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

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 d(6) 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 mu(3)-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.