Investigation of Electronic and Steric Properties of N-Heterocyclic Carbenes by NMR and Photoluminescence

Project no. 32347 · Status: Kapanmış (Yayın bekleniyor) · 2024

Summary

<p class="ql-align-justify">In&nbsp;this&nbsp;study,&nbsp;the&nbsp;σ-donor&nbsp;abilities&nbsp;of&nbsp;N-heterocyclic&nbsp;carbene&nbsp;(NHC)&nbsp;ligands&nbsp;were&nbsp;evaluated&nbsp;through&nbsp;the&nbsp;combined&nbsp;use&nbsp;of&nbsp;nuclear&nbsp;magnetic&nbsp;resonance&nbsp;(NMR)&nbsp;and&nbsp;photoluminescence&nbsp;spectroscopy.&nbsp;An&nbsp;electronically&nbsp;graded&nbsp;series&nbsp;of&nbsp;azolium&nbsp;salts&nbsp;was&nbsp;synthesised&nbsp;from&nbsp;imidazole,&nbsp;benzimidazole&nbsp;and&nbsp;5,6-dimethylbenzimidazole&nbsp;cores,&nbsp;and&nbsp;their&nbsp;structures&nbsp;were&nbsp;elucidated&nbsp;by&nbsp;¹H-NMR,&nbsp;proton-coupled&nbsp;¹³C-NMR&nbsp;and&nbsp;HMQC&nbsp;techniques.&nbsp;A&nbsp;nine-membered&nbsp;ligand&nbsp;series&nbsp;(<strong>L1</strong>–<strong>L9</strong>),&nbsp;combining&nbsp;three&nbsp;heterocyclic&nbsp;backbones&nbsp;with&nbsp;three&nbsp;wingtip&nbsp;groups,&nbsp;was&nbsp;obtained&nbsp;in&nbsp;68–92%&nbsp;yields.&nbsp;The&nbsp;one-bond&nbsp;coupling&nbsp;constants&nbsp;(¹<em>J</em>_CH)&nbsp;at&nbsp;the&nbsp;<em>C</em>2&nbsp;position&nbsp;spanned&nbsp;222–216&nbsp;Hz,&nbsp;while&nbsp;the&nbsp;energy&nbsp;differences&nbsp;from&nbsp;photoluminescence&nbsp;spectra&nbsp;ranged&nbsp;from&nbsp;1.03&nbsp;to&nbsp;1.55&nbsp;×&nbsp;10^–19&nbsp;J.&nbsp;As&nbsp;the&nbsp;number&nbsp;of&nbsp;electron-donating&nbsp;groups&nbsp;increased,&nbsp;¹<em>J</em>_CH&nbsp;decreased&nbsp;while&nbsp;Δ<em>E</em>&nbsp;increased.&nbsp;All&nbsp;ligands&nbsp;were&nbsp;screened&nbsp;<em>in&nbsp;situ</em>&nbsp;in&nbsp;the&nbsp;α-alkylation&nbsp;of&nbsp;acetophenone&nbsp;with&nbsp;benzyl&nbsp;alcohol,&nbsp;and&nbsp;the&nbsp;yields&nbsp;showed&nbsp;a&nbsp;strong&nbsp;inverse&nbsp;linear&nbsp;relationship&nbsp;with&nbsp;the&nbsp;¹<em>J</em>_CH&nbsp;values&nbsp;(R²&nbsp;=&nbsp;0.914).&nbsp;The&nbsp;highest&nbsp;yield&nbsp;(71%)&nbsp;was&nbsp;obtained&nbsp;with&nbsp;<strong>L9</strong>,&nbsp;predicted&nbsp;spectroscopically&nbsp;to&nbsp;be&nbsp;the&nbsp;strongest&nbsp;σ-donor,&nbsp;thereby&nbsp;confirming&nbsp;the&nbsp;hypothesis.&nbsp;Complex&nbsp;<strong>1</strong>,&nbsp;isolated&nbsp;from&nbsp;this&nbsp;ligand&nbsp;by&nbsp;Ag₂O-mediated&nbsp;transmetalation,&nbsp;markedly&nbsp;outperformed&nbsp;the&nbsp;<em>in&nbsp;situ</em>&nbsp;system&nbsp;at&nbsp;identical&nbsp;iridium&nbsp;loading&nbsp;(92%&nbsp;versus&nbsp;71%&nbsp;after&nbsp;120&nbsp;min).&nbsp;The&nbsp;study&nbsp;therefore&nbsp;introduces&nbsp;a&nbsp;rapid,&nbsp;low-cost&nbsp;and&nbsp;reproducible&nbsp;screening&nbsp;approach&nbsp;for&nbsp;the&nbsp;electronic&nbsp;properties&nbsp;of&nbsp;NHCs,&nbsp;one&nbsp;that&nbsp;operates&nbsp;directly&nbsp;on&nbsp;azolium&nbsp;salts&nbsp;and&nbsp;eliminates&nbsp;the&nbsp;need&nbsp;for&nbsp;the&nbsp;reference&nbsp;carbonyl&nbsp;complexes&nbsp;required&nbsp;by&nbsp;the&nbsp;Tolman&nbsp;Electronic&nbsp;Parameter.</p>

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