STABILIZATION OF UNUSUAL SIMULTANEOUS BINDING OF 4 CYTOSINE NUCLEOBASES TO COPPER(II) BY A NOVEL NETWORK OF BIFURCATED HYDROGEN-BONDING

The crystal structure of tetrakis(cytosine)copper(II) perchlorate dihy drate has been determined. All the hydrogen atoms were obtained from F ourier-difference synthesis. The geometry around. copper is a bicapped octahedron (4 + 2 + 2). The adjacent cytosine rings are oriented hea d-to-tail with respect to each other and are roughly at right angles t o the co-ordination plane. The exocyclic oxo groups form an interligan d, intracomplex hydrogen-bonding network above and below the co-ordina tion plane with the exocyclic amino groups of alternate cytosine bases . The EPR and electronic spectra are consistent with the retention of the solid-state structure in solution. The steric effect of the C(2)=O group of cytosine is offset by the presence of the intracomplex hydro gen-bonding network. The trend in Ei values of Cu-II-Cu-I couples for 1.4 complexes of cytosine, cytodine, pyridine, 2-methylpyridine and N- methylimidazole suggests that both steric effects and pi-delocalizatio n in imidazole and pyridine ligands and the steric effect of C(2)=O in pyrimidine ligands are important in stabilising Cu-I relative to Cu-I I.