The disordered structure of tetraferrocenyl-[3]-cumulene, (Fc)₂C=C=C=C(Fc)₂, by simulated annealing using synchrotron powder diffraction data

In the molecular structure of tetraferrocenyl-[3]-cumulene, (Fc)₂C=C=C=C(Fc)₂, four ferrocene molecules are connected via a linear bridge consisting of four carbon atoms. At room temperature, the crystal structure has space group P2₁/a (Z = 1) with a = 13.104 (5), b = 6.121 (2), c = 11.194 (4) Å, β = 114.922 (1)° and V = 814.3 (8) ų. A phase transition during cooling was not observed from room temperature to 75 K. From high-resolution X-ray powder diffraction data, the structure of the room-temperature phase was solved by the method of simulated annealing and refined by the Rietveld method using rigid bodies and restraints. The crystal structure was found to be highly disordered with the molecules occupying two orientations with equal probability and a 50% occupancy of the carbon atoms in the cumulene bridge. The disorder could be modelled by stacking faults in ordered structures. In contrast to other compounds of this class, the ferrocenyl groups are in a syn rather than in an up–down conformation with respect to the cumulene bridge.