J. Phys. Chem. 1984,88, 3842-3846
3842
Recent Developments in the Synthesis of Alkalides and Electrides James L. Dye Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 (Received: August 25, 1983; In Final Form: December 13, 1983) Two improved synthesis methods have permitted the isolation of a number of new crystalline alkalide salts as well as the first crystalline electride. One method uses dissolved lithium to stabilize solutions in 2-aminopropane, while the second uses mixtures of dimethyl ether with either trimethylamine or diethyl ether as crystallization solvents. The optical, electrical, and magnetic properties of a number of sodide salts are discussed. Relative stabilities of seven crystalline sodides, a ceside, and an electride were determined by differential scanning calorimetry. A new sodide, Rb+(15-~rown-5)~.Na-, is the most stable, melting at 75 "C to form a blue liquid which can be heated rapidly to 100 "C before decomposing irreversibly. The synthesis, characterization, and properties of a crystalline ceside, Cs+(18C6)2.Cs-, and a crystalline electride, Cs+(18C6),.e-, are described.
Alkalides are salts in which the negative species are alkali metal anions. The key to the stabilization of alkalides is complexation of the cation by a crown etherla or cryptand.lb Figure 1 gives the structural formulas of 18-crown-6 (1 8C6) and cryptand [pqr] (Cpqr) in which p,q,r represent the number of ether oxygens in each strand (p = m 1, q = n 1, r = o 1). Coordination of the alkali metal cation by the ether oxygens (and nitrogens of the cryptand) gives enough stability to prevent reduction of M + to the metal in tho presence of M- or e-. The "parent compound", N a +C222.Na-, has been fully characterized.2 Its crystal s t r ~ c t u r eoptical ,~ transmission spectrum,4g5powder conductivity,6 thermodynamics of f ~ r m a t i o nand , ~ 23NaN M R spectrums have been described. A number of other sodide salts, M+L.Na-, in which M is Li, K, Rb, or Cs and L is a crown ether or cryptand, have also been synthesized r e ~ e n t l y . ~ ,Other ~ * ~ ~alkalides, M+LaM-, in which M is K, Rb, or Cs, have been prepared as thin f i l m ~ ~ > ~and J ' - 'some ~ have been crystallized: but the generally poor stability of these compounds has hampered efforts to characterize them completely. Recently, crystalline solids of stoichiometry Cs18C614 and Cs( 18C6)215were synthesized. The former has now been shown by 133CsN M R spectro~copy'~ to be Cs+( 18C6)2.Cs-, the first crystalline ceside which is stable a t ambient temperatures. The new techniques described in this paper should allow the preparation of a number of other crystalline alkalides. Electrides are salts in which the anions are trapped electrons and the positive ions are complexed alkali metal cations. The simplest picture of their structure (which has not, however, been experimentally verified) is electron trapping at anion vacancies, as with F centers.I6 Since electrons are the only anions in the
+
+
+
(1). (a) Pedersen, C. J. J . Am. Chem. SOC.1967,89, 7017. 1970,92, 386. (b) Dietrich, B.; Lehn, J.-M.; Sauvage, J. P. Tetrahedron Lett. 1969, 2885, 2889. J . Am. Chem. SOC.1970, 92, 2916. (2) Dye, J. L.; Ceraso, J. M.; Lok, M. T.; Barnett, B. L.; Tehan, F. J. J. Am. Chem. SOC.1974,96, 608. (3) Tehan, F. J.; Barnett, B. L.; Dye, J. L. J . Am. Chem. SOC.1974, 96, 7203. ~ .
.
(4) Dye, J. L.; Yemen, M. R.;DaGue, M. G.; Lehn, J.-M. J. Chem. Phys. 1978. __ .-, 68. 1665. ( 5 ) Le, L. D.; Issa, D.; VanEck, B.; Dye, J. L. J. Phys. Chem. 1982, 7, 86. (6) Dye, J. L. Angew. Chem., Int. Ed. Engl. 1979, 18, 587. (7) Schindewolf, U.; Le, L. D.; Dye, J. L. J . Phys. Chem. 1982,86,2284. - - I
( 8 ) Ellaboudy, A.; Tinkham, M. L.; Smith, P. B.; VanEck, B.; Dye, J. L. J. Phys. Chem.; this issue. (9) Van Eck, B.; Le, L. D.; Issa, D.; Dye, J. L. Inorg. Chem. 1982, 21, 1966. (10) Dye, J. L. In "Progress in Inorganic Chemistry"; Lippard, S., Ed.;
Wiley-Interscience: New York, to be published. (11) Dye, J. L.; DaGue, M. G.; Yemen, M. R.; Landers, J. S.; Lewis, H. L. J . Phys. Chem. 1980,84, 1096. (12) DaGue, M. G.; Landers, J. S.; Lewis, H. L.; Dye, J. L. Chem. Phys. Lett 1979, 66, 169. (13) Dye, J. L. J. Phys. Chem. 1980,84, 1084. (14) Issa, D.; Dye, J. L. J . Am. Chem. SOC.1982, 104, 3781. (15) Ellaboudy, A.; Smith, P. B.; Dye, J. L. J. Am. Chem. SOC.1983,105, 6490. (16) Markham, J. J. "F-Centers in Alkali Halides"; Academic Press: New York, 1966.
salt, this view requires that all of the anionic sites contain electrons. Until very recently, electrides could be prepared only as reactive films or powders by solvent e ~ a p o r a t i o n ~ ~or~ vapor ~ J ~ 3depo~ition.~ '~ We have now dem~nstrated'~ that the recently prepared crystalline compound Cs( 18C6)2is an electride, Cs+(l8C6)2.e-. It is likely that other stable electrides can be synthesized by the same techniques. Because of electron-electron interactions and delocalization, electrides can show a variety of properties ranging from electron spin pairing17,19to apparent metallic or near-metallic conductivity,'2-33,34 Dysonian line shapes in the EPR spectra,12,15,19,33,34 and plasma-type absorption Crystalline alkalides and electrides provide the major focus of this paper. However, studies of films and powders prepared by solvent evaporation from solutions of the appropriate stoichiometry have provided much information about the nature of these compounds. In many cases, when the solution stoichiometry is (M')+L,M-, the transmission spectrum shows that the film produced by solvent evaporation is the corresponding alkalide.4*'lJ2 When the solution contains mainly M+L and e;, electrides are usually f0rmed!9"3'~?'~ The presence of both M- and e; in solution often leads to complex film spectra and the appearance of new or shifted absorption The preparation of alkalides and electrides by solvent evaporation has a number of disadvantages: (1) All nonvolatile solutes, including decomposition products and excess complexant or metal, are present in the sample. (2) Nonuniformity can result from selective precipitation: the solid formed first may not be the same as that which forms as the last traces of solvent are removed. (3) Films tend to be of nonuniform thickness so that absorption bandshapes are distorted. (4) Solids can be produced in nonequilibrium forms whose properties then change with time as the sample "anneals". The ability to prepare alkalide and electride films by codeposition of the complexant and metal5 promises to eliminate many of these problems. We have recently constructed a bell-jar evaporator18 which permits the preparation of uniform films of controlled composition and thickness for optical and electrical studies. Synthesis of Crystalline Alkalides and Electrides Were it not for the tendency of metal solutions in amines and ethers to decompose autocatalytically, the preparation of crystalline alkalides and electrides would be straightforward. Stoichiometric amounts of the complexant and metal are dissolved in ammonia or methylamine; most of the solvent is removed by evaporation and a suitable cosolvent or mixture is added. We have used 2-aminopropane, either alone or mixed with diethyl ether (and sometimes n-pentane), as the precipitation s ~ l v e n t . ~Although ,~~ (17) Landers, J. S.; Dye, J. L.; Stacy, A,; Sienko, M. J. J. Phys. Chem. 1981,85, 1096. (18) Faber, M.; Dawes, S.,unpublished results, this laboratory. (19) Issa, D. Ph.D. Dissertation, Michigan State University, 1982.
0022-3654/84/2088-3842%01.50/0 0 1984 American Chemical Society
Synthesis of Alkalides and Electrides
The Journal of Physical Chemistry, Vol. 88, No. 17, 1984 3843
A
(0) (b) Figure 1. Structural formulas of 18-crown-6 and a general cryptand.
these methods have been successful for a number of sodides and a few other alkalides? irreversible decomposition frequently occurred even in favorable cases, and always occurred in attempts to crystallize electrides. Since the decomposition reaction is catalyzed by impurities, rigorous cleaning of the preparation vessels was required. The procedures have been described in detailgJ0J3 and only recent modifications which have increased the stability and ease of synthesis will be considered here. Stabilization by Lithium. It was noted by I ~ s a ' ~that * ' ~solutions that contained cesium, lithium, and 18-crown-6 were much more stable than the corresponding solutions without lithium present. For example, the presence of lithium permitted addition of 2aminopropane and diethyl ether to the residue left by nearly complete evaporation of methylamine. The resulting solution was stable, in contrast to the rapid decomposition which occurred in the absence of lithium. The added stability allowed precipitation of stable, well-formed crystals upon cooling. Surprisingly, these dark blue crystals contained very little lithium and had the simple stoichiometry Cs18C6.14 W e have since found that solution stabilization by dissolved lithium is a rather general phenomenon. A puzzling feature of these results was the solubility behavior of lithium. Pure lithium is insoluble in 2-aminopropane and its mixtures with diethyl ether. Yet the solutions are clearly homogeneous and can have lithium concentrations as high as 0.5 M. Recent studies by FaberZ0have shown that lithium metal is insoluble in these solvents, but Li(CH3NH2)4is very soluble. This explains the decomposition behavior of certain solutions of lithium, cesium, and 18C6. When all of the methylamine was removed prior to the addition of 2-aminopropane, the lithium did not dissolve and the resulting solution of cesium and 18C6 decomposed rapidly. Similarly, attempts to redissolve Csl8C6 crystals in purified 2-aminopropane without lithium present resulted in immediate decomp~sition.'~ The role played by lithium in stabilizing the solutions has not been determined. Perhaps it removes free radicals which are responsible for autocatalytic decomposition. This might occur by the formation of lithium alkyls according to Li', e;
+ R-
-
LiR
(1)
Synthesis Apparatus. The synthesis apparatus used to prepare crystalline alkalides and electrides in sealed tubes is shown in Figure 2A. The metals are introduced into the side arms of flask B by using heat-shrinkable Teflon tubing as previously When lithium is used, a sintered glass frit is present in its side arm to keep the lithium pieces and glass fragments out of the flask until the lithium can be dissolved in methylamine and filtered through the frit. The complexant is introduced into flask A through the side arm which is then sealed off. After evacuation and distillation of the metals, (except lithium) into flask B, methylamine (or sometimes ammonia) is condensed in A and the solution of the complexant is poured through the frit into B. The metals dissolve to give a deep blue solution of the desired composition. Most of the methylamine is distilled into an external cooled flask until only a wet slurry remains in B. The cosolvent or solvent mixture is then distilled into B. The solvent composition (20) Faber, M., unpublished results, this laboratory.
B
Figure 2. A. Apparatus for the preparation of crystalline alkalides and electrides in sealed tubes. B. H cell for the synthesis of alkalides and
electrides. is adjusted until crystals just begin to form at -30 to 0 "C (depending on the sensitivity of the particular system to decomposition). The solution is filtered through the frit into A and cooled to yield the precipitate. At this stage additional solvent is sometimes removed by distillation in order to increase the yield, or additional diethyl ether is condensed into A. The mother liquor is then poured into B and frozen with liquid nitrogen. Flask B is then removed by making a seal off at the construction. The washing liquid (diethyl ether or n-pentane) is distilled into A and the slurry of crystals and liquid is poured into chamber C. By repeatedly pouring the supernatant into A and condensing fresh washing liquid into C by cooling it, the crystals can be thoroughly washed. Finally, the liquid in A is frozen with liquid nitrogen, torr for about 1 h (while keeping the system is pumped to the crystals cool if necessary), and chamber C is removed by making a seal off at the constriction. By inverting vessel C the polycrystalline sample can be distributed into the tubes which are then individually removed by making seal offs while the tubes are immersed in liquid nitrogen. The preparation of alkalides and electrides in this way is a complex procedure and requires construction of a new apparatus after each synthesis. Until the introduction of the less reactive crystallization solvents described below, however, it was the only procedure which proved to be even moderately successful. For the preparation of thermally unstable alkalides and electrides, it is still the method of choice because all manipulations can be performed while keeping the sample cold. New CrystallizationSolvents. Studies of metal solution stability by PezZ1showed that tertiary amines are much less subject to
-
3844
The Journal of Physical Chemistry, Vol. 88, No. 17, 1984
attack by alkali metal anions than are primary or secondary amines. Also, radical chain reactions are more easily initiated by hydrogen atom abstraction from carbon atoms which are /3 to oxygen or nitrogen than from those which are in a or y positions. These considerations suggested that solvents such as trimethylamine (Me,N) and dimethyl ether (MezO) might be superior to 2-propylamine and diethyl ether as crystallization cosolvents. None of the alkali metals (Na through Cs) is soluble in Me3N even in the presence of crown ethers. The alkali metals do not dissolve unassisted in Me20, but at least K, Rb, and Cs dissolve readily in the presence of 18C6 as does a 1:l mixture of N a and K (presumably to give K'18C6 Na-). The solutions are very stable and lithium is not needed to enhance this stability. Indeed, when Me,N was distilled onto Li(CH3NHz)4,decomposition occurred immediately,20 so the use of lithium in these solutions should be avoided. We have recently found that appropriate combinations of the solvents methylamine, trimethylamine, and dimethyl ether (and sometimes also diethyl ether) make excellent crystallization media. If methylamine can be avoided entirely, the stability is even better. These solvents are readily purified with benzophenone and an excess of NaK alloy, although predrying of Me3N over CaHz is required. Simplified Apparatus and Procedures. The enhanced stability of alkalides and electrides in MezO and Me,N has permitted considerable simplification of the apparatus and procedures used to synthesize those compounds which are stable a t ambient temperatures. These include Na+C222.Na-, M+18C6.Na-, in which M = K, Rb, or Cs, K+(15C5)2-Na-, Rb+(15C5)2.Na-, Rb+(15C5)2-Na-,Csl8C6,Cs( 18C6),, and others whose stoichiometry has not yet been established. The preparation vessel, shown in Figure 2B, is a slight modification of the Airless-Ware H cell available from KontesVz2After purification, the solvents are stored in stainless steel tanks equipped with vacuum-tight gauges which register both vacuum and pressurez3and which can be attached to a dual-vacuum manifoldz4 via flexible-bellowsstainless steel tubing and Ultra-Torr fittings.z5 The H cell is attached to the manifold in the same manner. The added flexibility and the ease with which the apparatus can be attached and removed from the vacuum line provide an extremely convenient system. We have found that ethylene-propylene 0 rings2z are satisfactory for use with these solvents. The dual manifold utilizes Teflon valveszzthroughout and each port (consisting of a 15 mm O-ring joint) can be connected to either the high-vacuum manifold or the second manifold through which solvents are distilled. Before condensing them into the H cells, the solvents are prepurified just before use by condensing them into a bulb (attached to the manifold) which contains potassium metal and 18-crown-6. This gives a concentrated metal solution which reacts rapidly with reducible impurities. The preparation of Rb+l8C6aNa- will illustrate the method. A weighed amount of vacuum-sublimed ether was introduced into compartment A (Figure lB), and measured lengths of Rb and N a in calibrated tubes were introduced into B in an inert-atmosphere box. After evacuation to torr, the metals were distilled out of the tubes to form a mirror on the walls of compartment B. Approximately 20 mL of dimethyl ether was condensed into A at -78 OC, the crown ether was dissolved by warming the solvent to --15 OC, and the solution was poured through the frit into B. (Caution: Since M e 2 0 boils at -24 "C it should be kept at 0 O C or below when in glass vessels and adequate protection should be provided in case of breakage.) A deep blue solution formed immediately and the entire metal film could be dissolved in 10-20 min at -10 to -20 O C . An approximately equal volume of Me3N was then distilled into B and the relative amounts of Me3N and MezO were adjusted by distilling solvent out of the cell (primarily M e 2 0 is lost) until crystals just began to form at --lo OC. The solution was then poured through (21) Pez, G., personal communication. (22) Kontes Corp., P.O.Box 729, Vineland, NJ 08360. (23) Matheson Corp., P.O. Box 85, East Rutherford, NJ 07073. (24) Wayda, A. L.;Dye, J. L., to be published. (25) Cajon Corp., 9760 Shepard Road, Macedonia, OH, 44056.
Dye the frit into A and cooled to -78 OC, yielding rod-shaped crystals whose color closely resembled that of metallic copper. (If desired, the yield can be increased by distilling out additional solvent at this stage.) The mother liquor was poured into B and solvent was removed by distillation until the remaining solution in B was only a faint blue color (nearly pure Me3N). This provided a washing solvent which was repeatedly distilled into A and poured back into B. When the crystals were free of residue, all of the solvent was torr for distilled out of the H cell and it was evacuated to about 1 h while the crystals were kept at -0 OC. The H cell was then closed and removed to an inert atmosphere box where the crystals were transferred to a storage container. Since no seal offs are required, the H cells can be used repeatedly. The entire procedure requires 4-5 h and is straightforward and reliable.
-
Crystalline Sodides Salts which contain Na- tend to be much more stable than other alkalides. The probable reason for this increased stability is a lower concentration of trapped electons and the intrinsically higher thermodynamic stability of Na- compared with other alkali metal anions. As shown by absorption spectra,6*1° in solution the equilibrium
M+
+ 2e;
M-
(2)
lies further to the right for sodium than for any other metal.6*10 This means that precipitated sodides tend to have lower concentrations of trapped electrons than do other alkalides. Since electrides are less stable than alkalides, trapped electrons can provide sites for the initiation of decomposition. In addition, excitation of an electron from Na- to the conduction band requires more energy than for the other alkalides, so the population of thermally excited electrons is also smaller. Their greater stability has led to the synthesis of a number of crystalline sodides. All sodides are very reflective crystals which range from gold to dark bronze in color. Except for Na+C222*Na-, salts in which cryptands are used tend to be less stable than those with crown ethers. (However, Na'18C6,Na- has not been synthesized to date.) The sodides Li+C21 l-Na-, Na+C221-Na-, and Cs'C322.Na- have been synthesized and analyzed: as has Rb+C222~Na-?~Stabilities were marginal, but since most of these salts were crystallized from solvent mixtures which contained 2-aminopropane, the intrinsic stabilities might not have been achieved. The recent synthesis of crystalline Cs+l8C6.Na-,9 K'18C6Na-:*z6 and Rb'l 8C6.Na-26 showed that stable sodides could be prepared by using crown ethers. Current efforts are focussing on "sandwich" complexes of Cs+ and Rb+ with 18-cr0wn-6~~ and 5 ~attempts ~ to synthesize sodides Rb+ and K+ with 1 5 - c r o ~ n - in with stoichiometry M+Lz.Na-. Many sodides are stable up to their melting points and decompose irreversibly and exothermically at higher temperatures. When melting precedes significant decomposition, both can be monitored by differential scanning calorimetry (DSC). For example, the DSC trace for the stable sodide Rb+( 15C5)2.Na- is shown in Figure 3. The small endothermic transition at 35 OC might be caused by the melting of excess crown ether, by evaporation of remaining traces of solvent, or by the presence of electride. The compound melts at 75 OC and then decomposes irreversibly. The onset of rapid decomposition occurs at 100 O C with a heating rate of loo min-'. Table I gives the results of such DSC studies for a number of sodides. Although the decomposition temperature may vary somewhat from sample to sample (note, for example, the difference between the melting point of Na+C222-Na- originally observed visually2 and that obtained by DSC) the general trends can be easily established by DSC methods. The crystal structure of Na+C222,Na- shows that the cryptated cations, which pack in an A, B, C,-repeat pattern within
-
(26) Ellaboudy, A., unpublished results, this laboratory. (27) Tinkham, M. L. M.S. Thesis, Michigan State University, 1982. (28) Tinkham, M. L., unpublished results, this laboratory.
The Journal of Physical Chemistry, Vol, 88, No. 17, 1984 3845
Synthesis of Alkalides and Electrides I N
"
I
1
I
1
I
"
'
I
1
'
120
1
'
1
'
I
0.4
I I II I
,
30
50
O
70
I
I
I
90 TEMPERATURE ("C)
(
#
/-,-------,\
/
\
I
I
,
'
,
,
110
Figure 3. Differential scanning calorimeter trace for a 3.1-mg sample of Rbf(15C5)z.Na- at a heating rate of 10" min-'. TABLE I: Temperature of "Melting""and Decomposition of Some Alkalides and an Electride from Differential Scanning Calorimetryb dec temp, "C sample "mp", "C 10% peak Na+C222.Na73" 89 105 Rb+C222.Na50d 51 63 K'18C6.Na3Se 53 86 Rb'18C6.Na66 82 92 Cs'lSC6.Na38 84 103 K+(15C5)z.Na45 91 108 Rbt( 15C5)2.Na19 113 121 CS'( 18C6)2*C~66 40 95 Cst( 18C6)z-e36 40 60
Endothermic peak is presumed to result from melting. Heating rate = 10" min-I. 'Visually observedZat 83 "C. dInflection during decomposition. 'Small endothermic peak which could result from excess crown ether, solvent, or the presence of electride. /Additional small endothermic peak observed at 36-38 "C. a hexagonal unit cell, are essentially in van der Waals contact and the sodium anions occupy the pseudooctahedral holes. The packing pattern is shown in Figure 4. The Na+ to Na- distance is 7.06 A and the closest in-plane Na- to Na- distances are 8.83 A while the distance from an Na- in one plane to the nearest Nain an adjacent plane is 11.O A. No other structures are known a t this time. Crystals of Cs+18C6.Na- are monoclinic with a = 13.90 A, b = 15.50 A, c = 8.93 A, p = 93.3O, and z = 4.29 Thin sodide films produced by the rapid evaporation of methylamine solutions all show the intense absorption band of Na-. The absorption maximum for Na+C222.Na- occurs at 15400 cm-'. For M+18C6.Na-, the absorption maximum occurs at 14 000, 13 800, and 14 600 cm-' for M = K, Rb, and Cs, respectively.8 This sensitivity to the environment is similar to that observed for Na- and K- in solution as one changes the solvent.30 All of the sodides studied to date have dc powder conductivities which obey Ohm's law and which decrease exponentially with 1/T:' suggesting that they are semiconductors. However, in most cases they appear to be extrinsic semiconductors in which trapped electrons, rather than electrons from Na-, are thermally excited to the conduction band. The apparent band gap depends on the preparation and on time, and the extrapolation of plots of log g vs. 1 / T to infinite temperature yield low conductivities ( u , C 1 ohm-' cm-I). An exception is Na+C222.Na-, which has a reproducible band gap of 2.4 0.2 eV and a value of g- > IO6ohm-' cm-', characteristic of an intrinsic semiconductor. Films of Na'C222.Na- from methylamine evaporation show photocond~ctivity~~ for photon energies above about 1.5 eV, with
*
(29) Issa, D.; Ward, D., unpublished results, this laboratory. (30) Lok, M. T.; Tehan, F. J.; Dye, J. L. J . Phys. Chem. 1972, 76,1975. (31) Pappaioannou, J.; Jaenicke, S.; Dye, J. L., to be published. (32) Jaenicke, S.; Yemen, M.; Le, L. D.; Dye, J. L., unpublished results, this laboratory.
Figure 4. Packing diagram of Na+C222.Na-. The large cryptated cations are essentially in van der Waals contact and the sodium anions (solid circles) are in the pseudo-octahedral holes between the Na+C222 layers. I
r !
A Figure 5. Powder EPR spectrum of Rb'18C6sNa- at 263 K. The sharp central line occurs at g = 2.0023. The hyperfine peaks of 85Rbare identified by the downward-pointing arrows while the positions of the *'Rb peaks are given by the upward-pointing arrows.
a low quantum yield The magnitude of the photoconductive response decreases nearly exponentially with time as the films "age" even though they show no apparent decomposition. The results are complex and not completely understood at this time, but they suggest that the photoconductivity is a surface phenomenon resulting from excitation of trapped electrons rather than bulk photoconductivity from photoexcitation of Na-. Proof that crystalline compounds of stoichiometry MLNa (M # Na) are sodides, M+L.Na-, rather than salts of M-, was obtained by 23Na magic angle spinning N M R as described in a companion paper.8 Pure crystalline sodides are diamagnetic as determined by measuring the static susceptibilities. Except for Na+C222.Na-, however, trapped electrons are also present in sufficient concentrations to give strong EPR signals. In most cases either a single line at the free-electron g value or strongly overlapping lines are observed. However, for K+18C6-Na-, in addition to a narrow single line, a four-line hyperfine pattern is observed,28presumably
3846
The Journal of Physical Chemistry, Vol. 88, No. 17, 1984
from electron coupling to 39K. For the case of Rb+18C6,Na-, hyperfine coupling to both 85Rband *'Rb is clearly evident, as shown in Figure 5.26 A striking dependence of the width and shape of the hyperfine components on m, is clearly evident. N o coupling to 23Nais seen. Additional studies with single crystals are needed to determine the anisotropies of the g tensor and hyperfine tensor which are responsible for this effect. Compounds of Cesium with 18-Crown-6 Either by using lithium to stabilize solutions of cesium and 18C6 in 2-aminopropane or by using Me20-Me3N mixtures as the crystallization solvent, it was possible to prepare crystalline precipitates of stoichiometry Cs18C613 and Cs(18C6)2.14 The former precipitates as dark blue- to dark bronze-colored, rodshaped crystals, while the latter yields shiny, black, flat crystals. Both are reasonably stable at room temperature (hours to days in the absence of air). Both melt to form dark blue liquids which decompose rapidly. Differential scanning calorimetry showed that a sample of Cs18C6 began to decompose at -40 OC before melting at -60 OC, while C ~ ( 1 8 C 6apparently )~ melted at -36 O C and decomposed irreversibly a t 40 to 60 OC. Properties of Cs18C6. We originally pointed that Cs18C6 could be either an electride, Cs+18C6-e-, or a ceside, Cs+( 18C6)2.Cs-. The former choice was preferred, primarily because of the optical spectrum of thin films produced by solvent evaporation from solutions of the crystals in methylamine. The initial film had an absorption maximum at 9.5 X lo3 cm-', attributed to Cs-, but also showed high absorbance at (5-9) X lo3 cm-I, attributed to trapped electrons. When the film was warmed to 12 OC, the peak of Cs- decreased and the absorbance at -7 X lo3 cm-I increased. Thus, the stable film appeared to be an electride rather than a ceside. Since a sodide had been synthesized with the stoichiometry Cs18C6Na, it also seemed reasonable that if a ceside existed it would have the stoichiometry Cs218C6 (Cs+l8C6Cs-) rather than Cs18C6. The magnetic susceptibility and EPR spectra, described in detail in a companion paper,33showed that Cs18C6 is nearly diamagnetic but contains 1-2% unpaired spins. This behavior could result either from extensive spin pairing in an electride or from partial occupancy of anionic sites in Cs+(18C6)2.Cs- by trapped electrons. Powder conductivity studies of C ~ 1 8 C yielded 6 ~ ~ a band gap of 0.8 eV, too low for an intrinic ceside. However, the intercept a t infinite temperature corresponds to a specific conductance of only about 2 ohm-' cm-l, indicating extrinsic semiconductivity by defect electrons. Thus, the conductivity behavior could result either from a ceside which contains a substantial concentration of trapped electrons in anionic sites or from an electride in which the spin-paired electrons are in deeper traps than are the unpaired electrons. The definitive experiment which showed that Cs18C6 is indeed the ceside, Cs+(18C6),.Cs-, rather than an electride, was the study of the 133CsN M R spectrum by magic angle spinning (MASS) technique^.'^ Two peaks were observed, one at -61 ppm (relative to aqueous Cs+), characteristic of Cs+ in the "sandwich" complex, C ~ + ( 1 8 C 6 )and ~ , the other at -228 ppm, which is shifted so far upfield from Cs+ that it can only be attributed to Cs-. Properties of C ~ ( l 8 C 6 ) The ~ . existence of Cs+(18C6)2.Csprompted usI5to attempt to synthesize the corresponding electride, Cs+(18C6),.e-. The resulting crystalline compound, with the stoichiometry Cs( 18C6)2, has all of the characteristics expected for an electride in which electrons are singly trapped at anionic sites. The optical spectrum of thin films consists of a single narrow peak in the IR at 6650 cm-', as expected for a localized electride. The powder c o n d ~ c t i v i t y ' ~yields J ~ a band gap of 0.9 eV and an intercept at infinite temperature of lo2 ohm-' cm-I, suggesting that this compound is an intrinsic semiconductor. The magnetic s ~ s c e p t i b i l i t yis~ ~nearly that expected for an electride with magnetically isolated electrons and the EPR spectrum33consists
-
Dye of a single asymmetric line at g = 2.0023 with an asymmetry ratio, A/B, which increases with increasing temperature, from 1.3 at 100 K to 1.8 at 260 K, without significant change in the line width (0.5 G) or g value. Finally, the 133CsMASS-NMR spectrum of C ~ ( 1 8 C 6 shows )~ only a single peak at 6 = +81 from Cs+(aq). The paramagnetic shift of 140 ppm from that expected for Cs+ in a sandwich complex probably results from unpaired electron density at Cs+. Summary and Conclusions A number of crystalline sodides have now been synthesized and characterized. The exceptional stability of Na- compared with that of other alkalides permits the synthesis of M'LNa-, relatively free of contamination by M-. Some sodides, such as K'18C6.Naand Rb+l8C6*Na-,can be made with appreciable concentrations of trapped electrons. EPR studies of mixed sodide-electride salts of stoichiometry M+L-(Na-),(e-),- could provide valuable information about the environment oftrapped electrons, since the effects of electron-electron interactions could be minimized. Until recently, the outlook for the preparation of stable salts of K-, Rb-, and Cs- was poor because of their extreme instability. However, the new synthetic methods described in this paper have already resulted in the preparation of a stable ceside. Preliminary results which rubidium and 18C6 also suggest the formation of a stable rubidide. In view of the unusual thermal stability of Rb+(15C5)2-Na- and K+(15C5)2-Na-, it will be interesting to explore other alkalides and electrides which contain these sandwich cations. Thin, solvent-free films of alkalides and electrides prepared by rapid solvent evaporation have provided much qualitative information about these compounds. With the ability to prepare films by direct deposition of the metal and the complexant from the vap0r~9'~ it should be possible to carry out quantitative studies of spectra, conductivities, and photoconductivities of thin alkalide and electride films of known composition and thickness. The isolation of the relatively stable crystalline electride, Cs+( 18C6)2.e-, promises to open a new class of compounds for study. In this case, the electrons are sufficiently isolated from each other to form essentially a Curie-law paramagnet. It would be interesting to study the magnetic interactions in other crystalline electrides. For example, powdered samples of lithium with C211 exhibit temperature-dependent spin pairing, approaching diamagnetic behavior at liquid-helium temperatures." Preliminary studiesz6 of the crystalline "electride" formed from cesium and C222 (which shows no N M R peak of Cs-) show that it is essentially diamagnetic. Thus, electrides exhibit a wide variety of magnetic properties. The optical, electrical, and magnetic properties of films and powders of Li2C211,34K + C 2 2 2 ~ e -and , ~ ~ C ~ ~ 1 8 suggest C 6 ~ ~the presence of metallic character, indicating that electrides are at or near the borderline between metals and semiconductors. If crystalline electrides of this type could be synthesized, they would provide much more information than can be obtained with powdered samples. A major goal of our current research is to determine the crystal structures of electrides so that structureproperty relationships can be determined.
Acknowledgment. This research was supported by the National Science Foundation under Grant No. DMR-79-21979. Some of the work was done while the author was on sabbatical leave at Bell Laboratories in Murray Hill, N J , where advice from D. W. Murphy and A. L. Wayda was most helpful. Most of the research described in this paper was done by the graduate students and postdoctoral research associates named in the references, to whom special thanks are due. Registry No. Na+C222*Na-, 53536-71-3; RbC222.Na-, 90065-15-9; K+18C6.Na-, 80737-33-3; Rb+l8C6*Na-, 89462-38-4; Cs+18C6.Na-, 80737-31-1; K+( 15C5)2.Na-, 89462-36-2; Rb+(15C5)2.Na-, 89462-37-3; Cs+(l8C6)yCs-, 87039-74-5; Cs+(l8C6)2C, 87039-73-4.
_____
(33) Issa, D.; Ellaboudy, A.; Janakiraman, R.; Dye, J. L. J . Phys. Chem., this issue.
(34) Landers, J. S. Ph.D. Dissertation, Michigan State University, 1981. (35) DaGue, M. G. Ph.D. Dissertation, Michigan State University, 1979.
