Dyes, Radicals and Nitrous Oxide Adducts Derived from N-Heterocyclic Carbenes

N-Heterocyclic carbenes are able to form covalent adducts with nitrous oxide (N2O) under ambient conditions. This thesis summarizes the studies of these adducts and their reactivity. A ditopic carbanionic N-heterocyclic carbene was found to react with nitrous oxide, resulting in a stable covalent adduct with two N2O groups attached to the heterocycle. Mesoionic N-heterocyclic carbenes derived from C2-arylated 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, as well as a triazolylidene, also react with N2O. Crystallographic analyses of all complexes reveal bent N2O groups, which can adopt a cis or trans configuration. Resulting complexes of imidazole- or triazole-based mesoionic carbenes and nitrous oxide can be converted into azo dyes by reaction with arenes in the presence of AlCl3 or HCl. The azo coupling can be achieved with electron-rich aromatic compounds such as mesitylene, trimethoxybenzene, azulene, or dibenzo-18-crown-6. The latter coupling reaction allows for an easy preparation of a colorimetric sensor for potassium or sodium ions. The putative intermediate of the reaction with acid, a rare diazohydroxide, has been isolated and structurally characterized. Using imidazolium compounds with two N2O groups, it is possible to prepare amine-substituted azo dyes, or dyes with two azoarene groups attached to the heterocycle. The synthesis and the characterization of a new class of neutral aminyl radicals is also reported. Monoradicals were obtained by the reduction of azoimidazolium dyes with potassium. Structural, spectroscopic and computational data suggest that the spin density is centered on one of the nitrogen atoms of the former azo group. The reduction of a dimeric dye with an octamethylbiphenylene bridge between the azo groups resulted in the formation a biradical with largely independent unpaired electrons. Both, the monoradicals and the biradical were found to display high stability in solution as well as in the solid state.

Severin, Kay
Lausanne, EPFL

Note: The status of this file is: EPFL only

 Record created 2018-06-18, last modified 2019-06-19

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