Diamine incorporated compounds derived from polymeric nickel(II) fumarates and oxalates: Crystal structure, spectral and thermal properties of [Ni(en)3](O2CCHCHCO2)·3H2O and [Ni(en)3](O2CCO2)

Pressure-Induced Ferroelastic Transition Drives a Large Shape Change in a Ni(II) Complex Single Crystal

Crystals with significant length reduction at an accessible low pressure are highly desirable for piezo-responsive devices. Here, we show a molecular crystal [Ni(en)₃](ox) (en = ethylenediamine and ox = oxalate anion) that exhibits an abrupt shape change with a contraction rate of ∼4.7% along its c axis near the phase transition pressure of ∼0.2 GPa. High-pressure single-crystal X-ray diffraction and Raman spectroscopy measurements reveal that this material undergoes a first-order ferroelastic transition from high-symmetry trigonal P3̅1c to low-symmetry monoclinic P2₁/n at ∼0.2 GPa. The oxalate anions serve as unique components, and their disorder-order transformation and rotation of 90° through cooperative intermolecular hydrogen bonding triggered unconventional anisotropic microsize contraction under compression, which can be appreciated visually. Such a prominent directional deformation at a low pressure driven by molecular motors of oxalate anions provides insights for the design of novel molecular crystal-based piezo-responsive switches and actuators in deep-sea environments.

Synthesis, crystal structure, infrared spectrum, and thermal properties of [Ni(1,10-phenanthroline)3](fumarate)·9H2O complex with hydrogen bonded supramolecular layers involving fumarate anions

From the aqueous-ethanolic system Ni(OH)2—H2 fum—phen (H2fum = fumaric acid, phen = 1,10-phenanthroline), novel complex [Ni(phen)3](fum)·9H2O (1) was isolated and characterized by chemical analyses and FT-IR spectroscopy. Results of single crystal X-ray structure analysis have shown that the ionic crystal structure of 1 is built of [Ni(phen)3]2+ complex cations, fumarate dianions and nine crystallographically independent water molecules of crystallization. The Ni(II) atom exhibits hexa-coordination by three phen ligands with mean Ni-N bond length of 2.090 Å. Water molecules form hydrophilic supramolecular layers with fumarate dianions via extended network of O—H···O type hydrogen bonds with O···O distances from the range of 2.676(2)—2. 916(2) Å; hydrophobic complex cations are embedded between these layers. Thermal study of 1 has shown that endothermic dehydration in the temperature range of 95—195 °C takes at least two steps of the process.

Graphical Abstract

Crystal structure of [Ni(phen)3]fum·9H2O (phen = 1,10-phenanthroline; H2 fum = fumaric acid) which is built of supramolecular layers formed by hydrogen bonded water solvate molecules and fum dianions and between the supramolecular layers embedded [Ni(phen)3]2+ complex cations is described here.

Molecular motor-driven abrupt anisotropic shape change in a single crystal of a Ni complex

Many molecular machines with controllable molecular-scale motors have been developed. However, transmitting molecular movement to the macroscopic scale remains a formidable challenge. Here we report a single crystal of a Ni complex whose shape changes abruptly and reversibly in response to thermal changes at around room temperature. Variable-temperature single-crystal X-ray diffraction studies show that the crystalline shape change is induced by an unusual 90° rotation of uniaxially aligned oxalate molecules. The oxalate dianions behave as molecular-scale rotors, with their movement propagated through the entire crystalline material via intermolecular hydrogen bonding. Consequently, the subnanometre-scale changes in the oxalate molecules are instantly amplified to a micrometre-scale contraction or expansion of the crystal, accompanied by a thermal hysteresis loop. The shape change in the crystal was clearly detected under an optical microscope. The large directional deformation and prompt response suggest a role for this material in microscale or nanoscale thermal actuators.