The synthesis and properties of selected transition metal-pyrazole complexes and their application towards homogeneous catalysis

  • Wynne Evans

    Student thesis: Doctoral Thesis


    The literature shows that M- 2 -pyrazolyl bridged complexes may find useful application in the field of homogeneous catalysis. Metal-containing, chelating ligands can theoretically be. generated by deprotonation of transition metal complexes containing n -pyrazole ligands. Reaction of such species with other metal complexes containing labile groups, should provide a route to the synthesis of homobimetallic or heterobimetallic complexes containing the bridging ligand.

    This study describes the preparation of Ni(II) and Ru(II) pyrazole and 3, 5-dimethyl pyrazole complexes by reaction of the pyrazoles with NiCl 2 (PPh3) 2 and RuCl 2 (PPh3)3 respectively. The reactions of the same pyrazoles with RuBr 2 (PPh3)3 and [RuI 2 (PPh3) 2 ] n/ have also been investigated. The preparations have yielded new Ru(II) pyrazole complexes and although the Ni(II) pyrazole products have been previously reported, the synthetic routes used are "clean" and novel. An explanation for the observed preferential coordination of pyrazole over phosphine in Ni(II) complexes is offered in terms of "Pearson's Principle". The reactivity of the resulting Ni(II) and Ru(II) pyrazole complexes towards bases, specifically; triethylamine, potassium hydroxide and 1, 8-bis (dimethylamino)napthalene, has also been explored. Whilst deprotonation of a Ni(II) pyrazole complex, trans-[NiCl 2 (pzH)4], has been found to result in the polymeric [Ni(pz) 2. H 20]^ deprotonation of the analogous 3, 5-dimethylpyrazole Ni(II) complex, has not been effected. An explanation is offered for this which involves the pKa ' s of the coordinated pyrazoles. Deprotonation of trans- [RuCl 2 (PPh3) 2 (pzH) 2 ]. %C5H}4, using triethylamine or potassium hydroxide has only been achieved in the presence of water and air. The product of this reaction has been proposed to contain both μ^2 -pyrazolyl and μ^2-oxo
    ligands. In the absence of air, reaction of [RuCl 2 (PPh3) 2 (pzH) 2 ]. hC^\i^
    with triethylamine in methanol, has been shown to yield trans-[RuH(CO)(PPh3 ) 2 (pzH)(MeOH)].

    In order to ascertain the ability of the Ru(II) pyrazole complexes to undergo reactions associated with the steps involved in a homogeneous catalytic cycle, the reactions of [RuCl 2 (PPh.3) 2 (pzH) 2 ]. kC^A^ with O 2, H 2/ Mel, CO and I 2 have been investigated and the products of those reactions characterized as far as possible.

    Attempts to generate heterobimetallic complexes containing palladium and the M-2 -pyrazolyl ligand have been made by reacting the complex [Pd(pz) 2 (dppe)] with NiCl 2 (PPh 3 ) 2 and RuCl 2 (PPh 3 ) 3 . Preliminary experiments indicate that the latter reaction may produce a Ru(II)- Pd(II) complex. The catalytic activity of this species towards the homogeneous hydrogenation of alk-1-enes has been investigated, using gas chromatography.

    The techniques used to. characterize complexes include; elemental analysis, infra-red and H, P nmr spectrescopy, uv/vis spectroscopy and measurement of magnetic moments. The account also includes descriptions of experimental techniques invoked during the handling of air sensitive materials.
    Date of AwardJun 1991
    Original languageEnglish
    Awarding Institution
    • Polytechnic of Wales

    Cite this