π Back-Bonding in Dibenzyl-β-diketiminato Copper Olefin Complexes

N,N'-Dibenzyl-4-amino-2-iminopent-3-ene, nacnacBnH, was obtained in a one-step synthesis starting from benzylamine and acetylacetone. Reaction of nacnacBnH with CuOtBu in the presence of various Lewis bases gave the corresponding copper complexes (nacnacBn)CuL (L: 2, styrene; 3, η2-acrylonitrile; 4, allyl phenyl ether; 5, stilbene; 7, xylylisonitrile; 8, triphenylphosphine). With diphenylacetylene (DPA) the dimeric complex {(nacnacBn)Cu}2(μ-DPA), 6, was obtained. In the presence of excess DPA, 6 coordinates additional acetylene to form the monomeric complex (nacnacBn)Cu(DPA), 6b. All complexes, with the exception of 4 and 6b, were characterized by X-ray diffraction studies. Structural and spectroscopic data indicate that π back-bonding in 2−8 is still weak when compared to other transition metals, but stronger than in most Cu(I) complexes. Olefin exchange experiments indicate preferred binding of electron-deficient olefins. Reaction of 3 with morpholine did not yield any hydroamination products, in agreement with significant π back-bonding toward the olefin.