A Monoaryllead Trichloride That Resists Reductive Elimination

Article abstract of DOI:10.1002/anie.201712944

Transmetallation of Pb(OAc)₄ with R₂Hg (1), followed by treatment with HCl in Et₂O, provided RPbCl₃ (2), the first kinetically stabilized monoorganolead trihalide that resists reductive elimination under ambient

Full text of DOI:10.1002/anie.201712944

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Accepted Article
Title: Monoaryllead Trichloride Defying Reductive Elimination
Authors: Marian Olaru, Ralf Kather, Emanuel Hupf, Enno Lork, Stefan
Mebs, and Jens Beckmann
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Monoaryllead Trichloride Defying Reductive Elimination
[
a]
[a]
[a]
[a]
[b]
[a]
Marian Olaru, Ralf Kather, Emanuel Hupf, Enno Lork, Stefan Mebs,* Jens Beckmann*
Abstract: Transmetallation of Pb(OAc)
4
with R
2
Hg (1), followed by
(already lowered in energy due to relativistic effects) making it
less available for bonding.
[
13]
treatment with HCl in Et O, provided RPbCl
2
3
(2), the first kinetically-
stabilized monoorganolead trihalide that defies reductive elimination
under ambient conditions. The kinetic stabilisation relies on an
[
7]
Starting from Pb(OAc)
4
,
we were now able to isolate the
(2) and utilize it as a
building block for the preparation of three unsymmetrically
substituted diaryllead dichlorides RR'PbCl (3a, R' = Ph; 3b, R' =
-MeOC ; 3c, R' = 4-Me NC ; R = 6-Ph P(O)-Ace-5-), using
diaryl mercurials, Hg (1) and R' Hg, as non-reductive
transmetallation reagents (Scheme 1).
first monoaryllead trichloride, RPbCl
3
2
intramolecularly coordinating O-donor substituent (R = 6-Ph P(O)-
Ace-5-). The gram scale preparation of 2 was the key for the
2
synthesis of unsymmetrically substituted diaryllead dichlorides
4
6
H
4
2
6
H
4
2
2 6 4 2 6 4
RR'PbCl (3a, R' = Ph; 3b, R' = 4-MeOC H ; 3c, R' = 4-Me NC H ).
R
2
2
The organometallic chemistry of lead is dominated by the
[
1]
oxidation state IV. Tetraethyllead, Et
4
Pb, is a prominent, air-
stable example that has been used as anti-knock additive in the
[
2]
petrol industry for almost a century. Plumbylenes R
2
Pb (R =
alkyl, aryl), on the other hand, are highly reactive congeners of
[
3]
carbenes and need to be stabilized by bulky substituents or
[
4]
intramolecularly coordinating ligands. In contrast, the inorganic
chemistry of lead is dominated by the oxidation state II, whereas
inorganic lead(IV) species are rare. Lead dichloride, PbCl
2
, is
commercially available, but lead tetrachloride, PbCl
4
,
[
5]
decomposes violently above 0 °C. Lead dihydroxide, Pb(OH)
exists in aqueous solution, but plumbic acid Pb(OH)
2
Scheme 1. Synthesis of monoaryllead trichloride 2 and unsymmetrically
substituted diaryllead dichlorides 3a–3c.
4
was only
detected in a solid argon matrix. Lead tetraacetate, Pb(OAc)
[
6]
4
,
[
7]
is reasonably stable, but a strong oxidizer. Lead tetrafluoride,
[
8]
PbF
4
,
the lead oxides PbO
2
9]
, Pb
2
O
3
, Pb
are stable, but exhibit extended
D-solid-state structures. More or less discrete lead(IV) anions,
3 4
O and the related
The monoaryllead trichloride 2 was obtained by gram scale
reactions in up to 83% yield as bright yellow to orange crystalline
solid that can be stored unchanged at room temperature for
[
oxoplumbates, e.g. Na
4 4
PbO ,
3
2−
[10]
such as [PbX
6
]
(X = F, Cl, IO
6
, N
3
, OH), are more stable than
[14]
several weeks. The diaryllead dichlorides 3a–3c were isolated
their neutral parents, e.g. PbCl
confirmed by the recent isolation of the [PbSe
is isoelectronic with the elusive PbBr
observations, the empirically found relative stability of
organolead halides decreases in the order R Pb > R PbX >
PbX >> RPbX (R = alkyl, aryl; X = halides). Particularly
noteworthy is the instability of all monoorganolead trihalides,
4
and Pb(OH)
4
–
. This trend is
anion, which
in 48–86% yield and obtained as indefinitely stable colourless
4
[11]
4
]
[15]
and yellow crystalline solids.
4
Unlike PbCl that possesses
4
. Consistent with these
[16]
tetrahedrally coordinated Pb atoms, the molecular structures
of 2 and 3c·DMSO show that the spatial arrangement around
[17]
4
3
the Pb atoms is distorted trigonal bipyramidal
octahedral, respectively, and defined by CCl
and distorted
Cl
[
1]
R
2
2
3
3
O and C
2
2 2
O
donor sets (Figure 1). The short Pb–O distances of 2 (2.314(2)
[
12]
RPbX
3
, even well below 0 °C.
These experimental findings
Å) and 3c (2.374(3), 2.481(4) Å) compare well with that of
have been confirmed by computational studies, which trace the
relative instability of lead(IV) to charge separation and relativistic
[18]
3
polymeric Ph PbOH (2.37(2) Å)
and are indicative for
significant intra- and intermolecular Pb···O donor-acceptor
interactions. As a result, the P–O bond lengths of 2 (1.501(6) Å)
and 3c (1.507(2) Å) are slightly elongated compared to that of 6-
[
13]
effects other than the superseded "inert-pair effect". Thus, the
increasing number of halides X within the series R PbX4–n (n =
–4) is associated with a build-up of positive charge on the Pb
atom, which results in greater contraction of the 6s orbital
n
1
Ph
acceptor interactions. For the same reason the axial Pb–Cl bond
2.535(1) Å) of 2 is slightly longer than the equatorial Pb–Cl
2
P(O)-Ace-5-Cl (1.489(2) Å) lacking intramolecular donor-
(
bonds (2.411(1) Å, 2.424(1) Å).
[
a]
Dr. M. Olaru, R. Kather, Dr. E. Hupf, Dr. E. Lork, Prof. Dr. J.
Beckmann
Institut für Anorganische Chemie und Kristallographie, Universität
Bremen
Leobener Straße 7, 28359 Bremen, Germany
E-mail: j.beckmann@uni-bremen.de
Dr. S. Mebs
Institut für Experimentalphysik, Freie Universität Berlin
Arnimallee 14, 14195 Berlin, Germany
E-mail: stebs@chemie.fu-berlin.de
Supporting information including experimental details, NMR spectra,
crystallographic data, and details regarding DFT calculations can be
found under https://doi.org/10.1002/anie.2016XXXXX.
In solution, 2 is reasonably stable at ambient conditions on
the laboratory time scale, however, at higher temperatures it is
5
4
12
thermally unstable and, like PbCl
and PhPbCl
Energy calculations show
that the reductive elimination reactions are only moderately
3
,
it also
[
19]
undergoes reductive elimination.
[
b]
b]
–1
–1
exothermic for PhPbCl
3
(–81.9 KJ·mol ) and 2 (–34.5 KJ·mol )
–1
and even endothermic for PbCl
4
(92.8 KJ·mol ) in the gas
However, the liberation of lattice energy (–2282
[
20]
[
phase.
–
1 [21]
KJ·mol )
associated with the crystallization of PbCl
2
outweighs these values by orders of magnitude and proves to
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be the thermodynamic driving force that compromises the
thermal stability.
energy density over ED ratio (G/ρ(r) = 0.98 a.u.) and a total
energy density over ED ratio close to zero (H/ρ(r) = −0.05 a.u.).
[
22]
Figure 1. Molecular structure of 2 (left) and 3c·DMSO (right); thermal
ellipsoids are set at 30% probability. Selected bond lengths [Å] and angles [°]:
for 2: C10–Pb1 2.202(2); O1–Pb1 2.314(2); Cl1–Pb1 2.5351(7); Cl2–Pb1
2.4244(7); Cl3–Pb1 2.4108(7); O1–P1 1.507(2); C10–Pb1–O1 89.14(8); C10–
Pb1–Cl3 136.87(7); O1–Pb1–Cl3 81.89(5); C10–Pb1–Cl2 120.76(7); O1–Pb1–
Cl2 87.08(5); Cl3–Pb1–Cl2 100.91(3); C10–Pb1–Cl1 97.98(7); O1–Pb1–Cl1
172.17(5); Cl3–Pb1–Cl1 90.64(3); Cl2–Pb1–Cl1 92.05(3); for 3c: Pb1–O1
2.374(3); Pb1–C40 2.164(5); Pb1–C10 2.187(4); Pb1–O2 2.481(4); Pb1–Cl2
2.610(1); Pb1–Cl1 2.658(1); P1–O1 1.505(3);C40–Pb1–C10 169.78(18); C40–
Pb1–O1 82.96(15); C10–Pb1–O1 87.99(14); C40–Pb1–O2 87.40(17); C10–
Pb1–O2 85.80(15); O1–Pb1–O2 75.99(13); C40–Pb1–Cl2 93.95(15); C10–
Pb1–Cl2 92.84(13); O1–Pb1–Cl2 103.98(9); O2–Pb1–Cl2 178.63(10); C40–
Pb1–Cl1 93.94(14);
Figure 2. a) AIM bond topology, b) NCI iso-surface (s = 0.5), c) ELI-D iso-
surface (Y = 1.3) and d) ELI-D lone-pair basin of the oxygen atom of 2.
The longer life span of 2 at ambient conditions, compared to
Notably, AIM atomic charges show comparable values for 2 and
PhPbCl
3
, suggests that the intramolecular Pb···O donor-acceptor
3
PhPbCl with Q(Pb) = 1.5 to 1.6 e, Q(Cl) ≈ −0.52 e, and Q(R) = 0
interaction provides significant kinetic stabilisation, which raises
the question regarding the nature of this bond. For a first
approximation of the bond energy, the -peri-interaction energy
to −0.1 e, which suggests that charge distribution differences
play a minor role for the kinetic stabilization. The predominately
ionic Pb···O bond character is further supported by the blue-
coloured disc-shaped NCI basin between the O and Pb atoms,
which is characteristic for basically non-covalent interactions
(Figure 2b). The localization domain representation of the ELI-D
(Figure 2c) shows that the O atoms lone pairs are closely
localized around the O atom, so that electron sharing between
the Pb atom and the O atom may be neglected (Figure 2c). This
is also nicely visible by mapping the ELI-D distribution on the
oxygen lone-pairs basin surfaces (Figure 2d). Here, the basin
surface was found to be in direction of the Pb atom and no
enhanced electron localizability is observed on the surface,
which also suggests a weak coordinative Pb···O contact. The
axial Pb–Cl interaction is characterized by a lower ED and less
23
(
-PIE) was calculated using an isodesmic reaction. The -PIE
–1
of 2 (–2.4 KJ mol ) is only very small; significantly smaller than
that of RTeCl
the PbCl moiety is much weaker than that of the TeCl
–1
3
(–36.0 KJ mol ). Apparently, the Lewis acidity of
moiety
3
3
or alternatively, the energy gain associated with the attractive
donor acceptor interaction is almost entirely compensated by
repulsion between the substituents. For a more detailed picture,
the bond situation of 2 was analyzed using a complementary set
of real-space bonding indicators (RSBIs). The RSBI analysis
comprises topological dissection of the electron density (ED) into
[24]
atomic basins according to the Atoms-In-Molecules (AIM)
space-partitioning scheme and dissection of the electron pair-
density according to the Electron Localization Indicator (ELI-
−
3
negative H/ρ(r) value at the axial Pb–Cl bcp (0.48 eÅ , −0.27
[
25]
−3
D) space-partitioning scheme. Non-covalent bonding aspects
are evaluated according to the recently introduced Non-Covalent
a.u.) than the equatorial Pb–Cl interaction (0.55 eÅ , −0.30 a.u.)
and by a more prominent NCI basin for the axial Pb–Cl bond
(Figure 2b).
[
26]
Interactions (NCI) index
by determination of the reduced
2
1/3
4/3
density gradient, s(r) = [1/2(3π ) ]|ρ|/ρ , which shows a
In conclusion, the first kinetically-stabilized monoaryllead
[
25]
complementary spatial distribution than ELI-D
spatial separation of covalent and non-covalent bonding aspects
Figure 2). Analysis of the Pb···O bond critical point in the
computed electron density (ED) suggests a weak interaction
Figure 2a), the ionic character of which is unravelled by its
suggesting
trichloride, RPbCl
ambient conditions, was prepared (R = 6-Ph
kinetic stabilization is provided by predominately ionic,
intramolecular Pb···O donor acceptor interaction that increases
3
(2), defying reductive elimination under
2
P(O)-Ace-5-). The
(
a
(
[
16]
the coordination number, compared to PbCl
4
,
from 4 to 5. The
−
3
topological properties, such as a low ED value of 0.34 eÅ , a
positive Laplacian of the ED (4.6 eÅ ), a strongly positive kinetic
preparation of 2 on a gram scale was instrumental for the
synthesis of unsymmetrically substituted diaryllead dichlorides
−
5
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Angewandte Chemie International Edition
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COMMUNICATION
RR'PbCl (3a, R' = Ph; 3b, R' = 4-MeOC
2
6
H
4
; 3c, R' = 4-
Frenking and S. Shaik), Wiley-VCH Verlag GmbH
&
Co. KGaA,
Cl
2
Weinheim, 2014, pp 1–24.
Me NC H ). We currently use 3a–3c as starting materials to
2
6 4
[
14] The reaction of Pb(OAc)
4
with R
2
Hg (1) at room temperature in CH
2
explore lead centred chirality.
2
07
provided an equimolar mixture of RPb(OAc)
3
(( Pb) = −1239 ppm,
3
1
207
31
199

−
( P) = 50.3 ppm, J( Pb– P) = 74 Hz) and RHgOAc (( Hg) =
31
1343 ppm, ( P) = 41 ppm), which could not be separated due to
Acknowledgements
similar solubility in common solvents. Treatment of this mixture in
2
07
2
toluene with HCl in Et O (2 M) caused the precipitation of 2 (( Pb) =
207 31
3
1
−
723 ppm, ( P) = 47.3 ppm, J( Pb– P) = 70 Hz). Solid 2 melts with
The Deutsche Forschungsgemeinschaft (DFG) is gratefully
acknowledged for financial support. We thank Sarah Gansemer
for performing the DSC analysis.
decomposition between 139 and 143 °C. In this temperature range, the
–1
DSC trace shows an exothermic event that accounts for −77 KJ mol
.
See ESI for details.
[
15] Transmetallation of
2
2 with R' Hg quantitatively afforded equimolar
2
07
31
Keywords: lead • transmetallation • reductive elimination •
mixtures RR'PbCl (3a, R = Ph, ( Pb) = −676.6 ppm, ( P) = 43.3
2
207
31
ppm; 3b, R' = 4-MeOC
6
H
4
, ( Pb) = −362.6 ppm, ( P) = 45.7 ppm,
kinetic stabilization • bond analysis
2
207
31
207
2 6 4
J( Pb– P) = 11 Hz; 3c, R' = 4-Me NC H , ( Pb) = −631.7 ppm,
3
1

( P) = 42.4 ppm) and R'HgCl. Similar solubility of 3b and 3c, and
[
1]
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details.
[
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4
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1
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Angewandte Chemie International Edition
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COMMUNICATION
COMMUNICATION
The kinetic stabilization provided by
an intramolecular Pb···O donor
acceptor interaction allowed the
isolation of the first monoaryllead
Marian Olaru, Ralf Kather, Emanuel
Hupf, Enno Lork, Stefan Mebs,* Jens
Beckmann*
trichloride, RPbCl
3
(R = 6-Ph
2
P(O)-
Page No. – Page No.
Ace-5-) that was used for the
synthesis of unsymmetrically
Monoaryllead Trichloride Defying
Reductive Elimination
substituted diaryllead dichlorides
RR'PbCl (R' = Ph, 4-MeOC , 4-
Me NC ).
2
6 4
H
2
6 4
H
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