X-ray crystal structure of tetrakis(1-methylcytosine)copper(II) perchlorate dihydrate: effect of 1-methyl substitution on cytosine on the spectral and redox behaviour

Abstract

Copper(II) tetrakis-complexes of cytosine (cyt), 1-methylcytosine (1-mcyt) and cytidine (cyd) have been isolated and their spectral and electrochemical properties investigated. The X-ray crystal structure of tetrakis(1-mcyt)copper(II) perchlorate dihydrate has been successfully determined. The co-ordination geometry around copper in the complex corresponds to square-based 4+4' co-ordination. In addition to the preferential Cu---N3 bonds, there is significant interaction between copper(II) and the exocyclic O2 of 1-mcyt rings. The mutually cis 1-mcyt rings are present in a head-tail-head-tail arrangement, which is stabilised by a network of bifurcated hydrogen-bonding between the exocyclic amine hydrogen atoms and the oxygen atoms of the adjacent carbonyl groups. The aqueous solution spectra of the complexes are slightly different from solid state spectra revealing that the solid state structures undergo slight changes on dissolution in water. The electronic and EPR spectral and electrochemical results are consistent with the retention of the solid state structure even in solution. The EPR spectra exhibit N-superhyperfine lines corresponding to the coordination of four N3 atoms of cytosines. The higher g|| values indicate decreased covalency in the metal---ligand bond and the range of g||/A|| quotient (119-121 cm) confirms the presence of CuN₄ square-planar co-ordination geometry even in solution. The plot of i_pc and E_1/2 values versus 1-mcyt concentration for the electrochemical titration of Cu(ClO₄)₂ with 1-mcyt reveals an inflection point indicating the formation of 1:4 species in solution. The trend in E_1/2 values of the complexes shows that the incorporation of electron releasing methyl group/ribose moiety at N1 position of cytosine ring enhances the stabilisation of Cu(II), in spite of the steric demand from O=C2.