Structural Chemistry & Crystallography Communication

Keywords

Phosphoric triamide; Hydrogen bond; Crystal structure; Graph set motsssif

Introduction

Phosphoramides constitute a well-studied sub-class of phosphorus(V)-nitrogen compounds due to the biological activity of some derivatives and growing applications in pharmacological and agricultural industry [1-3]. These compounds can bind to a metal cation as an oxygen-donor ligand [4-6]. Within this sub-class, compounds with a P(O)[NHC(O)][N]₂ skeleton are interesting for the preparation of chelating bidentate ligands and conformational studies of the C(O)―NH―P(O) fragment [7-12]. Here we report the synthesis, spectroscopic characterizations, and X-ray crystallography of N-(2-chlorobenzoyl)-N’, N"-bis(4- methylphenyl)-phosphoric triamide (C₂₁H₂₁ClN₃O₂P). The possibility of forming different hydrogen bonds is also discussed.

Experimental

X-ray crystallography

A single crystal of the investigated compound was measured with a SuperNova four-circle diffractometer of Rigaku Oxford Diffraction equipped with a 40W micro focus CuKα X-ray source collimated by mirrors, and CCD detector Atlas S2. Measurement was done at 95K using a Cryostream 800 Plus chiller. Measurement and data processing of the CCD images were done by program CrysAlisPro [13], structure was solved by Superflip [14] and refined by Jana 2006 [15]. All atoms except hydrogen were refined anisotropically. Hydrogen atoms belonging to carbon were kept at the expected positions while positions of hydrogen atoms belonging to nitrogen were refined using a restraint on N-H distances keeping them the same. Isotropic ADP of hydrogen atoms were constrained to 1.2 multiple of the U_eq of their parent atom. No unusual features were found during the structure solution and refinement.

Spectroscopic measurements

¹H, ¹³C and ³¹P NMR spectra were recorded on a Bruker Avance III-300 spectrometer. ¹H and ¹³C NMR spectra were referenced using the solvent CDCl₃ resonances (7.29 and 77.07 ppm for ¹H and ¹³C, respectively) for [2-Cl-C₆H₄C(O)NH]P(O)[Cl]₂ and using DMSO-d₆ resonances (2.50 ppm,¹H, and 39.52 ppm,¹³C) for [2-Cl- C₆H₄C(O)NH]P(O)[NHC₆H₄-4-CH₃]₂. The ³¹P NMR spectra were calibrated using the “absolute referencing” from the related ¹H spectra. IR spectra were recorded on a Thermo Nicolet Avatar 370 FTIR spectrometer and on a Buck 500 scientific spectrometer using KBr discs. The mass spectra were recorded with an MS model of the CH7A Varian Detector. Elemental analyses (C, H and N) were performed on a Thermo Finnigan Flash 1112EA elemental analyzer and the melting points were recorded with an Electrothermal IA 9000 apparatus.

Synthesis

Caution: Phosphorus pentachloride is very sensitive to the moisture and its partial hydrolysis gives phosphoryl trichloride, so, for the preparation of the [2-Cl-C₆H₄C(O)NH]P(O)[Cl]₂ reagent, dry CCl₄ solvent was used. Carbon tetrachloride was dried over P2O5 under reflux condition and distilled prior to use. A previous article mentioned the synthesis of [2-Cl-C₆H₄C(O)NH]P(O)[Cl]₂ reagent and its melting point (92–93°C) [16]. The procedure reported here is similar to the above-mentioned literature method with a few modifications. The modifications consist in replacing the dry C₆H₆ solvent by dry CCl₄ and also using PCl₅ as phosphorus-chlorine reagent instead of 2-Cl-C₆H₄C(O)N═PCl₃. A schematic protocol for the synthesis of [2-Cl-C₆H₄C(O)NH] P(O)[Cl]₂ reagent and [2-Cl-C₆H₄C(O)NH]P(O)[NHC₆H₄-4-CH₃]₂ phosphoric triamide is given in Scheme 1. It should be noted that the 2-Cl-C₆H₄C(O)N═PCl₃ reagent is also formed during the procedure used in this paper when PCl₅ and 2-Cl-C₆H₄C(O)NH₂ react (stage (i) in Scheme); however, to avoid the hydrolysis of sensitive 2-Cl-C₆H₄C(O)N═PCl₃ reagent, we preferred to continue the reaction with adding HCOOH to the solution containing 2-Cl- C₆H₄C(O)N═PCl₃ (stage (ii) in Scheme). We further studied the [2-Cl-C₆H₄C(O)NH]P(O)[Cl]₂ reagent with IR and NMR experiments and mass spectrometry. Moreover, the fusion point measured by us is a few more than that was reported in literature, probably due to different crystallinities of product obtained in different solvents or the more purity of sensitive reagent obtained by us.

Scheme 1: The synthesis procedure for the preparation of [2-Cl-C₆H₄C(O)NH]P(O)[Cl]₂ reagent and [2-Cl-C₆H₄C(O)NH] P(O)[NHC₆H₄-4-CH₃]₂ phosphoric triamide.

Synthesis of [2-Cl-C₆H₄C(0)NH]P(0)[Cl]₂

The [2-Cl-C₆H₄C(O)NH]P(O)[Cl]₂ reagent was prepared by reflux of phosphorus pentachloride and 2-chlorobenzamide in equimolar ratio in dry CCl₄ for 3 h (stage (i) in Scheme). The completion of this stage was monitored by stopping of the evolution of gas bubbles in an oil vessel. The reaction mixture was then cooled at an icebath and formic acid with similar ratio of the noted reactants was syringed drop-wise into the cold solution, and resulting colorless solution was stirred at 25°C for 2 h. After completion of the reaction, the stirring was stopped and the solvent was removed at a reduced pressure (stage (ii) in Scheme) to form a solid white product (yield: above 80%). Fusion point: 99°C. IR (cm^–1): 3091, 2834, 2676, 2553, 1712, 1673, 1592, 1477, 1431, 1405, 1292, 1264, 1227, 1162, 1105, 1046, 960, 901, 873, 776, 752, 716, 653, 592, 561, 509, 465, 405. ³¹P{¹H}NMR (acetone-d₆): δ=1.78 (s). ¹HNMR (CDCl₃): δ=7.41 (m, ¹H), 7.50 (m, 2H), 7.74 (m, ¹H), 9.62 (d²J(P,H)=8.4 Hz, ¹H, NH). ¹³CNMR (CDCl₃): δ=127.34, 130.51, 130.89,131.67, 131.80 (d³J(P,C)=10.4 Hz), 133.34, 165.64 (d²J(P,C)=3.6 Hz). MS (70 eV, EI): m/z (%)=238 (30) [M–³⁵Cl]⁺ (M is based on two ³⁵Cl and one ³⁷Cl), 139 (88) [C₇H₄ ³⁵ClO]⁺, 138 (100) [C₇H₅ ³⁵ClN]⁺, 101 (63) [P³⁵Cl2]⁺, 75 (95) [CH₂PNO]⁺, 47 (56) [PO]⁺. (C₇H₅Cl₃NO₂P) (%): C=30.83; H=1.84; N=5.14; found: C=30.83; H=1.72; N=5.21.

Synthesis of [2-Cl-C₆H₄C(O)NH]P(O)[NHC₆H₄-4-CH₃]₂

A solution of p-toluidine (10 mmol) in CHCl₃ (25 ml) was added dropwise to a solution of [2-Cl-C₆H₄C(O)NH]P(O)[Cl]₂ (2.5 mmol) in the same solvent (25 ml) at 273 K. After 4 h of stirring at an ice bath temperature, the process was stopped and the mixture kept at room temperature for a few days to remove the solvent. The solid obtained was washed with distilled water to remove the [NH₃C₆H₄(4-CH₃)]Cl salt (stage (iii) in Scheme). Suitable single crystals were obtained from a solution of the synthesized compound in CHCl₃/CH₃CN (1:4 v/v) at room temperature after a few days (yield: above 80%). Fusion point: 242°C. IR (cm^–1): 3336, 3288, 3081, 2889, 1670, 1517, 1446, 1385, 1289, 1216, 945, 821, 740. ³¹P{¹H}NMR (DMSO-d₆): δ=–6.17 (s). ¹HNMR (DMSO-d₆): δ=2.22 (s, 6H, Me), 7.02 (d, 3^J(H,H)=8.4 Hz, 4H), 7.08 (d³J(H,H)=8.7 Hz, 4H), 7.36 (m, 2H), 7.45 (m, 2H), 7.74 (d²J(P,H)=9.9 Hz, 2H, NH), 10.05 (s, ¹H, NH). ¹³CNMR (DMSO-d₆): δ=20.71, 127.33, 129.78, 131.76, 136.34 (d, 3^J(P,C)=9.1 Hz), 138.89, 168.35. MS (70 eV, EI): m/z (%)=415 (8) [M]⁺ (³⁷Cl), 413 (40) [M]⁺ (³⁵Cl), 412 (43) [M –1]⁺, 138 (65) [2-³⁵Cl-C₆H₄CNH]⁺, 137 (20) [2-³⁵Cl-C₆H₄CN]⁺, 107 (50) [C₇H₈NH]⁺. (C₂₁H₂₁ClN₃O₂P) (%): C=60.89; H=5.07; N=10.15; found: C=60.53; H=5.14; N=10.39.

Description of the crystal structure

The asymmetric unit of [2-Cl-C₆H₄C(O)NH]P(O)[NHC₆H₄-4-CH₃]₂ consists of one molecule, as shown in Figure 1. The crystal data and refinement parameters are listed in Table 1 and selected bond distances and angles are listed in Table 2. The P atom has a distorted tetrahedral configuration as has been noted for other phosphoric triamides [17], with bond angles around the P atom in the range of 99.54(5)–118.89(6)°. All P—N bonds in this compound are shorter than a typical phosphorus-nitrogen single bond (1.77 Å) and longer than a typical phosphorusnitrogen double bond (1.57 Å) [18], caused probably by the overlap of the electrostatic effects of the P—N polar bonds with their corresponding sigma bonds [19]. The P―N_P distances of 1.6335(11) and 1.6465(11) Å are significantly shorter than the related P―N_CP bond distance (1.6862(11) Å), resulting from the electronic effect caused by the C(O) group. This prolongation can also be found in the CSD for different types of P―N bonds in C(O)NHP(O)-based phosphoric triamides [20]. The phosphoryl and carbonyl groups, separated by the NH unit, adopt an antiposition with respect to each other, which is in agreement with previously reported acyclic phosphoric triamide compounds containing a C(O)NHP(O)(NH)₂ skeleton [21,22]. The P═O bond length is of standard value (1.4770(9) Å), and the bond-angle sums of about 355° for one nitrogen atom and about 360° for two other nitrogen atoms (P—N—C+C—N—H+H—N—P) confirm their sp² character. The criteria for distinguishing between planar and non-planar geometries from bond-angle sums are the same as previously proposed: N(planar) and N(pyramidal) refer to the cases with Σ ≥ 352.5° and Σ ≤ 339.0°, respectively, and the intermediate entries are the cases with Σ in the range 339.0° – 352.5° [23].

Figure 1: Displacement ellipsoids plot (50% probability) is shown for [2-Cl-C₆H₄C(O)NH]P(O)[NHC₆H₄-4-CH₃]₂ with atom numbering scheme. H atoms are drawn as spheres of arbitrary radii.

Table 1: Crystal data and structure refinement for [2-Cl-C₆H₄C(O)NH] P(O)[NHC₆H₄-4-CH₃]₂.

Table 2: Selected bond distances (Å) and angles (°).

The nitrogen atoms in the title structure do not take part in hydrogen bonding as an acceptor, because they have low Lewisbase character. So, two H-acceptors and three H-donors existing in the structure make three different hydrogen-bonded ring motifs. The more acidic NH of the C(O)NHP(O) moiety (N_CPH) participates in an R²₂(8) motif together with P(O), whereas the two other H atoms of the NHR units (NP) participate in an R²₂(12) motif combined with R¹₂(6) together with C(O). On the other hand, adjacent molecules are linked via a sequence of alternating R²₂(8) and R²₂(12)/R¹₂(6) ring motifs with together C¹₁(4) chain motif in a linear arrangement parallel to b axis (Figure 2). It was found that the strongest N_CP—H…O hydrogen bonds exist in the R²₂(8) motif (Table 3), as in recently published papers [24-33]. In addition to the hydrogen bonds noted, there are also the weak C—H…O interactions (between the phosphoryl oxygen with the neighboring aromatic C―H donor of the 2-Cl-C₆H₄ part) through discrete D¹₁(2) hydrogen bond motifs along the a axis. Further stabilization of this compound is achieved via chlorine atom participating in the C—H…Cl hydrogen bond along the c axis (through D¹₁(2) graph-set motifs). These interactions complete a 3D structure of the title compound.

Figure 2: The crystal packing diagram for [2-Cl-C₆H₄C(O)NH] P(O)[NHC₆H₄-4-CH₃]₂ is represented, showing a 1-D arrangement along the b axis. The H atoms bonded to C atoms were omitted for clarity. The N_P—H…O═C and CP—H…O═P hydrogen bonds are shown as dashed lines (symmetry codes: (i) –x + 2, –y + 1, –z; (ii) –x + 2, –y + 2, –z).

Table 3: Hydrogen bonds geometries for [2-Cl-C₆H₄C(O)NH]P(O)[NHC₆H₄- 4-CH₃]₂ (Å and °). Symmetry codes: (i) –x + 2, –y + 1, –z (ii) –x + 2, –y + 2, –z iii) x – 1, y, z (iv) –x + 2, y – 1/2, –z + 1/2.

Conclusion

In summary, we reported a new phosphoric triamide, namely, [2-Cl-C₆H₄C(O)NH]P(O)[NHC₆H₄-4-CH₃]₂ that was prepared in good yield and purity by treating 2-chlorobenzoyl phosphoramidic dichloride with p-toluidine. It crystallizes in the monoclinic system with P2₁/c space group. Adjacent molecules are linked through (N_P—H…)2O═C and (N_CP—H…) (C―H…) O═P hydrogen bonds in two-dimensional slabs parallel to ab plane. The slabs consist of chains of alternating R₂² (8) and R₂² (12) / R₂² (6) ring motifs (constructed from N―H…O hydrogen bonds) along b axis, which are connected with discrete D₁¹ (2) motifs (of C―H…O) along a axis. Weak C—H…Cl hydrogen bonds connect the slabs in the c axis direction, giving rise to a three – dimensional supramolecular structure.

Acknowledgements

Financial support of this work by Ferdowsi University of Mashhad is gratefully acknowledged (Project No. 28383/3). The X-ray part of the work was carried out with the support of Czech Science Foundation, grant GACR 15-12719S using instruments of the ASTRA lab established within the Operation program Prague Competitiveness - project CZ.2.16/3.1.00/24510.

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