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Gelest, Inc.
Applications of Germanium Compounds
Andrew E. Wille and Barry Arkles
Gelest Inc.
Germanium compounds have emerged as critically important materials in the fabrication of microelectronics,
optics and sensors. Potential new applications in organic transformations and polymer synthesis have also been
reported. This article highlights some of the chemistry associated with these applications. It also compares and
contrasts the chemistry of germanium with the more widely understood chemistry of silicon. For readers with a
deeper interest in the chemistry of germanium, comprehensive reviews provide a detailed description.1,2,3,4,5
Metallic Germanium and Metallization Chemistry
Germanium is a semiconductor. In fact, germanium was the first material to be fabricated into practical semiconductor devices, diodes. As a consequence, a significant body of literature relating to the electrical properties of
germanium has developed. The mobility of holes in germanium is greater than that of silicon and of any other
common semiconductor. The hole and electron mobilities are closer in germanium than other semiconductors,
particularly at low temperatures. This has made germanium an attractive candidate for high performance CMOS
technology. In high speed digital communications associated with broad-band and cell-phones, SiGe films are
used to fabricate heterojunction bipolar transistors (HBTs). In other high speed applications SiGe films are
deposited as molecular films which act as templates for the epitaxial deposition of silicon. The resulting lattice
mismatch creates a strained silicon layer which exhibits enhanced electron mobility, providing improved device
performance. SiGe technology has the potential to replace GaAs in many applications. Other electronic applications of metallic germanium and its alloys include quantum dots, amorphous films for solar energy and LEDs.6,7
Germane, GeH4, in combination with silane, SiH4, is the most widely used germanium precursor for chemical
vapor deposition (CVD) of SiGe films. Other precursors include methylgermane, n-butylgermane, t-butylgermane
and diethylgermane.8
A broader range of germanium compounds have been used to synthesize germanium nanowires including
diphenylgermane9,10 and germanium tetraiodide11,i. Amorphous a-GeCO:H films have been deposited from
tetramethylgermane by PECVD.12
Bulk germanium metal, fabricated from wafers, is currently of interest in the fabrication of monolithic
fiber-optic receivers, since germanium photodiodes perform well at 1.3-1.5 micron wavelengths.
Germanium metal is also of utility in infrared detectors, which comprise the metal’s largest current use.
Inorganic Germanium Chemistry
Bond energies of Ge-X bonds are generally considered to be about 10% less than corresponding Si-X bonds.
This is reflected in the somewhat lower thermal stability of germanium compounds. The electronegativities of
germanium and silicon are 2.01 and 1.90, respectively, on the Pauling scale; their configuration energies are 11.80
eV and 11.33 eV, respectively. While silicon is apparently more electropositive than germanium, reactivity
indicates that polarization parameters may be more equivalent. Valence states of +2 are readily accessible for
germanium and GeCl2, GeBr2 and GeI2 are isolable compounds. In contrast, silicon dihalides are known as reactive intermediates by studies of emission bands or trapping experiments. The chemistry of both elements proceeds
primarily in the +4 state, although hyper-coordinate states, particularly for fluorocompounds, are known.
Both silicon and germanium metal react with halogens and hydrogen halides to give analogous products.
Halides of both elements react rapidly with water to give oxides and hydroxides. Germanium oxides can be
converted back to the halides by reaction with the appropriate acid. In the case of silicon, only hydrofluoric acid
reacts with the oxides to form fluorosilanes.
Germanium compounds have higher optical densities than silicon compounds. This leads to the second major
application of germanium compounds: the controlled combustion of germanium tetrachloride with silicon tetrachloride in a hydrogen-oxygen flame in chemical vapor axial deposition (CVAD) for the formation of ingots from
which step-index fiber optics are drawn.,14,15 Tetraethoxy-germane is a liquid metal-organic precursor with a high
vapor pressure that is well-suited for CVD of germanium oxide in optical16 and dielectric17applications. Sol-gel
processing has also been employed as a method of preparing mixed germanium oxides. Again, tetraethoxygermane is the most widely used metal-organic precursor, but other germanium alkoxides containing methoxy and
isopropoxy groups are used.18
Reprinted from: Metal-Organics for Materials, Polymers and Synthesis © Gelest Inc., 2011
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Organic Chemistry
In contrast to silicon, sterically hindered organogermanium compounds in the +2 state are
readily isolated. Examples include dicyclopentadienylgermanium, di[bis(trimethylsilyl)methyl]
germanium and bis[bis(trimethylsilyl)amino]germanium.
(CH3)3Si
Si(CH3)3
N Ge
Ge
(CH3)3Si
(CH3)3Si
H C Ge
(CH3)3Si
N
Si(CH3)3
Si(CH3)3
C H
Si(CH3)3
The bis[bis(trimethylsilyl)amino]germanium forms poly(germanium enolates) upon reaction with
_,`-unsaturated ketones.19
O
(CH3)3Si
((CH3)3Si)2N
Si(CH3)3
N Ge
(CH3)3Si
N
Ge
((CH3)3Si)2N
+
Si(CH3)3
O
Di[bis(trimethylsilyl)amino]germanium also reversibly binds H2 and CO2 in 3-coordinate complexes.
Germanium dihalides, GeI2 and GeCl2 and its readily prepared complexes GeCl2-etherate and GeCl2dioxanate react with alkyl and aryl halides20 to yield alkyl and aryl germanium trihalides.
CH3CH2CH2CH2Cl
+ GeCl 2
CH3CH2CH2CH2GeCl 3
GeCl 3
Br
3 GeCl 2 ¥• O
O
+
3
2
GeBr 3
+
Most germanium proceeds in the +4 state and is comparable to silicon. The following enumeration is
extremely incomplete.
Grignards and alkali metal organics will displace halides bound to germanium and silicon to give the
organo-substituted compound.
Germanium forms Ge-Ge bonds readily through Grignard chemistry in addition to alkali metal coupling,
which is the most accessible route for Si-Si bond formation. While Si-Si bonds can be readily cleaved by
Na-K in ether, Ge-Ge are cleaved only under more rigorous conditions such as the use of higher boiling
solvents like tetrahydrofuran. In both cases multiple metal-metal bonds give photoreactive compounds.
Organogermanium and organosilicon compounds can both be prepared by the copper catalyzed direct
reaction of alkyl and aryl halides with the metal.
Silicon undergoes migration from carbon to oxygen in the Brook rearrangement of _-silyl ketones.
Germanium does not appear to undergo a similar migration.
Doubly bonded silicon-silicon and germanium-germanium compounds have been demonstrated, but are
not representative of large classes of chemistry for the elements.
Perfluorinated methyl and ethyl germanes are isolable and stable. Simple trifluoromethyl(methyl)silanes
have only recently been prepared. Other perfluorinated methyl and ethyl silanes are thermally unstable,
decomposing above 80°.
Repeating silicon-oxygen bonds tend to form extended linear polymers in preference to cyclic structures.
Repeating germanium-oxygen bonds tend to form cyclic structures in preference to extended linear polymers.
In contrast to silicon hydrides, germanium hydrides readily undergo metallation. For example, the reaction of triphenylgermane with butyl lithium followed by carbonation yields triphenylgermanecarboxylic acid.
Like organosilanes, organogermanes can be fluorodemetallated. Additionally, organogermanes can be
demetallated with bromine.
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Germanium hydrides and silicon hydrides react with olefins in hydrogermylation and hydrosilylation
reactions. Both are catalyzed by platinum compounds, but catalysis is not a requirement for hydrogermylation.
Trichlorogermane is particularly reactive and reaction products are associated with both hydrogermylation
and dichlorogermene insertion.21
R2CHCR2GeCl 3
RC CR’GeCl3
RGeCl 3
R2C=CR2
RX
RC CR’
HGeCl 3
RCOCl
R2O
R
RGeCl 3
RCOGeCl 3
R
GeCl 3
GeCl 3
Cl 3Ge
Germanium in Organic Synthesis
For a variety of reasons applications of organogermanes in organic synthesis are limited, while applications
of organosilanes in organic synthesis are well-developed. Until recently organogermanes have had relatively
poor availability compared to organosilanes. Also, the germanium oxygen bond is somewhat more susceptible to
hydrolysis compared to the silicon oxygen bond. Beyond these facts, the largest determinant for differences in
applications is the higher cost of germanium compounds. Consequently, only unique applications of organogermanium compounds are likely to be of importance. Potential applications include trimethylgermylacetonitrile as
a carbanion source, trimethylbromogermane and trimethylchlorogermane employed in the preparation of masked
dienolates in a variety of regioselective syntheses, trichlorogermane for ether cleavage, and germanium diiodide
and cesium trichlorogermanate for conversion of alkyl halides to alkylgermyltrihalides.
The palladium catalyzed cross-coupling reactions of germanium compounds have been investigated.22,23,24,
O
Pd(0), Ar 1X
GeH
3
Cs2CO3
Pd(0), Ar 2X
GeAr1
n-Bu4NF
3
O
Br
Ar1
Ar 2
R
N
Pd catalyst
+
Ge
O
R
O
O
Tri-n-butylgermane is an effective reducing agent. It reduces benzylic chlorides 70x faster than silyl
hydrides and reduces acyl chlorides to aldehydes in the presence of Pd(0) .
Cl
C 6H 5
C
H
H
C 6H 5
C
H
nBu
+
nBu
Ge
nBu
H
TiCl4
+
nBu
CH 3O
nBu
Ge
Cl
nBu
CH 3O
Polymers and Polymerization
The most commercially important application of germanium compounds are as catalysts in polymerization
of polyester. Non-yellowing polyester fibers and clear polyester beverage bottles are produced by germanium
dioxide and hydroxygermatrane catalyzed condensation of dimethyl terephthalate and selected diols.
Copolymers have been prepared from organogermanes with polymerizeable functionality, including allyl, vinyl27
and methacrylate28 groups.
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REFERENCES:
1. M. Lesbre, P. Mazerroles and J. Satge, The Organic Compounds of Germanium, Wiley, 1971
2. S. Patai, ed. “The Chemistry of Organic Germanium, Tin and Lead Compounds, Vol. 1, Wiley, 1995
3. Z. Rappoport, ed. “The Chemistry of Organic Germanium, Tin and Lead Compounds, Vol. 2, Wiley, 2002
4. B. J. Aylett, Organometallic Compounds, Volume 1, Part 2, Chapman & Hall, 1979
5. V. I. Davydov, Germanium, Gordon & Breach, 1966
6. W. DeBoer, et al, Appl. Phys. Lett., 1991, 58, 1226.
7. H. Kuhne et al, J. Mater. Res., 1993, 8, 131.
8. B. Kellerman et al, J. Vac. Sci & Tech. A., 1995, 13, 1819.
9. T. Hanrath et al, Adv. Mater., 2003, 15, 437
10. K. Ryan et al, J. Am. Chem. Soc., 2003, 125, 6284.
11. Y. Wu et al, Chem. Mat., 2000, 605, 12.
12. A. Grill et al, J. Mater. Res., 2002, 17, 367.
13. R. Maurer, US Pat. 3,737,293, 1973.
14. J. MacChesney et al, US Pat. 4,217,027, 1980.
15. C. Hung et al, J. Mater. Res., 1992, 7, 1870.
16. D. Secrist et al, Bull. Am. Ceram. Soc., 1966, 45, 784.
17. S. Fisher et al, Solid State Tech., September, 1993, 55.
18. see specific references in the germanium compound section following this article
19. S. Kobayashi et al, J. Am. Chem. Soc., 1992, 114, 4929.
20. F. Riedmiller et al, Organometallics, 1999, 18, 4317
21. W. Wolfsberger, J. Praktische Chem., 1992, 334, 453
22. T. Nakamura et al, Org. Lett., 2002, 4, 3165
23. M. Kosugi et al, J. Organomet. Chem., 1996, 508, 255
24. J. Faller et al, Organometallics, 2002, 21, 5911
25. H. Mayr et al, Angew. Chem. Int’l Ed Eng., 1992, 31, 1046.
26. L. Geng, X. Lu, Organomet. Chem., 1989, 376, 41.
27. S. Rafikov, Izv. Akad. Nauk. SSR, Ser. Khim., 1982, 920
28. D. Mixon et al, J. Vac. Sci. Technol., B, 1989, 7(6), 1723
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Stable Homonuclear Double and Triple Bonded Germanium Species and Their Reactions
Elke Hoppe and Philip P. Power
Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California
95616, USA. E-mail: pppower@ucdavis.edu
“Although germanium is only slightly larger than silicon, it is too large for any !-overlap and no
multiple bonded germanium compounds are known.”1
There was widespread agreement with this opinion until relatively recently,2 but in the last quarter
of the 20th century and the beginning decade of the 21st the synthesis of numerous stable compounds
that contain multiple bonds to germanium and other heavier main group elements has not only vitiated
this statement, but has also changed our perception of bonding itself. Germanium compounds have
played a key role in effecting this change. This is because group 14 element compounds have occupied
a central position in the study of multiple bonding among heavier main group elements and germanium
is the central element in group 14. In many respects its multiple bonded derivatives represent a
transition point in the geometrical, bonding and reactivity changes that occur with increasing atomic
number. In this article we provide a brief summary of two classes of multiple bonded germanium
compounds. These are the germanium congeners of the double bonded olefins (digermenes) or the
triple bonded alkynes (digermynes).
The first multiple bonded germanium species to be isolated had their origin in the work of Lappert
and his group in the 1970s. The original objective of their work was the preparation of stable heavier
group 14 element carbene analogues of formula: ER2 (E = Si, Ge, Sn, or Pb; R = amido or alkyl group).
The key element of the synthetic approach was simply to use large ligands such as the amido or alkyl
groups -N(SiMe3)2 or -CH(SiMe3)2 to achieve stability by preventing association via ligand bridging or
E-E bond formation. A very simple salt elimination route sufficed to isolate the first red tin and purple
lead dialkyl species: ER2 via reaction of the lithium salt of the ligand with the metal dihalide,3 but the
corresponding yellow germanium dialkyl :Ge{CH(SiMe3)2}2 was most conveniently obtained via the
reaction of :Ge{N(SiMe3)2}2 with two equivalents of LiCH(SiMe3)2. 4 ,5 Like its tin counterpart:
Ge{CH(SiMe3)2}2 existed as carbene-like monomers in solution5 and could behave as a Lewis base
donor toward transition metals.6 However, it was later shown to have a Ge-Ge double bonded dimeric
structure 7 of formula (Dis)2Ge=Ge(Dis)2, Dis = CH(SiMe3)2 8 (like its tin counterpart
(Dis)2Sn=Sn(Dis)2) in the solid state.4,5,7
Ge[N(SiMe3 ) 2 ]2 + 2 LiDis
- 2 LiN(SiMe 3) 2
Ge(Dis) 2
1/
2
Dis
Dis
Ge
Ge
Dis
Dis
Scheme 1
The most surprising aspect of the tin and germanium structures was that they were not planar like
ethylene but had a trans-pyramidalized structure with local C2h symmetry at the C2E=EC2 (E = Ge or
Sn) cores. Moreover the GeGe and SnSn bond lengths were only slightly shortened compared to single
bonds. Synthetic and structural results like these initiated a revolution not only in main group chemistry
but have also led to new insights in chemical bonding.9,10,11 Numerous classes of multiple bonded
germanium compounds now exist12,13,14,15,16,17 but the scope of this review is confined to stable species
with double or triple bonds between two germanium atoms.
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DIGERMENES
Syntheses of Digermenes
Whereas the first digermene was synthesized by Lappert via a salt elimination route, several
different synthetic approaches have since been developed. These include photochemical conversions,
various elimination reactions as well as insertion routes which are summarized in the following
sections.
Photochemical Conversions
A photochemical conversion led to the first structural characterization of a stable digermene in the
solid state in 1984 by the group of Masamune. Photolysis of (Ar2Ge)3 (Ar = 2,6–dimethylphenyl or 2,6diethylphenyl) afforded of Ar2Ge=GeAr2 as shown in Scheme 2.18
H3C
Ar2
Ge
Ar2Ge
h
Ar2Ge
GeAr2
GeAr2
C2H5
,
Ar =
H3C
C2H5
Scheme 2: Photochemical route to digermenes
The Ge-Ge bond length was found to be 2.213(2) Å which is considerably shorter than the single
bond distance of 2.44 Å and the coordination of the Ge atoms displayed pyramidal distortion in which
the germanium was displaced by 0.15 Å from the plane formed by the two ipso-carbons and the other
germanium. Similar structural data were found for tetrakis(2,4,6-triisopropylphenyl)digermene 19
(GeGe = 2.213(1) Å) whereas the tetrakis(mesityl)digermene,20 which was synthesized by the same
route in 2007, shows a slightly longer Ge-Ge distance of 2.2856(8) Å. These structures had a greater
pyramidal distortion (pyramidal displacement = 0.42 Å) than the corresponding disilenes with the same
ligands (e.g. Mes2Si=SiMes2, Mes = -C6H2-2,4,6-Me3: displacement = 0.22 Å)21 but a smaller one than
distannenes such (Dis)2Sn=Sn(Dis)2 (displacement = 0.61 Å)7.
Reductive Elimination Synthetic Routes
Reductive elimination reactions are a well established method for synthesis of multiple bonded
germanium species. These are usually effected by dehalogenation or salt elimination reactions.
Masamune and co-workers used lithium naphthalenide as a reducing agent for dichloro(2,6diisopropylphenyl)mesitylgermane (Dipp(Mes)GeCl2), Dipp = C6H3-2,6-iPr2. Under elimination of
lithium chloride the product was formed as two isomers with the Z isomer being more stable than the E
isomer but only the Z variant crystallized upon concentration of a solution of the reaction mixture.22
The GeGe bond length is 2.301(1) Å. It was shown by kinetic studies that isomerization proceeds in a
staightforward way via rotation around the GeGe bond axis and not through a thermal dissociationrecombination mechanism.
2 Dipp(Mes)GeCl2
Li naphthalenide
Dipp(Mes)Ge
Ge(Mes)Dipp
E and Z isomers
Scheme 3
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Reduction of dichlorobis(diisopropylmethylsilyl)germane with a sodium dispersion afforded the
tetrakis(trialkylsilyl)digermene. As shown by the group of Kira23 the trialkyl substituents enforce a
nearly planar geometry around the GeGe bond and no twisting is observed. The distance between the
two germanium atoms is 2.268(1) and 2.266(1) Å, respectively, and is thus shorter than in
Dis2GeGeDis2 (2.3458(7) Å)7 or Dipp(Mes)GeGe(Mes)Dipp (2.3011(9) Å)22.
2 (R 3Si)2 GeCl2
2 Na
(R3 Si) 2 Ge
Ge(SiR3 )2
Scheme 4
The use of lithium naphthalenide as reducing agent for Tbt(Mes)GeCl2 (Tbt = C6H2-2,4,6[CH(SiMe3)2]3) by Tokitoh and coworkers afforded MesTbtGeGeMesTbt.The GeGe bond length
(2.416(3) Å) in this compound is considerably longer than GeGe double bond lengths of other reported
digermenes with carbon substituents and is very close to a normal GeGe single bond of GeGe = 2.44 Å
in elemental germanium. Elongation of the GeGe double bond is consistent with the very large size of
the substituents. Considerable geometrical distortion, which also involves twisting of the geometry
around the GeGe axis, was also observed.24
2 Tbt(Mes)GeCl 2
Li naphthalenide
Tbt(Mes)Ge
Ge(Mes)Tbt
Scheme 5
Reaction of a tetrachloro digermane with 1,1-dilithiosilane R2SiLi2 (R = SiMetBu2) afforded a
heavier group 14 element cyclopropene along with a linear digermene via salt elimination.25,26 The
GeGe double bond in the three membered ring is a relatively short one (2.2429(5) Å).
RCl2GeGeCl2R
R2
E
R2ELi2 (xs)
- 2 LiCl
RGe
GeR
+
R2E
ER2
E = Si, Ge
Scheme 6
The use of terphenyl groups (Ar' = C6H3-2,6(C6H3-2,6-íPr2)2 or Ar* = C6H3-2,4,6(C6H3-2,6-iPr3)2) as
sterically encumbering ligands enabled halogeno substituted digermenes to be isolated. These species,
which are in equilibrium with the corresponding monomeric germylenes, allow further digermenes to
be readily synthesized by replacement of the chlorines.27 The chlorines can be substituted by methyl,
ethyl or phenyl groups in a salt elimination reaction with the corresponding lithium reagents. In
contrast the analogous terphenyl tin (II) chlorides display trans-bridged structures with no tin-tin
bond.27 Corresponding dihydrodigermenes can be easily obtained via reaction of the chloro derivatives
with LiB(s-Bu3)H.28 The product has crystallographic 2/m symmetry in which the C-Ge-Ge-C array is
incorporated in the mirror plane with a Ge-Ge distance of 2.3724(9) Å. The GeGe bond length is longer
than average (see below in Table 1) probably as a result of steric effects.
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2 LiAr
- 2 LiCl
2 :GeCl2 (dioxane)
ArClGe
2 ArGeCl
GeClAr
Ar = Ar', Ar*
Ph
Ge
H
Ar'
.
2 LiPh Et2O
Ge
2 Ar'GeCl
Ar'
Ph
2 Li(BsBu3H)
Ge
Ar'
2 RMgBr
R = Me, Et
R
Ge
Ar'
Ar'
Ge
H
Ar'
Ge
R
Scheme 7
Reaction of the cationic tricyclic species (GeSitBu3)3+ (with a B(ArF)4- counter anion; ArF = -C6F5)
with potassium halide afforded the halogen-substituted cyclotrigermene.29 The GeGe double bond
(2.2743(8) Å) and the GeGe single bonds (2.4191(9) and 2.4200(9) Å) are relatively short. These
effects are explained by introduction of an electronegative group at the endocyclic sp3 germanium atom
and accordingly with the ?*-aromaticity concept.29
SitBu3
Sit Bu3
Ge
Ge
t
+
B(Arf)4
Ge
Ge
KX
-KB(Arf)4
Ge
Sit Bu3
Bu3Si
X
Ge
t
Sit Bu3
Bu3Si
Scheme 8
Weidenbruch and co-workers showed that tetragermanium 1,3-butadiene could lead to digermene
when it is reacted with 2-methoxyphenyl isocyanate. 30 During the reaction an initial germylene
cleavage occurs. The four membered ring is planar but it is not clear if the neighboring GeGe and CN
double bonds are conjugated. The digermene also formed in this reaction Tripp2GeGeTripp2 (Tripp =
C6H2-2,4,6-iPr3) had been obtained previously.19,31,32 The GeGe bond lengths in this molecule were
reported to be 2.2894(6) and 2.2635(15) Å, respectively.
RGe
NC
N
OMe
+
Scheme 9
100
RGe
R2Ge
GeR
GeR2
GeR
GeR2
OMe
+
R2Ge
GeR2
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Synthesis via Insertion Reactions
Another interesting approach for the synthesis of ring species containing GeGe double bonds was
described by Sekiguchi's group. Reaction of a three membered ring with GeCl2.dioxane to afford an
insertion reaction and a four membered ring product as shown by.33
t Bu MeSi
2
E
E'
t
t
SiMet Bu2
+ GeCl2 dioxane
Si
Ge
Ge
Si
Si
Cl
t Bu MeSi
2
SiMet Bu2
Bu2MeSi
SiMet Bu2
Bu2MeSi
E = Ge, E' = Si
E = Si, E' = Ge
Cl
SiMet Bu2
Scheme 10
PROPERTIES
Digermenes are usually colored diamagnetic compounds with the n+ " n- or n " p electronic
transitions as the chromophore. Digermenes can dissociate to germylene monomers in solution when
the substituents are sufficiently large. Whereas disilenes are usually dimeric in solid state as well as in
solution. About half of the digermenes dissociate into monomers in solution whereas almost all
distannenes are dissociated when dissolved. They can act as electrophilic compounds.
Structural aspects and bonding
Ge
Ge
Ge
Scheme 11
In contrast to olefins which usually have planar structures, digermenes typically display
considerable distortion from this ideal. These distortions can be defined with use of a number of
different parameters that include not only the length of the double bond between germanium atoms, but
also the angular parameters ? and ? illustrated above in Scheme 11. Instead of ? also the geometrical
displacement of the germanium atom above the plane of the connected atoms can be used to describe
the molecular geometry accurately.
The strength of the double bond between the two germanium atoms is relatively low. This, and the
observed geometry can be interpreted as a double donor-acceptor bond or, alternatively, by the valence
bond representation.7
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The molecular orbital (MO) treatment of their bonding also affords valuable insights. In this
approach the geometrical distortions are explained on the basis of mixing of anti-bonding and bonding
levels within the molecule. This mixing is a second order Jahn-Teller (SOJT) effect25-27 which arises
from a symmetry allowed, intramolecular mixing of an anti-bonding orbital with a bonding orbital
(generally the HOMO (highest occupied molecular orbital) in multiple bonded species).28,29 This can
lead to very large deformations in molecular geometry because the orbital mixing introduces nonbonding, lone pair character in a HOMO (originally a purely !-orbital) which can have a drastic effect
on molecular shape.30 The extent of the mixing is inversely proportional to the energy separation of the
orbitals and this is maximized in the heavier elements because the weakened bonding often permits a
close approach (<4eV) of the molecular levels. The initial stages of the SOJT distortion and orbital
interactions are shown in Scheme 12.
(b3u)
(bu)
n+ (ag)
(b1g)
Ge
Ge
(b2u)
Ge
n- (bu)
Ge
(ag)
(ag)
D2h (planar)
C2h (trans pyramidalized)
Scheme 12
The GeGe bond lengths in digermenes (Table 1) can be strongly influenced by the spatial and
electronic properties of the coordinating ligands. However, a correlation between steric bulk around the
germanium and its double bond length is not feasible.34,35,36 The nature of the ligands also influences
the twisting and bending around the GeGe double bond. It can be seen from Table 1 that the GeGe
bond lengths in digermenes vary from ca. 2.21 to 2.50 Å and have an average value of ca. 2.32 Å. Thus
the shortest (and presumably the strongest) Ge-Ge double bonds are about 9.4 % shorter than the 2.44
Å single bond length in elemental germanium.
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Table 1
Compound
Ref.
GeGe [Å]
(Dis)2Ge=Ge(Dis)2
37
2.3458(7)
(Et2C6H3) 2Ge=Ge(C6H3 Et2 )
18
2.212(3)
Ar'HGe=GeHAr'
83
2.3025(3)
Mes2Ge=GeMes2
38
2.2855(7)
MesDippGe=GeMesDipp
22
Tripp2Ge=GeTripp2
19
Ar'HGe=GeHAr'
28
t
trans-bending [°]
twisting [°]
color
!max [nm]
32
0
bright yellow
not reported
15
11
yellow
263, 412
45
not reported
orange
434
33.4
2.9
yellow
not reported
2.3011(9)
36
not reported
yellow
280, 412
2.213(1)
12.3
13.7
pale yellow
not reported
2.3724(9)
20.5
not reported
orange
not reported
(N(R')C2H 4N(R')Si(R) 2Ge)2 R' = Bu
39
2.4541(15)
41.3, 42.3
not reported
dark red
475
(N(R')C2H 4N(R')Si(R) 2Ge)2 R' = tBu,i Pr
40
2.4602(8), 2.4528(9)
„much larger than in
any other“
63
violet
557
Ar*MeGe=GeMeAr*
27
2.316(2)
39.7
not reported
yellow-green
not reported
Ar*EtGe=GeEtAr*
27
2.347(2), 2.271(6)
37.9
not reported
yellow
not reported
Ar*PhGe=GePhAr*
27
2.318(3)
33.7
not reported
orange
not reported
Ar*ClGe=GeClAr*
27
2.363(2)
36.8
not reported
orange
not reported
BbtBrGe=GeBrBbt
41
2.5088(8)
44.6
64.2
orange
449
42
2.2521(8)
0
20.4
orange-yellow
not reported
(Me PrSi)2Ge=Ge(SiMe Pr 2)2
43
2.267(2), 2.270(2)
5.9, 7.1
0, 0
yellow
361, 413
(iPr3Si)2Ge=Ge(SiiPr3) 2
43
2.2957(2)
16.4
0
pale yellow
472, 432, 367, 332, 277
Ar'ClGe=GeClAr'
79
2.4626(4)
38.8
not reported
orange
not reported
(TrippGe)2(Tripp2Ge)2 Se
30
2.2975(5)
33.4, 35.1
not reported
yellow
399
(TrippGe)2(Tripp2Ge)2 S
44
2.2841(5)
29.9, 35.7
not reported
yellow
400
( Bu2Ar)2Ge=Ge(Ar Bu2)2
45
2.3644(4)
37.2, 42.6
not reported
red
425
(Tripp)Ge=Ge(Tripp)(Tripp)Ge=Ge(Tripp)
46
2.3568(7), 2.3439(5)
not reported
not reported
greenish black,
dark blue hexane
solution
405, 560
MesTbtGe=GeTbtMes
24
2.416(3)
not reported
16.5, 34.6
t
t
( BuMe3C6H) 2Ge=Ge(C6H BuMe 3)2
i
i
t
t
orange, hexane hexane solution: r.t. 575
solution: r.t. blue (germylene); low temp.
low temp. orange
439 (digermene)
yellow
GeGe double bonds in three-membered cycles
(tBu3SiGe)2Ge(Cl)SitBu 3
47
2.2723(9)
trans-bent, not
reported
not reported
dark red
not reported
(tBu3SiGe)2Ge(Br)SitBu3
47
2.2742(9)
cis-bent, 45.1, 10.2
not reported
dark red
not reported
( Bu3SiGe)2Ge(I)Si Bu3
47
2.2720(6)
cis-bent, 43.3, 8.0
not reported
dark red
not reported
(tBu3SiGe)2Ge(SitBu3)Si(SiMe 3)
48
2.264(2)
cis-bent: 12.5, 4.4*
not reported
red
not reported
48
2.2638(11)
0
0
dark red
not reported
25, 26
2.2429(5)
51.0
not reported
dark red
470, 403, 311, 236
25, 26
2.2429(5)
60.5
not reported
dark red
457, 407, 324, 234
49
2.241(4)
not reported
not reported
dark red
not reported
49
2.240(3)
not reported
not reported
dark red
not reported
t
t
t
t
( Bu3SiGe)2Ge(Si Bu3)2
t
t
( Bu2MeSiGe) 2Si(Si Bu2 Me)2
t
t
( Bu2MeSiGe) 2Ge(Si Bu2 Me)2
(tBu3SiGe)2Ge(SitBu3)2
t
t
(( Bu3Ge)Ge)2Ge(Ge Bu3)2
GeGe double bonds in four-membered cycles
((SitBu2 Me)Ge)2(Si(Cl)SitBu2 Me)2
33
2.291(6)
not reported
not reported
bright orange
not reported
(Ar'Ge)2(CPh)2
50
2.4710(8)
not reported
not reported
dark red
375
* No energy minimum was found for the trans-bent form. The introduction of electropositve substituents leads to rather small trans bent angles.
see " Ref. 43
gelest.com
103
Gelest, Inc.
REACTIVITY
Digermenes display a wide variety of reactions. The range of such reactions is quite diverse but
most of them can be classified as additions, coordination reactions or reductions. Products that ensue
from the reactions of dissociated digermenes (i.e. germylenes) are not considered in this section.
Addition Reactions
Addition of Lewis acids or multiple bonded species such as O2, isonitriles, azides, butadiene, N2O
and diazomethane, to double bonds are well known from the chemistry of olefins and more recently
from the chemistry of disilenes. Addition reactions of digermenes have also been extensively studied.
Oxygen
Reactions of digermenes with dioxygen have been investigated by several research groups. Very
shortly after their structures had been established, digermenes were shown to react with oxygen by a
1,2-addition process affording 3,4-digermadioxetanes which rearrange to 2,4-digermadioxetanes with
irradiation.51 The GeGe distance (2.441(2) Å) in the 3,4-digermadioxetane is essentially the same as a
normal GeGe single bond length (2.44 Å).52
R
O
O
Ar2Ge
GeAr2
Ar = Dipp, C6H3Et2
O2
Ar2Ge
O
Ge
GeAr2
Ar
R
OH
90 °C
GeAr2
R = H, Me
h
O
Ar2Ge
GeAr2
O
Scheme 13
In 2003 Tokitoh and coworkers showed that exposure of solid TbtMesGe=GeTbtMes to an oxygen
atmosphere in the absence of light led to the formation of trans-1,3,2,4-dioxadigermetanes in two
conformational isomers. Formation of the corresponding cis isomer was not observed. Interestingly,
when the reaction was conducted with a digermene in solution a mixture of one of the trans isomers
and the cis isomer were formed. Most likely this can be explained by a dissociation of the digermene
into two germylene fragments in solution which react with dioxygen.24 The Ge...Ge distances (2.653(2),
2.660(2) and 2.691(4) Å, respectively) are much longer than typical Ge-Ge single bonds (2.457-2.463
Å).53,54 This is in contrast to the behaviour of the analogous silicon species in which the separation can
be even shorter than typical Si-Si single bond lengths. This can be explained by the shorter Si-O bond
lengths which impose closer Si...Si approaches.
104
Tbt
Mes
Ge
Ge
Mes
Tbt
O2
Mes
O
Ge
solid state
Tbt
Gelest, Inc.
Ge
O
Mes
Tbt
solution
Tbt
Ge
O2
Mes
Tbt
Mes
O
Ge
Mes
+
Ge
O
Tbt
Ge
Mes
Tbt
Tbt
O
Ge
O
Mes
Scheme 14
Exposure of digermene stabilized by 1,4-di-tert-butylbenzene ligands afforded a trigerma-1,3dioxolane product as shown by Weidenbruch's group.55
R2
Ge
R2GeGeR2
O
O
R2Ge
GeR2
O2
Scheme 15
The geometry of the five membered ring is nearly planar. The Ge-Ge bond length of 2.513(1) Å is
slightly longer than a typical GeGe single bond. Earlier, Baines and coworkers had shown that the
addition of O2 to tetramesityldigermene resulted in the formation of a 3,4-digermadioxetane with
subsequent insertion of a germylene into the O-O single bond.56
O
Mes2(H)GeSiEt3
Mes2
Ge
Mes2Ge
GeMes2
+
Mes2Ge
GeMes2
Mes2
Ge
O
h
O2
Et3SiH
Mes2Ge
O
GeMes2
O
O
+ Mes2Ge
GeMes2 +
Mes2Ge
O
GeMes2
O
Scheme 16
The GeGe single bond (2.504(3) Å) is slightly shorter than that in the five-membered ring reported
by Weidenbruch. Formation of the 1,3-digermadioxetane results from rearrangement during irradiation
of the 3,4 digermadioxetane as already shown above for Ar2GeGeAr2 by Masamune.51
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Gelest, Inc.
O
Mes2Ge
GeMes2
O
h
O
O
Mes2Ge
GeMes2
Mes2Ge
Mes2
Ge
O
O
Mes2Ge
GeMes2
Scheme 17
Digermaoxirane was formed as part of the reaction mixture and proved to be relatively stable since
it could be stored for several months under ambient conditions without decomposition. Nevertheless,
attempts at a direct synthesis from digermene and m-chloroperbenzoic acid failed even though a similar
approach had been used for the preparation of disilaoxiranes by West and coworkers.57 Instead, only
1,3 digermadioxetane, the pentacycle and only in very small amounts the digermaoxirane were
obtained.56
A tetragermabuta-1,3-diene was reacted with dry air to yield in a tetraoxatetragermabicyclo[4.1.1]octane skeleton. The folded cyclodigermoxane ring can be seen as a result of a [2+4] addition of O2 to
the terminal germanium atoms, followed by a 1,2-addition of a second O2 molecule.43
R
GeR
RGe
2 O2
GeR2
O
GeR2
R2Ge
R
O
Ge
Ge
O
R2Ge
O
Scheme 18
Diazomethane
MesDipGe=GeDipMes reacted with diazomethane R'HCN2 (R' = H, SiMe3) to give a
azodigermirane in a [2+1] cycloaddition. No subsequent loss of nitrogen was observed, however.19 In
contrast Ando and coworkers,58 obtained a digermirane in low yield as shown in Scheme 19. The
azodigermirane features a GeGe bond with a distance of 2.4237(4) Å, which is typical for a single
bond. A similar ring system was isolated as a product of the reaction of tetramesityldisilene with
phenyldiazomethane.59
CHR'
N
Tripp2Ge
GeTripp2
CHR'N2
N
TrippGe
R2Ge
Scheme 19
106
GeR2
GeTripp
CH2N2
R2Ge
GeR2
Gelest, Inc.
1,3-Butadiene
1,3-butadienes are well known reagents in olefin chemistry, particularly in connection with DielsAlder reactions. The first use of 1,3-butadiene in digermene chemistry in 2000 by Sekiguchi involved
an attempt to trap the digermene tBu3Si(Cl)Ge=Ge(Cl)SitBu3 which decomposes during warming above
a temperature of -8 °C. Its reaction with isoprene and 2,3-dimethyl-1,3-butadiene, respectively,
afforded only a single stereoisomer of the addition product.60
t
tBu Si
3
Cl
Cl
Ge
Ge
Cl
t
Bu3Si
SitBu3
Cl
Ge
- NaCl
Cl
Na
Bu3Si
Cl
R
Ge
Ge
Cl
Sit Bu3
Ge
R = H, Me
t
Si Bu3
R
Scheme 20
Stereoselective Diels-Alder reactions are extensively studied.60 The addition of 1,3-butadienes to a
diastereotopic digermene afforded interesting results. For example it has been found that the cis-bent
geometry around the Ge=Ge double bond of halogen substituted cyclotrigermenes has an effect on
selectivity in the reaction with dimethyl butadiene. Thus, the dimethylbutadiene attacks the GeGe
double bond from the side having the bulkier tBu3Si and only one stereoisomer is formed during the
reactions. This is opposite to the selectivity of mesityl substituted cyclotrigermenes.61,62,52
Sit Bu3
Sit Bu3
X
Ge
Ge
Ge
t
Bu3Si
X
Ge
t
Sit Bu3
Bu3Si
Ge
Ge
Sit Bu3
Scheme 21
When Mes2Ge=GeMes2 was reacted with 1,3-butadiene, formation of mesityl(trimesitylgermyl)germacylopentene was observed. This was explained on the basis of a 1,2 shift of one mesityl group
from one to the second germanium atom followed by trapping with the diene. A trapping reaction using
Et3SiH led to the same result.7
Mes
Mes3Ge
Mes2Ge
GeMes2
Mes3Ge
Ge
GeMes
Mes
Et3SiH
Mes3Ge
Ge
SiEt3
Scheme 22
H
11
107
Gelest, Inc.
Reactions with Protic Lewis Acids and Halogen Reagents
Reactions of digermenes with protic reagents can either lead to addition products or eliminations.
For example, the reaction of digermenes with alcohols has been investigated by several groups.
Masamune and coworker observed the addition of methanol to tetra(diethylphenyl)digermene
(Ar2GeGeAr2).18 The same reactivity was found for Mes2GeGeMes2 and methanol, carbonic acids and
chloroform, respectively. In the case of chloroform the addition involved the C-Cl bond rather than the
C-H bond.7
Y
X
Ar2Ge
XY
GeAr2
Ar2Ge
GeAr2
X = OMe
X = OH
X = CHCl2
X = OMe, OOR
R = tBu, Mes
Ar = 2,6-Et2C6H3
Ar = 2,5-tBuC6H3
Ar = Mes
Ar = Mes
Y=H
Y=H
Y = Cl
Y=H
Scheme 23
The reaction of R2GeGeR2 (R = 2,5-tBu2C6H3) with water resulted in a digermanol.55 This is in
contrast to reactions of (Dis)2GeGe(Dis)2 with ethanol or Tbt(Mes)GeGe(Mes)Tbt with methanol,
respectively, which provided the corresponding alkoxides via an elimination reaction.24,63
Interestingly, the reaction of the hexakis(tripp)tetragerma-1,3-butadiene leads not to a direct addition
product but to a cyclic oxatetragermapentane as a result of an intial 1,2 addition of water to one of the
GeGe double bonds followed by 1,3-hydride shift and ring formation.30
RGe
GeR
H2O
RGe
GeR2
R2Ge
H
H
GeR
RGe
GeR2
R2Ge
GeR
R2Ge
GeR2
O
HO
R = Tripp
H
Scheme 24
Ring compounds featuring GeGe double bonds also react with methylene dichloride and tetrachloro
methane at room temperature. In the case of CCl4 a 1,2-addition occurs and the resulting
cyclodigermane can be isolated using an excess of dichloromethane the product of a ring expansion is
obtained. For chloroform only formation of a mixture of products was obtained.25,26
R
R
R
E
R
Ge
Cl
Scheme 25
108
E
CC l4
Ge
Cl
R
R
R Ge
R
E = Si , Ge
R = Si CH3(tBu)2
R
E
CH2C l2
GeR
R
Cl
Ge
R
E
R
Ge
Ge
R
Cl
Cl
R
Cl
Ge
R
Gelest, Inc.
Reactions with Heavier Chalcogens
Heating of tetragerma-1,3-butadiene with excess of selenium in the presence of a small amount of
Et3P as a chalcogen transfer reagent afforded the formation of selenatetragermacyclopentene as yellow
crystals.30 The same reactivity is observed in the presence of elemental sulfur. Tetragerma-1,3butadiene forms with sulfur a thiotetragermacyclopentane.43 Both structures are very similar. The five
membered ring products are almost planar. The Ge1Ge2 and Ge3Ge4 bond lengths which have an
average value mean 2.46 Å are typical for GeGe single bonds, but the Ge2Ge3 bonds, which are
2.2841(5) and 2.2975(5) Å are typical for a GeGe double bond.
RGe
R2Ge
GeR
RGe
+ Se
GeR2
GeR
R2Ge
(Et3P)
GeR2
Se
R = Tripp
1
/ 8 S8
RGe
GeR
R2Ge
GeR2
S
Scheme 26
Photolysis of trigermirane in the presence of elemental sulfur gave two reaction products. One is the
addition product of the formed digermene and sulfur whereas the second product can be viewed as a
result of the head-to-tail dimerized germathione, Mes2Ge=S, intermediate that is formed by reaction of
sulfur and the corresponding germylene.64
Mes2
Ge
Mes2Ge
GeMes2
h
GeMes2 + Mes2Ge
Mes2Ge
S8
S
Mes2Ge
GeMes2
+ Mes2Ge
S
S
Mes2Ge
GeMes2
S
Scheme 27
Reaction of digermene dihydride with PMe3 results in an isomerization reaction to afford a mixed
valent species. In this compound the GeGe distance of 2.5304(7) Å is at the longer end of the GeGe
single bond range.28,65
109
Gelest, Inc.
Ar'
2 LiBsBu3H
2 Ar'GeCl
Ar'
H
Ge
PMe3
Ge
H
Ge
Ar'
Ar'
PMe3
Ge
H
H
Scheme 28
Reduction Reactions
Reduction of the dihydrodigermene Ar'(H)GeGe(H)Ar' with alkaline metals (Li, Na or K) gave
highly reactive salts M2[Ar'(H)GeGe(H)Ar'] as deep red/black crystals.66 Depending on the nature of
the alkali metal counterions the nature of the structural type formed differed. In the potassium salt the
[Ar'(H)GeGe(H)Ar']2- core is trans pyramidal with terminal Ge-H bonds and a long GeGe distance of
2.6468(4) Å. In contrast in the sodium salt the [Ar'(H)GeGe(H)Ar']2- core has bridging hydrogen
atoms, the GeGe distance with 2.5904(5) Å is long.66 On the other hand the structure of the lithium salt
resembles that of the potassium salt, but has a much shorter GeGe distance of 2.395(2) Å. Theoretical
calculations show that the different isomeric structures are close in energy.67
Ar'
H
Ge
Ge
H
Ar'
M
Ar'(H)GeGe(H)Ar' 2
M = Li, Na, K
Scheme 29
DIGERMYNES
These compounds are analogous to their alkyne carbon counterparts. A stable example was first
reported in 2002 via the reduction of a digermene. Only a few examples of stable digermynes are
currently known although they have already been shown to have a rich chemistry.
Synthesis of digermynes
The synthesis of dichlorobis(terphenyl)digermanes offered not only the chance for further
substitution reactions leading to heteroleptic digermanes but also afforded an easy access route for
germanium analogues of alkynes. The synthesis of the first stable digermyne by this route was
described in 2002. 68 Reduction of Ar'(Cl)GeGe(Cl)Ar' with potassium furnished Ar'GeGeAr' as
orange-red crystals. This digermyne has a planar trans bent core and is centrosymmetric. The GeGe
distance with 2.2850(6) Å is considerably shorter than a GeGe single bond indicating considerable
multiple-bonding character. The Ge-C(ipso) is 1.996(3) Å and the GeGeC bending angle is 128.67(8)°.
Ar*ClGe
Scheme 30
110
GeClAr*
2K
Ar*Ge
GeAr*
Gelest, Inc.
A similar reaction was also reported in 2006 by Tokitoh and co-workers. 69 Starting with
BbtBrGeGeBrBbt, Bbt = C6H2-2,6-(CH(SiMe3)2)2-4-C(SiMe3)3, a reduction with KC8 led to
BbtGeGeBbt. For this compound two nonidentical molecules were found in the unit cell. The GeGe
bond lengths [2.2060(7) and 2.2260(7) Å] are shorter than that in Ar'GeGeAr'. The GeGeC bending
angles vary from 123.60(13) to 138.66(14)° and are in the same range observed for Ar'GeGeAr'
(128.67(8)°).
Bbt
Br
Ge
Bbt
KC8
Ge
Ge
Ge
Bbt
Br
Bbt
Scheme 31
Although Ar'GeGeAr' and BbtGeGeBbt are both digermynes, they show different reactivities which
is apparently caused by the different germanium substituents. In Ar'GeGeAr' the bond order is
decreased from idealized triply bonded to approximately two and contains some lone-pair character at
the germanium atoms. The lower bond order may be accounted for in terms of a second-order JahnTeller effect involving the mixing of a ?* orbital and the in plane ??orbital upon trans bending of the
linear skeleton (Scheme 32).70,71,72 The structure can also be written in several resonance forms as
shown on the left hand side of Scheme 32. One of these has a diradicaloid character form and this is
reflected in the reactivity of Ar'GeGeAr' as discussed below.
bu ( *)
bg ( *)
Ge
Ge
ag (n+ )
Ge
Ge
au ( )
bu (n-)
Ge
ag ( )
D
8
Ge
h
(linear)
C2h (trans-bent)
Scheme 32
111
Gelest, Inc.
Reactivity of Ar'GeGeAr'
Ar'GeGeAr' and BbtGeGeBbt displayed different reactivities. When BbtGeGeBbt was heated in
presence of Et3SiH with its reactive Si-H bond no reaction was observed, whereas, during addition
reactions with water or methanol formation of the 1,1-dimethoxydigermane and 1,1dihydroxydigermane, respectively, was observed. Reaction with 2,3-dimethyl-1,3-butadiene yielded the
product of a [4+1] addition.70
Bbt
Ge
Ge
Bbt
Bbt
Ge
Ge
Et3SiH
no reaction
Bbt
2 ROH
R = H, Me
Bbt(H)2Ge
Ge(OR)2Bbt
Scheme 33
This is in contrast to the observed reactivity of Ar'GeGeAr' and 2,3-dimethyl-1,3-butadiene. Three
molecules of 2,3-dimethyl-1,3-butadiene react with one germylene molecule to form two
germacyclopentenes bridged via an olefin.73 This difference in reactivity emphasizes the different
nature of the GeGe bond in Ar*GeGeAr* versus BbtGeGeBbt.
Ar*GeGeAr*
Ge
Ar*
Ar*
Ge
Scheme 34
Reductions
Digermynes can undergo further one and two step reductions with alkali metals. Experiments with
stoichiometric quantities of alkali metal reductant (1:1 ratio of alkali metal : Ar'/Ar*GeCl) (Ar* =
C6H3-2,6-(C6H2-2,4,6-Pri3)2) showed that the reduced mono- and dianions were produced in addition to
the neutral species, especially if short reaction times were used.74,75 Planarity of the core trans bent
geometry as seen in the precursors are preserved in the monoanionic radical and dianionic diradical
compounds. No dimerization like that seen for diarylacetylenes76,77 was observed, presumably because
of steric effects. The GeGe distances (NaAr*GeGeAr*: 2.3089(8) Å, KAr'GeGeAr': 2.3331(4) Å) are
slightly longer than observed in the neutral (2.2850(6) Å) compound but the GeGeC angles (112.60(5)115.32(12)°) are ca. 15° narrower than in Ar'GeGeAr' (128.67(8)°). These changes are probably caused
by an increase of the electropositive character of Ge after addition of one electron to the GeGe unit. As
a result, the electronegativity of the ligands relative to the central element is increased so that greater
bending is observed. The differences between the Ar' and Ar* derivatives are only minor.78
112
Gelest, Inc.
Reaction of Ar'GeCl and Ar*GeCl with an excess of lithium, sodium or potassium afforded the
doubly reduced salts.79 The alkali metal countercations are sandwiched between the flanking aryl rings
and the structures are very similar. The GeGe distances are somewhat increased (Li: 2.455(9) Å, Na:
2.394(1) Å, K: 2.3912(6) Å) in comparison to the monoanionic salts at 2.32 Å and the GeGeC angles
are narrower than those in the monoanions. This is consistent with the addition of the second electron
to the n+ orbital of Scheme 32 in the GeGe unit.79 Also the [Ar'GeGeAr']2- is isoelectronic to double
bonded Ar'SbSbAr'80 and the GeGe bond can be considered a double one.
Ar'GeGeAr'
Ar*GeGeAr*
2M
M
2M
M2[Ar'GeGeAr']
M = Li
M[Ar'GeGeAr']
M =K
M2[Ar*GeGeAr*]
M = Na, K
M[Ar*GeGeAr*]
M = Na
M
Scheme 35
Reactions with Unsaturated Molecules
Several of the reactions of Ar'GeGeAr' are suggestive of diradical character (Scheme 36-38). For
instance, very recently it has been shown the addition of two equivalents O2 to a bis terphenyl
digermyne yielding the unique species {Ar'Ge(#-O)2($1,$1:#2-O2)GeAr'}. In this species the two
germanium atoms are connected via an peroxo bridge and #2-oxo groups.80 The Ge...Ge distance is
quite short (2.4127(10) Å), although there is no GeGe bond.
O
Ar'GeGeAr'
2 O2
Ar'Ge
O
O
GeAr'
O
Scheme 36
In Scheme 37 reaction with the radical TEMPO (tetramethylpiperidineoxide) cleanly affords
:Ge(Ar')(TEMPO). Scheme 38 shows that upon reaction with Me3SiN3 the diradicaloid Ar'Ge(#NSiMe3)Ge(Ar') is obtained. In addition the reactions of Ar'GeGeAr' with tin reagents is consistent
with its radical character. Several of the reactions in Scheme 39 are consistent with the participation of
radicals as exemplefied by the activation of the flanking aryl ring of an Ar' group upon reaction with
Me3SiCCH or the one electron coupling to give a C-C bonded product upon reaction with PhCN.
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Gelest, Inc.
The reaction of Ar'GeGeAr' with different isonitriles (Scheme 37) showed different reactivity
depending on the nature of the isonitrile. The reaction with tBuNC afforded a 1:1 adduct in which the
isonitrile is coordinated to one of the Ge centers.81 As a consequence the GeGe distance increased
slightly (2.3432(8) Å) and has the nature of a double bond. The addition of a second Lewis base
molecule was not observed with tBuNC as Lewis base but when Ar'GeGeAr' was reacted with two
equivalents MesNC yielding a digermylene which is a characterized by a very long GeGe bond
(2.6626(8) Å).82
Ge
Ge
Ar'
H
Ar'
O
ai r
N
Ar'
O
Ge
Ge
Ar'
NH
O
ON
t
BuNC
Ar'
Ge
t
Bu NC
Ar'GeGeAr'
Ge
H
O
2 N2O
Ge
Ar'
Ar'
2 MesNC
MesNC
Ar'
Ge
+ 2 N2
O
H
2 TEMPO
CNMes
Ge
Ar'
2
Ge
O
Ar'
Ar'
N
Ge
Scheme 37
H
N
n
Bu3Sn
Ge
Ge
Ar'
Ge
Ar'
SiMe3
N
Ge
N
SiMe3
N
H
114
SnnBu3
nBu SnN
3
3
Ar'
Me3SiN3
H2
N
Me3SnN3
Ar'GeGeAr'
Ge
Ar'
PhSCH2N3
2 Ar'Ge(SPh)2N=CH2
Scheme 38
Ar'
AdN3
Ar'Ge
GeAr'
N
Ad
Ge
N
H2
Ar
'
Ar'
Ge
N
N
N
Ph
N
SiMe3
N
Ar'Ge
Me3Si
Ph
N
SiMe3
Me3SiCHN2
Ar'
Gelest, Inc.
GeAr'
GeAr'
GeAr*
PhCN
Ar'
Ge
Ge
N
N
Ph
PhC
Ph
*ArGe
Ar'GeGeAr'
PhNNPh
Me3SiC
CPh
Ar'
CH
Ar'
Ge
iPr
Ge
SiMe3
Ph
Ph
Ar'Ge
Ge
SiMe3
Dipp
Scheme 39
Addition of Hydrogen
The digermene Ar'GeGeAr' reacts spontaneously with hydrogen under ambient conditions to give,
depending on the stoichiometry, differing quantities of germanium hydrides Ar'(H)GeGe(H)Ar',
Ar'(H)2GeGe(H)2Ar' and ArGeH3. These compounds represent germanium hydrides with formal
oxidation states at the Ge atom of +2, +3 and +4, respectively.83 This was the first example of a
successful addition of hydrogen to a main group molecule at ambient conditions (25 °C and 1 atm).
Ar'(H)GeGe(H)Ar'
Ar'GeGeAr'
Ar'(H2)GeGe(H2)Ar'
Ar'GeH3
1 H2
21 %
10 %
9%
2 H2
2%
85 %
13 %
3 H2
0%
65 %
35 %
Scheme 40
CONCLUSIONS
Digermenes, of which more than thirty have been isolated and characterized, can be readily
synthesized by photochemical reactions, insertion routes and various elimination reactions. In contrast
to olefins, however, they do not have planar structures but show in most cases a trans pyramidalized
geometry and Ge-Ge bond lengths shorter than a single one. The germanium congeners of triple
bonded alkynes, digermynes, have been synthesized via reduction of digermenes. These have a transbent structure in contrast to their carbon congeners and Ge-Ge bond lengths similar to the “short”
Ge-Ge double bonds.
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Both digermenes and digermynes have very rich chemistries which have many differences from
unsaturated carbon species. The most notable feature is the very high reactivity that is generally
observed. This is exemplified by the fact that both digermenes and digermynes react directly with
hydrogen – the first example of a reaction between hydrogen and a main group molecule under ambient
conditions.
ABBREVIATIONS
Dis =
Si(CH3)3
Mes =
Si(CH3)3
Si(CH3)3
Si(CH3)3
(H3C)3Si
(H3C)3Si
Si(CH3)3
Tbt =
Si(CH3)3
Si(CH3)3
Si(CH3)3
Bbt =
Si(CH3)3
(H3C)3Si
(H3C)3Si
Si(CH3)3
Dipp =
Tripp =
Ar' =
Ar* =
TEMPO =
Scheme 41
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Si(CH3)3
O N
Gelest, Inc.
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