Synthesis and Reactivity of Alkylaluminum Adducts of Zirconium Ketene and Ketone Complexes (1987)

A family of alkylaluminum adducts of zirconium ketene and ketone complexes has been prepared. Treatment of dimeric bis(cyclopentadienyl)zirconium ketene complexes with alkylaluminum reagents affords trinuclear Zr2Al bridging ketene complexes of formula [Cp2Zr(C,O-η2-OCCHR)]2(μ-AlR2)(μ-X). X-Ray structural characterization of these complexes reveals several novel features. Two zirconium ketene ligands are spanned by symmetric dialkylaluminum and hydride, chloride, or methyl ligands to form slightly puckered 6-membered rings. A notable feature of these structures is the coordination of the bridging ligand X (H, Cl, or Me), which is characterized by a large Zr-X-Zr angle and an unusual hybridization for the bridging methyl group. The coordination of this methyl group represents a new bonding geometry for carbon, a trigonal-bipyramidal configuration between two metal centers. This geometry models intermediates in alkyl transmetallations that proceed with inversion and implies that transmetallations with inversion should be facile for electrophilic metal centers. The bridging methyl complex [CP2Zr(C,O-η2-OCCHCH2CMe3)]2(μ-AlMe2)(μ-CH3) reacts with acetylene to give an oxymetallacyclopentene. Carbonylation of the bridging methyl complex produces an acyl-enol complex—an unprecedented transformation for a group 4 ketene complex and one that is relevant to the behavior of ketene intermediates over Fischer-Tropsch catalysts. Zirconium chloro acyl complexes react with alkylaluminum reagents to give alkyl-aluminum adducts of zirconium ketone complexes. Mechanistic studies of this reaction provide strong support for a stepwise mechanism involving transmetallation to form a zirconium alkyl acyl complex followed by reductive coupling of the zirconium alkyl and acyl ligands. A significant feature of these studies is the observation that aluminum reagents dramatically accelerate the reductive coupling of zirconium alkyl and acyl ligands. The ketone complexes react readily with olefins, alkynes, and ketones in reactions that should prove useful in organic synthesis. Insertion of ethylene into the zirconium ketone ligand yields oxymetallacyclopentanes, which can be hydrolyzed to tertiary alcohols. Insertion of alkynes into the zirconium ketone ligand affords oxymetallacyclopentenes. Terminal alkynes react with high regioselectivity to give oxymetallacyclopentenes with the alkyl substituent α to the metal center. Hydrolysis of these complexes yields tertiary allyl alcohols. The ketone complexes couple with organic ketones to give diolates. Hydrolysis of the diolates affords 1,2-diols. These results demonstrate that zirconium ketone complexes, in contrast to later transition metal ketone complexes, induce the reductive coupling of organic carbonyl substrates to give diolates, a reaction that may prove useful for the preparation of 1,2-diols. Trinuclear Zr2Al ketone complexes are formed in the reaction of two equivalents of alkyl acyl zirconium complexes and one equivalent of an aluminum reagent. These complexes appear structurally similar to the trinuclear Zr2Al ketene complexes discussed above.