Co-ordination chemistry of adenine (HAde): synthesis and characterization of [CuII(tren)(nucleobase)]X2[tren = tris-(2-aminoethyl)amine, X = Cl or NO3] complexes and crystal structure of [CuII(tren)(Ade)]Cl·2H2O

Abstract

The five-co-ordinate complexes [Cu^II(tren)(H₂O)]Cl₂1, [Cu^II(tren)(HAde)]Cl₂2, [Cu^II(tren)(HAde)]-[NO₃]₂3 and [Cu^II(tren)(Ade)]Cl·2H₂O 4[tren = tris(2-aminoethyl)amine and HAde = neutral adenine] have been synthesized and characterized. The geometry and structures of the complexes were studied by electronic and IR spectra and, in addition, the structure of complex 4 has been determined by X-ray crystallography. The physicochemical data for complexes 2 and 3 support the presence of neutral adenine co-ordinated to Cu^II, whereas in complex 4 the adenine molecule is bound in its monoanionic form, as confirmed by the X-ray analysis [monoclinic, space group P2₁/a, a= 15.001(2), b= 8.422(1), c= 15.039(2)Å, β= 105.90(6)°, Z= 4; R= 0.065 for 2596 unique diffraction data]. The co-ordination polyhedron around the Cu²⁺ ion is approximately trigonal bipyramidal, with the equatorial sites occupied by the three primary amino nitrogen atoms and the axial positions occupied by the tertiary amino nitrogen and the imidazole N⁹ nitrogen from the adenine monoanion. The Cu–N(9) distance is rather short at 1.965(9)Å. Such selective metal bonding in adenine is very probably promoted by the trigonal-bipyramidal geometry around Cu^II and by the relatively low steric hindrance of the Cu^II(tren) moiety.