Tuning the gap: Electronic properties and radical-type reactivities of heteronuclear [1.1.1]propellanes of heavier group 14 elements

Article abstract of DOI:10.1021/om100977z

Two heteronuclear [1.1.1]propellanes of group 14, Ge₂Si ₃Mes₆ (1) and Sn₂Si₃Mes₆ (2) (Mes = 2,4,6-Me₃C₆H₂), were prepared by reductive coupling of Mes₂

Full text of DOI:10.1021/om100977z

ARTICLE
pubs.acs.org/Organometallics
Tuning the Gap: Electronic Properties and Radical-Type Reactivities of
Heteronuclear [1.1.1]Propellanes of Heavier Group 14 Elements
Dominik Nied,† Pascual O~na-Burgos,† Wim Klopper,‡ and Frank Breher*,†
†Institute of Inorganic Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstrasse 15, 76131 Karlsruhe, Germany
‡Institute of Physical Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany
S Supporting Information
b
ABSTRACT: Two heteronuclear [1.1.1]propellanes of group
14, Ge2Si3Mes6 (1) and Sn2Si3Mes6 (2) (Mes = 2,4,6-
Me3C6H2), were prepared by reductive coupling of Mes2SiCl2
and GeCl2 dioxane or SnCl2. Both compounds were character-
3
ized in detail, including X-ray structure analyses on single
crystals. In each case it was found that the E2Si3 cluster core
consists of three bridging {SiMes2} units and two ligand-free
bridgehead atoms (Eb). As a result of the different size of the
bridging units, the distances between the bridgehead atoms are
considerably shorter (0.10 Å for 1 and 0.27 Å for 2) than in the
homonuclear counterparts Ge5Mes6 and Sn5Dep6 (Dep = 2,6-
Et2C6H3) known from the literature. The stronger Eb Eb
3 3 3
interactions in 1 and 2 were confirmed by electrochemical
studies using cyclic voltammetry. UV/vis studies, together with
density functional theory (DFT) calculations, further supported
these findings. A correlation of the Eb Eb distances and the
singlet and triplet A2 transitions for a series of homo- and heteronuclear [1.1.1]propellanes revealed that higher A2 excitation
3 3 3
3
wavelengths, and thus lower ΔESfT energies, are obtained either by increasing the distances between the bridgehead atoms or by
arranging the involved orbitals in close spatial proximity. Reactivity studies on 1 and 2 using selected reagents showed that Me3SnH
or the disulfide FcS-SFc (Fc = ferrocenyl), which are prone to radical-type reactivity, can be readily added across the bridge (the tin
hydride reacts only with 1). The resulting 1,3-disubstituted bicyclo[1.1.1]pentane derivatives Me3Sn-Ge(SiMes2)3Ge-H (3) and
FcS-E(SiMes2)3E-SFc (4 (E = Ge) and 5 (E = Sn)) were characterized in detail, including X-ray structures of 4 and 5.
Interestingly, the homolytic S-S bond addition reactions were found to be susceptible to light. Even though the tin-containing
propellane 2 turned out to be more reactive than 1, both conversions can be drastically enhanced simply by using daylight in the lab.
’ INTRODUCTION
isolated and characterized only recently.14 Furthermore, one
heteronuclear metalla[1.1.1]propellane of the formula Ge2{Sn-
(Cl)R}3 (R = 2,6-Mes2C6H3; Mes = 2,4,6-Me3C6H2) is known
from the literature.15 In each case it was found that the central
distance between the “naked”16 bridgehead atoms within the
[1.1.1] scaffolds of heavy propellanes is considerably elongated
by between 13% and 20% of a regular single bond. On the basis of
this bond stretch, these molecules have frequently been de-
scribed as singlet biradicaloids.2,17 Although highly correlated
CASSCF calculations predicted the biradicaloid character18 to be
fairly small, experimental studies indeed revealed some radical-
type reactivities.12,13,19
Propellane molecules1 belong to the so-called “nonclassical”
structures, consisting of two ligand-free cluster constituents in an
umbrella-type conformation. The bonding between these inverted
tetrahedral bridgehead atoms (Eb) is not trivial and has been
extensively discussed in the literature.2,3 Even the “simplest” all-
carbon [1.1.1]propellane4 has attracted renewed interest, from
both experimentalists5 and theoreticians.6 The long discussed
heavy analogues of all-carbon propellanes, E5R6 (E = Si, Ge, Sn
with R = aryl ring; Chart 1), have continuously attracted con-
siderable interest in the last decades, not at least because these
heavy main group compounds are often fundamentally different7
from their carbon counterparts and have remained a challenge for
both experimentalists and theoreticians for a long time.2,8
’ RESULTS AND DISCUSSION
Although heavy [1.1.1]propellanes have been intensively
investigated by theory in the past,9 synthetically accessible and
structurally characterized species are very rare. While the tin
derivatives were synthesized two decades ago,10,11 the homo-
nuclear germanium12 and silicon13 propellanes have been
In order to extend our studies in this area and to relate the
Eb Eb separation in heavy [1.1.1]propellanes with their
3 3 3
Received: October 13, 2010
Published: March 04, 2011
r
2011 American Chemical Society
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Chart 1
isotopes (1J = 324 Hz). The 119Sn NMR chemical shift for the
bridgehead tin atoms in 2 of δ = -1348 ppm compares very well
to other tin propellanes known from the literature (cf. -1751
ppm for Sn5(2,6-Et2C6H3)6;10 -1893 ppm for Sn5{(2,6-iPrO)2-
C6H3}611). The most striking feature, however, is the compar-
ably large coupling of 3464 Hz (absolute value) between the
bridgehead tin atoms in 2, which is somewhat smaller than for the
pentastanna[1.1.1]propellanes (8145 and 3973 Hz) known from
Sita and Drost, respectively.
The molecular structures of both title compounds are shown
in Figure 1 (space group for both clusters Pbca).23 Relevant
structural parameters are given in the figure caption. In each case,
the E2Si3 core structures consist of three bridging {SiMes2} units
and two ligand-free bridgehead atoms. The distances and angles
within these scaffolds fall within the expected ranges.10-13 Most
importantly, the distance between the bridgehead germanium
atoms in 1 is 0.10 Å shorter (d = 2.767(1) Å) than in the
homonuclear germanium cluster Ge5Mes6.12 This is in accord
with the smaller radius of the bridging silicon atoms. Note that
Power and co-workers observed an opposite effect and reported a
Scheme 1. Synthesis of 1 and 2 (Mes = 2,4,6-Me3C6H2)
Geb Geb distance of 3.363 Å for Ge2{Sn(Cl)R}3 (R = 2,6-
3 3 3
Mes2C6H3), which is also a result of the different size of the
bridging units.15 The separation between the two “naked” tin
atoms in 2 (Figure 1) was found to be 3.097(1) Å, which is about
0.27 Å shorter than in the all-tin propellane Sn5Dep6 (Dep = 2,6-
Et2C6H3) known from the seminal work of Sita et al.10 With
these observations on the Eb Eb distances in the heteronuclear
3 3 3
electronic properties and their reactivities, we synthesized and
characterized two heteronuclear metalla[1.1.1]propellanes of the
general formula E2Si3Mes6 (E = Ge, Sn) featuring in each case
three flanking {SiMes2} entities and two ligand-free germanium
(1) or tin (2) bridgehead atoms. The synthesis of the novel
propellanes is outlined in Scheme 1.
propellanes 1 and 2 in mind, we anticipated considerable
differences in the electrochemistry and the electronic absorption
spectra as compared to the homonuclear analogues, i.e., Ge5Mes6
and Sn5Dep6.
Indeed, stronger Eb Eb interactions in 1 and 2 have been
3 3 3
confirmed by electrochemical studies using cyclic voltammetry
under strictly anaerobic and dry conditions. By analogy with the
heavy propellanes known from the literature, both title com-
pounds are quasi-reversibly reduced to the radical anions
[E2Si3Mes6]•- and dianions [E2Si3Mes6]2-. Due to the energe-
tically destabilized LUMO,24 both reduction events for 1 and 2
are slightly more cathodically shifted as compared to Ge5Mes6
and Sn5Dep6, respectively. We detected potential differences of
ΔE(1) = -0.11 V and ΔE(2) = -0.07 V for 1 (cf. Ge5Mes6,12
Figure 2) and ΔE(1) = -0.17 V and ΔE(2) = -0.29 V for 2 (cf.
Sn5Dep6,10 see Supporting Information). As expected, the shifts
to more negative potentials are much more pronounced for the
tin-containing propellanes due to the larger difference in the
Both compounds were prepared by reductive coupling of 3
equivs. of Mes2SiCl2 with 2 equivs. of either GeCl2 dioxane (1) or
3
SnCl2 (2) and a freshly prepared lithium naphthalenide solution
(10 equivs.). The detailed mechanism of the formation of the
target compounds remains the subject of speculation.20 1H NMR
spectroscopic monitoring of the crude reaction products, how-
ever, showed that the heteronuclear [1.1.1]propellanes with two
heavier group 14 elements, Ge (1) or Sn (2), in the bridgehead
positions and three Si atoms within the flanking units are formed
in 10% to 15% yield. It is important to note that no other
[1.1.1]propellanes with different connectivities within the mo-
lecular cores are formed under these conditions.21 The dihalides
GeCl2 dioxane and SnCl2 are therefore selective sources for the
Eb Eb separation.
3
3 3 3
bridgehead positions. After nonoptimized workup procedures, 1
was isolated in analytically pure form in low but reproducible
yield of ca. 4% as orange crystalline material. The tin homologue
(2) was isolated as dark purple crystals in 12% yield.22 Both
clusters are thermally stable under argon (melting points
>200 °C) and stable toward degassed water. The elemental
analyses and the EI mass spectra were consistent with the
compositions Ge2Si3Mes6 (1) and Sn2Si3Mes6 (2). Solutions
of both title compounds are EPR-silent. 1H NMR spectroscopic
investigations of C6D6 solutions of 1 (THF-d8 for 2) revealed, as
expected, two aryl-ring resonances at δ = 6.57 and 6.21 ppm (δ =
6.67 and 6.18 ppm for 2) and three further singlets for none-
quivalent Me groups at δ = 2.63, 2.41, and 2.05 ppm (δ = 2.44,
2.14, and 1.94 ppm for 2). The 29Si NMR chemical shifts of δ =
66 (1) and 98 (2) ppm differ only slightly. The latter shows non-
resolved satellites due to a one-bond coupling to the 117/119Sn
In order to shed some more light on the electronic character-
istics, we performed close analyses of the UV/vis spectra and
compared the results with those obtained for the homonuclear
analogues (Figure 3). By analogy with the electrochemical
findings, the shorter Geb Geb distance in 1 has an impact
3 3 3
on the singlet 1A2 excitation (HOMO-1 (a1) f LUMO (a2)),24
which is hypsochromically shifted by about 9 nm (cf. λ = 323 vs
332 nm for Ge5Mes6, Figure 3 top). We also noticed a significant
effect on the lowest energy absorption (HOMO (e) f LUMO
(a2), E) centered at λmax = 415 nm (cf. 437 nm for Ge5Mes6).
1
Interestingly, the A2 excitation for 2 is almost invariant as
compared to Sn5Dep6 (λ = 383 vs 381 nm, Figure 3, bottom).
A large hypsochromical shift was detected only for the HOMO-
LUMO transition (E, cf. 496 nm vs 558 nm). Furthermore, we
observed for both tin-containing propellanes the 3A2 transitions
at λ = 549 nm (2) and ca. 495 nm (shoulder for Sn5Dep6). These
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Figure 1. Molecular structures of 1 (a) and 2 (b); displacement ellipsoids drawn at the 30% probability level. Selected bond lengths (Å) and angles
(deg) for 1: Ge1 Ge2 2.767(1), Ge1-Si1 2.418(1), Ge1-Si2 2.428(1), Ge1-Si3 2.399(1), Ge2-Si1 2.422(1), Ge2-Si2 2.413(1), Ge2-Si3
3 3 3
2.439(1), Si-C 1.890(4)-1.902(4), Ge1-Si1-Ge2 69.8(1), Ge1-Si2-Ge2 69.7(1), Ge1-Si3-Ge2 69.8(1), Si1-Ge1-Si2 90.5(1), Si1-Ge1-
Si3 90.8(1), Si2-Ge1-Si3 91.1(1), Si1-Ge2-Si2 90.7(1), Si1-Ge2-Si3 89.7(1), Si2-Ge2-Si3 90.5(1); for 2: Sn1-Sn2 3.097(1), Sn1-Si1
2.632(2), Sn1-Si2 2.653(2), Sn1-Si3 2.645(2), Sn2-Si1 2.638(2), Sn2-Si2 2.603(2), Sn2-Si3 2.626(2), Si-C 1.899(9)-1.912(10), Sn1-Si1-
Sn2 72.0(1), Sn1-Si2-Sn2 72.2(1), Sn1-Si3-Sn2 72.0(1), Si1-Sn1-Si2 88.8(1), Si1-Sn1-Si3 88.7(1), Si2-Sn1-Si3 87.9(1), Si1-Sn2-Si2
89.8(1), Si1-Sn2-Si3 88.9(1), Si2-Sn2-Si3 89.4(1).
Figure 2. Cyclic voltammograms of 1 (red) and Ge5Mes6 (blue) at
room temperature in THF; redox-potential E0 vs the ferrocene/
1/2
ferrocenium couple (Fc/Fcþ). Scan rate: 100 mV s-1, Pt/[nBu4N]-
[PF6]/Ag. Both redox waves show completely the same behavior in a
second CV cycle; only one is shown here.
transitions are usually not visible in the UV/vis spectrum but
obviously are allowed due to spin-orbit coupling effects for the
heavier propellanes.19 Note that this triplet transition corres-
ponds to the lowest energy absorption for 2.
Figure 3. Experimental UV/vis spectra of 1 (0.06 mM, red, top) and 2
The experimental results were supported by (time-
dependent) density functional theory ((TD)-DFT) calculations
on the model compounds E2E03Dmp6 (Dmp = 2,6-Me2C6H3) at
the B3LYP/def2-TZVP level in D3 symmetry (see Supporting
Information). Table 1 lists relevant interatomic distances of the
DFT-computed geometries of the singlet ground states (1A1)
and the wavelengths of the corresponding vertical singlet (1A2)
and triplet (3A2) excitations.
(0.09 mM, purple, bottom) in THF; those obtained for Ge5Mes6 (blue,
top) and Sn5Dep6 (green, bottom) are given for comparison; see also
Table 1.
The geometries of the computed ground states are in very
good agreement with the experimentally observed structures of 1
and 2, and the TD-DFT-calculated vertical singlet and triplet A2
excitation wavelengths correspond very well to the absorptions
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ARTICLE
Table 1. Geometry Parameters (in Å) of the Propellanes
E2E03Dmp6 in their Ground States (1A1); Vertical Singlet
(1A2) and Triplet (3A2) Excitation Wavelengths (in nm)
Obtained at the B3LYP/def2-TZVP Level in D3 Symmetrya
by hetero- or homolytic X-Y bond breaking, should strongly
depend on the amount of interaction between the bridgehead
atoms in 1 and 2. In terms of bond energies alone, it is the subtle
balance between X-Y and Eb Eb bond cleavage and X-E and
3 3 3
Y-E bond formation that governs the overall reactivity. Further-
more, we speculated that the general reactivity of heavy propel-
lanes might be enhanced by light due to the facile accessibility
of the excited singlet (1A2) and triplet states (3A2) in the UV/vis
region (see above). Table 2 summarizes and compares the
obtained results.
1A1
1A2
3A2
entry
E2E03Dmp6
Eb Eb
vertical
vertical
3 3 3
1
2
3
4
5
6
7
8
9
E = Si, E0 = Si
E = Si, E0 = Ge
E = Si, E0 = Sn
E = Ge, E0 = Si
E = Ge, E0 = Ge
E = Ge, E0 = Sn
E = Sn, E0 = Si
E = Sn, E0 = Ge
E = Sn, E0 = Sn
2.654
2.761
3.004
2.810
2.911
3.136
3.143
3.231
3.441
325.7
333.0
347.2
333.0
341.1
351.9
389.4
392.1
398.4
546.8
548.3
554.0
498.0
490.2
492.3
556.4
532.7
522.1
In contrast to the pentasila[1.1.1]propellane Si5Mes6, which
shows closed-shell and radical-type reactivity, 1 and 2 are stable
against degassed water, and no reaction is observable with PhOH
and 9,10-dihydroanthracene (entries 1 to 3).26 For other
reagents such as Me3SnH or the disulfide FcS-SFc (Fc =
ferrocenyl), which are prone to radical-type reactivity, we ob-
served differences in the reactivities of 1 and 2. Only the
propellane 1 gave the addition product Me3Sn-Ge-
(SiMes2)3Ge-H (3) in quantitative yield (characteristic NMR
signals for an asymmetrically 1,3-disubstituted bicyclo-
[1.1.1]pentane derivative). By analogy with Sn5Dep6, the tin
compound 2 did not react under these conditions (room
temperature, C6D6; see Scheme 2). The disulfide FcS-SFc
undergoes homolytic S-S bond cleavage with both propellanes,
affording the symmetrical bicyclo[1.1.1]pentane derivatives
(4 and 5) depicted in Scheme 2 (reactions on larger scale in
THF).27
4 and 5 are thermally very stable (mp > 200 °C, dec) and form
analytically pure,28 orange-red crystals from a toluene/acetoni-
trile mixture. Solutions are intensively orange (4)/red (5) in
color, showing absorption maxima in THF at λmax(4) = 465 (sh),
369 (ε = 11 000 M-1 cm-1) and 290 (sh) nm and λmax(5) =
535-372 (br, ε = 6500 M-1 cm-1) and 311 (ε = 32 000 M-1
cm-1) nm, respectively (see Supporting Information). 1H NMR
spectra of both compounds showed the expected signals for a
symmetric substitution pattern of the bridgehead atoms. The 29Si
NMR resonances of the bridging Sibr atoms are shifted to lower
frequencies (δ = -40 ppm (4) and -54 ppm (5)) as compared
to the free propellanes 1 and 2 (cf. 66 and 98 ppm). The 119Sn
NMR resonance of 5 at δ = -177 ppm is considerably shifted to
higher frequencies (cf. -1348 ppm for 2) with additional
couplings to 29Sibr (154 Hz vs 324 Hz for 2) and 117Snb isotopes
(9341 Hz vs. 3464 Hz for 2).29
Electrochemical investigations on the novel bicyclo[1.1.1]-
pentane derivatives 4 and 5 revealed in each case only one Fc-
centered, (quasi)reversible oxidation event at a potential of
E01/2(4) = 0.1 V and E01/2(5) = 0.17 V (see Supporting Infor-
mation) and, thus, no redox communication between the two Fc
units. However, this is not surprising since only a weak commu-
nication was observed already for the starting material (FcS)2
(ΔE01/2 = 0.18 V)30 and the distance between the electroactive
Fc entities (ca. 13.6 Å) is much larger in 4 and 5. Furthermore,
we observed electrochemically irreversible (but chemically re-
versible) reduction events, which most likely can be attributed to
the stepwise reductive splitting of the E-S bonds within 4 and 5,
furnishing the dianions [1]2- or [2]2-. Similar processes have
been reported recently for transition metal terminated bicylo-
[1.1.1]pentastannane derivatives.19
a The computational results for Ge2Si3Dmp6 (cf. 1) and Sn2Si3Dmp6 (cf.
2) are highlighted in bold.
experimentally detected by UV/vis spectroscopy (1: λexp(1A2) =
3
323 nm, λcalc(1A2) = 333 nm; the A2 transition has not been
observed; 2: λexp(1A2) = 383 nm, λcalc(1A2) = 389 nm; λexp
-
(3A2) = 549 nm, λcalc(3A2) = 556 nm). In search of general
trends, we also calculated other (hypothetical) cluster core
combinations, and in Table 1 are also included the results of
the homonuclear propellanes reported previously.12,13 Overall,
we obtained three data sets consisting of all conceivable element
combinations from Si to Sn (see Table 1). As can be seen from
1
column 4, we found a linear increase of the A2 excitation
wavelength upon replacing a bridging entity (E0) by its heavier
congener. This is in nice accord with the increasing radius of E0
and, as a result of this, the increasing Eb Eb distance along each
3 3 3
series (see column 3). However, as can be seen from column 5,
the corresponding vertical 3A2 excitations showed some peculia-
rities without linear relationships. For instance, both title com-
pounds (1 and 2, cf. entries 4 and 7 in Table 1) show the highest
3A2 excitation wavelengths (lowest excitation energy) within each
set of data, although the Eb Eb distances are much shorter and
3 3 3
the first singlet 1A2 excitation wavelengths are lower than those of
its analogues within each set. Our current interpretation is that
two important factors mutually contribute to the 3A2 excitations
and, as a consequence, the ease of accessing the excited triplet A2
state of heavy propellanes. It appears that higher 3A2 excitation
wavelengths, and thus lower ΔESfT energies, are obtained by
either (a) elongating the Eb Eb distances or (b) arranging the
3 3 3
involved orbitals (HOMO-1(a1) and LUMO(a2))24 in close
spatial proximity. While the first effect, that is, the bond-stretch,
leads to a smaller energy difference between the involved orbitals
(HOMO-1(a1) and LUMO(a2)), a close proximity of the latter
furnishes a larger exchange interaction <a1a2|a2a1> and an extra
3
stabilization of the A2 state.,13 We also noticed that results
from calculations of natural orbital occupation numbers obtained
at the (6,4)CASSCF/def2-TZVP//B3LYP/def2-TZVP level
showed only slight differences, which can mainly be attributed
to varying Eb Eb separations (see Supporting Information).25
3 3 3
To shed more light on the reactivity of heavy
[1.1.1]propellanes in general, and to compare the novel hetero-
nuclear species 1 and 2 with the homonuclear analogues, we
performed reactivity studies on the NMR tube scale. In general,
the addition of a substrate X-Y to the bridgehead atoms, either
In order to unambiguously establish the structures of 4 and 5,
single crystals were grown from toluene/acetonitrile at room
temperature, and X-ray diffraction studies were performed
(space group for both compounds C2/c;23 Figure 4). Both
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Table 2. NMR Tube Scale Reactions (1H NMR) of 1 and 2 with Selected Reagents
reagent
Ge2Si3Mes6 (1)
Sn2Si3Mes6 (2)
product
H2O (degassed)
PhOH
-
-
-
þ
þ
-
-
-
-
þ
no reaction
no reaction
no reaction
9,10-dihydroanthracene
Me3SnH
Me3Sn-Ge(SiMes2)3Ge-H (3)
FcS-E(SiMes2)3E-SFc (4, 5)
FcS-SFc
Scheme 2. Reactions of 1 and 2 with Me3SnH and FcS-SFc (Fc = ferrocenyl)
bridgehead atoms are indeed bonded to SFc substituents. While
the addition of the SFc entities to the tin atoms in 5 has only a
limited geometric impact, the structural changes are somewhat
larger for 4. In particular, the trigonal-bipyramidal Ge2Si3 core
structure of 4 is stretched in the direction of the bridgehead
propellane 2 around 45% of the addition product 5 was formed
after ca. 40 min (Figure 5b), the germanium homologue 1
reacted much slower (Figure 5a). Most revealingly, both con-
versions can be drastically enhanced by light. Repeated exposure
of the samples to daylight in the lab for ca. 20 s (longer for 1)
without further shaking gave a very significant increase of the
yields (yellow areas in Figure 5). Again, the tin propellane 2 is
much more reactive under these conditions. It is important to
note that photochemically enhanced reactivity under these
conditions is most likely due to the excited states of the
propellanes and not of the disulfide FcS-SFc.33 Although free-
radical reaction channels are known for disulfides,34 FcS-SFc
does not give any photoproduct under our mild conditions
(daylight in the lab) upon treatment of a C6D6 solution with
several drops of the good radical trap CCl4.35 We note in passing
that other disulfides such as PhS-SPh react even faster with 2
and that the reaction is also enhanced by light. Currently we are
not able to comment on whether this biradicaloid behavior is due
to the excited singlet or triplet A2 state of 1 and 2. Nevertheless,
the present study serves as proof-of-principle that the electronic
properties and the reactivities of heavy propellanes can be
deliberately controlled and adjusted to a certain extent and there
atoms (e.g., d(Geb Geb) = 2.767(1) in 1 vs 2.884(1) Å in 4),
3 3 3
whereas the Sn2Si3 scaffold of 5 is only marginally changed as
compared to 2 (e.g., d(Snb Snb) = 3.097(1) in 2 vs 3.125(1) Å
3 3 3
in 5). The Ge-S and Sn-S bond lengths fall within the expected
ranges;31 the bond lengths and angles within the S-Fc substit-
uents are comparable to the starting material.32
It was of interest to compare the reactivities of 1 and 2 in
general and to check whether the homolytic S-S bond addition
reactions are susceptible to light. To this end, we prepared
separate NMR samples with 1:1 mixtures of the propellanes 1
and 2 and FcS-SFc in the dark (c = 8.7 mM). In order to
maximize the spectral window, we used quartz EPR tubes with an
outer diameter of 5 mm (typical for regular NMR tubes) for this
study. After addition of C6D6, the tubes were sealed (Young
Teflon valve) and immediately transferred into a 400 MHz NMR
spectrometer. Repeated 1H NMR measurements revealed clear
differences in the reactivity of both clusters. While for the tin
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were confirmed by electrochemical investigations. UV/vis stud-
ies together with DFT calculations unraveled the interrelated
nature of three main parameters, the Eb Eb distance and the
3 3 3
singlet and triplet A2 transitions, which were correlated for a
series of homo- and heteronuclear [1.1.1]propellanes. Two
3
important factors mutually contribute to the A2 excitations
and, as a consequence, the ease of accessing the excited triplet
A2 state of heavy propellanes. Higher 3A2 excitation wavelengths,
and thus lower ΔESfT energies, are obtained either by increasing
the distances between the bridgehead atoms or by arranging the
involved orbitals in close spatial proximity. Furthermore, this
work establishes that the electronic properties of this type of
biradicaloids can be related with their reactivity. In particular, the
photochemically controllable radical-type reactivity of 1 and 2
opened up exciting perspectives for future investigations.
’ EXPERIMENTAL SECTION
General Methods and Instrumentation. All manipulations
were carried out using standard Schlenk line or drybox techniques
under an atmosphere of argon. Solvents were freshly distilled under
argon from the appropriate drying agent. Deuterated solvents were
refluxed over the appropriate drying agent, distilled, and stored under
argon in Teflon valve ampules. NMR samples were prepared under an
inert atmosphere in 5 mm Wilmad 507-PP or 5 mm EPR tubes (thin wall
quartz 700LG; RototecSpintec), either flame-sealed or fitted with J.
Young Teflon valves. 1H, 13C{1H}, 29Si, and 119Sn NMR spectra were
recorded on Bruker Avance spectrometers operating at 1H Larmor
frequencies of 300 and 400 MHz, respectively, and are referenced
according to IUPAC recommendation.36 1H, 13C, and 29Si chemical
shifts are given relative to TMS, and 119Sn relative to Me4Sn. Coupling
constants J are given in hertz as positive values regardless of their real
individual signs. The multiplicity of the signals is indicated as s, d, or m
for singlets, doublets, or multiplets, respectively. The abbreviation br is
given for broadened signals. Assignments were confirmed as necessary
with the use of two-dimensional correlation experiments. Metalla-
[1.1.1]propellanes of the heavier group 14 elements show distinctive
resonances in their 1H NMR spectra. Due to the special arrangement of
the Mes ligands, three signals for the Me groups (each 18 H) and two
resonances for the aromatic protons in meta position (each 6 H) of the
Mes ligands are usually observed for unsubstituted [1.1.1]propellanes or
symmetrically substituted bicyclo[1.1.1]pentanes. For unsymmetrically
substituted bicyclo[1.1.1]pentanes, however, the number of resonances
is doubled and the integrals are bisected according to the nonequiva-
lence of the bridgehead atoms. In the case of the unsymmetrically
substituted Me3SnH adduct Me3Sn-Ge(SiMes2)3Ge-H (3) we did
not differentiate between ortho and para Me groups (abbreviated as o/p-
CH3). Cyclic voltammetry measurements were performed with an
EG&G potentiostat (PAR-model 263A) and an electrochemical cell
for sensitive compounds.37 We used a freshly polished Pt disk working
electrode, a Pt wire as counter electrode, and a Ag wire as (pseudo)
reference electrode ([nBu4N][PF6] (0.1 M) as electrolyte). Potentials
were calibrated against the Fc/Fcþ couple,38 which has a potential of
E01/2 = 0.35 V vs Ag/AgCl. IR spectra were recorded on a Bruker Vertex
70 spectrometer in the range from 4000 to 400 cm-1 using a KBr
beamsplitter. Samples were prepared by using the ATR technique
(attenuated total reflection) on bulk material, and the data are quoted
in wavenumbers (cm-1). The intensity of the absorption band is
indicated as vw (very weak), w (weak), m (medium), s (strong),
vs (very strong), and br (broad). UV/vis spectra were recorded on a Varian
Cary 100 Scan UV/vis spectrometer. Mass spectra and elemental
analyses were recorded by the institutional technical laboratories of
the Karlsruhe Institute of Technology (KIT). Due to varying solvent
Figure 4. Molecular structures of 4 (a) and 5 (b). Displacement ellipsoids
are drawn at the 30% probability level; hydrogen atoms are omitted and
carbon atoms of the Mes substituents are drawn with arbitrary radii for
clarity. Selected bond lengths (Å) and angles (deg) for 4: Ge1 Ge10
3 3 3
2.884(1), Ge1-Si1 2.449(1), Ge1-Si2 2.428(1), Ge1-S1 2.272(1), S1-
C27 1.744(3), Fe-Cpcentroid 1.637 Si-C 1.892(3)-1.898(3), Ge1-Si1-
Ge10 72.5(1), Ge1-Si2-Ge10 72.8(1), Si1-Ge1-Si2 88.6(1), Si1-
Ge1-Si10 88.7(1), Ge1-S1-C27 110.5(1); for 5: Sn1 Sn10
3 3 3
3.125(1), Sn1-Si1 2.618(1), Sn1-Si2 2.610(1), Sn1-S1 2.442(1), S1-
C27 1.741(4), Fe-Cpcentroid 1.638, Si-C 1.892(4)-1.903(4), Sn1-Si1-
Sn10 73.0(1), Sn1-Si2-Sn10 73.5(1), Si1-Sn1-Si2 88.2(1), Si1-Sn1-
Si10 88.2(1), Sn1-S1-C27 107.0(1) equivalent atoms generated by
3
1
-x þ /2, y, -z þ /2; Cpcentroid = centroid of the Cp ring.
can be little doubt that different aryl substituents should also
exert considerable effects. All this, including in-depth kinetic
studies, is work in progress.
’ CONCLUSIONS
Whereas our previous reports12,13 drew attention to the
synthetic accessibility of the pentagerma- and pentasilapropel-
lanes, the present account not only demonstrates the possibility
of modifying the inherent electronic properties of heavy
[1.1.1]propellanes in general but also reveals new facets of their
reactivity. The heteronuclear propellanes 1 and 2 possess shorter
Eb Eb distances than in the homonuclear analogues, which has
3 3 3
a considerable impact on their electrochemistry and electronic
absorption spectra. The stronger Eb Eb interactions in 1 and 2
3 3 3
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Organometallics
ARTICLE
Figure 5. 1H NMR spectroscopic monitoring of 1:1 mixtures (c = 8.7 mM) of the propellanes 1 (a) and 2 (b) and FcS-SFc (yields of 4 and 5 vs reaction
time). Yellow areas correspond to repeated exposure of the samples to daylight in the lab without further shaking.
content of the samples of 4 and 5, we determined prior to elemental
analyses the exact amount of residual toluene by 1H NMR spectroscopy.
Computational Details. Slightly modified model systems, i.e.,
E2E03Dmp6 (with Dmp = 2,6-dimethylphenyl), were used for all of the
calculations, which were performed with the TURBOMOLE program
package.39 The geometries were optimized in D3 symmetry at the
B3LYP40,41 level in the def2-TZVP basis42 using the grid m4 and
including weight derivatives. Sharp convergence criteria were used
(scfconv=9, denconv=1d-7, -energy 8, -gcart 5). For the singlet and
triplet excited states, 10 singlet and 10 triplet vertical excitation energies
were computed in each irreducible representation at the B3LYP/def2-
TZVP level by means of the TD-DFT approach (rpaconv=7). The
coordinates of the minimum structures of the ground states (B3LYP,
def2-TZVP) are compiled at the end of the Supporting Information
(Cartesian coordinates in bohr units). The (6,4)CASSCF calculations of
the ground states were performed in the def2-TZVP basis with the
program MOLPRO.43 The calculations were performed on the model
compounds E2E03Me6, which were obtained by replacing the dimethyl-
phenyl ligands by small methyl groups, keeping the B3LYP/def2-TZVP-
optimized ground-state geometry of the E2E03C6 core. A (6,4) active
space was used, obtained by distributing six electrons among four
orbitals: the highest occupied a1 level, the highest occupied e level (2-
fold degenerate), and the lowest unoccupied orbital of a2 symmetry.
300.1 MHz, ppm): δ 2.05 (s, 18 H, p-CH3), 2.41 (s, 18 H, o-CH3), 2.63
(s, 18 H, o-CH3), 6.21 and 6.57 (s, 12 H, m-H). 13C{1H} NMR (C6D6,
75.5 MHz, ppm): δ 21.8, 26.9, 28.0, 129.2, 129.6, 134.9, 138.5, 144.6,
144.9. 29Si NMR (C6D6, 79.5 MHz, ppm): δ 65.7. IR-ATR (cm-1): ν
409 (s), 439 (vs), 500 (vw), 550 (s), 567 (m), 598 (vs), 618 (m), 710
(vw), 816 (vw), 843 (s), 1025 (w), 1055 (vw), 1080 (vw), 1234 (vw),
1260 (vw), 1286 (vw), 1366 (w), 1375 (w), 1406 (w), 1445 (m), 1547
(vw), 1602 (w), 2852 (w), 2921 (m), 2954 (w), 3018 (vw). UV/vis
(nm): λmax 415 (ε = 5100 M-1 cm-1), 323 (ε = 7800 M-1 cm-1).
Preparation of Sn2Si3Mes6 (2). To a stirred THF solution
(20 mL) of Mes2SiCl2 (2.00 g, 5.93 mmol) and SnCl2 (0.750 g, 3.95
mmol) was added dropwise a freshly prepared lithium naphthalenide
solution (30 mL, 0.66 M in THF) at -78 °C. The dark red solution was
stirred for 1 h at this temperature and overnight at room temperature.
The mixture was treated with NH4Cl (0.53 g, 10 mmol). After the
solvent and naphthalene were removed in vacuo (50 °C) the residue was
washed with pentane (40 mL) and dried in vacuo. The black powder was
carefully exposed to air for several minutes. During this time, the color
changed to dark purple. The powder was transferred back to a Schlenk
vessel under argon after complete color change and extracted with
toluene (20 mL). The solvent was partially removed in vacuo and layered
with acetonitrile (12 mL) to afford analytically pure dark purple crystals
of 2. Yield: 220 mg (212 μmol, 12%). The procedure can be scaled up to
approximately 1 g of product (recrystallization for three times). Mp >
200 °C. EI-MS (70 eV) m/z (%): 1036.5 (2). Anal. Found: C, 62.06; H,
6.38. Calcd for [C54H66Sn2Si3] [1036.8 g/mol]: C, 62.56; H, 6.42. 1H
NMR (THF-d8, 300.1 MHz, ppm): δ 1.94 (s, 18 H, p-CH3), 2.14 (s, 18
H, o-CH3), 2.44 (s, 18 H, o-CH3), 6.18 and 6.67 (s, 12 H, m-H).
13C{1H} NMR (THF-d8, 75.5 MHz, ppm): δ 20.0, 25.2, 26.8, 127.9,
128.9, 137.0, 137.2, 142.8, 142.9. 29Si NMR (THF-d8, 59.6 MHz, ppm):
δ 98 ppm (1J(29Si,119Sn) = 324 Hz). 119Sn NMR (THF-d8, 149.0 MHz,
ppm): δ -1348 ppm (1J(119Sn,29Si) = 324 Hz, 2J(119Sn,117Sn) = 3464
Hz). IR-ATR (cm-1): ν 424 (vs), 474 (vw), 489 (vw), 507 (vw), 551
(s), 566 (w), 593 (s), 617 (w), 845 (m), 1027 (w), 1288 (vw), 1365
(vw), 1406 (w), 1442 (broad), 1606 (w), 2853 (vw), 2912 (vw), 2953
(vw), 3019 (vw). UV/vis (nm): λmax 550 (ε = 3800 M-1 cm-1), 496 (ε
= 4900 M-1 cm-1), 383 (ε = 10 300 M-1 cm-1), 283 (shoulder).
Preparation of Me3Sn-Ge(SiMes2)3Ge-H (3). To a toluene
(3 mL) solution of 1 (3 mg, 3.2 μmol) was added Me3SnH (excess), and
this solution was stirred at room temperature for 5 min. After the solvent
and the excess Me3SnH were removed in vacuo the residue was
redissolved in C6D6 (0.6 mL) and characterized by NMR spectroscopy.
1H NMR (C6D6, 400.1 MHz, ppm): δ 0.61 (s, 2J(1H,117Sn) = 46 Hz,
Starting Materials. Mes2SiCl2 (Mes
=
2,4,6-Me3C6H2),44
Me3SnH,45 GeCl2 dioxane,46 and (FcS)2 were prepared according
47
3
to literature methods.
Preparation of Ge2Si3Mes6 (1). To a stirred tetrahydrofuran
(THF) solution (20 mL) of Mes2SiCl2 (1.00 g, 3.0 mmol) and
GeCl2 dioxane (0.463 g, 2.0 mmol) was added a freshly prepared
3
lithium naphthalenide solution (25 mL, 0.4 M in THF) dropwise at
-78 °C. 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 (60 °C) the residue was extracted with hexane (2 ꢀ
10 mL). The solution was partially evaporated and applied to a pre-
equilibrated silica gel chromatography column (3 cm ꢀ7 cm). Elution
with the same solvent and toluene (gradient) at ambient pressure
provided a fraction from which 1 was obtained in analytically pure form
as orange crystals from a toluene (2 mL) solution layered with
acetonitrile (10 mL). In order to remove small amounts of byproduct
incorporated by the adhesion of the mother liquor, the crystals were
washed with acetone. Yield: 35 mg (37 μmol, 4%). Mp > 250 °C. EI-MS
(70 eV) m/z (%): 944.1(14). Anal. Found: C, 68.62; H, 7.08. Calcd for
1
[C54H66Ge2Si3] [944.6 g/mol]: C, 68.66; H, 7.04. H NMR (C6D6,
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Organometallics
ARTICLE
Table 3. Crystal Data, Data Collection, and Structure Refinement for 1, 2, 4, and 5
1
2
4
5
empirical formula
M
C
54H66Ge2Si3
C54H66Sn2Si3
1036.72
C74H84S2Fe2Ge2Si3 2 toluene
C74H84S2Fe2Sn2Si3 2 toluene
3
3
944.52
1562.95
monoclinic
C2/c (no. 15)
23.038(5)
15.932(3)
21.740(4)
107.82(3)
7597(3)
1.306
1655.15
cryst syst
space group23
a /Å
orthorhombic
Pbca (no. 61)
8.497(2)
orthorhombic
Pbca (no. 61)
8.551(2)
monoclinic
C2/c (no. 15)
23.325(5)
15.972(3)
21.830(4)
108.00(3)
7734(3)
b/Å
26.976(5)
41.646(8)
27.169(5)
c/Å
42.266(9)
β/deg
V/Å3
μ/mm-1
Dcalcd/g cm-3
cryst dimens/mm
Z
9546(3)
9819(3)
1.370
1.125
1.152
1.314
1.403
1.367
1.421
0.50 ꢀ 0.25 ꢀ 0.20
8
0.30 ꢀ 0.10 ꢀ 0.10
8
0.25 ꢀ 0.25 ꢀ 0.20
4
0.40 ꢀ 0.35 ꢀ 0.35
4
T/K
200
200
200
200
2θmax/deg
reflns measd
reflns unique
params/restraints
R1 [I g 2σ(I)]
wR2 (all data)
52.00
65 167
50.00
63 751
52.00
52.00
27 778
28 360
9368 (Rint = 0.0767) 8561 (Rint = 0.1765) 7427 (Rint = 0.0694)
7567 (Rint = 0.0597)
448/0
550/0
0.0617
0.1417
550/0
448/0
0.1035
0.0520
0.0460
0.1852
0.1412
0.1214
max./min. resid electron dens/e ꢀ 10-6 pm-3 0.734/-0.568
3.236/-1.143
1.073/-0.953
1.380/-0.651
2J(1H,119Sn) = 48 Hz, 9 H, Sn-CH3), 2.07, 2.09, 2.36, 2.49, 2.51, and 2.57
(s, each 9 H, o/p-CH3), 6.32 (s, 4J(1H,119Sn) = 305 Hz, 2J(1H,29Si) = 6
Hz, 1 H, Ge-H), 6.22, 6.23, 6.57, and 6.62 (s, each 3 H, m-H). 29Si NMR
(C6D6, 59.6 MHz, ppm): δ -36 (2J(29Si,117/119Sn) = 28 Hz). 119Sn
NMR (C6D6, 149.0 MHz, ppm): δ -37 (2J(29Si,117/119Sn) = 28 Hz).
Preparation of FcS-Ge(SiMes2)3Ge-SFc (4). A THF solution
(20mL) ofhexakis(2,4,6-trimethylphenyl)digermatrisila[1.1.1]propellane
(1) (35 mg, 37.1 μmol) and diferrocenyl disulfide (17 mg, 39.0 μmol) was
stirred at rt for one week. After the solvent was removed in vacuo, the
residue was redissolved in a minimum amount of toluene (4 mL) and
layered with acetonitrile (12 mL) to afford analytically pure 3 as orange
crystals. In order to remove small amounts of byproduct incorporated by
the adhesion of the mother liquor, the crystals were washed with acetone.
Yield: 45 mg (32.7 μmol, 89%). Mp > 200 °C (dec). Anal. Found: C,
remove small amounts of byproduct incorporated by the adhesion of the
mother liquor, the crystals were washed with acetone. Yield: 60 mg (40.8
μmol, 84%). Mp > 200 °C (dec). Anal. Found C, 62.80; H, 6.00; S, 4.06.
Calcd for [C74H84Fe2S2Si3Sn2 1.14C7H8] [1563.09 g/mol]: C, 62.48; H,
3
5.96; S, 4.07. 1H NMR (C6D6, 300.1 MHz, ppm): δ 2.07 (s, 18 H, p-CH3),
2.42 (s, 18 H, o-CH3), 2.60 (s, 18 H, o-CH3), 4.01 (m, 4 H, H3 and H4),
4.14 (s, 10 H, Cp), 4.58 (br, 4 H, H2 and H5), 6.32 and 6.68 (s, 12 H,
m-H). 13C{1H} NMR (C6D6, 75.5 MHz, ppm): δ 20.7 (p-CH3), 25.4 (o-
CH3), 27.4 (o-CH3), 68.5 and 68.6 (C3 and C4), 70.1 (Cp), 75.9 and 76.0
(C2 and C5), 79.9 (C1), 128.6 and 129.2 (m-CH), 132.0 (Si-C), 137.8 (p-
C(CH3)), 144.3 and 145.7 (o-C(CH3)). 29Si NMR (C6D6, 59.6 MHz,
ppm): δ -54 (1J(29Si,119Sn) = 154 Hz). 119Sn NMR (C6D6, 111.9 MHz,
ppm): δ -177 (1J(119Sn,29Si) = 154 Hz, 2J(119Sn,117Sn) = 9.341 Hz). IR
(ATR, cm-1): ν 404 (vs), 427 (vs), 450 (m), 464 (m), 487 (vs), 548 (s),
567 (w), 596 (s), 616 (m), 694 (m), 728 (s), 816 (s), 843 (s), 886 (m),
1002 (m), 1020 (m), 1054 (w), 1106 (w), 1158 (vw), 1179 (vw), 1238
(vw), 1287 (w), 1375 (w), 1406 (m), 1444 (br), 1495 (vw), 1544 (vw),
1602 (m), 2725 (vw), 2853 (w), 2915 (w), 2943 (w), 3018 (vw), 3087
(vw). UV/vis(THF, nm):λmax 535-372 (br, ε=6500M-1 cm-1), 311(ε
= 32 000 M-1 cm-1).
66.56; H, 6.32; S, 4.26. Calcd for [C74H84Fe2Ge2S2Si3 1.39C7H8]
3
1
[1470.95 g/mol]: C, 66.74; H, 6.36; S, 4.26. H NMR (C6D6, 300.1
MHz, ppm): δ 2.07 (s, 18 H, p-CH3), 2.44 (s, 18 H, o-CH3), 2.63 (s, 18 H,
o-CH3), 4.02 (m, 4 H, H3 and H4), 4.15 (s, 10 H, Cp), 4.63 (m, 4 H, H2
and H5), 6.32 and 6.64 (s, 12 H, m-H). 13C{1H} NMR (C6D6, 75.5 MHz,
ppm): δ 20.6 (p-CH3), 25.7 (o-CH3), 27.5 (o-CH3), 68.6 and 68.8 (C3
and C4), 70.2 (Cp), 76.4 and 76.6, (C2 and C5), 79.2 (C1), 128.3 and
128.7 (m-CH), 132.3 (Si-C), 137.8 (p-C(CH3)), 145.1 and 145.5 (o-
C(CH3)). 29Si NMR (C6D6, 59.6 MHz, ppm): δ-40. IR (ATR, cm-1): ν
411 (vs), 441 (vs), 489 (vs), 548 (s), 571 (m), 598 (vs), 615 (m) 694 (m),
728 (vs), 817 (s), 842 (s), 888 (w), 925 (vw), 1004 (m), 1022 (s), 1053
(w), 1082 (w), 1106 (m), 1158 (vw), 1241 (vw), 1260 (w), 1290 (vw),
1373 (w), 1406 (m), 1440 (m, br), 1494 (vw), 1545 (vw), 1603 (w), 2725
(wv), 2854 (vw), 2915 (w), 2958 (w), 3019 (vw), 3085 (vw). UV/vis
(THF, nm): λmax 465 (shoulder), 369 (ε = 11 000 M-1 cm-1), 290
(shoulder).
Preparation of FcS-Sn(SiMes2)3Sn-SFc (5). A THF solution
(20 mL) of hexakis(2,4,6-trimethylphenyl)distannatrisila[1.1.1]propellane
(2) (50 mg, 48.3 μmol) and diferrocenyl disulfide (22 mg, 50.7 μmol) was
stirred at rt for three days. After the solvent was removed in vacuo, the
residue was redissolved in a minimum of toluene (8 mL) and layered with
acetonitrile (25 mL) to afford analytically pure 4 as red crystals. In order to
Crystal Structure Determinations of 2, 3, 4, and 5. Crystal
data collection and processing parameters are given in Table 3. In order
to avoid degradation, the single crystals were mounted on glass fibers
using perfluoropolyether oil and cooled rapidly in a stream of cold N2
using an Oxford Cryosystems Cryostream unit. Diffraction data were
measured using a STOE STADI 4 diffractometer with CCD detector
and graphite-monochromated Mo KR (0.71073 Å) radiation. All
calculations were performed using the SHELXTL (ver. 6.12) program
suite.48,49 The structures were solved by direct methods and successive
interpretation of the difference Fourier maps, followed by full matrix
least-squares refinement (against either F or F2). All non-hydrogen
atoms were refined anisotropically. The contribution of the hydrogen
atoms, in their calculated positions, was included in the refinement using
a riding model. Upon convergence, the final Fourier difference map of
the X-ray structures showed no significant peaks. All details of the
structure solution and refinements are given in the Supporting
1426
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Organometallics
ARTICLE
Information (CIF data). A full listing of atomic coordinates, bond
lengths and angles, and displacement parameters for all the structures
have been deposited at the Cambridge Crystallographic Data Centre.
See Notice to Authors, Issue No. 1.
(7) (a) Power, P. P. Nature 2010, 463, 171. (b) Chivers, T.; Konu, J.
Comm. Inorg. Chem. 2009, 30, 131.
(8) Gr€utzmacher, H.; Breher, F. Angew. Chem. 2002, 114, 4178;
Angew. Chem., Int. Ed. 2002, 41, 4006.
(9) A summary of computational studies on [1.1.1]propellanes can
be found in: Karni, M.; Apeloig, Y.; Kapp, J.; Schleyer, P. von, R. In The
Chemistry of Organic Silicon Compounds, Vol. 3; Rappoport, Z.; Ape-
loig, Y., Eds.; Wiley: Chichester, 2001. See also refs 2 and 3.
(10) (a) Sita, L. R.; Kinoshita, I. J. Am. Chem. Soc. 1992, 114, 7024.
(b) Sita, L. R.; Kinoshita, I. J. Am. Chem. Soc. 1991, 113, 5070. (c) Sita,
L. R.; Kinoshita, I. J. Am. Chem. Soc. 1990, 112, 8839. (d) Sita, L. R.;
Bickerstaff, R. D. J. Am. Chem. Soc. 1989, 111, 6454. (e) Sita, L. R. Adv.
Organomet. Chem. 1995, 38, 189. (f) Sita, L. R. Acc. Chem. Res. 1994,
27, 191.
’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic data in
b
CIF format for the structure determinations of 1, 2, 4, and 5; UV/
vis spectra and cyclic voltammograms of 4 and 5; calculated
natural orbital occupation numbers and coordinates of the
calculated structures. This material is available free of charge
via the Internet at http://pubs.acs.org.
(11) Drost, C.; Hildebrand, M.; L€onnecke, P. Main Group Met.
Chem. 2002, 25, 93.
(12) Nied, D.; Klopper, W.; Breher, F. Angew. Chem. 2009,
121, 1439; Angew. Chem., Int. Ed. 2009, 48, 1411.
(13) Nied, D.; K€oppe, R.; Klopper, W.; Schn€ockel, H.; Breher, F. J.
Am. Chem. Soc. 2010, 132, 10264.
’ AUTHOR INFORMATION
Corresponding Author
*Fax: þ49 721 608 470 21. Tel: þ49 721 608 448 55. E-mail:
breher@kit.edu.
(14) For closely related cluster compounds of the general formula
[E8R6] see: (a) Fischer, G.; Huch, V.; Mayer, P.; Vasisht, S. K.; Veith, M.;
Wiberg, N. Angew. Chem. 2005, 117, 8096; Angew. Chem., Int. Ed. 2005,
44, 7884; (b) Schnepf, A.; K€oppe, R. Angew. Chem. 2003, 115, 940;
Angew. Chem., Int. Ed. 2003, 42, 911; (c) Schnepf, A.; Drost, C. Dalton
Trans. 2005, 3277; (d) Schnepf, A. Chem. Soc. Rev. 2007, 36, 745; (e)
Wiberg, N.; Lerner, H.-W.; Wagner, S.; N€oth, H.; Seifert, T. Z.
Naturforsch. B 1999, 54, 877; (f) Wiberg, N.; Power, P. P. In Molecular
Clusters of the Main Group Elements; Driess, M.; N€oth, H., Eds.; Wiley-
VCH: Weinheim, 2004. For a Si cluster of the formula Si5Tip6 (Tip =
2,4,6-iPr3C6H2) consisting of only one ligand-free Si atom see: (g)
Scheschkewitz, D. Angew. Chem. 2005, 117, 3014; Angew. Chem., Int. Ed.
2005, 44, 2954. See also: (h) K. Abersfelder, K.; White, A. J. P.; Rzepa,
H. S.; Scheschkewitz, D. Science 2010, 327, 564. (i) Klap€otke, T. M.;
Vasisht, S. K.; Mayer, P. Eur. J. Inorg. Chem. 2010, 3256.
’ ACKNOWLEDGMENT
P.O.B. thanks the Alexander von Humboldt foundation for
a postdoctoral fellowship. F.B. and W.K. thank the Deutsche
Forschungsgemeinschaft for financial support through SFB/
TRR 88.
’ DEDICATION
Dedicated to Prof. Dr. H. Schn€ockel on occasion of his 70th
birthday.
(15) (a) Richards, A. F.; Brynda, M.; Power, P. P. Organometallics
2004, 23, 4009. See also:(b) Richards, A. F.; Brynda, M.; Olmstead,
M. M.; Power, P. P. Organometallics 2004, 23, 2841.
’ REFERENCES
(1) Wiberg, K. B. Chem. Rev. 1989, 89, 975.
(16) It is quite common to describe ligand-free atoms of main group
element clusters as “naked”. See for instance: Schn€ockel, H. Chem. Rev.
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Organometallics
ARTICLE
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the reductive coupling reaction has to be exposed to air for several
minutes. During this time, the color changes to dark purple, indicative
for the formation of 2. Currently, we are not able to comment on the
reactions taking place in this step. This is work in progress.
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3 3 3
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and 5 gave only full decomposition of the compounds.
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constants for several addition products. Furthermore, even shorter
distances have been detected for some 1,3-disubstituted bicyclo-
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