Squares, molecular

The template-directed preparation of cycloi is(paraquat-4,4 -biphenylene (a molecular square ) has been achieved the use of a macrocyclic hydroquinone-based polyether template incorporating an ester moiety in one polyether chain afforded a 1 1 mixture of two topologically stereoisomeric [3]catenanes <96CEJ877>. 

Macrocyclic receptors made up of two, four or six zinc porphyrins covalently connected have been used as hosts for di- and tetrapyridyl porphyrins, and the association constants are in the range 105-106 M-1, reflecting the cooperative multipoint interactions (84-86). These host-guest complexes have well-defined structures, like Lindsey s wheel and spoke architecture (70, Fig. 27a), and have been used to study energy and electron transfer between the chromophores. A similar host-guest complex (71, Fig. 27b) was reported by Slone and Hupp (87), but in this case the host was itself a supramolecular structure. Four 5,15-dipyridyl zinc porphyrins coordinated to four rhenium complexes form the walls of a macrocyclic molecular square. This host binds meso-tetrapyridyl and 5,15-dipyridyl porphyrins with association constants of 4 x 107 M-1 and 3 x 106 M-1 respectively. 

Host-guest complexes such as (67) have been prepared from molecular squares involving Lewis base receptor sites, such as cyclobis[(cw-(dppp)Pt(4-ethynylpyridyl)2)(cM-LM)]Ag2 6+(OTf)6, where M = Pdn or Ptn and L = dppp or 2PEt3, by reaction with pyridine, pyrazine, phenazine, or 4,4 -dipyridyl ketone.519... 

In the field of porous supramolecular metal complexes, both molecular and extended-solid materials have been extensively studied in recent years. A particularly well-studied class of compounds is the metal-containing molecular squares, that is, square-shaped porous tetrameric structures (30,108). These have been prepared by several approaches, the most common being the reaction of an organic bridging ligand with a metal complex that has available cis-coordination sites (109-113) (Fig. 13). However, the resulting metal centers are usually coordinatively saturated, which makes it difficult for guest molecules to interact directly with the metal atoms. 

Recently, several groups reported molecular squares prepared by an alternative approach, in which bifunctional metalloligands serve as the linkers thereby placing the metal ions into the walls of the container molecules (114—116). Hupp and coworkers, for example, have designed square-shaped macrocycles based on salen-type components (117). Thus, the molecular square 15 was prepared by the directed assembly of cis- [(PEt3)2Pt(OTf)2] and... 

Fig. 13. Schematic illustration of metal-organic molecular squares, assembled from linear organic linkers and 90° metal units (left), linear metal units and organic comers (middle), or linear metal units and 90° metal units (113). The latter two classes have several inner-cavity binding sites and thus fit the definition of metalated container molecules.

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... 

Other molecular squares (327) have been obtained from cfr-Pt(CsCC=CH)2 (PR3)2 and cw-PtX2(PR 3)2 (X = Cl, OTf) in reactions carried out under high dilution conditions. These reactions have been extended to condensations with... 

J8. Cyanide-Bridged Iron(II)-M(II) Molecular Squares LgFe--------C=N-------ML2.1... 

Figure 1. Structure of a cyanide-bridged Fe(II)-M(II) molecular square (L = bpy M = Fe,Co,Cu).

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. 

Figure 4.6 Synthesis of chiral molecular squares 6a-d based on enantiopure linear l,l -binaphthyl-derived bipyridyl bridging ligands and /ac-Re CO)3CI corners.

Figure 4.9 Directed-assembly of Pt-alkynyl chiral molecular squares.

Oenyuk, B., Whiteford, J.A. and Stang, P.J. (1996) Design and study of synthetic chiral nanoscopic assemblies. Preparation and characterization of optically active hybrid, iodonium-transition-metal and all-transition-metal macrocyclic molecular squares. J. Am. Chem. Soc., 118 (35), 8221-8230. 

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. 

Fujita, M., Sasaki, 0., Mitsuhashi, T., et al., On the structure of transition-metal-linked molecular squares. Chem. Commun. 1996, 1535-1536. 

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

Figure 10.22 Speciation maps for Pd(en)2+ / 4,4 -bipyridyl mixtures at (a) 0.01 mM and (b) 5 mM total Pd(en)2+ concentration. The three numbers represent the numbers of Pd(en)2+ , 4,4 -bipyridyl and H+ components in the assembly, respectively. A —1 for the number ofH+ indicates OH-. The molecular square 10.10 is species 440. (Reproduced by permission of The Royal Society of Chemistry).

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