Ambidentate character of the 6-aminofulvene-2-aldiminate ligand containing both diimine and cyclopentadienyl donors

The 6-aminofulvene-2-aldiminate (AFA) ligand contains both cyclopentadienyl and diimine donors. The ligand preferentially coordinates to transition metals via the nitrogen atoms; however once these are occupied, the cyclopentadienyl ring may be coordinated to a Cp*Ru+ unit, providing bi- and trimetallic species. In the tetrahedral [Zn(Ph2AFA)2] (2) the metal is located approximately in the planes of the two ligands; however in the square-planar [Pd(Ph2AFA)2] (1) there is a severe distortion of the coordination and the metal is located almost 1.3 Å out of the planes of the two ligands. This situation arises to avoid the steric interaction of the ligand phenyl substituents. Treatment of 2 with [Cp*Ru(NCMe)3][BF4] gives [Cp*Ru(η5-Ph2AFA)] (3), providing the first example of the cyclopentadienyl coordination of an AFA ligand in which the nitrogen donors are vacant, and it is thought to form as a result of the fragmentation of an intermediate mixed Zn/Ru species. The X-ray structure of 3 provides some evidence for an interaction between the Ru center and one of the exocyclic carbon atoms, thus suggesting a fulvene form of the ligand in this complex. Similar treatment of 1 with [Cp*Ru(NCMe)3][BF4] provides a trimetallic PdRu2 species (4) in which both of the cyclopentadienyl rings in the complex 2 are coordinated to Ru. The complex [(Ph2AFA)Pd(Me)PPh3] (5) is formed on treatment of [(COD)PdMe(Cl)] with NaPh2AFA in the presence of PPh3, and preliminary investigations have shown that its activation with [Ni(COD)2] as a phosphine scavenger provides species active toward ethylene oligomerization and polymerization.