Unusual Reactivity of N,N,N′,N′-Tetramethylethylenediamine-Coordinated Neutral Nickel(II) Polymerization Catalysts

Abstract

Tmeda-coordinated species [(N,O)NiCH3(tmeda)] (tmeda = N,N,N′,N′-tetramethylethylenediamine), obtained by reaction of [(tmeda)NiMe2] with salicylaldimines, (NO)H, are reactive and versatile intermediates for olefin polymerization catalysis. Solution NMR spectroscopic studies of 1-tmeda (N,O= 2,6-(3,5-(F3C)2C6H3)2C6H3-N═CH-(3,5-I2-2-OC6H2)) revealed two major binding modes of the tmeda ligand, open κ1- and, unexpectedly, chelating κ2-fashion, which interconvert slowly on the NMR chemical shift time scale, and form equilibria with solvent complexes 1-L (L= dmso, methanol). Binding of tmeda is favored by 2−3 orders of magnitude at the temperatures studied (25 to 80 °C) over binding of solvent. Chelating κ2-coordination of tmeda renders the monoanionic bidentate salicylaldiminato ligand κ1-coordinate. Exposure of dmso solutions of 1-tmeda to excess ethylene in an NMR tube at 55 °C resulted in the very minor formation of propylene and an equilibrium mixture of Ni(II)-ethyl complexes 2-dmso and [(κ1-N,O)Ni(αCH...