Full text of DOI:10.1002/anie.201804787

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Accepted Article
Title: Shadow Mask Templates for Site-Selective Metal Exchange in
Magnesium Porphyrin Nanorings
Authors: Pernille S. Bols and Harry Laurence Anderson
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201804787
Angew. Chem. 10.1002/ange.201804787
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10.1002/anie.201804787
Angewandte Chemie International Edition
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Shadow Mask Templates for Site-Selective Metal Exchange in
Magnesium Porphyrin Nanorings
Pernille S. Bols and Harry L. Anderson*
Dedicated to Professor Jeremy Sanders on the occasion of his 70th birthday
Abstract: Molecular templates can be used in many different ways
to control the outcome of chemical reactions. Here we present a
new type of template-directed synthesis. We show that templates
can be used as shadow masks: the shape of the template becomes
imprinted on the product because reaction only occurs at sites not
masked by the template. We demonstrate this effect by using
oligopyridine templates to dictate the sites of demetallation when a
magnesium porphyrin nanoring is treated with acid. Magnesium
centers that are coordinated to the template are protected whereas
uncoordinated magnesium centers are removed. After site-selective
demetallation, the template can be removed and other cations, such
as zinc(II) and copper(II), can be inserted into the free-base
porphyrin centers. This strategy provides a simple route to a wide
range of heterometallated porphyrin arrays.
Stencils and shadow masks have been used to create patterns since
at about 60,000 years ago[1] and this strategy is widely exploited in
top-down nanofabrication.[2] Here we demonstrate the same effect
on a molecular scale, through the use of ‘shadow mask templates’.
Busch defined a chemical template as a species that organizes an
assembly of atoms, with respect to one or more geometric loci, in
order to achieve a particular linking of atoms.[3] A template
provides instructions for the formation of a single product from a
substrate or substrates which otherwise have the potential to
assemble and react in a variety of ways.[4] Recently, many new
types of template-directed synthesis have been invented, providing
routes to previously inaccessible molecular architectures.[5-7] There
are previous reports of the use of molecules as shadow masks,[8]
but to the best of our knowledge, this is the first time this principle
has been used to create precisely defined molecular structures.
The work presented here grew from a simple observation:
treatment of the magnesium complex of a butadiyne-linked six-
porphyrin nanoring, Mg6-P6, with acetic acid in dichloromethane
results in rapid removal of the magnesium cations from all six
porphyrin centers, whereas the complex of this nanoring with a
hexapyridyl template, Mg6-P6·T6, is not demetallated under these
conditions. We concluded that the template masks the magnesium
ions from acid. To test this hypothesis, we inserted a five-legged
template T5[7] into a magnesium porphyrin nanoring and submitted
it to demetallation conditions. As expected, the uncoordinated
Scheme 1. The template masking method applied to synthesize (H2)Mg5-P6
magnesium porphyrin is cleanly demetallated, whereas the
template-coordinated magnesium cations remain in place (Scheme
1). Analysis of crude reaction mixtures by H NMR spectroscopy
using the template T5. Ar = 3,5-bis(trihexylsilyl)phenyl.
1
To test the scope of this template effect, we investigated
whether it could be applied to four-legged templates, by comparing
three ligands with pyridine coordination sites in different positions
(T4a, T4b and T4c, Figure 1).[10,11] In each case, the complex of
the template with magnesium nanoring Mg6-P6 was treated with
10% acetic acid in dichloromethane. All three reactions give clean
site-selective demetallation, and the products are obtained in high
yield without need for chromatographic purification (Scheme 2).
The identities of the final compounds were confirmed by 1H NMR
and MALDI-ToF MS analysis. The NH protons in the free-base
porphyrins give distinctive proton resonances at chemical shifts of
−1.0 to −1.3 ppm, and the exact chemical shift is characteristic of
each isomer.
and MALDI-ToF mass spectrometry shows that site-selective
demetallation of Mg6-P6·T5 to (H2)Mg5-P6·T5 is a quantitative
reaction with no detectable trace of over-demetallated by-products.
The template is displaced from (H2)Mg5-P6·T5 by treatment with
a large excess of DABCO, and (H2)Mg5-P6 is isolated in 88%
yield from Mg6-P6.[6,7,9]
[*]
P. S. Bols and Prof. H. L. Anderson
Department of Chemistry, University of Oxford
Chemistry Research Laboratory, Oxford OX1 3TA (UK)
E-mail: harry.anderson@chem.ox.ac.uk
Homepage: http://hla.chem.ox.ac.uk/
Supporting information and the ORCID identification numbers for the
authors of is article can be found under …DOI…
1
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10.1002/anie.201804787
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N
N
N
N
N
demetallation process; treatment of the complex of Mg6-P6 and T2
with acetic acid gives only the fully demetallated ring (H2)6-P6.
N
Ph
Ph
N
N
N
N
N
N
T4a
T4b
T4c
N
N
N
Bu
Bu
Ph
Ph
N
Ph
Bu
Bu
N
N
Bu
Bu
N
N
N
T3a
T3b
T3c
C8H17
N
N
C8H17
T2
Figure 1. Structures of the oligopyridine ligands tested as masking templates.
(T3b and T2 were found not to be effective as masking templates.)
Next, we tested three-legged templates, starting with the 1,2,3-
isomer T3a (Figure 1).[11] Treatment of Mg6-P6 with one
equivalent of T3a followed by acetic acid results in site-selective
demetallation to form 1,2,3-(H2)Mg3-P6·T3a, together with about
7% of the fully demetallated ring (H2)6-P6. This fully demetallated
nanoring is easily separated from 1,2,3-(H2)Mg3-P6·T3a by
chromatography on silica. The formation of (H2)6-P6 as a
byproduct during the reaction of Mg6-P6·T3a with acetic acid
reflects the weaker association constant of this tridentate template
compared with T6, T5 and T4a-c.[11]
Scheme 2. The template masking method applied to synthesize five
heterometallated rings from Mg6-P6.
Disappointingly, treatment of Mg6-P6 with the symmetric
three-legged template T3b[12] (1:1 mole ratio) followed by acetic
acid does not result in site-selective demetallation. The main
products are unreacted starting material and fully demetallated ring
(H2)6-P6. 1H NMR titrations revealed the reason for this result:
when one equivalent of T3b is added to Mg6-P6, the main product
is the 1:2 complex Mg6-P6·(T3b)2, together with a small amount
of the 1:1 complex Mg6-P6·T3b, and some uncomplexed Mg6-P6
nanoring. It appears that formation of the 1:2 complex with T3b is
too favorable, making this ligand ineffective as a masking template.
In order to solve this problem, we designed a three-legged template
with bulky groups to prevent the stacking, T3c, based on a truxene
core.[13] Template T3c has a similar coordination geometry to T3b,
but the bulky n-butyl chains in the truxene are expected to hinder
Magnesium porphyrins are ready demetallated by acid, but
they do not easily undergo transmetallation when treated with other
metal salts,[14] which makes it possible to use shadow mask
templates to achieve site-selective transmetallation. For example,
treatment of 1,2-(H2)2Mg4-P6 with zinc(II) acetate results in clean
insertion of zinc(II) at the two free-base positions to give 1,2-
Zn2Mg4-P6; the remaining magnesium centers can then be
demetallated selectively with acetic acid to yield 1,2-Zn2(H2)4-P6
(Scheme 3a).[15] This compound is useful for testing the
cooperativity of ligand binding inside nanorings.[11] We have also
prepared a heterometallated nanoring containing two paramagnetic
copper(II) ions opposite each other (1,4-Cu2Mg4-P6·T4a Scheme
3b). In this reaction, the T4a template was left in place during
insertion of copper(II) into the two free-base centers of 1,4-
(H2)Mg4-P6·T4a. We removed T4a and inserted T6 to obtain 1,4-
Cu2Mg4-P6·T6 (see Supporting Information), which is a valuable
compound for investigating quantum interference through long-
range exchange coupling.[16] Many metal cations can be inserted
into porphyrins under mild conditions,[17] so the magnesium/free-
base nanorings reported here should be useful intermediates for
preparing diverse heterometallated systems.
1
stacking. H NMR spectroscopy shows that addition of T3c to the
nanoring only leads to formation of a 1:1 complex, Mg6-P6·T3c.
Addition of acetic acid to this complex results in site-selective
demetallation, together with formation the fully demetallated
nanoring (H2)6-P6. 1H NMR analysis of the crude reaction mixture
(Figure S2) shows that it consists of 83% 1,3,5-(H2)3Mg3-P6·T3c
and 17% (H2)6-P6. These two products are readily separated by
chromatography on silica. The weaker binding resulting from the
presence of only three pyridine coordination sites makes T3c less
effective that T4a–c as a masking template, but the bulk of the
truxene effectively blocks formation of the 1:2 complex. The two-
legged template T2 (Figure 1) binds too weakly to control the
2
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Scheme 3. Synthesis of heterometallated rings. a) Synthesis of a di-zinc(II)
porphyrin ring. (i) Zn(OAc)2, 2 h, 20 °C, 75% yield. (ii) 50% AcOH in CH2Cl2,
30 min, 20 °C, 69% yield. b) Synthesis of a di-copper(II) porphyrin ring. (iii)
CuOAc, air, 1 h, 60 °C, 87% yield. Ar = 3,5-bis(trihexylsilyl)phenyl.
In summary, we have demonstrated that oligopyridine ligands
can be used as shadow mask templates to achieve site-selective
demetallation of magnesium porphyrin nanorings. Selective
demetallation is quantitative with five- and four-legged templates,
and it provides useful selectivity with three-legged templates if the
binding of more than one template unit can be prevented, however
it does not work with two-legged templates. This new template
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[12] M. Fujita, H. Oka, K. Ogura, Tetrahedron. Lett. 1995, 36,
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a
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Acknowledgements
We thank the ERC (grant 320969), the EPSRC and the John
Templeton Foundation for support, Dr. Huawei Jiang, Dr. Jonathan
Cremers, Michael Jirasek and Isabell Grübner for providing
samples of starting materials, and the EPSRC National Mass
Spectrometry Service at Swansea University and Dr. Renée Haver
for mass spectra.
Keywords: template • macrocycles • porphyrin oligomers •
masking • magnesium
[16] S. Richert, J. Cremers, I. Kuprov, M. D. Peeks, H. L.
Anderson, C. R. Timmel, Nat. Commun. 2017, 8, 14842.
[17] J. K. M. Sanders, N. Bampos, Z. Clyde-Watson, S. L.
[1] a) D. L. Hoffmann, C. D. Standish, M. García-Diez, P. B.
Pettitt, J. A. Milton, J. Zilhão, J. J. Alcolea-González, P.
3
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Darling, J. C. Hawley, H.-J. Kim, C. C. Mak, S. J. Webb, in
The Porphyrin Handbook, Vol. 3 (Eds.: K. M. Kadish, K. M.
Smith, R. Guilard) Academic, San Diego, 2000, pp. 1–48.
4
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Entry for the Table of Contents
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Pernille S. Bols and Harry L. Anderson*
Page No. – Page No.
Shadow Mask Templates for Site-
Selective Metal Exchange in
Magnesium Porphyrin Nanorings
Text for Table of Contents
Hidden behind a mask: Magnesium atoms that are coordinated to a template are resistant to acid, so oligopyridine
templates can be used as shadow masks. The shape of the template becomes imprinted on the molecular structure of the
product because reaction only occurs at sites not masked by the template.
5
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