Synthesis and Reactivity of Transition Metal Complexes of Polypyridyl Ligands

The work presented in this thesis was conducted with the aim of developing new polypyridyl based ligands, establishing coordination chemistry of their metal (mainly d6 and d8) complexes and studying the reactivities of these complexes for a range of potential applications. Chapter 1 introduces the field of coordination chemistry and metal complexes. A brief overview on some key concepts in structural constitution, geometry, stability and reaction mechanism of metal complexes is provided. In addition, the advantages of employing pyridyl based ligands in the development of functionalised metal complexes are discussed. Recent literature examples of metal complexes of multidentate polypyridyl based ligands and their applications are given. Roles of polypyridyl based ligands in these complexes are also discussed. Chapter 2 presents the study of the reactivities of [Pt(diimine)2]2+ (diimine = 2,2′-bipyridine (bpy) and 4,4′-dimethyl-2,2′-bipyridine (4,4′-Me2bpy)) towards a range of monodentate ligands (pyridine and phosphine). Reaction intermediates and products of these reactions were characterised by spectroscopic techniques and, in some cases, X-ray crystallography. NMR studies showed that reaction of [Pt(bpy)2]2+ and one equivalent of pyridine or substituted pyridines gave multiple products (free bpy ligands and [Pt(bpy)(pyridine)2]2+) whereas analogous reactions with phosphine ligands gave solely the long-lived five-coordinate intermediates [Pt(diimine)2(phosphine)]n+. Reaction of [Pt(diimine)2]2+ with an excess amount of phosphine ligands gave the four-coordinate products [Pt(diimine)(phosphine)2]2+. Isolation and characterisation by X-ray crystallography of one of these intermediates provide definitive structure evidence that reaction of [Pt(diimine)2]2+ and phosphine ligands proceeds via the associative ligand substitution mechanism. Chapter 3 examines the synthesis and properties of inverse and regular 2-pyridyl-1,2,3-triazole complexes of Pd(II), Pt(II), Re(I) and Ru(II). A comparison of the structures, stability, photochemical, electrochemical and photophysical properties of the d6 and d8 metal complexes shows that despite the inverse and regular triazole complexes are structurally very similar, their chemical and physical properties are quite different. The stability of these complexes was examined by ligand exchange studies, solution stability studies and DFT calculations. These studies showed that inverse triazole ligands formed less stable complexes than the isomeric regular triazole ligands. In addition, inverse [Ru(bpy)2(triazole)]n+ complexes are photochemically active upon UV light irradiation, whereas regular [Ru(bpy)2(triazole)]n+ complexes are photochemically inert under identical conditions. Electronic properties of inverse triazole complexes have been examined using cyclic voltammetry, electronic absorption and emission spectroscopies and DFT calculations and were shown to be quite different from the regular triazole complexes. Chapter 4 describes a new purification procedure for the widely used pentadentate polypyridyl ligand N4Py and the synthesis of a family of [CoIII(N4Py)(X)]n+ complexes. Cyclic voltammetry and electronic absorption spectroscopic studies showed that electronic properties of [CoIII(N4Py)(X)]n+ complexes could be tuned by varying the nature of monodentate ligand X. Photocatalytic hydrogen production studies in aqueous solutions showed that all six tested [CoIII(N4Py)(X)]n+ complexes are catalysts for hydrogen production from water and displayed relative low catalytic activities, when compared to cobalt polypyridyl complexes reported in the literature. Results from a preliminary mechanistic study are also discussed. Chapter 5 describes the synthesis of a new quinolinyl based ligand 2PyN2Q, derived from N4Py, and the synthesis of octahedral zinc and copper complexes of 2PyN2Q and N4Py. The coordination chemistry of 2PyN2Q is compared to that of N4Py. Consequences of replacing two pyridyl groups in N4Py with two sterically bulky and more electronic withdrawing quinolinyl groups for the structures and electronic properties of metal 2PyN2Q complexes were examined by X-ray crystallography, cyclic voltammetry and electronic absorption spectroscopy