Thermal evolution and crystal structures of the 3,5-pyrazole dicarboxylic acid (hydrated form and anhydrous): Ligations with nickel II and barium II

A crystalline form of anhydrous 3,5-pyrazole dicarboxylic acid (H3pdc), H3pdc-α, which is the most stable form at room temperature, has been identified by a variable temperature diffraction study of the commercial form, H3pdc,H2O. Single crystals of H3pdc,H2O, anhydrous H3pdc-β, the high temperature form, as well as Ni(Hpdc),4H2O and Ba(H2pdc)2(H3pdc)2 have been prepared, and their crystal structures determined. The structures of H3pdc,H2O, and H3pdc-β consist of a planar central pyrazole ring, with two carboxylic groups slightly twisted from coplanarity, due to a complex hydrogen bonding scheme. The α form transforms into the β variety upon heating (>240 °C). The mononuclear complex, Ni(Hpdc),4H2O, in which deprotonated H3pdc serves as a bidentate ligand, has been characterized. The crystal structure of Ba(H2pdc)2(H3pdc)2 displays a 50% cationic partial occupancy, with double layers of [Ba0.5(H2pdc)(H3pdc)]∞ planar, polymeric type ribbons along the [010] direction, linked to each other via the Ba coordination scheme along the a axis of the unit cell, with a stacking of Ba(H2pdc)2(H3pdc)2 layers parallel to (001).