Donor−Acceptor Triazenes:  Synthesis, Characterization, and Study of Their Electronic and Thermal Properties

Abstract

A new class of 1,3-disubstituted-triazenes were synthesized by coupling functionalized benzimidazol-2-ylidenes, as their free N-heterocyclic carbenes or generated in situ from their respective benzimidazolium precursors, to various aryl azides in modest to excellent isolated yields (36-99%). Electron delocalization between the two coupled components was studied using UV-vis spectroscopy, NMR spectroscopy, and X-ray crystallography. Depending on the complementarity of the functional groups on the N-heterocyclic carbenes and the organic azides, the respective triazenes were found to exhibit lambda(max) values ranging between 364 and 450 nm. X-ray crystallography revealed bond alteration patterns in a series of triazenes characteristic of donor-acceptor compounds. Triazene thermal stabilities were studied using thermogravimetric analysis and found to be strongly dependent on the sterics of the benzimidazol-2-ylidene component and the electronics of the azide component. Triazenes possessing bulky N-substituents (e.g., neo-pentyl, tert-butyl, etc.) were stable in the solid-state to temperatures exceeding 150 degrees C, whereas analogues with small N-substituents (e.g., methyl) were found to slowly decompose at room temperature. Triazenes featuring electron-rich phenyl azide components decomposed at higher temperatures than their electron-deficient analogues. Products of the thermally induced triazene decomposition reaction were identified as molecular nitrogen and the respective guanidine. Using an isotopically labeled triazene, the mechanism of the decomposition reaction was found to be analogous to the Staudinger reaction.

Synthesis and Study of 5,5′‐Bibenzimidazolylidenes and Their Bimetallic Complexes

“…[33] The reaction mixture was stirred at ambient temperature for 12 h. Excess triethyloxonium tetrafluoroborate was quenched by adding ethanol (2 mL) followed by stirring for an additional 2 h. The resulting solution was poured into excess ether and white solids (186.1 mg, 85 %) were collected by vacuum filtration. [34] (41.8 mg, 0.157 mmol), dry CH 3 CN (3 mL), triethyloxonium tetrafluoroborate (41.7 mg, 0.219 mmol) and a stir bar. [33] The reaction mixture was stirred at ambient temperature for 12 h. Excess triethyloxonium tetrafluoroborate was quenched by adding ethanol (2 mL) followed by stirring for an additional 2 h. The resulting solution was concentrated and washed with methanol (3 mL) to afford 47.6 mg (95 % yield) of the desired compound as a white powder.…”

5-Azido-4-dimethylamino-1-methyl-1,2,4-triazolium Hexafluoridophosphate and Derivatives

“…Organic azides were occasionally converted to triazenes, essentially by reaction with N-heterocyclic carbenes. Thus, a number of crystal structures of neutral aryl or alkyl triazenes have been reported [13][14][15][16][17][18][19][20]. The use of quaternary azidoazolium cations for this transformation resulted in cationic triazatrimethine cyanine dyes, but only one crystal structure of this type was reported, although with benzimidazole [21].…”

Quinobis(imidazolylidene): Synthesis and Study of an Electron‐Configurable Bis(N‐Heterocyclic Carbene) and Its Bimetallic Complexes

“…[62,63] Given that ferrocene (FcH) can be functionalized [64] with NHC-reactive moieties and exhibits well-defined electrochemical behavior, we pursued derivatives bearing groups that would facilitate coupling to 1. Previously, we attached Fc-based electrochemical handles to BBI (R = tert-amyl) by using NHC-azide [82] and NHC-isothiocyanate [62] coupling chemistry to afford adducts II and III, respectively (Scheme 5). [83] Electrochemical anal- Table 1.…”