Structural Chemistry & Crystallography Communication

Keywords

Chloranilate dianion; Phosphonium cation; H-bond; 1D lattice; Crystal structure; Anilate; Layered structure

Introduction

The use of the chloranilate dianion (C₆O₄Cl₂M)^2- (where (C₆O₄Cl₂M)^2- =2,5?dihydroxy-1,4-benzoquinone derivative dianion, (Scheme 1) to prepare different coordination complexesand salts dates back to 1967 when E K Andersen reported the first structurally characterized chloroanilate salt: (NH₄)₂(C₆O₄Cl₂)·H₂O [1] and also of the chloranilic acid [2,3]. A search in the CCDC database, (updated Nov. 2014) shows a total of 328 different structures containing the chloroanilate molecule. In most of them (ca. 245 cases) chloranilate appears as the fully deprotonated (C₆O₄Cl₂)^2- dianion, in ca. 55 cases it appears as the monoprotonated (HC₆O₄Cl₂)^- anion and in ca. 30 cases it appears in the neutral H₂C₆O₄Cl₂ acid form. In eleven cases it is even possible to find it with two different protonation states in the same crystal. Interestingly, the three possible combinations (0/?1, -1/-2 and 0/- 2) have been observed. Thus, there are four examples containing a mixture of de di-acid and the monoanion (0/-1) [4-6], two cases with a mixture of the monoanion and the dianion (-1/-2) [7,8] and five compounds combining de di-acid and the dianion (0/-2) [8-11].

Scheme 1: Chloranilic acid, H2C6O4Cl2 (left) and [PPh3Et]+ cation (right).

If we make a survey of the coordinating ability of the chloranilate anion (either as monoanion or as dianion), we find that among the ca. 300 compounds containing any of the two anions, they appear coordinated to a metal ion in ca. 184 cases and isolated in ca. 116 compounds (half of them as the dianion and the other half as the monoanion). In all, except two, of these ca. 184 examples where the chloranilate anions act as ligands coordinating metallic centres, the anions present the expected chelating bidentate coordination mode(1:κ²O,O’). In some cases they also coordinate an additional metal ion through only one of the oxygen atoms (1:κ²O,O’; 2:κ¹O) and in most casesthey bridge two metal atoms acting as bis-chelates through both sides of the ligand (1:κ²O,O’; 2:κ²O”O’”). The two only compounds where chloranilate is not a chelating ligand are two Mo compounds where the chloranilate bridges two Mo2 dimers in a monodentate way (1:κ¹O; 2:κ¹O’; 3:κ¹O”; 4:κ¹O’”) [12]. Among the 182compounds where chloranilate acts as chelate, it acts as terminal ligand (1:κ¹O,O’) in only 42 cases and as bridge (1:κ²O,O’; 2:κ²O”,O’”) in 136 cases. There are four compounds where the dianion acts as terminal and as a bridge [13-15].

Among the 42 cases where chloranilate acts as terminal ligand the two most abundant metal ions are, by far, Fe [16] and Cu [17] with 13 compounds each. Other metals include Be, Cr, Co, Zr, Mo, Ru, Rh, Pd, In, Re and U. Among the 136 examples where chloranilate acts as bridging ligand there are only eight heterometallic compounds where chloranilate connects two different metals [18-21]. The remaining 128 compounds are homometallic and include all the s-block elements except Be and Mg, all the first row transition metals except Ti, many second and third row transition metals and up to ten rare earths [15]. The most abundant metals are Cu, Mn and Co with 21, 12 and 11 examples, respectively.

Among the ca. 116 compounds where chloranilate anions are not coordinated, 109 do not contain any metal but only organic cations. Most of them (ca. 75) with N?containingheterocycles as pyridinium, pyrazinium, pyrimidinium, imidazolium, pyrazoliumand ca. 30 with ammonium-type cations. Other cations include tetrathiafulvalene derivatives (4 examples) and metal complexes (7 examples). In these seven metalcontaining compounds presenting isolated chloranilate anions the metals form very stable complexes with other ligands and are not available to form complexes with the chloranilate anions [10,11,22-24]. Interestingly, although there are ca. 30 examples with ammonium derivatives cations, there is no example with any phosphonium derivative cation and, therefore, (PPh₃Et)₂(C₆O₄Cl₂)·2H₂O (1) represents the first example of any chloranilate anion with a phosphonium derivative cation. Here we present the synthesis and a detailed analysis of the crystal structure of compound (PPh₃Et)₂(C₆O₄Cl₂)·2H₂O (1), the first chloranilate salt with a phosphonium cation.

Results and Discussion

Structure of compound (PPh₃Et)₂(C₆O₄Cl₂)·2H₂O (1)

Compound 1 crystallizes in the monoclinic P21/c space group. Its asymmetric unit contains one [PPh₃Et]⁺ cation located on a general position, half (C₆O₄Cl₂)^2- anion, located on an inversion centre, and a water molecule. Figure 1 shows the asymmetric unit and the labelling Scheme 1. Table 1 lists the crystal data and structure refinement parameters. The structure of 1 is formed by layers of [PPh₃Et]⁺ cations parallel to the ab plane alternating along the c direction with layers containing chloranilate anions and water molecules (Figure 2).

Table 1: Crystal data and structure refinement for [PPh₃Et]₂(C₆O₄Cl₂)·2H₂O (1). ^[a]R₁(F)=S||F_o|–|F_c||/S|F_o|; ^[b]wR₂(F²)=[Sw(F_o²–F_c²)²/SwF_o⁴]½; ^[c]S(F^₂)=[Sw(F_o²–F_c²)^₂/(n + r – p)]^1/2.

Figure 1: Chloranilic acid, H2C6O4Cl2 (left) and [PPh3Et]+ cation (right).

Figure 2: View of the cationic and anionic layers alternating along the c direction. Colour code: C=grey, O=red, Cl=green, H=white and P=orange.

The cationic layers contain ordered [PPh₃Et]⁺ cations situated in rows along the b direction with alternating opposite orientations in order to optimize the space fillingsince there is no π-π interactions between the cations (Figure 3a). The anionic layers contain rows parallel to the direction formed by alternating chloranilate dianions and water molecules (Figures 3b and 4).

Figure 3: (A) View of the cationic layer in the ab plane. (B) View of the anionic layer in the same plane. Colour code: C=grey, O=red, Cl=green, H=white and P=orange.

Figure 4: View of the chain formed by the chloranilate anions and the water molecules. Thin red lines are the H-bonds. The H···O bond distances are indicated in red. Colour code: C=grey, O=red, Cl=green and H=white.

The anion/water chains are held thanks to the formation of two different H-bonds. The first one is formed between the O3 atom of the chloranilate anion and the H1A of the water molecule (O3···H1A = 2.00(3) Å, O1W-H1A···O3 = 167.6≡, O1W···O3 = 2.835(2) Å). The second H bond is formed between the O2 atom of the chloranilate anion and the other H atom (H1B) of the water molecule (O2···H1B = 2.09(3) Å, O1W?H1B···O2 = 149.8≡, O1W···O2 = 2.846(2) Å). This second H-bond is somewhat weaker than the first one as clearly seen by the O···H distances and the O-H-O1W bond angles. From the O···H distances and the O-H···O angles these two H-bonds can be classified as moderate [25,26].

A careful search in the CCDC database of all the compounds where the (C₆O₄Cl₂)^2-dianion appears non-coordinated to any metal, shows that there are only 54 of such compounds. In most cases (47) the (C₆O₄Cl₂)^2- dianion forms H-bonds with the cations, including ten compounds where the H bonds are also formed with water molecules and two where the H-bonds are also formed with chloranilic acid molecules that also appear in the compound [8,11].

Among the only 7 compounds where the (C₆O₄Cl₂)^2- dianion does not form any H-bond with the cations, in five cases it forms H-bond only with water molecules [10,27-30], as observed in compound 1, in one case it only forms H-bonds with chloranilic acid molecules [10] and finally, there is one compound where the (C₆O₄Cl₂)^2- dianion appears completely isolated without forming any H-bond [22].

A close inspection at the five compounds where, as in 1, the (C₆O₄Cl₂)^2- dianion only forms H-bonds with water molecules, shows H-bonded networks of different dimensionalities. Thus, there is one 0D case (each (C₆O₄Cl₂)^2- dianion is connected to two water molecules which are not further H-bonded to any other molecule) [28], there are three cases where the H-bonded chloranilate dianions and the water molecules form a 2D network [10,27,30] and only one example of 1D network [29] (as observed in compound 1). The main difference between this last compound (JIXCAU) and 1 is that in 1 in each chloranilate dianion all the oxygen atoms are H-bonded to water molecules but in JIXCAU only two opposite oxygen atoms are connected and each of the oxygen atom is connected to two water molecules [29]. We can, therefore, conclude that compound 1 has a unique H-bonded 1D topology where all the oxygen atoms of the chloranilate dianion are H-bonded to one water molecule each (Figure 5).

Figure 5: View of the two only known examples of 1D H-bonded lattices of chlroanilate dianions and water molecules: (A) compound 1, (B) compound JIXCAU. Thin red lines represent the H-bonds. C=grey, O=red, Cl=green and H=white.

The bond distances and angles in the (C₆O₄Cl₂)^2- dianion (Table 2) are similar to those found in other related compounds containing non-coordinated (C₆O₄Cl₂)^2- dianions [10,27-30]. These bond distances and angles confirm the dianionic character of the chloranilate anion in 1.

Table 2: Main bond lengths (Å) and angles (≡) for [PPh₃Et]₂(C₆O₄Cl₂)·2H₂O (1). Symmetry transformations used to generate equivalent atoms: #1-x+1, -y+1, -z.

Experimental

Synthesis of (PPh₃Et)₂(C₆O₄Cl₂)·2H₂O (1): A solution of CrCl₃·6H₂O (213 mg, 0.8 mmol) in H₂O (5 mL) was added drop-wise to an aqueous solution (45 mL) prepared by mixing three solutions: (i) H₂C₆O₄Cl₂ (502 mg, 2.4 mmol) in water (25 mL) at 60°C, (ii) KOH (2 g, 35.64 mmol) in water (10 mL) and (iii) [PPh₃Et]Br (891 mg, 2.4 mmol) in water (10 mL). The resulting solution was heated at 60°C for 1 h. The solution was cooled to room temperature and then extracted with three portions of CH₂Cl₂ (of 50 mL each). The resulting deep red lower CH₂Cl₂ fractions were collected and the solvent was removed by rotary evaporation. The obtained deep red lacquer solid was recrystallized with N, N-dimethylformamide (DMF) to afford 1 as red block crystals. These crystals were obtained while studying the influence of different synthetic conditions in the synthesis of tris (chloroanilate) metalate complexes of formula [M^III(C₆O₄Cl₂)₃]^3- (MIII=Cr, Fe or Ga). We think that the excess of KOH in this case resulted in the formation of the hydroxido chromium (III) anionic complex [Cr (OH)₄(H₂O)₂]^- which should not be attacked by chloranilate dianion, allowing the crystallization of this dianion with the bulky [PPh₃Et]⁺ cation whose solubility must be lower that that of the, also known, K⁺ salts of the chloranilate dianion [31].

4.2.2 X-ray structure determination of (PPh₃Et)₂(C₆O₄Cl₂)·2H₂O (1): A single crystal of compound 1 was covered with a layer of a viscous perfluoro-polyether (FomblinY), mounted on a cryoloop with the aid of a microscope and immediately placed in the low temperature nitrogen stream of the diffractometer. The intensity data sets were collected at 120 K on a Supernova diffractometer equipped with an Oxford Cryostream unit with mirror-monochromated and with Mo Kα radiation (λ=0.71073 Å). The structures were solved using the WINGX [32] package for CRYSALIS [33] by direct methods and refined by least squares against F² (SHELXL-97) [34]. Empirical absorption correction was performed using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. All non-hydrogen atoms were anisotropically refined. The hydrogen atoms were positioned geometrically and refined using a riding model, except for the H atoms of the water molecules that were located from the difference map. A summary of the data collection and structure refinement of compound 1 is provided in Table 1.

Conclusions

The exploration on new synthetic routes to prepare novel chloranilate-based complexes has led to the synthesis of a novel chloranilate salt containing the bulky [PPh₃Et]⁺ cation: (PPh₃Et)₂(C₆O₄Cl₂).2H₂O (1). This compound is the first chloranilate salt with any phophonium cation and shows the formation of a 1D H-bonded network implying only the water molecules and the chloranilate dianions. Compound 1 is the second example of a 1D H-bonded network formed between chloranilate dianions and water molecules although the topology of the chain formed in 1 is unique.

Acknowledgments

We thank the Spanish MINECO (Project CTQ-2011-26507) and the Generalitat Valenciana (Prometeo II/2014/076 and ISIC programs) for financial support. Thanks are given to the Consejo Superior de Investigaciones Cientifícas (CSIC) of Spain for the award of a license for the use of the Cambridge Crystallographic Data Base (CSD).

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