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(η 3 -C 4 H 7 )(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(SiMe 3 ) 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 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)(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.