iClick Reactions of Square-Planar Palladium(II) and Platinum(II) Azido Complexeswith Electron-Poor Alkynes: Metal-Dependent Preference for N1 vs N2Triazolate Coordination and Kinetic Studies with 1 H and 19 F NMR Spectroscopy

Two square-planar palladium(II) and platinum(II) azido complexes [M(N3)(L)] with L = N-phenyl-2-[1-(2-pyridinyl)ethylidene]hydrazine carbothioamide reacted with four different electron-poor alkynes R–C≡C–R′ with R = R′ = COOCH3, COOEt, COOCH2CH2OCH3 or R = CF3, R′ = COOEt in a [3 + 2] cycloaddition “iClick” reaction. The resulting triazolate complexes [M(triazolateR,R’)(L)] were isolated by simple precipitation and/or washing in high purity and good yield. Six out of the eight new compounds feature the triazolate ligand coordinated to the metal center via the N2 nitrogen atom, but fortuitous solubility properties allowed isolation of the N1 isomer in two cases from acetone. When the solvent was changed to DMSO, the N1 → N2 isomerization could be studied by NMR spectroscopy and took several days to complete. 19F NMR studies of the iClick reaction with F3C–C≡C–COOEt led to identification of a putative early linear intermediate in addition to the N1 and N2 isomers, however with the latter as the final produc...