Synthesis and co-ordination chemistry of the compartmental tetradentate ligand bis[3-(2-pyridyl)pyrazol-1-yl]methane

Abstract

Reaction of 3-(2-pyridyl)pyrazole with CH₂Br₂ and NaOH under phase-transfer conditions afforded the new ligand bis[3-(2-pyridyl)pyrazol-1-yl]methane (L), containing two bidentate pyrazolyl-pyridine arms linked to a methylene spacer. The crystal structure of L·4H₂O revealed a complicated network of hydrogen bonding between stacks of ligands and interspersed chains of water molecules. Reaction of L with Co^II, Ni^II, Cu^II and Zn^II gave in every case a centrosymmetric dinuclear complex of the type [M₂L₂(µ-X)₂]²⁺ (X = hydroxide or monodentate acetate). Three of the four complexes were crystallographically characterised and have the same basic core structure. The M₂(µ-X)₂ core is spanned by two bridging ligands L, each of which co-ordinates one bidentate arm to each metal. The two ligands are co-ordinated in an achiral ‘face-to-face’ mode rather than the alternative helical mode; this leaves space between the two metal ions for the monodentate ligands X, which would not be possible with a helical arrangement of ligands. The electronic spectral properties of the Co^II, Ni^II and Cu^II complexes are consistent with the pseudo-octahedral N₄O₂ co-ordination geometries seen in the crystal structures. Magnetic susceptibility studies on [Cu₂L₂(µ-OH)₂][PF₆]₂ show an antiferromagnetic interaction with a singlet–triplet splitting 2J of –110 cm^–1, very different from that expected on the basis of the structural parameters of the Cu₂(µ-OH)₂ core due to the additional pathway for magnetic exchange provided by the bridging ligands L. In contrast, in the mononuclear complex [PbL₂][ClO₄]₂ the ligands L are tetradentate chelates, giving the metal centre an eight-co-ordinate geometry which is very irregular due to the additional presence of a stereochemically active lone pair. In [In₂LCl₄(µ-OH)₂], L reverts to its more usual bridging mode. This complex contains two pseudo-octahedral In^III centres with cis,cis,cis-N₂O₂Cl₂ co-ordination environments, and is a rare example of hydroxide ligands bridging two In^III centres.