Organometallics, aggregate structures polar

The work reported here has shown that inclusion complexation of organic and organometallic chromophores by thiourea, TOT and cyclodextrins can induce second harmonic generation capability in the polar crystals which result, even when the original bulk materials are themselves incapable of SHG. Structural evidence has been presented to show tht the solid state inclusion structures are acentric, and a simple electronic picture t0 the polarization response of these materials within the two-state modeP ° has been discussed. In an earlier section we remarked that of the many complexes we have made, only one has NOT been acentric. This result was not anticipated. We postulate that it is a natural tendancy in such materials, rather that an exception. If we consider a dipolar molecule in isotropic solution, we can imagine that if it were to aggregate, it would do so in a head to tail fashion in order to minimize electrostatic repulsion. The situation is illustrated in Scheme 3. The arrangement that would result is centrosymmetric. 

It turns out that they generally form complex aggregates with two, four, six, or more molecules bonded together, often with solvent molecules, one reason why apparently polar compounds such as BuLi dissolve in hydrocarbons. In this book we shall not be concerned with these details and we shall represent organometallic compounds as simple monomeric structures. 

As stated at the outset, the organometallic ligand can transport alkali metal cations across phospholipid membranes. Since the molecular surface of the 1 1 complex is polar at one end and apolar at the other, it will be reasonable to assume that transfer of the cations through lipophilic media is more likely to occur in the form of polymeric cation-ligand aggregates than in the form of monomeric entities. The observed structures of the dinuclear Li and the trinuclear Na" " complexes may represent in fact two possible modes of such an aggregation during the process of transport. 

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