How Strained are [1]Ferrocenophanes?

A series of [1]­ferrocenophanes ([1]­FCPs) bridged by boron, carbon, silicon, phosphorus, and sulfur, respectively, were investigated by DFT calculations. A comparison of measured molecular structures with calculated structures showed that the applied B3PW91/6-311+G­(d,p) level of theory provides realistic molecular geometries. Geometry optimization of carbon-bridged [1]­FCPs (ERx = CH2, CMe2) revealed that these unknown species with tilt angles α of 38.5° may be sufficiently stable to allow isolation, given sufficient kinetic stability. In order to measure the amount of strain in [1]­FCPs, a hypothetical 1,2-addition of a C–H group of FeCp2 across the E–Cipso bond of a [1]­FCP to give a bis­(ferrocenyl)­species was investigated. The calculated reaction enthalpies were compared to experimental ΔHROP values as obtained from differential scanning calorimetry (DSC) measurements.