Half-rotation in a kinetically locked [2]catenane induced by transition metal ion substitution

Article abstract of DOI:10.1039/c2cc32418k

We report on a heterocircuit [2]catenane in which a reversible half-rotation of one ring about the other can be induced, and locked in place, by switching the coordination of the interlocked rings between Pd(ii) and Co(iii).

Full text of DOI:10.1039/c2cc32418k

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Half-rotation in a kinetically locked [2]catenane induced by transition
metal ion substitutionw
David A. Leigh,*ab Paul J. Lusby,a Alexandra M. Z. Slawinc and D. Barney Walkera
Received 4th April 2012, Accepted 21st April 2012
DOI: 10.1039/c2cc32418k
We report on a heterocircuit [2]catenane in which a reversible
half-rotation of one ring about the other can be induced, and
locked in place, by switching the coordination of the interlocked
rings between Pd(II) and Co(III).
Reversible control of large amplitude sub-molecular motions is a
feature of many of the structures considered to be prototypical
elements of synthetic molecular machines.1 Although many
methods to efficiently assemble mechanically interlocked molecules
have been developed,2 there are relatively few examples
through which the interlocked macrocycles can be ‘locked’ in
different co-conformations.3–7 Sauvage and colleagues have
exploited the different coordination preferences of Cu(I)
(tetrahedral) and Cu(II) or Zn(II) (trigonal bipyramidal) in the
switching of catenanes,3 rotaxanes4 and so-called ‘‘molecular
muscles’’.5,6 Here we report on a heterocircuit [2]catenane that
can be kinetically fixed in two different co-conformations
through binding of both macrocycles to non-labile transition
metal ions with different coordination requirements, specifically
square planar Pd(II) and octahedral Co(III). The mechanism of
formal rotation is a two stage process: (i) ‘unlocking’ of the system
by removing the chelated metal ion, allowing relative movement of
the interlocked rings, followed by (ii) addition of a different
metal ion to lock the molecule in its new co-conformational
arrangement.
Metal template synthesis is a powerful tool for the assembly
of mechanically interlocked molecular structures.8 A ‘3+1’
donor set enables square planar Pd(II) ions to form kinetically
robust heteroleptic complexes in which the ligands can be
preorganised for the synthesis of both rotaxanes9 and catenanes.10
We used this strategy to prepare [2]catenane Pd3H2 by coordina-
tion of Pd2(CH3CN) to the 2,6-bis(oxymethylene)pyridine moiety
of macrocycle 1H2 (Scheme 1, i) and subsequent ring closing
metathesis and hydrogenation (Scheme 1, ii, iii).
Scheme 1 Synthesis and reversible interconversion of Pd3H2, 3H4
and [Co3]Et4N.
a School of Chemistry, University of Edinburgh, The King’s Buildings,
West Mains Road, Edinburgh, UK EH9 3JJ
Comparison of the 1H NMR spectra of macrocycle 1H2 and
b School of Chemistry, University of Manchester, Oxford Road,
Manchester, UK M13 9PL. E-mail: David.Leigh@manchester.ac.uk
Pd3H2 (Fig. 1a and b) shows a significant upfield shift of
c School of Chemistry, University of St. Andrews, Purdie Building,
St. Andrews, Fife, UK KY16 9ST
w Electronic supplementary information (ESI) available: Experimental
procedures, NMR spectra and X-ray crystallographic data. CCDC
874997 and 874998. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cc32418k
several signals corresponding to aromatic resonances closest to
the 2,6-bis(oxymethylene)pyridine motif (He and Hf) in the
[2]catenane whereas signals close to the 2,6-dicarboxamide
unit (e.g. HD) are essentially unchanged. Although the large
macrocycle is bound to the metal centre by a single coordination
c
5826 Chem. Commun., 2012, 48, 5826–5828
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Fig. 1 Partial 1H NMR spectra (400 MHz, CD2Cl2, 298 K) of
(a) macrocycle 1H2, (b) Pd3H2, (c) 3H4 and (d) [Co3]Et4N. The
assignments of the signals correspond to those shown in Scheme 1.
Signals corresponding to residual solvent peaks, trace impurities and
the Et4N counterion are shown in grey.
Fig. 2 X-Ray crystal structures of (a) Pd3H2 (CCDC 874998) and
(b) 3H4 (CCDC 874997). Carbon atoms of the smaller macrocycle are
shown in light blue and those of the larger macrocycle in grey, oxygen
atoms are red, nitrogen dark blue, hydrogen gold, and palladium pink.
For clarity non-amide hydrogen atoms are not shown. Selected bond
lengths for Pd3H2 [A]: Pd-N2 2.06; Pd-N5 1.92; Pd-N11 2.04;
Pd-N41 2.08; N77-H74 2.45; N77-H83 2.01; other selected distance [A]:
N5-N41 4.00. Selected bond angles for Pd3H2 [1]:N2-Pd-N11 161.1;
H74N-N77-H83N 77.1. Selected bond lengths for 3H4 [A]: H2N-N5 2.45;
H11N-N5 2.34; H2N-N41 2.28; H11N-N41 2.43. Selected bond angles
for 3H4 [1]: H2N-N5-H11N 75.2; H2N-N41-H11N 76.8.
bond this motif is kinetically stable.10 A large downfield shift
of the amide protons (HC) of the 1H2 macrocycle in the
[2]catenane is also observed suggesting that intercomponent
hydrogen bonding occurs, probably to the carboxamido oxygens
of the other ring through folding in solution. The topology of
Pd3H2 was confirmed by X-ray crystallography of a single crystal
grown from slow cooling of a hot saturated solution of the
complex in acetonitrile (Fig. 2a). The intercomponent hydrogen
bonding interaction observed in solution is satisfied inter-
molecularly in the solid state (see ESIw).
Macrocycle 1H2 also contains a 2,6-pyridinedicarboxamide
binding site that in its deprotonated (carboxamido) form has
been previously shown to bind a selection of ‘hard’ transition
metal ions,11 including Co(III).12 Conversion of [2]catenane
Pd3H2 into the corresponding octahedral Co(III) complex was
carried out in two steps (Scheme 1, iv, v). Removal of the
Pd(II) ion with KCN furnished the free ligand 3H4 in 98%
yield (Scheme 1, iv). The downfield shift of the amide signal
undergone a significant shuttling motion in order to accom-
modate the trivalent metal ion. Resonances corresponding to
0
benzylic protons HD and HD are heavily shielded in
[Co3]Et4N with respect to Pd3H2 and 3H2, whilst the chemical
shifts of the Hc and Hd signals are similar to those in 3H2,
indicating that they are no longer in close proximity to the
aromatic groups of the small amide macrocycle. The calculated
minimum energy structure (Spartan 06, MMFF) of [Co3]Et4N,
1
showing the 1801 half-turn indicated by the H NMR data, is
shown in Fig. 3.
To return the heterocircuit catenane back to its original
state [Co3]Et4N was stirred first with Zn in acetic acid and
then treated with Pd(OAc)2 in a mixture of CH2Cl2 and
CH3CN (Scheme 1, vi, vii). We attribute the modest yield (71%)
0
HC and the pronounced shielding of signals Hc and Hd in
the 1H NMR spectrum (Fig. 1c) indicates that the general
co-conformation of Pd3H2 is retained in the metal-free catenane
3H4 in CD2Cl2 through an intercomponent pyridine-NH-
pyridine bifurcated H-bond motif.13 The X-ray structure of
a single crystal of the demetalated catenane, grown by slow
diffusion of diethyl ether into a saturated solution of 3H4 in
CH2Cl2, confirms that this arrangement is maintained in the
solid state (Fig. 2b).
Treatment of 3H4 with Co(OAc)2, Et4NOAc and sodium
ethoxide followed by stirring under air generated [Co3]Et4N in
81% yield (Scheme 1, v). Introduction of the Co(III) ion
introduces several pronounced changes to the 1H NMR spectrum
of [Co3]Et4N (Fig. 1d) verifying that the two macrocycles have
Fig. 3 Energy minimised structure of [Co3] (Spartan 06, MMFF).
The colour scheme is as for Fig. 2 with the addition of cobalt (green).
For clarity hydrogen atoms and the Et4N counterion are not shown.
c
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of the final step to small amounts of Pd(II) binding to the
2,6-pyridine dicarboxamide of the large macrocycle.
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In summary, a switchable [2]catenane has been prepared for
which the co-conformation is controlled by the coordination
preference of the transition metal ion the interlocked macro-
cycles bind to. The [2]catenane is co-conformationally locked
when bound to either metal; switching between the two sites is
permitted by first abstracting the transition metal ion (Pd(II) or
Co(III)), which allows the sub-molecular fragments to rotate
more freely, and then locking the structure again through
metal ion re-coordination. Easily operated and reversible
systems for controlling the co-conformation of heterocircuit
[2]catenanes could prove useful in the design of more complex
pieces of synthetic molecular machinery.
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We are grateful to the European Commission for a Marie
Curie Fellowship to D.B.W. and the Engineering and Physical
Sciences Research Council (EPSRC) for funding. P.J.L. is a
Royal Society University Research Fellow.
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5828 Chem. Commun., 2012, 48, 5826–5828
This journal is The Royal Society of Chemistry 2012