Preparation and molecular structures of stable bis(germylenes) with pincer topology

Article abstract of DOI:10.1002/ejic.200700295

Benzannulated N-heterocyclic bis(germylenes) with pincer ligand topology have been prepared by the reaction of N,N′,N″,N?- tetralithiated tetraamines with GeCl₂·1,4-dioxane or by the transamination reaction between a tetraamine and Ge[N(SiMe

Full text of DOI:10.1002/ejic.200700295

SHORT COMMUNICATION
DOI: 10.1002/ejic.200700295
Preparation and Molecular Structures of Stable Bis(germylenes) with Pincer
Topology
F. Ekkehardt Hahn,*[a] Alexander V. Zabula,[a] Tania Pape,[a] and Alexander Hepp[a]
Keywords: N-Heterocycles / Germylenes / Pincer ligands
Benzannulated N-heterocyclic bis(germylenes) with pincer
ligand topology have been prepared by the reaction of
that the bis(germylenes) exist as monomers in the solid state.
Significant intramolecular Ge···Ge and Ge···N interactions
N,NЈ,NЈЈ,NЈЈЈ-tetralithiated tetraamines with GeCl2·1,4-diox- have been observed for the lutidine-bridged bis(germylene).
ane or by the transamination reaction between a tetraamine
and Ge[N(SiMe3)2]2. X-ray diffraction studies have shown,
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
in the presence of Et3N. The bridging unit present in the
tetraamides determined which reducing agent had to be
used in the subsequent reaction. Reduction of the carbonyl
groups in the phenylene-bridged tetraamide 1a was
achieved with AlH3 in THF at ambient temperature. The
four carbonyl groups in tetraamide 1b were reduced in good
yield (81%) by an excess of freshly prepared BH3 in THF.
The bis(germylene) 3 was obtained by lithiation of the
corresponding tetraamine 2a at –78 °C in THF followed by
the reaction of the generated organolithium compound with
GeCl2·1,4-dioxane (Scheme 2). An analogous reaction se-
quence gave also the bis(germylene) 4, but the product was
quite impure and the overall yield was finally less than 30%
after multiple recrystallization steps. The yield of bis(ger-
mylene) 4 could be impoved significantly to 60–65% by re-
action of the tetraamine 2b with Ge[N(SiMe3)2]2 in boiling
THF.
Bis(germylene) 3 was crystallized from hexane. The
structure analysis shows monomeric units of the bis(germy-
lene) which exhibit no significant inter- or intramolecular
interactions (Figure 1). Bond lengths and angles in 3 are
similar to those reported for a monodentate benzannulated
germylene[9e] and other related bis(germylenes).[11] The ni-
trogen atoms are planarized as was observed for the analo-
gous benzannulated N-heteroclic carbenes.[12] The shortest
distance between two germanium atoms is the intermo-
lecular separation between Ge1 and Ge2 [4.226(5) Å] which
is too long to be regarded as an interaction. The lack of
Ge···Ge interactions in 3 is possibly caused by the sterically
demanding N-substituents. We have observed such Ge···Ge
interactions in bis(germylenes) with sterically less de-
manding N-substituents.[11]
Introduction
Polydentate N-heterocyclic carbene (NHC) ligands[1]
containing two imidazolin-2-ylidene[2] or benzimidazolin-2-
ylidene[3] donor groups, which are capable of forming che-
late complexes including tridentate derivatives with pincer
topology,[4] are known. In addition, some tris(imidazolin-2-
ylidenes)[5] and complexes with cyclic tetrakis(benzimidazo-
lin-2-ylidene)[6] and [11]ane-P2CNHC ligands[7] have been de-
scribed. Much less is known about polydentate silylenes,
germylenes and stannylenes which can act as chelate li-
gands, although the monodentate derivatives have been pre-
pared.[8–10]
Previously we described the preparation and coordina-
tion chemistry of germanium analogues of benzannulated
N-heterocyclic bis(carbenes) in which two benzimidazolin-
2-germylene moieties are linked together with an aryl or an
alkyl group.[11] It has been demonstrated that these bis(ger-
mylenes) can act as bidentate chelate ligands. Continuing
our investigations in this field, we report now on the prepa-
ration of the N-heterocyclic benzannulated bis(germylenes)
with pincer topology which are potentially tridentate li-
gands.
Results and Discussion
The N-heterocyclic bis(germylenes) were obtained from
the tetraamines 2a and 2b which were prepared by re-
duction of the carbonyl groups in the corresponding tet-
raamides 1a and 1b (Scheme 1). The tetraamides are ob-
tained by acylation of N-(ortho-aminophenyl)amides with
benzene- (1a) or lutidine-2,6-bis(carboxylic chlorides) (1b)
The molecular structure of bis(germylene) 4 containing
a lutidine bridge between the germylene units is depicted in
Figure 2. Its conformation is distincly different to that of 3.
While no significant intermolecular interactions were no-
ticed, short intramolecular Ge···Ge and Ge···N contacts
[a] Institut für Anorganische und Analytische Chemie der
Westfälischen Wilhelms-Universität Münster,
Corrensstrasse 36, 48149 Münster, Germany
Fax: +49-251-83-33108
E-mail: fehahn@uni-muenster.de
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F. E. Hahn, A. V. Zabula, T. Pape, A. Hepp
SHORT COMMUNICATION
Figure 1. Molecular structure of 3. Selected bond lengths [Å] and
angles [°]: Ge1–N1 1.861(2), Ge1–N2 1.852(2), Ge2–N3 1.851(2),
Ge2–N4 1.859(2), N1–C1 1.396(3), N1–C21 1.469(4), N2–C2
1.384(3), N2–C7 1.462(3), N3–C15 1.381(3), N3–C14 1.460(3), N4–
C16 1.395(3), N4–C26 1.469(3), C1–C2 1.415(4), C15–C16
1.408(4); N1–Ge1–N2 85.05(10), N3–Ge2–N4 85.05(10), Ge1–N1–
C1 114.0(2), Ge1–N2–C2 114.5(2), Ge2–N3–C15 114.2(2), Ge2–
N4–C16 114.0(2).
geometry of the N2Ge1···Ge2N2 moiety is similar to the
distorted trans-bent arrangement found for E=E double
bonds of heavier homologues of alkenes. The intramolecu-
lar distances Ge1···N3 [3.386(7) Å] and Ge2···N3
[3.154(7) Å] are again shorter than those found in bimolecu-
lar aggregates of bis(germylene) with intermolecular Ge···N
separations of about 3.53 Å.[11]
Scheme 1. Preparation of compounds 1a, 1b and 2a, 2b.
Figure 2. Molecular structure of 4. Selected bond lengths [Å] and
angles [°]: Ge1–N1 1.862(3), Ge1–N2 1.847(3), Ge2–N4 1.845(3),
Ge2–N5 1.862(3), N1–C1 1.386(5), N1–C20 1.462(5), N2–C2
1.372(5), N2–C7 1.455(5), N4–C14 1.377(5), N4–C13 1.456(5), N5–
C15 1.385(5), N5–C25 1.462(5), C1–C2 1.422(5), C14–C15
1.425(5); N1–Ge1–N2 84.88(13), N4–Ge2–N5 85.01(14), Ge1–N1–
C1 114.1(2), Ge1–N2–C2 114.9(2), Ge2–N4–C14 115.0(2), Ge2–
N5–C15 113.9(3); Ge2···Ge1 3.041(5), Ge1···N3 3.386(7), Ge2···N3
Scheme 2. Preparation of bis(germylenes) 3 and 4.
were found in the molecular structure of 4. The Ge1···Ge2
separation is 3.041(5) Å which is significantly shorter than
twice the van der Waals radius of the germanium atom.[13]
It is also shorter than the Ge···Ge separation which we
found for alkyl-bridged bis(germylenes) [3.577(2) Å].[11] The 3.154(7).
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Stable Bis(germylenes) with Pincer Topology
SHORT COMMUNICATION
C6H4), 3.85 [s, 4 H, NCH2–C(CH3)3], 0.86 [s, 18 H, C(CH3)3] ppm.
13C NMR (50.3 MHz, [D8]toluene): δ = 144.2, 142.2 (Ar–Cipso
GeN2–C6H4), 140.6 (Ar–Cipso NCH2–C6H4), 129.2, 128.3, 127.3
(NCH2–C6H4), 118.7, 118.1 (Ar–Cmeta GeN2–C6H4),110.5, 110.3
(Ar–Cortho GeN2–C6H4), 56.8 (NCH2–C6H4), 50.5 [NCH2–
C(CH3)3], 33.0 [C(CH3)3], 28.6 [C(CH3)3] ppm. MS (EI, 70 eV):
m/z (%) = 600 (68) [M]+, 543 (91) [M – tBu]+.
Conclusions
We have prepared the bis(germylenes) 3 and 4 containing
two N-heterocyclic germylene (NHGe) units bridged by a
1,3-phenylene- or 1,3-pyridinediylbis(methylene) group,
respectively. These compounds are germanium analogues of
N-heterocyclic pincer-type bis(carbene) ligands. Their use
1
in the complex formation with transition metal ions and the Selected Spectroscopic Data for 4: Yield 240 mg (64%). H NMR
(200 MHz, [D8]toluene): δ = 7.02–6.59 (m, 11 H, Ar–H), 5.18 (s, 4
H, NCH2–C5H3N), 3.77 [s, 4 H, NCH2–C(CH3)3], 0.90 [s, 18 H,
C(CH3)3] ppm. 13C NMR (50.3 MHz, [D8]toluene): δ = 159.3 (Cα-
C5H3N), 144.0, 142.1 (Ar–Cipso C6H4), 137.2 (Cγ-C5H3N), 119.8
(Cβ-C5H3N), 118.6, 118.1 (Ar–Cmeta C6H4), 110.4, 110.3 (Ar–Cortho
C6H4), 56.8 (NCH2–C5H3N), 51.9 [NCH2–C(CH3)3], 33.0
[C(CH3)3], 28.6 [C(CH3)3] ppm. MS (EI, 70 eV): m/z (%) = 601 (46)
[M]+, 544 (100) [M – tBu]+.
properties of such pincer complexes are currently investi-
gated.
Experimental Section
Starting Materials, Reaction Conditions and Instrumentation: All
manipulation were carried out under argon using Schlenk or glove-
box techniques. Toluene and n-hexane were dried with sodium/
benzophenone and were freshly distilled prior to use. [D8]Toluene
was dried with Na/K alloy. The preparation of the tetraamide 1a
and the tetraamine 2a have been described.[11] Compounds 1a and
1b were obtained by similar methods. However, 2b was prepared in
analogy to 2a by using BH3 instead of AlH3 as a reducing agent.
X-ray Diffraction Studies: Diffraction data for 3 and 4 were col-
lected with a Bruker AXS APEX CCD diffractometer equipped
with a rotation anode at 153(2) K using graphite-monochromated
Mo-Kα radiation (λ = 0.71073 Å). Diffraction data were collected
over the full sphere and were corrected for absorption. The data
reduction was performed with the Bruker SMART[15] program
package. The structures were solved with the SHELXS-97[16] pack-
age using the heavy-atom method and were refined with SHELXL-
97[17] against |F2| using first isotropic and then anisotropic thermal
parameters for all non-hydrogen atoms. Hydrogen atoms were
added to the structure models in calculated positions. CCDC-
641062 (3) and -641063 (4) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk./data/request/cif.
Ge[N(SiMe3)2]2 was prepared as described in the literature.[14] 1
H
and 13C NMR spectra were recorded with a Bruker AC-200 spec-
trometer.
1
Tetraamide 1b: Yield 75%, white solid. H NMR (200 MHz, [D6]-
DMSO): δ = 10.91 [s, 2 H, NH–C(O)–C5H3N], 9.07 [s, 2 H, NH–
C(O)–C(CH3)3], 8.45–8.34 (m, 3 H, Ar–H C5H3N), 7.73–7.68 (m,
2 H, Ar–H C6H4), 7.52–7.47 (m, 2 H, Ar–H C6H4), 7.31–7.26 (m,
4 H, Ar–H C6H4), 1.01 [s, 18 H, C(CH3)3] ppm. 13C NMR
(50.3 MHz, [D6]DMSO): δ = 176.9 [C(O)–C5H3N], 161.4 (Cα-
C5H3N), 148.2 [C(O)–C(CH3)3], 140.5 (Cγ-C5H3N), 131.4, 130.6
(Ar–Cipso C6H4), 126.2 (Cβ-C5H3N), 125.7, 125.5, 125.1, 125.0 (Ar–
Cmeta and Ar–Cortho C6H4), 38.7 [C(CH3)3], 26.9 [C(CH3)3] ppm.
MS (MALDI): m/z (%) = 516 [M + H]+, 538 [M + Na]+.
Crystal Data for Bis(germylene) 3: C30H38Ge2N4, M = 599.82, tri-
¯
clinic, P1, Z = 2, a = 9.4855(12), b = 9.8055(12), c = 15.744(2) Å,
α = 85.590(3), β = 89.526(2), γ = 77.424(2)°, V = 1427.7(3) Å3,
14127 measured reflections, 6544 unique reflections (Rint = 0.0371),
R = 0.0414, wR = 0.0905 for 4947 contributing reflections
[IՆ2σ(I)], refinement against |F2| with anisotropic thermal param-
eters for all non-hydrogen atoms and hydrogen atoms in calculated
positions.
Tetraamine 2b: A 1  solution of BH3 in THF (40 mmol, 40 mL)
was added to the solid tetraamide 1b (500 mg, 0.97 mmol) at 0 °C.
The mixture was stirred at room temperature for 3 h, than heated
under reflux for 4 h. Excess BH3 was hydrolyzed with CH3OH at
0 °C. The resulting boron compound was transformed into the tet-
raamine by hydrolysis in a THF/H2O mixture with NaOH. 2b was
extracted from the reaction mixture 3 times with CH2Cl2 (20 mL
each). The combined organic phases were dried with Na2SO4. Re-
moval of the solvent gave an oil, which was dissolved in hexane
and filtered. Evaporation of the hexane gave 2b as a colorless or
yellowish air-sensitive oil. Yield 360 mg (81%). 1H NMR
(200 MHz, CDCl3): δ = 7.52 (t, 1 H, Hγ-C5H3N), 7.11 (d, 2 H, Hβ-
C5H3N), 6.75–6.52 (m, 8 H, Ar–H C6H4), 4.37 (s, 4 H, NCH2–
C5H3N), 3.81 (s br, 4 H, NH), 2.79 [s, 4 H, NCH2–C(CH3)3], 0.96
[s, 18 H, C(CH3)3] ppm. 13C NMR (50.3 MHz, CDCl3): δ = 158.2
(Cα-C5H3N), 138.0, 137.2, 137.0 [Ar–Cipso for (NH)2C6H4 and
Cγ-C5H3N], 119.7 (Cβ-C5H3N), 119.4, 118.9 [Ar–Cmeta for (NH)2-
C6H4], 112.1, 111.8 [Ar–Cortho for (NH)2C6H4], 56.1 (NCH2–
C5H3N), 49.7 [NCH2–C(CH3)3], 31.4 [C(CH3)3], 27.8 [C(CH3)3]
ppm. MS (EI, 70 eV): m/z (%) = 459 (100) [M]+.
Crystal Data for Bis(germylene) 4: C29H37Ge2N5, M = 600.82, tri-
¯
clinic, P1, Z = 2, a = 9.690(2), b = 9.807(2), c = 14.792(3) Å, α =
91.743(4), β = 96.699(4), γ = 96.715(4)°, V = 1385.1(4) Å3, 11194
measured reflections, 4890 unique reflections (Rint = 0.0392), R =
0.0449, wR = 0.1143 for 3839 contributing reflections [IՆ2σ(I)],
refinement against |F2| with anisotropic thermal parameters for all
non-hydrogen atoms and hydrogen atoms in calculated positions.
Acknowledgments
The authors thank the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie for financial support. A. V. Z.
thanks the NRW Graduate School of Chemistry, Münster for a
predoctoral grant.
Bis(germylenes) 3 and 4: The bis(germylenes) were prepared from
GeCl2·1,4-dioxane or Ge[N(SiMe3)2]2 as described by us for related
derivatives.[11] Crystals for the X-ray analyses were grown by slow
diffusion of hexane into a saturated solution of 4 in toluene or
directly from an n-hexane solution of 3.
Selected Spectroscopic Data for 3: Yield 80%. 1H NMR (200 MHz,
[D8]toluene): δ = 7.14–7.00 (m, 12 H, Ar–H), 4.82 (s, 4 H, NCH2–
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Received: March 15, 2007
Published Online: May 3, 2007
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