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Tailoring the Electrochemical Profiles of Molecular Architectures Through Lewis Acid Coordination and Direct Functionalization
Kelsey, Shaun Ryan
Kelsey, Shaun Ryan
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Abstract
Redox reactions are vital to the processes and technologies that shape our daily lives, from charging batteries to sustaining plant life. These reactions, which involve the transfer of electrons, form the basis of many essential functions. In this context, the exploration of methods to adjust the electrochemical properties of molecular frameworks becomes crucial. Such investigations aim to simplify these processes and unveil the fundamental relationship between molecular structure and electrochemical behavior.
Chapter 1 serves to set the foundation for the work provided in this dissertation by addressing two distinct, yet interconnected, large scale issues. On each issue, the broader scientific landscape and societal relevance is addressed before the chapter gradually delves into specific research gaps and challenges. By elucidating how my research is designed to address these challenges, this chapter lays the groundwork for subsequent discussions.
In chapter 2, the electrochemical properties of a family of macrocyclic complexes that feature a zinc(II) center paired with a secondary redox-inactive metal cation in heterobimetallic (Na+, Ca2+, Nd3+, Y3+) motifs or homobimetallic (Zn2+) motifs have been investigated. The new complexes were prepared via a divergent strategy, isolated, and structurally characterized with single-crystal X-ray diffraction (XRD) analysis. XRD results show that the structures of the complexes are modulated by the identity of the incorporated secondary metal ions. Cyclic voltammetry data reveal that ligand-centered reduction is promoted in the bimetallic complexes and that the paired metal ions synergistically influence the redox properties of the complexes. Similar to prior work from our group and others, the bimetallic complexes containing stronger Lewis acids undergo more significant reduction potential shifts; contrasting with prior work on complexes containing redox-active metals, however, the zinc(II) complexes studied here display faster electron transfer (as judged by lower reorganization energies, λ) when incorporating di- or tri-valent Lewis acids in contrast to monovalent (and more weakly acidic) sodium. The quantified trends in these data offer insights that could help distinguish metal- versus ligand-centered reduction of bimetallic complexes.
In chapter 3, the E1/2 potential associated with reduction of the linearly-functionalized 6,6’-biazulenic scaffold is accurately correlated to the combined p Hammett parameters of the 2/2’-substituents over >600 mV range. X-ray crystallographic analysis of the 2,2’-dichloro-substituted derivative revealed unexpectedly short CCl bond distances, along with other metric changes, suggesting a non-trivial cycloheptafulvalene-like structural contribution.
Chapter 4 presents a convenient synthetic route to the theoretically appealing 2,2’-dimercapto-1,1’,3,3’-tetraethoxycarbonyl-6,6-biazulene. In addition, a family of unsymmetrically terminated 6,6’-biazulenes was synthesized via C-C coupling of differently functionalized monoazulenic units. In CH2Cl2/nBu4NPF6 solutions, all 6,6’-biazulenenic derivatives invariably exhibited one-step, 2-electron reversible reduction at mild potentials due to potential compression/inversion. Plotting 13C NMR chemical shifts δ(13COtrans) and δ(13COcis) vs. δ(13CN) for the octahedral [(NC)Cr(CO)5] core in a series of complexes [(OC)5Cr(2-isocyano-2’-X-1,1’,3,3’-tetraethoxycarbonyl-6,6’-biazulene)] (X = S-, SAuPPh3, SCH2CH2CO2CH2CH3, SH, NC-Cr(CO)5) revealed a nearly perfect inverse linear correlation. This analysis provided a remarkably sensitive spectroscopic tool for assessing electron delocalization across the ca. 12 Å-long 6,6’-biazulenic π-linker. Moreover, the [(NC)Cr(CO)5] moiety in [(OC)5Cr(2-isocyano-2’-mercapto-1,1’,3,3’-tetraethoxycarbonyl-6,6’-biazulene)] served as an instructive RAIR reporter to indicate approximately upright orientation of the molecular constituents upon self-assembly of the 2-mercapto-6,6’-biazulenic scaffold on metallic gold surfaces.
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2024-01-01
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University of Kansas
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Chemistry, Inorganic chemistry, chemistry, electrochemistry, functional materials, lewis acid, rectifier, synthesis