Recent Developments in the Chemistry of 3H-1,2,3,4-Triazaphosphole Derivatives

This microreview describes the progress that has been made during the last few years in the synthesis, coordination chemistry, and application of donor-functionalized 3H-1,2,3,4-triazaphospholes, the phosphorus analogues of the wellknown 1,2,3-triazoles. The latter class of compounds has attracted tremendous interest in various areas of chemistry due to their facile assembly through click chemistry. In contrast, the phosphorus-containing analogues have received much less attention, although their synthesis is fairly straightforward, starting from organic azides and phosphaalkynes, without the need for use of any catalyst. These fascinating low-coordinate phosphorus heterocycles have conjugated pi systems with high degrees of aromaticity and possess rather high p density at their phosphorus atoms, due to significant N-C=P <-> N+=C-Pconjugation. It has been shown recently that [3+2] cycloaddition between functionalized organic azides and phosphaalkynes also allows the incorporation of additional donor substituents into specific positions of the phosphorus heterocycle, and both chelating and polydentate triazaphospholes are nowadays synthetically accessible. These compounds have been used to prepare the first coordination compounds based on triazaphospholes, making use of the chelate effect. The recently used strategies in this area facilitate, for the first time, synthetic access to a completely new set of triazaphospholes as well as their corresponding metal complexes, leading to a much broader scope for potential applications. As a very recent highlight, experimental and theoretical findings have established triazaphospholes as a new class of phosphorus-containing extended pi systems for optoelectronic applications, because significant fluorescence emission of 2-pyridyl-functionalized triazaphosphole derivatives has been observed. New developments in the areas of homogeneous catalysis and materials sciences can consequently be foreseen in the near future.