A Study on the [3+2] Cycloaddition Reaction of Square Planar Ni(II) Azido Complexes: Structure, Properties, and Catalytic Applications of the Products

Two square planar Ni(II) azido complexes [Ni(N₃)(L1)] and [Ni(N₃)(L2)] (where L1 = N-phenyl-2-(pyridin-2-ylmethylene)hydrazine-1-carbothioamide; L2 = (E)-1-(((2-(diethylamino)ethyl)imino)methyl)naphthalen-2-olato) were used to study the effect of auxiliary ligands on the [3+2] cycloaddition reactions with different dipolarophiles. The reactivity of the complex [Ni(N₃)(L1)] was greater than that of the complex [Ni(N₃)(L2)]. [Ni(N₃)(L1)] gives an N2-triazolato product with an electron-deficient alkyne R₁─C≡C─R₂ with R₁ = R₂ = COOCH₃, COOEt, or R₁ = CF₃, R₂ = COOEt while [Ni(N₃)(L2)] gives a homobimetallic bis(μ-NN’-triazolato) bridged product only with F₃C─C≡C─COOEt. The complex [Ni(N₃)(L2)] reacts with dialkyl acetylene-dicarboxylate alkyne, yielding N1-triazolato products under strictly anhydrous conditions, whereas the same reaction under ambient conditions yielded a new unexpected octahedral complex in which the alkyne is converted into a novel O, O donor bidentate ligand. The nature of the predominant triazolato isomer (N1/N2) was experimentally confirmed by single-crystal X-ray diffraction analysis and also verified by DFT calculations. [Ni(N₃)(L1)] gives a homobimetallic bis(μ-tetrazolato) bridged product by the [3+2] cycloaddition reaction of 2-cyanopyridine and 2-cyanopyrimidine. [Ni(N₃)(L1)] also underwent a 1,3-dipolar cycloaddition with phenyl isothiocyanate at room temperature, giving the corresponding tetrazolato-thione complex, while the same reaction with [Ni(N₃)(L2)] does not proceed. Both complexes give Ni(II) isothiocyanate complexes by the reaction of carbon disulfide. The catalytic activities of all the Ni(II) complexes were evaluated for the synthesis of 2-amino-3-cyano-4H-pyran derivatives. [Ni(triazolate^COOMe,COOMe–N²)(L1)] (complex 3) emerged as a highly efficient catalyst, demonstrating performance significantly superior to previously reported catalysts at room temperature. 0.0001 mol % catalyst loading is sufficient to obtain the product, and the highest turnover number (680000) and turnover frequency (34000 min^–1) were achieved.