US REPORT R-612 Conference Paper November 1972
INVESTIGATIONS OF TH£ SYSTEMS Br2-F2-AsF5
AMD Br2-F2-BF3
A.ŠMALC
univerza v ljubljani institut "jožef Stefan" ljubljana, Jugoslavija • We regret that some of the pages in the microfiche copy of this report may not be up to the proper legibility standards, even though the best possible copy was used for preparing the master fiche. KEYWORDS
HALOGEN HEXAFLUOROARSENATES HALOGEN TETRAFLUOROBORATES SYNTHESES
Odobrila Oddelek za kemijo in Komisija za tisk IJS (Natisnjeno na IJS v 200 izvodih) US REPORT R-612
INVESTIGATIONS OF THE SYSTEMS Br2-F2-AsF5
AND Br2-F2-BF3*
A.Smalc
University of Ljubljana, Institute "J.Stefan"»Ljubljana, Yugoslavia
ABSTRACT, The reactions in the systems Br2-F2-AsF5 and Br2-F2-BF3
have been investigated measuring the amounts of AsF5 and BF3 absorbed in dependence of mole ratio of the reactants. The results
are summarized in a plot mole fraction of AsF5 vs. mole fraction
of F2. In the first system the existence of three compounds is
indicated - Brj AsFe , Br2 AsF6 , and BrF2 AsFe can be obtained depending on mole ratio of reactants used. In the second system
only two compounds can be obtained: Bra BF*, , and BrF2 BF«T, respectively. Br2+AsF6 , a new compound of Brz* cation, is at room temperature a dark red solid, stable only under pressure of a few mm Hg of ASF5. Br3 BF% , however, forms dark brown needles with a dissociation pressure which follows the equation log p « 11.808 - 2b^ .
In the recent years a number of papers appeared concerning an interesting field of halogene chemistry that is halogene and interhalogene cations. Some of these species for example bromine and chlorine cations were found to exist only a few years ago. Dibromine and tribromine cations were observed at first in the solutions whenever bromine was being oxidized with peroxy- disulfuryl difluoride in fluorosulfuric acid or in so called
* Presented at U l European Symposium on Fluorine Chemistry, Ljubljana, Aug. 28th - Sept. 1st, 1972
- 1 Superacid I:
Solution in HSQ3F (brown) + 3 Br2 + 3 S206F2-> 2 Br3 + 2 S03F~
Bra"*" + S03F~ e=* Br2 + BrS03F
Solution in Superacid I - HSOaF-SbFs-aSOa i + k Brt • S206F25= 6 Br2 + 2 S03F~ R.J.Gillespie and M.J.Morton, Chem.Commun. 1968, 1565
R.J.Gillespie and M.J.Morton, Quart.Rev. 2211» 553
The first compound cf dibromine cation was prepared by the reaction of an excessive amount of antimony pentafluoride with bromine and bromine pentafluoride:
+ U.5 Br2 • 15 SbF5 + BrFs-* 5 Br2 Sb3Fl6~ A.J.Edwards, G.R.Jones, R.J.C.Sills, Chem.Commun. 196b. 1527
The compounds of tribromine cation were prepared by the reaction between dioxygenyl hexafluoroarsenate or -antimonate and bromine:
02AsF6 + 1.5 Br2-* Br3*AsF6~ • 02 (') + 2 02SbF6 + 1.5 Br2-* Br3 SbF*~ •»• 02 ( )
(') O.Glemser and A.šmalc, Angew.Chem., Intern.Ed. Q, 517 (19*>9) (2) O.Glemser and A.šmalc, not published
Another possibility for the synthesis of these compounds is given by the reaction between arsenic or antimony pentafluoride and excess of -bromine in the presence of a proper fluorinating or oxidizing agent, respectively:
- 2 - + k Br2 • BrF3 * 3 MF5-* 3 Br3 MF6~
7 Br2 + BrF5 + 5 MF$-* 5 Br3*MF6~ M = As(M or Sb(2) (1) O.Glemser and A.šmalc,
Angew.Chem., Intern.Ed. Q% 517 (1969) (2) O.Glemser and A.Šraalc, not published
When the reaction between dioxygenyl hexafluoroarsenate and bromine was carried out stepwise by addition of bromine in small portions, three reaction steps were observed. We believe that the reaction course is as follows:
6 02AsF6 + Br2~* 6 02 + k AsF5 + 2 BrF2AsF6 yellowish + U AsF5 • 2 brF2AsF6 + 5 Bra-* 6 Br2 AsF$~ red + 6 Br2 AsF6~ + 3 Br2-> 6 Br3*AsF6~ brown
In the first step, six millimoles of oxygen and four millimoles of arsenic pentafluoride were set free by each millimole of bromine added. After the first step of the reaction bad been finished a yellowish solid with the composition corresponding to
the formula BrF2AsF* was obtained. After further amounts of bromine had been added the colour of the solid became red and then turned to brown. The red colour might be ascribed to the dibromine cation formed during the second step of the reaction. Addition of an excessive amount of bromine, however, leads to the formation of the brown tribromine hexafluoroarsenate. In order to confirm the proposed existence of dibromine hexafluoroarsenate (which, of course, chould be less stable than the corresponding antimony compound) the reactions in the system bromine - fluorine - arsenic pentafluoride were investi gated in detail. Fluorine was chosen as fluorinating agent since the measuring of the amount of it to be added to the reaction mixture is simple. Known amounts of fluorine were successively added to a
- 3 - known amount of bromine contained in a closed all Kel-F reaction tube. After each addition of fluorine and weighing of the re action tube arsenic pentafluoriue was absorbed in the obtained reaction mixture at -16 C. The amount of the later was determined after each absorption by weighing the reaction tube after the excessive arsenic pentafluoride had been pumped off at -lb C. The results obtained are summarized in a plot "mole fraction of arsenic pentafluoride absorbed versus mole fraction of fluorine added", Fig. 1.
0,50
*~ 0,40 0,30 - 1 5,67Bf2*F2*2A$F5 * 2BrjA$F; • 2.S78r2 2 3Br2*F2+2AsF5 • n\\k$\ 0,20 - 3 2Br2*F2*2AsF5 = MrfAsF^ 4 Br2+ F2* 1,2A$F5 = O.llrjAsFj •0,UrFjA$Fj 5 Bf2*3F2 + 2A$FS « 2BrfjA$F; 0.10 0,10 0,20 0,30 0,40 0,50 "fjitMi) f t %**%,t*htft Fig. 1. -1, - It is evident that in this system three compounds can be obtained depending on mole ratio of reactants used that is tri- bromine hexafluoroarsenate, dibromine hexafluoroarsenate and di- fluorobromonium hexafluoroarsenate, respectively. The curve drawn on the basis of the supposed reactions is designated by circular points. It coincides exceptionally well with the experi mentally obtained values marked with triangles. Following the obtained results we have synthetized dibromine hexafluoroarsenate by the absorption of arsenic pentafluoride in a mixture of bromine and bromine fluorides with a mole ratio Br2 : Fz= = 2:1 at -7a°C. Dibromine hexafluoroarsenate is a dark red solid stable at -76 C. However, at room temperature it shows a considerable dis sociation pressure of arsenic pentafluoride. If arsenic pentafluoride is pumped off the disproportionation takes place leading to the more stable tribromine compound, probably following the equation: + + 2 Br2 AsF6" - AsF5-* Br3 AsFe~ • BrF Br2 • BrF3 In addition we have investigated a related system bromine - fluorine - boron trifluoride in hope that bromine tetrafluoro- borates are possibly existing compounds. The same experimental approach vas employed as before. In contrast to arsenic penta fluoride system we have observed that only two compounds are existing in the boron trifluoride system that is tribromine tetrafluoroborate and difluorobromonium tetrafluoroborate, respectively. The results are presented here in a plot "mole fraction of boron trifluoride absorbed versus mole fraction of fluorine added" (Fig. 2). Tribromine tetrafluoroborate forms dark brown needles, stable at -7& C. Its infrared spectrum taken in a cell with internal window cooled with liquid nitrogen shows the bands characteristic for tetrafluoroborate anion. At room temperature, - 5 - 0.50- • - 5> / & o- 1 12.5 3r7. F7.2 BF3 > 7 Br* BF4% 5.5 Bt, 2 JE'I-FJOBF, «2lr*BF4- 12« 2Sir F, .2BF, «2Bfj 8F* 1 3 2 Br,. f2 .15BF, * 1.25 Br; 8F;.0.25 BrF, BF4 I 0.33Br, »F,. 0.67 BF,» 0.67 BrF, Bfk 5 133 Br, .F, = 0.67 81 Fj an 0.20 OJO 1.(0 Fig. 2. however, tribromine tetrafiuoroborate shows a considerable dis sociation pressure which follows the equation log p « ll.tiOB - ^—^- (251-301°K) The average molecular weight determined by weighing of a known volume of vapour phase at known pressure and temperature (about 25°C) is 116,5. This result confirms that the compound is practically completely dissociated into components at room temperature: Molecular lass of Br 3 BFi, bra+BFiT M - 326,56 + Br3 BF.T -+ Br2 + BrF • BF3 M - 108,06 ^Bra^BFi,"-^ UBr2 + BrF 3 + 3BF3 M • 122,U5 measured value (average of k measurements at room temperature) M 116,5 - 6 - As expected dibromine tetrafluoroborate is not stable at reaction conditions used and could not be isolated. Tribromine tetrafluoroborate is on the other hand less stable as compared with the corresponding arsenic compound and even less stable than dibromine hexafluoroarsenate. ACKNOWLEDGEMENT - This work was financed through the Research Community of Slovenia. - 7 -