Dynamic Conformational Behavior in Stable Pentaorganosilicates

Article abstract of DOI:10.1002/ejic.201900641

Silicates with five organic groups are conformationally dynamic even with two bidentate ligands. Symmetry breaking by incorporating a single nitrogen or phosphorus atom provides insight into their dynamic behavior. N-containing silicates with bidentate 2-

Full text of DOI:10.1002/ejic.201900641

A Journal of
Accepted Article
Title: Dynamic Conformational Behavior In Stable Pentaorganosilicates
Authors: Leon J.P. van der Boon, Jesper H. Hendriks, Danny Roolvink,
Sean J. O'Kennedy, Martin Lutz, J. Chris Slootweg, Andreas
W. Ehlers, and Koop Lammertsma
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900641
Link to VoR: http://dx.doi.org/10.1002/ejic.201900641

10.1002/ejic.201900641
European Journal of Inorganic Chemistry
FULL PAPER
Dynamic Conformational Behavior In Stable Pentaorganosilicates
Leon J.P. van der Boon,[a] Jesper H. Hendriks,[a] Danny Roolvink,[a] Sean J. O'Kennedy,[b] Martin Lutz,[c] J. Chris Slootweg,[a][d]
Andreas W. Ehlers,*, [a][d][e] and Koop Lammertsma*,[a][e]
Abstract: Silicates with five organic groups are conformationally
dynamic even with two bidentate ligands. Symmetry breaking by
Ph
O
N
Ph
N
incorporating a single nitrogen or phosphorus atom provides
insight into their dynamic behavior. N-containing silicates with
bidentate 2-phenylpyridine, biphenyl, and a Me (8), Et (9) or Ph
(10) ligand were studied comprehensively by NMR spectroscopy
and DFT theory to reveal two isoenergetic conformers with a
barrier of ca. 10 kcal.mol-1. P-containing silicate 14 with
bidentate triphenylphosphine, biphenyl, and Me ligands is
subject to multiple Berry pseudorotations, turnstile rotations, and
conformational flexibility of the P-center. The stability increased
by masking the P-center with a BH3 group (16). DFT and NMR
modeling reveal two isoenergetic conformers for 16 with a
barrier of ca. 19 kcal‧mol-1 for a complex interconversion
pathway. This barrier bodes well for the design of
configurationally stable chiral-at-metal transition metal catalysts.
Ph
Ph
Si
Si
Me
Me
Si
N
O
Ph
Ph
1
2a
2b
S
CF3
F3C
O
P
O
Bu
Si
Si
Si
SiH
Ph
Ph
O
F3C CF3
S
4
2c
2d
3
Scheme 1. Pentacoordinate species subject to Berry pseudorotations.
been observed in silicates containing a heteroatom, like 4.[10] By
using adequately sized bidentate ligands, such as two 2-
(phenyl)-naphthyl units (2c),[11] selected pseudorotations can be
hindered to enable the isolation of chiral silicates in spite of the
dynamic nature of the enantiomers. Silicates like 2c but also
phosphoranes like 3[12] are chiral at the central atom and do not
require asymmetric ligands as in, e.g., 1.
To advance the access to and to enhance the insight into
manipulating the dynamics of pentaorganosilicates that are
chiral-at-silicon it is desirable to expand the scope of bidentate
ligands. We opted for simple aromatic ones, because these can
be extended by, e.g., catenated phenyl groups or aromatic
substituents to increase congestion if needed. Earlier, we
reported on silicates carrying two N-phenylpyrroles[9] (2b) and
two 2-(phenyl)benzo[b]-thiophenes (2d),[11] but whereas their
conformers may be detectable by NMR spectroscopy, DFT
calculations revealed the chelation of the five-membered pyrrole
and thiophene rings to be less suited for inhibiting Berry
pseudorotations than the six-membered phenyl ring (2c).
Introduction
Silicates with five organic substituents are remarkably stable
compounds.[1–3] Their stabilization is typically attributed to the
electron withdrawing nature of the organic ligands or the
coordinating heteroatoms,[4] which are often oxygen and/or
nitrogen atoms (e.g., 1; Scheme 1), but silicates carrying only
carbon groups can also be quite stable (m.p. > 150 oC), provided
the presence of two aromatic bidentate ligands as in 2a.[5–7] For
example, two biphenyl ligands give so much stability that the fifth
group occupies an equatorial position even if it is a fluorine atom,
thereby seemingly violating the notion of apicophilicity.[8]
In our recent studies on all-carbon pentaorganosilicates we
focused on controlling the dynamic stereomutations that result
from Berry pseudorotations.[9] Similar permutations have also
In this work, we report on two novel pentaorganosilicate
templates that feature one six-membered heterocyclic ring with
either a nitrogen or a phosphorus atom in only one of the
bidentate ligands. Here we present their syntheses and describe
their complex dynamic properties using NMR spectroscopy and
DFT calculations. These analyses seek to provide criteria for
controlling the chirality of silicates and by inference how to
control the dynamics in chiral-at-metal catalysts.[13–17]
[a] Leon J.P. van der Boon, Jesper H. Hendriks, Danny Roolvink, Dr. J.
Chris Slootweg, Dr. Andreas W. Ehlers, Prof. Koop Lammertsma
Faculty of Sciences, Vrije Universiteit Amsterdam
De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
E-mail: a.w.ehlers@uva.nl, k.lammertsma@vu.nl
[b] Sean J. O'Kennedy
Department of Chemistry and Polymer Science
Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa
[c] Dr. M. Lutz
Crystal and Structural Chemistry
Bijvoet Center for Biomolecular Research
Cl2
Si
Utrecht University, Padualaan 8 3584 Utrecht, The Netherlands
[d] Dr. J. Chris Slootweg, Dr. Andreas W. Ehlers
Van ’t Hoff Institute for Molecular Sciences
University of Amsterdam, Science Park 904
1090 GD Amsterdam, The Netherlands
N
Q
RLi 8-11
(
)
+
11
NEt Br
(
)
4
Si
Si
R
5
6
n
BuLi
Br
(2eq.)
[e] Dr. A. W. Ehlers, Prof. K. Lammertsma
Department of Chemistry, Science Faculty
7
N
N
University of Johannesburg
PO Box 254, Auckland Park, Johannesburg, South Africa
=
=
11
Li 8 9 10
, ,
9
R
Q
Me
8
Et
Ph
10
( )
,
(
),
( ),
Br
n
11
(
), N( Bu)4
(
)
Scheme 2. Synthesis of pyridine bearing pentaorganosilicates.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201900641
European Journal of Inorganic Chemistry
FULL PAPER
Results and Discussion
We start with the silicate of which one of the bidentate ligands
contains a single pyridine ring. This silicate represents the
closest N-hetero analogue to 2.
N-containing silicates 8-11 – experimental. As starting
material we used dichlorobiphenylsilane 5, which can be
obtained from dilithiated 2,2’-dibromobiphenyl and SiCl4. The Li2-
salt is soluble in Et2O, reacts with SiCl4 in yields up to 71%, but
requires a product distillation step.[18,19] A THF suspension reacts
slower, but gives a higher
yield (86%) and requires
only product purification by
washing with n-pentane.
Reaction of 5 with the Li2-
Figure 2. 1H NMR of 8 measured at intervals from 20 to -80 °C.
salt
of
3-bromo-2-(2-
trigonal bipyramid. Its rather small torsion angle of 58.1(3)° with
the axial Si-C bond is likely due to both steric repulsion and the
unfavorable π-interaction of the phenyl ring with the axial bond.[4]
The Li+ counter ion is coordinated to the pyridine nitrogen atom
(dNLi 2.066(5) Å) and is further stabilized by three THF solvent
molecules. This behavior differs from non-coordinating silicates
like 2a that crystallize with a tetra-THF coordinated Li+ ion.
Pyridine silicates 8-10 are highly dynamic. Broad
resonances are observed at room temperature in both the
bromophenyl)-pyridine (6)
afforded the single pyridine
containing spirosilane 7 in
52%
isolated
yield
(Scheme 2).[20] Treatment
of the silane with MeLi, EtLi,
and PhLi gave the desired
silicates 8, 9, and 10,
respectively. The 29Si-NMR
1
aromatic and aliphatic regions of the H NMR spectra (THF-d8).
spectra
characteristic high field
resonance for each silicate Figure 1. Molecular structure 10 in the
(8 -107.2;
10 -102.2 ppm). All have
1H
NMR spectra as
show
the
Neither are all carbon resonances resolved in the 13C NMR
spectra. Lowering the temperature gradually to -80 °C has a
1
significant effect as is illustrated for the H NMR spectra of 8 in
crystal (ellipsoids are set at 50%
9
-101.6;
probability; hydrogen atoms are omitted for
clarity). Only the major component of the
disordered THF groups is shown. The
structure crystallizes as a racemic mixture
of enantiomerically pure crystals in space
group P21. Selected bond lengths [Å],
angles and torsion angles [°]: Si1–C12
1.935 (3), Si1–C11 2.016 (3), Si1–C13
1.926 (3), Si1-C121 1.940(3), N51–Li1
2.066 (5), C13–Si1–C121 117.63 (12),
C12–Si1–C121 122.89 (12), C11–Si1–C12
94.93 (11), C11–Si1–C13 88.72 (11),
C23–C13–Si1–C11 58.1 (3).
Figure 2 (see SI for more details). At -80 °C two resonances (δ
0.247 and 0.268 ppm) are observed for the Me group with the
major one being broadened, possibly resulting in overlap of two
signals as shown by deconvolution (see SI) and thereby
preventing a more detailed analysis.
We presume the Me resonances to originate from two
near-isoenergetic conformers of 8 that are in rapid equilibrium
with a coalescence temperature of ca. -10 to -20 °C. The
aromatic region shows likewise behavior in the same
temperature range. This dynamic behavior is reminiscent to that
reported for bispyrrole-phenyl silicate 2b (coalescence at ca. -12
oC, ∆G‡=15.5 kcal‧mol-1), which has been attributed to Berry
pseudorotation between near-isoenergetic conformers. The
dynamic exchange for pyridine silicate 8 appears to occur at a
slightly lower temperature and thus a little faster than for silicate
2b assuming similar chemical shift differences, suggesting
somewhat smaller conversion barriers between the near-
expected as do the exact
mass spectroscopic meas-
urements. The melting
points of the silicates are
noteworthy lower than that
for 2a (8 47.2; 9 77.6; 10
107.3 C (dec.; 2a, nBu4N+
o
counterion, 180 oC).
A single crystal X-ray structure confirms the identity of the
phenyl derivative 10 (Figure 1). Its molecular structure features a
biphenyl and a phenylpyridine ligand in a screw-like fashion that
underlies the inherent chiral property of this silicate; 10 is, of
course, present as a dynamically racemizing enantiomeric pair.
The phenyl substituent is located in the equatorial position of the
Figure 3. Isomerization of the conformers of 8 (black) and 10 (red) at B3LYP-D3/6-311++G(2p,d)//B3LYP/6-31G(d); energies in kcal‧mol-1-. On the left are
the enantiomeric pairs in which the pyridine group occupies an axial position and on the right where this group is located in an equatorial position.
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10.1002/ejic.201900641
European Journal of Inorganic Chemistry
FULL PAPER
isoenergetic conformers of 8. To obtain more insight into the
exchange of the Li+ cation of 8 for the non-coordinating
ammonium nBu4N+ cation did not show any change in the
aromatic or methyl regions in the 1H-NMR spectrum the resulting
silicate 11. Moreover, the 1H-NMR spectrum of silicate 9
recorded at -70 oC (see SI) showed two signals that are
assigned to the two CH2 protons for the ethyl group. Apparently,
at low temperatures the Berry pseudorotation and consequently
racemization is hampered on the NMR time scale due to steric
congestion, thereby causing different chemical environments for
the two methylene protons .[8] Both these observations support
the notion that the low-energy barriers for the Berry
pseudorotations that interconnect the near-isoenergetic
conformers are solely responsible for the broadening of the
NMR resonances.
dynamic behaviour we resorted to DFT calculations.
N-containing silicates 8 and 10 – computational. A survey
for 8 and 10 was conducted at the B3LYP-D3/6-311++G(2p,d)//
B3LYP/6-31G(d) level. Figure
3 shows the two pairs of
enantiomeric conformations (∆/Λ8a and ∆/Λ8b) and the
transition structures connecting them. The minimum energy
structure 8a has the pyridine ring in an axial position of a trigonal
bipyramid with the methyl group occupying an equatorial
position. With a barrier of 10.9 kcal‧mol-1 racemization will occur
readily via the Cs symmetrical transition structure 8-TS1.
Structure 8a is energetically favored by only 0.2 kcal‧mol-1 over
conformer 8b in which the pyridine group has been transferred
from an axial to an equatorial position. Racemization of this
conformer has an even lower barrier of 9.6 kcal‧mol-1; 8-TS2
represents the Cs symmetrical transition structure for this
process. Conformer ∆8a isomerizes into Λ8b and vice versa via
the asymmetric transition structure 8-TS3. This represents a
turnstile rotation (TR), which is a single-step double Berry
pseudorotation. Note that the mirror image of 8-TS3 reflects the
enantiomeric rearrangement of Λ8a into ∆8b. The 8a→8b
barrier amounts to a modest 10.5 kcal‧mol-1, which concurs with
the observed coalescence in the 1H NMR spectrum. For the
more sterically crowded 10, which carries a phenyl substituent
instead of a methyl, the isomerization barrier is even lower (9.5
kcal‧mol-1) due to the favorable electron-withdrawing properties
of the axial phenyl ring.
An alternative path by which 8a and 8b can isomerize
proceeds via a single-step Berry pseudorotation that has the
square pyramidal 8-TS4 as transition structure (Figure 3, middle
top). However, the barrier for this process is 4.3 kcal‧mol-1 higher
than it is for the turnstile rotation (8-TS3) because of friction
between the aromatic ortho-hydrogens in the transition structure.
For the more crowded 10 the corresponding difference in barrier
heights of 5.7 kcal‧mol-1 is even larger. These results concur with
earlier findings that showed inversion pathways to be hampered
by sterically more demanding ring systems[8] or with more
electron withdrawing substituents.[11]
P-containing silicates 14 and 16 – experimental. The next
target is a silicate in which one of the two bidentate ligands
contains a six-membered ring with a phosphorus atom. Instead
of incorporating an aromatic phosphinine ring in analogy to the
discussed pyridine approach, we opted for a different framework,
one in which the phosphorus atom is embedded in the central
ring of one of the bidentate ligands. In other words, the target
silane precursor is 13 (Scheme 3). An added advantage of this
choice is that it has the potential to provide insight into the
influence of six versus five membered ring coordination on the
conformational dynamics of the silicate.
Spirosilane 13 was obtained in 87% yield by reacting
dichlorobiphenylsilane 4 with the Li2-salt of bis(2-bromophenyl)-
phenylphosphane 12.[21] The molecular structure of 13, obtained
from a single crystal X-ray structure determination, is shown on
the left of Figure 4 and confirms the identity of the spirosilane.
As expected, the six-membered heterocycle with its sp3
hybridized phosphorus atom is slightly bent as reflected by the
torsion angle between the two adjacent phenyl rings (∠ C81–
P1–Si1–C21 151.27(4)°). The P-phenyl substituent is pointed in
the opposite direction and ‘hoovers’ over one side of the
1
biphenyl group. All of this gives cause to complex H and 13C
NMR spectra (see exp. section and SI).
We wondered whether associative/dissociative lithium-
nitrogen interactions might actually contribute to the dynamic
behavior of the silicates. This doesn’t appear to be the case as
Ph
Li
Cl2
Si
P
MeLi
Si
Si
Me
4
P
14
13
Ph
Br
Ph
P
n
BuLi
BH3SMe2
(2 eq.)
Figure 4. Molecular structures of 13 (left) and 15 (right) in their respective
crystals (ellipsoids are set at 50% probability; hydrogen atoms are omitted for
clarity). Both compounds crystallize as racemates in centrosymmetric space
groups. Selected bond lengths for 13 [Å] and angles [o]: Si1–C81 1.8661 (7),
Si1–C12 1.8713 (8), P1–C131 1.8417 (8), P1–C11 1.8291 (7), C12–Si1–C122
91.26 (3), C12–Si1–C21 111.50 (3), C41–C21–C81 164.58, C21–Si1–C81
107.80 (3), C131–P1–C11 99.57 (3). Selected bond lengths for 15 [Å], angles
and torsion angles [o]: Si1–C81 1.872 (9), Si1–C12 1.8641 (9), P1–B1 1.9149
(11), P1–C131 1.8150 (9), C12–Si1–C122 91.88 (4), C12–Si1–C21 111.24 (4),
C41–C21–C81 162.23, C21–Si1–C81 106.57 (4), C131–P1–C11 103.84 (4),
B1–P1–C131 111.02 (5), C41–C21–C81–C101 -2.3(3).
Li
H3B Ph
P
Br
12
MeLi
Si
Si
Me
16
15
P
Ph
BH3
Scheme 3. Synthesis of silicates bearing a phosphorus atom in the
scaffold.
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10.1002/ejic.201900641
European Journal of Inorganic Chemistry
FULL PAPER
we used a fitting program (NMRfit),[22] which revealed the sharp
signals at 0.92 and 0.24 ppm (-70 °C) to originate from the broad
signal at 0.55 ppm (20 °C). This isomerization process is well
described by first order kinetics but only above -30 °C. An Eyring
plot (see SI) suggests an activation barrier of 10.1 kcal‧mol-1 for
the isomerization and an energy difference of about 0.5
kcal‧mol-1 between the two conformers. The low activation
entropy of 1.6 cal‧mol-1‧K-1 agrees with an unimolecular
rearrangement. A second, higher order process gets involved at
temperatures below -30 °C, causing the rate constants to
deviate significantly from linearity. We interpret this behavior to
be caused by ion-pairing as the chemical shifts of both signals
move slightly upfield and the least squares regression of this
region in the Eyring plot shows a negative entropic contribution
to the activation Gibbs free activation energy. It then seems that
at decreasing temperatures (<-30 °C) ion exchange takes
prevalence over conformational interchange.
Figure 5. Aliphatic region of the 1H-NMR spectrum of 14 measured at
10 °C intevals from 20 °C to -90 °C
To eliminate any possible interference of the phosphorus
lone pair during the methylation of 13 with MeLi, we coordinated
the P-center with a BH3 group. Reaction of 13 with BH3‧SMe2
afforded the spirosilane precursor 15 in 91% yield as a white
crystalline solid. The molecular structure, obtained from a single
crystal X-ray structure determination, is shown on the right hand
side of Figure 4. The differences with the BH3 unprotected 13
are small. For example, the puckering angle of the phenyl
flanked heterocycle is reduced from 151.27(4)o for 13 to
147.33(5)o for 15, whereas its C-P-C bond angles are slightly
larger (13 99.57(3)-104.97(3)°; 15 103.84(4)-107.41(4)°). It is
then not surprising that the 1H and 13C NMR spectroscopic
differences between the two structures are only minimal.
Treatment of spirosilane 15 with MeLi provided silicate 16,
as a brown foam in ~90% purity, presumably due to a reaction of
MeLi with BH3. Unfortunately, despite many attempts, no
suitable crystals could be obtained for an X-ray crystal structure
analysis. Nevertheless, NMR spectroscopic and DFT analyses
provided much insight. The 1H29Si-HMBC spectrum, shown in
Figure 6, reveals the presence of two distinct silicates in a 1 : 1.7
ratio with characteristic signatures (δ1 δ 29Si -107.5 ppm; δ1δ 29Si
-110.6 ppm). We presume these silicates to be two conformers
of 16 since they have the same exact mass. To assign these
isomers, we discuss the results of the DFT analysis first (Figure
7).
Formation of pentaorganosilicate 14 was pursued by
reacting 13 with methyllithium but gave, however, a complex
reaction mixture. Whereas no crystals suitable for an X-ray
crystal structure analysis could be obtained from the extracted
solid material, analysis of the NMR spectra proved to be
1
revealing. The aliphatic region of the H NMR spectrum showed
two distinct signals at room temperature (Figure 5). Applying
1H29Si-HMBC revealed that the sharp 1H NMR signal at 0.39
ppm corresponds to a 29Si NMR chemical shift at 0.34 ppm,
which is indicative for a tetracoordinated silicon (c.f., δ29Si
13 -19.9 ppm). This spectroscopy further revealed that the broad
1H NMR singlet at 0.55 ppm showed only a Si-coupling at lower
o
1
temperatures. For example, at -90 C the broad H NMR signal
shifted downfield to a sharp singlet at 0.92 ppm that coupled
with the 29Si NMR chemical shift at -115.1 ppm, which we
presume to be from silicate 14. Cooling of the reaction mixture
also showed a gradual decrease in intensity of the resonance at
0.39 ppm in favor of one at 0.24 ppm that is coupled with a 29Si
NMR chemical shift at -108.2 ppm, which could indicate the
presence of an isomer of silicate 14. Unfortunately, the reaction
mixture proved to degrade rather quickly to an unknown
ensemble of products, thereby inhibiting a more detailed
analysis.
The variable temperature spectra shown in Figure 5 as
1
well as the coupling of the H NMR resonances with both tetra-
and pentacoordinate silicon atoms suggest the presence of
P-containing silicates 14 and 16 – computational. Single
point calculations were conducted for the silicates at B3LYP-
D3/6-311++G(2p,d) on structures optimized at B3LYP/6-31G(d).
The energy profiles for 14 (red) and the BH3-stabilized 16 (black)
are depicted in Figure 7. The very top of Figure 7 shows
Newman projections along the imaginary Si-P axis. Their
identifiers a, b, c, and d are associated with minima and those
labeled ab, bc, and cd represent transition structures for Berry
pseudorotations. Shown is the counterclockwise pseudorotation
of the methyl and biphenyl substituents on silicon with respect to
the dibenzo[b,e][1,4]phosphasilane unit; this motion represents
an enantiomeric rotation of the chiral silicates with the clockwise
motion representing the other. The upper energy diagram of
Figure 7 shows the profile for structures with the P-Ph group
oriented ‘orthogonally’ to the carbon frame of the dibenzo[b,e]-
[1,4]phosphasilane unit, whereas this orientation is ‘in-plane’ for
the bottom diagram; for simplicity they are labeled ‘Ph=axial’
and ‘Ph=equatorial’, respectively.
equilibria. To address the likely dynamic 1H-NMR behavior of 14
Figure 6. 1H29Si-HMBC spectrum of BH3-protected silicate 16.
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10.1002/ejic.201900641
European Journal of Inorganic Chemistry
FULL PAPER
Figure 7. B3LYP-D3/6-311++G(2p,d)//B3LYP/6-31G(d) isomerizations of 14 (red) and 16 (black) with Berry pseudorotation transition structures. All energies
(kcal‧mol-1) are relative to the lowest energy form. Silicon is colored light pink and phosphorus maroon. The P-Ph group is in an ‘axial’ position in the upper part
and in an ‘equatorial’ position in the lower part (see text). X refers to the phosphorus BH3 substituent (16) or its lone pair (14). The hatched lines that cross in
the center represent the barriers for the interconversion of ax-16(14)b and eq -16(14)c and of ax-16(14)c and eq -16(14)b.
Energy profile with axial P-Ph. The lowest energy silicate
structures ax-14a and ax-16a have Cs symmetry and a boat
conformation for the dibenzo[b,e][1,4]phosphasilane unit. The
‘side view’ of ax-16a in the top left of Figure 7 not only visualizes
this boat form but also shows the stabilizing aromatic π-stacking
of the P-phenyl substituent with the biphenyl ligand on Si. A
square pyramidal conformation for a silicate’s global energy
minimum is unusual since it is typically associated with the
transition structure for a Berry pseudorotation, but we presume
the favored boat form of the dibenzo[b,e][1,4]phosphasilane unit
in combination with advantageous dispersion forces to be
responsible for this geometrical behavior.
The counterclockwise rotation of the Si-groups represents
a Berry pseudorotation and gives the 3.7 (4.8) kcal‧mol-1 less
stable trigonal bipyramidal ax-16b (ax-14b) via a modestly
higher barrier of only 3.8 (5.1) kcal‧mol-1 (ax-16ab (ax-14ab)) as
visualized by the Newman projections and the upper energy
diagram in Figure 7. This Berry pseudorotation involves a
change from a square pyramidal to a trigonal bipyramidal
conformation that requires, of course, a change in C-Si-C angles
around the silicon. This change is reflected in the increase of the
angle between the methyl carbon, silicon, and one of the ipso
carbon atoms of the biphenyl ligand that changes from 100.3° in
ax-16a via 123.1° for ax-16ab to 134.4o in ax-16b.
Continuing the counterclockwise rotation results next in the
2.6 (4.0) kcal.mol-1 more favorable ax-16c (ax-14c) that has a
trigonal bipyramidal conformation around silicon with the two
bidentate ligands in apical/equatorial positions. Whereas this
conformer is just 1.1 (0.8) kcal‧mol-1 less stable than the global
minima, the Berry pseudorotation by which it is formed is the
rate determining step for isomerization of the silicate, requiring a
significant 14.9 (13.2) kcal‧mol-1. One phenyl ring of the biphenyl
ligand occupies the axial position of the distorted square
pyramidal transition structure ax-16bc in which the equatorial
methyl group is aligned almost parallel to the six membered P-
containing ring. The sizeable pseudorotation barrier is caused by
friction between the biphenyl ligand and the ‘axial’ P-phenyl
substituent.
Further continuing the rotation of the Si groups converts
ax-16c (ax-14c) into the 11.8 (12.0) kcal‧mol-1 less favorable ax-
16d (ax-14d), which is unstable as the reverse motion requires a
mere 0.4 kcal‧mol-1 (ax-16cd and ax-14cd). The axial methyl
group is aligned parallel to the P-Ph group in the square
pyramidal structure, which contrasts the global minimum ax-16a
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10.1002/ejic.201900641
European Journal of Inorganic Chemistry
FULL PAPER
that is stabilized by aromatic π-stacking (see the ‘side views’ on
the right and left of Figure 7). The pseudorotation causes a
decrease of the C-Si-C angle between the methyl carbon and
one of the ipso carbon atoms of the biphenyl ligand from 124.0°
in ax-16c via 108.3° for ax-16cd to 96.6° in ax-16d, which is the
opposite from that seen for ax-16a → [ax-16ab]‡ → ax-16b.
The described conversion of ax-16a (ax-14a) to ax-16d
(ax-14d) reflects a 180o rotation in the Newman projections and
shows little influence of whether or not the phosphorus atom
carries the stabilizing BH3 phenyl group. The rotation cycle is
completed by mirroring the energy diagram to return to the
global minimum. What still must be addressed is the P-Ph
transfer into an ‘equatorial’ position and its influence on the
energy profile (bottom of Figure 7). This profile is distinctly
different from that in which the P-Ph group is axially oriented.
Energy profile with equatorial P-Ph. In sharp contrast to
the discussed ax-P-Ph energy profile, square pyramidal eq-16a
(eq-14a) is a transition structure for the Berry pseudorotation
that converts eq-16b (eq-14b) into its mirror image (ΔE 7.7 (0.6)
kcal‧mol-1 (Figure 7, bottom). We assume this difference in
behavior to reflect the absence of stabilizing aromatic π-stacking.
The rest of the energy profile shows resemblance to that for the
conformers with ax-P-Ph orientations. Thus, the Berry
pseudorotation of eq-16b into eq-16c is also associated with a
sizeable barrier of 19.8 kcal‧mol-1 and the subsequent clockwise
rotation gives the energetically less favored square pyramidal
eq-16d, just as is the case for the ax-P-Ph analogues. However,
Berry pseudorotation of eq-14b into eq-14c has a distinctive
lower barrier of 10.4 kcal‧mol-1 (eq-14bc) probably due to less
repulsion between the phosphorus lone pair and the substituents
on Si. The most stable isomers on the eq-P-Ph energy profile
are eq-16b and eq-14c, which are, respectively, only 1.1 and 2.5
kcal‧mol-1 less stable than their global minima. The energy
profiles for eq-14 and eq-16 differ mostly with respect to the
Berry pseudorotation barriers which are significantly higher for
the P-BH3 stabilized silicate 16 due to steric repulsion.
series of Berry pseudorotations and ring inversions, i.e., via ax-
14b, eq-14c, and eq-14b.
The DFT calculations further enable the assignment of the
two conformers of 16 observed at room temperature NMR as ax-
16a and ax-16c; their computed energy difference amounts to
1.1 kcal‧mol-1 with an interconversion barrier of 18.6 kcal‧mol-1
(ax-16bc). Also the calculated 1H NMR chemical shifts of ax-16a
(0.76 ppm) and ax-16c (–0.71 ppm) agree well with the
observed ones for the methyl group in 16 (0.65 ppm and –0.64
ppm). The phenyl ring in ax-16c hovers over the methyl group to
cause significant shielding resulting in an upfield chemical shift
at –0.64 ppm. This is not the case for the ax-16a isomer which
has its Me resonance at a more common 0.65 ppm.
Conclusion
This study reports the first nonsymmetric single N and P atom
containing organosilicates. One has a C,C-bridging 2-phenyl-
pyridine group and the other a C,C-bridging triphenylphosphine
with as remaining ligands a bidentate biphenyl and a methyl,
ethyl, or phenyl group (8-10) or only a phenyl group (14). The
stability of 14 increases by BH3-substitution of its P-center (16).
All pyridine containing silicates are remarkably stable with
melting points ranging from 47 to 107o C for 8-10. No melting
point could be determined for foams 14, and 16. X-ray crystal
structures are presented for 10, 13 and 15.
Silicates 8-10 show dynamic conformational behavior in
solution like the all-carbon silicate 2a. They behave also similar
to both the N- and S-hetero homologues 2b and 2d that have
two hetero-containing C,C-bidentate ligands with five-membered
pyrrole and thiophene rings, respectively. Replacing one phenyl
group in 2a for a pyridine group appears not to hamper the
conformational dynamics of the system. VT NMR spectroscopic
analysis revealed a rapid interchange of two isoenergetic
conformers. DFT calculations estimate an energy difference of
less than 0.5 kcal‧mol-1 and an interconversion barrier of ca. 10
kcal‧mol-1, which is of the same magnitude as the racemization
barriers for both conformers.
The P-containing silicates 14 and 16 behave differently
because the phosphorus atom is embedded in the central six-
membered ring of the C,C-chelating triphenylphosphine. VT
NMR analysis revealed rapidly interchanging conformers for 14,
but also an equilibrium with its silane precursor. An Eyring plot
suggests an energy difference of about 0.5 kcal‧mol-1 between
the two conformers and an interconversion barrier of ca. 10
kcal‧mol-1. DFT calculations confirm the presence of two
isoenergetic conformers (ΔE < 1.0 kcal‧mol-1) and indicate a
barrier of 13.4 kcal‧mol-1 for a complex interconversion pathway
that includes Berry pseudorotations, turnstile rotations, and ‘ring
flips’ of the dichelating six-membered Si,P-ring. Stabilizing the
silicate by BH3-coordination of the P-center (16) raises the
lowest energy barrier for the complex interconversion to 18.6
kcal‧mol-1. This barrier is of the same general magnitude as
reported for 2c, which can be obtained as non-racemizing
enantiomers if the fifth ligand is an equatorial fluorine. This
bodes well for pursuing silicates with chiral integrity through
either enlarging the size of the C,C-coordinating ring (16) or
increasing the size of the ligand (2c).
Remains the question how the ax-P-Ph and eq-P-Ph
energy profiles are connected. It is unlikely that this occurs
through the square pyramidal conformers a and d as it would
cause friction between the ortho hydrogens of the biphenyl
ligand and the dibenzo[b,e][1,4]phosphasilane unit, but maybe
more importantly eq-16a (eq-14a) is a transition structure and
ax-16d (ax-14d) is a metastable conformer. The two energy
profiles are instead connected through the trigonal bipyramidal
conformers b and c. All what is needed is bending up the
aromatic ‘wings’ of the P-containing six-membered ring of ax-
16b (ax-14b) to give after reorientation eq-16c (eq-14c), which
requires only a modest 5.3 (5.3) kcal‧mol-1. A similar ring flip
converts eq-16b (eq-14b) into ax-16c (ax-14c) and has a
comparable reaction barrier of 8.7 (6.6) kcal‧mol-1.
NMR-DFT correlation. The presented DFT analysis of 14 is
in excellent agreement with the energetic data that could be
deduced from the limited VT H NMR experiments that suggest,
1
based on Eyring plots, an energy difference of ca. 0.5 kcal‧mol-1
between two conformers and a barrier of ca. 10 kcal‧mol-1 for
their interconversion. Whereas the DFT potential energy surface
for 14 might be complex, it identifies ax-14a and ax-14c as its
two most stable conformers with a similarly small energy
difference of only 0.8 kcal‧mol-1. The DFT calculations also give
a similarly sized barrier of 13.4 kcal‧mol-1 for the interchange of
the conformers and illustrate the process to occur through a
We feel that the concepts outlined in this study on dynamic
conformational behavior of nonsymmetric silicates may also be
This article is protected by copyright. All rights reserved.

10.1002/ejic.201900641
European Journal of Inorganic Chemistry
FULL PAPER
refluxed for 72 h. After cooling, the phases were separated and the
aqueous phase was extracted three times with chloroform. The combined
organic phases were dried over Na2SO4 and concentrated under vacuum.
The crude mixture was separated by column chromatography (SiO2,
cHex/EtOAc, 2:1, Rf = 0.38), giving 1.22 g (92%) of product 6 as a
colorless oil, which solidified overnight. 1H NMR (500.23 MHz, CD2Cl2,
296 K): δ 8.62 (dd, 3J(H,H) = 5.0 Hz, 4J(H,H) = 1.0 Hz, 1H; H6), 8.01 (dd,
applicable to pentacoordinated transition metals and that it may
contribute to enlarging the number chiral-at-metal catalysts.
Computational Section
Single-point density functional theory calculations were
performed at the B3LYP[23–25]-D3[26]/6-311++G(2p,d) //B3LYP[27–
29]/6-31G(d) level using the Gaussian09 suite of programs,
revision D.01.[30] All geometry optimizations were carried out
without symmetry restrictions and were performed at B3LYP/6-
31G(d). The signature of all stationary points, minima and
transition structures, were determined by through analytical
calculation of the Hessian. at the same level of theory. Cartesian
coordinates and total energies are provided for all structures in
the Supporting Information. NMR chemical shifts have been
calculated using the GIAO Method at the B3LYP/6-
311++G(2p,d) level.
4
3
3J(H,H) = 8.0 Hz, J(H,H) = 1.0 Hz, 1H; H4), 7.68 (dd, J(H,H) = 8.5 Hz,
4J(H,H) = 1.0 Hz, 1H; H12), 7.46–7.43 (m, 1H; H10), 7.35–7.31 (m, 2H;
4
H9, H11), 7.25 (dd, 3J(H,H) = 8.0 Hz, J(H,H) = 5.0 Hz, 1H; H5); 13C{1H}
NMR (125.78 MHz, CD2Cl2, 296 K): δ 158.65 (C2), 148.23 (C6), 141.52
(C8), 140.66 (C4), 132.89 (C12), 130.63 (C9 or C11), 130.39 (C9 or C11),
127.71 (C10), 124.60 (C5), 122.63 (C7), 121.49 (C3); HR-MS (ESI):
calcd for C11H9Br2N 311.9018, found 311.9017.
2-Pyridylphenyl-biphenyl-2,2’-silane 7: n-BuLi (8.24 mL,
13.23 mmol, 2.07 eq.) was added to a solution of 6 (2.0
g, 6.39 mmol, 1.0 eq.) in Et2O (80 mL) and cooled to –
78 °C, the yellow suspension was stirred at –78 °C for
90 min. In a separate Schlenk vessel, 5 (1.6 g, 6.39
mmol, 1.0 eq.) was dissolved in Et2O (60 mL), cooled
15 16
14 17
13 18
Si
12
11
10
7
9
3
4
6
8
2
5
Experimental Section
N
to 0 °C and the lithiated suspension of 6 was added dropwise to the
colorless solution of 5 at 0 °C. The resulting orange/red suspension was
stirred overnight at room temperature, quenched using water, extracted
into Et2O (x3), dried over Na2SO4, filtered and concentrated in vacuo.
The orange residue was purified using column chromatography
(cHex/EtOAc, first 10/0, then 10/1, 9/1, 8/1), concentrated in vacuo and
washed using n-pentane to obtain product 7 as a white solid. (558.5 mg,
1.67 mmol, 52%). 1H NMR (500.23 MHz, CD2Cl2, 296 K): δ 8.56–8.65
(m; H6), 8.33 (d, 3J(H,H) = 8.0 Hz, 1H; H9), 7.96 (d, 3J(H,H) = 7.5 Hz, 2H;
H15), 7.68 (dd, 3J(H,H) = 7.0 Hz, 4J(H,H) = 1.5 Hz, 1H; H4), 7.58 (t,
3J(H,H) = 7.5 Hz, 1H; H10), 7.53 (t, 3J(H,H) = 7.5 Hz, 2H; H16), 7.42–
7.38 (m, 3H; H12, H18), 7.33 (t, 3J(H,H) = 7.5 Hz, 1H; H11), 7.23 (t,
3J(H,H) = 7.5 Hz, 2H; H17), 7.12–7.10 (m, 1H; H5). 13C{1H} NMR (125.78
MHz, CD2Cl2, 296 K): δ 167.54 (C2), 152.10 (C6), 150.55 (C8), 150.44
(C14), 142.20 (C4), 134.50 (C18), 134.23 (C12), 133.39 (C7), 132.09
(C10), 132.01 (C16), 131.92 (C13), 129.97 (C11), 128.43 (C17), 127.21
(C3), 122.98 (C5, C9), 121.59 (C15); 1H-29Si-HMBC NMR (400.13, 79.49
MHz, CD2Cl2, 296 K): δ –9.21 (Si); IR (neat): 3040 (w, C-HAr), 820 (m,
CAr-H), 758 (s, CAr-H); C23H16NSi 334.1047, found 334.1043. Mp
216.1 °C.
General. Tetrahydrofuran (THF) was distilled subsequently from LiAlH4
and sodium/potassium alloy and diethyl ether from sodium/potassium
alloy. n-Butyllithium and methyllithium were purchased as 1.6 M solutions
in hexanes and in diethyl ether, respectively; ethyllithium was purchased
as a 0.5 M solution in benzene/cyclohexane (9:1) Phenyllithium was
purchased as a 1.9 M solution in di-tert-butylether. Tetrachlorosilane was
distilled and refluxed before use to remove HCl. The mass, NMR and
melting point samples of silicates were prepared and handled in the
purified N2 atmosphere of an MBRAUN Unilab glovebox; other syntheses
were performed using standard Schlenk techniques. NMR spectra were
recorded on a Bruker Avance 400 (1H, 13C, 31P{1H}, 11B{1H},29Si,2D
spectra) or Bruker Avance 500 (1H, 13C). NMR chemical shifts are
internally referenced to the solvent for 1H (CHCl3: 7.26, THF: 3.59,
CH2Cl2: 5.32 ppm) and 13C (CHCl3: 77.16, THF: 67.58, CH2Cl2: 53.84
ppm), and externally for 29Si, 31P{1H}, 11B{1H} to TMS, 85% H3PO4 and
BF3.OEt2, respectively. Melting points were measured on samples in
sealed capillaries and are uncorrected. HR-ESI-MS measurements of
silicates were measured on an AccuTof JMS-T 100LP mass
spectrometer.
Cl
Cl
1
5,5-Dichloro-5H-dibenzo[b,d]silole
(5).
2,2-dibromo-
Lithium methyl-2-pyridylphenyl-biphenyl-2,2’-silicate
8: MeLi (1.6 M in Et2O, 0.098 mL, 0.16 mmol, 1,05
Si
Li
15
biphenyl (6.14 g, 19.69 mmol, 1.0 eq.) was dissolved in
THF (55 mL) and cooled to –78 °C. To this colorless
solution, nBuLi (25.48 mL, 40.76 mmol, 2.07 eq.) was
6
14
13
16
17
5
2
eq.) was added to
a solution 2-pyridylphenyl-
12
9
4
7
8
Si
3
3
11
10
18
19
biphenyl-2,2’-silane 6 (50mg, 0.15 mmol, 1.0 eq.) in
THF (1 mL) at –78 °C. The resulting yellow solution
was stirred for 0.5 hour at this temperature and
allowed to warm to RT. The orange solution was
2
4
added and the mixture was stirred for one hour at -78 °C.
Silicontetrachloride (11.28 mL, 98.47 mmol, 5.0 eq.) was separately
dissolved in THF (30 mL) and cooled to –95 °C. The yellow suspension
of the 2,2-dilithumbiphenyl was added dropwise to this colorless solution
and the mixture was stirred overnight, gradually warming to room
temperature. Next, all volatiles were removed in vacuo and the product
was extracted into Et2O (3x 20 mL). The product was obtained as a
impure white solid after evaporation of residual solvent in vacuo. Yield:
4.44 g (17.0 mmol, 86%). 1H NMR (500.23 MHz, THF-d8): δ 7.92 (d,
3J(H,H) = 7.8 Hz, 2H; H3), 7.75 (d, 3J(H,H) = 7.1 Hz, 2H; H6), 7.56 (dt,
3J(H,H) = 7.7 Hz, 4J(H,H) = 1.2 Hz, 2H; H4), 7.39 (dt, 3J(H,H) = 7.4 Hz,
4J(H,H) = 0.9 Hz, 2H; H5). 13C{1H} NMR (125.78 MHz, THF-d8, 296 K): δ
146.38 (C2), 134.04 (C4), 133.11 (C6), 130.98 (C1), 129.76 (C5), 122.09
(C3). 1H-29Si-HMBC NMR (400.13, 79.49 MHz, THF-d8, 295 K): δ 6.16.
Mp: 92.1-101.6 °C.
N
5
6
concentrated in vacuo and the resulting orange foam was washed with n-
pentane to obtain 8 as a bright yellow powder (98.3 mg, quant. with
3
excess MeLi). 1H NMR (500.23 MHz, THF-d8, 269 K) δ 8.19 (dd, J(H,H)
4
3
= 4.7 Hz, J(H,H) = 1.7 Hz, 1H), 8.14 (d, J(H,H) = 7.6 Hz, 1H), 7.77 (br.
s, 1H), 7.70 (d, 3J(H,H) = 7.7 Hz, 2H; H18), 7.56 (d, 3J(H,H) = 7.0 Hz, 1H),
3
7.52 (d, J(H,H) = 6.3 Hz, 2H), 7.07–7.02 (m, 2H; H17), 6.90 (br. s, 4H),
0.35 (s, 3H; H19). 13C{1H} NMR (125.78 MHz, THF-d8, 296 K): δ 136.32
(CH), 124.96 (C18), 120.83 (bs), 119.40 (C17), 7.03 (C19), the signals
for C1 to C 16 were unresolved. 1H-29Si-HMBC NMR (400.13, 79.49 MHz,
THF-d8, 296 K): δ –107.2 (Si). HR-MS (ESI): calcd for C24H18NSi
348.1214, found 348.1214. Mp: >47.2 °C (decomp.).
Lithium methyl-2-pyridylphenyl-biphenyl-2,2’-silicate
3-Bromo-2-(2-bromophenyl)pyridine 6: prepared
Li
9: EtLi (0.5 M, 0.36 mL, 1,2 eq.) was added to a
solution of 2-pyridylphenyl-biphenyl-2,2’-silane 6 (50
mg, 0.15 mmol, 1.0 eq.) in THF (1 mL) at –78 °C.
The resulting yellow solution was stirred for 0.5 hour
at this temperature and allowed to warm to r.t The
orange solution was concentrated in vacuo and the
orange foam was washed using n-pentane to obtain
Br
15
14
13
literature procedure.[35] 2,3-
16
17
according to
a
12 7
11
10 9
N
3
6
4
12
7
8
Si
3
8
2
5
11
10
18
19 20
Dibromopyridine 7 (1.00 g, 4.22 mmol) and 2-
bromo-phenylboronic acid (854 mg, 4.22 mmol)
were dissolved in tetrahydrofuran (20 mL), aqueous
9
2
4
5
Br
N
6
K2CO3 (1 M, 15 mL), 5 drops of aqueous KOH (10%) and then degassed.
Pd(PPh3)4 (244 mg, 0.21 mmol) was added and the yellow mixture was
This article is protected by copyright. All rights reserved.

10.1002/ejic.201900641
European Journal of Inorganic Chemistry
FULL PAPER
9 as a slightly yellow powder (84.3 mg, 0.13 mmol, 85.4%). 1H NMR
(400.13 MHz, THF-d8, 269 K): δ 8.20 (dd, 3J(H,H) = 4.6 Hz, 4J(H,H) = 0.8
Hz, 1H), 8.16 (d, 3J(H,H) = 7.2 Hz, 1H), 7.87–7.82 (m, 1H), 7.70 (d,
3J(H,H) = 7.8 Hz, 2H; H18), 7.64 (d, 3J(H,H) = 7.0 Hz, 1H), 7.60 (d,
3J(H,H) = 6.0 Hz, 2H), 7.09–7.01 (m, 2H; H17), 6.90 (br. s, 4H), 0.93–
was unresolved. 31P NMR (162.0 MHz, CDCl3): δ –4.17. Mp: 123.7 °C
(decomp).
5'-Phenyl-5'H-spiro[dibenzo[b,d]silole-5,10'-dibenzo-
21
22
[b,e][1,4]phosphasilane] (13): n-BuLi (11.97 mL,
19.15 mmol, 2.07 eq.) was added dropwise to a
solution of bis(2-bromophenyl)phenylphosphane
(3.88 g, 9.25 mmol, 1.0 eq.) in THF (140 mL) at –
78 °C and the orange solution was stirred for one
hour. Separately, 5,5-dichlorodibenzosilole (2.42 g,
9.25 mmol, 1.0 eq.) was dissolved in THF (70 mL)
and cooled to –78 °C, after which the yellow solution
was added to the orange solution. The resulting dark
20
19
0.88 (m, 2H; H19), 0.73 (t, J(H,H) = 7.5 Hz, 3H; H20). 13C NMR (100.6
3
MHz, THF-d8, 269 K): δ 122.08 (CH), 118.43 (CH), 116.54 (CH), 13.84
(C19), 8.06 (C20), the signals for carbon 1 to 18 were unresolved. 1H-
29Si-HMBC NMR (400.13, 79.49 MHz, THF-d8, 296 K): δ –101.6 (Si). HR-
MS (ESI): calcd. for C25H20NSi 362.1370, found 362.1386. Mp: >77.6 °C
(decomp.).
14
P
18
13 15
13
17
16
18 16
17
14
Si
15
1 6
12 7
11
10 9
8 2
5
3 4
Lithium phenyl-2-pyridylphenyl-biphenyl-2,2’-silicate
10: PhLi (1.9 M, 0.082 mL, 0.16 mmol, 1,05 eq.)
Li
red solution was stirred overnight, gradually letting the mixture reach
room temperature. Afterwards, the product was filtrated through a plug of
alumina and solvents were removed in vacuo. The product was purified
by dissolving it in little amount of DCM, followed by precipitation in n-
pentane. The product was obtained as a pure white solid (3.56 g, 8.08
mmol, 87%). 1H NMR (500.23 MHz, CDCl3): δ 7.95 (dd, 3J(H,H) = 7.8,
15
14
13
16
17
was added to a solution 2-pyridylphenyl-biphenyl-
2,2’-silane 6 (50 mg, 0.15 mmol, 1.0 eq.) in THF (1
mL) at –78 °C. The resulting dark red solution was
stirred for 0.5 hour at this temperature and allowed
to warm to RT. The mixture was then allowed to
12
9
7
8
Si
3
11
10
18
20
21
22
19
2
4
5
N
6
3
warm to RT and became a dark brown solution, which gave a brown
foam-like solid after concentration in vacuo. The product was obtained
with some excess PhLi present. 1H NMR (400.13 MHz, THF-d8) δ 8.21 (d,
3J(H,H) = 7.5 Hz, 1H), 8.18 (dd, 3J(H,H) = 4.8 Hz, 4J(H,H) = 1.8 Hz, 1H),
7.74 (d, 3J(H,H) = 7.6 Hz, 2H), 7.47 (d, 3J(H,H) = 6.0 Hz, 1H), 7.43–7.42
(m, 1H), 7.30 (d, 3J(H,H) = 6.0 Hz, 2H), 7.10 (t, 3J(H,H) = 7.0 Hz, 1H).
7.06–7.02 (m, 4H), 6.90 (dt, 3J(H,H) = 7.1 Hz, 4J(H,H) = 0.9 Hz, 1H), 6.82
(t, 3J(H,H) = 7.0 Hz, 1H), 6.73–6.70 (m, 3H). 1H-29Si-HMBC NMR (400.13,
79.49 MHz, THF-d8, 296 K): δ –102.2 (Si). HR-MS (ESI): calcd for
C29H20NSi 410.1370, found 410.1373. Mp: >107.3 °C (decomp.).
4J(H,H) = 2.8 Hz, 2H; H3, H9), 7.61 (d, J(H,H) = 7.1 Hz, 1H; H12), 7.52
(t, 3J(H,H) = 9.0 Hz, 2H; H18), 7.42 (m, 2H; H4, H10), 7.35 (m, 9H; H17,
3
H15, H20, H21, H22), 7.18 (m, 3H; H11, H16), 7.05 (t, J(H,H) = 6.9 Hz,
3
1H; H5), 6.62 (d, J(H,H) = 7.1 Hz, 1H; H6). 13C{1H} NMR (125.78 MHz,
CDCl3): δ 149.86 (C2), 149.16 (C8), 146.75 (d, 1J(C,P) = 11.6 Hz;
C13),140.75 (d, 1J(C,P) = 15.0 Hz; C19), 136.51 (C7), 135.26 (d, 2J(C,P)
= 3.1 Hz; C20), 134.47–134.40 (m; C1, C14), 134.37 (C6), 134.07 (C12),
134.01 (d, 3J(C,P) = 3.8 Hz; C15), 133.48 (d, 2J(C,P) = 3.8 Hz; C18),
130.91 (C4 or C10) 130.85 (C4 or C10), 129.45 (d; 3J(C,P) = 10.0 Hz;
C17), 128.85–128.76 (m; C21, C22), 127.85 (C16), 127.56 (C5, C11),
121.02 (C3 or C9), 120.89 (C3 or C9).* 1H-29Si-HMBC NMR (400.13,
79.49 MHz, CDCl3, 295 K): δ –19.40. 31P NMR (162.0 MHz, THF d8): δ
–6.76. MP: 192.2–195.5 °C.
Tetraethylammonium
biphenyl-2,2’-silicate 11: MeLi (1.6 M in Et2O, 0.098
mL, 1,05 eq.) was added to solution 2-
pyridylphenyl-biphenyl-2,2’-silane (50 mg, 0.15
mmol, 1.0 eq.) in THF (1 mL) at –78 °C. The
resulting yellow solution was stirred for 0.5 hour at
this temperature and allowed to warm to RT.
methyl
2-pyridylphenyl-
NEt4
15
14
13
16
17
a
12
7
Si
3
11
10
18
19
6
Triphenylphosphinemethylsilicate 14: MeLi (1,6 M in
8
9
P
2
4
Et2O, 0.078 mL, 0.12 mmol, 1.1 eq.) was added
Li
N
5
dropwise to a solution of triphenylphosphinesilane 13
6
Si
(50.0 mg, 0.11 mmol, 1.0 eq.) in THF (1 mL) at –78 °C.
Me
Tetraethylammonium bromide was suspended separately in THF (2 mL)
and the yellow silicate solution was added to this mixture. The
suspension turned darker yellow and was stirred overnight at RT.
Volatiles were evaporated in vacuo and the white foam was washed with
Et2O to yield 11 as a white solid with 15% hydrolysis (quant.). 1H NMR
The pink solution was stirred for 30 minutes at this
temperature and for 30 minutes at RT. Volatiles were
removed in vacuo and the remaining red solid was washed with pentane
(3x 5 mL), obtaining a complex mixture of 14 in various forms as a brown
foam (quant). 1H NMR (400.13 MHz, THF-d8, 183 K): complex aromatic
region, 0.92 (H), 0.38 (H), 0.23 (H). 29Si NMR (162.0 MHz, THF-d8, 183
K): δ –115.1 (Si), –107.6 (Si), 0.9 (Si).
4
(400.13 MHz, THF-d8, 269 K) δ 8.22 (dd, 3J(H,H) = 4.4 Hz, J(H,H) = 1.6
Hz, 1H), 8.14 (d, 3J(H,H) = 7.6 Hz, 1H), 7.74–7.71 (m, 2H, H18), 7.51–
7.46 (m, 3J(H,H) = 7.0 Hz, 3H), 7.11–7.04 (m, 2H, H17), 6.90 (br. s, 4H),
3.32 (q, 2J(H,H) = 14.5 Hz, 3J(H,H) = 7.4 Hz, 8H), 1.23–1.19 (m, 12H),
0.31 (s, 3H; H19).
22
Borane triphenylphosphinesilane 15: BH3SMe2 (2.86 mL,
21
20
21
20
BH3
5.72 mmol, 4.0 eq.) was added dropwise to a colorless
solution of triphenylphosphinesilane 13 (0.63 g, 1.43
mmol, 1.0 eq.) in THF (60 mL) at 0 °C. The colorless
solution was stirred overnight, gradually warming to
room temperature. Afterwards, H2O was added, the
mixture was extracted into DCM and solvents were
19
P
14
14
Bis(2-bromophenyl)(phenyl)phosphane (12):[36] 1,2-
dibromobenzene (2.8 mL, 23.30 mmol, 2.03 eq.) was
dissolved in THF (40 mL) and Et2O (40 mL) and cooled
to –115 °C. To this colourless solution nBuLi (14.56 mL,
23.30 mmol, 2.03 eq.) was added dropwise at a rate of
0.15 mL/min after which the resulting white suspension
13 15
15
16
13
18
10
9
18 16
17
17
6
Si
8
7
12
7
Br
Br
1
1
2
11
5
P
8
2
3
6
5
10
9
4
3
removed in vacuo. Product was purified by washing with
4
1
pentane to obtain 15 as a pure white solid (0.59 g, 1.29 mmol, 91%). H
NMR (500.23 MHz, THF-d8): δ 8.31 (dd, 2J(H,P) = 12.1 Hz, 3J(H,H) = 7.7
was stirred for one hour. Dichlorophenylphosphine (1.55 mL, 11.50 mmol,
1.0 eq.) was added dropwise to the suspension and stirred for 30
minutes. The orange solution was stirred overnight, gradually increasing
the temperature to room temperature. Afterwards, aqueous NH4Cl was
added and the product was extracted into DCM, the organic layer was
dried with Na2SO4 and filtrated. After evaporation of the solvents, the
white product was purified by washing with pentane and cold methanol.
Yield: (2.79 g, 6.64 mmol, 58%). 1H NMR (500.23 MHz, CDCl3): δ 7.64–
7.60 (m, 2H; H6), 7.44–7.37 (m, 3H; H8, H10), 7.29 (td, 3J(H,H) = 8.1 Hz,
4J(H,H) = 1.4 Hz, 2H; H9), 7.24–7.20 (m, 4H; H4, H5), 6.77–6.73 (m, 2H;
H3). 13C{1H} NMR (125.78 MHz, CDCl3): δ 137.9 (d, 1J(C,P) = 11.0 Hz;
C2), 137.70 (C3), 134.54 (d, 3J(C,P) = 21.5 Hz; C9), 133.23 (d, 3J(C,P) =
2.5 Hz; C6), 130.56 (s; C5), 130.21 (d, 2J(C,P) = 32.4 Hz; C1), 129.58
(C10), 129.03 (d, 2J(C,P) = 7.7 Hz, C8), 127.71 (C4), the signal for C7
3
3
Hz, 2H; H14), 7.99 (d, J(H,H) = 7.8 Hz, 1H; H9), 7.91 (d, J(H,H) = 8.1
Hz, 1H; H3), 7.62 (tt, 3J(H,H) = 7.7 Hz, 3J(H,P) = 1.4 Hz, 2H; H13), 7.55–
7.49 (m, 3H; H10, H12, H22), 7.43–7.39 (m, 4H; H18, H21), 7.36–7.33
(m, 3H; H17, H4), 7.29–7.25 (m, 3H; H20, H11), 6.82 (t, 3J(H,H) = 7.2 Hz,
3
4
4J(H,H) = 0.6 Hz, 1H; H5), 5.82 (dd, J(H,H) = 7.3 Hz, J(H,H) = 0.5 Hz,
1H; H6), 1.37 (br.s, 3H; BH3). 13C{1H} NMR (125.78 MHz, THF-d8): δ
151.09 (C8), 150.42 (C2), 137.84 (d, 1J(C,P) = 56.9 Hz; C19), 137.50 (d,
2J(C,P) = 4.2 Hz; C16), 137.13 (d, 3J(C,P) = 7.6 Hz; C17), 136.43 (d,
1J(C,P) = 54.46 Hz; C19), 135.74 (C7), 135.72 (d, 2J(C,P) = 15.4 Hz;
C14), 135.48 (C22), 134.88 (C6), 133.38 (d, 2J(C,P) = 9.5 Hz; C20),
132.55 (C12), 132.04 (C4), 131.56–131.41 (m; C10, C13, C18), 130.02
(d, 2J(C,P) = 9.9 Hz; C21), 129.36 (C11), 128.57 (C5), 122.15 (C3),
122.04 (C9). H-29Si-HMBC NMR (400.13, 79.49 MHz, THF-d8): δ –21.6.
1
31P NMR (162.0 MHz, THF-d8): δ 10.53. 11B NMR (128.3 MHz, CDCl3): δ
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10.1002/ejic.201900641
European Journal of Inorganic Chemistry
FULL PAPER
–36.54. HR-MS (EI): calcd for C30H24BPSi 454.1478 and C30H21PSi
440.1150, found 454.1481 and 440.1116.
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J. C. Slootweg, A. W. Ehlers, K. Lammertsma, Inorg. Chem.
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give spirosilane 7 with two pyridine groups, but instead only a
complex product mixture, n.d.
[21] Reaction of 4 with 2 equiv of the Li2-salt of 12 did not give the
spirosilane with two equivalent bidentate P-containing ligands
but instead a complex mixture of products., n.d.
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Borane triphenylphosphinemethylsilicate 16:
21
BH3
22
21
13
20
14 18
MeLi (0.078 mL, 0.12 mmol, 1.1 eq.) was added
dropwise to solution of boron-
19
14
20
P
15 17
16
a
Li
6
13 15
triphenylphosphinesilane 15 (50.0 mg, 0.11
mmol, 1.0 eq.) in THF (1 mL) at –78 °C. The pink
solution was stirred for 30 minutes at –78 °C and
for 30 minutes at RT. Volatiles were removed in
vacuo and the red solid was washed with n-
5
18 16
17
1
Si
7
4
Me
2
3
12
11
8
9
10
pentane (3x 5 mL) to afford crude 16 as a syn:anti mixture in a 1:0.6
ratio; the ratio is based on integration of the respective 31P NMR signals).
The product was found to contain multiple impurities which could not be
removed by washing steps or by crystallization. As
a result, full
characterization by means of 1H and 13C NMR spectroscopy was
hampered; the recorded spectra are presented below and show multiple
overlapping signals in the aromatic region. However, the two 1H NMR
resonance signals corresponding to the −CH3 groups in 16-syn and 16-
anti could be located at −0.64 ppm and 0.65 ppm, respectively. These
values were found to be in good agreement with the values obtained
from NMR calculations using DFT (GIAO method; δ1H NMR calc −CH3
16-syn = −0.71 ppm; 16-anti = 0.76 ppm). The product was further
identified by heteronuclear NMR spectroscopy in combination with HR-
MS. 1H-29Si-HMBC NMR (400.13, 79.49 MHz, THF-d8): δ –110.6, –107.5.
31P NMR (162.0 MHz, THF-d8): δ 2.75 (s) and 0.20 (s) in a 1:0.6 ratio.
11B{1H} NMR (128.3 MHz, THF-d8): δ –37.16 (s). HR-MS (CSI): calcd. for
C31H27BPSi 469.1718 ([M]− –[CH3]−), found 469.1734; calcd. for
C31H24PSi 455.1390 ([M]− –[CH3]− –BH3), found 455.1415.
CCDC 1901693-1901695 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
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This work was supported by The Netherlands Organisation for
Scientific Research, Chemical Sciences (NWO-CW). We
acknowledge SARA Computing and Networking Services for
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Keywords: silicon • silicate • hypervalent compounds • Berry
pseudorotation • density functional calculations ∙
4373–4376.
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FULL PAPER
Entry for the Table of Contents
FULL PAPER
Dynamic Silicates
Leon J.P. van der Boon, Jesper
Hendriks, Danny Roolvink Sean J.
O'Kennedy, Martin Lutz, J. Chris
Slootweg, Andreas W. Ehlers* and Koop
Lammertsma*
Page No. – Page No.
Controlled dynamic behavior by the propeller-like motion of the substituents
around the silicon center of bidentate silicates is dictated by Berry pseudorotations,
turnstile rotations, and conformational flexibility. The bidentate triphenylphosphine
opens new perspectives.
Dynamic Conformational Behavior In
Stable Pentaorganosilicates
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