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Bent Metallocenes

Introduction

Bent Metallocenes were brought to the forefront organometallic chemistry shortly after the discovery of ferrocene in 1951. Ferrocene (I) is a transition metal complex in which an Iron (Fe) atom is sandwiched between two parallel cyclopentadienyl (Cp) rings. In this structure the metal coordinates to the π-electron system of the ring instead of a particular atom. These type compounds are now commonly referred to as metallocenes. In bent metallocenes, the ring systems coordinated to the metal are not pararell, but are tilted at an angle. How much the angle is tilted or bent is compound specific and is determined by steric and electronic properties. Some of the first bent metallocenes were introduced by Wilkinson and Birmingham, most notably their report of Cp2TiCl2 (II) and Cp2ZrCl2 (III).



Preparation

Bent metallocenes can be synthesized by a variety of different methods. In the original report by Wilkinson and Birmingham bis-cyclopentadienyl halides of Ti, Zr, V, Nb and Ta were made by the reaction of cyclopentadienylmagnesium chloride or bromide with the metal chloride or bromide in a benzene-ether mixed solvent solution. A more convenient alternative procedure, generally producing higher yields, was the reaction of cyclopentadienylsodium with the metal halide in either tetrahydrofuran or 1,2-dimethoxyethane. In the Grignard method, cyclopentadienylmagnesium chloride or bromide was prepared from ether solutions of either n-propylmagnesium chloride or ethylmagnesium bromide, respectively. Derivatizing the basic bent metallocene structure has led to many new structures and there are now thousands of known bent metallocenes. The aforementioned Titanium bent metallocene and Zirconium analogue are synthesized as follows:


 * 2 NaC5H5 + TiCl4 → (C5H5)2TiCl2 + 2 NaCl


 * ZrCl4(THF)2 + 2 NaCp → Cp2ZrCl2 + 2 NaCl + 2 THF

Reactivity

The reactivity of bent metallocenes is dependent upon the molecular orbital diagram and the number of d electrons the metal has in a particular compound. These compounds are versitible and therefore their reactivity is nearly unlimited.

Some important reactions and developments are shown below:


 * Bent Metallocenes can lead to other important reagents

(C5H5)2ZrCl2 + 1/4 LiAlH4 → (C5H5)2ZrHCl “Schwartz’s Reagent” + 1/4 "LiAlCl4"


 * Bent Metallocenes can also be used to create other metal-metal bonds




 * Bent Metallocenes have been functionalized at the cyclopentadienyl ring, this idea has created an entire host of new compounds. Rausch et al. used the same method Hafner et al. used to prepare functionalized fulvene systems and were able to prepare bent metallocenes (titanocenes and zirconocenes) with a variety of different functional groups at the Cp Ring. Example:




 * Bent metallocenes with methyl groups on the Cp rings (Cp*) can exhibit Tuck-in Chemistry

Major Utility

Olefin Polymerization Catalyst

Bent metallocenes as olefin polymerization catalysts date back to 1957 when Natta reported the polymerization of ethylene with the titanocene catalyst Cp2TiCl2 and the cocatalyst trimethyl aluminum, a cocatalyst traditionally used in Ziegler-Natta olefin polymerization systems. At this point it was believed that the metallocene was essentially useless in terms of the making the product available commercially due to the low catalytic activity of the metallocene with the Ziegler-Natta cocatylst. In the 1970’s in the laboratory of Professor Walter Kaminsky an accident led to a major discovery. When analyzing the Cp2Zr2Cl2/Al(CH3)3 polymerization reaction, trace amounts of water was introduced into the rector which led to a highly active ethylene polymerization system. After further analysis of the system it was realized that trimethyl aluminum Al(CH3)3, was being hydrolyzed producing the necessary cocatalyst methylaluminoxane (MAO or (Al(CH3)O)n). This sparked interest in the field and derivatives of the basic Kaminsky structures are still amongst the most active catalyst ever synthesized for the production of polyethylene and polypropylene.



References