DIRECT ELECTROCHEMICAL SYNTHESIS OF INORGANIC AND ORGANOMETALLIC COMPOUNDS

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.


2/17

16

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


3/17

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


4/17

18

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,


5/17

19

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


6/17

20

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


7/17

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


8/17

22

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)


9/17

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


10/17

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)


11/17

.

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


12/17

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.


13/17

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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53. Kumar, N. and Tuck, D.G. (1982) 'The direct electrochemical synthesis of neutral and

anionic complexes of thorium (IV)' ,

Can.

J.

Chem.

60, 2579-2582.

54. Bustos,

L.,

Green, J.H., Khan, M.A. and Tuck, D.G. (1983) 'The electrochemical synthesis

of j3-diketonato complexes of cadmium(II), and the crystal and molecular structure of

Cd(acachJ>hen',

Can. J.

Chem.

61, 2141-2146.

55. Matassa, L., Kumar, N.

and

Tuck, D.

G

(1985) 'Direct electrochemical synthesis

of

chelate

complexes of uranium(lV) and uranium(VI) Inorg. Chim. Acta 109, 19-21.

56. Mabrouk, H.E., Tuck, D.G. and Khan, M.A. (1987) 'The direct electrochemical synthesis

of zinc and cadmium catecholates and related compounds',

Inorg.

Chim. Acta 129, 75-80.

57. Annan, T.A., Peppe, C. and Tuck, D.G. (1990) 'The direct electrochemical synthesis

of

dlO

metal ion complexes

of

some anionic bidentate oxygen donors',

Can. J. Chem.

68, 423-

430.

58. Annan, T.A., Peppe, C. and Tuck, D.G. (1990) 'The direct electrochemical synthesis of

some metal derivatives

of

3-hydroxy-2-methyl-4-pyrone',

Can. J. Chem.

68, 1598-1605.

59. Kumar, N., Tuck, D.G. and Watson, K.D. (1987) 'The direct electrochemical synthesos

of

some transition metal carboxylates',

Can. J. Chem.

65, 740-743.

60. Mabrouk, H.E. and Tuck, D.G. (1988) 'The direct electrochemical synthesis of zinc and

cadmium derivatives

of

a,w-dithiols, and their reaction with carbon disulphide',

Inorg.

Chim. Acta 145, 237-241.

61. Geloso, C., Kumar, R., Lopez-Grado,

l.R.

and Tuck, D.G. (1987) The direct


of

dialkyldithiocarbamates and diethyldithiophosphate complexes

of Main Group and transition metals', Can. J. Chem. 65, 928-932.

62. Habeeb,

J.J.,

Said, F.F.

and

Tuck, D.G. (1981) 'The direct electrochemical synthesis

of

cationic complexes', J. Chem. Soc

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118 120.

63. Kumar,

N and

Tuck, D.G. 'The direct electrochemical synthesis

of

some thorium(lV)

nitrate complexes', Can. J.

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62, 1701-1704.

64. Mabrouk, H.E. and Tuck, D.G. (1988) 'The direct electrochemical synthesis

of

tin(ll)

derivtaives of aromatic 1,2-diols, and a study of their oxidative addition reactions', J. Chem.

Soc

.•

Dalton Trans. 2539-2543.

65. Barnard, G., Mabrouk, H.E.,

and

Tuck, D.G. Unpublished results.

66. Kickham, J.E., Taricani, L. and Tuck, D.G. Unpublished results.

67. Annan, T.A., Tuck, D.G., Khan, M.A. and Peppe, C. (1991) 'Direct electrochemical

synthesis of X

2

1nCH

2

X compounds X

=

Br I), and a study of their coordination chemistry,

Organometallics

10, 2159-2166.

68. Mabrouk, H.E. and Tuck, D.G. (1989) 'Coordination compounds of indium. Part 45.

Indium(l) derivatives of aromatic diols'

Can.

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67, 746-750.

69. Geloso, C., Mabrouk, H.E. and Tuck, D.G. (1989) 'Coordination compounds

of

indium.

Part 47. Indium(l) and thallium(l) derivatives

of

a1kanedithiols'

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17/17

3

Trans. 1759-1763.

70. Brady, O.L.,

and

Hughes, E.D. (1933) Coordination compounds

of

2,2 -diphenol ,

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71. Annan,

T.A.

Kickham, J.E.

and

Tuck, D.G. (1991) The direct electrochemical synthesis

of

the novel copper

I)

complex CU2[OC6CliOH)h[(C6H5hPCH2P(C6H5hh

Can.

J.

Chern.

69,251-256.

72. McElroy, A.D., Kleinberg, J. and Davidson, A.W. (1952) The anodic oxidation of higher

members

of

the aluminum family

in

liquid ammonia ,

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Am.

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74,735-739.

73. Davidson, A.W., and Kleinberg,

J

(1953) Unfamiliar oxidation states in liquid ammonia ,

J. Phys. Chern.

57, 571-576.

74. Annan, T.A. Gu, J., Tian, Z. and Tuck, D.G. Unpublished results.

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DIRECT ELECTROCHEMICAL SYNTHESIS OF INORGANIC AND ORGANOMETALLIC COMPOUNDS