Rhodium(I) and iridium(I) complexes of pyrazolyl-N-heterocyclic carbene ligands

Article abstract of DOI:10.1039/b603455a

Several Rh(i) and Ir(i) complexes containing an N-heterocyclic carbene-pyrazolyl chelate ligand have been synthesised. Determination of the single-crystal X-ray structure of the Ir(i) complex showed a novel binding mode with the iridium centre coordinated to two ligands via two carbene donors in preference to one ligand forming the entropically favoured chelate. The hydrogenation activity of several of these complexes was investigated along with that of previously synthesised Rh(i) and Ir(i) complexes containing an analogous phosphine-pyrazolyl chelate. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b603455a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Rhodium(I) and iridium(I) complexes of pyrazolyl-N-heterocyclic
carbene ligands
Barbara A. Messerle,*a Michael J. Pagea and Peter Turnerb
Received 7th March 2006, Accepted 25th May 2006
First published as an Advance Article on the web 9th June 2006
DOI: 10.1039/b603455a
Several Rh(I) and Ir(I) complexes containing an N-heterocyclic carbene-pyrazolyl chelate ligand have
been synthesised. Determination of the single-crystal X-ray structure of the Ir(I) complex showed a
novel binding mode with the iridium centre coordinated to two ligands via two carbene donors in
preference to one ligand forming the entropically favoured chelate. The hydrogenation activity of
several of these complexes was investigated along with that of previously synthesised Rh(I) and Ir(I)
complexes containing an analogous phosphine-pyrazolyl chelate.
Research into mixed donor NHC–N chelates, such as the NHC-
Introduction
oxazolyl ligand in 1, has attracted considerable attention. NHCs
are strong r-donor ligands that form very stable metal–ligand
bonds. N-Donor ligands on the other hand form weaker more
labile bonds.8 While it is often advantageous to incorporate these
weaker N donors into the metal catalyst, the ligand lability
can provide a route for catalyst decomposition. When tethered
to a stable NHC donor a relatively labile sp2 N donor will
become anchored to the metal centre thereby allowing only partial
ligand dissociation.9 Much early work in the development of
such heterotopic NHC–N ligands was aimed at the development
of Pd(0) complexes of NHC-pyridyl chelates (2 and 3)10 for
use as C–C bond forming catalysts. Later work has focused on
including non-heterocyclic imino groups into the ligand (4)11 that
increase the proximity of the sterically shielding Rꢀ group to the
N donor. More recently NHC-pyrazolyl ligands (5),12 related to
those reported here, and their complexes with Pd(II) and Ag(I)
have also been reported.
Herein we report the synthesis of the NHC-pyrazolyl ligand pre-
cursors: methyl-1-[(1-pyrazolyl)methyl]imidazolium tetraphenyl-
borate, [PzMeImH][BPh4] (6) and 3-methyl-1-[2-(1-pyrazolyl)-
ethyl]imidazolium tetraphenylborate, [PzEtImH][BPh4] (7). The
NHC derivatives PzMeIm and PzEtIm were generated in situ
to form complexes with Rh(I) and Ir(I): [Rh(PzMeIm)(COD)]-
[BPh4] (8), [Rh(PzEtIm)(COD)][BPh4] (9), [Rh(PzMeIm)(CO)2]-
[BPh4] (10), [Rh(PzEtIm)(CO)2][BPh4] (11) and [Ir(PzMeIm)2-
(COD)][BPh4] (12) where COD = 1,5-cyclooctadiene. Com-
plexes 8, 9 and 12 were investigated as potential hydrogena-
tion catalysts, as were the analogous phosphine-pyrazolyl com-
plexes [Rh(PyP)(COD)]BPh4 (13) and [Ir(PyP)(COD)]BPh4 (14)
(Chart 2).13
N-Heterocyclic carbene (NHC) ligands are known to be effective
and versatile donors in homogeneous transition-metal catalysts.1 It
was first noted by Hermann and co-workers that a close similarity
existed between NHCs and electron-rich organophosphines in
terms of their metal coordination chemistry.2 This led to an initial
application of NHCs as phosphine substitutes in homogeneous
catalysts of proven efficiency, most notably in the Pd(0) catalysed
Heck reaction3 and Ru(III) Grubbs type catalysts,4 where the use of
NHC donors resulted in a vast improvement to catalytic activity.
In contrast to this impact on C–C bond forming catalysts, early
work on incorporating NHC ligands into analogues of Wilkin-
son’s ([RhCl(PPh3)3]) and Crabtree’s ([Ir(PCy3)(pyridine)(COD)])
hydrogenation catalysts generally resulted in little improvement to
catalytic activity.5 Recent work by Burgess and co-workers, how-
ever, on chiral NHC-oxazolyl chelates has resulted in the develop-
ment of some highly active and enantioselective Ir(I) hydrogena-
tion catalysts with carbene donor ligands (1) (Chart 1).6 These sys-
tems showed an increased activity and much improved enantios-
electivity when compared to analogous Ir(I) phosphine-oxazolyl
systems,7 although the activity and selectivity of these Ir(I) systems
was highly dependent on the steric environment around iridium.
Results and discussion
Ligand precursor synthesis
Chart 1
The imidazolium salts 6 and 7 were synthesised via SN2 dis-
placement of the halide from 1-chloromethylpyrazole and 1-(2-
bromoethyl)pyrazole, respectively, with N-methylimidazole. From
a solution of toluene the halide salt precipitates as a pale yellow oil,
and counterion exchange with NaBPh4 in situ affords the desired
aSchool of Chemistry, University of New South Wales, Sydney, NSW, 2052,
Australia
bSchool of Chemistry, University of Sydney, Sydney, NSW, 2006, Australia.
E-mail: b.messerle@unsw.edu.au
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NMR spectroscopy, MS and microanalysis. Complex 8 was also
characterised by X-ray crystal structure determination (Fig. 2(a)).
In the 1H NMR spectra of 8 the COD resonances are observed as
four broad lines between 1.8 and 4.8 ppm due to fluxionality in
the conformation of the COD chelate. Conformational exchange
was no longer observed at 220 K at which temperature the COD
resonances sharpen up to form a set of 12 multiplets. At this
temperature a diastereotopic splitting of the bridgehead protons of
the NHC-pyrazolyl ligand and the COD CH2 protons is observed.
1
The H NMR spectra of 9 shows resolved COD resonances and
diastereotopic splitting of the ligand bridgehead protons and COD
CH2 protons, at room temperature. Conformational exchange
within complex 9 is therefore considerably slower than in complex
8, possibly due to an increased steric interaction between the COD
ligand and the NHC-pyrazolyl ligand.
Chart 2
The Ir(I) complex 12 was synthesised in an analogous fashion
to 8 and 9 via the reaction of two equivalents of ligand precursor 6
with [Ir(COD)(l-OEt)]2 and an excess of NaOEt. It was observed
that reacting [Ir(COD)(l-OEt)]2 with a sub-stoichiometric amount
of ligand precursor resulted in the formation of the same complex.
Complex 12 was isolated as an air stable orange solid in good
yield and characterised by 1H and 13C NMR spectroscopy,
MS, microanalysis and X-ray crystal structure determination
(Fig. 2(b)). The X-ray structure shows 12 to have a highly
unusual bonding mode with two potentially bidentate PzMeIm
ligands coordinated to the metal centre in a monodentate fashion
through the carbene donor alone, leaving both pyrazolyl donors
salt as an amorphous beige solid which can be recrystallised from
acetone–diethyl ether to yield fine white crystals of the product.
It was observed that nucleophiles such as DMF, acetonitrile and
triethylamine competed with N-methylimidazole in displacing the
halide from the pyrazolyl precursor.
The length of the alkyl chain bridging the ligand was increased
from one to two carbons (6 to 7) to investigate the influence of
the structure on the catalytic activity of the complex. Changing
the bridge length can influence the steric environment around the
metal centre by altering the bite angle of the chelate as well as
the orientation of the donor groups relative to the metal centre.
Ring-strain within the metallocycle can also affect the stability of
the chelate and the lability of the weaker pyrazolyl donor.
Unlike most analogous mixed donor NHC–N systems the
ligand derivatives of 6 and 7 are surrounded by very little steric
bulk leaving the coordinated metal centre largely unshielded.
However, the ligand design does allow for facile inclusion of bulky
substituents onto each of the pyrazolyl and the imidazolium rings
at the 3 position.14
1
uncoordinated. The H NMR spectra reveals two distinct sets
of resonances for each PzMeIm ligand and four distinct COD
ethylidene resonances. The methylene protons from each PzMeIm
ligand are also diastereotopic suggesting a strong steric interaction
restricting the mobility of the ligand arms. 2D 1H NOESY
experiments at 298 K revealed that the conformation of 12 was
still fluxional at this temperature with exchange peaks observed
between both sets of PzMeIm resonances and between COD
resonances (Fig. 1). At 200 K this exchange was no longer
observed.
Unlike the reaction with ligand precursor 6, the reaction
of ligand precursor 7 with [Ir(COD)(l-OEt)]2 under similar
conditions led to the formation of an amorphous red precipitate
which 1H NMR spectroscopy revealed to be a mixture of several
PzEtIm containing species, and which could not be sufficiently
purified and characterised. This result indicates the degree to
Complex synthesis
The Rh(I) complexes 8 and 9 were synthesised via deprotonation
of the corresponding imidazolium salt in situ by reaction with the
metal precursor [Rh(COD)(l-OEt)]2 in the presence of an excess
of NaOEt (Scheme 1). Both 8 and 9 were isolated as air-stable
bright yellow solids in high yield and characterised by 1H and 13
C
Scheme 1
3928 | Dalton Trans., 2006, 3927–3933
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Table 1 Crystallographic data for [Ir(PzMeIm)2(COD)]BPh4 (12) and
[Rh(PzMeIm)(COD)]BPh4 (8)
12
8
Empirical formula C48.50H53.25BIrN8O0.38
C40H42BN4Rh
692.50
M/g mol−1
957.25
Crystal system
Space group (no.)
Monoclinic
C2/c (15)
31.705(6)
13.512(3)
24.585(5)
117.508(3)
9341(3)
1.361
8
150(2)
0.71073
2.900
Monoclinic
P21/n (14)
9.396(5)
19.599(11)
18.476(11)
93.911(10)
3394(3)
1.355
4
150(2)
0.71073
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Dc/g cm−3
Z
T/K
˚
k(Mo-Ka)/A
l(Mo-Ka)/mm−1
Crystal size/mm
Crystal colour
0.537
0.347 × 0.204 × 0.049
0.366 × 0.119 × 0.078
Orange
Yellow
1
Fig. 1 2D H NOESY spectra of 12 at 298 K showing exchange peaks
Crystal habit
Blade
56.68
Prism
56.62
◦
2hmax
/
between COD ethylidene proton resonances.
hkl Range
−42 40, −17 17, −32 31 −12 12, −25 25, −23 24
N
44772
33922
8150 (0.0612)
6019
1.026
0.0324
0.0749
which complex structure and stability is influenced by altering
the alkyl bridge length within the ligand.
Nind (Rmerge
)
11157 (0.0666)
7636
1.213
Nobs (I > 2r(I))
GoF (all)
The rhodium dicarbonyl complexes 10 and 11 were synthesised
by the displacement of COD from complexes 8 and 9, respectively,
by stirring a suspension of the complex under an atmosphere of
CO. Both 10 and 11 were isolated as mildly air sensitive pale
R1 (F, I > 2r(I))
0.0375
wR2 (F2, all data) 0.0916
ꢀ
ꢀ
ꢀ
2
R1 =
ꢁFo| − |Fcꢁ/ |Fo| for Fo > 2r(Fo); wR2 = ( w(Fo
−
ꢀ
yellow solids. The complexes were characterised by H and 13C
1
Fc2)2/ (wFc2)2)1/2 all reflections, w = 1/[r2(Fo2) + (0.03P)2] where P =
2
(Fo + 2Fc2)/3
NMR spectroscopy, MS, microanalysis and FTIR spectroscopy.
Two strong bands were observed in the IR spectra at 2092 and
2017 cm−1, and 2093 and 2032 cm−1 for complexes 10 and 11
respectively, indicating the presence of two inequivalent, metal
bound, carbonyl groups. Attempted displacement of COD from
12 with CO resulted in complex decomposition.
maximised in such an orthogonal arrangement. The details of
these interactions, and in particular the validity of any dp–pp
back donation, have been discussed in detail elsewhere.17
On binding to Ir(I), the PzMeIm ligand is bound in a very
different manner to that observed for Rh(I). The crystal structure
of 12 shows a square planar geometry about iridium. The Ir(1)–
X-Ray crystal structure determination
˚
The solid-state structures of 8 and 12 were determined using single-
crystal X-ray diffraction analyses (Table 1). Crystals suitable for
analysis were obtained by vapour diffusion of hexane into a THF
solution of 8, and by vapour diffusion of diethyl ether into a DCM
solution of 12, under N2. ORTEP depictions of the cations of 8
and 12 are shown in Fig. 2.
The crystal structure of 8 reveals a square planar geometry
around the metal centre with one PzMeIm ligand bound in a
chelate fashion to rhodium as expected. The Rh(1)–C(15) bond
C(9) and Ir(1)–C(17) bond distances are 2.053(4) and 2.045(4) A,
respectively. The angle of 93.57(16)◦ for C(9)–Ir(1)–C(17) is
significantly larger than the ideal 90◦ and is probably due to steric
crowding between the uncoordinated pyrazolyl arms. The NHC
ring planes in 12 are oriented perpendicular to the square plane of
the complex, consistent with the binding anisotropy noted above
and free rotation of the ligand about the Ir–C bond.
We have previously observed pyrazolyl donors to have only
a slightly weaker coordinating strength to Rh(I) than to Ir(I).13
There should be a negligible difference between the steric nature
of PzMeIm when chelated to either Rh(I) or Ir(I). This suggests
that the orbital interactions described above are more favourable
for Ir(I) complexes than Rh(I) complexes (at least with COD as
coligand), thereby inducing the NHC to adopt an orthogonal
orientation to the plane of the metal complex and disrupting the
formation of the chelate. This phenomenon is not observed in
analogous chelates with an NHC ligand tethered to a stronger
donor such as phosphine or even pyridine, in which case the desired
chelate is always formed.18
5b,15,17c
˚
distance of 2.053(2) A is in the range expected for such bonds.
The bite angle of 85.14(7)◦ for N(4)–Rh(1)–C(15) is quite low due
to ring-strain within the metallocycle, which adopts a pseudo-boat
conformation. The bite angles of the six-membered metallocycles
formed by the analogous pyrazolylphosphine chelate (PyP in 13)13
and N,N-bispyrazolylmethane chelate (Bpm)16 ligands bound to
Rh(I) are significantly higher (88.9 and 88.4◦ respectively). The
increased ring-strain in 8 can be attributed to the anisotropy of
NHC coordination which directs the carbene ring towards an
orthogonal orientation relative to the plane of the complex. Two
factors contribute to this anisotropy, firstly in presenting the slim
axis of the carbene ring to the bulky plane of the complex any
steric interaction with the ligand is minimized. Secondly, orbital
interactions between the vacant carbene p-orbital and either a
free metal d-orbital and/or a metal–ligand r-bonding orbital are
Hydrogenation reactions
The Rh(I) and Ir(I) NHC complexes 8, 9 and 12 were investigated
as potential hydrogenation catalysts, as were the analogous
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the Rh(I) complex containing the phosphine-pyrazolyl chelate
(13) showed a significantly higher activity than the Rh(I) NHC-
pyrazolyl analogues (8 and 9) with the hydrogenation reactions
reaching completion in only 35 min in the case of 13 and 50 min
when using complexes 8 and 9. The biscarbene complex 12 showed
absolutely no activity after several hours. This was unexpected as
similar bisphosphine complexes reported in the literature exhibit
a moderate activity under similar conditions.19 It is possible that
in complex 12 the uncoordinated pyrazolyl donors interfere with
the reaction progress via a transient coordination to the iridium
centre.
The hydrogenation of simple olefins such as styrene is achieved
in less time and under milder conditions (e.g. at 25 ◦C and 1 atm
H2), using commercially available complexes such as Crabtree’s
catalyst and its analogues,5,19 than the rate of hydrogenation
reported here for complexes 8, 9, 12, 13 and 14. The hydrogenation
activity of complexes with chelating NHC–N ligands is strongly
influenced by the steric environment about the metal, with a
greater steric shielding of the metal centre generally leading to an
improved catalytic activity.6 This suggests that there is considerable
potential to improve the activity of these complexes, with the
possibility of developing these complexes as catalysts for the hydro-
genation of more substituted, sterically demanding substrates.
Conclusions
Two new mixed donor NHC-pyrazolyl ligand precursors 6 and 7
were synthesised and their coordination chemistry with Ir(I) and
Rh(I) was investigated. An unusual binding mode was observed in
complex 12 with two potentially chelating NHC-pyrazolyl ligands
binding to iridium in a monodentate fashion through the carbene
donor. Complexes 8, 9, 13 and 14 were shown to be efficient
catalysts for the hydrogenation of styrene. A more comprehensive
hydrogenation study is currently underway to fully investigate the
potential catalytic activity of these complexes and their derivatives.
Fig. 2 The ORTEP depictions of (a) [Rh(PzMeIm)(COD)]+ (8) and (b)
[Ir(PzMeIm)2(COD)]+ (12) at 20% thermal ellipsoids for non-hydrogen
atoms.
Experimental
complexes containing pyrazolylphosphine ligands (13 and 14). The
reactions were performed under 120 psi of H2 gas, at 55 ◦C, using
1 mol% of catalyst in THF; the results are summarized in Table 2.
Catalysts 8, 9, 13 and 14 all showed a moderate activity with
complete conversion of the substrate within 50 min in all cases. No
change in activity was observed between complexes 8 and 9, i.e. in
going from the six membered to the seven-membered metallocycle,
nor is any change in activity observed between the analogous
pair of rhodium and iridium complexes 13 and 14. Interestingly,
General procedures
All manipulations of metal complexes and air sensitive reagents
were carried out using standard Schlenk techniques. All solvents
were dried and distilled under nitrogen prior to use. 1-Chloro-
methylpyrazole,20 1-(2-Bromoethyl)pyrazole,21 [Ir(COD)(l-Cl)]2,22
[Rh(COD)(l-Cl)]2,23 [Ir(COD)(l-OEt)]2
and [Rh(COD)(l-
24
24
OEt)]2 were prepared by literature methods.
1H and 13C NMR spectra were recorded on Bruker DPX300
and DMX500 spectrometers, operating at 300 and 500 MHz (1H)
Table 2 Hydrogenation of styrene
and 75 and 125 MHz (13C), respectively. H and 13C chemical
1
shifts are referenced to internal solvent resonances. The following
subscripted abbreviations are used to assign resonances: Pz =
pyrazolyl, Im = imidazolyl, COD = 1,5-cyclooctadiene and
BPh = tetraphenylborate counterion. IR spectra were recorded
on an Avatar 370 FT-IR (Thermo Nicolet) spectrometer. Melting
points were determined using a Mel-Temp (Laboratory Devices)
apparatus. ESI-MS were carried out in the Mass Spectrometry
Unit, School of Chemistry, University of Sydney, Australia.
Microanalyses were carried out at the Campbell Analytical
Laboratory, the University of Otago, New Zealand. The X-ray
Catalyst
% Conversion (time/min)
12 [Ir(PzMeIm)2(COD)]BPh4
8 [Rh(PzMeIm)(COD)]BPh4
9 [Rh(PzEtIm)(COD)]BPh4
13 [Rh(PyP)(COD)]BPh4
14 [Ir(PyP)(COD)]BPh4
0 (180)
100 (50)
100 (50)
100 (35)
100 (35)
3930 | Dalton Trans., 2006, 3927–3933
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structures of 12 and 8 were obtained by Dr Peter Turner at the
X-ray crystallography centre, University of Sydney.
CCDC reference numbers 600823 and 600824.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b603455a
[Rh(COD)(l-OEt)]2 (0.102 g, 0.199 mmol) and sodium ethoxide
(0.028 g, 0.411 mmol) in methanol (20 mL) and the mixture stirred
for 3 h. A bright yellow solution formed and a beige solid remained
undissolved. The solid was filtered off and the filtrate reduced
in vacuo, to ca. 20 mL, to precipitate a bright yellow solid. This
precipitate was washed with hexane and dried in vacuo to yield 8
(0.253 g, 89%), mp 176–178 ◦C (decomp.).
Synthesis of 3-methyl-1-[(1-pyrazolyl)methyl]imidazolium tetra-
phenylborate, 6. A solution of 1-chloromethylpyrazole hydrogen
chloride (1.09 g, 7.12 mmol) in toluene (15 mL) was added to a
solution of 1-methylimidazole (1.50 mL, 18.8 mmol) in toluene
(15 mL) and the mixture was left to reflux for 24 h. NaBPh4
(2.50 g, 7.31 mmol) was then added and the mixture refluxed for a
further 3 h, after which time a beige precipitate had formed. The
precipitate was recrystallised from acetone–diethyl ether to yield
fluffy white crystals of 6 (2.22 g, 65%), mp 208–210 ◦C.
1H NMR (500 MHz, CD2Cl2, 220 K): d 7.38 (m, 8H, m-CHBPh),
7.32 (d, 3JH3–H4 = 2.2, 1H, H3Pz), 7.16 (d, 3JH5–H4 = 2.6, 1H, H5Pz),
3
3
7.03 (t, Jo-CH–m-CH = 7.3, 8H, o-CHBPh), 6.90 (t, Jp-CH–m-CH = 7.3,
4H, p-CHBPh), 6.63 (d, 3JH4–H5 = 2.0, 1H, H4Im), 6.57 (d, 3JH5–H4
=
3
2.0, 1H, H5Im), 6.23 (app. t, JH4–H3/H5 = 2.4, 1H, H4Pz), 5.44 (d,
2JCH–CH = 13.8, 1H, N–CHa), 4.92 (app. t, JH1–H2/H8 = 7.3, 1H,
3
H1COD-trans to C), 4.48 (dt, 3JH2–H1 = 7.3, 3JH2–H3 = 7.5, 1H, H2COD),
4.42 (app. t, 3JH5–H6/H4 = 7.3, 1H, H5COD), 4.27 (d, 3JCH–CH = 13.8,
1H NMR (300 MHz, acetone-d6): d 9.08 (br s, 1H, H2Im), 8.01
(d, 1H, 3JH3/5–H4 = 2.3, H3Pz or H5Pz), 7.81 (dd, 1H, 3JH4–H5 = 1.9,
1H, N–CHb), 4.18 (dt, JH6–H5 = 5.7, JH6–H7 = 7.3, 1H, H6COD),
3
3
3.65 (s, 3H, CH3), 2.70 (m, 1H, H8CODa), 2.54 (m, 1H, H4CODa),
4JH4/5–H2 = 1.5, H4Im or H5Im), 7.64 (dd, 1H, 3JH5–H4 = 1.9, 4JH5/4–H2
=
2.34 (m, 1H, H8CODb), 2.31 (m, 1H, H4CODb), 2.25 (m, 1H, H7CODa),
1.5, H5Im or H4Im), 7.62 (d, 1H, 3JH5/3–H4 = 1.9, H5Pz or H3Pz), 7.34
(m, 8H, o-CHBPh), 6.91 (dd, 8H, 3Jm-CH–o-CH = 7.5, 3Jm-CH–p-CH = 7.2,
m-CHBPh), 6.76 (t, 4H, 3Jp-CH–m-CH = 7.2, p-CHBPh), 6.23 (s, 2H, CH2),
6.38 (dd, 1H, 3JH4–H3/5 = 2.3, 3JH4–H5/3 = 1.9, H4Pz), 4.01 (s, 3H, CH3)
ppm. 13C NMR (300 MHz, acetone-d6): d 164.00 (q, 1JB–C = 49.4,
B–C), 141.85 (C3Pz or C5Pz), 136.75 (C2Im), 136.00 (o-CBPh), 130.95
(C5Pz or C3Pz), 125.05 (m-CBPh), 124.25 (C4Im or C5Im), 121.90 (C5Im
or C4Im), 121.25 (p-CBPh), 107.20 (C4Pz), 61.95 (CH2), 36.00 (CH3)
ppm. IR (KBr): m 3053, 1295, 1174, 1090, 843, 766, 707 cm−1.
MS (electrospray) m/z: (ES+) 163.1 (100%, [PzMeImH]+), (ES−)
319.6 (100%, [BPh4]−). Microanalysis: found: C 78.46, H 6.52, N
11.63%; calc.: C 79.67, H 6.48, N 11.61%.
2.11 (m, 1H, H3CODa), 1.81 (m, 1H, H7CODb), 1.77 (m, 1H, H3COD
)
b
ppm. 13C NMR (125 MHz, CD2Cl2, 220 K): d 174.72 (d, 1JC2–Rh
=
50.0, C2Im), 163.75 (q, 1JB–C = 49.4, B–C), 141.55 (C3Pz), 135.60 (m-
CBPh), 132.69 (C5Pz), 126.08 (o-CBPh), 122.72 (C4Im), 122.14 (p-CBPh),
120.99 (C5Im), 107.21 (C4Pz), 97.87 (C2COD), 96.87 (C1COD), 80.16
(C5COD), 70.75 (C6COD), 62.31 (CH2), 38.15 (CH3), 35.13 (C4COD),
30.78 (C8COD), 29.30 (C7COD), 26.92 (C3COD) ppm. IR (KBr): m
3052, 1477, 1269, 1221, 735, 709 cm−1. MS m/z: (ES+) 373.1 (12%,
[Rh(PzMeIm)(COD)]+), 371.0 (100%). Microanalysis: found: C
66.29, H 5.96, N 7.82%; calc.: (3 + 2MeOH) C 66.67, H 6.66, N
7.41%.
Synthesis of Rh complex 9. Complex 9 was synthesised in a
similar fashion to 8 from the ligand precursor 7. A bright yellow
powder of 9 was isolated in good yield (0.089 g, 86%), mp 178–
180 ◦C (decomp.).
Synthesis of 3-methyl-1-[2-(1-pyrazolyl)ethyl]imidazolium tetra-
phenylborate, 7. A solution of 1-(2-bromoethyl)pyrazole (1.50 g,
8.57 mmol) in toluene (15 mL) was added to a solution of 1-
methylimidazole (1.5 mL, 18.8 mmol) in toluene (15 mL) and the
mixture heated under reflux for 24 h. NaBPh4 (3.00 g, 8.77 mmol)
was then added and the mixture refluxed for a further 3 h, after
which time a beige precipitate had formed. The precipitate was
recrystallised from acetone–diethyl ether to yield fluffy white
crystals of 7 (4.17 g, 97%), mp 174–176 ◦C.
1H NMR (500 MHz, CD2Cl2): d 7.39 (m, 8H, m-CHBPh), 7.28
3
3
(d, JH5–H4 = 2.6, 1H, H5Pz), 7.20 (d, JH3–H4 = 1.8, 1H, H3Pz),
3
3
7.06 (t, Jo-CH–m-CH = 7.3, 8H, o-CHBPh), 6.91 (tt, Jp-CH–m-CH = 7.3,
4Jp-CH–o-CH = 1.3, 4H, p-CHBPh), 6.71 (d, JH4–H5 = 1.8, 1H, H4Im),
3
6.56 (d, 3JH5–H4 = 1.1, 1H, H5Im), 6.24 (app. t, 3JH4–H3/H5 = 2.2, 1H,
H4Pz), 6.14 (m, 1H, NIm–CHa), 4.91 (m, 1H, H1COD-trans to C),
4.42 (m, 1H, NPz–CHa), 4.38 (m, 1H, H2COD), 4.31 (m, 1H, H5COD),
4.22 (dt, 2JCH–CH = 15.2, 3JCH–CH2 = 4.8, 1H, NIm–CHb), 3.94 (s, 3H,
CH3), 3.79 (m, 1H, H6COD), 3.76 (m, 1H, NPz–CHb), 2.66 (m, 1H,
H8CODa), 2.56 (m, 1H, H4CODa), 2.46 (m, 1H, H7CODa), 2.33 (m, 1H,
H3CODa), 2.25 (m, 1H, H8CODb), 2.24 (m, 1H, H4CODb), 2.05 (m,
1H, H7CODb), 1.98 (m, 1H, H3CODb) ppm. 13C NMR (125 MHz,
CD2Cl2): d 163.90 (q, 1JB–C = 49.4, B–C), 140.36 (C3Pz), 135.83 (m-
CBPh), 133.41 (C5Pz), 125.60 (o-CBPh), 122.94 (C4Im), 122.86 (C5Im),
121.78 (p-CBPh), 107.90 (C4Pz), 97.48 (C1COD), 95.19 (C2COD), 80.04
(C5COD), 72.76 (C6COD), 50.02 (NIm–C), 48.30 (NPz–C), 38.60 (CH3),
33.59 (C4COD), 30.36 (C7COD), 29.97 (C8COD), 27.97 (C3COD) ppm
(C2Im undefined). IR (KBr): m 3054, 1477, 768, 735, 707 cm−1.
MS m/z: (ES+) 387.1 (29%, [Rh(PzEtIm)(COD)]+), 375.0 (100%).
Microanalysis: found: C 69.70, H 6.51, N 8.30%; calc.: C 69.70, H
6.28, N 7.93%.
1H NMR (300 MHz, acetone-d6): d 8.49 (br s, 1H, H2Im), 7.56
(dd, 1H, 3JH5–H4 = 1.9, 4JH5/4–H2 = 1.5, H5Im or H4Im), 7.54 (d, 1H,
3
3JH3/5-H4 = 2.3, H3Pz or H5Pz), 7.46 (d, 1H, JH5/3–H4 = 1.9, H5Pz
3
4
or H3Pz), 7.39 (dd, 1H, 1H, JH4–H5 = 1.9, JH4/5–H2 = 1.5, H4Im
3
or H5Im), 7.34 (m, 8H, o-CHBPh), 6.91 (dd, 8H, Jm-CH–o-CH = 7.5,
3Jm-CH–p-CH = 7.5, m-CHBPh), 6.76 (t, 4H, 3Jp-CH–m-CH = 7.5, p-CHBPh),
3
3
6.22 (dd, 1H, JH4–H3/5 = 1.9, JH4–H5/3 = 2.3, H4Pz), 4.77 (m, 2H,
NIm–CH2 or NPz–CH2), 4.66 (m, 2H, NPz–CH2 or NIm–CH2), 3.91
(s, 3H, CH3) ppm. 13C NMR (300 MHz, acetone-d6): d 164.00 (q,
1JB–C = 49.4, B–C), 139.80 (C5Pz or C3Pz), 136.00 (o-CBPh), 130.30
(C3Pz or C5Pz), 125.05 (m-CBPh), 123.70 (C5Im or C4Im), 122.65 (C4Im
or C5Im), 121.25 (p-CBPh), 105.65 (C4Pz), 50.70 (NIm–C or NPz–C),
49.45 (NPz–C or NIm–C), 35.65 (CH3) ppm. IR (KBr): m 3074,
1277, 1170, 1090, 843, 738, 712, 605 cm−1. MS (electrospray) m/z:
(ES+) 177.2 (100%, [PzEtImH]+), (ES−) 319.7 (100%, [BPh4]−).
Microanalysis: found: C 79.42, H 6.55, N 11.43%; calc.: C 79.84,
H 6.70, N 11.29%.
Synthesis of Rh complex 10. A suspension of 8 (0.093 g,
0.134 mmol) in hexane (10 mL) and methanol (1 mL) was degassed
via three freeze–pump–thaw cycles. An atmosphere of CO was
introduced over the reaction mixture, which was stirred at room
Synthesis of Rh complex 8. A suspension of 6 (0.198 g,
0.410 mmol) in methanol (20 mL) was added to a solution of
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temperature for 3 h. The bright yellow solid turned pale yellow,
this solid was collected and washed with hexane and dried in vacuo
to afford 10 (0.077 g, 90%), mp 125–126 ◦C (decomp.)
(C5Pzb), 125.50 (m-CBPh), 124.10 (C4Imb), 123.75 (C4Ima), 121.65
a
a
(p-CBPh), 120.10 and 119.95 (C5Im and C5Imb), 107.20 (C4Pz and
C4Pzb), 80.05 (C1COD), 78.35 (C5COD and C6COD), 77.05 (C2COD),
1H NMR (500 MHz, CDCl3): d 7.52 (m, 8H, m-CHBPh), 7.42 (d,
3JH3–H4 = 2.3, 1H, H3Pz), 7.00 (t, 3Jo-CH–m-CH = 7.2, 8H, o-CHBPh), 6.93
(d, 3JH5–H4 = 2.6, 1H, H5Pz), 6.88 (t, 3Jp-CH–m-CH = 7.2, 4H, p-CHBPh),
6.35 (m, 2H, H4Im and H5Im), 6.19 (t, 3JH4–H3/H5 = 2.6, 1H, H4Pz),
4.13 (br s, 2H, CH2), 3.46 (s, 3H, CH3) ppm. 13C NMR (125 MHz,
63.95 (CH2 ), 63.70 (CH2 ), 38.28 and 38.05 (CH3 and CH3 ),
a
b
a
b
a
32.35–29.90 (C3COD, C4COD, C7COD and C8COD) ppm. (C2Im and
C2Im undefined). IR (KBr): m 3054, 1387, 1224, 734, 707, cm−1.
b
MS m/z: (ES+) 463.4 (63%, [Ir(PzMeIm)(COD)]+), 517.1 (23%,
[Ir(PzMeIm)2]+), 625.1 (10%, [Ir(PzMeIm)2(COD)]+), 355.1 (2.5%,
[Ir(PzMeIm)]+), 456.9 (100%). Microanalysis: found: C 59.13, H
5.34, N 11.14%; calc.: C 61.07, H 5.55, N 11.87%.
1
CDCl3): d 164.02 (q, JB–C = 49.4, B–C), 145.33 (C3Pz), 136.03
(m-CBPh), 134.61 (C5Pz), 125.93 (o-CBPh), 122.78 (C4Im), 122.18
(p-CBPh), 122.07 (C5Im), 107.83 (C4Pz), 61.49 (CH2), 38.23 (CH3)
ppm (C2Im and C≡O’s undefined). IR (KBr): m 2092 (CO), 2017
(CO) cm−1. MS m/z: (ES+) 265.6 (37%, [Rh(PzMeIm)]+), 293.6
(76%, [Rh(PzMeIm)(CO)]+), 320.9 (100%, [Rh(PzMeIm)(CO)2]+).
Microanalysis: found: C 64.95, H 5.15, N 9.59%; calc.: C 63.77, H
4.72, N 8.75%.
General procedure for catalytic hydrogenation
A solution of 0.015 mmol of catalyst, 1.50 mmol of styrene
and 0.3 mmol of mesitylene in 5 mL of THF in a glass vial
was sealed inside a 320 ml steel bomb reactor (Parr Instrument
Co. USA) under nitrogen. The reactor was flushed three times
with 120 psi of H2 gas. The solutions were stirred under 120 psi
of H2 at 55 ◦C for the duration of the reaction. The solutions
were then concentrated to 0.5 ml under reduced pressure (water
Synthesis of Rh complex 11. Complex 11 was prepared in a
similar manner to 10 via displacement of COD from 9. 11 was
isolated as a pale yellow solid in high yield (0.115 g, 92%), mp
114–116 ◦C (decomp.).
1
1H NMR (500 MHz, CD2Cl2): d 7.40 (m, 9H, m-CHBPh and
aspirator), made up to 1.5 ml with CDCl3 and analysed via H
3
3
NMR spectroscopy. Percent conversions were reported relative
to remaining substrate. Comparison of substrate/product peak
integrals relative to mesitylene, used as an internal standard,
indicated approximately 85% of substrate/product was recovered
after workup. This difference was assumed to have a minimal
influence on reported conversions due to a similarity in boiling
points between ethylbenzene (136 ◦C) and styrene (145–146 ◦C).
H3Pz), 7.27 (d, JH5–H4 = 2.6, 1H, H5Pz), 7.05 (t, Jo-CH–m-CH = 7.3,
8H, o-CHBPh), 6.90 (m, H5, p-CHBPh and H4Im), 6.58 (d, 3JH5–H4
=
2.0, 1H, H5Im), 6.37 (app. t, 3JH4–H3/H5 = 2.6, 1H, H4Pz), 4.48 (br s,
2/3
2H, NIm–CH2), 3.86 (s, 3H, CH3), 3.63 (br t,
J
= 5.9,
CH–CH/CH2
2H, NPz–CH2) ppm. 13C NMR (125 MHz, CD2Cl2): d 163.87
1
(q, JB–C = 49.4, B–C), 142.87 (C3Pz), 135.79 (m-CBPh), 133.79
(C5Pz), 125.66 (o-CBPh), 123.90 (C5Im), 123.64 (C4Im), 121.83 (p-
CBPh), 109.07 (C4Pz), 49.24 (NIm–C), 49.12 (NPz–C), 39.72 (CH3)
ppm (C2Im and C≡O’s undefined). IR (KBr): m 2093 (CO), 2032
(CO) cm−1. MS m/z: (ES+) 279.6 (51%, [Rh(PzEtIm)]+), 307.6
(64%, [Rh(PzEtIm)(CO)]+), 334.9 (100%, [Rh(PzEtIm)(CO)2]+).
Microanalysis: found: C 63.98, H 5.01, N 8.60%; calc.: C 64.24, H
4.93, N 8.56%.
Acknowledgements
We gratefully acknowledge financial support from the University
of New South Wales, the Australian Research Council (ARC) and
the Australian government for an Australian Postgraduate Award
(M. J. P).
Synthesis of Ir complex 12. A suspension of 6 (0.200 g,
0.415 mmol) in methanol (15 mL) was added to a solution of
[Ir(COD)(l-OEt)]2 (0.070 g, 0.101 mmol) and sodium ethoxide
(0.040 g, 0.588 mmol) in methanol (15 mL) and the mixture stirred
for 3 h. The solution changed from yellow to orange and a beige
solid remained undissolved. The solid was filtered off and the
filtrate reduced in vacuo, to ca. 5 mL, to precipitate a pale orange
solid. This precipitate was washed with hexane and dried in vacuo
to yield 12 as a pale orange powder (0.056 g, 59%), mp 155–160 ◦C.
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