Triazaphospholes versus Triazoles: An Investigation of the Differences between “Click”-Derived Chelating Phosphorus- and Nitrogen-Containing Heterocycles

A new class of pyridyl-functionalized triazaphospholes bearing either tBu or SiMe3 substituents at the 5-position of the N3PC heterocycle have been prepared via the “click” reaction starting from 2-(azidomethyl)pyridine and the corresponding phosphaalkynes. In order to investigate the electronic structure and donor–acceptor properties of such novel chelating and low-coordinate phosphorus heterocycles, calculations at the DFT level have been carried out. Moreover, cyclic voltammetry measurements were performed and the results were compared with those for the structurally related triazole-based systems, demonstrating a significant influence of the phosphorus atom as well as the substitution pattern on the electronic properties of the novel compounds. The P,N hybrid ligands form Re(I) complexes of the type [(N∧mN)Re(CO)3Br] via coordination of the nitrogen atom N2 to the metal center rather than via the phosphorus atom, as verified crystallographically.