Two cocrystals of trans-NiII(hfac)2(H2O)2 containing cocrystallized water or 1,2-dimethoxybenzene were characterized by single-crystal X-ray diffraction. The hydrogen-bonding networks in each cocrystal were compared to previously reported structures, revealing that NiII(hfac)2(H2O)2 is capable of forming a variety of hydrogen-bond network motifs depending on the nature of the coformers in the crystal structure. Cocrystallization of 1,2-dimethoxybenzene was found to inhibit the formation of extended hydrogen-bond networks.
Keywords: crystal structure, cocrystal, hydrogen-bond network, solvate, chemical vapor deposition precursor
Abstract
Chemical vapor deposition (CVD) precursors often employ fluorinated β-diketonate ligands bonded to metal atoms; however, these are prone to the formation of hydrates under ambient conditions and form hydrogen-bond networks which hinder the CVD process. This work investigated the use of 1,2-dimethoxybenzene (DMB) as a ligand to prevent the formation of hydrogen-bonding networks. The single-crystal structures of two complexes of diaquabis(1,1,1,5,5,5-hexafluoro-4-oxopent-2-en-2-olato-κ2O,O′)nickel(II), trans-NiII(hfac)2(H2O)2, were determined, one as a 4/3 hydrate, [Ni(C5HF6O2)2(H2O)2]3·4H2O, and one as a 1,2-dimethoxybenzene (DMB) pentasolvate, [Ni(C5HF6O2)2(H2O)2]·5C8H10O2. The hydrogen-bond networks of these two materials were compared to previously reported hydrogen-bond networks seen in NiII(hfac)2(H2O)2. DMB was found to disrupt the formation of an extended hydrogen-bond network without coordinating to the Ni center.
Introduction
The drive for improved microelectronic circuits requires the development of methods to fabricate materials with finely tuned films of metal compounds. Chemical vapor deposition (CVD) has been used for decades in the production of microelectronics. For CVD precursors, ensuring thermal stability while allowing for low-temperature volatilization are ideal. A common strategy has been to employ fluorinated β-diketonate ligands, such as 1,1,1,5,5,5-hexafluoroacetylacetonate (hfac) (Mishra & Daniele, 2015 ▸). Under ambient conditions, these transition-metal and lanthanide complexes form hydrate adducts to the metal center (Binnemans, 2005 ▸; Malandrino et al., 1996 ▸; Malandrino et al., 2006 ▸; Zhang et al., 2008 ▸). These hydrates hinder the sublimation process, presumably through the formation of strong intermolecular hydrogen-bonding networks. Neutral bidentate ligands such as dimethoxyethane (DME) are capable of displacing coordinated water molecules (Malandrino et al., 1996 ▸; Fatila et al., 2012 ▸) and are capable of forming volatile mononuclear complexes, making them amenable to acting as CVD precursors. We sought to determine whether the 1,2-dimethoxybenzene (DMB) ligand could successfully displace the aqua ligands in trans-Ni(hfac)2(H2O)2. While the Ni(hfac)2(DME) and Mn(hfac)2(DME) complexes have been prepared (Ibrahim et al., 2025 ▸; Gray et al., 2026 ▸), to the best of our knowledge, analogous compounds containing the DMB ligand have not been isolated. Based on its reduced flexibility and the reduced donating ability of the methoxy group, we did not expect it to coordinate as strongly as DME. We were interested in the possibility that DMB could disrupt the hydrogen-bond network around the Ni(hfac)2(H2O)2 complex and what structural changes the DMB ligand could facilitate.
Experimental
Chemicals and materials
NaHCO3 (99.9%) and NiCl2·6H2O (99.5%) were purchased from Mallinckrodt Chemical Works. 1,1,1,5,5,5-Hexafluoropentane-2,4-dione (H-hfac) (98%) and n-heptane were obtained from Thermo Scientific Chemicals. 1,2-Dimethoxybenzene (DMB) (veratrole) was purchased from Acros Organics (99%). Ultrapure water (18 MΩ) was used as solvent.
Synthetic procedure
NaHCO3 (0.5934 g, 7.063 mmol) was dissolved in water (50 ml). An equimolar amount of H-hfac (1 ml, 7 mmol) was then added. The pH was measured to be approximately 6. To this solution, 0.1 g of NaHCO3 was added to raise the pH to approximately 7. NiCl2·6H2O (0.8397 g, 3.533 mmol) was added to the solution. A precipitate immediately formed. The precipitate was filtered off under vacuum. The crude yield was 0.6052 g [33%, 1.189 mmol, assuming a formula of NiII(hfac)2(H2O)2]. The green solid was immediately suspended in n-heptane (10 ml) and reacted with 1,2-dimethoxybenzene (0.9 ml, 7 mmol). Recrystallization via slow evaporation yielded colorless and green crystals in the same pot (the yield was not obtained). Throughout this article, the green crystals are referred to as 1 and the colorless crystals as 2 (see Scheme 1 and Fig. 1 ▸).
Figure 1.
Crystals of 1 (green, left) and 2 (colorless, right). A 150 µm MiTeGen loop is shown for scale. The specimens shown in this image are not the crystals used for the data collection.
Crystallographic data collection
Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. H atoms of water molecules were located in the difference map and refined freely with relative isotropic displacement parameters, with the exception of H3OA_2 and H3OB_2. These atoms were placed via the difference map but their positions were not allowed to refine, as otherwise H3OB_2 would refine to an unrealistic position. All other H atoms were placed in computed locations and refined as riding atoms with relative isotropic displacement parameters. Hirshfeld surface compositions were calculated using the CrystalExplorer software package (Spackman et al., 2021 ▸). The crystal of trans-NiII(hfac)2(H2O)2 and water, crystal 1, for which the data are reported, was found to be a 2-component non-merohedral twin with an approximate domain ratio of 89:11. The minor domain was rotated from the first domain by 179.9° about an axis in reciprocal space given by the following vector [0.484 0.000 1.000] and an axis in direct space given by the following vector [0.996 0.001 1.000]. The twin law relating the two domains is (in line format): [−0.350 0.004 0.653] [0.000 −1.000 0.000] [1.345 −0.002 0.350].
Table 1. Experimental details.
Experiments were carried out at 153 K with Mo Kα radiation using a Bruker D8 VENTURE diffractometer. H atoms were treated by a mixture of independent and constrained refinement.
| 1 | 2 | |
|---|---|---|
| Crystal data | ||
| Chemical formula | [Ni(C5HF6O2)2(H2O)2]3·4H2O | [Ni(C5HF6O2)2(H2O)2]·5C8H10O2 |
| M r | 1598.64 | 1199.65 |
| Crystal system, space group | Monoclinic, P21/n | Monoclinic, P21/c |
| a, b, c (Å) | 24.3968 (14), 7.2381 (4), 31.5461 (17) | 11.9278 (5), 15.7842 (6), 15.0115 (6) |
| β (°) | 106.300 (2) | 93.887 (1) |
| V (Å3) | 5346.7 (5) | 2819.7 (2) |
| Z | 4 | 2 |
| μ (mm−1) | 1.25 | 0.45 |
| Crystal size (mm) | 0.27 × 0.25 × 0.08 | 0.34 × 0.26 × 0.08 |
| Data collection | ||
| Absorption correction | Multi-scan (TWINABS; Sevvana et al., 2019 ▸; Sheldrick, 2012 ▸) | Multi-scan (SADABS; Krause et al., 2015 ▸) |
| Tmin, Tmax | 0.603, 0.745 | 0.679, 0.746 |
| No. of measured, independent and observed [I > 2σ(I)] reflections | 11657, 11657, 10792 | 66877, 7928, 5947 |
| R int | Not applicable | 0.060 |
| (sin θ/λ)max (Å−1) | 0.629 | 0.695 |
| Refinement | ||
| R[F2 > 2σ(F2)], wR(F2), S | 0.065, 0.147, 1.16 | 0.038, 0.106, 1.02 |
| No. of reflections | 11657 | 7928 |
| No. of parameters | 1020 | 558 |
| No. of restraints | 566 | 267 |
| Δρmax, Δρmin (e Å−3) | 0.98, −0.69 | 0.40, −0.42 |
Results and discussion
Crystal structure analysis
The crystal structure of 1 is a cocrystal of trans-NiII(hfac)2(H2O)2 and water, and it crystallized in the space group P21/n. The asymmetric unit contains two molecules of the complex, as well as two half-molecules in which the Ni atom is located on a crystallographic inversion center (Fig. 2 ▸). There are four cocrystallized water molecules, resulting in a trans-NiII(hfac)2(H2O)2 to water stoichiometry of 3:4. Eight of the twelve trifluoromethyl (CF3) groups in the asymmetric unit are disordered and modeled in two partially occupied parts. Geometric and rigid bond restraints were applied to both parts of all of the disordered CF3 groups to ensure reasonable geometries and anisotropic displacement parameters (ADPs). When necessary, ADP constraints were applied to the F atoms to prevent the refinement of prolate, oblate, or non-positive definite ADPs. Bond distance restraints were applied to the O—H and H⋯H distances for all of the aqua ligands to ensure similar bond lengths and angles between all of the ligands. Similar restraints were applied to the cocrystallized water molecules in the structure. The packing of crystal 1 consists of alternating layers parallel to the [101] direction featuring hydrogen-bonding interactions between the aqua ligands and free water molecules, and sheets of CF3⋯CF3 interactions (Fig. 3 ▸ and Table 2 ▸). There are two primary hydrogen-bonding motifs that make up the hydrogen-bonding layers, which will be categorized using graph-set notation (Etter et al., 1990 ▸). In the first type (Fig. 4 ▸, a), one of the aqua ligands on the trans-NiII(hfac)2(H2O)2 complex forms an 
(6) hydrogen-bonded ring with the two O atoms of a hfac ligand on an adjacent NiII(hfac)2(H2O)2 complex. The aqua ligand at the other end of the complex donates a hydrogen bond to two of the cocrystallized water molecules. In the second type of hydrogen-bond motif, both of the aqua ligands on a trans-NiII(hfac)2(H2O)2 unit act as hydrogen-bond donors to cocrystallized water molecules, forming 
(6) hydrogen-bond chains (Fig. 4 ▸, b). The cocrystallized water molecules in this layer also form (6) hydrogen-bonded rings with the O atoms of adjacent hfac ligands. This leads to a general packing scheme in which one layer of an aqua–hfac (6) hydrogen-bond network is followed by two layers of (6) aqua–water hydrogen-bond networks. All cocrystallized water molecules act as hydrogen-bond donors to the O atoms of hfac ligands and as acceptors from aqua ligands. This complex hydrogen-bonding network explains the large number of molecules in the asymmetric unit of the crystal. As shown in Fig. 1 ▸, crystals of 1 were large and it was necessary to cut the crystals for data collection. Obtaining a high-quality single crystal of a suitable size proved to be difficult, as the crystals tended to slip rather than split cleanly when being cut. This resulted in streaks in the diffraction pattern, and also made it difficult to select a quality single crystal. Data collection was attempted on several crystals before a satisfactory structure solution could be obtained. The crystal packing offers a possible explanation for the macroscopic slipping behavior of the crystals. Based on the weak intermolecular forces of CF3⋯CF3 contacts, it can be speculated that when the crystals were being cut for mounting the slipping occurred along the areas of CF3⋯CF3 interactions, although this was not confirmed by experiment.
Figure 2.
The asymmetric unit of 1. Only the majorly occupied parts of disordered CF3 groups are shown. H atoms not bonded to water have been hidden for clarity. Unless otherwise stated, the following applies to all images of crystal structures in this article: non-H atoms are drawn as displacement ellipsoids at the 50% probability level and H atoms are drawn as fixed size spheres.
Figure 3.
A 2 × 2 × 2 supercell of 1, viewed along the crystallographic b axis, highlighting the alternating layers of hydrogen-bond networks and CF3⋯CF3 contacts. The red and blue axes of the displayed unit cell are the a and c axes, respectively. Only the majorly occupied parts of disordered CF3 groups are shown.
Table 2. Hydrogen-bond geometry (Å, °) for 1.
| D—H⋯A | D—H | H⋯A | D⋯A | D—H⋯A |
|---|---|---|---|---|
| O3_1—H3OB_1⋯O1_1i | 0.81 (2) | 2.11 (4) | 2.824 (5) | 147 (6) |
| O3_1—H3OA_1⋯O4_1i | 0.81 (2) | 2.21 (4) | 2.879 (5) | 141 (6) |
| O6_1—H6OA_1⋯O1_5 | 0.80 (2) | 2.00 (2) | 2.796 (6) | 172 (6) |
| O6_1—H6OB_1⋯O4_5ii | 0.80 (2) | 1.94 (2) | 2.728 (6) | 166 (7) |
| O3_2—H3OA_2⋯O2_5iii | 0.78* | 2.01* | 2.786 (6)* | 173* |
| O3_2—H3OB_2⋯O3_5iv | 0.88* | 1.87* | 2.733 (6)* | 167* |
| O6_2—H6OB_2⋯O2_2ii | 0.81 (2) | 2.13 (4) | 2.837 (5) | 145 (6) |
| O6_2—H6OA_2⋯O5_2ii | 0.81 (2) | 2.18 (4) | 2.876 (5) | 145 (6) |
| O3_3—H3OA_3⋯O4_5 | 0.78 (5) | 2.07 (5) | 2.842 (6) | 173 (9) |
| O3_3—H3OB_3⋯O1_5ii | 0.78 (5) | 1.99 (5) | 2.766 (6) | 173 (8) |
| O3_4—H3OB_4⋯O3_5 | 0.81 (2) | 2.05 (2) | 2.842 (6) | 168 (8) |
| O3_4—H3OA_4⋯O2_5iii | 0.81 (2) | 1.97 (2) | 2.769 (6) | 171 (8) |
| O1_5—H1OA_5⋯O2_3v | 0.81 (2) | 2.05 (3) | 2.835 (6) | 161 (6) |
| O1_5—H1OB_5⋯O1_3ii | 0.81 (2) | 2.31 (5) | 2.943 (6) | 135 (6) |
| O2_5—H2OB_5⋯O1_4vi | 0.81 (2) | 2.34 (5) | 2.965 (6) | 135 (6) |
| O2_5—H2OA_5⋯O2_4 | 0.81 (2) | 2.09 (4) | 2.845 (6) | 154 (6) |
| O3_5—H3OA_5⋯O4_2 | 0.81 (2) | 2.16 (5) | 2.823 (6) | 139 (6) |
| O3_5—H3OB_5⋯O1_2 | 0.81 (2) | 2.12 (4) | 2.848 (6) | 150 (6) |
| O4_5—H4OA_5⋯O2_1ii | 0.82 (2) | 2.09 (4) | 2.835 (6) | 152 (8) |
| O4_5—H4OB_5⋯O5_1ii | 0.81 (2) | 2.10 (5) | 2.823 (6) | 147 (8) |
Symmetry codes: (i) 
; (ii) 
; (iii) 
, 
; (iv) 
; (v) 
; (vi) 
. Note: (*) these H atoms were initially placed via the difference map. They were refined using fixed positions, as otherwise H3OB_2 would refine to an unrealistic position.
Figure 4.
A layer of the hydrogen-bonding network present in 1, viewed in the [101] direction. In this figure and subsequent figures, hydrogen bonds are represented by dashed blue lines. Hydrogen bonds labeled ‘a’ involve an aqua ligand donor and hfac acceptors, while hydrogen bonds labeled ‘b’ involve aqua ligand donors and water acceptors. Portions of the hfac ligands are drawn in wireframe for clarity.
Crystal 2 is a cocrystal of trans-NiII(hfac)2(H2O)2 and DMB, and crystallized in the space group P21/c. The Ni atom is situated on a crystallographic inversion center, leading to half of the molecule being crystallographically unique. Within the asymmetric unit, the aqua ligand acts as a hydrogen-bond donor to two cocrystallized DMB molecules, with both of the O atoms on the DMB acting as acceptors to one of the H atoms of the aqua ligand in an 
(5) manner (Fig. 5 ▸ and Table 3 ▸). Each of the CF3 groups of the complex are disordered and were modeled in two partially occupied parts using geometric and rigid bond restraints to ensure realistic geometries and ADPs. An ADP constraint was used to prevent atom F3′_1 from having prolate ADPs. There is an additional cocrystallized DMB molecule that is located in a cavity on a crystallographic inversion center and is only half occupied within the asymmetric unit, giving a trans-NiII(hfac)2(H2O)2 to DMB stoichiometry of 1:5. The DMB molecule located on the inversion center is disordered and was modeled in two partially occupied general positions, with occupancies that add up to 50%. The two disordered parts were modeled using a geometric restraints based on the ordered DMB molecule in Residue 3. A rigid bond restraint was applied to both parts of the disordered DMB molecule. In the unit cell, the Ni atoms of the trans-NiII(hfac)2(H2O)2 complexes sit on the inversion centers located at 
, 0, 0 and , , , with all of the Ni atoms in a plane. The disordered free DMB molecule sits on the inversion centers between the metal complexes at , , 0 and , 0, (Fig. 6 ▸). In the packing of the crystal, the trans-NiII(hfac)2(H2O)2 complexes and associated hydrogen-bonded DMB molecules form contacts with the free DMB molecule in the crystallographic b and c directions, and contact adjacent complexes in the a, [011], and [0
1] directions (Fig. 6 ▸).
Figure 5.
The asymmetric unit of 2. The DMB molecule in Residue 4 is disordered. Only the majorly occupied disordered parts are shown. H atoms not bonded to water have been hidden for clarity.
Table 3. Hydrogen-bond geometry (Å, °) for 2.
| D—H⋯A | D—H | H⋯A | D⋯A | D—H⋯A |
|---|---|---|---|---|
| O3_1—H1_1⋯O1_2 | 0.80 (2) | 2.17 (2) | 2.8978 (17) | 152 (2) |
| O3_1—H1_1⋯O2_2 | 0.80 (2) | 2.33 (2) | 2.9854 (17) | 140 (2) |
| O3_1—H2_1⋯O1_3 | 0.84 (2) | 2.31 (2) | 2.9100 (16) | 128.8 (19) |
| O3_1—H2_1⋯O2_3 | 0.84 (2) | 2.23 (3) | 3.0377 (17) | 161 (2) |
Figure 6.
A 2 × 2 × 2 supercell of 2, viewed along the crystallographic a axis. The b and c axes are shown in green and blue, respectively. Minorly occupied portions of disordered parts have been hidden for clarity.
Like with 1, crystals of 2 were large and needed to be cut to obtain an appropriate size for data collection. Contrary to 1, the crystals of 2 cleaved cleanly, likely because the crystal structure lacks the layers of CF3⋯CF3 interactions present in compound 1. In fact, there are no F⋯F short contacts (defined as a distance less than the sum of the van der Waals radii of the two atoms) in the crystal packing of 2, and all of the close contacts to the trans-NiII(hfac)2(H2O)2 complex are with DMB molecules. The chemical environments of the metal complexes in the two crystal structures were investigated using Hirshfeld surface analysis, a useful method for enumerating the differences in packing environments between different molecules in crystal structures (Spackman & Jayatilaka, 2009 ▸). Hirshfeld surfaces were computed for the trans-NiII(hfac)2(H2O)2 complexes in 1 and 2. Table 4 ▸ gives the surface compositions for the metal complexes in each of the structures. Each of the four trans-NiII(hfac)2(H2O)2 molecules in the structure of 1 have very similar surface compositions, with F⋯F contacts being the largest contributor to the Hirshfeld surface. In contrast, the trans-NiII(hfac)2(H2O)2 complex in 2 has almost no F⋯F contacts, making up less than 1% of the Hirshfeld surface. The complex in 2 has about double the F⋯H/H⋯F contacts of the complexes in 1, and about triple the contribution from H⋯H contacts. Notably, there are no C⋯H/H⋯C contacts for any of the trans-NiII(hfac)2(H2O)2 complexes in 1, while the complex in 2 has a 4.4% contribution of these contacts to the Hirshfeld surface.
Table 4. Contributions (%) of different contact types to the Hirshfeld surfaces of each of the four trans-NiII(hfac)2(H2O)2 complexes in 1, the average values in 1, and for the trans-NiII(hfac)2(H2O)2 complex in 2.
All contacts include reciprocal contacts. Minor contributions have been omitted, leading to a sum of less than 100% for each complex.
| Hirshfeld surface contact type | 1, Residue 1 | 1, Residue 2 | 1, Residue 3 | 1, Residue 4 | 1, Average | 2 |
|---|---|---|---|---|---|---|
| F⋯F | 43.8 | 42.6 | 45.0 | 45.1 | 44.1 | 0.7 |
| F⋯H/H⋯F | 28.9 | 27.2 | 28.3 | 30.2 | 28.7 | 61.5 |
| F⋯C/C⋯F | 7.3 | 9.9 | 6.3 | 6.0 | 7.4 | 2.7 |
| F⋯O/O⋯F | 2.2 | 1.7 | 2.4 | 2.6 | 2.2 | 1.7 |
| C⋯H/H⋯C | 0 | 0 | 0 | 0 | 0 | 4.4 |
| H⋯H | 3.4 | 3.5 | 3.9 | 3.9 | 3.7 | 10.9 |
| O⋯H/H⋯O | 12.9 | 12.4 | 11.8 | 12.1 | 12.3 | 16.7 |
Comparison to literature structures
A search of the Cambridge Structural Database (CSD; Version 6.01, including May 2026 updates) (Groom et al., 2016 ▸) reveals only four crystal structures containing NiII(hfac)2(H2O)2 complexes: VOXRAB (Romero et al., 1992 ▸), KUMBEZ (Luneau et al., 1992 ▸), TUKPEU (Burdukov et al., 1996 ▸), and ECOJAI (Maekawa et al., 2006 ▸). VOXRAB is a neat form of the cis-NiII(hfac)2(H2O)2 complex, in which the two aqua ligands are cis rather than trans across the nickel center. The crystal packing of VOXRAB bears some similarities to that of 1, with (6) hydrogen-bonded rings between the aqua ligands and hfac ligands leading to chains of metal complexes in which the hydrogen-bonded rings are separated by Ni atoms [Fig. 7 ▸(a)]. Unlike in crystal 1, in which the hydrogen-bond network forms two-dimensional sheets separated by F⋯F contacts, the hydrogen-bond chains of metal complexes in VOXRAB are one-dimensional, and contacts between chains include F⋯F contacts, as well as F⋯H contacts. An examination of the Hirshfeld surface compositions between 1 and VOXRAB shows that VOXRAB has significantly more F⋯O contacts compared to 1, while having fewer H⋯O contacts (Table 5 ▸).
Figure 7.
(a) Hydrogen bonding between cis-NiII(hfac)2(H2O)2 complexes in VOXRAB. (b) 
(14) hydrogen-bonded rings in TUKPEU. (c) A view of the hydrogen-bond sheet in KUMBEZ. (d) (18) hydrogen-bonded rings in ECOJAI. The CIF files deposited in the CSD for structures VOXRAB, KUMBEZ, and TUKPEU do not contain displacement ellipsoid information, so the atoms are drawn as fixed size spheres.
Table 5. A comparison of Hirshfeld surface compositions (%) between 1 and VOXRAB.
| Hirshfeld surface contact | 1, average | VOXRAB |
|---|---|---|
| F⋯F | 44.1 | 46.6 |
| F⋯H/H⋯F | 28.7 | 25.1 |
| F⋯C/C⋯F | 7.4 | 8.1 |
| F⋯O/O⋯F | 2.2 | 9.8 |
| C⋯H/H⋯C | 0 | 0 |
| H⋯H | 3.7 | 1.8 |
| O⋯H/H⋯O | 12.3 | 8.6 |
The structures of KUMBEZ, TUKPEU, and ECOJAI are all cocrystals of trans-NiII(hfac)2(H2O)2 with 2,5-dihydroimidazole nitroxide-based coformers. For crystals KUMBEZ and TUKPEU, each H atom on the aqua ligands acts as a donor to a coformer, giving a trans-NiII(hfac)2(H2O)2 to coformer ratio of 1:4, similar to what is seen in crystal 2. Unlike in 2, the coformers in KUMBEZ and TUKPEU have multiple hydrogen-bond-accepting sites, resulting in the formation of hydrogen-bond networks. In TUKPEU, the hydrogen bonding forms (14) rings in which the aqua ligand acts as a hydrogen-bond donor to an imidazole N atom and a methoxy O atom of two separate coformers [Fig. 7 ▸(b)]. Similar to 1, the hydrogen-bond networks in KUMBEZ form two-dimensional hydrogen-bonded sheets parallel to the [101] direction [Fig. 7 ▸(c)]. Each trans-NiII(hfac)2(H2O)2 complex participates in two (14) hydrogen-bonded rings, and each hydrogen-bonded ring is separated from the next by the Ni atom of the metal complex.
ECOJAI includes a coformer that contains a trans-NiII(hfac)2 center, with a symmetric 2,5-dihydroimidazole nitroxide dimer acting as a bidentate ligand on the metal center. This ligand has two hydrogen-bond-accepting sites. Contrary to 2, in which each H atom of the aqua ligand acts as a donor to both acceptor atoms on the DMB coformer, each acceptor of the coformer in ECOJAI is hydrogen bonded to only one H atom from the aqua ligand. The crystal packing and hydrogen-bond network are quite different from crystal 1, consisting of (18) hydrogen-bonded rings in which each H atom of the aqua ligand of the trans-NiII(hfac)2(H2O)2 complex forms a hydrogen bond with the N-oxide O atom of the other complex, with a crystallographic inversion center located in the center of the hydrogen-bond network [Fig. 7 ▸(d)]. Similarly to TUKPEU, the trans-NiII(hfac)2(H2O)2 complex participates in two (18) hydrogen-bonded rings, and each hydrogen-bonded ring is separated frem the next by the Ni atom of NiII(hfac)2. While the sample size is small, it is observed that cocrystals of trans-NiII(hfac)2(H2O)2 are capable of forming different hydrogen-bond motifs depending on the identity of the coformer. Coformers which have two accepting sites on different ends of the molecule can form hydrogen-bonded sheets or rings linked by Ni centers. Water, a coformer with two donating sites and two accepting sites, enabled the formation of sheets which contained both ring and chain hydrogen-bond motifs. Notably, the crystal structure of 2 demonstrates that DMB, a coformer with two accepting sites located on the same side of the molecule, prevents the formation of a hydrogen-bond network, even when the DMB molecule itself does not coordinate to the metal center. This provides insight into methods by which cocrystal design can be used to manipulate the hydrogen-bond networks in trans-NiII(hfac)2(H2O)2 and structurally related compounds.
Conclusion
In this work, the crystal structures of two solvates of trans-NiII(hfac)2(H2O)2 are reported. The crystal structure containing water as a coformer formed layers of hydrogen-bond networks separated by layers of CF3⋯CF3 contacts, a packing motif that potentially results in the macroscopic tendency of the crystals to slip rather than break cleanly when cut. The DMB cocrystal formed discrete units of trans-NiII(hfac)2(H2O)2 hydrogen bonded to four DMB molecules, with the DMB molecules occupying all hydrogen-bond-accepting sites of the aqua ligands, preventing the formation of an extended hydrogen-bond network. These new structures increased the amount of reported NiII(hfac)2(H2O)2 crystal structures, with only four such structures having been reported previously. An examination of the hydrogen-bonding motifs in each of the structures revealed that despite the small sample size, trans-NiII(hfac)2(H2O)2 cocrystals can form a variety of hydrogen-bond network motifs, and that DMB is capable of disrupting the formation of a hydrogen-bond network in trans-NiII(hfac)2(H2O)2 without needing to coordinate to the metal center.
Supplementary Material
Crystal structure: contains datablock(s) 26169twin5revised, 26164revised, global. DOI: 10.1107/S2053229626009460/vx3026sup1.cif
Structure factors: contains datablock(s) 26169twin5revised. DOI: 10.1107/S2053229626009460/vx30261sup2.hkl
Structure factors: contains datablock(s) 26164revised. DOI: 10.1107/S2053229626009460/vx30262sup3.hkl
Supporting information file. DOI: 10.1107/S2053229626009460/vx30261sup4.mol
Supporting information file. DOI: 10.1107/S2053229626009460/vx30262sup5.mol
Acknowledgments
The authors would like to acknowledge Dr. Maren Pink of the Indiana University Molecular Structure Center for her assistance in the collecting and processing of the crystal structure data. Support for the acquisition of the Bruker VENTURE D8 diffractometer through the Major Scientific Research Equipment Fund from the President of Indiana University and the Office of the Vice President for Research is gratefully acknowledged. The authors thank the College of Engineering and Science and Department of Chemistry and Physics at Louisiana Tech University for their support of the project through a W. W. Chew #2 Professorship award to Elisabeth M. Fatila.
Conflict of interest
The authors declare no conflicts of interest.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Crystal structure: contains datablock(s) 26169twin5revised, 26164revised, global. DOI: 10.1107/S2053229626009460/vx3026sup1.cif
Structure factors: contains datablock(s) 26169twin5revised. DOI: 10.1107/S2053229626009460/vx30261sup2.hkl
Structure factors: contains datablock(s) 26164revised. DOI: 10.1107/S2053229626009460/vx30262sup3.hkl
Supporting information file. DOI: 10.1107/S2053229626009460/vx30261sup4.mol
Supporting information file. DOI: 10.1107/S2053229626009460/vx30262sup5.mol