A redetermination of the structure and Hirshfeld surface analysis of poly[diaquadi-μ-hydroxido-tetrakis(μ-nicotinato N-oxide)tricopper(II)]

The structure of the product obtained from the reaction of pyridine-2,3-dicarboxylic acid and hydrated copper(II) chloride in hot aqueous NaOH solution was shown to be {[Cu3(μ-OH)2(H2O)2(μ-nicNO)4]}n (nicNO is pyridine-3-carboxylate N-oxide), which evidently arose from decarboxylation and oxidation of the reactant acid. The structure has been reported previously using room-temperature data. Here, a more complete description of the intermolecular interactions, including a Hirshfeld surface analysis, is reported.


Chemical context
N-oxidation of the pyridine ring can significantly increase its electron-donating ability because the charge-polarized pyridine-N-oxide moiety can donate three pairs of electrons while a neutral nitrogen atom in pyridine only gives one pair of electrons. Therefore, it is expected that N-oxidation can increase the coordination capacities and flexibility of the ligand. Metal complexes of pyridine-N-oxide ligands have been found to be particularly useful in the selective adsorption and separation of gases (CO 2 over CH 4 ) and as anti-HIV and luminescent agents (Noro et al., 2015;Xiong et al., 2014;Balzarini et al., 2005;Lis et al., 2002). These features have motivated our interest in the chemistry of carboxylic acid derivatives of pyridine-N-oxide for investigating the influence of the N-oxide moiety on the coordination mode(s) in the crystal lattice Hosseini-Hashemi et al., 2018Bazargan et al., 2016Bazargan et al., , 2020Shahbazi et al., 2017;. Here, we report the isolation and X-ray crystal structure of the coordination polymer [Cu 3 (-OH) 2 (H 2 O) 2 (-nicNO) 4 ] n (1) (nicNO is pyridine-3-carboxylate N-oxide) as the unexpected product from the reaction of pyridine-2,3-dicarboxylic acid with hydrated Cu II chloride. It appears that oxidation and decarboxylation of the starting acid occurred during the reaction, as has been seen previously (Hosseini-Hashemi et al., 2018;. During the course of this work, we found two prior reports of this structure [NICTCU (Knuutilla, 1981) and NICTCU01 (Kang et al., 2020)], both obtained with room-temperature data. Overall, the present structure is the same as the previous ones, but with some differences in metrical parameters as a result of the lower temperature of the data collection used here, a lower R value [0.0250 for all reflections (3592) vs 0.0416 for 2525 with I ISSN 2056-9890 > 3(I) in NICTCU and 0.0538 for 3349 with I > 2(I) in NICTCU01. The present structure has slightly better s.u.'s on all derived parameters than obtained for NICTCU and significantly better ones than those obtained by Kang et al.. One deficiency of the NICTCU structure is the free refinement of hydrogen-atom parameters, a risky procedure with room-temperature data when heavy atoms are present, which led to C-H distances for the aromatic rings varying from 0.97 (2) to 0.84 (3) Å and O-H distances of 0.77 (3) to 0.41 (4) Å , the last three being particularly unrealistic. In addition, there was no absorption correction despite a linear absorption coefficient of 2.422 mm À1 . Kang et al. performed an absorption correction and treated hydrogen atoms appropriately, but with an R int of 0.0780 their data are clearly of poorer quality than in the present case (R int = 0.0208).

Structural commentary
The monomer unit plus one N-oxide atom from the bridging nicotinato-N-oxido ligand on each end copper atom (O3 ii and O3 iii ) is shown in Fig. 1. This moiety is centrosymmetric with Cu2 lying on the crystallographic center of symmetry. The coordination about Cu1 is square pyramidal with the N-oxide atom from the bridging nicotinato-N-oxido ligand (O3 ii ) in the apical site and the basal sites occupied by the bridging hydroxide (O7-H7) and the water molecule (O8) in trans positions, and a carboxylate oxygen atom from the bridging nicotinato-N-oxido ligand (O1) and the bridging nicotinato-Noxide ligand (O5 i ). The Cu1-O distances and bond angles are in line with those typically seen for tetragonally elongated, square-pyramidal Cu II . Cu2 is coordinated by the bridging hydroxide (O7-H7) and a carboxylate oxygen of the nicotinato-N-oxide ligand (O4) and their symmetry-related counterparts. Although rigorously planar, the coordination about Cu2 shows a rhombic distortion from square geometry due to the difference in the Cu2-O4 [1.9687 (11) Å ] and Cu2-O7 [1.9240 (11) Å ] bond lengths and the O4-Cu2-O7 angle of 87.69 (5) . This geometry is quite comparable to those in the previously reported structures (Table 1). One feature noted by Kang et al. (2020) but not by Knuutilla (1981) is a weak contact by the N-oxide oxygen atoms coordinated to Cu1 (O3 ii and O3 iii ) to Cu2 with the Cu2-O3 ii distance of 2.6828 (15) Å being considerably longer than the Cu1-O3 ii distance [2.4208 (13) Å ] but definitely shorter than the sum of the van der Waals radii (2.92 Å ), indicating a short contact. The O7-Cu2-O3 ii and O7 i -Cu2-O3 ii angles of 81.66 (5) and 98.34 (5) , which differ greatly from 90 , suggest the coordination of Cu2 should not be described as an elongated octahedron. There are close to 100 structures listed in the CSD (Groom et al., 2016) (7) 92.21 (15) Symmetry codes: (i) Àx + 1, Ày + 1, Àz + 1; (ii) x + 1, y + 1, z.
two, the distance is to a ligand oxygen atom bridging copper centers and so more comparable to the present work. Where commented on, the long distance is attributed to a Jahn-Teller distortion, but in our case the Cu2-O3 ii distance not only is long, but also its direction is tilted away from the Cu2 coordination plane normal by $8 . This suggests that O3 ii is close to Cu2 for sterical convenience, not due to the formation of a Cu2-O3 ii bond. The intramolecular O7-H7AÁ Á ÁO2 hydrogen bond (Table 2) belongs to a S 1 1 (6) graph set (Bernstein et al., 1995).

Hirshfeld surface analysis
with Crystal Explorer 17 (Turner et al., 2017). A detailed description of the use of Crystal Explorer 17 and the plots obtained has been published (Tan et al., 2019) so will not be given here. Fig. 4a presents the surface mapped over d norm over the range À0.7162 to 1.5102 arbitrary units in which the bright-red spots indicate the strong O-HÁ Á ÁO hydrogen bonds and the lighter red spots the weaker C-HÁ Á ÁO hydrogen bonds listed in Table 2. Mapping of the Hirshfeld surface over shape-index is illustrated in Fig. 4b and provides a picture of possible -stacking interactions. These are indicated by red-orange triangles surrounded by blue triangles, which occur over the pyridine rings, confirming the slippedstacking interaction discussed in Section 3. This is also indicated by the surface mapped over curvature (Fig. 4c) where the substantially flat regions of the plot again occur over the pyridine rings. Parsing the intermolecular interactions into specific types is accomplished with the fingerprint plots (Fig. 5). Fig. 5a shows the full fingerprint plot while Fig. 5b presents the HÁ Á ÁO/OÁ Á ÁH interactions which, not surprisingly, constitute the largest of the intermolecular interactions at 35.8% of the total. These are followed by HÁ Á ÁH (Fig. 5c, 25.9%), HÁ Á ÁC/ CÁ Á ÁH (Fig. 5d, 10.8%) and OÁ Á ÁCu (Fig. 5e, 10.8%) interactions. Not shown are the CÁ Á ÁC (7.9%) and HÁ Á ÁN/NÁ Á ÁH (2.5%) contacts, with the former corresponding primarily to the slipped -stacking interactions.

Synthesis and crystallization
An aqueous solution of CuCl 2 Á2H 2 O (0.034 g, 0.2 mmol in 3.5 mL) was added to an aqueous solution (3.5 mL) containing pyridine-2,3-dicarboxylic acid (0.04 g, 0.2 mmol) and NaOH (0.2 ml, 1 mol L À1 ), the mixture was stirred at 333 K for 2 h and then cooled to room temperature. After standing for a week, the light-blue precipitate that formed was filtered off and dried. Dark-blue, block-like crystals were obtained by slow evaporation of a solution of the precipitate in 5 mL of distilled water at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms attached to carbon were placed in idealized locations (C-H = 0.95 Å ) and were included as riding contributions with U iso (H) = 1.2U eq (C). Those attached to oxygen were placed in locations obtained from a difference map and were refined with DFIX O-H = 0.84 (1) Å restraints.

Funding information
JTM thanks Tulane University for support of the Tulane Crystallography Laboratory.   The Hirshfeld surface plotted over (a) d norm , (b) shape index and (c) curvature.

Special details
Experimental. The diffraction data were obtained from 3 sets of 400 frames, each of width 0.5° in ω, colllected at φ = 0.00, 90.00 and 180.00° and 2 sets of 800 frames, each of width 0.45° in φ, collected at ω = -30.00 and 210.00°. The scan time was 5 sec/frame. 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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. H-atoms attached to carbon were placed in calculated positions (C-H = 0.95 Å) while those attached to oxygen were placed in locations derived from a difference map and their coordinates adjusted to give O-H = 0.84 %A. The former were included as riding contributions with isotropic displacement parameters 1.2 times those of the attached atoms while the latter were refined subject to the restraint DFIX 0.84 (1).