Abstract
A number of novel tetraaza[14]annulene (TAA) ligands and their transition metal complexes have been synthesised, characterised and electrochemically investigated as monomer sub-units from which conducting polymer films can be prepared. Electrochemical analysis of the new ligand (p-NH2)2dptaaH2, prepared from the catalytic hydrogenation of (p-N02)2dptaaH2 at atmospheric pressure in DMF, shows three irreversible ligand-centred redox peaks in CH3CN electrolyte (L+0 = 0.48 V, L2+/+ = +1.02 V, L0/- = -1.44 V) with the oxidation peaks becoming reversible in CH2Cl2 electrolyte. Solutions of Ni(p-NH2)2dptaa and Co(p-NH2)2dptaa, prepared by insertion reaction of the new ligand and the metal(II) acetate in DMF, show reversible metal-centred couples in DMF electrolyte (Ni(I)/(II) = -1.44V, Co(I)/(II) = -1.14 V, Co(II)/(III) = +0.10 V) as well as reversible and irreversible ligand-centred processes. Films of poly(p-NH2)2dptaaH2, prepared from a solution of the ligand in CH2Cl2 electrolyte by repeatedly scanning over the first oxidation peak (DeltaE = -0.2 -+1.1 V), grow linearly until the maximum surface coverage (I~max ~ 6x10-8 mol/cm2, ~ 281 'layers') is reached. Characterisation of these films suggests that they form by aniline-like polymerisations involving the free NH2 group and the 2- and 3-positions of the 4- substituted aniline unit of the macrocycle. Analysis of the film in clean CH2Cl2 electrolyte (DeltaE = -0.2 - +1.6 V) shows single forward and reverse processes (Eap = +1.23 V, Ecp = +0.68 V) due to charging and discharging of the film. The response is stable to repeated scans and in aqueous acid electrolyte. Sweep rate analysis shows processes other than those in a mono-layer. Solutions of the nickel(II) and cobalt(II) complexes in DMF electrolyte, subject to the same electropolymerisation conditions, deposit films of polyNi(p-NH2)2dptaa and polyCo(p-NH2)2dptaa at the electrode surface, although the build-up in polymer is slow compared to that of the metal-free ligand. The value of the Co(II)/(III) couple (+0.10 V) for a solution of Co(p-NH2)2dptaa in DMF electrolyte is only slightly lower than that of Cotaa (+0.12 V) in DMF electrolyte and, based on this evidence, films of polyCo(p-NH2)2dptaa are expected to show similar oxygen reduction properties to those of polyCotaa. The new ligands (pyr)2dptaaH2 and (p-NH3Cl)2dptaaH2, have not been electrochemically analysed and the pyrrole-substituted ligand is insufficiently soluble to form conducting polymers from solution. The cobalt(II) complex Co(p-NO2)2dptaa could not be reduced by the method successful for reduction of the metal-free ligand, and reduction of 7,16-(NO2)taaH2 resulted in a break-up of the macrocyclic ligand. The new ligand 3,7,12,16-(Me)4taaH2 shows a trans arrangement of methyl groups and the structure of (p-NO2)2dptaaH2 has also been determined. Chapter 1 introduces the area of oxygen reduction in relation to fuel cell technology and reviews the types of oxygen reduction catalyst along with methods of incorporating them onto electrodes. Chapter 2 introduces macrocyclic ligand chemistry and gives a review of the TAAs concentrating on their structure and synthesis. Chapter 3 describes the synthesis of some TAA ligands and metal complexes, many of which have been used as starting materials. Chapter 4 discusses the synthetic strategy used when designing novel TAA ligands, and details attempted syntheses using these methods. Chapter 5 describes the synthesis of new TAA ligands and metal complexes. Chapter 6 introduces cyclic voltammetry, reviews the electrochemistry of the TAAs and conventional conducting polymers, and describes the electrochemical investigation of the new TAAs. Chapter 7 concludes by discussing both synthesis and electrochemistry and suggests areas for further work.