Rationalizing the formation and versatility of multinuclear metal complexes of bis(1-methyluracil-5-yl)methane as hybrids between classical calix[n]arenes and metallacalixaromatics

Metallacalix[n]arenes are a distinct class of metallacyclic compounds consisting of heteroaromatic rings L and square-planar cis-a2MII entities (M = Pt or Pd; a = NH3 or amine; or a2 = chelating diamine) instead of methylene bridges as in the classic calix[n]arenes. Here a series of hybrid compounds is described, which simultaneously have bridging –CH2– as well as cis-a2MII units, and uracil containing ligands L. Specifically, L = bis(1-methyluracil-5-yl)methane (1) and in one case a derivative of it, bis(1-methyluracil-5-yl)methylbenzene (2), have been reacted with cis-[a2M(H2O)2]2+ (with a = NH3 or a2 = 2,2′-bipyridine or bis(pyrazloyl-1-yl)propane) in water and products were isolated. Altogether X-ray crystal structures of eight metallacycles (complexes 3–6, 8–11) of M2L2, M4L2, and M6L6 stoichiometries have been determined as well as a second modification of 1. In all closed metallacycles the 1-methyluracil entities are deprotonated with metals coordinating via N3 positions, and without exception the uracil rings adopt 1,3-alternate conformations. A special feature of ligand 1 in its twofold deprotonated form is its propensity to bind additional metal ions through its exocyclic oxygen functionalities. While O4 sites appear to be favored as secondary metal binding sites, linkage isomerism and involvement of O2 is likewise possible (compounds 4, 9, 10). On the basis of the X-ray crystal structures, a reaction scheme is proposed which accounts for the different stoichiometries observed.