Passing Two Strings through the Same Ring Using an Octahedral Metal Center as Template: A New Synthesis of [3]Rotaxanes

Abstract

Octahedral transition metal centers such as Fe(II), Co(II), and Co(III) have been used as templates in the construction of [3]pseudorotaxanes and [3]rotaxanes from various acyclic and macrocyclic fragments. The species obtained consist of a ring threaded by two string-like compounds. Such systems are relatively uncommon in the [3]rotaxane family, the most usual form being made up of a single axis threaded through two rings. The key structural feature of the present systems is the coordinating unit incorporated in the various organic fragments and used in conjunction with the metal to gather and thread the two filaments through the ring. This bidentate chelate is derived from 8,8'-diphenyl-3,3'-bi-isoquinoline, a very rare example of an endotopic but nonsterically hindering ligand. The stoppered [3]rotaxanes were obtained by using an open-chain fragment bearing azide groups as end functions, followed by click chemistry using a propargyl ether attached to a very bulky group. A particularly attractive X-ray structure was obtained for a cobalt(III)-complexed [3]pseudorotaxane consisting of a 41-membered ring and two crescent-shaped threaded components. The Fe(II) and Co(III) complexes were characterized by (1)H NMR and ES-MS. By taking advantage of the markedly different kinetic properties of the two oxidation states, Co(II) and Co(III), it was possible to proceed to fast coordination or decoordination reactions (for the divalent state) or, when needed, to "freeze" the complexes due to the kinetic inertness of the trivalent state and to study them by (1)H NMR. Finally, demetalation of the two stoppered compounds prepared was performed. This demetalation reaction was fast for the Co(II)-complexed [3]rotaxane, whereas decomplexation of the Fe(II) equivalent required harsh conditions which were not compatible with the stability of the metal-free rotaxane. Interestingly, the thermal stability of the free [3]rotaxane toward unthreading and formation of its constitutive elements was only limited. (1)H NMR measurements showed that the half-life of the rotaxane is about one week at room temperature in dichloromethane. A variable-temperature study revealed that the unthreading reaction leading to dissociation of the [3]rotaxane has a remarkably high entropy of activation, in agreement with the intuitive view that the unthreading process involves a highly ordered transition state.

The application of CuAAC ‘click’ chemistry to catenane and rotaxane synthesis

“…37 This motif was used to create a [3]rotaxane 38 21, which unusually features two axles threaded through a single macrocycle (Scheme 6). 39,40 Two bis-isoquinoline rods derivatised with azide groups at each end (17b) were threaded through the cavity of macrocycle 18 to generate metal complexes M-20 (M = Fe(II) or Co(III)). Stoppering with bulky alkyne unit 8via the CuAAC reaction afforded Fe II -21 and Co III -21 in good yields (82% and 89%, respectively).…”

Transition metal-templated synthesis of catenanes and rotaxanes

“…Chemical structure of the [3]rotaxane of Figure 4(b) based on the modified 3,3′-biisoquinolinate chelate [27]. acts as a true catalyst.…”

“…Catenanes could also be synthesized this way either on Ru(II) [23,24] or Rh(III) [25] templates, one of the rings incorporating the bis-phenanthroline coordinating unit. The ML 3 -template strategy was also efficiently used to synthesize [3]rotaxanes of unique structure, two molecular threads passing through its ring, which was made possible thanks to the design of an original 3,3′-biisoquinoline chelate (Figure 4(b)) [26,27]. The chemical structure of the system built on octahedral Co(II), the latter template cation being easily extracted by EDTA, is displayed in Figure 5.…”

An Introduction to the Mechanical Bond

“…A similar concept involves two dumbbells encircled by a single ring-an architecture (Figure 1.28e, Section 4.2.1.2) first created by Anderson. 305 Sauvage 306 and Leigh 307 have also created examples of these doubly threaded architectures, the latter case, which was nicknamed a 'molecular sheaf', includes a crystal structure (Figure 1.29c) 265 Solvent molecules, counterions, and protons are omitted for the sake of clarity. molecular sheaf (Figure 1.29h) comprising two dumbbells, each encircled by two inorganic rings, has also been described by Leigh and Winpenny.…”