Synthesis and characterization of rhodium(I) 2-methylcupferrate complexes and their kinetic behaviour in iodomethane oxidative addition

Article abstract of DOI:10.1016/j.jorganchem.2012.12.005

A number of substituted monocarbonyl complexes [Rh(MeCupf)(CO)(PR ₃)] (where R = Ph, p-MeOph, p-Tol, o-Tol and Cy, MeCupf = N-nitroso-N-(2-methylphenyl)hydroxylaminato bidentate ligand) have been prepared from [Rh(MeCupf)(CO)₂] and identified by IR, UV/visible and NMR techniques. The kinetics and mechanism for the oxidative addition of iodomethane to [Rh(MeCupf)(CO)(PR₃)] complexes have been investigated through the variation of steric and electronic effects of the phosphine ligands, as well as through varying the solvent and temperature. The Rh^III-alkyl complex was the primary species formed as a result of oxidative addition, followed by a consecutive slower formation of the Rh^III-acyl species by carbonyl-insertion. A detailed mechanism is discussed.

Full text of DOI:10.1016/j.jorganchem.2012.12.005

Journal of Organometallic Chemistry 726 (2013) 14e20
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and characterization of rhodium(I) 2-methylcupferrate
complexes and their kinetic behaviour in iodomethane oxidative
addition
*
Stefan Warsink, Fanuel G. Fessha, Walter Purcell, Johan A. Venter
Department of Chemistry, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 November 2012
Accepted 4 December 2012
A number of substituted monocarbonyl complexes [Rh(MeCupf)(CO)(PR3)] (where R ¼ Ph, p-MeOph, p-
Tol, o-Tol and Cy, MeCupf ¼ N-nitroso-N-(2-methylphenyl)hydroxylaminato bidentate ligand) have been
prepared from [Rh(MeCupf)(CO)2] and identified by IR, UV/visible and NMR techniques. The kinetics and
mechanism for the oxidative addition of iodomethane to [Rh(MeCupf)(CO)(PR3)] complexes have been
investigated through the variation of steric and electronic effects of the phosphine ligands, as well as
through varying the solvent and temperature. The RhIIIealkyl complex was the primary species formed
as a result of oxidative addition, followed by a consecutive slower formation of the RhIIIeacyl species by
carbonyl-insertion. A detailed mechanism is discussed.
Keywords:
Oxidative addition
Rhodium
Cupferrate
Iodomethane
Kinetics
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
paper describes the synthesis and characterisation of a series of
rhodium complexes containing a methyl substituted cupferrate
The synthesis of rhodium organometallic complexes in our
laboratory is covered in a large number of articles [1e4]. A signifi-
cant number of these studies involve the synthesis of [Rh(LL0-
BID)(CO)(PPh3)] (where LL0-BID ¼ monoanionic bidentate ligands
containing different donor atoms L and L0) and other relevant
complexes. These complexes were synthesised to study the elec-
tronic and steric influence of the different bidentate (LL0) and
monodentate ligands on the rate of substitution and oxidative
addition reactions, as well as to evaluate the complexes as possible
catalysts in organic synthesis. One of these complexes, [Rh(Cupf)(-
CO)(PPh3)] (Cupf ¼ N-nitroso-N-phenylhydroxylaminato bidentate
ligand) indicated that it has the tendency to form a very stable 5-
coordinated complex as was illustrated by the characterisation of
[Rh(Cupf)(CO)(PPh3)2] [2]. The oxidative addition reaction between
[Rh(Cupf)(CO)(PPh3)] and iodomethane [1c] also clearly indicated
the catalytic participation of a five-coordinated solvent-stabilized
complex, [Rh(Cupf)(CO)(PPh3)(S)]. Another interesting result from
this study was the formation of a cis-[Rh(Cupf)(CO)(PPh3)(CH3)(I)]
oxidative addition complex compared to the final trans product for
the H3CeRheI moiety for a large number of those complexes such
as [Rh(ox)(CO)(PPh3)(CH3)(I)] [3] and [Rh(dmavk)(CO)(PPh3)(-
CH3)(I)] (ox ¼ oxine, dmavk ¼ dimethylaminovinylketone) [4]. This
derivative, MeCupf, as bidentate ligand. It also describes the
oxidative addition of iodomethane to these complexes and the
analysis of its kinetics.
2. Experimental
2.1. General procedures and instrumentation
Rhodium(III) chloride (Aldrich), 2-nitrotoluene (Merck), n-butyl
nitrite (Merck), triphenylphosphine (Merck) and iodomethane
(Merck) were used as purchased. Organic solvents used were
distilled prior to use and water was double distilled.
Infrared spectra of the solids were collected in the range of
2300e550 cmꢀ1 using a Digilab Merlin 3.0 spectrophotometer.
NMR spectra were obtained with Bruker 300 and 600 MHz spec-
trophotometers. 1H NMR data are listed in the order: chemical shift
(d
, reported in ppm and referenced to the residual solvent peak of
CD2Cl2
¼ 5.32 ppm]), multiplicity, coupling constant (J, in Hz),
number of hydrogens, assignment. Hydrogen decoupled 13C NMR
data are listed in the order: chemical shift ( , reported in ppm and
[
d
d
referenced to the residual solvent peak of CD2Cl2), multiplicity,
coupling constant (J, in Hz), assignment. Hydrogen and carbon
decoupled 31P NMR data are listed in the order: chemical shift (
d,
reported in ppm and referenced to triphenylphosphine oxide
¼ 29.9 ppm]), multiplicity, coupling constant (J, in Hz). Positive
* Corresponding author. Tel.: þ27 514013336; fax: þ27 514446384.
E-mail address: venterja@ufs.ac.za (J.A. Venter).
[d
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.12.005

S. Warsink et al. / Journal of Organometallic Chemistry 726 (2013) 14e20
15
1
chemical shifts (
d
) are denoted for high-frequency shifts relative to
¼ 174.2 Hz, minor isomer), 47.33 (d, JRheP ¼ 170.7 Hz, major
P
the solvent (13C) or H3PO4 capillary (31P).
isomer). IR nmax/cmꢀ1
: 1982 (CO). Analysis calculated for
All kinetic measurements were carried out in air and all solvents
were pre-dried over aluminium oxide and distilled. UV/visible
spectra were collected on a Varian Cary 50 double-beam spectro-
photometer equipped with temperature cell regulator (accurate
within 0.1 ꢁC) in a 1.000 ꢂ 0.001 cm quartz cuvette. Infrared spectra
of liquids were recorded in dry organic solvents (toluene or
dichloromethane) in a NaCl cell in the range 2100e1600 cmꢀ1 using
a Digilab Merlin 3.0 Spectrophotometer equipped with a tempera-
ture cell regulator (accurate within 0.3 ꢁC). Iodomethane is highly
volatile and the solutions were prepared in a fume hood and used
immediately after preparation. The Cary 50 double-beam spectro-
photometer was initially used to verify the stability of the
[Rh(MeCupf)(CO)(PR3)] complexes in different solvents. Suitable
wavelengths were consequently selected to study the reaction
between [Rh(MeCupf)(CO)(PR3)] complexes and iodomethane in
the different solvents used. Typical complex concentrations were
1.7 ꢃ 10ꢀ4 M for UV/visible and 0.02 M for IR kinetic measurements.
Iodomethane concentrations were varied between 0.2 and 1 M thus
ensuring good pseudo-first-order plots of ln(At ꢀ AN) vs time where
At and AN are the absorbencies at time t and infinity, respectively.
The mathematical calculations for the kinetic investigations were
performed using the Scientist programme [5]. Some of the reac-
tions were performed using both the IR and UV/visible spectro-
photometer to correlate the different reactions with all the physical
changes during the reactions.
C26H22N2O3PRh (544.34): C, 57.37; H, 4.07; N, 5.15. Found: C,
57.83; H, 4.39; N, 5.26.
2.3.2. Carbonyl tri-(4-tolyl)-phosphine (O,O0-(N-nitroso-N-oxido-
(2-tolyl)-amine))rhodium(I), [Rh(MeCupf)(CO)(P(p-Tol)3)]
The product was obtained as a yellow solid in a yield of 78%. 1H
NMR (300 MHz, CD2Cl2, 25 ꢁC):
d 7.6e7.45 (m, 6H, aryl-H), 7.45e7.3
(m, 4H, Cupf-H), 7.3e7.2 (m, 6H, aryl-H), 2.43 (s, 3H, aryl-CH3,
minor isomer), 2.41 (s, 3H, aryl-CH3, major isomer), 2.37 (s, 3H,
Cupf-CH3, minor isomer), 2.21 (s, 3H, Cupf-CH3, major isomer). 13
C
{1H} NMR major isomer (151 MHz, CD2Cl2, 25 ꢁC): 190.5 (dd, 1JRhe
d
¼ 77.0 Hz, 2JPeC ¼ 25.3 Hz, CO), 141.0 (s, p-aryl-C), 138.8 (s, 1-Cupf-
C
C), 133.9 (d, 2JPeC ¼ 11.9 Hz, o-aryl-CH), 133.1 (s, 2-Cupf-C), 131.7 (d,
1JPeC ¼ 4.0 Hz, i-aryl-C), 131.6 (s, 6-Cupf-CH), 130.1 (s, 4-Cupf-CH),
3
129.0 (d, JPeC ¼ 11.0 Hz, m-aryl-CH), 126.3 (5-Cupf-CH), 124.6 (3-
Cupf-CH), 21.1 (s, aryl-CH3), 18.4 (s, Cupf-CH3). 31P{1H, 13C} NMR
1
(121 MHz, CD2Cl2, 25 ꢁC):
d
46.22 (d, JRheP ¼ 173.2 Hz, minor
isomer), 44.93 (d, 1JRheP ¼ 169.4 Hz, major isomer). IR nmax/cmꢀ1
:
1969 (CO). Analysis calculated for C29H28N2O3PRh (586.42): C,
59.29; H, 4.98; N, 4.77. Found: C, 59.14; H, 5.36; N, 5.16.
2.3.3. Carbonyl tri-(4-methoxyphenyl)-phosphine (O,O0-(N-nitroso-
N-oxido-(2-tolyl)-amine))rhodium(I), [Rh(MeCupf)(CO)(P(p-
MeOPh)3)]
The product was obtained as a yellow solid in a yield of 93%. 1H
NMR (300 MHz, CD2Cl2, 25 ꢁC):
d 7.65e7.45 (m, 6H, aryl-H), 7.40e
2.2. Synthesis of biscarbonyl (O,O0-(N-nitroso-N-oxido-(2-tolyl)-
amine))rhodium(I)
7.25 (m, 4H, Cupf-H), 7.05e6.90 (m, 6H, aryl-H), 3.88 (s, 9H,
OCH3, minor isomer), 3.85 (s, 9H, OCH3, major isomer), 2.41 (s, 3H,
Cupf-CH3, minor isomer), 2.22 (s, 3H, Cupf-CH3, major isomer). 13
C
RhCl3$xH2O (300 mg, 1.12 mmol) was heated in DMF (25 ml) at
180 ꢁC to produce tetracarbonyl-dichloro-dirhodium(I) [6]. 2-
Methylcupferrate (190 mg, 1.26 mmol) was added slowly to the
cooled solution, after which ice water was added dropwise to
precipitate the complex. The product was obtained after filtration
and drying in vacuo as a brick-red solid in a yield of 84%. 1H NMR
{1H} NMR major isomer (151 MHz, CD2Cl2, 25 ꢁC): 190.6 (dd, 1JRhe
d
¼ 77.5 Hz, 2JPeC ¼ 25.3 Hz, CO), 161.5 (s, p-aryl-C), 138.8 (s, 1-Cupf-
C
C), 135.5 (d, 2JPeC ¼ 13.3 Hz, o-aryl-CH), 133.1 (s, 2-Cupf-C), 131.6 (d,
1JPeC ¼ 3.6 Hz, i-aryl-C), 130.1 (s, 6-Cupf-CH), 126.3 (s, 4-Cupf-CH),
3
124.6, 124.1 (2 s, 3-Cupf-CH, 5-Cupf-CH), 113.8 (d, JPeC ¼ 13.3 Hz,
m-aryl-CH), 55.3 (s, OCH3), 18.6 (s, Cupf-CH3). 31P{1H, 13C} NMR
1
(300 MHz, CD2Cl2, 25 ꢁC):
d
7.54e7.48 (2H, m, Cupf), 7.42e7.36 (m,
(121 MHz, CD2Cl2, 25 ꢁC):
d
44.00 (d, JRheP ¼ 172.9 Hz, minor
2H, Cupf), 2.37 (3H, s, CH3). 13C{1H} NMR (75 MHz, CD2Cl2, 25 ꢁC):
isomer), 42.84 (d, 1JRheP ¼ 169.0 Hz, major isomer). IR nmax/cmꢀ1
:
d
184.6 (d, 1JRheC ¼ 74.7 Hz, CO),184.4 (d, 1JRheC ¼ 73.1 Hz, CO),137.8
1965 (CO). Analysis calculated for C29H28N2O6PRh (634.42): C,
54.81; H, 4.60; N, 4.41. Found: C, 55.35; H, 4.75; N, 4.49.
(1-Cupf-C), 133.6 (2-Cupf-C), 131.8, 131.3, 126.8, 124.9 (4Cupf-CH),
18.0 (CH3). IR nmax/cmꢀ1: 2063, 2087 (CO). Analysis calculated for
C9H7N2O4Rh (310.07): C, 34.86; H, 2.28; N, 9.03. Found: C, 34.66; H,
2.73; N, 9.27.
2.3.4. Carbonyl tri-(2-tolyl)-phosphine(O,O0-(N-nitroso-N-oxido-
(2-tolyl)-amine))rhodium(I), [Rh(MeCupf)(CO)(P(o-Tol)3)]
The product was obtained as a yellow solid in a yield of 84%. 1H
2.3. Synthesis of [Rh(MeCupf)(CO)(PR3)] complexes
NMR (300 MHz, CD2Cl2, 25 ꢁC):
d 7.9e7.8 (m, 3H, 6-tolyl-H), 7.45e
7.35 (m, 4H, Cupf-H, tolyl-H), 7.35e7.2 (m, 9H, Cupf-H, tolyl-H), 2.4
(s, 9H, tolyl-CH3), 2.13 (s, 3H, CH3, minor isomer), 1.95 (s, 3H, CH3,
major isomer). 13C{1H} NMR major isomer (151 MHz, CD2Cl2, 25 ꢁC):
[Rh(MeCupf)(CO)2] (0.252 g, 0.814 mmol) was dissolved in 5 ml
acetone. The phosphine ligand (0.235 g, 0.895 mmol) was added
slowly to the solution. Ice water was then added dropwise to the
solution to precipitate the complex. The solid was then filtered and
dried at room temperature.
1
2
2
d
190.1 (dd, JRheC ¼ 79.1 Hz, JPeC ¼ 23.4 Hz, CO), 142.8 (d, JPe
¼ 8.7 Hz, 6-aryl-CH), 138.6 (s, 1-Cupf-C), 135.0 (br s, 2-aryl-C),
C
1
133.1 (s, 2-Cupf-C), 131.9 (d, JPeC ¼ 8.1 Hz, 1-aryl-C), 131.6 (s, 6-
Cupf-CH), 130.8 (s, 5-aryl-CH), 130.0 (s, 4-Cupf-CH), 129.1 (s,
3-aryl-CH), 126.3 (s, 5-Cupf-CH), 125.7 (s, 4-aryl-CH), 124.4 (s, 3-
2.3.1. Carbonyl triphenylphosphine (O,O0-(N-nitroso-N-oxido-
(2-tolyl)-amine))rhodium(I), [Rh(MeCupf)(CO)(PPh3)]
Cupf-CH), 23.3 (d, JPeC ¼ 6.6 Hz, aryl-CH3), 18.2 (s, Cupf-CH3). 31P
3
The product was obtained as a yellow solid in a yield of 94%. 1H
{1H, 13C} NMR (121 MHz, CD2Cl2, 25 ꢁC):
d
43.43 (d, JRhe
1
1
NMR (300 MHz, CD2Cl2, 25 ꢁC):
d
7.75e7.25 (m, 19H, Ph, Cupf), 2.41
¼ 173.7 Hz, minor isomer), 41.14 (d, JRheP ¼ 171.9 Hz, major
P
(s, 3H, CH3, minor isomer), 2.19 (s, 3H, CH3, major isomer). 13C{1H}
isomer). IR nmax/cmꢀ1
: 1970 (CO). Analysis calculated for
1
NMR major isomer (151 MHz, CD2Cl2, 25 ꢁC):
d
190.4 (dd, JRhe
C29H28N2O3PRh (586.42): C, 59.29; H, 4.98; N, 4.77. Found: C, 55.43;
H, 4.91; N, 4.39.
2
¼ 76.8 Hz, JPeC ¼ 25.2 Hz, CO), 138.7 (s, 1-Cupf-C), 134.1 (d,
C
2JPeC ¼ 11.9 Hz, o-Ph-CH), 133.1 (s, 2-Cupf-C), 131.9 (d, JPe
1
4
¼ 9.8 Hz, i-Ph-C), 131.6 (s, 6-Cupf-CH), 130.6 (d, JPeC ¼ 2.2 Hz,
2.3.5. Carbonyl tricyclohexylphosphine (O,O0-(N-nitroso-N-oxido-
(2-tolyl)-amine))rhodium(I), [Rh(MeCupf)(CO)(PCy)3]
C
3
p-Ph-CH), 130.2 (s, 4-Cupf-CH), 128.3 (d, JPeC ¼ 7.6 Hz, m-Ph-
CH), 126.4, 124.6 (2s, 3-Cupf-CH, 5-Cupf-CH), 18.4 (s, Cupf-CH3).
The product was obtained as a pink solid in a yield of 82%. 1H
31P{1H, 13C} NMR (121 MHz, CD2Cl2, 25 ꢁC):
d
48.25 (d, JRhe
NMR (300 MHz, CD2Cl2, 25 ꢁC):
d 7.60e7.50 (m, 2H, Cupf-H), 7.50e
1

16
S. Warsink et al. / Journal of Organometallic Chemistry 726 (2013) 14e20
7.30 (m, 2H, Cupf-H), 2.39 (s, 3H, CH3, minor isomer), 2.34 (s, 3H,
CH3, major isomer), 2.10e2.00 (m, 3H, 1-Cy-H), 1.90e1.45 (m, 12H,
2-Cy-H, 6-Cy-H), 1.40e1.10 (m, 18H, 3-Cy-H, 4-Cy-H, 5-Cy-H). 13C
{1H} NMR major isomer (151 MHz, CD2Cl2, 25 ꢁC):
d
191.5 (dd, 1JRhe
2
¼ 77.7 Hz, JPeC ¼ 23.2 Hz, CO), 139.2 (s, 1-Cupf-C), 133.2 (s, 2-
C
Cupf-C), 131.5 (s, 6-Cupf-CH), 130.1 (s, 4-Cupf-CH), 126.4 (5-
Scheme 2. Ligand substitution to introduce different phosphine ligands. R ¼ Ph (2), 4-
MePh (3), 4-OMePh (4), 2-MePh (5), Cy (6).
1
Cupf-CH), 124.8 (3-Cupf-CH), 34.8 (d, JPeC ¼ 25.7 Hz, 1-Cy-CH),
2
30.0 (s, 3-Cy-CH2, 5-Cy-CH2), 27.5 (d, JPeC ¼ 4.0 Hz, 2-Cy-CH2, 6-
Cy-CH2), 26.4 (s, 4-Cy-CH2), 18.6 (s, Cupf-CH3). 31P{1H, 13C} NMR
1
(121 MHz, CD2Cl2, 25 ꢁC):
d
62.43 (d, JRheP ¼ 164.0 Hz, minor
isomer), 61.82 (d, 1JRheP ¼ 161.6 Hz, major isomer). IR nmax/cmꢀ1
:
major isomer, as its bond to the rhodium is labilized most, although
the reverse has also been observed for electron-withdrawing
phosphite ligands which are sterically less demanding [9].
However, this does not need to be an electronic effect, as the
crystallization of the minor isomer is a realistic option. In the case
of the unsubstituted cupferrate ligand, the nitroso-oxygen has the
highest trans-influence; the methyl-substituent on the phenyl-ring
is not expected to change this to a significant degree.
1956 (CO). Analysis calculated for C26H40N2O3PRh (562.49): C,
55.52; H, 7.17; N, 4.98. Found: C, 55.91; H, 7.34; N, 4.62.
3. Results and discussion
3.1. Synthesis of rhodium(I) complexes
The pKa values of the free phosphine ligands are frequently used
as an index of basicity and are proportional to the electron donating
ability of the phosphine. According to Table 1, pKa values for the
different phosphine ligands increase from PPh3 to PCy3 which
indicates that there is an increase in electron donating capability
(strong basicity) of the substituent on the tertiary phosphine from
PPh3 to PCy3.
The corresponding values of the CO vibrations from the IR
spectra are also given in Table 1. It can be seen that the value
observed for the carbonyl ligand in the IR decreases with
increasing phosphine pKa. This can be explained by the electron-
donating properties of the phosphine ligands, which correlates
with the pKa value. The higher this value, the more nucleophilic
the phosphorus centre, which increases the electron density on
the rhodium upon coordination. This in turn reduces the bond
Using 2-methyl-substituted cupferron as ligand [7], the biscar-
bonyl rhodium(I) complex 1 was synthesized using the well-known
dinuclear precursor [Rh-m-Cl(CO)2]2 (Scheme 1) [6].
This complex was isolated in a yield of 84% and has character-
istic signals in the IR spectrum for the two carbonyl ligands at 2063
and 2087 cmꢀ1. This complex was then reacted with differently
substituted phosphines to generate sterically and electronically
varying monocarbonyl complexes in good to excellent yields
(Scheme 2) [8].
From 1H, 13C and 31P NMR it appeared that the substitution
reaction shown in Scheme 2 did not proceed with a high selectivity
as two structural isomers were routinely obtained in an approxi-
mate ratio of 1:2, irrespective of the phosphine ligand used. This
shows that the difference in trans-influence of the donor atoms of
the cupferrate ligand is not significant enough as to achieve
selective substitution of a specific carbonyl ligand. When the results
for this substituted ligand were compared with those for the parent
ligand, it appeared that no NMR data were reported. Therefore, the
parent [Rh(Cupf)(CO)(PPh3)] complex was synthesized according to
the same procedure as for complexes 2e6. It appeared that under
these conditions two isomers were also formed. From this it can be
concluded that this behaviour is typical of the rhodium cupferrate
complexes investigated, and results pertaining to the trans-influ-
ence of the two oxygen donors drawn solely from crystallographic
studies should be critically evaluated. All efforts to separate the two
complexes did not yield a single product in any significant yield.
Based on the small electronic difference between the two isomers,
very similar behaviour (to the point of equal) is expected in kinetic
experiments.
Characteristic for the complexes 2e6 is the occurrence of only
one carbonyl stretching signal in the IR spectra, although two
signals would be expected. Apparently, the resolution of the spectra
is not so high as to show the small difference in electronic density
imparted on the carbonyl ligand by the two different oxygen
donors. Furthermore, the occurrence of a single carbonyl signal
indicates that only one of the carbonyl ligands was substituted by
a phosphine ligand. The carbonyl trans to the donating moiety with
the largest trans-influence is expected to be substituted in the
order in the CO ligand through an increase in
p-backbonding,
leading to a lower wavenumber for its vibration in the infrared.
However, this trend does not hold for complex 5, bearing the
tris(o-tolyl)phosphine ligand. The pKa value would suggest that
a
nCO lying between that of complex 2 and 3 should be observed.
Instead, the observed value is almost identical to that of complex
4, bearing the much more nucleophilic tris(p-anisyl)phosphine.
The discrepancy arises from the fact that not only electronic
factors are important in this ligand series. For the phosphine
ligands in complex 5 and 6, the steric component is substantially
different than for those in the complexes 2e4. This does not
change the nucleophilicity of the ligand, but it does influence the
coordination environment around the rhodium centre to
a significant degree. The influence of the cupferrate substituent
can also be gauged by comparison with literature values for
rhodium(I) complexes bearing the unsubstituted cupferrate [1c].
This comparison shows that the 2-methylcupferrate ligand
induces nCO signals to be observed at slightly lower wavenumbers
than the parent ligand. This effect is expected to have a mostly
electronic nature, caused by the electron donating methyl
substituent.
Table 1
Carbonyl IR-signals of 2e6, pKa-values of the free phosphine and the cone angles of
the different phosphines as defined by Tolman [10].
Complex
PR3
pKa
nCO (cmꢀ1
)
Cone angle (ꢁ)
2
3
4
5
6
PPh3
2.73
3.84
4.57
3.08
9.65
1982
1970
1964
1965
1954
145
145
145
194
170
P(4-MePh)3
P(4-OMePh)3
P(2-MePh)3
PCy3
Scheme 1. Synthesis of [Rh(MeCupf)(CO)2].

S. Warsink et al. / Journal of Organometallic Chemistry 726 (2013) 14e20
17
3.2. Kinetic studies
Rh
1.61
Rh -alkyl
1.41
The stability of the [Rh(MeCupf)(CO)(PPh3)] complex in different
solvents was verified in solutions of acetone, chloroform, acetoni-
trile, ethyl acetate, methanol and benzene. The solutions were
monitored with UV/visible spectroscopy for a number of hours. No
significant amount of decomposition took place under the selected
experimental conditions for at least 13 h. The reaction between
[Rh(MeCupf)(CO)(PR3)] and iodomethane gave a broad absorption
maximum in the 340e450 nm region and all kinetic measurements
were done at a particular maximum absorption (Fig. 1).
1978 cm
2056 cm
1.21
1.01
Abs0.81
Rh -acyl
1722 cm
0.61
0.41
0.21
0.01
3.3. Reaction mechanism
2100 2050 20
10
1900 1850 1800 1750 1700 1650
Wavenumber
The IR and UV/visible spectra clearly indicate that all the
different RhIeMeCupf complexes react with iodomethane. These
results also point to the formation of the RhIIIealkyl product in the
first reaction (oxidative addition) and the subsequent acyl forma-
tion in the second reaction as indicated in equation (1).
Fig. 2. Consecutive IR scans (4 min intervals) for the oxidative addition of [Rh(Me-
Cupf)(CO)(PPh3)] with iodomethane in chloroform at 25.0 ꢁC, [Rh]
¼
0.02 M,
[CH3I] ¼ 0.2 M.
½RhðLL0ÞðCOÞðPR3ÞꢄðIÞ
½RhðLL0ÞðCOÞðPR3ÞðCH3ÞðIÞꢄ
ðIIÞ
½RhðLL0ÞðCOCH3ÞðPR3ÞðIÞꢄ
ðIIIÞ
first
second
reaction
(1)
þ
reaction
CH3I
The Rh(I)eCO stretching frequency at about 1978 cmꢀ1 for (I)
disappears with the simultaneous appearance of the RhIIIeCO peak
at about 2057 cmꢀ1 for (II). The peak formation at 1722 cmꢀ1 is
indicative of acyl formation for (III) (Fig. 2). Another important
aspect of all the IR spectra is the complete disappearance of the
RhIeCO peak which points to a large forward reaction (large
equilibrium constant) with little or no reverse reaction.
The kinetic study of the reaction between [Rh(MeCupf)(CO)(PR3)]
and different iodomethane concentrations showed a significant
intercept which could not be attributed to the reverse reaction. This
intercept can only be attributed to the involvement of solvent
molecules in the overall reaction. Using this information the
following mechanism is proposed for the reaction between
[Rh(MeCupf)(CO)(PR3)] and iodomethane (Scheme 3).
3.4. Rate laws
The rate law for the alkyl formation reaction, assuming that k3 is
a fast reaction, is given by equation (2).
h
i.
ꢀd RhI dt ¼ ðk1½CH3Iꢄ þ k2½SꢄÞ½RhðMeCupfÞðCOÞðPR3Þꢄ
(2)
Equation (2) simplifies to equation (3) under pseudo-first-order
conditions with [Rh(MeCupf)(CO)(PR3)] ꢅ [CH3I].
kobs ¼ k1½CH3Iꢄ þ k2½Sꢄ ¼ k1½CH3Iꢄ þ k20
The rate law for the acyl formation (final step in Scheme 3) is
given by equation (4).
(3)
½RhðMeCupfÞðCOCH3ÞðPR3ÞðIÞðSÞꢄ=dt
¼ k4½RhðMeCupfÞðCOÞðPR3ÞðCH3ÞðIÞꢄ½Sꢄ
þ k ½RhðMeCupfÞðCOÞðCH3ÞðPR3ÞðIÞꢄ
(4)
(5)
ꢀ4
1.2
1
Hence,
R ¼ ðk4½Sꢄ þ kꢀ4Þ½RhðMeCupfÞðCOÞðPR3ÞðCH3ÞðIÞꢄ
0.8
0.6
0.4
0.2
0
350
370
390
410
430
450
470
490
510
530
550
Fig. 1. Visible spectra (4 min interval) for the reaction between [Rh(Me-
Cupf)(CO)(PPh3)] and iodomethane in acetone at 25.0 ꢁC, [Rh] ¼ 1.7 ꢃ 10ꢀ4 M.
Scheme 3. Mechanism for the oxidative addition reaction between [Rh(Me-
Cupf)(CO)(PR3)] and iodomethane.

18
S. Warsink et al. / Journal of Organometallic Chemistry 726 (2013) 14e20
0.0050
0.0045
0.0040
0.0035
0.0030
0.0025
0.0020
0.0015
0.0010
0.0005
0.0000
0.0035
0.003
UV/visible result
Rh(III) formation, IR result
39.8 °C
0.0025
0.002
-1
kobs, s
Rh(I)-disappearance, IR result
0.0015
-1
29.0 °C
kobs, s
0.001
Acyl formation, IR result
0.0005
0
0
0.2
0.4
0.6
[CH I], M
0.8
1.0
1.2
24.5 °C
17.0 °C
1
Fig. 4. Comparison between IR and UV kinetic results for the reaction between
[Rh(MeCupf)(CO)(PPh3)] and iodomethane in chloroform at 25.0 ꢁC.
0
0.2
0.4
0.6
0.8
1.2
this study. This suggests that the model presented in Scheme 3 is a
fair reflection of the reaction that was experimentally investigated.
The UV/visible spectra on the other hand only showed one
reaction in the wavelength range that was used. All these spectra
showed a steady absorption increase at about 400 nm. The kinetic
results for the reaction between [Rh(MeCupf)(CO)(PR3)] with
iodomethane in chloroform (same conditions as those of the IR
[CH3I], M
Fig. 3. Kinetic results for the oxidative addition between [Rh(MeCupf)(CO)(PPh3)] and
iodomethane in acetone.
Integration of equation (5) yields equation (6) with
study) gave a rate constant of 2.31(3) ꢃ 10ꢀ3
M
ꢀ1sꢀ1, which
[S] [ [Rh(MeCupf)(CO)(PR3)(CH3)(I)].
corresponds very favourably with the rate constants for RhI
disappearance and RhIIIealkyl formation. It is difficult to assign the
UV/visible spectrum change to either the RhI disappearance or
RhIIIealkyl formation due to the fact that the two rate constants are
virtually the same. The absence of a second reaction in the UV/
visible area can be attributed to a very small extinction coefficient
for the acyl product as well as its rate of formation being slow
compared to that of the oxidative addition reaction.
kobs ¼ k4½Sꢄ þ k
¼ k0 þ k
(6)
ꢀ4
ꢀ4
4
Equation (6) predicts a rate constant which is independent of
the iodomethane concentration with an intercept of k
.
ꢀ4
3.5. Kinetic results
The linear dependence between the observed rate constant and
the iodomethane concentration at different temperatures is clearly
illustrated in Fig. 3 and the values of k1 and k02 are reported in
Table 2. The k1 (slope) and k02 (intercept) values obtained in acetone
The UV/visible kinetic study for a whole temperature range as
well as the IR kinetic study gave straight lines with relatively large
intercepts as predicted by equation (3). The fact that the IR spec-
trum clearly indicates the absence of an equilibrium for the RhI and
were also used to calculate the activation enthalpy (
D
#H) and
RhIIIealkyl reaction (disappearance of (CO) at 1978 cmꢀ1) leads to
n
entropy (
D
#S) using the Eyring equation. The values found for the
the interpretation of the intercept as indicative of a solvent assisted
pathway. The kinetics of the same reaction in different solvents
clearly pointed to a large difference in rate (slope) as well as solvent
pathway intervention (intercept), with the largest contribution by
acetonitrile.
oxidative addition are 48 kJ/mol for
kind of reaction. For
D
#H, which is normal for this
D
#S a value of ꢀ137 J/K/mol was found, which
corresponds with the associative character of the reaction.
The kinetic study of the disappearance and formation of these
different complexes in the IR region clearly indicates that the rate of
A quite significant solvent effect is evident from the values for k1,
which shows an increasing trend with increased solvent polarity,
with chloroform being the only exception (Table 2). The significant
solvent dependence of k1 strongly suggests a mechanism in which
a polar transition state is stabilised by more polar solvents and can
be taken as evidence that the function of the solvent is to ease the
charge separation during the rearrangement and formation of the
5-coordinate intermediate. This idea is also supported from the
large entropy of activation for the acetone medium. The relative
RhI disappearance (2.0(2) ꢃ 10ꢀ3
M
M
ꢀ1sꢀ1) and the rate of RhIIIe
ꢀ1sꢀ1) are the same within
alkyl formation (2.1(5) ꢃ 10ꢀ3
experimental error (Fig. 4). The rate for the formation of the RhIIIe
acyl product is approximately a factor 13 slower than the above
mentioned reactions.
The rate of RhIIIeacyl formation was also found to be [CH3I]
independent with an average rate constant of 1.5(9) ꢃ 10 Mꢀ1sꢀ1
(Fig. 4). From these discussions it is clear that there is a fairly good
correlation between the mechanism and the subsequent rate laws
for this system and the experimental results that were obtained in
small
D D
#H values, accompanied by large negative #S values,
indicate an associative mechanism including bond formation and/
or partial charge creation (electrostriction) during the formation of
the transition state.
Table 2
Solvent effect for the oxidative addition of [Rh(MeCupf)(CO)(PPh3)] with iodo-
Van Eldik et al. [12] conducted a combined solvent, temperature
and pressure dependence study on the oxidative addition of
methane at 25.0 ꢁC using UV/visible spectroscopy k2 ¼ k02=½Sꢄ.
Solvent
3
[11] D [11] 103 k1 (Mꢀ1 sꢀ1
)
104 k02 (sꢀ1
)
105 k2
k1/k2
(Mꢀ1 sꢀ1
)
δ
Benzene
Ethyl acetate
Acetone
Chloroform
Methanol
Acetonitrile 38
2.3
6
17
4.8
32.6
0.1
17.1
17
4
19
0.101(2)
0.658(7)
1.33(4)
2.31(2)
3.94(5)
5.18(6)
0.02(1)
0.94(3)
5.1(2)
8.5(1)
12.0(3)
15.7(4)
0.0178
0.919
3.75
6.8
4.86
8.2
567
71
35
33
81
63
δ
δ
δ
29.8
Fig. 5. Representation of a three-centre transition state (I) and linear transition state (II).

S. Warsink et al. / Journal of Organometallic Chemistry 726 (2013) 14e20
19
Table 4
[Rh(Sacac)(CO)(PPh3)] (Sacac ¼ thioacetylacetonate) and [Rh(Cupf)
(CO)(PPh3)] with iodomethane. Kinetic data for the [Rh(Sacac)
(CO)(PPh3)] complex exhibited no significant dependence on the
solvent and were interpreted in terms of a concerted three-centre
transition state (I) (Fig. 5). The [Rh(Cupf)(CO)(PPh3)] complex on
the other hand, showed a significant solvent effect in most polar
solvents. This observation was interpreted in terms of a linear
transition state with participation of an ion-pair intermediate (II).
In order to make a more realistic comparison between the k1
and k02 values, the latter was divided by the solvent concentrations
to obtain the second order constants, k2. The ratio k1/k2 is thus
a measure of competition between direct CH3I addition and solvent
dependent pathways. In the case of acetonitrile, the k2 path
becomes more pronounced when compared to k1 if compared with
benzene for example (Table 2). On the other hand, for ethyl acetate
and acetone, having nearly the same donocity, the ratio increases
almost twofold. The proposed rate-determining k2 path is exactly
the same as that for the solvent dependent path of square planar
substitution reactions. The rate of oxidative addition (k1-path)
changes with a factor of about 51 as the polarity of the solvent
increases from benzene to acetonitrile. Highly polar solvents favour
the solvent route (k2-path) since a factor 785 increase was found for
the k2-path while only a factor of 51 was noticed for the k1-path.
Kinetic data for the oxidative addition of selected [Rh(LL0-BID)(CO)(PPh3)]
complexes with iodomethane in chloroform at 25.0 ꢁC.
L-L0-BID
L
L0
Ring size
Rate constants
Alkyl (Mꢀ1 sꢀ1
Ref.
)
Acyl (sꢀ1
)
Cupf
MeCupf
acac
Sacac
hpt
macsm
O
O
O
O
O
N
O
O
O
S
S
S
5
5
6
6
5
6
0.0050(1)
0.00231(2)
0.0065(4)
Not observed
0.0083
0.0012(1)
0.00015(9)
0.0016(1)
>0.01
0.01
0.0078(4)
[1c]
This work
[15]
[16]
[17]
0.034(1)
[18]
PCy3 (9.65), it is anticipated that k1 for PCy3 should be larger
compared to P(p-MeOC6H4) (pKa of 4.57). Since the cone angle of
PCy3 is 170ꢁ, it has a much larger steric demand compared to PPh3,
P(p-Tol)3 and P(p-MeOC6H4)3 (145ꢁ), and the electronic effect is
overshadowed by the steric component resulting in a k1 value of
only 0.00335(4) for PCy3. This is approximately a twofold decrease
when compared to P(p-MeOPh)3.
A reaction mechanism consistent with the experimental results
is shown in Scheme 3. The k1 path implies a nucleophilic attack on
iodomethane giving the 5-coordinate intermediate for which the
degree of ion separation will be solvent dependent. During the
RheCH3 bond formation, the CH3 ligand will tend, based on
similar assumptions for square-planar substitution reactions [14]
to move towards the least strongly bound ligand (nitroso
oxygen) and away from the most strongly bound ligand (PPh3).
Once the RheC bond is established, the phosphorus atoms can
facilitate the simultaneous CeI bond breaking. The same effect will
also facilitate the fast nucleophilic attack of Iꢀ between the RheC
and RheO bonds thus leading to the proposed cis-addition product
[1c].
The rate of the oxidative addition of [Rh(MeCupf)(CO)(PPh3)]
with iodomethane is slightly faster than that of the corresponding
[Rh(Cupf)(CO)(PPh3)] complex (Table 4), but still appreciably
slower than the six-membered [Rh(acac)(CO)(PPh3)] complex. The
electronic effect of the addition of a methyl group to the cupferron
backbone in the current structure is relatively small. From these
results it appears that the methyl group donates electron density to
the bidentate ligand. This in turn increases the electron density on
the rhodium centre which renders the metal complex to be a better
Lewis base. The final result is better interaction between the metal
d-orbital and the s*-orbital of the iodomethane and finally an
increase in the oxidative addition rate (bond breaking of the CH3eI
bond).
The data in Table 4 indicate that in general, for the same donor
atoms L and L0, an increase in the rate of oxidative addition with an
increase in size of the chelate ring is observed. The increase in rate
of oxidative addition, correlating to the donor atoms of the
bidentate ligands, is found to be in the order O,O < O,S < O,N < N,S
while no meaningful correlation was found for the rate of acylation.
The increase in the s-donating ability of tertiary phosphines by
the introduction of electron donating substituents must always be
considered in conjunction with the effect that electron donating
groups will have on the
p-backbonding capabilities of the phos-
phine. This is especially true when tertiary phosphines are used as
ligands in transition metal complexes, since their electron donore
acceptor capabilities determine the electron density on the metal
and this will have an effect on other possible ligands such as CO.
The metal centre acts as a nucleophile when a transition metal
complex undergoes oxidative addition reactions. The ability of
a ligand to increase the electron density on the central metal, will
lead to an increase in the rate of oxidative addition, assuming all
other influences, factors and parameters remain constant. The
oxidative addition of [Rh(MeCupf)(CO)(PR3)] with iodomethane
illustrates the steric and electronic effect of tertiary phosphines on
the rate of oxidative addition.
According to Table 3, the first three phosphines are isosteric
with a common Tolman cone angle of 145ꢁ and therefore have the
same steric demand on the expected rate. However, electronically
there is an increase in their s-donating ability as predicted by their
respective Brönsted pKa values [13]. The second-order rate
constants for the oxidative addition (k1) show a corresponding
increase from PPh3 to P(p-MeO-Ph)3. This is in agreement with the
higher nucleophilicity of the respective RhI complexes. In accor-
dance, the k1 path shows a 5 fold increase from PPh3 to P(p-
MeOC6H4)3 which is an expected trend for a nucleophilic attack at
the sp3 carbon of iodomethane. The result k1(PCy3) > k1(P(o-
Tolyl)3) is in agreement with the electronic effect. The fact that both
values are less than that of P(p-MeOC6H4)3 indicates that the steric
effect is also operative. However, upon considering the pKa value of
4. Conclusions
Table 3
Sterically and electronically different rhodium(I) complexes
bearing the 2-methyl substituted MeCupf ligand have been syn-
thesised and fully characterised. It appeared that in the substitution
of one carbonyl ligand for a tertiary phosphine, a mixture of two
isomers was formed. In contrast to earlier reports, this appears to
be a general reaction pattern for this type of ligand. The oxidative
addition of iodomethane to the monophosphine complexes was
exhaustively investigated. From these studies it was concluded that
there is only a forward reaction in the oxidative addition, with no
observed reverse reaction. The subsequent carbonyl insertion was
Ligand effect for the oxidative addition of [Rh(MeCupf)(CO)(PR3)] with iodomethane
at 25.0 ꢁC using UV/visible spectroscopy in acetone. The nCO values are from this
study, the pKa values are those of the free phosphines, and the cone angles those of
Tolman [10].
PR3
pKa nCO (cmꢀ1) Cone angle (ꢁ) 103 k1 (Mꢀ1 sꢀ1) 104 k02 (sꢀ1
)
PPh3
P(p-Tol)3
P(p-MeOC6H4)3 4.57 1964
PCy3
2.73 1982
3.84 1970
145
145
145
170
194
1.33(4)
2.30(7)
7.05(7)
3.35(4)
0.615(5)
5.1(2)
3.5(3)
7.89(4)
5.2(2)
2.55(3)
9.65 1954
3.08 1965
P(o-Tol)3

20
S. Warsink et al. / Journal of Organometallic Chemistry 726 (2013) 14e20
[13] C.A. McAuliffe, G. Wilkinson, Comprehensive Coordination Chemistry, Perga-
mon Press, New York, 1987, p. 989.
[14] K.F. Purcell, J.C. Kotz, Inorganic Chemistry, Saunders College Publishing,
Philadelphia, 1977, p. 392.
[15] D.E. Graham, G.J. Lamprecht, I.M. Potgieter, A. Roodt, J.G. Leipoldt, Transition
Met. Chem. 16 (1991) 193e195.
[16] Y. M. Terblans, M.Sc. Thesis, Unpublished Results, Univ. of the Free State,
Bloemfontein, South Africa, 1993.
relatively slow. A large solvent effect was observed for the more
polar solvents, indicating that a solvento-species is involved on
those occasions. In addition to the electronic effect, the steric
influence of these monodentate donor ligands cannot be under-
estimated. Apparently, the steric bulk introduced by the phosphine
substituents hampers the approach of the incoming iodomethane,
lowering the rate of oxidative addition. This shows that an
optimum exists for the propensity of oxidative addition for this
type of complexes, with a fine balance to strike between electron
donating groups and steric bulk.
[17] H. Preston, Ph.D. Thesis, Unpublished Results, Univ. of the Free State,
Bloemfontein, South Africa, 1993.
[18] A. Roodt, G.J.J. Steyn, Recent Res. Dev. Inorg. Chem. 2 (2000) 1e23.
Stefan Warsink (1981) obtained his PhD in 2011 at the
University of Amsterdam under the supervision of Prof. C.J.
Elsevier. The focus of his dissertation was the synthesis
and characterization of palladium NHC complexes and
their application in Z-selective transfer hydrogenation of
alkynes. From early 2012, he moved to the research group
of Prof. A. Roodt at the University of the Free State,
Bloemfontein (South Africa) as a Postdoctoral Fellow. His
research focuses on rhodium complexes bearing hetero-
ditopic ligands for the carbonylation of methanol and the
kinetic studies thereof. He also joined the crystallographic
team of the group.
Acknowledgements
Financial assistance from the South African National Research
Foundation (NRF), THRIP and Research Fund of the University
of the Free State is gratefully acknowledged. Opinions, findings,
conclusions or recommendations expressed in this material
are those of the authors and do not necessarily reflect the views of
the NRF.
References
[1] (a) S.S. Basson, J.G. Leipoldt, J.T. Nel, Inorg. Chim. Acta 86 (1984) 167e172;
(b) S.S. Basson, J.G. Leipoldt, A. Roodt, J.A. Venter, T.J. van der Walt, Inorg.
Chim. Acta 119 (1986) 35e38;
Fanuel Gebremichael Fessha obtained his MSc in Inor-
ganic Chemistry. He is currently employed as scientist by
Life Technologies in the USA. He has years of industry
experience in making oligonuleotide and assays and has
a great interest in creating and designing protocols for
manufacturing processes. He is also involved in the
developing and optimizing of synthesis and purification of
DNA probes.
(c) S.S. Basson, J.G. Leipoldt, A. Roodt, J.A. Venter, Inorg. Chim. Acta 128 (1987)
31e37;
(d) S.S. Basson, J.G. Leipoldt, J.T. Nel, Inorg. Chim. Acta 84 (1987) 167;
(e) J.G. Leipoldt, S.S. Basson, L.J. Botha, Inorg. Chim. Acta 168 (1990) 215e220;
(f) J.G. Leipoldt, E.C. Steynberg, R. van Eldik, Inorg. Chem. 26 (1987) 3068e
3070;
(g) G.J. van Zyl, G.J. Lamprecht, J.G. Leipoldt, T.W. Swaddle, Inorg. Chim. Acta
143 (1988) 223e227;
(h) J.G. Leipoldt, G.J. Lamprecht, G.J. van Zyl, Inorg. Chim. Acta 96 (1985) L31e
L34;
(i) D.M.C. Smit, S.S. Basson, A. Roodt, E.C. Steynberg, Rhodium Express 7e8
(1994) 12e16;
(j) Y.M. Terblans, S.S. Basson, W. Purcell, G.J. Lamprecht, Acta Crystallogr. C51
(1995) 1748e1750;
(k) P. Ebenebe, S.S. Basson, W. Purcell, Rhodium Express 16 (1996) 11e16;
(l) M. Theron, E. Grobbelaar, W. Purcell, S.S. Basson, Inorg. Chim. Acta 358
(2005) 2457e2463;
(m) E. Grobbelaar, W. Purcell, S.S. Basson, Inorg. Chim. Acta 359 (2006)
3800e3860;
(n) J. Conradie, G.J. Lamprecht, A. Roodt, J.C. Swarts, Polyhedron 23 (2007)
5075e5087;
(o) M.M. Conradie, J. Conradie, Inorg. Chim. Acta 361 (2008) 2285e2295;
(p) J. Conradie, J.C. Swarts, Organometallics 28 (2009) 1018e1026;
(q) A.J. Muller, J. Conradie, W. Purcell, S.S. Basson, J.A. Venter, S. Afr. J.
Chem. 63 (2010) 11e19;
Walter Purcell has been in the academic environment for
the last 25 years and obtained his PhD in Inorganic
Chemistry. He has been promoter and co-promoter for
a number of Masters and Ph.D. students in both Analytical
and Inorganic Chemistry. He has numerous articles in
international journals and also contributed a number of
papers at national and international conferences. His
research interest includes the study of the reaction kinetics
of organometallic and inorganic substances in organic and
water mediums as well as the quantification and identifi-
cation of different elements in natural minerals and ores,
synthesis of organometallic complexes, crystallography,
ISO 17025, method development and statistic validation.
(r) M.M. Conradie, J. Conradie, Dalton Trans. 40 (2011) 8226e8237.
[2] S.S. Basson, J.G. Leipoldt, J.A. Venter, Acta Crystallogr. C46 (1990) 1324e1326.
[3] K.G. van Aswegen, J.G. Leipoldt, I.M. Potgieter, G.J. Lamprecht, A. Roodt,
G.J. van Zyl, Transition Met. Chem. 16 (1991) 369e371.
[4] L.J. Damoense, Ph.D. Thesis, Unpublished Results, Univ. of the Free State,
Bloemfontein, South Africa, 2000.
[5] Scientist for Windows, Least Square Parameter Estimation, Version 4, Micro-
math, 1990.
[6] J.A. McCleverty, G. Wilkinson, Inorg. Synth. 28 (1990) 84e86.
[7] A.T. Balaban, R.E. Garfield, M.J. Lesko, W.A. Seitz, Org. Prep. Proced. Int. 30
(1998) 439e446.
Johan A. Venter obtained his PhD in Inorganic Chemistry.
He specializes in the oxidative addition reactions of orga-
notransition metal complexes. He publishes internation-
ally and presented papers at several conferences, both
local and international. His interests are in the platinum
group chemistry, organometallic complexes, inorganic
synthesis, oxidative addition, reaction kinetics X-ray crys-
tallography and method validation in Analytical Chemistry.
[8] M. Ahmed, A.J. Edwards, C.J. Jones, J.A. McCleverty, A.S. Rothin, J.P. Tate, Dalton
Trans. (1988) 257e263.
[9] S.S. Basson, J.G. Leipoldt, W. Purcell, J.A. Venter, Acta Crystallogr. C48 (1992)
171e173.
[10] C.A. Tolman, Chem. Rev. 77 (1977) 313e348.
[11] U. Mayer, V. Gutman, Adv. Inorg. Chem. Radiochem. 17 (1975) 189e230.
[12] R. van Eldik, J.G. Leipoldt, J.A. Venter, Inorg. Chem. 30 (1991) 2207e2209.