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 CoII, NiII, CuII and ZnII 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 CoII, NiII and CuII 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 InIII centres with cis,cis,cis-N2O2Cl2 co-ordination environments, and is a rare example of hydroxide ligands bridging two InIII centres.