Iron complexes of dendrimer-appended carboxylates for activating dioxygen and oxidizing hydrocarbons

Article abstract of DOI:10.1021/ja076817a

The active sites of metalloenzymes are often deeply buried inside a hydrophobic protein sheath, which protects them from undesirable hydrolysis and polymerization reactions, allowing them to achieve their normal functions. In order to mimic the hydrophobic environment of the active sites in bacterial monooxygenases, diiron(II) compounds of the general formula [Fe ₂([G-3]COO)₄(4-RPy)₂] were prepared, where [G-3]COO^- is a third-generation dendrimer-appended terphenyl carboxylate ligand and 4-RPy is a pyridine derivative. The dendrimer environment provides excellent protection for the diiron center, reducing its reactivity toward dioxygen by about 300-fold compared with analogous complexes of terphenyl carboxylate ([G-1]COO^-) ligands. An Fe^IIFe^III intermediate was characterized by electronic, electron paramagnetic resonance, Moessbauer, and X-ray absorption spectroscopic analyses following the oxygenation of [Fe₂-([G-3]COO)₄(4-PPy)₂], where 4-PPy is 4-pyrrolidinopyridine. The results are consistent with the formation of a superoxo species. This diiron compound, in the presence of dioxygen, can oxidize external substrates.

Full text of DOI:10.1021/ja076817a

Published on Web 03/11/2008
Iron Complexes of Dendrimer-Appended Carboxylates for
Activating Dioxygen and Oxidizing Hydrocarbons
Min Zhao,† Brett Helms,‡ Elena Slonkina,§ Simone Friedle,† Dongwhan Lee,†
Jennifer DuBois,§ Britt Hedman,*,§ Keith O. Hodgson,*,§ Jean M. J. Fre´chet,*,‡ and
Stephen J. Lippard*,†
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139, Department of Chemistry, UniVersity of California, Berkeley,
and DiVision of Materials Sciences, Lawrence Berkeley National Laboratory, Berkeley,
California 94720-1460, and Department of Chemistry and Stanford Synchrotron Radiation
Laboratory, Stanford UniVersity, Stanford, California 94305
Received September 9, 2007; E-mail: frechet@berkeley.edu; lippard@mit.edu
Abstract: The active sites of metalloenzymes are often deeply buried inside a hydrophobic protein sheath,
which protects them from undesirable hydrolysis and polymerization reactions, allowing them to achieve
their normal functions. In order to mimic the hydrophobic environment of the active sites in bacterial
monooxygenases, diiron(II) compounds of the general formula [Fe2([G-3]COO)4(4-RPy)2] were prepared,
where [G-3]COO- is a third-generation dendrimer-appended terphenyl carboxylate ligand and 4-RPy is a
pyridine derivative. The dendrimer environment provides excellent protection for the diiron center, reducing
its reactivity toward dioxygen by about 300-fold compared with analogous complexes of terphenyl carboxylate
([G-1]COO-) ligands. An FeIIFeIII intermediate was characterized by electronic, electron paramagnetic
resonance, Mo¨ssbauer, and X-ray absorption spectroscopic analyses following the oxygenation of [Fe2-
([G-3]COO)4(4-PPy)2], where 4-PPy is 4-pyrrolidinopyridine. The results are consistent with the formation
of a superoxo species. This diiron compound, in the presence of dioxygen, can oxidize external substrates.
upon irradiation.4 The fourth-generation dendrimer diiron(III)
Introduction
compound used in this work was highly robust against alkaline
hydrolysis, demonstrating that the dendrons stabilize the active
site just as do hydrophobic protein frames.
The active sites of metalloenzymes are often surrounded by
a hydrophobic sheath of amino acids that shields them from
undesirable hydrolysis and polymerization reactions and facili-
tates their normal functions. Dendrimers are highly branched
molecules with well-defined 3-D structures. As in metalloen-
zymes, a site-isolated environment can be provided by dendrons
arranged to protect a core moiety from unwanted side reactions.1
This distinctive property of dendrimers makes them excellent
candidates to mimic a hydrophobic metalloprotein core frame-
work. Dendritically functionalized iron(II) porphyrins are able
to bind O2 reversibly and model the chemistry of heme proteins.2
In addition, a porphyrin compound with a dendrimer at its
periphery exhibited size and shape selectivity due to cavities
formed within the dendrons.3 A dendrimer-derived mimic of a
non-heme diiron protein was achieved when modified triaza-
cyclononanes were used as ligands to prepare diiron(III)
complexes, which could be reduced to their diiron(II) forms
Terphenyl carboxylate ligands recently developed in our
laboratory5,6 and elsewhere7 form diiron(II) complexes that
closely resemble the active site of the hydroxylase of soluble
methane monooxygenase (MMOH) in its reduced state. In the
presence of dioxygen, several of these complexes convert to
di(µ-hydroxo)diiron(III) species that mimic the oxidized state
of MMOH.6,8-12 In some of this chemistry, the formation of
peroxodiiron(III) and/or di(µ-oxo)diiron(IV) intermediates was
suggested, although no direct evidence for such species was
obtained.6
In the present study we have modified the terphenyl car-
boxylate ligand framework to incorporate a dendrimer sheath
by utilizing, in particular, a poly(benzylether) carboxylic acid
[G-3]-COOH (7). Because of the dendritic polybenzylether units
and terminal long-chain alkyl groups of 7, we anticipated that
† Massachusetts Institute of Technology.
(4) Enomoto, M.; Aida, T. J. Am. Chem. Soc. 2002, 124, 6099-6108.
(5) Lee, D.; Lippard, S. J. J. Am. Chem. Soc. 1998, 120, 12153-12154.
(6) Lee, D.; Pierce, B.; Krebs, C.; Hendrich, M. P.; Huynh, B. H.; Lippard, S.
J. J. Am. Chem. Soc. 2002, 124, 3993-4007.
‡ University of California, Berkeley.
§ Stanford University.
(1) Hecht, S.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2001, 40, 74-91.
(2) (a) Collman, J. P.; Fu, L.; Zingg, A.; Diederich, F. Chem. Commun. 1997,
193-194. (b) Jiang, D.-L.; Aida, T. Chem. Commun. 1996, 1523-1524.
(c) Jiang, D.-L.; Aida, T. J. Macromol. Sci. Pure Appl. Chem. 1997, A34,
2047-2055. (d) Zingg, A.; Felber, B.; Gramlich, V.; Fu, L.; Collman, J.
P.; Diederich, F. HelV. Chim. Acta 2002, 85, 333-351.
(3) Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick, K. S. J. Am. Chem. Soc.
1996, 118, 5708-5711.
(7) Tolman, W. B.; Que, L., Jr. J. Chem. Soc., Dalton Trans. 2002, 653-660.
(8) Lee, D.; Du Bois, J.; Petasis, D.; Hendrich, M. P.; Krebs, C.; Huynh, B.
H.; Lippard, S. J. J. Am. Chem. Soc. 1999, 121, 9893-9894.
(9) Lee, D.; Lippard, S. J. Inorg. Chem. 2002, 41, 2704-2719.
(10) Yoon, S.; Lippard, S. J. J. Am. Chem. Soc. 2004, 126, 2666-2667.
(11) Yoon, S.; Lippard, S. J. J. Am. Chem. Soc. 2005, 127, 8386-8397.
(12) Lee, D.; Lippard, S. J. Inorg. Chem. 2002, 41, 827-837.
9
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J. AM. CHEM. SOC. 2008, 130, 4352-4363
10.1021/ja076817a CCC: $40.75 © 2008 American Chemical Society

Iron Complexes of Dendrimer-Appended Carboxylates
A R T I C L E S
1572, 1497, 1454, 1437, 1392, 1369, 1337, 1246, 1226, 1198, 1157,
1089, 1047, 1027, 908, 843, 810, 791, 735, 697. LR-MS (EI), Calcd
for C25H24Br2O4: m/z 546. Found: m/z 546 ([M]+, 20), 494(30), 492-
(15), 475(80), 473(40), 458(20), 456(10), 402(60), 400(30), 385(40),
383(20), 91(100). HR-MS (EI), Calcd. for C25H24Br2O4: m/z 546.0041.
Found: m/z 546.0046. Anal. Calcd for C25H24Br2O4: C, 54.77; H, 4.41.
Found: C, 54.40; H, 4.65.
its diiron complexes would be highly soluble in a variety of
organic solvents and that the dimetallic center would be well
protected. Our previous research on diiron model compounds
revealed that oxygenated intermediates could often be observed
at low temperatures, T < -78 °C. With the dendritic shield,
we envisaged increased thermal stability, since solvent and
substrate access is more restricted, and that we would more
readily be able to investigate the structure and reactivity of these
intermediates.
tert-Butyl 3,5-Di(benzyloxy)-2,6-diphenylbenzoate (3). A reaction
mixture containing 2 (3.87 g, 7.05 mmol), phenylboronic acid (12.9 g,
106 mmol), Pd(PPh3)4 (2.03 g, 1.76 mmol), and K3PO4 (30.0 g, 141
mmol) in 150 mL of dry 1,2-dimethoxyethane was subjected to three
freeze-pump-thaw cycles before heating at 95 °C for 72 h under an
argon atmosphere. The crude mixture was diluted with diethyl ether
and then extracted successively with saturated NH4Cl, saturated
NaHCO3, water, and then brine. The organic layer was dried over
MgSO4 and concentrated. The product was obtained as a colorless solid
(2.75 g, 72%) after column chromatography using a gradient of 4:1
(v/v) chloroform/hexanes to chloroform. Recrystallization from hexanes
yielded colorless needles. 1H NMR (CDCl3) δ (ppm): 7.40-7.25 (m,
16H), 7.16 (d, 2H, J 6.8 Hz), 6.66 (s, 1H), 4.96 (s, 4H), 0.95 (s, 9H).
13C NMR (CDCl3) δ (ppm): 166.8, 155.6, 138.1, 136.9, 135.9, 130.5,
128.4, 127.6, 127.6, 127.0, 126.7, 122.1, 101.0, 81.5, 70.8, 27.2. FT-
IR (cm-1): 3087, 3061, 3030, 2974, 2931, 2875, 1723, 1602, 1585,
1497, 1454, 1368, 1325, 1303, 1255, 1222, 1169, 1083, 1051, 1029,
910, 849, 830, 818, 790, 762, 736, 698. LR-MS (FAB), Calcd for
C37H34O4: m/z 542. Found: m/z 543 ([M + H]+, 100), 487(40), 469-
(25). HR-MS (FAB), Calcd for C37H34O4: m/z 542.2457. Found: m/z
542.2459. Anal. Calcd for C37H34O4: C, 81.89; H, 6.32. Found: C,
81.81; H, 6.51.
Experimental Section
General Considerations. All reagents were purchased from com-
mercial sources and used as received except for Fe(OTf)2‚2MeCN,13
2,6-dibromo-3,5-dihydroxybenzoic acid,14 [FeII (µ-O2CArTol)2(O2CArTol)2-
2
(Py)2] (A),5 [FeII2(µ-O2CArTol)4(t-Bu-Py)2] (B),8 and [FeIII2(µ-OH)2(µ-
O2CArTol)2(O2CArTol)2(t-Bu-Py)2] (C),8 which were prepared according
to literature procedures. All solvents were saturated with nitrogen and
purified by passage through activated Al2O3 columns under nitrogen.
Air-sensitive manipulations were performed under nitrogen in an
MBraun glovebox.
Physical Measurements. 1H NMR spectra were recorded on Inova
300 MHz and Bruker 500 MHz DRX spectrometers. Chemical shifts
were referenced to residual deuterated solvent peaks. FT-IR spectra
were recorded on an Avatar 360 FT-IR instrument, and samples were
prepared as KBr pellets. UV-vis spectra were recorded on a Hewlett-
Packard 8453 diode array spectrophotometer. ESI-MS spectrometry was
performed with an Agilent 1100 Series LC/MSD system. MALDI-TOF
mass spectrometry was performed on a Voyager-DE instrument from
PerSeptive Biosystems using R-cyanohydroxycinnamic acid as a matrix.
FAB and EI mass spectrometry was performed by the UC Berkeley
mass spectrometry facility.
tert-Butyl 3,5-Dihydroxy-2,6-Diphenylbenzoate (4). Hydrogenoly-
sis of 3 (1.93 g, 3.55 mmol) was accomplished using a suspension of
10% Pd/C (200 mg) in 50 mL of a 1:1 (v/v) mixture of methanol and
dichloromethane. The reaction vessel was evacuated and back-filled
with hydrogen gas three times before stirring at ambient temperature
for 16 h. The Pd/C was filtered though Celite, which was then washed
several times with the same solvent mixture used in the reaction. After
removal of the volatiles in vacuo, the product was obtained as a colorless
solid (1.25 g, 97%) that required no further purification. 1H NMR
(CDCl3) δ (ppm): 7.48-7.39 (m, 10H), 6.68 (s, 1H), 5.11 (s, 2H),
0.95 (s, 9H). 13C NMR (CDCl3) δ (ppm): 166.6, 153.3, 136.5, 133.7,
130.6, 129.1, 128.4, 118.3, 102.7, 81.6, 27.2. FT-IR (cm-1): 3505,
3485, 3058, 3023, 3004, 2977, 2931, 2249, 1721, 1696, 1608, 1593,
1496, 1472, 1460, 1392, 1368, 1320, 1253, 1153, 1073, 1040, 968,
910, 848, 790, 764, 733, 702. LR-MS (FAB), Calcd for C23H22O4: m/z
362. Found: m/z 362 ([M]+, 100), 307(90), 289(50). HR-MS (FAB),
Calcd for C23H22O4: m/z 362.1518. Found: m/z 362.1521. Anal. Calcd
for C23H22O4: C, 76.22; H, 6.12. Found: C, 76.12; H, 5.94.
tert-Butyl 2,6-Dibromo-3,5-dihydroxybenzoate (1). A solution
containing 2,6-dibromo-3,5-dihydroxybenzoic acid (3.12 g, 10 mmol),
di-tert-butyl dicarbonate (8.72 g, 40 mmol), and magnesium perchlorate
(22 mg, 0.1 mmol) in 50 mL of nitromethane and 50 mL of acetone
was heated at 40 °C for 16 h. The reaction mixture was then diluted
with 200 mL of ethyl acetate and washed successively with saturated
NH4Cl, saturated NaHCO3, water, and brine. The organic layer was
dried over MgSO4 and concentrated. The residue was purified by
column chromatography using an ethyl acetate/hexanes gradient of 1:2
to 2:1 (v/v). The product was obtained as a colorless solid (1.84 g,
50%). 1H NMR (acetone-d6) δ (ppm): 9.2 (br, 2H), 6.75 (s, 1H), 1.60
(s, 9H). 13C NMR (acetone-d6) δ (ppm): 164.4, 154.2, 139.8, 103.5,
96.7, 82.9, 27.3. FT-IR (cm-1): 3469, 3158, 2980, 2930, 2810, 2666,
2579, 2493, 1690, 1576, 1476, 1453, 1424, 1385, 1368, 1337, 1294,
1248, 1157, 1065, 1045, 971, 834, 807, 739, 690. LR-MS (FAB), Calcd
for C11H12Br2O4: m/z 366. Found: m/z 367 ([M + H]+, 30), 312(30),
295(15), 154(100), 136(75). HR-MS (FAB), Calcd for C11H12Br2O4:
m/z 365.9102. Found: m/z 365.9110. Anal. Calcd for C11H12Br2O4: C,
35.90; H, 3.29. Found: C, 36.20; H, 3.36.
[G-3]-Ester (6). The second-generation bromide dendron 515 (1.62
g, 1.44 mmol) was dissolved in 10 mL of acetone along with 4 (250
mg, 0.691 mmol) and 18-crown-6 (18 mg, 0.068 mmol). To this solution
was added finely pulverized anhydrous K2CO3 (571 mg, 4.14 mmol).
After 16 h at gentle reflux, the reaction mixture was cooled and diluted
with ethyl acetate, filtered, and concentrated. The product was obtained
as a colorless glass (1.67 g, 99%) after column chromatography using
tert-Butyl 2,6-Dibromo-3,5-di(benzyloxy)benzoate (2). A portion
of benzyl bromide (2.08 g, 12.2 mmol), 1 (2.00 g, 5.42 mmol), and
18-crown-6 (284 mg, 1.08 mmol) were dissolved in 40 mL of acetone.
To this solution was added finely pulverized anhydrous K2CO3 (4.50
g, 32.6 mmol). After 16 h at gentle reflux, the reaction mixture was
cooled to room temperature and diluted with ethyl acetate, filtered,
and concentrated. The product was obtained as colorless needles (2.85
g, 96%) after recrystallization from a mixture of hexane and ethyl
1
a mixture of hexane and ethyl acetate (15:1, v/v) as eluent. H NMR
(CDCl3) δ (ppm): 7.44 (d, 4H, J 7.2 Hz), 7.37 (t, 4H, J 7.3 Hz), 7.29
(t, 2H, J 7.2 Hz), 6.75 (s, 1H), 6.53 (s, 8H), 6.49 (s, 2H), 6.44 (s, 4H),
6.41 (s, 4H), 4.98 (s, 4H), 4.79 (s, 8H), 3.93 (t, 16H, J 6.6 Hz), 1.76
(m, 16H), 1.42 (m, 16H), 1.26 (m, 128H), 0.94 (s, 9H), 0.88 (t, 24H,
J 6.8 Hz). 13C NMR (CDCl3) δ (ppm): 166.7, 160.5, 160.0, 155.8,
139.2, 138.9, 138.0, 136.0, 130.7, 127.6, 127.0, 122.0, 105.7, 104.9,
101.6, 100.7, 100.0, 81.5, 70.5, 70.0, 68.0, 31.9, 29.63, 29.60, 29.58,
29.56, 29.39, 29.31, 29.25, 26.0, 22.7, 14.1. FT-IR (cm-1): 2923, 2853,
1
acetate (15:1, v/v). H NMR (CDCl3) δ (ppm): 7.38-7.32 (m, 10H),
6.50 (s, 1H), 5.06 (s, 4H), 1.64 (s, 9H). 13C NMR (CDCl3) δ (ppm):
164.7, 155.1, 140.0, 135.8, 128.8, 128.3, 127.0, 101.2, 101.1, 84.1,
71.5, 28.1. FT-IR (cm-1): 3087, 3065, 3032, 2978, 2932, 2875, 1730,
(13) Hagen, K. S. Inorg. Chem. 2000, 39, 5867-5869.
(14) Borchardt, R. T.; Sinhababu, A. K. J. Org. Chem. 1981, 46, 5021-5022.
(15) Helms, B.; Liang, C. O.; Hawker, C. J.; Fre´chet, J. M. J. Macromolecules
2005, 38, 5411-5415.
9
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A R T I C L E S
Zhao et al.
1725, 1597, 1450, 1377, 1331, 1289, 1258, 1160, 1057, 830, 720, 698,
673. LR-MS (MALDI-TOF): Calcd. for C161H250O16: m/z 2439.9;
Found: m/z 2462.4 ([M + Na]+, 100). Anal. Calcd for C161H250O16:
C, 79.20; H, 10.32. Found: C, 79.53; H, 10.59.
a long needle for 30 s. Spectra were then recorded every 30 s for 15
min and with an increment of 20% in the cycle time thereafter. For
substrate reactions, this procedure was continued for 1.5 h to achieve
full conversion to the oxygenated intermediate 10. Ar was then bubbled
through the solution for 15-20 min to remove excess O2. At this point,
substrate was injected via an airtight syringe. The reaction temperature
was maintained at -29 °C for 2 h, and then the solution was gradually
allowed to warm up to room temperature overnight. During workup,
Chelex was added to the reaction mixture, which was then stirred for
two h to remove metal ions. The Chelex was filtered off and washed
with a copious amount of CH2Cl2. The filtrates were combined, and
the solvent was removed in vacuo. The products were analyzed by GC-
MS with 1-bromodecane as an internal standard.
GC-MS Analyses. All gas chromatography was performed on an
Agilent 6890 gas chromatograph connected to an Agilent 5973 Network
mass selective detector. An HP-5ms (5%-phenyl)-methylpolysiloxane
capillary column (30 m × 0.25 mm × 0.25 µm) was employed.
Conditions for the 9,10-dihydroanthracene (DHA) reaction were as
follows: initial temperature of 150 °C with initial time of 5 min, then
the temperature was raised at 15 °C/min to 250 °C and held for a final
time of 10 min. For the anthrone reaction: initial temperature ) 150
°C, initial time ) 5 min, ramp rate ) 50 °C/min to 230 °C, final time
) 10 min. Products were identified by their mass spectra and retention
times compared with those of authentic samples. Quantification was
made by comparison of the total ion count of the peaks interested with
that of the internal standard. Calibration plots for the detector response
were prepared from authentic samples of known concentration and with
an internal standard.
Stopped-Flow Kinetic Studies. Kinetics experiments were carried
out by using a Canterbury stopped-flow SF-40 and MG-6000 rapid
diode array system (Hi-Tech Scientific) under ambient pressure. To
avoid moisture and pre-oxidation, the instrument employs stainless-
steel flow lines and an argon-purged anaerobic attachment. The mixing
cell was maintained to (0.1 °C. All lines of the instrument were
extensively washed with dioxygen-free anhydrous solvent before
experiment. Solutions of 9-P (75.2 µM in CH2Cl2 before mixing) were
prepared in a drybox and stored in an airtight syringe prior to loading
into the stopped-flow apparatus. A saturated solution of dioxygen was
prepared by bubbling the gas through CH2Cl2 for 20 min in a septum-
sealed round-bottomed flask maintained at 20 °C, and the concentration
was taken as 5.8 mM before mixing.16 A total of 300 scans were taken
within 1050 s.
Mo1ssbauer Spectroscopy. Spectra were recorded on an MSI
spectrometer (WEB Research Co.) using a 57Co source in a Rh matrix.
All samples were prepared with 57Fe-enriched (40%) compounds. The
solid sample of 9-P was prepared by mixing ∼25 mg of the compound
in Apiezon N grease, coating the mixture on the lid of a nylon holder,
and freezing in liquid nitrogen. The frozen toluene solution sample of
10 was prepared by the following procedure. Dioxygen was introduced
into a toluene solution (400 µL) of 9-P (∼25 mg) at -29 °C. After 1.5
h, the reaction mixture was cooled in an acetone/CO2 bath and
transferred via pipet to a sample holder where the lid was permanently
assembled using epoxy. The sample was then immersed in liquid
nitrogen to freeze the liquid. After the measurement, the sample was
warmed in the holder to room temperature for 1 h to obtain the fully
oxidized product (11) and then frozen again for measurement. Data
were collected at 4.2 K. Calibration was done with natural iron foil at
room temperature for isomer shift values. The WMOSS plot and fit
program17 was used to fit the spectra to Gaussian lines.
[G-3]-COOH (7). Thermolysis of 6 (1.50 g, 0.614 mmol) was
performed in 10 mL of quinoline at 220 °C for 16 h. The reaction
mixture was diluted with 50 mL of ethyl acetate, washed with 2.0 M
HCl (5 × 25 mL), dried, and concentrated. The title compound was
obtained as a pale brown glass (1.40 g, 96%) after column chroma-
1
tography using a mixture of hexane and ethyl acetate (9:1, v/v). H
NMR (CDCl3) δ (ppm): 7.44 (d, 4H, J 7.2 Hz), 7.37 (t, 4H, J 7.2 Hz),
7.32 (t, 2H, J 7.2 Hz), 6.76 (s, 1H), 6.53 (s, 8H), 6.49 (s, 2H), 6.44 (s,
4H), 6.41 (s, 4H), 4.98 (s, 4H), 4.79 (s, 8H), 3.92 (t, 16H, J 6.6 Hz),
1.76 (m, 16H), 1.42 (m, 16H), 1.26 (m, 128H), 0.88 (t, 24H, J 6.8
Hz). 13C NMR (CDCl3) δ (ppm): 168.9, 160.5, 160.0, 155.9, 139.1,
138.9, 135.7, 135.4, 130.4, 127.9, 127.4, 122.3, 105.8, 105.0, 101.7,
100.8, 70.6, 70.1, 68.1, 32.0, 29.71, 29.67, 29.66, 29.63, 29.46, 29.39,
29.32, 26.1, 22.7, 14.2. FT-IR (cm-1): 2922, 2853, 1702, 1599, 1451,
1375, 1327, 1219, 1158, 1051, 920, 849, 830, 772, 704, 682. LR-MS
(MALDI-TOF), Calcd for C157H242O16: m/z 2383.8. Found: m/z 2410.7
([M + Na]+, 100), 2426.6 ([M + K]+, 20). Anal. Calcd for
C157H242O16: C, 79.09; H, 10.22. Found: C, 79.21; H, 10.37.
[G-3]-COONa (8). Compound 7 (130 mg, 54.5 µmol) was dissolved
in 4 mL of THF, and to this solution was added 19 µL of 3 N aq
NaOH, followed by 1 mL of MeOH to make the reaction mixture
homogeneous. The solution was allowed to reflux for 5 h. The solvents
were then removed under reduced pressure. The residue was dissolved
in pentane and filtered through Celite. Pentane was evaporated to afford
a light yellow solid, which was dried under vacuum at 80 °C overnight
to yield 136 mg of product (100% yield). FT-IR (cm-1): 2923 (s),
2853 (m), 1599 (s), 1574 (s), 1456 (m), 1375 (m), 1326 (w), 1169 (s),
1055 (m), 830 (m), 761 (w), 722 (w), 700 (w), 682 (w). 1H NMR
(CDCl3) δ (ppm): 7.47 m, (4H), 7.28 (m, 6H), 6.71 (s, 1H), 6.60 (d,
8H), 6.55 (s, 2H), 6.47 (s, 8H), 4.98 (s, 4H), 4.83 (s, 8H), 3.97 (t,
16H), 1.83 (m, 16H), 1.51 (m, 16H), 1.36 (m, 128H), 0.98 (t, 24H).
LR-MS (ESI), Calcd for C157H241O16Na: m/z 2406. Found: m/z 2383
([M - Na]-, 100)
[Fe2([G-3]COO)4(4-RPy)2] (9-R). Compound 8 (96.2 mg, 40.0
µmol) and Fe(OTf)2‚2MeCN (10.9 mg, 25.0 µmol) were mixed in THF
and stirred at room temperature for 10 h. The volatile fraction was
removed in vacuo, and the residue was dissolved in pentane. Insoluble
materials were filtered off. Excess 4-R-pyridine (4-cyanopyridine,
4-CNPy; or 4-pyrrolidinopyridine, 4-PPy) was then added to the yellow
filtrate, and the mixture was stirred vigorously for 2 h. Solvent was
removed, pentane (2 mL) was added to dissolve the dendrimer product,
and the solution was filtered. These procedures were repeated three
times to afford the diiron(II) products as a red (for 4-CNPy) or yellow
(for 4-PPy) sticky solid. 9-CN: Yield 95%. An 57Fe-enriched sample
of 9-P was prepared from 57Fe-enriched (40%) Fe(OTf)2‚2MeCN, which
was synthesized by reaction of triflic acid with a mixture of 57Fe and
natural Fe powder (2:3, w/w).13 FT-IR (cm-1): 2924 (s), 2853 (s), 2238
(w, νCtN), 1598 (s), 1454 (m), 1378 (m), 1323 (m), 1167 (s), 1054
(m), 830 (m) 765 (w), 721 (w), 703 (w), 682 (w). Anal. Calcd for
C
640H992N4O74Fe2 (9-CN + 10H2O): C, 76.57; H, 9.96; N, 0.56.
Found: C, 76.58; H, 9.86; N, 0.64. 9-P: Yield 90%. FT-IR (cm-1):
2923 (s), 2853 (s), 1599 (s), 1559 (w), 1527 (w), 1457 (m), 1378 (m),
1312 (w), 1296 (w), 1258 (w), 1226 (w), 1163 (s), 1055 (m), 1014
(w), 830 (m), 805 (w), 786 (w), 765 (w), 721 (w), 701 (w), 682 (w).
Anal. Calcd for C646H1008N4O74Fe2 (9-P + 10H2O): C, 76.62; H, 10.03;
N, 0.55. Found: C, 76.48; H, 9.80; N, 0.54.
Electron Paramagnetic Resonance (EPR) Spectroscopy. X-band
EPR spectra were recorded on a Bruker Model 300 ESP spectrometer.
The temperature was controlled with a liquid-helium EPR 900 cryostat
Dioxygen Reactivity Studies. Measurements were carried out in a
UV-vis cell equipped with a Dewar vessel for cooling. Diiron(II)
complexes were dissolved to a concentration of 0.2-0.4 mM in a
drybox and loaded into the cell, which was then sealed with a septum
and cooled to -29 °C in a nitromethane/dry ice bath. After the initial
spectrum was recorded, O2 gas was bubbled into the solution through
(16) Battino, R. Oxygen and Ozone, 1st ed.; Pergamon Press: Oxford and New
York, 1981.
(17) Kent, T. A. WMOSS, version 2.5; WEB Research Co.: Minneapolis, MN,
1998.
9
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Iron Complexes of Dendrimer-Appended Carboxylates
A R T I C L E S
Scheme 1. Synthetic Route to tert-Butyl-3,5-dihydroxy-2,6-diphenylbenzoate, 4
(Oxford Instruments). Samples of 9-P were prepared as a 1-1.5 mM
solution in toluene or CH2Cl2 in the drybox. One sample was frozen to
77 K immediately when it was brought outside of the drybox. Another
was cooled at -29 °C. O2 was bubbled for 30 s through the solution,
which was frozen to 77 K after 1.5 h. Spin quantification was done
with copper triflate as a concentration standard. Values obtained by
double integration of the signals were divided by a correcting factor
that is a function of the principal g values.18
distance (R) and the bond variance (σ2). Atom types and coordination
numbers were systematically varied during the course of the analysis,
but were not allowed to vary within a given fit. Data were fit over the
region of k ) 2-15 Å-1
.
The intensities and energies of the preedge features for all samples
were quantified by using the fitting program EDG_FIT (G. N. George,
SSRL) and established methodology.21 All spectra were fit over several
energy ranges (7108-7116, 7108-7117, 7108-7118, and 7108-7119
eV), with varied backgrounds to give nine fits per sample. Each preedge
feature was modeled with pseudo-Voigt line shapes of a fixed 1:1 ratio
of Lorentzian to Gaussian contributions. The backgrounds were chosen
to give a best fit to the preedge area while reproducing edge features.
Each fit was considered successful if it simultaneously reproduced the
data and the second derivative of the data over the entire energy range.
The total area is the sum of the areas of all preedge features, where the
area is approximated by peak height multiplied by the full width at
half-maximum, scaled by 100. Error was calculated by determining
the standard deviation for peak heights and half widths for each preedge
feature from all successful fits.
X-ray Absorption Spectroscopy (XAS). Samples of 9-P, 10, and
11 were prepared as 2.56 mM solutions in toluene and kept in liquid
N2 until used. Procedures similar to those used in the Mo¨ssbauer sample
preparation were adopted to generate 10 and 11, which were then
transferred to Lucite sample cells with a Kapton tape window via pipet.
Samples A-C were ground with BN and pressed into a 1 mm thick
Al sample holder under an inert atmosphere. The X-ray absorption
spectra were recorded at the Stanford Synchrotron Radiation Laboratory
(SSRL) on focused 20-pole wiggler beam line 7-3 for 9-P, 10, 11, and
C and the 2-3 beam line for A and B, with the ring operating at 3
GeV, 50-100 mA. A Si(220) double-crystal monochromator was
utilized for energy selection at the Fe K edge, detuned 50% at 8000
eV to minimize contamination of the radiation by higher harmonics
on beam line 2-3. On beam line 7-3, a Rh-coated mirror upstream of
the monochromator was used for harmonic rejection. The samples were
maintained at 10 K during data collection by using an Oxford
Instruments CF1208 continuous-flow liquid-helium cryostat. Data were
measured in fluorescence mode for 9-P, 10, and 11 with a Canberra
Ge 30-element array detector and in transmission mode for A-C, using
ionization chambers filled with N2. Internal energy calibration was
performed by simultaneous measurement of the absorption of Fe foil
placed between two ionization chambers located after the sample. The
first inflection point of the foil was assigned to 7111.20 eV. No
photoreduction effects were observed for any of the data sets. The
averaged data included 33 scans for 9-P, 21 scans for 10, 15 scans for
11, 11 scans for A, and 9 scans for B and C.
Results and Discussion
Design, Synthesis, and Characterization of Diiron Den-
drimer Complexes. The molecular design of dendritic ligands
capable of assembling uniquely only two iron moieties at a
central carboxylate-rich core was inspired by the family of
terphenyl carboxylates.5-7,9,10,22 The basic structure of this ligand
was modified so as to incorporate convenient points for
dendronization with C12-terminated Fre´chet-type poly(benzyl
ether)dendritic wedges using a convergent approach.23 A short
synthetic route to the central core unit 4 was devised from 2,6-
dibromo-3,5-dihydroxybenzoic acid as a starting material (Scheme
1).14 Magnesium perchlorate was employed as a Lewis acid
catalyst in the selective decarboxylative esterification of the
benzoate functionality (1).24 The phenolic moieties were then
alkylated with benzyl bromide in a Williamson-type etherifi-
cation facilitated by 18-crown-6 as a phase transfer catalyst.
The arylation of 2 via palladium-catalyzed cross-coupling was
The averaged data were normalized with the program XFIT19 by
first subtracting a polynomial background absorbance that was fit to
the preedge region and extended over the postedge with control points,
followed by fitting a three-region polynomial spline of orders 2, 3,
and 3 over the postedge region. The data were normalized to an edge
jump of 1.0 between the background and spline curves at 7130 eV.
Theoretical extended X-ray absorption fine structure (EXAFS) signals
ø(k) were calculated using FEFF (version 7.02)20 and fit to the data by
EXAFSPAK (G. N. George, SSRL). The experimental energy threshold,
E0, the point at which the photoelectron wavevector k ) 0, was chosen
as 7130 eV and was varied in each fit as a common value for every
(18) Aasa, R.; Va¨nngård, T. J. Magn. Reson. 1975, 19, 308-315.
(19) Ellis, P. J.; Freeman, H. C. J. Synchroton Radiat. 1995, 2, 190-195.
(20) Ankudinov, A. L.; Rehr, J. J. Phys. ReV. B: Condens. Matter Mater. Phys.
1997, 56, R1712-R1715.
(21) Westre, T. E.; Kennepohl, P.; DeWitt, J. G.; Hedman, B.; Hodgson, K. O.;
Solomon, E. I. J. Am. Chem. Soc. 1997, 119, 6297-6314.
(22) Tshuva, E. Y.; Lippard, S. J. Chem. ReV. 2004, 104, 987-1011.
(23) Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638-7647.
(24) Goossen, L.; Do¨hring, A. AdV. Synth. Catal. 2003, 345, 943-947.
2
component. The scale factor, S0 , was set to 1.0. The structural
parameters that were varied during the refinements included the bond
9
J. AM. CHEM. SOC. VOL. 130, NO. 13, 2008 4355

A R T I C L E S
Zhao et al.
Scheme 2. Synthesis of the Third-Generation Dendrimer-Appended Carboxylate Ligand
nontrivial given the hindered nature of the reactant. After several
attempts, Suzuki coupling with Pd(PPh3)4 as the catalyst
demonstrated superior yields. The role of the base in this step
was also noteworthy because neither Cs2CO3 nor K2CO3 gave
the desired product, whereas K3PO4 was able to furnish 3 readily
in 72% yield. The benzyl groups of 3 were removed after
hydrogenolysis using Pd/C to give the central core unit poised
for further dendronization.
A full comparison of compounds with both the third- and fourth-
generation ligands will be made in due course.
The standard synthetic protocol for the preparation of
carboxylate-rich diiron complexes involves deprotonation of
carboxylic acid to ensure efficient complexation.9 Compound
7 could not be deprotonated by NaOH at room temperature,
however. A modified procedure25 was therefore adopted,
utilizing a mixed-solvent system comprising THF, water, and
MeOH to make a homogeneous reaction mixture. The solution
was then brought to reflux for 5 h, during which time the acid
was fully deprotonated by NaOH to afford 8. Its formation was
demonstrated by loss of the CdO stretch of the free acid (1702
cm-1) and the appearance of two CdO stretches arising from
the carboxylate anion (1599 and 1574 cm-1) in the IR spectrum.
Compound 8 was then mixed with Fe(OTf)2‚2MeCN in THF
for over 10 h (Scheme 3), resulting in a clear yellow solution.
This behavior is the same as that observed for the simple
terphenyl carboxylate ArTolCO2-,9 indicating the formation of
the corresponding diiron complex with THF ligands. After
removing the THF solvent, pentane was added and the insoluble
C12-terminated Fre´chet-type poly(benzyl ether)dendritic wedges
were chosen for the dendronization of 4 so that the fully
assembled dendritic complex would be both highly globular and
soluble in a range of organic solvents (Scheme 2). Our previous
experience with these wedges and dendrimers thereof suggested
that they would be ideal candidates for the encapsulation and
protection of the diiron active site, for their performance in other
catalyst constructs has been similarly successful.15 The second-
generation dendritic wedges 5 were coupled to 4 by standard
methodologies,23 giving the tert-butyl-protected ligands 6 in high
yields. The tert-butyl group of 6 was then removed via
thermolysis in quinoline at 220 °C to give the carboxylic acid
derivative 7. We report here diiron complexes derived from 7.
(25) Kawa, M.; Fre´chet, J. M. J. Chem. Mater. 1998, 10, 286-296.
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Iron Complexes of Dendrimer-Appended Carboxylates
A R T I C L E S
Scheme 3. Synthesis of Dendrimer-Appended Diiron(II) Complexes 9-R
triflate salts were filtered off. Without further purification, the
filtrate was further exposed to solid 4-RPy suspended in
pentanes. Unreacted 4-RPy was removed by copious pentane
dissolution/filtration from the heterogeneous mixture.
Information). This assignment is also consistent with the
measurements described in the XAS section (vide infra). Thus,
taken together, the data indicate that dinuclear windmill
complexes form with the dendrimer-appended ligand. The use
of water in the synthesis of 8 and the elemental analysis are
consistent with the formula [Fe2([G-3]COO)4(4-RPy)2]‚10H2O.
Dioxygen Reactivity. Reaction of a toluene solution of 9-CN
with dioxygen at -29 °C led to a slow reduction in intensity of
the 475 nm absorption band with no indication of the formation
of an intermediate (Figure S2, Supporting Information). The
kinetics of this reaction were fit to a pseudo-first-order equation
with kobs ) 6.6(9) × 10-4 s-1. The reaction of the analogous
paddlewheel compound [Fe2(µ-O2CArTol)4(4-CNPy)2] with di-
oxygen occurs with a rate constant almost 300-fold larger at
this temperature.27 We attribute this difference to the dendrimer
framework, which limits diffusion of dioxygen into the diiron
core and provides an increased energy barrier for structural
rearrangements that may be required for the chemistry associated
with dioxygen binding.
The flexibility of the dendrimer unit makes it impossible to
obtain crystal structures to characterize the resulting products,
9-CN and 9-P, but we have a sufficiently good understanding
of related complexes through spectroscopic and other charac-
terization methods that we can assign the chemical composition
and probable structure with confidence. First, the elemental
analyses establish the presence of one N donor and two
dendrimer ligands per iron atom. Although from this information
alone we cannot rule out the possibility of a mononuclear iron
complex with two carboxylates and one N-donor ligand,
previous work on related mononuclear Fe(II) compounds
revealed that two carboxylate and two N-donor ligands are
typically associated with the metal center unless the N donor is
bidentate.26 In addition, no mononuclear iron(II) complex formed
when [Fe2(µ-O2CArTol)2(O2CArTol)2(THF)2] was allowed to react
with excess 4-cyanopyridine (4-CNPy). Second, the product of
the reaction of 4-CNPy with the dendrimer precursor has an
optical absorbance at 463 nm (ꢀ ) 2600 cm-1 M-1) in
anhydrous CH2Cl2. The value is quite similar to that (λmax 460
nm, ꢀ ) 2200 cm-1 M-1) of [Fe2(O2CArTol)2(µ-O2CArTol)2(4-
When O2 was bubbled into a CH2Cl2 solution of 9-P at -29
°C, an optical band at 442 nm (ꢀ ) 6000 cm-1 M-1) grew in,
reaching a maximum at about 1.5 h (Figure 1a). This oxygenated
intermediate, 10, was stable as long as the temperature was
maintained below -5 °C, judging by the intensity of this
absorbance. The same behavior was also observed in other
solvents, specifically chloroform and toluene. When Ar was used
to remove excess dioxygen, the intensity of the 442 nm peak
remained the same, indicating irreversible binding. Upon
warming to room temperature, the absorption maximum shifted
to 429 nm (ꢀ ) 5000 cm-1 M-1), which we attribute to
formation of the diiron(III) species 11, discussed below.
CNPy)2(H2O)2], a windmill-structured dinuclear species.11,27
A
more direct, third piece of evidence is provided by both X- and
Q-band EPR spectra, in which a broad g ) 16 signal
characteristic of a coupled diiron(II) center clearly indicates the
presence of a dinuclear complex (Figure S1, Supporting
(26) (a) Lee, D.; Lippard, S. J. Inorg. Chim. Acta 2002, 341, 1-11. (b) Hagadorn,
J. R.; Que, L., Jr.; Tolman, W. B. Inorg. Chem. 2000, 39, 6086-6090.
(27) Zhao, M.; Song, D.; Lippard, S. J. Inorg. Chem. 2006, 45, 6323-6330.
9
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A R T I C L E S
Zhao et al.
Figure 1. Optical changes observed when 9-P reacts with O2 in CH2Cl2. (a) At -29 °C (0.252 mM). Spectra were recorded every 30 s up to 15 min and
with an increment of 20% in the sampling thereafter. (b) At 20 °C (0.0376 mM). A total of 300 scans were taken in 1050 s.
Figure 2. Mo¨ssbauer spectra of (a) 9-P and (b) oxygenated intermediate 10. The experimental data are displayed as dots, and the fits are displayed as solid
lines.
O2
II 9
8
k2
98
Reaction of 9-P with O2 at 20 °C leads to the formation
of 10, which dominates the spectrum for about 100 s as indicated
by its absorbance at 442 nm (Figure 1b). Compound 10
subsequently decays to 11, overwhelming the spectral win-
dow. The intensity change at 442 nm was fit to an A f
B f C process as indicated by eq 1, where k1′ ) k1[O2],
since excess dioxygen ensures pseudo-first-order conditions.
II III
III III
FeIIFe
9-P
Fe Fe
11
(2)
Fe Fe -O2
k1
10
Mo1ssbauer and EPR Studies. The Mo¨ssbauer spectrum of
9-P (Figure 2a) displays a single sharp quadrupole doublet,
indicating the two iron sites to be indistinguishable. The isomer
shift (δ ) 1.15 mm/s) and quadrupole splitting (∆EQ ) 2.76
mm/s) are consistent with those of high-spin diiron(II) com-
pounds in an oxygen-rich coordination environment.22,28
The Mo¨ssbauer spectrum of 10 (Figure 2b) displays two
quadrupole doublets, indicating two different iron sites with the
isomer shift and quadrupole splitting values of δ ) 1.20 mm/s,
∆EQ ) 3.10 mm/s and δ ) 0.48 mm/s, and ∆EQ ) 0.71 mm/s,
respectively. The fit gave a 1:1 ratio for the two sites. By
comparison with the parameters reported previously, we could
assign the former as a high-spin iron(II) center and the latter as
a high-spin iron(III) center.22,28 These results strongly suggest
that the oxygenated intermediate 10 is a mixed-valent FeIIFeIII
species, lending further support to the assigned dinuclear
structure of 9-R. The EPR spectrum of 10 displays a weak broad
absorbance at g ) 2 and relatively strong signals at g ) 4.24
and 4.04 (Figure S4, Supporting Information), which is typical
Abs442(t) ) [9-P]0 ꢀ11 + (ꢀ9-P - ꢀ11) +
{
[
k1′
(ꢀ10 - ꢀ11)
exp(- k1′t) -
]
k2 - k1′
k1′
(ꢀ10 - ꢀ11) exp(- k2t) (1)
}
k2 - k1′
When ꢀ10 and ꢀ9-P were fixed at 6000 and 1000 cm-1 M-1
,
respectively, and the initial concentration of 9-P was 0.0376
mM, the fit (Figure S3, Supporting Information) yielded k1′ )
0.0132(8) s-1, k2 ) 0.04 (2) s-1, and ꢀ11 ) 4523(5) cm-1 M-1
.
The rate constants k1′ and k2 are of the same order of magnitude,
consistent with the fact that both 10 and 11 appeared within a
short period of time at this temperature. This chemistry is
attributed to eq 2, and the nature of 10 and 11 is further
addressed below.
(28) Mu¨nck, E. In Physical Methods in Bioinorganic Chemistry: Spectroscopy
and Magnetism; Que, L., Jr., Ed.; University Science Books: Sausalito,
CA, 2000; pp 287-319.
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Iron Complexes of Dendrimer-Appended Carboxylates
A R T I C L E S
for mononuclear Fe(III) in a rhombic environment.29 Quantifica-
tion indicated that the signals comprise less than 3.5% of total
iron, the remainder being EPR-silent. This intermediate is
capable of oxidizing substrates 9,10-dihydroanthracene (DHA)
to anthracene and anthraquinone and also anthrone to an-
thraquinone, as will be discussed later. An FeIIFeIII cation,
previously generated by oxygenation of a diferrous compound,
could not activate substrates because there was no oxygen-
containing species bound to Fe(III).6 We therefore believe that
the FeIIFeIII intermediate obtained here contains a reduced form
of dioxygen, presumably superoxide (O2-), since it can perform
multi-electron oxidations. In addition, the unpaired electron of
a bound superoxo ion would be strongly coupled to the Fe(III)
center, accounting for the absence of an EPR signal in the
X-band spectrum. Intensive efforts were undertaken to detect
an isotope-sensitive O-O stretching band by resonance Raman
spectroscopy, but no mode could be identified. This result may
reflect either a very efficient and possibly reversible photodis-
sociation process or the lack of an optical band involving the
superoxide ion for resonance enhancement. The fact that the
optical spectrum of a mixture of Et4N[FeCl4] and 4-PPy is
similar to that of 10 is consistent with the presence of an Fe-
Npyridine bond in the latter. Recently, an oxygenated diiron(III)
intermediate with no obvious optical absorbance was discovered
for a transient species formed in the reaction with dioxygen of
both a mutant and wild-type forms of reduced toluene/o-xylene
monooxygenase.30 Taken together, these results suggest that the
strong absorption band observed for intermediate 10 originates
from a ligand-to-metal charge transfer involving 4-PPy and
Fe(III) and that no optical band derives from the superoxide
ligand. Therefore, the 442 nm absorption would not provide
significant enhancement to vibrations implicating the Fe-O-O
fragment in a resonance Raman experiment.
Figure 3. Fe K-edge spectra for 9-P, 10, and 11. The inset shows the
magnified preedge region.
Table 1. XAS Preedge Fit Results for 9-P, 10, and 11
preedge peak
energy
preedge peak
intensitya
total preedge
peak intensitya
sample
9-P
7111.8 (0.01)
7113.5 (0.01)
7111.5 (0.01)
7112.9 (0.01)
7114.3 (0.01)
7113.0 (0.03)
7114.5 (0.01)
9.0 (0.1)
3.3 (0.2)
1.8 (0.1)
2.7 (0.1)
1.7 (0.1)
4.1 (0.1)
3.1 (0.6)
12.3 (0.2)
6.2 (0.1)
10
11
7.1 (0.5)
a Values reported for the preedge intensity are multiplied by 100 for
convenience.
extremely sensitive to the oxidation state and geometry at an
average iron site in a molecule.21 The preedge spectrum of 9-P
consists of two peaks centered at 7111.8 and 7113.5 eV (Figure
3, inset; Table 1). This preedge energy range is characteristic
of ferrous complexes, and the total preedge area of 12.3 units
and the area distribution between the two peaks indicate high-
spin five-coordinate iron centers in 9-P.21 The preedge fit for
11 reveals two peaks of comparable intensity at 7113.0 and
7114.5 eV with the total area of 7.1 units (Table 1). This energy
range and area distribution are typical for high-spin ferric
complexes with distorted-octahedral geometry.21 In addition, this
pattern is distinct from that observed for (µ-oxo)diiron(III)
complexes,21 and thus an oxo bridge can be ruled out for 11.
Moreover, this assignment is also consistent with the small
quadrupole splitting (0.66 mm/sec) of 11 determined by
Mo¨ssbauer spectroscopy; oxo bridges typically give rise to
significantly larger values.31 The preedge of 10 comprises three
peaks at 7111.5, 7112.9, and 7114.3 eV, encompassing the
energy regions of both iron(II) and iron(III), and thus further
supporting the mixed-valent description for 10. The relatively
low total preedge intensity of 6.2 units indicates that both iron
centers in 10 are in distorted-octahedral environments and
contain no bridging oxo group. Finally, the highest energy peak
for 10 at 7114.3 eV, which can be assigned to the iron(III)
center, is shifted by 0.2 eV from the analogous peak for 11,
suggesting that the iron(III) environments are different in 10
and 11. Hence, the Fe K preedge data indicate that both Fe(II)
and Fe(III) centers in 10 are different in nature than Fe(II) in
9-P and Fe(III) in 11, respectively.
When allowed to warm up, a solution of 10 decayed to the
high-spin diiron(III) species 11, for which the quadrupole
splitting ∆EQ is 0.66 mm/s and the isomer shift δ is 0.48 mm/
s. From the difference in the ∆EQ parameters, the Fe(III) atoms
in 10 and the final product must have different coordination
environments.
X-ray Absorption Spectroscopy (XAS). The Fe K-edge
X-ray absorption edge shifts to higher energy from 9-P to 10
to 11, indicating an increase in the average iron oxidation state
across the series (Figure 3). The main edge transition for 9-P
overlays well with those for the diiron(II) complexes A and B
(Figure S5, Supporting Information), whereas the spectrum of
11 overlays with that for the diiron(III) complex C (Figure S6,
Supporting Information), supporting the iron(II) and iron(III)
oxidation level for 9-P and 11, respectively. The midway edge
energy position for 10 between those for 9-P and 11 suggests
that 10 contains equal amounts of iron(II) and iron(III). A linear
combination of 50% of 9-P and 50% of 11, however, does not
reproduce the data for 10 completely (Figure S7, Supporting
Information), especially in the preedge region, indicating that
the iron centers in 10 are of different nature than those in the
precursor and decay species.
The preedge region of the X-ray absorption spectrum is
(29) Palmer, G. In Physical Methods in Bioinorganic Chemistry: Spectroscopy
and Magnetism; Que, L., Jr., Ed.; University Science Books: Sausalito,
CA, 2000, pp 121-185.
(30) Murray, L. J.; Garc´ıa-Serres, R.; Naik, S.; Huynh, B. H.; Lippard, S. J. J.
Am. Chem. Soc. 2006, 128, 7458-7459.
(31) Mizoguchi, T. J.; Davydov, R. M.; Lippard, S. J. Inorg. Chem. 1999, 38,
4098-4103.
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J. AM. CHEM. SOC. VOL. 130, NO. 13, 2008 4359

A R T I C L E S
Zhao et al.
Table 2. EXAFS Curve-Fitting Results for 9-P, 10, and 11a
σ2 × 106
(Å2)
sample
fit no.
R (Å)
∆E
0 (eV)
Fb
9-P
1
5 O/Nc
4 C
4 O/N
2 C/O
4 C
4 O/N
2 C/O
0.2 Fe
4 C
6 O/N
5 C
6 O/N
1 Fed
5 Cd
6 O/N
5 C
6 O/N
1 Fed
5 Cd
2.10
3.06
2.12
2.50
3.07
2.12
2.49
2.92
3.09
2.05
3.04
2.05
3.03
3.00
2.02
3.00
2.02
3.00
2.94
1200
660
900
830
600
900
860
650
510
1300
340
1280
400
1520
840
270
820
400
1160
-6.2
-3.9
0.497
2
3
0.400
0.393
-3.8
10
11
4
5
0.7
0.1
0.331
0.280
6
7
0.7
0.0
0.298
0.241
a Errors are estimated to be 25% for coordination numbers and 0.01-
0.03 Å for distances. b Error (F) is defined as F ) [∑k6(øexptl - øcalcd)2]/
∑k6øexptl2]. c Scatterers differing by Z ) (1 are not distinguishable by
EXAFS. The first element in a pair indicates the type of atom used to model
the backscattered wave in the theoretical fit. d Due to a high degree of
correlation, the errors for the Fe-Fe and Fe-C distances at ∼3.0 Å are
estimated to be around 0.05 Å.
low coordination number of 0.2 Fe and improves the fit error
only marginally in comparison to that of fit 2. Since this short
metal-metal distance is readily detected in the EXAFS analysis
of B (Table S2 and Figure S10, Supporting Information), we
conclude that a tetrabridged structure with a short Fe-Fe
separation is incompatible with the data for 9-P and may only
be valid for 20% or less of all species in the solution. Therefore,
the local structure around the iron centers of 9-P is more similar
to that of A than of B. The average Fe-ligand distances are
longer for 9-P (2.12 Å) than those for A (2.00 Å), reflecting
steric demands imposed by the dendrimer ligands.
Figure 4. (a) Fe K-edge EXAFS data for 9-P, 10, and 11. (b) Fourier
transforms of the EXAFS data.
The Fourier transforms of the EXAFS data shown in Figure
4 illustrate major structural differences between the samples.
In particular, the first-shell scattering is significantly less intense
for 9-P than that for 10 and 11, implying a broad distribution
of the first-shell bond lengths in 9-P. In addition, the Fourier
transforms for 10 and 11 display an intense second-shell peak
between 2 and 3 Å, similar to the one observed for diiron
complexes and proteins with a short Fe-Fe separation32,33 and
absent in the data for the precursor 9-P.
The first shell for the decay compound 11 can be modeled
with 6 O/N ligands at 2.02 Å (Table 2, fit 6), consistent with
expected bond lengths for a high-spin ferric complex. However,
the shell of five Fe-C(pyridine, carboxylate) interactions at 3.00
Å has a very low Debye-Waller parameter σ2, even when
additional shells at longer distances are modeled in the fit (Table
S3 and Figure S11, Supporting Information). Inclusion of an
Fe-Fe vector at 3.00 ( 0.05 Å improves the σ2 value and results
in reasonable parameters for the Fe-Fe component (fit 7). This
Fe-Fe distance is 0.15 Å longer than that in the tetrabridged
[FeIII2(µ-OH)2(µ-O2CArTol)2] complex C, suggesting that the
number, nature, and/or geometry of the bridging ligands are
different in 11 and C. The Fe-Fe separation of 3.00 Å in 11
matches well the Fe-Fe distance of 2.99 Å in the oxidized form
of MMOH, which features a µ-1,3 carboxylate and two hydroxo
bridging ligands.33
The EXAFS fits for 9-P yield an average first-shell Fe-ligand
distance of 2.10-2.12 Å and are significantly improved upon
inclusion of a scattering component at 2.50 Å (Table 2, fits 1
and 2; Figure S8, Supporting Information). A similar component
at 2.44 Å is required in the data for the dibridged [FeII (µ-O2-
2
CArTol)2(O2CArTol)2] complex A, where it can be attributed to
an average of the Fe-O and Fe-C distances of the terminal
bidentate carboxylate ligand in correspondence with the crystal
structure (Table S1 and Figure S9, Supporting Information). This
result suggests that one terminal bidentate carboxylate per iron
atom is present in 9-P. The data for 9-P do not accommodate a
short Fe-Fe vector (fit 3), which would be representative of a
tetrabridged structure similar to that of the [FeII (µ-O2CArTol)4]
The EXAFS data for the intermediate complex 10 are best
modeled with 6 O/N scatterers at 2.05 Å, consistent with the
intermediate valence state (Table 2, fit 4). As in the case of 11,
an Fe-Fe component at 3.05 ( 0.05 Å can be included in the
second shell of the data (fit 5), improving σ2 for the carbon
shell. Although more detailed fits to the data of 10 have also
been performed (Table S4 and Figure S12, Supporting Informa-
tion), it is not possible to assign any of the EXAFS-derived
2
complex B with an Fe-Fe separation of 2.82 Å. Particularly,
the addition of a short Fe-Fe vector in fit 3 results in a very
(32) (a) Hsu, H.-F.; Dong, Y.; Shu, L.; Young, V. G., Jr.; Que, L., Jr. J. Am.
Chem. Soc. 1999, 121, 5230-5237. (b) Hedman, B.; Co, M. S.; Armstrong,
W. H.; Hodgson, K. O.; Lippard, S. J. Inorg. Chem. 1986, 25, 3708-
3711.
(33) Rudd, D. J.; Sazinsky, M. H.; Merkx, M.; Lippard, S. J.; Hedman, B.;
Hodgson, K. O. Inorg. Chem. 2004, 43, 4579-4589.
9
4360 J. AM. CHEM. SOC. VOL. 130, NO. 13, 2008

Iron Complexes of Dendrimer-Appended Carboxylates
A R T I C L E S
Scheme 4. Possible Structure of the FeIIFeIII Superoxo
Intermediate 10
the UV-vis spectrum (Figure 5a). Upon warming, however,
the absorption maximum began to decrease (Figure 5b),
indicating reaction of the substrate with 10. As was the case
for decamethylferrocene, the diiron(II) center appears to be
protected by the dendrimer shell, reacting with dioxygen only
above -30 °C. The oxygenated intermediate is also very stable
and does not react with substrate without warming.
interactions specifically to a bound superoxo ligand because of
the broad range of Fe-O(carboxylate) and Fe-N(pyridine) bond
lengths when Fe(II) and Fe(III) centers are simultaneously
present.
After workup of the reaction mixture, the products (Scheme
5) were analyzed by GC-MS. As listed in Table 3, 13% of the
substrate converts to anthracene and 25% to anthraquinone. No
anthrone was detected in this experiment. Products were
identified by comparing their retention times and mass spectral
patterns with those of authentic standards. There were two
additional small peaks in the GC trace (Figure S13, Supporting
Information) that we could not identify. The mass spectrum of
one showed its most intense signal at 178+, identical to that of
anthracene, and very weak signals at 194+ and 372+, respec-
tively, which could arise from coupling of DHA and anthrone
radicals. However, the retention time is less than that of anthrone
itself. Considering the larger molecular mass expected for a
coupled compound, this assignment therefore seems unlikely.
The mass spectrum of another peak displayed numerous signals,
and no conclusion could be drawn concerning its identity.
Excluding these contributions, the mass balance was calculated
to be 62%. A control experiment using the exact procedure but
performed in the absence of 9-P showed no DHA oxidation
and 93% recovery of the substrate.
The generation of anthracene and anthraquinone most likely
occurs via separate reaction pathways because no anthracene
was oxidized to anthraquinone in a control experiment. To
supply more information about the mechanism, two similar
reactions were performed. In one case, Ar was not used to
provide an O2-free environment prior to substrate addition.
Although the same amount of anthracene formed, production
of anthraquinone was significantly decreased (Table 3). In
another experiment, the reaction was brought to room temper-
ature under Ar over a shorter period of time (15 min) after
substrate addition, and yields of 11% for anthracene and 5%
for anthraquinone were recorded. In an experiment where
anthraquinone was used as substrate, no reaction occurred. These
observations suggest that the formation of anthracene is not
affected by the presence of free dioxygen or the temperature,
but that the intermediate steps to form anthraquinone are greatly
influenced by these factors. The decrease in oxygenated product
in the presence of O2 also suggests that radical chain chemistry,
which was reported previously with a ferric bi-imidazoline
complex,41 is unlikely to be occurring here.
Although no FeIIFeIII superoxo intermediate has yet been
detected in experimental studies of the oxygenation of non-
heme diiron enzymes,30,34 theoretical work on dioxygen activa-
tion of soluble MMOH has suggested the existence of an FeIIFeIII
superoxo species, the structure of which is portrayed in Scheme
4.35,36 An Fe-Fe distance of 3.44 Å was computed recently by
QM/MM, reduced by about 0.3 Å compared with that previously
reported by a pure QM calculation.36 Although the Fe-Fe
distance from theoretical calculation for MMOHred agrees
reasonably well with that from crystallographic studies, the value
calculated for Q is significantly larger than that (2.46 Å)
obtained from XAS.36,37 It is possible to obtain shorter values
if other models are used. Therefore, intermediate 10 may bear
a core structure similar to that shown in Scheme 4. We stress,
however, that we do not rule out other possible binding modes
for the superoxide ion. We also note the absence of an optical
absorbance band at 442 nm in a published report of an
η1-superoxo iron(III) complex.38
Reaction of the Oxygenated Intermediate 10 with External
Reagents. a. Cp*2Fe. Cp*2Fe did not react with 10 at -29 °C,
judging by the failure to generate the characteristic spectrum
of Cp*2Fe+. Upon warming, however, a broad absorbance
around 780 nm grew in, which is diagnostic of the decameth-
ylferrocenium cation.39 The relative stability of 10 toward
reduction at the lower temperature may reflect the inability
of Cp*2Fe to penetrate the dendrimer sheath at low tempera-
tures.
b. Oxidation of 9,10-Dihydroanthracene (DHA). DHA is
a commonly used substrate for oxygenation because its weak
C-H bond (BDE ) 78 kcal/mol) is easy to activate.40 After
the FeIIFeIII intermediate 10 was generated and reached its
maximum concentration, Ar was bubbled through the reaction
mixture for 20 min to remove unreacted O2. DHA was then
injected through an airtight syringe. The temperature was
maintained at -29 °C for 2 h, and no change was observed in
(34) (a) Merkx, M.; Kopp, D. A.; Sazinsky, M. H.; Blazyk, J. L.; Mu¨ller, J.;
Lippard, S. J. Angew. Chem., Int. Ed. 2001, 40, 2782-2807. (b) Kopp, D.
A.; Lippard, S. J. Curr. Opin. Chem. Biol. 2002, 6, 568-576. (c) Stubbe,
J. Curr. Opin. Chem. Biol. 2003, 7, 183-188. (d) Moe¨nne-Loccoz, P.;
Baldwin, J.; Ley, B. A.; Loehr, T. M.; Bollinger, J. M., Jr. Biochemistry
1998, 37, 14659-14663. (e) Riggs-Gelasco, P. J.; Shu, L.; Chen, S.; Burdi,
D.; Huynh, B. H.; Que, L.. Jr.; Stubbe, J. J. Am. Chem. Soc. 1998, 120,
849-860. (f) Kryatov, S. V.; Chavez, F. A.; Reynolds, A. M.; Rybak-
Akimova, E. V.; Que, L., Jr.; Tolman, W. B. Inorg. Chem. 2004, 43, 2141-
2150.
c. Oxidation of Anthrone Substrate. To eliminate the
pathway to form anthracene, anthrone was introduced as the
substrate. Similar optical changes were observed upon the
introduction of dioxygen to 9-P, generating 10, which was stable
and unreactive at low temperature for 2 h (Figure 6a). Reaction
with substrate ensued when the solution was allowed to warm
up (Figure 6b). Approximately 75% conversion to anthraquino-
ne, and only this one product, was observed when Ar was used
to flush free O2 from the system prior to warm up. The higher
yield strongly implies that anthrone is more reactive than DHA,
perhaps because it can tautomerize to 9-hydroxyanthracene, the
O-H bond in which is more readily oxidized than the C-H
(35) Gherman, B. F.; Baik, M.-H.; Lippard, S. J.; Friesner, R. A. J. Am. Chem.
Soc. 2004, 126, 2978-2990.
(36) Rinaldo, D.; Philipp, D. M.; Lippard, S. J.; Friesner, R. A. J. Am. Chem.
Soc. 2007, 129, 3135-3147.
(37) Shu, L.; Nesheim, J. C.; Kauffmann, K.; Mu¨nck, E.; Lipscomb, J. D.; Que,
L., Jr. Science 1997, 275, 515-518.
(38) Shan, X.; Que, L., Jr. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 5340-
5345.
(39) Vaucher, S.; Charmant, J. P. H.; Sorace, L.; Gatteschi, D.; Mann, S.
Polyhedron 2001, 20, 2467-2472.
(40) Bordwell, F. G.; Cheng, J.; Ji, G. Z.; Satish, A. V.; Zhang, X. J. Am. Chem.
Soc. 1991, 113, 9790-9795.
(41) Roth, J. P.; Mayer, J. M. Inorg. Chem. 1999, 38, 2760-2761.
9
J. AM. CHEM. SOC. VOL. 130, NO. 13, 2008 4361

A R T I C L E S
Zhao et al.
Figure 5. Optical changes observed when 10 reacted with DHA (a) at -29 °C for two h and (b) upon warming to room temperature.
Scheme 5. Oxidation of DHA in the Presence of FeIIFeIII Species
to detect the product of a single turnover reaction, however,
and there is usually some anthraquinone impurity in anthrone.
A control experiment in which 18OH2 was used in the prepara-
tion of 9-P did not affect the outcome, indicating no exchange
with water. Taken together, these results prevent us from
assigning 10 as being responsible for the catalysis, although in
a preliminary double-mixing stopped-flow experiment, where
10 was generated at 20 °C in the first chamber and mixed with
anthrone after 100 s, the substrate significantly decreased the
lifetime of the intermediate compared with its decay in the
absence of the hydrocarbon. Further studies are planned to
establish the mechanism of catalysis.
10 and O2
Table 3. Product Distribution in the Reaction of DHA with 10,
Which Was Generated at -29 °C, Followed by Substrate Addition
at The Same Temperature
reaction conditions
anthracene (%)
anthraquinonea (%)
Ar, warm up from -29 °C to
RT over 12 h
13
25
O2, warm up from -29 °C to
13
11
9
5
Conclusion
RT over 12 h
Ar, warm up from -29 °C to
RT over 15 min
Dinuclear iron(II) complexes of general formula [Fe2([G-3]-
COO)4(4-RPy)2] were successfully prepared from dendrimer-
appended carboxylate ligands. The compounds were character-
ized by UV-vis, EPR, Mo¨ssbauer, and XAS spectroscopy in
addition to elemental analysis. The oxygenation of such
complexes is retarded by about 300-fold compared with that of
related compounds derived from the sterically less demanding
simple terphenyl carboxylate ([G-1]COO-). A metastable FeII-
FeIII intermediate was detected in the reaction of [Fe2([G-3]-
COO)4(4-PPy)2] with dioxygen. This intermediate was charac-
terized by Mo¨ssbauer and XAS spectroscopy. Although no
oxygen-sensitive band could be observed by resonance Raman
spectroscopy, we formulate this intermediate as an FeIIFeIII
superoxo species because it is EPR-silent and can react with
external substrates, such as DHA and anthrone, to form
oxygenated products. Catalytic oxidation was discovered for
anthrone reacting in the presence of the intermediate and excess
O2.
a Product yield was calculated based on 10.
bond.42 Such behavior would account for the failure to detect
anthrone in the DHA reaction.
When anthrone was allowed to react with 10 in the presence
of excess O2 and with gradual warm up, however, catalytic
oxidation of anthrone to anthraquinone was observed (Scheme
6), although we have no evidence that 10 is the catalyst. Table
4 lists the conditions and the product distribution for reactions
occurring in CH2Cl2. At a higher ratio of substrate to 10, there
is significantly higher turnover. The highest turnover was
obtained when the reaction was conducted at room temperature.
A control experiment in the absence of 10 and in the presence
of Fe(OTf)2‚2MeCN showed no conversion to product, although
when 4-PPy was present there was quantitative formation of
anthraquinone from anthrone. In experiments where toluene was
used as the solvent, catalytic reactivity was still observed (Table
5), so it is unlikely that a solvent-derived radical is involved in
the chemistry.
In order to test whether the mixed-valent intermediate 10 may
promote substrate oxidation, we generated 10 with 18O2 and
conducted similar experiments. When excess isotopically labeled
dioxygen was present, there was 100% incorporation of 18O into
the anthroquinone product. When the reaction was allowed to
continue under inert gas, surprisingly, we observed 16O incor-
poration, the source of which is unclear at this time. It is difficult
The present results provide one of few examples in non-heme
diiron chemistry where external substrate oxidation can be
achieved with dioxygen43 as the terminal oxidant instead of a
peroxide.44 In addition, the chemistry provides an FeIIFeIII
(43) (a) Dong, Y.; Me´nage, S.; Brennan, B. A.; Elgren, T. E.; Jang, H. G.; Pearce,
L. L.; Que, L., Jr. J. Am. Chem. Soc. 1993, 115, 1851-1859. (b) Carson,
E. C.; Lippard, S. J. Inorg. Chem. 2006, 45, 837-848.
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Commun. 2002, 1288-1289. (b) Jensen, M. P.; Lange, S. J.; Mehn, M. P.;
Que, E. L.; Que, L., Jr. J. Am. Chem. Soc. 2003, 125, 2113-2128. (c)
Costas, M.; Tipton, A. K.; Chen, K.; Jo, D.-H.; Que, L., Jr. J. Am. Chem.
Soc. 2001, 123, 6722-6723. (d) Oldenburg, P. D.; Shteinman, A. A.; Que,
L., Jr. J. Am. Chem. Soc. 2005, 127, 15672-15673. (e) Kim, J.; Larka, E.;
Wilkinson, E. C.; Que, L., Jr. Angew. Chem., Int. Ed. 1995, 34, 2048-
2051.
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2524-2526.
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4362 J. AM. CHEM. SOC. VOL. 130, NO. 13, 2008

Iron Complexes of Dendrimer-Appended Carboxylates
A R T I C L E S
Figure 6. Optical changes observed when 10 reacted with anthrone (a) at -29 °C for 2 h and (b) upon warming to room temperature.
Scheme 6. Catalytic Oxidation of Anthrone to Anthraquinone in
bridging superoxo structure, analogous to that predicted by
theory.36 This structure differs from others by its unique µ-η2,η1
binding mode.
the Presence of 10 and O2
Acknowledgment. This work was supported by grants from
the National Institute of General Medical Sciences (GM32134
to S.J.L.), the National Science Foundation (DMR-0317514 to
J.M.J.F.), and the Department of Energy, Office of Basic Energy
Sciences through the Biomolecular Science Program at LBNL
(DEAC02-05CH11231 to J.M.J.F.), and the National Institutes
of Health (P41 RR001209 to K.O.H.) The Structural Molecular
Biology program at SSRL is funded by the National Institutes
of Health, National Center for Research Resources, Biomedical
Technology Program, and the Department of Energy, Office of
Biological and Environmental Research. We thank Dr. Rayane
Moreira for some initial work and Dr. Shawn Burdette for
initiating the collaboration between the laboratories of S.J.L.
and J.M.J.F. We thank Dr. Roman Davydov and Professor Brian
M. Hoffman for measuring the X- and Q-band EPR spectra of
the diiron(II) compound 9-P. This project was in part made
possible by Grant Number 5 P41 RR001209 from the National
Center for Research Resources (NCRR), a component of the
National Institutes of Health (NIH). Its contents are solely the
responsibility of the authors and do not necessarily represent
the official view of NCRR or NIH.
Table 4. Product Distribution When Anthrone Was Allowed to
Reacted with 10 in CH2Cl2. The Yields Were Calculated Based on
Substrate
substrate/10
anthraquinone (%)
turnover
11:1
18:1
129:1
103:1 (RT)
53
53
11
43
5.6
9.5
14
44
Table 5. Product Distribution When Anthrone Was Allowed to
React with 10 in Toluene. The Yields Were Calculated Based on
Substrate
reaction condition
substrate/10
anthraquinone (%)
turnover
-29 °C, slowly warm
10.5:1
103:1
124:1
30
24
13
3.2
25
16
RT
71 °C
superoxo intermediate, which has been proposed in theoretical
work but not yet observed for protein systems. Apparently
dendritic encapsulation suppresses simple dioxygen-initiated
outer-sphere one-electron oxidation to yield FeIIFeIII mixed-
valent compounds with no bound O2 derivative, which has been
reported for “smaller” diiron(II) models.6,45 The spectroscopic
characterization of this intermediate leads us to propose a
Supporting Information Available: Optical changes during
oxygenation (Figures S2 and S3); physical characterizations of
reaction intermediate and products (Figures S1, S4, and S13);
and XAS analysis for 9-P, 10, 11, and A-C including Figures
S5-S12 and Tables S1-S4. This material is available free of
charge via the Internet at http://pubs.acs.org.
(45) (a) Lee, D.; DuBois, J. L.; Pierce, B.; Hedman, B.; Hodgson, K. O.;
Hendrich, M. P.; Lippard, S. J. Inorg. Chem. 2002, 41, 3172-3182. (b)
Hagadorn, J. R.; Que, L., Jr.; Tolman, W. B.; Prisecaru, I.; Mu¨nck, E. J.
Am. Chem. Soc. 1999, 121, 9760-9761.
JA076817A
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