Formation of 1,3-Diene zirconocenes and -hafnocenes

The potential surface of the parent conjugated diene system, 1,3-butadiene, exhibits two local minima. These correspond to the two possible planar conformations of the C4H6 molecule, distinguished by the arrangement of the vinyl moieties relative to their connecting tr-bond. 

Some years ago we observed that Group IV metallocenes, i.e., the elusive monomeric units bis(i7-cyclopentadienyl)zirconium and -hafnium, readily form 1 1 complexes with conjugated dienes in their 5-trans conformation (22). To our knowledge, these complexes (3) are the first and, at the time, only known examples of mononuclear (i-/ranj-i7 -diene)metal complexes, i.e., compounds in which both double bonds of the i-irans conformer of a conjugated diene are coordinated to the same transition metal center 21). Some examples of this class of transition metal complexes are stable, isolable compounds even at room temperature 23). 

Two major factors are likely to be responsible for the relative stability of (5-rrani-i7 -diene)zirconocene and -hafnocene complexes. It seems that the characteristic bonding features of the bent metallocene unit thermodynamically favor coordination of the 5-trans over the i-cis conjugated-diene 

In addition, the experimental observation of (s-/rani-ij -diene)metal-locene complexes is kineticcdly facilitated by the special appearance of the energy profile of the dienemetallocene system. For the parent system there is evidence (27-31) that the stable (butadiene)ZrCp2 isomers [about equal amounts of (s-trans- (3a) and (s-cis-T]i -butadiene)zirconocene (5a) are obtained under equilibrium conditions (22, 23)] are connected through a reactive (T -butadiene)zirconocene intermediate (4a) which rapidly equilibrates with 5-tra/u-diene complex 3a, but is separated from the (s-cis-diene)metallocene isomer 5a by a rather substantial activation barrier 

Valence orbitals of the bent metallocene unit (from ref. 24), and schematic representation of the energy profile of the (butadiene)ZrCp2 system. 

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