Jt-tt interactions

Ligand 34 acts as a tridentate ligand toward Zn(II), with the anilino nitrogen uncoordinated, but as a tetradentate hgand with Cu(II) (50). [Zn(34)] + binds dT with log K = 6.9 and the crystal structure of the l-methylth5miine adduct again shows evidence of tt-jt interaction. 

Donor-acceptor (Pirkle) H bonding, tt-jt interaction. Ability to tr- or H-bond Normal (usually)... 

Fig. 9 Structure model proposed for the self-assembly process of individual nanoparticles to form microspheres through tt-Jt interactions (left). The sizes of individual nanoparticles and micro-spheres can be determined directly from TEM images (right). Figure reproduced from Figs. 2 and 3 from [52]...

The use of tt—jt interactions to form a supramolecular polymer is not Hmited to water, however. Many groups (including that of the present authors) have developed structures that self-assemble in other solvents partly or mainly as a consequence of 7T—7T interactions (Figure 34.16) [74-77]. 

Metal Coordination and tt-jt Interactions as Light-Harvesting Antenna... 

In anion-jt interactions with electron-deficient (neutral) aromatic tt-acceptors, the halide lies preferentially over the periphery of the aromatic ring (as illustrated in Fig. 13a) and this is apparently related to the shape of the acceptor LUMO (presented for comparison in Fig. 13b). 

How do these cyclase enzymes control the precise regiochem-istry and stereochemistry of these multistep cyclizations The active site of pentalenene synthase consists of a hydrophobic cleft, which is lined with aromatic and nonpolar residues. It is thought that the carbocation intermediates might be stabilized by the formation of tt-cation interactions, with aromatic residues such as phenylalanine, tyrosine, and tryptophan. In pentalenene synthase, replacement of Phe-76 or Phe-77 by Ala gave > 10-fold reduction in activity, which suggests that they may stabilize carbocationic intermediates through Jt-cation interactions. 

Apart from phosphine ligands, polypyridine ligands have been incorporated into platinum(II) alkynyl moieties to give luminescent complexes. The use of polypyridine ligands is interesting because their planar structure and the square-planar coordination geometry of the d platinum(II) center could modify the excited state of these luminescent complexes by tt-jt and/or Pt- -Pt interactions. The complex... 

The cooperativity of cation-it and n-n interactions has also been demonstrated taking a series of mono-valent cationic species such as, Li+, Na" ", K+, NH4+,PH4+, OH3+ and SH3+ where systematic quantum chemical studies were performed to estimate the effects of cation-n and n-n interaction on each other in cation-Tt-7T systems (Fig. 18.6b) [123]. In this study all possible orientations of onium ions to form cation-jt complexes have been explored and the most stable conformation has been taken further to evaluate its effect on k-k interaction. The results from this study indicate a notable increase of 2-5 kcal/mol in the n-n interaction energy in the presence of the cations. The cation-jt interaction energy is also enhanced in the presence of n-n interaction albeit to a smaller extent. The wide varieties of cations (which include both metal and inorganic ions) employed here underline the generality of the results. 

Beer and coworkers have used pyridinium groups in the synthesis of anion-templated interlocked molecular species such as rotaxane 80. They observed that the yield of pyridine-containing macrocycle 81 was significantly improved in the presence of 3,5-bis-hexylamide-substituted pyridininm derivative 82 Cl, as shown in Scheme 14. This was attributed to favorable jt-tt stacking interactions between the electron-rich hydroquinone gronps of the precursor and the electron-poor pyridinium of 82 Cl. This inspired the analogous synthesis of 80 as depicted below. 

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