RGe(I)Ge(I)R compound (R = PhC(NtBu)2) with a Ge - Ge single bond and a comparison with the gauche conformation of hydrazine

Article abstract of DOI:10.1021/om800714f

This article reports the reduction of the chloride [PhC(NtBu) ₂]GeCl with potassium in THF to afford the reddish crystals of [PhC(NtBu)₂]₂Ge₂ (2). The molecule of 2 contains a Ge-Ge bond. The X-ray structure and DFT calculation indicate that the Ge-Ge bond possesses an unusual gauche-bent geometry. The Ge - Ge bond length in 2 is 2.570 A, which is very close to the single Ge - Ge interaction (2.61 A) but significantly longer than that for typical digermenes, R ₂GeGeR₂ (2.21 -2.51 A) and the two structurally characterized digermynes (2.2850 A and 2.2060 A), which proves that there is no multiple bond character in 2.

Full text of DOI:10.1021/om800714f

Organometallics 2008, 27, 5459–5463
5459
RGe(I)Ge(I)R Compound (R ) PhC(NtBu)2) with a Ge-Ge Single
Bond and a Comparison with the Gauche Conformation of
Hydrazine†
Selvarajan Nagendran,‡ Sakya S. Sen,‡ Herbert W. Roesky,*,‡ Debasis Koley,§
Helmut Grubmu¨ller,§ Aritra Pal,‡ and Regine Herbst-Irmer‡
Institut fu¨r Anorganische Chemie der UniVersita¨t Go¨ttingen, Tammannstrasse 4,
37077 Go¨ttingen, Germany, and Theoretische and Computergestu¨tzte Biophysik, Max Planck Institut
Go¨ttingen, Am Fassberg 11, 37077 Go¨ttingen, Germany
ReceiVed July 28, 2008
This article reports the reduction of the chloride [PhC(NtBu)2]GeCl with potassium in THF to afford
the reddish crystals of [PhC(NtBu)2]2Ge2 (2). The molecule of 2 contains a Ge-Ge bond. The X-ray
structure and DFT calculation indicate that the Ge-Ge bond possesses an unusual gauche-bent geometry.
The Ge-Ge bond length in 2 is 2.570 Å, which is very close to the single Ge-Ge interaction (2.61 Å)
but significantly longer than that for typical digermenes, R2GeGeR2 (2.21-2.51 Å) and the two structurally
characterized digermynes (2.2850 Å and 2.2060 Å), which proves that there is no multiple bond character
in 2.
Scheme 1
Introduction
Alkynes (RCt CR) are compounds with carbon-carbon triple
bonds. They contribute largely to the richness and diversity of
organic chemistry and portray the simplicity with which carbon
forms multiple bonds. Synthesis of similar compounds (REt ER)
with their heavier congeners (E ) Si-Pb) was not possible until
recently due to the poor overlap of the adjacent p orbitals.
Nevertheless, by means of a tailor-made starting material, the
first alkyne analogue of a heavier group 14 element, namely,
an amber-green diplumbyne (vide infra) [RPbPbR; R ) 2,6-
Trip2C6H3 (Trip ) 2,4,6-iPr3C6H2)] was isolated by Power and
co-workers1 in 2000. Following this, the first carmine red-orange
digermyne [RGeGeR; R ) 2,6-Trip2C6H3 (Trip ) 2,4,6-
iPr3C6H2)]2 and dark blue-green distannyne [RSnSnR; R ) 2,6-
Dipp2C6H3 (Dipp ) 2,4-iPr2C6H3)]3 were also reported by the
same group in 2002. The series was complete in 2004 with the
first isolation of an emerald-green disilyne [RSiSiR; R )
Si(iPr){CH(SiMe3)2}2] by Sekiguchi and co-workers.4 Quite
strikingly, all of the heavier alkyne analogues (REt ER; E )
Si-Pb) are trans-bent in contrast to the linear geometry of
alkynes as indicated by Jurkschat and co-workers.5 Also, when
the triple bond of HCt CH has a bond order of 3, the bond
order in the case of heavier alkyne analogues decreases as we
go down the group from Si to Pb. The calculations show that
the bond order has the following trend: disilyne ca. 2.6,
digermyne ca. 2, distannyne ca. 2 or 1 depending on the
environment, and diplumbyne ca. 1.6 Thus, a diplumbyne should
be considered as a diplumbylene with a Pb-Pb single bond
and a lone pair of electrons on each Pb atom. Analogous to
this is the recently reported amidinato and guanidinato germa-
nium (I) dimer by Jones and co-workers in 2006.7 These
compounds also possess Ge-Ge single bonds with a lone pair
of electrons on germanium atoms and exhibit trans-bent
geometry.
It is explicit from the aforementioned discussion that all of
the known heavier group 14 analogues of alkyne and their
valence isomers possess a trans-bent structure. In view of this,
we became interested in assembling a valence isomer of a
heavier alkyne with a structure that is different from that of the
usual trans-bent geometry. Accordingly, we report here the first
germanium(I) dimer 2 in a gauche-bent geometry stabilized by
means of an amidinato ligand with simple tBu substituents on
the nitrogen atoms. The dimer 2 was obtained by reducing the
amidinato germanium(II) chloride 1 with a slight excess of finely
divided potassium metal.
† Dedicated to Professor Edgar Niecke on the occasion of his 70th
birthday.
* Corresponding author. E-mail: hroesky@gwdg.de.
‡ Universita¨t Go¨ttingen.
§ Max Planck Institut Go¨ttingen.
Results and Discussion
(1) Pu, L.; Twamley, B.; Power, P. P. J. Am. Chem. Soc. 2000, 122,
3524–3525.
(2) Stender, M.; Phillips, A. D.; Wright, R. J.; Power, P. P. Angew.
Chem., Int. Ed. 2002, 41, 1785–1787.
The reaction of tert-butylcarbodiimide (tBuNdCdNtBu) with
one equivalent of PhLi in diethyl ether followed by treatment
(3) Philips, A. D.; Wright, R. J.; Olmstead, M. M.; Power, P. P. J. Am.
Chem. Soc. 2002, 124, 5930–5931.
(4) Sekiguchi, A.; Kinjo, R.; Ichinohe, M. Science 2004, 305, 1755–
1757.
(5) Jambor, R.; Kasˇna´, B.; Kirschner, K. N.; Schu¨rmann, M.; Jurkschat,
K. Angew. Chem., Int. Ed. 2008, 47, 1650–1653.
(6) (a) Power, P. P. Chem. Commun. 2003, 2091–2093. (b) Robinson,
G. H. Acc. Chem. Res. 1999, 32, 773–782. (c) Pu, L.; Senge, M. O.;
Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 1998, 120, 12682–12683.
(7) Green, S. P.; Jones, C.; Junk, P. C.; Lippert, K. -A.; Stasch, A. Chem.
Commun. 2006, 3978–3980.
10.1021/om800714f CCC: $40.75
2008 American Chemical Society
Publication on Web 10/08/2008

5460 Organometallics, Vol. 27, No. 21, 2008
Nagendran et al.
Figure 2. ORTEP view (30% ellipsoid probability) of compound
2. Hydrogen atoms have been removed for clarity. Selected bond
distances (Å) and bond angles (deg): Ge(1)-N(2) 2.005(2),
Ge(1)-N(1) 2.026(2), Ge(2)-N(3) 2.020(2), Ge(2)-N(4) 2.014(2),
Ge(1)-Ge(2) 2.569(5), C(1)-N(1) 1.326(4), C(1)-N(2) 1.337(4),
C(10)-N(3) 1.333(3), C(10)-N(4) 1.338(3), N(1)-C(80) 1.482(4),
N(2)-C(90) 1.479(4), N(3)-C(190) 1.481(4), N(4)-C(180) 1.475(4),
N(2)-Ge(1)-N(1)64.65(9),N(2)-Ge(1)-C(1)32.54(9),N(1)-Ge(1)-C(1)
32.27(9), N(2)-Ge(1)-Ge(2) 103.55(7), N(1)-Ge(1)-Ge(2)
97.84(7), C(1)-Ge(1)-Ge(2) 105.09(6), N(4)-Ge(2)-N(3) 64.85(9),
N(4)-Ge(2)-C(10)32.58(9),N(3)-Ge(2)-C(10)32.44(9),N(4)-Ge(2)-Ge(1)
104.62(7), N(3)-Ge(2)-Ge(1) 98.17(6), C(10)-Ge(2)-Ge(1)
105.96(6).
Figure 1. ORTEP view (30% ellipsoid probability) of compound
1. Hydrogen atoms and disordered atoms have been removed for
clarity. Selected bond distances (Å) and bond angles (deg):
Ge(1)-Cl(1) 2.2572(13) Ge(1)-N(1) 2.060 ( 0.012(2), N(1)-C(1)
1.330(3),N(1)-C(21)1.480(3),C(1)-C(11)1.483(4),N(1)-Ge(1)-N(1)
63.22(11), N(1)-Ge(1)-Cl(1) 92.10(8), N(1)-C(1)-N(1) 108.6(3),
N(1)-C(1)-C(11) 125.72(13).
with GeCl2 · dioxane afforded [PhC(NtBu)2]GeCl (1; Scheme
1). Compound 1 was obtained as a colorless crystalline solid in
75% yield, and its structure was confirmed by NMR spectro-
scopic, EI mass spectrometric, and elemental analyses.
1
The H NMR spectrum of compound 1 shows a singlet at
multiplet for 10 aromatic protons (7.34-7.39 ppm). Therefore,
1
1.08 ppm for the 18 protons of two tBu groups and another
multiplet for 5 aromatic protons (7.48-7.56 ppm). The most
abundant ion peak in the EI-mass spectrum appeared at m/z
338.5.
there is a very small shift in the H NMR spectrum from the
starting material. The most abundant ion peak in the EI-mass
spectrum appeared at m/z 608.5.
The molecular structure of 2 is shown in Figure 2. Compound
2 crystallizes in the orthorhombic space group Pbca. The
coordination environment of one of the Ge(I) atoms exhibits a
distorted tetrahedral geometry (Figure 2). The coordination sites
of the Ge(I) centers are each occupied by the N atoms of the
amidinato ligand and by the other Ge(I) atom. The lone pair of
the Ge(I) occupies the remaining coordination site of the
tetrahedron. The Ge-Ge bond length in 2 is 2.570 Å, which is
very close to the single Ge-Ge interaction (2.61 Å) but
significantly longer than that for typical digermenes, R2GeGeR2
(2.21-2.51 Å8) and the two structurally characterized diger-
mynes (2.2850 Å2 and 2.2060 Å9), which proves that there is
no multiple bond character in 2.
Maintaining a toluene solution of 1 overnight at -32 °C
resulted in colorless single crystals suitable for X-ray structural
analysis. Compound 1 crystallizes in the monoclinic space group
C2/c (Figure 1). The Ge(II) center exhibits distorted tetrahedral
geometry as the sum of the bond angles is 248°, which is
significantly smaller than 360°. The two sites of the Ge(II) center
are occupied by the N atoms from the amidinato ligand, and
the other site is occupied by a chlorine atom. The lone pair of
the Ge(II) occupies the remaining coordination site. The
structure is very similar to the structures of the recently reported
amidinato and guanidinato germanium(II) chlorides.
Treatment of 1 with 1.5 equivalents of finely divided
potassium in THF for 48 h afforded a deeply colored solution
of germanium dimer [PhC(NtBu)2]2Ge2 (2; Scheme 2). Recrys-
tallization of the crude product in toluene gave reddish crystals
of the germanium (I) dimer with 35% yield. Compound 2 was
isolated as a reddish crystalline solid with good solubility in
solvents such as diethyl ether, toluene, and THF. Moreover, it
is stable in solution or in the solid state at room temperature in
an inert atmosphere. It has been characterized by spectroscopic
methods and X-ray crystallography.
In order to elucidate the nature of the Ge-Ge interaction,
we performed DFT calculations on the isolated gauche species
2. Calculations were performed at both B3LYP/6-31G** and
BP86/6-31G** levels of theory. Structures and energies are
reported in the B3LYP level unless otherwise mentioned. The
DFT optimized geometry 3 shows a close resemblance with
the geometry of 2 (Figures 2 and 3), though the Ge-Ge bond
length is overestimated (2.702 Å in 3 vs 2.570 Å in 2), and
there are subtle differences in average Ge-N distances (2.091
Å in 3 vs 2.015 Å in 2). Moreover, the C1-Ge1-Ge2′-C10′
1
The H NMR spectrum of compound 2 shows a singlet at
1.16 ppm for the 36 protons of four tBu groups and another
Scheme 2

RGe(I)Ge(I)R Compound (R) PhC(NtBu)2)
Organometallics, Vol. 27, No. 21, 2008 5461
The drastic increase in the Ge1-Ge1′ distance occurs with the
increase of the dihedral angle from roughly 153° to 166°.
The end structure of the scan with trans oriented amidinate
ligands (C1-Ge1-Ge1′-C1′ 180°) was fully optimized. In-
terestingly, the unconstrained optimization furnished the gauche-
bent structure 3. In fact, all similar attempts to optimize the
trans configured geometry failed. This prompted us to conclude
the absence of any trans-variant of conformer 2.
Though previous theoretical investigations have pointed out
the stability of the planar-trans conformation for the Ge, Sn,
and Pb species,10 they have also emphasized the effect of the
bulky ligands in destabilizing the gauche conformer on steric
grounds and experimentally utilizing this strategy to isolate the
trans-bent conformer.
After the confirmation of the gauche-bent structure of the
germanium dimer 2, we were very interested to compare it with
the gauche structure of hydrazine. The number of electrons
around one Ge(I) center is the same as that of one nitrogen
atom in hydrazine. A selection of the structural data for 2, 3,
and hydrazine11 are shown in Figure 6.
Each nitrogen atom in hydrazine shows a distorted tetrahedral
geometry with no multiple bond character just like what Ge(I)
exhibits in 2. From the above results, we can argue that the
bonding and the geometry of Ge(I) is very similar to the
isoelectronic neutral neighboring group 15 element derivative.
In conclusion, we have prepared a germanium(I) dimer, which
is stabilized by bulky amidinate ligands. Theoretical studies
confirm that the dimer exhibits a gauche-bent geometry, and
the Ge-Ge bond shows no multiple bond character. We are
currently exploring the further chemistry of the Ge(I) dimer 2.
Figure 3. B3LYP/6-31G** optimized structure 3 (hydrogens are
omitted for clarity) with selected bond lengths (in Å) and angles
(in deg). Ge1-Ge1′ 2.702, Ge1-N1 (Ge1′-N1′) 2.094, Ge1-N2
(Ge1′-N2′) 2.091, N1-C1 (N1′-C1′) 1.335, N2-C1 (N2′-C1′)
1.345,C2-Ge1-Ge1′106.5,C2′-Ge1′-Ge1106.5,C1-Ge1-Ge1′-C1′
113.0.
Experimental Section
All manipulations were carried out in an inert atmosphere of
dinitrogen using standard Schlenk techniques and in a dinitrogen
filled glovebox. Solvents were dried and distilled prior to use. The
NMR spectra were recorded in THF-d8. The chemical shifts δ were
relative to SiMe4. EI mass spectra were obtained using a Finnigan
MAT 8230 instrument. Elemental analyses were performed by the
Institut fu¨r Anorganische Chemie, Universita¨t Go¨ttingen. Melting
points were measured in a sealed glass tube on a Bu¨chi B-450
melting point apparatus and were uncorrected.
Preparation of Compound 1. PhLi (6.86 mL, 13.72 mmol,
1.8 M in diethyl ether) was added to a solution of tBuNdCdNtBu
(2.12 g, 13.72 mmol) in diethyl ether (80 mL) at -78 °C. The
solution was raised to ambient temperature and stirred for 4 h. The
solution was added drop by drop to a stirred suspension of
GeCl2 · dioxane (3.18 g, 13.72 mmol) in diethyl ether (20 mL) at
-78 °C. The reaction mixture was warmed to room temperature
and was stirred for 24 h. The precipitate was filtered, and after the
removal of all volatiles, the residue was extracted with toluene (20
mL). Storage of the extract at -32 °C in a freezer for 1 day afforded
colorless crystals of 1. Mp: 145-150 °C. Elemental analysis (%)
calcd for C15H23ClGeN2 (339.45): C 53.18; H 6.77; N 8.27; found,
C,52.66; H, 6.63; N,8.06. 1H NMR (200 MHz, THF-d8, 25 °C): δ
Figure 4. Frontier orbitals of 3 (isodensity value ) 0.002 electron/
bohr3). (a) KS-HOMO (-3.564 eV). (b) KS-LUMO (-0.598 eV).
dihedral angle has increased by 8.1° (113.0° in 3 vs 104.9° in
2) during the course of the optimization.
In Figure 4, (a) shows the Kohn-Sham (KS) HOMO of the
optimized structure of 3, which is largely composed of σ-bond-
ing interaction between the Ge p-orbitals, whereas in KS-LUMO
(b), there is a strong Ge-Ge πy bonding overlap with an
additional Ge-N π* interaction (Figure 4). Similar types of
frontier orbitals are reported by Jones and co-workers in their
theoretical investigations of trans-bent Ge(I) dimers. Weinhold’s
NBO analysis indicates a strong Ge-Ge bonding interaction
in 3, with high p-character (s-character 14.4%; p-character
85.3%) and a Wiberg bond index of 0.915. The Ge-N bonds
are highly polarized (NPA charges, Ge; 0.524 e and N (mean);
-0.713 e) with an average bond order of 0.431.
)
1.08 ppm (s, 18H, tBu),7.41-7.48 ppm (m, 5H,Ph);
13C{1H}NMR (500 MHz, THF-d8, 25 °C): δ ) 31.8 (CMe3), 54.2
Unlike the previous studies, which investigated both the
structural and electronic features of trans-bent group 14 ele-
ments, herein we report a similar theoretical characterization
of a gauche-bent Ge(I) dimer 3. We were curious to understand
the relative stability of the optimized gauche-bent structure with
respect to its trans-variant. To this end, we performed a relaxed
potential energy scan of the C1-Ge1-Ge1′-C1′ dihedral angle
from 3. Figure 5 shows the energy profile along with the
concomitant change in dihedral angles and Ge1-Ge1′ distances.
(8) As determined by the survey of the Cambridge Crystallographic
Database.
(9) Sugijama, Y.; Sasamori, T.; Hosoi, Y.; Furukawa, Y.; Takagi, N.;
Nagase, S.; Tokitoh, N. J. Am. Chem. Soc. 2006, 128, 1023–1031.
(10) (a) Jung, Y.; Brynda, M.; Power, P. P.; Head-Gordon, M. J. Am.
Chem. Soc. 2006, 128, 7185–7192. (b) Lein, M.; Krapp, A.; Frenking, G.
J. Am. Chem. Soc. 2005, 127, 6290–6299.
(11) (a) Kohata, K.; Fukuyama, T.; Kuchitsu, K. J. Phys. Chem. 1982,
86, 602–606. (b) Schlegel, H. B.; Skanche, A. J. Am. Chem. Soc. 1993,
115, 7465–7471.

5462 Organometallics, Vol. 27, No. 21, 2008
Nagendran et al.
Figure 5. Potential energy scan for the isomerization of 3 with respect to the C1-Ge1-Ge1′-C1′ dihedral angle changes at the B3LYP/
6-31G** level of theory (refer to the text).
empirical absorption correction with SADABS was applied.12,13
The structure was solved by the Patterson method (SHELXS-97)14
and refined against all data by the full-matrix least-squares methods
on F2 (SHELXL-97).15 All non-hydrogen atoms were refined with
anisotropic displacement parameters. The hydrogen atoms were
refined isotropically on calculated positions using a riding model
with their Uiso values constrained to 1.5 Ueq of their pivot atoms
for terminal sp3 carbon atoms and 1.2 times for all other carbon
atoms.
Crystal Data for Compound 1. C15H23ClGeN2, Mr ) 339.39,
Figure 6. Structural features between isoelectronic hydrazine
monoclinic space group C2/c, a ) 14.037(3) Å, b ) 11.109(2) Å,
(gauche) and the gauche-Ge(I) skeleton of 2 and 3. For complete
c ) 11.847(2) Å, ꢀ ) 115.20(3)o V ) 1671.6(5) Å3, Z ) 4, Fcalcd
labeling, please refer to Figure 3. Ge-Ge-N1 96.4 (97.8),
) 1.349 Mgm-3, F(000) ) 704, T ) 100(2) K, µ (Cu KR) ) 3.862
Ge-Ge-N3 96.4 (98.2), Ge-Ge-N2 103.8 (103.6), Ge-Ge-N4
mm-1
.
103.8 (104.6), N1-Ge1-N2 64.0 (64.6), and N4-Ge2-N3 63.0
(64.8). N-N-Ho 107.8 (106 ( 2), N-N-Hi 112.2 (112 ( 2),
H-N-N-H (91 ( 2); Hi (inner angles) and Ho (outer angles).
The values within parenthesis refer to experimental observables.
Of the 8223 measured reflections, 1192 were independent (Rint
) 0.0346). The final refinement converged to R1 ) 0.0290 for I >
2σ (I), wR2 ) 0.0675 for all data. The final difference Fourier
synthesis gave a min/max residual electron density of -0.204/
+0.214 e · Å-3
.
(CMe3), 128.6, 128.9, 129.1, 129.9, 130.6, 135.6 (Ph), 173.3 ppm
(NCN); EI-MS: m/z(%): 338.5[M+], (100).
Crystal Data for Compound 2. C30H46Ge2N4, Mr ) 607.89,
orthorhombic space group Pbca, a ) 20.379(3) Å, b ) 14.120 (2)
Å, c ) 21.943(3) Å, V ) 6319.7(16) Å3.
Preparation of Compound 2. THF (50 mL) was added to a
mixture of 1 (1.42 g, 4.18 mmol) and finely divided potassium (0.25
g, 6.25mmol) at ambient temperature. The resulting red mixture
was stirred for 2 days. The solvent was then removed in Vacuo,
and the residue was extracted with toluene (50 mL). The insoluble
precipitate was filtered off, and the red filtrate was concentrated to
yield crystals of 2 (0.89 g, 35%). Mp: 220-225 °C. Elemental
analysis (%) calcd for C30H46Ge2N4 (607.99): C 59.26; H 7.63; N
9.22; found, C,57.52; H, 7.47; N,8.54. Due to the sensitivity of the
crystals the analytical values show some deviation from the calcd
Of the 56663 measured reflections, 4494 were independent (Rint
) 0.0686). The final refinement converged to R1 ) 0.0299 for I >
2σ (I), wR2 ) 0.0743 for all data. The final difference Fourier
synthesis gave a min/max residual electron density of -0.458/
+0.415 e · Å-3
.
In structure 1, the Ge and Cl atoms are disordered along the
2-fold axis. A similar disorder also appears in the recently reported
germanium(II) halides. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB21EZ,UK;fax:(+44)1223-336-033;ordeposit@ccdc.cam.ac.uk.
1
one. H NMR (200 MHz, THF-d8, 25 °C): δ)1.16 ppm (s, 36H,
tBu), 7.34-7.39 ppm (m, 10H,Ph); 13C{1H}NMR (500 MHz, THF-
d8, 25 °C): δ)31.9 (CMe3), 32.1 (CMe3), 53.5 (CMe3), 55.0
(CMe3), 123.0, 126.9, 127.8, 128.1, 128.3, 128.9, 129.1, 129.4,
129.9, 130.5, 131.5, 138.0 (Ph), 154.5 (NCN), 164.3 ppm (NCN);
EI-MS: m/z(%): 608.5[M+] (100).
(12) SAINT-NT; Bruker AXS Inc.: Madison, Wisconsin, 2000.
(13) Sheldrick, G. M. SADABS 2.0; Universita¨t Go¨ttingen, Germany,
2000.
(14) Sheldrick, G. M. SHELXS-90, program for structure solution. Acta
Crystallogr., Sect. A 1990, 46, 467–473.
(15) Sheldrick, G. M. SHELXL-97, program for crystal structure
refinement. Acta Crystallogr., Sect. A 2008, 64, 112–122.
Crystal Structure Determination. The data of both structures
were collected on a Bruker three-circle diffractometer equipped with
a SMART 6000 CCD detector and a mirror-system-monochromated
Cu KR source. The data were integrated with SAINT, and an

RGe(I)Ge(I)R Compound (R) PhC(NtBu)2)
Organometallics, Vol. 27, No. 21, 2008 5463
Computational Details. All calculations were performed with
the Gaussian 03 program package.16 The geometry of complex 2
was optimized at the BP8617 and B3LYP18 level using a quasi-
relativistic Stuttgart-Dresden ECP19 with a corresponding valence
basis set (31/31/1) for Ge, (augmented by a d-type polarization
function with an exponent of 0.246) and 6-31G* basis sets for C
and N, and an extra polarization function was added to H.20
Geometry optimizations were carried out without any symmetry
restriction. Harmonic force constants were computed at the
optimized geometries to characterize the stationary points as
minima. Weinhold’s NBO approach was used to calculate the NPA
charges, orbital populations and other bonding analyses.21 The
constrained potential energy scan of the dihedral angle (C1-Ge1-
Ge1′-C1′) was performed at the B3LYP level. The molecular
orbital plots were made using the GaussView program package.22
Acknowledgment. This work was supported by the
Goettinger Akademie der Wissenschaften and Deutsche
Forschungsgemeinschaft. S.N. thanks the Alexander von
Humboldt Stiftung for a research fellowship. D.K. is indebted
to GWDG for computational facilities.
Supporting Information Available: Crystal data (CIF) for 1
and 2, Cartesian coordinates of the optimized structure 3 in both
levels of theory, and the full list of authors of ref 16. This material
is available free of charge via the Internet at http://pubs.acs.org.
(16) Frisch, M. J. Gaussian 03, revision D.01; Gaussian, Inc.: Wall-
ingford, CT, 2004.
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