Substituent effects in digermanes: Electrochemical, theoretical, and structural investigations

Article abstract of DOI:10.1021/om400132z

The digermanes R₃GeGePh₃ (R₃ = Bu ⁱ₃, Hexⁿ₃, (C₁₈H ₃₇)₃, or Bu^tMe₂) were synthesized using the hydrogermolysis reaction, and the X-ray crystal structures of Bu ⁱ₃GeGePh₃and Bu^tMe ₂GeGePh₃ were determined. The isobutyl-substituted digermane contains two independent molecules in the unit cell with Ge-Ge bond distances of 2.4410(5) and 2.4409(5) A, and Bu^tMe ₂GeGePh₃ has a Ge-Ge bond distance of 2.4255(3) A. These four digermanes and the four additional digermanes R ₃GeGePh₃ (R₃ = Me₃, Bu ⁿ₃, Bu^s₃, or PhMe₂) were characterized by cyclic voltammetry, differential pulse voltammetry, and linear sweep voltammetry in order to determine the effects of varying substituent patterns on the oxidation potential of these systems. Digermanes having more inductively donating substituents exhibit more negative oxidation potentials than those having less inductively donating substituents. Density functional theory calculations were also performed on these eight systems, and the energies of their frontier orbitals were determined. The energy of the HOMO and LUMO in these systems was shown to depend on the electron-donating ability of the organic substituents.

Full text of DOI:10.1021/om400132z

Article
pubs.acs.org/Organometallics
Substituent Effects in Digermanes: Electrochemical, Theoretical,
and Structural Investigations
Erin K. Schrick,† Trevor J. Forget,† Kimberly D. Roewe,† Aaron C. Schrick,† Curtis E. Moore,‡
James A. Golen,‡,§ Arnold L. Rheingold,‡ Nicholas F. Materer,† and Charles S. Weinert*,†
†Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078, United States
‡Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California 92093-0358, United States
§University of Massachusetts Dartmouth, North Dartmouth, Massachusetts 02747, United States
S
* Supporting Information
ABSTRACT: The digermanes R3GeGePh3 (R3 = Bui3, Hexn ,
3
(C18H37)3, or ButMe2) were synthesized using the hydroger-
molysis reaction, and the X-ray crystal structures of Bui3GeGePh3
and ButMe2GeGePh3 were determined. The isobutyl-substituted
digermane contains two independent molecules in the unit cell
with Ge−Ge bond distances of 2.4410(5) and 2.4409(5) Å,
and ButMe2GeGePh3 has a Ge−Ge bond distance of 2.4255(3) Å.
These four digermanes and the four additional digermanes
R3GeGePh3 (R3 = Me3, Bun , Bus3, or PhMe2) were char-
3
acterized by cyclic voltammetry, differential pulse voltammetry,
and linear sweep voltammetry in order to determine the effects
of varying substituent patterns on the oxidation potential of these systems. Digermanes having more inductively donating
substituents exhibit more negative oxidation potentials than those having less inductively donating substituents. Density
functional theory calculations were also performed on these eight systems, and the energies of their frontier orbitals were
determined. The energy of the HOMO and LUMO in these systems was shown to depend on the electron-donating ability of the
organic substituents.
sequential oxidation steps, where n is the number of
catenated germanium atoms.10,13
INTRODUCTION
■
Oligogermanes are a class of catenated compounds that struc-
turally resemble saturated hydrocarbons but possess physical
properties that more closely mirror those of conjugated
unsaturated polyenes.1−3 However, as opposed to these π-conjugated
organic systems, oligogermanes exhibit σ-delocalization, where
the pair of electrons present in the highest occupied molecular
orbital (HOMO) are delocalized across the germanium−
germanium backbone, provided that the germanium atoms are
coplanar and are arranged in a sequential trans-conformation.4,5
Therefore, these molecules can exhibit σ → σ* electronic
transitions that are similar to the π → π* transitions observed
in their doubly or triply bonded carbon congeners, resulting in
the observation of broad but distinct absorbance peaks in their
UV/visible spectra. The σ-bonding electrons present in these
molecules have also been shown to interact with the available
π or π* molecular orbitals on aryl substituents attached to
the Ge−Ge backbone, which shifts their observed λmax values
to lower energy.6−10 Oligogermanes are also electrochemi-
cally active, and their oxidation potentials can be readily
obtained using cyclic voltammetry or differential pulse
voltammetry.9−15 The majority of these compounds exhibit
a single irreversible oxidation wave, although aryl-substituted
linear oligogermanes have been shown to undergo n − 1
We have developed a versatile method for the construction
of singly bonded oligogermanes that makes use of the
hydrogermolysis reaction as the key step in Ge−Ge bond
formation.7−10,13,14,16,17 This reaction proceeds in acetonitrile
solvent, which also doubles as a key synthetic reagent, as it is
involved in the conversion of the germanium amide starting
material R3GeNMe2 into an α-germyl nitrile R3GeCH2CN,
which is the active species in the Ge−Ge bond-forming process.
The hydrogermolysis reaction can be coupled with a hydride
protection/deprotection strategy that allows for sequential
chain buildup where the germanium atoms are added to the
chain one at a time. Using these methods, we are able to
control both the degree of catenation and the substituent
pattern, which was not possible using other previously
developed synthetic techniques.
We have characterized the majority of the oligogermanes
prepared in our laboratory using UV/visible spectroscopy and
cyclic voltammetry and have established a correlation between
the structures of these materials and their physical properties.
The energy of the HOMO−LUMO gap in these compounds
Received: February 14, 2013
Published: March 18, 2013
© 2013 American Chemical Society
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decreases with an increase in the number of catenated ger-
manium atoms and also with the incorporation of more
inductively donating organic substituents along the Ge−Ge
backbone.7,9 Variation of the degree of catenation has a
more prolific effect on the energies of the frontier orbitals in
these compounds than does variation of the substituents, but
changes in the substituent pattern do have a measurable effect.
In order to investigate the substituent effects on the oxi-
dation potentials of these systems in detail, we have
investigated a series of digermanes having the general formula
Scheme 2
R3GeGePh3 (R3 = Me3 (1),6,18,19 Bun (2),17 Bui3 (3), Bus3
3
(4),20 Hexn3 (5), (C18H37)3 (6), PhMe2 (7),20 or ButMe2 (8)),
where compounds 3, 5, 6, and 8 are newly synthesized.
Digermanes are the simplest systems containing single Ge−Ge
bonds, and the substituents are varied at only one of the
germanium atoms in the series of digermanes at hand. We have
found that more inductively electron-donating substituents
destabilize the HOMO in these molecules and therefore render
them easier to oxidize, and these effects are somewhat subtle.
For example, different oxidation potentials were observed for
the three isomeric butyl-substituted species 2−4. We have also
combined our electrochemical investigations with density
functional theory calculations that were used to determine
the energies of the frontier orbitals in these systems. We have
found excellent correlation between the experimental and
theoretical data, and these results are described below.
2037 and 2033 cm−1, respectively,23 since 12−14 contain alkyl
rather than aryl substituents. Both 12 and 13 are liquids, while
the octadecyl derivative 14 is a solid at room temperature. The
three organogermanes were converted to the corresponding
chlorides via the method of Kunai et al.24 and subsequently to
the amides 9−11 by salt metathesis with LiNMe2 (Scheme 2).
The 1H NMR spectra of the three amide compounds contained
a singlet for their −N(CH3)2 protons at 2.57 (9), 2.65 (10), and
2.64 (11) ppm. The amide ButMe2GeNMe2 (15) was synthesized
from commercially available ButMe2GeCl and exhibits a resonance
at 2.60 ppm corresponding to its −N(CH3)2 protons.
RESULTS AND DISCUSSION
The newly synthesized digermanes 3, 5, 6, and 8 were prepared
according to Scheme 1, and the corresponding germanium
■
Scheme 1
The amides 9−11 and 15, as well as the previously reported
amides R3GeNMe2 (R3 = Me3,25 Bun ,17 Bus3,20 PhMe220), were
3
treated with Ph3GeH in acetonitrile solvent to yield the diger-
manes 1−8. This reaction proceeds via the in situ conversion of
the germanium amide starting materials R3GeNMe2 to the
corresponding α-germyl nitriles R3GeCH2CN, as shown in
Scheme 1.16,17 All of the digermanes synthesized are crystalline
solids, with the exception of the trihexyl-substituted derivative
5, which is a viscous clear liquid, and the octadecyl-substituted
digermane 6, which is an oily solid. The new digermanes 3, 5, 6,
and 8 were characterized by NMR (1H and 13C) spectroscopy
and elemental analysis, and the X-ray crystal structures of 3 and
8 were also determined.
amides used for their synthesis (Bui3GeNMe2 (9), Hexn3GeNMe2
(10), and (C18H37)3GeNMe2 (11)) were prepared in four steps
starting from GeO2. A significant limitation in the area of
oligogermane chemistry is the lack of reliable preparative routes
for the synthesis of organogermane starting materials having
the general formula R3GeX or R2GeX2 (R = an alkyl or aryl
group, X = Cl or Br). However, triorganogermane compounds
R3GeH can be readily prepared by the method of Corriu
et al.,21,22 and we have used this preparative route for the
synthesis of the three germanium starting materials Bui3GeH
(12), Hex3GeH (13; Hexn = n-C6H13), and (C18H37)3GeH
(14), none of which are commercially available (Scheme 2). An
important advantage of this method is that it makes use of the
inexpensive starting material GeO2 instead of GeCl4 or GeBr4,
which are typically used in other preparative routes.
1
The H NMR spectrum of Bui3GeGePh3 (3) contains the
expected pattern for the three isobutyl groups with two
doublets at 0.88 (J = 6.6 Hz) and 1.25 (J = 6.6 Hz) ppm
corresponding to the methylene and methyl protons as well as a
multiplet at 1.90 ppm corresponding to the methine protons.
1
The H NMR spectrum for ButMe2GeGePh3 (8) contains two
singlets at 2.60 and 0.97 ppm arising from the protons of the
tert-butyl group and the methyl groups, respectively. The
chemical shift for the methyl groups of 8 differs from that for
the signal corresponding to the related phenyl-substituted
derivative PhMe2GeGePh3 (7)20 observed at 0.64 ppm. The
upfield shift of the methyl group protons in 7 versus those of 8
can be attributed to through-space shielding of these protons by
the π-electron cloud of the phenyl ring in 7,26,27 which is absent
in the tert-butyl-substituted derivative 8.
The three newly synthesized organogermanes 12−14 each exhibit
1
a characteristic Ge−H resonance in their H NMR spectra.
These were observed as septets at 3.53 (12), 4.08 (13), and 4.00
3
(14) ppm with JH−H coupling constants of 2.8 (12), 3.0 (13),
and 2.7 (14) Hz. These compounds also exhibit sharp νGe−H stretch-
ing bands in their infrared spectra at 2010 (12), 2005 (13), and
2007 (14) cm−1, which are observed at lower energy than those
for Ph3GeH and Mes3GeH (Mes = 2,4,6-trimethylphenyl) at
The 1H NMR spectrum of Hexn GeGePh3 (5) contains
3
several overlapping resonances for the methylene protons of
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Figure 1. ORTEP diagram of Bui3GeGePh3 (3). Thermal ellipsoids are drawn at 50% probability.
the n-hexyl group, but the 13C NMR spectrum contains six
Table 1. Selected Bond Distances (Å) and Angles (deg) for
Bui3GeGePh3 (3)
distinct resonances at 32.3, 31.9, 26.7, 23.0, 14.3, and 12.5 ppm.
The resonances are arranged in a progressing upfield pattern
based on their proximity to the germanium atom, with the
feature at 32.3 ppm corresponding to the α-carbon atom, while
that at 12.5 ppm is due to the terminal methyl group. Similarly,
molecule 1
Ge(1)−Ge(2)
molecule 2
2.4410(5) Ge(1′)−Ge(2′)
average
2.4409(5) 2.4410(5)
1.972(3) 1.967(3)
1.975(4) 1.974(4)
1.985(4) 1.978(4)
1.957(4) 1.957(4)
1.952(3) 1.955(4)
1.968(3) 1.963(3)
Ge(1)−C(1)
Ge(1)−C(5)
Ge(1)−C(9)
Ge(2)−C(13)
Ge(2)−C(19)
Ge(2)−C(25)
1.962(3) Ge(1′)−C(1′)
1.972(4) Ge(1′)−C(5′)
1.970(3) Ge(1′)−C(9′)
1.956(4) Ge(2′)−C(13′)
1.957(4) Ge(2′)−C(19′)
1.958(3) Ge(2′)−C(25′)
1
the H NMR spectrum of (C18H37)3GeGePh3 (6) contains a
single intense broad feature at 1.34 ppm arising from the
17 groups of methylene protons and a triplet at 0.91 ppm (J =
6.6 Hz) corresponding to the terminal methyl group. The 13C
NMR spectrum of 6 contains a single intense resonance at 30.3
ppm arising from 14 of the 18 carbon atoms present in the
octadecyl chain. Additional features were also observed and
correspond to the α-CH2 carbon atom (23.2 ppm), the two
carbon atoms of the methylene groups proximal to the
octadecyl chain terminus (32.4 and 29.9 ppm), and the
terminal methyl group (14.4 ppm). A similar chemical shift
C(1)−Ge(1)−C(5) 109.3(2) C(1′)−Ge(1′)−C(5′) 116.0(2) 112.7(2)
C(1)−Ge(1)−C(9) 109.5(2) C(1′)−Ge(1′)−C(9′) 110.0(1) 109.8(2)
C(5)−Ge(1)−C(9) 112.8(2) C(5′)−Ge(1′)−C(9′) 112.0(2) 112.4(2)
C(13)−Ge(2)−C(19) 106.4(2) C(13′)−Ge(2′)−C(19′) 109.6(1) 108.0(2)
C(13)−Ge(2)−C(25) 106.5(1) C(13′)−Ge(2′)−C(25′) 106.7(1) 106.6(1)
C(19)−Ge(2)−C(25) 106.9(2) C(19′)−Ge(2′)−C(25′) 106.4(1) 106.6(2)
C(1)−Ge(1)−Ge(2) 110.5(1) C(1′)−Ge(1′)−Ge(2′) 112.2(9) 111.4(1)
C(5)−Ge(1)−Ge(2) 106.7(1) C(5′)−Ge(1′)−Ge(2′) 102.63(9) 104.7(7)
C(9)−Ge(1)−Ge(2) 107.9(1) C(9′)−Ge(1′)−Ge(2′) 103.2(1) 105.5(1)
C(13)−Ge(2)−Ge(1) 115.72(9) C(13′)−Ge(2′)−Ge(1′) 110.94(9) 113.33(9)
C(19)−Ge(2)−Ge(1) 108.0(1) C(19′)−Ge(2′)−Ge(1′) 115.1(1) 111.5(1)
C(25)−Ge(2)−Ge(1) 112.84(9) C(25′)−Ge(2′)−Ge(1′) 107.63(9) 110.24(9)
pattern was reported for the germane Bun Ge(C11H23).28
3
The X-ray crystal structures of 3 and 8 have been
determined, and ORTEP diagrams are shown in Figures 1
and 2, respectively. Selected bond distances and angles are also
Table 2. Selected Bond Distances (Å) and Angles (deg) for
ButMe2GeGePh3 (8)
Ge(1)−Ge(2)
Ge(1)−C(1)
Ge(1)−C(7)
Ge(1)−C(13)
Ge(2)−C(19)
Ge(2)−C(23)
Ge(2)−C(24)
C(1)−Ge(1)−C(7)
C(1)−Ge(1)−C(13)
C(7)−Ge(1)−C(13)
2.4255(3)
1.952(2)
1.962(2)
1.949(2)
1.989(2)
1.951(2)
1.951(2)
106.73(7)
110.73(7)
106.63(8)
C(19)−Ge(2)−C(23)
C(19)−Ge(2)−C(24)
C(23)−Ge(2)−C(24)
Ge(1)−Ge(2)−C(19)
Ge(1)−Ge(2)−C(23)
Ge(1)−Ge(2)−C(24)
Ge(2)−Ge(1)−C(1)
Ge(2)−Ge(1)−C(7)
Ge(2)−Ge(1)−C(13)
108.02(9)
109.93(9)
107.8(1)
115.27(6)
107.37(7)
108.18(6)
110.51(5)
109.05(5)
112.91(5)
2.4410(5) and 2.4409(5) Å and an average distance of
2.4410(5) Å. The Ge−Ge bond distance in 3 is longer than
the 2.4208(8) Å Ge−Ge bond distance in the n-butyl-
Figure 2. ORTEP diagram of ButMe2GeGePh3 (8). Thermal ellipsoids
are drawn at 50% probability.
substituted analogue Bun GeGePh3 (2)17 due to the increased
3
steric effects of the branching present at the β-carbon of the
three isobutyl groups in 3. The Ge−Ge bond distance in 3 is
also elongated relative to those in Me3GeGePh3 (1)18 and
Et3GeGePh3,17 which measure 2.418(1) and 2.4253(7) Å,
respectively, despite the fact that the structures of 1 and
collected in Tables 1 (3) and 2 (8). The triisobutyl-substituted
digermane 3 crystallizes with two independent molecules in the
unit cell, having germanium−germanium bond distances of
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Et3GeGePh3 are C3-symmetric, where the alkyl and phenyl
groups on the two germanium atoms are in an eclipsed
conformation. However, the Ge−Ge bond distance in 3 is
shorter than those in the more sterically encumbered
The cyclic and differential pulse voltammograms, as well as the
linear sweep voltammograms, of the other seven digermanes
are nearly identical in appearance. The CV of 3 shown in Figure
3 contains a single irreversible oxidation wave at 1510 9 mV,
while the DPV contains an anodic current peak at 1410 7 mV
and the LSV contains an oxidation wave at 1545 13. The
oxidation waves for 1−8 in their CVs all appear at higher
potential than those in the corresponding DPVs, as expected
since the charging current present in the CV is suppressed in
the DPV experiment. In addition, the DPV oxidation waves
appear at the same potential within experimental error when
the voltammograms are acquired with the potential progressing
in either the positive or negative direction. The 10-cycle CV of
3 shown in Figure 5 is also nearly identical in appearance to
those for the other seven digermanes. The position of the
oxidation wave remains at the same potential within
experimental error for each cycle, and the anodic current
diminishes slightly with each progressive scan.
29
digermanes Ph3GeGePh3 and Pri3GeGePh3,16 which measure
2.446(1) and 2.4637(7) Å, respectively.
The increased steric effects of the isobutyl substituent are
also manifested in the Ge−Cα bond distances, which average
1.973(5) Å in 3 and are longer than the average bond distance of
1.951(6) Å in 2.17 The average C−Ge−C bond angle about the
Ge(1) is 111.6(2)°, while in 2 the corresponding bond angle is
108.6(2)°, and the average Cα−Ge−Ge bond angles in 3 and 2 are
107.2(1)° and 110.3(2)°, respectively. The more significant
deviation from an idealized tetrahedral environment in 3 is
again a result of the increased steric attributes of the isobutyl
substituents versus the n-butyl substituents in 2.
The Ge−Ge bond distance in ButMe2GeGePh3 (8) is
2.4255(3) Å and is only slightly longer than that in
PhMe2GeGePh3 (7) (2.4216(4) Å)20 but is nearly identical
to the Ge−Ge bond distance in Me3GeGePh3 (1) of 2.418(1) Å.18
Thus, the replacement of a single methyl group with either a
tert-butyl or phenyl substituent has little effect on the Ge−Ge
bond distance among these three molecules. The Ge(1)−
C(19) bond distance to the central carbon atom of the tert-
butyl group is 1.989(2) Å, which is longer than typical Ge−C
bond distances of 1.95−1.97 Å as a result of the steric
encumbrance of the tert-butyl group. The average C−Ge−C
bond angle about Ge(2) is 108.6(1)°, which is close to the ideal
tetrahedral bond angle and is also similar to the related C−Ge−C
bond angles in 7 (109.2(2)°) and 1 (108.7(1)°).
Variation of the substituents at the alkyl-substituted
germanium atom has a minimal effect on the geometry at the
second phenyl-substituted germanium atom. The environment
about the phenyl-substituted germanium atom in the five
crystallographically characterized digermanes 1−3, 7, and 8 is
essentially identical, with average Ge−Cipso bond distances
falling into the narrow range 1.955(4)−1.958(4) Å and the
average Cipso−Ge−Cipso bond angles in the range 107.1(2)−
108.7(2)°. The substituents on the two germanium atoms
adopt a staggered conformation in compounds 2, 3, 7, and 8,
while in 118 and Et3GeGePh317 the conformations are eclipsed.
The digermanes 1−8 were each characterized using cyclic
voltammetry (CV), differential pulse voltammetry (DPV), and
linear sweep voltammetry (LSV), and the experimental results
are collected in Table 3. A representative overlaid CV and DPV
The trend in oxidation potentials in these digermanes, from
easiest to most difficult to oxidize and based on their varying
substituents, is as follows:
PhMe2 (7) < (C18H37)3 (6) < Bus (4) < Bui3 (3)
3
< Hexn3 (5) < Bun3 (2) < ButMe2 (8) < Me3(1)
Of the eight digermanes investigated, the trimethyl-substituted
species 1 is the most difficult to oxidize, having a single
irreversible oxidation wave in its cyclic voltammogram at 1795
mV, while the most facile digermane to oxidize is
PhMe2GeGePh3 (7), which exhibits an oxidation wave at
1450 mV. The digermane 1 lacks the more electron-donating
substituents present in 2−6, and therefore 1 is expected to be
the most difficult to oxidize. The same trend in electron-
donating ability of the substituents and oxidation potential is
maintained in the series of three digermanes Me3GeGePh3 (1),
ButMe2 (8), and PhMe2 (7). Substituting a methyl group in 1
for a tert-butyl group in 8 diminishes the oxidation potential
of 8 by ca. 195 mV versus that for 1, while the oxidation
potential of 7 is ca. 345 mV more negative than that of 1.
The But group in 8 is more inductively electron donating
than a methyl group, and thus the energy of the HOMO,
which is taken to be the source of the electron removed from
the digermane upon oxidation, should be destabilized in 8
versus 1.
Although the ipso-carbon atom of the phenyl group present in 7
is effectively more electronegative and therefore less electron
donating than that of a germanium-bound alkyl substituent, the
π-system of the phenyl group serves as an electron donor to the
germanium atom and destabilizes the HOMO of 7 relative to
those in both 8 and 1, and thus 7 is the most facile of these three
compounds (and the most facile overall) to oxidize.
Table 3. Electrochemical Data for Digermanes 1−8
a
b
c
compound
CV (mV)
DPV (mV)
LSV (mV)
Me3GeGePh3 (1)
1795 11
1550 12
1510 10
1505 12
1515 11
1490 13
1605
1540
1410
1405
1410
8
9
9
7
8
1802 12
1532 11
1545 13
1535 10
1510 10
1485 14
1445 12
1610 11
Bun GeGePh3 (2)
3
Bui3GeGePh3 (3)
Bus3GeGePh3 (4)
This same trend is also observed among the three
digermanes Bun GeGePh3 (2), Hexn GeGePh3 (5), and
3
3
Hexn GeGePh3 (5)
3
(C18H37)3GeGePh3 (6). The n-butyl-substituted digermane 2
is the most difficult to oxidize, while the n-hexyl derivative 5 has
an oxidation potential that is ca. 45 mV more negative than that
of 2 and that for the octadecyl-substituted digermane 6 is ca. 25
mV more negative than that of 5. Furthermore, although the
values are very similar when considering experimental error, the
expected trend is also observed among the three tributyl-
substituted digermanes Bus3GeGePh3 (4), Bui3GeGePh3 (3),
(C18H37)3GeGePh3 (6)
PhMe2GeGePh3 (7)
ButMe2GeGePh3 (8)
1400 11
1450
1600
9
8
1330
1470
7
7
a
b
Conditions: 0.1 M [Bun N][PF6], scan rate = 100 mV/s. Conditions:
4
0.1 M [Bun4N][PF6], pulse period = 0.1 s, pulse width = 0.05.
c
Conditions: 0.1 M [Bun N][PF6], scan rate = 100 mV/s.
4
of 3 is shown in Figure 3, an LSV of Bui3GeGePh3 (3) is shown
in Figure 4, and a 10-cycle CV scan of 3 is shown in Figure 5.
and Bun GeGePh3 (2), where the n-butyl digermane 2 is the
3
most difficult to oxidize and the sec-butyl digermane 4 is the
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Figure 3. CV (scan rate = 100 mV/s) and DPV (pulse period = 0.1 s, pulse width = 0.05 s, sample time = 0.02 s) of Bui3GeGePh3 (3) in CH2Cl2
solvent with 0.1 M [Bu4N][PF6] as the supporting electrolyte.
Figure 4. LSV of Bui3GeGePh3 (3) in CH2Cl2 solvent with 0.1 M [Bu4N][PF6] as the supporting electrolyte. Sweep rate = 100 mV/s.
least difficult. This trend also correlates with the values
calculated for the inductive substituent constant σI for these
groups.30 The values of σI for an n-butyl, isobutyl, and sec-butyl
group are 0.0617, 0.0635, and 0.0688, respectively.
these species, a concrete reason for the absence of a reversible
or quasi-reversible process has not been offered. The
irreversibility of the oxidations of oligogermanes suggests that
a chemical reaction is occurring after the oligogermane is
oxidized. We have postulated that germylene extrusion might
be occurring that results in concomitant chain contraction,
Although we10,13 and others11,12 have observed only
irreversible oxidation waves in the cyclic voltammograms of
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Figure 5. Ten-cycle CV of Bui3GeGePh3 (3) in CH2Cl2 solvent with 0.1 M [Bu4N][PF6] as the supporting electrolyte.
Scheme 3
particularly since we have observed n − 1 oxidation waves of
was characterized by NMR (1H and 13C) spectroscopy.
However, there was no evidence for the formation of any of
the three possible trapping products shown in Scheme 3.39,40
In order to obtain a better understanding of the electronic
makeup of these molecules, density functional theory
calculations were performed on 1−8 using the 6-311G(d,p)
basis set.41 The energies of the frontier orbitals as well as the
calculated UV/visible absorbance maxima, experimental λmax
values, and CV oxidation potentials of these eight species are
collected in Table 4. These data are arranged in order of
increasing stabilization of the HOMO. The HOMO for 1 is the
most stabilized at −6.402 eV, while that for the sec-butyl
derivative 4 is the most destabilized at −6.277 eV, and the
HOMO energies among these eight digermanes lie within
0.125 eV of one another.
aryl-substituted oligogermanes having the general formula
GenAr2n+2.
10,13 Further evidence for the possibility of germylene
extrusion stems from UV irradiation of oligogermanes in the
presence of the trapping reagent CCl4 or 2,3-dimethyl-1,3-
butadiene, leading to the isolation of R2GeCl2 or 1,1-dialkyl-
3,4-dimethyl-1-germacyclopent-3-ene, although the formation
of germanium radicals resulting from homolytic scission of the
Ge−Ge bonds was also detected as a competing process.31−38
These investigations included the photolysis of the digermanes
PhMe2GeGeMe338 and PhMe2GeGeMe2Ph,35 which resulted in
the detection of germylenes and germyl radicals.
In order to attempt to ascertain if germylene extrusion is the
process occurring after oxidation of the digermanes, we
conducted bulk electrolysis experiments on Bun GeGePh3 (2)
3
in the presence of ca. 20 equiv of 1,3-dimethylbutadiene, which
was expected to yield one or more of the trapping products
shown in Scheme 3. Electrolysis of a 0.500 g sample of 2 over a
4 h period at a potential of 2100 mV was carried out in
dichloromethane. The current reached a plateau after 84 min,
the resulting mixture was evaporated to dryness, and the
products were extracted from the crude reaction mixture with
hexane. Evaporation of the hexane yielded a solid material that
The energy ordering shown in Table 4 deviates somewhat
from the order described above for the oxidation potentials of
these eight digermanes, but this is expected due to the
proximity in energy of the HOMO in these compounds. We
presume that the electron removed from the digermanes during
oxidation lies in the HOMO, although there are contributions
to this molecular orbital other than those involved in formation
of the germanium−germanium bond. In particular, the phenyl
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Table 4. Computational and Experimental Data for Digermanes 1−8
a
b
compound
HOMO (eV) LUMO (eV) HOMO−LUMO gap (nm) theor λmax (nm)
transition
λmax (nm) Eox (mV)
Bus3GeGePh3 (4)
−6.277
−0.804
227
242 (0.0845)
249 (0.0684)
252 (0.0541)
257 (0.0441)
241 (0.0530)
247 (0.0682)
249 (0.0724)
240 (0.0681)
247 (0.0710)
249 (0.0690)
247 (0.0679)
249 (0.0723)
251 (0.1693)
258 (0.0856)
244 (0.0520)
249 (0.0604)
245 (0.0813)
248 (0.0517)
238 (0.0739)
245 (0.0680)
246 (0.0679)
HOMO → LUMO+3
HOMO → LUMO+2
HOMO → LUMO+1
HOMO → LUMO
244
1505 12
(C18H37)3GeGePh3 (6)
−6.296
−6.308
−0.761
−0.770
224
224
HOMO → LUMO+3
HOMO → LUMO+2
HOMO → LUMO+1
HOMO → LUMO+3
HOMO → LUMO+2
HOMO → LUMO+1
HOMO → LUMO+2
HOMO → LUMO+1
HOMO → LUMO+1
HOMO → LUMO
236
241
1490 13
1515 11
1550 12
Hexn GeGePh3 (5)
3
Bun GeGePh3 (2)
−6.316
−6.324
−6.350
−6.370
−6.402
−0.768
-0.902
-0.788
-0.825
-0.821
224
229
223
224
222
232
244
232
238
230
3
PhMe2GeGePh3 (7)
Bui3GeGePh3 (3)
ButMe2GeGePh3 (8)
Me3GeGePh3 (1)
1450
1510 10
1600
1795 11
9
HOMO → LUMO+2
HOMO → LUMO+1
HOMO → LUMO+2
HOMO → LUMO+1
HOMO → LUMO+3
HOMO → LUMO+2
HOMO → LUMO+1
8
a
b
Oscillator strengths for the individual transitions are given in parentheses. Values are from cyclic voltammetry.
group on the second germanium in 7 is expected to interact
with the orbitals involved in Ge−Ge bonding and therefore
destabilize the HOMO, which was found in the DFT
computations.
seven digermanes as well. Of the eight digermanes investigated,
only 7, which contains phenyl substituents on each of the two
germanium atoms, has a significant contribution to the
UV/visible spectra from the HOMO → LUMO electronic
transition.
The digermanes 1 and 8 have the most stabilized HOMOs
among these eight molecules, which correlates with the
electrochemical results, and 4 and 6 have the most destabilized
HOMOs, which also correlates with the electrochemistry data.
In addition, the inductive donating effects of longer chain alkyl
groups are expected to be enhanced relative to shorter chain
substituents, and this is clearly indicated among the three
digermanes 2, 5, and 6. The n-butyl-substituted digermane 2 is
more difficult to oxidize than the n-hexyl-substituted species 5,
which are both in turn more difficult to oxidize than the
octadecyl 6 substituted digermane.
Time-dependent DFT calculations were also performed to
determine the theoretical UV/visible absorbance maxima, and
these data are shown in Table 4. The electronic transitions
having the highest oscillator strength are typically from the
germanium−germanium single-bond-based HOMO to three
orbitals localized on the phenyl groups that correspond to the
LUMO+1, LUMO+2, and LUMO+3. The HOMO, LUMO,
and LUMO+1 orbitals for digermane 1 are depicted in Figure 6.
The LUMO shown in Figure 6b is an antibonding orbital
localized along the germanium−germanium bond. Figure 6e
and f illustrate the LUMO and LUMO+1, respectively, with the
germanium−germanium bond directed into the page.
As found in other investigations, the LUMO and LUMO+n
orbitals are essentially degenerate, and so several electronic
transitions involving the LUMO and the next two or three
energy levels are typically observed. A calculated spectrum of
the Bus3GeGePh3 (4) is shown in Figure 7, which has four
predicted absorbance maxima at 242, 249, 252, and 257 nm,
with that at 242 nm being the most intense. As indicated by the
data in Table 4, the experimental absorbance maximum for 4
was observed at 244 nm, and there is excellent agreement
between the calculated and observed λmax values for the other
We were unable to obtain X-ray quality crystals of
(C18H37)3GeGePh3 (6) for a structural analysis; however, we
generated a calculated structure from the DFT computations,
which is illustrated in Figure 8. This structure is one of several
with a similar energy minimum that differ only in the
conformation of the octadecyl groups. The calculated Ge−Ge
bond distance is 2.492 Å, which is similar to the crystallographic
distance found in most digermanes.1,15−18 The average C−Ge−C
and C−Ge−Ge bond angles about the octadecyl-substituted
germanium atom Ge(2) are 109.8° and 109.1°, respectively,
while the C−Ge−C and C−Ge−Ge bond angles about the
phenyl-substituted Ge(1) atom are 107.8° and 111.1°,
respectively. The average Ge−C distances at Ge(1) and
Ge(2) are 1.984 and 1.998 Å. These metric parameters also
match closely with those found in digermanes that have been
crystallographically characterized. The calculated geometry
about Ge(2) in 6 is very similar to that in Bun GeGePh3 (2)
3
(dGe−Ge = 2.4212(8) Å)18 in that the first four carbon atoms of
the octadecyl chain are arranged in a similar fashion to the
n-butyl groups in 2, while the remaining 14 atoms of the
octadecyl chain are directed outward away from the Ge−Ge
bond.
In conclusion, we have prepared the four new digermanes
Bui3GeGePh3 (3), Hexn GeGePh3 (5), (C18H37)3GeGePh3 (6),
3
and ButMe2GeGePh3 (8) starting from the corresponding
germanes R3GeH (R = Bui (12), Hexn (13), C18H37 (14)) or
the chloride ButMe2GeCl. The X-ray crystal structures of 3 and
8 were obtained, and these compounds have Ge−Ge single-
bond distances of 2.4410(5) and 2.4255(3) Å, where the
former value is an average of two crystallographically
independent molecules. These four digermanes and the four
previously synthesized digermanes Me3GeGePh3 (1),
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Figure 6. (a) HOMO, (b) LUMO, and (c) LUMO +1 orbitals computed for species Me3GeGePh3 (1). (e and f) LUMO and LUMO +1,
respectively, with the Ge−Ge bond directed into the page. The LUMO+2 and LUMO+3 are similar to the LUMO+1.
Figure 7. Calculated UV/visible spectrum of Bus3GeGePh3 using time-dependent DFT computations.
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spectrometer. IR spectra were obtained using a Perkin-Elmer 1720
infrared spectrometer, and UV/visible spectra were recorded using a
Hewlett-Packard 8453 diode array spectrometer. Electrochemical data
(CV, DPV, LSV, BE) were obtained using a DigiIvy DY2312
potentiostat using a glassy carbon working electron, a platinum wire
counter electrode, and an Ag/AgCl reference electrode in CH2Cl2
solution using 0.1 M [Bu4N][PF6] as the supporting electrolyte. Bulk
electrolyses were conducted in a two-compartment cell using a
reticulated vitreous carbon electrode (BASi). Elemental analyses were
conducted by Galbraith Laboratories.
Synthesis of Bui3GeH (12). To a suspension of K2[(C4H8O2)3Ge]
(3.756 g, 9.051 mmol) in diethyl ether (100 mL) was added a solution
of BuiMgCl (10.9 mL, 2.5 M, 27.3 mmol) in THF. The reaction
mixture was stirred at room temperature for 2 h followed by the
addition of LiAlH4 (1.04 g, 27.3 mmol) using a solid addition funnel.
The reaction mixture was stirred for a further 2 h and was then
hydrolyzed using an aqueous 25% H2SO4 solution. The resulting
mixture was filtered, and the filtrate was extracted with diethyl ether
(3 × 25 mL). The Et2O layer was dried over anhydrous MgSO4 and
filtered, and the Et2O was removed in vacuo to yield compound 12 as a
colorless liquid (1.817 g, 82%). 1H NMR (25 °C, C6D6): δ 3.53 (sept,
J = 2.8 Hz, 1H, Ge-H), 1.92 (m, 3H, CH), 1.32 (d, J = 7.0 Hz, 6H,
−CH2−), 0.92 (d, J = 6.6 Hz, 18H, −CH3) ppm. 13C NMR (25 °C,
C6D6): δ 27.3 (CH3), 26.8 (CH), 26.3 (CH2) ppm. IR (Nujol): 2010
cm−1 (νGe−H). Anal. Calcd for C12H28Ge: C, 58.81; H, 11.52. Found:
C, 58.72; H, 11.58.
Figure 8. Calculated structure of (C18H37)3GeGePh3 (6) using time-
dependent DFT computations.
Synthesis of Bui3GeCl. To a solution of 12 (1.490 g, 6.085 mmol)
in diethyl ether (35 mL) was added CuCl2 (1.64 g, 12.2 mmol). The
reaction mixture was stirred at room temperature for 16 h. The
mixture was filtered through Celite, and the diethyl ether was removed
in vacuo to yield Bui3GeCl (1.34 g, 79%) as a colorless liquid. 1H NMR
(25 °C, C6D6): δ 1.94 (m, 3H, CH), 1.04 (d, J = 7.0 Hz, 6H, −CH2−),
0.92 (d, J = 7.0 Hz, 18H, −CH3) ppm. 13C NMR (25 °C, C6D6): δ
28.1 (CH2), 27.3 (CH3), 26.8 (CH) ppm. Anal. Calcd for C12H27ClGe:
C, 51.56; H, 9.74. Found: C, 51.22; H, 9.89.
Bun GeGePh3 (2), Bus3GeGePh3 (4), and PhMe2GeGePh3 (7)
3
were characterized by cyclic, differential pulse, and linear sweep
voltammetry. Each of the digermanes 1−8 exhibits a single
irreversible oxidation wave in their cyclic voltammograms.
Alteration of the substituent pattern at one germanium atom in
these molecules resulted in variation of the observed oxidation
potentials with a trend that corresponds to the inductive
electron-donating ability of these substituents. In general,
digermanes having more highly inductively donating sub-
stituents were found to be easier to oxidize, resulting from the
destabilization of the HOMO in these molecules, than those
having less donating substituents.
Density functional theory calculations were also performed
on these molecules, and the theoretical results were compared
to the electrochemical data. The DFT findings indicate that the
frontier orbitals among the eight digermanes are all similar in
energy, and there is very good correlation between the
theoretical findings and experimental electrochemical and
UV/visible data. As expected, inductively donating substituents
at germanium destabilize the HOMO and in general diminish
the oxidation potential of the corresponding digermane. The
UV/visible spectra of digermanes 1−8 were obtained and range
from 230 to 244 nm. The experimental UV/vis data are in
excellent agreement with the theoretical data obtained using
time-dependent DFT, and the electronic transitions that occur
are between the HOMO, which is composed primarily of the
germanium−germanium bonding orbitals, and the LUMO or
the nearly degenerate LUMO +1, LUMO +2, or LUMO +3, where
the latter three MOs are localized on the phenyl substituents.
Synthesis of Bui3GeNMe2 (9). To a solution of Bui3GeCl (1.005 g,
3.596 mmol) in benzene (25 mL) was added LiNMe2 (0.220 g, 4.31
mmol). The reaction mixture was stirred for 18 h and then was filtered
through Celite. The volatiles were removed in vacuo to yield 9 (0.902
g, 87%) as a pale yellow liquid. 1H NMR (25 °C, C6D6): δ 2.57 (s, 6H,
−N(CH3)2), 1.86 (m, 3H, CH), 1.00 (d, J = 7. Hz, 6H, −CH2−), 0.87
(d, J = 7.2 Hz, 18H, −CH3) ppm. 13C NMR (25 °C, C6D6): δ 41.2
(−N(CH3)2), 28.8 (CH2), 26.9 (CH3), 25.8 (CH) ppm. Anal. Calcd
for C14H33GeN: C, 58.35; H, 11.55. Found: C, 58.24; H, 11.77.
Synthesis of Bui3GeGePh3 (3). To a solution of 9 (0.503 g, 1.75
mmol) in acetonitrile (20 mL) was added a solution of Ph3GeH (0.639 g,
2.10 mmol) in acetonitrile (10 mL). The reaction mixture was sealed
in a Schlenk tube and heated at 85 °C for 48 h. The volatiles were
removed in vacuo to yield a light yellow solid. The crude product was
distilled in a Kugelrohr oven (125 °C, 0.05 Torr) to remove residual
Ph3GeH, yielding 3 (0.755 g, 79%) as a white solid. 1H NMR (25 °C,
C6D6): δ 7.69−7.66 (m, 6H, aromatics), 7.21−7.16 (m, 9H,
aromatics), 1.90 (m, 3H, CH), 1.25 (d, J = 6.6 Hz, 6H, −CH2−),
0.878 (d, J = 6.6 Hz, 18H, −CH3) ppm. 13C NMR (25 °C, C6D6): δ
135.9 (o-C6H5), 128.7 (m-C6H5), 128.6 (p-C6H5), 27.5 (CH3), 27.1
(CH), 26.4 (CH2) ppm. Anal. Calcd for C30H42Ge2: C, 65.75; H, 7.74.
Found: C, 65.58; H, 7.64.
Synthesis of Hexn3GeH (13). To
a suspension of
K2[(C4H8O2)3Ge] (2.500 g, 6.022 mmol) in diethyl ether (100 mL)
was added a solution of HexnMgBr (9.03 mL, 2.0 M, 18.1 mmol) in
THF. The reaction mixture was stirred at room temperature for 1 h
followed by the addition of LiAlH4 (0.687 g, 18.1 mmol) using a solid
addition funnel. The reaction mixture was stirred for a further 2 h and
was then hydrolyzed using an aqueous 25% H2SO4 solution. The
resulting mixture was filtered, and the filtrate was extracted with
diethyl ether (3 × 15 mL). The Et2O layer was dried over anhydrous
MgSO4 and filtered, and the Et2O was removed in vacuo to yield
EXPERIMENTAL SECTION
■
General Considerations. All reagents were handled under an
inert atmosphere of N2 using standard Schlenk, syringe, and glovebox
techniques unless otherwise specified. The compounds Me3GeGePh3
(1),19 Bun GeGePh3 (2),17 Bus3GeGePh3 (4),20 PhMe2GeGePh3
3
(7),20 and K2[(C4H8O2)3Ge]21,22 were prepared according to
literature procedures. The reagent ButMe2GeCl was purchased from
Gelest, Inc., and solutions of BuiMgCl, HexnMgBr, and C18H37MgCl
were purchased from Aldrich. 1H and 13C NMR spectra were recorded
at 300 and 75.46 MHz, respectively, using an INOVA Gemini 2000
1
compound 12 as a colorless liquid (0.317 g, 16%). H NMR (25 °C,
C6D6): δ 4.08 (sept, J = 3.0 Hz, 1H, Ge-H), 1.54−1.46 (m, 6H,
−CH2(CH2)4CH3), 1.39−1.29 (m, 18H, −CH2CH2CH2CH2CH2CH3),
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0.93−0.86 (m, 15H, −CH2(CH2)3CH2CH3) ppm. 13C NMR (25 °C,
C6D6): δ 32.2 (−CH2CH2CH2CH2CH2CH3), 31.9 (−CH2CH2CH2CH2-
CH2CH3), 26.7 (−CH2CH2CH2CH2CH2CH3), 23.0 (−CH2CH2-
CH2CH2CH2CH3), 14.3 (−CH2CH2CH2CH2CH2CH3), 12.5
(−CH2CH2CH2CH2CH2CH3) ppm. IR (Nujol): 2005 cm−1
(νGe−H). Anal. Calcd for C18H40Ge: C, 65.66; H, 12.25. Found: C,
65.87; H, 12.37.
and a catalytic amount of CuI (0.28 g, 1.47 mmol). The reaction
mixture was stirred at room temperature for 18 h. The mixture was
filtered through Celite, and the diethyl ether was removed in vacuo to
1
yield (C18H37)3GeCl (6.72 g, 92%) as a pale yellow solid. H NMR
(25 °C, C6D6): δ 1.34 (br m, 102H, −CH2(CH2)16CH3), 0.92 (t, J =
5.7 Hz, 9H, −CH2(CH2)16CH3) ppm. 13C NMR (25 °C, C6D6): δ
32.4, (−(CH2)15CH2CH2CH3), 30.3 (−CH2(CH2)14CH2CH2CH3),
29.9 (−(CH2)15CH2CH2CH3), 23.1 (−CH2(CH2)16CH3), 14.4
(−(CH2)15CH2CH2CH3) ppm. Anal. Calcd for C54H111ClGe: C,
74.66; H, 12.89. Found: C, 74.80; H, 12.76.
Synthesis of Hexn GeCl. To a solution of 13 (0.351 g, 1.07 mmol)
3
in diethyl ether (20 mL) were added CuCl2 (0.287 g, 2.13 mmol) and
a catalytic amount of CuI (0.006g, 0.03 mmol). The reaction mixture
was stirred at room temperature for 18 h. The mixture was filtered
Synthesis of (C18H37)3GeNMe2 (11). To a solution of
(C18H37)3GeCl (6.409 g, 7.379 mmol) in benzene (60 mL) was
added LiNMe2 (0.46 g, 9.0 mmol). The resulting suspension was
stirred for 18 h and then filtered through Celite. The volatiles were
through Celite, and the diethyl ether was removed in vacuo to yield
1
Hexn GeCl (0.313 g, 81%) as a colorless liquid. H NMR (25 °C,
3
C6D6): δ 1.55−1.47 (m, 6H, −CH2(CH2)4CH3), 1.29−1.25 (m, 18H,
−CH2CH2CH2CH2CH2CH3), 1.10−1.04 (m, 6H, −CH2-
(CH2)3CH2CH3), 0.89 (t, J = 6.6 Hz, 9H, −CH3) ppm. 13C NMR
(25 °C, C6D6): δ 34.3 (−CH2CH2CH2CH2CH2CH3), 32.1 (−CH2CH2-
CH2CH2CH2CH3), 27.2 (−CH2CH2CH2CH2CH2CH3), 23.3
(−CH2CH2CH2CH2CH2CH3), 14.4 (−CH2CH2CH2CH2CH2CH3),
12.7 (−CH2CH2CH2CH2CH2CH3) ppm. Anal. Calcd for
C18H39GeCl: C, 59.44; H, 10.82. Found: C, 59.62; H, 10.89.
1
removed in vacuo to yield 11 (4.19 g, 65%) as a pale yellow solid. H
NMR (25 °C, C6D6): δ 2.64 (s, 6H, −N(CH3)2), 1.31 (br m, 102H,
−CH2(CH2)16CH3), 0.92 (t, J = 6.9 Hz, 9H, −CH2(CH2)16CH3)
ppm. 13C NMR (25 °C, C6D6): δ 42.5 (−N(CH3)2), 32.4,
(−(CH2)15CH2CH2CH3), 30.2 (−CH2(CH2)14CH2CH2CH3), 29.9
(−(CH2)15CH2CH2CH3), 23.1 (−CH2(CH2)16CH3), 14.3
(−(CH2)15CH2CH2CH3) ppm. Anal. Calcd for C56H117GeN: C,
76.66; H, 13.45. Found: C, 76.33; H, 13.74.
Synthesis of Hexn GeNMe2 (10). To a solution of Hexn GeCl
3
3
Synthesis of (C18H37)3GeGePh3 (6). To a solution of 11 (1.47 g,
1.67 mmol) in CH3CN (20 mL) was added a solution of Ph3GeH
(0.522 g, 1.71 mmol) in CH3CN (10 mL). The reaction mixture was
sealed in a Schlenk tube and was heated at 85 °C for 48 h. The
volatiles were removed in vacuo, and the resulting viscous oil was
distilled in a Kugelrohr oven (145 °C, 0.07 Torr) to remove excess
(0.313 g, 0.860 mmol) in benzene (50 mL) was added LiNMe2 (0.053
g, 1.0 mmol). The resulting suspension was stirred for 18 h and then
filtered through Celite. The volatiles were removed in vacuo to yield 10
(0.266 g, 83%) as a pale yellow liquid. 1H NMR (25 °C, C6D6): δ 2.65
(s, 6H, −N(CH3)2), 1.65−1.60 (m, 6H, −CH2(CH2)4CH3), 1.52−
1.32 (m, 18H, −CH2(CH2)3CH3), 1.09−1.04 (m, 6H, −CH2(CH2)3-
CH2CH3), 0.95−0.90 (m, 9H, −CH2(CH2)4CH3) ppm. 13C NMR (25
°C, C6D6): δ 42.5 (−N(CH3)2), 37.4 (−CH2CH2CH2CH2CH2CH3),
32.8 (−CH2CH2CH2CH2CH2CH3), 26.8 (−CH2CH2CH2-
CH2CH2CH3), 23.9 (−CH2CH2CH2CH2CH2CH3), 14.6
(−CH2CH2CH2CH2CH2CH3), 13.1 (−CH2CH2CH2CH2CH2CH3)
ppm. Anal. Calcd for C20H45GeN: C, 64.52; H, 12.19. Found: C,
64.77; H, 12.25.
1
Ph3GeH, yielding 6 (0.953 g, 50%) as a pale yellow solid. H NMR (25
°C, C6D6): δ 7.48 (m, 6H, o-C6H5), 7.14 (m, 9H, m-C6H5 and p-
C6H5), 1.34 (br m, 102H, −CH2(CH2)16CH3), 0.92 (t, J = 6.9 Hz, 9H,
−CH2(CH2)16CH3) ppm. 13C NMR (25 °C, C6D6): δ 139.1 (ipso-
C6H5), 135.5 (o-C6H5), 129.4 (m-C6H5), 128.7 (p-C6H5), 32.4,
(−(CH2)15CH2CH2CH3), 30.3 (−CH2(CH2)14CH2CH2CH3), 29.9
(−(CH2)15CH2CH2CH3), 23.2 (−CH2(CH2)16CH3), 14.4
(−(CH2)15CH2CH2CH3) ppm. Anal. Calcd for C72H126Ge2: C,
76.04; H, 11.18. Found: C, 76.09; H, 11.22.
Synthesis of Hexn GeGePh3 (5). To a solution of 10 (0.266 g,
3
0.710 mmol) in CH3CN (10 mL) was added a solution of Ph3GeH
(0.240 g, 0.787 mmol) in CH3CN (10 mL). The reaction mixture was
sealed in a Schlenk tube and was heated at 85 °C for 48 h. The
volatiles were removed in vacuo, and the resulting viscous oil was
distilled in a Kugelrohr oven (130 °C, 0.05 Torr) to remove excess
Synthesis of ButMe2GeNMe2 (15). To a solution of ButMe2GeCl
(2.000 g, 10.24 mmol) in diethyl ether (25 mL) was added a solution
of LiNMe2 (0.630 g, 12.4 mmol) in diethyl ether (25 mL). The
resulting solution was stirred for 18 h and then was filtered through
Celite. The Et2O was distilled off under N2, and the resulting residue
was taken up in hexane and filtered through Celite. The hexane was
disilled off under N2 to yield 15 (1.600 g, 77%) as a pale yellow liquid.
1H NMR (25 °C, C6D6): δ 2.60 (s, 6H, −N(CH3)2), 0.97 (s, 9H,
−C(CH3)3), 0.14 (s, 6H, −CH3) ppm. 13C NMR (25 °C, C6D6): 43.4
(−N(CH3)2), 29.1 (−C(CH3)3), 24.2 (−C(CH3)3), −2.3 (−CH3)
ppm. Anal. Calcd for C8H21GeN: C, 47.11; H, 10.40. Found: C, 47.77;
H, 9.99.
1
Ph3GeH, yielding 5 (0.194 g, 43%) as a pale yellow liquid. H NMR
(25 °C, C6D6): δ 7.67−7.65 (m, 6H, o-C6H5), 7.21−7.14 (m, 9H,
m-C6H5 and p-C6H5), 1.48−1.42 (m, 6H, −CH2(CH2)4CH3), 1.26−1.16
(m, 24H, −CH2(CH2)4CH3), 0.85−0.81 (m, 9H, −CH2(CH2)4CH3)
ppm. 13C NMR (25 °C, C6D6): δ 135.7 (o-C6H5), 128.8 (m-C6H5),
128.6 (p-C6H5), 36.2 (−CH2CH2CH2CH2CH2CH3), 31.7 (−CH2CH2-
CH2CH2CH2CH3), 26.6 (−CH2CH2CH2CH2CH2CH3), 22.9
(−CH2CH2CH2CH2CH2CH3), 14.9 (−CH2CH2CH2CH2CH2CH3),
14.3 (−CH2CH2CH2CH2CH2CH3) ppm. Anal. Calcd for C36H54Ge2:
C, 68.39; H, 8.62. Found: C, 68.22; H, 8.55.
Synthesis of (C18H37)3GeH (14). To a suspension of
K2[(C4H8O2)3Ge] (6.66 g, 16.0 mmol) in diethyl ether (100 mL)
was added a solution of C18H37MgCl (100. mL, 0.5 M, 50.0 mmol) in
diethyl ether. The reaction mixture was stirred at room temperature
for 2 h followed by the addition of LiAlH4 (1.61 g, 42.4 mmol) using a
solid addition funnel. The reaction mixture was stirred for a further 2 h
and was then hydrolyzed using an aqueous 25% H2SO4 solution. The
resulting mixture was filtered, and the filtrate was extracted with
diethyl ether (3 × 25 mL). The Et2O layer was dried over anhydrous
MgSO4 and filtered, and the Et2O was removed in vacuo to yield
compound 14 as a white solid (7.43 g, 56%). 1H NMR (25 °C, C6D6):
δ 4.00 (sept, J = 2.7 Hz, 1H, Ge-H), 1.30 (br m, 102H, −CH2(CH2)16-
CH3), 0.91 (t, J = 6.6 Hz, 9H, −CH2(CH2)16CH3) ppm. 13C NMR (25 °C,
C6D6): δ 34.4, (−(CH2)15CH2CH2CH3), 30.3 (−CH2(CH2)14CH2-
CH2CH3), 29.9 (−(CH2)15CH2CH2CH3), 23.2 (−CH2(CH2)16CH3),
14.4 (−(CH2)15CH2CH2CH3) ppm. Anal. Calcd for C54H112Ge: C,
77.74; H, 13.54. Found: C, 77.82; H, 13.63.
Synthesis of ButMe2GeGePh3 (8). To a solution of 15 (0.870 g,
4.27 mmol) in CH3CN (10 mL) was added a solution of Ph3GeH
(1.060 g, 3.476 mmol) in CH3CN (10 mL). The reaction mixture was
sealed in a Schlenk tube and was heated at 85 °C for 48 h. The
volatiles were removed in vacuo, and the resulting viscous oil was
distilled in a Kugelrohr oven (145 °C, 0.07 Torr) to remove excess
Ph3GeH, yielding 8 (1.75 g, 89%) as a pale yellow solid. 1H NMR (25 °C,
C6D6): δ 7.63 (d, J = 7.2 Hz, 6H, o-C6H5), 7.17−7.14 (m, 9H, m-C6H5
and p-C6H5), 0.97 (s, 9H, −C(CH3)3), 0.39 (s, 6H, −CH3) ppm. 13
C
NMR (25 °C, C6D6): δ 139.1 (ipso-C6H5), 135.8 (o-C6H5), 128.8
(m-C6H5), 128.6 (p-C6H5), 28.9 (−C(CH3)3), 24.0 (−C(CH3)3), −3.9
(−CH3) ppm. Anal. Calcd for C24H30Ge2: C, 62.14; H, 6.53. Found:
C, 62.70; H, 6.46.
Computational Details. Gaussian 09 was utilized for all
computations.42 Energy calculations, geometry optimizations, and
frequency calculations are performed using the hybrid density
functional method including Becke’s three-parameter nonlocal-
exchange functional43 with the correlation functional of Lee−Yang−
Parr, B3LYP.44 Final optimizations were performed using the
6-311+G(d,p) basis set.41 All atomic positions were optimized without
geometry constraints to a minimum in the total force. Given that the
Synthesis of (C18H37)3GeCl. To a solution of 14 (6.98 g, 8.37
mmol) in diethyl ether (60 mL) was added CuCl2 (2.33 g, 1.71 mmol)
2254
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Article
HOMO and LUMO energies depend primarily on the local geometry
around the Ge atoms, no attempt was made to explore the
conformational space for the molecules having long alkyl chains.
Frequency calculations were performed at a lower level (6-31G*) to
confirm that the stable geometries have real vibrational frequencies.
The time-dependent density functional computations, as implemented
by Gaussian 09, were utilized to explore the excited manifold and
compute the possible electronic transitions and oscillator strengths.
GaussSum was used to compute the UV/visible spectra.45
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X-ray Crystal Structure Determinations. Diffraction intensity
data were collected with a Siemens P4/CCD diffractometer.
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SHELXL-97.
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S
* Supporting Information
155.
Crystallographic parameters and CIF files giving crystallo-
graphic data for 3 and 8. This material is available free of charge
via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: weinert@chem.okstate.edu.
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Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Dedicated to the memory of Prof. Duward F. Shriver (1934-
2013). Funding for this work was provided by a CAREER grant
from the National Science Foundation (No. CHE-0844758)
and is gratefully acknowledged. Computing for this project was
partially supported by the OSU High Performance Computing
Center at Oklahoma State University, funded in part through
instrumentation grant OCI-1126330 by the National Science
Foundation.
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