Syndiotactic and Isotactic Specific Metailocene Catalysts with Hapto-flexible Cyclopentadienyl-Fluorenyl Ligand

Abstract

The unbridged metallocenes can generally be classified as stereo-chemically non-rigid molecules. The fast rotation of the aromatic rings about their bond axis to the transition metal attributes a very high fluxionallity to these molecules. Non-rigid character can be considered for bridged metallocenes if one extends the notion of fluxionallity of the ring(s) to their capability of rapidly and reversibly changing their bonding order (hapticity) to the transition metal. The hapto-tropic behavior of metallocenes with substituted and unsubstituted cyclopentadienyl ring(s) is known as common occurrence in transition metal organometallic chemistry and homogeneous catalysis [1]. The hapto-flexible aromatic ligands bound to the transition metal can facilitate the ligand exchange reaction by lowering “temporarily” the hapticity in the transition state and permitting the increase of the formal co-ordination number without breaking the canonic electronic rules [2]. In this article we have reviewed the basic ideas of stereoselectivity in the light of recent metallocene structure discoveries revealing the presence of haptotropy. The quasi five fold increase of the molecular weight of the syndiotactic polypropylene produced with diphenylmethylidene-μ-(cyclopentadienyl-fluorenyl)ZrCl₂/MAO catalyst system with respect to the molecular weight of the syndiotactic polymer produced with the parent isopropylidene-μ-(cyclopentadienyl-fluorenyl)ZrCl₂ is brought in direct relation to the difference in hapticity of these molecules in solution and in their cationic forms as active species. The experimental proof for this assumption is given unequivocally through facile hydrogenation of the fluorenyl’s six-member rings in the former and complete inertness of the benzenic rings of the latter to the hydrogenation. In an extension of the same idea haptotropy is also proposed to be responsible for the formation of short blocks of syndiotactic sequences in predominantly isotactic chains formed with the zirconocene isopropylidene-μ-(3-trimethylsilylcyclopentadienyl-fluorenyl)zirconium dichloride. It is further demonstrated that a change in the size and nature of the catalyst’s substituents could increase the probability of occurrence of the haptotropic behavior. The possibility of haptotropy being involved in occasional isospecific/syndiospecific site transformation via a reversible η⁵↔η³↔η¹ mechanism is discussed. The argument is reinforced by introduction of a new syndiotactic specific monocyclic η⁵, η¹ metallocene structure exhibiting similar symmetry properties.