Abstract
To obtain information of the driving force of intramolecular aromatic ring stacking in ternary Cu(II) complexes, the effects of ring-substituted hydroxyl and nitro groups on stabilization due to stacking interactions have been investigated by potentiometric and spectroscopic methods for Cu(II)(DA)(L-AA) where DA =2, 2'-bipyridine, 1, 10-phenanthroline, 5-nitrophenanthroline, histamine, 2-(aminomethyl)pyridine, or ethylenediamine and AA=phenylalanine, tyrosine, p-nitrophenylalanine, tryptophan, 5-hydroxytryptophan 6-nitrotryptophan, alanine, or valine. Strong charge transfer bands at 350-450 nm in the difference spectra and circular dichroism (CD) spectral magnitude enhancements in the d-d, region were observed for the systems involving aromatic DA and AA and ascribed to the stacking interactions. The stability constants log βpqrs for Cup(DA)q(AA)r(H)s were determined at 25°C and I=0.1 mol.dm (KNO3). The stabilization due to intramolecular stacking was evaluated by calculating the constants K for the hypothetical equilibrium from the log βpqrs values obtained: Cu(DA)(L-ala)+Cu(en)(L-AA)_??_Cu(DA)(L-AA)+Cu(en)(L-ala)where ala=alanine and en=ethylenediamine. The nitro group in p-nitrophenylalanine contributes to the stacking with the imidazole ring of histamine, while that in 5-nitrophen, anthroline stabilizes with tryptophan. The CD magnitudes were found to be quantitatively related to the populations of the stacked species calculated from the log K values. The Cu(II)-aromatic ring interaction is not an important factor for the stabilization of the stac ked form. The stability enhancements due to the nitro group indicate that charge transfer due to the electron density difference plays an important role in specific stacking interactions. The structure-stability relationship was, discussed from possible charge, transfer modulated by ring substituents and the central metal ion.
