CRYSTAL-STRUCTURES AND SOLUTION CONFORMATIONS OF THE MESO AND RACEMICISOMERS OF (ETHYLENEBIS(1-INDENYL))ZIRCONIUM DICHLORIDE

The crystal and molecular structures of the C-2 symmetric rac-(EBI)ZrC l2 (1r) [monoclinic, space group I2/c, No. 15, a = 11.957(1) Angstrom, b = 10.627(1) Angstrom, c = 13.775(2) Angstrom, beta = 106.06(1)degre es] and of its meso isomer (1m) [monoclinic, space group P2(1)/n, No. 14, a = 11.119(3) Angstrom, b = 10.467(1) Angstrom, c = 14.949(2) Angs trom, beta = 100.94(2)degrees] have been solved. 1r is in the indenyl- forward (II) conformation, as is the case for most of the chiral ansa ethylene-bridged bisindenyl-type metallocenes. In solution however, al ready at room temperature there is a rapid (NMR time scale) interconve rsion between the two II and Y (indenyl-backward) conformations, as sh own by conformational analysis on the proton spectra of the bridge met hylenes. This equilibrium is shifted toward the lower energy conformat ion delta at lower temperatures and is influenced by both the solvent and the sigma-ligands. The solid state structure of Im shows that this meso form is actually in a chiral conformation (C-1 symmetry) because of the staggered placement of the two indenyl ligands (Ind boolean AN D Ind = 10.0 degrees). In solution this aspecific zirconocene gives a perfectly symmetric H-1 NMR spectrum, indicating that, as in the case of the rac-isomer, there is a rapid interconversion between the two eq uienergetic, mirror-image limit conformations.