Indian Journal of Chemistry Vol. 23A, December 1984, pp. 990-991 Reactions of Potassium Bifluoride with Phosphorus Halides R G KALBANDKERI, D K PADMA & A R VASUDEVA MURTHY· Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012 Received 7 May 1984; revised and accepted 5 June 1984 The reaction of potassium bifluoride with phosphorus trichloride/tribromide in acetonitrile medium gives phosphorus trifluoride (PF J). Phosphoryl chloridejbromide react with potassium bifluoride in acetonitrile at room temperature to give potassium difluorophosphate (KP02F 2) and potassium hexafluorophosphate (KPF 6)' Similarly, potassium difluorodithiophosphate and potassium hexafluorophosphate are obtained in the reaction between thiophosphoryl chloride and potassium bifluoride in acetonitrile medium. The products have been characterised by their IR spectra and chemical analyses. A suspension of sodium fluoride or potassium fluoride comparison of the IR spectral data reported in in boiling acetonitrile medium has been successfully literature 7 with those obtained presently (Observed: employed in our laboratory for the preparation of 490, 860, 895, 1195, 1710, 1750em -!. Reported: 487, silicon tetrafluoride", phosphorus trifluoride", 860,892,1195,1713, 1715cm -1). phosphoryl fluoride 3 , thiophosphoryl fluoride", (b) Reaction of phosphorus tribromide with potassium thionyl fluoride.' and sulphuryl fluoride". Exploratory bijluoride work in this laboratory has indicated that a suspension Phosphorus tribrornide (11.6 g) in acetonitrile of potassium bifluoride in boiling acetonitrile could (25 ml) was added to the boiling suspension of be a more efficient fluorinating reagent than alkali potassium bifluoride in acetonitrile as described fluorides in the preparation of phosphorus trifluoride. above. The phosphorus trifluoride gas formed was When the same medium is used to prepare the fluorine collected and purified; yield 90% (3.4 g)(Calc: P, 35.22; analogues of phosphoryl chloridejbromide and F, 64.77. Found: P, 35.02; F, 64.24%). thiophosphoryl chloride, the products obtained are found to be completely different from those expected. Reaction of phosphoryl chloride with potassium The resulting products are found to be potassium bijluoride: difluorophosphate and potassium hexafluorophos- Isolation of potassium dijluorophosphate and phate in the reaction of phosphoryl chloridejbromide potassium hexajluorophospha te with potassium bifluoride, while potassium difluoro- A mixture containing potassium bifluoride dithiophosphate and potassium hexafluorophosphate (3.2516 g), acetonitrile (15 ml) and phosphoryl chloride are obtained in the reaction of thiophosphoryl chloride (3,34 g) was taken in a polyethylene bottle, which was with potassium bifluoride. The results of investigation placed in a desiccator provided with calcium chloride of these reactions are reported in this paper. guard tube. The contents were stirred for 24 hr with a magnetic stirrer at room temperature (25°C). During Materials and Methods this interval hydrogen chloride gas was continuously Preparation of phosphorus trijluoride: evolved and a white solid residue was deposited. (a) Reaction of phosphorus trichloride with Acetonitrile was distilled off and the solid residue (A) potassium bijluoride was characterized as discussed below. The experimental set-up and the procedure for the The IR spectrum of the solid (A) in KBr pellet, preparation of phosphorus trifluoride used in the indicated that the product could be a mixture of present study were the same as described in our earlier potassium difluorophosphate and potassium hexa- paper:'. Phosphorus trichloride (8g) in acetonitrile fluorophosphate contaminated with potassium (25 ml) was slowly added to a boiling suspension of chloride. The products were separated and identified potassium bifluoride (30 g) in acetonitrile (100 ml) adopting the method of Lustig and Ruff". The solid (A) during 2 hr. The evolved gas was condensed in a trap was shaken with absolute ethyl alcohol (100 ml) and cooled by liquid nitrogen and then distilled at low filtered. The alcoholic solution (B) contained temperature (- 100°C). The yield of phosphorus potassium difluorophosphate contaminated with trifluoride was found to be 4.5035 g(87.7%). The purity potassium hexafluorophosphate. Dry ether was added of phosphorus trifluoride was checked by chemical to alcoholic solution (B) till cloudy precipitate of analysis (Calc: P, 35.22; F, 64.77. Found: P, 35.98; F, potassium hexafluorophosphate settled down. The 64.08%). Further confirmation was provided by a precipitate was filtered off. On evaporation of the 990 KALBANDKERIel al.: REACTIONS OF KHF2 WITH PHOSPHORUS HALIDES filtrate, a white solid (C) was left behind. This solid (C) represented by Eq.(J) can go to the extreme right more was identified as potassium difluorophosphate by easily. chemical analyses and by the comparison of the IR 2PX3+ 3KH F 2- 2PF 3+ 3KX + 3HX ... (I) spectral data with those reported? in literature (Found: (X=Ci or Br) P, 21.85; F, 28.16. Calc. for KP0 F : P, 22.12; F, 2 2 The reaction of potassium bifluoride with 27.12/"'.,)IR(KBr):; 480,510,535,835,860,1150,1310, phosphoryl chloride/bromide gives rise to, instead of 1335 cm -I (reported? 481, 512, 535, 835, 857, I phosphoryl fluoride, potassium difluorophosphate 1145, 1311, 1330cm- ). and potassium hexafluorophosphate. The reaction The solid residue (from A) was mixed with the may be visualised to take place in terms of the precipitate obtained by the addition of ether to following equations: alcoholic solution (B) and extracted with small quantity of cold water (0). Potassium hexafluorophos- 2POX3 + 4KHF 2 -+2POF 3 + 2KF + 2KX + 4HX (2) phate (E) owing to its limited solubility did not 2POF +2KHF -+KP0 F + KPF +2HF (3) dissolve, and could be recrystallised from hot water. 3 2 2 2 6 On evaporation of water solution (0) was found to contain potassium chloride. Potassium hexafluoro- 2POX3 + 4KHF 2 - KP02F 2 + I<PF 6 + 2KX phosphate was characterised by chemical analyses +4HX ... (2+3) (Found: P, 16.43; F, 62.15. Calc.: P, 16.84; F, 61.92%); (X=CI or Br) IR(KBr): 560, 840cm -I (reported" 555, 840cm -I). The formation of potassium difluorophosphate and potassium hexafluorophosphate can be accounted for Reaction of phosphoryl bromide with potassium in terms of further reaction of phosphoryl fluoride bijluoride: formed initially with potassium bifluoride at room Potassium dijluorophosphate and potassium hexaj1uo- temperature. rophosphate The reaction between potassium bifluoride and In a similar manner, potassium difluorophosphate, thiophosphoryl chloride, resulting in the formation of potassium hexafluorophosphate and potassium potassium difluorodithiophosphate and potassium bromide were obtained as solid residues when hexafluorophosphate, is similar to the one described potassium bifluoride (3.5014 g) in acetonitrile (15 ml) above. The reaction can be represented as follows: was treated with phosphoryl bromide (5.974 g)at room 2PSCI3 +4KHF2-+2PSF3 + 2KF +2KCI +4HCJ. .. (4) temperature. The products were separated and 2PSF +2KHF -+KPS F +KPF +2HF ... (5) identified as described above. 3 2 2 2 6 2PSCI +4KHF2-+KPS2F2 +KPF6 +2KCI Reaction of potassium bijluoride with thiophosphoryl 3 +4HCI .. (4 + 5) chloride: Preparation of potassium difluorodithiophosphate and It may be of interest to point out that similar potassium hexajluorophosphate products have been obtained in the reactions of On substituting phosphoryl chloride by thiophos- phosphoryl fluoride+"? and thiophosphoryl fluo- phoryl chloride (3.341 g) in the above reaction with rides, 11 with potassium fluoride and cesium fluoride in potassium bifluoride (3.5172 g), potassium difluorodi- acetonitrile medium. thiophosphate (KPS2F 2) and potassium hexafluoro- References phosphate (KPF 6) were formed. The products were I Padma D K & Vasudeva Murthy A R, J Fluorine Chern, 4(1974) separated and characterised as described previously: 241. 2 Kalbandkeri R G, Padma D K & Vasudeva Murthy A R, Indian KPS2F2 (Found: P, 17.66; F, 21.57. Calc.: P, 18.01;F, J Chern, 20A (1981) 83. 22.10/",,,);IR(KBr): 720, 780, 810cm -1 (reported? 318, 3 Kalbandkeri I R G, Ph.D. Thesis, Indian Institute of Science, 360,718, 805cm -1). KPF2 (Found: P, 16.43;F, 62.12. Bangalore, (1981). Calc.: P, 16.84; F, 61.92%); IR(KBr): 560, 840cm-1 4 Padma D K, Vijayalakshmi S K & Vasudeva Murthy A R, J (reported? 555, 840cm -1). Fluorine Chern, 8 (1976) 461. 5 Padma D K, Bhat V S & Vasudeva Murthy A R, J inorg nucl Results and Discussion Chern, 43 (1981) 3101. 6 Padma D K, Bhat V S & Vasudeva Murthy A R,lndian J Chern, The experimental results show that potassium 20A (1981) 777. bifluoride is an efficient fluorinating agent for the 7 Wilson M K & Polo S R, J chem Phys, 20 (1952) 1716. preparation of phosphorus trifluoride. One of the 8 Lustig M & Ruff J K, Inorg Chern, 6(1967) 2115. parameters responsible for the efficient conversion 9 Buhler K & Bues W, Z anorg allgem Chern, 308 (1961) 62. 10 Lange W, Ber, 628(1929) 786, may be the easy expulsion of hydrogen halide (H'Cl or II Roesky H W, Tebbe F N & Muetterties E L,Inorg Chern, 9 (1970) HBr) from the reaction mixture. The reaction, 831. 991.