Boron-Bridged Ferrocenophanes: Strained Monomers for Metallopolymers

Abstract

Incorporating metals in synthetic polymers can impose a diverse range of different new and valuable properties. Organometallic polymers containing three-coordinate boron have the potential for intriguing electronic and optical properties associated with the electron deficient nature of the boron centers. Boron-containing conjugated polymers can potentially be obtained via ring-opening polymerization (ROP) of strained boron-bridged [n]ferrocenophanes ([n]FCPs). The first three boron-bridged [1]FCPs (21a-c) were reported almost two decades ago. Unfortunately, thermal ROP of those highly strained monomers resulted only in some insoluble materials which brought the chemistry to a dormancy. This chemistry, which is described in this thesis, was revitalized by developing flexible approaches to synthesize new boron-bridged [1]FCPs (27a-cR1R2). The strategy of adding alkyl groups on the Cp rings was adopted to provide steric protection to the bridging moiety as well as to increase the solubility of the monomers and the resulting polymers. Detailed studies were performed by fine tuning of the bulk of alkyl groups (CHR1R2) on Cp rings as well as the reaction conditions in order to understand the mechanism of the formation of strained [1]FCPs (27a-cR1R2). Moreover, thermal ROP of these boron-bridged [1]FCPs gave soluble metallopolymers with moderate molecular weights (Mw ≈ 10 kDa). Synthesis and complete characterization of the first examples of azabora[2]ferrocenophanes (30a-c) with unsaturated BN moieties at the bridging position are also described in this thesis. Albeit large tilt angles (α ≈ 24°) these species showed thermal stability even up to 300 °C and did not ring open under such conditions. However, DFT studies revealed that these [2]FCPs (30a-c) are equally strained as the well-known SiMe2-bridged [1]FCP. Even though electronic stabilization from an amino substituent at the boron atom seemed essential to prepare isolable boron-bridged [n]FCPs, such stabilization resulted in decreased iii electrophilicity of the boron center and caused a lack of polymerizability of the monomers. To address this issue, the first examples of sterically protected bora[1]ferrocenophanes (27d-fMeMe) as highly reactive strained ferrocenophanes were synthesized. Species 27dMeMe and 27eMeMe showed exceptional bathochromic shifts in UV-Vis spectroscopy. Optimized geometry of 27eMeMe at the B3PW91/6-311+G(d,p) level of theory revealed this species to be the new record holder with respect to Cp ring tilting (α ≈ 34°). Moreover, thermal ROP of 27dMeMe resulted in poly(ferrocenylborane) which showed a helical secondary structure in solution.