Di-, Tri-, and Tetranuclear Complexes of Ni, Pd, and Zn with Oxalamidinato Bridges: Syntheses, Structures and Catalytic Reactions

The reaction of oxalic amidines R¹N=C(NHR²)−C(NHR²)=NR¹ (oxamH<INF>2</INF>) with 2 equiv. (allyl)<INF>2</INF>M or (acac)<INF>2</INF>M (M: Ni, Pd) results in the formation of dinuclear complexes [(T)M(oxam)M(T)] containing dianionic bridging oxalamidinate and η³-allyl or acac as terminal ligands T. The complexes were characterized by elemental analyses, mass spectrometry, and ¹H and ¹³C NMR spectroscopy. In addition, the solid-state structures of 1 (M: Ni; T: η³-allyl; R¹ = p-tolyl; R² = mesityl), 2 (M: Ni; T: η³-allyl; R¹ = R² = mesityl), 9 (M: Pd; T: acac; R¹ = phenyl; R² = p-tolyl), and 10 (M: Pd; T: acac; R¹ = R² = p-tolyl) were determined by X-ray single-crystal diffraction analyses. All complexes contain the metal ions in a square-planar environment. Furthermore, in the η³-allyl complexes two isomers are found in which the two allyl groups are oriented in syn and anti positions. Trinuclear complexes of the composition [(T)M(oxam)M¹(oxam)M(T)] (M¹: Zn; M: Ni or Pd) are obtained by treating Et<INF>2</INF>Zn with 2 equiv. of an oxalic amidine in toluene, followed by addition of 2 equiv. M(acac)<INF>2</INF> or Pd(allyl)<INF>2</INF>. The synthesis and solid-state X-ray analysis of the tetranuclear complex [(T)M(oxam)M¹(oxam)M¹(oxam)M(T)] (16: M = M¹: Zn; oxam: R¹ = p-tolyl; R² = mesityl) is also described. A number of Ni^II complexes oligomerize or polymerize ethylene upon activation with MAO or Et<INF>3</INF>Al. The catalyses can be adjusted by the bulkiness of the nitrogen substituents, the nature of the cocatalyst, and the nuclearity of the complexes. The most active and selective polymerization catalyst is the heterotrinuclear NiZnNi complex 12 which contains two tetramesityloxalamidinato bridges and acac as terminal ligands T. Complex 12 can be activated by using only 2 equiv. Et<INF>3</INF>Al per Ni. Initial efforts show that the palladium complexes are catalysts for the Heck coupling.