From Mono- to Polynuclear Coordination Complexes with a 2,2'-Bipyrimidine-4,4'-dicarboxylate Ligand.

The coordination properties of the ligand 2,2'-bipyrimidine-4,4'-dicarboxylic acid (H2bpd) with lanthanide(III) ions (Ln = Eu, Tb, or Lu) were investigated. The syntheses of the H2bpd ligand and its salts, [K2(bpd)(H2O)2] (1) and [(AlkNH)Lu(bpd)2] (Alk = Et, Hex, or en), are described. In the presence of LnCl3 salts (Ln = Lu, Eu, or Tb), the formation of [Ln(bpd)2]- and [Ln(bpd)(H2O)x]+ species was assessed by 1H nuclear magnetic resonance (NMR), spectrophotometry, and spectrofluorometric titrations in aqueous solution. The solid state structure of 1, [K(H2O)2][Lu(bpd)2] (2), and [(Et3NH)Lu(bpd)2] (3) could be determined by X-ray diffraction, showing the ligand to act as a tetradentate unit with formation of three five-membered chelate rings around the central Ln(III). With the aim of building polynuclear assemblies, the coordination between [Lu(bdp)2]- and [Lu(tta)3(H2O)] units (tta = thenoyltrifluoroacetylacetonate) was also investigated. In methanol, 1H NMR titration experiments revealed the formation of complex mixtures from which two new species could be identified, [Lu2(bpd)(tta)4] (4) and H[Lu(bpd)(tta)2] (5), as confirmed by their solid state structure analysis. Using highly lipophilic cations in chloroform, the octametallic complex [enH]4[Lu8(bpd)4(tta)18] (6) could be isolated and its X-ray structure determined.