Assembly of dynamic heterometallic oligoporphyrins using cooperative zinc-nitrogen, ruthenium-nitrogen, and tin-oxygen coordination [9]

Full text of DOI:10.1021/ja9915652

8120
J. Am. Chem. Soc. 1999, 121, 8120-8121
Scheme 1
Assembly of Dynamic Heterometallic
Oligoporphyrins Using Cooperative Zinc-Nitrogen,
Ruthenium-Nitrogen, and Tin-Oxygen
Coordination
Hee-Joon Kim, Nick Bampos, and Jeremy K. M. Sanders*
UniVersity Chemical Laboratory, Lensfield Road,
Cambridge CB2 1EW, U.K.
ReceiVed May 11, 1999
The construction of supramolecular architectures1 through
noncovalent self-assembly has led to a resurgence of interest in
coordination chemistry. For example, the ligand coordination
properties of metalloporphyrins have resulted in a vast array of
supramolecular systems, such as, cyclic oligomers,2 linear oli-
gomers3 and polymers,4 squares,5 tapes,6 and other geometries.7
However, each of these constructions has employed only a single
type of metal-ligand interaction. This report describes what we
believe to be the first association under conditions of thermody-
namic reversibility of heterometallic porphyrin oligomers driven
by the synergistic coordination of at least two different metal-
loporphyrin building blocks designed to exhibit complementary
geometries and cooperative binding properties.
To achieve our goal, we chose Zn(II), Ru(II), and Sn(IV)
porphyrins for the contrast in their coordination chemistry. Zn
porphyrins prefer nitrogen donor ligands, adopt 5-coordinate
square pyramidal geometry, and are kinetically very labile, while
Ru(CO) porphyrins form stable and inert complexes with nitrogen
donor ligands and adopt 6-coordinate geometry. Sn(IV) porphy-
rins, on the other hand, prefer oxygen donor ligands, adopt
6-coordinate octahedral geometry, and exchange carboxylate
ligands rather slowly.8 When complementary binding functions
are introduced into appropriately designed Zn(II), Ru(II), and Sn-
(IV) porphyrins, it becomes possible to assemble multiple
components in a controlled manner.
For example, a two-component system can be readily envis-
aged: Zn porphyrin Zn-1 was designed with two carboxylic acid
groups for recognizing Sn(IV), while Sn(IV) porphyrin Sn-2
possesses a single pyridyl group for recognizing Zn, as shown in
Scheme 1.9 Zn-1 is obtained as a highly insoluble mixture of
interconverting cis and trans atropisomers, which, upon addition
of 1 equiv of Sn-2,10 are converted quantitatively (over 24 h due
to the insolubility) to the heterodimer [Sn-2/Zn-1] (Scheme 1).
The resonances of [Sn-2/Zn-1] are sharp and well resolved in
the 1H NMR spectrum, allowing assignment and characterization
by 2D NMR techniques (COSY, NOESY). The most diagnostic
signals are the pyridyl resonances which are sharp and shifted
significantly to higher field (∆δ HR -6.23 and Hâ -1.44 ppm).
Usually, resonances of pyridyl ligands bound to simple Zn
porphyrins are difficult to detect at room temperature as the ligand
undergoes fast exchange on the NMR time scale; however, for
[Sn-2/Zn-1] in the presence of excess Sn-2, slow exchange
between free and bound ligands on the NMR chemical shift time
scale allows observation of sharp coordinated ligand resonances.
Structurally diagnostic nOe connectivities between the sharp
pyridine ligand resonances of Sn-2 and the meso and the aryl
protons of the Zn porphyrin (pointing into the cavity) are also
observed in the NOESY spectrum of [Sn-2/Zn-1], confirming
the integrity of the Zn-pyridyl bond. These spectroscopic features
provide strong evidence for the intramolecular interaction between
the Zn porphyrin and the pyridine of the Sn porphyrin in [Sn-2/
Zn-1]. Furthermore, in a 2 mM solution of [Sn-2/Zn-1], displace-
ment of the intramolecularly bound Sn pyridine requires the
addition of more than 1000 equiv of neat pyridine. This suggests
that the effective molarity for the intramolecular Zn-N binding
event is more than 2 M.11 The Sn porphyrin unit becomes free to
rotate about the Sn-carboxylate bond, as indicated by the disap-
pearance of the upfield shifted R and â resonances of the pyridine
unit of [Sn-2/Zn-1]. When the solvent is removed by evaporation
and the residue reconstituted in pyridine-free CDCl3, the original
conformation of [Sn-2/Zn-1] is restored as confirmed by 1H NMR
spectroscopy, with the intramolecular Zn-pyridyl bond intact.
Recognition of the pyridyl group of Sn-2 by the Zn center of
Zn-1 is kinetically the first event in the assembly process, even
though the Zn-pyridyl binding affinity is significantly weaker than
that between the Sn porphyrin of Sn-2 and the carboxylate
component of Zn-1. The evidence for this is the observation that
* E-mail: jkms@cam.ac.uk.
(1) Lehn, J.-M., Ed.; Sauvage, J.-P., Hosseini, W., volume Eds. Compre-
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F.; Devynck, J. Inorg. Chem. 1990, 29, 2734. (b) Martensson, J.; Sandros,
K.; Wennerstrom, O. Tetrahedron Lett. 1993, 34, 541. (c) Burrell, A. K.;
Officer, D. L.; Reid, D. C. W.; Wild, K. Y. Angew. Chem., Int. Ed. 1998, 37,
114. (d) Kobuke, Y.; Miyaji, H. J. Am. Chem. Soc. 1994, 116, 4111. (e)
Stibrany, R. T.; Vasudevan, J.; Knapp, S.; Potenza, J. A.; Emege, T.; Schugar,
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(9) The synthesis of Zn-1 and Sn-2 will be described elsewhere.
(10) All NMR experiments were performed at 2-5 mM concentration.
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10.1021/ja9915652 CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/19/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 35, 1999 8121
Scheme 2
Scheme 3
a mixture of oligomers is obtained in the reaction of the free-
base analogue H2-1, which lacks the recognition site for the
pyridyl group, with Sn-2. The poor solubility of Zn-1 in CDCl3,
1
3). In the resulting H NMR spectrum, the meso proton of the
Sn-porphyrin and the R-protons of the coordinated carboxylate
(H2 in Scheme 3) of [Zn-5/Sn-3/Zn-1] were shifted upfield
relative to the case of [Sn-3/Zn-1] in the absence of Zn-5, while
the diagnostic pyridyl protons were difficult to identify due to
exchange. This observation at 300 K implies a weak contribution
from the ring current of coordinated Zn-5. The pyridyl signals
of [Sn-3/Zn-1] sharpen (at 245 K) in the presence of Zn-5 at a
temperature which is higher than when Zn-5 is absent (at 220K),
suggesting that the spinning motion of the Sn-porphyrin can be
retarded by the cooperativity of the intra- and intermolecular co-
ordination of two Zn porphyrins. In effect, the weak binding of
Zn-5 converts [Sn-3/Zn-1] into a complex with dynamic proper-
ties resembling those of the monopyridyl analogue [Sn-2/Zn-1].
We also assembled a trinuclear heterometallic oligoporphyrin
[Ru-5/Sn-3/Zn-1] consisting of Zn, Sn, and Ru porphyrins. [Ru-
5/Sn-3/Zn-1] is quantitatively prepared by the addition of 1 equiv
of the Ru porphyrin Ru-5 to [Sn-3/Zn-1] (Scheme 3). Ru
porphyrins exhibit a significantly larger association constant than
Zn porphyrins, so that in [Ru-5/Sn-3/Zn-1] all binding interactions
are amplified relative to [Zn-5/Sn-3/Zn-1], effectively “locking”
the whole structure into a rigid conformation. Most remarkably,
the same heterotrimer assembles when the three individual
components Zn-1, Sn-3, and Ru-5 are simply mixed in solution.
To the best of our knowledge, [Ru-5/Sn-3/Zn-1] represents the
first example of a thermodynamically constructed heterometallic
oligoporphyrin containing three different metals.12 The rigidity
of the structure and the favorable geometry of [Ru-5/Sn-3/Zn-1]
make this molecule ideal for characterization by 2D NMR
techniques; nOe cross peaks (NOESY) were identified throughout
the molecule, linking the three porphyrins of the assembly
(represented by double-headed arrows in Scheme 3).
1
resulting in a lack of signals in its H NMR spectrum, ensures
that the molarity of Sn-2 is in effect much higher than that of
Zn-1 during the early stages of the reaction period (about 24 h),
over which no intermediate species were detected, implying that
the assembly of [Sn-2/Zn-1] is rapid. It has been established
previously that addition of 2 equiv of benzoic acid to Sn(TPP)-
(OH)2 results in the complete formation of Sn(TPP)(benzoate)2
in about 30 min via Sn(TPP)(OH)(benzoate).8 In sharp contrast,
while a combination of complex oligomers can be envisaged for
the reaction in Scheme 1 where the carboxylate can bind
intermolecularly to other Sn units, the accelerated rate and
exclusive formation of the Sn-carboxylate component of [Sn-
2/Zn-1] is driven by the intramolecular recognition of the pyridine
group of Sn-2 by the Zn ion of Zn-1.
By exploiting the binding affinities resulting in the cooperative
construction of [Sn-2/Zn-1], it has been possible to specifically
design the structure of the building blocks to construct structurally
diverse but well-defined assemblies. For example, the heterotrimer
[Sn-3/(Zn-4)2] is quantitatively prepared from the combination
of 2 equiv of Zn-4 and 1 equiv of Sn-3, Scheme 2. While the
heterodimer [Sn-2/Zn-1] is remarkably robust, [Sn-3/(Zn-4)2] is
coordinatively saturated at each site but is less robust overall, as
it lacks the cooperativity inherent in [Sn-2/Zn-1]; as a result [Sn-
3/(Zn-4)2] displays dynamic NMR behavior analogous to that of
the [Sn-2/Zn-1] complex only at low temperatures.
In a series of experiments designed to investigate the coopera-
tive Sn-carboxylate binding, treatment of the dichloroacetate
derivative of Sn-3 with Zn-4 yields [Sn-3/(Zn-4)2] in a matter
of minutes, while treatment of the analogous dichloroacetate
derivative of Sn-5 with benzoic acid fails to produce any of the
dibenzoate product, as expected from the relative pKa values.
These competition experiments demonstrate that coordination of
the Zn-pyridyl and Sn-carboxylate components is highly syn-
ergistic, resulting in a dramatic increase in the effective concen-
tration of carboxylate at the Sn center.
The heterodimer [Sn-3/Zn-1] is obtained quantitatively from
the addition of the Zn-diacid porphyrin Zn-1 to the Sn di-pyridyl
porphyrin Sn-3 (1:1 mixture, Scheme 1). Complex [Sn-3/Zn-1]
possesses both Zn-bound and uncoordinated pyridyl groups,
prompting us to investigate the time scale of the “spinning” Sn-
porphyrin and the ability to use the free pyridine to construct a
trimetallic assembly. The structure of [Sn-3/Zn-1] in solution was
confirmed by its 1H NMR spectrum: diagnostic signals (HR, HR′,
Hâ, and Hâ′) for both the Zn-bound pyridine and the unbound-
pyridine are observed only at temperatures below 250 K, sug-
gesting a temperature-dependent rotation of the Sn-porphyrin
component of [Sn-3/Zn-1]. We exclude intermolecular competi-
tion on the basis that we failed to detect a concentration depend-
ence on the appearance of the pyridyl signals of [Sn-3/Zn-1].
To further probe these interactions, we added 1 equiv of a
simple Zn porphyrin Zn-5 to a solution of [Sn-3/Zn-1]10 (Scheme
In summary, we have prepared the heterometallic dynamic
oligoporphyrins which possess a noncoordinated pyridine and
have shown that competition experiments are able to provide new
insights into the intramolecular and intermolecular coordination
processes. We also report the first thermodynamically driven
trinuclear heterometallic oligoporphyrin assembly, based on a
combination of covalent and noncovalent interactions which may
provide a new approach for the preparation of complex porphyrin
assemblies such as catenanes and rotaxanes.
Acknowledgment. We thank EPSRC, the Foreign & Commonwealth
Office (U.K.), and Korea Research Foundation for financial support, Dr
C. C. Mak for MALDI spectra, Dr J. C. Hawley for advice with the
Sn(IV) chemistry, and Prof. M. J. Gunter for valuable discussions.
Supporting Information Available: Synthetic procedure and spec-
troscopic characterization of [Sn-2/Zn-1], [Sn-3/(Zn-4)2], [Sn-3/Zn-1],
and [Ru-5/Sn-3/Zn-1]. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA9915652
(12) Heteronuclear trimetallic complexes in nonporphyrin system: Carde-
nas, D. J.; Sauvage, J.-P. Inorg. Chem. 1997, 36, 2777.