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[α-(CH3CO)C4H3N], afforded the dinuclear {V2[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.