Quantitative Synthesis of (C6H5)2AsF2+AsF6- and (C6H5)2A8F3 via Interaction of Benzene with Arsenic Pentafluoride

Francis L. Tanzella and Neil Bartlett* Department of Chemistry, University of California and the Materials and Molecular Research Division, Lawrence Berkeley Laboratory, Berkeley, California 94720, USA Dedicated to Prof. Dr. Drs. h. c. Oskar Glemser on the occasion of his 70th birthday Z. Naturforsch. 36b, 1461-1464 (1981); received May 18, 1981 Arylfluoroarsonium, Arsenic, Benzene, Substitution

Benzene reacts quantitatively with AsFs to give (C6H5)2AsF2+AsF6-, the first reported aryl or alkyl fluoroarsonium(V) salt. This reacts quantitatively with CsF to give (CeH5)2AsF3 and CsAsF6. Interaction of (C6H5)2AsF3 with AsF5 reconstitutes the fluoro- arsonium salt. Variable temperature 19F NMR studies show (CÖHS^ASFS to be a rigid trigonal bipyramid, with the phenyl groups in equatorial positions.

Introduction Interaction of benzene with arsenic pentafluoride to give (C6H5)2ASF2+ASF6~ (I) The reaction of benzene with 02+AsF6~ or CöFß+AsFe- gives a mixture of products [1]. In Arsenic pentafluoride (6.22 mmole, measured tensimetrically) was condensed into a Kel-F tube order to clarify that chemistry, the reaction of Ö Ö C H provided with a magnetic spinbar and subsequently with ASF5 was undertaken on the assumption that SO2CIF (5 ml) was condensed (from its purification the reactions with O2ASF6 and CeFeAsFö generated vessel held at —45 °C). A molar excess of benzene some ASFÖ which could also interact with benzene. was condensed on to the frozen solvent. With the No products have heretofore been reported for bottom half of the tube held at —78 °C the benzene in the upper half was allowed to melt at room tem- the reaction of C6H6 with either PF5, AsF5 or SbF5, perature forming a green color upon solution. This although C6H5ASF4 and C6H5PF4 have each been was, (within a few seconds), followed by the forma- synthesized [2, 3] by other routes: tion of a copious colorless precipitate. To ensure completion of the reaction, the tube was held at C6H5As(0)(0H)2+3SF4-^C6H5AsF4+3S0F2+2HF 0 °C (with stirring) for one hour. Removal of solvent C6H5P(0)(0H)2+3SF4->C6H5PF4+3S0F2+2HF and drying under vacuum gave a white free-flowing crystalline solid, (1.41 g, 6.2 mmole "C6H5AsF4"). When ASFÖ is passed into a solution of CeHß in either HF or SO2CIF a white crystalline product Analysis for C&H^AsF^ forms quickly and quantitatively. This is identical Calcd C 31.60 H 2.21 As 32.9 F 33.3, to one of the products from 02+ or CeFö"1" salt inter- Found C 31.77 H 2.33 As 33.1 F 32.9. actions with benzene. The physical and chemical evidence shows that it is the salt (C6H5)2AsF2+AsF6~. The 19F NMR spectrum (DMSO) is shown in Fig. 1. IR: (Nujol mull, cm-i) 1575 (w), 1342 (w), Experimental 1272 (w), 1185 (w), 1078 (w), 1065 (m), 995 (m), 742 (s), 709 (s), 675 (s), 462 (m), 398 (m), 380 (m). General experimental procedures are similar to The mass spectrum of the volatiles from the solid those described in a related paper [1]. The acquisi- at 125 °C showed the Ci2HioAsF2+ species to be the tion and preparation of reagents and solvents are highest molecular weight fragment and of high also as described in that paper with the following abundance. The NMR and infrared data sup- additions: (CeHsAsOOH, supplied by Research port the formulation (C6H5)2AsF2+AsF6_ for the Organic/Inorganic (Sun Valley, California) was re- solid. This was confirmed by the following chemistry. crystallized from CCI4. CsF was used as received from Alfa Ventron (Beverly, Massachusetts). (CH3)2SO, supplied by Matheson, Coleman and Bell Interaction of (CsH5)2AsF2+AsFtr with CsF (Norwood, Ohio), was dried and stored over molec- to give (C6H5)2ASF3(II) ular sieves. (C6H5)2AsF2+AsF6- (0.46 g, 1 mmole) from above was mixed with cesium fluoride (0.15 g, 1 mmole) in a Teflon tube. Anhydrous hydrogen fluoride (5 ml) was distilled on to the solids, and the mixture

Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung This work has been digitalized and published in 2013 by Verlag Zeitschrift in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der für Naturforschung in cooperation with the Max Planck Society for the Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Advancement of Science under a Creative Commons Attribution Creative Commons Namensnennung 4.0 Lizenz. 4.0 International License. allowed to stand at room temperature for one hour, after which the solvent was removed under vacuum. The resultant white, solid was transferred to a Teflon filtration apparatus and extracted with several batches of dry toluene. Removal of the toluene from the filtrate, under vacuum, gave a colorless solid (0.29 g, 1.0 mmole). The 19F NMR spectrum (toluene) is given in Table I. + Mass spectrum: m\e 286 (4.2) (C6H5)2AsF3 , 264 + (24.3) (C6H5)2ASF2 , 248 (5.0) (C6H5)2AsF+, 227 (1.6) + (C6H4)2As+, 209 (11.0) C6H5ASF3 , 190 (36.5) + C6H5ASF2 , 171 (14.8) C6H5ASF+. The toluene- insoluble by-product of the reaction was shown to be CSASF6 by its characteristic X-ray powder pattern.

Interaction of (C^H^j^AsFz with AsF*>

(C6H5)2ASF3 (0.210 g, 0.73 mmole), obtained as described above, was placed in a Teflon tube and anhydrous hydrogen fluoride (5 ml) condensed on top of it. A molar excess of AsFs was condensed into the tube and the vessel was warmed to room tem- perature. The solid dissolved and the supply of 40 50 60 70 80 ASF5 was maintained until uptake ceased. The excess of AsFs was removed under vacuum at ppm Upfield of CFCI3 — 78 °C and the solvent at room temperature to give a colorless solid g, mmole). An Shift Multiplicity (0.331 0.73 Integration X-ray powder pattern of this product was identical assignment to that obtained from the benzene/AsFs reaction. — 61.7 equal intensity quartet (930 Hz) 3 ASF6- — 62.8 singlet 1 (C6H5)2ASF2+ Interaction of (C&H^)2AsOOH with SFA

SF4 (9.2 g, 85 mmoles) was condensed on to Fig. 1. 19F NMR Spectrum of (C6H5)2AsF2+AsF6-. (CeHs^AsOOH (4.4 g, 17 mmoles) in a Monel bomb.

Table I. NMR of (C6H5)2AsF3.

a) (C6H5)2AsF2+AsF6- + CsF -* (C6H5)2ASF3 + CsF Shift* Multiplicity (JF-F) Integration Assignment — 69.8 doublet (71 Hz) 2 Faxial — 92.4 triplet (71 Hz) 1 Fequatorial

b) (C6H5)2ASOOH + 2 SF4 -> (C6H5)2AsF3 + 2 SOF2 + HF Shift* Multiplicity (JF-F) Integration Assignment

— 70.0 doublet (72 Hz) 2 Faxial — 92.5 triplet (72 Hz) 1 Fequatorial c) Variable temperature NMR data for b)

Temperature Shift* (FAXIAL) Shift* (Fequatorial) JFa-Fe (Hz) — 50 — 72.0 — 91.8 72 0 — 70.7 — 92.4 72 31 — 70.1 — 92.6 72 40 — 70.0 — 92.6 70 50 — 69.9 — 92.6 69 61 — 69.6 — 92.8 59 71 — 69.5 — 92.8 48 After being heated at 200 °C for 12 h, the solid grey- doublet and triplet of relative intensity 2:1 with a colored product was placed in a 50 ml flask with 4 g common coupling constant of 67 Hz, suggesting a of NaF. The product was dissolved in 20 ml petro- leum ether, filtered and recrystallized to give color- rigid trigonal bipyramid at room temperature. less crystalline (CeH5)2AsF3 (2 g, 7 mmoles). The Littlefield and Doak fluorinated (CeHs^AsH or solid which melts with decomposition ~135°C, (CeHs^AsCl with SF4 to obtain a crystalline solid gives the same mass spectrum and 19F NMR as the 19F NMR of which consisted of a singlet (at described for II above. The NMR was unchanged in 69.1 ppm upfield of relatively unchanged the temperature range —50 to -f- 71 °C (see Table I). CFCI3) from —90 °C to room temperature. On the basis Results and Discussion of elemental analyses this material was described 19 as (C6H5)2ASF3 and the F NMR findings were Benzene reacts with arsenic pentafluoride to give attributed to pseudo-rotation. In order to settle + the colorless, crystalline solid (CeH5)2AsF2 AsFe- this controversy Muetterties' original preparation (1) in quantitative yield: was repeated.

+ 2C6H6 + 2 ASF5 -> (C6H5)2ASF2 ASF6- + 2HF (1) The reaction [7]: 1 (C6H5)2 AsOOH+2 SF4 ->(C6H5)2 ASF3+2 S OF2+HF 1 is a non-volatile solid that melts with decompo- (3) sition at ~ 150 °C and the analytical data indicate was run using a large excess of SF4 in a Monel an empirical formula CeHsAsF4. However, bomb. The crystalline product (approximately 40% C6H5ASF4 is reported [2] to be a yellow liquid at yield) gave a mass spectrum identical to the one ordinary temperatures. The 19F NMR spectrum of 1 reported for 2 above. The 19F NMR spectrum in (in DMSO with as reference) is given in Fig. 1. CFCI3 toluene (see Tab. I) consisted of a doublet and a The quartet at —61.7 ppm is characteristic [4] of triplet of relative intensity 2:1 with a coupling AsFe- and the singlet of one-third the intensity at constant of 70 Hz. The spectrum showed no sign of —62.8 ppm suggests the cation (CeH5)2AsF2+. The coalescence in the temperature range —50 to 71 °C. shifts are independent of concentration. The mass This spectrum is identical with that of 2. This spectrum obtained from the decomposition of 1 in leaves no doubt of the correctness of the original the sample chamber of the mass spectrometer shows 19F NMR studies of Muetterties and their inter- peaks at mje corresponding to (C6H5)2AsF2+, pretation. Evidently, Littlefield and Doak did not + (C6H5)2AsF+, ( 6 4 6 5 2 and AsF2+. C H )AS+, C H ASF have (CeH5)2AsF3 as they thought. Perhaps, be- This evidence all indicates the formulation cause they derived their so-called (CeHs)2AsF3 from + - 6 5)2 2 AsFe . (C H ASF arsenic(III) species (CeHs^AsH or (CeHs^AsCl), In order to characterize 1 chemically, the ar- they may not have produced an arsenic(V) ma- sonium salt was reacted with CsF: terial.

+ (CEH5)2ASF2 ASF6- + CsF -> (C6H5)2AsF3 + CSASF6 To further the chemical characterization of 2 (2) (CeH5)2AsF3, it was treated with AsFs and this Reaction (2) gives 2 in quantitative yield. This has resulted in an alternative synthesis of 1: 19 + the same F NMR spectrum (see Tab. I) as that (C6H5)2ASF3 + ASF5 -> (C6H5)2AsF2 AsF6- (4) reported by Muetterties [5] for 5)2AsF3. The (CEH Brownstein and Schmutzler [8] have reported a mass spectrum has (mje) peaks corresponding to similar reaction: + (CEHS^AsFa, (C6H5)2AsF2 , (CEH5)2AsF+, (CEH4)As+, + (CH3)2PF3 + 5 -> (CH3)2PF2+ AsFe" (5) CEH5AsF3, C6H5ASF2 and C6H5AsF+ as well as ASF some smaller fragments. 2 is soluble in aromatic 3 solvents and deliquesces quickly in moist air. where 3 was identified by its 19F NMR spectrum Although these results are consistent with the but not isolated. These spectroscopic and chemical formulation (CeHs)2AsF3, a small controversy in studies serve to fully characterize the reaction of the literature prevented definite verification. benzene with AsFs. Littlefield and Doak [6] questioned the published Having identified 1 as the crystalline component 19F NMR spectrum. Muetterties had reported [5] in the mixture resulting from the reaction of that the spectrum for (C6Hs)2AsF3 consisted of a benzene with either 02+ or CeFe+, the solubility of this salt in a variety of solvents was examined. It ments [9] that the CH3CN • AsFs adduct dismuta- was found to be very soluble in anhydrous HF at tes to give (CH3CN)2AsF4+AsF6-. HF elimination room temperature. Large needle-shaped crystallites products from this cation have not been seen but were formed on cooling solutions to —78 °C. the elimination of HF from such a cation may be

Since C6H5ASF4 is not observed in the product of less likely as the protons of the CG3 group are the interaction of benzene with AsFs and since further from the F atoms of the AsFs entity than in Smith [2] did not report 1 in the synthesis of the benzene/AsF5 case. C6H5ASF4, it seems unlikely that reaction (1) pro- Reactions (1) and (2) can be used as a quantitative ceeds via a C6H5ASF4 intermediate: route for the synthesis of (CeH5)2AsF3 (100% yield + based on benzene, 50% yield based on AsFs). When 2 C6H5ASF4 -> (C6H5)2ASF2 ASF6- (6) compared to the 40% yield from the high pressure Although some by-product or impurity in the and temperature method of reaction (3) this new CßHe/AsFs reaction might catalyze the above dis- route seems to be a better general method for the proportionation, Smith has shown CeHsAsF4 to be clean synthesis of (C6H5)2AsF3. In fact, other stable in the presence of both CeHß and HF. organic molecules may react with AsFs in a similar Another possible route for the CeHö/AsFs re- manner to give the synthesis of various types of action is via reaction (4) with (CeHs^AsFs as inter- R2ASF3 molecules. Furthermore it is possible that mediate. However, since no (C6Hs)2AsF3 was seen PF5 and SbFs may also react in a similar manner to in reaction (1) even when a tenfold excess of ben- provide a method for the efficient syntheses of zene over AsF5 was used this reaction route seems R2PF3 and R2SbF3 species. Therefore this route unlikely. based on reactions (1) and (2) could possibly be a A plausible mechanism for the production of 1 is general method for the synthesis of pentasub- via the formation of CeHß • AsFs adduct followed stituted group V compounds of the type R2MF3. by dismutation and HF elimination, the last two processes perhaps occuring concertedly. This work was supported in part by the Com- 2 C6H6 + 2 AsFs -> 2 C6H6 • AsFs mittee on Research of the University of California, 2 C6H6 • ASF5 -* (C6H6)2AsF4+ ASF6~ Berkeley and by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical + (C6H6)2ASF4+ ASF6- -> (C6H5)2ASF2 ASF6- + 2 HF Sciences Division of the U. S. Department of Energy It has been suggested from conductivity measure- under Contract Number W-7405-ENG-48.

[1] F. L. Tanzella and N. Bartlett, to be published. [6] L. B. Littlefield and G. O. Doak, J. Am. Chem. [2] W. C. Smith, J. Am. Chem. Soc. 82, 6176 (1960). Soc. 98, 7881 (1976). [3] E. L. Muetterties, W. Mahler, and R. Schmutzler, [7] E. L. Muetterties, private communication. Inorg. Chem. 2, 613 (1963). [8] M. Brownstein and R. Schmutzler, J. Chem. Soc. [4] E. L. Muetterties and W. D. Phillips, J. Am. Chem. Commun. 1975, 278. Chem. Soc. 81, 1084 (1959). [9] F. N. Tebbe and E. L. Muetterties, Inorg. Chem. [5] E. L. Muetterties, W. Mahler, K. S. Packer, and 6, 129 (1967). R. Schmutzler, Inorg. Chem. 3, 1298 (1964).