Generation of a Dynamic System of Three‐Dimensional Tetrahedral Polycatenanes†

Supramolecular chemistry explores the effects of non-covalent interactions on the self-organization of matter. One strand of supramolecular enquiry has led to the creation of a variety of structurally and topologically nontrivial mechanically interlocked molecules. In the pursuit of deepening the complexity of such structures we report herein the formation of a new class of mechanically interlocked molecules in a system of three-dimensional tetrahedral catenanes. Three distinct reversible processes were used in parallel to enable the creation of metal–organic tetrahedral polycatenanes, namely imine bond formation, 3] metal–ligand coordination, and donor–acceptor interactions. A tetrahedral cageM4L6 (A; Scheme 1) was prepared through the reaction of a suspension (owing to poor solubility) of NDI diamine (6 equiv) with 2-formylpyridine (12 equiv) and iron(II) bis(trifluoromethane)sulfonimide (4 equiv) in acetonitrile. This discrete metallosupramolecular architecture, capable of reversible catenation with an organic macrocycle, was designed to provide a pathway to polycatenated species. NDI moieties have been widely used in the self-assembly of mechanically interlocked molecules because of their planar, electron-poor aromatic surfaces, which engender favorable aromatic donor–acceptor interactions. A dynamic combinatorial library (DCL) of polycatenated cages was thus formed by allowing equilibration of the coordinatively dynamic M4L6 tetrahedral cage in the presence of an excess of bis-1,5-(dinaphtho)-[38]crown-10 (B; Scheme 1). H NMR analysis of equilibrated DCLs revealed the binding of B to the cage to be best described by a non-cooperative model, yielding a crown ether to cage-incorporated NDI binding constant of 794œ 34 Lmol¢1. Controlling crown ether and cage concentrations was found to determine the constitution of the library, and allowed the preparation of the fullysaturated tetrahedral [7]catenane. The H NMR spectrum of cage A (Figure 1b) is consistent with the presence of a mixture of diastereomers with T, S4, and C3 point symmetries in solution, as has been previously observed for analogous cages. The superposition of diastereomer H resonances lead to broadened and overlapping signals, giving the overall appearance of a T-symmetric complex. The expected M4L6 type structure was confirmed by ESI-MS, COSY, and NOESY NMR spectroscopy (see Supporting Information). For A to be catenated by B, we hypothesized a decoordination–threading–re-coordination mechanism to occur at the pyridylimine metal chelate linkages (Supporting Information, Figure S11). An analogous mechanism is thought to operate in similar supramolecular hosts during the encapsulation of guests with larger radii than the hostsÏ pores. To investigate cage/crown ether binding, an excess of B (10 equiv) was allowed to equilibrate with A (1 equiv, Figure 1c); a minimum of 12 h was found necessary for equilibration (see the Supporting Information). Crown ether signals corresponding to catenation around an NDI axle of cage Awere observed. The exchange between free and bound crown ether was observed to be slow on the NMR timescale at 298 K as peaks for the catanated complexes appeared as new resonances and did not shift with changes in crown ether concentration. The H resonances of bound crown ether molecules (Figure 1c, peaks H, H, and H) appeared upfield of resonances attributed to free crown ether (peaks H, H, and H) owing to the close proximity of the shielding aromatic NDI. We attribute the broadening of the H resonances of cage A in the DCL to loss of symmetry, with the exception of the new NDI peak (peak H in Figure 1c). The NDI resonance (H), a singlet at 8.76 ppm in A, split into two signals in the DCL, one encompassing all free NDI binding sites (H*) at 8.70 ppm and one assigned to all crown etherbound NDIs (H), the latter signal being shifted significantly upfield at 8.22 ppm. This peak was integrated relative to the resonances H, H, and H of bound crown ether to confirm the expected one-to-one stoichiometry. Variable-temperature H NMR experiments on the DCL showed only small changes to line shapes and resonance shifts; however, specific catenated species and topological isomers of AB2, AB3, and AB4 remained indistinguishable. In an attempt to drive the [*] S. P. Black, Dr. A. R. Stefankiewicz, Dr. M. M. J. Smulders, Dr. J. R. Nitschke, Prof. J. K. M. Sanders University of Cambridge, Department of Chemistry Lensfield Road, Cambridge CB2 1EW (UK) E-mail: [email protected] [email protected] Homepage: http://www-jrn.ch.cam.ac.uk