Tris-Pyridyl Ligands and Beyond: Modifications Derived from the Bridgehead and Donor Vector

Abstract

Abstract

Tris-Pyridyl Ligands and Beyond: Modifications Derived from the Bridgehead and Donor Vector – Jessica Ellen Waters

The development of new classes of ligands is a fundamental and ongoing challenge in modern chemistry, and a cornerstone in modern catalysis, supramolecular assembly and biomimetics. The current world of coordination chemistry is dominated by ligands based on organic frameworks. However, there is a developing field of coordination and supramolecular chemistry using inorganic frameworks, and this thesis is focused on fundamental research in the synthesis and design of ligand molecules of this type.

In Chapter 1, a concise account of previous studies of tris 2-, 3- and 4-pyridyl based ligands E(py)3 (where E = a main group bridgehead atom or group, and py = a pyridyl donor arm) is presented. The main conclusion from this survey of the literature is that, despite recent developments in this area, there are some significantly unexplored fields. This is particularly the case for metal-based Group 13, 14 and 15 pyridyl ligands containing 4-pyridyl functionality, which may open new avenues of supramolecular coordination chemistry. In addition, the elaboration of this type of metal-bridgehead by the incorporation of other polycyclic aromatic N-donor groups has been almost completely ignored so far. The aims of the thesis (Chapter 2) are primarily to expand our understanding of the structural effects of moving the donor-N atoms in ligands of this type to different positions in the ring substituents – closer to or further away from the main group bridgehead atoms. Chapter 3 explores one of the primary aims of the program of research, focusing on tris- and bis-8-quinolyl ligand arrangements containing Group 13 (Al), Group 14 (Sn) and Group 15 (Sb) bridgehead atoms, in which the N-donor atoms are more remote from the bridgeheads than in pyridyl counterparts. A highlight in this area is the development of bis-8-quinolyl ligands and the potential interaction of the bridgehead alkyl substituents with coordinated metals – illustrated by the structure of [{Me2Al(2-Me-8-qy)2}Li(THF)] [(4a)Li(THF)]. In Chapter 4, the synthesis of the first examples of heavy Group 14 and 15 4-pyridyl ligands E(4-py)3 (E = PhSn, Sb, Bi) and their coordination chemistry are reported. It is shown that moving the N-donor atoms within the pyridyl ring to the 4-position results in the ability to build an extensive range of new metal organic frameworks (MOFs). An important conclusion from this work is that underlying periodic behaviour (s-p separation and the inert pair effect) has a major effect on the ligand coordination modes, providing the basis for modulation of ligand character (N,N,N- vs N,N,N,E-donor behaviour). Chapter 5 describes more complex research in which, instead of using a bridgehead atom, a bridgehead group is employed (in this case a cyclodiphosphazane ring unit), to separate pyridyl donor functionality and expand the ligand bite. General conclusions and suggestions for future work are gathered in Chapters 6 and 7. Overall, the research provides new types of main group metal-based ligands which have potential applications in catalysis and in the building of porous networks. As such, it develops new aspects of coordination and supramolecular chemistry which have significant promise for future studies.