Iodomethane oxidative addition and CO migratory insertion in monocarbonylphosphine complexes of the type [Rh((C6H5)COCHCO((CH2)n CH3))(CO)(PPh3)]: Steric and electronic effects

Article abstract of DOI:10.1016/j.jorganchem.2008.10.040

The chemical kinetics, studied by UV/Vis, IR and NMR, of the oxidative addition of iodomethane to [Rh((C₆H₅)COCHCOR)(CO)(PPh₃)], with R = (CH₂)_nCH₃, n = 1-3, consists of three consecutive reaction steps that involves isomers of two distinctly different classes of Rh^III-alkyl and two distinctly different classes of Rh^III-acyl species. Kinetic studies on the first oxidative addition step of [Rh((C₆H₅)COCHCOR)(CO)(PPh₃)] + CH₃I to form [Rh((C₆H₅)COCHCOR)(CH₃)(CO)(PPh ₃)(I)] revealed a second order oxidative addition rate constant approximately 500-600 times faster than that observed for the Monsanto catalyst [Rh(CO)₂I₂]^-. The reaction rate of the first oxidative addition step in chloroform was not influenced by the increasing alkyl chain length of the R group on the β-diketonato ligand: k₁ = 0.0333 ([Rh((C₆H₅)COCHCO(CH₂CH₃))(C O)(PPh₃)]), 0.0437 ([Rh((C₆H₅)COCHCO(CH₂CH₂CH ₃))(CO)(PPh₃)]) and 0.0354 dm³ mol^-1 s^-1 ([Rh((C₆H₅)COCHCO(CH₂CH₂ CH₂CH₃))(CO)(PPh₃)]). The pK_a^′ and keto-enol equilibrium constant, K_c, of the β-diketones (C₆H₅)COCH₂COR, along with apparent group electronegativities, χ_R of the R group of the β-diketones (C₆H₅)COCH₂COR, give a measurement of the electron donating character of the coordinating β-diketonato ligand: (R, pK_a^′, K_c, χ_R) = (CH₃, 8.70, 12.1, 2.34), (CH₂CH₃, 9.33, 8.2, 2.31), (CH₂CH₂CH₃, 9.23, 11.5, 2.41) and (CH₂CH₂CH₂CH₃, 9.33, 11.6, 2.22).

Full text of DOI:10.1016/j.jorganchem.2008.10.040

Journal of Organometallic Chemistry 694 (2009) 259–268
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Iodomethane oxidative addition and CO migratory insertion in
monocarbonylphosphine complexes of the type
[Rh((C6H5)COCHCO((CH2)nCH3))(CO)(PPh3)]: Steric and electronic effects
*
Nomampondomise F. Stuurman, Jeanet Conradie
Department of Chemistry, University of the Free State, Nelson Mandela, Bloemfontein 9301, 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 April 2008
Received in revised form 17 October 2008
Accepted 22 October 2008
Available online 30 October 2008
The chemical kinetics, studied by UV/Vis, IR and NMR, of the oxidative addition of iodomethane to
[Rh((C6H5)COCHCOR)(CO)(PPh3)], with R = (CH2)nCH3, n = 1–3, consists of three consecutive reaction
steps that involves isomers of two distinctly different classes of RhIII-alkyl and two distinctly different
classes of RhIII-acyl species. Kinetic studies on the first oxidative addition step of [Rh((C6H5)COCH-
COR)(CO)(PPh3)] + CH3I to form [Rh((C6H5)COCHCOR)(CH3)(CO)(PPh3)(I)] revealed a second order oxida-
tive addition rate constant approximately 500–600 times faster than that observed for the Monsanto
catalyst [Rh(CO)2I2]ꢀ. The reaction rate of the first oxidative addition step in chloroform was not influ-
enced by the increasing alkyl chain length of the R group on the b-diketonato ligand: k1 = 0.0333
Keywords:
b-Diketone
Rhodium
Oxidative addition
pKa
([Rh((C6H5)COCHCO(CH2CH3))(CO)(PPh3)]), 0.0437 ([Rh((C6H5)COCHCO(CH2CH2CH3))(CO)(PPh3)]) and
0
0.0354 dm3 molꢀ1 sꢀ1 ([Rh((C6H5)COCHCO(CH2CH2CH2CH3))(CO)(PPh3)]). The pKa and keto–enol equilib-
rium constant, Kc, of the b-diketones (C6H5)COCH2COR, along with apparent group electronegativities, vR
Carbonyl
Phosphine
of the R group of the b-diketones (C6H5)COCH2COR, give a measurement of the electron donating charac-
0
ter of the coordinating b-diketonato ligand: (R, pKa , Kc,
vR) = (CH3, 8.70, 12.1, 2.34), (CH2CH3, 9.33, 8.2,
2.31), (CH2CH2CH3, 9.23, 11.5, 2.41) and (CH2CH2CH2CH3, 9.33, 11.6, 2.22).
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
The reactivity of square planar rhodium(I) monocarbonylphos-
phine complexes [Rh(b-diketonato)(CO)(PPh3)] in the oxidative
addition Reaction (1) with iodomethane was studied with regard
to the phosphine basicity [8], the electronic effect of differentꢀsub-
stituents R0 and R on the b-diketonato ligand = R0COCHCOR [9]
and the solvent polarity and donicity effects [10].
Rhodium complex compounds are one of the most widely
spread industrial homogeneous catalysts for organic raw material
processing. Classic examples of efficacious catalyst systems are
methanol carbonylation to give acetic acid in the presence of
[Rh(CO)2I2] (Monsato process) [1], alkene hydroformylation by
the [RhHCO(PPh3)2] catalyst [2], hydrogenation of olefins and
acetylenes with the help of [RhCl(PPh3)3] (Wilkinson’s catalyst)
[3] and the use of [Rh(CH3COCHCOCH3)(CO)2] in the hydroformyl-
ation of olefins [4]. In the field of olefin polymerization, metal
complexes with a coordinatively unsaturated Lewis acid metal
center are generally required, whereas for transformations such
as the carbonylation of methanol, electron-rich metal centers are
necessary to favor oxidative addition of iodomethane to RhI [5,6].
High catalytic reactivity of these rhodium complexes is in many
respects due to the nature of ligand surroundings [7]. It is of inter-
est to note how the steric and electronic properties of the ancillary
ligands can affect the rates of the oxidative addition and migratory
insertion steps, which is a vital step in the functioning of many of
these compounds as homogeneous catalysts.
[RhI( -diketonato)(CO)(PPh )] + CH I
β
3
3
ð1Þ
[RhIII( -diketonato)(CH )(CO)(PPh )(I)]
β
3
3
Steric factors are important in controlling the rate of oxidative
addition reactions [11]. Previous studies on [Rh(b-diketo-
nato)(CO)(PPh3)] complexes did not focus on the steric influence
of the R or R0 groups of the b-diketonato ligand R0COCHCORꢀ on
the reactivity of Reaction (1). In this paper, we report the synthesis
and characterization of monocarbonylphosphine complexes
[Rh((C6H5)COCHCO((CH2)nCH3))(CO)(PPh3)], n = 1–3, and the oxi-
dative addition reactions of these complexes with iodomethane.
It is found that the electron donating (electronic) properties of
the phenyl and alkyl groups on the b-diketonato ligand R0COCH-
COR speed up the oxidative addition reaction, but the size of the
alkyl group (CH2)nCH3 (steric property) did not restrain the rate
of oxidative addition. The rate of iodomethane oxidative addition
* Corresponding author. Tel.: +27 51 4012194; fax: +27 51 4446384.
E-mail address: conradj.sci@ufs.ac.za (J. Conradie).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.10.040

260
N.F. Stuurman, J. Conradie / Journal of Organometallic Chemistry 694 (2009) 259–268
O
O
to the complexes of this study was found to be faster than all pre-
viously published values according to Reaction (1).
R
2
DMF
OC
OC
Cl
Cl
CO
CO
2
RhCl3.3H2O
Rh
Rh
Rh
2. Results and discussion
(1) R = CH2CH3
(2) R = CH2CH2CH3
(3) R = CH2CH2CH2CH3
2.1. Synthesis and identification of complexes
R
O
R
R
The route utilized to obtain the b-diketones of general formula
(C6H5)COCH2COR, R = (CH2)nCH3, n = 1–3, is given in Scheme 1. b-
Diketones exist in solution and in gas phase as mixtures of keto
and enol tautomers. In the solid state, the enol form is often the
form mostly observed [12]. From a 1H NMR study, by comparing
the relative intensities of the CH2 (keto at ca 4.1 ppm) and CH (enol
at ca 6.2 ppm) signals in solution, the percentage of the keto tau-
tomers in CDCl3 solution at 25 °C of the b-diketones Hba
(R = CH3), Hbap (1) (R = CH2CH3), Hbab (2) (R = CH2CH2CH3) and
Hbav (3) (R = CH2CH2CH2CH3) was established as 7.5%, 10.9%,
8.0% and 7.9%, respectively. The pKa values obtained for the conju-
gated keto–enol system of the different phenyl-containing b-dike-
tones 1, 2 and 3 are 9.33(3), 9.23(5) and 9.33(5), respectively. The
basic character of these ligands should increase the electron den-
sity at the rhodium center, making it a stronger nucleophile and
therefore more reactive towards oxidative addition [13].
O
O
O
O
Kc1
CO
PPh3
CO
CO
2 PPh3
2
Rh
Rh
CO
PPh3
O
- 2 CO
isomer A
isomer B
(4) R = CH2CH3
(7) R = CH2CH3
(5) R = CH2CH2CH3
(8) R = CH2CH2CH3
(6) R = CH2CH2CH2CH3
(9) R = CH2CH2CH2CH3
Scheme 2. Synthetic route for the synthesis of [Rh(b-diketonato)(CO)2] complexes
4–6 and [Rh(b-diketonato)(CO)(PPh3)] complexes 7–9 from RhCl3 ꢁ 3H2O and
appropriate b-diketones 1–3.
difference in the position, dH in ppm, of the signals of the protons
of the alkyl group of the b-diketonato ligand. The two isomers
are the isomer with the PPh3 group trans to the oxygen nearest
to the more electron donating phenyl group of the chelate ring
and the isomer with the PPh3 group cis to the oxygen nearest to
the phenyl group. The former is labeled as isomer A and the latter
as isomer B, as defined in Scheme 2. Isomer A is expected to be the
predominant isomer due to the trans-influence where the PPh3
group is trans to the oxygen near an electron donating group (phe-
nyl) which has a larger trans-influence compared to an alkyl group.
This is in agreement with the polarization theory, since the oxygen
atom nearest to an alkyl group will be least polarisable due to the
electron attracting property of the alkyl group [16].
The equilibrium constant, defined as Kc = [isomer B]/[isomer A]
and applicable to the equilibrium shown in Scheme 2, may be
determined by calculating the ratio of peak integral values of
non-overlapping corresponding signals of each isomer and averag-
ing all answers. DrH values in CDCl3 at 25 °C for the [Rh(b-diketo-
nato)(CO)(PPh3)] complexes 7–9 were determined with Eq. (15)
and are summarized in Table 1, together with the Kc values and
the thermodynamic data. (see Fig. S3 in Supplementary material
for the variation of Kc with temperature for 7, 8 and 9). The enthal-
py effect of the interconversion of the [Rh(b-diketo-
nato)(CO)(PPh3)] isomers of 7–9, is smaller than was found for
[Rh(b-diketonato)(CO)(PPh3)] complexes with b-diketones con-
taining a ferrocenyl group [17] or a thienyl group [18].
It has previously been shown that accurate apparent group elec-
tronegativities, vR
, of the R group for esters of the type
R(C@O)(OCH3) can be obtained from a linear fit between vR and
the IR carbonyl stretching frequency [14]. The straight line gener-
ated by the fit of
vR and
m(C@O) (see Fig. S1 in Supplementary mate-
rial) fits the equation
m
(C@O)R = 74.53 R + 1561 and was used to
v
determine the effective or apparent group electronegativities of
the R groups = CH3, CH2CH3, CH2CH2CH3 and CH2CH2CH2CH3 as
2.34 [15], 2.31, 2.41 and 2.22, respectively. These electronegativities
are considered to be electron-donating (vCF3 = 3.31) [15], making
the b-diketonates 1–3 a good choice to be complexed to rhodium
in order to increase the electron density at the metal. The increased
electron density at the rhodium center should promote oxidative
addition and consequently the overall rate of production during cat-
alytic processes.
The new yellow-orange complexes of [Rh(b-diketonato)(CO)2]
with b-diketonato = bap (4) (27% yield), bab (5) (17% yield) and
bav (6) (18% yield) and the new light yellow [Rh(b-diketo-
nato)(CO)(PPh3)] complexes with b-diketonato = bap (7) (16%
yield), bab (8) (13% yield) and bav (9) (14% yield) were synthesized
as shown in Scheme 2. 1H NMR spectra showed that for each of the
[Rh(b-diketonato)(CO)(PPh3)] complexes synthesized, two isomers
exist in solution (see Fig. S2 in Supplementary material). The differ-
ence between the two isomers is manifested especially in the big
O
O
keto
form
R
O
Kc
O
slow
CH2-Li+
R
OEt
O
OH
O
LDA
R
CH3
(1) R = CH2CH3
O-Li+
CH2
enol
(2) R = CH2CH2CH3
(3) R = CH2CH2CH2CH3
fast
forms
OH
O
R
Scheme 1. Synthetic route for the synthesis of phenyl containing b-diketones Hbap (1-phenylpentane-1,3-dione, propanylacetophenone, C6H5COCH2COCH2CH3) (1), Hbab
(1-phenylhexane-1,3-dione, buterylacetophenone, C6H5COCH2COCH2CH2CH3) (2) and Hbav (1-phenylheptane-1,3-dione, valerylacetophenone, C6H5COCH2-
COCH2CH2CH2CH3) (3). The keto–enol equilibrium of the b-diketones is indicated on the right.

N.F. Stuurman, J. Conradie / Journal of Organometallic Chemistry 694 (2009) 259–268
261
Table 1
0.7
RhI
The equilibrium constant Kc in CDCl3 at 25 °C and for the equilibrium shown in
Scheme 2 and the thermodynamic data relevant to this equilibrium for [Rh(ba)-
(CO)(PPh3)], [Rh(bap)(CO)(PPh3)] (7), [Rh(bab)(CO)(PPh3)] (8) and [Rh(bav)
(CO)(PPh3)] (9).
0.6
0.5
0.4
0.3
0.2
0.1
0.0
DrH (kJ molꢀ1
)
DrGa (kJ molꢀ1
)
DrS (J molꢀ1 Kꢀ1
)
a
RhIII
alkyl 1
-
Complex
Kc
RhIII
acyl 1
-
[Rh(ba)(CO)(PPh3)]
0.47
–
–
–
[Rh(bap)(CO)(PPh3)] 0.70 1.6
[Rh(bab)(CO)(PPh3)] 0.77 2.0
[Rh(bav)(CO)(PPh3)] 0.80 1.7
0.9
0.6
0.5
2.3
4.7
4.0
a
At 25 °C.
2150
2050
1950
1850
1750
1650
wave number / cm-1
2.2. Kinetics
(a) First reaction
2.2.1. Infrared study
Kinetic measurements for the reaction of Rh(bap)(CO)(PPh3)]
(7), [Rh(bab)(CO)(PPh3)] (8) or [Rh(bav)(CO)(PPh3)] (9) with iodo-
methane in chloroform were first carried out using IR spectroscopy
by monitoring the changes in absorbance of peaks at different
RhIII
alkyl 2
-
RhIII
acyl 1
-
RhIII
alkyl 1
-
0.3
0.2
0.1
0.0
m
(CO) bands. This technique is ideal to distinguish between
CO bonds in metal–CO complexes that resonate at ꢂ1980–
2000 cmꢀ1 for RhI-monocarbonylphosphine complexes and at
ꢂ2050–2100 cmꢀ1 for RhIII-alkyl complexes and CO bonds in me-
tal–COCH3 complexes that resonate at ꢂ1700–1750 cmꢀ1 for
RhIII-acyl complexes [17]. All three RhI complexes followed the
same reaction sequence in reaction with iodomethane. Fig. 1 is a
representative example, illustrating the infrared spectroscopic
observations made during the oxidative addition of iodomethane
to [Rh(bav)(CO)(PPh3)] (9), as well as the following carbonyl inser-
tion and deinsertion steps. Fig. 2 gives the absorbance-time data
obtained for this reaction.
2150
2050
1950
1850
1750
1650
wave number / cm-1
(b) Second reaction
0.4
RhIII
-
The first step for the reaction between iodomethane and
[Rh(bav)(CO)(PPh3)] (9) shows that the disappearance of the RhI
complex (signal at 1982 cmꢀ1, kobs = 0.0066(2) sꢀ1) leads to the
alkyl 2
RhIII
acyl 2
-
0.3
0.2
0.1
0.0
immediate formation of
2075 cmꢀ1, kobs = 0.037(1) sꢀ1) and the simultaneous formation of
the RhIII-acyl1 species at
slightly slower rate (signal at
a
RhIII-alkyl1 species (signal at
a
1720 cmꢀ1, kobs = 0.0033(2) sꢀ1) as a result of CO insertion. The rate
of formation of the RhIII-alkyl1 species at 2075 cmꢀ1 is virtually
higher than the rate of disappearance of RhI due to the fact that
RhIII-alkyl1 is simultaneously being converted to RhIII-acyl1 at a
rate of 0.0033(2) sꢀ1. When utilizing the consecutive reaction ki-
netic model on the time-absorbance data of RhIII-alkyl1, it was
found that the RhIII-alkyl1 species appeared at practically the same
rate as the RhI disappearance, see Section 2.2.2 below. The ob-
served slower rate of RhIII-acyl1 appearance is considered to imply
that the equilibrium involving the CO insertion step, is too slow to
be maintained during the oxidative addition of iodomethane to RhI.
The first reaction that can thus be presented as follow:
2150
2050
1950
1850
1750
1650
wave number / cm-1
(c) Third reaction
Fig. 1. Illustration of the oxidative addition reaction between 0.1730 mol dmꢀ3
iodomethane and 0.0003 mol dmꢀ3 [Rh(bav)(CO)(PPh3)] (9) as monitored on the IR
spectrophotometer between 1650 and 2150 cmꢀ1 in chloroform at 25 °C. The first
reaction is indicated by the disappearance of RhI and the simultaneous appearance
of RhIII-alkyl1 and RhIII-acyl1. The second reaction is indicated by the simultaneous
disappearance of RhIII-alkyl1 and RhIII-acyl1 and the formation of RhIII-alkyl2
species. The third reaction is indicated by the disappearance of RhIII-alkyl2 and the
formation of RhIII-acyl2 species.
I + CH3I
Rh
oxidative
addition
k-1 K1, k1
ð2Þ
K2, k2
much slower than the first reaction set (t1/2 ꢃ 4 h). During the
second reaction set, the RhIII-acyl1 species at 1720 cmꢀ1
(kobs = 0.000049(1) sꢀ1) and the RhIII-alkyl1 species at 2075 cmꢀ1
(kobs = 0.000053(5) sꢀ1) disappear as the formation of a new RhIII-
alkyl 2 species at 2056 cmꢀ1 (kobs = 0.000052(2) sꢀ1) occurs. This
wavenumber of the new RhIII-alkyl 2 is 19 cmꢀ1 lower than the
wavenumber of the RhIII-alkyl1. The disappearance of the RhIII-al-
kyl1 species at 2075 cmꢀ1 and the RhIII-acyl1 species at
1720 cmꢀ1 at kinetically the same rate, supplies further evidence
that there exists an equilibrium between the RhIII-alkyl1 and the
RhIII-acyl1 species that is fast enough to be maintained during
the formation of the RhIII-alkyl2. This second reaction was found
[RhIII-alkyl1]
[RhIII-acyl1]
k-2
CO insertion
slow equilibrium
In Eq. (2) above a typical structure of a RhIII-alkyl complex
would be [RhIII(b-diketonato)(CH3)(CO)(PPh3)(I)] while for RhIII-
acyl [RhIII(b-diketonato)(COCH3)(PPh3)(I)] is representative (b-
diketonato = bap, bab or bav). Based on infrared measurements,
however, it is not possible to offer any structural data on a RhIII-
alkyl or a RhIII-acyl.
The second reaction set, for the reaction between iodomethane
and [Rh(bav)(CO)(PPh3)] (9) in chloroform as observed on the IR, is

262
N.F. Stuurman, J. Conradie / Journal of Organometallic Chemistry 694 (2009) 259–268
0.8
0.6
0.4
0.2
0
0.3
0.2
0.1
0
0.3
RhIII-alkyl1
RhIII-acyl1
0.3
2nd Reaction
RhIII-alkyl2
RhIII-alkyl2
RhIII-acyl2
0.2
0.1
0
2
nd Reaction
0.2
0.1
0
3rd Reaction
1000
6000
11000
200000
250000
RhIII-alkyl1
1st Reaction
RhIII-alkyl1
RhIII-acyl1
200
RhI
1st Reaction
400
0
600
800
1000
0
1000
2000
3000
4000
relative time/s
time/s
Fig. 2. IR absorbance vs. time data of the reaction between iodomethane (0.1730 mol dmꢀ3) and [Rh(bav)(CO)(PPh3)] (9) (0.0003 mol dmꢀ3) in chloroform at 25 °C. Left: the
data obtained for the three reaction sets. Right: the solid line is obtained when experimental kinetic data of RhIII-alkyl1 were fitted to the consecutive reaction model of Eq. (14).
to be first order and independent of [CH3I]. The kinetic data of the
second reaction are consistent with
The overall reaction sequence for this oxidative addition reac-
tion, obtained by combining Reactions (2)–(4), may therefore be
represented as
K2, k2
[RhIII-alkyl1]
[RhIII-acyl1]
CO
RhI + CH3I
{
}
k-2
ð3Þ
k-1 K1, k1
First
reaction
k-3 k3
deinsertion
K2, k2
[RhIII-alkyl2]
[RhIII-alkyl1]
[RhIII-acyl1]
{
}
k-2
The third reaction set for the reaction between iodomethane
Second
ð5Þ
k-3 k3
reaction
and [Rh(bav)(CO)(PPh3)] (9) in chloroform, as observed on the IR,
includes the very slow (t1/2 ꢃ 38 h) disappearance of the RhIII-al-
kyl2 species at 2059 cmꢀ1 (kobs = 0.000005(1) sꢀ1) and the appear-
ance of a new RhIII-acyl2 species at 1716 cmꢀ1 at the same rate.
The third reaction set is also first order and [CH3I] independent.
The kinetic data of the third reaction are consistent with
[RhIII-alkyl2]
Third
k-4
k4
reaction
[RhIII-acyl2]
[RhIII-alkyl2]
The reaction between iodomethane and [Rh(bap)(CO)(PPh3)] (7)
or [Rh(bab)(CO)(PPh3)] (8) followed the same reaction sequence as
described above for the reaction between iodomethane and
[Rh(bav)(CO)(PPh3)] (9) in chloroform. Table 2 gives a summary
of the rate constants determined.
k-4
k4
ð4Þ
[RhIII-acyl2]
Table 2
Kinetic rate constants for the oxidative addition of iodomethane to [Rh(b-diketonato)(CO)(PPh3)] complexes 7,
8 and 9 in chloroform at 25 °C, as monitored by IR
spectrophotometry.
Complex
[CH3I]/mol dmꢀ3
RhI disappearance
kobs (sꢀ1
RhIII-alkyl1 formation
kobs (sꢀ1
RhIII-acyl1 formation
kobs (sꢀ1
)
k1 (dm3 molꢀ1 sꢀ1
)
)
)
First reaction
[Rh(bap)(CO)(PPh3)]
[Rh(bab)(CO)(PPh3)]
[Rh(bav)(CO)(PPh3)]
0.2085
0.1500
0.2143
0.0071(4)
0.0071(2)
0.0066(2)
0.034(1)
0.047(2)
0.031(2)
0.036(5)
0.0152(1)
0.037(1)
0.0049(1)
0.0042(3)
0.0033(2)
RhIII-alkyl1 disappearance
kobs (sꢀ1
RhIII-acyl1 disappearance
kobs (sꢀ1
RhIII-alkyl2 formation
)
)
kobs (sꢀ1
)
Second reaction
[Rh(bap)(CO)(PPh3)]
[Rh(bab)(CO)(PPh3)]
[Rh(bav)(CO)(PPh3)]
0.00033(3)
0.00106(1)
0.000053(5)
0.00037(8)
0.00089(2)
0.000049(1)
0.00041(10)
0.00113(1)
0.000052(2)
RhIII-acyl2 formation
kobs (sꢀ1
RhIII-alkyl2 disappearance
)
kobs (sꢀ1
)
Third reaction
[Rh(bap)(CO)(PPh3)]
[Rh(bab)(CO)(PPh3)]
[Rh(bav)(CO)(PPh3)]
0.00015(2)
0.000005(2)
0.000005(2)
0.00013(2)
0.000006(1)
0.000005(1)

N.F. Stuurman, J. Conradie / Journal of Organometallic Chemistry 694 (2009) 259–268
263
2.2.2. Consecutive reaction treatment
If k and k are very small in Reaction (5) and if the equilib-
0.008
-8.6
T
ꢀ1
ꢀ2
k
rium K2 was found to be fast enough to be maintained during the
formation of RhIII-alkyl2, then the first two steps in Reaction (5)
can be approximated by
-9.4
0.006
0.004
0.002
0
35.0°C
-10.2
0.0032
0.0034
0.0036
1/T / K-1
k
ð6Þ
24.7°C
10.0°C
In this case, the formation and depletion of the RhIII-alkyl1 can
be studied by using the consecutive reaction treatment in Eq. (13).
Upon performing a least squares fit of the available IR absorbance
vs. time data of the RhIII-alkyl1 peak for the reaction in chloroform
to Eq. (13) (graph shown for RhIII(bav)-alkyl1 in Fig. 2, right) the
rate constants were determined as k1obs = 0.0128(6) sꢀ1
(k1 = 0.0597 dm3 molꢀ1sꢀ1) and k3 = 0.000047(5) sꢀ1. The value of
k1obs corresponds kinetically close with the value obtained for
the disappearance of the RhI from data treatment utilizing Eq.
(12), k1 = 0.031(2) dm3 molꢀ1 sꢀ1. The value of k3, utilizing Eq.
(13), was practically the same as the value of 0.000049(1) sꢀ1
which was determined by treating the depletion of the RhIII-alkyl1
during the second reaction in isolation (Eq. (12)).
0.00
0.02
0.04
0.06
0.08
[CH3I] / mol dm-3
0.006
0.004
0.002
0
-8.0
-8.6
-9.2
-9.8
T
35.0°C
k
0.0032
0.0034
0.0036
1/T / K-1
26.0°C
9.0°C
k
2.2.3. UV/Vis study
The reaction between iodomethane and [Rh(bap)(CO)(PPh3)]
(7), [Rh(bab)(CO)(PPh3)] (8) or [Rh(bav)(CO)(PPh3)] (9) was also
monitored on a UV/Vis spectrophotometer and all three reaction
steps could be uniquely identified for each complex. The reaction
rate constant obtained for the first step corresponded to the rate
constant for the disappearance of the RhI species as observed on
the IR. The rate constant for the second and third steps also corre-
sponded to the rate constant for the second and third reaction sets
as observed on the IR spectrophotometer. The second step is the
formation of the RhIII-alkyl2 species and the third step is the for-
mation of the final reaction product, the RhIII-acyl2 species.
The temperature and iodomethane concentration dependence
of the oxidative addition (first) reaction step as given by Reaction
(2) is illustrated in Fig. 3 for [Rh(bap)(CO)(PPh3)] (7), [Rh(bab)-
(CO)(PPh3)] (8) and [Rh(bav)(CO)(PPh3)] (9). Plots of kobs vs.
[CH3I] are linear with non zero intercepts (Fig. 3), indicating the
oxidative addition reactions to be first order in iodomethane and
hence second order overall. This linear dependence on concentra-
tion of iodomethane is characteristic of oxidative addition to d8
transition-metal complexes [19]. The non zero intercept implies
0
0.01
0.02
0.03
0.04
0.05
0.06
[CH3I] / mol dm-3
0.006
0.004
0.002
0
-8.5
T
k
-9.0
-9.5
35.0°C
-10.0
0.0032 1/T0/.0K03-14
25.0°C
13.7°C
k
0.00
0.02
0.04
0.06
0.08
[CH3I] / mol dm-3
Fig. 3. Temperature and iodomethane concentration dependence for the oxidative
addition of iodomethane to [Rh(bap)(CO)(PPh3] (7) (top), [Rh(bab)(CO)(PPh3] (8)
(middle) and [Rh(bav)(CO)(PPh3] (9) (bottom) as monitored on the UV/Vis
spectrophotometer in chloroform at 340 nm for the first reaction {RhI +
CH3I ꢀ [RhIII-alkyl1 ꢀ RhIII-acyl1]} (oxidative addition step). Inset: Linear depen-
dence between ln (k1/T) and 1/T, as predicted by the Eyring equation.
that the k step in Reaction (5) exists. The rate constants and
ꢀ1
the activation parameters, which were obtained from the temper-
ature dependence study utilizing Eq. (14), are summarized in Table
3. The large negative values obtained for the first step for
D
S# are
consistent with an interpretation that the transition state for the
forward step 1 reaction occurs via an associative mechanism,
where the coordination number changes from 4 to 6.
alkyl complexes and ca 150 ppm for RhIII-acyl complexes
[18,20,21]. Selected fragments of 1H and 31P NMR spectra, recorded
during the oxidative addition of iodomethane to [Rh(bap)-
(CO)(PPh3)] (7) in CDCl3, are given in Figs. 4 and 5, respectively.
The same reaction sequence, as observed on IR and UV/Vis, was ob-
served on the 1H NMR and the 31P NMR. The new feature intro-
duced by the NMR study, is the existence of two structural
isomers for each reaction intermediate in Reaction (5). This is ex-
pected, since the RhI complexes exist of two structural isomers A
and B (Scheme 2) which are in a fast equilibrium with each other.
The two observed isomers of each intermediate will be referred to
as A and B, e.g. RhIII-alkyl1A and RhIII-alkyl1B. The choice of the la-
bels is arbitrary and has no significance, see Figs. 4 and 5. The rate
constants, obtained for the A and B structural isomers of a specific
reaction intermediate, were within experimental error the same
suggesting that a fast equilibrium between the A and B structural
isomers exists for each reaction intermediate, according to Reac-
tion (5). Taking into account that two main isomers exist for each
2.2.4. 1H and 31P NMR study
The reaction between iodomethane and the rhodium(I) com-
plexes of this study was also monitored by 1H and 31P NMR in order
to obtain additional insight into this reaction. NMR spectroscopy is
an excellent technique for the identification of the different iso-
mers resulting from the oxidative addition reaction of [Rh(b-dike-
tonato)(CO)(PPh3)] with iodomethane. On the1H NMR, RhIII-alkyl
complexes of the type of [Rh(b-diketonato)(CH3)(CO)(PPh3)(I)]
can be identified by the resonance signal of the CH3-group at ca
1.4–1.7 ppm. This signal is a doublet of doublets due to coupling
of the H with Rh (spin 1/2) and P (spin 1/2). RhIII-acyl complexes
of the type of [Rh(b-diketonato)(COCH3)(PPh3)(I)] can be identified
by the sharp resonance signal of the acyl COCH3-group at ca 3 ppm
[20]. The 31P NMR of the rhodium complexes gives a doublet with
1J(31P–103Rh) = ca 170 ppm for RhI complexes, ca 120 ppm for RhIII-

264
N.F. Stuurman, J. Conradie / Journal of Organometallic Chemistry 694 (2009) 259–268
Table 3
Temperature dependent kinetic rate constants and activation parameters for the oxidative addition of iodomethane to [Rh(b-diketonato)(CO)(PPh3)] complexes 7, 8 and 9.
b-Diketonato
10 °C
25 °C
35 °C
D
H#
D
S#
D
G#a
k1 (dm3 molꢀ1 sꢀ1
)
105
k
ꢀ1
(sꢀ1
)
k1 (dm3 molꢀ1 sꢀ1
)
104
k
ꢀ1
(sꢀ1
)
k1 (dm3 molꢀ1 sꢀ1
)
104 (sꢀ1
k
ꢀ1
)
(kJ molꢀ1
)
(J Kꢀ1 molꢀ1
)
(kJ molꢀ1
)
bap
bab
bav
0.0133(6)
0.0179(4)
0.017(4)
7.1(1)
0.43(1)
4.1(1)
0.0333(9)
0.0437(1)
0.0354(8)
1.8(3)
1.5(3)
2.2(3)
0.0693(2)
0.082(1)
0.053(3)
2.9(7)
3.2(4)
2.8(1)
45.1
39.1
30.7
ꢀ121.3
ꢀ139.4
ꢀ170.0
81.3
80.7
81.3
a
At 25 °C.
reactant and reaction product, the complete reaction sequence for
the oxidative addition of iodomethane to 7, 8 or 9 is therefore
[RhI(β-diketonato)(CO)(PPh3)] isomers + CH3I
k-1 K1, k1
First
reaction
K2, k2
RhIII-alkyl1
isomers
RhIII-acyl1
isomers
k-2
Second
reaction
ð7Þ
k-3 k3
RhIII-alkyl2 isomers
Third
reaction
k-4
k4
RhIII-acyl2 isomers
The proposed chemical structures of the RhIII-products in Eq. (7)
are presented in Scheme 3. The stereochemistry of the rhodium(III)
reaction products stems from information obtained from single
crystal X-ray crystallographic, NMR and DFT computational studies
on related complexes that generally react in the same way as the
monocarbonylphosphine rhodium complexes reported in this study
[10,21]. The proposed geometry of the RhIII-alkyl1 isomers is in
agreement with the geometries of [Rh((C4H3S)COCHCOR)(CH3)-
(CO)(PPh3)(I)]-alkyl1 where R = Ph, CF3 or C4H3S which were ob-
tained by solution-NMR techniques and DFT calculations [22]. The
alkyl1 isomers thus result from trans addition of CH3I to rhodium(I),
i.e. with the CH3 group and the iodide above and below the square
planar plane of the rhodium(I) reactant. Only a possible geometry
of the RhIII-acyl1 isomers is proposed, since no corresponding struc-
ture or geometry for any related b-diketonato complex has to date
been determined by any technique. The structure of one of the
ppm
6.20 6.15 6.10 5.55 5.51 3.05 3.00 2.95 1.80 1.73 1.50 1.45
Fig. 4. Fragments of the 1H NMR spectra in CDCl3, illustrating the reaction sequence
during the oxidative addition and the following carbonyl insertion and deinsertion
reactions of 0.0238 mol dmꢀ3 [Rh(bap)(CO)(PPh3)], (7) reacting with 0.1799
mol dmꢀ3 iodomethane in CDCl3 (T = 25 °C) at the indicated times. 6.10–
6.25 ppm: The spectra illustrate the decrease (1st Reaction) of the signal of the
methine proton of the b-diketonato ligand of the RhI-A and RhI-B isomers with the
simultaneous formation (1st Reaction) and decrease (2nd Reaction) of the signals of
the methine proton of the b-diketonato ligand of the RhIII-alkyl1A and 1B, as well as
the RhIII-acyl1A and 1B isomers. 5.51–5.55 ppm: The spectra illustrate the increase
(2nd Reaction) of the signal of the methine proton of the b-diketonato ligand of the
RhIII-alkyl2A and 2B at 5.7 ppm. 2.95–3.05 ppm: The reaction sequence is also
illustrated by the increase (1st Reaction) and decrease (2nd Reaction) of the signal
of the CH3-group of the RhIII-acyl1A and 1B isomers, followed by the increase (3rd
Reaction) of the signal of CH3-group of the RhIII-acyl2A and 2B isomers. 1.7–
1.80 ppm: The increase (2nd Reaction) in the signal of the CH3-group of the RhIII
-
alkyl2A and 2B isomers). 1.40–1.50 ppm: The formation (1st Reaction) and
depletion (2nd Reaction) of the signal of CH3 group of RhIII-alkyl1A and 1B isomers.
Note the multiplet of the CH3 group of the RhIII-alkyl1 and RhIII-alkyl2 isomers is
due to coupling of the H with Rh (spin 1/2) and P (spin 1/2).
RhIII-alkyl2 isomers of
a related [Rh(b-diketonato)(CH3)(CO)
(PPh3)(I)]-alkyl2 complex, [Rh(FcCOCHCOCF3)- (CH3)(CO)(PPh3)(I)]-
alkyl2 (Fc = ferrocenyl), was crystallographically characterized [10].
The spectroscopic and spectrophotometric properties of [Rh(FcC-
OCHCOCF3)(CH3)(CO)(PPh3)(I)]-alkyl2 and the alkyl2 complexes of
this study are in agreement with each other (see Table S1 in Supple-
mentary material). This geometry is also in agreement with solu-
tion-NMR techniques and DFT calculations on the RhIII-alkyl2
isomers [Rh((C4H3S)COCHCOR)(CH3)(CO)- (PPh3)(I)]-alkyl2 with
R = Ph, CF3 or C4H3S [22]. The proposed geometries of the RhIII-acyl2
isomers where the acyl moiety is in the apical position, are in
analogous to the geometries of [Rh((C4H3S)COCHCOR)(COCH3)
(PPh3)(I)]-acyl2 and [Rh(FcCOCHCOCF3)(COCH3)(PPh3)(I)]-acyl2,
which were determined by DFT calculations [22,23]. No [Rh(b-dike-
tonato)(COCH3)(PPh3)(I)]-acyl2 has crystallographically been char-
acterized, but the crystal structures of related [Rh(L,L0-BID)
(COCH3)(PPh3)(I)]-acyl products (L,L0-BID = bidentate ligand =
(PhCOCHPPh2)ꢀ [24], mnt (HMnt = maleonitriledithiolate) [25],
dmavk (Hdmavk = dimethylaminovinylketone) [26] or stsc
(Hstsc = salicylaldehydethiosemicarbazose) [27]) all containing
the acyl moiety in the apical position indicates this geometry as
thermodynamically preferred.
50 49 48 38 37 36 35 34 33 30 29 ppm
Fig. 5. Selected 31P NMR spectra illustrating doublet 31P signals of the indicated
reactants and products during the three consecutive reactions for the oxidative
addition of iodomethane to [Rh(bap)(PPh3)(CO)], (7), in CDCl3 at 25 °C. The doublet
of each signal is due to coupling of P with Rh (spin 1/2).

N.F. Stuurman, J. Conradie / Journal of Organometallic Chemistry 694 (2009) 259–268
265
I
COCH3
I
I
O
O
O
O
O
O
O
O
O
O
CO
CO
COCH3
PPh3
CO
I
Rh
Kc1
Rh
Rh
Rh
Rh
PPh3
PPh3
CH3
PPh3
CH3
Kc1
PPh3
Kc1
R
R
R
R
R
R
R
R
R
R
k1
k2
k3
k4
Kc1
Kc1
+ CH3I
k−1
k−2
k−3
k−4
COCH3
I
I
I
O
O
O
O
O
O
O
O
O
O
PPh3
CO
PPh3
PPh3
CH3
PPh3
Rh
Rh
Rh
Rh
Rh
I
CO
CH3
COCH3
CO
PPh3
RhIII-alkyl 1
isomers
RhIII-acyl 1
isomers
RhI
isomers
RhIII-alkyl 2
isomers
RhIII-acyl 2
isomers
R = CH2CH3, CH2CH2CH3 or CH2CH2CH2CH3
Scheme 3. Mechanism for iodomethane oxidative addition and CO migratory insertion in monocarbonylphosphine complexes of the type [Rh((C6H5)COC-
HCO((CH2)nCH3))(CO)(PPh3)] indicating the proposed structures of the rhodium(III) reaction intermediates and –products.
2.2.5. Reactivity correlation: iodomethane oxidative addition to [Rh(b-
diketonato)(CO)(PPh3)] complexes vs. addition to the Monsanto
catalyst
The rate-determining step in the rhodium-iodide catalyzed
reaction of methanol to acetic acid, is the oxidative addition of
iodomethane to [Rh(CO)2I2]ꢀ to produce the [Rh(CH3)(CO)2I3]ꢀ al-
kyl reaction intermediate that rapidly converts to an acyl
[Rh(CH3CO)(CO)I3]ꢀ according to the reaction scheme
For all complexes of the type [Rh(R1COCHCOR2)(CO)(PPh3)] it is
clear that a decrease in the group electronegativity of R1 and R2
(better electron donating power) leads to an increased reactivity
of RhI with iodomethane (Table 4), with complexes 7–9 being the
most reactive. Complexes 7–9 revealed a second order oxidative
addition rate approximately 500–600 times faster than that ob-
served for [Rh(CO)2I2]ꢀ. The rate-determining step for the oxida-
tive addition reaction of 7–9, the carbonyl insertion step, is ca 60
times faster than the rate-determining oxidative addition step of
iodomethane to [Rh(CO)2I2]ꢀ [1]. An increase in the size of the alkyl
group on the b-diketonato of [Rh((C6H5)COCHCOR)(CO)(PPh3)],
with R = (CH2)nCH3, n = 1–3, did not hamper the rate of oxidative
addition.
ð8Þ
The corresponding reaction scheme of the current study is
3. Conclusions
The reaction rate of the first oxidative addition step or the
following carbonyl insertion and deinsertion steps of [Rh((C6H5)
COCHCOR)(CO)(PPh3)] + CH3I (with R = (CH2)nCH3, n = 1–3) in
chloroform, were not influenced by the increasing alkyl chain
length of the R group on the b-diketonato ligand. However, the
electron donating properties of the R chain accelerate the oxidative
addition step 500–600 times faster than the rate-determining
oxidative addition step of iodomethane to the Monsanto catalyst
[Rh(CO)2I2]ꢀ under similar conditions [1].
ð9Þ
Table 4
Kinetic rate constants for the oxidative addition step in chloroform at 25 °C of [Rh(R1COCHCOR2)(CO)(PPh3)], as well as group electronegativities of the R substituents of the b-
diketonato ligand.
Abbreviation of b-diketonato ligand (R1COCHCOR2)
R1
R2
(vR1
+
v
R2)/(Gordy scale)a
k1f/molꢀ1 dm3 sꢀ1
k2f/ sꢀ1
Ref.
e
e
e
bab
bap
bav
dtm
bth
acac
dbm
ba
fctfa
tta
tfaa
tfba
C6H5
C6H5
C6H5
C4H3S
C4H3S
CH3
C6H5
C6H5
Fc
CF3
CF3
C6H5
CF3
CH2CH2CH3
CH2CH3
CH2CH2CH2CH3
C4H3S
Ph
CH3
C6H5
CH3
CF3
4.62
4.52
4.43
4.20
4.31
4.68
4.42
4.55
4.88
5.11
5.35
5.22
6.02
0.0437(1)
0.0333(9)
0.0354(8)
0.029(1)
0.0042(3)
0.0049(1)
0.0033(2)
f
[21]
f
0.0265(6)
0.024(3)c
0.00961b
0.00930b
0.00611(1)
0.00171(4)
0.00146b
0.00112b
0.00013(1)
0.000068d
0.0065(5)
[8]
f
[35]
[31]
[10]
[18]
[31]
[31]
[9]
–
f
f
C4H3S
CH3
CF3
–
–
hfaa
CF3
0.00048(9)
0.13
Monsanto catalyst
[1]
a
Group electronegativities of this study and Ref. [15].
Value in acetone.
Value in 1,2-dichloroethane.
In CH2Cl2 at 35 °C.
Rate constants from this study.
(k2)obs = k1[CH3I] due to fast equilibrium K2 in Eq. (9).
b
c
d
e
f

266
N.F. Stuurman, J. Conradie / Journal of Organometallic Chemistry 694 (2009) 259–268
4.2.2. [Rh(b-diketonato)(CO)2] complexes 4–6
4. Experimental
4.1. Materials and apparatus
The general procedure was as follows: [Rh2Cl2(CO)4] was pre-
pared in situ by refluxing RhCl3 ꢁ 3H2O (0.1001 g, 0.38 mmol) in
DMF (3 ml) until the color changed from red to yellow (ca
30 min) [29]. The dimer-containing solution was allowed to cool
on ice and an equivalent amount of solid b-diketone (0.38 mmol)
was slowly added while stirring. After 30 min of stirring at room
temperature, the crude product [Rh(b-diketonato)(CO)2], for com-
plex [Rh(bap)(CO)2] (4), was precipitated with an excess of water,
extracted with ether, washed with water, dried with MgSO4 and
solvent removed under reduced pressure. Recrystallization was
done with hot hexane. The crude product [Rh(b-diketonato)(CO)2],
for complexes [Rh(bab)(CO)2] (5) and [Rh(bav)(CO)2] (6), was pre-
cipitated with an excess of water, filtered, air dried and recrystal-
lized with hot hexane.
Solid reagents used in preparations (Merck, Aldrich and Fluka)
were used without further purification. Liquid reactants and sol-
vents were distilled prior to use; water was doubly distilled. Or-
ganic solvents were dried according to published methods [28].
4.2. Synthesis
4.2.1. b-Diketones 1–3
The general procedure to synthesize the phenyl-containing
b-diketones Hbap (1-phenylpentane-1,3-dione, propanylacetophe-
none, C6H5COCH2COCH2CH3) (1); Hbab (1-phenylhexane-1,
3-dione, buterylacetophenone, C6H5COCH2COCH2CH2CH3) (2) and
Hbav (1-phenylheptane-1,3-dione, valerylacetophenone, C6H5CO-
CH2COCH2CH2CH2CH3) (3) was as follows: a Schlenk setup was
used. In a three-necked round bottomed flask, with a magnetic stir-
rer bar, acetophenone (1.2015 g, 10 ml) was added in dried, freshly
distilled and degassed THF (1.0 ml). The flask was attached to a gas
flow adapter and N2 bubbler. The solution was degassed while
being stirred for 30 min. Lithium diisopropylamide (LDA) (6.0 ml
of 1.8 mol dmꢀ3 solution in hexane, 10.8 mmol) was added to the
solution at 0 °C with a syringe, under nitrogen. A color change to
clear orange showed that a slight excess of LDA was added. The
solution was allowed to stir for 10 min before an appropriate ester
(10 mmol) was slowly added into the solution at 0 °C while stir-
ring. The reaction mixture was stirred overnight under N2 atmo-
sphere. Ether (30 ml) was added into the reaction mixture and
stirred for 20 min. The resulting precipitate was filtered and
washed with ether (2 ꢄ 30 ml). Ether (20 ml) was added to the pre-
cipitate, and 0.3 M HCl (20 ml) dropped by while stirring till a pH
lower than 4 was reached. The product was extracted with ether
(2 ꢄ 50 ml). The combined ether extracts were dried with anhy-
drous MgSO4 and removed by evaporation. Silica gel column chro-
matography was used to separate the product.
[Rh(bap)(CO)2] (4): Yield: 0.0899 g, 27%. M.p. 79–81 °C. IR
(cmꢀ1) = 2065 and 2008. 1H NMR (d/ppm, CDCl3): 1.2 (3H, t,
CH3), 2.4 (2H, q, CH2), 6.2 (1H, s, CH) 7.4–7.9 (5H, m, C2H5). Ele-
mental Anal. Calc. for RhC13H11O4: C, 46.7; H, 3.3. Found: C, 46.6;
H, 3.1%.
[Rh(bab)(CO)2] (5): Yield: 0.0615 g, 17%. M.p. 53–55 °C. IR
(cmꢀ1) = 2084 and 2014. 1H NMR (d/ppm, CDCl3): 1.0 (3H, t,
CH3), 1.7 (2H, m, CH2), 2.4 (2H, t, CH2), 6.3 (1H, s, CH), 7.4–7.9
(5H, m, C2H5). Elemental Anal. Calc. for RhC14H13O4: C, 48.3; H,
3.8. Found: C, 48.2; H, 3.7%.
[Rh(bav)(CO)2] (6): Yield: 0.0657 g, 18%. M.p. 45–50 °C. 1H NMR
(d/ppm, CDCl3) 0.9 (3H, t, CH3), 1.4 (2H, m, CH2), 1.7 (2H, m,
CH2), 2.4 (2H, t, CH2), 6.3 (1H, s, CH), 7.4–7.9 (5H, m, C2H5). Ele-
mental Anal. Calc. for RhC15H15O4: C, 49.7; H, 4.2. Found: C, 49.5;
H, 4.0%.
4.2.3. [Rh(b-diketonato)(CO)(PPh3)] complexes 7–9
The general procedure was as follows: to a solution of [Rh(b-
diketonato)(CO)2] (0.2 mmol) in hot n-hexane (3 cm3) PPh3
(0.2 mmol) was added. The resulting reaction mixture was stirred
for ca 1 min, until no more CO gas was released, and filtered. Silica
gel column chromatography was used to purify the product where
necessary. Crystallographic pure crystals of the desired complexes
[Rh(bap)(CO)(PPh3)] (7), [Rh(bab)(CO)(PPh3)] (8) and [Rh(bav)-
(CO)(PPh3)] (9), were obtained after recrystallization from acetone.
[Rh(bap)(CO)(PPh3)] (7): Yield: 0.0955 g, 16%. M.p. 139–144 °C.
IR (cmꢀ1) = 1982. 1H NMR (d/ppm, CDCl3) isomer A: 1.2 (3H, t,
CH3), 2.5 (2H, q, CH2), 6.2 (1H, s, CH), 7.0–8.0 (20H, m, aromatic);
isomer B: 0.6 (3H, t, CH3), 2.0 (2H, q, CH2), 6.1 (1H, s, CH), 7.0–
8.0 (20H, m, aromatic); ratio isomer A: isomer B = 1.00: 0.69. 31P
NMR (d/ppm, CDCl3) isomer A: 48.88 (1J(31P–103Rh) = 176 Hz) iso-
mer B: 48.36 (1J(31P–103Rh) = 177 Hz). Elemental Anal. Calc. for
RhC30PH26O3: C, 63.4; H, 4.6. Found: C, 63.5; H, 4.5%.
[Rh(bab)(CO)(PPh3)] (8): Yield: 0.0732 g, 13%. M.p. 135–138 °C.
IR (cmꢀ1) = 1981. 1H NMR (d/ppm, CDCl3) isomer A: 1.0 (3H, t,
CH3), 1.8 (2H, m, CH2), 2.5 (2H, t, CH2), 6.2 (1H, s, CH), 7.0–8.0
(20H, m, aromatic); isomer B: 0.6 (3H, t, CH3), 1.2 (2H, m, CH2),
2.0 (2H, t, CH2), 6.1 (1H, s, CH), 7.0–8.0 (20H, m, aromatic); ratio
isomer A: isomer B = 1.00: 0.78. 31P NMR (d/ppm, CDCl3) isomer
A: 48.80 (1J(31P–103Rh) = 175 Hz) isomer B: 48.56 (1J(31P–103Rh) =
175 Hz). Elemental Anal. Calc. for RhC31PH28O3: C, 63.9; H, 4.8.
Found: C, 63.8; H, 4.6%.
Characterization data for Hbap (1): 1H NMR (d/ppm, CDCl3): 1.1
(3H, t, keto CH3), 1.2 (3H, t, enol CH3), 2.5 (2H, q, enol CH2), 2.6
(2H, q, keto CH2), 4.1 (2H, s, keto CH2), 6.2 (1H, s, enol CH), 7.4–
7.9 (5H, m, 2 ꢄ C6H5); 13C NMR (d/ppm, CDCl3): 198.5 (s,
COCH2CH3), 183.4 (COPh), 135.4, 132.6, 129.2, 129.0, 127.4 (s,
Ph), 95.9 (s, COCCO), 32.8 (s, CH2CH3), 10.1 (s, CH2CH3). Rf = 0.39
(ether:hexane = 1: 4). Elemental Anal. Calc. for C11H11O2: C,
75.41; H, 6.33. Found: C, 75.23; H, 6.31%.
Characterization data for Hbab (2): 1H NMR (d/ppm, CDCl3): 0.9
(3H, t, keto CH3), 1.0 (3H, t, enol CH3), 1.6 (2H, q, keto CH2), 1.7
(2H, q, enol CH2), 2.4 (2H, t, enol CH2), 2.6 (2H, t, keto CH2), 4.1
(2H, s, keto CH2), 6.2 (1H, s, enol CH), 7.5–8.0 (5H, m, 2 ꢄ C6H5);
13C NMR (d/ppm, CDCl3): 197.1 (s, COCH2CH2CH3), 179.0 (COPh),
135.5, 133.6, 129.2, 129.0, 127.4 (s, Ph), 96.6 (s, COCCO), 41.5 (s,
CH2CH2CH3), 19.7 (s, CH2CH2CH3), 14.2 (s, CH2CH2CH3). Rf = 0.34
(ether:hexane = 1:4). Elemental Anal. Calc. for C12H13O2: C, 76.17;
H, 6.92. Found: C, 76.05; H, 6.85%.
Characterization data for Hbav (3): 1H NMR (d/ppm, CDCl3): 0.9
(3H, t, keto CH3), 1.0 (3H, t, enol CH3), 1.3 (2H, m, keto CH2), 1.4
(2H, m, enol CH2), 1.6 (2H, m, keto CH2), 1.7 (2H, m enol CH2),
2.4 (2H, t, enol CH2), 2.6 (2H, t, keto CH2), 4.1 (2H, s, keto CH2),
6.2 (1H, s, enol CH), 7.5–8.0 (5H, m, 2 ꢄ C6H5); C NMR (d/ppm,
CDCl3): 197.1 (s, COCH2CH2CH2CH3), 183.4 (COPh), 135.5, 132.4,
129.0, 129.2, 127.4 (s, Ph), 96.5 (s, COCCO), 39.3 (s,
CH2CH2CH2CH3), 28.3 (s, CH2CH2CH2CH3), 22.2 (s, CH2CH2CH2CH3),
14.1 (s, CH2CH2CH2CH3). Rf = 0.58 (ether:hexane = 1:4). Elemental
Anal. Calc. for C13H15O2: C, 76.82; H, 7.44. Found: C, 76.83; H,
7.45%.
[Rh(bav)(CO)(PPh3)] (9): Yield: 0.0805 g, 14%. M.p. 139–145 °C.
IR (cmꢀ1) = 1983. 1H NMR (d/ppm, CDCl3) isomer A: 1.0 (3H, t,
CH3), 1.4 (2H, m, CH2), 1.7 (2H, m, CH2), 2.5 (2H, t, CH2), 6.2 (1H,
s, CH), 7.0–8.0 (20H, m, aromatic); isomer B: 0.7 (3H, t, CH3), 1.2
(4H, m, 2 ꢄ CH2), 2.0 (2H, t, CH2), 6.1 (1H, s, CH), 7.0–8.0 (20H,
m, aromatic); ratio isomer A: isomer B = 1.00: 0.78. 31P NMR
(d/ppm, CDCl3) isomer A: 48.84 (1J(31P–103Rh) = 176 Hz) isomer B:
48.53 (1J(31P–103Rh) = 175 Hz). Elemental Anal. Calc. for
RhC32PH30O3: C, 64.4; H, 5.1. Found: C, 64.2; H, 5.1%.

N.F. Stuurman, J. Conradie / Journal of Organometallic Chemistry 694 (2009) 259–268
267
order rate constants, k1, by determining the slope of the linear plots
of kobs against the concentration of the incoming iodomethane li-
gand. Non-zero intercepts implied that kobs = k1[CH3I] + kꢀ1 and that
4.3. Spectroscopy and spectrophotometry
NMR measurements at 25 °C were recorded on a Bruker Ad-
vance II 600 NMR spectrometer [1H (600.130 MHz) and 31P
(242.937 MHz)]. The chemical shifts were reported relative to
SiMe4 (0.00 ppm) for the 1H spectra and relative to 85% H3PO4
(0 ppm) for the 31P spectra. Positive values indicate downfield shift.
IR spectra were recorded from neat samples on a Digilab FTS 2000
infrared spectrophotometer. UV/Vis spectra were recorded on a
Cary 50 Probe UV/Vis spectrophotometer.
the k step in the proposed reaction mechanism exists.
ꢀ1
Kinetic data for the consecutive reaction treatment [28] of
k1
k3
IR data applicable to the general reaction RhI ! RhII-alkyl1 !
RhII-alkyl2 were fitted to Eq. (12) when monitoring the disappear-
ance of RhI and to Eq. (13) for monitoring the appearance and dis-
appearance of RhIII-alkyl1.
k1obs½RhIŠ
½RhIII ꢀ alkyl1Š ¼
½expðꢀk1obstÞ ꢀ expðꢀk3tފ
ð13Þ
0
t
k3 ꢀ k1obs
4.4. Acid dissociation constant (Ka) determinations
All kinetic mathematical fits were done utilizing the fitting program
MINSQ [27]. The error of all the data is presented according to crystal-
lographic conventions, for example kobs = 0.0236(1) sꢀ1 implies
The pKa values were determined by measuring the absorbance
of 0.07 mmol dmꢀ3 b-diketone solutions at different pH’s during
an acid–base titration in acetonitrile:water mixtures, 1:9 by vol-
ume,
ously [30].
kobs = (0.0236 0.0001) sꢀ1
.
l
= 0.100 mol dmꢀ3 (NaClO4) at 25.0(5) °C as described previ-
The activation parameters were determined from the Eyring
relationship [28],
4.5. Kinetic measurements
k
T
D
RT
H#
D
R
S#
kB
h
ln ¼ ꢀ
þ þ ln
ð14Þ
Oxidative addition reactions were monitored on the IR (by mon-
itoring formation and disappearance of the carbonyl peaks), on the
UV/Vis (by monitoring the change in absorbance at the indicated
wavelength) spectrophotometers and on the NMR (by monitoring
the change in integration units of the specified signals) spectro-
meter. All kinetic measurements were monitored under pseudo-
first-order conditions with [CH3I] 10–400 times the concentration
of the [Rh(b-diketonato)(CO)(PPh3)] complex in the specified
solution. The concentration [Rh(b-diketonato)(CO)(PPh3)] ffi
0.0003 mol dmꢀ3 for UV/Vis measurements, ffi0.03 mol dmꢀ3 for IR
measurements and ffi0.03 mol dmꢀ3 for NMR measurements.
Kinetic measurements, under pseudo-first-order conditions for
different concentrations of [Rh(b-diketonato)(CO)(PPh3)] at a con-
stant [CH3I], confirmed that the concentration of [Rh(b-diketo-
nato)(CO)(PPh3)] did not influence the value of the observed
kinetic rate constant. The observed first-order rate constants were
obtained from least-square fits of absorbance (IR and UV/Vis) or
integration units (NMR) vs. time data [31].
with
D
H
# = activation enthalpy,
D
S
# = activation entropy, k = rate
constant, kB = Boltzmann’s constant, T = temperature, h = Planck’s
constant, R = universal gas constant and the activation free energy
D
G#
=
D
H# ꢀ T
D
S#.
4.6.3. Calculation of thermodynamic quantities
The reaction enthalpy, DrH [33,34], for the equilibrium shown
in Scheme 2 is calculated by the van’t Hoff equation
ꢀ
ꢁ
Dr
R
H
1
ðiiÞ
1
ln KcðiiÞ ¼ ln KcðiÞ ꢀ
ꢀ
T
ðiÞ
ð15Þ
T
where ln Kc(ii) and ln Kc(i) are the equilibrium constants at temper-
atures T(ii) and T(i), R = 8.314 J Kꢀ1 molꢀ1. The thermodynamic quan-
tity ‘‘Gibbs free energy”, DrG, and reaction entropy, DrS, can be
calculated from the equations DrG = ꢀ RT ln Kc and DrG = DrH ꢀ T
DrS [30].
The [Rh(b-diketonato)(CO)(PPh3)] complexes 7, 8 and 9 were
tested for stability in chloroform by means of overlay IR and UV/
Vis spectra for at least 24 h. A 1H NMR spectrum of complexes 7, 8
and 9, after 48 h in solution of CDCl3, confirmed stability in solution.
Acknowledgements
Financial assistance by the South African National Research
Foundation under Grant number 2067416 and the Central
Research Fund of the University of the Free State is gratefully
acknowledged.
4.6. Calculations
4.6.1. Calculations of % keto isomer and Kc value of the b-diketones
The % keto tautomer and equilibrium constant, Kc applicable to
Scheme 1, were obtained from
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2008.10.040.
% keto tautomer ¼ ðI of keto signalÞ=fðI of keto signalÞ
þ ðI of enol signalÞg ꢄ 100%
ð10Þ
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