Phenoxy-Linked Mesoionic Triazol-5-ylidenes as Platforms for Multinuclear Transition Metal Complexes

Abstract

The preparation and full characterization of a variety of mono- ([L1-H]), di- ([L2-H2], [L2A-H2]), and tri- ([L3-H3]) 1,2,3-triazolium salts constructed form “clicked” hydroxybenzene derivatives are reported. Deprotonation with potassium hexamethyldisilazide, followed by in situ metalation, allowed for the synthesis of a series of mono- (L1·[M]), di- (L2·[M]2, L2·[M]2), and trinuclear (L3·[M]3) group 9–11 (M = [Rh(CO)2Cl], [Pd(allyl)Cl], and [AuCl]) triazol-5-ylidene metal complexes. In solution, all metal complexes feature symmetrical patterns displaying C2 and C3 fold axes when supported by di- and tritriazol-5-ylidene ligands. The vibration frequencies of Ln·[Rh(CO)2Cl]n (n = 1–3) complexes indicate that the electron-donor properties of the new ligands are comparable to those for previously reported MIC complexes and superior to classical NHCs. Prompting coordination of the vicinal phenoxy group to the metal centers proved unsuccessful after treatment of the Ln·[Rh(CO)2Cl]n and Ln·[Pd(allyl)Cl]n (n = 1–3) precursors with AgBF4; the expected chelated cationic complexes were highly unstable, indicating a weak or no coordination availability through the oxygen atom. Crystal structures of the complexes L1·[AuI] and L2A·[Pd(allyl)I]2 illustrated the metal center geometrical environment and confirmed the lack of coordination through the phenoxy moiety of the ligand. Preliminary catalytic trials established the enhanced performance of di- and trimetallic palladium complexes in cross-coupling reactions and the intramolecular cyclization of enynes catalyzed by gold complexes.