J. Phys. Chem. 1980, 8 4 , 1084-1090
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Preparation and Analysis of Metal/Solvent Solutions and the Formation of Alkali Metal Anions James L. Dye Department of Chemistty, Michigan State University, East Lansing, Michigan 48824 (Received July 16, 1979) Publication costs assisted by the U.S. Nationai Science Foundation
The techniques used to prepare metal solutions in ammonia, amines, and ethers in the presence and absence of crown ethers or cryptands are described in detail. This includes purification and introduction of measured amounts of metal, complexing agent, and solvent,vacuum-line techniques, and glassware cleaning. The procedures for preparing crystalline salts of alkali metal anions and films and powdered samples of “electrides” are also described. Measurement techniques used to determine NMR, ESR, and optical spectra of these materials are described.
Introduction Much has been written about the nature of metal-ammonia solutions and the properties of solvated ele~trons.l-~ The identification of centrosymmetric alkali metal anions5-10 and cation-electron ion pairsl1-l7 in amine and ether solvents has resulted in a number of articles about the species present in such systems.18-21By contrast, the experimental techniques used to study metal solutions have only been described in fragmentary form, scattered among a number of papers. Because this is an experimentally difficult area of research, especially for a new investigator, the present paper emphasizes experimental techniques and problems. Only enough background information about the nature of the species present is included to provide a framework for discussion of the methods and results. Also included is a highly speculative section on the possible species present in solid “alkalide” and “electride” salts and a description of possible new directions which such research might take. Most of the techniques decribed here are those currently in use in our laboratories. They have originated from sources too numerous to acknowledge in detail. It is likely that some of the methods are inferior to those used elsewhere, and some of the procedures may be unnecessary. However, we have a tendency to continue to use procedures which are convenient and which seem to work for us. The central experimental problems in this research area are the inherent thermodynamic instability of metal solutions, their sensitivity to the presence of reducible impurities and decomposition catalysts, and the apparent autocatalytic nature of many of the decomposition reactions. Because of these problems, we find it necessary to pay careful attention to experimental details such as glassware cleaning, solvent and metal purification, and vacuum-line techniques. While this makes it possible to prepare kinetically stable metal solutions in ammonia, ethylenediamine, methylamine, etc., we have found that a change in the solvent used, the addition of a crown or cryptand, or some change in procedures is often accompanied by new decomposition problems. The techniques described here have been designed to minimize these problems. Metal Solution Model In planning new experiments or attempting to interpret results, it is convenient to have a model which describes the nature of the system. While not everyone agrees with the model outlined here, we have found it to be useful and
in accord with the known facts about metal solutions. The origin of the model is the ion-cluster model of metal-ammonia solution^^^^^^ expanded to include the presence of alkali metal anions and contact ion pairs in less polar solvents. The various species in metal solutions are based upon three distinct entities: the solvated cation, M+, the solvated electron, e;, and the centrosymmetric alkali metal anion, M-. The addition of a crown ether24i25or cryptand%;” permits formation of the complexed cation, M+C.% As with any electrolyte in a medium of low dielectric constant, ion pairs form as the concentration is increased. This leads to solvent-separated or solvent-shared ion pairs between M+ and e; in ammonia and contact pairs in the less polar amines and ethers. The contact pair, M+.e-, is also referred to as a “monomer” and may result in considerable electron density at the metal nucleus as detected by EPR ~ p e c t r o s c o p y . ~ ~ - ~ ~ A major difficulty with this model is the nature of the spin-paired species. Both static susceptibilities and EPR spectra of metal-ammonia solutions require electron spin pairing. The ion-cluster model presumes the formation of a triple ion, e-.MS.e-, but this species must have extra stabilization compared with normal triple ions in ammonia. An alternative model assumes the formation of a dielectron, eZ2-,presumably paired with a ~ a t i o n . ~ Although ~-~~ it has been proposed that the spin-paired species in ammonia is the centrosymmetric alkali metal anion,48the metal-dependent absorption band of this species seen in a variety of solvents and solid films is absent in ammonia.49 In poorer donor solvents than ammonia, association of esolv-and M+ leads to the formation of both contact ion pairs (monomers) and alkali metal anions. The equilibrium 2e; + M+ e M(1) shifts to the right for a given metal as the solvation energies of the cation and electron decrease.50 Although this is difficult to quantify, it parallels the decrease in metal sol~bility~l and follows roughly the order NH3 > HMPA > ethylenediamine = methylamine > ethylamine > polyethers > THF. The relative concentration of M- compared to e; also depends upon the metal. For example, in ethylenediamine we estimateN values of K1 of 3 X lo”, 2 x IO7, 5 x lo5, 3 x lo4,and < 3 X lo3 M-2 for Na, Rb, K, Cs, and Li respectively. The tendency of lithium solutions to form Li+ and e; is so large that the existence of Li- has never been conclusively demonstrated. By contrast, K1 is so large for sodium that, except in NH, and HMPA, so0 1980 American Chemical Society
Analysis of MetaVSolvent Solutions
Figure 1. Apparatus for the preparationof metal samples in glass tubes
of measured inside diameter.
lutione of sodium contain predominantly Na+ and Na-. Metal Solution Preparation and Handling Purification of Metals. The metals Na, K, Rb, and Cs can be vacuum-distilled in borosilicate glass or fused silica and can be purchased in break-seal ampules. However, sodium can bte extracted from sodium borosilicate glass at high temperatures in the presence of K, Rb, or Cs or by solutions of these metals at low temperature^.^^ Therefore, it is recommended that, except for work with sodium, the final distillation should be made in a fused silica apparatus. A problem wjhich is not always appreciated when working with Rb and Cs is that the oxides of these metals decompose when heated and may re-form as the metal is distilled. Therefore, prior oxidation of the metal should be avoided if possible. The methods described below not only avoid oxide contamination but also permit easy measurement of the quantity of metal used. By measuring the length of metal in a glass tube of known inside diameter it is possible to determine the amount of metal present.53 We use the “trombone” apparatus shown in Figure 1to take advantage of this technique. Three long extensions, each with three different diameter glass tubes (measured at both ends before construction), are sealed to a manifold. Opposite each extension is a tube of sufficient diameter and length to hold about one third of the total sample. A break-seal ampule containing the metal” is sealed to the apparatus which also contains a glass-encased magnet. After evacuation, the break-seal is broken and the metal is heated until it melts and runs into the tubes. The three sections are then sealed-off under vacuum and separated. When the inverted tube is; heated, the metal can be forced into each of the “trombone” sections which are sealed-off for storage. The total length of each tube should be at least twice the length of metal. When a sample of a particular mass is needed, a tube of the appropriate diameter is selected and heated to drive the desired length to one end of the tube. After vacuum seal-off, the metal column is driven to the center of the sealed-off portion and the length is measured. To introduce the measured metal sample into the final apparatus, we utilize Teflon heat-shrinkable tubing.55 The glass tube containing the metal is scored with a glass knife and placed into a tube of the same diameter as a metaldistillation side arm (fused silica if desired). This assembly is attached to the side arm with heat-shrinkable tubing to provide a vacuum-tight connection. After evacuation of the apparatus, the metal-containing tube is moved to the
The Journal of Physical Chemistry, Vol. 84, No. 10, 1980 1085
region of the Teflon connection and, by bending the flexible connection, the tube is broken and allowed to slide into the side arm up to a constriction. A vacuum seal-off is then made behind the metal sample. (Caution! Cesium melts slightly above room temperature and reacts with Teflon, so that the metal-containing tube must be cooled before it is broken.) We have used this procedure for several years and find it to be superior to previous meithods except when large metal samples are needed. It is considerably more difficult to work with lithiunn and the alkaline earths since these metals cannot be distilled in glass or quartz apparatus and the alkaline earths react readily with nitrogen as well as with oxygen. When pieces are cut from a metal in an inert atmosphere box there is generally more solution decomposition than when the metals can be distilled. I t has been suggestedMthat contamination can be reduced by dissolving an alkaline earth metal or lithium in liquid ammonia, filtering the solution through a fritted glass or quartz disk, and then evaporating the ammonia (or use the solution directly). We have used this procedure several times and find it to be a convenient way to introduce lithium or alkaline earth metals intio the apparatus. However, it is not certain that this resulted in improved stability of the solutions. In an inert atmosphere box under argon from which nitrogen has not been removed but which has a very low concentration of oxygen, pure lithium foil remains shiny for long periods of time but freshly cut barium darkens quickly. We have taken advantage of the slow contamination of lithium foil and the ease of cutting a measured area of the foil to prepare small samples of lithium in thin glass tubing for later use. Thin-walled glass tubes of 3-5 mm o.d., sealed at one end, are preweighed and provided with a cap to which irradiated polyolefin heat-shrinkable tubing5’ has been attached. The lithium foil is cut into appropriate sized pieces in an inert atmosphere box and inserted into the glass tubing, the cap is put on and the heat-shrinkable tubing is heated with a soldering gun to form a gas-tight seal. After removal from the box, the glass tube containing the lithium foil is cooled with liquid nitrogen and flame-sealed. By cutting the heat-shrinkable plastic tubing away with a razor blade, the glass tube and sealed-off piece can be weighed to accurately determine the mass of metal. The individual tubes are stored in standard-wall 9-mm lengths of tubing which are flamesealed under vacuum. When needed, the lithium can be introduced into a side arm on the final apparatus in the same manner as for the other metals, but, of course, it cannot be distilled before use. We have also prepared pure lithium and barium by vacuum distillation from a tantalum cup onto the walls of a cooled quartz tube.58 The procedures were, however, cumbersome and time consuming and are currently being modified. We are also introducing a nitrogen-removal train on the inert atmosphere box to complement the oxygenremoval system. Construction and Cleaning of Apparatus. Because of problems caused by extraction of sodium from borosilitcate glass by metal solutions,52it is often necessary to construct from fused silica all parts of the apparatus which will come into contact with solutions. This is not necessary when large quantities of material are used, as in synthesis, for example, but it is required for most spectroscopic studies. Although the best technique utilizes break seals which can be degassed by heating under vacuum, we have foiund it very convenient to use Teflon valves and joints. After extensive testing of a number of vacuum valves, we have found those produced by the Kontes Corporations9to be
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LJ Figure 2. Vessel for purification and storage of solvents.
best suited to our needs. The Teflon vacuum connectors produced by Fischer and Porter Co.'j0 have been very satisfactory. The methods which we use for glassware cleaning were developed some time ago61 and have not been changed. After cleaning with a detergent solution if necessary, the glassware is filled (in an exhaust hood) with an HF cleaner. This cleaner consists of 33% "OB, 5% HF, 2% acidstable detergent, and 60% HzO by volume. After thorough rinsing, the apparatus is filled with aqua regia and heated until fumes are given off. Usually the aqua regia is allowed to remain in the apparatus overnight. The acid cleaning is followed by thorough rinsing with doubly distilled water and oven drying in an oven reserved for this purpose. To prepare solutions with the best stability, the portion of the apparatus which will come into contact with solution can be prerinsed with a metal solution and then with pure solvent. By following these procedures we have been able M solutions of cesium in ethylenediamine to prepare which showed less than 10% decomposition over a period of 4 h at room temperature. Purification of Solvents and Complexing Agents. Since our major concern is metal solution stability, the most important criterion of purity is the absence of reducible impurities and decomposition catalysts. Thus, the solvents may not be completely free of other nonreducible, volatile impurities. The solvent purification schemes used vary from one solvent to another and generally begin with procedures recommended in the literature for that particular solvent. In most cases, a preliminary drying over CaO under reflux is followed by refluxing over BaHz although the former step may be eliminated if the solvent is reasonably dry as received. The solvent is then vacuum distilled into glass vessels of the type shown in Figure 2. For solvents such as the primary amines, tetrahydrofuran (THF), dimethoxyethane (DME), diglyme, etc., in which Na-K alloy is soluble, the solvent, after drying over BaHz, can be distilled directly onto a mirror of Na-K. This has the advantage of serving as an indicator of purity. If a stable blue color is not obtained within a few hours, the solvent is redistilled onto a fresh Na-K mirror. To avoid the buildup of decomposition products and the carryover of metals during final distillation, the dry solvent is stored away from metals in a clean vessel. We have found it convenient to dry and store ethers, including THF, over excess Na-K alloy in the presence of benzophenone. The benzophenone ketyl and dianion which form are excellent drying agents, and the purple-blue color of the solution indicates the absence of reducible impurities. The Na-K alloy must be present in excess to prevent the distillation of benzophenone along with the solvent. When solvent is distilled from a storage vessel which contains either Na-K
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alloy or the benzophenone ketyl, it is advisable to first distill it into an empty trap or through a medium porosity fritted disk to prevent spray carryover of nonvolatile materials. All distillations are performed through "tees" external to the vacuum manifold to prevent its contamination by solvent or spray carryover. Both 18-crown-6(18C6) and 2,2,2-cryptand ((2222) are crystalline solids which can be prepurified by recrystallization. The former is recrystallized several times from acetonitrile and then sublimed under vacuum while the latter is usually just sublimed as received. Both can be zone refined if desired. Since polyethers are air and light sensitive, the crown ethers and cryptands are stored in vacuo in the dark. The crown ethers are volatile enough that it is necessary to protect the vacuum manifold with a cold trap when drying samples under vacuum. To prevent sample loss when pumping for long periods of time, the samples are cooled to 0 "C or below. Cryptands are so nonvolatile at room temperatures that no sample loss has been observed while pumping. Because both crown ethers and cryptands can trap moisture, we routinely evacuate the apparatus to a manifold pressure of 10" torr or less and pump overnight before introducing the solvent. Cryptands 211,221, and 322 and the crown ethers 15C5 and 12C4 are liquids and the latter two are rather volatile. The purification and handling of the liquids is more difficult than with the crystalline complexing agents. Because of the high cost of the liquid cryptands ( ~ $ 1 5 0 / gfor C211 and C221; C322 is not commercially available at present) it is not practical to subject them to extensive purification. Fortunately, the boiling points are so high that vacuum distillation ("molecular distillation") into a trap kept at room temperature removes water and other volatile impurities and produces colorless liquids which can be used directly. We have found it best to distill the sample again directly into the measuring apparatus just before use. We have not used 12C4 but have worked with 15C5. This liquid crown ether was first dried over BaHz and then vacuum distilled. Stable solutions of sodium in methylamine in the presence of up to a tenfold excess of 15C5 were made with this material. It is interesting (and disappointing) that 15C5 is a poorer complexing agent for Nat in this solvent than is 18C6 even though the hole size of the former is optimal for Na+. Solution Decomposition. The single most important experimental advance that could be made in the metalsolution field would be the elimination or control of solution decomposition. As indicated above, under favorable circumstances the decomposition can be very slow. For example, we once tried to decompose a dilute solution ( - 5 X M) of sodium in ethylenediamine after finishing a study. After standing at room temperatures for about 1 week there was still no sign of decomposition, and we found it necessary to heat the sample to -60 "C for several hours to effect a decomposition. On the other hand, samples which were ostensibly prepared in the same way sometimes decomposed in a few minutes. In our experience, the presence of cryptands in solution always leads to more rapid decomposition than in their absence. Whether this is because of the reactivity of the cryptand itself or because of impurities in the cryptand is unknown. Generally, solutions made with crown ethers are more stable than those made with cryptands. The progress of solution decomposition is variable. In some cases, especially with ammonia and ethylenediamine, the decomposition proceeds at a more-or-less uniform rate. However, in most solvents, especially when crown ethers or cryptands are present, the products of decomposition
Analysis of Metal/Solvent Solutions
seem to catalyze the decomposition process so that once decomposition begins it proceeds at a very rapid rate. In extreme cases it becomes impossible to make ti blue solution again even in the presence of excess metal. This autocatalytic nature of the decomposition in many systems seems to be the reason that the preparation of stable solutions is so nonreproducible. The easiest way to avoid decomposition is to cool the solution. Storage a t dry ice temperatures is usually sufficient to prevent decomposition. This suggests that freezing the solution with liquid nitrogen might be even better but, curiously, this is not always the case. Evidently the presence of solid solvent and/or solid metal can sometimes lead to rapid decomposition. Solutions which contain primarily alkali metal anions are generally much more stable than those which contain solvated electrons. Thus, sodium solutions are usually an order-of-magnitude more stable than, for example, cesium solutions in most solvents. It is possible that the presence of cation-electron pairs is conducive to hydrogen atom abstraction, leaving behind a reactive free radical. However, in the absence of detailed mechanistic studies, or even complete piroduct analyses, the decomposition process remains a mystery. Studies of the decomposition pathway would be very helpful. Preparation of Crystalline Alkalide Salts. Since the preparation and characterization of crystalline Na+C222.Na- by precipitation from ethylamine or THF solutions having this stoichiometry,6>*we have been able to precipitate EL number of compounds from solutions which have the stoichiometry M+C222,N-in which M and N may be the same (Na, K, Rb, or Cs) or different (M = K, Rb or Cs with N = Na). By appropriate choice of solvent it has been possible to grow small crystals which range in color from ydlow-gold for Na+C222.Na- to copper-bronze for Cs +C222Cs-. Often the solubility in ethylamine is too high to yield precipitates, in which case isopropylamine can sometimes be used effectively. However, solutions in this solvent decompose readily. When a precipitate has been formed it is important to rapidly remove the mother liquor and wash the crystal with a solvent in which the solubility of the alkalide is low but which can remove trapped electrons. We have found diethyl ether to be a useful washing solvent. The dry crystals appear to be much more stable than crystals which are wet with solvent. However, compared with Na+C222.Na-, all other alkalide salts are much less stable. For example, we have kept samples of Na'C222.Na- in the freezer at -10 to -20 "C for over 3 years without noticeable change, but the seemingly similar compound K+C222-Na- must be stored at dry- ice temperatures or below. The type of apparatus used to prepare and store precipitatod alkailide salts is shown in Figure 3. The cryptand is dissolved iin the appropriate solvent and the solution poured over ia metal mirror. A major problem is the slow rate of dissolution of the metal. For example, to dissolve enough Na-l( alloy in 10 mL of a C222-isopropylamine solution a t -30 "C to prepare a 0.04 M solution of K+C222,Na- took 3 h with continuous agitation. The process might be faster at higher temperatures but at the risk of excessive decomposition. If some ammonia can be tolerated in the solution, it is possible to first dissolve the metals and cryptand in ammonia, evaporate most of the ammonia, and then add the desired solvent. This method is promising but has not been tested extensively. Once the powder or crystals have been washed several times with diethyl ether and dried by evaporation of the residual ether, they tend to be stable at low temperatures.
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POWDER
METAL $ ~ ~ ~ # c T I o METAL N CMMBER WITH GLASS-COVERED STIR BAR MAW CHAMBER
Flgure 3. Apparatus for the preparation of alkalkie salts. The storage vessel on the side arm is often replaced by an array of glass tubes for the preparatlon of multiple samples.
However, another problem can arise if a vacuum seal-off is made near the preparation bulb. On numerous occasions, a seal-off under dynamic vacuum while the crystals were kept cold with liquid nitrogen led to rapid and irreversible decomposition of d of the solid sample. Evidently some residual material at the seal-off point was volatilized and started the decomposition process. To eliminate this problem, we found it necessary to transfer the powdered sample into a clean storage vessel as shown in Figure 3. This vessel, which alternately can contain a number of sample tubes, is never exposed to the liquid solution. Vacuum seal-off of individual sample tubes with the contents kept cold with liquid nitrogen generally does not lead to decomposition, Even when some of the sample wm near the seal-off point and was therefore decomposed by the heat, the bulk of the sample remained unchanged. The best crystals of Na+C222*Na-have been obtained by recrystallization of powered samples prepared as described above. When this was tried with a sample precipitated from a solution of stoichiometry K+C2224a- in isopropylamine, metallic sodium formed while all of the potassium and some of the sodium remained in solution. The solution retained its dark blue color so that, if solution decomposition were responsible for the disappearance of Na-, it was not complete. Problems with decomposition have so far prevented the preparation of crystals suitable for structure determintation except for the compound Na+C222.Na-. This material is much more stable than the other compounds and is easier to prepare. Although crystals of Na+C222*Na-are sensitive to photodecomposition they are stable for at least several days in the dark at room temperature. We have measured the conductivity of packed powders of this compound at temperatures up to 45 "C over a period of hours without noticeable decomposition. Although the crystals are reactive toward moisture, they do not seem to be very reactive toward dry air, especially when cold. Crystals which were accidentally exposed to a dry ice-2-propanol bath remained unchanged in appearance for several hours. Even the condensation of frost on cold crystals does not result in decomposition unless the temperature is raised.
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the potassium NMR spectra of K'C222,K- in THF have been unsuccessful, perhaps because of exchange with paramagnetic species. Only the peak of K T 2 2 2 was observed.67 The study of mixed metal systems by NMR promises to be a useful technique for identification of species in solution. For example, when Na and C222 in a 2:l ratio were dissolved in ethylamine, sodium-23 NMR peaks were observed for both Na+C222 and Na-. However, when K, Na, and C222 were dissolved in equimolar amounts, only the peak of Na- was observed.67This is in accord with the formation of K+C222,Na- in solution. Optical spectra of solutions also verify that K- reacts readily with Na+ to yield Na-.@ The sodium-23 NMR signal of Na"C222 in solutions of Na+C222,Rr- in ethylenediamine disappears when KBr is added showing that K+ can replace Na+ in the cryptand .69 In order to determine the relative stabilities of M+C222 and M- for various alkali metals in solution, we examined the alkali metal NMR spectra of solutions prepared by contact with two metals both present in excessSB7When K-Cs or Rb-Cs alloys were used with C222 in THF, Cswas readily detected but Cs'C222 was not. We conclude that WC222,Cs- or Rb+C222,Cs-were the dominant solute species. When Na-Cs alloys were used, Cs+C222,Cs-and Na- were all detected but, surprisingly, Na+C222 was not. When the solution first contacted Cs metal and then Na metal, only Cs+C222,Na- was observed. However, when the solution first contacted Na metal and then Cs metal, Cs'C222 and Cs- were observed with only a small signal for Na-. These results are complicated by the slow dissolution rate of Na metal and the low solubility of Na+C222aNa- in THF. We have recently obtained the solid-state sodium-23 spectrum of a polycrystalline sample of Na+C222,Na-."O The signals of both types of sodium species were detected although the line widths were very broad, presumably because of dipolar couplings to protons. Line widths of 2800 and 4700 Hz (full width at half-height) were observed for Na- and Na'C222, respectively. By using appropriate techniques to minimize dipolar broadening or by utilizing Measurement Techniques and Some Recent saturation transfer methods one might be able to use Results solid-state NMR methods to identify species in crystalline Measurement techniques used in our laboratory for the and powdered samples of M+C-N-. study of solution conductances, emf, optical spectra, EPR Optical Spectra. The most informative data about solid spectra, alkali metal NMR spectra, etc. have been de~ ~will ~ not ~ ~be" ~ ~alkalides ~ ~ ~ and ~ ~electrides ~ ~ ~ as ~ well ~ as concentrated solutions scribed in detail p r e v i o ~ s l y and has come from the transmission spectra of thin films deconsidered here. posited on the windows of quartz optical cells. This Alkali Metal N M R Studies. Alkali metal NMR studtechnique and our most recent results are described in ies7p9of solutions containing Na-, Rb-, or Cs- show that detail in a companion paper.49 these species do not consist of a solvated cation in the Magnetic Studies. By evaporating solutions of M+vicinity of a pair of electrons but rather are genuine cenC222,e; or M+18C6,e; in ammonia or methylamine, it is trosymmetric anions. possible to obtain dark blue powders or films whose optical The presence of M- in solution, as detected by optical spectra suggest the presence of either trapped or free spectroscopy, does not guarantee that it can be detected electrons (or both). We have examined several of these by alkali metal NMR spectroscopy. For example, solutions systems by electron paramagnetic resonance (EPR) and of Na+l8CG,Na- in methylamine show two peaks only at magnetic susceptibility techniques. The apparatus used low temperature^.^ Above about -5 "C a single peak is to produce samples for magnetic susceptibility studies is observed because of exchange of the sodium nucleus beshown in Figure 4. Static susceptibilities show that the tween N a W C 6 and Na-. Solutions of cesium in ethyl"electride" prepared from cesium and 18-crown-6 is amine in the presence of C222 at a mole ratio (cs)/(c222) paramagnetic while that from potassium and 2,2,2-cryp= 2 contain high relative concentrations of Cs- compared tand is diamagnetic. The former has a film spectrum with e; as indicated by the optical spectrum. However, characteristic of trapped electrons while the spectrum of while the (25-133 NMR peak of Cs'C222 can be easily the latter resembles the plasma absorption of a metal.4g detected, no peak of Cs- has been seen in this solvent.67 Both systems have strong EPR absorptions centered at the Presumably, exchange with paramagnetic species such as free-electron g value which show the low-temperature Cs+-e; broadens the NMR signal to such an extent that characteristics of a powder pattern.71 The results are it cannot be detected. On the other hand, in THF both preliminary in nature but agree with the presence of peaks are observed at low temperatures. Attempts to study By contrast, crystals containing K-, Rb-, or Cs- are much more reactive. Another way to prepare solid samples of MT222.N- or M'C222-e- as well as solids which contain crown ethers is to rapidly evaporate the solvent from a concentrated solution. Because of inevitable heterogeneities which result, this procedure is not as satisfactory as the precipitation technique, but when the latter method fails, it can provide powdered samples for the study of magnetic properties, electrical conductivity, etc. By using this technique to form thin, transparent films, suitable for transmission spectroscopy, we have obtained considerable information about the species present.62 This method is described in detail in a companion paper.49 Analysis Methods. When powders or films are prepared by solvent evaporation, the overall analysis is determined by the initial stoichiometry so that only tests for decomposition and the presence of solvent are important. However, when precipitates or crystals can be isolated, then elemental analysis becomes useful. Because of the ease of decomposition of these materials and their tendency to react with air and moisture, one must be certain that commercial analysts handle them properly in an inert atmosphere box during weighing and sample introduction. We have had some problems with apparent oxidation of reactive samples prior to or during analysis. One of the best ways to tell whether a sample has decomposed is to first degas the sample, and then measure the volume of hydrogen produced when the sample is decomposed with water or an ammonium salt. This technique has been previously described in detail.63 Having decomposed a sample with water, one can use flame emission or atomic absorption to determine the total alkali metal content and to identify the metals present. By adding standard HC1 and back-titrating with NaOH it is possible to determine the amount of metal hydroxide formed and the amount of cryptand present. Finally, we have found proton NMR analysis to be useful for determining the quantity of cryptand, crown, and solvent present in a sample.62
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The Journal of Physical Chemistry, Vol. 84, No. 10,
E“ lr’
A
FUSED O U k T Z ESR TUBE
11
Figure 4. Fused silica apparatus for the preparation of samples by solvent evaporation. The unlformity of fused silica EPR tubes permits the use of such lubes in magnetic susceptibility studies with a separate tube used for cnllbration.
localized traps for Cs+18C6.e- and delocalization of the electrons for K+C222.e-. Samples of NatC222.Na- are diamagnetic, show no appreciable EPli absorption, and have the temperature-dependent powder conductivity expected for a semiconductor with a band gap of 2.4 eV. The resistivity of packed powders varies exponentially with 1/T over at least six orders of magnitude of r e s i ~ t a n c e . ~ ~
Possible Areas of Future Research Although the existence of solid salts containing alkali metal anions has been conclusively demonstrated, and the general properties of a few of them have been determined, much remains to be done in this area. The existence of “electride” salts, in which the positive charge on a complexed cation is balanced by trapped electrons, is strongly indicated by optical and EPR spectra, but the structure of such ,systems is a matter of speculation at present. We need to be able to precipitate “electride” salts from solution in order to determine their true stoichiometry. The ultimate goal of such studies would be to grow single crystals for X-ray structure determination. Several of the systems made by evaporating solutions of stoichiometry M+C,e- have optical and magnetic properties which suggest that they are “expanded metals” with the charge on the complexed cations balanced by electrons in a conduction band. Obviously we need to determine the conductivities of these films or powders in order to test this hypothesis. By attaching thin-strip electrodes to the inner windows of quartz optical cells it should be possible to measure both the conductivity and the photoconductivity of thin [solid films of alkalides and electrides. It would also be informative to study the photoelectric behavior of such films. The optical spectra of films suggest the presence of both transient and permanent electron traps. It would be of interest to study the changes in spectra accompanying light absorption by M- for example. Both steady-state and flash photolysis methods should be explored. All of the solid materials described in this paper utilize a complexed alkali cation as the counterion for M- or e-. It might be possible to replace M+C by some other cation.
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Such a cation must be resistant to reduction, which severely limits the choice. It appears at this time that among nonmetallic cations, tetraalkylammonium ions, R4N+,are the best candidates. Thermodynamic estimates of the stability of salts such as Ba2+C222.(Na-)2 and 13a2+C2224e-)2indicate50that cryptated divalent ions 01’ this type should be good candidates for the synthesis of new salts. Potential Uses for Alkalide Salts. The only potential use for salts of the alkali metal anions that seems certain is as reducing agents in aprotic solvents. The salt Na+C222.Na- can be readily prepared, stored indefinitely in vacuo at low temperatures, and dissolved in a solvent such as THF when needed to provide a two-electron reducing agent. The cost of C222 is so high, however, that this will probably not be practical until a substitute cation cain be found. The light-absorbing and semiconducting properties of Na+C222.Na- and the ease of forming thin films from solution would probably be useful if the compound $were more stable. Perhaps the instabilities result from impurities so that alkalide salts produced from very ,pure materials might be stable enough to take advantage of their electrical properties. Because electrons are only weakly bound in alkalide and “electride” salts it is possible that photoelectron emimion might occur at long wavelengths. For example, Deluhay and co-workers have found73that solutions of sodium in hexamethylphosphorictriamide (HMPA) emit electrons thermionically at and below room temperatures. Once again, the usefulness of these substances as active phlotoelectron emitters would depend upon their stability. Finally, there is much current interest in batteries which utilize alkali metals such as sodium or lithium to provide a cathodic reaction. In one approach, liquid solutions are used. Such solutions must provide both high conductivities and resistance to reduction by the metals. Research on alka€i metal solutions in a variety of solvents should be useful in the attempt to find such solutions. Acknowledgment. This research was supported by Grant No. DMR-77-22975from the U.S.National Science Foundation. I am most grateful to my students and other co-workers who have developed the methods describeid in this paper and whose names appear in the references. Particular credit for some of the unpublished methods and observations described here is due to M. G. DaGue, M. R. Yemen, P. B. Smith, B. Van Eck, H. L. Lewis, and R. C. Phillips.
References and Notes (1) Foc a detailed discussion of the properties of metal-ammonia solultbns see J. C. Thwnpm, “Electrons in Uquld Ammonia”, Oxford University Press, Oxford, 1976. (2) Review articles on metal-ammonia solutions include the follourina: (a) C. A. Kraus, J. Chem. Educ., 30,83 (1953); (b) W. L. Jolly, I w g . Chem., 1, 235 (1959); (c) M. C. R. Symons, 0. Rev. &em. SOC., 13, 99 (1959); (d) U. Schindewolf, Angew. Chem., 80, 165 (1968); Angew. Chem., Int. Ed. Engl., 7, 190 (1968); (e) T. P. [)as, Adv. Chem. Phys., 4, 303 (1962); (f) J. L. Dye, Sei. Am., 218, 77 (Feb. 1967); (9) M. H. Cohen and J. C. Thompson, Adv. Phys., 17, 857 (1968). (3) A collection of “landmark papers” in the metal-ammonia field has been given by W. L. Jolly, “Metal-Ammonla Solutions”, Dowclen, Hutchinson, Ross, Stroudsberg, Pa., 1972. (4) Papers resulting from conferences in the field of metal-ammonia solutions and the broader general area of electrons in fluids are (a) G. Lepoutre and M. J. Sienko, Ed., “Metal-Ammonia Solutlons”, Colloque Weyl I, W. A. Benjamin, New York, 1964; (b) R. F. Gould, A&. Chem. Ser., No. 50 (1965); (c) J. J. Lagowskl and M. J. Slenko, Ed., “Metal-Ammonla Solutions”, Colloque Weyl 11, IUPAC, Butterworths. London, 1970; (d) U. Schindewolf, Ber. Bunsenges. Phys. Chem., 75, (1971); (e) J. Jortner and N. R. Kestner, Ed., “Electr~Dns in Fluids”, Colloque Weyl 111, Springer-Verlag, Berlin, 1973; (f) “Cdbque Weyl N. Electronsin Fluids--The Nature of MetaCAmmcmia
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Solutions”, J. Phys. Chem., 79,no. 26 (1975);(9) Can. J. Chem., 55, 1796-2277 (1977),“International Conference on Electrons in Fluids, Banff, Alberta, Canada, Sept 5-1 1, 1976. (5) S. Matalon, S. Golden, and M. Ottolenghi, J. Phys. Chem., 73,3098 (1969). (6) J. L. Dye, J. M. Ceraso, M. T. Lok, B. L. Barnett, and F. J. Tehan, J. Am. Chem. Soc., 96, 60811974). (7) J. M. Ceraso and J. L. Dye, J. Chem. Phys., 61, 1585 (1974). (8)F. J. Tehan, B. L. Barnett, and J. L. Dye, J. Am. Chem. Soc., 96, 7203 (1974). (9)J. L. Dye, C. W. Andrews, and J. M. Ceraso, J . Pbys. Chem., 79, 3076 (1975). (10)J. L. Dye, C. W. Andrews, and S. E. Mathews, J . Pbys. Cbem., 79, 3065 (1975). (11) L. J. Gilling, J. G. Kloosterboer, R. P. H. Rettschnick, and J. D. W. van Voorst, Chem. Phys. Lett., 8,457,462 (1971). (12)L. M. Dorfman, F. Y. Jou, and R. Wagemen, Ber. Bunsenges. Phys. Chem., 75, 681 (1971). (13)J. W. Fletcher and W. A. Seddon, J. Phys. Chem., 79,3055 (1975). (14) R. Catterali, J. Slater, W. A. Seddon, and J. W. Fletcher, Can. J. Chem., 54, 3110 (1976). (15) W. A. Seddon, J. W. Fletcher, F. C. Sopchyshyn, and R. Catterall, Can. J. Chem., 55,3356 (1977). (16) W. A. Seddon, J. W. Fletcher, and R. Catterall, Can. J. Cbem., 55, 2017 (1977). (17) J. L. Dye, Pure Appl. Chem., 49,3 (1977). (18)J. L. Dye, M. T. Lok, F. J. Tehan, R. B. Coolen,N. Papadakis, J. M. Ceraso, and M. DeBacker, ref 4d, p 659. (19) M. T. Lok, F. J. Tehan, and J. L. Dye, J. Phys. Chem., 76,2975 (1 972). (20)J. L. Dye in ref 4e,p 77. (21) J. L. Dye, J. Chem. fduc., 54,332 (1977). (22) M. Gold, W. L. Jolly, and K. S.Pitzer, J. Am. Chem. Soc.,84,2264 (1962). (23) J. L. Dye in ref 4c, p 1. (24) C. J. Pedarsen, J. Am. Chem. Soc., 89,7017(1967);92,386(1970). (25) C. J. Pedersen, Fed. Proc., 27, 1305 (1968). (26) B. Dietrich, J.-M. Lehn, and J. 8. Sauvage, TetrahedronLett., 2885, 2889 (1969). (27) J.-M. Lehn, J. P. Sawage, and B. Dietrich, J. Am. Chem. Soc., 92, 2916 (1970). (28) The trivial name 18-crown-6 (18C6)will be used for the cyclic polyether, 1,4,7,10,13,16-hexaoxacyclooctadecane, while 2,2,2cryptand (‘2222) refers to the macrobicyclic polyether 4,7,13,16,21,24-hexaoxa-lI lO-diazabicycio[8.8.8]hexacosane.Other cryptands such as C211 are similar with the numbers referring to the number of ether oxygens in each strand.
(29) K. D. Vos and J. L. Dye, J. Chern. Phys., 38, 2033 (1963). (30) K. Bar-Eli and T. R. Tuttle, Jr., Bull. Am. Phys. Soc., 8,352 (1963). (31) M. Ottolenghi, K. Bar-Eli, H. Linschitz, and T, R. Tuttle, Jr., J. Chem. Phys., 40,3729 (1964). (32) K. Bar-Eli and T. R. Tuttle, Jr., J. Chem. Phys., 40,2508 (1964). (33) L. R. Dalton, J. D. Rynbrandt, E. M. Hansen, and J. L. Dye, J. Chem. Phys., 44,3969 (1966). (34) R. Catterall, M. C. R. Symons, and J. W. Tipping, J . Chern. Soc. A , 1529 (1966);1234 (1967). (35) J. L. Dye and L. R. Dalton, J . Phys. Chem., 71, 184 (1967). (36) R. Catterall, J. Slater, and M. C. R. Symons, J . Chem. Phys., 52, 1003 (1970).
We
(37) V. A. Nicely and J. L. Dye, J . Chem. Phys., 53, 119 (1970). (38)R. Catterall, M. C. R. Symons, and J. W. Tipping in ref 4c,p 317. (39)R. Catterall, J. Slater, and M. C. R. Symons in ref 4c, p 329. (40)R. Catterall, I. Huriey, and M. C. R. Symons, J . Chem. Soc., Dalton Trans., 139 (1972). (41) R. Catterall and P. P. Edwards, J. Phys. Chem., 79,3010 (1975). (42) For references to theoretical papers prior to 1975 see M. D. Newton, J . Phys. Cbem., 79,2795 (1975). (43) R. H. Land and D. E. O’Rellly, J. Chem. Phys., 46,4496 (1967). (44)K. Fueki, J. Chem. Pbys., 50,5381 (1969). (45)D.+. Feng, K. Fuekl, and L. Kevan, J. Chem. Phys., 58,3281 (1973). (46)D. A. Copehnd and N. R. Kestner, J. Chem. Phys., 58,3500 (1973). (47)N. R. Kestner and J. Logan, J. Phys. Chem., 79, 2815 (1975). (48) S.Golden, C. Guttman, and T. R. Tuttle, Jr., J. Am. Chem. Soc., 87, 135 (1965);J . Chem. Phys., 44, 3791 (1966). (49) J. L. Dye, M. G. D a m , M. R. Yemen, J. S. Landers, and H. L. Lewis, J. Pbys. Cbem., paper In this issue. (50) J. L. Dye, Angew. Chem., Int. Ed. fngl., 18,587 (1979). (51)J. L. Dye in “Progress in Macrocyclic Chemistry”, Vol. 1, J. J. Christensenand R. M. Izatt, Ed., Wiley-Interscience, New Yo&, 1979. (52) I. Hurley, T. R. Tuttle, Jr., and S.Golden, J. Chem. Phys., 48,2816
(1968). (53) F. J. Tehan and J. L. Dye, Anal. Chem., 47, 1876 (1975). (54)Alkali metals under argon in break-seal ampules are available from Alfa Div., Ventron Corp., 152 Andover St., Danvers, Mass. 01923. (55) Heat-shrinkableTeflon tuMng (Fbtke) is avallable in various dhmeters from Pope Scientific Co., P. 0. Box 495,Menomonee Falls, Wisc. 53051. (56)M. J. Sienko, private communication. (57) Irradiated polyolefin heat-shrinkable tubing is available from most electronics supply houses. We use type 221 from Alfa Wire Corp., 711 Lidgerwood Ave., Elizabeth, N.J. 07207. (58) V. A. Nicely, Ph.D. Dissertation, Michigan State University, 1969. (59)Teflon Vacuum Valves are available from Kontes of Illinois, 1916 Greenleaf St., Evanston, Ill. 60204. (60) Solv-Seal Teflon vacuum joints are available in various sizes from Fischer & Porter Co, Lab-Crest Scientific Division, Warminster, Pa. 18974. The external plastic couplings used to hold the joints together are superior to ball-jdnt clamps for this purpose. However, the inserts for these couplings have recently been made of a plastic which is so stiff that we have had to fabricate our own from Teflon in order to prevent glassware breakage. (61) L. H. Feldman, R. R. Dewald, and J. L. Dye, Adv. Chem. Ser., No.
50, 163-172 (1965). (62) J. L. Dye, M. R. Yemen, M. G. DaGue, and J.-M. Lehn, J. Chem. Phys., 68, 1665 (1978). (63) R. R. Dewaid and J. L. Dye, J. Pbys. Chem., 68, 128 (1964). (64) R. C. DouthS and J. L. Dye, J . Am. Chem. Soc., 82,4472 (1960). (65) J. L. Dye, R. F. Sankuer, and G. E. Smlth J . Am. Chem. Soc., 82, 4797 (1960). (66) J. L. Dye, G. E. Smith, and R. F. Sankuer, J. Am. Chem. Soc., 82, 4803 (1960). (67)P. B. Smith, Ph.D. Dissertation, Michigan State University, 1978. (68)M. G. DeBacker and J. L. Dye, J. Phys. Chem., 75,3092 (1971). (69)J. M. Ceraso, Ph. D. Dissertation, Michigan State University, 1975. (70) B. Van Eck, unpublished observations, this laboratory. (71)M. G. DaGue, unpublished observations, this laboratory. (72)M. R. Yemen, unpublished observations, this laboratory. (73)B. Baron, P. Delahay, and R. Lugo, J. Chem. Phys., 53,1399 (1970).
