MONITORING THE ELECTRONIC FEATURES OF RIGID C-1-BRIDGED GROUP-4 ANSA-METALLOCENES BY USING THE DYNAMIC AND STRUCTURAL-PROPERTIES OF THEIR S-CIS-ETA(4)-BUTADIENE COMPLEXES

The six-membered ring annulated C-1-bridged ansa-metallocene [(4-cyclopentadienylidene)-4,7,7-trimethyl-4,5,6,7-tetrahydroindenyl]zirconium dichloride (6a) was treated with ''butadiene-magnesium'' to yield the corresponding (s-cis-eta(4)-butadiene)ansa-zirconocene complex as a mixture of two diastereoisomers (8a/8a'). Their mutual interconversion was monitored by dynamic H-1 NMR spectroscopy at high temperatures. The Gibbs activation energy of the thermally induced 8a reversible arrow 8a' (butadiene)metallocene ring topomerization is Delta G(inv)(not equal) (368 K) = 20.2 +/- 0.5 kcal mol(-1). This is about 8 kcal mol(-1) higher than observed for the analogous (sigma(2), pi-butadiene)metallocene ring inversion process in the parent bis(cyclopentadienyl)zirconium system. The corresponding (s-cis-eta(4)-butadiene)ansa-hafnocene complex (9/9') was prepared analogously. Its (eta 4-butadiene)Hf inversion barrier (Delta G(inv)(not equal) (268 K) = 14.9 +/- 0.5 kcal mol(-1)) is about 7 kcal mol(-1) higher than that of the (s-cis-eta 4-butadiene)HfCp(2) reference. The (s-cis-eta 4-butadiene)[7-n-butyl-(4-cyclopentadienylidene)4,7-dimethyl-4,5, 6,7-tetrahydroindenyl]zirconium complex 8b was characterized by X-ray diffraction. The eta-butadiene ligand exhibits only a small C-C bond alternation (1.38(2), 1.36(2) and 1.40(2) Angstrom and a relatively small difference of the butadiene C-terminal-Zr (2.324(12) and 2.301(13) Angstrom) vs. C-internal-Zr (2.445(13) and 2.422(13)Angstrom bond lengths. This points to an increased a complex character of the rigid ansa-metallocene complex compared with the parent (s-cis-eta 4-butadiene)zirconocene complex 2a whose structure was determined as a reference (2a: butadiene C-terminal-Zr, 2.332(5) and 2.302(5) Angstrom; C-internal-Zr, 2.491(6) and 2.480(5) Angstrom. A theoretical extended Huckel theory analysis has revealed that the pronounced increase in the butadiene metallocene topomerization barrier on going from the ''normal'' bent metallocene (D(1)-Zr-D(2), 126.2 degrees, where D(1) and D(2) denote the centroids of the Cp rings) to the strained C-1-bridged ansa-metallocene complex 8 containing a markedly opened bent metallocene wedge (8b: D(1)-Zr-D(2), 114.7 degrees) is electronic in origin, probably caused by closing the energy gap between the 2a(1) and b(1) frontier orbitals. We conclude that the stereoelectronic features of the bent metallocenes can be controlled by the application of specific steric constraints.