Dispersion energy enforced dimerization of a cyclic disilylated plumbylene

Article abstract of DOI:10.1021/ja300654t

By reaction of 1,4-dipotassio-1,1,4,4-tetrakis(trimethylsilyl) tetramethyltetrasilane with PbBr₂ in the presence of triethylphosphine a base adduct of a cyclic disilylated plumbylene could be obtained. Phosphine abstraction with B(C₆F₅)₃ led to formation of a base-free plumbylene dimer, which features an unexpected single donor-acceptor PbPb bond. The results of density functional computations at the M06-2X and B3LYP level of theory indicate that the dominating interactions which hold the plumbylene subunits together and which define its actual molecular structure are attracting van der Waals forces between the two large and polarizable plumbylene subunits.

Full text of DOI:10.1021/ja300654t

Article
pubs.acs.org/JACS
Dispersion Energy Enforced Dimerization of a Cyclic Disilylated
Plumbylene
Henning Arp, Judith Baumgartner,* and Christoph Marschner*
Institut fur Anorganische Chemie, Technische Universitat Graz, Stremayrgasse 9, A-8010 Graz, Austria
̈
̈
Patrick Zark and Thomas Muller*
̈
Institut fur Reine und Angewandte Chemie, Carl von Ossietzky Universitat Oldenburg, Carl von Ossietzky-Str. 9-11, D-26111
̈
̈
Oldenburg, Germany
S
* Supporting Information
ABSTRACT: By reaction of 1,4-dipotassio-1,1,4,4-tetrakis-
(trimethylsilyl)tetramethyltetrasilane with PbBr2 in the presence
of triethylphosphine a base adduct of a cyclic disilylated
plumbylene could be obtained. Phosphine abstraction with
B(C6F5)3 led to formation of a base-free plumbylene dimer,
which features an unexpected single donor−acceptor PbPb bond.
The results of density functional computations at the M06-2X
and B3LYP level of theory indicate that the dominating
interactions which hold the plumbylene subunits together and which define its actual molecular structure are attracting van
der Waals forces between the two large and polarizable plumbylene subunits.
a bidentate oligosilanylene ligated stannylene, we wanted to
extend this study to lead.
INTRODUCTION
■
The isolation of the first stable germylenes and stannylenes was
accomplished by Lappert and co-workers1,2 approximately 35
years ago, and still the chemistry of these heavier carbene
analogues continues to attract the attention of both
experimentally and theoretically oriented chemists.3,4
RESULTS AND DISCUSSION
■
Synthesis. The synthesis of the plumbylene phosphine
adduct 2 was accomplished by addition of 1,4-dipotassiote-
trasilane 110,11 to a suspension of PbBr2 and PEt3 (Scheme 1).
Generation of the “base free” plumbylene dimer 4 could then
be achieved by abstraction of triethylphosphine from 2 with the
strong Lewis acid B(C6F5)3 (Scheme 1). The thus formed
phosphine adduct of the borane12 could be separated by
fractional crystallization, but nevertheless several attempts of
recrystallization from pentane were required to grow crystals of
plumbylene dimer 4 suitable for X-ray diffraction (XRD). On
one occasion together with much amorphous black material
(shown by NMR spectroscopy to be 4) a few green
symmetrically shaped crystals were found. By X-ray crystal
structure analysis these were found to correspond to the
plumbylene-B(C6F5)3 adduct 3 (Figure 2). However, even
when 2 equiv of B(C6F5)3 were used in the reaction with 2 only
trace amounts of 3 below the NMR detection limit were
formed. This behavior toward B(C6F5)3 differs from our
recently published results of the same reaction sequence
applied to the corresponding stannylene phosphine adduct.9 In
the tin case the stannylene borane adduct could be formed
selectively suppressing a competing dimerization process. The
dimeric structure of compound 4 was proven by XRD (Figure
One major reason for this interest is the fundamental
differences in electronic ground states, structures, and
reactivities between carbenes and their dimers, i.e. alkenes,
and their heavier counterparts the metallylenes and dimetal-
lenes. Heavy metallylenes exhibit singlet ground states with an
increasing singlet−triplet gap with rising atomic number.5 This
is caused by the increasing energy difference between their s-
and p-orbital levels and the consequential lack of orbital mixing.
However, by attaching large electropositive substituents to the
divalent group 14 atom to some extent they can be forced into
mixing their s- and p-orbitals and thus significantly lower the
singlet−triplet gap. This behavior is illustrated by Sekiguchi’s
distannene (tBu2MeSi)2SnSn(SiMetBu2)2 that despite bear-
ing bulky groups on the tin atoms does not dissociate into
monomers in solution.6
The tendency to form monomeric compounds is even more
pronounced on descending group 14 to lead. This is well
exemplified by the difference between bis[tris(trimethylsilyl)-
silyl]tin and -lead.7 Both compounds exist as monomers in
solution, but the tin compound crystallizes as a distannene type
dimer while the lead compound retains the monomeric
plumbylene structure in the solid state.7,8 After our recent
synthesis9 of a bicyclic distannene, utilizing the dimerization of
Received: January 20, 2012
Published: March 28, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of Plumbylene Adducts 2 and 3 and Plumbylene Dimer 4 Which Dissociates to Plumbylene 6 in Solution
3). While the phosphine complex 2 was found to be infinitely
stable in solution as well as in the solid state under exclusion of
moisture and air, plumbylene dimer 4 decomposed in solution
at room temperature to elemental lead and the corresponding
cyclotetrasilane, resulting from the reductive elimination of
lead.
typical for tetrylenes which have small energy separations
between occupied and virtual molecular orbitals with large
coefficients at the magnetic active nuclei.15 Furthermore, the
strongly deshielded 207Pb NMR signal clearly indicates that
dimer 4 exists as plumbylene monomer, 6, in solution (see
Scheme 1). When the 207Pb shifts of related plumbylenes such
as PbAr*[Si(SiMe3)3] (Ar* = C6H3-2,6-Mes2) (δ = +10510
ppm)16 and Pb{(Me3Si)2(CSiMe2CH2)}2 (δ = +10050 ppm)
are considered,17 a chemical shift of this order of magnitude
seems reasonable. With respect to the 29Si chemical shift for the
NMR Spectroscopy. For plumbylene phosphine adduct 2
at rt, no 207Pb NMR signal could be detected. This is probably
due to a dissociation−association process of the phosphine.
This is also indicated by the 31P NMR spectrum, where a broad
signal at δ = −60.0 ppm without resolved coupling to 207Pb is
observed. In the 29Si NMR spectrum sharp signals for the
SiMe2 units at δ = −10.7 and the quaternary silicon atoms at δ
= −87.3 ppm were found as expected while the SiMe3
resonances appeared as a badly resolved broad signal at δ =
−1.5 ppm. This can also be rationalized by the said
silicon atom attached to lead Klinkhammer and co-workers
7,16
reported resonances of δ = +198.6 for Pb[Si(SiMe3)3]2
and
δ = +156.5 for PbAr*[Si(SiMe3)3].16 However, we did not find
any signal in this area. Comparison with the 29Si NMR chemical
shifts of related stannylene derivatives18 suggests a 29Si NMR
chemical shift for this silicon atom in plumbylene 6 close to δ =
0 ppm. This assumption was further supported by theoretical
investigations, which predict for the α-silicon atom in
plumbylene 6 a chemical shift of δ = −35 ppm.19 Our
measurement at −40 °C eventually led to the observation of a
signal at δ = −8.5 ppm which we tentatively assign to the α-Si
resonance.
1
dissociation−association process of the phosphine. In the H
and 13C NMR spectra the expected pattern of signals was
found. Cooling to −60 °C allows the observation of a 207Pb
resonance at δ = +1139 ppm as a doublet with a coupling
1
constant of J(PbP) = 3083 Hz which was also observed in the
31P NMR spectrum at the same temperature. This chemical
shift agrees fairly well with an expected value of δ = +1595 ppm
based on Wrackmeyer’s empirical correlation13 of NMR
chemical shifts of Sn(II) compounds to analogous Pb(II)
compounds.13
X-ray Crystallography. Compounds 2, 3, and 4 were
subjected to single-crystal X-ray diffraction analysis, and the
structural features are listed in Table S1. The structure of 2
(Figure 1) is the first plumba-cyclopentasilane to be structurally
characterized. Similar to the recently published cyclopentasi-
lanyl stannylene phosphine adduct9 it adopts an envelope
conformation with one of the quaternary Si atoms on the flap.
The donor−acceptor interaction with the phosphine is clearly
indicated by the strong pyramidalization of the Pb atom in 2
[pyramidalization angle β(Pb) = 71.2°]20 and by the Pb−P
bond (274.0 pm) which is significantly longer than the sum of
the covalent radii (255 pm).21
The plumbylene borane adduct 3 (Figure 2) is an analogue
to the stannylene borane adduct reported by us earlier.9 The
lead atom in compound 3 is in an approximate trigonal planar
coordination environment with only a small deviation from
planarity [β(Pb) = 7.9°] and a relatively long B−Pb bond
For the plumbylene dimer 4 again no 207Pb NMR signals
could be observed at rt, as decomposition of 4 in solution
proved to be faster than acquisition of the 207Pb NMR
spectrum. The decomposition takes place in an analogous way
to bis[tris(trimethylsilyl)silyl]lead (5),7 but with a shorter
lifetime. After 30 min in a benzene solution, complete
decomposition to 1,1,2,2-tetrakis(trimethylsilyl)-
tetramethylcyclotetrasilane14 and elemental lead occurred.
Nevertheless, acquisition of spectra at −40 °C allowed
observation of a 207Pb resonance with a chemical shift of δ =
+19516 ppm, which to the best of our knowledge is by far the
most downfield shifted resonance of a lead compound ever
recorded. This strong paramagnetically deshielded resonance is
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pm covered by Klinkhammer’s heteroleptic plumbylene dimers
of the type {[(Me3Si)3Si]ArPb}2.8 The two plumbylene units of
4 show different arrangements around the lead atoms. The
Pb(1) atom in Figure 3 shows some stereochemical activity of
Figure 1. Molecular structure of 2 (thermal ellipsoid plot drawn at the
30% probability level). Hydrogen atoms omitted for clarity (bond
lengths in pm, angles in deg). Pb(1)−Si(4) 272.3(2), Pb(1)−Si(1)
272.8(2), Pb(1)−P(1) 274.0(2), Si(1)−Si(2) 237.1(3), P(1)−C(19)
182.9(10), Si(2)−C(2) 189.8(9), Si(4)−Pb(1)−Si(1) 95.67(8),
Si(4)−Pb(1)−P(1) 97.86(7), Si(1)−Pb(1)−P(1) 106.01(7).
Figure 3. Molecular structure of 4 (thermal ellipsoid plot drawn at the
30% probability level). Hydrogen atoms omitted for clarity (bond
lengths in pm, angles in deg). Pb(1)−Si(4) 270.9(4), Pb(1)−Si(1)
273.2(3), Pb(1)−Pb(2) 306.40(8), Pb(2)−Si(12) 269.7(3), Pb(2)−
Si(9) 273.7(3), Si(1)−Si(2) 233.4(6), Si(2)−C(1) 187.5(13), Si(4)−
Pb(1)−Si(1) 94.38(12), Si(4)−Pb(1)−Pb(2) 109.54(11), Si(1)−
Pb(1)−Pb(2) 101.83(7), Si(12)−Pb(2)−Si(9) 96.49(10), Si(12)−
Pb(2)−Pb(1) 101.80(7), Si(9)−Pb(2)−Pb(1) 156.09(9).
the lone pair, and it is therefore highly pyramidalized with both
SiPb(1)Pb(2) bond angles being close to the tetrahedral angle
of 109.5° and a pyramidalization angle β(Pb(1)) = 66.0°.20
Pb(2) to the contrary shows a distorted planar geometry, the
sum of bond angles around Pb(2) being 354.3° concomitant by
a pyramidalization angle β of 15.1°. This very unusual structural
arrangement can be rationalized by assuming a single donor−
acceptor interaction in the solid state between the two
plumbylene units with the planar Pb(2) being the donor and
the pyramidalized Pb(1) acting as the acceptor. A somewhat
similar situation is found in distannene (Mebp2Sn)2 (Mebp =
2,3,4-methyl-6-tert-butylphenyl)24 and, in particular, in Weiden-
bruch’s cyclotriplumbane (Tep2Pb)3 (Tep =2,4,6-triethylphen-
yl), where each lead atom acts as an electron pair acceptor for
one of its neighbors and as a donor to the other one.25 As the
geometry of the core in Weidenbruch’s compounds depends on
subtle changes in the steric bulk of the ligands, plumbylene 6
might adopt an intermediate position.25−27 The electronic
situation would favor trimerization, while the silyl ligands
employed do not allow higher aggregates than a dimer for steric
reasons.
Computational Study. Quantum mechanical calculations
at the M06-2X/SDD(Pb) 6-31G(d)(P, Si, F, C, B, H) level,
here denoted as M06-2X/A, provided a more detailed picture
of the bonding in 4, the dimeric form of plumbylene 6, and the
related plumbylene complexes with PEt3 (2) and B(C6F5)3
(3).28,29 The molecular structures, which were predicted by the
calculations for compounds 2−4, are in good qualitative
agreement with the data obtained from XRD measurements
(Figure 4). In addition, the results of the calculations reveal for
the free plumbylene 6 a half-chair conformation of the
plumbacyclopentasilane ring with an endocyclic SiPbSi bond
angle α = 90.5° and Pb(II)Si(IV) bonds which are by 4−5 pm
longer than reported for Pb−Si linkages in other plumbylenes
(Figure 4).30 The substitution with the electropositive silyl
Figure 2. Molecular structure of 3 (thermal ellipsoid plot drawn at the
30% probability level). Hydrogen atoms omitted for clarity (bond
lengths in pm, angles in deg). Pb(1)−B(1) 243.4(7), Pb(1)−Si(1)
266.54(17), Pb(1)−Si(4) 266.91(18), B(1)−Pb(1)−Si(1)
131.06(16), B(1)−Pb(1)−Si(4) 125.26(16), Si(1)−Pb(1)−Si(4)
102.80(6).
[243.5 pm (3) vs 229 pm (sum of the covalent radii)].21 Both
features are in agreement with the predominant plumbylene/
borane donor/acceptor interaction. In addition, the plumbylene
is also acting as a Lewis acid by accepting electron donation
from one of the ortho-fluorines of the BArf moiety. This is
indicated in the solid state by a relative close F···Pb contact of
277.8 pm, halfway between the sum of the covalent and van der
Waals radii (208 and 349 pm).21,22
In contrast to Klinkhammer’s disilylated monomeric
plumbylene ([(Me3Si)3Si]2Pb) (5) the crystal structure analysis
of 4 (Figure 3) revealed a dimeric arrangement with a Pb−Pb
separation of 306.4 pm. This is substantially longer than a
typical Pb−Pb single bond of 284 pm such as found for
hexaphenyldiplumbane,23 but well within the range of 284−354
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Lewis base/acid complex 3, the almost trigonal planar
coordination of the Pb atom and the close contact between
the ortho-fluorine and the Pb-atom, are also present in the
calculated gas phase structure of 3. This finding excludes the
possibility that the close F···Pb contact found in the solid state
structure is a consequence of crystal packing effects, and it
suggests the presence of a structure shapening C−F→ Pb
donor−acceptor interaction. The results of the NBO analysis
reveal an overall electron donation from the plumbylene to the
borane in 3 of 0.95 au, and the donor−acceptor bond strength
BE of the Pb−B bond is calculated to be −65.1 kJ mol−1.29,31
According to the results of the computations the overall
molecular structure of the plumbacyclopentsilane ring of
plumbylene 6 is conserved nearly unchanged in the acceptor
complex 2 and in the donor adduct 3 and also in the
plumbylene dimer 4. In agreement with the experimental data
Pb atoms with two significantly different coordination environ-
ments were found in the computed structure of dimer 4
(Figures 3 and 4). The structural data suggest a single donor−
acceptor interaction between the plumbylene subunits with a
distorted trigonal planar Pb(2) atom (β(Pb(2)) = 14.8°),20
acting as the donor and a strongly pyramidalized second Pb(1)
atom (β(Pb(1)) = 74.4°)20 (see Figures 4, 5). This interaction
results in a relative long Pb(2)Pb(1) bond (d(Pb(1)Pb(2) =
309.9 pm). The conformation around this bond with a dihedral
angle of 74.5° between the planes spanned by the Pb atoms and
their adjacent Si atoms excludes a second donor−acceptor
interaction (Figure 5a). Consequently, the frontier orbitals of
dimer 4 are the stereochemically active electron pair at Pb(1)
(HOMO) and the vacant p-type orbital at Pb(2) (LUMO)
(Figure 5b). The NBO analysis suggests a significant electron
transfer from the donor plumbylene to the acceptor
plumbylene of 0.24 au which results in a calculated Wiberg
bond index (WBI) for the donor−acceptor bond in dimer 4 of
Figure 4. Comparison between experimental (XRD, black) and
calculated [M06-2X/A, red] structural parameter of compounds 2, 3,
the central part of dimer 4 and computed structural data of 6. Bond
angles α and pyramidalization angles β in deg (italic), bond lengths in
pm (R = SiMe3).
groups decreases markedly the computed singlet−triplet energy
difference, ΔE(ST), for tetrylene 6 compared to the parent
plumbylene PbH2 [ΔE(ST) = −215.4 kJ mol−1 (PbH2),
ΔE(ST) = −145.4 kJ mol−1 (6)]. Due to the small endocyclic
SiPbSi bond angle α, ΔE(ST) is however larger than that
predicted for Klinkhammer’s plumbylene 5 [ΔE(ST) = −128.9
kJ mol−1 (5)]. According to an NBO analysis29 the donor−
acceptor interaction between plumbylene 6 and the phosphine
molecule in complex 2 is accompanied by an electron transfer
of 0.25 au from the phosphine to the plumbylene and results in
a calculated donor−acceptor bond strength BE of −76.0 kJ
mol−1.29,31 Both distinct features which are found in the
experimental solid state structure of the plumbylene/borane
Figure 5. (a) Schematic view of the donor−acceptor bond in plumbylene dimer 4. Left: View in the direction orthogonal to the Pb(2)Si(9)Si(12)
plane. Right: After rotation by 90° around the Pb(1)Pb(2) bond. (b) Calculated surface diagrams of HOMO (left) and LUMO (right) of dimer 4
(isodensity value: 0.05; color code: dark gray: Pb; blue gray: Si; light gray: C; H-atoms omitted).
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0.70. This value is smaller than those computed for the Pb−Pb
single bond in staggered Pb2H6 [point group (PG): D3d,
d(PbPb) = 288.8 pm, WBI = 0.88] and in trans bent Pb2H4
[PG: C2h, d(PbPb) = 290.9 pm, WBI = 0.87]. According to the
NBO analysis the Pb−Pb linkage in dimer 4 is supported by
negative hyperconjugation which involves the lone pair at
Pb(1) and the σ* orbital of the Pb(2)−Si(9) bond (see Scheme
2). As a result from this interaction the bond angle α(Pb(1),
addition free optimization of the molecular structure of dimer 4
using the B3LYP/A method results in a Pb(1)−Pb(2) bond,
which is by 19.7 pm longer than determined by XRD.
Furthermore, the computed binding energy BE for the free
optimized dimer 4 at B3LYP/A is considerably decreased
compared to the M06-2X value [B3LYP/A//B3LYP/A: BE(4)
= −26.5 kJ mol−1].
A rotational isomer of dimer 4, the diplumbylene 7, with an
approximate trans bent configuration of the constituent
plumbylenes 6 and a slightly smaller Pb−Pb separation was
located on the potential energy surface [Pb−Pb = 298.2 pm,
β(Pb) = 52.2° and 36.9°]. In this conformer both plumbylenes
are connected by a conventional double donor−acceptor
interaction. The binding energy for trans bent dimer 7 is
however significantly smaller than calculated for dimer 4 [BE =
−76.4 kJ mol−1 (7) vs −110.8 kJ mol−1 (4) at M06-2X/A].
Interestingly, B3LYP computations applied at both plumbylene
dimers at their M06-2X equilibrium structures predict both
dimers to be nearly equal in energy with dimer 7 being slightly
more stable by 0.5 kJ mol−1. The results of these model
calculations suggest that it is solely the optimization of the
dispersion energy as interplay between attracting and repelling
forces between the plumbylene subunits which determines the
actual shape of the plumbylene dimer 4.
The analysis of the computational results indicates that the
dominant attracting force that holds together dimer 4 in its
inmost folds40 is van der Waals interaction, which is supported
by a comparatively weak donor−acceptor interaction. In
addition, it is the stronger dispersion energy contribution that
prefers the single donor−acceptor dimer 4 over the trans bent
dimer 7 with an double donor−acceptor interaction between
the constituent plumbylenes. At this stage of the discussion we
feel that it is appropriate to point out that in the past many
elaborate analyses of bonding in organometallic compounds,41
in particular in systems with important donor−acceptor
interactions, the contributions of the attractive dispersion
interactions between the experimentally often unavoidable large
substituents are neglected although they might be decisive. This
realization calls for a computational reinvestigation of these
systems.
Scheme 2. Canonical Structures Which Describe the
Negative Hyperconjugation in Dimer 4
Pb(2)Si(9)) is significantly widened [α(Pb(1)Pb(2)Si(9)) =
155.4°] and the Pb(2)Si(9) (276.6 pm) bond is longer than the
comparable Pb(2)Si(12) bond (272.4 pm). Consequently, also
the computed WBIs of these two bonds differ markedly [WBI
(Pb(2)Si(9)) = 0.64) and WBI (Pb(2)Si(12)) = 0.71].32
The overall binding energy BE between both plumbylene
subunits in dimer 4 is computed to be −110.8 kJ mol−1.31,33
Therefore, the PbPb linkage in 4 is considerably stronger than
the PbPb bond in the parent diplumbene Pb2H4 in its trans
bent conformation [BE(Pb2H4) = −62.4 kJ mol−1], although
on the basis of the computed WBIs (see above) a stronger
PbPb bond in Pb2H4 is expected. In addition, on the basis of
the computed singlet/triplet separations E(ST) for plumby-
lenes 5 and 6 (see above) and the nonexistence of a dimer of
plumbylene 5, the formation of dimer 4 is completely
unexpected. This suggests that it is not the conventional
donor−acceptor bonding interaction that ties both plumbylene
subunits in dimer 4 together. A qualitative assessment of the
attractive coulomb potential, EC, between the two Pb-atoms
which results from charge transfer between both plumbylene
subunits indicates that this factor is of only minor importance
as it accounts for less than 25% of the computed BE (EC =
−25.8 kJ mol−1).34 This leaves the attractive dispersion
potential between the large and polarizable substituents of
plumbylene 6 as the decisive force for an understanding of the
bonding situation. A related situation is found in the sterically
EXPERIMENTAL SECTION
■
t
overloaded disilane Bu3Si−SitBu3 which is marked by an
General Remarks. All reactions involving air-sensitive compounds
were carried out under an atmosphere of dry nitrogen or argon using
either Schlenk techniques or a glovebox. All solvents were dried using
a column based solvent purification system.42 Potassium tert-
butanolate was purchased from Merck. All other chemicals were
obtained from different suppliers and used without further purification.
1H (300 MHz), 13C (75.4 MHz), 31P (124.4 MHz), 207Pb (62.8
MHz), and 29Si (59.3 MHz) NMR spectra were recorded on a Varian
INOVA 300 spectrometer. For all samples C6D6 was used as solvent if
not stated otherwise. To compensate for the low isotopic abundance
of 29Si the INEPT pulse sequence43,44 was used for the amplification of
the signal.
extremely long Si−Si bond but shows a comparatively high
thermostability.35,36 Recently, dispersion forces were also
recognized as important factors that explain the high stability
of organometallic compounds37 and that of hydrocarbons with
extremely long alkane C−C bonds.38 The here applied M06-2X
functional properly accounts for noncovalent van der Waals
interactions, while the most prominent deficit of the popular
B3LYP functional is the nearly complete negligence of
dispersion.39 Therefore, the difference in the calculated bond
energies using these two functionals allows estimating the
contribution of noncovalent bonding in dimer 4.37,38 As
expected for the parent Pb2H4 nearly the same Pb−Pb bond
energy BE is computed using the two different functionals
[M06-2X/A//M06-2X/A: BE(Pb2H4) = −62.4 kJ mol−1;
B3LYP/A//M06-2X/A: BE(Pb2H4) = −54.4 kJ mol−1]29
indicating that attracting dispersion forces are not significant.
In sharp contrast in the case of dimer 4 dispersion forces are
decisive as the B3LYP functional predicts an even positive Pb−
Pb bond energy BE [M06-2X/A//M06-2X/A: BE(4) = −110.8
kJ mol−1; B3LYP/A//M06-2X/A: BE(4) = +1.0 kJ mol−1]. In
X-ray Structure Determination. For X-ray structure analyses the
crystals were mounted onto the tip of glass fibers, and data collection
was performed with a BRUKER-AXS SMART APEX CCD
diffractometer using graphite-monochromated Mo Kα radiation
(0.71073 Å). The data were reduced to F2 and corrected for
o
absorption effects with SAINT45 and SADABS,46,47 respectively. The
structures were solved by direct methods and refined by the full-matrix
least-squares method (SHELXL97).48 If not noted otherwise all non-
hydrogen atoms were refined with anisotropic displacement
parameters. All hydrogen atoms were located in calculated positions
to correspond to standard bond lengths and angles. All diagrams were
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drawn with 30% probability thermal ellipsoids, and all hydrogen atoms
were omitted for clarity.
Industrie (FCI) for a scholarship (No.183191) and the High-
Performance Computing centre of CvO university is thanked
for computer time.
Crystallographic data (excluding structure factors) for the structures
of compounds 2, 3, and 4 reported in this paper have been deposited
with the Cambridge Crystallographic Data Center as supplementary
publication no. CCDC-831746 (2), 831750 (3), and 831749 (4).
Copies of data can be obtained free of charge at http://www.ccdc.cam.
ac.uk/products/csd/request/.
REFERENCES
■
(1) Goldberg, D. E.; Harris, D. H.; Lappert, M. F.; Thomas, K. M. J.
Chem. Soc., Chem. Commun. 1976, 261−262.
(2) Goldberg, D. E.; Hitchcock, P. B.; Lappert, M. F.; Thomas, K. M.;
Thorne, A. J.; Fjeldberg, T.; Haaland, A.; Schilling, B. E. R. J. Chem.
Soc., Dalton Trans. 1986, 2387−2394.
1,1,1,4,4,4-Hexakis(trimethylsilyl)tetramethyltetrasilane11 and B-
49
(C6F5)3 were prepared according to literature procedures.
Plumbylene Phosphine Adduct 2. After stirring a solution of
1,1,1,4,4,4-hexakis(trimethylsilyl)tetramethyltetrasilane (612 mg, 1.0
mmol) and KOtBu (236 mg, 2.1 mmol) in THF (5 mL) for 18 h at 60
°C the solution was cooled to rt and added dropwise to a stirred
suspension of PbBr2 (367 mg, 1.0 mmol) and PEt3 (120 mg, 1.0
mmol) in THF (5 mL). On addition a color change from green to red
appeared, and the resulting red suspension was stirred for 2 h. All
volatiles were removed under reduced pressure, and the residue was
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def2TZVP(Pb)//M06-2X/A level of theory; see Supporting Informa-
tion for further details.
ASSOCIATED CONTENT
* Supporting Information
■
S
Details for the calculated structures of compounds 2−7, as well
as X-ray crystallographic information for compounds 2, 3, and 4
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
baumgartner@tugraz.at;
■
christoph.marschner@tugraz.at;
thomas.mueller@uni-oldenburg.de.
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Notes
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̈
The authors declare no competing financial interest.
F.; Marsmann, H. Organometallics 1998, 17, 4425−4428.
(27) Sturmann, M.; Saak, W.; Marsmann, H.; Weidenbruch, M.
̈
ACKNOWLEDGMENTS
This work is dedicated to Prof. Akira Sekiguchi on the occasion
of his 60th birthday. Support of the study was provided by the
■
Angew. Chem., Int. Ed. 1999, 38, 187−189.
(28) The Gaussian 09 program was used. Gaussian 09 Revision B. 01;
Gaussian, Inc.: Wallingford, 2010.
Austrian Fonds zur Forderung der Wissenschaften (FWF) via the
project P-21346 (J.B.). P. Zark thanks the Fonds der Chemischen
̈
(29) For detailed description of the computations, see the Supporting
Information.
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in the Supporting Information (Figure S8). Therefore, the here
reported BE for dimer 4 is in agreement with detection of the
monomer 6 in toluene solution.
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