Geometric and electronic structures of phenoxyl radicals hydrogen bonded to neutral and cationic partners

Article abstract of DOI:10.1002/chem.201102854

Two di-tert-butylphenols incorporating an N-methylbenzimidazole moiety in the ortho or para position have been synthesised (^MeOH and ^pMeOH, respectively). Their X-ray structures evidence a hydrogen bond between the phenolic proton an

Full text of DOI:10.1002/chem.201102854

DOI: 10.1002/chem.201102854
Geometric and Electronic Structures of Phenoxyl Radicals Hydrogen Bonded
to Neutral and Cationic Partners
Maylis Orio,[a] Olivier Jarjayes,[b] Benoit Baptiste,[b] Christian Philouze,[b]
Carole Duboc,[c] Jenny-Lee Mathias,[d] Laurent Benisvy,*[d] and Fabrice Thomas*[b]
Abstract: Two di-tert-butylphenols in-
corporating an N-methylbenzimidazole
moiety in the ortho or para position
and pMeOH yield the corresponding
phenolate species (MeO)À and (pMeO)À,
respectively, whilst that of the previ-
ously reported HOH (analogous to
MeOH but lacking the N-methyl group)
produces an unprecedented hydrogen-
bonded phenol benzimidazolate spe-
cies, as evidenced by its X-ray struc-
ture. The latter is believed to be in
equilibrium in solution with its tauto-
meric phenolate form, as suggested by
NMR, electrochemistry and DFT stud-
ies. The one-electron oxidations of the
anions occur by a simple ET process
affording phenoxyl radical species,
whose electronic structure has been
studied by HF-EPR spectroscopy and
DFT calculations. In particular, analy-
sis of the g1 tensor shows the order
have been synthesised
(
MeOH and
pMeOH, respectively). Their X-ray
structures evidence a hydrogen bond
between the phenolic proton and the
iminic nitrogen atom, whose nature is
intra- and intermolecular, respectively.
The present studies demonstrate that
MeOH is readily oxidised by an intra-
molecular PET mechanism to form the
2.0079>2.0072>2.0069>2.0067
for
Me
H
Me
H
C
C
C+
C+
(
O) , ( O) , ( OH) and ( OH) ,
respectively. (MeO) exhibits the largest
C
g1 tensor (2.0079), consistent with the
absence of intramolecular hydrogen
H
C
hydrogen-bonded phenoxyl-N-methyl-
bond. The g1 tensor of ( O) is inter-
Me
C+
C+
mediate between those of (MeOH)
benzimidazolium system
(
OH) ,
Keywords: density functional calcu-
lations · electronic structure · elec-
tron transfer · hydrogen bonds ·
phenoxyl radicals
whereas oxidation of pMeOH occurs by
and (MeO) (g1 =2.0072), indicating that
the phenoxyl oxygen is hydrogen-
bonded with a neutral benzimidazole
partner.
C
intermolecular PET, affording the neu-
pMe
C
O)
tral phenoxyl benzimidazole
(
system. The deprotonations of MeOH
Introduction
Tyrosyl radicals are of widespread importance in biological
systems, since they are involved in many enzymatic process-
es.[1–7] A prototypical example of a tyrosyl radical protein is
[a] Dr. M. Orio
Laboratoire de Spectrochimie Infrarouge et Raman
Bꢀtiment C5 - UMR CNRS 8516
Universitꢁ des Sciences et Technologies de Lille
59655 Villeneuve d’Ascq Cedex (France)
C
the water-oxidising photosystem II (PSII), in which TyrZ
acts as a relay in the transfer of electrons between the P680
chlorophylls and the water-oxidising manganese cluster, as
well as being directly potent in water oxidation through H-
atom abstraction.[1–3] Interestingly, PSII contains a second
[b] Dr. O. Jarjayes, Dr. B. Baptiste, Dr. C. Philouze, Prof. Dr. F. Thomas
Dꢁpartement de Chimie Molꢁculaire
Chimie Inorganique Redox Biomimꢁtique (CIRE)
UMR-5250, Universitꢁ Joseph Fourier
BP 53, 38041 Grenoble Cedex 9 (France)
Fax : (+33)476-51-4836
C
tyrosyl radical, namely, TyrD , which has been found to be
C
C
more stable than TyrZ . The biological function of TyrD is
C
unknown, but it has clearly been shown that TyrD is not in-
E-mail: Fabrice.Thomas@ujf-grenoble.fr
volved in water oxidation in PSII during photosynthesis.
Another example of importance is galactose oxidase, in
which the tyrosyl radical is coordinated to a CuII ion.[4] It is
directly involved in catalysis by abstracting an H atom from
the alcohol substrate. Among the other enzymes which are
believed to generate tyrosyl radicals on their own peptidic
chain, the best known are ribonucleotide reductase,[1,5] cyto-
chrome c oxidase[1,6] and prostaglandin H synthase.[1,7] Mod-
ulation of tyrosyl radical formation and its reactivity is be-
lieved to be achieved through metal coordination (if appli-
cable) or the existence of hydrogen bonds with a neighbour-
ing base (e.g., His). During the last few years chemical
models for hydrogen-bonded tyrosyl radicals have been
[c] Dr. C. Duboc
Laboratoire National des Champs Magnꢁtiques Intenses (LCNMI)
UPR CNRS 3228, BP 166, 38042 Grenoble Cedex 9 (France)
Present adress: Dꢁpartement de Chimie Molꢁculaire
Chimie Inorganique Redox Biomimꢁtique (CIRE), UMR-5250
Universitꢁ Joseph Fourier
BP 53, 38041 Grenoble Cedex 9 (France)
[d] J.-L. Mathias, Dr. L. Benisvy
Laboratory of Biomimetic Chemistry
Department of Chemistry, Bar-Ilan University
Ramat Gan 52900 (Israel)
Fax: (+972)3-7384053
E-mail: benisvl@mail.biu.ac.il
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102854.
5416
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Chem. Eur. J. 2012, 18, 5416 – 5429

FULL PAPER
characterised.[8–17] Evidence has then been gathered that oxi-
dation of tyrosine, if not coordinated to a metal ion, occurs
by a proton-coupled electron-transfer (PET) mechanism.[18]
thermodynamic and kinetic investigations on HOH have pro-
vided additional important insights into the redox mecha-
nism[12] and have elegantly shown that such compounds are
[10–12,15]
Indeed, the oxidation potential of tyrosine (TyrOH +/TyrOH
among the best models of the TyrZ –His pair of PSII.
C
C
couple) is high (1.4 to 1.5 V vs. NHE in water),[18] and is not
compatible with the fact that enzymes operate with small re-
action free energies. Recent electrochemical and kinetic in-
vestigations performed on phenolic compounds that are in-
tramolecularly hydrogen-bonded to an adjacent base (e.g.
imidazole, pyridine) have revealed that coupling of the oxi-
dation reaction with proton-electron transfer (PET) im-
proves the driving force of the reaction.[11,18] In addition, the
oxidation process is concerted (CPET) in order to avoid un-
favourable formation of charges during the reaction. “Con-
certed” in this case implies that both the electron and the
proton move in a single kinetic step.[11]
One of the most powerful techniques to probe the local
environment of tyrosyl radicals in proteins is high-field EPR
(HF-EPR), which allows for determination of the g-tensor
anisotropy and thus the three principal g values.[3] More spe-
cifically, an electropositive environment like the presence of
a hydrogen bond induces a shift of the low-field component
g1 that can be experimentally detected in enzymatic systems.
We recently reproduced the lowering of g1 as a result of hy-
drogen-bonding interaction between the phenoxyl oxygen
atom and a cationic partner in biomimetic models.[10,17] To
date the effect of the charge of the donor on the g tensor in
hydrogen-bonded phenoxyl radicals has been poorly docu-
mented, although it is of major importance for characteris-
ing intermediates and better understanding biological pro-
cesses that involve tyrosyl radical species (Figure 1).[18–19] It
has been previously shown by combined spectroscopic and
theoretical methods that oxidation of phenol–imine conju-
gates,[9–12] and especially HOH (Figure 2),[10] occurs by an in-
tramolecular PET mechanism, affording a phenoxyl radical
hydrogen-bonded to an iminium cationic partner. Detailed
In an effort to better under the parameters that influence
the electronic structure and the g-tensor anisotropy of hy-
drogen-bonded phenoxyl radicals we herein complement
our previous works by investigating the effect of the charge
of the partner. Because of their relatively high stability we
investigate phenol–benzimidazole compounds[10] rather than
Mannich bases.[17] With the aim of tuning the charge of the
hydrogen-bond partner (benzimidazole group) and control-
ling the mechanism of radical formation (CPET or ET), we
herein extend the electrochemical, spectroscopic and theo-
retical studies that we performed on HOH[10] to the depro-
tonated derivative (HO)À[16] (Figure 2). In addition, we intro-
duce the new N-methylated phenol MeOH and its corre-
sponding phenolate (MeO)À (Figure 2) in order to gain in-
sight on the effect of hydrogen-bonding on the properties of
the corresponding phenoxyl radicals.
To discriminate the effect of delocalisation from pure
electrostatic effects, we introduce the new compound pMeOH
and phenolate (pMeO)À, which have an N-methylbenzimida-
zole group connected to the phenol group at the para posi-
tion, and thus are unable to establish an intramolecular hy-
drogen bond between the phenolic proton and the N-meth-
ylbenzimidazole.
We herein demonstrate that phenoxyl radical properties
(i.e., oxidation potential, electronic structure and their g
tensors) are subtly modulated by the charge of the hydro-
gen-bonded partner, the strength of the hydrogen-bonding
interaction and their deviation from planarity.
Results and Discussion
Phenolic compounds HOH, MeOH and pMeOH were synthes-
ised in one step, as described in the Experimental Section.
The corresponding phenolates (HO)À, (MeO)À and (pMeO)À
were synthesised by treating the parent phenol with a stoi-
chiometric amount of base nBu4NOH, isolated and charac-
terised (see Experimental Section).
C
Figure 1. Proposed mechanism for the formation of the stable TyrD radi-
cal in photosystem II.
X-ray crystal structures of HOH, MeOH, pMeOH, (MeO)À and
(HO)À: The X-ray crystal structure of HOH·0.5H2O has been
reported previously.[10] Herein we describe briefly a new un-
solvated structure for this compound and present in details
the structures of unprecedented MeOH, pMeOH, (MeO)À and
(HO)À (Figure 3, Table 1).
In unsolvated HOH the iminic nitrogen atom N1 is in-
volved in a bifurcated hydrogen-bond network. One is intra-
molecular with phenolic hydrogen atom H1 (O1···N1 2.74 ꢃ
and ]OH···N 1498), while the other is intermolecular with
the amino nitrogen atom N2 of another molecule
À
(N1···H26' N2' 2.99 ꢃ), leading to the formation of poly-
meric chains (see Supporting Information). In contrast to
Figure 2. Compounds of interest in this study.
the structure of HOH·0.5 H2O, in which the three independ-
Chem. Eur. J. 2012, 18, 5416 – 5429
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www.chemeurj.org
5417

F. Thomas, L. Benisvy et al.
The crystal structure of MeOH (Figure 3c) shows that the
phenol and N-methylbenzimidazole rings are not coplanar,
but twisted by 338. This deviation from planarity is most
likely due to a steric clash between the N-methyl group and
one of the meta-H atoms of the phenol group. Yet, stabilis-
ing intramolecular hydrogen-bonding between the phenolic
hydrogen atom and the N-methylbenzimidazole iminic nitro-
gen atom is established, as evidenced by the short O1···N1
À
distance (2.65 ꢃ), and the O1 H1···N1 angle of 1508. Inter-
À
estingly, the C1 O1 bond length of 1.366(2) ꢃ is consistent
with a phenol and resembles that of HOH (1.367(3) ꢃ). The
structure of pMeOH is shown in Figure 3e. The phenol and
N-methylbenzimidazole rings are twisted by 428. In addition,
À
the C1 O1 bond length (1.361(3) ꢃ) is similar to those of
MeOH and HOH. An intermolecular hydrogen bond exists
between phenolic hydrogen atom H1 and the N-methylbenz-
imidazole iminic nitrogen atom N1’ of another molecule, as
À
judged by the O1···N1’ distance of 2.91 ꢃ and the O1
H1···N1’ angle of 1548.
The corresponding anions (HO)À and (MeO)À have both
been structurally characterised as tetrabutylammonium salts
(Figure3b,d). Both X-ray structures contain water molecules
Figure 3. ORTEP diagrams of the molecular structures of a) HOH,
b) (HO)À in (HO)[NnBu4]·H2O, c) MeOH, d) (MeO)À·H2O in MeO)-
[NnBu4]·2.4H2O, and e) pMeOH shown with 30% thermal ellipsoids. Hy-
A
(
([HO][nBu4N]·H2O and [MeO]
ACTHNUTRGENNGU CAHTUNGTRNEN[UGN nBu4N]·2.4 H2O), which estab-
ACHTUNGTRENNUNG
drogen atoms are omitted for clarity except the phenolic and amino ones.
lish intermolecular hydrogen bonds with imidazole N ac-
ceptors to form dimers {HO···H2O···H2O···OH} in the former
or with both phenolate OÀ and imidazole N acceptors yield-
ing a linear polymer chain {MeO···H2O···H2O···OMe···}n in the
latter salt (see Supporting Information).
ent molecules appear to be quasi-planar (phenol/imidazole
twist angles of 7–178),[10] the new unsolvated HOH structure
deviates significantly from planarity with a phenol/imidazole
twist angle of 298. This polymorphism thus suggests that
solid-state structures should be used with special care when
estimating the strength of the hydrogen bond between
phenol protons and imino acceptor, as recently emphasised
by Mayer et al.[12]
The structure of (MeO)À differs significantly from that of
its corresponding phenol parent MeOH. Indeed, the N-Me
group is now pointing toward the phenolate OÀ atom
(phenol/imidazole twist angle ca. 1398), whereas in MeOH it
points in the opposite direction (see Figure 3c, d).
Table 1. Crystallographic data for HOH, (HO)
HOH
G
ACHTUNGTRENNUNG
E
MeOH
E
MeO)
G
HLpMe
formula
M
crystal system
space group
a [ꢃ]
b [ꢃ]
c [ꢃ]
a [8]
C21H26N2O
322.45
monoclinic
P21/a
10.145(7)
11.854(3)
15.930(3)
90
C37H63N3O2
581.92
monoclinic
P21/c
16.153(5)
32.515(9)
19.087(4)
90
C22H28N2O
336.48
monoclinic
P21/n
6.085(2)
36.08(7)
9.225(1)
90
C38H67.8N3O3.4
621.17
monoclinic
P21/c
14.02(1)
24.70(1)
16.65(1)
90
C22H28N2O
336.46
monoclinic
P121/c1
15.449(7)
9.911(2)
12.052(6)
90
b [8]
91.96(3)
90
1915(1)
4
132.69(2)
90
7369(4)
8
103.05(3)
90
1973(1)
4
137.77(3)
90
3875(4)
4
92.44(5)
90
1843.8(13)
4
g [8]
V [ꢃ3]
Z
T [K]
293
1.180
0.534
200
1.049
0.064
293
1.133
0.069
200
1.065
0.067
200
1.212
0.74
1calcd [gcmÀ3
]
m [cmÀ1
]
monochromator
radiation
graphite
CuKa
graphite
MoKa
graphite
MoKa
graphite
MoKa
graphite
MoKa
l [ꢃ]
reflns collected
1.5418
4278
0.71073
55699
0.71073
23012
0.71073
30003
0.71073
3202
independent reflns (Rint
observed reflns
R
Rw
)
4248 (0.09616)
2522 [I>2s(I)]
0.0746
12816 (0.09642)
8423 [I>2s(I)]
0.0599
3496 (0.11680)
2138 [I>2s(I)]
0.0556
6935 (0.15944)
4268
0.0626
0.0766
2228 (0.1225)
3202 [I>2s(I)]
0.0626
0.1084
0.0700
0.0725
0.1162
5418
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Chem. Eur. J. 2012, 18, 5416 – 5429

H-Bonded Phenoxyl Radicals
FULL PAPER
The structure of (HO)À is in great contrast to that of its N-
methylated analogue (MeO)À. First, the anion is quasi-planar
with phenol/imidazole twist angles of about 1 and 128 for
relatively large phenol–imidazole twist angles preclude accu-
rate determination of the strength of this hydrogen-bonding.
In solution in CDCl3, H NMR spectroscopy is informative.
1
the two molecules in the asymmetric unit. Second, the C1
O1 bond lengths of the two molecules of 1.363(3) and
In particular, the OH proton resonances for HOH and MeOH
appear at 13.60[10] and 12.48 ppm, respectively. Such a high
chemical shifts of the phenolic OH protons unambiguously
indicate relatively strong hydrogen-bonding in both com-
pounds, and that the latter hydrogen bond is weaker than
the former. The hydrogen bond is intramolecular, as evi-
denced by the fact that the OH resonance remains unaffect-
ed even at high dilution. Thus, for all phenol compounds,
the intramolecular OH···N hydrogen bond is clearly main-
À
À
1.364(3) ꢃ are too long for a phenolate C O bond, and are
À
characteristic of
a
protonated phenol (e.g., C1 O1
H
À
1.367(3) ꢃ in OH). Third, the two C N bonds of the benz-
imidazole ring of (HO)À (1.355(3)/1.351(3) ꢃ and 1.361(3)/
1.345(3) ꢃ for the two molecules in the assymmetric unit)
are significantly different to the corresponding bonds in the
phenol parent, which displays a typical imidazole pattern
with a short C=N bond (1.326(3) ꢃ in HOH and 1.328(2)
1
tained in solution. In contrast, in the H NMR spectrum of
and 1.329(2) ꢃ for the two molecules in HOH·0.5H2O[10]
)
(HO)À no OH resonance was observed. Attempts to discrim-
inate between the imidazolate and phenolate forms of
(HO)À in solution by variable-temperature NMR failed, as
neither the OH nor the NH proton could be located even at
233 K in CDCl3. This certainly indicates fast prototropic ex-
change between the two forms.
and a long C N bond (1.368(3) ꢃ in HOH and 1.363(2) and
1.358(2) ꢃ for the two molecules in HOH·0.5H2O[10]). There-
fore, the structural data are consistent with (HO)À being best
described as a phenol–imidazolate conjugate anion. The
latter establishes OH···N hydrogen-bonding between the
phenol OH group and the adjacent imidazolate N atom, as
À
Electrochemistry: The electrochemical behaviour of MeOH,
pMeOH, (MeO)À, (HO)À[16] and (pMeO)À was studied by cyclic
voltammetry (CV) in the non-coordinating solvent CH2Cl2
in the presence of 0.1m tetrabutylammonium perchlorate
(TBAP) as supporting electrolyte. All potentials are given
relative to the ferrocene/ferrocenium (Fc/Fc+) redox couple,
which was used as reference (Table 2).
À
evidenced by the O···N distance of 2.527 ꢃ and the O1
H1···N1 angles of 148.0 and 149.88. This OH···NÀ tautomeric
form is unexpected and unprecedented. Considering the pKa
of a free phenol/phenolate of about 10 and that of a free
imidazole/imidazolate of about 14.5,[20] deprotonation would
occur at the most acidic site, that is, the phenol site, to pro-
duce a phenolate–imidazole conjugate, as assigned recently
by Moore et al.[16] for (HO)À in solution state. We show here
that the presence of a hydrogen bond significantly alters the
pKa values, making the phenol-imidazolate form isolable.
Further reaction of the anion with iodomethane in stoichio-
metric amount leads to methylation at the N position and
not the O position, confirming that the imidazolate form is
relevant in solution. The discrepancy between this structure
and the previous assignment of (HO)À as a phenolate–benz-
imidazole compound in solution[16] prompted us to investi-
gate the relevance of the two forms by DFT methods (see
Supporting Information). We optimised the structures of
(HO)À with the proton removed from either the phenol or
the benzimidazole group and calculated the energy differ-
ence between the two forms. The computed energy gap be-
tween the two forms is small (5.0 kJmolÀ1 in the favour of
the phenol–imidazolate form), and thus suggests that both
forms are relevant and might exist in equilibrium in solu-
tion, as further suggested by NMR and electrochemical
studies.
Table 2. Oxidation potentials of the neutral and anionic compounds.[a]
Neutral
compounds
E1/2 [V]
(DEp)[b]
Anionic
E1/2 [V]
ACHTUNGRTEN(NUNG DEp)
N
compounds[c]
HOH
+0.46 (0.14)
+0.48 (0.18)
(HO)À
À0.23 (0.12)
À0.42 (0.15)
À0.51 (0.14)
MeOH
pMeOH
(
MeO)À
Eap = +0.33
A
pMeO)À
Ecp =À0.27[d]
[a] At 298 K in CH2Cl2 +0.1m TBAP versus Fc/Fc+. [b] The DEp for the
Fc/Fc+ couple under our experimental conditions is 0.10 V. [c] In the
presence of one equivalent of nBu4NOH. [d] Irreversible process.
The CV of MeOH is shown in Figure 4a and compared to
that of the previously described HOH.[10] In both cases the
CV consists of a quasi-reversible one-electron oxidation pro-
cess attributed to a proton-coupled electron-transfer process
[PET, see Eq. (1)][11,12] leading to formation of cation
R
C+
( OH) , which is stable on the timescale of the experiment
Therefore, the oxygen atom is systematically involved in
hydrogen-bonding interactions in the series, and only the
nature of this bond differs from one compound to another:
Intramolecular with OH as the donor in HOH, MeOH and
(HO)À, intermolecular with OÀ as acceptor in (MeO)À·2.4H2O
and intermolecular with OH as donor in pMeOH.
(at 298 K). The E1/2 value obtained for MeOH (0.48 V vs. Fc/
Fc+) is very close to that reported for HOH (0.46 V vs. Fc/
Fc+, see Table 2). Thus, as for the oxidation of HOH, that of
MeOH involves transfer of the phenolic proton to the benzi-
midazole moiety, and the resulting species is a phenoxyl rad-
ical whose oxygen atom is hydrogen-bonded to the benz-
imidazolium proton. Based on the similarities between HOH
and MeOH, from both structural and electrochemical (similar
E1/2 values) points of view, it is reasonable to assume that
oxidation of MeOH also occurs in a CPET process affording
NMR studies: The solid-state structures of phenolic com-
pounds HOH·0.5H2O,[10] HOH and MeOH all involve a intra-
molecular hydrogen bond between the phenol OH group
and benzimidazole N atom. However, polymorphism and
Me
C+
the hydrogen-bonded phenoxyl radical
(
OH)
[see
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5419

F. Thomas, L. Benisvy et al.
electrons. The fact that no more than 0.5 electrons could be
removed was confirmed by amperometric titration with a ro-
tating disc electrode (RDE; see Supporting Information).
Importantly, the colour of the solution at the end of electrol-
ysis was found to be similar (although less intense) to that
obtained after electrolysis in the presence of one equivalent
of nBu4NOH (see below). We therefore suggest that the
redox process at +0.33 V corresponds to formation of phe-
noxyl radical species (pMeO) in an intermolecular PET pro-
C
cess. Here the N-methylbenzimidazole moiety acts as an in-
termolecular base,[21] affording half an equivalent of (pMeO)
C
and half an equivalent of the harder to oxidise N-methyl-
benzimidazolium (pMeOH2)+ form of the phenolic compound
[Eq. (2)].
Figure 4. Cyclic voltammetry curves of 0.8–1 mm solutions of HOH (top),
MeOH (middle), pMeOH (bottom) in CH2Cl2 (+0.1m TBAP) in the ab-
sence (a) or presence (b) of one molar equivalent of [nBu4N][OH] at
298 K (iR-compensated); scan rate: 0.1 VsÀ1. Potentials are referred to
Fc/Fc+.
Addition of one equivalent of nBu4NOH to the phenolic
compounds results in formation of the anions (MeO)À,
(
pMeO)À and (HO)À. In contrast to the PET mechanism char-
acterising oxidation of hydrogen-bonded phenols, oxidation
of the phenolate compounds occurs in a simple ET process
[Eq. (3)]. A one-electron reversible oxidation process corre-
sponding to the generation of a phenoxyl radical is obtained
at E1/2 =À0.23 V and À0.42 V versus Fc/Fc+ for (HO)À[16]
and (MeO)À, respectively. Identical results were obtained for
the synthetically prepared (HO)À and (MeO)À.
Eq. (1)]. This indicates that the presence of the N-methyl
group at the benzimidazole ring does not significantly affect
the formation of the radical. This is consistent with the fact
that the intramolecular hydrogen-bonding interaction is pre-
served in solution for MeOH, as evidenced by NMR data,
which is crucial for the PET process to proceed in a concert-
ed manner and at relatively low potential. However, the
slightly lower E1/2 value for HOH compared to MeOH, may
account for the difference in strength of the hydrogen
bonds, which is weaker for MeOH than for HOH.
These values are much lower than that obtained for the
phenol–benzimidazole parent as a result of the negative
charge of the compounds[20] and differences in the oxidation
mechanism (ET versus CPET). Most importantly, whilst the
oxidation potentials of HOH and MeOH were found to be
almost the same, this is not the case for the corresponding
The CV of pMeOH contrasts sharply with that of MeOH
since the oxidation process is not reversible. It displays an
anodic peak at Eap = +0.33 V versus Fc/Fc+ which is associ-
ated with a cathodic peak at the much lower potential value
of Ecp =À0.27 V versus Fc/Fc+. This evidences either slow
electrode kinetics or the existence of a coupled chemical re-
action (e.g., a proton transfer to the N-methylbenzimidazole
base). To gain insight into the redox process, we performed
an electrolysis at +0.60 V at 233 K. The experiment was
stopped when the intensity of the current was less than 5%
of the initial value, which corresponds to removal of 0.5
anions (HO)À and (MeO)À (DE1/2 =E
G
1/2ACHTUNGTRENNUNG(
MeOÀ)=
0.19 V). Since (MeO)À cannot establish an intramolecular hy-
drogen bond as it lacks an NH group [Eq. (3)], but (HO)À
can, for example, in its OÀ···HN tautomeric form, the magni-
tude of DE1/2 reflects, at least partially, the change in hydro-
gen-bond strength between the oxidised and reduced forms
(see thermodynamic cycle in Figure S1 of the Supporting In-
formation).[12] The DE1/2 of 0.19 V corresponds to
17.6 kJmolÀ1. Such a value is within the range of the energy
5420
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H-Bonded Phenoxyl Radicals
FULL PAPER
Table 3. Electronic properties of the radical compounds.[a]
Compound
lmax [nm] (e [mÀ1 cmÀ1])
of a hydrogen bond, and thus suggests that the hy-
drogen bond of one form is very weak. This obser-
vation is consistent with the fact that hydrogen-
bonding is expected to be stronger for the pheno-
late form (reduced form) than for the phenoxyl rad-
ical form (oxidised form). Indeed, since hydrogen-
bond strength is related to the difference in pKa
values between donor and acceptor, the hydrogen-
bond strength is expected to decrease upon oxida-
tion of the phenolate (pKa(PhOH/PhOÀ)ꢀ10) to
H
+
C
( OH)
368 (3810), 398 (3780), 434 (1520), 790 (820)
Me
+
C
A
OH)
353 (5750), 392sh (2520), 416 (2350), 671 (830)
374 (4040), 395 (4440), 467 (1270), 910 (1080)
371sh (3840), 386sh (2930), 404 (2245), 468 (1420), 728 (1640)
347 (14190), 367 (17390), 384 (28250), 600br (6520)
H
C
( O)
Me
C
ACHUTGTNRENUNNG( O)
pMe
C
O)
A
[a] In CH2Cl2 +0.01m TBAP (UV/Vis).
C+
C
the phenoxyl radical (pKa(PhOH /PhO )ꢀ0). Thus, hydro-
gen-bonding interaction clearly has an effect on the oxida-
tion potential of these phenolate compounds. However,
other factors may contribute to the difference in oxidation
potential between (MeO)À and (HO)À, such as 1) the possible
contribution of the phenol–benzimidazolate tautomer of
(HO)À in solution, and 2) the difference in geometry be-
tween (HO)À and (MeO)À, particularly in the phenolate/benz-
imidazole twist angle, which would thereby induce differen-
ces in resonance stabilisation. The latter contribution is fur-
ther evidenced by the E1/2 difference (ca. 0.09 V) between
the energy of the HOMO upon deprotonation of the radical
cation and thereby a decrease in the HOMO–LUMO
energy gap. Such a red shift is also observed for the longer-
wavelength absorption (650–900 nm). Thus the neutral phe-
noxyl radical is clearly spectroscopically distinguishable
from its cationic counterpart.
The spectra obtained after electrochemical oxidation of
solutions of (pMeOH) and (pMeO)À are shown in Figure 6.
(
MeO)À and (pMeO)À, which reflects changes in resonance sta-
bilisation as described in detail below.
One-electron oxidised species: The one-electron oxidised
species were generated electrochemically in CH2Cl2 at
243 K and characterised by EPR and UV/Vis spectroscopy.
Upon oxidation the colourless solutions of the phenol and
phenolate compounds turn green or blue. The UV/Vis spec-
tra (Figure 5) of the electrochemically generated species
H
[10] Me
H
Me
C+
C+
C
C
( OH) ,
(
OH) , ( O) and ( O) all display a sharp
absorption in the 370–470 nm region, together with a broader
and less intense band at longer wavelengths (650–900 nm);
see Table 3. These are typical p–p* features of phenoxyl
radical compounds[10,14,17,22] and confirm the phenoxyl radical
Figure 6. UV/Vis spectra after electrochemical oxidation at 243 K of solu-
tions of pMeOH (a) and (pMeO)À (b). Eapplied was +0.40 V and À0.20 V, re-
spectively. Electrolysis was stopped when the current was less than 5%
of the original value. The spectra were recorded at 243 K (l=1.000 cm)
in 0.07 mm solutions, and the molar extinction coefficients calculated on
the basis of the initial concentration of the phenol compound.
character of the oxidised species. Interestingly, the band at
H
C+ [10]
about 420 nm observed for the radical cations ( OH)
and (MeOH) [434 (1520) and 416 nm (2350mÀ1 cmÀ1), re-
C+
spectively] appears to be red-shifted for the corresponding
Both spectra are remarkably similar in the 330–1100 nm
region, which suggests formation of the same chromophore
in both cases. Both spectra exhibit a broad band at 600 nm
and sharp absorptions at 384 and 367 nm, which correspond
to the p–p* transitions of phenoxyl radicals.[22] However, all
of the transitions are of double the intensity for electrolysed
H
Me
C
C
neutral radicals ( O) and ( O) [468 (1420) and 467 nm
(1270mÀ1 cmÀ1), respectively], which indicates an increase in
(
pMeO)À than for electrolysed pMeOH. Considering that elec-
trolysis in the presence of base is achieved with a >90%
yield (based on RDE voltammetry, see Supporting Informa-
tion), the spectrum obtained after oxidation of (pMeO)À is at-
tributed to (pMeO) . Since pMeOH does not absorb significant-
C
ly (compared to phenoxyl radical species) above 350 nm, the
UV/Vis data indicate that only half of the amount of pMeOH
is oxidised to (pMeO) in the absence of exogenous base. The
C
remaining part of the compound is likely in the phenol–
benzimidazolium form. This confirms the electrochemical
behaviour of (pMeOH) presented above [see Eq. (2)], that is,
oxidation of pMeOH is accompanied by intermolecular
proton transfer.
Figure 5. UV/Vis spectra of 0.1–0.18 mm solutions of electrochemically
+
H
Me
generated (MeOH) (a, solid), ( O) (b, dashes) and ( O) (c, dots) at
C
C
C
288 K in CH2Cl2.
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5421

F. Thomas, L. Benisvy et al.
The X-band EPR spectra of fluid solutions of electro-
H
+
[10]
Me
+
H
C
C C
chemically generated ( OH)
,
( OH) , ( O) and
Me
C
(
O) are remarkably similar. They consist of an unresolved
S=1= radical signal centred at g=2.005, with a line width
2
(peak-to-peak) of about 0.4 mT (see Supporting Informa-
tion). Though the lack of hyperfine features precludes infor-
mation on the geometric (such as in phenoxyl–tertiary am-
monium systems)[17] and electronic structure of the phenoxyl
radicals, the relatively small line width most likely indicates
that the unpaired electron is not (or to a very little extent)
delocalised at the benzimidazole ring. In contrast, the spec-
Figure 7. X-band EPR spectrum of (pMeO) at 298 K in CH2Cl2:DMSO
trum of (pMeO) displays hyperfine structure due to the inter-
C
C
(1:9). Modulation amplitude 0.05 mT, modulation frequency 100 KHz,
microwave frequency 9.42 GHz, power 10 mW. Solid line: experimental
spectrum; dotted line: simulation performed by using the hyperfine con-
stants indicated in the inset (in MHz). The values in parentheses are
computed values obtained from B3LYP calculations.
action of the electron spin with nuclei of the benzimidazole
moiety.[23] Therefore, the spin density is more delocalised
over the benzimidazole rings when this substituent is located
in para position of the phenoxyl group. The spectrum of
pMe
C
O) (Figure 7) could be simulated using the hyperfine
(
coupling constants AH =6.0 (for 2 H atoms), AH =2.4, AH =
2.9 and AN =5.5 MHz. Density functional calculations at the
B3LYP level of theory (see below) satisfactorily reproduce
this set of values and give further insight into the sign of the
hyperfine constants. The computed values are AH =6.0,
AH =6.4, AH =À3.0, AH =À4.8, AN =4.6 and AN =2.9 MHz.
Their assignment is as follows: The hydrogen hyperfine con-
stants higher than 6.0 MHz correspond to the two meta hy-
drogen atoms of the phenoxyl groups, and the others to H
atoms located on resonant positions of the benzimidazole
ring (Figure 7). Finally, the
form II; Figure 8). The energy calculations (including elec-
tronic and solvation terms) reveal that form II is more
C+
stable than form I by 33.1 kJmolÀ1. Therefore, (MeOH)
could be described as a phenoxyl radical hydrogen-bonded
H
C+[10]
to a benzimidazolium group, as for ( OH)
and other
phenoxyl–tertiary ammonium systems.[17] Examination of
the geometry optimised form II shows a quinonoid pattern
À
in the phenoxyl ring, that is, the Cortho Cmeta bonds are short
(1.381 and 1.392 ꢃ) with a strong double-bond character,
larger nitrogen hyperfine con-
stant corresponds to the iminic
nitrogen atom N1.[23]
Geometric and electronic struc-
tures of the radical species by
DFT methods: The geometric
structures of the oxidised radi-
cal species were investigated by
theoretical calculations at the
B3LYP level of theory (Fig-
ures 8 and 9). The atom num-
bering used in the text is shown
in Table 4. The nature of the
SOMO and the corresponding
unpaired spin-density distribu-
tion were also investigated for
all optimised structures. (Fig-
ures 10 and 11, Table 4).
H
C+
Radical cations ( OH)
and
Me
C+
(
OH) : Similarly to previous
C+[10]
H
works on ( OH)
we consid-
ered two forms for (MeOH) , in
which the proton is either locat-
ed on the phenoxyl oxygen
C+
C+
(i.e., OH ···N, form I) or on
+
+
Figure 8. Optimised structures and selected bond lengths [ꢃ] for a) (MeOH) form I, b) (MeOH) form II,
the N-benzimidazole imine ni-
C
C
+
Me
trogen atom, (i.e., O ··· HN, c) (MeO) form III, d) ( O) form IV.
C
C
C
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Chem. Eur. J. 2012, 18, 5416 – 5429

H-Bonded Phenoxyl Radicals
FULL PAPER
0.08) is characteristic of hydro-
gen-bonded phenoxyl radi-
cals.[26]
In contrast, in the least stable
form I, significant delocalised of
the SOMO on the N-methyl-
benzimidazole ring indicates
a non-negligible benzimidazolyl
radical character.
The ortho- and para- substituted
Me
C
neutral radicals
( O) and
pMe
C
(
O) : For the neutral radical
C
O) , we considered two
Me
(
forms, namely, III and IV,
which differ in the orientation
of the benzimidazole ring
(Figure 8). In form III the phe-
noxyl and benzimidazole rings
À
are coplanar and the N CH3
À
and C O bonds are trans to
one another, whereas in
form IV the phenoxyl and ben-
zimidazole rings are orthogonal
to one another. Form IV is
more stable by 32.0 kJmolÀ1.
This is consistent with the struc-
turally characterised parent
anion (MeO)À showing a signifi-
cant phenol/benzimidazole twist
(see Figure 3d). Thus, as for
(
MeO)À, the large energy differ-
ence between the two radical
forms of (MeO) (twisted and
C
planar) likely arises from steric
hindrance between the methyl
group and the adjacent phenox-
yl meta hydrogen atom. It is in-
structive to compare the distri-
bution of bond lengths within
the phenoxyl ring in forms III
and IV. In the twisted form IV,
Figure 9. Optimised structures and selected bond lengths [ꢃ] for a) (pMeO) form V, b) (pMeO) form VI),
C
C
H
H
H
H
C
C
C
C
c) ( O) form VII, d) ( O) form VIII, e) ( O) form IX, f) ( O) form X.
a
quinonoid distribution of
À
whereas the other C C bonds are long (1.409–1.488 ꢃ) with
bond lengths is observed with almost identical and short
À
À
À
significant single-bond character. The C O bond length
C2 C3 and C5 C6 bond lengths (1.384 and 1.381 ꢃ, respec-
À
À
À
À
(1.261 ꢃ) is significantly shorter than those of the neutral
and zwitterionic forms of MeOH (1.356 and 1.293 ꢃ, respec-
tively; see Supporting Information). These results are in
agreement with the few available X-ray structures for phe-
noxyl radicals.[25] The phenoxyl radical character of form II
tively) and long C1 C2, C3 C4, C4 C5 and C1 C6 bond
lengths (1.478, 1.421, 1.417 and 1.468 ꢃ respectively). In the
planar form III, the same overall pattern is observed, al-
À
though it is less symmetrical. For instance, the C2 C3 and
À
C5 C6 bond lengths differ by 0.025 ꢃ (1.378 and 1.403 ꢃ re-
of (MeOH) , as for form II ( OH) , is further confirmed
by the SOMO, which is mainly located on the phenoxyl ring.
Consequently most of the positive spin density resides on
the phenoxyl ring, and is distributed at the O, Cortho and Cpara
atoms (i.e., O 0.28, C2 and C4 0.18, C6 0.21), as expected
for phenoxyl radical species (Figure 10 and Table 4). The
relatively high spin density at the ipso carbon atom (C1
spectively), and the C3 C4 and C4 C5 bond lengths by
H
[10]
C+
C+
À
À
À
0.023 ꢃ (1.423 and 1.400 ꢃ respectively). The C O bond
length in form III is shorter than that in IV (1.253 and
À
1.261 ꢃ respectively). This dissymetrisation in the C C
bond lengths is a consequence of planarity and subsequent
resonance effects in form III. Indeed, the SOMO of form III
is significantly delocalised on the benzimidazole ring (87%
Chem. Eur. J. 2012, 18, 5416 – 5429
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5423

F. Thomas, L. Benisvy et al.
Table 4. Spin populations on selected atoms and numbering.
Compound
O
C1
C2
0.02
0.18
0.15
0.18
0.16
0.13
0.18
0.16
0.18
C4
C6
Me
(
OH) +, form I
0.15
0.28
0.30
0.32
0.31
0.30
0.32
0.30
0.32
0.30
0.10
0.18
0.08
0.06
0.08
0.06
0.06
0.08
0.06
0.08
0.06
0.12
0.18
0.21
0.20
0.22
0.21
0.20
0.22
0.20
0.23
0.20
0.06
0.15
0.18
0.15
0.18
0.16
0.17
0.18
0.16
0.18
0.17
0.09
C
C
Me
Me
Me
Me
A
OH) +, form II
C
C
C
A
O) , form III
O) , form V
O) , form IV·H2O
O) , form V
A
(
(
pMe
pMe
H
C
C
A
O) , form VI
C
C
C
( O) , form VII
H
( O) , form VIII
H
( O) , form IX
0.13
<0.02
H
C
( O) , form X
Figure 11. Composition of the SOMO of a) (pMeO) form V, b) (pMeO)
C
C
C
H
H
H
H
C
C
C
form VI), c) ( O) form VII, d) O) form VIII, e) ( O) form IX, f) ( O)
form X.
distribution of bond lengths is observed within the phenoxyl
ring in both cases. A remarkable symmetrical pattern is ob-
À
served along the C1 C4 axis, with the bond lengths in each
À
À
À
À
À
À
pair C1 C2/C1 C6, C2 C3/C5 C6 and C3 C4/C4 C5 being
quasi-equal for form V and equal for form VI (see Figure 9).
À
Interestingly, the C O bond length is barely affected by the
orientation of the N-methylbenzimidazole ring (1.260 ꢃ in
+
+
Figure 10. Composition of the SOMO of a) (MeOH) form I, b) (MeOH)
C
C
Me
form II, c) (MeO) form III, d) ( O) form IV.
C
C
form V versus 1.261 ꢃ in form VI). This contrasts with
Me
C
O) , in which a significant difference was observed be-
(
tween planar and orthogonal forms (1.253 and 1.261 ꢃ, re-
C
phenoxyl and 13% benzimidazole contributions), whereas
that of form IV is mainly (99%) located on the phenoxyl
ring (see Figure 10). Nevertheless, both forms preserve
a phenoxyl radical quinoid pattern with positive spin popu-
lations mainly distributed on the O, C1, C2, C4 and C6
atoms, but the individual atom contributions are slightly
lower for form III than for form IV (0.30/0.32, 0.06/0.08,
0.15/0.18, 0.20/0.22, 0.15/0.18 respectively; see Table 4).
Also, a non-negligible contribution (0.06) of the iminic ni-
trogen atom of the N-methylbenzimidazole moiety could be
evidenced in the planar structure (form III).
spectively). The composition of the SOMO of (pMeO) in its
most stable planar form V is 82% phenoxyl and 18% N-
methylbenzimidazole, whereas it is almost exclusively phe-
noxyl (99%) in the orthogonal form VI (Figure 11). The
SOMO of the planar form of (pMeO) is more delocalised
C
C
than that of the planar form of (MeO) (87% phenoxyl and
13% benzimidazole), that is, the contribution of the conju-
gated N-methylbenzimidazole moiety is enhanced when it is
located at the para rather than at the ortho position
(Figure 11). Consequently, lower spin populations are found
at the phenoxyl O, C1, C2, C4 and C6 atoms in form V
(0.27, 0.06, 0.13, 0.24, 0.13, respectively, with a 0.06 contribu-
tion of the iminic nitrogen atom of the N-methylbenzimida-
zole moiety) than in form VI (0.32, 0.08, 0.18, 0.22, 0.18 re-
spectively, without significant influence of the atoms of the
The optimised structures of (pMeO) in which the N-meth-
C
ylbenzimidazole moiety is coplanar with (form V) and or-
thogonal to (form VI) the phenoxyl ring are shown in
Figure 9. In contrast to (MeO) the lowest-energy form is
C
planar form V (more stable by 9.2 kJmolÀ1), which eviden-
ces stronger stabilisation by resonance effect. A quinonoid
N-methylbenzimidazole moiety; see Table 4). The increasing
Me
delocalisation of the SOMO in (pMeO) compared to ( O)
C
C
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Chem. Eur. J. 2012, 18, 5416 – 5429

H-Bonded Phenoxyl Radicals
FULL PAPER
correlates well with the experimental facts that (pMeO) is
zole ring. It is noteworthy that the spin populations in for-
ms VII and VIII are very close to those obtained for the cor-
C
formed at a lower potential than (MeO) and that the EPR
C
Me
spectrum of (pMeO) is more expanded than that of ( O) .
responding forms III and IV of (MeO) (Table 4), and this
C
C
C
suggests a negligible effect of the methyl substituent on the
benzimidazole properties. For the most stable planar hydro-
gen-bonded form IX, we observed minor changes compared
to planar form VII, the main difference being the slightly
lower spin population at the C2 atom.
H
H
C
C
Neutral radical ( O) : For the radical ( O) we considered
four forms VII–X (Figure 9). Forms VII and VIII are analo-
gous to forms III and IV of (MeO) , that is, the phenoxyl and
C
À
benzimidazole rings are either coplanar with the N H and
C O bonds being trans (form VII), or orthogonal (form
VIII) to one another. In form IX the phenoxyl and benz-
imidazole rings are coplanar with the N H and C O bonds
being cis to one another, thus allowing the formation of an
O ···HN hydrogen bond. Form X represents the extreme sit-
uation in which the proton is located at the phenol group
(i.e., OH ··· N), as if it derived directly from one-electron
À
Form X is the highest in energy and represents an ex-
treme case in which a very acidic phenoxyl radical cation is
at hydrogen-bonding distance from a basic imidazolate
group. This is not a chemically realistic situation, and one
would expect a spontaneous intramolecular proton transfer
to give form IX. Thus, form X could be regarded as “transi-
À
À
C
tion state” upon oxidation of (HO)À (in OH···ÀN form) to
À
C+
H
removal from the parent anion (HO)À in its crystallographi-
cally characterised OH···ÀN form. The optimised forms VII,
VIII and X were found to be of much higher energy (by
31.0, 34.7 and 41.7 kJmolÀ1, respectively) than optimised
form IX. The stabilisation of form IX may be explained by
the interplay of several factors, including the lack of steric
hindrance with the phenoxyl meta hydrogen atom, the exis-
tence of resonance stabilisation and the formation of a hy-
drogen bond between the phenoxyl oxygen and the N-H
( O) in form IX (as O ···HN). The existence of form X may
result from stabilisation of this “transition state” by signifi-
cant delocalisation of the unpaired electron. This indicates
that electronics play a crucial role in the location of the
proton in the compound. This also poses the question of the
possibility of an unprecedented PET mechanism for oxida-
tion of a phenol–benzimidazole pair in the deprotonated
form (see below). The structure of form X differs notably
C
C
H
À
C
from the other forms of ( O) . The C O bond is much
À
À
group. As expected, the variation of C C bond lengths and
longer (1.335 ꢃ), and the phenoxyl C C bonds do not show
a quinonoid pattern (see Figure 9 f). Therefore ( O) in
H
H
À
C
C
the computed C O bond lengths in ( O) in forms VII and
VIII are quite similar to that obtained for forms III and IV
form X exhibits a much smaller phenoxyl radical character.
This is further confirmed by the SOMO analysis, which
shows a quasi-equal contributions of the phenoxyl (48%)
and benzimidazolyl (52%) rings. Accordingly, the spin pop-
ulations on the O, C1, C2, C4 and C6 atoms (0.10, 0.12,
<0.02, 0.06 and 0.09, respectively) are much smaller than
those of the other forms (Figure 11f and Table 4).
of (MeO) and thus will not be commented further. The qui-
C
nonoid structure of planar hydrogen-bonded form IX resem-
bles that of planar form VII, with similar C2 C3, C3 C4,
C4 C5 and C5 C6 bond lengths. However, noticeable dif-
ferences arising from hydrogen-bonding could be evidenced
by careful examination of the C1 C2, C1 C6 and C O
bond lengths: The first two bonds are shortened by 0.01 ꢃ,
whereas the last-named is significantly lengthened
(+0.015 ꢃ) in form IX. Interestingly, the O···H N distance
of 1.926 ꢃ is significantly larger than those obtained for
compounds in which the hydrogen bond is established be-
À
À
À
À
À
À
À
High-field EPR: In the absence of hyperfine couplings, X-
band EPR spectroscopy is inefficient in probing the spin dis-
tribution of the phenoxyl radical and the electrostatic envi-
ronment around the phenoxyl O atom. It is known that the
anisotropy of the g tensor in organic radicals is too small to
be detected at 9.4 GHz, but it can be resolved at higher fre-
À
tween a phenoxyl oxygen atom and a cationic benzimidazo-
H
C+
lium (or N-methylbenzimidazolium) group, like ( OH)
and (MeOH) (1.815 ꢃ), that is, the hydrogen-bonding inter-
quencies.[3] Thus, we performed EPR measurements at
C+
H
C+
C
action is stronger when the donor is a cationic partner.
The composition of the SOMO of ( O) is affected by the
285 GHz for frozen CH2Cl2 solutions of (MeOH) , ( O) and
H
Me
C
C
( O) at 10 K (Table 5). The EPR spectra are shown in
relative orientation of the phenoxyl ring with respect to the
Figure 12. For all radical compounds, a rhombic (S=1= )
2
benzimidazole ring in a similar way to (MeO) : For an or-
signal is observed, from which the three principal g values
g1, g2 and g3 (see Figure 12 inset) are resolved and can be ex-
tracted by simulation (see Table 5). Regardless of the com-
pounds investigated, the obtained g2 and g3 values of about
2.0044 and 2.0023, respectively, are invariable, and are iden-
C
thogonal orientation of the rings (form VIII) the contribu-
tions of the phenoxyl and benzimidazole moieties are 99
and 1% respectively, that is, similar to (MeO) . When the
C
phenoxyl and benzimidazole rings are coplanar a significant
contribution of the benzimidazole ring to the SOMO could
be evidenced: The SOMO has 88% phenoxyl and 12%
benzimidazole composition when the N H and C O bonds
are trans to one another (form VII), and 85% phenoxyl and
H
[10]
C+
tical to those reported for ( OH) , the tri-tert-butylphe-
noxyl radical[15] and phenoxyl radical hydrogen-bonded to
a neighbouring ammonium group.[17] In contrast, the g1
value varies significantly (Table 5). This behaviour is consis-
À
À
À
À
À
15% benzimidazole ring when the N H and C O bonds are
cis and hydrogen-bonded to one another (form IX). There-
fore, the intramolecular hydrogen bond in form IX appears
to increase delocalisation of the SOMO over the benzimida-
tent with the g1 tensor (oriented along the C O axis,
Figure 12) being sensitive to the local environment around
the oxygen atom, that is, any changes in the spin density at
the oxygen atom, as well as in the energy gap between the
Chem. Eur. J. 2012, 18, 5416 – 5429
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5425

F. Thomas, L. Benisvy et al.
Table 5. Experimental[a] and calculated[b] g tensors of the radical com-
pounds.
are within the range of those reported for crystals of strong-
ly hydrogen-bonded g-irradiated tyrosine·HCl (2.0067),[27]
the transient TyrD radical (2.0066, Figure 1)[2] and other phe-
Compound
g1
g2
g3
C
H
( OH) +, exptl
2.0067
2.0069
2.0041
2.0065
2.0079
2.0083
2.0079
2.0071
2.0073
2.0071
2.0080
2.0072
2.0084
2.0080
2.0069
2.0037
2.0043
2.0044
2.0038
2.0043
2.0044
2.0043
2.0045
2.0044
2.0043
2.0046
2.0046
2.0044
2.0044
2.0045
2.0045
2.0026
2.0022
2.0023
2.0000
2.0023
2.0023
2.0026
2.0023
2.0023
2.0023
2.0024
2.0024
2.0023
2.0023
2.0023
2.0022
2.0023
noxyl radicals hydrogen-bonded to iminium nitrogen
atoms.[9] Importantly, they are higher than those reported
for strongly intramolecularly hydrogen-bonded phenoxyl–
tertiary ammonium systems in which the positive charge is
mainly located on the nitrogen atom (2.0061–2.0066).[17] De-
localisation of the positive charge over the benzimidazolium
ring thus lowers the charge density on the nitrogen atom
C
Me
Me
Me
Me
Me
Me
Me
OH) +, exptl
C
N
OH) +, form I calcd
C
N
OH) +, form II calcd
C
N
C
N
O) , exptl
O) , form III calcd
O) , form IV calcd
O) , form IV·H2O calcd
C
N
C
N
[c]
C
N
pMe
pMe
pMe
H
C
A
O) , exptl
and makes the oxygen environment less electropositive in
C
O) , form V calcd
A
H
C+
C+
( OH) and (MeOH) than in phenoxyl–tertiary ammonium
C
(
O) , form VI calcd
( O) , exptl
systems. The calculated g1 values of (MeOH) for forms I
C
C
C
C
C+
H
( O) , form VII calcd
(proton on the phenoxyl oxygen atom) and II (proton on
the N-methylbenzimidazole moiety) are quite different (g1 =
2.0041 and 2.0065 respectively; Table 5). Clearly, only the
latter set of values reproduces satisfactory the experimental
H
( O) , form VIII calcd
H
( O) , form IX calcd
H
C
( O) , form X calcd
[a] In CH2Cl2 +0.1m TBAP; T=10 K. The confidence level on the experi-
mental g values is 0.0001. [b] EPR parameters were obtained from addi-
tional single-point calculations with the hybrid functional B3LYP and the
EPR-II basis set (see Experimental Section). [c] At the hydrogen-bond
distance of 1.67 ꢃ deduced from the X-ray crystal structure of [MeO]-
C+
data, and further supports the assumption that (MeOH) is
a phenoxyl radical hydrogen-bonded to a cationic partner.
The highest g1 value (2.0079) is obtained for (MeO) , that
C
is, the compound for which the phenoxyl oxygen atom
ACHTUNGTRENNUNG[nBu4N]·2.4H2O.
cannot establish any intramolecular hydrogen bond. The
computed g1 value for (MeO) in forms III and IV (in which
C
the N-methylbenzimidazole and phenoxyl rings are coplanar
and orthogonal, respectively) are 2.0083 and 2.0079, respec-
tively (Table 5). The inclusion of a water molecule hydro-
gen-bonded to the phenoxyl oxygen atom (form IV·H2O) re-
sults in a dramatic decrease of the g1-tensor component
(2.0071) which does not account for experimental data. Al-
though the difference in anisotropy is small between for-
ms III and IV, the experimental data are in better agreement
with form IV. This indicates that the phenoxyl and N-meth-
ylbenzimidazole rings are orthogonal in (MeO) , in agreement
C
with the energy calculations, and the fact that the oxygen
atom establishes no or a very weak hydrogen bond with
Figure 12. High-field (285 GHz) EPR spectra of electrochemically gener-
a water molecule.
+
H
Me
ated a) (MeOH) , b) ( O) , c) ( O) and d) (pMeO) recorded as 5 mm
CH2Cl2 solutions at 5 K. The star denotes an unidentified paramagnetic
impurity.
C
C
C
C
H
C
The g1 value of 2.0072 measured for ( O) is intermediate
between those of (MeO) and ( OH) . The electrostatic en-
Me
C
C+
H
C
vironment around the oxygen atom in ( O) is therefore less
electropositive than in (MeOH) (or ( OH) ) but more
H
C+
C+
C
oxygen pz (contributing to the SOMO) and py (lone pair) or-
bitals will affect the g1 value.[3,10] In the present case, forma-
tion of an in-plane hydrogen bond stabilises the py lone pair
of the oxygen atom, thereby increasing the energy gap be-
tween this orbital and the SOMO. The net result is a de-
crease in the spin–orbit coupling due to hydrogen-bonding
and thus a lowering of the g1 value.
Thus, in this series of compounds, only the g1 value is in-
formative for probing local environmental changes on the
phenoxyl radical. Consistently, the experimentally deter-
mined g1 value varies as such: 2.0079>2.0073>2.0072>
electropositive than in (MeO) . This fact is consistent with the
presence of an intramolecular hydrogen bond between the
phenoxyl radical oxygen and a neutral partner (benzimid-
H
C
azole group) in ( O) . This simple analysis was confirmed by
H
C
theoretical calculations: The computed g1 values for ( O) in
forms VII, VIII and IX (in which the N-methylbenzimid-
azole and phenoxyl rings are coplanar without a hydrogen
bond and orthogonal or coplanar with a hydrogen bond, re-
spectively) are 2.0084, 2.0080 and 2.0069, respectively
(Table 5). Clearly, hydrogen-bonding interactions between
the phenoxyl oxygen atom and the N-methylbenzimidazole
NH group in form IX decrease significantly the g1 compo-
nent. The experimental g1 value is far from the calculated
pMe
H
Me
2.0069>2.0067[10] for (MeO) , ( O) , ( O) , ( OH) and
C
C
C
C+
H
C+
( OH) respectively (see Table 5). The lowest g1 values,
2.0067[10] and 2.0069, are obtained for ( OH)
and
value for forms VII and VIII but matches well with that of
H
C+
Me
H
C+
C
(
OH) , respectively. This evidences a hydrogen bond with
form IX. These results are thus consistent with ( O) being
a cationic partner which provides a very electropositive en-
vironment around the phenoxyl oxygen atom. These values
a neutral phenoxyl radical species intramolecularly hydro-
gen-bonded to the benzimidazole NH moiety. Calculations
5426
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Chem. Eur. J. 2012, 18, 5416 – 5429

H-Bonded Phenoxyl Radicals
FULL PAPER
on form X, in which the proton is located on the phenol
group in place of the N-benzimidazole moiety, give a g1
value of 2.0037, which clearly excludes it as experimentally
relevant.
benzimidazole species by X-ray diffraction. Its oxidation po-
tential is much lower than that of the phenol parent as
a result of increased electron density at the oxygen atom
(negative charge) and differences in the oxidation mecha-
The g1 value obtained for (pMeO) of 2.0073, is lower than
nism (simple ET in this case). Electrochemically generated
C
that of (MeO) (2.0079) (Table 5). The lower g1 value in
Me
C
C
( O) exhibits a high g1 value, in accordance with the ab-
pMe
pMe
C
C
(
O) is consistent with ( O) being more delocalised
sence of hydrogen-bonding and a preferred twisted structure
which prevents delocalisation of the radical onto the N-
methybenzimidazole ring. In contrast, the analogous para-
than (MeO) , since a decrease of the spin density at the O
atom decreases this g component. The calculated g1 values
of 2.0071 and 2.0080 for the planar form V and the orthogo-
C
substituted radical (pMeO) appears to be planar and more
C
nal form VI of (pMeO) , respectively, confirm this fact. Thus,
conjugated than (MeO) , as evidenced by the experimental
C
C
both the experimental and calculated results suggest that
electrochemical and EPR data as well as DFT calculations.
In contrast to (MeO)À, X-ray diffraction analysis reveals
that (HO)À in the solid state consists of an unprecedented
phenol–benzimidazolate and not a phenolate–benzimidazole
compound. The energy gap between these two tautomeric
forms is small (5.0 kJmolÀ1) and suggests that they are most
the para-substituted radical (pMeO) is planar, whereas the
C
ortho-substituted radical (MeO) is orthogonal.
C
Combined HF-EPR data and DFT calculations have thus
+
H
provided strong evidence that (MeOH) and ( O) are phe-
noxyl radical species in which the oxygen atom establishes
an intramolecular hydrogen bond with either a cationic part-
C
C
likely in equilibrium in solution. Remarkably, the one-elec-
H
C
ner or a neutral partner, whereas no such interaction exists
tron oxidised ( O) could be unambiguously identified as
pMe
in (MeO) and ( O) .
a phenoxyl–benzimidazole radical species. Its g1 value is in-
C
C
C+
C
termediate between those obtained for (MeOH) and (MeO) ,
and thus evidences an intermediate electrostatic environ-
ment around the phenoxyl oxygen atom, that is, the pres-
ence of a hydrogen bond with a neutral partner. Considering
that (HO)À may exist in solution in both phenol–benzimid-
azolate and phenolate–benzimidazole forms,[28] either
a simple ET or an alternative unprecedented PET mecha-
nism may be considered for its one-electron oxidation
[Eq. (4)], as in the case of the tyrosinate–histidine pair of
PS II suggested by Faller et al. (Figure 1).[19]
Conclusion
We have shown that MeOH contains a phenolic proton which
is intramolecularly hydrogen-bonded to a N-methylbenz-
imidazole group both in solution and at the solid state, simi-
larly to HOH. Its one-electron oxidation occurs by an intra-
molecular CPET mechanism similar to that proposed for
C
the generation of TyrD in PSII and affords the phenoxyl N-
C+
methylbenzimidazolium radical species (MeOH) . HF-EPR
spectroscopy, which allows for resolution of the g-tensor an-
isotropy, reveals a small g1 value (2.0069) consistent with the
existence of an intramolecular hydrogen bond between the
phenoxyl oxygen atom and the protonated iminic nitrogen
atom of the N-methylbenzimidazole moiety, as confirmed by
complementary DFT calculations.
pMeOH, which differs from MeOH in the position of the N-
methylbenzimidazole substituent (para instead of ortho),
lacks the possibility to establish an intramolecular hydrogen
bond between the phenolic proton and the benzimidazole
group. Its oxidation occurs in an unprecedented PET mech-
anism leading to an equimolar mixture of the N-methyl-
benzimidazole–phenoxyl radical (pMeO) and N-methylbenz-
C
imidazolium–phenol (pMeOH2)+ conjugates. Thus, during oxi-
dation, intermolecular proton transfer occurs to the N-meth-
ylbenzimidazole base, leaving half of the phenol compound
(in its acidic (pMeOH2)+ form) unoxidised. These results
show the importance of the spatial separation between the
pro-phenoxyl oxygen atom and the acceptor nitrogen atom
in determining the mechanism of the oxidation process, as
well as the nature of the oxidation products formed. This il-
lustrates the need for a short distance between TyrD and the
His to improve radical formation in biological systems
(Figure 1).
In summary we have shown that phenoxyl radical proper-
ties (oxidation potential, geometric and electronic struc-
tures) as well as oxidation mechanism are subtly modulated
by the distance/charge of the hydrogen-bonded benzimid-
azole moiety and thus indirectly by the strength of this hy-
drogen bond. Furthermore, the correlation observed be-
tween the g tensor and the radical structure is of particular
importance in unravelling biological processes involving ty-
rosyl radicals by HF-EPR, as exemplified by the works of
Faller et al. on PSII.
Addition of one equivalent of nBu4NOH to MeOH affords
(
MeO)À, which has been identified as a phenolate–N-methyl-
Chem. Eur. J. 2012, 18, 5416 – 5429
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5427

F. Thomas, L. Benisvy et al.
at room temperature under N2 atmosphere. The solvent was then re-
moved under vacuum to give an off-white crystalline powder. Colourless
block single crystals suitable for X-ray crystallography were obtained by
slow evaporation of an Et2O solution. Yield: 83%. 1H NMR (300 MHz,
CDCl3): d/ppm=8.55 (d, 1H, PhOH), 7.58 (m, 2H, Bzim), 7.19 (d, 1H,
PhOH),6.94 (dd, 2H, Bzim), 2.88 (t, 8H, NBu4), 1.48 (s, 9H, tBu), 1.34
(s, 9H, tBu), 1.27 (m, 16H, NBu4), 0.91 (t, 12H, NBu4); MS (ESI(À)): m/
z 423 [M]À; elemental analysis (%) calcd for C43H63N5O·H2O: C 75.50, H
9.58, N 10.24, O 4.68; found: C 75.21, H 9.37, N 10.19, O 4.04.
Experimental Section
General: X-Band EPR spectra were recorded on a Bruker ESP 300E
spectrometer at 293 K on 50 mL samples. Spectra were simulated with the
SIMFONIA software (Bruker). High-field EPR spectra were recorded at
285 GHz by using
a home made spectrometer (LNCMI, Grenoble,
France). NMR spectra were recorded on a Bruker AM 300 (1H at
300 MHz, 13C at 75 MHz). Chemical shifts are given relative to tetrame-
thylsilane (TMS). Mass spectra were recorded on a Thermofinnigan (EI/
DCI) apparatus. Microanalysis was performed by the Service Central
d’Analyse du CNRS (Lyon, France). UV/Vis spectra at 298 K were re-
corded on a Perkin-Elmer Lambda 2 spectrophotometer equipped with
a temperature controller unit set at 298 K. The quartz cell path length
was 1.000 cm. UV/Vis spectra 238 K were recorded on a Cary 50 spectro-
photometer equipped with a Hellma low-temperature immersion probe
(1.000 cm path length quartz cell). The temperature was controlled with
a Lauda RK8 KS cryostat.
N-Methyl-2-(2’-hydroxy-3’,5’-di-tert-butylphenyl)benzimidazole
(
MeOH):
To 3,5-di-tert-butylsalicylaldehyde (411 mg, 1.76 mmol) was added 6 mL
of 40% NaHSO3(aq). The mixture was stirred for 5 h at 258C. N-Methyl-
1,2-phenylenediamine (215 mg, 1.76 mmol) and ethanol (5 mL) were
added. The mixture was heated to reflux for 24 h and the reaction mix-
ture was then poured into 150 mL of water. The pale yellow precipitate
was filtered off and recrystallised from CH2Cl2. Yield : 83%. Single crys-
tals were obtained by slow evaporation of a toluene solution. 1H NMR
(300 MHz, CDCl3): d/ppm=12.48 (s, 1H, OH), 7.75 (m, 1H, Bzim), 7.50
(d, 1H, PhOH), 7.44 (d, 1H, PhOH),7.42 (m, 1H, Bzim), 7.34 (m, 2H,
Bzim), 4.03 (s, 3H, CH3), 1.49 (s, 9H, tBu), 1.37 (s, 9H, tBu); MS (DCI,
NH3/isobutane): m/z 337 [M+H]+; elemental analysis (%) calcd for
C22H28N2O: C 78.53, H 8.39, N 8.33; found: C 78.32, H 8.27, N 8.27.
Electrochemistry: Cyclic voltammetry curves were recorded on a CHI
660 potentiostat in a standard three-electrode cell under argon atmos-
phere. An Ag/AgNO3 (0.01m) reference electrode was used. All the po-
tentials given in the text are referred to the regular Fc/Fc+ redox couple
used as external reference. A vitreous carbon disc electrode (5 mm diam-
eter) polished with 1 mm diamond paste was used as working electrode.
Electrolysis was performed on a PAR 273 potentiostat under argon at-
mosphere at À408C with a carbon-felt working electrode.
2,4-Di-tert-butyl-6-(1-methyl-1H-benzo[d]imidazol-2-yl)phenolate tetra-
ACTHNUTRGNEUNG
butylammonium salt ((MeO)[nBu4N]): MeOH (50 mg, 0.15 mmol) was dis-
solved in Et2O (6 cm3), after which 1 equiv of NBu4OH (155 mL of a 1m
solution in MeOH) was added, and the reaction mixture was left stirring
for 1 h at room temperature under N2 atmosphere. The solvent was then
Crystal structure analysis: For all structures, collected reflections were
corrected for Lorentz and polarisation effects but not for absorption. The
structures were solved by direct methods and refined with TEXSAN
removed under vacuum yielding
a brown oily material, which was
washed several times with pentane to give light yellow-green solid. Yield:
86%. Yellowish block single crystals suitable for X-ray crystallography
were obtained by slow evaporation of a pentane solution. 1H NMR
(200 MHz, CDCl3): d/ppm=7.75 (m, 1H, Bzim), 7.49 (s, 1H, PhOH),
7.44 (s, 1H, PhOH), 7.40 (s, 1H, Bzim), 7.35 (m, 2H, Bzim), 4.02 (s, 3H,
CH3), 3.38 (t, 8H, NBu4), 1.64 (m, 8H, NBu4), 1.49 (s, 9H, tBu), 1.36 (s,
9H, tBu), 1.39 (m, 8H, Nbu4), 0.97 (t, 12H, NBu4); MS (ESI(À)): m/z
423 [M]À; elemental analysis (%) calcd for C43H63N5O·H2O: C 75.50, H
9.58, N 10.24, O 4.68; found: C 75.21, H 9.37, N 10.19, O 4.04%.
(HOH, MeOH, (HO)
(
A
( ACHTUNTGNRUEGN
MeO)[nBu4N])[28] and OLEX2 software
mal parameters. Hydrogen atoms were generated in idealised positions,
riding on the carrier atoms, with isotropic thermal parameters except the
hydroxyl ones, which were localised on the Fourier map and fixed.
CCDC-236867 (HOH), CCDC-236868
(
MeOH), CCDC-824137 ((HO)-
ACHTUNGTRENNUNG
[nBu4N]) and CCDC-832162 (pMeOH)
N
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallograph-
ic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
N-Methyl-2-(4’-hydroxy-3’,5’-di-tert-butylphenyl)benzimidazole
(
pMeOH):
HLpMe was prepared in a similar way to HLMe from 3,5-di-tert-butyl-4-hy-
droxybenzaldehyde and N-methyl-1,2-phenylenediamine. Single crystals
were grown by slow evaporation of a concentrated CH2Cl2:toluene (1:5)
solution. Yield: 91%.1H NMR (300 MHz, DMSO): d 1.46 (18H, s), 3.87
(3H, s), 7.28–7.18 (2H, m), 7.57–7.54 (3H, m), 7.70 (1H, d); MS (DCI,
NH3/isobutane): m/z 337 [M+H]+; elemental analysis (%) calcd for
C22H28N2O: C 78.53, H 8.39, N 8.33; found: C 78.42, H 8.22, N 8.19.
Computational details: Theoretical calculations were based on DFT and
were performed with the ORCA program package.[29] Geometry optimi-
sations were carried out by using the GGA functional B3LYP[30–31] in
combination with the TZV/P[32] basis set for all atoms and by taking ad-
vantage of the resolution of the identity (RI) approximation in the Split-
RI-J variant[33] with the appropriate Coulomb fitting sets. Increased inte-
gration grids (Grid4 in ORCA convention) and tight SCF convergence
criteria were used. For both geometry optimisation and molecular-prop-
erty calculations, solvent effects were accounted for according to the ex-
perimental conditions. For that purpose, we used CH2Cl2 (e=9.08) as sol-
vent within the framework of the conductor-like screening (COSMO) di-
electric continuum approach.[34] EPR parameters were obtained from ad-
ditional single-point calculations with the hybrid functional B3LYP and
the EPR-II[35] basis set. The g tensor was calculated as a second deriva-
tive property of the energy with respect to the external magnetic field
and the electron magnetic moment. For that purpose, the coupled-per-
turbed Kohn–Sham equations were employed in conjunction with a para-
metrised one-electron spin–orbit operator.[36–38] Hyperfine coupling con-
stants were calculated directly from Fermi contact terms and dipolar con-
tributions as the expectation values of the appropriate operator over the
spin density. The spin–orbit contribution (SOC) of the hyperfine interac-
tion was also calculated and its isotropic part was added to the Fermi
contact term whereas its anisotropic part was added to the dipolar contri-
bution.[39]
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ACHTUNGTRENNUNG
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in MeOH) was added, and the reaction mixture was left stirring for 1 h
5428
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ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 5416 – 5429

H-Bonded Phenoxyl Radicals
FULL PAPER
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Received: September 13, 2011
Published online: March 13, 2012
Chem. Eur. J. 2012, 18, 5416 – 5429
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