Order-disorder phase transition with associated cell-tripling in the (octamethylferrocene)(2,3-dichloro-1,4-naphthoquinone)2 charge-transfer complex

The structural change associated with the first-order phase transition (TC = 155.1 K) in a mixed-stack charge-transfer complex (octamethylferrocene)(2,3-dichloro-1,4-naphthoquinone)2 was investigated crystallographically. X-ray structure determination at 90 K revealed that the transition is associated with the order-disorder of the C5Me4H ring in octamethylferrocene. In the low temperature phase, tripling of the unit cell occurred, accompanied by a change in spacegroup from P21/c to P−1, and every third octamethylferrocene molecule twists out of the plane to minimize steric interactions.