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

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.