Molecular loops

Shearman LP, Sriram S, Weaver DR et al 2000 Interacting molecular loops in the mammalian circadian clock. Science 288 1013-1019... 

As high doses these internal links would be expected to produce looped structures, with a marked effect on viscosity. No such effect is observed. To explain this, we must assume that the radicals, trapped in the crystalline regions, migrate to the surface where the molecular loops are found (corresponding to some extent to the old amorphous... 

Work by Paul Beer et al. in Oxford on dithiocarbamate-functionalised resorcarenes has resulted in resorcarene trimers termed molecular loops and molecular tetrahedra composed of four resorcarene units. The molecular loops (pyridine-capped Cd(II) and Zn(II) complexes) can bind C60 which fits neatly into the 16.4 A wide central cavity.35 The slightly larger tetrahedral tetramer which has a 19.4 A edge length, compared to 19.1 A for the loop, is bridged by square planar copper(III) ions, a very unusual oxidation state of copper that is obtained from iodine oxidation of an intermediate copper(II) complex. The solid-state structures of examples of the loop and tetrahedron are shown in Figure 10.35. 

Figure 10.35 X-ray molecular structures of a molecular loop and tetrahedron derived from the self-assembly of 10.39 with Zn(II) or Cu(III), respectively.35...

Scheme 4.7 A molecular loop lock that responds to chemical and redox stimuli.

Shiu K-B, Lee H-C, Lee G-H, Wang Y (2002) Synthesis, structures, and solvent-occlusion properties of a molecular loop and a molecular square using tetracarbonyl- and diphosphine-ligated diruthenium(I) as building blocks and dicarboxylates as linkers. Organometallics 21 4013 1016... 

Fox DO, Drew MGB, Beer PD (2000) Resorcarene-based nanoarchitectures metal-directed assembly of a molecular loop and tetrahedron. Angew Chem Int Ed 112 135... 

Metal-organic-framework (MOF) three-dimensional architectures composed of metal ions and organic ligands Molecular loop dinuclear molecules with two bridging ligands with an approximate geometry resembling that of a loop... 

An important issue in controlled radical polymerizations is radical-radical coupling. Due to the high local radical concentrations around the initiators, this may cause intra- or inter-molecular coupling of chain ends. When it happens intra-molecularly, loops of arms are formed. When it happens inter-molecularly, star-star coupling or even networks are generated. In order to obtain star polymers with defined architectures, measures should be taken to avoid these radical couplings in the polymerization process (see below). 

A very simple way of stoppering a bisammonium thread by using NaBPhi was proposed by Lawrence and Rao. [8a] They managed to obtain a threaded molecular loop - the per-2,6-dimethyl-j5-CD [2]rotaxane 7 - in an excellent 71 % yield by doing the self-assembly in aqueous solution (Fig. 2). Furthermore, the same methodology has been employed successfully [8b] for the construction of the [3]rotaxane 8 incorporating a tetraarylporphyrin unit. 

Analysis of molecular loops is based on the features that differentiate between curves in space. In its simplest form, curves can be classified into two classes according to their possible embedding in the plane. A molecular chain that can, after allowed deformations, be drawn ( embedded ) in two dimensions is essentially different from one that cannot. In this latter case, all possible two-dimensional projections of the curve will exhibit overcrossings and never be reduced to a planar structure. 

Figure 6 Examples of simplest knots in molecular loops. The trefoil knot exhibits chirality. The handedness is determined by the right-hand rule. All three overcrossings in the (T+) knot have handedness of +1, whereas this is —1 in the case of the (T ) knot.

In their work with metal-linked resorcarenes. Beer and co-workers generated a unique hex-anucelar zinc molecular loop. Two bis-dithiocarbamate chelated ions link adjacent resorcarenes, with a terminal pyridine completing the square-pyramidal coordination of each metal ion. 

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