Unique Bora[1]ferrocenophanes with Sterically Protected Boron: A Potential Gateway to Helical Polyferrocenes.

Silicon-bridged [1]ferrocenophanes are a versatile class of monomers to obtain well-defined metallopolymers, however, their boron-bridged analogues are far less utilized despite being significantly higher strained. We assumed that the reactivity of known bora[1]ferrocenophanes towards ring-opening polymerization is hampered by pi-donating R2N groups at the bridging boron atom and therefore prepared the first bora[1]ferrocenophanes lacking such electronic stabilization. The new, isolated ferrocenophane with a 2,4,6-triisopropylphenyl group attached to the bridging boron atom exhibits the most tilted Cp rings among all isolated strained sandwich compounds [alpha(DFT)=33.3 degrees] with a measured record value of the bathochromic shift (lambda(max)=516 nm). Attempts to purify the mesityl analogue by vacuum sublimation transformed this monomer to a purple-colored polymer that resulted in Cotton effects in circular dichroism spectroscopy. DFT calculations revealed a left-handed helical structure for this polymer. This is the first evidence for a polyferrocene with a chiral secondary structure.