A R T I C L E S
Wang et al.
which explained why the curve in Figure 4 decreased signifi-
cantly when small amounts of THF were used. An understanding
of this behavior was obtained from the properties of LiAlH4 in
ethereal solutions.
The physical and chemical properties of LiAlH4 in ethereal
solutions have been investigated by conductometric, ebullo-
scopic, and spectroscopic techniques.20-22 These studies showed
that the ion pairs of LiAlH4 in THF are solvent separated and
exist in two concentration-dependent equilibria: an equilibrium
between ion pairs and free ions at low concentration (<0.1 M
THF), and the formation of triple ions at higher concentrations
(∼0.4 M THF). In contrast, LiAlH4 in diethyl ether was reported
to be concentration-independent and form only contact ions,20,21
which might explain the lack of rehydrogenation of LiAlH4 in
Et2O. Moreover, the NMR results showed that the LiAlH4 is
solvated by four molecules of THF, thereby indicating the
formation of a LiAlH4‚4THF adduct.20 Infrared and Raman
results suggested that the trend in the ordering of the cation
solvatioin goes as: Li-Et2O , Li-THF < Li-diglyme, and
that THF strongly attaches to Li+ in LiAlH4, forming a four-
coordinated lithium solvate.20,22 Moreover, the change in the
formation enthalpy of LiAlH4 as solvent separated ion pairs in
THF was estimated to be -32 kJ/mol lower than that associated
with LiAlH4 as contact ion pairs in Et2O, in the temperature
range from -70 to 25 °C.20,23,24 Also, the ion aggregation of
LiAlH4 in THF from lower to higher concentrations was
estimated to be exothermic.22 Therefore, it was surmised that
LiAlH4 was more stable in THF than in Et2O due to a -30 to
-40 kJ/mol enthalpy change.
Furthermore, the value of the standard free energy of the
reaction in eq 3, that is, the formation of LiAlH4 from LiH and
Al, ranges from 21.7 kJ/mol25 to 34.27 kJ/ mol;26 and that
associated with the reverse reaction in eq 1, that is, the formation
of LiAlH4 from Li3AlH6 and Al, ranges from 18.7 kJ/mol25 to
27.68 kJ/mol.26 Therefore, these reactions do not occur spon-
taneously. To make these reactions more thermodynamically
favorable, the enthalpies and or entropies of these reactions must
change. Although the entropies can be changed by increasing
the hydrogen pressure, it requires a very high pressure to make
the reaction in eq 3 thermodynamically favorable; and for the
reverse reaction in eq 1, it requires at least 1000 bar.27 On the
other hand, if the enthalpies of these reactions can become more
exothermic, they may become thermodynamically favored.
Based on the results of this study, it was surmised that this did
in fact occur, as a result of the solvation effect associated with
LiAlH4 forming solvent separated ion pairs in THF. With no
LiAlH4 regeneration occurring even at hydrogen pressures up
to 100 bar, with no regeneration occurring in diethyl ether, and
with the conversion of these reactions limited by the formation
of a LiAlH4‚4THF adduct, all of these results suggested that
the enthalpy change associated with the solvation of THF with
LiAlH4 to form the LiAlH4‚4THF adduct made these reactions
Figure 4. Effect of the amount of THF used on the rehydrogenation kinetics
of LiAlH4 in terms of conversion of LiAlH4. All of these experiments were
carried out using the same procedure as described in Materials and Methods
with the rehydrogenation pressure at 97.5 bar. The inset shows a linear
correlation between the amount of THF used and that predicted, which was
obtained from knowing the amount of regenerated LiAlH4 in the THF
solution and by varying x in the complex formula LiAlH4‚xTHF until the
data aligned with the diagonal. The results in the inset verified not only
that the conversion was limited by the amount of THF, but also that the
LiAlH4‚4THF adduct formed, in agreement with that reported in the
literature.20
present in the residue sample (curve f). Clearly, all of the
Li3AlH6 in the decomposed sample (see curves b and c in Figure
2) necessarily produced LiAlH4 through the reverse reaction of
eq 1.
It is noteworthy that these experimental results are in
agreement with those reported elsewhere13,14 and discussed in
the Introduction. For example, no reaction occurred in the
absence of THF; nor did any reaction take place in Et2O under
the same conditions used with THF (i.e., ambient temperature
and up to 100 bar of hydrogen). These results also suggested
that the THF played the critical role in fostering the rehydro-
genation of LiAlH4, through the key step in the physiochemical
pathway, that is, the HPBM step in the presence of both THF
and hydrogen.
The effect of the amount of THF on the rehydrogenation
kinetics in terms of the conversion of LiAlH4 is displayed in
Figure 4. The conversion increased with an increase in the THF
to just over 90% when 20 mL of THF was used. It then dropped,
but only by about 10% down to 80% or so, even when as little
as 5 mL of THF was used. However, the conversion dropped
much more quickly down to about 50% and nearly linearly with
the THF decreasing from 5 to 2.5 mL. These results show that
very high conversions can be obtained with THF at ambient
temperature and a reasonable but somewhat high pressure.
Recall that, in the absence of THF, the conversion was zero at
these conditions (Figure 2, curve b). It is shown below that much
lower pressures can also be used.
The inset shows a linear correlation between the amount of
THF used and that predicted, which was obtained from knowing
the amount of regenerated LiAlH4 in the THF solution and by
varying x in the adduct formula LiAlH4‚xTHF until the data
aligned with the diagonal. This correlation not only verified that
the conversion was limited by the amount of THF, but also that
the LiAlH4‚4THF adduct formed between LiAlH4 and THF,20
(21) Ashby, E. C.; Dobbs, F. R.; Hopkins, H. P. J. Am. Chem. Soc. 1975, 97,
3158-3162.
(22) Shirk, A. E.; Shriver, D. F. J. Am. Chem. Soc. 1973, 95, 5904-5912.
(23) Hogen-Esch, T. E.; Smid, J. J. Am. Chem. Soc. 1966, 88, 307-318.
(24) Hogen-Esch, T. E.; Smid, J. J. Am. Chem. Soc. 1966, 88, 318-324.
(25) Chase, M. W., Jr. NIST-JANAF Themochemical Tables, ed.
4
(J. Phys. Chem. Ref. Data, Monograph. 1998, 9, 1-1951) (NIST
1951&Units)SI&Mask)2).
(26) Dymova, T. N.; Aleksandrov, D. P.; Konoplev, V. N.; Silina, T. A.;
Sizareva, V. S. Russ. J. Coord. Chem. 1994, 20, 263-268.
(20) Ashby, E. C.; Dobbs, R. R.; Hopkins, H. P. J. Am. Chem. Soc. 1973, 95,
2823-2829.
(27) Jang, J.; Shim, J.; Cho, Y. W.; Lee B. J. Alloys Compd. 2006, in press.
9
5952 J. AM. CHEM. SOC. VOL. 128, NO. 17, 2006