Aerobic oxidation of tetrahydrofuran by a series of iron (III) containing POSS compounds

Article abstract of DOI:10.1016/j.poly.2009.04.014

A series of iron (III) containing POSS compounds, [Bu₄N][(R)₇Si₇O₁₂FeCl] (R = isobutyl- (1), ethyl- (2), phenyl- (3), and cyclopentyl- (4)) were investigated as potential catalysts for aerobic oxidation of tetra

Full text of DOI:10.1016/j.poly.2009.04.014

Polyhedron 28 (2009) 2183–2186
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Aerobic oxidation of tetrahydrofuran by a series of iron (III)
containing POSS compounds
Michael T. Hay a, , Steven J. Geib b,1, Derek A. Pettner a,2
*
a Penn State Beaver, 100 University Drive, Monaca, PA 15061, USA
b University of Pittsburgh, 309 Chevron Building, Pittsburgh, PA 15260, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of iron (III) containing POSS compounds, [Bu4N][(R)7Si7O12FeCl] (R = isobutyl- (1), ethyl- (2), phe-
nyl- (3), and cyclopentyl- (4)) were investigated as potential catalysts for aerobic oxidation of tetrahydro-
furan (THF). The oxidation products for this reaction were characterized by IR, GC and GC–MS. The major
Received 30 October 2008
Accepted 15 April 2009
Available online 23 April 2009
oxidation product was c-butyrolactone (C4H6O2, GBL); while the two minor products were 2-hydroxy-
THF and its tautomer, 4-hydroxybutanal. A third minor oxidative product is believed to be an isomer
of dihydroxy-THF. THF free syntheses of 2–4 have been developed and all three compounds were char-
acterized by elemental analysis, IR, and X-ray diffraction studies. The crystallographic data for 1–4 dem-
onstrate a similar iron coordination sphere between the four compounds, but the different substitutents
on the various POSS. The corresponding turnover numbers for the aerobic oxidation of THF suggest that
smaller substitutents on the POSS ligands improve the stability of the catalytic species. When compared
to other published catalytic iron systems, these turnover numbers appear to be the largest for the aerobic
conversion of THF to GBL.
Keywords:
POSS
Iron complexes
Aerobic oxidation
X-ray crystal structures
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
THF in the presence of four different iron (III) chloro-POSS
compounds (1–4) and a control of iron (III) chloro complex that
Metal containing polyhedral oligomeric silsesquioxanes (POSS)
have been used to model a variety of industrial catalysts consisting
of silica (SiO2) supported metals [1,2]. As a result, our laboratory
has been investigating the use of POSS ligands as potential models
for the Fe-ZSM-5 catalyst [3]. During this process, we synthesized
and fully characterized four different iron (III)-containing POSS
compounds, [Bu4N][(R)7Si7O12FeCl] (R = isobutyl- (1), ethyl- (2),
phenyl- (3), and cyclopentyl- (4)) (Fig. 1).
does not contain the POSS, [Bu4N][FeCl4] (5). In addition, we pres-
ent the synthesis and structural characterization of the ethyl- (2)
and phenyl- (3) derivatives, and an improved synthesis and struc-
tural characterization for the cyclopentyl- derivative (4) [15].
2. Experimental
2.1. Materials and instrumentation
In the crystal structure of the isobutyl derivative (1) an equiva-
lent of
c-butyrolactone (C4H6O2, GBL) had crystallized with the
All procedures were performed under aerobic conditions. Hex-
ane and toluene were dried over Na and stored over 4 Å molecular
sieves. Acetonitrile was dried over CaH2 and stored over 4 Å molec-
ular sieves. IR spectra were recorded on a Nicolet Impact 410 FTIR
Spectrometer using a KBr pellet or NaCl plates. Elemental analysis
was performed by Galbraith Laboratories Inc. (Knoxville, TN) using
a Perkin–Elmer 240 CHN Analyzer. Gas chromatography (GC) was
recorded on a GOW-MAC Series 400 Thermal Conductivity GC
using a carbowax column. Gas chromatography–mass spectrome-
try (GC–MS) analysis was performed on an Agilent 6890 gas chro-
complex [4]. Since GBL had not been added to the reaction mixture,
we hypothesized that it was formed from the aerobic oxidation of
the solvent, tetrahydrofuran (C4H8O, THF), during the synthesis of
1 (Reaction 1).
The aerobic oxidation of ethers, such as THF, has been facilitated
by a variety of transition metal complexes [5–12]. However, oxida-
tion of ethers using an iron containing complex is especially signif-
icant because of iron’s importance in biological reactions [13,14].
We offer here our results of the controlled aerobic oxidation of
matograph coupled to
spectrometer using a Varian VF5-MS column (20 m, 0.18 mm i.d.,
0.15 m) programmed from 40 (1 min hold) to 300 °C at 15 °C/
min) operating in 20 to split injection mode (Injector
T = 250 °C). The compounds were ionized via electron ionization.
The X-ray diffraction data were collected under a cooled N2 stream
a Waters GCT time-of-flight mass
l
* Corresponding author. Tel.: +1 724 773 3876; fax: +1 724 773 3557.
E-mail addresses: mth7@psu.edu (M.T. Hay), geib@pitt.edu (S.J. Geib).
1
1
Tel.: +1 412 624 1131; fax: +1 412 624 8611.
2
Tel.: +1 724 773 3876; fax: +1 724 773 3557.
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.04.014

2184
M.T. Hay et al. / Polyhedron 28 (2009) 2183–2186
(KBr,
m
(cmꢀ1)): 3137w, 3071s, 3049s, 2965s, 2931s, 2874s,
Cl
Fe
R
O
2719w, 1961m, 1891m, 1824m, 1776m, 1593s, 1480s, 1431s,
1381s, 1068vs, 879s, 744vs, 694vs, 645s, 594m, 551s.
Si
O
NBu4+
O
R
O
O
O
R
Si
Si
R
O
Si
Si O
O
O
R
O
2.3.2. Compound 4
Yield: 73.8%. Anal. Calc. for C51H99FeClNO12Si7 (1206.24): C,
50.78; H, 8.27; N, 1.16. Found: C, 50.61; H, 8.72; N, 1.26%. IR
Si
Si
O
R
R
m
(cmꢀ1)): 2948vs, 2864s, 1481m, 1471m, 1454m, 1383m,
Fig. 1. Iron (III) chloro-POSS compounds, R = isobutyl- (1), ethyl- (2), phenyl- (3),
and cyclopentyl- (4)
(KBr,
1243s, 1099vs, 1040vs, 943s, 906w, 736m, 496vs.
H
H
H
O
O
O
H
Air and 1
H
H
2.4. X-ray analysis of iron (III) chloro-POSS compounds
H
C
C
C
H
C
C
C
C
H
C
H
H
H
Crystallization of
2 from a minimum amount of toluene
H
H
afforded yellow crystals of 2. C30H71ClFeNO12Si7, M = 925.81,
monoclinic space group, P21/c, a = 9.8962(5), b = 29.5660(15),
THF
GBL
c = 16.1237(8) Å, b = 90.7710(10)o, V = 4717.2(4) Å3, Z = 4, Dcalc
=
Reaction 1. Oxidation of tetrahydrofuran to c-butyrolactone.
1.302 Mg/m3; F(0 0 0) = 1976,
l
Mo = 0.422 mmꢀ1; crystal dimen-
sions: 0.32 ꢁ 0.28 ꢁ 0.25 mm. The structure was solved via direct
methods. Least-squares refinement on F2 using all reflections con-
verged to RF2 ¼ 15:29%. One of the ethyl methylene carbons was
disordered 50:50 over two sites.
at 150 K on a Bruker Apex CCD diffractometer (sealed-tube Mo
radiation) which is controlled via Bruker Smart software. Trisila-
nolethyl-POSS, (CH3CH2)7Si7O12H3, the trisilanolphenyl-POSS,
(C6H5)7Si7O12H3, and the trisilanolcyclopentyl-POSS, (cyclo-C5H9)7-
Si7O12H3 were all obtained from Hybrid Plastics or as a gift from
Dr. Andre Lee from Michigan State University and used as received.
Triethylamine, anhydrous tetrahydrofuran (THF), c-butyrolactone
(GBL), and dodecane were all obtained from Sigma–Aldrich chem-
ical and used as received. Compound 1 was synthesized as de-
Crystallization of
3 from a minimum amount of toluene
afforded yellow crystals of 3. C58H71ClFeNO12Si7–2(C6H5CH3),
M = 1446.36, monoclinic space group, P21/n, a = 15.079(3),
b = 20.224(4), c = 25.423(4) Å, b = 100.642(4)o, V = 7619(2) Å3,
Z = 4, Dcalc = 1.261 Mg/m3; F(0 0 0) = 3052,
l ;
Mo = 0.401 mmꢀ1
crystal dimensions: 0 0.22 ꢁ 0.34 ꢁ 0.35 mm. The structure was
solved via direct methods. Least-squares refinement on F2 using
all reflections converged to RF2 ¼ 27:52%. There are two toluene
solvate molecules per asymmetric unit which were constrained
to fit approximately rigid hexagons. The crystal was very weakly
diffracting which caused problems in the refinement including
poorly shaped ADP’s, unexpected variations in Ueq values, and
some atypical bond lengths. These problems are noted in the
checkcif report. Crystallization of 4 from a minimum amount of
toluene afforded yellow crystals of 4. C51H99ClFeNO12Si7,
M = 1206.24, monoclinic space group, P21/c, a = 15.4303(16),
b = 22.127(2), c = 40.539(4) Å, b = 93.408(2)°, V = 13817(2) Å3,
scribed in the literature [16]. Compound
according to the literature procedure [17].
5 was prepared
2.2. Synthesis of compound 2
A colorless solution of the trisilanol-POSS, (CH3CH2)7Si7O12H3,
(0.200 g, 0.336 mmol) in toluene (4.0 mL) was treated with 3 equiv.
of triethylamine (141 lL, 1.01 mmol). The mixture was stirred for
5 min at room temperature before 1 equiv. of solid 5 (0.148 g,
0.336 mmol) was added. The yellow solid did not immediately dis-
solve. After stirring for 10 min, the solution had turned yellow and
a precipitate had formed. A white precipitate was collected by fil-
tration. The yellow filtrate was concentrated in vacuo giving a yel-
low oil, which was then extracted with acetonitrile. A majority of
the yellow oil dissolved except for a small amount of insoluble
white precipitate. These were removed by filtration, and the fil-
trate was concentrated in vacuo to give yellow crystalline material,
which was then extracted with toluene and then concentrated in
vacuo to afford a yellow powder after being dried under vacuum.
Z = 8, Dcalc = 1.159 Mg/m3; F(0 0 0) = 5192,
l ;
Mo = 0.428 mmꢀ1
crystal dimensions: 0.21 ꢁ 0.28 ꢁ 0.34 mm. The structure was
solved via direct
methods. Least-squares refinement on F2 using all reflections con-
verged to RF2 ¼ 23:54%. Crystals diffracted very weakly which
caused problems in the refinement including poorly shaped ADP’s,
unexpected variations in Ueq values, and some atypical bond
lengths. These problems are noted in the checkcif report. One of
the cyclopentyl groups was disordered over two 50% occupancy
orientations.
2.2.1. Compound 2
Yield: 83.9%. Anal. Calc. for C30H71FeClNO12Si7 (925.81): C,
38.92; H, 7.73; N, 1.51. Found: C, 38.89; H, 7.63; N, 1.54%. IR
(KBr,
m
(cmꢀ1)): 2967s, 2877s, 1481m, 1461m, 1415w, 1383w,
1252s, 1086vs, 940s, 881m, 753s, 696vs, 492s, 458s.
2.5. Aerobic oxidation of the tetrahydrofuran
2.3. Synthesis of compounds 3 and 4
In a 10-mL micro-scale round bottom flask, 0.0500 mol% of the
iron complex was dissolved in 2.00 mL of THF. The resulting solu-
tion was golden yellow. Three stacked micro-scale water con-
densers were added to the round bottom flask to prevent
evaporation of the solvent. The mixture was stirred for 5 days
at room temperature. Afterwards, the reaction mixture was ana-
lysed by infrared spectroscopy (IR) and gas chromatography
(GC). Dodecane was used as an internal standard during the GC
analysis, and the turnover number from four separate trials were
averaged to give the reported values. On separate occasions, sim-
ilar reaction mixtures were analysed by gas chromatography–
Compounds 3 and 4 were prepared following a procedure anal-
ogous to the synthesis of 2. Similar molar quantities of the appro-
priate trisilanol-POSS were used, but due to their lower solubility
in toluene, twice the volume of solvent was used as well as longer
reaction times were necessary.
2.3.1. Compound 3
Yield: 82.2%. Anal. Calc. for C58H71FeClNO12Si7 (1262.09): C,
55.19; H, 5.67; N, 1.11. Found: C, 55.12; H, 5.49; N, 1.12%. IR

M.T. Hay et al. / Polyhedron 28 (2009) 2183–2186
2185
Cl
R
O
R
O
Si OH
OH
OH
+
Si
O
Fe
O
NBu4
O
R
O
O
O
R
3 Et3N /Toluene
O
R
Si
Si
O
R
Si
Si
O
+ [NBu4][FeCl4]
R
Si
Si
R
Si
Si
O
O
- 3 Et3NHCl
O
O
R
R
O
O
O
O
Si
Si
Si
Si
O
O
R
R
R
R
Reaction 2. THF free synthesis of 1–4.
mass spectrometry (GC–MS) to determine the identities of the
THF oxidation products.
3.2. X-ray analysis of iron (III) chloro-POSS compounds
The iron (III) center in 2–4 all have the same coordination
geometry previously observed in 1 [15,16], a distorted tetrahedral
geometry composed of a single chlorine atom and three trisilanate
oxygen atoms. The Fe–Cl bond distances in 1–4 are very similar
[Fe–Cl: 1, 2.2536(10) Å; 2, 2.2534(5) Å, 3, 2.227(2) Å, and 4,
2.256(1) Å], while the average Fe–Otrisilanate bond length of 1–4
are also very close [Fe–Otrisilanate: 1, 1.842 ( 0.008) Å; 2, 1.84
( 0.01) Å, 3, 1.835( .009) Å, and 4, 1.825( .009) Å]. Clearly, the four
complexes are structurally comparable within the coordination
environment of the iron and differ only in the type and thus the
size of the hydrocarbon substitutents on the POSS ligand.
3. Results and discussion
3.1. Synthesis of iron (III) chloro-POSS compounds
The conversion of THF to GBL was originally detected when 1
was synthesized and crystallized in a solution of THF under aerobic
conditions [4]. A general THF free synthesis was therefore devel-
oped in order to prepare 1–4 (Reaction 2). The corresponding POSS
trisilanol was treated with 3 equiv. of triethylamine in toluene.
Although 5 was insoluble in toluene, the reaction between 5 and
the POSS ligand still occurred albeit slower and with a considerable
amount of stirring. After reaction times ranging from 5 to 60 min,
depending on the POSS, triethyl ammonium chloride salt and a
small amount of unreacted 5 were removed by filtration. The
resulting yellow toluene solution was concentrated under reduced
pressure to give a yellow oil, which was then extracted with aceto-
nitrile. The trisilanol POSS ligands are insoluble in acetonitrile,
while 1–4 are soluble, therefore unreacted trisilanol POSS ligand
can be easily removed from the reaction mixture increasing the
purity of the product. A final extraction with either hexane or
toluene, depending on the product, resulted in the formation of
1–4 in relatively high yields.
3.3. Aerobic oxidation of the tetrahydrofuran
A yellow solution of 0.0500 mol% 1–5 in anhydrous THF was
stirred open to the atmosphere for five days. The IR spectrum of
the reaction mixtures of each sample was compared to the IR spec-
trum of pure THF (Fig. 2). New bands were detected that are indic-
ative of oxidation products: a shoulder at 3416 cmꢀ1, a broad band
at 3293 cmꢀ1, and two sharper bands at 1778 cmꢀ1 and 1726 cmꢀ1
.
The peaks in the 3000 cmꢀ1 range are characteristic of hydroxyl
(–OH) functionality, while the peaks in the 1700 cmꢀ1 range sup-
port the presence of carbonyl (C@O) groups [18]. The 1778 cmꢀ1
peak is consistent with the IR of pure GBL obtained in our labora-
tory as well as the literature value [19].
According to the GC–MS results, GBL was confirmed as the ma-
jor oxidation product for the aerobic oxidation of THF. In addition,
two minor products were also detected. The more abundant of the
two minor species was 2-hydroxy-THF. The shoulder at 3416 cmꢀ1
in the IR also supports its presence [20]. It can undergo a ring-chain
tautomerism to form 4-hydroxybutanal [21], however, the GC–MS
did not detect the ring opened species (Reaction 3). In the liquid
phase, however, the 1726 cmꢀ1 peak in the IR is consistent with
4-hydroxybutanal [22]. Therefore, it is possible that either the
gas phase equilibrium favoured the cyclic tautomer in the MS or
the ring-opened form did not elute from the GC column. The third
most abundant species detected by the GC–MS is believed to be a
dihydroxy-THF molecule. An M+ peak of 104 in the mass spec is
consistent with a molecular formula of C4H8O3, and the broad band
at 3293 cmꢀ1 in the IR would also support the presence of a second
hydroxyl containing species. We were unable to definitively con-
firm which dihydroxy-THF isomer was present in the reaction
mixture.
THF
Reaction Mixture
1778 cm-1
3416 cm-1
H
H
O
O
C
1726 cm-1
3293 cm-1
H2
C
OH
H
H
C
C
C
OH
H
C
H2
C
H2
C
H
H
3400
2400
1400
400
wavenumber (cm-1)
H
2-hydroxy THF
4-hydroxybutanal
Fig. 2. Infrared spectral data of pure THF (top) and a typical reaction mixture
(bottom).
Reaction 3. Ring-chain tautomerism of 2-hydroxy-THF and 4-hydroxybutanal.

2186
M.T. Hay et al. / Polyhedron 28 (2009) 2183–2186
Table 1
dissolving them in THF, we have demonstrated that all four com-
Turnover numbers for THF oxidation products.
plexes are capable of either catalysing this oxidation reaction or
acting as pre-catalysts. The low turnover numbers observed with
5 supports that the presence of the POSS ligand significantly stabi-
lizes the catalysts allowing for dramatically higher yields of GBL. A
comparison of the turnover numbers for 1–4 also shows that by
varying the substitutent on the POSS, the stability of the catalyst
can be improved: the POSS with the smallest substitutent, 2, pro-
vides the most stable catalyst.
Iron (III) chloro complex
THF to GBL turnover
number
Total oxidation turnover
number
Isobutyl POSS (1)
Ethyl POSS (2)
Phenyl POSS (3)
Cyclopentyl POSS (4)
No POSS (5)
74 ( 7)
114 ( 9)
89 ( 7)
82 ( 7)
6 ( 2)
100 ( 8)
170 ( 10)
130 ( 10)
120 ( 9)
14 ( 3)
Supplementary data
As of this study, it is not clear that the iron (III) chloro-POSS
complexes (1–4) remain intact during the oxidation of THF, and
are thus responsible for the catalytic conversion of THF to GBL. In
our previous report [4], we showed that an equivalent of GBL co-
crystallized with 1, suggesting that 1 remains intact during the oxi-
dation of THF. In this work, though, we were unable to isolate 1–4
from any of the THF reaction mixtures, possibly due to its low con-
centration. Also, the iron (III) center is paramagnetic [4], making
simple spectroscopic methods for characterizing the catalytic iron
species difficult. It is possible that the iron (III) chloro-POSS com-
plexes could function as a pre-catalyst by potentially loosing the
chloride ligand or reacting with water or oxygen to generate the
catalytic species. As a result, we calculated turnover numbers for
the THF oxidation reaction based on the initial number of moles
of iron (III) in 1–5.
CCDC 706342, 706344 and 706343 contain the supplementary
crystallographic data for 2, 3 and 4. 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: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgements
We thank the University College of the Pennsylvania State Uni-
versity for their financial support. Special thanks go to Dr. Andre
Lee (Michigan State University) for providing several samples of
the trisilanol POSS and Dr. Robert Minard (Pennsylvania State Uni-
versity) for his assistance in obtaining and interpreting the GC–MS
data.
Since the reaction mixture contained multiple oxidation prod-
ucts of THF, we calculated both the THF to GBL turnover numbers
as well as the turnover numbers for the total oxidation of THF
(Table 1). The iron (III) chloro-POSS complexes (1–4) generated be-
tween 74 and 114 equiv. of GBL per mole of iron (III), while the to-
tal number of oxidation equivalents per iron (III) ranged from 100
to 170. As a control, 5 was dissolved in THF under the same condi-
tions and only 6 turnovers of GBL and 14 total oxidation turnovers
were measured. Apparently, the POSS ligand enhances the stability
of the catalytic species resulting in the higher turnover numbers.
This may be a result of the POSS ligand’s electron withdrawing
ability, which would result in a more Lewis acidic iron center [23].
To place the iron (III) chloro-POSS system’s turnover numbers in
context, a review of the literature was conducted revealing only
two previously published examples of the catalytic oxidation of
THF to GBL by dioxygen in the presence of an iron complex. The
first system used FeCl2ꢂ(1.5 THF) under a pressure of dioxygen
and carbon dioxide [14]. The second system used a carboxylate-
bridged diiron complex in the presence of a triarylphosphine in
air-saturated THF [13]. Both systems produce multiple THF oxida-
tion products, including GBL. Approximately 2.5 turnovers of GBL
were obtained after a reaction time of about 8 days from the
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Clearly, the GBL observed in the crystal structure of 1 is formed
by the aerobic oxidation of THF [4]. By first synthesizing the four
iron (III)-containing POSS compounds (1–4) in toluene and then