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 alpha of 38.5 degrees 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-CIPipso bond of a [1]FCP to give a bis(ferrocenyl)species was investigated. The calculated reaction enthalpies were compared to experimental Delta H-ROP values as obtained from differential scanning calorimetry (DSC) measurements.