Kinetically-Controlled Metallocycles

Acyclic oligomers can polymerize to increase the number ofbonds formed. However, the donor and acceptor sites at each end of the polymer will always remain uncoordinated. Thus, theidealscenarioof one bond perbuilding block cannot be achieved unless cyclization occurs. When cyclization is unfavorable, it becomes likely that the oligomers will polymerize until higher oligomers precipitate as kinetic products. 

The above discussion is based on the reversible reactions that produce the most thermodynamically stable products. In contrast, irreversible reactions produce kinetically-controlled products. In such cases, the self-correction mechanism is not operative. Well-defined building blocks with appropriate geometrical features are even more important for the synthesis of kinetically-controlled macrocycles. It is possible to obtain larger cycles because different sizes of linear species form in the reaction, which will cyclize to give a mixture ofdifferent sizes of cycles. Smaller cycles will still be favored over larger cycles in such kinetically-controlled reactions owing to kinetic factors. 

Assuming a fundamental reaction, shown in Eq. (4.1), to generate the cyclic species [LM] from ligand L and metal M, the reaction rate is r= 

Since most cyclization reactions are carried out in highly dilute conditions, [L] and [M] are usually much smaller than 1 M. The reaction rate thus is faster for smaller cycles than larger cycles, and, as a result, higher yields of smaller cycles are typically obtained. 

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Figure 4.3 Schematic for directed assembly of large kinetically-controlled metallocycles.

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