Platinum(II)-Complexed Tetrahydroimidazo[1,2-<i>b</i>][1,2,4]oxadiazoles Derived from Metal-Mediated 1,3-Dipolar Cycloaddition. Novel Type of Heterocycles, Which Do Not Exist without the Metal Center

A novel type of heterocyclic ligands, viz., 2-R-3a-R′-5,5,6,6-tetramethyl-3a,4,5,6-tetrahydroimidazo[1,2-b][1,2,4]oxadiazole, were generated by an intermolecular PtII-mediated 1,3-dipolar cycloaddition between the imidazoline N-oxides ONC(R′)NHCMe2CMe2 (R′ = H, Me) and the coordinated nitriles in the complexes cis- and trans-[PtCl2(RCN)2] (R = NMe2, NC5H10, Et). The reaction proceeds smoothly under mild conditions (CH2Cl2, 20−25 °C, 0.5−3 h) and gives the complexes [PtCl2{NC(R)ONC(R′)NHCMe2CMe2}2] (R/R′ = NMe2/Me, cis- and trans-1; NMe2/H, cis- and trans-2; R/R′ = N(C5H10)/Me, cis- and trans-3; N(C5H10)/H, cis- and trans-4; Et/H, cis- and trans-5) in 50−84% yields. These species were characterized by elemental analyses (C, H, N), ESI+-MS, IR, 1H NMR, 13C and NMR spectroscopies, and X-ray data (for trans-2, trans-4, and cis-5). In CDCl3 solution in the light, trans-1−5 and cis-1−5 undergo retro-conversion to furnish the corresponding protonated imidazoline N-oxide and [PtCl2(RCN)2] along with yet unidentified products. Tetrahydroimidazo[1,2-b][1,2,4]oxadiazoles exist only in the coordinated state, and an attempt of liberation of the heterocyclic ligands from trans-3, trans-4, cis-5, and trans-5 by treatment with 2 equiv of 1,2-(diphenylphosphino)ethane at 50−60 °C (1 day for R = Et and 3−5 days for R = NMe2) leads to formation of the free parent imidazoline N-oxides and the nitriles. DFT (B3LYP) calculations allowed the interpretation of the experimental observations and indicated that the cycloaddition reactions are controlled by thermodynamic rather than kinetic factors.