1

2. Preparative inorganic and organometallic electrochemistry

Advantages:

extremely high oxidation and reduction power maintained by controlling the electrode potential ε

no contamination derived from ox. /red. agents or by-products

selectively driven electrode reaction by adjusting ε

in the case of some substances there are no other synthetic methods (e.g. CrH, layers of numerous metals and alloys)

Disadvantages:

relative slowness

special equipments

2.1. THEORETICAL BACKGROUND

2.1.1. CURRENT EFFICIENCY

Faraday law:

where m is the mass of product, M is its molar mass, z is the charge number of the electrode reaction, F is the Faraday constant (96487 C mol-1), I is the current intensity, t is the time of electrolysis

MIt

mzF

2

2.1.2. THE CURRENT VS VOLTAGE CURVE (Fig. 1)

2.1.3. THE ROLE OF THE OVERVOLTAGE (Fig. 2, Table 1, Table 2)

In aqueous medium it is imperative to push back the undesired gas evolution

Possibilities to avoid it:

Use of electrodes with high overvoltage with respect to formation of H2 and O2

E.g. Reduction of V(V)/V(IV) salts to V(III) or V(II):

VO2+ + 2 H+ + e- = VO2+ + H2O ε0 = +1.0 V

VO2+ + 2 H+ + e- = V3+ + H2O ε0 = +0.34 V

V3+ + e- = V2+ ε0 = -0.20 V

Increase of the reactant concentrations

Decrease of the temperature

2.2. EXPERIMENTAL CONDITIONS

2.2.1. SELECTION OF THE PROPER ELECTRODE

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Criteria:

inertness/starting material (see also Table 3)

overvoltage (in aqueous solutions!)

conductivity

crystal structure

shape

2.2.2. SOLVENTS AND SUPPORTING ELECTROLYTES

Criteria:

conductivity

In aqueous medium: mineral acids for acidic, alkaline metal hydroxides for basic, alkaline metal halogenides, sulphates, nitrates, perchlorates for neutral solutions

In non-aqueous medium: perchlorates of R4N+, Na+ and Li+, furthermore, I-, Br-, PF6

- and BF4

- salts

control of pH (big challenge in non-aqueous medium!)

in the case of non-aqueous media there are often no counterpart of the electrochemical reaction in aqueuos medium, more resistant to oxidation/reduction, wider range of Δε (Table 4)

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2.2.3. THE ELECTRODE POTENTIAL

upon changing ε the reaction belonging to the most positive potential occurs in the cathode and most negative in the anode, resp.

reference electrode rquired for precise measurement of ε (most widely used: in aq. sol. calomel, in acetonitrile Ag/Ag+(0,01M) el.; Table 5, Table 6, Fig. 3)

2.2.4. THE EFFECT OF THE ELECTROLYTE CONCENTRATION

E.g. anodic electrolysis of cold H2SO4 sol. on Pt (Table 7); 3 reactions:

2 H2O = O2 + 4 H+ + 4 e-

2 SO42- = S2O8

2- + 2 e-

3 H2O = O3 + 6 H+ + 6 e-

(at 0˚C the last one can be neglected)

2.2.5. THE EFFECT OF THE TEMPERATURE

resistivity of the cell, changes of the reaction rates (Table 8)

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2.2.6. DEPOSITION OF METAL COATINGS

Infuenced by:

current density (low j: coarse crystals, high j: finely particled crystals, very high j: tree-

like sructures + H2 evolution, staining)

electrolyte concentration (sufficiently high required)

temperature (has effects of different direction and extent)

additives (e.g. sugars, camphor, gelatine, glue, casein; their adsorption on the surface

may result fine-particled coating)

formation of metal complexes (e.g. deposition of Ag from AgNO3 sol.: badly adherent,

coarse-particled coating, however, from [Ag(CN)2]- sol.: smooth durable coating)

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2.2.7. METHODS OF THE ELECTROLYSIS (Fig. 4)

const. U (= IR + Δε, where U is the applied voltage, Δε is the potential difference

between the working and auxiliary electrodes, I is the current intensity, R is the cell

resistivity); advantageous when only little amount of the electroactive component is

deposited

const. I (Fig. 5)

const. ε (= potential difference between working and reference electrodes; advantage

opposite to the former two: selectivity, disadvantage: sophisticated equipment)

2.2.8. CELL DESIGNS (Fig. 6, 7 and 8)

undivided compartment (no diaphragma)

divided compartment (diaphragma between the working and auxiliary electrodes;

usually more advantageous)

Interesting case

The product of the cathodic reduction of benzene in methyl amine sol. containing LiCl is

1,4-cyclohexadiene in undivided cell

cyclohexene in divided cell.

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2.3. ELECTROLYTIC SYNTHESES

2.3.1. Preparation of gases

H2, O2: yielded in high purity by the electrolysis of KOH sol. of 30%

O3: O3/O2 mixture of composition of 12.5% obtained by anodic current of high density from H2SO4 sol. of 1.08 g cm-3 below temperature of -10˚C; much higher yield from HClO4 sol. Of 40% at -56˚C

Cl2: pure, O2-free gas from HCl of higher conc. than 23% by medium j

F2: by the electrolysis of KF/HF

GeH4: on Pb cathode from ice-cooled H2SO4 sol. containing GeO2

SbH3: on Pt-Ir cathode at 0°C from a H2SO4 sol. (1.7 L of vol., 2 M of conc.) cont. 8 g of Sb and 80 g of tartaric acid

PbH4, BiH3: give formation in lower conc. on Pb or Bi cathode from H2SO4 sol.

2.3.2. Deposition of metals

In a fairly pure state (C-, S-, P- and N-free) metals can be obtained from their salt sol.-s by cathodic reduction:

most easily the more noble metals,

under certain conditions from aq. sol.: Zn, Cd, Ga, In, Tl, Pb, Sb, Ni, Co, Fe, Mn, Cr

from aq. citrate sol.-s: Mo, W, Ta and Nb,

on mercury cathode (H2 overvoltage! + form. of amalgam) from aq. sol.-s the alkali and alkali earth metals

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Non-aqueous solvents more rarely give good results, however, e.g.

very pure Li from LiCl sol. of abs. pyridine or acetone,

Na, Sr, Cd, Sn, Sb, Bi and As from electrolyte sol. of acetone,

La, Nd and Ce from their salts of abs. ethanolic sol. on Hg cathode, and

Al from xylenic sol. of its organometallic compounds, resp.

can be obtained.

2.3.3. Cathodic reductions without metal deposition

Smooth metal surfaces as cathode (H2 overvoltage!) are used (e.g. Hg, Pb, Tl, Zn, Cd and Sn, furthermore their amalgams).

Especially applied for the preparation of hydrides, organometallics and compounds bearing metal of low oxidation state (e.g. NH4V(SO4)2.12 H2O, (NH4)2V(SO4)2.6 H2O,

CrSO4.5 H2O, Ag2F, K3MoCl6, K3W2Cl9, EuSO4, YbSO4).

2.3.4. Anodic oxidations

Unless the metal of the anode is oxidized, smooth Pt or Pt-Ir (high O2 overvoltage + resistivity to oxidation) and, in sulphuric acid, Pb anodes are preferred.

Examples: Cu2O, AgO, (NH4)2S2O8, KClO3, KClO4, KBrO3, Co2(SO4)3.18 H2O,

BaFeO4.H2O, Pb(OCOCH3)4, (NH4)2PbCl6.

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2.3.5. Electrolysis of molten salts

Frequently applied in industry (e.g. large-scale preparation of Na, K, Be, Mg, Ca and

Al), however, in general rarely in the lab due to its uncomfortness and insufficient

yields.

It has greater importance in the laboratory-scale preparation of pure Li, Ta, Th, U,

La and other rare earth metals.

The molten electrolyte is usually a halogenide, especially some fluorides (capable of

dissolving the oxide well).

It is advantageous to carry out the electrolysis at the lowest temperature (e.g. use of

eutectic mixture of salt) but above the melting point of the metal.

Problems arise if the deposited metal dissolves back into the molten salt or react with

the anode gas (e.g. Cl2, CO or fluorine compounds).

Solution: efficient separation of the cathode and anode space, use of a guard tube

around the anode.

The crucible for the electrolysis is usually made of porcelain, glass, quartz, corundum

or graphite.

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2.3.6. Preparation of organometallic compounds

The species generated in the course of the electrolysis of organic compounds often result in the formation of organometallic compound when reacting with the electrode metal.

In numerous cases, however, new derivatives can be obtained by the electrolysis of organometallics.

Cathodic methods (radical mechanism; R is an organic group, M is a metal):

R+ + e- → R•

R• + M → RM

Anodic methods (the organometallic often prepared by the anodic oxidation of another one: indirect metallation):

R- → R• + e-

R• + M → RM

Possible side-reactions: dimerization of R• or cleavage of H atom.

Problem: medium has low electric conductivity!

E.g. the R2M organometallics (M = Mg, Be, Zn, Cd, Hg) are not only bad conductors in

pure state but in their sol. with donor-type solvents (e.g. ethers) as well.

Solution: use of salts, metal hydrides and organomet. reagents as additivies

E.g. addition of EtNa to Et2Zn a salt-like adduct forms: NaZnEt3 → Na+ + ZnEt3-

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2.3.6.1. Cathodic reductions

2.3.6.1.1. Alkali and alkaline earth metal compoundsUsually they are generated in situ in sol. for the preparation of organic compounds.

There are more efficient methods than the electrolytic ones.

Electrolytic initiation of anionic polymerizations, e.g.

Cathodic red. of 1,1-diphenyl ethylene in CaI2/HMPA medium

2 Ph2C=CH2 + Ca2+ + 2 e- → (Ph2C–CH2–CH2–CPh2)2-Ca2+

2.3.6.1.2. Reduction of onium-type cations

Main application fields:

Preparation of organic amalgams via electroreduction of onium ions

R4N+ + e- + Hg → R4N•/Hg

(cf. Na+ + e- + Hg → Na•/Hg)

Organic amalgams bear metallic characters as well.

Requirements of the successful synthesis: in gen. T below 0ºC, solvents being less sensitive to reduction than water

E.g. tetramethyl ammonium amalgam (silverish-white crystalline substance, very reactive, liberates Me3N at room temp.) can be obtained from the red. of abs. ethanolic sol. of Me4N

+Cl- on Hg cathode at temp. of. -10ºC.

The analogue S- or P-containing amalgams have similar properties.

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Electrolysis of onium ions with the participation of the cathode material as reactant

Above room temp., following the neutralization of the onium cation, an organometallic compound forms by the cleavage of the hetero atom-carbon bond (A = hetero atom, e.g. S, P, I, Sb, N; M = electrode metal, e.g. Hg, Pb; R = organic group)

RnA+ + e- + M → RM + Rn-1A

E.g. Reduction of the aq. sol. of the salt BzMe2S+Tos- on Hg cathode at 90ºC gives

crystalline Bz2Hg in 94% yield (Bz ≡ benzyl, Tos ≡ tosylate).

Electrolysis of organometallic onium ions

E.g. Reduction of the aq. sol. of the salt Bz3MeAs+Cl- on Hg cathode gives Bz2MeAs in 95% yield.

2.3.6.1.3. Metallation of organic compounds on the cathode metal

Example 1: Reduction of the aq. sol. of cyclohexanone cont. 5% H2SO4 on Hg cathode at 55ºC gives Ch2Hg (Ch = cyclohexyl) in ca. 30% yield.

Example 2: Red. of the aq. NaOH sol. of acryl nitrile on Sn cathode at 15ºC gives Sn(CH2CH2CN)4.

Example 3: Red. of the acetone-water mixture of MeBr cont. Bu4N+Br- on Pb cathode at

room temp. gives PbMe4 in 98% yield.

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2.3.6.1.4. Reduction of organomercury salts (RHgX)

Preparation of radical-type product (R = Me, Et, n-Pr) with metallic character (good el. conductivity, black colour):

RHgCl + e- → RHg• + Cl-

RHg• may be isolable showing metallic luster in compressed state; upon heating disproportion occurs:

2 RHg• → R2Hg + Hg

Example: Ph2Hg can be obtained by the red. of the aq. sol. of PhHg(CH3COO) cont. KNO3 on Hg cathode.

2.3.6.1.5. Reduction of organothallium compounds

2.3.6.1.6. Reduction of organosilicon, -germanium, -tin and -lead compounds

E.g. red. of Ph3MX (M = Si, Ge, Sn, Pb; X = Cl, CH3COO) in dimethoxy ethane sol. on Hg cathode.

Different processes can occur; in all cases first:

Ph3MX + e- → Ph3M• + X-

followed by the formation of:

if M = Si, Ge, then Ph3MH (H abstraction),

if M = Sn, then Ph3MMPh3 (dimerization), and

if M = Pb, then Ph2Hg (arylation of the cathode metal).

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2.3.6.1.7. Reduction of organoarsenic, -antimony and -bismuth compounds

Similar processes as in Group 14, however, the dimerization of the intermediate radicals are much more often.

2.3.6.2 Anodic oxidations

2.3.6.2.1. Preparation of the p-block organometallics

The corresponding organic derivative of the anode metal forms.

Methods can be divided into two main groups:

Electrolysis of the diethyl ether sol. of Grignard-type magnesium compounds

E.g. PbEt4 on Pb anode from EtMgCl, Et3Al on Al anode from EtMgI and Ph3B on the

metallic form of B anode from PhMgCl, resp., can be obtained (trialkyl phosphines can be also prepared in an analogous way).

Anodic ox. of complex organic „ate” salts cont. alkaline metal and Al or B

E.g. PbEt4 can be obtained from KAlEt4 on Pb anode with Hg as counter electrode:

4 KAlEt4 + Pb → PbEt4 + 4 AlEt3 + 4 K(Hg)x

E.g. SbEt3 and InEt3 obtained from NaF.2 AlEt3 on Sb and In anode, resp., in 85% yield.

E.g. Et2Mg obtained from aq. sol. of NaBEt4 on Mg anode with low j in 73% yield.

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2.3.6.3. Electrosynthesis of organic derivatives of transition metals

Some examples:

Preparation of Ti(cot)Cl.solv (cot ≡ cyclooctatetraene, solv ≡ solvent) complexes in

70% yield from the pyridinic or thf sol. of TiCl4 and cyclooctatetraene on Al cathode

at 20-40ºC by reduction

Prep. of Cr(CO)6 in 60% y. from the Bu4NBr/pyridine sol. of Cr(acac)3 (acac ≡ acetyl

acetonato) on steel cathode under CO pressure at 81ºC (analogously obtained:

Mn2(CO)10, V(CO)6, Fe(CO)5, Co2(CO)8, Ni(CO)4)

Prep. of ferrocene in 90% y. from the dimethyl formamide sol. of TlCp on Fe anode

Prep. of Rh(PPh3)4 in 70% y. from the acetonitrile-toluene solv. mixt. of RhCl(PPh3)3

on Pt cathode

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Thank you

for

your attention!

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Fig. 1

18

Fig. 2

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Table 1

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Table 2

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Table 3

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Table 4

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Table 5

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6. táblázat: Standard elektródpotenciálok vizes oldatokban 25°C-on Table 6: Standard potentials in aq. sol. at 25°C

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Fig. 3

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Table 7

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Table 8

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Fig. 4

29

Fig. 5

30

Fig. 6

31

Fig. 7

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Fig. 8