Spectroscopic and Electrochemical Characterization of an Aqua Ligand Exchange and Oxidatively Induced Deprotonation in Diiron Complexes

Article abstract of DOI:10.1021/ic030192+

Reaction of the unsymmetrical phenol ligand 2-((bis(2-pyridylmethyl)amino)methyl)-6-(((2-pyridylmethyl)benzylamino)-methyl) -4-methylphenol (HL-Bn) or its 2,6-dichlorobenzyl analogue (HL-BnCl ₂) with Fe(H₂O)₆(ClO₄)₂ in the presence of disodium m-phenylenedipropionate (Na₂(mpdp)) followed by exposure to atmosphere affords the diiron(II,III) complexes [Fe ₂(L-Bn)(mpdp)(H₂O)](ClO₄)₂ and [Fe₂(L-BnCl₂)(mpdp)(CH₃OH)](ClO ₄)₂, respectively. The latter complex has been characterized by X-ray crystallography. It crystallizes in the monoclinic system, space group P2₁/n, with a = 13.3095(14) A, b = 20.1073(19) A, c = 19.4997(19) A, α = 90°, β = 94.471(2)°, γ = 90°, V = 5202.6(9) A³, and Z = 4. The structure of the compound is very similar to that of [Fe ₂(L-Bn)(mpdp)(H₂O)](BPh₄)₂ determined earlier, except for the replacement of a water by a methanol on the ferrous site. Magnetic measurements of [Fe₂(L-Bn)(mpdp)(H ₂O)](BPh₄)₂ reveal that the two high-spin Fe ions are moderately antiferromagnetically coupled (J = -3.2(2) cm^-1). Upon dissolution in acetonitrile the terminal ligand on the ferrous site is replaced by a solvent molecule. The acetonitrile-water exchange has been investigated by various spectroscopic techniques (UV-visible, NMR, Moessbauer) and electrochemistry. The substitution of acetonitrile by water is clearly evidenced by Moessbauer spectroscopy by a reduction of the quadrupole splitting value from 3.14 to 2.41 mm/s. In addition, it causes a 210 mV downshift of the oxidation potential of the ferrous site and a similar reduction of the stability domain of the mixed-valence state. Exhaustive electrolysis of a solution of [Fe₂(L-Bn)(mpdp)(H₂O)] ²⁺ shows that the aqua diferric species is not stable and undergoes a chemical reaction which can be partly reversed by reduction to the mixed-valent state. This and other electrochemical observations suggest that upon oxidation of the diiron center to the diferric state the aqua ligand is deprotonated to a hydroxo. This hypothesis is supported by Moessbauer spectroscopy. Indeed, this species possesses a large quadrupole splitting value (ΔE_Q ≥ 1.0 mm·s^-1) similar to that of analogous complexes with a terminal phenolate ligand. This study illustrates the drastic effects of aqua ligand exchange and deprotonation on the electronic structure and redox potentials of diiron centers.

Full text of DOI:10.1021/ic030192+

Inorg. Chem. 2004, 43, 1638−1648
Spectroscopic and Electrochemical Characterization of an Aqua Ligand
Exchange and Oxidatively Induced Deprotonation in Diiron Complexes
Sylvie Chardon-Noblat,† Olivier Horner,‡ Barbara Chabut,‡ Fre´de´ric Avenier,‡ Noe1lle Debaecker,‡
Peter Jones,‡ Jacques Pe´caut,§ Lionel Dubois,‡ Claudine Jeandey,‡ Jean-Louis Oddou,‡
,‡
Alain Deronzier,† and Jean-Marc Latour*
Laboratoire d’Electrochimie Organique et de Photochimie Re´dox, UMR CNRS 5630,
ICMG FR CNRS 2607, UniVersite´ Joseph Fourier Grenoble 1, BP 53,
38041 Grenoble Ce´dex 9, France, CEA-De´partement de Recherche Fondamentale sur la Matie`re
Condense´e, SerVice de Chimie Inorganique et Biologique, UMR CEA-CNRS-UJF 5630,
Laboratoire des Me´talloprote´ines, UMR CEA-CNRS-UJF 5046, CEA/Grenoble,
38054 Grenoble Ce´dex 9, France, and CEA-De´partement Re´ponse et Dynamique Cellulaires,
Laboratoire de Physicochimie des Me´taux en Biologie, FRE CEA-CNRS-UJF 2427,
CEA/Grenoble, 38054 Grenoble Ce´dex 9, France
Received June 10, 2003
Reaction of the unsymmetrical phenol ligand 2-((bis(2-pyridylmethyl)amino)methyl)-6-(((2-pyridylmethyl)benzylamino)-
methyl)-4-methylphenol (HL-Bn) or its 2,6-dichlorobenzyl analogue (HL-BnCl2) with Fe(H O)6(ClO ) in the presence
2
4 2
of disodium m-phenylenedipropionate (Na2(mpdp)) followed by exposure to atmosphere affords the diiron(II,III)
complexes [Fe2(L-Bn)(mpdp)(H O)](ClO ) and [Fe2(L-BnCl2)(mpdp)(CH OH)](ClO ) , respectively. The latter complex
2
4 2
3
4 2
has been characterized by X-ray crystallography. It crystallizes in the monoclinic system, space group P21/n, with
a ) 13.3095(14) Å, b ) 20.1073(19) Å, c ) 19.4997(19) Å, R ) 90°, â ) 94.471(2)°, γ ) 90°, V ) 5202.6(9)
3
Å , and Z ) 4. The structure of the compound is very similar to that of [Fe2(L-Bn)(mpdp)(H O)](BPh4)2 determined
2
earlier, except for the replacement of a water by a methanol on the ferrous site. Magnetic measurements of [Fe2(L-
Bn)(mpdp)(H O)](BPh4)2 reveal that the two high-spin Fe ions are moderately antiferromagnetically coupled (J )
2
−3.2(2) cm-1). Upon dissolution in acetonitrile the terminal ligand on the ferrous site is replaced by a solvent
molecule. The acetonitrile−water exchange has been investigated by various spectroscopic techniques (UV−visible,
NMR, Mo¨ssbauer) and electrochemistry. The substitution of acetonitrile by water is clearly evidenced by Mo¨ssbauer
spectroscopy by a reduction of the quadrupole splitting value from 3.14 to 2.41 mm/s. In addition, it causes a 210
mV downshift of the oxidation potential of the ferrous site and a similar reduction of the stability domain of the
mixed-valence state. Exhaustive electrolysis of a solution of [Fe2(L-Bn)(mpdp)(H O)]2+ shows that the aqua diferric
2
species is not stable and undergoes a chemical reaction which can be partly reversed by reduction to the mixed-
valent state. This and other electrochemical observations suggest that upon oxidation of the diiron center to the
diferric state the aqua ligand is deprotonated to a hydroxo. This hypothesis is supported by Mo¨ssbauer spectroscopy.
Indeed, this species possesses a large quadrupole splitting value (∆EQ g 1.0 mm‚s-1) similar to that of analogous
complexes with a terminal phenolate ligand. This study illustrates the drastic effects of aqua ligand exchange and
deprotonation on the electronic structure and redox potentials of diiron centers.
Introduction
ally known protein with a dinuclear iron active site, it was
long considered as the prototype of this class. However,
subsequent structure resolutions of other proteins of the class
have shown that instead it is rather an exception.2 Since the
Hemerythrin (Hr) is a dioxygen carrier found in sipunculid
worms and other marine invertebrates.1 As the first structur-
* To whom correspondence should be addressed. E-mail: jlatour@cea.fr.
† Universite´ Joseph Fourier Grenoble 1.
(1) Wilkins, P. C.; Wilkins, R. G. Coord. Chem. ReV. 1987, 79, 195-
‡ Laboratoire de Physicochimie des Me´taux en Biologie, CEA/Grenoble.
§ Service de Chimie Inorganique et Biologique, CEA/Grenoble.
214.
(2) Kurtz, D. M. JBIC 1997, 2, 159-167.
1638 Inorganic Chemistry, Vol. 43, No. 5, 2004
10.1021/ic030192+ CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/05/2004

Aqua Ligand Exchange in Diiron Complexes
Chart 1. Representation of (a) the Ligand HL-Bn (and Substituted
early 1980s, the structures of several Hr forms have been
solved by X-ray crystallography.3-5 The protein backbone
furnishes the iron pair with five terminal histidines and two
bridging carboxylates from an aspartate and a glutamate. In
the deoxy form, the two Fe2+ ions are bridged by a hydroxide
resulting in an asymmetric coordination: one iron is hexa-
coordinated with a N3O3 environment while the other one is
only pentacoordinated with a N2O3 environment and pos-
sesses a vacant coordination site for dioxygen interaction.3
Upon dioxygen binding, the diiron pair is oxidized to the
diferric state while the hydroxide bridge is deprotonated. The
resulting hydroperoxide is bound to the Fe3+ ion in the N2O3
site and forms a hydrogen bond with the oxo bridge. The
hydroperoxo ligand of the oxy form can be replaced by
anions (e.g. azide) leading to the corresponding met forms,
which have been structurally characterized.3-5
Derivatives) and (b) the Cation [Fe2(L-Bn)(mpdp)(H2O)]2+
Apart from the diferrous and diferric states, two mixed-
valent [FeIIFeIII] semimet forms have been prepared in vitro,
either by reduction of the met form (semimetR Hr) or by
oxidation of the deoxy form (semimetO Hr). None of them
has been structurally characterized by X-ray crystallography
yet. Nevertheless, extensive spectroscopic studies have
identified a [FeII(µ-OH)FeIII] unit6,7 and shown that the two
forms differ by the localization of the iron valences in the
two distinct protein coordination sites: in semimetO Hr the
FeII is in the N2O3 site and accessible to solvent molecules
while in semimetR Hr the FeIII occupies the N2O3 site and is
bound by a hydroxide ion.8,9
(µ-Oxo)bis(µ-acetato)diiron(III) complexes of tridentate
amine ligands (hydrotris(pyrazolyl)borate, triazacyclononane)
were first developed as models of methemerythrins and
indeed reproduced their physical properties.10,11 Nevertheless,
the terminal ligands saturated the iron coordination spheres
and could not allow the binding of an exogeneous ligand.
This was achieved by Wieghardt et al.12 by combining a
diamine and a triamine to bind the diiron core and more
recently by Lippard et al. using 1,8-naphthyridine-based
ligands13 or diamines in conjunction with sterically encum-
bered dicarboxylates.14 We used a different approach on the
basis of an asymmetrical phenolato ligands to mimic the
asymmetrical histidine coordination in Hr, the phenoxo
bridge playing the role of the hydroxide.15,16 The same
approach was used by Balch, Satcher, et al.17,18 with similar
ligands providing the iron atoms with an alkoxo bridge.
In the course of our studies, we used the asymmetrical
hexadentate pentaamine ligand HL-Bn (Chart 1), which does
not saturate the coordination of one iron of the pair. Indeed,
its mixed-valent complex [FeIII(L-Bn)(mpdp)FeII(H2O)]-
(BPh4)2, in which the two irons are bridged by the two
carboxylate groups of 1,3-benzenedipropionate (mpdp), pos-
sesses a water molecule completing the coordination of the
Fe2+ ion.15 In this article, we report extensive spectroscopic
and electrochemical studies of the exchange of this terminal
aqua ligand. In addition, we report the X-ray structure of
the methanol adduct of the mixed-valent complex of a similar
ligand HL-BnCl2 (Chart 1), illustrating the easy exchange
of the terminal ligand. Furthermore, we show that electro-
chemical oxidation of the FeIIIFeII aqua complex to the
diferric state induces the deprotonation of the aqua ligand.
Mo¨ssbauer spectroscopy is sensitive to coordination changes
in these kinds of complexes19,20 and proved very helpful to
characterize these ligand exchanges and transformations. The
biological relevance of these reactions will be discussed.
(3) Stenkamp, R. E. Chem. ReV. 1994, 94, 715-726.
(4) Martins, L. J.; Hill, C. P.; Ellis, W. R. J. Biochemistry 1997, 36, 7044-
7049.
(5) Farmer, C. S.; Kurtz, D. M. J.; Liu, Z. J.; Wang, B. C.; Rose, J.; Ai,
J.; Sanders-Loehr, J. JBIC 2001, 6, 418-429.
(6) Scarrow, R. C.; Maroney, M. J.; Palmer, S. M.; Que, L. J.; Roe, A.
L.; Salowe, S. P.; Stubbe, J. J. Am. Chem. Soc. 1987, 109, 7857-
7864.
Experimental Section
Materials. Acetonitrile (HPLC grade S) for electrochemistry was
obtained from Rathburn Chemicals Ltd., stored under inert atmo-
(14) Mizoguchi, T. J.; Kuzelka, J.; Spingler, B.; DuBois, J. L.; Davydov,
R. D.; Hedman, B.; Hodgson, K. O.; Lippard, S. J. Inorg. Chem. 2001,
40, 4662-4673.
(7) Thomann, H.; Bernardo, M.; McCormick, J. M.; Pulver, S.; Andersson,
K. K.; Lipscomb, J. D.; Solomon, E. I. J. Am. Chem. Soc. 1993, 115,
8881-8882.
(15) Kanda, W.; Moneta, W.; Bardet, M.; Bernard, E.; Debaecker, N.;
Laugier, J.; Bousseksou, A.; Chardon-Noblat, S.; Latour, J. M. Angew.
Chem., Int. Ed. Engl. 1995, 34, 588-590.
(8) Pearce, L. L.; Kurtz, D. M. J.; Xia, Y. M.; Debrunner, P. G. J. Am.
Chem. Soc. 1987, 109, 7286-7293.
(16) Lambert, E.; Chabut, B.; Chardon-Noblat, S.; Deronzier, A.; Chottard,
G.; Bousseksou, A.; Tuchagues, J. P.; Bardet, M.; Laugier, J.; Latour,
J. M. J. Am. Chem. Soc. 1997, 119, 9424-9437.
(9) McCormick, J. M.; Solomon, E. I. J. Am. Chem. Soc. 1990, 112, 2005-
2007.
(10) Wieghardt, K.; Pohl, K.; Gebert, W. Angew. Chem., Int. Ed. Engl.
1983, 22, 727-728.
(17) Satcher, J. H.; Balch, A. L.; Olmstead, M. M.; Droege, M. W. Inorg.
Chem. 1996, 35, 1749-1750.
(11) Armstrong, W.; Spool, A.; Papaefthymiou, G.; Frankel, R.; Lippard,
S. J. Am. Chem. Soc. 1984, 106, 3653-3667.
(18) Satcher, J. H.; Droege, M. W.; Olmstead, M. M.; Balch, A. L. Inorg.
Chem. 2001, 40, 1454-1458.
(12) Mauerer, B.; Crane, J.; Schuler, J.; Wieghardt, K.; Nuber, B. Angew.
Chem., Int. Ed. Engl. 1993, 32, 289-291.
(19) Borovik, A. S.; Papaefthymiou, V.; Taylor, L. F.; Anderson, O. P.;
Que, L., Jr. J. Am. Chem. Soc. 1989, 111, 6183-6195.
(20) Maeda, Y.; Tanigawa, Y.; Hayami, S.; Takashima, Y. Chem. Lett.
1992, 591-594.
(13) He, C.; Barrios, A. M.; Lee, D.; Kuzelka, J.; Davydov, R. D.; Lippard,
S. J. J. Am. Chem. Soc. 2000, 122, 12683-12690.
Inorganic Chemistry, Vol. 43, No. 5, 2004 1639

Chardon-Noblat et al.
sphere, and used without further purification. Acetonitrile for
spectroscopic measurements was distilled over CaH2 and stored
within an inert-atmosphere glovebox. Dichloromethane was passed
over alumina and stored under argon. All other reagents were of
reagent grade quality and used as received. 1,3-Benzenedipropionic
acid was prepared as described in the literature.21
Syntheses. The asymmetric binucleating phenol ligand HL-Bn
and the corresponding diiron(II,III) complexes [FeIII(L-Bn)(mpdp)-
FeII(H2O)](BPh4)2 or [FeIII(L-Bn)(mpdp)FeII(H2O)](ClO4)2 (Chart
1) were synthesized as described previously.15 The 2,6-dichloro-
benzyl ligand (HL-BnCl2) and its diiron(II,III) complex were
prepared similarly.16 Anal. Calcd for [Fe2(L-Bn)(mpdp)(H2O)]-
(BPh4)2‚2H2O, Fe2C94H92N5O8B2: C, 72.69; H, 5.97; N, 4.51; Fe,
7.19. Found: C, 72.9; H, 5.9; N, 4.4; Fe, 6.9. Calcd for [Fe2(L-
BnCl2)(mpdp)(H2O)](ClO4)2‚2H2O, Fe2C46H50N5O16Cl4: C, 46.73;
H, 4.26; N, 5.92; Fe, 9.45. Found: C, 46.1; H, 4.3; N, 5.7; Fe, 9.5.
FAB(+) mass spectrometry: m/z ) 1027, [Fe2 (L-BnCl2)(mpdp)-
(ClO4)]+.
Physicochemical Studies. Electronic absorption spectra were
recorded on a Lambda 9 Perkin-Elmer spectrometer in acetonitrile.
NMR spectra were recorded in deuterated acetonitrile on a Varian
Unity 400 spectrometer with a 5 mm indirect detection operating
at 400 MHz for the proton as described previously.16
Magnetic susceptibility experiments were done on a MPMS
Quantum Design magnetometer operating at 0.5-5 T in the 2-300
K temperature range. The polycrystalline sample (ca. 15 mg) was
contained in a kel F bucket which was calibrated independently.
The susceptibility of the compound was corrected from the
diamagnetism calculated using Pascal’s constants.22 The simulation
of the data was conducted in two steps. First the data were fitted
to the Van Vleck equation for two interacting spins S ) 2 and S )
5/2 on the basis of the Heisenberg exchange Hamiltonian H )
-2JS1S2.22 Then a full treatment based on matrix diagonalization
using the Hamiltonian
Figure 1. Structure of the cation [FeIII(L-BnCl2)(mpdp)FeII(CH3OH)]2+
with thermal ellipsoids at the 50% level.
used as supporting electrolyte. All potentials reported are relative
to Ag|10 mM Ag+ for acetonitrile solutions or Ag|AgCl (saturated
LiCl in EtOH) for CH2Cl2 solutions and can be converted to the
NHE by adding 0.548 or 0.22 V, respectively.24 For cyclic
voltammetry and exhaustive electrolyses the working electrode was
a platinum disk (area ) 0.2 cm2) or plate (area ) 5 cm2),
respectively. The spectroelectrochemical experiments were ran
within a drybox in conventional cuvettes which were inserted into
the optical translator connected through a fiber optic system
(Photonics Spectrofip system) to a Hewlett-Packard 8452A diode
array spectrophotometer.
Results
Part I: Syntheses and Solid-State Studies. Syntheses.
[FeIII(L-Bn)(mpdp)FeII(H2O)](BPh4)2 was obtained by fol-
lowing a literature procedure25 from the reaction in methanol
of the ligand HL-Bn with 2 equiv of Fe(NO3)3‚9H2O and 1
equiv of disodium 1,3-benzenedipropionate followed by
precipitation with Na(BPh4) and crystallization from an
acetonitrile/methanol mixture.15 Alternatively, the reaction
can be performed under anaerobic conditions with Fe(ClO4)2‚
6H2O and then exposed to air;19 the desired complex [FeIII(L-
Bn)(mpdp)FeII(H2O)](ClO4)2 crystallizes upon standing. Crys-
tals of [FeIII(L-BnCl2)(mpdp)FeII(CH3OH)](ClO4)2 suitable
for an X-ray analysis have been obtained by slow evaporation
of an acetonitrile/methanol mixture. The structures of the
two compounds are very similar except for the replacement
of the terminal aqua ligand on FeII by a methanol molecule
(see below).
X-ray Structure of [FeIII(L-BnCl2)(mpdp)FeII(CH3OH)]-
(ClO4)2. Crystals of [FeIII(L-BnCl2)(mpdp)FeII(CH3OH)]-
(ClO4)2 consist of discrete cations [FeIII(L-BnCl2)(mpdp)-
FeII(CH3OH)]2+ and perchlorate anions. The structure of the
cation is illustrated in Figure 1. The crystal parameters are
listed in Table 1, and important bond distances and angles
are summarized in Table 2. The two iron ions are bridged
H ) -2JS1S2 + Σi{Di[(Siz)2 - (1/3)(Si)(Si + 1) +
Ei[(Six)2 - (Siy)2]}
was used to obtain the zero-field splitting of the Fe2+ ion.23 The R
index {R ) Σ[(øTexp) - (øTcal)]2/NΣ(øTexp)2} was 6.6 × 10-6
.
57Fe Mo¨ssbauer experiments were performed using a 30 mCi
source of 57Co(Rh). The 14.4 keV γ-rays were detected by means
of a proportional counter and Mo¨ssbauer spectra recorded on a 512
multichannel analyzer working in the multiscaling mode. The
system was calibrated with a metallic iron absorber at room
temperature, and all velocity scales and isomer shifts are referred
to the iron standard. The temperature of the sample was measured
with a Pt resistor. A conventional liquid-nitrogen, horizontal
transmission cryostat was used; the source was maintained at room
temperature and moved by a constant-acceleration electromechanical
drive system under feedback control. The minimum observed line
width (fwhm) was 0.25 mm/s.
Electrochemical measurements were done using an EG&G
Princeton Applied Research model 173 potentiostat-galvanostat
equipped with a Sefram TGM 164 X-Y recorder. All voltammo-
grams were obtained in a conventional three-electrode cell under
an argon atmosphere in a drybox (Jaram) with a dioxygen content
lower than 5 ppm. Tetrabutylammonium perchlorate (TBAP) was
(21) Beer, R. H.; Tolman, W. B.; Bott, S. G.; Lippard, S. J. Inorg. Chem.
1989, 28, 4557-4559.
(22) Kahn, O. Molecular Magnetism; VCH: New York, 1993.
(23) Atta, M.; Scheer, C.; Fries, P. H.; Fontecave, M.; Latour, J. M. Angew.
Chem., Int. Ed. Engl. 1992, 31, 1513-1515.
(24) Pavlishchuk, V. V.; Addison, A. W. Inorg. Chim. Acta 2000, 298,
97-102.
(25) Suzuki, M.; Uheara, A.; Oshio, H.; Endo, K.; Yanaga, M.; Kida, S.;
Saito, K. Bull. Chem. Soc. Jpn. 1987, 60, 3547-3555.
1640 Inorganic Chemistry, Vol. 43, No. 5, 2004

Aqua Ligand Exchange in Diiron Complexes
Table 1. Crystal and Structure Refinement Data for
the analogous bis(µ-propionato) complex in the mixed-valent
FeIIFeIII state, the average Fe-ligand distance to the ferric
site is shorter than that to the ferrous one (2.056 vs 2.131
Å), as expected. From this 0.075 Å difference it was
concluded that the valences of the two ions are localized. A
similar difference 0.073 Å is observed for 1, which indicates
that the valences of the two iron atoms are localized and
that Fe1 is the Fe3+ site and Fe2 the Fe2+ one. The distance
Fe2-O71 between the Fe2+ ion and the methanol oxygen is
2.118 Å, which is very similar to the distance (2.156 Å)
between the Fe2+ ion and the oxygen of the aqua ligand in
[FeIII(L-Bn)(mpdp)FeII(H2O)](BPh4)2.15
[FeIII(L-BnCl2)(mpdp)FeII(CH3OH)](ClO4)2
empirical formula
fw
C48H52Cl4Fe2N5O15
1192.45
temp
193(2) K
wavelength
cryst system
space group
unit cell dimens
0.710 73 Å
monoclinic
P21/n
a ) 13.3095(14) Å; R ) 90°
b ) 20.1073(19) Å; â ) 94.471(2)°
c ) 19.4997(19) Å; γ ) 90°
5202.6(9) Å3, 4
V, Z
d(calcd)
abs coeff
1.522 Mg/m3
0.835 mm-1
F(000)
2460
cryst size
0.08 × 0.2 × 0.3 mm
Magnetic Susceptibility. The magnetic properties of
[FeIII(L-Bn)(mpdp)FeII(H2O)](BPh4)2 were investigated over
the range 2-300 K at six magnetic fields between 0.5 and
5 T. The product of the molar susceptibility by temperature
decreases smoothly from ca. 7 cm3‚K‚mol-1 at 300 K to ca.
5.7 cm3‚K‚mol-1 at 100 K and then more abruptly to reach
0.4 cm3‚K‚mol-1 at 2 K (Figure S1, Supporting Information).
Such a behavior is characteristic of antiferromagnetically
coupled systems. This temperature dependence of øT could
be simulated at 0.5 T by using the Van Vleck equation
derived from the Heisenberg exchange Hamiltonian for two
interacting spins S1 ) 2 and S2 ) 5/2. The best fit was
obtained for an exchange interaction J ) -3.2 cm-1.
Nevertheless, below 30 K the simulation was of poorer
quality. This was due to the fact that the anisotropy of the
system was neglected in the calculation. Indeed, owing to
spin-orbit coupling, Fe2+ ions can have substantial axial
zero-field splittings, up to 20 cm-1,28 such that the condition
D , J is not fulfilled and Van Vleck’s equation is not valid.
Therefore, a matrix diagonalization procedure was used.23
To avoid an overparametrization of the system, it was
assumed that the Fe3+ ion has an isotropic g tensor and no
zero-field splitting (g3 ) 2.0, D3 ) E3 ) 0) and that the
zero-field splitting tensor of the Fe2+ ion was not rhombic
(E2 ) 0). The best fit parameters thus obtained are J )
-3.2(2) cm-1, g2 ) 2.07(3), and D2 ) 9.2(8) cm-1.
θ range for data collcn
limiting indices
reflcns collcd
indpndt reflcs
abs corr
refinement method
data/restraints/params
goodness-of-fit on F2
1.84-28.79°
-17 e h e 11, -21 e k e 21, -18 e l e 26
12 993
9004 [R(int) ) 0.0348]
none
full-matrix least-squares on F2
9004/0/875
1.072
final R indices [I > 2σ(I)] R1 ) 0.0666, wR2 ) 0.1521
R indices (all data)
R1 ) 0.1135, wR2 ) 0.2013
largest diff peak and hole 1.416 and -1.144 e‚Å-3
Table 2. Important Bond Lengths and Angles for
[FeIII(L-BnCl2)(mpdp)FeII(CH3OH)](ClO4)2
Fe(1)-O(51)
Fe(1)-O(1)
Fe(1)-O(61)
Fe(1)-N(4)
Fe(1)-N(3)
Fe(1)-N(1)
1.9429(18)
1.9490(15)
1.9630(18)
2.1426(19)
2.171(2)
Fe(2)-O(62)
Fe(2)-O(1)
Fe(2)-O(52)
Fe(2)-O(71)
Fe(2)-N(5)
Fe(2)-N(2)
2.070(2)
2.0928(16)
2.1055(18)
2.1180(19)
2.143(2)
2.200(2)
2.276(2)
O(51)-Fe(1)-O(1)
O(51)-Fe(1)-O(61)
O(1)-Fe(1)-O(61)
O(51)-Fe(1)-N(4)
O(1)-Fe(1)-N(4)
O(61)-Fe(1)-N(4)
O(51)-Fe(1)-N(3)
O(1)-Fe(1)-N(3)
O(61)-Fe(1)-N(3)
N(4)-Fe(1)-N(3)
O(51)-Fe(1)-N(1)
O(1)-Fe(1)-N(1)
O(61)-Fe(1)-N(1)
N(4)-Fe(1)-N(1)
N(3)-Fe(1)-N(1)
101.38(7) O(62)-Fe(2)-O(1)
99.30(8) O(62)-Fe(2)-O(52)
93.25(7) O(1)-Fe(2)-O(52)
93.53(8) O(62)-Fe(2)-O(71)
164.88(8) O(1)-Fe(2)-O(71)
86.75(8) O(52)-Fe(2)-O(71)
89.61(8) O(62)-Fe(2)-N(5)
87.99(7) O(1)-Fe(2)-N(5)
170.55(8) O(52)-Fe(2)-N(5)
89.61(7) O(71)-Fe(2)-N(5)
163.61(7) O(62)-Fe(2)-N(2)
89.27(7) O(1)-Fe(2)-N(2)
92.41(8) O(52)-Fe(2)-N(2)
75.63(7) O(71)-Fe(2)-N(2)
78.23(8) N(5)-Fe(2)-N(2)
90.44(7)
94.12(7)
90.22(7)
90.36(8)
176.76(7)
86.59(7)
95.40(8)
91.64(7)
170.29(8)
91.41(8)
172.96(8)
87.45(7)
92.61(7)
92.11(7)
77.96(8)
Part II: Acetonitrile/Aqua Ligand Exchange. The
present and past X-ray characterization of these mixed-valent
complexes shows that a ligand exchange on the Fe2+ site
can be easily achieved. Since this process is likely to modify
the electronic properties of the diiron center, we decided to
investigate the aqua/acetonitrile exchange through spectro-
scopic and electrochemical methods.
by the phenolate oxygen O1 and the two carboxylate groups
of the mpdp ligand. The coordination of Fe1 is comple-
mented by the three nitrogen atoms of the bis(picolyl)amine
branch. The overall N3O3 coordination around Fe1 corre-
sponds to a rhombically distorted octahedron. The coordina-
tion of Fe2 is complemented by the two nitrogen atoms N2
and N5 from the tertiary amine and the pyridine group,
respectively, and by O71 from the methanol molecule.
Examination of the bond distances in Table 2 shows that
they average 2.061 Å around Fe1 and 2.134 Å around Fe2.
The latter is close to that found in diferrous bis(µ-
propionato)26 and bis(µ-diphenylphosphato)27 complexes of
the tetrapyridyl ligand: 2.151 and 2.138 Å, respectively. In
Electronic Absorption Spectroscopy. The electronic
absorption spectra of mixed-valent FeIIFeIII complexes of
µ-phenolato ligands are dominated in the visible region by
an intense band (ꢀ ) 900-1000 mol-1‚L‚cm-1) close to 550
nm which is assigned to the charge transfer from the bridging
phenolate ligand to the Fe3+ ion (LMCT pπ f dπ*).19,25
When the complexes [FeIII(L-Bn)(mpdp)FeII(H2O)](BPh4)2
or [FeIII(L-BnCl2)(mpdp)FeII(CH3OH)](ClO4)2 are dissolved
in acetonitrile, their solutions exhibit the same absorption
band at 578 nm (17 300 cm-1, ꢀ ) 1300 mol-1‚L‚cm-1)
(26) Borovik, A. S.; Hendrich, M. P.; Holman, T. R.; Mu¨nck, E.;
Papaefthymiou, V.; Que, L., Jr. J. Am. Chem. Soc. 1990, 112, 6031-
6038.
(27) Jang, H.; Hendrich, M.; Que, L., Jr. Inorg. Chem. 1993, 32, 911-
(28) Solomon, E.; Pavel, E.; Loeb, K.; Campochiaro, C. Coord. Chem.
ReV. 1995, 144, 369-460.
918.
Inorganic Chemistry, Vol. 43, No. 5, 2004 1641

Chardon-Noblat et al.
Figure 2. Electronic absorption spectrum of [FeIII(L-Bn)(mpdp)FeII(CH3-
CN)]2+ in acetonitrile (1) with successive additions of water: 400, 800,
1200, and 1600 molar equiv (5).
(Figure 2, curve 1). This indicates that, upon dissolution in
acetonitrile, the exogeneous aqua/methanol ligand is substi-
tuted by an acetonitrile molecule leading to the formation
of [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+. Progressive addition
of water causes this band to move from 578 to 569 nm
(17 575 cm-1, ꢀ ) 1050 mol-1‚L‚cm-1) with a slight loss of
intensity (Figure 2). This transformation is quantitative after
addition of ca. 1500 molar equiv of water (Figure 2, curve
5) and occurs with an isosbestic point at 947 nm (Figure
S2). These observations are consistent with an initial
replacement of the exogeneous ligand present in the solid
state (H2O or CH3OH) by acetonitrile and then a simple
transformation of the acetonitrile complex cation [FeIII(L-
Bn)(mpdp)FeII(CH3CN)]2+ into the aqua one [FeIII(L-Bn)-
(mpdp)FeII(H2O)]2+ (eq 1).
Figure 3. NMR spectrum of [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+ in aceto-
nitrile in the absence (a) and in the presence (b) of 1500 molar equiv of
D2O.
The spectrum of [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+ (Fig-
ure 3a) bears a strong resemblance with those of the [FeIIFeIII]
complexes of the asymmetrical ligands (HL-BnOH and HL-
PhOH) providing a terminal phenolate to one iron ion.16 This
allows one to assign quite confidently the three extreme
downfield resonances at 555, 375, and 265 ppm to the
equatorial methylenic protons32 of the branch bound to the
Fe3+ ion.30 The three very wide resonances at 219, 210, and
137 ppm correspond to the ortho-pyridyl protons: their short
distances to the metal ions is responsible for their short T2.
The observation of two resonances more downfield shifted
than the third one indicates that the Fe3+ ion with the larger
spin is bound in the branch possessing two pyridines, thereby
confirming the valence localization found in the solid state.
The peculiar upfield shifted resonance at -48 ppm is
assigned to the equatorial methylenic proton of the third
picolyl group bound to the Fe2+ ion. [FeIII(L-BnCl2)(mpdp)-
FeII(CH3CN)]2+ possesses a closely similar spectrum except
for the lack of a peak at 95 ppm which we assign there-
fore to the ortho-benzylic protons of [FeIII(L-Bn)(mpdp)-
FeII(CH3CN)]2+.
Progressive addition of D2O causes important shifts (10-
30 ppm) of several signals which stabilize after addition of
ca. 1500 molar equiv of D2O (Figure 3b). The signals
experiencing the most important shifts are those associated
with the ortho-pyridinyl and methylenic protons which are
closer to the iron atoms. It is especially noteworthy that while
most signals exhibit upfield shifts, two of them located at
74 and -48 ppm in [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+ and
78 and -45 ppm in [FeIII(L-BnCl2)(mpdp)FeII(CH3CN)]2+
exhibit downfield shifts of more than 20 ppm. These signals
are assigned, respectively, to the axial and equatorial
methylenic protons of the picolyl group bound to the Fe2+
ion. Their peculiar behavior may therefore result from the
anisotropy of the Fe2+ ion. As a consequence of these shifts,
H2O
[FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+
{
}
CH3CN
[FeIII(L-Bn)(mpdp)FeII(H2O)]2+ (1)
The study of the temperature dependence of the equilib-
rium showed that formation of the acetonitrile complex is
favored at low temperature. Nevertheless, owing to the small
spectroscopic changes (see Figure 2), it was impossible to
quantitate precisely this effect.
NMR Spectroscopy. The NMR spectrum of [FeIII(L-Bn)-
(mpdp)FeII(CH3CN)]2+ in acetonitrile extends over more than
600 ppm, from 555 to -48 ppm (Figure 3a). Such a behavior
departs drastically from that of the mixed-valent complexes
of the tetrapyridyl19 (Hbpmp) and tetraimidazolyl29 ligands
whose spectral range covers only 400 ppm and is restricted
to positive chemical shifts. The wide spectral range of
[FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+ results from a combina-
tion of (i) the very weak magnetic exchange interaction
between the high-spin Fe2+ (S ) 2) and Fe3+ (S ) 5/2) ions
and (ii) the fact that their valences are localized on the NMR
time scale.15 Assignments of the signals were made by
chemical substitution (replacement of mpdp by acetate), T1
measurements, and COSY experiments as well as by
comparisons with related compounds.19,30,31
(29) Mashuta, M.; Webb, R. J.; McCluster, J. K.; Schmitt, E. A.;
Oberhausen, K. J.; Richardson, J. F.; Buchanan, R. M.; Hendrickson,
D. N. J. Am. Chem. Soc. 1992, 114, 4-115, 3815-3827.
(30) Ming, L. J.; Jang, H. G.; Que, L., Jr. Inorg. Chem. 1992, 31, 359-
364.
(31) Wang, Z.; Holman, T. R.; Que, L., Jr. Magn. Reson. Chem. 1993, 31,
S78-S83.
(32) Axial and equatorial methylenic protons are defined by analogy with
cyclohexane.
1642 Inorganic Chemistry, Vol. 43, No. 5, 2004

Aqua Ligand Exchange in Diiron Complexes
sponds to an Fe3+ center with Mo¨ssbauer parameters δ/Fe
) 0.47(1) mm/s and ∆EQ ) 0.36(1) mm/s. These parameters
are similar to those obtained for the Fe3+ center in [FeIII(L-
Bn)(mpdp)FeII(CH3CN)]2+ (see trace A2 of Figure 4A) and
in [FeIII(L-Bn)(mpdp)FeII(H2O)](BPh4)2 (δ/Fe ) 0.475(4)
mm/s and ∆EQ ) 0.329(6) mm/s).16 The second and third
sites (trace B2, 31(1)%, and trace B3, 16(1)%, respectively)
correspond to two Fe2+ centers with similar isomer shift
values δ/Fe ) 1.21(1) and 1.19(1) mm/s but different
quadrupole splitting values ∆EQ ) 3.02(2) and 2.41(2) mm/
s, respectively. The parameters associated with the second
site are close to those obtained for the Fe2+ center in [FeIII(L-
Bn)(mpdp)FeII(CH3CN)]2+ (see trace A1 of Figure 4A) while
those obtained for the third site are close to those published
for the FeIIN2O3(OH2) site in [FeIII(L-Bn)(mpdp)FeII(H2O)]-
(BPh4)2 (δ/Fe ) 1.196(4) mm/s and ∆EQ ) 2.657(7) mm/
s).15 The latter observation suggests that the three iron sites
are associated with two dinuclear species. Indeed, the second
iron site (trace B2) corresponds to the Fe2+ center in the
remaining starting [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+ com-
plex, whereas the third iron site (trace B3) is associated with
the Fe2+-OH2 center in [FeIII(L-Bn)(mpdp)FeII(H2O)]2+.
Under this assumption, the first iron site (trace B1) corre-
sponds to the Fe3+ center in the starting [FeIII(L-Bn)(mpdp)-
FeII(CH3CN)]2+ complex (≈62%) as well as the Fe3+ center
in a new complex (namely [FeIII(L-Bn)(mpdp)FeII(H2O)]2+,
≈32%) generated from [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+
by water substitution of the CH3CN ligand. We did not notice
any improvement in the fitting procedure by considering four
iron sites, i.e. by considering slightly different Mo¨ssbauer
parameters for each of the Fe3+ centers in [FeIII(L-Bn)-
(mpdp)FeII(CH3CN)]2+ and [FeIII(L-Bn)(mpdp)FeII(H2O)]2+
complexes. The Mo¨ssbauer parameters obtained for all iron
sites in [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+ and [FeIII(L-Bn)-
(mpdp)FeII(H2O)]2+ are summarized in Table 3.
Figure 4. Mo¨ssbauer spectra at 77 K of acetonitrile solution of [FeIII(L-
Bn)(mpdp)FeII(CH3CN)]2+ in the absence (A) and in the presence (B) of
1500 equiv of water.
a significant decrease of the spectrum range to ca. 550 ppm
is observed.
In conclusion, it appears that the values of the isomer shifts
of the Fe2+ sites for the aqua and acetonitrile complexes
are quite similar whereas the values of their quadrupole
splittings are significantly different. Indeed, acetonitrile
substitution of the terminal water results in a significant
increase of the quadrupole splitting value of the Fe2+ site
(+0.5 mm/s).
Mo1ssbauer Spectroscopy. The zero-field Mo¨ssbauer
spectrum of [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+ in acetonitrile
solution at 77 K is shown in Figure 4A together with its
deconvolution into individual iron sites (solid lines above
the experimental spectrum).
It consists of two quadrupole doublets (traces A1 and A2)
of equal relative areas with isomer shifts δ/Fe ) 1.20(1) and
0.48(1) mm/s and quadrupole splittings ∆EQ ) 3.14(1) and
0.39(1) mm/s. A high-spin Fe3+ impurity (trace A3, δ/Fe )
0.64(2) mm/s, ∆EQ ) 1.0(1) mm/s) is also present (6(1)%).
The first doublet (trace A1) is attributed to the high-spin
Fe2+ center, while the second one (trace A2) corresponds to
the high-spin Fe3+ center of the mixed-valent complex.
Figure 4B shows the zero-field Mo¨ssbauer spectrum
obtained after addition of 1500 equiv of water to an
acetonitrile solution of [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+.
At first we note the presence of a high-spin Fe3+ impurity
Electrochemical Study. To further characterize the ter-
minal exchange reaction, the electrochemical properties of
[FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+ and [FeIII(L-Bn)(mpdp)-
FeII(H2O)]2+ have been investigated in acetonitrile and
methylene chloride solutions, respectively. Cyclic voltam-
mograms of a solution of [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+
at a platinum disk in CH3CN are depicted in Figure 5. In
the anodic region (Figure 5, curve a) a reversible couple is
observed at E1/2(ox) ) 0.53 V vs Ag|10 mM Ag+ (Epa
)
0.57 V, Epc ) 0.49 V, ∆Ep ) 0.08 V) corresponding to the
reversible one-electron oxidation of the Fe2+ center of the
mixed-valent FeIIFeIII complex (eq 2).
II
III
(trace B4, 6%) as found in the [Fe (L-Bn)(mpdp)Fe (CH3-
CN)]2+ sample (see trace A3 of Figure 4A). Apart from this
impurity, the spectrum can be fitted by considering three
distinct iron sites. The first site (trace B1, 47(1)%) corre-
[FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+
a
[FeIII(L-Bn)(mpdp)FeIII(CH3CN)]3+ + e- (2)
Inorganic Chemistry, Vol. 43, No. 5, 2004 1643

Chardon-Noblat et al.
Table 3. Mo¨ssbauer Data for Mixed-valent and Diferric Complexesa
site 1 (FeIIIN3O3)
∆EQ
Mixed-Valent Complexes
site 2
compd
δ/Fe
G
δ/Fe
∆EQ
G
[Fe2(L-Bn)(mpdp)(CH3CN)]2+
0.48(1)
0.47(1)
0.475(4)
0.47
0.39(1)
0.36(1)
0.329(6)
0.51
0.39(1)
1.20(1)
1.19(1)
1.196(4)
1.16
3.14(1)
2.41(2)
2.657(7)
2.42
0.39(1)
0.38(1)
0.217(7)
0.41
[Fe2(L-Bn)(mpdp)(H2O)]2+
0.38(1)
0.198(6)
0.33
15
[Fe2(L-Bn)(mpdp)(H2O)](BPh4)2
19
[Fe2(bpmp)(OAc)2(BF4)2
[Fe2(L-BnO)(dpp)2](PF6)16
1.183(4)
3.358(7)
0.296(8)
0.550(5)
1.220(7)
0.296(8)
Diferric Complexes
[Fe2(L-Bn)(mpdp)(CH3CN)]3+
[Fe2(L-Bn)(mpdp)(H2O)]3+
[Fe2(L-Bn)(mpdp)(OH)]2+
0.47(1)
0.48(1)
0.48(1)
0.447(5)
0.40(1)
0.44(1)
0.44(1)
0.31(2)
0.48(1)
0.40(1)
0.55(1)
0.55(1)
0.57(2)
0.25(1)
0.60(1)
1.51(1)
0.99(2)
0.48(1)
0.41(1)
0.41(1)
0.53(2)
0.41(1)
0.41(1)
0.53(2)
16
[Fe2(L-BnO)(dpp)2](PF6)2
a δ/Fe ) isomer shift (mm/s) with respect to metallic iron at room temperature. ∆EQ ) quadrupole splitting (mm/s). Γ ) width at half-maximum of the
Lorentzian lines (mm/s). Standard deviations are given in parentheses, in units of the last digit of quoted value.
Exhaustive electrolysis of the solution at 0.80 V requires
1 mol of electrons/mol of [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+
and generates quantitatively the diiron(III,III) complex as
judged from analysis of the resulting voltammogram. An
exhaustive electrolysis at 0.35 V of this oxidized solution
regenerates the starting complex showing the total revers-
ibility of the system.
In the cathodic domain (Figure 5, curve b), the cyclic
voltammogram exhibits a reversible one-electron reduction
of the Fe3+ center of the mixed-valent complex (eq 3) with
E1/2(red) ) -0.35 V (Epa ) - 0.31 V; Epc ) -0.39 V, ∆Ep
) 0.08 V).
The exhaustive reduction at -0.70 V leads quantitatively
to the diferrous complex (λmax ) 420 nm).26 The complete
regeneration of [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+ by reoxi-
dation at 0 V proves the reversibility of the one-electron-
transfer process.
[FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+ + e- a
[FeII(L-Bn)(mpdp)FeII(CH3CN)]+ (3)
In methylene chloride, a solution of [FeIII(L-Bn)(mpdp)-
Figure 5. Cyclic voltammograms of [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+
in acetonitrile in the absence (a, b) and in the presence (c, d) of 1500 molar
equiv of water.
FeII(H2O)]2+ exhibits the same electrochemical pattern with
two reversible redox processes, but significant shifts are
observed in half-wave potentials at E1/2(ox) ) 0.80 V and
E1/2(red) ) 0.13 V vs Ag/saturated AgCl/LiCl (which
Scheme 1
corresponds to E1/2(ox) ) 0.48 V and E1/2(red) ) -0.19 V
vs Ag/10 mM Ag+).24
The ligand exchange (CH3CN/H2O) reaction can be also
studied by electrochemistry. Progressive addition of water
in a CH3CN + 0.1 M TBAP solution containing [FeIII(L-
Bn)(mpdp)FeII(CH3CN)]2+ causes a decrease of the potential
(E1/2(ox)) of the FeIIIFeII/FeIIIFeIII couple (Figure 5, curve c,
at 1500 molar equiv of H2O) to a maximum of ∆E1/2(ox) )
-0.21 V. At the same time, the variation (∆E1/2(red)) of the
potential of the FeIIFeII/FeIIIFeII couple is small (-0.03 V),
which is consistent with the coordination of the N3O3 site
being mostly unchanged (Figure 5, curve d).
The reverse exchange (H2O/CH3CN) was studied by
progressive addition of CH3CN in CH2Cl2 + 0.1 M TBAP
electrolyte containing [FeIII(L-Bn)(mpdp)FeII(H2O)]2+. As
expected, the potential of the anodic peak of the FeIIIFeII/
FeIIIFeIII system increases with the concentration of CH3CN.
It reaches a stable value of 0.95 V after addition of 7200
molar equiv. However, the redox process is more complicated
than a simple acetonitrile/water exchange. Actually, on the
reverse scan the potential of the corresponding cathodic peak
decreases indicating that the process becomes less reversible
(see below). This observation suggests that a chemical
reaction is occurring at this stage. Scheme 1 summarizes the
transformations of the mixed-valent complex occurring
during the exogen ligand exchanges and the redox processes.
1644 Inorganic Chemistry, Vol. 43, No. 5, 2004

Aqua Ligand Exchange in Diiron Complexes
Scheme 2
The formation of the hydroxo complex is a chemically
reversible process since the reduction at -0.10 V of the
oxidized solution regenerates quantitatively the starting aqua
complex after consumption of 1 Faraday/mol.
[FeIII(L-Bn)(mpdp)FeIII(H2O)]3+
a
[FeIII(L-Bn)(mpdp)FeIII(OH)]2+ + H+ (4)
It appears from the cyclic voltammograms (Figure 6,
curves c and d) that the electrogenerated mixed-valence
species FeIIIFeII(OH-) is unstable on the cyclic voltammetry
time scale. On the reverse scan the intensity of the anodic
peak of the FeIIIFeII(H2O)/FeIIIFeIII(H2O) couple is markedly
larger than that of the aqua diferric complex remaining after
exhaustive oxidation of [FeIII(L-Bn)(mpdp)FeII(H2O)]2+. This
indicates that the mixed-valent hydroxo species [FeIII(L-Bn)-
(mpdp)FeII(OH)]+ is rapidly protonated (Scheme 2). This
observation is supported by the fact that the ratio of the
respective anodic peak intensities of the FeIIIFeII(H2O)/
FeIIIFeIII(H2O) and FeIIIFeII(OH)/FeIIIFeIII(OH) couples in-
creases when the scanning rate decreases (from 500 to 20
mV‚s-1). It must be noted also that in the reduction domain
only the FeIIFeII(H2O)/FeIIIFeII(H2O) couple is observed,
which suggests that the reduction of FeIIIFeII(OH) is slower
than the protonation rate of either [FeIII(L-Bn)(mpdp)-
FeII(OH)]+ or its reduced species FeIIFeII(OH).
The diferric hydroxo complex can be directly obtained
through an oxygen ligand exchange at the FeIIIFeIII redox
level by adding water to an electrogenerated FeIIIFeIII
acetonitrile complex in CH3CN solution. Obviously, the
proportion of the equilibrium between the FeIIIFeIII(H2O) and
FeIIIFeIII(OH) species depends on the quantity of added water.
The amount of the latter complex increases with the water
content and reaches a maximum close to 70% for 8000 molar
equiv of water added (data not shown). As shown in eq 4,
the formation of the hydroxo diferric complex was ac-
companied by a release of protons. The presence of these
protons in electrolyte solution is easily detected on the cyclic
voltammogram by an increase of the irreversibility of the
[FeIII(L)FeII(OH2)]2+/[FeII(L)FeII(OH2)]+ redox system at E1/2
) -0.39 V. Indeed, addition of HClO4 (2 molar equiv) to a
solution of the FeIIIFeIII complex affects the voltammogram
in the manner just described above (data not shown). This
result confirms the existence of a dynamic equilibrium
between the aqua and hydroxo complexes. Scheme 2
summarizes the different redox and chemical coupled reac-
tions between the characterized species present after an
exhaustive oxidation of the aqua mixed-valent complex.
Figure 6. Cyclic voltammograms of [FeIII(L-Bn)(mpdp)FeII(CH3CN)]3+
in acetonitrile + 7% water before (a, b) and after (c, d) electrolysis.
Part III: Oxidatively-Induced Deprotonation. Electro-
chemical Study. As detailed below, by studying the oxida-
tion reaction of the mixed-valent aqua complex [FeIII(L-
Bn)(mpdp)FeII(H2O)]2+, we have shown that the diferric aqua
complex is not stable on the time scale of an exhaustive
electrolysis. A bulk electrolysis conducted at 0.50 V of an
hydroorganic solution (7% H2O in CH3CN + 0.1 M TBAP)
of [FeIII(L-Bn)(mpdp)FeII(H2O)]2+ requires 1 electron/
molecule. The electrochemical response of the resulting
solution presents a new intense reversible system at E1/2(ox)
) 0.00 V (Epa ) 0.04 V; Epc ) -0.04 V; ∆Ep ) 0.08 V
(Figure 6, curves c and d) besides the redox systems at
E1/2(ox) ) 0.33 V and E1/2(red) ) -0.39 V of the remaining
aqua complex (Figure 6, curves a and b, or Figure 5, curves
c and d). Moreover, the formation of these two diferric
complexes is clearly demonstrated by voltamperometry
experiments using a Pt rotating disk electrode. Comparison
of the wave heights of voltamperograms shows that the ratio
of the two species in the mixture is ca. 90:10 in favor of the
new species.
The new species exhibits a reduction potential less positive
than the aqua and acetonitrile diferric complexes. The
resulting solution has an absorption band located at 598 nm,
while that of the acetonitrile diferric complex is at 604 nm,
as seen previously. These observations are consistent with
the presence of a bis FeIII complex having an increase of the
electron density on the two metal ions. Indeed, E1/2(ox) values
of the mixed-valent complexes with a terminal phenolate in
place of the terminal water ligand are also located around 0
V.16 The similarity of the new species with the latter
complexes is clearly evidenced by Mo¨ssbauer spectroscopy
which shows that the external Fe3+ ion is bound by an anionic
ligand (see next section). Electrochemical behavior and
spectroscopic properties are thus in agreement with the
existence of a diferric hydroxo complex in chemical equi-
librium with the diferric aqua complex (eq 4).
Inorganic Chemistry, Vol. 43, No. 5, 2004 1645

Chardon-Noblat et al.
Bn)(mpdp)FeII(CH3CN)]2+ (therefore containing [FeIII(L-Bn)-
(mpdp)FeIII(CH3CN)]3+) and (B) the solution obtained after
water addition to the previous solution (the solid lines above
the experimental spectrum correspond to its deconvolution
into individual iron sites).
Spectrum A in Figure 7 is poorly resolved in the -1.0 to
1.0 mm/s range. Nevertheless, it can be fitted by considering
three distinct iron sites. The first iron site (trace A1, 42(1)%)
corresponds to an Fe3+ center with Mo¨ssbauer parameters
δ/Fe ) 0.47(1) mm/s and ∆EQ ) 0.40(1) mm/s, which are
very close to those obtained for the Fe3+ center in [FeIII(L-
Bn)(mpdp)FeII(CH3CN)]2+ (Table 3 and Figure 4, trace A2).
The second site (trace A2, 42(1)%) also corresponds to an
Fe3+ center with Mo¨ssbauer parameters δ/Fe ) 0.40(1) mm/s
and ∆EQ ) 0.25(1) mm/s. These two sites are attributed to
the [FeIII(L-Bn)(mpdp)FeIII(CH3CN)]3+ species. The ad-
ditional site (trace A3, 16(1)%) is attributed to a high-spin
Fe3+ impurity (δ/Fe ) 0.32(1) mm/s and ∆EQ ) 0.98(1)
mm/s), probably a degradation product of the electrochemi-
cally oxidized [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+ complex.
After addition of water, spectrum B is obtained that
extends over a larger velocity range (wings appear at -0.4
and 1.2 mm/s) and looks more complicated but better
resolved than spectrum A (Figure 7). This suggests that at
least one new iron species is involved in the sample. This
spectrum can be fitted by considering essentially three dis-
tinct iron sites. The first iron site (trace B1, 48(1)%) corre-
sponds to an Fe3+ center with δ/Fe ) 0.48(1) mm/s and ∆EQ
) 0.44(1) mm/s; these parameters are close to those obtained
for the Fe3+ centers in [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+ and
[FeIII(L-Bn)(mpdp)FeIII(CH3CN)]3+ (Table 3 and traces A2
in Figure 4 and A1 in Figure 7, respectively) and are likely
to be associated with the Fe3+ ion in the [N3O3] site. The
second and third sites (trace B2, 26(1)%, and trace B3,
22(1)%, respectively) correspond to two Fe3+ centers with
the same isomer shift value (δ/Fe ) 0.55(1) mm/s) and
quadrupole splitting values ∆EQ ) 0.60(1) and 1.51(1) mm/
s, respectively. These three iron sites are proposed to be
associated with two dinuclear Fe3+ species. Under this
assumption, the second iron site is likely to be the Fe3+-OH2
center in the aqua complex [FeIII(L-Bn)(mpdp)FeIII(H2O)]3+
(≈52% abundance). One can notice a significant increase
of the quadrupole splitting value when going from the second
to the third iron site (0.60 to 1.51 mm/s). This latter feature
is reminiscent of the parameters observed for the Fe3+ center
linked to the terminal phenolate ligand in related complexes
(δ ) 0.570(5) mm/s and ∆EQ ) 0.99(2) mm/s for [FeII(L-
Figure 7. Mo¨ssbauer spectra at 77 K of an acetonitrile solution of [FeIII(L-
Bn)(mpdp)FeIII(CH3CN)]3+ before (A) and after (B) addition of 1500 equiv
of water.
Attempts to prepare the mixed-valent hydroxo complex by
chemical deprotonation with an organic base or by adding
OH- ion to [FeIII(L-Bn)(mpdp)FeII(H2O)]2+ have failed so
far to give the hydroxo mixed-valent complex. On the
contrary, hydroxide treatment invariably led to demetalation
of the complex while addition of triethylamine gave quan-
titative reduction to the diferrous state as judged from
electronic absorption and NMR spectroscopies.26
Mo1ssbauer Spectroscopy. We recently published a Mo¨ss-
bauer study of diiron complexes of the asymmetrical ligand
HL-BnOH (Chart 1) which provides one iron of the pair with
a terminal phenolate ligand.16 A drastic increase of the
quadrupole splitting value (from 0.30 to ca. 1.0 mm/s) was
noticed for the Fe3+ ion bound to the terminal phenolate
ligand, with respect to that bound to the three nitrogen donors
of the bis(picolyl) branch. This is due to the change in the
asymmetry of the charge distribution introduced by the
anionic ligand. It is therefore possible, using Mo¨ssbauer
spectroscopy, to distinguish a complex with a neutral ligand,
acetonitrile or water, from one having a hydroxo ligand.
Indeed, the latter complex will exhibit a larger quadrupole
splitting value compared to the former ones. Figure 7 shows
the zero-field Mo¨ssbauer spectra recorded at 77 K for (A)
the electrochemically oxidized acetonitrile solution of [FeIII(L-
BnO)(dpp)2FeIII]2+ and δ ) 0.550(4) mm/s and ∆EQ
)
1.220(7) mm/s for [FeIII(L-BnO)(dpp)2FeIII]+16 (where dpp
stands for diphenyl phosphate). Such a large quadrupole
splitting is consistent with the presence of an anionic ligand
on the third iron site. This anion results from water addition
to the acetonitrile solution of [FeIII(L-Bn)(mpdp)FeIII(CH3-
CN)]3+. Therefore, it is likely to be a hydroxo ligand. We
propose that the second dinuclear species (≈44%) corre-
sponds to the [FeIII(L-Bn)(mpdp)FeIII(OH)]2+ species. In
addition, a high-spin Fe3+ impurity is present (trace B4,
≈4%). It must be noted that all attempts to increase the
1646 Inorganic Chemistry, Vol. 43, No. 5, 2004

Aqua Ligand Exchange in Diiron Complexes
Table 4. Electrochemicala Data for [FeIII(L-Bn)(mpdp)FeII(S)]2+
Complexes
number of Fe3+ sites did not improve significantly the fit of
the spectrum shown in Figure 7. The Mo¨ssbauer param-
eters obtained for all iron sites in [FeIII(L-Bn)(mpdp)-
FeIII(CH3CN)]3+, [FeIII(L-Bn)(mpdp)FeIII(H2O)]3+, and [FeIII(L-
Bn)(mpdp)FeIII(OH)]2+ are summarized in Table 3.
complex
E
1/2(red)b
E
1/2(ox)b ∆E (V)
Kcom
refs
S ) phenolate
S ) CH3CN
S ) pyridine
S ) H2O
-0.35
+0.20
+0.15
+0.18
0.63
1.08
0.88
0.86
0.98
0.88
0.73
0.68
3.8 × 1016 16
7.6 × 1014 this work
2.2 × 1012 20
3.1 × 1011 this work
Discussion
a Vs NHE, determined by cyclic voltammetry at 100 mV s-1 in CH3CN
or CH2Cl2 + 0.1 M TBAP at a platinum disk electrode. E1/2 ) (Epa
Epc)/2, where Epa and Epc are the anodic and the cathodic peak potentials.
b
Acetonitrile/Water Exchange. The replacement of the
acetonitrile terminally bonded to the Fe2+ ion by a water
molecule brings a significant perturbation of the electronic
structure of the diiron center which has been probed by
various spectroscopic and electrochemical techniques. Indeed,
electronic absorption spectroscopy reveals a 275 cm-1
increase in the energy of the phenolate to FeIII charge-transfer
transition. Such an energetic difference is small when
compared to those observed for axial ligand variations in
ferric salen complexes33 or in phenolato-substituted tris-
(picolyl)amine complexes.34 In the latter case, addition of
excess water to the Fe3+ complex in acetonitrile causes an
energy increase of 3850 cm-1 of the charge-transfer transi-
tion. This effect was attributed to the substitution of
acetonitrile by water. A ca. 15 times smaller effect is
observed in the present study. This is due to the fact that
the coordination change occurs on the Fe2+ while it is
observed on the phenolate to Fe3+ charge-transfer transition.
Substitution of the weak acceptor acetonitrile by the weak
donor aqua ligand will decrease the strength of the phenolate-
FeII bond, thereby enabling the phenolate to engage in a
stronger bond with the Fe3+ ion.
Owing to the intrinsic anisotropy of the Fe2+ ion it is
difficult to analyze the changes in the NMR chemical shifts
of the various protons induced by the terminal ligand
exchange. Nevertheless, one observes an overall decrease
of the NMR spectrum width upon going from acetonitrile
to aqua ligand amounting to ca. 10% (600 to 550 ppm). Such
a reduction of the spectrum width may be related to a small
strengthening of the antiferromagnetic exchange interaction
within the (µ-phenolato)bis(µ-carboxylato)FeIIFeIII core. Within
the theoretical framework developed by Hoffmann35 and
Kahn,36 the magnetic exchange interaction is proportional
to the overlap of the magnetic orbitals and inversally
proportional to their energy splitting. The above discussion
suggests that the substitution of acetonitrile by an aqua ligand
will reduce the overlap between the orbitals of the phenolic
oxygen and those of the Fe2+ ion while increasing the overlap
with those of the Fe3+ ion, the two effects counterbalancing
each other. On the other hand, it will increase the energy of
the orbitals of the Fe2+ fragment and therefore decrease the
energy gap between the magnetic orbitals. The latter effect
may overcome the former leading to an increase in the
antiferromagnetic exchange, but the overall effect is small.
+
A change in the anisotropy of the Fe2+ ion is likely to
contribute also to the narrowing of the spectrum. Indeed,
the most important changes are observed in the methylenic
protons of the picolyl group bound to FeII.
The electrochemical methods are able to probe both
coordination sites and the diiron center as a whole. Upon
replacement of acetonitrile by water, the oxidation potential
decreases by 210 mV while the reduction potential decreases
by 30 mV. Both the sign and the magnitude of these potential
shifts are consistent with the stronger electron-donating
character of the aqua ligand and its binding to the Fe2+ ion.
The half-wave potential difference between the oxidation
(eq 2) and the reduction (eq 3) of the mixed-valent species
(∆E ) E1/2(ox) - E1/2(red)) can be related to the equilibrium
constant of the comproportionation (Kcom) reaction as follows:
∆E ) (RT/nF) ln Kcom
[FeII(L-Bn)(mpdp)FeII(L)]+ +
[FeIII(L-Bn)(mpdp)FeIII(L)]3+
a
2[FeIII(L-Bn)(mpdp)FeII(L)]2+ (5)
L ) H2O or CH3CN
This relationship establishes that the larger the potential
difference, the larger the stability domain of the mixed-valent
species. Examination of Table 4 shows that (i) the stability
domain of the FeIIIFeII complexes is strongly dependent on
the terminal ligand of the Fe2+ ion and (ii) the substitution
of acetonitrile by an aqua ligand reduces the stability domain
by 210 mV corresponding to a decrease of more than 3 orders
of magnitude of the comproportionation constant. Gagne´37
has identified the various factors influencing the stability of
mixed-valent forms of dinuclear metal compounds. These
are (i) structural changes associated with the redox reactions,
(ii) Coulombic interactions, (iii) magnetic exchange between
the metal centers, and (iv) electronic delocalization which
is expected to stabilize the mixed-valent form. In our previous
study of analogous systems possessing a terminal phenolate
on one iron of the pair, we showed that the asymmetry of
the charge distribution was the major contributor to the
stability of the mixed-valent species.16 In this respect, it is
noteworthy that Kcom for the aqua complex is in the range
observed for the mixed-valent complexes of the tetra-
pyridyl19,25 and tetraimidazole29 phenol ligands (Kcom ∼ 1012
(33) Pyrz, J. W.; Roe, A. L.; Stern, L. J.; Que, L., Jr. J. Am. Chem. Soc.
1985, 107, 614-620.
(34) Jensen, M. P.; Lange, S. J.; Mehn, M. P.; Que, E. L.; Que, L., Jr. J.
Am. Chem. Soc. 2003, 125, 2113-2128.
(35) Hay, P. J.; Thibeault, J. C.; Hoffmann, R. J. Am. Chem. Soc. 1975,
97, 4884-4899.
(36) Kahn, O.; Briat, B. J. Chem. Soc., Faraday Trans. 2 1976, 72, 268-
281.
(37) Gagne´, R. R.; Spiro, C. L.; Smith, T. J.; Hamann, C. A.; Thies, W.
R.; Shiemke, A. K. J. Am. Chem. Soc. 1981, 103, 4073-4081.
Inorganic Chemistry, Vol. 43, No. 5, 2004 1647

Chardon-Noblat et al.
and 1011, respectively). This observation highlights the
similarity of the electron-donating properties of H2O and
these nitrogen donors. Acetonitrile, on the other hand, differs
quite substantially from these donors and contributes sig-
nificantly to the asymmetry of the charge distribution. Our
present Mo¨ssbauer spectroscopic studies support this inter-
pretation. Indeed, they have shown that the quadrupole
splitting is a sensitive probe of the coordination of the Fe2+
site. In the aqua complex ∆EQ ) 2.68 mm‚s-1, a value close
to those of the mixed-valent complexes of the symmetric
tetrapyridyl ligand (∆EQ ) 2.2-2.7 mm‚s-1, Table 3),19,38
where the Fe2+ ion is bound by a pyridine. In the acetonitrile
complex ∆EQ ) 3.14(1) mm‚s-1 in agreement with a more
asymmetric electronic distribution.
Similar ESIMS experiments on the electrochemically oxi-
dized solution of [FeIII(L-Bn)(mpdp)FeII(H2O)]2+ could not
be done owing to the presence of a large excess (at least
10-fold) of supporting electrolyte.
The observation that triethylamine does not deprotonate
[FeIII(L-Bn)(mpdp)FeII(H2O)]2+ but reduces it to the diferrous
state is at odds with the behavior of the mononuclear Fe2+
complex of a poly(amino)pyridyl ligand whose aqua ligand
is readily deprotonated.40 This difference in behavior may
be related to the different structures of the two complexes.
Indeed, in the mononuclear compound the Fe-O(water) bond
distance is exceptionally short, 1.919 Å vs 2.156 Å in
[FeIII(L-Bn)(mpdp)FeII(H2O)](BPh4)2, where the water is
trans to the phenolate.15
Aqua Ligand Deprotonation. Electrochemical oxidation
of an hydrated acetonitrile solution of [FeIII(L-Bn)(mpdp)-
FeII(H2O)]2+ or addition of water to [FeIII(L-Bn)(mpdp)-
FeIII(CH3CN)]3+ lead to the formation of the same compound
that we formulate as the diferric hydroxo complex [FeIII(L-
Bn)(mpdp)FeIII(OH)]2+. We have not been able so far to
obtain a direct characterization of its nature, and its formula-
tion rests on accumulated indirect but convergent evidence:
(i) The overall chemical reversibility of the oxidation of
[FeIII(L-Bn)(mpdp)FeII(H2O)]2+ indicates that the chemical
reaction following the oxidative electron transfer does not
imply a drastic chemical change. (ii) The liberation of protons
during the oxidation was shown. (iii) The hypsochromic shift
of the electronic absorption of the oxidized solution indicates
the presence of an electron donating ligand. (iv) The value
of the reduction potential of the resulting diferric species
matches those of complexes possessing a terminal phenolate
in the same position. (v) The Mo¨ssbauer parameters of this
species match those of complexes possessing a terminal
phenolate, which indicates that the iron is bound by an
anionic ligand. The viability of this diferric hydroxo complex
[FeIII(L-Bn)(mpdp)FeIII(H2O)]2+ was demonstrated by react-
ing either the diferrous complex with tert-butyl hydro-
peroxide or 1 with m-chloroperbenzoic acid and analyzing
the reaction mixture by electrospray ionization mass spec-
trometry.39 In both cases, the dication [FeIII(L-Bn)(mpdp)-
FeIII(OH)]2+ is observed by a peak at m/z ) 438.5, which
moves to m/z 439.5 in the presence of H218O (Figure S3).
Summary and Conclusion
Combined spectroscopic and electrochemical studies have
shown that the protonation state of the water-derived terminal
ligand in the present diiron complexes is strongly dependent
on the redox state of the iron pair. Indeed, while a hydroxide
is favored for the diferric state, an aqua ligand is favored
for the mixed-valent state and strictly obtained for the
diferrous state. This behavior is consistent with the known
deprotonation of the hydroxo bridge upon formation of
oxyhemerythrin. In addition, as well the acetonitrile/water
exchange as the aqua/hydroxo transformation induces a very
strong perturbation of the electronic structure of the diiron
center causing a change of several hundred millivolts in the
redox potential values. This illustrates how deeply interrelated
proton and electron transfers are in these biologically relevant
diiron centers.
Supporting Information Available: Tables of X-ray crystal-
lographic details (refined positional coordinates, anisotropic thermal
parameters, complete bond lengths and angles) for complexes 1-3
in CIF format, temperature dependence of the øT product of [FeIII(L-
Bn)(mpdp)FeII(H2O)](BPh4)2 (Figure S1), UV-visible spectra of
the reaction of [FeIII(L-Bn)(mpdp)FeII(CH3CN)]2+ with H2O (Figure
S2), and ESI-MS spectra of [FeIII(L-Bn)(mpdp)FeIII(OH)]2+ in the
absence or presence of H218O (Figure S3). This material is available
free of charge via the Internet at http://pubs.acs.org.
IC030192+
(39) A 0.2 mM solution of 1 in acetonitrile is reacted at -40 °C with 4
equiv of mCPBA in the presence of 2000 equiv of H218O and the
solution analyzed by ESI-MS: Avenier, F.; Dubois, L.; Latour, J.-M.
Submitted for publication.
(38) Manago, T.; Hayami, S.; Oshio, H.; Osaki, S.; Hasuyama, H.; Herber,
(40) Bernal, I.; Jensen, I. M.; Jensen, K. B.; McKenzie, C. J.; Toftlund,
H.; Tuchagues, J. P. J. Chem. Soc., Dalton Trans. 1995, 3667-3675.
R. H.; Maeda, Y. J. Chem. Soc., Dalton Trans. 1999, 1001-1011.
1648 Inorganic Chemistry, Vol. 43, No. 5, 2004