Rhodium(i) complexes bearing N-donor ligands: Catalytic activity towards intramolecular cyclization of alkynoic acids and ligand lability

Article abstract of DOI:10.1039/c1nj20094a

The structures of rhodium(i) complexes bearing tris(pyrazol-1-yl)toluidine (tpt) and tris(N-methylimidazol-2-yl)methanol (tim) ligands were examined in the solid state using single crystal X-ray diffraction, and in the solution state using variable temperature NMR spectroscopy. The solid state structures of the rhodium(i) tpt and rhodium(i) tim complexes showed that the ligands are bound to the rhodium(i) centre in the κ² binding mode, rather than the κ³ binding mode. In the solution state, rhodium(i) complexes bearing the tpt ligand undergo fluxional behaviour at room temperature, which was attributed to rotation of the toluidine substituent about the C-C bond or the equilibrium between κ² and κ³ binding modes. At low temperatures, rhodium(i) complexes bearing the tpt ligand adopted the κ² binding mode, consistent with the coordination mode in the solid state structures. The efficiency of the complexes as catalysts for the intramolecular hydroalkoxylation of 4-pentynoic acid and 5-hexaynoic acid to form the corresponding lactone was established. The presence of the third unbound N-donor was shown to reduce the catalytic efficiency of the complexes with tridentate ligands when compared to their counterparts bearing bidentate ligands, due to either the steric hindrance or competitive binding of the third N-donor with the substrate during the catalytic cycle.

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PAPER
Rhodium(I) complexes bearing N-donor ligands: catalytic activity
towards intramolecular cyclization of alkynoic acids and ligand labilityw
Bradley Y.-W. Man,a Mohan Bhadbhadeab and Barbara A. Messerle*a
Received (in Gainesville, FL, USA) 5th February 2011, Accepted 2nd June 2011
DOI: 10.1039/c1nj20094a
The structures of rhodium(I) complexes bearing tris(pyrazol-1-yl)toluidine (tpt) and
tris(N-methylimidazol-2-yl)methanol (tim) ligands were examined in the solid state using single
crystal X-ray diffraction, and in the solution state using variable temperature NMR spectroscopy.
The solid state structures of the rhodium(I) tpt and rhodium(I) tim complexes showed that the
ligands are bound to the rhodium(I) centre in the k2 binding mode, rather than the k3 binding
mode. In the solution state, rhodium(I) complexes bearing the tpt ligand undergo fluxional
behaviour at room temperature, which was attributed to rotation of the toluidine substituent
about the C–C bond or the equilibrium between k2 and k3 binding modes. At low temperatures,
rhodium(I) complexes bearing the tpt ligand adopted the k2 binding mode, consistent with the
coordination mode in the solid state structures. The efficiency of the complexes as catalysts for
the intramolecular hydroalkoxylation of 4-pentynoic acid and 5-hexaynoic acid to form the
corresponding lactone was established. The presence of the third unbound N-donor was shown to
reduce the catalytic efficiency of the complexes with tridentate ligands when compared to their
counterparts bearing bidentate ligands, due to either the steric hindrance or competitive binding
of the third N-donor with the substrate during the catalytic cycle.
bis(pyrazol-1-yl)methane (bpm) ligands and bis(N-methyl-
imidazol-2-yl)methane (bim) ligands are well studied.5Although
Introduction
Late transition metal complexes with polydentate N-donor
Group 9 complexes of the bidentate pyrazolyl and imidazolyl
ligands have been successfully used as catalysts for a wide range
methane containing ligands are well studied, the complexes
of transformations, including the formation of new bonds
of their corresponding tridentate counterparts are not. The
such as C–X bonds (X = N, O or S)1 and hydroformylation.2
presence of the additional donor in the tridentate ligands could
Where phosphine and carbene donors are widely used as
lead to different reactivities of the metal complexes due to
donors in late transition metal catalysts, the advantages of
changes in the coordination chemistry. Complexes bearing
N-donor ligands lie in their synthetic accessibility and stability.
the tris(pyrazol-1-yl)toluidine ligand (1) have the potential to
be immobilised onto macroscopic scaffolds such as polymers,6
silicon7 and proteins.8 Liddle et al.9 recently published a conve-
Many of the N-donor ligands used in successful catalysts contain
sp2-hybridised nitrogen donor atoms, including N-heterocyclic
and diimine compounds. They are generally coordinated as
nient synthesis for an amine functionalised tris(pyrazol-1-yl)alkane
multidentate ligands in order to reduce the lability of the
ligand, tris(pyrazol-1-yl)toluidine (tpt, Fig. 1).
metal–nitrogen bond.
Cationic rhodium(I) and iridium(I) complexes bearing biden-
The anionic poly(pyrazol-1-yl)borate ligands containing the
pyrazolyl N-donor are widely used in coordination chemistry,3
tate N-donor ligands such as bpm are efficient catalysts for the
formation of C–N10 and C–O11 bonds. More specifically,
and the poly(pyrazol-1-yl)methane and poly(imidazole-2-yl)-
methane ligands are neutral analogues of these.4 The coordination
chemistry and catalytic activity of Group 9 complexes bearing
a School of Chemistry, University of New South Wales, Sydney,
NSW 2052, Australia. E-mail: b.messerle@unsw.edu.au;
Fax: +61-2-9385-6141
b Mark Wainwright Analytical Centre, University of New South
Wales, Sydney, NSW 2052, Australia
w Crystallographic data in CIF format for complexes 4b (CCDC
Fig. 1 The structures of tris(pyrazol-1-yl)toluidine ligands, p-tris-
782080), 6 (CCDC 782081), 2a ((CCDC 782082, 782083), 5 (CCDC
(pyrazol-1-yl)toluidine (p-tpt, 1a) and o-tris(pyrazol-1-yl)toluidine (o-tpt, 1b)
782085), 3b (CCDC 782086), 3a (CCDC 782087). For crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1nj20094a
respectively.
c
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Scheme 1 Catalysed intramolecular cyclization of: (a) 4-pentynoic
acid (8); and (b) 5-hexynoic acid (10) to the corresponding cyclic
lactones 9 and 11, respectively.
Rh(I) complexes with sp2-N donor ligands12 and cationic Rh/Ir(I)
complexes of the tris-carbene (TIMEN) tris[2-(3-isopropyl-
imidazol-2-ylidene)ethyl]amine ligand system13 are efficient
catalysts for the intramolecular cyclisation of acetylenic carboxylic
acids. The transition metal catalysed intramolecular cyclisation
of acetylenic carboxylic acids offers an atom efficient approach
to the synthesis of five and six-membered ring systems containing
oxygen.14 Industrially, this catalytic process is particularly
useful for the preparation of pharmaceuticals, flavours and
fragrances.15 The Rh,12,13,16 Pd16a and Hg17 catalysed intra-
molecular cyclisation of alkynyl carboxylic acids forming the
five-membered or six-membered exocyclic enol lactones has
been performed previously (Scheme 1).
Scheme 2 Synthesis of rhodium (I) complexes 2–4 bearing the tpt
ligand (1).
n-pentane into a concentrated solution of the desired complex
in dichloromethane at room temperature. The crystals were
analysed using single crystal X-ray diffraction and the crystallo-
graphic parameters are summarized in Table 1. Selected bond
lengths and angles for the complexes 2a, 2b, 3a, 3b and 4b are
shown in Table 2. ORTEP depictions of the cationic fragments
of the complexes are shown in Fig. 2 (2a and 3a), Fig. 3 (2b),
Fig. 4 (3b) and Fig. 5 (4b).
Here, the structures of a series of rhodium(I) tris(pyrazol-1-yl)-
toluidine complexes bearing a range of co-ligands were investi-
gated. The structures of the analogous Rh(I) complexes bearing
tris(N-methylimidazol-2-yl)-methanol ligands were also investi-
gated. In order to establish the effect the presence of a third
sp2 N-donor or a toluidine substituent on the ligand may have
on the catalytic activity of the Rh(I) complexes, we compared
the catalytic efficiency of the rhodium(I) complexes bearing the
tridentate ligands described here with that of their counterparts
bearing bidentate ligands for the intramolecular cyclization of the
aliphatic alkynoic acids 8 and 10.
All of the rhodium(I) complexes bearing the tpt ligand (1)
and cyclic bisolefins as co-ligands adopted a distorted square
planar geometry around the rhodium(I) centre. The molecular
structure (Fig. 2(a)) of [Rh(COD)(p-tpt)][BArF] (2a) shows the
p-tpt ligand (1a) coordinated to the rhodium(I) centre via the
k2 binding mode. The unbound pyrazolyl donor in complex 2a
is positioned away from the rhodium centre with the phenyl ring
of the p-tpt ligand (1a) above the rhodium centre. To reduce the
steric interactions between the co-ligand and the p-tpt ligand (1a)
the smaller and more constrained co-ligand norbornadiene was
also used. The molecular structure of the resulting complex
[Rh(NDB)(p-tpt)][BArF] (3a) shows that the p-tpt ligand (1a)
was also bound to the rhodium centre via k2 binding mode in
this complex. Increased N–Rh–X bite angle values observed
for the NBD complex 3a compared to those of the COD
complex 2a are attributed to the reduced Rh–N bond lengths
and the more constrained nature of the NBD co-ligand.
In the tris(3,5-dimethylpyrazol-1-yl)methane complexes of
Rh(I) reported previously by Hallett et al.,18 the use of a
smaller and more constrained co-ligand such as NBD led to
a significant change in the coordination geometry of the tris-
(pyrazol-1-yl)methane ligands, with the tridentate ligand changing
from the k2 binding mode to the k3 binding mode. This was
attributed to a reduction in the steric interaction between the
N-donor ligand and bisolefin co-ligand. This was also observed
in the complexes reported by Adams et al.19 where the smaller
CO co-ligand led to the k3 binding mode of the tridentate
N-donor ligands in the solution phase. These observations
suggest that in the case of the tpt ligand under investigation here,
steric interactions between the co-ligand and the N-donor
ligand are not solely responsible for the dominance of the k2
binding mode. This in turn suggests that the presence of an
Results and discussion
Synthesis of rhodium(I) complexes bearing
tris(pyrazol-1-yl)toluidine ligands (1)
The rhodium(I) tris(pyrazol-1-yl)toluidine olefin complexes
(2 and 3) were synthesised by stirring a solution of [Rh(L)2]-
[BArF] (L = 1,5-cyclooctadiene, COD; or norbornadiene,
NBD) with the appropriate ligand (1) in tetrahydrofuran
(THF) at room temperature (Scheme 2). The bisolefin complexes
(2 and 3) were isolated and purified by the removal of THF
in vacuo, followed by recrystallisation from a mixture of dichloro-
methane and n-pentane. The corresponding dicarbonyl complexes 4
(a and b) were synthesised by stirring a degassed solution of the
olefin complex 2 in dichloromethane under a carbon monoxide
(CO) atmosphere. The resulting dicarbonyl complexes 4 were
isolated by the addition of n-pentane followed by recrystalli-
sation from dichloromethane and n-pentane.
Solid state structures of rhodium (I) complexes bearing the
tris(pyrazol-1-yl)toluidine ligand (1)
Single crystals of complexes 2a, 2b, 3a, 3b and 4b suitable for
X-ray diffraction analysis were grown by the slow diffusion of
c
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electron donating group, in the form of the primary amine on
the tpt ligand (1) has influenced the k2 2 k3 equilibrium. To
examine the influence of the location of the primary amine
group on the toluidine substituent on the N-donor ligands, the
structures of analogous complexes bearing the o-tpt ligand
(1b) were examined.
The molecular structure (Fig. 3) of [Rh(COD)(o-tpt)][BArF]
(2b) shows that the asymmetric unit contains two complex
molecules, A and B (Fig. 3(a) and (b) respectively), which
differ in the mode of complexation of the ligand. Both
structures have the o-tpt (1b) ligand coordinated to the
rhodium centre via the k2 binding mode. The only significant
difference between structures A and B is the orientation of the
third unbound pyrazolyl donor. In structure A (Fig. 3(a)), the
unbound pyrazolyl donor is positioned away from the rhodium
centre, while in structure B (Fig. 3(b)), the pyrazolyl donor is
positioned above the rhodium centre. Selected bond lengths and
angles (Table 2) show that in both structures the metal centre
adopts a distorted square planar geometry, and no significant
variation in the Rh–N and Rh–X bond lengths are observed.
On comparing the bond lengths and angles for the structures
of 2a and 2b with the amine substituent in the para- and
ortho-positions respectively, it is clear that the position of
the amine on the toluidine substituent has little influence on
the structure of the complex. The complex bearing the NBD
co-ligand [Rh(NDB)(o-tpt)][BArF] (3b) only contained one
structure in the unit cell. The molecular structure (Fig. 4) of
the complex 3b shows the rhodium centre bound to the ligand
in a k2 binding mode with the unbound pyrazolyl donor
positioned above the metal centre. On comparing the bond
lengths and angles of structure B of 2b and 3b bearing the COD
and NBD co-ligands respectively, it is apparent that there are
no significant deviations in the nature of the coordination
chemistry of the o-tpt ligand (1b) in the complexes on changing
the nature of the olefin co-ligand. Small deviations in the bond
angles are observed, which can be attributed to the more
constrained nature of the NDB co-ligand compared to the
COD co-ligand.
To further reduce the steric interactions around the metal centre,
the cyclic diene co-ligands were replaced with CO co-ligands,
forming the rhodium(I) complexes [Rh(CO)2(p-tpt)][BArF] (4a)
and [Rh(CO)2(o-tpt)][BArF] (4b). The rhodium centre in
complex 4b is bound to the o-tpt ligand (1b) in a k2 binding
mode (Fig. 5). Here, the unbound pyrazolyl donor is positioned
above the rhodium(I) centre, which is similar to the positioning
of the unbound pyrazolyl donor in structure B of the COD
complex 2b (Fig. 3). A distorted square planar geometry was
adopted around the rhodium centre in 4b (Table 2). Comparing
the selected bond lengths of the complex 4b with those of
the parent COD complex 2b, shows that the change to the
carbon monoxide co-ligands does not significantly influence
the coordination chemistry of the o-tpt ligand (1b).
Solution state structure of rhodium(I) complexes bearing the
tris(pyrazol-1-yl)toluidine ligand
The 1H NMR spectrum of [Rh(COD)(p-tpt)][BArF] (2a) at
room temperature (Fig. 6(a)) contains broad resonances due
to the pyrazolyl protons. The line broadening was attributed
c
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Table 2 Selected bond lengths and angles for the complexes 2a, 2b, 3a, 3b and 4b
[Rh(COD)(o-tpt)][BArF] (2b)
[Rh(COD)(p-tpt)]- [Rh(NDB)(p-tpt)]-
[BArF] (2a)
[Rh(NBD)(o-tpt)]-
[BArF] (3b)
[Rh(CO)2(o-tpt)]-
[BArF] (4b)
A
B
[BArF] (3a)
Atom Pairs Bond lengths (A)
Rh–N1
Rh–N2
Rh–X1a
Rh–X2a
2.104(3)
2.102(3)
2.015
2.056(6)
2.036(7)
2.001
2.065(6)
2.099(7)
2.018
2.099(6)
2.086(7)
2.018
2.056(6)
2.036(7)
1.995
Rh–N1
Rh–N2
Rh–C1
Rh–C2
2.052(3)
2.086(3)
1.851(5)
1.847(4)
2.024
2.009
2.008
2.002
1.983
Bond angles (1)
N1–Rh–X1 93.06
N2–Rh–X2 94.89
N1–Rh–N2 84.89(11)
X1–Rh–X2 87.28
101.05
100.82
86.28(13)
71.72
95.60
93.10
86.10(3)
86.57
95.32
93.27
86.40(2)
86.65
98.87
N1–Rh–N2 85.71(11)
C1–Rh–C2 88.03(17)
N1–Rh–C1 92.16(16)
N2–Rh–C2 94.04(14)
101.97
88.20(2)
71.16
a
X1 and X2 are defined as the centroids for the alkene bonds of the COD and NBD co-ligands.
Fig. 3 ORTEP depictions showing the cationic fragment of the two
co-crystallised structures for the complex [Rh(COD)(o-tpt)][BArF]
(2b): (a) Structure A; and (b) Structure B, at 50% thermal ellipsoid
for non-hydrogen atoms.
Fig. 2 ORTEP depictions of the cationic fragments of (a) [Rh(COD)-
(p-tpt)][BArF] (2a) and (b) [Rh(NBD)(p-tpt)][BArF] (3a) at 50%
thermal ellipsoid for non-hydrogen atoms.
structure of the complex 2a. The same observations were made
for the corresponding NBD complex (3a) and the dicarbonyl
complex (4a) of the p-tpt ligand.
to either exchange between the k3 and k2 binding mode or atrop-
isomerism, which would involve the rotation of the toluidine
ring about the C–C bond connecting the toluidine ring to the
bridging carbon. Upon cooling to ꢀ50 1C, the signals in the
NMR spectrum become sharper with the resonances due to
the pyrazole H3, H4 and H5 protons each resolving into two
resonances integrating to a ratio of 1 : 0.5 (Fig. 6(b)). This
shows that there are two equivalent pyrazolyl donors in one
environment, with the third pyrazolyl donor in a different
chemical environment, which is consistent with the solid state
The 1H NMR spectrum of [Rh(COD)(o-tpt)][BArF] (2b),
was similar to that of [Rh(COD)(p-tpt)][BArF] (2a), with broad
resonances due to the pyrazolyl protons at room temperature,
which sharpened upon cooling. However, in the case of the
spectrum of 2b, each of the resonances due to pyrazolyl protons
resolved at low temperature into three distinct resonances,
indicating that at low temperature there is no symmetry about
the rhodium centre in 2b, leading to three distinct pyrazolyl
donor environments. In addition to the asymmetric pyrazolyl
c
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Fig. 4 ORTEP depiction of the cationic fragment of [Rh(NBD)(o-tpt)]-
[BArF] (3b) at 50% thermal ellipsoid for non-hydrogen atoms.
Fig. 7 1H NMR (600 MHz) spectrum of the complex 2b at ꢀ55 1C in
CDCl3 showing only the aromatic region.
as the H20 proton on the aniline ring at 5.8 ppm. The resonances
were fully assigned for both the major and minor products. An
analogous minor species was also observed for the dicarbonyl
complex 4b. Elemental analysis of the complexes 2b, 3b and 4b
confirmed the purity of each of the complexes, suggesting that
these minor species were the result of conformational exchange.
The NOESY spectrum of 2b at ꢀ50 1C (not shown) contained
cross peaks due to exchange between the major and minor
species showing that the second minor species arises due to
conformational exchange. The two possible exchange processes
giving rise to the second species could be interconversion
between the k2 and k3 or atropisomerism.
In the work by Hallett et al.,18 it was shown for rhodium(I)
coordinated to the tris(3,5-dimethylpyrazol-1-yl)methane ligand
in a k2 binding mode, the 13C chemical shift of the bridging
carbon lies between 74–75 ppm, while for complexes where the
rhodium is bound in the k3 binding mode the 13C chemical
shift of the bridging carbon lies between 67–70 ppm. Using
an 1H-13C HMBC correlation experiment, the resonances due
to the bridging carbon for both the major and minor product
of the complex 4b were assigned. The 13C chemical shifts of
the bridging carbons for the major (92.8 ppm) and minor
(93.0 ppm) products respectively differ by only 0.2 ppm. When
compared to the 7 ppm difference observed by Hallett et al.18
between the rhodium(I) k2 and k3 complexes bearing the
tris(3,5-dimethylpyrazol-1-yl)methane ligand, this suggests
that the exchange being observed in the 1H NMR spectra
for complexes 2b, 3b and 4b is due to atropisomerism, not
exchange between k2 and k3,with rotation of the toluidine
substituent about the C–C bond between the bridging carbon
of the ligand and the aryl carbon of the toluidine ring.
Fig. 5 ORTEP depiction of the cationic fragment of [Rh(CO)2(o-tpt)]-
[BArF] (4b) at 50% ellipsoid for non-hydrogen atoms.
Synthesis of rhodium(I) complexes bearing
tris(N-methylimidazol-2-yl)methanol ligands
The complexes [Rh(COD)(tim)][BArF] (5) and [Rh(CO)2(tim)]-
[BArF] (6) were synthesised using the same general synthetic
strategy described above shown in Scheme 1 in 70% and 57%
yield, respectively.20
Fig. 6 1H NMR (600 MHz) spectrum of the complex 2a: (a) at 25 1C;
and (b) at ꢀ55 1C in CD2Cl2 showing only the aromatic region.
Solid and solution state structures of rhodium(I) complexes
donor environment, a second minor species with similar chemical
shifts and multiplicity was observed in the 1H NMR spectrum
at low temperatures (Fig. 7). This was indicated, for example,
by the appearance of a low intensity second doublet, assigned
bearing the tris(N-methylimidazol-2-yl)methanol ligands
Single crystals of the complexes 5 and 6 were grown by the
slow diffusion of n-pentane into a concentrated solution of
c
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the complex in dichloromethane. The molecular structures for
both complexes showed the rhodium centre coordinated to the
tim ligand in k2 binding mode (the molecular structure of 5 is
not shown). Unlike the rhodium(I) complexes bearing the tpt
ligands, the NMR spectra of 5 did not show any fluxional
behaviour at room temperature.
Based on the solid state structure of the imidazolyl
complexes 5 and 6, two unique imidazolyl donor environments
were expected in the solution state. However, the 1H NMR
spectrum at room temperature only shows one set of resonances
due to all three imidazolyl donors which suggests that the
interconversion between the two binding modes (i.e. k2 and k3)
is sufficiently fast measured on the NMR timescale such that
The solid state structure of [Rh(CO)2(tim)][BArF] (6)
(Fig. 8, Table 3) shows that the rhodium centre binds to the
tim ligand in k2 binding mode. The unit cell for the crystal
structure of 6 shows two of the cationic fragments associated
in space as indicated by the ORTEP representation viewed
from the top (Fig. 8(a)) and the side (Fig. 8(b)). The two
rhodium centres are located directly above each other with the
distance between the two rhodium centres at 3.2 A. This
distance is longer than literature reported Rh–Rh bonds which
are of the order of 2.3 A.21
1
all three imidazolyl donors become equivalent. The H NMR
spectrum of the complex [Rh(CO)2(tim)][BArF] (6) at ꢀ90 1C
also did not resolve the resonances due to the imidazolyl
donors in different chemical environments.
Intramolecular cyclization of aliphatic alkynoic acids to
alkylidene lactones
It has been reported that changes in the steric environment
induced by the three-dimensional arrangement of the ligand in
a complex can induce changes in the selectivity and activity of
the complex as a catalyst.22 In the previous sections, we have
shown that in the solution state, the rhodium(I) complexes
exist in a k2 binding mode, that is with two of the N-donors
bound, while the third remains free in solution. The presence
of the unbound N-donor could have a significant impact on
the activity and selectivity of the complexes with tridentate
ligands as catalysts relative to the activity of their counterparts
with bidentate ligands.
The relative activity of the Rh(I) complexes 4a, 4b and 6
with tridentate pyrazole and imidazole ligands as catalysts was
established for the intramolecular cyclization of 4-pentynoic
acid (8) (Scheme 1 (a), Table 4). A solution of 4-pentynoic acid
and 2 mol% of the catalyst in d6-benzene was heated at 80 1C
and the progress of the reaction monitored using 1H NMR
spectroscopy. Complexes 4a and 4b were not as active as
catalysts for the intramolecular cyclization of 8 when compared
to the imidazole-based complex 6. In the case of 4a, 1.4 h was
required before complete conversion of starting material was
observed (Table 4, entry 3), and for 4b, 5.5 h was required
before 78% conversion was observed (Table 4, entry 4). In the
case of the imidazolyl complex (6), only 0.5 h was required
under the same conditions to achieve complete conversion.
The above results also suggest that the position of the amine
group on the tpt complexes (i.e. 4a, para; 4b, ortho) has a
significant influence on the activity of the complex as a catalyst
for the intramolecular cyclization of 4-pentynoic acid (8). The
reduced activity of the complex 4b as a catalyst compared to
Fig. 8 ORTEP representation of the cationic fragment of the
complex 6 at 50% ellipsoid for non-hydrogen atoms viewed from:
(a) the side; (b) the top.
Table 3 Selected bond lengths and angles for the complexes 5 and 6a
[Rh(COD)(tim)][BArF] (5)
[Rh(CO)2(tim)][BArF] (6)
Bond lengths (A)
Bond lengths (A)
Atom pair
Atom pair
Rh–N1
Rh–N2
Rh–X1
Rh–X2
2.088 (4)
2.084 (4)
2.135
Rh–N1
Rh–N2
Rh–C1
Rh–C2
2.048 (9)
2.072 (7)
1.839 (11)
1.851 (13)
Bond angles (1)
86.8(3)
85.5(5)
94.2(4)
93.7(4)
2.124
Bond angles (1)
86.23 (14)
86.42
94.24
92.80
N1–Rh–N2
X1–Rh–X2
N1–Rh–X1
N2–Rh–X2
N1–Rh–N2
C1–Rh–C2
N1–Rh–C1
N2–Rh–C2
a
X1 and X2 are defined as the centroids for the alkene bonds of the COD co-ligand.
c
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Table 4 Comparison of catalytic activity of rhodium(I) complexes
and literature complexes as catalyst for the intramolecular cyclization
of 4-pentynoic acid (8) to the corresponding lactone 9 (Scheme 1(a))
Table 5 Comparison of catalytic activity of rhodium(I) complexes
and literature complexes as catalyst for the intramolecular cyclization
of 5-hexynoic acid (10) to the corresponding lactone 11
Mol% Temperature,
metal solvent
Mol% Temperature,
metal solvent
Entry Catalyst
Timea
0.3 h
Entry Catalyst
Timea
C6D6,
1
2
[Rh(bim)(CO)2]BArF (7)
[Rh(bim)(CO)2]BPh4
1
C6D6, 70 1C
13.3 h
192 h12
(80%)
1
[Rh(bim)(CO)2]BArF (7)
2
80 1C
—
—
0.7
d6-acetone 50 1C
2
3
[Rh(tim)(CO)2]BArF (6)
[Rh(p-tpt)(CO)2]BArF (4a)
2
2
0.5 h
1.4 h
5.5 h
(78%)
15.5 h12
(88%)
4
5
[Rh(o-tpt)(CO)2]BArF (4b)
2
—
3
5
CD3CN, 25 1C
72 h24
d6-acetone,
50 1C
[Rh(bim)(CO)2]BPh4
0.35
84 h24
(B80%)
d6-acetone,
50 1C
15.5 h12
(98%)
4
5
5
1
2
CD3CN, 25 1C
CD3CN, 50 1C
CDCl3, 60 1C
6
0.40
192 h13
CD3CN,
50 1C
7
8
1
2
8 h13
168 h26
(15%)
6
a
Time required to achieve 100% conversion unless otherwise stated.
CDCl3,
60 1C
3 h23
The presence of the third unbound imidazolyl limits
the ability of the substrate to bind to the catalytic centre, or
the nucleophilic attack of the oxygen nucleophile onto the
activated alkyne bond. Despite the steric hindrance of the third
unbound imidazolyl donor, the complex 6 is still active as
a catalyst for the intramolecular cyclization of 4-pentynoic
acids (8) compared to other complexes published previously
(Table 4).
CD3CN,
25 1C
9
5
5
12 h24
Based on the high activity of complex 7 as a catalyst for the
intramolecular cyclization of 4-pentynoic acid (8), we examined
the activity of complex 7 as a catalyst for the intramolecular
cyclization of 5-hexynoic acid (Scheme 1 (b)). 5-Hexynoic acid
(10) was heated at 70 1C in d6-benzene in the presence of
1 mol% of 7 and the reaction monitored using 1H NMR
spectroscopy. Complete conversion of the alkynoic acid 10
to the cyclic lactone 11 was observed after 13.3 h. This result
is a considerable improvement over other literature reports
of rhodium(I) complexes as catalysts for the same trans-
formation (Table 5). In particular, this result also highlights
the ability of a weakly coordinating counterion such as
BArF (Table 5, entry 1) to improve catalytic performance of
complexes compared to a coordinating counterion such as
BPh4 (Table 5, entry 2).14
CD3CN,
25 1C
10
24 h24
CD3CN,
24 1C
11
AuCl + 10 mol% K2CO3 10
2 h25
a
Time required to achieve 100% conversion unless otherwise stated.
4a was attributed to steric interactions between the amine
group and the substrate during the catalytic cycle, as both
compounds are isoelectronic.
To determine the effect of the of the third unbound N-donor
on catalytic activity, we studied the efficiency of the most active
complex, [Rh(tim)(CO)2][BArF] (6), and its counterpart, with
a bidentate ligand [Rh(bim)(CO)2][BArF] (7) as catalysts for
the intramolecular cyclization of 4-pentynoic acid (8). Based
on the percentage conversion of 4-pentynic acid (8) to the
corresponding lactone 9, the rhodium(I) complex 7 with the
bidentate ligand was the more efficient catalyst with complete
conversion of substrate observed after 0.3 h compared to the
0.5 h required when complex 6 with the tridentate ligand was used
as a catalyst. Presumably, the difference in the catalytic activity is
due to the presence of the third unbound imidazolyl donor.
Conclusion
A series of rhodium(I) complexes bearing the p- (1a) and
o-tris(pyrazol-1-yl)toluidine ligands (1b) were synthesised and
their coordination chemistry studied in the solid and solution
state using X-ray crystallography and NMR spectroscopy, respec-
tively. Despite the potential of the tris(pyrazol-1-yl)toluidine
ligands to bind to the rhodium centre in either a k2 (bidentate)
c
1736 New J. Chem., 2011, 35, 1730–1739
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or a k3 (tridentate) binding mode, the k2 binding mode was
shown to be the preferred binding mode in the solid state for
all of the complexes considered. Despite significant changes to
the nature of the co-ligands (norbornadiene, 1,5-cyclooctadiene
and carbon monoxide), it was found that the tris(pyrazol-1-yl)-
toluidine ligand still favoured the k2 binding mode.
The numbering of the atoms within BArFꢀ and BPh4
ꢀ
counterions used is shown below.
To investigate the influence of the donor strength of the
tridentate ligands on the coordination chemistry of ligands,
the analogous tris(N-methylimidazole-2-yl)methanol complexes
[Rh(tim)(COD)][BArF] (5) and [Rh(tim)(CO)2][BArF] (6) were
synthesised and the structures of these complexes were also
studied using X-ray crystallography and NMR spectroscopy.
The rhodium(I) complexes 5 and 6 also adopted square planar
geometries with a k2 binding mode. The bidentate binding mode
of all of the complexes studied has important implications
for their activity as catalysts, as the binding mode suggests
that the complexes with tridentate ligands could behave in a
similar fashion to the corresponding complexes with bidentate
ligands.
General synthesis of rhodium bisolefin tris(pyrazol-1-yl)toluidine
complexes bearing the BArF counterion. (2)
A
solution of desired tris(pyrazol-1-yl)toluidine and
[Rh(L)2][BArF] (L = COD or NBD) in THF was stirred at
room temperature for 20 to 30 min. The resulting solution was
reduced in vacuo and the desired compound was recrystallised
from a mixture of dichloromethane and n-pentane. Single
crystals were grown via slow diffusion of n-pentane into a
concentrated solution the desired complex in dichloromethane
overnight at room temperature.
Of the three rhodium(I) complexes with tridentate N-donor
ligands, the most active catalyst for the cyclisation of 4-pentynoic
acid was the complex with the tridentate imidazolyl ligand,
[Rh(tim)(CO)2]BArF (6). The activity of the complex 6 for the
intramolecular cyclization of aliphatic alkynoic acids was
compared with that of its counterpart bearing a bidentate
ligand [Rh(bim)(CO)2]BArF (7) , and it was found that the
presence of the third unbound N-donor reduces the catalytic
activity of the complex. This loss of activity is presumably
due to the steric hindrance induced by the presence of an
additional N-donor with the substrate during the catalytic
cycle. Despite this loss of activity, the rhodium(I) complexes
bearing the imidazolyl donor were shown to still be highly
active in the intramolecular cyclization of aliphatic alkynoic
acids compared to previously reported catalysts. We are
currently also investigating the effect of the third ligand
donor on the regioselectivity of the catalysed intramolecular
hydroalkoxylation reaction.
[Rh(p-tpt)(COD)][BArF] (2a). p-tris(pyrazol-1-yl)toluidine
(1a) (28.4 mg, 0.10 mmol), [Rh(COD)2][BArF] (103.6 mg,
0.09 mmol) to yield 2a (103.6 mg, 75%) as yellow crystals
(Found: C, 49.1; H, 3.1; N, 7.0%. C56H39BF24N7Rh requires
C, 48.8; H, 2.6; N, 7.1%); dH (600 MHz; CDCl3; ꢀ50 1C)
d 7.97 (br, 1H, H5P(2) or H3P(2)), 7.69 (br, 8H, H2BArF), 7.49
(br, 4H, H4BArF), 7.45 (br, 2H, H5P(1) or H3P(1)), 7.21 (br, 2H,
H3P(1) or H5P(1)), 7.00 (br, 1H, H3P(2) or H5P(2)), 6.71
(d, J = 7.8 Hz, 1H, H30), 6.38 (br, 2H, H4P(1)), 6.37 (br,
1H, H4P(2)), 6.12 (d, J = 7.8 Hz, 1H, H20), 4.20 (br, 2H,
H1,COD), 3.64 (br, 2H, H1,COD), 2.47 (br, 2H, H2,COD or
H3,COD), 1.82 (br, 2H, H2,COD or H3,COD), 1.62 (br, 4H,
H2,COD and H3,COD).13C{1H} NMR (150 MHz; CDCl3;
ꢀ55 1C) 162.07 (q, J = 50 Hz, C1BArF), 151.1 (C10), 146.1
(C3P(2) or C5P(2)), 143.6 (C3P(1) or C5P(1)), 135.63 (C3P(1) or
C5P(1)), 135.14 (C2BArF), 134.56 (C3P(2) or C5P(2)), 129.2
(q, J = 31 Hz, C3BArF) 129.1 (C30), 124.9 (q, JC-F = 270
Hz, CF3), 123.9 (C40) 118.6 (C4BArF), 117.9 (C1BArF), 114.9
(C20), 108.9 (C4P2), 108.1 (C4P1), 94.2 (Ca), 84.2 (C1COD),
83.60 (C1COD), 30.63 (C2,COD and C3, COD), 29.67 (C2,COD and
C3,COD).
Experimental
The preparation, purification and reaction of all complexes
described were carried out using Schlenk techniques under an
atmosphere of dry nitrogen using solvents dried using the
PuraSolv solvent purifer system. The compounds
[Rh(COD)2][BArF],27 [Rh(COD)Cl]2,28 [Rh(NBD)2][BArF],27
[Rh(bim)(CO)2][BArF],10 5-hexynoic acid,12 o-tpt,9 p-tpt9 and
tim29 were prepared using literature methods. All organic
compounds were purchased from Sigma Aldrich and used as
received. Iridium(III) and rhodium(III) chloride hydrate were
purchased from Precious Metals Online and used as received.
X-ray diffraction studies were performed on a Bruker
APEXII diffractometer. All calculations were made with
programmes of the SHELXTL system.30 NMR spectra were
recorded on Bruker Avance 400 or 600 MHz spectrometer
using the residual solvent peak as reference. Microanalyses
and high resolution mass spectrometry were carried out at the
Mass Spectrometry service of the Research School of Chemistry,
Australian National University, Canberra, Australia.
[Rh(o-tpt)(COD)][BArF] (2b). o-tris(pyrazol-1-yl)toluidine
(1b) (27.8 mg, 0.10 mmol), [Rh(COD)2][BArF] (102.9 mg,
0.09 mmol) to yield 2b (72.5 mg, 58%) as yellow crystals
(Found: C, 48.6; H, 3.0; N, 6.5. C56H39BF24N7Rh requires C,
48.8; H, 2.6; N, 7.1%); dH (600 MHz; CDCl3; ꢀ55 1C) 7.86
(br, 1H, H5P3), 7.68 (br, 1H, H5P2), 7.72 (br, 1H, H5P1), 7.67
(s, 8H, H3BArF), 7.57 (br, 1H, H3P2), 7.52 (br, 1H, H3P1), 7.48
(s, 4H, H1BArF), 7.38 (t, J = 8.2 Hz, 1H, H40), 6.73 (t, J = 8.2 Hz,
1H, H30), 6.72 (br, 1H, H50), 6.69 (br, 1H, H4P3), 6.52 (br, 1H,
H4P2), 6.40 (br, 1H, H4P1), 5.52 (d, J = 8.2 Hz, 1H, H20), 4.31
(br, 1H, H1COD), 4.26 (br, 1H, H1COD), 3.90 (br, 1H, H1COD),
3.73 (br, 1H, H1COD), 2.66–2.51 (m, 1H, H2COD), 2.50–2.40
(m, 1H, H2COD), 2.08–1.67 (m, 1H, H2COD), 1.96–1.77 (m, 4H,
H2COD), 1.77–1.62 (m, 1H, H2COD). dC (150 MHz; CDCl3;
ꢀ50 1C) 161.2 (q, J = 49.3 Hz, C4BArF), 144.9 (C3P2), 144.8
(C3P1), 143.4 (C10), 142.6 (C5P3), 137.4 (C5P2), 136.5 (C5P1),
134.3 (C40), 134.0 (C3BArF), 129.3 (C3P3), 128.3 (q, J = 31 Hz,
C2BArF), 127.9 (C20), 124.0 (q, J = 271 Hz, CF3), 119.6 (C30),
c
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New J. Chem., 2011, 35, 1730–1739 1737

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117.4 (C50), 117.1 (C3BArF), 114.6 (C10), 109.3 (C4P3), 108.9
(C4P1), 108.4 (C4P2), 92.1 (Ca), 86.3 (C1COD), 83.8 (C1COD),
82.8 (C1COD), 30.4 (C2COD), 29.9 (C2COD), 29.3 (C2COD), 28.8
(C2COD).
133.8 (C3P2), 128.3 (C2BArF), 124 (CF3), 117.2 (C3BArF), 114.7
(C30), 108.4 (C4P2), 107.9 (C4P1), 93.0 (Ca); m/z (ES+)
464.0350 (M+. C18H15N7O2Rh requires 464.0432).
n
max/cmꢀ1 (KBr) 3492(m), 3399(m), 3161(m), 3141(m),
2107(vs, nCO), 2046(vs, nCO), 1630(s), 1608(s), 1519(s).
[Rh(p-tpt)(NBD)][BArF] (3a). p-tris(pyrazol-1-yl)toluidine
(1a) (44.0 mg, 0.10 mmol), [Rh(NBD)2][BArF] (101.6 mg,
0.09 mmol) to yield 3a (119 mg, 98%) as yellow crystals
(Found: C, 48.7; H, 2.7; N, 7.0. C55H35BF24N7Rh requires
C, 48.5; H, 2.6; N, 7.2.); dH (600 MHz; CDCl3; ꢀ55 1C) 7.99
(br, 1H, H5P(2)), 7.69 (s, 8H, H2BArF), 7.49 (s, 4H, H4BArF),
7.17 (br, 2H, H5P(1)), 7.14 (br, 2H, H3P(1)), 7.09 (br, 1H,
H3P(2)), 6.69(d, J = 8.2 Hz, 2H, H30), 6.38 (br, 1H, H4P(2)),
6.32 (br, 1H, H4P(1)), 5.89 (d, J = 8.2 Hz, 2H, H20), 4.03
(br, 2H, H1NBD), 3.89 (br, 1H, H2NBD), 3.51 (br, 2H, H1NBD),
3.45 (br, 1H, H2NBD), 1.17 (br, 2H, H3NBD). dC (150 MHz;
CDCl3; ꢀ50 1C) 161.2 (q, JBC = 49 Hz, C1BArF), 149.9 (C40),
145.0 (C5P(2)), 142.1 (C3P(1)), 134.4 (C5P(1)), 134.3 (C2BArF),
133.5 (C3P(2)), 128.5 (q, J = 32 Hz, C3BArF), 127.6 (C20), 123.9
(q, JCF = 271 Hz, CF3), 123.4 (C10), 117.2 (C4BArF), 114.1
(C30), 108.1 (C4P(2)), 107.2 (C4P(1)), 93.2 (Ca), 60.9 (C1NBD),
59.3 (C1NBD), 50.9 (C2NBD), 50.4 (C2NBD), 31.3 (C3NBD).
[Rh(o-tpt)(CO)2][BArF] (4b). [Rh(COD)(o-tpt)][BArF] (2b)
(171 mg, 0.12 mmol) to yield 4b (131 mg, 80%) as yellow
crystals. (Found: C, 45.0; H, 2.2; N, 6.9. C50H27BF24N7Rh
requires C, 45.2; H, 2.1; N, 7.4%) dH (400 MHz; CDCl3;
ꢀ55 1C) 7.92 (d, J = 1.9 Hz, 1H, H3P(1)), 7.86 (d, J = 3.0 Hz,
1H, H5P(1)), 7.83 (d, J = 1.9 Hz, 1H, H3P(2)), 7.81 (d, J = 3.0
Hz, 1H, H5P(2)), 7.70 (d, J = 3.0 Hz, 1H, H5P(3)), 7.67 (br, 4H,
H1BArF), 7.49 (br, 8H, H3BArF), 7.40 (t, J = 8.2 Hz, 1H, H40),
6.73 (t, J = 8.2 Hz, 1H, H30), 6.63 (d, J = 2.5 Hz, 1H, H50),
6.63 (t, J = 2.5 Hz, 1H, H4P(1)), 6.50 (t, J = 2.5 Hz, 1H,
H4P(2)), 6.47 (t, J = 2.5 Hz, 1H, H4P(3)), 6.44 (d, J = 2.5 Hz,
1H, H3P(3)), 5.45 (d, J = 8.2 Hz, 1H, H20). dC (100 MHz;
CDCl3; ꢀ55 1C) 181.8 (CO), 181.11 (CO), 161.7 (q, J1
=
BC
49 Hz, C4BArF), 150.3 (C3P1), 150.2 (C3P2), 143.7 (C5P3), 143.4
(C60), 138.8 (C5P1), 137.9 (C5P2), 135.3 (C40), 134.7 (C1BArF),
129.3 (C3P3), 128.9–128.3 (m, C2BArF), 124.5 (q, J1CF = 273 Hz,
CF3), 120.2 (C30), 118.0 (C50), 116.4 (C3BArF), 113.5 (C10),
110.3 (C4P3), 109.9 (C4P1), 109.4 (C4P2).nmax/cmꢀ1 (KBr)
3504(m), 3415(m), 3165(m), 3145(m), 2104(vs, nCO), 2046(vs,
nCO), 2015(m, nCO), 1632(s), 1609(s), 1575(m) and 1522(m).
[Rh(o-tpt)(NBD)][BArF] (3b). o-tris(pyrazol-1-yl)toluidine
(1b) (43.5 mg, 0.10 mmol), [Rh(NBD)2][BArF] (109 mg, 0.09
mmol) to yield 3b (111 mg, 91%) as yellow crystals (Found: C,
48.6; H, 2.4; N, 7.2. C55H35BF24N7Rh requires C, 48.5; H, 2.6;
N, 7.2%.); dH (400 MHz; CDCl3; ꢀ55 1C) 7.75 (br, 1H,
H5P(3)), 7.74 (br, 1H, H5P(2)), 7.65 (br, 1H, H5P(3)), 7.69
(s, 8H, H2BArF), 7.49 (s, 4H, H4BArF), 7.36 (t, J = 8.2 Hz,
1H, H40), 7.14 (br, 2H, H3P(3) & H3P(2)), 6.81 (d, J = 8.2 Hz,
1H, H50), 6.70 (t, J = 8.2 Hz, 1H, H30), 6.55 (br, 1H, H4P(1)),
6.40 (br, 1H, H4P(2)), 6.39 (1H, H4P(1)), 6.20 (br, 1H, H3P(3)),
5.55 (d, J = 8.2 Hz, 1H, H20), 4.00 (s, 4H, H1NBD), 3.81 (s, 2H,
H2NBD), 1.34 (s, 2H, H3NBD). dC (100 MHz; CDCl3; ꢀ55 1C)
161.9 (q, JBC = 74 Hz, C1BArF), 145.4 (C3P(3) & C3P(2)), 143.9
(C60), 142.9 (C5P(3)), 138.3 (C5P(2)), 137.2 (C5P(1)), 134.8
(C2BArF), 134.6 (C40), 128.7 (m, C3BArF), 128.9 (C3P(3)),
128.3 (C20), 124.5 (q, JCF = 271 Hz, CF3), 119.6 (C30),
117.9 (C50), 117.7 (C4BArF), 114.6 (C10), 109.3 (C4P(1)), 109.2
(C4P(3)), 108.5 (C4P(2)), 92.7 (Ca), 63.3 (C3NBD), 61.0 (C1NBD),
51.1 (C2NBD).
[Rh(tim)(COD)][BArF] (5). A solution of tris(N-methyl-
imidazol-2-yl)methanol (26.8 mg, 0.1 mmol) and [Rh(COD)2]-
[BArF] (101 mg, 0.09 mmol) in THF (5 mL) was stirred at
room temperature for 20 to 30 min. The resulting pale yellow
solution was reduced in vacuo and recrystallised from a
mixture of dichloromethane and n-pentane to yield the 5
(81 mg, 70%) as a pale yellow solid. (Found: C, 47.2; H, 3.1;
N, 6.4%. C53H40BF24N6ORh requires C, 47.3; H, 3.00; N,
6.2). dH (400 MHz; MeOD) d 7.61 (12H, H2BArF and H4BArF),
7.25 (br, 3H, H2), 7.05 (br, 3H, H3), 4.10 (br, 4H, H1COD),
3.76 (br, 9H, N-CH3), 2.38–2.21 (m, 4H, H2COD), 1.95–1.76
(m, 4H, H2COD); dC (100 MHz; MeOD) 161.5 (q, J = 49 Hz,
C1BArF), 145.2 (C1), 134.4 (C2BArF), 129.7–128.5 (m, C3BArF),
126.0 (C3), 124.9 (C2), 124.4 (q, J = 271 Hz, CF3),
117.6–116.6 (m, C4BArF), 82.4 (d, J = 13.3 Hz, C1COD), 35.0
(N-CH3), 29.9 (C2COD).
General synthesis of rhodium dicarbonyl tris(pyrazol-1-yl)-
aniline complexes (4a) and (4b)
[Rh(tim)(CO)2][BArF] (6). A solution of 6 (59.3 mg, 0.04 mmol)
in dichloromethane (2 mL) was degassed using three consecutive
cycles of freeze-pump-thaw was stirred under a carbon monoxide
atmosphere for 30 to 60 min. To the solution, n-pentane was
added and the resulting precipitate filtered and washed with
n-pentane to yield the compound 6 (32.5 mg, 57%) as a bright
yellow solid. dH (400 MHz; MeOD) 7.70–7.57 (m, 12H, H4BArF
& H2BArF), 7.29 (d, J = 1.5 Hz, 3H, H2), 7.23 (d, J = 1.5 Hz, 3H,
H3), 3.75 (s, 9H, N-CH3); dC (100 MHz; MeOD) d 183.9 (CO),
183.2 (CO), 161.4 (q, JBC=50 Hz, C1BArF), 144.8 (C2),
134.4 (C2BArF), 129.5–128.5 (m, C3BArF), 128.8 (C3), 125.5 (C2),
124.4 (q, J = 271 Hz, CF3), 117.3–116.9 (C2BArF), 73.9 (Ca), 34.7
(N-CH3). m/z (ESI+) 1295.1077 (M+H+. C47H29B11N6O3F24Rh
requires 1295.1066).nmax/cmꢀ1 (KBr) 3319(br), 3166(w), 3139(w),
2094(vs, nCO), 2032(vs, nCO), 1610(m), 1555(m), 1506(m),
1609(s).
A solution of the rhodium COD complex in dichloromethane
was degassed using three consecutive cycles of freeze-pump-thaw
and stirred under a carbon monoxide atmosphere for 30 to
60 min. To this solution, n-pentane was added and the
resulting precipitate filtered and washed with n-pentane.
[Rh(p-tpt)(CO)2][BArF] (4a). [Rh(COD)(p-tpt)][BArF] (2a)
(185 mg, 0.14 mmol) to yield 4a (125 mg, 71%) as yellow
crystals. dH (600 MHz; CDCl3; ꢀ55 1C) 7.97 (br, 1H, H5P2),
7.77 (br, 2H, H5P1), 7.67 (s, 8H, H3BArF), 7.48 (s, 4H, H1BArF),
7.31 (br, 2H, H3P1), 6.97 (br, 1H, H3P2), 6.57 (d, J = 7.2 Hz,
2H, H30), 6.44 (br, 2H, H4P1), 6.35 (br, 1H, H4P2), 6.00
(d, J = 7.2 Hz, 2H, H20); dC (150 MHz; CDCl3; ꢀ55 1C)
181.7 (CO), 181.2 (CO), 161.2 (q, J = 49 Hz, C4BArF), 150.4
(C10), 146.7 (C5P1), 145.4 (C5P2), 135.6 (C3P1), 134.3 (C1BArF),
c
1738 New J. Chem., 2011, 35, 1730–1739
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Generalised procedure for metal catalysed intramolecular
cyclisation of alkynoic acids. Catalysed intramolecular cyclisation
of alkynoic acids was conducted on a small scale in NMR tubes
fitted with a concentric Teflon valve. The catalyst precursor, the
substrate and, if required, 2,6-dimethoxytoluene as an internal
standard were dissolved in deuterated solvent. The temperature
in the NMR spectrometer was calibrated using neat ethylene
glycol and a K-type thermocouple. The identity of the product
was confirmed by comparison with literature and/or identified
using standard 2D NMR spectroscopic techniques. Unless other-
wise stated in the results and discussion, the values quoted are the
result of a single catalysis experiment.
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1
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Acknowledgements
We would like to thank the University of New South Wales,
the Australian Research Council, the Australian Government
(APA) and Baxter Family Trust for funding. We would also
like to acknowledge Dr Vicki Tolhurst and Dr Danielle
Kennedy for their contributions.
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