Mechanistic aspects of the template effect

The template centre plays the key role in matrix reactions. If the steric course of macrocyclisation, or other multistep reaction, is directed and facilitated by the ligsons spatial structures, and their activation by coordination to the metal ion or another centre, then the process is controlled by the kinetic coordination template effect. If the metal ion (or another centre) sequesters one of the components from an equilibrium mixture (starting ligsons and oilier competing molecular species) and, as a consequence, shifts the position of the equihbriiun towards the formation of the desired product in the form of its metal complex, then the thennodynamic (or equilibrium) coordination template effect is considered to be operative for the reaction. 

Two distinct classes of template effect have been distinguished since 1964 [39]. Both types of effect are, chemically, manifestations of molecular organisation by means of coordination to the template centre [14], This is their common feature. 

Reactions with a pronounced kinetic coordination template effect are found, in particular, in cases of preparation of macrocycUc compounds by ring-closure of chelate precursors. For example, when the complex [Ni(L4)] is electrophilically alkylated with l,2-bis(bromomethyl)benzene, the macrocyclic product [Ni(L5)Br2] is isolated ( q. 1.1). 

In the above examples of the kinetic coordination template effect, the metal ion serves to transfer information to interacting compoimds, so the structure of the reaction product is predetermined. In such cases the strict geometry of the metal ion eoordination sphere [2, 11] may be considered as an information source. 

Mesitylene-Mo(CO)3 reacts with allylphosphine in benzene to produce fac-(CO)3Mo(H2PCH2CH=CH2)3, which under the action of a free-radical initiator 2,2 -azobis(isobutyronitrile) in toluene is converted into the macrocyclic triphosphine complex/ac-(CO)3Mo(L8) [43] (Eq. 1.3). 

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