Recycle of tin thiolate compounds relevant to ammonia-borane regeneration

Article abstract of DOI:10.1039/b919383a

The use of benzenedithiol as a digestant for ammonia-borane spent fuel has been shown to result in tin thiolate compounds which we demonstrate can be recycled, yielding Bu₃SnH and ortho-benzenedithiol for reintroduction to the ammonia-borane regeneration scheme.

Full text of DOI:10.1039/b919383a

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Recycle of tin thiolate compounds relevant to ammonia–borane
regenerationw
Andrew D. Sutton,*a Benjamin L. Davis,a Koyel X. Bhattacharyya,a Bobby D. Ellis,b
John C. Gordon*a and Philip P. Power*b
Received (in Berkeley, CA, USA) 17th September 2009, Accepted 3rd November 2009
First published as an Advance Article on the web 13th November 2009
DOI: 10.1039/b919383a
The use of benzenedithiol as a digestant for ammonia–borane
spent fuel has been shown to result in tin thiolate compounds
which we demonstrate can be recycled, yielding Bu3SnH and
ortho-benzenedithiol for reintroduction to the ammonia–borane
regeneration scheme.
Our focus in this regard was centered on the conversion of 3
and 4, i.e., species containing Sn–SR bonds, to a species
containing a formate moiety, (Bu3Sn–OC(O)H), as this
compound has been described as a precursor to Bu3SnH via
thermal decarboxylation.12 Our initial investigations focused
on a relatively simple system, i.e., Bu3SnSPh (5), which can be
formed by refluxing (Bu3Sn)2O and PhSH in a 1 : 2 molar ratio
in toluene with a Dean–Stark apparatus (to collect the water
generated). After refluxing for 12 h the toluene was removed
in vacuo and the product distilled under reduced pressure to
yield 5 in near quantitative yield. Direct reaction of 5 with
formic acid does not result in a clean reaction, and although
some evidence does exist for the production of Bu3Sn–OC(O)H,
an equilibrium appears to exist which predominantly favors
A necessary target in realizing a hydrogen (H2) based economy,
especially for the transportation sector, is its storage for
controlled delivery, presumably to an energy producing fuel
cell.1 Chemical hydrogen storage has been dominated by one
appealing material, ammonia–borane (H3N–BH3, AB), due to
its high gravimetric capacity of hydrogen (19.6 wt%) and low
molecular weight (30.7 g molÀ1). In addition, AB has both
hydridic and protic moieties, yielding a material from which
H2 can be readily released in contrast to the loss of H2 from
C2H6, which is substantially endothermic.2 A number of
publications have described H2 release from amine boranes,
yielding various rates depending on the method applied.3–6
The viability of any chemical hydrogen storage system is
critically dependent on efficient recyclability, but reports on
the latter subject are sparse, invoke the use of high energy
reducing agents, and suffer from low yields.1,7–10
We recently described an energy efficient regeneration process
for polyborazylene (PB) spent fuel.11 In this scheme, PB is
digested with ortho-benzenedithiol to yield [NH4][B(C6H4S2)2]
(1) and (C6H4S2)BHÁNH3 (2). 1 can then be converted into 2
using Bu3SnH as a reductant with C6H4SH(S–SnBu3) (3) as a
by-product, thus resulting in a single boron containing
compound that can subsequently be converted to AB using a
slight excess of Bu2SnH2.11 Modification of this process due to
the relative instability of Bu2SnH2 enables us to carry out the
reduction chemistry with Bu3SnH via an amine exchange step
with Me3N. In such a scheme, the use of Bu3SnH as the
reductant produces C6H4(S–SnBu3)2 (4) as a by-product. In
order to maximize the overall efficiency of an AB regeneration
process which utilizes the Sn–H moiety in a reduction step,
recycle of the tin-containing by-products is essential, and
herein we report a recycle scheme for the regeneration of both
Bu3SnH and the digesting reagent ortho-benzenedithiol from 3
and 4 (Fig. 1).
a Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
E-mail: adsutton@lanl.gov; Fax: (+1) 505 667 9905;
Tel: (+1) 505 665 2931
b University of California, Davis, One Shield Ave, Davis, CA 96363,
USA. E-mail: pppower@ucdavis.edu; Fax: (+1) 530 752 8995;
Tel: (+1) 530 752 6913
w Electronic supplementary information (ESI) available: Full experi-
mental data. See DOI: 10.1039/b919383a
Fig. 1 Regeneration of AB with tin recycle and ortho-benzenedithiol
recovery.
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17 torr). Attempts to achieve accurate isolated yields of
ortho-benzenedithiol on removal of the solvent were hampered
due to its volatility under dynamic vacuum and slight loss of
the isolated compound following the rigorous washing and
drying of the resultant Bu3SnCl.
After the ortho-benzenedithiol has been removed, the
resulting Bu3SnCl that is subsequently formed can then be
converted to Bu3SnOC(O)H (6) via the reaction of Bu3SnCl
with formic acid in the presence of either NaOH or Et3N
which presumably proceeds via the formation of (Bu3Sn)2O.
6 is quantitatively formed as shown by 119Sn NMR spectroscopy
(Fig. 2) at room temperature. 6 has also been synthesised
independently by refluxing a toluene solution of (Bu3Sn)2O
with formic acid, followed by removal of solvent and distillation
of the product giving 78% yield of 6. The distillation of 6
through a column of 3 mm Raschig rings under reduced
pressure (112 1C, 0.3 torr) afforded 60% isolated yield of
Bu3SnH, which can re-enter the AB regeneration process.
In conclusion, we have demonstrated that Bu3SnH and
ortho-benzenedithiol components can be recycled following
their use in a scheme to recycle the tin reductant for AB
regeneration which is a necessary component for a viable AB
regeration scheme.
Fig. 2 119Sn {1H} NMR spectra of Bu3SnCl (bottom) conversion to
Bu3SnOC(O)H (top) following reaction with base and formic acid.
the presence of 5 at ambient temperature. No enhancement of
Bu3Sn–OC(O)H is observed by NMR spectroscopy at
increased temperatures although a slight increase of 5 is
observed with increasing concentrations of formic acid. In
support of this fact is the observation that independently
prepared Bu3SnOC(O)H and PhSH react instantly to form 5
at room temperature, which indicates that the equilibrium lies
heavily in favor of the Sn–SPh species.
This work was funded by the US Department of Energy,
Office of Energy Efficiency and Renewable Energy.
However, when Bu3SnSPh was dissolved in an ethereal
solution of HCl (1.0 M, 0.5 mL), this resulted in clean
conversion to Bu3SnCl and PhSH. Upon removal of solvent
and subsequent redissolution in deutero solvent, this gave
an approximately 1 : 1 ratio of these two species according to
1H NMR spectroscopy with no evidence for any residual 5,
indicating near quantitative conversion to the desired products.
After satisfactorily studying a model system, we then
pursued the same conversion with the by-products of AB
regeneration. Initial studies of C6H4SH(S–SnBu3)11 (3) in
HCl–Et2O in the above manner indeed yielded Bu3SnCl and
C6H6S2 in a 1 : 1 ratio, as expected. On scale-up 3 can be
converted to Bu3SnCl (83% isolated yield) following washing
and drying the resultant solid several times with Et2O. 3 was
not observed in either the 1H or 11B NMR on redissolution of
the solid. Compound 4 can be readily synthesised via a salt
metathesis route in THF from [C6H4S2][Na]2 and two equi-
valents of Bu3SnCl in high yield (93%). Similarly to the model
compound 3, 4 can also be converted to Bu3SnCl and
ortho-benzenedithiol as expected with Bu3SnCl being isolated
(87% yield), again with no indication of residual 4 present.
This shows that both the Sn–SR by-products of digestion, 3
and 4, can be readily converted to Bu3SnCl, which allows the
following stages to be applied to either digestion scheme.
The ortho-benzenedithiol can also be recovered by vacuum
distillation at this stage under mild conditions (b.p. 119–120 1C,
Notes and references
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