TUNING THE ELECTRONIC PROPERTIES OF CYCLAM DERIVATIVES: ENHANCED INTERMETALLIC COUPLING AND CATALYSIS
My thesis work revolves around the ability to modify the 1,4,8,11-tetraazacyclotetradecane (cyclam) framework in order tune the electronic properties of resulting metal complexes towards real life applications. A huge direction for science and engineering is the pursuit of Moore’s Law, to constantly miniaturize electronic processes while improving their performance. With the physical limits of copper wiring being reached on nanoscale levels, alternative resources must be utilized. Naturally, the absolute limit of wiring would be on the single molecular scale. It is this idea that Chapters 1-3 are founded upon. Moving forward, I deemed three key concepts are important for success of this project: (1) the ability for modification of the molecule to be incorporated into existing technologies, (2) redox stability of the molecular complexes to allow multiple charges to pass through without losing integrity, and (3) the ability to function as a wire and allow current to pass through. Requirement (1) has been proven possible in previous work on cyclam, however (2) and (3) were yet to be shown for any cobalt tetraazamacrocyclic complex until this work.
Chapter 1 covers my first successful exploration into modification of the cylcam ligand in order to obtain favorable electronic properties. Cobalt complexes utilizing the MPC ligand (5,12-dimethyl-7,14-diphenyl-1,4,8,11-tetraazacyclotetradecane) show stability upon reduction, whereas the cyclam analogues did not. In fact, [Co(MPC)(C2Ph)2]+ was the first cobalt based tetraazamacrocyclic alkynyl complex to show such redox stability without the use of heavily electron withdrawing axial ligands. It was found that this improvement of redox stability is a result of the weakened equatorial ligand field caused by the steric bulk of the phenyl substituents of the cyclam framework. This in turn led to improved axial ligand bonding and hence greater stability. This work shows the CoIII(MPC) framework can satisfy requirement (2).
Based on the results of Chapter 1, Chapter 2 realizes the idea that with improved axial ligand bond strengths in CoIII(MPC) complexes, the possibility for electronic delocalization between cobalt and the axial ligand performing as the wire is opened. A series of dinuclear CoIII(MPC) complexes, with cobalt centers linked through a butadiyndiyl bridge, were prepared. With each cobalt being identical, theoretically each should behave electrochemically similar and reduction of the complex should be a single two electron event. It is however shown that this two electron event was, in fact, split into two single electron events. The source of this result is the delocalization of the first added electron between both cobalt centers, effectively making two half-reduced metals. Therefore, the ability for CoIII(MPC) complexes to satisfy requirement (3) has been proven.
Chapter 3 expands on the results shown in Chapters 1 and 2. Where Chapter 2 showed delocalization of an electron between cobalt centers, Chapter 3 shows delocalization of a hole through cobalt between ethynylferrocene ligands. With this, all three requirements are met and the ability as Co(MPC) to function as a wire has been proven for both oxidation and reduction, both between cobalt and through cobalt.
Chapter 4 takes a new direction, however applies the same basic principle as the previous three in modifying the cyclam ligand to achieve desired properties. Where application in electronic devices are made stable by use of the bulky MPC ligand, application towards catalysis requires an open catalytic site and weak enough axial coordination to allow the substrate to leave once reduced. Through the alkyl substitution of the cyclam ligand in NiII(CTMC) (5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradecane) in place of the MPC ligand, electronically donating properties of the macrocycle were maintained while opening the axial catalytic site. In this work, it was shown that reduction in steric bulk of the ligand from phenyl to ethyl to methyl, while maintaining electron donating properties, improved catalytic efficiency and all complexes were superior to NiII(cyclam).