Inorganic Chemistry
Article
4
2
high-valent iron−oxo oxidant. On the other hand, a
supporting reductant is necessary for an oxygen-dependent
catalyst that would help to make the “ready oxidant” from aerial
oxygen. Examples of high-valent iron−oxo complex formation
from molecular oxygen in the presence of a proton and electron
Fourier transform infrared spectroscopy on KBr pellets was
performed on a Shimadzu FT-IR 8400S instrument. Elemental
analyses were performed on a Perkin-Elmer 2400 series II CHN
analyzer. Electrospray ionization mass spectrometry (ESI-MS) spectra
were recorded with a Waters QTOF Micro YA263 instrument.
Solution electronic spectra (single and time-dependent) were
measured on an Agilent 8453 diode-array spectrophotometer. All
room temperature NMR spectra were recorded on a Bruker Avance
500 MHz spectrometer. Gas chromatography (GC)−MS measure-
ments were carried out on a Perkin-Elmer Clarus 680 gas
chromatograph coupled with a Clarus SQ8T mass spectrometer with
a maximum temperature of 300 °C using an Elite-5MS (30 m × 0.25
mm × 0.25 μm) column. Labeling experiments were carried out with
43−47
donor are, however, rare.
While cis-dihydroxylation of olefin is commonly performed
by a biomimetic iron complex and H O , the use of molecular
oxygen to perform this reaction remains a challenging task. In
this direction, we have recently shown that benzilic acid, an α-
hydroxy acid, underwent decarboxylation to form benzophe-
none in the reaction of an iron(II) benzilate complex with
2
2
18
4
8
Ph2
O gas (99 atom %) from Icon Services Inc..
[Fe (Tp )(mandelate)(H O)] (1). To a white slurry of KTp
2
2
dioxygen. The iron(II) benzilate complex, [(Tp )-
Fe (benzilate)], was capable of cis-dihydroxylation of cyclo-
II
Ph2
Ph2
II
(
0.35 g, 0.5 mmol) and Fe(ClO ) ·6H O (0.181 g, 0.5 mmol) in
4 2 2
hexene with the incorporation of both of the oxygen atoms of
molecular oxygen into the diol product. In the reaction, the
benzilate anion acts as the sacrificial reductant for dioxygen
reduction on the iron center. To gain deeper insight into the
role of α-hydroxy acids as sacrificial reductants and also to
understand the nature of dioxygen-derived oxidant, we have
explored the dioxygen reactivity of a series of iron(II) α-
hydroxy acid complexes using a monoanionic facial trinitrogen
donor ligand, hydrotris(3,5-diphenylpyrazole-1-yl)borate
Tp ). In this Article, we report the dioxygen reactivity of
three iron(II) α-hydroxy acid complexes, [(Tp )-
Fe (mandelate)(H O)] (1), [(Tp )Fe (benzilate)] (2), and
methanol (5 mL) was added a methanolic solution (5 mL) of
mandelic acid (0.076 g, 0.5 mmol) and triethylamine (70 μL). The
resulting milky-white suspension was stirred at room temperature for 2
h. A white solid precipitated and was isolated by filtration and dried. X-
ray-quality crystals of 1 were grown from a solvent mixture of
dichloromethane and methanol (1:1) at room temperature. Yield: 0.30
48
g (68%). Elem anal. Calcd for C H BFeN O (894.60 g/mol): C,
53 43
6
4
7
3
1.16; H, 4.84; N, 9.39. Found: C, 70.88; H, 4.91; N, 9.50. IR (KBr):
400(br), 3034(m), 2623(m), 1717(vs), 16010(br) 1545(m),
Ph2
1477(m), 1462(m), 1413(m), 1358(m), 1300(m), 1232(m),
(
1
7
190(m), 1171(m), 1065(s), 1009(m), 939(m), 891(m), 764(s),
Ph2
−1
31(s), 698(vs), 611(m), 532(m), 517(m) cm .
II
Ph2
II
II
Ph2
2
[Fe (Tp )(mandelate-d)] (1-d). 1-d was synthesized according
Ph2
II
[
(Tp )Fe (HMP)] (3) (HMP = 2-hydroxy-2-methylpropa-
noate; Scheme 1). We also report the synthesis and reactivity of
to the protocol described for 1 except that mandelic-α-d acid was used
instead of mandelic acid.
II
Ph2
[
Fe (Tp )(benzilate)] (2). Complex 2 was synthesized according
48
to the literature procedure.
Scheme 1. Synthesis of Biomimetic Iron(II) Complexes
[
Fe (TpPh2)(HMP)] (3). A mixture of 2-hydroxy-2-methylpropanoic
II
acid (0.052 g, 0.5 mmol) and triethylamine (70 μL, 0.5 mmol) in
Ph2
methanol (2 mL) was added to a stirring mixture of the ligand KTp
(
0.35 g, 0.5 mmol) and Fe(ClO ) ·6H O (0.181 g, 0.5 mmol) in
4 2 2
methanol (5 mL). The resulting white slurry was allowed to stir at
room temperature for 2 h and filtered. The complex was recrystallized
from a dichloromethane/methanol solvent mixture. Yield: 0.32 g
(
77%). Elem anal. Calcd for C H BFeN O ·CH OH (860.58 g/
49 41 6 3 3
mol): C, 69.78; H, 5.27; N, 9.77. Found: C, 69.59; H, 5.30; N, 10.07.
IR (KBr): 3583(m), 3462(br), 3063(m), 2623(m), 1545(m), 1477(s),
1
1
6
464(s), 1431(m), 1414(m), 1358(m), 1335(m), 1236(m), 1172(vs),
068(m), 1032(m), 1009(m), 968(m), 916(m), 810(m), 764(vs),
−1
98(vs), 671(m), 619(m), 567(m) cm .
II
Ph2
[
Fe (Tp )(MPA)] (4). Complex 4 was synthesized according to
the protocol described for 1 except that α-methoxyphenylacetic acid
was used instead of mandelic acid. The crude compound was
recrystallized from a solvent mixture of dichloromethane/methanol at
room temperature. Yield: 0.32 g (71%). Elem anal. Calcd for
C H BFeN O (890.61 g/mol): C, 72.82; H, 4.87; N, 9.44.
Ph2
II
two iron(II) α-methoxy acid complexes, [(Tp )Fe (MPA)]
(
Ph2
II
4) and [(Tp )Fe (MMP)] (5) (where MPA = 2-methoxy-2-
5
4
43
6
3
phenylacetate and MMP = 2-methoxy-2-methylpropanoate), to
compare their reactivity with the iron(II) α-hydroxy acid
complexes. The formation of an iron−oxygen intermediate
upon dioxygen activation at the metal center of biomimetic
iron(II) complexes together with its reactivity toward different
substrates is discussed.
Found: C, 72.86; H, 4.73; N, 9.27. IR (KBr): 3591(m), 3061(m),
2
1
1
5
924(m), 2824(m), 2606(m) 1676(m), 1597(s), 1545(m), 1477(s),
414(m), 1377(m), 1360(m), 1335(m), 1236(m), 1167(vs), 1119(s),
068(s), 1007(m), 812(m), 764(vs), 696(vs), 669(m), 631(m),
−1
65(m) cm .
FeII(TpPh2)(MMP)] (5). To a stirring solution of KTpPh2 (0.35 g,
[
0
.5 mmol) and Fe(ClO ) ·6H O (0.181 g, 0.5 mmol) in methanol (5
4
2
2
mL) was added dropwise a mixture of 2-methoxy-2-methylpropanoic
acid (0.059 g, 0.5 mmol) and triethylamine (70 μL, 0.5 mmol) in
methanol (5 mL). The white slurry was allowed to stir at room
temperature for 2 h. The precipitated solid was filtered out, dried
under a vacuum, and recrystallized from a dichloromethane/methanol
solvent mixture. Yield: 0.28 g (67%). Elem anal. Calcd for
C H BFeN O (842.57 g/mol): C, 71.27; H, 5.14; N, 9.97.
EXPERIMENTAL SECTION
■
All reagents were purchased from commercial sources and used
without further purification, unless otherwise noted. Solvents were
distilled and dried prior to use. The preparation and handling of air-
sensitive materials were carried out under an inert atmosphere by
using standard Schlenk techniques or in a glovebox. Although no
problem was encountered during the synthesis of these complexes from
Fe(ClO ) hydrate, perchlorate salts are potentially explosive and should
5
0
43
6
3
Found: C, 71.39; H, 4.77; N, 10.14. IR (KBr): 3445(br), 3059(m),
2978(m), 2625(m), 1666(vs), 1545(m), 1479(s), 1464(s), 1414(m),
1377(m), 1030(m), 1009(m), 947(m), 914(m), 874(m), 804(m),
4
2
Ph2
be handled with care! KTp and mandelic-α-d acid were prepared
according to literature procedures.
49
−1
764(vs), 696(vs), 669(m), 567(m) cm .
2
811
dx.doi.org/10.1021/ic402443r | Inorg. Chem. 2014, 53, 2810−2821