The effect of pressure on the crystal structure of [Gd(PhCOO)(3)(DMF)](n) to 3.7 GPa and the transition to a second phase at 5.0 GPa

The effect of pressure on the crystal structure of the coordination polymer [Gd(PhCOO)(3)(DMF)](n) has been studied to 5.0 GPa. At ambient pressure the structure is tetragonal (space group P4(2)/n) with the polymers extending along the c-direction of the unit cell; successive Gd atoms are alternately bridged by four benzoates and by two benzoates; the coordination spheres of the metal atoms are completed by DMF ligands. This results in two different Gd center dot center dot center dot Gd repeats, measuring 3.8953(3) and 5.3062(3) angstrom, respectively. The polymer chains interact with each other via dispersion interactions, including a number of CH center dot center dot center dot pi contacts to phenyl rings in which the H center dot center dot center dot ring-centroid distances are 3.19 to 3.28 angstrom. Up to 3.7 GPa the crystal remains in a compressed form of its ambient-pressure phase. The a-axis shortens by 7.7%, and the c-axis by 2.9%, the difference reflecting the greater ease of compression along the crystallographic directions mediated by weak intermolecular interactions. At ambient pressure the Gd-O distances span 2.290(2)-2.559(2) angstrom, with an average of 2.39(3) angstrom. At 3.7 GPa the corresponding parameters are 2.259(3) to 2.509(4) and 2.36(3) angstrom. The Gd center dot center dot center dot Gd distances shortened by 0.0467(4) and 0.1851(4) angstrom, and the CH center dot center dot center dot pi distances span the range 2.76-2.90 angstrom. During compression a number of H center dot center dot center dot H contacts develop, the shortest measuring 1.84 angstrom at 3.7 GPa. On increasing the pressure to 5.0 GPa a phase transition occurred in which the shortest H center dot center dot center dot H contact is relieved by conversion of an edge-to-edge phenyl-phenyl contact into a pi center dot center dot center dot pi stacking interaction. The new phase is also tetragonal, space group P (4) over bar 4, the inversion symmetry present in phase-I being lost in phase-II. The phase transition allows more efficient packing of ligands, and while the a-axis decreases in length the c-axis increases. This leads to Gd center dot center dot center dot Gd distances of 3.8373(4) and 5.3694(4) angstrom, the latter being longer than at ambient pressure. Gd-O distances at 5.0 GPa span the range 2.265(5) to 2.516(5) angstrom, with a mean of 2.36(2) angstrom.