Oxidative addition of CH3I and CO migratory insertion in a series of ferrocene-containing carbonyl phosphine β-diketonato rhodium(I) complexes

Article abstract of DOI:10.1021/om800655j

A kinetic study of the oxidative addition reaction between CH₃I and a series of ferrocene-containing β-dIketonato carbonyl phosphine rhociium(I) complexes [Rh(FcCOCHCOR)(CO)(PPh₃)j, with R = CF ₃ (1), CH3 (2), Ph = phenyl

Full text of DOI:10.1021/om800655j

1018
Organometallics 2009, 28, 1018–1026
Oxidative Addition of CH3I and CO Migratory Insertion in a Series
of Ferrocene-Containing Carbonyl Phosphine ꢀ-Diketonato
Rhodium(I) Complexes
Jeanet Conradie and Jannie C. Swarts*
Department of Chemistry, UniVersity of the Free State, P.O. Box 339,
Bloemfontein, 9300, Republic of South Africa
ReceiVed July 10, 2008
A kinetic study of the oxidative addition reaction between CH3I and a series of ferrocene-containing
ꢀ-diketonato carbonyl phosphine rhodium(I) complexes [Rh(FcCOCHCOR)(CO)(PPh3)], with R ) CF3
(1), CH3 (2), Ph ) phenyl (3), and Fc ) ferrocenyl (4), showed that the mechanism consists of three
definite sets of reactions involving isomers of at least two distinctly different classes of RhIII-alkyl and
two different classes of RhIII-acyl species. The first reaction set involves oxidative addition itself, while
the second and third reaction sets lead to different isomerization products involving carbonyl insertion or
deinsertion in the Rh-CH3 or Rh-COCH3 bond. Second-order rate constants for the first oxidative addition
step in CHCl3 at 25 °C were ki ) 0.0061(1), 0.065(1), 0.077(2), and 0.16(1) dm3 mol-1 s-1 for 1, 2, 3,
and 4, respectively. The latter is the fastest reaction of this type reported to date for any [Rh(ꢀ-
diketonato)(CO)(PPh3)] complex. Activation parameters for all reactions from a variable temperature
kinetic study resulted in 29 < ∆H# < 45 kJ mol-1 and -188 < ∆S# < -116 J mol-1 K-1. Utilizing
kinetic results from this and several previously reported studies involving 13 different [Rh(ꢀ-
diketonato)(CO)(PPh3)] complexes, it was possible to quantify the relationship between
[Rh(R1COCHCOR2)(CO)(PPh3)] reactivity toward oxidative addition and the propensity of ꢀ-diketonato
ligands to donate electrons to the Rh center as ln k1 ) -3.14(ꢁR1 + ꢁR2) + 10.0, where ꢁR is the group
electronegativity on the Gordy scale of ꢀ-diketonato R groups. The pKa of the free ꢀ-diketone may be
estimated from the relationship pKa ) -3.484(ꢁR1 + ꢁR2) + 24.6.
groups having ꢁFc ) 1.87 as measured on the Gordy scale.3
Gordy scale ꢁR values are empirical numbers that express the
Introduction
The oxidative addition of CH3I to rhodium(I) carbonyl centers
is a fundamental process in organometallic chemistry with
significant implications in catalysis, especially when followed
by carbonyl insertion in a Rh-CH3 bond to generate an acyl
derivative, Rh-COCH3. A classic example of such a system is
the oxidative addition of CH3I to the [Rh(CO)2(I)2]- catalyst in
the Monsanto process of converting methanol to acetic acid.1
We have focused our attention on oxidative addition studies
of [RhI(R1COCHCOR2)(CO)(PPh3)] complexes. It is well-
known that electron-rich RhI complexes undergo faster oxidative
addition than electron-poor complexes.2 The electron density
on the RhI center of a [Rh(R1COCHCOR2)(CO)(PR3)] complex
can be manipulated by selecting R, R1, and R2 groups, which
are either electron-withdrawing or electron-donating. The CF3
combined tendency of not only one atom but a group of atoms,
like CF3 in the present ꢀ-diketones, to attract electrons (including
those in a covalent bond) as a function of the number of valence
electrons, n, and the covalent radius, r (in Å), of groups, as
discussed elsewhere.4
Much interest is now devoted to the effects of the ancillary
ligands on the rates of oxidative addition and migratory carbonyl
insertion steps. Previous studies have described in detail the
effect of electron-withdrawing R1 and R2 groups on the rate of
oxidative addition of CH3I to [Rh(R1COCHCOR2)(CO)(PPh3)].5
Only one report6 exists on the effect of the highly electron-
donating ferrocenyl group on the rate of oxidative addition.
There, a study of the reaction between CH3I and the rhodium(I)
carbonyl complex [Rh(FcCOCHCOCF3)(CO)(PPh3)] led to the
following reaction sequence for the oxidative addition of CH3I
to any ꢀ-diketonato complex of the type [Rh(ꢀ-diketonato)(CO)-
(PPh3)]:6
group with group electronegativity ꢁCF ) 3.01 represents an
3
example of a very powerful electron-withdrawing group, while
the ferrocenyl group is one of the strongest electron-donating
* To whom correspondence should be addressed. E-mail:
swartsjc.sci@ufs.ac.za.
(3) (a) du Plessis, W. C.; Vosloo, T. G.; Swarts, J. C. J. Chem. Soc.,
Dalton Trans. 1998, 2507. (b) Du Plessis, W. C.; Erasmus, J. J. C.;
Lamprecht, G. J.; Conradie, J.; Cameron, T. S.; Aquino, M. A. S.; Swarts,
J. C. Can. J. Chem. 1999, 77, 378.
(4) (a) Wells, P. R. Progress in Physical Organic Chemistry; John Wiley
& Sons, Inc.: New York, 1968; Vol. 6, p 111. (b) Kagarise, R. E. J. Am.
Chem. Soc. 1955, 77, 1377.
(5) (a) Basson, S. S.; Leipoldt, J. G.; Nel, J. T. Inorg. Chim. Acta 1998,
84, 167. (b) Leipoldt, J. G.; Steynberg, E. C.; van Eldik, R. Inorg. Chem.
1987, 26, 3068.
(6) Conradie, J.; Lamprecht, G. J.; Roodt, A.; Swarts, J. C. Polyhedron
2007, 23, 5075.
(1) (a) Maitlis, P. M.; Haynes, A.; Sunley, G. J.; Howard, M. J. J. Chem.
Soc., Dalton Trans. 1996, 2187. (b) Cavallo, L.; Sola, M. J. Am. Chem.
Soc. 2001, 123, 12294. (c) Haynes, A.; Maitlis, P. M.; Morris, G. E.; Sunley,
G. J.; Adams, H.; Badger, P. W.; Bowers, C. M.; Cook, D. B.; Elliot, P. I. P.;
Ghaffer, T.; Green, H.; Griffin, T. R.; Payne, M.; Pearson, J. M.; Taylor,
M. J.; Vickers, P. W.; Watt, R. J. J. Am. Chem. Soc. 2007, 126, 2847.
(2) (a) Gonsalvi, L.; Adams, H.; Sunley, G. J.; Ditzel, E.; Haynes, A.
J. Am. Chem. Soc. 2002, 124, 13597. (b) Rankin, J.; Benyei, A. C.; Poole,
A. D.; Cole-Hamilton, D. J. J. Chem. Soc., Dalton Trans. 1999, 3771. (c)
Gonsalvi, L.; Adams, H.; Sunley, G. J.; Ditzel, E.; Haynes, A. J. Am. Chem.
Soc. 1999, 121, 11233.
10.1021/om800655j CCC: $40.75
2009 American Chemical Society
Publication on Web 01/23/2009

Kinetics of Ferrocene-Containing Rhodium(I) Complexes
Organometallics, Vol. 28, No. 4, 2009 1019
This reaction sequence proved to be a completely general
mechanism, with all previously reported mechanisms just being
a simplification of eq 1 that is controlled by the size of ki, where
i ) (4, (3, (2, or (1. The end scripts 1 and 2 in RhIII-alkyl1,
RhIII-acyl1, RhIII-alkyl2, and RhIII-acyl2 denote the first- or
second-formed alkyl or acyl species during the course of the
reaction. In addition, two geometrical isomers of the RhI and
each of the four RhIII species were detected by 1H NMR
spectroscopy. The equilibrium between the two RhI isomers has
been discussed in detail;7 the single-crystal X-ray-determined
structures of [RhI(FcCOCHCOCF3)(CO)(PPh3)]8 and [RhIII-
(FcCOCHCOCF3)(CO)(PPh3)(CH3)(I)], an isomer of a RhIII-
alkyl2 species,6 have been reported; and the solution geometry
of a RhIII-alkyl1 intermediate isomer, [Rh(PhCOCHCOTh)-
(CO)(PPh3)(CH3)(I)] with Th ) thienyl ) C4H3S, has been
determined by NMR techniques.9 Density functional theoretical
(DFT) computations of the equilibrium geometry of the 12
possible [RhIII(RCOCHCOTh)(CO)(PPh3)(CH3)(I)]-alkyl reac-
tion products (R ) CF3, Ph, or Th) revealed that the first alkyl1
product results from trans addition to RhI and that the second
thermodynamic stable alkyl2 product adopts an octahedral
geometry with the PPh3 group and the iodide above and below
the square planar plane.10
We report here on the reactivity toward oxidative addition
of a series of ferrocene-containing [Rh(FcCOCHCOR)(CO)(P-
Ph3)] complexes where the R group on the ꢀ-diketonato ligand
is successively replaced with more electron-donating groups
CF3, CH3, Ph, and Fc. We also unify our own studies and that
of previous studies for the first time in a mathematical model
to predict the reactivity of [Rh(R1COCHCOR2)(CO)(PPh3)]
complexes toward oxidative addition by relating electron density
of the RhI center expressed in terms of the sum of R1 and R2
group electronegativities, ꢁR1 + ꢁR2, with kinetic rate constants
irrespective of whether R1 and R2 are electron-withdrawing or
electron-donating. Finally, utilizing present structural knowledge
obtained from single-crystal X-ray structure determinations,
structural NMR experiments, and density functional theory
structure optimizations on related complexes, the geometry of
intermediates in the complete reaction sequence is proposed.
Figure 1. Electron density of the RhI nucleus is a function of R
side groups on the ꢀ-diketonato ligand. The RhI nucleus of 4 is
much more electron-rich than that of any other complex studied
herein by virtue of the smaller group electronegativity, ꢁR, of the
ferrocenyl group.
relevant R groups.3a,11 When applied to the present series of
complexes, the change in electron density of the RhI center in
1-4 as a result of changes in the ꢀ-diketonato ligand should
be a function of ꢁR + ꢁFc, where R and Fc are the pendant side
groups of the ꢀ-diketonato ligand. Should the Fc group be
replaced by another group, say R2, then the change in the
electron density of the RhI center should be a function of ꢁR +
ꢁR2.
The complete reaction between all of the [Rh(FcCOCHCOR)-
(CO)(PPh3)] complexes 1-4 and CH3I proceeds via three sets
of reactions according to eq 1. However, each individual
complex exhibits small mechanistic variations due to the size
of especially k1 and k2. The first of the three sets of reactions
for all four complexes involves the oxidative addition of CH3I
to the RhI center to form an easily observable intermediate RhIII-
alkyl1 complex followed by a near simultaneous CO migratory
insertion to yield the corresponding RhIII-acyl1 complex ac-
cording to eq 2.
The RhIII-alkyl and RhIII-acyl reaction products for the first
set of reactions are distinguished from later-formed, second RhIII-
alkyl and RhIII-acyl reaction products by the nomenclature RhIII-
alkyl1 and RhIII-acyl1 as opposed to RhIII-alkyl2 and RhIII-acyl2.
The RhIII-alkyl1 and RhIII-acyl1 complexes were identified by
IR and NMR spectroscopy. IR spectroscopy can clearly dif-
ferentiate the various species observed in these reactions. Thus,
terminal Rh(I) carbonyls, terminal Rh(III) carbonyls, and Rh(III)
acyl complexes reveal characteristic bands in the 1980-2000,
2050-2100, and 1700-1750 cm-1 regions, respectively. IR
spectroscopy is ideal to distinguish between CO groups that
vibrate at ∼1980-2000 cm-1 in RhI-carbonyl complexes, at
∼2050-2100 cm-1 for RhIII-alkyl-carbonyl complexes, H3C-
RhIII-CO, and acyl CO groups in metal-acyl complexes that
resonate at ∼1700-1750 cm-1 for RhIII-COCH3 complexes.7
Figure 2a illustrates the course of the first set of reactions
for the oxidative addition of CH3I to 2 in chloroform as
Results and Discussion
To determine how different RhI electron densities in mixed-
metal ferrocene-containing ꢀ-diketonato complexes influence
the rate of oxidative addition of CH3I to [Rh(FcCOCHCOR)-
(CO)(PR3)] complexes, we focused our attention on the
complexes shown in Figure 1. RhI electron densities were
manipulated by choosing ꢀ-diketonato R groups, which are
highly electron-withdrawing (complex 1), highly electron-
donating (4), or intermediate in electron-withdrawing nature
(complexes 2 and 3). We have previously shown that a measure
of the relative electron density on a molecule due to a ligand
or a particular molecular side group on a molecule can be
expressed as the sum of group the electronegativities ꢁR of the
monitored by IR spectroscopy in the range 1690-2140 cm-1
.
The disappearance of the RhI-carbonyl complex [signal at 1983
cm-1 in CHCl3; observed pseudo first-order rate constant kobs
) 0.0038(1) s-1] shows that the first step of the reaction leads
to the formation of a RhIII-alkyl1 species [peak at 2077 cm-1
,
kobs ) 0.0056(2) s-1] followed by the near simultaneous
(7) Conradie, J.; Lamprecht, G. J.; Otto, S.; Swarts, J. C. Inorg. Chim.
Acta 2002, 328, 191.
(8) Lamprecht, G. J.; Swarts, J. C.; Conradie, J.; Leipoldt, J. G. Acta
Crystallogr. 1993, 49, 82.
(9) Conradie, M.; Conradie, J. Inorg. Chim. Acta 2008, 361, 2285.
(10) Conradie, M.; Conradie, J. Inorg. Chim. Acta 2008, 362, 519.
(11) (a) Kemp, K. C.; Fourie, E.; Conradie, J.; Swarts, J. C. Organo-
metallics 2008, 27, 353. (b) Conradie, J.; Cameron, T. S.; Aquino, M. A. S.;
Lamprecht, G. J.; Swarts, J. C. Inorg. Chim. Acta 2005, 358, 2530. (c)
Auger, A.; Muller, A. J.; Swarts, J. C. J. Chem. Soc. Dalton Trans. 2007,
3623.

1020 Organometallics, Vol. 28, No. 4, 2009
Conradie and Swarts
Figure 2. Infrared monitoring of the oxidative addition of CH3I (0.0517 mol dm-3) to [Rh(FcCOCHCOCH3)(CO)(PPh3)] (0.0068 mol
dm-3) in chloroform (T ) 25.0 °C). The inserts give the absorbance vs time data of the indicated species. (a) The first set of reactions: RhI
+ CH3I h {RhIII-alkyl1 h RhIII-acyl1}. Here, the formation of the RhIII-alkyl1 species [k1 ) 0.100(3) dm3 mol-1 s-1] is followed by the
formation of the RhIII-acyl1 complex at a slower rate [k1 ) 0.032(1) dm3 mol-1 s-1]. The two products are actually in equilibrium with each
other, although the equilibrium is too slow to be maintained on the time scale of the first reaction set. (b) The second set of reactions:
{RhIII-alkyl1 h RhIII-acyl1} h RhIII-alkyl2 [k3 ) 8(1) × 10-5 s-1]. (c) The third set of reactions: RhIII-alkyl2 h RhIII-acyl2 [k4 ) 4(1) ×
10-6 s-1]. (d) kobs for the first set of reactions as a function of CH3I concentration. kobs as determined by 1H NMR at [CH3I] ) 0.0707 mol
dm-3 are also shown with points O, ), and 4.
formation of the RhIII-acyl1 species at a slightly slower rate
[peak at 1719 cm-1, kobs ) 0.0018(2) s-1] as a result of CO
insertion. The rate of formation of the RhIII-alkyl1 species at
2077 cm-1 was slightly higher than the rate of disappearance
of RhI, but they are close enough to each other to be considered
kinetically equivalent. The observed slower rate of RhIII-acyl1
appearance is considered to imply that the equilibrium involving
the carbonyl insertion step, that is, the k2 and k-2 steps in eq 2,
is too slow to be maintained during the oxidative addition of
CH3I to RhI. Compelling evidence that the carbonyl insertion
step involves an equilibrium will be forthcoming from the
discussion below involving the second general reaction step.
Figure 2d illustrates kobs for the first set of reactions involving
2 as a function of CH3I concentration. The RhI disappearance
represents the k1 step and the RhIII-acyl1 appearance the k2 step
in eq 2. Because the graph of kobs for the RhI disappearance vs
[CH3I] passes through the origin, k-1 is about 0 for 2. Second-
order rate constants for the first oxidative addition step, k1 )
kobs/[CH3I], are summarized in Table 1.
signal of the acyl CH3-group at ca. 3 ppm.12 By carefully
comparing the positions and integrals of the different signals
in the course of the reaction as followed by H NMR, the
1
spectral parameters of different reaction products could be
identified, and their appearance and disappearance could be
monitored as a function of time. The new feature introduced
by the NMR study is the existence of two structural isomers
for each reaction intermediate, referred to as A and B, for
example, RhIII-alkyl1A and RhIII-alkyl1B in fast equilibrium with
each other. The choice of the labels is arbitrary and has no
significance. These isomers are expected, since the RhI com-
plexes also exist as two structural isomers in a fast equilibrium
with each other.7 The equilibria are labeled “fast” as compared
to the half-life of each of the three reaction steps, but it is slow
on the NMR time scale. This enabled us to observe them by 1H
NMR. The ratio between the two structural isomers A and B
of each reaction intermediate, the Kc values as indicated in the
reaction scheme in Figure 3 and tabulated in Table 2, did not
change during the course of each reaction step, implying that
the two isomers of each reaction intermediate exist in a fast
equilibrium with each other.
The same reaction sequence for the first set of reactions for
1
2, as observed by IR, was also observed by H NMR and 31P
Figure 3 illustrates selected 1H NMR signals of the reaction
products of the reaction between CH3I and 2. The appearance
and disappearance of these and other signals gave kinetic data
in agreement with IR data; see Table 1 and Figure 2d for the
NMR spectroscopy. RhIII-alkyl complexes of the type [Rh(ꢀ-
diketonato)(CO)(PPh3)(CH3)(I)] can inter alia unambiguously
be identified by the 1H NMR resonance signal of the CH3 group
at ca. 1.4-1.7 ppm. This signal is a doublet of doublets due to
coupling with Rh (spin 1/2) and P (spin 1/2). RhIII-acyl
complexes of the type [Rh(ꢀ-diketonato)(COCH3)(PPh3)(I)] can
1
combined IR and H NMR data for the first set of reactions.
(12) Varshavsky, Y. S.; Cherkasova, T. G.; Buzina, N. A.; Bresler, L. S.
J. Organomet. Chem. 1994, 464, 239.
1
conveniently be identified by the sharp H NMR resonance

Kinetics of Ferrocene-Containing Rhodium(I) Complexes
Organometallics, Vol. 28, No. 4, 2009 1021
Oxidative addition of CH3I to 1, 3, and 4 observed by 1H NMR
all followed the same reaction patterns as 2 given by eq 2, and
the results are summarized in Table 1.
31P NMR of carbonyl phosphine rhodium complexes gives a
doublet with 1J(31P-103Rh) ) ca. 170 ppm (ca. 0.70 Hz) for RhI
complexes, ca. 120 ppm (ca. 0.50 Hz) for RhIII-alkyl complexes,
and ca. 150 ppm (ca. 0.60 Hz) for RhIII-acyl complexes.9,12,13
When following the oxidative addition reaction of CH3I to 2,
two structural isomers of RhI and each RhIII reaction intermediate
according to eq 2 are observed, giving kinetic data in agreement
1
with H NMR and IR data; see Figure 4 and Table 1.
Comparison of the influence of different side groups on the
ꢀ-diketonato ligands of 1-4 showed that complex 4 reacted
the fastest; RhI disappeared ca. 30 times faster in 4 than in 1
(Table 1). Only 1 reacted slow enough that the equilibrium K2
(eq 2) could be maintained during RhI disappearance, because
only for 1 was the rate of RhIII-alkyl1 and RhIII-acyl1 formation
equal. RhI disappearance was ca. 10 times faster for 2 than for
1 and 12 times faster for 3 than for 1. It is clear from these
results that the reactivity toward oxidative addition of the
[Rh(FcCOCHCOR)(CO)(PPh3)] complexes is in the order (half-
lives in seconds of the first reaction with [CH3I] ) 1.000 mol
dm-3 are given in brackets):
R ) Fc(4) > Ph(9) ≈ CH3(11) > CF3(114)
The faster reaction rate of complexes 2-4 as compared to 1
resulted in the conclusion that in none of these three complexes
could equilibrium K2 be maintained during RhI disappearance
because for all three of these complexes, RhIII-acyl1 formation
was noticeably slower than RhIII-alkyl1 formation. For 4, the
difference was the greatest with RhIII-acyl1 formation, almost
seven times slower than RhIII-alkyl1 formation (IR data, Table
1). However, that RhIII-acyl1 and RhIII-alkyl1 is indeed involved
in an equilibrium, stems clearly from the results involving the
second reaction sequence that is described in the next paragraph.
The 30-fold variation in rate constants for the first reaction
sequence is directly a consequence of the group electronegativity
of the ꢀ-diketonato R groups influencing the electron density
on the RhI nucleus of 1-4. To independently verify that the
RhI nucleus of 1-4 becomes more electron-rich as ꢁR decreases,
we note from Table 2 that the decreasing ꢁR values3 from 3.01
(complex 1) to 1.87 (complex 4) led to a decrease in [Rh(Fc-
COCHCOR)(CO)(PPh3)] infrared carbonyl stretching wave-
number from υ ) 1986 till 1977 cm-1. This lowering in υco is
consistent with a more electron-rich RhI center and explains
the increase in oxidative addition reaction rate in moving from
1 to 4.
The second set of reactions as observed by IR and NMR
spectroscopy involves a much slower (t1/2 ≈ 4 h for 2) CH3I-
independent CO deinsertion of the RhIII-acyl1 to yield a new
RhIII-alkyl2 according to eq 3:
This second, slower set of reactions as observed by IR
spectroscopy is illustrated in Figure 2b for 2. Here, the RhIII-
alkyl1 species at 2077 and the RhIII-acyl1 species at 1719 cm-1
start to disappear at the same rate [k3 ) 4.4(2) × 10-5 s-1 and
5(1) × 10-5 s-1, respectively] with the simultaneous formation
of a second alkyl product, the RhIII-alkyl2 species at 2059
cm-1salso at approximately the same rate [k3 ) 7.1(4) × 10-5

1022 Organometallics, Vol. 28, No. 4, 2009
Conradie and Swarts
Figure 3. Selected ꢀ-diketonato methine (at ∼5.8 ppm) and CH3 1H NMR peaks in CDCl3 of the indicated reactants and products during
the three sets of reactions for the oxidative addition of 0.2034 mol dm-3 CH3I to 0.01918 mol dm-3 [Rh(FcCOCHCOCH3)(CO)(PPh3)] in
CDCl3 at 25.0 °C. Signals for two geometrical isomers for each reaction intermediate can be identified.
Table 2. Rate and Equilibrium Constants Applicable to the Equation in Figure 3 for the Oxidative Addition of CH3I to
[Rh(FcCOCHCOR)(PPh3)(CO)] at 25 °C in Chloroforma
pKa of
ꢀ-diketoneb mol-1 s-1
k1 (dm3
∆H#
∆S#
104
106
complex
ꢁR
υco(cm-1
)
)
(k1) (kJ mol-1
)
(k1) (J mol-1 K-1
)
k3 (s-1
)
k4 (s-1
)
Kc1
Kc2
Kc3
Kc4
Kc5
c
1: R ) CF3 3.01
2: R ) CH3 2.34
1986
1980
1977
1977
6.56
10.01
10.41
13.10
0.00611(1)
29(3)
40(2)
45(1)
-188(9)
-133(5)
-116(4)
-147(2)
1.7(2) 4.4(1) 0.68 2.57 1.00 4.56
0.87(5) 7.7(5) 0.22 0.52 2.13 0.82 1.50
d
0.065(1)
0.077(2)
0.157(2)
3: R ) Ph
4: R ) Fc
2.21
1.87
1.2(1) 4.8(9) 0.56 0.48 1.52 0.83
1.3(1) 18(2)
*
e
33.6(7)
e
e
e
e
a Experimentally, k-1 ) k-3 ) k-4 ≈ 0, except for the CF3 complex, where k-1 ) 0.0005 s-1. Kci ) [RhB species]/[RhA species] with i ) 1, 2,..., 5
were determined from NMR data. b From ref 3a. c Data from ref 6. d Not determined. e Only one isomer.
Figure 4. 31P NMR spectra illustrating doublet 31P peaks of the indicated reactants and products during the three reaction sets for the
oxidative addition of 0.500 mol dm-3 CH3I to 0.018 mol dm-3 [Rh(FcCOCHCOCH3)(CO)(PPh3)] in CDCl3 at T ) 25 °C and times (a) t
) 0, (b) t ) 1753 s (toward the end of the first reaction set), and (c) t ) 7925 s (during the second reaction set).
s-1]. This observation, that the RhIII-alkyl1 species disappears
at the same rate as the RhIII-acyl1 species, supplies strong
evidence that an equilibrium exists between the RhIII-alkyl1 and
the RhIII-acyl1 species. The equilibrium is clearly fast enough
to be maintained on the time scale of the RhIII-alkyl2 formation
but still slow enough on the NMR time scale to observe both
species. Surprisingly, the second reaction sequence is almost
independent of R group on the ꢀ-diketonato ligands of 1-4. A
variation of barely a factor of 2 was observed in the rate
constants of this CH3I-independent reaction step; see Table 1.
fast first set of reactions, the RhIII-alkyl2 products of 1-4 could
be isolated and have previously been characterized.7 X-ray
crystallography revealed that the CF3-containing RhIII-alkyl2
product of 1 adopts an octahedral geometry with the PPh3 group
and the iodide above and below the square planar plane.6
Theoretical DFT calculations of the equilibrium geometry of
the possible reaction products of complexes of the type
[Rh(ThCOCHCOR)(CO)(PPh3)] with Th ) thionyl and R )
Ph, CF3, or Th revealed that the RhIII-alkyl1 product results from
Because of the long half-life of the second (and third, t1/2
4 and 48 h for 2, respectively) set of reactions in contrast to the
≈
(13) Conradie, M.; Conradie, J. Inorg. Chim. Acta 2008, 361, 208.

Kinetics of Ferrocene-Containing Rhodium(I) Complexes
Organometallics, Vol. 28, No. 4, 2009 1023
trans CH3I addition to RhI, that is, with the CH3 group and the
iodide above and below the square planar plane.10
The third set of reactions as observed by IR and NMR
methods involves a very slow CO migratory insertion (t1/2
≈
48 h for 2) to yield a new RhIII-acyl2 complex according to eq
4:
This third, very slow set of reactions is illustrated in Figure
2c for the IR monitoring of 2 and demonstrates the slow
disappearance of the RhIII-alkyl2 species [k4 ) 5(2) × 10-6 s-1]
at 2059 cm-1 and the appearance at approximately the same
rate of a new RhIII-acyl2 species at 1714 cm-1 [k4 ) 3(1) ×
10-6 s-1]. The long half-life of this reaction implies that it could
not be followed with great accuracy due to, for example,
evaporation problems of the solvent.
As was found for reaction set 2, the variation in ꢀ-diketonato
R groups had almost no influence on the isomerization rate
constant k4 associated with reaction set 3 (Table 1). At most, in
moving from 1 to 4, k4 increased 4-fold. This contrasts sharply
with k1 of reaction set 1, which increased 30-fold in moving
from 1 to 4.
UV/vis results of the oxidative addition of CH3I to 1-4 also
show all three general reaction steps with reaction rates in
accordance with the IR and NMR results. Figure 5a illustrates
the absorbance-time data of the oxidative addition of CH3I to
complex 2. All three reactions could be followed at 493 nm.
The second reaction, however, underwent a small change in
absorbance at 493 nm (due to the influence of the third reaction)
as opposed to the large change in absorbance at 410 nm. Reliable
k3 values could be determined at 410 nm, because of the large
change in absorbance for the second reaction at this wavelength
and the small change in absorbance for the third reaction and
because the first step is more than 70 times slower than the rate
of the second reaction steps. The first reaction step represents
the oxidative addition process whereby the RhI nucleus converts
to the RhIII-alkyl1 reaction intermediate in equilibrium with the
RhIII-acyl1 reaction intermediate. The second reaction step
represents formation of the RhIII-alkyl2 reaction intermediate,
and the third reaction step represents formation of the final RhIII-
acyl2 reaction product. Plots of kobs for the first reaction of 1-4
vs [CH3I] are linear (see, for example, Figure 5b for 4),
indicating the reaction to be first order in CH3I and hence second
order overall. The second and third reactions followed by UV/
vis spectrophotometer are independent of [CH3I]; see, for
example, Figure 5c for the second reaction of 4. The obtained
rate constants are listed in Table 2. The activation entropies
(Table 2) for the first oxidative addition step were determined
from a variable temperature (15-40 °C) study. They are all
large and negative, as frequently found for oxidative addition
of CH3I to square planar d8 metal complexes,14 which is
generally thought to proceed via an SN2 mechanism.15
Taking into account that two main isomers exist for each
reactant and reaction product, the complete reaction sequence
for the oxidative addition of CH3I to 1-4 can be presented by
Figure 5. (a) Absorbance vs time data for the UV monitoring of
the reaction between 0.0004 mol dm-3 [Rh(FcCOCH-
COCH3)(CO)(PPh3)], 2, and 0.0942 mol dm-3 CH3I at 493 and
410 nm. (b) Temperature and CH3I concentration dependence of
the oxidative addition reaction of CH3I to [Rh(FcCOCHCOFc)-
(CO)(PPh3)], 4, as monitored by UV/vis spectrometry in chloroform
at 525 nm for the first reaction RhI + CH3I h {RhIII-alkyl1 h
RhIII-acyl1} and (c) at 525 and 380 nm for the second reaction
{RhIII-alkyl1 h RhIII-acyl1} h RhIII-alkyl2.
Scheme 1. In Scheme 1, the proposed molecular geometries of
the RhIII products are based on single-crystal X-ray crystal-
lographic, NMR, and DFT computational chemistry studies
either on 1-4 themselves or on related complexes that react
(14) (a) van Zyl, G. J.; Lamprecht, G. J.; Leipoldt, J. G.; Swaddle, T. W.
Inorg. Chim. Acta 1988, 143, 223. (b) Venter, J. A.; Leipoldt, J. G.; van
Eldik, R. Inorg. Chem. 1991, 30, 2207. (c) Leipoldt, J. G.; Basson, S. S.;
Botha, L. J. Inorg. Chim. Acta 1990, 168, 215. (d) Wilson, J. M.; Sunley,
G. J.; Adams, H.; Haynes, A. J. Organomet. Chem. 2005, 690, 6089.
(15) Purcell, K. F.; Kotz, J. C. Inorganic Chemistry; W.B. Saunders
Company: Philadelphia, 1977; pp 938-948.

1024 Organometallics, Vol. 28, No. 4, 2009
Scheme 1. Proposed Geometries in the Reaction Sequencea
Conradie and Swarts
I
a
The proposed geometries stem from the following information. Rh isomers: The single-crystal X-ray structure of one of the isomers of 1 was
solved and discussed in ref 8. RhIII-alkyl1 isomers: Geometry proposed in analogy to the geometries of [Rh(TfCOCHCOR)(CO)(PPh3)] with Tf )
thionyl and R ) Ph, CF3, or Tf that were obtained by solution NMR techniques and DFT calculations.9,10 RhIII-acyl1 isomers: Only a possible
geometry; no corresponding structure or geometry for any related ꢀ-diketonato complex has to date been determined by any technique. RhIII-alkyl2
isomers: One of the alkyl2 isomers of 1 was crystallographically characterized.6 RhIII-acyl2 isomers: Proposed geometries are in analogy to the
geometries of products of oxidative addition of CH3I to [Rh(ThCOCHCOR)(CO)(PPh3)] and [Rh(FcCOCHCOCF3)(CO)(PPh3)]; these were determined
by DFT calculations.10,16
Figure 6. Some [Rh(ꢀ-diketonato)(CO)(PPh3)] complexes with their CH3I oxidative addition rate constants (25 °C in CH3Cl except for
5-7 and 10, which are measured in acetone). In 12 and 13, R is either ꢀ-diketonato pendant substituent CH3 or thionyl, C4H3S. Rate
constants are from refs 6, 9, 13, and 17.
according to the same general reaction sequence as the ferro-
cenyl-containing rhodium complexes of this study.6,7,10,16
function of group electronegativity, ꢁR, of the ꢀ-diketonato R
groups. Figure 7 illustrates that the sum of the group electrone-
gativities of R1 and R2 of the ꢀ-diketonato ligand
(R1COCHCOR2)- coordinated to [Rh(R1COCHCOR2)(CO)-
(PPh3)] is linearly dependent on ln k1, k1 being the second-order
rate constant for the oxidative addition of CH3I to these rhodium
complexes. The sum of the group electronegativities of R1 and
R2 of the ꢀ-diketonato ligand (R1COCHCOR2)- coordinated to
complexes of the type [Rh(R1COCHCOR2)(CO)(PPh3)] there-
fore gives a good indication of the electron density (nucleo-
philicity) of rhodium in each complex. The effect of the pKa
value of the various ꢀ-diketones on the oxidative addition
reaction rate is also shown in Figure 7. A lower pKa value
implies that the ꢀ-diketonato ligand will remove electron density
from the Rh nucleus, making it a weaker nucleophile and thus
less reactive toward oxidative addition. The relationship between
pKa and (ꢁR1 + ꢁR2) should thus also be linear, and this effect
is demonstrated in Figure 7c.
In an attempt to quantify the oxidative addition rate constants
of a series of complexes [Rh(R1COCHCOR2)(CO)(PPh3)] in
terms of group electronegativity, ꢁR, reaction rate constants of
1-4 were compared with those obtained for the related
complexes6,9,13,17 shown in Figure 6.
Because the Rh(I) nucleus acts as a nucleophile when it
undergoes oxidative addition, it is 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. Thus, any ligand bonded to the metal that
will increase the electron density on the metal center will lead
to an increased rate of oxidative addition, assuming all other
influences, factors, and parameters remain constant. In the
present series of ꢀ-diketonato ligands, the capability of the ligand
to manipulate the electron density on the metal is considered a
(16) Conradie, M.; Conradie, J. S. Afr. J. Chem. 2008, 61, 102.
(17) (a) Basson, S. S.; Leipoldt, J. G.; Roodt, A.; Venter, J. A.; van der
Walt, T. J. Inorg. Chim. Acta 1986, 119, 35. (b) Lamprecht, D. Electro-
chemical, kinetic and molecular mechanic aspects of rhodium(I) and
rhodium(III) complexes. Ph.D. Thesis, University of the Orange Free State
R.S.A., 1998.
(18) Conradie, M. M.; Muller, A. J.; Conradie, J. S. Afr. J. Chem. 2008,
61, 13.
(19) Ellinger, M.; Duschner, H.; Starke, K. J. Inorg. Nucl. Chem. 1978,
40, 1063.

Kinetics of Ferrocene-Containing Rhodium(I) Complexes
Organometallics, Vol. 28, No. 4, 2009 1025
Figure 7. (a) Linear relationship between ln k1, the second-order rate constant for the first step of oxidative addition of CH3I to
[Rh(R1COCHCOR2)(CO)(PPh3)], and the sum of the group electronegativities of R1 and R2, (ꢁR1 + ꢁR2), of the ꢀ-diketonato ligand coordinated
to rhodium complexes. Rate data for various complex are shown in Figure 6. (b) Relationship between ln k1 and the pKa of the corresponding
free ꢀ-diketone. (c) Relationship between ꢀ-diketone pKa and (ꢁR1 + ꢁR2). Rate constants were obtained from refs 6, 9, 13, and 17. Individual
group electronegativities were ꢁCF3 ) 3.01, ꢁCH ) 2.34, ꢁPh ) 2.21, ꢁTh ) 2.10, and ꢁFc ) 1.87 and were obtained from refs 3 and 18. pKa
3
values were from refs 3b, 5, and 18-20.
Figure 7 reveals that the ferrocene-containing complex 4
exhibits a remarkably high nucleophilicity. Indeed, 4 has the
highest propensity toward oxidation addition of any known
[Rh(ꢀ-diketonato)(CO)(PPh3)] complex and has a higher reac-
tivity than many Rh(I) carbonyl complexes. It is ca. 2000 times
more reactive than the anionic Monsanto carbonylation catalyst,
[Rh(CO)2I2]-.1 Two of the few known RhI complexes that
undergo faster oxidative addition by CH3I than 4 include the
[RhIIII(bis-iminocarbazolido)(I)(CO)CH3] complex that reacts ca.
5 × 104 times faster21 and [Rh(methyl(2-methylamino-1-
cyclopentene-1-dithiocarboxylato))(CO)(PPh3)] that reacts ca.
two times faster than 4.22
complexes was found to be a function of electron density of
the RhI center, and the latter may conveniently be expressed as
a function of the sum of ꢀ-diketonato R substituent group
electronegativities, ꢁR1 + ꢁR2. Utilizing this principle, CH3I
oxidative addition reaction rates to any complex of the type
[Rh(R1COCHCOR2)(CO)(PPh3)] may be estimated by the
equation ln k1 ) -3.14(ꢁR1 + ꢁR2) + 10, while the acidity of
the free ꢀ-diketone ligand may be estimated by the equation
pKa ) -3.484(ꢁR1 + ꢁR2) + 24.6.
Experimental Section
Materials and Apparatus. Complexes 1-4 were synthesized
as described earlier.7 NMR measurements, at 298 K unless
otherwise stated, were recorded on a Bruker Advance DPX 300
NMR spectrometer [1H (300.130 MHz) and 31P (121.495 MHz)].
Chemical shifts are reported as δ values relative to SiMe4 at 0 ppm
The data from Figure 7 can be fitted to eqs 5-7.
ln k1 ) -3.14(ꢁR1 + ꢁR2) + 10.0
ln k1 ) 0.83pKa - 11.6
(5)
(6)
(7)
1
for the H spectra or relative to 85% H3PO4 at 0 ppm for the 31P
pKa ) -3.484(ꢁR1 + ꢁR2) + 24.6
spectra. Infrared spectra were recorded on a Hitachi 270-50 infrared
spectrometer either in a KBr matrix or in chloroform solutions.
UV measurements were recorded on a GBC-916 UV/vis spectrom-
eter. Liquid reactants and solvents were distilled prior to use, and
water was double distilled.
and allows prediction of CH3I oxidative addition rate constants
of simple [Rh(ꢀ-diketonato)(CO)(PPh3)] complexes as well as
pKa values of free ꢀ-diketone ligands if the group electronega-
tivities of the R1 and R2 side groups are known.
Kinetic Measurements. The kinetic rate constants were deter-
mined utilizing UV/vis (by monitoring the change in absorbance),
IR (by monitoring formation and disappearance of the carbonyl
peaks), and 1H and 31P NMR spectroscopy (by monitoring the
change in integration units of the various signals with time). A
linear relationship between UV absorbance, A, and concentration,
C, confirmed the validity of the Beer-Lambert law (A ) εCl with
l ) path length ) 1 cm) for the complexes 1-4 at experimental
wavelengths mentioned in Figure 7 for 2 and 4. For 1, λexp ) 375,
400, 530, and 580 nm, and for 3, λexp ) 380 nm for all three reaction
steps. For 1-4, λmax (nm) values [ε (dm3 mol-1 cm-1 are given in
brackets)] in chloroform are as follows: 1, 498 [2060(10)]; 2, 493
[880(10)]; 3, 500 [1520(10)]; and 4, 379 [12500(200)] and 473
[3780(20)]. All kinetic measurements were monitored under pseudo
first-order conditions with CH3I concentrations 7-10000 times the
concentration of the [Rh(FcCOCHCOR)(CO)(PPh3)] (R ) CH3,
Ph, and Fc) complex. At least five different concentrations within
thisrangewereutilized.Theconcentrationof[Rh(FcCOCHCOR)(CO)-
(PPh3)] was e0.0004 mol dm-3 for UV/vis measurements and
e0.01 mol dm-3 for IR and NMR measurements. The activation
Conclusion
Kinetic studies on a series of [Rh(FcCOCHCOR)(CO)(PPh3)]
complexes show that the oxidative addition reactions are first
order in both the Rh(I) complex and the CH3I, with second-
order rate constants significantly larger than those reported for
related [Rh(R1COCHCOR2)(CO)(PPh3)] complexes. The high
nucleophilicity of complex 4 (R ) Fc) caused it to undergo the
fastest oxidative addition with CH3I (k1 ) 0.16 dm3 mol-1 s-1
)
yet observed for any [Rh(ꢀ-diketonato)(CO)(PPh3)] complex.
This is 30 times faster than that of the poorer nucleophilic
complex 1 having an electron-withdrawing substituent, CF3, on
the ꢀ-diketonato ligand. The rate of oxidative addition of all
(20) Stary´, J. The SolVent Extraction of Metal Chelates; MacMillan
Company: New York, 1964; Appendix.
(21) Gaunt, J. A.; Gibson, V. C.; Haynes, A.; Spitzmesser, S. K.; White,
A. J. P.; Williams, D. J. Organometallics 2004, 23, 1015.
(22) Roodt, A.; Steyn, G. J. J. Recent Res. DeV. Inorg. Chem. 2000, 2,
1.
#
parameters ∆H# and ∆S for the reaction in chloroform were

1026 Organometallics, Vol. 28, No. 4, 2009
Conradie and Swarts
obtained from kinetic experiments performed at least at four
temperatures between 15.0 and 35.0 °C. The [RhI(ꢀ-diketonato)(CO)-
(PPh3)] complexes 1-4 were tested for stability in chloroform by
determining the slope of the linear plots of kobs against the
concentration of the incoming CH3I ligand. Nonzero intercepts
implied that kobs ) k1[CH3I] + k-1 and that the k-1 step in the
proposed reaction mechanism exists.24 Zero intercepts indicated
k-1 ) 0. The activation parameters were determined from the Eyring
relationship ln k/T ) -∆H#/RT + ∆S#/R + ln kB/h with ∆H# )
activation enthalpy, ∆S# ) activation entropy, k ) rate constant,
kB ) Boltzmann’s constant, T ) temperature, h ) Planck’s constant,
and R ) universal gas constant. The activation free energy may be
calculated from ∆G# ) ∆H# - T∆S#.24
1
means of overlay IR and UV/vis spectra for at least 24 h. A H
NMR spectrum of complexes 1-4 after 48 h in solution of CDCl3
confirmed solution stability.
Calculations. Pseudo first-order rate constants, kobs, were
calculated by fitting23 kinetic data to the first-order equation24 [A]t
obst)
) [A]0 e(-k with [A]t and [A]0 the absorbance of the indicated
species at time t and at t ) 0 (UV/vis or IR experiments) or integral
values for suitable peaks, see Figures 3 and 4, in 1H and 31P NMR
spectra. The experimentally determined pseudo first-order rate
constants were converted to second-order rate constants, k1, by
Acknowledgment. Financial assistance by the South
African NRF under Grant 2054243 and the Central Research
Fund of the University of the Free State is gratefully
acknowledged.
(23) MINSQ, Least Squares Parameter Estimation, Version 3.12; Mi-
croMath: Salt Lake City, UT, 1990.
(24) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd
ed.; McGraw-Hill: New York, 1995; pp 15, 49, 70-75, 156.
OM800655J