N-heterocyclic Carbene Synthesis Towards Self-Assembled Monolayers on Gold

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McLeod, Jennifer
Keyword
Chemistry , Self-Assembled Monolayer
Abstract
Modifying surfaces with self assembled monolayers (SAMs) is a simple and effective way to create functional materials. Traditionally compounds containing a thiol head group are used to generate SAMs on gold surfaces. However, a recent discovery by the Crudden lab demonstrated increased bond strength and surface stability of n heterocyclic carbenes (NHCs) to a gold substrate. Here in we report the synthesis and modification of gold surfaces with NHCs containing unique functionality. Chemical synthesis was characterized using nuclear magnetic resonance spectroscopy techniques and SAM formation was analyzed using x-ray photoelectron spectroscopy (XPS). N,N’-Ditolyl-4,5(R,R)-diphenylimidazolium tetrafluroborate (19) is a known molecule with chiral functionality about the backbone and successfully formed a SAM on gold with a calculated carbon to nitrogen ratio of 29:2 from XPS analysis results. Iodine (31), triflate (34), hydrogen bicarbonate (35), and tetrafluoroborate (37) salts of N,N’-Diisopropyl[2,3-d]imidazoliumnaphthalene were synthesized in order to investigate the π-stacking of the naphthalene backbone when assembled on a gold surface. Unfortunately, from XPS analysis a full monolayer was never achieved. Finally, N,N’-Diisopropylnaphtho[2,3-d]imidazolium-4,9-dione salts: iodine (48), triflate (50), and tetrafluoroborate (49), were synthesized in an attempt to electrochemically probe the redox active quinone moiety once a SAM had formed on a gold surface. Compound 49 formed a monolayer on the gold surface with a carbon to nitrogen ratio of 16:2 from the XPS analysis results and compound 50 has a carbon to nitrogen ratio of 36:2. The electrochemistry experiments were performed using a monolayer of 50 on a gold electrode. The cyclic voltammogram revealed very minor features characteristic of the quinone moiety which indicates low molecular consistent with XPS results.
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