Pentagerma[1.1.1]propellane: A combined experimental and quantum chemical study on the nature of the interactions between the bridgehead atoms

Article abstract of DOI:10.1002/anie.200805289

Ge, whiz! A detailed study of the synthesis, structure, redox chemistry, and bonding properties of pentagerma[1.1.1]propellane (1, see picture) examines fundamental aspects of the interactions between the bridgehead germanium atoms. DFT and CASSCF calcula

Full text of DOI:10.1002/anie.200805289

Angewandte
Chemie
DOI: 10.1002/anie.200805289
Metallapropellanes
Pentagerma[1.1.1]propellane: A Combined Experimental and
Quantum Chemical Study on the Nature of the Interactions between
the Bridgehead Atoms**
Dominik Nied, Wim Klopper,* and Frank Breher*
The question about the interactions between the bridgehead
atoms of metallapropellanes has been extensively discussed in
the literature.[1] Efforts mainly aim to clarify fundamental
aspects of the nature of the interaction between the
“inverted” bridgehead atoms. Even the “simplest” [1.1.1]pro-
pellane of Group 14, the all-carbon propellane [C5R6],[2] has
attracted renewed interest. Recent experimental and quan-
tum chemical investigations performed by Luger and co-
workers revealed the existence of a bonding path between the
two bridgehead carbon atoms.[3] By applying modern valence-
bond theory, Wu, Shaik, Hiberty et al. described this two-
electron s bond as a charge-shift bond—neither classically
covalent nor classically ionic.[4] For the heavier congeners of
carbon, that is, for metalla[1.1.1]propellanes [E5R6] (E = Si,
Ge, Sn), the situation is even more complicated.[1,5] Quantum
Nevertheless, the inherent singlet ground state of these
species points towards noticeable Eb···Eb interactions,[13] and
the energy splittings between their lowest-energy singlet and
triplet state (DES–T) were calculated to be quite high (around
50 kcalmolꢀ1). Since “true” singlet biradicals usually show
small DES–T values,[6] metallapropellanes unquestionably have
less biradical and more closed-shell characteristics,[14] which
makes their designation as “biradicaloids” more appropri-
ate.[1,5,15]
Although numerous quantum chemical calculations have
been performed for metalla[1.1.1]propellanes as well as
hetero[1.1.1]propellanes,[16] synthetically accessible and struc-
turally characterized species are very rare (Scheme 1).[17]
Seminal work was performed by Sita and Kinoshita in the
early 1990s; they succeeded in isolating the pentastanna-
[1.1.1]propellane [Sn5R6] (A) and the derivative [Sn7R8] (B,
R = 2,6-Et2C6H3).[18] The experimentally determined Snb···Snb
separations of 336.7 and 334.8 pm are 20% longer than a
chemical
calculations
concerning
the
pentasila-
[1.1.1]propellane [Si5H6] performed by Schleyer and Jano-
schek suggested substantial singlet biradical character[6] and
that “it would be misleading to represent the structure by
drawing a line between the bridgehead atoms”.[7] Schoeller
et al. have also predicted a long Sib···Sib separation[8] in the
pentasila compound.[9] Nagase and Kudo have presented
evidence that the overall biradical character of [Sn5H6] is very
small and is comparable to that of the carbon homologue.[10]
On the other hand, Gordon et al.[11] cast doubt on the
existence of an “inverted” metal–metal bridgehead bond for
metallapropellanes of the general formula [E5H6].[12] They
suggested to assign a small biradical character to the ground
state of the latter, consisting of a maximum of 14% within the
Group 14 element series for the silicon derivative, which is in
accordance with the assumption of Schleyer and Janoschek.[7]
ꢀ
regular Sn Sn single bond. Structurally characterized com-
pounds such as [Sn5R6] (C, R = 2,6-(iPrO)2C6H3) and
[Ge2{Sn(Cl)R}3] (D, R = 2,6-Mes2C6H3) were reported later
by the groups of Drost[19] and Power,[20] respectively. To date,
[*] D. Nied, Prof. Dr. F. Breher
Institut fꢀr Anorganische Chemie, Universitꢁt Karlsruhe (TH)
Engesserstrasse 15, 76131 Karlsruhe (Germany)
Fax: (+49)721-608-7021
E-mail: breher@aoc1.uni-karlsruhe.de
Prof. Dr. W. Klopper
Institut fꢀr Physikalische Chemie, Universitꢁt Karlsruhe (TH)
Engesserstrasse 15, 76131 Karlsruhe (Germany)
Fax: (+49)721-608-3319
E-mail: klopper@chem-bio.uni-karlsruhe.de
[**] Partial financial support by the Fonds der Chemischen Industrie is
gratefully acknowledged. We are deeply grateful to Prof. Dieter
Fenske for the donation of an X-ray diffractometer. We thank
Alexander Baldes for the calculation of the WBI and PABOON
indices.
Scheme 1. Synthetically accessible metalla[1.1.1]propellanes of heavier
Group 14 elements. R1 =2,6-Et2C6H3, R2 =2,6-(iPrO)2C6H3, R3 =2,6-
Mes2C6H3; Mes=2,4,6-Me3C6H2.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805289.
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however, metallapropellanes solely composed of silicon[21] or
germanium[22] atoms are unknown, and experimental studies
on the reactivity of metallapropellanes are still in their
infancy. To shed more light on the accessibility and stability of
metallapropellanes of this type and to investigate fundamen-
tal aspects of their bonding, we have specifically synthesized
the first pentagerma[1.1.1]propellane [Ge5Mes6] (1)—one of
the missing members within the Group 14 element series
closing the gap to the theoretical predictions and discussions.
The title compound [Ge5Mes6] (1) was initially prepared
in very low yield from [Ge3Mes6] (2) with 6.0 equivalents
lithium powder and 3.0 equivalents GeCl2·dioxane as oxidiz-
ing agent (E01=2 = ꢀ0.76 V vs. ferrocene/ferrocenium (Fc/Fc+);
Scheme 2)[23a] in THF and purified by repeated crystallization
from toluene/hexane at ꢀ308C. During our attempts to
optimize the yield, we noticed that 1 can directly be prepared
from Mes2GeCl2 (3) using 3.3 equivalents freshly prepared
lithium naphthalenide (Li+[Naphth]Cꢀ; 1.0m in THF) in the
presence of GeCl2·dioxane in THF. Note that GeCl2·dioxane
Figure 1. Experimental UV/Vis spectrum of 1 in THF and TD-DFT
calculated UV/Vis transitions of A2 (c) and E symmetry (b). The
calculated transitions are hypsochromically shifted by about 10 nm
and calibrated to the experimental spectrum.
symmetric cluster com-
pound and a hindered rota-
ꢀ
tion of the Ge Cipso bond
(non-equivalent
methyl protons).
ortho-
The molecular structure
of 1 is shown in Figure 2 and
confirms the formation of
the
first
pentagerma-
[1.1.1]propellane featuring
two ligand-free bridgehead
germanium atoms (Geb;
Ge1 and Ge2 in Figure 2).
Scheme 2. Synthesis of 1.
The latter are bonded to
three bridging germanium
as an additional germanium source and/or oxidizing agent is
crucial for the successful synthesis of 1.[23b,c] 1H NMR spec-
troscopic monitoring of the crude reaction product showed
that the target compound (1) is formed in about 10% yield.
After a non-optimized workup procedure, 1 was isolated in
analytically pure form in low but reproducible yield of
approximately 4% as orange crystalline material. Compound
1 is sensitive to air but thermally very stable under argon
(m.p. > 3008C) and stable towards degassed water. The
elemental analysis was consistent with the composition
[Ge5Mes6]. The EI mass spectra showed no peaks above the
molecular ion envelope centered at m/z 1078, which corre-
sponds to the expected value and isotope distribution for
[Ge5Mes6] (see the Supporting Information). The lowest
energy absorbance lmax was observed in the UV/Vis spectrum
of 1 in THF at 437 nm (e = 670 mꢀ1 cmꢀ1), alongside a
characteristic shoulder at l = 332 nm (Figure 1; see below
for the assignment of the absorption bands). Compound 1 is
atoms (Gebr) at distances between 245.3(1) and 250.1(1) pm.
Of particular interest is the separation of 286.9(2) pm
between the two bridgehead atoms,[24] which is approximately
1
EPR-silent at room temperature and 100 K. H NMR spec-
troscopic investigations of C6D6 solutions of 1 revealed two
aryl-ring resonances at d = 6.61 and 6.26 ppm, whereas three
further singlets at d = 2.53, 2.26, and 2.07 ppm were detected,
which belong to the methyl substituents of the mesityl ligand.
These data strongly suggested both the formation of a
Figure 2. Molecular structure of [Ge5Mes6] (1). Displacement ellipsoids
are drawn at the 30% probability level. Selected bond lengths [pm]:
Ge1···Ge2 286.9(2), Ge1–Ge3 248.9(2), Ge1–Ge4 247.6(1), Ge1–Ge5
245.3(1), Ge2–Ge3 246.6(1), Ge2–Ge4 248.5(2), Ge2–Ge5 250.1(1).
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ꢀ
45 pm longer than usually observed for a regular Ge Ge
singlet and triplet states of these clusters are particularly
ꢀ
single bond. The quotient q{Geb···Geb/av(Geb Gebr)} = 1.19 is
interesting from a bonding perspective. They feature much
longer Eb···Eb separations (331.2 pm for SQ1* and 305.8 pm
for TQ1*) than the ground state. These findings suggest
considerable Eb···Eb interactions in the latter. The calculated
vertical singlet excitation energy of 341.1 nm (1A2) for SQ1*
directly corresponds to the experimentally observed shoulder
in the UV/Vis spectrum of 1 at l = 332 nm, whereas the lowest
energy absorbance at lmax = 437 nm belongs to electronic
transitions between the cluster bonding HOMO of e symme-
try and the LUMO of a2 symmetry. Furthermore, our DFT
calculations predicted an almost linear decrease of the singlet
excitation energy upon descending the group (see entries for
SQX* in Table 2).
While comparing the singlet and triplet excitation ener-
gies within the homologous series, we noticed that the linear
decrease is not reproduced for the corresponding triplet
excitations. We essentially found a maximum for the germa-
nium model system. The excited triplet state TQ1* (3A2) was
revealed to be DE = 56.7 kcalmolꢀ1 higher in energy than the
ground state, whereas only DE = 49.6 kcalmolꢀ1 was found
for the silicon analogue TQ4*. On the basis of these DFT
findings within the homologous series, and in terms of DES–T
energies alone, we would assign (if at all) the highest amount
of biradical(oid) character to the silicon derivative and the
smallest to the title compound 1.
equal to that of the analogous tin propellane A reported by
Sita et al. (q = 1.18).[18] In accordance with the 1H NMR
spectra, the mesityl ligands are symmetrically arranged
around the cluster core. Owing to the propeller-like arrange-
ment of the mesityl substituents, 1 adopts an almost ideal D3-
symmetric structure.
To gain further insight into the amount of interaction
between the bridgehead germanium atoms in 1, and to
compare with the silicon (4) and tin (5) homologues, we
carried out a series of DFT calculations (see the Supporting
Information for details). In each case we found the following
ordering of the frontier molecular orbitals: a) HOMOꢀ1:
Eb···Eb bonding orbital of a1 symmetry; b) HOMO: degener-
ꢀ
ate set of Eb Ebr cluster bonding orbitals of e symmetry;
c) LUMO: Eb···Eb antibonding orbital of a2 symmetry. Table 1
Table 1: Geometry parameters (in pm) of Q1 and its silicon (Q4) and tin
(Q5) homologues in their ground (1A1) as well as excited singlet and
triplet A2 states, obtained at the B3LYP/def2-TZVP level in D3 symmetry.
E (X)
QX (1A1)
SQX* (1A2)
TQX* (3A2)
ꢀ
Eb Ebr
ꢀ
Eb Ebr
ꢀ
Eb Ebr
Eb···Eb
Eb···Eb
Eb···Eb
Si (4)
Ge (1)
Sn (5)
265.4
291.1
344.1
239.3
254.4
292.0
307.4
331.2
389.1
241.9
259.0
296.5
283.4
305.8
362.5
242.2
256.8
294.4
To address the bonding properties of 1 a little further, we
performed electrochemical studies using cyclic voltammetry
(see the Supporting Information). The known tin cluster
[Sn5R6] (A, R = 2,6-Et2C6H3) is readily reduced in two
separated, quasireversible one-electron reduction waves
centered at E01=2 = ꢀ1.96 and ꢀ2.48 V vs. Fc/Fc+.[18c] These
electrochemical findings support the inherent electron defi-
ciency of the bridgehead tin atoms in A. Overall, it is the
change from a “through-cage” interaction via a partially lifted
Eb···Eb interaction to separated, eight-valence-electron
(ER2)3Eꢀ entities that appears to influence the redox
potentials DE for cluster compounds of this type. At room
temperature, 1 is quasireversibly reduced to the radical anion
[Ge5Mes6]Cꢀ (6) at a potential of E10=2 = ꢀ2.18 V vs. Fc/Fc+. A
second quasireversible reduction wave is observed for the
lists relevant interatomic distances of the DFT computed
geometries of the singlet ground state (QX, a model for the
(real) compounds X = 1, 4, and 5 obtained by omitting the 4-
methyl group from the mesityl ligand; D3 symmetry) as well
as for the excited singlet (SQX*) and triplet states (TQX*) of
these model compounds that involve a single excitation from
the highest occupied a1 orbital (HOMOꢀ1) into the lowest
unoccupied molecular orbital (LUMO, a2 symmetry).
Table 2 displays the wavelengths of the corresponding
vertical and adiabatic excitation energies obtained from time-
dependent density functional theory (TD-DFT) at the
B3LYP/def2TZVP level (the optimization of the equilibrium
1
3
geometries of the A2 and A2 states was performed in D3
process [Ge5Mes6]Cꢀ (6) + eꢀ Q[Ge5Mes6]2ꢀ (7) at E01=2
=
symmetry).
ꢀ2.61 V. Compared to A, both half-wave potentials of 1 are
slightly more negative (DE(1) = ꢀ0.22 V, DE(2) = ꢀ0.13 V).
Thus, in terms of electrochemical potentials alone, the
inherent electron deficiency of the bridgehead atoms—
although of comparable extent in the two compounds—
should be slightly more pronounced for the tin case.
Table 2: Vertical and adiabatic excitation energies to the singlet and
triplet A2 states (wavelengths in nm) of Q1 and its silicon (Q4) and tin
(Q5) homologues, obtained at the B3LYP/def2-TZVP level in D3
symmetry.
It was of further interest to support the electrochemical
findings by EPR spectroscopy. The disproportionation con-
stant Kdisp for the reaction 2[Ge5Mes6]Cꢀ (6)Q[Ge5Mes6] (1) +
[Ge5Mes6]2ꢀ (7) was calculated to be 4.0 ꢀ 10ꢀ8. This observa-
tion indicates the thermodynamic stability of the radical anion
6. Indeed, freshly prepared samples of 6 (1 equiv sodium
powder in THF) show strong signals in a frozen solution upon
cooling to 100 K. Figure S2 in the Supporting Information
summarizes the continuous-wave EPR spectra of 6 in THF
recorded at the X band, along with the corresponding
simulations for a system with an electron spin of S = 1/2. As
SQX* (1A2)
TQX* (3A2)
E (X)
vertical
adiabatic
vertical
adiabatic
Si (4)
Ge (1)
Sn (5)
325.7
341.1
398.4
337.0
366.1
420.2
546.8
490.2
522.1
576.6
504.5
538.4
The agreement between the computed ground state (Q1,
Eb···Eb = 291.1 pm) and the experimentally observed geome-
try of 1 (Eb···Eb = 286.9(2) pm) is very good. The excited
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ꢀ
expected, an axial g tensor with g values close to the free-
bridgehead and bridging atoms (Eb Ebr) of the cluster. The
electron value and with hyperfine couplings on all compo-
nents was observed. The spin Hamiltonian parameters used
for the simulations are g? = 1.980 and gk = 1.999,[25] with
hyperfine couplings to two equivalent germanium atoms
(I(73Ge) = 9/2, 7.73% n.a.) of A? ꢁ 20 MHz and Ak = 41 MHz.
This result is in accordance with DFT calculations, which
predicted the spin density to reside mainly on the bridgehead
germanium atoms. Since the SOMO is Geb···Geb antibonding
in nature, the Geb···Geb separations are considerably longer in
the reduced form (d = 290.6 pm (Q1)!328.9 pm (Q6); BP86/
def2-TZVP level), meaning that the “through cage” inter-
action is gradually lifted upon one-electron reduction (see the
Supporting Information).
To address the central question of the amount of
biradicaloid character in a more sophisticated way, we have
performed additional quantum chemical calculations using
the complete active space self-consistent field (CASSCF)
method for the model compounds [E5Me6] (qX; see the
Supporting Information). A (6,4) active space was used,
obtained by distributing six electrons among four orbitals: the
highest occupied a1 level, the highest occupied (two-fold
degenerate) e level, and the LUMO of a2 symmetry. Table 3
smallest quotients z were found for the all-tin propellane q5.
This finding is in line with results from NBO analyses, which
revealed more delocalized Eb···Eb interactions upon descend-
ing the group. This qualitative description is further supported
by the shape of the bonding (6,4)CASSCF natural orbital for
q1 (Figure 3, left), providing evidence for “delocalized tails”
within the cluster core.
The CASSCF results show that occupation numbers of
about 1.9 and 0.1 are found for all three systems. They are
effectively independent of the Group 14 element within this
series. To put these occupation numbers into perspective, we
note that similar ones are obtained in (2,2)CASSCF calcu-
lations (with two electrons in the active space of the 1sg and
1su orbitals) of the H2 molecule at an internuclear separation
of 5/3re, where re is the equilibrium bond length. This is
roughly the distance beyond which a restricted Hartree–Fock
calculation shows a triplet instability for H2. Clearly, in view
of the computed excitation energies, the singlet ground-state
wave function for 1 is stable, but it appears that the Eb···Eb
interactions in metallapropellanes of this type are perturbed
to such an extent (“stretched bonds”) that highly correlated
quantum chemical methods provide evidence for some
biradicaloid characteristics. This finding nicely illustrates the
continuous nature of the conversion of an ordinary closed-
shell molecule into a biradicaloid and eventually into a
perfect biradical by the introduction of a suitable perturba-
tion.[14] On the basis of our findings, approximately 10%
biradicaloid character could be assigned to these systems. This
value is in accord with earlier studies, although our results did
not show a distinct maximum for the silicon propellane.[7,11]
Our CASSCF findings prompted us to investigate the
biradicaloid behavior of 1 in more detail.[26] As far as we are
aware, no such biradicaloid reactivity has yet been reported
for any metallapropellane known to date, and we speculated
that typical reagents for radical-type reactivity would provide
a suitable entry point. On reaction of 1 with an excess of
Me3SnH in toluene at room temperature, a spontaneous and
clean reaction was observed within 15 minutes. The color of
Table 3: Natural orbital occupation numbers of q1 and its silicon (q4)
and tin (q5) homologues, obtained at the (6,4)CASSCF/def2-TZVP//
B3LYP/def2-TZVP level. Calculations of Wiberg bond indices (WBI) and
population analyses based on occupation numbers (PABOON) were
performed at the B3LYP/def2-TZVP level (see the Supporting Informa-
tion).
E (qX)
Occupation
number
WBI
ꢀ
PABOON
Eb···Eb Eb Ebr z[a]
Eb···Eb Eb Ebr z[a]
ꢀ
Si (q4) 1.900 0.102 0.67
Ge (q1) 1.902 0.101 0.55
Sn (q5) 1.907 0.097 0.39
0.88
0.81
0.82
0.76 0.82
0.68 0.60
0.48 0.44
1.28
1.11
1.14
0.64
0.54
0.39
ꢀ
[a] z=(Eb···Eb)/(Eb Ebr).
1
reports the natural orbital occupation numbers of the frontier
orbitals, obtained from (6,4)CASSCF calculations in the def2-
TZVP basis at the B3LYP/def2-TZVP optimized geometries.
The natural orbitals for the pentagerma[1.1.1]propellane q1
are depicted in Figure 3. Furthermore, Wiberg bond indices
(WBI) and results from population analyses based on
occupation numbers (PABOON) are given in Table 3. Both
the solution changed from orange to colorless. The H NMR
spectrum of the crude product showed the characteristic
signals for the addition product bicyclo[1.1.1]pentagermane
[H Ge(GeMes2)3Ge SnMe3] (8, Scheme 3). Along with the
singlet for the SnMe3 group, overall six methyl and four aryl
resonances were found for the mesityl substituents of the
ꢀ
ꢀ
ꢀ
unsymmetrically substituted cluster. The Ge H moiety was
methods reveal
a much weaker interaction (39–76%)
between the bridgehead atoms (Eb···Eb) than between the
Figure 3. (6,4)CASSCF natural orbitals of q1 with occupation numbers
1.902 (left) and 0.101 (right). Only the C atoms of the Me ligands are
shown for clarity.
Scheme 3. Reaction of 1 with Me3SnH.
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[3] a) M. Messerschmidt, S. Scheins, L. Grubert, M. Pꢁtzel, G.
detected as singlet at d = 6.85 ppm (J117Sn,H = 294 Hz, J
=
119Sn,H
Szeimies, C. Paulmann, P. Luger, Angew. Chem. 2005, 117, 3993;
Angew. Chem. Int. Ed. 2005, 44, 3925; b) P. Coppens, Angew.
Chem. 2005, 117, 6970; Angew. Chem. Int. Ed. 2005, 44, 6810.
[4] a) W. Wu, J. Gu, J. Song, S. Shaik, P. C. Hiberty, Angew. Chem.
2009, DOI: 10.1002/ange.200804965; Angew. Chem. Int. Ed.
2009, DOI: 10.1002/anie.200804965; For a nice explanation of
“charge-shift bonding” in general, see b) S. Shaik, D. Danovich,
B. Silvi, D. L. Lauvergnat, P. C. Hiberty, Chem. Eur. J. 2005, 11,
6358.
307 Hz). We are currently not able to comment on any
mechanistic details, but work to clarify these points, as well as
general reactivity studies, is currently in progress.[27] Overall,
the predictions from the (6,4)CASSCF calculations on the
biradicaloid character of 1 are consistent with the exper-
imental observations on its behavior, and this result suggests
that the pentagerma[1.1.1]propellane 1 features some radical-
type reactivity.[28]
[5] H. Grꢂtzmacher, F. Breher, Angew. Chem. 2002, 114, 4178;
Angew. Chem. Int. Ed. 2002, 41, 4006, and references therein.
[6] a) W. T. Borden in Enzyclopedia of Computational Chemistry
(Ed.: P. von R. Schleyer), Wiley, New York, 1998, 708; b) Bir-
adicals (Ed.: W. T. Borden), Wiley, New York, 1982; c) J. A.
Berson, Acc. Chem. Res. 1997, 30, 238; d) D. A. Dougherty, Acc.
Chem. Res. 1991, 24, 88; e) L. Salem, C. Rowland, Angew. Chem.
1972, 84, 86; Angew. Chem. Int. Ed. Engl. 1972, 11, 92; f) W. T.
Borden, H. Iwamura, J. A. Berson, Acc. Chem. Res. 1994, 27,
109; g) A. Rajca, Chem. Rev. 1994, 94, 871.
[7] a) P. von R. Schleyer, R. Janoschek, Angew. Chem. 1987, 99,
1312; Angew. Chem. Int. Ed. Engl. 1987, 26, 1267.
[8] The abbreviation Eb···Eb (with E = Si, Ge, Sn) denotes the
distance between the bridgehead atoms in metalla-
[1.1.1]propellanes.
Experimental Section
Details concerning general synthetic techniques, NMR spectroscopic
investigations, and X-ray crystal structure determinations, as well as
computational details, are compiled in the Supporting Information.
Preparation of [Ge5Mes6] (1):
A freshly prepared lithium
naphthalenide solution (1m in THF, 10 mL, 10 mmol) was added
dropwise at ꢀ788C to a stirred solution of Mes2GeCl2 (1.146 g,
3.0 mmol) and GeCl2·dioxane (0.463 g, 2.0 mmol) in THF (10 mL).
The dark red solution was stirred for 1 h at this temperature and
overnight at room temperature. After the solvent and naphthalene
were removed in vacuo (608C), the residue was washed with n-hexane
(2 ꢀ 10 mL) and acetonitrile (2 ꢀ 10 mL) and extracted with toluene
(10 mL). The solution was partially evaporated, layered with aceto-
nitrile (10 mL), and stored at ꢀ408C to afford orange crystals of 1. To
remove small amounts of by-products incorporated by the adhesion
of the mother liquor, the crystals were washed with acetone. By
repeated recrystallization of the mother liquor and flash chromatog-
raphy (thoroughly dried silica gel, pentane/toluene 9:1) of the n-
hexane/acetonitrile solution, another crop of 1 could be isolated in
analytical pure form. Yield: 40 mg (37 mmol, 4%); m.p. > 3008C; EI-
MS (70 eV) m/z (%): 1078 (25); elemental analysis (%) calcd for
C54H66Ge5: C 60.15, H 6.17; found C 61.08, H 6.14; 1H NMR
(400.1 MHz, C6D6): d = 2.07 (s, 18H, p-CH3), 2.26 (s, 18H, o-CH3),
2.53 (s, 18H, o-CH3), 6.26 and 6.61 ppm (s, 12H, Mes-H). 13C{1H}
NMR (100.6 MHz, C6D6): d = 21.7, 26.8, 28.1 (o- and p-CH3), 129.2,
129.4, 137.8, 142.4, 142.5, 143.4 ppm (C6H2Me3); IR (ATR): n˜ =
464(vw), 497(vw), 548(vs), 585(w), 707(vw), 728(vw), 848(vs),
880(vw), 888(vw), 926(vw), 1022(m), 1241(vw), 1262(vw), 1296(w),
1367(m), 1413(w), 1438(s), 1556(vw), 1602(w), 2724(vw), 2855(vw),
2914(w), 2939(w), 2968(w), 3019 cmꢀ1 (vw); UV/Vis: lmax = 437 (e =
670 mꢀ1 cmꢀ1), 332 nm (shoulder).
[9] W. W. Schoeller, T. Dabisch, T. Busch, Inorg. Chem. 1987, 26,
4383.
[10] a) S. Nagase, Polyhedron 1991, 10, 1299; see also b) S. Nagase, T.
Kudo, Organometallics 1987, 6, 2456; c) S. Nagase, T. Kudo,
Organometallics 1988, 7, 2534.
[11] M. S. Gordon, K. A. Nguyen, M. T. Carroll, Polyhedron 1991, 10,
1247.
[12] Evidence against significant Eb···Eb bonding is corroborated,
among other aspects, by the similarity of the Eb···Eb distances in
the singlet and triplet states of [E5H6] and by the fact that no
bond critical point (bcp) has been located along the E···E
interaction lines for the metallapropellane systems (E = Si, Ge,
Sn). But note that slight differences in the charge densities of
ꢀ
these systems can affect the absence or presence of Eb Eb bcp in
these species. As was pointed out recently by Ottosson and co-
ꢀ
workers, the absence of Eb Eb bcpꢃs may also result from
reversed order of the s(E···E) and s*(E···E) orbitals, so that the
latter becomes the HOMO for metallapropellanes with M = Si,
Ge, and Sn; see reference [16b].
Reaction of 1 with Me3SnH: A solution of 1 (5 mg, 4.6 mmol) and
Me3SnH (excess) in toluene (3 mL) was stirred for 1 h at room
temperature. The colorless solution was evaporated to dryness, and
the excess Me3SnH was removed in vacuo. The residue was
redissolved in C6D6 and analyzed by 1H NMR spectroscopy.
[13] N. Gallego-Planas, M. A. Whitehead, J. Mol. Struct. (Theochem)
1992, 260, 419.
ˇ ´
´
´
[14] a) V. Bonacic-Koutecky, J. Koutecky, J. Michl, Angew. Chem.
1987, 99, 216; Angew. Chem. Int. Ed. Engl. 1987, 26, 170. For the
theoretical treatment of (multi)radical species, see b) J. Cullen,
Chem. Phys. 1996, 202, 217; c) T. Van Voorhis, M. Head-Gordon,
J. Chem. Phys. 2000, 112, 5633; d) T. Van Voorhis, M. Head-
Gordon, J. Chem. Phys. 2002, 117, 9190; e) A. D. Dutoi, Y. Jung,
M. Head-Gordon, J. Phys. Chem. A 2004, 108, 10270; f) A. Sodt,
G. J. O. Beran, Y. Jung, B. Austin, M. Head-Gordon, J. Chem.
Theory Comput. 2006, 2, 300; g) Y. M. Rhee, M. Head-Gordon,
J. Am. Chem. Soc. 2008, 130, 3878.
1H NMR (400.1 MHz, C6D6): d = 0.59 (s, 2J117Sn,H = 47 Hz, 2J
=
119Sn,H
49 Hz, 9H, SnMe3), 2.09, 2.10, 2.31, 2.45, 2.48, and 2.51 (s, each 9H,
CH3), 6.26, 6.28, 6.61, and 6.64 (s, each 3H, Mes-H), 6.85 ppm (s,
J117Sn,H = 294 Hz, J119Sn,H = 307 Hz, 1H, Ge-H).
Received: October 29, 2008
Published online: January 15, 2009
Keywords: biradicaloid species · cluster compounds ·
[15] “A biradicaloid is a closed-shell species derived from a singlet
biradical by a weak interaction between the radical centers”
M. J. S. Dewar, E. F. Healy, Chem. Phys. Lett. 1987, 141, 521.
[16] See, for example, a) K. A. Nguyen, M. T. Carroll, M. S. Gordon,
J. Am. Chem. Soc. 1991, 113, 7924; b) N. Sandstrꢄm, H.
Ottosson, Chem. Eur. J. 2005, 11, 5067; c) Y. Wang, J. Ma, S.
Inagaki, Tetrahedron Lett. 2005, 46, 5567; d) A. Ebrahimi, F.
Deyhimi, H. Roohi, J. Mol. Struct. (Theochem) 2003, 626, 223;
e) D. B. Kitchen, J. E. Jackson, L. C. Allen, J. Am. Chem. Soc.
1990, 112, 3408.
.
density functional calculations · germanium ·
metallapropellanes
[1] a) F. Breher, Coord. Chem. Rev. 2007, 251, 1007; b) M. Karni, Y.
Apeloig, J. Kapp, P. von R. Schleyer in The Chemistry of Organic
Silicon Compounds, Vol. 3 (Eds.: Z. Rappoport, Y. Apeloig),
Wiley, Chichester, 2001.
[2] K. B. Wiberg, Chem. Rev. 1989, 89, 975.
Angew. Chem. Int. Ed. 2009, 48, 1411 –1416
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1415

Communications
[17] For closely related cluster compounds [E8R6], see a) [Si8R6]: G.
Richards, M. Brynda, M. M. Olmstead, P. P. Power, Organo-
metallics 2004, 23, 2841.
Fischer, V. Huch, P. Mayer, S. K. Vasisht, M. Veith, N. Wiberg,
Angew. Chem. 2005, 117, 8096; Angew. Chem. Int. Ed. 2005, 44,
7884; b) [Ge8R6]: A. Schnepf, R. Kꢄppe, Angew. Chem. 2003,
115, 940; Angew. Chem. Int. Ed. 2003, 42, 911; c) [Ge8R6]: A.
Schnepf, C. Drost, Dalton Trans. 2005, 3277; d) [Sn8R6]: N.
Wiberg, H.-W. Lerner, S. Wagner, H. Nꢄth, T. Seifert, Z.
Naturforsch. B 1999, 877.
[23] a) V. Y. Lee, A. A. Basova, I. A. Matchkarovskaya, V. I. Faustov,
M. P. Egorov, O. M. Nefedov, R. D. Rakhimov, K. P. Butin, J.
Organomet. Chem. 1995, 499, 27; b) A. Sekiguchi, T. Fukawa, M.
Nakamoto, V. Ya. Lee, M. Ichinohe, J. Am. Chem. Soc. 2002, 124,
9865; c) E. Rivard, J. Steiner, J. C. Fettinger, J. R. Giuliani, M. P.
Augustine, P. P. Power, Chem. Commun. 2007, 4919.
[24] Note that Power and co-workers (reference [20]) reported a
Geb···Geb distance of 336.3 pm for [Ge2{Sn(Cl)R}3] (D), which is
an obvious result of the size difference of the cluster constitu-
ents.
[25] Only some minor features of the experimental spectrum could
not be reproduced by the simulation. The value for A? can only
be given as an approximation owing to signal overlap and a
relatively large line width.
[26] It is important to note that the quantity of radical character is
fundamentally different from the theory of radical behavior.
Radical character serves as theoretical measure, whereas radical
behavior is experimentally observed for such species. See
reference [14] for more details.
[18] a) L. R. Sita, I. Kinoshita, J. Am. Chem. Soc. 1992, 114, 7024;
b) L. R. Sita, I. Kinoshita, J. Am. Chem. Soc. 1991, 113, 5070;
c) L. R. Sita, I. Kinoshita, J. Am. Chem. Soc. 1990, 112, 8839;
d) L. R. Sita, R. D. Bickerstatt Kinoshita, J. Am. Chem. Soc.
1989, 111, 6454.
[19] C. Drost, M. Hildebrand, P. Lꢄnnecke, Main Group Met. Chem.
2002, 25, 93.
[20] A. F. Richards, M. Brynda, P. P. Power, Organometallics 2004, 23,
4009.
[21] The recently published silicon cluster [Si5R6] (R = 2,4,6-tri(iso-
propyl)phenyl) formally represents a valence isomer of a
pentasila[1.1.1]propellane showing only one ligand-free silicon
cluster atom: D. Scheschkewitz, Angew. Chem. 2005, 117, 3014;
Angew. Chem. Int. Ed. 2005, 44, 2954. A cage compound
resembling the analogous bicyclo[1.1.1]pentasilane [Si5R6H2] is
also known: Y. Kabe, T. Kawase, J. Okada, O. Yamashita, M.
Goto, S. Masamune, Angew. Chem. 1990, 102, 823; Angew.
Chem. Int. Ed. Engl. 1990, 29, 794.
[27] Under the conditions chosen (excess Me3SnH), other conceiv-
able
addition
products
such
as
[H2Ge5Mes6]
or
[(Me3Sn)2Ge5Mes6] were not observed.
[28] For the same radical-type reactivity of a 1,3-dibora-2,4-diphos-
phoniocyclobutane-1,3-diyl see H. Amii, L. Vranicar, H. Gor-
nitzka, D. Bourissou, G. Bertrand, J. Am. Chem. Soc. 2004, 126,
1344.
[22] Power and co-workers reported on a [Ge5R4] cluster for which a
singlet biradicaloid resonance form was postulated: A. F.
1416
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Angew. Chem. Int. Ed. 2009, 48, 1411 –1416