Main Group Hydrogen-Bond Donors and Pyridyl Ligands for Supramolecular and Coordination Chemistry
The thesis involves the investigation of two main areas in which new ligands based on main group frameworks are developed. In the first part of this thesis we explore the host-guest chemistry of a range of non-carbon frameworks based on P-N bonds. In the first chapter we develop strategies for enhancing the anion binding properties of phosphazanes of the type [(RNH)(E)P(µ-NtBu)]2 (E = O, S, Se) which are bench stable, H-bond receptors that can be regarded as inorganic analogues of squaramides (a key class of organic anion receptor). We find that there are distinct advantages of these inorganic receptors over organic counterparts such as the ease by which their functionality and electronic character can be altered (by means of the R-groups and chalcogenide present). Selenium substitution at the phosphine-centres, the presence of electron withdrawing R-groups and metal coordination to the soft donor centres can be used to modulate and enhance anion binding. The water stability and superior anion binding properties of the selenophosph(V)azanes gives them applications as synthetic anion transporters through phospholipid bilayers. Next, we establish synthetic methodologies for the rational design of bimetallic phosphazane coordination complexes of the type [LnM(PyridylNH)P(µ-NtBu)]2 (M = transition metal, L = supporting ligand). By optimisation of the NH bond polarity we achieve halide binding approximately one order of magnitude stronger than for organic analogues. We find that metal coordination renders these complexes robust enough to function as synthetic anion transporters through phospholipid bilayers and reveal how metal coordination affects the transport activity. We also investigate the general coordination chemistry of the parent ligand and the effect of quinolyl substitution. Finally, we investigate the host-guest chemistry of pentameric phosphazane macrocycle {[NHP(µ-NtBu)]2}5. The polar coordination site of this system promotes new modes of guest encapsulation via H-bonding with the π-systems of unsaturated bonds, and with reactive anionic phosphorus centres. Guests can be kinetically locked within the structure, with halide anions displaying stronger effective binding than in any classical organic or metal-organic receptor reported to date. The second part of the thesis moves on to explore the coordination properties of main group bridged trispyridyl ligands. The new 6-methyl substituted ligands E(6-Me-2-py)3 (E = As, Sb, Bi) are synthesised, allowing the assessment of the effects of bridgehead modification alone on descending a single group in the periodic table. We show that the primary influence on coordination behaviour is the increasing Lewis acidity and electropositivity of the bridgehead atom as group 15 is descended, which not only modulates the electron density on the pyridyl donor groups but also introduces the potential for anion selective coordination behaviour. We then continue to investigate the structural consequences of bridgehead change and derivatisation in a range of transition metal complexes and also demonstrate the effect of bridgehead oxidation going from E = As to As=O. Having revealed that the presence of a 6-Me group is crucial for the synthesis of trispyridyl main group ligands we explore the coordination chemistry of the silicon derivative E= PhSi, for which coordination studies were previously hindered by the inefficient synthesis of the ligand. In the final chapter we expand the scope of ligand modification of the trispyridyl aluminate E = EtAl by reaction with organic acids and aldehydes.
Main group chemistry, Coordination chemistry, Supramolecular Chemistry
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