C-OH bond cleavage initiated by electron transfer: Electroreduction of 9-fluorenol

Article abstract of DOI:10.1016/j.electacta.2016.01.127

Cyclic voltammetry, chronoamperometry, coulometry, electrolysis, digital simulation, quantum chemical calculations of 9-fluorenol as an example, were used to show that the electroreduction of aryl derivatives of methanol in 0.1 M Bu₄NClO₄/DMF proceeds via the ECE mechanism (including the stages of radical anion formation and the C-OH bond cleavage in the radical anion) complicated by the reactions of the depolarizer with the anionic products. Among these reactions are the deprotonation of 9-fluorenol and its monoanions by hydroxide anion and fluorenyl anion. The thermodynamic parameters of the reactions have been estimated both theoretically and experimentally. It was found that the equilibrium constants of the fluorenyl anions deprotonation are close (C-anion) or higher (O-anion) than that of fluorenol. As a result the total equilibrium is shifted towards the side of the dianion of 9-fluorenone. The unusual ratio of the equilibrium constants was explained by lower basicity of π?-dianion compare with other anions.

Full text of DOI:10.1016/j.electacta.2016.01.127

Accepted Manuscript
Title: C-OНbond cleavage initiated by electron transfer:
electroreduction of 9-fluorenol
Author: Andrey S. Mendkovich Mikhail A. Syroeshkin Darya
V. Nasybullina Mikhail N. Mikhailov Vadim P. Gultyai
Mikhail N. Elinson Alexander I. Rusakov
PII:
S0013-4686(16)30129-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2016.01.127
EA 26509
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Electrochimica Acta
Received date:
Revised date:
Accepted date:
29-10-2015
27-12-2015
17-1-2016
Please cite this article as: Andrey S.Mendkovich, Mikhail A.Syroeshkin, Darya
V.Nasybullina, Mikhail N.Mikhailov, Vadim P.Gultyai, Mikhail N.Elinson, Alexander
I.Rusakov, rmC-OНbond cleavage initiated by electron transfer: electroreduction of
9-fluorenol, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.01.127
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С-ОН bond cleavage initiated by electron transfer: electroreduction of 9-fluorenol
Andrey S. Mendkovicha*, Mikhail A. Syroeshkina, Darya V. Nasybullinaa, Mikhail N.
Mikhailovа, Vadim P. Gultyaiа, Mikhail N. Elinsonа, Alexander I. Rusakovb
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. Fax: +7 499 135 5328; e-mail: asm@free.net
bP. G. Demidov Yaroslavl State University, 150000 Yaroslavl, Russian Federation
Graphical abstract
Highlights
• The electroreduction of 9-Fluorenol in aprotic media was studied.
• The process proceeds via С–O bond cleavage in the radical anion.
• The substrate is deprotonated by anionic products giving rise 9-fluorenone dianion.
Abstract
Сyclic voltammetry, chronoamperometry, coulometry, electrolysis, digital simulation,
quantum chemical calculations of 9-fluorenol as an example, were used to show that the
electroreduction of aryl derivatives of methanol in 0.1 M Bu4NClO4/DMF proceeds via the ECE
mechanism (including the stages of radical anion formation and the C–OH bond cleavage in the
radical anion) complicated by the reactions of the depolarizer with the anionic products. Among
these reactions are the deprotonation of 9-fluorenol and its monoanions by hydroxide anion and
fluorenyl anion. The thermodynamic parameters of the reactions have been estimated both
theoretically and experimentally. It was found that the equilibrium constants of the fluorenyl
anions deprotonation are close (C-anion) or higher (O-anion) than that of fluorenol. As a result
the total equilibrium is shifted towards the side of the dianion of 9-fluorenone. The unusual ratio
of the equilibrium constants was explained by lower basicity of π*-dianion compare with other
anions.
Keywords:9-Fluorenol; cyclic volammetry; chronoamperometry; radical anion;
dissociative electron transfer.
1. Introduction

It is known that the presence of an electron on an antibonding molecular orbital
destabilizes bonds, transfer of an electron to a neutral molecule could result in dissociation of the
latter [1]. A considerable number of papers deal with studies on bond cleavage processes
initiated by the electron transfer [2-9]. One of the main reasons of this special attention to these
reactions is that they are of undoubted interest both for organic synthesis [10-13] and for
theoretical organic chemistry, e.g., in interpretation of results of experimental studies on
nucleophilic substitution processes [14-17]. Furthermore, bond cleavage reactions in biological
systems attracted the serious attention of researchers in recent years [18-21].
Many examples are known in literature for dissociation reactions of various types of
bonds initiated by electron transfer resulting in elimination of various functional groups.
The following examples can be specially mentioned: elimination of nitrite anions from
radical anions of nitroaromatic compounds [22-23], phenoxide anions [24] and benzoate anions
[25] from radical anions of ethers and esters, arylsulfonate anions from the corresponding
olephine derivatives [26] thiophenolate anions from triphenylmethane derivatives [27-28] and
other derivatives [29], alkylsulfide anions from disulfide radical anions [30-31], alkoxy anions
from radical anions of organic peroxides [32-46], substituted benzyl anions from radical anions
of bicumenes [47] etc. The regioselectivity of bond cleavage in benzylthiocyanates (C-SCN or
CS-CN) [48] and sulfenate esters (S-O or C-O) [49] depending on radical anion structure was
discussed. Researchers paid particular attention to the elimination of a halide anion from radical
anions of halogen derivatives, both aliphatic [50-56] and aromatic [57-62]. The reactions of
radical anions of these compounds have been studied most thoroughly, both experimentally and
theoretically [2, 3] [63]. This is primarily due to the fact that formation of radical anions of this
class of compounds plays an important role in many reactions of practical interest, e.g., in
formation of Grignard reagents [64-67] and in electrochemical generation of carbenes [68].
On the other hand, only few papers are available in literature that describe the elimination
of an OH group initiated by the electron transfer. Cleavage of an N-OH bond was observed in
electroreduction of oximes [69] and N-arylhydroxylamines [70]. It was also shown [71] that
electroreduction of methanol derivatives containing unsaturated and aromatic substituents in
aprotic solvents could be described by the scheme:
ROH + 2e + H+ → RH + OH−
An attempt to elaborate this scheme was made in subsequent studies [72-73]. However, the
conclusions concerning the primary products of C-OH bond dissociation due to electron transfer
were contradictory. For example, the mechanism proposed for electroreduction of 9-

hydroxybifluorenyl [72] involved a step of elimination of an ОН radical from its radical anion,
whereas the same researchers assumed elimination of a hydroxide anion from the radical anion
of 9-fluorenol 1˙¯ with similar structure [73]. It also remained unclear whether dissociative
electron transfer in the case of 1 and related compounds is a stepwise or synchronous process,
since curves of cyclic voltammetry for compound 1 at potential scan rates up to 100 V·s-1
contained no anodic peaks corresponding to oxidation of its radical anion [73]. Likewise, no
radical anion formation was detected in the reaction of 1,1-diphenylmethanol with solvated
electron [74]. Therefore, we undertook an electrochemical and quantum-chemical study of C-OH
bond cleavage initiated by electron transfer for compound 1 as an example.
2. Experimental
2.1. Electroanalytical and electrolysis instrumentation, experimental techniques and reagents
Chronoamperometry (CA), cyclic voltammetry (CV), coulometry and controlled potential
electrolysis were implemented on an IPC-Pro computer-assisted potentiostat manufactured by
Econix (potential scan rate error 1.0%; the potential is set to within 0.25 mV). Experiments were
performed in a 10-mL five-neck glass conic electrochemical cell with a water jacket for
thermostatting. Polarization curves were recorded using a three-electrode scheme. The working
electrode was a glassy-carbon disc electrode (d = 1.7 mm). A graphite rod was utilized as a
cathode for the coulometry and electrolysis. A platinum wire (insulated by a ceramic membrane
in coulometry and electrolysis) served as the auxiliary electrode. A saturated calomel electrode
(SCE) was used as the reference electrode and was connected to the solution by a bridge with a
porous ceramic diaphragm filled with the background electrolyte (0.1 M Bu4NClO4 solution in
DMF). The tested solutions were thermostatted at 25±0.5 °С. Deaeration of solutions was
performed by passing argon. To prevent the solution surface from contact with ambient air
during the experiment, argon was constantly fed to the cell free space above the solution surface.
In a typical case, 5 mL of a solution was utilized. The working electrode was polished and the
solution was agitated vigorously with argon before recording each CA and CV curve.
To analyze CA and CV curves, the current values in the presence of the substrate were
corrected for the current of the background electrolyte at this potential. The depolarizer
concentration (C) was 2.5, 5, 7.5, 10, 15, 20, and 25 mmol·L–1. In the case of CV, the cathodic
and anodic peaks currents at the potential scan rates (v) of 0.05, 0.1, 0.225, 0.4, 0.65, 1, 2 and 3
V·s–1 were used as response functions. The above-mentioned variations of C and v correspond
the change in the kinetic parameter λ [75-76] by more than four orders of magnitude. The
sampled current voltammograms [77] (a subtype of steady-state voltammograms [78]) were

plotted using the values of the current from the CA curves at a transient time (t) of 4 s at the
corresponding potentials. To compare the experimentally obtained and simulated CA curves, the
values of the current at the potentials of the limiting current of the first wave in the sampled
current voltammograms (i1lim) in the range of t = 1-4 s were used as the response function.
As the effects of uncompensated resistance (Ru) were significant in the present work, its
value was measured accurately. The CV curves of ferrocene electrooxidation [79] were used for
this purpose. Ru was calculated as the slope of the peak potential vs peak current plot for anodic
and cathodic peaks from CV curves of 2.5, 5, 10, 15, 20, 25 and 30 mmol·L–1 solutions of
ferrocene at scan rates of 0.025, 0.05, 0.1, 0.225, 0.4, 0.65, 1, 2, and 3 V·s–1. The resulting curves
are strictly linear, and their slopes correspond to Ru = 840 ± 10 Ω. This value is specific to the
working electrode used in this work and the arrangement of the electrodes in the cell, which was
carefully reproduced in each experiment. The exact active area of the working electrode (equal to
2.30 mm2) was determined by simulation of the CV curves of ferrocene electrooxidation using
the known ferrocene diffusion coefficient in DMF [80].
9-Fluorenol,
9-fluorene,
9-fluorenone,
tetrabutylammonium
perchlorate,
tetrabutylammonium hydroxide, MeCN and DMF (“extra dry” grade) were supplied by Acros
Organics.
100 mg of 1 was electrolyzed in MeCN at the potential -2.6 V vs SCE. After
consumption of one mole equivalents of charge the electrolysis was stopped and catholyte was
exposed to air to oxidize 32- and the solvent was evaporated. Thin-layer chromatography on
silica with hexane-ethyl acetate 12:1 as eluent revealed the presence in catholyte of two
products: 2 (Rf, 2 = 0.76) and 3 (Rf, 3 = 0.40). No spot corresponding to 1 (Rf, 1 = 0.13) was
observed. The products separation by column chromatography on silica (0.04-0.063 mm) with
hexane-ethyl acetate 12:1 as eluent gave 3 in 50% yield and 2 in 20%. Intensely colored layer of
the insoluble in polar solvent material at the top of the column indicated the formation of the
resins. The molecular ion peaks in mass spectra of the isolated compounds are 166 and 180 m/z,
i.e. identical to those in spectra of 2 and 3.
2.2. Digital simulations
Digital simulations of the CA and CV curves were carried out using DigiElch
Professional, version 4.0 (Build 3.008), from ElchSoft. The computation of the model curves
took into account the edge effect and uncompensated resistance. In case of CV curves of 1 the
effect gives rise to changes in the current for low scan rates and long transient times up to 4% for
cathodic peak and up to 19% for anodic peak corresponds to the oxidation of 3˙¯ . The rate
constants were determined from the CA and CV data using the procedure described previously

[70, 81] involving the variation of concentrations as well as scan rate or transient time in the case
of CV and CA, respectively.
The diffusion coefficient D for 1 was taken to be 10-5 cm2·s-1. It was based on the value
which we found for 3, compounds structurally similar to 1, whose anion radical is stable under
these the conditions. In the case of 3, the use of the value of D = 10-5 cm2·s-1 enabled us to
achieve a good agreement between the experimental (ip ) and theoretical (itpheor ) first peak
currents in the entire range of concentrations and potential scan rates studied. The mean ratio of
peak currents was ip /itpheor = 0.99±0.03
.
The values of the transfer coefficients (α) were taken to be 0.5. Proton transfer reactions
(4-10, 13 and 14) and rate constant of bond cleavage (2) considered to be fast. Theoretical values
of the homogeneous equlibrium constats (K) have been used as the initial approximations. The
standard potentials (Eº) and heterogeneous electron transfer constants (ks) as well as K
magnitudes were determined by optimization using standard DigiElch Professional techniques to
attain the best match of the model and the experimental curves over the ranges of potential scan
rates and concentrations indicated above, followed by determination of the average values. The
values was used for calculation of itpheor , and ip /itpheor ratio was used to assess the overall quality
of the results of the fitting process.
When optimizing it was taken into account that K4/K5=K7/K6=K9/K10=K13/K14=K15 and,
as it was found earlier [82], K9/K5 = 1.5·102. In case of electroreduction of 3 in the presence of 2
the best agreement between simulation and the experimental voltammograms were obtained for
the following set of simulation parameter: Eº11= -1.23 V, Eº12 = -1.87 V K6=9.5·10-2, K13= 1.5,
K15=15.0. The values were used in simulation of 1 electroreduction process to reduce the number
of simulation parameters. The best coincidence with the experimental results in this case was
attained when using the following values: Eº1 = -2.55 V, K4 = 9.6·104, K9 = 9.7·105, K14 =
9.47·10-2.
2.3. Quantum chemical calculations
The quantum chemical calculations were performed in the framework of density
functional theory (DFT) using the B3LYP exchange correlation functional [83-85]. As we
showed earlier [86], the calculated energies of the radical anions of compounds containing π-
bonds depend fairly strongly on the presence of diffuse functions on hydrogen atoms in the basis
set, whereas these effects are insignificant for compounds without π-bonds. Therefore, in this
work, all calculations were performed using the 6-311++G(d,p) basis set. All quantum chemical
calculations were performed using the Gaussian 03 [87] and Gaussian 09 [88] program packages.

Geometry optimization was carried out for all species under investigation. The character of the
stationary points found (minimum or saddle point on the PES) was determined by calculation of
the eigenvalues of the matrix of the second derivatives of energy with respect to nuclear
coordinates. The influence of solvation was taken into account in all optimizations in the
framework of the reactive field continuum model theories: PCM [89-91] and CSC-PCM [92],
using the parameters for DMSO, which are similar to those of DMF that was used in the
experiment. Energy profile (Fig. 1) for reaction (3) has been calculated by geometry optimized at
fixed C-O distances. Equlibrium constats were calculated using Gibbs free energy values listed
in the Table 1.
3. Results and discussion
3.1. Quantum chemical calculations
Figure 1 demonstrates the profile of 1˙¯ dissociation reaction obtained by calculation of
its potential energy surface (PES). As one can see in Fig. 1, 1˙¯ in aprotic medium with a
dielectric constant close to that of DMF can, in fact, exist as a kinetically independent species, as
indicated by the corresponding minimum on its PES. Calculations show that the formation of 1˙¯
involves considerable structural changes. In particular, the C-O bond length increases by 0.06 Å
and the C4a-C4b bond length decreases by 0.05 Å.
Though transfer of an electron to 1 gives the corresponding radical anion, the latter is
thermodynamically and kinetically unstable and should decay to give a hydroxyl anion ОН¯ and
fluorenyl radical 2˙ (Fig. 1). The Gibbs free energy of 1˙¯ dissociation are -18.9 kcal·mol-1. The
activation energy calculated as the difference between energies of optimized structures of
transition state and 1˙¯ equal 6.4 kcal·mol-1. The latter value is approximately two times smaller
than the activation energy of dissociation of a carbon-halogen bond in aryl halide radical anions
[93]. It is known [1] that the relatively high activation energies of the latter are due to the fact
that the carbon-halogen bond lies in the plane of the aromatic system, hence its dissociation is
forbidden for symmetry reasons. Therefore, the carbon-halogen bond should not only be
elongated but should also deviate from the plane for this reaction to occur. As shown in [93], this
deviation can be up to ~30° in the transition state and can decrease the activation energy 2-3
fold.
Unlike aryl halides, the hydroxy group that undergoes elimination in 1˙¯ is located
outside the aromatic system plane, so there are no symmetry limitations for 1˙¯ dissociation.
However, as one can see from Fig. 2, an increase in the length of the dissociating bond is also
accompanied by considerable structural changes. It is interesting that the process that occurs in

this case is opposite to that observed for anion radicals of aryl halides, namely, planarity
increases due to a change in C9 atom hybridization from sp3 to sp2.
Thus, the results of quantum-chemical calculations make it possible to believe that the
electron transfer to molecule 1 produces 1˙¯ (1), which decomposes to fluorenyl radical 2˙ and a
hydroxide anion (2). Quantum-chemical calculations show that the electron affinity of 2˙ is 1.7
eV higher than that of 1. Therefore, it should be expected that it would undergo reduction to the
corresponding anion at potentials of 1 reduction (3). In other words, the electroreduction of 1
should follow the ЕСЕ mechanism (1-3).
(1)
(2)
(3)
However, taking into consideration the basicity of ОН¯ and 2¯ anions formed in
reactions (2) and (3) and the acidic properties of the starting 1, it can be assumed that the
mechanism of its electroreduction would also involve protolytic equilibria (4)-(7). The results of
calculations on the thermodynamics of these reactions shows that the equilibrium will be shifted
to the right in all cases except (6) (Table 2). Nevertheless, earlier, insufficient attention was
given to the effect of the specified reactions on the overall electroreduction mechanism. In the
paper [71] dealing with electroreduction of methanol derivatives, the possibility of proton
transfer between the starting compound and the products was not considered at all. The role of
some mentioned reactions in the electroreduction mechanism of 1 derivatives was discussed in
[73]. However, the conclusions on the general process scheme were made in [73] solely on the
basis of a qualitative examination of CV curves and were mostly assumptions. Therefore, in
order to determine reliably the mechanism of C-OH bond cleavage initiated by the electron
transfer, it appeared expedient to study the kinetics of this process by electroanalytical methods.
More generally, it may be interesting because in contrast to “real” selfprotonation reaction
between initial anion radical and the starting molecule (e.g. [94]) there are not too many
examples of proton transfer between starting molecule and the products of its anion radical
dissociation [95-102].
(4)
(5)
(6)
(7)
3.2. Electrochemical Investigations

The figures demonstrate a CV curve (Fig. 3-5) and a current-potential curve of 1
electroreduction (Fig. 6) built from chronoamperometric data. The CV curves of 1 exhibit two
cathodic peaks (Fig. 3). The potential of the second peak, corresponds to the electroreduction of
2, which should be the product of ЕСЕ mechanism. As we might have expected based on the low
activation energy of reaction (2), the electroreduction peak of 1 on CV curves in the range of
potential scan rates studied (from 0.025 to 3 V·s-1) is chemically irreversible and the anodic peak
corresponding to 1˙¯ oxidation is not observed (Fig. 4). As noted above, it could be expected that
electroreduction of 1 should be described by the ЕСЕ mechanism and the apparent number of
electrons participating in electroreduction (napp) should be 2. However, the napp values found
from CA data as the ratio of the limiting electroreduction current of 1 to the limiting current of
an one-electron diffusion process, or in the case of CV as the ratio of the cathodic peak current to
the peak current of a one-electron process not complicated by near-electrode reactions, are much
smaller than 2. In the case of CA, in the range from 2 to 4 s, napp = 1.19±0.02 and nearly does not
depend on time and concentration of 1. In the case of CV, the mean napp value equals 0.98±0.05
and does not depend on the depolarizer concentration, either. Such behavior corresponds to the
achievement of pure kinetic conditions. This is not surprising, since the activation energy of the
reaction (2) is extremely low, and the anions ОН¯ and 2¯ are highly basic.
The low napp values allow us to assume that, along with reactions (1)-(3), reactions of the
starting compounds with its electroreduction products also take place and lead to
electrochemically inactive compounds. As noted above, it appears most likely that these
reactions include the protonation of ОН¯ (4)-(5) and 2¯ anions (6)-(7). In favor of this
hypothesis is the fact that the addition of equimolecular amount of phenol increases the peak
current to a level corresponding to two electron ECE process.
At the same time the value of napp for the mechanism (1)-(7) in pure kinetic conditions
should be equal 2/3. Moreover, the CV curves do not show peaks that could be assigned to
oxidation of carbon-centered (1¯ С) or oxygen-centered anions (1¯ О) of 1 (Fig. 5). The last feature
of 1 electroreduction was already noted previously [73] and was explained by the reaction (8) of
proton transfer between 1¯ С and 1¯ О to give neutral 1 and 32-. The peaks of 1¯ is also absent in
the cyclic voltammograms of a solution of 1 which contains an equimolar amount of Bu4NOH,
although there are peaks corresponding to 3 and its dianion [82]
(8)
As comparison of CV curves for 1 and 3 shows, the curves of the former actually contain
peaks of successive oxidation of 32¯ and 3˙¯ (Fig. 4-5). Furthermore, we detected the formation
of 32¯ due to electroreduction of 1 upon preparative electrolysis of 1 at a limiting current

potential. Figure 5 shows the CV curves of the catholyte before and after the electrolysis. The
initial part (from 0 to -0.4 V) of the last CV curve is not shown at the Fig. 5 because of
considerable anode current corresponding to the oxidation of the 32¯ .
However, the results of quantum-chemical calculations for the thermodynamics of
protolytic reactions involving 1 and its electroreduction products (Table 2) indicate that the
equilibrium constant of proton transfer between 1¯ С and 1¯ O (8) is a few orders smaller than
those for the dianion formation in reactions of OH¯ with 1¯ С and 1¯ O (reactions 9 and 10).
(9)
(10)
Furthermore, one can see from Table 2 that the equilibrium constant of 32¯ formation in
the case of 1¯ С (K10) is close to the constant of 1¯ С formation from a neutral molecule (K4),
whereas in the case of 1¯ O, K9 is by 2 orders higher than K5, which explains the lack of oxidation
peaks of C- or O- anions.
An experimental confirmation of an unusual ratio of equilibrium constants for reactions
(5) and (9) was obtained by us previously [82]. For this purpose, we studied the protonation of
cathodically generated 32¯ (reactions 11-12) with water and determined the equilibrium
constants of reverse reactions with respect to reactions (4-5) and (9-10). An advantage of this
approach is that alcohol 1 is electrochemically inactive at potentials of 32¯ formation, hence
reactions (1)-(3) can be excluded from consideration. Therefore, we used a similar approach in
this study in order to determine the parameters of protolytic equilibria (6)-(7) and (13)-(14).
(11)
(12)
(13)
(14)
As shown previously [82], two cathodic peaks are observed on the CV curves of 3 (Fig.
5), the first of which (-1.23 V) is related to 3˙¯ formation, whereas the second one (-1.87 V)
corresponds to 32¯ formation (curve с). The shape of the second peak indicates that it is of quasi-
reversible nature that is observed for a number of electroreduction processes of anion radicals of
carbonyl compounds to the corresponding dianions [103]. Comparison of experimental and
theoretical cyclic voltammograms allowed us to estimate the heterogeneous constant of electron
transfer rate: ks, 12 = 0.01 cm·s-1. The low value of the constant may be caused by high energy of
reorganization upon second electron transfer (the C-O bond length increases by 0.06 Å.). On the

anodic branch of the CV curve, peaks of 3˙¯ (-1.16 V) и 32¯ (-1.76 V) anodic oxidation are
observed.
Addition of 2 to a solution of 3 does not affect the current values corresponding to the
formation and oxidation of 3˙¯ even at a small potential scan rate (0.025 V·s-1) and with a
tenfold excess of 2. This fact allows us to believe that 2 does not protonate 3˙¯ . On the other
hand, the ratio of the anodic and cathodic branches of the second peak decreases with an increase
in the concentration of 2 (Fig. 7, curves b-d). Furthermore, an anodic peak at -0.54 V
corresponding to oxidation of fluorene anion 2¯ appears in the presence of 2 (Fig. 7, curves b-d).
It is of note that peaks of monoanions 1¯ С and 1¯ O do not appear on the CV curve of 3 (Fig. 7)
upon addition of 2 (Fig. 7, curves b-d), like in the case of water addition [82]. Using the results
of quantum-chemical calculations on the thermodynamics of 1¯ С and 2¯ oxidation and the
parameters of the correlation relationship [104], it can be concluded that the oxidation potential
of 1¯ С should be about -0.3 V. However, no oxidation peaks are observed in this region (Fig. 7).
It should also be noted that the shapes of the cathodic and anodic peaks corresponding to
the formation and oxidation of 32¯ on the CV curves of solutions containing a proton donor (Fig.
8) differ considerably from the shape typical of irreversible protonation of 32¯ and the 1¯ С and
1¯ O anions formed (Fig. 8, curve с). At the same time, the theoretical curves calculated for the
mechanism (6)-(7) and (11)-(14) using the theoretical value K15 = 15.4 of the constant of
equilibrium between the 1¯ С and 1¯ O anions and parameters listed in Section 2.2 (Fig. 7, curve b)
have a shape similar to that observed experimentally.
(15)
The best fit between experimental and theoretical CV curves (Fig. 8, curves a and b) is
reached at the ratio of constants K13/K6 and K14/K7 predicted on the basis of quantum-chemical
calculations of the thermodynamics of the reactions in question (Table 2) but at the values of
these constants approximately one to two orders lower than the theoretical ones. The latter fact
may result from the use of the continual model to describe the solvation effects; however, this
model does not account for the formation of hydrogen bonds with a solvent, which is typical of
hydroxy derivatives. As shown below, the data obtained allowed us to give an adequate
description of 1 electroreduction.
Thus, experimental results confirm the conclusion on the unusual ratio of constants,
K13/K6, made on the basis of quantum-chemical calculations. This ratio, like the unusual ratio of
the equilibrium constants of reactions (5) and (9) that we found previously [82], is caused by a
special feature of π*-dianions, in particular, their lower basicity in comparison with other

negatively charged species [105], and can be observed in those cases where the corresponding
protolytic equilibria give a π*-dianion.
Unlike the electroreduction mechanism considered above at potentials of 32¯ formation,
the mechanism of 1 electroreduction, along with bulk reactions (4-5) and (8-10), should involve
protonation of the fluorene anion formed, (6-7) and (13-14). Like in the previous case, the use of
theoretical values of the ratio of equilibrium constants for the reactions mentioned above makes
it possible to describe the electroreduction of 1 correctly in the entire range of concentrations and
potential scan rates used. As one can see from Fig. 9, the shape of theoretical CV curves is in
good agreement with the experimental curves of 1 electroreduction, and the experimental and
theoretical peak currents are similar in the entire range of concentrations and potential scan rates:
ip /itpheor =1.06 ± 0.09
.
Comparison of the shape of experimental cyclic voltammograms with that of theoretical
CV curves obtained by numeric simulation methods also allowed us to estimate the
heterogeneous constant of electron transfer rate: ks1 = 0.1 cm·s-1. The low value of the constant is
apparently caused by high energy of reorganization upon electron transfer due to the fact that
radical anion formation involves considerable structural changes. It should be noted that this
constant has a similar value of 0.2 cm·s-1 in the case of N-(4-nitrophenyl)hydroxylamine [70],
where formation of the radical anion is accompanied by elongation of the N-O bond by 0.03 Å.
The electrolysis results also confirm the mechanism under discussion. The main products
isolated from catholyte are 2 and 3. The yield of 3 is approximately equal to the theoretical value
50%, when the yield of 2 is about 20%. The decrease in the yield of 2 may be due to its
homogeneous reduction by 32¯ which gives rise 3˙¯ and 2˙¯ . The contribution of this reaction
should increase with increasing of 32¯ and 2 concentrations. The formation of 3˙¯ , which is
reduced at the potential of electrolysis, explains why the initial decreasing of the current changes
later for it growing. The presence of resin material among the products of the electrolysis (see
2.1) may be caused by the reactions of 2˙¯ and the products of its protonation.
4. Conclusion
The data of our experiments and calculations indicate that the electroreduction of
fluorenol is accompanied by cleavage of the C-ОН bond in its anion radical and by formation of
bases, namely, fluorenol anion 2¯ and hydroxide anion ОН¯ , which participate in the protolytic
equilibrium described by Scheme 2. The shift of equilibrium toward dianion formation is due to
an unusual ratio of constants, KII/KI░>░1, caused by a lower basicity of the π*-dianion in
comparison with other anions.

Conflict of interest
The authors declare that there is no conflict of interest with any financial organization
regarding the material discussed in the manuscript.
Acknowledgements
Financial support of the Russian Science Foundation is acknowledged (RSF Grant 14-50-00126).
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Figure Caption
Fig. 1. Profile of С-О bond dissociation in 1˙¯ (DFT B3LYP/6-311++G(d,p) CSC-PCM)
∆G=G2˙-GOH-
Fig. 2. Variation of angle θ between the С9-Н bond and the plane of the aromatic
system along the coordinate of С-О bond dissociation reaction in 1˙¯ (DFT
B3LYP/6-311++G(d,p) CSC-PCM)
Fig. 3. CV curves for 5 mmol·L-1 1 and 2 in 0.1 М Bu4NClO4 in DMF at a
potential scan rate of 0.1 V·s-1.

Fig. 4. CV curves for 5 mmol·L-1 1 and 3 in 0.1 М Bu4NClO4 in DMF at a potential scan rate of
0.1 V·s-1
Fig. 5. CV curves for 5 mmol·L-1 solution of 1 before and after electrolysis, and 5
mmol·L-1 solution of 3 in 0.1 М Bu4NClO4 solution in DMF at a potential scan
rate of 0.1 V·s-1.
Fig. 6. Currents on CA curves (in 2 s after applying an impulse) for 7.5
mmol·L-1 1 in 0.1 М Bu4NClO4 solution in DMF (circles) in comparison with a
simulated curve (line).
Fig. 7. CV curves for 5 mmol·L-1 solution of 3 in the absence (a, black) and in
the presence of 5 mmol·L-1 (b, blue), 10 mmol·L-1 (c, red), and 26 mmol·L-1 2
(d, green) in 0.1 М Bu4NClO4 solution in DMF at a potential scan rate of 0.1
V·s-1.
Fig. 8. CV curve of a mixture of 5 mmol·L-1 3 and 50 mmol·L-1 2 in 0.1 М
Bu4NClO4 solution in DMF at a potential scan rate of 0.4 V·s-1 (a, black),
theoretical curves for mechanisms (6)-(7) and (11)-(14) (see Section 2.2 for
simulation parameters) (b, blue), and (11)-(12) with irreversible protonation
(kprot = 2·102 M-1·s-1) of 32¯ (c, red).
Fig. 9. CV curve of 20 mmol·L-1 1 in 0.1 М Bu4NClO4 solution in DMF (—) in
comparison with the theoretical curve (○) (see Section 2.2 for simulation
parameters).
Scheme 1. Structures and designations of compounds and intermediates studied
Scheme 2. Protolytic equilibrium between 1 and a base (В- is a base).
Table 1. Theoretical values of Gibbs free energy (DFT B3LYP/6-311++G(d,p) CSC-PCM)
Species G, Kcal/mol
-361838
1
-361533
1¯ c
-361532
1¯ o
-314627
2
2-
-314320
-314235
2˙
32-
-361228
OH-
-47669
-47981
H2O

Table 2. Theoretical values of the constants of protolytic equilibria. (DFT B3LYP/6-
311++G(d,p) CSC-PCM)
Reaction
(4)
K
1.2E+05
4.9E+03
1.0E+00
2.4E+01
1.2E-01
9.8E+05
4.1E+04
2.0E+02
8.4E+00
(5)
(6)
(7)
(8)
(9)
(10)
(13)
(14)