Strongly Electron-Donating Triazolylidene Ligands: Cationic Metal Carbonyl Complexes of 1‑Methyl-1,2,3-triazole as Triazolium Surrogates

A new strategy for tuning the electronic
properties of 1,2,3-triazol-5-ylidene
metal complexes is reported using {Mo­(η3-C4H7)­(bipy)­(CO)2} or {Re­(bipy)­(CO)3} fragments as substituents at the triazole N3 atom. The reaction
of cationic molybdenum­(II) and rhenium­(I) 1-methyl-1,2,3-triazole
compounds with the strong base KN­(SiMe3)2 in
the presence of electrophilic metal fragments, such as AgOTf (OTf
= trifluoromethanesufonate) or [CuCl­(IPr)] [IPr = 2,6-(diisopropyl)­phenylimidazol-2-ylidene]
affords a new type of 1,2,3-triazol-5-ylidene complexes. For silver­(I)
cationic bis­(triazolylidene), [Ag­(tzNHCM)2]­OTf
(M = [Mo], 2; [Re], 4), complexes are obtained,
whereas in the case of Cu­(I) mixed normal/mesoionic NHC, [Cu­(IPr)­(tzNHCM)]­OTf (M = [Mo], 7; [Re], 8) complexes
are formed. This special type of mesoionic N-heterocyclic carbenes
bear a metal fragment at the N3 atom of the 1,2,3-triazole moiety,
showing notable enhancement of the carbene electron donor ability
compared to conventional alkyl-substituted analogues. Transmetalation
from cationic silver bis­(triazolylidene) complexes 2 and 4, prepared using this methodology, has proven to be very
efficient toward [M′Cl­(cod)]2 (M′ = Rh, Ir;
cod = 1,5-cyclooctadiene), affording the corresponding cationic bis­(triazolylidene)
[M′(cod)­(tzNHCM)2]­OTf (9–12) complexes. A subsequent reaction with CO­(g)
easily produces substitution of the diene ligand, affording the corresponding cis-dicarbonyl [M′(CO)2(tzNHCM)2]­OTf (13–16) compounds.