"Directed" assembly of metallacalix[n]arenes with pyrimidine nucleobase ligands of low symmetry: interchanging metals in mixed-metal metallacalix[4]arenes and incorporating additional metals at the exocyclic groups

The pyrimidine (pym) nucleobase cytosine (H₂C) forms cyclic ring structures (“metallacalixn]arenes”) when treated with square‐planar cis‐a₂M^II entities (M=Pt, Pd; a=NH₃ or a₂=diamine). The number of possible linkage isomers for a given n and the number of possible rotamers can be substantially reduced if a “directed” approach is pursued. Hence, two cytosine ligands are bonded in a defined way to a kinetically robust platinum corner stone. In the accompanying paper (Part I: A. Khutia, P. J. Sanz Miguel, B. Lippert, Chem. Eur. J. 2010 , 17, DOI: 10.1002/chem.2010002722) we have demonstrated this principle by allowing cis‐Pta₂(H₂C‐N3)₂]²⁺ to react with (en)Pd^II to give cycles of (N1,N3 ? N3,N1?)_x (with x=2 or 3; ? represents Pt^II and ? represents Pd^II). In an extension of this work we have now prepared cis‐Pta₂(HC‐N1)₂] ( 1 ; HC=monoanion of cytosine) and treated it with (bpy)Pd^II (bpy=2,2′‐bipyridine) to give the Pt₂Pd₂ cycle cis‐{Pt(NH₃)₂(N1‐HC‐N3)₂Pd(bpy)}₂](NO₃)₄ ? 13H₂O ( 5 ) with the coordination sites of the metals inverted; hence, platinum is bonded to N1 and palladium is bonded to N3 sites. Again, not only the expected single linkage isomer is formed, but at the same time the solid‐state structure and ¹H NMR spectroscopy reveal the preferential occurrence of a single rotamer (1,3‐alternate). The addition of (bpy)Pd^II to 5 led to the formation of Pd₆Pt₂ complex 6 in which the exocyclic N4H₂ groups of the cytosine ligands have undergone deprotonation and chelate four more (bpy)Pd^II entities through the O2 and N4H sites. With a large excess of (bpy)Pd^II over 5 (4:1), cis‐(NH₃)₂Pt^II is eventually substituted by (bpy)Pd^II to give the Pd₈ complex 7 . In both 6 and 7 stacks of three (bpy)Pd^II entities occur. The linkage isomer of 5 , cis‐{Pt(NH₃)₂(N3‐HC‐N1)₂Pd(bpy)}₂](NO₃)₄ ? 9H₂O ( 8 ), has been structurally characterized and the two complexes compared. The acid/base properties of cis‐Pt(NH₃)₂(H₂C‐N1)₂] ( 1 ) have been determined and compared with those of the corresponding N3 isomer. The complexation of AgCl by 1 is reported.