Crystal structure of aqua-trans-bis(dimethyl sulfoxide-κO)(pyridine-2,6-dicarboxylato-κ3 O 2,N,O 6)nickel(II)

The title complex is situated on a twofold rotation axis and forms an alternating layered structure with a a three-dimensional hydrogen-bonding network.


Chemical context
Crystal engineering plays an important role in the research of molecule-based functional materials by providing an effective approach towards the rational design and preparation of compounds with special structural features (Robin & Fromm, 2006;Cook et al., 2013;Wang et al., 2013). The crystallization of coordination polymers involves both the formation of a local coordination geometry and the propagation and packing of extended polymeric structures in the three-dimensional space. The competition among various types of intermolecular interactions plays a critical role in this process and is strongly influenced by synthetic conditions such as the choice of solvent, temperature, and the molecular features of the starting materials (Li & Du, 2011;Du et al., 2013). Although much has been learned about how the synthetic conditions affect intermolecular interactions and the final crystal structures, the targeted synthesis of a coordination polymer with a particular crystal structure is still a challenge. We recently reported an Ni II -containing one-dimensional coordination polymer based on the tridentate 2,6-pyridine dicarboxylic acid dianion (dpa 2À ) and bridging pyrazine molecules that was prepared by using DMSO as the solvent (Liu et al., 2016). The one-dimensional polymeric structure exhibitsinteractions that were not previously observed when water was used as the solvent under the same preparation conditions. In order to explore the bridging effect of substituted pyrazine, we have repeated this preparation using 2-chloropyrazine to replace pyrazine under the same synthetic conditions. We report herein the synthesis and crystal structure of the resulting title compound for which incorporation of 2-chloropyrazine was not observed.

Structural commentary
The title complex crystallizes in the monoclinic space group C2/c with half of the molecule in the asymmetric unit, the other half being generated by twofold rotation symmetry. The tridentate 2,6-pyridine dicarboxylic acid dianion coordinates to the Ni II cation in a meridional fashion via the pyridine nitrogen atom and two carboxylate oxygen atoms (Fig. 1). The reactant 2-chloropyrazine is not found in the structure of the title complex. Instead, the Ni II cation is further coordinated by two trans-positioned DMSO molecules and a water molecule through their oxygen atoms. Water molecules may have been produced as a result of the reaction between 2,6-pyridine dicarboxylic acid and nickel carbonate. The two Ni1-O1 dpa bonds have the same length 2.1130 (7) Å ] and the two Ni1-O4 DMSO bonds have the same length [2.0934 (7) Å ]. The Ni1-N1 bond length is 1.9613 (12) Å and the Ni1-O3 water bond length is 2.0040 (11) Å , both being significantly shorter than the other four bonds, resulting in a distorted octahedral NO 5 coordination environment of the Ni II cation. These bond lengths are very similar to those observed in the pyrazinebridged one-dimensional structure reported previously (Liu et al., 2016).  Table 1 Hydrogen-bond geometry (Å , ). (17 Symmetry codes: (i) Àx þ 1 2 ; y þ 1 2 ; Àz þ 1 2 ; (ii) x À 1 2 ; Ày þ 1 2 ; z þ 1 2 ; (iii) Àx þ 1 2 ; Ày þ 1 2 ; Àz þ 1.

Figure 2
The crystal packing of the title compound, showing hydrogen bonds as dashed lines.

Figure 1
The molecular structure of the title compound, with displacement ellipsoids drawn at the 50% probability level. Unlabeled atoms are related by the symmetry transformation Àx, y, 1 2 À z.
water molecules and the hydrogen-bond acceptors are the non-coordinating O2 atoms of the 2,6-pyridine dicarboxylic acid dianion and the O4 atoms of the DMSO molecules (Table 1, Fig. 2). In the crystal packing, layers of the Ni IIdpa 2À complexes alternating with layers of DMSO molecules are formed parallel to (001) (Fig. 2).

Database survey
A search of the Cambridge Structural Database (Groom et al., 2016) returned eight structures that are related to the title complex. These structures incorporate some or all of the ligands in the title complex and include mononuclear and binuclear complexes, as well as coordination polymers. One of the structures is mer- (Felloni et al., 2010) that crystallizes isotypically with the title complex. Therefore bond lengths and bond angles surrounding the Co II are very similar to those in the title complex. Another mononuclear complex is aquachlorido(dimethyl sulfoxide-O)(pyridine-2,6-dicarboxylato-N,O,O 0 )iron(III) (Rafizadeh et al., 2006). In the crystal, this complex also forms alternating layers parallel to (001) due to the interdigitation of DMSO molecules. Other complexes in the search results involve either coordinating or non-coordinating DMSO molecules and one or more 2,6-pyridine dicarboxylate dianions coordinating to a metal ion.

Synthesis and crystallization
Anhydrous NiCO 3 (0.33 mmol, 39.56 mg), 2,6-pyridine dicarboxylic acid (0.33 mmol, 55.71 mg), and 2-chloropyrazine (0.50 mmol, 57.26 mg) were mixed in 10 ml dimethyl sulfoxide under stirring for 30 minutes. The resulting mixture was placed in a stainless steel autoclave. The autoclave was then sealed and heated to 373 K for 24 h and cooled to room temperature at a rate of 0.1 K per minute. The resulting green crystals were collected by filtration (yield 30.0%). Selected IR bands (KBr,

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The title complex is located on a twofold rotation axis, thus half of it occupies the asymmetric unit. The coordinating water molecule lies on the symmetry axis which requires one hydrogen atom to be located while the other is related by symmetry. This hydrogen atom was obtained from a difference-Fourier map and was refined freely. The other hydrogen atoms were positioned geometrically (C-H = 0.93/1.00 Å ) and allowed to ride with U iso (H)= 1.2/1.5U eq (C). Methyl hydrogen atoms were allowed to rotate but not to tip. Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: XP in SHELXTL-Plus (Sheldrick, 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

Aqua-trans-bis(dimethyl sulfoxide-κO)(pyridine-2,6-dicarboxylato-κ 3 O 2 ,N,O 6 )nickel(II)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. All H atoms were positioned geometrically ( C-H = 0.93/1.00 Å) and allowed to ride with U iso (H)= 1.2/1.5U eq (C). Methyl ones were allowed to rotate around the corresponding C-C. The Ni complex is located on a 2-fold rotational axis of symmetry thus half of it occupies the asymmetric unit. The coordinated water molecule lies on the symmetry axis which requires one proton to be located while the other is related by the symmetry. The water proton was obtained from a Difference Fourier map and refined freely.