Structure and bonding of KSiH3 and its 18-crown-6 derivatives: Unusual ambidentate behavior of the SiH3- anion

Article abstract of DOI:10.1021/ic201774x

Density functional theory (DFT) calculations of [K(18-crown-6)SiH ₃] (1) and KSiH₃ (2) have shown that both the classical tet and non-classical inv coordination modes of the [SiH₃] ^- anion to the K⁺ ion are energetically accessible. Single-crystal X-ray structures of the tet and inv derivatives [K(18-crown-6)SiH₃?THF] (1a) and [K(18-crown-6)SiH ₃?HSiPh₃] (1b) confirm this conclusion, showing that small changes in the coordination sphere of the metal are sufficient to alter the orientation of the anion. A topological analysis of the calculated electron densities for 1 and 2 reveals that the K???Si interaction in the tet conformer of 2 possesses a significant amount of covalent character. In contrast, the inv form of 2 displays primarily electrostatic character for the K???Si and K???H interactions. Incorporation of the 18-crown-6 ligand in 1 reduces the polarizing power of the K⁺ cation, hardening the cation-anion interaction in both conformers. The experimental structures of 1a and 1b bear out these conclusions, with the strongly bound tetrahydrofuran (THF) ligand softening the K⁺ ion in 1a and favoring the tet conformer, while the weakly interacting HSiPh ₃ ligand in 1b has minimal effect on the K⁺ center, resulting in an inv orientation.

Full text of DOI:10.1021/ic201774x

ARTICLE
pubs.acs.org/IC
Structure and Bonding of KSiH3 and Its 18-Crown-6 Derivatives:
Unusual Ambidentate Behavior of the SiH3 Anion
ꢀ
David J. Wolstenholme,*,† Paul D. Prince,‡ G. Sean McGrady,*,† Michael J. Landry,† and Jonathan W. Steed*,§
†Department of Chemistry, University of New Brunswick, P.O. Box 4400, Fredericton, New Brunswick E3B 5A3, Canada
‡Department of Chemistry, King’s College London, Strand, London WC2R 2LS, United Kingdom
§Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, United Kingdom
S Supporting Information
b
ABSTRACT: Density functional theory (DFT) calculations of [K(18-
crown-6)SiH3] (1) and KSiH3 (2) have shown that both the classical
tet and non-classical inv coordination modes of the [SiH3]ꢀ anion to
the K+ ion are energetically accessible. Single-crystal X-ray structures of
the tet and inv derivatives [K(18-crown-6)SiH3 THF] (1a) and
3
[K(18-crown-6)SiH3 HSiPh3] (1b) confirm this conclusion, showing
3
that small changes in the coordination sphere of the metal are sufficient
to alter the orientation of the anion. A topological analysis of the
calculated electron densities for 1 and 2 reveals that the K Si
3 3 3
interaction in the tet conformer of 2 possesses a significant amount
of covalent character. In contrast, the inv form of 2 displays primarily electrostatic character for the K Si and K H interactions.
3 3 3
3 3 3
Incorporation of the 18-crown-6 ligand in 1 reduces the polarizing power of the K+ cation, hardening the cationꢀanion interaction
in both conformers. The experimental structures of 1a and 1b bear out these conclusions, with the strongly bound tetrahydrofuran
(THF) ligand softening the K+ ion in 1a and favoring the tet conformer, while the weakly interacting HSiPh3 ligand in 1b has
minimal effect on the K+ center, resulting in an inv orientation.
’ INTRODUCTION
attributed the added stability of this unusual conformation to the
presence of significant negative charge on the hydrogen atoms,
resulting in strong electrostatic interactions with the Li+ ions.7 In
contrast, density functional theory (DFT) calculations by Pacios
et al. have shown that NaSiH3 adopts a tet geometry (ΔE = +2.4
kcal/mol), notwithstanding the possibility of similar inter-ion
interactions.8,9 The difference in the coordination modes of these
two MSiH3 complexes was believed to result from the much
shorter anion-to-cation distances in the Li derivative, which
maximizes the ionic effects.9 However, a more detailed analysis
of the factors that underlie the geometry adopted by these MSiH3
systems was not undertaken.
In spite of the early predictions of Schleyer and Clark, only a
single well-defined MSiH3 crystal structure has been reported in
the Cambridge Structural Database (CSD). In this instance, the
[SiH3]ꢀ moiety of an oligomeric sodium alcoholate (HAZCUG)
displays an inv orientation of the anion.6 This dearth of experi-
mental structural data has restricted our understanding of the
chemical bonding in metal silyl hydrides to theoretical modeling.
In light of the underdeveloped and poorly understood coordina-
tion chemistry of these important main group systems, we have
carried out a detailed experimental and theoretical study of the
structure and bonding displayed by [K(18-crown-6)SiH3] (1)
In contrast with the behavior of CꢀH bonds that constitute
the passive framework of most organic molecules and materials,
the difference in electronegativity between carbon and silicon
(2.5 vs 1.8; cf. hydrogen 2.1),1 in tandem with the significantly
weaker SiꢀH bond (76 vs 98 kcal/mol),2 conspire to endow the
hydridic SiꢀH moiety with a chemical umpolung (polarity
inversion) and enhanced reactivity. The bonding motifs ex-
hibited by alkali metal silyl hydrides (MSiH3) are also quite
different from their organic counterparts, since these systems
can adopt either a “classical” tetrahedral (tet) or “non-classical”
inverted (inv) anion-to-cation orientation. The simplest MSiH3
(M = K, Rb, and Cs) salts crystallize in the rock salt lattice
(Fm3m).3,4 However, poorly defined hydrogen atom positions
and the crystallographic symmetry of this lattice conspire to
prevent explicit assignment of a tet or inv geometry. Conversely,
a number of alkali metal tris(trimethylsilyl)silanide complexes
have been shown to crystallize as dimers that exhibit pseudo
tetrahedral coordination of the Si(Me3Si)3 moieties.5 In light of
these issues, deeper insights into the structural proclivities of
MSiH3 complexes are needed to augment our limited under-
standing of the coordination chemistry of this intriguing class of
inorganic systems.
Some 25 years ago, ab initio calculations by Schleyer and Clark
predicted the inv structure of LiSiH3 to be more stable than its tet
counterpart (ΔE = Einv ꢀ Etet = ꢀ2.4 kcal/mol).7 These authors
Received: August 15, 2011
Published: October 07, 2011
r
2011 American Chemical Society
11222
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|

Inorganic Chemistry
ARTICLE
and its derivatives, along with a comparison with its parent
compound KSiH3 (2). We find that ΔE for both 1 and 2 is
remarkably small, and that subtle changes in the local chemical
environment of the K+ ion in 1 can switch the [SiH3]ꢀ
coordination mode from tet to inv.10 We have also carried out
an analysis of the electron distribution for these benchmark
systems, which has assisted our understanding of the behavior of
the [SiH3]ꢀ anion. Our aim was to identify the factors that
underlie the preference for a tet or inv coordination, and hence to
obtain a deeper understanding of this fundamental area of main
group structural chemistry.
Figure 1. Single-crystal X-ray diffraction structures of (a) 1a and (b) 1b,
with thermal ellipsoids at 50% probability. Salient bond lengths (Å) and
angles (deg) are as follows: 1a: K(1) Si(1) 3.592(1); K(1) O(1S)
’ RESULTS AND DISCUSSION
A recent MP2/6-31+G* study predicted the tet geometry to be
the more stable orientation of the anion in KSiH3 (2) (ΔE = +1.3
kcal/mol).11 However, our high level calculations (see Support-
ing Information) indicate that the inv form represents the ground
state geometry of this system, although the two conformers are
almost degenerate (ΔE = ꢀ0.3 kcal/mol). In a similar fashion,
coordination of the K+ ion in 1 by the 18-crown-6 ligand results
in two energy minima, with a marginal preference for the inv
conformation (ΔE = ꢀ0.9 kcal/mol). This feature is also
apparent in the NMR spectra of 1a and 1b, in which the [SiH3]ꢀ
anion environment is almost identical, pointing to a low energy
barrier between the two conformations. The almost identical
energies associated with these coordination modes in 1 and 2
makes these systems ideal testbeds for exploring the factors that
contribute to the structures adopted by MSiH3 compounds.
3 3 3
3 3 3
2.835(3); Si(1) H(1) 1.55; Si(1) H(2) 1.25; Si(1) H(3) 1.43;
3 3 3 3 3 3 3 3 3
Si(1)ꢀK(1)ꢀO(1S) 160.95(6). 1b: K(1) Si(1) 3.515(2); K(1) H-
3 3 3
3 3 3
(1S) 2.99; K(1) H(2S) 3.16; Si(1)ꢀH(1S) 1.49; Si(2)ꢀH(2S) 1.39;
3 3 3
Si(1)ꢀK(1)ꢀSi(2) 180.0.
Solid-State Structures of [K(18-crown-6)SiH3 THF] (1a)
and [K(18-crown-6)SiH3 HSiPh3] (1b). The single crystal struc-
3
3
ture of [K(18-crown-6)SiH3 THF] (1a) reveals a tet orientation
3
of the [SiH3]ꢀ moiety, with the ancillary tetrahydrofuran (THF)
ligand occupying a position trans to this anion (Figure 1a). This
neutral ligand binds to K+ with a comparable strength as the
oxygen donors of the crown ether moiety (dK O = 2.835(3) Å
3 3 3
for THF versus 2.805(3)ꢀ2.911(2) Å for 18-crown-6). The K+
ion is situated 0.39 Å above the mean plane of the crown ether
ligand and faces a tet [SiH3]ꢀ moiety canted about 12° from the
normal. The K Si distance (3.592(1) Å) in 1a is slightly longer
3 3 3
than in its inv counterpart, [K(18-crown-6)SiH3 HSiPh3] (1b)
3
(3.515(2) Å), consistent with the inv coordination of the
[SiH3]ꢀ anion reported for HAZCUG and NaSiH3 complexes.6
In a similar vein, the calculated K Si bonds in the inv form of 2
3 3 3
is also shorter than its tet counterpart. However, the optimized
structure of 1, which contains no ancillary ligand, displays
Figure 2. Complete set of bond paths for (a) inv and (b) tet forms of 1
and 2. The bond critical points (BCP) are represented by small solid red
circles.
essentially identical K Si distances for both orientations of
the anion (see Supporting Information).
3 3 3
In contrast, the solid-state structure of [K(18-crown-6)SiH3
axis and mirror plane defined by the rhombohedral space group
3
HSiPh3] (1b) reveals a K[(18-crown-6)]+ moiety with the metal
displaced 0.65 Å (cf. 0.39 Å in 1a) above the mean plane of the
crown ether toward an inv [SiH3]ꢀ moiety (Figure 1b). The
ancillary HSiPh3 ligand again occupies the available site trans to
R3m:r (Z0 = 1/6), with the axis extending along the Ph3SiꢀH
3 3 3
K
SiH3 directrix. The most prominent features of this structure
3 3 3
are the K Si and K H distances (3.515(2) and 2.99 Å,
3 3 3 3 3 3
respectively), which define the inv orientation of the ion pair.
Bonding in [K(18-crown-6)SiH3] (1) and KSiH3 (2). In an
attempt to gain insight into the versatile chemical bonding
displayed by these MSiH3 complexes, we have analyzed the
electron distribution in the tet and inv isomers of 1 and 2
according to the concepts developed in the theory of “Atoms
in Molecules” (AIM).13 The molecular graphs for the inv
structures reveal both K Si and K H interactions
this anion. The K HꢀSi distance (3.16 Å) for the HSiPh3
3 3 3
ligand in 1b invites comparison with that reported for the related
hypervalent silicon hydride system [K(18-crown-6)][H2SiPh3].12
in which the K H distance is ∼15% shorter (2.69 Å). We
3 3 3
conclude that the ancillary HSiPh3 moiety in 1b exerts little
influence on the local chemical environment of the K+ ion, in
contrast to the strongly bound THF ligand in 1a, which draws the
cation back toward the plane of the crown ether ligand by some
0.26 Å. The three subcomponents in 1b share a common 3-fold
3 3 3
3 3 3
(Figure 2a), consistent with the bonding motif exhibited by the
inv form of NaSiH3.9 In these instances, the electron density
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Inorganic Chemistry
Table 1. Topological Properties of the Calculated Electron Density for the Optimized tet and inv Structures of 1 and 2
ARTICLE
tetrahedral
Fb(r) (e Åꢀ3
inverted
Fb(r) (e Åꢀ3
2
2
compound
parameter
d (Å)
)
r Fb(r) (e Åꢀ5
)
d (Å)
)
r Fb(r) (e Åꢀ5
)
1
SiꢀH
SiꢀH
SiꢀH
1.543
1.526
1.524
3.329
0.676
0.708
0.711
0.099
3.976
4.249
4.279
0.747
1.560
1.560
1.560
3.334
2.784
2.782
2.782
2.857
2.861
2.888
2.886
2.870
2.866
1.565
3.111
2.576
0.655
0.655
0.655
0.073
0.073
0.073
0.073
0.020
0.020
0.019
0.019
0.020
0.020
0.653
0.107
0.110
3.513
3.505
3.514
0.842
0.837
0.838
0.838
0.181
1.180
0.173
0.173
0.178
0.179
3.343
1.221
1.219
K
K
K
K
H
H
H
H
H
H
Si
H
H
H
H
H
H
H
H
H
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
2.556
2.547
0.034
0.034
0.305
0.310
2
SiꢀH
K
K
1.512
3.162
0.733
0.143
4.601
0.921
Si
3 3 3
3 3 3
H
accumulated at the bond critical points (BCP), Fb(r), for both
the K Si and K H interactions turn out to be nearly
3 3 3
3 3 3
identical, indicating that the electron density in the vicinity of the
anionꢀcation interface is flat and delocalized (Table 1). This
feature indicates that the [SiH3]ꢀ moieties interact globally with
the K+ ions, in an analogous manner as the [BH4]ꢀ anions in
[Na(15-crown-5)BH4] and [K(18-crown-6)BH4].14,15 In con-
trast, the tet structures of 1 and 2 display a single K Si bond
3 3 3
path between the cation and the anion (Figure 2b). The Fb(r)
values for the K Si interactions in this conformation are
3 3 3
considerably larger than those of their inv counterparts, revealing
a stronger interaction between the K and Si atoms.
The presence of the 18-crown-6 ligand in the inv and tet
structures of 1 results in the formation of a number of weak inter-
Figure 3. Extended structure of 1a, highlighting SiꢀH HꢀC and
3 3 3
CꢀH HꢀC interactions between adjacent ion pairs. Salient bond
3 3 3
ion SiꢀH HꢀC interactions, which may be described as
3 3 3
lengths (Å) and angles (deg) are as follows: SiꢀH HꢀC =
3 3 3
unconventional dihydrogen bonds between the hydridic SiꢀH
bonds and the hydrogen bond donor ꢀOCH2.16ꢀ18 The SiꢀH
bonds in the inv conformation of 1 each engage in bifurcated
2.46ꢀ2.69; SiꢀH H = 111ꢀ127; H HꢀC = 137ꢀ155; SiꢀH
3 3 3
3 3 3
3 3 3
HꢀC = 13ꢀ68; CꢀH HꢀC = 2.33; CꢀH H = 150;H HꢀC =
3 3 3
3 3 3
3 3 3
116; SiꢀH HꢀC = 68; CꢀH HꢀC = 2.35; CꢀH H = 138;
3 3 3 3 3 3 3 3 3
SiꢀH HꢀC interactions with the crown ether, while the tet
H
HꢀC = 124; SiꢀH HꢀC = ꢀ166. (see Supporting Informa-
3 3 3
3 3 3
3 3 3
conformer displays only two such interactions, with a unique
SiꢀH bond. These weak interactions provide a possible explana-
tion for the canting of the anion, a feature which is reproduced in
the crystal structure of 1a, albeit less pronounced. It is note-
tion for further details).
stabilize the extended structures of these systems will play a role
in determining whether a tet or inv geometry is adopted.
Accordingly, we have explored the supramolecular chemistry of
these MSiH3 complexes 1a and 1b, which differ in the orientation
of the [SiH3]ꢀ anion as a result of small changes in the local
coordination environment of the K+ ions (i.e., the presence of
different neutral ancillary ligands).
worthy that similar SiꢀH HꢀC interactions were also identi-
3 3 3
fied in the solid-state structure of [K(18-crown-6)][H2SiPh3].12
The charge redistribution within 2 attendent on coordination of
the crown ether ligand to form 1 leads to a weakening of the
primary inter-ion interactions in both the tet and inv confor-
mers, as reflected in their distinctly smaller Fb(r) values. The
geometry adopted by the [SiH3]ꢀ moiety thus appears to be
controlled by different factors in KSiH3 and in its crown ether
adducts.
The SiꢀH bonds of the anion in 1a each engage in weak
intermolecular SiꢀH HꢀC interactions with a methylene
3 3 3
hydrogen atom of an adjacent crown ether ligand (Figure 3).
These interactions are characterized by H H contacts ranging
Secondary Interactions in [K(18-crown-6)-SiH3 THF] (1a)
3 3 3
3
from 2.46 to 2.69 Å, which exceed the sum of the van der Waals
radii for two interacting neutral hydrogen atoms (2.4 Å).19
However, the hydridic nature of the SiꢀH moieties increases the
effective size of the hydrogen atom to about 1.39 Å, making 2.59 Å
and [K(18-crown-6)SiH3 HSiPh3] (1b). The driving force be-
3
hind the preference of a tet or inv orientation for the [SiH3]ꢀ
moiety in the solid-state remains unclear, on account of only a
handful of reported experimental and theoretical structures.
However, it is clear that the intermolecular interactions which
a more appropriate upper limit for identifying SiꢀH HꢀC
3 3 3
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Inorganic Chemistry
ARTICLE
Table 3. Theoretical Atomic Charges (q) for the tet and inv
Forms of 1 and 2
tetrahedral
inverted
compound
atom
q
atom
q
1
K
0.862
1.265
K
0.878
1.247
Si
H
H
H
K
Si
H
H
H
K
ꢀ0.712
ꢀ0.707
ꢀ0.709
0.736
ꢀ0.709
ꢀ0.709
ꢀ0.709
0.817
2
Si
H
1.353
Si
H
1.316
ꢀ0.696
ꢀ0.711
The crystal packing displayed by these MSiH3 complexes
reveals additional supramolecular interactions. Several close
Figure 4. Extended structure of 1b, highlighting the supramolecular
SiꢀH HꢀC and CꢀH O interactions. Salient bond lengths (Å)
CꢀH HꢀC contacts (2.33 and 2.35 Å) between the methy-
3 3 3
3 3 3
3 3 3
and angles (deg) are as follows. SiꢀH HꢀC = 2.55; SiꢀH H =
lene hydrogen atoms of the THF and/or the crown ether in 1a
result in favorable electrostatic interactions, commonly referred
to as HꢀH bonding.22 These HꢀH bonds connect adjacent
3 3 3
3 3 3
91; H HꢀC = 152; SiꢀH HꢀC = ꢀ116; CꢀH O = 2.50;
3 3 3 3 3 3 3 3 3
CꢀH O = 165; CꢀH O = 2.62; CꢀH O = 148.
3 3 3 3 3 3 3 3 3
crown ether molecules, and together with the SiꢀH
HꢀC
3 3 3
dihydrogen interactions described above, effectively extending
the structure in three dimensions. In contrast, the oxygen atoms
of the crown ether in 1b are involved in more conventional
Table 2. Theoretical Delocalization Indices (δ) for the tet
and inv Forms of 1 and 2
CꢀH
O hydrogen bonds, which appear to be solely respon-
sible for the supramolecular architecture adopted by this
tetrahedral
parameter
inverted
parameter
3 3 3
compound
δ
δ
system. Overall, the weak CꢀH
HꢀC and CꢀH
O
3 3 3
3 3 3
interactions in these systems appear to play no role in directing
the orientation of the [SiH3]ꢀ moieties, since these interactions
do not directly involve the anions.
1
SiꢀH
SiꢀH
SiꢀH
0.685
0.701
0.700
0.167
SiꢀH
SiꢀH
SiꢀH
0.657
0.658
0.657
0.057
0.052
0.052
0.052
0.016
0.016
0.015
0.015
0.016
0.016
0.650
0.157
0.103
HardꢀSoft Interactions in [K(18-crown-6)SiH3] (1) and
KSiH3 (2). Although the geometry and topological analysis of
the electron density presented above for 1 and 2 provides
fundamental insights into the chemical bonding in these MSiH3
systems, they reveal scant clues as to the controlling factors that
determine whether a tet or inv geometry is adopted. The
delocalization index, δ(A,B), which measures the number of
electron pairs shared between two atoms, provides an alternative
means of interpreting their chemical bonding.23 A large δ value
K
K
K
K
H
H
H
H
H
H
Si
K
K
K
K
H
H
H
H
H
H
Si
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
0.027
0.027
for the K Si interaction in the tet structure of 2 (Table 2)
implies a certain degree of covalent character associated with the
inter-ion bonding in this system. Conversely, the K Si and
3 3 3
2
SiꢀH
K
K
0.700
0.410
SiꢀH
K
K
3 3 3
K
H interactions in the related inv form are characterized by
3 3 3
Si
Si
3 3 3
3 3 3
3 3 3
3 3 3
smaller δ values, consistent with primarily electrostatic stabiliza-
H
H
tion of the ion pair. It is noteworthy that the sum of the individual
K
Si and K H interactions contributes approximately the
3 3 3
3 3 3
interactions.20,21 In contrast, the SiꢀH moieties in 1b form
same amount to the overall stabilization of the inv complex as
does the single K Si interaction in its tet counterpart.
In terms of Pearson’s hardꢀsoft (Lewis) acidꢀbase (HSAB)
Principle,24,25 the anion-to-cation bonding in the inv form of 2
(consisting of both K Si and K H contributions) is a
bifurcated SiꢀH HꢀC interactions with the para-H atoms of
3 3 3
3 3 3
the phenyl groups of HSiPh3 (Figure 4). The crystallographic
symmetry of the R3m space group results in a single unique
SiꢀH HꢀC interaction, and the H H distance for this
3 3 3
3 3 3
3 3 3
3 3 3
contact is 2.55 Å, which once again falls at the upper limit identified
chemically harder interaction than is the more covalent (softer)
Si interaction in the tet version of 2. This conclusion is
for this type of interaction. The inter-ion SiꢀH HꢀC interac-
K
3 3 3
3 3 3
tions present in these two systems (vide supra) are characterized
supported by an analysis of the atomic charges for these systems
(Table 3). The K+ ion in the inv conformer of 2 is significantly
more electron deficient than is the cation in its tet counterpart,
and interacts preferentially with the more negatively charged
face of the ambidentate [SiH3]ꢀ anion, through electrostatic
by experimental H
H distances greater than those of their
3 3 3
supramolecular counterparts (2.87 Å in 1a and 3.23 Å in 1b),
which highlights the role that the intermolecular interactions
play in stabilizing the structure and geometry of these metal
hydrides.
K
H and K Si interactions. In contrast, the somewhat less
3 3 3
3 3 3
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Inorganic Chemistry
ARTICLE
electron-deficient K+ ion in the tet structure of 2 interacts with
provides an additional impetus for the [SiH3]ꢀ anion to orient
in certain conformation.
the less negative portion of the anion through a single K Si
3 3 3
interaction. Hence, the geometry adopted by 2 appears to be
strongly correlated with the hard or soft nature of the metal
center to which the anion coordinates. This feature directly
’ EXPERIMENTAL DETAILS
controls the amount of covalent character present in the K Si
bonds, with a tet structure being preferentially adopted as the
covalency of this interaction increases.
General Considerations. All manipulations were carried out
under an inert atmosphere in either a nitrogen-filled glovebox or using
an argon Schlenk line. The KH used throughout was purchased from
Sigma Aldrich as a 60% suspension in mineral oil, which was transferred
directly to a flask fitted with a Young’s valve under an argon atmosphere.
The mineral oil was then removed by washing with several portions of
pentane, removing each aliquot in turn by cannula. The 18-crown-6 was
dissolved in a minimum quantity of dry degassed Et2O and stored over
predried molecular sieves (4 Å) for at least 24 h. This solution was
decanted from the sieves, and the solvent removed under reduced
pressure. The dried KH and 18-crown-6 were each stored in the
glovebox. Other reagents were purchased from commercial sources
and used without further purification. Solvents were purified and dried
by distillation from sodium metal using benzophenone as an indicator,
and degassed by sparging with argon for 15ꢀ20 min. THF-d8, was
purchased from Goss and stored over sodium metal for a period of 2ꢀ3
d, followed by degassing using a freezeꢀpumpꢀthaw method. The
THF-d8 was then decanted and stored over fresh sodium or predried
molecular sieves (4 Å).
3 3 3
The bonding scenario in the crown ether adducts is somewhat
more complicated, since the coordinating oxygen atoms lower
the positive charge on the K+ ion, reducing its polarizing power
and weakening its interaction with the [SiH3]ꢀ moiety. Accord-
ingly, the δ values for the K Si and/or K H interactions are
3 3 3
3 3 3
considerably smaller than in the isolated parent species 2
(Table 2), resulting in the primary inter-ion interactions for both
conformations of 1 being largely electrostatic in nature. The
reduced polarizing power of the cation results in a chemically
harder interaction between the K and Si atoms (Table 3). This
feature is also revealed in the crystal structure of 1a, where the
THF donor ligand results in less charge transfer occurring
between K+ and [SiH3]ꢀ, giving rise to a softer anion-to-cation
interaction and a more stable tet geometry. The electrostatic
bonding between K+ and [SiH3]ꢀ in both conformers of 1
indicates a shift in the behavior of these two coordination modes
toward a common bonding motif. However, the inter-ion and
Synthesis and Characterization of [K(18-crown-6)SiH3]
(1). Method 1. Potassium metal (0.11 g, 2.56 mmol) was cut into small
chunks (2 mm3) and placed in a Schlenk vessel. In a separate Schlenk, 18-
crown-6 (0.69 g, 2.61 mmol) was dissolved in THF (30 mL), and the
resulting solution was transferred via cannula onto the potassium with
constant stirring. The metal dissolved gradually to give an intense blue
solution. Gaseous SiH4, generated in situ by the reaction of Si(OEt)4 with
LiAlH4 in nBu2O, was transferred into the reaction vessel under a
constant stream of N2, which acted as a carrier gas, and condensed onto
the walls of the reaction vessel, which was maintained at ꢀ196 °C. The
frozen solution of potassium and 18-crown-6 in DME was cautiously
allowed to warm to ambient temperature with swirling, resulting in a
color change from blue through colorless to yellow, and finally to amber
(the surface of the metal changed from blue through gold to green). The
mixture became cloudy after 1 h, and was left to stir for 24 h at room
temperature. After standing for approximately 5 d, the mixture had
separated into an off-white precipitate and a deep yellow colored
solution. The 1H NMR spectrum of the white precipitate was consistent
with [K(18-crown-6)SiH3] (1). In addition, trace amounts of a hyperva-
lent product were also observed from this reaction, which we have
tentatively assigned as [K(18-crown-6)(p-MeC6H4)3SiH2]. Diffraction
quality crystals were obtained by dissolving this product in the minimum
amount of warm THF, followed by layering with cold pentane.
supramolecular SiꢀH HꢀC interactions appear also to play a
3 3 3
role in stabilizing these MSiH3 complexes. The sum of the δ
values for the bifurcated SiꢀH HꢀC interactions in the inv
3 3 3
structure of 1 is greater than the analogous contribution from the
corresponding interactions in the tet isomer, a feature which
serves to stabilize further the inv geometry. We conclude that the
experimental and theoretical geometries adopted by the tet and
inv crown ether adducts of 1 are determined by the interplay of
several factors, including the electronic character of the metal
binding site, the ambidentate nature of the [SiH3]ꢀ anion, and
the formation of non-classical inter-ion and supramolecular
SiꢀH HꢀC interactions with the crown ether moiety.
3 3 3
’ SUMMARY
DFT calculations for 1 and 2 indicate that the classical tet and
non-classical inv coordination modes of the [SiH3]ꢀ anion are
both energetically accessible. This conclusion is borne out by the
single crystal X-ray structures of 1a and 1b, which highlight that
small changes in the local coordination environment of the metal
center can reverse the orientation of the anion. A topological
analysis of the electron distribution for these systems has
provided insight into the features that dictate the preference of
a tet or inv geometry. The tet structure of 2 is stabilized by a
considerable degree of covalent character associated with the
1
1
1H NMR δ(THF-d8): 1.2 (s, J(29Siꢀ H) = 73.64 Hz, 3H), 3.62
(s, 36H). 13C NMR δ (THF-d8): 70.57 (s, CH2). 29Si NMR δ(THF-d8):
1
ꢀ169.4 (q, 1J(29Siꢀ H) = 73.66 Hz. 29Si{1H} NMR δ(THF-d8):
ꢀ169.4 (s).
Crystal Data for 1a at 120(2) K (CCDC = 821709). Mr = 406.63, with
MoKα radiation (0.71073 Å); monoclinic space group Cc (no. 9), a =
13.8625(8), b = 10.0064(8), c = 16.7283(10) Å, β = 106.967(4)°, V =
K
Si interaction, whereas the K Si and K H interactions
3 3 3
3 3 3 3 3 3
for the inv conformation are predominantly electrostatic in
nature, resulting in a global interaction of the [SiH3]ꢀ moiety
with the K+ ion. Incorporation of the 18-crown-6 ligand in 1
reduces the polarizing power of the cation, causing the anion-to-
cation interactions in both conformers to become more electro-
static in nature. The [SiH3]ꢀ moiety hence behaves in a sꢀimilar
manner as established ambidentate anions such as NO2 and
SCNꢀ, whose coordination modes are dictated by the hard or
soft character of the metal binding site.2 The presence of weak
2219.4(3) Å3, Z = 4, 2θmax = 50.0°, 3697 unique reflections [Rint
=
0.0823], μ = 0.323 mmꢀ1, GOF = 1.008, R1 (I > 2σ) = 0.0409, wR2 (all
data) = 0.0754, largest diff. peak and hole 0.245 and ꢀ0.325 e Åꢀ3
.
Method 2. Potassium metal (0.18 g, 4.62 mmol) cut into chunks
(2ꢀ3 mm3) and placed in a Schlenk vessel. 18-crown-6 (1.22 g, 4.66
mmol) was dissolved in THF (45 mL) in a separate vessel and added to
the potassium via cannula while stirring to give an intense blue colored
solution. PhSiH3 (1.7 mL, 1.49 g, 15.74 mmol) was then added to the
reaction vessel via syringe, resulting in the same color changes as
observed using method 1. After standing for 2 d, colorless crystals had
inter-ion and supramolecular SiꢀH HꢀC interactions in the
3 3 3
solid-state structure of the crown ether adducts 1a and 1b
11226
dx.doi.org/10.1021/ic201774x |Inorg. Chem. 2011, 50, 11222–11227

Inorganic Chemistry
ARTICLE
1
formed, and these were characterized by H NMR spectroscopy as
’ ACKNOWLEDGMENT
[K(18-crown-6)SiH3 HSiPh3] (1b), in which the SiH3ꢀ and HSiPH3
3
We are grateful to the EPSRC (U.K.) and NSERC (Canada)
for financial support of this research.
ligands were produced through a ligand redistribution reaction involving
PhSiH3. The 1H NMR spectra also revealed the presence of comparable
amounts of the hypervalent product [K(18-crown-6)Ph3SiH2] (see
Supporting Information).11
1H NMR δ(THF-d8): 1b f 1.2 (s, 1J(29Siꢀ H) = 73.5 Hz, 3H), 3.5
1
’ REFERENCES
(s), 5.4 (s, J(29Siꢀ H) = 200 Hz,), 7.4 (m), 7.7 (m); [K(18-crown-
1
1
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1
7.98 (d). 13C NMR δ (THF-d8): 1b f 70.48 (s, CH2). 29Si NMR
δ(THF-d8): ꢀ170.3 (q, 1J(29Siꢀ H) = 73.5 Hz; [K(18-crown-
1
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(para), 156.24 (ipso). 29Si{1H} NMR δ(THF-d8): 1b f ꢀ170.3 (s);
[K(18-crown-6)Ph3SiH2] f ꢀ73.99 (t, 1J(29Siꢀ H) = 131 Hz).
1
Crystal Data for 1b at 115(2) K (CCDC = 821708). Mr = 594.92, with
MoKα radiation (0.71073 Å); rhombohedral space group R3m (no. 160),
a = b = c = 9.4687(10) Å, α = β = γ = 98.201(10), V = 820.06(15) Å3, Z =
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,
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parameter = 0.31(7), largest diff. peak and hole 0.408 and ꢀ0.562 e Åꢀ3
.
’ COMPUTATIONAL DETAILS
Geometry optimizations were performed at the (DFT)ꢀB3LYP/6-
311++G(d,p) level of approximation using the Gaussian09 software
suite.26 C3v symmetry was imposed for KSiH3 2, while calculation on
[K(18-crown-6)SiH3] 1 involved no such constraints. Frequency calcu-
lations confirmed that the structures thus obtained were stable minima
on their potential energy surfaces, with the exception of the tet conformer
of 1, which displayed a low imaginary frequencyassociated withthetilting
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of the SiꢀH bond involved in the inter-ion SiꢀH HꢀC interactions.
3 3 3
The final optimized geometries were then used to obtain wave functions
for each system. The topological analysis of the electron distributions
and atomic properties were carried out using a combination of the
AIM2000 and AIMALL software packages.27,28
The reliability of these calculations was determined through a
comparison of the energies, geometries, and electron distributions for
tet and inv structures of 2 at various levels of approximation (see
Supporting Information). The B3LYP/6-311++(d,p) calculations were
found to provide reasonable values relative to higher level MP2 and
CCSD calculations, with the exception of the overall energy difference
between the two conformations. However, the calculations consistently
converged on the same orientation of the [SiH3]ꢀ anion, irrespective of
changes in the functional or basis set. Accordingly, all values reported
here refer to the B3LYP/6-311++(d,p) calculations (see Supporting
Information for details concerning the MP2 and CCSD calculations).
(22) (a) Matta, C. F.; Hernꢀandez-Trujillo, J.; Tang, T.-H.; Bader,
R. F. W. Chem.—Eur. J. 2003, 9, 1940. (b) Wolstenholme, D. J.; Cameron,
T. S. J. Phys. Chem. A 2006, 110, 8970. (c) Wolstenholme, D. J.; Matta,
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’ ASSOCIATED CONTENT
(23) Fradera, X.; Austen, M. A.; Bader, R. F. W. J. Phys. Chem. A
1999, 103, 304.
S
Supporting Information. Details concerning the ener-
b
gies, geometries, and topologies for the B3LYP, MP2, and CCSD
calculations at 6-311G(d,p), 6-311++G(d,p), and 6-311++G-
(3df,pd) level of approximations. We also present the B3LYP/6-
311++G(d,p) geometries for the [K(18-crown-6)SiH3] and the
experimental geometries for the intermolecular interactions
present in [K(18-crown-6)SiH3 THF] and [K(18-crown-6)-
SiH3 HSiPh3]. This material is available free of charge via the
(24) Pearson, R. G. Chemical Hardness; John Wiley-VCH:Weinheim,
Germany, 1997.
(25) Pearson, R. G. J. Chem. Sci. 2005, 117, 269.
(26) Frisch, M. J. et al. Gaussian 09, Revision A.02; Gaussian, Inc.:
Wallingford, CT, 2009.
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2000, 21, 1040. (b) Biegler-K€onig, F. W.; Sch€onbohm, J. J. Comput.
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(28) Keith, T. A. AIMALL, version 09.04.23; Gristmill Software:
Overland Park, KS, 2009.
3
3
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: dwolsten@unb.ca (D.J.W.), smcgrady@unb.ca (G.S.M.),
jon.steed@durham.ac.uk (J.W.S.).
11227
dx.doi.org/10.1021/ic201774x |Inorg. Chem. 2011, 50, 11222–11227