Design and synthesis of rigid space-separated phenanthrolines based on the MOLRAC concept

The work presented in this thesis describes the synthesis of a new class of rigid molecules which incorporate at least one polypyridyl ligand separated by a well defined distance and geometry from either a redox active site (I) or a second ligating moiety (II). This group of ligands has been prepared as part of a study of energy and electron transfer in structurally organised polypyridyl complexes of ruthenium and osmium.

Synthetic approaches towards initial target molecules employed sequential cycloaddition strategies, and newly developed cycloaddition reagents: 7,8-diazaphencyclone III (DAPC), and 9-diazo-4,5-diazafluorene IV (DADAF). Representative products from this work include mono-topic ligands with attached quenching functionality eg. V, and space-separated di-topic ligands eg VI.

The wide range of products generated by this approach show variations which include: relative orientations between ligating and/or attached effector groups which may be of varying redox character, and the number and configuration of bonds separating the groups.

The second phase of the project employed convergent cycloaddition strategies which allowed the preparation of rigid spacer systems in short, direct assembly protocols. The synthesis of appropriate ligand BLOCKs^t, eg. VII-X, was pivotal to this approach due to each coupling method utilising the reactivity of norbornene pi-bonds.

3,6-Di-(2'-pyridyl)-s-tetrazine XI was used as a coupling reagent for norbornenes in a two-step reaction sequence exemplified by Scheme I. Tetrazine XI was reacted with ligand BLOCK VII under basic conditions to generate the dihydropyridazine XII, which was reacted with a further equivalent of VII under high pressure to yield homo-coupled species XIII. A number of homo- and hetero-linked species have been generated using this general procedure.

1,3-Dipolar cycloadditions with norbornene BLOCKs were achieved by thermal activation of suitably functionalised BLOCK epoxides, eg XV (Scheme II) to afford products including quencher-functionalised mono-topic ligands (XVIIa), symmetrical and unsymmetrical space-separated di-topic ligands (XVIIb), and porphyrin-ligand structures (XVIIc).

The convergent approach has also been successfully employed in the coupling of metal complex sub-units to give homo- and hetero-metallic dinuclear complexes eg XX. This technique allows a high degree of control in the preparation of large structurally complex inorganic species and represents a significant advance in this topical field (Scheme III).

Electrochemical and photophysical testing has been performed on a selection of ruthenium polypyridyl complexes prepared or derived from this study. Quenching of Ru^ll(bpy) based MLCT excited states in some quinone functionalised species was observed due to intramolecular electron transfer which appears to proceed via a through-space mechanism. Efficient quenching of Ru^ll(bpy) MLCT excited states was also observed in large Ru^II-Os^ll dinuclear complexes as a result of intramolecular energy transfer.