Molecular Loops, Triangles, and Squares

In general, much less is known about the chemistry and spectroscopy of molecular loops, triangles, and squares. 

As was noted in the introduction, it was not the purpose of this chapter to be comprehensive but rather to introduce the reader to the rich structural and spectroscopic properties of these linked MM quadruply bonded species. Cotton, Murillo, and their coworkers have prepared many related compounds, dimers of dimers, molecular loops, triangles, and squares involving Rh and Ru " units, but their chemistries have been less developed [12,44-48]. Also of note has been the elegant syntheses and spectroscopies of Ren and his coworkers on Ru"" -linked units [49-52]. These too are fascinating because of electronic communication between redox active centers. They do not contain MM quadruple bonds and so have not been discussed either. 

An array of closed stmctures, either polygons or polyhedra, may be formed with [cw-Mo2(DAniF02(MeCN)4]. The simplest of these polygons, called loops (Fig. 3a), have two Mo2" units joined by two dicarboxylate linkers. Molecular triangles and squares (Fig. 3b and c) are formed by procedures similar to that used to form loops. It should be noted that for loops, triangles, and squares, the ratio of the Mo2" comer pieces to the dicarboxylate is the same, that is, 1 1. The formation of one polygon type versus another is determined principally by the shape of the linker, namely, the disposition of the carboxylate chelates relative to one another. Linear dicarboxylates favor the squares, while bent dicarboxylates tend to produce loops. Finally, closed polyhedra, or cages, may form when [cis-Mo2(DAniF)2(MeCN)4] is used... 

It is interesting to speculate upon how a molecular loop and triangle or a triangle and square manage to equilibrate and apparently so readily. One can perhaps consider a catalytic process involving some impurity, and Cotton suggested that water may play a key role in these interconversions. 

Fig. 1 Examples of a dimer of dimers (a) a molecular loop (b) a molecular triangle (c) tmd a molecular square (d). In (b) and (c) the p-anisole groups have been omitted. Adapted from [12]...

Fig. 11 Imposed strain rates plotted against critical molecular weight for chain fracture for PEO. The triangles are for experiments with aqueous PEO solutions performed using the pressure driven flow apparatus and the squares are from experiments carried out in the pump driven flow loop. The circles are for the viscous solvent (water/glycerol 50/50 = 6.4 cP) passed through the flow cell...

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