Structural Chemistry & Crystallography Communication

Keywords

Crystal structure; N,N,N′,N′-Tetramethylethylenediammonium; Dichromate; Hirshfeld surface analysis; Fingerprint plots

Introduction

In the last three decades, chromium(VI) reagents in combination with amines have been widely used for the oxidation of alcohols to the corresponding carbonyl compounds [1]. It has been shown that the nature of the amine determines the oxidizing power of the dichromate anion and this is inversely related to the donor strength of the associated amine ligand [1,2]. As part of an ongoing research program, our group has been involved in the synthesis and structural characterization of new organic dichromates over several years, which can be used for oxidation of organic substrates [3-7]. As a continuation of our studies in this area, we report here the synthesis of a new organic dichromat salt, (C₆H₁₈N₂)[Cr₂O₇], (I). The chemical composition and crystal structure were determined by energy-dispersive X-ray spectroscopy (EDX) analysis (Figure 1) and single-crystal X-ray diffraction; the proposed structural model is supported by validation tools by means of bond-valence-sum (BVS) calculations for chromium and oxygen atoms [8,9]. In order to evaluate the nature and energetic associated with intermolecular interactions in the crystal packing, the detailed analyses of Hirschfeld surface and fingerprint plots calculations were performed [10-13].

Figure 1: The EDX spectrum of (I).

Experimental

Synthesis and crystallization of (C₆H₁₈N₂) [Cr₂O₇]

The title compound was prepared by dissolving 10 mmol of chromium trioxyde (1 g, purity 99.99%, Sigma-Aldrich) in 20 ml of distilled water and 10 mmol of N,N,N′,N′- tetramethylethylenediamine (1,5 ml, purity 99.0%, Merck) in 15 ml of ethanol (96%) with a molar ratio of 1:1. The mixture was stirred for 20 minutes and the solution is allowed to stand at room temperature. Brown single crystals of suitable dimensions for crystallographic study were formed in the reactionnel midle after 5 days of slow evaporation of the solvent. The reactional mechanism is as follows :

First step : dissolution of CrO₃

CrO₃ + H₂O →H₂CrO₄

H₂CrO₄ + H₂O = HCrO₄^- + H₃O⁺ pka1 =1, 14

Second step : condensation of HCrO₄^-

2 HCrO₄^- = Cr₂O₇^2- + 2 H₂O 2< pH < 6

Third step : protonation of base

C₆H₁₆N₂ + 2 H₃O⁺ → C₆H₁₈N₂2+ + 2 H₂O

C₆H₁₈N₂²⁺ + Cr₂O₇^{2‐} → (C₆H₁₈N₂) [Cr₂O₇]

X-ray structure determination

Single crystal X-ray diffraction data for the compound at room temperature was collected on a CAD-4 Enraf-Nonius diffractometer equipped with graphite monochromated MoKα radiation (λ=0.71073 Å). Cell constants and an orientation matrix for data collection were obtained from a least-squares refinement using the setting angles of 25 carefully centered reflections. The cell parameters were then refined using the full set of collected reflections in the range 2°< 2θ< 44°. The half sphere of data was collected at 293 K giving rise to 2392 unique reflections, of which 1626 were observed with I/σ(I)>2. The linear absorption coefficient for MoKα is 1.68 mm^-1. The data were corrected for Lorentz and polarization effects. The structure was solved by direct methods and successive Fourier difference syntheses and refined on (F²) by full-matrix least-squares methods respectively with the SHELXS-97 and SHELXL-97 programs [14]. All non-H atoms were refined anisotropically H atoms attached to CH₃, CH₂ and N₁ atoms were placed geometrically and refined using a riding model: C-H=0.96 Å for CH₃ group with Uiso(H)=1.5Ueq(C); C-H=0.97 Å of CH2 group with Uiso(H)=1.2 Ueq(N); N1-H=0.98 Å withUiso(H) = 1.2 Ueq (N1). The positions of H atoms attached to N₂ and N₃ atoms were localized in difference Fourier maps, the distances were restrained with N-H=0.86 Å and the hydrogen atom were refined isotropically with Uiso(H)=1.2 Ueq of parent atom. The final cycle of full-matrix least-squares refinement was based on 1626 observed reflections [I>2.00σ(I)] and 242 variable parameters and converged with agreement factors of R (F²>2σ(F²))=0.079 and wR(F²)=0.219 (All data). The structure graphics were drawn with Diamond Version 3.2e supplied by Crystal Impact [15]. The crystal data and refinement details are summarized in Table 1. Atomic coordinates with equivalent isotropic displacement parameters for non-hydrogen atoms are given in Table 2. Selected bond distances and angles are listed in Table 3. Hydrogen bond scheme and C-H···O interactions are described in Table 4. Crystallographic data (excluding structure factors) for the structural analysis has been deposited with the Cambridge Crystallographic Data Centre, No. CCDC-1491800. Copies of this information may be obtained free of charge from web: www.ccdc.cam.ac.uk

Table 1: Crystal data, data collection and refinement details of (I).

Table 2: Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (A²) of (I).

Table 3: Geometric parameters (Å, °) of (I) Symmetry codes: (i) −x, −y, −z; (ii) −x+1, −y+1, −z+1.

Results and Discussion

Crystal structure of (C₆H₁₈N₂)[Cr₂O₇]

The molecular structure of (I) is shown in Figure 2. In this structure, the asymmetric unit is composed of one and half of a dichromate anions and one and half of a N,N,N′,N′- tetramethylethylenediammonium dications. One dication is organized around an inversion centre located at the centre of the –CH₂-CH₂– bridge and the two dimethylamine groups are anti with respect to one another.

Figure 2: The molecular structure of (I), showing the atom-labeling scheme Displacement ellipsoids are drawn at the 50% probability level. Symmetry-related atoms are Generated by a crystallographic inversion centres [Symmetry codes: (i) x, y + 1, z: (ii) 1 − x, 1 − y, 1 − z].

In the cations, the nitrogen atoms are protonated. Examination of the organic cations shows that the bond distances and angles have no significant differences from those in other compounds involving the same organic groups: organic acide base from N,N,N′,N′- tetramethylethylenediammonium [16], (C₆H₁₈N₂)3[Cr₂O₇]2.H₂O and (C₆H₁₈N₂)3[C₂O₄][Cr₂O₇]2.4H₂O [3,4] (C₆H₁₈N₂)[HPO₄]2.4H₂O [17] (C₆H₁₈N₂)2[P₄O₁₂].4H₂O [18] and C₆H₁₈N₂)[H₂P₂O₇].2H₂O [19]. The [Cr₂(3)O₇]²⁻ anion exhibits disorder of the bridging O11 oxygen atom around the inversion symmetry over two positions O₁₁ and O₁₁ (i) -x, −y, −z) in a ratio 0.5:0.5. Contrary to the quasi eclipse conformation generally encountered in alkali dichromates [20,21] this one exhibits a surprising staggered conformation: the tetrahedral twist about 60° away from the exactly eclipsed conformation. In dichromate anions, the Cr-O terminal bond lengths are in the range 1.577 (9)-1.625 (8) Å and the bridging Cr-O bonds are longer and in the range 1.783 (7)-1.791 (19) Å. Similar geometrical features have also been noticed in other crystal structures of organic dichromates [3,4,6,7,22-26]. The terminal Cr1-O3 distance is considerably longer (1.625 (8) A) than the other therminal Cr-O bond lengths. This lengthening can be explained on the basis of strenth of different O…H bonding interactions: The O3 oxygen atom accepts one strong hydrogen bond of type N-H…O [27,28].

The bond-valence-sum (BVS) calculations for chromium (Cr₁, Cr₂, Cr₃) and oxygen (O₁, O₂, O₅, O₆, O₈, O₉) atoms are close to their oxidation states: Cr₁: 5.94, Cr₂: 5.96, Cr₃: 6.04, O₁: 1.84, O₂: 1.83., O₅: 1.93, O₆: 1.80, O₈: 1.94, O₉:1.83 v.u. (valence unit). The bond valence sum found for O₃, O₄, O₇, O₁₀ and O₁₁differes significantly from the expected value of 2.0 vu: O₃: 1.78., O₄: 2.28, O₇: 1.77, O₁₀: 1.76, O₁₁: 2.33 v.u. This can be explained by strenth N-H…O bonds for O₃ and O₇, by the existence of a significant bond valence of H…O bond lengths of intermolecular C-H…O interactions that we did not calculated here, for O4 and O10 and by the disorder for O11.

The organic cations and inorganic anions are each arranged in columns parallel to c axis and alternate with each other along (110) direction, forming a layered arrangement parallel to (110) plane (Figure 3). Each organic dication, involving NH+, CH2 and CH3 groups, is connected to six different dichromate anions via for N-H…O and nine C-H…O hydrogen bonds (Table 4), forming a three-dimensional supramolecular network. Only two of the N-H …O hydrogen bonds are considered as strong according to the Blessing and Brown criteria [27,28]. A detailed packing analysis of the crystal structure of (I) revealed the formation of {(C₆H₁₈N₂)[Cr₂O₇]}n infinite undulating chains lying parallel to the [110], which generate R22(11), R22(9) and R12(4) ring motifs [29,30] (Figure 4). These one-dimensional chains extending on the (110) plane formed a layer.

Figure 3: The packing of the molecular entities in the crystal structure of (I).

Table 4: Hydrogen-bondgeometry(Å,°)of(I)Symmetrycodes(i)−x,−y,−z;(ii)−x+1,−y+1,−z+1;(iii)x+1,y,z;(iv)x+1,y,z+1v)−x,−y+1,−z;(vi)−x+1,−y,−z+1;(vii)−x,−y+1,−z+1.

Figure 4: A perspective view of one chain of (I), showing R22 (11), R22 (8) and R12 (4) ring Motifs along [1-10] direction. Hydrogen bonds are represented by dashed lines C-H…O (red) N-H…O (blue).

Hirshfeld surface analysis

Organic small molecule crystal packings are often dominated by the hydrogen bonding patterns. However, a crystal structure is determined by a combination of many forces, and hence all of the intermolecular interaction of a structure should be taken into account. Visualization and exploration of intermolecular close contacts of a structure is invaluable, and this can be achieved using the Hirshfeld surface [10,11]. A large range of properties can be visualized on the Hirshfeld surface with the program Crystal Explorer [31] including de and di, in which de and di represent the distances from a point on the HS to the nearest atoms outside (external) and inside (interna) the surface, respectively. The intermolecular distance information on the surface can be condensed into a two-dimensional histogram of de and di, which is a unique identifier for molecules in a crystal structure, called a fingerprint plot [12,13]. Instead of plotting de and di on the Hirshfeld surface, contact distances are normalized in CrystalExplorer using the van der Waals radius of the appropriate internal and external atom of the surface:

d_norm=(d_i−r_i^vdw)/r_i^vdw+(d_e−r_e^vdw)/r_e ^dw

For the two dications of (I), the three-dimensional Hirshfeld surface (3D-HS) that has been mapped over d_norm is given in Figure 5 contacts with distances equal to the sum of the van der Waals radii are shown in white (label 3), and contacts with distances shorter than or longer than the related sum values are shown in red (labels 1 and 2) (highlighted contacts) or blue, respectively. The interaction between N-H and oxygen atoms can be seen in the Hirshfeld surface as the bright-red area (label 1) (Figure 5). The light-red spots are due to C—H…O interactions (Label 2). The shapes of the HSs of the two dications in the structure of (I) are similar, reflecting similar intermolecular contacts and similar conformations (anti conformation of the dimethylamine groups).

Figure 5: View of the three dimensional Hirshfeld surfaces (3D-HS) for the two dications of (I). (3D-HS mapped with d_norm).

Figure 6 illustrates the analysis of the two-dimensional fingerprint plots (2D-FP) for the two dications of (I). 2D-FP are the twodimensional representations of the information provided by visual inspection of the 3D-HS, which are plotted on an evenly spaced grid formed by (d_e, d_i) pairs. Each grid point is coloured according to the frequency of occurrence of the (d_e,d_i) pair on the 3D-HS, from blue for small Contributions, through green to red for maximum contributions, if present. The 2D-FP in Figure 6 are quite asymmetric; this is expected, since interactions occur between two different species (cation and anion) [32]. For the title salt, H…O contacts, which are attributed to N-H…O and C-H…O hydrogen-bonding interactions, appear as one sharp spike in the two-dimensional-FP with a prominent long spike at d_e + d_i = 1.8 Å. They have the most significant contribution to the total Hirshfeld surfaces (63%). The H…H contacts appear in the middle of the scattered points in the two-dimensional fingerprint maps with a single broad peak at d_e = d_i = 1.25 Å and a percentage contribution of 37%.

Figure 6: Two-dimensional full fingerprint plots (2D-FP) for the two dications of (I) (a), and two- dimensional-FP resolved into H…O (b) and H…H (c) close contacts.

Conclusion

In summary, we reported one new organic dichromate, (C₆H₁₈N₂) [Cr₂O₇] using a facile slow evaporation method and it crystallizes in the triclinc space group P . In the structure the organic cations and inorganic anions are each arranged in rows parallel to c axis and alternate with each other along (1-10) direction, forming a layered arrangement parallel to (110) plane. The stability and the cohesion of the structute is ensured by N-H…O hydrogen bonds and weak C-H…O interactions. 3D-HS and 2D-FP reveal that the structure is dominated by H…O and H…H contacts. Our future research efforts will be devoted to the synthesis of differents organic dichromates with new organic base such as amine or diamine to exploit this related systems and study the relationship between the crystal structures and properties of this related materials.

Acknowledgements

The authors are grateful to Dr. R. Ben Smail, Universite de Charthage, Institut Preparatoire des Etudes d'ingenieurs de Nabeul, for fruitful discussions.

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