H-NMR Study of Ternary Platinum(II) Complexes with N-(1- Naphtyl)methyl-1,2-ethanediamine or N-(9-Anthrylmethyl)-1,2- ethanediamine and Bipyridine or Phenanthroline and Restricted Single Bond Rotation Due to Aromatic–Aromatic Ring Interaction

sive study because of its role in (i) the packing of aromatic molecules in the crystalline state, (ii) base-stacking interactions of DNA, (iii) three-dimensional structures of proteins, (iv) molecular recognition of pharmaceuticals by biological receptors or enzymes, and (v) template-directed synthesis. Ternary complexes of platinum(II) or palladium(II) containing 2,2 -bipyridine (bpy) or 1,10-phenanthroline (phen) and an amino acid bearing an aromatic side chain such as tyrosine and tryptophan have been reported to show aromatic p–p interactions. The crystal structures of ternary complexes of platinum(II), bpy, and N-benzyl-1,2-ethanediamine (Been), N-(1-naphthylmethyl)-1,2-ethanediamine (Npen), or N-(9-anthrylmethyl)-1,2-ethanediamine (Aten) indicate that intramolecular aromatic p–p interactions occur in the cases of Npen and Aten complexes. The structure of Been complexes in an aqueous solution was examined by H-NMR spectroscopy and this cation has a structure in which the phenyl ring and bpy are partially stacked in aqueous solution although the phenyl and bpy rings participate in only intermolecular stacking in the crystalline state. In order to clarify the nature of the aromatic–aromatic interaction in various amino acids or proteins in solution as well as solid state, we used ternary platinum(II) complexes having two aromatic rings as a model compound. The present paper describes the results of some detailed H-NMR measurements of [Pt(bpy)(Npen)] and [Pt(bpy)(Aten)] along with [Pt(phen)(Npen)] and [Pt(phen)(Aten)] . The aromatic p–p interactions became stronger as the ring size increased from pheny to naphthyl and anthryl groups. The characteristics of the intermolecular interactions between them and the restricted rotation about Cipso–C–N–Pt (Cipso denotes the ipso carbon of the aromatic ring; see Fig. 1) for [Pt(bpy)(Aten)] are delineated in this paper.