Polymerization of Racemic 2,2′-Dialkyl-Sila[1]ferrocenophanes: DFT‑Assisted Polymer Analysis by 29Si NMR Spectroscopy Using Model Compounds

The described research aimed at the preparation of racemic, C2 symmetric sila[1]ferrocenophanes with alkyl groups in 2,2′-positions to complement their known enantiopure counterparts. As the synthetic approach, the well-known Ugi amine chemistry was chosen to introduce planar chirality into the ferrocene framework. The challenge in this multistep process is the separation of a rac and meso mixture of diols obtained through the reduction of 1,1′-dialkanoylferrocenes by LiAlH4 or NaBH4. From the three tested alkanoyl groups, only one led to a significant excess of the rac diol, which could be separated by crystallizations and converted to rac-2,2′-diisobutyl-dimethylsila[1]ferrocene. Thermal ring-opening polymerization of this new monomer gave a poly(ferrocenylsilane) that consists of two types of diads, as revealed by two sets of peaks in its 29Si NMR spectrum centered at −5.66 and −7.74 ppm. 29Si NMR chemical shifts could be predicted with density functional theory (DFT) methods for planar-chiral bis(ferrocenyl)dimethylsilanes that were used to model these meso and racemo diads of the polymer. These calculations realistically predict that silicon atoms of racemo diads are higher shielded than those of meso diads. Surprisingly, the 29Si NMR peaks of both diads are split into a set of peaks, revealing a sensitivity of δ values beyond diads.