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

Article abstract of DOI:10.1002/chem.19970030807

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][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. Binding occurs through simultaneous first- and second-sphere coordination. This involves three separate interactions: first-sphere σ donation from an aromatic N atom to the Pd centre. second-sphere hydrogen bonds between the NH₂ group and polyether O atoms, and π stacking between the electron-poor aromatic rings of the substrate and the electron-rich aromatic spacing units of the receptor ¹HNMR spectra exhibit chemical shift changes indicative of the H-bonding and π-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.