A stable silicon(ii) monohydride

Article abstract of DOI:10.1039/c0dt01675f

A stable silicon(ii) monohydride is accomplished through a covalent shared interaction of the silylene lone-pair and a sp³-hybridized boron atom of the Lewis acidic BH₃. Experimental charge density investigations reveal a central pos

Full text of DOI:10.1039/c0dt01675f

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An international journal of inorganic chemistry
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Volume 40 | Number 20 | 28 May 2011 | Pages 5365–5632
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COVER ARTICLE
Roesky, Stalke et al.
A stable silicon(II) monohydride

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PAPER
A stable silicon(II) monohydride†‡
Anukul Jana, Dirk Leusser, Ina Objartel, Herbert W. Roesky* and Dietmar Stalke*
Received 1st December 2010, Accepted 9th March 2011
DOI: 10.1039/c0dt01675f
A stable silicon(II) monohydride is accomplished through a covalent shared interaction of the silylene
lone-pair and a sp3-hybridized boron atom of the Lewis acidic BH3. Experimental charge density
investigations reveal a central positively charged silicon atom bound to a negatively charged hydrogen
atom. The positively charged H–Si–BH3 moiety is coordinated by the lone-pairs of electrons of the
benzamidinate ligand. This coordination is reinforced by a transannular Si1 ◊ ◊ ◊ C1 privileged exchange
channel.
their importance in manufacturing amorphous silicon. The latter
is used for advanced discrete electronic devices such as power
transistors, and in the development of integrated circuits such as
computer chips.
Introduction
The chemistry of silicon(II) dihydride has been studied at high
temperature or in a matrix at very low temperatures, but it has been
so far elusive at room temperature. Here we report the synthesis
of a stable Lewis acid base stabilized silicon(II) monohydride,
LSiH(BH3) [where L indicates PhC(NtBu)2] in good yield: starting
from the corresponding silicon(II) chloride, LSiCl(BH3), by the
reaction with potassium K-selectride (K[B(s-Bu)3]H). The 29Si
NMR spectrum of this compound confirms the presence of a
H–Si–BH3 moiety. Charge density investigations from a high-
resolution low-temperature diffraction experiment reveals that
there seems to be only one consistent interpretation of the
electronic structure: LSiH(BH3) is the first silicon(II) monohydride,
containing a central Si atom. It is stabilized through a covalent
shared interaction to a sp3-hybridized boron atom. The positively
charged H–Si–BH3 moiety is coordinated by the lone-pairs of
the benzamidinate ligand. These non-shared interactions allow a
much more flexible coordination geometry at the silicon atom.
Group 14 hydrides are of practical interest as a result of their
widespread use in synthetic chemistry,1 and their employment as
precursors for high purity elements as well as alloys for electronic
devices.2 Several hydrides are known from silicon, the sister
element of carbon, derived from oxidation state +4. However, the
corresponding stable silicon compound of oxidation state +2 is elu-
sive to date. The parent member of the silylene family is silicon(II)
dihydride, SiH2. Silylene (SiH2) plays a central role in the field
of silicon chemistry and it is considered as a transient species. It
appears as the most common intermediate during decomposition
reaction of silanes, which have attracted much attention because of
SiH2 is generated for example by photolysis of phenylsilane
(PhSiH3) at 193 nm in the gas phase and is only stable in an
argon matrix at temperatures below -190 ◦C.3 SiH2 is unstable at
room temperature and polymerizes or disproportionates to give
insoluble products, which are of limited use. However, theoretical
studies of SiH2,4 a Lewis donor acceptor stabilized SiH2,5a as well
5b
as a H3B-coordinated SiH2 were carried out. Consequently, we
aimed to synthesize a stable silicon(II) monohydride species at
room temperature, which is important for the development of a
new field in silicon chemistry.
Results and discussion
Herein, we report the syntheses (Scheme 1) of Lewis acid base
stabilized monochlorosilylene, LSiCl(BH3) (2) and monohydrosi-
lylene, LSiH(BH3) (3) by employing the chelating benzamidinate
ligand L [where L indicates PhC(NtBu)2].
Scheme 1 Preparation of compounds 2 and 3.
In 2006, we reported the synthesis of a chlorosilylene, LSiCl
(1) with a stereoactive lone-pair present at the silicon atom.6 We
tried to prepare silicon(II) hydride using compound 1, like germa-
nium(II) hydride and tin(II) hydride from L¢GeCl and L¢SnCl (L¢ =
HC(CMeNC6H3-iPr2)2),7 with the reaction of K-selectride (K[B(s-
Bu)3]H). Unfortunately we did not obtain the expected product.
Recently So et al. mentionedthe intermediate [{PhC(NtBu)2}SiH],
which is formed in solution.8 Consequently we employed the
lone-pair of 1 to the Lewis acid BH3 to yield the Lewis acid
Institut fu¨r Anorganische Chemie, Universita¨t Go¨ttingen, Tammannstrasse
4, 37077, Go¨ttingen, Germany. E-mail: hroesky@gwdg.de;dstalke@chemie.
uni-goettingen.de; Fax: +49-551-393373
† Electronic supplementary information (ESI) available: Details of the
charge density determination. CCDC reference number 778885 for 3. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt01675f
‡ Dedicated to Professor Richard F. W. Baader on the occasion of his 80th
birthday.
5458 | Dalton Trans., 2011, 40, 5458–5463
This journal is
The Royal Society of Chemistry 2011
©

base stabilized chlorosilylene, LSiCl(BH3) (2). Compound 2 was
isolated as a white crystalline solid with good solubility in solvents
such as diethyl ether, toluene, and THF. Furthermore, 2 is stable
in solution or in the solid state at room temperature in an inert
atmosphere. It has been characterized by elemental analysis and
spectroscopic methods. The 29Si NMR spectrum of 2 exhibits one
1
quartet (d = 46.33 ppm), with a coupling constant of J(29Si–
11B) = 58.93 Hz, due to the 11B nucleus (I = 3/2). In the literature
there are also reports on the synthesis of transition metal hydrides,
namely iron and nickel hydride by the reaction of metal halides
with potassium borohydride, K(BEt3)H.9 Therefore 2 was reacted
with the hydrogenating agent K[B(s-Bu)3]H in toluene at -30 ◦C to
afford the stable monohydrosilylene, LSiH(BH3) (3) in good yield.
A solution of 3 in benzene-D6 did not show any evidence
of oligomerization or decomposition after 48 h of heating
in an oil bath at 80 ◦C. The 1H NMR spectrum of 3 ex-
hibits a broad resonance at d = 6.12 ppm for the Si–H
proton. This NMR signal is shifted upfield when compared
to that of the transition metal hydrido hydrosilylene com-
plexes, such as Cp*(Me2PCH2CH2PMe2)Mo(H)Si(H)Ph (d =
9.45 ppm)10a or Cp*(CO)Ru(H)Si(H)C(SiMe3)3 (d = 9.14 ppm)10b
or Cp*(CO)2W(H)Si(H)C(SiMe3)3 (d = 10.39 ppm).10c This was
expected, because the electron density is higher at the silicon(II)
atom coordinated to transition metal hydrido complexes with
respect to the silicon(II) atom of monohydrosilylene, LSiH(BH3)
(3). In the latter the lone-pair of electrons at the silicon atom
coordinates to the Lewis acid BH3. The 29Si NMR spectrum of 3
exhibits a quartet in the 1H decoupled spectrum (d = 54.31 ppm,
and J(29Si–11B) = 56.00 Hz) and shows a doublet of quartets in a
1H coupled spectrum with a coupling constant of 235.12 Hz. The
IR spectrum exhibits a band at 2107 cm-1, which is assigned to the
Si–H stretching frequency.
Fig. 1 (a) Molecular structure of 3 (anisotropic displacement parameters
at the 50% level). Carbon bound hydrogen atoms are omitted for clarity.
Displayed hydrogen atom positions have been located and refined freely
and (b) Canonical Lewis diagrams of the silylene 3 emphasizing different
feasible bonding modes.
The central structural element of LSiH(BH3) (3) is a planar
SiN2C four-membered ring,11 with the silicon atom basically in
the plane of the chelating benzamidinate monoanionic ligand.
Above and below that plane a hydride atom and the BH3 Lewis
acidic moiety is bonded to the silicon atom. The Si–H bond
˚
˚
length of 1.47342(11) A, the Si–B distance of 1.9624(5) A, and
Direct inspection of the ED, r(r), is precluded by the huge
density contributions of the core densities. Since we are interested
in the fine details and changes of the ED in the vicinity of atoms
(e.g. lone-pairs, valence shell polarization) and between atoms
(bonding characteristics), analysis of the first (gradient field) and
second derivative of the ED, the Laplacian field, L(r), is the method
of choice (see Fig. 2). Negative values of L(r) refer to charge
concentrations, while areas of positive values in the Laplacian
field show charge depletions.
˚
the two equidistant Si–N distances of 1.8288(8) A on average
are in the expected range. The latter are longer than Si–N single
bonds in silicon(IV) amines and represent lone-pair-driven dative
N→Si bonds.12 3 comprises a non-crystallographic mirror plane
in the CN2Si- backbone and a two-fold axis along Si1 ◊ ◊ ◊ C1 for
the [PhC(NtBu)2]- anion. The two tertiary carbon atoms of the
tBu-substituents are only marginally out of plane. The negative
charge in the benzamidinate ligand gives rise to a shortening of
˚
the N–C1 bonds to 1.339(5) A on average, which is close to the
Analysis of the gradient leads to a characterization of bonds
by the inspection of the bond path (BP), a line of maximum
density between two nuclei, in terms of its length and bending,
and the topological criteria at the bond critical point (BCP)
(local extremum), like its position on the path relative to the two
bonded atoms, the density, r(rBCP), the second derivative of the
˚
expected value of a C N double bond (1.29 A). Simple geometric
considerations would suggest sp2-hybridization for the nitrogen
atoms and sp3-hybridization of the boron and the silicon atom.
However, electron counting leads to a conflicting interpretation of
the molecular structure (see Fig. 1(b)).
2
Therefore, the molecular structure of 3 was determined by high-
resolution single crystal X-ray structure analysis (100 K, d =
0.47 A, Fig. 1(a)). Detailed insight into the bonding resulted from
a multipole refinement13 and a subsequent topological analysis of
the electron density (ED, overall experimental noise below 0.2 eA
to a resolution of 0.55 A) based on Bader’s Quantum Theory of
Atoms in Molecules (QTAIM).14
density, — r(rBCP), and the ellipticity, e(rBCP), the ratio of the two
curvatures of the density perpendicular to the bond. General rules
facilitate the classification of bonds via the topological criteria
at the BCP.15 Strength and multiple bond character rises with
˚
-3
2
˚
r(rBCP), negative — r(rBCP) is typical for shared, positive values for
˚
closed shell interactions, non-zero ellipticity can be caused by p-
contributions or coupling of lone-pair density into the bond.16
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 5458–5463 | 5459
©

Fig. 2 L(r) in the SiN2- (a), Si1, B1, H100- (b), Si1, C1, H100-plane (c), negative values ranging from -1 to -70 eA-5 (charge concentrations) are plotted
˚
-5
-5
˚
˚
in blue and positive values ranging from +1 to +10 eA (charge depletions) in red contour lines; isosurface of -L(r) at the -5 eA level (d), plots (e) and
(f) show the L(r) along selected bond paths and the line connecting C1 and Si1 (black dotted, relative to the interatomic midpoint), with dBCP being the
distance from the respective BCP.
In the quantum chemical framework of QTAIM, discrete atomic
volumes, the atomic basins, are defined by the zero-flux surface,
given by —r(r)·n(r), where n(r) is the normal vector to the surface.
This allows the determination of physical-based atomic charges
by integration of the density over the atomic basins, providing an
extremely powerful tool to judge the bonding in a molecule.
In 3 we found the typical two-dimensional Laplacian distribu-
tion for a covalent carbon–carbon and a polar nitrogen–carbon
bond with the valence shell charge concentrations (VSCC) point-
ing towards the bonding partners (Fig. 2(a) and (c)). The analyses
along the bond paths, given in Fig. 2(e) and (f), support the features
for these shared covalent interactions. Shared density at both
sides of the respective BCPs over a wide range in the interatomic
regions is detected. The individual shape depends on the degree
of polarization. The minima of L(r) are more pronounced for the
more electronegative partner and the Laplacian is asymmetrically
distributed relative to the position of the BCP. In polar bonds
it is shifted away from the more electronegative bonding partner
to give an increased atomic volume and charge at the expense of
the more electropositive atom (also see the integrated charges in
Table 1).
For the more interesting bonds, foremost those at the silicon
atom, the findings are remarkable. The Si–B bond is far from
a silicon-centred lone-pair driven Si→B dative bond (Fig. 2(b)
and (f)) as most of the silylene canonical forms might suggest
(Fig. 1(b)). In contrast, charge is concentrated over more than
˚
0.7 A in the interatomic region (Fig. 2(f)), slightly polarized
towards the boron atom (Fig. 2(b) and (d)), but always negative at
Table 1 Topological parameters of the bond critical points and integrated atomic charges11
-3
2
-5
˚
˚
˚
˚
r(rBCP) [eA ]
— r(rBCP) [eA ]
eBCP
dBP [A]
d1BCP [A]
Q [e]
Si–B
Si–N1
Si–N2
Si–H
N1–C8
N1–C1
N2–C1
C1–C2
C2–C3
B–H103
B–H101/2
0.87(3)
0.83(4)
0.74(4)
0.80(6)
1.75(2)
2.52(4)
2.42(3)
1.92(3)
2.06(3)
1.00(2)
0.99(1)
1.96(6)
1.97(6)
-5.07(5)
+5.17(9)
+6.10(9)
+9.02(13)
-10.21(8)
-21.18(15)
-17.64(13)
-13.60(8)
-13.84(7)
+1.76(6)
+0.1(4)
0.10
0.19
0.31
0.50
0.01
0.09
0.13
0.09
0.23
0.54
0.46(1)
0.07(4)
0.05(2)
1.966
1.843
1.834
1.480
1.475
1.337
1.346
1.483
1.396
1.198
1.22(1)
1.08(1)
1.07(1)
0.983
0.825
0.817
0.776
0.870
0.770
0.757
0.755
0.723
0.497
0.50(1)
0.64(2)
0.66(1)
+1.68/+1.21
+1.68/-1.31
+1.68/-1.20
+1.68/-0.52
-1.31/+0.48
-1.31/+0.60
-1.20/+0.60
+0.60/-0.19
-0.19/+0.06
+1.21/-0.54
+1.21/-0.54
+0.03/ -0.02
-0.06/+0.11
C
Me–H
+21(2)
+22(2)
CPh–H
eBCP is the ellipticity (eBCP = l2/l1 - 1), dBP the total length of the BP, d1BCP the distance of the first named atom to the BCP, Q the charge of the two
involved atoms, derived by the difference of atomic number Z and r(r) integrated over the atomic basin.
5460 | Dalton Trans., 2011, 40, 5458–5463
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The Royal Society of Chemistry 2011
©

both sides of the BCP, which supports rating the Si–B bond as a
covalent interaction with shared density.
This spatial distribution fits a hybridization state half way between
sp2 and sp3 at the nitrogen atoms. L(r) is negative close to the
nitrogen atom due to the charge concentration caused by the
In severe contrast to this bonding mode we found the Si–H bond
to be extremely polarized (Fig. 2(b), (c) and (f)). Especially when
compared to the L(r) of the other hydrogen bonds, displaying e.g.
classical polarized but shared C–H bonds, the difference is striking.
In the B–H bonds L(r) is marginally shifted to the hydrogen-basin
and the steep ascent of the Laplacian shows strong depletion as
soon as the boron-basin is reached. In the Si–H bond we find
concentrations solely in the vicinity of the hydrogen atom H100,
which makes the distribution qualitatively more comparable to
the coordinating lone-pair at the nitrogen atoms of the anionic
benzamidinate ligand (Fig. 2(d)). These findings together with the
huge concentration around H100 and the distinct negative charge
require the classification of the silicon hydrogen bond as that of a
hydride.
˚
coordinating lone-pair (extremum of the VSCC about 0.4 A from
the core) but the concentration is not reaching out far. Already
˚
0.3 A ahead of the BCP, L(r) turns positive, hence to charge
depletion and stays positive for the whole bond path until reaching
the inner core of the silicon atom (Fig. 2(a) and (f)). This is the
typical shape found for dative or ionic bonds. There is no shared
density to give rise to an interatomic charge concentration and
there is not much difference in L(r) along the BP between the Si–
N bonds and the Si–H100 bond. The shape of the basin around
silicon in the direction of the ligand and H100 is that of an ion:
distinct and spherical charge depletion. The silicon atom seems
to have two faces in 3: covalent towards boron and closed shell
towards the benzamidinate ligand and the hydride H100.
The topological analysis of the benzamidinate ligand reveals
very interesting features, providing some reasoning why it is such
a versatile ligand in stabilizing complexes with silicon in low
oxidation state.17 The idea of the negative charge being delocalized
in the N2C1-backbone is supported by the high densities and
negative Laplacians at the BCPs. Even between C1 and the ipso
We found four VSCCs around the boron atom B1 which form
almost ideal tetrahedral angles between 108.7◦ and 110.7◦, in
contrast to the classical bond angles from straight interatomic
lines. Those range from 100.9◦ to 116.4◦ in a quite unsystematic
appearance. Similarly, looking at the angles between the VSCCs
at the carbon atom C1 as well provides a more consistent picture
than referring to the classically quoted bond angles: 126.2◦
-3
˚
carbon atom of the phenyl ring r(rBCP) is higher (1.921 eA )
than for the single bonds between the tertiary carbon atoms and
(CCN1–CCC1–CCN2), 114.8◦, and 118.9◦ (CCC2–CCC1–CCN1/N2
)
the methyl groups (av.: 1.798 eA ) and only 0.3 eA below the
vs. 106.24(3)◦ (N1–C1–N2), 128.16(3)◦ (N1–C1–C2), 125.59(3)◦
(N2–C1–C2), respectively.
-3
-3
˚
˚
-3
˚
mean value of the aromatic C–C bonds (av.: 2.231 eA ). This
is remarkable, because the phenyl ring is oriented perpendicular
to the p-system of the N2C1-unit, which precludes conjugation
with the ring. More striking, however, is the presence of a flat
Conclusive evidence for the electronic interpretation of 3 as
the first Si(II)-hydride was gained by integration of all symmetry
independent atomic basins. In Table 1 the atomic charges display
prominent positive values for silicon (+1.68 e), boron (+1.21 e),
and even C1 (+0.60 e), mainly counterbalanced by N1 (-1.31 e),
N2 (-1.21 e), and the hydrogen atoms bound to silicon (-0.53 e)
and boron (av.: -0.53 e). Even more meaningful are the group
charges, which are displayed in Scheme 2a.
-5
˚
(-5.132 eA ) but well resolved charge concentration (CC) at
C1 pointing towards the transannular silicon atom (Fig. 2(c)).
This critical point in the Laplacian distribution is of the (3,+1)
type and therefore different from the (3,+3) critical points (local
minima of the Laplacian, respectively local maxima in the negative
Laplacian distribution) found in the VSCCs of shared interactions.
It is important to state that this feature is not caused by the p-
density of the N2C1-backbone, which can be easily verified by
comparison with the distribution of the valence shells around the
aromatic carbon atoms of the phenyl ring. However, no BP with
the associated BCP could be found between C1 and Si1, even
though the curvature of L(r) along the interatomic line is akin to
that of a bond.18 Although there certainly is no classical Lewis
two-center-two-electron bond between Si1 ◊ ◊ ◊ C1 the interaction
induced charge concentration in the valence shell at C1 might
indicate a privileged exchange channel.19 Similar polarization
patterns are well understood in metal–ligand interactions, where
ligand induced charge concentrations at the metal atoms are
described and quantified.20
More prominent than this weak interaction is the ligand
coordination to the electropositive silicon atom via the nitrogen-
centered lone-pair densities. Interestingly, the extrema of the
nitrogen lone-pair VSCCs are shifted away from the direct line
between N and Si (Fig. 2(a)), evidence for electronic and steric
strain in the molecule.21 The shape of the three-dimensional
distribution (Fig. 2(d)) is not typical for a well defined sp2-lone-
pair. The isosurface representation of a single free or coordinating
lone-pair is generally formed like a convex plate.22 The distribution
in 3 is leaping out perpendicular to the SiN2-plane, reminiscent to
oxygen atoms, where two lone-pairs merge to one broad VSCC.23
Scheme 2 (a) Group charges in 3 gained from the integration of atomic
basins as sum over the integrated atomic charges and (b) canonical formula
that contributes most to explain the bonding.
From there the intramolecular charge transfer is obvious. Even
the BH3 group bears negative charge. The positive charge at the
boron atom itself is overcompensated by the negative hydrogen
atoms. As expected, we found a neutral phenyl ring and the whole
negative charge of the ligand to be concentrated in the C(NtBu)2-
backbone. Surprisingly, it is exclusively accumulated in the nitro-
gen basins while the C1-basin gains positive charge, because the
electronegative nitrogen atoms polarize the C–N bonding density.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 5458–5463 | 5461
©

In addition, the hydride atom H100 counterbalances in part the
positive charge at Si1, but the SiH-core remains positive.
(100) [M+ - BH3]. Anal. calcd for C15H26BN2Si (274.20): C, 65.68;
H, 9.92; N, 10.21. Found C, 64.92; H, 9.86; N, 10.12.
Crystallographic details for 3. The high-resolution data for
the multipole refinement were collected from an oil-coated shock-
cooled crystal24 on a BRUKER TXS diffractometer with D8
goniometer and INCOATEC Helios mirror optics (Mo-Ka radia-
Experimental section
General considerations
˚
tion, l = 0.71073 A) equipped with an open stream liquid nitrogen
All manipulations were performed in a dry and oxygen-free N2
atmosphere by using Schlenk-line and M-Braun MB 150-GI
glove-box techniques. Solvents were purified with the M-Braun
solvent drying system. The H, 11B, 13C, and 29Si NMR spectra
were recorded on a Bruker Avance DRX 500 MHz spectrometer
and referenced to the deuterated solvent in the case of the H
and 13C, BF3·OEt2 for 11B, and SiMe4 for 29Si used as reference.
EI-MS were measured on a Finnigan Mat 8230 instrument.
Elemental analyses were performed by the Analytisches Labor
des Instituts fu¨r Anorganische Chemie der Universita¨t Go¨ttingen.
Infrared spectral data were recorded on a Perkin–Elmer PE-1430
instrument. Melting points were measured in sealed glass tubes
with a Bu¨chi B 540 melting point instrument. Benzene-D6 was
dried by distillation after drying with potassium under reflux in the
presence of benzophenone. All commercially available compounds
(Aldrich, Across) were used as received unless stated otherwise.
The starting material LSiCl (1) was synthesized according to the
literature procedure.17
cooling device and an APEXII detector. The data for the multipole
refinement were collected in an omega-scan mode (Dw = 0.3◦) at
fixed j-angles with a detector distance of 5 cm (low and mid-
order data) and 4 cm (high-order data) at exposure times between
10 (low-order) and 180 s (high-order data). This procedure led to
a high-resolution data set (for details see ESI, Table S1†), which
was corrected for absorption, scaled and merged with SADABS-
2008/2.25
The structure was solved with SHELXS,26 and a conventional
Independent Atom Model (IAM) refinement using all data was
performed with SHELXL27 to check the data quality and to
determine the absolute structure. The refined IAM served as the
starting model for the subsequent multipole refinement.
The following strategy was applied: the positional and
anisotropic displacement parameters of the non-hydrogen atoms
1
1
˚
were refined with the high-order data (dmax = 0.60 A). These
parameters were kept fixed during the subsequent refinement
steps. The hydrogen atoms were identified by a difference Fourier
˚
Synthesis of LSiCl(BH3) (2). BH3·THF (2 mL, 1 M, 2 mmol)
was added to a toluene solution (35 mL) of 1 (0.59 g, 2 mmol)
at -30 C. The reaction mixture was allowed to warm slowly to
analysis using the low-order data (dmin = 1.00 A). Based on the
same subset of data, the hydrogen atom positions were refined
with a SADI-restraint for the boron-bound hydrogen atoms and
an isotropic riding model (default values of SHELXL) for the sp3-
(methyl, BH3) and sp2-bound hydrogen atoms was applied. Then
the hydrogen atoms were shifted along their bonding vectors to
distances of 1.085 A for those bound to sp -hybridized carbon
atoms, 1.076 A for those bound to sp -hybridized carbon atoms,
1.200 A for the BH3, and 1.480 A for H100, which is bonded to
the silicon atom.28,29
◦
room temperature and stirred further for 1 h at this temperature.
After that all the volatiles were removed in a vacuum. The residue
was dissolved in toluene (40 mL), and the solution was filtered
over celite. The resulting solution was concentrated (to about
20 mL), and was stored overnight in a freezer at -30 ◦C to afford
colorless crystals (0.49 g, 80% yield). Mp: 242 ◦C. 1H NMR (C6D6,
500 MHz): d 1.02 (s, 18H, C(CH3)3), 1.39 (q, J = 93.95 Hz, 3H,
BH3), 6.70–6.93 (m, 5H, C6H5) ppm. 11B NMR (C6D6, 160 MHz):
d -41.96 ppm. 13C NMR (C6D6, 125 MHz): d 30.75 (C(CH3)3),
55.24 (C(CH3)3), 127.49, 128.08, 128.25, 128.32, 129.63, 130.87
(C6H5), 174.93 (NCN) ppm. 29Si NMR (C6D6, 99 MHz): d 46.33
(q, J(29Si–11B) = 58.93 Hz) ppm. EI-MS (70 eV): m/z (%): 294 (100)
[M+ - BH3]. Anal. calcd for C15H26BClN2Si (308.16): C, 58.36; H,
8.49; N, 9.07. Found C, 58.15; H, 8.95; N, 8.65.
3
˚
2
˚
˚
˚
Multipole refinement. The multipole refinement using the
atom-centered multipole model of Hansen and Coppens30 was
carried out on F2 with the full-matrix-least-squares refinement
program XDLSM implemented in the XD200631 program pack-
age. The core and the spherical valence densities were composed of
relativistic Dirac–Fock wave functions reported by Su, Coppens
and Macchi (SCM bank file).32 Single-zeta orbitals with energy-
optimized Slater exponents were used for the deformation density
terms.33 The radial fit of these functions was optimized by
refinement of the expansion-contraction parameters k and k¢.
The expansions over the spherical harmonics were truncated at
the hexadecapolar level for all heteroatoms and all multipoles
(nl = 1 to 4) of each atom shared the same k¢-set. The deformation
densities of the hydrogen atoms were represented by bond directed
dipoles and quadrupoles. To derive adequate parameters for the
contraction of the hydrogen atoms, k and k¢ values suggested
by Volkov et al. were introduced and kept fixed during the
refinement.34 In the final cycles all parameters except k¢ were
refined together using all positive reflections (no I/s exclusion to
avoid bias) until convergence was reached. Several chemical and
non-crystallographic symmetry constraints were applied (details
are given in the ESI†). The electron density is not biased by and
is well separated from the thermal motion of the non-hydrogen
Synthesis of LSiH(BH3) (3). A solution of K[B(s-Bu)3]H in
THF (2 mL, 1 M, in THF) was slowly added drop by drop to
a sti◦rred solution of 2 (0.61 g, 2 mmol) in toluene (30 mL) at
-30 C. The reaction mixture was warmed to room temperature
and stirred for an additional 2 h. After removal of all the volatiles,
the residue was extracted with toluene (20 mL) to yield colorless
compound 3. Suitable crystals for single crystal X-ray structural
analysis were obtained from a saturated hot n-hexane solution
◦
1
(0.35 g, 65%). Mp: 182 C. H NMR (C6D6, 500 MHz): d 1.00
(s, 18H, C(CH3)3), 1.28 (q, J = 92.38 Hz, 3H, BH3), 6.12 (br, 1H,
SiH), 6.77–6.91 (m, 5H, C6H5) ppm. 11B NMR (C6D6, 160 MHz):
d -42.88 ppm. 13C NMR (C6D6, 125 MHz): d 30.84 (C(CH3)3),
54.12 (C(CH3)3), 127.91, 128.29, 128.32, 128.38, 130.51, 131.34
(C6H5), 170.85 (NCN) ppm. 29Si NMR (C6D6, 99 MHz): d 54.31
(J(29Si–1H) = 235.12 Hz and J(29Si–11B) = 56.00 Hz) ppm. IR
(Nujol, KBr): n = 2107 cm -1 (Si–H). EI-MS (70 eV): m/z (%): 260
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5462 | Dalton Trans., 2011, 40, 5458–5463
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The Royal Society of Chemistry 2011
©

atoms. This was justified by the rigid bond test (DMSDA test)
according to Hirshfeld.35
6 C.-W. So, H. W. Roesky, J. Magull and R. B. Oswald, Angew. Chem.,
2006, 118, 4052–4054, (Angew. Chem., Int. Ed., 2006, 45, 3948–3950).
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Stalke, J. Am. Chem. Soc., 2009, 131, 1288–1293; (b) A. Jana, H. W.
Roesky, C. Schulzke and A. Do¨ring, Angew. Chem., 2009, 121, 1126–
1129, (Angew. Chem., Int. Ed., 2009, 48, 1106–1109).
8 S.-H. Zhang, H.-X. Yeong, H.-W. Xi, K. H. Lim and C.-W. So, Chem.–
Eur. J., 2010, 16, 10250–10254.
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Crystallographic data for compound 3. C15H27BN2Si, M =
274.29 g mol-1, T = 100(2) K, orthorhombic, space group P212121,
3
˚
˚
a = 8.516(2), b = 11.588(3), c = 17.098(5) A, V = 1687.2(8) A ,
Z = 4, rcalc. = 1.080 Mg m-3, m = 0.129 mm-1, F(000) = 600, 86 541
reflections measured, 15 218 independent (R(int) = 0.0253), R1 (all
data) = 0.0320, R2 (all data) = 0.0399, Flack parameter: 0.01(4).
10 (a) B. V. Mork, T. D. Tilley, A. J. Schultz and J. Cowan, J. Am. Chem.
Soc., 2004, 126, 10428–10440; (b) M. Ochiai, H. Hashimoto and H.
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11 Details of the charge density determination are available in the ESI for
this article and from the CCDC (778885).
12 N. Kocher, J. Henn, B. Gostevskii, D. Kost, I. Kalikhman, B. Engels
and D. Stalke, J. Am. Chem. Soc., 2004, 126, 5563–5568.
13 A. Volkov, P. Macchi, L. J. Farrugia, C. Gatti, P. R. Mallinson, T.
Richter and T. Koritsanszky, XD2006, 2006.
Conclusion
Conclusively, there seems to be only one consistent interpretation
of the electronic structure. 3 is the first silicon(II) monohydride,
containing a Si1.68+ central atom (Scheme 2b). It is stabilized
through a covalent shared interaction to a sp3-hybridized boron
atom of the Lewis acid BH3. The positively charged H–Si–BH3
moiety is coordinated by the lone-pairs of the benzamidinate
ligand. The orientation of the VSCCs associated with those lone-
pair densities seem to be first of all caused by hybridization
requirements and not by a directed shared interaction with
the silicon atom. Non-shared interactions allow a much more
flexible coordination response of the ligand at the silicon atom
since the bonding is not predominantly orbital-controlled. We
conclude that the interaction between the silicon atom and
the ligand is mainly of a closed shell non-covalent N-lone-
pair character, reinforced by a transannular Si1 ◊ ◊ ◊ C1 privileged
exchange channel. The negative charge of the ligand is spread
over the C1N2-backbone by a merge of two extreme electronic
situations: either in a p-system formed by p-orbitals perpendicular
to the sp2-hybridized atoms C1, N1, and N2, or lone-pair density
coupling back into the C1N2-unit of the two negatively charged
sp3-hybridized nitrogen atoms. However, the density is distinctly
polarized, leading to a negative charge at the nitrogen atoms, which
counterbalances the positive silicon(II) hydride by an interaction
of the closed shell type.
14 R. F. W. Bader, Atoms in Molecules – A Quantum Theory, Oxford
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17 S. S. Sen, H. W. Roesky, D. Stern, J. Henn and D. Stalke, J. Am. Chem.
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Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft, in
particular to the DFG Priority Program 1178 Experimental charge
density as the key to understand chemical interactions and the
DNRF funded Center of Materials Crystallography, Aarhus.
30 N. K. Hansen and P. Coppens, Acta Crystallogr., Sect. A: Cryst. Phys.,
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Richter and T. Koritsanszky, XD2006, A Computer Program Package
for Multipole Refinement, Topological Analysis of Charge Densities
and Evaluation of Intermolecular Energies from Experimental or
Theoretical Structure Factors, 2006.
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©