Experimental evidence and bond characterization of a cyclopropenylgermylene

Article abstract of DOI:10.1016/j.jorganchem.2005.09.041

The reduction of p-anisyl(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)dichlorogermane (1) with potassium in the presence of an excess of tert-butyldimethylsilane in benzene under reflux gave p-anisyl(tert-butyldimethylsilyl)(1,2,3-tri-tert-butylcycloprop-2-en-1-yl) germane (4) in 15% yield. The formation of 4 indicates that p-anisyl(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)germylene (2), which is the first example of a (cycloprop-2-en-1-yl)germylene derivative, was generated and trapped by the hydrosilane. The DFT calculations revealed that the cis-2-p-anisyl-1,3,4-tri-tert-butyl-2-germabicyclo[1.1.0]butane-2,4-diyl structure cis-5 is 8.0 kJ/mol more stable than cis-2. The NBO analysis revealed that cis-5 has a 2-germabicyclo[1.1.0]butane diradical character.

Full text of DOI:10.1016/j.jorganchem.2005.09.041

Journal of Organometallic Chemistry 691 (2006) 595–603
www.elsevier.com/locate/jorganchem
Experimental evidence and bond characterization
of a cyclopropenylgermylene
a,
a
a
Shinobu Tsutsui *, Hiromasa Tanaka , Eunsang Kwon ,
a
a,b,
*
Shigeki Matsumoto , Kenkichi Sakamoto
a
Photodynamics Research Center, RIKEN (The Institute of Physical and Chemical Research), 519-1399 Aoba, Aramaki, Aoba-ku, Sendai 980-0845, Japan
b
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
Received 27 June 2005; received in revised form 12 September 2005; accepted 21 September 2005
Available online 4 November 2005
Abstract
The reduction of p-anisyl(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)dichlorogermane (1) with potassium in the presence of an excess of
tert-butyldimethylsilane in benzene under reflux gave p-anisyl(tert-butyldimethylsilyl)(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)germane (4)
in 15% yield. The formation of 4 indicates that p-anisyl(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)germylene (2), which is the first example of
a (cycloprop-2-en-1-yl)germylene derivative, was generated and trapped by the hydrosilane. The DFT calculations revealed that the cis-2-
p-anisyl-1,3,4-tri-tert-butyl-2-germabicyclo[1.1.0]butane-2,4-diyl structure cis-5 is 8.0 kJ/mol more stable than cis-2. The NBO analysis
revealed that cis-5 has a 2-germabicyclo[1.1.0]butane diradical character.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Organogermanium compound; Germylene; Cyclopropene; Density functional theory; Natural bond orbital analysis
1. Introduction
by the reduction of tribromo(1,2,3-tri-tert-butylcycloprop-
2-en-1-yl)silane (Scheme 1) [7–9]. Although the reaction
mechanism for the formation of the disilene is unclear, the
disilene is one of the formal tetramers of (1,2,3-tri-tert-butyl-
cycloprop-2-en-1-yl)silylyne.
Cyclopropene is one of the smallest ring compounds and
has a high reactivity due to its strained structure [1,2]. Some
unique isomerizations of silicon compounds with a cyclo-
prop-2-en-1-yl group have been reported [2j–7]. In particu-
lar, the reactivities of the unsaturated silicon compounds
with the cycloprop-2-en-1-yl group are interesting. Fink
and coworkers [3] reported that the photolysis of 2-mesityl-
1,1,1,3,3,3-hexamethyl-2-(1,2,3-tri-tert-butylcycloprop-2-en-
1-yl)trisilane produces mesityl(1,2,3-tri-tert-butylcycloprop-
2-en-1-yl)silylene, which isomerizes to the corresponding
silacyclobutadiene. Very recently, we reported the structure
and reactions of the lattice-framework disilene, 2,3,4,6,7,8,
20,30,40,60,70,80-dodeca-tert-butyl-[5,50]bi{1,5-disilatricyclo-
[4.2.0.01,4]octylidene}-2,7,20,70-tetraene, which is prepared
On the other hand, a germanium compound with the
cycloprop-2-en-1-yl group has not yet been reported. We
have been interested in the chemistry of such compounds,
especially a germylene with the cycloprop-2-en-1-yl group.
During the course of this study, we succeeded in prepar-
ing p-anisyl(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)dichlo-
rogermane (1). Compound 1 should be regarded as a
useful precursor for the corresponding germylene, p-anisyl-
(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)germylene (2). In
this paper, we report the generation of 2 by the reduction
of 1, which was prepared from tri-p-anisyl(1,2,3-tri-tert-
butylcycloprop-2-en-1-yl)germane (3a). Germylene 2 is
the first example of a cyclopropenylgermylene derivative.
The intermediate 2 was trapped by tert-butyldimethylsi-
lane. The structure and bonding characteristics of 2 were
investigated by DFT calculations.
*
Corresponding authors. Tel.: +81 22 795 6588; fax: +81 22 795 6589
(K. Sakamoto).
E-mail addresses: stsutsui@riken.jp (S. Tsutsui), sakamoto@si.chem.
tohoku.ac.jp (K. Sakamoto).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.09.041

596
S. Tsutsui et al. / Journal of Organometallic Chemistry 691 (2006) 595–603
t-Bu
t-Bu
MeO
t-Bu
t-Bu
SiBr3
t-Bu
12 KC8
4
HCl / cat. AlCl3
-12 KBr / -graphite
3a
GeCl2
t-Bu
1 (74%)
toluene
t-Bu
t-Bu
t-Bu t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
Si Si Si Si
Scheme 3.
t-Bu t-Bu
t-Bu
t-Bu
74% yield (Scheme 3). The structure of 1 was established
1
by mass spectrometry and H and 13C NMR spectrosco-
Scheme 1.
pies. The reaction of 3c with HCl/cat.AlCl3 did not give
the corresponding aryl(1,2,3-tri-tert-butylcycloprop-2-en-
1-yl)dichlorogermane.
2. Results and discussion
2.1. Preparation of triaryl(1,2,3-tri-tert-butylcycloprop-2-
en-1-yl)germanes
2.3. Generation and trapping of p-anisyl(1,2,3-tri-tert-
butylcycloprop-2-en-1-yl)germylene (2)
The triaryl(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)germ-
anes 3a–c were prepared by the reaction of tri-tert-butyl-
cyclopropenium tetrafluoroborate with the corresponding
triarylgermyllithiums (Scheme 2). Compounds 3a–c are sta-
ble in air with the corresponding melting points (3a: 178–
180 ꢁC, 3b: 262–263 ꢁC, 3c: 216–219 ꢁC). The structures
of 3a–c were established by mass spectrometry and 1H
and 13C NMR spectroscopies.
The reduction of 1 with potassium in the presence of an
excess of tert-butyldimethylsilane in benzene under reflux
gave p-anisyl(tert-butyldimethylsilyl)(1,2,3-tri-tert-butylcy-
cloprop-2-en-1-yl)germane (4) in the isolated yield of 15%
(Scheme 4). Compound 4 was isolated by GPC (toluene
as the eluent) and the structure of 4 was established by
mass spectrometry and 1H, 13C, and 29Si NMR spectrosco-
pies. The other compounds in the reaction mixture were
1
not identified. The H NMR spectrum of 4 in C6D6 shows
2.2. Halodearylation of triaryl(1,2,3-tri-tert-butylcycloprop-
2-en-1-yl)germanes
three kinds of peaks for the tert-butyl groups and two
kinds of peaks for the methyl groups on the silicon atom.
The magnetic non-equivalence of the peaks is caused by
the chiral germanium atom. The formation of 4 indicates
that germylene 2 was generated and trapped by the hydros-
ilane. Germylene 2 is the first example of a cyclopropenyl-
germylene derivative. The reduction of 1 with potassium
graphite in the presence of an excess triethylsilane in
THF at ꢀ80 ꢁC gave a complex mixture including the unre-
acted 1. Also, the reduction of 1 with potassium in benzene
As previously reported, the reaction of triphenyl(1,2,3-
tri-tert-butylcycloprop-2-en-1-yl)silane with HBr gas in
the presence of a catalytic amount of AlBr3 (HBr/cat.
AlBr3) produces tribromo(1,2,3-tri-tert-butylcycloprop-2-
en-1-yl)silane in 94% yield [7]. In contrast, the halodephe-
nylation of 3b with HX/cat.AlX3 (X = Br, Cl) produced
a reaction mixture due to the cleavage of the Ge-cyclop-
ropenyl bond. This result indicates that the elimination of
the cyclopropenyl group and phenyl group simultaneously
occurred during the course of the reaction. The bromode-
arylation of 3a, which has a greater electron-donating
group on the phenyl ring, with HBr/cat.AlBr3 also pro-
duced a reaction mixture due to the cleavage of the Ge-
cyclopropenyl bond. Interestingly, the chlorodearylation
of 3a with HCl/cat.AlCl3 selectively gave p-anisyl(1,2,3-
tri-tert-butylcycloprop-2-en-1-yl)dichlorogermane (1) in
MeO
K
t-BuMe2SiH
SiMe2(t-Bu)
1
t-Bu
t-Bu
Ge
THF / reflux
H
t-Bu
4 (15%)
K
MeO
t-Bu
t-Bu
t-Bu
t-Bu
Ar3GeLi
THF
GeAr3
t-Bu
t-BuMe2SiH
t-Bu
-
Ge:
[BF4 ]
t-Bu
3a (Ar = p-anisyl, 77%)
3b (Ar = phenyl, 85%)
t-Bu
t-Bu
3c (Ar =o-Me2N-phenyl, 56%)
2
Scheme 2.
Scheme 4.

S. Tsutsui et al. / Journal of Organometallic Chemistry 691 (2006) 595–603
597
under reflux in the absence of a trapping reagent produced
a complex mixture.
2.4. Theoretical calculations
2.4.1. Structures and relative energies
To discuss the structure and relative energy of 2 compared
with the isomeric structures, we performed density func-
tional theory (DFT) calculations using the GAUSSIAN 98 pro-
gram package [10,11]. The bonding character of the
calculated structures were investigated by means of the nat-
ural bond orbital (NBO) analysis.[12] The initial structures
for optimization were chosen based on the optimized struc-
´
tures of SiC3H4 reported by Veszpremi et al. [13] and Maier
et al. [14] As shown in Fig. 1, five stable structures cis-2,
trans-2, cis-5, trans-5, and 6 were found at the B3LYP/6-
31G(d) level. The selected geometric parameters of cis-2,
trans-2, cis-5, trans-5, and 6 are shown in Fig. 2. Fig. 3 shows
the electronic structures of cis-2, trans-2, cis-5, trans-5, and 6
based on the results of the NBO analysis, together with the
electron occupancy assigned to each bond or lone-pair orbi-
tal in the structures. The cis-2-p-anisyl-1,3,4-tri-tert-butyl-2-
germabicyclo[1.1.0] butane-2,4-diyl structure cis-5, in which
CD and CE locate on the same side of the GeC3 skeleton, is
the most stable among all the structures optimized in this
study. Another germabicyclo[1.1.0]butanediyl structure
trans-5, in which CD and CE are on opposite sides of the
GeC3 skeleton is 18.3 kJ/mol higher in energy than cis-5.
cis- and trans-p-Anisyl(1,2,3-tri-tert-butylcycloprop-2-en-
1-yl)germylenes (cis- and trans-2) are 8.0 and 17.9 kJ/mol
less stable than cis-5, respectively. 1-p-Anisyl-2,3,4-tri-tert-
Fig. 2. Selected geometric parameters of cis-2, trans-2, cis-5, trans-5, and
6. All the methyl groups and the p-anisyl group are omitted for clarity.
butyl-1-germacyclobutadiene 6, which has a planar GeC3
ring, is 124.2 kJ/mol less stable than cis-5.
The optimized structures of cis- and trans-2 apparently
contain the divalent germanium atom. As shown in Fig. 2,
the geometric parameters for the cyclopropenyl ring in both
structures are almost identical. The CA–Ge–CD bond angles
in cis-2 (109.0ꢁ) and trans-2 (118.2ꢁ) are comparable to the
corresponding values for the solid-state structure of
bis(2,4,6-tri-tert-butylphenyl)germylene (108.0ꢁ) [15] and
[bis(trimethylsilyl)methyl][tris(trimethylsilyl)methyl]germyl-
ene(111.3ꢁ) [16], while this bond angle is larger than the value
in dimethylgermylene (95.9ꢁ) optimized at the B3LYP/6-
311+G(d,p) level. The large CA–Ge–CD bond angle in 2
would be caused by steric repulsion between the bulky sub-
stituents on the germanium atom. The CA–Ge–CD and
CE–CA–Ge bond angles in trans-2 are larger than those in
cis-2. The larger bond angles as well as the lower thermody-
namic stability of trans-2 indicate that the steric repulsion be-
tween the p-anisyl group and the tert-butyl group on CA is
more significant in trans-2 than that in cis-2. The NBO
Fig. 1. Optimized structures and relative energies of cis-2, trans-2, cis-5,
trans-5, and 6 obtained at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d)
level. Hydrogen atoms are omitted for clarity.

598
S. Tsutsui et al. / Journal of Organometallic Chemistry 691 (2006) 595–603
tively. Although the corresponding triplet structure is also
1.95 (σ)
1.90 (π)
1.96 (σ)
1.90 (π)
0.78 (LP)
1.95
a local minimum on the potential energy surface of cis-5,
the triplet structure is 191.8 kJ/mol less stable than the sin-
glet structure.
CA
CA
1.89
CC
CA
1.89
CC
1.95
1.89
1.89
1.79
CC
CB
CB
CB
For trans-5, the CB–CC distance (1.854 A) is 0.201 A
longer than that in cis-5. The geometric parameters for the
GeC3 skeleton in trans-5 are close to those in cis-5, except
for the CB–CC distance. The benzene ring of the p-anisyl
group in trans-5 locates nearly perpendicular to the CA–
CB–CC triangle in the GeC3 skeleton unlike cis-5. The
NBO structure of trans-5 is described as a pair of resonant
structures, as shown in Fig. 3. For the resonant structure
trans-5, the number of electrons on the CA–CB, CB–CC,
Ge–CB bonds is calculated to be 1.96, 1.53, and 1.67, respec-
tively. The number of electrons accommodated on the r and
p bonds between CA and CC is 1.95 and 1.34, respectively. No
unpair electron is assigned on the 2p orbital of CA. The va-
lence electrons of 1.84 on the germanium atom are assigned
as the lone-pair electrons. Thus, the bonding character of
trans-5 is close to that of a germylene with the cyclopropenyl
ring, despite its germabicyclobutanediyl geometry.
˚
˚
1.96
Ge
1.95 (LP)
1.96
Ge
1.96 (LP)
1.71
1.71
Ge
1.93
1.79 (LP)
CD
D 1.95
D 1.94
C
C
cis-5
cis-2
trans-2
CA 1.95 (σ)
1.34 (π)
1.95 (σ) CA
1.34 (π)
1.97 (σ)
1.95 (π)
CA
1.95
1.96
CB
1.96
CC
CC
CC
CB
CB
1.90
1.53
1.53
1.94 (σ)
1.91 (π)
1.67
1.67
Ge
Ge
Ge
1.94
CD
1.84 (LP)
1.94
1.84 (LP)
1.94
CD
CD
trans-5
6
Fig. 3. Structures of cis-2, trans-2, cis-5, trans-5, and 6 described on the
basis of the result of the NBO analysis. The values shown in the structures
are the number of electrons assigned to each bond. LP represents a lone-
pair orbital on an atom. All the tert-butyl groups and the p-anisyl group
are omitted for clarity.
Structure 6, in which the GeC3 ring is fixed as planar,
B
A
C
˚
˚
has the long Ge–C (2.008 A) and C –C (1.622 A) bonds
C
A
B
˚
˚
analysis also suggests that the electronic structure of both
cis- and trans-2 can be regarded as a germylene. As shown
in Fig. 3, the number of electrons assigned to the lone-pair
orbital of the germanium atom is greater than 1.95 for both
structures, which are very close to the value (1.99) for dim-
ethylgermylene. For the CA–CC bond in cis-2, the number
of electrons assigned on the r and p bonds is 1.95 and
1.90, respectively. The number of electrons on the CA–CB,
CB–CC, Ge–CB, and Ge–CD bonds in cis-2 is as many as
1.89, 1.89, 1.96, and 1.95, respectively. The result of the
NBO analysis for trans-2 is very similar to cis-2.
as well as the short Ge–C (1.790 A) and C –C (1.357 A)
bonds. The CB–CC distance is calculated to be 2.375 A,
˚
implying that no bond exists between CB and CC unlike
the case of cis- and trans-5. From the viewpoint of geome-
try, 6 is a germacyclobutadiene. Re-optimization of 6 with-
out maintaining the planar GeC3 skeleton gave trans-5.
The NBO analysis result also indicates that 6 has a germa-
cyclobutadiene character. Two double bonds and two sin-
gle bonds are assigned to the GeC3 skeleton of 6. The
number of electrons assigned to each bond is greater than
1.9, which is high enough to confirm the presence of a
chemical bond. Namely, structure 6 can be regarded as a
germacyclobutadiene derivative.
For cis-5, the CB–CC distance is calculated to be
˚
1.653 A. This bond distance is longer than the normal C–
A
B
C
˚
C bond (ca. 1.5 A). The C –C –C –Ge dihedral angle in
cis-5 is as small as 115.6ꢁ and the plane containing the
CA–CB–CC triangle faces toward the plane containing the
six carbon atoms of the benzene ring. It is noteworthy that
re-optimization of cis-5 followed by a 90ꢁ rotation of the
benzene ring along the Ge–CD bond gave the initial struc-
ture of cis-5. The NBO analysis revealed that cis-5 has the
bonding character of a bicyclobutane diradical derivative.
As shown in Fig. 3, the valence electron of 0.78 is assigned
to the 2p orbital of CA perpendicular to the plane contain-
ing the CA–CB–CC triangle, while the germanium atom has
1.79 electrons on the lone-pair orbital (70.7% s-character
and 29.2% p-character). It seems interesting that a large
number of electrons is assigned on the lone-pair orbital
of the germanium atom, because a bicyclobutane structure
should formally have one unpaired electron on each CA
and the germanium atom. The large electron occupancy
on the lone-pair orbital of the germanium atom results in
weak r bonds between CB, CC and the germanium atom.
The number of electrons on the CB–CC, Ge–CB, and Ge–
CC bonds is calculated to be 1.79, 1.71 and 1.71, respec-
To elucidate the substituent effects on the geometry of the
GeC3 skeleton, the structures of GeC3H4 were investigated.
As shown in Fig. 4, five structures cis-7, trans-7, cis-8, trans-
8, and 9 were optimized at the B3LYP/6-311+G(d,p) level.
The trans-2-germabicyclo[1.1.0]butane-2,4-diyl structure
trans-7 is the most stable of the five structures. The cis-ger-
mabicyclobutanediyl structure cis-7 is 28.8 kJ/mol less stable
than trans-7. The germylene structures cis- and trans-8 are
32.5 and 31.0 kJ/mol higher in energy than trans-7, respec-
tively. The planar germacyclobutadiene 9 is a transition-
state structure. The result of the NBO analysis for cis-7,
trans-7, cis-8, trans-8, and 9 is shown in Fig. 5. It is interesting
that the NBO structures of cis- and trans-7 are almost iden-
tical and have a germylene character similar to trans-5. The
NBO structures of cis-8, trans-8, and 9 are similar to those
of cis-2, trans-2, and 6, respectively. Only cis-5 has an NBO
structure different from the corresponding hydrogen-substi-
tuted structure (cis-7). Another finding to be pointed out is
that cis-5 is more stable than trans-5, while the trans structure
is energetically more favorable in the hydrogen-substituted
compound. These results indicate that the tert-butyl groups

S. Tsutsui et al. / Journal of Organometallic Chemistry 691 (2006) 595–603
599
and/or p-anisyl group in 5 would strongly influence their
electronic structure and relative stability. What factors influ-
ence the electronic structure and relative stability of cis-5?
The spatial distribution of the HOMO of cis-5 would provide
a clue to answer this question.
The HOMOs of cis-5, trans-5, cis-7, and trans-7 are de-
picted in Fig. 6. For trans-5, cis-7, and trans-7, the HOMOs
are comprised of the non-bonding orbital of the germa-
nium atom and the p orbital delocalized over the CA–
CB–CC triangle in the GeC3 skeleton. In the HOMO of
cis-5, on the other hand, the p orbital of the benzene ring
interacts with the orbitals of the GeC3 skeleton. In the orbi-
tals of the GeC3 skeleton, the CA atom has a larger lobe
compared with CB and CC, implying that CA possesses
Fig. 4. Optimized structures and relative energies of isomers of GeC3H4
cis-7, trans-7, cis-8, trans-8, and 9 optimized at the B3LYP/6-311+G(d,p)
level.
Fig. 6. Side view images of the HOMOs of cis-5, trans-5, cis-7, and trans-
7. Hydrogen atoms are omitted for clarity.
1.98 (σ)
1.67 (π)
1.98 (σ)
1.67 (π)
1.98 (σ)
1.69 (π)
1.98 (σ)
1.69 (π)
1.98
1.72
1.98
1.72
1.98
1.70
1.98
1.70
1.61
Ge
1.60
Ge
1.61
Ge
1.60
Ge
1.89 (LP)
1.98 (LP)
1.96
1.89 (LP)
1.97
1.98 (LP)
1.97
1.96
H
H
H
H
trans-7
cis-7
1.97 (σ)
1.94 (π)
1.97 (σ)
1.94 (π)
1.99 (σ)
1.99 (π)
1.97
1.90
1.90
1.90
1.90
1.99
Ge
1.98
1.99
Ge
1.98
1.96 (σ)
1.92 (π)
1.93
Ge
1.95
1.99 (LP)
1.99 (LP)
H
H
H
trans-8
cis-8
9
Fig. 5. Structures of cis-7, trans-7, cis-8, trans-8, and 9 described on the basis of the NBO analysis results. The values shown in the structures are the
number of electrons assigned to each bond. LP represents a lone-pair orbital on an atom. All the tert-butyl groups and the p-anisyl group are omitted for
clarity.

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S. Tsutsui et al. / Journal of Organometallic Chemistry 691 (2006) 595–603
0.05
0.00
0.10
cis-2
671
0.05
660
680
Ge
0.00
0.05
0.00
0.10
trans-2
559
0.05
550
570
0.00
0.10
cis-5
393
0.05
0.00
0.10
trans-5
347
350
0.05
0.00
0.10
Fig. 7. Schematic diagram of the through-space interaction in the HOMO
of cis-5.
0.05
0.00
7
692
0.05
0.00
680
700
an unpaired electron as suggested by the NBO analysis. A
schematic diagram of the through-space interaction in the
HOMO of cis-5 is presented in Fig. 7. This interaction
plays an essential role in the high stability of cis-5. Of
course, the steric repulsion between the p-anisyl group
and the tert-butyl groups on CB and CC would be an
important factor in determining the relative stability of
cis- and trans-5. Also, the presence of this interaction rea-
sonably explains the optimized structure of cis-5, in which
the plane containing the CA–CB–CC triangle faces toward
the plane containing the six carbon atoms of the benzene
ring. The optimized structure of trans-5 might be governed
by the steric repulsion, because the CACD distance in trans-
250
300
400
450
500
Wavelength / nm
Fig. 8. Absorption wavelengths of cis-2, trans-2, cis-5, trans-5, and 7
calculated at the TD-B3LYP/6-311+G(d,p) level.
transition at 559 nm. The TD calculation predicts the lon-
gest-wavelength absorption of 6 at 692 nm.
3. Conclusion
We succeeded in generating the first example of a
cyclopropenylgermylene, p-anisyl(1,2,3-tri-tert-butylcyclo-
prop-2-en-1-yl)germylene (2), by reduction of the corre-
sponding dichlorogermane 1. The intermediary 2 was
trapped by tert-butyldimethylsilane to give the correspond-
ing tert-butyldimethylsilyl(hydro)germane 4. DFT calcula-
tions revealed that the cis-2-p-anisyl-1,3,4-tri-tert-butyl-
2-germabicyclo[1.1.0]butane-2,4-diyl structure cis-5 is
8.0 kJ/mol more stable than cis-2. The NBO analysis re-
vealed that cis-5 has a diradical character, although the ger-
manium atom possesses 1.78 electrons in its lone-pair
orbital. The electronic structure of the trans-germa-
bicyclobutanediyl structure trans-5 can be described by
two resonance structures, which have a germylene charac-
ter. For GeC3H4, on the other hand, the trans-germa-
bicyclobutanediyl structure trans-7 is more stable than
cis-7 and both structures have a germylene character. The
high stability of cis-5 can be rationalized by a through-
space interaction in the HOMO between the p orbital of
the benzene ring in the p-anisyl group and the orbitals of
the GeC3 ring. Introduction of the p-anisyl group on the
divalent germanium atom dramatically changes the elec-
tronic structure of the GeC3 skeleton from a germylene
character to a diradical character. The TD-DFT calcula-
tion for the five structures predicts that their longest-wave-
length absorptions strongly depend on the geometry
around the germanium atom. This structure dependence
of the absorption wavelengths would provide useful infor-
˚
˚
5 (4.257 A) is much longer than that in cis-5 (3.105 A).
In conclusion, the GeC3 skeleton tends to adopt a 2-
germabicyclo[1.1.0]butane-2,4-diyl structure. The germa-
bicyclobutanediyl structures basically have the bonding
character of a cyclopropenyl-substituted germylene. How-
ever, introducing an aryl substituent on the germanium
atom dramatically changes the relative stability and the
bonding character of the germabicyclobutanediyl structure.
2.4.2. Spectroscopic properties
We describe the spectroscopic properties of cis-2, trans-
2, cis-5, trans-5, and 6 based on the time-dependent DFT
(TD-DFT) calculation. Recently, the applicability of the
TD-DFT method has been proved for unsaturated silicon
[7,9,17] and germanium [18] compounds. To evaluate the
electron transition energies of cis-2, trans-2, cis-5, trans-5,
and 6, the TD calculations were carried out at the
B3LYP/6-311+G(d,p) level. The results of the calculations
are shown in Fig. 8. The TD calculations predict quite
different spectral patterns for the five structures. The
absorption wavelengths of cis-5 and trans-5 for the
HOMO–LUMO transition are calculated to be 393 nm
and 347 nm, respectively. In contrast, cis-2 has the
HOMO–LUMO transition at 671 nm, which is signifi-
cantly red-shifted relative to dimethylgermylene (430 nm)
[19] and diphenylgermylene (466 nm) [20]. Another germyl-
ene structure trans-2 also has a red-shifted HOMO–LUMO

S. Tsutsui et al. / Journal of Organometallic Chemistry 691 (2006) 595–603
601
mation on the structure of germylene 2 and its related com-
pounds, if the UV–vis spectral data for the compounds are
available.
4.2.2. Preparation of triphenyl(1,2,3-tri-tert-butylcycloprop-
2-en-1-yl)germane (3b)
A solution of triphenylgermyllithium [prepared by mix-
ing lithium (2.90 g, 417 mmol) and triphenylchlorogermane
(6.46 g, 19.0 mmol) in THF (200 ml) at room temperature]
was added to a mixture of tri-tert-butylcyclopropenyl tetra-
fluoroborate (4.56 g, 15.5 mmol) and THF (200 ml) at
0 ꢁC. After stirring at 0 ꢁC for 1 h, the mixture was hydro-
lyzed with water. The organic layer was separated, and the
aqueous layer was extracted with hexane. The organic layer
and the extracts were combined, washed with water and
brine, dried over magnesium sulfate, and filtered. The fil-
trate was concentrated under reduced pressure, and the res-
idue was chromatographed on silica gel using hexane as the
eluent. The eluate was concentrated under reduced pressure
to give 3b (6.72 g, 13.1 mmol, 85%). 3b: Colorless crystals;
m.p. 262–263 ꢁC; 1H NMR (CDCl3, d) 0.95 (s, 9H), 1.06 (s,
18H), 7.25–7.3 (m, 9H), 7.5–7.6 (m, 6H); 13C NMR
(CDCl3, d) 30.63 (CH3), 31.39 (C), 31.48 (CH3), 38.58
(C), 42.24 (C), 127.23 (C), 127.48 (CH), 128.00 (CH),
136.67 (CH), 140.57 (C); MS (70 eV) m/z (%) 455
(M+ ꢀ 57, 2), 207 (100). Anal. Calc. for C32H42Ge: C,
77.51; H, 8.28%. Found: C, 77.12; H, 8.34%.
4. Experimental
4.1. General methods
The 1H, 13C, and 29Si NMR spectra were recorded using
a Varian INOVA 300 FT-NMR spectrometer at 300, 75.4,
and 59.6 MHz, respectively. The mass spectra were re-
corded using Shimadzu GCMS-QP5050A and Hitachi M-
2500 mass spectrometers. Gel permeation chromatography
(GPC) was conducted using a LC908 recycling high-perfor-
mance liquid chromatography (Japan Analytical Instru-
ments Co., Ltd.) using JAIGEL-1H (20 mm · 600 mm)
and JAIGEL-2H (20 mm · 600 mm) columns, using tolu-
ene as the eluent.
4.2. Materials
Toluene, tert-BuLi, triphenylchlorogermane, lithium,
HCl gas, aluminum chloride, potassium, trimethylchloros-
ilane, CHCl3, and CDCl3 were commercially available
and used as supplied. THF, hexane, C6D6 and tert-buty-
ldimethylsilane were freshly distilled over potassium.
Tri-tert-butylcyclopropenium tetrafluoroborate [21], tri-p-
anisylgermane [22], and tris[o-(dimethylamino)phenyl]ger-
mane [23] were prepared according to the reported
procedures.
4.2.3. Preparation of tris[o-(dimethylamino)phenyl](1,2,3-
tri-tert-butylcycloprop-2-en-1-yl)germane (3c)
A pentane solution of tert-BuLi (1.45 N, 5.8 ml,
8.41 mmol) was added to a THF (30 ml) solution of tri-
s[(o-dimethylamino)phenyl]germane (3.00 g, 6.91 mmol) at
ꢀ40 ꢁC. After stirring at ꢀ40 ꢁC for 3 h, tri-tert-butylcyc-
lopropenyl tetrafluoroborate (2.56 g, 8.70 mmol) was
added to the mixture at ꢀ40 ꢁC. After stirring overnight
at room temperature, the solvents were concentrated under
reduced pressure. Dry hexane was added, and the resulting
salt was removed by filtration. The filtrate was concen-
trated under reduced pressure (200 ꢁC/0.05 mmHg), and
3c (2.50 g, 3.90 mmol, 56%) was obtained as colorless solid.
4.2.1. Preparation of tri-p-anisyl(1,2,3-tri-tert-butylcyclo-
prop-2-en-1-yl)germane (3a)
A solution of tri-p-anisylgermyllithium [prepared by mix-
ing tert-BuLi (1.45 N, 7.7 ml, 11.2 mmol) and tri-p-anisyl-
germane (4.24 g, 10.7 mmol) in THF (100 ml) at ꢀ40 ꢁC]
was added to a mixture of tri-tert-butylcyclopropenyl tetra-
fluoroborate (3.13 g, 10.6 mmol) and THF (200 ml) at
ꢀ40 ꢁC. After stirring at ꢀ40 ꢁC for 2 h and at room temper-
ature for 21 h, the mixture was hydrolyzed with water. The
organic layer was separated, and the aqueous layer was ex-
tracted with hexane. The organic layer and the extracts were
combined, washed with water and brine, dried over magne-
sium sulfate, and filtered. The filtrate was concentrated
under reduced pressure, and the residue was chromato-
graphed on silica gel using toluene as the eluent. The eluate
was concentrated under reduced pressure to give 3a
(4.89 g, 8.13 mmol, 77%). 3a: Colorless crystals; m.p.
1
3c: m.p. 216–219 ꢁC; H NMR (C6D6, d) 1.166 (s, 9H),
1.169 (s, 18H), 2.18 (s, 18H), 6.98–7.20 (m, 9H), 8.28–
8.31 (m, 3H); 13C NMR (C6D6, d) 31.40 (CH3), 32.12
(C), 33.11 (CH3), 40.48 (C), 46.03 (CH3), 47.56 (C),
121.14 (CH), 122.40 (CH), 128.95 (CH), 129.30 (C),
141.04 (CH), 141.76 (C), 159.12 (C); MS (30 eV) 640
(M+). Anal. Calc. for C39H57GeN3: C, 73.14; H, 8.97; N,
6.56. Found: C, 73.35; H, 9.00; N, 6.59%.
4.2.4. Preparation of p-anisyl(1,2,3-tri-tert-butylcycloprop-
2-en-1-yl)dichlorogermane (1)
Dry HCl gas was bubbled through a mixture of 3a
(1.57 g, 2.60 mmol), aluminum chloride (0.126 g, 0.948
mmol), and dry toluene (30 ml) for 2.5 h. After adding ace-
tone (0.05 ml, 1.09 mmol), the resulting mixture was con-
centrated under reduced pressure, followed by the
addition of hexane and decantation. The mixture was con-
centrated under reduced pressure. Bulb-to-bulb distillation
[140–180 ꢁC/0.05 mmHg (bath temp.)] gave 1 (750 mg,
1.93 mmol, 74%). 1: Colorless crystals; m.p. 98.2–99.6 ꢁC;
1
178.2–179.8 ꢁC; H NMR (CDCl3, d) 0.93 (s, 9H), 1.06 (s,
18H), 3.78 (s, 9H), 6.83 (d, J = 8 Hz, 6H), 7.43 (d,
J = 8 Hz, 6H); 13C NMR (CDCl3, d) 30.69 (CH3), 31.38
(C), 31.43 (CH3), 38.40 (C), 41.87 (C), 54.93 (CH3), 113.14
(CH), 127.07 (C), 131.81 (C), 137.77 (CH), 159.46 (C); MS
(70 eV) m/z (%) 545 (M+ ꢀ 56, 1) 207 (100). Anal. Calc.
for C36H48GeO3: C, 71.90; H, 8.04%. Found: C, 71.72; H,
8.12%.

602
S. Tsutsui et al. / Journal of Organometallic Chemistry 691 (2006) 595–603
1H NMR (CDCl3, d) 0.93 (s, 9H), 1.33 (s, 18H), 3.82 (s,
3H), 6.96 (d, J = 8.7 Hz, 2H), 7.71 (d, J = 8.7 Hz, 2H);
13C NMR (CDCl3, d) 30.33 (CH3), 30.78 (CH3), 31.40
(C), 37.90 (C, overlapping), 55.19 (CH3), 114.23 (CH),
125.60 (C), 132.58 (C), 134.22 (CH), 161.27 (C). Anal.
Calc. for C22H34Cl2GeO: C, 57.69; H, 7.48%. Found: C,
58.08; H, 7.56%.
(c) K. Komatsu, Z. Yoshida, in: A. de Meijere (Ed.), Methods of
Organic Chemistry (Houben-Weyl), vol. E 17d, Thieme, Stuttgart,
1996, pp. 3079–3192;
(d) W. Billups, in: S. Patai, A. Rappoport (Eds.), The Chemistry of
the Cyclopropyl Group, Wiley Interscience, New York, 1987;
(e) J.J. Gajewski, Hydrocarbon Thermal Isomerization, Academic
Press, New York, 1981.
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1146;
(b) H. Hopf, H. Priebe, R. Walsh, J. Am. Chem. Soc. 102 (1980)
1210;
(c) H. Hopf, G. Wachholz, R. Walsh, Chem. Ber. 118 (1985) 3579;
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4.2.5. Reduction of 1a with potassium in the presence of tert-
butyldimethylsilane
A mixture of 1 (51.3 mg, 1.1 · 10ꢀ4 mol), potassium
(47.6 mg, 1.2 mmol), tert-butyldimethylsilane (379 mg,
3.3 mmol), and benzene (10 ml) was stirred for 6 h under
reflux. After removing potassium by filtration, the filtrate
was concentrated under reduced pressure. After adding
hexane, the resulting salt was removed by filtration. After
evaporating the solvent, separation of the residue using
recycling GPC (toluene as an eluent) gave p-anisyl(tert-
butyldimethylsilyl)(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)
germane (4, 8.4 mg, 1.7 · 10ꢀ5 mol, 15%). 4: Colorless oil;
1H NMR (C6D6, d) 0.34 (s, 3H), 0.45 (s, 3H), 0.92 (s,
9H), 1.11 (s, 9H), 1.13 (s, 9H), 1.19 (s, 9H), 3.26 (s,
3H), 4.25 (s, 1H), 6.82 (d, J = 8.4 Hz, 2H), 7.62 (d,
J = 8.4 Hz, 2H); 13C NMR (C6D6,d) ꢀ3.59 (CH3),
ꢀ2.62 (CH3), 19.27 (C), 27.51 (CH3), 30.80 (CH3), 30.93
(CH3), 31.23 (C), 31.71 (CH3), 31.83 (C), 37.86 (C),
44.12 (C), 54.32 (CH3), 113.98 (CH), 130.12 (C), 130.57
(C), 133.63 (C), 138.44 (CH), 159.93 (C); 29Si NMR
(C6D6,d) 0.1; MS (40 eV) m/z (%) 504 (M+, 0.2), 447
(7), 341 (7), 207 (100), 151 (10). Anal. Calc. for
C28H50GeOSi: C, 66.80; H, 10.01%. Found: C, 66.47; H,
9.60%.
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All calculations were carried out using the GAUSSIAN 98
program package [10,11]. The structures of 2, 5, and 6 were
optimized using the B3LYP hybrid functional [24] with the
6-31G(d) basis sets. The 6-311+G(d,p) basis sets were used
for 7–9. For all the compounds, the relative energies were
calculated at the B3LYP/6-311+G(d,p), including the
zero-point energy correction obtained at the B3LYP/6-
31G(d) level. The NBO analysis was performed at the
B3LYP/6-311+G(d,p) level.
(c) T. Veszpremi, M. Takahashi, B. Hajgato, J. Ogasawara, K.
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Chem. Int. Ed. 43 (2004) 4610;
Appendix A. Supplementary data
(b) S. Tsutsui, E. Kwon, H. Tanaka, S. Matsumoto, K. Sakamoto,
Organometallics 24 (2005) 4629.
[8] S. Tsutsui, H. Tanaka, E. Kwon, S. Matsumoto, K. Sakamoto,
Organometallics 23 (2004) 5659.
[9] H. Tanaka, E. Kwon, S. Tsutsui, S. Matsumoto, K. Sakamoto, Eur.
J. Inorg. Chem. (2005) 1235.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.
2005.09.041.
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