Molecular squares, self-assembly

Most helicates have linear axes, though a few helicates with circular axes are known - indeed the chiral (D4) molecular squares formed from Zn2+ and 2,5 -bis(2,2 -bipyridin- 6 -yl)pyrazine, 22, may be regarded as circular helicates (450). The formation of circular or linear forms seems to depend on balances between kinetic and thermodynamic control iron(II)-poly-2,2/-diimine systems with their substitutionally-inert metal centers provide useful systems for disentangling thermodynamic and kinetic contributions. The mechanism of formation of circular helicates of this type is believed to entail a kinetically favored triple helicate intermediate (484). Self-assembly of chiral-twisted iron(III)-porphyrin dimers into extended polynuclear species takes place through the intermediacy... 

Porphyrin-based self-assembled molecular squares 389 can form mesoporous thin films in which the edge of a square, thus the size of the cavity, can be adjusted by appropriate choice of substituents [8]. Fibers that form coil-coiled aggregates with distinct, tunable helicity are built from crown ethers bearing porphyrins 390 [9]. In addition to the porphyrin applications discussed in Sections 6.3.2.2 and 6.4, dendrimer metalloporphyrins 391 to be applied in catalysis [10] and the water-soluble dendritic iron porphyrin 319 modelling globular heme proteins [11] can be mentioned. 

Lee, S,J., Kim, J.S. and Lin, W. (2004) Chiral molecular squares based on angular bipyridines Self-assembly, characterization, and photophysical properties. Inorg. Chem., 43 (21), 6579-6588. 

Scheme 10.4 Self-assembly of molecular square 10.10 from Pd (en)2+ and 4,4 bipyridine in competition with protonation reactions.

Examples of ditopic and tetratopic macrocycles have been described by Stang and coworkers by combining the calixarene chemistry with the metal-directed self-assembly of molecular squares [61]. In this case the calixarene units coordinate to the Pt through their P(Ph)2 ligands at the lower rim and assume a diverging spatial orientation in the macrocycle. These calixarene metallamacrocycles are used in transport experiments to carry sulfonate salts from one aqueous phase into another through chloroform. 

Square structures are prepared from metal complexes containing the required 90° angle. Shown in Scheme 4, neutral-charged molecular square 37 is prepared by spontaneous self-assembly of complex 30 and 36 <1997JA2524>. Synthesis of precursor complex 30 is accomplished by coupling of Pt(ll) chloride 34 and ethynylpyridine anion 35. 

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