Cp*Rh-based heterometallic metallarectangles: Size-dependent borromean link structures and catalytic acyl transfer

Article abstract of DOI:10.1021/ja402630g

A series of CpRh-based functional metallarectangles have been synthesized from metallaligands. Enlargement of one linker leads to the isolation of two novel Borromean link architectures. All these complexes are intact in solution, as evident from ESI-MS spectroscopic analysis. Arising from the combination of open Cu centers and favorable cavity space, {(CpRh)₄(bpe) ₂[Cu(opba)·2MeOH]₂}4(OTf)·6MeOH shows extraordinary catalytic abilities with high efficiency and wide substrate selectivity in the acyl-transfer reaction.

Full text of DOI:10.1021/ja402630g

Communication
pubs.acs.org/JACS
Cp*Rh-Based Heterometallic Metallarectangles: Size-Dependent
Borromean Link Structures and Catalytic Acyl Transfer
Sheng-Li Huang,† Yue-Jian Lin,† T. S. Andy Hor,‡ and Guo-Xin Jin*,†
†Advanced Materials Laboratory, Department of Chemistry, Fudan University, Shanghai 200433, P. R. China
‡Institute of Materials Research & Engineering, Agency for Science, Technology, and Research (A*STAR), 3 Research Link,
Singapore 117602, and Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
S
* Supporting Information
link structures (Scheme 1). To the best of our knowledge, in
the absence of a metal ion template, these one-pot self-
assembled molecular Borromean rings are previously unknown.
ABSTRACT: A series of Cp*Rh-based functional metal-
larectangles have been synthesized from metallaligands.
Enlargement of one linker leads to the isolation of two
novel Borromean link architectures. All these complexes
are intact in solution, as evident from ESI-MS spectro-
scopic analysis. Arising from the combination of open Cu
centers and favorable cavity space, {(Cp*Rh)4(bpe)2[Cu-
(opba)·2MeOH]2}4(OTf)·6MeOH shows extraordinary
catalytic abilities with high efficiency and wide substrate
selectivity in the acyl-transfer reaction.
Scheme 1. Size-Dependent Formation of Borromean Link
Structures
n the past two decades, metallasupramolecular compounds
Ihave attracted a great deal of attention not only because of
their intriguing structures1 but also for their potential
electronic,2 magnetic,3 host−guest,4 drug delivery,5 and
catalytic properties.6,7 These have led to significant interest in
the construction of various molecular architectures1 with
desirable shapes, sizes, and ultimately function. In particular,
the molecular flask-like catalytic prototypes6 are developing
rapidly due to their special suprastructures and the associated
catalytic characteristic. Furthermore, the cavity size of frame-
works could be easily regulated by modulating the linker length,
which may give the opportunity to study the size-dependent
interlocked architectures8 and their related catalytic efficiency.7
In discrete system, the molecular species consist of
interlinked ring-like molecules which are of great interest due
to their fascinating structures, the topological importance, and
potential applications in smart materials and nanoscale devices.9
More specifically, the molecular Borromean link represents one
of the most intriguing entangled bodies based on its structural
integrity and aesthetic beauty.10 For chemists, the topological
complexity of discrete Borromean systems poses a formidable
synthetic challenge. The only reported syntheses have
originated in DNA11 and metal ion templates.12 It is important
to note that the clever use of metal−ligand interactions could
facilitate the assembly of Borromeates;12d further demetalation
results in metal-template-free Borromeands with real Borro-
mean link structures.13 Encouraged by the enhanced stability of
interpenetrating structures compensating weak coordination
bonds, we focused on understanding size-dependent interlinked
metallarectangles with two varying linkers offering more flexible
length regulation. Herein we report a surprising observation
that enlarging just one arm of the rectangle led to the isolation
of two nontrivial 3-interlocked frameworks with Borromean
The most popular methods for the preparation of
coordination cycles are ligand substitution with building blocks
of limited space. Since the first report on the organometallic
Cp*Rh (Cp* = η5-C5Me5) fragment used as a building block
for the construction of metallasupramolecular frameworks in
1998,14 Cp*Rh-based supramolecular chemistry has grown
rapidly.15 In the assembly of these structures from metal-
laligands,16 the frameworks are readily functionalized with
Lewis acid sites. Here, the molecular precursor LCu
=
[Cu(opba)]2− [opba = o-phenylenebis(oxamato)] was used as
a metallaligand complex which provides not only the bis-
chelating coordination sites (oxalate-like) but also catalytic
copper centers. Each Cp*Rh-based metal corner comprises
three available coordination sites, besides the bis-chelating
ligand LCu, and a monodentate bifunctional pyridine derivative
(for example, 4,4′-bipyridine) is also needed. The two types of
arms (spacers) are joined at the metal corners to give the
rectangular architectures.
We used a different synthetic methodology from the reported
method for Cp*Rh-based metallasupramolecular complexes15f
from the same starting material, viz. [Cp*RhCl2]2 (Scheme 2).
A stoichiometric (1:1) mixture of [Cp*RhCl2]2 and the
Received: March 14, 2013
© XXXX American Chemical Society
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Communication
one of the other rings and threaded by the other one (1→2,
2→3, 3→1). Although no rectangle is interlocked with any
other individual rectangle, all three constitute an inseparable
ensemble; scission of any one of the rings severs the union of
the three (Figure 2). Another noteworthy feature of this
Scheme 2. Synthesis of Complexes 1−5
corresponding monodentate bifunctional pyridine derivative
L1−L5 [L1 = pyrazine (pz); L2 = 4,4′-bipyridyl (bpy); L3 = 1,2-
bis(4-pyridyl)ethylene (bpe); L4 = N,N′-4-dipyridyloxalamide
(dpo); L5 = 1,4-bis(4-pyridyl)benzene (bpb)] in MeOH at
room temperature gave the binuclear complexes with four
coordination sites as linear connecting units in almost
quantitative yields. Chloride abstraction by AgOTf (Tf =
O2SCF3) followed by addition of Na2LCu gave rise to green
solutions containing molecular rectangles. Complexes 6 and 7
Figure 2. Borromean link structures. View of the X-ray structures of 4
(a,c) and 5 (b,d) as ball-and-stick and space-filling presentations.
Counteranions are omitted for clarity.
−
were obtained through guest exchange of 1 with NO3 and 2
with H2O (Figures S5 and S6), respectively.
All the basic units of complexes 1−5 are metallarectangles
based on the common [4Rh+2Cu] nuclear core with different
component units. The three coordination sites of the half-
sandwich Rh fragment are occupied by pyridyl donor L and
chelating group LCu. A representative structure, 3 (Figure S2),
shows three crystallographically independent metal centers, viz.
Rh(1), Rh(2), and Cu. The Cu(II) has approximately square
pyramidal surroundings, with two nitrogen and two oxygen
atoms from the opba group in the equatorial plane and two
MeOH oxygen atoms in the axial positions. In these complexes,
the lengths of pyridyl arms change significantly from 7.00 to
15.62 Å (Figure 1). Complexes 1−3 are monomeric rings;
upon enlargement of pyridyl arms through the use of the longer
linker N,N′-4-dipyridyloxalamide, trimeric complex 4 was
obtained. It was surprising that no two rectangles are
interlocked (i.e., catenated), yet each is threading through
structure is the presence of amide hydrogen bonds between N
atoms of the outer rectangle and N−H moieties of the inner
rectangle, which is reminiscent of some reported amide H-
bond-directed catenanes.17 To determine whether the main
driving force for the formation of this molecular Borromean
ring is the longer pyridyl arms or the templated amide
hydrogen bonds, we tested 1,4-bis(4-pyridyl)benzene, with
nearly equivalent N···N distance of the two 4-pyridyl groups
but without an obvious hydrogen-bonding unit, as a ligand.
This resulted in the formation of complex 5 also with
Borromean link structure like 4 (Figure 2), which clearly
pointed to the pyridyl arm length as the key factor that
promoted interpenetration.
In support of the solid-state structures as evidenced from the
X-ray diffraction analysis, the electrospray ionization mass
spectrometry (ESI-MS) data of complexes 2−5 also point to
the same species as their trifluoromethanesulfonate salts in
solution. Peaks on the parent complexes 2 and 3 minus some
counteranions, e.g., [2 × 2 − 2OTf]2+ (m/z = 2335.46, Figure
S8) and [3 × 2 − 3OTf]3+ (m/z = 1541.68, Figure S9), were
observed, strongly suggesting that the rectangular structures are
intact in solution. The experimental peaks were all isotopically
resolved and in good agreement with their theoretical
distributions. Do the intact rectangular structures of 2 and 3
in solution imply that the structures of Borromean link
complexes 4 and 5 are also preserved? The ESI-MS spectrum
indicated 4 and 5 in solution would not split into monomeric
rectangles but would preserve their 3-interlocked trimeric
structures: [4 − 3OTf]3+ (m/z = 2507.65) and [5 − 3OTf]3+
(m/z = 2487.70) (Figure 3).
To demonstrate the size effect on catalytic activities of these
metallacycle catalysts, we conducted the acyl-transfer reaction
between N-acetylimidazole (NAI) and x-pyridylcarbinol (x-PC;
x = 2, 3, 4).18 The NAI and x-PC substrates could achieve
suitable orientation and alignment for highly efficient acyl
transfer through binding within the cavity of the metallacycle by
Figure 1. Single-crystal X-ray structures of complexes 6 (a), 2 (b), 3
(c), 4 (d), and 5 (e). All hydrogen atoms have been omitted for clarity.
B
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rectangles, thus leaving no space for substrate entry into
these Borromean link structures. But for 1, its cavity is
inherently narrow. These findings lead to the postulate that this
acyl-transfer reaction proceeded through the constrained
proximity of two reactive species and a tightly bound
intermediate (Figure 5). The nitrogen atoms of NAI and 3-
Figure 3. Calculated (bottom, blue) and experimental (top, red) ESI-
MS spectra (3+) of (A) 4 and (B) 5.
Figure 5. Tetrahedral intermediate doubly bound inside the cavity of
complex 3.
open Cu centers. Furthermore, only the combination of
substrates with the right distance can span the cavity and
react at an accelerated rate. Their significantly different cavity
sizes provided a basis for evaluation of the efficiency of different
metallacycle catalysts in acyl-transfer reaction. For 3-PC, both 2
and 3 significantly accelerate the reaction rate. Complex 3 is
almost 3 times more active than 2 (Figure 4), which may be
PC could be easily coordinated to the inner Cu centers of
metallarectangle 3, forming the cooperatively bound inter-
mediate with low activation energy, thus facilitating the
subsequent catalytic cycle based on the weak binding ability.
In conclusion, we have prepared a series of cavity-specific
Cp*Rh-based heterometallic metallarectangles from a metal-
laligand. Using an enlarged linker led to interpenetrating
structures as a compensation of the weak coordination bonds.
The two coordination-driven self-assembled metallasupramole-
cules with real Borromean link structures are unprecedented.
The synthetic simplicity and readily tunable and easily
modifiable modular design are distinct advantages of this
function-oriented assembly. With appropriate cavity size,
catalyst 3 shows remarkable catalytic abilities with high
efficiency and wide substrate selectivity in the acyl-transfer
reaction between N-acetylimidazole and x-pyridylcarbinol.
These exciting results suggest the potential biomimicking effect
̀
of metallasupramolecular catalysis, but its development vis-a-vis
understanding of interpenetration phenomenon and purpose-
driven design of such suprastructures is still in its infancy.
ASSOCIATED CONTENT
* Supporting Information
Figure 4. Progress curves for the acyl-transfer reaction between N-
acetylimidazole and 3-pyridylcarbinol.
■
S
Experimental details, crystallographic data, IR and ESI-MS
spectra, and progress curves for the acyl-transfer reaction for
1−5. This material is available free of charge via the Internet at
http://pubs.acs.org.
ascribed to 3 having highly favorable cavity size for
accommodating the two reactants. 4-PC has longer distance
between the N atom and carbinol group than 3-PC, and hence
needs a more favorable and enlarged cavity size; complex 3
catalyzed the acyl-transfer reaction with medium rate (Figure
S14 and S15). For 2-PC, all the rates drop significantly relative
to 3- and 4-PC (Figure S12). This can be understood as the
Cu-bound carbinol group of 2-PC and the imidazole N-acetyl
group bound to the other side are positioned in opposite
directions, resulting in an unfavorable transition state for acyl-
transfer reaction. It is also noticeable that, regardless of 2-, 3-, or
4-PC, complexes 1, 4, and 5 catalyzed the reaction with low
rate, only a little stronger than without catalyst. This is not
surprising since 4 and 5 cannot split into monomeric
AUTHOR INFORMATION
Corresponding Author
gxjin@fudan.edu.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Science Foundation of China
(91122017), the Program for Changjiang Scholars and
Innovative Research Team in University (IRT1117), the
C
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Journal of the American Chemical Society
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