REORIENTATIONAL MOTIONS OF PERMETHYLATED CYCLOPENTADIENYL RINGS IN POLYCRYSTALLINE ORGANOMETALLIC COMPOUNDS

The dynamic behavior in the solid state of (C5Me5)2Fe (I), (C5Me5)Rh(CO)2 (II), (C5Me5)2Cr2(CO)4 (III), (C5Me5)Fe2(mu-2-CO)2(CO)2 (IV), and (C5Me5)2Rh2(mu-2-Cl)2Cl2 (V) has been investigated by variable-temperature H-1 spin-lattice relaxation time measurements and potential energy barrier calculations within the pairwise atom-atom approach. In all the cases two different processes have been detected corresponding to the rotation of the methyl group about its C3 axis and to the reorientation of the cyclopentadienyl ligand about its C5 axis, respectively. The separate intramolecular and intermolecular contributions to the total reorientational barriers have been evaluated. It has been shown that in crystalline III and IV a synchronous motion of the methyl groups during ring reorientation reduces intramolecular repulsions between the H atoms of the methyl groups and the CO ligands and affords a low-energy reorientational path. C-13 spin-lattice relaxation times have also been determined for the individual carbon atoms by using Torchia's pulse sequence. The C-13-T1 values observed for ring carbons have been rationalized on the basis of two relaxation interactions (dipole-dipole and chemical shift anisotropy) modulated by the motions involving the permethylated cyclopentadienyl rings. The chemical shift anisotropies of cyclopentadienyl C-13 resonances have been determined on chloroform solutions of compounds II-V at different magnetic field strengths. A qualitative comparison between solution- and solid-state C-13 relaxation data shows that the same relaxation mechanisms are operative in both physical states.