The replacement of chlorides by cytosines in copper(II) complexes containing guanidine derivatives

Two copper(II) chloride complexes of amidino-O-methylurea (L1), [Cu(L1)Cl2] (1), and (N-benzyl)-amidino-O-methylurea (L2), [Cu(L2)Cl2] (2), were prepared and characterized by elemental analysis, infrared, diffuse reflectance, electron spin resonance and electrospray ionization mass spectra. Their cytosine binding abilities has been studied and found that two cytosine molecules are able to coordinate with the copper centers by replacing the chloride ligands to yield the bifunctional binding adducts [Cu(L1)(cyt)2]Cl2 (1c) and [Cu(L2)(cyt)2]Cl2 (2c), respectively. The shift of the CO band of cytosine in both cytosine-bound products to higher energy suggested that the N(3)–cytosine atom coordinates to the copper center. The large blue shifts of the d–d absorbance maxima and the nine superhyperfine splitting from the CuN4 chromophore were also observed in their electronic and EPR spectra. Their thermal decompositions have also supported the interaction of cytosine with complexes 1 and 2. Density functional calculations have also been performed and revealed that square planar coordination geometry is more stable for both 1c and 2c. The binding energy of 1c is found to be ∼20% lower than that of 2c, indicative of the higher binding potential of 1c.