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 Zr Cl4(THF)2 gave the three diastereomeric bis(1-neomenthylindenyl)zircon ium 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 d ichloride diastereoisomers (4b-A,B,C) was obtained from (1-neoisomenth ylindenyl)lithium and the zirconium tetrachloride-bis(tetrahydrofuran) adduct. Recrystallization from methylene chloride furnished the (opti cally active) pure diastereomers 4a-A and 4b-A, respectively. Catalyti c hydrogenation (PtO2, 50-60 bar of H-2, CH2Cl2) converted them into t he pure (p-S, s(1-neomenthyl-4,5,6,7-tetrahydroindenyl)zirconium dichl oride (5a-A) and -neoisomenthyl-4,5,6,7-tetrahydroindenyl)zirconium di chloride (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) ang strom, c = 12.421(1) angstrom, beta = 108.18(1)degrees, Z = 1, R = 0.0 37, R(w) = 0.041; 4b-A P2(1), a = 7.559(2) angstrom, b = 9.046(2) angs trom, 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, an d 5b-A exhibit C2-symmetric bent metallocene conformations in the crys tal which are characterized as having the bulky terpenyl substituents oriented antiperiplanarly toward the lateral sectors of the bent metal locene wedge (''bis-lateral:anti'' conformation). In contrast, the bis (neoisomenthylindenyl)zirconium dichloride complex 4b-A has only one h ydrocarbyl substituent arranged laterally whereas the other is oriente d to the open front side of the metallocene backbone (C1-symmetric ''c entral,lateral:gauche'' conformation). In solution the conformational behavior of these complexes is different. By dynamic temperature-depen dent H-1 and C-13 NMR spectroscopy both neoisomenthyl-substituted comp lexes 4b-A and 5b-A appear as single C2-symmetric species in solution. In contrast, the neomenthyl-substituted complexes 4a-A and 5a-A exhib it equilibrating conformational mixtures. In the case of the s(1-neome nthyl-4,5,6,7-tetrahydroindenyl)zirconium dichloride system (5a-A) it was shown that two conformational isomers are present in a ca. 80:20 r atio. 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 polymerizatio n to a considerable extent, which was carried out at homogeneous bent metallocene/methylalumoxane Ziegler-type catalysts derived from the co mplexes 4 and 5. Both neoisomenthyl-substituted systems produce isotac tic high molecular weight polypropylene with purely enantiomorphic sit e control at low temperature. The isomeric neomenthyl-substituted syst ems are much less stereoselective. It is assumed that the bis(1-neomen thyltetrahydroindenyl)zirconium catalyst switches back and forth betwe en the C2- and C1-symmetric conformations of the bent metallocene back bone during the catalytic process, thereby probably giving rise to the formation of alternating isotactic and near to atactic sequences alon g the growing polymer chain.