Cooperative Noncovalent Interactions Induce Ion Pair Separation in Diphenylsilanides

Article abstract of DOI:10.1002/chem.201704217

This crystallographic and computational study describes an unusual potassium silanide structure. A contact ion pair is expected in the solid state between potassium and silicon, yet the potassium cation binds an aromatic ring and the anionic silanide inte

Full text of DOI:10.1002/chem.201704217

A Journal of
Accepted Article
Title: Cooperative Noncovalent Interactions Induce Ion Pair Separation
in Diphenylsilanides
Authors: Eric A. Marro, Eric M. Press, Tapas K. Purkait, Daniel
Jimenez, Maxime A. Siegler, and Rebekka S. Klausen
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201704217
Link to VoR: http://dx.doi.org/10.1002/chem.201704217
Supported by

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COMMUNICATION
Cooperative Noncovalent Interactions Induce Ion Pair Separation
in Diphenylsilanides
[
a]
[a]
[a]
[a]
[a]
Eric A. Marro, Eric M. Press, Tapas K. Purkait, Daniel Jimenez, Maxime A. Siegler, Rebekka S.
[
a]
Klausen *
Abstract: This crystallographic and computational study describes
an unusual potassium silanide structure. A contact ion pair is
expected in the solid state between potassium and silicon, yet the
potassium cation binds an aromatic ring and the anionic silanide
interacts with CH bonds on neighboring crown ether molecules.
These structure-bonding phenomena are attributed to strong soft-
soft interactions.
the cooperative effect of these noncovalent interactions exceeds
the strength of the Si-K ionic bond. Our results are consistent
with a body of literature showing significant differences in the
recognition of hard and soft anions and suggest opportunities for
the further development of silanide nucleophiles in synthesis.
As part of a research program focused on the materials
[
11–14]
chemistry of polymeric and molecular silanes,
we identified
disilanide
2
as
a
a
key intermediate in the synthesis of
polymer inspired by crystalline silicon
We report the structural characterization of silyl anions
poly(cyclosilane),
[
14]
(
silanides) separated from
a
potassium countercation by
(Scheme 1).
2
is obtained by potassium tert-butoxide
[
1]
[15–17]
unexpected noncovalent
interactions.
Silanides,
like
and inorganic
and these materials represent an intriguing set of
mediated desilylation of 1 in the presence of 18-cr-6,
yielding a deep red solid which was used without further
purification in the synthesis of cyclosilane 3. Intrigued by 2’s
potential as a versatile intermediate in the preparation of new
poly(cyclosilane) precursors, we sought to isolate and
structurally characterize this silanide.
[
2–4]
carbanions, are versatile reagents in organic
[
5–7]
synthesis
[
8]
hard/soft congeners. Controlling ion pair separation in solution
is a challenge with profound consequences for reactivity and
selectivity: strong Coulombic attraction between oppositely
charged species ties the nucleophilic anion to its countercation,
[
9]
inhibiting reaction with a desired electrophile. Solvation is not
the only mechanism for ion pair separation. Synthetic anion
receptors employ the cooperative influence of multiple
noncovalent interactions to separate an ion pair and selectively
[
10]
bind the anion. Understanding the nature of the intermolecular
interactions leading to ion pair separation provides insight into
fundamental structure-bonding phenomena, as well as avenues
for designing and optimizing new reactivity.
K
Si1
Si1
O
O
O
O
K
O
K
O
(18-cr-6)K
H
−
Structure & bonding?
Implications for
Si
R
Si
R
R
R
−
reactivity & selectivity? contact ion pair crown ether separated ion pair
•
Si-K ionic bond
• cation-π
CH-anion
•
Figure 1. Crystal structure of 2. Cen-K distance: 3.190(2) Å; Si1-K distance:
4.6897(7) Å. Displacement ellipsoids shown at 50% probability level;
hydrogens and minor component of the disorder for the 18-cr-6 omitted for
clarity. Black = carbon, blue = silicon, red = oxygen, dark purple = potassium.
The center of the ring (Cen) is shown in magenta.
Ph Ph
Si
Me Me Ph
Si Si
Me Me Ph
2 KOt-Bu
TMS
Ph
Cl(SiMe2)2Cl
2
18-cr-6
Ph
Me Si
SiMe2
Ph
Si
Si
2
Si
Si
Ph
Si
Si
−
2 TMSOt-Bu
Me2Si
SiMe2
TMS
PhMe Me
PhMe Me
Si
2
[K(18-cr-6)]+
1
Ph Ph
3
2
The crystal structure of 2 does not show a contact ion pair:
the Si1-K distance is very long (4.6897(7) Å) and Si1 is
pyramidal, not tetrahedral (Figure 1). While crown ethers are
well known to suppress ion-pairing in solution, the approximately
two-dozen crown ether-ligated potassium silanides included in
Scheme 1. Contact and separated ion pairs. Synthesis of disilanide 2 and
conversion to cyclosilane. 18-cr-6 = eighteen-crown-six.
Here we show that a crown ether macrocycle engages both
a cation and an anion in noncovalent interactions, forming a
loose ion pair in the solid state. In this crystal packing structure,
[
15–22]
the Cambridge Structural Database are contact ion pairs.
The triphenylsilyl anion is representative with a tetrahedral
[
23]
central silicon atom and a Si-K distance of 3.456(3) Å.
Precedence for separated ion pairs are limited to strong donor
[
1]
substituents and solvents,
like Krempner’s polyether-
and hexamethylphosphoramide-
[a]
Mr. Eric A. Marro, Mr. Eric M. Press, Dr. Tapas K. Purkait, Mr.
Daniel Jimenez, Dr. Maxime A. Siegler, Prof. Rebekka S. Klausen
Department of Chemistry
[
24,25]
substituted silanides
coordinated barium silanides.
[
26]
Johns Hopkins University
3
400 N. Charles St, Baltimore, MD 21218
Instead of a contact ion pair, each potassium engages in a
cation-π interaction with one of two phenyl groups substituting
the terminal silicon atom (Si1). The observed geometry, in which
the cation is nearly centered over the phenyl ring (θ = 14.7°) and
E-mail: Klausen@jhu.edu
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
1
the Si-centroid distance is 3.190(2) Å, is consistent with
indicate that Si1 is not protonated. Additionally, H NMR spectra
+
[27]
calculated attractive benzene-K interactions.
We note that if
of the crystals are inconsistent with an authentic sample of
steric effects arising from the phenyl ring were important, then a
tetrahedral contact ion pair maximizing distance from both Ph
rings would be expected. The silanide is not a free anion, as Si1
forms close contacts (<3.30 Å, the sum of the van der Waals
radii of silicon and hydrogen) with C-H bonds on two different
adjacent crown ethers (Figure 2). The crown ether binds both
the cation and the anion at distinct sites.
HSiPh
while HSiPh
color is observed during data collection which is conducted
2
(SiMe
2
)
2
SiPh
2
H (Figure S1). Crystals of 2 are dark-red
2
(SiMe
2
)
2
SiPh H is colorless; no change in crystal
2
under a cold stream (ca. 110 K) of inert N2(g)
.
Figure 2. Crystal packing structure of 2. For clarity, one phenyl ring, most
hydrogens, and minor component of the disorder in the 18-cr-6 are omitted.
Black = carbon, blue = silicon, red = oxygen, dark purple = potassium; pink =
hydrogen. The center of the phenyl ring (Cen) is shown in magenta. Solid
green lines indicate the Cen-K distance. Dashed light blue lines indicate anion-
CH contacts.
This result surprises in several ways. It is not intuitive that a
cation-binding structure like a crown ether participates in anion
binding. Cation-π interactions are nearly unprecedented in the
presence of charged aromatic donors. Benzhydryl potassium
2
-
Figure 3. [SiPh
2
(SiMe
2
)
2
SiPh
2
]
calculations suggest terminal lone pairs. DFT
CAM-B3LYP/6-31+G(d). a) HOMO, geometry optimized in gas phase. b) ESP
2
-
-1
-1
map of [SiPh
2 2 2 2
(SiMe ) SiPh ] . -149 kcal mol (red) to 63 kcal mol (blue). c)
(
Ph
2
CHK) crystallizes as a coordination polymer in which
NBO of each terminal silicon atom, electron occupancy listed below. d) NBO of
potassium is sandwiched between the anionic central carbon of
−
the benzhydryl anion Ph
view of the crystal structure of 2 highlighting phenyl ring bending attributed to
lone pair repulsion. The torsional angle ∠Si1-C1-C -C (numbering adapted
2
CH showing a planar, delocalized structure. e) A
[
28]
one benzhydryl unit and a benzene ring of a second.
o
m
In light of these unusual features, we considered if
inadvertent protonation of the oligosilane chain termini arising
from either adventitious water or cavitand deprotonation might
have occurred. If the silanide is protonated, the charge balance
is not obvious, but disorder in the crown ether may obscure
either a deprotonated carbon or an associated hydroxide
from ref [31]) is 173.7°. Potassium, 18-cr-6, hydrogens, and one phenyl ring
omitted for clarity. See SI for further details.
We investigated if cation-arene binding is due to de-
localization of negative charge from silicon onto the adjacent
phenyl ring. Prior work on lithiated phenylsilanides does not
[
29,30]
ion.
However, several experiments suggest that the
[
8,32]
structure proposed in Figure 1 is correct. While hydrogens can
be difficult to detect accurately by X-ray diffraction, the
oligosilane chain itself is well ordered and hydrogen atoms at
other positions have been readily identified. Difference Fourier
maps show that atoms C14 → C18 (phenyl ring) are all
protonated. The structure refinement of 2 shows no residual
support resonance delocalization
and quantum chemical
2
−
calculations on the oligosilanyl core ([SiPh
2
2 2 2
(SiMe ) SiPh ] ) of 2
do not support this hypothesis either. The highest occupied
molecular orbital (HOMO) was calculated for the optimized
geometry (Figure 3a, see Figures S2-S3 for other geometries
and MO’s). In all cases, the HOMO is delocalized across the
oligosilane, with minimal electron density found on the phenyl
rings. The molecular orbital calculations are more consistent
with delocalization across the σ-conjugated oligosilane than the
benzene rings. The calculated electrostatic potential (ESP)
−
3
electron density peaks larger than 0.45 e Å (the highest peak
being found at 0.87 Å from K1). Residual electron density peaks
found near Si1 are always found along the Si−Si and Si−C
bonds, and there are no peaks pointing outward, which strongly
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Figure 4. a) Crystal structure of contact ion pair 4. Hydrogens omitted for clarity. b) Crystal structure of 5ŸTHF. Cation interactions are shown with a solid green
line and include both a contact ion pair (Si5-K) and a cation-π interaction (Cen-K2). Si5-K1 distance: 4.0882(13) Å; Cen-K2 distance: 3.112(3) Å. Hydrogens and
minor component of the disorder in one 18-cr-6 and THF are omitted for clarity. c) Crystal structure of 5 expanded to show all neighboring 18-cr-6 molecules.
Anion-CH interactions are shown with a dashed light blue line. Potassium contacts are shown with a solid green line. One phenyl ring on each terminal Si atom is
omitted for clarity. Solvent molecules and hydrogens not involved in contacts are omitted for clarity. Each terminal Si atom contacts three crown ether molecules.
See the SI for additional details.
map of the disilanide core also shows anionic character
concentrated on the terminal Si atom, and not on the Ph ring
contributor to the cation-π interaction in second row materials.
Negative charge on silicon is not free, as the non-classical
hydrogen bonds with ethereal CH groups on adjacent crown
ethers stabilize the anion (Figure 2). The cooperative effect of
multiple CH-anion contacts is suggested by calculations on the
full structure of 2, which predict contact ion pairing in the gas
phase (Figure S5).
(
Figure 3b). We conclude that 2 is most accurately described as
having a lone pair of electrons on each terminal Si atom. The
picture is further supported by natural bond orbital (NBO)
calculations (Figure 3c). Comparison of 2 to a carbon analog
shows a marked contrast and supports the argument that the
silanide negative charge does not mix with the aromatic
[
37]
−
The soft silanide may prefer diffuse interactions in contrast
to hard anions that prefer charge-controlled interactions. Recent
progress in the molecular recognition of large multinuclear
anions suggests significant differences between hard and soft
species. While hosts for hard anions like fluoride typically feature
substituent. The NBO of the benzhydryl anion (Ph
2
CH ) is
delocalized across two coplanar atoms, the benzylic site and an
adjacent carbon (Figure 3d). Finally, phenyl ring bending
observed in the crystal structure of 2 (Figures 3e and S4) further
supports the assignment of terminal lone pairs on silicon as
Strohmann et al. have observed similar bending in lithium
silanides attributed to repulsion from the high s-character of the
−
classical hydrogen bond donors (e.g. NHŸŸŸF ), high binding
constants are measured between large anions and CH donors.
[
38,39]
[
31]
The binding cavities of Flood’s polyaryl macrocyclic hosts
are lined only with CH bonds and show exceptional affinity for
silicon lone pair.
[
40]
We suggest that in the solid state 2 is a loose ion pair in
which cation-π and CH-anion interactions contribute to ion pair
separation. To the best of our knowledge, cation-π
large nonspherical anions like perchlorates.
Johnson et al.
−
report that the thiolate anion (HS ) is more tightly bound by a
[
41]
benzenoid than pyridinyl host.
Sindelar has shown strong
[
33,34]
[42]
interactions
in these systems, electrostatic attraction is
contributor to the strength of the cation-π interaction.
crystal structure of 2 suggests that polarizability is a significant
have only been studied with neutral π-donors;
binding of large anions in water by aliphatic CH donors.
If
a
dominant
hard-soft theory rationalizes the binding modes observed in 2,
then crown ether-driven ion pair separation might be observed in
other potassium silanides. Anions 4 and 5 were synthesized by
[
35,36]
The
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COMMUNICATION
the same procedure as 2.
the forces implicated in these binding events are not just
electrostatic in nature, but must also involve induced dipole
interactions and other events dependent on strongly polarizable
molecular species.
The crystal structures of monoanion 4 and dianion 5 show
that not only do both contact and crown ether-separated ion
pairs exist but that these can coexist within a single structure.
Monoanion 4 adopts a tetrahedral geometry about the central
silicon atom and the Si1-K distance of 3.6142(7) Å is within the
In conclusion, we report a set of crystal structures that
challenge intuitive ideas about electrostatic attraction. Our
results have implications for understanding and predicting ion
recognition events with structurally complex soft cations and
anions. The insights from the structural characterization of
silanides 2, 4, and 5 and the evidence of soft anion character
suggest avenues for hypothesis-driven optimization of reactivity
and selectivity. Ongoing studies focus on synthetic applications
of diphenylsilanides.
+
typical range for [K(18-cr-6)] silanides (Figure 4a). In the crystal
structure of pentasilane 5 each terminal silicon participates in a
different interaction with potassium (Figure 4b). At Si1, a cation-
π separated ion pair is observed. The K2-centroid distance is
short (3.112(3) Å) and the cation is centered over the aromatic
ring (θ = 8.5°). At the other chain terminus, an Si5-K contact ion
pair is observed. The differences between tetrasilane 2 and
pentasilane
5 may reflect odd-even effects in oligosilane
[
43]
CCDC 1568512-1568514
contain
the
supplementary
packing.
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre.
Anion-CH contacts are observed in 5 (Figure 4c). At the
cation-π terminus of 5, the lone pair bearing atom Si1 contacts
four different hydrogens on three crown ether molecules with
distances ranging from 3.166-3.409 Å. Two of the Si1-CH
+
contacts arise from the [K(18-cr-6)] complex bound to a phenyl
Acknowledgements
ring. Close contacts between ion-paired Si5 and ethereal CH’s
are also observed (3.255-3.262 Å) at the other chain terminus.
Interestingly, the solution phase NMR spectra of 5 are consistent
with a symmetric structure, suggesting a change in structure to a
solvent separated ion pair.
Chemical synthesis is supported by U.S. Department of Energy
(
DOE), Office of Science, Basic Energy Sciences (BES), under
Award DE-SC0013906. R. S. K. thanks the American
#
Academy of Arts and Sciences for the Mason Award for Women
in the Chemical Sciences and the Alfred P. Sloan Foundation for
a
Sloan Research Fellowship. We thank Johns Hopkins
University for a Catalyst Award.
Keywords: anions • crown compounds • ion pairs • pi
interactions • silicon
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This crystallographic and computational study describes an unusual separated ion
pair in which crown ether molecules bind both cation and anion. These structure-
bonding phenomena are attributed to strong soft-soft interactions.
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