A Flexible Approach to Strained Sandwich Compounds: Chiral [1]Ferrocenophanes with Boron, Gallium, Silicon, and Tin in Bridging Positions

Abstract

The enantiomerically pure dibromoferrocene 3 [(Sp,Sp )-1,1'-dibromo-2,2'-di(isopropyl)ferrocene], equipped with two iPr groups in α positions, was prepared using known "Ugi amine" chemistry. Species 3 was targeted in order to gain access to new [1]ferrocenophanes ([1]FCPs) to be used as monomers for ring-opening polymerization. The iPr groups on the sandwich unit were introduced to stabilize bridging moieties, as well as to increase solubilities of targeted metallopolymers. The planar chiral dibromide 3 can quantitatively be lithiated at 0°C [2 equiv nBuLi, hexanes/thf (9:1), 30 min]. Salt-metathesis reactions with respective element dichloride species gave chiral [1]FCPs with a variety of bridging moieties [ERx =Ga[2-(Me2NCH2)C6H4] (4 a), SiMe2 (4 b), SntBu2 (4 c), BNiPr2 (4 d)]. The new [1]FCPs were fully characterized including single-crystal X-ray analysis. The stabilizing iPr groups on the Cp rings increase the thermal stabilities of 4 b-d compared to known [1]FCPs, equipped with the same bridging moieties. All three compounds 4 b-d are volatile and could be isolated by vacuum sublimation. Our new approach to [1]FCPs has the potential to overcome many of the existing difficulties in ferrocenophane chemistry, such as limited stability of starting monomers and low solubilities of resulting polyferrocenes.