First row late d-block metal pincer complexes: exploring their coordination chemistry and catalysis

In this thesis, the synthesis and characterisation of a series of sterically and electronically distinct N,Npy,O and N,NPy,N pincer complexes based on the late 3d metals, Zn, Co, Ni and Fe, are described; some reaction chemistry including catalysis is also disclosed. Chapter 1 presents an overview of the development and application of first row transition metal pincer complexes and their role in modern homogeneous catalysis. In Chapter 2, the synthesis of a series of novel unsymmetrical N,Npy,O and N,Npy,N (pro)ligands and their reactions with zinc(II) chloride and zinc(II) tetrafluoroborate are documented. The resulting complexes are all diamagnetic and adopt both mono- and bimetallic structures based on neutral species or cation-anion pairs; in one case a bis(pincer ligand) complex has been isolated. The amenability of selected BF4-containing zinc complexes to undergo BF4-coordination or B-F bond cleavage/hydrolysis has been demonstrated. Chapters 3 and 4 report the preparation of cobalt(II) and nickel(II) chloride/tetrafluoroborate complexes of the same (pro)ligands. A wide variety of structural types are exhibited including those based on square planar or square pyramidal geometries with steric and electronic effects proving influential. Where appropriate the paramagnetic behaviour of selected Co(II) and Ni(II) complexes has been the subject of a 1H NMR spectroscopic investigation. As with the zinc work, evidence for B-F bond hydrolysis has been noted with the N,N,N-family of pincer complexes. As an underlying theme across these three synthetic chapters, chloride exchange reactions by using CsF, AgF AgOAc and AgBF4 have been explored. Chapter 5 discusses the synthesis of one N,N,O-iron(II) chloride along with two iron(II) tetrafluoroborate complexes bearing symmetrical and unsymmetrical N,N,N-ligands. Investigations reveal that the N,N,N-complexes are suitable for use as ethylene polymerisation precatalysts when treated with either MAO or MMAO as co-catalyst. Indeed, the catalytic activity of the symmetrical N,N,N-iron(II) tetrafluoroborate catalyst has proved exceptionally high affording highly linear high molecular weight polyethylene. All experimental details for Chapters 2 – 5, including the single crystal X-ray diffraction parameters, are provided in Chapter 6.