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Nature of the mixed hydrides between aluminum chloride and lithium aluminum hydride in anhydrous tetrahydrofuran
J. inorg,nucl.Chem., 1972,Vol.34, pp. 2439-2448. PergamonPress. Printedin Great Britain
NATURE ALUMINUM HYDRIDE
OF THE MIXED HYDRIDES BETWEEN CHLORIDE AND LITHIUM ALUMINUM IN ANHYDROUS TETRAHYDROFURAN
MASAKI YOSHIO, N O B U H I K O 1SHIBASHI, H I R O H I K O WAKI and TETSU RO SEIYAMA Applied analytical Chemistry, Faculty of Engineering, Kyushu University, Fukuoka, Japan (Received 19 November 1971 )
A b s t r a c t - T h e concentration dependence of the specific conductivity of aluminum chloride, chloroderivatives of aluminum hydride and lithium aluminum hydride in tetrahydrofuran were measured. Two types of the conductivity-concentration curve were obtained; the one is linear and the other is concave upward, which may be due to a disproportionate ionization process and due to a variation in the association number with a change in concentration, respectively. The reaction of aluminum chloride with lithium aluminum hydride is also investigated by the conductivity method. The formation of the species AIHCI2 and AIH2CI and of ionized complex Li+AIHCI3 - and Li+AIH2CI2- in tetrahydrofuran is proposed to account for the maxima in the conductivity curves. The formation of the complex in diethyl ether is also briefly discussed.
INTRODUCTION
the conductivity of diethyl ether solutions of aluminum chloride and lithium aluminum hydride was initiated by G. G. Evans et al. [1]. They proposed the formation of ionic species, Ai2C15- and AIH4-, which were confirmed by Arkhiv and Mikheeva[2]. Hayashi and Ishida[3] assumed the formation of A1HCi2 and AIH2CI in various mixed ratios of aluminum chloride to lithium aluminum hydride. The formation of AIHCI2 and AIH2CI was also confirmed by the analysis of the i.r. spectra[4]. Recent studies[5] postulated the existence of several complex compound and three or four conducting ions based on the infrared analysis. The exact nature of the ionic species present in the solution has still, however, not been definitely proved. In this paper, the conductivity of aluminum chloride, aluminum dichlorohydride and aluminum chlorodihydride in the tetrahydrofuran solution has been investigated. Two types of the ionization process have been found. There is an increasing interest in the use of tetrahydrofuran as a non-aqueous solvent for the electrochemical studies [6-8] and for the electrodeposition of aluminum[9-11]. A STUDY of
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
G. G. Evans,T. P. R. Gibb, J. K. Kennedy and F. P. DelGreco,J. inorg, nucl. Chem. 4, 40 (1957). S. M. Arkhpov and V. I. Mikheeva, Zh. neorg. Khim. 11, 2006 (1966). T. Hayashi and T. lshida, Bull. Univ. Osaka Pref. A7, 55 (1959). E. C. Ashby and J. Prather, J. A m. chem. Soc. 88,729 (1966). F. A. Clay, W. B. Harding and C. J. Stimetz, Plating 56, 1027 (1969). J. Comyn, F. S. Dainton and K, J. Ivin, Electrochim. A cta 13, 1851 (1968). J. Badz-Lambling and M. Sato, A cta chim. Hung. Tomus 32, 191 (1962). J. Perichon and R. Buvet, Bull. Soc. chim. Fr. 1279 (1968). N. lshibashi, Y. Hanamura, M. Yoshio and T. Seiyama, Denki Kagaku 37, 73 (1969). G. Hikita, M. Yoshio, N. Ishibashi and T. Seiyama, Denki Kagaku 38.199 (1970). N. lshibashi and M. Yoshio, Electrochim. Acta 17. In press (1972). 2439
2440
M. YOSHIO, N. ISHIBASHI, H. WAKI and T. S E I Y A M A
A high solubility of lithium chloride in tetrahydrofuran, although insoluble in diethyl ether, makes it a convenient solvent to examine the ionization state. The conductimetric studies of aluminum dichl0rohydride and of aluminum chlorodihydride with lithium chloride in tetrahydrofuran have been carded out with a view to examine the nature of the complex ions formed between aluminum chloride and lithium aluminum hydride. The formation of different complex species in tetrahydrofuran from those in diethyl ether has been confirmed conductimetrically, as pointed out in the previous paper[9]. The composition of mixed hydrides in diethyl ether has been also discussed. EXPERIMENTAL Diethyl ether and tetrahydrofuran were refluxed over sodium for several hours and were then distilled. Commercial aluminum chloride was purified by sublimation. A tetrahydrofuran solution of lithium aluminum hydride was obtained by refluxing tetrahydrofuran in suspension of commercial lithium aluminum hydride (Merck A. G.) for 4 or 5 hr and filtering insoluble residue under a nitrogen gas. A tetrahydrofuran solution of lithium chloride was dried over lithium hydride during 24 hr. Aluminum dichlorohydride AIHCI~ and aluminum chlorodihydride AIHzCI were prepared by the mixing of aluminum hydride and diethyl ether solutions of aluminum chloride in the theoretical ratio and by the mixing of diethyl solutions of lithium aluminum hydride and of aluminum chloride, according to the methods of Evans[l] and Mikheev[2], respectively. Excess diethyl ether was removed at a room temperature under a reduced pressure and the trace residual ether was replaced by tetrahydrofuran. After the removal and the replacement were repeated several times, diethyl ether was completely removed from the tetrahydrofuran solution. The conductivity measurements were carried out under an atmosphere of nitrogen gas by using an assembly consisted of a 70 ml conductivity cell and an adapter with glass joint, to which were attached a burette. The solutions, which were diluted by the addition of the solvent from the burette, were mixed with a magnetic stirrer under a temperature 20_+0"1°C and its conductivity was measured by using Yanagimoto conductivity apparatus type MY-5. THEORY, RESULTS A N D D I S C U S S I O N
Conductimetric studies on aluminum chloride, aluminum dichlorohydride and aluminum chlorodihydride in tetrahydrofuran
Several authors reported that solvents of a low base strength should, in general, favour coordination disproportionation of metal halides [12-14]. Different equilibria, however, were frequently suggested by other authors[15-17] in their interpretation of similar systems. There are two possible processes, dissociation and disproportionation as represented by Equations (1) and (2), in which the n-th polymerized molecule of the chloro-derivative of aluminum hydride AI,CI3mH3~,-m)can be ionized by chloride anion transfer because of the larger ionic radius of chloride anion than that of hydride anion:
12. 13. 14. 15. 16. 17.
Al,,ClamH:~n-m~ ~---AlnCl3s-lH~n-mj + + CI-
(1)
2AlnClamHatn-m) ~- AlnClsm-lH3tn-m) + + AlnCl3ra+lHatn-m~-.
(2)
L. I. Katzin, J. chem. Phys. 36, 3034 (1962). W. Lihus and D. Puchalska, J. phys. Chem. 71, 3549 (1967). T. B. Swanson and V. W. Laurie, J. phys. Chem. 69, 244 (1965). G. Jander and K. Kraffczyk, Z. anorg, allg. Chem. 282, 121 (1955). R.C. Paul and K. C. Malhotra, Z. anorg, aug. Chem. 321, 56 (1963). W. G. Movius and N. A. Matwiyott, lnorg. Chem. 6, 847 (1967).
Aluminum chloride and lithium aluminum hydride
2441
The equilibrium constants expressed in concentration units for Equations (1) and (2) are written as:
(3)
k = [A1.Clam_IH~(n_m) +] [CI-]/[AInCIzmH3(n_m)]
k' = [AlnCl3m-lH3(.-m) +][AI.CI3,n+IH3(._m) -- ]/[AI.Cl3mHa(.-m)]. 2
(4)
In a solvent of the low dielectric constant, most of the solute may be dissolved as an unionized species AI.CI3,.H3(._m) and the main ionic species may be a monovalent one. Under the assumption that the n-th polymerized complex is alone present, the specific conductivity of the solution is written as Equations (5) and (6) according to the either ionization process (1) or (2): 103K = hAi.Cl,._,n .... + • [AlnClam_lH3(n_m) +] + hct_ [CI-] + =
[AlnClam_lHa(n_m)+] {h+A[.C[3.__I~ . . . . . + +
kfl
}
(5)
1 OaK ' = k+Al.Cl., J-I~..~ • [AlaCl3m_lHz(n_m) +] --t-k~,I.CI~_IH..... [AInCI3m+lHa(n_m)- ]
= [AlnClam+lHatn-m) +] {X+t.O~.+,H......- + hxt.Cl.. ,n......+ } (6) where K is the specific conductivity and h is the ion conductivity. From Equations (3) and (5), 1 0 3 K ~_
+ k 1/2 {hAI.CI~_, H . . . . . + + hCl- }
[AlnClamHa(n_m)] llz.
(7)
Similarily, from (4.) and (6), +
1oar' = k'{ h a~,cl3,lr~......++ h At.C~,.+,H ......- } [AI,CI3mH3(n_m) ].
(8)
We can approximately put [Al]total = [A1.ClzmH~._,.)], where [Al]tot~l is the analytical concentration of aluminum. The equilibrium constant and the ion conductivity can be assumed to be unchanged over the range of concentrations investigated without introducing serious ~rrors. Then the following relations can be obtained: K oc [Al]total, 1/2 or log K = const. + ½log [AI]total
(9)
K' oe [Al]total, or log K' = const. + log [Al]tota~.
(10)
Accordingly, the ionization process expressed in Equation (1) gives a linear relation between the logarithm of the specific conductivity and the logarithm of aluminum concentration, with a slope of 0.5, while under the condition of equation (2) the specific conductivity is proportional to the concentration of aluminum. Figure 1 shows the typical r--[Al]tota~ plots for aluminum chloride. The curve B in Fig. 1 shows much lower conductivities, that is, lower molar conductance than does the curve A. These lower conductivities supposedly are partly due
2442
M. YOSHIO, N. ISHIBASH1, H. WAKI and T. SEIYAMA
'E
o/
~j "T 0
o
j.1.
u
g
o. (/)
I
I 0.05 [ AlCl3],
t
I 0.10
M
Fig. 1. Typical plots of the specific conductivity, K, of tetrahydrofuran solutions of aluminum chloride at 20.0°C; the concentration of the stock solution is, (A) O 0.100 M and (B)O0.190 M. to the change in a solvation state which is caused by the effect of the concentration o f the stock solution, age and temperatures attained in the preparation, and partly due to the presence of the dimeric species. In the tetrahydrofuran solution of aluminum chloride, the presence of the dimer is recognized within a day after the preparation, although it dissociates eventually to a monomer[18]. As the curves give straight lines, the solvation state and the association number are not varied by the dilution during the measurements at least. It is likely that aluminum dichlorohydride and aluminum chlorodihydride, like aluminum chloride, exist as a m o n o m e r in the presence of tetrahydrofuran. T h e linear relation between r and [Al]total is also obtained in Fig. 2. T h e ionization of these salts is also disproportionate equilibrium such as: 2A1HCI2 ~ AIHCI + + AIHC132AIH2CI ~ AIH2+,+ AIH2Ci2-. A n o t h e r type of a dissociation is in the case of lithium aluminum hydride. T h e log K--log [Al]total slope should be 0.5, when LiAIH4 ~ Li ÷ + AIH4-. T h e slope of the curve, as shown in Fig. 3, is actually larger than 0.5 and concave upward. It is reasonable to assume that the association number of the solute is varied by the dilution in this case. (2n + 1)LiAIH4 ~--- (LiAIH4)z.+I -----~Li(LiAIH4) ~+ + (LiAIH4) ~AIH418. H. Waki and N. Uemura, Bull. chem. Soc. Japan 41, 1740 (1968).
Aluminum chloride and lithium aluminum hydride
'E
_o
% x
I
t
0
I
I
005
0.10
[ A i] totol'
M
Fig. 2. Specific conductivity of solutions of AIH~CI (©) and AIHCIz (Q) in tetrahydrofuran at 20-0°C.
-3
/
-4
//
-5 _A IE u 7 -6
g
_J
-7
-8 I
-3
-2
I
I
-I L0g[AI]tota I ,
-0 M
Fig. 3. Conductivity of tetrahydrofuran solutions of lithium aluminum hydride (O) and that of diethyl ether solutions of aluminum chloride (O) at 20.0°C.
2443
2444
M. YOSHIO, N. ISHIBASHI, H. WAKI and T. SEIYAMA
log K = const. + ~
log [LiAIH4].
(11)
When the association number increases with an increase in concentrations of lithium aluminum hydride, the conductivity curve becomes concave upward according to Equation (11). The conductance behaviour of aluminum chloride diethyl ether is similar to that of lithium aluminum hydride in tetrahydrofuran, as shown in Fig. 3. By the anion exchange study, the association number of aluminum chloride in diethyl ether is increased with its increasing concentration[19]. As the slope, however, approaches to 1.0 at a low aluminum content, the ionization state may be represented as disproportion products, AICI2÷ and AICI4-. It is recognized that the dissociation equilibria in ethereal solutions may be the coordination disproportionation such as [AlnCl3m_lH3~n_m)L4] + [Al,,Clam+r H3¢n-m)]-, where L is tetrahydrofuran or diethyl ether molecule.
Conductimetric titration o f aluminum dichlorohydride and of aluminum chlorodihydride with lithium chloride in tetrahydrofuran In the continuous variations method for determining the complex composition, the specific conductivity has been plotted against a molar fraction of lithium chloride at the constant total concentrations of both solutes as shown in Figs. 4 and 5. The pure tetrahydrofuran solution of lithium chloride is negligibly small. With an
T
~o
Molar fraction of lithium chloride
Fig. 4. Continuous variation plots for AIH2CI-LiCI mixture in tetrahydrofuran; specific conductivity plotted against a molar fraction of lithium chloride under the condition of the constant concentration of solutes, (©) [AIHzCI] + [LiCl] = 0.063 M 20.0°C, (0) 0.100 M 26-0°C and (A) 0.252 M 20.0°C, respectively. 19. M. Yoshio, Y. Nishiyama and N. Ishibashi, J. inorg, nucl. Chem. 34, 2033 (1972).
Aluminum chloride and lithium aluminum hydride
2445
f\ T
\
4~ o
x ~t
0.5 MolQr
froction
of
lithium
~O chloride
Fig. 5. Continuous variation plots for AIHCI2-LiCI mixture in tetrahydrofuran; ((2)) [AIHCI2]+ [LiCI] = 0-051 M 20.0°C and (0) 0.100 M 26-0°C. addition of small amount of lithium chloride to the tetrahydrofuran solution of the solutes, AIH2CI or AIH C12, the conductivity increased remarkably to the maximum point indicating the complex formation. T h e conductivities of the mixtures always showed a maximum at molar fraction x = 0.4-0.5 in each system. Accordingly, the maximum points of the conductivity curve indicate the formation of new conducting ions at the molar ratio 1 : l, because the conductivity of electlyte is not strictly additive [20, 21]. A I H e C I + LiCI ~ LiAIH2CI2 ~--- L i + + AIH2CI2-
(12)
AIHCI~ + LiCI ~ LiAIHCI~ .-~ L i + + AIHCla -.
(13)
Conductimetric study on the reaction o f aluminum chloride with lithium aluminum hydride
A solution of lithium aluminum hydride is titrated with aluminum chloride by using the molar ratio method. When the specific conductivity has been plotted against the molar ratio of lithium to aluminum under the condition of constant lithium aluminum hydride concentration, the conductivity curve has two inflection points where the molar ratios of A1CI3/LiA1H4 are 1 and 3, as shown in Fig. 6. A conductance behaviour in tetrahydrofuran shows the formation of AIH2CI and of AIHCI,,, which was already recognized in diethyi ether by i.r. analysis. Since lithium chloride is considerably soluble in tetrahydrofuran (up to 1.3 M at 15°C), 20. c. w. Davies, Ion Association, p. 27. Butterworths, London (1967). 21. J. F. Tate and M. M. Jones, J. inorg, nucl. Chem. 21.241 (1960).
I I N C V~,I. 34No. 8 - C
2446
M. YOSHIO, N. ISHIBASHI, H. WAKI and T. S E I Y A M A
2.0
I-5
'E o
q, 0 x 1.0
0'5
0
I
I
I
I
2
3
t 4
I 5
[AlCl s] / [LiAIH 4 ]
Fig. 6. Molar ratio plots for AICIa-LiAIH4 mixture in tetrahydrofuran; conductivity plotted against the molar ratio of AICIa to LiAIH4 under the condition of the concentration of LiAIH4 = 0.100 M at 20.0°C.
although insoluble in diethyl ether, the reaction of lithium chloride with AIH2CI or with AIHCIz is not negligible. The complex species formed at AICla/LiAIH4 molar ratio 1 may be represented as follows: LiAIH4 + AICIa ~ 2AIH2CI + LiCI
(14)
AIH~CI+ LiCI ~ LiAIH~CI~ ~ Li++ AIH2CI~ -.
(15)
An increase in the conductivity in Fig. 6 up to the molar ratio 1 is mainly due to the formation of Li ÷ cation and of AIH~CI2- anion. As shown in Fig. 7, the curve of the specific conductivity of the solution of AIH2CI/LiCI molar ratio 1 against total aluminum concentration is in agreement well with that of tetrahydrofuran solution of the AICI3/LiAIH4 molar ratio 1. These mean that Equations (14) and (15) show reasonable ionization processes. From the results of the conductimetric titration of aluminum dichlorohydride with lithium chloride, next reactions occur at the molar ratio 3: LiAIH4 + 3AICIs ~ 4AIHCIz + LiCI
(16)
AIHCI2 + LiCI ~ LiAIHCIa ~.~ Li + + AIHCI3-.
(17)
The main conducting ions are Li + and AIHC13- in the range of molar ratio from 1 to 3. Beyond the molar ratio 3, the conductivity increases with an increase in concentration of Li+AICI4 -. The complex compounds in diethyl ether is briefly discussed. Figure 8 shows
Aluminum chloride and lithium aluminum hydride
2447
I o
6
V
~o x
/ n
0
I
I
,
02
OI
[ A l l to*ol
M
'
Fig. 7. Comparison of the conductivities of the tetrahydrofuran solutions of the AICI3to-LiAIH4 molar ratio 1 (O) with those of the solutions of the AIHzCI-to-LiCI molar ratio 1 (O) and with those of the AIHCI2-to-LiCI molar ratio 1 (A), at 20.0°C.
E u T
%
o T O x
1
I
J
M o l a r fraction
, ~e'-,,--=,,e=qD 05
of
lithium
aluminum
0 iO
hydride
Fig. 8. Conductimetric titration of solutions of LiAIH4 with AICIa in diethyl ether, and vice versa, under the condition of the constant total aluminum concentration, (C)) [AICl3] + [LiA1H4] = 1.53 M and (O) 0.300 M, at 20.0°C.
2448
M. YOSHIO, N. I S H I B A S H I , H. WAKI and T. S E I Y A M A
the conductivity of the diethyl ether solution at the various molar ratios of aluminum chloride to lithium aluminum hydride. At A1CIJLiAIH4 molar ratio 3, the reaction LiAIH4+ 3AICI3 ~ 3AIHCI2+ LiAIHCI3, occurs and lithium chloride does not precipitate. The LiAIHCI3 ionizes the Li + and AIHCIa-. However, the further addition of aluminum chloride brings about the precipitation of lithium chloride and leads to the destruction of the ionic species completed at molar ratio 1. LiAIH4+ AICI3 ~ LiCI~+ 3AIH2Ci. Compared with the conductance behaviour of tetrahydrofuran solution, the greatest difference is the low conductivity of the diethyl ether solution at the equi-molar solution, resulting from the precipitation of lithium chloride. At molar ratio 1 : 3 and 1 : 7, the next reactions are postulated; 3 LiAIH4 + AICIa --~ AIH3 + 3 LiCIt, 7LiAIH4 + AICI3 --* 4LiAIH4A1H3 + 3 LiCIt,. No detailed explanation for ionic species formed, however, can be given for it in the case of a molar ratio lower than 1. Acknowledgement-The authors wish to express their thanks to the Nisshin Steel Co. Ltd. for financial support of this work.
NATURE ALUMINUM HYDRIDE
OF THE MIXED HYDRIDES BETWEEN CHLORIDE AND LITHIUM ALUMINUM IN ANHYDROUS TETRAHYDROFURAN
MASAKI YOSHIO, N O B U H I K O 1SHIBASHI, H I R O H I K O WAKI and TETSU RO SEIYAMA Applied analytical Chemistry, Faculty of Engineering, Kyushu University, Fukuoka, Japan (Received 19 November 1971 )
A b s t r a c t - T h e concentration dependence of the specific conductivity of aluminum chloride, chloroderivatives of aluminum hydride and lithium aluminum hydride in tetrahydrofuran were measured. Two types of the conductivity-concentration curve were obtained; the one is linear and the other is concave upward, which may be due to a disproportionate ionization process and due to a variation in the association number with a change in concentration, respectively. The reaction of aluminum chloride with lithium aluminum hydride is also investigated by the conductivity method. The formation of the species AIHCI2 and AIH2CI and of ionized complex Li+AIHCI3 - and Li+AIH2CI2- in tetrahydrofuran is proposed to account for the maxima in the conductivity curves. The formation of the complex in diethyl ether is also briefly discussed.
INTRODUCTION
the conductivity of diethyl ether solutions of aluminum chloride and lithium aluminum hydride was initiated by G. G. Evans et al. [1]. They proposed the formation of ionic species, Ai2C15- and AIH4-, which were confirmed by Arkhiv and Mikheeva[2]. Hayashi and Ishida[3] assumed the formation of A1HCi2 and AIH2CI in various mixed ratios of aluminum chloride to lithium aluminum hydride. The formation of AIHCI2 and AIH2CI was also confirmed by the analysis of the i.r. spectra[4]. Recent studies[5] postulated the existence of several complex compound and three or four conducting ions based on the infrared analysis. The exact nature of the ionic species present in the solution has still, however, not been definitely proved. In this paper, the conductivity of aluminum chloride, aluminum dichlorohydride and aluminum chlorodihydride in the tetrahydrofuran solution has been investigated. Two types of the ionization process have been found. There is an increasing interest in the use of tetrahydrofuran as a non-aqueous solvent for the electrochemical studies [6-8] and for the electrodeposition of aluminum[9-11]. A STUDY of
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
G. G. Evans,T. P. R. Gibb, J. K. Kennedy and F. P. DelGreco,J. inorg, nucl. Chem. 4, 40 (1957). S. M. Arkhpov and V. I. Mikheeva, Zh. neorg. Khim. 11, 2006 (1966). T. Hayashi and T. lshida, Bull. Univ. Osaka Pref. A7, 55 (1959). E. C. Ashby and J. Prather, J. A m. chem. Soc. 88,729 (1966). F. A. Clay, W. B. Harding and C. J. Stimetz, Plating 56, 1027 (1969). J. Comyn, F. S. Dainton and K, J. Ivin, Electrochim. A cta 13, 1851 (1968). J. Badz-Lambling and M. Sato, A cta chim. Hung. Tomus 32, 191 (1962). J. Perichon and R. Buvet, Bull. Soc. chim. Fr. 1279 (1968). N. lshibashi, Y. Hanamura, M. Yoshio and T. Seiyama, Denki Kagaku 37, 73 (1969). G. Hikita, M. Yoshio, N. Ishibashi and T. Seiyama, Denki Kagaku 38.199 (1970). N. lshibashi and M. Yoshio, Electrochim. Acta 17. In press (1972). 2439
2440
M. YOSHIO, N. ISHIBASHI, H. WAKI and T. S E I Y A M A
A high solubility of lithium chloride in tetrahydrofuran, although insoluble in diethyl ether, makes it a convenient solvent to examine the ionization state. The conductimetric studies of aluminum dichl0rohydride and of aluminum chlorodihydride with lithium chloride in tetrahydrofuran have been carded out with a view to examine the nature of the complex ions formed between aluminum chloride and lithium aluminum hydride. The formation of different complex species in tetrahydrofuran from those in diethyl ether has been confirmed conductimetrically, as pointed out in the previous paper[9]. The composition of mixed hydrides in diethyl ether has been also discussed. EXPERIMENTAL Diethyl ether and tetrahydrofuran were refluxed over sodium for several hours and were then distilled. Commercial aluminum chloride was purified by sublimation. A tetrahydrofuran solution of lithium aluminum hydride was obtained by refluxing tetrahydrofuran in suspension of commercial lithium aluminum hydride (Merck A. G.) for 4 or 5 hr and filtering insoluble residue under a nitrogen gas. A tetrahydrofuran solution of lithium chloride was dried over lithium hydride during 24 hr. Aluminum dichlorohydride AIHCI~ and aluminum chlorodihydride AIHzCI were prepared by the mixing of aluminum hydride and diethyl ether solutions of aluminum chloride in the theoretical ratio and by the mixing of diethyl solutions of lithium aluminum hydride and of aluminum chloride, according to the methods of Evans[l] and Mikheev[2], respectively. Excess diethyl ether was removed at a room temperature under a reduced pressure and the trace residual ether was replaced by tetrahydrofuran. After the removal and the replacement were repeated several times, diethyl ether was completely removed from the tetrahydrofuran solution. The conductivity measurements were carried out under an atmosphere of nitrogen gas by using an assembly consisted of a 70 ml conductivity cell and an adapter with glass joint, to which were attached a burette. The solutions, which were diluted by the addition of the solvent from the burette, were mixed with a magnetic stirrer under a temperature 20_+0"1°C and its conductivity was measured by using Yanagimoto conductivity apparatus type MY-5. THEORY, RESULTS A N D D I S C U S S I O N
Conductimetric studies on aluminum chloride, aluminum dichlorohydride and aluminum chlorodihydride in tetrahydrofuran
Several authors reported that solvents of a low base strength should, in general, favour coordination disproportionation of metal halides [12-14]. Different equilibria, however, were frequently suggested by other authors[15-17] in their interpretation of similar systems. There are two possible processes, dissociation and disproportionation as represented by Equations (1) and (2), in which the n-th polymerized molecule of the chloro-derivative of aluminum hydride AI,CI3mH3~,-m)can be ionized by chloride anion transfer because of the larger ionic radius of chloride anion than that of hydride anion:
12. 13. 14. 15. 16. 17.
Al,,ClamH:~n-m~ ~---AlnCl3s-lH~n-mj + + CI-
(1)
2AlnClamHatn-m) ~- AlnClsm-lH3tn-m) + + AlnCl3ra+lHatn-m~-.
(2)
L. I. Katzin, J. chem. Phys. 36, 3034 (1962). W. Lihus and D. Puchalska, J. phys. Chem. 71, 3549 (1967). T. B. Swanson and V. W. Laurie, J. phys. Chem. 69, 244 (1965). G. Jander and K. Kraffczyk, Z. anorg, allg. Chem. 282, 121 (1955). R.C. Paul and K. C. Malhotra, Z. anorg, aug. Chem. 321, 56 (1963). W. G. Movius and N. A. Matwiyott, lnorg. Chem. 6, 847 (1967).
Aluminum chloride and lithium aluminum hydride
2441
The equilibrium constants expressed in concentration units for Equations (1) and (2) are written as:
(3)
k = [A1.Clam_IH~(n_m) +] [CI-]/[AInCIzmH3(n_m)]
k' = [AlnCl3m-lH3(.-m) +][AI.CI3,n+IH3(._m) -- ]/[AI.Cl3mHa(.-m)]. 2
(4)
In a solvent of the low dielectric constant, most of the solute may be dissolved as an unionized species AI.CI3,.H3(._m) and the main ionic species may be a monovalent one. Under the assumption that the n-th polymerized complex is alone present, the specific conductivity of the solution is written as Equations (5) and (6) according to the either ionization process (1) or (2): 103K = hAi.Cl,._,n .... + • [AlnClam_lH3(n_m) +] + hct_ [CI-] + =
[AlnClam_lHa(n_m)+] {h+A[.C[3.__I~ . . . . . + +
kfl
}
(5)
1 OaK ' = k+Al.Cl., J-I~..~ • [AlaCl3m_lHz(n_m) +] --t-k~,I.CI~_IH..... [AInCI3m+lHa(n_m)- ]
= [AlnClam+lHatn-m) +] {X+t.O~.+,H......- + hxt.Cl.. ,n......+ } (6) where K is the specific conductivity and h is the ion conductivity. From Equations (3) and (5), 1 0 3 K ~_
+ k 1/2 {hAI.CI~_, H . . . . . + + hCl- }
[AlnClamHa(n_m)] llz.
(7)
Similarily, from (4.) and (6), +
1oar' = k'{ h a~,cl3,lr~......++ h At.C~,.+,H ......- } [AI,CI3mH3(n_m) ].
(8)
We can approximately put [Al]total = [A1.ClzmH~._,.)], where [Al]tot~l is the analytical concentration of aluminum. The equilibrium constant and the ion conductivity can be assumed to be unchanged over the range of concentrations investigated without introducing serious ~rrors. Then the following relations can be obtained: K oc [Al]total, 1/2 or log K = const. + ½log [AI]total
(9)
K' oe [Al]total, or log K' = const. + log [Al]tota~.
(10)
Accordingly, the ionization process expressed in Equation (1) gives a linear relation between the logarithm of the specific conductivity and the logarithm of aluminum concentration, with a slope of 0.5, while under the condition of equation (2) the specific conductivity is proportional to the concentration of aluminum. Figure 1 shows the typical r--[Al]tota~ plots for aluminum chloride. The curve B in Fig. 1 shows much lower conductivities, that is, lower molar conductance than does the curve A. These lower conductivities supposedly are partly due
2442
M. YOSHIO, N. ISHIBASH1, H. WAKI and T. SEIYAMA
'E
o/
~j "T 0
o
j.1.
u
g
o. (/)
I
I 0.05 [ AlCl3],
t
I 0.10
M
Fig. 1. Typical plots of the specific conductivity, K, of tetrahydrofuran solutions of aluminum chloride at 20.0°C; the concentration of the stock solution is, (A) O 0.100 M and (B)O0.190 M. to the change in a solvation state which is caused by the effect of the concentration o f the stock solution, age and temperatures attained in the preparation, and partly due to the presence of the dimeric species. In the tetrahydrofuran solution of aluminum chloride, the presence of the dimer is recognized within a day after the preparation, although it dissociates eventually to a monomer[18]. As the curves give straight lines, the solvation state and the association number are not varied by the dilution during the measurements at least. It is likely that aluminum dichlorohydride and aluminum chlorodihydride, like aluminum chloride, exist as a m o n o m e r in the presence of tetrahydrofuran. T h e linear relation between r and [Al]total is also obtained in Fig. 2. T h e ionization of these salts is also disproportionate equilibrium such as: 2A1HCI2 ~ AIHCI + + AIHC132AIH2CI ~ AIH2+,+ AIH2Ci2-. A n o t h e r type of a dissociation is in the case of lithium aluminum hydride. T h e log K--log [Al]total slope should be 0.5, when LiAIH4 ~ Li ÷ + AIH4-. T h e slope of the curve, as shown in Fig. 3, is actually larger than 0.5 and concave upward. It is reasonable to assume that the association number of the solute is varied by the dilution in this case. (2n + 1)LiAIH4 ~--- (LiAIH4)z.+I -----~Li(LiAIH4) ~+ + (LiAIH4) ~AIH418. H. Waki and N. Uemura, Bull. chem. Soc. Japan 41, 1740 (1968).
Aluminum chloride and lithium aluminum hydride
'E
_o
% x
I
t
0
I
I
005
0.10
[ A i] totol'
M
Fig. 2. Specific conductivity of solutions of AIH~CI (©) and AIHCIz (Q) in tetrahydrofuran at 20-0°C.
-3
/
-4
//
-5 _A IE u 7 -6
g
_J
-7
-8 I
-3
-2
I
I
-I L0g[AI]tota I ,
-0 M
Fig. 3. Conductivity of tetrahydrofuran solutions of lithium aluminum hydride (O) and that of diethyl ether solutions of aluminum chloride (O) at 20.0°C.
2443
2444
M. YOSHIO, N. ISHIBASHI, H. WAKI and T. SEIYAMA
log K = const. + ~
log [LiAIH4].
(11)
When the association number increases with an increase in concentrations of lithium aluminum hydride, the conductivity curve becomes concave upward according to Equation (11). The conductance behaviour of aluminum chloride diethyl ether is similar to that of lithium aluminum hydride in tetrahydrofuran, as shown in Fig. 3. By the anion exchange study, the association number of aluminum chloride in diethyl ether is increased with its increasing concentration[19]. As the slope, however, approaches to 1.0 at a low aluminum content, the ionization state may be represented as disproportion products, AICI2÷ and AICI4-. It is recognized that the dissociation equilibria in ethereal solutions may be the coordination disproportionation such as [AlnCl3m_lH3~n_m)L4] + [Al,,Clam+r H3¢n-m)]-, where L is tetrahydrofuran or diethyl ether molecule.
Conductimetric titration o f aluminum dichlorohydride and of aluminum chlorodihydride with lithium chloride in tetrahydrofuran In the continuous variations method for determining the complex composition, the specific conductivity has been plotted against a molar fraction of lithium chloride at the constant total concentrations of both solutes as shown in Figs. 4 and 5. The pure tetrahydrofuran solution of lithium chloride is negligibly small. With an
T
~o
Molar fraction of lithium chloride
Fig. 4. Continuous variation plots for AIH2CI-LiCI mixture in tetrahydrofuran; specific conductivity plotted against a molar fraction of lithium chloride under the condition of the constant concentration of solutes, (©) [AIHzCI] + [LiCl] = 0.063 M 20.0°C, (0) 0.100 M 26-0°C and (A) 0.252 M 20.0°C, respectively. 19. M. Yoshio, Y. Nishiyama and N. Ishibashi, J. inorg, nucl. Chem. 34, 2033 (1972).
Aluminum chloride and lithium aluminum hydride
2445
f\ T
\
4~ o
x ~t
0.5 MolQr
froction
of
lithium
~O chloride
Fig. 5. Continuous variation plots for AIHCI2-LiCI mixture in tetrahydrofuran; ((2)) [AIHCI2]+ [LiCI] = 0-051 M 20.0°C and (0) 0.100 M 26-0°C. addition of small amount of lithium chloride to the tetrahydrofuran solution of the solutes, AIH2CI or AIH C12, the conductivity increased remarkably to the maximum point indicating the complex formation. T h e conductivities of the mixtures always showed a maximum at molar fraction x = 0.4-0.5 in each system. Accordingly, the maximum points of the conductivity curve indicate the formation of new conducting ions at the molar ratio 1 : l, because the conductivity of electlyte is not strictly additive [20, 21]. A I H e C I + LiCI ~ LiAIH2CI2 ~--- L i + + AIH2CI2-
(12)
AIHCI~ + LiCI ~ LiAIHCI~ .-~ L i + + AIHCla -.
(13)
Conductimetric study on the reaction o f aluminum chloride with lithium aluminum hydride
A solution of lithium aluminum hydride is titrated with aluminum chloride by using the molar ratio method. When the specific conductivity has been plotted against the molar ratio of lithium to aluminum under the condition of constant lithium aluminum hydride concentration, the conductivity curve has two inflection points where the molar ratios of A1CI3/LiA1H4 are 1 and 3, as shown in Fig. 6. A conductance behaviour in tetrahydrofuran shows the formation of AIH2CI and of AIHCI,,, which was already recognized in diethyi ether by i.r. analysis. Since lithium chloride is considerably soluble in tetrahydrofuran (up to 1.3 M at 15°C), 20. c. w. Davies, Ion Association, p. 27. Butterworths, London (1967). 21. J. F. Tate and M. M. Jones, J. inorg, nucl. Chem. 21.241 (1960).
I I N C V~,I. 34No. 8 - C
2446
M. YOSHIO, N. ISHIBASHI, H. WAKI and T. S E I Y A M A
2.0
I-5
'E o
q, 0 x 1.0
0'5
0
I
I
I
I
2
3
t 4
I 5
[AlCl s] / [LiAIH 4 ]
Fig. 6. Molar ratio plots for AICIa-LiAIH4 mixture in tetrahydrofuran; conductivity plotted against the molar ratio of AICIa to LiAIH4 under the condition of the concentration of LiAIH4 = 0.100 M at 20.0°C.
although insoluble in diethyl ether, the reaction of lithium chloride with AIH2CI or with AIHCIz is not negligible. The complex species formed at AICla/LiAIH4 molar ratio 1 may be represented as follows: LiAIH4 + AICIa ~ 2AIH2CI + LiCI
(14)
AIH~CI+ LiCI ~ LiAIH~CI~ ~ Li++ AIH2CI~ -.
(15)
An increase in the conductivity in Fig. 6 up to the molar ratio 1 is mainly due to the formation of Li ÷ cation and of AIH~CI2- anion. As shown in Fig. 7, the curve of the specific conductivity of the solution of AIH2CI/LiCI molar ratio 1 against total aluminum concentration is in agreement well with that of tetrahydrofuran solution of the AICI3/LiAIH4 molar ratio 1. These mean that Equations (14) and (15) show reasonable ionization processes. From the results of the conductimetric titration of aluminum dichlorohydride with lithium chloride, next reactions occur at the molar ratio 3: LiAIH4 + 3AICIs ~ 4AIHCIz + LiCI
(16)
AIHCI2 + LiCI ~ LiAIHCIa ~.~ Li + + AIHCI3-.
(17)
The main conducting ions are Li + and AIHC13- in the range of molar ratio from 1 to 3. Beyond the molar ratio 3, the conductivity increases with an increase in concentration of Li+AICI4 -. The complex compounds in diethyl ether is briefly discussed. Figure 8 shows
Aluminum chloride and lithium aluminum hydride
2447
I o
6
V
~o x
/ n
0
I
I
,
02
OI
[ A l l to*ol
M
'
Fig. 7. Comparison of the conductivities of the tetrahydrofuran solutions of the AICI3to-LiAIH4 molar ratio 1 (O) with those of the solutions of the AIHzCI-to-LiCI molar ratio 1 (O) and with those of the AIHCI2-to-LiCI molar ratio 1 (A), at 20.0°C.
E u T
%
o T O x
1
I
J
M o l a r fraction
, ~e'-,,--=,,e=qD 05
of
lithium
aluminum
0 iO
hydride
Fig. 8. Conductimetric titration of solutions of LiAIH4 with AICIa in diethyl ether, and vice versa, under the condition of the constant total aluminum concentration, (C)) [AICl3] + [LiA1H4] = 1.53 M and (O) 0.300 M, at 20.0°C.
2448
M. YOSHIO, N. I S H I B A S H I , H. WAKI and T. S E I Y A M A
the conductivity of the diethyl ether solution at the various molar ratios of aluminum chloride to lithium aluminum hydride. At A1CIJLiAIH4 molar ratio 3, the reaction LiAIH4+ 3AICI3 ~ 3AIHCI2+ LiAIHCI3, occurs and lithium chloride does not precipitate. The LiAIHCI3 ionizes the Li + and AIHCIa-. However, the further addition of aluminum chloride brings about the precipitation of lithium chloride and leads to the destruction of the ionic species completed at molar ratio 1. LiAIH4+ AICI3 ~ LiCI~+ 3AIH2Ci. Compared with the conductance behaviour of tetrahydrofuran solution, the greatest difference is the low conductivity of the diethyl ether solution at the equi-molar solution, resulting from the precipitation of lithium chloride. At molar ratio 1 : 3 and 1 : 7, the next reactions are postulated; 3 LiAIH4 + AICIa --~ AIH3 + 3 LiCIt, 7LiAIH4 + AICI3 --* 4LiAIH4A1H3 + 3 LiCIt,. No detailed explanation for ionic species formed, however, can be given for it in the case of a molar ratio lower than 1. Acknowledgement-The authors wish to express their thanks to the Nisshin Steel Co. Ltd. for financial support of this work.