New Optically Active N-Heterocyclic Carbene Complexes for Hydrogenation: A Tale with an Atropisomeric Twist

A synthesis of 1,2,4-triazolium salts 1 and 2 from three different, easily varied, components was developed to facilitate access to a diverse set of N -heterocyclic carbene complexes. Salts 1 epimerized in the synthesis, so they were not investigated further. A coordinated chlorine atom was retained on reaction of 2 with [Ir(COD)Cl] 2, and this resulted in two atropisomeric complexes, 3 and 4, which were both characterized via X-ray diffraction studies. Neither of these complexes mediated hydrogenation of E -1,2-diphenylethene, but both 3 and 4 were reacted with NaBARF to give the chlorine-free complex 5, which was catalytically active in this reaction.