Pyrazolyl-N-heterocyclic carbene complexes of rhodium as hydrogenation catalysts: The influence of ligand steric bulk on catalyst activity

Article abstract of DOI:10.1039/b906724h

A series of bidentate 1-(1-pyrazolylmethyl)-substituted NHC ligands (13a-c, 14a-c and 15a-c) were synthesised with substituents of varying steric bulk incorporated adjacent to the donor atoms. These ligands were coordinated to rhodium(I) to give a series of complexes of the general formula [Rh(L)(COD)]BPh₄ (where L = a mixed-donor pyrazolyl-NHC ligand and COD = 1,5-cyclooctadiene). The solid state structures of [Rh(13b)(COD)]BPh ₄ (16b), [Rh(13c)(COD)]BPh₄ (16c), [Rh(14a)(COD)]BPh ₄ (17a), [Rh(14b)(COD)]BPh₄ (17b), [Rh(15a) ₂(COD)]BPh₄ (18a), and [Rh(15b)(COD)]BPh₄ (18b) were determined by single crystal X-ray diffraction. The complex [Rh(15a) ₂(COD)]BPh₄ (18a) is unusual in that two of the pyrazolyl-NHC ligands (15a) are coordinated to the metal through the NHC donor instead of one ligand forming the expected chelate. These complexes (with the exception of 18a) were found to be effective catalysts for the hydrogenation of styrene. The catalytic activity was correlated with complex structure, and it was found that the greater the steric bulk of the metal bound ligand, the slower the rate of the hydrogenation.

Full text of DOI:10.1039/b906724h

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www.rsc.org/dalton | Dalton Transactions
Pyrazolyl-N-heterocyclic carbene complexes of rhodium as hydrogenation
catalysts: The influence of ligand steric bulk on catalyst activity†
Michael J. Page,a Jo¨rg Waglerb and Barbara A. Messerle*a
Received 2nd April 2009, Accepted 17th June 2009
First published as an Advance Article on the web 21st July 2009
DOI: 10.1039/b906724h
A series of bidentate 1-(1-pyrazolylmethyl)-substituted NHC ligands (13a–c, 14a–c and 15a–c) were
synthesised with substituents of varying steric bulk incorporated adjacent to the donor atoms. These
ligands were coordinated to rhodium(I) to give a series of complexes of the general formula
[Rh(L)(COD)]BPh4 (where L = a mixed-donor pyrazolyl-NHC ligand and COD = 1,5-cycloocta-
diene). The solid state structures of [Rh(13b)(COD)]BPh4 (16b), [Rh(13c)(COD)]BPh4 (16c), [Rh(14a)-
(COD)]BPh4 (17a), [Rh(14b)(COD)]BPh4 (17b), [Rh(15a)2(COD)]BPh4 (18a), and [Rh(15b)(COD)]-
BPh4 (18b) were determined by single crystal X-ray diffraction. The complex [Rh(15a)2(COD)]BPh4
(18a) is unusual in that two of the pyrazolyl-NHC ligands (15a) are coordinated to the metal through
the NHC donor instead of one ligand forming the expected chelate. These complexes (with the
exception of 18a) were found to be effective catalysts for the hydrogenation of styrene. The catalytic
activity was correlated with complex structure, and it was found that the greater the steric bulk of the
metal bound ligand, the slower the rate of the hydrogenation.
ylidene) and [Rh(IMe)2(COD)]Cl(6, IMe= 1,3-dimethylimidazol-
2-ylidene) were also shown to be moderately active catalysts for
the reduction of simple alkenes.15 The stability and activity of
these complexes was, however, strongly dependent on the addition
of phosphine ligands to the system, which suggested that the
active catalyst was in fact a Rh-NHC-phosphine species similar
to 3. Extensive hydrogenation studies by Buriak et al. on NHC
analogues of Crabtree’s catalyst proved more successful.16,17 Sub-
stitution of the pyridine ligand in 2 with an NHC was observed to
yield catalysts of comparable hydrogenation activity. Optimisation
of this catalyst motif by incorporating an alternative phosphine
ligand and less coordinating counter-anion led to the development
of the highly active catalyst system [Ir(IMe)(PnBu3)(COD)]BARF
(7, BARF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate). Im-
portantly, this complex proved to be stable indefinitely under the
catalytic conditions. This represented a significant advantage over
Crabtree’s catalyst which forms an inactive hydride-bridged trimer
species after 1 h under hydrogenation conditions.9
Surprisingly the investigation of hydrogenation catalysts con-
taining a more complicated ligand motif has received little atten-
tion. The use of a chelating ligand motif, in particular, where the
NHC donor is tethered toa separate donor group, has thepotential
to moderate the lability of each donor group and influence the
stability and reactivity of the complex.18 Very recently, outstanding
results were achieved by Burgess and coworkers with the Ir(I)
hydrogenation catalysts 8, which contain a chiral NHC-oxazolyl
chelate.19 These complexes were found to exhibit excellent activity
and enantioselectivity (>98% ee) for the reduction of highly
substituted prochiral alkenes. However, the hydrogenation activity
of these species was highly dependent on the steric properties
of the NHC-oxazole ligand. A much enhanced stability of these
complexes was observed compared to the related phosphine-
oxazole systems.20 Only two other hydrogenation catalysts (921
and 1022) have been reported with such mixed donor NHC ligands,
Introduction
N-heterocyclic carbenes (NHCs) are now recognised as effective
and versatile donors in homogeneous transition metal catalysis.1–4
In terms of their coordination chemistry they are frequently
compared to electron rich phosphine ligands, yet they are generally
stronger s-donors and form complexes of higher thermal and
hydrolytic stability.1 In a desire to exploit these properties, NHCs
were initially applied as phosphine substitutes in homogeneous
catalysts of proven efficiency. This strategy was most successful
in Pd(0) Heck reaction catalysts5,6 and Ru(II) Grubbs metathesis
catalysts,7 where the use of NHC donors resulted in a vast
improvement to catalytic activity and catalyst stability.
In an attempt to extend this success to catalytic Rh(I) and Ir(I)
hydrogenation systems a number of NHC analogues of Wilkin-
sons (Rh(PPh3)3Cl, 1)8 and Crabtrees ([Ir(PCy3)(py)(COD)]PF6,
2, COD = 1,5-cyclooctadiene, py = pyridine)9 hydrogenation
catalysts have since been reported.10 In early work, Nolan et al.
reported that the NHC complexes Rh(IMes)(PPh3)2Cl (3, IMes =
1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and [Ir(SIMes)-
(py)(COD)]PF6 (4, SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-
dihydroimidazol-2-ylidene) were much less active than the original
phosphine systems 1 and 2, respectively.11,12 However, Crudden
et al. found that in the presence of a phosphine scavenger
such as CuCl the hydrogenation activity of 3 was significantly
enhanced and TOFs (turnover frequencies) exceeding those of
1 could be obtained.13,14 The mono and bis NHC Rh com-
plexes Rh(ICy)(COD)Cl (5, ICy = 1,3-dicyclohexylimidazol-2-
aSchool of Chemistry, University of New South Wales, Sydney, NSW, 2052,
Australia. E-mail: b.messerle@unsw.edu.au
bResearch School of Chemistry, Australian National University, Canberra,
A.C.T. 0200, Australia
† CCDC reference numbers 724356–724362. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b906724h
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The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7029–7038 | 7029
©

and both were found to be effective for the reduction of a-amido
alkenes. The NHC-phosphanyl complex 10, in particular, was
found to afford high conversions and excellent enantioselectivities
(up to 98% ee) under relatively mild reaction conditions.
Scheme 1
We have previously reported the activity of the Rh(I) mixed
donor NHC-pyrazolyl chelate complexes 11 and 12 as hydro-
genation catalysts.23 Both complexes showed good activity for the
hydrogenation of styrene under relatively mild reaction conditions,
and in an effort to further develop the chemistry of 11, a series of
mixed-donor imidazolium-pyrazolyl ligand precursors of varying
steric bulk (13–15) and their corresponding NHC complexes with
rhodium have been prepared.
eliminate a corresponding equivalent of the imidazolium HCl
salt. Anion metathesis with NaBPh4 and recrystallization of the
mixture afforded the products as white crystalline powders. In this
context the solid state structure of one of these compounds (13c)
was determined crystallographically (for details see Experimental
section).
The rhodium N-heterocyclic carbene complexes 16–18 were
synthesized according to Scheme 2. The complex [Rh(COD)(m-
OMe)]2 and the desired pyrazolyl-imidazolium salt (13–15) were
added to a solution of sodium methoxide in methanol and the
mixture refluxed for several hours. A bright yellow powder of the
complex precipitated upon concentration of the solution in vacuo,
which was collected and recrystallized from dichloromethane and
hexane.
An anomalous result was observed for the ligand precursor
15a which contains a bulky tert-butyl substituent on the pyrazolyl
donor and a methyl substituent on the NHC. The reaction of only
one equivalent of the ligand precursor per rhodium centre led to
the isolation of the unusual biscarbene adduct 18a, which has two
NHC ligands coordinated to the rhodium centre with the pendant
pyrazolyl donors remaining uncoordinated.
Several attempts were made to coordinate the bulky ligand
precursor 15c, which has tert-butyl substituents on both the
pyrazole and NHC donor, to rhodium with no success. Despite
refluxing a methanol solution of 15c and [Rh(COD)(m-OMe)]2 in
the presence of excess sodium methoxide for 48 h, only unreacted
ligand was recovered.
Results and discussion
The imidazolium-pyrazolyl ligand precursors (13–15) were syn-
thesized according to Scheme 1.23 The pyrazole moiety was
reacted with formaldehyde under basic conditions to generate a
hydroxymethylpyrazole species which upon reaction with thionyl
chloride afforded a chloromethylpyrazole intermediate that was
isolated as the hydrochloride salt. The high susceptibility of the
chloro group to hydrolysis prevented isolation of the free base
which was therefore used without further purification. Reaction
of two equivalents of the N-substituted imidazole was required
to yield the desired imidazolium-pyrazolyl products (13–15) and
X-Ray crystal structure analysis
The solid state structures of 16b and c (Fig. 1), 17a and b
(Fig. 2), and 18a and b (Fig. 3) were determined by single crystal
X-ray diffraction. Crystals suitable for analysis were obtained via
slow diffusion of hexane into CH2Cl2 solutions of the complexes.
Complexes 16b and 16c crystallized as pairs of crystallographically
inequivalent formula moieties (16b and 16b*, 16c and 16c*). Fig. 1
Scheme 2
7030 | Dalton Trans., 2009, 7029–7038
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©

previously23 are also included for comparison. An interesting trend
is observed on comparing the rhodium–carbene Rh1–C1 bond
˚
˚
˚
lengths in the structures 16a (2.053(2) A), 16b (2.0363(14) A),
˚
˚
16b* (2.0224(13) A), 16c (2.0829(13) A), and 16c* (2.0746(13) A).
These complexes vary only in relation to the size of the substituent
on the NHC donor, and the Rh1–C1 bond lengths increase
according to the nature of the NHC donor substituent in the
order mesityl < methyl < tert-butyl. This order suggests that
the steric influence of the methyl substituent is greater than
that exerted by the mesityl substituent. The lesser apparent
steric impact of the mesityl substituted ligand may in part be
due to an interaction of the COD olefinic hydrogens with the
aromatic p-system of the mesityl ring (16b: CHCOD ◊ ◊ ◊ mesityl
Fig. 1 ORTEP diagrams with 50% ellipsoids and partial crystal structure
numbering scheme of the cationic fragments of complexes 16b and 16c.
˚
ring centroid = 3.103 A, 16b*: CHCOD ◊ ◊ ◊ mesityl ring centroid =
˚
2.947 A). A similar interaction was also observed in the crystal
˚
structures of 17b (CHCOD ◊ ◊ ◊ mesityl ring centroid = 2.697 A)
˚
and 18b (CHCOD ◊ ◊ ◊ mesityl ring centroid = 2.728 A). However,
the rhodium–carbene Rh1–C1 bond length for complexes 17a
˚
˚
(2.0196(11) A) and 17b (2.0487(16) A) (Fig. 2) follow the reverse
order with the bond lengths for the substituents methyl <
mesityl. This result suggests that factors other than the steric
bulk of the NHC substituent, such as ring strain within the
ligand metallocycle and the size of the pyrazolyl substituent, also
influence the length of the rhodium–carbene bond.
Fig. 2 ORTEP diagrams with 50% ellipsoids and partial crystal structure
numbering scheme of the cationic fragments of complexes 17b and 17c.
When comparing the rhodium–pyrazolyl Rh1–N1 bond lengths
˚
˚
˚
in the structures 16b (2.1152(12) A), 16b* (2.1243(13) A), 17b
˚
(2.1433(14) A), and 18b (2.2091(9) A), which have identical NHC
donor groups and vary only with respect to the substituents on
the pyrazolyl donor, a clearer trend is observed. In this case, as
expected, the Rh1–N1 bond lengths vary in proportion to the size
of the pyrazolyl substituent in the order hydrogen < phenyl <
tert-butyl. When comparing the bite angle (C1–Rh1–N1) formed
by the ligand, no clear trend is observed in relation to the steric
bulk of the substituents.
depicts only one of the conformationally very similar cations for
each structure.
In all of the crystal structures a square planar geometry around
the metal centre is observed, with the metallocycle formed by
the NHC-pyrazolyl chelate adopting a pseudo-boat conforma-
tion. A list of relevant bond lengths and angles is shown in
Table 1. The metrics of complex 16a, with the methyl substituted
NHC and unsubstituted pyrazolyl donor, which was reported
The tertiary-butyl pyrazolyl substituent in combination with
the carbene methyl substituent of ligand 15a leads to a different
coordination, with a bis-monodentate arrangement of the ligand
in complex 18a. The crystal structure of 18a shows the two
NHC rings oriented perpendicular to the plane of the complex
(Fig. 3). The Rh1–C1 and Rh1–C2 bond distances are 2.047(2)
◦
˚
Table 1 Relevant bond lengths (A) and angles ( ) for Rh NHC complexes
Complex Rh1–C1
Rh1–N1
C1–Rh1–N1
16a23
16b
16b*
16c
2.053(2)
2.0962(19)
2.1152(12)
2.1243(13)
2.1075(11)
2.1132(11)
2.1209(9)
2.1433(14)
2.045(2) (Rh1–C2)
2.2091(9)
85.14(7)
85.02(5)
84.25(5)
84.03(5)
84.30(5)
82.09(4)
85.43(6)
89.51(9) (C1–Rh1–C2)
83.88(4)
˚
2.0363(14)
2.0224(13)
2.0829(13)
2.0746(13)
2.0196(11)
2.0487(16)
2.047(2)
and 2.045(2) A, respectively, and the C1–Rh1–C2 bite angle of
89.51(9)◦ approaches the ideal angle of 90◦ for a square planar
geometry about the metal centre.
16c*
17a
The preference of 18a to coordinate two NHC ligands instead
of coordinating a single chelating ligand can be attributed to two
factors. Firstly, by adopting an orthogonal orientation of the NHC
ring relative to the square plane of the metal complex, any steric
17b
18a
18b
2.0351(11)
Fig. 3 ORTEP diagrams with 50% ellipsoids and partial crystal structure numbering scheme of the cationic fragments of complexes 18a and 18b.
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interactions within the complex are minimized. It is interesting
to note that the analogous mesityl substituted NHC ligand in 18b
forms the desired chelate. This suggests that the methyl substituent
on the NHC ring of 18a has a slightly greater steric interaction
with the Rh(COD) fragment than the planar mesityl substituent
in 18b. This is consistent with the observation that the Rh1–C1
bond length increases in the order mesityl < methyl < tert-butyl
for complexes 16a–c.
The mesityl rings of complexes 17b and 18b undergo restricted
rotation about the N–C bond as evidenced by the presence of
two distinct resonances in the 1H NMR spectrum for each of the
ortho-methyl groups and each of the meta-protons on the mesityl
ring. The observation of exchange peaks between each pair of
resonances in the 2D NOESY spectra indicated that slow rotation
of the mesityl ring about the N–C bond was occurring on the
NMR time scale.
The strength of the N-donor also contributes to the formation
or otherwise of the N–C chelate. In the case of complexes 16a
and 17a, which have a hydrogen and phenyl substitutent adjacent
to the pyrazolyl N-donor atom, respectively, the Rh–N bond is
strong enough to form the chelate. However, for 18a, which has
a very bulky tert-butyl group next to the N-donor atom, the
Rh–N bond is much weaker and the preference to form the chelate
in 18a is reduced. This unusual type of bis-NHC bonding was
previously reported by this group during the synthesis of the
analogous iridium complex of ligand 13a.23
In solution, the bis-NHC complex 18a exists in two isomeric
forms in a ratio of ca. 1:0.9, as was determined by the H NMR
1
spectrum of the complex at 298 K. In the solid state structure
of 18a, described above, the two NHC ligands bind with the
plane of the NHC ring oriented perpendicular to the square
plane of the metal complex. The two isomers observed in solution
most probably arise from a syn or anti orientation of these two
NHC ligands, with a syn orientation indicating the methyl NHC
substituents are on the same face of the metal complex and
an anti orientation indicating that they are on opposite faces.
In the 2D NOESY spectrum of 18a (Fig. 4), exchange peaks
between resonances of the two isomers indicate that the isomers
are exchanging over the time frame of the NMR experiment. NMR
spectroscopic investigations of similar Rh complexes showing
hindered rotation of the NHC have previously been reported.24
A strong coupling was observed between the NHC C2 carbon
and the spin half 103Rh nucleus in the 13C NMR spectra of each
of the complexes 16a (50.0 Hz), 16b (51.5 Hz), 17a (61.6 Hz), 17b
(52.5 Hz), 17c (49.3 Hz), and 18a (58.1 Hz). This coupling was
not determined for complexes 16c and 18b due to insufficient
resolution of the C2 carbon resonance from the baseline of
the spectrum. When correlated with the crystal structure of the
complexes it is apparent that the size of the coupling constant is
inversely proportional to the length of the Rh–C bond.
Structural analysis of the Rh complexes using NMR spectroscopy
All complexes were fully characterised by 1H and 13C NMR
spectroscopy. In the 1H NMR spectra of complexes 16a23 and 16b,
the resonance due to the COD protons and the protons on the
methylene bridge of the NHC-pyrazolyl chelate were significantly
broadened due to fluxionality of the complex. The remaining
complexes, 16c, 17a–c, and 18b, however, which contain ligands of
greater steric bulk, did not show an averaging of the diastereotopic
1
proton resonances in their H NMR spectra at 298 K. This is
most likely because conformational exchange within each of these
complexes is inhibited by steric interactions between the bulky
NHC-pyrazolyl ligands and the Rh(COD) fragment.
Fig. 4 NOESY spectrum of 18a showing the COD ethenyl, and N-methyl protons of the two isomers, with the resonances due to the minor isomer
labelled (*). A selection of the exchange peaks between the resonances of the two isomers is highlighted.
7032 | Dalton Trans., 2009, 7029–7038
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Table 2 TOFs for the Rh NHC catalyzed hydrogenation of styrene at
50 psi H2 pressure, 300 K, and 0.5 mol% catalyst loading for each of the
catalysts 16a–c, 17a–c and 18ba
substrate, whereas 17c achieved complete reduction of styrene
within 73 min of reaction. A similar catalytic activity was obtained
with complex 18b, which contains a tert-butyl substituent on the
pyrazolyl donor. This catalyst required approximately 84 min
to achieve complete reduction of styrene. These results indicate
that the bulky tert-butyl substituents in the complexes 16c, 17c,
and 18b severely limit the activity of the catalysts. This effect is
most probably due to the inhibition of substrate coordination
due to steric interaction with the bulky tert-butyl substituent.
Interestingly, an unusually low catalytic activity was also observed
for complex 17b. In the solid state structure of 17b (Fig. 2), the two
planar phenyl and mesityl substituents can be seen to shield one
face of the complex. While these substituents may not be bulky
enough to inhibit coordination of the styrene substrate to the metal
centre, they may obstruct the initial approach of the substrate to
the complex and thereby inhibit the reaction rate of the catalyst.
The bis-NHC complex 18a was shown to be entirely inactive
under the chosen reaction conditions and no conversion of the
substrate was observed even after several hours of reaction. This
observation is consistent with studies on analogous bis-NHC
systems reported in the literature.21,23
Catalyst
16a
16b
16c
17a
17b
17c
18b
TOF/h-1
484
476
161
328
142
243
187
a TOF = mol product/mol catalyst/time at 50% conversion
Hydrogenation studies
The efficiency of complexes 16–18 as catalysts for the hydrogena-
tion of styrene to ethylbenzene was investigated. The reactions
were performed with a catalyst loading of 0.5 mol% under 50 psi
of H2 gas at 300 K. A plot of the substrate conversion (%) versus
time (min) is shown in Fig. 5, and the corresponding TOFs are set
out in Table 2.
It can be seen from Fig. 5 that the complexes 16a, 16b, and
17a, which contain the smallest degree of steric bulk on the
NHC-pyrazolyl ligand, were the fastest catalysts of the series. The
complexes 16a, 16b, and 17a all had a very similar catalytic activity,
with >98% conversion achieved within approximately 50 min
of reaction. However, the initial rate of reaction for complexes
16a and 16b was faster than the initial rate of reaction for 17a.
This difference is reflected in the TOFs of 484, 476, and 328 h-1
for the catalysts 16a, 16b, and 17a, respectively. It is possible
that the slower initial reaction rate of 17a is due to the large
phenyl substituents on the pyrazolyl donor inhibiting the initial
hydrogenation of the COD coligand.
Conclusions
A series of new NHC-pyrazolyl ligands of varying steric bulk and
their RhCOD complexes were synthesized. The structure of the
complexes was investigated in solution using NMR spectroscopy,
and in the solid state using X-ray crystallography, with the solid
state structures of 16b–c, 17a–b, and 18a–b obtained. It was
observed that the steric bulk of the ligands had a pronounced
effect on the structure of the complexes. The size of the ligand
substituents was observed to strongly influence the length of the
In comparison, the complexes 16c and 17c, which contain a
tert-butyl group on the NHC donor, had much slower reaction
rates. This was particularly the case for 16c which required
approximately 115 min to achieve complete conversion of the
Fig. 5 Time courses for the catalysed hydrogenation of styrene to ethylbenzene at 50 psi H2 pressure, 300 K, and 0.5 mol% catalyst loading for each of
the catalysts 16a–c, 17a–c and 18b.
This journal is
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Dalton Trans., 2009, 7029–7038 | 7033
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Rh–C(NHC) and Rh–N bonds. Surprisingly, for complexes 16a–c
the rhodium–carbene bond lengths increased according to the
nature of the NHC donor substituent in the order mesityl <
methyl < tert-butyl, which suggested a lesser steric interaction
between the planar mesityl ring and the Rh(COD) fragment
compared to the smaller methyl substituent. Evidence for a
positive interaction between the COD olefinic hydrogen atoms
and the aromatic p-system of the mesityl ring was observed in
the crystal structures of 16b, 17b and 18b. The small difference
in steric impact of the two substituents (methyl and mesityl) was
observed to have a pronounced effect on the complex structure for
ligands 15a and 15b. Where coordination of the methyl substituted
NHC ligand 15a to rhodium gave the unusual bis-NHC complex
18a, the mesityl substituted NHC ligand 15b preferred to form
the more sterically demanding chelate complex 18b. The size
of the ligand substituents also had a pronounced affect on the
solution behaviour of the complexes, with the complexes of lesser
steric bulk (16a, b) displaying fluxionality on the NMR time
scale.
The steric bulk of the ligands had a significant effect on the
hydrogenation activity of the complexes. It was observed that an
increased steric shielding of the metal centre resulted in a greatly
reduced activity of the catalyst. The highest reaction rates were
obtained with complexes 16a, b and 17a, which contain a very
small degree of steric shielding about the metal centre. However,
for complexes 16c, 17c and 18b, which contain a bulky tert-butyl
substituent on one of the donor groups, a much slower reaction
rate was observed. This effect was most probably due to the
increased bulk of the ligand inhibiting the coordination of the
alkene substrate.
Experimental
All manipulations or metal complexes and air sensitive reagents
were carried out using standard Schlenk techniques25 or in a
nitrogen filled glove box. For the purpose of air sensitive manip-
ulations and in the preparation of metal complexes, solvents were
dried and distilled under an atmosphere of argon using standard
procedures.26 The ligand 13a,23 the rhodium complex 16a,23
and all pyrazole,27 hydroxymethylpyrazole,28,29 and imidazole30–32
compounds were synthesized according to published procedures.
The 1H and 13C NMR spectra were recorded on a Bruker
DPX300 spectrometer. The spectra were recorded at 298 K
unless otherwise specified. Chemical shifts (d) are quoted in ppm.
1H NMR and 13C{ H} NMR chemical shifts were referenced
1
internally to residual solvent resonances. Coupling constants (J)
are given in Hz. In assigning the NMR spectra the superscripts
im and pz are used to denote resonances from imidazolyl and
pyrazolyl rings, respectively with positions on the ring are denoted
in the standard way (1-5). The superscripts o (ortho), m (meta),
and p (para) are used to denote positions on phenyl moieties and
the superscrits a and b used to distinguish between diastereotopic
proton environments. Elemental analyses were carried out at
the Campbell Microanalytical Laboratory, University of Otago,
New Zealand. X-ray diffraction data were collected in phi and
omega scans on a Nonius KappaCCD diffractometer using
Mo-Ka radiation. The structures were solved with direct methods
(SHELXS97) and refined by full-matrix least-squares refinement
of F2 using SHELXL97.33–35 (For details see Table 3.) Crystals of
13c, 16b, (16c)2·CH2Cl2, 17a, 17b, 18a and 18b·CH2Cl2 suitable
for X-ray crystallography were grown from layering hexane over a
Table 3 Summary of crystallographic data for 13c, 16b, (16c)2·CH2Cl2, 17a, 17b, 18a and 18b·CH2Cl2
13c
16b
(16c)2·CH2Cl2
17a
17b
18a
18b·CH2Cl2
Empirical formula C35H37BN4
C48H50BN4Rh
796.64
Monoclinic
P21
16.1877(2)
9.2908(1)
26.4808(3)
90
93.842(1)
90
C87H98B2Cl2N8Rh2 C52H50BN4Rh
C60H58BN4Rh
948.82
Monoclinic
P21/n
12.9939(3)
18.8994(3)
20.5054(4)
90
108.262(1)
90
C58H72BN8Rh
994.96
Orthorhombic
Pbca
25.2736(4)
12.7805(2)
31.9609(6)
90
C54H62BCl2N4Rh
951.70
Triclinic
P-1
13.4959(2)
13.5060(2)
14.0357(2)
82.846(1)
70.731(1)
86.942(1)
2396.04(6)
1.319
M (g mol-1)
Crystal system
Space group
524.50
1554.07
Triclinic
P-1
844.68
Triclinic
P-1
Triclinic
P-1
˚
a (A)
9.6457(3)
11.0776(6)
14.2768(8)
75.011(2)
84.346(3)
80.266(3)
1449.97(12)
1.201
12.3719(2)
14.3921(2)
21.9391(3)
97.527(1)
98.504(1)
96.085(1)
3798.74(10)
1.359
10.4027(2)
12.9461(2)
16.4453(2)
70.599(1)
80.016(1)
84.831(1)
2056.11(6)
1.364
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
90
90
3
˚
V (A )
Dc (g cm-3)
Z
3973.67(8)
1.332
4
4782.02(16)
1.318
4
10323.7(3)
1.280
8
2
2
2
2
T (K)
100(2)
100(2)
100(2)
100(2)
100(2)
100(2)
100(2)
Crystal size (mm) 0.21 ¥ 0.16 ¥ 0.15 0.55 ¥ 0.33 ¥ 0.20 0.53 ¥ 0.21 ¥ 0.19 0.54 ¥ 0.18 ¥ 0.12 0.57 ¥ 0.28 ¥ 0.07 0.23 ¥ 0.19 ¥ 0.07 0.56 ¥ 0.54 ¥ 0.22
q range (◦)
Completeness (%) 99.8
Index ranges
2.66–25.00
2.59–35.00
99.2
-25≥h≥26
-14≥k≥14
-42≥l≥40
98011
34298
0.0437
1.020
-0.017(7)
0.0301
0.0665
0.0360
2.55–33.00
99.8
-18≥h≥18
-22≥k≥21
-33≥l≥33
106162
28579
2.55–35.00
98.9
-16≥h≥16
-20≥k≥20
-26≥l≥26
64528
2.71–29.00
99.9
-17≥h≥17
-25≥k≥25
-27≥l≥27
65743
12682
0.0634
1.057
2.60–26.00
99.9
-31≥h≥29
-15≥k≥14
-39≥l≥39
74638
10136
0.0696
1.034
2.56–35.00
99.8
-21≥h≥21
-21≥k≥21
-22≥l≥22
79673
21076
0.0637
1.041
-11≥h≥11
-13≥k≥13
-16≥l≥16
Reflns measured 18020
Unique reflns
Rint
5095
0.0653
1.008
—
17905
0.0442
1.056
—
0.0581
1.055
—
GoF (all)
Flack parameter
R1 (I > 2s(I))
—
—
—
0.0463
0.0312
0.0698
0.0480
0.0740
0.0333
0.0810
0.0413
0.0838
0.0340
0.0819
0.0494
0.0868
0.0388
0.0807
0.0700
0.0892
0.0354
0.0760
0.0490
0.0799
wR2 (I > 2s(I)) 0.1025
R1 (all data)
0.0807
0.1135
wR2 (all data)
0.0685
7034 | Dalton Trans., 2009, 7029–7038
This journal is
The Royal Society of Chemistry 2009
©

concentrated CH2Cl2 solution of the respective compound. Mass
spectra were acquired at the Bioanalytical Mass Spectroscopy
Facility (BMSF), University of New South Wales, on a Finnigan
Micromass QToF spectrometer.
6.96 (s, 1H, H4pz), 6.92 (t, 8H, 3J(m-o/p-BPh4) = 7.3 Hz, m-BPh4),
3
6.77 (t, 4H, J(p-m-BPh4) = 7.3 Hz, p-BPh4), 6.56 (s, 2H, CH2),
1
3.87 (s, 3H, CH3) ppm. 13C{ H} NMR (75 MHz, acetone-d6): d
165.0 (q, 1J(C–B) = 49.4 Hz, B-C), 154.0 (C3pz), 147.5 (C5pz), 137.4
(C2im), 137.0 (brq, 2J(C–B) = 1.5 Hz, o-BPh4), 133.4 (PhC), 130.6
(PhC), 130.3 (PhC), 129.9 (PhC), 129.8 (PhC), 129.6 (PhC), 129.4
(PhC), 126.6 (PhC), 126.1 (q, 3J(C–B) = 2.7 Hz, m-BPh4), 125.2
(C4im), 122.8 (C5im), 122.3 (p-BPh4), 106.2 (C4pz), 61.6 (CH2), 36.9
(CH3) ppm. MS (ESI+) m/z (%): 315.2 [M]+ (100).
Synthesis of pyrazolyl-imidazolium ligand precursors
General procedure. To
a solution of the desired 1-
hydroxymethylpyrazole (3.0 mmol) in chloroform (20 mL) was
added thionyl chloride (1 mL, ca. 14 mmol) and the solution was
stirred at room temperature for several hours. A small amount
of white solid precipitated from solution which was reduced to
dryness under vacuum without filtering to give a beige residue
of the corresponding 1-chloromethylpyrazole hydrochloride salt.
The product was suspended in toluene (40 mL), the desired
imidazole (6.0 mmol) was added, and the mixture was refluxed
for 2 h to form a brown to yellow oily suspension of the pyrazolyl-
imidazolium chloride salt and the imidazolium hydrochloride side
product. NaBPh4 (6.0 mmol) was added to the suspension and the
reaction mixture was refluxed for 5 min, and then stirred at room
temperature for 1 h resulting in the formation of a beige residue.
The residue was filtered and recrystallized from either acetone, or
ethyl acetate, and diethyl ether to yield the pyrazolyl-imidazolium
salt.
1
14b (66%): H NMR (300 MHz, acetone-d6): d 9.30 (t, 1H,
4J(H2im–H4/5im) = 1.6 Hz, H2im), 7.97 (m, 2H, PhH), 7.91 (brd,
3J(H5im–H4im) = 1.9 Hz, H5im), 7.78 (brd, 3J(H4im-H5im) = 1.9 Hz,
H4im), 7.60 (m, 5H, PhH), 7.40 (m, 3H, PhH), 7.34 (m, 8H, o-
3
BPh4), 7.12 (s, 2H, Hm), 7.02 (s, 1H, H4pz), 6.91 (t, 8H, J(m-
o/p-BPh4) = 7.3 Hz, m-BPh4), 6.83 (s, 2H, CH2), 6.76 (t, 4H,
3J(p-m-BPh4) = 7.3 Hz, p-BPh4), 2.34 (s, 3H, p-CH3), 1.99 (s,
1
6H, o-CH3) ppm. 13C{ H} NMR (75 MHz, acetone-d6): d 165.0
1
(q, J(C–B) = 49.4 Hz, B-C), 154.2 (C3pz), 147.6 (C5pz), 142.1
(Cp), 138.3 (C2im), 137.0 (brq, 2J(C–B) = 1.4 Hz, o-BPh4), 135.4
(C◦), 133.4 (PhC), 131.9 (N-C), 130.6 (PhC), 130.4 (Cm), 130.3
(PhC), 129.8 (PhC), 129.7 (PhC), 129.6 (PhC), 129.5 (PhC), 126.6
(PhC), 126.0 (q, 3J(C–B) = 2.7 Hz, m-BPh4), 125.7 (C4im), 123.6
(C5im), 122.3 (p-BPh4), 106.4 (C4pz), 62.4 (CH2), 21.0 (p-CH3), 17.3
(o-CH3) ppm. MS (ESI+) m/z (%): 419.2 [M]+ (100).
13b (65%): Anal. found: C, 81.72; H, 6.76; N, 9.70. C40H39BN4
requires: C, 81.90; H, 6.70; N, 9.55. 1H NMR (300 MHz, acetone-
d6): d 9.17 (t, 1H, 4J(H2im–H4/5im) = 1.5 Hz, H2im), 8.01 (d, 1H,
3J(H5pz–H4pz) = 2.3 Hz, H5pz), 7.94 (apparent t, 1H, 3/4J(H5im–
14c (61%): Anal. found: C, 83.38; H, 6.78; N, 8.42. C47H45BN4
requires: C, 83.42; H, 6.70; N, 8.28. 1H NMR (300 MHz, acetone-
d6): d 9.00 (t, 1H, 4J(H2im–H4/5im) = 1.6 Hz, H2im), 7.93 (m, 2H,
3
PhH), 7.76 (brd, J(H4im–H5im) = 1.9 Hz, H4im), 7.63–7.31 (m,
H4/2im) = 1.6 Hz, H5im), 7.67 (d, 1H, J(H3pz–H4pz) = 1.8 Hz,
9H, PhH and H5im), 7.36 (m, 8H, o-BPh4), 6.94 (s, 1H, H4pz), 6.93
3
H3pz), 7.65 (apparent t, 1H, 3/4J(H4im–H5/2im) = 1.6 Hz, H4im),
(t, 8H, J(m-o/p-BPh4) = 7.3 Hz, m-BPh4), 6.78 (t, 4H, J(p-m-
BPh4) = 7.3 Hz, p-BPh4), 6.51 (s, 2H, CH2), 1.63 (s, 9H, C(CH3)3)
3
3
3
7.35 (m, 8H, o-BPh4), 7.13 (2, 2H, Hm), 6.92 (t, 8H, J(m-o/
p-BPh4) = 7.3 Hz, m-BPh4), 6.77 (t, 4H, 3J(p-m-BPh4) = 7.3 Hz,
p-BPh4), 6.63 (s, 2H, CH2), 6.41 (t, 1H, 3J(H4pz–H3/5pz) = 1.8 Hz,
ppm. 13C{ H} NMR (75 MHz, acetone-d6): d 164.9 (q, 1J(C–B) =
1
49.6 Hz, B-C), 153.7 (C3pz), 147.3 (C5pz), 137.0 (brq, J(C–B) =
2
1
H4pz), 2.36 (s, 3H, p-CH3), 2.00 (s, 6H, o-CH3) ppm. 13C{ H}
1.4 Hz, o-BPh4), 135.0 (C2im), 133.4 (PhC), 130.6 (PhC), 130.2
(PhC), 129.9 (PhC), 129.8 (PhC), 129.6 (PhC), 129.4 (PhC), 126.6
(PhC), 126.0 (q, 3J(C–B) = 2.8 Hz, m-BPh4), 123.0 (C5im), 122.3
(p-BPh4), 121.7 (C4im), 106.3 (C4pz), 61.8 (CH2), 61.6 (C(CH3)3),
29.5 (C(CH3)3) ppm. MS (ESI+) m/z (%): 357.2 [M]+ (100).
15a (61%): Anal. found: C, 80.38; H, 7.73; N, 10.26. C37H41BN4
requires: C, 80.43; H, 7.48; N, 10.14. 1H NMR (300 MHz, acetone-
d6): d 9.02 (brs, 1H, H2im), 7.69 (apparent t, 1H 3/4J(H5im–
H2/4im) = 1.7 Hz, H5im), 7.58 (apparent t, 1H 3/4J(H4im–H2/5im) =
1.7 Hz, H4im), 7.35 (m, 8H, o-BPh4), 6.92 (t, 8H, 3J(m-o/
p-BPh4) = 7.3 Hz, m-BPh4), 6.77 (t, 4H, 3J(p-m-BPh4) = 7.3 Hz,
p-BPh4), 6.45 (s, 2H, CH2), 6.06 (s, 1H, H4pz), 3.93 (s, 3H, NCH3),
NMR (75 MHz, acetone-d6): d 165.2 (q, 1J(C–B) = 49.4 Hz, B–C),
143.1 (C3pz), 142.1 (Cp), 138.3 (C2im), 137.0 (q, 2J(C–B) = 1.4 Hz,
o-BPh4), 135.4 (N–C), 132.1 (C5pz), 131.9 (C◦), 130.4 (Cm), 126.1
(q, 3J(C–B) = 2.8 Hz, m-BPh4), 125.6 (C4im), 123.6 (C5im), 122.3
(p-BPh4), 108.3 (C4pz), 63.4 (CH2), 21.0 (p-CH3), 17.3 (o-CH3)
ppm. MS (ESI+) m/z (%): 267.1 [M]+ (100).
13c (62%): Anal. found: C, 80.08; H, 7.14; N, 10.73. C35H37BN4
requires: C, 80.15; H, 7.11; N, 10.68. 1H NMR (300 MHz, acetone-
d6): d 9.52 (t, 1H 4J(H2im–H4/5im) = 1.6 Hz, H2im), 8.02 (d, 1H,
3J(H5pz–H4pz) = 2.3 Hz, H5pz), 7.92 (apparent t, 1H 3/4J(H4im–
H2/5im) = 1.9 Hz, H4im), 7.81 (apparent t, 1H 3/4J(H5im–H2/4im) =
1.9 Hz, H5im), 7.62 (d, 1H, 3J(H3pz–H4pz) = 1.6 Hz, H3pz), 7.35 (m,
8H, o-BPh4), 6.92 (t, 8H, 3J(m-o/p-BPh4) = 7.3 Hz, m-BPh4), 6.78
2.38 (s, 3H, CCH3), 1.24 (s, 9H, C(CH3)3) ppm. 13C{ H} NMR
1
(75 MHz, acetone-d6): d 165.1 (q, 1J(C–B) = 49.3 Hz, B–C), 164.2
(C3pz), 141.4 (C5pz), 137.3 (Cy]2im), 137.1 (brs, o-BPh4), 126.1 (q,
3J(C–B) = 2.9 Hz, m-BPh4), 125.3 (C4im), 122.7 (C5im), 122.4 (p-
BPh4), 104.9 (C4pz), 60.8 (CH2), 37.0 (NCH3), 32.8 (C(CH3)3),
30.6 (C(CH3)3), 10.9 (CCH3) ppm. MS (ESI+) m/z (%): 233.2
[M]+ (100).
3
(t, 4H, J(p-m-BPh4) = 7.3 Hz, p-BPh4), 6.59 (s, 2H, CH2), 6.37
3
(t, 1H, J(H4pz–H3/5pz) = 2.1 Hz, H4pz), 1.73 (s, 9H, C(CH3)3)
1
1
ppm. 13C{ H} NMR (75 MHz, acetone-d6): d 165.0 (q, J(C–
B) = 49.4 Hz, B-C), 142.8 (C3pz), 137.1 (q, J(C–B) = 1.4 Hz,
2
o-BPh4), 132.0 (C5pz), 126.0 (q, 3J(C–B) = 2.8 Hz, m-BPh4), 123.4
(C5im), 122.3 (p-BPh4), 121.8 (C4im), 108.2 (C4pz), 63.1 (CH2), 61.7
(C(CH3)3), 29.5 (C(CH3)3) ppm. MS (ESI+) m/z (%): 205.1 [M]+
(100).
15b (69%): Anal. found: C, 82.29; H, 7.73; N, 8.62. C45H49BN4
requires: C, 82.30; H, 7.52; N, 8.53. 1H NMR (300 MHz, acetone-
d6): d 9.41 (t, 1H 4J(H2im–H4/5im) = 1.6 Hz, H2im), 8.05 (apparent
t, 1H 3/4J(H5im–H2/4im) = 1.8 Hz, H5im), 7.83 (apparent t, 1H
3/4J(H4im–H2/5im) = 1.9 Hz, H4im), 7.34 (m, 8H, o-BPh4), 7.14 (s,
14a (69%): Anal. found: C, 79.92; H, 6.54; N, 7.99.
C44H39BN4·EtOAc requires: C, 79.88; H, 6.70; N, 7.60. 1H NMR
(300 MHz, acetone-d6): d 8.75 (brs, 1H, H2im), 7.93 (m, 2H, PhH),
7.66–7.29 (m, 10H, PhH, H4im and H5im), 7.35 (m, 8H, o-BPh4),
3
2H, Hm), 6.92 (t, 8H, J(m-o/p-BPh4) = 7.3 Hz, m-BPh4), 6.77
3
(t, 4H, J(p-m-BPh4) = 7.3 Hz, p-BPh4), 6.65 (s, 2H, CH2), 6.11
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7029–7038 | 7035
©

(s, 1H, H4pz), 2.47 (s, 3H, CCH3), 2.36 (s, 3H, p-CH3), 2.04 (s,
1H NMR (300 MHz, DCM-d2): d 7.40 (m, 8H, o-BPh4), 7.29
3 3
1
6H, o-CH3), 1.24 (s, 9H, C(CH3)3) ppm. 13C{ H} NMR (75 MHz,
(d, 1H, J(H3pz–H4pz) = 1.9 Hz, H3pz), 7.22 (d, 1H, J(H5pz–
1
3
acetone-d6): d 165.0 (q, J(C–B) = 49.3 Hz, B-C), 164.5 (C3pz),
H4pz) = 2.4 Hz, H5pz), 7.05 (t, 8H, J(m-o/p-BPh4) = 7.4 Hz,
142.1 (Cp), 141.4 (C5pz), 138.2 (C2im), 137.0 (q, 2J(C–B) = 1.4 Hz,
o-BPh4), 135.4 (C◦), 132.0 (N–C), 130.4 (Cm), 126.0 (q, 3J(C–B) =
2.9 Hz, m-BPh4), 125.6 (C4im), 123.5 (C5im), 122.3 (p-BPh4), 104.8
(C4pz), 61.3 (CH2), 32.8 (C(CH3)3), 30.6 (C(CH3)3), 21.0 (p-CH3),
17.3 (o-CH3), 10.8 (CCH3) ppm. MS (ESI+) m/z (%): 337.2 [M]+
(100).
m-BPh4), 6.91 (t, 4H, J(p-m-BPh4) = 7.4 Hz, p-BPh4), 6.90 (d,
3
1H, 3J(H5im–H4im) = 1.9 Hz, H5im), 6.66 (d, 1H, 3J(H4im-H5im) =
1.9 Hz, H4im), 6.48 (d, 1H, 2J(Ha-Hb) = 13.8 Hz, NCHaHb), 6.24
(t, 1H, 3J(H4pz-H3/5pz) = 2.4 Hz, H4pz), 5.11 (d, 1H, 2J(Hb-Ha) =
13.8 Hz, NCHaHb), 4.88 (t, 1H, J = 7.3 Hz, COD1), 4.64 (t, 1H,
J = 6.4 Hz, COD5), 4.20 (q, 1H, J = 7.6 Hz, COD2), 4.10 (q,
1H, J = 7.6 Hz, COD6), 2.92–1.62 (m, 8H, COD3/4/7/8), 1.74
15c (66%): Anal. found: C, 80.71; H, 8.14; N, 9.51. C40H47BN4
requires: C, 80.79; H, 7.97; N, 9.42. 1H NMR (300 MHz, acetone-
d6): d 9.36 (t, 1H 4J(H2im–H4/5im) = 1.6 Hz, H2im), 7.90 (apparent
t, 1H 3/4J(H4im–H2/5im) = 1.8 Hz, H4im), 7.72 (apparent t, 1H
3/4J(H5im–H2/4im) = 1.8 Hz, H5im), 7.34 (m, 8H, o-BPh4), 6.92
1
(s, 9H, C(CH3)3) ppm (C2im undefined). 13C{ H} NMR (75 MHz,
1
DCM-d2): d 165.5 (q, J(C–B) = 49.5 Hz, B-C), 142.2 (C3pz),
3
136.6 (brs, o-BPh4), 133.2 (C5pz), 126.4 (q, J(C–B) = 2.9 Hz,
m-BPh4), 122.6 (p-BPh4), 121.5 (C4im), 120.2 (C5im), 108.3 (C4pz),
97.2 (d, 1J(C–Rh) = 8.8 Hz, COD2), 94.7 (d, 1J(C–Rh) = 7.6 Hz,
3
3
(t, 8H, J(m-o/p-BPh4) = 7.3 Hz, m-BPh4), 6.77 (t, 4H, J(p-m-
BPh4) = 7.3 Hz, p-BPh4), 6.41 (s, 2H, CH2), 6.06 (s, 1H, H4pz), 2.37
(s, 3H, CCH3), 1.72 (s, 9H, NC(CH3)3), 1.25 (s, 9H, CC(CH3)3)
1
1
COD1), 81.9 (d, J(C–Rh) = 13.9 Hz, COD5), 73.0 (d, J(C–
Rh) = 12.7 Hz, COD6), 64.9 (CH2), 59.1 (C(CH3)3), 36.1 (COD),
32.6 (C(CH3)3), 32.5 (COD), 29.1 (COD), 27.1 (COD) ppm
(C2im undefined). MS (ESI+) m/z (%): 415.9 [M]+ (100), 1149.3
[2M + BPh4]+ (48).
ppm. 13C{ H} NMR (75 MHz, acetone-d6): d 165.5 (q, 1J(C–B) =
1
49.3 Hz, B–C), 164.7 (C3pz), 141.9 (C5pz), 137.6 (brs, o-BPh4),
135.5 (C2im), 126.6 (q, 3J(C–B) = 2.9 Hz, m-BPh4), 123.6 (C5im),
122.8 (p-BPh4), 122.3 (C4im), 105.2 (C4pz), 62.1 (NC(CH3)3), 61.3
(CH2), 33.3 (CC(CH3)3), 31.1 (CC(CH3)3), 30.1 (NC(CH3)3), 11.4
(CCH3) ppm. MS (ESI+) m/z (%): 275.2 [M]+ (100).
17a (74%): Anal. found: C, 73.50; H, 5.98; N, 7.26.
C52H50BN4Rh requires: C, 73.94; H, 5.97; N, 6.63. 1H NMR
(300 MHz, DCM-d2): d 7.96 (m, 2H, PhH), 7.63 (m, 6H, PhH),
3
7.38 (m, 2H, PhH), 7.31 (m, 8H, o-BPh4), 6.99 (t, 8H, J(m-o/
2
p-BPh4) = 7.3 Hz, m-BPh4), 6.90 (d, 1H, J(Ha–Hb) = 13.4 Hz,
Synthesis of rhodium complexes
NCHaHb), 6.86 (t, 4H, 3J(p-m-BPh4) = 7.3 Hz, p-BPh4), 6.60 (s,
3
General procedure. A freshly prepared solution of NaOMe
(35 mg, 1.5 mmol, of Na in 20 mL of methanol) was filtered into a
methanol (10 mL) solution of the metal precursor [Rh(COD)(m-
OMe)]2 (73 mg, 0.15 mmol) and the desired pyrazolyl-imidazolium
ligand precursor (0.30 mmol). The solution was refluxed for
several hours to form a yellow solution that was cooled to room
temperature and the volume reduced under vacuum until a bright
yellow precipitate formed. The precipitate was filtered, washed
with diethyl ether, and dried in vacuo to yield the rhodium NHC
complex which was further purified via recrystallization from
DCM and hexane. The products were observed to decompose
in solutions of chloroform upon prolonged standing, therefore all
spectra were acquired in DCM-d2.
1H, H4pz), 6.58 (d, 1H, J(H4im–H5im) = 1.9 Hz, H4im), 6.52 (d,
3
2
1H, J(H5im–H4im) = 1.9 Hz, H5im), 5.93 (d, 1H, J(Hb-Ha) =
13.4 Hz, NCHaHb), 4.73 (brq, 1H, J= 6.3 Hz, COD1), 4.60 (brt,
1H, J= 6.3 Hz, COD5), 4.30 (brt, 1H, J= 6.3 Hz, COD6), 3.72
(brq, 1H, J= 6.3 Hz, COD2), 3.70 (s, 3H, CH3), 2.62–1.78 (m,
8H, COD3/4/7/8) ppm. 13C{ H} NMR (75 MHz, DCM-d2): d
1
177.4 (d, 1J(C-Rh) = 61.5 Hz, C2im), 164.6 (q, 1J(C–B) = 49.5 Hz,
B–C), 156.1 (C3pz), 148.6 (C5pz), 136.5 (brs, o-BPh4), 131.3 (PhC),
131.2 (PhC), 131.0 (PhC), 130.3 (PhC), 129.5 (PhC), 129.4 (PhC),
129.3 (PhC), 127.9 (PhC), 126.2 (q, 3J(C–B) = 2.9 Hz, m-BPh4),
123.9 (C4im), 122.3 (p-BPh4), 121.2 (C5im), 108.0 (C4pz), 101.7
1
1
(d, J(C–Rh) = 8.6 Hz, COD1), 95.8 (d, J(C–Rh) = 6.7 Hz,
1
1
COD2), 78.9 (d, J(C–Rh) = 11.9 Hz, COD6), 74.8 (d, J(C–
Rh) = 12.6 Hz, COD5), 62.4 (CH2), 38.2 (CH3), 33.2 (COD), 32.3
(COD), 29.2 (COD), 28.8 (COD) ppm. MS (ESI+) m/z (%): 526.0
[M]+ (100).
16b (89%): Anal. found: C, 68.56; H, 6.17; N, 6.70.
C48H50BN4Rh·0.66CH2Cl2 requires: C, 68.54; H, 6.07; N, 6.57.
1H NMR (300 MHz, DCM-d2): d 7.44 (m, 8H, o-BPh4), 7.35 (d,
1H, 3J(H3pz–H4pz) = 2.3 Hz, H3pz), 7.27 (d, 1H, 3J(H5pz–H4pz) =
2.3 Hz, H5pz), 7.06 (t, 8H, 3J(m-o/p-BPh4) = 7.2 Hz, m-BPh4), 7.03
(s, 2H, Hm), 6.92 (t, 4H, 3J(p-m-BPh4) = 7.2 Hz, p-BPh4), 6.71 (d,
1H, 3J(H5im-H4im) = 1.8 Hz, H5im), 6.64 (d, 1H, 3J(H4im–H5im) =
1.8 Hz, H4im), 6.28 (t, 1H, 3J(H4pz–H3/5pz) = 2.4 Hz, H4pz), 5.28
(brs, 2H, CH2), 4.65 (brs, 2H, COD1/2), 3.68 (brs, 2H, COD5/6),
2.50–1.70 (brm, 8H, COD3/4/7/8), 2.37 (s, 3H, p-CH3), 1.99 (s,
17b (71%): Anal. found: C, 76.08; H, 6.21; N, 6.07.
C60H58BN4Rh requires: C, 75.95; H, 6.16; N, 5.90. 1H NMR
(300 MHz, DCM-d2): d 8.02 (m, 2H, PhH), 7.62 (m, 6H, PhH),
7.42 (m, 2H, PhH), 7.30 (m, 8H, o-BPh4), 7.16 (s, 1H, Hm), 6.98
(t, 9H, 3J(m-o/p-BPh4) = 7.3 Hz, m-BPh4 and Hm¢), 6.85 (m, 5H,
p-BPh4 and NCHaHb), 6.66 (brs, 2H, H4im and H5im), 6.63 (s, 1H,
2
H4pz), 6.01 (d, 1H, J(Hb–Ha) = 13.7 Hz, NCHaHb), 4.80 (brt,
1
6H, o-CH3) ppm. 13C{ H} NMR (75 MHz, DCM-d2): d 177.0
1H, J= 7.2 Hz, COD1), 4.07 (brq, 1H, J= 6.1 Hz, COD5), 3.50
(m, 2H, COD2 and COD6), 2.52–1.42 (m, 8H, COD3/4/7/8),
2.41 (s, 3H, p-CH3), 2.10 (s, 3H, o-CH3), 1.99 (s, 3H, o-CH3¢)
1
1
(d, J(C–Rh) = 51.5 Hz, C2im), 164.6 (q, J(C–B) = 49.0 Hz,
B-C), 142.4 (C3pz), 140.6 (Cp), 136.6 (brs, o-BPh4), 135.5 (C◦),
133.6 (C5pz), 129.8 (Cm), 126.5 (q, J(C–B) = 2.8 Hz, m-BPh4),
ppm. 13C{ H} NMR (75 MHz, DCM-d2): d 177.7 (d, 1J(C-Rh) =
3
1
123.7 (C4im), 122.6 (p-BPh4), 121.9 (C5im), 108.2 (C4pz), 97.4 (brs,
COD1/2), 63.6 (CH2), 32.7 (brs, COD4/7), 29.4 (brs, COD3/8),
21.4 (p-CH3), 18.5 (o-CH3) ppm (COD5/6 and N-C undefined).
MS (ESI+) m/z (%): 477.6 [M]+ (100), 1273.4 [2M + BPh4]+ (26).
16c (71%): Anal. found: C, 67.59; H, 6.56; N, 7.19.
C43H48BN4Rh·0.5CH2Cl2 requires: C, 67.24; H, 6.36; N, 7.21.
52.5 Hz, C2im), 164.6 (q, 1J(C–B) = 49.6 Hz, B-C), 156.3 (C3pz),
148.4 (C5pz), 140.8 (Cp), 136.6 (brs, o-BPh4), 135.3 (C◦), 135.2
(C◦), 131.4 (PhC), 131.0 (PhC), 130.9 (PhC), 130.4 (PhC), 130.0
(Cm), 129.6 (PhC), 129.6 (PhC), 129.1 (PhC), 127.8 (PhC), 126.2
(q, 3J(C–B) = 2.9 Hz, m-BPh4), 124.4 (C4im), 122.3 (p-BPh4), 121.4
1
(C5im), 108.5 (C4pz), 101.9 (d, J(C–Rh) = 7.3 Hz, COD1), 94.0
7036 | Dalton Trans., 2009, 7029–7038
This journal is
The Royal Society of Chemistry 2009
©

1
1
(d, J(C–Rh) = 7.3 Hz, COD2), 78.2 (d, J(C–Rh) = 12.9 Hz,
18b (87%): Anal. found: C, 70.51; H, 6.99; N, 5.97.
C53H60BN4Rh·0.5CH2Cl2 requires: C, 70.67; H, 6.76; N, 6.16. 1H
NMR (300 MHz, DCM-d2): d 7.37 (m, 8H, o-BPh4), 7.04 (m, 10H,
m-BPh4, NCHaHb, and Hm), 6.98 (brs, 1H, Hm¢), 6.88 (t, 4H, 3J(p-
m-BPh4) = 7.4 Hz, p-BPh4), 6.78 (d, 1H, 3J(H5im–H4im) = 1.9 Hz,
1
COD6), 77.7 (d, J(C–Rh) = 12.9 Hz, COD5), 62.4 (CH2), 34.2
(COD), 31.2 (COD), 31.0 (COD), 27.2 (COD), 21.5 (p-CH3), 18.8
(o-CH3), 18.0 (o-CH3¢) ppm. MS (ESI+) m/z (%): 630.1 [M]+
(100).
3
17c (91%): Anal. found: C, 74.44; H, 6.44; N, 6.45. C55H56BN4Rh
requires: C, 74.49; H, 6.37; N, 6.32. 1H NMR (300 MHz, DCM-
d2): d 7.84 (m, 2H, PhH), 7.61 (m, 6H, PhH), 7.38 (m, 2H,
H5im), 6.58 (d, 1H, J(H4im-H5im) = 1.9 Hz, H4im), 6.03 (s, 1H,
H4pz), 5.65 (d, 1H, 2J(Hb–Ha) = 14.1 Hz, NCHaHb), 5.11 (t, 1H,
J = 7.3 Hz, COD1), 4.75 (q, 1H, J = 7.6 Hz, COD2), 3.88 (brq, 1H,
J = 6.8 Hz, COD5), 3.46 (brt, 1H, J = 6.8 Hz, COD6), 2.72–1.30
(m, 8H, COD3/4/7/8), 2.37 (s, 3H, p-CH3), 2.28 (s, 3H, CCH3),
1.93 (s, 3H, o-CH3), 1.82 (s, 3H, o-CH3¢), 1.42 (s, 9H, C(CH3)3)
PhH), 7.31 (m, 9H, o-BPh4 and NCHaHb), 6.99 (t, 8H, J(m-o/
3
p-BPh4) = 7.3 Hz, m-BPh4), 6.91 (d, 1H, 3J(H4im–H5im) = 2.1 Hz,
H4im), 6.85 (t, 4H, 3J(p-m-BPh4) = 7.3 Hz, p-BPh4), 6.60 (d,
1H, 3J(H5im–H4im) = 2.1 Hz, H5im), 6.59 (s, 1H, H4pz), 6.03
(d, 1H, 2J(Hb-Ha) = 13.7 Hz, NCHaHb), 4.53 (brt, 1H, J =
7.0 Hz, COD5), 4.37 (brq, 1H, J= 7.6 Hz, COD1), 4.24 (brq,
1H, J = 7.0 Hz, COD6), 3.86 (brt, 1H, J = 7.6 Hz, COD2),
2.67–1.58 (m, 8H, COD3/4/7/8), 1.82 (s, 9H, C(CH3)3) ppm.
1
ppm. 13C{ H} NMR (75 MHz, DCM-d2): d 166.6 (C3pz), 164.6
(q, 1J(C–B) = 49.4 Hz, B–C), 143.6 (C5pz), 140.6 (Cp), 136.6 (brs,
o-BPh4), 135.4 (C◦), 135.1 (N–C), 134.8 (C◦¢), 129.8 (Cm), 129.7
3
(Cm¢), 126.3 (q, J(C–B) = 2.9 Hz, m-BPh4), 124.4 (C4im), 122.5
(p-BPh4), 121.2 (C5im), 106.9 (C4pz), 101.0 (d, 1J(C–Rh) = 7.3 Hz,
COD1), 94.2 (d, 1J(C-Rh) = 7.0 Hz, COD2), 75.7 (d, 1J(C–Rh) =
13C{ H} NMR (75 MHz, DCM-d2): d 176.8 (d, 1J(C–Rh) =
1
49.3 Hz, C2im), 164.6 (q, 1J(C–B) = 49.6 Hz, B–C), 155.5 (C3pz),
148.6 (C5pz), 136.5 (brs, o-BPh4), 131.3 (PhC), 130.9 (PhC), 130.3
(PhC), 129.5 (PhC), 129.4 (PhC), 129.4 (PhC), 129.0 (PhC), 127.9
14.5 Hz, COD5), 74.2 (d, J(C–Rh) = 12.4 Hz, COD6), 61.8
1
(CH2), 35.8 (COD), 32.7 (COD), 32.2 (C(CH3)3), 32.1 (C(CH3)3),
29.7 (COD), 26.3 (COD), 21.4 (p-CH3), 18.7 (o-CH3), 18.0
(o-CH3¢), 12.1 (CCH3) ppm (C2im undefined). MS (ESI+) m/z
(%): 548.1 [M]+ (100), 1413.6 [2M + BPh4]+ (18).
3
(PhC), 126.2 (q, J(C–B) = 2.9 Hz, m-BPh4), 122.3 (p-BPh4),
121.2 (C5im), 120.5 (C4im), 107.7 (C4pz), 98.8 (d, 1J(C–Rh) =
8.9 Hz, COD1), 93.9 (d, 1J(C-Rh) = 7.3 Hz, COD2), 80.6 (d, 1J(C–
1
Rh) = 13.8 Hz, COD5), 74.0 (d, J(C–Rh) = 13.01 Hz, COD6),
General procedure for catalytic hydrogenations
63.4 (CH2), 59.3 (C(CH3)3), 35.2 (COD), 32.5 (C(CH3)3), 30.9
(COD), 29.7 (COD), 27.5 (COD) ppm. MS (ESI+) m/z (%): 568.1
[M]+ (100).
The hydrogenation reactions were carried out in a 40 mL steel
bomb reactor (Parr Instrument Co. USA) equipped with a
stainless steel cannular dip tube. The temperature of the bomb
was maintained by positioning the bomb on a magnetic stirrer
hotplate, which also facilitated mixing of the solution through
a magnetic stirrer bar placed inside the bomb. A THF (20 mL)
solution of styrene (54.4 mM), and an internal standard toluene
(71.4 mM) was added to the catalyst (0.5 mol%), the bomb was
sealed and purged with H2 gas for 30 s and then pressurized under
50 psi of H2 gas at 27 ◦C. Aliquots (0.2 mL) of the reaction solution
were taken at regular intervals through the stainless steel dip tube,
which was purged with solution (1.0 mL) prior to collecting the
aliquot.
The aliquots were analyzed using capillary electrophoresis using
the micellar electrokinetic capillary chromatography (MECC)
technique. Prior to analysis, 15 mL of the aliquot was diluted
with methanol (235 mL), SDS, (500 mM, 100 mL), and water
(650 mL). Separation of the analytes, styrene, ethylbenzene, and
toluene was achieved using an aqueous buffer solution of sodium
borate (50 mmol, pH = 8.5), SDS (50 mM), and MeOH (5% v/v).
The samples were run through an 8.5 cm capillary, at a 15 kV
separation voltage, and 5 s injection time, with elution of the
analytes complete within 10 min of injection. A UV detector was
used to detect the aromatic analyte species with peak integrals
being measured at 195 nm.
18a (26%): Anal. found: C, 69.50; H, 7.33; N, 11.26.
C58H72BN8Rh requires: C, 70.01; H, 7.29; N, 11.26. (18aa) H
1
NMR (300 MHz, DCM-d2): d 7.31 (m, 8H, o-BPh4), 7.00 (t, 8H,
3J(m-o/p-BPh4) = 7.3 Hz, m-BPh4), 6.85 (t, 5H, 3J(p-m-BPh4) =
7.3 Hz, p-BPh4 and C4im), 6.65 (d, 1H, J(Ha–Hb) = 13.3 Hz,
2
NCHaHb), 6.57 (d, 1H, J(H5im–H4im) = 1.9 Hz, H5im), 6.37 (d,
3
1H, J(Hb–Ha) = 13.3 Hz, NCHaHb), 6.01 (s, 1H, H4pz), 5.01
2
(brs, 2H, COD1/5), 4.39 (brs, 2H, COD2/6), 3.96 (s, 3H, NCH3),
2.71–2.11 (m, 8H, COD3/4/7/8), 2.34 (s, 3H, CCH3), 1.19 (s, 9H,
C(CH3)3) ppm. 13C{ H} NMR (75 MHz, DCM-d2): d 181.7 (d,
1
1J(C–Rh) = 58.1 Hz, C2im), 164.6 (q, J(C–B) = 49.3 Hz, B-C),
1
164.3 (C3pz), 140.4 (C5pz), 136.5 (brs, o-BPh4), 126.2 (q, J(C–
3
B) = 2.7 Hz, m-BPh4), 124.7 (C4im), 122.3 (p-BPh4), 120.0 (C5im),
104.5 (C4pz), 92.1 (d, 1J(C–Rh) = 8.4 Hz, COD1/5), 91.5 (d, 1J(C-
Rh) = 8.4 Hz, COD2/6), 62.3 (CH2), 39.4 (NCH3), 32.4 (COD),
31.4 (COD), 30.6 (C(CH3)3), 30.4 (C(CH3)3), 11.6 (CCH3) ppm.
1
(18ab) H NMR (300 MHz, DCM-d2): d 7.31 (m, 8H, o-BPh4),
7.00 (t, 8H, 3J(m-o/p-BPh4) = 7.3 Hz, m-BPh4), 6.85 (t, 5H, 3J(p-
m-BPh4) = 7.3 Hz, p-BPh4 and C4im), 6.59 (d, 1H, J(Ha–Hb) =
2
13.3 Hz, NCHaHb), 6.52 (d, 1H, 3J(H5im–H4im) = 1.9 Hz, H5im),
6.07 (d, 1H, 2J(Hb–Ha) = 13.3 Hz, NCHaHb), 6.02 (s, 1H, H4pz),
5.09 (brt, 2H, J = 6.8 Hz, COD1/5), 4.25 (brq, 2H, J = 6.2 Hz,
COD2/6), 4.13 (s, 3H, NCH3), 2.71–2.11 (m, 8H, COD3/4/7/8),
2.28 (s, 3H, CCH3), 1.21 (s, 9H, C(CH3)3) ppm. 13C{ H} NMR
1
(75 MHz, DCM-d2): d 181.7 (d, 1J(C-Rh) = 58.1 Hz, C2im), 164.6
1
(q, J(C–B) = 49.3 Hz, B–C), 164.2 (C3pz), 140.3 (C5pz), 136.5
Acknowledgements
(brs, o-BPh4), 126.2 (q, 3J(C–B) = 2.7 Hz, m-BPh4), 124.5 (C4im),
122.3 (p-BPh4), 120.2 (C5im), 104.4 (C4pz), 93.1 (d, J(C-Rh) =
We gratefully acknowledge financial support from the University
of New South Wales, the Australian Research Council (ARC),
the Australian government for an Australian Postgraduate Award
(M. J. P.), and the German Academic Exchange Services (DAAD)
for a postdoctoral scholarship (J. W.).
1
1
8.4 Hz, COD1/5), 90.6 (d, J(C–Rh) = 8.4 Hz, COD2/6), 62.2
(CH2), 39.6 (NCH3), 32.7 (COD), 31.4 (COD), 30.6 (C(CH3)3),
30.4 (C(CH3)3), 11.4 (CCH3) ppm. MS (ESI+) m/z (%): 676.2
[M]+ (100).
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7029–7038 | 7037
©

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The Royal Society of Chemistry 2009
©