A cobalt(ii) bis(salicylate)-based ionic liquid that shows thermoresponsive and selective water coordination

Article abstract of DOI:10.1039/c4cc01023j

A metal-containing ionic liquid (MCIL) has been prepared in which the [Co^II(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. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01023j

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A cobalt(II) bis(salicylate)-based ionic liquid that
shows thermoresponsive and selective water
coordination†
Cite this:DOI: 10.1039/c4cc01023j
Received 7th February 2014,
Accepted 2nd May 2014
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 have revealed properties previously unobserved for neat ILs.3
the [CoII(salicylate)2]2À anion is able to selectively coordinate two For example, Mochida and co-workers reported the reversible
water molecules with a visible colour change, even in the presence coordination of several organic vapours and light gases by
of alcohols. Upon moderate heating or placement in vacuo, the Ni(II)- and Cu(II)-containing MCILs, affording a change between
hydrated MCIL undergoes reversible thermochromism by releasing square-planar and octahedral geometries in the metal centers
the bound water molecules.
along with changes in colour and other physical properties.3b
Such interactions and changes in the metal sites show the
Ionic liquids (ILs) (i.e., neat salts with melting points below potential of MCILs for specific functional applications such as
100 1C) have attracted a great deal of interest as new functional selective chemical binding or sensing.
materials in recent years due to their unique combination of
One particular area of MCIL–solvent research that has not
physico-chemical properties (i.e., vanishingly low vapour pressure, received much attention is the interaction of MCILs with
broad liquid range, ionic conductivity).1 ILs having a metal water. This is likely due to the chemical instability and/or low
complex as a component ion (i.e., metal-containing ILs (MCILs)) miscibility of many MCILs with water.3b,4 Only a handful of
have been widely investigated in the literature.2 MCILs are studies have reported on MCILs in which water is coordinated
considered a distinct subclass of ILs because the inclusion of a to the metal ion.2c,f However, to date, no studies have examined
metal ion potentially allows for incorporation of metal-based the reversibility or selectivity of these MCIL–water coordinative
optical, magnetic, catalytic, or molecular binding properties.
interactions. In general, the exclusion of water molecules from
A variety of transition-metal complexes have been used in MCILs is considered desirable because some MCIL properties
the design of MCILs.2 Most of the studies on MCILs so far have (e.g., their optical properties) can be altered compared to the
been focused on characterising the fundamental coordination anhydrous state.5 In contrast to traditional ‘protection’ from
chemistry and properties of the MCILs in their neat states.2 water, the recent work of Ohno and co-workers has revealed that
Very little work has been performed on investigating their mixtures of some non-metal-based ILs with a small amount of
behaviour in, or interaction with, solvents. In the studies water can yield interesting properties such as enhanced dissolution
reported to date, the coordination number of the constituent of biopolymers,6 liquid-crystalline properties,7 and dynamic phase
metal ion has generally been unresponsive to external stimuli, transition behaviour.8 This work hints at enticing possibilities
rendering it difficult to promote isomerisation of the metal for the discovery and development of interesting and useful
complexes via changes in external conditions. However, several phenomena in MCIL–water mixtures, achievable by careful
recent studies on interactions of MCILs with molecular liquids design of the constituent ions.
Herein, we report a new type of MCIL based on a Co(II)
bis(salicylate) ([CoII(Sal)2]2À) anion and two tetra-n-butylphos-
a Dept. of Chemical & Biological Engineering, University of Colorado, Boulder,
phonium ([P4444]+) cations that shows highly selective and thermally
CO 80309, USA. E-mail: nobler@colorado.edu, douglas.gin@colorado.edu
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
reversible coordination of two water molecules (Fig. 1).
Upon exposure to liquid or gaseous water, the [CoII(Sal)2]2À
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]2À anion with accompanying visible colour
and metal-center geometry changes. Upon heating to moderate
temperatures or placement in vacuo at ambient temperature,
† Electronic supplementary information (ESI) available: Detailed synthesis pro-
cedures 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
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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]2À anion, likely forming multi-
nuclear Co(II) complexes or oligomers (see the ESI† for details
and analysis). Work is in progress to more definitively char-
acterise 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
Fig. 1 Structure of [P4444]2[CoII(Sal)2]; its reversible, highly selective coor-
dination with water; and its accompanying thermochromism.
the resulting [P4444]2[CoII(Sal)2(H2O)2] MCIL undergoes reversi- colour change from deep blue to magenta (i.e., red-purple). The
ble thermochromism via release of the metal-bound water UV-visible spectra were then measured for [P4444]2[CoII(Sal)2]
molecules to re-form the original [P4444]2[CoII(Sal)2] complex. after mixing with different molar equivalents of water mole-
This type of highly selective and reversible thermochromic cules per mole of MCIL (i.e., Mwater). As Mwater was increased,
water coordination is unprecedented in MCILs and illustrates the intensity of the peak at 573 nm decreased compared to that
the potential of such systems for applications such as water at 533 nm. When the Mwater value in the mixture exceeded two,
sensing and water–alcohol sorptive separations. A small number and the spectrum no longer underwent changes upon further
of solid-state materials has been reported to be able to perform addition; only the peak at 533 nm remained (see ESI,† Fig. S17).
selective H2O–alcohol separations (e.g., poly(vinyl alcohol), Similar colour changes have been observed when Co(II) com-
zeolites)9,10 as well as colorimetric sensing and/or binding of plexes undergo a four-coordinate tetrahedral to a six-coordinate
H2O in the presence of alcohols (e.g., MOFs).11 However, to our octahedral geometrical change.11b,14 The absorption intensity
knowledge, a liquid-phase material that is able to do both has at 573 nm of the mixtures as a function of different Mwater
not been reported, especially one that has the unique combi- values was then plotted to estimate the ratio of six-coordinate to
nation of physical properties inherent to ILs.
four-coordinate species at different water loadings and the
[P4444]2[CoII(Sal)2] was prepared by adding aq. solutions of number of water molecules preferentially coordinated per
[P4444][HSal] and LiHSal to an aq. CoII(H2O)6Cl2 solution at a MCIL. In this study, the absorption decreased rapidly with increas-
molar ratio of 0.5 CoII(H2O)6Cl2 : 1.0 [P4444][HSal] : 1.0 LiHSal. ing Mwater and reached a plateau after two equivalents of added
After workup and isolation, the resulting dark blue liquid water molecules (Mwater = 2.0), which is consistent with the
was characterised as [P4444]2[CoII(Sal)2] by a combination of formation of [CoII(Sal)2(H2O)2]2À for the hydrated MCIL anion
elemental analysis; solution magnetic moment measurements (see ESI,† Fig. S18). Also, FTIR analysis of the MCIL–water mixtures
via 1H NMR spectroscopy; ATR-FTIR and UV-visible spectro- showed that the typical bands corresponding to coordinated water
scopies; and variable-temperature magnetic susceptibility studies molecules were found upon increasing the Mwater value (see ESI,†
on the neat material. Because the product was paramagnetic, it Fig. S19). These results strongly suggest that the [CoII(Sal)2]2À
was not possible to perform traditional structural characterisa- anion in the original MCIL binds two water molecules as
tion by 1H and 13C NMR spectroscopy (see the ESI† for full details co-ligands and that the Co(II) center undergoes a four-coordinate
and spectra). Characteristic changes in the FTIR spectra of the to a six-coordinate geometry change.
starting materials vs. that of the formed product indicated that
The colour change upon contact with water can be reversed
the salicylate ligands in the product anion were coordinated in a by evaporating the coordinated water vapour in vacuo at room
bidentate fashion to the Co(II) center (see ESI†). In the solution temperature over 30 min. In addition, the colour change can
state, [P4444]2[CoII(Sal)2] was established to be in the S = 1/2 low- also be readily reversed by mild heating at ambient pressure.
spin state (m = 2.1 Bohr magnetons at 20 1C) by measuring its Fig. 2 shows photographs of the hydrated form of the MCIL,
solution magnetic moment in dichloromethane using the Evans [P4444]2[CoII(Sal)2(H2O)2], upon varying the temperature
method (see ESI,† Table S1).12 The geometrical implication of between 25 and 55 1C. As can be seen in Fig. 2 (left), the
this result is that the anionic Co(II) complex exists in solution as a hydrated sample is magenta at 25 1C, suggesting that the
low-spin square planar.13 UV-visible spectra of [P4444]2[CoII(Sal)2] in [CoII(Sal)2(H2O)2]2À anion exists as six-coordinate (presumably
dichloromethane and in its neat liquid form both show absorption octahedral) complex. Upon heating to 55 1C at ambient pres-
bands at 533 and 573 nm (see ESI†), suggesting that the magnetic sure, the sample becomes deep blue (Fig. 2, right), consistent
and metal coordination environments of [P4444]2[CoII(Sal)2] in with release of the Co(II)-bound H2O molecules to re-form
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species present at different solvent loadings. A detailed descrip-
tion 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).
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 mix-
tures 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) com-
plexes 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.
In summary, a new functional MCIL ([P4444]2[CoII(Sal)2]) has been
prepared that shows selective coordination of water with an asso-
ciated 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 discus-
sions, elemental analysis, and some spectroscopic studies.
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
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Solvent bound
Keq, neat MCIL (MÀ2
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Keq, MCIL as CH CN soln (MÀ2
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3
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205 Æ 7
14 Æ 3
0.4 Æ 0.1
0.15 Æ 0.02
1.6 Æ 0.1
0.74 Æ 0.02
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