Directed Ortho Calciation of 1,3-Bis(3-isopropylimidazol-2-ylidene)benzene

Article abstract of DOI:10.1021/acs.organomet.7b00303

The deprotonation of 1,3-bis(3-isopropylimidazol-2-ylidene)benzene with Me₃SiCH₂CaX (X = Br, I) in tetrahydrofuran (THF) yields the ether adducts of the corresponding 2,6-bis(3-isopropylimidazol-2-ylidene)phenylcalcium halides (X = B

Full text of DOI:10.1021/acs.organomet.7b00303

Article
pubs.acs.org/Organometallics
Directed Ortho Calciation of 1,3-Bis(3-isopropylimidazol-2-
ylidene)benzene
Alexander Koch, Sven Krieck, Helmar Gorls, and Matthias Westerhausen*
̈
Friedrich Schiller University Jena, Institute of Inorganic and Analytical Chemistry Humboldtstrasse 8, D-07743 Jena, Germany
S
* Supporting Information
ABSTRACT: The deprotonation of 1,3-bis(3-isopropylimida-
zol-2-ylidene)benzene with Me3SiCH2CaX (X = Br, I) in
tetrahydrofuran (THF) yields the ether adducts of the
corresponding 2,6-bis(3-isopropylimidazol-2-ylidene)-
phenylcalcium halides (X = Br (1·2thf), I (2·2thf)). The
crystallization behavior of 2 can be improved via substitution of
ligated thf molecules by tetrahydropyran (thp) ligands, leading
to 2·2thp. These heteroleptic complexes 1·2thf and 2·2thp show
very small Ca−Cipso bond lengths to the ipso-carbon atoms of
the aryl groups. Calciation of 1,3-bis(3-isopropylimidazol-2-
ylidene)benzene with Ca(CH2SiMe3)2 in THP leads to the
formation of ether-free homoleptic bis[2,6-bis(3-isopropylimi-
dazol-2-ylidene)phenyl]calcium (3). Intramolecular steric strain
causes an elongation of the Ca−Cipso bonds to the aryl groups. In all of these complexes, the Ca−Ccarbene distances are
significantly larger than those to the ipso-carbon atoms of the aryl groups.
reactivity of solid calcium turnings or calcium powders is too
low. Nevertheless, undesired side reactions had been observed,
leading to degradation of the substrate; thus, the reduction of
2,6-bis(diphenylphosphanylmethyl)phenyl halide with calcium
led to P−C bond cleavage and formation of diphenylphosphi-
nates in addition to other products.2 Very acidic organic
substrates such as cyclopentadienes3 can be calciated, but less
acidic compounds such as amines4−6 and thiols7 can only be
deprotonated in the presence of anhydrous ammonia but not in
common organic solvents. In these latter cases, organo-
potassium reagents were reacted with calcium iodide in a
metathetical approach, yielding insoluble KI and organocalcium
complexes. Other strategies favor transamination or metalation
protocols, as achieved for the synthesis of bis(alkynyl)calcium
derivatives.8 For straightforward calciations, organocalcium
reagents are required and the success of this strategy depends
on the kinetic pKa values9 and on the availability of suitable
reagents. The straightforward and multigram synthesis of
bis(trimethylsilylmethyl)calcium provides such a powerful
calciation reagent.10 If a substrate contains several hydrogen
atoms of comparable acidity, functional groups often serve as
anchors (binding sites) for the metalation reagent and lead to
directed ortho metalation (DOM), a typical domain for
organolithium and to a lesser extent organomagnesium
reagents.11 Such directed metalations have already been studied
using phenylcalcium complexes; these reagents deprotonated
1,3-dimethoxybenzene at the 2-position, yielding tris(2,6-
INTRODUCTION
■
Metalation reactions are the substitution of a hydrogen atom by
a metal atom. There are two different procedures. In the direct
metalation, the metal itself is employed for the deprotonation
and, commonly, this reaction is a heterogeneous procedure.
The “classic” metalation uses an organometallic reagent for the
deprotonation, and advantageously, this reaction is performed
under homogeneous conditions (Scheme 1). However, the
organometallic reagent has to be prepared in an initial step if it
is not commercially available.
In the form of commercially available calcium turnings, the
metal itself is rather inert and only soluble in liquid ammonia,
forming a deep blue solution. These solutions are extremely
reactive with strongly reducing properties. Therefore, Birch-
type reductions often represent the major reaction pathway if
aromatic substrates are used.1 For many direct metalations the
Scheme 1. Direct and “Classic” Metalation Reactions of
Organic Substrates with Acidic Hydrogen Atoms (DMG =
Directed Metalation Group)
Received: April 20, 2017
© XXXX American Chemical Society
A
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dimethoxyphenyl)dicalcium iodide.12 In another example, the
deprotonation of 3-bromothiophene gave 3-bromothien-2-yl-
calcium complexes which could be crystallized and structurally
characterized as the 18-crown-6 adduct of bis(3-bromothien-2-
yl)calcium.13 However, in contrast to the easily achieved ortho
lithiation of phenyloxazolines and N,N-dimethylbenzamide,11
ortho calciation proved to be quite challenging and far from a
straightforward procedure. Due to our finding that the
coordination pocket of 2,6-bis(imidazol-2-ylidene)pyridine
exhibits the ideal size to tightly bind calcium ions,14 we studied
the deprotonation of isoelectronic 1,3-bis(imidazol-2-ylidene)-
benzene (Scheme 2) and the coordination behavior of its anion
Scheme 3. Synthesis of the [2,6-Bis(3-isopropylimidazol-2-
ylidene)phenyl]calcium Halides (X = Br (1·2thf), I (2·2thp))
via Metalation of 1,3-Bis(3-isopropylimidazol-2-
ylidene)benzene
Scheme 2. Comparison of the Isoelectronic Pyridyl and
Phenyl Backbones at a Metal Center (Shown in Blue) of the
Fragments Ca{NC5H3-2,6-(NHCR)2} (Left) and Ca{C6H3-
2,6-(NHCR)2} (Right)
Scheme 4. Synthesis of Bis[2,6-bis(3-isopropylimidazol-2-
ylidene)phenyl]calcium (3) via Reaction of 1,3-Bis(3-
isopropylimidazol-2-ylidene)benzene with Ca(CH2SiMe3)2
at calcium ions to elucidate the suitability of carbene side arms
as directing groups. Furthermore, formation of side products
should be excluded as have been reported for alkali-metal-
mediated magnesiation reactions leading to metalation at
various sites, introduced as normal, abnormal, and “para-
normal” reactivity patterns of N,N′-disubstituted carbenes.15
RESULTS AND DISCUSSION
■
sensitive toward moisture and air and very soluble, hampering
a higher crystalline yield.
The NMR spectra show characteristic low-field shifts of the
carbene carbons around 200 ppm (Table 1). For the starting
The heavy Grignard reagents Me3SiCH2CaX (X = Br, I)16 are
easily accessible on a multigram scale with good yields
according to literature procedures, and the dialkylcalcium
10
reagent Ca(CH2SiMe3)2 can be readily prepared in situ. Due
Table 1. Comparison of the 13C{1H} Chemical Shifts (ppm)
of 1,3-Bis(3-isopropylimidazol-2-ylidene)benzene and Its
Lithiated and Calciated Derivatives
to the straightforward syntheses of these reagents, isolation and
purification via crystallization was not necessary. The
concentrations of these alkylcalcium solutions were determined
by acid−base titration of aliquots. These solutions were used
for the calciation studies.
compound
δ(Ccarbene
)
δ(CiC) δ(CoC
)
δ(CmC
)
δ(CpC)
C6H4-1,3-(NHCiPr
)
213.2
200.0
197.3
128.1
169.8
166.7
142.5
110.8
112.5
108.9
115.5
124.6
124.9
The reaction of 1,3-bis(3-isopropylimidazol-2-ylidene)-
benzene, 1,3-(NHCiPr)2C6H4, with a slight excess of (trime-
thylsilylmethyl)calcium bromide and iodide in tetrahydrofuran
gave the corresponding bis(tetrahydrofuran) adducts of the
[2,6-bis(3-isopropylimidazol-2-ylidene)phenyl]calcium halides
([(thf)2Ca(X)C6H3-2,6-(NHCiPr)2]; X = Br (1·2thf), I (2·
2thf)) with moderate yields (Scheme 3). To improve the
crystallization behavior in order to obtain single crystals, [2,6-
bis(3-isopropylimidazol-2-ylidene)phenyl]calcium iodide was
recrystallized from tetrahydropyran (THP), leading to an
exchange of the ligated ether bases, and the bis(thp) complex
(2·2thp, [(thp)2Ca(I)C6H3-2,6-(NHCiPr)2]) was isolated.
Calciation of 1,3-bis(3-isopropylimidazol-2-ylidene)benzene
with Ca(CH2SiMe3)2, prepared in tetrahydropyran according to
a literature protocol10 without intermediate isolation of this
dialkylcalcium reagent, led to the formation of ether-free
bis[2,6-bis(3-isopropylimidazol-2-ylidene)phenyl]calcium (3,
[Ca{C6H3-2,6-(NHCiPr)2}2]; Scheme 4). The NMR spectro-
scopic monitoring of the reaction verified a complete
conversion, but this diarylcalcium complex is extremely
2
Li-C6H3-2,6-(NHCiPr
)
153.6
150.4
2
[(thf)2Ca(Br)C6H3-2,6-
(NHCiPr)2] (1)
[(thp)2Ca(I)C6H3-2,6-
197.1
199.9
165.8
170.2
150.2
151.3
108.9
108.7
125.0
124.3
(NHCiPr)2] (2)
[Ca{C6H3-2,6-
(NHCiPr)2}2] (3)
1,3-bis(3-isopropylimidazol-2-ylidene)benzene, a chemical shift
of δ(13Ccarbene{1H}) 213.2 ppm was observed, whereas a high-
field shift of approximately 15 ppm was found upon
coordination to calcium and lithium ions. The chemical shifts
of the ipso-carbon atoms of the phenyl ring are again very
similar for lithium and calcium complexes, and resonances
around 170 ppm were detected. Furthermore, the phenyl
carbon atoms, bound to the N-heterocyclic carbenes, also
experience a significant low-field shift in comparison to 1,3-
bis(3-isopropylimidazol-2-ylidene)benzene. In contrast to the
phenyl group, the 4- and 5-positioned carbon atoms of the N-
B
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heterocyclic carbene moieties are not affected by the
coordination to these s-block metal ions.
The crystal structures of the calcium complexes 1−3 were
determined to elucidate steric requirements and shielding of the
alkaline-earth metal. The molecular structure and numbering
scheme of [2,6-bis(3-isopropylimidazol-2-ylidene)phenyl]calci-
um bromide (1·2thf, [(thf)2Ca(Br)C6H3-2,6-(NHCiPr)2]) are
depicted in Figure 1. The Ca1−C9ipso bond length of 247.0(4)
Figure 2. Molecular structure and numbering scheme of [2,6-bis(3-
isopropylimidazol-2-ylidene)phenyl]calcium iodide (2·2thp). Ellip-
soids represent a probability of 30%, and H atoms are omitted for
clarity. Selected parameters: bond lengths (pm), Ca1−I1 308.40(13),
Ca1−O1 243.4(5), Ca1−O2 242.1(6), Ca1−C1 261.3(7), Ca1−C9
244.6(7), Ca1−C12 259.1(7), N1−C1 134.6(9), N2−C1 137.5(9),
N3−C12 138.6(9), N4−C12 137.0(9); angles (deg), C1−Ca1−C9
66.1(2), C1−Ca1−C12 133.2(2), C9−Ca1−C12 67.1(2), C9−Ca1−
I1 172.7(2).
Figure 1. Molecular structure and numbering scheme of [2,6-bis(3-
isopropylimidazol-2-ylidene)phenyl]calcium bromide (1·2thf). Ellip-
soids represent a probability of 30%, and H atoms are neglected for the
sake of clarity. Selected parameters: bond lengths (pm), Ca1−Br1
287.81(8), Ca1−O1 240.7(5), Ca1−O2 241.6(3), Ca1−C1 266.1(4),
Ca1−C9 247.0(4), Ca1−C12 264.4(4), N1−C1 136.1(4), N2−C1
137.1(5), N3−C12 137.2(5), N4−C12 135.7(5); angles (deg), C1−
Ca1−C9 65.76(11), C1−Ca1−C12 132.14(11), C9−Ca1−C12
66.38(11), C9−Ca1−Br1 170.34(9), Ca1−C9−C4 121.2(2), Ca1−
C9−C8 119.5(2), C4−C9−C8 113.7(3).
longer Ca−C bonds of 248.3(5) pm.19 Furthermore, longer
Ca−C bond lengths are also observed for monomeric and
dimeric calcium methanediides due to the stabilization of these
complexes by charge delocalization into peripheric diphenyl-
phosphoryl substituents.20 This short Ca1−C9ipso bond in 2·
2thp has the effect of pulling the calcium atom into the
coordination pocket, also enforcing small Ca1−Ccarbene values
(average 260.2 pm) and large N1−C1−Ca1 and N4−C12−
Ca1 bond angles of 142.6(5) and 144.2(5)°, respectively. The
Ca1−I1 bond length of 308.4(1) pm lies in the large range of
characteristic values;13,17,21 the quite large variation of Ca−I
distances is caused by the softness of the iodide and, hence, its
polarizability.
The molecular structure and numbering scheme of bis[2,6-
bis(3-isopropylimidazol-2-ylidene)phenyl]calcium (3) is de-
picted in Figure 3. The asymmetric unit consists of two
molecules A and B; due to the similarity of these molecules
only molecule B is shown in this representation. Due to the
rather small bite of the tridentate aryl base, the calcium is in a
distorted-octahedral environment and is bound exclusively to
carbon atoms. The Ca1−Cipso distances are longer (average
values: A, 249.3 pm; B, 251.8 pm) than those observed for the
heteroleptic arylcalcium halides, verifying enhanced intra-
molecular strain. This finding is in agreement with elongated
Ca1−Ccarbene distances with average values of 266.3 and 266.7
pm for molecules A and B, respectively. A comparison of
[Ca{C6H3-2,6-(NHCiPr)2}2] (3) with [(thf)Ca{NC5H3-2,6-
(NHCMes)2}2]14 shows significant differences. The Ca−NPy
distances to the pyridyl backbones exhibit only slightly larger
values of 252.4(3) and 260.7(3) pm (in comparison to Ca−
Cipso of 3); however, intramolecular π stacking of the mesityl
groups attached to the NHC fragments opens a coordination
gap for an additional thf molecule. The outer N-bound
isopropyl groups attached to the NHC moieties hinder such
an approach between the ligands.
pm is rather short and significantly shorter than the Ca−Ccarbene
distances with an average value of 265.3 pm. The calcium atom
is strongly pulled toward the ipso-carbon atom, leading to large
N1−C1−Ca1 and N4−C12−Ca1 bond angles of 143.6(3) and
144.6(3)°, respectively. The Ca1−Othf distances lie in the
expected range due to lack of severe intramolecular strain. The
Ca1−Br1 bond length of 287.81(8) pm is a characteristic value,
as also found for other arylcalcium bromides with six-
coordinate calcium atoms in distorted-octahedral environ-
ments.13,17 A remarkable feature, however, is the fact that the
calcium atom deviates from the aryl plane, leading to an angle
sum around the ipso-carbon atom of only 354.4°.
The molecular structure and numbering scheme of [2,6-
bis(3-isopropylimidazol-2-ylidene)phenyl]calcium iodide (2·
2thp) are shown in Figure 2. In this complex, tetrahydropyran
molecules are bound to the calcium atom due to an improved
crystallization behavior from this ether. Due to weaker Lewis
basicity, slightly larger Ca1−Othp distances with an average
value of 242.7 pm are observed. This weaker donor strength of
the ether ligands leads to an even shorter Ca1−C9ipso bond
length of 244.6(7) pm, among the smallest Ca−C bond lengths
ever found in molecular organocalcium compounds. Even in
ether-free [Ca{C(SiMe3)3}2] with a two-coordinate calcium
atom and a slightly bent C−Ca−C moiety (C−Ca−C
149.7(6)°), longer Ca−C distances of 245.9(9) pm have
been observed.18 The dialkylcalcium complex [(diox)2Ca{CH-
(SiMe3)2}2] with a four-coordinate calcium center also exhibits
In Table 2, selected bond lengths of arylcalcium complexes
are given. For comparison, selected alkylcalcium derivatives and
C
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Article
smaller coordination numbers lead to shorter Ca−C and Ca−X
bonds. However, this influence is reduced because smaller
coordination numbers are usually stabilized with bulky
substituents at the periphery of the molecule, which also
cause intramolecular strain. This intramolecular steric repulsion
between the bulky ortho substituents causes an elongation of
the Ca−C bonds of diarylcalcium 3. In addition, poorly
shielded calcium ions which are highly Lewis acidic often
undergo agostic interactions with σ bonds of organic
substituents.
CONCLUSION
■
In this project, we demonstrated that (trimethylsilylmethyl)-
calcium halides (heavy Grignard reagents) as well as
homoleptic bis(trimethylsilylmethyl)calcium are valuable metal-
ation (calciation) reagents. The metalation force of these
alkylcalcium derivatives allows efficient deprotonation of
substituted arenes in a directed ortho metalation, using N-
heterocyclic carbenes as directing groups. Side reactions, found
for alkali-metal-mediated magnesiation reactions, have not been
observed. In addition, ether cleavage reactions or attack of the
carbene side arms does not represent a severe challenge in
these deprotonation reactions. Therefore, a straightforward
calciation of 1,3-bis(3-isopropylimidazol-2-ylidene)benzene in
tetrahydrofuran allows the synthesis of [2,6-bis(3-isopropyli-
midazol-2-ylidene)phenyl]calcium derivatives. In all of these
complexes, the calcium centers are in distorted-octahedral
environments; two ether molecules (thf or thp) complete the
coordination spheres of the heteroleptic arylcalcium halides 1
and 2, whereas diarylcalcium 3 crystallizes without additional
ether molecules.
The 2,6-bis(3-isopropylimidazol-2-ylidene)phenyl groups
show the smallest known Ca−Cipso bond lengths in the
bis(ether) adducts of heteroleptic [2,6-bis(3-isopropylimidazol-
2-ylidene)phenyl]calcium halides (1·2thf and 2·2thp). In the
homoleptic complex bis[2,6-bis(3-isopropylimidazol-2-
ylidene)phenyl]calcium (3), the rather bulky 2,6-bis(3-
isopropylimidazol-2-ylidene)phenyl groups induce intramolec-
ular strain that can be recognized at the elongated Ca−Cipso
Figure 3. Molecular structure and numbering scheme of bis[2,6-bis(3-
isopropylimidazol-2-ylidene)phenyl]calcium (3). Ellipsoids represent a
probability of 30%, and H atoms are neglected for the sake of clarity.
Selected parameters of molecule A [molecule B]: bond lengths (pm),
Ca1−C1 269.6(3) [270.4(3)], Ca1−C9 248.7(3) [251.5(3)], Ca1−
C12 265.6(3) [263.8(3)], Ca1−C19 265.6(3) [263.2(3)], Ca1−C27
249.8(3) [252.0(3)], Ca1−C30 264.3(3) [269.3(3)]; angles (deg),
C1−Ca1−C9 65.34(10) [64.71(11)], C1−Ca1−C12 130.68(10)
[129.73(10)], C9−Ca1−C12 65.35(10) [65.35(10)], C19−Ca1−
C27 65.41(10) [64.84(10)], C19−Ca1−C30 130.07(10)
[129.00(10)], C27−Ca1−C30 65.18(11) [64.55(10)], C9−Ca1−
C27 156.45(10) [158.24(11)].
ether adducts of calcium dihalides are included. From these
values the influence of the donor strength of the ether coligands
is evident. Even though the Ca−O bond lengths vary in a rather
narrow range, a characteristic influence on the distances
between calcium and the anion (halide or aryl) is observed.
The weaker Lewis basicity of thp in comparison to thf leads to a
significant shortening of the corresponding Ca−X (X = Br, I)
and Ca−C bond lengths. Furthermore, also the influence of the
halide (Br or I) on the Ca−C bond lengths in heteroleptic
Grignard-type complexes can easily be elucidated. In
arylcalcium halides, the Ca−C distance is smaller for the
iodides, whereas this tendency seems to be reversed for
alkylcalcium halides. In addition to these electronic effects,
steric requirements cannot be neglected completely. Generally,
Table 2. Comparison of Structural Data of Selected Aryl- And Alkylcalcium Compounds (Bond Lengths (pm) and Angles (deg);
pTol = 4-Methylphenyl, p-Tolyl)
compound
Ca−C
Ca−X
Ca−O
C−Ca−X
ref
[(thf)2Ca(Br)C6H3-2,6-(NHCiPr)2] (1)
[(thp)2Ca(I)C6H3-2,6-(NHCiPr)2] (2)
[Ca{C6H3-2,6-(NHCiPr)2}2] (3)
[(thf)4Ca(Br)C6H5]
247.0
244.6
249.3
258.3
257.4
251.0
257.4
251.7
250.0
252.7
258.6
249.3
245.9
287.8
308.4
240.7, 241.6
242.1, 243.4
170.3
172.7
this work
this work
a
156.5
this work
22
289.0
317.8
312.1
320.8
307.5
289.8
319.1
238.4−239.5
237.0−239.2
240.2−244.7
239.3−241.9
234.4−239.0
236.9−241.3
238.9−241.1
178.5
177.4
171.8
177.4
109.2
168.7
170.5
[(thf)4Ca(I)C6H5]
22
[(thp)4Ca(I)C6H5]
23
[(thf)4Ca(I)C6H2-2,4,6-Me3]
[(thf)3Ca(I)C6H3-2,6-pTol2]
[(thp)4Ca(Br)CH2SiMe3]
[(thp)4Ca(I)CH2SiMe3]
[(tmeda)2Ca(CH2SiMe3)2]
[(thf)2Ca{CH(SiMe3)2}2]
[Ca{C(SiMe3)3}2]
24
25
13
16
b
c
261.4
177.3
26
c
233.9
135.5
27
c
149.7
18
d
[(thf)4CaBr2]
288.5
284.0
311.3
307.8
236.4, 236.9
236.6−237.9
234.9, 239.5
235.7−237.5
180
28
d
[(thp)4CaBr2]
179.3
21
d
[(thf)4CaI2]
180
29
d
[(thp)4CaI2]
177.7
21
a
b
c
d
C9−Ca1−C27 bond angle. Average Ca−N bond length. C−Ca−C bond angle. X−Ca−X bond angle.
D
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Synthesis of [2,6-Bis(3-isopropylimidazol-2-ylidene)phenyl]-
calcium Bromide (1·2thf). (Bromomethyl)trimethylsilane was
reduced with activated calcium in THF at 0 °C. The reaction
suspension was filtered, yielding a 0.11 M solution of [(thf)4Ca(Br)-
(CH2SiMe3)] in THF. From this solution, 5 mL (1.32 equiv) was
added to a solution of 121 mg of 2,6-bis(3-isopropyl)imidazol-2-
ylidene)benzene (0.4 mmol, 1 equiv) in 4 mL of THF at −78 °C.
After it was warmed to room temperature, the reaction mixture was
filtered and the volume reduced to 4 mL and a few milliliters of
pentane were added. Storing of this mother liquor at −40 °C led to the
crystallization of 146 mg of [(thf)2Ca(Br)C6H3-2,6-(NHCiPr)2]·3THF
bonds. In summary, (trimethylsilylmethyl)calcium halides
(heavy Grignard reagents) and homoleptic bis-
(trimethylsilylmethyl)calcium represent valuable metalation
reagents with a strong deprotonation power; nevertheless,
side reactions do not dominate the reactivity and, therefore,
isolation of calciated complexes can be achieved in a
straightforward manner. N-Heterocyclic carbene side arms are
effective directing groups for ortho deprotonation of arenes and
are not attacked by these strong metalation reagents. This class
of tridentate CCC-NHC pincer ligands has gained more and
more importance as ligands in catalytic synthesis:30 e.g.,
hydroelementation reactions, paying special attention to hard
Lewis acid catalyzed hydroamination reactions of unactivated
alkenes.31,32
1
([1·2thf]·3THF, 47%). H NMR (300.13 MHz, 25 °C, [D8]THF): δ
7.62 (2H, d, 3JH−H = 1.6 Hz), 7.16 (2H, d, 3JH−H = 1.6 Hz), 6.99 (3H,
3
m), 4.83 (2H, sept, JH−H = 6.6 Hz), 3.64 (28 H, m), 1.79 (28H, m),
3
1.55 (12H, d, JH−H = 6.6 Hz) ppm. 13C{1H} NMR (100.6 MHz, 25
°C, [D8]THF): δ 197.3, 166.7, 150.4, 124.9, 116.2, 115.1, 108.9, 67.2,
52.6, 25.4, 22.2 ppm. Anal. Calcd: Ca, 5.15. Found: Ca, 5.23. Single
crystals were obtained by recrystallization in THF/pentane and
storage at −40 °C.
EXPERIMENTAL SECTION
■
General Remarks. All manipulations were carried out under an
inert nitrogen atmosphere using standard Schlenk techniques, if not
otherwise noted. THF, THP, and pentane were dried over KOH and
subsequently distilled over sodium/benzophenone under a nitrogen
atmosphere prior to use. DMSO was dried over CaH2. Deuterated
solvents were dried over sodium, distilled, degassed, and stored under
Synthesis of [2,6-Bis(3-isopropylimidazol-2-ylidene)phenyl]-
calcium Iodide (2·2thp). A 0.14 M THP solution of [(thp)4Ca(I)-
(CH2SiMe3)] (4.5 mL, 0.65 mmol, 1.05 equiv) was added at −78 °C
to a solution of 179 mg of 2,6-bis(3-isopropylimidazol-2-ylidene)-
benzene (0.6 mmol, 1 equiv) in 10 mL of THF. After it was warmed to
room temperature, the reaction mixture was filtered, the solvent was
changed to THP, and this THP solution was stored at −40 °C, leading
to crystallization of 213 mg of [(thp)2Ca(I)C6H3-2,6-(NHCiPr)2]·
1.5THP (44%) as long needles. 1H NMR (300.13 MHz, 25 °C,
1
nitrogen over sodium. H and 13C{1H} NMR spectra were recorded
on Bruker Avance 400 and Fourier 300 spectrometers. Chemical shifts
are reported in parts per million relative to SiMe4 as an external
standard referenced to the solvent residual proton signal. All substrates
were purchased from TCI, ABCR, or Alfa Aesar and used without
further purification. The yields given are not optimized. Calcium metal
was activated according to a standard procedure.17e The starting 2,6-
bis(3-isopropylimidazolium)benzene diiodide was prepared according
to literature protocols,32,33 as also described with slight variations in
the Supporting Information. Purity was monitored by NMR
spectroscopic measurements. Due to the air and moisture sensitivity
of the compounds analytical characterizatio, e.g. combustion analyses,
were challenging. Despite the fact that V2O5 was added, the values of
the combustion analysis occasionally deviate from theoretical values,
presumably due to carbonate formation and partial loss of intercalated
solvent during handling and weighing.
3
3
[D8]THF): δ 7.63 (2H, d, JH−H = 1.6 Hz), 7.18 (2H, d, JH−H = 1.6
Hz), 6.99 (3H, m), 4.91 (2H, sept, 3JH−H = 6.6 Hz), 3.56 (14.4 H, m),
1.62 (7.3 H, m), 1.52 (27H, m) ppm. 13C{1H} NMR (100.6 MHz, 25
°C, [D8]THF): δ 197.1, 165.8, 150.2, 125.0, 116.1, 115.2, 108.9, 68.0,
52.6, 26.6 ppm. Anal. Calcd: Ca, 5.16. Found: Ca, 5.45.
Synthesis of Bis[2,6-bis(3-isopropylimidazol-2-ylidene)-
phenyl]calcium (3). KCH2SiMe3 (87 mg, 0.68 mmol, 1.05 equiv)
was dissolved at 0 °C in 5 mL of THP. A 0.088 M THP solution of
[(thp)4Ca(I)(CH2SiMe3)] (7.45 mL, 0.65 mmol, 1 equiv) was added
to this solution, immediately forming a suspension. This mixture was
stirred at 0 °C for 1 h before 370 mg of 2,6-bis(3-isopropylimidazol-2-
ylidene)benzene (1.25 mmol, 1.92 equiv) was added at −40 °C. The
suspension was warmed to room temperature, and all solids were
removed by filtration. The volume of the filtrate was reduced to ca. 3
mL, and this mother liquor was stored at −40 °C, leading to
crystallization of 38 mg of [Ca{C6H3-2,6-(NHCiPr)2}2]·1.1THP (8%)
in the shape of long needles. 1H NMR (300.13 MHz, 25 °C,
[D8]THF): δ 7.53 (2H, d, 3JH−H = 1.7 Hz), 7.00 (3H, m), 6.93 (2H, d,
3JH−H = 1.7 Hz), 4.04 (2H, sept, 3JH−H = 6.5 Hz), 3.56 (7.1H, m), 1.62
Synthesis of 2,6-Bis(3-isopropylimidazol-2-ylidene)benzene.
KN(SiMe3)2 (1.85 g, 9.25 mmol, 2.05 equiv), dissolved in THF, was
added at −78 °C to a suspension of 2.48 g of 2,6-bis(3-
isopropylimidazolium)benzene diiodide (4.5 mmol) in 40 mL of
THF. Warming to room temperature and stirring overnight led to a
light orange suspension. Depending on the quality of applied
KN(SiMe3)2, a deep red suspension is sometimes observed. All solids
were removed by filtration. Afterward all volatiles were removed under
vacuum, leading to a cream-colored solid. In the case of a red
suspension the solid was further washed with diethyl ether, finally
leading to 780 mg of 2,6-bis(3-isopropylimidazol-2-ylidene)benzene
3
(3.6H, m), 1.52 (7.1H, m), 1.06 (12H, d, JH−H = 6.5 Hz) ppm.
13C{1H} NMR (100.6 MHz, 25 °C, [D8]THF): δ 199.9, 170.2, 151.3,
124.3, 116.2, 114.4, 108.7, 68.8, 52.1, 26.6, 23.3, 23.1 ppm. Anal.
Calcd: Ca, 5.49. Found: Ca, 5.55.
1
(60%). H NMR (400.13 MHz, 25 °C, [D8]THF): δ 8.34 (1H, t,
3
3JH−H = 2.1 Hz), 7.70 (2H, dd, JH−H = 2.1 Hz, 3JH−H = 8.1 Hz), 7.53
Crystal Structure Determinations. 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; absorption was taken into account
on a semiempirical basis using multiple scans.34−36 The structures
were solved by direct methods (SHELXS37) and refined by full-matrix
3
3
(2H, d, JH−H = 1.8 Hz), 7.31 (1H, t, JH−H = 8.1 Hz), 7.11 (2H, d,
3JH−H = 1.8 Hz), 4.49 (2H, sept, 3JH−H = 6.9 Hz), 1.43 (12H, d, 3JH−H
= 6.9 Hz) ppm. 13C{1H} NMR (100.6 MHz, 25 °C, [D8]THF): δ
213.2, 142.5, 128.1, 116.5, 116.0, 115.5, 110.8, 51.5, 22.4 ppm.
Synthesis of [2,6-Bis(3-isopropylimidazol-2-ylidene)phenyl]-
lithium. nBuLi solution (1.7 mL of a 1.6 M hexane solution, 2.7
mmol, 1.1 equiv) was added to a suspension of 0.73 g of 2,6-bis(3-
isopropylimidazol-2-ylidene)benzene (2.45 mmol, 1 equiv) in 30 mL
of pentane. The suspension was stirred for 3 days. The solid was
collected and dried, yielding quantitatively 2,6-[bis(3-isopropylimida-
zol-2-ylidene)phenyl]lithium. 1H NMR (300.13 MHz, 25 °C,
least-squares techniques against Fo (SHELXL-9737 and SHELXL-
2
201638). All hydrogen atoms were included at calculated positions with
fixed thermal parameters. All non-hydrogen, nondisordered atoms
were refined anisotropically.37,38 The crystal of 3 contains large voids,
filled with disordered solvent molecules. The size of the voids is 2236
Å3/unit cell. Their contribution to the structure factors was secured by
back-Fourier transformation using the SQUEEZE routine of the
program PLATON,39 resulting in 621 electrons/unit cell. Crystallo-
graphic data as well as structure solution and refinement details are
summarized in Table S1 in the Supporting Information. XP40 and
POV-Ray41 were used for structure representations.
3
saturated [D8]THF solution): δ 7.25 (2H, d, JH−H = 1.5 Hz), 7.02
(1H, dd, 3JH−H = 1.6 Hz), 6.95 (2H, dd, 3JH−H = 1.6 Hz), 6.80 (2H, d,
3JH−H = 1.5 Hz), 3.96 (2H, sept, 3JH−H = 6.7 Hz), 1.05 (12H, d, 3JH−H
= 6.7 Hz) ppm. 13C{1H} NMR (100.6 MHz, 25 °C, saturated
[D8]THF solution): δ 200.0, 169.8, 153.6, 124.4, 116.0, 115.7, 112.4,
51.6, 23.0 ppm.
E
DOI: 10.1021/acs.organomet.7b00303
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(10) In situ preparation is described in the experimental details for
compound 3. See also: Koch, A.; Krieck, S.; Westerhausen, M. In
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00303.
■
S
gesattigten Kohlenwasserstoffen losliches Calcium-bis(trimethylsilyl-
̈
̈
methanid) und dessen Verfahren zur Herstellung. Patent DE 10 2017
002966A1.
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NMR spectra of new compounds, experimental details of
the crystal structures and refinement details, and
m o l e c u l a r r e p r e s e n t a t i o n o f 2 , 6 - b i s ( 3 -
isopropylimidazolium)benzene diiodide (PDF)
Accession Codes
CCDC 1540814−1540817 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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(13) Westerhausen, M.; Koch, A.; Gorls, H.; Krieck, S. Chem. - Eur. J.
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2017, 23, 1456−1483.
AUTHOR INFORMATION
Corresponding Author
*M.W.: e-mail, m.we@uni-jena.de; tel, +49 3641 9 48110; fax,
+49 3641 9 48132; web, http://www.lsac1.uni-jena.de.
■
(14) Koch, A.; Krieck, S.; Gorls, H.; Westerhausen, M. Organo-
̈
metallics 2017, 36, 994−1000.
́ ́
(15) Martínez-Martínez, A. J.; Fuentes, M. A.; Hernan-Gomez, A.;
Hevia, E.; Kennedy, A. R.; Mulvey, R. E.; O’Hara, C. T. Angew. Chem.,
Int. Ed. 2015, 54, 14075−14079.
ORCID
(16) Kohler, M.; Koch, A.; Gorls, H.; Westerhausen, M. Organo-
̈
̈
Matthias Westerhausen: 0000-0002-1520-2401
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
A.K. thanks the Fonds der Chemischen Industrie im Verband
der Chemischen Industrie e.V. (FCI/VCI, Frankfurt/Main,
Germany; fund no. 510259) for a generous Ph.D. stipend and is
also grateful to the Graduate Academy of the Friedrich Schiller
University Jena for a concluding grant.
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