Selective iron-catalyzed oxidation of benzylic and allylic alcohols

Article abstract of DOI:10.1002/adsc.201100210

A convenient and selective oxidation of alcohols with hydrogen peroxide to give aldehydes and ketones has been developed. Using in situ generated iron chloride complexes [Fe(L3)₂Cl_n] [n=0-1, L3 =6-(N-phenylbenzimidazoyl)-2-pyridinecarboxylic acid], aldehydes and ketones were obtained in good yield and excellent selectivity after a short reaction time at room temperature. Copyright

Full text of DOI:10.1002/adsc.201100210

FULL PAPERS
DOI: 10.1002/adsc.201100210
Selective Iron-Catalyzed Oxidation of Benzylic and Allylic
Alcohols
Benoꢀt Join,a Konstanze Mçller,a Carolin Ziebart,a Kristin Schrçder,a
Dirk Gçrdes,b Kerstin Thurow,b Anke Spannenberg,a Kathrin Junge,a
and Matthias Bellera,*
a
Leibniz-Institut fꢁr Katalyse e.V. an der Universitꢂt Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany
Fax : (+49)-381-1281-5000; e-mail: matthias.beller@catalysis.de
Center for Life Science Automation (CELISCA), Universitꢂt Rostock, Friedrich-Barnewitz-Str. 8, 18119 Rostock,
b
Germany
Received: March 24, 2011; Revised: June 30, 2011; Published online: October 20, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100210.
Abstract: A convenient and selective oxidation of al- yield and excellent selectivity after a short reaction
cohols with hydrogen peroxide to give aldehydes and time at room temperature.
ketones has been developed. Using in situ generated
iron chloride complexes [Fe(L3)2Cln] [n=0–1, L3
=6-(N-phenylbenzimidazoyl)-2-pyridinecarboxylic
Keywords: alcohols; catalysis; hydrogen peroxide;
acid], aldehydes and ketones were obtained in good iron; oxidation
Introduction
Results and Discussion
The selective oxidation of alcohols remains an impor- During our study, we chose the oxidation of benzyl al-
tant transformation for organic synthesis and the cohol as benchmark reaction (Table 1 and Figure 1).
chemical industry.[1] In order to prevent the formation According to our previously published results on
of unwanted waste and toxic reagents, the use of inex- arene oxidations[8a] and epoxidations,[13] we first inves-
pensive and less toxic iron complexes together with tigated the performance of a catalyst generated from
hydrogen peroxide represents an ideal combination FeCl3·6H2O and pyridine-2,6-dicarboxylic acid
for such oxidations. The only by-product formed in (H2Pydic, L1) as ligand in the presence of pyrrolidine
this atom efficient transformation is water.[2]
as base in t-amyl alcohol (Table 1, entries 1–5). As ex-
During the last decade, iron has become broadly pected all components of this catalyst system were
used for a multitude of redox processes. Apart from necessary to get activity even if the desired benzalde-
reductions,[3] various iron systems have been reported hyde was only obtained in moderate conversion and
for epoxidation,[4] sulfoxidation,[5] hydroxylation,[4d,6] selectivity (63% and 67%, respectively, Table 1,
as well as oxidation of arenes to phenols[7] and entry 5).
quinones.[8] In general, chemoselectivity is the key
Previously, we demonstrated that the epoxidation
issue for these reactions due to potential non-selective of olefins can be easily performed using FeCl3·6H2O/
Fenton- or Gif-type chemistry.[9] Notwithstanding, so- imidazole catalysts.[4b,14]
called oxygenated Fenton chemistry is still
debatable.[9d,f]
Unfortunately, in our model reaction only a very
low conversion of 10% was detected (Table 1,
Although alcohol oxidations catalyzed by iron com- entry 6). However, the selectivity was excellent and
plexes have been already investigated,[10] until now no no degradation of the substrate or the product was
general method has been discovered and improve- observed. In order to take advantage of both systems,
ments are desired. Several systems have been de- we investigated the effect of a ligand combining both
scribed employing different kinds of oxidants with moieties. Employing 6-(N-phenylbenzimidazoyl)-2-
[11]
[12]
H2O2 and O2 as the most interesting ones.
pyridinecarboxylic acid as ligand (L3), which is easily
accessible from 2,6-pyridinedimethanol,[15] resulted in
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Table 1. Oxidation of benzyl alcohol: optimization of the
Table 2. Oxidation of benzyl alcohol: influence of the metal
catalytic system.[a]
source on both conversion and yield.[a]
Entry
Catalyst
Conv. [%][b]
Selec. [%][c]
Entry
[Fe]
L
Base
[mmol]
Conv.
[%][b]
Selec.
[%][c]
1
2
3
4
5
6
7
8
9
FeCl3·6H2O
FeCl2
85
81
46
56
35
54
31
–
21
3
16
18
86
96
>99
95
>99
>99
>99
–
–
–
71
–
1
2
3
4
5
6
7
8
9
–
–
–
–
–
–
0.05
0.05
0.05
0.05
0.05
0.05
0.05
<5
<5
<5
<5
63
10
78
15
>99
11
–
–
–
–
67
>99
97
>99
55
Fe
G
FeCl3·6H2O
FeCl3·6H2O L1
FeCl3·6H2O
FeCl3·6H2O L1
FeCl3·6H2O L2
FeCl3·6H2O L3
FeCl3·6H2O L4
FeCl3·6H2O L5
ACHTUNGTRENNUNG
G
–
FeACHTUNTGRENNUNG
ACHTUNGTRENNUNG
MnCl2
CuCl2
NiCl2
10
11
12
RuCl3·6H2O
CrCl3
10 FeCl3·6H2O L6
>99
[a]
[b]
Reaction conditions: see Experimental Section.
Conversion and yield were determined by GC analysis;
average of 2 runs.
Selectivity refers to the chemoselectivity of aldehyde
from alcohol.
[a]
Reaction conditions: see Experimental Section.
Conversion and yield were determined by GC analysis.
Selectivity refers to the chemoselectivity of aldehyde
from alcohol.
[b]
[c]
[c]
dant were varied. Best results were obtained employ-
ing 2 equivalents of H2O2, which were added to the
reaction mixture within 30 min (83% conversion, 73%
yield).
In order to prove the specificity of our system, we
tested several iron sources as well as other metal salts
(Table 2). Among the different iron sources (Table 2,
entries 1–7), best results were observed using
FeCl3·6H2O or FeCl2. The latter even showed a simi-
lar activity coupled with a better selectivity (96% in-
stead of 86%, Table 2, entries 1 and 2). On the other
hand, any other metal gave detrimental results under
these conditions. For example, using ruthenium or
chromium (Table 2, entries 11 and 12), two metals
widely used in oxidations, only low conversion was
achieved (<20%). When manganese is employed no
reaction at all took place (Table 2, entry 8). Clearly,
this specificity for iron chloride salts is a priori in con-
tradiction with a metal-induced auto-oxidation pro-
cess. Further optimization of the reaction parameters
revealed that the nature of the base has no influence
on reactivity nor on selectivity which is in contradic-
Figure 1. Ligands used for the iron-catalyzed oxidation of
benzyl alcohol.
an improvement of conversion (78%) and chemose- tion to our previous observation for iron-catalyzed ox-
lectivity (97%) (Table 1, entry 7). Further modifica- idation systems.[8a,13a] Furthermore, it should be noted
tions of the benzimidazole fragment of the ligand (L4 that the amine cannot act as buffer due to the low
or L5) decreased conversion or chemoselectivity amount of base used (5 mol%). Therefore, we postu-
(Table 1, entries 8 and 9). Notably, the synthesized late that under these conditions pyrrolidine does not
symmetrical ligand L6 derived from ligand L3 showed act as a co-ligand and simply deprotonates the car-
low activity (Table 1, entry 10).
boxylic acid moiety of the tridentate ligand (vide
Next, we investigated the effect of the concentra- infra). Indeed, inexpensive sodium carbonate can be
tion of the oxidant. Here, the rate of addition of hy- used without problems instead of pyrrolidine. Apply-
drogen peroxide as well as the total amount of oxi- ing optimized conditions (FeCl2/L3/Na2CO3, dichloro-
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Selective Iron-Catalyzed Oxidation of Benzylic and Allylic Alcohols
Table 3. Oxidation of different benzyl alcohols: scope and
Table 4. Oxidation of secondary alcohols.[a]
limitations.[a]
Entry Product
t
Conv.
Yield
[%][b]
Selec.
[%][c]
Entry
Product
Conv.
[%][b]
Yield
[%][b]
Selec.
[%][c]
[min] [%][b]
1
2
3
84
92
93
76
90
93
90
98
1
60
30
65 (93)[d] 65 (93)[d] >99
2[e]
80
76[f]
95
95
95
>99
4
80
60
75
3[g]
30
30
>99
5
6
92
80
97
94
91
78
61
90
77
71
85
77
92
82
78
4[e]
56
56
>99
7[d]
[a]
Reaction conditions: see Experimental Section.
Conversion and yield were determined by GC analysis.
Selectivity refers to the chemoselectivity of ketone from
[b]
[c]
8
alcohol.
[d]
[e]
Using 0.04 mmol of FeCl2 and 3 equiv. of hydrogen per-
oxide.
9
Using 0.04 mmol of metal and 2 equiv of hydrogen per-
oxide.
Isolated yield.
[f]
10
92
76
83
[g]
Using 0.04 mmol of FeCl2 and 4 equiv. of hydrogen per-
oxide.
[a]
Reaction conditions: see Experimental Section.
[b]
[c]
Conversion and yield were determined by GC analysis.
Selectivity refers to the chemoselectivity of aldehyde
from alcohol.
the selectivity (98% and >99%, respectively)
(Table 3, entries 2 and 3). Electron-withdrawing
groups like the trifluoromethyl-substituent did not
show any effect (Table 3, entry 5). On the other hand
p-methoxybenzyl alcohol gave somewhat lower selec-
[d]
Using 0.04 mmol of FeCl2.
methane, 2 equivalents H2O2) gave benzaldehyde in tivity (77%). However, performing the reaction with
76% yield and high selectivity after 30 minutes
(Table 3, entry 1).
a
higher loading of FeCl2 (8 mol% instead of
5 mol%) restored a good selectivity (92%, Table 3,
Next, we focused on the scope of the aromatic alco- entry 7). Testing 2-, 3-, and 4-methyl-substituted
hol. Adding electron-withdrawing substituents as well benzyl alcohols showed no major differences, proving
as electron-donating groups on the aromatic moiety the absence of significant steric influence on the reac-
did not significantly influence the efficiency of the tion (Table 3, entries 8–10).
system, except for the nitro group where the yield
After demonstrating the selective oxidation of pri-
dropped to 60% with 75% selectivity (Table 3, mary benzyl alcohols, we became interested in the re-
entry 4). This is rather due to a lower solubility in di- action of secondary alcohols (Table 4). Applying our
chloromethane than an effect from the electron-with- standard procedure on 1-phenylethanol 10 we ob-
drawing group. Slightly deactivating groups, such as tained a moderate conversion of 42% with an excel-
chloride or fluoride, had a moderate positive effect on lent selectivity of 99% for acetophenone 10a. Obvi-
both the conversion (92% and 93%, respectively) and ously, the secondary alcohol reacted more slowly than
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Table 5. Oxidation of allylic alcohols: scope and limita-
strate cyclohexanol 13 was oxidized to the corre-
sponding cyclohexanone 13a in 56% yield with high
selectivity (99%) after 30 min using 2 equivalents of
H2O2 (Table 4, entry 4).
ACHTUNGTRENNUNG
tions.[a]
After dealing with various benzylic alcohols, which
can be easily transformed with a good to very good
selectivity to the corresponding aldehydes and ke-
tones, we proceeded to investigate the oxidation of al-
lylic alcohols. The resulting conjugated ketones and
aldehydes are present in a large number of com-
pounds used by perfumery and food industry like car-
vone, citral or perillaldehyde. In order to apply our
methodology to these substrates (Table 5) we first in-
vestigated the reactivity of cinnamyl alcohol 14
(Table 5, entry 1). Using our standard conditions
(5 mol% FeCl2/L3/Na2CO3, 2 equiv. of H2O2, 30 min)
we obtained a high conversion of 94% but a moderate
selectivity for the product 14a of 73%. Attempts to
characterize the side-products were not successful.
Using 3-cyclohexenol the cyclohex-3-enone 15a was
obtained in 76% selectivity with complete conversion
(Table 5, entry 2). Furthermore, the oxidation of gera-
niol 17, perillyl alcohol 18 and carveol 19 proceeded
smoothly under our standard conditions. Here, con-
versions were good (83 to 89%) and selectivities re-
mained moderate to good for this kind of sensitive
substrates (58 to 76%) (Table 5, entries 4–6). Again,
in all cases no side-products could be identified.
We also tested the reusability of the catalyst
system. Indeed, it is possible to use the catalyst for
second run (75% conversion). However, after third
addition of benzyl alcohol and hydrogen peroxide no
conversion was observed.
Entry
Product
Conv.
[%][b]
Yield
[%][b]
Selec.
[%][c]
1
2
94
72
76
73
76
>99
3
>99
59
68
59
76
4[d,e]
89
5[d,e]
83
84
48
53
58
63
6[e]
In order to understand the mechanism of our
system, we established a concentration/time diagram
for the oxidation of the benzyl alcohol 1 (see Support-
ing Information). After addition of the oxidant, the
formation of benzaldehyde 1a started immediately.
Then, a linear consumption of 1 occurred followed by
formation of 1a. After 10 min reaction time, some
degradation is observed decreasing the selectivity.
[a]
Reaction conditions: see Experimental Section.
[b]
[c]
Conversion and yield were determined by GC analysis.
Selectivity refers to the chemoselectivity of aldehyde or
ketone from alcohol.
Direct addition of the hydrogen peroxide solution.
Using 0.04 mmol of FeCl2.
[d]
[e]
the primary one. Increasing the reaction time to one Unfortunately, it was not possible to detect potential
hour resulted in the improvement of the conversion side products like benzoic acid because of the low
(65%) with a remaining selectivity of 99% (Table 4, concentration. Next, we performed competitive reac-
entry 1). An increased loading of FeCl2 (8 mol% in- tions between 1 and 10 to evaluate the selectivity of
stead of 5 mol%) in the presence of 3 equivalents of the catalyst towards primary and secondary benzyl al-
H2O2 gave the optimal result with 93% conversion cohols (Figure 2). Initially, we used an equal ratio of
and 99% selectivity. When using phenylethylene 0.5 mmol of both 1 and 10. After 30 min of reaction
glycol 11 as substrate only a single oxidation product using 1 mmol of hydrogen peroxide a conversion of
11a was observed (Table 4, entry 2). In this case, stan- 93% of 1 as well of 65% from 10 was measured
dard conditions (30 min, 2 equivalents of H2O2) are [Figure 2, (A)]. Taking into account that 1 mmol of
optimal. The a-hydroxy ketone 11a was isolated in H2O2 is enough to get a complete conversion of both
good yield (76%) with an excellent selectivity (95%). substrates we performed a second attempt using only
Xanthenol 12 gave similar results with a complete 0.5 mmol of H2O2. In this case, we observed 44% con-
conversion after 30 min. The desired ketone 12a was version for 1 compared to 14% for 10 [Figure 2, (B)].
isolated in 95% yield (Table 4, entry 3). In addition to An additional experiment using 1 and 0.5 mmol of hy-
aromatic secondary alcohols, also the aliphatic sub- drogen peroxide led to the formation of benzaldehyde
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Selective Iron-Catalyzed Oxidation of Benzylic and Allylic Alcohols
lower. Only 0.14–0.15 deuterium is present (for a
maximum of 0.5). This difference between the result-
ing starting material and the product can be explained
by an exchange between hydrogen and deuterium
during the catalytic cycle. Thus, we investigated the
H-D exchange of water with the product.[11b] Adding
different amounts of D2O to the reaction mixture
(i.e., 50 mL, 200 mL or 400 mL) did not lead to an in-
creased deuterium incorporation in the product. Simi-
lar experiments using H218O (50 mL or 150 mL added)
led to no incorporation of 18O into the product. Appa-
rently, water does not play an important role in the
catalytic cycle.
Next, we performed a reaction with ethylbenzene, a
substrate highly sensitive to radical oxidation espe-
cially on its benzylic position. Under our reaction
conditions no oxidation products have been detected
and the starting material is completely recovered. Ad-
ditional reactions using radical inhibitors, i.e., duro-
quinone, and (2,2,6,6-tetramethylpiperidinyl)-N-oxyl
(TEMPO) have been performed to exclude a radical
pathway, too. Using 1 equivalent of duroquinone with
respect to benzyl alcohol led to a slight reduction of
reactivity compared to normal conditions. Hence,
78% conversion and 81% selectivity were observed,
which are close to the results obtained before (84%
and 90%, respectively). In the presence of TEMPO,
we observed a higher influence on the reactivity.
Here, benzaldehyde is obtained in 39% yield with
Figure 2. Competitive reaction between primary and secon-
dary alcohols: A using 1 mmol of H2O2; B using 0.5 mmol of
H2O2; C using 0.5 mmol of H2O2, without 1-phenylethanol
10; conversion and yield were determined by GC analysis;
selectivity refers to the chemoselectivity of aldehyde or
ketone from alcohol.
1a with 44% conversion and 77% selectivity, which is 96% selectivity. However, TEMPO is a well known
in agreement with previous results [Figure 2, (C)].
ligand in iron-catalyzed oxidations.[12a,e] Hence, we
To get some information on the limiting step of the postulate that the decrease in reactivity is due to a
catalytic cycle, we measured the kinetic isotope effect change in the iron complex rather than an inactiva-
(KIE) using an equimolar mixture of benzyl alcohol tion of radical species. In summary, under our condi-
and deuterated benzyl alcohol-a,a-d2 at 2 different tions, the oxidation of alcohols should not be induced
substrate concentrations (Scheme 1). The mixture of by a Fenton free radical diffusion pathway.
the starting materials contained an average amount of
To gain more insights into the molecular nature of
0.8–0.87 deuterium, which was calculated by mass the active catalyst and its precursors we performed
spectrometry. At the end of the reaction the resulting also ESI-MS measurements. The spectrum of a freshly
benzyl alcohol had a similar amount of deuterium prepared solution of FeCl2/L3 (1/1) in dichlorome-
(0.84–0.87 deuterium) incorporated. This shows a kH/ thane showed in positive ion mode a series of signals
kD of 1.1 and 1, respectively, confirming that the limit- at m/z=370.0273, 685.1277, 1054.1476 corresponding
ing step of the catalytic cycle is not the dehydrogena- to [Fe(L3)]+ (chlorine abstracted), [Fe(L3)2 +H]+ and
tion of the alcohol. More surprisingly, the amount of [Fe2(L3)3]+ (chlorine abstracted), respectively where
deuterium present in benzaldehyde is drastically [Fe(L3)2 +H]+ is the most intense signal. The same
Scheme 1. Determination of the KIE: Influence of deuterium on the reactivity.
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experiment was performed with FeCl3·6H2O as the
iron precursor leading to identical signals. Addition of
pyrrolidine (pyr) did not change significantly the
signal patent of the ESI-mass spectrum. The afore-
mentioned compounds were accompanied with addi-
tional low intense ones of m/z=756.2022 and
1440.3194 corresponding to [Fe(L3)
(pyr)+H]+ and
2ACHTUNGTRENNUNG
[Fe2(L3)3A
CHTUNGTRENNUNG
used instead of pyrrolidine as base, no coordinated
base was detected but an additional signal at m/z=
707.1096 appeared. This can be attributed to an
[Fe(L3)2 +Na]+ species which is a result of the excess
of sodium cations in the solution. Then, we subjected
samples of reaction mixtures to ESI-MS measure-
ments. After 15 min of reaction, the presence of the
ligand (m/z=316.1096, [L3+H]+ and m/z=338.0894,
[L3+Na]+) is detected as well as the formerly identi-
fied complex [Fe(L3)2]+ (m/z=684.1200, chloride ab-
stracted) and [Fe(L3)2 +Na]+ (m/z=707.1095). At the
end of the reaction, i.e., 30 min, appropriate signals
are detected. Furthermore, a less abundant signal was
observed at m/z=272.1176. It corresponds to the de-
carboxylated ligand L3. Taking in consideration that
this species could also be bound to the iron centre
and result in an active catalyst, we used an authentic
sample as a ligand and performed the oxidation of
benzyl alcohol. However, in the presence of the de-
carboxylated ligand no conversion was observed.
Figure 3. ORTEP representation of the tetra-iron cluster
[Fe4(L3)4Cl8]. Displacement ellipsoids are drawn at the 30%
probability level. Hydrogen atoms are omitted for clarity.
Hence, we conclude that this compound is inactive. Conclusions
All these results let us postulate that [Fe(L3)2], or its
chlorinated form [Fe(L3)2Cl], is the stabilized form of In conclusion, we report a novel iron-catalyzed oxida-
the active catalyst.
tion of benzylic and allylic alcohols using hydrogen
To our delight crystallization experiments from a peroxide as oxidant under air and mild conditions.
solution of FeCl2 and L3 in dichloromethane/tert-amyl This methodology can also be applied to more sensi-
alcohol (5/1) at À208C resulted in the preparation of tive compounds like perillyl alcohol, geraniol or car-
a molecular-defined tetra-iron cluster linked by four veol. After short reaction time, 30 min up to 1 hour,
ligands L3 (Figure 3). Each iron atom is directly coor- the desired oxo compounds are obtained in moderate
dinated by 2 nitrogen atoms of a single ligand as well to very good yields (48–95%) and excellent selectivity
as by 2 oxygen atoms from two different ligands L3. 2 (58–>99%). Mechanistic investigations reveal that
chloride ions are completing the octahedral coordina- free diffusion radicals are not significantly involved
tion mode of the iron centre. The octahedral coordi- during the oxidation. A molecular-defined tetra-iron
nation geometry is slightly distorted, which is ex- cluster linked by four ligands L3 could be isolated,
À
À
À
pressed by angles N4 Fe2 O3 of 147.9(1)8 and N1
Fe1 O1 of 148.0(1)8. Subjected to ESI-MS, the crystal catalyst precursor of the active species.
showed a single signal at m/z 684.1214 corresponding
which forms [Fe(L3)2Cln] (n=0 or 1) in solution as
À
to the [Fe(L3)2]+ (chlorine abstracted) species as ob-
served for the in situ catalyst.[16]
Experimental Section
Finally, the isolated iron cluster was used in the oxi-
dation of our original model reaction. Although the
yield of benzaldehyde was lower compared to the in
situ generated catalyst, the obtained chemoselectivity
(84%) is similar. Apparently, the tetra-iron cluster ap-
pears to act as a reservoir for [Fe(L3)2Cl] or [Fe(L3)2]
species, precursors for the active species. The differ-
ence of reactivity might be explained by the difficulty
to get a mononuclear species in solution.
General Method for the Oxidation of Alcohols
In a 50-mL round-bottomed flask, the iron salt (5 mol%,
0.025 mmol) followed by the ligand (5 mol%, 0.025 mmol)
and the base (5 mol%, 0.025 mmol) were added in sequence.
Then 9 mL of solvent were added and the mixture stirred at
room temperature for 10 min under air. After addition of
the substrate (0.5 mmol) and the internal standard (dodec-
ane or hexadecane, 100 mL) the mixture was vigorously
stirred under air and the reaction started by addition of the
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Selective Iron-Catalyzed Oxidation of Benzylic and Allylic Alcohols
first drop of hydrogen peroxide. X equivalents of the oxi-
dant were added through a syringe pump. After a time t, the
reaction was quenched by the addition of sodium sulphite
(Na2SO3) and a sample was directly taken and subjected to
GC analysis.
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Purification of Isolated Compounds
1-Phenyl-1,2-ethanediol (11): Purification was realized by
silica gel chromatography using a mixture of hexane and
1
ethyl acetate as eluent (gradient from 3:1 to 2:1). H NMR
(300 MHz, CDCl3): d =7.95–7.90 (m, 2H), 7.67–7.59 (m,
1H), 7.54–7.46 (m, 2H), 4.89 (s, 2H), 3.55 (br, 1H); HR-MS
(ESI-TOF): m/z=136.05191, calcd. for [M]+: 136.05188
(0.2 ppm).
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Amali, K. Junge, X. Ribas, M. Costas, M. Beller,
Angew. Chem. 2011, 6, 1461–1465; Angew. Chem. Int.
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9H-Xanthen-9-ol (12): Purification was realized by silica
gel chromatography using a mixture of hexane and ethyl
acetate as eluant (10:0.3 to remove impurities followed by
1
10:0.6 to get the product). H NMR (300 MHz, CDCl3): d=
3
3
8.34 (dd, JH,H =1.71 Hz, JH,H =7.95 Hz, 2H), 7.76–7.67 (m,
2H), 7.53–7.46 (m, 2H), 7.41–7.34 (m, 2H); HR-MS (ESI-
TOF): m/z=196.05188, calcd. for [M]+: 196.05169 (1.0 ppm).
Acknowledgements
B. J. appreciates financial support provided by the Alexand-
er-von-Humboldt Stiftung. K.S. thanks the Max-Buchner-For-
schungsstiftung. We thank M. Heyken for technical assis-
tance.
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[16] Crystal data for [Fe4(L3)4Cl8]·5.2CH2Cl2:, M=2205.88,
monoclinic, space group C2/c, a=31.0557(8), b=
9.3022(1),
c=31.8441(7) ꢇ,
b=92.501(2)8,
V=
9190.6(3) ꢇ3, T=150(2) K, Z=4, m=1.214 mmÀ1, ab-
sorption correction: numerical (max. and min. trans-
mission: 0.8719 and 0.7248), 40191 reflections meas-
AHCTUNGTRENNGuUN red, 8649 independent reflections (Rint =0.0307), final
R values (I> 2s(I)): R1 =0.0421, wR2 =0.1197, final R
values (all data): R1 =0.0524, wR2 =0.1239, 587 refined
parameters. Data were collected on a STOE IPDS II
diffractometer using graphite-monochromated Mo Ka
radiation. The structure was solved by direct methods
and refined by full-matrix least-squares procedures on
F2 with the SHELXTL software package.[17] CCDC
816722 contains the supplementary 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.
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3030
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Adv. Synth. Catal. 2011, 353, 3023 – 3030