Arylcalcium iodides in tetrahydropyran: Solution stability in comparison to aryllithium reagents

Article abstract of DOI:10.1021/om300508w

Reduction of para-substituted iodobenzene in tetrahydropyran (THP) with finely dispersed calcium powder yields arylcalcium iodides of the type [(THP)₄Ca(C₆H₄-4-R)I] with R = CH₃ (1), Cl (2), Br (3), I (4), OCH₃ (5). A 2-fold insertion of calcium into dihalobenzenes was not observed. The β-naphthylcalcium iodide [(THP)₄Ca(β-Naph)I] (6) is also accessible by direct synthesis in THP. The durability of arylcalcium compounds in THP was studied in comparison to that in THF, and a slightly enhanced lifetime in THP at ambient temperature was observed. Furthermore, the relative reactivity and selectivity of 1 and its lithium counterpart [{(THP)₂Li}₂(μ-Tol)(μ-Br)] (7) in the reaction with THP and THF were studied. α-Metalation and subsequent cycloreversion was the major pathway observed for THF in both cases. In the degradation reaction induced by 7, several byproducts arising from carbolithiation and, surprisingly, from β-metalation reactions were identified, while 1 was found to be more selective. The related [(THP) ₂Li(μ-Ph)]₂ (9) was prepared and used to unambiguously identify some of the products. In order to verify the formation of benzyllithium as one of the byproducts, an authentic sample of [(dme)Li(μ-CH ₂Ph]₂ (8) was prepared. In THP, an inversion of the relative reactivity of 1 and 7 was observed and the calcium compound was found to be more reactive than its lithium analogue. The crystal structures of 1-9 were determined by X-ray diffraction studies, and a trans arrangement of the anionic ligands due to electrostatic reasons was observed in case of the hexacoordinated calcium complexes.

Full text of DOI:10.1021/om300508w

Article
pubs.acs.org/Organometallics
Arylcalcium Iodides in Tetrahydropyran: Solution Stability in
Comparison to Aryllithium Reagents
Jens Langer, Mathias Kohler, Reinald Fischer, Feyza Dundar, Helmar Gorls, and Matthias Westerhausen*
̈
̈
̈
Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena, Humboldtstrasse 8, D-07743 Jena, Germany
S
* Supporting Information
ABSTRACT: Reduction of para-substituted iodobenzene in
tetrahydropyran (THP) with finely dispersed calcium powder yields
arylcalcium iodides of the type [(THP)4Ca(C6H4-4-R)I] with R =
CH3 (1), Cl (2), Br (3), I (4), OCH3 (5). A 2-fold insertion of
calcium into dihalobenzenes was not observed. The β-naphthylcal-
cium iodide [(THP)4Ca(β-Naph)I] (6) is also accessible by direct
synthesis in THP. The durability of arylcalcium compounds in THP
was studied in comparison to that in THF, and a slightly enhanced
lifetime in THP at ambient temperature was observed. Furthermore,
the relative reactivity and selectivity of 1 and its lithium counterpart
[{(THP)2Li}2(μ-Tol)(μ-Br)] (7) in the reaction with THP and THF were studied. α-Metalation and subsequent cycloreversion
was the major pathway observed for THF in both cases. In the degradation reaction induced by 7, several byproducts arising
from carbolithiation and, surprisingly, from β-metalation reactions were identified, while 1 was found to be more selective. The
related [(THP)2Li(μ-Ph)]2 (9) was prepared and used to unambiguously identify some of the products. In order to verify the
formation of benzyllithium as one of the byproducts, an authentic sample of [(dme)Li(μ-CH2Ph]2 (8) was prepared. In THP, an
inversion of the relative reactivity of 1 and 7 was observed and the calcium compound was found to be more reactive than its
lithium analogue. The crystal structures of 1−9 were determined by X-ray diffraction studies, and a trans arrangement of the
anionic ligands due to electrostatic reasons was observed in case of the hexacoordinated calcium complexes.
(iii) Iodoarenes represent the most suitable substrates;
bromoarenes give lower yields, whereas chloro- and
fluoroarenes show no reaction with activated calcium.
(iv) Heating during direct synthesis should be avoided in
order to limit side reactions such as ether degradation.
INTRODUCTION
■
The year 2005 marked the occurrence of the first structurally
characterized arylcalcium derivatives, independently prepared
by the groups of Niemeyer,1 Harder,2 and Westerhausen,3 using
different synthetic strategies. During the following years
arylcalcium derivates emerged from elusive academic curiosities
to an easily accessible group of substances.4
These compounds show tremendous potential and might be
able to challenge the well-established aryllithium derivatives in
organic and organometallic syntheses. The worldwide accessi-
bility of calcium, its low price, and its nontoxic behavior
regardless of concentration further encourage subsequent
investigations. Given its position in the periodic table, the
alkaline-earth metal calcium is expected to combine typical
properties of s block metals (saltlike behavior, highly
heteropolar metal−carbon bonds) and early transition metals
(Lewis acidity of cations, d orbital participation, and catalytic
activity) within its compounds. Considering a few precondi-
tions, convenient high-yield syntheses of solutions of these
post-Grignard reagents have been developed recently.5 These
preconditions are as follows:
THF was found to be essential to obtain well-defined
products. A number of THF-ligated derivatives were structur-
ally characterized, and hexacoordinated calcium complexes of
the general formula [(THF)4Ca(Ar)X] with the aryl ligand in a
position trans to the halide anion were commonly observed.5−7
Deviating coordination numbers and/or geometries of the THF
complexes were found for sterically crowded aryl substituents8
or dinuclear complexes.9,10
In order to further develop arylcalcium derivatives into
successful substitutes of broadly applied aryllithium com-
pounds, it is crucial to identify reactions and conditions
under which calcium derivatives show superior reactivity
compared to the lithium derivatives. The perfect arylcalcium
derivative should fulfill the following criteria:
(i) stable at ambient temperature in the solid state and
solution
(ii) reactivity equal to or even higher than that of the lithium
counterpart
(iii) enhanced selectivity
(i) Activation of calcium succeeds with ammonia, yielding a
finely divided highly reactive metal powder.
(ii) The direct synthesis has to be performed in ethereal
Received: June 8, 2012
solutions, preferably in THF.
Published: August 13, 2012
© 2012 American Chemical Society
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(iv) definite composition and ether content in order to
provide an exact stoichiometry for reactions
(v) rather high solubility to ensure homogeneous reaction
conditions
in the case of 1 and identified as a second arylcalcium species.13
The low-field shift found for the ortho hydrogen atoms of the
aromatic ring to δ 8.48 in the H NMR spectrum of this
1
species, relative to the signals of 1 (δ 7.62) or its THF
counterpart (δ 7.60), is indicative of a bridging tolyl group.
This interpretation is further supported by the high-field shift of
the ipso carbon atom to δ 180.1 in the 13C NMR spectrum,
relative to terminally bound tolyl groups (δ 184.2). A similar
trend was also observed for other arylcalcium species
containing bridging aryl groups.14,15 Additionally, an analogous
shift was also found for related phenyllithium species
containing terminal or bridging phenyl groups, respectively.16
This calcium species most likely represents another example
of an oxo-centered cluster species, occasionally observed for the
aryl derivatives of the heavier alkaline-earth metals.2,3,17−20 An
equilibrium between this compound and 1 can be excluded,
since the concentration of this impurity is dependent on the
route of the synthesis of 1. When the THP complexes were
prepared from the corresponding THF species, containing this
impurity, by ligand exchange, the concentration of this second
arylcalcium species was higher than that in crops observed via
the direct synthesis route.
RESULTS AND DISCUSSION
■
Synthesis. Addressing the shortcomings of the known
THF-stabilized arylcalcium halides with respect to solvent
degradation, the synthesis and reactivity of post-Grignard
reagents in THP were investigated. The smaller ring strain of
this solvent molecule compared to that of the commonly used
THF should make this cyclic ether less accessible to
degradation reactions. However, the higher melting point of
THP (mp −49.2 °C) in comparison to that of THF (mp
−108.4 °C) limits its use as a solvent at very low temperatures.
In a general synthesis, excess of activated calcium (30 mmol)
was suspended in THP and 25 mmol of iodoarene was added at
−20 °C (Scheme 1). After several hours of shaking at ambient
Scheme 1. Synthesis of Post-Grignard Reagents of the Type
[(THP)4Ca(Aryl)I]
Stability and Reactivity. THF degradation by arylcalcium
halides was identified earlier as one of the challenges for further
applications of these compounds. Thus far, a comprehensive
NMR investigation with respect to lifetimes and product
spectrum is not available, although some products of the
decomposition of arylcalcium iodides in THF were
claimed.8,10,21,22 Therefore, a comparative study of the
decomposition of a representative member of the type
[(THP)4Ca(C6H4-4-R)I], namely the p-tolyl derivative 1
(Figure 1), was undertaken not only in THP but also in
THF to be able to evaluate the stability of arylcalcium species in
these solvents under similar conditions. The use of the same
batch of starting material ensures the comparability of the runs.
On dissolution in THF, the THP ligands of 1 are immediately
replaced by THF, which is also present in large excess.
However, these experiments not only serve to establish and
compare the relative stabilities of the arylcalcium species in
those cyclic ethers but also provide valuable data about the
selectivity and reactivity in comparison to those of aryllithium
species. Therefore, [{(THP)2Li}2(μ-Tol)(μ-Br)] (7) (see
Figure 2) was included in this study, to allow the first direct
comparison of the reactivity of well-defined arylcalcium iodides
and aryllithium compounds. The selection of 7 over a
homoleptic derivative takes the often dramatic enhancement
of the reactivity of organolithium derivatives in the presence of
lithium halides into account23 and makes the lithium
compound more comparable to the calcium systems.
When initially studying the reactivity of the THF complexes,
generated in situ by addition of an excess of THF to crystalline
samples of 1 and 7 in order to obtain reference data for
comparison to the THP systems, we found that half of the
primary p-tolylcalcium derivative was consumed after 8 days at
ambient temperature, while the lithium complex was more
reactive and half of the starting concentration was already
reached after 4 days (see Figure 3). As judged by the formation
of ethene, the major pathway of the decomposition reaction in
both cases seems to be the α-metalation of THF followed by its
[3 + 2] cycloreversion, in agreement with earlier studies on the
fission of THF by strong bases.24 The expected products of
these processes are in our cases ethene, calcium or lithium
ethenolate, toluene, and [(THF)4CaI2] or LiBr, respectively.
temperature, excess calcium was removed by filtration and the
conversion was determined by acidic titration of an aliquot of
the filtrate. A variety of substrates was tested successfully, and
conversions comparable to the THF systems were obtained
under these conditions.5,6 The products are readily crystallizing
substances, facilitating the isolation of the post-Grignard
reagents of the type [(THP)4Ca(C6H4-4-R)I] (R = CH3 (1),
Cl (2), Br (3), I (4), OCH3 (5)) as well as of the β-naphthyl
derivative [(THP)4Ca(C10H7)I] (6). Acidimetric titration to
determine the alkalinity as well as complexometric titration for
the calcium content justifies the description of most of the
derivatives by the general formula [(THP)4Ca(Aryl)I]. Only
the isolated crop of 3 shows overly high alkalinity and calcium
content as a result of a lack of selectivity between the C−I and
C−Br bonds in the direct synthesis.11 Nevertheless, the isolated
compound contains exclusively the 4-bromophenyl substituent,
as judged by NMR experiments. The deviating analytical data
are the result of partial substitution of the iodide anion by
bromide.
The crude products obtained were investigated for by-
products. [(THP)4CaI2],12 formed during the direct synthesis
to a minor extent, was found as an impurity of the crude
reaction products in some cases, indicated by alkalinity that is
slightly too low relative to the calcium content. However, this
impurity is present in related THF complexes as well and can
be removed easily by recrystallization.
Another type of impurity, commonly observed for the THF
systems investigated for comparison, was detected to a minor
extent when THP was used, marking the first advantage of this
solvent. This impurity was studied in detail by NMR techniques
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Figure 3. Durability of [(THP)4Ca(I)(p-Tol)] (1) and
[{(THP)2Li}2(μ-Tol)(μ-Br)] (7) in tetrahydrofuran (THF) and
tetrahydropyran (THP). The contents of the organometallics were
Figure 1. Molecular structure and numbering scheme of [(THP)4Ca-
(I)C6H4-4-CH3] (1). Ellipsoids represent a probability of 40%, and H
atoms are omitted for reasons of clarity. The parameters of
[(THP)4Ca(I)C6H4-4-OCH3] (5) are added in brackets. Selected
bond lengths (Å): Ca1−C1 = 2.647(5) [2.643(9)], Ca1−I1 =
3.1571(9) [3.136(2)], Ca1−O1 = 2.384(3) [2.394(5)], Ca1−O2 =
2.402(3) [2.400(6)], Ca1−O3 = 2.438(3) [2.405(5)], Ca1−O4 =
2.383(3) [2.437(5)]. Selected bond angles (deg): C1−Ca1−I1 =
176.4(1) [175.0(2)], C1−Ca1−O1 = 88.2(1) [90.7(2)], C1−Ca1−
O2 = 91.5(1) [87.5(2)], C1−Ca1−O3 = 96.6(1) [91.5(2)], C1−
Ca1−O4 = 91.8(1) [95.8(2)], O1−Ca1−O2 = 90.1(1) [95.3(2)],
O1−Ca1−O3 = 175.2(1) [174.8(2)], O1−Ca1−O4 = 94.0(1)
[85.4(2)], O2−Ca1−O3 = 90.4(1) [89.6(2)], O2−Ca1−O4 =
174.8(1) [176.6(2)], O3−Ca1−O4 = 85.3(1) [89.6(2)], I1−Ca1−
O1 = 88.77(8) [86.4(1)], I1−Ca1−O2 = 90.39(7) [88.7(1)], I1−
Ca1−O3 = 86.41(7) [91.8(1)], I1−Ca1−O4 = 86.49(8) [88.0(1)].
1
determined by integration of H NMR spectra.
apparent, although in very low overall concentration (see the
Experimental Section and Figure 4).
1
Figure 4. H NMR spectrum of the reaction mixture of 1 and THF:
(+) unidentified product; (−) residual 1; (*) residual signal of
○
benzene-d6; ( ) toluene; (#) ethene; (×) calcium ethenolate.
On studying the minor byproducts, in order to gain
information about the selectivity of the reactions, we were
surprised to find a complete signal set of LiOCH2CH2CH
CH2 in case of the reaction of 7. This product presumably
arises from THF, β-attacked by the aryllithium species.
Although the fission of THF by organolithium derivatives is a
very well studied chemistry24,28 due to the importance of these
compounds, we are not aware of any direct observation of this
species, though its presence was rightly concluded from the
products observed by quenching experiments or subsequent
reactions.29−31
Another occurring side reaction is the attack of formed
toluene by residual p-tolyllithum species, leading to benzyl-
lithium derivatives. [(dme)Li(CH2C6H5)]2 (8) was independ-
ently prepared by transmetalation of chlorotribenzylstannane
with lithium for comparison reasons to ensure the formation of
benzyllithium. The molecular structure of 8 is shown in Figure
5. This complex crystallized as a dimer with a central Li2C2 ring.
The tetracoordinate lithium atoms are in significantly distorted
tetrahedral environments, with the deviations stemming from a
small bite angle of the bidentate bases.
Figure 2. Molecular structure and numbering scheme of
[{(THP)2Li}2(μ-Tol)(μ-Br)] (7). Ellipsoids represent a probability
of 40%, and H atoms are neglected for reasons of clarity. Selected
bond lengths (Å): Li1−C1 = 2.207(3), Li2−C1 = 2.210(4), Li1−Br1
= 2.530(3), Li2−Br1 = 2.527(3), Li1−O1 = 1.942(3), Li1−O2 =
1.974(3), Li2−O3 = 2.011(3), Li2−O4 = 1.974(3), Li1···Li2 =
2.677(4). Selected bond angles (deg): Li1−C1−Li2 = 74.6(1), C1−
Li1−Br1 = 108.8(1), Li1−Br1−Li2 = 63.9(1), Br1−Li2−C1 =
108.8(1), O1−Li1−O2 = 102.5(1), O3−Li2−O4 = 101.9(2), Li1−
C1−C2 = 112.8(1), Li1−C1−C6 = 112.0(1), Li2−C1−C2 =
120.6(2), Li2−C1−C6 = 118.6(2), C2−C1−C6 = 112.2(2).
Both lithium compounds present in solution, p-tolyllithium
and benzyllithium, additionally insert ethene,32 formed via the
dominating α-elimination reaction of THF, into the lithium−
carbon bond, as indicated by the formation of 1-ethyl-4-
While in the case of 7 only one set of signals was observed for
the ethenolate species in the 1H and 13C NMR spectra,25 three
distinct sets of signals of calcium ethenolates26,27 became
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Figure 5. Molecular structure and numbering scheme of [(dme)Li(μ-
CH2Ph)]2 (8). The ellipsoids represent a probability of 40%, and H
atoms are omitted for reasons of clarity. Selected bond lengths (Å):
Li1−C1 = 2.202(7), Li1−C8 = 2.286(6), Li1−O1 = 1.973(6), Li1−
O2 = 1.983(6), Li2−C1 = 2.283(6), Li2−C8 = 2.229(7), Li2−O3 =
1.966(6), Li2−O4 = 1.996(6), C1−C2 = 1.454(4), C8−C9 =
1.451(4). Selected bond angles (deg): C1−Li1−C8 = 112.6(2),
C1−Li1−O1 = 126.2(3), C1−Li1−O2 = 115.2(3), C8−Li1−O1 =
107.2(3), C8−Li1−O2 = 107.1(3), O1−Li1−O2 = 84.4(2), C1−Li2−
C8 = 111.7(2), C1−Li2−O3 = 104.4(3), C1−Li2−O4 = 113.5(3),
C8−Li2−O3 = 127.6(3), C8−Li2−O4 = 112.9(3), O3−Li2−O4 =
84.1(2).
Figure 6. Molecular structure and numbering scheme of [{(THP)2Li}-
(μ-Ph)]2 (9). The ellipsoids represent a probability of 40%, and H
atoms are omitted for reasons of clarity. Selected bond lengths (Å):
Li1−C1 = 2.227(2), Li1−C7 = 2.213(2), Li2−C1 = 2.197(2), Li2−C7
= 2.245(2), Li1−O1 = 1.981(2), Li1−O2 = 1.983(2), Li2−O3 =
1.966(2), Li2−O4 = 1.979(2), Li1···Li2 = 2.428(3). Selected bond
angles (deg): Li1−C1−Li2 = 66.56(8), C1−Li1−C7 = 108.14(9),
Li1−C7−Li2 = 65.98(8), C1−Li2−C7 = 108.03(9), O1−Li1−O2 =
97.63(10), O3−Li2−O4 = 99.62(9), Li1−C1−C2 = 111.6(1), Li1−
C1−C6 = 114.04(10), Li2−C1−C2 = 141.10(11), Li2−C1−C6 =
101.57(10), C2−C1−C6 = 113.0(1), Li1−C7−C8 = 105.5(1), Li1−
C7−C12 = 137.35(10), Li2−C7−C8 = 116.40(10), Li2−C7−C12 =
109.98(9), C8−C7−C12 = 112.9(1).
methylbenzene as well as n-propylbenzene. The identification
of those substances is based on the directly observed signals of
the ethyl group at δH 1.05 (t, CH3), 2.42 (q, CH2) and two of
the three signals of the n-propyl group at δH 0.79 (t, CH3) and
2.42 (t, CH2) in the 1H NMR spectrum, while the other signals
were only observed by two-dimensional NMR experiments
because they overlap with the signals of toluene and THF. In
order to ensure their formation, a control experiment was
conducted using PhLi as starting material, the THP adduct
[(THP)2Li(μ-Ph)]2 (9) of which is shown in Figure 6. This
complex crystallized as a dimer with a four-membered Li2C2
ring and the alkali metal in distorted-tetrahedral environments.
The Li−C bond lengths of 2.20−2.25 Å lie in the expected
region for organilithium compounds.
Scheme 2. Observed Reaction Pathways during Degradation
of 7 in THF
a
When PhLi was employed in this reaction, the formation of
ethylbenzene was expected, containing an ethyl group which
should give rise to signals similar to those observed for 1-ethyl-
4-methylbenzene, while no formation of benzyllithium is
possible and therefore the signals assigned to propylbenzene
should not appear. The experiment confirmed these
assumptions. Scheme 2 summarizes the occurring reactions in
the system 7/THF.
The three side reactions described for the organolithium
compound 7 were not observed in case of the calcium
derivative 1. However, here the very low overall concentration
of the calcium ethenolate might suggest a subsequent reaction
of this species, but further products such as oxo-centered
clusters were not detected. Only two additional doublets at δ
8.19 and 4.99 were observed in the 1H NMR spectrum, related
to signals at δ 161.3 and 136.2 in the 13C NMR spectrum. It
was confirmed by DEPT and two-dimensional NMR experi-
ments that these signals belong to two CH groups, coupled to
each other. The observed shifts make a 1,2-disubstituted alkene
derivative more likely than other possibilities,33,34 but this trace
product remained essentially unidentified. Having thoroughly
a
Framed substances were observed by NMR measurements: red,
major product; blue, minor product; green, trace amounts.
studied the THF systems, we focused our attention on the
THP systems. THP as a ligand or solvent is known to be more
resistant than THF toward degradation reactions induced by
different organolithium compounds. Both protophilic35 and
nucleophilic36 fission is considerably slowed down: e.g., n-BuLi
shows a half-life time of 107 3 min in THF but a significantly
increased half-life time of 1257 111 min in THP at ambient
temperature.35 The products of THP degradation are unknown,
but a ring-opening reaction followed by polymerization was
assumed.35 α-Metalation is likely to be the first step, as
suggested by the isolation of complexes containing α-metalated
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THP from the reaction mixture of [(tmeda)Na(μ-TMP)(μ-
CH2SiMe3)Zn(CH2SiMe3)] and THP.37
equilibrium in this solvent should be shifted toward the less
reactive dimer. Even the formation of smaller amounts of
tetrameric species in equilibrium, as observed in case of diethyl
ether as ligand,42 seems likely. These structural differences
between lithium and calcium compounds in solution seem to
result in enhanced reactivity of mononuclear p-tolylcalcium
iodide relative to oligonuclear p-tolyllithium in the less polar
THP. Furthermore, this advantage in reactivity does not come
at the price of lowered selectivity. In contrast, the lithium
system was found to be less selective and substantial amounts
of benzyllithium were formed by the reaction of p-tolyllithium
and liberated toluene (see Figure 8).
Against our expectations and also in marked contrast to the
observations reported for organolithium derivatives, the use of
THP as solvent does not positively affect the durability of the
primary calcium species [(THP)4Ca(C6H4-4-CH3)I]. Quite the
contrary, in early stages of the reaction the decay of this
compound is even somewhat more pronounced than in THF
(see Figure 3). However, broad overlapping signals in the
ranges of δ 8.40−7.90, 6.92−6.78, and 2.08−1.98 in THP point
to the formation of secondary p-tolylcalcium-containing
products, possibly with bridging p-tolyl groups. Therefore, the
remaining overall content of calcium-bound p-tolyl groups is
probably higher than that indicated in Figure 3. Assuming that
the initial step of the fission of both THF and THP by p-
tolylcalcium complexes is deprotonation at the α-position (or
less likely at the β-position), regardless of the bonding mode of
the tolyl substituent or the nuclearity of the compound, the
time-dependent formation of toluene would be a good
indicator for the overall decrease of p-tolylcalcium species.
Figure 7 illustrates a slightly slower accumulation of toluene in
1
Figure 8. Aromatic region of the H NMR spectrum of the reaction
mixture of 7 and THP: (+) residual 7; (*) residual signal of benzene-
○
d6; (−) toluene; ( ) benzyllithium.
In the case of the calcium complex [(THP)4Ca(C6H4-4-
CH3)I] no formation of benzylcalcium derivatives, although
well-known,8,43 was detected via NMR measurements under
the applied conditions.
Figure 7. Time-dependent formation of toluene by 1 (and 7) in THP
and THF.38
These results raise the hope that arylcalcium compounds
provide increased reactivity in comparison to aryllithium
derivatives, at least in less polar solvents.
the reaction mixtures of 1 in THP in comparison to the THF
system, which can be interpreted as a slightly stabilizing effect
of THP on arylcalcium species relative to THF. Other
degradation products, arising from the THP, could not be
detected by NMR measurements, but observation of an
amorphous precipitate, separating from the reaction mixture
in later stages, supports the hypothesis of polymer formation
mentioned earlier.35
More important than the achieved minor enhancement of
the lifespan of p-tolylcalcium species in THP is the inversion of
the relative reactivity compared to the lithium derivative 7 in
this solvent. The calcium compound clearly exceeds the
reactivity of its lithium counterpart under similar conditions,
as illustrated in Figures 3 and 7.
A reasonable explanation for these differences might be
found in the structures of the compounds in solution. While
arylcalcium iodides exist predominately as monomers in THF
solutions, as judged by the NMR data, and stay this way in
THP as well, a much more complex situation is found for
aryllithium compounds. In addition to the predominant dimer,
containing bridging aryl groups, also substantial amounts of
monomers are present in THF.16,39 Keeping in mind that THP
is a less basic40,41 but slightly more bulky donor ligand, the
Molecular Structures. In order to use arylcalcium
compounds as reagents in organometallic chemistry, a well-
defined composition of these complexes is desirable to ensure
exact stoichiometries in reactions. Crystallization is a powerful
tool to achieve this goal, but unfortunately, only a few of the
known THF complexes were obtained in the form of well-
defined crystals and, hence, the crystal structures for instance of
the p-halogen-substituted phenyl derivatives are not available.
Especially for the most interesting derivative, p-iodophenylcal-
cium iodide, a composition different from that commonly
observed was reported for the THF-stabilized complex,5
indicating an impure product. The use of THP as ligand and
solvent was found to be superior and provides a straightforward
access to crystals of 1−6, suitable for X-ray diffraction
experiments.
The molecular structures of these para-substituted phenyl-
calcium iodides are rather similar, due to comparable steric
demand of the aryl groups in complexes 1−6. As representative
examples the structure models of 1, 2, 4, and 6 are displayed in
Figures 1 and 9−11, respectively, and selected values of 3 and 5
are added in the figure legends. In addition, bond lengths and
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Figure 11. Molecular structure and numbering scheme of [(THP)4Ca-
(I)(β-Naph)] (6). Ellipsoids represent a probability of 40%, and H
atoms are neglected for reasons of clarity. Selected bond lengths (Å):
Ca1−C1 = 2.534(6), Ca1−I1 = 3.166(1), Ca1−O1 = 2.442(4), Ca1−
O2 = 2.386(4), Ca1−O3 = 2.405(4), Ca1−O4 = 2.414(4). Selected
bond angles (deg): C1−Ca1−I1 = 176.9(1), C1−Ca1−O1 = 89.7(2),
C1−Ca1−O2 = 94.2(2), C1−Ca1−O3 = 89.3(2), C1−Ca1−O4 =
89.0(2), O1−Ca1−O2 = 92.1(1), O1−Ca1−O3 = 176.8(2), O1−
Ca1−O4 = 91.0(1), O2−Ca1−O3 = 84.9(1), O2−Ca1−O4 =
175.5(1), O3−Ca1−O4 = 92.0(1), I1−Ca1−O1 = 92.9(1), I1−
Ca1−O2 = 87.4(1), I1−Ca1−O3 = 88.3(1), I1−Ca1−O4 = 89.2(1).
Figure 9. Molecular structure and numbering scheme of [(THP)4Ca-
(I)C6H4-4-Cl] (2). Ellipsoids represent a probability of 40%, and H
atoms are neglected for reasons of clarity. Selected bond lengths (Å):
Ca1−C1 = 2.643(9), Ca1−I1 = 3.136(2), Ca1−O1 = 2.394(5), Ca1−
O2 = 2.400(6), Ca1−O3 = 2.405(5), Ca1−O4 = 2.437(5). Selected
bond angles (deg): C1−Ca1−I1 = 175.0(2), C1−Ca1−O1 = 90.7(2),
C1−Ca1−O2 = 87.5(2), C1−Ca1−O3 = 91.5(2), C1−Ca1−O4 =
95.8(2), O1−Ca1−O2 = 95.3(2), O1−Ca1−O3 = 174.8(2), O1−
Ca1−O4 = 85.4(2), O2−Ca1−O3 = 89.6(2), O2−Ca1−O4 =
176.6(2), O3−Ca1−O4 = 89.6(2), I1−Ca1−O1 = 86.4(1), I1−
Ca1−O2 = 88.7(1), I1−Ca1−O3 = 91.8(1), I1−Ca1−O4 = 88.0(1).
with only slight deviations from linearity. However, significant
distortions result from strongly different proximal and distal
Ca−C−C angles, impeding molecular C2 symmetry. This kind
of distortion results from steric strain, because the aryl ring
pushes two ether ligands apart and slightly slips between these
two coligands. This arrangement enforces significantly different
O−Ca−O angles between neighboring THP ligands varying
from 83 to 99°. The Ca−C bond lengths of the compounds do
not show an obvious trend in relation to the para substituent.
However, a long Ca−C bond is always accompanied by a less
squeezed C6−C1−C2 angle. There is some indication that
elongation of this bond might require only small amounts of
energy, due to a rather shallow potential surface. Taking a
closer look at the molecular structure of 2, cigar-shaped thermal
ellipsoids were observed for C1, C2, and C6 in the direction of
the Ca−C1, C2−C1, and C6−C1 bonds, respectively. This can
be interpreted not only as thermal motion but also as an
unresolved disorder due to the superposition of at least two
slightly different aryl substituents or due to very small amounts
of iodine on the aryl positions. A very shallow potential surface
with respect to the Ca−C bond length also offers an
explanation, and a similar reason was reported for the variation
of the Ca−I bond of [(THF)4CaI2], where elongation by 15
pm only requires 1.7 kcal mol−1.44
Figure 10. Molecular structure and numbering scheme of [(THP)4Ca-
(I)C6H4-4-I] (4). Ellipsoids represent a probability of 40%, and H
atoms are neglected for reasons of clarity. Only molecule A of two
crystallographically independent molecules is displayed. The param-
eters of [(THP)4Ca(I)C6H4-4-Br] (3) are added in brackets. Selected
bond lengths (Å): Ca1−C1 = 2.565(4) [2.605(9)], Ca1−I1 =
3.1512(8) [3.060(2)], Ca1−O1 = 2.453(3) [2.378(6)], Ca1−O2 =
2.380(3) [2.382(5)], Ca1−O3 = 2.384(3) [2.414(5)], Ca1−O4 =
2.390(3) [2.438(5)]. Selected bond angles (deg): C1−Ca1−I1 =
176.3(1) [174.5(2)], C1−Ca1−O1 = 91.0(1) [90.1(2)], C1−Ca1−
O2 = 90.5(1) [85.9(2)], C1−Ca1−O3 = 92.8(1) [92.1(2)], C1−
Ca1−O4 = 87.8(1) [94.9(2)], O1−Ca1−O2 = 87.8(1) [98.6(2)],
O1−Ca1−O3 = 175.4(1) [172.0(2)], O1−Ca1−O4 = 97.2(1)
[83.7(2)], O2−Ca1−O3 = 89.5(1) [89.3(2)], O2−Ca1−O4 =
174.8(1) [177.5(2)], O3−Ca1−O4 = 85.6(1) [88.4(2)], I1−Ca1−
O1 = 87.54(7) [87.1(2)], I1−Ca1−O2 = 92.78(7) [89.9(1)], I1−
Ca1−O3 = 88.91(8) [91.4(1)], I1−Ca1−O4 = 89.01(8) [89.5(1)].
The Ca−I bond length of [(THP)4Ca(C6H4-4-Br)I] (3)
exhibits an unexpected small value, giving another hint toward
partial occupation of this position by bromine. In case of 4,
cocrystallization of small amounts of calcium diiodide was
observed, but only one of the two independent molecules
within the crystal was affected. Selected structural parameters of
the unaffected molecule of 4 together with the other THP
adducts of arylcalcium iodides are listed in Table 1.
Tetrahydropyran is the weaker base and more bulky than
tetrahydrofuran. Both features support slightly larger Ca−O
distances to the THP ligands and averaged values between
angles are given in Table 1. In all of these complexes the
anionic ligands are positioned trans, for electrostatic reasons,
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Table 1. Selected Structural Parameters (Average Values, Bond Lengths (Å) and Angles (deg), Coordination Number CN of the
Metal Atom) of THP Adducts of Aryllithium and Arylcalcium Iodides
1
2
3
4
5
6
7
9
M
Ca
Ca
Ca
Ca
Ca
Ca
Li
Li
aryl
p-Tol
I
C6H4Cl
I
C6H4Br
I
C6H4I
I
C6H4OMe
I
β-Naph
I
p-Tol
Br
Ph
X
L
THP
6
THP
6
THP
6
THP
6
THP
6
THP
6
THP
4
THP
4
CN
M−C1
M−X
2.647
3.157
2.402
176.4
112.6
129.5
117.6
2.643
3.136
2.409
175.0
115.3
126.4
118.3
2.605
3.060
2.403
174.5
116.5
129.1
114.3
2.565
3.151
2.402
176.3
117.1
130.0
112.5
2.550
3.156
2.416
176.3
123.8
125.6
112.5
2.534
3.166
2.412
176.9
117.1
129.0
113.8
2.209
2.529
1.975
108.8
2.221
M−O
C1−M−X
M−C1−Cprox
M−C1−Cdist
C−C1−C
1.977
112.2
113.0
Table 2. Selected Observed and Calculated 13C{1H} NMR Data of Arylcalcium Iodides in THP/Benzene-d6
a
a
a
a
aryl
p-tolyl
δ(Cipso) obsd (calcd)
δ(Cortho) obsd (calcd)
δ(Cmeta) obsd (calcd)
δ(Cpara) obsd (calcd)
183.0 (183.4)
185.6 (184.5)
186.3 (185.5)
186.8 (186.1)
176.3 (178.4)
141.2 (140.9)
142.3 (141.9)
142.9 (143.1)
143.3 (143.5)
141.5 (141.8)
126.3 (125.8)
125.1 (125.4)
127.9 (128.5)
133.9 (135.1)
111.7 (110.2)
130.7 (132.5)
129.3 (129.6)
118.2 (117.8)
90.0 (90.9)
p-Cl-C6H4
p-Br-C6H4
p-I-C6H4
p-MeO-C6H4
157.3 (154.5)
a
δi = 128.5 + ∑I; increments I for the para substituents were taken from ref 46.
2.402 and 2.416 Å in complexes 1 to 6 were observed. In four
related THF complexes, the average Ca−O distances range
from 2.38 to 2.406 Å.5,6,45
CONCLUSION
■
Solutions of arylcalcium iodides can be prepared with good
yields in tetrahydropyran (THP). The arylcalcium iodides are
soluble in this solvent, despite the large electronegativity
difference between calcium and carbon and the expected
saltlike behavior. This finding should allow application of these
post-Grignard solutions in homogeneous calciation, group
transfer, and other reactions. The observed arylcalcium iodides
are monomeric in THP solution as well as in the solid state, and
diverse substituents can be introduced at the para position. The
anionic aryl and iodide ligands always are arranged trans to each
other for electrostatic reasons. Due to similar steric crowding all
of these complexes of the type [(THP)4Ca(Aryl)I] contain
hexacoordinate alkaline-earth-metal centers in distorted-octahe-
dral environments.
The durability of arylcalcium iodide solutions is slightly
extended by substitution of the five-membered cyclic ether
THF by less strained THP. In comparison to aryllithium
compounds, the THP-ligated calcium complexes exhibit an
altered reactivity in the deprotonation of THP, proving that
calcium complexes indeed are able to outbid well-defined
organolithium derivatives under certain reaction conditions. In
THF solutions the opposite order of reactivity between lithium
and calcium derivatives was observed.
NMR Spectroscopy. In order to establish the influence of
the neutral ligand THP on the chemical shift of calcium-bound
aryl substituents, a 2/1 mixture of THP and benzene-d6 was
used for 13C NMR measurements, since the commonly used
THF-d8 would immediately replace the THP ligands. These
NMR data were compared to those reported for the related
THF complexes in THF-d8 solutions, in which the predom-
inant species probably contains four coordinated THF-d8
ligands. The most significant differences were obtained for
the calcium-bound ipso carbon atoms, which show a character-
istic high-field shift of approximately 2 ppm in case of the THP-
ligated complexes. When the conditions of the measurements
are made even more comparable by using a 2/1 mixture of
THF and benzene-d6, this difference shrinks to little more than
1 ppm. For example, the calcium-bound carbon atom appeared
as a singlet in the 13C NMR spectrum of [(THP)4Ca(C6H4-4-
CH3)I] (1) at δ 185.3 in THF-d8, δ 184.2 in THF/benzene-d6,
and δ 183.0 in THP/benzene-d6. These similar data suggest
that both the THP- and THF-ligated systems have very similar
structures in solution and most likely maintain their solid-state
structures.
Prediction of the 13C NMR chemical shifts of the carbon
atoms of such calcium-substituted benzenes is possible using
well-known empirical increment systems.46 From the data of
1−5, the empirical increments of the [(THP)4CaI] substituent
can be extracted, being +58.0 (ipso), +12.4 (ortho), −3.3
(meta), and −5.3 ppm (para). Although most increments used
in such empirical systems are based upon data obtained in
CDCl3, the calculated values show acceptable deviations from
the measured values observed for the calcium complexes 1−5
(see Table 2). Therefore, the increments reported will help to
predict and assign the NMR data of hitherto unknown
arylcalcium derivatives.
A large benefit of calcium-based organometallics is the metal
itself. Calcium is inexpensive, is absolutely nontoxic, is available
worldwide, and is easily activated with liquid ammonia, and in
addition (in contrast to lithium powder) a nitrogen atmosphere
is sufficient for handling of even activated calcium powder.
Arylcalcium complexes (post-Grignard reagents) are easily
prepared via direct synthesis in THP, are monomeric with
hexcoordinate metal centers, have good crystallization proper-
ties for purification purposes, have definite compositions of the
type [(THP)4Ca(Aryl)I], have high solubility and reactivity
toward substrates, and have a sufficient durability in THP
solutions, enabling a rich organometallic chemistry. The only
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26.3%). Anal. Calcd for C26H44CaClIO4 (623.07 g mol−1): Ca, 6.43.
Found: Ca, 6.52. H NMR (THP/benzene-d6 2/1, 400 MHz): δ 1.33
remaining advantage of aryllithium species is their commercial
availability, but it is not too temerarious to predict that
arylcalcium compounds will also be in trade in the near future.
1
(m, CH2 THP), 1.40 (m, CH2 THP), 3.46 (t, J = 5.2 Hz, CH2 THP),
7.01 (AA′ part of an AA′BB′ spin system, 2H, m-CH Ph), 7.63 (BB′
part, 2H, o-CH Ph). 13C{1H} NMR (THP/benzene-d6 2/1, 100.6
MHz): δ 23.9 (CH2 THP), 27.0 (CH2 THP), 68.7 (OCH2 THP),
125.1 (2C, m-C), 129.3 (1C, C−Cl), 142.3 (2C, o-C), 185.6 (1C, C−
Ca).
EXPERIMENTAL SECTION
■
General Remarks. All manipulations were carried out under an
inert atmosphere (argon or nitrogen) using standard Schlenk
techniques. The solvents were dried according to common procedures
and distilled under an argon atmosphere; deuterated solvents were
dried over sodium, degassed, and saturated with argon. The yields
given are not optimized. 1H and 13C{1H} NMR spectra were recorded
on Bruker AC 200 MHz, AC 400, and AC 600 spectrometers.
Chemical shifts are reported in parts per million. The residual signal of
benzene-d6 (1H NMR, δ 7.16; 13C NMR, δ 128.0) was used as the
internal standard. The calcium content was determined by
complexometric titration of a hydrolyzed aliquot with 0.05 M EDTA
using Eriochrome BlackT as indicator.47 PhLi and p-tolyllithium−
lithium bromide were prepared according to known procedures.48,49
The coordinated diethyl ether was removed under vacuum, and the
residue was recrystallized from THP.
Crystals of [(p-chlorophenyl)CaI((THP)4] suitable for X-ray
diffraction experiments were obtained by cooling a saturated solution
of the complex in THP from ambient temperature to −20 °C.
Synthesis of [(p-bromophenyl)CaI(THP)4] (3). Activated
calcium (1.2 g, 29.9 mmol) was suspended in tetrahydropyran (40
mL), and 4-bromo-1-iodobenzene (7.07 g, 25 mmol) was added
slowly at −20 °C. The resulting suspension was shaken for 1 h at 0 °C
and for an additional 4 h without cooling. Then the formed precipitate
was removed by filtration through a Schlenk frit, covered with
diatomaceous earth. The residue on the filter was dried in vacuo and
suspended in THP (60 mL), and the suspension was stirred for 1 h at
ambient temperature. The solution obtained after filtration was
combined with the mother liquor. A conversion of 60.7% was
determined by acidimetric consumption of an aliquot of the combined
solutions. After 1 day of storage of the brown solution at −40 °C the
colorless precipitate that formed was collected on a Schlenk frit and
dried in vacuo. Yield: 3.05 g (4.6 mmol, 18.3%). Anal. Calcd for
C26H44CaBrIO4 (667.52 g mol−1): Ca, 6.00. Found: Ca, 6.58. 1H
NMR (THP/benzene-d6 2/1, 400 MHz): δ 1.33 (m, CH2 THP), 1.40
(m, CH2 THP), 3.47 (t, J = 5.2 Hz, CH2 THP), 7.17 (AA′ part of an
AA′BB′ spin system, 2H, m-CH Ph), 7.61 (BB′ part, 2H, o-CH Ph).
13C{1H} NMR (THP/benzene-d6 2/1, 100.6 MHz): δ 23.9 (CH2
THP), 27.0 (CH2 THP), 68.8 (OCH2 THP), 118.2 (1C, C−Br),
127.9 (2C, m-C), 142.9 (2C, o-C), 186.3 (1C, C−Ca).
Synthesis of [(p-tolyl)CaI(THP)4] (1). Method a. Activated
calcium (1.0 g, 25 mmol) was suspended in tetrahydropyran (40
mL), and 4-iodotoluene (4.36 g, 20 mmol) was added at −20 °C. The
resulting suspension was shaken for 6 h without cooling. Thereafter,
unreacted calcium was removed by filtration and the conversion (84%)
was determined by acidic consumption of an aliquot. The reddish
brown solution was stored at −40 °C over 4 days, resulting in a
colorless crystalline precipitate of [(p-tolyl)CaI(THP)4]. This solid
was collected on a cooled Schlenk frit and dried under vacuum. Yield:
4.87 g (8.1 mmol, 40%, crude product).
Method b. Solid [(p-tolyl)CaI(THF)4] was dissolved in the
minimum amount necessary of THP. The resulting solution was
evaporated to dryness under reduced pressure to give [(p-tolyl)CaI-
(THP)4] in essentially quantitative yield.
Synthesis of [(p-iodophenyl)CaI(THP)4] (4). 1,4-Diiodobenzene
(8.25 g, 25 mmol) was slowly added to a suspension of activated
calcium (1.2 g, 29.9 mmol) in tetrahydropyran (40 mL) at −20 °C.
Afterward the resulting suspension was shaken for 1 h at 0 °C and for
an additional 4 h without cooling. Additional tetrahydropyran (20 mL)
was added after 4 h of shaking. Then the unreacted calcium was
removed by filtration through a Schlenk frit, covered with diatoma-
ceous earth. A conversion of 71.3% was determined by acidimetric
consumption of an aliquot of the filtrate. The brown solution was
stored for 1 week at −40 °C. The colorless precipitate that formed was
collected on a cooled Schlenk frit and dried in vacuo. Yield: 3.5 g (4.9
mmol, 19.6%). Anal. Calcd for C26H44CaI2O4 (714.52 g mol−1): Ca,
5.61. Found: Ca, 5.51. 1H NMR (THP/benzene-d6 2/1, 400 MHz): δ
1.34 (m, CH2 THP), 1.40 (m, CH2 THP), 3.46 (t, J = 5.2 Hz, CH2
THP), 7.30 (AA′ part of an AA′BB′ spin system, 2H, m-CH Ph), 7.43
(BB′ part, 2H, o-CH Ph). 13C{1H} NMR (THP/benzene-d6 2/1,
100.6 MHz): δ 23.9 (CH2 THP), 27.0 (CH2 THP), 68.7 (OCH2
THP), 90.0 (1C, C−I), 133.9 (2C, m-C), 143.3 (2C, o-C), 186.8 (1C,
C−Ca).
Anal. Calcd for C27H47CaIO4 (602.65 g mol−1): Ca, 6.65. Found:
1
Ca, 6.62. H NMR (THF/benzene-d6 2/1, 400 MHz): δ 1.34 (m,
16H, CH2 THP), 1.41 (m, 8H, CH2 THP), 2.13 (s, 3H, CH3 tolyl),
3
3.44 (t, 16H, JH,H = 5.2 Hz, OCH2 THP), 6.79 (AA′ part of an
AA′BB′ spin system, 2H, m-CH tolyl), 7.60 (BB′ part, 2H, o-CH
tolyl). 13C{1H} NMR (THF/benzene-d6 2/1, 400 MHz): δ 21.5 (CH3
tolyl), 23.9 (4 × CH2 THP), 27.1 (8 × CH2 THP), 68.6 (8 × CH2
THP), 126.1 (2 × m-CH tolyl), 130.4 (p-C tolyl), 141.0 (2 × o-CH
tolyl), 184.2 (Ca-i-C tolyl). An additional signal set of low intensity of
1
a calcium bound tolyl group was observed: H NMR (THF/benzene-
d6 2/1, 400 MHz): δ 2.02 (s, 3H, CH3 tolyl), 6.80 (AA′ part of an
AA′BB′ spin system, 2H, m-CH tolyl), 8.48 (BB′ part, 2H, o-CH
tolyl). 13C{1H} NMR (THF/benzene-d6 2/1, 400 MHz): 21.2 (CH3
tolyl), 127.0 (2 × m-CH tolyl), 133.9 (p-C tolyl), 143.9 (2 × o-CH
tolyl), 180.1 (Ca-i-C tolyl).
1H NMR (THP/benzene-d6 2/1, 400 MHz): δ 1.35 (m, CH2
3
Recrystallization of the crude product from THP and heptane in a
ratio of 1/1 led to crystals suitable for X-ray diffraction experiments
(ambient temperature to −40 °C).
THP), 1.42 (m, CH2 THP), 2.15 (3H, s, CH3 tolyl), 3.47 (t, JH,H
=
5.2 Hz, OCH2 THP), 6.83 (2H, AA′ part of an AA′BB′ spin system,
m-CH tolyl), 7.62 (2H, BB′ part, o-CH tolyl). 13C{1H} NMR (THP/
benzene -d6 2/1, 400 MHz): δ 21.5 (CH3 tolyl), 23.9 (CH2 THP),
27.0 (CH2 THP), 68.7 (OCH2 THP), 126.3 (2 × m-CH tolyl), 130.7
(p-C tolyl), 141.2 (2 × o-CH tolyl), 183.0 (Ca-i-C tolyl).
Synthesis of [(p-methoxyphenyl)CaI(THP)4] (5). Method a. 4-
Iodoanisole (4.53 g, 19.3 mmol) was slowly added to a suspension of
activated calcium (1.2 g, 29.9 mmol) in tetrahydropyran (35 mL) at
−20 °C. Afterward the resulting suspension was shaken for 1 h at 0 °C
and for an additional 4 h without cooling. Then the unreacted calcium
was removed by filtration through a Schlenk frit, covered with
diatomaceous earth. A conversion of 61% was determined by
acidimetric consumption of an aliquot of the filtrate. The residue on
the filter was extracted with additional THP (20 mL), yielding further
4% of the product, adding up to 65% overall conversion. The washing
solution was discarded while the brown mother liquor was stored for 1
week at −40 °C. The colorless precipitate that formed was collected
on a cooled Schlenk frit and dried in vacuo. Yield: 1.94 g (3.13 mmol,
16.2%).
Crystals of [(p-tolyl)CaI(THP)4] suitable for X-ray diffraction
experiments were obtained by cooling a saturated solution of the
complex in tetrahydropyran from ambient temperature to −20 °C.
Synthesis of [(p-chlorophenyl)CaI(THP)4] (2). Activated calcium
(1.2 g, 29.9 mmol) was suspended in tetrahydropyran (40 mL), and 4-
chloro-1-iodobenzene (5.96 g, 25 mmol) was added slowly at −20 °C.
The resulting suspension was shaken for 1 h at 0 °C and for an
additional 4 h without cooling. Afterward the unreacted calcium was
removed by filtration through a Schlenk frit, covered with diatoma-
ceous earth. A conversion of 59.4% was determined by acidimetric
consumption of an aliquot. After 2 weeks of storage of the brown
solution at −40 °C the colorless precipitate that formed was finally
collected on a Schlenk frit and dried in vacuo. Yield: 4.1 g (6.6 mmol,
Method b. Solid [(p-methoxyphenyl)CaI(THF)4] (0.53 g, 0.94
mmol) was dissolved in THP (20 mL). The resulting yellowish
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solution was stirred for 30 min and afterward slowly reduced to
dryness under reduced pressure. The off-white residue was dried in
vacuo. Then the solid was suspended in n-hexane (20 mL), the
suspension was stirred for 20 min, and the solid was finally collected
on a Schlenk frit. The off-white solid was washed with n-hexane (10
mL) and dried under vacuum. Yield: 0.55 g (0.89 mmol, 94%). Anal.
Calcd for C27H47CaIO5 (618.65 g mol−1): Ca, 6.48. Found: Ca, 6.53.
1H NMR (THP/benzene-d6 2/1, 400 MHz): δ 1.35 (m, CH2 THP),
1.42 (m, CH2 THP), 3.46 (t, J = 5.2 Hz, CH2 THP), 6.65 (AA′ part of
an AA′BB′ spin system, 2H, m-CH Ph), 7.61 (BB′ part, 2H, o-CH Ph).
13C{1H} NMR (THP/benzene-d6 2/1, 100.6 MHz): δ 23.9 (CH2
THP), 27.0 (CH2 THP), 54.1 (1C, O−CH3), 68.7 (OCH2 THP),
111.7 (2C, m-C), 141.5 (2C, o-C), 157.3 (1C, C−O), 176.3 (1C, C−
Ca).
colorless needles that formed were collected on a Schlenk frit, washed
with cold toluene (10 mL) and cold diethyl ether (10 mL), and dried
under vacuum. Yield: 1.16 g (2.48 mmol; 30%) of [(dme)Li(μ2-
CH2Ph)]2·PhCH3. 1H NMR (benzene-d6, 200.1 MHz): δ 1.65 (4H, s,
CH2Ph), 2.11 (3H, s, CH3), 2.86 (8H, s, O−CH2), 2.94 (12H, s, O−
CH3), 6.53 (2H, t, J = 7.0 Hz, p-CH), 6.90 (4H, d, J = 7.6 Hz, o-CH),
7.10 (4H, dd, J = 7.6 Hz, J = 7.0 Hz, m-CH). 13C NMR (benzene-d6,
50.3 MHz): δ 30.4 (1J(13C−1H) = 121.4 Hz, Li−CH2), 58.5
(1J(13C−1H) = 142.6 Hz, O−CH3), 70.5 (1J(13C−1H) = 144.0 Hz,
O−CH2), 111.7 (1J(13C−1H) = 157.1 Hz, p-CH), 120.8 (1J(13C−1H)
= 156.4 Hz, CH), 128.2 (1J(13C−1H) = 163.9 Hz, CH), 159.2 (i-CH).
Synthesis of [(THP)4Li2(μ-Ph)2] (9). A sample of [(Et2O)Li(μ-
Ph)]4, prepared according to a known procedure,48 was dried under
vacuum to remove coordinated diethyl ether. Subsequent recrystalliza-
tion from a THP/heptane mixture (ratio 1/2) gave crystals of 9. The
crystals were suitable for X-ray diffraction experiments. A yield was not
1
In the H NMR spectra, very broad signals of a second arylcalcium
species were observed. The chemical shift points to a bridging p-
1
1
determined. H NMR (THF/benzene-d6 2/1, 400 MHz): δ 1.34 (m,
methoxyphenyl moiety. H NMR (THP/benzene-d6 2/1, 400 MHz):
3
16H, CH2 THP), 1.41 (m, 8H, CH2 THP), 3.44 (t, 16H, JH,H = 5.2
δ ∼6.73 (v br), 8.21 (v br).
Hz, OCH2 THP), 6.91 (m, 2H, p-CH Ph), 7.01 (m, 4H, m-CH Ph),
8.02 (m, 4H, o-CH, Ph).
Crystals of [(p-methoxyphenyl)CaI(THP)4] suitable for X-ray
diffraction experiments were obtained by cooling a saturated solution
of the complex in tetrahydropyran from ambient temperature to −20
°C.
Reactivity Studies. A sample of 1 or 7 was dissolved in 0.4 mL of
THF or THP in a sealable NMR tube. Benzene-d6 (0.2 mL) was
added, and the tube was sealed with a Teflon screw cap afterward. The
sample was immediately measured, stored in a room kept at 22 °C,
and remeasured approximately every 24 h. The residual signal of
benzene-d6 was used as an internal standard for both integration and
chemical shift. The decay of aryl species was followed by integrating
the downfield part of the AA′BB′ spin system of the p-tolyl
substituent.
Observed Calcium-Containing Species. “XCa(O−CHCH2)”:
three different species A−C (ratio approximately 1/2.5/1) in THF;
signals marked with an asterisk overlap with other signals and were
observed via HSQC and HMBC experiments.
Synthesis of [(β-naphthyl)CaI((THP)4] (6). Activated calcium
(0.4 g, 10.0 mmol) was suspended in tetrahydropyran (35 mL), and β-
iodonaphthalene (2.11 g, 8.3 mmol) was added slowly at −20 °C. The
suspension was shaken for 1 h at 0 °C and for an additional 4 h
without cooling. Then the precipitate that formed was removed by
filtration through a Schlenk frit, covered with diatomaceous earth. The
residue on the filter was dried in vacuo and suspended in THP (40
mL), and the suspension was stirred for 1 h at ambient temperature.
The solution obtained after filtration was combined with the mother
liquor. A conversion of 75.6% was determined by acidimetric
consumption of an aliquot of the combined solutions. After 1 week
of storage of the dark violet solution at −40 °C the pale yellow
precipitate that formed was finally collected on a Schlenk frit and dried
in vacuo. Yield: 1.3 g (2.0 mmol, 24.5%). Anal. Calcd for C30H47CaIO4
(638.68 g mol−1): Ca, 6.28. Found: Ca, 6.16. 1H NMR (THP/
benzene-d6 2/1, 600 MHz): δ 1.33 (m, CH2 THP), 1.40 (m, CH2
THP), 3.46 (t, J = 5.2 Hz, CH2 THP), 7.11 (t, 1H, H7), 7.20 (t, 1H,
H6), 7.47 (d, 1H, H4), 7.59 (d, 1H, H5), 7.63 (d, 1H, H8), 7.95 (d,
1H, H3), 8.17 (s, 1H, H1). 13C{1H} NMR (THP/benzene-d6 2/1,
100.6 MHz): δ 23.9 (CH2 THP), 27.0 (CH2 THP), 68.7 (OCH2
THP), 122.5 (2C, C4+C7), 123.6 (1C, C6), 127.1 (1C, C8), 127.9
(1C, C5), 132.6 (1C, C9), 133.6 (1C, C10), 139.5 (1C, C1), 140.4
(1C, C3), 186.8 (1C, C−Ca).
1H NMR (THF/benzene-d6 2/1, 400 MHz): A, δ 3.58 (1H, 
CH2, *), 3.98 (d, JH,H = 13.7 Hz, 1H, CH2), 7.05 (1H, O−CH,
*); B, δ 3.60 (1H, CH2, *), 3.87 (d, JH,H = 13.7 Hz, 1H, CH2),
7.30 (dd, JH,H = 13.8 Hz, JH,H = 5.3 Hz, O−CH); C, δ 3.73 (d, JH,H
= 5.8 Hz, 1H, CH2), 4.08 (d, JH,H = 13.9 Hz, 1H, CH2), 7.18
(1H, O−CH, *). 13C{1H} NMR (THF/benzene-d6 2/1, 400
MHz): A, δ 86.5 (CH2), 156.1 (O−CH); B, δ 85.9 (CH2),
157.1 (O−CH); C, δ 87.7 (CH2), 156.0 (O−CH).
Observed Lithium-Containing Species. “LiOCHCH2”: 1H NMR
(THF/benzene-d6 2/1, 400 MHz) δ 3.31 (ill-resolved dd, 1H, 
CHH′), 3.67 (ill-resolved dd, JH,H = 1.2 Hz, CHH′), 7.00 (dd, JH,H
=
13.4 Hz, JH,H = 5.2 Hz, O−CH); 13C{1H} NMR (THF/benzene-d6
2/1, 400 MHz) δ 81.6 (=CH2), 158.7 (−CH).
Crystals of [(β-naphthyl)CaI((THP)4]·THP suitable for X-ray
diffraction experiments were obtained by cooling a saturated solution
of the complex from ambient temperature to −40 °C.
“LiOCH2CH2CHCH2”: 1H NMR (THF/benzene-d6 2/1, 400
MHz) δ 2.19 (m, 2H, JH,H = 7.2 Hz, JH,H = 7.2 Hz, JH,H = 1.3 Hz,
CH2), 3.78 (t, JH,H = 7.2 Hz, O−CH2), 4.85 (ddt, 1H, JH,H = 10.2 Hz,
JH,H = 2.5 Hz, JH,H = 1.1 Hz, CHH′), 4.94 (ddt, 1H, JH,H = 17.2 Hz,
JH,H = 2.4 Hz, JH,H = 1.3 Hz, =CHH′), 5.89 (m, 1H, =CH−); 13C{1H}
NMR (THF/benzene-d6 2/1, 400 MHz) δ 43.6 (CH2), 65.3 (OCH2),
114.2 (CH2), 138.4 (−CH).
Synthesis of [(THP)4Li2(μ-Br)(μ-tolyl)] (7). [(p-tolyl)Li·LiBr] (0.5
g, 2.7 mmol) was suspended in n-heptane (10 mL). THP (1.5 mL)
was slowly added to the stirred suspension to give a clear yellowish
solution. Then the reaction mixture was stored at −20 °C for 2 days.
The crystals that formed were isolated by decantation and dried
1
1
“LiCH2C6H5”: H NMR (THF/benzene-d6 2/1, 400 MHz) δ 1.66
undervacuum. Yield: 1.02 g (1.93 mmol, 71.5%). H NMR (THP/
(s, 2H, CH2), 5.59 (m, 1H, p-CH Ph), 6.21 (m, 2H, o-CH Ph), 6.45
(m, 2H, m-CH Ph); H NMR (THP/benzene-d6 2/1, 400 MHz) δ
benzene-d6 2/1, 400 MHz): δ 1.34 (m, CH2 THP), 1.41 (m, CH2
THP), 2.14 (3H, s, CH3 tolyl), 3.43 (t, JH,H = 5.1 Hz, OCH2 THP),
1
3
1.62 (s, 2H, CH2), 5.71 (m, 1H, p-CH Ph), 6.32 (m, 2H, o-CH Ph),
6.54 (m, 2H, m-CH Ph); 13C{1H} NMR (THP/benzene-d6 2/1, 400
MHz) δ 35.7 (CH2), 106.2 (p-CH Ph), 117.5 (2 × o-CH Ph), 128.2 (2
× m-CH Ph), 161.1 (i-C Ph).
6.85 (2H, AA′ part of an AA′BB′ spin system, m-CH tolyl), 7.87 (2H,
BB′ part, o-CH tolyl). 13C{1H} NMR (THP/benzene-d6 2/1, 400
MHz): δ 21.6 (CH3 tolyl), 24.0 (CH2 THP), 27.1 (CH2 THP), 68.6
(OCH2 THP), 126.0 (2 × m-CH tolyl), 131.6 (p-C tolyl), 144.5 (2 ×
o-CH tolyl), 179.6 (Li-i-C tolyl).
Suitable crystals of 6 for X-ray diffraction experiments were
obtained directly from the reaction mixture at −20 °C.
Synthesis of [(dme)Li(μ-CH2Ph]2·(toluene) (8). In analogy to a
known procedure,50 a solution of benzyllithium was prepared from
2.35 g (5.5 mmol) of tribenzyltin chloride and 4 equiv of
methyllithium in diethyl ether (25 mL). The precipitate of LiCl that
formed was removed by filtration, and the mother liquor was reduced
to dryness in vacuo. The resulting residue was recrystallized using a
mixture of toluene and DME (ambient temperature to −40 °C). The
X-ray Structure Determination. The intensity data for the
compounds were collected on a Nonius KappaCCD diffractometer
using graphite-monochromated Mo Kα radiation. Data were corrected
for Lorentz and polarization effects but not for absorption effects.51,52
The structures were solved by direct methods (SHELXS53) and
2
refined by full-matrix least-squares techniques against Fo (SHELXL-
9753). The hydrogen atom bonded to the methylene groups C1 and
C8 of compound 8 and all hydrogen atoms of compound 10 were
located by difference Fourier synthesis and refined isotropically. All
other hydrogen atoms were included at calculated positions with fixed
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Organometallics
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thermal parameters. All nondisordered, non-hydrogen atoms were
refined anisotropically.53 Crystallographic data as well as structure
solution and refinement details are summarized in Table SI1
(Supporting Information). XP (SIEMENS Analytical X-ray Instru-
ments, Inc.) was used for structure representations.
(8) Krieck, S.; Gorls, H.; Westerhausen, M. J. Am. Chem. Soc. 2010,
̈
132, 12492−12501.
(9) Fischer, R.; Gartner, M.; Gorls, H.; Yu, L.; Reiher, M.;
̈
̈
Westerhausen, M. Angew. Chem. 2007, 119, 1642−1647; Angew.
Chem., Int. Ed. 2007, 46, 1618−623.
(10) Gartner, M.; Gorls, H.; Westerhausen, M. J. Organomet. Chem.
̈
̈
ASSOCIATED CONTENT
* Supporting Information
2008, 693, 221−227.
■
(11) Mochida, K.; Yamanishi, T. J. Organomet. Chem. 1987, 332,
247−252.
S
Figures giving NMR spectra, Table SI1, giving crystal data and
refinement details for the X-ray structure determinations of
compounds 1−9, and CIF files giving crystallographic data for
1−9. This material is available free of charge via the Internet at
http://pubs.acs.org. Crystallographic data (excluding structure
factors) has also been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication
CCDC-866599 for 1, CCDC-866600 for 2, CCDC-866601 for
3, CCDC-866602 for 4, CCDC-866603 for 5, CCDC-866604
for 6, CCDC-866607 for 7, CCDC-866610 for 8, and CCDC-
866606 for 9. Copies of the data can be obtained free of charge
on application to the CCDC, 12 Union Road, Cambridge CB2
1EZ, U.K. (e-mail: deposit@ccdc.cam.ac.uk).
(12) Langer, J.; Krieck, S.; Fischer, R.; Gorls, H.; Westerhausen, M. Z.
̈
Anorg. Allg. Chem. 2010, 636, 1190−1198.
(13) This species was misleadingly assigned earlier on as a calcium
ethenolate.6
(14) Langer, J.; Krieck, S.; Gorls, H.; Westerhausen, M. Angew.
̈
Chem., Int. Ed. 2009, 48, 5741−5744; Angew. Chem. 2009, 121, 5851−
5854.
(15) Fischer, R.; Langer, J.; Krieck, S.; Gorls, H.; Westerhausen, M.
̈
Organometallics 2011, 30, 1359−1365.
(16) Reich, H. J.; Green, D. P.; Medina, M. A.; Goldenberg, W. S.;
Gudmundsson, B. O.; Dykstra, R. R.; Phillips, N. H. J. Am. Chem. Soc.
̈
1998, 120, 7201−7210.
(17) Gartner, M.; Gorls, H.; Westerhausen, M. Organometallics 2007,
̈
̈
26, 1077−1083.
(18) Langer, J.; Gorls, H.; Westerhausen, M. Organometallics 2010,
̈
AUTHOR INFORMATION
Corresponding Author
*E-mail: m.we@uni-jena.de. Fax: +49 (0) 3641 948132.
■
29, 2034−2039.
(19) Fischer, R.; Gorls, H.; Westerhausen, M. Organometallics 2007,
̈
26, 3269−3271.
(20) Langer, J.; Gartner, M.; Fischer, R.; Gorls, H.; Westerhausen, M.
̈
̈
Notes
Inorg. Chem. Commun. 2007, 1001−1004.
(21) Krieck, S.; Gorls, H.; Westerhausen, M. J. Organomet. Chem.
2009, 694, 2204−2209.
(22) Krieck, S.; Gorls, H.; Yu, L.; Reiher, M.; Westerhausen, M. J.
Am. Chem. Soc. 2009, 131, 2977−2985.
(23) Hevia, E.; Mulvey, R. E. Angew. Chem., Int. Ed. 2011, 50, 6448−
6450; Angew. Chem. 2011, 123, 6576−6578.
The authors declare no competing financial interest.
̈
ACKNOWLEDGMENTS
■
̈
We thank the German Research Foundation (DFG, Bonn/
Germany) for financial funding. F.D. is also grateful to the
German Academic Exchange Service (DAAD, Bonn, Germany)
for support within the International Association for the
Exchange of Students for Technical Experience (IAESTE
program).
(24) For a review on ether fission by organoalkalimetal compounds
see: Maercker, A. Angew. Chem., Int. Ed. Engl. 1987, 26, 972−989;
Angew. Chem. 1987, 99, 1002−1019.
(25) For NMR data of lithium ethenolates see: (a) Andrews, P. C.;
Bull, S. D.; Koutsaplis, M. New J. Chem. 2010, 34, 1678−1691.
(b) Wen, J. Q.; Grutzner, J. B. J. Org. Chem. 1986, 51, 4220−4224.
(c) Oakes, F. T.; Yang, F.-A.; Sebastian, J. F. J. Org. Chem. 1982, 47,
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