direct electrochemical synthesis of inorganic and organometallic compounds
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8/16/2019 DIRECT ELECTROCHEMICAL SYNTHESIS OF INORGANIC AND ORGANOMETALLIC COMPOUNDS
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DIRECT ELECTROCHEMIC L SYNTHESIS
OF INORG NIC
AND
ORGANOMETALLIC COMPOUNDS
DENNIS
G. TUCK
Department
o Chemistry
and Biochemistry
University
o
Windsor
Windsor,
Ontario
N9B 3P4
Canada
ABSTRACT.
The
method
of direct
electrochemical synthesis consists
of
oxidizing
a metal anode
in a
non-aqueous
solution containing a ligand or ligand precursor) to
produce the
appropriate
inorganic or organometallic
compound.
In many cases, the product precipitates directly in the
cell, making for easy
isolation,
so
that
the
technique
is
both
direct
and simple, and
in addition
the product yields
are
very high. One advantage of the technique is that the products
are often
derivatives of a low
oxidation
state of the metal;
examples
of this include
chromium III) bromide,
tin lI)
and
lead lI)
diolates and thiolates, hexahalogenodigallate lI) anions, thorium diiodide,
copper l) thiolate
complexes, and indium l) derivatives
of thiols, dithiols,
and diols. In
some
systems,
the low
oxidation state
compound undergoes
subsequent reaction;
for
example,
in
the
synthesis
of
RInX2
the reaction sequence involves the
oxidation
of indium
metal to
give InX,
which then
reacts
with
RX to give RInX
2
.
Another
possible post-electrolysis process
is
disproportionation. Examples of these
various
preparative routes will
be
discussed.
Details of two
recent investigations are
also reported. One of these depends
on
the
oxidation
of indium
in solutions
containing CH
2
X
2
X
Br
,I), to give X
2
InCH
2
X or X
2
InCX
2
InX
2
species
as the
final
products.
A
different system involves the oxidation
of
indium
in
liquid ammonia
solutions
of
NH
4
X or aromatic
1,2-diols,
and the
reactions
in liquid ammonia
are discussed.
1. Introduction
A basic
tenet
of many of
the
papers
presented at
this
meeting is that the electrochemical
technique represents the optimum
method of carrying out
oxidation
or
reduction reactions,
in
large
measure because
the removal
or addition
of
electrons to
a
given
solute
species
can be
achieved
without
the complications attendant
upon
the addition of
redox
reagents to the reaction
mixture.
In
this paper,
we
are
particularly
concerned
with preparative electrochemistry, which
is a
subject
which has developed considerably in
recent
years in
inorganic, organometallic
and
biochemistry. The
work which we have done in
Windsor
has concentrated
on
the use of non
aqueous solvents, and we have been able to prepare a wide
range
of compounds by the use of
some simple
apparatus,
and some
equally
simple
ideas. In particular, we
are
concerned with
electrochemical systems
in
which
the anode
serves
not only
as
a
sink
for
electrons but
also
as
a
reagent towards species which are present
in solution,
or which
are
generated
in
solution as the
electrolysis
proceeds. The
use of a
sacrificial electrode
is particularly
important, because
a
high
purity metal
serves
as the starting point of the
synthesis.
In addition,
we
have established that
15
A.
J
L
Pombeiro and J A. McCleverty eds.),
Molecular Electrochemistry
o
Inorganic Bioinorganic and OrganomettaJJic
Compounds,
15-31.
© 993 Kluwer Academic Publishers.
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the absence of detailed electrochemical parameters
such as Eo in
preparative non-aqueous
electrochemistry is no more a hindrance
to
the use of the
method
than
is
the absence of
thermodynamic data
to the
chemist
who
simply wishes
to
heat substances together
in
a flask.
Inorganic chemists were rather
slow to
recognize the advantages inherent
in
the use of
metals
as
reagents, which
is
surprising given the fact that organozinc halides were first synthesized
130
years
ago from
Zn
RX, and
that the
use
of
magnesium to yield
Grignard reagents
is
fundamental in organic
and
organometallic chemistry. The use
o
high vacuum techniques,
which
allow the evaporation of
gram
quantities of
metals into
a high temperature vapour,
and
the
development of the associated equipment for collecting the products, led to a new interest in the
use of
metals as
synthetic reagents.
Our use
of
metals in
direct electrochemical synthesis
was
entirely coincidental, but it is worth noting that some of the compounds
which we
have prepared
have
also been
obtained
by
the
more
exacting
methods
of vapour phase synthesis.
In
principle,
one
can
use the metal
as
either cathode or anode in direct electrochemical synthesis, but most of
our work
has
been concerned with anodic oxidation, and the discussion
in
this paper will
be
confined
to
such experiments. This
is
not
to
ignore the many interesting reports on the use of
sacrificial cathodes, some of
which have been
reviewed elsewhere
[1].
I shall
also
outline
the
general directions which our research
has
taken,
and
identify the particular ligand systems
for
which we believe direct electrochemical synthesis provides advantages of yield and purity .
. The final point to
be
emphasized is that because one starts with a
metal
which is
by
definition
in the zero oxidation state, the experimental technique
will
necessarily give preferential
access
to
the lower oxidation states if these can
be
stabilized in the solvent system in question. In
particular, for a number of
metals in
the
Main Group
section of the Periodic Table, direct
electrochemical synthesis is a simple
and
attractive way of getting
to
compounds which otherwise
may not
easily prepared,
and
hence provides
an
entree
to
the study of their chemistry.
2. Experimental Outline
As with most
electrochemical systems, our work
has
been
conducted at or near
room
temperature
and
with solvent systems
which
are readily accessible in most laboratories. The cells
which we
have
used
are unsophisticated in the extreme, and the electrical power
can be
derived
either from a relatively cheap CIDC rectifier power pack, or in the extreme case from chemical
storage batteries
[2].
The typical simple cell shown in Fig. 1 is based on a
100
mL tall-form
beaker; a stream of dry nitrogen passing through the solution allows one
to work in oxygen- and
moisture-free conditions. The apparatus can
be
modified if the products or reagents are very air
sensitive,
and
one
such
system
has
been described by
Casey
et al
[3].
The favoured solvent
in
our laboratory is acetonitrile, but other basic solvents, or mixtures thereof, have also been used.
The
main
criteria are solubility
for
ligand
and
background electrolyte, little or no reactivity
towards the product, ease of purification,
and
stability
to an
applied voltage
o ca. 20
V cm-
I
.
The background electrolyte has generally
been
Et
4
NCl0
4
ca.
5
mg
per 5 mL of solution , but
tetraalkylammonium salts of PF
6
-
or
BF
4
-
have
also been
used, especially
when
oxidation by
CI0
4
- is
a
real
or suspected problem.
We normally
run
experiments at a current of 20-30 rnA, and the applied voltage is then that
required
to
achieve
such
a current; typical
values
would be in
the range
10-30
V,
depending
on
the solutes in the electrolyte phase. The applied voltage required obviously depends
on
the
electrode potential
for
the reaction, but more importantly
on
the EMF
needed
to drive the current
carriers through a
medium
of
low
dielectric constant.
Given
the use of electrodes
with
surface
areas of a
few
cm
2
,
the
current density
at
the
anode is in
the order
10-20 rnA cm-
2
.
These
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N
in
out
Pt leads
Figure 1
Diagrammatic
represent tion of
electrochemical cell
Scheme 1
RMX
RMXL 4 1
RMX
n
X
n
m-
Xn
M NCS)n
7
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parameters are
by
no means unique, but serve as
an
illustration of the practical
conditions which
we have found convenient. The most critical factor is the current, since t low a value gives
only
small
quantities
of product,
while
a
high
current
density
can
cause excessive heating
of
the
cell.
3. Direct Synthesis of Halides and Related Compounds
The first systematic experiments which we
carried
out involved the
oxidation
of metal anodes
in
non-aqueous
solutions of halogen, especially bromine and iodine. We found that the metal
halides
themselves
are
easily
accessible, and that anionic complexes or
adducts with neutral
ligands can also be readily prepared
by
adding the
appropriate
species to the
solution.
This work
is
summarized
in Scheme 1, which shows a range of different syntheses, all of which were
carried out with the
simple apparatus described above.
An
obvious extension
was
to investigate
the reactions of organic halides,
and
successful
syntheses of
RMX,
RMX;.-, etc., were
found to
be
possible, with
good yields of
product.
We were also able
to prepare
compounds
by
inserting
a
metal
into an existing
metal-halogen
bond, and derivatives of the type Ph
3
SnZnCl.tmed (tmed
= N,N,N ,N -tetramethylethanediamine)
were obtained
from Ph
3
SnCI
[4].
Another extension is that
pseudohalogens
can replace halogen, so that neutral and anionic
isothiocyanates are also accessible
by
this
method
[5], as are heterometallic carbonyls [6]. A
simple
preparation of
(ph3PHnCoCI4
makes a useful undergraduate laboratory
experiment
[2].
All of
these
processes can
be
represented as the electrochemically driven oxidative
addition
of
a
metal atom
to
an
X-X,
R-X,
M -X or M-M
bond. The metals for which such reactions have
been observed in our
laboratory
in one or all of
these
systems include Mg, Ti, Zr, Hf,
V, Cr,
Mo, Mn,
Fe,
Co, Ni,
Pd, Cu,
Ag, Au,
Zn,
Cd, Hg,
Ga,
In, Sn,
Th
and
U [7]-[31].
We have not
followed this
work recently as
much as
we would
have
wished, but
the
range
of
experiments, which are discussed in more
detail
in an earlier paper [1], already shows that both
transition
metal
and
Main
Group
elements can be
used as the anode
in
such
systems,
and
that
a
wide
range of useful products can be prepared in a very simple
and
straightforward way. I do
not propose
to discuss this
area further, other
than
to
emphasize the
particular
advantage
of
such
syntheses, which are
that one
can
prepare
the anhydrous halides without resorting
to
the high
temperature methods which are otherwise required: that removal of water from
hydrated
materials is not necessary:
that
the derivatives e.g.,
M X ~ -
R M X ~ - RMXnLm) are
as
easily
obtained in
the
one-step synthesis
as
are the
parent
halides themselves;
and that the
products are
obtained
in
high
yield
and
purity.
4. Thiolates and Related Compounds
It has
been
known for many years that thiols or disulfides can be reduced electrochemically to
the corresponding RS-
anions. This
is the first
step in
the direct
electrochemical synthesis
of
metal
thiolates and their derivatives,
since these anions,
or more probably the radicals
produced
when the anions discharge at the anode, react with a variety of metals.
We have
carried
out
successful
syntheses with
the elements Co, Ni, Cu, Ag, Au, Zn, Cd, Hg, In, TI, Sn and
Pb
to
give M(SR)n with R
=
Et, t-Bu, n-Bu, C
S
H11
Ph, Q- m-, p-tolyl, 2-naphthyl,
etc.
(not all
combinations) [32]-[37].
As with
the
halide systems,
one can equally well
produce
the
compounds themselves, or their derivatives, by appropriate
adjustment
of the solution
phase
conditions. The synthesis of these
substances
is experimentally simple and straightforward,
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especially since the products are often insoluble in the solvent systems used, and so can be
conveniently collected beneath the anode.
This is a convenient point at which to introduce the method of chemical accounting which gives
some insight into the mechanism
of
these and related electrochemical syntheses. For a thiol, the
sequence
is
anode: nRS- M
- M(SR)n
. Ie
giving
as
the overall stoichiometry
If
a disulfide
is
used, eq. (1)
is
replaced by
and the overall process
is
1)
2)
3)
la)
3a)
There
is one noteworthy difference between the two systems, in that none of the metals listed
above react directly with thiols, whereas a number
of
Main Group metals will undergo a thermal
reaction with Ph
2
S
2
to give reactions which are stoichiometrically equivalent to eq. 3a) [38].
The sum
of
eqs.
(1)
2), or la) 2), corresponds to the obvious fact that electrons flow
through the cell, and leads to
an
important experimental parameter which we term the
electrochemical efficiency
Ep),
defined
as
moles
of
metal dissolved from the anode per Faraday
of electricity flowing through the cell. The weight loss at the anode is readily determined; the
total quantity of electricity can be measured either by placing a silver coulometer in series with
the cell and power supply, or by maintaining a constant current by manual control for a given
period of time. Under the typical conditions which we have used, a current of 20
rnA
over 2 h
leads to the dissolution of approx. 50-200
mg
of metal, depending on the atomic mass of the
latter, so that
Ep is
easily determined to
±
0.02 mol
F-
i
. For the elements listed above, the
Ep
values are invariably 0.5
mol
F-
i
for Zn, Cd, Sn and Pb, and 1.0 mol F-
i
for Cu, Ag, In
and
Tl. We shall return to a discussion
of
some
of
the values below, but for the case
of
Zn or Cd,
this result is in accord with the formation of M SRh compounds or their derivatives.
Following the syntheses
of
thiolato compounds,
we
also successfully prepared some analogous
M SePh)n compounds
(M =
Cu, Ag, Zn, Cd, Tl, Sn) and adducts such
as
Cd SePh)z.phen and
CuSePh.1.5Ph
3
P by electrolysis with solutions of Ph2Sez in toluene/CH
3
CN mixtures [39].
Similarly, solutions
of
Ph
2
PH in
CH
3
CN gave M PPh
2
)n (M =
Co, Cu, Ag, Au, Zn, Cd), and
in an extension of this work, solutions of Ph
2
PH and
S8 in
toluene/CH
3
CN yielded derivatives
of
M S2PPh2h for Co, Ni, Zn and
Cd
[40]. These straightforward syntheses lead to studies
of
the oxidative and structural investigation of some of these compounds. Attempts to extend the
diphenylphosphido work by using
(£-C
6
H
iihP
were not successful, since the products were
extremely unstable.
These syntheses are all characterized by simplicity
of
procedure, by high yield, and by the
formation
of
pure products. Since the one-step method requires the use
of
essentially only metal
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plus
ligand
precursor, the chances of
contaminating
the product are
minimized,
and
we
believe
that this approach is
an
improvement on
those
in
the literature for thiolates and selenolates.
The
generality of the methods
for
phosphido complexes is open to doubt
in
view of the problems
encountered with
{£-C
6
H
l1
}zPH,
and further
work
is obviously required here. The
potential
significance of these
syntheses
lies
not only
in
their simplicity, or
in
the interesting structures
identified,
but
also because some of these
products
may prove to
be
precursors of the III-V
and
IV-VI
compounds
which
are
so important in the microelectronics industry.
One interesting and unexpected aspect of these
syntheses is that the products
of the
electrochemical oxidation of copper included some molecules with unusual
cage
structure based
on CU4S4
CU2S2 CU4P4
etc.,
rings [35], [41]-[44].
This is not
the
place to attempt a discussion
of such structures, nor
those
of the [M
4
(SPh)IO]2- anions (M
=
Zn,
Cd), since
good reviews have
already
been
published
[45,46],
but
their easy
accessibility
by direct
electrochemical
synthesis
is
yet
another feature
of this versatile method.
Some
special
mention should also be made
of
the electrochemical
synthesis of amido
complexes.
In
some
early
unpublished work,
we
used
solutions of i-Pr2NH, 2,2,6,6-
tetramethylpiperidine or M ~ S i } z N H attempts to prepare
M(NR2h
compounds
(M
= Zn, Cd,
Hg), but although
solid
products were
obtained,
their properties were not those of the expected
properties, and this work was not pursued in the light of competing interests. With the amine
pY2NH,
on
the other
hand,
the
experiment
proceeded smoothly to
give
M(NPY2)n
(M
=Cu, Ag,
Zn, Cd, Tl) [47].
t seems
likely that the anions of the
amines used
earlier, or the radicals
derived from them, react with CH
3
CN to give species of the type R
2
NC(CH
3
)N-, and
that
the
products are in
fact
derivatives of
such
ligands.
Finally, we may note that if
one
views the parent ligand precursor
e.g.,
thiol,
phosphine,
etc.)
as a weak
acid which
is reduced cathodically to yield the corresponding
anion,
then R
3
CH
compounds
can also
be included
in this
group of syntheses. The successful production of
PhCCCu, which is itself a
useful
synthetic
reagent in
organic chemistry,
from
PhCCH (PK -
25)
is one
example
of this [48], and
in
related
studies of wider implication,
Casey
[3]
and Lehmkuhl
[49,50]
have obtained
cyclopentadienyl and related
compounds. The range of syntheses
achieved
with such weak acids, which do not
react
with
metals
under non-electrochemical conditions, is
illustrated
in
Scheme 2.
s.
Bidentate Ligands
A different group of
weak acids
which lend themselves readily to the direct electrochemical
synthesis
of metallic derivatives is illustrated in Scheme 3. These differ from those
in
Scheme
2 essentially in that the acidic
group
(OH, SH, etc.) is part of a molecule which contains a second
donor
atom, so
that
cathodic
reduction gives
an anionic ligand which is a potential chelating
agent.
In
early
studies [51,52], it was found
that acac- (2,4-pentandionate) derivatives were
readily accessible
by this
route,
and as in the work
described
above, the
products
may be
M(acac)n
or
M(acac)nLm depending
on
the solutes present during
the
electrolysis [53]-[55]. A
number of related
bid.entate
oxygen donors
have been
studied, including catecholates [53] and
other aromatic 1,2-diolates
[57,58] and
carboxylic
acids [59]. Bidentate
sulphur
donor
ligands
represent a
simple extension
of this aspect of direct electrochemical synthesis, and dithiolato [60],
[44],
dialkylthiocarbamato [61] and diethyldithiophosphato
[61]
derivatives have been prepared.
The
metals
which have
been
successfully
used in
this aspect of the
work
include Cr, Mn, Fe, Co,
Ni,
Cu,
Ag, Zn, Cd,
Hg,
Ga, In, Tl Sn, Th and U
Once
again,
it is appropriate to emphasize that the simplicity and directness of the method offer
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Scheme
Scheme 3
Scheme 4
M(SeR)n
M S2
R
n
M S SH)R]n
M acac)n
M acac)n4n
/
Hacac. L
M[O OH)R]n
R OH)2
M 02
R
n
7
M[O OH)R]n
El
3
NH[rn0
2
R]
t
N
In[O(OH)R] 1
2
In[O(OH)R]
l . . Q 0 2 c 6 ~ r 4
In[O(OH)R](02
C
6
8r
4)
21
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considerable
advantages, both
in the procedure itself and in the isolation of pure products. For
example, in the synthesis of
metal
carboxylates the products are water-free, since aqueous media
are not used, and
so do
not require extensive washing
to
remove soluble contaminants. Equally,
in
the
synthesis
of M acac)n compounds, the direct transformation of metal into complex
avoids
dissolution, extraction,
etc.
as
in
the
more
classical
approach.
Large
scale
preparations
have
been
successfully
carried out by
Lehmkuhl
et al
[51]
6. Strong
Acid
Systems
Since many metals
react
directly
with
aqueous solutions of strong mineral
acids,
there
has
been
little
incentive
to apply electrochemical methods for the preparation of derivatives of these
acids.
A brief
investigation showed that
[Cr dmso)6]Br3 can be
obtained
by
this one-step
route [15], and
later efforts showed that other
metals
can serve as the starting point for dmso or CH
3
CN
complexes
of
the type
[ML61 BF4)n
for
M
=
V,
Cr,
Mn,
Fe, Co, Ni,
Zn,
Cd and In
[62].
Some work
which
we
were not able to develop fully as
would
have been wished, using the
heavy elements thorium
and
uranium,
showed
that the application
of
the
method
is
not restricted
to lighter metals, since [Th(dmso)s](N0
3
)4 could
be
prepared
from
solutions of nitric acid in tri
n-butyl phosphate; other
media used in this work
included
N
0
4
/EtOAc/CH
CN
mixtures [63].
An
efficient and compact method of treating
spent
fuel rods from a nuclear reactor might
be
developed around the electrochemical
oxidation
and dissolution of the metal
fuel
element.
7.
Low
Oxidation State Products
One of the
most
interesting aspects of the
work,
as noted earlier, is that
low
oxidation
state
compounds are
often
produced by the dissolution of a metal
anode.
A list of some
such
syntheses
is given
in
Table 1, and the appropriate papers should be consulted for details. The particular
interest in Main Group metals in our laboratory has
again
lead us to concentrate
on
the elements
Ga, In, TI, Sn and Pb, although the early syntheses on copper l) species [25], of
CrBr3
[15],
and
of ThI2
[28],
show that
these are
examples of interest in transition and
heavy element chemistry.
The contrast between
the electrochemical method and the method in the
literature
is very
striking
in one
particular
case, namely
the
formation
of the tin(lI) compounds
by the
electrochemical
oxidation of the
metal in
solutions of 1,2-aromatic diols
in
acetonitrile R OH}z
=
catechol, 2,3-dihydroxynaphthalene,
Br4C6 OH}z,
2,2 -dihydroxybiphenyl). The
room
temperature,
high
yield, electrochemical
synthesis
of Sn OzR) compounds is a great improvement
over the high temperature methods
used in
the earlier
syntheses
of
such
compounds [64].
The
ready
accessibility of the
Sn OzR)
materials
lead to a study of their redox reactions, and their
coordination chemistry.
Not
surprisingly, the direct synthesis of Pb OzR),
and its
redox
chemistry, follow from the tin(ll) system [65]. In each case the
Ep
value of 0.5 mol F-
i
can
be
understood
by the sequence
cathode: R OH}z 2e - R ~ H2
anode:
~
M - M OzR) 2e
giving
the
overall
stoichiometry
(4)
(5)
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23
T BLE 1
Low oxidation state products
Product
Ref.
CrBr3
15
G ~ X l
27,31
Ti acach
52
InX,InI
2
- 7,67
CuX
23
In SR)n n = 1,2)
37
CuSR
35,41-45
In[O OH)R]
68
T h ~
28
In[S SH)R]
69
Sn SR
33
TISR
37
S n ~ R )
64
T l 2 ~ R
69
Pb 02
R
)
65
T BLE
2.
Electrochemical efficiencies
in
mol
F-
1)
for
the oxidation of
elemental indium.
Solute
EF
Product
Ref.
RX
1
RInX
2
, etc.
20
~ N S m
1.0 ± 0.1
InL3
61
RSH
1.00 ± 0.03
In SR)n 37
n
=
1,2,3
P h 2 S ~
1.00
In SePhh ?)
39
Y
NH
1.10
no
stable product 47
R OH)2
1.01
±
0.01
In[O OH)R]
68
R SH
1.00 ± 0.03
In[S SH)R]
69
R O)OH
1.02
In 02
R
h
57,58
Ph
2
PH
1.05
no
stable product
40
CH
2
X
2
1.02
±
0.01
X
2
InCH
2
X, etc.
67
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24
R OHh
+
M
.... M ~ R ) + H2
6)
As
with
the
cases
discussed above, this reaction
does
not
occur
under
normal
conditions.
We
also found that with
the aromatic
thiols RSH);
the
primary
products were
Sn SRh or
Pb SRh,
although subsequent unidentified) oxidation processes in
the presence of bidentate
donors gave
Sn SR)4.bpy
etc. for tin, despite the
fact
that the
Ep
of
0.5
mol F-
1
showed that the
electrochemical processes
are
essentially
unchanged by
these
donors from eqs.
1)
2) [33].
The
results
with
the
elements
gallium,
indium and
thallium
are particularly
interesting in this
context.
With thallium
RSH
or R SHb as with other
ligands, the
products are
invariably
those of
thallium I), and Ep
= 1.0 mol F-
1
[37],
which is
in keeping with the
known
stability of
this oxidation state;
in
the
case
of solutions of R OHh, the product
is
contaminated
with
thallium
metal
formed
by cathodic reduction of the slightly soluble product
[66]. With
gallium, one can
obtain
either
the
Gll:J.xi
anionic
complexes
of
gailium Il)
from solutions containing HX
X
=
Cl,
Br, I),
with Ep
- 0.6, or GaX
4
-
from
solutions of
X-
X
2
,
although
the Ep
values
show
that a gailium Il)
species
is
again formed at the anode, with subsequent
oxidation
to
gallium
III)
in the electrolyte
phase
[27].
Indium yields indium lIl)
compounds
as the products
in
a number of cases, but the
Ep
value
of 1.0 indicates clearly that the product of
anodic
oxidation is
an
indium l) species. See Table
2.)
Some special cases are worth
discussing.
In
the
oxidation
of this
element in solutions
of
RX
R =
Me,
Et, Ph,
Bz,
C FS; X = CI, Br,
I;
not all combinations) the isolated products are
RInX
2
,
RInX
2
.bpy
or ~ N [ R I n X 3 ] depending on
the composition of the electrolyte, but
in every
instance one
finds Ep
= 1.0
mol
F-
1
, implying the sequence
cathode: RX
e ....
· X-
anode: X-
In .... InX e
followed
by oxidative
insertion and complexation
InX +
RX
....
RInX2
7)
8)
9)
10)
Similarly,
solutions
of CH
/CH
CN X = CI, Br,
I) give Ep
=
1.0 mol
F-
1
, and in
the
case
of X
=
Br or I,
the products
are
derivatives of
~ I n C H 2 X ; InCI disproportionates
in these
systems to give In
0
InX
3
.
The
oxidative reactions
of InX have been sufficiently well studied
in
non-electrochemical work for
the
chemical
processes 9) and 10), and the disproportionation,
to be
well
understood [67].
A
similar
situation
has been found
in attempts to synthesize
indium complexes
of a
number
of
the ligands
identified
in Schemes
3
and
4.
With
solutions
of bidentate oxygen
donors
[57,58],
or
~ N C S H
[61],
Ep
= 1.0
mol F-
1
,
but
in each
case the product recovered
from
the reaction
mixture is InL
3
, while for Ph
2
PH
or
pY2NH but the presumed InL species is too reactive to
allow
any identifiable
product
to
be isolated. These results suggest
that there are again post-electrolytic
oxidative processes,
possibly
of the type
InL
HL
....
~
H I ~ H
i ~
11)
12)
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.
h i ~
HL .... H I ~
h i ~
....
I ~ L
.
2H
2InL
....
2HL
2In
25
13)
14)
15)
Research into
such reactions,
in which one-electron transfer is the essence of the presumed
mechanism,
is planned. The only
conclusion
possible at the moment is that indium l)
species are
the favoured product of the
anodic
oxidation of this
element. One
system in which
InL,
~ L
and InL3 species
were identified
was
for
L = SR,
where the
nature of R
appears to have
a strong
influence
on the
sequence
of
reactions such as eqs.
11)- 15) [37].
Finally, we note the unusual compounds prepared from indium
and
solutions of aromatic 1,2-
diols [68]
or aliphatic dithiols
[69], where the
interest is
on
both
the low
oxidation state of the
metal and
the
structure of the
ligand.
In each system, the product is an
indium I)
complexes of
the type
In[O OH)R]
or
In[S SH)R]
in
which
only one of the
two
acidic hydrogens of the parent
has
been
lost. The electrochemical
efficiency
is unity, which can be explained in at least
two
ways. In the first of these,
the
sequence
eq. 4)
5) see above) is followed by
Inz OZR) R OH}z .... 2In[O OH)R]
16)
with an overall stoichiometry analogous to eq. 6), namely
R OH)z
In
.... In[O OH)R] IhH
z
17)
and as before
it should be
noted
that this reaction only occurs in the electrochemical cell.
An
alternative
to
eqs. 16)- 18) is
to
replace
eq. 4)
by
cathode: R OH)z e
....
R OH)O-
IhH
z
18)
and the anode
process
is
then obviously
R OH)O-
In ....
In[O OH)R]
19)
This would
entail
a revision of eq. 5) to
allow for
a
sequence such
as
solution: R OH)O- R OH)z .... [R OH)O OH)zRr 20)
anode: 2[R OH)O OH)zRr M
....
M OzR) 2e 3R OH)z 21)
and
a number of other variants
can also be invented.
There is no
experimental evidence
as to
the species in
solution,
but
since
the existence of the products is unquestionable,
further
work on
this matter is indicated.
The chemistry of
these species
involves two types of reaction,
namely
removal of
the
hydrogen
to
give e.g.)
Et
3
NH+[InOzRr and
the oxidation to
the corresponding indium Ill)
species
by
iodine or Q-quinone. These are summarized in Scheme
4.
We have not been able to obtain
crystals of
either
In[O OH)R] or In[S SH)R]
compounds,
but fortunately a related thallium l)
compound, Tl
z
[O OH)C
1Z
H
s
1z
was
prepared
many
years
ago
[70],
and
this has
a
dimeric
structure
based
on a TlzOz ring with a
lone
pair of electrons on thallium l) [66]. Generally,
the
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26
chemistry of In[O(OH)R] and In[S(SH)R] compounds can be satisfactorily explained on the basis
of an analogous structure, especially since compounds with I n 2 ~ rings have been identified in
other studies. There is still much to be done to
develop
the chemistry of indium(l), and direct
electrochemical
synthesis hllS provided a most
useful
entry in a
new aspect
of this
work.
The structure of the singly protonated ligand,
as
well
as
the nature of the species generated the
electrolyte
phase, is another challenging aspect of this work. The indium(l) compounds are not
unique in
this respect, since
studies
with
copper anodes
and solutions of
aromatic
1,2-diols
gave
a copper(l) derivative ofR OH)O- as the adduct C ~ [ O C 6 C I 4 O H ) h . d i p h o s [71]. Further
work
involving
other
metals
and such
ligands
is
planned.
8. Solutions in Liquid Ammonia
We have not
attempted
to carry
out
any
direct
electrochemical syntheses in aqueous media, but
there are reports
in
the literature of work carried out some 40 years
ago
on the electrochemical
oxidation of a number of metals in liquid ammonia
[72,73],
and since two of the metals in
question were gallium
and
indium,
this
seemed
a natural area
for
further investigation.
The
immediate
conclusion, which we established
by
measuring p, is
that
indium
is
oxidized in liquid
ammonia solutions
of
ammonium halides
at -35°C to the state, but unfortunately
we
were
not able to isolate any compounds of this oxidation state from the resultant solution, although in
one particular case we were able to show
by
Raman spectroscopy that species with the
characteristic p(ln-In) stretching
mode
were
present
in the solution [74]. When we attempted
to
work
up these solutions
both
indium I) and III) halide derivatives of
ammonia were
obtained,
and a mass balance, taking into account the quantity of material isolated and the quantity of
indium
dissolved,
showed
that the typical disproportionation reactions of
indium
I
were
indeed
being reproduced under these conditions. We concluded
that
the overall
stoichiometry is
(24)
but that unfortunately
the
inherent instability of ~ X 4 in these solutions, even in the presence of
ligands known to stabilize these
species
in other circumstances, means that this is not a useful
preparative route to indium(Il) complexes.
We
also
investigated solutions
of quinones and substituted catechols,
in
the latter case using
mixtures of ammonia
and
an organic solvent
to achieve sufficient
solubility,
and here
the
electrochemical efficiency
shows
that
indium goes
to
the oxidation state. In the presence
of an Q-quinone, oxidation of the
lower
oxidation state halide leads to InX(catecholate), and
hence
by substitution
to
InX
3
•
When a 1,2-diol is used, there are again questions as to the solute
species which are generated at the cathode,
but
the overall reaction can be
written
as
(25)
with Ep = 0.33 mol p-l. The indium(III) derivative of
3,5-di-tert-butyl-catechol
is in
fact the
dimeric
anion
[In2(dbcatechoIMNH
3
)4]2-, whose crystal structure confirms the presence of
the
substituted catecholate ligand,
and
shows that the dimer is
dependent
upon an I n 2 ~ ring.
It
seems
likely that the use of liquid
ammonia media
may
offer
some
advantages
in
the direct
electrochemical synthesis of low oxidation
state
complexes. We hope to investigate this
matter
in the future.
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27
9. Conclusions
I have tried to show that the use
of
a metal
as
the sacrificial anode in electrochemical
oxidation can form the basis of a very wide range of syntheses of inorganic and organometallic
compounds. We believe that the method
is
indeed a choice one for a series
of
ligands derived
from weak mono- and dibasic organic acids, and that while we have not investigated all metals
ourselves, there seems every reason to assume that the range of syntheses could be extended to
most areas of the Periodic Table.
The emphasis in our work on the metallic elements Zn, Cd, Hg, Ga, In, TI, Sn and Pb is
a logical consequence of the synergistic effect
of
other work going on in our laboratory. These
electropositive metals do not generally react with weak organic acids, but the driving force
provided by the applied potential brings about reactions which proceed with an understandable
stoichiometry, with high yield, and under mild conditions. I believe the range of syntheses
discussed in this paper has served to establish direct electrochemical synthesis as a useful and
readily accessible experimental technique for those who are not afraid to use electricity
in
chemistry.
10. Acknowledgement
Much of the work reported in this and other papers has been supported by Operating Grants
from the Natural Sciences and Engineering Research Council
of
Canada. It
is
also a pleasure to
acknowledge
my
gratitude
to
the large number
of
co-workers whose names are recorded
in
the
papers which I have quoted here and elsewhere.
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