Formation of Ni(I) Species upon Activation of 5,6,7-Trihydroquinolin-8-imine Nickel(II) Dihalides with MAO and MMAO

Abstract

Reactions between (E)-N-(2,6-diisopropylphenyl)-2-methyl-6,7-dihydroquinolin-8(5H)-imine nickel(II) dihalides and organoaluminum compounds AlMe₃, AlⁱBu₃, MAO, and MMAO were monitored using EPR spectroscopy in situ. It was found that rapid one-electron reduction of the Ni(II) center results in formation of a variety of monovalent nickel species which are stable at room temperature in an oxygen-free atmosphere. On the basis of the EPR data obtained, these compounds were assigned to the heterobinuclear nickel-aluminum complexes of the type LNi^I(μ-R)₂AlX₂, where L = N,N′-donor ligand, R = Me, ⁱBu, H, Br, or Cl bridge, and X = Me, ⁱBu, Br or, Cl, depending on conditions. Formation of the dihydride-bridged LNi^I(μ-H)₂AlX₂ complex was confirmed using deuterium-labeled compounds. In the presence of an excess of ethylene or 1-hexene, LNi^I(μ-R)₂AlX₂ eliminates RAlX₂ to afford LNi^IR(C₂H₄) or LNi^IR(C₆H₁₂) species, respectively. Structural assignments of the Ni^I species are supported by DFT calculations of their g-tensor values.