Substituent effects in linear organogermanium catenates

Article abstract of DOI:10.1021/om800240w

The trigermanes EtOCH₂CH₂Ge(R₂)Ge(Ph ₂)Ge(R₂)CH₂CH₂OEt (R = Et, Bu, Ph) and Ph₃GeGe(Bu₂)-Ge(Ph₂)CH₂CH ₂OEt as well as the two tetragermanes Ph₃GeGe(Bu ₂)Ge(Ph₂)Ge(R₂)CH₂CH₂OEt (R = Et, Bu) have been prepared and characterized. The absorption and electrochemical attributes of these species, along with the butylated oligogermane series Ph₃Ge(GeBu₂)_nCH ₂CH₂OEt (n = 1-3) and the digermanes Ph ₃GeGeR₃ (R = Et, Bu, Pr', Ph), have been investigated using UV/visible spectroscopy and cyclic voltammetry. In general, the position of the absorption maximum shifts to lower energy and the oxidation potential decreases with increasing chain length. Variation of the organic substituents at germanium was also found to have a measurable effect on these spectral and electrochemical features. The experimental results were correlated with the energies of the HOMO and LUMO in these molecules, which were determined by density functional theory (DFT) calculations.

Full text of DOI:10.1021/om800240w

Organometallics 2008, 27, 3753–3760
3753
Substituent Effects in Linear Organogermanium Catenates
Monika L. Amadoruge,† James R. Gardinier,*,‡ and Charles S. Weinert*,†
Departments of Chemistry, Oklahoma State UniVersity, Stillwater, Oklahoma 74078, and Marquette
UniVersity, Milwaukee, Wisconsin 53201-1881
ReceiVed March 14, 2008
The trigermanes EtOCH2CH2Ge(R2)Ge(Ph2)Ge(R2)CH2CH2OEt (R ) Et, Bu, Ph) and Ph3GeGe(Bu2)-
Ge(Ph2)CH2CH2OEt as well as the two tetragermanes Ph3GeGe(Bu2)Ge(Ph2)Ge(R2)CH2CH2OEt (R )
Et, Bu) have been prepared and characterized. The absorption and electrochemical attributes of these
species, along with the butylated oligogermane series Ph3Ge(GeBu2)nCH2CH2OEt (n ) 1-3) and the
digermanes Ph3GeGeR3 (R ) Et, Bu, Pri, Ph), have been investigated using UV/visible spectroscopy and
cyclic voltammetry. In general, the position of the absorption maximum shifts to lower energy and the
oxidation potential decreases with increasing chain length. Variation of the organic substituents at
germanium was also found to have a measurable effect on these spectral and electrochemical features.
The experimental results were correlated with the energies of the HOMO and LUMO in these molecules,
which were determined by density functional theory (DFT) calculations.
metallic complexes.14–17 The possibility of using catenated
compounds of the heavier group 14 elements has also been
investigated, and wires based on arrays of Si, Ge, or Sn centers
might be expected to have interesting properties and could be
used as electronic models for enhancing the understanding of
one-dimensional semiconducting nanowires of these elements.
As has been addressed with silicon18–28 and tin29–46 oligomers
and polymers, as well as in some sporadic reports on the related
germanium congeners,47–52 the optical and electronic properties
of these compounds are intimately related to their structure. The
Introduction
The development of molecular wires having tunable properties
and size is of significant interest in the areas of molecular
electronics and nanotechnology.1–4 Most attention has centered
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of electronic communication.5–8 A variety of transition-metal-
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from discrete self-assembled moieties and complexes connected
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* To whom correspondence should be addressed. E-mail: weinert@
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† Oklahoma State University.
‡ Marquette University.
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3754 Organometallics, Vol. 27, No. 15, 2008
Amadoruge et al.
Scheme 1
electronic properties of linear chains of R2E units (E ) Si, Ge,
Sn) have been suggested to arise from σ conjugation of sp3
hybrid orbitals.53–55 Therefore, one can “coarse-tune” the
electronic properties of these molecules by changing the number
of bonded group 14 atoms in the backbone of the molecule.
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perethylated germanes Et(GeEt2)nEt (n ) 2-6) undergoes a red
shift with increasing chain length, varying from 202 nm for the
digermane to 258 nm for the hexagermane.56 Similarly, in the
series Me(GeMe2)nMe (n ) 2-6) the absorption maximum
varies from 194 nm for the dimer (n ) 2) to 255 nm for the
hexamer (n ) 6), and the oxidation potential for the series was
also shown to decrease with increasing Ge-Ge chain length,
from 1.28 V for the dimer to 0.53 V for the hexamer.47 For the
same series of permethylated oligomers the ionization potential
was also shown to decrease in energy with increasing chain
length.48
yet been fully established to give full control over all possible
substitution patterns at the germanium atoms. Furthermore, the
impact of the organic supporting ligands on the electronic
properties of these catenates has not yet been fully addressed.
For instance, it is highly desirable to understand how the
variation of the organic substituents affects the fine-tuning of
the electronic and electrochemical behavior properties of these
systems.
We have recently demonstrated that the hydrogermolysis
reaction involving an R-germyl nitrile and a germanium hydride
(eq 1), where the R3GeCH2CN species is generated in situ from
the amide R3GeNMe2 and CH3CN, is a versatile synthetic route
to oligogermanes which allows control over both the length of
the Ge-Ge backbone and the peripheral organic substituents.50–52
This method can be used for the synthesis of a wide variety of
oligogermanes in good to excellent yields, thus permitting a
detailed survey of their properties. We now wish to expound
on our previous findings concerning the synthesis of these
systems and describe our findings on the impact of the variation
of the composition of these molecules on their optical and
electronic attributes by considering a combination of experi-
mental data and density functional calculations.
Several issues have hindered widespread investigation into
these compounds as viable candidates for molecular wires,
particularly in the case of the germanium-containing species.
Although several methods have been reported for the preparation
of oligogermanes,56–77 a viable synthetic methodology has not
R3GeNMe2 CH CN, 85°C8 R3GeCH2CN R′ GeH, CH CN, 85°C8
3
3
3
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Linear Organogermanium Catenates
Organometallics, Vol. 27, No. 15, 2008 3755
Scheme 2
as the germanium amides react with this solvent to furnish
R-germyl nitriles which are the active species in hydrogermoly-
sis reactions and react with the germanium hydrides to generate
Ge-Ge bonds.50–52 The R-germyl nitriles have been described
to contain a labile Ge-C bond,78,79 although this assumption
was based on the reactivity of Et3GeCH2CN.78 We have recently
demonstrated that the lability of the Ge-C bond in these species
depends on the steric and electronic attributes of the other
organic substituents bound to germanium.51
intermediate hydrides generated by the reactions of the oligo-
mers with diisobutylaluminum hydride (DIBAL-H) were neither
isolated nor purified but, rather, were treated in crude form with
the germanium amides in CH3CN solution in order to provide
the desired products 5 and 6 in high yields. All Ge-Ge bond
forming reactions proceed by initial conversion of the germa-
nium amides to R-germyl nitriles by reaction with the CH3CN
solvent.
The reaction of Ph3GeGePh2CH2CH2OEt with DIBAL-H was
previously found to be unsuccessful,50 but the ethoxyethyl group
of 5 can readily be cleaved by DIBAL-H to furnish the
intermediate trigermane hydride 7, which was subsequently
converted to the tetragermanes 6a,b. The difference in reactivity
of Ph3GeGePh2CH2CH2OEt and 5 therefore appears to be
electronic rather than steric in nature, since compound 5 contains
electron-donating n-butyl groups located between the two
phenyl-substituted Ge centers. The oligogermanes prepared in
these investigations have been characterized by NMR (1H and
13C) spectroscopy, elemental (C, H) analyses, cyclic voltam-
metry, and UV-visible spectroscopy. All of these materials are
soluble in typical organic solvents and are viscous oils which
are air- and moisture-sensitive.
Unlike most other preparative routes that have been previ-
ously described, our synthetic method allows for the stepwise
preparation of oligogermanes, where the organic substituents
attached to the germanium center can be systematically varied.
We have used this method to prepare the three trigermanes 1a-c
in excellent yields starting from Ph2GeH2 and the three synthons
R2Ge(NMe2)CH2CH2OEt (R ) Et, Bun, Ph)50 (Scheme 1). The
ethyl-substituted derivative 1a is volatile, and care must be taken
when distilling the crude product under vacuum to remove
excess Ph2GeH2. The synthesis of the digermane 2 was
previously reported, and compound 2 was employed for the
preparation of the trigermane 3 and the tetragermane 4 in good
to excellent yields (Scheme 2).50 The trigermane 5 was also
prepared from 2, and compound 5 was subsequently used for
the preparation of the two tetragermanes 6a,b (Scheme 2). The
Computational Studies. In order to facilitate the discussion
of the electronic properties of the oligogermanes, it is useful to
examine the results of computational studies performed on a
comprehensive set of 51 derivatives ranging from digermanes
to octagermanes that are either known or hypothetical com-
pounds (see the Supporting Information). In these studies, all
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3756 Organometallics, Vol. 27, No. 15, 2008
Amadoruge et al.
where the germanium component is constructed from a mixture
of mainly (68%) two out-of-phase 4s orbitals mixed with a
significant contribution (32%) of two in-phase Ge 4pz orbitals
(top left, Table 1).
For the higher oligogermanes of the form GenH2n+2, the
HOMO is germanium-based and is comprised mainly (85-91%)
of a molecular orbital composed of out-of-phase 4px (see Table
1 for coordinate system) atomic orbitals. These are mixed with
5-12% of a molecular orbital comprised of partly out-of-phase
in-plane 4pz atomic orbitals (see Table 1 for coordinate system)
that have one node in the yz plane and also with 2-5% of a
molecular orbital containing n 4s atomic orbitals that are partly
“out of phase” with n - 2 nodes. The LUMO of the higher
oligogermanes, GenH2n+2, is also a mixture of three components.
In this case the major component (ca. 65%) is a molecular orbital
comprised of out-of-phase 4s atomic orbitals mixed with
22-30% of a molecular orbital constructed form in-phase 4pz
atomic orbitals. The smallest component (6-13%) of the LUMO
is a set of partly out-of-phase 4px orbitals (x directed along the
internuclear axis) that has n - 1 nodes. To a first approximation,
with the smallest component of mixing being ignored, the
HOMO is essentially out-of-phase 4px in character while the
LUMO represents an in-phase combination of sp hybrid orbitals.
Figure 1. Relative energies (eV) of frontier orbitals for
R3Ge(GeR2)nGeR3 (n ) 0-6): (red lines) R ) H; (blue lines) R )
Me. The orbital plots for n ) 0, 2, 6 are also shown.
compounds were subject to semiempirical quantum mechanical
PM3-geometry optimization prior to single-point density func-
tional calculations (DFT) using the B3LYP/6-31G* basis set.
Higher level calculations on several derivatives were also
explored (ab initio HF/3-21G geometry optimization followed
by DFT B3LYP/6-311G**), which gave identical trends but
were significantly more computationally expensive (in some
cases, prohibitively). Therefore, only the results of the smaller,
but complete, set using the B3LYP/6-31G* basis will be
addressed, as these are also in qualitative agreement with the
experimental absorbance and voltammetry investigations. Se-
lected frontier orbitals (HOMO and LUMO) are shown for some
parent oligogermanes in Figure 1, along with the calculated
energies of the orbitals for R3Ge(GeR2)nGeR3 (n ) 0-6; R )
H, Me) derivatives.
The relative energies of the HOMO and LUMO vary in the
expected manner according to chain length, the Ge-Ge
distances which are guided by the steric bulk of substituents
and by the inductive effects of peripheral groups bound to the
oligogermane core. Thus, the HOMO energy increases (becomes
destabilized) as the proportion of electron-rich R2GeII centers
increases relative to the terminal R3GeIII centers. Electron-
donating groups bound to germanium destabilize the HOMO
by making the chain more electron rich, as exemplified by
comparing the relative energies of the HOMO in
R3Ge(GeR2)nGeR3 (n ) 0-6; R ) H versus R ) Me) in Figure
1. Similarly, the energy of the HOMO increases along the series
Me < Et < Prn < Bun due to the inductive effects of replacing
C-H with C-alkyl groups.
With increasing chain length, the LUMO becomes stabilized
via conjugation, as expected from the σ* character. As found
for the HOMO, the substitution of electron-donating substituents
destabilizes the LUMO via inductive effects, as indicated by a
comparison of the CH3 versus H groups in Figure 1. It is also
noteworthy that for the new compounds described here replacing
a germanium-alkyl group with the CH2CH2OEt group has only
a very small stabilizing effect on the energies of the frontier
orbitals, an exception being the replacement of the CH3 group
with the CH2CH2OEt group, where the inductive (destabilizing)
effects become important.
With the exceptions noted below for aryl derivatives, the main
features of the corresponding frontier orbitals for the substituted
oligogermanes mirror those for the simple H3Ge(GeR2)nGeH3
(n ) 0-6) series in that the HOMO is σ bonding while the
LUMO is σ* antibonding in nature. As will be elaborated on,
the relatively low symmetry of the oligogermanes (giving rise
to a large number of molecular orbitals of the same symmetry)
combined with the close energy separation of the valence 4s
and 4p orbitals on germanium results in extensive mixing. Of
the compounds studied, the homoleptic digermanes Ge2R6 can
be differentiated from the higher oligomers by symmetry
considerations. For instance, the Ge2R6 series has idealized D3d
symmetry which renders one 4p orbital on each germanium atom
(pz, along the internuclear C3 axis) available for σ-bonding
interactions and a pair of degenerate 4p orbitals per germanium
(px,y orthogonal to the C3 axis) available for π-bonding interac-
tions. In contrast, for the homoleptic R3Ge(GeR2)nGeR3 (n )
1-6) series, the highest symmetry is C2V for odd values of n
and C2h for even values of n. In this manner, only one 4p orbital
which is orthogonal to the plane of the molecule participates in
π-bonding interactions, while the other two 4p orbitals partici-
pate in σ-bonding interactions (Vide infra).
Phenyl substitution has a significant impact on the frontier
orbitals of oligogermanes, since the phenyl group both is a better
σ donor than either methyl groups or hydrogens and is
sufficiently bulky to increase the Ge-Ge bond distance.
Therefore, this substitution is expected to significantly raise the
energy of the HOMO; however, conjugation with the phenyl
group orbitals partially offsets the expected destabilization.
Furthermore, the LUMO and next-higher virtual orbitals of the
aryl-substituted oligogermanes, which consist of two group
orbitals per phenyl ring, are almost exclusively composed of
linear combinations of phenyl-based π* orbitals rather than
being germanium-based σ*, as these are in-phase sp hybrid
orbitals which are higher in energy. Thus, the variation in
LUMO energy is very small throughout the series of aryl-
substituted oligogermanes.
The main contributions to the frontier orbitals (HOMO and
LUMO) for
a representative series of oligogermanes
H3Ge(GeR2)nGeH3 (n ) 0-3) are summarized in Table 1. The
HOMO of Ge2H6 is a mainly germanium-based σ orbital with
only 10% bonding contribution from an A1g hydrogen group
orbital, where the germanium component is comprised mainly
(96%) of two out-of-phase Ge 4pz orbitals mixed with a small
amount (4%) of two in-phase Ge 4s orbitals (bottom left, Table
1). The LUMO of Ge2H6 is a germanium-based σ* orbital (only
1% bonding contribution from the A2u hydrogen group orbital)

Linear Organogermanium Catenates
Organometallics, Vol. 27, No. 15, 2008 3757
Table 1. Summary of LUMO and HOMO Composition from DFT Calculations
a Nonstandard coordinate system employed for simplicity (x, y normally in molecular plane).
As expected, the HOMO-LUMO energy gap in oligoger-
manes can be coarsely tuned by varying the degree of catenation,
with longer chains giving rise to smaller energy gaps. Changing
the nature of substituents bound to the oligogermane core
changes the relative energy of the HOMO to a greater extent
than the energy of the LUMO and therefore provides a simple
means for fine-tuning the HOMO-LUMO energy gap of these
compounds. Both of these conclusions were also observed
experimentally, as described below.
Absorption and Electrochemical Characteristics. Cyclic
voltammograms for the various oligogermanes were obtained
in CH3CN solution using 1.0 M [Bu4N][PF6] as the supporting
electrolyte. Irreversible oxidation waves were observed in all
cases, as exemplified for the Ph3Ge(GeBu2)nGeBu2CH2CH2OEt
(2, n ) 0; 3, n ) 1; 4, n ) 2) series shown in Figure 2. The
reported values for the oxidations found in Table 2 are for the
anodic waves, as the expected cathodic return waves were
absent, and are average values of four independent measure-
ments which were generally reproducible with errors of less
than (30 mV. The irreversibility of the oxidation waves is in
Figure 2. Cyclic voltammograms for CH3CN solutions of
Ph3Ge(GeBu2)nGeBu2CH2CH2OEt obtained at 150 mV/s using
(nBu4N)(PF6) as the supporting electrolyte: (black line) n ) 0 (2);
(red line) n ) 1 (3); (blue line) n ) 2 (4).
accord with previous findings of electrochemical measurements
on permethyloligogermanes.47,49 Chain contraction of oligoger-
manes has also been reported to occur via germylene extrusion

3758 Organometallics, Vol. 27, No. 15, 2008
Amadoruge et al.
Table 2. Absorption, Electrochemical Data, and Calculated HOMO/
LUMO Energy Levels (B3LYP/6-31G*) for Oligogermanes 1-6 and
8 (R ) CH2CH2OEt)
compd
formula
λmax (nm) Eox (mV) HOMO LUMO
1a
1b
1c
2
3
4
REt2GeGePh2GeEt2R
RBu2GeGePh2GeBu2R
RPh2GeGePh2GePh2R
Ph3GeGeBu2R
243
243
247
224
232
245
232
248
248
240
235
231
232
1577 ( 22 -5.23 -0.19
1500 ( 18 -5.18 -0.10
1609 ( 24 -5.49 -0.52
1590 ( 19 -5.45 -0.36
1546 ( 16 -5.41 -0.37
1474 ( 21 -5.20 -0.38
1525 ( 26 -5.43 -0.35
1483 ( 17 -5.22 -0.38
1462 ( 19 -5.19 -0.38
1576 ( 13 -5.45 -0.66
1635 ( 12 -5.56 -0.30
1587 ( 17 -5.46 -0.35
1588 ( 11 -5.38 -0.34
Ph3Ge(GeBu2)2R
Ph3Ge(GeBu2)3R
Ph3GeGeBu2GePh2R
Ph3GeGeBu2GePh2GeEt2R
Ph3GeGeBu2GePh2GeBu2R
Ph3GeGePh3
5
6a
6b
8a
8b
8c
8d
Pri3GeGePh3
Et3GeGePh3
Bu3GeGePh3
and heterolytic Ge-Ge bond cleavage,80 and similar reactions
may be responsible for the irreversible processes in the
compounds described here. Regardless, the relative oxidation
potentials of the series of oligogermanes measured in these
studies parallel the results found from the DFT calculations, in
that the oxidation potential decreases with an increasing
proportion of R2Ge centers along the oligogermane backbone.
Thus, the oxidation potentials of the series 2-4 decrease on
traversing from the digermane (1589 mV) to the trigermane
(1546 mV) and to the tetragermane (1474 mV).
Figure 3. UV/visible spectra in CH3CN solution: (black line)
EtOCH2CH2Ph2GeGePh2GePh2CH2CH2OEt (1c); (red line) EtOCH2-
CH2Et2GeGePh2GeEt2CH2CH2OEt (1b); (blue line) EtOCH2CH2-
Bu2GeGePh2GeBu2CH2CH2OEt (1a).
Several other trends in the oxidation potentials of these
systems are noteworthy. First, the observed oxidation potentials
of the trigermanes 3 (1546 mV) and 5 (1525 mV) agree with
the results predicted from the DFT calculations, that the presence
of a phenyl substituent raises the relative energy of the HOMO
compared to the presence of an alkyl substituent. Additionally,
for the two tetragermanes 6a,b, there is a small decrease in the
oxidation potential with increasing inductive effects on exchang-
ing ethyl substituents in 6a (1483 mV) with the butyl groups
of 6b (1462 mV), which was also expected on the basis of the
DFT calculations. Finally, for the four digermanes R3GeGePh3
investigated in this study, the oxidation potentials of 8a (R )
Ph), 8c (R ) Et), and 8d (R ) Bu) are all lower than that of 8b
(R ) Pri). This result also agrees with the DFT calculations in
that 8b has the lowest lying HOMO in the series, which is
presumably a steric consideration. Compound 8b was calculated
to have the longest Ge-Ge distance among the four digermanes
(see the Supporting Information). This has also been observed
experimentally, as the Ge-Ge distance in 8b is 2.4637(7) Å51
versus those for 8a (2.437(2) Å),67 8c (2.4253(7) Å),50 and 8d
(2.4212(8) Å, average of two independent molecules).50
Absorption data for oligogermanes 1-6 and 8 are collected
in Table 2, and UV/visible spectra for the three related series
1a-c, 2-4, and 8a-d are shown in Figures 3–5, respectively.
The absorption bands for the digermanes 8a-d and the butylated
series 3-5 are broad, and the absorbance maxima (λmax) range
from 221 to 245 nm. As expected on the basis of similar studies
conducted for a series of permethylated47 and perethylated56
germanium oligomers as well as a related group of butylated
tin species,30 the position of the absorbance maximum among
the oligomers 3-5 undergoes a red shift with increasing chain
length. These findings agree with previous observations on
related systems and with the magnitude of the HOMO/LUMO
gap calculated by DFT (Vide supra). The relative position of
the LUMO remains approximately the same among the three
molecules, but increasing the number of germanium atoms in
the chain results in an overall destabilization of the energy of
Figure 4. UV/visible spectra in CH3CN solution: (black line)
Ph3GeGeBu2CH2CH2OEt (2); (red line) Ph3Ge(GeBu2)2CH2CH2OEt
(3); (blue line) Ph3Ge(GeBu2)3CH2CH2OEt (4).
the HOMO, thus shifting the energy of the electronic transition
to lower energy.
For the series of digermanes R3GeGePh3, the absorbance
maximum of the phenyl-substituted derivative 8a (R ) Ph) is
significantly red-shifted relative to the alkyl-substituted species
8b-d. This trend parallels the results found from DFT calcula-
tions (see the Supporting Information) and can be attributed to
the lower energy and greater number of low-lying virtual orbitals
from the phenyl group substituents in 8a relative to the alkyl-
substituted derivatives 8b-d. The three compounds 8b-d all
have similar absorption characteristics, as predicted from the
DFT calculations, where the energetic differences between the
frontier orbitals on traversing the series of these three com-
pounds is negligible. Likewise, the series of trigermanes 1a-c
all have approximately the same HOMO/LUMO separation and
their λmax values fall into the narrow range of 243-247 nm.
However, the absorbance bands 1a-c are all significantly
broader and tail off into the visible region when compared to
those of compounds 1, 4-6, and 8. This results in the
trigermanes 1a-c being slightly pale yellow while the remaining
nine species are colorless.
(80) Mochida, K.; Chiba, H.; Okano, M. Chem. Lett. 1991, 109–112.

Linear Organogermanium Catenates
Organometallics, Vol. 27, No. 15, 2008 3759
glovebox techniques.81 The compounds R2Ge(NMe2)CH2CH2OEt
(R ) Et, Bu, Ph),50 2-4,50 8a,51 8b,51 8c,50 and 8d50 were prepared
by literature methods. The reagents DIBAL-H (1.0 M in hexanes)
(Aldrich) and Ph2GeH2 (Gelest) were purchased from commercial
sources and used without further purification. Solvents were purified
using a Glass Contour Solvent Purification System. NMR spectra
were recorded using a Varian Gemini 2000 spectrometer operating
at 300.0 MHz (1H) or 75.5 MHz (13C) and were referenced to the
C6D6 solvent. Cyclic voltammograms were obtained using a
Bioanalytical Systems Epsilon Electrochemical Workstation with
a glassy-carbon-disk working electrode, a platinum-wire counter
electrode, and a Ag/AgCl reference electrode using 1.0 M
[Bu4N]PF6 in CH3CN as the supporting electrolyte. UV/visible
spectra were obtained using a Hewlett-Packard Agilent UV/visible
spectroscopy system. Elemental analyses were conducted by Desert
Analytics or Midwest Microlabs.
Preparation of EtOCH2CH2(GeEt2)(GePh2)(GeEt2)CH2CH2-
OEt (1a). To a solution of Et2Ge(NMe2)CH2CH2OEt50 (0.535 g,
2.16 mmol) in acetonitrile (20 mL) in a Schlenk tube was added
Ph2GeH2 (0.250 g, 1.09 mmol) in acetonitrile (10 mL). The tube
was sealed and heated in an oil bath at 85 °C for 48 h, after which
time the volatiles were removed in Vacuo. Residual Ph2GeH2 was
removed by Kugelrohr distillation (110 °C, 0.05 Torr) to yield 0.498
g (72%) of 1a as a thick colorless liquid. 1H NMR (C6D6, 25 °C):
δ 7.68 (d, J ) 6.3 Hz, 4 H, m-H), 7.23-7.13 (m, 6 H, p- and o-H),
3.49 (t, J ) 7.8 Hz, 4 H, GeCH2CH2O-), 3.24 (q, J ) 7.2 Hz, 4
H, -OCH2CH3), 1.54 (t, J ) 7.8 Hz, 4 H, GeCH2CH2O-),
1.15-1.06 ppm (m, 26 H, aliphatics). 13C NMR (C6D6, 25 °C): δ
140.2 (ipso-C), 135.9 (o-C), 128.4 (m-C), 128.0 (p-C), 68.7
(-OCH2CH3), 65.6 (-GeCH2CH2O-), 15.8 (-OCH2CH3), 15.5
(Ge(CH2CH3)2), 10.2 (GeCH2CH2O-), 7.4 ppm (Ge(CH2CH3)2).
UV/visible: λmax 243 nm (v br, ꢀ ) 2.05 × 104 cm-1 M-1). Anal.
Calcd for C28H48Ge3O2: C, 53.01; H, 7.63. Found: C, 52.93; H,
7.25.
Figure 5. UV/visible spectra for in CH3CN solution: (black line)
Ph3GeGePh3 (8a); (red line) Pri3GeGePh3 (8b); (blue line)
Et3GeGePh3 (8c); (purple line) Bu3GeGePh3 (8d).
Conclusions
The synthesis of oligogermanes with controllable nuclearity
and organic substitution patterns has been described. Examina-
tion of the experimental electronic properties and the results of
density functional calculations reveal that the HOMO in each
of these molecules is a σ orbital resulting mainly from the out-
of-phase linear combination of p orbitals on germanium, but
with a small contribution from mixing of the 4s and the
orthogonal 4p orbitals. For oligogermanes without phenyl
substituents, the LUMO is σ* in nature but is extensively mixed,
mainly between the out-of-phase linear combination of 4s
orbitals and the in-phase linear combination of 4pz orbitals. The
net result is that the LUMO can be adequately described as
being due to an in-phase but spatially inverted linear combina-
tion of sp hybrid orbitals.
Preparation of EtOCH2CH2(GeBu2)(GePh2)(GeBu2)CH2CH2-
OEt (1b). To a solution of Bu2Ge(NMe2)CH2CH2OEt50 (1.505 g,
4.950 mmol) in acetonitrile (25 mL) in a Schlenk tube was added
Ph2GeH2 (0.569 g, 2.49 mmol) in acetonitrile (10 mL). The tube
was sealed and heated in an oil bath at 80 °C for 48 h, after which
time the volatiles were removed in Vacuo. Residual Ph2GeH2 was
removed by Kugelrohr distillation to yield 1.535 g (83%) of 1b as
a thick pale yellow liquid. 1H NMR (C6D6, 25 °C): δ 7.73 (d, J )
7.8 Hz, 4 H, m-H), 7.23-7.11 (m, 6 H, p- and o-H), 3.57 (t, J )
7.5 Hz, 4 H, GeCH2CH2O-), 3.28 (q, J ) 7.2 Hz, 4 H,
-OCH2CH3), 1.62 (t, J ) 7.5 Hz, 4 H, GeCH2CH2O-), 1.48 (m,
8 H, Ge(CH2CH2CH2CH3)2), 1.35 (pent, J ) 6.8 Hz, 8 H,
The HOMO/LUMO gap can be tuned in a predictable way
by changing the length of the Ge-Ge chain in these compounds,
as well as by altering the organic groups along the germanium-
germanium backbone, where increasing the chain length is the
most effective means for decreasing the HOMO/LUMO gap.
Variation of the σ-donor abilities of the groups bound to
germanium provides a means to more finely tune this energy
difference, since the relative energy of the HOMO is more
affected by such a change than that of the LUMO. For phenyl-
substituted oligogermanes, the LUMO is derived from linear
combinations of phenyl group π* orbitals, rather than being
germanium-based, which significantly alters the electronic
properties of these compounds. However, the electronic tun-
ability according to the findings of the aliphatic series is still
preserved, as seen experimentally from UV/visible spectroscopic
and electrochemical measurements. For consideration of the use
of oligogermanes as viable candidates for molecular wires, it
would be desirable to improve the air and electrochemical
stability and, to this end, we are currently investigating new
ligand systems that would promote reversibility by constraining
Ge-Ge bond dissociation.
Ge(CH2CH2CH2CH3)2), 1.25 (t,
J
)
7.5 Hz,
8
H,
Ge(CH2CH2CH2CH3)2), 1.13 (t, J ) 7.5 Hz, 6 H, -OCH2CH3),
0.89 ppm (t, J ) 6.8 Hz, 12 H, Ge(CH2CH2CH2CH3)2). 13C NMR
(C6D6, 25 °C): δ 140.4 (ipso-C), 138.0 (o-C), 128.4 (m-C), 128.1
(p-C), 68.9 (-OCH2CH3), 65.7 (-GeCH2CH2O-), 28.8
(-OCH2CH3), 27.0 (Ge(CH2CH2CH2CH3)2), 16.6 (Ge(CH2CH2CH2-
CH3)2), 15.8 (Ge(CH2CH2CH2CH3)2), 15.4 (GeCH2CH2O-), 13.8
ppm (Ge(CH2CH2CH2CH3)2). UV/visible: λmax 243 nm (v br, ꢀ )
1.57 × 104 cm-1 M-1). Anal. Calcd for C36H64Ge3O2: C, 57.91;
H, 8.64. Found: C, 58.06; H, 8.78.
Preparation of EtOCH2CH2(GePh2)(GePh2)(GePh2)CH2-
CH2OEt (1c). To a solution of Ph2GeH2 (0.510 g, 2.23 mmol) in
acetonitrile (15 mL) in
a
Schlenk tube was added
Ph2Ge(NMe2)CH2CH2OEt50 (1.52 g, 4.42 mmol) in acetonitrile (10
mL). The tube was sealed and heated in an oil bath at 85 °C for
48 h, after which time the volatiles were removed in Vacuo to yield
a thick viscous liquid, which was distilled in a Kugelrohr oven
(140 °C, 0.05 Torr) to yield 1.681 g (92%) of 1c as a white solid.
Experimental Section
General Considerations. All manipulations were carried out
under an inert N2 atmosphere using standard Schlenk, syringe, and
(81) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air SensitiVe
Compounds, 2nd ed.; Wiley: New York, 1986.

3760 Organometallics, Vol. 27, No. 15, 2008
Amadoruge et al.
1H NMR (C6D6, 25 °C): δ 7.68-7.64 (m, 4 H, m-H), 7.49-7.45
(m, 8 H, m-H), 7.17 (m, 6 H, p- and o-H), 7.11-7.05 (m, 12 H, p-
and o-H), 3.45 (t, J ) 7.8 Hz, 4 H, GeCH2CH2O-), 3.02 (q, J )
6.9 Hz, 4 H, -OCH2CH3), 1.93 (t, J ) 7.8 Hz, 4 H,
H, -OCH2CH3), 1.48 (t, J ) 7.2 Hz, 2 H, GeCH2CH2O-),
1.44-1.28 (m, 4 H, aliphatics), 1.19-1.13 (m, 4 H, aliphatics),
1.09-0.98 (m, 14 H, aliphatics), 0.73 (t, J ) 6.8 Hz, 3 H,
-OCH2CH3), 0.71 ppm (t, J ) 7.2 Hz, 6H, Ge(CH2CH2CH2CH3)2).
13C NMR (C6D6, 25 °C): δ 139.1 (ipso-C), 139.0 (ipso-C), 136.0
(o-C), 135.6 (o-C), 128.6, 128.5 (2 m- and 2 p-C), 68.7
(-OCH2CH3), 65.6 (-GeCH2CH2O-), 28.5 (-OCH2CH3), 26.6
(Ge(CH2CH2CH2CH3)2), 14.0 (Ge(CH2CH3)2), 13.7 (Ge(CH2CH2CH2-
CH3)2), 10.3 (Ge(CH2CH2CH2CH3)2), 8.6 (GeCH2CH2O-), 7.0
(Ge(CH2CH2CH2CH3)2), 5.6 ppm (Ge(CH2CH3)2). UV/visible: λmax
248 nm (v br, ꢀ ) 2.97 × 104 cm-1 M-1). Anal. Calcd for
C46H62Ge4O: C, 59.97; H, 6.78. Found: C, 60.10; H, 6.90.
GeCH2CH2O-), 0.95 ppm (t, J ) 6.9 Hz, 6 H, -OCH2CH3). 13
C
NMR (C6D6, 25 °C): δ 138.2 (ipso-C), 136.5 (o-C), 135.6
(o-C),139.0 (ipso-C), 128.8, 128.5, 128.4, 128.3 (m- and p-C), 68.0
(-OCH2CH3), 65.4 (-GeCH2CH2O-), 17.3 (-OCH2CH3), 15.3
ppm (GeCH2CH2O-). UV/visible: λmax 247 nm (v br, ꢀ ) 1.98 ×
104 cm-1 M-1). Anal. Calcd for C44H48Ge3O2: C, 63.93; H, 5.85.
Found: C, 63.51; H, 5.69.
Preparation of Ph3Ge(GeBu2)(GePh2)CH2CH2OEt (5). To a
solution of Ph3Ge(GeBu2)CH2CH2OEt50 (0.672 g, 1.19 mmol) in
benzene (15 mL) was added a solution of 1.0 M DIBAL-H in
hexane (1.31 mL, 1.31 mmol). The reaction mixture was refluxed
under N2 for 24 h, after which time the solvent was removed in
Vacuo to yield a viscous oil. The oil was dissolved in acetonitrile
(20 mL), transferred to a Schlenk tube, and treated with a solution
of Ph2Ge(NMe2)CH2CH2OEt50 (0.409 g, 1.19 mmol) in acetonitrile
(10 mL). The tube was sealed, and the reaction mixture was heated
at 90 °C for 4 days. The volatiles were removed in Vacuo, and the
crude product mixture was washed through a 1 in. × 3 in. silica
gel column using benzene (35 mL). The solvent was removed in
Preparation of Ph3Ge(GeBu2)(GePh2)(GeBu2)CH2CH2OEt
(6b). To a solution of Ph3Ge(GeBu2)(GePh2)CH2CH2OEt (5; 0.211
g, 0.267 mmol) in benzene (10 mL) was added a solution of 1.0 M
DIBAL-H in hexane (0.28 mL, 0.28 mmol). The reaction mixture
was refluxed under N2 for 24 h, after which time the solvent was
removed in Vacuo to yield a thick opaque oil. The oil was dissolved
in acetonitrile (10 mL), transferred to a Schlenk tube, and treated
with a solution of Bu2Ge(NMe2)CH2CH2OEt50 (0.083 g, 0.27
mmol) in acetonitrile (10 mL). The tube was sealed and the reaction
mixture was heated at 90 °C for 48 h. The volatiles were removed
in Vacuo, and the crude product mixture was washed through a 1
in. × 3 in. silica gel column using benzene (45 mL). The solvent
was removed in Vacuo to yield 6b (0.224 g, 86%) as a thick pale
yellow oil. 1H NMR (C6D6, 25 °C): δ 7.60-7.56 (m, 10 H, m-H),
7.14-7.07 (m, 25 H, o- and p-H), 3.42 (t, J ) 7.6 Hz, 2 H,
-CH2CH2O-), 3.09 (q, J ) 6.8 Hz, 2 H, -OCH2CH3), 1.48 (t, J
) 6.8 Hz, 2 H, GeCH2CH2O-), 1.40-1.26 (m, 4 H, aliphatics),
1.21-0.97 (m, 13 H, aliphatics), 0.71 ppm (m, 12 H,
(GeCH2CH2CH2CH3)4). 13C NMR (C6D6, 25 °C): δ 139.1 (ipso-
C), 139.0 (ipso-C), 135.6 (o-C), 135.3 (o-C), 128.6 (2 m- and 2
p-C), 68.7 (-OCH2CH3), 65.6 (-GeCH2CH2O-), 28.6
(-OCH2CH3), 26.6 (2 -CH2CH2CH2CH3), 16.1 (-CH2CH2-
CH2CH3), 15.3 (-CH2CH2CH2CH3), 14.9 (-CH2CH2CH2-
CH3), 13.9 (-CH2CH2CH2CH3), 13.6 (GeCH2CH2O-), 10.2
(GeCH2CH2CH2CH3), 6.9 ppm (GeCH2CH2CH2CH3). Anal. Calcd
for C50H70Ge4O: C, 61.44; H, 7.22. Found: C, 64.41; H, 7.42.
1
Vacuo to yield 5 (0.595 g, 63%) as a thick colorless oil. H NMR
(C6D6, 25 °C): δ 7.65-7.61 (m, 10 H, aromatics, m-H), 7.20-7.08
(m, 15 H, aromatics, o-H and p-H), 3.48 (t, J ) 7.5 Hz, 2 H,
-CH2CH2O-), 3.14 (q, J ) 6.6 Hz, 2 H, --OCH2CH3), 1.53 (t, J
) 7.5 Hz, 2 H, -CH2CH2O-), 1.39 (m, 4H, aliphatics), 1.15 (m,
8H, aliphatics), 0.77 (t, 3H, J ) 6.6 Hz, -OCH2CH3), 0.76 ppm
(t, J ) 7.2 Hz, 6H, -CH2CH2CH2CH3). 13C NMR (C6D6, 25 °C):
δ 139.3 (ipso-C), 139.2 (ipso-C), 136.0 (o-C), 135.7 (o-C), 128.7,
128.5 (2 m- and
2
p-C), 68.8 (-OCH2CH3), 65.7
(-GeCH2CH2O-), 28.7 (-OCH2CH3), 26.8 (-CH2CH2CH2CH3),
14.0 (-CH2CH2CH2CH3), 13.8 (-CH2CH2CH2CH3), 10.4
(GeCH2CH2O-), 7.1 ppm (-CH2CH2CH2CH3). UV/visible: λmax
232 nm (br, ꢀ ) 4.02 × 104 cm-1 M-1). Anal. Calcd for
C42H52Ge3O: C, 63.80; H, 6.63. Found: C, 64.11; H, 7.15.
Preparation of Ph3Ge(GeBu2)(GePh2)(GeEt2)CH2CH2OEt
(6a). To a solution of Ph3Ge(GeBu2)(GePh2)CH2CH2OEt (5; 0.525
g, 0.664 mmol) in benzene (20 mL) was added a solution of 1.0 M
DIBAL-H in hexane (0.75 mL, 0.75 mmol). The reaction mixture
was refluxed under N2 for 24 h, after which time the solvent was
removed in Vacuo to yield a thick opaque oil. The oil was dissolved
in acetonitrile (25 mL), transferred to a Schlenk tube, and treated
with a solution of Et2Ge(NMe2)CH2CH2OEt50 (0.165 g, 0.666
mmol) in acetonitrile (15 mL). The tube was sealed, and the reaction
mixture was heated at 90 °C for 48 h. The volatiles were removed
in Vacuo, and the crude product mixture was washed through a 1
in. × 3 in. silica gel column using benzene (50 mL). The solvent
was removed in Vacuo to yield 6a (0.508 g, 83%) as a thick
colorless oil. 1H NMR (C6D6, 25 °C): δ 7.70-7.54 (m, 10 H,
aromatics, m-H), 7.19-7.03 (m, 15 H, aromatics, o-H and p-H),
3.42 (t, J ) 7.2 Hz, 2 H, GeCH2CH2O-), 3.09 (q, J ) 6.8 Hz, 2
Acknowledgment. C.S.W. is grateful to the Department
of Chemistry at Oklahoma State University for funding this
work. J.R.G. thanks Marquette University and the donors of
the Petroleum Research Fund, administered by the American
Chemical Society, for financial support.
Supporting Information Available: Text giving computational
details, tables giving calculated HOMO/LUMO energy levels,
figures giving frontier orbital diagrams, and tables giving Cartesian
(xyz) coordinates for compounds 1-6 and 8. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM800240W