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.