Synthesis of tripodal phenol porphyrins: A modular approach to mimic enzymes with a cross-linked tyrosine

Article abstract of DOI:10.1055/s-2004-825589

The synthesis of various tren [tris(2-aminoethyl)amine]-capped porphyrins incorporating a substituted phenol into different positions is reported. The conformations of the different ligands have been studied in solution by means of NMR spectroscopy.

Full text of DOI:10.1055/s-2004-825589

1
158
LETTER
Synthesis of Tripodal Phenol Porphyrins: A Modular Approach to Mimic
Enzymes with a Cross-Linked Tyrosine
S
D
ynthesis of
N
ew T
a
r
ipodal Ph
v
enol Porphyr
i
ins d Gueyrard, Amandine Didier, Christian Ruzié, Arnaud Bondon, Bernard Boitrel*
Université de Rennes1, UMR CNRS 6509, Institut de Chimie, 35042 Rennes Cedex, France
Fax +33(2)23235637; E-mail: Bernard.Boitrel@univ-rennes1.fr
Received 11 March 2004
Abstract: The synthesis of various tren [tris(2-aminoethyl)amine]-
capped porphyrins incorporating a substituted phenol into different
positions is reported. The conformations of the different ligands
have been studied in solution by means of NMR spectroscopy.
Key words: porphyrins, spectroscopy, biomimetic synthesis,
phenols, enzymes
The exergonic slow reductive cleavage of molecular oxy-
gen through the four electron-four proton mechanism is
1
essential for aerobic life on Earth. This reduction results
in the synthesis of ATP from ADP and inorganic phos-
phorus, the electrons flowing from NADH to O2.2 It is
catalyzed by cytochrome c oxidase (CcO), a copper he-
moprotein whose structure has been initially resolved by
,3
4
5
X-ray in 1995 and later refined. The active site of the en-
zyme consists of two major components, a five-coordinat-
ed iron(II) porphyrin and a copper(I) complexed by three
histidine residues, by which most of the synthetic models
Figure 1 Active site of CcO.
posttranslationally modified tyrosines in nature. Indeed,
in CcO, Tyr244 is cross-linked to one of the copper-ligated
histidines, His240, indicative of radical chemistry at the
6
are inspired. Accordingly, elucidating the fundamental
aspects of dioxygen binding with heme and copper centers
through the design and synthesis of functional analogs of
CcO is crucial. In this respect, numbers of biomimetic
models cleverly designed and skillfully made, have been
reported, all of them having two proximal metal coordina-
tion sites favoring the binding of an O molecule. We
1
3
active site. This covalent linkage is expected to perturb
both the redox potential of the tyrosyl radical (Y)14 and
pK of the phenol/phenolate as occurs for the redox-ac-
a
tive, covalently cross-linked tyrosine in galactose oxidase
15
7
(GO). Nevertheless, in GO, the tyrosine is cross-linked
with a cysteine residue and coordinates the copper, where-
2
8
have developed a series of ligands, which resulted in the
as it does not in CcO. The significant decrease of pK for
a
initiation and the verification of the idea that iron-only
porphyrins can catalyze the clean reduction of dioxygen to
water, when adsorbed at the surface of an electrode at
physiological pH. In fact, this crucial observation, initial-
ly controversial, but later confirmed and clearly ex-
plained, demonstrates that the postulated m-peroxo
complex is not an essential intermediate for the four-elec-
tron reduction of oxygen, as long as electrons and protons
can be rapidly delivered to the catalytic site of a synthetic
ligands able to mimic the Cu coordination sphere has
B
1
6
been reproduced but so far, no synthetic model of CcO
bearing a mimic of the triad heme a -Cu -Tyr244 has been
9
3
B
designed and prepared. This observation could stem from
both the synthetic difficulties and the complicated confor-
mational control of the complete assembly, particularly
for the orientation of the phenol group, as depicted in
Figure 1 for the enzyme. Accordingly, the availability of
copper-coordinating molecules bearing a mimic of cross-
linked tyrosines, with a tunable pK and for which the co-
ordination of the mimic to the copper cation could be al-
lowed or not, becomes an exciting challenge.
1
0
1
1
model.
a
Our conclusion, in agreement with recent spectroscopic
results, is also supported by Yoshikawa et al. who sug-
gested that the peroxo-derivative could be protonated by
the low pK Tyr244, located in the closed environment of
the coordination sphere of copper, forming a hydroperoxo
1
2
In our ongoing projects devoted to the understanding of
enzymatic catalytic cycles, we herein report the design
and preparation of new ligands as potential models of en-
zymes that include a cross-linked tyrosine mimic such as
CcO or GO, as well as the first insights of their conforma-
tional study.
a
5
adduct. Besides, this Tyr represents an example of the
2
44
SYNLETT 2004, No. 7, pp 1158–1162
0
3.
06.
2
0
0
4
Advanced online publication: 10.05.2004
DOI: 10.1055/s-2004-825589; Art ID: G07604ST
©
Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of New Tripodal Phenol Porphyrins
1159
NH2
H2N
the hydroxy group is in meta-position to the amide linkage
position 6 in Scheme 1), this coordination should be
possible.
(
H2N
N
NH2
NH
HN
N
Additionally, the two aromatic rings of the picket, conju-
gated through the amide bond on the porphyrin, are ex-
pected not to be coplanar as previously reported but to be
TAPP
i
O
O
NH2
20
NH
HN
H2N
twisted with a angle ranging from ca. 20 ° to 40 °. Ac-
O
NH
H2N
cordingly, the OH group should be slightly displaced on
the side of the porphyrin as the Tyr244 OH is within the en-
zyme, maintained by a hydrogen bond with the farnesyl,
instead of pointing straight to the center of the heme
(Figure 1).
N
ii
N
NH
NH2
NH
NHTr
HN
HN
N
N
1
(50%)
iii -> 2
TAPPTr (41%)
iv -> 3
R
R
3
Thus, two new ligands have been prepared in which the
phenol should not be able to coordinate the copper cation,
once this one inserted in the tripodal cap, and so, these
ligands appear to be potential synthetic models of CcO.
They are closely related to one another as they only differ
by the nature of their link to the meso aromatic ring of the
porphyrin. As illustrated in Scheme 1, this link is benzylic
for 4 and amidic in the case of 5. We believe that this vari-
ation could have a significant influence on the spatial ar-
rangment of the OH function. The synthesis of these two
ligands begins with the preparation of porphyrin 1 bearing
5
N
6
NH
HN
NH
HO
HN
O
O
X
O
NH
NH
HO
HN
O
NH
NH
O
X
O
N
N
NH
NH
v
NH
NH
HN
HN
N
N
2
3
X = CH (85%)
4 X = CH2 (26%)
5 X = CO (49%)
2
X = CO (52%)
Scheme 1 Synthesis of tren-capped phenol porphyrins. R = NO2;
Reagents: i) trityl bromide, THF, Et N, 0 °C; ii) CH =CHCOCl, THF,
Et N, –15 °C then TFA; iii) 2-hydroxy-5-nitrobenzaldehyde,
NaBH CN, TFA, MeCN, r.t.; iv) 2-hydroxy-5-nitrobenzoic acid,
DCC, DMAP, pyridine, r.t.; v) tren, MeOH/CHCl , reflux.
3
2
2
1
three acrylamide pickets and one amino group, which is
allowed to react either with 2-hydroxy-5-nitrobenzalde-
hyde in a reductive amination reaction or with 2-hydroxy-
3
3
3
22
5
-nitrobenzoic acid activated by DCC in pyridine. The
These new molecules have the obvious advantage of final capping reaction with tren reacting on the three
being closely related to our previously studied iron-cop- acrylamide pickets is a common step for both porphyrins
1
7
per analogs. Recently, two pertinent examples of the to afford compounds 4 and 5.
His -Tyr-Cu motif have been published and their crystal
3
On the other hand, when 2-hydroxy-5-nitrobenzyl bro-
mide was grafted on porphyrin 6, a second series of
ligands was obtained. Depending on the ratio of benzyl
bromide reagent vs. porphyrin, two porphyrins 7 and 8
were easily prepared with acceptable yields. However, in
these two ligands, according to molecular mechanic mod-
elling, the substituted phenol could coordinate the copper
atom, once complexed in the tren because there is no
means to control the spatial position of the phenol, that is
1
8
structures reported. Unfortunately, their assembly with a
porphyrin remains a difficult task to be realized. Further-
more, on observing the active site of CcO in Figure 1, it
appears that both metals (Cu and Fe ) are separated from
B
a3
the tyrosine hydroxyl by the same distance of 5.7 Å. On
the other hand, Cu and the Tyr hydroxyl are diametri-
B
244
cally opposed relative to the iron, and this spatial arrange-
ment is quite difficult to reproduce.
To do so, we chose the methodology that consists of teth- presumably in free rotation around the tripodal cap. As al-
ering a phenol moiety bearing, for instance, an electron- ready mentioned, such a coordination of a phenolate resi-
withdrawing group in the para-position as described in due on copper exists in GO. In this topology, the
Scheme 1. Indeed, if we were able to control the orienta- porphyrin, with a redox and coordination inactive metal
tion of the hydroxyl group vs. the center of the cavity built such as nickel(II), could be employed only as a structural
by the tripodal architecture, this would provide a conve- framework, thereby providing a convenient opportunity
nient way of organizing the surroundings of both metals. of studing the reactivity of copper(I) –phenolate complex-
Moreover, two other structural features should be influ- es, for which no intermolecular assembly will be possible.
enced by this stratagem. Firstly, owing to the substitution To obtain a ligand in which the conformation of the nitro-
pattern of the porphyrin itself, the nitro phenol is attached phenol is dictated by the superstructure, we also tried to
to the macrocycle by meso-position 20 where the tripodal prepare a molecule that consists of the same type of
cap is off-center of diametrally opposed meso-position 5. substituted phenol (R = NO in the present example) but
2
Secondly, the further coordination of the copper cation by attached on the tren motif by two points (Scheme 2).
the phenol (or phenoxide) could be manipulated by the
Indeed, the nitrophenol would be strapped between the
two secondary amines of porphyrin 6 with the OH group
pointing directly toward the center of the cavity. Unfortu-
nately, up to now, this synthesis failed even employing
conditions as severe as heating porphyrin 6 with 2,6-di-
position of the hydroxy group on the aromatic picket. Our
1
9
molecular mechanic modelling studies indicate that this
aromatic picket would have to come in close contact with
the tren to allow the hydroxy group, in ortho-position to
the amide linkage, to coordinate. On the other hand, when
Synlett 2004, No. 7, 1158–1162 © Thieme Stuttgart · New York

1
160
D. Gueyrard et al.
LETTER
particularly important for porphyrins 4 and 5, for which
the steric hindrance of the tripodal cap could eject the OH
to the outside of the cavity. To perform a comparison be-
tween the different superstructures, including compounds
N
NH
N
HN
O
O
NH
HN
O
O
7 and 8, all the spectra were recorded in DMSO-d at a
6
NH
N
NH
NH
HN
temperature between 333 K and 363 K, depending on the
N
best resolution obtained on a 500 MHz spectrometer. For
6
(59%)
instance, in the porphyrins 7 and 8, the proton H is found
6
i
to be shifted upfield by an average value of d = 0.4 ppm in
comparison with 2-hydroxy-5-nitrobenzyl bromide. This
slight shift is due to the lateral position of the aromatic
N
N
N
N
3
ring attached to the tren motif. Moreover, H experiences
N
N
3
N
HN
O
OH
R
O
O
O
OH
the same upfield shift, indicating that in fact, the benzyl
residue is in free rotation with no privileged conforma-
tion. The same comparison is more instructive in the case
of porphyrins 4 and 5. Indeed, in comparison with the raw
NH
5
NH
HN
6
O
HN
O
O
O
NH
NH
N
N
N
NH R
NH
NH
NH
HN
HN
N
ii
nitrophenol compound, H is shifted upfield by d = 0.7
6
7
(66%)
ppm and 1.8 ppm in 4 and 5, respectively. This observa-
tion is consistent with the fact that H is oriented closer to
the center of the cavity in 5 than in 4. Additionally, the
6
N
N
3
N
3
N
O
HO
R
R
chemical shift of H corroborates this observation as this
O
3
OH
5
6
NH
HN
5
6
proton, maintained in the periphery of the porphyrin, is
not – or almost not – shifted upfield in porphyrins 5 and 4.
This first approach has been confirmed by the analysis of
O
O
NH
N
NH
NH
HN
N
2
D ROESY spectra, from which more accurate informa-
8
(56%)
tion can be obtained.
Scheme
2
Synthesis of substituted tren-capped porphyrins.
R = NO ; Reagents: 2-bromomethyl-4-nitrophenol (i: 1.1 equiv; ii:
b
2
2
.2 equiv), Et N, THF, 60 °C.
3
e
f
d
O
N a c
a
l
N
b
j
N
bromomethyl-4-nitrophenol in DMF at 100 °C, but re-
mains an interesting target.
N
O
O
g
d
c
6
k
O
i
N
h
N
O
3
Nevertheless, these properties need to be confirmed
through studies of the coordination chemistry of these
new molecules. In a first approach, their conformations in
solution have been scrutinized by proton NMR spectros-
copy. A simple investigation consists of comparing the
chemical shift of representative protons for both the raw
nitrophenol derivative and the porphyrin (Table 1).
j
i
N
l
k
O
O
N
N
N
N
N
N
Table 1 Chemical Shifts of the Phenol Picket Protons
Figure 2 Detailed labeling of the aliphatic protons of 5 (for clarity,
multiple bonds are omitted and protons exhibiting a ROE with H are
circled).
H (ppm) H (ppm) H (ppm)
6
3
5
6
2
5
2
4
7
8
-Hydroxy-5-nitrobenzoic acid
8.55
8.33
7.15
8.56
7.39
8.10
7.74
7.90
7.77
5.38
7.02
6.33
6.77
6.37
Indeed, for porphyrins 4 and 5, if Figure 2 is representa-
tive of the actual conformation in solution, H should in-
6
-Hydroxy-5-nitrobenzyl bromide 8.34
duce strong ROE on the protons of the cap. As a matter of
fact, for porphyrins 4 and 8, there are only a few cross
peaks in the spectra (data not shown) without any correla-
tion between the protons of the phenyl ring bearing the hy-
droxy- and the nitro groups and the cap protons. Such an
absence is in good agreement with a free rotation of this
moiety. In contrast, with the more constrained porphyrin
8.02
7.84
7.59
Obviously, the phenolic proton would be a good candidate
but is not a reliable probe as it is mobile, exchangeable
and temperature dependent. On the other hand, the chem-
ical shift of a proton such as H in the ortho-position of the
hydroxy function, is indicative of its location in the shield-
ing ring current of the macrocycle. This information is
5
a ROESY spectrum is obtained displaying several cross
peaks (Figure 3). A more complete description of the
molecular structure requires the full assignment of the
protons, which has been performed using the combination
of standard 2D experiments such as COSY, TOCSY,
6
Synlett 2004, No. 7, 1158–1162 © Thieme Stuttgart · New York

LETTER
Synthesis of New Tripodal Phenol Porphyrins
1161
heteronuclear H-C HSQC. The first comment concerns desired product was eluted with 0.6% MeOH/CH
the high symmetry of the molecule as shown by the equiv-
Cl . After evapo-
2
ration to dryness, 330 mg of 1 were collected (yield = 50%).
2
alence of the two branches of the tren surrounding the sub- In a 100 mL round bottom flask under argon, 80 mg (0.096 mmol)
of 1, DCC (39 mg, 0.191 mmol), DMAP (1 mg, 0.01 mmol) and 2-
hydroxy-5-nitrobenzoic acid (35 mg, 0.191 mmol) were dissolved
stituted phenol. Also remarkable is the strong high field
shift of all the resonances of the methylene protons of the
in 10 mL of freshly distilled pyridine. Stirring was maintained over-
tren. This shielding must be related to a large movement
night and then, the mixture was dried under vacuum. The product
of the tren toward the porphyrin plane. Finally, evidence
for a major conformation with the hydroxy pointing to-
was dissolved in CHCl and poured onto a 15 mm silica gel column.
3
The desired product was eluted with 0.7% MeOH/CHCl . After
3
ward the center of the macrocycle is given by the labelled evaporation to dryness, 50 mg of 3 were collected (yield = 52%).
strong ROE cross-peaks between the H and four methyl-
ene groups of the tren (see Figure 2 for labelling of the
molecule).
6
In a 250 mL two-neck round bottom flask under argon, 250 mg
0.090 mmol) of 3 were dissolved in 90 mL of a mixture CHCl3–
(
MeOH (1:5) degazed during 2 h. The solution was heated at 55 °C.
With a syringe, 16 mL (0.096 mmol) of tris-2-aminoethylamine
were slowly added. Stirring was maintained during 5 h and then, the
mixture was dried under vacuum. The product was dissolved in
CH Cl and poured onto a 15 mm silica gel column. The desired
2
2
product was eluted with 5–10% MeOH/CH Cl /NH . After evap-
2
2
3 (g)
1
oration to dryness, 50 mg of 5 were collected (yield = 49%). H
NMR (500 MHz, DMSO-d , 363 K): d = 11.03 (broad s, 3 H, NH),
6
8
.75 (d, 4 H, J = 4.5 Hz, H ), 8.73 (d, 2 H, J = 4.8 Hz, H ), 8.71
bpyr bpyr
(
d, 2 H, J = 4.8 Hz, Hbpyr), 8.67 (broad d, 2 H, H ), 8.60 (d, 2 H,
aro
J = 8.0 Hz, H ), 8.56 (d, 1 H, J = 3.2 Hz, H ), 7.83–7.45 (m, 4 H,
aro
3
H ), 7.73 (dd, 1 H, Jo = 7.0 Hz, Jm = 1.5 Hz, Haro), 7.52 (dd, 2 H,
aro
Jo = 7.5 Hz, Jm = 1.8 Hz, Haro), 7.43 (td, 2 H, Jo = 7.5 Hz, Jm = 1.8
Hz, H ), 7.37 (td, 2 H, Jo = 7.5 Hz, Jm = 1.8 Hz, H ), 7.35 (dd, 1
aro
aro
H, Jo = 9.0 Hz, Jm = 3.2 Hz, H ), 5.38 (d, 2 H, J = 9.0 Hz, H ), 2.06
5
6
(
m, 4 H, H /H ), 1.88 (m, 2 H, H ), 1.80 (m, 4 H, H /H ), 1.66 (m, 2
g h l j k
H, H ), 0.62 (t, 2 H, J = 9.0 Hz, H ), –0.06 (t, 2 H, J = 9.0 Hz, H ),
i
f
d
–
–
0.20 (m, 2 H, H ), –0.36 (t, 2 H, J = 9.0 Hz, H ), –1.23 (m, 2 H, H ),
c
e
b
1.46 (m, 2 H, H ), –2.52 (s, 2 H, –NH ). HRMS (ESI-MS): calcd
a
pyr
+
m/z = 1148.4895 for C H N O [M + H] ; found: 1148.4877. UV/
Vis (CHCl /MeOH 10%): l = 423 (e/dm mol cm 323400),
5
6
6
62 13
7
3
–1
–1
3
max
19 (16800), 550 (3900), 590 (4700) and 646 (1900) nm.
Synthesis of 7: In a 100 mL round bottom flask under argon, 200
1
1
mg (0.193 mmol) of 6, 2-bromomethyl-4-nitrophenol (53.6 mg,
.231 mmol) and 0.2 mL of Et N were dissolved in 50 mL of freshly
0
3
distilled THF. The solution was heated at 65 °C overnight and then
dried under vacuum. The product was dissolved in CHCl and
3
Figure 3 500 MHz ROESY spectrum of porphyrin 5.
poured onto a 15 mm silica gel column. Compound 8 was eluted first
with 5.6% MeOH/CHCl then the desired product 7 was eluted with
3
To conclude, four new tripodal phenol porphyrins bearing 6.4% MeOH/CHCl
. After evaporation to dryness, 168 mg of 7
3
1
were collected (yield = 66%). H NMR (500 MHz, DMSO-d , 363
a cross-linked tyrosine mimic at different positions of the
ligand, and with several orientations have been synthe-
sized and studied by NMR spectroscopy. Owing to the
simple design of these compounds, we should be able to
6
K): d = 10.93 (broad s, 1 H, NH), 9.42 (broad s, 1 H), 9.24 (broad s,
1
8
H), 8.90 (d, 1 H, J = 5.0 Hz, H ), 8.89 (d, 1 H, J = 4.5 Hz, H ),
bpyr bpyr
.79 (d, 1 H, J = 5.0 Hz, Hbpyr), 8.76 (d, 1 H, J = 5.0 Hz, Hbpyr), 8.69
(
broad s, 1 H), 8.67 (d, 1 H, J = 4.5 Hz, Hbpyr), 8.63 (d, 1 H, J = 4.5
modulate the position and the conformation of the substi- Hz, Hbpyr), 8.55 (d, 1 H, J = 4.5 Hz, Hbpyr), 8.51 (d, 1 H, J = 4.5 Hz,
tuted phenol moiety. Furthermore, the electron-withdraw- Hbpyr), 8.24 (dd, 1 H, Jo = 7.5 Hz, Jm = 1.5 Hz, H ), 8.01 (d, 1 H,
aro
J = 8.0 Hz, H ), 7.97 (d, 1 H, J = 8.0 Hz, H ), 7.90 (dd, 1 H,
ing NO group was employed, but we are currently
aro
aro
2
Jo = 9.0 Hz, Jm = 3.0 Hz, H ), 7.87 (td, 1 H, Jo = 7.5 Hz, Jm = 1.5
preparing the analogs with an electron-donating OMe
group, as the latter is expected to stabilize a radical in the
para-position without conformational change in the
superstructure.
5
Hz, H ), 7.84 (d, 1 H, J = 3.0 Hz, H ), 7.83–7.75 (overlapping m,
aro
3
6
H, H ), 7.70 (td, 1 H, Jo = 7.5 Hz, Jm = 1.5 Hz, H ), 7.64 (td, 1
a
r
o
a
r
o
H, Jo = 7.5 Hz, Jm = 1.5 Hz, H ), 7.71 (d, 1 H, J = 6.5 Hz, H ),
aro
aro
7
.48 (td, 1 H, Jo = 7.5 Hz, Jm = 1.5 Hz, H ), 7.45 (td, 1 H, Jo = 7.5
aro
Hz, Jm = 1.5 Hz, Haro), 7.41 (td, 1 H, Jo = 7.5 Hz, Jm = 1.5 Hz,
H ), 7.64 (td, 1 H, Jo = 7.5 Hz, Jm = 1.5 Hz, H ), 6.77 (d, 2 H,
Typical Experiments
aro
aro
Synthesis of 5: In a 500 mL round bottom flask under argon, 650
J = 9.0 Hz, H ), 3.18 (s, 2 H, CH
), 2.43 (m, 1 H, -CH -), 2.24
2
3
6
2benz
2
mg (0.708 mmol) of TAPPTr and 1 mL of Et N were dissolved
3
(
m, 1 H, -CH -), 2.10–1.75 (overlapping m, 7 H, -CH -), 1.62 (m, 4
2
2
in 250 mL of freshly distilled THF, then acryloyl chloride (208 mL,
H, -CH -), 1.18 (m, 2 H, -CH -), 0.87 (m, 2 H, -CH -), 0.72 (m, 1 H,
2
2
2
2
–
.55 mmol) dissolved in 20 mL of THF was added dropwise at
15 °C over 10 min. Stirring was maintained during 10 min and
-
CH -), 0.50 (m, 1 H, -CH -), –0.08 (m, 1 H, -CH -), –0.22 (m, 1 H,
2
2
2
-
CH -), –0.48 (m, 2 H, -CH -), –0.58 (m, 1 H, -CH -), –1.14 (m, 1
2
2
2
then, the mixture was dried under vacuum. The residue was dis-
solved in 30 mL of CH Cl , and 3 mL of trifluoroacetic acid was
added. After 2 h, the mixture was washed twice with 10 mL of aq
NaOH (5%). The organic phase was concentrated by rotary evapo-
ration and poured onto a 15 mm silica gel column (3 × 10 cm). The
H, -CH -), –1.20 (m, 1 H, -CH -), –1.63 (m, 1 H, -CH -), –2.45
2
2
2
2
2
(
m, 1 H, -CH -), –2.57 (s, 2 H, -NH ). HRMS (ESI-MS): calcd
2 pyr
+
m/z = 1188.5208 for C H N O [M + H] ; found: 1188.5195 (1
ppm). UV/Vis (CHCl /MeOH 10%): l = 423 (e/dm mol cm
3
6
9
66 13
7
3
–1
–1
3
max
01200), 517 (15400), 550 (4200), 589 (4900) and 645 (2000) nm.
Synlett 2004, No. 7, 1158–1162 © Thieme Stuttgart · New York

1
162
D. Gueyrard et al.
LETTER
Acknowledgment
(10) Collman, J. P.; Shiryaeva, I. M.; Boulatov, R. Inorg. Chem.
2003, 42, 4807.
The authors thank MENRT (A.D.) and Région Bretagne (C.R.) for
financial support.
(
(
(
(
11) Ricard, D.; L’Her, M.; Richard, P.; Boitrel, B. Chem.–Eur.
J. 2001, 7, 3291.
12) Fabian, M.; Wong, W. W.; Gennis, R. B.; Palmer, G. Proc.
Natl. Acad. Sci. U.S.A. 1999, 96, 13114.
13) Blomberg, M. R. A.; Siegbahn, P. E. M.; Wikstrom, M.
Inorg. Chem. 2003, 42, 5231.
14) (a) Proshlyakov, D. A.; Pressler, M. A.; Babcock, G. T.
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