A series of isopropyl-substituted oligogermanes Pri3Ge(GePh2)nGePri3 (n = 0-3) featuring a luminescent and dichroic pentagermane Pri3Ge(GePh2)3GePri3

Article abstract of DOI:10.1021/acs.organomet.6b00596

The series of oligogermanes Prⁱ₃Ge(GePh₂)_nGePrⁱ₃ (n = 0, 2; n = 1, 3; n = 2, 4; n = 3, 5) were prepared by the hydrogermolysis reaction. All four species were characterized by NMR (¹H and ¹³C) and UV/visible spectroscopy as well by cyclic voltammetry and differential pulse voltammetry. The oligogermanes exhibited a successive red shift in their absorbance maxima as the length of the Ge-Ge chain was increased, and these molecules also became easier to oxidize as the chain length was increased. The X-ray crystal structures of 4 and 5 as well as the trigermane HPh₂GeGePh₂GePh₂H (6) were determined. The pentagermane 5 was shown to be dichroic in the solid state due to the packing of the individual molecules in a columnar fashion in the crystal. Pentagermane 5 is also luminescent in solution, exhibiting an emission maximum at 380 nm. The physical properties of 5, like the hexagermane Prⁱ₃Ge(GePh)₄GePrⁱ₃, mimic the properties of the larger polygermane systems.

Full text of DOI:10.1021/acs.organomet.6b00596

Article
pubs.acs.org/Organometallics
A Series of Isopropyl-Substituted Oligogermanes
Pri3Ge(GePh2)nGePri3 (n = 0−3) Featuring a Luminescent and Dichroic
Pentagermane Pri3Ge(GePh2)3GePri3
Sangeetha P. Komanduri,† F. Alexander Shumaker,† Kimberly D. Roewe,† Melanie Wolf,§ Frank Uhlig,§
Curtis E. Moore,‡ Arnold L. Rheingold,‡ 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
§6330 Institut fur Anorganische Chemie, TU Graz, Graz, Austria
̈
S
* Supporting Information
ABSTRACT: The series of oligogermanes Pri3Ge-
(GePh2)nGePri3 (n = 0, 2; n = 1, 3; n = 2, 4; n = 3, 5) were
prepared by the hydrogermolysis reaction. All four species
were characterized by NMR (1H and 13C) and UV/visible
spectroscopy as well by cyclic voltammetry and differential
pulse voltammetry. The oligogermanes exhibited a succes-
sive red shift in their absorbance maxima as the length of
the Ge−Ge chain was increased, and these molecules also
became easier to oxidize as the chain length was increased. The
X-ray crystal structures of 4 and 5 as well as the trigermane
HPh2GeGePh2GePh2H (6) were determined. The pentager-
mane 5 was shown to be dichroic in the solid state due to the packing of the individual molecules in a columnar fashion in the
crystal. Pentagermane 5 is also luminescent in solution, exhibiting an emission maximum at 380 nm. The physical properties of 5,
like the hexagermane Pri3Ge(GePh)4GePri3, mimic the properties of the larger polygermane systems.
Several long-chain oligogermanes have been reported,
including Ge10Me22,34 GenMe2n(SMe)2 (n = 1−10),28
Ge11Me22Ph2,28 Ge5Ph12,35,36 and Pri3Ge(GePh2)4GePri3.37,38
The hexagermane Pri3Ge(GePh2)4GePri3 was shown to have an
absorbance maximum and an emission peak akin to those
observed for polygermanes and is also dichroic in the solid
state. On the basis of these findings, it was postulated that
oligogermanes having five or six germanium atoms arranged such
that σ-delocalization can occur39,40 likely represent the arrange-
ment of germanium atoms in polygermanes, meaning that these
larger systems might have ordered regions of five, six, or more
germanium atoms interspersed with regions of structural dis-
order. To further investigate this postulate, we have synthesized
the pentagermane Pri3Ge(GePh2)3GePri3 and also have prepared
the lower oligogermanes in the series Pri3Ge(GePh2)nGePri3
(n = 0, 1, or 2) for comparison. It has been shown that, like the
hexagermane Pri3Ge(GePh2)4GePri3, the pentagermane Pri3Ge-
(GePh2)3GePri3 is luminescent and dichroic.
INTRODUCTION
■
Polygermanes1−12 and germanium nanomaterials, including
nanoparticles, nanowires, and quantum dots,13−26 represent
two important classes of compounds due to their potentially
useful optical and electronic properties. It is expected that
germanium-based materials will continue to increase in impor-
tance as the limitations of silicon-containing materials are
realized, and both polygermanes and germanium nanomaterials
will likely serve as replacements for silicon in semiconducting and
optical devices. For example, it has been recently shown both
theoretically27 and experimentally28 by the SEM-break junction
method29 that oligogermanes can function as molecular wires.
Several polygermane systems have also been shown to have
emissive and/or nonlinear optical attributes.1,2,6,7,30−32 How-
ever, with the polygermane systems the exact composition of the
materials is not known. The properties of these systems are
tunable based on the number of catenated germanium atoms and
also, albeit to a lesser extent, based on the nature of the organic
substituents attached to the germanium−germanium back-
bone.33 The smaller oligogermane systems are discrete
molecules where the exact composition and structure can be
ascertained, and these molecules can serve as small-molecule
mimics or models for the larger polygermane systems and
also potentially for germanium nanomaterials, provided a suffi-
cient number of catenated germanium atoms are present in the
oligogermane.
RESULTS AND DISCUSSION
■
The chlorogermane Pri3GeCl (1), which is normally commer-
cially available, is used as a starting material to prepare the
oligogermanes Pri3Ge(GePh2)nGePri3 (2, n = 0; 3, n = 1; 4, n = 2;
Received: July 27, 2016
© XXXX American Chemical Society
A
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5, n = 3). However, a production shortage required the devel-
opment of a reliable synthetic method for its preparation. Pre-
viously reported methods for the synthesis of 1 involved the
action of the Grignard reagent PriMgCl on GeCl4 to yield
Pri2GeCl2 and Pri3GeCl in a 5:1 mixture in low yield41 or by the
treatment of GeCl4 with PriMgBr, which yielded a mixture of
the germanes Pri4−nGeCln (n = 1−3).42 Three other reported
procedures were faced with similar complications.43−45 We have
found that Ge(OEt)4 is a useful starting material for the pre-
paration of 1 in high yield and purity (Scheme 1). The critical
worked up after the reaction times indicated in Scheme 3, and the
products were purified using Kugelrohr distillation. For the
synthesis of 2, the chlorogermane 1 was converted to Pri3GeH in
49% yield using lithium aluminum hydride (Scheme 2). The 1H
NMR spectrum of Pri3GeH exhibits a singlet at δ 3.74 ppm
corresponding to the germanium-bound hydrogen atom and
a peak at 1997 cm−1 (νGe−H) in its IR spectrum. These spectral
features match those previously reported in the literature.47 The
synthesis of the digermane Pri6Ge2 (2) was then carried out by
reacting Pri3GeH with Pri3GeNMe2 (Scheme 3).
The trigermane 3 was prepared in 45% yield starting with
Ph2GeH2 and 2 equiv of Pri3GeNMe2 (Scheme 3). The yields of
2 and 3 are both low, and this was also observed in the previ-
ously described synthesis of 2 that was prepared from Pri3GeX
(X = Cl, Br), where SmI2 was used as the reducing agent,48 and
also for 3, which was synthesized from Pri3GeCl and Ph2GeBr2
again using SmI2 as the reducing agent.48,49 The synthesis of the
tetragermane 4 was achieved by the reaction of HPh2GeGePh2H46
with 2 equiv of Pri3GeNMe2, yielding 4 in 44% yield based on
HPh2GeGePh2H (Scheme 3). Pentagermane 5 was prepared
from HPh2GeGePh2GePh2H (6) and 2 equiv of Pri3GeNMe2 in
52% yield after a reaction time of 7 days (Scheme 3).
Scheme 1
step in obtaining 1 in pure form is the use of 2 equiv of PriLi in the
reaction with Ge(OEt)4, where the Ge(OEt)4 is added to the
solution of PriLi. If the expected 3 equiv of PriLi is instead used or
the order of addition is reversed, an intractable mixture of
products results that includes the formation of polymeric
material. We were able to reproducibly obtain 1 in overall yields
of ca. 50% starting from Ge(OEt)4.
The synthesis of HPh2GeGePh2GePh2H (6) was carried out
according to the published procedure by the reaction of ButLi
with Ph2GeH2.50 The 1H NMR of 6 exhibits a singlet at δ 5.68
ppm for the two magnetically equivalent germanium-bound
hydrogen atoms. The X-ray crystal structure of 6 was obtained;
an ORTEP diagram of 6 is shown in Figure 1, and selected bond
distances and angles are collected in Table 1.
Compound 1 was converted to the amide Pri3GeNMe2
(Scheme 2),46 and this compound was used to prepare four
Scheme 2
new oligogermanes with the general formula Pri3Ge-
(GePh2)nGePri3 (2, n = 0; 3, n = 1; 4, n = 2; 5, n = 3), as shown
in Scheme 3. The hydrogermolysis reactions for the preparation
Scheme 3
Figure 1. ORTEP diagram of HPh2GeGePh2GePh2H (6). Thermal
ellipsoids are drawn at 50% probability.
The two germanium−germanium single-bond distances mea-
sure 2.4338(3) and 2.4308(3) Å with an average value of
2.4323(3) Å. The bond distances in 6 are similar to those in
the aryl-substituted trigermanes Ge3Ph8 (2.438(2) and 2.441
(2) Å),51 Ge3Tol8 (Tol = p-CH3C6H4; 2.4450(4) and 2.4359
(5) Å),52 Tol3GeGePh2GeTol3 (2.4318(5) and 2.4338(4) Å),52
and Ph3GeGeMe2GePh3 (2.429(1) and 2.429(1) Å).53 The
Ge−Ge−Ge bond angle in 3 measures 118.94(1)°, which is
also similar to that in the trigermanes Ge3Ph8 (121.3(1)°),51
Ge3Tol8 (117.54(1)°),52 Tol3GeGePh2GeTol3 (114.80(2)°),52
and Ph3GeGeMe2GePh3 (120.3(1)°).53 Thus, the presence
of 2−5 are substantially more sluggish than typically observed
and required reaction times of 4−7 days. The reactions were
B
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Table 1. Selected Bond Distances (Å) and Angles (deg) for HGePh2GePh2GePh2H (6)
Ge(1)−Ge(2)
Ge(2)−Ge(3)
Ge(1)−C(1)
2.4338(3)
2.4308(3)
1.951(2)
1.962(2)
1.960(2)
1.958(2)
1.951(2)
1.947(2)
118.94(1)
109.33(9)
C(13)−Ge(2)−C(19)
C(25)−Ge(3)−C(31)
C(1)−Ge(1)−Ge(2)
C(7)−Ge(1)−Ge(2)
C(13)−Ge(2)−Ge(1)
C(19)−Ge(2)−Ge(1)
C(13)−Ge(2)−Ge(3)
C(19)−Ge(2)−Ge(3)
C(25)−Ge(3)−Ge(2)
C(31)−Ge(3)−Ge(2)
110.29(9)
111.12(9)
109.63(7)
112.15(6)
108.54(6)
108.78(6)
106.42(6)
103.66(6)
112.64(7)
113.77(7)
Ge(1)−C(7)
Ge(2)−C(13)
Ge(2)−C(19)
Ge(3)−C(25)
Ge(3)−C(31)
Ge(1)−Ge(2)−Ge(3)
C(1)−Ge(1)−C(7)
of a hydrogen atom and two aryl groups at the termini of the Ge3
chain rather than three aryl groups has very little effect on the
structural metrics of these trigermanes.
The X-ray crystal structures of 4 and 5 were also determined.
An ORTEP diagram of 4 is shown in Figure 2, and selected bond
The longer of the two Ge−Ge bond distances is at the center of
the molecule, which is typically observed for oligogermanes. The
single crystallographically unique Ge−Ge−Ge bond angle is
116.51(3)°, and these structural attributes are similar to those
in the perphenyl-substituted tetragermane Ge4Ph10.51 Like 4,
Ge4Ph10 has an inversion center between the two central ger-
manium atoms and all four germanium atoms are thus copla-
nar. The germanium−germanium bonds in Ge4Ph10 measure
2.464(2) and 2.461(2) Å, and the Ge−Ge−Ge bond angle is
117.8(1)°. The presence of the three isopropyl groups at the
ends of the Ge4 chain in 4 results in a slight elongation of both
germanium−germanium single bonds as well as a contraction of
the Ge−Ge−Ge bond angles due to their steric bulk.
An ORTEP diagram of pentagermane 5 is shown in Figure 3,
and selected bond distances and angles are collected in Table 3.
The environment at the terminal germanium atoms in 5 is
distorted from tetrahedral more significantly than in 4, with
an average C−Ge−C bond angle of 111.2(2)° at Ge(1) and
111.0(2)° at Ge(5). The average Ge−Ge bond distance in 5 is
2.4710(8) Å, and the average Ge−Ge−Ge bond angle is
117.01(3)°. The structure of 5 can be compared to that of the
structurally characterized pentagermane Ge5Ph12, which has
been analyzed at 29535 and 100 K.36 The pentagermane Ge5Ph12
has an average Ge−Ge bond length of 2.460(4) Å and an aver-
age Ge−Ge−Ge bond angle of 115.6(2)° at 295 K, and these
parameters are 2.4502(6) Å and 115.52(2)°, respectively, in the
structure analyzed at 100 K. The terminal Ge−Ge bonds in 5 are
longer than those in both structures of Ge5Ph12 by an average of
0.03 Å due to the presence of three isopropyl groups rather than
three phenyl substituents. The most significant structural
difference, however, is that four of the five germanium atoms
in Ge5Ph12 are in the same plane, while in 5 only the three central
germanium atoms are coplanar, as shown in Figure 4. One of the
terminal germanium atoms in 5 is canted above the Ge3 plane,
and the other is canted below the Ge3 plane. The structure of
5 resembles that of the hexagermane Pri3Ge(GePh2)4GePri3
(7),37,38 where the four central germanium atoms are coplanar
Figure 2. ORTEP diagram of Pri3Ge(GePh2)2GePri3 (4). Thermal
ellipsoids are drawn at 50% probability.
distances and angles are collected in Table 2. All four germanium
atoms in 4 are arranged in a trans-coplanar environment, as
enforced by the inversion center disposed between the two
central germanium atoms. The environment at Ge(1) in 4 is very
slightly distorted from a tetrahedral environment due to the
steric encumbrance of the three isopropyl substituents with an
average C−Ge−C bond angle of 108.5(2)°. The two crystallo-
graphically unique germanium−germanium bonds in 4 measure
2.4669(7) and 2.472(1) Å, with an average value of 2.469(1) Å.
Table 2. Selected Bond Distances (Å) and Angles (deg) for Pri3Ge(GePh2)2GePri3 (4)
Ge(1)−Ge(2)
Ge(2)−Ge(2′)
Ge(1)−C(1)
Ge(1)−C(2)
Ge(1)−C(3)
Ge(2)−C(4)
Ge(2)−C(5)
Ge(1)−Ge(2)−Ge(2′)
C(1)−Ge(1)−C(2)
C(1)−Ge(1)−C(3)
2.4669(7)
2.472(1)
1.985(5)
1.995(5)
1.993(5)
1.962(5)
1.974(5)
116.51(3)
107.6(2)
109.6(2)
C(2)−Ge(1)−C(3)
C(4)−Ge(2)−C(5)
C(1)−Ge(1)−Ge(2)
C(2)−Ge(1)−Ge(2)
C(3)−Ge(1)−Ge(2)
C(4)−Ge(2)−Ge(1)
C(4)−Ge(2)−Ge(2′)
C(5)−Ge(2)−Ge(1)
C(5)−Ge(2)−Ge(2′)
108.2(2)
104.9(2)
110.6(2)
110.5(1)
110.2(1)
105.1(1)
109.5(1)
111.2(1)
108.9(1)
C
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Figure 3. ORTEP diagram of Pri3Ge(GePh2)3GePri3 (5). Thermal ellipsoids are drawn at 50% probability.
Table 3. Selected Bond Distances (Å) and Angles (deg) for Pri3Ge(GePh2)3GePri3 (5)
Ge(1)−Ge(2)
Ge(2)−Ge(3)
Ge(3)−Ge(4)
Ge(4)−Ge(5)
Ge(1)−C(2)
Ge(1)−C(5)
Ge(1)−C(8)
Ge(2)−C(10)
Ge(2)−C(22)
Ge(3)−C(16)
Ge(3)−C(28)
Ge(4)−C(34)
Ge(4)−C(40)
Ge(5)−C(47)
Ge(5)−C(50)
Ge(5)−C(53)
Ge(1)−Ge(2)−Ge(3)
Ge(2)−Ge(3)−Ge(4)
Ge(3)−Ge(4)−Ge(5)
C(2)−Ge(1)−C(5)
C(2)−Ge(1)−C(8)
C(5)−Ge(1)−C(8)
C(10)−Ge(2)−C(22)
2.4775(8)
2.4654(8)
2.4769(8)
2.4641(8)
1.990(5)
1.988(5)
1.986(5)
1.974(5)
1.972(5)
1.969(5)
1.964(5)
1.955(5)
1.968(5)
1.999(5)
1.990(5)
1.990(5)
118.92(3)
114.35(3)
117.76(3)
111.1(2)
112.6(2)
109.9(2)
104.8(2)
C(16)−Ge(3)−C(28)
C(34)−Ge(4)−C(40)
C(47)−Ge(5)−C(50)
C(47)−Ge(5)−C(53)
C(50)−Ge(5)−C(53)
C(2)−Ge(1)−Ge(2)
C(5)−Ge(1)−Ge(2)
C(8)−Ge(1)−Ge(2)
C(10)−Ge(2)−Ge(1)
C(22)−Ge(2)−Ge(1)
C(10)−Ge(2)−Ge(3)
C(22)−Ge(2)−Ge(3)
C(16)−Ge(3)−Ge(2)
C(28)−Ge(3)−Ge(2)
C(16)−Ge(3)−Ge(4)
C(28)−Ge(3)−Ge(4)
C(34)−Ge(4)−Ge(3)
C(40)−Ge(4)−Ge(3)
C(34)−Ge(4)−Ge(5)
C(40)−Ge(4)−Ge(5)
C(47)−Ge(5)−Ge(4)
C(50)−Ge(5)−Ge(4)
C(53)−Ge(5)−Ge(4)
106.0(2)
108.0(2)
109.2(2)
111.9(2)
111.9(2)
103.8(2)
107.4(2)
111.9(2)
105.3(2)
110.5(2)
110.8(2)
105.8(2)
108.7(2)
109.0(2)
114.0(2)
104.3(2)
108.8(2)
104.3(2)
106.0(2)
111.7(2)
110.3(2)
108.9(2)
104.8(2)
Figure 4. Structure of 5 in the solid state (a) and packing diagram of 5 viewed along the b-axis (b).
and the terminal germanium atoms are disposed in a similar
fashion to those of 5.
The packing diagram of 5 is shown along the crystallographic
b-axis in Figure 4, where the individual molecules of 5 stack in a
columnar fashion with a terminal germanium atom canted above
and below the plane of three germanium atoms in a repeating
fashion. This is observed along the other two unit cell axes as
well and was also present in the crystallographic packing of 7.
This type of crystallographic packing results in dichroism in both
5 and 7. As shown in Figure 5, single crystals of 5 appear colorless
D
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Figure 5. Single crystal of 5 viewed under ambient light (left) and under cross-polarized light in two orientations (center and right).
Table 4. Electrochemical (CV and DPV) and UV/Vis Absorbance Data for 2−6
2
3
4
5
6
Eox (CV, mV)
n/a
1555
1525
1560
1545
1860
2055
1695 (sh)
1875
1865
2095
Eox (DPV, mV)
n/a
1480
1400
1820
2030
1380
1530
1890
1500
1650 (sh)
1800
2020
λmax (nm)
ε (M−1 cm−1
231
8.7 × 103
242
2.1 × 104
273
4.6 × 104
300
5.7 × 104
240
3.0 × 104
)
Figure 6. CV and DPV of 5 in CH2Cl2 solution.
under ambient light but pale yellow under cross-polarized light
in one orientation. When rotated by 30°, the crystals appear
purple/blue in color.
of 3 (Table 4). The CV and DPV of 3 are similar to those of
the trigermane Bun GeGePh2GeBun (8), which has a single
3
3
oxidation wave at 1540 mV in its CV and 1490 mV in its DPV,
and this similarity is expected since the inductive donating
effects of an n-butyl and isopropyl group are similar.54 Other
alkyl-substituted oligogermanes, including the series of com-
pounds Me(GeMe2)nMe (n = 2−6),55,56 exhibit only one irre-
versible oxidation wave in their CVs regardless of chain length.
However, the voltammograms of 3 and 8 differ from those of
other trigermanes that have aryl substituents at all three of the
Oligogermanes 2−5 were characterized using cyclic vol-
tammetry (CV) and differential pulse voltammetry (DPV) in
dichloromethane solvent using [Bun N][PF6] as the supporting
4
electrolyte. No discernible oxidation wave for the digermane 2
was observed under the experimental conditions. However, the
CV and DPV of trigermane 3 each exhibited a single irreversible
oxidation wave at 1555 mV in the CV and 1480 mV in the DPV
E
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Figure 7. Overlaid UV/visible spectrum (blue) and emission spectrum (red) of 5 in CH2Cl2 solution.
germanium atoms, which exhibit two successive oxidation waves
rather than one.52 This was observed for trigermane 6 (Table 4),
which contains two oxidation waves in its CV at 1545 and
1865 mV and in its DPV at 1530 and 1890 mV. Similarly, two
oxidation waves were observed in the CV of the trigermane
Ph8Ge3 at 1696 and 2052 mV.52
chain length increases. In addition the first oxidation wave for the
hexagermane 7 was observed at 1100 mV in its DPV, indicating
the 7 is easier to oxidize than the pentagermane 5.
The UV/visible spectra of oligogermanes 2−6 were recorded
in dichloromethane solvent, and the absorbance maxima of these
species were found to undergo a successive red shift as the length
of the germanium−germanium chain increased. The observed
absorption peaks correspond to the σ → σ* electronic transition
in these species. The λmax for 2 was observed as a shoulder at
231 nm, while the trigermane 3 and tetragermane 4 exhibited
clearly defined λmax peaks at 242 and 273 nm, respectively.
The pentagermane 5 has a λmax at 300 nm that is blue-shifted by
10 nm from that of the hexagermane 7. As was observed for 7, the
pentagermane 5 is luminescent in dichloromethane solution
and exhibits a broad emission band at 380 nm when excited at
302 nm. The absorbance and emission spectra of 5 are shown in
Figure 7, and it was found that the emission maximum of 5 is
red-shifted by 10 nm from that of hexagermane 7. Therefore, it
appears that the emission maximum shifts to lower energy as
the number of catenated oligogermanes decreases, which will
be investigated for other oligogermanes prepared in the future.
The absorbance and emission spectra of both 5 and 7 are in the
range of those reported for several polygermanes, including
(R2Ge)n (R = Et, Prn, Bun, Ph, p-Tol),4,10 which have absorbance
maxima ranging from 293 to 326 nm, and ((Me3SiOC6H4)2Ge)n,
which has an emission maximum at 369 nm.2 Thus, it is likely that
the polygermanes might have regions of order consisting of five,
six, or more germanium atoms arranged in a trans-coplanar
arrangement such that σ-delocalization can occur. The chain-
length dependence of the emission maxima in 5 and 7 also
indicates that this attribute is tunable by variation of the length of
the germanium−germanium backbone in these systems.
The presence of aryl substituents presumably stabilizes the
species that are generated after the oxidation event takes place
such they can also be oxidized as the sweep continues to more
positive potential. Photochemical investigations of oligoger-
manes indicate that both homo- and heterolytic cleavage of the
germanium−germanium bond can occur to generate radicals and
germylenes, respectively.9,34,57−59 Recently, it has been shown
that the germanium−germanium bonds in the trigermane 8 and
the digermane Bun GeGePh3 undergo homolytic cleavage upon
3
irradiation with UV−C light to generate germanium radicals.59 It
is possible that a similar process may be occurring electrochemi-
cally, where homolytic Ge−Ge bond scission is occurring after
oxidation of the oligogermanes to yield germanium-based radical
species. However, efforts to trap the electrochemically generated
products formed from 3 or higher oligogermanes have, to date,
not been fruitful.
The CV and DPV of 4 each contain three irreversible oxidation
waves at 1525, 1860, and 2055 mV (CV) and 1400, 1820, and
2030 mV (DPV). The CV and DPV of 5 are shown in Figure 6,
and there are four irreversible waves for this species in the DPV
that were observed at 1380, 1500, 1800, and 2020 mV (Table 4).
There is also a shoulder that is visible at 1650 mV that is likely
due to the formation of a minor species that is generated
electrochemically. In the CV of 5, the first and second oxidation
waves were not resolved, and thus three features at 1560, 1875,
and 2095 mV and a shoulder at 1695 mV were observed.
Comparing the DPV data for 3−5, it can be seen that the
oligogermanes become easier to oxidize as the chain length
increases from three to five germanium atoms. This has been
observed in several other series of oligogermanes as well and
indicates that the HOMO of these molecules, which is where the
electron is abstracted, becomes destabilized in energy as the
CONCLUSIONS
■
The series of triisopropyl-terminated oligogermanes Pri3Ge-
(GePh2)nGePri3 (n = 0−3) exhibit a successive red shift in their
absorbance maxima (λmax) as the length of the germanium−
germanium chain increases. In addition, these molecules become
F
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Article
was returned to room temperature and was filtered though Celite.
The organic layer was separated, and the aqueous layer was washed with
Et2O (3 × 5 mL). The combined organic layer and extracts were dried
over anhydrous MgSO4, the mixture was filtered, and the volatiles were
removed in vacuo to yield Pri3GeH (0.15 g, 49%) as a colorless oil. 1H
NMR (C6D6, 25 °C): δ 3.74 (s, 1H, Ge−H), 1.29 (sept, J = 8.0 Hz, 3H,
−CH(CH3)2), 1.15 (d, J = 8.0 Hz, 18H, −CH(CH3)2) ppm. 13C NMR
(C6D6, 25 °C): δ 21.1 (−CH(CH3)2), 13.8 (−CH(CH3)2) ppm. Anal.
Calcd for C9H22Ge: C, 53.27; H, 10.93. Found: C, 53.11; H, 10.98.
Synthesis of Pri3GeGePri3 (2). To a solution of Pri3GeH (0.15 g,
0.74 mmol) in CH3CN (10 mL) was added a solution of Pri3GeNMe2
(0.15 g, 0.61 mmol) in CH3CN (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, and the crude product mixture was distilled in a
Kugelrohr oven (0.05 Torr, 125 °C). The remaining material was
identified as 2 as a colorless oil (0.176 g, 72%). 1H NMR (C6D6, 25 °C):
δ 1.41 (sept, J = 8.8 Hz, 6H, −CH(CH3)2), 1.08 (d, J = 8.8 Hz, 36H,
−CH(CH3)2) ppm. 13C NMR (C6D6, 25 °C): δ 20.1 (−CH(CH3)2),
19.5 (−CH(CH3)2) ppm. Anal. Calcd for C18H42Ge2: C, 53.54; H,
10.48. Found: C, 53.41; H, 10.39.
easier to oxidize as the chain length increases, as shown by their
CV and DPV data. The X-ray crystal structures of 4 and 5 were
determined, and all four germanium atoms in the tetragermane 4
are arranged in a trans-coplanar fashion. The structure of the
pentagermane 5 has the three central germanium atoms arranged
in a coplanar fashion with the one terminal triisopropylgermane
group canted above and the other canted below the Ge3 plane.
The individual molecules of 5 stack in a columnar fashion along
all three crystallographic axes and as a result, crystals of 5 are
remarkably dichroic, appearing either pale yellow or blue under
cross-polarized light in different orientations. In addition, 5
is luminescent in dichloromethane and exhibits an emission
maximum at 380 nm when excited near its absorbance maximum
(300 nm). The emission maximum of 5 is red-shifted relative to
that of the hexagermane Pri3Ge(GePh2)4GePri3 (7) by 10 nm.
Both 5 and 7 exhibit physical properties that mimic the larger
polygermane systems, and it is likely that other long-chain olig-
ogermanes will have similar physical properties. The absorbance,
emission, and electrochemical properties of these systems can be
tuned by variation of their composition, and this should lead to
their being employed as useful optical and electronic materials.
Synthesis of Pri3GeGePh2GePri3 (3). To a solution of Ph2GeH2
(0.145 g, 0.634 mmol) in CH3CN (15 mL) was added a solution of
Pri3GeNMe2 (0.360 g, 1.46 mmol) in CH3CN (15 mL). The reaction
mixture was sealed in a Schlenk tube and was heated in an oil bath at
85 °C for 48 h. The volatiles were removed from the reaction mixture
in vacuo, and the crude product mixture was distilled in a Kugelrohr oven
(0.04 Torr, 135 °C). The material remaining in the pot was identified
as 3 and was isolated as a colorless oil (0.180 g, 45%). 1H NMR (C6D6,
25 °C): δ 7.48 (d, J = 6.4 Hz, 4H, o-C6H5), 6.99−6.90 (m, 6H, m-C6H5
and p-C6H5), 1.33 (sept, J = 7.2 Hz, 6H, −CH(CH3)2), 0.96 (d, J =
7.2 Hz, 36H, −CH(CH3)2) ppm. 13C NMR (C6D6, 25 °C): δ 138.5
(ipso-C6H5), 135.9 (o-C6H5), 128.6 (m-C6H5), 128.5 (p-C6H5), 21.3
(−CH(CH3)2), 16.8 (−CH(CH3)2) ppm. Anal. Calcd for C30H52Ge3:
C, 57.14; H, 8.31. Found: C, 57.07; H, 8.25.
EXPERIMENTAL SECTION
■
General Considerations. The reagents Ge(OEt)4 (Gelest),
Ph2GeH2 (Gelest), PriLi (0.70 M in pentane, Aldrich), HCl (1.0 M in
Et2O, Aldrich), and LiNMe2 (Aldrich) were purchased from commercial
sources and were used as received. The reagents HGePh2GePh2H52 and
HGePh2GePh2GePh2H were prepared according to literature proce-
dures.50 All manipulations were carried out using standard Schlenk,
syringe, and glovebox techniques, and solvents were dried using a
Glass Contour solvent purification system. NMR spectra were recorded
on a Bruker Inova spectrometer operating at 400.00 MHz (1H) or
100.57 (13C) MHz. IR spectra were obtained using a PerkinElmer
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 electrode, a platinum
wire counter electrode, and a Ag/AgCl reference electrode in CH2Cl2
solution using 0.1 M [Bu4N][PF6] as the supporting electrolyte. Ele-
mental analyses were conducted by Galbraith Laboratories or Midwest
Microlabs.
Synthesis of Pri3Ge(GePh2)2GePri3 (4). To a solution of
HGePh2GePh2H (0.42 g, 0.92 mmol) in CH3CN (15 mL) was added
a solution of Pri3GeNMe2 (0.48 g, 1.94 mmol) in CH3CN (10 mL). The
reaction mixture was sealed in a Schlenk tube and was heated in an oil
bath at 100 °C for 6 d. The volatiles were removed from the reaction
mixture in vacuo, and the crude product mixture was distilled in a
Kugelrohr oven (170 °C, 0.005 Torr). The material remaining in the pot
1
was identified as 4 as a yellow solid (0.35 g, 44%). H NMR (C6D6,
25 °C): δ 7.56 (d, J = 6.8 Hz, 8H, o-C6H5), 6.97 (m, 8H, m-C6H5), 6.90
(t, J = 7.2 Hz, 4H, p-C6H5), 1.53 (sept, J = 7.6 Hz, 6H, −CH(CH3)2),
1.16 (d, J = 7.6 Hz, 36H, −CH(CH3)2) ppm. 13C NMR (C6D6, 25 °C): δ
138.2 (ipso-C6H5), 137.1 (o-C6H5), 128.5 (m-C6H5), 128.3 (p-C6H5),
21.3 (−CH(CH3)2), 16.7 (−CH(CH3)2) ppm. Anal. Calcd for
C42H62Ge4: C, 58.83; H, 7.29. Found: C, 58.75; H, 7.32.
Synthesis of Pri3GeOEt. To a solution of PriLi (47.0 mL, 0.70 M in
pentane, 33 mmol) was added Ge(OEt)4 (4.16 g, 16.4 mmol) in pentane
(10 mL) dropwise over 20 min. The reaction was very exothermic,
and gelatinous material formed immediately upon addition of
Ge(OEt)4. The reaction mixture was stirred for 18 h, and the volatiles
were removed in vacuo. The crude reaction mixture was dissolved in
benzene (75 mL) filtered through Celite, and the solvent was removed
from the filtrate in vacuo to yield Pri3GeOEt as a pale yellow oil (0.82 g,
Synthesis of Pri3Ge(GePh2)3GePri3 (5). To a solution of 6 (0.39 g,
0.57 mmol) in CH3CN (10 mL) was added a solution of Pri3GeNMe2
(0.29 g, 1.19 mmol) in CH3CN (5 mL). The reaction mixture was sealed
in a Schlenk tube and was heated in an oil bath at 130 °C for 7 d, after
which a white precipitate had formed. The reaction mixture was filtered,
and the solid was washed with hexane (25 mL). The white solid was
isolated to provide 5 (0.32 g, 52%). 1H NMR (C6D6, 25 °C): δ 7.80 (d, J
= 8.1 Hz, 4H, o-C6H5), 7.44 (d, J = 8.2 Hz, 8H, o-C6H5), 7.20−7.08 (m,
14H, p-C6H5 and m-C6H5), 7.02 (t, J = 8.0 Hz, 4H, p-C6H5), 1.54 (sept, J
= 8.3 Hz, 6H, −CH(CH3)2), 0.96 (d, J = 8.3 Hz, 36H, −CH(CH3)2)
ppm. 13C NMR (C6D6, 25 °C): δ 140.8 (ipso-C6H5), 140.4 (ipso-C6H5),
137.2 (o-C6H5), 136.9 (o-C6H5), 128.5 (m-C6H5), 128.2 (m-C6H5),
128.1 (p-C6H5), 127.9 (p-C6H5), 21.4 (−CH(CH3)2), 18.4 (−CH-
(CH3)2) ppm. Anal. Calcd for C54H72Ge5: C, 59.81; H, 6.69. Found: C,
59.47; H, 6.60.
1
60% based on PriLi). H NMR (C6D6, 25 °C): δ 3.72 (q, J = 8.1 Hz,
2H, −OCH2CH3), 1.38 (sept, J = 8.0 Hz, 3H, −CH(CH3)2), 1.26 (t, J =
8.1 Hz, 3H, −OCH2CH3), 1.17 (d, J = 8.0 Hz, 18H, −CH(CH3)2) ppm.
13C NMR (C6D6, 25 °C): δ 59.5 (−OCH2CH3), 18.8 (−CH(CH3)2),
18.1 (−CH(CH3)2), 15.3 (−OCH2CH3) ppm. Anal. Calcd for C11H26-
GeO: C, 53.50; H, 10.61. Found: C, 53.67; H, 10.69.
Synthesis of Pri3GeCl (1). To a solution of Pri3GeOEt (0.54 g,
2.19 mmol) in hexane (20 mL) was added HCl (10.0 mL, 1.0 M in Et2O,
10 mmol). The reaction mixture was stirred for 1.5 h, and the vol-
atiles were removed in vacuo to yield Pri3GeCl (1) as a colorless oil
1
(0.43 g, 82%). H NMR (C6D6, 25 °C): δ 1.40 (sept, J = 8.1 Hz, 3H,
−CH(CH3)2), 1.09 (d, J = 8.1 Hz, 18H, −CH(CH3)2) ppm. 13C NMR
(C6D6, 25 °C): δ 19.1 (−CH(CH3)2), 18.9 (−CH(CH3)2) ppm. Anal.
Calcd for C9H21ClGe: C, 45.54; H, 8.92. Found: C, 45.57; H, 8.89.
Synthesis of Pri3GeH. To a solution of Pri3GeCl (0.36 g, 1.5 mmol)
in Et2O (25 mL) was added a suspension of LiAlH4 (0.07g, 2 mmol) in
Et2O (20 mL). The reaction mixture was stirred overnight and then
was quenched with degassed deionized water at −78 °C. The mixture
X-ray Crystal Structure Determinations. Diffraction intensity
data were collected with a Siemens P4/CCD diffractometer. Crystallo-
graphic data for the X-ray analysis of 4−6 are collected in Supporting
Information (Table S1). The crystal-to-detector distance was 60 mm,
and the exposure time was 20 s per frame using a scan width of 0.5°.
The data were integrated using the Bruker SAINT software program.
Solution by direct methods (SIR-2004) produced a complete heavy
G
DOI: 10.1021/acs.organomet.6b00596
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Article
atom phasing model consistent with the proposed structures. All non-
hydrogen atoms were refined anisotropically by full-matrix least-squares
(SHELXL-97). All hydrogen atoms were placed using a riding model.
Their positions were constrained relative to their parent atom using the
appropriate HFIX command in SHELXL-97.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.6b00596.
■
S
Table of crystallographic data (Table S1) for compounds
4, 5, and 6; NMR spectra for all newly prepared com-
pounds (PDF)
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Crystallographic data for compounds 4, 5, and 6 (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: Charles.s.weinert@okstate.edu. Tel: (405) 744-6543.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by a grant from the National Science
Foundation (CHE-1464462) and is gratefully acknowledged.
■
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