Ultrafast Ligand Self-Exchanging Gadolinium Complexes in Ionic Liquids for NMR Field Probes

Article abstract of DOI:10.1021/acs.inorgchem.7b03191

Concurrent magnetic field monitoring in MRI with an array of NMR field probes allows for reducing image imperfections. High ¹⁹F concentrations together with short relaxation times are basic probe properties. We present the NMR properties of [Gd

Full text of DOI:10.1021/acs.inorgchem.7b03191

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Ultrafast Ligand Self-Exchanging Gadolinium Complexes in Ionic
Liquids for NMR Field Probes
Anna Looser,† Christoph Barmet,‡,§ Thomas Fox,† Olivier Blacque,† Simon Gross,‡ Jennifer Nussbaum,‡
Klaas P. Pruessmann,*,‡ and Roger Alberto*,†
†Department of Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
‡Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Gloriastrasse 35, CH-8092 Zurich, Switzerland
§Skope Magnetic Resonance Technologies AG, Gladbachstrasse 105, CH-8044 Zurich, Switzerland
S
* Supporting Information
ABSTRACT: Concurrent magnetic field monitoring in MRI with an array of NMR field probes allows for reducing image
imperfections. High 19F concentrations together with short relaxation times are basic probe properties. We present the NMR
properties of [Gd(NTf2)4]− dissolved in an ionic liquid consisting of an imidazolium cation and the [NTf2]− anion. These
solutions achieve fluorine concentrations as high as 26 M and rapid relaxation times in the sub-ms time range. The second order
self-exchange rate of coordinated [NTf2]− with [NTf2]− as determined with [Y(NTf2)4]− is 4.5 × 105·M−1·s−1. X-ray structure
analyses confirm the coordination number of eight around Gd3+ and Y3+. These ionic-liquid solutions are thus excellent
candidates for dynamically probing the NMR fields in MRI.
another nucleus, the best option being 19F due to its favorable
INTRODUCTION
■
gyromagnetic ratio.5 To yield coherent output of an adequate
Magnetic resonance imaging (MRI) is a powerful and widely
signal-to-noise ratio, 19F nuclei in the probe liquid must be all
deployed tool for medical diagnostics. It is based on nuclear
magnetically equivalent and of high density on the order of tens
magnetic resonance (NMR) of 1H nuclei in magnetic
of M. In existing implementations, this is achieved with
background fields of 1 T and above. For spatial encoding,
NMR in the human body is modulated by magnetic gradient
fields generated by driving high currents through suitable coils.
Deviations from nominal gradient dynamics are caused by
various hardware imperfections such as delays, eddy currents,
mechanical oscillations, and thermal drifts. By perturbing the
signal encoding, such imperfections cause image artifacts.
Among others, deficiencies of the encoding fields form one
perfluorinated organic compounds such as hexafluorobenzene,
doped with a relaxation agent to accelerate probe recovery after
each field readout. Concurrent field measurement with such
probes has recently enabled a range of promising applica-
tions.5−9
In efforts to improve further the versatility of field recording
during MRI, the next key challenge is to render it fully
continuous. The need for probe recovery between excitations
causes dead time, and probe signals may fade upon dephasing
main hindrance to fast MRI with demanding gradient schemes
such as echo-planar1 and spiral2 readouts.
in strong, sustained gradient fields. A system that overcomes
Encoding errors can be addressed by means of image
reconstruction if the actual magnetic field dynamics are known
with high fidelity. A recent general means of obtaining such
1
these interruptions was recently created from H field probes
using water droplets.10 It achieves continuity of field measure-
ment by rapid alternation between probes with very short
information is concurrent magnetic field recording with an
array of NMR field probes.3,4 The principle of these probes is
relaxation times (<1 ms). In principle, this approach should
hold equally for 19F probes and their operation during MRI
to acquire NMR signals from small (μL) droplets of a sample
liquid and determine field dynamics from their phase time
courses. To avoid interference with MRI at the H resonance
1
Received: December 19, 2017
frequency, concurrent NMR in field probes must be based on
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b03191
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Scheme 1. Synthesis of [NnBu4][Gd(NTf2)4] (3)
procedures. However, unlike for water, for most fluorine-rich
liquids it is difficult to bring relaxation times down to the sub-
ms range. For effective relaxation enhancement, the 19F atoms
must interact with paramagnetic centers at close range, ideally
in the first coordination sphere. However, paramagnetic salts
and metal complexes are hardly soluble in fluorocarbons. Metal
complexes with homoperfluorinated ligands, on the other hand,
are typically solid. For use in a field probe, they would need to
be dissolved, reducing the 19F concentration below useful levels.
To overcome this impasse, in the present work we explore a
novel strategy for reconciling high 19F density with rapid
relaxation. The basic idea is to form a homoperfluorinated ionic
liquid (IL), in which the anions also act as ligands for a
dissolved paramagnetic-metal complex. We show that such a
system can be implemented with the new Gd(III) complex
a
n
(i) THF, BuLi, HN(SiMe3)2, 0°C, 24 h, 80%; (ii) toluene, Tf2NH,
r.t. 71%; and (iii) CH2Cl2 or C2H2Cl4, 24 h, 95%, solvent evaporation.
For synthetic details, see Supporting Information.
[NnBu4][Gd(NTf2)4] (NTf2 = bis(trifluoromethylsulfonyl)-
−
eight to form the homoleptic complex [NnBu4][Gd(NTf2)4]
imide), obtained from the reaction of its precursor [Gd-
(NTf2)3] with [NnBu4](NTf2). A solution of [Gd(NTf2)3] in
(3). An ORTEP diagram is shown in Figure 1.
ILs of the triflimide type [cat+][NTf2 ] (cat+ = amim = 1-allyl-
−
The four NTf2 ligands coordinate to Gd3+ in a transoid
fashion with respect to the CF3 groups. While crystal structures
of binary complexes with the NTf2 ligand are rare (vide
supra), and only a few examples have been reported in
−
3-methylimidazolium, emim = 1-ethyl-3-methylimidazolium) is
shown to yield a single 19F signal, reflecting rapid ligand self-
exchange, and relaxation times in the sub-ms range as targeted.
−
−
literature, anhydrous lanthanide NTf2 compounds are even
rarer.17,21,22 The hydrated compounds reported by Bhatt et al.23
RESULTS AND DISCUSSION
■
Gadolinium, as a middle lanthanide element, is of typical hard
character and displays generally complexes with coordination
numbers of 8−9. Its complexes with a multidentate ligand and
one or more coordinated water molecules are key as MRI
contrast agents.11,12 The principle behind the applicability of
these agents is the rapid exchange of coordinated water
molecules for allowing magnetization transfer to bulk water.13
In contrast to in vivo applications of Gd3+ complexes, NMR
field probes as described in the introduction require complexes
that exchange their homoperfluorinated ligands rapidly with the
bulk ionic-liquid solvent. Therefore, Gd3+ complexes with
weakly coordinating anionic ligands are key for our purpose.14
At the same time, these anions should be perfluorinated and
form ionic liquids15 in order to ensure one single 19F signal.
These criteria reduce the selection of anions substantially.
and Hoogerstraete et al.24 show either nine-coordinated
−
lanthanide cations with three NTf2 ligands in a transoid
orientation for the early lanthanides or the eight-coordinated
structure with a monodentate coordination of the NTf2 and
with five H2O ligands as found for the middle and heavier
−
lanthanides. The only known eight-coordinated structure
containing four NTf2 ligands is that of [YbII(NTf2)4]2−
−
exhibiting a cisoid conformation of the ligands.17 This structure
corroborates that Gd3+, similar to reported transition metal
centers,21 will extend its coordination sphere in an IL
comprising [NTf2]−. For achieving exclusive coordination to
weakly coordinating anions, the presence of stronger O-donors,
−
which compete with NTf2 coordination, must be omitted.
Accordingly, when reacting Gd(O-iPr)3 instead of [Gd(bta)3] 1
with Tf2NH or by crystallizing 2 in the presence of small
amounts of isopropanol, crystals of the nine-coordinate
complex [Gd(NTf2)3(HO-iPr)3] (4) could be grown. An X-
ray structure analysis reveals that the coordination environment
−
Bis(trifluoromethylsulfonyl)imide (NTf2 ) is particularly con-
venient due to its potential bidentate coordination mode. Only
a limited number of homoleptic d- or f-element complexes with
−
−
NTf2 as ligands have been reported, among them one with
in this complex consists of three bidentate NTf2 ligands in a
Co2+,16 Yb2+ and with Nd3+, Tb3+, Tm3+, and Lu3+.17,18
cisoid arrangement (see Figure 1 and Supporting Information).
Together with the three isopropanol ligands, the coordination
number is 9 in this complex. Similar ternary complexes with
water instead of isopropanol display transoid arrangements,
probably due to steric reasons.23,24
Binary gadolinium complexes with weakly coordinating
anions (A−) as ligands can be prepared by protonation of
basic L in GdL3 type precursors with the respective conjugate
acid HA. Concomitant ligand exchange leads to desired
GdA3.19 Accordingly, we prepared in a first step [Gd(bta)3]
(1, bta = bis(trimethylsilyl)amide) from an anion metathesis
reaction in THF between GdCl3 and Li[bta].20 The reaction
between 1 and the strong acid Tf2NH gave the complex
[Gd(NTf2)3] (2), which could be isolated as a white powder in
a clean reaction (Scheme 1). Complex 2 is very soluble in
triflimide-based IL (vide infra). Due to its coordinatively
unsaturated state, we hypothesized that 2 will form in the IL a
complex with a coordination number higher than six, as implied
by the stoichiometric formula. To support this hypothesis, we
reacted 2 with 1 eq of [NnBu4](NTf2) in dichloromethane or
tetrachloroethane.21 After very slow evaporation of the solvent
over days in the glovebox, crystals of quality for X-ray
diffraction structure analysis could be grown, which confirmed
its structure; 2 extended its coordination sphere from six to
The NTf2− anion is a very common constituent of ILs with a
broad range of physicochemical properties and applica-
tions.25−27 For the purpose as the NMR field probe, suitable
ILs should have low viscosity and dissolve 2 at the highest
possible concentration to reach the attempted >25 M [19F]
limit. Among the described ILs, imidazolium-based ones with
NTf2− approach these properties best.28 The IL [amim][NTf2]
(amim+ = 1-allyl-3-methyl-1H-imidazol-3-ium) has a viscosity η
= 40 mPa·s (20 °C) and a fluoride concentration [F] of 22.7 M
according to its density of ρ = 1.524 g/mL. We focused our
studies on this IL, and some additional results with [emim]-
[NTf2] (emim+ = 1-ethyl-3-methyl-1H-imidazol-3-ium) are
given in the Supporting Information.
Ln3+ complexes undergo generally fast dissociative ligand
self-exchange, especially with weakly coordinating anions or
B
DOI: 10.1021/acs.inorgchem.7b03191
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. ORTEP of [NnBu4][Gd(NTf2)4] (3, left) and [Gd(NTf2)3(HO-iPr)3] (4, right). Cations omitted and fluorides shown as spheres for
−
clarity. The transoid and cisoid configurations in 3 and 4, respectively, are visible for the [NTf2 ] ligands. Ellipsoids except for fluorides are drawn on
the 30% probability level (see Supporting Information for details).
neutral molecules such as water.29−32 For an estimation of the
components around a chemical shift of −81.7 ppm (Figure 3).
The high field component could be attributed to the complex
ligands based on the comparison with mixtures containing
other complex/IL ratios. At 185 K, the calculations indicate two
different 19F shifts for the complex ligands with a separation of
0.46 ppm and an inherent line width of around 180 Hz. It is
remarkable that the higher populated complex ligands exhibit
an unambiguously broader line than the less populated free
ligands (population ratio 4:1.5). One possible explanation is
still that fast intramolecular dynamic processes occur between
different complex sites, whereas intermolecular exchange with
the free ligands is already slow. Anisotropy effects can be
another reason, because they might be stronger in the larger
and less symmetric complex as compared with the free ligands.
With regard to these uncertainties and especially to the
unknown exchange mechanism, the calculated rates as well as
the resulting activation barrier (see below) can just be
approximate values. However, at any time, the observed 19F
line patterns did not exceed a total width of 450 Hz. This value
is small, compared to the line broadening effects within
paramagnetic complexes, which are in addition resolved in
highly viscous IL. Therefore, line splittings, which are, in
analogy to 6, potentially also expected for solutions of 2 in IL,
are unlikely and actually not observed, since they are wiped out
by much stronger effects of paramagnetism and viscosity. This
results in single 19F resonances and complies well with an
important criterion for accurate MRI calibration (for a
temperature dependence study with [emim][NTf2], see
Supporting Information).
NTf2 self-exchange rate within the paramagnetic Gd3+
−
complex 3 via NMR, we chose the analog but diamagnetic
Y3+ complex, since both lanthanoides have comparable ionic
radii.12 The complex [Y(NTf2)3] (6) was prepared in the same
way as 2 via [Y(bta)3] (5) and crystallized in the presence of
excess [NnBu4][NTf2]. The structure of the resulting complex
[NnBu4][Y(NTf2)4] (7) could be solved and is, in agreement
with the similarity of Y3+ and Gd3+, isostructural to 3 (see
Supporting Information). Thus, we recorded temperature
dependent 19F NMR spectra of the model complex 7 in
diluted mixtures with the IL in order to monitor the ligand
exchange and to calculate the exchange rates on the base of line
shape analysis with the program gNMR. Solutions of the
gadolinium complex 3 in pure IL are not suitable for this
purpose, since its strong paramagnetism and the temperature
dependent viscosity of the IL solvent wipe out the influence of
dynamic effects onto the line shapes.
The room temperature 19F NMR spectra of solutions of 6
and [amim][NTf2] in d6-acetone show different signals at
−80.35 and −80.41 ppm, respectively (Figure 2a,b). A 1:1
An Arrhenius analysis of the obtained rate constants
according to Figure 3 yields a pre-exponential constant of
about 109 s−1, an activation barrier of ∼24 kJ/mol, and a second
order rate constant kex at r.t. of 4.5 × 105·M−1·s−1. In undiluted
IL, the pseudo first order exchange constant is thus well below
the μs range (see Supporting Information).
Figure 2. Room temperature, 470.67 MHz 19F NMR spectra (d6-
acetone) of 0.1 M Y(Tf2N)3 6 (a), 0.1 M ionic liquid [amim][NTf2]
(b), and a (1:1) 0.1 M mixture of both (c).
For evaluating the utility of Gd3+/IL solutions for an NMR
field probe application, we investigated their T1 and T2
dependencies on concentration and viscosity. When [Gd3+] is
kept constant at 0.1 M under successive decrease of [F−]IL and
viscosity by dilution with CH2Cl2, relaxation times display the
trends that are depicted in Figure 4 (left). Without dilution
with CH2Cl2 ([F] = 23 M), the observed T1 time of 1.8 ms is
long compared with T2 of 0.17 ms. Upon dilution, viscosity
decreases, and T1 reduces to 0.4 ms, accompanied by just a
mixture displays just one sharp resonance for free and bound
NTf2− at −80.36 ppm (Figure 2c), indicating the expected fast
exchange between these two components on the NMR time
scale.
The 19F resonances of diluted solutions of 6 and IL in
CD2Cl2 after cooling from 233 to 185 K show successive line
broadening, coalescence, and splitting into two distinguishable
C
DOI: 10.1021/acs.inorgchem.7b03191
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. 19F NMR (282.39 MHz) spectra of a mixture of 6 with [amim][NTf2] (1:1.5) in CD2Cl2 at different temperatures. Top parts: observed
spectra; bottom parts: calculated line shapes at appropriate exchange rate [Hz].
Figure 4. 19F T1 and T2 relaxation times of [Gd(NTf2)3] (2) in the ionic liquid [amim][[NTf2]; 0.1 M [Gd3+] in IL diluted with dichloromethane to
the indicated fluoride concentration (left) and over a range of concentrations of 2 in pure ionic liquid at 298 and 313 K (right).
slight T2 increase. However, after dilution, [F−] amounts to just
8 M, which is too low for useful field probes. Alternatively, a
variation of the complex concentration in undiluted IL has been
checked in order to accelerate T1 relaxation (Figure 4, right).
Due to increasing complex concentrations up to 0.46 M and
growing paramagnetic influences, T1 and T2 reach values of 0.5
and 0.04 ms, respectively, along with a total fluorine
concentration of up to 25.6 M (density 2.125 g/cm3, for
details, see Supporting Information)
are steered by molecular reorientation rates, which directly
depend on the viscosity of the medium. At reduced
temperatures and a lower degree of IL dilution, viscosity
increases with the consequence of reduced molecular mobility.
This leads to less efficient T1 relaxation, in particular for high
frequency NMR nuclei like 19F (ω = 282 MHz): the decreasing
molecular tumbling frequency τc−1 then reaches values far
below ω/0.62 (i.e., average tumbling rate for most efficient T1
relaxation). On the other hand, at low tumbling rates,
electron−nuclear dipolar couplings are less efficiently sup-
pressed by averaging effects, and therefore, T2 relaxation is
accelerated. Moreover, at high Gd concentrations, the observed
T2 relaxation times show no significant temperature depend-
ence (Figure 4, right). This is consistent with the assumed
pseudo first order nature of the ligand self-exchange, which is
not diffusion controlled and, therefore, less dependent on the
viscosity, since the free ligands of the IL solvent are densely
packed in the second (and third) coordination sphere of the
metal center.
Overall, the proposed Gd3+/IL solutions offer a highly
interesting system for use as NMR field probes. With achieved
T1 and T2 times down to the sub-ms range, and fluoride
concentrations of up to 25 M, it enables rapid probe re-
excitation and continuous field observation with alternating
sets. In comparison, previously deployed systems based on
Cr(TMHD)3 dissolved in hexafluorobenzene exhibited minimal
T1 and T2 times of 77 and 47 ms, respectively, and were thus
unsuitable for such use.3 Moreover, the proposed system offers
the flexibility of fine-tuning both T1 and T2 over a wide range to
match the requirements of specific applications. Here, lower
signal intensity due to the reduction of fluoride concentration
must not necessarily decrease net field sensitivity, since its effect
can be compensated by equally lowered T1/T2 ratios, exhibiting
favorable steady-state signal intensity at rapid probe operation.
Figure 4 reflects an expected T1 and T2 relaxation behavior
upon temperature and concentration changes. Both T1 and T2
CONCLUSION
■
Solutions of Gd3+ complexes with homoperfluorinated, anionic
ligands in ILs that comprise the fluorinated ligands achieve T1/
T2 relaxation times in the sub-ms range. Therefore, the concept
of dissolving ultrafast ligand self-exchanging paramagnetic
complexes in undiluted IL is more promising than the use of
D
DOI: 10.1021/acs.inorgchem.7b03191
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(3) De Zanche, N.; Barmet, C.; Nordmeyer-Massner, J. A.;
Pruessmann, K. P. NMR probes for measuring magnetic fields and
field dynamics in MR systems. Magn. Reson. Med. 2008, 60, 176−186.
(4) Barmet, C.; De Zanche, N.; Pruessmann, K. P. Spatiotemporal
magnetic field monitoring for MR. Magn. Reson. Med. 2008, 60, 187−
197.
(5) Vannesjo, S. J.; Wilm, B. J.; Duerst, Y.; Gross, S.; Brunner, D. O.;
Dietrich, B. E.; Schmid, T.; Barmet, C.; Pruessmann, K. P.
Retrospective Correction of Physiological Field Fluctuations in
High-Field Brain MRI Using Concurrent Field Monitoring. Magn.
Reson. Med. 2015, 73, 1833−1843.
(6) Wilm, B. J.; Nagy, Z.; Barmet, C.; Vannesjo, S. J.; Kasper, L.;
Haeberlin, M.; Gross, S.; Dietrich, B. E.; Brunner, D. O.; Schmid, T.;
Pruessmann, K. P. Diffusion MRI with Concurrent Magnetic Field
Monitoring. Magn. Reson. Med. 2015, 74, 925−933.
(7) Kasper, L.; Haeberlin, M.; Dietrich, B. E.; Gross, S.; Barmet, C.;
Wilm, B. J.; Vannesjo, S. J.; Brunner, D. O.; Ruff, C. C.; Stephan, K. E.;
Pruessmann, K. P. Matched-filter acquisition for BOLD fMRI.
NeuroImage 2014, 100, 145−160.
(8) Gross, S.; Barmet, C.; Dietrich, B. E.; Brunner, D. O.; Schmid, T.;
Pruessmann, K. P. Dynamic nuclear magnetic resonance field sensing
with part-per-trillion resolution. Nat. Commun. 2016, 7, 13702.
(9) Gross, S.; Vionnet, L.; Kasper, L.; Dietrich, B. E.; Pruessmann, K.
P. Physiology Recording with Magnetic Field Probes for fMRI
Denoising. NeuroImage 2017, 154, 106−114.
(10) Dietrich, B. E.; Brunner, D. O.; Wilm, B. J.; Barmet, C.;
Pruessmann, K. P. Continuous Magnetic Field Monitoring Using
Rapid Re-Excitation of NMR Probe Sets. IEEE T. Med. Imaging 2016,
35, 1452−1462.
(11) Hermann, P.; Kotek, J.; Kubicek, V.; Lukes, I. Gadolinium(III)
complexes as MRI contrast agents: ligand design and properties of the
complexes. Dalton Trans. 2008, 23, 3027−3047.
(12) Helm, L.; Merbach, A. E. Inorganic and bioinorganic solvent
exchange mechanisms. Chem. Rev. 2005, 105, 1923−1959.
(13) Caravan, P.; Ellison, J. J.; McMurry, T. J.; Lauffer, R. B.
Gadolinium(III) chelates as MRI contrast agents: Structure, dynamics,
and applications. Chem. Rev. 1999, 99, 2293−2352.
(14) Krossing, I.; Raabe, I. Noncoordinating anions - Fact or fiction?
A survey of likely candidates. Angew. Chem., Int. Ed. 2004, 43, 2066−
2090.
(15) Freudenmann, D.; Wolf, S.; Wolff, M.; Feldmann, C. Ionic
Liquids: New Perspectives for Inorganic Synthesis? Angew. Chem., Int.
Ed. 2011, 50, 11050−11060.
(16) Nockemann, P.; Pellens, M.; Van Hecke, K.; Van Meervelt, L.;
Wouters, J.; Thijs, B.; Vanecht, E.; Parac-Vogt, T. N.; Mehdi, H.;
Schaltin, S.; Fransaer, J.; Zahn, S.; Kirchner, B.; Binnemans, K.
Cobalt(II) Complexes of Nitrile-Functionalized Ionic Liquids. Chem. -
Eur. J. 2010, 16, 1849−1858.
(17) Mudring, A.-V.; Babai, A.; Arenz, S.; Giernoth, R. The
“Noncoordinating” Anion Tf2N− Coordinates to Yb2+: A Structurally
Characterized Tf2N− Complex from the Ionic Liquid [mppyr][Tf2N].
Angew. Chem., Int. Ed. 2005, 44, 5485−5488.
(18) Babai, A.; Mudring, A. V. The first homoleptic bis-
(trifluoromethanesulfonyl)amide complex compounds of trivalent f-
elements. Dalton T. 2006, 1828−1830.
(19) Beck, W.; Suenkel, K. Metal complexes of weakly coordinating
anions. Precursors of strong cationic organometallic Lewis acids. Chem.
Rev. 1988, 88, 1405−1421.
saturated solutions of complex ligand mixtures in innocent
solvents, which are in addition volatile and less convenient for
practical use. Ternary mixtures of cation, ligand, and para-
magnetic centers achieve physical properties and fluorine
concentrations that are indispensable for reliable NMR field
sensing. The actually described complex [Gd(NTf2)3]
possesses in this respect the most favorable properties reported
so far, as its ligand self-exchange in [amim][NTf2] as well as in
[emim][NTf2] is fast, resulting in sub-ms 19F relaxation times
together with high fluorine concentrations and the appearance
of just one 19F signal, and enables a range of applications based
on short-lived fluorine sensors including continuous field
observation. The desired high fluoride concentrations require
small cation sizes and/or more −CF3 groups per anion.
Compared with the described allyl- and ethyl-methyl-
imidazolium cations, [PR3R′]+ or other alkyl-methyl-imidazo-
lium cations are associated with significantly smaller fluorine
concentrations due to their larger size. Fluorine concentrations
are then in the range of just 13 M as for, e.g.,
[P(nBu)3(C2H4OC2H4OCH3)+], compared to 23 M for
[amim][[NTf2].
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b03191.
General information concerning experimental procedures
and syntheses and characterization data, additional
figures for T1/T2 dependencies on viscosity and
temperature for [emim][NTf2], and crystal data for
complexes 3, 4, and 7 (PDF)
Accession Codes
CCDC 1531945, 1557761, and 1587081 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: ariel@chem.uzh.ch (R.A.)
*E-mail: pruessmann@biomed.ee.ethz.ch (K.P.P.)
ORCID
Roger Alberto: 0000-0001-5978-3394
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(20) Bradley, D. C.; Ghotra, J. S.; Hart, F. A. Low co-ordination
numbers in lanthanide and actinide compounds. Part I. The
preparation and characterization of tris{bis(trimethylsilyl)-amido}-
lanthanides. J. Chem. Soc., Dalton Trans. 1973, 1021−1023.
(21) Bortolini, O.; Chiappe, C.; Ghilardi, T.; Massi, A.; Pomelli, C. S.
Dissolution of Metal Salts in Bis(trifluoromethylsulfonyl)imide-Based
Ionic Liquids: Studying the Affinity of Metal Cations Toward a
“Weakly Coordinating” Anion. J. Phys. Chem. A 2015, 119, 5078−
5087.
■
The authors acknowledge the Swiss National Science
Foundation SNFS (project 200021_155996/1) and the
University of Zurich for financial support.
REFERENCES
■
(1) Mansfield, P. Multi-Planar Image-Formation Using Nmr Spin
Echoes. J. Phys. C: Solid State Phys. 1977, 10, L55−L58.
(2) Ahn, C. B.; Kim, J. H.; Cho, Z. H. High-Speed Spiral-Scan Echo
Planar Nmr Imaging 0.1. IEEE T. Med. Imaging 1986, 5, 2−7.
(22) Babai, A.; Mudring, A.-V. Anhydrous Praseodymium Salts in the
Ionic Liquid [bmpyr][Tf2N]: Structural and Optical Properties of
E
DOI: 10.1021/acs.inorgchem.7b03191
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
[bmpyr]4[PrI6][Tf2N] and [bmyr]2[Pr(Tf2N)5]. Chem. Mater. 2005,
17, 6230−6238.
(23) Bhatt, A. I.; May, I.; Volkovich, V. A.; Collison, D.; Helliwell,
M.; Polovov, I. B.; Lewin, R. G. Structural Characterization of a
Lanthanum Bistriflimide Complex, La(N(SO2CF3)2)3(H2O)3, and an
Investigation of La, Sm, and Eu Electrochemistry in a Room-
Temperature Ionic Liquid, [Me3NnBu][N(SO2CF3)2]. Inorg. Chem.
2005, 44, 4934−4940.
(24) Vander Hoogerstraete, T.; Brooks, N. R.; Onghena, B.; Van
Meervelt, L.; Binnemans, K. Crystal structures of hydrated rare-earth
bis(trifluoromethylsulfonyl)imide salts. CrystEngComm 2015, 17,
7142−7149.
(25) Sheldon, R. Catalytic reactions in ionic liquids. Chem. Commun.
2001, 2399−2407.
(26) Welton, T. Ionic liquids in catalysis. Coord. Chem. Rev. 2004,
248, 2459−2477.
(27) Parvulescu, V. I.; Hardacre, C. Catalysis in ionic liquids. Chem.
Rev. 2007, 107, 2615−2665.
(28) Chiappe, C.; Malvaldi, M.; Melai, B.; Fantini, S.; Bardi, U.;
Caporali, S. An unusual common ion effect promotes dissolution of
metal salts in room-temperature ionic liquids: a strategy to obtain ionic
liquids having organic-inorganic mixed cations. Green Chem. 2010, 12,
77−80.
(29) Dessapt, R.; Helm, L.; Merbach, A. E. DMSO exchange on
[Gd(DMSO)8]3+ a variable pressure 17O NMR study. J. Phys.:
Condens. Matter 2004, 16, S1027.
(30) Weingartner, H. NMR studies of ionic liquids: Structure and
̈
dynamics. Curr. Opin. Colloid Interface Sci. 2013, 18, 183−189.
(31) Richens, D. T. Ligand Substitution Reactions at Inorganic
Centers. Chem. Rev. 2005, 105, 1961−2002.
(32) Rollet, A.-L.; Bessada, C. NMR Studies of Molten Salt and
Room Temperature Ionic Liquids. Annu. Rep. NMR Spectrosc. 2013,
78, 149−207.
F
DOI: 10.1021/acs.inorgchem.7b03191
Inorg. Chem. XXXX, XXX, XXX−XXX