Effect of crystal packing on the structures of polymeric metallocenes.

The pressure dependencies of the crystal structures of the polymeric metallocenes lithium cyclopentadienide (LiCp) and potassium cyclopentadienide (KCp) have been determined by synchrotron X-ray powder diffraction. The decrease of the volume of LiCp by 34% up to a pressure of p = 12.2 GPa and of KCp by 23% at p = 5.3 GPa as well as the bulk moduli of K = 7.7 GPa for LiCp and 4.9 GPa for KCp indicate a high compressibility for these compounds. The crystal structures of KCp have been determined up to p = 3.9 GPa. An increase of the bend angle is found from 45 degrees at p = 0 GPa up to 51 degrees at p = 3.9 GPa. This variation is completely explained by a model invoking attractive K+ Cp- interaction and repulsive nonbonded carbon-carbon interactions. It is proposed that the bend angle in the polymeric alkali metal metallocenes is the result of the optimization of the crystal packing.

Disordered crystal structure of pentamethylcyclopentadienylsodium as seen by high-resolution X-ray powder diffraction.

The crystal structure of pentamethylcyclopentadienylsodium, [NaC10H15] (NaCp*), has been determined from high-resolution X-ray powder diffraction. The compound crystallizes in space group Cmcm with lattice parameters a = 4.61030 (3), b = 16.4621 (3), c = 14.6751 (2) A, V = 1113.77 (4) A(3) (Z = 4). NaCp* forms polymeric multidecker chains along the a axis. The Rietveld refinement (R(p) = 0.050 and R(F) = 0.163) shows that the Cp* moieties occupy, with disorder, two different orientations rotated away from the eclipsed conformation by +/-13.8 degrees.

Disorder determined by high-resolution powder diffraction: structure of pentamethylcyclopentadienyllithium.

The crystal structure of pentamethylcyclopentadienyllithium, [Li(C(10)H(15))] (LiCp*), has been determined from a high-resolution powder pattern by modelling and the maximum entropy method (MEM). The compound crystallizes in space group R3m with lattice parameters a = b = 14.7711 (5), c = 3.82206 (6) Å and V = 722.19 (4) Å(3) (Z = 3). LiCp* forms polymeric 'multidecker' chains along the c axis. The pentamethylcyclopentadienyl anions are coplanar with each other and show threefold rotational disorder. The MEM calculations did not only confirm the structural model and the type of disorder, but also discovered additional symmetry compared with the Rietveld analysis. This is the first solid-state structure of a Lewis-base-free alkali metal Cp* compound.