Structural Properties of Nickel Dimethylglyoxime at High Pressure: Single-Crystal X‑ray Diffraction and DFT Studies
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Abstract
Structural
changes in nickel dimethylglyoxime (Ni(dmg)<sub>2</sub>) 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 (∼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)<sub>2</sub> 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