Synthesis and co-ordination chemistry of the tetradentate chelating ligand 1,3-bis[3-(2-pyridyl)pyrazol-1-yl]propane: crystal structures of complexes with FeII, CuII, ZnII, AgI and PbII

Abstract

The new ligand 1,3-bis[3-(2-pyridyl)pyrazol-1-yl]propane (L) was prepared by reaction of 3-(2-pyridyl)pyrazole with 1,3-dibromopropane under phase-transfer conditions, and contains two bidentate chelating pyridyl/pyrazolyl fragments linked by a flexible trimethylene chain, which permits the angle between the chelating fragments to vary in its complexes. It acts as a tetradentate chelate to a single metal ion, forming a variety of mononuclear complexes which have been structurally characterised. The complex trans-[FeL(dmf)₂][ClO₄]₂ is a high-spin d⁶ complex in which L co-ordinates equatorially. In [CuL][BF₄]₂ the [CuL]²⁺ cations have a significant tetrahedral distortion arising from the inability of the ligand to be planar; chains of [CuL]²⁺ cations are bridged by [BF₄]^– anions, with each bridging anion forming semi-co-ordinate interactions with the axial sites of the two [CuL]²⁺ fragments on either side of it and also hydrogen bonds to the methylene protons of each of the two metal fragments. The complex [(ZnL)₃(CO₃)][ClO₄]₄ contains a rare example of the symmetric µ₃-bridging mode of carbonate, which has arisen from efficient fixation of atmospheric CO₂ by a solution containing L and zinc(II) acetate. Complexes containing the [AgL]⁺ cation, with either nitrate or perchlorate as the anions, form columnar stacks in the crystal because of weak intermolecular Ag· · ·Ag interactions; the basic geometry of the [AgL]⁺ cations is very similar to that of [CuL]²⁺, despite the different stereoelectronic preferences of Ag^I and Cu^II. The complex [PbL(NO₃)₂] is eight-co-ordinate with two bidentate nitrate ligands in addition to L; the lone pair of Pb^II is stereochemically active, and results in a clear distortion of one square face of the approximately square antiprismatic geometry.