Triethylphosphine as a molecular gear — phase transitions in ferrocenyl–acetylide–gold(I)

A sequence of two discontinuous phase transitions, occurring just above 125 K and 148 K, has been observed for a ferrocenyl–acetylide–gold(I) complex with triethylphosphine, structure (1), by means of a multi-temperature single-crystal X-ray diffraction technique. Three distinct phases have been identified. The high-temperature α and low-temperature γ phases share the same space group Pbca, whereas the intermediate β phase is in the Pb21a subgroup of Pbca. In all phases molecules of (1) form well defined double layers, with PEt3 groups interlocking in planes perpendicular to c. On the molecular level, both phase transitions involve almost uniquely a conformational change of triethylphosphine: a gear-like rotation around the P—Au axis and concerted flips of the ethyl moieties. The mechanism of these transitions may be imagined as initiated by a rotation of a single PEt3 group in a double layer (a single gear movement), followed by adjacent phosphines adjusting their conformations as a result of steric strain. The structural changes underlying phase transitions are sequential, occurring layer-wise, the γ→β transition involving approximately every other layer in the crystal lattice, the β→α yielding a total conformation change. The sequence of phase transitions results in a noticeable contraction of the crystal cell volume.