Access to well-defined isolated Fe(ii) centers on silica and their use in oxidation

Article abstract of DOI:10.1039/b711015d

Well-defined Fe^II isolated sites are obtained by reaction of diaryl-N,N′-diazadiene bis(neosilyl) iron (1) with an aerosil silica, SiO_2-(700). This system can be used as a precursor for the catalytic oxidation of cyclohexene into cyc

Full text of DOI:10.1039/b711015d

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www.rsc.org/dalton | Dalton Transactions
Access to well-defined isolated Fe(II) centers on silica and their use in oxidation
Charbel Roukoss,a Steven Fiddy,b Aimery de Mallmann,a Nuria Rendo´n,a Jean-Marie Basset,a Emile Kuntza
and Christophe Cope´ret*a
Received 18th July 2007, Accepted 8th August 2007
First published as an Advance Article on the web 3rd September 2007
DOI: 10.1039/b711015d
Well-defined FeII isolated sites are obtained by reaction of diaryl-N,Nꢀ-diazadiene bis(neosilyl) iron (1)
with an aerosil silica, SiO2-(700). This system can be used as a precursor for the catalytic oxidation of
cyclohexene into cyclohexene oxide, cyclohexenol and cyclohexenone in the presence of H2O2.
was loaded within a nitrogen filled glove-box in a double-airtight
Introduction
sample holder equipped with Kapton windows. The sample was
The active sites of several oxygenases1,2 and heterogeneous oxida-
studied at room temperature, at the iron K-edge, in the transmis-
tion catalysts are based on Fe centers.3–6 Notably, Fe-based zeolitic
sion mode and using a double-crystal Si-(111) monochromator
materials selectively oxidize benzene into phenol by N2O, but the
detuned to eliminate most of the higher harmonics content of the
structure of the active site is still a matter of debate: isolated
beam. Additionally, the spectra were recorded between 11 600 and
12 400 eV. The analysed spectrum was the result of the averaging of
three acquisitions. The data analyses were performed by standard
procedures using the programs developed by Alain Michalowicz,
Fe center vs. binuclear Fe system (probably bonded through
a l-oxo bridge).7 Recently, several bio-inspired homogenous
catalysts have appeared using either mono- or binuclear Fe centers
wrapped in a stabilizing ligand.8–13 Using surface organometallic
chemistry,14 we report herein the reaction of 2,3-dimethyl-1,4-
[(2ꢀ,6ꢀ)-diisopropylphenyl]-N,Nꢀ-diazadiene bis(neosilyl) iron (1)15
with silica as a route to well-defined silica supported isolated Fe
centers (2). Their activity as epoxidation catalyst precursors has
been tested.
in particular the EXAFS fitting program RoundMidnight.16 The
background absorption, l0, was calculated by using a theoretical
expression developed by Lengeler and Eisenberger and the single
atomic absorption of the absorber, l1 was interpolated by a sixth
degree polynomial between 11 600 and 12 400 eV.17
Fitting of the spectrum was done on the k3 weighted data using
2
the following EXAFS equation where S0 is the scale factor; Ni
is the coordination number of shell i; Fi is the EXAFS scattering
function for atom i; Ri is the distance to atom i from the absorbing
atom; k is the photoelectron mean free path; ri is the Debye–Waller
factor; Ui is the EXAFS phase function for atom i; and Uc is the
EXAFS phase function for the absorbing atom:
Experimental
All experiments were carried out by using standard air free
methodology in a glove box under Ar, on a Schlenk line,
or in Schlenk-type apparatus interfaced to high vacuum line
(10−5 Torr). Ether and THF were distilled over a sodium–
benzophenone mixture. CH2Cl2 was distilled over P2O5 and
ꢁ
ꢂ
n
ꢀ
NiSi (k, Ri) Fi (k, Ri)
−2Ri
2
0
∼
=
v (k)
S
exp
2
˚
kRi
k (k, Ri)
stored over 3 A molecular sieves. All solvents were degassed
i=1
by four freeze–pump–thaw cycles. The 2,3-dimethyl-1,4-[(2ꢀ,6ꢀ)-
diisopropylphenyl]-N,Nꢀ-diazadiene bis(neosilyl) iron (1) was pre-
pared according to the literature procedure.15 Gas-phase analysis
was performed on a Hewlett-Packard 5890 series II gas chromato-
graph equipped with a flame ionization detector and Al2O3–KCl
on a fused silica column (50 m × 0.32 mm).
ꢃ
ꢄ
× exp −2r2i k2 sin [2kRi + Ui (k,Ri) + Uc (k)]
The program FEFF7 was used to calculate theoretical values
for Si, Fi, k, Ui, and Uc based on model clusters of atoms. The
refinements were performed by fitting the structural parameters
Ni, Ri, ri and the energy shift, DE0. The fit residue, q, was calculated
by the following formula:
Elemental analysis was performed at the CNRS Central Anal-
ysis Service of Solaize and at the Laboratory of Organometallic
Synthesis and Electrosynthesis at Dijon. Infrared spectra were
recorded on a Nicolet 550-FT by using an infrared cell equipped
with CaF2 windows, allowing in situ studies. Typically 16 scans
were accumulated for each spectrum (resolution, 2 cm−1).
The X-ray absorption spectra were acquired at the SRS of the
CCLRC at Daresbury, UK, on beam line 7.1. The sample studied,
a solid resulting from the grafting of 1 onto SiO2-(700) (vide infra),
ꢅ
[k3vexp (k) − k3vcal (k)]2
k
ꢅ
q =
[k vexp (k)]2
3
k
Where vexp(k) and vcal(k) designate respectively the experimental
and simulated oscillatory parts of the absorption coefficient. The
smaller this parameter is (in % in Table 1), the better is the
agreement between experimental and simulated spectrum.18
Grafting of 1 on SiO2-(700) monitored by infrared spectroscopy.
Representative procedure
aLaboratoire de Chimie Organometallique de Surface, CPE Lyon 43 Bd
du 11 Novembre 1918, F-69616 Villeurbanne Cedex, France. E-mail:
coperet@cpe.fr.
bSynchrotron Radiation Department, Beam-line 7.1, CCLRC Daresbury
Laboratory, Warrington, UK WA4 4AD
In an IR-cell 50–100 mg of silica were pressed into a 18 mm self-
supporting disk, which was put into a sealed glass high-vacuum
5546 | Dalton Trans., 2007, 5546–5548
This journal is
The Royal Society of Chemistry 2007
©

reactor equipped with CaF2 windows, and partially dehydroxy-
lated under vacuum (500 ◦C, 12 h and 700 ◦C, 4 h). The silica
disk was then immersed in a pentane solution of 1 at 25 C for
3 h followed by two pentane washings and a drying step under
dynamic vacuum at room temperature. After this step an IR
spectrum was recorded.
silanol groups at 3747 cm−1 disappeared (Fig. 1), but a broad band
(m0) appeared in agreement with the presence of remaining silanols
interacting with organometallic moieties (3740–3550 cm−1, ∼15%
of the original SiOH intensity). Moreover, two groups of bands
appeared in the 3000–2850 cm−1 and 1470–1350 cm−1 regions.
These bands, m2–3 and d6–7, are assigned to m(CH) and d(CH) vibrations
of the alkyl ligands respectively. Additionally, two other bands
appeared at 3065 (m1) and 1586 cm−1 (m5), which were respectively
◦
Preparation of [1–SiO2-(700)] by impregnation. Representative
procedure
assigned to m(=C–H) and m(C vibrations.
=
N)
A mixture of 1 (80 mg, 0.13 mmol, 1.15 equiv. per surface silanols)
and SiO2-(700) (500 mg) in pentane (8 mL) was stirred at 25 ◦C for
3 h. After filtration, the solid was washed 3 times with pentane,
and all volatile compounds were condensed into another reactor
(of known volume) in order to quantify the TMS evolved during
grafting. The resulting green powder was dried under vacuum
(10−5 Torr, 2 h) to yield 535 mg of 2 [1–SiO2-(700)]. Analysis by gas
chromatography indicated the formation of 0.09 mmol of Me4Si
during the grafting (0.85 Me4Si–Fe). Elemental analysis of 2: Fe:
1.13%wt, C: 6.95%wt, H: 0.87%wt, N: 0.67%wt.
Cyclohexene epoxidation. Representative procedure
Cyclohexene was degassed by four freeze–pump–thaw cycles and
˚
dried over freshly activated molecular sieves (3 A). In a 10 mL
batch reactor, 60 mg (0.012 mmol Fe) of catalyst were loaded
followed by 1 mL of cyclohexene (substrate : catalyst ∼750). At
t = 0, 200 ll of a 30% solution of H2O2 in H2O (substrate :
oxidant = 5, oxidant : catalyst ∼150) were added, and the reaction
mixture was stirred at 60 ◦C. Small aliquots (40 ll) were analyzed
at different times by GC using dodecane as an external standard.
Fe lixivation was investigated by titrating the Fe present in the
solution using colorimetric measurements: the reaction mixture
was filtered, 1 mL of supernatant was brought to 10 mL with a
0.1 M nitric acid aqueous solution, and this resulting solution was
diluted 250 times, treated with the indicator HI93746 and the Fe
content was measured with a HANNA colorimeter instrument
indicating the presence of less than 2% of initial Fe content.
Fig. 1 Monitoring of the grafting of 1 on SiO2-(700) by IR spectroscopy.
(a) SiO2-(700) pellet (50 mg). (b) After immersing SiO2-(700) in a pentane
solution of 2,3-dimethyl-1,4-[(2ꢀ,6ꢀ)-diisopropylphenyl]-N,Nꢀ-diazadiene
bis-neosilyl iron at 25 ◦C for 3 h followed by two washings with pentane
and drying under vacuum.
Grafting of 1 was also carried out by impregnation in pentane at
25 ◦C with larger quantities of SiO2-(700). Iron elemental analysis on
the resulting solid varies between 1.09–1.13%wt, which corresponds
to ∼0.19–0.20 mmol Fe g−1, confirming that most surface silanols
have reacted (80% based on 0.26 mmol OH g−1 of SiO2-(700)),
which is consistent with what has been observed by in situ IR
spectroscopy (vide supra). During grafting, 0.9
0.1 equiv. of
Me4Si/grafted Fe was released. Moreover, carbon and nitrogen
elemental analyses are 6.95 and 0.53%wt, respectively, which
Results and discussion
correspond to 29
3 C/Fe (expected 32 for 2) and 1.9
0.2
◦
After immersing a silica disk partially dehydroxylated at 700 C
N/Fe (expected 2.0 for 2). Furthermore, hydrolysis of the solid
gives 0.8 0.1 equiv. of Me4Si (expected 1.0). All these data are
consistent with the formation of 2 as the major surface species
(Scheme 1).
Iron K-edge EXAFS studies on [1–SiO2-(700)] provided further
insight into the structure of the Fe surface species (Fig. 2): an
(SiO2-(700)) for 3 h in a purple solution of 2,3-dimethyl-1,4-
[(2ꢀ,6ꢀ)-diisopropylphenyl]-N,Nꢀ-diazadiene bis(neosilyl) iron (1)15
in pentane at 25 ◦C (1.2 equiv. with respect to surface silanols;
0.26 mmol accessible OH g−1 of silica), the silica disk turned green;
it was washed twice with pentane to remove the excess complex and
then dried under vacuum (10−5 Torr, 2 h). Monitoring the reaction
by IR spectroscopy showed that the band attributed to isolated
˚
˚
average of one oxygen atom at 1.85 A, one carbon atom at 1.97 A
˚
and two nitrogen atoms at 2.10 A in the coordination sphere of
Scheme 1 Proposed structure of the major surface species 2 (Ar = 2,6-diisopropylphenyl).
The Royal Society of Chemistry 2007 Dalton Trans., 2007, 5546–5548 | 5547
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©

Scheme 2 Reactivity of FeII centers with H2O2 (X = CH2SiMe3 in 2 and
X = OH during the catalytic cycle).
Fig. 2 Iron K-edge k3-weighted EXAFS (left) and Fourier transform
(right) of solid (2). Dashed lines: spherical wave theory, solid lines:
experimental.
been fully characterised by elemental analysis, IR and EXAFS
spectroscopies. In the presence of H2O2, this system catalyses
the epoxidation of cyclohexene, but Fenton chemistry is the
major pathway of oxidation leading to 2-cyclohexenol and 2-
cyclohexenone. Further studies are currently under way to gen-
erate stable single-site Fe oxidation catalysts.
iron, which is consistent with 2 being the major species (Table 1).
The Fe–C and Fe–N bond distances measured by EXAFS are in
good agreement with those obtained by X-ray crystallography for
15,19,20
˚
˚
1 (Fe–C ∼ 2.042 A and Fe–N ∼ 2.025 A).
Finally, the Fe–
OSi bond length is also very close to measured values of similar
5,21
˚
Fe molecular complexes (1.86–1.91 A).
Acknowledgements
Table 1 EXAFS Parameters for the solid 2.a The errors generated by the
EXAFS fitting program “RoundMidnight 2005”16 are indicated between
parentheses
NR is grateful to the Spanish MEC for a postdoctoral fellowship.
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Distance from
Fe/A
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2
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˚
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t
CH3CO3H with or without catalyst gave cyclohexene oxide in
95% yield within 15 min. With H2O2 (30% in H2O, 150 equiv.)
in CH3CN, cyclohexene (750 equiv.) is slowly converted into
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in the presence of 2 (1 equiv.), while no conversion was observed
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A route to well-defined isolated FeII centers dispersed onto a
silica surface has been described. The alkyl derivative 2 has
22 S. Goldstein and D. Meyerstein, Acc. Chem. Res., 1999, 32, 547.
5548 | Dalton Trans., 2007, 5546–5548
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The Royal Society of Chemistry 2007
©