Nickel complexes with click-derived pyridyl-triazole ligands: weak intermolecular interactions and catalytic ethylene oligomerisation

The ligands 1-(cyclohexyl)-4-(2-pyridyl)-1,2,3-triazole (1), 1-(2,6-diisopropylphenyl)-4-(2-pyridyl)-1,2,3-triazole (2), 1-(4-butoxyphenyl)-4-(2-pyridyl)-1,2,3-triazole (3) and 1-(methyl)-4-(2-pyridyl)-1,2,3-triazole (4) were synthesized by the Cu(I) catalyzed Click reaction between 2-pyridylacetylene and the corresponding azides. The ligands were then reacted with NiBr2 center dot 3H(2)O to generate the complexes (1)(2)NiBr2 (1a), (2)(2)NiBr2 (2a), (3)(2)NiBr2 (3a) and (4)(2)NiBr2 (4a). Structural characterization of 1a confirmed the mononuclear and distorted octahedral environment around the Ni(II) center, with the pyridyl-triazole ligands coordinating in a bis-chelating fashion. Bond length analysis inside the 1,2,3-triazole ring shows a short N=N double bond that is flanked by two longer C-N and N-N bonds pointing to the existence of azo character in the ring. The highly polar five-membered 1,2,3-triazole ring makes its C-H bond acidic, and these bonds participate in an extended weak intermolecular C-H center dot center dot center dot Br interactions with the Br-groups of neighboring molecules, resulting in a 3-D network. The nickel complexes with these Click ligands were tested as pre-catalysts for ethylene oligomerization, and the complexes showed moderate activity in that reaction with good selectivity towards C4 oligomers.