Dichlorophosphoranides Stabilized by Formamidinium Substituents

Dichlorophosphoranides featuring N,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,Ndimethyl-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.


Introduction
Phosphoranides A are hypervalent anionic phosphorus(III) compounds formally possessing a 10-electron valence shell and a distorted pseudotrigonal bipyramidal arrangement at the phosphorus atom. e electronegative ligands at phosphines make nucleophilic addition possible to afford stable phosphoranides (Figure 1). e 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]. Nheterocyclic carbenes are known to be suitable for stabilization of high-coordinated P atoms. e reaction of a sterically hindered N-heterocyclic carbene with PCl 3 in hexane affords a high yield of phosphoranide B. e 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]. e final step of the proposed mechanism is the reaction of dibromophosphine 2 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.

Materials and Methods
All procedures with air-and moisture-sensitive compounds were performed under an atmosphere of dry argon in flamedried glassware. Solvents were purified and dried by standard methods. Melting points were determined with an electrothermal capillary melting point apparatus and were uncorrected. 1 H spectra were recorded on a Bruker Avance DRX 500 (500.1 MHz) or Varian VXR-300 (299.9 MHz) spectrometer. 13  were performed at 123K on a Bruker Smart Apex II diffractometer operating in the ω scans mode. e intensities of 30253 reflections were collected (4944 unique reflections, R merg = 0.039). Convergence for 11 was obtained at R 1 = 0.0287 and wR = 0.0484, GOF = 0.9187 for 4259 observed reflections with I ≥ 3σ(I); GOF = 0.9187, R 1 = 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/Å 3 . e 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].
To a frozen solution of PCl 3 (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. e mixture was stirred for 15 min, and then, selenium (1.9 mmol) was added in two portions over 10 min. e resulting suspension was stirred overnight. e insolubles were filtered off and washed with benzene (2 × 2 mL), and the filtrate was evaporated. e 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%. 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. e reaction mixture was stirred for 30 min, and then, the solvent was removed under reduced pressure until dryness. e 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.  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. 31 P NMR (202 MHz, CDCl 3 ): δ � 30 ppm [7].

Results and Discussion
We started the synthesis of derivatives bearing one formamidine substituent. us, compounds 4a 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. ey are stable, distillable in high-vacuum compounds. While the 31 P 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/antiisomers.
e reaction of phosphonous diamide 6b with phosphorus trichloride in a 1 : 2 ratio produced dichlorophosphine 7b (δ p = 134 ppm), which was isolated by distillation as an individual compound (Scheme 2). e compound is stable in the solid state, but in solution, it decomposes quite promptly, in a few hours. Monitoring this process by 31 P NMR reveals formation of numerous signals including phosphorus trichloride. e 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. e 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, PCl 3 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).
Indeed, by adding phosphorus trichloride to a mixture of formamidine 1 and its silylated derivative 8, the target dichlorophosphoranide 9 was prepared. e reaction mixture was monitored by 31 P NMR spectroscopy, and it exhibited only one 31 P 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.
e molecular structure of 9 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. e P-Cl bond lengths in dichlorophosphoranide 9 are very different (P1-Cl1 2.8509(6)Å; P1-Cl2 2.2058(6)Å). e second value is close to P-Cl bond lengths ranging from 2.295 to 2.469Å 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. 31 P NMR chemical shifts of phosphoranide 3 (δ p � 56.8 ppm in CDCl 3 ) and 9 (δ p � 124.7 ppm in CDCl 3 ) 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 (δ p � 92.3 ppm in CDCl 3 ) was prepared by addition of 2,2,6,6tetramethylpiperidinedichlorophosphine 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 silylformamidine 4a reacts with phosphorus trichloride in a 2 : 1 ratio producing chlorophosphine 10 [7].
Monitoring by 31 P NMR, a solution of chlorophosphine 10 (δ p = 31 ppm) showed that its signal gradually disappears and a signal in a strong field (δ p = − 102 ppm) grows, which became predominant over time. When triethylamine was added to the solution, the signal (δ p = − 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). e reaction is reversible and, when triethylamine is added, phosphoranide 11 is converted to chlorophosphine 10. e 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. e 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). e 31 P resonance of 11 (δ p = − 102 ppm in CDCl 3 ) is substantially shifted to a higher field, but it is very close to that of the related phosphoranide C (δ p = − 98.9 ppm in CD 2 Cl 2 ). 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.

Conclusions
We confirmed experimentally the mechanism for formation of dichloro(dibromo)-phosphoranides 3 and 9 previously proposed on the basis of DFT calculations. Dichlorophosphoranide 9 was prepared by a threecomponent reaction between C-trimethylsilyl-N,N-dimethyl-N′-phenylformamidine, N,N-dimethyl-N′-phenylformamidine, and phosphorus trichloride. At first, Ctrimethylsilyl-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. e molecular structures of phosphoranides 9 and 11 were determined by single-crystal X-ray diffractometry.
Data Availability e 1 H, 13 C, 31 P NMR instrumental data and elemental analysis data used to support the findings of this study are included within the article.