Structural Properties of Nickel Dimethylglyoxime at High Pressure: Single-Crystal X-ray Diffraction and DFT Studies

Structural changes in nickel dimethylglyoxime (Ni(dmg)₂) were followed by single-crystal X-ray diffraction in a diamond-anvil cell (DAC) at pressures up to 5.1 GPa, that is, in the pressure range through the major color change point (2 GPa), but before the phase transition at 7.4 GPa. Significant average compression (similar to 4%/GPa) was observed, with anisotropic, but continuous and monotonic lattice strain. The maximum compression was observed for the direction perpendicular to planar layers of Ni(dmg)₂ and thus corresponds to decreasing the shortest contacts between nickel cations. Compression within the layers was not so pronounced as the compression between the layers. The structure and dynamics of the short O—H···O hydrogen bond connecting the adjacent dimethylglyoxime ligands were investigated by periodic DFT calculations and showed evidence of a flat, asymmetric single-well proton potential facilitating large-amplitude proton oscillations. The proton motion appears to be coupled to the dynamics of the adjacent methyl groups, resulting in the increased asymmetry of the hydrogen bond at higher pressures.