Bis(1,2,3-triazol-5-ylidenes) (i-bitz) as Stable 1,4-Bidentate Ligands Based on Mesoionic Carbenes (MICs)

Abstract

Direct metalation of bis(1,2,3-triazolium) salts affords mononuclear rhodium(I) complexes, which feature a 1,4-bidentate bis(1,2,3-triazol-5-ylidene) (i-bitz) ligand. The topology of the ligand is similar to 2,2′-bipyridines (bpy) and their congeners, as well as bis(1,2,4-triazol-5-ylidenes) (bitz). As the former, but in contrast to the latter, the free i-bitz can be isolated, which paves the way for various applications.

Application of 1,2,3-triazolylidenes as versatile NHC-type ligands: synthesis, properties, and application in catalysis and beyond

“…It is apparent that the presence of aryl substituents on N1 and N3 enhances the stability of the free triazolylidenes, which will have implications if free 1,2,3-triazolylidenes are employed as organocatalysts. 40 In some cases metallation has been accomplished in the presence of KOtBu (Scheme 8). 29,40 Sarkar et al have utilised this method in the formation of triazolylidene copper(I) complexes 27, 29 and Bertrand and co-workers have reported a base-mediated metallation of a ditriazolium salt to yield the cationic 45 ditriazolylidene rhodium complex 28.…”

“…40 In some cases metallation has been accomplished in the presence of KOtBu (Scheme 8). 29,40 Sarkar et al have utilised this method in the formation of triazolylidene copper(I) complexes 27, 29 and Bertrand and co-workers have reported a base-mediated metallation of a ditriazolium salt to yield the cationic 45 ditriazolylidene rhodium complex 28. 40 This reactivity is distinctly different from related 1,2,4-ditriazolylidenes, which do not form related chelating rhodium(I) complexes.…”

“…In CH 2 Cl 2 , the corresponding silver carbene is formed, which is sufficiently stable for being used for triazolylidene transfer (cf section on synthesis). In THF, however, an unstable intermediate, presumably an [Ag(carbene)(OH)] species, is generated, which 40 demetallates to yield the mesoionic oxide 49.…”

“…This suggests electronic flexibility, e.g. The potential of the 1,2,3-triazole product to act as a ligand for 40 copper and to participate in the catalytic cycle has been addressed by preparing a representative N3-bound triazole copper complex (R = Ph, R' = Bn). 29 This complex is indeed a catalyst precursor, however its activity (67% after 7 h) is two orders of magnitude lower than that of complexes 48 and is thus not competitive.…”

“…The latter aspect is obviously compromised to some extent by the inductive effect of the two nitrogen atoms adjacent to the carbene. 7 Efforts have thus been directed towards the 40 development of carbene systems which feature the stabilising heteroatoms in more remote positions in order to further increase the donor properties of NHCs, leading to so-called remote carbenes (rNHCs; no heteroatom adjacent to the carbene, e.g. F, G), 8 and to abnormal/mesoionic carbenes (aNHCs, MICs; 45 carbenes for which no uncharged resonance structure can be drawn, e.g.…”