Self-assembly of palladium(II) and platinum(II) complexes of 2-hydroxypyrimidine to novel metallacalix[4]arenes. Receptor properties through multiple H-bonding interactions

Reaction of enM(H2O)2](NO3)2 (en = ethylenediamine, M = PdII, PtII) with 2-hydroxypyrimidine (Hpymo) in water results in self-assembly to cyclic complexes of type enM(pymo-N1,N3)]4(NO3)4 (1, M = PdII; 2, M = PtII) which are structurally analogous to calix4]arenes. The tetranuclear cations in compounds 1 and 2 adopt, in the solid state, a 1,3-alternate orientation of the pymo residues. Attempts to coordinate either soft (PdII, PtII), borderline (CuII, ZnII), or hard (LaIII, BeII, NaI) metal ions to the oxo surface of the metallacalix4]arene lower rim were fruitless. In fact, X-ray studies performed on enPt(pymo-N1,N3)]4(ClO4)4]2.Cu(H2O)6](ClO4)2.9H2O (4) showed that multiple H-bonding interactions between the water coordination sphere of the copper center and the oxo surface of two tetranuclear cations take place instead of a direct interaction between the heterometal and the oxo surface of the metallacalix4]arene. Encapsulation of Cu(H2O)6]2+ is also responsible for the stabilization of the pinched-cone conformation of the tetranuclear cations in 4. pH* dependent 1H NMR spectra of compounds 1 and 2 indicate a very low basicity of the O-donor group of the pymo residues, revealing no protonation of this position down to pH* 0. Moreover, recrystallization of 2 from strongly acidic aqueous solution (approximately 1 M HClO4) affords the adduct enPt(pymo-N1,N3)]4(ClO4)4]2.H20O8](ClO4)4 (6). Analogously to 4, the cationic H20O8]4+ species is sandwiched between two tetranuclear cations as a result of multiple H-bonding interactions with the oxo surface of two metallacalix4]arenes. Again a pinched-cone orientation of the pymo residues is realized.