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(eta(3)-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(tzNHC(M))(2)]OTf (M = [Mo], 2; [Re], 4), complexes are obtained, whereas in the case of Cu(I) mixed normal/mesoionic NHC, [Cu(IPr)(tzNHC(M))]OTf (M = [Mo], 7; [Re], 8) complexes are formed. This special type of mesoionic N-heterocyclic carbenes bear a metal fragment at the N-3 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)(tzNHC(M))(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)(tzNHC(M))(2)]OTf (13-16) compounds.