On the relationship between crystallographic and spectroscopic evidence of dynamic processes in the solid state. The case of the osmium cluster ‘helicopters’

Abstract

The reorientational motion of the C₆H₆ and CH₂CH₂ ligands in [Os₃(CO)₈(η²-CH₂CH₂)(µ₃-η²: η²: η²-C₆H₆)], evidenced in the solid by ¹³C cross-polarization magic angle spinning n.m.r. spectroscopy, has been examined by means of the atom–atom potential energy method. The results show that the motion of the ethene fragment is correlated to that of the benzene fragment and controlled primarily by intramolecular energy terms, while the crystal packing does not create relevant intermolecular barriers. The possibility of similar reorientational processes in solid [Os₃(CO)₇(µ₃-σ : η² : σ-C₂Me₂)(η⁶-C₆H₆)] has also been explored and the results compared with the X-ray crystallographic indication of extensive in-plane librational motion of the C₆H₆ ligand. In neither case does reorientation of the Os(CO)₃ groups appear to be a possibility.