Reactions of ferricinium salts with Lewis bases

Article abstract of DOI:10.1007/s11172-011-0318-1

A reaction of the ferricinium radical cation with Lewis bases leads to substitution in the cyclopentadienyl ring with the formation of ferrocene derivatives only when the radicals are formed from the Lewis bases in the process of the reaction.

Full text of DOI:10.1007/s11172-011-0318-1

Russian Chemical Bulletin, International Edition, Vol. 60, No. 10, pp. 2081—2087, October, 2011
2081
Reactions of ferricinium salts with Lewis bases
V. N. Babin, Yu. A. Belousov, T. A. Belousova, Yu. A. Borisov, V. V. Gumenyuk, and Yu. S. Nekrasov
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (495) 135 5085. Eꢀmail: belyur@ineos.ac.ru
A reaction of the ferricinium radical cation with Lewis bases leads to substitution in the
cyclopentadienyl ring with the formation of ferrocene derivatives only when the radicals are
formed from the Lewis bases in the process of the reaction.
Key words: ferricinium radical cation, Lewis bases, redox disproportionation, substitution
in cyclopentadienyl ring, intermediate complexes, quantum chemical calculations, density
functional theory (DFT), B3LYP functional.
the corresponding heterocycles and sodium hydride in THF unꢀ
der argon. Traces of the starting heterocycle were removed by
decanting a benzene solution after reflux of the heterocycle salts.
Precipitates were dried in vacuo.
It is known that ferricinium salts are convenient, mild
oxidants (E1/2 = 0.31—0.47 V (saturated calomel elecꢀ
trode (s.c.e.)),1,2 E1/2 = 0.34 V (90% aqueous EtOH,
s.c.e.)3) and they are frequently used for the generation of
radical products.4—6 In addition, ferricinium cation in the
presence of various neutral (diethylamine, Py, DMF,
DMSO, hexametapol, 1,10ꢀphenanthroline, 2,2´ꢀbipyriꢀ
dine) or charged (Cl–, Br–, OH–) Lewis bases is reduced
to ferrocene.7—14
In many cases, for example for Cl– and OH–, thermoꢀ
dynamic description of the direct reduction process does
not seem possible, since the oxidation potential of a proꢀ
posed donor (a Lewis base) is too high. For Cl– and OH–
anions in acetonitrile it is 2.24 and 0.92 V (MeCN,
Ag/AgCl), respectively.15 In some cases, substitution in
cyclopentadienyl ring takes place in addition to reduction
to ferrocene. For example, the reaction of ferricinium catꢀ
ion with CN– anion leads to the formation of cyanoꢀ
ferrocene.9
To sum up, two questions are principal for the reacꢀ
tions of the ferricinium radical cation with Lewis bases
proceeding with the redox transformations: 1) which cases
are responsible for substitution in cyclopentadienyl ring
with the formation of ferrocene derivatives and 2) how the
reduction of the ferricinium radical cation to ferrocene
takes place in the reactions occurring "against a potential"
with oxidationꢀresistant bases?
Reactions were carried out on a preparative scale under arꢀ
gon or in the sealed tubes directly in the resonator of an ESR
spectrometer in the temperature region from –80 to +40 °C.
ESR spectra were recorded on a ERSꢀ221 (ZWG DDR) specꢀ
trometer in the Xꢀregion with the HFꢀmodulation 100 kHz. The
reactants, ferricinium salt and bases, were taken in equimolar
ratios. The ESR studies were performed both in the presence of
a spin trap (ButNO) and without it. Elemental analysis of comꢀ
pounds obtained was performed in the Laboratory of Microanalꢀ
ysis of INEOS RAS. Melting points were measured in open capꢀ
illaries and were not corrected. Mass spectra (EI, 70 eV, injecꢀ
tion temperature 80—220 °C) were recorded on a Varian CHꢀ8
instrument.
Quantum chemical calculations for ferrocene, ferricinium
radical cation, pyrazolide anion, pyrazolyl radical, pyrazole,
hydroxy anion, hydroxyl radical, and intermediates of their reꢀ
actions were performed in the framework of density functional
theory (DFT) with the B3LYP functional.21—23 Optimization of
geometric structures of molecular systems was performed using
procedures given in the works24,25 and the GAUSSIANꢀ98 proꢀ
gram26 on a SC760D minisupercomputer. For calculations of
enthalpies of the processes in the gaseous phase, the calculated
total energies were used, as well as corrections for the energy
of zero vibrations of the corresponding molecules, ions, and
radicals.
Reaction of sodium pyrazolide with ferricinium hexafluoroꢀ
phosphate. A. A mixture of ferricinium hexafluorophosphate
(0.5 g, 1.5 mmol) and sodium pyrazolide (0.135 g, 1.5 mmol) in
THF (75 mL) was stirred for 45 min in a Schlenk flask at room
temperature. The color of the solution turned from dark blue to
reddish brown. Then the solvent was evaporated in vacuo, the
residue was extracted with freshly distilled THF and subjected to
chromatography on a column with Al2O3. Ferrocene was eluted
with hexane (yellow band), the yield was 0.11 g (38%), m.p.
173 °C (cf. Ref. 27: m.p. 173 °C). MS, m/z: 186 [M]+. NꢀFerroꢀ
cenylpyrazole was eluted with THF (yellowish red band), the
Experimental
The reactions of Lewis bases as azole sodium salts, sodium
alkoxides (NaOR, R = Me, Et) and amides (NaNR2, R = H, Et,
Pri) with ferricinium hexafluorophosphate in THF, dichloroꢀ
methane, acetonitrile, and acetone solutions were studied.16—19
Ferricinium hexafluorophosphate was obtained according to the
known procedure.20 Azole sodium salts were synthesized from
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2044—2050, October, 2011.
1066ꢀ5285/11/6010ꢀ2081 © 2011 Springer Science+Business Media, Inc.

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Babin et al.
yield was 0.09 g (23%), m.p. 63 °C. Found (%): C, 62.31; H, 5.02;
N, 10.87; Fe, 21.17. C13H12N2Fe. Calculated (%): C, 61.90;
H, 4.76; N, 11.11; Fe, 22.20. MS, m/z: 252 [M]+, 225 [M – HCN]+,
214 [M – C2N]+, 199 [M – C2N2H]+, 186 [M – C3N2H2]+.
Further elution with THF gives pyrazole (0.04 g, 39%) with
m.p. 70 °C (cf. Ref. 28: m.p. 70 °C). The residue in the Schlenk
flask left after the extraction with THF was washed with ethaꢀ
nol, water, and acetone and dried in vacuo. According to
the elemental analysis data, these were coordination polymers of
the composition [Fe(C3H3N2)2]n. Found (%): C, 37.53; H, 3.52;
N, 30.05. C6H6N4Fe. Calculated (%): C, 37.89; H, 3.16;
N, 29.47.
B. A mixture of ferricinium hexafluorophosphate (0.5 g,
1.5 mmol) and sodium pyrazolide (0.14 g, 1.5 mmol) in THF
(75 mL) was stirred for 15 min at 40 °C, the solvent was evapoꢀ
rated in vacuo, the mixture was treated similarly to that in methꢀ
od A to isolate ferrocene (0.14 g, 50%), Nꢀferrocenylpyrazole
(0.14 g, 37%), and pyrazole ( 0.04 g, 39%).
Reaction of sodium 3,5ꢀdimethylpyrazolide with ferricinium
hexafluorophosphate was carried out similarly to method B with
the use of sodium 3,5ꢀdimethylpyrazolide (0.18 g, 1.5 mmol)
and ferricinium hexafluorophosphate (0.5 g, 1.5 mmol) to isoꢀ
late ferrocene (0.12 g, 43%), Nꢀferrocenylꢀ3,5ꢀdimethylpyrazole
(0.17 g, 41%) with m.p. 46 °C, and 3,5ꢀdimethylpyrazole (0.03 g,
21%) with m.p. 105 °C (cf. Ref. 28: m.p. 107 °C).
NꢀFerrocenylꢀ3,5ꢀdimethylpyrazole. Found (%): C, 64.12;
H, 5.64; Fe, 20.35. C15H16N2Fe. Calculated (%): C, 64.30;
H, 5.83; Fe, 19.93. MS, m/z: 280 [M]+, 274 [M – 6 H]+, 272
[M – 8 H]+, 270 [M – 10 H]+, 254 [M – C2H2]+ and/or
[M – CN]+, 239 [M – CH3CN]+.
Reaction of sodium imidazolide with ferricinium hexafluoroꢀ
phosphate was carried out similarly to method B with the use of
sodium imidazolide (0.14 g, 1.5 mmol) and ferricinium hexaꢀ
fluorophosphate (0.5 g, 1.5 mmol) to isolate ferrocene (0.03 g,
10%), Nꢀferrocenylimidazole (0.02 g, 4.7%) with m.p. 57 °C
(decomp.), and coordination polymers (Im2Fe)n (0.11 g, ~37%).
NꢀFerrocenylimidazole. MS, m/z: 252 [M]+, 225 [M – HCN]+,
198 [M – 2 HCN]+, 186 [M – C3N2H2]+.
Reaction of sodium benzotriazolide with ferricinium hexafluoroꢀ
phosphate. A mixture of ferricinium hexafluorophosphate (1.0 g,
3.0 mmol) and sodium benzotriazolide (0.42 g, 3.0 mmol) in
THF (100 mL) was stirred for 30 min, concentrated, and subꢀ
jected to chromatography on a column with Al2O3. A colorless
fraction, which preceded a yellow band on the column, was
eluted with hexane. According to the mass spectrometric data,
this fraction contains cyclopentadiene resins and tetrahydroꢀ
furan oligomers. Elution with the hexane—THF (1 : 1) yielded
a fraction, from which ferrocene (0.28 g, 50%) was isolated
with m.p. 172 °C (cf. Ref. 27: m.p. 173 °C). Further elution with
THF gave a fraction corresponding to the red band on the
column. After the solvent was evaporated, reddish orange crysꢀ
tals of Nꢀferrocenylbenzotriazole (0.37, 40%) with m.p. 113 °C
were obtained. Collection of the red band was followed
by collection of a colorless eluate, evaporation of the solvent
from which gave benzotriazole (0.08 g, 23%) with m.p. 101 °C
(cf. Ref. 28: m.p. 100 °C).
Results and Discussion
Reactions of azole salts with ferricinium hexafluoroꢀ
phosphate are accompanied with a rapid change of color
of the reaction mixture from blue to dark red already withꢀ
in the first few minutes. Chromatographic separation of
the reaction mixture formed after 5—75 min at 20—40 °C
on Al2O3 showed that it contains ferrocene (a reduced
form of ferricinium) (~50%), Nꢀferrocenylazole (5—40%),
azole (the heterocycle itself, not its salt) (10—40%), coorꢀ
dination polymers (FeL2)n (~10%), small amount of
cyclopentadiene resins (~2%), and tetrahydrofuran oligoꢀ
mers (traces). The yields of Nꢀferrocenylazoles decrease
in the order benzotriazole > pyrazole ~ 3,5ꢀdimethylpyrꢀ
azole > imidazole.
No aminoꢀ or alkoxyferrocenes are formed in the reacꢀ
tions of sodium amides and alkoxides with ferricinium
hexafluorophosphate. Rather partial reduction to ferrocene
and destruction with the formation of cyclopentadiene
resins and inorganic iron compounds take place.
Studies of reactions of ferricinium hexafluorophosꢀ
phate with the azole sodium salts by ESR spectroscopy
showed that, irrespective of the nature of the starting
azolide anion, a very broad signal (ΔH1/2 ≈ 250 mT) with
the gꢀfactor close to that of electron (g = 2.0026—2.0028)
appears in the spectrum at the beginning at –80 °C, which
cannot be assigned neither to the ferricinium radical catꢀ
ion (the signal for ferricinium is found only at very low
temperatures, g|| = 4.40 and g⊥ = 1.39),29 nor to the azolyl
radicals having a multiplet structure.30,31 It does not corꢀ
respond to an inorganic compound of FeII (g = 3.30—3.43
and g = 6.58—6.86)32 either. Apparently, the spectrum
can belong to the charge transfer complexes [Fc• Az–]
+
(Fc is the ferrocenyl, Az– is the azolide anion). Intensity
of this signal rapidly decreases on temperature elevation
with simultaneous appearance of a signal, which is a tripꢀ
let of triplets of the spin adducts of azolyl radicals if
a ButNO spin trap is present in the system. Parameters of
the spectra of the azolyl radical spin adducts are given in
Table 1. Figure 1 shows the ESR spectrum of pyrazolylꢀ
Table 1. Parameters of the ESR spectra of nitroxyl radiꢀ
cals in the ButNO—AzNa system in THF at 20 °C
Radical
gꢀFactor
aN
aN
2
1
mT
ButN(O•)Pz
ButN(O•)Im
ButN(O•)Bt
2.0060
2.0060
2.0060
2.0060
2.0060
1.45
1.48
1.38
1.39
1.54
0.20
0.18
0.15
0.18
—
NꢀFerrocenylbenzotriazole. Found (%): C, 63.85; H, 4.38;
N, 13.51. C16H13N3Fe. Calculated (%): C, 63.37; H, 4.29;
N, 13.86. MS, m/z: 303 [M]+, 275 [M – N2]+, 247
[M – C2N2H4+], 218 [M – C4N2H9]+, 217 [M – C4N2H10]+,
186 [M – C6N3H3]+.
ButN(O•)ꢀ3,5ꢀ(Me)2Pz
But2NO•
Note. Pz is the pyrazolyl, Im is the imidazolyl, Bt is the
benzotriazolyl.

Reactions of ferricinium with Lewis bases
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 10, October, 2011
2083
Scheme 1
a
b
Fig. 1. The ESR spectrum of PzN(O•)But (a) and But2NO• (b)
radicals observed in the reaction of ferricinium hexafluorophosꢀ
phate with sodium pyrazolide in the presence of ButNO in THF
at 20 °C (see Table 1).
tertꢀbutylnitroxyl radical formed in the reaction of sodium
pyrazolide with ferricinium hexafluorophosphate.
Since formation of these azolylꢀtertꢀbutylnitroxyl radꢀ
icals is also observed in the direct reaction with ButNO,33
the concentration ratio of azolylꢀtertꢀbutylnitroxyl and
But2NO• radicals is of principal importance. In the reacꢀ
tions of ferricinium hexafluorophosphate with the azole
sodium salts, the concentration of azolylꢀtertꢀbutylnitrꢀ
oxyl radicals formed is significantly higher than that of
But2NO•, which indicates the generation of azolyl radiꢀ
Nꢀferrocenylazoles (see Scheme 1, Eqs (2) and (3)), the
second route leads to decomposition of the ferrocene core
and formation of coordination polymers (FeAzx)n. Secꢀ
ond, the azolyl radicals which come into the solvent bulk
can be terminated by entering various reactions with the
solvent. Apparently, this explains appearance of tetraꢀ
hydrofuran oligomers among the reaction products, which
were detected by mass spectrometry.
+
cals directly in the system [Fc• Az–]. It should be emꢀ
phasized that the use of the 2,4,5ꢀtriphenylimidazole soꢀ
dium salt (lophine) in the reaction with ferricinium
hexafluorophosphate leads to the formation of stable lophiꢀ
nyl radical (g = 2.0036), which can be observed directly.
A combination of chemical and spectroscopic data alꢀ
lows us to conclude that the reactions under study follow
the mechanism of a direct reduction of the ferricinium
radical cation to ferrocene with the azole anions (Scheme 1,
Eq. (1)). Calculations of enthalpy of the first step in
the direct generation of azolyl radicals for the pyrazolide
anion showed that this process is exothermic (ΔH =
= –81.0 kcal mol–1) (Table 2).
The results in the work11 are important for the underꢀ
standing the mechanism of substitution in the cyclopentaꢀ
dienyl ring, in which the reaction of the ferricinium radiꢀ
cal cation with the radical obtained by oxidation of a Lewis
base is considered as a key step of the process. Thus (see
Scheme 1, Eqs (2) and (3)), cyanoferrocene (CN• instead
of Az•), phenylferrocenyl sulfone (PhSO2• instead of Az•),
Table 2. Relative total energies with correction for the zero viꢀ
brations (E´) and with correction for the zero vibrations and
entropy members (G), as well as entropies (S) of ferrocene, pyrꢀ
azole, and particles formed in their reactions
The azolyl radicals, which came out of the cell, possess
high enough reactivity and are apparently consumed in
several ways. First, they can react with another ferriciniꢀ
um radical cation at the site of localization of the unpaired
electron. The ground electron state of the ferricinium radꢀ
ical cation is 2E2g with the configuration (a21g e32g).34 The
Compound
E´
G
S
/cal mol–1 deg–1
kcal mol–1
Ferrocene
105.73
106.30
84.69
84.71
92.070
93.292
Ferricinium
radical cation
Pyrazolide anion
Pyrazolyl radical
Pyrazole
thermal population of the first excited state 2A1g (a11g e4
)
2g
35.52
36.71
44.99
19.08
19.89
28.49
64.674
66.253
65.028
is significant at room temperature, since the electron tranꢀ
2
2
sition E2g → A1g occurs at ~200 cm–1, to which the kT
corresponds under these conditions.35 Depending on the
electron state of ferricinium cation, either attack on the
cyclopentadienyl ring or formation of the coordination
Fe—N bond takes place. The first pathway leads to the
replacement of the hydrogen atom and formation of
Note. The total energy (–E/Hartreess) for ferrocene is 510.4390,
for the ferricinium radical cation is 510.2030, for pyrazolide anꢀ
ion is 225.5698, for pyrazolyl radical is 225.4639, and for pyrꢀ
azole is 226.1594.

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Babin et al.
pꢀtolylferrocenyl sulfide (CH3C6H4S• instead of Az•),
αꢀcyanoisopropylferrocene (C(CH3)2CN• instead of Az•)
were obtained. One can consider that a direct cyanation of
ferrocene9 with liquid cyanic acid in the presence of
a strong oxidant FeCl3 also proceeds by the mechanism of
the reaction of the ferricinium radical cation with the CN•
radical. The same mechanism operates in the direct elecꢀ
trochemical alkylation of ferrocene with alkyl radicals in
the anode decarboxylation of the carboxylic acid anꢀ
ions.36,37
1.080
1.391
1.079
1.429
1.080
1.383
1.082
1.082
1.406
1.407
1.082
N
1.517
1.460
1.081
1.432
1.233
1.397
1.349
N
1.514
1.713
Scheme 1 seems the most probable, however, an alterꢀ
native Scheme 2 of the oneꢀelectron redox process with
a preliminary "activating complexation" is quite acceptable.
In this case, the role of reducing agent belongs to the
preliminary formed complex of the ferricinium radical catꢀ
ion and the azolide ion (Lewis base) rather than to the
azole anion (the Lewis base in the anionic form). Then,
the process proceeds according to Eq. (3). If applied to the
reactions with the azole salts, this should lead to the fact
that azolyl radicals are not generated in the process of
formation of Nꢀferrocenylazoles and do not come out of
the cells, which poorly agrees with the facts of rapid accuꢀ
mulation of the spin adducts AzN(O•)But, comparatively
low yield of Nꢀferrocenylazoles, and appearance of tetraꢀ
hydrofuran oligomers.
To answer the question on the structure of the interꢀ
mediate complex and on how the attack on the ferriciniꢀ
um radical cation takes place, we performed model quanꢀ
tum chemical calculations.* Figure 2 shows possible strucꢀ
tures of the complex emerging in the reaction of the ferriꢀ
cinium radical cation with the pyrazolyl radical. The strucꢀ
tures of 1a and 1b are formed upon the attack by the
pyrazolyl radical on the cyclopentadiene ring, whereas 1c,
on the iron atom. All three intermediates correspond to
the local minima on the potential energy surface, with the
complex 1a being energetically more preferable (Table 3).
Based on the data given in Tables 2 and 3, enthalpies
for the reactions (2) and (3) with the formation of the
intermediates 1a—c were calculated. It turned out that all
the steps of the process are exothermic:
1.081
1.081
1.441
1.444
1.443
1.082
1.081
1.442
1.081
1.443
1.083
1a
1.537
1.098
1.396
1.081
1.469
1.082
1.472
N
1.382
1.537
1.084
1.365
1.080
N
1.397
1.403
1.083
1.418
1.080
1.408
1.081
1.081
1.424
1.449
1.081
1.459
1.467
1.082
1.425
1.081
1b
1.083
1.440
1.084
1.461
1.083
1.467
1.083
1.462
1.435
1.082
1.079
N
1.400
1.358
1.381
N
1.080
1.436
Intermediate
–ΔH(2)
–ΔH(3)
2.007
kcal mol–1
1.385
1.084
1.455
1.083
1a
1b
1c
30.7
25.4
14.2
164.5
169.9
/181.1.
1.079
1.423
1.419
1.073
1.460
1.085
1.465
1.084
To sum up, in the reaction of the ferricinium radical
cation with the azolide anions, substitution in the cycloꢀ
1c
Fig. 2. The intermediate cationic complexes 1a—c formed in the
reaction of the ferricinium radical cation with the pyrazolyl radꢀ
ical (see Scheme 1, Eq. (2)).
* Quantum chemical calculations for compounds, their interꢀ
mediates and transformations were carried out for the gaseous
phase. This puts certain limits on the use of these data for the
description of the processes proceeding in solutions. Nevertheꢀ
less, it is considered reasonable to use these calculated data,
since they agree with the results of the studies of the process
obtained by chemical and spectroscopic methods.
pentadienyl ring with the formation of Nꢀferrocenylazoles
occurs through the azolyl radicals, which then directly
react with the ferricinium radical cation.

Reactions of ferricinium with Lewis bases
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 10, October, 2011
2085
Table 3. Relative total energies (E) with correction for the
zero vibrations (E´) and with correction for the zero vibraꢀ
tions and entropy members (G), as well as entropies (S) of the
intermediates 1a—c
Scheme 2
Form
E
E´
G
S/cal mol–1 deg–1
kcal mol–1
1a
1b
1c
0.00
4.21
16.80
0.00
5.34
16.55
0.00
4.94
16.27
113.203
116.662
117.182
Scheme 4
Let us consider the cases when anionic Lewis bases are
not directly oxidized to radicals. To explain the mechaꢀ
nism of this type of reactions, a concept of "activating
complexation" has been suggested,38 which consists in the
following (Scheme 3). If donor L (for example, a Lewis
base in the anionic form L–) cannot directly reduce acꢀ
ceptor Fc•+, then a complex is formed, which plays the
role of the reducing agent.
Scheme 3
L– + Fc•+
L– + Fc•+
FcL• + Fc•+
L• + Fc
FcL•
The cation formed in (7) reacts with the Lewis bases to
be decomposed to the inorganic iron complexes: Fe(OH)3
is formed with alkalis,7,13 [Fe(SCN)6]3– anion is formed
with NH4SCN,10 [FeCl4]– and [FeBr4]– anions are formed
with Cl– and Br–, respectively.10,12 In the work,12 it was
shown that decomposition of the ferricinium cation reꢀ
sults in disproportionation to initially form FeII coordinaꢀ
tion complexes, which in the secondary processes are
oxidized with the starting ferricinium cation to FeIII comꢀ
plexes. Therefore, finally the greater part of the ferriciniꢀ
um is reduced to ferrocene, whereas the inorganic iron
compounds formed are the FeIII complexes.
To answer the question on the structure of the interꢀ
mediate complex and on how the attack on the ferriciniꢀ
um radical cation occurs, we performed model (see
above) quantum chemical calculations. Figure 3 shows
possible structures of the radical emerging it the reaction
of the ferricinium radical cation with hydroxide ion. All
three intermediates 2a—c correspond to the minima on
the potential energy surface, with the radical 2c being the
FcL+ + Fc
As a rule, the Lewis bases used are quite simple moleꢀ
cules (ions) with the localized σꢀbonds and without
πꢀelectron system of conjugation. Therefore, abstraction
of an electron leads to a strong change in the charge on the
key atom, which is energetically unfavorable. Conversely,
a complex formed acquires donor properties and readily
donates electron.
Such a scheme is characteristic of a Lewis base in the
anionic form. An important example is the reduction of
ferricinium salts in alkaline medium, when L– = OH–.
Due to the thermodynamic reasons mentioned at the beꢀ
ginning of the paper, the hydroxide ion cannot be directly
oxidized to a radical with the ferricinium radical cation,
but their complexation takes place to furnish ferricinium
hydroxide, which can be isolated.7 Its reaction with the
ferricinium salt leads to the rapid reaction (7) with the
formation of ferrocene7 (Scheme 4).

2086
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Babin et al.
1.081
1.082
1.440
1.388
Table 4. Relative total energies (E) with correction for the zero
vibrations (E´) and with correction for the zero vibrations and
entropy members (G), as well as entropies (S) of the intermediꢀ
ates 2a—c
1.418
1.081
1.453
0.979
1.433
2.281
1.451
1.080
Form
E
E´
G
S/cal mol–1 deg–1
kcal mol–1
1.488
1.081
1.082
1.444
2a
2b
2c
26.58
5.13
0.0
25.80
6.80
0.0
25.12
6.36
0.0
106.617
104.737
102.173
2a
1.423
2.206
1.442
1.082
1.444
1.080
1.442
1.081
most stable (Table 4). From this it follows that the attack
of the ferricinium radical cation by the anion having small
size can proceed at the iron atom to form the ironꢀcenꢀ
tered intermediate complex.
In conclusion, the reactions of the ferricinium radical
cation with Lewis bases lead to the substitution in the
cyclopentadienyl ring with the formation of ferrocene deꢀ
rivatives only in such cases when the Lewis bases are transꢀ
formed to radicals in the course of the reaction, which
then directly react with the ferricinium radical cation. Forꢀ
mation of radicals from the bases can occur by oxidation
either directly with ferricinium salts, or with strong oxiꢀ
dants introduced into the reaction medium, for example,
Cu2+ or Fe3+ salts, or electrochemically.
1.108
1.083
1.435
1.082
1.450
1.082
1.530
1.473
1.538
0.981
1.437
1.085
1.082
1.443
1.081
2b
1.436
1.082
When the Lewis bases are not oxidized, reduction of
the ferricinium radical cation follows apparently the
scheme of "activating complexation" involving a complex
of the ferricinium radical cation with the base with further
electron transfer to another ferricinium radical cation.
During this process, some ferricinium radical cations are
reduced to ferrocene, whereas some by further reaction
with the Lewis base are transformed to the iron coordinaꢀ
tion compound.
1.438
1.081
1.433
1.450
1.082
1.084
1.085
1.427
1.427
1.413
1.085
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 09ꢀ03ꢀ00535).
1.085
1.461
1.443
1.795
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1.085
0.983
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Received December 30, 2009;
in revised form July 15, 2011