Catalytic hydrogenation of norbornadiene by a rhodium complex in a self-folding cavitand

Article abstract of DOI:10.1002/anie.201003026

It's a wrap! The inclusion of [Rh(nbd)₂]BF₄ (nbd=norbornadiene) in a deep-cavity cavitand produces a catalytically active species that promotes the hydrogenation of norbornadiene to norbornene (see picture). The structure of the cavitand acts as a second-sphere ligand and modifies the stability, selectivity, and reactivity observed for the free organometallic complex in solution.

Full text of DOI:10.1002/anie.201003026

Angewandte
Chemie
DOI: 10.1002/anie.201003026
Supramolecular Catalysis
Catalytic Hydrogenation of Norbornadiene by a Rhodium Complex in
a Self-Folding Cavitand**
Maria Angeles Sarmentero, Hꢀctor Fernꢁndez-Pꢀrez, Erik Zuidema, Carles Bo, Anton Vidal-
Ferran,* and Pablo Ballester*
Reactions mediated by supramolecularly encapsulated tran-
sition metal complexes are rare,[1–3] as cavities large enough to
encapsulate the transition structure are required. Natural
proteins,[4,5] and hydrogen-bonded,[6,7] metal-mediated,[8,9] or
hydrophobically driven[10,11] synthetic multimolecular assem-
blies provide such spaces. For example, Raymond, Bergman
et al. used M4L6 assemblies as vessels for cationic metal
sandwich complexes[12] and controlled the reactivity of the
corresponding half-sandwich.[3,13] Herein we show that deep
cavitands can also perform chemistry in this regard and
present their unprecedented behavior in a catalytic hydro-
genation mediated by a rhodium(I) complex.
freely within 2 even at low temperatures. In contrast, the
bisnorbornadiene rhodium(I) cation 1+ is elongated along the
axis that goes through the two methylene bridge carbon atoms
and the rhodium atom (Figure 1), and thus its inclusion in 2
should show orientation preferences.
Cavitands are synthetic unimolecular receptors derived
from resorcin[4]arene that contain an aromatic-lined cavity
and are open at one end.[14] Some of us[15,16] recently showed
that deep-cavity cavitands, synthesized originally by Rebek
et al. for the inclusion of neutral molecules[17] and ferrocene
derivatives,[18] form inclusion aggregates with several cationic
organometallic complexes. We prepared an inclusion complex
of the organometallic [Rh(nbd)2]BF4 (1-BF4; nbd = norbor-
nadiene) and Rebekꢀs self-folding octaamide cavitand 2.
Compound 1-BF4 is extensively used as a precursor for
coordinatively unsaturated rhodium cationic species having
interesting catalytic properties, such as the hydrogenation of
unsaturated organic substrates.[19] Substitution of one of the
norbornadiene units by mono- or bidentate ligands gives
heteroleptic complexes prior to hydrogen exposure. How-
ever, inclusion of 1-BF4 in 2 provides an alternative to the
ligand exchange.
Figure 1. Chemical structures of self-folding cavitand 2, norbornadiene
4, and organometallic cationic complexes 1+ and 3+. CAChe minimized
structures of 1+ and 3+ shown with their van der Waals surface
embedded into a sphere with a radius of 3.5 ꢀ centered on the metal
atom.
1
The H NMR spectrum of 2 in CD2Cl2 shows that the
signals of the amides (NH) and the aromatic protons (H4) of
the diamidobenzene ring are very broad, which is due to the
chemical exchange between cycloenantiomers occurring in
Cobaltocenium cation 3+ forms a stable inclusion complex
with 2.[16] The included metallocene 3+ spins and tumbles
1
this solvent at a rate that is similar to the H NMR time-
scale.[17,20] The signal of the methine proton (H1) is sharp and
well-resolved; it appears at d = 5.80 ppm, indicating that 2 is
in a vase-like conformation. The addition of 0.4 equiv of the
cation 1-BF4 to a 5 mm solution of 2 in CD2Cl2 produces a new
set of proton signals for the bound host. The amide NH
groups and the protons of the diamidobenzene (H4) in bound
2 are sharp singlets at d = 9.63 ppm and d = 7.61 ppm,
respectively. The inclusion of cation 1+ in 2 induces the fast
interconversion of the cycloenantiomers of the octaamide.
The signals of included 1+ appear in two different regions
of the 1H NMR spectrum: two singlets are in the upfield
region and are assigned to the protons Hb’’ (d = 0.46 ppm) and
Hc’’ (d = À2.91 ppm) of the norbornadiene unit deep in the
aromatic cavity of 2; two other singlets are assigned to the
protons Ha’ (d = 4.13 ppm) and Hb’ (d = 3.60 ppm) of the
“upper” norbornadiene unit that is not included in the
aromatic cavity (Supporting Information, Figure S2). A
remaining singlet at d = 2.85 ppm corresponds to the proton
[*] Dr. M. A. Sarmentero, Dr. H. Fernꢁndez-Pꢂrez, Dr. E. Zuidema,
Dr. C. Bo, Prof. A. Vidal-Ferran, Prof. P. Ballester
Institute of Chemical Research of Catalonia (ICIQ)
Avgda. Paꢃsos Catalans 16, 43007 Tarragona (Spain)
Fax: (+34)97-792-8228
E-mail: avidal@iciq.es
pballester@iciq.es
Homepage: http://www.iciq.es/portal/359/default.aspx
http://www.iciq.es/portal/325/default.aspx
Prof. A. Vidal-Ferran, Prof. P. Ballester
Catalan Institution for Research and Advanced Studies (ICREA)
Passeig Lluꢄs Companys, 23, 08010 Barcelona (Spain)
[**] We thank MICINN (CTQ2008-00222/BQU, CTQ2008-00950/BQU
Consolider Ingenio 2010 Grant CSD2006-0003), DURSI
(2009SGR6868, 2009SGR623), and the ICIQ Foundation for gen-
erous financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003026.
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Ha’’ of the deeply bound norbornadiene. The observation of
separated proton signals for the two norbornadiene units of
included 1+ supports axial inclusion geometry for the 1+ꢀ2
complex. An EXSY experiment revealed the existence of
weak cross-peaks between the proton signals of the two
norbornadiene units of bound 1+ (Ha’Ha’’, Hb’Hb’’ and Hc’Hc’’),
a result indicative of a slow tumbling motion on the 1H NMR
timescale (kexch = 0.4 sÀ1, DG = 18 kcalmolÀ1).
Further analysis of the EXSY experiment revealed the
presence of other two minor species involved in chemical
exchange processes with the 1+ꢀ2 complex. One is free
norbornadiene 4 and its presence indicates that 1+ is partially
dissociated. The dissociation process should afford the
cationic species [Rh(nbd)(Sn)]+ (where n = 2 or 3 and S =
H2O or CH2Cl2).[21] Broad signals, which can be assigned to
the protons of [Rh(nbd)(Sn)]+ included in 2, are observed at
d = À0.04 ppm and À3.24 ppm in the EXSY experiment. The
integral ratios of the Haf proton pertaining to the free 4, to
protons Hb’’’ and Hc’’’ of [Rh(nbd)(Sn)]+ꢀ2 are in agreement
with this hypothesis (Figure 2). Figure 3 depicts the mini-
mized structures of the two cationic inclusion complexes
formed from 1+ and octaamide 2.
The addition of incremental amounts of 1-BF4 produced
the expected increase in the intensities of the signals
corresponding to the 1+ꢀ2 complex at the expense of those
of free 2 and a significant increase of the proton signals of 4
and [Rh(nbd)(Sn)]+ꢀ2. Significant broadening and chemical
shift changes were also observed in the signals of the
complexes during the titration experiment. Concomitantly,
two new signals resonating at d = 3.70 ppm and d = 3.51 ppm
Figure 3. CACHe minimized structures of the cationic inclusion com-
plexes 1+ꢀ2 and [Rh(nbd)(CH2Cl2)2]+ꢀ2.
appeared (Figure 2, top; Supporting Information, Figure S3).
We surmise that a binuclear metallo complex is formed, in
accord with proposals of Chen and Feder,[22] with three
norbornadiene ligands. The structure of this putative complex
is unknown, but our subsequent experiments used an excess
of host 2 with respect to 1-BF4 to suppress such high-order
complexes.
Next, the effect on hydrogenation reactions of the two
cationic complexes inside of 2 was assessed. We divided a
3 mm solution of 1-BF4 in CD2Cl2 in two aliquots of equal
volume. One was directly shaken under 1 bar of H2. After two
to five minutes, depending on the intensity of the shaking, a
black precipitate of rhodium metal was formed and the
solution completely lost its original red color. Analysis of the
solution showed the total absence of 1+.
The spectrum contained several proton signals with
complex multiplicities, suggesting the formation of a mixture
of compounds from the H2 treatment. When the solution was
analyzed by GC-MS, an intense peak corresponding to a mass
of 188 Da was observed. This compound is most likely formed
by the hydrogenation of the partially reduced dimer 6
(MW 186) that Schrock and Osborn described to be the
major product of the catalytic hydrogenation of 4 in acetone
promoted by 1+.[23] The structure of dimer 6 was elucidated by
Roth and Katz.[24] In our case, the production of stoichio-
metric amounts of rhodium(0) with respect to the partially
reduced dimer 6 during the hydrogenation of 1+ may be
responsible for the effective overreduction to 7.[25]
In striking contrast with these findings, the exposure of the
second aliquot of the CD2Cl2 solution of 1-BF4 to which
1.2 equiv of octaamide 2 was added to 1 bar of hydrogen did
not produce the formation of detectable amounts of black
rhodium(0) to the naked eye. Instead, a change in the color of
the solution from orange to pale yellow occurred under the H2
Figure 2. Changes in the 1H NMR spectra (400 MHz, CD2Cl2) acquired
at 298 K during the titration of 2 with 1-BF4 ([2]=5.0 mm). Primed and
second primed letters correspond to the protons in the 1+ꢀ2 complex
and triple-primed letters to the [Rh(nbd)(Sn)]+ꢀ2 complex. ?=impur-
ities.
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1
atmosphere. The H NMR spectrum (Figure 4) showed that
for hours under 1 bar of hydrogen at room temperature. The
the signals of the protons of 2 were much broader than in the
precursor solution, and we could not observe separate proton
signals for free and bound 2, and only one single set of signals,
coordination of an additional norbornadiene molecule is
required to reduce the activation energy of the rate limiting
step of the hydrogenation catalytic cycle.[27] Accordingly, we
added 10 equiv of norbornadiene to the NMR tube containing
the mixture under 1 bar of hydrogen and connected the NMR
tube to an H2 reservoir. The immediate change in color from
yellow to orange indicated the formation of the complexes
1+ꢀ2 and [Rh(nbd)(Sn)]+ꢀ2. A significant increase of 8 and
the disappearance of 4 were also observed by NMR. New
signals for dimer 6 also became evident, with the ratio of 8:6
as 80:20 with 10% mol of the supramolecular catalyst.
Apparently, small quantities of 1+ are released to the solution
from the dissociation of 1+ꢀ2 and catalyze the dimerization of
norbornadiene to 6.
In a laboratory-scale experiment, we exposed a 3 mm
CH2Cl2 solution of 1-BF4 (1.0 mol%) and octaamide 2
(1.2 mol%) to 1 bar of H2 for several minutes, then added
neat norbornadiene (100 equiv) in one portion. The resulting
mixture was stirred for 1 h under H2 and subsequently
analyzed by GC. The resulting mixture contained 58% of 8,
39% of dimer 6, and 3% of 9.[28] The fact that norbornene 8 is
the major product testifies to the catalytic activity of the
supramolecular, encapsulated transition metal complex. The
included metal complex mainly catalyzes the hydrogenation
of 4 to norbornene 8, while non-included metal complex
converts 4 to dimer 6 (Scheme 1). The hydrogenation of 4 to
norbornene 8 involves included catalytic metal centers, but
the transition state of the dimerization process is not
attainable in the constrained environment of 2.
Figure 4. Observed changes in the proton signals of a CD2Cl2 solution
of 2 ([2]=3.0 mm) with of 1-BF4 (0.8 equiv) after hydrogen treatment.
that of the norbornadiene unit buried inside the aromatic
cavity of 2, was detected. These signals do not coincide with
any of the signals previously assigned to the included
norbornadiene units of 1+ꢀ2 and [Rh(nbd)(Sn)]+ꢀ2.
In short, the cavitation slows down the hydrogenation
reaction of the included norbornadiene ligand and thus
stabilizes the rhodium cationic species towards reduction to
rhodium(0). Furthermore, we detected the signals for the
norbornadiene ligand that is near the top of the cavity,
indicating that 2 also prevents the fast reduction of the
“upper” norbornadiene molecule. An EXSY experiment
acquired on the mixture under 1 bar of hydrogen showed
the existence of another set of proton signals for included
norbornadiene (Supporting Information, Figure S8). This
1
new set is not detected in the 1D H NMR spectrum but it
can be inferred from the cross peaks owing to chemical
exchange involving the protons of the external norbornadiene
unit and the sharper set of protons of included norbornadiene.
We also noticed that the integration value of the sharper
proton signals of included norbornadiene is greater than for
the external norbornadiene protons. Taken together, these
data indicate that when the mixture of inclusion complexes
1+ꢀ2 and [Rh(nbd)(Sn)]+ꢀ2 is treated with H2, neither their
norbornadiene ligands nor the rhodium metal centers are
readily reduced as occurred in the case of free 1+. Whatever
the structures of these complexes are, they must be highly
kinetically stable to prevent the formation of rhodium(0) in
solution. Their dissociation would produce exposed cationic
rhodium species that are readily reduced to rhodium(0) under
H2 atmosphere.
Scheme 1. a) Behavior of 1-BF4 under a hydrogen atmosphere (1 bar)
in dichloromethane solution. Rhodium-catalyzed hydrogenation of
norbornadiene 4 in absence (b) or in presence (c) of the cavitand 2.
In summary, we have demonstrated the formation of two
inclusion complexes, 1+ꢀ2 and [Rh(nbd)(Sn)]+ꢀ2 from 1+
and cavitand 2. Under hydrogen and an excess of norborna-
diene, the complexes are catalytically active and produce
norbornene 8 as the major reaction product. Although the
structure of the catalysts is unknown, its behavior is very
different from that of free 1+, which converts norbornadiene
into dimer 6. This work is an example of an organic
transformation catalyzed by supramolecularly encapsulated
The 1H NMR spectrum of the mixture under 1 bar of
hydrogen showed that the proton signals of free norborna-
diene (present before H2 treatment) disappeared and were
replaced by signals of norbornene 8. Presumably, this
reduction occurs through the intermediacy of putative
dihydrido rhodium inclusion complex.[26] After the free
norbornadiene is reduced, the system appears to be stable
Angew. Chem. Int. Ed. 2010, 49, 7489 –7492
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7491

Communications
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metal complex in which host 2 acts as a second-sphere ligand
that modifies the stability, selectivity, and reactivity of
organometallic complex 1+ relative to the free form in
solution. We are exploring its potential application as catalyst
for other organic transformations.
Received: May 18, 2010
Published online: August 30, 2010
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[27] Homogeneus Catalysis: Understanding the Art, (Ed.: P. W. N. M.
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Keywords: homogeneous catalysis · host–guest systems ·
hydrogenation · rhodium · supramolecular chemistry
.
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