Synthetic models of the deoxy and oxy forms of the non-heme dioxygen- binding protein hemerythrin [24]

Full text of DOI:10.1021/ja982417z

11022
J. Am. Chem. Soc. 1998, 120, 11022-11023
Scheme 1
Synthetic Models of the Deoxy and Oxy Forms of the
Non-Heme Dioxygen-Binding Protein Hemerythrin
Tadashi J. Mizoguchi and Stephen J. Lippard*
Department of Chemistry
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
ReceiVed July 9, 1998
The three prototypical reversible dioxygen-binding proteins
comprise hemoglobin (Hb), hemocyanin (Hc), and hemerythrin
(Hr) and are distinguished by the presence of active-site heme-
iron, dicopper, and non-heme diiron centers, respectively.1
Pioneering work2 has shown that an appropriately designed
synthetic iron porphyrin3 can function as a fully operational Hb
mimic without the need for a protein matrix. The µ-η2:η2-
peroxodicopper(II) active center of oxyHc has similarly been
modeled.4,5 Despite the extensive body of published material in
the area of synthetic non-heme diiron chemistry,6-9 small molecule
complexes that reproduce the functionally relevant active-site
structures and properties of the deoxy and oxy forms of Hr have
yet to be reported.
DeoxyHr contains a (µ-hydroxo)bis(µ-carboxylato)diiron(II)
core to which are ligated five imidazole groups of histidine
residues, affording one 5- and one 6-coordinate iron atom.10
Dioxygen binding occurs at the coordinatively unsaturated
iron(II) center in coupled electron-proton transfer chemistry that
yields a terminally bound hydroperoxide ligand hydrogen bonded
to the bridging oxygen atom of the resulting (µ-oxo)bis-
(µ-carboxylato)diiron(III) core.10 From recent studies of the
protein11 and from synthetic work,12 the task of mimicking the
functional chemistry of Hr outside of the protein environment
would appear to be formidable. In the present paper we describe
our recent efforts to meet this challenge through the synthesis of
an asymmetric (µ-hydroxo)bis(µ-carboxylato)diiron(II) com-
plex which, upon exposure to dioxygen in the presence of
N-methylimidazole (N-MeIm), generates a species exhibiting
spectroscopic properties that closely match those of oxyHr.
To facilitate the assembly and stabilization of the desired bis-
(µ-carboxylato)diiron Hr model complexes, a new dinucleating
dicarboxylate ligand, dibenzofuran-4,6-bis(diphenylacetate)
(Ph4DBA), was designed. Computer modeling of this ligand
revealed the carboxylate groups to have the ability to orient
themselves orthogonal to one another for bridging two metals in
a syn-syn bidentate13 mode. Multigram quantities of the free
acid of Ph4DBA were readily obtained from dibenzofuran (1) in
three steps in ∼40% overall yield (Scheme 1). Double depro-
tonation of 114 followed by trapping with benzophenone yielded,
upon treatment with acid, diol 2. Subsequent reduction of 2 with
triethylsilane/boron trifluoride15 led to nearly quantitative isolation
of compound 3. Addition of carbon dioxide to the dianion of
316 followed by an acidic workup generated diacid 4. The crude
product, washed with water and Et2O, proved pure enough by
NMR spectroscopy17 for use in subsequent metalation chemistry.
The neutral, air-sensitive diiron(II) compound [Fe2(µ-OH)-
(µ-Ph4DBA)(TMEDA)2(OTf)] (5), where TMEDA ) N,N,N′,N′-
tetramethylethylenediamine and OTf ) triflate, was assembled
in THF/CH3CN solution at room temperature by mixing diacid
4, Et3N, Fe(OTf)2‚2CH3CN,18 TMEDA, and H2O in a 1:3:2:2:
1.5 molar ratio. Recrystallization of the crude product from
CH2Cl2/Et2O led to analytically pure material and afforded X-ray
quality crystals sufficient to characterize 5 as a (µ-hydroxo)bis-
(µ-carboxylato)diiron(II) complex with two bidentate TMEDA
ligands, one per metal center, and one terminally bound triflate
anion, cis to the bridging hydroxide, completing the coordination
spheres of one 5- and one 6-coordinate iron.19 The 77 K zero-
field Mo¨ssbauer spectrum of the resulting asymmetric complex
5 (Figure S1) displayed a very broad doublet (Γ ≈ 0.70 mm s-1
)
that was fit as two overlapping quadrupole doublets (δ1 ) 1.04,
∆EQ1 ) 2.81; δ2 ) 1.33, ∆EQ2 ) 2.81 mm s-1), reflecting the
inequivalence of the two iron centers. These isomer shift and
quadrupole splitting parameters are indicative of high-spin
Fe(II) and, when averaged, are identical to those reported for
deoxyHr (δ ) 1.19, ∆EQ ) 2.81 mm s-1).20
When the triflate ligand in 5 was replaced in a metathesis
reaction with the noncoordinating tetraphenylborate (BPh4) anion,
[Fe2(µ-OH)(µ-Ph4DBA)(TMEDA)2(CH3CN)]BPh4 (6) was ob-
tained following recrystallization of the crude product from
CH2Cl2/CH3CN/Et2O. An X-ray crystal structure analysis of 6
showed that the {Fe2(µ-OH)(µ-Ph4DBA)(TMEDA)2}+ unit of 5
was preserved and that, in place of the coordinated triflate in 5,
was an acetonitrile molecule (Figure 1). The metric parameters
of the dimetallic core of 6 are very similar to those of other
crystallographically characterized (µ-hydroxo)bis(µ-carboxylato)-
diiron(II) complexes.21,22 The Fe-Obridge bond distances range
from 1.946(2) to 2.043(2) Å, and the Fe-Obridge-Fe bond angle
averages to 105.65(8)° for the two chemically equivalent mol-
ecules of 6 in the crystallographic asymmetric unit. The
difference in coordination number of the two iron atoms in 6 is
(1) Lippard, S. J.; Berg, J. M. Principles of Bioinorganic Chemistry;
University Science: Mill Valley, 1994; pp 284-302.
(2) Collman, J. P. Inorg. Chem. 1997, 36, 5145-5155.
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(9) LeCloux, D. D.; Barrios, A. M.; Mizoguchi, T. J.; Lippard, S. J. J. Am.
Chem. Soc. 1998, 120, 9001-9014.
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(16) Eisch, J. J. In Organometallic Syntheses; Eisch, J. J., King, R. B.,
Eds.; Academic: New York, 1981; Vol. 2, pp 97-98.
(17) Recrystallization from CH2Cl2/THF/Et2O afforded analytically pure
4: 1H NMR (acetone-d6, 250 MHz) δ 8.01 (d, 2H), 7.33-7.13 (m, 22H),
6.99 (d, 2H). Anal. Calcd for C40H28O5 C, 81.62; H, 4.79. Found C, 81.81; H,
4.81.
(18) Diebold, A.; Hagen, K. S. Inorg. Chem. 1998, 37, 215-223.
(19) A complete structure determination was hampered by severe disorder
in the TMEDA ligands and lattice solvent molecules. Anal. Calcd for
C53H59N4O9F3SFe2 C, 58.04; H, 5.42; N, 5.11. Found C, 57.89; H, 5.33; N,
5.11.
(10) Stenkamp, R. E. Chem. ReV. 1994, 94, 715-726.
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Williams, R. J. P. Biochemistry 1969, 8, 1951-1958.
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(13) Rardin, R. L.; Tolman, W. B.; Lippard, S. J. New J. Chem. 1991, 15,
417-430.
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10.1021/ja982417z CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/13/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 42, 1998 11023
ꢀ500 ) 2200 M-1 cm-1, and ꢀ330/ꢀ500 ) 3.1. The 500-nm band of
oxyHr has been attributed to a hydroperoxide-to-Fe(III) charge-
transfer transition,24 which is considerably higher in energy than
the peroxo-to-Fe(III) charge-transfer transitions (λmax ≈ 600 nm)
of µ-1,2-peroxodiiron(III) complexes.25 The 470-nm band of 7
is, therefore, tentatively assigned to the ligand-to-metal charge-
transfer transition(s) of a terminally bound hydroperoxide-
iron(III) unit.
Direct evidence for a dioxygen-bound moiety in 7 was obtained
through resonance Raman spectroscopy. Excitation at 514.5 nm
into the low-energy side of the 470-nm band of 7 prepared with
16O2 yielded a resonance-enhanced Raman band at 843 cm-1
(Figure 2B), which falls within the range expected for the O-O
stretch of a metal-bound peroxide unit.26 This assignment was
confirmed by preparing a sample of 7 using 18O2, wherein the
O-O stretch was observed at 797 cm-1 (Figure 2B), a value
consistent with that expected for a largely uncoupled harmonic
oscillator. These O-O stretching frequencies match those of
oxyHr,27 thereby indicating that O2 binds to both 7 and oxyHr in
a very similar manner, probably as a terminal hydroperoxide.
Frozen solution (THF, 77 K) Mo¨ssbauer spectra of both 5 plus
3 equiv of N-MeIm and of 7 indicate retention of dinuclear
structures. Fits of the data to quadrupole doublets revealed
parameters for the solution of 5 + 3N-MeIm (δ ) 1.23, ∆EQ )
2.85 mm s-1) very close to the values for 5 in the solid state
(vide supra). Addition of dioxygen to form 7 shifted δ to ∼0.50
mm s-1 and ∆EQ to ∼1.42 mm s-1, values which compare
favorably to those for oxyHr (δav ≈ 0.50, ∆EQav ≈ 1.48 mm s-1)20
and support the presence of a (µ-oxo)diiron(III) unit.6 Hydrogen
bonding of this putative hydroperoxide ligand to the µ-oxo atom
in a manner similar to that proposed for oxyHr28 remains to be
established.
Figure 1. ORTEP diagram of [Fe2(µ-OH)(µ-Ph4DBA)(TMEDA)2-
(CH3CN)]+ showing 30% probability thermal ellipsoids for all nonhy-
drogen atoms. Selected interatomic distances (Å): Fe21‚‚‚Fe22, 3.1638-
(5); Fe21-O201, 1.959(2); Fe21-O203, 2.102(2); Fe21-O205, 2.040(2);
Fe21-N202, 2.172(2); Fe21-N203, 2.254(2); Fe22-O201, 2.026(2);
Fe22-O204, 2.143(2); Fe22-O206, 2.140(2); Fe22-N201, 2.336(2);
Fe22-N204, 2.219(2); Fe22-N205, 2.282(2). The Fe21-O201-Fe22
bond angle is 105.12(8)°.
In summary, we have synthesized a new dinucleating dicar-
boxylate ligand that has allowed for the preparation and charac-
terization of novel iron complexes having functionally relevant
structural and spectroscopic features of deoxy- and oxyHr. Two-
metal oxidation12 is neither a thermodynamically nor kinetically
hindered process in Hr, and we have devised a synthetic system
that mimics this chemistry. Efforts to characterize further the
putative hydroperoxide adduct 7 and to understand its formation
and decomposition chemistry are currently in progress.
Figure 2. [Fe2(µ-OH)(µ-Ph4DBA)(TMEDA)2(OTf)] (5) in CH2Cl2
containing 3 equiv of N-MeIm at -78 °C. (A) Electronic absorption
spectra before (- - -) and after (s) addition of dioxygen. (B) Resonance
Raman spectra (excited at 514.5 nm) after oxygenation with 16O2 (s)
and 18O2 (- - -).
Acknowledgment. This work was supported by grants from the NSF
and NIH. We thank Professor W. M. Reiff of Northeastern University
for allowing us to use his Mo¨ssbauer spectrometer, Ms. A. M. Barrios
for assistance in obtaining the resonance Raman spectra, and Dr. J. Du
Bois for helpful discussions.
reflected by the systematically shorter bond lengths for the
5-coordinate, distorted trigonal bipyramidal Fe21 compared to
the 6-coordinate, pseudooctahedral Fe22 (Figure 1).
Supporting Information Available: Details of the syntheses, X-ray
crystallographic study, and physical characterization of 5-7 (34 pages,
print/PDF). See any current masthead page for ordering and Internet
access instructions. See any current masthead page for ordering informa-
tion and Web access instructions.
Compounds 5 and 6 represent the first synthetic diiron
complexes that reproduce not only the (µ-hydroxo)bis(µ-carboxy-
lato)diiron(II) core structure of deoxyHr but also the coordination
numbers at each iron(II) center, thereby affording an open site
for potential binding of dioxygen. When dioxygen was bubbled
for ∼30 s into a colorless solution of 5 in CH2Cl2 containing 3
equiv of N-MeIm at -78 °C, the solution turned red-orange.
Optical absorptions at 336 and ∼470 nm reached their maximum
intensity (ꢀ336 ≈ 7300; ꢀ470 ≈ 2600 M-1 cm-1; ꢀ336/ꢀ470 ) 2.8)
after approximately 5 min (Figure 2A). Under these conditions,
formation of this initial species 7 was irreversible, and the product
was not stable indefinitely, the intensity of the visible bands slowly
diminishing over the course of several hours. The UV-vis
spectrum of 7 is remarkably similar to that of oxyHr,23 which
exhibits bands centered at 500 and 330 nm with ꢀ330 ) 6800,
JA982417Z
(23) Keresztes-Nagy, S.; Klotz, I. M. Biochemistry 1965, 4, 919-931.
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Stephens, P. J.; Musselman, R. L.; Solomon, E. I. J. Am. Chem. Soc. 1989,
111, 4688-4704.
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E. I. J. Am. Chem. Soc. 1998, 120, 5674-5690.
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