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

<p>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 (<strong>I</strong>) or a second ligating moiety (<strong>II</strong>). 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.</p> <p>Synthetic approaches towards initial target molecules employed sequential cycloaddition strategies, and newly developed cycloaddition reagents: 7,8-diazaphencyclone <strong>III</strong> (DAPC), and 9-diazo-4,5-diazafluorene <strong>IV</strong> (DADAF). Representative products from this work include mono-topic ligands with attached quenching functionality eg. <strong>V</strong>, and space-separated di-topic ligands eg <strong>VI</strong>.</p> <p>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.</p> <p>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. <strong>VII-X</strong>, was pivotal to this approach due to each coupling method utilising the reactivity of norbornene pi-bonds.</p> <p>3,6-Di-(2'-pyridyl)-s-tetrazine <strong>XI</strong> was used as a coupling reagent for norbornenes in a two-step reaction sequence exemplified by Scheme I. Tetrazine <strong>XI</strong> was reacted with ligand BLOCK <strong>VII</strong> under basic conditions to generate the dihydropyridazine <strong>XII</strong>, which was reacted with a further equivalent of <strong>VII</strong> under high pressure to yield homo-coupled species <strong>XIII</strong>. A number of homo- and hetero-linked species have been generated using this general procedure.</p> <p>1,3-Dipolar cycloadditions with norbornene BLOCKs were achieved by thermal activation of suitably functionalised BLOCK epoxides, eg <strong>XV</strong> (Scheme II) to afford products including quencher-functionalised mono-topic ligands (<strong>XVIIa</strong>), symmetrical and unsymmetrical space-separated di-topic ligands (<strong>XVIIb</strong>), and porphyrin-ligand structures (<strong>XVIIc</strong>).</p> <p>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 <strong>XX</strong>. 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).</p> <p>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.</p>