Vanadium-promoted aldol condensation and pinacolic coupling of acetylpyrrole: Formation of two new potent dinuclear catalysts for olefin copolymerization

Reaction of VCl3(THF)(3) with 3 equiv of the potassium salt of acetylpyrrole, K[alpha-(CH3CO)C4H3N], afforded the dinuclear {V-2[K(THF)](2)[CH3CO(C4H3N)](2)} [(C4H3N)C(O)CH2C(C4H3N)(CH3)O](2) (1), where four acetylpyrrolide anions from two vanadium moieties have undergone an aldol condensation. The two pyrrolyl-acetylacetonate units so generated work as tetradentate binucleating ligands holding together the dinuclear structure. A similar reaction carried out on the divalent [V2Cl3(THF)(6)](2)[Zn2Cl6] salt afforded the dinuclear and trivalent {VK(THF)[(C4H3N)C(O)CH3]}(2)[(C4H3N)(CH3)(O)CC(O)(CH3)(C4H3N)].0.5(toluene) (2), where two acetylpyrrolide ligands, one per vanadium atom, have undergone a reductive coupling to form a pinacol. Even in this case the dipyrrolylpinacol works as a tetradentate binucleating ligand holding together the dinuclear frame. Attempts to perform the same reaction on another V(II) salt such as VCl2(TMEDA)(2) (TMEDA = N,N,N',N'-tetramethylethylenediamine) afforded instead the corresponding divalent derivative V(TMEDA)[(C4H3N)C(O)CH3](2) (3), where the acetylpyrrolide anions remained intact. Both 1 and 2 are potent olefin copolymerization catalysts upon activation with diethyl aluminum chloride, while 3 displays low but not negligible activity.