Crystal Structures and Solution Conformations of the Meso and Racemic Isomers of (Ethylenebis (1-indenyl)) zirconium Dichloride

The crystal and molecular structures of the C-2 symmetric rac-(EBI)ZrCl2 (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)degrees] 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) Angstrom, 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, already at room temperature there is a rapid (NMR time scale) interconversion between the two II and Y (indenyl-backward) conformations, as shown by conformational analysis on the proton spectra of the bridge methylenes. This equilibrium is shifted toward the lower energy conformation 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 AND 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 equienergetic, mirror-image limit conformations.