Synthesis and electrochemical studies of diiron complexes of 1,8-naphthyridine-based dinucleating ligands to model features of the active sites of non-heme diiron enzymes

Article abstract of DOI:10.1021/ic000975k

A bis(μ-carboxylato)(μ-1,8-naphthyridine)diiron(II) complex, [Fe₂(BPMAN)(μ-O₂CPhCy)₂](OTf)₂ (1), was prepared by using the 1,8-naphthyridine-based dinucleating ligand BPMAN, where BPMAN = 2,7-bis[bis(2-pyridylmethyl)-aminomethyl]-1,8-naphthyridine. The cyclic voltammogram (CV) of this complex in CH₂Cl₂ exhibited two reversible one-electron redox waves at +296 mV (ΔE_p = 80 mV) and +781 mV (ΔE_p = 74 mV) vs Cp₂Fe⁺/Cp₂Fe, corresponding to the Fe^IIIFe^II/Fe^IIFe^II and Fe^IIIFe^III/Fe^IIIFe^II couples, respectively. This result is unprecedented for diiron complexes having no single atom bridge. Dinuclear complexes [Fe₂(BPMAN)(μ-OH)(μ-O₂CPhCy)]-(OTf)₂ (2) and [Mn₂(BPMAN)(μ-O₂CPhCy)₂](OTf)₂ (3) were also synthesized and structurally characterized. The cyclic voltammogram of 2 in CH₂Cl₂ exhibited one reversible redox wave at -22 mV only when the potential was kept below +400 mV. The CV of 3 showed irreversible oxidation at potentials above +900 mV. Diiron(II) complexes Fe₂(BEAN)(μ-O₂CPhCY)₃](OTf) (4) and [Fe₂(BBBAN)(μ-OAC)₂(OTf)](OTf) (6) were also prepared and characterized, where BEAN = 2,7-bis(N,N-diethylaminomethyl)-1,8-naphthyridine and BBBAN = 2,7-bis-{2-[2-(1-methyl)benzimidazolylethyl]- N-benzylaminomethyl}-1,8-naphthyridine. The cyclic voltammograms of these complexes were recorded. The Moessbauer properties of the diiron compounds were studied.

Full text of DOI:10.1021/ic000975k

1414
Inorg. Chem. 2001, 40, 1414-1420
Synthesis and Electrochemical Studies of Diiron Complexes of 1,8-Naphthyridine-Based
Dinucleating Ligands to Model Features of the Active Sites of Non-Heme Diiron Enzymes
Chuan He and Stephen J. Lippard*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
ReceiVed August 25, 2000
A bis(µ-carboxylato)(µ-1,8-naphthyridine)diiron(II) complex, [Fe2(BPMAN)(µ-O2CPhCy)2](OTf)2 (1), was prepared
by using the 1,8-naphthyridine-based dinucleating ligand BPMAN, where BPMAN ) 2,7-bis[bis(2-pyridylmethyl)-
aminomethyl]-1,8-naphthyridine. The cyclic voltammogram (CV) of this complex in CH2Cl2 exhibited two
reversible one-electron redox waves at +296 mV (∆Ep ) 80 mV) and +781 mV (∆Ep ) 74 mV) vs Cp2Fe+/
Cp2Fe, corresponding to the FeIIIFeII/FeIIFeII and FeIIIFeIII/FeIIIFeII couples, respectively. This result is unprecedented
for diiron complexes having no single atom bridge. Dinuclear complexes [Fe2(BPMAN)(µ-OH)(µ-O2CPhCy)]-
(OTf)2 (2) and [Mn2(BPMAN)(µ-O2CPhCy)2](OTf)2 (3) were also synthesized and structurally characterized.
The cyclic voltammogram of 2 in CH2Cl2 exhibited one reversible redox wave at -22 mV only when the potential
was kept below +400 mV. The CV of 3 showed irreversible oxidation at potentials above +900 mV. Diiron(II)
complexes [Fe2(BEAN)(µ-O2CPhCy)3](OTf) (4) and [Fe2(BBBAN)(µ-OAc)2(OTf)](OTf) (6) were also prepared
and characterized, where BEAN ) 2,7-bis(N,N-diethylaminomethyl)-1,8-naphthyridine and BBBAN ) 2,7-bis-
{2-[2-(1-methyl)benzimidazolylethyl]-N-benzylaminomethyl}-1,8-naphthyridine. The cyclic voltammograms of
these complexes were recorded. The Mo¨ssbauer properties of the diiron compounds were studied.
Introduction
step, the oxidation potential of the diiron(II) core would deter-
mine the O2 reactivity. It therefore would be valuable to
understand how the redox properties of the synthetic model
complexes varied with their compositions and structures. The
study of the electrochemical behavior of diiron complexes is
limited, however, due to decomposition pathways that occur
under redox conditions.16
To address this problem, we have developed a series of new
dinucleating ligands based on 1,8-naphthyridine. The 1,8-
naphthyridine unit coordinates to and bridges two metal ions,
as we have demonstrated previously.17 Dimetallic compounds
prepared from these “masked carboxylate” ligands exhibit a wide
range of metal-metal separations and flexible geometry.
Different oxidation states of the metal ions can be accom-
modated while still maintaining a stable dinuclear core.
In the present report we describe the synthesis and charac-
terization of several diiron(II) compounds that contain the
dinucleating ligands BPMAN, BEAN, and BBBAN,18 where
BPMAN ) 2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naph-
thyridine, BEAN ) 2,7-bis(N,N-diethylaminomethyl)-1,8-naph-
thyridine, and BBBAN ) 2,7-bis{2-[2-(1-methyl)benzimida-
The enzymes methane monooxygenase (MMO), ribonucleo-
tide reductase (RNR), and ∆-9 desaturase (∆9D) belong to a
class of non-heme diiron-containing proteins that have been the
subjects of extensive investigation in recent years.1-4 Carboxy-
late-bridged diiron clusters at the active sites of these enzymes
activate O2 for different biological functions. The reduced forms
of the proteins have been characterized by macromolecular
X-ray crystallography.5-7 They all contain diiron(II) units with
carboxylate-only bridges at the active sites (Figure 1) that react
with O2 to give intermediates responsible for their respective
functions. In at least one case, a diiron(II) center bridged only
by carboxylate groups in ribonucleotide reductase was suggested
to function as an electron supplier during enzyme turnover.8
Many carboxylate-bridged diiron(II) complexes have been
prepared as models for the active sites of the reduced enzymes.
Some of these complexes also react with O2 to give intermedi-
ates that mimic features observed in the enzymatic systems.9-15
These reactions all involve oxidation of the diiron centers.
Knowledge of the redox potentials of diiron(II) complexes may
help explain the O2 reactivity in some cases. If outer sphere
electron transfer from diiron(II) to O2 were to occur as the first
(9) Me´nage, S.; Zang, Y.; Hendrich, M. P.; Que, L., Jr. J. Am. Chem.
Soc. 1992, 114, 7786-7792.
(10) Kim, K.; Lippard, S. J. J. Am. Chem. Soc. 1996, 118, 4914-4915.
(11) Ookubo, T.; Sugimoto, H.; Nagayama, T.; Masuda, H.; Sato, T.;
Tanaka, K.; Maeda, Y.; Okawa, H.; Hayashi, Y.; Uehara, A.; Suzuki,
M. J. Am. Chem. Soc. 1996, 118, 701-702.
(1) Solomon, E. I.; Brunold, T. C.; Davis, M. I.; Kemsley, J. N.; Lee,
S.-K.; Lehnert, N.; Neese, F.; Skulan, A. J.; Yang, Y.-S.; Zhou, J.
Chem. ReV. 2000, 100, 235-349.
(2) Valentine, A. M.; Lippard, S. J. J. Chem. Soc., Dalton Trans. 1997,
3925-3931.
(12) Dong, Y.; Yan, S.; Young, V. G., Jr.; Que, L., Jr. Angew. Chem., Int.
Ed. Engl. 1996, 35, 618-620.
(3) Que, L., Jr. J. Chem. Soc., Dalton Trans. 1997, 3933-3940.
(4) Wallar, B. J.; Lipscomb, J. D. Chem. ReV. 1996, 96, 2625-2657.
(5) Rosenzweig, A. C.; Nordland, P.; Takahara, P. M.; Frederick, C. A.;
Lippard, S. J. Chem. Biol. 1995, 2, 409-418.
(13) LeCloux, D. D.; Barrios, A. M.; Mizoguchi, T. J.; Lippard, S. J. J.
Am. Chem. Soc. 1998, 120, 9001-9014.
(14) 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.
(15) Hagadorn, J. R.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1998,
120, 13531-13532.
(16) Armstrong, W. H.; Spool, A.; Papaefthymiou, G. C.; Frankel, R. B.;
Lippard, S. J. J. Am. Chem. Soc. 1984, 106, 3653-3667.
(17) He, C.; Lippard, S. J. J. Am. Chem. Soc. 2000, 122, 184-185.
(18) He, C.; Lippard, S. J. Tetrahedron 2000, 56, 8245-8252.
(6) Logan, D. T.; Su, X.-D.; A° berg, A.; Regnstro¨m, K.; Hajdu, J.; Eklund,
H.; Nordlund, P. Structure 1996, 4, 1053-1064.
(7) Lindqvist, Y.; Huang, W.; Schneider, G.; Shanklin, J. EMBO J. 1996,
15, 4081-4092.
(8) Miller, M. A.; Gobena, F. T.; Kauffman, K.; Mu¨nck, E.; Que, L., Jr.;
Stankovich, M. T. J. Am. Chem. Soc. 1999, 121, 1096-1097.
10.1021/ic000975k CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/13/2001

Studies of [Fe2(BPMAN)(µ-O2CPhCy)2](OTf)2
Inorganic Chemistry, Vol. 40, No. 7, 2001 1415
Figure 1. Schematic representation of the active site structures of reduced MMOH, RNR and ∆-9 desaturase.
Figure 2. Schematic representation of ligands BPMAN, BEAN, and BBBAN.
1445 (m), 1396 (s), 1350 (w), 1276 (s), 1223 (m), 1152 (s), 1100 (w),
zolylethyl]-N-benzylaminomethyl}-1,8-naphthyridine (Figure 2).
A diiron complex containing additional carboxylate bridges,
[Fe2(BPMAN)(µ-O2CPhCy)2](OTf)2 (1), prepared with the use
of BPMAN exhibited two reversible redox waves in a cyclic
voltammetric experiment. The complexes [Fe2(BPMAN)(µ-
OH)(µ-O2CPhCy)](OTf)2 (2), [Mn2(BPMAN)(µ-O2CPhCy)2]-
(OTf)2 (3), [Fe2(BEAN)(µ-O2CPhCy)3](OTf) (4), [Fe2(BEAN)(µ-
O2CCPh3)3](OTf) (5), and [Fe2(BBBAN)(µ-OAc)2(OTf)](OTf)
(6) were also synthesized and their cyclic voltammograms
recorded. The Mo¨ssbauer properties of all the diiron compounds
were studied as well.
1031 (s), 905 (w), 793 (w), 766 (m), 736 (m), 702 (w), 637 (s). UV-
vis (CH2Cl2, λmax, nm (ꢀ, M-1 cm-1)) 560 (660). Anal. Calcd for 1,
C62H62N8O10S2F6Fe2: C, 54.48; H, 4.56; N, 8.18. Found: C, 54.36; H,
4.42; N, 8.23.
[Fe2(BPMAN)(µ-OH)(µ-O2CPhCy)](OTf)2 (2). To a solution of
Fe(OTf)2‚2CH3CN3 (150 mg, 0.361 mmol) in acetonitrile (5 mL) was
added sodium 1-phenylcyclohexanecarboxylate (41 mg, 0.181 mmol),
BPMAN (100 mg, 0.181 mmol), Bu3N (43 µL, 0.181 mmol), and water
(5 µL). The mixture was allowed to stir for 1 h and the solvent was
removed under vacuum. The residue was dissolved in CH2Cl2 and
filtered. Crystallization from Et2O/CH2Cl2 yielded green blocks suitable
for X-ray crystallography (129 mg, 60%). FTIR (KBr, cm-1): 3072
(m), 2932 (s), 2857 (m), 1604 (s), 1568 (s), 1513 (w), 1480 (m), 1445
(s), 1392 (s), 1349 (w), 1275 (s), 1224 (s), 1154 (s), 1100 (m), 1053
(w), 1031 (s), 978 (w), 960 (w), 906 (w), 888 (w), 867 (w), 848 (w),
Experimental Section
General Procedures and Methods. All reagents were obtained from
commercial suppliers and used without further purification unless
otherwise noted. THF was distilled from sodium benzophenone ketyl
under nitrogen. Pentane and diethyl ether were purified by passage
through activated alumina columns under nitrogen. Dichloromethane,
acetonitrile, and propionitrile were distilled from CaH2 under nitrogen.
Fourier transform infrared spectra were recorded on a Bio-Rad FTS135
instrument and UV-vis spectra on a Varian 1-E spectrophotometer.
Mo¨ssbauer spectra were recorded on an MS1 spectrometer (WEB
Research Co.) with a 57Co source in a Rh matrix maintained at room
temperature in the MIT Department of Chemistry Instrumentation
Facility. Ligands BPMAN, BEAN, and BBBAN were synthesized
according to procedures reported elsewhere.18 Fe(OTf)2‚2CH3CN3, Mn-
(OTf)2‚2CH3CN3, and 1-phenylcyclohexanecarboxylic acid were pre-
pared by following known procedures.19,20 All air-sensitive manipula-
tions were carried out either in a nitrogen-filled Vacuum Atmospheres
drybox or by standard Schlenk line techniques.
[Fe2(BPMAN)(µ-O2CPhCy)2](OTf)2 (1). To a solution of Fe(OTf)2‚
2CH3CN3 (60 mg, 0.145 mmol) in acetonitrile (3 mL) was added
sodium 1-phenylcyclohexanecarboxylate (33 mg, 0.145 mmol) and
BPMAN (40 mg, 0.072 mmol). The mixture was allowed to stir for 1
h and the solvent was removed under vacuum. The residue was
dissolved in CH2Cl2 and filtered. Crystallization from Et2O/CH2Cl2
yielded blue blocks suitable for X-ray crystallography (62 mg, 63%).
FTIR (KBr, cm-1): 3070 (w), 2935 (s), 2859 (m), 1605 (s), 1593 (s),
793 (w), 764 (s), 737 (w), 702 (w), 637 (s). UV-vis (CH2Cl2, λmax
,
nm (ꢀ, M-1 cm-1)) 600 (569). Anal. Calcd for 2, C49H48N8O9S2F6Fe2:
C, 49.76; H, 4.09; N, 9.47. Found: C, 49.66; H, 4.00; N, 9.59.
[Mn2(BPMAN)(µ-O2CPhCy)2](OTf)2 (3). The procedure used to
prepare 1 was followed except that Mn(OTf)2‚2CH3CN3 was substituted
as the metal salt. Compound 3 was obtained as yellow blocks from
Et2O/CH2Cl2 in 57% yield. Single crystals suitable for X-ray crystal-
lography were grown at -30 °C from saturated EtO2/CH2Cl2. FTIR
(KBr, cm-1): 3067 (w), 3033 (w), 2935 (s), 2859 (m), 1604 (s), 1590
(s), 1446 (m), 1401 (s), 1350 (w), 1277 (s), 1224 (m), 1152 (s),1099
(w), 1031 (s), 1015 (w), 905 (w), 793 (w), 766 (m), 736 (m), 702 (w),
637 (s). Anal. Calcd for 3, C62H62N8O10S2F6Mn2: C, 54.47; H, 4.57;
N, 8.20. Found: C, 54.37; H, 4.36; N, 8.13.
[Fe2(BEAN)(µ-O2CPhCy)3](OTf) (4). A portion of sodium 1-
phenylcyclohexanecarboxylate (90.3 mg, 0.399 mmol) in THF (0.5 mL)
was added to a solution of Fe(OTf)2‚2CH3CN3 (109.7 mg, 0.266 mmol)
in THF (1 mL). After 15 min of stirring, a portion of BEAN (40 mg,
0.133 mmol) in THF (1 mL) was added. The yellow solution was
allowed to stir for 1 h and the solvent was removed under vacuum.
The residue was dissolved in CH2Cl2 and filtered. Crystallization from
pentane/Et2O/CH2Cl2 yielded light yellow blocks suitable for X-ray
crystallography (86 mg, 55%). FTIR (KBr, cm-1): 3065 (m), 3040
(w), 3022 (w), 2978 (m), 2934 (s), 2862 (s), 1609 (s), 1579 (s), 1449
(s), 1396 (s), 1346 (m), 1277 (s), 1224 (m), 1149 (s), 1031 (s), 914
(m), 874 (m), 844 (m), 769 (m), 732 (s), 697 (m), 637 (s). Anal. Calcd
for 4, C58H73N4O9SF3Fe2: C, 59.49; H, 6.28; N, 4.78. Found: C, 59.37;
H, 6.48; N, 4.79.
(19) Bryan, P. S.; Dabrowiak, J. C. Inorg. Chem. 1975, 14, 296-299.
(20) Calderon, S. N.; Newman, A. H.; Tortella, F. C. J. Med. Chem. 1991,
34, 3159-3164.

1416 Inorganic Chemistry, Vol. 40, No. 7, 2001
He and Lippard
[Fe2(BEAN)(µ-O2CCPh3)3](OTf) (5). The same procedure to
prepare 4 was followed, except that sodium triphenylacetate was used
as the carboxylate group. Crystallization from pentane/Et2O/benzene
yielded 5 as light yellow blocks (32 mg, 22%). FTIR (KBr, cm-1):
3060 (m), 3035 (w), 3022 (w), 2979 (m), 2944 (w), 2876 (w), 1610
(s), 1595 (s), 1571 (w), 1561 (m), 1542 (w), 1508 (w), 1492 (m), 1383
(s), 1277 (s), 1262 (s), 1223 (m), 1150 (s), 1031 (s), 870 (w), 768 (m),
637 (s). Anal. Calcd for 5, C79H73N4O9SF3Fe2: C, 66.67; H, 5.17; N,
3.94. Found: C, 66.27; H, 5.18; N, 3.74.
[Fe2(BBBAN)(µ-OAc)2(OTf)](OTf) (6). A portion of BBBAN (30.3
mg, 0.044 mmol) in acetonitrile (2 mL) was added to a solution of
Fe(OTf)2‚2CH3CN3 (19 mg, 0.046 mmol) in acetonitrile (3 mL),
followed by Fe(OAc)2 (7.7 mg, 0.044 mmol). This solution was allowed
to stir for 6 h and all the insoluble Fe(OAc)2 powder dissolved. The
color of the solution turned purple-pink. To this solution was added
Et2O until it became cloudy. The precipitates were filtered off. Layering
Et2O on top of the solution at -30 °C gave purple-pink plates suitable
for X-ray crystallographic studies (22 mg, 41%). FTIR (KBr, cm-1):
3065 (w), 3031 (w), 2959 (m), 2928 (m), 2876 (w), 1617 (m), 1612
(s), 1578 (m), 1571 (m), 1561 (m), 1523 (m), 1490 (m), 1458 (s), 1413
(m), 1332 (w), 1278 (s), 1224 (m), 1152 (s), 1031 (s), 892 (w), 857
(m), 796 (m), 753 (s), 707 (m), 638 (s). Anal. Calcd for 6,
C50H50N8O10S2F6Fe2: C, 49.52; H, 4.16; N, 9.24. Found: C, 49.30; H,
4.35; N, 9.26.
X-ray Crystallography Studies. X-ray crystallographic studies were
carried out on a Bruker (formerly Siemens) CCD diffractometer with
graphite-monochromatized Mo KR radiation (λ ) 0.71073 Å) controlled
by a Pentium-based PC running the SMART software package.21 Single
crystals were mounted at room temperature on the ends of quartz fibers
in Paratone N oil, and data were collected at 193 K in a stream of cold
N2 maintained by a Bruker LT-2A nitrogen cryostat. Data collection
and reduction protocols are described in detail elsewhere.22 The
structures were solved by direct methods and refined on F2 by using
the SHELXTL software package.23 Empirical absorption corrections
were applied with the SADABS program,24 and the structures were
checked for higher symmetry by PLATON.25 All non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were assigned idealized
locations and given isotropic thermal parameters 1.2 times the thermal
parameter of the carbon atoms to which they were attached. In the
structure of 2, one triflate anion is disordered and was refined
accordingly. In the structure of 6, two half-occupied triflate anions were
found in the lattice. A solvent acetonitrile molecule is half-occupied.
Another acetonitrile molecule is disordered and was refined as C56 at
0.5 occupancy, C57(A) and N11(A) at 0.25 occupancy, and C57(B)
and C11(B) at 0.25 occupancy.
Electrochemistry. Cyclic voltammetric measurements were per-
formed in a Vacuum Atmospheres drybox under N2 with an EG&G
Model 263 potentiostat. A three-electrode configuration consisting of
a 1.75 mm2 platinum working electrode, a Ag/AgNO3 (0.1 M in
CH3CN) reference electrode, and a coiled platinum wire auxiliary
electrode was used. The supporting electrolyte was 0.5 M (Bu4N)(PF6).
Sample concentrations of 2 mM were typically employed. All cyclic
voltammograms were externally referenced to Cp2Fe.
Mo1ssbauer Spectroscopy. Powdered solid samples (∼0.03 mol)
were suspended in Apiezon N grease and packed into nylon sample
holders in the drybox. The samples were removed from the drybox
and rapidly precooled to 77 K. All data were collected at 4.2 K. Isomer
shift (δ) values are reported with respect to iron foil that was used for
velocity calibration at room temperature. The spectra were fit to
Lorentzian lines by using the WMOSS plot and fit program.26
Figure 3. ORTEP diagram of [Fe2(BPMAN)(µ-O2CPhCy)2](OTf)2 (1)
showing the 40% probability thermal ellipsoids for all non-hydrogen
atoms.
(µ-O2CPhCy)](OTf)2 (2) and [Mn2(BPMAN)(µ-O2CPhCy)2]-
(OTf)2 (3). Compound 1 was prepared by reacting Fe(OTf)2‚
2CH3CN3, NaO2CPhCy, and BPMAN in a 2:2:1 ratio in
acetonitrile and crystallized as blue blocks from CH2Cl2/Et2O
at room temperature. The structure of 1 is shown in Figure 3,
and selected bond lengths and angles are listed in Table 2. The
two iron(II) atoms are bridged by two carboxylate groups and
the 1,8-naphthyridine unit of BPMAN. Both iron atoms are six-
coordinate with considerably distorted pseudooctahedral geom-
etry. The two 1-phenylcyclohexanecarboxylate ligands coordi-
nate the two iron atoms in an asymmetric manner with one
Fe-O distance longer than the other (Table 2). The Fe‚‚‚Fe
distance is 3.738(4) Å.
Compound 2 was synthesized by a similar procedure, and
green crystals were obtained from CH2Cl2/Et2O at room
temperature. A single-atom hydroxide bridge draws the two iron
atoms together, the Fe‚‚‚Fe distance being 3.221(6) Å. The
carboxylate group and the 1,8-naphthyridine unit of BPMAN
also link the iron atoms, the resulting Fe-O-Fe angle being
107.8(3)°. Each iron atom is six-coordinate with distorted
pseudooctahedral geometry. The structure of 2 is shown in
Figure 4, and selected bond lengths and angles are listed in Table
2. Compound 3, a dimanganese(II) analogue of 1, was also
prepared and characterized. The two manganese atoms are
3.829(4) Å apart from each other, and details of the structure
are included in Supporting Information.
Preparation and Structural Characterization of Paddle-
Wheel Diiron(II) Complexes [Fe2(BEAN)(µ-O2CPhCy)3]-
(OTf) (4) and [Fe2(BEAN)(µ-O2CPh3)3](OTf) (5). The
diiron(II) compound 4 was synthesized by allowing Fe(OTf)2‚
2CH3CN3, NaO2CPhCy, and BEAN to react in a 2:3:1 ratio in
tetrahydrofuran and crystallized as yellow blocks from pentane/
Et2O/CH2Cl2. X-ray crystallographic analysis revealed a paddle-
wheel structure with two iron atoms bridged by three carboxylate
groups and the 1,8-naphthyridine unit of ligand BEAN, as
indicated in Figure 5. Each iron has a five-coordinate, square
pyramidal stereochemistry and the Fe‚‚‚Fe distance is 2.8486-
(6) Å. The bridging 1-phenylcyclohexanecarboxylate ligands
coordinate the two iron atoms with average Fe-O distances of
2.041(2) Å for Fe(1) and 2.030(2) Å for Fe(2) (Table 3). When
NaO2CCPh3 was used as the carboxylate ligand, compound 5
Results and Discussion
Preparation and Structural Characterization of [Fe2-
(BPMAN)(µ-O2CPhCy)2](OTf)2 (1), [Fe2(BPMAN)(µ-OH)-
(21) SMART V5.05; Bruker AXS: Madison, WI, 1998.
(22) Feig, A. L.; Bautista, M. T.; Lippard, S. J. Inorg. Chem. 1996, 25,
6892-6898.
(23) Sheldrick, G. M. SHELXTL97-2: Program for the Refinement of Crys-
tal Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(24) Sheldrick, G. M. SADABS: Area-Detector Absorption Correction;
University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(25) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands. 1998.
(26) Kent, T. A. WMOSS: Mo¨ssbauer Spectral Analysis Software. 2.5;
Minneapolis, MN, 1998.

Studies of [Fe2(BPMAN)(µ-O2CPhCy)2](OTf)2
Inorganic Chemistry, Vol. 40, No. 7, 2001 1417
Table 1. Summary of X-ray Crystallographic Data
1
2‚CH2Cl2
3
4‚CH2Cl2
6‚CH3CN
formula
fw
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
C62H62N8O10F6S2Fe2 C50H50N8O9Cl2F6S2Fe2 C62H62N8O10F6S2Mn2 C59H75N4O9Cl2F3SFe2 C52H53N9O10F6S2Fe2
1369.02
P21/c
1267.70
P212121
1367.20
P1h
1255.89
P21/n
1253.85
P21212
14.2124(3)
22.3326(3)
19.5692(4)
10.2490(3)
21.7275(5)
24.6524(7)
13.5590(4)
14.6503(4)
18.3921(6)
82.0690(10)
70.0740(10)
64.6370(10)
3103.54(16)
2
10.89270(10)
38.5562(5)
14.9577(2)
15.6105(5)
32.7813(11)
11.6641(4)
95.4630(10)
110.0740(4)
6183.0(2)
4
5489.7(3)
4
5900.32(12)
4
5968.9(3)
4
T, °C
F
-85
-85
-85
-85
-85
calcd, g/cm3
1.471
0.620
3-50
31849
10866
7035
1.534
1.463
1.414
1.395
µ(Mo KR), mm-1
2θ range, deg
total no. of data
no. of unique data
observed dataa
no. of parameters
Rb
0.784
0.557
3-46
13760
8572
6151
811
0.0548
0.686
3-50
43904
10376
8284
0.636
3-45
23386
7155
5906
629
0.0769
0.2038
0.885, -0.948
3-46
26553
8307
7129
727
0.0631
0.1558
0.963, -0.609
804
0.0658
0.1660
721
0.0445
0.1254
0.421, -0.652
wR2c
0.1415
0.777, -0.455
max, min peaks, e/Å3 0.828, -0.801
a Observation criterion: I > 2σ(I). b R ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) {∑[w(Fo - Fc2)2]/∑[w(Fo )2]}1/2
.
2
2
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1 and
a
2‚CH2Cl2
bond length
Fe(1)‚‚‚Fe(2) 3.738(4) O(1)-Fe(1)-O(3) 108.25(13)
90.48(13)
bond angle
1
Fe(1)-O(1) 2.083(3) O(1)-Fe(1)-N(1)
Fe(1)-O(3) 1.997(3) O(1)-Fe(1)-N(5) 161.43(14)
Fe(1)-N(1) 2.301(4) O(3)-Fe(1)-N(3) 159.60(14)
Fe(1)-N(3) 2.240(4) N(1)-Fe(1)-N(3)
76.11(14)
Fe(1)-N(4) 2.217(4) N(1)-Fe(1)-N(4) 150.04(14)
Fe(1)-N(5) 2.210(4) O(2)-Fe(2)-O(4) 102.73(14)
Fe(2)-O(2) 1.989(3) O(2)-Fe(2)-N(2) 119.10(13)
Fe(2)-O(4) 2.087(3) O(2)-Fe(2)-N(6) 161.29(13)
Fe(2)-N(2) 2.363(4) O(4)-Fe(2)-N(7) 165.26(14)
Fe(2)-N(6) 2.244(4) N(2)-Fe(2)-N(6)
74.50(14)
Fe(2)-N(7) 2.159(4) N(2)-Fe(2)-N(8) 147.93(15)
Fe(2)-N(8) 2.228(4)
2‚CH2Cl2 Fe(1)‚‚‚Fe(2) 3.221(6) Fe(1)-O(1)-Fe(2) 107.8(3)
Fe(1)-O(1) 1.952(6) O(1)-Fe(1)-O(2) 99.1(2)
Fe(1)-O(2) 2.131(6) O(1)-Fe(1)-N(1) 104.6(3)
Fe(1)-N(1) 2.318(8) O(1)-Fe(1)-N(3) 174.8(3)
Fe(1)-N(3) 2.252(7) O(2)-Fe(1)-N(4) 162.0(3)
Figure 4. ORTEP diagram of [Fe2(BPMAN)(µ-OH)(µ-O2CPhCy)]-
(OTf)2 (2) showing the 40% probability thermal ellipsoids for all non-
hydrogen atoms.
Fe(1)-N(4) 2.216(7) N(1)-Fe(1)-N(3)
74.5(3)
Fe(1)-N(5) 2.187(8) N(1)-Fe(1)-N(5) 149.7(3)
The two iron atoms are 3.504(4) Å apart and bridged by two
acetates and the 1,8-naphthyridine unit of BBBAN. One acetate
bridges two iron in a syn-syn bidentate fashion. The other
acetate is monodentate bridging with coordination of the other
oxygen atom O(4) to Fe(2). One triflate is terminally bound to
Fe(1). The two acetate groups bridge two iron atoms such as
those of residues Glu-144 and Glu-243 between the two iron-
(II) atoms in reduced MMOH (Figure 1).
Fe(2)-O(1) 2.034(6) O(1)-Fe(2)-O(3)
Fe(2)-O(3) 2.059(6) O(1)-Fe(2)-N(2)
93.3(2)
87.7(3)
Fe(2)-N(2) 2.272(9) O(1)-Fe(2)-N(8) 165.9(3)
Fe(2)-N(6) 2.277(7) O(3)-Fe(2)-N(6) 152.9(3)
Fe(2)-N(7) 2.174(8) N(2)-Fe(2)-N(6)
Fe(2)-N(8) 2.248(9) N(2)-Fe(2)-N(7) 149.3(3)
75.4(3)
a Numbers in parentheses are estimated standard deviations of the
last significant figure. Atoms are labeled as indicated in Figures 3
and 4.
Electrochemistry of 1. The cyclic voltammogram (CV) of
1, recorded in CH2Cl2, exhibits two reversible redox waves with
E1/2 values of +296 mV (∆Ep ) 80 mV) and +781 mV (∆Ep
) 74 mV) vs Cp2Fe+/Cp2Fe (Figure 7). This result implies that
the diiron(II,III) and diiron(III) states may be accessed chemi-
cally and electrochemically from 1. Self-assembled diiron
centers typically dissociate when undergoing redox reactions,
although one-electron reversible redox waves have been previ-
ously reported for carboxylate-bridged diiron centers with or
without single-atom bridge(s).27-30 The observation of two
reversible redox waves is unprecedented to our knowledge for
a carboxylate-supported diiron center without any single-atom
bridge, however.
was obtained but only in low yield. There are two molecules in
the asymmetric unit, one having severely disordered phenyl
rings. Both molecules have the paddle-wheel diiron core and
the structure of the ordered molecule is depicted in the
Supporting Information.
Preparation and Structural Characterization of [Fe2-
(BBBAN)(µ-OAc)2(OTf)]-(OTf) (6). The reaction of equal
molar quantities of Fe(OTf)2‚2CH3CN3, Fe(OAc)2, and BBBAN
in acetonitrile, followed by crystallization from Et2O/CH3CN,
yielded crystals of 6, the structure of which is shown in Figure
6. Selected bond lengths and angles are included in Table 3.
Each iron is six-coordinate with pseudooctahedral geometry.

1418 Inorganic Chemistry, Vol. 40, No. 7, 2001
He and Lippard
Figure 6. ORTEP diagram of [Fe2(BBBAN)(µ-OAc)2(OTf)](OTf) (6)
showing the 40% probability thermal ellipsoids for all non-hydrogen
atoms.
Figure 5. ORTEP diagram of [Fe2(BEAN)(µ-O2CPhCy)3](OTf) (4)
showing the 40% probability thermal ellipsoids for all non-hydrogen
atoms.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
4‚CH2Cl2 and 6‚CH3CNa
bond length
4‚CH2Cl2 Fe(1)‚‚‚Fe(2) 2.8486(6) O(1)-Fe(1)-O(3)
Fe(1)-O(1) 2.0370(19) O(1)-Fe(1)-O(5) 157.57(8)
Fe(1)-O(3) 2.056(2) O(3)-Fe(1)-O(5) 91.81(8)
bond angle
91.73(8)
Fe(1)-O(5) 2.0312(19) O(1)-Fe(1)-N(3) 105.13(9)
Fe(1)-N(1) 2.179(2)
Fe(1)-N(3) 2.167(2)
O(3)-Fe(1)-N(1) 171.79(9)
O(3)-Fe(1)-N(3) 109.38(9)
Fe(2)-O(2) 2.0496(19) O(2)-Fe(2)-O(4)
90.30(8)
Figure 7. Cyclic voltammogram of 2 mM [Fe2(BPMAN)(µ-O2-
CPhCy)2](OTf)2 (1) in CH2Cl2 with 0.5 M (Bu4N)(PF6) as supporting
electrolyte and a scan rate of 100 mV/s.
Fe(2)-O(4) 2.034(2)
O(2)-Fe(2)-O(6) 163.68(8)
Fe(2)-O(6) 2.0051(19) O(4)-Fe(2)-O(6)
87.81(8)
86.92(8)
O(4)-Fe(2)-N(2) 152.78(9)
O(4)-Fe(2)-N(4) 129.15(9)
Fe(2)-N(2) 2.210(2)
Fe(2)-N(4) 2.153(2)
O(2)-Fe(2)-N(2)
oxidation with an E1/2 value of -216 mV vs Cp2Fe+/Cp2Fe in
CH2Cl2.29 A mixed-valence diiron(II,III) complex with two five-
coordinate iron atoms was obtained from this oxidation. A short
metal-metal distance of 2.6982(13) Å suggested direct interac-
tion between two iron centers in the mixed-valence form. With
two additional anionic carboxylate groups, the oxidation po-
tential of the diiron(II) center in this system is 512 mV lower
than that of 1, suggesting that the addition of each anionic
carboxylate group causes an approximately -250 mV change
of the redox potential of a diiron center. Further oxidation of
the tetracarboxylate-bridged diiron(II,III) species was not ob-
served by cyclic voltammetry, however. The presence of the
second redox couple in the CV of 1 therefore suggests that a
diiron(III) species with carboxylate-only anionic bridges could
be feasible if the diiron unit were stabilized by the use of a
multidentate ligand system.
By using alkoxide-bridged heptadentate dinucleating ligands
BPMP and BIMP, two reversible redox waves were previously
observed for diiron(II) complexes having two additional car-
boxylate bridges (Table 4).31-34 The E1/2 values for the
FeIIIFeII/FeIIFeII and FeIIIFeIII/FeIIIFeII couples of these complexes
measured in acetonitrile are several hundred millivolts lower
than those of 1 when converted to approximately the same scale
(Table 4). The alkoxide bridge, not yet found in a biological
6‚CH3CN Fe(1)‚‚‚Fe(2) 3.504(4)
Fe(1)-O(1) 2.050(5)
Fe(1)-O(3) 2.112(4)
Fe(1)-O(5) 2.130(5)
Fe(1)-N(1) 2.315(5)
Fe(1)-N(3) 2.245(6)
Fe(1)-N(4) 2.165(5)
Fe(2)-O(2) 1.981(5)
Fe(2)-O(3) 2.268(4)
Fe(2)-O(4) 2.160(4)
Fe(2)-N(2) 2.304(5)
Fe(2)-N(6) 2.189(5)
Fe(2)-N(7) 2.155(5)
Fe(1)-O(3)-Fe(2) 106.17(18)
O(1)-Fe(1)-O(3)
90.30(17)
O(1)-Fe(1)-O(5) 177.4(2)
O(1)-Fe(1)-N(1) 102.15(19)
O(3)-Fe(1)-N(3) 167.78(19)
N(1)-Fe(1)-N(3)
74.65(19)
N(1)-Fe(1)-N(4) 162.4(2)
O(2)-Fe(2)-O(3) 106.38(17)
O(2)-Fe(2)-O(4) 165.27(18)
O(3)-Fe(2)-O(4)
O(2)-Fe(2)-N(2)
58.90(16)
94.67(19)
O(3)-Fe(2)-N(6) 146.00(18)
N(2)-Fe(2)-N(7) 169.11(19)
a Numbers in parentheses are estimated standard deviations of the
last significant figure. Atoms are labeled as indicated in Figures 5
and 6.
The presence of two anionic carboxylate groups and the
neutral 1,8-naphthyridine moiety in bridging positions between
the diiron centers in 1 is consistent with the high oxidation
potentials exhibited. The self-assembled tetracarboxylate
diiron(II) compound [Fe2(µ-O2CArTol)4(4-tBuC5H4N)2], recently
reported from our laboratory, undergoes a reversible one-electron
(27) Hartman, J. R.; Rardine, R. L.; Chaudhuri, P.; Pohl, K.; Wieghardt,
K.; Nuber, B.; Weiss, J.; Papaefthymiou, G. C.; Frankel, R. B.;
Lippard, S. J. J. Am. Chem. Soc. 1987, 109, 7387-7396.
(28) Holz, R. C.; Elgren, T. E.; Pearce, L. L.; Zhang, J. H.; O’Connor, C.
J.; Que, L., Jr. Inorg. Chem. 1993, 32, 5844-5850.
(29) Lee, D.; Krebs, C.; Huynh, B. H.; Hendrich, M. P.; Lippard, S. J. J.
Am. Chem. Soc. 2000, 122, 5000-5001.
(30) Que, L., Jr.; True, A. E. In Progress in Inorganic Chemistry; Lippard,
S. J., Eds.; John Wiley & Sons: New York, 1990; Vol. 38, pp 97-
200.
(31) Suzuki, M.; Uehara, A.; Oshio, H.; Endo, K.; Yanaga, M.; Kida, S.;
Saito, K. Bull. Chem. Soc. Jpn. 1987, 60, 3547-3555.
(32) Borovik, A. S.; Papaefthymiou, V.; Taylor, L. F.; Anderson, O. P.;
Que, L., Jr. J. Am. Chem. Soc. 1989, 111, 6183-6195.
(33) Mashuta, M. S.; Webbs, R. J.; Oberhausen, K. J.; Richardson, J. F.;
Buchanan, R. M.; Hendrickson, D. N. J. Am. Chem. Soc. 1989, 111,
2745-2746.

Studies of [Fe2(BPMAN)(µ-O2CPhCy)2](OTf)2
Inorganic Chemistry, Vol. 40, No. 7, 2001 1419
Table 4. Redox Potential of Diiron Centers in Complexes 1 and 2 and Selected Previous Models
FeIIIFeIII/FeIIIFeII
(mV vs SCE)a
FeIIIFeII/FeIIFeII
complex
(mV vs SCE)a
ref
[Fe2O(OAc)2(Me3TACN)2](PF6)
[Fe2O(OAc)2(TPA)2](ClO4)2
[Fe2(BPMP)(OAc)2](BF4)2
[Fe2(BPMP)(OAc)2(OH)](BF4)2
[Fe2(BPMP)(OPr)2](BPh4)2
[Fe2(BIMP)(OAc)2](ClO4)2
[Fe2(µ-O2CArTol)4(4-tBuC5H4N)2]
1
-370b
-90
+680
+670
+692
+435
-1500c
27
28
31
31
32
33
29
-30
-80
-10
-245
+244
+758
+438
+1241
this work
this work
2
a Cp2Fe+/Cp2Fe ) +460 mV vs SCE in CH2Cl2. b Quasireversible. c Irreversible.
reversible wave at -22 mV may correspond to the Fe(III)-
OH-Fe(II)/Fe(II)-OH-Fe(II) couple. The electrochemistry
becomes irreversible when both iron atoms are oxidized at
higher potentials, possibly due to core disassembly. In the
presence of excess base, the hydroxide bridge may be depro-
tonated upon oxidation of one of the iron atoms, rendering the
previously reversible process irreversible. The oxidation poten-
tial for the FeIIIFeII/FeIIFeII couple of 2 is approximately 318
mV lower than that of 1. The presence of a more donating
hydroxide bridge in 2 makes the diiron center easier to oxidize.
The CV of 3, a dimanganese(II) analogue of 1, was also
recorded in dichloromethane. The dimetallic center was only
oxidized at around 960 mV, and an irreversible process was
observed (Supporting Information).
Compounds 4 and 5 resemble the tetracarboxylate-bridged
paddle wheel complex [Fe2(µ-O2CArTol)4(4-tBuC5H4N)2] re-
ported recently,29 with one carboxylate bridge being replaced
by the 1,8-naphthyridine unit of BEAN. The CV of 4 showed
only irreversible electrochemistry with oxidation only at po-
tentials above +400 mV vs Cp2Fe+/Cp2Fe (Supporting Informa-
tion). This result may be contrasted with the reversible redox
behavior of [Fe2(µ-O2CArTol)4(4-tBuC5H4N)2] in CH2Cl2 at an
Figure 8. Cyclic voltammogram of 2 mM [Fe2(BPMAN)(µ-OH)-
(µ-O2CPhCy)](OTf)2 (2) in CH2Cl2 with 0.5 M (Bu4N)(PF6) as
supporting electrolyte and a scan rate of 100 mV/s. The insert shows
the reversible wave observed when the experiment was conducted at
potentials below +400 mV vs Cp2Fe+/Cp2Fe.
system, significantly decreases the redox potential of the diiron
core.
E
1/2 value of -216 mV vs Cp2Fe+/Cp2Fe. The additional carbox-
A similar bis(µ-carboxylato)diiron(II) complex, [Fe2(µ-
O2CCH3)2(TPA)2](BPh4)2, was synthesized to model the reduced
forms of diiron-containing enzymes.9,35 This compound, pre-
pared by self-assembly from an iron(II) precursor and TPA, is
very O2 reactive. By contrast, a CH2Cl2 solution of 1 reacts
with O2 very slowly (several days) at room temperature. Thus
the dinucleating ligand used here, while stabilizing the dinuclear
iron core under different redox conditions, limits its O2
reactivity, possibly due to its more rigid geometry, which
prevents it from forming monomers in solution. It was suggested
that [Fe2(µ-O2CCH3)2(TPA)2](BPh4)2 does dissociate in solution,
which may contribute to its greater reactivity.
ylate group may play an important role in stabilizing the five-
coordinate mixed-valence diiron(II,III) oxidation product in the
tetracarboxylate-bridged system. Complex 4 is also unreactive
toward O2 in CH2Cl2 at room temperature, although potential
coordination sites are available. The rigid ligand scaffold may
limit access of a potential O2-derived ligand to the available
coordination site. The CV result also implies that compound 4
cannot be oxidized by O2 by an outer sphere electron transfer.
The diiron(II) complex [Fe2(µ-O2CArTol)4(4-tBuC5H4N)2], with
a lower oxidation potential, reacts with O2 to give a mixture of
diiron(II,III) and diiron(III,IV) species at -78 °C.14
Electrochemistry of 2 and 3. A carboxylato- and hydroxo-
bridged diiron(II) complex 2 was prepared in which a hydroxide
group has replaced one of the two carboxylate bridges in 1.
The cyclic voltammogram of 2 recorded in CH2Cl2 exhibited a
reversible redox wave at -22 mV (∆Ep ) 84 mV) when the
CV experiment was conducted at potentials below +400 mV
vs Cp2Fe+/Cp2Fe (Figure 8, inset). When the scan range was
extended to higher potentials, a second reversible wave was not
observed. Instead, the reduction wave of the original reversible
couple at -22 mV split into three components, as shown in
Figure 8. An irreversible redox process occurred when CV
experiments were conducted in the presence of 5 equiv of Et3N
at potentials below +400 mV. These results suggest that the
The CV of 6 was also recorded in CH2Cl2. An irreversible
process was observed at potentials above +400 mV vs
Cp2Fe+/Cp2Fe (Supporting Information). The hexadentate ligand
BBAAN appears not to stabilize the diiron core as effectively
as the octadentate ligand BPMAN when the oxidation states of
the metals change. A solution of compound 6 in acetonitrile or
CH2Cl2 is O2 sensitive, presumably because dissociation of
triflate provides easy access of O2 to the diiron unit.
Mo1ssbauer Spectroscopic Studies. Mo¨ssbauer spectra of 1,
2, 4, 6, and 7 in the solid state were recorded at 4.2 K.
Parameters are summarized in Table 5. A fit of the data for 1
gives values for the diiron(II) state of δ ) 1.18(2) mm/s and
∆EQ ) 3.01(2) mm/s. The Mo¨ssbauer parameters of 2, with
average values of δav ) 1.15(2) mm/s and ∆EQav ) 2.80(2)
mm/s, compare very well with those of deoxyHr (δ ) 1.14 mm/s
and ∆EQ ) 2.76(2) mm/s).30 The spectra of 1 and 2 are
illustrated in Figure 9.
(34) BPMP ) 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol
and BIMP ) 2,6-bis[(bis((1-methylimidazolyl)methyl)amino)methyl]-
4-methylphenol.
(35) TPA ) tri(2-pyridylmethyl)amine.

1420 Inorganic Chemistry, Vol. 40, No. 7, 2001
He and Lippard
Summary and Conclusions
Several carboxylato- and 1,8-naphthyridine-bridged
diiron(II) complexes were prepared and characterized by using
1,8-naphthyridine-based dinucleating ligands. The cyclic vol-
tammograms and Mo¨ssbauer spectra of these compounds were
recorded. The bis(µ-carboxylato)(µ-1,8-naphthyridine)diiron(II)
complex [Fe2(BPMAN)(µ-O2CPhCy)2]-(OTf)2 (1) exhibited two
one-electron reversible redox waves, which are assigned to the
FeIIIFeII/FeIIFeII and FeIIIFeIII/FeIIIFeII couples, respectively. Such
behavior has only been previously encountered for diiron
complexes derived from alkoxide-bridged multidentate ligands
and is unprecedented for carboxylate-linked complexes having
no single-atom bridge. The results suggest that diiron(II) sites
bridged only by carboxylate groups in biology can act to supply
one to two electrons without a significant change of geometry.
The oxidation potentials of 1 are quite high. Comparison with
results from a previously reported tetracarboxylate-bridged
diiron(II) compound indicates that addition of two anionic
carboxylate groups shifts the first oxidation potential of the
diiron(II) unit by approximately -512 mV. With additional
imidazole donor ligands and in the protein environment, the
diiron(II) cores at the active sites of the enzymes have much
lower oxidation potentials that fall into the physiological range,
by comparison to these model complexes.
The cyclic voltammograms of the carboxylato- and hydroxo-
bridged diiron complex [Fe2(BPMAN)(µ-OH)(µ-O2CPhCy)]-
(OTf)2 (2) exhibited one reversible redox wave at -22 mV,
only when the CV experiment was conducted at potentials below
+400 mV vs Cp2Fe+/Cp2Fe in CH2Cl2. The much lower
oxidation potential of 2 compared to 1 is consistent with
replacement of one carboxylate bridge with a more donating
hydroxide ion. The dimanganese(II) complex [Mn2(BPMAN)
(µ-O2CPhCy)2](OTf)2 (3), an analogue of 1, could not be
oxidized at potentials below +900 mV. At higher potentials,
an irreversible process occurred. Thus the two reversible redox
waves below +900 mV exhibited by 1 can be attributed to the
diiron core; ligand oxidation most likely can be excluded.
Diiron(II) complexes [Fe2(BEAN)(µ-O2CPhCy)3](OTf) (4),
[Fe2(BEAN)(µ-O2CCPh3)3](OTf) (5), and [Fe2(BBBAN)(µ-
OAc)2(OTf)](OTf) (6) were also synthesized and characterized.
No oxidation occurs in CH2Cl2 solutions of complexes 4 and 6
at potential below +400 mV vs Cp2Fe+/Cp2Fe, as judged from
CV experiments. Complex 4 is O2 inert, whereas 6 is sensitive
to O2 in solution. The rigid geometry that restrains access of
O2 to the diiron core may account for the lack of O2 reactivity
of 4, whereas an open coordination site in 6 may provide an O2
binding site.
Figure 9. Mo¨ssbauer spectra at 4.2 K of (A) [Fe2(BPMAN)(µ-
O2CPhCy)2](OTf)2 (1) and (B) [Fe2(BPMAN)(µ-OH)(µ-O2CPhCy)]-
(OTf)2 (2). Lower curves: experimental data (‚) and calculated fit (-).
Upper curves: subsets for the calculated spectrum of 2.
Figure 10. Mo¨ssbauer spectra at 4.2 K of [Fe2(BEAN)(µ-O2CPhCy)3]-
(OTf) (4). Lower curves: experimental data (‚), calculated fit (-). Upper
curves: two subsets for the calculated spectrum.
Table 5. Summary of Mo¨ssbauer Parameters for 1, 2, 4, and 6
Recorded at 4.2 K (mm/s) in the Solid State
complex
d
∆EQ
G
Acknowledgment. This work was supported by grants from
the National Science Foundation and National Institute of
General Medical Sciences. We thank Mr. Dongwhan Lee and
Ms. Jane Kuzelka for help in acquiring and fitting the Mo¨ssbauer
spectra.
1 (powder)
2 (powder)
1.18(2)
1.13(2)
1.18(2)
1.05(2)
1.08(2)
1.20(2)
1.24(2)
3.01(2)
2.66(2)
2.93(2)
1.95(2)
2.79(2)
3.28(2)
3.03(2)
0.24(2)
0.33(2)
0.37(2)
0.31(2)
0.32(2)
0.35(2)
0.35(2)
4 (powder)
6 (powder)
Supporting Information Available: Figure S1, showing the
ORTEP diagram of 3; Figure S2, the connectivity of 5; Figures S3-
S7, the cyclic voltammograms of 1, 2 in the presence of excess Et3N,
3, 4, and 6; and Figure S9, the Mo¨ssbauer spectrum of 6; and X-ray
crystallographic files, in CIF format, for complexes 1-4 and 6. This
material is available free of charge via the Internet at http://pubs.acs.org.
The spectrum of 4 shows two distinct iron sites, one having
δ ) 1.05(2) mm/s and ∆EQ ) 1.95(2) mm/s and the other
having δ ) 1.08(2) mm/s and ∆EQ ) 2.79(2) mm/s (Figure
10). The large difference of the quadrupole splitting values
between two iron sites, although not fully understood, may arise
from the more asymmetric environment of Fe(2) (Table 3). The
Mo¨ssbauer parameters of 6, listed in Table 5, are closer to those
exhibited by the active site of Hred of MMO (δ ) 1.30 mm/s
and ∆EQ ) 2.87-3.14 mm/s), probably because they have
similar carboxylate bridging modes.36,37
IC000975K
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D. E.; Salifoglou, A.; Lippard, S. J. J. Am. Chem. Soc. 1995, 117,
10174-10185.
(37) Fox, B. G.; Surerus, K. K.; Mu¨nck, E.; Lipscomb, J. D. J. Biol. Chem.
1988, 263, 10553-10556.