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 = Pd-II, Pt-II) with 2-hydroxypyrimidine (Hpymo) in water results in self-assembly to cyclic complexes of type [enM(pymo-N-1,N-3)](4)(NO3)(4) (1, M = Pd-II; 2, M = Pt-II) which are structurally analogous to calix[dr]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 (Pd-II, Pt-II), borderline (Cu-II, Zn-II), or hard (La-III, Be-II, Na-I) metal ions to the oxo surface of the metallacalix[4]arene lower rim were fruitless. In fact, X-ray studies performed on {[enPt(pymo-N-1,N-3)](4)(ClO4)(4)}(2).[Cu(H2O)(6)](ClO4)(2). 9H(2)O (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 metallacalix[4]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 H-1 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 (similar to 1 M HClO4) affords the adduct {[enPt(pymo-N-1,N-3)](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 metallacalix[4]arenes. Again a pinched-cone orientation of the pymo residues is realized.