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,] —.