Structural Properties of Nickel Dimethylglyoxime at High Pressure

Article pubs.acs.org/JPCC

Structural Properties of Nickel Dimethylglyoxime at High Pressure: Single-Crystal X‑ray Diffraction and DFT Studies Ian F. Bruce-Smith,†,‡ Boris A. Zakharov,*,†,§ Jernej Stare,∥ Elena V. Boldyreva,†,§ and Colin R. Pulham‡ †

Novosibirsk State University, Pirogova Str. 2, 630090, Novosibirsk, Russia School of Chemistry and Centre for Science at Extreme Conditions, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, United Kingdom § Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch of the Russian Academy of Sciences, Kutateladze Str. 18, 630128, Novosibirsk, Russia ∥ National Institute of Chemistry, Hajdrihova 19, SI-1000, Ljubljana, Slovenia ‡

S Supporting Information *

ABSTRACT: 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 (∼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 largeamplitude 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.

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INTRODUCTION High-pressure studies are widely used to obtain new compounds and materials, to study structure−property relationships, and to follow the coupling between intramolecular distortions, intermolecular interactions, and macroscopic strain (see ref 1 and refs therein). Coordination compounds are of special interest for such studies on account of the occurrence of phase transitions associated with substantial structural rearrangement and the possibility of obtaining quenchable highpressure phases (see refs 2 and 3 and refs therein). Diamondanvil cells (DACs) make it possible to collect X-ray diffraction and spectroscopic data in situ (see refs 4−16 and refs in these books and reviews), and this also enables monitoring the changes in physical (optical, magnetic, electric) properties as a function of crystal-field splitting, distortion of hydrogen bonds, and changes in intramolecular geometry, which result from the pressure-induced anisotropic structural strain. Transition-metal complexes have attracted attention since the very first years of high-pressure research.17 Nickel dimethylglyoxime (Ni(dmg)2, Figure 1) was first synthesized by Chugaev in 1905.18 It forms a distinctive, fine, red powder that is highly insoluble in most solvents, despite the complex containing two OH functional groups. The compound is mainly known for its application in analytical chemistry as a test compound to detect qualitatively the presence of nickel and also for quantitative © 2014 American Chemical Society

Figure 1. Labeled 50% probability displacement ellipsoid plot of Ni(dmg)2 molecule at 0.23 GPa.

analysis of Ni using gravimetric methods. This compound also attracted the attention of the high-pressure research community in the middle of the previous century because of its ability to change color continuously from orange to yellow via violet, blue, and green (if observed in transmission mode) in the pressure range from ambient up to 5 GPa.19 Before the adoption of the ruby fluorescence technique for determining pressure,20 it was therefore used to estimate pressure through simple observation of the compound’s color, even without using a calibrated absorption spectrum. Davies calibrated Ni(dmg)2 as a pressure Received: September 4, 2014 Revised: September 26, 2014 Published: September 26, 2014 24705

dx.doi.org/10.1021/jp508939g | J. Phys. Chem. C 2014, 118, 24705−24713

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marker for DACs up to pressures of 2 GPa.21 The difficulty in obtaining high quality single crystals of Ni(dmg)2 meant that Davies produced thin films of powder using a DAC, often utilizing an inert solid such as NaCl or AgCl to improve the film’s strength and measured how its optical absorption spectrum changed with pressure. Drickamer noted that metal glyoxime complexes exhibit much more intense absorption peaks in the visible spectrum when in the solid state than when in solution.22 Anex and Krist explained the enhanced absorption in the solid state in terms of a transition between the 3dz2 and 4pz orbitals of the nickel ions.23 Zahner and Drickamer measured the absorption spectrum as a function of pressure for several metal dimethylglyoxime complexes including Ni(dmg)2. A very large shift to lower wavenumbers was observed with the application of pressure for the band which is assumed to correspond to transitions between the 3dz2 and 4pz.24 There have been several attempts to determine the structure of Ni(dmg)2 at ambient conditions. The shape of the dimethylglyoxime ligand was known and the coordinates of the nonhydrogen atoms in the crystal structure could be determined reasonably well even in 1936 by Milone.25 In 1953, Godycki and Rundle used new refinement techniques to find a more accurate structure using Milone’s data.26 In their model the nonmethyl hydrogen atoms were originally equidistantly placed between the two oxygen atoms. The formation of this strong O−H···O bond was proposed to account for the low solubility of this compound in water because the hydrogen appears to be held so tightly that the O−H group does not interact appreciably with water, and hence inhibits dissolution of the complex.26 The value of the stretching frequency for O−H bonds involved in short H-bonds in Ni(dmg)2 is ∼1800 cm−1; this is significantly lower than could be expected for “normal” hydrogen bonds.27 This fact is related to decreasing the distance between O atoms in the H-bond and the corresponding elongation of the O−H distance resulting in lower stretching vibration frequencies. Moreover, the authors noticed that the N−O stretching vibrations show only a single band the infrared spectra of Ni(dmg)2 suggesting that all of the N−O bonds are equivalent, and hence, the H-bonds should be symmetric.27 However, in 2003, Li et al. accurately determined the structure of Ni(dmg)2 using single-crystal X-ray diffraction, but these more precise data were unable to confirm that the O−H···O bond is symmetric.28 The same conclusion was reached based on a single crystal structure refinement by Czapik and Gdaniec in 2009.29 The relationship between the structural and spectral changes for Ni(dmg)2 remains unknown, presumably because of the difficulty of obtaining single crystals of this compound. Up to now, high pressure studies of Ni(dmg)2 have only been performed using powdered polycrystalline samples. The optical, structural, and electrical properties of polycrystalline Ni(dmg)2 were studied by Takeda et al. up to pressures of around 25 GPa.30 The effect of pressure on the cell parameters and volume were described, and a reversible phase change at 7.4 GPa was reported. The powder diffraction data did not allow for structural determination of the second phase, or the structural refinement of the first phase under pressure. While anisotropic changes in the cell parameters of Ni(dmg)2 were measured, the structural effects responsible for this macroscopic strain were not determined. The results also suggested that the particularly strong color change observed around 2 GPa is not a result of a phase transition. The aim of the present study was to follow structural changes in Ni(dmg)2 in the pressure range through the major color

change point (2 GPa), but before the phase transition at 7.4 GPa by single-crystal X-ray diffraction, in order to investigate which changes in interatomic distances account for the pronounced anisotropic continuous lattice strain.

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MATERIALS AND METHODS Preparation of Single Crystals. It was found that the best crystals were produced using NiCl2·6H2O dissolved in an aqueous ethanol solution (40% ethanol by volume) to form a Ni2+ solution (0.01M). A solution of (dmg)H2 in ethanol (0.01M) was prepared using an ultrasonic bath. A small test tube was placed vertically in a holder and the NiCl2·6H2O solution (3 cm3) was added to the bottom. Aqueous ethanol (40%, 3 cm3) was added down the side of the test tube using a hypodermic needle. This was followed by the addition of the (dmg)H2 solution (3 cm3), also using a hypodermic needle. This produced two distinct layers with boundary between the NiCl2 aqueous ethanol solution and the (dmg)H2 ethanol solution. A red tinge was initially observed at the boundary of the (dmg)H2 solution. After standing at 22 °C for 60 h, small red crystals were observed to have grown suspended in the aqueous NiCl2 solution. These were filtered off and dried in air. High Pressure Generation and Measurement. Pressure was generated using a Boehler-Almax DAC31 with a diamond culet size of 600 μm. A stainless steel gasket with initial thickness of 200 μm was preindented to ∼90 μm. A hole of 300 μm in diameter was drilled in the gasket using the spark erosion technique. Methanol−ethanol (4:1) mixture was used as the pressure-transmitting medium,32,33 which ensured hydrostatic conditions over the entire pressure range of the experiment. Two rubies were added to the gasket to provide pressure calibration within ±0.05 GPa using the fluorescence technique.20 The dimensions of the selected crystal were measured to be 0.14 mm × 0.06 mm × 0.04 mm. High-Pressure Single-Crystal X-ray Diffraction. Diffraction data sets were collected using Mo−Kα radiation on an Oxford Diffraction Gemini R Ultra X-ray diffractometer with a CCD area detector at 0.2, 0.6, 1.2, 1.9, 2.8, 3.2, 3.9, 4.5, and 5.1 GPa. Data collection, cell determination and data integration were performed using CrysAlisPro software.34 Those reflections from the sample that overlapped with the diamond and gasket reflections were excluded manually. Absorption correction by Gaussian integration was achieved using the Absorb-7 with Absorb-GUI.35 The initial crystal structure model for refinement was obtained using SHELXS9736 and diffraction data collected from a free crystal of Ni(dmg)2 without a DAC. Refinement was carried out with SHELXL36 using X-Step 3237 as the GUI. Hydrogen atom parameters were constrained (AFIX 33 with Uiso(H) = 1.5 Ueq(C), C−H = 0.96 Å for CH3groups; Uiso(H) = 1.2 Ueq(O) with H atom lying equidistantly from O atoms in H-bonds for O−H groups). The SIMU restraint (which assumes that the Uij values on neighboring atoms are similar, with default deviation values) was applied to all non-H atoms when refining the structure at 5.1 GPa; if the structure at this pressure was refined anisotropically without restraints, the displacement ellipsoids had nonsatisfactory flattened shape. Mercury38 and Platon39 were used for structure visualization and analysis. In addition, the voids were calculated at 0.2 and 4.5 GPa using the “rolling ball” method included in Mercury38 with a probe radius of 0.2 Å and a grid of 0.1 Å. Data collection and refinement parameters, as well as crystal data, are summarized in Table 1. 24706


pressure (GPa)

24707

a

0.100 0.659 h = −19 → 19, k = −10 → 10, l = −8 → 8

0.116 0.660 h = −19 → 18, k = −10 → 9, l = −8 → 8

0.049, 0.090, 1.14 426 0 0.35, −0.42

0.089 0.664 h = −18 → 18, k = −10 → 10, l = −7 → 7

0.076 0.664 h = −18 → 18, k = −10 → 10, l = −7 → 7 0.043, 0.087, 1.11 429 0 0.43, −0.43

0.336, 0.407 2805, 426, 328

16.0380 (11), 9.939 (1), 5.9397 (3) 946.79 (12) 2.06

0.378, 0.464 2787, 429, 325

16.1082 (14), 9.9613 (12), 5.9848 (3) 960.31 (15) 2.03

0.044, 0.086, 1.12 465 0 0.34, −0.31 3.2

0.324, 0.406 3201, 465, 318

0.336, 0.407 2344, 470, 264

0.053, 0.095, 1.05 470 0 0.59, −0.52 2.8

16.5690 (18), 10.3439 (16), 6.4085 (4) 1098.3 (2) 1.78

0.2

16.5699 (16), 10.4295 (13), 6.4616 (4) 1116.66 (19) 1.75

0

For all pressure values: crystal system, orthorhombic, space group Ibam, and size of 0.14 × 0.06 × 0.04 mm3.

Refinement R[F2 > 2σ(F2)], wR(F2), S No. of reflections No. of restraints Δρmax, Δρmin (e Å−3)

V (Å3) μ (mm−1) Data Collection Tmin, Tmax No. of measured, independent, and observed [I > 2σ(I)] reflections Rint (sin θ/λ)max (Å−1) range of h, k, l

Crystal Data a, b, c (Å)

Refinement R[F2 > 2σ(F2)], wR(F2), S No. of reflections No. of restraints Δρmax, Δρmin (e Å−3) pressure (GPa)

V (Å3) μ (mm−1) Data Collection Tmin, Tmax No. of measured, independent, and observed [I > 2σ(I)] reflections Rint (sin θ/λ)max (Å−1) range of h, k, l

Crystal Data a, b, c (Å)

Table 1. Data Collection, Structure Refinement, and Crystal Parameters for a Nickel Dimethylglyoximea

0.040, 0.096, 1.07 422 0 0.38, −0.36

0.095 0.665 h = −18 → 18, k = −10 → 10, l = −7 → 7

0.367, 0.444 2723, 422, 314

15.9507 (13), 9.9145 (11), 5.8872 (3) 931.03 (14) 2.10

0.045, 0.084, 1.09 450 0 0.30, −0.27 3.9

0.098 0.661 h = −19 → 19, k = −10 → 10, l = −8 → 8

0.374, 0.465 3067, 450, 321

16.4739 (16), 10.2024 (14), 6.2972 (4) 1058.41 (19) 1.84

0.6

0.038, 0.090, 1.08 424 0 0.36, −0.43

0.081 0.665 h = −18 → 18, k = −10 → 10, l = −7 → 7

0.336, 0.407 2699, 424, 317

15.8529 (13), 9.8959 (10), 5.8379 (3) 915.85 (13) 2.13

0.061, 0.231, 1.23 445 0 1.14, −0.86 4.5

0.110 0.664 h = −19 → 18, k = −9 → 10, l = −8 → 8

0.384, 0.464 2987, 445, 319

16.3688 (17), 10.0853 (15), 6.1843 (4) 1020.93 (19) 1.91

1.2

0.045, 0.105, 1.04 423 42 0.52, −0.43

0.106 0.663 h = −17 → 18, k = −10 → 10, l = −7 → 7

0.383, 0.461 2673, 423, 317

15.74120 (13), 9.88870 (12), 5.7878 (3) 900.93 (5) 2.17

0.041, 0.085, 1.06 435 0 0.42, −0.37 5.1

0.096 0.664 h = −18 → 18, k = −10 → 9, l = −8 → 8

0.336, 0.407 2905, 435, 312

16.2653 (17), 10.0195 (14), 6.0897 (4) 992.44 (19) 1.97

1.9

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Computational Methods. Ni(dmg)2 was treated by planewave periodic DFT calculations, as implemented in the program package VASP v. 5.2.40−43 The Perdew−Burke− Ernzerhof exchange-correlation functional (PBE)44 was used together with a plane-wave basis set with a cutoff of 400 eV and Projector Augmented Wave (PAW) atomic pseudopotentials.45,46 The integrals in the reciprocal space were calculated on a 2 × 2 × 2 mesh of k-points. The DFT-D2 dispersion corrections of Grimme were added to the original PBE functional.47 The models were built on the basis of experimental crystallographic data collected at 0.2 and 5.1 GPa, representing the lowest and highest pressure considered in this study. It should be noted that the pressure effects are modeled solely by varying the unit cell parameters. Geometry optimization following the symmetry constraints of the Ibam space group was performed in order to refine the position of hydrogen atoms; all other atoms and the unit cell parameters were kept fixed to the experimental positions and values. This approach was tested in one of the previous studies,48 yielding reasonable results. A constant-volume molecular dynamics (MD) simulation was performed for both pressure extremes using a time step of 1 fs; the temperature of 300 K was controlled by a Nosé chain thermostat49 with a coupling frequency of about 225 cm−1. The total simulation time was 30 ps of which the first 10 ps were considered as equilibration and not used in the analysis. A minor part of our calculations was the optimization of an isolated Ni(dmg)2 complex performed at the B3LYP/6-31+G(d,p) level of theory by the Gaussian 09 program.50

Figure 3. Response of cell parameters (a) and cell volume (b) of Ni(dmg)2 to pressure. Error bars do not exceed symbol size.

crystallographic axis (coinciding with the direction of the Ni−Ni contacts) to form infinite chains in which the complexes are rotated by 90° with respect to their immediate neighbors (Figure 4a). The chains are further packed without any

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RESULTS AND DISCUSSION Single-Crystal X-ray Diffraction. The single crystal remained intact across the pressure range of this experiment. According to previously reported data,19,30 pressure induces continuous color changes from orange to yellow via blue, violet, and green if the sample is observed in the transmission mode. In our experiments we could see continuous crystal darkening with increasing pressure but not the detailed changes in its color (Figure 2), presumably because the crystal was thick and

Figure 2. Single crystal of Ni(dmg)2 at 0.2 and 5.1 GPa in the DAC.

the DAC had yellow artificial diamonds. The crystal structure of nickel dimethylglyoxime at all the pressures could be refined in the orthorhombic crystal system, space group Ibam (Table 1), with cell parameters and volume decreasing monotonically with increasing pressure (Table 1, Figure 3). The asymmetric unit contains half a molecule of Ni(dmg)2 located on a site of symmetry 2/m, with all nonhydrogen atoms lying on the mirror plane, resulting in a four-coordinate nickel ion and a short intramolecular OH···O hydrogen bond associated with the ligands (Figure 1). There are four molecules per unit cell. The molecules are stacked along the c

Figure 4. Packing of Ni(dmg)2 parallel to the c axis (a), the packing of Ni(dmg)2 viewed along the a axis (b), and the packing of Ni(dmg)2 viewed along the c axis (c) at 0.2 GPa.

intermolecular bonds being formed between the molecules (Figure 4b,c). The crystal packing is consistent with the crystal morphology: thin needles elongated along the c axis. 24708



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The changes in cell volume and cell parameters vs. pressure were continuous and monotonic (Figure 3). The major compression was observed along the c-direction (a decrease of 10% up to 5.1 GPa, corresponding to a 20% reduction in cell volume). Along a and b, a clear, but smaller, compression of 5% was observed. In general, these data were in reasonable agreement with those reported by Takeda et al.30 based on powder diffraction. However, extrapolating the trends observed in the decrease of a and b cell parameters versus pressure shows that the a (P) and b (P) curves intersect at around 5.5 GPa, while Takeda et al.30 suggested linear dependencies of all cell parameters with pressures up to ∼7 GPa. The directions of major and minor compression are clearly related to the orientation of the molecular chains and the packing of these chains in the crystal. The Ni−Ni distances along the a and b axes are the same as the length of the corresponding cell parameters, while the Ni−Ni distance along c is half the length of cell parameter c. The direction of major compression (0.67 Å or 10.4% at 5.1 GPa) is along the c axis, normal to the planar layers of Ni(dmg)2 and, thus, corresponds to shortening of the contacts between nickel cations along the chains of the molecules, which are the shortest Ni−Ni distances already at ambient pressure (3.2308(2) Å). The distances between nickel atoms were also analyzed for the other compounds from CSD. The distribution of the shortest Ni−Ni contacts and bonds is shown on Figure 5 and one can see that the title compound in selected pressure range shows Ni−Ni distances values which are also characteristic for cluster compounds with Ni−Ni bonds.51 Compression within the layers themselves (along axes a and b) is not as significant, as compression between the layers due to the location of “free” voids, most of which lie between the layers but not within the layers. The Ni−Ni interactions appear to account for the compressibility of the structure along the chains (c-axis). They are also responsible for the color changes observed on compression on account of their strong influence on the separation of the 3dz2 and 4pz orbitals. The observed red shift with increasing pressure reflects the decreasing energy separation between these two orbitals caused by the preferential stabilization of the 4pz orbitals with decreasing Ni−Ni distance. Anisotropy of compression in the plane normal to the chain axes can be interpreted, if the changes in the size and distribution of voids in the structure are analyzed (Figure 6). As the pressure is increased, the major compression along the c axis causes methyl groups of different layers to move closer to each

Figure 6. Distribution of voids in the structure of Ni(dmg)2 at 0.2 GPa (a) and at 4.5 GPa (b).

other. This accounts for the difference in the relative compressibility along b and along a. The methyl groups are able to approach each other with minimum interaction along a, but compression along b is sterically hindered. The relative compressibilities along a and b become equal at the higher pressures achieved in this study. This can be explained assuming that the size of the voids can no longer be reduced at these pressures without a more significant structural rearrangement. One might expect that the phase transition that was reported in the literature at 7.4 GPa30 could be due to rotation of the molecules in the chains, to minimize steric stress within and between the chains, similar to what has been reported for carbonyl complexes.52,53 Further experiments at higher pressures are needed, possibly supported by calculations on models with unit cell parameters extrapolated to high pressures, to test this hypothesis. The existing MD simulations already demonstrate that the methyl torsional motion becomes more constrained on

Figure 5. Distribution of Ni−Ni distance values for the compounds from CSD V. 5.34 (February 2014).51 Rectangle shows an area for Ni−Ni contacts in Ni(dmg)2. Arrow shows direction corresponding to distance change on increasing pressure. 24709



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Figure 7. Response to pressure of the Ni1−N2 (a), O1−N1 (b), O2−N2 (c) bonds, and O1··· O2 (d) distance in the H-bonds.

increasing pressure, suggesting increased steric repulsion between methyl groups. Most of the bond lengths within the molecules exhibit no statistically significant changes with pressure (a full table of bond lengths at different pressures can be found in the Supporting Information; Figure 7). One aspect of potential interest is the possibility of a pressure-induced proton shift along the O−H···O bond in the complex to make the hydrogen bond more symmetrical. Such an effect has been reported previously for several compounds.54−56 The quality of the diffraction data collected at high pressure did not allow the position of the hydrogen atom to be followed directly, but some conclusions could be drawn based on the comparison of the two N−O distances (Figure 7b). The N1−O1 and N2−O2 distances remained unchanged within experimental error. This suggests that in the case of nickel dimethylglyoxime no significant proton transfer along the O−H···O hydrogen bond takes place with increasing pressure. The O1−N1 bond can be supposed to exhibit less of a response to pressure due to the attached hydrogen atom. The distance between the O1 and O2 atoms remained constant within the experimental error, which agrees with the fact that this O−H···O hydrogen bond is very strong. The observed stiffness of the O1−H···O2 hydrogen bond and the absence of experimental evidence of proton transfer motivated the use of quantum calculations for the precise location of the hydrogen-bonded proton and a study of its dynamical behavior. X-ray diffraction techniques often fail in the precise location of protons, particularly in short hydrogen bonds. In addition, it has been found in a previous study that in a similar hydrogen bond the proton dynamics are significantly coupled to the torsional motion of the adjacent methyl groups, resulting in peculiar structural features.57 Quantum Calculations. Optimization of the two periodic models corresponding to the pressure extremes (0.2 and 5.1 GPa) yields refined positions of hydrogen atoms while keeping the rest of the structure the same as measured by diffraction.

Table 2. Calculated and Measured Geometric Parameters of the Intramolecular Hydrogen Bond method

P (GPa)

RO1H (Å)

RH···O2 (Å)

RO1···O2 (Å)

H optimization

0.2 5.1 0.2 5.1 0.2 5.1

1.138 1.144 1.206 1.190 0.872 1.199

1.339 1.314 1.285 1.291 1.596 1.260

2.467a 2.448a 2.473 2.462 2.467 2.448

MD average experimental a

Fixed to the experimental value.

The calculated O−H distances are displayed in Table 2. While the OH distances calculated by optimization of hydrogen positions exhibit the trend common to hydrogen bonds (that is, shifting the proton toward the center on compressing the donor··· acceptor distance), the MD-averaged O−H and O···H distances show the reverse trend−the proton is localized closer to the donor atom even when the O···O distance is reduced. This trend appears to be counterintuitive at the first glance and is evidently due to dynamical effects. It can be shown that the increased asymmetry of the hydrogen bond at higher pressure (denser packing) arises from the dynamical coupling between the proton motion and dynamics of the adjacent methyl groups. First, it should be noted that at both pressure extremes the proton dynamics features very large amplitudes (at least 0.6 Å), resulting in frequent passing of the proton from the donor to the acceptor oxygen and back. This is a common feature of hydrogen-bonded systems with a flat, single well proton potential.57,58 Figure 8 shows a 2 ps profile of geometric parameters of the hydrogen bond. The average distances displayed in Table 2 therefore result from a multitude of diverse contributions corresponding to proton location at the donor and the acceptor site as well as in-between. The proton location nearer to O1 is slightly prevalent, and the prevalence is slightly more pronounced at high pressure/density. 24710



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Figure 10. Probability density of methyl torsional orientation obtained from MD at different pressures.

Figure 8. Dynamic profile of the hydrogen bond geometry, extracted from MD simulation.

for methyl groups both near O1 and O2. Rotation remains possible, but orientations of 0° are several times less probable, indicating the existence of a rotational barrier most likely caused by steric repulsion between the neighboring molecules. As the rotation of methyl groups is notably hindered, symmetrization of the proton potential cannot be fully achieved; consequently, the proton is preferentially located closer to O1. This effect is more pronounced at high pressure, because the steric repulsion between the neighboring molecules increases, rendering methyl torsional motion more confined; for instance, the ratio between the lowest and highest probability density drops from 0.38 at 0.2 GPa to 0.19 at 5.1 GPa. This results in the shrinking of the O−H distance, an effect opposing and fully compensating the otherwise established trend of symmetrizing the hydrogen bond with the decreasing donor···acceptor distance.

But which factor controls the prevalence? Inspection of the optimized geometry of the isolated Ni(dmg)2 complex, a fully static calculation, reveals an interesting feature: torsional orientation of the methyl groups adjacent to the donor (O1) and acceptor (O2) site is different, as shown in Figure 9. While

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CONCLUSIONS High quality single crystals of Ni(dmg)2 have been grown and single-crystal X-ray diffraction data recorded up to pressures of 5 GPa. There were no detectable phase changes in this region, with the color change being attributed to the influence of pressure on the Ni−Ni distance in the chains formed by the molecules. The strong anisotropy of structural strain results from an interplay of Ni−Ni interactions and the steric effects imposed by the repulsion of the methyl groups. As the use of complementary experimental and theoretical methods offers several advantages in the structural studies of new compounds, the experimental diffraction study was assisted by periodic DFT calculations, yielding improved refinement of hydrogen atoms and important information about the structure and dynamics of the short hydrogen bond. A peculiar feature is the significant coupling between methyl torsions and the proton potential, resulting in an asymmetric, single-well hydrogen bond. Due to the pressure-driven steric repulsion the proton exhibits the tendency to be located nearer to the donor site at higher pressures. This high-pressure study complements previous work on organic molecular crystals, for which coupling of methyl-group dynamics and adjacent O−H···O and N−H···O hydrogen bonds was analyzed as a function of temperature.59 It also shows a new effect of pressure on the short O−H···O bond, as compared with previously reported symmetrization,54−56 or, on the contrary, a complete ionization of the two components resulting from a shift of the donor H atom to the acceptor.60

Figure 9. Optimized structure of the isolated Ni(dmg)2 complex with methyl torsional orientations.

at the donor site the methyl hydrogen is eclipsed with the hydrogen bond plane, it assumes staggered conformation at the acceptor site. Bearing in mind chemical symmetry of the system, it is evident that rotation of both methyl groups is required in order to obtain an equivalent structure on proton transfer. The orientation of methyl groups may cause considerable asymmetry of the proton potential, up to about 1 kcal/mol as demonstrated earlier for the case of tetraacetylethane.57 If for whatever reason the methyl torsional motion is hindered, it may impair the symmetry of the hydrogen bond. For Ni(dmg)2, the possibility of O−H distance elongation is closely related to the ability of effective symmetrization of the proton potential. In turn, this is crucially dependent on the torsional flexibility of the CH3 groups. The latter is demonstrated by the statistics of torsional orientation displayed in Figure 10. It can be seen that the methyl groups do not rotate entirely freely. Instead, they preferentially form a torsional angle of about 60° relative to the plane of the hydrogen bond; this holds 24711



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ASSOCIATED CONTENT

S Supporting Information *

A full table of bond lengths and CIF and FCF files, as well as checkcif report at different pressures. Complete structural data were deposited in CSD51 with refcodes CCDC 1022677−1022686. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the support of the Year Abroad Programme of The University of Edinburgh (I.F.B.-S.), the Royal Society of Edinburgh (C.R.P.), The Ministry of Education and Science of the Russian Federation, The Russian Academy of Sciences and The grant from the President of Russia for State support of Russian leading Scientific Schools, project NSh-279.2014.3 (B.A.Z., E.V.B.), and the Slovenian Research Agency (Program Code P1-0012; J.S.).

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Structural Properties of Nickel Dimethylglyoxime at High Pressure

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