Neutral complexes, structural aspects

Structural aspects of the actinide carboxylate complexes have been reviewed235 from which the structures of many adducts that are likely to be involved in the extraction with carboxylic acid - neutral donor mixtures may be predicted. 

Le Cointe, M., Lemee-Cailleau, M. H., Cailleau, H. and Toudic, B. (1996). Structural aspects of the neutral-to-ionic transition in mixed stack charge-transfer complexes. 

Cram and co-workers studied thermodynamic and structural aspects of the interaction in depth. /-Butylamine was typically used as a standard guest, the association of which with a huge variety of crown ethers having different ring sizes, numbers and positions of oxygen atoms, and different substituents was measured in chloroform as a solvent. Picrate, thiocyanate, or hexafluorophosphate extraction was a common tool (for the later discussion of the role of the counterion in complexation see Ref. 47 as concerns pH dependence, it is determined typically by the ranges of existence of the ammonium ion in the protonated and the host in the neutral forms). Some illustrative examples of K ss for t-butylamine thiocyanate, extracted into chloroform at 24 C [9], are listed below ... 

Of course, unities are themselves labile at neutral pH, and therefore in principle one should be able to obtain an identical result by simply mixing salicylaldehyde, amines, and zinc in buffer in the presence of DNA. In a second report in 1999, we examined exactly this possibility [6]. Once again as expected, the pyrrolidine was the amine most retained by the DNA resin in the presence of salicylaldehyde and zinc. An inherent complication of this strategy, however, is that the labile library constituents made analysis and understanding of the results of library selection difficult. Furthermore, while useful as a proof of concept study, there is no clear pathway from the metal complexes identified in these studies to compounds suitable for use in structural studies or in vivo. As we will discuss later, consideration of such issues has been an important aspect of the design of more recent libraries targeting RNA recognition. 

---