Oxidation-promoted activation of a ferrocene C-H bond by a rhodium complex

Article abstract of DOI:10.1039/c3dt50240f

The oxidation of a rhodium(i) complex containing a ferrocene-based heterodifunctional phosphine N-heterocyclic carbene (NHC) ligand produces a stable, planar chiral rhodium(iii) complex with an unexpected C-H activation on ferrocene. The oxidation of rhodium(i) to rhodium(iii) may be accomplished by initial oxidation of ferrocene to ferrocenium and subsequent electron transfer from rhodium to ferrocenium. Preliminary catalytic tests showed that the rhodium(iii) complex is active for the Grignard-type arylation of 4-nitrobenzaldehyde via C-H activation of 2-phenylpyridine.

Full text of DOI:10.1039/c3dt50240f

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Oxidation-promoted activation of a ferrocene C–H
bond by a rhodium complex†
Cite this: DOI: 10.1039/c3dt50240f
Agnès Labande,*a,b Nathalie Debono,a,b Alix Sournia-Saquet,a,b Jean-Claude Darana,b
and Rinaldo Polia,b,c
The oxidation of a rhodium(I) complex containing a ferrocene-based heterodifunctional phosphine
N-heterocyclic carbene (NHC) ligand produces a stable, planar chiral rhodium(III) complex with an un-
expected C–H activation on ferrocene. The oxidation of rhodium(I) to rhodium(III) may be accomplished
by initial oxidation of ferrocene to ferrocenium and subsequent electron transfer from rhodium to ferro-
cenium. Preliminary catalytic tests showed that the rhodium(III) complex is active for the Grignard-type
arylation of 4-nitrobenzaldehyde via C–H activation of 2-phenylpyridine.
Received 4th January 2013,
Accepted 13th February 2013
DOI: 10.1039/c3dt50240f
www.rsc.org/dalton
Introduction
Cyclometalation via C–H activation has been well known for
decades, especially with the use of pincer ligands on a large
variety of metals.1 The mechanisms for C–H activation involve
principally oxidative addition on a low oxidation state metal,
electrophilic bond activation or σ-bond metathesis.1e C–H acti-
vation on rhodium complexes has been widely studied,2–4
however, only two examples in the literature describe the C–H
activation by oxidative addition on rhodium(I) complexes with
PCP pincer ligands bearing a ferrocenyl unit in place of a
phenyl.5 One example of C–H activation has been observed on
an iridium(I) complex bearing a heterodifunctional ferrocenyl-
phosphine–pyridine ligand, although the corresponding
rhodium(I) complex only shows a Rh⋯HC agostic interaction.6
Activation on rhodium(III) complexes covers Caryl–H7 as well as
Fig. 1 Rhodium(I) complex 1 studied in this work.
ligands.11 Based on the pioneering work of Wrighton12 and
Mirkin13 and the more recent one of Long14 and Bielawski15
on the influence of redox-active ligands on the reactivity of the
metal center, we have investigated the behavior of rhodium(I)
complex 111a (Fig. 1) upon oxidation. The phosphine group in
the ferrocene-based heterodifunctional phosphine–NHC
ligand is directly linked to the ferrocenyl unit, whereas the
NHC is moved away from it by a methylene spacer. As the
phosphine group is connected to both ferrocene and rhodium,
we envisioned that the electronic changes on ferrocene would
be efficiently transmitted to the metal center,12c,16,17 making it
more electrophilic upon oxidation. In the process of studying
the redox chemistry of compound 1, we have discovered an
unprecedented C–H activation of a ferrocene C–H bond on Rh
triggered by ferrocene oxidation, eventually leading to a planar
chiral rhodium(III) complex, and we present the results of this
investigation in this contribution.
C
alkyl–H8 bonds. In this context, half-sandwich rhodium(III)
complexes such as [Cp*RhCl2]2 and [Cp*Rh(MeCN)3]X2
proved particularly useful for the catalyzed functionalization of
C–H bonds,4,9 but very few contributions deal with chiral com-
plexes9g,h and we only found one example reporting catalytic
C–H activation with a non-half-sandwich complex.10
Our group has a strong interest in the coordination chemi-
stry and catalytic activity of functionalized N-heterocyclic
carbene (NHC) ligands, among them redox-active ferrocenyl
aCNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne,
BP 44099 F-31077 Toulouse Cedex 4, France. E-mail: agnes.labande@lcc-toulouse.fr;
Fax: (+33) 561553003; Tel: (+33) 561333158
Results and discussion
bUniversité de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France
cInstitut Universitaire de France, 103 bd Saint-Michel, F-75005 Paris, France
†Electronic supplementary information (ESI) available: Synthetic, spectroscopic,
A cyclic voltammetry (CV) analysis of 1 in CH2Cl2 showed a
reversible redox wave for the ferrocene unit at E°1/2 = 0.35 V vs.
[FcH/FcH+]18 and a second irreversible process at Ep,a = 0.95 V
vs. [FcH/FcH+] that could be assigned to the oxidation of Rh(I)
(Fig. 2, top). A square wave voltammetry analysis at the
crystallographic, electrochemical and computational details. CCDC 905985. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt50240f
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Fig. 3 Square-wave voltammograms recorded during the electrolysis of
complex 1 at 0.49 V vs. [FcH/FcH+] in MeCN–nBu4NBF4 (0.1 M); frequency 20
Hz, step potential 5 mV, amplitude 20 mV. Dashed curve: Rh(I) complex 1; plain
curve: intermediate solution, 15 min after interruption of electrolysis.
Fig. 2 Cyclic voltammograms on a Pt microelectrode of complex 1, top: 1 mM
in CH2Cl2 with nBu4NBF4 (0.1 M) at a scan rate of 0.1 V s−1; bottom: 1 mM in
MeCN with nBu4NBF4 (0.1 M) at a scan rate of 0.2 V s−1
.
potential of the first oxidation wave (0.35 V vs. [FcH/FcH+])
gave a total of 0.91 F mol−1, which indicated the transfer of
one electron per molecule.19
Fig. 4 Square-wave voltammograms recorded during the electrolysis of
complex 1 at 1.19 V vs. [FcH/FcH+] in MeCN–nBu4NBF4 (0.1 M); frequency 20
Hz, step potential 5 mV, amplitude 20 mV. Dotted curve: Rh(I) complex 1;
dashed-dotted curve: solution immediately after interruption of electrolysis (2 F
mol−1); dashed curve: intermediate solution, 2 h after interruption of electro-
lysis; plain curve: final solution, 20 h after interruption of electrolysis.
The CV of 1 in acetonitrile showed a different behavior.
Besides a shift of the two waves to less positive potentials (E1°/2
=
0.31 V and Ep,a = 0.8 V vs. [FcH/FcH+] for the reversible ferro-
cene and the irreversible Rh(I) oxidations, respectively), two
additional reduction waves, not present in CH2Cl2, now appear consumption of 2 F mol−1 of the Rh complex, decomposition
at 0.53 V and −0.47 V (see Fig. 2, bottom).
occurred resulting in partial loss of the ferrocene wave.
The isolation and characterization of the product were then
The controlled potential electrolysis of 1 in MeCN at 0.49 V
vs. [FcH/FcH+], a potential once again sufficient to oxidize the attempted by chemical oxidation experiments. The oxidants
ferrocene unit selectively, resulted in transfer of greater 1,1′-diacetylferrocenium tetrafluoroborate [FcAc][BF4] (E1°/2
=
charge. After 165 min of electrolysis, 2 F mol−1 had passed 0.49 V vs. [FcH/FcH+] in MeCN) and thianthrenium tetrafluoro-
through the cell and 1 was completely transformed into a new borate [Th][BF4] (E°1/2 = 0.86 V vs. [FcH/FcH+] in MeCN) were
species 2, which shows a redox process at 0.58 V vs. [FcH/FcH+] selected because of their suitable oxidation potentials, their
(Fig. 3). Upon interrupting the current flow at this stage, 2 “innocent” character and the expected ease of purification of
evolved spontaneously and slowly to product 3 (0.32 V vs. the oxidized Rh complex (Fig. 5).20 As calculated from the vol-
[FcH/FcH+]) which contains ferrocene in its reduced state, as tammogram of 1 in MeCN, [FcAc][BF4] should oxidize only
indicated by its electrochemical properties (Fig. 3, plain curve). ferrocene while [Th][BF4] should be capable of oxidizing
When the electrolysis was carried out at 1.19 V vs. [FcH/FcH+] ferrocene and rhodium. The use of d6-MeCN as a solvent
(thus beyond the oxidation potential of rhodium) and inter- allowed 1H and 31P NMR monitoring.
rupted after counting 2 F mol−1, the same intermediate
When using [FcAc][BF4], only minute amounts of 3 were
species 2 was again generated selectively after only 50 min, fol- detected by 31P NMR as a doublet at ca. δ 41 (JRh–P = 138 Hz)
lowed by conversion to 3, which was almost complete 20 h after 12 h.21 Greater insight was obtained from the experiment
after interruption of electrolysis (Fig. 4).
carried out with 1.5 equivalents of [Th][BF4]. The 31P NMR
This is a first indication that ferrocene can act as an elec- spectrum after 10 min showed a large signal at δ 28.5 that
tron relay for oxidation of the Rh center. When, on the other evolved after 4 h 15 min to a well-defined major doublet at δ
hand, the same electrolysis was continued beyond the 28.7 assigned to the intermediate 2, along with some starting
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Fig. 5 Chemical oxidants [FcAc][BF4] (left) and [Th][BF4] (right).
Scheme 2 Synthesis of complexes 4 and 5.
Scheme 1 Oxidation of rhodium(I) complex 1 to rhodium(III) complex 3.
Hence, intermediate 2 contains the unactivated ferrocenyl
ligand while the COD ligand is no longer present.
material (doublet at δ 19.5) and a small doublet at δ 41.6
The crystallization of 3 proved unsuccessful whatever the
(JRh–P = 140 Hz, complex 3). The NMR monitoring showed the solvent/technique, but the addition of one equivalent of 2,2′-
gradual disappearance of 2 in favor of the growth of 3. The bipyridine (bipy) to this complex in CH2Cl2 gave an orange-red
transformation was complete after 48 h, with ca. 25% of 1 solid after purification. Slow evaporation of a MeCN–C6F6 solu-
remaining unreacted. The observation of these 31P resonances tion of the latter led to X-ray quality crystals of 4, where one
suggests that compounds 2 and 3 are diamagnetic. The final bipy and one Cl− ligand (coming from the solvent) occupy the
1H NMR spectrum also shows the resonances of reduced thi- three MeCN coordination positions in 3 (Scheme 2 and Fig. 6).
anthrene and free 1,5-cyclooctadiene. The latter is released Selected bond distances and angles are shown in Table 1.
during the first step of the process leading from 1 to 2. Use of
The Rh(1)–C(1) distance is in the range of previously
an excess amount (2.1 equivalents) of [Th][BF4] led to initial described Rh complexes with pincer-type σ-ferrocenyl
broadening of the 31P resonance of 2 and did not allow the ligands,5b and the other distances are within the expected
detection of the resonance of 3, presumably because of over- range for this type of complex, yet this structure displays a few
oxidation and rapid self-exchange between 3 and 3+ (Scheme 1). remarkable features. Firstly, it confirms the presence of a
The orange product 3 was isolated in 75% yield by repeated Rh–C bond at ferrocene with a fac arrangement of the tridentate
precipitation and washing with diethyl ether, and character- ligand. A second interesting aspect is the generation of both
ized by NMR and mass spectrometry.22 It proved stable at planar chirality at ferrocene and central chirality at the metal,
room temperature and even air-stable as a solid. To our sur-
prise, 1H and 13C NMR analyses revealed only seven proton reso-
nances for the ferrocene unit when we expected eight.
Additional 13C NMR and 2D experiments (COSY, HSQC and
HMBC) confirmed the seven C–H bonds and 13C NMR revealed
the presence of three quaternary carbon signals in the ferro-
cene region, one of which appeared as a doublet of doublets
with a large coupling constant of 30 Hz to the Rh atom.23 This
suggested that ferrocene C–H activation and creation of a
C–Rh bond has occurred, hence generating planar chirality at
ferrocene. The carbenic carbon was found at 147.2 ppm,
almost 30 ppm upfield relative to 1 (176.9 ppm), indicating
binding of the NHC to a rhodium(III) center.24 Additional NMR
data acquired in CD2Cl2 account for three MeCN molecules,
one of which is in rapid exchange with free acetonitrile.
Complex 2 could not be isolated, since it evolved spon-
1
taneously to 3 even in the solid state, but its H and 13C NMR
Fig. 6 ORTEP view of the cation in compound 4. Ellipsoids are shown at the
properties indicate the presence of eight ferrocene C–H bonds. 50% probability level. All hydrogens are omitted for clarity.
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Table 1 Selected bond distances (Å) and angles (°) for the experimental struc-
ture of 4 and for the optimized geometries of 4, 5 and 6
X-Ray
4
DFT
4
5
6
Distances (Å)
Rh(1)–Cl(1)
Rh(1)–P(1)
Rh(1)–N(3)
Rh(1)–N(4)
Rh(1)–C(1)
Rh(1)–C(11)
2.4313(6)
2.3210(6)
2.104(2)
2.156(2)
2.028(2)
2.022(2)
2.499
2.454
2.116
2.226
2.042
2.061
2.577
2.463
2.153
2.112
2.041
2.062
2.474
2.482
2.237
2.140
2.046
2.073
Angles (°)
Cl(1)–Rh(1)–P(1)
Cl(1)–Rh(1)–N(3)
Cl(1)–Rh(1)–N(4)
Cl(1)–Rh(1)–C(1)
Cl(1)–Rh(1)–C(11)
P(1)–Rh(1)–N(3)
P(1)–Rh(1)–N(4)
P(1)–Rh(1)–C(1)
P(1)–Rh(1)–C(11)
N(3)–Rh(1)–N(4)
N(3)–Rh(1)–C(1)
N(3)–Rh(1)–C(11)
N(4)–Rh(1)–C(1)
N(4)–Rh(1)–C(11)
C(1)–Rh(1)–C(11)
C(6)–P(1)–Rh(1)
101.98(2)
82.78(5)
83.56(5)
87.57(7)
168.68(7)
175.08(5)
101.58(6)
87.00(7)
88.92(7)
77.56(8)
94.51(8)
86.40(8)
168.75(8)
97.38(8)
89.97(9)
113.85(8)
94.94
83.63
80.19
89.76
173.51
178.09
101.50
88.88
91.47
77.03
92.37
89.99
166.07
99.60
89.29
112.75
95.06
80.56
84.07
168.20
99.85
101.02
178.64
91.69
87.75
77.82
88.67
171.17
88.99
93.43
90.06
110.65
175.27
83.52
83.63
91.07
86.33
100.82
99.24
84.94
90.95
74.98
169.26
101.26
95.24
169.62
87.59
109.76
Fig. 7 Relative electronic energies (in kcal mol−1) and figures of the optimized
structures for the three possible isomers of the crystallographically characterized
compound 4.
geometry of 4 on the basis of the experimental structure as a
starting geometry, and then the other two possible isomers by
transposing the positions of the Cl and bipy ligands. The suit-
ability of the computational level (see ESI†) is verified by the
relatively good match between experimental and computed
geometric parameters of 4, as shown in Table 1. The computed
distances between the metal and the donor atoms are slightly
longer, within 0.1 Å of the experimental value except for the P
donor for which the difference is 0.13 Å, as is commonly
found for this level of theory. The match in the angular para-
meters is less satisfactory with a deviation up to ca. 7° for the
P–Rh–Cl angle, whereas all the other angles are in better
agreement.
The energy results are shown in Fig. 7. For isomer 5 with
the Cl atom placed trans to the metallated Cp ring, a lower
energy was obtained in agreement with the NMR assignment.
For the third isomer (6), containing the Cl ligand trans to the P
donor, construction of the starting geometry immediately
revealed a severe steric limitation caused by the short van der
Waals contacts between the NHC methyl substituent and the
o-H atom of the bipy ligand. The geometry optimization con-
verged to a final structure where the NHC is bent away from
the ferrocene Cp plane, on one side of the Rh equatorial
coordination plane, while the adjacent bipy ring is bent away
in the opposite direction (see Fig. 7), resulting in significant
distortion of the preferred octahedral coordination geometry
at the metal and of the preferred coplanarity of the bipy ligand
and in a much higher energy minimum, 8.4 kcal mol−1 higher
than the most stable isomer 5. The difference between the cal-
culated electronic energies of 4 and 5 (3.7 kcal mol−1) is too
high to rationalize the observed relative 4 : 3 population of the
two isomers, for which a ΔG of only 0.17 kcal mol−1 is
expected. However, the prediction of relative stability is in the
right direction. More sophisticated calculations including the
zero point vibrational energy and the thermal corrections to
enthalpy and entropy were not carried out because they were
prohibitive in this case, given the large size of the calculation
(all atoms were treated quantomechanically).
hence the potential production of up to three diastereoisomers
in this case (given the rigidity of the tridentate ligand and the
strength of the bonds with the rhodium center, imposing the
fac stereochemistry). Finally, the tridentate ligand adopts a
severely distorted geometry to accommodate the octahedral
coordination of rhodium. Indeed, the octahedral coordination
is also distorted (see Fig. 6). The metal coordination forces the
phosphorus atom to deviate significantly from the plane of
the Cp ring to which it is attached by 0.357(1) Å, and tilts the
two ferrocene cyclopentadienyl units to a dihedral angle of
10.9(2)°.
A solution of the orange-red solid in acetone-d6 displays
two 31P NMR doublets at δ 35.8 (JRh–P = 128 Hz) and δ 31.5
(JRh–P = 127 Hz) in a 4 : 3 ratio, and two sets of 1H NMR
signals, which we attributed to the existence of two diastereo-
isomers. Hence, one of the three possible diastereoisomers is
not observed. The carbenic carbon atom in both isomers
appears around 154.4 ppm. However, the newly formed Rh-
bonded quaternary carbon atom is observed at δ 62.94 for the
major isomer and at δ 76.61 for the minor one. This important
shift may be imputed to a different trans influence of the Cl−
and bipy ligands.2e Finally, a ROESY experiment showed
dipolar coupling between a bipyridyl proton and two ferrocenyl
protons (one on each Cp) for the major species, which does
not fit the geometrical features of the structure represented in
Fig. 6. Therefore, we tentatively attribute the minor species to
the crystallographically characterized diastereoisomer 4 and
the major species to the diastereoisomer bearing the Cl−
ligand trans to the Cp ligand, 5.
In order to confirm this assignment, DFT calculations were
carried out on the three possible isomers, optimizing first the
The mechanism of formation of 3 probably involves initial
oxidation of rhodium(I) in 1 to rhodium(III), facilitating the
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Scheme 4 Reaction of 2-phenylpyridine with 4-nitrobenzaldehyde.
compound 1. Unfortunately, it was not possible to determine
whether a rhodium(II) species was indeed involved in the
process, as RPE experiments conducted at 113 K immediately
after addition of the oxidant to complex 1 did not show any
signal corresponding to a metal-centered radical.
The electronic structure of the new complex 3 is similar to
that of [Cp*Rh(MeCN)3](BF4)2, which is known to be a good
Scheme 3 Proposed mechanism for the formation of complex 3.
Csp2–H activation catalyst. It was therefore tested for the
Grignard-type arylation of 4-nitrobenzaldehyde via the C–H
activation of 2-phenylpyridine (Scheme 4).27 To our delight,
preliminary catalytic tests with complex 3 showed 33% conver-
sion into the expected alcohol after 24 h at 65 °C in THF
(1H NMR measurement, non-optimised conditions). The reac-
tion was also carried out in 1,2-dichloroethane at 60 °C and
gave 24% conversion after 24 h.
release of the COD ligand and leading to the tricationic inter-
mediate 2 (Scheme 3), which is stabilized by solvent coordi-
nation. The rhodium center becomes thus very electrophilic
and promotes electrophilic aromatic substitution at the ferro-
cene ring to give the stable complex 3.13c,25 The C–H activation
step is slower than for previously described ferrocenyl PCP
pincer ligands,5 consistent with the high strain generated in
the new rhodium(III) complex.
Conclusions
An interesting point is that, although the oxidation of
rhodium(I) to rhodium(III) may be accomplished directly by
electrolysis at 1.19 V/[FcH/FcH+] or by chemical oxidation with
[Th][BF4], the same transformation is also realized by initial
oxidation of ferrocene to ferrocenium by electrolysis at 0.49 V
vs. [FcH/FcH+] or more sluggishly by chemical oxidation with
[FcAc][BF4]. This probably involves a relay mechanism where
one or two electrons are transferred from rhodium to ferro-
cenium. This phenomenon has already been observed on
rhodium(I) complexes bearing ferrocenyl ligands.17 The differ-
ence between the oxidation potentials of ferrocene and
rhodium (ca. 0.5 V) means that, after generating the Fe(III)–
Rh(I) oxidation product which presumably involves minimal
structural rearrangement, the electron transfer from Rh(I) to
Fe(III) is endoergic by ca. 11.5 kcal mol−1, an activation barrier
that can be easily overcome at room temperature. It therefore
seems likely that the slow step of the process is this endoergic
electron transfer yielding an Fe(II)–Rh(II) intermediate that can
subsequently be more easily oxidized with intervention of the
MeCN coordination. It is known that, in the presence of struc-
tural rearrangement, a second electron transfer can be facili-
tated.26 The second electron transfer may occur either before
or after losing the COD ligand. Therefore, the transformation
of 1 to 2 should be a relatively fast process in agreement with
the cyclic voltammogram shown in Fig. 2. The intervention of
the solvent before the second electron transfer is suggested by
the reversibility of the CV in CH2Cl2. Evidently, the absence of
stabilization of the Fe(II)–Rh(II) intermediate in CH2Cl2 further
raises the activation barrier for the oxidative decomposition of
In summary, the oxidation of rhodium(I) complex 1 produced
a stable, planar chiral rhodium(III) complex with an unex-
pected C–H activation on ferrocene that shows some activity
for the catalytic C–H activation of 2-phenylpyridine. More work
will be done to optimize the reaction conditions and expand
the substrate scope. Given the growing importance of rhodium(III)-
catalyzed C–H activation/functionalization and the fact that
there are very few examples of chiral rhodium(III) complexes
bearing three available coordination sites, our efforts will also
focus on the resolution of planar chiral complex 3 to give an
enantiopure catalyst.
Acknowledgements
This work was supported by the Agence Nationale de la
Recherche (ANR-07-JCJC-0041, postdoctoral grant to ND).
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+°
= 0.54 V/SCE in CH2Cl2–n-Bu4BF4, 0.1 V s−1, 25 °C,
FcH ]
and E°[FcH/FcH ] = 0.41 V/SCE in MeCN–n-Bu4BF4, 0.2 V s−1
,
+°
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Paper
J. G. Osteryoung and J. J. Ondea, in Electroanalytical Chemi- 24 (a) M. Poyatos, M. Sanaú and E. Peris, Inorg. Chem., 2003,
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−
20 N. G. Connelly and W. E. Geiger, Chem. Rev., 1996, 96, 25 BF4 acts as the Lewis base to take the proton from ferro-
877.
cene in the Wheland-type intermediate.
21 A blank experiment was carried out by dissolving complex 26 “If one or more electron-transfer steps involve significant
1 in d6-MeCN. After 1 week, the 31P NMR only showed the
signal of the initial complex at 19.3 ppm.
structural change such as a rearrangement or a large
change in solvation, the standard potentials of the electron
transfer reactions can shift to promote the second electron
transfer and produce an apparent multielectron wave”;
A. J. Bard and L. R. Faulkner, in Electrochemical Methods:
Fundamentals and Applications, John Wiley & Sons, Inc.,
New York, 2nd edn, 2001, ch. 12, p. 475.
22 The purification of complex 3 can also be done by column
chromatography on silica gel (MeCN–MTBE 1/3 then pure
MeCN), albeit giving 3 in lower yield due to complex trail-
ing on silica.
23 Selective 1H{31P} and 13C{31P} NMR experiments allowed us
to assign unambiguously the quaternary carbon signals in 27 L. Yang, C. A. Correia and C.-J. Li, Adv. Synth. Catal., 2011,
the ferrocene region.
353, 1269.
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Dalton Trans.