Synthesis of Electron-Rich, Planarized Silicon(IV) Species and a Theoretical Analysis of Dimerizing Aminosilanes

Article abstract of DOI:10.1002/chem.201703649

Equipping silicon(IV) with electron-rich, geometrically constrained NNN- and ONO-tridentate substituents leads to aminosilanes with increased Lewis acidity—expressed through the formation of Si₂N₂ rings by head-to-tail dimerization.

Full text of DOI:10.1002/chem.201703649

A Journal of
Accepted Article
Title: Synthesis of electron-rich, planarized silicon(IV) species and the
theoretical analysis of dimerizing aminosilanes
Authors: Nina Kramer, Christoph Jöst, Alexandra Mackenroth, and Lutz
Greb
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To be cited as: Chem. Eur. J. 10.1002/chem.201703649
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Synthesis of electron-rich, planarized silicon(IV) species and the
theoretical analysis of dimerizing aminosilanes
Nina Kramer, Christoph Jöst, Alexandra Mackenroth, Lutz Greb*
t
Abstract: Equipping silicon(IV) with electron-rich, geometrically
constrained NNN- and ONO-tridentate substituents leads to
aminosilanes with increased Lewis acidity – expressed through the
and HN[CH
2
(O) Bu]
2
, with significant charge reorganization from
[
7]
the initial Sn(IV) to a formal stannylene Sn(II) species. An analog
intramolecular electron transfer was described by Driess et al. for
formation of Si
N
2 2
rings by head-to-tail dimerization. Depending on the
germanium(IV), even though the presence of
a
Ge(II)
substituents, the dimerization can be controlled for the first time,
yielding monomeric, structurally reversible and dimeric states. The
monomeric species display substantial distortions from tetrahedral
towards planar geometry at silicon. The dimerization and the Lewis
acidiy of aminosilanes are rationalized by (conceptual) DFT, NBO,
ETS-NOCV and QTAIM methods. The preorganization at silicon,
London dispersion between the substituents and resonance
intermediate could be supported only by secondary reaction
products.[8] Very surprisingly, the realization of corresponding
monomeric silicon-based compounds is still pending. One
[
9]
exclusive attempt was reported. No intramolecular charge
transfer was found, but a surprising dimerization of the primary
reaction products was observed (2, figure 1b). The proposed
parent monomeric species have neither been detected nor could
the dimerization process be prevented by installation of a bulky
group at silicon. Instead, this modification led to a radical type
dimerization in the ligand backbone, indicating particular reactivity
of the eventual monomeric units. It should be stressed, that the
ability for hypercoordination at silicon[ in a monomeric state,
combined with the electron releasing nature of redox-active
substituents,[11]
2 2
phenomena inside the formed Si N tetracycles are identified as
driving forces for the dimerization. Comparison with selected
aminosilanes permits general conclusions on the Lewis acidity of
silicon species and on the aggregation of amphiphilic compounds.
10]
Introduction
Molecular main-group element compounds with unusual
geometries are of longstanding interest, owing to their particular
(
physico)chemical features, which are enabled by the deformation
of the central element.[1] Such species have mainly been studied
for group 13-16 elements and revealed peculiar structures and
unexpected reactivity. The increase of Lewis acidity in group 13
species by pyramidalization has been proposed theoretically[2]
and confirmed by experiment.[3] Seminal studies by Arduengo et
al. demonstrated the electronic and geometrical effects of
electron-rich and geometrically constrained substituents on group
15
elements (e.g. in unusual T-shaped phosphorous
[
4]
compounds). An essential feature of the electron-rich ligands is
the transfer of electron density to the central atom, leading to a
formally reduced state with specific reactivity.[4c, 5] The capability
of such compounds for main-group element mediated reversible
bond activations lead to a recent burst of research activities and
catalytic applications.[6] For group 14 elements, the corresponding
chemistry is less developed. Intramolecular electron transfer was
observed in the tin compound 1 (figure 1a) derived from SnCl
4
[
a]
Nina Kramer, Christoph Jöst, Alexandra Mackenroth, Dr. Lutz Greb
Anorganisch-Chemisches Institut
Ruprecht-Karls-Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg
E-mail: greb@uni-heidelberg.de
Figure 1. a/b) Previously studied electron-rich, geometrically constrained group
Supporting information for this article (experimental and computation
details) is given via a link at the end of the document. CCDC
t
14 compounds based on the HN[CH
2
(O) Bu]
2
ligand show an intramolecular
electron transfer for tin (a) or irreversible dimerization for silicon (b), c)
amphiphilic silicon-Lewis base compounds which tend to dimerize in the solid
state and d) the herein described electron-rich, geometrically constrained Si(IV)-
NNN and -ONO compounds.
1565850-1565854 and 1566286 contain(s) the supplementary
crystallographic data for this paper. These data are provided free of
charge by the Cambridge Crystallographic Data Centre.
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is promising for new bond activation processes mediated by
CH
3
CN respectively and the desired products were easily
silicon
–
the most abundant element in the earth crust.
obtained on gram scale with up to 91 % yield. Initial attempts to
apply the NEt based method (suitable for the preparation of
compounds 2) starting with the neutral H ONO ligand 10 failed.
Instead, the very efficient formation of a neutral, hexacoordinate
silicon(IV) complex Si[ONOH] (11) was observed for HSiCl and
PhSiCl as electrophiles, with the concomitant liberation of H or
, respectively (scheme 1, see SI for the structural analysis of
11, reminiscent of similar neutral SiO
complexes).[16] Such mild
Understanding and prevention of the dimerization leading to
compounds like 2 would thus mean a major step towards the
exploration of new modes of ligand-element cooperativity with
silicon(IV) compounds. In comparison to other silicon
donor/acceptor complexes, the dimerization of 2 in solution is
astonishing, since aminosilanes are not considered as effective
Lewis acids towards neutral donors. However, the driving forces
for the dimerization of compounds 2, as well as for other
aminosilanes like 3 and 4 (figure 1c) have not been considered in
detail.[12],[13] The present work will serve several objectives in a
combined synthetic and computational approach (figure 1d): I)
The synthesis of a variety of silicon(IV) compounds with
3
3
2
3
3
2
6 6
C H
4
N
2
conditions for the cleavage of Si-C bonds are remarkable and
have already been observed during the reaction of diols with
carbon substituted siloxanes.[17] Obviously, the relatively weak
3
base NEt leads to insufficient deprotonation of 10 (or of silicon
bound intermediates) and conditions acidic enough for a fast and
irreversible intramolecular protonolysis of the substituent (H or
Ph) at silicon. In contrast, such acidic protons in the adjacencies
3
-
geometrically constrained triamido (NNN , 5/6) and amido-
diphenolato (ONO3-
7) substituents is achieved and permits the
,
unprecedented control over the dimerization process leading to
either monomeric, structurally reversible or dimeric species.
Acute distortions from tetrahedral towards planar geometries in
the monomeric compounds disclose the amplification of silicon
Lewis acidity by preorganization. II) The solvation-corrected
Gibbs free energies of the dimerization process are calculated by
state-of-the art computational theory. Strong dispersive forces
which act as driving force in the dimerization process are found.
A detailed analysis employing conceptual DFT, NBO, ETS-NOCV
and QTAIM rationalizes the effect of planarization and inspects
the nature of Si-N bonding in this new class of compounds.
Unique stabilizing factors in the dimers are identified and
compared with aminosilanes 3 and 4, allowing for more general
conclusions on Lewis acidity at silicon and the aggregation of
amphiphilic species.
of silicon are not present with the diketo-amine ligand
t
(HN[CH
2
(O) Bu]
2
) in the synthesis of 2, where deprotonation
occurs at the enolic positions. Indeed, upon application of the
much stronger base benzyl potassium in toluene and the
subsequent addition of PhSiCl
3 3
or TipSiCl (Tip = 2,4,6-
triisopropyl) respectively, the desired compounds 7a-b were
obtained in overall good yields (scheme 1). The obtained
colorless compounds vary from highly to slightly air sensitive with
fast and intense coloration upon exposure to air, indicating redox-
reactions due to the strong electron donating ability of the redox-
active substituents. The moisture sensitivity of NNN compounds
2
5/6 (readily reacting with traces of H O) is much more pronounced
compared to the ONO species 7. Comprehensive aspects of the
highly promising (redox)-reactivity of 5-7 will be given in future
contributions from our laboratory.
Results and Discussion
Synthesis
The synthesis of triamido-silicon (MeNNN-SiR’, 5a-d, iPrNNN-SiR’,
6
a-b, scheme 1) or amido-diphenolato-silicon (ONO-SiR’, 7a-b)
compounds based on bis(2-methylaminophenyl)amine (H
MeNNN,
),[6c] bis(2-isopropylaminophenyl)amine (H iPrNNN, 9)[6c] and
N,N-bis(3,5-di-tert-butyl-2-phenoxy)amine (H
ONO, 10)[14] are
3
8
3
3
described first. Spectroscopic and structural discussion of the
products will be given subsequently in a comparative perspective.
Hydro- and chlorosilanes 5a,b/6a,b (scheme 1) were obtained by
reaction of the corresponding arylamines 8/9 and HSiCl
in the presence of stoichiometric amounts of NEt in toluene or
CH CN, showing full conversion at NMR scale after one hour at
rt. Preparative scale reactions yielded the desired products after
removal of the salts by extraction with toluene or Et O and
recrystallization from toluene or Et O. Derivatives with R’ = Ph,
nHexyl (5c,d) were readily accessible by a convenient B(C
catalyzed (2 mol%) dehydrogenative coupling of a 1:1 mixture of
and R’SiH
in toluene (scheme 1).[15] Reaction profiling indicated
3 4
or SiCl
3
3
2
2
6 5 3
F )
8
3
the internal Si-N bond coupling as the final step toward the
products and full conversion without any byproduct except
Scheme 1. Synthesis of NNN-SiR’ compounds (5/6) by nucleophilic reaction or
dehydrogenative coupling, the undesired reaction to the octahedral silicon
complex 11 and synthesis of ONO-SiR’ (7) compounds after deprotonation with
BnK.
dihydrogen (see figure SI1). The catalytic amount of B(C
6
F
5
)
3
was
O or
removed by crystallization/precipitation of 5c/5d from cold Et
2
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Spectroscopy
Table 1. Measured and calculated 29Si-NMR chemical shifts for 5-7.
The structures in solution were studied by multi-nuclear /
δ(29Si) exp.[a]
δ(29Si) calc.[b]
δ(29Si) calc. [b]
Compound
R‘=)
a(H)
b(Cl)
c(Ph)
d(Hex)
a(H)
2
9
1
-
dimensional solution NMR spectroscopy.
Si/ H NMR
(
(CD
2
Cl
2
)
monomer
-7.8
dimer
-61.1
-80
HSQC/HMBC spectroscopy serves as a useful tool for the
determination of the Si-coordination, as the chemical shift strongly
depends on the coordination state.[18] The experimentally
5
-61
-80.4
-3.4
9.8
5
-15
5
-6
-37.5
-62.4
-57.6
2
9
determined Si-NMR chemical shifts of compounds 5-7 are
presented in comparison with the DFT calculated values of the
monomeric and dimeric species in table 1 (spin-orbit relativistic
and solvent corrected DFT, ZORA-SO-PBE0(COSMO,
5
7.6
6
-14.9 (25°C)/
-10.5
-59.0 (-90°C)
)/TZ2P, for further details, see SI).[19] For the MeNNN
6b(Cl)
7a(Ph)
7b(Tip)
-14.8
-66.5
0
-17
-7.8
-4.1
-80.9
-69.7
N. N.
CH
derived
(
2 2
Cl
29Si-NMR
compounds
5a,b,
the
shifts
5
-61.0 ppm, -80.4 ppm, typical for λ -coordinate silicon) indicated
dimerization in solution. A head-to-tail type arrangement of 5a,b
in solution was supported by H NOESY spectroscopy, showing
[a]experimental (CD
[b]
2
Cl
2
at RT)
DFT calculation (ZORA-SO-
1
PBE0(COSMO, CH Cl )/TZ2P) relative to δ(29Si) calc. TMS = 0 ppm
2 2
cross-peaks between the NMe-groups and aromatic protons H6
This observation is surprising, as one would expect the fourfold
tBu-substituted ONO-substituent to be more hindered towards
dimerization by Pauli repulsion. Also, the aggregation of the ONO
class of compounds was successfully controlled by installation of
the bulky 2,4,6-triisopropylphenyl-group (Tip) at silicon (7b), with
the 29Si-chemical shift of 0.0 ppm now lying well in the range of
tetracoordinate silicon.
By consequence, the aggregation state of electron-rich
aminosilanes in solution can be either controlled by the choice of
substituents directly bound to silicon, or by modification of the
NNN groups. This flexibility allows for the generation of ‘self-
protecting’ (reversible dimerizing) systems and the future
investigation of the influence of aggregation on the reactivity
towards various substrates and the redox chemistry.
(
ortho to central N) as well as between Si-H and H6 (see figures
SI2/3). Such a spatial proximity is impossible in a monomeric state
and confirms a symmetric aggregation structure as proposed in
scheme 1. In marked contrast, compounds 5c,d exist as
monomers in solution, indicated by the less negative 29Si NMR
shifts of -3.4 ppm and 9.8 ppm, respectively. Mild liquid injection
field desorption ionization spectrometry (LIFDI) was performed to
gain insight into the gas phase aggregation state.[20] The dimeric
nature of 5a persists in the gas phase, indicated by a dimer
molecular mass peak (m/z = 506.2). Interestingly though, a peak
at m/z = 253.1 also revealed the presence of the monomeric
species, hence indicating a certain reversibility of the aggregation
process for 5a. Species 5b-d were found monomeric also in the
gas phase. With the iPrNNN derived compound 6a, reversible
dimerization was observed by H and 29Si-HMBC/HSQC VT-NMR
1
spectroscopy. The 29Si-HSQC NMR spectra at rt suggested the
Solid-state structures
presence of monomeric species (-14.9 ppm). However, upon
gradually cooling the solution to -40°C, the formation of a new
species was detected, with increasing proportion at lower
temperatures (up to 1:1 at -90 °C, see figures SI4-7). The 29Si-
Solid-state structures were obtained by X-ray diffraction for
compounds 5a-c, 6a-b, 7a-b (figure 2, for 5a and 6b see SI).
Selected bond lengths and angles can be found in table 2. In all
cases, the metric oxidation state of the aryl rings of the
substituents confirmed the fully reduced, trianionic nature of the
triamido or amido-diphenolato groups – in line with its colorless
HSQC NMR spectra at low temperature revealed a chemical shift
of -59.0 ppm for the new species, in good agreement to the
calculated shift of the dimeric form. The process was entirely
reversible, giving back the pristine monomeric compound at room
appearance.
The
observed
aggregation
states
temperature.[
21]
(monomeric/dimeric) were in full agreement with the structural
interpretations based on spectroscopy, as will be discussed in the
following, grouped by monomeric (5c, 7b) and dimeric structures
(5a, 5b, 6a, 7a).
The molecular structure of phenyl-substituted triamido silane 5c
is shown in figure 2a, revealing a tetravalent silicon. The Si-C1
bond (1.850 Å) and the central Si-N1 bond (1.745 Å) show no
Although there exist
a variety of neutral
intramolecular and/or ionic intermolecular temperature dependent
coordination-equilibria in silicon compounds,[22] there is only one
example of a neutral intermolecular coordination equilibrium.[23]
By consequence, this is the first demonstration of a reversible
dimerization of an amphiphilic silicon compound in solution.
Chlorosilane 6b prevailed monomeric in solution (-14.8 ppm),
thus standing in contrast to the related MeNNN chlorosilane
derivative 5b, which was observed only as a dimer at rt. Similarly,
ONO based compounds 7 were evaluated in solution. In this case,
exceptional lengths, whereas the two outer silicon nitrogen bonds
24]
(
Si-N2/N3) are shortened (1.722 Å).[
The most remarkable
features of the structure are the N1-Si-C1 and the N2-Si-N3
angles: they are significantly enlarged from the ideal tetrahedral
angle to 126.2°/129.8°, respectively. A structural survey in the
CCSD revealed those angles as record values for tetravalent
silicon(IV).[25] [26] No close contacts to neighboring molecules or
solvent molecules were observed in the packing structure,
the phenylsilane 7a was observed as dimer, based on negative
peak in the 29Si-HMBC NMR spectra (-66.5 ppm), but which is
monomeric in the case of the MeNNN-substituent (5c).
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Figure 2. Molecular structures of compounds a) 5c, b) 5b, c) 6a, d) 7b, e) 7a; probability level of displacement ellipsoids 50%, bond length and bond angles can be
found in table 2. For 5a and all further details of X-ray structure determination, see SI.
Table 2. X-ray structural determined, selected bond length (in Å) and bond angles (in °) for 5a-c, 6a and 7a-b.
NNN
5a (R' = H)
2.0385 (15)
1.7403 (16)
1.7423 (17)
1.446 (19)
1.8782 (15)
178.31(80)
132.94 (8)
82.32(7)
5b (R' = Cl)
2.0399(15)
1.7263(15)
1.7350(15)
2.1542(8)
1.8526(15)
175.22(4)
124.77(7)
79.80(7)
5c (R' = Ph)
1.7458(14)
1.7222(14)
1.7268(15)
1.8495(17)
---
6a (R' = H)
1.996(3)
ONO
7a (R' = Ph)
2.6061(33)
1.6355(16)
1.6365(17)
1.873(3)
7b (R' = Tip)
1.749(2)
1.6612(17)
1.6635(17)
1.854(2)
---
Si-N1
Si-N1
Si-N2
1.749(2)
Si-O2
Si-N3
1.748(2)
Si-O3
Si-R
1.437(24)
1.893(2)
Si-R
Si-N1'
N1-Si-R
N2-Si-N3
N1-Si-N1'
Si-N1'
N1-Si-R
O1-Si-O2
N1-Si-N1'
1.7459(18)
171.05(5)
122.09(9)
78.47(8)
126.17(7)
129.67(12)
---
173.70(90)
129.03(11)
81.05(11)
128.35(10)
121.07(8)
---
excluding solid state effects as the reason for the obtuse bond
angles. Rather, the distortion is caused by strong π-donation and
the constrained geometry of the substituent, as will be considered
more in detail in the theoretical section. The ONO-derived 7b
the bond distances towards silicon increase slightly, as is usually
observed when going from tetra- to pentacoordination. The most
notable changes between the monomeric and dimeric species
occurred in the “intramolecular” (axial) Si-N1 bonds (1.99-2.61 Å)
which are increased considerably and are in all cases longer than
the newly formed “intermolecular” (equatorial) Si-N1’ bond (1.75-
(
figure 2d) also crystallizes as a monomer and shows similarity to
c: Si-N1, Si-O2/O3 and Si-C1 bond lengths are unexceptional,
5
whereas again the N1-Si-C1 angle at the tetravalent silicon atom
is distorted to 128.4°. The O2-Si-O3 angle is not as much
enlarged as the N2-Si-N3 in 5c (121.1° vs. 129.8°), in agreement
with a lesser ability of the oxygen atoms to act as π-donors, hence
underpinning an electronic origin of the distortion from tetrahedral
geometry towards planarity. X-ray structural analysis of
compounds 5a, 5b, 6a, 7a revealed dimeric structures with
pentacoordinate silicon atoms. Two formally monomeric units are
dimerized in head-to-tail fashion and are horizontally displaced
2 2
1.89 Å). In general, the Si-N bond lengths within the Si N rings
are shorter (for Si-N1’) or longer (for Si-N1) than the sum of Si and
N covalent radii (1.82 Å),[28] but for both cases within the range of
experimentally measured values of intramolecular Si-N
donor/acceptor complexes.[
10a]
Regarding bond lengths, the
dimerization can be considered as double donor/acceptor N1’Si
interaction between hypothetical monomeric units. The equatorial
Si-N1’ bonds that have been formed during dimerization go at the
expense of the previous Si-N1 bond in the monomers. A detailed
description of the dimerization, the origin of the geometrical
distortion and the bonding situation in those compounds will be
given in the next section.
2 2
towards each other, forming rhomboid Si N -rings as the central
motif (N1-Si-N1’ angle: 78.4-82.3°). The coordination geometry
around silicon in all dimeric compounds can be described as
trigonal-bipyramidal with the N1 and R substituents occupying the
axial positions, slightly distorted towards tetragonal-pyramidal
geometry with the N1’ substituent at the apical position on a
N1/N2(O2)/N3(O3)/R plane. Compounds 5a (see SI), 6a (figure
Quantum theoretical analysis of the dimerization process of
amphiphilic silicon compounds
2
c) and 7a (figure 2d) have C
inversion center lying in the center of the planar Si
whereas 5b (figure 2b) posseses C symmetry with the rotation
i
molecular point symmetry, with the
From a very limited Lewis acidity and basicity perspective, the
dimerization in compounds like 2 and 5-7 is unexpected. Even
though several silicon donor-acceptor complexes with strong
donors (e.g. NHCs) are known, common tetravalent silicon(IV)
compounds are not considered as potent Lewis acids and are
rather disfavored for the formation of stable bimolecular
(intermolecular) adducts with weak neutral donors in solution.[
Especially aminosilanes should feature as both weak donors and
2
N
2
-ring,
2
axis in the middle of the slightly folded (22.1°) tetracycle. The Si-
N2/N3 and Si-O2/O3 bond lengths match that of
hypercoordinated silicon compounds like for example the
structurally related (aza)silatranes, as do the Si-R bond
lengths.[27] In comparison to the monomeric species 5c and 7b,
10]
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Chemistry - A European Journal
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weak acceptors since nitrogen atoms attached to silicon have
decreased basicity (charge delocalization towards silicon).[29] This
delocalization should increase the electron density at silicon and
thus decrease its Lewis acidity.[30] Thermodynamic data on the
-1
Table 3. Calculated energies (in kcal mol ) for the dimerization process of
3-7.
ΔG b
ΔE(PW6B95-
D3(BJ,abc)/QZVPP)
a
exp.
dimer
ΔΔE(B3LYP
(D3)-B3LYP)
a
d
3
4
dissociation of SiF
4
-nitrogen adducts show, that it is the crystal
F
3
SiCH
2
NMe
2
-12.1
-4.1
8.9
12.1
-10.9
-2.3
no
no
-16.8
-3.0
lattice energy of the adducts and not the formation enthalpy in the
gas phase, which is crucial for the stability of the complexes in the
solid state.[31] Theoretical studies on the dissociation energies of
neutral, pentacoordinate silicon compounds with dative NSi
bonds found indeed, that the dissociation enthalpy usually ranges
Me NSiH
2
2
Cl
5a
MeNNNSiH
MeNNNSiCl
MeNNNSiPh
MeNNNSiEt
iPrNNNSiH
iPrNNNSiCl
-35.0
-27.5
-4.7
yes
yes
no
-31.2
-37.6
-43.0
-39.1
-38.3
-36.7
-52.5
5b
5c
5d
6a
6b
7a
21.0
20.2
-6.8
-7.2
-33.6 (-36.0c)
no
-1 [12c, 32]
below 10 kcal mol ,
and that it is not the Lewis acidity of the
yes
no
silicon but rather the lattice energy that drives the formation of the
dative bonds.[33] The precipitation of stable silicon-nitrogen
-13.7
-40.9
16.5
-9.2
ONOSiPh
yes
adducts from solution has been demonstrated, but it is unclear, to
which extend the lattice energy is responsible.[34] The only
[
a]
pure electronic energies, [b] Gibbs free energy including thermal and
solvent correction (COSMO-RS), [ by SCS-MP2/QZVPP, difference of
electronic energies obtained at B3LYP/TZVPP level with and without
dispersion correction (D3(BJ)).
c]
[d]
example of
a
reversible solution state complexation with
.[23]
moderate neutral nitrogen donors was reported for PhCCSiF
3
Amphiphilic silicon species with amine donor groups may in
principle dimerize in a head-to-tail fashion by the formation of two
intermolecular donor/acceptor interactions. However, various
aminosilanes have been found to be monomeric in the gas phase,
For example, the experimentally measured, large C1-Si-N1-bond
angle of 5c (126.1°) was reproduced very well (126.4°), thus
confirming also the absence of crystal packing effects that lead to
such an obtuse bond angle. The solvent-corrected, association
Gibbs free energies for the monomers were obtained at the
in solution as well as in the solid state.[35] In contrast, H
4, scheme 1c) crystallizes in head-to-tail dimeric
arrangement, but the dimerization was not observed in solution
2
ClSi-NMe
2
(
a
[
36]
accurate
RS(CH Cl
meta-hybrid
)/def2-QZVPP level (ΔG
a
PW6B95-D3(BJ,abc),COSMO-
, table 3).[42] The values agree
< 0 for
[
37]
2
2
and the gas phase. Even the amphiphilic silicon compound 3,
with formally stronger Lewis acidic (-SiF ) and basic (-NMe ) sides
as compared to 2, 5-7, forms adducts only in the solid state.
perfectly with the experimental observations (ΔG
a
3
2
[
12d]
dimerization observed in solution). Comparison with the pure
electronic energies shows, that the inclusion of entropy and
solvation disfavors the dimerization. In fact, the pure electronic
energies are all attractive and spread over a range of up to
2 3
Even the potentially more Lewis acidic compound TfOSi(NMe )
does not dimerize in solution.[38]
Fleischer[39] and others[40] argued, that the reason for the
5
0 kcal mol-1 difference in association energy. Certainly, this large
moderate Lewis acidity of silicon compounds is the high
deformation energy from the tetrahedral to trigonal-bipyramidal
form. Early theoretical studies on the inversion barrier of
silicon(IV) demonstrated, that π-donor/σ-acceptor ligands lower
the energy of the planar transition state.[26n, 41] Consequently, one
variance and the relative ordering cannot be explained by general
Lewis-acidity trends (e.g. more electronegative substituents at
silicon lead to stronger binding). To estimate the influence of
London dispersion, energies were obtained with and without
dispersion correction at the B3LYP/def2-TZVPP level, since the
B3LYP functional does not account for medium-range electron
correlation effects responsible for dispersion.[ The differential
values ΔΔE(B3LYP(D3)-B3LYP) show that dispersion forces play
a massive role in the associations in favor of the dimers 5-7,
amounting for up to 52 kcal mol-1 for ONOSiPh 7a. Dispersion has
a significant effect even for 3, but is only marginal for 4.
Calculations of a hypothetical ONO derivative without tBu-groups
could conjecture, that increased association energies can
indirectly be achieved by deliberate lowering of the deformation
energy of the silicon species through π-donor/σ-acceptor ligands
and geometrical preorganization / planarization. The strategy of
planarization has never been stressed nor used in the context of
silicon Lewis acids. By consequence, the careful inspection of the
origin in the surprising aggregation process of amphiphilic silicon
species 5-7 in solution is highly beneficial.
It appeared, that the bonding situations and driving forces in 5-7
were difficult to describe with a single quantum theoretical tool.
However, to keep the matter simple and readable, the stabilizing
factors will be emphasized only by the most relevant methods in
the following, even if alternative tools confirmed the hypotheses
redundantly (summarized in figure 5). For the full computational
details, see SI.
43]
revealed
a
contribution of 15 kcal mol-1 by the dispersive
attraction between these polarizable groups (see SI). The
attractive, non-covalent forces between the substituents were
further justified by an NCI-plot (see figure SI8). That the
association of densely substituted Lewis acid/Lewis base pairs is
strongly influenced by dispersion interaction has been recognized
for several classes of compounds.[
42b,
44]
However, the herein
observed magnitude of dispersive stabilization is remarkable for
a dative association, and was thus identified as the first influence
on the surprising stability of dimers 5-7 (see figure 5).
Exact energies and the influence of dispersion
The first requirement for a meaningful theoretical analysis was the
assessment of the exact energies of the dimerization process of
compounds 3-7. Comparison with available solid state structural
data distinguished the B3LYP-D3(BJ)/def2-TZVPP level of theory
ideal for geometry optimizations.
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Chemistry - A European Journal
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Influence of preorganization and electrostatic attraction
(57 %) to 6a (50 %). Accordingly, the importance of orbital
interactions increases in the opposite order.
A closer look at the intrinsic Lewis acidity of the monomers was
considered by comparison of 3, 4 and 6a (for atom numbering,
Specific orbital interaction in the dimers
2 2 3
see figure 3a). Especially, compound 3 (Me NCH SiF ) seems
abnormal, as it shows a much weaker tendency towards
dimerization as one would expect based on the electronegativity
of the substituents in a SiF moiety – even after the neglect of
3
dispersion.[45] To assess the orbital situation in the monomers,
The increasing contribution of orbital interactions in order of 3/4 to
6a indicated the possibility of specific stabilizing orbital
interactions in the dimers of 6a, which were thus analyzed by the
ETS-NOCV scheme (extended transition state (ETS) method for
aminosilanes 4Mon, 3Mon and 6aMon were analyzed by conceptual
DFT and NBO theory. Visual inspection of the frontier orbitals
energy decomposition analysis combined with the natural orbitals
[
46]
[48]
for chemical valence (NOCV) theory).
The dimers were
for 6aMon revealed a significant deformation of the HOMO from the
nitrogen lone pair towards silicon and a localized LUMO with p -
z
fragmented into closed shell species similar to the EDA, yielding
NOCV deformation densities, which qualify and quantify the types
of the orbital interactions in the dimerization process. Only the
main conclusions obtained by the ETS-NOCV analysis shall be
sketched here in a schematic representation (figure 3a); for all
details of ETS-NOCV and fragment orbitals, see SI. For 3, only
the σ-expected type donor/acceptor interaction between N’ and Si
was found (blue arrows). For 4, the same σ-type donor/acceptor
interaction was found (NOCV 2), but a second, more critical
deformation density revealed charge depletion in the entire Si-N
bonding region, by the participation of the Si-N σ-bonding
electrons in the monomer during the dimerization (NOCV 1, green
arrows). In 6a, the orbital interaction is even more complex,
revealing three significant NOCVs, which are difficult to express
in terms of a simple orbital picture. Like in 4, the strongest
interaction is characterized by the depletion of charge density in
the Si-N1 bonding region. (NOCV 1, 129.1 kcal mol , green
arrows).
shape at silicon (see SI for all FMOs and Fukui dual descriptors).
This deformation is much less pronounced for 4Mon, and virtually
absent for 3Mon. The observed orbital perturbations in 6aMon bear
a strong resemblance to the features found in planar silicon
transition states.[41a] The geometrical distortion from the
tetrahedral angle (experimentally observed in the N1-Si-C1 angle
of 126° for 5c, see above) describes indeed the situation of silicon
deformation towards planarity and results from the ability of the
ONO and NNN ligand frameworks to act as a π-donor/σ-acceptor,
additionally supported by geometric strain of the ligand. Second
order perturbation energies obtained by NBO analysis in 6aMon
reveal that the threefold nitrogen lone- pair donation into the
-1
σ*(SiN)- and σ*(SiH)-NBOs amount to 40.5 kcal mol , thus
providing energy to compensate for the deformation at silicon.
The perturbation energies do not have any true physical meaning
but are helpful for the relative ranking of stabilizing orbital
-1
2
3
interactions. The Si-N /N σ-NBOs in 6aMon are located to 79 % at
nitrogen and show considerable π-back-donation of 4 % from the
nitrogen lone-pair into the silicon acceptor orbitals. These findings
indicate high ionicity of the Si-N bonds and rationalize the
2
3
experimentally observed shortening of the Si-N /N bonds in 5c.
The planarization of 6aMon has a tremendous effect on the Lewis
acidity: the species is preorganized and should have a reduced
deformation energy during adduct formation. This hypothesis was
confirmed by energy decomposition analysis (EDA) for 3, 4 and
6
a, with the intuitive fragmentation into two closed shell
monomeric species (for full details, see SI).[47] The preparation
energy for the deformation of the relaxed monomeric fragments
-
to the geometries in the dimers is much larger for 3 (46.8 kcal mol
1
-1
)
, compared to 6a (32.0 kcal mol ). The interaction energy
between the prepared fragments in 3 is substantial, but it is
compensated by the high preparation energy. By consequence,
the preorganization and the low deformation energy at silicon was
identified as the second contribution to the stability of dimers 5-7
(
see figure 5).
EDA also revealed, that the electrostatic attraction between the
fragments is the most dominant contribution in all studied dimers.
The favorable Coulomb attraction is obvious, if the high positive
atomic charges at silicon and negative ones at nitrogen are
considered (NBO or QTAIM, see SI). Thus, the electrostatic
attraction was disclosed as the third stabilizing factor for the
dimerization. However, the importance of electrostatic attraction
is decreasing in going from species
3
(58 %) over
4
Figure 3. a) Schematic representation of the electron charge deformations and
energies for 3, 4 and 6a as obtained by ETS-NOCV, b) proposed resonance
phenomenon in compounds like 6a.
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The second NOCV represents the expected donor/acceptor
interaction between N1 and Si’ (NOCV 2, 31.8 kcal mol , blue
The overlap diagram of the preorthogonalized NBOs (PNBOs) of
-1
1
1
σ-(Si-N ) with σ*-(Si-N ’) reveals, that the overlap integral is
1
1
arrows). A last, a rather unintuitive charge deformation channel
2 2
maximized by tilting of the Si N -ring towards a smaller N -Si-N ’-
-1
was identified (NOCV 3, 14.9 kcal mol , red arrows), in which the
acceptor orbital is entirely delocalized across the aromatic rings,
angle, as is indeed expressed experimentally for all species by
the rhomboid instead of a square tetracycle (figure 4a). This kind
1
1
1
but accepts electron density through the nitrogen atom N .
of interaction is occurring between all σ/σ*-(Si-N )/(Si-N ’) as well
1
1
1
Importantly, this latter interaction describes a N /N ’ electron
exchange path – an interaction that is also observed by QTAIM
analysis (see below). Simply speaking, the coordinating nitrogen
atoms in 4 and 6a deliver electron density to the “intermolecular”
silicon (Si’), that it forwards to the neighboring N’ and that can be
used again for coordination to Si, closing the cycle. Such a
situation could also be described as resonance phenomenon
between interchanging formally dative and covalent bonds inside
as the σ/σ*-(Si-H)/(Si-N ’) combinations (eightfold), amounting for
the large total energy gain. Such intermolecular perturbations can
be found to a certain degree also in the NBO analysis of 4, but
are not pronounced in 3.
2 2
With particular regard to the proposed Si N -bond delocalization,
the electron density topologies were investigated using Bader’s
theory of atoms in molecules (QTAIM).[50] The explanation and a
full set of bond descriptors for 3, 4 and 6a can be found in the SI.
Recent topological studies on hypercoordinate silicon species
emphasized the ionic bond character of silicon to electronegative
substituents.[51] Accordingly, closed shell character for all bonds
to silicon and highly positive atomic charges, with small covalent
contributions was found (table 4). The electron densities of the
the Si
2 2
N -ring (see figure 3b), that should stabilize any
(
semi)metal amine dimer by a corresponding resonance mixing of
the respective wavefunctions. Is this phenomenon expressed in
some other quantum theoretically derived orbital or topological
signature? Indeed, second order perturbation energies from NBO
-1
1
analysis of dimeric 6a reveal a huge energy gain (212 kcal mol )
newly formed Si-N ’ bonds in 3 and 4 are lower than any other Si-
[
49]
1
by hyperconjugation from the in-plane Si-N σ-bonds into the
E bond in the monomers, and all other descriptors are consistent
with its weak, dative nature and closed shell character of
intermediate type.[ The most interesting results can be drawn
from the comparison of the bonds to silicon in monomeric and
1
geminal σ*(Si-N ’) NBOs (figure 4a, for individual energetic
52]
contributions, see SI).
1
dimeric species of 6a. All Si-E bonds, except the Si-N bond in
6aMon, are little affected by the dimerization, generally revealing
increased ionicity upon pentacoordination. In the monomeric form
of 6a, the three Si-N bonds are almost identical. In contrast, after
dimerization, the Si-N1 and Si-N ’ bonds inside the Si
1
N
2
ring
2
become almost similar, but differ substantially from the four Si-
2
N2/3 bonds. The visual inspection of ∇ ρ in the plane of the Si
2
N
2
1
-
ring (figure 4b) in dimeric 6a supports the picture of similar Si-N
1
and Si-N ’ bonds, with non-negligible charge transfer from the
nitrogen lone-pair regions towards silicon, but the electron density
concentrations still located in the nitrogen atomic basin. The two
nitrogen lone-pairs unify to one charge concentrated donation
area with proliferations towards silicon (see SI for VSCCs). The
1
1
nature of the Si-N /Si-N ’ bonds is still predominantly closed shell,
yet a remarkable decrease in the G(rBCP)/ρ(rBCP) ratio (Lagrangian
kinetic energy per electron) is observed upon dimerization. Since
2 2
ρ(rBCP) in the Si N
ring bonds gets lower if compared to the Si-N1
bond in the monomer, the major proportion in the lowering of
G(rBCP)/ρ(rBCP) upon dimerization must arise from the reduction in
G(rBCP) itself. It is well known, that the kinetic energy is directly
related to the amount of delocalization. Thus, a reason could be
2 2
delocalization occurring inside the Si N ring electrons, as already
indicated by the ETS-NOCV and NBO analyses. The proposed
resonance between the four internal Si-N bonds is further
substantiated by the delocalization indices, based on the pair-
density (Fermi-hole) integration over the QTAIM derived atomic
basins. The delocalization index quantifies the magnitude of
electron sharing between different atoms and reveals
extraordinary exchange pathways.[53] The three center
delocalization indices (δ (A-B-C)) for the coordinating N towards
selected atoms over silicon are given in table 4 (for 6a also in
figure 4b, for atom naming in compounds 3, 4 see figure 3a).
Figure 4. a) Most relevant NBO interactions for the stabilization of dimeric
species 6a as revealed by second order perturbation analysis and the contour
1
1
1
diagram of the PNBOs for geminal hyperconjugation from σ(Si-N )  σ*(Si-N ‘),
2
2 2
b) Laplacian of the electron density in the Si N plane of 6a, positive ∇ ρ (charge
depletion) in blue and negative (charge concentration) in red, together with
values of delocalization indices (red numbers) all relating to nitrogen N1 (upper
right).
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Chemistry - A European Journal
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rationalized their particular properties. By variation of the
substituents at silicon or at the NNN substituents, the inevitable
dimerization, which was recently observed in related species,
could be controlled for the first time, yielding either monomeric,
structurally reversible or dimeric species. The monomeric species
revealed substantial distortion at silicon towards planarity – a
record value reported for Si(IV) thus far. Solution-corrected
association Gibbs free energies of the dimerization reaction
perfectly reproduced the experimental results. Attractive London
dispersion between the large substituents was disclosed as the
first crucial factor for the efficient dimerization. Planarized silicon
compounds like 5-7 feature a preformed coordination side with
kinetically favorable localized electrophilicity and low deformation
energy at silicon. The low deformation energy in 5-7 was identified
as the second factor for the efficient dimerization. EDA and atomic
charges revealed electrostatic attractions between the monomers
as the third dominant factor, however with increasing contribution
of orbital interactions upon planarization at silicon. Besides the
expected σ-donor/acceptor interaction, additional stabilization
through participation of the σ-bond electrons of the central Si-N1
bond was found. The indication of a σ-bond delocalization was
supported by significantly stabilizing geminal hyperconjugations
Table 4. QTAIM derived descriptors for selected bonds in monomeric and
dimeric species of 3, 4 and 6a.
∇2(rBCP
Compound
ρ(rBCP
)
)
G(rBCP)/ρ(rBCP
)
δ (A-B-C)
6a
Mon
SiNiPr
SiN
0.1333
0.1324
0.5358
0.4958
1.485
1.436
SiH
0.1254
0.1485
0.939
6a
6a
1
SiNiPr
SiN
SiH
0.1251
0.0782
0.1238
0.4610
0.1642
0.1507
1.403
1.000
0.947
0.004 (N -Si-H)
1
1
0.181 (N -Si-N ')
1
2
0.129 (N -Si-N )
1
2
SiN'
0.0975
0.2466
1.121
0.060 (N -Si'-N ')
4
Mon
SiN
SiCl
SiH
0.1405
0.0927
0.1272
0.6031
0.1831
0.1612
1.550
1.015
0.958
4
4
SiN
SiCl
SiH
SiN'
0.1200
0.0815
0.1292
0.0545
0.3592
0.1349
0.1629
0.0575
1.258
0.925
0.964
0.708
0.018 (N-Si'-Cl')
0.169 (N-Si'-N')
0.136 (N-Si-Cl)
0.108 (N-Si'-H')
0.059 (N-Si-H)
3
Mon
SiF (avg)
SiC
0.1372
0.1365
0.9216
0.1122
2.012
0.897
3
3
inside the Si
electron delocalization indices as well as the decrease in the
kinetic energy density of the electrons involved in the central Si N
2
2 2
N -ring and by biased QTAIM derived three center
SiFeq (avg)
SiFax
SiC
0.1280
0.8082
1.910
0.003 (N-Si'-F'ax
0.119 (N-Si'-F'eq
0.094 (N-Si'-C')
)
)
0.1255
0.1246
0.0587
0.7797
0.1029
0.0612
1.889
0.866
0.739
2
SiN'
ring. This resonance phenomenon is thus identified as last
stabilizing factor for the efficient dimerization for this specific
example, and might similarly play a decisive role for other
amphiphilic dimerizing species.
For a full set of descriptors, see SI. All values are given in atomic units. The
last column contains the three-center delocalization indices between A and
C in A-B-C.
Two observations are remarkable: 1) In all cases (3, 4 and 6a),
only marginal exchange is found between the nitrogen and the
substituent placed trans to this nitrogen – a result similarly
obtained by Molina et al., which accounts for the fact that the
bonding in such hypercoordinate SiX
5
species may not be
described by the classical 3c-4e bonding model, and consistent
with our findings of ETS-NOCV.[54] 2) There is a strong
delocalization between the equatorial and axial substituents.
Remarkably, the delocalization index for N1-Si-N1’ inside of the
2 2
Si N ring of 6a exceeds the value for the other two eq.-ax. (N1-
Si-N2/3) significantly (0.181 vs. 0.129). This observation
1
substantiates the interpretation that the Si-N bonds inside the
2 2
central Si N exhibit a stabilizing σ-type delocalization. To a
slightly weaker extent, the same signature is found in 4, but is
absent in 3, in which such a delocalization is impossible.[55] [56]
Thus, the effect of resonating dative and covalent bonds in
dimeric aminosilanes was identified as the fourth effect on the
remarkable stability of aminosilanes 5-7 (figure 5). Importantly,
this effect should be effective for other amphiphilic species like for
example aluminum amides.[57]
Figure 5. Summarized factors contributing to the efficient dimerization of
geometrically constrained, electron-rich silicon compounds 5-7.
In light of the usually rather moderate Lewis acidity of silicon
species, the herein described efficient dimerization is remarkable.
The present work gives the explanation and presents a combined
experimental and theoretical account of the strategy of
preorganization in silicon Lewis acids. The disclosed influences of
Conclusions
In the present work, we described the first synthesis of electron-
rich, geometrically constrained silicon(IV) compounds with the
triamido- (NNN) and amido-diphenolato- (ONO) substituents and
dispersion,
geometrical
strain,
deformation
energy
(
preorganization) and charge delocalization/resonance should
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Chemistry - A European Journal
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extend the perception of Lewis acidity at silicon and contribute to
the currently active field of neutral silicon Lewis acids.[58] Given
the redox-activity of the electron-rich substituents in combination
with the unquenched nature of the now accessible monomeric
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Geometrically constrained, electron-
rich aminosilanes are synthesized,
imparting control over a unique
dimerization process. Quantum
theoretical analyses of the process
permit general conclusions on the
Lewis acidity of silicon compounds
and on dimerizing amphiphilic
species.
Nina Kramer, Christoph Jöst, Alexandra
Mackenroth, Lutz Greb*
Page No. – Page No.
Synthesis of electron-rich, planarized
silicon(IV) species and the theoretical
analysis of dimerizing aminosilanes
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