Date of Award

Spring 2020

Project Type

Dissertation

Program or Major

Chemistry

Degree Name

Doctor of Philosophy

First Advisor

Richard P Johnson

Second Advisor

Arthur Greenberg

Third Advisor

Erik Berda

Abstract

Triangulenes are the homologous series of triangle-shaped polycyclic aromatic hydrocarbons, first contemplated by chemists nearly 100 years ago. Computational models predict that triangulenes will be polyradicals, with high-spin electronic ground states. Recent technological advances have allowed the molecular-scale synthesis and on-surface characterization of the first three members of this series using atomic force microscopy techniques. In this work, a short and scalable synthesis of the [3]triangulene ring system was developed. Cascade cyclization of a tetra-benzyl alcohol precursor in trifluoromethanesulfonic acid solution gave the planar and three-fold symmetrical 4,8,12-trihydro[3]triangulenium carbocation. This new species has been characterized by NMR (nuclear magnetic resonance) and optical spectroscopies and is highly fluorescent. Quenching of the cation into basic solutions or by hydride transfer from triethylsilane provides access to stable dihydro and tetrahydro[3]triangulenes. Quenching with triethylamine gave isomerically pure 1,8-dihydrotriangulene, a known precursor to [3]triangulene. These neutral species interconvert with carbocations in a complex series of proton and hydride transfers. The presence and distribution of these cationic intermediates are determined by acid concentration and time spent in solution. With the help of density functional theory (DFT) calculations, a logical pathway to each isomer was proposed through a series of proton and hydride transfers.

This route provides several important [3]triangulene precursors. Preliminary experiments designed to generate [3]triangulene in the solution phase were performed. Reaction of 1,8-dihydrotriangulene with p-chloranil in solution was followed by NMR, optical spectroscopy, and LDI-TOF (laser-desorption-ionization time-of-flight) spectrometry. These experiments provided evidence for the formation and rapid oligomerization of [3]triangulene, consistent with the expectation of its high-spin ground state. The electrochemistry of 1,8-dihydrotriangulene was investigated for the first time. Experiments to date demonstrate a complex series of redox and chemical processes.

A related series of topologically interesting structures can be conceptually derived from triangulene by carving out one side and the center ring, or from phenalene by growing rings along two ring faces. When flipped on end, the resulting structure is V-shaped; herein, we refer to these structures as "victorenes." Viewed as phenalenyl homologues, the victorenes should not have a simple Kekulé aromatic structure. Also by analogy to the phenalenyl ring system, both the cations and anions in this series are likely to be aromatic. These structures are predicted by our DFT computations to be chiral, with low barriers to interconversion of enantiomers. A new and general synthetic route to victorenes was developed that allowed the preparation of two [3]victorene ketone derivatives. Slight modification of this route enabled the incorporation of substituents that block the reactive sites and afforded clean intramolecular cyclization to several hydro[3]victorenes. The relative free energies of hydro[3]victorenes were calculated using DFT and matched well with the experimental observations. Generation of the first victorenium cations was achieved in a TfOD/DCE-d4 solution, with successful observation by 1H NMR.

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