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Electrochemistry 191

electron-donating substituents reduce the potential by as
much as 0.7 V. Although a rigorously base-free solvent
, 2.6 V vs SCE; , 2.1 V; , 1.9 V; matrix should result in a reversible oxidation of benzene,
+
the resulting cation radical (PhH ·) is a strong electrophile
and Lewis acid that will react with water (complete sepa-
ration of water from benzene is difficult; 18 ppm H 2 Ois
a 1-mM concentration) to give PhOH and H OinaCEC
+
3
process.
, 1.8 V

E. Organometallic Molecules
The defining characteristic of organometallic molecules is
4. Brønsted Acids
the presence of one or more metal-carbon bonds. In con-
Carboxylic acids, phenols, and alcohols are electrochem- trast to the acid/base character of coordination complexes
ically reduced via their Brønsted acidity at a reduction of metal ions (with their ligand-centered redox chemistry,
potential that is a direct measure of their acidity (pK a )in see Section II), the metal-carbon center is highly covalent
a given solvent. with limited polarity (similar to carbon-carbon, carbon-
nitrogen, or carbon-oxygen centers). As a result, the elec-
HA + e − 1/2H 2 + A −
trochemistry of organometallic molecules is more clos-
(E 1/2 ) Pt ≈−0.059 (pK a )VvsNHE. (178) edly related to that of organic molecules than inorganic
coordination complexes.
The “foundation stone” of organometallic chemistry is
5. Oxidation of Nucleophilic Substrates II
bis(cyclopendienyl) iron(II) [ferrocene, (Cp)Fe (Cp)], an
and Lewis Bases
ironatomsandwichedbetweentwofive-memberedcarbon
All molecules with nonbonding electron pairs (e.g., H 2 O, rings [Cp, C 5 H 5 ·; each carbon with a p electron to give (1)
ROH, ROR, RNH 2 , RSH, RSR, etc.) are, by defini- two pi-bonds delocalized around the carbon ring and (2) an
tion, Lewis bases with a degree of nucleophilicity. Their unpaired electron to give a covalent bond that is shared by
II
electrochemical oxidation potential is a measure (1) of the five carbons of the ring]. Thus, the Fe (Cp) 2 molecule
the ease of removal for one of the electron pair of elec- has the iron on a line that connects the centers of two
trons and (2) of relative nucleophilicity (the less positive parallel planar Cp. groups to give an “iron sandwich.”
the potential, the more nucleophilic). Aromatic molecules Figure 7 illustrates the electrochemial redox chem-
with Lewis-base substituents are easier to oxidize than the istry in acetronitrile for several coordination com-
II
III
III
2+
aliphatic forms of the substituents (e.g., PhOMe, +1.75 V plexes of iron [Fe (MeCN) ,Fe Cl 3 , and Fe (acac) 3
4
vs SCE; MeOH, +2.5 V vs SCE) because the aromatic (acac = acetylacetonate)] in relation to that for two
II
ring provides a means to delocalize the positive charge iron organometallics [Fe (Cp) 2 and Fe VIII (CO) 5 (iron-
and electron spin that would result from electron removal pentacarbonyl); both stable 18-electron systems]. In
II
(in the case of PhOMe, there are five additional hydro- MeCN, Fe (MeCN) 2+ is the only charged species of the
4
gen atoms to share the positive charge and six unsaturated group. It is reversibly oxidized (II/III couple; E 1/2 , +1.6V
III
carbon centers to share the spin density). Within this con- vs SCE). The uncharged Fe Cl 3 molecule is reversibly re-
II
−
text, the water molecule in a nonbasic solvent matrix is duced(III/IIcouple; E 1/2 , +0.2VvsSCE)togiveFe Cl ,
3
the most resistant to electron removal, which is reduced by an irreversible two-electron process
to iron metal (E p,c , −1.5 V vs SCE). The more basic

+

+
2H 2 O − e − (H 2 O)H O· HO·+ H O. (179) III
2 3 Fe (acac) 3 molecule is reversibly reduced (III/II couple;
E 1/2 , −0.7 V vs SCE), but does not exhibit a second re-
Because aliphatic alcohols can be viewed as “organic wa-
duction peak. The III/II reduction potentials for these three
ter” (but with a greater basicity and a weaker O H bond),
coordination complexes are a measure of their relative
they are almost as difficult to oxidize [MeOH(H 2 O) →
electrophilicity (Lewis acidity).
MeO·+ H O + e ; E p,a , +2.5VvsSCE].Otheraliphatic
+
−
3
bases (amines and thiols) are oxidized by similar
pathways.
1. Ferrocene
The pi-electron cloud of the aromatic ring is much more
II
susceptable to electron removal than a saturated hydro- The Fe (Cp) 2 molecule is resistant to reduction, but ex-
carbon. Thus, benzene is oxidized at +2.45 V vs SCE; hibits a highly reversible one-electron oxidation,