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

Abstract

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 P4₂/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⋯Gd repeats, measuring 3.8953(3) and 5.3062(3) Å, respectively. The polymer chains interact with each other via dispersion interactions, including a number of CH⋯π contacts to phenyl rings in which the H⋯ring-centroid distances are 3.19 to 3.28 Å. 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) Å, with an average of 2.39(3) Å. At 3.7 GPa the corresponding parameters are 2.259(3) to 2.509(4) and 2.36(3) Å. The Gd⋯Gd distances shortened by 0.0467(4) and 0.1851(4) Å, and the CH⋯π distances span the range 2.76–2.90 Å. During compression a number of H⋯H contacts develop, the shortest measuring 1.84 Å at 3.7 GPa. On increasing the pressure to 5.0 GPa a phase transition occurred in which the shortest H⋯H contact is relieved by conversion of an edge-to-edge phenyl-phenyl contact into a π⋯π stacking interaction. The new phase is also tetragonal, space group P, 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⋯Gd distances of 3.8373(4) and 5.3694(4) Å, 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) Å, with a mean of 2.36(2) Å.