Silicon-bridged [1]Metallocenophanes: Strained Monomers for Metallopolymers

Abstract

[n]Metallocenophanes possess an ansa [n] bridge (an inter-ring bridge composed of n bridging atoms) which links the Cp rings and tilts them about the metal centre. [n]Metallocenophanes with a large dihedral angle between the two Cp rings (tilt angle, α) are highly strained and susceptible to strain release by ring-opening polymerization (ROP). The strain may be varied by changing the metal (M) and/or the bridging element (E). Amongst the ones known so-far, [1]ferrocenophanes ([1]FCPs) represent the most widely explored class of [n]metallocenophanes with silicon-bridged [1]ferrocenophanes being the most well-researched. The initiation of ROP in these strained molecules may be induced thermally (thermal ROP) or by using anionic reagents (anionic ROP) such as n-butyllithium. Research in the Müller group has been directed towards the development of new synthetic strategies and monomers for ROP. To expand the family of polymerizable [1]metallocenophanes, new [1]FCPs and [1]ruthenocenophanes ([1]RCPs) were prepared. Species rac-106R was prepared following a one-pot synthesis from the readily synthesized 1,1´-dibromoferrocene, and polymerized via thermal ROP to give soluble polymers. New sila[1]ferrocenophanes rac-25 and rac-110 with tBu and iPr substituents in the 3,3´-position to the bridging moiety were synthesized and polymerized thermally or anionically. Thermal ROP of rac-25 gave a high-molecular-weight polymer, however, anionic ROP resulted in the formation of oligomers only. Finally, a large knowledge gap was filled in [1]RCP chemistry by synthesizing the first known sila[1]ruthenocenophanes 126 and Sp,Sp-143. The α angle of both species measured from X-ray structural analysis revealed that the tilt in both is around 6° higher than in the corresponding [1]FCPs. The more pronounced structural distortion in [1]RCPs was also evident in the observed difference in chemical shifts of 126 and Sp,Sp-143 compared to their [1]FCP analogues. The enantiopure Sp,Sp-143 was polymerized thermally and gave a soluble polymer. Salt-metathesis reactions are kinetically controlled, therefore a distinct difference in product ratios was observed in the synthesis of [1]FCP and the corresponding [1]RCPs. An alternative strategy for synthesizing [1]FCPs was attempted from an understanding of the reaction kinetics of salt-metathesis reactions.