“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?

摘要

The pyrimidine (pym) nucleobase cytosine (H₂C) forms cyclic ring structures (“metallacalix[n]arenes”) when treated with square-planar cis-a₂MII 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)PdII to give cycles of (N1,N3N3,N1▪)_x (with x=2 or 3; represents PtII and ▪ represents PdII). 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)PdII (bpy=2,2′-bipyridine) to give the Pt₂Pd₂ cycle cis-[{Pt(NH₃)₂(N1-HC-N3)₂Pd(bpy)}₂](NO₃)₄13H₂O (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)PdII 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)PdII entities through the O2 and N4H sites. With a large excess of (bpy)PdII over 5 (4:1), cis-(NH₃)₂PtII is eventually substituted by (bpy)PdII to give the Pd₈ complex 7. In both 6 and 7 stacks of three (bpy)PdII entities occur. The linkage isomer of 5, cis-[{Pt(NH₃)₂(N3-HC-N1)₂Pd(bpy)}₂](NO₃)₄9H₂O (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.