Activation of an α-Diimine Ni(II) Precatalyst with AlMe<sub>3</sub> and Al<sup>i</sup>Bu<sub>3</sub>: Catalytic and NMR and EPR Spectroscopy Studies
Igor E. Soshnikov, Nina V. Semikolenova, Konstantin P. Bryliakov, Artem A. Antonov, Wen‐Hua Sun, Evgenii P. Talsi
Abstract
The nature of Ni(II) and Ni(I) species formed upon the activation of the N,N-α-diimine Ni(II) precatalyst LNiIIBr2 (1) with AlMe3, MMAO, and AliBu3 has been studied by NMR and EPR spectroscopy (L = 1,4-bis(2,4,6-trimethylphenyl)-2,3-dimethyl-1,4-diazabuta-1,3-diene). It has been shown that the paramagnetic complex 3AlMe3 with the proposed structure [LNiI(μ-Me)2AlMe2] or [(L•–)NiII(μ-Me)2AlMe2] predominates in the reaction solution of 1/AlMe3 at Al/Ni > 10. Upon activation of 1 with MMAO (Al/Ni > 20–30), nickel predominantly exists in the form of the structurally similar complex 3MMAO ([LNiI(μ-Me)2AliBu2] or [(L•–)NiII(μ-Me)2AliBu2]). The reaction of 1 with AliBu3 leads to rapid ligand scrambling, affording the EPR-active complex 4 ([(L•–)AliBu2]). The latter process is accompanied by Ni reduction to the zerovalent state. Possible roles of the observed species in the catalytic systems are discussed.