Mechanical Bond Formation by Radical Templation

Abstract

One of the more promising entries into molecular nanotechnology has followed in the wake of the advent of the mechanical bond [1] in chemistry. Mechanically interlocked molecules [2] (MIMs), such as bistable [2]catenanes [3] and [2]rotaxanes, [4] have been integrated into nanoelectromechanical systems [5] (NEMs) in the context of, for example, molecular electronic devices [6] (MEDs) with a future generation of computers in mind, and mechanized silica nanoparticles [7] (MSNPs) with finely tuned and precisely targeted drug delivery capsules in prospect. The initial forays into the making of the mechanical bond relied on either statistical events [8] intervening during the course of classical chemical reactions or on the meticulously scripted and synthetically demanding use of covalent templation [9] in the midst of multistep reaction sequences. These approaches to the synthesis of MIMs are often limited in their practical utility on account of low yields of products and the tedious nature of the synthetic protocols, not to mention the close to total absence of any discrete and potentially useful intramolecular intercomponent interactions existing between their mechanically interlocked components.The emergence of noncovalent templates, which rely upon metal-ligand coordination, [10] donor-acceptor interactions, involving both charged [11] and neutral [12] recognition sites, hydrophobic forces [13] expressed in aqueous solutions, hydrogen bonding also operative in neutral [14] as well as charged [15] settings, electrostatic forces, [16] and anion binding [17] to provide [18] the supramolecular assistance [19] to covalent and hence mechanical bond formation, have transformed the chemical landscape and brought mechanostereochemistry [20] to the forefront as a means of introducing integrated nanosystems based on "smart" molecules into nano-and biotechnology. These templation processes, which make use of noncovalent bonding interactions, however, have not relied previously upon the remarkable stabilization which can ensue from the radical-pairing interactions.In the domain of donor-acceptor MIMs, viologen-(1,1'-dialkyl-4,4'-bipyridinium) dications [21] V 2+ -and cyclobis-(paraquat-p-phenylene) (CBPQT 4+ ) [22] have been investigated [23] extensively as examples of p-electron-deficient guests and hosts, respectively. Recently, we have demonstrated [24] that viologen radical cations (VC + ) form strong inclusion complexes with the tetracationic cyclophane in its reduced diradical dicationic state (CBPQT 2( C +) ) as a consequence of radical-pairing interactions. In view of the remarkable stabilization [25] associated with this supramolecular entity containing three [p···p]-stacked bipyridinium radical cations (BIPYC + ), [26] we set out to assemble a [2]rotaxane incorporating a viologen derivative and the tetracationic cyclophane in its dumbbell and ring components, respectively, by means of a radical template-directed protocol. Assuming this objective can be met, we can contemplate a [2]rotaxane existing in a rela...

Symbiotic Control in Mechanical Bond Formation

“…Despite having alternative ways [10] to incorporate BIPY 2+ units into MIMs,t he advantages of radical templation [8] are such that it allows the construction of catenanes and rotaxanes in the absence of electron-rich complementary units (Scheme 1), so that the resulting MIMs can have much simpler constitutions. [8b] More importantly,w hile the coconformations of the catenanes and rotaxanes,u nder reducing conditions,a re controlled by stabilizing radical-pairing interactions,C oulombic repulsions play an important role in dictating the high-energy states adopted by the fully oxidized MIMs.T his structure-dependent redox behavior allows us to access and explore the properties of systems far from equilibrium.…”

“…[14] This particular copper-free procedure,h owever, has several drawbacks,i ncluding slow reactions that can take up to several weeks to go to completion, resulting in low yields (commonly 30 %), especially when reaction time periods are limited. Moreover, because the alkynes are internal ones in this particular approach, it follows that the constitutional options available to the alkyne components are limited, meaning that it becomes difficult to functionalize the resulting rotaxanes and it is nigh impossible to synthesize catenanes.C onsequently,n ew methodologies for the efficient production of MIMs,employing radical templation, [8] are sorely needed.…”

Radical-radical rotaxanes with tetracationic cyclophane ring, and quaternary azaaromatic rotaxanes with cage macrocycles

“…2,11,36,38 For elucidation of this process, the use of the 1,1-dialkyl-4,4'-bipyridinium radical cationic BIPY …”

“…The interactions of radical dimers have been used as a recognition motif to create a mechanically interlocked system. This combination of templation with radical-pairing is promising for preparation of mechanically interlocked molecules (MIMs) composed of species which have little or no binding affinities in their ground states 38 (Scheme 1). It should be mentioned that the structures of radical cations given in Scheme 1 and the following are a short way of representing these species; they do not show their valence-bond forms.…”

Mechanostereochemistry and the mechanical bond

“…Upon oxidation of the three bipyridinium units of the host CBPQT 4+ and the guest 58 to their radical cationic states, radical-radical interactions overcome the Coulombic repulsion between the now dicationic host and monocationic guest and the inclusion complex 59 is formed. Combining this source of radical templation with light-induced oxidation and copper-free 1,3-dipolar azide-alkyne cycloaddition, [2]rotaxanes such as 60 (figure 13a) can be synthesized (Li et al 2010a), which show, upon aerobic oxidation to their non-radical ground states, little or no recognition between the thread and ring components. The recently solved solid-state structure (figure 13b) of the tris-radical complex 61 formed between CBPQT 2+· and MV +· reveals significant radical-radical interactions (Fahrenbach et al 2012).…”