THE ROLE OF TORSIONAL ISOMERS OF PLANARLY CHIRAL NONBRIDGED BIS(INDENYL)METAL TYPE COMPLEXES IN STEREOSELECTIVE PROPENE POLYMERIZATION

Reaction of enantiomerically pure (1-neomenthylindenyl)lithium with ZrCl4(THF)2 gave the three diastereomeric bis(1-neomenthylindenyl)zirconium dichloride complexes 4a-A,-B,and -C, in a 93:2:5 ratio. Similarly, a 82:7:11 mixture of the three bis(1-neoisomenthylindenyl)zirconium dichloride diastereoisomers (4b-A,B,C) was obtained from (1-neoisomenthylindenyl)lithium and the zirconium tetrachloride-bis(tetrahydrofuran) adduct. Recrystallization from methylene chloride furnished the (optically active) pure diastereomers 4a-A and 4b-A, respectively. Catalytic hydrogenation (PtO2, 50-60 bar of H-2, CH2Cl2) converted them into the pure (p-S, p-S)-bis(1-neomenthyl-4,5,6,7-tetrahydroindenyl)zirconium dichloride (5a-A) and (p-R,p-R)-bis(1-neoisomenthyl-4,5,6,7-tetrahydroindenyl)zirconium dichloride (5b-A) complexes. All four metallocenes were characterized by X-ray crystal structure analyses. Complex 4a-A crystallizes in space group P2 with cell parameters a = 10.510(1) angstrom, b = 6.975(1) angstrom, c = 12.421(1) angstrom, beta = 108.18(1)degrees, Z = 1, R = 0.037, R(w) = 0.041; 4b-A P2(1), a = 7.559(2) angstrom, b = 9.046(2) angstrom, c = 25.015(3) angstrom, beta = 96.72(1)degrees, Z = 2, R = 0.037, R(w) = 0.040; 5a-A P2(1)2(1)2, a = 15.422(1) angstrom, b = 18.716(1) angstrom, c = 7.118(1) angstrom, Z = 2, R = 0.062, R(w) = 0.074; 5b-A P2(1)2(1)2, a = 10.103(1) angstrom, b = 26.626(5) angstrom, c = 6.762(1) angstrom, Z = 2, R = 0.030, R(w) = 0.037. Complexes 4a-A, 5a-A, and 5b-A exhibit C2-symmetric bent metallocene conformations in the crystal which are characterized as having the bulky terpenyl substituents oriented antiperiplanarly toward the lateral sectors of the bent metallocene wedge (''bis-lateral:anti'' conformation). In contrast, the bis(neoisomenthylindenyl)zirconium dichloride complex 4b-A has only one hydrocarbyl substituent arranged laterally whereas the other is oriented to the open front side of the metallocene backbone (C1-symmetric ''central,lateral:gauche'' conformation). In solution the conformational behavior of these complexes is different. By dynamic temperature-dependent H-1 and C-13 NMR spectroscopy both neoisomenthyl-substituted complexes 4b-A and 5b-A appear as single C2-symmetric species in solution. In contrast, the neomenthyl-substituted complexes 4a-A and 5a-A exhibit equilibrating conformational mixtures. In the case of the (p-S,p-S)-bis(1-neomenthyl-4,5,6,7-tetrahydroindenyl)zirconium dichloride system (5a-A) it was shown that two conformational isomers are present in a ca. 80:20 ratio. These are C2- (major isomer) and C1-symmetric (minor isomer) and probably correspond to the ''bis-lateral:anti'' and ''central,lateral:gauche'' conformational types, respectively, that were identified by X-ray diffraction in the crystal. These conformational properties seem to determine the outcome of the stereoselective propene polymerization to a considerable extent, which was carried out at homogeneous bent metallocene/methylalumoxane Ziegler-type catalysts derived from the complexes 4 and 5. Both neoisomenthyl-substituted systems produce isotactic high molecular weight polypropylene with purely enantiomorphic site control at low temperature. The isomeric neomenthyl-substituted systems are much less stereoselective. It is assumed that the bis(1-neomenthyltetrahydroindenyl)zirconium catalyst switches back and forth between the C2- and C1-symmetric conformations of the bent metallocene backbone during the catalytic process, thereby probably giving rise to the formation of alternating isotactic and near to atactic sequences along the growing polymer chain.