Mechanostereochemistry and the mechanical bond

Abstract

The chemistry of mechanically interlocked molecules (MIMs), in which two or more covalently linked components are held together by mechanical bonds, has led to the coining of the term mechanostereochemistry to describe a new field of chemistry that embraces many aspects of MIMs, including their syntheses, properties, topologies where relevant and functions where operative. During the rapid development and emergence of the field, the synthesis of MIMs has witnessed the forsaking of the early and grossly inefficient statistical approaches for template-directed protocols, aided and abetted by molecular recognition processes and the tenets of self-assembly. The resounding success of these synthetic protocols, based on templation, has facilitated the design and construction of artificial molecular switches and machines, resulting more and more in the creation of integrated functional systems. This review highlights (i) the range of template-directed synthetic methods being used currently in the preparation of MIMs; (ii) the syntheses of topologically complex knots and links in the form of stable molecular compounds; and (iii) the incorporation of bistable MIMs into many different device settings associated with surfaces, nanoparticles and solid-state materials in response to the needs of particular applications that are perceived to be fair game for mechanostereochemistry.

Figure 1.

The creation of a mechanical bond, in the form of a catenane or a rotaxane, from a 1:1 complex that we call (Ashton et al. 1991) a pseudorotaxane, underlines the importance of non-covalent bonding interactions in the synthesis of MIMs, which, by definition, are molecules and not supermolecules because the parting of their components requires the breaking of a covalent bond.

Scheme 1.

A list of timelines illustrating the progress in the field of mechanically interlocked molecules (MIMs) relating to their structures, preparations and applications as well as providing the names of some of the key players.

Figure 2.

Two statistical approaches to the synthesis of MIMs. (a) The purported synthesis of a [2]catenane 3 by carrying out a macrocyclization in the presence of a pre-formed ring (Wasserman 1960). (b) The synthesis of a [2]rotaxane 7 by repetitive threading and stoppering of a ring bound to a solid support, followed by its release (Harrison & Harrison 1967).

Figure 3.

The significant transformations in a 20-step directed synthesis of a [2]catenane 13 wherein the two rings are held together by covalent bonds until the final steps (Schill & Lüttringhaus 1964) when these covalent bonds are cleaved.

Figure 4.

The Cu(I)-templated synthesis (Dietrich-Buchecker et al. 1983) of a [2]catenate comprising two 1,10-phenanthroline-based rings together with the crystal structures (Cesario et al. 1985) of both the Cu(I) [2]catenate 17 and its demetallated analogue, the [2]catenand 18.

Figure 5.

(a) The ‘all-in-one’ synthesis of a benzylic imine [2]catenate templated by transition metal coordination. (b) A tubular representation of the solid-state structure of a Zn(II)-templated benzylic imine [2]catenate illustrates the topological node generated by the meridional octahedral metal cation (Leigh et al. 2001).

Figure 6.

(a) The equilibrium that exists in aqueous solution between the Pd(II)-templated metallomacrocycle 22 and the dimeric [2]catenane 23 (Fujita et al. 1994). The dynamic coordinative bonds between the Pd(II) cation and the pyridyl ligands allows the reversible self-assembly of the thermodynamic product. (b) A tubular representation of the crystal structure of a triply interlocked [2]catenane 24 prepared by Pt(II) templation (Fujita et al. 1999).

Figure 7.

The stoppering of cyclodextrin-based pseudorotaxanes to form [2]rotaxanes can be carried out through coordinative bonding (a) or by covalent bond formation (b).

Figure 8.

(a) The template-directed synthesis of the donor–acceptor [2]catenane 35 comprising the macrocycle cyclobis(paraquat-para-phenylene) and bis-para-phenylene[34]crown-10 (Ashton et al. 1989). Tubular renderings of the solid-state structures of (b) the previously described [2]catenane 35, (c) a branched [7]catenane 36 (Amabilino et al. 1997) and (d) a folded [3]pseudorotaxane 37 (Basu et al. 2011).

Figure 9.

(a) The synthesis of the amide hydrogen-bonded [2]catenanes 41 discovered independently by Hunter (1992) and Vögtle et al. (1992). (b) A smaller [2]catenane 42 and (c) a [2]rotaxane 44 prepared using a similar strategy by Leigh and co-workers (Johnston et al. 1995, 1996).

Figure 10.

(a) The formation of a pseudorotaxane 47 between dibenzylammonium hexafluorophosphate (45) and dibenzo[24]crown-8 (46) with its solid-state structure portrayed alongside. (b) Reversible formation of a [2]rotaxane 51 by a clipping protocol wherein a diimino-crown ether self-assembles around a secondary dialkylammonium ion centre, which can then be kinetically trapped by reduction of its imine bonds to form the stable [2]rotaxane 52. Its crystal structure is displayed alongside.

Figure 11.

The chloride-templated synthesis of (a) a [2]rotaxane 54 (Wisner et al. 2002), and (b) a [2]catenane 56 (Sambrook et al. 2004) with their respective crystal structures positioned alongside in tubular format with the chloride template rendered as a green sphere.

Figure 12.

Structural representations (a) and solid-state structures (b) of the ground, mixed-valence and radical cation dimer states, in turn, of the [3]catenane 57⁴⁺ (Spruell et al. 2010).

Figure 13.

(a) The synthesis of a frustrated [2]rotaxane 60 based on the recognition of a monocationic bipyridine-based radical thread by the dicationic diradical CBPQT²⁺, followed by copper-free click chemistry (Li et al. 2010a). (b) Tubular representation of the crystal superstructure of the tris-radical host–guest complex 61 formed between CBPQT^2+⋅ and MV^+⋅ (Fahrenbach et al. 2012).

Figure 14.

(a) A stepwise synthesis under kinetic control of a molecular Trefoil knot 64 by cyclization of a dinuclear double helical complex 63, with the solid-state structure of one of the topological enantiomers displayed (Dietrich-Buchecker & Sauvage 1989; Dietrich-Buchecker et al. 1990). (b) All-in-one synthesis under thermodynamic control of a molecular Borromeate 67 displayed as its solid-state structure (Chichak et al. 2004).

Figure 15.

A switchable donor–acceptor molecular shuttle 68 (Bissell et al. 1994). The CBPQT⁴⁺ ring prefers to be located on the benzidine (orange) station until its oxidation or protonation moves the ring to the biphenol (red) station. Deprotonation or reduction results in the return of the ring to the benzidine station.

Figure 16.

(a) A photoswitchable [2]rotaxane 69 that changes its physical properties upon switching, from fluorophilic to polarophilic. When 69 is (b) deposited on a surface and covered with droplets of CH₂I₂, photo-induced switching moves the droplet uphill, constituting an example of a nanoscale molecular machine carrying out work on the macroscale (Berná et al. 2005).

Figure 17.

(a) A switchable [2]rotaxane 70 that can be incorporated into (b) a 160000-bit molecular memory chip (Adapted from Nature Publishing Group). (c) The read/write cycle is achieved by oxidation of the green TTF station of 70, upon which the blue CBPQT⁴⁺ macrocycle moves to the red DNP station. Re-reduction of the TTF unit generates the metastable state co-conformation (MSCC), which represents the ‘1’ state, and subsequent relaxation to the ground state co-conformation (GSCC) regenerates the ‘0’ state (Green et al. 2007).

Figure 18.

A redox-active mechanized silica nanoparticle (MSNP) decorated with bistable rotaxanes. Target molecules are loaded (step 1) in the ‘open’ state of the rotaxane, and the valve closed (step 2) by switching the rotaxane to the ‘closed’ state. Switching back to the ‘open’ state (step 3) under specific stimulus results in release (step 4) of the target molecule (Nguyen et al. 2005).

Figure 19.

(a) A 2 nm MOF strut 71 containing electron-rich crown ethers can generate a cubic MOF capable of binding electron-poor molecules, such as methyl viologen, within its pores (from Li et al. 2009). Adapted from AAAS. (b) A related strut 72 wherein the crown ether is now catenated generates a dense two-dimensional network of catenanes in the solid state (Li et al. 2010c) (adapted from The Royal Society of Chemistry), whereas (c) a longer derivative 73 results in a three-dimensional array of MIMs (Li et al. 2010b).