Supramolecular squares of dirhodium(ii) tetracarboxylate: Combining carboxylate-exchange and metal-ligand coordination for self-assembly

Article abstract of DOI:10.1039/c2cc35206k

Square self-assemblies are obtained from dirhodium(ii) tetracarboxylate complexes using an isonicotinate-type ligand to act as an equatorial ligand to one dirhodium unit and an axial ligand to another. It is shown that the supramolecular squares are formed selectively out of a number of possible compounds in the dynamic carboxylate exchange library. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc35206k

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COMMUNICATION
Supramolecular squares of dirhodium(II) tetracarboxylate: combining
carboxylate-exchange and metal–ligand coordination for self-assemblyw
a
a
a
a
b
´ ´
Lok H. Tong, Sarah Clifford, Antoine Gomila, Sylvain Duval, Laure Guenee and
Alan F. Williams*a
Received 19th July 2012, Accepted 15th August 2012
DOI: 10.1039/c2cc35206k
Square self-assemblies are obtained from dirhodium(II) tetra-
carboxylate complexes using an isonicotinate-type ligand to act
as an equatorial ligand to one dirhodium unit and an axial ligand
to another. It is shown that the supramolecular squares are
formed selectively out of a number of possible compounds in the
dynamic carboxylate exchange library.
for multichromophore assembly about the dirhodium(II) cores10
and to control peptide structure.11 Axial adduct formation by
dirhodium(II) tetracarboxylate has been used to create liquid
crystalline polymers,12,13 in catalyst immobilisation,14 molecular
sensing,15,16 and to assemble nanometric spheres from pentagonal
units.17
The synthesis of square metallomacrocycles has been a subject of
interest for more than twenty years.18,19 Dirhodium units have
previously been used to yield molecular squares only by employing
‘‘cis-protected’’ dirhodium precursors as corner components which
allows di-substitutions to occur in a ‘‘cis’’fashion at equatorial
sites.20–22 Herein we report a one-pot synthesis of two nano-sized
supramolecular squares 2 and 3 based on dirhodium(II) tetra-
carboxylates using carboxylate exchange and metal–ligand coordi-
nation in the axial position. This work demonstrates the use of a
new combination of two reversible orthogonal interactions for
construction of self-sorting supramolecular assemblies.
The use of multiple reversible orthogonal interactions is a
fascinating synthetic route for the construction of complex multi-
component assemblies.1 Reversible interactions such as hydrogen
bonds,2 p–p stacking,3,4 ion–dipole interactions,5 and reversible
covalent bond formation6,7 have been employed concurrently
with metal–ligand interactions to form a range of supramolecular
structures. The combination of three independent interactions,
aldehyde–amine condensations, metal–ligand interactions and
formation of boronic esters has recently been demonstrated as
a route to obtain a complex structure.8
A new dirhodium(II) tetra-3,5-di-tert-butylbenzoate precursor 1
which possesses high solubility in organic solvents was obtained in
quantitative yield by reacting dirhodium(II) tetraacetate with 8-fold
excess of 3,5-di-tert-butylbenzoic acid in refluxing toluene, with
continuous removal of the liberated acetic acid from the reaction
mixture (Scheme 1). The crystal structure of 1 is typical for
dirhodium(II) tetracarboxylates (Fig. S1, ESIw).23
If compound 1 was reacted with one equivalent of isonicotinic
acid, under reflux, a single product 2 was isolated in 83% yield.
Compound 2 was confirmed as a tetramer from the crystal
structure.z The carboxylate of the isonicotinate bridges one dirho-
dium unit while the pyridine binds to an axial site of an adjacent
dirhodium (Fig. 1). The complex is distorted from planarity, with a
S4 axis passing through the centre of the pseudo-square, so that the
asymmetric unit consists of one monomer. The centroids of the
dirhodium units lie 1.02 A from the mean plane, and the distance
between neighbouring dirhodium centroids is 10.20 A. The vacant
axial coordination sites of the dirhodium units are occupied by
methanol molecules which are linked by hydrogen bonding to
neighbouring tetramers (Fig. S2, ESIw). The Rh–Rh and Rh–O-
(acetate) distances are normal for dirhodium(II) tetracarboxylates.9
The axial Rh–N(isonicotinate) distance is 2.151(13) A.
Dimetal-tetracarboxylates, M2(O2CR)4 are found for many
transition metal elements in the +II oxidation state. The four
carboxylates bridge the two metal ions and will be referred to
as equatorial ligands, while two further ligands can bind along
the metal–metal axis (the axial ligands). These two interactions
are orthogonal: the carboxylate ion occupying the equatorial
bridging site will not occupy the axial site, while a simple
nitrogen donor will only bind to the axial site, and cannot
bridge equatorially. Carboxylate ligand substitution and adduct
formation with axial ligands are useful properties of dirhodium(II)
tetracarboxylates Rh2(O2CR)4 (R = organic substituents).9 Partial
or complete exchange of the carboxylate ligands of Rh2(O2CR)4
with other carboxylic acids R0CO2H occurs in a stepwise fashion
with replacement of O2CR by O2CR0 to generate a library of six
differently substituted products [Rh2(O2CR)4ꢀn(O2CR0)n], n = 0–4.
Carboxylate ligand substitution has been employed as a scaffold
a Department of Inorganic and Analytical Chemistry,
University of Geneva, Sciences II, 30, quai Ernest Ansermet,
1211 Geneva 4, Switzerland. E-mail: alan.williams@unige.ch;
Fax: +41 (0)22 379 68 30
b Crystallography Laboratory, University of Geneva, School of
Physics, 24, quai Ernest Ansermet, 1211 Geneva 4, Switzerland
w Electronic supplementary information (ESI) available: Full synthetic
and experimental details. CCDC 862373 and 862374. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2cc35206k
The structure of 2 in solution is confirmed by the 1H NMR
spectrum showing (i) two signals for each aryl proton of the di-tert-
butyl aromatic group in a 2 : 1 ratio (Fig. 2) and two signals for
the tert-butyl protons as expected from the crystal structure
c
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Fig. 2 The aromatic region of the 1H NMR spectra in CDCl3 of
compounds 1 to 5.
only a mono-adduct 4 whose 1H NMR and electronic spectra
show axial coordination, but no carboxylate exchange. On the
other hand, if the pyridine is blocked by substitution by phenyl
groups at the 2 and 6 positions no axial coordination is
observed at room temperature. Refluxing overnight in toluene
gave scrambling of carboxylates (Fig. S7, ESIw): mono-substituted
Rh2[(O2CR1)3(O2CR2)] (R2 = 2,6-(C6H5)2C5H2N) (25%),
two inseparable di-substituted cis/trans isomers Rh2[(O2CR1)2-
(O2CR2)2] (combined yield of 20%), and tri-substituted
Rh2[(O2CR1)(O2CR2)3] (4%). Formation of the fully substituted
product Rh2(O2CR2)4 was not observed.
Scheme 1 Synthesis of the dirhodium tetracarboxylate species (1–5).
Reagent and conditions: (i) 3,5-di-tert-butylbenzoic acid (8 eq.), toluene,
overnight reflux; (ii) isonicotinic acid (1 eq.), toluene, overnight reflux;
(iii) 4-(pyridin-4-yl)benzoic acid (1 eq.), benzene, overnight reflux; (iv)
methyl isonicotinate (1 eq.), CHCl3, r.t.; (v) methyl 4-(pyridin-4-yl)benzoate
(1 eq.), CHCl3, r.t.
From this we deduce that when the exchange of the carboxylate
ligands occurs under reflux but the axial coordination of the
pyridine is blocked, a library of substituted compounds is
obtained. However, when the pyridine is available for axial
Rh–N coordination, the dynamic combinatorial library26 of
six possible scrambled products in the carboxylate exchange is
perturbed. Formation of the mono-substituted product is
amplified from 25% to 83% because it can self-tetramerise
to yield the discrete assembly 2, and the four rhodium–nitrogen
bonds formed in 2 shift the reversible carboxylate exchange
towards mono-substitution. If the reaction between 1 and
isonicotinic acid was carried out at room temperature only an
orange brown insoluble material along with traces of the mono-
adduct of 1ꢂL (L = 4-CO2H–C5H4N) and isonicotinic acid
were obtained. It is attributable to the formation of a mixture of
polymers, the thermodynamically stable product being hindered
by slow kinetics. An insoluble product was also obtained if
two equivalents of isonicotinic acid were used in refluxing
toluene, and this was attributed to multiple substitutions
leading to polymer formation.
Fig. 1 X-ray crystal structure of 2. The tert-butyl groups and hydrogen
atoms are omitted for clarity (key: Rh pink, O red, N blue, C grey).
and (ii) a downfield shift of the pyridyl protons (Fig. S3, ESIw)
showing the coordination of the pyridyl group to the dirhodium
centre and comparable to shifts seen for the mono-adduct 4. The
electronic spectrum (Table S1, ESIw) also shows evidence for axial
coordination since the absorption maximum for the dirhodium
species attributable to the p*(Rh2) - s*(Rh2) transition9,24,25
shows a hypsochromic shift in comparison with precursor 1
(Fig. S4, ESIw). The UV absorption spectrum of 2 in CHCl3
is concentration independent within the range 1.0 ꢁ 10ꢀ3 M to
4.9 ꢁ 10ꢀ7 M, suggesting that the dirhodium chromophores and
the assemblies remain intact (Fig. S5, ESIw).
The strategy used for 2 was repeated using an elongated
analogue of isonicotinic acid, 4-(pyridinyl)benzoic acid which
gave with 1 in refluxing benzene the tetramer 3 in 49% yield.
Despite the structural similarity with square 2, compound 3
only shows moderate solubility in CHCl3 and CH2Cl2. The
1H NMR spectrum of 3 shows two sets of aryl protons in a
2 : 1 ratio in agreement with the mono-substituted nature of
the dirhodium moiety (Fig. 2). The pyridyl protons in 3 appear
in downfield region (9.55, 8.27 ppm), compared to methyl
4-(pyridine-4-yl)benzoate (8.68, 8.11 ppm) (Fig. S3, ESIw)
showing coordination of the pyridyl group to dirhodium centre.
The MALDI-TOF mass spectrum of 2 shows the molecular
ion (m/z = 4109.24, calculated 4109.20), along with peaks at
m/z = 2054.88 and m/z = 1027.34 corresponding to the frag-
mented dimeric and monomeric units respectively (Fig. S6, ESIw).
Finally the diffusion coefficient for the tetramers 2 determined by
1H DOSY experiments at 25 1C in CDCl3 is 430 mm2 sꢀ1
,
compared with that of the monomer 1 which is about 650 mm2 sꢀ1
If the carboxylate function of isonicotic acid is blocked by
.
esterification, its reaction with 1 under reflux in toluene gives
c
9892 Chem. Commun., 2012, 48, 9891–9893
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1
This is supported by the observation of similar H NMR and
show electrochemical behaviour compatible with four indepen-
dent redox centres and could act as four electron reservoirs.
We gratefully thank the Swiss National Science Foundation
for their support of this work.
electronic spectra for 3 and the model compound 5 (prepared
by stoichiometric mixing of 1 with methyl 4-(pyridine-4-yl)
benzoate). Despite numerous efforts, only an average mass for
3 was observed in the MALDI TOF spectrum, but exact
masses were observed for monomeric, dimeric and trimeric
fragments of 3.
Notes and references
As expected for the greater size of the square the diffusion
coefficient of 3 determined by 1H DOSY experiments at 25 1C
z Crystal data for 2: C59H78NO9.5Rh2 was collected on a IPDS II
diffractometer at 180(2) K using MoKa radiation (l = 0.71073 A).
Full-matrix, least-squares refinements on F2 using all data. Total data
128 659, unique data 8666 (Rint = 0.2164), Mr = 1159.04, cubic, space
in CDCl3 is 370 mm2
s
ꢀ1. Although the Stokes–Einstein
equation is not suitable for determination of the hydro-
dynamic radius of these complexes due to their non-spherical
shape, the order of the diffusion coefficients 1 > 2 > 3 is in
good agreement with the expected size progression.
%
group Ia3d, a = b = c = 59.616(3) A, a = b = g = 901, V =
211 879(17) A3, Z = 96, R1 [I > 2s(I)] = 0.1141, wR2 = 0.2877.
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The monomer 1 has two vacant axial sites for binding
ligands, and UV-visible titrations (Fig. S8–S10, ESIw) with a
series of pyridine bases showed two-step binding with typical
values of log10K1 of 6 and log10K2 of 3. The tetramers 2 and 3
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a single quasi-reversible system (0.01 o v o 5 V sꢀ1). In the
case of monomeric complexes, we observed the one-electron
oxidation of the dirhodium(II) species: [Rh2]4+ - [Rh2]5+ +eꢀ
in the range observed for other tetracarboxylate dirhodium
complexes.27,28 The addition of an axial ligand to one of the two
metal centres of compound 1 causes a negative shift of about
ꢀ100 mV of the potential E1/2 (Fig. S11, ESIw). This shift
reflects stabilization of the oxidized state Rh(III)–Rh(II) due to
increased electron density on the metal centres with the presence
of an additional N-donor. For 2 and 3, the cyclic voltammograms
may be interpreted in terms of four independent one-electron
oxidations showing negligible interaction. The peak observed
for 2, where the rhodium–rhodium distance is shorter, is slightly
broader than that seen for 3 which may indicate slightly greater
interaction (Fig. S12, ESIw). This affords a notable contrast with
the systems where the dirhodium units are linked by equatorial–
equatorial interactions (using terephthalate or oxalate21,22) where
significant interaction was observed.
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In conclusion, we have shown the facile one-step synthesis
of nanometre-sized squares in high yields using a new combi-
nation of reversible orthogonal interactions. Two bonds
have been used: dirhodium–carboxylate, and axial rhodium–
nitrogen binding. Heating is required for carboxylate exchange,
but when this occurs and axial coordination is possible,
the mono-substituted product is selected out of the dynamic
mixture as a result of the tetramerisation of the exchanged
product. The resulting assemblies show Lewis acidity, binding
up to four Lewis bases in the vacant axial positions, and are
potentially available for the construction, via Lewis acid–Lewis
base interactions, of more elaborate structures. They further
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9891–9893 9893