Complexation behaviour of LiCl and LiPF6-model studies in the solid-state and in solution using a bidentate picolyl-based ligand

Article abstract of DOI:10.1039/d0cc05682k

Structural knowledge on ubiquitous lithium salts in solution and in the crystalline state is of paramount importance for our understanding of many chemical reactions and of the electrolyte behaviour in lithium ion batteries. A bulky bidentate Si-based ligand (6) was used to create simplified model systems suitable for correlating structures of LiCl and LiPF6 complexes in the solid-state and in solution by combining various experimental, spectroscopic, and computational methods. Solution studies were performed using 1H DOSY, multinuclear variable temperature NMR spectroscopy, and quantum chemical calculations. [Ph2Si(2-CH2Py)2·LiCl]2 (3) dissociates into a monomeric species (9) in THF. For [Ph2Si(2-CH2Py)2·LiPF6]2 (11), low temperature NMR studies revealed an unprecedented chiral coordination mode (12) in non-coordinating solvents. This journal is

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Complexation Behaviour of LiCl and LiPF6 – Model Studies in the
Solid-State and in Solution Using a Bidentate Picolyl-Based Ligand
Noel Angel Espinosa-Jalapa,a Nele Berg,b Michael Seidl,a Ilya G. Shenderovich,b Ruth M. Gschwindb
Received 00th January 20xx,
Accepted 00th January 20xx
and Jonathan O. Bauer*a
DOI: 10.1039/x0xx00000x
Structural knowledge on ubiquitous lithium salts in solution and in correlations between solid-state and solution are only possible
the crystalline state is of paramount importance for our understan- to a limited extent.9 Structural investigations on lithium
ding of many chemical reactions and of the electrolyte behaviour in compounds in solution using diffusion-ordered NMR spectro-
lithium ion batteries. A bulky bidentate Si-based ligand (6) was used scopic methods have therefore become an increasingly import-
to create simplified model systems suitable for correlating structur- ant field, but have been reported mainly for lithium amide,
es of LiCl and LiPF6 complexes in the solid-state and in solution by mixed-metal, and organolithium reagents.10 Simplified model
combining various experimental, spectroscopic, and computational systems that use bulky and specially designed ligands have
1
methods. Solution studies were performed using H DOSY, multi- proven to be helpful tools for obtaining in-depth information on
nuclear variable temperature NMR spectroscopy, and quantum structure-forming principles of fundamentally important metal
chemical calculations. [Ph2Si(2-CH2Py)2 • LiCl]2 (3) dissociates into a compounds, which are either used in preparative chemistry or
monomeric species (9) in THF. For [Ph2Si(2-CH2Py)2 • LiPF6]2 (11), found in nature.11 Especially in solution, the structural space
low temperature NMR studies revealed an unprecedented chiral often shows great versatility and flexibility and can be efficiently
coordination mode (12) in non-coordinating solvents.
restricted using bulky ligands with diagnostic molecular
patterns in order to achieve information on coordination modes
that are otherwise difficult to identify.12
This gave us the idea to introduce diphenylbis(2-picolyl)-
silane [Ph2Si(2-CH2Py)2] (6) (Scheme 1) because we expected
this to be a promising backbone for the design of new bulky
bidentate ligands that would enable detailed model studies of
widely used synthetically and technically important lithium salts
by showing defined structural patterns in solution. Different to
phosphorus-based bis(2-picolyl) NPN ligands,13 tetravalent
bis(2-picolyl)-substituted group 14 elements do not have an
additional free donor site capable of coordination, but sterically
demanding substituents can easily be incorporated.14 This
Structural knowledge on synthetically and technically relevant
lithium salts like LiX (X = halogenide) and LiPF6 in the solid-state
and in solution in the presence of coordinating solvents or
ligands is particularly important for a better understanding of
salt effects both in chemical reactions and technical applications
such as in lithium ion batteries.1-4 LiCl is very often a by-product
in chemical reactions with lithiated species involved and has
long been known to be a potential key to reactivity and
selectivity in reactions mediated by organometallic reagents.5
Combinations of inorganic salts or donor ligands with certain
organometallic reagents (e.g. organolithium, -magnesium, and
-zinc compounds) have gained broad applications in preparative
chemistry.6,7 Significant progress has recently also been made
in the field of LiCl catalysis.8 Understanding cation–anion
coordination modes between Li+ and PF6– in LiPF6 electrolytes is
of great importance and has contributed significantly to the
improvement of lithium ion batteries.2 The development of
defined [ligand • metal salt] combinations can open up entirely
new paths in synthetic chemistry and provide hitherto unknown
information on cation–anion interactions. However, the
structural behaviour in solution often remains a black-box and
a.Institut für Anorganische Chemie, Fakultät für Chemie und Pharmazie, Universität
Regensburg, Universitätsstraße 31, D-93053 Regensburg, Germany.
b.Institut für Organische Chemie, Fakultät für Chemie und Pharmazie, Universität
Regensburg, Universitätsstraße 31, D-93053 Regensburg, Germany.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Scheme 1. Synthesis of [Ph2Si(2-CH2Py)2 • LiCl]2 (3) (route A), the free ligand 6 via the
borane-protected intermediates 4 and 5 (route B), and the lutidine derivative 8 (route
C).
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makes them proper target molecules for use as bulky bidentate picolyllithium with dichlorosilane
1 (Scheme 1, C). Both com-
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ligand systems that can limit structural flexibility in coordination pounds and
6
8
were obtained in crystaDlliOnIe: 1f0o.1r0m39,/sDu0iCtaCb0l5e68f2oKr
modes. With this ligand in hands, we shed light on the structural single-crystal X-ray diffraction analysis (for details, see the SI).
7
behaviour of an LiCl and an LiPF6 complex both in the solid-state
Solid-state Li NMR spectroscopy of dimer
3
shows a signal
7
and in solution by combining various NMR spectroscopic at δ = 2.6 ppm. In DCM, the Li NMR chemical shift of complex
methods with quantum chemical calculations.
3 (δ = 2.5 ppm) fits perfectly with the results of the solid-state
We first set out to investigate a straightforward synthetic NMR spectroscopy, which led us to the conclusion that the
route to the new bidentate ligand (
pyridines (2-picolines) that are functionalized by main group However, in a THF solution of
elements at the methyl position have long been the subject of shifted to δ = 0.8 ppm while the H NMR chemical shift of the
intensive research due to their interesting electronic properties pyridinyl-H atom in 6-position (δ = 8.50 ppm) still shows an
and coordinating abilities.15 However, only little is known about interaction of the ligand with the lithium salt when compared
6) (Scheme 1). 2-Methyl- dimeric structure is retained in this weakly coordinating solvent.
3
, the 7Li NMR signal is highfield-
1
the coordination abilities of multidentate, unsubstituted (2- with the 1H NMR spectrum of the free ligand Ph2Si(2-CH2Py)2 (
picolyl)silanes.16 Direct reaction of dichlorodiphenylsilane (
(δ = 8.34 ppm). LiCl, dissolved in THF without any ligand, shows
with two equiv. of 2-picolyllithium17 ) resulted in the format- a 7Li NMR signal at δ = 0.5 ppm. We therefore assumed that in
ion of a dimeric LiCl complex ( ) coordinated by the new biden- THF, dimer breaks up into a monomeric species ( ) as a result
tate ligand diphenylbis(2-picolyl)silane ( ). Compound could of the good coordination properties of the solvent (Scheme 2).
easily be isolated by extraction of the formed solids with DCM This was further supported by quantum chemical calculations at
in 68% yield (Scheme 1, ) and was characterized by single- the B3LYP/6-31+G(d) level of theory using the Polarizable
crystal X-ray diffraction analysis. The LiCl complex
crystallized Continuum Model (PCM) (solvent: THF),19 which showed that
6)
1
)
(2
3
3
9
6
3
A
3
from acetonitrile/THF as colourless blocks in the triclinic crystal this deaggregation is indeed a slightly exothermic process in
system, space group P1 (Figure 1, top). The LiCl core forms a THF (ΔH = –5 kJ mol–1), whereas the complete dissociation into
dimeric rhombic structural unit in which each lithium atom is the free ligand
additionally coordinated by the two pyridyl nitrogen atoms of thermodynamically unfavorable process (ΔH = +10 kJ mol–1
one ligand each (average N–Li bond length: 2.073(5) Å). An ideal with respect to monomer ). Compound 10 was previously
6 (10) is a
and a plausible LiCl • THF species1b,20
9
tetrahedral geometry is retained around the silicon atom so identified as a crucial species in equilibrium processes of
that no significant structural changes of the bidentate ligand are organometallic reagents in the presence of LiCl in THF
required for an efficient coordination.
Interestingly, Ph2Si(2-CH2Py)2
solutions.20 The same calculations for the lutidine derivative
(6
) is quite similar to the show a highly exothermic process (ΔH = –63 kJ mol–1 in total) for
phosphorus analog18 PhP(2-CH2Py)2 concerning its coordination the dissociation of a hypothetical dimer into the free ligand
8
behaviour to LiCl.18c In the molecular structure of the latter, only and [LiCl • 2 THF]2 (10) (for details, see the SI), so that even the
the two pyridyl nitrogen atoms are involved in the coordination formation of a monomeric LiCl species can be excluded if there
7
to LiCl forming a similar dimer as reported herein.18c
is a methyl group in the 6-position of the pyridyl ring. Li NMR
This prompted us to investigate the structure of [Ph2Si(2- spectroscopy of
CH2Py)2 • LiCl]2 ( ) in more detail in organic solvents. For this at δ = 0.5 ppm in accordance with free LiCl in THF. DFT
purpose, we developed a two-step synthesis to obtain Ph2Si(2- calculations of the 7Li NMR chemical shift at the B3LYP/6-
CH2Py)2(
) in a LiCl-free fashion using BH3 as protecting agent 311+G(2d,p)//B3LYP/6-31+G(d) level21 strengthen the
(Scheme 1,
available to study the influence of a methyl group in the 6-posi- monomeric LiCl complex
8 with one equiv. LiCl in THF shows one signal
3
6
B
). In addition, we made the lutidine derivative
8
dissociation of dimer
3
into monomer
9
in THF. For the
7
9
, a Li NMR chemical shift of δ = 1.3
tion of the pyridyl ring on the coordination behaviour (Scheme1, ppm was calculated, which matches well with the
). In contrast to the picoline derivative
, the bis(2,6- experimentally observed 7Li NMR signal (δ = 0.8 ppm) of the LiCl
lutidinyl)silane
could easily be obtained without complexation adduct in THF. The calculated 7Li NMR chemical shift of dimer
C
6
8
3
of LiCl when carrying out the direct reaction of two equiv. 2- was clearly downfield-shifted both in THF (δ = 1.9 ppm) and in
the gas phase (δ = 2.3 ppm). Although the differences in the
7Li NMR chemical shifts are only marginal, the trend agrees well
with the proposed model.
We finally performed 1H-diffusion-ordered NMR spectrosco-
pic (1H DOSY)22 measurements to get additional support for the
formation of a monomeric Ph2Si(2- CH2Py)2 • LiCl adduct (9) in
THF solution. The hydrodynamic radius (rH) of ligand
was determined to be 4.70 Å. For a solution of complex
the 1H DOSY experiment gave a radius of rH = 6.12 Å. This radius
6
3
in THF
in THF,
corresponds much more to a monomeric LiCl complex such as
9
, as assumed in THF (see Scheme 2), than to the intact dimer
(for details on the measurement, see the SI). For comparison,
1H-diffusion measurements of the lutidine derivative
with and
3
Figure 1. Molecular structures of complexes 3 (top) and 11 (bottom) in the crystal
(displacement ellipsoids set at the 50 % probability level, hydrogen atoms and included
solvent molecules are omitted for clarity).
8
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0CC05682K
Scheme 3. Synthesis of [Ph2Si(2-CH2Py)2 • LiPF6]2 (11) from the LiCl complex 3 via anion
exchange with TlPF6.
ture. The formation of such a species can open new possibilities
for generating polar chiral environments in solution. Further-
more, knowledge on ionic association in the presence of
coordinating additives on a molecular level can provide helpful
information concerning the conductivity behavi-our of LiPF6-
containing nonaqueous electrolyte solutions.26
In summary, we presented a combined experimental, spect-
roscopic, and computational model study concerning structures
of fundamentally important lithium salts in the solid-state and
in solution in the presence of a new bidentate silicon-based
Scheme 2. Calculated reaction enthalpies (ΔH) for the proposed dissociation of dimer 3
into a monomeric LiCl complex (9) and for the unlikely dissociation into the free ligand 6
and a dissolved LiCl • THF complex (10) in THF [B3LYP/6-31+G(d)] (PCM calculations;
solvent: THF).
without addition of LiCl gave hydrodynamic radii of rH = 4.08 Å
(without LiCl) and rH = 4.88 Å (addition of one equiv. LiCl), respe-
ctively, which indicates interactions between compound
8 and
LiCl in THF, but the formation of a stable complex can be
excluded.
In order to get more insight into the coordinating abilities of
ligand (
dissociation and association processes in coordinating and non-
coordinating solvents. It was shown that complex breaks up
into a monomeric LiCl adduct ( ) in THF solution, but remains a
6). We have gained detailed insights into fundamental
ligand
med a ligand exchange reaction on adduct
could easily be converted into the dinuclear LiPF6 complex 11
6
towards technically important lithium salts, we perfor-
3
. The LiCl complex
3
3
9
by anion exchange with TlPF6 (Scheme 3). 11 crystallized as
colourless plates in the monoclinic crystal system, space group
P21/n. The molecular structure of 11 was determined by single-
crystal X-ray crystallography (Figure 1, bottom) and is another
example for the rare bridging coordination of two lithium
cations by two PF6– anions, each of which providing two fluorine
atoms.23 The P–F–Li angles of the dinuclear eight-membered
core range from 136.10(10)° [P(1)–F(1)–Li(1)] to 157.80(12)°
[P(2)–F(8)–Li(1)] and differ greatly from an almost linear
dimer in less-coordinating DCM. The chloride ions could easily
be exchanged by PF6 ions, while maintaining the dinuclear
–
structure in the solid-state. [Ph2Si(2-CH2Py)2 • LiPF6]2
(11)
shows a rare bridging coordination of the lithium centers in the
solid-state and forms a unique chiral species in solution, which
was elucidated by variable temperature NMR measurements. In
our ongoing studies we will address the targeted functionalizat-
ion of ligand
6 at the silicon α-position. This might lead to new
prototypes of multifunctional bulky bidentate C2-symmetric
ligands,27 which can be used as stereochemical probes to reveal
unusual coordination spheres in solution.
The Elite Network of Bavaria (ENB), the Bavarian State
Ministry of Science and the Arts (StMWK), and the University of
Regensburg are gratefully acknowledged for financial support.
We also thank Prof. Manfred Scheer and Prof. Jörg Heilmann for
generous and continuous support.
arrangement, as found in the [N,N,N',N'',N''-pentamethyldieth-
ylenetriamine • LiPF6]2 complex (mean P–F–Li angle: 175.5°).23
7
Solid-state Li NMR spectroscopy of 11 shows a signal at δ =
0.7 ppm. In DCM at room temperature, dimer 11 can still be
detected in very small amounts [δ(7Li) = 0.6 ppm], but is mainly
converted into a new species [δ(7Li) = 1.7 ppm]. In view of the
–
high structural flexibility of Li+···PF6 interactions,24 we carried
out preliminary and for the first time variable temperature
multinuclear NMR studies on a LiPF6 complex, which gave inter-
esting information on the structural behaviour in solution (Fig-
ure 2). In DCM, a defined major species [δ(7Li) = 1.2 ppm] beco-
mes more and more evident starting from –30 to –80 °C, while
at the same time formation of a minor species [δ(7Li) = 3.3 ppm]
2
occurs. Remarkably, as the temperature decreases, JHH coup-
ling (13.2 Hz) within the CH2 groups of the main compound
1
becomes increasingly clear in the H NMR spectrum. The dia-
Figure 2. Left: Variable temperature NMR measurement (400 MHz) of complex 11 in
DCM-d2 showing the diastereotopic splitting of the CH2 groups in the 1H NMR spectrum
and the 7Li NMR signals. Right: Suggestion of a plausible structure (12) of the LiPF6
complex in solution.
stereotopicity of the methylene hydrogen atoms at –80 °C is a
stereochemical probe that gives a clear hint to the formation of
a defined and unprecedented complex exhibiting a chiral coord-
ination sphere. A suggestion of a plausible structure (12) of the
LiPF6 complex in DCM in accor-dance with the solution NMR
studies is also shown in Figure 2. Two lithium cations are each
Conflicts of interest
There are no conflicts to declare.
–
chelated by two fluorine atoms of one PF6 anion in a chiral
complexation mode.25 A flipping coor-dination of the lithium
centers and a rapid exchange between the coordinating and
non-coordinating counteranion can be ex-pected and matches
with the 19F and 31P NMR spectra at ambient and low tempera-
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DOI: 10.1039/D0CC05682K
Using a new bulky bidentate ligand and combining various structure elucidation methods, coordination
modes of [ligand • LiX] (X = Cl, PF6) complexes both in solid-state and in solution have been revealed.