Isomerism with metallacalix[4]arenes of the nonsymmetrical pyrimidine nucleobase cytosine: how connectivity and rotamer state determine the topology of multinuclear derivatives.

Two cyclic octanuclear complexes, 2 and 3, of cation composition {Pd(bpy)}(8)C(4) (bpy = 2,2'-bipyridine) form side by side when Pd(bpy)(H(2)O)(2) and cytosine (H(2)C) are reacted in water. The two complexes are isomers, composed of central metallacalix[4]arene backbones to which four additional Pd(bpy) units are bonded pairwise to exocyclic groups of the C(2-) ligands. As a consequence of differences in the N1-N3 connectivity patterns of the two central Pd(4)C(4) rings and 1,3-alternate rotamer states of cytosinate in both compounds, the spatial arrangements of exocyclic groups are distinctly different, leading to two Pd(3) stacks and two Pd(1) entities in 2, yet to four Pd(2) stacks in 3.