A stable and crystalline triarylgermyl radical: Structure and EPR spectra

Article abstract of DOI:10.1002/anie.200805328

(Figure Presented) A radical thing: After being obtained unexpectedly in low yields, the synthesis of the first stable triarylgermyl radical·Ge[3, 5-tBu₂-2,6-(EtO)₂C₆H]₃ (1; C gray, O blue) was considerably opti

Full text of DOI:10.1002/anie.200805328

Communications
DOI: 10.1002/anie.200805328
Stable Germanium Radicals
A Stable and Crystalline Triarylgermyl Radical: Structure and EPR
Spectra**
Christian Drost,* Jan Griebel, Reinhard Kirmse, Peter Lꢀnnecke, and Joachim Reinhold
Dedicated to Professor Michael F. Lappert on the occasion of his 80th birthday
There is much current interest in the chemistry of stable and
isolable radicals of the heavier Group 14 elements germa-
nium, tin, and lead.[1] Since the discovery of persistent heavier
Group 14 element radicals almost 30 years ago,[2] some
germanium- or tin-centered radicals with aryl,[3] alkyl,[4] or
amido ligands[5] with half lives varying from minutes to years
have been detected and studied by EPR spectroscopy. These
compounds were generated either by UV irradiation of
solutions containing stable low-valent precursors, hydrogen
abstraction from metallanes, or reduction of halide deriva-
tives. Despite the fact that these persistent radicals have been
successfully detected in solution, they have successfully defied
any attempt of isolation in the solid state for a long time.
The first remarkably stable germanium-, tin-, and lead-
centered free radicals have been described, including the
reported germyl and stannyl radicals have a more or less
pronounced pyramidal geometry at the metal center.[1]
Herein we report the isolation, characterization, and an
optimized synthesis of the first stable triarylgermyl radical
with an almost planar geometry at germanium in solid state.
In the course of our studies into mixed-valence compounds of
the heavy Group 14 elements,[8] reaction of Li[3,5-tBu2-2,6-
(EtO)2C6H] (LiR) with [GeCl2(dioxane)] in a 2:1 stoichiom-
etry led to the germyl radical CGeR3 (1) as the only crystalline
compound in 9% yield [Equation (1)]. Bright orange crystals
of the remarkably thermally stable and thermochromic
multinuclear
cyclotrigermenyl
radical
C[Ge(2,6-
Mes2C6H3)]3,[6a] the radical species MAr*GeGeAr* and
M2Ar*GeGeAr* (M = Na, K; Ar* = terphenyl),[6b] and the
mononuclear trisilylgermyl/stannyl CM(SiMetBu2)3 (M = Ge,
Sn),[7a,b] and trisilylplumbylradical CPb[(Si(SiMe3)2Et]3.[7c]
Whereas the former radicals were synthesized by reduction
of the corresponding heteroleptic terphenylgermanium(II)
chlorides, the generation of the persilylated germanium and
tin radicals was achieved by oxidation of the sodium
trisilylmetallates NaM(SiMetBu2)3, formed in situ, with
[MCl2(dioxane)]. The latter is both oxidizing agent and
substrate for metallate formation in the reaction with
NaSiMetBu2. The silylated plumbyl radical was synthesized
by a similar approach, but using Pb(2,6-tBu2C6H3O)2 as the
oxidant for potassium trisilylplumbanide.[7c] According to X-
ray crystallography and EPR solution studies, the geometry
around the radical centers of CM(SiMetBu2)3 (M = Ge, Sn) is
strictly planar, both in the solid state and in solution.[7a,b] In
contrast, EPR studies showed that all other previously
compound 1 were obtained from a n-hexane solution. These
crystals turned dark red in color upon heating in a sealed
capillary under nitrogen to 2008C, but decomposed rapidly at
temperatures above 2008C, with concurrent decolorization.
The molecular structure of radical 1 is illustrated in
Figure 1.[9] It resembles that of structurally characterized
triarylmethyl analogues, with
a general propeller-like
arrangement of the aryl groups.[10] In compound 1, three
[*] Dr. C. Drost, J. Griebel, Prof. Dr. R. Kirmse, Dr. P. Lꢁnnecke
Institut fꢂr Anorganische Chemie der Universitꢃt Leipzig
Johannisallee 29, 04103 Leipzig (Germany)
Fax: (+49)341-973-6199
E-mail: cdrost@rz.uni-leipzig.de
Prof. Dr. J. Reinhold
Wilhelm-Ostwald-Institut fꢂr Physikalische und Theoretische
Chemie der Universitꢃt Leipzig
Linnestrasse 2, 04103 Leipzig (Germany)
Fax: (+49)341-973-6399
Figure 1. Molecular structure of 1 with ellipsoids set at 30% proba-
bility; hydrogen atoms are omitted for clarity. Selected bond lengths [ꢀ]
and angles [8]: Ge1–C1 1.943(3), Ge1–C19 1.946(4), Ge1–C37 1.927(4);
C1-Ge1-C19 119.36(15), C1-Ge1-C37 119.69(14), C19-Ge1-C37
120.42(13).
[**] C.D. gratefully acknowledges Sogem Deutschland GmbH for a
donation of germanium tetrachloride and Prof. E. Hey-Hawkins for
support.
1962
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Chemie
twisted aryl rings radiate from the germanium center, with
only a slight deviation from 1208 observed in the Cipso-Ge1-
Cipso angles. The dihedral angles between the mean plane of
2,6-dimethoxyphenyl group as a model substituent were
performed with geometry optimization.
The experimentally obtained and the calculated 73Ge hfs
each aryl ring and the central (Cipso
)
reference plane are
coupling constants for a planar Ge(Caryl) unit and for a
3
3
30.23(13)8 for C1, and 27.19(17)8 for C19 and 26.81(18)8 for
C37. The coordination at germanium is essentially planar
pyramidal structure (the energy minimum from the geometry
optimization) are given in Table 1. Moreover, the germanium
(sum of angles = 359.478), and its deviation from the (Cipso
)
3
ꢀ
reference plane is marginal (0.082(2) ꢀ). The Ge1 Cipso
bonds (C1 1.943(3), C19 1.946(4), C37 1.927(4) ꢀ) are some-
what shorter than those reported for hexaphenyldigermane
(average 1.96(1) ꢀ).[11]
Table 1: Experimental[a] and calculated[b] 73Ge hyperfine couplings (in mT)
for 1 and spin densities of the germanium s, p, and d orbitals.
Experimental
Calculated
planar
4.23
1.06
2.11
2.12
–
0.009
0.760
0.033
0.802
pyramidal
Radical 1 was also investigated by variable-temperature
EPR studies. The room-temperature X-band EPR spectrum
of 1 in n-hexane exhibits a central line, caused by interaction
of the unpaired electron with the germanium isotopes having
I = 0, which is flanked by a nonsymmetrical low-intensity
hyperfine structure (hfs) dectet arising from interaction with
the 73Ge nucleus (I = 9/2, natural abundance 7.8%). Owing to
the low natural abundance of 73Ge, the intensity of an
individual 73Ge hfs line amounts only 0.8% of that of the
central line. The spectrum was simulated, yielding the values
Ak
A?
2Adip
8.07
4.39
2.45
5.62
6.13
0.068
0.718
–
8.75
5.75
2.00
6.75
–
0.057
0.673
0.044
0.774
hAavi
[c]
a0
s density
p density
d density
total density
0.786
[a] Measured in n-hexane; ꢁ0.10 mT. [b] Calculated for the 2,6-dime-
thoxyphenyl substituent. [c] Obtained from the EPR spectrum obtained
in liquid solution.
g0 = (2.0095 ꢁ 0.0010) and a0Ge = (6.13 ꢁ 0.10) mT. However,
13
neither hyperfine interactions arising from the
C
atoms
ipso
1
bonded to the germanium atom nor H hfs couplings with
H
para atoms of the aryl rings were observed, even for very low
radical concentrations (ca. 10ꢀ5 m); in liquid solution, the
minimum linewidth was DBpp ꢂ 0.38 mT. The X-band spec-
trum of a frozen solution of 1 at 130 K is shown in Figure 2
s and p spin densities calculated from the experimental data
using the MORTON/PRESTON formalism[13] and those
gained from the DFT calculations are given. From these
data, the following conclusions can be drawn:
a) The isotropic 73Ge hfs coupling appears to be the key
factor in the evaluation of the structure of 1 in solution.
Therefore, a satisfactory agreement between the exper-
imentally observed and the calculated values is achieved
only for the pyramidal Ge(Caryl)3 core structure.
b) The DFT-optimized Cipso-Ge-Cipso angle for the pyramidal
structure is 116.58; the deviation of the Ge(Caryl)3 unit
from planarity and the calculated torsion angle amount to
ca. 118 and ca. 348, respectively (X-ray analysis: 28.18).
The pyramidal structure is favored over the planar
structure by 8.5 kJmolꢀ1.
c) The calculated dipolar part of the 73Ge hfs tensor (2Adip
)
appears not to be essentially influenced by the geometry
of the Ge(Caryl)3 unit. Therefore, it cannot be used as a
criterium.
The formation of radical 1 in the reaction of Li[3,5-tBu2-
2,6-(EtO)2C6H] with [GeCl2(dioxane)] [Equation (1)] was
entirely unforeseen.[14] In contrast to this reaction, from which
no germanium(II) derivative was isolated, the reaction of the
Grignard compound BrMg[3,5-tBu2-2,6-(EtO)2C6H] with
[GeCl2(dioxane)] gave the crystalline digermene [Ge{3,5-
tBu2-2,6-(EtO)2C6H}2]2 (2) in 85% yield.[15] Involvement of
the lithium germanate LiGe[3,5-tBu2-2,6-(EtO)2C6H]3 in the
formation of 1, in analogy to the synthesis of CGe-
(SiMetBu2)3,[7a] however appears unlikely, as attempts to
prepare the lithium germanate from 2 and Li[3,5-tBu2-2,6-
(EtO)2C6H] failed, and the solution remained EPR-silent.
Instead, radical 1 was isolated in 62% yield after stepwise
treatment of digermene 2 with an equimolar amount of
Figure 2. a) Experimental and b) simulated X-band EPR spectrum of 1
in n-hexane at 130 K.
together with its simulation. It is axially symmetric, with gk =
Ge
2.004, g? = 2.012 (gi = ꢁ 0.001), AkGe = 8.07, and A?
=
4.39 mT (Ai = ꢁ 0.10 mT).
These EPR parameters are in good agreement with those
reported for other triarylgermyl radicals that were only
observed in solution,[1,12] and are also indicative of a non-
planar structure of 1 in solution. To investigate this structural
feature and to obtain information on the spin density
distribution in compound 1, DFT-MO calculations using the
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Communications
[GeCl2(dioxane)] followed by two equivalents of Li[3,5-tBu2-
Keywords: density functional calculations · EPR spectroscopy ·
germanium · radicals · structure elucidation
.
2,6-(EtO)2C6H] (see Experimental Section). However, the
constitution of the intermediate from the reaction of diger-
mene 2 with [GeCl2(dioxane)], and thus the formation of 1,
remains an open question; trapping reactions have so far been
unsuccessful, and further experiments are in progress.
In summary, we have synthesized the first crystalline
triarylgermyl radical 1. The compound was characterized by
X-ray analysis and variable-temperature EPR spectroscopy.
The results obtained support an almost planar structure for 1
in the solid state and a pyramidal geometry in solution.
Furthermore, DFT calculations show that a pyramidal
structure is energetically slightly favored.
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2369; b) M. Lehnig, H. U. Buschhaus, W. P. Neumann, T.
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[5] M. J. S. Gynane, D. H. Harris, M. F. Lappert, P. P. Power, P.
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Experimental Section
All manipulations were carried out under vacuum or nitrogen by
using standard Schlenk techniques.
Optimized synthesis for 1: [GeCl2(dioxane)] (0.07 g, 0.30 mmol)
was added in small portions to a solution of 2 (0.190 g, 0.15 mmol) in
THF (20 mL) at 08C. The pale yellow reaction mixture was stirred for
approximately 2 min at 08C.
A solution of Li[3,5-tBu2-2,6-
(EtO)2C6H] (0.176 g, 0.62 mmol) was then added (this compound
was prepared from 1-Br-3,5-tBu2-2,6-(EtO)2C6H and nBuLi (2.5m in
n-hexane) at ꢀ208C in Et2O; all volatile compounds were subse-
quently removed, and after washing with hexane, THF (10 mL) was
added). The mixture was stirred for 10 h. All volatile compounds were
removed in vacuo, and the residual solid was extracted into n-hexane.
After filtration and concentration, the solution was set aside for 12 h
at ambient temperature and a further 12 h at ꢀ308C to give 1 as bright
orange crystals (0.17 g, 62%). M.p. (nitrogen, sealed capillary): 2018C
(decomp.); elemental analysis (%) calcd for C54H87O6Ge: C 71.70,
H 9.70, O 10.60; found: C 72.2, H 9.8, O 11.3. EI-MS: m/z (%): 905
(53) [M]+, 876 (100) [MꢀEt]+, 627 (100) [MꢀR]+. UV/Vis (n-hexane)
lmax (e): 274 (24430), 353 (43200), 464 nm (9500).
2: A solution of [GeCl2(dioxane)] (0.70 g, 3.02 mmol) in THF
(5 mL) was added dropwise to a cooled (ꢀ788C) solution of the
Grignard compound BrMg[3,5-tBu2-2,6-(EtO)2C6H] (prepared from
1-Br-3,5-tBu2-2,6-(EtO)2C6H (2.2 g, 6.16 mmol) and magnesium shav-
ings (0.5 g, 20.75 mmol) in 40 mL of THF). The reaction mixture was
allowed to warm to room temperature and stirred for 12 h. All
volatile compounds were removed under vacuum and the residue was
extracted into hot n-hexane and subsequently filtered. Concentration
of the filtrate until the onset of crystallization and storage at ambient
temperature gave yellow 2 (1.61 g, 85%). M.p. (nitrogen, sealed
capillary): 1718C; elemental analysis (%) calcd for C36H58O4Ge:
C 68.91, H 9.32, O 10.20; found: C 68.78, H 9.13, O 10.33; NMR
(C6D6, tetramethylsilane, 300 K): 1H (400 MHz): d = 1.31 (t, J =
7.2 Hz, 6H, OCH2Me), 1.41 (s, 18H, tBu), 4.14 (q, J = 7.2 Hz, 4H,
OCH2Me), 7.23 ppm (s, 1H, aryl); 13C{1H} (100.6 MHz): d = 15.03
(CH2Me), 31.0 (CMe3), 34.6 (CMe3), 71.9 (CH2Me3), 125.7, 134.2,
143.8, 160.0 ppm (aryl); EI-MS: m/z (%): 628 (100) [M]+, 613
[MꢀMe]+.
[8] C. Drost, M. Hildebrand, P. Lꢃnnecke, Main Group Met. Chem.
2002, 25, 93.
[9] X-ray analysis data for 1: C54H87GeO6, Mr = 904.83, orthorhom-
bic, space group Pbca: a = 14.9264(10), b = 16.7882(11), c =
43.273(5) ꢀ, V= 10843.7(15) ꢀ3, Z = 8, m(MoKa) = 0.71073 ꢀ,
m = 0.609 mmꢀ1. Data were collected at 213(2) K on a Stoe-IPDS
imaging plate diffractometer; 30458 reflections collected, of
which 6937 were independent (Rint = 0.1201). The structure was
solved by direct methods; all non-hydrogen atoms were refined
anisotropically, and were included in a riding mode. Final R
indices: [I > 2s(I)]: R1 = 0.0472, wR2 = 0.0900; all data: R1 =
0.0900, wR2 = 0.0969. CCDC-264413 contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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EPR spectra of 1 were recorded in n-hexane solutions (concen-
tration < 10ꢀ3 m) at T= 295 and 130 K with a BRUKER ESP 300 E
spectrometer. The spectra were simulated using the program package
XSophe.[16]
DFT calculations were carried out using the Gaussian03 program
package[17] with B3LYP functionals[18,19] and the standard 6-31g(d)
basis set.[20]
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Received: October 31, 2008
Revised: January 3, 2009
Published online: February 4, 2009
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Angewandte
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[14] Formation of germanium radicals from reactions of lithium
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(SiMe3)2 with GeI2.[4a]
[15] The discussion of the molecular structure of 2 will be the subject
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Angew. Chem. Int. Ed. 2009, 48, 1962 –1965
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1965