M. Palaniandavar et al., STABILIZATION OF UNUSUAL SIMULTANEOUS BINDING OF 4 CYTOSINE NUCLEOBASES TO COPPER(II) BY A NOVEL NETWORK OF BIFURCATED HYDROGEN-BONDING, Journal of the Chemical Society. Dalton transactions, (7), 1996, pp. 1333-1340
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.