Slow exchange of bidentate ligands between rhodium(I) complexes: Evidence of both neutral and anionic ligand exchange

Article abstract of DOI:10.1002/ejic.201402810

The phosphine double exchange process involving [RhCl(COD)(TPP)] and [Rh(acac)(CO)(TMOPP)] (TPP = PPh₃, TMOPP = P(C₆H₄-4-OMe)₃) to yield [RhCl(COD)(TMOPP)] and [Rh(acac)(CO)(TPP)] is very rapid but is followed by a much slower process where the bidentate ligands are exchanged to yield [Rh(acac)(COD)] and a mixture of [RhCl(CO)(TPP)₂], [RhCl(CO)(TMOPP)₂], and [RhCl(CO)(TPP)(TMOPP)]. The exchange involving [RhCl(COD)(L)] and [Rh(acac)(CO)(L)] yields [Rh(acac)(COD)] and [RhCl(CO)(L)₂], where the reaction is much faster when L = TPP than when L = TMOPP. The mixed-metal system comprising [IrCl(COD)(TPP)] and [Rh(acac)(CO)(TPP)] yields all four complexes [M(acac)(COD)] and [MCl(CO)(TPP)₂], where M = Rh and Ir. This illustrates that both a neutral ligand exchange and an anionic ligand exchange occur. Possible pathways for these processes are discussed.

Full text of DOI:10.1002/ejic.201402810

FULL PAPER
DOI:10.1002/ejic.201402810
Slow Exchange of Bidentate Ligands between Rhodium(I)
Complexes: Evidence of Both Neutral and Anionic Ligand
Exchange
Si Chen,[a,b] Eric Manoury,[a,b] and Rinaldo Poli*[a,b,c]
Keywords: Rhodium / Ligand exchange / Phosphine ligands / Acetylacetonate ligands / Cyclooctadiene ligands
The phosphine double exchange process involving and
[Rh(acac)(CO)(L)]
yields
[Rh(acac)(COD)]
and
[RhCl(COD)(TPP)] and [Rh(acac)(CO)(TMOPP)] (TPP = PPh3,
TMOPP = P(C6H4-4-OMe)3) to yield [RhCl(COD)(TMOPP)]
and [Rh(acac)(CO)(TPP)] is very rapid but is followed by a
much slower process where the bidentate ligands are ex-
[RhCl(CO)(L)2], where the reaction is much faster when L =
TPP than when L = TMOPP. The mixed-metal system com-
prising [IrCl(COD)(TPP)] and [Rh(acac)(CO)(TPP)] yields all
four complexes [M(acac)(COD)] and [MCl(CO)(TPP)2],
where M = Rh and Ir. This illustrates that both a neutral li-
gand exchange and an anionic ligand exchange occur. Pos-
sible pathways for these processes are discussed.
changed to yield [Rh(acac)(COD)] and
a mixture of
[RhCl(CO)(TPP)2], [RhCl(CO)(TMOPP)2], and [RhCl(CO)-
(TPP)(TMOPP)]. The exchange involving [RhCl(COD)(L)]
through monomer–monomer interactions.[5] A very rapid
ligand exchange process was also observed for [Rh(acac)-
(CO)(PPh3)] (acac = acetylacetonate) with free PPh3.[6]
We have recently embarked on an investigation of the
hydroformylation reaction catalyzed by Rh complexes sup-
ported on precise phosphine-functionalized macromolec-
ular architectures built by controlled radical polymeriza-
tion.[7–9] These polymers were obtained by copolymeriza-
tion of styrene and 4-diphenylphosphinostyrene (DPPS);
they can be considered as having polystyrene-linked tri-
phenylphosphine ligands. With regard to these catalytic
studies, we have explored double exchange processes where
one phosphine ligand (P1) bonded to one type of Rh com-
plex (Rh1) exchanges with a second phosphine ligand (P2)
bonded to a second type of Rh complex (Rh2), see Equa-
tion (1).
Introduction
The mechanism of ligand exchange processes has long
been a topic of interest for coordination chemists.[1–3] Li-
gand exchange dynamics is of importance in all catalytic
processes, whether industrially or biologically relevant. Li-
gand exchange in square-planar d8 complexes has occupied
a dominant position, given the large number of catalytic
reactions promoted by d8 metal centers such as RhI, IrI,
NiII, PdII, PtII and AuIII. Most of the ligand exchange in-
vestigations have dealt with PtII complexes, given their rela-
tive inertness and stereochemical stability that bring the re-
actions within a suitable half-life range for convenient stud-
ies by classical mixing and monitoring methods.[3]
Ligand exchange in RhI complexes has been studied to
a lesser extent. It is nevertheless well appreciated that it oc-
curs predominantly via an associative pathway, as for the
other d8 systems and as anticipated by the “16 and 18 elec-
tron” rule.[4] For instance, NMR spectroscopic investi-
Rh1–P1 + Rh2–P2 i Rh1–P2 + Rh2–P1
(1)
This double exchange process on polymer-supported
gations on [RhCl(COD)(L)] (COD = 1,5-cyclooctadiene; L phosphine ligands has provided important information in
= PPh3, AsPh3) in the presence of excess L have revealed relation to polymer dynamics, which will be described sepa-
very fast (signal coalescence on the NMR timescale) and rately in a specialized polymer journal. Here, we report our
associative (first order in free L) ligand exchange. Further- results on the model system using the regular (nonpolymer-
more, fast exchange still occurs in the absence of free L supported) phosphine ligands since they provide interesting
new information on the coordination chemistry of rhodium
[a] CNRS, Laboratoire de Chimie de Coordination (LCC),
205 route de Narbonne, BP 44099, 31077 Toulouse Cedex 4,
France
and notably on the mechanism of ligand exchange involving
both neutral and anionic bidentate ligands.
E-mail: rinaldo.poli@lcc-toulouse.fr,
http://www.lcc-toulouse.fr/equipe_g/pages/poli/index.html
[b] Université de Toulouse, UPS, INPT,
31077 Toulouse Cedex 4, France
[c] Institut Universitaire de France,
Results
(a) The Phosphine Double Exchange Process
103, Bd. Saint Michel, 75005 Paris, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402810.
In order to conveniently follow the double exchange reac-
tion in Equation (1) by NMR spectrometry, we searched for
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a combination of two Rh systems and two phosphine li- δ1(31P NMR) = 29.90 ppm, δ2(31P NMR) = 24.82 ppm,
gands allowing us to individually detect all four compounds δ(103Rh NMR) = –8159 ppm, JP1Rh = 125.0, JP2Rh
in the mixture, at least by the better resolved 31P NMR 126.5 Hz, and JP1P2 = 361.3 Hz.
spectroscopic technique. A suitable combination was iden-
=
tified as [RhCl(COD)(L)] and [Rh(acac)(CO)(L)] with L =
PPh3 and P(C6H4-4-OMe)3. These ligands will be hence-
forth abbreviated as TPP for triphenylphosphine and
TMOPP for tris(4-methoxyphenyl)phosphine. The double
exchange process in Equation (1) was investigated by mix-
ing [RhCl(COD)(TPP)] (1) and [Rh(acac)(CO)(TMOPP)]
(2) in CDCl3. Relevant 31P NMR spectra were collected
in Figure 1. They demonstrate that the double phosphine
exchange process is very rapid; essentially an equilibrated
1:1:1:1 mixture of the four compounds is obtained rapidly
after mixing [see Figure 1, spectrum (c)]. The observed
Figure 2. 31P{1H} NMR spectrum (CDCl3) of the crystallized solid
after the complete reaction between 1 and 2.
chemical shifts and JPRh coupling constants for each com-
pound are in agreement with those reported in the litera-
ture: 1, δ = 30.68 ppm, JPRh = 150.6 Hz (ref. δ = 31.3 ppm,
JPRh = 150.1 Hz);[10] 2, δ = 44.14 ppm, JPRh = 173.7 Hz (ref.
δ
=
43.5 ppm, JPRh
=
175.6 Hz);[11] [Rh(acac)-
(CO)(TPP)] (3), δ = 48.67 ppm, JPRh = 175.0 Hz (ref. δ =
(b) Simpler Exchange Processes on Rh Complexes
48.6 ppm, JPRh = 179.7 Hz);[12] [RhCl(COD)(TMOPP)] (4),
δ = 27.00 ppm, JPRh = 149.4 Hz (ref. δ = 27.7 ppm, JPRh
=
In order to determine the precise nature of products 5,
148.7 Hz).[10] However, additional resonances were already 6, and 7, additional experiments were carried out by mix-
visible after 30 min in the δ = 32–23 ppm region and in- ing, on the one hand, pairs of Rh complexes having the
creased slowly, indicating the formation of additional prod- same ligand set except for the phosphine (i.e. 1 and 4 and
ucts.
separately 2 and 3) and, on the other hand, pairs of Rh
complexes having different ligand sets and the same phos-
phine (i.e. 1 and 3 and separately 2 and 4). The former two
experiments did not lead to any spectral evolution, whereas
the latter ones led to the evolutions illustrated in Figure 3
and Figure 4. The reaction between 1 and 3 led selectively
to the resonance of compound 5, whereas that between 2
and 4 led selectively to the resonance of compound 6. The
formation of compound 5 (Figure 3) is already quantitative
after mixing and immediately recording the spectrum, the
resonance of 1 at δ = 30.68 ppm (Figure 1, a) having com-
pletely disappeared. A small residual resonance of 3 at δ =
48.67 ppm remains present because this compound was
used in slight excess. The formation of compound 6 from 2
and 4 was much slower, since the resonances of both rea-
gents are still visible with a small intensity after 24 h (Fig-
ure 4, b). Both 5 and 6 were isolated from the final mixtures
Figure 1. 31P{1H} NMR spectra recorded for the reaction between
[RhCl(COD)(TPP)] (1) and [Rh(acac)(CO)(TMOPP)] (2) solvent =
CDCl3, room temperature. (a) complex 1; (b) complex 2; (c–e) 1:1
mixture, spectra recorded after the indicated time from mixing.
Further evolution of the mixture at room temperature by crystallization. Comparison with the literature[13,14] indi-
led essentially to the complete disappearance of the reso- cated that they correspond to trans-[RhCl(CO)(TPP)2] and
nances of the four above-mentioned complexes, indicating trans-[RhCl(CO)(TMOPP)2], respectively. The identity of
the irreversibility of the process. Crystallization of the final compound 6 was further confirmed by determination of the
solution by pentane vapor diffusion led to the deposition unit cell parameters of a single crystal, which matched those
of a crystalline solid that, after redissolution into CDCl3, reported for [RhCl(CO)(TMOPP)2].[15] The full spectral
afforded the 31P{1H} NMR spectrum shown in Figure 2. characterization of compounds 5 and 6 (see SI) also in-
Subsequent P–P COSY, P–Rh HMQC, and HSQC analyses cluded the 13C{1H} NMR spectrum, which has apparently
(see Supporting Information, Figures S1–S3) indicated that not been previously reported. The carbonyl C atom gives a
the mixture consists of three different compounds: one (5) complex multiplet from the coupling to the 103Rh and the
with δ(31P NMR)
=
28.97 ppm, δ(103Rh NMR)
=
two equivalent 31P nuclei. It is around δ = 187 ppm for both
–8169 ppm, and JPRh = 124.7 Hz, a second one (6) with compounds but the spectra are distinguished by the dif-
δ(31P NMR) = 24.76 ppm, δ(103Rh NMR) = –8149 ppm, ferent pattern for the aromatic C atoms and by the reso-
and JPRh = 123.9 Hz, and a third one (7) characterized by nance of the OMe C atom at δ = 55.4 ppm for compound
an ABX (P2Rh) system in the 31P NMR spectrum with 6.
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Figure 3. 31P{1H} NMR spectra recorded for the reaction between
1 and 3; solvent: CDCl3, room temperature. (a) immediately after
mixing; (b) redissolved crystallized product.
Scheme 1. Summary of the observed exchange processes.
Several mixed phosphine complexes of type
[RhCl(CO)(L1)(L2)], but not compound 7, have previously
been reported. Their 31P NMR properties closely parallel
those reported here for 7. However, they have been obtained
by phosphine exchange processes from [RhCl(CO)(L1)2]
and free L2, and therefore only in a phosphine-rich environ-
ment.[18] These reactions were described as very fast asso-
ciative processes, whereas the process described here entails
phosphine exchange from a Rh complex to another, ac-
companied by simultaneous rearrangement of the other
ligands and notably bidentate ones. The exchange of bi-
dentate ligands between two different RhI complexes has
not been the subject of extensive investigations.
Figure 4. 31P{1H} NMR spectra recorded for the reaction between
2 and 4; solvent: CDCl3, room temperature. (a) Immediately; (b)
after 24 h; (c) redissolved crystallized product.
The identification of the trans-[RhCl(CO)L2] products in
these simpler experiments implies the simultaneous forma-
tion of [Rh(acac)(COD)] (8), Equation (2) (L = TPP or
TMOPP). Clearly, when the phosphine exchange is carried
out on the mixture of both systems (L = TPP and TMOPP,
as in Figure 1), the mixed phosphine derivative
[RhCl(CO)(TPP)(TOMPP)] can also be generated by rapid
phosphine exchange. This mixed phosphine complex must
therefore correspond to product 7 [Equation (3)].
(c) Ligand Exchange between Rh and Ir Complexes
A final experiment consisted of running the same reac-
tion as in Equation (2) (L = TPP) except that one complex
contained Rh, 1, whereas the other one contained Ir,
[IrCl(COD)(TPP)] (9). The latter complex was generated in
situ by adding 1 equiv. of TPP per Ir atom to compound
[IrCl(COD)]2. This experiment was expected to lead to
either the products of Equation (4) or to those of Equa-
tion (4Ј), depending on the way in which the ligands are
exchanged. Exchanging the neutral ligands (CO and TPP
on the Rh complex with COD on the Ir complex) yields the
products of Equation (4), whereas exchanging the anionic
ligands (acac on the Rh complex with Cl and TPP on the
Ir complex) leads to the products of Equation (4Ј). Thus,
the results of this experiment provide useful information on
the ligand exchange mechanism.
[RhCl(COD)(L)] + [Rh(acac)(CO)(L)]Ǟ
[RhCl(CO)(L)2] + [Rh(acac)(COD)] (2)
[RhCl(CO)(TPP)2] + [RhCl(CO)(TMOPP)2]
i 2 [RhCl(CO)(TPP)(TMOPP)] (3)
The experimental study of this phenomenon was com-
pleted with the identification of the byproduct 8. Since this
compound is phosphorus free, its detection could only be
performed from its 1H NMR and 13C NMR properties. Af-
ter removal of most of the less soluble [RhCl(CO)(L)2] co-
products by crystallization, the residual solution indeed ex-
hibited spectral properties in agreement with those reported
for complex 8.[16,17] The observed ligand exchange processes
can be summarized as shown in Scheme 1.
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The 31P{1H} NMR results of this experiment are shown worth noting that the exchange of Cl and acac ligands be-
in Figure 5. After 7 h at room temperature, the starting ma- tween different metals is not unprecedented, and is reported
terial resonances are still visible (doublet at δ = 48.67 ppm for the exchange between various M(acac)2 and MЈCl2 com-
with JPRh = 175.0 Hz for 1 and singlet at δ = 21.93 ppm for plexes (M, MЈ = Mg, V, Fe, Co, Ni, Zn), leading in many
9). However, a new singlet at δ = 24.20 ppm can be assigned cases to the observation of bimetallic intermediates.[21–23]
to 10, the expected phosphine-containing products of
Equation (4) (literature: 23.40 ppm in CDCl3[14]) and a new
small doublet at δ = 29.27 ppm (JRhP = 126.8 Hz) can be
assigned to 5, the expected product of Equation (4Ј). The
Discussion
integrated intensities of complex 10 and 5 are in a 2.8:1
It is of interest to speculate on the mechanism of the slow
ratio. The 13C{1H} spectrum confirmed the presence of bidentate ligand exchange processes of Equation (2) and
both complex 8[16,17] and 11[19] through the characteristic Equation (4/4Ј). The redistribution, which will be repre-
resonances of the metal-bonded COD carbon atoms: doub- sented for a generic phosphine ligand L, must involve either
let at δ = 76.24 ppm (literature: 76.76 ppm),[16,17] JCRh
=
exchange of neutral ligands – a cyclooctadiene on the M
14 Hz for complex 8 and singlet at δ = 58.87 ppm (litera- complex, where M = Rh for Equation (2) or Ir for Equa-
ture: 59.3 ppm)[19] for complex 11. This spectrum is shown tion (4), with L and CO on the other Rh complex – or ex-
in the SI (Figure S4). The intensity of the resonance of com- change of the anionic ligands – acac on the Rh complex
plex 8 is greater than that of 11 by a factor of 15.0. Since with Cl, accompanied by L, on the M complex, where M
the reaction was carried out with equimolar amounts, the = Rh for Equation (2) or Ir for Equation (4Ј). In order to
10/5 and 8/11 ratios should be identical, but identical inten- experimentally distinguish between the two possibilities for
sity ratios are not to be expected because the NMR integra- the Rh-only system of Equation (2), it would be necessary
tion for the Overhauser-enhanced resonances of the slow- to carry out an isotope labeling experiment where the label
relaxing 31P{1H} and 13C{1H} nuclei does not carry quanti- is on the metal atom, which is impossible with naturally
tative information. The proton environment of these nuclei occurring isotopes because the metal is 100% 103Rh. How-
in the two compounds should be rather similar, suggesting ever, a related reaction where one compound was labeled
that the Overhauser effect may not be significantly different using the Ir congener [Equation (4) and Equation (4Ј)],
in each pair of related compounds. However, the relaxation showed the occurrence of both exchange pathways. Obvi-
times could be significantly different because in one case ously, this result only proves that the mixed-metal system is
the observed nucleus is bonded to a magnetically dipolar able to follow both exchange pathways. The Rh-only system
103Rh nucleus, whereas in the other case it is bonded to a of Equation (2) could undergo the slow bidentate ligand ex-
quadrupolar (I = 3/2) Ir nucleus. In order to reconcile the change by only one of the two possible schemes. However,
different observed intensities, we must consider that the res- it seems reasonable to extrapolate the result of the mixed-
onance intensity of the Rh complex is underestimated rela- metal system to the Rh-only system. Importantly, the op-
tive to that of the Ir complex in at least one (but probably erating mechanism must be able to rationalize the large rate
both) of the NMR spectra. At any rate, the two NMR mea- difference observed when L = TPP or TMOPP.
surements consistently indicate that the exchange proceeds
We start by analyzing the “neutral ligand exchange”
preferentially through Equation (4), by a factor between the pathway. The system does not contain any free neutral li-
lower (31P{1H} NMR) and upper (13C{1H} NMR) inte- gand capable of triggering an associative exchange pathway,
grated intensity limits of 2.8 and 15.0.[20]
since the solvent chloroform has no significant coordinating
properties. As mentioned in the introduction, ligand ex-
change processes in RhI complexes are generally associative,
but a few examples where the metal reactivity (ligand ex-
change or other) is triggered by ligand dissociation exist,
including dissociation of N2 trans to an aryl group[24] and
SiPr2 trans to an amido donor.[25] The dynamic behavior of
complexes [RhX(PPh3)3] (X = Cl, CF3, H, CH3, Ph) is a
peculiar example of a dissociative self-exchange process.[26]
However, all of these processes deal with monodentate li-
gands.
A reasonable dissociative pathway for the exchange of
neutral ligands may be conceived as shown in Scheme 2.
Given the known trends of trans effects of ligand bond
dissociation energies, and of chelating effects, the most
likely initial dissociation is that of L trans to one of the
COD double bonds in complex [RhCl(COD)L] yielding in-
termediate A, but this process is unproductive. Dissociation
Figure 5. 31P{1H} NMR spectra recorded for the reaction between
1 (blue spectrum) and 9 (red spectrum). The violet spectrum was
recorded 7 h after mixing; solvent: CD2Cl2, room temperature. The
resonance marked with an asterisk corresponds to an impurity
(Ph3PO).
Hence, the reaction occurs through both mechanisms, of L trans to one acac O atom in the other reagent yields
but the neutral ligand exchange pathway prevails. It is intermediate B. Next, compound [RhCl(COD)(L)] may re-
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act with additional L, generated during the reversible for- leading to a similar dinuclear complex EЈ, also seems pos-
mation of either A or B, to open the COD chelate and yield sible but would be unproductive. In order to satisfy first
intermediate C, possibly via an associative pathway. This principles, all elementary steps envisaged for this “anionic
intermediate may then react with B to afford the COD- ligand exchange” mechanism are such that they produce
bridged bimetallic complex D. There are many ways in neutral systems (i.e. no charge separation) and maintain a
which this exchange may further proceed to the final prod- square-planar configuration around each RhI center in all
ucts, but the important points are the formation of the bi- intermediates. Although pentacoordination is possible in
metallic intermediate and the initial dissociation of L. On RhI chemistry, square-planar complexes are preferred when
the basis of this hypothesis, the observed trend of reactivity potential π-donor atoms such as Cl or O are present in the
(much faster rate when L = TPP) appears consistent with coordination sphere. Thus, in intermediate E, for instance,
the literature. Indeed, through calorimetric studies, Nolan the positive charge of the Rh atom on the left hand side is
et al. reported that the reaction between [RhCl(COD)]2 and saturated by the covalent interaction with the bridging Cl
L to yield [RhCl(COD)L] is more favorable for TMOPP atom, whereas the bond of this Cl atom to the Rh center
(58.7Ϯ0.3 kcal/mol) than for TPP (51.7Ϯ0.3 kcal/mol).[27] at the right is dative. The charge of the Rh atom on the
Hence, the dissociation of TMOPP from RhI is expected to right hand side is saturated by the enolate of the mono-
be much slower than that of TPP.
dentate acac ligand. In the next step, the Cl and acac ligand
swap positions through an exchange reaction that involves
attack of the Rh atom on the left hand side by the lone pair
of the free acac carbonyl function, as suggested in
Scheme 3, for a net charge change of zero and formation of
intermediate F. From here onward, it is easy to see how the
exchange may continue, with either associative or dissoci-
ative processes, to complete the ligand exchange.
Scheme 2. Possible mechanism for the neutral ligand exchange
leading from [MCl(COD)L] and [Rh(acac)(CO)L] to [Rh(acac)-
(COD)] and [MCl(CO)L2] (M = Rh, Ir).
Turning now to the “anionic ligand exchange” pathway,
it is clear that a dissociative process involving dissociation
of the anionic ligands and charge separation would be diffi-
cult, especially in a low polarity solvent such as chloroform.
However, an associative process seems feasible. It is also
possible that the association via formation of bridged dinu-
clear intermediates is triggered by dissociation of a neutral
ligand. This is suggested by the literature report of rapid
halide scrambling between [RhBr(CO)(TPP)2] and
[IrCl(CO)(TPP)2].[18] The dynamic exchange on the 1H
NMR timescale of the two inequivalent halves of the COD
ligand in compound [RhCl(COD)L] (L = PPh3, AsPh3),
which is kinetically second order in the metal complex,
might also involve halide-bridged intermediates.[5] We can
Scheme 3. Possible mechanism for the anionic ligand exchange
leading from [MCl(COD)L] and [Rh(acac)(CO)L] to [M(acac)-
(COD)] and [RhCl(CO)L2] (M = Rh, Ir).
The O atoms in [Rh(acac)(CO)L] are also centers of nu-
thus propose that the first step of the reaction between cleophilic reactivity. Therefore, it is possible in principle to
[RhCl(COD)L] and [Rh(acac)(CO)L] is the formation of a envisage another anionic ligand exchange pathway, starting
dinuclear halide bridged complex (E in Scheme 3) with the with attack of a rhodium complex by one O lone pair of the
elimination of a ketone group of the acetylacetonate, which acac ligand in the second complex. However, compounds of
rearranges to monodentate coordination. The associative the type [Rh(acac)(CO)L] have been reported not to lead to
pathway is shown in Scheme 3, but the dissociative variant coalescence of the asymmetric acac resonances,[28] even
would of course lead to the same result. The alternative upon warming, although the phenomenon is observed in
exchange (associative or dissociative) of a phosphine ligand, the presence of excess L.[12] This suggests that ligand ex-
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ated solvent as internal standard and are reported in ppm (δ) rela-
tive to tetramethylsilane. 31P NMR chemical shifts are reported
relative to external 85% H3PO4. Peaks are labeled as singlet (s),
doublet (d), triplet (t), multiplet (m), and broad (br). The proton
and carbon assignments were assisted by 1H–13C HMQC experi-
ments. Complexes [RhCl(COD)(TPP)],[29] [Rh(acac)(CO)(TPP)],[30]
[Rh(acac)(CO)(TMOPP)],[11] and [IrCl(COD)(TPP)][31] were syn-
thesized by procedures closely related to those reported in the lit-
erature (details in SI).
change by self-association, if it occurs, is a slower process
for [Rh(acac)(CO)L] than for [RhCl(COD)L].
Note that the first two exchange processes in the “anionic
ligand exchange” pathway up to intermediate F do not in-
volve L dissociation, therefore they do not account for the
marked reactivity difference in rate when L = TPP or
TMOPP. L dissociation only occurs in the further steps go-
ing from F to the products. Therefore, the pathway of
Scheme 3 can be reconciled with the experimentally ob-
served trend only if intermediate F is generated by fast pre-
equilibrium processes, relative to the L dissociation process
that occurs in a later step and would be rate limiting.
Finally, it is necessary to comment on the difference in
rate between Equation (2) when L = TPP, which is very ra-
pid as shown in Figure 3, and Equations (4/4Ј) where the
Isolation of a Mixture of [Rh(CO)Cl(TPP)2] (5), [Rh(CO)Cl-
(TMOPP)2] (6), and [Rh(CO)Cl(TPP)(TMOPP)] (7): The two
separately prepared solutions of 2 (35 mg, 0.06 mol) in CH2Cl2
(1 mL) and 1 (30.5 mg, 0.06 mmol) in CH2Cl2 (1 mL) were com-
bined at room temperature. The resulting mixture was stirred over-
night. The resulting solution was concentrated to ca. half the vol-
ume and then diffusion of pentane vapors yielded a crystalline so-
ligand is again TPP, which is on the other hand much lid, yield 29 mg. The solid was characterized by 31P{1H} NMR (see
Figure 2) and by 31P–31P COSY, 31P–103Rh HMQC, and 31P–103Rh
slower. This difference may be explained by the stronger Ir–
ligand bonds relative to the corresponding Rh–ligand
HSQC (see SI) in CDCl3.
bonds. For the “neutral ligand exchange” pathway of
Scheme 2, the initial TPP dissociation would not be dis-
criminating since it always occurs on the Rh complex, but
Reaction Between [RhCl(COD)(TPP)] (1) and [Rh(acac)(CO)
(TPP)] (3): Generation of [Rh(CO)Cl(TPP)2] (5) and [Rh(acac)-
(COD)] (8). Two separately prepared solutions of 1 (30.5 mg,
the COD dissociation step leading to intermediate C is 0.06 mmol) in CH2Cl2 (1.5 mL) and 3 (29.5 mg, 0.06 mmol) in
CH2Cl2 (1.5 mL) were combined and the resulting mixture was
stirred at room temperature, progressively depositing a yellow pre-
cipitate. After 5 h, the solid was filtered, washed with pentane, and
dried under vacuum. Pentane (20 mL) was added to the filtrate to
yield an additional yellow crystalline precipitate, which was again
filtered off, washed, and dried. These solids were identified as com-
plex 5 by NMR spectroscopy (see below) and compared with those
in the literature.[13] The residual yellow solution was evaporated
under reduced pressure to yield a yellow-brown solid, identified by
NMR spectroscopy as complex 8 by comparison of its 1H and
13C{1H} NMR spectra (see below) with those in the literature.[16,17]
[Rh(CO)Cl(TPP)2]: 1H NMR (400 MHz, CDCl3): δ = 7.78–7.74
(m, 6 H, CHAr), 7.44–7.4 (m, 9 H, CHAr) ppm. 31P{1H} NMR
(162 MHz, CDCl3, 298 K): δ = 28.97 (d, JP-Rh = 126.4 Hz) ppm.
likely to be much slower for the iridium complex. For the
“anionic ligand exchange” pathway of Scheme 3, it is the
Ir–TPP bond dissociating in one of the later rate-limiting
steps that would make the difference in the observed ex-
change rates.
Conclusions
The rapid phosphine double exchange of Equation (1),
using the 1/2 combination, has unveiled an unexpected side
reaction consisting of the slow exchange of the bidentate
ligands, leading to the formation of complex 8 and a statis-
tical mixture of 5, 6, and 7. Control experiments involving
13C{1H} NMR (101.5 MHz, CDCl3): δ = 187.6–186.6 (m, CO),
1
the reactions between [RhCl(COD)(L)] and [Rh(acac)- 134.73, 132.96, 130.09, 128.12 (CHAr). [Rh(acac)(COD)] ppm. H
NMR (400 MHz, CDCl3, 298 K): δ = 5.34 (s, 1 H, CHacac), 4.09
(s, 4 H, CHcod), 2.49–2.46 (m, 4 H, CH2 cod), 1.95 (s, 6 H, CH3 acac),
1.87–1.81 (m, 4 H, CH2 cod) ppm. 13C{1H} NMR (101.5 MHz,
CDCl3, 298 K): δ = 186.64 (s, COacac), 134.8–134.4, 131.2–131.9,
130.24, 128.49, 128 (CHAr), 99.76 (d, J = 2.03 Hz, CHacac), 76.47
(d, J = 14.7 Hz, CHcod), 30.24 (s, CH2 cod), 27.36 (s, CH3 acac) ppm.
(CO)(L)] for L = TPP or TMOPP, as well as involving the
mixed-metal system 1 and 9 have provided useful infor-
mation on the mechanism of this process. It has been dem-
onstrated that both the neutral ligands (bidentate COD
with CO and L) and the anionic ligands (bidentate acac
with Cl and L) can be exchanged, at least for the mixed-
metal system.
Reaction Between [RhCl(COD)(TMOPP)] (4) and [Rh(acac)(CO)
(TMOPP)] (2): Generation of [Rh(CO)Cl(TMOPP)2] (6) and
[Rh(acac)(COD)] (8). This reaction was carried out according to
the same protocol described in the previous section for the corre-
sponding TPP complexes, starting from complex 2 (34.9 mg,
0.06 mmol) in CH2Cl2 (1.5 mL) and 4 (0.06 mmol) in CH2Cl2
(1.5 mL). The latter was generated in situ from [Rh(COD)Cl]2
(14.8 mg, 0.03 mmol) and TMOPP (21.14 mg, 0.06 mmol). The re-
Experimental Section
General: All manipulations were performed under an inert atmo-
sphere of dry argon by using a vacuum line and Schlenk-tube tech-
niques. Acetylacetonatodicarbonylrhodium(I), [Rh(acac)(CO)2]
(99% Strem),
chloro(1,5-cyclooctadiene)rhodium(I) dimer, covered yellow precipitate (same workup as above) was identified
[Rh(COD)Cl]2 (98%, Strem), chloro(1,5-cyclooctadiene)iridium(I)
dimer, [Ir(COD)Cl]2 (99%, Strem), tris(4-methoxyphenyl)phos-
phine, TMOPP (Ͼ95%, TCI), and triphenylphosphine, PPh3
(Ͼ98.5%, Fluka) were used as received. Solvents were dried by
standard procedures and distilled under argon prior to use. 1D-
and 2D-NMR spectra were recorded in 5-mm tubes at 297 K with
Bruker Avance 400 and 500 spectrometers. 1H NMR and 13C NMR
chemical shifts were determined using the residual peak of deuter-
as complex 6 by comparison of its NMR properties with those
in the literature,[14] while the residue recovered from the solution
corresponded again to complex 8. [Rh(CO)Cl(TMOPP)2]: 1H
NMR (400 MHz, CDCl3): δ = 7.67–7.62 (m, 6 H, CHAr), 6.92 (d,
6 H, CHAr), 3.83 (s, 9 H, CH3 OMe) ppm. 31P{1H} NMR (162 MHz,
CDCl3, 298 K): δ = 24.8 (d, JP-Rh = 124.74 Hz) ppm. 13C{1H}
NMR (101.5 MHz, CDCl3): δ = 187.2 (m, CO), 160.86 (Cq), 136.09
(CHAr), 124.89 (Cq), 113.7 (CHAr), 55.4 (CH3 OMe) ppm. In ad-
Eur. J. Inorg. Chem. 2014, 5820–5826
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
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Acknowledgments
The authors are grateful to the Agence Nationale de la Recherche
(ANR) for support of this work through grant “BIPHASNANO-
CAT” (ANR-11-BS07-025-01). Additional support from the Cen-
tre National de la Recherche Scientifique (CNRS) and from the
Institut Universitaire de France (IUF) is also gratefully acknowl-
edged. Dr. Y. Coppel is thanked for useful discussions on the NMR
spectroscopic results.
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Received: August 26, 2014
Published Online: October 30, 2014
Eur. J. Inorg. Chem. 2014, 5820–5826
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim