Two silver(I) complexes {[1-R-4-($N$-t4-butylacetamido)-1,2,4-triazol-5-ylidene]₂Ag}⁺ Cl⁻ [R = Et (1b), 𝑖-Pr (2b)] of $N/O$-functionalized N-heterocyclic carbenes derived from 1,2,4-triazoles are reported. The silver complexes, 1b and 2b, have been synthesized from the reaction of the $N/O$-functionalized triazolium chloride salts namely, 1-R-4-(N-t-butylacetamido)-1,2,4-triazolium chloride [R = Et (1a), 𝑖-Pr (2a)] by treatment with Ag₂O in 53-56% yield. The 1,2,4-triazolium chloride salts 1a and 2a were prepared by the alkylation reaction of 1-R-1,2,4-triazole (R = Et, 𝑖-Pr) with $N-t$-butyl-2-chloro acetamide in 47-63% yield. The molecular structures of the silver(I) complexes, 1b and 2b, have been determined by X-ray diffraction studies. The density functional theory studies on the silver 1b and 2b complexes suggest that the 1,2,4-triazole derived N-heterocyclic carbenes to be strong 𝜎−donating ligands similar to the now much recognized imidazolebased N-heterocyclic carbenes. The reactivity studies with (SMe₂)AuCl and (SMe₂)CuBr indicated the silver complexes, 1b and 2b, to be good transmetallating agents.

Synthesis, structure and density functional theory (DFT) studies of a series of new ruthenium complexes, [1-(R)-3-𝑁-(benzylacetamido)imidazol-2-ylidene]RuCl(𝑝-cymene) [R = Me (1c), 𝑖-Pr (2c), CH₂Ph (3c); 𝑝-cymene = 4-𝑖-propyltoluene] supported over $N/O$-functionalized N-heterocyclic carbene (NHC) ligands are reported. In particular, the ruthenium (1-3)c complexes were synthesized from the respective silver complexes, [1-(R)-3-𝑁-(benzylacetamido)imidazol-2-ylidene]₂Ag⁺Cl− [R = Me (1b), 𝑖-Pr (2b), CH₂Ph (3b)] by the treatment with [Ru(𝑝-cymene)Cl₂]₂ in 65-76% yields. The molecular structures of (1-3)c revealed the chelation of the N-heterocylic carbene ligand through the carbene center and an amido sidearm of the ligand in all of the three complexes. The density functional theory studies on the ruthenium (1-3)c complexes indicated strong binding of the NHC ligand to the metal center as was observed from the deeply buried NHC-Ru 𝜎-bonding molecular orbitals.

A new type of tricyclic triazolooxazine derived N-heterocyclic carbene precursors were developed by the alkylation reaction of a tricyclic triazolooxazine framework. In particular, the reaction of 5a,6,7,8,9,9ahexahydro-4𝐻-benzo[𝑏][1,2,3]triazolo[1,5-𝑑][1,4]oxazine with methyl iodide and ethyl iodide yielded the tricyclic triazolooxazine derived N-heterocyclic carbene precursors, (1−2)a, in 67−84% yield. The tricyclic triazolooxazinium iodide salts, (1−2)a, underwent metallation in a straight forward manner upon treatment with PdCl₂ in the presence of K₂CO₃ in pyridine to give the trans-{3-(R)-5a,6,7,8,9,9a-hexahydro-4𝐻-benzo[𝑏][1,2,3]triazolo[1,5-𝑑][1,4]oxazin-4-ylidene} PdI₂(pyridine) [R = Me (1b), Et (2b)] complexes in 23−25% yield. The new tricyclic triazolooxazine derived N-heterocyclic carbene moiety, as stabilized upon binding to palladium in the (1−2)b complexes, was structurally characterized by the X-ray single crystal diffraction studies.