Cobalt(III) Complexes For Surface Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis addresses the potential for use of cobalt(III) complexes for functionalisation of lightly oxidised iron surfaces. In Chapter 2 the preparation of cobalt(III) complexes of a series of ligands based on 1,1,1-tris(aminomethyl)ethane is described. The synthesis was approached in two ways. Firstly, preparation of functionalised triol molecules as precursors to functionalised triamine ligands was investigated. This approach utilised the Tollens condensation of aldehydes with formaldehyde. In a second approach, the functionalisation of tetrakis(aminomethyl)methane in which one amine arm has been differentiated was used. The tetraamine was reacted with benzaldehyde and reduced with borohydride ion to give a secondary amine molecule that was then functionalised using alkyl or aryl sulfonyl chloride molecules. Chapter 3 describes the measurement of the binding of some cobalt(III) complexes to the surface of high surface area goethite. It was observed that complexes that have three exchangeable ligands bind more strongly than those with two exchangeable ligands. This is rationalised as being due to there being more bonds to the surface formed by complexes with three exchangeable ligands. It was also observed that complexes with three exchangeable ligands give greater surface coverage than those with two. This is likely due to the larger cross sectional area of the complexes with two exchangeable ligands in comparison to that of those with three, which blocks potential adjacent sites. Preliminary experiments on the use of the contact angle, SEM, EDS and QCM to characterise complex binding are explored in Chapter 4 . The results from the EDS and QCM experiments show that these may be valuable tools for measuring this binding and the subsequent surface properties, but have not yielded detailed results at this point. In Chapter 5 the use of cobalt(III) complexes as inhibitors of corrosion of iron in hydrochloric acid is investigated. All the complexes tested, even those that showed no binding to goethite surfaces, inhibit the corrosion to some degree. The level of inhibition is dependent on the complex, with [Co(tren)Cl2]Cl showing maximum inhibition of 81% and [Co(tame)Cl3] showing maximum inhibition of 53%. For some of the complexes, their concentration in solution over the course of the experiment was monitored by UV-vis. It was found that the complex disappears in a zero order reaction, the rate of which is dependent on the complex. However, the exact nature of this reaction is unknown. Furthermore, it was observed that inhibition of corrosion continues after the complex is no longer observed in solution. There is a difficulty in rationalising the inhibition being dependent on the complex identity, but not its continued presence in solution. Consequently, the mechanism of corrosion inhibition that explains all of these observations is still not known.