A minimalist approach to C-H activation by copper

Article abstract of DOI:10.1002/hlca.200690086

The complex [Cu₂(1)₂]²⁺ (1=1,3-bis(1-methyl-1H-benzimidazol-2-yl)benzene) undergoes slow oxidation by dioxygen in DMF solution to give the hydroxylated product [Cu ₂(2-H)₂]²⁺ (2=2,6-bis(1-methyl-1H-benzimidazol- 2-yl)phenol) characterized by an X-ray crystal-structure analysis. The oxidation occurs much faster when Cu^II is mixed with 1 in the presence of H₂O₂, with 80% hydroxylation observed within a few minutes. The mononuclear complex formed with 1-methyl-2-phenyl-1H-benzimidazole (3) shows no hydroxylation under these conditions. It is concluded that the hydroxylation requires the presence of a ligand capable of stabilizing a binuclear species, but no special coordinative activation of the copper is required.

Full text of DOI:10.1002/hlca.200690086

Helvetica Chimica Acta – Vol. 89 (2006)
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A Minimalist Approach to CꢀH Activation by Copper
by Anita Kuebel-Pollaka), StØphane Rüttimanna), Nichola Dunna), Xavier Melicha), Alan F. Williams*a),
and GØrald Bernardinellib)
a) Department of Inorganic, Analytical and Applied Chemistry, University of Geneva,
30 quai Ernest Ansermet, CH-1211 Gen›ve 4 (phone: +41223796425; fax: +41223796830;
e-mail: Alan.Williams@chiam.unige.ch)
b) Laboratory of X-Ray Crystallography, University of Geneva, 24 quai Ernest Ansermet,
CH-1211 Gen›ve 4
Dedicated to Professor Andreas Zuberbühler on the occasion of his retirement
The complex [Cu2(1)2]2+ (1=1,3-bis(1-methyl-1H-benzimidazol-2-yl)benzene) undergoes slow oxi-
dation by dioxygen in DMF solution to give the hydroxylated product [Cu2(2-H)2]2+ (2=2,6-bis(1-meth-
yl-1H-benzimidazol-2-yl)phenol) characterized by an X-ray crystal-structure analysis. The oxidation
occurs much faster when CuII is mixed with 1 in the presence of H2O2, with 80% hydroxylation observed
within a few minutes. The mononuclear complex formed with 1-methyl-2-phenyl-1H-benzimidazole (3)
shows no hydroxylation under these conditions. It is concluded that the hydroxylation requires the pres-
ence of a ligand capable of stabilizing a binuclear species, but no special coordinative activation of the
copper is required.
Introduction. – Although the CꢀH bond is traditionally regarded as difficult to oxi-
dize in the laboratory, Nature has developed a large number of enzymes capable of acti-
vating the CꢀH bond in an efficient and controlled way. The enzyme tyrosinase is a
good example of this and uses a dicopper centre to transform the phenol of tyrosine
to an ortho-catechol [1][2]. Tyrosinase is structurally similar to the oxygen-carrying
protein hemocyanin [3], and both have been the subject of intensive modelling with
low-molecular-weight coordination compounds. Hydroxylation of a model complex
of CuI containing a ligand with two tridentate centres separated by a meta-xylyl spacer
was reported as long ago as 1981 by Karlin et al. [4], and some years later Feringa and
co-workers showed a similar effect using a bis-bidentate ligand [5]. Since then, a num-
ber of other workers have published similar results using a variety of bis-tridentate
ligands [6][7] and bis-bidentate ligands [8–10].
In most cases, molecular oxygen acts as the source of the O-atom, although Karlin
and co-workers have shown that H2O2 will also effect hydroxylation of a dicopper(II)
complex where the CuI complex shows oxygenase activity [11]. The mechanism has
been studied in some detail by Karlin, Zuberbühler and co-workers showing the initial
formation of a dicopper(II) peroxo-species which then hydroxylates the ligand [12]. In
a detailed study, they showed the presence of a NIH shift which suggested that the m-
(h2 :h2)-peroxo complex carries out an electrophilic attack on the aromatic spacer, and
this was confirmed by the effect of substituents on the aromatic ring [13]. This hypoth-
esis is generally accepted for peroxo complexes. The pressure dependency was meas-
ꢀ 2006 Verlag Helvetica Chimica Acta AG, Zürich

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ured recently [14], and Casella et al. have shown that these complexes may also pro-
mote hydroxylation of exogeneous substrates [6][15][16]. Casella and co-workers
have shown recently that H2O2 can induce the double hydroxylation of a system con-
taining two tridentate binding sites separated by a 1,3-dimethylenephenyl spacer, and
have deduced from spectroscopic data that a m-1,1-hydroperoxo species is formed
and is the active species [17]. Such a species has since been characterized by X-ray crys-
tallography by Suzuki and co-workers [18].
We have previously studied the binuclear copper(I) complex [Cu2(1)2]2+ in connec-
tion with the formation of helicates [19][20] and observed that it slowly turned green
upon standing in DMF solution. In the present manuscript, we characterize the prod-
uct, show it to be the result of ligand hydroxylation, and present qualitative observa-
tions on the mechanism. We discuss their significance for the hydroxylation of aromatic
CꢀH bonds.
Results. – Synthesis. For the purpose of comparison with the ligand 2 produced by
hydroxylation, a purely organic route to 2 was developed (Scheme) using the nitro
amine reduction method of Piguet et al. [21]. Ligands 3 and 4 were synthesized from
benzene-1,2-diamine, and benzoic acid and salicylic acid, respectively, followed by
methylation involving deprotonation with NaH and treatment with MeI. For ligand
4, the anisole group thereby formed was deprotected with BBr3. The complex
[Cu2(2-H)2](TsO)2 ·3 H2O was prepared by mixing stoichiometric amounts of ligand,
copper(II) p-toluenesulfonate, and Et3ACHTRENUNG .
Characterization of the Hydroxylated Product. The oxygenated product was initially
formed by stirring a solution of [Cu2(1)2]2+ in DMF at 808 under O2 for 18 h. Evapora-
tion of the solvent gave the product as a green solid, which could be recrystallized from
DMF by vapor diffusion of Et
and showed the complex to be [Cu2(2-H) CTHREUNG
O. The crystals were suitable for X-ray crystallography,
2A(DMF)2](ClO4)2 ·2 DMF. The complex could
equally be prepared by using stoichiometric amounts of ligand 1 and copper(II) per-
chlorate under the same conditions.
The electrospray mass spectrum (ES-MS) shows the peaks expected for the com-
(DMF)2]2+ with adduct anions and with loss of weakly bound DMF.
plex [Cu2(2-H)2ACHTREUNG
The isotopic distribution corresponds to a dinuclear copper species. The UV/VIS spec-
trum of the hydroxylated product is quite characteristic. Ligand 1 shows essentially only

Helvetica Chimica Acta – Vol. 89 (2006)
Scheme. Synthesis of Ligand 2
843
one band in the UV at 300 nm (e 35500 cmꢀ1
two bands at 284 nm (e 21500 cmꢀ1 ꢀ1) and 324 nm (e 20400 cmꢀ1
hydroxylated complex [Cu2(2-H)2A
(DMF)2]2+, the second band is shifted into the visible
353 nm (e 16000 cmꢀ1 ꢀ1) and is responsible for the strong green color of the solution.
M
ꢀ1), but in ligand 2 the spectrum shows
M
M
ꢀ1). In the
CTHREUNG
M
This band has been attributed by Casella et al. to a ligand-based transition to which
some ligand-to-metal charge-transfer character is added [8]. The band is slightly
blue-shifted in EtOH solution. This band is a useful marker of hydroxylation and has
been used to follow the reaction. Not surprisingly, in view of the double phenolate
bridge, which would be expected to give rise to strong antiferromagnetic coupling,
1
the complex is EPR-silent. A H-NMR spectrum in (D7)DMF may be observed in
the range 0–21 ppm, but the peaks are severely broadened and could not be assigned.
Structure of [Cu2(2-H)2(DMF)2](ClO4)2 ·2 DMF. The structure of the complex cat-
ion shows two CuII ions linked by two ligands which have been hydroxylated and depro-
tonated, with the phenolate functions bridging the two metals (Fig. 1). The cation lies
on a crystallographic centre of inversion, and so the two Cusites are identical, with
square pyramidal coordination (SPY-5). The base of the pyramid is formed by two
N-atoms of benzimidazoles and two bridging phenolate functions; the apex of the pyr-
amid is formed by the O-atom of a coordinated DMF molecule. The CuꢀN bond dis-
tances of 1.952(4) and 1.951(4) Š are slightly shorter than the Cuꢀbenzimidazole dis-
tances observed for another square pyramidal copper(II) complex (2.04 Š [22]), but
longer than the bonds to two-coordinate copper(I) in [Cu2(1)2]2+ (1.89 Š) [19]. The
copper-phenolate distances of 1.974(3) and 2.009(3) Š are slightly longer than those
observed by Karlin et al. [4] and Casella et al. [8] in complexes where there is only

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Helvetica Chimica Acta – Vol. 89 (2006)
Fig. 1. View of the cation [Cu2(2-H)2(DMF)2]2+. There is a crystallographic centre of inversion
between the two CuII ions. Ellipsoids are drawn at 40% probability.
one bridging phenolate. The axial CuꢀO(DMF) distance of 2.271(3) Š is, as expected
for CuII, much longer. The CuꢀCudistance of 3.0489(7) Š is less than half that
observed in [Cu2(1)2]2+ (7.140 Š) [19]. The conformation of the ligands is interesting:
the phenolate moiety is significantly distorted from planarity, and may be regarded
as slightly folded about the axis of the CꢀO bond. In consequence, the two atoms on
the axis lie roughly 0.08 Š above the least-squares plane, and the others 0.01–0.06 Š
below it. The two benzimidazole moieties are twisted at ca. 428 to the phenolate
plane. The ligand as a whole appears to be curved, and the plane of the phenol is
inclined by ca. 478 with respect to the Cu2O2 plane (Fig. 2).
The cations form layers parallel to the bc plane and show stacking interactions
between benzimidazole moieties of neighboring complexes as observed previously in
benzimidazolonium salts [23]. The perchlorate anions and included DMF molecules
show no features of interest and lie in sheets in between the layers of cations.
The Hydroxylation Reaction. The observation of hydroxylation of [Cu2(1)2]2+ in
DMF solution was somewhat surprising since the two CuI ions are over 7 Š apart in
this complex [19], and the CuI is in no way activated for dioxygen binding. The initial
preparation used quite concentrated solutions (0.025M) and heating. If the procedure
was repeated at room temperature and followed by UV/VIS spectroscopy, the forma-
tion of hydroxylated product was very slow, with only 30% conversion after 3 weeks. It

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845
Fig. 2. Another view of [Cu2(2-H)2(DMF)2]2+ roughly along the CꢀO bonds of the phenolate moiety
showing the distortion of the phenolate and its inclination to the Cu2O2 plane
could be speeded up somewhat by addition of an acetate buffer, and stopped by addi-
tion of 2,6-lutidine. Measurement of the oxygen uptake showed that the volume of O2
necessary to oxidize completely the Cu(I) to Cu(II) was taken up over a timescale of
minutes, but that the hydroxylated product was formed much more slowly. In MeCN
solution, no oxidation was observed at all. The slowness of the reaction in DMF, cou-
pled to the apparent rapid oxidation to CuII deduced from the O2-uptake experiment,
led us to suspect that partial decomposition of the solvent (to CO and Me
be involved.
2ACHTREUNNG H) might
We then studied the effect of H2O2 as an oxidant on a mixture of CuII and ligand 1.
The hydroxylation was much faster in this case. In DMF with a concentration of 1
around 10ꢀ3 M, a 20% excess of CuII, and 10 equiv. of H2O2, 60% conversion, as judged
by the absorbance, was achieved in under 1 h. The yield could be increased slightly to
ca. 80% in the presence of a lutidine buffer. Fig. 3 shows the growth of the band at 350
nm and a clean isosbestic point. The reaction could also be effected in EtOH, although
the low solubility of the product in this solvent forces the use of lower concentrations to
avoid precipitation of the product. A clean reaction with an isosbestic point is again
observed. The hydroxylation reaction using H2O2 should in principle lead to the liber-
ation of protons according to Eqn. 1.
2 Cu2+ +2 1+2 H2O2 ! [Cu2(2-H)2]2+ +2 H
3A
(1)
The pH of the solution does indeed fall upon addition of H2O2, and titration of the
resulting solution with lutidine suggests that 1 equiv. of protons is liberated per mol of
copper. The presence of lutidine slightly increases the yield by preventing protonation
of free ligand.
The nature of the species formed in solution by CuII and 1 is not clear. Ligand 1 is
not chelating and may act as a monodentate or as a bis-monodentate ligand in a binu-

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Fig. 3. Spectral changes with time in a solution of [Cu(TsO)2]=9.6·10ꢀ4 M, [1]=8·10ꢀ4 M in DMF
(lutidine buffer 0.025M) to which a tenfold excess of H2O2 is added. The growth of the transition at
350 nm is clearly visible. The dashed line is that of a solution containing [Cu(TsO)2]=9.6·10ꢀ4 M and
[2]=8·10ꢀ4 M.
clear complex. A monodentate benzimidazole moiety is not expected to be a strongly
binding ligand with a typical stability constant of 103 [24]. Under the conditions used for
hydroxylation, only partial complexation of copper would be expected. Electrospray
mass spectra confirmed this showing peaks due to free ligand, [Cu(1)] and [Cu(1)2].
Spectroscopic titrations, either adding ligand to CuII or CuII to ligand suggested the for-
mation of [Cu(1)] and [Cu(1)2] with log b values of 2.9(2) and 7.0(1), respectively, but
these values should only be considered as indicative since the absorbance changes were
small.
The final experiment was to investigate whether a more simple ligand such as 3
could be hydroxylated under the same conditions. Ligand 3 has an aromatic CꢀH
bond in the same position with respect to the coordination site of the benzimidazole,
but cannot form a binuclear complex in the way that ligand 1 can. Oxidation of ligand
3 was attempted in four ways: a) by simple reaction with 0.5 or 1 equiv. of CuII in DMF
in air; b) as for a but with 0.5 equiv. of copper only for 5 h at 608; c) as for b, but in pres-
ence of 3 equiv. of AcOH for 4 h at 808; d) as for a but in presence of 5 equiv. of H2O2.
In no case was the band at 356 nm expected for hydroxylation observed, although
ligand 1 would have been at least partially hydroxylated under these conditions. The
expected product of hydroxylation 4 was synthesized directly, and reacted with CuII.
One equiv. of CuII reacted with 2 equiv. of ligand 4 and 2 equiv. of Et
3ACHTRENUGN in DMF solu-
tion showed the intense band expected for the phenolato complex at 356 nm (emax
11000 cmꢀ1
tions.
M
ꢀ1). We may, therefore, deduce that 3 is not oxidized under these condi-

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847
Discussion. – Although the hydroxylation of aromatic substrates catalyzed by cop-
per is by now well-established, the work described here shows a number of unusual fea-
tures. The traditionally studied systems contain two good complexation sites for the CuI
ions, usually bidentate or tridentate, which lower the redox potential of the CuII/CuI
couple, and thereby favor reaction with O2, together with a suitable bridging unit link-
ing the two coordinate sites. In this work, ligand 1 is a simple bis-monodentate ligand,
which complexes CuI only weakly [20] giving a linear coordination of the copper in
[Cu2(1)2]2+ [19] which is not favourable for CuII. [Cu2(1)2]2+ does not in fact react
with oxygen in MeCN solution. Furthermore, complexation of CuII by ligand 1 has
also been shown to be weak.
The benzene-1,3-diyl bridge does not appear suitable for dioxygen activation since
the CuꢀCudistance in [Cu (1)2]2+ is over 7 Š, but examination of models and the
2
results presented here show that rotation about the benzimidazol-phenyl bonds will
bring copper ions bound to the benzimidazoles much closer to each other. Such confor-
mational reorganization is now well-established for m-xylyl bridges in the tyrosinase
model systems. The conformational change will approach the dicopper site to the Cꢀ
H bond which is to be hydroxylated. This will of course be energetically costly, but
can be compensated by the binding of a peroxide between the two coppers. Since 1
is only weakly complexed by CuII, we may assume that the formation of the bridging
peroxide group (which is well-precedented) also plays an important role in stabilizing
the dinuclear complex. The binding of the peroxide to two copper ions enhances its
electrophilicity [25], and the aryl CꢀH bond is brought close to the now electrophilic
peroxide by the conformational rearrangement of the bridging ligand. The way is thus
opened for a rapid hydroxylation, as we observe. The need for peroxide to stabilize the
dinuclear complex would also explain the greatly increased rate in the reaction with
H2O2.
The mononuclear ligand 3 does not show this hydroxylation, presumably since the
ligand bridge which brings the CꢀH bond close to the peroxide is no longer present. It
may be noted that a bridging ligand is not essential for the stabilization of a peroxide
[26–28], especially when the Cu-coordination sphere is such as to favor electron trans-
fer to dioxygen, but this is not the case with ligand 3.
The initial observation of the hydroxylation with [Cu2(1)2]2+ remains somewhat
mysterious. The O2-uptake experiments showed that the copper was oxidized to CuII
quite rapidly, and long before the characteristic band of the hydroxylated product
was observed. The long timescale and the fact that this reaction is only seen in DMF
leads us to associate it with partial decomposition of the solvent. DMF is well-known
to have reducing properties, and may thereby generate trace amounts of CuI which
can then generate traces of peroxide by reaction with O2. This may then react as for
the CuII/peroxide system discussed above. We may also note that dicopper products
similar to those of the reactions presented here have been shown to accelerate the
hydrolysis of DMF to formate [29].
Finally, we may note that kinetic studies of these reactions were inconclusive. Since
the complexes are relatively weak and are substitutionally labile, the speciation in sol-
ution is a non-trivial matter. Both the reaction of [Cu2(1)2]2+ with dioxygen, and that of
CuII with 1 and hydrogen peroxide are non-complementary, in that the first requires 6
and the second 4 oxidizing equiv. to reach the product [Cu2(2-H)2]2+, while the oxidizing

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agents supply 4 and 2 equiv., respectively. We, therefore, suspect that [Cu2(2-H)2]2+ is to
be regarded as the final, thermodynamic product, and that it is not the initial product of
the hydroxylation reaction. It would be more logical for one ligand at a time to be
hydroxylated.
In conclusion, the results presented here show that efficient hydroxylation may be
observed in systems where the copper is not specifically activated for dioxygen binding,
and indeed where the copper complex involved in hydroxylation is not very stable. On
the other hand, the fact that the ligand positions the CꢀH bond close to the postulated
bridging peroxide does appear to be important as shown by the different reactivity of 1
and 3.
We thank the Swiss National Science Foundation for their support of this work.
Experimental Part
General. Solvents and starting materials were purchased from Fluka AG (CH-Buchs) and were used
without further purification unless otherwise stated. DMF was distilled from CaH2. UV/VIS Spectra:
Cary IE or Perkin-Elmer Lambda-5 UV/VIS/NIR spectrometers; quartz cells of 1- or 0.1-cm path
lengths; lmax (e) in nm. Spectroscopic titrations were carried out using a J&M diode array spectrometer
(Tidas series) connected to an external computer. Data were analyzed using the SPECFIT program [30].
IR Spectra: Perkin-Elmer Spectrum-One or 883 instruments, KBr discs; n in cmꢀ1. NMR Spectra: Varian
~
Gemini-300 or Bruker Advance-400 at 300 or 400 (1H), and 74.44 MHz (13C) at 228; chemical shifts d in
ppm with respect to Me4ACTHRESUNG i, J in Hz. MS: electron impact (EI) at 70 eV with VG 7000E and Finnigan 4000
instruments, electrospray ionisation (ESI) with Finnigan Mat-SSQ7000 instrument of the Mass Spec-
trometry Laboratory, University of Geneva; in m/z (rel.%). Elemental analyses were performed by
Dr. H. Eder, University of Geneva.
Synthesis of Ligands. 1,3-Bis(1-methyl-1H-benzimidazol-2-yl)benzene (1) was obtained according to
the procedure described in [19].
2,6-Dimethylanisole (=2-Methoxy-1,3-dimethylbenzene). KOH (85%; 21.08 g, 0.32 mol, 4 equiv.)
was added to 120 ml of DMSO. 2-Hydroxy-m-xylene (9.76 g, 0.08 mol) was added, and the soln. was stir-
red for 5 min. The flask was cooled in ice, and when the temp. reached 08, 12.26 ml of MeI (0.19 mol, 2.4
equiv.) were added. After 15 min, the flask was taken out of the ice and allowed to warm to r.t. H2O (100
ml) was added, and the product was extracted with CH2Cl2. The fractions of CH2Cl2 were collected and
washed with H2O to eliminate the remaining DMSO. The product, which is a liquid, was purified by col-
umn chromatography (CC) (SiO2 (200 g); CH2Cl2). The final product was then distilled under reduced
pressure: 8.8 g (0.065 mol; 81%) of liquid product. B.p. 170–1718. IR(KBr): 2943vs, 1475vs, 1416m,
1
1262s, 1214vs, 1170m, 1091s, 1017vs, 811m, 768vs. H-NMR (CDCl3, 300 MHz): 2.26 (s, 6 H); 3.69 (s,
MeO); 6.86–6.91 (dd, 1 arom. H); 6.97–6.99 (d, 2 arom. H). EI-MS: 136 (100, M+), 121 (97), 105
(17), 91 (47), 77 (50), 65 (13), 51 (14).
2-Methoxyisophthalic Acid (=2-Methoxybenzene-1,3-dicarboxylic Acid). 2,6-Dimethylanisole (8.8 g,
0.056 mol) was dissolved in 120 ml of H2O and heated to 608 with mechanical stirring. KMnO4 (43 g, 0.273
mol, 4.2 equiv.) was added in two portions. The first portion was added, and, when the violet colour dis-
appeared, the rest was poured into the soln. The soln. was left overnight at 608 and filtered while still
warm. The black precipitate (MnO2) was washed with hot H2O. The filtrate was concentrated to ca.
100 ml, then neutralized with HCl 4M to pH 3.3. A precipitate formed, and it was filtered and dried:
5.72 g (0.029 mol, 52%). The product is soluble in MeOH and H2O, insoluble in CH2Cl2. M.p. 1848.
IR: 3100–2400vs (br.), 1669vs (br.), 1583s, 1504m, 1468s, 1400s, 1296vs, 1240vs, 1168m, 1139m, 1092s,
1
1000s, 955m, 884m, 846m, 781m, 713m, 683vs. H-NMR (CD
3A
(t, 1 arom. H); 7.86–7.89 (d, 2 arom. H). EI-MS: 196 (22, M+), 179 (15), 178 (83), 166 (12), 165 (65),

Helvetica Chimica Acta – Vol. 89 (2006)
849
150 (16), 149 (100), 148 (38), 147 (67), 138 (18), 133 (17), 132 (51), 122 (15), 121 (39), 120 (82), 119 (45),
105 (41), 104 (23), 93 (10), 92 (34), 91 (30), 79 (23), 77 (30), 76 (32), 75 (15), 74 (16), 65 (51), 64 (17), 63
(57), 62 (24), 53 (30), 52 (27), 50 (24), 45 (17).
N,N’-Dimethyl-N,N-bis(2-nitrophenyl)benzene-1,3-dicarboxamide. 2-Methoxyisophthalic acid (1.6 g,
0.008 mol) was poured into 150 ml of CH2Cl2, the soln. was dried with CaH2, and a few drops of DMF
were added to the soln. SOCl2 (11 ml; 0.15 mol, 20 equiv.) was mixed with the soln. It was allowed to
reflux protected by a tube of CaCl2 for 90 min, until the product had completely dissolved. The solvent,
the HCl, and the SOCl2 which had not reacted were evaporated, and the flask was dried for 1 h under
vacuum. The resulting solid was slightly beige. The acid chloride is not very stable, and the synthesis
was continued without its isolation.
A soln. of 2.5 equiv. of N-methyl-2-nitroaniline (3 g, 0.02 mol) and 8 equiv. of Et3ACHTRENUNG (8.9 ml, 0.064
mol) was prepared in 30 ml of dried CH2Cl2. The acid chloride was dissolved in 150 ml of dried
CH2Cl2 and added dropwise into the first soln. The addition must last at least 15 min. The soln. was
then allowed to stay 15 h at r.t. with stirring. The solvent was evaporated, and the solid was put into a
mixture of 300 ml of CH2Cl2 and 200 ml of a aq. sol. of half-sat. NH4Cl. The aqueous phase was washed
with two 100 ml portions of CH2Cl2. The org. phases were assembled, dried (Na2SO4), and then the sol-
vent was evaporated. The solid was purified by CC (SiO2; initially 3% hexane/97% CH2Cl2 then 100%
CH2Cl2, and finally 5% MeOH/95% CH2Cl2): 2.07 g (4.46 mmol, 56%) of the product. M.p. 92–958.
IR: 3075w, 2950s, 1727s, 1657vs (br.), 1601s, 1526vs, 1464s, 1417s, 1350s, 1292m, 1237m, 1140m, 1079m,
1043m, 998m, 851m, 837m, 816m, 784m, 756m, 705m. 1H-NMR (CDCl3, 300 MHz): 2.7 (s); 3.2 (s);
3.25 (s); 3.32 (s); 3.37 (s); 3.45 (s); 3.53 (s); 3.85 (s); 3.95–3.99 (dd); 7.0–7.12 (m, 1 H); 7.3–7.53 (m, 7
H); 7.69–7.74 (m, 1 H); 7.89 (m, 1 H); 8.02–8.05 (m, 1 H). The NMR spectrum of this compound
shows a series of ss between 2.7 and 3.99 ppm because of blocked rotation around the OCꢀN,
C(aryl)ꢀCO, and NꢀC(aryl)NO2 bonds at r.t. ES-MS: 487 (68, M+ +23 (Na)), 146 (100).
2-Methoxy-1,3-bis(1-methyl-1H-benzimidazol-2-yl)benzene. N,N’-Dimethyl-N,N’-bis-(2-nitrophe-
nyl)benzene-1,3-dicarboxamide (1.4 g, 3 mmol) was dissolved in a soln. containing 600 ml of EtOH,
150 ml of H2O, and 18.9 ml of HCl (37%; 0.226 mol, 75 equiv.). Activated Fe (30 equiv., 5.05 g, 0.09
mol) were then poured into the mixture. The soln. was left in an atmosphere of N2 and allowed to reflux
for 20 h. The soln. was colourless after 1 h, but, after 20 h, it became red brown, and no more iron powder
remained in suspension. H2O (120 ml) was added to the soln., and EtOH was then evaporated. The soln.
was poured into 600 ml of CH2Cl2, then 30 g of [Na2H2(edta)] dissolved in 120 ml of H2O was mixed in.
The pH was brought to 8.5 with NH4OH (12%). [Na2H2(edta)] complexes FeIII more strongly than FeII.
Therefore, 6 ml of H2O2 (30%) were added, under vigorous stirring, to oxidize the remaining FeII. After
15 min of stirring, the org. phase was collected, and the aq. phase was washed twice with 300 ml of
CH2Cl2. The product was purified by CC (SiO2; CH2Cl2/MeOH: 100% ! 93%/0% ! 7% eluent: 0.55
g (1.5 mmol, 50%) of the product. M.p. 1858. IR: 3060m, 3050m, 2945m, 1668m, 1651m, 1610m,
1600m, 1506m, 1458vs, 1441s, 1426s, 1413vs, 1386vs, 1325s, 1282s, 1254m, 1234m, 1177w, 1155w, 1127w,
1
1098w, 1081m, 1054m, 1019w, 996s, 960w, 903w, 869m, 819m, 748vs. H-NMR (CDCl3, 300 MHz): 3.26
(s, MeO); 3.78 (s, 2 MeN); 7.34–7.40 (m, 3 H); 7.43–7.47 (m, 3 H); 7.79–7.82 (dd, 3J=7.65, 2 H);
7.85–7.88 (m, 3 H). EI-MS: 369 (20, [M+1]+), 368 (91), 367 (100), 353 (37), 351 (28), 337 (21), 207
(16), 184 (26), 77 (21).
2,6-Bis[1-methyl-1H-benzimidazol-2-yl]phenol (2). 2-Methoxy-1,3-bis(1-methyl-1H-benzimidazol-
2-yl)benzene (0.5 g, 1.3 mmol) was dissolved in 200 ml of dry CH2Cl2 (distilled on CaH2). The flask
was put into ice, and 5 equiv. (6.5 ml) of BBr3 were added to the mixture. The flask was kept in ice
until it melted and was left 15 h at r.t. under N2 while being stirred magnetically. The BBr3 was then
hydrolyzed with 100 ml of MeOH, the soln. stirred for 10 min, the solvent was evaporated, and the
solid was dried 1 h in low vacuum. The solid was mixed into 100 ml of H2O, as a suspension, and the
pH was neutralized to 6 with 2M NaOH. The precipitate was filtered, and the aq. phase was washed
with CH2Cl2. The org. phase was dried, and the solvent was evaporated. The beige solid was purified
by CC (SiO2; 99.5% CH2Cl2/0.5% MeOH). 0.30 g (0.87 mmol, 67%) of 2. The compound is soluble in
CH2Cl2, DMF, CHCl3, MeOH, EtOH, MeCN; insoluble in Et2CAHTREOUNG , hexane, H2O. M.p. >1908. UV
(DMF): 324 (20400), 284 (21500). UV (EtOH): 283 (21000), 324 (18500). IR: 3473–3418m (br.),
3062m, 2929m, 2875m, 2366m, 1629m, 1515m, 1501m, 1490vs, 1430m, 1407s, 1385s, 1324m, 1270s,

850
Helvetica Chimica Acta – Vol. 89 (2006)
1260s, 1151m, 1118m, 1097m (sh), 1055m, 1003m, 931w, 864m, 799m, 624m, 604m. 1H-NMR (CDCl3, 300
MHz): 3.98 (s, 2 Me); 7.14–7.18 (t, 3J=7.7, 1 H); 7.31–7.39 (m, 4 H); 7.44–7.47 (dd, 3J=7.08, 4J=1.7, 2
3
3
H); 7.79–7.81 (dd, J=6.75, 2 H); 7.83–7.86 (d, J=7.8, 2 H). EI-MS: 353 (100), 337 (10), 250 (8), 207
(10), 193 (10), 177 (30), 133 (15), 104 (15), 92 (13), 90 (13), 77 (100), 65 (26), 63 (30), 51 (66).
2-Phenyl-1H-benzimidazole. Benzene-1,2-diamine (2.03 g, 0.018 mol) and 3.31 g (0.027 mol) of
PhCOOH were mixed in 50 ml of H3PO4 (85%). The soln. was heated 2 h at 908 and then 4 h at 2008
under mechanical stirring. The mixture was poured into 200 ml of H2O and neutralized to pH 6 with
2M NaOH. The precipitate was filtered and suspended in 200 ml of an aq. soln. containing 10% of
Na2CO3. The suspension was heated to 608 for 1 h under magnetic stirring. The pink solid was then
dried under vacuum at 908. It was dissolved in MeOH and treated with charcoal for 1 h. The solid was
then recrystallized in MeOH: 2.44 g (0.0125 mol, 70%) of the product. M.p. >1908. 1H-NMR (CD
3ACHTREOUNG D,
300 MHz): 7.2–8.1 (m, 9 H). EI-MS :194 (100, M+), 97 (100), 90 (10), 77 (100), 63 (10).
1-Methyl-2-phenyl-1H-benzimidazole. 2-Phenyl-1H-benzimidazole (1.12 g, 5.7 mmol) was dissolved
in 50 ml of dry DMF (distilled over CaH2). The flask was then put into ice before 492 mg (ca. 10 mmol) of
NaH (50–60%) were added to the soln. The mixture was allowed to return to r.t. under magnetic stirring.
It stood for 1 h before 0.55 ml (8.8 mmol) of MeI was added. The soln. was kept under magnetic stirring at
r.t. for one night. During all the manipulations, the flask was kept under N2. The solvent was then evapo-
rated, the solid was dried and purified by CC (SiO2; CH2Cl2/MeOH 97:3). Yield: 60%. Soluble in MeOH,
EtOH, DMF, MeCN, acetone, CH2Cl2, CHCl3, and toluene; insoluble in Et2ACHTEROUNG , H2O, and hexane. M.p.
92–938. UV/VIS (DMF): 286 (15300). IR: 3060m, 2947m, 1943w, 1901w, 1777w, 1680w, 1610m, 1523m,
1466s, 1440s, 1380vs, 1327s, 1275m, 1250w, 1240m, 1177w, 1155m, 1126m, 1100m, 1076m, 1058m,
1020m, 1002m, 973w, 927m, 876w, 850w, 817m, 778s, 754vs, 700s, 679m, 540m, 484m. 1H-NMR (CD
3ACHTREOUNG D,
300 MHz): 3.88 (s, Me); 7.38–7.27 (m, 2 arom. H); 7.54–7.61 (m, 4 arom. H); 7.66–7.69 (m); 7.76–7.79
(m, 2 arom. H). EI-MS: 208 (75, M+), 207 (100), 129 (6), 104 (19), 90 (16), 77 (22), 63 (9), 51 (13).
2-(1H-Benzimidazolyl-2-yl)phenol. Benzene-1,2-diamine (4 g, 0.037 mol) and 5.5 g (0.04 mol) of sal-
icylic acid were dissolved in 50 ml of H3PO4 (85%). The salicylic acid sublimes below 1408; therefore, a
slight excess is needed. The soln. was heated 1 h at 1808 and 2 h at 2308. It was then allowed to cool and
poured into 200 ml of H2O. It was neutralized to pH 6 with 5M NaOH. The black precipitate was filtered
and suspended in 100 ml of an aq. soln. containing 40% Na2CO3. The mixture was heated at 608 for 1 h.
The solid was then treated with charcoal in MeOH and recrystallized from a mixture of EtOH and H2O.
Yield: 12%. M.p. >1808. IR: 3260m, 2918m, 1628w, 1588m, 1532m 1513s, 1486s, 1458s, 1434s, 1414m,
1395s, 1368m, 1317m, 1294m, 1276s, 1257s, 1223m, 1188m, 1157m, 1142m, 1129m, 1112m, 1081m,
1036w, 1003w, 960w, 904w, 839m, 820m, 798s, 738vs, 697m. 1H-NMR (CD
3ACHTREOUNG D, 300 MHz): 6.93–7.02
(m, 2 H); 7.22–7.27 (m, 2 H); 7.3–7.35 (m, 1 H); 7.57–7.69 (m, 2 H); 7.82–7.89 (dd, 1 H). EI-MS: 210
(100, M+), 182 (52), 156 (10), 143 (8), 129 (4), 105 (9), 91 (28), 68 (20), 65 (24), 63 (20), 51 (12).
1-Methoxy-2-(1-Methyl-1H-benzimidazol-2-yl)benzene. 2-(1H-Benzimidazol-2-yl)phenol (0.5 g, 2.38
mmol) was dissolved in 40 ml of dry DMF (distilled over CaH2). The flask was put into ice and 0.21 g (ca.
5 mmol) of NaH (50–60%) were added. The soln. was allowed to return to r.t. under magnetic stirring.
After 1 h, the soln. was again put into ice, and 0.74 g (5 mmol) of MeI was added. The mixture warmed up
to r.t. and left for a further 2 h under magnetic stirring. The product was purified by CC (SiO2; initially
100% of CH2Cl2 and then with CH2Cl2/MeOH 99.5:0.5). Yield: 58%. M.p. 98–1008. IR: 3381s, 3062s,
2945s, 2845s, 1647m, 1602s, 1577s, 1552m, 1520s, 1485s, 1457s, 1430s, 1380vs, 1324s, 1276s, 1252vs,
1235s, 1177s, 1149m, 1130m, 1120m, 1094s, 1053s, 1022vs, 1004m, 969m, 951m, 928m, 823m, 799m,
1
781s, 750vs. H-NMR (CDCl3, 300 MHz): 3.66 (s, MeN); 3.88 (s, MeO); 7.05–7.l (d, 1 H); 7.29–7.33
(m, 2 H); 7.39–7.42 (m, 1 H); 7.48–7.51 (m, 1 H); 7.57–7.6 (dd, 1 H); 7.81–7.84 (m, 1 H). EI-MS: 238
(95, M+), 237 (73), 209 (14), 208 (17), 207 (100), 77 (24), 51 (13).
2-(1-Methyl-1H-benzimidazol-2-yl)phenol. 1-Methoxy-2-(1-methyl-1H-benzimidazol-2-yl)benzene
(0.338 g, 1.4 mmol) was dissolved in 40 ml of dry CH2Cl2 (distilled over CaH2). The flask was put into
ice, and 7 ml (7 mmol) of a soln. of BBr3 1M in CH2Cl2 were added dropwise to the soln. The flask was
left, under magnetic stirring, until the ice melted and the H2O warmed to r.t. It was kept under magnetic
stirring over night. The excess of BBr3 was first hydrolyzed with MeOH (puriss.) and stirred for 10 min.
The solvent was then evaporated, and the solid was dried 1 h under vacuum. H2O (60 ml) was then
poured onto the solid, and the mixture was neutralized to pH 6 with 2M NaOH. The product was then

Helvetica Chimica Acta – Vol. 89 (2006)
851
extracted from this aq. soln. with CH2Cl2. The solid was purified by CC (SiO2; 100% with CH2Cl2 at first
until a first product eluted, then CH2Cl2/MeOH increasing gradually from 100 :0 to 90 :10, until the prod-
uct came out: 78 mg (0.348 mmol, 25%) of the product. M.p. 157–1588. UV/VIS (DMF): 286 (17050), 324
(15600). IR: 2552s (br.), 2360s (br.), 1825s, 1729m, 1691m, 1680m, 1632m, 1610s, 1586s, 1555m, 1518s,
1502m, 1453vs, 1383vs, 1326s, 1292s, 1265s, 1236s, 1162m, 1129m, 1112m, 1099m, 1060m, 1030m,
1
1004m, 969w, 942m, 908m, 838w, 801m, 764m, 752vs, 667w. H-NMR (CDCl3, 300 MHz): 4.07 (s, Me);
3
6.96–7.01 (t, J=7.17, 1 H); 7.34–7.42 (m, 6 H); 7.72–7.79 (m, 2 H). EI-MS: 225 (16, [M+1]+, ), 224
(96 M+), 223 (100), 222 (11), 208 (9), 207 (55), 195 (15), 169 (9), 78 (10), 77 (20), 63 (10), 51 (14).
Synthesis of Complexes. Bis[2,6-bis(1-methyl-1H-benzimidazol-2-yl)phenolate]dicopper(II) p-Tolu-
ACHTREUNG
CTHREUNG
CTHREUNG
precipitate were formed on the bottom of the flask (0.011 mmol, 46%), the soln. was colorless. The pre-
cipitate was dried 5 h at 408 under vacuum. UV/VIS (DMF): 286 (26000), 353 (16000), 755 (108). UV/VIS
(EtOH): 285 (26500), 346 (17000), 726 (180). IR: 3364m, 3057m, 2952m, 1675m, 1609m, 1569m, 1521m,
1479s, 1421s, 1280s, 1213s, 1185s, 1118s, 1087m, 1030m, 1008s, 931w, 875w, 859w, 819w, 749s, 712w, 678s.
1H-NMR ((D7)DMF, 400 MHz; 293 K): 3.74 (s); 3.95 (s); 6.56 (s); 6.91 (s); 7.25 (s); 7.33 (s); 9 (v. br.);
15.2 (s); 20.5 (s). All peaks are broadened by paramagnetic relaxation, and the feature around 9 ppm cor-
responds to several signals. ES-MS (in DMF): 1187.5 (22), 1185.5 (18, [Cu2(2-H)
H2O]+), 1004.9 (80), 1002.9 (74, [Cu2(2-H) (TsO)]+), 879 (14), 877 (13), 490 (35) 489 (52, [Cu2(2-H)2-
2A
(DMF)2]2+), 453.9 (63), 452.9 (70, [Cu2(2-H) DMF]2+), 417.5 (5), 416.3 (20, [Cu2(2-H)2]2+). Anal. calc.
2A
2ACHTRE(UGN TsO)·2 DMF·2
CTHREUNG
A
CHTREUNG
for [Cu2(2-H)2](TsO)2 ·3 H2O (C58Cu2H54N8O11S2; 1230.32): Cu10.33, C 56.62, H 4.42, N 9.10, S 5.21;
found: Cu 10.12, C 56.80, H. 4.49, N 9.12, S 5.05.
The compound could also be synthesized from ligand 1. 1,3-Bis-(1-methyl-1H-benzimidazol-2-
yl)benzene (1; 170 mg, 0.5 mmol) and Cu(ClO4)2 ·6 H2O (180 mg, 0.5 mmol) were dissolved in DMF
(20 ml). The flask was filled with O2 and heated to 808 for 2 d. The soln. turned from pale green to
dark green and the perchlorate salt bis[2,6-bis[1-methyl-1H-benzimidazol-2-yl)phenolate]bis(dimethyl-
formamide)dicopper(II) diperchlorate dimethylformamide solvate could be isolated as dark green crys-
tals by slow diffusion of Et
wise addition, under stirring, of 250 ml of H2O2 (30%), followed by the addition of 50 ml of Et
aliquots. The formation of [Cu2(2-H)2]2+ was established by the UV/VIS and ES mass spectra.
O vapor. Synthesis using H2O2 replaced the saturation with O2 by the drop-
CTHREUNG
Oxygen-uptake experiments were performed in a thermostated room. The apparatus consisted of a
flask containing DMF and a magnetic stirrer, solid [Cu2(1)2](ClO4)2 in a sidearm, a 20-ml burette (Met-
rohm), and a U-tube manometer. The apparatus was flushed with O2 and closed. The reaction was started
by tipping the [Cu2(1)2](ClO4)2 into the DMF and stirring. As the reaction progressed, the pressure was
maintained constant (as measured by the manometer) by advancing the piston of the burette. The soln.,
initially colourless, turned light green within a few min. Absorption was essentially complete within 1 h,
and the volume of O2 absorbed corresponded to that required to oxidize the CuI into CuII. The volume
required to form [Cu2(2-H)2]2+ would be three times larger, and the formation of this complex could
equally be ruled out by the UV/VIS spectrum.
X-Ray Crystal-Structure Determination: Bis[2,6-bis(1-methyl-1H-benzimidazol-2-yl)phenolate]bis-
(dimethylformamide)dicopper(II) Diperchlorate Dimethylformamide Solvate. Crystals were obtained
by vapor diffusion of Et2CATHEROUGN into a DMF soln., Cu2(C22H17N4O)2AHCERT(NUG ClO4)2ACHTRE(NUG C3H7NO)6, MR 735.8, dark
green crystal, 0.154”0.21”0.26 mm, monoclinic, P21/c, a=13.0308(9), b=15.7813(12), c=15.9187(11)
Š, b=100.112(8)8, V=3222.7(4) Š3, Z=2, m=0.823 mmꢀ1 (Tmin =0.8115, Tmax =0.8950), Dx =1.516 g
cmꢀ3. Intensities were measured at 200 K on a Stoe IPDS diffractometer with MoKa radiation
(l=0.7107 Š) and corrected for absorption. 37210 reflections were measured of which 6529 were unique
(Rint =0.098). The structure was solved by direct methods using MULTAN [31], and all other calculations
were performed with XTAL system [32] and ORTEP [33] programs. Full-matrix least-squares refinement
based on |F| using weight of 1/(s2(Fo)+0.00015(F2o)) gave final values R=0.042, wR=0.38, and
S=1.18(1) for 449 variables and 3403 contributing reflections. The H-atoms were placed in calculated
positions.

852
Helvetica Chimica Acta – Vol. 89 (2006)
CCDC-284148 contains supplementary crystallographic data for this paper. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +441223336033; e-
mail: deposit@ccdc.cam.ac.uk)).
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Received February 9, 2006