Synthesis and characterisation of polynuclear complexes with macrocyclic and related ligands
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis is in two distinct but related parts. The first deals mainly with manganese complexes of multinucleating Schiff-base macrocycles derived by condensation of 2,6-diacetylpyridine (DAP) and 1,3-diamino-2-hydroxypropane (H₂L1, H₄L1'), or 1,3-diaminopropane (L13). Six tetramanganese(II) complexes of H₂L1 and H₄L1', three pentamanganese(II) complexes of H₂L1 and one mixed-valence manganese(II)₂manganese(III)₃ complex of L13 were synthesised and characterised by single crystal X-ray structure analysis. A mechanism for the observed ring expansion reaction from the (2+2) macrocycle (H₂L1) to the (4+4) macrocycle (H₄L1') is proposed and supported by the results of ²⁵²Cf PDMS, conductivity and EPR measurements. Possible reasons for the isolation of three pentamanganese(II) complexes as well as a tetramanganese(II) complex of H₂L1 in the presence of acetate ions are discussed. The mixed-valence complex of L13 has led to the proposal of a new mechanism for water oxidation by the OEC (oxygen evolving complex) of photosystem II in green plants. The complexes are discussed in terms of their relevance as models for the OEC. This includes discussion of the results of EXAFS (Extended X-ray Absorption Fine Structure), XANES (X-ray Absorption Near Edge Spectroscopy), cyclic voltammetric and magnetic susceptibility studies. An unusual eight-coordinate manganese(II) complex of DAP was prepared and structurally characterised, as was a macrocyclic dilead complex with unusual single atom bridging via the nitrogen atom of a thiocyanate ligand. Less constrained noncyclic ligands are described in the second Section. These are related to the macrocyclic systems, and are derived from the condensation of formyl- or acetylpyridine derivatives with 1,n-aminoalcohols. Particular emphasis was given to manganese complexes because of their potential relevance to the OEC. Two ligand rerrangement reactions, involving the pyridine nitrogen and resulting in the formation of five-membered rings, were observed and possible mechanisms are proposed. Sixteen single crystal X-ray structure determinations were performed including those of eight manganese(II) complexes.