Dinuclear Nickel and Palladium Complexes with Bridging 2,5-Diamino-1,4-benzoquinonediimines: Synthesis, Structures, and Catalytic Oligomerization of Ethylene

Dinuclear, divalent acetylacetonato (acac) complexes of the type [M(acac){μ-C6H2(NR)4}M(acac)] (M = Ni, Pd)
have been prepared by the reaction of the corresponding bis(acac) metal precursor with 2,5-diamino-1,4-benzoquinonediimines C6H2(NHR)2(NR)2 (4a, R = CH2-t-Bu; 4b, R = CH2Ph; 4c, R = Ph), which are metalated
and become bridging ligands, also like in the complex [(C8H11)Pt{μ-C6H2(NCH2-t-Bu)4}Pt(C8H11)] (6) obtained
by the reaction of 4a with [PtCl2(COD)]. The complexes were fully characterized, including by X-ray diffraction for
[Ni(acac){μ-C6H2(NCH2Ph)4}Ni(acac)] (9b) and [Pd(acac){μ-C6H2(NCH2-t-Bu)4}Pd(acac)] (10a). The coordination geometry around the metal ions is square-planar, and a complete electronic delocalization of the quinonoid π
system occurs between the metal centers over the two NCCCN halves of the ligand. The nature of the
N substituent explains the differences between the supramolecular stacking arrangements found for [Ni(acac){μ-C6H2(NR)4}Ni(acac)] (9a; R = CH2-t-Bu; 9b, R = CH2Ph). The Ni complexes were evaluated as catalyst precursors
for ethylene oligomerization in the presence of AlEtCl2 or MAO as the cocatalyst, in particular in order to study
possible cooperative effects resulting from electronic communication between the metal centers and to examine
the influence of the N substituent on the activity and selectivity. These catalysts afforded mostly ethylene dimers
and trimers.