Unusual Reactivity of N,N,N ,N -Tetramethylethylenediamine-Coordinated Neutral Ni(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, (O)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(αCH2βCH3)(κ2-tmeda)] (2-κ2-tmeda). Ethylene is primarily dimerized to butenes, which qualitatively parallels the reactivity observed for tmeda-free solutions of 2-dmso, but tmeda-coordinated Ni(II)-alkyl complexes appeared unreactive, i.e., dormant, toward ethylene. Carrying out the aforementioned reaction under aqueous conditions revealed that hydrolysis of Ni(II)-Me species to methane is a relevant deactivation pathway of the catalyst precursor, which clearly contrasts the reactivity observed in the absence of tmeda. Observed pseudo-first-order rate constants of overall disappearance of 1-tmeda split into two independent contributions according to kobs,Me,1-tmeda= kins,Me,1-tmeda + khydr,1-tmeda[water], khydr,1-tmeda= 1.9 × 10−4 M−1 s−1, and kins,Me,1-tmeda ≈ 2.4 × 10−4 M−1 s−1 (0.1 M [C2H4]). Determination of activation parameters of the bimolecular elimination of ethane from 1-tmeda (ΔH= 97 ± 7 kJ mol−1 and ΔS ≈ −5 J mol−1 K−1), a generally relevant deactivation mechanism of Ni(II)-methyl complexes, points out that tmeda-coordinated Ni(II)-methyl complexes, despite being inactive toward activation with ethylene, are actively involved in decomposition reactions.