Acetylacetonate and Acetate Complexes of Nickel(II) Catalyse the Air Oxidation of Phosphines

The polymeric complex trans-[Ni(acac)₂(µ₂-dppe)]_n·2MeCN {1a, dppe = 1,2-ethylenebis(diphenylphosphine)} is sometimes transiently deposited from the reaction of [Ni(acac)₂] with dppe in MeCN prior to its facile onwards air oxidation to final crystallization of a doubly-oxygenated relative, cis-[Ni(acac)₂(µ₂-dppeO₂)]_n {2, dppeO₂ = 1,2-ethylenebis(diphenylphosphineoxide)}. A similar unsolvated phase of the initial polymer, trans-[Ni(acac)₂(µ₂-dppe)]_n (1b), can be isolated from toluene. The oxidation of dppe by O₂ is catalytic and dppeO₂ was isolated in close to stoichiometric yields from solutions containing 5 % Ni(acac)₂ relative to dppe. The reaction rate slows after a few turnovers due to inhibition by the product. The relative yields of dppeO₂ are higher than those from catalytic air oxidation of methyldiphenylphosphine and triphenylphosphine and we speculate that a pathway for this reaction involves a dimetallic cooperativity enabled specifically by dppe.