Group 9 complexes of an N-heterocycle carbene bearing a pentafluorobenzyl substituent: Attempted dehydrofluorinative coupling of cyclopentadienyl and N-heterocycle carbene ligands

Article abstract of DOI:10.1016/j.jfluchem.2004.10.002

Treatment of N-methylimidazole with pentafluorobenzyl bromide produces 1-pentafluorobenzyl-3-methylimidazolium bromide (1), which reacts with silver(I) oxide to give the N-heterocycle carbene (NHC) complex 1-pentafluorobenzyl-3- methylimidazolin-2-ylidene

Full text of DOI:10.1016/j.jfluchem.2004.10.002

Journal of Fluorine Chemistry 126 (2005) 451–455
www.elsevier.com/locate/fluor
Group 9 complexes of an N-heterocycle carbene bearing a
pentafluorobenzyl substituent: attempted dehydrofluorinative
coupling of cyclopentadienyl and N-heterocycle
carbene ligands
*
Sarah McGrandle, Graham C. Saunders
School of Chemistry, Queen’s University Belfast, David Keir Building, Belfast BT95AG, UK
Received 30 August 2004; received in revised form 5 October 2004; accepted 8 October 2004
Available online 18 November 2004
Dedicated to Prof. Dick Chambers on the occasion of his 70th birthday.
Abstract
Treatment of N-methylimidazole with pentafluorobenzyl bromide produces 1-pentafluorobenzyl-3-methylimidazolium bromide (1),
which reacts with silver(I) oxide to give the N-heterocycle carbene (NHC) complex 1-pentafluorobenzyl-3-methylimidazolin-2-ylidene
silver(I) bromide (2). Complex 2 acts as a carbene transfer reagent giving the complexes [(h5-C5Me5)MCl2(NHC)] (3a, M = Rh; 3b M = Ir)
on reaction with [(h5-C5Me5)MCl(m-Cl)]2. An attempt to use intramolecular dehydrofluorinative coupling methodology to link the carbene
and the pentamethylcyclopentadienyl ligands of [(h5-C5Me5)RhCl(CNtBu)(NHC)]BF4 was unsuccessful.
# 2004 Elsevier B.V. All rights reserved.
Keywords: Imidazole; N-heterocycle carbene; Dehydrofluorinative carbon–carbon coupling
1. Introduction
phosphine ligands [6]. This route involves the intramole-
cular dehydrofluorinative coupling of coordinated h5-
pentamethylcyclopentadienyl and phosphine ligands. The
latter must bear a fluorinated aryl group susceptible to
nucleophilic attack at an ortho position, such as penta-
fluorophenyl or tetrafluoropyridyl [6–8]. The development
of such an approach to ligands containing an NHC
functionality would obviate the problematic synthesis of a
non-coordinated cyclopentadienyl-NHC ligand and its
complexation. We therefore sought to investigate the
possibility of using the intramolecular dehydrofluorinative
coupling to prepare a cationic complex of a group 9 metal
containing a chelating cyclopentadienyl-NHC ligand. Here
we report the synthesis of pentamethylcyclopentadienyl
rhodium and iridium complexes of an NHC bearing a
pentafluorobenzyl substituent and the attempted intramole-
cular dehydrofluorinative carbon–carbon coupling of the
two ligands.
N-heterocycle carbenes (NHCs) are strong s-donors that,
in some cases, have been found to be more effective ligands
for organometallic catalysts than other two electron donor
ligands, such as phosphines [1–4]. Increasingly, chelating
ligands containing one or more NHC units and another
functionality are finding importance [5]. Although com-
plexes of h5-cyclopentadienyl ligands are exceedingly
common and important in organometallic chemistry, a
complex containing a chelating h5,kC-cyclopentadienyl-
NHC ligand is yet to be reported. We have developed
a convenient route to cationic rhodium and iridium com-
plexes of chelating tri- and bi-functional cyclopentadienyl-
* Corresponding author. Tel.: +44 28 90974682; fax: +44 28 90273333.
E-mail address: g.saunders@qub.ac.uk (G.C. Saunders).
0022-1139/$ – see front matter # 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2004.10.002

452
S. McGrandle, G.C. Saunders / Journal of Fluorine Chemistry 126 (2005) 451–455
2. Results and discussion
the formulation as a salt is based on that of the other silver
NHC complexes [1,9,10]. Although 2 was not obtained
analytically pure it was found amenable to further reaction
and was used without further purification.
The pentafluorobenzyl-substituted imidazolinium salt 1
was prepared in 87% yield by treatment of N-methylimi-
dazole with pentafluorobenzyl bromide in dichloromethane
(Scheme 1). Salt 1 was characterized by elemental analysis
and NMR spectroscopy, and the cation by mass spectro-
Treatment of the rhodium and iridium dimers [(h5-
C5Me5)MCl(m-Cl)]2 with 2 gave the NHC complexes 3a and
3b in 95 and 90% yield, respectively. The complexes
displayed similar NMR spectra. The two heterocycle
hydrogen resonances are at ca. d 7.0 and 6.5 with a mutual
coupling of ca. 2 Hz, and the methyl and pentamethylcy-
clopentadienyl hydrogen resonances occur at ca. d 4.0 and
1.7, respectively. The methylene hydrogen atoms are
diastereotopic and give broadened doublet resonances at
ca. d 6.0 and 5.5, with mutual coupling of ca. 15 Hz. These
resonances sharpen on cooling and broaden on warming.
The data are consistent with restricted rotation about the M–
C(NHC) bond and a conformation of the carbene ligand with
Cpy–Rh–C–N angles of ca. Æ908 (Fig. 1) leading to the non-
equivalence of the methylene hydrogen atoms. Simple
molecular models reveal that steric reasons can explain the
preference for this conformation and the restricted rotation.
Studies of other NHC complexes have revealed that any
restriction of rotation is for steric reasons alone [9,11].
Variable temperature NMR spectra of 3a and 3b were
recorded in order to obtain the energy barriers to rotation,
1
metry. As expected the H NMR spectrum exhibits five
resonances. The imidalozinium methylene and ethylene,
benzyl and methyl hydrogen resonances occur at d 10.81,
7.67, 7.61, 5.85 and 4.12, respectively, which are similar to
those of the non-fluorinated analogue [2]. The three
expected fluorine resonances occur at
d
ꢀ142.85,
ꢀ153.51 and ꢀ161.79, which are assigned to ortho, para
and meta fluorine atoms, respectively.
Silver NHC complexes, which are readily prepared from
imidazolium salts and silver(I) oxide, act as NHC transfer
reagents [1], removing the need to prepare free carbenes
which are highly sensitive to moisture. The silver complex 2
was prepared by treatment of 1 with silver(I) oxide and
isolated in 71% yield. The 1H NMR spectrum of 2 exhibited
only two higher frequency resonances indicative of the loss
of the ring methylene hydrogen atom, and is consistent with
the spectra of similar silver NHC complexes [9]. Attempts to
characterize 2 by mass spectrometry were unsuccessful, and
Scheme 1.

S. McGrandle, G.C. Saunders / Journal of Fluorine Chemistry 126 (2005) 451–455
453
3. Conclusion
Pentafluorobenzyl-substituted NHC complexes of rho-
dium and iridium have been conveniently prepared by the
use of a silver transfer reagent. In both complexes there is
restricted rotation about the M–C bond, which is proposed to
arise from the bulk of the ligands. A cation was generated
from the neutral rhodium complex by abstraction of a
t
chloride and addition of butylisonitrile. No intramolecular
dehydrofluorinative coupling was observed on addition of
proton sponge. The restricted rotation about the M–C bond
may account for the lack of reaction, since this may prevent
the pentafluorophenyl group from adopting a conformation
necessary for the reaction. Further investigations into the
intramolecular dehydrofluorinative coupling of penta-
methylcyclopentadienyl to NHC ligands are in progress.
Fig. 1.
DGz, about the M–C(NHC) bonds. Significant broadening of
the benzyl resonances was observed in the 1H NMR spectra
up to 323 K (at a spectrometer frequency of 300 MHz), but
the coalescence temperatures could not be reached in
CDCl3. Unfortunately the compounds were insufficiently
soluble in toluene, even at elevated temperature, for their
NMR spectra to be recorded. A minimum value of DGz of
62 kJmolꢀ1 for both complexes was obtained from the
spectra in CDCl3 [12–14]. Values of DGz of 60 kJ molꢀ1
upwards have been obtained for four coordinate group 9
metal(I) benzyl-NHC complexes with ligands less bulky
than h5-pentamethylcyclopentadienyl [9]. The ortho, para
and meta fluorine resonances occur at ca. d ꢀ141, ꢀ152 and
ꢀ160, respectively, showing that rotation about the C–C6F5
bond is not hindered.
4. Experimental
4.1. Instrumentation
1
The H and 19F NMR spectra were recorded in CDCl3
using a Bruker DPX300 or DRX500 spectrometer. 1H
(300.01 or 500.13 MHz) were referenced internally using
the residual protio solvent resonance relative to SiMe4 (d 0)
and 19F (282.26 MHz) externally to CFCl3 (d 0). All
chemical shifts are quoted in d (ppm), using the high
frequency positive convention, and coupling constants in
Hz. Infrared spectra were recorded on a Perkin-Elmer RX.1
FT-IR spectrophotometer. Mass spectra were recorded on a
VG Autospec X-series mass spectrometer. Elemental
analyses were performed by A.S.E.P., The School of
Chemistry, Queen’s University Belfast.
Treatment of complex 3a with t-butylisonitrile in the
presence of an excess of sodium tetrafluoroborate gave the
salt 4, in 75% yield (Scheme 1). The identity of the salt was
confirmed by elemental analysis, and that of the cation by
mass spectrometry. The IR spectrum exhibits n(CBN) at
2200 cmꢀ1
,
which is the same as that of [(h5-
C5Me5)RhCl2(CNtBu)] [15]. The H and 19F NMR spectra
are also consistent with this formulation, but show the
presence of two isomers in a ca. 1:1 ratio. The 19F spectra
show two pairs of resonances for the ortho, para and meta
fluorine atoms. The 1H NMR spectrum exhibits three
broadened singlets at d 3.93, 1.84 and 1.60 assigned to the
N-methyl, pentamethylcyclopentadienyl and butyl hydrogen
atoms, and six resonances in the region d 5.0–7.5, with a total
integration of four hydrogen atoms relative to that of three
for the N-methyl resonance, which can be assigned to the
NCH2 and HCCH hydrogen atoms. The data are consistent
with the two pairs of enantiomers shown in Scheme 1. These
arise from the stereogenic rhodium atom and rotamers from
the restriction of the rotation about the M–C(NHC) bond. It
is presumed that it is the bulk of the tBu group that increases
DGz significantly relative to 3a.
4.2. Materials
1
N-methylimidazole, benzyl chloride, silver(I) oxide and
tbutylisonitrile (Aldrich) were used as supplied. [(h5-
C5Me5)MCl(m-Cl)]2 (M = Rh or Ir) were prepared as
previously described [16].
4.3. 1-Pentafluorobenzyl-3-methylimidazolium
bromide (1)
Pentafluorobenzyl bromide (3.92 g, 15.0 mmol) was
added to N-methylimidazole (1.23 g, 15.0 mmol) in
dichloromethane (100 cm3) and the solution left at ambient
temperature for 20 h, after which time the colourless solid
that formed was collected by filtration and washed with
toluene (2 Â 10 cm3). Concentration of the filtrate produced
more solid, which was treated as before. The product was
obtained as a colourless crystalline solid (4.37 g, 87.4%).
Anal. Calcd. for C11H8N2BrF5Á0.5CH2Cl2: C, 35.8%; H,
2.35%; N, 7.3%. Found C, 36.9%; H, 2.7%; N, 7.7%.
LSIMS: 263 (M+), 245 ([M ꢀ F + H]+), 181 ([C6F5CH2]+).
In an attempt to induce dehydrofluoroinative carbon–
carbon coupling of the pentamethylcyclopentadienyl and
NHC ligands salt 4 was treated with proton sponge. No
reaction was observed by NMR over several days or at
elevated temperatures (up to 323 K).

454
S. McGrandle, G.C. Saunders / Journal of Fluorine Chemistry 126 (2005) 451–455
HRLSIMS: C11H8F5N2 requires 263.06077; found: M+
Calcd. for C21H22N2Cl2F5Ir.CHCl3: C, 33.9%; H, 3.0%; N,
3.6%. Found C, 34.0%; H, 2.65%; N, 3.8%. HRLSIMS:
C21H22N2ClF5193Ir requires 625.10211; found: M+
1
263.06014. H NMR: d 10.81 (1H, s, NCHN), 7.67 (1H, s,
HCCH), 7.61 (1H, s, HCCH), 5.85 (2H, s, NCH2), 4.12 (3H,
s, NCH3). 19F NMR: d ꢀ142.85 (2F, s, Fo), ꢀ153.51 (1F, s,
Fp), ꢀ161.79 (2F, s, Fm).
1
3
625.10487. H NMR: d 6.93 [1H, d, J(HH) 1.9, HCCH0],
6.50 [1H, d, 3J(HH) 1.9, HCCH0], 6.05 [1H, d, 2J(HH) 15.4,
2
NCHH0], 5.47 [1H, d, J(HH) 15.4, NCHH0], 3.99 (3H, s,
4.4. 1-Pentafluorobenzyl-3-methylimidazolin-2-ylidene
silver(I) bromide (2)
NCH3), 1.66 (15H, s, C5CH3). 19F NMR: d ꢀ141.19 (2F, m,
3
Fo), ꢀ151.88 [1F, t, J(FF) 20.4, Fp], ꢀ160.54 (2F, m, Fm).
4.7. [h5-Pentamethylcyclopentadienylrhodium-
(1-pentafluorobenzyl-3-methylimidazolin-2-ylidene)
tbutylisonitrilechloride]tetrafluoroborate (4)
Silver oxide (0.15 g, 0.65 mmol) was added to 1 (0.21 g,
0.61 mmol) in dichloromethane (100 cm3) and the mixture
stirred in the absence of light at ambient temperature for 2 h.
The reaction mixture was filtered through celite, which was
then washed with dichloromethane (2 Â 10 cm3). The
combined filtrate and washings were concentrated by rotary
evaporation to yield a pale yellow solid (0.41 g, 70.7%).
Complex 2 was not characterized by elemental analysis, but
Complex 3a (0.10 g, 0.185 mmol) was treated with tert-
butylisocyanide (0.015 g, 0.175 mmol) and NaBF4 (0.18 g,
1.73 mmol) in a solution of dichloromethane (30 cm3) and
methanol (20 cm3). After stirring for 1 h, the solvent was
removed by rotary evaporation and the product extracted
into dichloromethane. The solvent was removed from the
filtrate by rotary evaporation to yield a yellow solid (0.09 g,
75%). Anal. Calcd. for C26H31N3BClF9Rh: C, 44.25%; H,
4.4%; N, 5.95%. Found C, 43.5%; H, 4.4%; N, 5.6%.
HRLSIMS: C26H31N3ClF5Rh requires 618.11817; found:
M+ 618.11495. 1H NMR: d 7.35 (1H, br s, HCCH), 7.26 (1H,
br s, HCCH), 6.77 (2H, br s, HCCH), 6.12 [1H, br d, 3J(HH)
16.7, NCHH0], 5.36 (2H, br m, NCH2), 5.09 [1H, br d,
3J(HH) 16.7, NCHH0], 3.93 (3H, br s, NCH3), 1.84 (15H, br
s, C5CH3), 1.60 (9H, br s, tBu). 19F NMR: d ꢀ141.15 (2F, d, J
21.5, Fo), ꢀ141.82 (2F, d, J 15.6, Fo), ꢀ151.25 (1F, s, Fp),
ꢀ152.00 (1F, s, Fp), ꢀ153.01 (1.6F, s, 10BF4ꢀ), ꢀ153.07
(6.4F, s, 11BF4ꢀ), ꢀ160.25 (2F, m, Fm), ꢀ161.14 (2F, m, Fm).
1
its identity is confirmed by complexes 3a, 3b and 4. H
NMR: d 7.26 (1H, s, HCCH), 7.02 (1H, s, HCCH), 5.46 (2H,
s, NCH2), 3.87 (3H, s, NCH3). 19F NMR: d ꢀ140.84 (2F, s,
Fo), ꢀ151.12 (1F, s, Fp), ꢀ159.92 (2F, s, Fm).
4.5. h5-Pentamethylcyclopentadienylrhodium(1-
pentafluorobenzyl-3-methylimidazolin-2-ylidene)
dichloride (3a)
A solution of [{(h5-C5Me5)RhCl(m-Cl)}2] (0.068 g,
0.11 mmol) and 2 (0.10 g, 0.11 mmol) in dichloromethane
(100 cm3) was left at an ambient temperature for 15 h. The
reaction mixture was filtered through celite, which was
washed with dichloromethane (20 cm3). The combined
filtrate and washings were concentrated by rotary evapora-
tion to give an orange oil, which was recrystallized from
chloroform to yield an orange solid (0.06 g, 95.2%). Anal.
Calcd. for C21H22Cl2N2RhÁ0.5CHCl3: C, 40.9%; H, 3.6%;
N, 4.4%. Found C, 41.2%; H, 3.75%; N, 4.6%. LSIMS:
535 [M ꢀ Cl]+. HRLSIMS: C21H22N2ClF5Rh requires
n(CBN) 2200 cmꢀ1
.
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