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Aluminum ansa-Indenyl Compounds. Synthesis, Structures, Dynamic Properties, and Application in the Synthesis of Group 4 ansa-Metallocenes

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journal contribution
posted on 10.11.1999, 00:00 by B. Thiyagarajan, Richard F. Jordan, Victor G. Young
The synthesis, structures, dynamic properties, and indenyl transfer reactions of aluminum ansa-bis(indenyl) compounds are described. The reaction of 2 equiv of AlMe2Cl with 1 equiv of Li2[(1-indenyl)2SiMe2], Li2[(2-Me-1-indenyl)2SiMe2], Li2[(2-Me-4,5-benz-1-indenyl)2SiMe2]·Et2O, Li2[(2-Me-4-Ph-1-indenyl)2SiMe2]·Et2O, or Li2[1,2-(3-indenyl)2-C2H4] in Et2O followed by treatment with the appropriate Lewis base (L) affords {AlMe2(THF)(indenyl)}2SiMe2 (1), [{1-AlMe2(1,4-dioxane)0.5-2-Me-1-indenyl}2SiMe2]n (2a), {1-AlMe2(Et2O)-2-Me-4,5-benz-1-indenyl}2SiMe2 (3), {1-AlMe2(Et2O)-2-Me-4-Ph-1-indenyl}2SiMe2 (4a), {1-AlMe2(THF)-2-Me-4-Ph-1-indenyl}2SiMe2 (4b), or 1,2-{3-AlMe2(THF)-1-indenyl}2-C2H4 (5), respectively, as colorless to pale yellow solids in 41−70% isolated yields. Compounds 2a and 4b are isolated as the rac isomers, whereas 1 and 3 are isolated as rac/meso mixtures from which the rac isomer can be separated by recrystallization. Compound 5 was isolated as a single diastereomer of 1,2-{3-AlMe2(THF)-1-indenyl}2-C2H4. The molecular structures of rac-2a and rac-3 have been determined by X-ray crystallography. Low-temperature NMR studies establish that, in toluene-d8, rac-1 exists as a 2/1 mixture of two isomers, ({1-AlMe2(THF)-1-indenyl}{1-AlMe2(THF)-3-indenyl})SiMe2 (rac-1a) and {1-AlMe2(THF)-1-indenyl}2SiMe2 (rac-1b), which interconvert rapidly on the NMR time scale at room temperature. In contrast, similar studies establish that rac-2b (in THF-d8) and rac-3 and rac-4b (in toluene-d8) exist as the rac-{1-AlMe2L-1-indenyl}2SiMe2 isomers; in these cases no other isomers are detected. rac-1, rac-3, and rac-4b isomerize to rac/meso mixtures slowly (days) at ambient temperature and more rapidly (minutes) at 70 °C. Compounds 1 and 5 undergo slow partial disproportionation by ligand redistribution (e.g. 1:  28% conversion, 2 days, 23 °C, benzene); the more highly substituted indenyl compounds 3 and 4b are more resistant to this process. Compounds 1, 2, 3, 4a, and 5 react with Zr(NMe2)4 and Hf(NMe2)4 in benzene or toluene under mild conditions to yield the corresponding ansa-metallocenes {(1-indenyl)2SiMe2}M(NMe2)2 (6, M = Zr, rac/meso = 4/1; 7, M = Hf, rac/meso = 10/1), {(2-Me-1-indenyl)2SiMe2}Zr(NMe2)2 (8, rac/meso = 3/4), {(2-Me-4,5-benz-1-indenyl)2SiMe2}Zr(NMe2)2 (9, rac/meso = 9/10), {(2-Me-4-Ph-1-indenyl)2SiMe2}Zr(NMe2)2 (10, rac/meso = 2/3) and {1,2-(3-indenyl)2-C2H4}M(NMe2)2 (11, M = Zr, rac/meso = 7.3/1; 12, M = Hf, rac/meso = 7/1) in 70−90% NMR yields.