Influence of the Ligand's Electronic Effect on iClick Reaction of Ni(II)–Azido Complexes with Alkynes: Synthesis, Structure, Characterization, and Kinetic Studies

The synthesis and characterization of two new square planer Ni(II)-azido complexes [Ni(N₃)(L)]^-1 of 1-N₃ and 2-N₃ using electronically varied N/S/O donor tridentate ligands (where H₂L^OMe: (E)-2-(((2-mercaptophenyl)imino)methyl)-6-methoxyphenol), H₂L^t ^Bu: (E)-2,4-di-tert-butyl-6-(((2-mercaptophenyl)imino)methyl)phenol) were reported. Azido moiety in Ni-azido complexes was found as more dipolar than free azide ion. Six number of [3+2] cycloaddition coupling product of 1-N₃/2-N₃ and three different alkynes were isolated and well characterized as [Ni(triazolate)(L)]^-1 (1^/ ^t ^Bu-T) complexes. In triazolate complexes originated from terminal alkyne, the triazolate ligand was coordinated via the N1-T atom whose (1-T²) crystal structure was presented. The triazolato products of symmetrical and unsymmetrical nonterminal alkynes were isolated as N2-T mode of binding. The conversion of N1-T/N3-T to N2-T as well as the stability in a particular mode of binding was proposed as thermal control of their equilibrium based on the variable temperature ¹⁹F NMR studies on 1-T³/2-T³. The detail kinetic studies resulted that the reaction with methyl propiolate follows zero-order rate law, whereas other two alkynes showed first-order rate. Mostly, the electron-poor ethyl-4,4,4-trifluorobut-2-ynoate always react faster than dimethyl but-2-ynedioate, and Ni(II)-azido complex with L^t ^Bu ligated always react with a particular alkyne in a faster rate than the same of L^OMe.