Synthesis, structure, and solution dynamics of pentamethylcyclopentadienyl nickel complexes bearing N-heterocyclic carbene ligands

Article abstract of DOI:10.1021/om800376q

Neutral pentamethylcyclopentadienyl nickel complexes of general formula [Ni(NHC)X(η⁵-C₅Me₅)] [NHC = 1,3-dimethylimidazol-2-ylidene (Me-NHC), 1,3-bis(2,4,6,-trimethylphenyl) imidazol-2-ylidene (Mes-NHC), 1,3-bis(2,6-diisopr

Full text of DOI:10.1021/om800376q

Organometallics 2008, 27, 4223–4228
4223
Synthesis, Structure, and Solution Dynamics of
Pentamethylcyclopentadienyl Nickel Complexes Bearing
N-Heterocyclic Carbene Ligands
Vincent Ritleng, Caroline Barth, Eric Brenner, Sandra Milosevic, and Michael J. Chetcuti*
Laboratoire de Chimie Organome´tallique Applique´e, UMR CNRS 7509, Ecole Europe´enne de Chimie,
Polyme`res et Mate´riaux (ECPM), UniVersite´ Louis Pasteur, 25 Rue Becquerel, 67087 Strasbourg, France
ReceiVed April 29, 2008
Neutral pentamethylcyclopentadienyl nickel complexes of general formula [Ni(NHC)X(η5-C5Me5)]
[NHC ) 1,3-dimethylimidazol-2-ylidene (Me-NHC), 1,3-bis(2,4,6,-trimethylphenyl)imidazol-2-ylidene
(Mes-NHC), 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (iPr-NHC); X ) Cl, I] were prepared from
the reaction of pentamethylcyclopentadienyl acetylacetonate nickel(II) with 1 equiv of the corresponding
imidazolium salt (NHC · HX). The new complexes [Ni(Me-NHC)I(η5-C5Me5)] 1a, [Ni(Mes-NHC)Cl(η5-
C5Me5)] 2, and [Ni(iPr-NHC)Cl(η5-C5Me5)] 3 were obtained in moderate to good yields and were fully
1
characterized by 13C and H NMR spectroscopy, and in the cases of 1a and 2 by single-crystal X-ray
crystallography. The related cyclopentadienyl complex [Ni(Me-NHC)I(η5-C5H5)] 1b was also synthesized
and structurally characterized; its geometry and spectroscopic data are comparable to those of complex
1a. The variable-temperature (VT) 1H NMR spectra of the sterically constrained complexes 2 and 3 are
consistent with restricted rotation about the nickel-carbene carbon bond. The free energy of activation
for the dynamic processes, in both cases, was determined to be on the order of 65-67 kJ mol-1 by VT
NMR experiments.
and degradative cleavage.17–20 Applications of organometallic
complexes bearing NHC ligands in catalysis are now well
recognized.2,4–6,21–24
Introduction
The synthesis and isolation of the first N-heterocyclic carbene
(NHC), or imidazol-2-ylidene, was reported in 1991.1 Since
then, NHCs have become an important class of ligands in
organometallic chemistry.2–6 These ligands are often compared
to tertiary phosphines because of similarities in their bonding
to transition metals. Nevertheless, substantial differences do
exist, notably in their electron-donating power. In general NHCs
are stronger Lewis bases7,8 and have reduced π-back-bonding
with respect to tertiary phosphines.9–12 Consequently, complexes
thatincorporatetheseligandsaremorestabletowarddissociative13–16
Nickel-NHC complexes and their catalytic applications have
attracted some recent attention.25–38 The utility of nickel-NHC
complexes in catalysis has been eclipsed by the far wider range
of applications found for their palladium and ruthenium
analogues.2,39 Nevertheless, nickel complexes offer significant
potential advantages in contrast to palladium NHC species, most
notably in their much lower cost and in their reduced tendency
to deposit nanoparticles of metallic nickel.39 Despite the relative
diversity of ligand frameworks found in NHC nickel complexes,
no examples of such species bearing electron-rich and sterically
demanding η5-pentamethylcyclopentadienyl, η5-C5Me5 (Cp*),
ligands have been reported to date. An electron-rich and bulky
Cp* ligand linked to an electron-rich nickel(NHC) group may
impart substantial benefits in a catalytic cycle. Such Ni(NHC)-
Cp* complexes should be capable of (i) better stabilizing
* To whom correspondence should be addressed. E-mail: chetcuti@
chimie.u-strasbg.fr. Tel: + 33 3 90 24 26 31. Fax: + 33 3 90 24 26 39.
(1) Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991,
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10.1021/om800376q CCC: $40.75
2008 American Chemical Society
Publication on Web 07/22/2008

4224 Organometallics, Vol. 27, No. 16, 2008
Ritleng et al.
Scheme 1. Syntheses of Complexes 1a, 2, and 3
coordinatively unsaturated sites through steric protection and
electronic donation and (ii) subsequently enhancing a reductive
elimination step via the steric bulkiness of the Cp* ligand.36
We believed that it was of interest to develop a convenient route
to Ni(NHC)Cp* complexes.
Herein, we report the facile “one-pot” synthesis of [Ni(NH-
C)XCp*] (X ) Cl, I) complexes by reaction of the correspond-
ing imidazolium salt with [NiCp*(acac)] (acac ) acetylaceto-
nate), prepared in situ. The structures of two of the resulting
species were established by single-crystal X-ray diffraction
studies. The synthesis and structure of the new related cyclo-
pentadienyl complex [Ni(Me-NHC)ICp] is also described. Bulky
aryl-substituted NHC ligands lead to complexes in which there
is hindered rotation about the nickel-carbene bond at room
temperature. The activation free energy of rotation for these
1
processes was established by VT H NMR spectroscopy.
Scheme 2. Synthesis of Complex 1b
Results and Discussion
Synthesis of the Neutral Complexes [Ni(NHC)XCp*]. Most
common methods of attaching a NHC onto a metal center
require the prior synthesis of either the free carbene40–42 or a
NHC silver complex,8,42,43 before subsequent complexation or
transmetalation. This procedure is avoided here by the direct
addition of the imidazolium salt onto the nickel precursor.44,45
The cyclopentadienyl complexes [Ni(NHC)XCp] can be
prepared from nickelocene and the corresponding imidazolium
salt.44,45 Nevertheless, attempts to prepare the Ni(NHC)Cp*
complexes, using a parallel synthetic method, with NiCp*2 were
not fruitful. We therefore developed a new synthesis for the
Ni(NHC)Cp* complexes.
according to the procedure outlined by Manriquez.46 It proved
to be unnecessary to isolate the moderately stable [Ni(a-
cac)Cp*],47 and a suspension of the in situ prepared nickel
complex was reacted with the imidazolium salts in THF, in a
“one-pot” procedure shown in Scheme 1. The neutral complexes
[Ni(Me-NHC)ICp*] 1a, [Ni(Mes-NHC)ClCp*] 2, and [Ni(iPr-
NHC)ClCp*] 3 (Me-NHC ) 1,3-dimethylimidazol-2-ylidene,
Mes-NHC ) 1,3-bis(2,4,6,-trimethylphenyl)imidazol-2-ylidene,
iPr-NHC ) 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)
were all isolated as air-stable red-violet solids in moderate to
good yields (37-57% after workup).48
The hitherto unknown complex [Ni(Me-NHC)ICp] 1b was
obtained from nickelocene and the imidazolium salt (see
Experimental Section) by modification of a previously reported
synthesis that yielded other Ni(NHC)Cp complexes,44,45 but not
this one. The synthesis is shown in Scheme 2. All compounds
were characterized by 1H and 13C{1H} NMR spectroscopy (Vide
infra).
The nickel precursor used was [Ni(acac)Cp*], prepared by
the reaction of anhydrous [Ni(acac)2] with LiCp* in THF
(25) Lee, C.-C.; Ke, W.-C.; Chan, K.-T.; Lai, C.-L.; Hu, C.-H.; Lee,
H. M. Chem.-Eur. J. 2007, 13, 582.
(26) Ng, S.-S.; Jamison, T. F. J. Am. Chem. Soc. 2005, 127, 7320.
(27) Duong, H. A.; Cross, M. J.; Louie, J. J. Am. Chem. Soc. 2004,
126, 11438.
(28) Mahandru, G. M.; Liu, G.; Montgomery, J. J. Am. Chem. Soc. 2004,
126, 3698.
(29) Sawaki, R.; Sato, Y.; Mori, M. Org. Lett. 2004, 6, 1131.
(30) Omar-Amrani, R.; Thomas, A.; Brenner, E.; Schneider, R.; Fort,
Y. Org. Lett. 2003, 5, 2311.
Structural Studies. Crystals of 1a, 1b, and 2 suitable for
X-ray structure determination were grown from cold THF/
diethyl ether (1a, 1b) or toluene (2) solutions. The molecular
structures of 1a and 1b are shown in similar orientations in
parts a and b of Figure 1, respectively. Figure 2 shows the
geometry of complex 2. Crystallographic data and data collection
parameters are listed in Table 1, and a list of selected bond
lengths and angles for all three complexes appear in Table 2.
The molecular structures of these complexes are similar. All
feature a nickel atom bonded to a η5-Cp (1b) or Cp* (1a, 2)
group, a NHC moiety, and a halide ligand. If one considers the
Cp or Cp* group as a single ligand, the nickel atom lies at the
center of a trigonal plane formed by the ring centroid, the halide,
and the carbenoid carbon atom C(1) of the NHC ligand (∑(bond
angles) ) 360°). However, there are significant departures from
(31) Bo¨hm, V. P. W.; Weskamp, T.; Gsto¨ttmayr, C. W. K.; Herrmann,
W. A. Angew. Chem., Int. Ed. 2000, 39, 1602.
(32) McGuinness, D. S.; Cavell, K. J.; Skelton, B. W.; White, A. H.
Organometallics 1999, 18, 1596.
(33) Xi, Z.; Zhang, X.; Chen, W.; Fu, S.; Wang, D. Organometallics
2007, 26, 6636.
(34) Inamoto, K.; Kuroda, J.-I.; Hiroya, K.; Noda, Y.; Watanabe, M.;
Sakamoto, T. Organometallics 2006, 25, 3095.
(35) Chiu, P. L.; Lai, C.-L.; Chang, C.-F.; Hu, C.-H.; Lee, H. M.
Organometallics 2005, 24, 6169.
(36) Matsubara, K.; Ueno, K.; Shibata, Y. Organometallics 2006, 25,
3422.
(37) Matsubara, K.; Ueno, K.; Koga, Y.; Hara, K. J. Org. Chem. 2007,
72, 5069.
(38) Malyshev, D. A.; Scott, N. M.; Marion, N.; Stevens, E. D.;
Ananikov, V. P.; Beletskaya, I. P.; Nolan, S. P. Organometallics 2006, 25,
4462.
(39) Sommer, W. J.; Weck, M. Coord. Chem. ReV. 2007, 251, 874.
(40) Jafarpour, L.; Nolan, S. P. Organometallics 2000, 19, 2055.
(41) Herrmann, W. A.; Elison, M.; Fischer; J.; Ko¨cher, C.; Artus, G. R. J.
Chem.-Eur. J. 1996, 2, 772.
(42) Peris, E. Top. Organomet. Chem. 2007, 21, 83.
(43) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972.
(44) Kelly, R. A.; Scott, N. M.; D´ıez-Gonza´lez, S.; Stevens, E. D.; Nolan,
S. P. Organometallics 2005, 24, 3442.
(46) Bunel, E. E.; Valle, L.; Manriquez, J. M. Organometallics 1985,
4, 1680.
(47) Smith, M. E.; Andersen, R. A. J. Am. Chem. Soc. 1996, 118, 11119.
(48) Palladium(II) acetylacetonate has been used to prepare Pd(NHC)
species, but in this case, the free NHC was used, rather than the imidazolium
salt. Navarro, O.; Marion, N.; Scott, N. M.; Gonzalez, J.; Amoroso, D.;
Bell, A.; Nolan, S. P. Tetrahedron 2005, 61, 9716–9722.
(45) Abernethy, C. D.; Cowley, A. H.; Jones, R. A. J. Organomet. Chem.
2000, 596, 3.

Pentamethylcyclopentadienyl Ni Complexes of NHCs
Organometallics, Vol. 27, No. 16, 2008 4225
centroid, the nickel atom, and the halide atom and the plane
that contains the five-membered imidazolylidene ring. Molecules
of 1b sit on a crystallographically imposed mirror plane in which
this interplanar angle is set by symmetry at 90°. In 1a, the two
planes are also practically perpendicular (interplanar angle )
87°). In the more sterically demanding bis-mesityl NHC
complex 2, the imidazolylidene ring is tilted at an angle of 73.5°
with respect to the Cp*cent-Ni-Cl plane, presumably because
of the steric congestion present in the molecule.
The angle formed between the two least-squares mesityl ring
planes in 2 is 34°; that is, the two rings are twisted with respect
to each other, presumably to minimize steric interactions with
other ligands. This interplanar angle is comparable to the values
observed in the two other NHC complexes bearing mesityl
substituents, in which the values found are 34.7(8)-33.34(70)44
and 39.0°.45 The two mesityl rings both make angles of 78°
with respect to the five-membered imidazolylidene ring.
The molecular structure of 2 suggests that free rotation about
the nitrogen-mesityl bonds is not feasible. A space-filling
molecular model of 2 clearly shows that complete rotation about
the nitrogen-mesityl bonds is not allowed without severe
molecular deformations, due to the steric bulk of the Cp* ligand
and its interaction with the NHC group.
Figure 1. (a) ORTEP plot showing the molecular structure of
[Ni(Me-NHC)I(η5-C5Me5)] 1a. Key atoms are labeled. Ellipsoids
are shown at the 50% probability level. Only one of the two
(disordered) positions of the Cp* methyl carbon atoms is shown;
hydrogen atoms are omitted for clarity. (b) ORTEP plot showing
the molecular structure of [Ni(Me-NHC)I(η5-C5H5)] 1b. Ellipsoids
are are shown at the 50% probability level. Key atoms are labeled
and hydrogen atoms are omitted for clarity.
1
VT NMR Studies. The H and 13C{1H} NMR spectra of
complexes 1 at ambient temperature are straightforward: both
show the presence of one η5-Cp or Cp* group and the Me-
NHC ligand. The spectra reveal that an effective plane of
symmetry that bisects the molecule exists in solution on the
NMR time scale. This effective mirror plane, rigorously present
in the solid-state structure of 1b, contains the halogen, the nickel,
and the NHC carbene carbon atom, as well as the Cp or Cp*
centroid. It is also revealed in the NMR spectra of the more
sterically congested complexes 2 and 3, but the 1H NMR spectra
of these molecules are more complex and deserve further
comment.
The 1H NMR spectrum of 2 at ambient temperature displays
two singlets in a 1:1 integrated ratio for the four meta-hydrogen
atoms of the two mesityl groups, as well as three singlets, in a
3:3:3 relative integrated ratio, for the four ortho- and the two
para-methyl groups. The two alkene protons of the carbene
ligand are isochronous, indicating that a molecular mirror plane
is present on the 1H NMR time scale. If one considers
the molecular structure of 2, only a moderate rotation about
the Ni-NHC carbene-carbon bond, coupled with a minor twist
about the two nitrogen-aryl carbon bonds, is required to
Figure 2. ORTEP plot showing the molecular structure of
[Ni(Mes-NHC)Cl(η5-C5Me5)] 2. Ellipsoids are shown at the 50%
probability level. Key atoms are labeled and hydrogen atoms
are omitted for clarity. The molecule of toluene, found in the
unit cell, is not shown.
the idealized 120° angles of a trigonal structure for all three
molecules (Table 2). In all three structures, the X-Ni-NHC
angles are 94.5 ( 1°.
The nickel-carbene carbon bond lengths are not significantly
different from each other [Ni-C(1) ) 1.887(3), 1.880(4), and
1.906(3) Å for 1a, 1b, and 2, respectively]. The value for 2 is
comparable to those reported for the related analogues
[Ni(1,3-bis(2,4,6,-trimethylphenyl)dihydroimidazol-2-ylidene)-
ClCp] (1.85(2)-1.89(2) Å)44 and [Ni(Mes-NHC)ClCp] (1.917(9)
Å).45
The nickel-halide distances are normal in all three com-
plexes: the Ni-I bond length of 2.5212(5) Å in 1a is marginally
longer than the value 2.5006(6) Å found in the less sterically
congested Cp complex 1b. Both values are similar to the value
of 2.5269(9) Å found in [Ni(1,3,4,5-tetramethylimidazol-2-
ylidene)I(η3-C3H5)].49 The Ni-Cl distance of 2.1962(9) in 2 is
also close to the Ni-Cl distances observed in the related nickel
NHC complexes bearing mesityl substituents, where values of
2.199(5)-2.183(6) Å44 and 2.185(2) Å45 have been registered.
One significant difference between structures 1 and 2 is the
angle subtended between the plane formed by the Cp/Cp*
1
generate the mirror plane that bisects the molecule on the H
NMR time scale. We believe that these motions best explain
the observed symmetry in the spectrum.
Nevertheless, the two ortho-methyl and the two meta-
hydrogen signals are somewhat broad at ambient temperature.
A variable-temperature 1H NMR experiment was thus performed
on a toluene-d8 solution of 2 between 299 and 373 K. As the
temperature was increased, the ortho-methyl resonances became
even broader and eventually coalesced at a temperature (TC) of
ca. 353 K. Similar coalescence was observed for the aromatic
meta-hydrogen atom signals at 323 K. The free energy of
activation (∆Gq) for this fluxional process (based on the
coalescence temperature of the ortho-methyl groups and,
independently, of the meta-aromatic protons) is 67 ( 2 kJ
mol-1 50
.
1
The H NMR spectrum of complex 3 also displays a single
signal for the two alkene protons of the NHC double bond and
(49) Silva, L. C.; Gomes, P. T.; Veiros, L. F.; Pascu, S. I.; Duarte, M. T.;
Namorado, S.; Ascenso, J. R.; Dias, A. R. Organometallics 2006, 25, 4391.
(50) Gutowsky, H. S.; Holm, C. H. J. Chem. Phys. 1956, 25, 1228.

4226 Organometallics, Vol. 27, No. 16, 2008
Ritleng et al.
Table 1. X-ray Crystallographic Data and Data Collection Parameters for Complexes 1 and 2
parameter
1a
1b
2a
empirical formula
fw
C15H23IN2Ni
416.96
C10H13IN2Ni
346.83
C31H39ClN2Ni · C7H8
625.94
cryst syst
space group
unit cell dimens
a (Å)
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
triclinic
P1
monoclinic
P21/m
monoclinic
P21/c
j
8.2210(5)
8.3762(6)
14.2071(5)
86.125(4)
77.031(3)
62.850(2)
847.63(9)
2
7.1384(5)
8.9168(6)
9.3335(4)
90
103.872(4)
90
576.77(6)
2
1.997
16.3570 (7)
14.2962 (6)
16.8901 (7)
90
118.715 (2)
90
3463.9(3)
4
1.200
Z
Dcalcd (g cm-3
)
1.634
radiation, λ (Å)
absorp coeff (mm-1
temperature (K)
cryst size (mm)
h, k, lmax
Mo KR, 0.71073
Mo KR, 0.71073
Mo KR, 0.71073
)
2.956
4.321
0.664
173(2)
173(2)
173(2)
0.20 × 0.15 × 0.10
10, 10, 18
0.568, 0.784
3892
0.0393 (3095)
0.0899 (3892)
1.023
0.20 × 0.15 × 10
9, 11, 12
0.452, 0.662
1396
0.0270 (1234)
0.0609 (1396)
1.121
0.40 × 0.35 × 0.15
21, 18, 21
0,770, 0.868
7937
0.0617 (4731)
0.1742 (7937)
1.036
Tmin, Tmax
no. of reflns collected
R (reflections)
wR2 (reflections)
GOF on F2
a One molecule of toluene is present in the asymmetric unit.
Scheme 3. Dynamic Behavior of Complex 2a
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1a, 1b,
and 2
bond length or angle
1a
1b
2
Ni-C1
1.887(3)
2.5212(5)
1.758
94.53(11)
136.0
1.880(4)
2.5006(6)
1.755
93.63(13)
134.6
1.906(3)
2.1962(9)
1.782
95.27(9)
142.0
Ni-Xa
Ni-Cp†
cent
C1-Ni-Xa
C1-Ni-Cp†
X-Ni-Cp†
a
cent
a
129.4
87.2
131.7
90
122.7
73.5
cent
(NHC)b - (X-Ni-Cp†
)
a
cent
a
Rotation about the Ni-Ccarbene bond coupled with oscillations of
a X ) I for 1a, 1b and Cl for 2. Cp† ) Cp (1b) or Cp* (1a, 2).
b (NHC) ) best least-squares plane through five-membered imidazolyidene
ring.
the mesityl groups about the N-CAr bond of 2 leads to conformation
2a (shown from a different perspective to emphasize its symmetry). In
2a, the mirror plane renders Me groups 1 and 2 and also 3 and 4
equivalent; rotation about the Ni-Ccarbene bond equalizes Me groups 1
and 4 and, hence, all ortho-Me groups.
for the para protons of the aromatic rings. Thus an effective
1
mirror plane also bisects this molecule on the H NMR time
scale. Two doublets (for the meta aromatic ring protons) and
two pseudoseptets (for the CH protons of the isopropyl groups)
are observed. The central carbon atoms of the isopropyl groups
are diastereotopic, and this results in four doublets being
observed for the two sets of isopropyl methyl signals. All signals
are sharp at 263 K.
Variable-temperature 1H NMR data, from 263 to 373 K, were
collected on a toluene-d8 solution of 3. As the temperature
increased, the resonances of the two sets of meta aromatic ring
protons and those of the two isopropyl groups broadened. Four
distinct signal coalescences were observed as the temperature
was raised. These all gave the same free energy of activation
(∆Gq) of ca. 65 ( 2 kJ mol-1, which strongly suggests that all
four coalescences are linked to the same dynamic process in
compound 3.50
These results indicate that there is an effective mirror plane
of symmetry present in solution for all these molecules. In
addition, in complexes 2 and 3, the ortho substituents of the
aromatic rings, which are not equivalent at room temperature,
become equivalent on the NMR time scale when the solutions
of these species are heated moderately. The same effect is seen
for the meta aromatic ring protons.
The most likely dynamic process that could account for these
signal equivalences on the 1H NMR time scale at elevated
temperatures is free rotation about the Ni-Ccarbene bond, coupled
with oscillations about the N-CAr axes, as shown in Scheme
3. Space-filling models clearly show that even in complex 2
full rotation of the aryl rings about the N-CAr axis is not
possible owing to the strong steric interactions between the ortho
substituents and the Cp* group. This steric effect is enhanced
in complex 3, which contains larger isopropyl groups in the
ortho positions of its phenyl rings. However, small-angle
oscillations of the two rings are certainly possible, and this,
coupled with rotation about the Ni-Ccarbene bonds, would make
the two ortho groups of the aromatic rings equivalent; the two
meta substituents also become equivalent. The activation
energies of the dynamic processes for complexes 2 and 3 are
essentially the same. As these energies are unlikely to be equal
if the dynamic processes involved full rotation about the N-CAr
axes (the steric footprints of the aryl groups in 2 and 3 are quite
different), this also suggests that the dynamic process involves
Ni-Ccarbene rotation and not full N-CAr rotation.
In summary, it appears that the sterically constrained com-
plexes 2 and 3 both exhibit oscillations about the N-CAr bonds
and a comparable rotational barrier about the Ni-Ccarbene bond.
The absence of rotation about the nitrogen-aryl bonds is clearly
due to steric effects. Conjugation with the aromatic ring may
also result in the N-CAr bond having partial double-bond

Pentamethylcyclopentadienyl Ni Complexes of NHCs
Organometallics, Vol. 27, No. 16, 2008 4227
character.51 However, the origin of the restricted rotation
observed about the metal-NHC bond, which is seen here and
has also been observed elsewhere, is less obvious. Restricted
rotation about the metal carbene carbon bond was not observed
in the closely related Cp analogues [Ni(NHC)ClCp].44,45 Previ-
ous studies on the nature of the rotational barrier initially
invokedstericratherthanelectroniceffects,sincethemetal-carbene
bond was believed to have predominantly single-bond charac-
ter.8,49,52–55 However, more recent studies suggest that π-back-
bonding in NHC-metal bonds may not be negligible,9,10 and
indeed π-back-donation was found to make around a 10%
contribution to the Pt-NHC bond in [cis-Pt(NHC)(DMSO)Cl2]
complexes.11
It is noteworthy that the carbene carbon atoms in complexes
1a, 2, and 3 appear at 175 (in CDCl3) and 177 and 180 ppm
(both in C6D6), respectively, in the 13C NMR spectrum of these
complexes. These signals are downfield of the signals seen at
165, 166,45 and 16944 ppm, respectively (all in CDCl3), for their
corresponding Cp derivatives, and likely indicate increased
π-back-donation from the more electron-rich nickel atoms in
the Cp* complexes as compared to their Cp analogues.56 The
rotational barrier about the Ni-Ccarbene bond of 2 and 3 is thus
probably mainly steric in nature, but there is possibly a minor
electronic component.
the Institut de Chimie, Universite´ Louis Pasteur in Strasbourg.
Commercial compounds were used as received. 1,3-Dimethylimi-
dazolium iodide,57 1,3-bis(2,4,6,-trimethylphenyl)imidazolium
chloride,58,59 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride,58
and bis(2,4-pentanedionato)nickel(II)60,61 were prepared according
to published methods.
Synthesis of [Ni(Me-NHC)ICp*], 1a. n-Butyllithium (6.25 mL,
1.6 M solution in hexanes, 10.0 mmol) was added dropwise at -25
°C to a solution of pentamethylcyclopentadiene (1.61 mL, 10.0
mmol) in tetrahydrofuran (20 mL). After a few minutes, the
resulting white suspension was added to a green suspension of
bis(2,4-pentanedionato)nickel(II) (2.57 g, 10.0 mmol) in tetrahy-
drofuran (20 mL) at -25 °C. The reaction mixture was allowed to
reach room temperature and was stirred for 1 h at this temperature
to afford a dark red suspension of [NiCp*(acac)].46 The reaction
medium was again cooled to -25 °C, and a suspension of 1,3-
dimethylimidazolium iodide (2.24 g, 10.0 mmol) in tetrahydrofuran
(20 mL) was added. The mixture was again allowed to reach room
temperature and was then refluxed for 2 h. The resulting red-violet
suspension was filtered on Celite, and this was rinsed with
dichloromethane until the solvent ran colorless. The solvents were
removed under vacuum. The residue was washed with pentane (3
× 5 mL) to afford 1a as a red solid (2.37 g, 57% yield). Anal.
Calcd for C15H23N2INi: C, 43.21; H, 5.56; N, 6.72. Found: C, 42.89;
1
H, 5.43; N, 6.34. H NMR (toluene-d8, 298 K, 300.13 MHz): δ
6.04 (s, 2H, NCH); 3.58 (s, 6H, Me); 1.60 (s, 15H, C5Me5). 13C{1H}
NMR (CDCl3, 298 K, 75.47 MHz): δ 175.1 (NCN); 123.1 (NCH);
100.9 (C5Me5); 38.8 (Me); 10.8 (C5Me5).
Conclusion
Synthesis of [Ni(Me-NHC)ICp], 1b. A solution of nickelocene
(567 mg, 3.00 mmol) in DME (30 mL) was added to 1,3-
dimethylimidazolium iodide (699 mg, 3.12 mmol). The mixture
was refluxed for 63 h, during which time the solution color slowly
turned from a dark green to a dark red. The solvent was then
removed under vacuum and the reddish-black residue dissolved in
thf (20 mL), filtered on a Celite pad, and rinsed with tetrahydrofuran
(3 × 15 mL). The solvent was then removed under vacuum, and
the residue redissolved in dichloromethane and filtered on a 4 × 5
cm column of silica. Solvent was removed from the red filtrate,
and the resulting powder was washed with pentane (3 mL), dried,
and recrystallized from a diethylether/tetrahydrofuran mixture at
-28 °C to afford pure 1b (501 mg, 48% yield). Anal. Calcd for
C10H13N2INi: C, 34.63; H, 3.78; N 8.08. Found: C 34.87; H 3.93;
Neutral Cp* complexes of formula [Ni(NHC)XCp*] (NHC
) Me-NHC, X ) I, 1a; NHC ) Mes-NHC, X ) Cl, 2; NHC
) iPr-NHC, X ) Cl, 3) were isolated from the reaction of
[NiCp*(acac)], prepared in situ, with the corresponding imida-
zolium halides. The previously unreported complex [Ni(Me-
NHC)ICp] 1b was also prepared starting from nickelocene.
Single-crystal X-ray diffraction studies established the molecular
geometries of complexes 1a, 1b, and 2. Complexes 2 and 3
show a barrier to nickel-carbene bond rotation (∆Gq ) 65-67
kJ. mol-1) that was elucidated from VT 1H NMR studies. The
barrier is believed to be predominantly steric in nature. Studies
of the reactivity of these complexes are currently under way.
1
N 7.98. H NMR (CDCl3, 298 K, 300.13 MHz): δ 6.91 (s, 2H,
Experimental Section
NCH); 5.34 (s, 5H, C5H5), 4.15 (s, 6H, Me). 13C{1H} NMR (CDCl3,
298 K, 75.47 MHz): δ 165.1 (NCN); 123.9 (NCH); 91.9 (C5H5);
39.8 (Me).
General Comments. All reactions were carried out using
standard Schlenk techniques under an atmosphere of dry argon.
Solvents were distilled from appropriate drying agents under argon
prior to use. Solution NMR spectra were recorded on FT-Bruker
Ultra Shield 300 and FT Bruker-Spectrospin 400 spectrometers
operating at 300.13 or 400.14 MHz for 1H and at 75.47 or 100.61
MHz for 13C {1H}. DEPT 13C spectra were obtained for all
complexes to help in the 13C signal assignments. The chemical shifts
are referenced to the residual deuterated solvent peaks. Chemical
shifts (δ) and coupling constants (J) are expressed in ppm and Hz,
respectively. The 1H NMR variable-temperature experiments were
recorded at 400 MHz in toluene-d8, from 26 to 100 °C for complex
2 and from -10 to 100 °C for complex 3. Elemental analyses were
performed by the Service Central de Microanalyse du CNRS, at
Synthesis of [Ni(Mes-NHC)ClCp*], 2. [NiCp*(acac)]46 was
prepared as described for 1a using pentamethylcyclopentadiene (161
µL, 1.00 mmol) and n-butyllithium (625 µL of a 1.60 M hexanes
solution) in tetrahydrofuran (2 mL), followed by cold addition of
this slurry to a suspension of bis(2,4-pentanedionato)nickel(II) (257
mg, 1.00 mmol) in tetrahydrofuran (2 mL). The resulting dark red
reaction medium was maintained at room temperature, and a
suspension of 1,3-bis(2,4,6,-trimethylphenyl)imidazolium chloride
(341 mg, 1.00 mmol) in tetrahydrofuran (5 mL) was added. The
mixture was stirred at room temperature for 1 h, during which time
the color changed to violet. The solvent was then removed under
vacuum, and the resulting residue was extracted with toluene (10
mL) and filtered through a Celite pad. This was rinsed with toluene
(3 × 5 mL) until the washings were colorless. Concentration to
∼3 mL under vacuum followed by addition of pentane (10 mL)
(51) Gallagher, M. M.; Rooney, A. D.; Rooney, J. J. J. Organomet.
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(52) Newman, C. P.; Deeth, R. J.; Clarkson, G. J.; Rourke, J. P.
Organometallics 2007, 26, 6225.
(53) Enders, D.; Gielen, H. J. Organomet. Chem. 2001, 617, 70.
(54) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.; Herrmann, W. A.
Angew. Chem., Int. Ed. 1998, 37, 2490.
(55) Doyle, M. J.; Lappert, M. F. J. Chem. Soc., Chem. Commun. 1974,
679.
(57) Schaub, T.; Backes, M.; Radius, U. Organometallics 2006, 25, 4196.
(58) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R.; Craig, H. A.;
Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999, 55, 14523.
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4228 Organometallics, Vol. 27, No. 16, 2008
Ritleng et al.
afforded a violet solid when the solution was allowed to stand
overnight at -28 °C. The mother liquor was removed via syringe
and the solid washed with pentane (2 × 1 mL) and dried under
vacuum to give spectroscopically pure 2 (291 mg, 55% yield). 1H
NMR (toluene-d8, 298 K, 400.13 MHz): δ 6.92 (br, 2H, m-H); 6.80
(br, 2H, m-H); 6.22 (s, 2H, NCH); 2.69 (br, 6H, o-Me); 2.21 (s,
6H, p-Me); 1.81 (br, 6H, o-Me); 1.17 (s, 15H, C5Me5). 13C{1H}
NMR (C6D6, 298 K, 75.47 MHz): δ 177.1 (NCN); 138.6, 134.8
(o-C, C6H2Me3); 138.4 (p-C, C6H2Me3); 137.8 (ipso-C, C6H2Me3);
130.5, 128.4 (m-C, C6H2Me3); 124.1 (NCH); 102.1 (C5Me5); 21.4
(p-Me); 20.6 (o-Me); 18.6 (o-Me); 9.9 (C5Me5). A satisfactory
graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). A
summary of crystal data, data collection parameters, and structure
refinements is given in Table 1. Cell parameters were determined
from reflections taken from one set of 10 frames (1.0° steps in phi
angle), each at 20 s exposure. The structures were solved using
direct methods (SHELXS97) and refined against F2 for all reflec-
tions using the SHELXL97 software. Multiscan absorptions cor-
rections (MULscanABS in PLATON) were applied. All non-
hydrogen atoms were refined anisotropically in structures. The Cp*
rings in complex 1a exhibit disorder, and the Me groups of this
ligand were refined using a double-occupancy site model. Complex
1b lies on a crystallographic mirror plane, and there is only half of
the molecule in the asymmetric unit. Complex 2 contains a slightly
disordered molecule of toluene in the crystal lattice, and this part
of the structure was refined using fixed C-C distances. Hydrogen
atoms in all structures were generated according to stereochemistry
and refined as fixed contributors using a riding model in
SHELXL97.62,63
1
elemental analysis could not be obtained for complex 2; H and
13C{1H} spectra are included in the Supporting Information.
[Ni(iPr-NHC)ClCp*], 3. [NiCp*(acac)]46 was prepared as
described for 1a, using pentamethylcyclopentadiene (258 µL, 1.60
mmol) and n-butyllithium (1.00 mL, 1.6 M solution in hexanes) in
tetrahydrofuran (3 mL), followed by cold addition of this slurry to
a suspension of bis(2,4-pentanedionato)nickel(II) (411 mg, 1.60
mmol) in tetrahydrofuran (3 mL). The resulting dark red reaction
medium was cooled to -25 °C, and a suspension of 1,3-bis(2,6-
diisopropylphenyl)imidazolium chloride (679 mg, 1.60 mmol) in
tetrahydrofuran (8 mL) was added. The mixture was allowed to
reach room temperature and was then refluxed for 1 h. A color
change to violet occurred. The solvent was removed under vacuum,
and the resulting residue was extracted with toluene (15 mL) and
filtered over Celite. Concentration to dryness and several washings
with pentane gave 3 as a violet solid (365 mg, 37% yield).
Analytically pure solid was obtained by recrystallization from a
toluene/pentane solution. Anal. Calcd for C37H51N2ClNi: C, 71.91;
H, 8.32; N, 4.53. Found: C, 71.93; H, 8.10; N, 4.49. 1H NMR
Acknowledgment. We thank the CNRS and the Universite´
Louis Pasteur (University of Strasbourg) for financial support.
C.B. and S.M. thank the CNRS for M.Sc. research scholar-
ships. Mr. Michel Schmitt is gratefully acknowledged for
his assistance in obtaining the VT NMR data, and we
appreciate the work of Dr. Lydia Brelot in the X-ray structure
resolutions.
Supporting Information Available: Assigned 1H and 13C{1H}
and 13C DEPT NMR spectra of 2. CIF files giving X-ray structural
data, including data collection parameters, positional and thermal
parameters, and bond distances and angles for complexes 1a, 1b,
and 2. This material is available free of charge via the Internet at
http://pubs.acs.org. Crystallographic data (excluding structure fac-
tors) have also been deposited in the Cambridge Crystallographic
Data Centre as supplementary publication nos. CCDC 686287,
686288, and 686289, respectively. Copies of the data can be
obtained free of charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44 1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk; www:http://www.ccdc.cam.ac.uk).
3
(toluene-d8, 263 K, 400.13 MHz): δ 7.39 (d, 2H, m-H, J ) 7.6);
7.30 (t, 2H, p-H); 7.08 (d, 2H, m-H); 6.64 (s, 2H, NCH); 4.19 (m,
2H, CHMe2); 2.29 (m, 2H, CHMe2); 1.61 (d, 6H, 3J ) 6.8, CHMe2);
1.23 (d, 6H, 3J ) 6.8, CHMe2); 1.12 (s, 15H, C5Me5); 1.06 (d, 6H,
3J ) 6.8, CHMe2); 0.88 (d, 6H, 3J ) 6.8, CHMe2). 13C{1H} NMR
(C6D6, 298 K, 75.47 MHz): δ 180.0 (NCN); 149.6 and 145.8 (o-
CAr); 138.0 (ipso-CAr); 130.1 and 125.9 (m-CAr); 125.7 (p-CAr);
123.1 (NCH); 102.1 (C5Me5); 28.7 (CHMe2); 27.2 and 26.9
(CHMe2); 23.9 and 22.7 (CHMe2); 10.1 (C5Me5).
X-ray Diffraction Studies. Structure Determination and
Refinement. Single crystals of 1a and 1b suitable for X-ray
diffraction studies were selected from batches of crystals obtained
from thf/diethyl ether solutions at -25 °C. Crystals of complex 2
where harvested from toluene at -25 °C. Diffraction data for all
crystals were collected on a Kappa CCD diffractometer using
OM800376Q
(62) Kappa CCD Operation Manual; Delft, The Netherlands, 1997.
(63) Sheldrick, G. M. SHELXL97, Program for the refinement of crystal
structures; University of Gottingen: Germany, 1997.