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Choices of Iron and Copper Cooperative Selection during Self-Assembly.

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Angewandte
Chemie
contain more self-assembly information than presynthesized
ligands, since “assembly instructions” for both the ligands and
supramolecular structure must be included. This problem
becomes acute when sets of “non-orthogonal” subcomponents are employed, for example, two amines that could
condense with two different aldehydes to give a dynamic
library[5] of four imines, with the size of this library growing
for larger collections or multivalent starting materials.
We have previously described a system consisting of a set
of ligand components that combine together in all possible
ways in the absence of copper(i) ions, but which undergo a
thermodynamic self-sorting process in the presence of the
metal ion, thereby eliminating all mixed ligands from the
system [Eq. (1)].[6]
Herein, we describe a more-complex self-organizing
system,[7] in which a larger dynamic library collapses during
the simultaneous formation of iron(ii) and copper(i) complexes [Eq. (2)]. Each one of the six initially present chemical
Self-Assembly
DOI: 10.1002/ange.200504447
Choices of Iron and Copper: Cooperative
Selection during Self-Assembly**
David Schultz and Jonathan R. Nitschke*
The technique of subcomponent self-assembly is emerging at
the intersection of dynamic covalent[1] and metallo-supramolecular[2] chemistry. In subcomponent self-assembly two
hierarchical self-assembly reactions[3] occur as parts of the
same overall process: Intraligand (generally C=N) bonds
form at the same time as metal–ligand bonds, thus bringing
molecular and supramolecular structures into being at the
same time.[4]
The use of subcomponents rather than presynthesized
ligands in programmed self-assembly presents a particular
challenge to the “programmer/chemist”: for a given level of
structural complexity, one must employ subcomponents that
[*] D. Schultz, Dr. J. R. Nitschke
Department of Organic Chemistry
University of Geneva
30 Quai Ernest Ansermet, 1211 Gen0ve 4 (Switzerland)
Fax: (+ 41) 22-379-3215
E-mail: jonathan.nitschke@chiorg.unige.ch
[**] This work was supported by the Swiss National Science Foundation.
We thank Profs. A. Hauser, J. Lacour, and C. Piguet for critical
comments on the manuscript, and P. Perrottet for mass spectrometric analyses.
Supporting information for this article (the syntheses of 1, 2, and 3,
as well experimental details for the reactions shown) is available on
the WWW under http://www.angewandte.org or from the author.
Angew. Chem. 2006, 118, 2513 –2516
species was directed to its place in one of the two product
complexes by the dynamic interplay of steric and electronic
factors on both covalent and coordinative levels. We are not
aware of another example of a clean sorting of chemically
non-orthogonal components into two well-separated “baskets”. The driving forces behind this selectivity were examined individually, and it was found that selection preferences
expressed by the two metals acted in concert to deconvolute
the initial library of ligands. Although these preferences do
not lead to quantitative selection in the absence of one of the
metals, they reinforce each other in the full mixture to select
the observed products from all the possible products.
Although this model system consists of only two mononuclear
complexes, the methodology demonstrated here may thus
prove useful in the development of hierarchically selfassembled systems of greater complexity and function.
Mixing pyridine-2-carbaldehyde (A, 3 equiv), 6-methylpyridine-2-carbaldehyde (B, 3 equiv), tris(2-aminoethyl)amine (C, 1 equiv), and ethanolamine (D, 3 equiv) in aqueous
solution affords a dynamic library of imines (Scheme 1) in
equilibrium with the starting materials, as observed by ESIMS and NMR spectroscopy. The addition of copper(i)
tetrafluoroborate (1.5 equiv) and iron(ii) sulfate (1 equiv)
resulted in the dynamic library of imines collapsing within
12 h at 323 K to leave 1 and 2 as the sole products. This sorting
process is thermodynamic in nature. NMR spectra of the
reaction mixture during the hours following the addition of
the metals revealed the presence of kinetic products, which
disappeared during equilibration.
Certain factors play a clear role in winnowing down the
number of observed product structures: The template effect[8]
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2513
Zuschriften
Scheme 1. The formation of a dynamic combinatorial library of ligands, and the collapse of this library following addition of CuI and FeII ions.
should eliminate all partially formed ligands and ligand
subcomponents from the mixture, the chelate effect[9] should
favor structures containing ligands that bear the highest
number of bound donor atoms possible, and the iron(ii) and
copper(i) ions should be bound to six and four donor atoms,
respectively. Within these limits, a variety of different product
structures might nonetheless be envisaged. We discuss herein
our investigations of three distinct preferences exhibited by
copper(i) and iron(ii) ions. These preferences act in concert to
select 1 and 2 alone as the products of the reaction shown in
Scheme 1.
Firstly, copper(i) ions formed complexes that incorporated
methylated aldehyde B in preference to A. ESI-MS analysis
of the reaction mixture following the addition of copper(i)
tetrafluoroborate (1 equiv) to an aqueous mixture of aldehydes A and B as well as amine D (2 equiv each) showed the
presence of all the three possible products shown in Scheme 2.
We were not able to quantify them by NMR spectroscopic
analysis because of overlapping signals, however the concentrations of the free aldehydes A and B could be quantified.
Aldehydes A and B were incorporated into the product
mixture in a ratio of 30:70, which indicates a slight (ca. 6.3 kJ
Scheme 2. The choice of copper: aldehyde B was incorporated in
preference to A during the formation of 2 in aqueous solution.
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mol1) thermodynamic preference (assuming an equilibrium
of the type [Cu(B)2] + 2 AÐ[Cu(A)2] + 2 B) for the incorporation of B. This preference may be attributed to the presence
of an electron-donating methyl group on B, which allows the
ligands to better stabilize the cationic copper(i) center.
Secondly, iron(ii) ions were observed to form pseudooctahedral complexes that incorporate triamine C in preference to monoamine D. Complex 1 and amine D were the only
products observed by NMR spectroscopy and ESI-MS when
complex 3, prepared as a mixture of fac and mer isomers, was
mixed with an equimolar amount of triamine C in aqueous
solution (Scheme 3). On the basis of the sensitivity of the
Scheme 3. The choice of iron: the entropically driven displacement of
monoamine D by triamine C.
NMR spectrometer at the concentration studied (63 mm), we
concluded that C and 3 were present at concentrations of less
than 1 %. We estimate therefore that the energetic driving
force for this reaction is greater than 22 kJ mol1 (assuming an
equilibrium of the type [Fe(D)3] + CÐ[Fe(C)] + 3 D). The
chelate effect[9] may be understood to drive this substitution:
The incorporation of one equivalent of amine C results in the
liberation of three equivalents of amine D, which provides the
entropic driving force for this reaction.
A third driving force for the observed selectivity, complementing and amplifying the choice of the copper center for
B over A, is the preference of iron(ii) ions to incorporate
aldehyde A instead of B into complexes of type 1. As shown
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 2513 –2516
Angewandte
Chemie
in Scheme 4, the addition of iron(ii) ions (1 equiv) to a
mixture of triamine C (1 equiv) and aldehydes A and B
(3 equiv of each) gave a product mixture in which free A and
B were present in a 3:97 ratio following equilibration at 323 K
Scheme 4. The choice of iron: the preferential incorporation of the
less-hindered aldehyde A into 1 and 4. The steric clash between methyl
group and ring is shown for 4.
over 4 days. This ratio suggests that the incorporation of
aldehyde A over B was favored by about 14.1 kJ mol1
(assuming an equilibrium of the type [Fe(A)3] +
BÐ[Fe(A)2(B)] + A). The broad NMR and the ESI-MS
spectra of the product mixture were assigned to a mixture
of 1, incorporating uniquely aldehyde A, and 4, incorporating
two equivalents of A and one equivalent of B. The reaction
shown in Scheme 4 deviates substantially from a statistical
mixture of products; indeed, only two of the expected four
products are observed to form. No evidence was found of
complexes incorporating two or three equivalents of B.
Mixtures of products were obtained when either cobalt(ii)
or zinc(ii) sulfate was used in place of the iron(ii) salt in the
reactions shown in Schemes 1 or 4. The crystal structures
show that the iron(ii) ion is bound more tightly in 1 (mean
rFe-N = 1.95 A)[10] than is the cobalt(ii) ion (mean rCo-N =
2.15 A)[10] or zinc(ii) ion (mean rZn-N = 2.18 A)[10] in complexes
with the same ligand. This observation correlates with the
finding that the FeII ion is the only divalent first-row transition
metal to have a low-spin ground state with this ligand.[10] The
research groups of Drago,[11] Hendrickson,[12] and Hauser[13]
have investigated the magnetic behavior of 1 and 4 as well as
the other two congeners incorporating two and three equivalents of B, respectively. They determined that although 1
remains in the low-spin 1A1 state over the temperature range
30–450 K,[12, 13] the complexes containing B subcomponents
undergo spin crossover to the high-spin 5T2 state as the
temperature increases. The more B subcomponents a complex contains, the lower the temperature at which it undergoes spin crossover. A steric clash between the methyl groups
and the facing pyridyl rings, as shown in Scheme 4, appears to
destabilize the low-spin state relative to the high-spin state by
elongating the FeN bonds.
The strong thermodynamic preference of iron(ii) ions to
incorporate aldehyde A in preference to B during the
formation of 1 might thus be attributed to the high energy
Angew. Chem. 2006, 118, 2513 –2516
penalty paid for a steric clash in iron(ii) complexes of this
type. Since the presence of high-spin iron(ii) centers may be
inferred in complexes incorporating one or more B subcomponent, one might also invoke the possibility of a “spinselection” phenomenon, whereby the formation of short,
strong bonds between low-spin iron(ii) centers and sp2
nitrogen atoms serves as a thermodynamic driving force for
the preferential incorporation of sterically unhindered A.
In summary, we have examined individually the factors
leading to the observed high degree of selectivity in this
simple model system. The preference for iron(ii) complexes to
incorporate triamine C in preference to monoamine D may be
considered as “strong”—in excess of 22 kJ mol1. The preferences of copper for subcomponent B (ca. 6.3 kJ mol1) and
iron for subcomponent A (ca. 14.1 kJ mol1) were both
weaker, but these preferences complemented each other
and reduced the concentrations of mixed products to
undetectable levels (estimated to be < 1 %, based on ESIMS and NMR spectroscopic measurements).
This deconvolution methodology may prove useful in
determining the thermodynamic preferences of other systems
incorporating mixed sets of subcomponents, particularly in
cases where selective self-assembly is desired. The concurrent
formation of complexes 1 and 2 might also be harnessed to
drive the self-assembly of larger, more complex structures:
the covalent attachment of other self-assembling groups to A,
B, C, or D could allow the self-assembly “program” that
generates 1 and 2 to be used as a “subroutine” within the
assembly of a superstructure incorporating 1 and 2 as
subunits.
Received: December 14, 2005
Published online: March 9, 2006
.
Keywords: coordination modes · copper ·
dynamic combinatorial chemistry · iron · self-assembly
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