Superalkali-Alkalide Interactions and Ion Pairing in Low-Polarity Solvents

Article abstract of DOI:10.1021/jacs.1c00115

The nature of anionic alkali metals in solution is traditionally thought to be "gaslike"and unperturbed. In contrast to this noninteracting picture, we present experimental and computational data herein that support ion pairing in alkalide solutions. Concentration dependent ionic conductivity, dielectric spectroscopy, and neutron scattering results are consistent with the presence of superalkali-alkalide ion pairs in solution, whose stability and properties have been further investigated by DFT calculations. Our temperature dependent alkali metal NMR measurements reveal that the dynamics of the alkalide species is both reversible and thermally activated suggesting a complicated exchange process for the ion paired species. The results of this study go beyond a picture of alkalides being a "gaslike"anion in solution and highlight the significance of the interaction of the alkalide with its complex countercation (superalkali).

Full text of DOI:10.1021/jacs.1c00115

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Superalkali−Alkalide Interactions and Ion Pairing in Low-Polarity
Solvents
René Riedel,* Andrew G. Seel,* Daniel Malko, Daniel P. Miller, Brendan T. Sperling, Heungjae Choi,
Thomas F. Headen, Eva Zurek, Adrian Porch, Anthony Kucernak, Nicholas C. Pyper, Peter P. Edwards,*
and Anthony G. M. Barrett*
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ABSTRACT: The nature of anionic alkali metals in solution is
traditionally thought to be “gaslike” and unperturbed. In contrast
to this noninteracting picture, we present experimental and
computational data herein that support ion pairing in alkalide
solutions. Concentration dependent ionic conductivity, dielectric
spectroscopy, and neutron scattering results are consistent with the
presence of superalkali−alkalide ion pairs in solution, whose
stability and properties have been further investigated by DFT
calculations. Our temperature dependent alkali metal NMR
measurements reveal that the dynamics of the alkalide species is
both reversible and thermally activated suggesting a complicated
exchange process for the ion paired species. The results of this
study go beyond a picture of alkalides being a “gaslike” anion in
solution and highlight the significance of the interaction of the alkalide with its complex countercation (superalkali).
width. Considering that the alkali metals all possess
quadrupolar nuclei, these features have been ascribed to the
high shielding and high symmetry of an unperturbed “gaslike”
anion in solution, with little to no interaction with its local
environment.9−14 However, the high polarizability of the
alkalides and ready electron dissociation into solvated-electron
species with increasing solvent polarity imply that the genuine
alkalide could be significantly perturbed in certain cases.
Indeed, the “gaslike” picture of alkalides in solution has
recently been questioned by ab initio calculations, which have
instead provided two insights in favor of a picture of an alkalide
interacting with its environment: First, it was suggested that
the most stable species were formed via the association of the
alkalide anion with solvated/complexed alkali cations in a
known alkalide-forming solvent, 1,2-ethylenediamine,15 and it
was shown that the simulated absorption spectra for such
interacting species in the visible and ultraviolet ranges were in
agreement with experimental observations. The complexed
alkali metal cations have been termed “superalkali” cations
because their LUMOs are highly expanded but retain similar
INTRODUCTION
■
Anionic forms of the electropositive Group I metals, with the
exception of lithium, can be generated in condensed phases.1
Termed alkalides, these monoanions are chemically highly
reducing and possess a diffuse, closed-shell ns2 configuration,
resulting in an exceptionally high electronic polarizability. The
formation and stabilization of alkalide species requires
stringent chemical environments and involves either a
disproportionation or the reduction of one elemental alkali
metal by another.2 The dissolution and reduction process is
facilitated by strong stabilization of the alkali cation by
preorganized complexants such as crown ethers and crypt-
ands.3 This enables alkali metals to be dissolved in even weakly
polar solvents such as tetrahydrofuran (THF) by formation of
alkalide anions that persist in the absence of any better electron
receptor, as with ammonia or small amines (metal−ammonia
solutions4−6 and solutions containing solvated electrons7,8),
functional groups in organic or organometallic molecules (the
dissolving metal reduction), or simply a different, more
reducible metal. Cryogenic temperatures are also necessitated
to avoid reduction and decomposition of solvent.
Received: January 5, 2021
Published: March 4, 2021
Perhaps the most puzzling aspect of alkalides, which has
persisted almost from the time of their discovery, is an
understanding of the precise nature of their diffuse ns2 orbital
in solution. The NMR signatures of an alkalide species in
solution, and in the crystalline solids they form, are
significantly shielded and exhibit an exceptionally narrow line
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mixture was monitored in set intervals of 0.5 min. Macrocycle
concentration was doubled when the concentration of dissolved metal
reached approximately 70% saturation to maintain a sufficient metal
dissolution rate. The ratio between metal and macrocycle
concentration at a given metal concentration was found to have no
effect on the conductivity.
Small Angle Neutron Scattering (SANS). SANS spectra were
collected at the neutron diffractometer NIMROD18 at the ISIS
Neutron and Muon Source Facility, U.K. Samples were prepared by
mixing of all the components (HMHC (1 equiv) or 15-crown-5 (2
equiv), d8-THF, NaK (n/n 1:1, 3 equiv)) in a quartz NMR tube inside
an argon glovebox with a cryogenic and inert atmosphere being
maintained. Samples were then warmed to 240 K, and equilibrated for
10 h prior to measurement to ensure complete metal dissolution up to
a point of saturation. Data were reduced using standardized
procedures within the GudrunN software.19 Density values were
precisely determined for all relevant metal-free complexant solutions
in protiated THF using a 4 place digital LiquiPhysics Excellence
density meter DM40 over the temperature range of 273−303 K.
Density values at 243 K were determined by linear extrapolation
(Details are provided in the Supporting Information.).
symmetries to those of the uncomplexed alkali metal cations
and they are able to accept electron density from the alkalide in
a weak donor−acceptor sense. Second, molecular dynamics
simulations on explicitly solvated sodide (Na−) ions suggested
that the expanded 3s2 orbital is perturbed by its environment,
but the NMR response for the sodium nucleus is negligibly
affected, despite its quadrupolar nature.16
Here, we provide experimental evidence that alkalides
interact with their environment through the formation of ion
paired species in solution (see Figure 1). Further support is
Density Functional Theory. The DFT calculations used the
Perdew−Burke−Ernzerhof (PBE) functional20 with the Amsterdam
Density Functional (ADF)21−23 software package. TZP basis sets
from the ADF basis set library were used for the Na, K, C, N, and O
atoms, while the QZ3P + 1 diffuse function basis set was used for the
H atoms.24 The core electronic states were kept frozen for all atoms
except Na and K. Further computational details are provided in the
Supporting Information, section S7.
NMR Spectroscopy. 23Na NMR spectra were acquired at 106
MHz on a Bruker DRX-400 spectrometer. 39K NMR spectra were
acquired at 28 MHz on a Bruker Avance 600 MHz NMR
spectrometer. Chemical shifts are reported as δ-values in ppm relative
to the cation signal from external aqueous solutions of the respective
chloride salt at room temperature. Samples were prepared by addition
of all components (complexant HMHC (1 equiv) or 15-crown-5 (2
equiv), d8-THF, NaK (3 equiv)) to an oven- and flame-dried
borosilicate NMR tube with a J. Young valve inside an argon
glovebox. The sealed tubes were removed from the glovebox and
cooled to 195 K before exposure of the metal alloy to the solution.
The samples were stored at 195 K and warmed to 240 K for 10 h in
preparation for the respective NMR experiment to ensure complete
metal dissolution up to a point of saturation. The probe of the NMR
spectrometer was cooled to 200 K prior to quick sample loading. The
steady reduction in signal intensity upon thermal cycling is reversible,
while taking into account a slight loss of intensity over time due to
minor decomposition processes. All measurements were corrected for
any loss in signal intensity due to a shift of the Boltzmann distribution
of spin states.
Figure 1. Illustration of potential components of a sodide solution in
HMHC/THF. Separately solvated complexed potassium (pink)
cation (top right) and alkalide anion (top left) with its diffuse 3s2
valence orbital (blue) and a contact ion pair (bottom) in a medium of
THF molecules (red) are indicated.
provided by density functional theory (DFT) calculations that
suggest a nature beyond that of classic ion associates.17 Such
weakly covalent interactions between the alkalide and the
counter superalkali cation reflect a subtle chemistry for
alkalides, which has previously not been reported. As such,
our findings have implications for future control of alkalide
properties and their potential use in photo- and electro-
chemical applications.
RESULTS AND DISCUSSION
■
Design and Control of Chemical Composition. The
accurate preparation and investigation of alkalide solutions,
especially at concentrations as low as those shown in Figure 2a,
requires the use of complexing agents that are resistant to
irreversible reductive ring scission. A milestone in the
development of more stable alkalide systems was the
introduction of per-alkylated polyamine ligands to the field.25
This showed that the hexa-aza-crown 1,4,7,10,13,16-hexam-
ethyl-1,4,7,10,13,16-hexaazacyclooctadecane (hexamethyl-hex-
acyclen, HMHC, see Figure 2b) significantly outperforms the
reportedly most stable crown ether 15-crown-526 in its
resistance to reductive decomposition in the presence of
alkalides. THF was found to be the most suitable organic
solvent as it allowed for a comparably rapid metal dissolution
and for the preparation of alkalide solutions with exceptionally
high metal concentrations, long-lived stability, and persistence.
Figure 2b highlights the stark contrast in stability between the
EXPERIMENTAL SECTION
■
Synthesis. Full experimental details and characterization regarding
the preparation of HMHC are provided in the Supporting
Information. Crude HMHC and commercially available 15-crown-5
were purified by Kugelrohr distillation from NaK, kept under an argon
atmosphere and handled inside an argon glovebox.
Ionic Conductivity Measurements. Impedance measurements
were carried out using a potentiostat Gamry Reference 600 and using
platinized platinum electrodes (cell constant 0.98 cm). The Walden
product was calculated using the shear viscosity of metal-free
solutions of 15-crown-5 in THF at all relevant concentrations as an
approximation of the viscosity of metal solutions. The shear viscosity
was determined over the temperature range of 15−40 °C using a
RheoSense m-VROC viscometer. In a typical experiment, NaK was
added in small portions (<20 mg) to a solution of 0.06 M 15-crown-5
or 0.03 M HMHC in dry THF at 243 K while the conductivity of the
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(Figure 2a) and the accurate determination of their metal
content by ion chromatography, enabling the investigation of
conductivity across a wide concentration range. Plots of the
relationship between concentration and the associated Walden
product for solutions of NaK in 15-crown-5/THF and
HMHC/THF (Figure 3) show that both exhibit a rapid
decrease followed by a slight increase of the derived Walden
product with increasing metal concentration. While the
decrease in molar conductivity is a familiar consequence of
ion association, the presence of a minimum at ∼15 mM
distinguishes the Walden product trend of the alkalide systems
from that of a classic weak electrolyte. Such conductance
minima have been reported for several solutions of electrolytes
and ionic liquids in low-polarity solvents27−37 and agree with
the theory of a feedback between the ion association
equilibrium and the overall relative permittivity of mixtures
in low-polarity solvents.31,38−42 Examination of the frequency-
dependent dielectric spectra and the static permittivity over the
period of metal dissolution yields results that are consistent
with an increase of the overall relative permittivity of the
solution due to an increasing number of ion pairs upon metal
dissolution (Supporting Information, Figure SI-4).
We employed neutron scattering measurements to examine
any structural signatures for such associations. The length
scales of the macrocycle-encapsulated cation or the envisaged
contact-ion/ion paired superalkali−alkalide correspond to a
range in reciprocal space that is intermediary between that
probed in small and wide-angle neutron scattering, with
intramolecular distances overlapping with intermolecular,
solvent−solvent, or solvent−solute distances. The use of a
protiated macrocycle and deuterated solvent overcomes this
problem due to the large difference in coherent neutron
scattering length of the proton (−3.74 fm) and the deuteron
(6.67 fm). This enables the macrocyclic complex to behave as
a scattering object in the limit of low scattering angle Q, and
any change to its size/solvation via the association of the
alkalide should be apparent. Within the low-Q limit, the
coherent differential neutron scattering cross section, dσ/dΩ,
is given by
dσ
dΩ
2
= NVP (Δρ)2P(Q)
(1)
where N is the number of scattering objects of volume VP, Δρ
is the contrast between the scattering-length density of the
object and the average scattering-length density of the solvent,
and P(Q) is the form- or shape-factor for the object. This
expression holds only for the dilute limit, where there are no
significant “object−object” correlations in the solution.
Figure 4 presents neutron scattering data for both alkalide
solutions (15-crown-5 or HMHC in THF with and without
dissolved K+ Na−) and control solutions of either 15-crown-5
or HMHC in D2O both with and without dissolved K+ I−.
These control solutions were chosen to represent fully ion-
dissociated systems of separately solvated macrocycle-K+ and
I−. Furthermore, the iodide and sodide anions are almost
identical in size and possess similar coherent neutron scattering
lengths.
It is apparent from Figure 4 that the alkalide solutions both
possess an increase in small-angle intensity whereas the control
systems containing iodide do not exhibit such an increase.
Importantly, this indicates that the scattering behavior of the
macrocycle complex does not change in the control (as
expected from simply binding K+ with its extremely low
Figure 2. Alkalide solutions in low-polarity solvent tetrahydrofuran
(THF). (a) Relation between concentration, conductivity, and color
intensity of characteristically blue sodide solutions in 50 mL of THF
at extremely high dilution. (b) Stability of solutions of NaK in 40 mM
15-crown-5/THF (left) and 20 mM HMHC/THF (right) in the
presence of NaK after storage at room temperature for 24 h (top) and
48 h (bottom). Structures of aza-crown HMHC and crown ether 15-
crown-5 (15C5) used in this work.
metal solutions using HMHC and the more labile complexant
15-crown-5 in THF, which was ascribed to the decreased
reactivity of C−N bonds as compared to C−O bonds under
strongly reducing conditions.
Ionic Conductivity and Small Angle Neutron Scatter-
ing (SANS). Investigation of the concentration dependence of
ionic conductivity of electrolyte solutions is the acknowledged
technique to interrogate the nature of ion association in
solution. Our experimental methodology allowed for the
reproducible preparation of highly dilute metal solutions
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Figure 3. Walden product vs square-root of the concentration of sodide solutions. (a) Concentration dependent Walden product of solutions of
NaK in HMHC in THF at 243 K. (b) Concentration dependent Walden product of solutions of NaK in 15-crown-5 in THF at 243 K. Dashed lines
represent guides to the eye.
Figure 4. Small angle neutron scattering spectra for macrocycle solutions of both NaK in THF and KI in D2O. Coherent differential cross section
of blank (pink) and ion-containing (blue) solutions involving HMHC in THF-d8 (0.05 M, a), HMHC in D2O (0.05 M, c), 15-crown-5 in THF-d8
(0.1 M, b), and 15-crown-5 in D2O (0.1 M, d).
neutron scattering length and negligible contribution to
scattering from the macrocycle complex), whereas in the
alkalide systems the scattering volume and contrast has clearly
increased. Simple ellipsoidal form factor models are reported in
the Supporting Information in Figure SI-5 and are concordant
with an association of the alkalide with the macrocyclic
superalkali, effectively increasing the effective volume of the
scattering object in solution from that of a solvated macrocycle
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Table 1. Binding Energies and Select Molecular Properties of Superalkali and Superalkali−Alkalide Pair Models from DFT
a
Calculations
b
binding energy
(kJ/mol)
M−M distance
superalkali Hirshfeld charge
(e)
alkalide Hirshfeld charge
(e)
dipole moment
(D)
(Å)
Superalkali M-15-crown-52 Models
Na-15-crown-52
K-15-crown-52
−84.5
−109.7
+0.26
+0.23
4.5
2.7
Superalkali-Alkalide Pair M-15-crown-52 (M) Models
Na-15-crown-52 (Na)
−147.4
−178.2
5.71
5.59
+0.28
+0.28
−0.45
−0.42
17.1
15.6
K-15-crown-52 (Na)−
equatorial
K-15-crown-52 (Na)−axial
Na-15-crown-52 (K)
K-15-crown-52 (K)
−177.7
−132.9
−166.7
6.20
6.38
7.08
+0.27
+0.28
+0.28
−0.43
−0.42
−0.43
18.3
17.0
17.9
Superalkali M-HMHC Models
Na-HMHC
K-HMHC
−62.7
−90.9
+0.31
+0.26
0
0
Superalkali−Alkalide Pair M-HMHC (M) Models
Na-HMHC (Na)
K-HMHC (Na)−chair
K-HMHC (Na)−boat
Na-HMHC (K)
K-HMHC (K)
−152.0
−177.0
−183.5 (−150.6)
−130.8
−159.0
3.61
4.11
4.00 (4.50)
4.36
4.73
+0.22
+0.24
+0.21 (+0.27)
+0.25
+0.25
−0.23
−0.29
−0.29 (−0.33)
−0.21
−0.24
9.3
10.6
10.5 (12.6)
9.8
9.9
cd
,
cd
,
a
M is the symbol for the alkali metal, to represent either Na or K. Each HMHC system given was modeled with the chair conformation, except for
the “K-HMHC (Na)−boat” where the most stable and least stable positions of the sodide are given outside and within the parentheses,
respectively. Each 15-crown-52 system was modeled as the equatorial pair unless labeled differently. M-HMHC and M-15-crown-52 are referred to
b
as superalkalis. The binding energies (BE in kJ/mol), calculated in the gas-phase by subtracting the energy of the alkali metal(s) and 15-crown-5/
c
HMHC from the total system energy (see the Supporting Information, eqs S1, S2, S4, and S5). The BEs were calculated using the HMHC
d
conformation adopted in the M-HMHC(M) model. E[K-HMHC (Na)−chair] − E[K-HMHC (Na)−boat] = 0.84 kJ/mol.
to a solvated superalkali−alkalide ion pair. Moreover, a larger
aggregation of ions in solution would lead to a small angle
signal orders of magnitude higher than those witnessed in the
alkalide solutions, thus enabling us to rule out any macroscopic
phase separation or higher alkalide agglomerates in the
solutions.
to the chair conformation in the crystalline salt of K-HMHC
with tetraphenylborate43, or a boat conformation as is the case
for the crystalline sodide K-HMHC (Na)44,45 (Figure 5). For
the 15-crown-5 systems, an alkalide can approach either axial
or equatorial to the superalkali complex (Figure 6). These two
systems are essentially isoenergetic, although the equatorial
approach is statistically preferred. The chair and boat forms of
K-HMHC (Na) are also isoenergetic, but pure HMHC favors
the chair conformation by 5.9 kJ/mol. Calculations exploring
the potential energy landscape associated with flipping the
HMHC between the chair and boat conformations, both for
pure HMHC and within the superalkali−alkalide complex,
concluded that the chair form is the dominant species in
solution (Supporting Information, section S16). Therefore, we
focus upon the chair HMHC in this discussion.
Turning to the NMR parameters, the computed shielding
constant for the sodide in K-HMHC (Na) is 40−46 ppm
larger than for the sodium cation in Na-HMHC (K) or Na-
HMHC (Na). The Hirshfeld charges capture the loss of
electron density from the encapsulated metal atom and an
increase in electron density on the surrounding metal for all
ion paired systems, as expected for the alkalide state being
maintained even in a close ion pair. Both K-HMHC (Na) and
K-15-crown-52 (Na) possess significant dipole moments, in
line with the rationale for the experimentally observed increase
in molar conductivity at higher alkalide concentrations (Figure
3). This further rules out weakly dipolar superalkali dimers
(Supporting Information Table SI-4).
Ab Initio Calculations. To assess the possible species that
may form in alkalide solutions, DFT calculations were
performed on sodium and potassium superalkalis of HMHC
and 15-crown-5, superalkali−alkalide ion pairs, and superalkali
dimers. As shown in Table 1, K was calculated to interact more
strongly than Na with both HMHC and 15-crown-5. The
computed binding energies (BEs) suggest that the metal atoms
prefer to be coordinated to two 15-crown-5 molecules as
opposed to a single complexant (Supporting Information,
section S15). In agreement with prior DFT calculations,15 the
superalkali dimers were found to be less stable than the
superalkali−alkalide ion pairs when solvation effects were
considered (Supporting Information, section S14). Therefore,
in this discussion we focus on the superalkali−alkalide ion pairs
whose computed BEs and selected properties are given in
Table 1, with optimized geometries and molecular orbital
isosurfaces illustrated in Figures 5 and 6. Out of the four metal
combinations that are possible, the encapsulated potassium
and anionic sodide models, K-HMHC (Na), and K-15-crown-
52 (Na), each possess the largest BEs, in agreement with the
experimental observation of a predominance of potassium
cations and sodide anions.
The flexibility of the macrocycles and approach of the
second metal ion allows for several conformationally and
topologically different superalkali−alkalide contact ion pairs. In
its complexes with alkali cations, HMHC can adopt a
conformation with all six nitrogen atoms in one plane, similar
The chair HMHC superalkali Ci symmetry SOMO extends
from the metal symmetrically surrounding the superalkali
above and below, in contrast to the 15-crown-52 superalkali C1
symmetry SOMO that persists about the ether rings but does
not have much character near the alkali metal (cf. Figures 5 and
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agreement with our neutron scattering results and with the
crystallographically derived radii of alkalides in the solid state.
These HOMOs are formed from a bonding in-phase
interaction between the SOMO of the sodium atom with the
SOMO of the potassium superalkali, which itself contains
character from the LUMOs of the complexant, as well as 4s
and 5s K orbitals. The bond orders of 0.65 and 0.57 computed
between K and Na highlight the partial covalency of the
interaction in K-HMHC (Na) and K-15-crown-52 (Na),
respectively. Additional stability in these superalkali and
superalkali−alkalide complexes, alongside coordination and
Mδ+Mδ‑ pairing, originates from intramolecular H⇜⇝H
interactions first introduced in ref 5. The H⇜⇝H interaction
is mediated through orbital overlap between neighboring
hydrogen atoms, as seen in the superalkali SOMOs and
superalkali−alkalide HOMOs (Figures 5, 6, and Supporting
Information Figures SI-9−SI-14, SI-17). This H⇜⇝H
bonding arises from the partial population of the LUMO of
the organic species when alkali metals are introduced to the
system (Supporting Information Figures SI-7 and SI-8) and is
reminiscent of Rydberg bonding proposed by Simons.46,47 One
consequence of this H⇜⇝H interaction is that the distance
between pairs of hydrogen atoms is ∼0.1 Å shorter in the
superalkali systems as compared to pure HMHC and 15-
crown-5.
A fragment orbital analysis was performed using the (spin-
restricted) distorted K-HMHC/K-15-crown-52 superalkalis (as
found in the optimized superalkali−alkalide species) and the
(spin-restricted) Na atoms as the fragments.21 This revealed
the composition of the molecular orbitals of the complex in
terms of the occupied and unoccupied orbitals of the
fragments. As shown in Table SI-12, the Na 3s SOMO is
the dominant contribution to the all-important HOMO of the
chair K-HMHC (Na) and equatorial K-15-crown-52 (Na)
complexes (51.8% and 67.9%, respectively) as expected. The
SOMO of the superalkali provides the second largest
contribution (34.2% and 20.0%), but diffuse unoccupied
orbitals on the superalkali are also an important component,
suggesting this may be another manifestation of Rydberg
bonding (Supporting Information, section S19).46,47
Figure 5. Computed SOMOs/HOMOs of superalkali K-HMHC and
superalkali−alkalide (K-HMHC)δ+(Na)δ‑. Isosurfaces (isovalue =
0.010 au) for superalkali models K-HMHC in the chair (a) and
boat (b) conformations and for the superalkali−alkalide models K-
HMHC (Na) in the chair (c) and boat (d) conformations. Ion pairs
in the chair (e) and boat (f) conformations illustrated in the form of a
cartoon.
Variable Temperature NMR Spectroscopy. 23Na and
39K NMR spectra under cryogenic conditions exhibit the
expected narrow, shielded signals in concentrated solutions of
macrocycle/NaK and macrocycle/K in THF, respectively
(Figure 7 and Supporting Information Figure SI-1). The
dynamics of the alkalide species and their interactions with
their local environment or with other components in solution
are evidenced through comparison of 23Na spectra at different
temperatures over an accessible range of more than 60 K
(Figure 7). Increasing the temperature results in a rapid and
reversible decrease in the total integrated area of signals in the
chemical shift range of (−61) − (−63) ppm for both
macrocycle systems.
Considering the lack of any additional signals in the
chemical shift range of (+100) − (−200) ppm, we attribute
the unusual behavior of alkalide species in NMR to an
exchange process between an NMR-visible and an NMR-
invisible species, each involving the alkali metal nucleus of
interest. The line width of the alkalide signal increases only
slightly from 2.9 to 9.0 Hz from 235 to 290 K despite the
drastic loss in signal intensity. This suggests a thermally
activated transformation of the NMR-visible species in the
slow exchange limit.48 Very minor changes in the chemical
Figure 6. Computed SOMO/HOMOs of superalkali K-15-crown-52
and superalkali−alkalide (K-15-crown-52)δ+(Na)δ‑. Isosurfaces (iso-
value = 0.010 au) for superalkali model K-15-crown-52 (a) and
superalkali−alkalide K-15-crown-52 (Na) with the alkalide in the axial
(c) and equatorial (d) positions with respect to the sandwich complex
(as illustrated in the form of a cartoon (b)).
6). The HOMOs of the superalkali−alkalide complexes are
concentrated on the sodide component, and they have a large
spatial extent past the alkalide portion of the ion pair, in
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Figure 7. Temperature dependent 23Na NMR spectroscopy of sodide solutions in THF. (a) Upfield range of 23Na spectra showing the temperature
dependent change in shape and total signal intensity of the alkalide signal of a solution of 0.3 M NaK in HMHC/THF in the chemical shift range
between (−61) − (−65) ppm. (b) Upfield range of 23Na spectra showing the temperature dependent change in shape and total signal intensity of
the alkalide signal of a solution of 0.4 M NaK in 15-crown-5/THF in the chemical shift range between (−61) − (−65) ppm. Spectra are referenced
to 1 M aqueous NaCl solution at room temperature according to IUPAC recommendations. Peak heights in both sets are normalized to their
respective lowest temperatures. Insets show zooms of the signal onsets.
shift of the signals cannot be conclusively demonstrated here
due to the sensitive temperature dependence of the reference
signal.
or, more likely, a conformational difference between both
NMR-visible species without a loss of its integrity.
Mindful of our above discussion concerning the effect of a
perturbation of the alkalide on its NMR signature, we suggest
that this is due to the conformational flexibility of the
superalkali−alkalide complex of HMHC that is absent in the
15-crown-5 case, bearing in mind that both chairlike and
boatlike conformations for the HMHC macrocycle exist in
crystalline systems. Clearly, further work remains as to
identifying the precise nature, and mechanisms, of these
intriguing exchange processes.
If the relaxation of the quadrupolar alkalide nucleus was
purely due to the approach of the superalkali countercation, it
must be induced by a sufficiently large electric field gradient
(EFG) at the alkalide nucleus. However, our computed EFG,
Vzz, of the isolated sodide does not differ from that in the ion
pair and is essentially 0 relative to the computed Vzz for Na and
K in the superalkali (Supporting Information Tables SI-5 and
SI-7). Recent results from ab initio molecular dynamics showed
that the sodide anion may appear in NMR experiments as if it
were an unperturbed, spherical ion, despite the polarizable 3s
orbital being strongly affected by the surrounding species in
solution.16 We note that the solvent modeled in ref 16 was
methylamine, a solvent of higher dielectric than the THF
solutions used in this study and which would be expected to
perturb the alkalide species to an even greater extent. Hence,
we conclude that the NMR-visible alkalide species is subject to
a fast and reversible exchange process with an NMR invisible
species, which is particularly short-lived. This may be a
paramagnetic species or a charge-transfer state but is not due
to the association and dissociation of the superalkali−alkalide
complex.
CONCLUSION
■
Alkalides have a unique place in the history and chemistry of
the s-block elements.1 Since their discovery by J. L. Dye and
colleagues, the characterization of the alkalide species in
condensed matter systems has led to the fascinating discussion
of their diffuse, yet localized, electronic states in weakly polar
solvents and indeed to the application of alkalides as highly
reductive species across chemistry.25,49−51 The existence of ion
paired species has been mooted since the very earliest
discussions of alkalides but has never been experimentally
demonstrated conclusively.
Herein, we have provided experimental evidence of the
observation and effect of ion pairing of alkalides in solution,
both from examination of their macroscopic conductivity and
dielectric properties to the local disruption of solvent scattering
density witnessed in coherent neutron scattering. To suggest
what form these ion pairs may take in solution, we have carried
out DFT calculations on a number of possible superalkali−
alkalide complexes in addition to superalkalis and superalkali
dimers. In agreement with recent ab initio results,16 our models
Another interesting and exclusive feature of the HMHC
sodide system observed upon close inspection of the 23Na
NMR is the existence of a small shoulder in the major signal on
the low-field side (Figure 7a). This shoulder becomes more
pronounced as the signals drift apart with decreasing
temperature. The small difference in chemical shift between
the primary and secondary signals around the −60 ppm
chemical shift range suggests the involvement of a coordinative
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Article
implicate that the superalkali−alkalide complex, which we
suggest is the dominant species in solution, may be
indistinguishable in NMR from an isolated solvated alkalide
and so we have revisited the classical interpretation of such
data in the literature of alkalides. We attach great importance
to the temperature dependence of the conformationally
dynamic HMHC system, indicating that there is much more
to be understood as to the kinetics of alkalides in solution: The
reversible perturbation and possible disintegration of the
NMR-visible species (that may well be the contact ion pair)
highlights the significance of the interaction of alkalides with
their complex countercations.
Our studies paint a picture of an alkalide species being far
beyond a “gaslike” ion in solution, but instead one that could
be “chemically” controlled and developed by considering
superalkali−alkalide interactions of the sort delineated here.
We believe that the interactions of alkalides in solution is
merely the most recent in a long line of surprising and unique
aspects of s-block chemistry1 and certainly one that has the
potential to affect how these systems are extended and applied
in the future.
Heungjae Choi − School of Engineering, Cardiff University,
Cardiff CF24 3AA, U.K.; orcid.org/0000-0003-1108-
293X
Thomas F. Headen − ISIS Neutron and Muon Source, Science
and Technology Facilities Council, Rutherford Appleton
Laboratory, Didcot OX11 0QX, U.K.; orcid.org/0000-
0003-0095-5731
Eva Zurek − Department of Chemistry, State University of
New York at Buffalo, Buffalo, New York 14260-3000, United
States; orcid.org/0000-0003-0738-867X
Adrian Porch − School of Engineering, Cardiff University,
Cardiff CF24 3AA, U.K.
Anthony Kucernak − Department of Chemistry, Imperial
College London, Molecular Sciences Research Hub, London
W12 0BZ, U.K.; orcid.org/0000-0002-5790-9683
Nicholas C. Pyper − University Chemical Laboratory,
Cambridge CB2 1EW, U.K.
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c00115
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
■
ACKNOWLEDGMENTS
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c00115.
■
We thank GSK for support to A.G.M.B. with the Glaxo
endowment. This work is part-funded by the European
Regional Development Fund through the Welsh Government.
Experiments at the ISIS Neutron and Muon Source were
supported by a beamtime allocation RB181075453 from the
Science and Technology Facilities Council. D.P.M. and E.Z.
thank the Center for Computational Research at the University
at Buffalo, SUNY for computational support.52 D.P.M. thanks
Hofstra University for funding through a Faculty Research and
Development Grant (FRDG). E.Z. acknowledges the NSF
(Grant DMR-1827815) for financial support and Jochen
Autschbach for valuable discussions.
Experimental protocols and full characterization, NMR
data, description of ionic conductivity methodology,
dielectric spectroscopy methodology and data, computa-
tional details and data, isosurfaces, and structure
coordinates (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
René Riedel − Department of Chemistry, Imperial College
London, Molecular Sciences Research Hub, London W12
0BZ, U.K.; orcid.org/0000-0003-1501-0538;
Email: rene.riedel13@alumni.imperial.ac.uk
Andrew G. Seel − Department of Physics and Astronomy,
University College London, London WC1E 6BT, U.K.;
Inorganic Chemistry Laboratories, University of Oxford,
Oxford OX1 3QR, U.K.; orcid.org/0000-0003-0103-
8388; Email: a.seel@ucl.ac.uk
Peter P. Edwards − Inorganic Chemistry Laboratories,
University of Oxford, Oxford OX1 3QR, U.K.; orcid.org/
0000-0001-7340-4856; Email: peter.edwards@
chem.ox.ac.uk
ABBREVIATIONS
■
15C5, 15-crown-5; DFT, density functional theory; EFG,
electric field gradient; HMHC, 1,4,7,10,13,16-hexamethyl-
1,4,7,10,13,16-hexaazacyclooctadecane; HOMO, highest occu-
pied molecular orbital; LUMO, lowest unoccupied molecular
orbital; NMR, nuclear magnetic resonance; SANS, small angle
neutron scattering; SOMO, singly occupied molecular orbital;
THF, tetrahydrofuran
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