Galactose oxidase models: Creation and modification of proton transfer coupled to copper(II) coordination processes in pro-phenoxyl ligands

Article abstract of DOI:10.1002/ejic.200600286

Two tripodal ligands containing two pyridyl and one 4-(benzimidazol-2-yl)- 2-tert-butylphenol (HL^H) and one 2-tert-butyl-4-(N- methylbenzimidazol-2-yl)phenol (HL^Me) unit have been synthesized. They possess a N₃O donor set that is known to stabilize phenoxyl radicals more efficiently than the corresponding N₂O₂ donor unit. Reaction of one molar equiv. of Cu(ClO₄) ₂·6H₂O with HL^H or HL^Me affords the zwitterionic benzimidazolium phenolate complexes [Cu ^IIHL^H)]²⁺ and [Cu^II(HL ^Me)]²⁺ via a proton transfer reaction coupled to copper(II) coordination. Addition of HClO₄ to [Cu ^II(HL^H)]²⁺ and [Cu^II(HL ^Me)]²⁺ results in the formation of the benzimidazolium phenol complexes [Cu^II(H₂L^H)]³⁺ and [Cu^II(H₂L^Me)]²⁺, while addition of NEt₃ affords the benzimidazol-phenolate complexes [Cu ^II(L^H)]²⁺ and [Cu^II(L ^Me)]²⁺, respectively. The phenol's pK_a is remarkably low due to the strong withdrawing effect of the benzimidazolium substituent. X-ray crystallographic analysis of the copper(II) complexes shows that deprotonation of the axial phenol forces the metal to move out of the square plane towards the oxygen atom, and one (or two) Cu-N_pyridine equatorial bond length increases. The copper(II) phenoxyl species [Cu ^II(HL^H)]^.3+ and [Cu^II(HL ^Me)]^.3+ were prepared electrochemically, or by addition of two molar equiv. of copper(II) to HL^H or HL^Me. Under these conditions, radical formation has never been observed for tripodal ligands containing two pyridyl and one 2,4-di-tert-butylphenol group. This difference is explained in terms of the proton transfer mechanism and redox potentials. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600286

FULL PAPER
DOI: 10.1002/ejic.200600286
Galactose Oxidase Models: Creation and Modification of Proton Transfer
Coupled to Copper(II) Coordination Processes in Pro-Phenoxyl Ligands
Fabien Michel,[a] Fabrice Thomas,*[a] Sylvain Hamman,[a] Christian Philouze,[a]
Eric Saint-Aman,[b] and Jean-Louis Pierre[a]
Keywords: Copper / Phenoxyl / Tripodal ligands / Proton transfer / Radicals
Two tripodal ligands containing two pyridyl and one 4-
drawing effect of the benzimidazolium substituent. X-ray
crystallographic analysis of the copper(II) complexes shows
that deprotonation of the axial phenol forces the metal to
move out of the square plane towards the oxygen atom, and
one (or two) Cu–Npyridine equatorial bond length increases.
The copper(II) phenoxyl species [CuII(HLH)]·3+ and
[CuII(HLMe)]·3+ were prepared electrochemically, or by ad-
dition of two molar equiv. of copper(II) to HLH or HLMe. Un-
der these conditions, radical formation has never been ob-
served for tripodal ligands containing two pyridyl and one
2,4-di-tert-butylphenol group. This difference is explained in
terms of the proton transfer mechanism and redox potentials.
(benzimidazol-2-yl)-2-tert-butylphenol (HLH) and one 2-tert-
butyl-4-(N-methylbenzimidazol-2-yl)phenol (HLMe
)
unit
have been synthesized. They possess a N3O donor set that is
known to stabilize phenoxyl radicals more efficiently than
the corresponding N2O2 donor unit. Reaction of one molar
equiv. of Cu(ClO4)2·6H2O with HLH or HLMe affords the zwit-
terionic benzimidazolium phenolate complexes [CuII(HLH)]2+
and [CuII(HLMe)]2+ via a proton transfer reaction coupled to
copper(II) coordination. Addition of HClO4 to [CuII(HLH)]2+
and [CuII(HLMe)]2+ results in the formation of the benzimid-
azolium phenol complexes [CuII(H2LH)]3+ and [CuII(H2LMe)]2+
,
while addition of NEt3 affords the benzimidazol-phenolate
complexes [CuII(LH)]2+ and [CuII(LMe)]2+, respectively. The
phenol’s pKa is remarkably low due to the strong with-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
and sometimes are able to model GO’s reactivity towards
alcohol oxidation.[17,25,27,28,35,38,41,45] It has also been shown
that tripodal ligands possessing a N3O donor set stabilize
more efficiently copper(II) coordinated phenoxyl radicals
than the corresponding N2O2 set.
Introduction
Galactose Oxidase (GO) is a copper(II) enzyme that ca-
talyses the oxidation of primary alcohols to the correspond-
ing aldehydes, with concomitant reduction of dioxygen to
hydrogen peroxide.[1–11] This two electron chemistry is pro-
moted by a single copper atom, working in synergy with a
tyrosyl radical from the protein. Galactose Oxidase pos-
We have recently developed a biomimetic approach to
radical cofactor formation. In this context, we have studied
the solution chemistry of several tripodal ligands,[49] involv-
ing one pro-phenoxyl group (mimicking the amino acids
of the GO active site), under various copper(II), base, and
dioxygen reagent conditions.[50] We have shown that CuII
phenoxyl radical species could be obtained by adding two
equiv. of copper(II) to ligands possessing a N3O coordina-
tion moiety, such as HLtBu (Scheme 1), in the presence of
one equiv. of triethylamine (O2 is not involved in the reac-
tion). In the absence of base, only a small amount of radical
species is formed. In order to bypass this base requirement
for radical formation, and thus to obtain more biologically
relevant systems, we have synthesized the ligands HLH and
HLMe (Scheme 1). Their structures are also tripodal, with
two pyridyl and one phenol moiety joined together by a
pivotal amino nitrogen atom. The 2-tert-butylphenol unit is
substituted at its 4-position (with respect to the coordinat-
ing oxygen atom) by a benzimidazole (HLH) or N-methyl-
benzimidazole (HLMe) group in order to control the proton
transfer processes, as do some non-coordinating histidine
·
sesses a N2O2 donor set: One oxygen atom, from the Tyr272
radical residue, is the organic redox center, while the other
(from Tyr495) controls the proton transfer process. The
mechanism by which the radical is generated is of major
interest. It has been shown recently that mixing apo-GO
with copper (I or II) in the presence of O2 affords the CuII
radical enzyme.[12–15] During the last few years, several
model compounds for the GO active site have been devel-
oped.[16–48] Tripodal ligands have been widely used, and it
has been shown that they are good structural models, which
reproduce quite well the spectroscopic signatures of GO,
[a] Laboratoire de Chimie Biomimétique, LEDSS, UMR CNRS
5616, ICMG FR CNRS 2607, Université J. Fourier,
B. P. 53, 38041 Grenoble cedex 9, France
Fax: +33-4-7651-4836
E-mail: Fabrice.Thomas@ujf-grenoble.fr
[b] Pr. E. Saint-Aman, LEOPR, UMR CNRS 5630, ICMG FR
CNRS 2607, Université J. Fourier,
B. P. 53, 38041 Grenoble Cedex 9, France
3684
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Proton Transfer, and Copper(II) Coordination Processes in Pro-Phenoxyl Ligands
FULL PAPER
residues in enzymes. This substituent is also known to stabi- paper, and compared to that of an already published system
lize phenoxyl radicals.[51–52] Another aspect highlighting the based on the ligand HLtBu (Scheme 1).[50]
originality of HLH and HLMe is their ability to manage the
two proton/two electron shuttle process (like GO), unlike
Results and Discussion
other copper(II) complexes containing N3O ligands. The
phenoxyl oxygen atom and the copper(II) ion may control
the two electron transfer, while the benzimidazolium/benz-
imidazole and phenol/phenoxyl acidobasic couples may be
involved in the transfer of two protons. Their chemistry is
therefore close to that of copper complexes of ligands pos-
sessing a N2O2 donor set, while their N3O donor set is ex-
pected to enhance the stability of the phenoxyl radicals.
Synthesis of the Ligands
The ligands HLH and HLMe were obtained by using a
Mannich reaction: In a one-pot synthesis, HLH is obtained
by mixing bis(2-pyridylmethyl)amine and 4-(1H-benzoimid-
azol-2-yl)-2-tert-butylphenol in the presence of one equiv.
of formaldehyde. Ligand HLMe is obtained by mixing bis(2-
pyridylmethyl)amine and 2-tert-butyl-4-(1-methyl-1H-
benzoimidazol-2-yl)phenol in the presence of one equiv. of
formaldehyde.
The phenol benzimidazole (and not phenolate benzimid-
azolium) protonation state was confirmed by 1H NMR
([D6]DMSO) due to the hydroxy proton resonance (δ =
11.73 ppm and 11.76 ppm for HLH and HLMe respectively).
Its structural attributes were confirmed by GHMBC and
QHMQC sequences, showing 3J correlations between the
C2 carbon atom (δ = 136.9 ppm and 136.8 ppm for HLH
and HLMe respectively), the tert-butyl protons (δ =
1.50 ppm and 1.49 ppm for HLH and HLMe respectively)
and the hydroxy proton (see ESI).
Structures of the Copper(II) Complexes
The copper(II) phenolate complexes [CuII(HLH)]2+ and
[CuII(HLMe)]2+ were obtained by mixing stoichiometric
amounts of the ligands and Cu(ClO4)2·6H2O in acetonitrile.
Dark blue single crystals were obtained on slow diffusion
of di-iso-propyl ether into acetonitrile solutions of the com-
Scheme 1. Formulae for the ligands.
The coordination and solution chemistry of the cop- plexes (Table 1). The ORTEP views of the phenolate com-
per(II) complexes of HLH and HLMe are described in this plexes [CuII(HLH)]2+ and [CuII(HLMe)]2+ are depicted in
Table 1. Crystallographic data for [CuII(H2LH)]3+, [CuII(HLH)]2+, [CuII(H2LMe)]3+ and [CuII(HLMe)]2+
.
[CuII(H2LH)]3+
[CuII(HLH)]2+
[CuII(H2LMe)]3+
[CuII(HLMe)]2+
Formula
M
C34H38Cl3CuN7O13
922.62
C36H40Cl2CuN8O9
863.21
C37H43Cl3CuN8O15.33
1015.03
C35H39Cl2CuN7O9.66
846.80
Crystal system
Space group
monoclinic
C2/c
triclinic
P-1
triclinic
P-1
monoclinic
P21/n
a [Å]
b [Å]
c [Å]
α [°]
32.92(4)
15.38(2)
19.91(2)
90
121.13(1)
90
8.142(2)
11.656(2)
21.504(4)
96.99(2)
95.82(2)
105.53(2)
1932.4(8)
2
8.487(2)
15.076(2)
19.185(3)
94.57(1)
101.33(1)
100.05(1)
2353.3(8)
2
8.460(3)
22.356(4)
20.324(7)
90
101.5(3)
90
β [°]
γ [°]
V [Å3]
8630(16)
8
3766(1)
4
Z
T [K]
150
1.420
0.758
150
1.483
0.768
150
1.432
0.707
150
1.493
0.787
Dc [gcm–3]
µ [cm–1]
Monochromator
Wavelength
Reflections collected
Independent reflections (Rint
Observed reflections
R
graphite
Mo-Kα (0.71073 Å)
47630
11386 (0.10131)
11386 [IϾ2σ(I)]
0.0922
graphite
Mo-Kα (0.71073 Å)
48235
11277 (0.08692)
7923 [IϾ2σ(I)]
0.0403
graphite
Mo-Kα (0.71073 Å)
40692
7530 (0.11620)
4975 [IϾ2σ(I)]
0.0931
graphite
Mo-Kα (0.71073 Å)
27910
6410 (0.09244)
4624 [IϾ2σ(I)]
0.0887
)
Rw
0.1366
0.0530
0.1264
0.1310
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FULL PAPER
Figure 1. ORTEP view showing 35% displacement ellipsoids, and partial atomic labelling for (a) [CuII(HLH)]2+, (b) [CuII(HLMe)]3+, (c)
[CuII(H2LH)]2+ and (d) [CuII(H2LMe)]3+. Hydrogen atoms, except the phenolic proton as well as the iminium protons, have been omitted
for clarity. Weakly coordinating perchlorates are not shown (see text). Selected distances and angles are reported in Table 2.
Figure 1 (see parts a and b, respectively), and selected bond atom, with a Cu–O1 bond length of 2.133(4) Å [the Cu–O8
lengths and angles listed in Table 2. The geometry around distance is 2.875(5) Å].
the metal center is octahedral, with the copper(II) ion coor-
In both [CuII(HLH)]2+ and [CuII(HLMe)]2+, the benzimid-
dinated in a square plane by one tertiary nitrogen atom, azolium (or N-methylbenzimidazolium) rings of two sepa-
N1, two pyridine nitrogen atoms, N2 and N3, and one ace- rate molecules, of the same complex, are π-stacked head-
tonitrile nitrogen atom, N4. The phenolate oxygen atom, to-tail, with distances between the phenyl and imidazolium
O1, occupies one axial position, and one perchlorate oxy- centroids of 3.45 and 3.53 Å for [CuII(HLH)]2+ and
gen atom, O8, is very weakly coordinated at the opposite [CuII(HLMe)]2+ respectively. In both [CuII(HLH)]2+ and
axial position.
[CuII(HLMe)]2+ the iminium hydrogen atom, H1, is hydro-
In [CuII(HLH)]2+ (Figure 1, a), the bond lengths are gen bonded to the O9 oxygen atom of a perchlorate mole-
2.021(1) Å (Cu–N1), 1.982(2) Å (Cu–N2), 1.978(1) Å (Cu– cule.
N3), 1.973(2) Å (Cu–N4), 2.156(1) Å (Cu–O1) and
Adding one equiv. of HClO4 to acetonitrile solutions of
2.813(2) Å (Cu–O8), while the angles N1–Cu–N2, N1–Cu– [CuII(HLH)]2+ and [CuII(HLMe)]2+ affords the copper(II)
N3, N2–Cu–N4 and N3–Cu–N4 differ only slightly from phenol complexes [CuII(H2LH)]3+ and [CuII(H2LMe)]3+ that
90° [83.8(1)°, 82.9(1)°, 96.2(1)° and 95.1(1)° respectively]. were recrystallized by diffusion of diisopropyl ether into the
The mean deviation from the least square plane defined by solution (Table 1). In [CuII(H2LH)]3+ and [CuII(H2LMe)]3+
atoms N1, N2, N3, N4, and Cu is 0.0801 Å, and the copper (Figure 1, c and d, respectively) the copper(II) atoms have
atom is displaced 0.205(2) Å above the basal plane towards similar octahedral geometry. The square plane is defined by
the axial O1 oxygen atom.
one tertiary nitrogen atom, N1, two pyridine nitrogen
In [CuII(HLMe)]2+ (Figure 1, b), the Cu–N1, Cu–N2, atoms, N2 and N3, and one acetonitrile nitrogen atom, N4.
Cu–N3, and Cu–N4 bond lengths are 2.039(5), 1.995(5), The phenolic oxygen atom, O1, and one perchlorate oxygen
1.999(5) and 1.981(3) Å respectively. The mean deviation atom (O7 for [CuII(H2LMe)]3+ and O20 for [CuII(H2LH)]3+
)
from the least square plane defined by atoms N1, N2, N3, occupy the apical positions.
N4, and Cu is 0.036 Å. The copper atom is displaced
The Cu–N1, Cu–N2 and Cu–N4 bond lengths in
0.245(6) Å above the basal plane towards the axial oxygen [CuII(H2LH)]3+ (phenol copper) are similar to those ob-
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Proton Transfer, and Copper(II) Coordination Processes in Pro-Phenoxyl Ligands
FULL PAPER
Table 2. Selected bond lengths [Å] and angles [°] for [CuII(HLH)]2+, [CuII(H2LH)]3+, [CuII(HLMe)]2+, [CuII(H2LMe)]3+ and [CuII-
(HLtBu)]2+
.
[CuII(HLH)]2+
Cu–N1
Cu–N4
N1–Cu–N2
N2–Cu–N3
O1–Cu–N1
O1–Cu–N4
2.021(1)
1.973(2)
83.77(6)
159.37(6)
93.50(5)
93.65(6)
Cu–N2
Cu–O1
N1–Cu–N3
N2–Cu–N4
O1–Cu–N2
1.982(2)
2.156(1)
82.90(6)
96.18(6)
92.86(5)
Cu–N3
Cu–O8
N1–Cu–N4
N3–Cu–N4
O1–Cu–N3
1.978(1)
2.813(2)
172.84(6)
95.10(6)
103.60(5)
[CuII(H2LH)]3+
Cu–N1
Cu–N4
N1–Cu–N2
N2–Cu–N3
2.021(5)
1.972(6)
83.4(2)
Cu–N2
Cu–O1
N1–Cu–N3
N2–Cu–N4
1.980(6)
2.509(5)
82.8(2)
96.6(2)
Cu–N3
Cu–O20
N1–Cu–N4
N3–Cu–N4
1.964(5)
2.432(6)
174.9(2)
97.5(3)
165.7(2)
[CuII(HLMe)]2+
Cu–N1
Cu–N4
N1–Cu–N2
N2–Cu–N3
O1–Cu–N1
O1–Cu–N4
2.039(5)
1.981(6)
84.6(2)
160.2(2)
94.2(2)
97.8(2)
Cu–N2
Cu–O1
Cu–N3
N2–Cu–N4
O1–Cu–N2
1.995(5)
2.133(4)
82.5(2)
95.9(2)
96.1(2)
Cu–N3
Cu–O8
N1–Cu–N4
N3–Cu–N4
O1–Cu–N3
1.999(5)
2.875(5)
167.9(2)
93.7(2)
99.7(2)
[CuII(H2LMe)]3+
Cu–N1
Cu–N4
N1–Cu–N2
N2–Cu–N3
2.031(5)
1.984(5)
83.6(2)
Cu–N2
Cu–O1
N1–Cu–N3
N2–Cu–N4
1.967(6)
2.401(5)
83.2(2)
97.6(2)
Cu–N3
Cu–O7
N1–Cu–N4
N3–Cu–N4
1.972(6)
2.714(7)
172.3(2)
94.8(2)
165.6(2)
[CuII(HLtBu)]2+
Cu–N1
Cu–N4
N1–Cu–N2
N2–Cu–N3
2.037(4)
1.974(6)
83.4(2)
Cu–N2
Cu–O1
N1–Cu–N3
N2–Cu–N4
1.987(4)
2.456(3)
83.6(2)
95.4(2)
Cu–N3
1.977(4)
N1–Cu–N4
N3–Cu–N4
178.5(2)
97.4(2)
164.4(2)
served for [CuII(HLH)]2+ (phenolate copper). This is not the rings of two [CuII(H2LMe)]3+ complexes, the rings are
case for the Cu–N3 bond length that is 1.964(5) Å for stacked head-to-tail, as in [CuII(HLMe)]2+. Hydrogen bonds
[CuII(H2LH)]3+
,
which is 0.015 Å shorter than in are present between the two iminium hydrogen atoms, H2
[CuII(HLH)]2+ [1.978(1) Å]. One can also notice that the and H3, of [CuII(H2LH)]3+ and the perchlorate oxygen
Cu–O1 bond length in [CuII(H2LH)]3+ is much longer than atoms, O4 and O14. One hydrogen bond is present between
in [CuII(HLH)]2+, it is 2.509(5) Å in [CuII(H2LH)]3+ but only the iminium hydrogen atom, H2, of [CuII(H2LMe)]3+ and
2.156(1) Å in [CuII(HLH)]2+. The Cu–Operchlorate distance in the perchlorate oxygen atom, O15, while another is present
[CuII(H2LH)]3+ [Cu–O20, 2.432(6) Å] is shorter than ob- between the phenolic proton, H1, and the oxygen atom, O2,
served in [CuII(HLMe)]2+ [Cu–O8, 2.813(2) Å].
of one perchlorate molecule.
Even more marked geometric variations, were observed
In these structures, another important point of note is
for [CuII(H2LMe)]3+. The Cu–N1 and Cu–N4 bond lengths the torsion angle between the phenol(ate) and the para sub-
in [CuII(H2LMe)]3+ are similar to those observed in stituent of the ring. It is significantly higher for the N-meth-
[CuII(HLMe)]2+, while both the Cu–N2 and Cu–N3 bond ylbenzimidazole (40° and 35° for [CuII(H2LMe)]3+ and
lengths are 0.02 Å shorter (1.967(6) and 1.972(6) Å, respec- [CuII(HLMe)]2+, respectively) than for the benzimidazole (4°
tively for [CuII(H2LMe)]3+, 1.995(5) and 1.999(5) Å, respec- and 27° for [CuII(H2LH)]3+ and [CuII(HLH)]2+, respec-
tively for [CuII(HLMe)]2+). The Cu–O1 bond length in tively), this may be explained, at least partially, by consider-
[CuII(H2LMe)]3+ is significantly longer than in [CuII- ation of the steric hindrance of the methyl group. The con-
(HLMe)]2+ (2.401(5) Å in [CuII(H2LMe)]3+, 2.133(4) Å in sequence of this difference will be weaker electronic com-
[CuII(HLMe)]2+). The copper(II) atom is less displaced munication between the groups in the former case.
above the mean basal plane towards the axial oxygen atom
Complexes [CuII(HLH)]2+ and [CuII(HLMe)]2+ belong to
in [CuII(H2LMe)]3+ [0.121(7) Å] than in [CuII(HLMe)]2+ the small class of structurally characterized mononuclear
[0.245(6) Å]. The Cu–O7 distance in [CuII(H2LMe)]3+ copper(II) complexes of tripodal ligands, in which the phe-
[2.714(7) Å] is shorter than observed in [CuII(HLMe)]2+. A nolate group occupies the unusual axial position. The com-
weak π-stacking interaction (rings are slightly eclipsed) ex- plexes that belong to this class usually involve only one co-
ists between the phenol unit and the imidazole ring of ordinating phenolate, and exhibit a relatively long Cu–O
[CuII(H2LH)]3+, and between the N-methylbenzimidazolium bond (higher than 2.17 Å).24 In [CuII(HLH)]2+, and espe-
Eur. J. Inorg. Chem. 2006, 3684–3696
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F. Michel, F. Thomas, S. Hamman, C. Philouze, E. Saint-Aman, J.-L. Pierre
FULL PAPER
cially in [CuII(HLMe)]2+, the Cu–O1 distances are shorter of frozen CH3CN solutions of [CuII(H2LH)]3+ and
[2.156(1) and 2.133(4) Å, respectively], making them the [CuII(H2LMe)]3+ are typical of axial mononuclear cop-
shortest bonds reported for axially coordinated phenolates. per(II) complexes, and similar to that of [CuII(HLtBu)]2+
We have previously reported the X-ray crystal structures (Table 4).[49] In all three complexes, the copper(II) atoms
of both the copper(II) phenol and the copper(II) phenolate have roughly similar geometry, in agreement with the X-
complexes of HLNO2, the structures of which are related to ray diffraction analysis. The influence of the phenol para
HLH (they bear a nitro instead of a benzimidazole substitu- substituent cannot be visualized in the EPR spectra, as a
ent).[49] Unfortunately, the exogenous ligand was different consequence of the very weak phenolic oxygen–copper(II)
in these structures, an acetonitrile molecule in the phenol bond (see above). Complexes [CuII(HLH)]2+ and
form, and a chloride ion in the phenolate form. Thus, the [CuII(HLMe)]2+ exhibit, in their electronic spectra, the phe-
influence of the protonation of the axial phenolate on the nolate-to-copper(II) CT transition centered at 524 and
equatorial bonds could not be visualized. Here, we report 508 nm, respectively (Figure 3, Table 3). The low energy tail
X-ray structures in which the phenolate and phenol forms in the 700–900 nm region originates from d-d transitions.
have been crystallized with the same exogenous ligand (ace- On addition of one equiv. of NEt3 to CH3CN solutions of
tonitrile molecule) coordinated to the metal center. To the
best of our knowledge, this is the first example in which
changes in the copper(II) ion geometry, induced by depro-
tonation of the axial phenol, could be visualized so clearly:
the metal moves out of the basal plane towards the oxygen
atom as a consequence of a stronger axial bond, and one
(or both) Cu–Npyridine bond length(s) increase, reflecting the
weakening of the equatorial bonds (Figure 2).
Figure 2. Influence of the phenol protonation state on the Cu–O
and Cu–N bond lengths in [CuII(HLMe)]2+ (left) and [CuII(H2LMe)]3+
(right).
Spectroscopic Properties of the Copper(II) Complexes
The electronic spectra of acetonitrile solutions of
[CuII(H2LH)]3+ and [CuII(H2LMe)]3+ show d-d transitions at
around 600 nm (Table 3, Figure 3). This suggests that the
phenol subunit remains protonated in solution, and that the
Figure 3. 298 K electronic spectra of CH3CN solutions of: (a)
copper(II) atom resides within an octahedral (with a very
weakly coordinated perchlorate) or square pyramidal coor-
0.1 m [CuII(H2LH)]3+ (solid line), [CuII(HLH)]2+ (dashed line) and
[CuII(LH)]+ (dotted line); (b) 0.2 m [CuII(H2LMe)]3+ (solid line),
dination sphere (without the perchlorate). The EPR spectra [CuII(HLMe)]2+ (dashed line) and [CuII(LMe)]+ (dotted line).
Table 3. Electronic and electrochemical properties of the copper(II) complexes in CH3CN solutions.[a]
λmax (nm) [ε (–1 cm–1)]
E1/2
[a]
Complex
[CuII(H2LH)]3+
[CuII(HLH)]2+
[CuII(LH)]+
601 br [85]
1.01[b]
0.50[c]
0.23
415 sh [2240], 524 [1160], 700 sh [220], 900 br [165]
405 sh [1180], 556 [1415], 700 sh [510], 900 br [245]
615 br [85]
508 [1180], 664 sh [210], 900 br [135]
521 [1100], 682 sh [340], 900 br [135]
600 [180]
[CuII(H2LMe)]3+
[CuII(HLMe)]2+
[CuII(LMe)]+
0.95[b]
0.60
0.31
[CuII(HLtBu)]2+
Ͼ0.8[b,d]
0.15
[CuII(LtBu)]+
553 [935], 950 br [180]
[a] Vvs.Fc/Fc+. [b] Irreversible, the value given is Epa. [c] Shoulder. [d] Taken from ref.[49]
.
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Proton Transfer, and Copper(II) Coordination Processes in Pro-Phenoxyl Ligands
FULL PAPER
Scheme 2. Protonation equilibria for [CuII(HLMe)]2+ and [CuII(HLH)]2+
.
[CuII(HLH)]2+ and [CuII(HLMe)]2+ the phenolate-to-cop- shift of the λmax band that occurs when CH3CN is replaced
per(II) CT transition is displaced towards lower energy re- by CH3CN/H2O (1:1) suggests that the coordinating aceto-
gions (556 nm and 521 nm, respectively). Such a shift can nitrile in [CuII(H2LH)]3+ and [CuII(H2LMe)]3+ is replaced by
be explained by the increased donor ability of the p-phenol- a water molecule from the solvent mixture. Increasing the
s
ate substituent, and the subsequent deprotonation of the
wpH to 5 results in the appearance of a phenolate-to-cop-
N-methylbenzimidazolium and benzimidazolium groups by per(II) CT in the 475–500 nm region, showing that the phe-
NEt3, affording the phenolate benzimidazole complexes nolate benzimidazolium complexes are formed. The first
[CuII(LH)]+ and [CuII(LMe)]+. Both the phenolate benzimid- pKa values, 3.40 0.01 and 3.55 0.02 for the copper(II)
azolium and phenolate benzimidazole forms exhibit, in complexes of HLH and HLMe respectively, are thus attrib-
their EPR spectra, a signal typical of mononuclear cop- uted to the deprotonation of the phenol group. Such values
per(II) complexes (Table 4). The phenolate oxygen atom are remarkable, as they are much lower than those reported
does not bridge two copper atoms (as observed when the
phenolate ortho substituent is not sufficiently sterically hin-
dered). The protonation equilibria involving [CuII(HLH)]2+
and [CuII(HLMe)]2+ are summarized in Scheme 2.
Table 4. EPR parameters of the copper(II) complexes of HLMe and
HLH in CH3CN at 100 K.
[a]
[a]
[a]
gxx = gyy gzz
2.083 2.248
Axx
Ayy
Azz
[CuII(LMe)]+
0.5
0.5
1
0.5
0.5
1
17.2
17.2
18.4
[CuII(HLMe)]2+ 2.067
[CuII(H2LMe)]3+ 2.060
2.240
2.229
[CuII(LH)]+
broad
2.056
[CuII(HLH)]2+
2.235
2.233
1/1.5
1/1.5
1.5
1.5
18.0
18.0
[CuII(H2LH)]3+ 2.060
[a] Values in mT.
Protonation Constants of the Complexes
The protonation constants were determined from pH-
metric titrations that were monitored by UV/Vis spec-
troscopy (Figure 4). After dissolution of single crystals of
[CuII(H2LH)]3+ and [CuII(H2LMe)]3+ into a CH3CN/H2O
(1:1) mixture (the complexes are poorly soluble in pure
s
H2O), the wpH was increased by addition of NaOH. The
thermodynamic constants were obtained from refinement
of the UV/Vis data by the commercial SPECFIT 32 soft-
ware (Spectrum Software Associated). Their calculation
was based on the spectral changes induced by addition of
s
base. At wpH = 2, the visible spectra of both complexes
s
Figure 4. wpH dependence of the electronic spectra of 0.1 m
consist of CuII d-d transitions at around 650 nm (ε Ͻ
100 –1 cm–1), consistent with the phenol benzimidazolium
CH3CN/H2O (1:1) solutions of [CuII(H2LH)]3+ (a) and
[CuII(H2LMe)]3+ (b). wpH was increased by addition of NaOH, T
s
complexes being the major species present at this swpH. The = 298 K, l = 1.000 cm, I = 0.1  (NaClO4).
Eur. J. Inorg. Chem. 2006, 3684–3696
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FULL PAPER
for phenol copper(II) complexes of tripodal ligands (6.66–
The CV curves of the phenolate complexes [CuII-
7.31),[50] and for GO itself (7.9).[53] This can be explained (HLH)]2+ and [CuII(HLMe)]2+ in CH3CN display quasi re-
by the strong electron-withdrawing effect of the benzimid- versible electrochemical signals centered at E1/2 = 0.50 V
a
a
azolium (or N-methylbenzimidazolium) substituent that (Ip /Ipc 2.5) and E1/2 = 0.59 Vvs.Fc+/Fc (Ip /Ipc = 5), respec-
very efficiently stabilizes the phenolate form, and therefore tively (Figure 5, b). Coulometric titrations reveal that one
lowers its pKa. As a consequence, deprotonation of the phe- electron redox processes are occurring in both cases, sug-
nol group occurs prior to deprotonation of the coordinated gesting that these signals correspond to the oxidation of the
water molecule. This is in contrast with GO, for which de- phenolate to give a phenoxyl radical (vide infra). The lower
a
protonation of the tyrosine occurs after deprotonation of Ip /Ipc ratio obtained for [CuII(HLH)]2+ suggests that its oxi-
the coordinated water molecule (in the absence of sub- dation product is more stable than the oxidation product of
strate). When the swpH is raised to 7.5, a shift of the LMCT [CuII(HLMe)]2+. Resonance effects may contribute to this,
transition towards lower energy regions is observed, while as the planarity between the phenolate ring and its para
the ε value increases by a factor of 1.7 for the copper(II) substituent is greater for [CuII(HLH)]2+ than for
complex of HLH, and a factor of 1.1 for the HLMe complex. [CuII(HLMe)]2+ in the solid state. The E1/2 values obtained
This structural evolution is close to that observed during for [CuII(HLH)]2+ and [CuII(HLMe)]2+ fall within the range
addition of NEt3 to neat acetonitrile solutions of for related copper(II) complexes in which the phenolate
[CuII(HLH)]2+ and [CuII(HLMe)]2+. We therefore attribute para substituent is either NO2 or CF3,[49,55] namely, an elec-
these spectral changes to deprotonation of the benzimida- tron-withdrawing group.
zolium (or N-methylbenzimidazolium) group, rather than
Complexes [CuII(LH)]+ and [CuII(LMe)]+ exhibit, in their
a
c
deprotonation of a coordinated water molecule. The corre- CV curve, oxidation waves centered at E1/2 = 0.23 V (Ip /Ip
a
c
sponding deprotonation constants, 6.56 0.02 for the cop- = 1.6) and E1/2 = 0.32 Vvs.Fc+/Fc (Ip /Ip = 3.9), respec-
per(II) complex of HLH and 6.22 0.06 for the HLMe com- tively (Figure 5, a). As for [CuII(HLH)]2+ and [CuII-
plex are within the range of those reported for free benzimi- (HLMe)]2+, these signals correspond to the oxidation of the
dazolium and N-methylbenzimidazolium (5.63 and 5.67 phenolate moieties to give phenoxyl radicals. The Ip /Ipa ra-
c
s
respectively).[54] Further increasing the wpH results in the tio, as well as the E1/2 values, obtained for the N-methyl- or
precipitation of the copper(II) complex of HLMe. A shift of benzimidazolyl-phenolate complexes are lower than those
the λmax band is observed for the copper(II) complex of found for the corresponding phenolate benzimidazolium
HLH, which may be attributed to the deprotonation of a (or N-methylbenzimidazolium) complexes. This reflects an
coordinated water molecule (pKa = 8.03 0.10).
increase in the stability of the oxidation products of the
former complexes with respect to the latter. Deprotonation
of the benzimidazolium (or N-methylbenzimidazolium)
substituent makes the group less electron-withdrawing (in-
ductive effect), and thus easier to be oxidized. These results
Electrochemistry of the Copper(II) Complexes
The CV curves of [CuII(H2LH)]3+ and [CuII(H2LMe)]3+ in nicely illustrate how an external perturbation (protonation)
CH3CN (0.1  TBAP) at 298 K display an oxidation wave can influence the value of the phenoxyl/phenolate redox
a
at Ep = 1.01 and 0.95 V (referred to the Fc/Fc+ system) couple.
respectively, attributed to the oxidation of the phenol moi-
ety (Figure 5, c). It is irreversible, attesting to the primary
oxidation products not being stable over the timescale of
One Electron Oxidized Copper(II) Complexes
the measurement. Therefore, the redox processes will not be
investigated further.
Upon one electron electrochemical oxidation, the violet
solutions of each copper(II) phenolate complexes turn blue.
Figure 5. Cyclic voltammetry curves recorded on a vitrous carbon disc of 1 m CH3CN (+0.1  TBAP) solutions of, on the left: [CuII-
(LH)]+ (a), [CuII(HLH)]2+ (b), and [CuII(H2LH)]3+ (c). On the right: [CuII(LMe)]+ (a), [CuII(HLMe)]2+ (b), and [CuII(H2LMe)]3+ (c). Scan
rate 0.1 Vs–1, T = 298 K, reference electrode Fc+/Fc.
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Proton Transfer, and Copper(II) Coordination Processes in Pro-Phenoxyl Ligands
FULL PAPER
Two intense absorption bands at 400 and 435 nm and a
lower intensity band at ca. 700 nm dominate the visible
spectra, at 233 K, of the electrochemically generated
[CuII(LH)]·2+, [CuII(HLH)]·3+ and [CuII(HLMe)]·3+ com-
plexes in CH3CN (Figure 6 and Figure 7).[56] Such bands,
which have been previously reported for copper(II) phen-
oxyl complexes[57–59] and [CuII(LtBu)]·2+ [49]
the π-π* transitions of the radical.[60–67]
,
are attributed to
Figure 8. X-Band EPR spectra of a 1 m CH3CN solution of
[CuII(LH)]·2+. Microwave frequency 4485 GHz, power 1 mW, mod.
frequency 100 kHz, amp. 1.0 mT, T = 4 K.
The temperature dependence of the UV/Vis spectrum of
[CuII(LMe)]·2+ precludes any investigation of its stability at
298 K, while [CuII(HLMe)]·3+ was too unstable and de-
graded significantly even at 233 K. Complex [CuII(LH)]·2+
has a decomposition rate constant kdecay of 0.067 min–1
(t1/2 = 10.4 min at 298 K). This is 2.5 times weaker than
that of [CuII(LtBu)]·2+ (kdecay = 0.155 min–1), this is likely to
be due to resonance stabilization in [CuII(LH)]·2+. More-
over, [CuII(LH)]+ exhibits an E1/2 value (0.23 Vvs.Fc+/Fc)
Figure 6. 233 K electronic spectra of a 0.1 m CH3CN solution of
[CuII(LH)]·2+ (l = 1.000 cm).
that is roughly similar to that of [CuII(LF)]+ (E1/2
=
0.20 V),[4] the structure of which differs from that of
[CuII(LH)]+ by the nature of phenolate para substituent,
which is a fluorine atom. As the radical species [CuII-
(LF)]·2+ decomposed during electrolysis at 233 K, [CuII-
(LH)]·2+ it should be, in principle, too unstable to be charac-
terized. This is not true, showing that charge delocalization
over the benzimidazolium group contributes significantly to
the chemical stability of the radical species [CuII(LH)]·2+
.
This also shows that the usual correlation between the
chemical stability of copper(II) phenoxyl radical species,
and the potential values of the phenoxyl/phenolate redox
couple, is not so evident when charge delocalization occurs.
Complex [CuII(HLH)]·3+ was much less stable than
[CuII(LH)]·2+ (kdecay = 0.99 min–1, t1/2 = 0.7 min at 298 K),
this is in agreement with the spectroscopic and electrochem-
ical data. Protonation of benzimidazole, to produce a posi-
tively charged benzimidazolium group, makes it electron-
withdrawing, which destabilizes the phenoxyl radical.
Figure 7. 233 K electronic spectra of a 0.1 m CH3CN solution of
[CuII(HLH)]·3+ (l = 1.000 cm).
The 4 K X-Band EPR spectra of the electrogenerated
species [CuII(LH)]·2+ and [CuII(HLH)]·3+ in CH3CN show
broad ∆MS = 1 transitions at 250 and 390 mT. The associ-
ated ∆MS = 2 signal is seen at 150 mT. This signal is split
into four hyperfine lines separated by 8 mT (Figure 8),
Copper(II) Titration: Addition of 0 to 1 Molar Equivalent
The solution chemistry of each ligand has been studied
which are attributed to the interaction of the electronic spin in acetonitrile by increasing the ratio of copper(II) perchlo-
with the nuclear spin of the copper(II) (I = 3/2). This is rate to HLH and HLMe. When one molar equiv. of cop-
clear evidence of ferromagnetic coupling between the spins per(II) perchlorate is slowly added to an acetonitrile solu-
of the radical and the copper(II). An additional mononu- tion of HLH, an absorption band appears gradually in the
clear copper(II) signal, corresponding to degradation prod- visible spectra at 526 nm (Figure 9), and EPR reveals the
ucts, is also present in each EPR spectrum. [CuII(LMe)]·2+ formation of a mononuclear copper(II) complex. These
exhibits a weak transition at 160 mT, while [CuII(HLMe)]·3+ spectroscopic features well match those reported for the iso-
was too unstable to be prepared in sufficient amounts to lated copper(II) phenolate benzimidazolium species
enable its EPR spectrum to be recorded.
[CuII(HLH)]2+. Likewise, the addition of one molar equiv.
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FULL PAPER
Figure 9. Titration of a 0.1 m CH3CN solution of HLH against
copper(II) perchlorate at 233 K: from 0 to 1 molar equiv. of copper
(a), and from 1 to 2 molar equiv. of copper (b); arrows indicate
spectral changes upon the addition of copper (l = 1.000 cm).
Figure 10. Titration of a 0.3 m CH3CN solution of HLMe against
copper(II) perchlorate at 233 K: from 0 to 1 molar equiv. of copper
(a), and from 1 to 2 molar equiv. of copper (b); arrows indicate
spectral changes upon the addition of copper (l = 1.000 cm).
Scheme 3. Solution chemistry of HLMe and HLH (a), HLtBu (b), in the presence of copper(II) perchlorate in CH3CN.
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Proton Transfer, and Copper(II) Coordination Processes in Pro-Phenoxyl Ligands
FULL PAPER
of copper(II) to HLMe affords the copper(II) phenolate N- and then vanishes, while new intense transitions appear at
methylbenzimidazolium complex [CuII(HLMe)]2+ (Fig- around 400, 450 and 700 nm. The UV/Vis spectrum of the
ure 10). Thus, the coordination of the ligands to copper(II) final species exhibits intense absorption bands at 401, 435
induces a transfer of the phenolic proton to the benzimid- and 700 nm [396, 440 (shoulder) and 700 nm for HLMe].
azole (or N-methylbenzimidazole) substituent (Scheme 3, These features are similar to those of the electrogenerated
a). This behavior is in sharp contrast with that of HLtBu [CuII(HLH)]·3+ complex (and [CuII(HLMe)]·3+). The
(Scheme 3, b), which after addition of one molar equiv. of copper(II) phenolate complexes [CuII(HLH)]2+ and
copper(II) affords the copper(II) phenol complex [CuII(HLMe)]2+ are thus quantitatively oxidized by excess
[CuII(HLtBu)]2+ [50]
Moreover, formation of this phenol copper(II) to give the corresponding copper(II) phenoxyl
.
complex was more complicated than the simple reaction be- complexes [CuII(HLH)]·3+ and [CuII(HLMe)]·3+, releasing
tween the ligand HLtBu and copper(II). As 0 to 0.5 molar free copper(I) according to Scheme 3 (a).[68]
equiv. of copper(II) perchlorate was added, a progressive
When one molar equiv. of copper(II) was added to an
increase in the phenolate to copper(II) CT transition was acetonitrile solution of HLtBu, the copper(II) phenol com-
observed, showing that once the copper is chelated to by plex [CuII(HLtBu)]2+ was obtained quantitatively. Continu-
some of the available ligand in solution, the tertiary amine ing to add copper(II) results in the oxidation of a small
of the remaining free ligand deprotonates the weakly coor- amount of the [CuII(HLtBu)]2+ complex (less than 8%) to
dinated phenol (i.e. the pKa of the coordinated phenol is give the radical species [CuII(LtBu)]·2+, this is consistent with
lower than that of the tertiary ammonium, Scheme 3, b). the oxidation potential of the phenol in [CuII(HLtBu)]2+ be-
When 0.5 to one molar equiv. of copper(II) is added to ing too high to allow it to be oxidized by copper(II) to
HLtBu, the spectrum of the copper(II) phenolate complex produce the corresponding phenoxyl radical.[50]
[CuII(LtBu)]+ vanishes and is replaced by that of the cop-
After addition of one molar equiv. of copper(II) to HLH
per(II) phenol complex [CuII(HLtBu)]2+. Complexation of or HLMe deprotonation of the phenol occurs, induced by
copper(II) to the free protonated ligand H2LtBu+ induces proton transfer to the benzimidazole or N-methylbenzimid-
deprotonation of its tertiary ammonium group; the proton azole substituent, which lowers its redox potential, making
is transferred to the coordinated phenolate of [CuII- oxidation by exogenous copper(II) possible in the absence
(LtBu)]+, affording the copper(II) phenol complex of exogenous base.[69] Deprotonation of the phenol group
[CuII(HLtBu)]2+ (Scheme 3, b).
of HLtBu does not occur in the presence of one molar equiv.
Thus proton transfer, coupled to copper(II) complex- of copper(II), thus preventing its oxidation by excess cop-
ation, occurs in all cases (HLtBu, HLH and HLMe). Never- per(II).
theless, a major difference exists between HLtBu and HLH
On the other hand, if free copper(II) oxidizes the cop-
(and HLMe) namely, the endogenous base: proton transfer per(II) phenolate complex, then one should expect the for-
is managed either by the tertiary amine of HLtBu, or by the mation of a copper(II) phenoxyl complex at a copper(II)/
benzimidazole moiety in HLH or HLMe. In all these tripo- ligand ratio lower than 1:1. This is not the case, showing
dal ligands, the tertiary amine is involved in copper(II) co- that a comproportionation reaction consumes the radical
ordination. When the proton is transferred from the tertiary once it is formed (as in the case of GO[70] and other N3O
amine (ammonium form), it does not chelate the metal thus, ligands[50]) according to Equation 1.
a mixture of the protonated ligand and the copper(II) phe-
copper(II) phenoxyl+copper(I) Ǟ 2 copper(II)–phenolate
(1)
nolate complex is obtained when 0.5 molar equiv. of cop-
per(II) is added to HLtBu. The benzimidazolium group does
not interfere with the complexation processes. Thus, when
the proton is transferred from the benzimidazole (or N-
methylbenzimidazole) group of HLH and HLMe, all of the
ligand in solution chelates to the metal, and no free ligand
Conclusion
Herein we have reported a fine structural approach to
remains, even when only 0.5 molar equiv. of copper(II) is the protonation/deprotonation sequence of the axial Tyr495
added to the ligand. The existence, and location, of the pro- in the GO active site. We have also provided evidence of a
ton acceptor in the ligand is therefore of crucial importance proton transfer reaction coupled to a copper(II) coordina-
in dictating the mechanism by which copper(II) is chelated tion process occurring in the ligands: in the presence of one
by tripodal ligands.
molar equiv. of copper(II) the phenolic proton moves
towards the p-benzimidazole (or N-methylbenzimidazole)
substituent affording p-benzimidazolium phenolate com-
plexes. In the presence of excess copper(II) (in absence of an
exogenous base), a copper(II) phenoxyl radical is obtained,
Copper(II) Titration: Addition of 1 to 2 Molar Equivalents
The acetonitrile solutions of HLH and HLMe were ti- which was not observed for related ligands possessing a p-
trated against copper(II), at 233 K, and up to two molar tert-butyl group. The existence and location of the proton
equiv. of copper(II) was added. When one to two molar acceptor (tertiary amine or benzimidazole) in the ligand is
equiv. of copper(II) was added, the phenolate to copper(II) therefore of crucial importance in dictating the mechanism
CT transition at 524 nm (and 508 nm) of [CuII(HLH)]2+ by which copper(II) is chelated, and influences the reactiv-
(and [CuII(HLMe)]2+) progressively decreases in intensity ity of the complex towards an exogenous oxidizer.
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F. Michel, F. Thomas, S. Hamman, C. Philouze, E. Saint-Aman, J.-L. Pierre
FULL PAPER
mixture was extracted with CHCl3 and the combined organic layers
were dried with Na2SO4 and then concentrated. Column
chromatography on silica gel [ethyl acetate:pentane (1:20) was the
eluent] yielded 3-tert-butyl-4-hydroxybenzaldehyde (24 g, 40%) as
an orange solid. 1H NMR (200 MHz, CDCl3): δ = 1.44 (s, 9 H),
The control of the proton transfer and the metal coordi-
nation, are of prime importance in stabilizing phenoxyl rad-
icals in proteins.[71–79] Recently, each phenomenon has been
explored independently in biomimetic systems. This lead to
the characterization of metal coordinated phenoxyl radi-
cals,[16–50] or hydrogen bonded phenoxyl radicals generated
by proton coupled to electron transfer.[80–83] Ligands HLH
and HLMe are the first systems where both proton transfer
and metal coordination occur simultaneously, contributing
3
3
6.85–6.89 [d, JH,H = 8.2 Hz, 1 H], 7.62–7.67 [dd, JH,H = 8.2 Hz,
4
4JH,H = 1.7 Hz, 1 H], 7.84–7.85 [d, JH,H = 1.7 Hz, 1 H], 9.84 ppm
(s, 1 H). 13C NMR (75 MHz, CDCl3): δ = 29.7 (t, 3C), 35.1 (q),
117.5 (s), 129.9 (q), 130.6 (s), 137,6 (q), 161.2 (q), 192.3 ppm (s).
M.p. 125 °C. C11H14O2 (178.23): calcd. C 74.13, H 7.92; found C
to the stabilization of the phenoxyl radical (by deproton- 73.45, H 7.85.
ation, lowering of the redox potential by at least 0.35 V,
2-tert-Butyl-4-(1-methyl-1H-benzoimidazol-2-yl)phenol: To 3-tert-
metal coordination, and by resonance).
Butyl-4-hydroxybenzaldehyde (1.04 g, 5.8 mmol) in MeOH (20 mL)
was added N-methyl-1,2-phenylenediamine (0.709 g, 5.8 mmol) at
0 °C. The mixture was stirred for 0.5 h at 0 °C, and then for 3 h at
room temperature. Benzofuroxane (0.795 g, 5.8 mmol) was dis-
solved in CH3CN (5 mL), and added to the reaction mixture at
room temperature. The mixture was stirred for 12 h at 60 °C, then
cooled to 0 °C, and 10% NaOH (10 mL) was added. The reaction
was poured into 200 mL of water, and concentrated HCl was added
to neutralize the solution. The brown precipitate was filtered,
washed with cold water and CHCl3, and then dried under vacuum.
Yield: 50%. 1H NMR (300 MHz, [D6]DMSO): δ = 1.46 (s, 9 H),
3.89 (s, 3 H), 7.22–7.31 (m, 2 H), 7.54–7.60 (m, 2 H), 7.66–7.69 (m,
2 H), 10.00 ppm (s, 1 H). 13C NMR (75 MHz, [D6]DMSO): δ =
30.1 (t, 3C), 32.6 (t), 35.3 (q), 111.1 (s), 117.0 (s), 119.4 (s), 121.2
(q), 122.5 (s), 122.7 (s), 128.8 (s), 128.9 (s), 137.5 (q), 143.4 (q),
154.7 (q), 158.3 ppm (q). M.p. Ͼ 250 °C. MS (DCI, NH3/isobut-
ane) m/z (%): 280 (100) [M+H]+. C18H20N2O (280.36): calcd. C
77.11, H 7.19, N 9.99; found C 75.40, H 7.21, N 9.76.
Experimental Section
General: All chemicals were of reagent grade and used without pu-
rification. Microanalyses were performed by the Service Central
d’Analyses du CNRS (Lyon, France).
Low-temperature visible spectra were recorded on a CARY 50
spectrophotometer equipped with a low temperature Hellma im-
mersion probe (1.000 cm path length quartz cell). The temperature
was controlled with a Lauda RK8 KS cryostat.
X-band EPR spectra were recorded on a BRUKER ESP 300E
spectrometer equipped with a BRUKER nitrogen flow cryostat,
and a BRUKER EMX spectrometer equipped with an ESR 900
helium flow cryostat (Oxford Instruments). Spectra were treated
using the WINEPR software and simulated using the BRUKER
SIMFONIA software.
HLMe
:
2-tert-Butyl-4-(1-methyl-1H-benzoimidazol-2-yl)phenol
Rate constants for the self-decomposition of the radical species
were obtained spectrophotometrically. The absorbance decay at
450 nm (298 K) was fitted using the Biokine software (Bio Logic
Co, Claix, France).
(1.016 g, 3.6 mmol), bis(2-pyridylmethyl)amine (722 mg, 3.6 mmol)
and formaldehyde (1.4 mL of a 37% aqueous solution) in EtOH/
H2O (5:1, 100 mL) were refluxed together for 12 h. The reaction
mixture was then extracted with ethyl acetate, dried with Na2SO4
and then concentrated. Column chromatography on silica gel [ethyl
acetate/methanol (9:1)+1% isopropylamine was the eluent] yielded
The cyclic and differential pulse voltammograms of each com-
pound (1 m) in CH3CN, containing 0.1  tetra-n-butylammo-
nium perchlorate (TBAP) as the supporting electrolyte, were re-
corded on a CHI potentiostat at 298 K. The working electrode was
a glassy carbon disc, and the secondary electrode was a Pt wire,
and the reference electrode was 0.01  Ag/AgNO3. The potential
of the regular ferrocenium/ferrocene (Fc+/Fc) redox reaction,
+0.087 V under our experimental conditions, was used as an in-
ternal reference. Electrolysis was performed at 233 K at a carbon
felt electrode using a PAR 273 potentiostat.
1
HLMe (600 mg, 35%) as a pale orange solid. H NMR (300 MHz,
[D6]DMSO): δ = 1.49 (s, 9 H), 3.89 (s, 3 H), 3.91 (s, 4 H), 3.95 (s,
2 H), 7.21–7.34 (m, 4 H), 7.42–7.44 (d, 3JH,H = 7.8 Hz, 2 H), 7.52–
4
7.53 (d, JH,H = 2.0 Hz, 1 H), 7.56–7.60 (m, 1 H), 7.63–7.64 (d,
3
4JH,H = 2.0 Hz, 1 H), 7.65–7.68 (m, 1 H), 7.78–7.84 (m, JH,H
=
4
3
4.7 Hz, JH,H = 1.7 Hz, 2 H), 8.59–8.60 (d, JH,H = 4.7 Hz, 2 H),
11.76 ppm (s, 1 H). 13C NMR (75 MHz, [D6]DMSO): δ = 32.6 (t),
35.4 (q), 57.2 (d), 59.2 (d), 111 (s), 119.4 (q), 120.5 (q), 122.5 (q),
122.7 (s), 123.4 (s, 2C), 124.0 (s, 2C), 128.1 (s), 129.8 (s), 136.8 (q),
137.5 (q), 137.7 (s, 2C), 149.7 (s, 2C), 154.6 (q), 158.4 ppm (q, 2C).
MS (DCI, NH3/isobutane) m/z (%): 492 (100) [M+H]+. M.p.
150 °C. C31H33N5O (491.63): calcd. C 75.73, H 6.77, N 14.25;
found C 75.65, H 7.01, N 14.19.
Crystal Structure Analysis: For all structures, collected reflections
were corrected for Lorentz and polarization effects, but not for
absorption. The structures were solved by direct methods and re-
fined using the TEXSAN software. All non-hydrogen atoms were
refined with anisotropic thermal parameters. Hydrogen atoms were
generated at idealized positions, and were modeled as riding on the
carrier atoms, and refined with isotropic thermal parameters.
[CuII(HLMe)](ClO4)2: Ligand HLMe (132 mg, 0.269 mmol) was dis-
solved in CH3CN (10 mL) and Cu(ClO4)2·6H2O (99 mg,
0.267 mmol) in CH3CN (2 mL) was then added dropwise. The solu-
tion was stirred for 15 min and then the volume was reduced to
2 mL. Single crystals of [CuII(HLMe)](ClO4)2 were obtained by slow
diffusion of diisopropyl ether into the CH3CN solution (102 mg,
yield: 68%). ESI MS (m/z): 553 [M – H – (CH3CN) – 2(ClO4)]+.
C33H36Cl2CuN6O9 (795.13): calcd. C 49.85, H 4.56, N 10.57, Cu
7.99; found C 49.57, H 4.82, N 10.11, Cu 7.45.
CCDC-259962, -259841, -275709 and -275881 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
3-tert-Butyl-4-hydroxybenzaldehyde:
2-tert-Butylphenol
(20 g,
133 mmol) was dissolved in MeOH (80 mL), NaOH (80 g) was dis-
solved in water (80 mL) and added dropwise to the reaction mix-
ture. Then CHCl3 was added (during the course of 1 h) at 60 °C.
The reaction mixture was stirred for 3 h, then cooled to 0 °C and
hydrolyzed with 4  HCl until the solution reached pH 5–6. The
[CuII(H2LMe)](ClO4)3: Compound [CuII(HLMe)](ClO4)2 (23 mg,
0.029 mmol) was dissolved in CH3CN (1 mL), and HClO4 (70%,
2.4 µL) was added. The solution was stirred for 5 min. Single crys-
3694
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 3684–3696

Proton Transfer, and Copper(II) Coordination Processes in Pro-Phenoxyl Ligands
FULL PAPER
tals of [CuII(H2LMe)](ClO4)3 were obtained by slow diffusion of
isopropyl ether into the CH3CN solution (18 mg, yield: 90%). ESI
MS (m/z): 653 [M – H – (CH3CN) – 2(ClO4)]+. C33H37Cl3CuN6O13
(895.58): calcd. C 44.26, H 4.16, N 9.13, Cu 7.10; found C 44.18,
H 4.13, N 9.13, Cu 6.88.
Dr. Stephane Ménage for technical assistance with these experi-
ments.
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1
vacuum. Yield: 52%. H NMR (300 MHz, [D6]DMSO): δ = 1.48
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(s, 9 H), 6.95 (d, J = 8.1 Hz, 1 H), 7.17 (dd, 3J = 5.9 Hz, 4J =
3
4
3
3.2 Hz, 2 H), 7.57 (dd, J = 5.7 Hz, J = 3.2 Hz, 2 H), 7.87 (dd, J
4
4
= 8.1 Hz, J = 1.5 Hz, 1 H), 8.05 (d, J = 1.5 Hz, 1 H), 9.94 (s, 1
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(t), 34.9 (Cq), 114.9 (CH), 116.8 (CH), 121.0 (Cq), 121.9 (CH),
125.6 (CH), 125.9 (CH), 136.2 (Cq), 139.7 (Cq), 152.6 (Cq),
158.2 ppm (Cq). M.p. Ͼ 250 °C. MS (DCI, NH3/isobutane)
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H 6.81, N 10.52; found C 75.78, H 6.94, N 9.37.
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HLH:
2-tert-Butyl-4-(1H-benzoimidazol-2-yl)phenol (0.500 g,
1.88 mmol), bis(2-pyridylmethyl)amine (374 mg, 1.88 mmol) and
formaldehyde (0.7 g of a 37% aqueous solution, 9.4 mmol) in
EtOH/H2O (2:1, 100 mL) were refluxed together for 12 h. The reac-
tion mixture was then extracted with ethyl acetate, washed with
a saturated NaCl aqueous solution, dried with Na2SO4 and then
concentrated. Column chromatography on silica gel [ethyl acetate/
methanol (5:1)] yielded HLH (270 mg, 30%) as a beige solid. 1H
NMR (300 MHz, [D6]DMSO): δ = 1.50 (s, 9 H), 3.90 (s, 4 H), 3.94
(s, 2 H), 7.19 (dd, 3J = 5.9 Hz, 4J = 3.2 Hz, 2 H), 7.30 (d, 3J =
3
6.5 Hz, 1 H), 7.32 (d, J = 7.1 Hz, 1 H), 7.41 (d, J = 7.8 Hz, 2 H),
3
4
7.50–7.62 (m, 2 H), 7.78 (td, J = 7.6 Hz, J = 1.6 Hz, 2 H), 7.87
4
4
3
(d, J = 1.8 Hz, 1 H), 8.01 (d, J = 1 Hz, 1 H, 8 Hz), 8.58 (d, J =
4.0 Hz, 2 H), 11.73 (s, 1 H), 12.70 ppm (s, 1 H). 13C NMR
(75 MHz, [D6]DMSO): δ = 30.3 (tBu), 35.6 (Cq), 57.6 (CH2), 59.1
(CH2Py), 121.0 (Cq), 122.4 (CH), 123.4 (CH), 123.9 (CH), 124.5
(Cq), 125.4 (CH), 127.6 (CH), 135.9 (Cq), 137.2 (Cq), 137.7 (CH),
144.9 (Cq), 149.7 (CH), 153.0 (Cq), 158.5 (Cq), 158.8 ppm (Cq).
M.p. 200 °C. MS (DCI, NH3/isobutane) m/z (%): 478 (100)
[M+H]+. C30H31N5O (477.60): calcd. C 75.44, H 6.54, N 14.66;
found C 75.16, H 6.55, N 14.50.
[CuII(HLH)](ClO4)2: [CuII(HLH)](ClO4)2 was obtained in a similar
way to [CuII(HLMe)](ClO4)2 by using HLH instead of HLMe (yield:
75%). ESI MS (m/z): 539 [M – H – (CH3CN) – 2(ClO4)]+.
C32H34Cl2CuN6O9·H2O·CH3CN (781.10): calcd. C 48.61, H 4.68,
N 11.67, Cu 7.56; found C 47.92, H 4.51, N 11.68, Cu 7.50.
[CuII(H2LH)](ClO4)3: Complex [CuII(H2LH)](ClO4)3 was obtained
in a similar way to [CuII(H2LMe)](ClO4)3 by using HLH instead of
HLMe (yield: 90%). ESI MS (m/z): 539 [M – H – (CH3CN) –
2(ClO4)]+. C32H35Cl3CuN6O13 (881.56): calcd. C 44.16, H 4.38, N
9.36, Cu 7.08; found C 44.51, H 4.35, N 9.70, Cu 6.75.
Acknowledgments
The authors thank the Institute of Metals in Biology of Grenoble
who provided facilities for performing the 4 K EPR studies, and
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Eur. J. Inorg. Chem. 2006, 3684–3696
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3695

F. Michel, F. Thomas, S. Hamman, C. Philouze, E. Saint-Aman, J.-L. Pierre
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Received: March 31, 2006
Published Online: August 3, 2006
3696
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 3684–3696