Visiting the Limits between a Highly Strained 1-Zirconacyclobuta-2,3-diene and Chemically Robust Dizirconacyclooctatetraene

Article abstract of DOI:10.1002/chem.201800465

The reaction of the allene precursor Li₂(Me₃SiC₃SiMe₃) with [Cp₂ZrCl₂] (Cp=cyclopentadienyl) was examined. The selective formation of hitherto unknown linear, allene-bridged dizirconocene complexes [(Cp₂ZrCl)₂{?μ-(Me₃Si)C₃(SiMe₃)?}] and [(Cp₂Zr)₂{?μ-(Me₃Si)C₃(SiMe₃)?}₂] was observed. Upon σ coordination of the allenediyl unit to {Cp₂Zr}, pyrophoric Li₂(Me₃SiC₃SiMe₃) is tamed stepwise to yield a surprisingly robust 1,5-dizirconacyclooctatetra-2,3,6,7-ene with cumulated double bonds. This complex is unexpectedly inert against moisture, air, water and acetone. Surprisingly, it degrades under MS conditions to give the highly strained 1-zirconacyclobuta-2,3-diene. All compounds isolated have been fully characterised and the molecular structures are discussed. The stability and reactivity of these complexes are rationalised by DFT computations.

Full text of DOI:10.1002/chem.201800465

A Journal of
Accepted Article
Title: Visiting the limits between a highly strained 1-
zirconacyclobuta-2,3-diene and chemically robust
dizirconacyclooctatetraene
Authors: Fabian Reiß, Melanie Reiß, Anke Spannenberg, Haijun Jiao,
Wolfgang Baumann, Perdita Arndt, Uwe Rosenthal, and
Torsten Beweries
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Chemistry - A European Journal
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Visiting the limits between a highly strained 1-zirconacyclobuta-
,3-diene and chemically robust dizirconacyclooctatetraene
2
[
a]
[a]
[a]
[a]
[a]
Fabian Reiß,* Melanie Reiß, Anke Spannenberg, Haijun Jiao, Wolfgang Baumann, Perdita
[
a]
[a]
[a]
Arndt, Uwe Rosenthal, and Torsten Beweries*
Dedicated to the memory of Prof. Dr. Dr. h. c. mult. Günther Wilke in recognition of his contributions to the development of LIKAT.
ABSTRACT: The reaction of the allene precursor Li
with Cp ZrCl was examined. We observed selective formation of
hitherto unknown linear allene bridged dizirconocene complexes
(Cp ZrCl) {-μ-(Me Si)C (SiMe )-}] and [(Cp Zr) {-μ-
Me Si)C (SiMe )-} ]. Upon  coordination of the allenediyl unit to
Cp Zr] pyrophoric Li (Me SiC SiMe ) is tamed stepwise, yielding a
2
(Me
3
SiC
3
SiMe
3
)
1a) containing cumulated double bonds is not reported to date.
Computational studies on biradicaloids (B, E = CH , NH, O and
2
2
2
[7]
[8]
S; R = H) and beryllium analogues (B, E = Be; R = H) shed
light on the theoretical stability and the singlet-triplet states of
this class of compounds. In the synthesis of 1,3,4,6-tetrakis(t-
butylthio)thieno[3,4-c]thiophene a dithiocyclooctatetraene (B, E
= S, R = t-Bu-S) was mentioned as an intermediate which shows
[
2
2
3
3
3
2
2
(
3
3
3
2
[
2
2
3
3
3
surprisingly robust 1,5-dizirconacyclooctatetra-2,3,6,7-ene with
cumulated double bonds. This complex is unexpectedly inert against
moisture, air, water and acetone. Surprisingly it degrades under MS
conditions to give the highly strained 1-zirconacyclobuta-2,3-diene.
All isolated compounds were fully characterised and the molecular
structures are discussed. The stability and reactivity of these
complexes are rationalised by DFT computations.
[
9]
a C-C coupling of the central allene carbon atoms. To the best
of our knowledge, the disilacyclooctatetraene (structure of type
B, E = SiMe
1993, using in situ generated Li
Me SiCl , is the only example of this class of compounds that
has been fully characterised and structurally investigated to
2
, R = SiMe
3
), which was described by Barton in
2
(Me SiC SiMe (1) and
3
3
3
)
2
2
[
10]
date.
Introduction
As early as 1911 Willstätter isolated cyclooctatetra-1,3,5,7-ene
(
COT, A, Figure 1a) for the first time in a multi-step synthesis
1
starting from pseudopelletierine which was later confirmed by
Cope’s repetition of the original protocol.
[
2 ]
Later, Reppe
Figure 1. a) Willstätter’s COT (A), COT containing cumulated double bonds
B). b) Schematic representation of selected exotic stable metallacycles of
group 4 metals.
developed a nickel catalysed process for the synthesis of COT
(
[
3]
from acetylene. COT is unstable and easily forms explosive
organic peroxides. However, stabilization of this compound is
possible by coordination to transition metal centres to yield COT
complexes. With respect to the four C-C double bonds it shows
m n
In analogy to  complexation in Wilke’s M [COT] compounds 
coordination of unsaturated substrates was also found to result
in the formation and stabilization of unusual cyclic structures,
which can formally be regarded as the products of isolobal ring-
2
4
6
8
a broad chemistry with  ,  ,  and  -type coordination
[
4]
modes and can be regarded as classical olefin ligand. COT
itself is non-aromatic species in contrast to the
cyclooctatetraenide [C
system and forms sandwich like complexes analogous to
a
2
substitution of a CH group in hypothetical highly ring-strained
2

[
11]
8
H
8
]
dianion, which is a classical Hückel
compounds by a group 4 metal fragment. In this regard, Erker
and Suzuki as well as our group have reported on the
stabilization of a variety of exotic structural motifs such as 1-
metallacyclopent-3-ynes, 1-metallacyclopenta-2,3,4-trienes, and
[
5]
cyclopentadienyl ligands. Seminal work by Wilke established
this chemistry as he described a series of M [COT] derivatives
using disodium cyclooctatetraenide Na [C ] and metal halides
as starting materials (e.g. M[COT]: M = V, Ni, Co; M[COT] : M =
m
n
[
12 ]
2
8
H
8
1-metallacyclopenta-3,4-dienes (Figure 1b).
Very recently,
2
Beckhaus reported on the stabilization of a hexapentaene unit
[6]
[
13]
Ni, Ti, Zr; M
In contrast to the well examined and versatile chemistry of A, its
derivative cyclooctatetra-1,2,5,6-ene (B, E = CH ; R = H, Figure
2 3
[COT] : M = Cr, Mo, W).
as a binuclear zig-zag titanium complex.
As a part of our long-standing interest in the realization of
unusual metallacycles of Ti, Zr, and Hf we have very recently
attempted to access a hitherto unknown four-membered 1-
2
[
14 ]
titanacyclobutadi-2,3-ene (C, Scheme 1)
theoretical studies of these structures.
following up on
[
a]
Dr. F. Reiß, Dr. M. Reiß, Dr. A. Spannenberg, Dr. H. Jiao, PD Dr. W.
[15]
Baumann, Dr. P. Arndt, Prof. Dr. U. Rosenthal, PD Dr. T. Beweries
Leibniz-Institut für Katalyse e.V. an der Universität Rostock (LIKAT)
Albert-Einstein-Str. 29a, 18059 Rostock, Germany
E-mail: fabian.reiss@catalysis.de; torsten.beweries@catalysis.de
Supporting information for this article is given via a link at the end of
the document.
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The calculations clearly reveal the combination of the
zirconocenes and t-butyl substituents at the allene unit as
inappropriate due to the endergonic Gibbs Free Energies for the
considered reactions (Table 1). With a closer look to the
2
computed values, Cp
2
Zr( -Me
3
SiC
3
SiMe
3
) appeared as the
1-
metallacyclobuta-2,3-diene structure with the Gibbs Free Energy
most promising candidate for the formation of
a
2 2 3 2
Scheme 1. Possible products of a reaction of Li R C and [M]Cl .
-1
for its formation of -8.97 kcal·mol being exergonic but
challenging and compared to the corresponding 1-
titanacyclobuta-2,3-diene (-11.8 kcal·mol ) this value is slightly
-1
In this context, as all previous attempts to access structures of
type C failed, we investigated alternative allene precursors. In
this contribution, we present the synthesis of a dilithioallene
[
14b]
lower.
precursor and its reactions with Cp
reaction pathways to 1-metallacyclobutadi-2,3-ene (C) or
dimetallacyclooctatetraene (D) (Scheme 1).
2
ZrCl
2
to evaluate possible
Synthesis of the dilithioallene precursor Li
2
(Me
as well as structures of
have been well studied and
3 3 3
SiC SiMe ).
[
17 ]
Although propargylic metalation
[
18 ]
organolithium compounds
understood for decades, no structural investigation of a suitable
dilithioallene precursor is known to date. Based on our
experience with trimethylsilyl substituted substrates, supported
by the aforementioned theoretical studies of the stability of
Results and Discussion
[14b]
corresponding derivatives of C,
we decided to focus on the
) (1). In two previous studies, the
was performed by deprotonation of 1,3-
Motivation. To get a deeper insight into the chemistry of these
synthesis of Li
synthesis of
2 3 3 3
(Me SiC SiMe
1
zirconacyclobuta-2,3-dienes (C) we considered
isodesmic equation between the Cp’ ZrMe complex (Cp’= Cp =
-cyclopentadienyl, Cp’= Cp*=  -pentamethylcyclopentadienyl)
and the allene precursors Me(Me Si)C=C=C(SiMe )Me as well
a
simple
2
2
[
19 ]
5
5
bis(trimethylsilyl)prop-1-yne in Et
2
O and THF with n-BuLi

(
Scheme 3). Compound 1 was not isolated, however, its
3
3
presence was confirmed by addition of chlorosilanes and
subsequent analysis of the residues obtained. From analogous
reactions we have attempted to isolate 1 as a solid material, but
only colourless to yellowish oils were obtained. For this reason,
we modified the procedure and tested deprotonation of 1,3-
as Me(t-Bu)C=C=C(t-Bu)Me for comparison (Scheme 2).
[20]
bis(trimethylsilyl)prop-1-yne using Lochmann-Schlosser Base
but were only able to isolate the potassium salt
K(THF) (Me SiC(H)C SiMe )] which forms a linear coordination
,
[
2
3
2
3
[
21]
polymer in the solid state as confirmed by X-ray analysis.
West et al. reported the synthesis of Li in n-hexane with n-
we thus revisited the lithiation of 1,3-bis(trimethylsilyl)-
Scheme 2. Isodesmic equation to zirconacyclobuta-2,3-dienes.
4
C
3
[
22]
BuLi;
prop-1-yne with n-BuLi in aprotic, non-polar solvents toluene, n-
hexane, and benzene. Eventually, benzene was the solvent of
choice; after a reaction time of one week we obtained 1 in good
yields (62 %) as an analytically pure white pyrophoric solid
which decomposes at 239 °C under Ar atmosphere (Scheme 3).
Furthermore
we
considered
the
corresponding
metallacyclopenta-2,3,4-trienes for the analogue isodesmic
equation (Table 1). The metallacyclopenta-2,3,4-trienes
2
[16a]
2
[16b]
Cp
2
Zr( -t-BuC
4
t- Bu)
and Cp*
2
Zr( -Me
3
SiC
4
SiMe
3
)
are
experimentally accessible, nicely expressed by their exergonic
Gibbs Free Energy of -18.94 kcal·mol and -14.55 kcal·mol ,
therefore we chose theses as benchmark values.
-
1
-1
-
1
Table 1. Calculated Gibbs Free Energies (kcal·mol ) for the
syntheses of 1-zirconacyclobuta-2,3-dienes and
zirconacyclopenta-2,3,4-trienes.
Scheme 3. Optimised synthesis of 1 in benzene.
precursor
Cp
2
ZrMe
2
2 2
Cp* ZrMe
Me(Me
3
Si)C
Si)C
3
(SiMe
3
)Me
)Me
-8.97
-22.06
+0.74
-18.94
+3.87
-14.55
+22.24
-3.55
The NMR characterization of
1 was done at ambient
Me(Me
3
4
(SiMe
3
temperature in [D ]benzene solution (data see below). Whereas
6
the tetrameric aggregate completely lacks symmetry in the
crystal (eight inequivalent lithium sites and SiMe
Me(t-Bu)C
Me(t-Bu)C
3
(t-Bu)Me
(t-Bu)Me
3
groups, Figure
2), higher symmetry is found in solution. There are two silyl
4
groups and (at least) two lithium sites. This does not necessarily
imply dissociation of the aggregates but indicates a dynamic
13
29
behaviour which is also obvious from the C and Si NMR
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spectra (broad resonances for the methyl groups and the
metalised quart. carbon atom, lack of spin-spin coupling fine
structure, Figure S20). The strongest broadening is found for the
metalised allene carbon atoms whose signals hardly show up in
the 1D spectra but were identified by indirect detection (Figure
S19) with remarkable chemical shifts of δ = 44.6 and 37.8 ppm
Both values clearly indicate double bond character (cf. rcov C=C
[
25]
1.34; C≡C 1.20 Å)
Schleyer for a dilithiobutatriene dimer structure {(Li•THF)
BuC t-Bu)} (cf. C=C=C=C: 1.321(5), 1.292(5) and 1.336(5)
For 1benz angles between the planes defined by
Si,C,C/C,C,Si of the SiC Si units indicate roughly orthogonality
and correspond well to data reported by
2
(t-
4
2
[
26 ]
Å).
3
2
at much lower frequency than expected for metalised sp carbon
of the cumulated double bonds (64.7(2)-72.7(2)°, Table S4).
Notably, in 1thf one of the two allenediyl ligands in the
asymmetric unit deviates significantly from this behaviour with a
surprisingly small value (26.0(4)° vs. 83.7(2)°).
We conceived 1 as an ideal stoichiometric synthon for inorganic
as well as organic reactions, were lithium halide elimination at
atoms ( cf. phenyllithium, (CLi) 176 to 198 ppm depending on
conditions: data compiled in ref. 23 ). Although we did not
attempt to investigate the dynamics, we find it remarkable that it
does not lead to a complete signal averaging (for the monomeric
entity one would expect only one set of signals). Usually,
organolithium compounds in solution undergo fast inter- and
intra-aggregate exchange processes that require NMR
2
compounds like R-EX (R = substituent; E = element e.g. B, P, S,
Sn; X = halogen) can be used as driving force leading to a series
of interesting, new allenediyl bearing organoelement and/or
organometallic compounds.
[
24]
investigations to be done at very low temperatures if structural
features such as symmetry, coordination behaviour and degree
of association are to be determined. This aggregation behaviour
was however not further studied. The signal for the central
carbon atom is found downfield at 174.1 ppm. Furthermore we
Reactions of the dilithioallene precursor Li (Me SiC SiMe )
2
3
3
3
with Cp ZrCl . With compound 1 in hand we evaluated its
2
2
-1
detected a weak antisymmetric C=C=C vibration at 1751 cm in
the Raman spectrum which is in agreement with the calculated
reaction with Cp ZrCl2 using different stoichiometric ratios of
2
dilithioallene and zirconocene precursor. In a first experiment
-1
frequency (ṽcalc./uncor. = 1802 cm ).
with 1:1 stoichiometry in benzene we observed formation of
some by-products but good turnover into a main component
1
The molecular structure of 1 is shown in Figure 2 and reveals
the connectivity in this allene precursor. It should be mentioned
that 1 dissolves moderately in benzene and very well in THF, the
molecular structures obtained after crystallization from these
solvents differ considerably, but reveal in both cases the
which shows two resonances in the H NMR spectrum in 18:10
ratio at 0.39 and 6.04 ppm. This corresponds exactly to the
expected ratio for a hypothetic 1-zirconacyclobutadi-2,3-ene (C)
or a dizircona-cyclooctatetraene (D) (Scheme 1). With the help
1
3
of C NMR analysis, which shows four resonances, we could
[
21]
formation of a tetramer in the solid state. In THF solvate (1thf), exclude the presence of a compound of type C since the
the peripheral lithium atoms are saturated by THF coordination.
In benzene solvate (1benz) there is no direct interaction with the
solvent but we detected short contacts between Li atoms and
resonance for the central carbon atom was found at 171.4 ppm
and thus only slightly deviates from the value of linear 1. In
comparison, the only reported examples of 1-metallacyclobutadi-
1
3
SiCH
3
groups of neighboring tetramers (cf. Li4-C23A 2.450(3)
2,3-enes show C resonances of the central carbon atom that
and Li7-C32B 2.438(4) Å). The benzene solvate crystallises in
the triclinic space group P and thus we found four slightly
are shifted to much lower field (cf. 257 ppm in (i-
[27]
PrO)
2
28 ]
(pyridine)
2
Mo(t-BuC
3
t-Bu), 220 ppm in Cp(Cl)W(t-BuC
3
t-
[
different C
3
units in the tetramer which forms the asymmetric unit. Bu)
and 205 ppm in (Ph
3
SiO) (phen)Mo(p-MeOPhC -Ph-p-
2
3
[
29]
In each C=C=C group two different C-C bonds are found, one
shorter (e.g. C2-C3 1.304(2) Å) and one slightly longer (e.g. C1-
C2 1.323(2) Å).
OMe) ).
Since it was difficult to separate the by-products and obtain
suitable crystals for X-ray diffraction analysis of the assumed
2 2
product, we then performed the reaction of 1 with Cp ZrCl in 1:2
stoichiometry. After reaction in benzene at ambient temperature
clean conversion into a single main product was observed
(
Scheme 4).
Scheme 4. Optimised synthesis of 2.
The bright orange solid of 2 is highly air and moisture sensitive,
decolourises upon decomposition and degrades at 161 °C under
1
Figure 2. Molecular structure of compound 1benz crystallised from benzene.
Thermal ellipsoids correspond to 30% probability. Hydrogen atoms as well as
co-crystallised benzene were omitted for clarity.
Ar atmosphere. The H NMR spectrum of 2 in [D
6
]benzene
Si
shows three resonances in 18:10:10 ratio, one for the Me
3
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+
+
group (0.37 ppm) and two signals for inequivalent Cp ligands
Notably, the MS-CI spectrum of 3 shows the [M+H] at m/z 807
as well as an intense peak at m/z 403 which suggests the
presence of the corresponding 1-metallacyclo-butadi-2,3-ene (C)
1
3
(
6.08, 6.16 ppm). C NMR analysis reveals a signal for the
central allene carbon atom at 174.7 ppm which is in the same
range as for the linear dilithioallene 1. In addition we detected
the antisymmetric C=C=C vibration as strong resonance at 1776
2
+
or a dicationic dimetallacyclooctatetraene [3] . ESI-TOF/HRMS
analysis unambiguously confirmed the presence of 1-
zirconacyclo-butadi-2,3-ene which formed under MS conditions
a
-
1
-1
cm in the IR spectrum and as weak signal at 1781 cm in the
Raman spectrum which is in agreement with the calculated
+
(m/z 403 [M+H] ) (Figure 4).
-
1
frequency (ṽcalc./uncor. = 1806 cm ). On the basis of these data in
combination with the molecular mass of m/z 697 we assume 2
as an allenediyl bridged dinuclear zirconocene monochloride
complex. This is confirmed by X-ray analysis (Figure 3). It
3
should be noted that similar compounds with bent C units were
reported, e.g. Jensen and Messerle described on the formation
of a dinuclear Ta allenediyl complex via double C-H activation of
[
30]
an allene ligand.
Figure 4. HRMS spectra for the peaks at m/z 403 of the 1-zirconacyclo-butadi-
2,3-ene (top) and for the peak at m/z 807 of compound 3 (bottom).
Figure 3. Molecular structure of compound 2. Thermal ellipsoids correspond
to 30% probability. Hydrogen atoms and the second molecule of the
asymmetric unit were omitted for clarity.
The low Gibbs free energy of a dimerization process from two
molecules of type C to give 3 supports this observation (-2.4
-1
kcal·mol ). However, attempts to sublime a compound of this
[
21]
type from 3 in vacuo at 207 °C failed. Nevertheless, this, and
the stability towards the eluent (MeOH/0.1% HCOOH in H
0:10) during the ESI-TOF/HRMS experiment demonstrates the
We used this unusual complex 2 as synthon for the formation of
the dizirconacyclooctatetraene which was postulated after initial
experiments described above. After optimization of the reaction
procedure we observed full conversion in a cooled diethyl ether
solution (30 °C) that can be followed by a slow colour change
from bright orange to deep red within a reaction time of one day
2
O
9
impressive stability of 3 (Tdec. = 315 °C under Ar). The molecular
structure of 3 confirms the unusual dimetallacyclooctatetraene
motif (Figure 5).
(
Scheme 5). After separation of LiCl, NMR spectra exclusively
showed the resonances that were assigned to structure D before
after reaction of 1 and Cp ZrCl in 1:1 stoichiometry in benzene.
We thus assign this compound as a 1,5-dizirconacyclooctatetra-
,3,6,7-ene (3) which to the best of our knowledge represents
Discussion of structural data. Both complexes 2 and 3 can be
crystallised from saturated solutions at ambient temperature.
While compound 2 crystallises from benzene (Figure 3), crystals
of 3 were grown from toluene (Figure 5). In 2 and 3 the Zr1-C1
distances are in the range of Zr-C  bonds (Table 2) which
2
2
2
the first dimetallacyclooctatetraene. The presence of an
antisymmetric C=C=C vibration is illustrated by strong signals in
the IR spectrum at 1784 and 1772 cm and weak signals at
corresponds to the data reported for Cp
2.280(5)/2.273(5) Å). The Zr-Cl distances in 2 are in the range
2
ZrMe
2
(cf.
[31]
-1
-1
2.4432(11) to 2.4562(11) Å, which is in excellent agreement to
1
787 and 1774 cm in the Raman spectrum which is in good
[
32]
values found for Cp
2
ZrCl
2
(cf. 2.446(5)/2.436(5) Å).
It should
agreement with the calculated frequencies (ṽcalc./uncor. = 1811 and
1
-1
be mentioned that on the basis of quantum chemical
calculations, the inner/outer orientation of the Cl atoms found in
the molecular structure of 2 does not correspond to the
thermodynamically most favorable orientation.
794 cm ).
Scheme 5. Optimised synthesis of 3.
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To gain a deeper insight into the bonding situations of 2 and 3
we calculated the Natural Localised Molecular Orbitals (NLMOs)
[
33]
with the NBO6.0 package.
In the calculated gas phase the
structure of complex 3 is C2 symmetric and 2 is C1 symmetric,
therefore we observed for 3 a set of degenerated NLMOs from
which only one part of these pairs is shown in Figure 6. With a
closer look to the HOMO-1 orbitals of 3 these are best described
as classical C-C -bonds which mainly consist of two pure p-
orbitals of the allenediyl carbon atoms with just a small
contribution of about 4 % from the d orbitals of the zirconium
centre. The HOMO orbitals show nearly
a rectangular
arrangement to the HOMO-1 orbitals and have lower zirconium
contribution. In contrast to this the LUMOs are best described as
empty d orbitals of the zirconium centre with minor antibonding
contributions from the Cp and allenediyl units. The NLMOs of
complex 2 show almost the same features and can be found in
the Supporting Information (Figure S45).
Figure 5. Molecular structure of compound 3. Thermal ellipsoids correspond
to 30% probability. Hydrogen atoms were omitted for clarity.
We found that orientation of both Cl atoms towards the centre of
the molecule (inner/inner) is preferred; compared to this, the
inner/outer and the outer/outer orientations are endergonic by
-1
-1
[21]
1
.48 kcal·mol and 3.43 kcal·mol , respectively. In contrast to
1benz, all C-C distances in the allenediyl units in complexes 2
and 3 are almost identical (1.303(6) to 1.319(6) Å) and are best
described as slightly shortened C-C double bonds (cf. C=C 1.34
[
25]
Å).
Barton’s disilacyclooctatetraene (E) (1.311(3) Å). A closer look
at the C-C-C angle shows that the C units in 2 and 3 do not
This is in agreement with the C1-C2 distance found in
[
10]
[
34]
Figure 6. Representation of NLMOs of compound 3.
3
differ considerably. The dilithioallene is significantly more bent,
most likely as a consequence of its tetrameric structure (Table 2).
For both zirconocene complexes the angles between the planes
General comments on the reactivity of complexes 2 and 3.
The structural features of the central allenediyl units in
complexes 2 and 3 are almost identical; however their thermal
and chemical behaviour differs considerably. Compound 2
decolourises within a few minutes when exposed to air. The
colourless powdery decomposition product can be dissolved in
benzene and two different crystal types were obtained from this
solution. Both crystal types were analysed by X-ray diffraction
defined by Si,C,C/C,C,Si of the SiC
orthogonal orientation of the cumulated double bonds (2:
6.9(3)° 3: 71.8(2)/69.8(2)°). As in Barton’s
3
Si units suggest almost
8
[
10]
disilacyclooctatetraene E) the allenediyl units in 3 are twisted
about 61.8(4)°, as defined by the planes C1-C2-C3 and C4-C5-
C6 (cf. 78.1° in E).
(
unit cell determination) and were both identified as (µ
2
-oxo)-
Table 2. Selected bond distances in Å and angles in °.
5
bis(chloro-(ŋ -cyclopentadienyl)-zirconium)
ZrCp ] in two different literature known unit cells. Furthermore
we identified [Cp Zr(Cl)-O-(Cl)-ZrCp ] by mass spectrometry, IR
and NMR spectroscopy. In addition to this we identified 1,3-bis-
trimethylsilyl)prop-1-yne as second product of this
[Cp Zr(Cl)-O-(Cl)-
2
[35]
Compound
E-Call.
C1-C2
C2-C3
C1-C2-C3
175.2(2)
2
2
2
[
a]
[b]
E (E = Si)
1.881(2)
1.311(3)
1.323(2)
-
(
[
d]
1benz
2.189(3)
2.220(3)
1.304(2)
173.65(15)
174.80(15)
1
13
[
c]
decomposition reaction via H and C NMR spectroscopic
analysis of the benzene solution. In contrast, the
dizirconacyclooctatetraene 3 is stable for hours under these
conditions. During analysis of compound 3 we noticed its
remarkable stability to air and humidity. For this reason we have
decided to carry out some stability tests. For this purpose, a
small amount of compound 3 was mixed with an excess neat
acetone, water and dichloromethane. After an exposure time of
(E = Li)
[
e]
f]
2
3
(E = Zr)
(E = Zr)
2.265(4)-
.288(4)
1.306(6)
1.303(6)
1.310(6)
1.319(6)
176.8(4)
178.0(4)
2
[
2.283(3)-
.306(3)
1.313(4)
1.309(4)
1.310(4)
1.306(4)
176.7(3)
177.3(3)
2
[
a] Data taken from ref. [10]; [b] Data not reported; [c] Shortest Li-C1 bonds,
1
13
for a detailed bond analysis see Table S5; [d] Only the largest and smallest
value is given here; [e] Two different molecules in the asymmetric unit; [f]
Values of both allenediyl units are reported, second set Zr2-C4-C5-C6.
2
4 hours, H and C NMR spectra of the dried solids were
collected in [D6]benzene (Figures 7 and S2). As can be seen
from the H NMR spectra, 3 is completely inert. It dissolves well
1
in dichloromethane, moderately in acetone and hardly in water.
In addition to this neat acetone, water and dichloromethane
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were added to solutions of 3 in [D6]benzene and again no
reaction or decomposition was observed.
General: All manipulations were carried out in an oxygen- and moisture-
free argon atmosphere using standard Schlenk and drybox techniques.
The solvents were purified with the Grubbs-type column system “Pure
Solv MD-5” and dispensed into thick-walled glass Schlenk bombs
equipped with Young-type Teflon valve stopcocks. Commercially
available n-BuLi solution (1.6 M and 2.5 M in n-hexane, Acros),
chlorotrimethylsilane
(Me
3
SiCl,
98
%,
Acros),
bis(cyclopentadienyl)zirconium(IV) dichloride (Cp
2
ZrCl , >98 %, Aldrich),
2
and trimethylsilylacetylene (98 %, TCI) were transferred in Schlenk
Tubes stored under argon and used as received. N,N,N',N'-
Tetramethylethylenediamine (99 %, Acros) was pre-dried over molecular
sieves (4 Å) and dried by sodium/benzophenone distillation. Potassium
[
38]
[39]
tert-butanolate (KOtBu)
and lithium diisopropylamine (LDA)
were
freshly prepared according to literature procedures and isolated as white
solids. 1,3-bis(trimethylsilyl)prop-2-yne was synthesised according to
modified literature procedure (see above). NMR spectra were determined
1
13
on Bruker AV300 and AV400.
H
and
]benzene (δ
Chemical shifts of Li are given relative
C chemical shifts were
[40]
referenced to the solvent signal: [D
6
H
7.16, δ
C
128.06)
and
[41]
7
8 H C
[D ]toluene (δ 2.09, δ 20.4)
1
to a reference frequency calculated from the actual H reference
frequency and( Li) = 38.863 797 MHz
D O, 9.7 mol/kg, at 0 ppm). Accordingly, chemical shifts of Si are given
2
7
[42]
(which corresponds to LiCl in
1
Figure 7. H NMR spectra of pure 3 (bottom) and of the reaction solutions
29
after 24 h exposure time (25 °C, [D6]benzene, 300.2 MHz).
29
relative to SiMe
4
, ( Si) = 19.867 187 MHz. Raman spectra were
recorded on a LabRAM HR 800 Raman Horiba spectrometer equipped
with an Olympus BX41 microscope with variable lenses was used. The
samples were excited by different laser sources: 633 nm (17 mW, air
cooled), 784 nm Laser diode (100 mW, air-cooled) or 473 nm Ar+ Laser
(20 mW, air-cooled). All measurements were carried out at ambient
temperature. IR spectra were recorded on a Bruker Alpha FT-IR, ATR
Spectrometer, spectra are not corrected. MS analysis was done using a
This behaviour is very unusual for metallacycles of early
transition metals as with these substrates typically formation of
[
36]
oxido or chlorido complexes is observed.
Also, with carbonyl
compounds, facile insertions into the M-C bonds are a common
[
37 ]
reactivity motif.
Complex 3 is highly non-polar; as a
+
-
Finnigan MAT 95-XP (Thermo-Electron), CI /CI Isobutane. The
measurement of the ESI-TOF/HRMS of 3 was carried out at a 6210
consequence it floats on water and is difficult to wet. This is
nicely expressed by the space filling model of 3 which shows, in
contrast to 1 and 2, the metal centre perfectly shielded by the Cp
Agilent Tec. device equipped with
MeOH/0.1 % HCOOH in H O 90:10 as eluent. CHN analysis was done
using Leco Tru Spec elemental analyser. Melting points are
uncorrected and were determined in sealed capillaries under Ar
atmosphere using Mettler-Toledo MP 70. Diffraction data for
K(THF) (Me SiC(H)C SiMe )], 1benz, 1thf, 2 and 3 were collected on a
a 1200 HPLC system using
2
and Me
the decreasing reactivity of these compounds towards air (1 > 2
3) can be rationalised.
3
Si groups (Figures S8/S11/S14). Having this in mind,
a
>
a
[
2
3
2
3
Bruker Kappa APEX II Duo diffractometer. The structures were solved by
2
direct methods and refined by full-matrix least-squares procedures on F
Conclusion
[ 43 ]
with the SHELXTL software package.
XP (Bruker AXS) and
were used for graphical representations. All calculations
[
44 ]
Diamond
In summary, we have revisited the chemistry of the previously
described, yet in our eyes very interesting and underrated allene
precursor 1. Upon reaction of 1 with Cp ZrCl hitherto unknown
2 2
unusual linear allenediyl bridged dizirconocene complexes,
namely a dinuclear monochlorido complex 2 as well as the first
dimetallacyclooctatetraene 3 are formed. Initial reactivity test
were carried out with the Gaussian 09 package of molecular orbital
[45]
programs. In our calculations, we have used the real-size molecules at
the BP86 level with the TZVP basis set for non-metal elements and the
effective core potential LANL2DZ basis set for Zr. Vibrational frequencies
were also computed, to include zero-point vibrational energies in
thermodynamic parameters and to characterise all structures as minima
on the potential energy surface.
showed that exchange of Li for [Cp
stabilization of the highly reactive C
2
Zr] results in a remarkable
precursor. As stepwise
3
2 3 2 3
Synthesis of [K(THF) (Me SiC(H)C SiMe )]: The mixture of potassium
synthesis of 3 from 1 and 2 was successful we are currently
investigating the reactivity of 2 to explore the frontiers of the
chemistry of more strained allenediyl bridged dizirconocenes.
Furthermore we are evaluating the potential of the highly
unusual dimetallacyclooctatetraene 3. In this report we show the
unexpected degradation of 3 under MS conditions to the highly
strained 1-zirconacyclobuta-2,3-diene.
tert-butanolate (2.46 g, 22 mmol) and LDA (2.32 g, 22 mmol) was
dispersed in 70 mL of benzene and stirred for two hours at ambient
temperature.
bis(trimethylsilyl)propyne (2.00 g, 10.8 mmol) was added dropwise within
5 minutes and stirred for further 18 hours. The viscous yellow mixture
To
the
resulting
pale
yellow
suspension
1
was freed of the supernatant by filtration and the solid was washed with
benzene (2 x 10 mL). The resulting solid was dried in vacuo for three
hours, redispersed in cooled THF (-78 °C, approx. 10 mL), filtered
through celite, concentrated to approximately 4 mL and stored at -78 °C
2 3 2 3
for three days. Colourless crystals of [K(THF) (Me SiC(H)C SiMe )] thus
Experimental Section
obtained were washed with a minimum of cooled THF and dried in vacuo
1
for three hours. m.p. (dec. Ar): 178 °C. H NMR (25 °C, [D8]THF, 400.13
1
1
MHz): δ = 0.62 (s, 1H,
3 3 H,C
JH,C = 142 Hz), -0.07 (s, 9H, CSi(CH ) , J =
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Chemistry - A European Journal
10.1002/chem.201800465
FULL PAPER
2
1
1
J
17 Hz,
J
H,Si = 7 Hz), -0.11 ppm ( s, 9H, CHSi(CH
3
)
3
,
J
H,C = 117 Hz,
75.49 MHz): δ = 171.4 (s, C=C=C), 110.1 (s, Cp), 105.4 (s, C=C=C), 3.5
2
13
1
29
H,Si = 6 Hz). C{ H} NMR (25 °C, [D8]THF, 100.63 MHz): δ = 150.1 (s,
CCCH), 78.1 (s, CCCH), 18.0 (s, CCCH), 3.3 (s, CSi(CH ), 2.4 ppm (s,
). Si-inept NMR (25 °C, [D8]THF, 79.49 MHz): -9.3 (dec), -
6.4 (ddec). IR (Nujol, 64 scans): 1996 (s), 1245 (m), 837 (s), 682 (w),
(s, SiCH
(dec, Si(CH
[M-4 methane ] 741 (24), [Cp
3
). Si-inept NMR (25 °C, [D6]benzene, 79.49 MHz): δ = –8.16
2
1
29
+
+
3
)
3
3
)
3, J( H- Si) = 6.2 Hz). MS-CI (isobutane): [M+H ] 806 (28),
29
+
+
CHSi(CH
3
)
3
2
Zr(Me
ESI-TOF/HRMS:
3 3
SiC SiMe
3
)+H ] 403 (100),
+
+
2
6
(
[(Me
[Cp Zr(Me
2895 (w), 1784 (s), 1772 (s), 1444 (w), 1397 (w), 1370 (w), 1240 (m),
3
Si)
C H
2 3 2
]
185
(20).
[M+H ]
807,
-
1
+
-
+
64 (w), 641 cm (w). MS-CI (isobutene): 364 (100) [(Me
5) [(Me Si) C H] .
3 2 3
3
Si)
C
4 6
] , 184
2
3 3
SiC SiMe
3
)+H ] 403. IR (ATR, 16 scanns): 3095 (w), 2950 (w),
-
1
4
2
132 (w), 1015 (m), 834 (s), 781 (s), 740 (s), 675 (s), 618 (m), 538 (w),
83 cm (m). RAMAN (784 nm,50 sec, 10 scans): 3104 (vw), 2961 (vw),
896 (vw), 1786 (vw), 1443 (vw), 1371 (vw), 1362 (vw), 1259 (vw), 1246
-
1
2 3 3 3 4
Synthesis of {Li (Me SiC SiMe )} (1): To a colourless solution of 1,3-
bis-(trimethylsilyl)prop-1-yne (1.89 g, 10.3 mmol) in 30 mL of benzene a
n-BuLi solution in n-hexane (2.5 M, 8.64 mL, 21.6 mmol) was added
dropwise within 20 minutes at ambient temperature. The clear
champagne coloured solution was cooled to -78 °C, degassed and
heated to 60 °C for one week. The resulting solution was then freed of all
volatile components and dried in vacuo for three hours. The colourless
(
(
vw), 1226 (vw), 1131 (vs), 1124 (m), 1072 (w), 893 (vw), 848 (w), 831
w), 806 (w), 774 (m), 728 (w), 679 (w), 620 (w), 538 (w), 502 (w), 387
-
1
(
w), 366 (m), 311 (s), 291 cm (s). Elemental analysis calcd (%) for
-
1
4 2
M(C38H56Si Zr ) = 807.65 g mol : C 56.24, H 7.03; found: C 56.51, H
6.99.
solid was washed with pentane (3 x 7 mL) and dried in vacuo to obtain 1
1
(
(
1
1.26 g, 1.6 mmol (Tetramer), 62 %). m.p. (dec. Ar): 239 °C. H NMR
1
3 3 H,C
25 °C, [D6]benzene, 400.13 MHz): δ = 0.31 (s, 18H, CSi(CH ) , J =
17 Hz, JH,Si = 6 Hz). C{ H} NMR (25 °C, [D6]benzene, 100.63 MHz):
δ = 174.1 (s, C=C=C), 44.6, 37.8 (br, C=C=C), 3.7, 3.3 (br, SiCH ). Si-
3
inept NMR (25 °C, [D6]benzene, 79.49 MHz): δ = -9.3, -14.3. Li{ H}
Acknowledgements
2
13
1
29
7
1
We thank our technical and analytical staff for assistance. We
would like to particularly thank Dr. Christine Fischer for help with
the MS analysis of compound 3 and Dr. Jonas Bresien (ibidem)
for the help with software- and computer cluster related
problems. Financial support by LIKAT is gratefully
acknowledged.
+
NMR (25 °C, [D6]benzene, 155.5 MHz) δ = 1.6, 2.6 (Figure S21). MS-CI
+
+
+
(
isobutene): [M ] 785 (100), [(Me
3 4 6 3 2 3 3
Si) C ] 365 (5), [(Me Si) C H ] 185 (5).
IR (Nujol, 64 scans): decomposition while measuring. RAMAN (632nm,
1
1
0 sec, 20 scans): 2952 (m), 2890 (s), 1751 (vw), 1408 (w), 1262 (vw),
246 (vw), 1237 (w), 1214 (vw), 831 (w), 754 (vw), 677 (w), 628 (vs), 565
-
1
(
w), 535 (w), 423 (vw), 386 (w), 344 (vw), 283 (vw), 221 cm (w).
-
1
8 8
Elemental analysis calcd (%) for M(C36H72Li Si ) = 785.17 g mol : C
55.07, H 9.24; found: C 55.09, H 9.00.
Keywords: metallacycles • X-ray analysis • bridging ligands •
zirconium • cyclopentadienyl ligands
2 2 3 3 3
Synthesis of [(Cp ZrCl) {-μ-(Me Si)C (SiMe )-}] (2): The colourless
solids Cp ZrCl (1.6 mmol, 0.464 g) and 1 (0.2 mmol, 0.157 g) were
2
2
[
[
[
1]
2]
3]
R. Willstätter, E. Waser, Ber. Dtsch. Chem. Ges. 1911, 44, 3423-3445.
A. C. Cope, C. G. Overberger, J. Am. Chem. Soc. 1948, 70, 1433-1437.
W. Reppe, O. Schlichting, K. Klager, T. Toepel, Justus Liebigs Ann.
Chem. 1948, 560, 1-92.
mixed, dissolved in benzene (10 mL) and stirred for 12 hours. The
resulting orange mixture was cannula filtered and the colourless
precipitate was washed with benzene (3 x 2 mL). The combined solutions
were concentrated and stored at ambient temperature. Fractional
[
4]
Selected examples: a) H. Dietrich, H. Dierks, Angew. Chem. 1966, 78,
crystallization and drying in vacuo yields orange crystals of 2 (0.412 g,
9
43-943. b) P. A. Kroon, R. B. Helmholdt, J. Organomet. Chem. 1970,
1
7
5 %). m.p. 161 °C (dec. Ar). H NMR (25 °C, [D6]benzene, 300.20
25, 451-454. c) A. Salzer, T. Egolf, L. Linowsky, W. Petter, J.
1
1
13
2
1
29
3
MHz): δ = 0.37 (s, 18H, CH , J( H- C) = 118.4 Hz, J( H- Si) = 6.3 Hz),
Organomet. Chem. 1981, 221, 339-349. d) J. H. Bieri, T. Egolf, W. Von
Philipsborn, U. Piantini, R. Prewo, U. Ruppli, A. Salzer,
Organometallics 1986, 5, 2413-2425. e) I. Bach, K.-R. Pörschke, B.
Proft, R. Goddard, C. Kopiske, C. Krüger, A. Rufińska, K. Seevogel, J.
Am. Chem. Soc. 1997, 119, 3773-3781. f) A. Hosang, U. Englert, A.
Lorenz, U. Ruppli, A. Salzer, J. Organomet. Chem. 1999, 583, 47-55. g)
T. K. Mukhopadhyay, M. Flores, R. K. Feller, B. L. Scott, R. D. Taylor,
M. Paz-Pasternak, N. J. Henson, F. N. Rein, N. C. Smythe, R. J.
Trovitch, J. C. Gordon, Organometallics 2014, 33, 7101-7112.
13
6.08 (s, 10H, Cp), 6.16 (s, 10H, Cp). C NMR (25 °C, [D6]benzene,
7
5.49 MHz): δ = 174.7 (s, C=C=C), 113.8 (s, Cp), 113.3 (s, Cp), 106.5 (s,
29
3
C=C=C), 3.2 (s, SiCH ). Si-inept NMR (25 °C, [D6]benzene, 79.49
MHz): δ = –5.41 (dec, Si(CH ) , J( H- Si) = 6.3 Hz). MS-CI (isobutane):
3 3
2
1
29
+
+
+
+
[M ] 697 (4), [M-Cp ] 627 (95), [Cp
2
Zr(Me
3 3 3
SiC SiMe ) ] 403 (100). IR
(ATR, 16 scanns): 3105 (w), 2948 (w), 2891 (w), 1776 (s), 1438 (m),
1
399 (w), 1364 (w), 1254 (m), 1236 (m), 1128 (w), 1066 (w), 1013 (m),
-1
830 (m), 795 (s), 738 (s), 677 (m), 622 (m), 520 (m), 508 (w), 400 cm
(
m). RAMAN (632 nm, 10 sec, 20 scans): 3110 (vw), 2964 (vw), 2948
[
[
5]
6]
a) E. Hückel, Z. Phys. 1931, 70, 204-286. b) P. v. R. Schleyer, C.
Maerker, A. Dransfeld, H. Jiao, N. J. R. van E. Hommes, J. Am. Chem.
Soc. 1996, 118, 6317-6318. c) J. Dominikowska, M. Palusiak, New J.
Chem. 2010, 34, 1855-1861.
(vw), 2902 (vw), 1781 (vw), 1439 (vw), 1363 (vw), 1126 (vs), 1064 (vw),
8
(
43 (vw), 816 (vw), 779 (w), 676 (vw), 624 (w), 522 (vw), 507 (vw), 361
-
1
w), 317 (w), 292 cm (m). Elemental analysis calcd (%) for
-
1
2 2 2
M(C29H38Cl Si Zr ) = 696.14 g mol : C 50.04, H 5.50; found: C 48.82, H
5
a) H. Breil, G. Wilke, Angew. Chem. 1966, 78, 942-942. b) B.
Bogdanović, M. Kröner, G. Wilke, Justus Liebigs Ann. Chem. 1966, 699,
.46 (best value of six measurements).
1-23. c) H.-J. Kablitz, G. Wilke, J. Organomet. Chem. 1973, 51, 241-
Synthesis of [(Cp
mmol, 210 mg) and 1 (0.075 mmol, 60 mg) were mixed as powders,
dissolved in Et O (10 mL) at -78 °C, stirred for one hour at this
2
Zr)
2
{-μ-(Me
3
Si)C
3
(SiMe
3
)-}
2
] (3): The solids of 2 (0.3
271.
[7]
[8]
S. Zilberg, Y. Haas, J. Phys. Chem. A 2006, 110, 8397-8400.
A. A. Tabares, E. L. Waters, R. W. Zoellner, Heteroat. Chem. 2016, 27,
37-43.
2
temperature and for additional 24 hours at -30 °C. The solvent was then
removed in vacuo, the resulting red raw product was dissolved in toluene
[9]
N. Matsumura, H. Tanaka, Y. Yagyu, K. Mizuno, H. Inoue, K. Takada,
M. Yasui, F. Iwasaki, J. Org. Chem. 1998, 63, 163-168.
(10 mL) and cannula filtered. The solution were concentrated and stored
at -78 °C. After fractional crystallization and drying the red crystals of 3 in
[10] J. Lin, Y. Pang, V. G. Young, Jr., T. J. Barton, J. Am. Chem. Soc. 1993,
115, 3794-3795.
1
vacuo yield (0.183 g, 75 %). m.p. (Ar, dec): 315 °C. H NMR (25 °C,
1
1
13
[
D6]benzene, 300.20 MHz): δ = 0.39 (s, 36H, CH
3
, J( H- C) = 118.2 Hz,
[11] U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg,
Organometallics 2005, 24, 456-471.
2
1
29
13
J( H- Si) = 6.2 Hz), 6.04 (s, 20H, Cp). C NMR (25 °C, [D6]benzene,
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201800465
FULL PAPER
[
12] Selected examples: a) U. Rosenthal, A. Ohff, W. Baumann, R. Kempe,
A. Tillack, V. V. Burlakov, Angew. Chem., Int. Ed. Engl. 1994, 33, 1605;
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V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, U. Rosenthal, P.
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Hashizume, T. Chihara, J. Am. Chem. Soc. 2004, 126, 60; f) J. Ugolotti,
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Int. Ed., 2008, 47, 2622; g) J. Ugolotti, G. Kehr, R. Fröhlich, S. Grimme,
G. Erker, J. Am. Chem. Soc. 2009, 131, 1996; h) G. Bender, G. Kehr, C.
G. Daniliuc, B. Wibbeling, G. Erker, Dalton Trans. 2013, 42, 14673; i) S.
K. Podiyanachari, G. Bender, G. Kehr, C. G. Daniliuc, G. Erker,
Organometallics 2014, 33, 3481.
[34] Only one NLMO of pairs of degenerated NLMOs is represented, for a
full set see Supporting Information Figure S46 .
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1965, 18, 173; b) L. J. Higham, A. A. Sanchez-Cid, P. G. Waddell CSD
Communication (Private Communication), 2015.
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J. Am. Chem. Soc. 1984, 106, 5472; b) P.-M. Pellny, V. V. Burlakov, W.
Baumann, A. Spannenberg, U. Rosenthal, Z. Anorg. Allg. Chem. 1999,
625, 910. Selected examples of chlorido complexes: a) S. L. Buchwald,
S. J. LaMaire, R. B. Nielsen, B. T. Watson, S. M. King, Tetrahedron Lett.
1987, 28, 3895; b) B. E. Bosch, G. Erker, R. Fröhlich, O. Meyer,
Organometallics 1997, 16, 5449.
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1987, 109, 2544.; b) M. Akita, K. Matsuoka, K. Asami, H. Yasuda, A.
Nakamura, J. Organomet. Chem. 1987, 327, 193; c) J. Scholz, M.
Dlikan, K. Thiele, Journal für Praktische Chemie 1988, 330, 808.
[38] M. H. Chisholm, S. R. Drake, A. A. Naiini, W. E. Streib, Polyhedron
1991, 10, 337.
[
[
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Chemistry - A European Journal
10.1002/chem.201800465
FULL PAPER
FULL PAPER
F. Reiß*, M. Reiß, A. Spannenberg, H.
Jiao, W. Baumann, P. Arndt, U.
Rosenthal, T. Beweries*
Page No. – Page No.
Visiting the limits between a highly
strained 1-zirconacyclobuta-2,3-diene
and chemically robust
The taming of a shrew: Reaction of a pyrophoric dilithioallene with zirconocene
results in a remarkable stabilisation and stepwise formation of a highly unusual
dizirconacyclooctatetraene species.
dizirconacyclooctatetraene
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