Synthesis and characterization of 14-electron cyclopentadienyl chromium(II) complexes containing a heterocyclic carbene ligand

Article abstract of DOI:10.1021/om980799b

Chromocene reacts with 1,3-dimesitylimidazolium chloride in THF to yield cyclopentadiene and CpCrCl(1,3-dimesitylimidazolin-2-ylidene) (1), which contains an N,N-heterocyclic carbene ligand. This new complex is a relatively rare example of a CpCr(II) complex containing four unpaired electrons. The reaction of 1 with phenylmagnesium bromide yields CpCrPh(1,3-dimesitylimidazolin-2-ylidene) (2). An X-ray crystallographic investigation of 2 reveals a two-legged piano stool structure with a planar central Cr atom.

Full text of DOI:10.1021/om980799b

Organometallics 1999, 18, 529-533
529
Syn th esis a n d Ch a r a cter iza tion of 14-Electr on
Cyclop en ta d ien yl Ch r om iu m (II) Com p lexes Con ta in in g a
Heter ocyclic Ca r ben e Liga n d
Mark H. Voges,1a Christian Rømming, and Mats Tilset*,1b
University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway
Received September 23, 1998
Chromocene reacts with 1,3-dimesitylimidazolium chloride in THF to yield cyclopentadiene
and CpCrCl(1,3-dimesitylimidazolin-2-ylidene) (1), which contains an N,N-heterocyclic
carbene ligand. This new complex is a relatively rare example of a CpCr(II) complex
containing four unpaired electrons. The reaction of 1 with phenylmagnesium bromide yields
CpCrPh(1,3-dimesitylimidazolin-2-ylidene) (2). An X-ray crystallographic investigation of 2
reveals a two-legged piano stool structure with a planar central Cr atom.
In tr od u ction
bene ligands include their ease of synthesis6,9,10 and the
robustness of the complexes they form: excellent ther-
mal stabilities at temperatures as high as 290 °C have
been noted.11 Both characteristics are deemed essential
for the potential large-scale, industrial catalytic ap-
plications of these compounds. For example, organo-
chromium complexes are interesting alternatives12 to
the ubiquitous group 4 metallocenes13 in the area of
homogeneous olefin polymerization.
In 1968, O¨ fele and co-workers made use of the acidic
nature of imidazolium salts to synthesize the first Cr-
diaminocarbene complex (eq 1).14 We have employed
a similar strategy in our approach and will report
here the details about novel 14-electron CpCr(II) com-
plexes containing the 1,3-dimesitylimidazol-2-ylidene
ligand.
The chemistry of organochromium compounds con-
taining the Cp and Cp* ligands2 has received consider-
able attention, especially during the past decade. Much
of the recent work has focused on the Cr(III) oxidation
state.3 A number of complexes containing the CpCr(II)
fragment have also been reported, but most of these rely
on carbonyl or other strongly π-accepting coligands to
impart stability to the low oxidation state.4 In contrast,
far less is known about the chemistry of CpCr(II)
complexes in the absence of such coligands.5 We have
started our exploration of CpCr(II) complexes with the
recently popularized class of ligands: N,N-heterocyclic
carbenes (1,3-disubstituted imidazol-2-ylidenes). These
were first reported as stable entities by Arduengo and
co-workers in 19916 and have since then found signifi-
cant uses as ligands in organometallic complexes.7 They
are best considered neutral two-electron-donor ligands
with insignificant π-back-bonding tendency. As such,
they can be viewed as appealing alternatives to phos-
phines.7,8 Some of the advantages of such diaminocar-
(1) (a) Current address: Bayer AG, OC-Forschung, ZeTO, Geb. H5,
D-51368 Leverkusen, Germany. (b) E-mail: mats.tilset@kjemi.uio.no.
(2) Abbreviations: Cp ) η5-C5H5; Cp* ) η5-C5Me5.
Resu lts a n d Discu ssion
(3) For leading references see: (a) Poli, R.; Mattamana, S. P.
Organometallics 1997, 16, 2427-2433. (b) Poli, R. Chem. Rev. 1996,
96, 2135-2204. (c) J olly, P. W. Acc. Chem. Res. 1996, 29, 544-551.
(d) Theopold, K. H. Eur. J . Inorg. Chem. 1998, 15-24. (e) Theopold,
K. H. Acc. Chem. Res. 1990, 23, 263-270.
The reaction of chromocene with 1,3-dimesitylimida-
zolium chloride in THF at ambient temperature yields
CpCrCl(1,3-dimesitylimidazoline-2-ylidene) (1) as a dark
purple powder in 67% isolated yield (eq 2). The novel
complex 1 is highly air sensitive; solid samples of 1
immediately turn black upon exposure to air. On the
other hand, it possesses a surprising thermal stability
(4) For a leading reference, see: Theopold, K. H. In Encyclopedia
of Inorganic Chemistry; King, R. B., Ed.; J ohn Wiley & Sons: New
York, 1994; Vol. 2, pp 666-694.
(5) Examples in this class are reported by: (a) Fryzuk, M. D.;
Leznoff, D. B.; Rettig, S. J . Organometallics 1995, 14, 5193-5202. (b)
Bhandari, G.; Kim, Y.; MacFarland, J . M.; Rheingold, A. L.; Theopold,
K. H. Organometallics 1995, 14, 738-745. (c) Heintz, R. A.; Ostrander,
R. L.; Rheingold, A. L.; Theopold, K. H. J . Am. Chem. Soc. 1994, 116,
11387-11396. (d) Thomas, B. J .; Noh, S. K.; Schulte, G. K.; Sendlinger,
S. C.; Theopold, K. H. J . Am. Chem. Soc. 1991, 113, 893-902. (e)
Chisholm, M. H.; Cotton, F. A.; Extine, M. W.; Rideout, D. C. Inorg.
Chem. 1979, 18, 120-125.
(9) Arduengo, A. J ., III; Rasika Dias, H. V.; Harlow, R. L.; Kline,
M. J . Am. Chem. Soc. 1992, 114, 5530-5534.
(10) Herrmann, W. A.; Ko¨cher, C.; Goossen, L. J .; Artus, G. R. J .
Chem. Eur. J . 1996, 2, 1627-1636.
(11) Herrmann, W. A.; Elison, M.; Fischer, J .; Ko¨cher, C.; Artus, G.
R. J . Angew. Chem., Int. Ed. Engl. 1995, 34, 2371-2374.
(12) Theopold, K. H.; Heintz, R. A.; Noh, S.-K.; Thomas, B. J . In
Homogeneous Transition Metal Catalyzed Reactions; Moser, W. R.,
Slocum, D. W., Eds.; American Chemical Society: Washington, DC,
1992; p 591.
(6) Arduengo, A. J ., III; Harlow, R. L.; Kline, M. J . Am. Chem. Soc.
1991, 113, 361-363.
(7) For a recent review, see: Herrmann, W. A.; Ko¨cher, C. Angew.
Chem., Int. Ed. Engl. 1997, 36, 2162-2187.
(8) O¨ fele, K.; Herrmann, W. A.; Mihalios, D.; Elison, M.; Herdtweck,
E.; Scherer, W.; Mink, J . J . Organomet. Chem. 1993, 459, 177-184.
(13) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143-1170.
10.1021/om980799b CCC: $18.00 © 1999 American Chemical Society
Publication on Web 01/20/1999

530 Organometallics, Vol. 18, No. 4, 1999
Voges et al.
Ta ble 1. Su m m a r y of Cr ysta llogr a p h ic Da ta for
Com p lex 2
and melts without decomposition at 250 °C. The carbene
complex 1 is soluble in aromatic solvents, chlorinated
solvents, and THF. It is slightly soluble in diethyl ether,
but virtually insoluble in aliphatic solvents. Exposure
of a toluene solution to air results in an immediate color
change to green, followed by a slower change to black.
The products of the air oxidation have not been char-
acterized.
empirical formula
fw
C32H34CrN2
498.61
cryst color, habit
cryst dimens
cryst syst
brown, cubic
0.60 × 0.55 × 0.40 mm
triclinic
θ range for data collection
unit cell dimens
2.00-33.05°
a ) 8.37150(10) Å
b ) 8.49780(10) Å
c ) 20.4745(3) Å
R ) 94.1140(10)°
â ) 93.8870(10)°
γ ) 107.0170(10)°
V ) 13833.15(3) Å3
P1h
space group
Z
Dcalcd
2
1.197 g cm-3
wavelength
temp
no. of reflns collected
no. of ind reflns
abs coeff
0.710 73 Å
150(2) K
19 689
The 1H NMR spectrum (C6D6) of 1 shows broad peaks
characteristic of a paramagnetic material. The following
peak assignments are made primarily on the basis of
9879 [R(int) ) 0.0356]
0.435 mm-1
1
the integrated intensities of the H NMR signals. Upon
F(000)
528
complexation, the imidazolin-2-ylidene ring protons are
shifted upfield (δ -4.4), while the Cp protons resonate
as a very broad signal far downfield (δ 205). The
m-protons of the mesityl rings also experience a down-
field shift (δ 11.4), but much less so as they are further
removed from the metal center. The o- and p-methyl
resonances are shifted to δ 3.7 and δ 7.2, respectively.
When the reaction (eq 2) is performed in THF-d8 in an
NMR tube, the initial spectrum shows that product
formation is immediate and also confirms that cyclo-
pentadiene is formed in addition to 1.
A mass spectrometry analysis of the solid indicates
the M+ ion at m/e 456 with the correct isotopic distribu-
tion. There are no indications of dimers or higher
oligomers in the mass spectrum. The magnetic suscep-
tibility of 1 was measured between 5 and 300 K. It
displays the Curie-Weiss behavior of a simple para-
magnet. The solution magnetic susceptibility (Evan’s
method, 298 K, µeff ) 4.62 µB)15 was found to be
consistent with the solid-phase measurements, indicat-
ing a high-spin d4 ion with four unpaired electrons. The
presence of four unpaired electrons is unusual for a
CpCr(II) derivative. Most known examples are low-spin
with two unpaired electrons.5 However, in all these
examples, the Cr ion is six-coordinate, and the valence
electron count is either 15 or 16. In this regard, the 14-
electron complex 1 has more in common with non-Cp-
containing Cr(II) complexes, especially the recently
reported substituted hydridotris(pyrazolyl)borate (Tp)
complexes.16,17
limiting indices
-12 e h e 12, -12 e k e 13,
-31 e l e 27
refinement method
full-matrix least-squares on F2
9879/0/453
no. of data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
extinction coeff
largest diff peak and hole
1.074
R1 ) 0.052, wR2 ) 0.123
R1 ) 0.063, wR2 ) 0.131
0.009(2)
0.872 and -0.534 e Å-3
Complex 1 reacts with PhMgCl in THF at -30 °C to
give a brown solution. Workup of the reaction mixture
yields CpCrPh(1,3-dimesitylimidazolin-2-ylidene) (2) as
brown crystals in 55% yield (eq 3).
1
The Cp and carbene ligands in 2 displayed H NMR
characteristics similar to those of 1. The broad Cp signal
was located at δ 209, whereas the carbene ring protons
came to resonate at δ -6.5. The phenyl ring protons
appeared at δ -14.8 (m), -111 (p), and -164 (o). Full
details are given in the Experimental Section. The mass
spectrum of 2 showed a parent ion at m/e 498 with the
correct isotopic distribution.
The molecular structure of complex 2 was confirmed
by X-ray crystallography. Crystallographic data are
summarized in Table 1, and selected bond distances and
angles are summarized in Table 2. Figure 1 shows an
ORTEP plot of the crystal structure. Complex 2 is the
first structurally characterized Cr compound to employ
an N,N-heterocyclic carbene ligand. The only other Cr-
(N,N-heterocyclic carbene) complexes described in the
literature are Cr(0) examples. These are Cr(CO)5-
(carbene) and Cr(CO)4(biscarbene) species; the latter
employs a bidentate chelating carbene.8,14
Structural characterization of 1 by X-ray crystal-
lography proved impossible. The one-dimensional crys-
tals that were obtained from a variety of solvent systems
were invariably disordered. Currently, the best evidence
that 1 is a monomer is the combined mass spectrometry
and magnetic susceptibility data, along with the fact
that the phenyl derivative (2, see below) is monomeric.
(14) O¨ fele, K. J . Organomet. Chem. 1968, 12, P42-P43.
(15) (a) Evans, D. F. J . Chem. Soc. 1959, 2003-2005. (b) Sur, S. K.
J . Magn. Reson. 1989, 169-173. (c) Grant, D. H. J . Chem. Educ. 1995,
39-40.
(16) (a) Hermes, A. R.; Morris, R. J .; Girolami, G. S. Organometallics
1988, 7, 2372-2379. (b) Hermes, A. R.; Girolami, G. S. Inorg. Chem.
1988, 27, 1775-1781.
(17) Kersten, J . L.; Kucharczyk, R. R.; Yap, G. P. A.; Rheingold, A.
L.; Theopold, K. H. Chem. Eur. J . 1997, 3, 1668-1674.
The phenyl complex 2 adopts a two-legged piano stool
structure in which the phenyl and carbene ligands
constitute the legs. The Cr, Cp centroid, phenyl-C6, and
carbene-C12 atoms are located in a plane. The sum of
the Cp(centroid)-Cr-C12, C6-Cr-C12, and Cp(cen-

14-Electron Cyclopentadienyl Cr(II) Complexes
Organometallics, Vol. 18, No. 4, 1999 531
Ta ble 2. Selected In ter a tom ic Dista n ces a n d
An gles for Cp Cr P h (ca r ben e) (2)a
Distances (Å)
Cr1-C6
Cr1-C12
Cr1-C1
Cr1-C3
Cr1-C5
Cr1-C4
Cr1-C2
N1-C12
N1-C13
N1-C15
N2-C12
N2-C14
N2-C24
2.089 (2)
2.1232 (13)
2.316 (2)
2.343 (2)
2.347 (2)
2.351 (2)
2.352 (2)
1.371 (2)
1.393 (2)
1.446 (2)
1.373 (2)
1.395 (2)
1.447 (2)
C1-C5
C1-C2
C2-C3
C3-C4
C4-C5
C6-C7
C6-C11
C7-C8
C8-C9
C9-C10
C10-C11
C13-C14
Cr1-Ctr
1.380 (4)
1.401 (4)
1.444 (4)
1.400 (4)
1.368 (4)
1.416 (2)
1.416 (2)
1.403 (2)
1.392 (3)
1.398 (3)
1.403 (2)
1.357 (2)
2.017
F igu r e 2. ORTEP view of the molecular structure of [1,3-
bis(2,4,6-trimethylphenyl)imidazolium][Cp*CrCl3].
dimesitylimidazolin-2-ylidene) (3), which is in agree-
ment with spectroscopic evidence that has been obtained.
Monitoring the progress of this reaction by 1H NMR
(CDCl3) clearly shows the decrease in resonances at-
tributed to 1 and the growth of new resonances at-
tributable to 3. These are also broad and paramagnet-
ically shifted (δ 219 for the Cp ligand). Subsequent
removal of the solvent gives a blue residue. Dissolution
Angles (deg)
97.98 (5)
C6-Cr1-C12
C12-N1-C13
C12-N1-C15
C13-N1-C15
C12-N2-C14
C12-N2-C24
C14-N2-C24
C7-C6-C11
C7-C6-Cr1
C8-C7-C6
122.9 (2)
122.8 (2)
103.26 (11)
128.66 (10)
128.06 (10)
106.64 (13)
106.50 (13)
126.3
111.85 (12)
125.99 (11)
122.16 (12)
111.76 (12)
124.36 (12)
123.77 (12)
115.05 (14)
120.61 (11)
124.33 (11)
C10-C11-C6
N1-C12-N2
N1-C12-Cr1
N2-C12-Cr1
C14-C13-N1
C13-C14-N2
C6-Cr1-Ctr
C12-Cr1-Ctr
1
of the blue product in C6D6 and analysis by H NMR
confirm that a new species, clearly distinct from 1, has
formed. The solution magnetic susceptibility of 3 (Evan’s
method, 298 K, µeff ) 3.39 µB) is in good agreement with
that expected for a Cr(III) species with three unpaired
electrons. Further characterization is provided by its
UV-vis spectrum and mass spectrometry. The oxidation
of the chloride complex 1 to the dichloride 3 is not
unexpected. Early work by E. O. Fischer and co-workers
offers precedent for the oxidation of low-valent CpCr
compounds by halocarbons to various CpCr(III) ha-
lides.19
135.5
C11-C6-Cr1
a
Ctr ) centroid of Cp ligand.
F igu r e 1. ORTEP view of the molecular structure of
CpCrPh(carbene) (2).
Complexes 1 and 2 are EPR silent, both in solution
and in frozen toluene glasses.20 These observations are
consistent with those reported for other paramagnetic
Cr(II) complexes.21
troid)-Cr-C6 bond angles is 359.8°, and the maximum
deviation, for Cr, from the least-squares plane is 0.032
Å. The central Cr atom is bonded only to carbon atoms
with Cr-C bond lengths of Cr-C(Cp) average 2.342 Å,
Cr-C(Ph) 2.089 Å, and Cr-C(carbene) 2.123 Å. The
Cr-C(carbene) bond distance is within the range com-
monly observed for Fischer carbenes bonded to the Cr-
(CO)5 fragment.
The phenyl derivative 2 also bears structural similar-
ity to the above-mentioned TpCr species. This coordi-
natively unsaturated complex is most likely stabilized
by the sterically demanding mesityl groups. These
would also serve to keep the complexes monomeric by
preventing ligands from bridging and metal-metal
bonding interactions from forming.5c,e It remains to be
seen whether higher-nuclearity species are formed when
sterically less demanding N-substituents at the carbene
ligand are employed.
Unlike Cp2Cr, Cp*2Cr does not readily react with 1,3-
dimesitylimidazolium chloride. A deep blue solution is
formed only after 48 h of reflux in THF. Addition of
pentane yields a blue solid, [1,3-bis(2,4,6-trimethyl-
phenyl)imidazolium+][Cp*CrCl3-]. Its structure was
determined by X-ray crystallography; however, the
quality of the structure was poor and did not allow for
satisfactory refinement. The structure is shown here
(Figure 2) only as a proof of chemical connectivity.22 We
note that the constituent ions of this salt have been
previously characterized by X-ray crystallography as the
(18) Fettinger, J . C.; Mattamana, S. P.; Poli, R.; Rogers, R. Orga-
nometallics 1996, 15, 4211-4222.
(19) Fischer, E. O.; Ulm, K.; Kuzel, P. Z. Anorg. Allg. Chem. 1963,
319, 253-265.
(20) A small, sharp, low-intensity signal was detected around g ) 2
in many, but not all samples of 1 and 2. This feature is believed to
arise from an impurity or decomposition product.
In chloroform, 1 reacts immediately with the solvent
at ambient temperature to give a blue homogeneous
solution. The color is highly characteristic of CpCrCl2L
compounds, where L is a neutral or anionic two-electron-
donor ligand.18 A likely candidate (eq 4) is CpCrCl2(1,3-
(21) For a leading reference, see: Fryzuk, M. D.; Leznoff, D. B.;
Rettig, S. J . Organometallics 1995, 14, 5193-5202.
(22) Crystal data for [1,3-bis(2,4,6-trimethylphenyl)imidazolium]-
[Cp*CrCl3]‚toluene (Mr ) 704.15): monoclinic, space group P2/e, a )
16.5453(4) Å, b ) 14.1803(4) Å, c ) 15.9328(4) Å, â ) 96.148(1)°, V )
3716.6(2) Å3, Z ) 4, dcalc ) 1.258 Mg m-3, µ(Mo KR) ) 0.552 mm-1
.

532 Organometallics, Vol. 18, No. 4, 1999
Voges et al.
salts [Cp*CrCl3-][Cp*2Cr+]23a and [1,3-bis(2,4,6-trimethyl-
phenyl)imidazolium+]Cl-.23b
(15 mL). Upon filtration, 1,3-bis(2,4,6-trimethylphenyl)imida-
zolium chloride was isolated as a pure (by 1H NMR) white solid
in 40% yield.
The lack of reactivity of Cp*2Cr when exposed to 1,3-
bis(2,4,6-trimethylphenyl)imidazolium chloride was sur-
prising. It was anticipated that the higher basicity of
Cp*2Cr would make it more reactive. Since this was not
the case, we tentatively suggest that the coordination
of 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene is
prevented for steric reasons. The use of less sterically
demanding substituents on the imidazolium ring may
allow a reaction analogous to eq 2 to proceed with Cp*2-
Cr. This possibility and further studies of these and
related chromium complexes are in progress and will
be reported in due time.
P r ep a r a t ion of Cp Cr Cl(1,3-d im esit ylim id a zolin e-2-
ylid en e) (1). Solid 1,3-dimesitylimidazolium chloride (1.16 g,
3.40 mmol) was added to a dark solution of Cp2Cr (617 mg,
3.39 mmol) in THF (30 mL). After stirring for 1.5 h, the
resulting violet solution was filtered through a fine glass frit
to remove dark insoluble impurities. Diethyl ether (30 mL)
was added to the filtrate, and the solution was stored overnight
at -35 °C. The product was obtained as a violet powder upon
1
filtration (1.04 g, 67%). H NMR (C6D6, 200 MHz): δ 205 (br,
ω1/2 ) 2,800 Hz, 5H, Cp), 11.4 (br, ω1/2 ) 250 Hz, 4H, m-H),
7.2 (br, ω1/2 ) 50 Hz, 6 H, p-CH3) 3.7 (br, ω1/2 ) 790 Hz, 12 H,
o-CH3), -4.4 (br, ω1/2 ) 300 Hz, 2H, NCH). UV/vis (THF, nm
(ꢀ, M-1 cm-1)): λmax ) 535 (120), 360 (680). µeff ) 4.62 µB (Evan’s
method). Mp: 248-254 °C (no dec). Anal. Calcd for C26H29
-
Exp er im en ta l Section
ClCrN2: C, 68.34; H, 6.40; N, 6.13; Cr, 11.38. Found: C, 66.27;
H, 6.33; N, 6.01; Cr, 11.50 and 11.53. MS (EI): m/e 456 (M+),
391 (M+ - Cp), 356 (M+ - Cp - Cl).
Gen er a l P r oced u r es. All manipulations involving orga-
nometallic compounds were carried out with use of vacuum
line, Schlenk, syringe, and/or drybox techniques. Dichlo-
romethane, dichloromethane-d2, and chloroform-d were dis-
tilled from CaH2. THF was distilled from sodium benzophe-
none ketyl. 1H NMR spectra were recorded on Bruker DPX
200 and 300 instruments. Chemical shifts are reported in ppm
relative to tetramethylsilane, with the residual solvent proton
resonance as an internal standard. Melting points were
measured in capillary tubes sealed under vacuum. Elemental
analyses were performed by Ilse Beetz Mikroanalytisches
Laboratorium, Kronach, Germany, and by Canadian Mi-
croanalytical Service, Delta, B.C., Canada. The solid-state
magnetic susceptibility study of 1 was conducted on a Quan-
tum Design MPMS with a 5.5 T superconducting magnet and
a SQUID detection system. Solution magnetic moments were
measured by a modification of the Evan’s method (C6D5H as
a reference).15 EPR spectra were taken on a Bruker 200D-SRC
instrument.
P r ep a r a tion of Cp Cr P h (1,3-d im esitylim id a zolin e-2-
ylid en e) (2). Solid 1 (419.1 mg, 0.917 mmol) was dissolved in
THF (20 mL). After the solution was cooled at -35 °C, 1 equiv
of PhMgCl (0.510 mL, 1.8 M) was added via syringe. The
solution was observed to darken quickly and become brown.
THF was removed under vacuum, and the brown solid residue
was extracted with toluene (2 × 10 mL). A grayish, fine solid
was removed when the toluene mixture was filtered through
Celite. Pentane (50 mL) was added to the solution, causing
the appearance of more fine powder. This too was removed by
filtration through Celite. Large, brown, X-ray quality product
crystals, in addition to smaller microcrystals, were formed
upon cooling the solution at -35 °C overnight. Additional
cooling resulted in the formation of more product. The product
(240 mg, 49%) was isolated by decanting the mother liquor.
1H NMR (C6D6, 200 MHz): δ 209 (br, ω1/2 ) 3,660 Hz, 5 H,
Cp), 14.8 (br, ω1/2 ) 210 Hz, 2H, Cr-Ph m-H), 11.2 (br, ω1/2
)
The elemental analyses for neither 1 nor 2 were satisfactory
despite several attempts, including adding oxidants to the
samples, using separately synthesized material, and using two
different microanalytical laboratories. For 2, X-ray quality
crystals as well as microcrystalline powder were subjected to
analysis. Despite the effort, both 1 and 2 gave carbon analysis
results that were too low by 2-3%; at the same time, the
nitrogen and hydrogen contents were within an acceptable
range. Metal-carbide formation is known to be a common
problem for early transition metals. In our case, the carbene
ligand may make these compounds especially prone toward
carbide formation under combustion conditions.
1,3-Bis(2,4,6-tr im eth ylp h en yl)im id a zoliu m Ch lor id e.
Synthesis of 1,3-bis(2,4,6-trimethylphenyl)imidazolium chlo-
ride followed a modification of the procedure given for 1,3-bis-
(4-methylphenyl)imidazolium chloride.24 2,4,6-Trimethyla-
niline (13.5 g, 100 mmol), paraformaldehyde (1.50 g, 50 mmol),
and toluene (40 mL) were combined in a round-bottom flask,
resulting in an orange slurry (the color of the trimethylaniline
that was used). Heating to 100 °C for 1 h under inert
atmosphere caused the mixture to become homogeneous. After
cooling the reaction mixture to 40 °C, concentrated HCl (37%
in H2O, 4.93 g, 50 mmol) was added, resulting in the immedi-
ate precipitation of copious amounts of white solid. To this
suspension, glyoxal (40% in H2O, 7.26 g, 50 mmol) was added
and caused a color change to yellow. The mixture was heated
to reflux for 1.5 h, during which it turned black. Cooling the
mixture and removing the volatiles in vacuo left a sticky black
tar. The substance was triturated and washed with acetone
255 Hz, 4H, mesityl m-H), 7.96 (br, ω1/2 ) 70 Hz, 6H, p-CH3),
3.8 (br, ω1/2 ) 1,100 Hz, 12H, o-CH3), -6.5 (br, ω1/2 ) 350 Hz,
2H, NCH), -111 (br, ω1/2 ) 1100 Hz, 1H, Cr-Ph p-H), -164
(br, ω1/2 ) 5,600 Hz, 2H, Cr-Ph o-H). UV/vis (THF, nm (ꢀ,
M-1 cm-1)): λmax ) 505 (100), 385 (380). Anal. Calcd for C32H34
-
CrN2: C, 77.08; H, 6.87; N, 5.62. Found: C, 74.59; H, 6.79; N,
5.53. MS (EI): m/e 498 (M+), 496 (M+ - 2H), 421 (M+ - Ph),
420 (M+ - Ph-H), 356 (M+ - Ph - Cp).
Sp ectr oscop ic Evid en ce for Cp Cr Cl2(1,3-d im esitylim i-
d a zolin e-2-ylid en e) (3). Compound 1 was observed to im-
mediately react with CDCl3, changing the color of the solution
from purple to blue. Evaporation of the solvent left a blue
residue. 1H NMR (C6D6, 200 MHz): δ 219 (br, ω1/2 ) 8130 Hz,
5H, Cp), 8.4 (br, ω1/2 ) 110 Hz, 4H, m-H), 3.70 (br, ω1/2 ) 55
Hz, 12H, o-CH3), 2.16 (br, ω1/2 ) 55 Hz, 6H, p-CH3), -7.0 (br,
ω1/2 ) 220 Hz, 2H, NCH). UV/vis (THF, nm (ꢀ, M-1 cm-1)):
λmax ) 685 (470), 540 (60). MS (EI): m/e 491 (M+). µeff ) 3.39
µB (Evan’s method).
X-r a y Cr ysta llogr a p h ic Str u ctu r e Deter m in a tion of 2.
Brown cubic crystals were obtained by cooling a toluene/
pentane solution at -35 °C. A crystal of dimensions 0.60 ×
0.55 × 0.40 mm was mounted on a glass fiber using paratone
oil. X-ray data was collected on a Siemens SMART CCD
diffractometer25 using graphite-monochromated Mo KR radia-
tion. Data collection method: ω-scan, range 0.6°, crystal-to-
detector distance 5 cm; further information is given in Table
1. Data reduction and cell determination were carried out with
the SAINT and XPREP programs.25 Absorption corrections
were applied by the use of the SADABS program.26
(23) (a) Aldridge, S.; Shang, M.; Fehlner, T. P. Acta Crystallogr. C
1998, C54, 47-49. (b) Arduengo, A. J ., III; Gamper, S. F.; Tamm, M.;
Calabrese, J . C.; Davidson, F.; Craig, H. A. J . Am. Chem. Soc. 1995,
117, 572-573.
(25) SMART and SAINT Area-detector Control and Integration
Software; Siemens Analytical X-ray intruments Inc.: Madison, WI,
1996.
(24) Arduengo, A. J ., III; U.S. Patent 5 077 414, 1991.
(26) Sheldrick, G. M. Private communication, 1996.

14-Electron Cyclopentadienyl Cr(II) Complexes
Organometallics, Vol. 18, No. 4, 1999 533
The structure was determined and refined using the SHELX-
TL program package.27 The non-hydrogen atoms were refined
with anisotropic thermal parameters; hydrogen positions were
calculated from geometrical criteria and given isotropic ther-
mal parameters.
tered by the Norwegian Academy of Science and Letters.
We thank Professor Einar Sagstuen for assisting with
the EPR studies, and Per Fostervoll for collecting the
SQUID data. We also thank Dr. Richard Blom and Dr.
Klaus J ens for helpful discussions.
Ack n ow led gm en t is made to Borealis AS for gener-
ous support of this work. We also acknowledge the
support of Statoil under the VISTA program, adminis-
Su p p or tin g In for m a tion Ava ila ble: Complete list of
bond angles and distances, as well as atomic coordinates and
thermal displacement parameters for 2 (8 pages). Ordering
information is given on any current masthead page.
(27) Sheldrick, G. M. SHELXTL, Version 5; Siemens Analytical
X-ray Instruments Inc.: Madison, WI, 1996.
OM980799B