Quantitative formation of [4]pseudorotaxanes from two rods and two bis-macrocycles incorporating porphyrinic plates between the rings

Abstract

[4]Pseudorotaxanes consisting of two very large coordinating bis-macrocycles and rigid rods incorporating two side-by-side chelates have been obtained quantitatively utilising the gathering and threading effect of copper(I); the assemblies obtained are several nanometres long and they contain two face-to-face zinc porphyrins which will be used to complex various organic substrates.

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Catenanes and rotaxanes¹ are particularly promising in relation to synthetic molecular machines and motors.² Recently, spectacular examples of dynamic molecules leading to real devices have been proposed.³ Multicomponent rotaxanes have been used in the elaboration of sophisticated molecular machines such as muscle-like compounds^4,5 or a molecular elevator.⁶ The synthesis of multi-rotaxanes incorporating several rings and several thread-like fragments is still far from being trivial⁷ although the copper(I) templated strategy proposed long ago^1b has recently been successfully applied to the preparation of 4-copper(I) atoms cyclic[2]pseudorotaxane tetramers which can also be described as [4]pseudorotaxanes.^8,9 Porphyrinic rotaxanes such as the [3]rotaxane reported by our group¹⁰ display novel functions in relation to host–guest chemistry, with possible extensions towards the making of molecular presses, able to compress given guests captured between the two plates of the [3]rotaxanereceptor.¹¹ In order to test the ability of copper(I) to act as an assembling and gathering element in the elaboration of large functional assemblies, it appeared as particularly attractive to prepare porphyrin-containing [4]pseudorotaxanes whose general structure is represented in a schematic fashion in Scheme 1.

Scheme 1 Copper(I) templated synthesis of [4]pseudorotaxanes from two bis-macrocycles containing a porphyrin bridge between the chelating units incorporated in the rings and two coordinating rods, each rod containing two rigidly connected bidentate chelates.

Transition metals have been used by several groups as assembling elements in the elaboration of spectacular non-covalent edifices such as squares, polygons, catenanes and cages.¹² In contrast, very few transition metal-based multicomponent threaded or interlocking systems have been made so far and, probably more remarkably, the use of very long bridges between the coordinating atoms, such as the porphyrinic linkers used in the present study, has never been reported. The organic fragments utilised in the copper(I)-driven assembling reaction are depicted in Fig. 1.

Fig. 1 The chemical structures of compounds 1, 2 and 3 used for the synthesis of 4⁴⁺ and 5⁴⁺.

The [4]pseudorotaxanes4⁴⁺ and 5⁴⁺ (i.e.[1₂·2₂·Cu₄]⁴⁺ and [1₂·3₂·Cu₄]⁴⁺, respectively) were obtained quantitatively from stoichiometric amounts of the corresponding components, as represented in Fig. 2. In compound 1, both N⁁N coordinating units of the 1,10-phenanthroline chelates are separated by 24 atoms, with an approximate distance of 30 Å according to CPK models. The rods used, 2^9b and 3, contain either a trimethylsilyl (TMS) or an acetylenic group as terminal functions respectively. The synthesis of 1 and 3 will be reported in a forthcoming paper. 2 and 3 were prepared in a few steps from 3,8-dibromo-4,7-phenanthroline¹³ whereas the synthesis of compound 1 was particularly difficult and involved many steps.

Fig. 2 The copper(I) driven quadruple threading reaction and the chemical structure of the [4]pseudorotaxanes4⁴⁺ and 5⁴⁺.

In a typical procedure, the two copper(I) [4]pseudorotaxanes4⁴⁺ and 5⁴⁺ were prepared following the same threading strategy. Bis-macrocycle 1 was first dissolved in degassed chloroform, while [Cu(MeCN)₄⁺](PF₆⁻) was separately dissolved in degassed acetonitrile. The two solutions were mixed to afford a precursor consisting of one bis-macrocycle and two metal centres. This complex was subsequently added to stoichiometric amounts of 2 or 3 suspended in chloroform. After one week of stirring at room temperature, a dark purple solution of the corresponding [4]pseudorotaxane was obtained quantitatively. The two compounds were isolated as pure PF₆⁻ salts. [4⁴⁺](PF₆⁻)₄ and [5⁴⁺](PF₆⁻)₄ are red brownish solids. It is remarkable that the quadruple threading process is at the same time quantitative and slow. The long reaction time required in order to reach completion of the reaction owes to the poor solubility of the rods and to the fact that the kinetic products which are formed at first have to partly or totally dissociate and subsequently reassemble so as to afford the final [4]pseudorotaxanes.

The two threaded complexes [4⁴⁺](PF₆⁻)₄ and [5⁴⁺](PF₆⁻)₄ were characterised by ¹H NMR, using one-dimensional and two-dimensional (COSY, ROESY) spectroscopy techniques. The very strong upfield chemical shift observed for the aromatic protons of the phenyl groups of the macrocycles as well as the various interfragment interactions which are observed by ROESY NMR experiments§ clearly prove threading of the two rods through the two bis-macrocycles. The chemical structure of these assemblies was confirmed by electrospray mass spectrometry (ES-MS). Indeed, the ES-MSspectrum of [4⁴⁺](PF₆⁻)₄ shows a single set of well-resolved peaks centered at [M]/4 = 1634.82 (calcd [M]/4 = 1635.13). In a similar fashion, the spectrum of [5⁴⁺](PF₆⁻)₄ consists of well-resolved set of peaks centered at [M + 3MeOH]/4 = 1510.81 (calcd [M + 3MeOH]/4 = 1510.83). The fact that both [4]pseudorotaxanes lead to clean ES-MS spectrums with no other peaks than the m/4 one tends to indicate that the edifices are highly stable even in the gas phase for both 4⁴⁺ and 5⁴⁺. This exceptional stability is likely to originate from the tetranuclear nature of the complexes and from the pronounced rigidity of both the rod and the aromatic linker connecting the two rings.

It is remarkable that copper(I) is able to induce the quantitative formation of the complex assemblies represented in Fig. 2 in spite of the large bis-macrocyclic building block used. This example illustrates once more the power of the translational entropy factor which is much more favourable to the formation of “small” discrete species rather than coordination oligomers. The present compounds will be used in the construction of new porphyrin-containing receptors and dynamic molecular systems able to modify the conformation of complexed substrates.

The 25-year old “gathering-and-threading” approach¹⁴ based on copper(I) and an appropriate set of ligands, consisting of rings and rods, turned out to be extremely efficient, even in the construction of gigantic threaded structures incorporating several organic fragments and copper(I) centres. The present case is particularly illustrative since, in spite of the length of the bis-macrocyclic unit 1, the final structure is obtained quantitatively. Particularly remarkable is the formation of a 66-membered ring, incorporating 4 metal centres, nitrogen and carbon atoms. The presence of zinc atoms in the central porphyrin coordination sites will allow the building of more sophisticated structures, in relation to molecular machines, by taking advantage of the Lewis acidity of these metal centres.

We thank the CNRS and the Agence Nationale de la Recherche (ANR no. 07-BLAN-0174, MOLPress) for their support as well as the Ministry of Education for fellowships to JF, CT and YT. CT acknowledges the Luxemburg Ministry of Research. We are also grateful to Lionel Allouche and Roland Graff (Service Commun de RMN, Université de Strasbourg, Strasbourg, France) for the NMR spectra.

Notes and references

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Footnotes

† This paper is dedicated to Professor Roeland Nolte on the occasion of his 65^th birthday.

‡ Electronic supplementary information (ESI) available: NMR and MS characterisation of compounds [4⁴⁺](PF₆⁻)₄ and [5⁴⁺](PF₆⁻)₄. See DOI: 10.1039/b901482a

§ Particularly noteworthy are, for instance in compound [5⁴⁺](PF₆⁻)₄, the correlations between aromatic protons of rod 3 and protons of the polyethylene glycol chain of the bis-macrocycle 1.

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