Heteroatom-Bridged Carba[2]ferrocenophanes with a Surprising Chemical Shift Pattern of Cp Protons

[2]Ferrocenophanes bridged by a carbon and a heteroatom
were targeted
in the hope that such an asymmetric bridge gives rise to monomers
suitable for ring-opening polymerization. Three strained sandwich
compounds with bridging C–Si (13SiMe2), C–P (13PPh),
or C–B (13BMes) moieties
were prepared using a lithium–bromine exchange followed by
a common salt-metathesis reaction. Single-crystal X-ray structural
analyses revealed an increase of the degree of Cp ring tilting, commonly
expressed as the α angle, from 12.58(9) (13SiMe2) to 16.63(2) (13PPh) to 19.66(11)° (13BMes). Despite being strained, all three species turned out to
be resistant toward thermal ring-opening polymerization. The characterization
of the three [2]ferrocenophanes by one- and two-dimensional 1H NMR spectroscopy disclosed that Cp protons in α position
to the bridging elements are deshielded relative to Cp protons in
the β position. Experimental assignments were supported by DFT
calculations of chemical shifts. The determined order of α and
β Cp protons is opposite to that known for [1]ferrocenophanes.
Using a set of known [n]ferrocenophanes (n = 1, 2) with significant tilted Cp rings, DFT calculations
revealed that both families exhibit a different order of Cp peaks
in 1H NMR spectra.