Imidazolium Cyclopentadienide Salts and their Use as Cp-Transfer Reagents

Article abstract of DOI:10.1002/ejic.201900047

The reaction of N-heterocyclic carbenes, 1a–c, towards cyclopentadienes, 2a–c, was studied. N,N′-diisopropyl-substituted carbene 1a acts as a Br?nsted base and deprotonates cyclopentadiene, 2a, and isopropylcyclopentadiene, 2b, to yield the corresponding imidazolium cyclopentadienide salts, 3a,b, whereas there is no reaction towards 1,2,3,4,5-pentamethylcyclopentadiene. Imidazolium cyclopentadienide salts, 3a,b, were characterized in solution by ¹H and ¹³C NMR spectroscopy, as well as in the solid state by single-crystal X-ray diffraction. In addition, it was demonstrated that imidazolium cyclopentadienide salt 3a can be used as a Cp-transfer reagent in the synthesis of different cyclopentadienyl transition metal complexes.

Full text of DOI:10.1002/ejic.201900047

DOI: 10.1002/ejic.201900047
Full Paper
Imidazolium Cyclopentadienide Salts
Imidazolium Cyclopentadienide Salts and their Use as
Cp-Transfer Reagents
Inga-Alexandra Bischoff,[a] Carsten Müller,[a] Volker Huch,[a] Michael Zimmer,[a] and
André Schäfer*[a]
1
Abstract: The reaction of N-heterocyclic carbenes, 1a–c, to- salts, 3a,b, were characterized in solution by H and 13C NMR
wards cyclopentadienes, 2a–c, was studied. N,N′-diisopropyl- spectroscopy, as well as in the solid state by single-crystal X-ray
substituted carbene 1a acts as a Brønsted base and de- diffraction. In addition, it was demonstrated that imidazolium
protonates cyclopentadiene, 2a, and isopropylcyclopentadiene, cyclopentadienide salt 3a can be used as a Cp-transfer reagent
2b, to yield the corresponding imidazolium cyclopentadienide in the synthesis of different cyclopentadienyl transition metal
salts, 3a,b, whereas there is no reaction towards 1,2,3,4,5- complexes.
pentamethylcyclopentadiene. Imidazolium cyclopentadienide
Introduction
Metallocenes are of tremendous interest for numerous reasons,
among others for their application as catalysts,[1] as metal pre-
cursors in chemical vapor deposition and pyrolysis tech-
niques,[2] or as bioactive molecules in medical applications.[3]
Scheme 1. Reaction N-heterocyclic carbenes 1a with cyclopentadienes 2a–b.
While some metallocenes, like ferrocene, can be synthesized
directly from the corresponding metal element and cyclopenta-
diene,[4] the far more common route, is to use Cp-transfer rea-
This is in agreement with a pKa value for N-heterocyclic carb-
ene 1a of 24 and a pKa value for cyclopentadiene of 18,[7] and
a report of a similar reaction between 1a and 2,3-benzo-
fluorene.[7a] Furthermore, DFT calculations at the B3LYP-D3/
def2-TZVPP level of theory predict the reaction of 1a with 2a
to be strongly exergonic (ΔG298 = 349.2 kJ mol–1).[8,9]
Both the substitution pattern on the N-heterocyclic carbene
and on the cyclopentadiene have strong influence on the pro-
ton transfer reaction. This is shown by the fact that N-hetero-
cyclic carbene 1a acts as a potent Brønsted base in the reaction
with cyclopentadienes, 2a, while there is no reaction between
1a and 1,2,3,4,5-pentamethylcyclopentadiene. This can be at-
tributed to the lower acidity of 1,2,3,4,5-pentamethylcyclo-
pentadiene, compared to cyclopentadiene, 2a.[10]
gents like alkali metal cyclopentadienide compounds, usually
obtained from the corresponding cyclopentadiene and a suit-
able metallabase.[5] Hence, the synthesis of metallocenes are
often multi step reactions and sometimes suffer from low
yields, due to side reactions because of the redox chemistry
of the involved metal precursors. Therefore, mild Cp-transfer
reagents are of great interest for the preparation of many cyclo-
pentadienyl metal complexes.[6]
Herein, we report on the use an N-heterocyclic carbene, 1a,
as a Brønsted base in the synthesis of imidazolium cyclopenta-
dienide salts, 3a,b, and their application as Cp-transfer reagents
in the synthesis of cyclopentadienyl metal complexes.
Furthermore, more bulky substituted N-heterocyclic carb-
enes 1b,c do not react with cyclopentadiene, 2a, at room tem-
perature.
Results and Discussion
When monomeric cyclopentadienes 2a,b were added to a solu-
tion of N-heterocyclic carbene 1a at 273 K, an immediate Brøn-
sted acid base reaction took place to give the corresponding
imidazolium cyclopentadienide salts 3a,b (Scheme 1).
[a] Faculty of Natural Sciences and Technology, Department of Chemistry,
Saarland University,
66123 Saarbrücken, Germany
E-mail: andre.schaefer@uni-saarland.de
https://www.uni-saarland.de/forschung/emmy-noether-chemie
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201900047.
As many cyclopentadienes undergo Diels-Alder dimerization
at room temperature, heating of the reaction mixture to pro-
mote a proton transfer reaction is not viable. Imidazolium cyclo-
pentadienide salts 3a,b can be isolated as air-sensitive colorless
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crystalline solids and can be stored at 248 K under an inert Alder reaction, and metal(II) chloride proceeds at room temper-
atmosphere for several days. Crystals suitable for single-crystal ature to give the corresponding metal complex 4 and 5. The
X-ray diffraction were obtained from THF solutions at 248 K, only by-product of the reaction is 1,3-diisopropyl-4,5-dimethyl-
allowing for a solid state characterization of these species, for imidazolium chloride, 1a·HCl, which is easily recovered by filtra-
the first time.[11] Imidazolium cyclopentadienide salts 3a,b show tion and recycled into carbene 1a, by treatment with a suitable,
similar structures in the solid state, which are solvent free crys- commercially available base, such as potassium tert-butox-
tals with the imidazolium H-atom exhibiting a hydrogen bond ide.[16]
to the π-system of the cyclopentadienide anion (Figure 1).
Similar structural motives are known for ammonium and phos-
phonium cyclopentadienide salts.[12]
When nickel(II) chloride is used as a metal precursor, cyclo-
pentadienylnickel chloride NHC complex 5 is obtained, rather
than nickelocene (Figure 2).
Figure 1. Molecular structure of 3a (a) and 3b (b) in the crystal (H-atoms
except for imidazolium-H omitted for clarity, thermal ellipsoids set at 50 %
probability level).
Figure 2. Molecular structure of 5 in the crystal (H-atoms omitted for clarity,
thermal ellipsoids set at 50 % probability level).
The (Im)H–Cpcentroid bonding distance varies somewhat be-
tween 3a and 3b, with 238.7–246.8 pm[13] in 3a and 240.4 pm
in 3b. In both cases, relatively uniform C–C bonding distance
within the Cp-moieties are observed (C2–C6: 3a: 137.6–
140.5 pm;[13] 3b: 139.3–141.5 pm[13]), indicating a conjugated
6π-electron aromatic system. The dihedral angle between the
imidazolium ring plane and the cyclopentadienyl ring plane is
67.8–77.3°[13] in case of 3a and 78.1° in case of 3b. The struc-
tural characteristics of the imidazolium cation in 3a and 3b are
very similar to the corresponding tetrafluoroborate salt,[14] with
N1–C1–N2 angles of 108.5–109.0°[13] (3a) / 108.7° (3b) and
C1–N1/C1–N2 bond length of 132.8–133.6 pm[13] (3a) / 132.5–
133.0 pm[13] (3b).
This is in agreement with the fact that cyclopentadienyl-
nickel chloride NHC complexes of this kind can be obtained by
treatment of nickelocene with imidazolium chlorides.[16a] The
herein described route, however, presents a more convenient
access to these complexes, which is of interest with regards to
their catalytic applications.[16b] The nickel atom in 5 exhibits a
trigonal planar coordination (sum of angles = 359.8°) with an
η5 bonded cyclopentadienyl substituent and a Ni–CNHC bond of
188.6 pm, which is in line with related complexes of this kind.[16]
Interestingly, complex 5 exhibits a very downfield shifted reso-
nances for the methine protons of the isopropyl groups of the
1
NHC moiety in the H NMR spectrum (δ1H = 7.31), compared
Even though an isolation of imidazolium cyclopentadienide
salts, 3a,b, is possible, it is not required when using them as
Cp-transfer reagents in the preparation of transition metal
cyclopentadienyl complexes, as they can be generated and
used in-situ, in one-pot reaction sequences. (Scheme 2).
to 1a (δ1H = 4.00). In the solid-state structure, these methine
protons can be observed pointing towards the nickel atom with
Ni–H distance of 264.1 pm and 271.4 pm, C–H bond lengths of
94.1 pm and 96.7 pm and C–H-Ni angles of 120.7° and 123.5°.
However, the C–H bond lengths are actually a bit shorter than
in 1a (100 pm) and there is no lowering of the 1J1 C coupling
13
H-
1 1 13
constant. In fact, a coupling constant of J
= 140 Hz for
H-
C
the methylylidene groups in 5 is detected, which is slightly
13
larger than what is observed in 1a (1J1
= 135 Hz). This
H-
C
is further supported by DFT calculations at the M06-L/IGLO-
III(C,H,Cl,N);def2-TZVPP(Ni)//B3LYP-D3/def2-TZVPP[9] level of
Scheme 2. One-pot synthesis of ferrocene, 4, and cyclopentadienyl nickel
chloride-NHC complex 5.
13
theory, suggesting coupling constants of 1J1 C = 168 Hz (1a),
H-
165 Hz (5), which is in qualitative agreement with the experi-
The one-pot synthesis of ferrocene, 4, and cyclopentadienyl mental observation that there is no significant difference in the
13
nickel chloride-NHC complex, 5, starting from readily available 1J1
coupling constant in 1a and 5. This suggests that the
H-
C
N-heterocyclic carbene 1a[15] and cyclopentadiene, obtained interaction is mostly electrostatic in nature (anagostic interac-
from commercially available dicyclopentadiene by Retro-Diels- tion).
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added the corresponding cyclopentadiene 2a,b (2a: 110 mg,
1.7 mmol; 2b: 180 mg, 1.7 mmol) at 273 K. The solution was
warmed to room temperature and stirred overnight. Evaporation of
all volatiles in vacuo gave imidazolium cyclopentadienide salts,
Conclusions
The reaction of different N-heterocyclic carbenes 1a–c towards
cyclopentadiene and of N-heterocyclic carbene 1a towards vari-
ably substituted cyclopentadienes, 2a,b and 1,2,3,4,5-penta- 3a,b, as colorless to beige solids. Recrystallization from THF at 248 K
afforded crystals suitable for single-crystal X-ray diffraction analysis.
methylcyclopentadiene, was explored. N-heterocyclic carbene
1a was shown to react rapidly with cyclopentadienes, 2a,b, in
proton transfer Brønsted acid base reactions, to give imid-
azolium cyclopentadienide salts 3a,b. These species can be iso-
lated and stored as stable crystalline solids and were character-
ized in solution and in the solid state but may also be gener-
ated in-situ when used as Cp-transfer reagents in the synthesis
of metallocenes and metallocene derivatives, such as 4 and 5.
Yield: 3a: 245 mg, 60 %; 3b: 241 mg, 50 %.
3a: 1H-NMR (400.13 MHz, 296.9 K, C6D6) δ = 1.18 (d, J = 6 Hz, 12 H,
CH3), 1.54 (s, 6 H, CH3), 4.50 (sept, J = 7 Hz, 2 H, CH), 6.17 (s, 1 H,
Im-H), 6.44 (s, 5 H, Cp-H); 13C{1H}-NMR (100.61 MHz, 296.8 K, C6D6)
δ = 8.6 (CH3), 21.7 (CH3), 51.1 (CH), 104.8 (Cp), 125.9 (C=C), 137.7
(Im-C). Elemental analysis: calculated: C: 78.31 %, H: 10.27 %, N:
11.42 %; found C: 78.11 %, H: 10.08 %, N: 11.40 %.
3b:[8] 1H-NMR (300.13 MHz, 239.8 K, C6D5CD3) δ = 1.29 (br s, 18 H,
CH3), 1.48 (br s, 6 H, CH3), 1.69 (d, J = 7 Hz, 6 H, CH3), 3.46 (sept,
J = 7 Hz, 1 H, CH), 4.45 (br s, 3 H, CH & Im-H), 6.26 (m, 4 H, Cp-H);
1H-NMR (300.13 MHz, 293.1 K, C6D5CD3) δ = 1.05 (d, J = 7 Hz, 3 H,
CH3), 1.11 (d, J = 7 Hz, 3 H, CH3), 1.40 (d, J = 7 Hz, 18 H, CH3), 1.65
(s, 6 H, CH3), 2.48–2.60 (m, 1 H, CH), 2.67–2.75 (m, 2 H, CH2), 4.09
(m, 2 H, CH), 5.89–6.48 (m, 3 H, Cp-H); 13C{1H}-NMR (75.48 MHz,
240.1 K, C6D5CD3) δ = 8.4 (CH3), 27.1 (CH3), 30.3 (CH3), 102.0 (Cp),
103.5 (Cp). Elemental analysis: calculated: C: 79.11 %, H: 11.18 %, N:
9.71 %; found C: 77.07 %, H: 10.68 %, N: 9.97 %.
Experimental Section
N-heterocyclic carbenes 1a–c and 6,6-dimethylfulvene were pre-
pared according to literature known procedures.[15,17–19] All manip-
ulations were carried out under an argon inert gas atmosphere
(argon 5.0), using either Schlenk line techniques or a glovebox. Di-
cyclopentadiene (90 %) was cracked and distilled freshly prior to
use.
NMR spectra were recorded on Bruker Avance III 300 and Bruker
Avance III 400 spectrometers. 1H and 13C NMR spectra were refer-
One-pot synthesis of ferrocene 4:
To a solution of 1a (171 mg, 1.0 mmol) and FeCl2 (60 mg, 0.5 mmol)
in THF was added 2a (63 mg, 1.0 mmol) via syringe at 273 K. After
stirring overnight and gradual warming to room temperature, all
volatiles were removed in vacuo and the solid residue was extracted
with hexane. After evaporation of the solvent, the corresponding
enced using the solvent signals (δ 1H(C6HD5)
=
7.16;
(δ 1H(C6D5CHD2) = 2.09; δ 13C(C6D6) = 128.06; δ 13C(C6D5CD3) =
20.43). Elemental analyses were performed on an Elementar vario
micro cube.
Single crystal X-ray diffraction analysis were carried out at low tem- metal complex was obtained as a crystalline solid.
peratures on a Bruker AXS X8 Apex CCD diffractometer operating
Yield: 118 mg, 67 %.
with graphite monochromated Mo-Kα radiation. Structure solution
1H-NMR (400.13 MHz,296.0 K, CDCl3) δ = 4.14 (s).
and refinement were performed using SHELX.[20]
The hexane-insoluble residue of the reaction with iron(II) chloride
CCDC 1885159 (for 3a), 1885160 (for 3b), and 1885161 (for 5) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre.
was shown to be 1,3-diisopropyl-4,5-dimethylimidazolium chloride,
1
1a·HCl, by H NMR spectroscopy.
1a·HCl: 1H-NMR (400.13 MHz, 296.2 K, CDCl3) δ = 1.68 (brs, 12 H,
iPr-CH3), 2.24 (brs, 6 H, CH3), 4.51 (br s, 2 H, iPr-CH), 10.79 (br s, 1
H, Im-H).
All DFT-calculations were performed using the Gaussian 09, Revi-
sion D.01 package of programs.[9]
One-pot synthesis of cyclopentadienyl nickel chloride-NHC
complex 5:
Isopropylcyclopentadiene (2b):
Isopropylcyclopentadiene, 2b, was prepared by a modified litera- To a solution of 1a (557 mg, 3.1 mmol) and NiCl2 (200 mg,
ture procedure.[21] To a solution of LiAlH4 (3.75 g, 98.9 mmol) in THF
at 273 was added 6,6-dimethylfulvene (10.0 g, 11.4 mL,
1.5 mmol) in THF was added one equivalent of 2a (102 mg,
1.5 mmol) via syringe at room temperature. After stirring for 3 h,
K
94.2 mmol). After stirring for 10 min, 25 mL of methanol were another equivalent of 2a was added. After stirring overnight, all
added slowly, and the reaction mixture was stirred for 1.5 h at
273 K. 2 M aqueous hydrochloric acid (47.1 mL, 94.2 mmol) was
added and the mixture was stirred for an additional 30 min at 273 K.
Diethyl ether was added and the aqueous phase was extracted
three times with diethyl ether. The organic layers were combined,
washed with saturated aqueous NaHCO3 solution, dried with
MgSO4 and the solvent was removed under reduced pressure.
volatiles were removed in vacuo and the solid residue was washed
with hexane and subsequently extracted with toluene. After evapo-
ration of all volatiles, 5 was obtained as a crystalline purple solid.
Yield: 110 mg, 21 %.
1H-NMR (300.13 MHz, 296.0 K, C6D6) δ = 1.18 (d, J = 7 Hz, 6 H, iPr-
CH3), 1.42 (d, J = 7 Hz, 6 H, iPr-CH3), 1.60 (s, 6H, CH3), 5.27 (s, 5 H,
Cp), 7.27 (sept, J = 7 Hz, 2 H, iPr-CH); 13C{1H}-NMR (100.62 MHz,
298.0 K, C6D6) δ = 9.9 (CH3), 21.8 (CH3), 21.9 (CH3), 55.3 (iPr-CH),
91.8 (Cp), 126.7 (C=C), 158.0 (Carbene-C). Elemental analysis: calcu-
lated: C: 56.60 %, H: 7.42 %, N: 8.25 %; found C: 57.33 %, H: 8.08 %,
N: 8.67 %.
If necessary, 2b can be purified by lithiation and subsequent hydrol-
ysis.
Yield: 5:98 g, 58 %.
1H NMR (400.13 MHz, 300.0 K, C6D6) δ = 1.04 (d, J = 7 Hz, 2 H, CH3),
1.11 (d, J = 7 Hz, 4 H, CH3), 2.49–2.61 (m, 1 H, CH), 2.69–2.83 (m, 2
H, CH2), 5.90–6.51 (m, 3 H, Cp-H).
Acknowledgments
Financial support from the Deutsche Forschungsgemeinschaft
1,3-Diisopropyl-4,5-dimethyl-imidazolium cyclopentadienide
salts 3a–b: To a solution of 1a (300 mg, 1.7 mmol) in THF was (DFG) (Emmy Noether program – SCHA 1915/3-1) and the
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1a + 2b and 3b, as indicated by VT-NMR spectroscopy. At temperatures
below 243 K, 3b can be observed as the predominate species in solution,
while 1a and 2b can be observed as predominate species at tempera-
tures above 313 K. See supporting information for 1H NMR spectra at
239.8 K and 293.1 K.
Chemical Industry Fund (FCI) is gratefully acknowledged.
Susanne Harling is thanked for elemental analysis and Nils
Steinbrück is thanked for UV/Vis spectroscopy.
[9] DFT calculations were carried out using the Gaussian 09 - Revision D.01
package of programs. See supporting information for optimized geome-
tries, references.
Keywords: Imidazolium salts · Cyclopentadienyl ligands ·
Carbene ligands · Metallocenes · Cp-transfer reagents
[10] 1H and 13C NMR spectroscopy at room temperature suggest there is no
reaction between 3c and carbenes 1a and its N,N′-dimethyl-substituted
derivative. Attempts to synthesize decamethylferrocene in a similar man-
ner to 4 failed.
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Received: January 14, 2019
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Imidazolium Cyclopentadienide
Salts
I.-A. Bischoff, C. Müller, V. Huch,
M. Zimmer, A. Schäfer* .................... 1–5
Imidazolium
Cyclopentadienide
Salts and their Use as Cp-Transfer
Reagents
Different imidazolium cyclopentadi- their use as Cp-transfer reagents in the
enide salts were synthesized and struc- synthesis of transition metal cyclopen-
turally characterized. Furthermore, tadienyl complexes was demonstrated.
DOI: 10.1002/ejic.201900047
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