The Isolation of Lithium Rhenide (Preliminary communication)

By A. v. GROSSE Contribution from the Research Institute of Temple University, Philadelphia, Pa. (Z. Naturforschg. 8 b. 533—536 [19531; eingegangen am 23. Juni 1953)

To Dr. Walt er Nod clack on his 60th birthday

Das einwertig-negative Rhenid-Ion, Re , ist vor 16 Jahren von Lundell und K n o w 1 e s in sehr verdünnten, sauren, wässerigen Lösungen beobachtet worden. Diese Lösungen wurden seither mehrfach von physikalisch-chemischer Seite untersucht, ohne daß jedoch ein Weg zur Konzentrierung und Isolierung der Rhenide beschrieben wurde. Anzunehmen war, daß salzähnliche binäre Verbindungen mit Alkalimetallen, wie z. B. Natriumrhenid — Na+ Re-, existenzfähig sein würden. Es wurde festgestellt, daß wässerige Kaliumperrhenat-Lösungen in einfacher Weise von Lithiummetall zu Lithiumrhenid reduziert werden können; das nebenbei in größeren Mengen gebildete Li0H-H.,0 kann vom leicht löslichen Lithiumrhenid durch fraktionierte Kristallisation glatt abgetrennt werden. Das feste, salzähnliche Lithiumrhenid ist in Abwesenheit von Sauer- stoff bei Zimmertemperatur relativ stabil; somit eröffnet sich jetzt die Möglichkeit zum Stu- dium der Alkalimetallrhenide.

he element rhenium was discovered in 1925 by Na+ Re- T would be expected to exist and it would be particu- Ida Tacke and Walter Noddack, with larly interesting to study both its similarities and O.Berg. In the intervening 28 years the very inter- differences from a typical salt, such as: esting chemistry of this metal has been extensively studied and many valuable observations recorded 1. Na+ Cl— As is well known, this metal is a member of the The dilute aqueous solutions of rhenide ion were subgroup of the halogen family in the Periodic System studied from a physical-chemical standpoint repeat- and the second higher homologue of manganese or edly 3—7 after their discovery, but no successful at- Mendelejeff's dvimanganese. In our opinion one of tempt to concentrate and isolate pure rhenides from the most striking discoveries in rhenium chemistry them is described in the literature. The highest con- was the observation made sixteen years ago by centration obtained in aqueous solution was 2.6 milli- 2 Lundell and Knowles at the U.S. Bureau of Stan- moles rhenide ion per liter in experiments of R u 1 f s dards, that rhenium can exist as a uninegative rhenide and E 1 v i n g 8. ion, or Re", in very dilute aqueous solutions. We became interested in this field of rhenium chem- This is the first time that a metal has been observed istry in connection with our program of preparing as a negative ion and would imply that salt-like com- volatile asymetric compounds of various elements for pounds of rhenium and other metals, for example, the study of their microwave spectra in the Depart- the alkalis or alkali earths, should be possible. For ment of Physics of Columbia University9. If alkali example, sodium rhenide or 1 See I. and W. Noddack, "Das Rhenium", Leo- 8 C. L. R u 1 f s and P. J. E 1 v i n g , J. Amer. chem. pold Voss Verlag, Leipzig 1933. Soc. 73, 3287—3292 [1951], 2 G. E. F. Lundell and H. B. K n o w 1 e s , J. Res. 9 See microwave spectrum of MnO.,F, A. J a v a n and Nat. Bur. Standards 18, 629 [1937]. A. v. Grosse, Physic. Rev. 87, 227 [1952]; of Re «sOjCl 3 O. T o m i c e k and F.Tomicek, Collect, czecho- and Re i"03Cl, A. J a v a n , G. S i 1 v e y , C. H.Tow- slov. chem. Commun. 11, 626 [1939]. nes and A. v. Grosse, Physic. Rev. 91, 222 [1953]; 4 J. J. L i n g a n e, J. Amer. diem. Soc. 64, 1001 [1942]. isolation of Mn03F, A. E n g e 1 b r e c h t and A. v. G r o s s e, 5 J. J. Lingane, J. Amer. chem. Soc. 64, 2182 [1942]. General Program of the 118th Meeting of the American r> E. K. Maun and N.Davidson, J. Amer. diem. Association for the Advancement of Science in Philadel- Soc. 72, 3509 [1950]. phia, Pa., 1951, p. 140; also isolation of Cr02F2, A. Engel- " C. L. R u 1 f s and P. J. E 1 v i n g , J. Amer. chem. Soc. brecht and A. v. Grosse, J. Amer. chem. Soc. 74, 73, 3284—3286 [1951]. 5262 [1952]. rhenides can be isolated it would be of interest to The main reason why we chose lithium in pref- explore whether a volatile hydrogen rhenide, HRe, erence to the other alkali metals, however, was the or volatile alkyl and aryl rhenides, such as CH3 • Re fact that the main reaction product, namely, lithium

and C6H5 • Re, would exist. hydroxide, crystallizes nicely and easily in the form L u n d e 11 and K n o w 1 e s2 prepared rhenide ion of LiOH • 1 HoO and has solubility characteristics solutions by reducing perrhenate ion with amalgam- which are advantageous for methods of separation by ated zinc in dilute sulfuric acid in a Jones reductor fractional crystallization (see experimental part). Fur- using ice-cold solutions and in the complete absence thermore, lithium reacts much less violently with of oxygen. These results were confirmed first by water than the other alkali metals. O. Tomicek and F. Tomicek3 and later in two The lithium reduction does not form any of the polargraphic investigations by Lingane4'5. L i n - colored intermediate valence stages of rhenium in g a n e 4 also reduced perrhenate ion to rhenide at the observable amounts. The solution contains only rhe- dropping mercury electrode. Numerous studies were nide ion and unreacted perrhenate ion. made on the reduction of perrhenate ion to rhenide We found that we can separate the easily soluble ion and the latter's oxidation to various positive and stable lithium rhenide from even a large excess valence states of rhenium 3~8. R u 1 f s and E 1 v i n g s of lithium hydroxide by fractional crystallization in

indicate that coordination complexes are formed with aqueous solution. Unreduced KRe04 is very sparingly ethylene diamine and pyridine in hydrohalic acid soluble in LiOH solution and there is no difficulty at media. All of the above investigations, however, do all in separating most of it from LiOH and even less not materially advance our knowledge of rhenide from the easily soluble lithium rhenide. compounds from the standpoint of inorganic chemis- We have obtained pure lithium rhenide, probably try beyond the original discovery of L u n d e 11 and as a hydrate, in the form of a practically white crystal- 2 Knowles . Thus S i d g w i c k in his two well line solid (or pale yellow in color in thicker layers). known monumental volumes on "The Chemical Ele- It is very easily soluble in water. Both the solid and 10 ments and Their Compounds" does not discuss the concentrated or dilute aqueous alkali solutions are rhenides and refers the reader to the literature "for stable for many weeks at room temperature, if kept 2> 5 evidence of Re~ in solution" . under a nitrogen atmosphere. 11 Last year Griswold, Kleinberg and Bravo These solutions and lithium rhenide crystals are reported that they were able to reduce potassium not particularly sensitive to air or oxygen. Flasks con- perrhenate in ethylenediamine-water solutions by taining these crystals can be opened to the atmos- means of potassium metal and obtain a solid mixture phere, for example, for taking a sample, without of potassium hydroxide containing rhenide. A method noticeable loss in rhenide content, provided they are of separating rhenide from KOH and concentrating evacuated quiddy, flushed with pure nitrogen and left it was not described, however. under nitrogen. This procedure can be repeated a In the present investigation our immediate objective number of times. However, if these solutions are per- was to see whether rhenides could be concentrated mitted to stand in the air for many hours or days the and whether they were stable enough to be isolated rhenide is oxidized completely. Thus the oxygen in the form of a pure compound. sensitivity of alkaline rhenide solutions is vastly dif- We found that pure aqueous solutions of potassium ferent from such oxygen-sensitive substances as, for perrhenate can be readily reduced with metallic lith- example, sodium triphenylmethyl; its deep red solu- ium. Lithium is the strongest reducing agent known tion in ether is discolorized in a few seconds when in aqueous solution and tops the list of other reducing exposed to air or oxygen. agents with a standard electrode potential, E0, of the A lithium rhenide solution readily reduces a hot electrode reaction: acidified potassium permanganate solution. Titration

Li —• Li+ + e~ of rhenide solutions with KMn04 affords a ready method to determine rhenide quantitatively. equalling 2.9578 volts at 25°C1-. The rhenide ion is oxidized by excess standard per- 10 Oxford, at the Clarendon Press, England, 1950. p. 1314. 12 See, for example, H.S.Taylor, "A Treatise of 11 E. G r i s w o 1 d , J. K 1 e i n b e r g and J. B. Br avo, Physical Chemistry", Vol. I, 1931. p. 827 (D. Van Nos- Science [Washington] 115, 375—376 [1952]. trand Co., New York. N. Y.). manganate in boiling dilute sulfuric acid, to perrhenic c) Pure nitrogen (02 content = 0.001%) in a cylinder. acid, i.e. Reduction of KRe04 Solution with Lith- ium: A typical reduction was carried out as follows: Re— + 4 [O]—• [ReO,] —. 1.50 g KRe04 (= 5.18 millimoles) were dissolved in warm Thus one equivalent of rhenide consumes eight 150 cc H20 (solubility of KReOa in H.,0 at 20° C 1.01 g/100 g sol., at 90° C = 9.50 g/100 g sol.) " in an Er- equivalents of oxygen and forms one equivalent of lenmeyer flask. This flask had two glass-ground joints; one perrhenate ion. joint was capped and used for the addition of lithium, A sensitive test for rhenide ion is a hydrochloric while the second led to a gas delivery tube, dipping under acid solution of bismuth trichloride. A rhenide solu- mercury. tion reduces it in a few seconds to a black bismuth 7.45 g Li-wire (= 1.08 g atom, about 102 cm), that is, a very large excess, were added in pieces 3—4 cm long to precipitate. the solution. The lithium reacted rapidly but not violently, We can also confirm the thallium test of G r i s - in contrast to the other alkali metals, forming mainly hy- wold, Kleinberg and Bravo11. We used a drogen and lithium hydroxide. After a while the solution nearly saturated aqueous solution of thallous hy- began to boil. It was kept close to boiling and a little later, as the concentration of LiOH in the solution in- droxide. The LiBe solutions containing also LiOH, creased, the reaction of lithium with the solution slowed produced in it, at room temperature, nearly instantly, down; one observed various colors on the reacting lithium a white precipitate of probably thallous rhenide, surface and after a while white crusts of LiOH began to Tl+Re". It turned brown in a few seconds and then appear. After a couple of hours the reaction was complete; became black due to reduction. In cold solutions, at the hot solution was rapidly filtered through a glass frit- tered plate funnel under nitrogen, from small amounts 0° C or below, the thallous rhenide precipitate was of a flocculent brownish precipitate into a second flask stabler, but also turned completely blade in a minute and allowed to cool under nitrogen. or so l3. A titration of an aliquot sample of the solution showed After the stability of lithium rhenide was demon- that 10% of the perrhenate was reduced to rhenide ion. strated we made a preliminary experiment of reacting or that the rhenide ion concentration was about 3.5 milli- molar (as mentioned previously, no colored intermediate rhenium metal powder with potassium. On heating, valence stages of rhenium appeared in this solution). No the two metals reacted "unter Feuererscheinung" attempt was made at this time to determine optimum con- (this may have been due to slight oxidation of Dr. ditions for maximum rhenide yields. The batdi can be Noddack's sample of rhenium!); on decomposing increased by a factor of IV2 to 2, but to insure ready the reaction product carefully, in the absence of air control of the reaction it is not recommended to go sub- stantially beyond this point. and under reduced pressure, with water, a potassium hydroxide solution was obtained which gave a strong 2. Concentration and Isolation 14 positive test for rhenide ion . of Lithium Rhenide by Fractional Crystallization Experimental Part Several reduction batdies can be conveniently com- 1. Preparation of Lithium bined for the production of larger quantities of lithium Rhenide Solution by Reduction of rhenide. Potassium Perrhenate Solution with The clear colorless solution on cooling and standing, Metallic Lithium preferably over night, deposited first the characteristic Chemicals Used: white bipyramids of unreacted KRe04 which is only sparingly soluble in LiOH solution. These crystals can a) Rhenium; 10 g of rhenium powder donated to the be recovered at this stage by decantation or filtration writer by Dr. W. Noddack in 1931 from some through a glass-frittered funnel or later and in higher of his first supply of rhenium, were used in this yields as described below. The crystallization of research. Furthermore, chemically pure KRe04 from LiOH • H.,0 and concentration of rhenide is preferably the Department of Chemistry, University of Ten- carried out by means of standard glass ground joint flasks nessee, Knoxville, Tennessee, was also used. connected by means of T's, V's and stopcocks to a b) Lithium; lithium wire of the Metalloy Corpor- vacuum line and to pure nitrogen and mounted on a ation, Minneapolis, Minnesota was used. 100 cm wire weighed 7.38 g or 73.8 mg/cm. It was cleaned 14 On repeating the test with lithium and rhenium in a and washed under anhydrous ether and used dry. platinum crucible it was observed that platinum reacts with molten lithium, in the course of a few minutes (!) at 13 C. L. R u 1 f s and P. J. E 1 v i n g , J. Amer. chem. a temperature slightly above its melting point (186° C!). Soc. 72, 3304 [1950], attempted to measure the solubility We intend to studv this unusual reaction between Li of TIRe in water without anticipating that it could be and Pt. unstable. 15 See I. and W. N o d d a c k , cit.p. 47. 16 Schlenk cross stand . Filtration of the solution can the KRe04 originally used in the reduction can be re- be conveniently carried out through a glass-frittered plate covered; it obviously can be used "as is" for subsequent (for example, as shown on drawing 37) 16. reductions with lithium. The rhenium left as ReOr The solubility of lithium hydroxide in water is as in solution can be recovered by any suitable method follows (in g LiOH/lOOg H,0): (Re2S_, nitron perrhenate, etc.!). The rhenide filtrate is again concentrated (distilling off at 0° C = 12.7 g 80c -- 15.3 g alcohol and H.,0) and the crystallization repeated. On the 30° = 13.1 g 100° 17.5 g third or fourth cycle (with a recrystallization of the head fractions of LiOH • H.,0 crystals if necessary!) and by The solubility in ethanol is substantially lower. diminishing suitably the scale of the apparatus one The rhenide-containing solution was first concentrated by obtains highly concentrated lithium rhenide solutions distillation to V2—V3 of its original volume in an 02-free which are practically free from lithium hydroxide. By evap- atmosphere and then allowed to crystallize so that well orating water slowly from them in a vacuum one obtains developed crystals of LiOH • H.,0 formed in the mother pure lithium rhenide with properties as described on p. 534. liquor. Formation of crusts on the walls is to be avoided. Analytical data to establish the formula of this compound The crystallization was completed by ice cooling. The simply could not be obtained in time to meet the deadline crystals were sucked off rapidly but completely on a of Dr. W. Noddack's sixtieth birthday celebration. frittered glass filter and washed with small portions of Our starting solutions were 2.5—7.5 millimolar in ice-cold ethyl alcohol, or, if the presence of alcohol is rhenide ion; by the procedure outlined they were concen- undesired, with smaller portions of ice-cold water. The trated to practically pure lithium rhenide in yields of up LiOH • H.,0 crystals, if the operations are carried out to about 90% of the total rhenide content. properly, are practically free from rhenide. They give Now that the stability and method of concentration only a yellow color in the bismuth test and only a trace of lithium rhenide has been demonstrated, it is evident of brown coloration with thallous hydroxide. If the that other methods of reduction of [ReOJ" to Re" may crystals should contain noticeable quantities of rhenide be used; after neutralization with lithium hydroxide, the batch should be recrystallized. suitable lithium salts can be separated from the lithium

About V2 to 3 of the LiOH can be separated in the rhenide by fractional crystallization. first batch of crystals. The clear filtrate contains practi- It is obvious that this is just the beginning of the study cally all of the rhenide, as can be easily ascertained by of the alkali rhenides. The study of their chemical and the above tests or by permanganate titration. physical properties, their reactions, methods of formation From the above description it is obvious that the from rhenium and other metals and decompositions at separation proposed here is in no way similar to Mme higher temperatures promises to be of fundamental impor- Marie Curie's tedious process of separating radium tance to inorganic chemistry. It is our hope, on Dr. Wal- chloride from barium chloride. Although the term "frac- ter Nodddack's sixtieth birthday, that the most tional crystallization" is used by us, it is meant in the interesting part of rhenium chemistry will just begin to sense of manual operation only. The lithium rhenide is unfold itself and that the discoverers of rhenium may see not isomorphous with LiOH • Ho0 and is not built into its their gift to science fascinate chemists on both sides of crystal lattice. The separation described here is much the Atlantic for many years to come.

more similar to the easy separation of protactinium from 16 7 See, for example, E. K r a u s e and A. v. G r o s s e , ZrOCl, • 8 H20 crystals 1 . 'Die Chemie der metall-organisehen Verbindungen", Ver- The" batch of LiOH • H20 crystals contain the KRe04 lag von Gebrüder Borntraeger, Berlin (1937), p. 810, crystals referred to previously. By dissolving the alkali drawing 37. in a requisite amount of water and filtering, 50—60% of 17 A. v. G r o s s e , Ber. dtsch. chem. Ges. 61, 238 [1928],