Steric vs. electronic anomaly observed from iodomethane oxidative addition to tertiary phosphine modified rhodium(i) acetylacetonato complexes following progressive phenyl replacement by cyclohexyl [PR3 = PPh3, PPh2Cy, PPhCy2 and PCy3]

Article abstract of DOI:10.1039/b922083f

Rhodium(i) acetylacetonato complexes of the formula [Rh(acac)(CO)(PR ₃)] [acac = acetylacetonate, PR₃ = PPh₃1, PCyPh₂2, PCy₂Ph 3, PCy₃4] were synthesized and the iodomethane oxidative addition to these complexes were studied. Spectroscopic and low temperature (100 K) single crystal X-ray crystallographic data of the rhodium complexes (1-4) indicate a systematic increase in both steric and electronic parameters of the phosphine ligands as phenyl groups on the tertiary phosphine are progressively replaced by cyclohexyl groups in the series. Second order rate constants for the alkyl formation in the oxidative addition of iodomethane in dichloromethane at 25 °C vary with approximately one order-of-magnitude from 6.98(6) × 10^-3 M^-1s ^-1 (PCyPh₂2) to 55(1) × 10^-3 M ^-1 s^-1 (PCy₂Ph 3) and do not follow the expected electronic pattern from 1 to 4, which indicates a flexibility of the cyclohexyl group, significantly influencing the reactivity. Activation parameters for the reactions range from 35(3) to 44(1) kJ mol^-1 for ΔH^≠ and -140(5) to -154(9) J K^-1 mol^-1 for ΔS^≠, and are supporting evidence for an associative activation for the oxidative addition step.

Full text of DOI:10.1039/b922083f

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www.rsc.org/dalton | Dalton Transactions
Steric vs. electronic anomaly observed from iodomethane oxidative addition to
tertiary phosphine modified rhodium(I) acetylacetonato complexes following
progressive phenyl replacement by cyclohexyl [PR3 = PPh3, PPh2Cy, PPhCy2
and PCy3]†
Alice Brink, Andreas Roodt,* Gideon Steyl and Hendrik G. Visser*
Received 22nd October 2009, Accepted 23rd April 2010
First published as an Advance Article on the web 17th May 2010
DOI: 10.1039/b922083f
Rhodium(I) acetylacetonato complexes of the formula [Rh(acac)(CO)(PR3)] [acac = acetylacetonate,
PR3 = PPh3 1, PCyPh2 2, PCy2Ph 3, PCy3 4] were synthesized and the iodomethane oxidative addition
to these complexes were studied. Spectroscopic and low temperature (100 K) single crystal X-ray
crystallographic data of the rhodium complexes (1–4) indicate a systematic increase in both steric and
electronic parameters of the phosphine ligands as phenyl groups on the tertiary phosphine are
progressively replaced by cyclohexyl groups in the series. Second order rate constants for the alkyl
formation in the oxidative addition of iodomethane in dichloromethane at 25 ◦C vary with
approximately one order-of-magnitude from 6.98(6) ¥ 10-3 M-1s-1 (PCyPh2 2) to 55(1) ¥ 10-3 M-1 s-1
(PCy2Ph 3) and do not follow the expected electronic pattern from 1 to 4, which indicates a flexibility of
the cyclohexyl group, significantly influencing the reactivity. Activation parameters for the reactions
range from 35(3) to 44(1) kJ mol-1 for DHπ and -140(5) to -154(9) J K-1 mol-1 for DSπ, and are
supporting evidence for an associative activation for the oxidative addition step.
Over the years, numerous studies have been undertaken to
Introduction
investigate the formation of various products in the oxida-
The rhodium- and iodide-catalysed process for the carbonylation
tive addition reactions between iodomethane and [Rh(BID)-
(CO)(PR3)].3,4,7–10 Both the cis and trans addition products of
of methanol to produce acetic acid was developed in the late
sixties by the Monsanto company.1 To date, it is still one of the
most successful industrial applications of homogeneous catalysis
and produces several million tons of acetic acid per year. The
iodomethane on the rhodium metal centre have been isolated.
[RhI(cupf)(CO)(CH3)(PPh3)]8 formed the cis addition product
while [RhI(OX)(CO)(CH3)(PPh3)]4 formed the trans alkyl species.
iodomethane oxidative addition reaction is the rate-determining
When the bidentate ligand contained a sulfur atom, as in the case
of [Rh(Sacac)(CO)(PPh3)],9,10 the primary product reported was
an acyl species. In this case the rate at which the Rh(I) complex
disappeared was equal to the formation of the acyl species. It was
later shown that the same mechanism is operative;11 the process
being significantly influenced by the steric properties induced by
the bidentate ligand.5 The oxidative addition of iodomethane to
1 has previously been reported by Basson et al.3 and the spectral
characteristics discussed by Varshavsky et al.7
Although different bidentate and phosphine ligand variations
were reported during the past two decades as presented above,5,6,12
the stepwise interchange of phenyl for cyclohexyl groups has not
been studied to date. Thus, we report here the oxidative addition
of iodomethane to [Rh(acac)(CO)(PR3)] (acac = acetylacetonate)
complexes by systematically manipulating the steric and electronic
parameters of the phosphorous ligands in a series of phosphines
ranging from triphenylphosphine (PPh3) to tricyclohexylphos-
phine (PCy3) (Fig. 1).
step of the catalytic cycle and a desire to obtain a faster catalyst
by substituting a carbonyl ligand in [Rh(CO)2I2]- with a more
electron-donating ligand, such as phosphines, is often expressed.2
The manipulation of the reactivity of the Rh(I) metal centre
in [Rh(BID)(CO)(PR3)] type of complexes (BID = different
monocharged O,O; N,O; O,S and N,S donor atom bidentate
ligands3–6 PR3 = different monodentate tertiary phosphine lig-
ands) in oxidative addition reactions has been reported previously.
The rates of these reactions are influenced by the steric and
electronic parameters of the non-participating ligands. Large,
sterically demanding ligands bonded to the Rh(I) metal centre can
inhibit the ease of the iodomethane moiety entering the rhodium
co-ordination sphere and decrease the catalytic activity, while
ligands with strong electron-donating abilities enhance the rates
of these reactions.2,3,5
Acetylacetonate was selected as symmetric bidentate ligand
to ensure the minimum number of isomers present during the
reaction process.
Crystallographic determinations of the [Rh(acac)(CO)(PR3)]
complexes with PR3 = PPh3 and PCy3 have been reported before
by Leipoldt et al.13 and Trzeciak et al.14 Recently, we also reported
Department of Chemistry, University of the Free State, P.O. Box 339,
Bloemfontein, 9300, South Africa. E-mail: roodta@ufs.ac.za; Fax: +27
(0)51 4446384; Tel: +27 (0)51 401 2547
† Electronic supplementary information (ESI) available: Additional tables
and figures. CCDC reference numbers 752115 and 752116. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b922083f
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The use of IR spectroscopy in determining the electron-donor
ability of phosphorous ligands have been illustrated with a variety
of phosphite and phosphine rhodium and iridium complexes.
Phosphite ligands, such as P(OPh)3 are known as better p-
acceptors and poorer s-donors compared to PPh3.17,22 The v(CO)
value of [Rh(b-diketonato)(CO)(P(OPh)3)] is observed around
2000 cm-1, while the values of analogous phosphine complexes17
are found at about 1970 cm-1. The use of even stronger p-acceptor
ligands18,19 such as P(NC4H4)3, increases the v(CO) frequency
even further to 2012 cm-1. The weaker s-donor and better p-
acceptor properties of P(OPh)3 may be further illustrated by the
formation of [Rh(b-diketonato)(P(OPh)3)2] complexes. Both CO
groups of [Rh(b-diketonato)(CO)2] complexes may be substituted
by P(OPh)3 due to the weakened Rh–C bond in the [Rh(b-
diketonato)(CO)(P(OPh)3)] complexes17,18 and since P(OPh)3 has
much less steric demand than PPh3, it allows the Rh(I) metal centre
to more easily accommodate two P(OPh)3 ligands in this fashion.
Fig. 1 Monodentate tertiary phosphine ligands utilized in [Rh(acac)-
(CO)(PR3)] for complexes 1 to 4.
NMR spectra
31P-NMR spectra of all the complexes were obtained in
dichloromethane at 25.0 ◦C. The value of the coupling constant,
d, increases systematically from complex 1 to 4 (Table 1), whereas
the crystal structures of the complexes of 2 and 3,15,16 but to ensure
a proper solid state comparison, all four the complexes’ structures
were redetermined at low temperature to carefully evaluate bond
length changes. These crystallographic results are also briefly
reported here.
1
1
the JRh–P shows a corresponding decrease. The value of JRh–P
,
according to the Fermi contact term, is very sensitive to changes
in the overlap between the 5s(Rh)-3s(P) orbitals.5 The decreasing
1JRh–P value suggests that the overlap between the orbitals decrease
and the Rh–P bond distance will increase with the gradual
substitution of a phenyl group by a more spatially demanding
cyclohexyl ring.6
Results and discussion
Synthesis
The 31P-NMR data, together with the IR results thus clearly
show that the systematic replacement of a phenyl group by a
cyclohexyl group increases the electron-density on the metal centre
for the systematic progression from 1 to 4.
The tertiary phosphine series PPh3, PCyPh2, PCy2Ph and PCy3
react with [Rh(acac)(CO)2] in a similar way as previously
reported,17,18 and rapid substitution of the first CO ligand
occurs. High yields were obtained with the synthesis of the
[Rh(acac)(CO)(PR3)] complexes, using octane as a solvent. The
use of acetone as a solvent, with regards to complexes 2 and 3,
resulted in decomposition to an oil except when synthesized under
Schlenk conditions.
X-ray crystallography
The crystal structures for 1 to 4 were determined at 100 K and the
basic data collection parameters are reported in the S. Table 1 (see
electronic supplementary information, ESI).† The steric demand
of the phosphine ligands in this study, was quantified by the
effective Tolman cone angle (qE), using the actual Rh–P bond
IR spectra
Infrared data for 1 to 4 were recorded in both the solid and
solution state and are presented in Table 1. The carbonyl stretching
frequencies, v(CO), decrease (1977.6 cm-1 to 1945.3 cm-1) for
complexes 1 to 4, indicating the expected systematic increase
in electron density on the rhodium(I) metal centre (increased
d-p backbonding) ability as the phenyl is stepwise replaced by
cyclohexyl.
distance.20 A van der Waals radius of 1.2 A for hydrogen and C–H
˚
˚
˚
bond distances of 0.97 A for CH2 and 0.93 A for CH were used. In
solution, the ligand substituents orientation might differ, resulting
in a variation in cone angle size.21 The effective cone angles (qE),
all indicate an expected increase in steric demand of the phosphine
ligands from 1 to 4 (Table 2). The qE values of 149.3 and 169.5◦ for
PPh3 and PCy3 ligands correspond well to the published Tolman22
cone angles, qT, of 145 and 170◦, determined according to the
Table 1 Summary of spectroscopic data for [Rh(acac)(CO)(PR3)]
complexes
˚
definition using a Ni–P bond distance of 2.28 A.
Table 2 Selected structural properties of [Rh(acac)(CO)(PR3)]
PR3
(1)
(2)
(3)
(4)
PR3 Effective Cone angle, qE/◦ Rh–P/A
Rh-O2/A
v(CO)/cm-1a
v(CO)/cm-1b
d (ppm)
1977.6
1977.0
48.6
1959.3
1971.2
53.3
1948.8
1967.4
58.8
1945.3
1959.7
59.3
˚
˚
˚
C1-O1/A
IR
31P NMR
(1)
(2)
(3)
(4)
149.3
151.2
163.5
169.5
2.2418(9) 2.0747(16) 1.153(3)
2.2327(6) 2.0764(17) 1.149(4)
2.2425(9) 2.0788(16) 1.151(3)
2.2537(4) 2.0879(10) 1.157(2)
1JRh–P/Hz
177.0
171.3
168.3
164.3
a Neat samples, ATR. b In CH2Cl2.
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The calculated density of the reported structures decrease
systematically from 1.565 to 1.400 g cm-1 from 1 to 4 and confirms
further, a progressive increase in overall spatial occupation in
the unit cell and asymmetric unit as the ligand size increases.
This is further manifested in the increase in volume of the
asymmetric unit; = [(unit cell volume)/Z], from 523, 557, 587,
3
˚
to 606 A , for PPh3, PCyPh2, PCy2Ph and PCy3, respectively
(S.Table 1†).
The Rh-O2 bond distances increase, although not strictly
systematically, from complex 1 to 4 (Fig. 2, Table 2), indicating
the increase in the trans influence of the phosphine ligands with
the increase in steric demand. The Rh–P bond distance varies
˚
≡
between 2.2418(9) and 2.2537(4) A, and the C O bond distance
˚
between 1.149(4) and 1.157(2) A. The rest of the crystallographic
≡
results from the crystal structure data, i.e. Rh–P and C O bond
distances, do not show the same systematic tendencies as were
observed with the IR and 31P-NMR spectroscopy. This could be
ascribed to packing effects and hydrogen bonding interactions.15,16
It is however interesting to note that the Rh–P bond distance in
Fig. 3 Infrared spectral changes observed during oxidative addition of
MeI to [Rh(acac)(CO)(PPh3)] in CH2Cl2. Spectra indicate the disappear-
ance of Rh(I) species with simultaneous increase of the alkylI peak. The
acyl peak formation corresponds to the disappearance of the alkylI species.
[Rh(acac)(CO)(PPh3)] = 3.12 ¥ 10-3 M, [CH3I] = 0.165 M, Dt = 16.5 s,
kobs = 6.20(9) ¥ 10-3 s-1.
˚
[Rh(acac)(CO)(PCyPh2)] is the shortest at 2.2327(6) A of the four
complexes 1–4, potentially supporting an electron rich rhodium(I)
centre.
Fig.
2
Atom numbering scheme for crystallographic data of
[Rh(acac)(CO)(PR3)] complexes as indicated by complex 3.
Kinetics
Fig. 4 Observed 31P-NMR spectra for the disappearance of the four
co-ordinated [Rh(acac)(CO)(PPh3)] species and the formation of the alkylI ,
acyl and alkylII intermediate products in CH2Cl2. [Rh(acac)(CO)(PPh3)] =
3.583 ¥ 10-2 M, [CH3I] = 0.156 M, Dt = 3 min, kobs = 5.3(8) ¥ 10-3 s-1.
The kinetic results of the four [Rh(acac)(CO)(PR3)] complexes
were surprisingly different. The formation of the expected Rh(III)-
alkylI species for the four [Rh(acac)(CO)(PR3)] complexes were
observed with all the spectroscopic methods reported. However,
after this first step, the reactions proceed via slightly different
pathways (S.Fig. 1†).
The oxidative addition of CH3I, as determined by IR and
31P NMR, to [Rh(acac)(CO)(PPh3)] (1977.6 cm-1) (Fig. 3 and 4)
resulted in the rapid formation of an alkylI product (2071.5 cm-1;
32.8 ppm), followed by a slower reaction attributed to acyl
formation (1720.5 cm-1, 37.7 ppm). As the alkylI started to
disappear, the acyl was still increasing to a maximum absorbance.
31P NMR scans of the same reaction indicated that after acyl
formation (37.7 ppm), a Rh(III)-alkylII (29.5 ppm) slowly began to
form after ca. 28 min; this was not observed by the IR spectra.
The geometry of these two alkyl species cannot be unequivocally
defined nor concluded from the current NMR spectra. However,
all solid state structures of reactants and products to date suggest
that the carbonyl ligand always lies in the equatorial plane
as defined by the bidentate ligand, and trans to one of the
bidentate ligand (O,O for acac types or O,N for Schiff base types)
donor atoms, with the iodido ligand in the apical position.8,12,23–29
Moreover, this solid state data indicate that the P-donor ligand
and the methyl group sometimes switch positions, as expected
for either a cis (via a concerted three-centred process) or a trans
(via a linear transition state/ionic process) oxidative addition
reaction. This observed positional interchange as observed in the
thermodynamic ground state structures, might be either due to the
mode of initial coordination or subsequent formal isomerisation
via e.g. an acyl species. Although there are significant changes in
the chemical shift values of the Rh(III)-alkylI and Rh(III)-alkylII
species in this study, the 1JRh–P values differ only by approximately
3–6 Hz, see S.Table 2.† This suggests the exclusion of a trans
CH3–Rh–P orientation in the alkyl species in this study observed
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by 31P NMR, since the methyl group has a much larger trans
influence than an oxygen donor atom from the acac backbone or
an iodido ligand. This will most probably result in a smaller 1JRh–P
than the ca. 116–124 Hz for all the species found in this study.5,6
Furthermore, the conversion from alkylI to finally alkylII suggests
moving to a more thermodynamic stable state and thus a lower
energy for alkylII , in agreement with preliminary computational
studies,30,31 which showed a ca. 10 to 28 kJ mol-1 energy difference
between the geometry of the Rh(III)-alkylI compared to that
of the Rh(III)-alkylII , as depicted in Fig. 6. However, it must
again be noted from these preliminary computational studies that
seemingly small steric differences may have a significant influence
on relative distributions of species. The complete computational
study is underway, but for the purpose of this paper, the geometries
for Rh(III)-alkylI (apical trans I–Rh-CH3 orientation) and Rh(III)-
alkylII (apical trans I–Rh–P orientation) are assumed as indicated
in Fig. 6.
formation of an alkylI species (2067.7 cm-1; 35.8 ppm) which was
not succeeded by acyl formation (Fig. 5). NMR spectra showed
the formation of an alkylII species (46.3 ppm) after 24 h. No acyl
species formation occurred during this time period.
The reaction of [Rh(acac)(CO)(PCy2Ph)] indicated the slow
formation of an alkylI intermediate (2052.3 cm-1; 43.0 ppm)
followed by an even slower forming acyl species after ca. 37 min
(38.1 ppm). After 3 h another alkyl species attributed to a Rh(III)-
alkylII isomer (41.7 ppm) was observed (Fig. 6).
The observed data for the reaction of [Rh(acac)(CO)(PCy3)]
with CH3I suggests the formation of an alkylI intermediate
(2052.3 cm-1; 50.2 ppm) followed by the formation of a second
alkylII isomer (45.0 ppm) after 12 min, and then the formation of
an acyl species (40.2 ppm) after 28 min. (S.Fig. 1†)
The oxidative addition of iodomethane to Rh(I) complexes
proceed in general as an equilibrium reaction, confirmed by
Roodt et al.5 At low iodomethane concentrations a small amount
of alkylI is formed from the Rh(I) reactant, while at higher
[CH3I], more of the alkylI species is formed and at a much faster
rate.
Plots of the observed first-order rate constants (kobs) versus
iodomethane concentrations, for the oxidative addition to the
[Rh(acac)(CO)(PR3)] complexes, yielded linear relationships with
non-zero intercepts for the tertiary phospine series. A typical
example of such a plot for the formation of the alkylI species
for complex 3 is shown in Fig. 7. Similar plots were obtained for
the other PR3 complexes. (S.Fig. 2–4†)
Fig. 5 Observed 31P-NMR spectra for the disappearance of the four
co-ordinated [Rh(acac)(CO)(PCyPh2)] species and the formation of the
alkylI and alkylII products in CH2Cl2 at 25 ◦C. Free phosphine, PCyPh2
was added as an internal standard at 33.6 ppm. [Rh(acac)(CO)(PCyPh2)] =
3.44 ¥ 10-2 M, [CH3I] = 0.156 M, Dt = 0.9 min, kobs = 9.7(5) ¥ 10-3 s-1.
Fig. 7 Temperature and [CH3I] dependence of the pseudo-first order
rate constant for the formation of [Rh(acac)I(CH3)(CO)(PCy2Ph)] in
dichloromethane, UV/vis; (l = 323 nm) [Rh(acac)(CO)(PCy2Ph)] = 9.10 ¥
10-5 M.
The solid lines represent the least-squares fits of the kobs data
versus [CH3I] at three different temperatures. The results are
consistent with the following rate expression of a simple two step
reversible reaction (Fig. 6):5,11
kobs = k1[CH3I] + k-1
(1)
where k1 and k-1 are the rate constants for the oxidative addi-
tion and reductive elimination steps, respectively. Similar linear
relationships were obtained for iodomethane oxidative addition
to complexes 1, 2 and 4. The disappearance of the Rh(I) species
and formation of the Rh(III)-alkylI intermediate was monitored
Fig. 6 General simplified reaction scheme for the iodomethane oxidative
addition to complexes (1) and (3).
IR spectra for the oxidative addition of CH3I to
[Rh(acac)(CO)(PCyPh2)], on the other hand, indicated the
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Table 3 UV-Vis kinetic data for the oxidative addition of CH3I
to [Rh(acac)(CO)(PR3)] complexes at different temperatures in
dichloromethane
˚
(at 2.2327(6) A) of the four complexes 1–4, potentially supporting
an electron rich rhodium(I) centre (Table 2).
Other examples of manipulating electron density and steric
interaction have been explored with [Rh(cacsm)(CO)(PR3)]5 com-
plexes (cacsmH = cyclohexyl 2-(methylamino)-1-cyclopentene-
1-dithiocarboxylato) with different mono-dentate phosphine
ligands where R = phenyl (Ph), para-chlorophenyl (p-cPh), para-
methoxyphenyl (p-mPh) and cyclohexyl (Cy). The first three
phosphines in this latter series P(p-cPh)3, PPh3 and P(p-mPh)3 are
sterically virtually identical with the same cone angle22 of 145◦,
but have increasing Brønsted pKa values33 (1.03, 2.73 and 4.57)
indicating an increase in their s-donating ability. The values for
k1 increased across the series, indicating that the driving factor
for the oxidative addition depended on the nucleophilicity of the
phosphines. The greater nucleophilicity of the PCy3 ligand (pKa =
9.7) suggests that k1 should be significantly larger than the other
three complexes, butitis ca. three-four orders of magnitude smaller
than the other three complexes. In this case it seemed as if the larger
steric demand of the PCy3 ligand (cone angle = 170◦) completely
overshadowed the electronic effect.
Rate Constant
103 k1/M-1s-1
103 k-1/s-1
T/◦C
IRa
UV-Vis
IRa
UV-Vis
K1b/M-1
(1)
(2)
(3)
(4)
24.9
15.1
5.3
26.0
14.0
6.0
25.6
14.5
5.5
25.6
14.3
5.9
30.9(3)
30.8(5)
17.2(4)
9.71(4)
55(1)
27.2(4)
18.6(2)
6.98(6)
3.38(4)
1.88(4)
27.1(2)
13.8(2)
9.1(1)
1.0(2)
1.4(4)
0.07(2)
0.6(4)
1.1(2)
0.5(1)
0.30(1)
0.9(4)
1.2(1)
0.64(8)
0.08(2)
0.11(2)
0.07(2)
0.29(9)
0.45(6)
0.15(5)
27(4)
31(7)
33(1)
52.9(8)
6.92(4)
26.4(8)
59(26)c
22(2)
29(4)
90(24)c
31(4)
27(6)
92(29)c
31(4)
62(20)c
a IR kinetic data determined at 25 ◦C. b Values calculated for UV-Vis data;
K1 = k1/k-1. c Since the intercept indicated a large esd, values should be
treated as estimations.
It is therefore concluded from the current study too that even
small changes at or around the metal centre may significantly affect
the rate of oxidative addition and therefore the catalytic ability of
these types of catalysts, sometimes in orders-of-magnitude. For
example Roodt et al.5 varied the ring-size of the bidentate ligand
in different [Rh(BID)(CO)(PR3)] and observed that a small change
of ca. 10◦ decrease in bite angle from the six-membered to the five-
membered chelates significantly affected the stability of the alkyl
intermediate.5
by IR and NMR spectroscopy and gave identical observed rate
constants within experimental error. The kinetics as studied by
UV/Vis spectroscopy gave similar results (Table 3).
A recent paper of Muller et al.32 reported the oxidative addition
of SeCN- to tertiary phosphine ligands with the corresponding
formation of SePVR3. A systematic increase in k1 for the forward
rate of almost 50 times, moving progressively from the weaker
electron donating PPh3 to PCy3, was observed. This is in agreement
with the electronic properties as discussed above and illustrated
However, the significant effect observed in this current study is
still quite surprising.
An interesting additional observation concerns the stability
constants (K1) for the formation of the intermediate Rh-alkylI
species. Although not that convincing due to the large e.s.d.’s of
the reductive elimination constants (k-1; Y-intercepts), it seems as
if there is not a similar variation in the K1 values (Table 4). If
this had been the case, one would have expected a much larger
steric effect on the thermodynamic equilibrium than the virtually
independence of the K1 values shown.
The activation parameters of the [Rh(acac)(CO)(PR3)] com-
plexes in this study range from 35(3) to 44(1) kJ mol-1 for DHπ and
-140(5) to -154(9) J K-1 mol-1 for DSπ, and the latter in particular,
supports an associative activation for the formation of the Rh-
alkylI (Fig. 8, Table 4).
1
by both the v(CO) and JRh–P data, see Table 1. Our results are
surprising and do not show the same systematics at all and the
steric vs. electronic correlation is severely distorted. In fact, the
value of k1 for [Rh(acac)(CO)(PCyPh2)] is the highest at 52.9(8) ¥
10-3 M-1s-1 and that of [Rh(acac)(CO)(PCy2Ph)] is the lowest at
6.92(4) ¥ 10-3 M-1s-1. The value of k1 for [Rh(acac)(CO)(PCy3)]
of 26.4(8) ¥ 10-3 M-1s-1 is the second slowest, completely op-
posite to the expected. This could of course be attributed to
the fact that there is a metal centre and other ligands with
possible inter- and intra-molecular interactions involved in our
case.
Clearly, the steric effect by the stepwise introduction of the
cyclohexyl group is more subtle and allow also these cyclohexyl
groups to utilize its flexibility to assume sterically favourable/or
unfavourable conformations, thereby significantly influencing the
reactivity at the rhodium(I) centre. Furthermore, the subtle
differences in the steric and electronic properties of the tertiary
phosphine ligand also shows a significant influence with regard to
product formation (see 31P NMR data). The latter observation
is however the subject of another study, which also includes
significant theoretical calculations on the energies associated with
this process.
The significant difference in relative reactivities of the
[Rh(acac)(CO)(PR3)] complexes toward iodomethane oxidative
addition is also clearly evident from the Eyring plots shown in
Fig. 8.
Table 4 Summary of spectroscopic and kinetic data for the four-
coordinated [Rh(acac)(CO)(PR3)] complexes in solution and associated
k1 values at 25 ◦C in dichloromethane
v(CO)/
d
31P
1JRh–P
/
103 k1/ DHπ (k1)/ DSπ (k1)/
cm-1
NMR/ppm Hz
M-1s-1
kJ mol-1
J K-1mol-1
It is however quite significant that an almost one order-of-
magnitude difference is observed from [Rh(acac)(CO)(PCyPh2)]
to that of [Rh(acac)(CO)(PCy2Ph)]. Moreover, as indicated above,
it is interesting to note that the Rh–P bond distance obtained
from the structural data of [Rh(acac)(CO)(PCyPh2)] is the shortest
(1) 1977.6 48.6
(2) 1959.3 53.3
(3) 1948.8 58.8
(4) 1945.3 59.3
177.0
171.3
168.3
164.3
30.8(5) 38(1)
55(1) 35(3)
6.98(6) 44(1)
27.1(2) 36(3)
-146(4)
-152(9)
-140(5)
-154(9)
5576 | Dalton Trans., 2010, 39, 5572–5578
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[Rh(acac)(CO)(PPh3)] (1)
Synthesised from [Rh(acac)(CO)2] (50 mg, 0.19 mmol) and free
phosphine, PPh3 (56 mg, 0.21 mmol), following general ligand
substitution method to yield (1) (72 mg, 77%). vmax(CH2Cl2)/cm-1
1977.0 (CO); (neat) 1977.6 (CO); dH(300 MHz; CDCl3; Me4Si)
2.08 (3H, s, Me), 1.60 (3H, s, Me), 5.42 (1H, s, 6-H), 7.66, 7.42,
7.37 (15H, m, Ph); dP(121.495 MHz; CH2Cl2; H3PO4) 48.6 (d, J
176.9).
[Rh(acac)(CO)(PCyPh2)] (2)
Synthesised from [Rh(acac)(CO)2] (80 mg, 0.31 mmol) and free
phosphine, PCyPh2 (96 mg, 0.35 mmol), following general ligand
substitution method to yield (2) (102 mg, 66%). vmax(CH2Cl2)/cm-1
1971.2 (CO); (neat) 1959.3 (CO); dH(300 MHz; CDCl3; Me4Si) 2.05
(3H, s, Me), 1.77 (3H, s, Me), 5.44 (1H, s, 6-H), 7.72, 7.39 (10H,
m, Ph), 1.75–1.12, 2.72 (11H, m, Cy); dP(121.495 MHz; CH2Cl2;
H3PO4) 53.3 (d, J 171.3).
Fig. 8 Eyring plots of the k1 rate constant (formation of [Rh(acac)I-
(CH3)(CO)(PR3)]) in CH2Cl2.
Conclusion
[Rh(acac)(CO)(PCy2Ph)] (3)
A systematic study on the effect of gradual substitution of phenyl
rings by cyclohexyl groups in rhodium acetylacetonato phosphine
complexes is reported. The spectroscopic results show a systematic
increase of the electronic and steric effects of the phosphine ligands
when moving from complex 1 to 4. However, the kinetic results of
the oxidative addition of CH3I to these complexes did not follow
a systematic pattern. This suggests that very small changes to the
steric and electronic properties of Rh(I) complexes and catalysts
could significantly (by orders-of-magnitude) affect the behaviour
of these systems. A complete computational study of these factors
are underway.
Synthesised from [Rh(acac)(CO)2] (62 mg, 0.24 mmol) and free
phosphine, PCy2Ph (73 mg, 0.26 mmol), following general ligand
substitution method to yield (3) (97 mg, 80%). vmax(CH2Cl2)/cm-1
1967.4 (CO); (neat) 1948.8 (CO); dH(300 MHz; CDCl3; Me4Si) 2.09
(3H, s, Me), 1.81 (3H, s, Me), 5.46 (1H, s, 6-H), 7.79, 7.39 (5H,
m, Ph), 1.70–1.14, 2.41 (22H, m, Cy); dP(121.495 MHz; CH2Cl2;
H3PO4) 58.8 (d, J 168.3).
[Rh(acac)(CO)(PCy3)] (4)
Synthesised from [Rh(acac)(CO)2] (52 mg, 0.20 mmol) and free
phosphine, PCy3 (62 mg, 0.22 mmol), following general ligand
substitution method to yield (4) (91 mg, 89%). vmax(CH2Cl2)/cm-1
1959.7 (CO); (neat) 1945.3 (CO); dH(300 MHz; CDCl3; Me4Si)
2.05 (3H, s, Me), 1.86 (3H, s, Me), 5.43 (1H, s, 6-H), 2.11, 1.99,
1.83–1.26 (33H, m, Cy); dP(121.495 MHz; CH2Cl2; H3PO4) 59.3
(d, J 164.3).
Experimental
General procedures
All reagents used for synthesis and characterisation were of
analytical grade. If nothing else is stated, all commercially available
reagents were used as received from Sigma-Aldrich. All organic
solvents were dried and distilled before use. Rhodium trichloride
hydrate (RhCl3·xH2O) was purchased from Next Chimica, South
Africa, and used without further purification.
X-ray diffraction analysis
The reflection data were collected on a Bruker X8 Apex II 4 K
Kappa CCD diffractometer with APEX2 software35 utilizing
COSMO36 for optimum collection of more than a hemisphere
of reciprocal space. The frames were integrated using a narrow-
frame integration algorithm and reduced with the Bruker SAINT-
PLUS37 and XPREP37 software packages. Data were corrected
for absorption effects using the multi-scan technique SADABS.38
The structure was solved by the direct methods package SIR9739
and refined using the WINGX software package40 incorporating
SHELXL.41 The program DIAMOND42 was used for all graphical
representation of the crystal structures. All structures are shown
with thermal ellipsoid drawn at 50% probability level.
Synthesis of [Rh(acac)(CO)(PR3)] (1–4)
Tetracarbonyldichloridodirhodium, [Rh(m-Cl)(CO)2]2, was syn-
thesised according to the method developed by McCleverty
and Wilkinson.34 [Rh(acac)(CO)2] was synthesized by mixing a
solution of acetylacetone (0.2 g, 2.2 mmol) in DMF to a similar
solution of [Rh(m-Cl)(CO)2]2 (0.4 g, 1.0 mmol). Upon addition of
ice water, the complex precipitated and filtered (0.4 g, 79%). Ligand
substitution of the [Rh(acac)(CO)2] complex was performed
by dissolving in octane followed by the slow addition of the
appropriate PR3 ligand as previously described.14–16 The mixture
was stirred and the light yellow precipitate filtered. Samples
suitable for X-ray diffraction were obtained by recrystallization
from acetone.
Kinetic measurements
The oxidative addition of iodomethane to complexes 1 to 4 were
monitored by repetitive IR and 31P NMR scans at 25 ◦C. IR
spectra were recorded on a Digilab FTS 2000 Fourier transform
spectrometer utilizing a He–Ne laser at 632.6 nm equipped with a
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temperature cell regulator ( 0.3 ◦C). UV-Vis kinetic measure-
ments were conducted on a Varian Cary 50 Conc. spectrometer
equipped with a Julabo F12-mV temperature cell regulator
(accurate within 0.1 ◦C) in a 1.00 cm quartz tandem cuvette
cell. Temperature variations for UV-Vis kinetic experiments were
performed between 5 and 25 ◦C. The 31P NMR spectra were
obtained on a Bruker AXS300 spectrometer at 25 ◦C. 31P
chemical shifts are reported relative to 85% H3PO4 (0 ppm) as
external standard; positive shifts are downfield. The observed
first-order rate constants were obtained from least-squares fits of
absorbance versus time data, under pseudo-first order conditions,
with [Rh]tot. = 1 ¥ 10-4 M. All kinetic measurements were
conducted in dichloromethane unless otherwise indicated.
12 J. Conradie, G. J. Lamprecht, A. Roodt and J. C. Swarts, Polyhedron,
2007, 26, 5075.
13 J. G. Leipoldt, S. S. Basson, L. D. C. Bok and T. I. A. Gerber, Inorg.
Chim. Acta, 1978, 26, L35.
14 A. M. Trzeciak, B. Borak, Z. Ciunik, J. J. Zio´łkowski, M. F. C. Guedes
da Silva and A. J. L. Pombeiro, Eur. J. Inorg. Chem., 2004, 1411.
15 A. Brink, A. Roodt and H. G. Visser, Acta Crystallogr., Sect. E: Struct.
Rep. Online, 2007, 63, m2831.
16 A. Brink, A. Roodt and H. G. Visser, Acta Crystallogr., Sect. E: Struct.
Rep. Online, 2006, 63, m48.
17 A. M. Trzeciak and J. J. Zio´łkowski, Inorg. Chim. Acta, 1985, 96, 15.
18 W. Simanko, K. Mereiter, R. Schmid, K. Kirchner, A. M. Trzeciak and
J. J. Zio´łkowski, J. Organomet. Chem., 2000, 602, 59.
19 A. M. Trzeciak, T. Głowiak, R. Grzybek and J. J. Zio´łkowski, J. Chem.
Soc., Dalton Trans., 1997, 1831.
20 S. Otto, A. Roodt and J. Smith, Inorg. Chim. Acta, 2000, 303, 295.
21 G. Ferguson, P. J. Roberts, E. C. Alyea and M. Khan, Inorg. Chem.,
1978, 17, 2965.
22 C. A. Tolman, Chem. Rev., 1977, 77, 313.
Acknowledgements
23 K. G. van Aswegen, J. G. Leipoldt, I. M. Potgieter, G. J. Lampecht, A.
Roodt and G. J. van Zyl, Transition Met. Chem., 1991, 16, 369.
24 A. Elduque, Y. Garee´s, L. A. Oro, M. T. Pinillos, A. Tiripicchio and F.
Ugozzoli, J. Chem. Soc., Dalton Trans., 1996, 2155.
25 L. J. Damoense, Ph.D. Thesis, University of the Free State, 2000.
26 Y. S. Varshavsky, M. R. Galding, T. G. Cherkasova, I. S. Podkorytov,
A. B. Nikol’skii, A. M. Trzeciak, Z. Olejnik, T. Lis and J. J. Zio´lkowski,
J. Organomet. Chem., 2001, 628, 195.
Financial assistance from the South African National Research
Foundation (NRF), THRIP, SASOL, and Research Fund of the
University of the Free State is gratefully acknowledged. Part of
this material is based on work supported by the South African
National Research Foundation (GUN 2038915). Opinions, find-
ings, conclusions or recommendations expressed in this material
are those of the authors and do not necessarily reflect the views of
the NRF.
27 M. Cano, J. V. Heras, M. A. Lobo, E. Pinilla and M. A. Monge,
Polyhedron, 1992, 11, 2679.
28 L. J. Damoense, W. Purcell and A. Roodt, Rhodium Express, 1995, 14,
4.
29 G. Steyl, Polyhedron, 2007, 26, 5324.
30 G. Steyl. Ph.D. Thesis, Rand Afrikaans University, 2004.
31 M. M. Conradie and J. Conradie, Inorg. Chim. Acta, 2009, 362, 519.
32 A. Muller, S. Otto and A. Roodt, Dalton Trans., 2008, 650.
33 C. A. McAuliffe, in Comprehensive Coordination Chemistry, ed.
G. Wilkinson, R.D. Gillard and J.A. McCleverty, Pergamon Press,
Oxford, 1st edn, 1987, vol. 2, ch. 14, pp. 989–1037.
34 J. A. McCleverty and G. Wilkinson, Inorg. Synth., 1990, 28, 84.
35 APEX2 (Version 1.0-27), Bruker AXS Inc., Madison, Wisconsin, USA,
2005.
36 COSMO, Version 1.48, Bruker AXS Inc., Madison, Wisconsin, USA,
2003.
37 SAINT-PLUS, Version 7.12 (including XPREP), Bruker AXS Inc.,
Madison,Wisconsin, USA, 2004.
Notes and references
1 P. M. Maitlis, A. Haynes, G. J. Sunley and M. J. Howard, J. Chem. Soc.,
Dalton Trans., 1996, 2187.
2 P. W. N. M. van Leeuwen, in Homogeneous Catalysis: Understanding the
Art, Kluwer Academic Publishers, Dordrecht, 2004, ch. 6, pp. 109–124.
3 S. S. Basson, J. G. Leipoldt, A. Roodt, J. A. Venter and T. J. Van Der
Walt, Inorg. Chim. Acta, 1986, 119, 35.
4 K. G. van Aswegen, J. G. Leipoldt, I. M. Potgieter, G. J. Lamprecht, A.
Roodt and G. J. van Zyl, Transition Met. Chem., 1991, 16, 369.
5 A. Roodt and G. J. J. Steyn, Recent Res. Devel. Inorganic Chem., 2000,
2, 1.
6 G. J. J. Steyn, A. Roodt, I. Poletaeva and Y. S. Varshavsky, J. Organomet.
Chem., 1997, 536–537, 197.
38 SADABS, Version 2004(1), Bruker AXS Inc., Madison, Wisconsin,
USA, 1998.
39 A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C.
Giacovazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori and R.
Spagna, J. Appl. Crystallogr., 1999, 32, 115.
7 Y. S. Varshavsky, T. G. Cherkasova, N. A. Buzina and L. S. Bresler,
J. Organomet. Chem., 1994, 464, 239.
8 S. S. Basson, J. G. Leipoldt, A. Roodt and J. A. Venter, Inorg. Chim.
Acta, 1987, 128, 31.
9 J. G. Leipoldt, S. S. Basson and L. J. Botha, Inorg. Chim. Acta, 1990,
168, 215.
10 C. H. Cheng, B. D. Spivack and R. Eisenberg, J. Am. Chem. Soc., 1977,
99, 3003.
11 G. J. J. Steyn, A. Roodt and J. G. Leipoldt, Inorg. Chem., 1992, 31, 3477.
40 L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837.
41 G. M. Sheldrick, SHELXL97 Program for Crystal Structure Refine-
ment, University of Gottingen, Germany, 1997.
42 K. Brandenburg and H. Putz, DIAMOND, Release 3.1a. Crystal
Impact GbR, Bonn, Germany, 2005.
¨
5578 | Dalton Trans., 2010, 39, 5572–5578
This journal is
The Royal Society of Chemistry 2010
©