Cooperative catalysis: Large rate enhancements with bimetallic rhodium complexes

Article abstract of DOI:10.1021/om400613b

A large increase in reaction rate was observed for the catalyzed mono- and dihydroalkoxylation of alkynes when structurally constrained bimetallic Rh(I) catalysts with imidazolyl-imine ligands were compared to their monometallic analogues. The "cooperative" enhancement was vastly improved by restricting the conformational freedom of the bimetallic structure and minimizing the intermetallic Rh···Rh distance.

Full text of DOI:10.1021/om400613b

Communication
pubs.acs.org/Organometallics
Cooperative Catalysis: Large Rate Enhancements with Bimetallic
Rhodium Complexes
Sandra W. S. Choy, Michael J. Page, Mohan Bhadbhade, and Barbara A. Messerle*
School of Chemistry, University of New South Wales, Sydney, Australia 2052
S
* Supporting Information
ABSTRACT: A large increase in reaction rate was observed
for the catalyzed mono- and dihydroalkoxylation of alkynes
when structurally constrained bimetallic Rh(I) catalysts with
imidazolyl-imine ligands were compared to their monometallic
analogues. The “cooperative” enhancement was vastly
improved by restricting the conformational freedom of the
bimetallic structure and minimizing the intermetallic Rh···Rh
distance.
imetallic complexes have the potential to yield remarkable
1
Breaction efficiencies in homogeneous catalysis. In certain
instances the bimetallic catalyst is significantly more active than
the related monometallic catalyst with a similar structure at the
metal center. This increase in catalytic activity is attributed to a
“cooperative” interaction of the two metal centers in the
bimetallic catalyst. One of the more versatile approaches to
bimetallic catalyst design is to link two discrete metal
complexes by a covalently bound tether.1e Such an approach
has been used with great effect for a number of diverse
reactions such as the Co2, Cu2, or Zn2 catalyzed hydrolysis of
phosphate esters,2−4 the use of Cr2, Al2, or Co2 catalysts for the
asymmetric ring opening of epoxides5 and various Ni2, Ti2, and
Figure 1. Bimetallic catalyst design: from a fluxional bpm ligand
scaffold (a) to a rigid imidazolyl-imine chelate (b, this work) that
allows a closer metal−metal separation.
Zr2 polymerization catalysts.6,7
We have recently demonstrated that bimetallic Rh2 and Ir2
bis(1-pyrazolyl)methane (bpm) complexes are highly effective
catalysts for the dihydroalkoxylation of alkynediols.8 The
degree of cooperative rate enhancement observed was found
to be highly dependent on the structure of the linking group,
with the highest catalytic activity obtained when the two metal
centers were supported on a 1,8-anthracenyl scaffold (Figure
1a). Despite the rigidity of the aromatic linking group, these
complexes still contain a high degree of conformational
flexibility due to rotation about the bpm−arene bond and a
fluxional pseudoboat conformation of the bpm metallacycle. As
a result, the intermetallic distances are poorly defined.
In an effort to develop more effective bimetallic catalysts, we
report here a series of new bis-rhodium(I) complexes
containing structurally rigid imidazolyl-imine chelates attached
directly to a linking scaffold via the imine nitrogen donor
(Figure 2). This arrangement significantly reduces the degree of
conformational freedom and allows for a shorter intermetallic
distance through alignment of the complex planes (Figure 1b).
The resulting bimetallic complexes achieved an enormous
increase in reaction rate when used as catalysts to promote the
addition of OH bonds to alkynes relative to that obtained using
an analogous monometallic catalyst.
Catalyst Synthesis. A series of three bimetallic ligand
scaffolds were synthesized with two imidazolyl-imine donors
connected by either a rigid phenylene linker via the meta (m-
diimine, 2a) and ortho (o-diimine, 3a) positions or a more
flexible ethylene linker (Et-diimine, 4a) (Figure 2). For
comparison the monometallic ligand Ph-imine (1a) was also
prepared. The corresponding mono- and bimetallic rhodium(I)
1,5-cyclooctadiene (COD) complexes (1b−4b, Figure 2) were
formed by reaction of the ligands 1a−4a with [Rh(COD)2]-
BArF (where BArF = tetrakis[3,5-bis(trifluoromethyl)phenyl]-
1
borate) in THF at room temperature. In the H NMR spectra
of 2b−4b only a single set of resonances was observed for the
imidazolyl-imine chelate, indicating equivalent rhodium com-
plex environments.
Introducing a CO atmosphere to a DCM solution of the
COD complexes 1b, 2b, and 4b gave the carbonyl complexes
1c, 2c, and 4c (Figure 2), which were identified in situ using 1H
NMR spectroscopy as the sole products of the reaction. Initial
Received: June 26, 2013
Published: August 16, 2013
© 2013 American Chemical Society
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Communication
Figure 3. ORTEP depictions of o-diimine-[Rh(CO)2]2 (3c) viewed
perpendicular (a) and parallel (b) to the phenylene axis.
Rh centers have equivalent coordination environments. 1H
NMR spectra were also acquired in the temperature range
203−323 K for both 2c and the mixture 3c,d. The spectra
showed no significant changes and retained narrow line widths
as the temperature was lowered from 323 K to 223 K. At
temperatures below 223 K a slight broadening of the
resonances was observed. The consistency of the spectra over
a wide range of temperatures indicates that the complex
structure is not fluxional on the NMR time scale at these
temperatures and that the distance between the Rh atoms is
well-defined.
Catalysis. We have long been interested in using rhodium-
(I) catalysts for the synthesis of oxygen-containing heterocycles
via the intramolecular cyclization of alkynoic acids and alkyne
diols.11 The resulting lactone and spiroketal products are
prevalent in a wide range of biologically active natural products,
and the development of tools that can facilitate their efficient
and selective synthesis is of broad significance.12
To explore the utility of our new bimetallic catalyst design for
hydroalkoxylation reactions, we first investigated the cyclization
of 4-pentynoic acid (5) to α′-angelica lactone (6), a
monohydroalkoxylation reaction, using the Rh(COD) com-
plexes 1b−4b as catalysts. The reactions were carried out in
benzene-d6 at 60 °C using 2 mol % of the monometallic catalyst
1b or 1 mol % of the bimetallic catalysts 2b−4b to give an
equivalent metal loading in all systems (Table 1). The
monometallic catalyst Ph-imine-[Rh(COD)] (1b) and the m-
phenylene-linked bimetallic catalyst m-diimine-[Rh(COD)]2
(2b) were both found to be largely inactive for this
transformation, achieving less than 4% conversion after 24 h
(entries 1 and 2). Conversely, the o-phenylene-linked bimetallic
Figure 2. Imidazolyl-imine ligands (1a−4a) and their corresponding
Rh(COD) (1b−4b), Rh(CO)2 (1c−4c), and Rh(CO)(COD) (3d)
complexes. (*) Complexes 3c,d were formed as an inseparable mixture
in a ca. 1:1 ratio.
attempts to isolate 1c, 2c, and 4c by either crystallizing the
complexes through the slow addition of pentane or removing
the volatiles in vacuo led to a complex mixture of
decomposition products. These decomposition products
appeared to contain recoordinated COD, indicating a high
lability of the CO ligand in the absence of a CO atmosphere.
Isolation of the desired products 1c, 2c, and 4c could be
achieved cleanly in high yield by rapidly crystallizing the
complexes from a CO-saturated solution. This was done by
adding the reaction solution to a vast excess of pentane and
collecting the resulting pale yellow powders. Once isolated, 1c,
2c, and 4c appeared to be stable in solution over prolonged
periods with no CO atmosphere present.
Introducing a CO atmosphere to a solution of the o-
phenylene-linked complex 3b gave a mixture of the expected
product o-diimine-[Rh(CO)2]2 (3c) and the partially carbony-
lated complex o-diimine-[Rh(CO)(COD)]2 (3d) in a ca. 1:1
ratio. Adding the reaction solution to a vast excess of pentane
gave an inseparable mixture of the two products 3c,d. A
fortuitous crystal of 3c was grown over 1 month from a CO-
saturated solution of the complex in DCM layered with
pentane; X-ray diffraction analysis gave the solid-state structure
shown in Figure 3. The two metal centers in 3c can be seen to
lie on opposite sides of the phenylene plane; however, an
inward twist of the complex planes (CPhN-CPh−N−C =
137.1(6) and 140.4(6)°) results in a relatively short Rh···Rh
distance of 3.325(1) Å. An intermolecular contact of 3.325
0.1 Å is not uncommon between Rh complexes containing π-
basic carbonyl or isonitrile ligands and a sterically flat
coordination sphere.9 This distance is consistent with a weak
metal−metal interaction between the two Rh centers in 3c, as
described previously by Mann et al.10
Table 1. Catalyzed Cyclization of 4-Pentynoic Acid (5) to
a
α′-Angelica Lactone (6) using 1b−4b
b
entry
catalyst
conversion, % (time)
1
2
3
4
Ph-imine-[Rh(COD)] (1b)
m-diimine-[Rh(COD)]2 (2b)
o-diimine-[Rh(COD)]2 (3b)
Et-diimine-[Rh(COD)]2 (4b)
<4 (24 h)
<4 (24 h)
99 (2.3 h)
48 (24 h)
a
Conditions: 4-pentynoic acid (5) (0.310 mmol), 6.2 μmol of Rh (2
1
The H and 13C NMR spectra of the bimetallic carbonyl
b
mol %), and C6D6 (0.5 mL). Determined by a comparison of product
and substrate integrals in the H NMR spectrum.
1
containing complexes 2c, 3c,d, and 4c indicated that the two
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complex o-diimine-[Rh(COD)]2 (3b) was a highly active
catalyst for this reaction, achieving quantitative conversion of
the substrate within 2.3 h (entry 3). Clearly the shorter Rh···Rh
separation that is possible in complex 3b, in comparison to the
similar phenylene-linked complex 2b, is essential to achieve a
cooperative bimetallic enhancement of the reaction rate. The
catalyst Et-diimine-[Rh(COD)]2 (4b), which contains a short
ethylene linker, was also considerably more reactive than its
monometallic counterpart 1b, achieving 48% conversion after
24 h (entry 4). Notably the reactivity of 4b is slower than that
of the o-phenylene-linked catalyst 3b. While both catalysts
contain a short two-carbon linking group, the flexible ethylene
chain in 4b would allow the two metal centers to adopt a more
remote conformation. The population of these remote states
would therefore limit effective cooperation between the metal
centers.
Encouraged by these results, we investigated the dihydroal-
koxylation of the alkyne diol substrate 7 to yield the spiroketal
products 8a,b (Figure 4). We have previously shown that
Rh(CO)2 complexes are much more effective catalysts for this
reaction than Rh(COD) complexes when using bidentate
N,N′-donor ligands.9 We therefore began our investigation
e
using the isolated CO-containing complexes 1c, 2c, and 4c
(Figure 4, top). The reaction was performed with a total
rhodium loading of 2 mol % in 1,1,2,2-tetrachloroethane-d2
(TCE-d2) at 25 °C, with reported conversions indicating the
sum of both regioisomers 8a,b. Both the monometallic complex
Ph-imine-[Rh(CO)2] (1c) and the m-phenylene-linked com-
plex m-diimine-[Rh(CO)2] (2c) achieved high conversions
after 3.5 h, 88% and 99%, respectively, with the bimetallic
complex 2c slightly more active than its monometallic congener
1c. The bimetallic ethylene-linked complex Et-diimine-[Rh-
(CO)2] (4c) was a much poorer catalyst for this reaction,
showing a rapid initial reaction rate followed by deactivation of
the catalyst.
In an alternative approach to studying the catalytic activity of
the complexes with CO coligands, the dihydroalkoxylation of 7
was performed under 1 atm of CO with the Rh(COD)
complexes 1b−4b (Figure 4, bottom), thereby generating the
CO-containing complexes in situ. Under these conditions the
activity of all bimetallic catalysts was dramatically enhanced
relative to the monometallic catalyst 1b. The bimetallic
cooperativity was observed to increase as both the separation
between metal centers decreased (3b (+CO) ≫ 2b (+CO))
and the flexibility of the linking scaffold decreased (3b (+CO)
≫ 4b (+CO)), consistent with our observations above on the
catalyzed cyclization of 4-pentynoic acid (5) using the
Rh(COD) complexes 1b−4b without CO present. The o-
phenylene-linked complex 3b was in fact an exceptionally
reactive catalyst in the presence of a CO atmosphere, achieving
quantitative conversion of the substrate within 12 min.
The level of intermetallic cooperativity observed for the
dihydroalkoxylation of 7 using the o-phenylene-linked complex
3b under 1 atm of CO is remarkably high. The turnover
frequency calculated at 50% conversion (TOF50) for 3b (1370
h−1) is over 6800 times greater than that obtained with the
monometallic complex 1b (0.2 h−1) under identical conditions
(Table 2, entries 2 and 4). The high catalytic activity obtained
with 3b is even more extraordinary when comparing the
efficiency of the other in situ generated catalysts 1b, 2b, and 4b
(+CO) with that of the isolated catalysts 1c, 2c, and 4c. Such a
comparison reveals the reaction to be normally impeded by a
CO atmosphere. For example, quantitative conversion of the
Figure 4. Catalyzed dihydroalkoxylation of 7 to yield spiroketals 8a,b:
(top) reactions performed under N2 for the preformed CO-containing
complexes 1c, 2c, and 4c; (bottom) carbonylated complexes generated
in situ using complexes 1b−4b under a CO atmosphere.
Table 2. Catalyzed Dihydroalkoxylation of 7 using 1c under
a
N2 or 1b−4b under 1 atm of CO
b
entry
catalyst
1c
conversion, % (time)
8b/8a
TOF50 (h‑1)
1
2
3
4
5
99 (5 h)
0.10
0.23
0.36
0.25
0.35
13.9
0.2
1b + CO
2b + CO
3b + CO
4b + CO
60 (143 h)
99 (35 h)
99 (12 min)
99 (145 h)
2.2
1370
1.0
a
Conditions: diol (7) (0.200 mmol), 4.0 μmol of Rh (2 mol %), and
TCE-d2 (0.6 mL). For reactions performed under CO, a solution of
the catalyst was degassed and 1 atm of CO added prior to addition of
b
7. Determined by a comparison of product and substrate integrals in
1
the H NMR spectrum.
substrate was achieved within 5 h using the isolated complex
1c, whereas the in situ generated catalyst 1b (+CO) obtained a
maximum conversion of only 60% after 143 h (Table 2, entries
1 and 2). The o-phenylene-linked bimetallic catalyst 3b (+CO)
is still 98 times faster than the isolated monometallic catalyst 1c
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Organometallics
Communication
(TOF50 = 13.9 h−1). The regioselectivity of the reaction was
found to be independent of the catalyst rate and did not vary
significantly between catalysts. In all reactions the spiroketal 8a
was formed as the most favored regioisomer (Table 2).
The dramatic increase in reaction efficiency reported here
using the o-phenylene-linked bimetallic catalyst 3b is rarely
observed in bimetallic catalysis. Our previous best result
obtained with a bpm-ligated dirhodium complex linked by a
1,8-anthracenyl scaffold (Figure 1a, M = Rh) was found to be
only 34 times faster than its monometallic analogue for the
dihydroalkoxylation of 7.8 Complex 3b appears uniquely
structured to facilitate such massive levels of cooperative rate
enhancement. The principles of minimizing conformational
flexibility and intermetallic distance have led to a highly
cooperative system for the addition of OH bonds to alkynes.
We have previously proposed a mechanism for the rhodium-
catalyzed dihydroalkoxylation of 7 which involves attack of the
benzylic alcohol on the π-coordinated alkyne as the first step.11d
A more detailed investigation of how this mechanism applies to
a bimetallic-catalyzed process is ongoing.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, a table, and CIF files giving experimental
procedures for the synthesis of all compounds, the preparation
of catalytic reactions, and crystallographic data for compounds
1b,c and 3c. This material is available free of charge via the
Internet at http://pubs.acs.org. Crystallographic data for the
structures reported in this paper have also been deposited with
the Cambridge Crystallographic Data Centre (CCDC) as
supplementary publication nos. 944983−944985.
(12) (a) Togni, A.; Grutzmacher, H. Catalytic Heterofunctionalization;
̈
Wiley-VCH: Weinheim, Germany, 2001. (b) Beller, M.; Seayad, J.;
Tillack, A.; Jiao, H.-J. Angew. Chem., Int. Ed. 2004, 43, 3368.
(c) Nakamura, I.; Yamamoto, Y. Chem. Rev. 2004, 104, 2127.
(d) Alonso, F.; Beletskaya, I. P.; Yus, M. Chem. Rev. 2004, 104, 3079.
(e) Weiss, C. J.; Marks, T. J. Dalton Trans. 2010, 39, 6576.
AUTHOR INFORMATION
Corresponding Author
*E-mail for B.A.M.: b.messerle@unsw.edu.au.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Australian Research Council (ARC)
and the University of New South Wales is gratefully
acknowledged. S.W.S.C. thanks the Australian Government
for an Australian Postgraduate Award (APA).
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