44077-G1
Exploring the Chemical Potential of 2-Iminoimidazolines in Small Molecule and Macromolecular Synthesis
The overarching goal of this research project was to develop new and efficient synthetic routes to functional materials containing N-heterocycles and other structurally-advanced compounds rich in nitrogen. In the first year of this ACS PRF grant, we discovered that imidazolylidenes, benzimidazolylidenes, and other N-heterocyclic carbenes (NHCs) react with alkyl, aryl, and acyl azides to afford 1,3-disubstituted triazenes in excellent yields (>99%). Considering the unusual structure of these compounds and the efficient method of their preparation, subsequent efforts have been focused on exploring the potential of this novel class of substrates as functional small molecule and polymeric materials, particularly in electronics and biological applications. Considering the high nitrogen content of these materials, early efforts focused on studying their thermal stabilities using thermogravimetric analysis. In general, their stabilities were found to be strongly dependent on the sterics of the NHC component, particularly the size of its N-substituents, as well as the electronic properties of the azide component. For example, triazenes possessing bulky N-substituents (e.g., tert-butyl) were stable in the solid-state to temperatures exceeding 150 degrees Celsius, 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. The products of the thermally-induced triazene decomposition reaction were found to be molecular nitrogen and the respective guanidine. To determine the mechanism of decomposition, an isotopically-labeled triazene was prepared and exposed to elevated temperatures. The products of this decomposition reaction were consistent with that of the Staudinger reaction, where phosphines are combined with organic azides to form phosphatriazenes and ultimately phosphazenes (and nitrogen). To evaluate the electronic properties of these materials, a systematic series of substituted triazenes were synthesized by coupling various functionalized NHCs with aryl azides. Depending on the complementarily of the functional groups on the NHCs and the organic azides, the UV-vis spectra respective triazenes were found to exhibit lamba max values ranging between 364 and 450 nm. Additional support for efficient electronic communication across the triazeno linkage was obtained using NMR spectroscopy and X-ray crystallography, which revealed bond alteration patterns in a series of triazenes characteristic of donor-acceptor compounds. Attention then shifted toward exploring the potential of this NHC/azide coupling reaction in accessing unique macromolecular materials. To demonstrate the potential of this reaction as a convenient method of installing new or altering existing functional groups on polymeric materials, poly(para-azidomethylstyrene) was first prepared using standard methods. Subsequent addition of a commercially-available NHC afforded a polymer with pendant triazeno moieties, as determined by NMR spectroscopy, IR spectroscopy, and size exclusion chromatography. The coupling reaction was found to be high-yielding and the extent of functionalization linearly correlated with the amount of free NHC added. This versatility was further elaborated by combination with Cu(I) catalyzed “click” chemistry. Thus, after poly(para-azidomethylstyrene) was partially modified with phenylacetylene (via click chemistry), residual azide moieties were reacted with free NHCs. This sequence effectively enabled the synthesis of a random copolymer bearing triazenes and triazoles. Thermogravimetric analysis revealed that the NHC-modified polymer cleanly loses molecular nitrogen at 135 degrees Celsius. Thus, thermal treatment proved to be an effective method for transforming polymers with pendant triazenes to their respective guanidines, and holds great potential for accessing polyionomers with cytotoxic and other biological activities. Recently, we developed a variety of complementary methods for preparing new classes of Janus-type bis(NHC)s, where two NHCs are annulated to a common linker. Combination of such bis(NHC)s with bis(azide)s was found to produce new classes of polytriazenes. Despite their relatively high nitrogen contents, these polymers were found to be stable up to 275 Celsius in the solid-state, at which point they cleanly lose nitrogen to form their respective polyguanidines. Notably, due to the synthetic versatility used to access bis(NHC)s, these polymers could be rendered soluble in a range of organic solvents by simply changing the N-substituents on the NHC moieties. The molecular weights of these materials were large enough (> 20 kDa) to enable the formation of mechanically-robust thin films. Doping these films with small amounts (<2 wt%) of iodine caused them to be electrically-conductive (sigma = 0.001 S/cm), as determined using four-point probe measurements. This research project has positively impacted the careers of involved researchers (three students and one PI) in three unique ways: (1) Thus far, efforts related to this project have resulted in seventeen (17) publications. (2) The students involved have learned new synthetic skills and characterization techniques in organic, organometallic, and polymer chemistry. (3) Data generated during this project have been used as preliminary results for successful proposals sent to the National Science Foundation, the Department of Defense, and a number of private funding organizations.
