Synthesis, crystal structures and properties of six new mixed ligand cobalt(II) 4-nitrobenzoates

Abstract The synthesis, spectral characteristics, thermal properties and single crystal structures of [Co(N-meim)2(4-nba)2] (N-meim = N-methylimidazole; 4-nba = 4-nitrobenzoate) (1), [Co(2-meim)2(4-nba)2] (2-meim = 2-methylimidazole) (2), [Co(pyr)3(4-nba)2] (pyr = pyrazole) (3), [Co(H2O)2(form)2(4-nba)2]·form (form = formamide) (4), [Co(H2O) (aceta)2(4-nba)2] (aceta = acetamide) (5) and [Co(H2O)3(N-mepyr)(4-nba)](4-nba) (N-mepyr = N-methylpyrazole) (6) are reported. The central Co(II) exhibits tetrahedral geometry in 1 and 2, trigonal bipyramidal geometry in 3 and octahedral geometry in 4–6. In the anhydrous compounds 1–3 as well as the diaqua compound 4, the monoaqua compound 5 and the triaqua compound 6, the neutral N-donor co-ligands or the O-donor amides function as terminal ligands. The aromatic 4-nba ligand exhibits monodentate binding mode in 1–4 and monodentate and bidentate binding in 5, while 6 contains an uncoordinated 4-nba in addition to a bidentate 4-nba ligand. A single C-H···O interaction links molecules of 1 into an infinite chain while the N-H···O bonds in 2 result in a two-dimensional network. Compounds 4 and 5 exhibit three varieties of H-bonding. Thermal decomposition of 1–6 results in the formation of a spinel oxide. A comparative study of thirty-one structurally characterized cobalt-4-nitrobenzoates is described. Graphical Abstract


Introduction
As part of our long-standing research interest, we are investigating the chemistry of 4nitrobenzoic acid (4-nbaH) with both d-block [1][2][3] and s-block metals [4,5]. Our studies using 4-nitrobenzoate (4-nba) as an organic linker have resulted in the structural characterization of molecular solids of Co(II) [1], Ni(II) [2,3], Mg(II) [4,6,7] and Ca(II) [8] as well as several coordination polymers of Ca(II) [9][10][11][12][13], Sr(II) [4] and Ba(II) [5,14]. The isomeric nitrobenzoic acids (Scheme S1) and their corresponding phthalic acid counterparts obtained by replacement of a -NO 2 group by the -COOH group are isoelectronic. However, in terms of their reactivity characteristics towards metal ions nitrobenzoic and phthalic acid exhibit altogether different behavior. For example, terephthalic acid (benzene-1,4-dicarboxylic acid, 1,4-BDC) has been extensively used as a linker to assemble metal-organic framework (MOF) materials since the report on [Zn 4 O(1,4-BDC) 3 (DMF) 8 (C 6 H 5 Cl)] (DMF ¼ dimethylformamide) referred to as MOF-5 [15]. The reaction of 1,4-BDC with Co(II) has been shown to result in the formation of [Co(1,4-BDC)DMF] designated as MOF-71, which is a framework constructed from rodshaped secondary building units [16]. In contrast to 1,4-BDC which extends the structure due to metal binding by both the carboxylate moieties disposed trans to each other, only the -COOH group of 4-nitrobenzoic acid (4-nbaH) is involved in metal binding. Although nitro oxygen binding has been reported for some s-block metals [17][18][19], the oxygen of the nitro group is generally not involved in metal binding in the case of transition metals. One example of a Cu(II) compound showing nitro oxygen coordination has been reported [20]. In 4-nbaH, the oxygen atoms of the nitro group disposed trans to the -COOH moiety function as H-acceptors resulting in interesting supramolecular network structure as reported by us in a very early study on [Co(H 2 O) 4 (4-nba) 2 ]Á2H 2 O (1a) [1].
In a recent article, we reported that an attempted synthesis of anhydrous Co(II)bis(4-nitrobenzoate) via a mechanochemical route starting from the tetraaquadihydrate compound 1a resulted in the formation of a diaqua compound viz.
[Co(H 2 O) 2 (DMSO) 2 (4-nba)](4-nba) containing coordinated as well as free 4-nba [21]. A later synthesis aimed at substitution of the cis(diaqua) ligands by a bidentate N-donor ligand viz. 2,2-bipyridine (bpy) afforded a mixed ligand binuclear Co(II) compound containing monodentate and free 4-nba in addition to bridging and terminal aqua ligands as well as bidentate bpy ligand [22]. In continuation of this theme, we have undertaken the present study to investigate the reactivity characteristics of in situ generated 1a towards a few N-donor and O-donor ligands (Figure 1 3 (N-mepyr)(4-nba)](4-nba) (6). Details of these investigations are described in this article. Scheme 1. Synthetic methodology employed for the synthesis of 1-6.

Experimental
All chemicals were used as received from commercial sources (Table S1) without any further purification. The starting materials and reaction products are air stable and hence were prepared under normal laboratory conditions in air. The tetraaqua dihydrate compound [Co(H 2 O) 4 (4-nba) 2 ]Á2H 2 O (1a) was prepared by a reported procedure [1]. Elemental analyses (C, H and N) were performed on a Variomicro cube CHNS analyzer. Infrared (IR) spectra of the solid samples diluted with KBr were recorded on a Shimadzu (IR Prestige-21) FT-IR spectrometer from 4000-400 cm À1 . Diffuse-reflectance spectra were recorded using a Shimadzu UV-2450 double beam spectrophotometer using BaSO 4 as reference (100% reflectance). Absorption data were calculated from the reflectance data using the Kubelka-Munk function (a/S ¼ (1ÀR)2/2R, where a is the absorption coefficient, R the reflectance, and S the scattering coefficient. TG-DTA study was performed in flowing air in Al 2 O 3 crucibles at a heating rate of 10 min À1 using a STA-409 PC simultaneous thermal analyzer from Netzsch. A temperature-controlled furnace was used for pyrolysis. X-ray powder patterns were recorded on a Rigaku SmartLab powder diffractometer using Cu-Ka radiation with Ni filter. The morphology of the cobalt oxides was investigated by a scanning electron microscope (FEI-Quanta FEG 200 F). The crystal structures of 1-6 were determined using a Bruker D8 Quest Eco X-ray diffractometer. Intensity data were collected using monochromated Mo (Ka) (k ¼ 0.7107 Å) radiation. The program suite APEX3 (Version 2019.1) was used (i) to integrate the frames, (ii) to perform absorption correction and (iii) to determine unit cell [23]. The structures were solved with SHELXT [24] and subsequent refinements were performed with SHELXL [25]. In 3, one unique pyrazole ligand exhibits substitutional disorder. The N22 and C21 atoms attached to N21 as well as C22 and C23 are disordered over two positions in the ratio of 50:50 due to the two-fold axis along the Co1-N21 bond. In 6, the unique N-methylpyrazole ligand is disordered over two positions (0.7:0.3) and was refined using a split model. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms attached to the aromatic ring were introduced in calculated positions and refined by riding on their respective parent C atoms. Crystallographic details of data acquisition and selected crystal refinement results are given in Table 1.   (11) 26.0215 (19) 24.789 (7) 7.3542 (4) 7.469 (3) 7.4606 (7) b (Å) 6.6107 (3) 7.4081 (5) 8.700 (2) 7.5325 (4) 11.643 (4) 9.2330 (10) c (Å) 15.2447 (6) 27.556 (2) 14.490 (4) 22.9600 (14) 13.829 (5  A mixture of commercial cobaltous carbonate (0.250 g, 2.10 mmol) and 4-nitrobenzoic acid (0.668 g, 4.00 mmol) were taken in water (40 mL) and heated on a water bath. On heating, both insoluble reactants slowly dissolved with the evolution of carbon dioxide resulting in the formation of a red solution. After $1 h, the above solution containing little undissolved carbonate was filtered and concentrated to 20 mL by heating over a steam bath followed by the addition of neutral ligands. Nmethylimidaole (2.00 mL) for 1 or 2-methylimidazole (0.328 g, 4.00 mmol) for 2 dissolved in $5 mL water was added to the filtrate. The beaker was left undisturbed at room temperature for crystallization. Within a day, purple crystals were observed. The crystals were washed with diethyl ether and dried to obtain [Co(N-meim) 2 (4nba) 2 ] (1) or [Co(2-meim) 2 (4-nba) 2 ] (2) in 68.1 (0.756 g) or 64.5% (0.716 g) yield. The use of pyrazole (0.544 g, 8.00 mmol) for 3, 5 mL of formamide for 4, acetamide (0.236 g, 4.00 mmol) or 2.00 mL of N-methylpyrazole for 6 afforded crystals of 3 to 6 in 71.3% (0.849 g), 78.4% (0.881 g), 76.5% (0.807 g) and 67.5% (0.712 g) yields, respectively, isolated as above. Alternatively, addition of N-methylimidazole (1.00 mL), formamide (2.50 mL) or N-methylpyrazole (1.00 mL) to a finely powdered sample of tetraaquadihydrate compound 1a (0.500 g, 1.00 mmol) resulted in the formation of a clear solution on addition of water ($8 mL). From this, 2, 4 or 6 could be isolated as above in high yield. Mechanical grinding of 1a (0.500 g, 1.00 mmol) with 2-methylimidazole (0.164 g, 2.00 mmol), pyrazole (0.340 g, 5.00 mmol) or acetamide (0.118 g, 2 mmol) for $20 minutes followed by addition of water ($8 mL) afforded 1, 3 or 5 which were isolated as above in $80% yield.

Description of crystal structures of 1-6
In all six Co(II) compounds described in this section, the geometric parameters (Table  S2)  The Co-N bond lengths at 2.026(2) Å are slightly longer than the Co-O bond distances at 1.991(2) Å in 1 ( Table 2). The O-Co-O, N-Co-N and O-Co-N bond angles scatter in a wide range between 92.57 (14) and 124.55 (11) , indicating that the fCoN 2 O 2 g tetrahedron is highly distorted ( Table 2). The hydrogen atom (H14B) bonded to C14 is linked via H-bonding (C14-H14BÁÁÁO2) with the carboxylate oxygen atom of an adjacent molecule (Table 3). This single C-HÁÁÁO interaction accompanied by a large DHA angle results in the formation of a hydrogen bonded chain of 1 extending along the c-axis ( Figure 2).
In [Co(2-meim) 2 (4-nba) 2 ] (2) all atoms are located in general positions and the structure consists of a unique Co(II) ion, two crystallographically independent monodentate 2-meim ligands and two unique 4-nba ligands ( Figure 3). As in 1, the Co-O bond lengths (1.9550 (19) Table 2). Unlike the single C-HÁÁÁO interaction in 1, a total of three N-HÁÁÁO interactions are observed involving the H22 and H32 atoms attached to N22 and N32, respectively, of the unique imidazole ligands. The H22 atom is involved in a bifurcated hydrogen bond and is bonded to the uncoordinated carboxylate oxygen O12 of two symmetry-related molecules of 2 (Table 3), while the H32 atom is hydrogen-bonded with the free carboxylate oxygen O2 of a neighboring molecule. The result of H-bonding is the formation of a two-dimensional hydrogen-bonded network in the crystallographic bc-plane (Figure 3, Figure S1).
In addition to the central metal Co1 and N21 of a pyrazole (pyr) ligand located on a two-fold axis, the structure of [Co(pyr) 3 (4-nba) 2 ] (3) consists of each of a crystallographically unique 4-nba and pyr ligands. The N22 and C21 atoms attached to N21 as well as C22 and C23 are disordered over two positions. Unlike 1 (or 2) the central metal exhibits five coordination in 3 and is bonded to three pyrazole ligands via the imine N atoms (N11, N11 i and N21) and two oxygen atoms of 4-nba ligands resulting in a trigonal bipyramidal fCoN 3 O 2 g polyhedron ( Figure 4, Figure S2).   (6) O11-Co1-O12 59.65 (9) Symmetry transformations used to generate equivalent atoms: (i) Àx þ 1,y,Àz The pyrazole N atoms occupy the equatorial sites while the symmetry related 4-nba ligands occupy the axial sites. The Co-O bond distance at 2.145(13) Å is slightly longer compared with the Co-N bond lengths (Table 2)   are similar and consist of a unique Co(II), two crystallographically independent terminal formamide (or acetamide in 5) ligands and two unique 4-nitrobenzoate ligands. In addition to two terminal aqua ligands (O1W, O2W) disposed trans to each other instead of a single aqua ligand in 5, compound 4 additionally has a lattice formamide molecule ( Figure 5). Both the unique 4-nba ligands (O21, O31) in 4 are bonded to Co(II) in a monodentate fashion, while the unique 4-nba ligands in 5 exhibit both monodentate and bidentate coordination. The pair of unique formamide ligands in 4 are disposed trans to each other while in 5 the amide ligands adopt a cis geometry. It is interesting to note that the pair of aqua ligands as well as the unique 4-nba ligands in 4 are also trans to each other, resulting in an all-trans orientation ( Figure 5). The       (3,11,17-triazatricyclo[11.3.1.15,9] To determine the ring centroid to ring centroid distances (CgÁÁÁCg) between adjacent aromatic rings in 1, 2, 4 and 5, Platon software [27,28] (Table S3). As it has been documented that stacking interactions between aromatic rings can exist at a very long CgÁÁÁCg distance [29], the observed data reveal pÁÁÁp stacking in 1-2 and 4-5.

Structural chemistry of cobalt 4-nitrobenzoates
In addition to the compounds described in this work, the structures of many cobalt compounds containing 4-nitrobenzoate are reported [1,22, and archived in the Cambridge Structural Database ( Table 4). The compounds in Table 4 are organized based on the increasing coordination number of cobalt. For the sake of completeness, all known examples (seven entries) containing Co(III) are included. It is interesting to note that all the known compounds crystallize in the centrosymmetric triclinic (space group no. 2) or monoclinic (space group no. 14 and 15) space groups with the central cobalt exhibiting tetra, penta or hexacoordination. The octahedral geometry is the most common (twenty entries), followed by tetrahedral (nine examples) and trigonal bipyramidal geometry. Compound 3 described in this study is a new example of a pentacoordinate Co(II)-4-nitrobenzoate in this series. In all the compounds, excepting entry no. 30 which is organometallic, the central Co is bonded to O and or N donor ligands. Five examples include 4 and 5 containing exclusively fCoO 6 g coordination Figure 6. The C25-H25ÁÁÁO22 and C35-H35ÁÁÁO32 interactions link the [Co(H 2 O) 2 (form) 2 (4nba) 2 ] units into an infinite chain extending along the a-axis (top). The N1-H1BÁÁÁO22, N11-H11AÁÁÁO1W, N11-H11AÁÁÁO1 and N11-H11BÁÁÁO2W (x, y À 1, z; x À 1, y þ 1, z; x À 1, y þ 1, z and x, y þ 1, z) interactions in 4 result in a 2D network (bottom).
sphere (entry nos. 13, 14, 17, 18 and 21) and four compounds (entry nos. 12 and 25-27) contain exclusively fCoN 6 g coordination spheres. In all these compounds, 4nitrobenzoate functions not only as a charge balancing anion but also is coordinated to Co in many cases. In all the Co(III) compounds (entry nos. 25-31) 4-nba functions as a charge balancing anion and one example of a Co(III) compound containing both coordinated and free 4-nba is known. Two examples of Co(II) compounds (entry nos. 12 and 13) containing only free uncoordinated 4-nba anions are known.
Compound 6 is an example that contains both coordinated and free 4-nba. A survey of the binding modes of 4-nba reveals that the ligand exhibits monodentate (g 1 ) or bidentate (g 2 ) or bridging bidentate (m 2 -g 1 :g 1 ) binding modes. The bridging mode results in the formation of a dinuclear compound (entry no. 11). All the compounds excepting entry nos. 1, 2 and 18 exhibit discrete structures and are zero-dimensional molecular solids. In entry nos. 1 and 2, which are two-dimensional coordination polymers as well as entry no. 18 which is a 1 D polymer, 4-nba exhibits monodentate binding. It is interesting to note that none of the tetracoordinate Co(II) compounds (entry nos. 1-9) contain a coordinated aqua ligand. However, entry no. 3 is a hemihydrate. A study of the secondary interactions (H-bonding) reveals that these compounds can exhibit a maximum of four varieties of H-bonding interactions. Compounds 4 and 5 exhibit three varieties of hydrogen bonding.

Synthetic aspects, spectral and thermal studies
Tetaaquadihydrate compound [Co(H 2 O) 4 (4-nba) 2 ]Á2H 2 O (1a) was first generated in situ by reaction of a 1:2 mixture of cobaltous carbonate with 4-nitrobenzoic acid in boiling water. Without isolating 1a, the respective neutral ligand viz. N-meim, 2-meim, pyr, form, aceta and N-mepyr was added to the reaction mixture resulting in the formation of 1-6 (Scheme 1). Although the reactions were performed in aqueous medium, only anhydrous compounds 1-3 are obtained in the case of N-meim, 2-meim and pyr. Compounds 1-6 can also be synthesized in very high yields by reaction of a finely powdered sample of 1a with N-meim or from N-mepyr or a grinding of 1a with 2meim or pyr or aceta followed by dissolution in water to isolate the product. A Figure 7. The crystal structure of 6 showing the atom labelling scheme and the coordination sphere of Co(II). Displacement ellipsoids are drawn at the 30% probability level for all non-hydrogen atoms. For the disordered N-methylpyrazole ligands see Figure S4.
comparison of the X-ray powder pattern of the bulk samples ( Figure S6) with the calculated pattern from the single crystal data reveals that 1-6 are monophasic.
The infrared (IR) spectra of all the compounds described in this study exhibit several bands in the mid-IR region, indicating the presence of organic moieties ( Figure S7). The anhydrous nature of 1-3 can be evidenced by the absence of a band in the -OH region (3500 cm À1 ). Bands at 3275 and 3317 cm À1 , respectively, in 2 and 3 can be assigned to -NH stretching vibrations, which is absent in 1 due to the methylation of the imidazole N. The hydrated compounds (4-6) exhibit a strong and broad absorption band in the region 2500-3600 cm À1 which can be assigned to the -OH stretching vibrations. The symmetric stretching vibrations of the -NO 2 groups are observed at $1340 cm À1 in all compounds. The diffused reflectance spectra of 1-6 exhibit two sets of bands in the 200-400 nm region and the 450-650 nm region ( Figure S8). The broad band in the longer wavelength region can be attributed to a d-d transition of Co(II) while the absorption around 280 nm can be assigned for the absorption of 4-nba.
In addition to TG-DTA experiments, a bulk sample (0.250 g) was pyrolyzed in a furnace at 800 C to obtain the residue (Table S4). The IR spectrum ( Figure S9) of the residue is devoid of signals due to the organic moieties, revealing formation of an oxidic phase. The powder pattern of the residue obtained upon pyrolysis at 800 C reveals that Co 3 O 4 is formed (matched with JCPDS card no. 00-042-1467) ( Figure S10), which exhibits a spherical morphology of homogeneously distributed particles as evidenced from its SEM image ( Figure S10). Thermal studies reveal that all compounds exhibit a very strong exothermic event at around 400 C which can be assigned for the decomposition of 4-nba [13][14][15]. Anhydrous compounds 1-3 do not exhibit any significant mass loss till 200 C. Between 200-320 C the first endothermic event is observed accompanied by a mass loss which may be due to the loss of the neutral N-meim in 1 ( Figure S11) and 2-meim in 2 ( Figure S11). The absence of mass spectral analysis of the emitted fragments precludes a detailed discussion of the exact nature of the decomposition processes. The strong exothermic peak at $400 C in 1 and 2 can be attributed to the decomposition of 4-nitrobenzoate giving cobalt oxide, (Co 3 O 4 ), as the final residue which is also confirmed by the pyrolysis study. Similarly, in 3, loss of pyrazole is observed by an endothermic event followed by the exothermic decomposition at 400 C to form residue ( Figure S11). Compounds 4-6 exhibit initial endothermic events that can be attributed to the loss of water followed by removal of neutral ligands. The exothermic signal centred around 400 C can be assigned for the decomposition of 4-nba resulting in the formation of a spinel oxide.

Conclusion
The synthesis, spectra and structures of six new mixed ligand cobalt(II)-4-nitrobenzoate compounds are reported. The study demonstrates a series of Co(II) 4-nitrobenzoate compounds exhibiting variable coordination sphere and hydration depending on the type of the neutral ligands employed. H-bonding interactions in the compounds yield interesting supramolecular architectures. A comparative study of thirty-one cobalt compounds reveals a rich coordination chemistry of this group of compounds.