Co-ordination engineering: when can one speak of an “understanding”? Case study of the multidentate ligand 2,2′-dimethyl-4,4′-bipyrimidine†

Abstract

The mode of co-ordination of the multidentate ligand 2,2′-dimethyl-4,4′-bipyrimidine (L) was found to depend on the metal ion, the crystallization conditions, the metal-to-ligand ratio, and the anion. With nickel a chelating co-ordination through the endo-dentate nitrogen donor set is observed in the molecular complex [NiCl₂(L)(H₂O)]·CH₃NO₂, derived from hot CH₃NO₂. With Cu(NO₃)₂ and CuI–CH₃CN one- and two-dimensional (1-D and 2-D) co-ordination polymers of formula _∞¹[Cu(NO₃)₂(µ-L)] and _∞²[Cu₂(µ₃-I)₂(µ-L)] are obtained, where the bipyrimidine ligand is solely bridging through the two exo-dentate nitrogen atoms. On the other hand, a synthesis from CuI and crystallization from hot dimethyl sulfoxide leads to a 1-D iodide-bridged co-ordination polymer _∞¹[Cu(µ-I)(L)] with a chelating ligand. With AgNO₃ two different types of co-ordination polymers were found, depending on the silver-to-ligand ratio. At a 1∶1 ratio in the presence of a co-ordinating anion a 2-D network, _∞²[Ag(µ-NO₃)₂(µ-L)], with only bridging bipyrimidine ligands is observed. At a metal excess, a 3-D framework, _∞³[Ag₃(µ₃-NO₃)₃(µ₃-L)₂], forms where L functions both as a chelating and as bridging ligand. A tetradentate co-ordination mode of L towards silver is also found with non-co-ordinating anions, such as BF₄^– and PF₆^–, and gives rise to the isostructural 2-D co-ordination polymers of formula _∞²[Ag₃(CH₃CN)₃(µ₃-L)₂]X₃ (X = BF₄ or PF₆).