Synthesis and study of redox-active acyclic triazenes: toward electrochromic applications.

Coupling of various 4-substituted phenyl azides with two distinct quinone-containing N-heterocyclic carbenes (NHCs) afforded the respective mono- and ditopic 1,3-disubstituted acyclic triazenes in moderate to excellent yields (38-92%). Depending on their pendant substituents (derived from the azides), the acyclic triazenes exhibited intense absorptions in the visible spectrum (359-428 nm), which were bathochromically shifted by up to Δλ=68 nm upon reduction of the quinone moiety on the component derived from the NHC. Cyclic voltammetry confirmed that the aforementioned redox processes were reversible, and a related set of UV-vis spectroelectrochemical experiments revealed that bulk electrolysis may also be used to switch reversibly the colors exhibited by these triazenes.

Quinobis(imidazolylidene): synthesis and study of an electron-configurable bis(N-heterocyclic carbene) and its bimetallic complexes.

Reaction of bromanil with N,N'-dimesitylformamidine followed by deprotonation with NaN(SiMe(3))(2) afforded 1,1',3,3'-tetramesitylquinobis(imidazolylidene) (1), a bis(N-heterocyclic carbene) (NHC) with two NHC moieties connected by a redox active p-quinone residue, in 72 % yield of isolated compound. Bimetallic complexes of 1 were prepared by coupling to FcN(3) (2) or FcNCS (3; Fc=ferrocenyl) or coordination to [M(cod)Cl] (4 a or 4 b, where M=Rh or Ir, respectively; cod=1,5-cyclooctadiene). Treatment of 4 a and 4 b with excess CO(g) afforded the corresponding [M(CO)(2)Cl] complexes 5 a and 5 b, respectively. Analysis of 2-5 by NMR spectroscopy and X-ray diffraction indicated that the electron-deficient quinone did not significantly affect the inherent spectral properties or coordination chemistry of the flanking imidazolylidene units, as compared to analogous NHCs. Infrared spectroscopy and cyclic voltammetry revealed that decreasing the electron density at ML(n) afforded an increase in the stretching energy and a decrease in the reduction potential of the quinone, indicative of metal-quinone electronic interaction. Differential pulse voltammetry and chronoamperometry of the metal-centered oxidations in 2-4 revealed two single, one-electron peaks. Thus, the metal atoms bound to 1 are oxidized at indistinguishable potentials and do not appear electronically coupled. However, the metal-quinone interaction was used to increase the electron density at coordinated metal atoms. Infrared spectroelectrochemistry revealed that the average nu(CO) values for 5 a and 5 b decreased by 14 and 15 cm(-1), respectively, upon reduction of the quinone embedded within 1. These shifts correspond to 10 and 12 cm(-1) decreases in the Tolman electronic parameter of this ditopic ligand.

Adapting N-heterocyclic carbene/azide coupling chemistry for polymer synthesis: enabling access to aromatic polytriazenes.

A click by any other name: Coupling bis(N-heterocyclic carbene)s with bis(azide)s afforded a novel class of conjugated polytriazenes. These polymers were rendered electrically conductive upon doping, and fluorene-containing variants exhibited luminescence. This adaptation of N-heterocyclic carbene (NHC)/azide coupling chemistry to polymer synthesis reveals the potential of NHCs as building blocks for accessing polymers having useful electronic properties.

Donor-acceptor triazenes: synthesis, characterization, and study of their electronic and thermal properties.

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.

Highly efficient synthesis and solid-state characterization of 1,2,4,5-tetrakis(alkyl- and arylamino)benzenes and cyclization to their respective benzobis(imidazolium) salts.

[reaction: see text] New synthetic methodology to a variety of 1,2,4,5-tetraaminobenzenes and their corresponding benzobis(imidazolium) salts has been accomplished. Palladium-catalyzed coupling of various 1,2,4,5-tetrabromo- or 1,2,4,5-tetrachlorobenzenes with aryl- or tert-alkylamines afforded the respective tetrakis(N-substituted)aminobenzenes in excellent yields. This enabled comparative solid-state structural analyses of this elusive class of electron-rich arenes with their oxidized derivatives. The tetraamines were found to undergo formylative cyclization to the corresponding benzobis(imidazolium) salts in good to excellent yields.

Triazene formation via reaction of imidazol-2-ylidenes with azides.

Treatment of N-heterocyclic carbenes (as their free carbenes or generated in situ) with alkyl, aryl, acyl or tosyl azides afforded the respective substituted triazenes in excellent yields.