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

Abstract

N,N'-Dibenzyl-4-amino-2-iminopent-3-ene, nacnac Bn H, was obtained in a one-step synthesis starting from benzylamine and acetylacetone. Reaction of nacnac Bn H with CuOtBu in the presence of various Lewis bases gave the corresponding copper complexes (nacnac Bn )CuL (L: 2, styrene; 3, η 2acrylonitrile; 4, allyl phenyl ether; 5, stilbene; 7, xylylisonitrile; 8, triphenylphosphine). With diphenylacetylene (DPA) the dimeric complex {(nacnac Bn )Cu} 2 (μ-DPA), 6, was obtained. In the presence of excess DPA, 6 coordinates additional acetylene to form the monomeric complex (nacnac Bn )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 backbonding toward the olefin.

Synthesis and structures of isopropyl-β-diketiminato copper(I) complexes

“…While bridging coordination of acetonitrile is rare, it is not unknown. 27 Monomer-dimer equilibria similar to equilibrium [3] in Scheme 2 have been observed for nacnac Bn Cu, which forms a diphenylacetylenebridged dimer in the solid state and a monomeric complex in solution when excess diphenylacetylene is present, 12 and for nacnac Ar Cu complexes, which display an equilibrium between a monomeric and a dimeric benzene-coordinated complex in solution. 7,25 While single-crystal diffraction studies confirmed the formation of 4, we were unable to obtain satisfactory elemental analyses, even from the crystalline material.…”

“…For copper(I), they are limited to applications in atomic layer deposition 9 or analyses of copper-ligand bonding. 10 In continuation of previous work on diketiminate copper complexes with N-alkyl substituents, 11,12 we report here the syntheses and characterization of N,N'-diisopropyl nacnac Cu I complexes. Just prior to submission of this manuscript, Arii et al reported nacnac i-Pr Cu germylene complexes, using the same ligand.…”

“…Only one resonance was observed for MeCN. Fast exchange of coordinated and free Lewis base was observed before in nacnacCuL complexes, 11,12,25 and the amount of coordinated MeCN in the two species thus cannot be derived from NMR. Only one peak for n CN was observed at 2259 cm -1 in the IR spectrum of 4 in toluene solution, 5 cm -1 higher than that of free MeCN.…”

“…We employed a one-step procedure for condensation of acetylacetone with isopropylamine in the presence of equimolar amounts of para-toluenesulfonic acid, 16 which afforded 1 directly, albeit in a relatively low yield of 23% after 5 days of reflux (Scheme 1). The analogous ligands nacnac Bn H and nacnac i-Bu H, bearing primary alkyl groups, have been obtained in 1-2 days using the same procedure, 12,17 while the preparation of nacnac CH(Me)Ph H required 5 days. 11 Longer reaction times thus seem to be necessary for secondary amines.…”

“…11 On the other hand, Cu-styrene complexes are readily obtained with nacnac Mes (Mes = 2,4,6-Me 3 C 6 H 3 ) 18 or nacnac dipp (dipp = 2,6-i-Pr 2 C 6 H 3 , see Experimental section), one of the most sterically encumbered N-aryl diketiminates, as well as with nacnac Bn or nacnac i-Bu . 12,17 In-plane coordination of the two carbon atoms of an olefin is thus possible for diketiminate ligands with aryl or primary alkyl substituents, but not if secondary alkyls are present on the nitrogen. The implied increased steric demand of nacnac i-Pr compared with nacnac dipp has to be set into contrast to observations for other systems.…”