SYNTHESIS AND COORDINATION CHEMISTRY OF THE COMPARTMENTAL TETRADENTATE LIGAND BIS[3-(2-PYRIDYL)PYRAZOL-1-YL]METHANE

Reaction of 3-(2-pyridyl)pyrazole with CH2Br2 and NaOH under phase-tra nsfer 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 . 4H(2)O revealed a complicated network of hydrogen bonding between stacks of ligands and interspersed chains of water molecules. Reaction of L with Co-II, Ni-I I, Cu-II and Zn-II gave in every case a centrosymmetric dinuclear comp lex of the type [M2L2(mu-X)(2)](2+) (X = hydroxide or monodentate acet ate). Three of the four complexes were crystallographically characteri sed and have the same basic core structure. The M-2(mu-X)(2) core is s panned by two bridging ligands L, each of which co-ordinates one biden tate arm to each metal. The two ligands are co-ordinated in an achiral 'face-to-face' mode rather than the alternative helical mode; this le aves space between the two metal ions for the monodentate ligands X, w hich 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 geometri es seen in the crystal structures. Magnetic susceptibility studies on [Cu2L2(mu-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 Cu-2(mu-OH) (2) core due to the additional pathway for magnetic exchange provided by the bridging ligands L. In contrast, in the mononuclear complex [Pb L2][ClO4](2) the ligands L are tetradentate chelates, giving the metal centre an eight-co-ordinate geometry which is very irregular due to t he additional presence of a stereochemically active lone pair. In [In2 LCl4(mu-OH)(2)], L reverts to its more usual bridging mode. This compl ex contains two pseudo-octahedral In-III centres with cis,cis,cis-N2O2 Cl2 co-ordination environments, and is a rare example of hydroxide lig ands bridging two In-III centres.