Molecular receptors for adenine and guanine employing metal coordination, hydrogen-bonding and pi-stacking interactions

Thiacyclophane ligands 1 and 2, containing a meta-xylyldithiaether unit, an aromatic spacing unit and a polyether chain, were prepared in good yield in a three-step synthesis, The macrocyclic organopalladium complexes [Pd(L)(MeCN)][BF4] (3: L = 1, 4: L = 2) were prepared through palladation of the respective thiacyclophane ligand by reaction with [Pd(MeCN)(4)]I[BF4](2). These complexes act as metalloreceptors to aromatic amines such as p-aminopyridine (pap), m-aminopyridine (map) and the DNA nucleobases adenine and guanine. second-sphere coordination. This involves three separate interactions: first-sphere a donation from an aromatic N atom to the Pd centre, second-sphere hydrogen bonds between the NH2 group and polyether O atoms, and pi stacking between the electron-poor aromatic rings of the substrate and the electron-rich aromatic spacing units of the receptor, H-1 NMR spectra exhibit chemical shift changes indicative of the H-bonding and pi-stacking interactions in solution. X-ray structures for thiacyclophane 1, metalloreceptor [Pd(1)(MeCN)][BF4] (3), metalloreceptor/model substrate complexes [Pd(1)(pap)][BF4] (5) and [Pd(2)(pap)][BF4] (7), and metalloreceptor/nucleobase complexes [Pd(1)(adenine)][BF4] (13), [Pd(2)(adenine)][BF4] (14) and [Pd(1)(guanine-BF3)][BF4] (15b) show details of these interactions in the solid state.