Elegant approach to spacer arranged silagermylene and bis(germylene) compounds

Article abstract of DOI:10.1039/c1cc12205c

Herein we report on the reactions of the stable LSiCl (1) and LGeCl (2) [L = PhC(NtBu)₂] with L¹Ge, [L¹ = CH{(CCH ₂)(CMe)(2,6-iPr₂C₆H₃N)₂}] (3) to yield 1-sila-5-germylene (4) and a 1,5-bis(germylene) (5). The reactions proceed through the 1,4 nucleophilic addition of the M-Cl (M = Si or Ge) to 3 without any modification of the oxidation state although the change of the oxidation state is thermodynamically more favorable. Compounds 4 and 5 were investigated by single crystal X-ray structural analyses, multi-nuclear NMR spectroscopy, and micro-analysis. Treatment of L¹AlMe·thf (6) with 1 resulted in the formation of the 1-sila-5-aluminium complex (7). The complex contains a Si(ii) and an Al(iii) atom in the molecule. All reported reactions proceed without changing the oxidation states at the metal centers.

Full text of DOI:10.1039/c1cc12205c

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COMMUNICATION
Elegant approach to spacer arranged silagermylene and bis(germylene)
compoundsw
Sankaranarayana Pillai Sarish, Sakya S. Sen, Herbert W. Roesky,* Ina Objartel and
Dietmar Stalke*
Received 17th April 2011, Accepted 6th May 2011
DOI: 10.1039/c1cc12205c
Herein we report on the reactions of the stable LSiCl (1)
and LGeCl (2) [L = PhC(NtBu)2] with L1Ge, [L1
=
CH{(CQCH2)(CMe)(2,6-iPr2C6H3N)2}] (3) to yield 1-sila-
5-germylene (4) and a 1,5-bis(germylene) (5). The reactions proceed
through the 1,4 nucleophilic addition of the M–Cl (M = Si or Ge)
to 3 without any modification of the oxidation state although the
change of the oxidation state is thermodynamically more
favorable. Compounds 4 and 5 were investigated by single
crystal X-ray structural analyses, multi-nuclear NMR spectro-
scopy, and micro-analysis. Treatment of L1AlMeꢀthf (6) with 1
resulted in the formation of the 1-sila-5-aluminium complex (7).
The complex contains a Si(II) and an Al(III) atom in the
molecule. All reported reactions proceed without changing the
oxidation states at the metal centers.
Chart 1 Pictorial diagram of interconnected and spacer separated
bis(carbene) analogues.
molecules. There are a few examples of spacer separated
bis-silylene7 and bis-germylene8 known. The synthesis of a
compound where two different elements are present in low
oxidation states, and each possesses a lone pair of electrons, is
of prime interest. Such a goal is definitely challenging and
requires a suitable precursor.9 The prototypical approach to
synthesize type B is to vary the substituents on the nitrogen
atom or modify the backbone skeleton of the precursors.7a,8a
However, these methods are not appropriate if two different
centers have to be present in a single molecule both in
low oxidation states. So a different approach is required.
Recently, we were successful in isolating a monochloro-silylene
Silylenes and germylenes are the silicon and germanium
analogues of carbenes and like carbon in carbene, they exhibit
the +2 oxidation state. The first stable N-heterocyclic silylene
was reported by West et al. in 1994,1 whereas the first
N-heterocyclic germylene was isolated by Veith and Grosser
(PhC(NtBu)2SiCl)
(1)10
and
monochloro-germylene
in 1982.2 Since then
a number of room temperature
(PhC(NtBu)2GeCl) (2)6 in high yield. Besides, Driess et al. and
we independently isolated a new type of N-heterocyclic-germylene
L1Ge (L1 = CH{(CQCH2)(CMe)(2,6-iPr2C6H3N)2}) (3).11,12
DFT calculation and reactivity patterns divulged that the
zwitterionic betaine resonance structure of 3 determines its
intriguing reactivities. For instance the reaction of 3 with
stable silylenes3a–f and germylenes3g–m were synthesized and
structurally characterized. The advent of these compounds
opened a fertile research field in main group chemistry with
increasing focus towards compounds with two such low valent
centers in a single molecule as adjacent elements (Chart 1, type A)
or separated by a spacer (Chart 1, type B).
trimethylsilyl triflate (Me3SiOTf, Tf
= SO2CF3) gives
In type A each E possesses a formal +1 oxidation state
whereas in type B each E adopts +2 oxidation state. The
syntheses, structures, and reactivities of type A have shown a
versatile nature.4 Previous studies from our laboratory also
exclusively a 1,4-addition product with the Me3Si group at
the nucleophilic carbon atom of the terminal methylene
moiety and OTf at the electrophilic Ge(II) site.11 Now, if we
look into the electronic structures of 1 and 2, they also possess
electrophilic Si and Ge atoms, respectively, because DFT
calculations showed that the lone pair of silylene and
germylene becomes concentrated in an orbital that exhibits
‘‘s’’ character. Inspired by these findings we treated 3 with
1 and 2, respectively, and synthesized a unique silagermylene
(4) and a bis(germylene) (5). To the best of our knowledge this
is the first example of a spacer separated sila-germylene and a
rare example of a spacer separated bis(germylene).
5
led to the isolation of bis(silylene) (PhC(NtBu)2Si)2 and
6
bis(germylene) (PhC(NtBu)2Ge)2 which belong to type A
molecules. Considering the success that type A showed, it
seemed appropriate to attempt the synthesis of type B
Institut fur Anorganische Chemie, Universitat Gottingen,
¨
¨
¨
¨
Tammannstrasse 4, 37077 Gottingen, Germany.
E-mail: hroesky@gwdg.de, dstalke@chemie.uni-goettingen.de;
Fax: +49 551-39-3373
The reactions of L1Ge (3) with LSiCl (1) and LGeCl (2, L =
PhC(NtBu)2) at room temperature in toluene afford the spacer
separated silagermylene complex 4 and bis(germylene) complex
w Electronic supplementary information (ESI) available: Full synthetic,
spectroscopic details for 4, 5, and 7 and X-ray crystallographic details
for 4 and 5. CCDC 819683 and 819684. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1cc12205c
c
7206 Chem. Commun., 2011, 47, 7206–7208
This journal is The Royal Society of Chemistry 2011

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also distorted tetrahedrally coordinated. The Ge atom in 4 is
threefold coordinated and exhibits distorted tetrahedral
geometry with the lone pair of electrons (Fig. 1). Two sites
are occupied by the chelating nitrogen atoms from the
b-diketiminato ligand with Ge–N bond lengths of 1.9467(14) A
and 1.9832 (14) A, respectively. The Ge–N bond distances are in
good accordance with those reported in the literature14 but
slightly longer than those in the precursor 3 (1.8658(17) A and
1.8650(18) A).11 Another site is occupied by the chlorine atom
and the lone pair of electrons fill the remaining coordination site
of the tetrahedron. The Ge–Cl bond length in 4 (2.3337(5) A)
Scheme 1 Syntheses of complexes 4 and 5.
5 (Scheme 1). 4 is obtained as a yellow crystalline solid in 88%
yield, whereas 5 is isolated in 90% yield. Both 4 and 5 show
good solubility in common organic solvents such as benzene,
toluene, diethyl ether, and THF. The mechanism of the
reactions seems to be obvious. In fact, due to the zwitterionic
nature of 3, the electrophilic addition of LSi+ or LGe+ takes
place at the exocyclic methylene group in 3, whereas the
nucleophile Clꢁ migrates to the b-diketiminato Ge(II) atom.
Single crystals of 4 for X-ray structural analysis are obtained
by storing a solution (n-pentane/toluene) at ꢁ32 1C in a
freezer. 4 crystallizes in the monoclinic space group P21/n.
The molecular structure of 4 is shown in Fig. 1 and the
selected bond lengths and angles are listed in the legend of
Fig. 1. The amidinate ligand is bonded in a N,N0-chelating
fashion to the Si atom and displays a distorted tetrahedral
geometry (with two nitrogen atoms of the ligand L, the
methylene carbon atom of the ligand L1, and the lone pair
of electrons at the Si atom). The four-membered
Si1–N1–C1–N2 ring is nearly planar, and the phenyl group
is orthogonally arranged to this plane. The Si1–C16 bond
(1.9798(19) A) is longer than standard Si–C single bonds
reported in the literature.13 It also indicates that the lone pair
at Si is not involved in multiple bonding and suggests that
more p-character is involved in the Si hybrid orbitals which are
used for the formation of the Si–C bond. The bridging C16 is
matches well with that of
2
(2.2572(13) A)6 and
[HC(CMeNAr)2]GeCl (Ar = 2,6-iPr2C6H3) (2.295(12) A)15a
but is significantly longer than that in the recently reported
[{TMSNC(Ph)NTMS}GeCl] (2.1556(8) A).15b
Single crystals of 5 suitable for X-ray structural analysis are
obtained from a n-pentane/toluene solution stored at ꢁ5 1C in
a freezer for one day. 5 crystallizes in the monoclinic space
group P21/n. Like 4 the coordination environments of each
amidinato Ge atom and b-diketiminato Ge atom in 5 is
threefold coordinated and resides in distorted tetrahedral
environments with one vertex occupied by a lone pair of
electrons (see ESIw for molecular structure of 5). The
amidinato Ge2–C29 bond length (2.085(2) A) is 0.08 A longer
than that of [HC(CMeNAr)2]GeMe (Ar = 2,6-iPr2C6H3).16
The bond lengths and angles of the GeN2C and GeN2C3
skeletons in 5 are similar to those of 4. One interesting
feature is the Ge1–Cl1 bond length (2.3340(7) A) in 5,
which is slightly longer than that in [HC(CMeNAr)2]GeCl
(Ar = 2,6-iPr2C6H3)15 but matches well with that of 4.
Additionally, 4 and 5 are characterized by multinuclear
NMR spectroscopy, EI-MS spectrometry, and elemental
analysis which are in agreement with the constitution of 4
and 5 as derived from X-ray structural analysis. In the 1H
NMR spectra of 4 and 5 the tBu protons appear as two sets of
singlets (for 4: d 1.03 and 0.70 ppm and for 5: d 0.94 and
0.67 ppm). The two non-equivalent CH protons of the methylene
bridge of 4 appear at d 2.79 and 2.39 ppm and the g-CH
and methyl protons of the b-diketiminato Ge part resonate
as singlets (d 5.64 and d 1.84 ppm). The g-CH proton of 5
also resonates as a singlet at d 5.64 ppm like that of 4.
Furthermore, in 5 the two non-equivalent CH2 protons of
the methylene bridge appear at d 2.88 and d 2.71 ppm, which
are shifted downfield compared to those of 4. In the 29Si NMR
spectrum of 4 a resonance exhibits at d 42.45 ppm which
corresponds to the three-coordinate silicon atom7c,17 of 4
and is shifted considerably downfield compared to that of
1 (d 14.57 ppm).10 The molecular ion peak corresponding to
4 and 5 was observed with a relatively low intensity at
m/z 784.5 and 830.3, respectively, in the EI-mass spectra.
To test the generality of this method we prepared the 1-sila
5-aluminium complex (7) by treatment of 1 with L1AlMeꢀthf
(6)18 in toluene, which afforded a compound containing silicon
and aluminium with an oxidation state of +2 and +3
Fig. 1 Molecular structure of 4. The anisotropic displacement para-
meters are depicted at the 50% probability level. All hydrogen atoms
are omitted for clarity. Selected bond lengths (A) and bond angles (1):
Si1–N1 1.8609(15), Si1–N2 1.8800(17), Si1–C16 1.9798(19), Ge1–N3
1.9832(14), Ge1–N4 1.9467(14), Ge1–Cl1 2.3337(5); N1–Si1–N2
69.31(7), N1–Si1–C16 100.39(8), N2–Si1–C16 95.83(8), N3–Ge1–N4
90.27(6), N4–Ge1–Cl1 96.25(4), N3–Ge1–Cl1 93.03(4). Selected bond
lengths (A) and bond angles (1) for 5: Ge1–N1 1.948(2), Ge1–N2
1.9811(19), Ge1–Cl1 2.3340(7), Ge2–N3 1.988(2), Ge2–N4 2.012(2),
Ge2–C29 2.085(2); N1–Ge1–N2 90.42(8), N1–Ge1–Cl1 96.20(6),
N2–Ge1–Cl1 93.29(6), N3–Ge2–N4 65.36(8), N4–Ge2–C29 93.52(9),
N3–Ge2–C29 97.97(9).
1
(Scheme 2). The H NMR spectrum of 7 exhibits an upfield
shifted singlet (d 5.71 ppm) compared to 6 (d 5.38) which can
be assigned to the g-CH proton. The two CH2 protons of the
methylene bridge display a resonance at d 2.46 ppm. The
methyl proton resonances of Al-Me and the ligand backbone
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 7206–7208 7207

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compared with those of 6 (d ꢁ0.99 and d 1.58 ppm). In the
13C NMR spectrum of 7 Al-Me and g-C atom of the
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(d 41.16 ppm) which corresponds to the three-coordinate
silicon atom and also matches excellently with that of 4. The
27Al NMR spectrum of 7 is silent due to the quadrupole
moment of the aluminium atom. Despite several attempts we
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In summary, complex 4 containing Si(II) and Ge(II) has been
prepared by the reaction of 1 with 3. Furthermore, a bis(ger-
mylene) 5 is also obtained by following the same synthetic
protocol. The synthetic strategy was also extended to prepare
complex 7 containing a Si(II) and an Al(III) center for the first
time. This synthetic approach opens an access to Si(II) com-
pounds in combination with other metals excluding the oxidative
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This work was supported by the Deutsche Forschungsge-
meinschaft. D. S. thanks the DNRF funded Center for Materials
Crystallography (CMC) for support and the Land Niedersachsen
for providing a fellowship in the Catalysis for Sustainable
Synthesis (CaSuS) PhD program.
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c
7208 Chem. Commun., 2011, 47, 7206–7208
This journal is The Royal Society of Chemistry 2011