Recycling of the Supporting Electrolyte Tetra(N-Butyl)Ammonium Hexafluorophosphate from Used Electrolyte Solutions

Recycling of the Supporting Electrolyte Tetra(n-butyl)ammonium Hexafluorophosphate from Used Electrolyte Solutions

Stefan Dümmling, Procedures are designed for the recycling of the supporting electrolyte Emerich Eichhorn, Susanne Schneider, tetra(n-butyl)ammonium hexafluorophosphate (TBAHFP) from Bernd Speiser*, and electrolyte solutions used for electroanalytical or electrosynthetic Marc Würde Institut für Organische purposes. The quality of the recycled product is assessed from cyclic Chemie, Universität voltammograms and is comparable to TBAHFP from the original Tübingen, Auf der Morgenstelle 18, synthesis. D - 72076 Tübingen, Germany

* Corresponding author. E-mail: [email protected]. chemie.uni-tuebingen.de

A supporting electrolyte (SE) is the precursors are much cheaper lyte batches containing TBAHFP used to increase conductivity in the than the commercial products. from two series of experiments majority of all electroanalytical or In our laboratory, large quanti- were used: a) from investigations of electrosynthetic experiments in ties of used electrolytes from elec- ferrocene (fc), N,N’-bis(sali- non-aqueous solutions (1,2). While troanalytical and electrosynthetic cylidene)ethylene-diaminato-cobalt some experiments have recently experiments had to be disposed of, (II)/(III), Co(salen) 1, and its one- been performed with microelec- so we tested the possibility of sepa- electron oxidation product [Co trodes under conditions where no rating and recycling the SE from (salen)]+,1+, in various solvents (5); SE is deliberately added (3), it is these solutions. If the quality of b) from analyses of hexaaminoben- common practice to have present a such a recycled SE was satisfactory zene derivatives [e.g. hexakis(di- ≈ methylamino)benzene, 2 (see struc- large excess of these salts (cSE for use in further experiments, and 0.1 M) relative to the substrates in- if the recycling procedure was not tures below)] in CH2Cl2 and vestigated (typical for electroana- significantly more involved than the CH3CN (6). We will show that the ≈ -4 quality of the recycled product is lytical work: csubstrate 10 M). synthesis of a new SE, it would be Excess SE affects the current in the possible to decrease not only the similar to that of TBAHFP from the bulk of the solution, which is main- costs for the purchase of the salts or original synthesis by comparing tained mostly by the ions of the SE, their precursors but also the amount background voltammograms and and migration effects on charged of liquid wastes produced. current potential curves of several substrates can be neglected (4). The In this manuscript, we describe redox active compounds. SE can also have effects on the dou- how the widely used SE tetra(n-bu- ble layer. tyl)ammonium hexafluorophos- n Although some SEs are avail- phate (TBAHFP) can be recycled N N able commercially, they are often from dimethylformamide (DMF), Co O O routinely synthesized in electro- acetonitrile, dichloromethane, or chemical laboratories because the pyridine solutions of organometal- + procedures are usually simple and lic and organic substrates. Electro- 11: Co = CoII ,nn = 0; : Co = Co III , =1

53 Current Separations 15:2 (1996) CH33 NCH it was passed through an Al2O3 col- was recrystallized four times from a umn (activation as given above). mixture of ethanol/water (3:1, v/v) (CH32 ) N N(CH32 ) and finally dried for 48 h at 100°C Preparation of Electrolyte in vacuo. Yield: 90-102 g (75-85% Solutions of the theoretical amount). In most cases, the electrolyte (CH32 ) N N(CH32 ) was prepared from 1.94 g TBAHFP Recycling of the Supporting and placed in a Schlenk flask. Air Electrolyte N(CH32 ) was removed by evacuating and From Co(salen) solutions.A 2 filling with Ar three times before 50 volume of 1.75 L electrolyte (mix- mL of the solvent (dried as de- ture of solutions in acetonitrile, scribed above) were added in an Ar CH2Cl2, DMF, pyridine, and pro- Experimental counter current. The electrolyte was pylene carbonate; concentration of deoxygenated by three freeze- TBAHFP: 0.1 M; contaminated General pump-thaw cycles. with fc, Co(salen), [Co(salen)]+, its Cyclic voltammetry was per- derivatives and possibly decompo- Cell Assembly sition products such as the salen li- formed in a three-electrode cell The glass cell was usually gand, salicylic aldehyde, and equipped with a Pt working elec- dried at 140°C, equipped with the ethylenediamine, in concentrations trode tip (BAS, electroactive area A counter electrode, evacuated, and ≈ 2 up to 1 mM) was concentrated by 0.08 cm ), a Pt wire counter elec- filled with Ar three times. A volume distillation to approximately 200 trode (fitted with a ground glass of 20 mL of electrolyte was added + mL. After cooling to room tempera- joint) and a Ag/Ag reference (0.01 against an Ar flow. Then, the work- ture, the SE was precipitated by the MAgClO4 in acetonitrile/0.1 M ing and the reference electrode addition of 400 mL water or ethanol TBAHFP) connected to the cell by were introduced. After background (12) forming a dark brown amor- a classical or double-reference (7) curves were recorded, weighed phous mass, which was collected, Haber-Luggin capillary. All poten- samples of the substrates were air-dried and recrystallized from tials are referred to an external added against an Ar flow. fc/fc+ standard in the respective 200 mL ethanol. The mother liquor electrolyte. A BAS 100A electro- Synthesis of Tetra(n-butyl)ammo- was rejected. The crystalline prod- chemical analyzer or a BAS nium Hexafluorophosphate. uct was recrystallized from ethanol 100B/W electrochemical worksta- We synthesized TBHFP based until the mother liquor became col- tion were used to generate the po- on a procedure cited by Fry (2,9) orless (usually 2 or 3 further recrys- tential sweep and to record the cur- and improved and optimized by tallizations; allow to cool down rent signal. In the latter case, data Salbeck (10), Fry (11), as well as in slowly; collect mother liquor, see were collected on a personal com- our laboratory. below). Finally, three additional re- puter. BAS 100A plots were digit- A 100 g sample (0.31 mol) of crystallizations from ethanol ized later. Plots were generated NBu4Br (Fluka, purum) was dis- yieldedafter48hofdryingat from all digital data with the X solved in 250 mL acetone (for syn- 100°C in vacuo 35-40 g colorless Window system-based program thesis) and then mixed with 50 g crystals (50-60% of the SE con- XEASI (8) on a UNIX workstation . (0.4 mol) NH4PF6 (Fluka, purum) tained in the electrolyte solution). in 350 mL acetone. Colorless From the collected solutions, Purification of Solvents NH4Br precipitated and was fil- 5-10 g more of raw TBAHFP could N,N-Dimethylformamide was tered off (glass funnel with folded be separated by evaporation of the fractioned three times in vacuo filter or Büchner funnel). The clear solvent to a volume of 200 mL. (P2O5 was added before the first solution was concentrated to a vol- This raw SE was usually purified fractionation) and stored in the ume of approximately 200 mL with together with the product of the dark. a rotary evaporator. The desired next batch of electrolyte recycling. Pyridine was kept over KOH TBAHFP was then precipitated by The evaporated ethanol may be re- for several weeks and distilled just the addition of water until a volume used in further recrystallizations. before use. of approximately 2 L was reached. From hexakis(dimethylamino)- Acetonitrile was distilled from After separation, the precipitate benzene solutions. The solution [≈ P2O5, NaH, and P2O5 again. The was redissolved in a solution of 5 g 1 L; mixture of CH2Cl2 and ace- resulting solvent was passed (0.04 mol) NH4PF6 in 200 mL of tonitrile; concentration of through a column with Al2O3 (neu- acetone (in some preparations more TBAHFP: 0.1 M; contaminated tral, dried at 140°C, and stored un- acetone had to be added in order to with fc and with hexakis(dimethyl- der Ar). dissolve all of the solid). Again, amino)benzene and derivatives, as Dichloromethane was distilled water was added and the precipitate well as their oxidation products] from P2O5 and then K2CO3 before collected. The colorless product was evaporated to dryness with a

Current Separations 15:2 (1996) 54 F1 Cyclic voltammograms in electrolytes prepared 100 from TBAHFP recycled from Co(salen) experi- 100 ment solutions; Pt elec- 2 i trode, A ≈ 0.08 cm ,all ( /nA) potentials vs. fc/fc+ in 0 the respective solvent; 0 top, left: DMF/0.1 M TBAHFP background voltammogram, v = Current 0.05 V/s; top, right: pyri- -100 dine/0.1 M TBAHFP background voltammo- -100 gram, v = 0.05 V/s; bottom, left: 0.25 mM -0.25 0 0.25 0.5 -1.25 -1.0 -0.75 -0.5 -0.25 Co(salen)ClO4 in DMF/0.1 M TBAHFP, 5 v = 1.0 V/s; bottom, 5 right: 0.25 mM Co(salen)ClO4 in DMF/0.1 M TBAHFP, v = 1.0 V/s, with basic i impurity. ( /µA) 0 0

Current -5 -5

-1.0 -0.75 -0.5 -0.25 0 -1.0 -0.75 -0.5 -0.25 0 Potential (E /V) Potential (E /V)

rotary evaporator. The solid residue water can be easily removed during sis and the recycling procedure (F1 was recrystallized from etha- the final drying stage. and F2). nol/water and the resulting crystals The recycling procedures used The background curves in were washed with 150 mL water. involve only a minor overhead DMF and pyridine electrolytes pre- Three further recrystallizations compared to the original synthesis pared from recycled TBAHFP (F1, from ethanol/water yielded after of TBAHFP. The main time-con- top) do not show any signals due to drying for 48 h at 100°C and 3 x suming manipulations, both in the electroactive contaminants. Cyclic 10-2 mbar 32.3 g pure TBAHFP synthesis and the recycling proto- voltammograms of [Co(salen)]+ (85% of the amount in the starting cols, are the recrystallization steps. consist of a simple, well-defined, solution). Fortunately, TBAHFP yields pure quasi-reversible peak couple (F1, and good quality crystals upon slow bottom left). These were identical cooling of the saturated ethanol or to current/potential curves recorded Results and Discussion ethanol/water solutions. in electrolytes prepared from origi- The starting solutions were nal TBAHFP. The cation TBAHFP can be recycled from highly colored in both cases due to [Co(salen)]+ is a Lewis acid (14), used electrolyte solutions with the contaminants [Co (salen), dicat- and it is extremely susceptible to good yields. The procedures de- ion of hexakis(dimethylamino)ben- reactions with bases. Presence of scribed differ mainly in the removal zene (13)]. Consequently, any re- basic impurities, even in small con- of the electrolyte solvents and in maining impurities could easily be centrations, causes a splitting of the the solvent applied for recrystalliza- detected by visual inspection. reduction peak (5). An example is tion. Distillation or evaporation of Of course, the quality of the re- shown in F1, bottom right, where the solvent can be used success- cycled TBAHFP for use in electro- DMF of a non-satisfactory purity fully. Distillation may be advanta- chemical experiments can finally be was used. The voltammograms geous if solvents with high boiling assessed by recording the cyclic clearly indicate the absence of any points are present (e.g., propylene voltammetric curves and comparing basic impurities in the electrolytes carbonate in the first procedure). them with those from electrolyte prepared from the recycled SE. TBAHFP is essentially insol- solutions prepared with the original In F2A, background cyclic uble in water. Therefore, the addi- SE. Thus, both background voltam- voltammograms of electrolytes pre- tion of water to the solvent during mograms and voltammograms of paredwithnewandrecycled recrystallization increases the yield substrate solutions were recorded TBAHFP in mixtures of CH2Cl2 of the recycled SE. Any residual for the SE from the original synthe- and acetonitrile (1:1) are presented,

55 Current Separations 15:2 (1996) F2 cation to the history of his citation and making available the procedure Cyclic voltammo- 5.0 grams in for the optimized TBAHFP synthe- CH2Cl2/CH3CN (1:1) (A) electrolytes, Pt elec- sis from his laboratory. Financial trode, A ≈ 0.08 cm2, support by the Deutsche For- all potentials vs. fc/fc+; a) background 2.5 schungsgemeinschaft, the Volk- i voltammograms, elec- ( /µA) swagen-Stiftung, and the Fonds der trolyte from new TBAHFP (0.1 M; Chemischen Industrie is gratefully solid line) and acknowledged. B.S., thanks for the

TBAHFP recycled Current 0 from hexakis(dimethyl- award of a Heisenberg fellowship. amino)benzene experi- ment solutions (0.1 M; broken line); b) vol- References tammograms of 2; -2.5 solid line: c = 0.06 mM 2, v = 0.1 V/s, -1.5 -1.0 -0.5 00.5 1.0 new TBAHFP (0.1 1. H. Lund and M.M. Baizer (Eds.) in M); broken line: c = “Organic Electrochemistry: An Intro- 0.24 mM 2, v = 0.1 duction and a Guide,” 3rd edition, V/s, recycled Marcel Dekker, New York, 1991. TBAHFP (0.1 M). (B) 2. A.J. Fry in “Laboratory Techniques 5 in Electroanalytical Chemistry,” 2nd edition, (P.T. Kissinger and W.R. Heineman, Eds.) Marcel Dekker,

( /µA) New York, 1996, 469-485. 3. A.M. Bond, Analyst, 119 (1995) R1 - R21.

Current 4. C. Amatore, in Lund and Baizer (1), 0 11 - 119. 5. E. Eichhorn, A. Rieker and B. Speiser, Angew. Chem., 104 (1991) 1246 - 1248; Angew. Chem. Int. Ed. Engl., 31 (1992) -1.5 -1.0 -0.5 0 0.5 1.0 1215 - 1217. Potential (E /V) 6. B. Speiser and M. Würde, unpublished results. 7. B. Gollas, B. Krauß, B. Speiser and extending over the entire accessible particularly, if the solubility behav- H. Stahl, Curr. Sep., 13:2 (1994) potential window. In this case, ior of the contaminants were simi- 42 - 44. again, the curves of the recycled SE lar to that of TBAHFP. In such 8. B. Speiser, unpublished. do not indicate any remaining im- cases, one trial recycling run can 9. The original procedure described by purities. easily show the quality of the recy- Fry (2) was referenced as: J.A. Cyclic voltammograms of cled SE from cyclic voltammo- Ferguson, Interface 6(2), Aug. 10, 1970. Interface was a newsletter hexakis(dimethylamino)benzene in grams. edited by graduate student Pete this solvent system (F2B; oxidation Kissinger (11). occurs in three stages up to a Conclusion 10. J. Salbeck, Elektronentransferreak- tetrakation (6)) also do not reveal tionen nichtalternierender chinoider significant differences between Good quality TBAHFP can be und hydrochinoider Verbindungen, Dissertation, Univ. Regensburg, electrolytes prepared from new or recycled from used solutions of this FRG, 1988. recycled SE. The slight potential SE after electroanalytical or elec- 11. A.J. Fry, personal communication. trosynthetic experiments. Only mi- shifts of the peaks on the reverse 12. In some cases, precipitation with scan are within the reproducibility nor overhead is involved as com- water yielded a black, highly vis- observed with this system. pared to the original synthesis of cous oil, which could not be fil- TBAHFP. Thus, the recycling pro- tered. Then, ethanol proved to be The recycling procedure de- a better solvent for the process. scribed here is successful since cedure can be recommended to de- We recommend to test the use of TBAHFP crystallizes very well crease the purchase and disposal water with a small sample of the from ethanol. Addition of water de- costs of TBAHFP as well as the electrolyte. creases the solubility of this SE and volume of liquid wastes produced. 13. J.M. Chance, B. Kahr, A.B. Buda, J.P. Toscano and K. Mislow, J. increases the yield. Org. Chem., 53 (1988) 3226-3232. If other contaminants were pre- Acknowledgments 14. B. Gollas, B. Speiser, H. Stahl, sent in the electrolyte solutions, the J. Sieglen and J. Strähle, Z. Natur- recycling procedure may not be as The authors thank A.J. Fry, forsch., 51b (1996) 388 - 398. effective as in the cases discussed, Middletown, CT , USA, for clarifi-

Current Separations 15:2 (1996) 56