Crystal structure and Hirshfeld surface analysis of poly[tris(μ4-benzene-1,4-dicarboxylato)tetrakis(dimethylformamide)trinickel(II)]: a two-dimensional coordination network

The structure of catena-[tris(μ4-benzene-1,4-dicarboxylato)-tetrakis(μ1– dimethylformamide-κ1 O)-trinickel(II)], C36H40N4Ni3O16, has been determined in the monoclinic P21/n space group. The compound has a two-dimensional coordination network structure and it is of interest with respect to lithium-ion battery applications. Hirshfeld surface analysis was performed to characterize interplanar interactions. The structure exhibits disorder of coordinated solvent molecules.

The crystal structure of the title compound, [Ni 3 (C 8 H 4 O 4 ) 3 (C 3 H 7 NO) 4 ], is a twodimensional coordination network formed by trinuclear linear Ni 3 (tp) 3 (DMF) 4 units (tp = terephthalate = benzene-1,4-dicarboxylate and DMF = dimethylformamide) displaying a characteristic coordination mode of acetate groups in polynuclear metal-organic compounds. Individual trinuclear units are connected through tp anions in a triangular network that forms layers. One of the DMF ligands points outwards and provides interactions with equivalent planes above and below, leaving the second ligand in a structural void much larger than the DMF molecule, which shows positional disorder. Parallel planes are connected mainly through weak C-HÁ Á ÁO, HÁ Á ÁH and HÁ Á ÁC interactions between DMF molecules, as shown by Hirshfeld surface analysis.

Chemical context
Extended hybrid organic-inorganic materials composed by transition metals and bridging carboxylates are interesting compounds that include the well-known metal-organic frameworks (MOFs), coordination polymers (CP) and coordination networks (CN) (Batten et al., 2013). In the last decade, much of the research into this kind of compounds has focused in the design of materials looking for tunability of potential industrial applications such as lithium-ion batteries (Shin et al., 2015;Maiti et al., 2015;Tian et al., 2016), substitutes for dye-sensitized solar cells (DSSCs) (Zhang et al., 2018;Yan et al., 2018;Jeevadason et al., 2014), luminescent compounds (Kara et al., 2018;Igoa et al., 2019) and magnetic materials (Mesbah et al., 2014) among others. In the search for new extended hybrid materials based on Ni and terephthalate (terephthalate = tp = benzene-1,4-dicarboxylate), the title compound [Ni 3 (C 8 H 4 O 4 ) 3 (C 3 H 7 NO) 4 ] was synthesized by a solvothermal process in dimethylformamide (DMF) and is currently under study for application as an anode material in lithium-ion batteries. In order to perform an adequate structure-property correlation, the crystal structure of the compound was determined and supramolecular features of potential interest for understanding Li-ion intercalation and migration were analysed using the Hirshfeld surface (HS). ISSN 2056-9890

Structural commentary
The title compound is a two-dimensional coordination polymer formed by linear trinuclear centrosymmetric Ni 3 (tp) 3 (DMF) 4 units connected through tp anions, which crystallizes in the monoclinic P2 1 /c space group. Two distinct hexacoordinated Ni 2+ cations (Ni1 in a special position with occupancy factor 0.5), two DMF ligands and two tp anions (anion B in a special position with occupancy factor 0.5) exist in the asymmetric unit (Fig. 1). The central Ni atom, located on an inversion centre, displays an octahedral coordination to O atoms from three pairs of carboxylate units belonging to three symmetry-related tp anions with Ni1-O bond distances in the range 2.0205 (14)-2.0868 (14) Å and a maximum deviation of 4.85 from the expected O-Ni1-O octahedral bond angles. The two terminal Ni2 cations also coordinate the carboxylate units of three symmetry-related tp units, one of them in bidentate mode, and two independent dimethylformamide ligands (one of them showing positional disorder) in a significantly distorted octahedron (Fig. 2). Ni2-O bond distances are in the range 2.0090 (15)-2.0791 (15) Å for terephthalate and 2.042 (12)-2.1853 (16) Å for DMF oxygen atoms respectively (including the lower occupancy disordered ligands). The O1B-Ni2-O2B angle of 61.52 (6) corresponding to a tridentate carboxylate, acting as bidentate towards Ni2, is very far away for the expected octahedral 90 angle. However, the coordination is still octahedral since O1B, O2B, O1C and O3A form a clear equatorial plane with Ni deviating by just 0.1202 (7) Å from the plane and the rest of the equatorial bond angles [O2B-Ni2-O3A = 99.18 (6), O3A-Ni2-O1C = 99.13 (7) and O1C-Ni2-O1B = 98.91 (7) ] are increased by about 10 to compensate for the very small angle from the bidentate ligand (O3A is in position 1 2 + x, 3 2 À y, 1 2 + z). Additionally the two apical atoms O1A and O1D lie 2.026 (6) and 2.1269 (16) Å , respectively, from the equatorial plane, forming an O1BD-Ni2-O1A angle of 176.0 (6) . The carboxylate that is bidentate towards Ni2 is also monodentate towards Ni1, with the O2B atom being the link between corner-sharing Ni1 and Ni2 octahedra, which explains the longer Ni-O2B bond distances of 2.0868 (14) and 2.0791 (15) Å to Ni1 and Ni2 respectively, compared with all other Ni-O tp bond distances (see Fig. 2). The trinuclear octahedral arrangement with the three Ni atoms coordinated exclusively by O has only been observed in one 1,3benzenedicarboxylate catena-[bis( 4 -isophthalato)bis( 3isophthalato)trinickel(II) bis(3-ethyl-1-methyl-1H-imidazol-3-ium)] (Chen et al., 2011) where the Ni cations are connected through the same number and coordination modes of carboxylate moieties. In that compound, however, two additional carboxylates complete the coordination spheres of the terminal Ni cations, instead of DMF molecules, giving a threedimensional connected network. Ni1Á Á ÁNi2 distances of 3.4414 (4) Å are observed, also found in the 1,2-benzenedicarboxylate (Ni central Á Á ÁNi terminal = 3.442 Å ). This coordination mode is frequently found in other trinuclear transition metal carboxylates, with and without different ligands bonded to the terminal cations.    Each terephthalate ion links two nearby trinuclear units forming a slightly distorted two-dimensional hexagonal arrangement along the crystallographic (101) plane as shown in Fig. 3. Since the central Ni atom (Ni1) of the trinuclear arrangement is located at (0, 0, 0) and equivalent ( 1 2 , 1 2 , 1 2 ) coordinates, the hexagonal arrangement shows a 2 + 4 distance pattern with two opposite nearby units at 9.6335 (11) Å (equal to the b-axis length) and four at 10.1407 (9) Å (equal to half of the short body diagonal of the unit cell) defining isosceles triangles with one small [56.718 (8) ] and two larger [(61.641 (4) ] angles. The tp anions link nearby units in two different modes. The longest interunit distance corresponds to tp anions connecting the top or bottom parts of the unit, parallel to the plane (terephthalate anion A), while the shorter distance corresponds to a tp unit that is located over a centre of symmetry (anion B), which connects the top/bottom part of one unit to the bottom/top part of the next unit. This diagonal connection produces a tilt in the linear trinuclear units that are rotated by 11.82 from the normal to the plane of the network, in a direction slightly away from the b axis.
The ordered DMF molecules (labelled C) point outwards at both sides of the planes providing a polar surface that allows for the interaction of parallel planes of the coordination polymer. The disordered DMF ligands (labelled D) occupy part of the void space between consecutive planes (see Section 3) and were modelled over three different positions rotated by 180 and displaced respectively, which strongly suggests that both static and dynamic disorder are present.

Supramolecular features and Hirshfeld surface analysis
Parallel planes do not stack in a typical hexagonal arrangement, where a layer projects over the voids of the poly[tris( 4benzene-1,4-dicarboxylato)tetrakis(dimethylformamide)trinickel(II)], but in this case one layer projects over the center of the short inter-unit distance of the next layer, or is shifted by half of the b-axis length. This is again a consequence of the position of the Ni1 atoms at the corners and the centre of the unit cell, forming planes along (101). Fig. 4a shows two parallel planes along the [101] direction (compare with Fig. 3) where it is shown that the projection of one plane falls away from the voids in the next one. Fig. 4b shows the same two planes along the [010] direction where the relative position of the ordered DMF ligands in consecutive layers is shown.
In order to visualize the interplanar interactions, Hirshfeld surface (HS) analysis (Hirshfeld, 1977;Spackman & Jayatilaka, 2009) was performed by using Crystal Explorer 17.5 (Turner et al., 2017). In the HS plotted over d norm ( sum of van der Waals radii, and the red and blue colours indicate distances shorter (in close contact) or longer (distant contact) than the van der Waals radii, respectively (Venkatesan et al., 2016). Since bonds from Ni1 to O atoms and from C2B and C4B to C atoms are not included in the asymmetric unit, bright-red spots appear over them. The following stronger short contacts shown as light-red spots correspond to weak C-HÁ Á ÁO hydrogen bonds shown in Table 1. It is interesting to note that the ordered DMF-C molecule shows one intramolecular C1C-H1CÁ Á ÁO1A and one interplanar C2C-H2CBÁ Á ÁO1B i hydrogen bond [symmetry code: (i) Àx + 3 2 , y + 1 2 , Àz + 1 2 ]. The former limits the rotation of the DMF group and the latter the orientation. This fixes the DMF-C molecules and provides the main interaction between parallel network planes. The DMF-D molecule, disordered over three positions, participates in no hydrogen bonds to the aldehyde carbon (C1D, C1AD or C1BD) but only to methyl H atoms, giving the molecule rotational freedom. Additionally, the DMF molecule is smaller than the void in which it sits, allowing for additional positional freedom. Removing DMF-C and DMF-D from the structural model, allowed the volume these molecules occupy in the crystal structure to be calculated. The void-calculation routine in PLATON (Spek, 2009) was used, with a probe radius of 1.2 Å (enough to place small monoatomic ions). Voids arising from removing DMF-C and DMF-D are 110.18 and 167.93 Å 3 per molecule, respectively (two of the voids are connected around 1 4 , À0.07, 3 4 and 3 4 , À0.02, 1 4 for DMF-C and 1/2,0.003,0 and 0,0.496,1/2 for DMF-D), again showing that the DMF-D molecule is located over a much larger void than its own size, justifying the observation of positional disorder. Moreover, performing the same void calculation procedure using each of the DMF-D positions individually (as is the real case for each appearance of the molecule in the crystal), it is observed that the highest occupied position of DMF-D leaves only 21.75 Å 3 free volume per molecule (in two smaller 10.88 Å 3 voids) while DMF-AD and DMF-BD leave larger 53.1 and 37.7 Å 3 voids, respectively. Besides the described hydrogen-bond interactions, contacts between H atoms from both DMF molecules and neigh-bouring H, O and C atoms from surrounding DMF and tp anions dominate the interactions in the crystal structure, as depicted in Fig. 6, where the two-dimensional fingerprint plots (McKinnon et al., 2007) are shown. HÁ Á ÁH interactions from the DMF ligands are the most relevant, covering 45% of the Hirshfeld surface The presence of voids and a significant number of weak interlayer interactions may well explain the possibility of using this material for Li-ion batteries, as will be discussed elsewhere.

Database survey
The May 2019 update of the CSD (Groom et al., 2016) contains six coordination networks comprising Ni and a terephthalate anion as the sole linker; however, none of them contains only O in the coordination sphere. Additionally, there are eight trinuclear linear Ni compounds formed by carboxylates and other oxygenated ligands, none of them coordination networks except for DAFHID (Chen et al., 2011), which is discussed above.

Synthesis and crystallization
The compound was synthesized by solvothermal method via reaction between NiCl 2 Á6H 2 O (0.6143 g, 2.58 mmol), terephthalic acid (0.8587 g, 5.20 mmol) and N,N-dimethylformamide   View of the three-dimensional Hirshfeld surface of the title complex plotted over d norm in the range À0.7548 to 1.5398 a.u.
(DMF)(50 ml) as a solvent; the reactants were dissolved in DMF and transferred to a steel autoclave at 423 K for 24 h.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were placed at geometrically suitable positions and refined riding with U iso (H) = 1.2 or 1.5 times the U eq of the parent C atom. There are two sites occupied with N,N-dimethylformamide (DMF) molecules; one of them showing disorder that was modelled in three different positions with relative occupancies of 0.502, 0.286 and 0.212. This causes C atoms from the DMF methyl groups to have very large thermal displacement parameters that required the use of similarity restraints to converge to reasonable values.  program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008) and VESTA (Momma & Izumi, 2011); software used to prepare material for publication: publCIF (Westrip, 2010).