Synthesis, characterization and magnetic properties of some transition metal pyrazolate complexes

• Main Author: Ehlert, Martin Kurt
• Format: Others
• Language: English
• Published: 2008
• Online Access: http://hdl.handle.net/2429/2965

A number of binary, divalent transition metal (substituted)pyrazolate compounds have been prepared of the general formula, [M(pz*)2]x (M = Cu (eleven compounds), Co (six compounds), Ni (one compound), Zn (one compound); pz* = pyrazolate and C-substituted pyrazolates). [Cu(4-Clpz)]x (4-Clpz = 4-chioropyrazolate) was prepared in green and brown forms. Three of the copper(ll) pyrazolates prepared, namely, the pyrazolate, 4-methylpyrazolate, and 4-chioropyrazolate (green) derivatives, have been characterized by single crystal X-ray diffraction which revealed them to consist of linear chains containing copper(II) ions doubly bridged by pyrazolate ligands. By analogy with the structures of these compounds and using physical evidence from all of the [M(pz*)2]x species, it has been concluded that all such compounds considered in this work possess the same polymeric linear chain motif. In addition to these compounds, a material with the formula .Cu(4-IpzHz) (4-Ipz = 4-iodopyrazolate; 4-IpzH = 4-iodopyrazole) was synthesized and characterized. The binary copper(II) and cobalt(ll) (substhuted)pyrazolates are paramagnetic and their magnetic susceptibilities were measured over the 2-300 K temperature range. All of these compounds exhibit antiferromagnetic exchange coupling (J, the exchange coupling constant, ranges from -58 to -105 c)1m. The [2]Cu(pz*) (pz*= pyrazolate, 4-methylpyrazolate, 4-chioropyrazolate (green), 4-bromopyrazolate) species are thermochromic; their colours change from green to blue upon cooling from 300 K to 77 K. Low temperature diffraction studies have shown the thermochromism to be concomitant with subtle structural changes in the compounds. These changes may be responsible for anomalies observed in the magnetic properties of the 4-Cl (green) and 4-Br derivatives. The analogous cobalt(ll) complexes exhibit anisotropic magnetic susceptibilities and quantitative analysis of their magnetic behaviour indicates that the compounds undergo Ising-like, 1-dimensional, antiferromagnetic exchange. The Ising-like behaviour is likely due to single-ion, zero-field splitting effects. In addition to polymeric, binary transition metal pyrazolates, several oligometallic, pyrazolate complexes were prepared and characterized. Most of these compounds were subjected to single crystal X-ray diffraction studies. Seven trimetallic copper(I) or copper(IIlI) species and a dimeric copper(ll) complex were prepared. Two isostructural zinc(ll) and cobalt(ll) dimers were prepared and studies have indicated that these compounds can undergo incipient polymerization reactions to form oligometallic chain compounds. Trimetallic and tetrametallic cobalt(II) complexes were also synthesized. Finally, two novel, octametallic molybdenum clusters were prepared and subjected to preliminary characterization. === Science, Faculty of === Chemistry, Department of === Graduate