with a magnetic stirring bar. The
flask was flushed with N2 and the reaction was allowed to stir at
80 °C for 17 h. The reaction was removed from heat and allowed
to cool to room temperature. The solution was dried under vacuum
at 95 °C to afford a dark blue liquid (4.65 g, 83%). Anal. Calc. for
C22H42Cl4N4Ni: C, 46.93; H, 7.52; N, 9.95. Found: C, 46.04; H,
7.57; N, 9.80%.
[3] I. Newington, J.M. Perez-Arlandis, T. Welton, Org. Lett. 9 (2007) 5247.
[4] N.V. Plechkova, K.R. Seddon, Ionic liquids: designer solvents for green
chemistry, in: A. Perosa, F. Zecchini, P. Tundo (Eds.), Methods and Reagents
for Green Chemistry: An Introduction, John Wiley and Sons, Hoboken, NJ, 2007.
[5] M.-J. Deng, I.W. Sun, P.-Y. Chen, J.-K. Chang, W.-T. Tsai, Electrochim. Acta 53
(2008) 5812.
[6] T.-I. Leong, I.W. Sun, M.-J. Deng, C.-M. Wu, P.-Y. Chen, J. Electrochem. Soc. 155
(2008) F55.
[7] D.-X. Zhuang, M.-J. Deng, P.-Y. Chen, I.W. Sun, J. Electrochem. Soc. 155 (2008)
D575.
3.4. Preparation of bis(1-octyl-3-methylimidazolium)
tetrachloronickelate(II), [C8mim]2[NiCl4]
[8] P. Wasserscheid, W. Keim, Angew. Chem., Int. Ed. 39 (2000) 3772.
[9] C.M. Gordon, J.D. Holbrey, A.R. Kennedy, K.R. Seddon, J. Mater. Chem. 8 (1998)
2627.
[10] J.D. Holbrey, K.R. Seddon, J. Chem. Soc., Dalton Trans. (1999) 2133.
[11] P. Bônhote, A.-P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Grätzel, Inorg.
Chem. 35 (1996) 1168.
[12] A.P. Abbott, G. Capper, D.L. Davies, R. Rasheed, Inorg. Chem. 43 (2004) 3447.
[13] K. Binnemans, Chem. Rev. 107 (2007) 2592.
[14] M.S. Sitze, E.R. Schreiter, E.V. Patterson, R.G. Freeman, Inorg. Chem. 40 (2001)
2298.
[15] C. Zhong, T. Sasaki, A. Jimbo-Kobayashi, E. Fujiwara, A. Kobayashi, M. Tada,
Y. Iwasawa, Bull. Chem. Soc. Jpn. 80 (2007) 2365.
1-Octyl-3-methylimidazolium chloride (4.54 g, 19.7 mmol) and
NiCl2(H2O)6 (2.34 g, 9.84 mmol) were placed in a 100 mL round
bottom flask equipped with a magnetic stirring bar. The flask
was flushed with N2 and the reaction was allowed to stir at 90 °C
for 9 h. The reaction was removed from heat and allowed to cool
to room temperature. The solution was dried under vacuum at
90 °C for 2 h to afford a dark blue liquid (5.17 g, 89%). Anal. Calc.
for C24H46Cl4N4Ni: C, 48.72; H, 7.84; N, 9.48. Found: C, 47.91; H,
7.90; N, 9.38%.
[16] R.E.D. Sesto, T.M. McCleskey, A.K. Burrell, G.A. Baker, J.D. Thompson, B.L. Scott,
J.S. Wilkes, P. Williams, Chem. Commun. (2008) 447.
[17] R.J. Gale, B. Gilbert, R.A. Osteryoung, Inorg. Chem. 18 (1979) 2723.
[18] P.B. Hitchcock, K.R. Seddon, T. Welton, J. Chem. Soc., Dalton Trans. (1993) 2639.
[19] C.J. Bowlas, D.W. Bruce, K.R. Seddon, Chem. Commun. (1996) 1625.
[20] J.D. Holbrey, R.D. Rogers, Physicochemical properties of ionic liquids: melting
points and phase diagrams, in: P. Wasserscheid, T. Welton (Eds.), Ionic Liquids
in Synthesis, Wiley-VCH, Weinheim, 2008.
3.5. Preparation of (1-butyl-3-methylimidazolium)(1-hexyl-3-
methylimidazolium) tetrachloronickelate(II), [C4mim][C6mim][NiCl4]
[21] The melting points of the single-chain liquids were below 0 °C, but attempts to
measure the exact temperatures with DSC were unsuccessful.
[22] J.S. Wilkes, J.A. Levisky, R.A. Wilson, C.L. Hussey, Inorg. Chem. 21 (1982) 1263.
[23] J.R. Sanders, E.H. Ward, C.L. Hussey, J. Electrochem. Soc. 133 (1986) 325.
[24] J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A. Broker, R.D.
Rogers, Green Chem. 3 (2001) 156.
[25] L.C. Branco, J.N. Rosa, J.J.M. Ramos, C.A.M. Afonso, Chem. Eur. J. 8 (2002) 3671.
[26] D.M.P. Mingos, A.L. Rohl, J. Chem. Soc., Dalton Trans. (1991) 3419.
[27] H. Jin, B. O’Hare, J. Dong, S. Arzhantsev, G.A. Baker, J.F. Wishart, A.J. Benesi,
M. Maroncelli, J. Phys. Chem. B 112 (2008) 81.
[28] B.K.M. Chan, N.-H. Chang, M.R. Grimmett, Aust. J. Chem. 30 (1977) 2005.
[29] K.J. Baranyai, G.B. Deacon, D.R. MacFarlane, J.M. Pringle, J.L. Scott, Aust. J.
Chem. 57 (2004) 145.
[30] D.M. Blake, L. Moens, D. Rudnicki, H. Pilath, J. Sol. Energy Eng. 128 (2006) 54.
[31] In preliminary experiments, the reaction of 1-heptyl-3-methylimidazolium
iodide and NiI2 was used to prepare [C7mim]2NiI4. The dark red liquid was
heated to 700 °C for 30 min under N2. Powder XRD indicated that the residue
1-Butyl-3-methylimidazolium chloride (2.25 g, 12.9 mmol), 1-
hexyl-3-methylimidazolium chloride (2.61 g, 12.9 mmol), and
NiCl2(H2O)6 (3.06 g, 12.9 mmol) were placed in a 100 mL round
bottom flask equipped with a magnetic stirring bar. The flask
was flushed with N2 and the reaction was allowed to stir at 90 °C
for 23 h. The reaction was removed from heat and allowed to cool
to room temperature. The solution was dried under vacuum at
100 °C to afford a dark blue liquid (5.97 g, 91%).
Acknowledgment
We thank the Petroleum Research Fund, administered by the
American Chemical Society, for support of this research.
consisted only of elemental nickel;
a Scherrer analysis of peak widths
suggested an average particle size of 44 nm.
[32] J.G. Huddleston, R.D. Rogers, Chem. Commun. (1998) 1765.
References
[1] N.V. Plechkova, K.R. Seddon, Chem. Soc. Rev. 37 (2008) 123.