Different ways of interaction between binary copper(II)-Schiff bases (Cu–N-salicylideneserinato) and pyrimidine derivatives

Five ternary complexes of Cu(II)–(N-salicylideneserinato) [Cu(sal-ser)] with different pyrimidine ligands have been prepared and the crystal structures determined: [Cu(sal-ser)(cytosine)] · 2H2O (1), [Cu(sal-ser)(1-methylcytosine)] · 1.25H2O (2), [Cu(sal-ser)(isocytosine)] · 1.25H2O (3), [Cu(sal-ser)(2,4-diamino-6-hydroxypyrimidine)] · 2H2O (4) and [Cu(sal-ser)(2-aminopyrimidine)(H2O)] (5). In all cases, the copper(II) ion presents a nearly square-planar basal coordination with the tridentate N-salicylideneserinato ligand and one donor atom of the pyrimidine base. Different ways of interaction between binary copper(II)–N-salicylideneserinato and cytosine or isocytosine derivatives have been found. Thus, cytosine derivatives interact via N(3) while the isocytosine ligand yields a N(1)-complex. Moreover, and surprisingly, the 2,4-diamino-6-hydroxypyrimidine (6-aminoisocytosine) ligand is able to form a ternary complex via the exocyclic oxygen O(4). The coordination geometry about the copper in the 2-aminopyrimidine complex is completed with an axial O(W) atom of a water molecule. Moreover, in complexes 3 and 4 additional weaker axial interactions of Cu(II) allows the formation of pairs of complex units. Thus, in complex 3, the copper atom of one unit interacts axially with the exocyclic amino group of an adjacent one. Similarly, in complex 4, the copper atom of one complex unit interacts with a second unit through the oxygen of the phenol group. A comparative study between these compounds has been made in order to understand the different coordination behaviour and particularly the influence of additional interactions, e.g., H-bonds [NH2(ligand)⋯O(binary complex)] which facilitate the stabilisation of the complexes versus steric effects [CO(ligand)⋯O(binary complex)] that disturb their formation.

Graphical abstractFive ternary complexes of Cu(II)–(N-salicylideneserinato) with different pyrimidine ligands have been prepared and the crystal structures determined. Different ways of interaction between binary copper(II)–N-salicylideneserinato and cytosine or isocytosine derivatives have found. Cytosine derivatives interact via N(3), isocytosine yields a N(1) complex and the 6-aminoisocytosine ligand is coordinated via the exocyclic oxygen O(4).Figure optionsDownload full-size imageDownload as PowerPoint slide