Crystal packing principles for ferrocenyl groups linked by polyyne chains: dimorphism of Fc-C4-Fc

Abstract

The crystal structure and packing of a new dimorph (2B) of 1,4-diferrocenyl-1,3-butadiyne (Fc-C₄-Fc, 2) is described and analysed, and compared with the different crystal packing of dimorph 2A. This analysis is made in the context of the polymorphic crystal packing of ferrocene (1), and the crystal packing of 1,8-diferrocenyl-octatetrayne, Fc-C₄-Fc. In 2B the packing is layered, allowing the Fc groups to be organised in approximately square arrays of well-developed edge-to-face (EF) motifs, very similar to the arrangement in the monoclinic and triclinic polymorphs of ferrocene. The C₄ chains are sandwiched between the Fc layers in 2B. The same molecule in 2A is packed with some offset-face-to-face (OFF) motifs between Fc groups, and with a motif in which C₄ chains occupy the grooves between cyclopentadienyl rings of Fc. In FcC₈Fc the packing is dominated by the C₈ chains, surrounded by six molecules, two of which present cyclopentadienyl faces to the C₈ chain, and four present the groove in the ferrocenyl group, with no EF or OFF motifs. In this progression of structure types and intermolecular motifs for molecules Fc-C_n-Fc, the members with C₄ occur at the transition between the ferrocenyl dominant intermolecular motifs and the C_n chain dominant features. Dimorph 2A is like the longer chain structure, while 2B is like ferrocene.