Formation od (Ar* = C6H3-2,6-Trip2; Trip = C6H2-2,4,6-i-Pr3) via reaction of Ar*GeGeAr* with 2,3-dimethyl-1,3-butadiene: evidence for the existence of a germanium analogue of an alkyne

Article abstract of DOI:10.1039/b203403d

The reduction of Ar*GeCl (Ar* = C6H3-2,6-Trip2; Trip = C6H2-2,4,6-i-Pr3) with one equivalent of potassium leads to the formation of a germanium analogue of an alkyne Ar*GeGeAr* 1; reaction of 1 with 2,3-dimethyl-1,3-butadiene yields 2, which was structurally characterized.

Full text of DOI:10.1039/b203403d

Formation of [Ar*Ge{CH2C(Me)C(Me)CH2}CH2C(Me)N]2 (Ar* =
C6H3-2,6-Trip2; Trip = C6H2-2,4,6-i-Pr3) via reaction of Ar*GeGeAr*
with 2,3-dimethyl-1,3-butadiene: evidence for the existence of a
germanium analogue of an alkyne
Matthias Stender, Andrew D. Phillips and Philip P. Power*
Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616,
USA. E-mail: pppower@ucdavis.edu
Received (in Cambridge, UK) 9th April 2002, Accepted 9th May 2002
First published as an Advance Article on the web 20th May 2002
The reduction of Ar*GeCl (Ar* = C6H3-2,6-Trip2; Trip =
C6H2-2,4,6-i-Pr3) with one equivalent of potassium leads to
the formation of a germanium analogue of an alkyne
Ar*GeGeAr* 1; reaction of 1 with 2,3-dimethyl-1,3-buta-
diene yields [Ar*Ge{CH2C(Me)C(Me)CH2}CH2C(Me)N]2 2,
which was structurally characterized.
In order to derivatize 1, a reaction was performed with a
stoichiometric amount of 2,3-dimethyl-1,3-butadiene in toluene
at room temperature (Scheme 1). This afforded the product 2 as
colorless crystals that are thermally stable up to the melting
point of ca. 250 °C. No Ge–C bond cleavage, which could have
resulted in the loss of the coordinated olefin, was observed up to
its melting point. Such retro reactions are known to occur with
Ge compounds synthesized by the addition of butadienes to
germylenes.10 The X-ray crystal structure‡ (Fig. 1) shows that
Ge has a distorted tetrahedral geometry (interligand angles
91.2–120.8°; av. 109.1°) and Ge–C bond distances that span the
range 1.954(5)–1.986(4) Å (av. 1.973 Å). The separation of the
two Ge centers in the molecule is 6.534(2) Å. The Ge–C bond
distances within the germacyclopent-3-ene ring are 1.954(5)
and 1.970(5) Å which are within the range (1.950–1.991 Å) of
the Ge–C distances in published germacyclopent-3-ene com-
The synthesis and characterization of compounds with multiple
bonds between heavier main group elements are of fundamental
interest due to their unusual structures and bonding.1 In
particular, numerous alkene analogues of the heavier group 14
elements with the general formula R2ENER2 (E = Si–Pb; R =
bulky organic group) have been isolated and characterized.2 The
structure and bonding of these compounds are very different
from the olefins R2CNCR2, and they usually have a trans
pyramidal (C2h) geometry and an ENE bond that is prone to
dissociate in solution in the case of the Ge, Sn or Pb
derivatives.1,2 Much less is known about the heavier group 14
congeners of alkynes REER (E = Si–Pb), although theoretical
studies on various hypothetical molecules REER (E = Si–Sn; R
= H, alkyl, aryl, silyl) predict a planar, trans-bent (C2h)
structure for alkyl or aryl substituted species instead of the
linear geometry of the alkynes.3 A recent paper has described
the synthesis and X-ray structure of Ar*PbPbAr* which is the
only currently known stable heavier group 14 element analogue
of an alkyne.4 It has a planar trans-bent CPbPbC core (Pb–Pb–C
94.2°) and a long, single Pb–Pb bond (Pb–Pb 3.188 Å) that is in
agreement with calculations on model species.5 A number of
reports have indicated the existence of REER species (E = Si or
Ge) as reactive intermediates, but they have yet to be isolated as
stable entities.6 Here, the synthesis and spectroscopic character-
ization of Ar*GeGeAr*, 1 (Ar* = C6H3-2,6-Trip2, Trip =
C6H2-2,4,6-i-Pr3), which is a stable Ge analogue of an alkyne
are reported. Crystals of 1 suitable for X-ray crystallography
have not yet been obtained. However, treatment of 1 with two
equivalents of 2,3-dimethyl-1,3-butadiene afforded [Ar-
Scheme 1
*Ge{CH2C(Me)C(Me)CH2}CH2C(Me)N]2 2, which has an
unusual structure, derived from the interaction of 1 with three
equivalents of 2,3-dimethyl-1,3-butadiene.
Reduction of Ar*GeCl7 with one equivalent of K in THF at
ambient temperature for 24 h afforded a dark red solution. After
suitable work-up and recrystallization from toluene, 1 was
obtained as analytically pure, red microcrystals in moderate
yield (ca. 35%). It is thermally stable to about 200 °C at which
the color begins to darken. The existence of 1 was supported by
1H and 13C NMR spectroscopy and C,H analysis.† A cyclic
voltammogram of 1 in THF–toluene displayed a quasi reversi-
ble reduction at ca. 21.46 V vs. SCE. Attempts to obtain X-ray
quality crystals of the ‘digermyne’ 1 for a structure determina-
tion have not yet been successful in spite of the use of several
different solvents and crystallization methods. Previous work
has shown that the reduction of Ar*GeCl with excess Na or K
in THF afforded the reduced species Na2Ar*GeGeAr*,
K2Ar*GeGeAr* or the radical NaAr*GeGeAr*.8,9 These reac-
tions strongly suggest that 1 is generated initially and is
subsequently reduced by excess alkali metal.
Fig. 1 Schematic drawing of 2. H atoms are not shown. Selected bond
distances (Å) and angles (°): Ge(1)–C(1) 1.986(4), Ge(1)–C(40) 1.970(5),
Ge(1)–C(43) 1.954(5), Ge(1)–C(37) 1.982(4), C(37)–C(38) 1.507(6),
C(38)–C(38A) 1.329(10), C(1)–C(2) 1.406(6), C(1)–C(6) 1.413(6); C(2)–
C(1)–C(6) 117.9(4), Ge(1)–C(1)–C(43) 121.8(3), Ge(1)–C(1)–C(6)
119.8(3), C(40)–Ge(1)–C(43) 91.2(3), C(1)–Ge(1)–C(37) 114.2(2), C(1)–
Ge(1)–C(40) 120.8(2), C(37)–Ge(1)–C(40) 107.2(2).
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CHEM. COMMUN., 2002, 1312–1313
This journal is © The Royal Society of Chemistry 2002

pounds.11,12 The angle at the Ge within the ring is 91.2(3)°
which is also in good agreement with literature values [ca.
88–93°].11,12 The five-membered ring incorporating the Ge
atom shows a puckered Cs ring conformation. The degree of
puckering, d, of germacyclopent-3-enes is defined by the angle
between the plane containing the four carbon atoms of the ring
and that containing Ge and the two adjacent ring carbons. In the
case of 2 the value for d is 16.3° and the Ge atom shows an out
of plane deviation of 0.3861 Å. The interplanar angle d lies in
the middle of the literature values which can vary between 6.0
and 30.2°. These angles depend on steric interactions of the
other Ge substituents with the germacyclopent-3-ene ring which
can have a large effect on d since there is only a small difference
in energy between the puckered (Cs) and planar (C2v) ring
conformation.12 The formation of a ‘bridged’ product such as 2
on addition of a diene to germylenes was previously unknown.
Initially, it was thought that a digermacyclohexadiene product
would be obtained as illustrated in Scheme 1. However this
involves a very sterically disfavored cis arrangement of Ar*
groups which could weaken the Ge–Ge bond to such an extent
that cleavage takes place to give a bis germylene species. This
may add further butadiene to form 2. Alternatively, since it is
probable that 1 has a trans bent structure with lone pair electron
density at each germanium, addition of butadiene could afford
a cyclopentene moiety at each germanium. The resulting
molecule could be so crowded that dissociation to give
Stirring for an additional 12 h resulted in a colorless solution. The volatile
materials were removed under reduced pressure and the white, solid residue
was dissolved with hexane (20 ml). The colorless solution was then
concentrated to incipient crystallization (ca. 10 ml). Storage at 5 °C for
12 h yielded [Ar*Ge{CH2C(Me)C(Me)CH2}CH2C(Me)N]2 2, as large,
colorless cubes. The supernatant solution afforded further traces of
crystalline 2 along with a pale yellow oil whose exact composition has not
been identified. Yield of 2 (M = 1355.19): 0.16 g, 0.12 mmol, 37.5%, mp
249–251 °C (no decomposition). 1H NMR (C6D6): d 7.12–7.19 (m, 14H, m-
Trip + p-C6H3, overlap), 2.89 (m, 12H, p-CH(CH3)2 + o-CH(CH3)2,
overlap, 3J 6.8 Hz), 2.75 (br, 12H, CH2 butadiene), 1.64 (s, 18H, CH3
butadiene), 1.32 (d, 12H, o-CH(CH3)2, 3J 6.8 Hz), 1.31 (d, 12H, o-
3
3
CH(CH3)2, J 6.4 Hz), 1.09 (d, 24H, p-CH(CH3)2, J 6.8 Hz); 13C NMR
(C6D6): d 155.0 (i-C6H3), 148.7 (o-C6H3), 147.0 (o-Trip), 139.7 (p-Trip),
130.0 (CMe butadiene), 130.5 (i-Trip), 129.6 (m-C6H3), 129.1 (p-C6H3),
120.7 (m-Trip), 38.8 (CH2 butadiene), 34.9 (p-CH(CH3)2), 31.0 (o-
CH(CH3)2), 27.2 (p-CH(CH3)2), 26.0 (o-CH(CH3)2), 24.5 (o-CH(CH3)2),
19.2 (CH3 butadiene); IR (Nujol): n/cm21 1925(w), 1870(w), 1760(w),
1590(m), 1575(m), 1555(m), 1530(w), 1320(sh), 1055(m), 750(m),
660(sh), 600(w), 575(w), 560(w). Compounds 1 and 2 gave satisfactory C,H
analysis.
‡ Crystal data for 2 at 90 K with Mo-Ka radiation (l = 0.71073 Å):
C90H128Ge2, M = 1355.19, colorless cube, monoclinic, space group P21/c,
a = 12.8612(8), b = 19.4797(12), c = 16.8275(10) Å, b = 106.8280(10)°,
Z = 4, Dc = 1.115 g cm21, m = 0.787 mm21, R1 = 0.0671 for 5196 [I >
2s(I)] data. CCDC reference number 185422. See http://www.rsc.org/
suppdata/cc/b2/b203403d/ for crystallographic data in CIF or other
electronic format.
Ar*Ge{CH2C(Me)C(Me)CH2}· radicals could occur which
could then add further 2,3-dimethyl-1,3-butadiene to yield 2.
In summary a stable germanium analogue of an alkyne, 1, has
been isolated and characterized by NMR spectroscopy, ele-
mental analysis and by its reaction with 2,3-dimethyl-1,3-buta-
diene to yield the unique product 2.
We thank the National Science Foundation (CHE-0094913)
for Financial support. The Bruker SMART 1000 diffractometer
was funded in part by NSF Instrumentation Grant CHE-
9808259. We thank Professor Frank Osterloh for his help in
recording the electrochemical data.
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Notes and references
† All manipulations were carried out under anaerobic and anhydrous
conditions. 1: An orange solution of 1.68 g (2.85 mmol) of Ar*GeCl7 in
THF (30 ml) was added to 0.112 g (2.85 mmol) of finely divided potassium
in THF (30 ml) at ambient temperature under vigorous stirring. After ca. 2
h the reaction mixture began to turn red, and upon stirring for further 24 h
the solution had become deep red. The volatile materials were removed
under reduced pressure and the red solid residue was extracted with hexane
(50 ml). The remaining potassium chloride was allowed to settle and the
supernatant solution was decanted off. The deep red solution was then
concentrated to incipient crystallization (ca. 30 ml). Storage at 5 °C for 12
h yielded Ar*GeGeAr* 1, as red microcrystals. Yield (M = 1108.92): 0.55
g, 0.49 mmol, 35%, mp 244–246 °C (darkens above 200 °C; stays glassy
after melting and cooling). 1H NMR (C6D6): d 7.21–6.90 (m, 14H, m-Trip
and m- and p-C6H3), 2.95 (sept, 4H, p-CH(CH3)2, 3J 6.8 Hz), 2.75 (sept, 4H,
3
3
o-CH(CH3)2, J 6.8 Hz), 1.38 (d, 12H, o-CH(CH3)2, J 7.2 Hz), 1.30 (d,
12H, o-CH(CH3)2, 3J 7.2 Hz), 1.09 (d, 12H, p-CH(CH3)2, 3J 7.2 Hz); 13
C
NMR (C6D6): d 156.8 (i-C6H3), 148.6 (o-C6H3), 146.1 (o-Trip), 139.1 (p-
Trip), 128.6 (i-Trip), 128.2 (m-C6H3), 127.5 (p-C6H3), 121.7 (m-Trip), 34.6
(p-CH(CH3)2), 31.3 (o-CH(CH3)2), 25.7 (p-CH(CH3)2), 25.4 (o-
CH(CH3)2), 24.4 (o-CH(CH3)2); IR (Nujol): n/cm21 1755(w), 1600(m),
1555(m), 1545(m), 1530(w), 1235(sh), 1180(w), 1125(w), 1075(sh),
1065(w), 1045(w), 870(m), 845(sh), 640(m), 575(w), 530(w); UV/Vis
(toluene): lmax = 490 nm, e = 1500 L mol21 cm21. 2: To a red solution of
0.35 g (0.32 mmol) of Ar*GeGeAr* in toluene (10 ml), 0.040 ml (0.35
mmol) of 2,3-dimethyl-1,3-butadiene was added via a microsyringe at
ambient temperature under vigorous stirring. A few minutes after the
addition the color of the reaction mixture faded from deep red to orange.
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