<p></p><p> 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.</p><p> 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)(C<sub>2</sub>Ph)<sub>2</sub>]<sup>+</sup> 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 Co<sup>III</sup>(MPC) framework can satisfy
requirement (2).</p><p> Based
on the results of Chapter 1, Chapter 2 realizes the idea that with improved
axial ligand bond strengths in Co<sup>III</sup>(MPC) complexes, the possibility
for electronic delocalization between cobalt and the axial ligand performing as
the wire is opened. A series of dinuclear Co<sup>III</sup>(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 Co<sup>III</sup>(MPC) complexes to satisfy
requirement (3) has been proven.</p><p> 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.</p><p>
</p><p> 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 Ni<sup>II</sup>(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 Ni<sup>II</sup>(cyclam).</p><p></p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/13262657 |
| Date | 16 December 2020 |
| Creators | Brandon L Mash (9662924) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/TUNING_THE_ELECTRONIC_PROPERTIES_OF_CYCLAM_DERIVATIVES_ENHANCED_INTERMETALLIC_COUPLING_AND_CATALYSIS/13262657 |
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