Iodomethane oxidative addition to β-diketonatobis(triphenylphosphite) rhodium(I) complexes: A synthetic, kinetic and computational study

Article abstract of DOI:10.1016/j.poly.2011.06.017

New rhodium(I)- and rhodium(III)-β-diketonato complexes of the type [Rh(FcCOCHCOR)(P(OPh)₃)₂] and [Rh(FcCOCHCOR)(P(OPh) ₃)₂(CH₃)(I)], with Fc = ferrocenyl and R = Fc, CH₃ and CF₃

Full text of DOI:10.1016/j.poly.2011.06.017

Polyhedron 30 (2011) 2345–2353
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Iodomethane oxidative addition to b-diketonatobis(triphenylphosphite)rhodium(I)
complexes: A synthetic, kinetic and computational study
Marrigje M. Conradie a, Johannes J.C. Erasmus a, Jeanet Conradie a,b,
⇑
a Department of Chemistry, University of the Free State, P.O. Box 339, 9300 Bloemfontein, South Africa
b Department of Chemistry, Centre for Theoretical and Computational Chemistry, University of Tromsø, N-9037 Tromsø, Norway
a r t i c l e i n f o
a b s t r a c t
Article history:
New rhodium(I)- and rhodium(III)-b-diketonato complexes of the type [Rh(FcCOCHCOR)(P(OPh)3)2] and
[Rh(FcCOCHCOR)(P(OPh)3)2(CH3)(I)], with Fc = ferrocenyl and R = Fc, CH3 and CF3, have been synthesized.
The reactivity of complexes of the type [Rh(b-diketonato)(P(OPh)3)2] increase in the order: b-diketo-
Received 6 April 2011
Accepted 21 June 2011
Available online 28 June 2011
nato = (CF3COCHCOCF3)ꢀ < (CF3COCHCOPh)ꢀ < (CF3COCHCOCH3)ꢀ < (PhCOCHCOPh)ꢀ < (CF3COCHCOFc)ꢀ
<
(CH3COCHCOPh)ꢀ
< < <
(CH3COCHCOCH3)ꢀ (CH3COCHCOFc)ꢀ (FcCOCHCOFc)ꢀ, giving linear relation-
Keywords:
b-Diketones
Rhodium
Oxidative addition
Phosphite
DFT
ships between the kinetic parameter ln k2 and the parameters that are related to the electron density
0
on the rhodium centre; the sum of the group electronegativities of the b-diketonato side groups (vR
+
vR
)
and the pKa of the uncoordinated b-diketone RCOCH2COR0. The large negative values of the volume and
entropy of activation indicated a mechanism which occurs via a polar transition state. A density func-
tional theory study, at the PW91/TZP level of theory, indicates that oxidative addition of iodo methane
to [Rh(FcCOCHCOCF3)(P(OCH3)3)2] occurs via a two-step mechanism. This mechanism involves a nucle-
ophilic attack by the metal on the methyl carbon to displace iodide to form a metal-carbon bond and
the coordination of iodide to the five-coordinated intermediate to give a six-coordinated trans alkyl
product.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
[13] confirmed the experimentally observed trans addition of iodo-
methane via a polar transition state, consistent with the SN2 mech-
Oxidative addition of covalent molecules (such as alkyl halides)
to unsaturated transition metal complexes, a fundamental process
in organometallic chemistry, plays a key role in many important
catalytic reactions [1]. Rhodium(I) complexes were made famous
by the use of [Rh(CO)2(I)2]ꢀ as catalyst in the Monsanto process
of converting methanol to acetic acid [2–6]. b-Diketonato com-
plexes of rhodium(I) can facilitate the formation of acyl species
during oxidative addition of iodomethane to complexes of the type
[Rh(b-diketonato)(CO)(PPh3)] [7]. However, as these complexes
contain a carbonyl ligand, it was found that most of the oxidative
addition reactions were complicated by more than one alkyl-acyl
conversion reactions [8–11]. Van Zyl [12] and co-workers reported
the first study concerned with the oxidative addition of iodometh-
ane to bisphosphite complexes of the type [Rh(b-diketo-
anism [14].
The ferrocenyl group (Fc) is a very good electron donating group
with an apparent group electronegativity Fc of 1.87 [15]. Introduc-
ing ferrocene as part of the ligand system should thus lead to an
increase in electron density on the Rh centre, enhancing the rate
of oxidative addition. The current study describes the synthesis
of, and oxidative addition of iodomethane to, three novel ferro-
v
cene-containing
complexes
of
the
type
[Rh(FcCOCH-
COR)(P(OPh)3)2] with R = CF3 (1), CH3 (2) and Fc (3). It is found
that the electron donating (electronic) properties of the ferrocenyl
group on the b-diketonato ligand FcCOCHCORꢀ speed up the
oxidative addition reaction with second-order rate constants
significantly larger than that previously reported for related
[Rh(b-diketonato)(P(OPh)3)2] complexes [12].
nato)(P(OPh)3)2]. The absence of
a carbonyl ligand in these
complexes led to an expected less complex mechanism in terms
of the absence of the carbonyl insertion step. A theoretical study
of the oxidative addition of iodomethane to [Rh(acac)(P(OPh)3)2]
2. Experimental
2.1. Materials and apparatus
⇑
Corresponding author at: Department of Chemistry, P.O. Box 339, University of
the Free State, 9300 Bloemfontein, South Africa. Tel.: +27 51 4012194; fax: +27 51
4446384.
Solid reagents used in preparations (Merck, Aldrich and Sigma)
were used without further purification. Liquid reactants and sol-
E-mail address: conradj@ufs.ac.za (J. Conradie).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.06.017

2346
M.M. Conradie et al. / Polyhedron 30 (2011) 2345–2353
H#Þ
vents were distilled prior to use, water was double distilled. DMF
was dried according to published methods [16].
r
r
ð
D
¼ Ta
ð5Þ
v
ðD
S#Þ
The activation Gibbs energy was calculated from equation [19]
2.2. Spectroscopy and spectrophotometry
D
G#
¼
D
H# ꢀ T
D
S#
ð6Þ
ð7Þ
NMR measurements, at 298 K were recorded on a Bruker
Avance DPX 300 NMR spectrometer [1H(300 MHz)]. Chemical
shifts are reported as d values relative to SiMe4 (0 ppm). UV/vis
spectra, kinetic measurements and reactions at high pressure were
followed on a scanning GBC model 916 UV/vis spectrophotometer
equipped with a thermostated Nova Swiss High pressure system
[17].
The volume of activation
D
V# is determined by [22]
lnðk2Þ ¼ ꢀð
with P = pressure.
D
V#=RTÞP þ constant
2.5. Computational chemistry
2.3. Kinetic measurements
Density functional theory (DFT) calculations were carried out
using the ADF (Amsterdam Density Functional) 2009 programme
[23–25] with the GGA (Generalized Gradient Approximation) func-
tional PW91 (Perdew-Wang) [26]. The TZP (Triple f polarised) basis
set, with a fine mesh for numerical integration, a spin-restricted
formalism and full geometry optimization with tight convergence
criteria, was used for minimum energy and transition state (TS)
searches. Approximate structures of the TS have been determined
with linear transit (LT) scans, with a constrained optimization
along a chosen reaction coordinate, to sketch an approximate path
over the TS between reactants and products. Numerical frequency
analyses [27,28], where the frequencies are computed numerically
by differentiation of energy gradients in slightly displaced geome-
tries, have been performed to verify the TS geometries. A TS has
one imaginary frequency. Finally, the TS was allowed to relax after
displacing the atoms according to the reaction coordinate as deter-
mined by the eigenvectors. Throughout, all calculations have been
performed with no symmetry constraint (C1) and all structures
have been calculated as singlet states.
Oxidative addition reactions were monitored on a UV/vis spec-
trophotometer, by monitoring the change in absorbance at the
indicated wavelength. All kinetic measurements were monitored
under pseudo-first-order conditions with the iodomethane 100–
2200 times the concentration of the [Rh(b-diketonato)(P(OPh)3)2]
complex in the specified solution. Kinetic measurements, under
pseudo-first-order conditions for different concentrations of
[Rh(b-diketonato)(P(OPh)3)2] at a constant iodomethane concen-
tration, confirmed that the concentration of [Rh(b-diketo-
nato)(P(OPh)3)2] 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 vs time data with the
aid of the fitting program MINSQ [18].
The [Rh(b-diketonato)(P(OPh)3)2] complexes (1), (2) and (3)
were tested for stability in acetone by means of overlay UV/vis
spectra for at least 24 h.
2.4. Calculations
Zero point energy and thermal corrections (vibrational, rota-
tional and translational) were made in the calculation of the ther-
modynamic parameters. The enthalpy (H) and Gibbs energy (G)
were calculated from [19]
Pseudo-first-order rate constants, kobs, were calculated by fit-
ting [18] kinetic data to the first-order equation [19]
obsꢁtÞ
At ꢀ A ¼ ðA0 ꢀ A Þeðꢀk
ð1Þ
1
1
U ¼ ETBE þ EZPE þ EIE
ð8Þ
ð9Þ
H ¼ U þ RTðgas phaseÞ or H ¼ UðsolutionÞ
G ¼ H ꢀ TS
with At , A and A0 = the absorbance of the indicated species at time
1
t, infinity and 0, respectively. The experimentally determined pseu-
do first order rate constants were converted to second order rate
constants, k2, by determining the slope of the linear plots of kobs
against the concentration of iodomethane. The error of all the data
is presented according to crystallographic conventions, for example
ð10Þ
where U is the total energy, ETBE is total bonding energy, EZPE is zero
point energy, EIE is internal energy (sum of vibrational, rotational
and translational energies), R is the gas constant, T is temperature
and S is entropy. The entropy (S) was calculated from the tempera-
ture dependent partition function in ADF at 298.15 K.
Solvent effects were taken into account for all calculations re-
ported here. The COSMO (Conductor like Screening Model) model
of solvation [29–31] was used as implemented [32] in ADF. The
COSMO model is a dielectric model in which the solute molecule
is embedded in a molecule-shaped cavity surrounded by a dielec-
kobs = 0.0236(1) sꢀ1, implies kobs = (0.0236 0.0001) sꢀ1
.
The activation parameters were determined by a least square fit
to the Eyring equation [19]
!
TkB
h
D
RT
H#
D
S#
k2 ¼
exp
ꢀ
þ
R
ð2Þ
tric medium with a given dielectric constant (
used is Esurf [33] and the solvents used are methanol (
and acetone (e0 = 20.7).
e
0). The type of cavity
as well as from two linear forms of the equation [20]
e0 = 32.6)
k2
T
D
RT
H#
D
R
S#
kB
h
ln ¼ ꢀ
þ
þ ln
ð3Þ
ð4Þ
2.6. Synthesis
and
!
k2
T
kB
h
D
R
S#
D
H#
R
2.6.1. [Rh(b-diketonato)(P(OPh)3)2] complexes 1–3
T ꢁ ln ¼ Tx ln
þ
ꢀ
The ferrocene containing b-diketones [34], [Rh(FcCOCH-
COR)(cod)] (cod = cyclooctadiene) [35] and [Rh(FcCOCHCOR)(CO)2]
complexes with R = CF3, CH3 and Fc (Fc = ferrocenyl) were prepared
as described previously [36,37]. The [Rh(FcCOCHCOR)(P(OPh)3)2]
complexes with R = CF3 (1) and CH3 (2) were prepared according
to previously published method [38] for [Rh(CF3COCH-
COPh)(P(OPh)3)2] (6). Complex [Rh(FcCOCHCOFc)(P(OPh)3)2] (3)
was prepared as follow. Solid [Rh(FcCOCHCOFc)(cod)] (0.09 mmol)
with
DH DS
# = activation enthalpy, # = activation entropy, k2 = sec-
ond order rate constant, kB = Boltzmann’s constant, T = temperature,
h = Planck’s constant, R = universal gas constant. Values determined
for
calculated standard errors obtained of
average experimental temperature [20,21] within 3 K:
DH DS
# and # using Eqs. (2)–(4) were the same, see Fig. 3(c). The
D
H# and S# relate to the
D

M.M. Conradie et al. / Polyhedron 30 (2011) 2345–2353
2347
was slowly added to a stirred solution of triphenylphosphite
(0.18 mmol) in 5 cm3 dry hexane. A few drops of THF was added
to dissolve the emulsion and stirred for another 15 min. The sol-
vent was evaporated at room temperature and complex 3 is recrys-
tallized from hexane.
2.6.2. [Rh(FcCOCHCOCF3)(P(OPh)3)2] (1)
Yield: 85%. 1H NMR (acetone-d): d 3.98 (5H, s, C5H5), 4.43 (2H, t,
C5H4), 4.63 (2H, t, C5H4) and 5.92 (1H, s, CH), 7.13–7.44 (30H, m,
6 ꢁ C6H5). Anal. Calc. for RhFeC50P2H40O8F3: C, 57.4; H, 3.9. Found:
C, 57.3; H, 3.6%.
2.6.3. [Rh(FcCOCHCOCH3)(P(OPh)3)2] (2)
Yield: 77%.1H NMR (acetone-d): d 1.50 (3H, s, CH3), 3.96 (5H, s,
C5H5), 4.24 (2H, t, C5H4), 4.53 (2H, t, C5H4) and 5.58 (1H, s, CH),
7.10–7.45 (30H, m, 6 ꢁ C6H5). Anal. Calc. for RhFeC50P2H43O8: C,
60.5; H, 4.4. Found: C, 60.5; H, 4.5%.
2.6.4. [Rh(FcCOCHCOFc)(P(OPh)3)2] (3)
Yield: 81%. 1H NMR (acetone-d): d 3.99 (10H, s, 2 ꢁ C5H5), 4.28
(4H, t, 2 ꢁ C5H4), 4.65 (4H, t, 2 ꢁ C5H4) and 6.07 (1H, s, CH), 7.20–
7.39 (30H, m, 6 ꢁ C6H5). Anal. Calc. for RhFe2C59P2H49O8: C, 61.0; H,
4.2. Found: C, 61.1; H, 4.0%.
2.6.5. [Rh(FcCOCHCOCF3)(P(OPh)3)2(CH3)(I)] complex (10)
Fig. 1. The PW91/TZP/methanol calculated minimum energy geometries of the
rhodium(I) [Rh(FcCOCHCOCF3)(P(OCH3)3)2] and the two rhodium(III) [Rh(FcCOCH-
COCF3)(P(OCH3)3)2(CH3)(I)]-alkyl reaction products, resulting from trans addition of
iodomethane to [Rh(FcCOCHCOCF3)(P(OCH3)3)2]. H atoms are removed for clarity
(except for the CH3 group bonded to the rhodium). Selected bond angles (°) and
bond lengths (Å) are as indicated.
Iodomethane (0.32 mmol,
a 2-fold excess) was added to
[Rh(FcCOCHCOCF3)(P(OPh)3)2] (1) (150 mg, 0.14 mmol) dissolved
in acetone (5 ml) at 25 °C. N2 was purged through the reaction ves-
sel before it was sealed and left at room temperature in the dark.
After 12 h the mixture was concentrated by purging with N2 and
the product (10) precipitated with 1 ml water. The precipitate
(0.161 g, 97%) was dried over phosphorous pentoxide under
vacuum.
2.6.6. [Rh(FcCOCHCOCF3)(P(OPh)3)2(CH3)(I)] (10)
Yield: 0.161 g, 97%. 1H NMR (acetone-d): d 1.87 (3H, s, CH3),
4.31 (5H, s, C5H5), 4.33 (H, m, C5H4), 4.38 (H, m, C5H4), 4.43 (H,
m, C5H4), 4.63 (H, m, C5H4), and 6.13 (1H, s, CH), 7.05–7.31 (30H,
m, 6 ꢁ C6H5). Anal. Calc. for RhFeC50P2H43O8F3I: C, 51.0; H, 3.7.
Found: C, 49.9; H, 3.6%.
3. Results and discussion
3.1. Synthesis
The synthesis and oxidative addition kinetics of iodomethane to
three novel ferrocene-containing [Rh(b-diketonato)(P(OPh)3)2]
complexes is described and compared to a series of six known
[Rh(b-diketonato)(P(OPh)3)2] complexes, see Scheme 1 for the
complex numbering.
[Rh(FcCOCHCOR)(P(OPh)3)2] complexes with R = CF3 (1), CH3
(2) and Fc (3) were prepared as shown in Scheme 2. The [Rh(FcC-
OCHCOR)(P(OPh)3)2] complexes with R = CF3 (1) and CH3 (2) were
prepared from the corresponding [Rh(FcCOCHCOR)(CO)2] com-
plexes by dropwise adding a solution of [Rh(FcCOCHCOR)(CO)2]
in acetone to a stoichiometic amount of freshly distilled trip-
henylphosphite in hexane. Complex [Rh(FcCOCHCOFc)(P(OPh)3)2]
(3) can only be obtained in low yields utilizing this route. Complex
(3) was obtained in high yield by slowly adding solid [Rh(FcCOCH-
COFc)(cod)] to a stoichiometic amount of freshly distilled trip-
henylphosphite in hexane. The rhodium(III) complex (10) was
obtained adding a 2-fold excess of iodomethane to (1). No solid
state X-ray crystal structure has been solved for any [Rh(b-diketo-
nato)(P(OPh)3)2(CH3)(I)] products [39]. The stereochemistry of a
related complex [Rh(CH3COCHCOCH3)(P(OPh)3)2(CH3)(I)] on
Fig. 2. The DFT calculated relative energies of the 12 possible [Rh(FcCOCH-
COCF3)(P(OCH3)3)2(CH3)(I)]-alkyl reaction products of the [Rh(FcCOCH-
COCF3)(P(OCH3)3)2] + CH3I reaction. Values calculated in methanol as solvent are
in bold followed by the relative energies calculated in acetone as solvent in bold
italics. A1 and A2 (lowest energies) result from trans addition, B, C and D represent
the various cis isomers. The energy of the reactants is taken as zero.

2348
M.M. Conradie et al. / Polyhedron 30 (2011) 2345–2353
0.006
0.004
0.002
0
-3.1
(a)
(b)
35.5°C
-3.4
25.0°C
k
-3.7
k
11.1°C
-4.0
-4.3
0.00
0.05
0.10
0.15
0.20
0.25
0
20
40
60
80
100
106P/Pa
[CH3I] / mol dm-3
(c)
-9.0
-9.5
-2800
-2900
-3000
-3100
ΔH# =40.00(53) kJmol-1
ΔH# =40.02(52) kJmol-1
ΔS# =-145.4(1.8) Jmol-1K-1
ΔS# =-145.3(1.8) Jmol-1K-1
T
T
k
k
-10.0
-10.5
-11.0
0.04
0.03
0.02
0.01
T
0.00
k
T/K
280 290 300 310 320
0.0032
0.0033
0.0034
0.0035
0.0036
280
290
300 310 320
T / K
1/T / K-1
Fig. 3. (a) Temperature and iodomethane concentration dependence for the oxidative addition of iodomethane to [Rh(FcCOCHCOCF3)(P(OPh)3)2] (1), as monitored on the UV/
vis spectrophotometer in acetone at 353 nm. [Rh(FcCOCHCOCF3)(P(OPh)3)2] = 1.022 ꢁ 10ꢀ4 mol dmꢀ3. (b) Pressure dependence of ln k2 for the reaction of [Rh(FcCOCH-
COCF3)(P(OPh)3)2] (1) with iodomethane in acetone at 25.4(1) °C at 353 nm. [Rh(FcCOCHCOCF3)(P(OPh)3)2] = 1.128 ꢁ 10ꢀ4 mol dmꢀ3 and [CH3I] = 0.120 mol dmꢀ3. (c) Eyring
plots according to Eqs. (2)–(4).
ground of 1H NMR data, is found to adopt an octahedral geometry
in which the b-diketonato ligand and the two triphenylphosphite
groups are located in an equatorial plane with the methyl and io-
dide ligands in the axial positions [12]. The trans addition of iodo-
methane to [Rh(CH3COCHCOCH3)(P(OPh)3)2] was supported by a
density functional theory (DFT) computational study of the transi-
tion state and product of the oxidative addition reaction [13]. The
DFT study also showed that simplified model systems for the oxi-
dative addition reaction [Rh(CH3COCHCOCH3)(P(OPh)3)2] + CH3I,
viz. [Rh(CH3COCHCOCH3)(P(OCH3)3)2] and [Rh(CH3COCHC-
OCH3)(P(OH)3)2], give a good account of the Rh-L experimental
bond lengths and relative stability of both the rhodium(I) and rho-
dium(III) b-diketonatobis(triphenylphosphite) complexes. Results
of a DFT study of the simplified rhodium(I) complex [Rh(FcCOCH-
COCF3)(P(OCH3)3)2] (1s) of complex (1) is thus presented to give
more insight in the oxidative addition reaction, transition state
and reaction products of ferrocene containing [Rh(b-diketo-
nato)(P(OPh3)3)2] complexes.
isomers of the formula [Rh(FcCOCHCOCF3)(P(OCH3)3)2(CH3)(I)]
are possible, see Fig. 2. Two isomers, A1 and A2 result from trans
addition of iodomethane, entering at the top or the bottom side
of the square planer plane formed by the two O atoms of the (FcC-
OCHCOCF3)ꢀ ligand and the two P(OCH3)3 groups. Ten isomers re-
sult from cis addition or from re-arrangement of a trans addition
product.
The ADF optimized relative electronic energies of the 12 possi-
ble rhodium(III) oxidative addition product isomers of the formula
[Rh(FcCOCHCOCF3)(P(OCH3)3)2(CH3)(I)] are also displayed in Fig. 2.
These energies indicate (in agreement with experimental and DFT
results for [Rh(CH3COCHCOCH3)(P(OR)3)2] with R = H, CH3 or Ph)
that the trans alkyl product isomers (Alkyl-A1 and Alkyl-A2) are
the most stable products of oxidative addition of iodomethane to
R'
R
R
Fc CF3
Fc CH3
Fc Fc
CF3 CF3
CF3 CH3
CF3 Ph
CH3 CH3
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
O
O
P(OPh3)
P(OPh3)
RhI
3.2. DFT study
R'
The ADF optimized geometry of the simplified complex (1s) is
presented in Fig. 1. Complex 1s is square planar with the ferrocenyl
Fc =
FeII
CH3 Ph
Ph Ph
moiety oriented with its (g
5–C5H4) ring approximately co-planar
with the Rh coordination plane. Oxidative addition of iodomethane
to [Rh(FcCOCHCOCF3)(P(OCH3)3)2] leads to a rhodium(III) alkyl
product. Twelve possible rhodium(III) oxidative addition product
Scheme 1. The [Rh(b-diketonato)(P(OPh)3)2] complexes described in this study, b-
diketonato = (RCOCHCOR0)ꢀ with R and R0 as indicated.

M.M. Conradie et al. / Polyhedron 30 (2011) 2345–2353
2349
significant amount of steric repulsion between the iodide and
methyl ligands and the P(OCH3)3 groups, similar as was found for
the I–Rh–I angle of the rhodium(III) alkyl complex [RhIII(b-diketo-
nato)(P(OPh)3)2(I)2] (169.4°) [42] and for the CH3–Rh–I angle
RhCl .3H O
3
2
DMF
of
trans-iodo-methyl-(N-benzoyl-N-phenylhydroxyamino-O,O0)-
bis(triphenylphosphite)-rhodium(III) (174.2°) [41].
OC
OC
Cl
Cl
CO
CO
Cl
Cl
1/2
Rh
Rh
Fe
1/2
Rh
Rh
3.3. Kinetics of ferrocene containing [Rh(b-diketonato)(P(OPh)3)2]
complexes
O
O
R
O
O
R
The UV/vis spectra of the oxidative addition reactions between
the metal complex and iodomethane showed only one reaction for
(1)–(3). The kinetic results showed very simple second order kinet-
ics indicating the direct formation of the rhodium(III) alkyl product
from the rhodium(I) complex. All the results showed a linear
dependence on the iodomethane concentration and all the inter-
cepts were calculated to be zero within experimental error (see
Figs. 3–5). These results are consistent with a very simple reaction
mechanism:
Fe
R
O
CO
CO
R
Rh
CO(g)
O
O
I
Rh
O
Fe
Fe
2 P(OPh)
P(OPh)
- 2 CO
R
3
2 P(OPh)
R
R
3
CH
Rh
3
O
O
O
O
P(OPh)
P(OPh)
P(OPh)
3
3
k
2
O
Rh
R
3
3
+CH I
3
Rh
P(OPh)
3
3
O
O
P(OPh)
P(OPh)
3
P(OPh)
O
I
Rh
3
Fe
Fe
Fe
(1) R = CF
Fe
3
(3) R = Fc
(2) R = CH
3
CH I
3
The rate law for this reversible reaction is given by Eq. (11)
R ¼ k2½Rhðb ꢀ diketonatoÞðPðOPhÞ3Þ2ꢂ½CH3Iꢂ
The observed rate constant, after integration and using pseudo-
first-order conditions is given by Eq. (12)
ð11Þ
R
CH
3
O
O
P(OPh)
3
3
Rh
I
P(OPh)
kobs ¼ k2½CH3Iꢂ
ð12Þ
Fe
The values for k2 was calculated from plots of kobs vs [CH3I] for
the different complexes and are reported in Table 1. Kinetic rate
constants and activation parameters for the oxidative addition of
iodomethane to [Rh(b-diketonato)(P(OPh)3)2] complexes (4)–(9)
(Scheme 1) are also included for comparative reasons.
(10) R = CF
3
Scheme 2. Synthetic route for the synthesis of [Rh(b-diketonato)(P(OPh)3)2)]
complexes (1)–(3) and [Rh(FcCOCHCOCF3)(P(OPh)3)2(CH3)(I)] (10) from RhCl3.3H2O
and appropriate b-diketone.
[Rh(FcCOCHCOCF3)(P(OCH3)3)2]. Trans addition of iodomethane to
the experimental complex [Rh(FcCOCHCOCF3)(P(OPh)3)2] (1) is
thus also expected. Trans addition of iodomethane to (N-benzoyl-
N-phenylhydroxyamino-O,O0)-bis(triphenylphosphite)-rhodium(I)
[40] was confirmed by the X-ray crystal structure of trans-iodo-
methyl-(N-benzoyl-N-phenylhydroxyamino-O,O0)-bis(trip-
henylphosphite)-rhodium(III) [41]. Oxidative addition of I2 to
[Rh(CH3COCHCOCF3)(P(OPh)3)2] also gave the trans product [42].
DFT calculations on the structurally similar rhodium–b-diketonato
complexes, [Rh(b-diketonato)(CO)(PPh3)] with b-diketone = 4,4,4-
3.4. Activation parameters
The activation parameters
D D
H# and S#, obtained from a vari-
able temperature study (see Fig. 3(c) and the insets in Figs. 4 and
5 and Table 1) show no marked variation with the character of
the b-diketone.
except complex (3) with a larger
D D
H# and S# values for all complexes are similar,
D
H# and a less negative S#.
D
The average experimental temperature of (3) is 15° while the aver-
age experimental temperature of all the other complexes of Table 1
is ca. 25°. The variations in
DH D
# and S# did not have an effect on
D
The
tive
G
# that varies between 68 and 94 kJ molꢀ1 for complexes (1)–(9).
DH
trifluoro-1-(2-thenoyl)-1,3-propanedione,
1-phenyl-3-(2-the-
# values are relatively small, compensating by highly nega-
# values, both of which may be considered as characteristics
S# values indi-
noyl)-1,3-propanedione, 1,3-di(2-thenoyl)-1,3-propanedione [43]
1-ferrocenyl-4,4,4-trifluorobutane-1,3-dione [44] or acetylacetone
[45] in agreement with experimental observations [9–11], also
found trans addition of CH3I to the square planar rhodium(I)
complexes.
Fig. 1 visualizes the optimized geometries of the simplified rho-
dium(III) [Rh(FcCOCHCOCF3)(P(OCH3)3)2(CH3)(I)]-alkyl A1 and A2
complexes. The geometry of rhodium(III) is octahedral as expected.
Alkyl A1 (Alkyl A2) has an I (CH3) in the apical position on the same
side as the ferrocenyl group. The calculated CH3-Rh-I angle of A1
and A2 is 173.1° and 170.4°, respectively, orientated in a direction
away from the OPh groups. These angles indicate that there is a
D
S
of an associative mechanism. The large negative
D
cate an increase in the coordination number during the formation
of the transition state.
The volumes of activation (
(1), (2) and (3) with iodomethane in acetone, is determined from
pressure dependent study. The results are illustrated in
Figs. 3(b)–5(b) and summarized in Table 1.
# may be considered
as the sum of two components i.e.
V#intr, which represents the
change in volume due to changes in the coordination number,
bond lengths and bond angles; and
V#solv, which represents the
D
V#), for the reaction of complexes
a
D
V
D
D

2350
M.M. Conradie et al. / Polyhedron 30 (2011) 2345–2353
The DFT calculated Gibbs energy profile of the simplified model
reaction [Rh(FcCOCHCOCF3)(P(OCH3)3)2] + CH3I is presented in
Fig. 6 and the activation parameters are summarized in Table 2. Val-
change in volume due to changes in solvation (electrostriction),
during formation of the transition state. During associative oxida-
tive addition, an overall volume decrease is expected because of
bond formation with the metal centre (
D
V#intr) and increasing elec-
ues calculated in methanol (
e0 = 32.6) or acetone (e0 = 20.7) as sol-
trostriction (
D
V#solv), due to a polar transition state involving het-
vent, are very similar. The calculated activation Gibbs energies,
D
G#, of the two transition states of the simplified model leading
erolytic cleavage of the H3C–I bond. The relatively small value of
to alkyl A1 and A2 (80 and 62 kJ molꢀ1, respectively, in acetone as
solvent) are in good agreement with the experimental value of
D
V
# of ca ꢀ20 cm3 molꢀ1 (and corresponding smaller
D
V#intr values)
is in agreement with a transition state where only one bond breaks.
A mechanism via a three centre concerted mechanism leads to
84 kJ molꢀ1. The calculated
D
G# for the simplified [Rh(CH3COCHC-
OCH3)(P(OCH3)3)2] + CH3I reaction, 60 kJ molꢀ1, was also in good
D
V#intr values of ꢀ30 cm3 molꢀ1 and larger [46]. Results of this
agreement with the experimental value, 78 kJ molꢀ1
,
for the
study with
D
V
# between ꢀ23 and ꢀ17 cm3 molꢀ1 is thus in agree-
[Rh(CH3COCHCOCH3)(P(OPh)3)2] + CH3I reaction, although the cal-
culated values for the full experimental system gave the exact
experimental value of 78 kJ molꢀ1 [12,13].
ment with the intimate two-step SN2 mechanism similar to that
previously presented van Zyl and co-workers [12] for the oxidative
addition of iodomethane to complexes (4)–(9).
R
R
CH
Rh
3.5. Relationships involving the chemical oxidation of [Rh(b-
diketonato)(P(OPh)3)2] complexes
+
3
O
O
P(OPh)
P(OPh)
3
3
k
-
O
O
2
P(OPh)
3
3
Rh
I
+ CH I
3
slow
P(OPh)
R'
The Rh(I) nucleus acts as a nucleophile when undergoing oxida-
tive addition. It is thus to be expected that anything that affects the
nucleophilicity (or electron-richness) of the central coordinating
metal, will influence the rate of oxidative addition reactions. From
the results reported in Table 1, it is clear that complex (4) having
two electron withdrawing CF3 side groups on the b-diketonato li-
gand exhibit the slowest rate of oxidative addition, while with
the rate of oxidative addition of complex (3) with two electron
donating ferrocenyl side groups on the b-diketonato ligand is ca
740 times faster than that of (4). In fact, the rate of oxidative addi-
tion of complex (3) is faster than that of all known [Rh(b-diketo-
nato)(P(OPh)3)2] (Table 1) and [Rh(b-diketonato)(CO)(PPh3)]
complexes [47]. It is ca 2600 times more reactive than the anionic
R'
fast
CH
R
3
O
O
P(OPh)
3
3
Rh
I
P(OPh)
R'
The proposed first step may be represented as:
R
R
[Rh(CO)2I2]ꢀ
with
δ+
δ−
Monsanto
carbonylation
catalyst,
O
O
P(OPh)
P(OPh)
O
O
P(OPh)
3
3
k2 = 0.000068 molꢀ1 dm3 sꢀ1 in CH2Cl2 at 35 °C [1]. The effect of
the electron donating properties of the substituents R and R0 of
the b-diketonato ligand on the reaction rate is shown in Fig. 7(a).
The sum of the group electronegativities of R and R0 of the b-dike-
tonato ligand (RCOCHCOR0)ꢀ, coordinated to complexes of the type
[Rh(b-diketonato)(P(OPh)3)2], therefore gives a good indication of
the electron density (nucleophilicity) of rhodium in each complex.
The effect of the pKa value of the various b-diketones on the oxida-
tive addition reaction rate is shown in Fig. 7(b). A b-diketonato
Rh
+ CH I
Rh
CH
3
I
3
3
P(OPh)
3
R'
R'
R
CH
3
+
I
-
O
O
P(OPh)
3
3
Rh
P(OPh)
R'
0.012
0.008
0.004
0
-7.0
-8.0
(a)
-0.8
(b)
T
25.0°C
k
-9.0
-1.1
-10.0
0.0033
0.0035
0.0037
1/T / K-1
-1.4
k
15.5°C
5.8°C
k
-1.7
-2.0
0.00
0.02
0.04
0.06
0.08
0
20
40
60
80
100
[CH3I] / mol dm-3
106P/Pa
Fig. 4. (a) Temperature and iodomethane concentration dependence for the oxidative addition of iodomethane to [Rh(FcCOCHCOCH3)(P(OPh)3)2] (2), as monitored on the UV/
vis spectrophotometer in acetone at 360 nm. [Rh(FcCOCHCOCH3)(P(OPh)3)2] = 1.023 ꢁ 10ꢀ4 mol dmꢀ3. Inset: linear dependence between ln(k2/T) and 1/T, as predicted by the
Eyring equation. (b) Pressure dependence of ln k2 for the reaction of [Rh(FcCOCHCOCH3)(P(OPh)3)2] (2) with iodomethane in acetone at 25.1(1) °C at 360 nm.
[Rh(FcCOCHCOCH3)(P(OPh)3)2] = 1.209 ꢁ 10ꢀ4 mol dmꢀ3 and [CH3I] = 0.007 mol dmꢀ3
.

M.M. Conradie et al. / Polyhedron 30 (2011) 2345–2353
2351
0.020
0.015
0.010
0.005
0.000
-1.0
(a)
-6.5
-7.0
-7.5
-8.0
-8.5
(b)
T
34.8°C
k
-1.2
0.0032
0.0034
0.0036
1/T / K-1
k
-1.4
-1.6
-1.8
25.0°C
k
16.3°C
0
20
40
60
106P/Pa
80
100
120
0.00
0.02
0.04
[CH3I] / mol dm-3
0.06
Fig. 5. (a) Temperature and iodomethane concentration dependence for the oxidative addition of iodomethane to [Rh(FcCOCHCOFc)(P(OPh)3)2] (3), as monitored on the UV/
vis spectrophotometer in acetone at 373 nm. [Rh(FcCOCHCOFc)(P(OPh)3)2] = 1.204 ꢁ 10ꢀ4 mol dmꢀ3. Inset: linear dependence between ln(k2/T) and 1/T, as predicted by the
Eyring equation. (b) Pressure dependence of ln k2 for the reaction of [Rh(FcCOCHCOFc)(P(OPh)3)2] (3) with iodomethane in acetone at 25.0(1) °C at 373 nm.
[Rh(FcCOCHCOFc)(P(OPh)3)2] = 1.204 ꢁ 10ꢀ4 mol dmꢀ3 and [CH3I] = 0.008 mol dmꢀ3
.
Table 1
Kinetic rate constants and activation parameters for the oxidative addition of iodomethane to [Rh(b-diketonato)(P(OPh)3)2] complexes, as monitored on the UV/vis
spectrophotometer in acetone at 25 °C.
H#/
D
S#/
D
G#/
D
V#/
Reference for
k2
b
R0
(
vR
+
v
R )/(Gordy
pKa
k2/
D
0
b-
R
Diketonato
scale)a
dm3 molꢀ1 sꢀ1
kJ molꢀ1
JKꢀ1 molꢀ1
kJ molꢀ1
cm3 molꢀ1
dfcm
fca
dbm
acac
ba
fctfa
tfaa
tfba
hfaa
Fc
Fc
Fc
CH3
3.74
4.21
13.1
0.176(6)
46.4(5.6)
62.31(83)
49(2)
40(3)
30(3)
40.00(53)
47(3)
51(2)
ꢀ104(19)
ꢀ51.6(2.9)
ꢀ115(7)
77
78
83
78
68
84
85
88
94
ꢀ17.4(9)
ꢀ23(1)
–
this study
this study
[12]
[12]
[12]
this study
[12,48]
[12]
10.01 0.153(3)
9.35 0.0149(3)
8.95 0.096(2)
8.70 0.0342(5)
6.56 0.0157(3)
C6H5 C6H5 4.42
CH3
C6H5 CH3
Fc
CF3
C6H5 CF3
CF3 CF3
CH3
4.68
4.55
4.88
5.35
5.22
6.02
ꢀ128(9)
ꢀ18.7(10)
ꢀ127(20)
ꢀ145.4(1.8)
ꢀ129(8)
–
CF3
CH3
ꢀ21(1)
ꢀ14.5(8)
ꢀ19.3(6)
–
6.3
0.0056(1)
6.30 0.00263(5)
4.71 0.00024(3)
ꢀ123(8)
56(3)
ꢀ126(10)
[12]
a
0
The group electronegativity (vR
+
v
R ) is calculated on the Gordy scale, with
vCF3 = 3.01 [34],
vCH3 = 2.34 [49],
vPh = 2.21 [34],
v
Fc = 1.87 [34].
b
Acid dissociation constant pKa from Refs. [50,51,34].
Fig. 6. The PW91/TZP/methanol calculated Gibbs energies of the two transition states of trans addition and the trans-iodomethane-[Rh(FcCOCHCOCF3)(P(OCH3)3)2(CH3)(I)]-
alkyl reaction products A1 and A2 of the [Rh(FcCOCHCOCF3)(P(OCH3)3)2] + CH3I reaction. The energy of the reactants is taken as zero.
ligand with a lower pKa will remove electron density from the Rh
nucleus, making it a weaker nucleophile and thus less reactive to-
wards oxidative addition. Linear relationships are observed be-
tween the kinetic parameter ln k2 and the parameters that are
0
related to the electron density on the rhodium centre, vR
and pKa:
+ vR

2352
M.M. Conradie et al. / Polyhedron 30 (2011) 2345–2353
Table 2
DFT calculated thermodynamic data of the reactants, TS and products during the oxidative addition reaction [Rh(FcCOCH-
COCF3)(P(OCH3)3)2] + CH3I and [Rh(CH3COCHCOCH3)(P(OCH3)3)2] + CH3I. All energy values are given compared to the reactant.
Values calculated in methanol as solvent are given first, followed by calculated values in acetone as solvent in bold italics.
Experimental activation parameters for [Rh(FcCOCHCOCF3)(P(OPh)3)2] + CH3I are given in brackets.
D
H
298K/kJ molꢀ1a
D
G
298K/kJ molꢀ1b
DS
298K/JKꢀ1 molꢀ1c
[Rh(FcCOCHCOCF3)(P(OCH3)3)2] + CH3I
Reactant
TS_A1
TS_A2
Product (alkyl-A1)
Product (alkyl-A2)
0 0
0 0
0 0
30 29 {38.6(6)}d
21 18 {38.6(6)}d
ꢀ70 ꢀ72
81 80 {84}d
65 62 {84}d
ꢀ3 ꢀ5
ꢀ170 ꢀ170 {ꢀ151(2)}d
ꢀ148 ꢀ148 {ꢀ151(2)}d
ꢀ226 ꢀ226
ꢀ77 ꢀ79
ꢀ13 ꢀ16
ꢀ212 ꢀ212
[Rh(CH3COCHCOCH3)(P(OCH3)3)2] + CH3Ie
Reactant
TS
Product (alkyl-A)
0
0
0
22 {40(3)}f
60 {78}f
ꢀ125 {ꢀ128(9)}f
ꢀ180
ꢀ56
ꢀ2
a
b
c
d
e
f
Enthalpy, calculated by Eq. (9).
Gibbs energy, calculated by Eq. (10).
Entropy, calculated from the temperature dependent partition function in ADF at 298 K.
Experimental values from this study.
Calculated thermodynamic data from Ref. [13].
Experimental activation parameters from Ref. [12].
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
0
-1
(a)
(b)
(3)
(3)
(2)
(2)
(7)
-2
(7)
-3
(8)
(8)
(9)
-4
(1)
(6)
(1)
(9)
k
k
-5
(5)
(5)
(6)
-6
-7
(4)
-8
(4)
-9
-10
-10
4
6
8
10
12
14
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
p
Ka
(χR1+χR2)(Gordy scale)
Fig. 7. (a) Linear relationship between ln k2, the second-order rate constant for the oxidative addition of iodomethane to [Rh(RCOCHCOR0)(P(OPh)3)2] and the sum of the
group electronegativities of R and R0, (vR v0R), of the b-diketonato ligand coordinated to rhodium complexes. Complex numbers are shown in Scheme 1. (b) Relationship
+
between ln k2 and the pKa of the corresponding free b-diketone. Results of this study are indicated in red (online version). Data is in Table 1. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)
ln k2 ¼ ꢀ2:92ðvR
þ
vR0 Þ þ 9:86 ðR2 ¼ 0:86Þ
Acknowledgements
lnk2 ¼ 0:73 pKa ꢀ 10:13 ðR2 ¼ 0:75Þ
The above relationships allow for the design of ligands that can
enhance the rate of oxidative addition.
The National Research Foundation of South Africa and the Cen-
tral Research Fund of the University of the Free State (Bloemfon-
tein) are acknowledged for funding.
Appendix A. Supplementary data
4. Conclusions
Cartesian coordinates (in Å) of the structures associated with
this study. Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.poly.2011.06.017.
The rate of oxidative addition of iodomethane to [Rh(b-diketo-
nato)(P(OPh)3)2] complexes is related to the electron density on
the rhodium(I) centre as expressed by the sum of the group
electronegativities of the b-diketonato side groups or the pKa of
the uncoordinated b-diketone. The order of the effect of the b-
diketonato ligand on the reactivity of the complexes is (slow-
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