Octa- and Nonamethylfluorenyl Complexes of Zirconium(IV): Reactive Hydride Derivatives and Reversible Hydrogen Migration between the Metal and the Fluorenyl Ligand

Reaction of the zirconocene dichloride Cp′′Flu*ZrCl2 (Cp′′ = 1,3-(SiMe3)2C5H3, Flu* = C13Me9) with iBuLi (iBuLi = LiCH2CHMe2) resulted in elimination of isobutylene and formation of Cp′′(η5:η3-C13Me9H)ZrH (1-syn-1,2-DHF*D), possessing an η5:η3-dihydrofluorenediyl ligand derived from a metal-to-benzo ring hydride transfer. This species undergoes reversible hydride transfer and exists in equilibrium with only one of its three other possible isomers (1-syn-3,4-DHF*D). Compound 1-syn-1,2-DHF*D catalyzes the cyclization of 1,5-hexadiene to methylenecyclopentane, and its reaction with excess isobutylene leads to the elimination of isobutane and formation of the cyclometalated zirconocene isobutyl species (η5:η1-C5H3-1-SiMe2CH2-3-SiMe3)(η5-C13Me9)ZriBu (2). Reaction of Cp′′Flu′′ZrCl2 with iBuLi directly generated the cyclometalated zirconocene species (η5:η1-C5H3-1-SiMe2CH2-3-SiMe3)(η5-C13Me8H)ZriBu (3); however, reaction of the dichloride Cp′′Flu′′ZrCl2 with iBuLi in the presence of hydrogen generated the dihydrofluorenediyl monohydride derivative Cp′′(η5:η3-C13Me8H2)ZrH (4). Treatment of the cyclometalated isobutyl species 3 with H2 led to partial hydrogenation of the Flu′′ ligand and formation of the monohydride Cp′′(η5:η3-C13Me8H6)ZrH (5), which contains a hexahydrofluorenediyl ligand. Partial hydrogenation of the Flu′′ ligand proceeded exclusively via an intramolecular pathway, as evidenced by the all-exo configuration of the methyl groups on the saturated benzo ring. Structural characterization of 1-syn-1,2-DHF*D, 2, 3, and 5 revealed a highly strained η5:η3-coordination mode for the dihydro- and hexahydrofluorenediyl ligands.