NMR spectroscopic identification of Ni(ii) species formed upon activation of (α-diimine)NiBr2 polymerization catalysts with MAO and MMAO

Abstract

The nature of Ni(II) species formed upon the activation of the Brookhart's α-diimine polymerization pre-catalyst LNiBr₂ with MAO and MMAO (L = 1,4-bis-2,4,6-dimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene) has been established using ¹H and ¹³C NMR spectroscopy. The heterobinuclear ion pair [LNi^II(μ-Me)₂AlMe₂]⁺[MeMAO]⁻ is observed at the initial stage of the reaction of LNiBr₂ with MAO at −40 °C, whereas the ion pair [LNi^II–^tBu]⁺[MeMMAO]⁻ predominates at the initial stage of the reaction of LNiBr₂ with MMAO under the same conditions. At higher temperatures, both ion pairs transform into a Ni(I) species displaying an axially anisotropic EPR spectrum (g_‖ = 2.21, g_⊥ = 2.06, A_⊥ = 1.06 mT).