Alkali Salts of Nitranilic and Cyanochloranilic Acids

Series of novel alkali salts of nitranilic acid (3,6-dinitro-2,5-dihydroxyquinone) and cyanochloranilic acid (3-cyano-6-chloro-2,5dihydroxyquinone), and also of neutral cyanochloranilic acid dihydrate, were prepared and their structures were studied. The nitranilate dianion revealed a considerable conformational flexibility of nitro groups. Steric and inductive effects exerted by different substituents (nitro, cyano and chloro) and their influence on molecular geometry and crystal packing are discussed.


2
There is even less data on cyanochloranilic acid (3cyano-6-chloro-2,5-dihydroxyquinone, Scheme 1b).Its ability to coordinate transition metals is well known, [40] however, only a single recent publication describes a few metal complexes. [41]s a part of our study of crystal chemistry of quinones and semiquinone radicals, we present two novel alkali salts of nitranilic acid, neutral cyanochloranilic acid and its two alkali salts.Compared to hydrogen [20] and halogen substituents, [19,21,22] the nitro and cyano groups are bulky and very strongly electron-withdrawing substituents, so it is interesting to assess their steric and electronic influence on the molecular conformation and crystal packing, especially on the quinoid π-stacking.

Preparation
All reagents used were purchased from commercial sources (Merck, Sigma-Aldrich, Kemika), were of p.a. grade and were used without further purification.
Sodium nitranilate hexahydrate was prepared according to a modified procedure by Nef: [32] saturated acetone solution of tetrachloroquinone (chloranil, 4.00 g) was heated to 80 -90 °C with constant stirring; saturated aqueous solution of sodium nitrite (10.00 g) was slowly added.The yellow solution quickly turned dark red and orange-yellow crystalline powder (sodium nitranilate) began to precipitate.After ca. 1 hour, the solution became colourless.The orange-yellow precipitate was filtered off, and a little solid NaOH was added to the solution to precipitate sodium nitranilate which was still dissolved, and then it was again filtered off.2.51 g of raw sodium nitranilate dihydrate was obtained (η = 98.8 %).Pure crystalline sodium nitranilate dihydrate was obtained by recrystallization from boiling water.
Nitranilic acid hexahydrate was prepared by dropwise addition of conc.nitric acid to a wet solid sodium nitranilate.The acid is added until all solid is dissolved; after ca. 1 hour, orange needle-like crystals of nitranilic acid hexahydrate are formed, and are then scooped up and dried.It is necessary to remove the crystals from the solution, because nitranilic acid is unstable in highly acidic media and slowly decomposes into oxalic acid.
Sodium nitranilate dihydrate was prepared by addition of equivalent amount of sodium carbonate into a fresh aqueous solution of nitranilic acid, while for preparation of lithium caesium nitranilate monohydrate a half-equivalent of lithium carbonate and a half-equivalent of caesium carbonate were used.The crystals were formed upon slow evaporation at room temperature.
Sodium cyanochloranilate trihydrate was prepared according to a modified method of Gräbe: [42] 1.00 g of 5,6dichloro-2,3-dicyanoquinone was added to a solution of sodium hydroxide (0.9 g in 30 mL of water) and heated to 80 -90 °C for 2 h.The colour of the solution turned dark purple.Purple sodium cyanochloranilate trihydrate was precipitated upon addition of 2 g NaCl; the precipitate was washed with 10 % aqueous NaOH until the filtrate turned colourless.
The neutral cyanochloranilic acid dihydrate was prepared by addition of conc.hydrochloric acid to a solid sodium cyanochloranilate until all solid is dissolved.The orange prismatic crystals of cyanochloranilic acid dihydrate were formed upon slow evaporation at room temperature.Potassium salt of cyanochloranilic acid was prepared by addition of excess of potassium carbonate into a fresh methanol solution of sodium cyanochloranilate.The crystals were formed upon slow evaporation at room temperature.

X-ray Structural Analysis
Single-crystal X-ray measurements were performed on an Oxford Diffraction Xcalibur Nova R diffractometer with a microfocus Cu-tube using mirror-monochromated CuKα radiation (λ = 1.54179Å).Program package CrysAlis PRO [43] was used for data reduction and multi-scan absorption correction.
The structures were solved using SHELXS97 [44] and refined with SHELXL-2017. [44]The structures were refined using the full-matrix least squares refinement; all non-hydrogen atoms were refined anisotropically.Hydrogen atoms were located from difference Fourier maps and refined using the following restraints: for water molecules 50 Å; hydrogen atoms bound to the neutral cyanochloranilic acid were refined without restraints.
Molecular geometry calculations were performed by PLATON, [45] and molecular graphics were prepared using ORTEP-3, [46] and CCDC-Mercury. [47]Crystallographic and refinement data for the structures reported in this paper are shown in Table 1.
Supplementary crystallographic data for this paper can be obtained free of charge via www.ccdc.cam.ac.uk/ conts/retrieving.html(or from the Cambridge Crystallographic

RESULTS AND DISCUSSION
We have prepared and structurally characterised two alkali salts of nitranilic acid, orthorhombic polymorph of sodium nitranilate dihydrate (Na2NA

Molecular Geometries
The molecular geometries of the nitranilate anions (Table 2) are in a good agreement with the previously determined ones [30,31,33,34] and also dianions of other substituted 2,5dihydroxyquinones. [2,[19][20][21] Therefore, the resonance depicted in Scheme 1a is well-justified.In LiCsNA•H2O the molecular symmetry is Cs, and the mirror plane is normal to the ring plane; C-N bonds are located on the mirror plane (Figure 1a).In orthorhombic Na2NA •2H2O the anion is located in a general position, with C1 symmetry (Figure 1b).
In LiCsNA•H2O the anion is planar within the experimental error -the nitro groups are located in the ring plane (Figure 1a, Table 3), while in the orthorhombic Na2NA•2H2O both nitro groups are twisted outside of the ring plane (Figure 1b, Table 3) and the ring is slightly puckered [its Cremer-Pople [48] puckering amplitude Q is 0.057(5) Å].Dihedral angles between the quinoid ring plane and a nitro group for the previously known structures range between 3.9 ° [34] and 56.6 °; [36] in the nitranilic acid hexahydrate it is 23 °.[31,31] Apparently, rotation about the C-N bond is easy and the orientation of the nitro group depends on the molecular environment.
Molecular geometry of the cyanochloranilate anion in K2CNCA•H2O (Table 2) is also in a good agreement with the literature data, [40,41] and the π electrons are delocalised as in Scheme 1b; the anion is located in a general position (Figure 2a).However, the centroid of the cyanochloranilate in Na2CNCA •3H2O is located at an inversion centre (Figure 2b), so the anion is disordered over two positions (due to symmetry, p.p. is 0.5 for both components).In both cases the anion is planar within experimental error.
Geometry of the cyanochloranilate dianion in K2CNCA •H2O revealed a slight asymmetry due to a stronger electron-withdrawing capability of the cyano group compared to the chlorine: the double C=C bond next to the Figure 1.ORTEP-3 [46] drawings of nitranilate anions from a) LiCsNA• H2O, and b) orthorhombic Na2NA•2H2O with atom numbering schemes.Displacement ellipsoids are drawn for the probability of 50 %.2) is by 0.015 Å shorter than the one next to chlorine atom (C5-C6, Table 2).This effect could not be observed in Na2CNCA•3H2O due to the disorder, while in the neutral cyanochloranilic acid (Table 2, Figure 2c) it is within 3 e.s.d.'s.4]

Crystal Packings
In orthorhombic Na2NA•2H2O two symmetry-independent sodium ions are present, both having a distorted octahedral coordination typical for sodium.Na1 bonds to three carbonyl and one nitro oxygen, from two anions; in addition there are two water molecules in a trans-arrangement.Coordination of Na2 is similar, however, the water molecules are in a cis-arrangement.The packing is layered, with alternating layers of cations and anions parallel to (001) (Figure 3).Water molecules further stabilise the packing by hydrogen bonding to nitro-and carbonyl oxygens (Table 4).
In LiCsNA•H2O Cs ions are coordinated by five carbonyls, five nitro oxygens (from five different anions) and a single water molecule.The shape of the coordination polyhedron may be described as a severely distorted tricapped square antiprism.Li cations have a usual square pyramidal coordination, being surrounded by four carbonyl oxygens (from two anions) and a single water molecule occupying the apical position.3D packing is achieved through cation-anion interactions and water-carbonyl oxygen hydrogen bonds (Table 4, Figure 4).
In Na2CNCA •3H2O the sodium cation has distorted trigonal bipyramidal coordination (CN 6); it forms three contacts with O atoms from the dianion and two with the water molecule.The crystal packing is layered with cationanion layers parallel to (001), which are connected by hydrogen bonds (Figure 5).Asymmetric unit comprises one and a half water molecules (three proton donors) which form four symmetry-independent hydrogen bonds with water molecules, anion O atoms and cyano groups as acceptors (Table 4).[21] Asymmetric unit of K2CNCA• H2O contains two potassium cations with different coordinations.K1 is heptacoordinated, in a shape of a capped distorted octahedron and forms contacts with 3 oxygens from the dianions, N and Cl atoms and a water molecule; K2 has a severely distorted hexacoordinated environment which could be best described as a badly distorted capped square pyramid.A single water molecule links two neighbouring cyanochloranilate anions through three symmetry-independent hydrogen bonds (Table 4, Figure 6), forming chains parallel to [001].][21][22] Thus, 3D packing is achieved (Figure 6).
Asymmetric unit of H2CNCA •2H2O comprises six strong proton donors (two hydroxyl groups and two water molecules) forming seven symmetry-independent hydrogen bonds (Table 4).The main motive are 2D hydrogen-bonded double layers lying in the plane (001) (Figure 7), and only dispersion interaction exist between those double layers.
Bulky, out-of-plane nitro groups effectively prevent π-stacking, so no stacking was observed in the structures of nitranilates.On the other hand, in salts of planar cyanochloranilates the dianions stack in an offset fashion, similar to other planar 2,5-dihydroxyquinones. [2,19,21,22]According to geometric parameters, these interactions are much weaker than in face-to-face stacks of hydrogen chloranilates, [23] and can be compared to aromatic stacks.
Inductive effect of the substituents affects not only acidity of hydroxyl groups, but also molecular geometry: double C=C bond and also the delocalised C-C bond next to the cyano group (C2-C3, Table 2) are shorter than the corresponding bond next to the chlorine substituent (C5-C6, Table 2).This effect may be used to fine-tune proton-donor and complexation capabilities of the 2,5-dihydroxyquinones, and could be therefore used in design and construction of coordination polymers and metal-organic frameworks. [1]ble 4. Geometric parameters of hydrogen bonds (Å, °)

Scheme 1 .
Scheme 1.The dissociation of nitranilic acid (a) and cyanochloranilic acid (b).The dianion has two delocalised π-systems separated by single C-C bonds.

Figure 3 .
Figure 3. Crystal packing of orthorhombic Na2NA•2H2O viewed in the direction [100].Sodium cations are shown as spheres of arbitrary radii and hydrogen bonds are shown as dashed lines.

Figure 4 .
Figure 4. Crystal packing of LiCsNA•H2O viewed in the approximate direction [100].Lithium and caesium cations are shown as violet and magenta spheres of arbitrary radii and hydrogen bonds are shown as dashed lines.

Figure 5 .
Figure 5. Crystal packing of Na2CNCA•3H2O viewed in the direction [010].Sodium cations are shown as spheres of arbitrary radii and hydrogen bonds are shown as dashed lines.

Figure 6 .
Figure 6.Crystal packing of K2CNCA• H2O viewed in the direction [100].Potassium cations are shown as spheres of arbitrary radii and hydrogen bonds are shown as dashed lines.

1 -
x, 1 -y, 3 -z (a) Cg = centre of gravity of the aromatic ring; (b) α = angle between planes of two interacting rings; (c) β = angle between Cg•••Cg line and normal to the plane of the first interacting ring.

Table 3
op. on A