A Cyclic [4]rotaxane that Behaves as a Switchable Molecular Receptor: Formation of a Rigid Scaffold from a Collapsed Structure by Complexation with Copper(I) Ions

Abstract

The most efficient molecular receptors are usually rigid edifices with a hollow part that is able to accommodate the complexed species through an electronic and geometrical complementarity between the substrate and the complexing parts of the host. By analogy with biological processes related to induced fit, other host–guest processes are based on flexible hosts that are able to adapt their geometry to that of the species to be recognized. In the very active field of catenanes, rotaxanes, and molecular machines, very few systems have been considered as interesting receptors for molecular guests. One of the main contributions to this subfield of research is that of anion recognition by various interlocking compounds. Our research group has also recently described a [3]rotaxane that is able to act as an adjustable receptor. The system consists of two rings threaded by an axis on which the rings can move freely. The complexing units are zinc porphyrins that are firmly attached to the rotaxane rings and are able to interact with given substrates that consist of doubly end functionalized compounds bearing 4-pyridyl groups. It was shown that the marked geometrical adaptability of the host in its metal-free form allows interaction with guests of very different sizes. Herein we report the properties of a related compound, namely a cyclic bisporphyrin [4]rotaxane, the behavior of which is totally different from that of a previously studied linear [3]rotaxane. Contrary to the latter compound, the metal-free [4]rotaxane collapses completely and does not show any complexation properties, whereas the copper(I)complexed compound is a good and selective receptor for diamine and dipyridyl substrates because of the scaffolding effect of the four metal centers (Figure 1). The recognition process can thus be switched on and off by complexing the free ligand to four Cu ions or demetalating the metalcomplexed species, respectively. The synthesis of rotaxane 1 [7] as well as that of its related [4]pseudorotaxane have already been reported. 1 was demetalated using a large excess of KCN (ca. 50 equivalents) at room temperature. The reaction mixture was stirred for 2.5 hours, and the crude product was then purified by chromatography on silica gel to afford the demetalated rotaxane 2 in 88% yield (Scheme 1). Rotaxane 2 was characterized by H NMR spectroscopy (COSY, ROESY, DOSY), electrospray mass spectrometry and UV/Vis spectroscopy. Very surprisingly, the H NMR spectrum of rotaxane 2 shows considerable loss of symmetry compared to the metalated system 1, in which all the Cu centers as well as the four stoppers were chemically equivalent. A quarter of the rotaxane 1 only had thus to be considered for complete NMR assignment. By contrast, the H NMR signals of 2 were doubled compared to those of rotaxane 1. Two distinct stoppers with significantly different H NMR signals and thus markedly different environments can be identified. NOE interactions between some parts of 2 were clearly detected, whereas these fragments were too far away from one another in 1 to show any interaction. For instance, some protons of the tBu groups of one type of stopper (H-re’; r= “rod”) correlate to protons H-b4’ and Hb7’ (b=bismacrocycle) of the 1,10-phenanthroline unit in 2, thus indicating that this stopper and a given 1,10-phenanthroline unit are located very close to one another. As the bismacrocycles and the axles are both rigid, the proximity between two such fragments tends to indicate that the topography of 2 is markedly different from that of 1, and that 2 has a totally collapsed structure. This hypothesis was confirmed by additional observations. For instance, one proton of the (CH2)3 linker (H-r9) is strongly shielded upon decomplexation; the corresponding signal moves from Figure 1. Principle of the recognition process that is switched on and off by metalation or demetalation. Small gray dots: Cu ions; gray squares: porphyrins; black double arrow: guest compound. The chemical structures of 1 and 2 are shown in Scheme 1.