Synthesis and coordination chemistry of doubly-tridentate tripodal pyridazine and pyrimidine-derived ligands: structural interplay between M2L and M2L2 (M=Ni and Pd) complexes and magnetic properties of iron(II) complexes

Abstract

The coordination chemistry of three bridging doubly-tridentate ligands, including the known compound 3,6-bis(di-2-pyridylmethyl)pyridazine (1), which is structurally similar to 1,4-bis(di-2-pyridylmethyl)phthalazine (2), and two pyrimidine-linked compounds 4,6-bis(di-2-pyridylmethyl)pyrimidine (3), and 4,6-bis(di-2-pyridylamino)pyrimidine (4), was investigated with FeII, NiII, and PdII metal salts. Ligands 3 and 4 were synthesized in one-pot reactions from easily obtained starting materials; compound 3 was synthesized from di-2-pyridylmethane and 4,6-diiodopyrimidine in 48% yield, while ligand 4 was prepared by reacting di-2-pyridylamine with 4,6-dichloropyrimidine in 27% yield. During the synthesis of 4, an additional compound, 4-chloro-6-(di-2-pyridylamino)pyrimidine (5), with only one tridentate binding site was obtained in 30% yield. Reactions of 1, 3, and 4 with Fe II or NiII salts gave two types of complexes, either discrete M2L or M2L2 assemblies. The Pd II complexes obtained were also characterized as discrete M 2L complexes. The compounds were characterized by a combination of NMR and IR spectroscopy, microanalysis and X-ray crystallography. Noticeable differences in the structures obtained for NiII coordination complexes with the carbon-linked (3) and nitrogen-linked (4) ligands were observed, whereby the nitrogen linker adopted a trigonal planar geometry and prevented tridentate facial coordination of the octahedral metal centres. The magnetic properties of dinuclear FeII complexes of 1 were examined to see if they showed spin-crossover effects, a feature recently observed by others in other dinuclear helicate complexes, but the complexes remained high-spin at all temperatures between 300 and 2 K. © CSIRO 2009.