Covalent heterogenization of a discrete Mn(II) Bis-Phen complex by a metal-template/metal-exchange method: An epoxidation catalyst with enhanced reactivity

Article abstract of DOI:10.1021/ja0742030

Considerable attention has been devoted to the immobilization of discrete epoxidation catalysts onto solid supports due to the possible benefits of site isolation such as increased catalyst stability, catalyst recycling, and product separation. A synthetic metal-template/metal-exchange method to imprint a covalently attached bis-1,10-phenanthroline coordination environment onto high-surface area, mesoporous SBA-15 silica is reported herein along with the epoxidation reactivity once reloaded with manganese. Comparisons of this imprinted material with material synthesized by random grafting of the ligand show that the template method creates more reproducible, solution-like bis-1,10-phenanthroline coordination at a variety of ligand loadings. Olefin epoxidation with peracetic acid shows the imprinted manganese catalysts have improved product selectivity for epoxides, greater substrate scope, more efficient use of oxidant, and higher reactivity than their homogeneous or grafted analogues independent of ligand loading. The randomly grafted manganese catalysts, however, show reactivity that varies with ligand loading while the homogeneous analogue degrades trisubstituted olefins and produces trans-epoxide products from cis-olefins. Efficient recycling behavior of the templated catalysts is also possible.

Full text of DOI:10.1021/ja0742030

Published on Web 03/20/2008
Covalent Heterogenization of a Discrete Mn(II) Bis-Phen
Complex by a Metal-Template/Metal-Exchange Method: An
Epoxidation Catalyst with Enhanced Reactivity
Tracy J. Terry and T. Daniel P. Stack*
Department of Chemistry, Stanford UniVersity, Stanford, California 94305
Received June 8, 2007; E-mail: stack@stanford.edu
Abstract: Considerable attention has been devoted to the immobilization of discrete epoxidation catalysts
onto solid supports due to the possible benefits of site isolation such as increased catalyst stability, catalyst
recycling, and product separation. A synthetic metal-template/metal-exchange method to imprint a covalently
attached bis-1,10-phenanthroline coordination environment onto high-surface area, mesoporous SBA-15
silica is reported herein along with the epoxidation reactivity once reloaded with manganese. Comparisons
of this imprinted material with material synthesized by random grafting of the ligand show that the template
method creates more reproducible, solution-like bis-1,10-phenanthroline coordination at a variety of ligand
loadings. Olefin epoxidation with peracetic acid shows the imprinted manganese catalysts have improved
product selectivity for epoxides, greater substrate scope, more efficient use of oxidant, and higher reactivity
than their homogeneous or grafted analogues independent of ligand loading. The randomly grafted
manganese catalysts, however, show reactivity that varies with ligand loading while the homogeneous
analogue degrades trisubstituted olefins and produces trans-epoxide products from cis-olefins. Efficient
recycling behavior of the templated catalysts is also possible.
Introduction
The first covalently attached epoxidation catalysts involved
1
3
titanium embedded in silica supports and many variations on
The epoxidation of olefins is of great interest due to the
importance of epoxides in the manufacture of both bulk and
fine chemicals. Epoxides serve as useful starting materials in
the synthesis of a variety of functionalized organic compounds,
as the epoxide ring reacts readily with a wide range of
nucleophiles with high regioselectivity.1,2 The facile and re-
giospecific opening of terminal epoxides makes this class of
epoxide particularly useful in the production of industrially
important products such as surfactants, corrosion protection
agents, and additives. While a wide range of homogeneous
catalysts for olefin epoxidation exist,4 heterogeneous catalysts
offer many potential advantages including easy product separa-
tion, long lifetime, and durability. For these reasons, much
IV
VI
VI
this theme of Lewis acid transition metals (Ti , W , Mo ,
V
VI
IV
V , Cr , Zr ) incorporated into silica supports have followed
1
4-24
over the past 40 years.
The high oxidation state of these
metals, steric constraints of the zeolite supports, and long
reaction times of these catalysts generally restrict the substrate
scope to small, electron-rich olefins that form stable epoxides
able to withstand the reaction conditions. Inspiration for many
of the newer generation of heterogeneous epoxidation catalysts
derives from recent advances in discrete homogeneous oxidation
catalysts.
3
-6
Immobilization of discrete metal complexes has led to several
efficient and reusable epoxidation catalysts, which oxidize
7
8-11,25-27
electron-rich olefins.
Few of these materials, however,
attention has focused on the development of heterogeneous
catalysts for epoxidation reactions.7
maintain high reactivity with terminal olefins and are even less
-12
(
13) Wulff, H. P.; Wattimena, F. US Patent 4367342, 1970.
(
(
(
1) Smith, J. G. Synthesis 1984, 629-656.
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1887-1888.
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New York, 1997.
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Am. Chem. Soc. 2007, 129, 7680-7689.
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Chem. Commun. 1992, 8, 589-590.
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Ricchiardi, G.; Lamberti, C.; Zulian, R.; Zecchina, A. J. Catal. 1998, 179,
64-71.
(
(
(
7) De Vos, D. E.; Sels, B. F.; Jacobs, P. A. Immobilization of Homogeneous
Oxidation Catalysts; Academic Press, Inc.: 2001; Vol. 46.
8) De Vos, D. E.; Sels, B. F.; Jacobs, P. A. AdV. Synth. Catal. 2003, 345,
(19) Tuel, A.; Gontier, S.; Teissier, R. J. Chem. Soc., Chem. Commun. 1996,
651-652.
4
57-473.
(20) Dongare, M. K.; Singh, P.; Moghe, P. P.; Ratnasamy, P. Zeolites 1991,
11, 690-693.
9) De Vos, D. E.; Jacobs, P. A. Catal. Today 2000, 57, 105-114.
(
(
(
10) Kamata, K.; Yonehara, K.; Sumida, Y.; Yamaguchi, K.; Hikichi, S.; Mizuno,
(21) Singh, A.; Selvam, T. J. Mol. Catal. A 1996, 113, 489-497.
(22) Rigutto, M. S.; Bekkum, H. V. J. Mol. Catal. 1993, 81, 77-98.
(23) Ulagappan, N.; Rao, C. N. R. J. Chem. Soc., Chem. Commun. 1996, 9,
1047-1048.
N. Science 2003, 300, 964-966.
11) De Vos, D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Chem. ReV. 2002,
1
02, 3615-3640.
12) Terry, T. J.; Dubois, G.; Murphy, A.; Stack, T. D. P. Angew. Chem., Int.
Ed. 2007, 46, 945-947.
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8560.
10.1021/ja0742030 CCC: $40.75 © 2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 4945-4953
9
4945

A R T I C L E S
Terry and Stack
effective with more electron-deficient olefins such as R,â-
unsaturated ketones and esters. The few heterogenized transition
metal catalysts that epoxidize electron-deficient olefins, such
as a covalently tethered Ru-porphyrin system,24 have limitations
including multiple reaction byproducts and a narrow substrate
tion.35 To construct coordination sites with two tetherable 1,-
10-phenanthroline ligands, hereafter referred to as a bis-phen
coordination, we have used a metal-template/metal-exchange
method at low surface loadings to ensure correlated pairs of
covalently attached phenanthroline ligands. These templated
materials loaded with manganese show enhanced epoxidation
reactivity with PAA over a range of ligand loadings compared
to materials prepared with randomly grafted phenanthroline
ligands. The latter are less selective epoxidation catalysts with
variable reactivity dependent on loading. A similar templating
approach led to an imprinted ferrous bis-phen species, which
possesses a coordination that is thermodynamically unstable in
a homogeneous solution. These iron materials were shown to
2
5,26
scope.
No reported catalysts effectively combine high
productivity and reusability with simple oxidants and a wide
substrate scope.
II
[
Mn (phen)2]X2, in which X is a weakly coordinating anion
and phen is 1,10-phenanthroline, was recently reported as a
highly active, homogeneous, electrophilic epoxidation catalyst
with peracetic acid (PAA) as the oxidant.28 The high reactivity,
broad substrate scope, simple oxidant, and convenient reaction
II
2+
12
conditions make [Mn (phen)2] and PAA an attractive epoxi-
be active epoxidation catalysts but do not approach the
efficiency, selectivity, substrate scope, or recyclability achieved
with the manganese catalyst materials reported herein.
II
2+
dation system. While the [Mn (phen)2] /PAA system is ef-
ficient with many unfunctionalized disubstituted and R-olefins
as well as R,â-unsaturated ketones and esters, limitations exist
possibly related to radical processes and/or side reactions. [Mn -
phen)2] requires 2 equiv of PAA per alkene at low catalyst
Experimental Section
II
General Considerations. The following chemicals were used as
received: P123 (Aldrich), tetraethyl orthosilicate (TEOS) (Aldrich),
2
+
(
loading (<0.05 mol %) for high conversions, degrades electron-
rich tri- and tetrasubstituted olefins, and cis-olefins are partially
isomerized yielding trans-epoxide products. As reactivity studies
thionyl chloride (SOCl
Gelest, Inc.), pentanes (Aldrich), sodium diethyldithiocarbamate
(Et NCS Na) (Acros), anhydrous acetonitrile (MeCN) (Aldrich), acetic
acid (J. T. Baker), 50% H O (EMD), and methanol (MeOH) (VWR).
2 2
Anhydrous tetrahydrofuran (THF) was obtained from a packed bed
solvent purification system using an alumina column. Amberlite IR-
2
) (Fluka), 3-mercaptopropyl triethoxysilane
(
2
2
29
suggest that the active catalyst is monomeric, immobilized
II
2+
[
Mn (phen)2] was targeted for further study. We now report
II
2+
improvement in the catalytic activity of [Mn (phen)2] by
covalently attaching this species onto a porous SBA-15 silica
support.
1
20 resin (Fluka) was rinsed with acetic acid before use. All syntheses
were performed under a N atmosphere using standard Schlenk line
2
techniques unless otherwise specified. NMR spectra were obtained on
an Inova 300 MHz NMR spectrometer with a Varian Inova console
using Solaris 2.7 software.
Recent advances in the heterogenization of discrete homo-
geneous catalysts predominantly employ tethering of single,
multidentate ligands such as a porphyrin, salen, or 1,4,7-
triazacyclononane, which provide the requisite metal coordina-
Synthesis of SBA-15. Micelle templated silica SBA-15 was prepared
according to the literature using a triblock copolymer as the surfactant
7,8
36
tion for catalytic activity. Random grafting of such complexes
onto a support ensures the retention of the homogeneous
coordination sphere and catalytic reactivity. Reports of im-
mobilized coordination environments for a single metal created
template. In a typical synthesis, 12 g of P123 was dissolved in 90
mL of distilled water and 360 g of 2 M HCl with stirring at 40 °C.
Once the solution was visibly homogeneous, 27 mL of TEOS were
added dropwise to the solution over 1 min. The mixture was stirred at
40 °C for 24 h, transferred to a glass pressure vessel, and heated in an
oven at 100 °C for 24 h. The resulting material was filtered with
deionized water and ethanol, air-dried, and calcined at 550 °C in air
for 5 h to remove the surfactant template.
II
2+
from several independent ligands, such as [Mn (phen)2] , are
limited. Immobilization into zeolites without covalent attachment
II
2+
of [Mn (bpy)2] (bpy ) bipyridine) significantly improved the
30
catalytic epoxidation reactivity with H2O2. This encapsulation
Ligand Syntheses. Ethyl 4-(3-(triethoxysilyl)propylthio)-1,10-
II
2+
strategy is suggested to site-isolate [Mn (bpy)2] , preventing
formation of polynuclear µ-oxo or µ-hydroxo complexes that
efficiently disproportionate H2O2.31 Covalent imprinting of
coordination sites composed of several independent ligands is
also limited with the greatest success in the area of organic
H
phenanthroline-3-carboxylate, 1 (Scheme 1), was prepared by adding
37
6
g of 4-hydroxy-[1,10]phenanthroline-3-carboxylic acid ethyl ester
to 20 mL of SOCl and a catalytic amount of DMF for 1 h at 85 °C.
2
After cooling, the SOCl
separated as a solid from a mixture of 500 mL of 10% K
mL of ethyl acetate to give 5.3 g of crude 4-chloro-[1,10]phenanthroline-
-carboxylic acid ethyl ester. This material was recrystallized from hot
heptanes (75% yield). The product was stirred with 1.4 equiv of
-mercaptopropyl triethoxysilane and K CO in 100 mL of anhydrous
THF under N for 12 h at 65 °C. The mixture was cooled and filtered,
2
was removed under vacuum and the product
2
CO and 500
3
3
2
supports. Borovik et al. report a particularly interesting
example of a covalently attached multiligand coordination
complex for nitric oxide delivery in which both the metalated
porphyrin and the axial ligand are maintained upon immobiliza-
tion.3
3
3
2
3
2
3,34
and the THF was removed under vacuum to give a yellow oil.
A significant challenge in grafting organic molecules onto
porous materials is controlling their concentration and distribu-
Dissolution of the oil in pentane followed by cooling to -115 °C
separated 1H in a pure form. H NMR (300 MHz, CDCl
1
1
3
, δ): 9.20 (s,
H), 9.17 (d/d, 1H), 8.59 (d, 1H), 8.23 (d/d, 1H), 7.86 (d, 1H), 7.63
d/d, 1H), 4.74 (m, 2H), 3.67 (q, 6H), 2.96 (t, 2H), 1.61 (m, 2H), 1.43
(
25) Yu, X.-Q.; Huang, J.-S.; Yu, W.-Y.; Che, C.-M. J. Am. Chem. Soc. 2000,
(
1
22, 5337-5342.
+
+
(
(
(
(
(
26) Sasidharan, M.; Wu, P.; Tatsumi, T. J. Catal. 2002, 205, 332-338.
27) Holbach, M.; Weck, M. J. Org. Chem. 2006, 71, 1825-1836.
28) Murphy, A.; Pace, A.; Stack, T. D. P. Org. Lett. 2004, 6, 3119-3122.
29) Murphy, A.; Stack, T. D. P. J. Mol. Catal. A 2006, 251, 78-88.
30) Knopsgerrits, P. P.; De Vos, D.; Thibaultstarzyk, F.; Jacobs, P. A. Nature
(t, 3H), 1.08 (t, 9H), 0.64 (m, 2H). HRMS-EI [M ], calculated m/z )
4
88.1801, found m/z ) 488.1787. Anal. Calcd for C24
32 2 5
H N O SSi: C,
5
8.99; H, 6.60; N, 5.73; O, 16.37; S, 6.56. Found: C, 57.62; H, 6.31;
N, 5.72; S, 5.92. The phen derivative 2 was prepared in a similar manner
1
994, 369, 543-546.
(31) Okawa, H.; Sakiyama, H. Pure Appl. Chem. 1995, 67, 273-280.
(32) Becker, J. J.; Gagne, M. R. Acc. Chem. Res. 2004, 37, 798-804.
(33) Welbes, L. L.; Borovik, A. S. Acc. Chem. Res. 2005, 38, 765-774.
(34) Mitchell-Koch, J. T.; Padden, K. M.; Borovik, A. S. J. Polym. Sci., Part
A: Polym. Chem. 2006, 44, 2282-2292.
(35) Corriu, R. J. P.; Mehdi, A.; Reye, C.; Thieuleux, C. Chem. Mater. 2003,
16, 159-166.
(36) Zhao, D. Y.; Huo, Q. S.; Feng, J. L.; Chmelka, B. F.; Stucky, G. D. J. Am.
Chem. Soc. 1998, 120, 6024-6036.
(37) Markees, D. G. HelV. Chim. Acta 1983, 66, 620-626.
4946 J. AM. CHEM. SOC.
9
VOL. 130, NO. 14, 2008

An Epoxidation Catalyst with Enhanced Reactivity
A R T I C L E S
1
in 70% overall yield. H NMR (300 MHz, CDCl
3
, δ): 9.25 (s, 1H),
GC. The catalyst material was washed with acetone, and after drying
in air, 10 mg were reserved for sulfur and manganese ICP analysis.
The remaining catalyst used in subsequent trials was assumed to fully
retain all of the initial manganese.
Inductively Coupled Plasma Atomic Emission Spectroscopy
(ICP). Each 10-20 mg sample of vacuum-dried material was dissolved
in ca. 1 mL of boiling 5% KOH and diluted to 10 mL with deionized
(DI) water. A 5 mL aliquot was acidified with 1 mL of concentrated
9
1
.21 (d/d, 1H), 8.66 (d, 1H), 8.27 (d/d, 1H), 7.90 (d, 1H), 7.67 (d/d,
H), 4.51 (m, 2H), 3.50 (m, 1H), 1.46 (t, 3H), 1.23 (d, 6H). HRMS-
EI [M ], calculated m/z ) 326.1089, found m/z ) 326.1084.
Templating and Cu Removal Procedure. In a typical synthesis,
1H (500 mg, 1.02 mmol) was combined with 190 mg (0.51 mmol) of
+
+
I
[
1
Cu(CH
50 mL of MeCN were added to form a deep brown solution before
the addition of 7.5 g of SBA-15. The solution was stirred overnight at
3 4 6 2
CN) ]PF in a 200 mL Schlenk flask under N . Approximately
3
HNO before dilution to 10 mL with DI water. Each basic solution
I
7
0
0 °C and filtered in air to give Cu T (Scheme 1) with a loading of
.11 mmol g of 1 (1 ) 1 after covalent attachment to the silica
surface). Approximately 75% of 1 was immobilized to the silica by
this procedure as determined by sulfur analysis; the ligand/copper ratio
was filtered through a 0.45 µm polytetrafluoroethylene filter and
submitted for sulfur analysis, while each acidic solution was filtered
through a 0.45 µm polyethersulfone filter and submitted for metal
analysis. ICP analysis was conducted on a TJA IRIS Advantage 1000
Radial ICAP spectrometer with a solid-state CID detector.
-
1
C
C
H
H
I
I
was 2.2(1):1. Removal of Cu from Cu T was accomplished with several
metal complexing agents including EDTA or sodium diethyldithiocar-
bamate. In a typical procedure, 7 g of Cu T was stirred with 300 mL
Porosimetry Analysis. Dinitrogen adsorption and desorption iso-
therms at -196 °C were measured using a Micromeritics ASAP 2010
porosimeter. Surface area calculations were conducted using the BET
(Brunauer-Emmett-Teller) method,39 and the pore diameter calcula-
tions were conducted using the BJH (Barrett-Joyner-Halenda)
I
of 0.1 M sodium diethyldithiocarbamate in CH
and rinsed with CH OH and acetone. This process was repeated three
times to give the bis-phen templated SBA-15, T, with less than 0.002
3
OH for 2 h, filtered,
3
-
1
40
mmol g Cu.
method.
Grafting Procedure. In a typical synthesis of the randomly grafted
1H in SBA-15 to give G, 810 mg (1.65 mmol) of 1 and 2 g of SBA-
EPR Analysis. EPR spectra were collected on a Bruker EMX
spectrometer with a Bruker ER 041XG QR microwave bridge and ER
4102ST cavity. For solid samples, ca. 6 mg of each material was
suspended in ca. 0.1 mL of solvent and cooled to -196 °C. Similar
spectra were obtained in ethanol or a 2:1 mixture of 2-methyltetrahy-
drofuran and propionitrile. For homogeneous samples, ca. 0.5 mM
samples in a 2:1 mixture of 2-methyltetrahydrofuran and propionitrile
H
1
5 were stirred overnight in 100 mL of MeCN in a 200 mL Schlenk
flask under N at 70 °C. Filtering gave G with a loading of 0.10 mmol
2
-
1
C
H
g
1 ; only ca. 15% of 1 used in the reaction was covalently attached
H
to the silica as determined by sulfur analysis, but the unreacted 1 in
the filtrate could be reused.
II
II
Grafting Procedure for High Ligand Density Materials. In a
typical synthesis of the high ligand density materials, 1.60 g (3.30
were cooled to -196 °C. The Mn complexes [Mn (1,10-phenanthro-
line)]2 , [Mn (1,10-phenanthroline)
+
II
] , and [Mn (1,10-phenanthro-
2+
II
2
H
line)3]2+ were prepared in situ by mixing of appropriate molar quantities
mmol) of 1 and 0.35 g of SBA-15 were stirred for 48 h in 100 mL of
II
II
MeCN in a 200 mL Schlenk flask under N
with a loading of 0.91 mmol g 1 .
Metalation Procedure. Approximately 0.2 mmol of [M(CF
2
at 70 °C. Filtering gave G
of Mn (CF
3
SO
)
3 2
and 1,10-phenanthroline while [Cu (2)
2
](CF
3
SO
3
)
2
+
-
1
C
was isolated before sample preparation {[CuII(2) ](CF SO ) : MS-ES
2
3
3 2
M
+], calculated m/z ) 357.6, found m/z ) 358.0; MS-ES+ [M ],
2
+
[
3
C
3 2
SO ) ]
II
II
II
calculated m/z ) 864.1, found m/z ) 864.4}. Simulations were
performed using Simfonia software.
(
M ) Mn or Cu ) per gram of material (2 equiv M per 1 ) were
stirred in CH OH overnight before filtering. To remove any weakly
coordinated metal, the metalated materials were stirred in refluxing
CH OH for 2 h, filtered and washed with copious amounts of CH OH.
3
Results
3
3
Peracetic Acid Preparation. In a 20 mL Nalgene bottle equipped
with a magnetic stir bar, 50% hydrogen peroxide (1.75 mL) was added
to a slurry of acetic acid (15 g) and Amberlite IR-120 (0.5 g). The
slurry was stirred overnight in an ice bath and filtered through a glass
microfiber filter (1.2 µm pores). The PAA mol % was determined by
Derivatization of the 1,10-phenanthroline ligand backbone
was necessary to allow immobilization and quantification of
the ligand. The propyltrialkoxysilane moiety allows for covalent
attachment of 1H to the silica surface while the sulfur atom,
purposefully incorporated, allows quantitative comparisons of
the covalently attached ligand to metal and counteranion content
from a single digested sample by Inductively Coupled Plasma
1
3
integration of the C NMR peaks of acetic acid and peracetic acid in
D
2
O or by a standard titration.38 This preparative method yields ca. 8
mol % PAA solutions, which were stored at -20 °C in a Nalgene bottle
and used within a month of synthesis.
II
spectroscopy (ICP). Ligand 2 and [Mn (2)2(CF3SO3)2] provide
a homogeneous comparison to the spectroscopic and reactivity
properties of the heterogenized ligand and catalysts.
Representative Epoxidation Conditions. In a representative reac-
II
-1
C
tion, a mixture of 85 µL of MeCN, 5 mg of Mn T (0.11 mmol g 1 ,
H
0
1
.30 µmol Mn, 0.5 mol %), 8 µL of vinylcyclohexane (58 µmol), and
µL of n-decane (internal standard, 0.51 µmol) was prepared in a 10
Two methods of covalent attachment of 1 on the SBA-15
silica were investigated: random grafting and metal-templating
mL test tube at 2 °C with a stir bar. 66 µL of an 8% PAA (1.5 equiv)
were added dropwise over 2 min. The reaction was stirred for an
additional 8 min, diluted with diethyl ether, and filtered through a basic
alumina plug in a glass pipet. GC analysis of the solution provided the
substrate conversion and product yield relative to the internal standard.
to create materials G and T, respectively (Scheme 1). Grafting
involved random attachment of 1 to the support surface in the
H
absence of any transition metal to form an anticipated uncor-
C
related distribution of covalently attached ligands, 1 . Metalation
II
C
of this material with 2 equiv of [Mn (CF3SO3)2] per 1 in
1
The epoxide product was identified by H NMR and by a comparison
of the GC retention time to an authentic sample.
II
methanol gave Mn G, after removal of any weakly associated
manganese with methanol washings. Metal-templating involved
Recycling Conditions. Each trial was setup as described above (0.5
mol % Mn, 0.4 M vinylcyclohexane, MeCN, 1.5 equiv 8% PAA,
I
H
+
an initial formation of an air stable [Cu (1 )2] complex
II
II
followed by covalent attachment to the SBA-15 silica to yield
2
2
°C) except that 100 mg of Mn T or Mn G were used initially in a
I
Cu T. Removal of the copper gave T, with an anticipated
0 mL scintillation vial. The PAA, prepared on the day of the
experiment, was added dropwise over ca. 5 min to ensure that the heat
of the reaction was efficiently dissipated. After 10 min, the catalyst
material was filtered and an aliquot of the solution was analyzed by
correlated distribution of pairs of covalently attached ligands.
(
39) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309-
19.
3
(
40) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73,
373-380.
(38) Dixon, W. T. Talanta 1966, 13, 1199-1200.
J. AM. CHEM. SOC.
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VOL. 130, NO. 14, 2008 4947

A R T I C L E S
Terry and Stack
lower pH (1 M nitric acid) over 4 h demonstrated the overall
stability of the material under acidic conditions, as the former
increased the surface area by 42%, while the latter resulted in
only a 4% change. The preferred demetalation procedure of 4
h at ambient conditions with a 0.1 M aqueous solution of sodium
diethyldithiocarbamate only resulted in a small change in the
surface area (+3%).
Material Analysis. A maximum loading of 0.9 mmol g-
1
1C could be achieved under random grafting conditions using a
ca. 10-fold excess of 1 over extended reaction times (48 h) at
H
7
0 °C in acetonitrile. This loading is close to the standard full
-
1
loading of ca. 1.2 mmol g of an alkyl triethoxysilane on
mesoporous silica with a surface area of ca. 600 m g . At
1
to allow a single metal center to be ligated by two 1 .
Assuming a tether length of ca. 10 Å for 1 , the distance
2
-1 43
-1
mmol g loadings, however, the ligands are sufficiently close
C 42
C 42
between the silicon atom and the center of the two coordinating
nitrogen atoms of the ligand in an extended alkyl chain
conformation, an average separation of ca. 20 Å is needed to
II
II
Metalation of T with [Mn (CF3SO3)2] gave Mn T. The catalytic
C
achieve site isolation of a single 1 on the surface. Such site
II
II
epoxidation reactivity of Mn T and Mn G are compared
herein, each with several concentrations of covalently attached
phenanthroline ligands.
Material Stability. SBA-15 is a readily synthesized, micelle-
templated, high surface area, mesoporous silica that is stable to
acidic conditions.41 The material used in this investigation had
C
2
-1
isolation of 1 on a 600 m g material requires a ligand loading
-1
of no more than ca. 0.30 mmol g . Significantly more efficient
and selective reactivity and consistent material synthesis (vide
infra) are possible at the lower ligand loadings of 0.3, 0.1, and
-
1
C
0.025 mmol g
of 1 .
Copper removal and reloading from the templated materials
was a fully reversible process as assessed by copper analysis
(Table 1, entries 3-5). Manganese uptake by the templated
an average pore diameter of ca. 65 Å and an average surface
2
-1
area of ca. 650 m g and was found to be stable to the PAA
12
oxidation conditions used herein. The surface area and pore
diameter were determined after the material was subjected to
various reaction conditions employed during the synthesis,
modification, and catalytic epoxidation trials.42 Only basic
solutions of EDTA significantly altered the pore structure and
surface area of the support; exposure of the materials for 1 h to
a 0.1 M EDTA at pH 5 and 8 resulted in a 17% decrease and
C
materials at various 1 loadings (0.30, 0.11, and 0.025 mmol
-
1
C
II
g
) consistently yielded ca. 2:1 ratios of 1 /Mn (Table 1,
entries 6-8). By contrast, the manganese uptake by randomly
C
grafted materials, at various 1 loadings (0.90, 0.10, and 0.025
-
1
mmol g ) yielded materials with less predictable ligand to
C
manganese ratios. At low loadings, a 1 /metal ratio near 1:1
II
II
70% increase, respectively, in the surface area. Further analysis
was observed consistently for Mn G and Cu G (Table 1,
entries 11-13). Control experiments indicate that the unmodi-
of the SBA-15 stability at both higher pH (neat pyridine) and
H
Scheme 1. Schematic Representation of the Covalent Attachment of 1 to the SBA-15 Silica via Random Grafting and Metal-Templating
II
II
a
Methods To Form Mn G and Mn T, Respectively
a
H
II
C
I
H
+
(
a) Random grafting of 1 , (b) metalation with an excess of [Mn (CF3SO3)2] per 1 followed by washing with CH3OH, (c) formation of [Cu (1 )2]
followed by covalent attachment, (d) demetalation with Et2NCS2Na.
4
948 J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008
9

An Epoxidation Catalyst with Enhanced Reactivity
A R T I C L E S
Table 1. Metal and Ligand Content in Materials from ICP Analysis
ligand 1C
metal
(
mmol g-1)
(mmol g-1)
entry
material
±
0.01a
±
0.002a
1C/metal
b
c
1
2
3
4
5
6
7
8
9
0
1
2
3
SBA-15
SBA-15
Cu T
0.00
0.00
0.11
0.11
0.11
0.30
0.11
0.025
0.10
0.90
0.10
0.025
0.10
0.001
0.005
0.051
0.001
0.052
0.14
0.055
0.013
0.001
0.10
I
2.2
T
Cu T
Mn T
Mn T
II
2.1
2.1
2.0
1.9
II
II
II
Mn T
G
Mn G
Mn G
Mn G
II
1
1
1
1
9.0
1.2
1.1
1.7
II
0.085
0.028
0.057
II
II
Cu G
a
Standard deviations determined from a minimum of three ICP measure-
b
ments on the same material. Manganese concentration in unmodified SBA-
c
1
[
5. Manganese concentration in unmodified SBA-15 after metalation with
II
Mn (CF3SO3)2] and washing with CH3OH.
fied silica retains no appreciable metal after the metalation/
washing conditions (Table 1, entry 2).
II
X-band EPR spectroscopy at -196 °C of the all the Mn
II
materials yielded nearly identical spectra. The spectra of Mn T,
II
II
2+
II
Mn G, and the homogeneous complexes [Mn (2)1] , [Mn -
Figure 1. X-band EPR signals in 2-methyltetrahydrofuran/propionitrile (2/
2+
II
2+
(
2)2] , and [Mn (2)3] each displayed a six-line, g ≈ 2 signal
II
-1
C
II
-1
1
) at -196 °C of (a) Cu T (0.1 mmol g 1 ), (b) Cu T (0.1 mmol g
II
II
42
C
typical of a high-spin Mn species. The X-band EPR spectra
1 ) with 2 equiv of (n-Bu4N)Cl per copper, (c) [Cu (2)2](OTf)2 (1 mM),
II
-1
C
II
-1
C
II
(d) Cu G (0.08 mmol g 1 ), and (e) Cu G (0.08 mmol g 1 ) with 2
equiv of (n-Bu4N)Cl per copper.
of the Cu materials were a more sensitive probe of the
coordination environments. The spectra of [Cu (2)2] , Cu G,
and Cu T all displayed characteristic mononuclear axial
signals consistent with a dx -y electronic ground state. The
spectra of [Cu (2)2](CF3SO3)2 and Cu T are comparable with
similar signals at g⊥ ) 2.06 and g| ) 2.270, while Cu G shows
II
2+
II
II 12
II
Table 2. Epoxidation Reactivity of Mn Catalysts with
Vinylcyclohexanea
2
2
II
II
II
a significant shift of the parallel feature to lower field (g| )
II
II
3%)b,d
selectivity (±
3%)c,d
2
.295). None of the EPR spectra of Cu G and Cu T at 0.1
catalyst
yield (
±
-
1
C
mmol g 1 shifted with a change in solvent conditions from
[MnII(phen) ]2+ e
95
72
95
72
2
II
2+ e
ethanol to 2-methyltetrahydrofuran/propionitrile (2:1). The EPR
[Mn (phen)1]
II
II
II
2
2+ e
2+ e
spectra of Cu T and [Cu (2)2](CF3SO3)2 were not affected by
the addition of 2 equiv of (n-Bu4N)Cl per copper in 2-meth-
[Mn (2) ]
80
73
85
73
II
[
Mn (2)1]
II
-1
C
yltetrahydrofuran/propionitrile (2:1), while a second signal (g|
Mn T (0.30 mmol g 1 )
Mn T (0.11 mmol g 1 )
Mn T (0.025 mmol g 1 )
97
98
98
97
98
98
II
-1
C
II
)
2.255) appeared in the EPR spectrum of Cu G (Figure 1).
II
-1
C
II
II
Substrate Scope and Epoxide Selectivity. Mn T, Mn G,
II
-1
C
II
2+
Mn G (0.90 mmol g 1 )
Mn G (0.11 mmol g 1 )
Mn G (0.025 mmol g 1 )
80
83
72
84
83
72
and [Mn (2)2] were investigated initially for their ability to
II
-1
C
II
epoxidize vinylcyclohexane (Table 2), and Mn T was found
II
-1
C
to be the most reactive and selective of all the catalysts. The
II
a
high reactivity and epoxide selectivity of Mn T is constant at
0.5 mol % Mn, 0.4 M vinylcyclohexane, 2 °C, 10 min, MeCN; 1.8
equiv of PAA were used with the homogeneous catalysts, and 1.5 equiv of
PAA were used with the immobilized catalysts. Each catalyst converts
95% of the vinylcyclohexane. Epoxide yield relative to an internal
standard as determined by GC. Epoxide selectivity indicates the percentage
C
each ligand loading of 1 , while the lower epoxide selectivity
II
with Mn G parallels that of the homogeneous system if only 1
b
>
II
c
equiv of 2 or 1,10-phenanthroline (phen) per Mn is used in
d
of vinylcyclohexane that is converted to epoxide. Average of five reactions;
the catalyst preparation.
the results were reproducible with different preparations of catalysts at the
II
The reaction rate with Mn T is ca. 5× faster than that with
same ligand loadings. e The homogeneous catalysts were prepared in situ.
II
Mn G at a similar catalyst loading while maintaining higher
selectivity for the epoxide (Figure 2). The reaction profiles
reported with 1-octene parallel the reactivity presented in Table
epoxide selectivities (Table 3). Since reasonable epoxide yields
II
2+
and selectivities are obtained for [Mn (2)2] with disubstituted
and terminal olefins (Table 3, entries 3-11), the advantages of
covalent attachment of the catalysts to the silica are most evident
with the trisubstituted electron-rich olefins (Table 3, entry 1)
and the more electron-deficient olefins (Table 3, entries 12-
2
with vinylcyclohexane.
Mn T functions with a wider range of substrates than the
II
homogeneous catalysts and requires less oxidant for higher
(
41) Cassiers, K.; Linssen, T.; Mathieu, M.; Benjelloun, M.; Schrijnemakers,
II
2+
1
6). [Mn (2)2] fully oxidizes trisubstituted olefins without
K.; Van Der Voort, P.; Cool, P.; Vansant, E. F. Chem. Mater. 2002, 14,
2
317-2324.
appreciable epoxide production and isomerizes ca. 5% of internal
(
42) See Supporting Information.
43) Zhao, X. S.; Lu, G. Q.; Whittaker, A. K.; Millar, G. J.; Xhu, H. Y. J.
Phys. Chem. B 1997, 101, 6525-6531.
II
cis-olefins to the trans-epoxide products. By contrast, Mn T
(
provides nearly a quantitative yield of trisubstituted epoxide and
J. AM. CHEM. SOC.
9
VOL. 130, NO. 14, 2008 4949

A R T I C L E S
Terry and Stack
rable manganese loadings. Even with the extended reaction time,
II
the epoxide yields with Mn G were lower. ICP analysis of the
II
II
used catalysts, both Mn G and Mn T, showed no significant
loss in the sulfur content, and reloading of the used catalysts
with [Mn(CF3SO3)2] reestablished their original reactivity.
Discussion
SBA-15 silica materials are attractive as supports for oxidation
catalysts as the materials are readily synthesized and are
2
-1
oxidatively stable. The large surface area (>600 m g ) and
large average pore diameter (>50 Å) of these materials also
provide for potential site isolation of covalently attached
catalysts and facile diffusion of reagents and products, respec-
tively. Our previous work with non-heme manganese epoxida-
tion catalysts identified [Mn(II)(1,10-phenanthroline)2]2 as a
very active catalyst for a wide range of electron-deficient olefins
using peracetic acid as an oxidant. A metal-template/metal-
exchange procedure was developed to create bis(1,10-phenan-
throline) metal coordination sites on the SBA-15 surface. The
metal template was a stable 2:1 complex of ethyl 4-(3-
II
Figure 2. Conversion of 1-octene to 1,2-epoxyoctane by Mn T (b) (0.11
+
-
1
C
II
-1
C
mmol g 1 ) and Mn G (9) (0.10 mmol g 1 ) in MeCN at 2 °C with
.5 equiv of PAA.
1
no measurable isomerization of cis-olefins (Table 3, entries 1
and 2). Mn G is also more selective than the homogeneous
catalysts with ca. 90% epoxide yields of trisubstituted olefins,
yet with ca. 5% isomerization of cis-2-heptene to its trans-
epoxide product. Mn T mediates efficiently epoxidation of R,â-
unsaturated ketones and esters, whereas the conversion of
electron-deficient olefins is incomplete with [Mn (2)2] at 0.5
mol % Mn and 1.8 equiv of PAA per olefin. Increasing the
number of equivalents of PAA with [Mn (2)2] did not improve
the epoxide yields. In the cases of highly electron-deficient
olefins that were not fully epoxidized with 1.5 equiv of PAA
per olefin and Mn T, higher yields and selectivities were
possible by the addition of more oxidant (Table 3, entries 13-
II
H
(triethoxysilyl)propylthio)-1,10-phenanthroline-3-carboxylate (1 )
II
I
H
+
with Cu(I), [Cu (1 )2] , and was initially formed in solution
I
C
+
and subsequently attached to the silica to give [Cu (1 )2]
II
2+
(Scheme 2). This four-coordinate, tetrahedral Cu(I) complex
H
exists as a single diastereomer with the nonsymmetric 1 ligand.
II
2+
By contrast, a metal with a hexacoordinate tendency such as
Mn(II) would almost certainly create a diastereomeric mixture
of metal complexes, leading potentially to an increased hetero-
geneity of the templated sites. Another advantageous property
of the copper is its substitution lability. Copper is removed
II
4
4,45
1
6).
readily with an appropriate chelating agent.
Metalation of
the material with manganese completes the metal-template/
metal-exchange procedure.
Chemoselectivity. The electrophilic chemoselectivity of these
Mn oxidants is evident from the reactivity with ethyl sorbate
Table 3, entry 14); the more electron-rich olefin is oxidized
II
(
As measured by the pore size and surface area, the SBA-15
materials were not altered significantly by either the metal-
template/metal-exchange procedure or the aggressive epoxida-
tion reaction conditions used herein. Lower ligand loadings by
preferentially yielding the 4,5-monoepoxide product before
significant formation of the diepoxide product. Intermolecular
competition experiments with 1-octene and allyl acetate, at low
conversions, further highlight the electrophilic nature of the
active oxidants (Table 4). The bis-coordinated homogeneous
-
1
C
this template procedure (<0.3 mmol g 1 ) resulted in more
efficient and selective manganese catalysts, presumably because
of the greater site isolation of the templated sites allowing for
a greater homogeneity of formed manganese complexes. The
II
2+
II
2+
catalysts, [Mn (phen)2] and [Mn (2)2] , show the highest
selectivity for the more electron-rich 1-octene substrate, while
II
II
2+
II
C
the monoligated Mn complexes, [Mn (phen)1] and [Mn -
metal-templated materials yielded consistently 2:1 1 /metal
2
+
(
2)1] , exhibit lesser chemoselectivity in the epoxidation
ratios upon metal reloading with a variety of divalent metal ions
C
reaction. The chemoselectivity in these competition experiments
(Fe, Mn, Cu). This coincides with the imprinted bis-1
II
2+
I
C
+
with [Mn (phen)1] were invariant over a 10-fold range of Mn
concentrations, consistent with a mononuclear active oxidant
coordination environment from [Cu (1 )2] , which is retained
upon metal exchange. Random grafting of the free ligand at
C
(
Table 4, entries 2-4).
low loadings exhibited 1 /metal ratio less than 2:1 after
II
II
Catalyst Recycling. With careful control of the reaction
metalation with either Mn or Cu . Such low ratios are
consistent with a distribution of bis-1 and mono-1C ligated
metals.
II
C
conditions, Mn T is a durable, reusable catalyst. Under the
II
conditions noted in Table 1, a sample of Mn T was reused
X-band EPR analysis of the CuII complexes provided a
five times without a significant change in the manganese content
in the materials, in the epoxide yields, and in the required
amount of oxidant. Efficient control of the heat of the reaction
was necessary to ensure that manganese did not leach from
sensitive probe into the environments of ligated Cu(II) in the
II
II
templated (Cu T) and randomly grafted (Cu G) materials. The
axial EPR spectra of both copper-loaded materials at 0.1 mmol
II
-1
C
Mn T, which reduced the catalyst efficiency in subsequent runs.
g
of 1 is consistent with on-average site isolation, as tight
II
The recycling behavior of the Mn T materials with ligand
packing of copper complexes would result in broad isotropic
-
1
C
loadings of 0.33 and 0.13 mmol g 1 were similar (Table 5).
II
In contrast, Mn G suffered from significant loss of reactivity
(44) Smith, R. M. Martell, A. E. Critical Stability Constants; Plenum Press:
New York, 1975; Vol. 2.
and metal leaching upon reuse (Table 5), and consecutive
reactions required longer times to reach completion at compa-
(
45) Dietrich-Buchecker, C. O.; Sauvage, J. P.; Kern, J. M. J. Am. Chem. Soc.
1984, 106, 3043-3045.
4950 J. AM. CHEM. SOC.
9
VOL. 130, NO. 14, 2008

An Epoxidation Catalyst with Enhanced Reactivity
Table 3. Epoxidation Activity of MnIIT a and [Mn (2)2]2+ b
A R T I C L E S
II
a
-1
C
b
0
.1 mmol g 1 , 0.5 mol % Mn, 1.5 equiv of PAA, 0.4 M substrate, 2 °C, 10 min, MeCN. 0.5 mol % Mn, 1.8 equiv of PAA, 0.4 M substrate, 2 °C,
c
d
e
1
0 min, MeCN. No appreciable trans epoxide product was detectable by GC or GC/MS. GC and GC/MS showed ca. 5% trans epoxide. 2.8 equiv of
PAA were added for full conversion. 3.0 equiv of PAA gave 98% diepoxide product. 4.4 equiv of PAA yielded 55% 4,5-monoepoxide and 45% diepoxide
after 30 min. Yield indicates the diepoxide product with 2.8 equiv of PAA. 1.8 equiv of PAA were required for full conversion. Isolated yield of 80%.
f
g
h
i
j
Table 4. Chemoselectivity of Mn Catalysts in Intermolecular Competition Experimentsa
II
a
b
Reaction conditions: 0.43 µmol Mn, 32 µmol of 1-octene, 64 µmol of allyl acetate, 18 µmol of PAA, 0.4 M olefin, 2 °C, MeCN. Averaged over five
c
II
reactions. Generated in situ with the appropriate [Mn (CF3SO3)2]/ligand ratio.
II
2+
signals. A more detailed comparison of the spectra suggests that
the copper coordination in Cu T more closely resembles the
studies of [Cu (bpy)2] (bpy ) bipyridine) show clearly that
II
exchange of the dinitrogen bpy chelate for oxygen ligand(s)
such as oxalate or water yields larger g| values (Table 6).
II
2+
48
49
tetra-nitrogen environment of homogeneous [Cu (2)2] than
II
II
that in Cu G. The larger g| values (ca. 2.29) of Cu G as
compared to Cu T (ca. 2.27) suggest an increase in the oxygen
atom ligation at the expense of nitrogen atom ligation. Solution
II
(
46) Peisach, J.; Blumberg, W. E. Arch. Biochem. Biophys. 1974, 165, 691-
46
708.
J. AM. CHEM. SOC.
9
VOL. 130, NO. 14, 2008 4951

A R T I C L E S
Terry and Stack
a
II
Table 5. Recycling Trials: Mn Concentration and Epoxide Yield of
Table 6. X-Band EPR Parameters of Some Cu Complexes
II
II
a
Mn T and Mn G Materials
4
complex
g
|
g
⊥
g
⊥
A (10- cm-1)
ref
II
II
II
Mn T
Mn T
Mn G
II
2+
2
C
-
1
0.13 1C (mmol g-1)
0.090 1C (mmol g-1)
[Cu (2) ]
2.270 2.065 2.055 160
2.267 2.060 2.060 155
2.295 2.060 2.060 155
2.220 2.060 2.060 NA
2.228 2.126 2.066 NA
2.315 2.072 2.072 166
herein
herein
herein
48
48
49
0
.
3
3
1
(
m
m
o
l
g
)
II
b
Cu T
trial Mn (mmol g-1)b yield (%) Mn (mmol g-1)b yield (%) Mn (mmol g-1)b yield (%)
II
b
Cu G
II
2+
0
1
2
3
4
5
0.16
0.16
0.15
0.15
0.14
0.15
0.075
0.075
0.075
0.066
0.061
0.075
0.065
0.060
0.030
[Cu (bpy)2]
II
96
96
96
94
96
96
96
96
96
75
70
65
[Cu (bpy)(oxalate)]
II
2+
[Cu (bpy)(OH2)2]
a
NA ) not available. b Materials with loadings of 0.1 mmol g 1C are
-1
II
II
reported. Results are consistent with Cu G and Cu T materials with
-
1
C
loadings from 0.3 to 0.03 mmol g 1 .
a
0
.5 mol % Mn, 1.5 equiv of PAA, 0.4 M vinylcyclohexane, 2 °C, 10
b
min, MeCN. ICP analysis.
optimal for epoxide product selectivity (Table 2). Not surprising
II
is that Mn T materials maintain high reactivity and epoxide
selectivity over a range of ligand loadings while Mn G
Scheme 2
II
materials exhibit a loss of epoxide selectivity as the ligand
loading decreases. The electrophilic chemoselectivity of the
II
homogeneous Mn catalysts varies with the number of coor-
dinating ligands, as assessed through intermolecular competition
II
experiments (Table 4). Mn T materials exhibit consistent
II
chemoselectivity over a range of ligand loadings while Mn G
materials vary with ligand loading. Immobilization and site
isolation of a bis-1 coordination sphere confer a greater
consistency in the epoxidation reactivity of the templated
materials than is available from the homogeneous catalysts or
the randomly grafted materials.
Very few electrophilic oxidants reported are as effective as
II
Mn T/PAA at converting electron-deficient olefins in R,â-
unsaturated ketones and esters to their corresponding epoxides,
II
and yet Mn T/PAA is also able to convert electron-rich
trisubstituted olefins to their respective epoxides in high yields.
II
2+
This latter result is striking given that [Mn (phen)2] with PAA
in a homogeneous solution only degrades trisubstituted olefins
to a variety of oxidized products, none of which is the epoxide.
The lesser degree of isomerization determined with internal cis-
II
olefins to trans-epoxide products with Mn T as compared to
II
2+
[
Mn (phen)2] (∼5% trans-epoxide) suggests that surface
immobilization reduces radical-like reactivity. Whether this is
due to a reduced lifetime of a transition state with radical
character or a reduction in the number and type of active
manganese oxidant(s) in the materials is unknown. The greater
efficiency exhibited by Mn T as compared to [Mn (2)2] also
The residual silica silanol groups, with a pKa near that of a
50
47
carboxylic acid (∼5), are potential oxygen ligands, though
II
II
2+
water is also possible.
The addition of (n-Bu4N)Cl to Cu G (2 equiv/Cu(II))
significantly alters the EPR spectrum with the appearance of
multiple features in the g| region. Such behavior is consistent
with variable ligation of chloride to copper yielding a hetero-
II
emphasize the benefits of catalyst site isolation by precluding
biomolecular catalyst deactivation processes. Mn T efficiently
II
converts oxygen-functionalized olefins such as ethyl sorbate to
the epoxide product (Table 3, entry 14) with only a slight excess
II
2+
II
of PAA, while [Mn (2) ] yields significantly less epoxide
geneous distribution of copper sites. Similar titrations with Cu T
2
while requiring a greater amount of oxidant.
at comparable ligand loadings only resulted in a slight shifting
of the g| signals to a lower field. The copper coordination
appears to remain homogeneous with well-formed bis-phenan-
II
Recycling studies with Mn T confirm the robust nature of
C
the bis-1 templated materials. However, dropwise addition of
freshly prepared PAA over 5 min is necessary to achieve high
product yields, which ensures that the heat of the reaction is
dissipated in the larger recycling runs. Reaction conditions that
tholine coordination.
Beyond the metal/ligand ratios and CuII EPR studies, the
catalytic epoxide yields and substrate chemoselectivity further
II
C
II
II
lead to manganese leaching from Mn T do not damage the
support a predominance of bis-1 Mn species in Mn T and
C
II
II
covalently attached ligands, as reconstitution of this material
to full manganese loading recovers full catalytic activity. Control
experiments with unmodified material and Mn(CF3SO3)2 show
no measurable substrate conversion with R-olefins under the
mono-1 Mn species in Mn G. Reactivity of the homogeneous
II
II
2+
II
2+
Mn catalysts, [Mn (phen)n] , and [Mn (2)n] (n ) 1 or 2)
reveals that two coordinated 1,10-phenanthroline ligands are
(
(
(
47) Rosenholm, J. M.; Czuryskiewicz, T.; Kleitz, F.; Rosenholm, J. B.; Linden,
M. Langmuir 2007, 23, 4315-4323.
(50) Another explanation of the cis-olefin isomerization difference between the
II
2+
48) Walker, F. A.; Sigel, H.; McCormick, D. B. Inorg. Chem. 1972, 11, 2756-
Mn T and [Mn(phen)
2
]
is different mechanisms of epoxidation. However,
2
763.
the similarity of the product distributions in the olefin competition
experiments argues against such a postulate.
49) Walker, F. A.; Sigel, H. Inorg. Chem. 1972, 11, 1162-1164.
4
952 J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008
9

An Epoxidation Catalyst with Enhanced Reactivity
A R T I C L E S
epoxidation reaction conditions used. Neither the production of
epoxide nor the selectivity of the materials may be attributed
to leaching metal.
prove beneficial for the immobilization of other catalysts that
require multiple ligands in their coordination spheres and may
provide access to coordination geometries not accessible in a
homogeneous solution.
Summary
We have developed a metal-template/metal-exchange method
to control the distribution of covalently attached independent
ligands in mesoporous silica at low loadings. Covalent attach-
ment of a correlated distribution of closely positioned 1,10-
phenanthroline ligands allows a bis-phen coordination to MnII
to be achieved and maintained in the material at various ligand
loadings. These imprinted manganese complexes are efficient
and recyclable heterogeneous epoxidation catalysts using PAA
and exhibit a greater substrate scope, more efficient oxidant use,
and higher product selectivity than either its homogeneous or
randomly grafted analogue. The correlated distribution of two
Acknowledgment. This work was supported by the NIH
Grant GM-50730. The authors thank Dr. G. Li from the Soil
and Environmental Biogeochemistry Department at Stanford for
ICP analysis and Prof. E. I. Solomon for the EPR spectrometer
use. The UCSF Mass Spectrometry Facility, supported by the
Biomedical Technology Resource Centers Program of the
National Center for Research Resources, NIH NCRR RR01614,
provided the HRMS data.
Supporting Information Available: Surface area and pore
diameter measurements of SBA-15 materials exposed to various
reaction conditions are provided along with calculations of the
estimated surface area coverage of a single tethered 1,10-
1
,10-phenanthroline ligands is proposed to be critical to the
consistent reactivity at various ligand loadings. Indeed, random
grafting of the identical ligand allows the formation of competent
catalysts, but the yield and the selectivity of the reaction are
not as favorable and vary with ligand loading. The side reactions
that occur with the homogeneous analogue leading to isomer-
ization of cis-substrates and deactivation of the catalyst are
attenuated significantly by covalent attachment and site isolation
into the imprinted materials. Similar template processes may
C
phenanthroline ligand, 1 , and the average surface area available
on the material for various ligand loadings. EPR spectra of the
II
Mn complexes and materials are also available. This material
is available free of charge via the Internet at http://pubs.acs.org.
JA0742030
J. AM. CHEM. SOC.
9
VOL. 130, NO. 14, 2008 4953