Rac/meso Transformations of Disiloxane-bis(1-indenyl)-ansa-ferrocenes:  An X-ray Crystallographic and NMR Study

Both stereoisomers of disiloxane-bis(1-indenyl)-ansa-ferrocene, Fe(η⁵-inden-1-yl-SiMe₂)₂O, 1, were prepared by the reaction of disiloxane-bis-1,3-inden-1‘-yl anion with FeCl₂; the diastereomers were separated by column chromatography and recrystallized from n-heptane. A single-crystal X-ray diffraction study confirmed that each stereoisomer, racemic 1_r and meso 1_m, involves parallel bis-indenyl coordination to the Fe metal center. These isomers were also characterized by ¹H, ¹³C, and ²⁹Si NMR and IR spectroscopy. Compounds 1_r and 1_m could each be transformed into the other diastereomer:  the conversion was retarded by base, but accelerated under weakly acidic conditions. The mechanism of this process, investigated using deuterium labeling experiments, is consistent with a series of successive 1,5-suprafacial, sigmatropic silicon shifts. Compound 1 was also copolymerized with 1,1,3,3,5,5,7,7-octamethylcyclotetrasiloxane (D₄) and 1,1,1,3,3,5,5,5-octamethyltrisiloxane (MDM) to produce green organometallic polymers.