# Dichlorophosphoranides Stabilized by Formamidinium Substituents

Authors: Anatoliy Marchenko; Georgyi Koidan; Anastasiya Hurieva; Eduard Rusanov; Alexander B. Rozhenko; Aleksandr Kostyuk
Journal: Heteroatom Chemistry
 (2020)

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## Abstract

Dichlorophosphoranides featuringN,N-dimethyl-N′-arylformamidine substituents were isolated as individual compounds. Dichlorophosphoranide 9 was prepared by the multicomponent reaction of C-trimethylsilyl-N,N-dimethyl-N′-phenylformamidine and N,N-dimethyl-N′-phenylformamidine with phosphorus trichloride. Its molecular structure derived from a single-crystal X-ray diffraction was compared to the analogous dibromophosphoranide prepared previously by us by the reaction of phosphorus tribromide with N,N-dimethyl-N′-phenylformamidine. It was shown that a chlorophosphine featuring two N,N-dimethyl-N′-mesitylformamidine substituents reacted with hydrogen chloride to form dichlorophosphoranide 11. Its molecular structure was also determined by X-ray analysis and compared with that of closely related dichlorophosphoranide C.


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## Body

## 1. Introduction

PhosphoranidesA are hypervalent anionic phosphorus(III) compounds formally possessing a 10-electron valence shell and a distorted pseudotrigonal bipyramidal arrangement at the phosphorus atom. The electronegative ligands at phosphines make nucleophilic addition possible to afford stable phosphoranides (Figure 1).Figure 1
Examples of phosphoranides stabilized byN-heterocyclic carbene ligands.The first isolated phosphoranide has been prepared by the reaction of tetrapropylammonium bromide with phosphorus tribromide, and its structure has been unambiguously determined by a single-crystal X-ray diffraction study [1, 2]. Later, tetrachlorophosphoranides and tetrafluorophosphoranides were prepared, with tetrafluorophosphoranide being the most stable derivative [3]. N-heterocyclic carbenes are known to be suitable for stabilization of high-coordinated P atoms. The reaction of a sterically hindered N-heterocyclic carbene with PCl3 in hexane affords a high yield of phosphoranide B. The imidazoliumyl substituent efficiently stabilizes phosphoranides. Another example is phosphoranide C in which the imidazolium moieties serve for stabilization [4, 5]. In our previous publication, we have described the synthesis of dibromophosphoranide 3 by the reaction of N,N-dimethyl-N′-phenylformamidine 1 with phosphorus tribromide in a 3 : 1 ratio. Its structure was established by X-ray diffraction analysis. Based on DFT calculations, the mechanism for formation of phosphoranide 3 has been suggested (Scheme 1) [6].Scheme 1
Mechanism proposed for formation of dibromophosphoranide3.The final step of the proposed mechanism is the reaction of dibromophosphine2 with N,N-dimethyl-N′-phenylformamidine. It should be noted that other P(III) halides, such as phosphorus trichloride, dichlorophosphines, and monochlorophosphines, do not react with the formamidines. Earlier, we were unable to check the mechanism, as dibromo(dichloro)phosphines featuring the formamidine substituent were unavailable. Recently, we have developed a method for the synthesis of C-trimethylsilyl-N,N-dialkyl-N′-arylformamidines and studied their reactions with phosphorus trichloride and chlorophosphines. A set of chlorophosphines featuring two formamidine substituents were isolated as stable compounds [7]. We assumed that dichlorophosphines featuring the formamidine substituents can be prepared by this method as well. It will allow investigation of the proposed mechanism and development of a method for the synthesis of phosphoranides.


## 2. Materials and Methods

All procedures with air- and moisture-sensitive compounds were performed under an atmosphere of dry argon in flame-dried glassware. Solvents were purified and dried by standard methods. Melting points were determined with an electrothermal capillary melting point apparatus and were uncorrected.1H spectra were recorded on a Bruker Avance DRX 500 (500.1 MHz) or Varian VXR-300 (299.9 MHz) spectrometer. 13C NMR spectra were recorded on a Bruker Avance DRX 500 (125.8 MHz) spectrometer. 31P NMR spectra were recorded on a Varian VXR-300 (121.4 MHz) spectrometer. Chemical shifts (δ) are reported in ppm downfield relative to internal TMS (for 1H, 13C) and external 85% H3PO4 (for 31P). Chromatography was performed on silica gel Gerudan SI60. Elemental analyses were performed at the Microanalytical Laboratory of the Institute of Organic Chemistry of the National Academy of Sciences of Ukraine.


### 2.1. X-ray Structure Determination

Crystal data for9: (C18H23Cl2N4P), M = 397.29, triclinic, space group P-1, а = 9.3377(2), b = 9.9530(2), c = 12.3654(3) Å, α = 108.414(1), β = 106.412(1), γ = 101.860(1)°, V = 989.57(4) Å3, Z = 2, dc = 1.33, μ 0.418 mm−1, F(000) 416, crystal size ca. 0.33 × 0.47 × 0.54 mm. All crystallographic measurements were performed at 123K on a Bruker Smart Apex II diffractometer operating in the ω scans mode. The intensity data were collected using Mo-Kα radiation (λ = 0.71078 Å). The intensities of 22667 reflections were collected (4183 unique reflections, Rmerg = 0.033). Convergence for 9 was obtained at R1 = 0.0294 and wR = 0.058 for 3520 observed reflections with I ≥ 3σ(I); GOF = 0.9332, R1 = 0.0360, and wR = 0.0617 for all 4167 data, 226 parameters, and the largest and minimal peaks in the final difference map 0.34 and −0.22 e/Å3.Crystal data for11: (C24H35Cl2N4P1), M = 481.45, orthorhombic, space group Pna21, а = 11.8655(3), b = 14.1280(3), c = 15.3685(3) Å, V = 2576.31(10) Å3, Z = 4, dc = 1.241, μ 0.333 mm−1, F(000) 1024, crystal size ca. 0.25 × 0.27 × 0.48 mm. All crystallographic measurements were performed at 123K on a Bruker Smart Apex II diffractometer operating in the ω scans mode. The intensities of 30253 reflections were collected (4944 unique reflections, Rmerg = 0.039). Convergence for 11 was obtained at R1 = 0.0287 and wR = 0.0484, GOF = 0.9187 for 4259 observed reflections with I ≥ 3σ(I); GOF = 0.9187, R1 = 0.0363, and wR = 0.0525 for all 4923 data, 285 parameters, the largest and minimal peaks in the final difference map 0.41 and −0.31 e/Å3. The structures were solved by direct methods and refined by the full-matrix least-squares technique in the anisotropic approximation for non-hydrogen atoms using the SIR97 and Crystals program package [8, 9].1,1-Bis(dimethylamino)-N,N-diisopropyl-N′-4-mesityl-phosphine-carboximidamide selenide (5a): To a frozen solution of 4a (1.31 g, 5 mmol) in Et2O (15 mL), a solution of PCl3 (0.69 g, 5 mmol) in diethyl ether (15 mL) was added. The reaction mixture was allowed to warm to ambient temperature (15°C) with stirring. The solvent was evaporated. Benzene (5 mL) was added to the residue, and then, a solution of dimethylamine (900 mg, 20 mmol) in benzene (6 mL) was added. The mixture was stirred for 15 min, and selenium (500 mg, 6 mmol) was added. The resulting suspension was stirred for 1 h at 15°C. The insolubles were filtered off and washed with benzene (2 × 5 mL), and the filtrate was evaporated. The residue was purified by silica gel plate chromatography. Yield, 60%. Rf 0.2–0.45 (CH2Cl2–hexane 1 : 1), m.p. 116–117°C; 31P {1H} NMR (202 MHz, CDCl3): δ = 65.7 (JPSe = 793 Hz) ppm; 1H NMR (300 MHz, CDCl3): δ = 2.10 (s, 6 H, CH3), 2.23 (s, 3 H, CH3), 2.84 (d, J = 2.7 Hz), 2.87 (s, 18 H, NCH3), 6.77 (s, 2 H, CH) ppm; 13C NMR (125.7 MHz, C6D6): δ = 18.6 (s, CH3), 20.2 (s, CH3), 37.9 (s, CH3), 39.8 (s, CH3), 126.1 (s, ipso-C), 127.6 (s, CH), 129.9 (s, ipso-C), 145.3 (d, J = 21 Hz, ipso-C), 150.5 (d, J = 155 Hz, CN); EI-MS 387–100% [M + 2]+; elemental analysis calcd (%) for C16H29N4PSe (387.37) C 49.61, H 7.55, N 14.46, P 8.00; found: C 49.84, H 7.37, N 14.62, P 8.26.1,1-Bis(dimethylamino)-N,N-dimethyl-N′-(4-mesityl)-phosphinecarboximidamide selenide (5b): To a frozen solution of PCl3 (0.27 g, 2 mmol) in benzene (2 mL), a solution of C-silylformamidine 4b (0.61 g, 1.9 mmol) in benzene (4 mL) was added with stirring. In 1 h, the reaction mixture was concentrated under vacuum. Benzene (3 mL) was added to the residue, and then, a solution of dimethylamine (0.41 g, 9 mmol) in benzene (3 mL) was added. The mixture was stirred for 15 min, and then, selenium (1.9 mmol) was added in two portions over 10 min. The resulting suspension was stirred overnight. The insolubles were filtered off and washed with benzene (2 × 2 mL), and the filtrate was evaporated. The residue was extracted with hexane (2 × 5 mL), the solvent was removed under reduced pressure, and the residual solid was purified by silica gel plate chromatography. Yield, 49%. Rf 0.5–0.8 (CH2Cl2 –hexane 1 : 1); m.p. 157–158°C (pentane); 31P NMR {1H} (202 MHz, CDCl3): δ 60.0 ppm (JPSe = 793 Hz). 1H NMR (300 MHz, CDCl3): δ = 1.38 (d, J = 6.3 Hz, 12 H, CH3), 2.13 (s, 6 H, CH3), 2.20 (s, 3 H, CH3), 2.50 (d, J = 10 Hz, 12 H, NCH3), 4.36 (br s, 2 H, CH), 6.78 (s, 2 H, CH) ppm; 13C NMR (125.7 MHz, CDCl3): δ = 19.1 (s, CH3), 20.2 (s, CH3), 21.2 (s, CH3), 37.8 (s, CH), 48.7 (s, CH3), 124.75 (s, ipso-C), 127.8 (ipso-C), 127.9 (s, CH), 143.8 (d, J = 8.8 Hz, ipso-C), 145.8 (d, J = 88 Hz, N=C); EI-MS 445-98.2% [M + 2]+; elemental analysis calcd (%) for C20H37N4PSe (443.48): C 54.17, H 8.41, N 12.63, P 6.98; found: C 53.89, H 8.88, N 13.01, P 7.32.General procedure for synthesis of compounds (6a and b): To a solution of phosphineselenide 7 (1.9 mmol) in benzene (4 mL), a solution of tris(morpholino)phosphine (2 mmol) in benzene (8 mL) was added. The reaction mixture was stirred for 30 min, and then, the solvent was removed under reduced pressure until dryness. The residue was dissolved in pentane (10 mL), and the obtained solution was cooled to −12°C. After several hours, the precipitated solid was filtered off, the filtrate evaporated under vacuum, and the residue distilled to produce compound 6.1,1-Bis(dimethylamino)-N,N-dimethyl-N′-mesityl-phosphine-carboximidamide (6a): Yield, 93%. B.p. 120–122°C/0.05 Torr; m.p. 26–28°C; 31P NMR (81 MHz, CDCl3): δ = 92.4, 88.7 (10 : 1) ppm; 1H NMR (300 MHz, C6D6): δ = 2.22 (s, 6 H, CH3), 2.27 (s, 3 H, CH3), 2.57 (s, 6 H, NCH3) and 2.64 (d, J = 8.7 Hz, 12 H, NCH3), 6.86 (s, 2 H, CH) ppm; 13C NMR (125.7 MHz, C6D6) δ = 18.8 (s, CH3), 20.2 (s, CH3), 38.1 (d, J = 10 Hz, CH3), 40.6 (d, J = 15 Hz, CH3), 125.3 (s, ipso-C), 127.7 (s, CH), 127.9 (s, ipso-C), 147.3 (s, ipso-C), 160.0 (d, J = 15 Hz, N=C); elemental analysis calcd (%) for C16H29N4P (308.4): C 62.31, H 9.48, N 18.17, P 10.04; found: C 62.02, H 9.71, N 18.42, P 9.86.1,1-Bis(dimethylamino)-N,N-diisopropyl-N′-mesityl-phosphine-carboximidamide (6b): Yield, 98%. B.p. 130°C/0.05 Torr, m.p. 59–60°C (pentane, −28°C); 31P NMR (81 MHz, CDCl3): δ = 90.1 ppm; 1H NMR (500 MHz, C6D6): δ = 1.34 (br s, 12 H, CH3), 2.26 (s, 6 H, CH3), 2.27 (s, 3 H, CH3), 2.34 (d, J = 8.5 Hz, 12 H, NCH3), 3.93 (br s, 2 H, CH), 6.85 (s, 2 H, CH) ppm; 13C NMR (125.7 MHz, C6D6): 19.1 (d, J = 4 Hz, CH3), 20.2 (s, CH3), 20.9 (s, CH3), 40.9 (d, J = 15 Hz, CH3), 47.8 (d, J = 13 Hz, CH), 124.1 (s, ipso-C), 126.0 (s, ipso-C), 127.9 (s, CH), 145.9 (s, ipso-C), 157.5 (d, J = 40 Hz, N=C); elemental analysis calcd (%) for C20H37N4P (364.52): C 65.90, H 10.23, N 15.37, P 8.50; found: C 66.32, H 9.97, N 15.67, P 8.31.1,1-Dichloro-N,N-diisopropyl-N′-mesityl-phosphine-carboximidamide (7b): To a solution of 6b (360 mg, 1 mmol) in benzene (4 mL), PCl3 (305 mg, 2.2 mmol) was added. The reaction mixture was stirred at 20°C for 25 min and then concentrated under vacuum. The oily residue was kept at 60°C under vacuum for 25 min and then distilled, b.p. 120°C/0.05 Torr to give 7b of 340 mg (99%). 31P NMR (81 MHz, C6D6): δ = 134.1 ppm; 1H NMR (300 MHz, C6D6): δ = 1.30 (d, J = 5.4 Hz, 12 H, CH3), 2.16 + 2.18 (2xs, 9 H, CH3), 4.03 (br s, 2 H, CH), 6.78 (s, 2 H, CH) ppm; 13C NMR (125.7 MHz, C6D6): δ = 18.6 (d, J = 2.5 Hz, CH3), 19.8 (s, CH3), 20.2 (s, CH3), 48.6 (s, CH), 125.0 (s, i-C), 128.3 (s, CH), 131.2 (s, i-C), 143.6 (d, J = 30 Hz, i-C), 154.1 (d, J = 99 Hz, C=N); elemental analysis calcd (%) for C16H25Cl2N2P (347.27): Cl 20.42, P 8.92; found: Cl 20.06, P 9.05.Dichlorophosphoranide (9): To a solution of silylformamidine 8 (1.0 g, 4.5 mmol) and 1 (670 mg, 4.5 mmol) in CH2Cl2 (10 mL), cooled to freezing, PCl3 (730 mg, 5.3 mmol) was added. The reaction mixture was allowed to warm at ambient temperature (16°C) with stirring. The solvent was removed under vacuum. The residue was extracted with Et2O (15 mL). The insoluble powder was filtered under argon, washed with Et2O (3 × 10 mL), and dried under vacuum. The collected solid was shaken in THF (26 mL), insoluble part was collected by filtration and washed with THF (5 mL), and the filtrate was evaporated under vacuum. The residue was recrystallized from CH3CN (7 mL) to give 9 of 330 mg (18%). M.p. 141–144°C (decomp); 31P NMR (202 MHz, CDCl3): δ = 124.7 ppm. 1H NMR (500 MHz, C6D6): δ = 1.23 (br s, 6 H, CH3), 2.49 (br s, 6 H, CH3), 4.67 (br s, 1 H, CH), 7.07 (br s, 6 H, Ph), 8.26 (br s, 4 H, Ph). Elemental analysis calcd (%) for C18H23Cl2N4P (397.29): Cl 17.85, P 7.80; found: Cl 18.11, P 7.69.Chlorophosphine (10): To a solution of silylformamidine 4a (0.96 g, 3.7 mmol) in benzene (2.5 mL), phosphorus trichloride (0.25 g, 1.8 mmol) in benzene (1 mL) was added. A slight exothermic effect was observed. In 1 h, all solvents evaporated to give a white solid. 31P NMR (202 MHz, CDCl3): δ = 30 ppm [7].Dichlorophosphoranide (11): To a solution of chlorophosphine 10 (0.6 g, 1.4 mmol) in benzene (5 mL), a solution of hydrogen chloride (0.05 g, 1.4 mmol) in ether (3 mL) was added. The precipitated solid was collected by filtration. The solid was washed with ether. The solid was recrystallized from benzene to give white crystals of 0.52 g, 80%. M.p. 181–182°C. 31P NMR (81 MHz, CDCl3): δ = −102 ppm; elemental analysis calcd (%) for C24H35Cl2N4P (481.45): Cl 14.73, P 6.43; found: Cl 14.38, P 6.04.


## 2.1. X-ray Structure Determination

Crystal data for9: (C18H23Cl2N4P), M = 397.29, triclinic, space group P-1, а = 9.3377(2), b = 9.9530(2), c = 12.3654(3) Å, α = 108.414(1), β = 106.412(1), γ = 101.860(1)°, V = 989.57(4) Å3, Z = 2, dc = 1.33, μ 0.418 mm−1, F(000) 416, crystal size ca. 0.33 × 0.47 × 0.54 mm. All crystallographic measurements were performed at 123K on a Bruker Smart Apex II diffractometer operating in the ω scans mode. The intensity data were collected using Mo-Kα radiation (λ = 0.71078 Å). The intensities of 22667 reflections were collected (4183 unique reflections, Rmerg = 0.033). Convergence for 9 was obtained at R1 = 0.0294 and wR = 0.058 for 3520 observed reflections with I ≥ 3σ(I); GOF = 0.9332, R1 = 0.0360, and wR = 0.0617 for all 4167 data, 226 parameters, and the largest and minimal peaks in the final difference map 0.34 and −0.22 e/Å3.Crystal data for11: (C24H35Cl2N4P1), M = 481.45, orthorhombic, space group Pna21, а = 11.8655(3), b = 14.1280(3), c = 15.3685(3) Å, V = 2576.31(10) Å3, Z = 4, dc = 1.241, μ 0.333 mm−1, F(000) 1024, crystal size ca. 0.25 × 0.27 × 0.48 mm. All crystallographic measurements were performed at 123K on a Bruker Smart Apex II diffractometer operating in the ω scans mode. The intensities of 30253 reflections were collected (4944 unique reflections, Rmerg = 0.039). Convergence for 11 was obtained at R1 = 0.0287 and wR = 0.0484, GOF = 0.9187 for 4259 observed reflections with I ≥ 3σ(I); GOF = 0.9187, R1 = 0.0363, and wR = 0.0525 for all 4923 data, 285 parameters, the largest and minimal peaks in the final difference map 0.41 and −0.31 e/Å3. The structures were solved by direct methods and refined by the full-matrix least-squares technique in the anisotropic approximation for non-hydrogen atoms using the SIR97 and Crystals program package [8, 9].1,1-Bis(dimethylamino)-N,N-diisopropyl-N′-4-mesityl-phosphine-carboximidamide selenide (5a): To a frozen solution of 4a (1.31 g, 5 mmol) in Et2O (15 mL), a solution of PCl3 (0.69 g, 5 mmol) in diethyl ether (15 mL) was added. The reaction mixture was allowed to warm to ambient temperature (15°C) with stirring. The solvent was evaporated. Benzene (5 mL) was added to the residue, and then, a solution of dimethylamine (900 mg, 20 mmol) in benzene (6 mL) was added. The mixture was stirred for 15 min, and selenium (500 mg, 6 mmol) was added. The resulting suspension was stirred for 1 h at 15°C. The insolubles were filtered off and washed with benzene (2 × 5 mL), and the filtrate was evaporated. The residue was purified by silica gel plate chromatography. Yield, 60%. Rf 0.2–0.45 (CH2Cl2–hexane 1 : 1), m.p. 116–117°C; 31P {1H} NMR (202 MHz, CDCl3): δ = 65.7 (JPSe = 793 Hz) ppm; 1H NMR (300 MHz, CDCl3): δ = 2.10 (s, 6 H, CH3), 2.23 (s, 3 H, CH3), 2.84 (d, J = 2.7 Hz), 2.87 (s, 18 H, NCH3), 6.77 (s, 2 H, CH) ppm; 13C NMR (125.7 MHz, C6D6): δ = 18.6 (s, CH3), 20.2 (s, CH3), 37.9 (s, CH3), 39.8 (s, CH3), 126.1 (s, ipso-C), 127.6 (s, CH), 129.9 (s, ipso-C), 145.3 (d, J = 21 Hz, ipso-C), 150.5 (d, J = 155 Hz, CN); EI-MS 387–100% [M + 2]+; elemental analysis calcd (%) for C16H29N4PSe (387.37) C 49.61, H 7.55, N 14.46, P 8.00; found: C 49.84, H 7.37, N 14.62, P 8.26.1,1-Bis(dimethylamino)-N,N-dimethyl-N′-(4-mesityl)-phosphinecarboximidamide selenide (5b): To a frozen solution of PCl3 (0.27 g, 2 mmol) in benzene (2 mL), a solution of C-silylformamidine 4b (0.61 g, 1.9 mmol) in benzene (4 mL) was added with stirring. In 1 h, the reaction mixture was concentrated under vacuum. Benzene (3 mL) was added to the residue, and then, a solution of dimethylamine (0.41 g, 9 mmol) in benzene (3 mL) was added. The mixture was stirred for 15 min, and then, selenium (1.9 mmol) was added in two portions over 10 min. The resulting suspension was stirred overnight. The insolubles were filtered off and washed with benzene (2 × 2 mL), and the filtrate was evaporated. The residue was extracted with hexane (2 × 5 mL), the solvent was removed under reduced pressure, and the residual solid was purified by silica gel plate chromatography. Yield, 49%. Rf 0.5–0.8 (CH2Cl2 –hexane 1 : 1); m.p. 157–158°C (pentane); 31P NMR {1H} (202 MHz, CDCl3): δ 60.0 ppm (JPSe = 793 Hz). 1H NMR (300 MHz, CDCl3): δ = 1.38 (d, J = 6.3 Hz, 12 H, CH3), 2.13 (s, 6 H, CH3), 2.20 (s, 3 H, CH3), 2.50 (d, J = 10 Hz, 12 H, NCH3), 4.36 (br s, 2 H, CH), 6.78 (s, 2 H, CH) ppm; 13C NMR (125.7 MHz, CDCl3): δ = 19.1 (s, CH3), 20.2 (s, CH3), 21.2 (s, CH3), 37.8 (s, CH), 48.7 (s, CH3), 124.75 (s, ipso-C), 127.8 (ipso-C), 127.9 (s, CH), 143.8 (d, J = 8.8 Hz, ipso-C), 145.8 (d, J = 88 Hz, N=C); EI-MS 445-98.2% [M + 2]+; elemental analysis calcd (%) for C20H37N4PSe (443.48): C 54.17, H 8.41, N 12.63, P 6.98; found: C 53.89, H 8.88, N 13.01, P 7.32.General procedure for synthesis of compounds (6a and b): To a solution of phosphineselenide 7 (1.9 mmol) in benzene (4 mL), a solution of tris(morpholino)phosphine (2 mmol) in benzene (8 mL) was added. The reaction mixture was stirred for 30 min, and then, the solvent was removed under reduced pressure until dryness. The residue was dissolved in pentane (10 mL), and the obtained solution was cooled to −12°C. After several hours, the precipitated solid was filtered off, the filtrate evaporated under vacuum, and the residue distilled to produce compound 6.1,1-Bis(dimethylamino)-N,N-dimethyl-N′-mesityl-phosphine-carboximidamide (6a): Yield, 93%. B.p. 120–122°C/0.05 Torr; m.p. 26–28°C; 31P NMR (81 MHz, CDCl3): δ = 92.4, 88.7 (10 : 1) ppm; 1H NMR (300 MHz, C6D6): δ = 2.22 (s, 6 H, CH3), 2.27 (s, 3 H, CH3), 2.57 (s, 6 H, NCH3) and 2.64 (d, J = 8.7 Hz, 12 H, NCH3), 6.86 (s, 2 H, CH) ppm; 13C NMR (125.7 MHz, C6D6) δ = 18.8 (s, CH3), 20.2 (s, CH3), 38.1 (d, J = 10 Hz, CH3), 40.6 (d, J = 15 Hz, CH3), 125.3 (s, ipso-C), 127.7 (s, CH), 127.9 (s, ipso-C), 147.3 (s, ipso-C), 160.0 (d, J = 15 Hz, N=C); elemental analysis calcd (%) for C16H29N4P (308.4): C 62.31, H 9.48, N 18.17, P 10.04; found: C 62.02, H 9.71, N 18.42, P 9.86.1,1-Bis(dimethylamino)-N,N-diisopropyl-N′-mesityl-phosphine-carboximidamide (6b): Yield, 98%. B.p. 130°C/0.05 Torr, m.p. 59–60°C (pentane, −28°C); 31P NMR (81 MHz, CDCl3): δ = 90.1 ppm; 1H NMR (500 MHz, C6D6): δ = 1.34 (br s, 12 H, CH3), 2.26 (s, 6 H, CH3), 2.27 (s, 3 H, CH3), 2.34 (d, J = 8.5 Hz, 12 H, NCH3), 3.93 (br s, 2 H, CH), 6.85 (s, 2 H, CH) ppm; 13C NMR (125.7 MHz, C6D6): 19.1 (d, J = 4 Hz, CH3), 20.2 (s, CH3), 20.9 (s, CH3), 40.9 (d, J = 15 Hz, CH3), 47.8 (d, J = 13 Hz, CH), 124.1 (s, ipso-C), 126.0 (s, ipso-C), 127.9 (s, CH), 145.9 (s, ipso-C), 157.5 (d, J = 40 Hz, N=C); elemental analysis calcd (%) for C20H37N4P (364.52): C 65.90, H 10.23, N 15.37, P 8.50; found: C 66.32, H 9.97, N 15.67, P 8.31.1,1-Dichloro-N,N-diisopropyl-N′-mesityl-phosphine-carboximidamide (7b): To a solution of 6b (360 mg, 1 mmol) in benzene (4 mL), PCl3 (305 mg, 2.2 mmol) was added. The reaction mixture was stirred at 20°C for 25 min and then concentrated under vacuum. The oily residue was kept at 60°C under vacuum for 25 min and then distilled, b.p. 120°C/0.05 Torr to give 7b of 340 mg (99%). 31P NMR (81 MHz, C6D6): δ = 134.1 ppm; 1H NMR (300 MHz, C6D6): δ = 1.30 (d, J = 5.4 Hz, 12 H, CH3), 2.16 + 2.18 (2xs, 9 H, CH3), 4.03 (br s, 2 H, CH), 6.78 (s, 2 H, CH) ppm; 13C NMR (125.7 MHz, C6D6): δ = 18.6 (d, J = 2.5 Hz, CH3), 19.8 (s, CH3), 20.2 (s, CH3), 48.6 (s, CH), 125.0 (s, i-C), 128.3 (s, CH), 131.2 (s, i-C), 143.6 (d, J = 30 Hz, i-C), 154.1 (d, J = 99 Hz, C=N); elemental analysis calcd (%) for C16H25Cl2N2P (347.27): Cl 20.42, P 8.92; found: Cl 20.06, P 9.05.Dichlorophosphoranide (9): To a solution of silylformamidine 8 (1.0 g, 4.5 mmol) and 1 (670 mg, 4.5 mmol) in CH2Cl2 (10 mL), cooled to freezing, PCl3 (730 mg, 5.3 mmol) was added. The reaction mixture was allowed to warm at ambient temperature (16°C) with stirring. The solvent was removed under vacuum. The residue was extracted with Et2O (15 mL). The insoluble powder was filtered under argon, washed with Et2O (3 × 10 mL), and dried under vacuum. The collected solid was shaken in THF (26 mL), insoluble part was collected by filtration and washed with THF (5 mL), and the filtrate was evaporated under vacuum. The residue was recrystallized from CH3CN (7 mL) to give 9 of 330 mg (18%). M.p. 141–144°C (decomp); 31P NMR (202 MHz, CDCl3): δ = 124.7 ppm. 1H NMR (500 MHz, C6D6): δ = 1.23 (br s, 6 H, CH3), 2.49 (br s, 6 H, CH3), 4.67 (br s, 1 H, CH), 7.07 (br s, 6 H, Ph), 8.26 (br s, 4 H, Ph). Elemental analysis calcd (%) for C18H23Cl2N4P (397.29): Cl 17.85, P 7.80; found: Cl 18.11, P 7.69.Chlorophosphine (10): To a solution of silylformamidine 4a (0.96 g, 3.7 mmol) in benzene (2.5 mL), phosphorus trichloride (0.25 g, 1.8 mmol) in benzene (1 mL) was added. A slight exothermic effect was observed. In 1 h, all solvents evaporated to give a white solid. 31P NMR (202 MHz, CDCl3): δ = 30 ppm [7].Dichlorophosphoranide (11): To a solution of chlorophosphine 10 (0.6 g, 1.4 mmol) in benzene (5 mL), a solution of hydrogen chloride (0.05 g, 1.4 mmol) in ether (3 mL) was added. The precipitated solid was collected by filtration. The solid was washed with ether. The solid was recrystallized from benzene to give white crystals of 0.52 g, 80%. M.p. 181–182°C. 31P NMR (81 MHz, CDCl3): δ = −102 ppm; elemental analysis calcd (%) for C24H35Cl2N4P (481.45): Cl 14.73, P 6.43; found: Cl 14.38, P 6.04.


## 3. Results and Discussion

We started the synthesis of derivatives bearing one formamidine substituent. Thus, compounds4a and b react consecutively with phosphorus trichloride, dimethylamine, and selenium in a one-pot procedure affording stable derivatives 5a and b which were isolated and fully characterized. Phosphineselenides 5a and b were purified by silica gel plate chromatography. Phosphineselenides 5a and b were reduced by tris(morpholino)phosphine to give phosphonous diamides 6a and b. They are stable, distillable in high-vacuum compounds. While the 31P NMR spectrum of highly sterically hindered compound 6b involves only one signal at 90.1 ppm, compound 6a exhibits two signals at 92.4 and 88.7 ppm in a ratio 10 : 1 corresponding to syn/anti-isomers. The reaction of phosphonous diamide 6b with phosphorus trichloride in a 1 : 2 ratio produced dichlorophosphine 7b (δp = 134 ppm), which was isolated by distillation as an individual compound (Scheme 2). The compound is stable in the solid state, but in solution, it decomposes quite promptly, in a few hours. Monitoring this process by 31P NMR reveals formation of numerous signals including phosphorus trichloride. The reaction of phosphonous diamide 6a under the same conditions also afforded dichlorophosphine 7a, which cannot be isolated as a pure compound, but it is possible to obtain its derivatives. The method of dichlorophosphine synthesis being available, it was possible to validate the proposed mechanism for the formation of dibromophosphoranide (Scheme 1). It is known that formamidines do not react with phosphorus trichloride. It allowed us to carry out a three-component reaction of formamidine 1, its trimethylsilylated derivative 8 with phosphorus trichloride. Initially, PCl3 would react with silylated formamidine 8 to form the corresponding dichlorophosphine, which, according to the proposed mechanism, should react with formamidine 1 to form dichlorophosphoranide 9 in the next stage (Scheme 3).Scheme 2
Synthesis of dichlorophosphines and their derivatives.Scheme 3
Synthesis of dichlorophosphoranide9.Indeed, by adding phosphorus trichloride to a mixture of formamidine1 and its silylated derivative 8, the target dichlorophosphoranide 9 was prepared. The reaction mixture was monitored by 31P NMR spectroscopy, and it exhibited only one 31P NMR signal at 124 ppm. Nevertheless, we separated phosphoranide 9 only during 18% yield. Its structure was confirmed by X-ray diffractometry. Compound 9 crystallizes in the space group P−1 with 2 molecules in the unit cell. Figure 2 shows the molecular structure and contains key interatomic distances and bond angles.Figure 2
The perspective view of the molecule9 with 50% probability ellipsoids for non-H-atoms. The selected bond lengths (Å) and angles (°): Cl(1)–P(1) 2.2058(6), Cl(2)–P(1) 2.8509(6), P(1)–N(4) 1.6621(13), P(1)–C(1) 1.8501(15), N(1)–C(1) 1.313(2), N(2)–C(1) 1.3255(19), N(2)–C(10) 1.5135(18), N(3)–C(10) 1.4115(19), N(4)–C(10) 1.4624(19); N(4)P(1)C(1) 89.10(7), P(1)C(1)N(2) 110.67(11), C(1)N(2)C(10) 116.88(12), N(2)C(10)N(4) 103.16(11), P(1)N(4)C(1).119.37(10).The molecular structure of9 shows a distorted, ψ-trigonal bipyramidal coordination of the P atom. Two chlorine atoms occupy the axial positions, while a lone electron pair and the cycle are located in the equatorial positions. The P–Cl bond lengths in dichlorophosphoranide 9 are very different (P1–Cl1 2.8509(6) Å; P1–Cl2 2.2058(6) Å). The second value is close to P–Cl bond lengths ranging from 2.295 to 2.469 Å in related phosphorus compounds, and the first value is far beyond that range and is intermediate between the covalent P–Cl bond and cationic-anionic distances in crystals [4, 10]. In comparison, the P–Br bond lengths in dibromophosphoranide 3 are very similar in length: 2.6945(16) and 2.5792(15) Å. Other structural parameters of both phosphoranides 3 and 9 are quite close. 31P NMR chemical shifts of phosphoranide 3 (δp = 56.8 ppm in CDCl3) and 9 (δp = 124.7 ppm in CDCl3) are indicative of their phosphoranide structures. While a high-field shift of phosphoranide 3 testifies that in a solution, it does not dissociate, a low-field shift of phosphoranide 9 attests to a high degree of dissociation. An analogous acyclic dichlorophosphoranide (δp = 92.3 ppm in CDCl3) was prepared by addition of 2,2,6,6-tetramethylpiperidinedichlorophosphine to cyclic (alkyl)(amino)carbene. Although X-ray was not available, it was presented as a phosphonium salt [11].In our previous work, we have shown that silylformamidine4a reacts with phosphorus trichloride in a 2 : 1 ratio producing chlorophosphine 10 [7].Monitoring by31P NMR, a solution of chlorophosphine 10 (δp = 31 ppm) showed that its signal gradually disappears and a signal in a strong field (δp = −102 ppm) grows, which became predominant over time. When triethylamine was added to the solution, the signal (δp = −102 ppm) disappeared and the signal of chlorophosphine 10 was restored. We carried out a quantitative experiment in which an equivalent amount of hydrogen chloride was added to a solution of chlorophosphine 10. It transformed into dichlorophosphoranide 11(Scheme 4). The reaction is reversible and, when triethylamine is added, phosphoranide 11 is converted to chlorophosphine 10. The molecular structure of phosphoranide 11 was unambiguously determined by single-crystal X-ray diffractometry (Figure 3). Compound 11 crystallizes in the Pna21 space group with 4 molecules in the unit cell. Figure 3 shows that the molecular structure contains some interatomic distances and bond angles. The molecular structure of phosphoranide 11 shows that P–Cl bond lengths are almost the same (Cl(1)–P(1) 2.3444(9), Cl(2)–P(1) 2.3303(9) Å). The 31P resonance of 11 (δp = −102 ppm in CDCl3) is substantially shifted to a higher field, but it is very close to that of the related phosphoranide C (δp = −98.9 ppm in CD2Cl2). Such a substantial highfield shift correlates with a smaller degree of dissociation into phosphine and hydrogen chloride [12, 13]. CCDC 1938108 (9) and 1938107 (11) contain the supplementary crystallographic data for this paper.Scheme 4
Synthesis of dichlorophosphoranide11.Figure 3
The perspective view of the molecule11 with 50% probability ellipsoids for non-H-atoms. The selected bond lengths (Å) and angles (°): Cl(1)–P(1) 2.3444(9), Cl(2)–P(1) 2.3303(9), P(1)–C(1) 1.893(2), P(1)–C(10) 1.887(2), N(1)–C(1) 1.331(3), N(2)–C(1) 1.296(3), N(3)–C(10) 1.342(3), N(4)–C(10) 1.292(3); C(1)P(1)C(10) 103.99(10), N(1)C(1)N(2) 128.2(2), N(3)C(10)N(4) 127.4(2).


## 4. Conclusions

We confirmed experimentally the mechanism for formation of dichloro(dibromo)-phosphoranides3 and 9 previously proposed on the basis of DFT calculations. Dichlorophosphoranide 9 was prepared by a three-component reaction between C-trimethylsilyl-N,N-dimethyl-N′-phenylformamidine, N,N-dimethyl-N′-phenylformamidine, and phosphorus trichloride. At first, C-trimethylsilyl-N,N-dimethyl-N′-phenylformamidine reacts with phosphorus trichloride to give the corresponding dichlorophosphine bearing the formamidine substituent, followed by addition of N,N-dimethyl-N′-phenylformamidine to afford the target dichlorophosphoranide 9. It was shown that chlorophosphine 10 reacts with hydrogen chloride to form dichlorophosphoranide 11. In the presence of triethylamine, the reaction is reversible and gives chlorophosphine 10. The molecular structures of phosphoranides 9 and 11 were determined by single-crystal X-ray diffractometry.



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