Racemic−Meso Interconversion for ansa-Scandocene and ansa-Yttrocene Derivatives. Molecular Structures of rac-{Me_2Si[η^5-C_5H_2-2,4-(CHMe_2)_2]_2}ScCl·LiCl(THF)_2, [meso-{Me_2Si[η^5-C_5H_2-2,4-(CHMe_2)_2]_2}Y(μ_2-Cl)]_2, and meso-{Me_2Si[η^5-C_5H_2-2,4-(CHMe_2)_2]_2}Zr(NMe_2)_2

Abstract

The dilithium bis(cyclopentadienide) species Li_2{Me_2Si[C_5H_2-2,4-(CHMe_2)_2]_2} (Li_2Ip) reacts with ScCl_3(THF)_3 to afford rac-IpScCl·LiCl(THF)_2 (1) and [meso-IpSc(μ_2-Cl)]_2 (2) and with YCl_3(THF)_(3.5) to afford rac-IpYCl·LiCl(THF)_2 (3) and [meso-IpY(μ_2-Cl)]_2 (4). Metalation with both scandium and yttrium chlorides yields the metallocene chlorides in approximately 3:1 racemic:meso ratios. Reaction of IpH_2 with Zr(NMe_2)_4 yields exclusively meso-IpZr(NMe_2)_2 (9). Treatment of 1 or 2 with allylmagnesium bromide affords the allyl complexes rac-IpSc(η^3-C_3H_5) (5) and meso-IpSc(η^3-C_3H_5) (6) and with crotylmagnesium chloride affords rac-IpSc(η^3-C_3H_4Me) (7) and meso-IpSc(η^3-C_3H_4Me) (8). Diastereomerically pure rac dichlorometalate compounds (1 or 3) or pure meso chloro dimers (2 or 4) undergo spontaneous isomerization upon dissolution in THF-d_8 with reversion back to a 3:1 racemic:meso ratio. Isomerization of 5 and 6 is observed in THF-d_8, above 55 °C, affording an equilibrium ratio of ∼2:1 racemic:meso isomers. While spontaneous isomerization of 5, 6, 7, or 8 is very slow at room temperature, various salts and Grignard reagents promote isomerization even at room temperature; the isomerizations are not accelerated by light. The proposed mechanism for racemic−meso isomerization involves heterolytic dissociation of one cyclopentadienide ligand from the metal, rotation around that Si−Cp- bond, and recoordination on the opposite face, effecting net epimerization. X-ray diffraction studies have been performed on rac-IpScCl·LiCl(THF)_2 (1), [meso-IpY(μ_2-Cl)]_2 (4), and meso-IpZr(NMe_2)_2 (9).

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