Christopher W. Jones et al.
pore volume, 1.03 cm3 gÀ1; average pore diameter, 80 ꢂ)[14] were synthe-
sized according to published procedures.
as well. First, these iron catalysts usually give very low turn-
over numbers (TONs), with typical TONs ranging from two
to six.[6a,c,d,7a] Additionally, these catalysts often utilize rela-
tively expensive organic ligands. One approach to potential-
ly address these shortcomings is to develop supporting strat-
egies for these molecular iron–ligand complex catalysts. The
use of solid-supported catalysts can offer the possibility of
simplified catalyst recovery and recycling, along with the po-
tential to employ flow reactors, leading to enhanced
TONs.[8] In the literature, there are many examples of poly-
mer- and silica-supported iron catalysts for various reac-
tions.[9] Furthermore, if the catalyst deactivates through bi-
molecular catalyst–catalyst interactions, catalyst supporting
strategies that isolate the catalysts from each other may lead
to enhanced catalyst longevity.
To this end, we considered methodologies for the immobi-
lization of molecular iron catalysts on various solid supports.
BPMEN, a common ligand in homogeneous iron catalysi-
s,[6a,b] was modified to contain two different functional
groups that would facilitate straightforward immobilization
on solid supports. By employing two immobilization meth-
odologies, olefin coupling[10] and click chemistry,[11] on two
different supports, a polymer resin and porous silica, four
new supported iron catalysts were prepared. The reactivity
of these supported catalysts was thoroughly studied in four
different oxidation reactions by using aqueous hydrogen
peroxide; catalyst recycling was also considered.
Synthesis of the Modified Ligands 1 and 2
The ligand S-BPMEN, 1, was synthesized through a nucleophilic substitu-
tion reaction. 4-Vinylbenzyl chloride (0.96 mL, 1 equiv) and potassium
carbonate (8.4 g, 10 equiv) were added to a solution of N1-methyl-N1,N2-
bis(pyridin-2-ylmethyl)ethane-1,2-diamine (1.56 g, 6.1 mmol) in acetoni-
trile (50 mL). The resulting suspension was stirred for 3 days. The solid
was filtered and the organic solution was concentrated under vacuum.
Pure 1 was obtained in 95% yield by column chromatography with 5%
MeOH in ethyl acetate. 1H NMR (400 MHz, CDCl3): d=8.56 (t, 2H),
7.64 (m, 3H), 7.39 (m, 5H), 7.18 (t, 3H), 6.76 (dd, 1H), 5.78 (d, 1H),
5.22 (d, 1H), 3.81 (s, 2H), 3.63 (d, 4H), 2.70 (dt, 4H), 2.24 ppm (s, 3H);
13C NMR (100 MHz, CDCl3): d=160.3, 159.5, 149.0, 148.8, 139.1, 136.6,
136.3, 136.2, 136.1, 128.9, 126.1, 122.9, 122.7, 121.8, 121.7, 113.3, 64.3,
64.2, 60.6, 60.3, 58.8, 55.6, 51.8, 42.9 ppm.
Ligand T-BPMEN, 2, was synthesized through reductive amination, ac-
cording to a published procedure.[15] N1-Methyl-N1,N2-bis(pyridin-2-ylme-
thyl)ethane-1,2-diamine (1.6 g, 6 mmol) and 4-ethynylbenzaldehyde
(0.81 g, 6 mmol) were mixed in tetrahydrofuran (THF; 50 mL) and then
treated with sodium triacetoxyborohydride (2 g, 9 mmol). The mixture
was stirred at room temperature under argon for 72 h. A solution of
sodium hydroxide (100 mL 10%) was added to quench the reaction, and
the aqueous solution was extracted by dichloromethane (3ꢁ50 mL). The
organic phase was collected and reduced under vacuum. Pure product
was obtained in 92% yield by column chromatography with 30% metha-
nol in ethyl acetate. 1H NMR (400 MHz, CDCl3): d=8.49 (tm, 2H), 7.59
(m, 2H), 7.48 (d, 1H), 7.39 (dm, 2H), 7.29 (t, 3H), 7.11 (dd, 2H), 3.74 (s,
2H), 3.61 (d, 4H), 3.03 (s, 1H), 2.62 (ddd, 4H), 2.18 ppm (s, 3H);
13C NMR (100 MHz, CDCl3): d=160.0, 159.4, 149.9, 148.8, 140.5, 136.3,
136.2, 132.0, 128.6, 122.9, 122.7, 121.9, 120.5, 83.7, 77.1, 76.9, 76.8, 64.2,
60.6, 58.8, 55.5, 51.8, 42.9 ppm.
Synthesis of Homogenous Complexes [Fe(1)
[Fe(2)(OTf)2] (4)
ACHTUNGRTENUN(NG OTf)2] (3) and
AHCTUNGTRENNUNG
Experimental Section
[Fe(1)(OTf)2] (3) was obtained by a two-step synthesis based on a proce-
ACHTUNGTRENNUNG
dure from the literature.[16] FeCl2 (0.085 g, 0.67 mmol) was mixed with
1 (0.25 g, 0.67 mmol) in dry THF (2 mL) in a glove box filled with N2.
The mixture was stirred for 24 h, and a yellow precipitate was formed.
Complex [Fe(1)Cl2] was collected in 97% yield as a yellow solid by filtra-
tion and dried under vacuum.
General
All chemicals were used as received unless specified otherwise. Divinyl-
benzene (DVB) and 4-vinylbenzyl chloride were passed through basic
alumina immediately prior to use. Cyclohexene was passed through silica
gel twice before use. 1H and 13C NMR spectra were acquired with
a Varian Mercury 400 MHz spectrometer, and chemical shifts were re-
ported in ppm with reference to the corresponding residual nuclei of the
deuterated solvents. Cross-polarization magic-angle spinning (CP-MAS)
solid-state NMR spectra were collected on a Bruker DSX 300 MHz in-
strument. Samples were packed in 7 mm zirconia rotors and spun at
10 kHz. Typical 13C CP-MAS parameters were 10000 scans, a 908 pulse
length of 4 ms, and recycle times of 4 s. FTIR spectra were recorded on
a Bruker IFS 66V/S spectrometer by dispersing samples in potassium
bromide pellets. Elemental analyses were performed by ALS Environ-
mental (Tucson, AZ, USA) and Atlantic Microlab (Norcross, GA, USA).
The reaction selectivity and conversion were determined by using capilla-
ry gas-phase chromatography on a Shimadzu GC 2010 equipped with
A mixture of [Fe(1)Cl2] (161.1 mg, 0.32 mmol) and AgOTf (165.8 mg,
0.64 mmol) was stirred overnight in CH2Cl2 (5 mL) in a glove box. The
AgCl precipitate was removed by filtration, and the filtrate was reduced
in volume and layered with Et2O. The colorless crystals thereby obtained
were isolated by filtration, washed with Et2O, and dried under vacuum to
give the product as a white solid (90%). See below for details regarding
X-ray crystallographic analysis for crystal data and structure refinement
in Table 1. Elemental analysis calcd (%) for C26H28F6FeN4O6S2: C 42.98,
H 3.88, N 7.71; found: C 42.56, H 3.82, N 7.36.
Single crystals of 3 were recrystallized from a mixture of CH2Cl2 and di-
ethyl ether by slow evaporation.
A suitable crystal (0.434ꢁ0.361ꢁ
0.359 mm3) was selected and mounted on a loop with paratone oil on
a profile Bruker APEX2 MoKa diffractometer. The crystal was kept at
173(2) K during data collection. By using Olex2,[17] the structure was
solved with the XS[18] structure solution program with the direct methods
solution method. The model was refined with the ShelXL[19] refinement
package by using least squares minimization.
a
flame ionization detector (FID) and
0.25 mmꢁ0.25 mm). Nitrogen physisorption experiments were conducted
at 77 K by using Tristar II (Micromeritics) system. Each sample
a DB-1701 column (30 mꢁ
a
(ꢀ100 mg) was degassed at 1108C overnight under vacuum. The surface
area of each sample was calculated by using the Brunauer–Emmett–
Teller (BET) equation and the pore diameter and total pore volume
were calculated by using the Broekhoff–de Boer method with the Fren-
kel–Halsey–Hill (BdB-FHH) modification. Only data from the adsorp-
tion branches were used in the calculations. Thermogravimetric analysis
(TGA) was conducted on a Netzsch STA409 instrument under a stream
of nitrogen diluted air (30 sccm nitrogen in 90 sccm air). Samples were
[Fe(2)ACHTUNGTNRE(NUG OTf)2] (4) was synthesized in a similar way. FeCl2 (0.051 g,
0.4 mmol) was mixed with 2 (0.15 g, 0.4 mmol) in dry THF (2 mL) in
a glove box. The mixture was stirred for 24 h, and a yellow precipitate
was formed. Complex [Fe(2)Cl2] was collected in 95% yield as a yellow
solid by filtration and dried under vacuum.
A
mixture of [Fe(2)Cl2] (153 mg, 0.3 mmol) and AgOTf (158 mg,
heated from 30 to 9008C at a rate of 108CminÀ1. Fe
A
0.6 mmol) was stirred overnight in CH2Cl2 (5 mL) in a glove box. The
AgCl precipitate was removed by filtration, and the filtrate was reduced
in volume and layered with Et2O. The colorless crystals thereby obtained
(OTf=triflate),[12] N1-methyl-N1,N2-bis(pyridin-2-ylmethyl)ethane-1,2-dia-
mine,[13] and mesoporous SBA-15 silica (BET surface area, 837 m2 gÀ1
;
&
&
2
Chem. Asian J. 2014, 00, 0 – 0
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!