New intermediate-spin chloroiron(III) complex with a mixed nitrogen-sulfur co-ordination sphere

Article abstract of DOI:10.1039/a706664c

A new tetradentate proligand, 6,6′-bis(2,2-diphenyl-2-sulfanylethyl)-2,2′-bipyridine (H₂L), involving two nitrogen bases and two thiols, has been synthesized. The crystal structure of its chloroiron(III) complex [FeL(Cl)] exhibits a five-co-ord

Full text of DOI:10.1039/a706664c

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New intermediate-spin chloroiron(III) complex with a mixed nitrogen–
sulfur co-ordination sphere‡
Marie-Aude Kopf,a Daniel Varech,a Jean-Pierre Tuchagues,b Daniel Mansuya and
Isabelle Artaud*,†,a
a Laboratoire de Chimie et Biochimie Pharmacologiques et Toxicologiques (URA 400 CNRS),
Université René Descartes, 45 rue des Saints-Pères, 75270 Paris Cédex 06, France
b Laboratoire de Chimie de Coordination du CNRS et de l’Université Paul Sabatier,
205 route de Narbonne, 31077 Toulouse Cédex, France
A new tetradentate proligand, 6,6Ј-bis(2,2-diphenyl-2-sulfanylethyl)-2,2Ј-bipyridine (H2L), involving two nitrogen
bases and two thiols, has been synthesized. The crystal structure of its chloroiron() complex [FeL(Cl)] exhibits a
five-co-ordinate iron in a distorted square-pyramidal structure including a long Fe᎐Cl axial bond. Its effective
magnetic moment (µeff = 3.8 µB from 10 to 300 K) and its Mössbauer parameters [δ = 0.328(5) mm sϪ1 and
∆EQ = 2.543(9) mm sϪ1 at 293 K] are consistent with a pure intermediate S = ³ spin state for the iron(). This
¯
²
complex exhibits a sulfur-to-iron charge transfer absorption at 490 nm and a one-electron reduction wave at
Ϫ415 mV (vs. SSCE). Upon reaction with tetrabutylammonium hydroxide it is converted into the µ-oxo complex
[Fe2L2O], which has been characterized by mass spectral and elemental analyses. Its formation is reversible in
dmf upon addition of trifluoroacetic acid in the presence of chloride anion, as shown by 1H NMR and UV/VIS
spectroscopy and electrochemistry. The [FeL(Cl)] complex appears to be the first example of an iron() complex
with a mixed nitrogen–sulfur co-ordination sphere in a pure S = ³ ground spin state.
¯
²
With the exception of some monomeric iron-()1,2 or -()3,4
complexes derived from thiosalicylideneimine, the non-heme
iron thiolate complexes known before 1993 were essentially
iron–sulfur clusters involving iron in mixed-valence states.5 The
synthesis of mononuclear iron() complexes with thiolate
ligands has been hampered by the easy FeIII-dependent oxid-
ation of thiolates to disulfides and the propensity of metal thi-
olates to give sulfur-bridged compounds.6 Since 1993 several
non-heme iron() complexes with a mixed nitrogen–sulfur co-
ordination sphere have been reported.7–10 One of the main
objectives of their synthesis was to build chemical models of
the newly characterized iron-containing nitrile hydratases.
These enzymes which catalyse the hydration of nitriles to
amides are the first non-heme iron enzymes with a low-spin
iron() active site. On the basis of EPR,11 resonance
Raman,12,13 EXAFS12,14 and ENDOR15,16 studies, their iron
was first assumed to be six-co-ordinated with a N3S2O donor
set, but a recent crystal structure17 reveals that the iron center is
co-ordinated to two nitrogens and three cysteines. The iron()
complexes reported so far with a mixed nitrogen–sulfur co-
ordination sphere such as N2S3,10 N3S3,9 or N4S2 8 are character-
ized either by a low-8,9 or a high-spin9,10 iron() state.
The electrode system consisted of a NaCl-saturated calomel
electrode (SSCE) as reference, a platinum electrode as auxiliary,
and a glassy carbon electrode as working electrode. The salt
NBu4BF4 was used as supporting electrolyte (0.1  in the
appropriate solvent). The potential sweep rate was 50 mV sϪ1
.
Ferrocene was used as an internal standard (under our experi-
₁
mental conditions, the ferrocenium–ferrocene couple is at E =
₂
1
450 mV vs. SSCE). The H NMR spectra were recorded at
300 K on a Bruker ARX-250 spectrometer driven by UXNMR
software (Bruker) on an Aspect Station 1. Chemical shifts are
reported in ppm downfield from SiMe4. The T1 relaxation time
measurements used an inversion–recovery pulse sequence with
a relaxation delay of 0.5 s and averaging at 512 scans into a
32 K data block. The experiment was repeated for 15 values
ranging from 10 ms to 1 s. The T1 values were calculated using
DISNMR software (Bruker). The calibrated value of the 360Њ
pulse was 26 ms. X-Band EPR spectra were recorded on a
Bruker ESP300 spectrometer operating at a 9.5 GHz microwave
frequency, at 4 K, with a 100 kHz modulation frequency and a
1 G (10Ϫ4 T) modulation amplitude. The microwave power
was 20 mW. Variable-temperature magnetic susceptibility data
were obtained on powder polycrystalline samples with a
Quantum Design MPMS SQUID susceptometer. Diamagnetic
corrections were applied by using Pascal’s constants.
Mössbauer measurements were made on a conventional
constant-acceleration spectrometer with a 25 mCi source of
57Co (rhodium matrix). Isomer shift values (δ) throughout the
paper are given with respect to metallic iron at room temper-
ature. The absorber was a sample of microcrystalline powder
(100 mg) enclosed in a cylindrical plastic sample holder (2 cm
diameter), the size of which had been determined to optimize
the absorption. Variable-temperature spectra were obtained
in the 300–4.2 K range, by using a MD 306 Oxford cryostat,
thermal scanning being monitored by an Oxford ITC4 servo-
control device ( 0.1 K accuracy). A least-squares computer
program18 was used to fit the Mössbauer parameters and
determine their standard deviations of statistical origin (given
in parentheses). Chemical ionization mass spectra were
recorded at the Ecole Normale Supérieure in Paris and fast
Herein we describe the synthesis of a new N2S2 tetradentate
compound and the complete characterization of its chloro-
iron() complex, which appears to have a mixed nitrogen–
sulfur co-ordination sphere in a pure S = ³ ground spin state.
¯
²
Experimental
Spectroscopic measurements
The UV/VIS spectra were recorded at room temperature on an
Uvikon 820 spectrophotometer. Cyclic voltammograms were
obtained with an EGG-PAR model 173 potentiostat and model
276 interface instruments at room temperature under argon.
† E-Mail: artaud@bisance.citi2.fr
‡ Non-SI units employed: Ci = 3.7 × 1010 Bq, µB ≈ 9.27 × 10Ϫ24 J TϪ1
,
Oe = 103 A mϪ1
.
J. Chem. Soc., Dalton Trans., 1998, Pages 991–998
991

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atom bombardment at the Laboratoire de spectrométrie de
Masse Bioinorganique in Strasbourg, France. Elemental
analyses were carried out by the microanalysis service at Paris
VI University and at the Institut de Chimie des Substances
Naturelles (Gif-sur-Yvette, France). The crystal structure was
solved by the X-ray service of Paris VI University.
6.75 and 6.73 (2 H, d, J = 7.5 Hz, H3, H3Ј), 4.32 and 4.26 (2 H,
7
br s, H22, H22Ј), 4.18–3.87 (6 H, m, CH2 , CH27Ј, H24 and H24Ј),
26
3.23 (2 H, m, H24, H24Ј) and 1.9–1.2 (12 H, m, CH225, CH2
,
CH227, CH225Ј, CH226Ј, CH227Ј). The signals at δ 6.73 and 4.32
refer to diastereoisomer A and those at 6.75 and 4.26 to B.
6,6Ј-Bis(2,2-diphenyl-2-sulfanylethyl)-2,2Ј-bipyridine (H2L).
To a suspension of compound V (3 g) in ethyl acetate (80 cm3)
and methanol (60 cm3) was added a solution of AgNO3 (1.5 g,
2.5 equivalents) in pyridine (1 cm3) and methanol (100 cm3).
The solid material dissolved immediately and the solution
became yellow. After 5 h at room temperature the solvent was
removed under vacuum yielding a yellow powder. The material
was dissolved in deaerated dichloromethane (50 cm3) and H2S
bubbled through the solution. The black precipitate was filtered
off and washed with dichloromethane. The combined filtrates
were washed with water and the aqueous layer extracted with
ether. The combined organic layers were dried over MgSO4 and
the solvent was removed to yield 1.91 g (92%) of the product as
a white powder, m.p. 202 ЊC (Found: C, 78.47; H, 5.6; N, 4.7.
Calc. for C38H32N2S2: C, 78.6; H, 5.56; N, 4.82%). Mass spec-
trum (CI ϩ NH3): m/z 581 (Mϩ, 56), 547 (12, M Ϫ SH Ϫ 1),
383 [17, M Ϫ (C6H5)2CSH ϩ 1], 199 (100, M Ϫ CH2bipyCH2)
and 183 [10%, M Ϫ 2(C6H5)2CSH ϩ 1]. 1H NMR, [(CD3)2SO]:
δ 7.72 (2 H, d, J = 7.5, H5, H5Ј), 7.55 (2 H, t, J = 7.5 Hz, H4, H4Ј),
7.45–7.3 (8 H, m, o-H of Ph), 7.3–7.15 (12 H, m, m- and p-H
of Ph), 6.79 (2 H, d, H3 and H3Ј), 4.1 (2 H, br s, 2 SH) and 4.05
Crystallography
A selected crystal of [FeL(Cl)] 1 [H2L = 6,6Ј-bis(2,2-diphenyl-2-
sulfanyl ethyl)-2,2Ј-bipyridine] was set up on a four-circle
Enraf-Nonius CAD4 diffractometer. Graphite-monochrom-
atized Mo-Kα radiation (λ 0.710 69 Å) was used. Unit-cell
dimensions with estimated standard deviations were obtained
from least-squares refinements of the setting angles of 25 well
centered reflections. Two standard reflections were monitored
periodically and showed no change during data collection.
Crystallographic data and other information is summarized in
Table 1. Corrections were made for Lorentz-polarization effects.
Computations were performed by using the personal computer
version of CRYSTALS.19 Atomic form factors for neutral Fe, S,
Cl, N, C and H were taken from ref. 20. Real and imaginary
parts of anomalous dispersion were taken into account. The
structure was solved by direct methods (SHELXS)21 and
successive Fourier maps. Only some hydrogens could be found
on difference maps, so they were all geometrically located and
given an overall isotropic thermal parameter. Owing to the
small number of reflections, only Fe, S and Cl atoms were
anisotropically refined. Full-matrix least-square refinements
were carried out by minimizing the function Σw(|Fo| Ϫ |Fc|)2
where Fo and Fc are the observed and calculated structure fac-
tors. Models reached convergence with R = Σ( Fo| Ϫ |Fc )/Σ|Fo|
7
(4 H, s, CH2 , CH27Ј). The resonance at δ 4.1 disappeared upon
addition of D2O.
[6,6Ј-Bis(2,2-diphenyl-2-sulfanylethyl)-2,2Ј-bipyridinato]-
chloroiron(III) 1. A solution of H2L (100 mg, 0.172 mmol, 1
equivalent), freshly prepared by deprotection of compound V,
in anhydrous dmf (10 cm3), was added under argon to a suspen-
sion of sodium hydride (8.2 mg, 2 equivalents) in anhydrous
dmf (10 cm3). The solid material dissolved within 15 min at
room temperature. A solution of FeCl3 (27.9 mg, 1 equivalent)
in anhydrous dmf (5 cm3) was then added under argon. A deep
red solution was obtained. The mixture was stirred for 1 h at
room temperature under argon and then exposed to the air. The
volume was then reduced to about 10 cm3. Addition of this
solution to ether (100 cm3) led to the precipitation of a brown
powder, which was filtered off and washed with ether. The
powder complex was dissolved in anhydrous CH2Cl2, in which
the NaCl salt formed during the metal insertion was insoluble
and thus removed by filtration. The solvent was then slowly
evaporated to yield 70 mg (60%) of complex 1 as black
microcrystals (Found: C, 67.98; H, 4.59; Cl, 5.53; N, 4.05;
S, 9.53. Calc. for C38H30ClFeN2S2: C, 68.11; H, 4.51; N, 4.18;
Cl, 5.29; S, 9.57%). Mass spectrum (FABϩ): m/z 634, (76,
FeN2S2Cl Ϫ Cl), 601 (67, FeN2S2Cl Ϫ Cl Ϫ HS) and 570
(100%, FeN2S2Cl Ϫ Cl Ϫ 2S).
2
2 ¹
²
and RЈ = [Σw(|Fo| Ϫ |Fc|) /Σw(Fo) ] = 0.060 and 0.070. Criteria
for a satisfactory complete analysis were the ratio of the root-
mean-square shift to standard deviation being less than 0.2 and
no significant feature in the last difference map.
CCDC reference number 186/870.
Synthesis
Diphenylmethanethiol I was synthesized according to the
procedure described by Klenk et al.22 Thiol I was protected
according to the procedure described by Berg and Holm,23
giving III. 6,6ЈBis-(bromomethyl)-2,2Ј-bipyridine II was syn-
thesized according to a previously described procedure.24–26
6,6Ј-Bis[2,2-diphenyl-2-(tetrahydropyran-2-ylsulfanyl)ethyl]-
2,2Ј-bipyridine V. A solution of compound III (650 mg, 2.3
mmol) in anhydrous diethyl ether (10 cm3) was cooled to
Ϫ78 ЊC and 2.5  LiBun in hexane (0.9 cm3) added dropwise.
The reaction mixture was allowed to warm to 0 ЊC over 1 h and
then cooled again to Ϫ78 ЊC. A solution of 6,6Ј-bis(bromo-
methyl)-2,2Ј-bipyridine (342 mg, 1 mmol) in thf (20 cm3) and
P(NMe2)3O (2 cm3), was added dropwise yielding a lightening
of the reaction mixture from brown to yellow. The mixture was
allowed to warm to 0 ЊC over 4 h and then added to water. The
organic layer was set apart and the aqueous layer extracted with
ethyl acetate (3 × 20 cm3). The combined organic layers were
dried over Na2SO4, and the solvent removed to give a yellowish
oil which crystallized after addition of acetone. The product
was filtered off and washed with methanol to afford 612 mg
(80%) of a 1:1 mixture of diastereoisomers A and B. Recrystal-
lization of the product from a CH2Cl2–MeOH mixture afforded
540 mg of a 3:2 mixture of A and B. Any further recrystalliz-
ation enriched the mixture in diastereoisomer A. M.p. 198–
200 ЊC (A:B = 3:2) (Found: C, 77.09; H, 6.31; N, 3.58. Calc. for
C48H48N2O2S2: C, 76.98; H, 6.46; N, 3.74%). Mass spectrum
(CI ϩ NH3): m/z 749 (Mϩ, 72), 663 (5, M Ϫ C5H9O Ϫ 1) and
ì-Oxo-bis{[6,6Ј-bis(2,2-diphenyl-2-sulfanylethyl)-2,2Ј-
bipyridinato]iron(III)} 2. To a solution of complex 1 (25 mg) in
anhydrous CH2Cl2 (10 cm3) was added under argon 1 
tetrabutylammonium hydroxide (37.3 µl) in methanol. Complex
2 precipitated as a brown powder, which was filtered off and
washed with water, yield 70% (Found: C, 68.41; H, 5.03;
N, 4.01; S, 9.33. Calc. for C76H60Fe2N4OS4ؒ2H2O: C, 69.09;
H, 4.85; N, 4.24; S, 9.67%). Mass spectrum (FABϩ): m/z
1269 (20, Fe2N4S4O Ϫ O ϩ 1), 1236 (6, Fe2N4S4O Ϫ O Ϫ S),
634 (30, FeN2S2), 601 (100, FeN2S2 Ϫ HS) and 568 (60%,
FeN2S2 Ϫ 2HS).
Reversibility of complex 2 formation
The addition of NBu4OH in dmf at room temperature directly,
in the electrochemical cell, UV cuvette or NMR tube, to com-
plex 1 at the concentration required for each spectroscopic
technique was carried out. Then trifluoroacetic acid (tfa) was
1
180 [100%, M Ϫ 2(C6H5)2CSC5H9O Ϫ2]. The H NMR spec-
trum (CDCl3) indicated the presence of two diastereoisomers:
δ 7.62–7.55 (4 H, m, H4, H4Ј, H5, H5Ј), 7.26–7.11 (20 H, m, Ph),
992
J. Chem. Soc., Dalton Trans., 1998, Pages 991–998

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O
Ph
Ph
(i)
Ph
Ph
Table 1 Crystallographic data for complex 1
S
CH
SH
CH
III
Formula
M
C38H30ClFeN2S2
670.09
I
Crystal system
Space group
a/Å
b/Å
c/Å
Monoclinic
P21/c
14.695(3)
15.135(5)
16.141(6)
116.24(3)
3220(4)
(ii)
Ph
O
β/Њ
BrH2C
N
N
CH2Br
C
S
U/Å3
Ph
II
IV
Z
4
Crystal shape
Crystal colour
µ/cmϪ1
Parallelepiped
Dark blue
7.04
+
Dc/g cmϪ3
1.38
(iii)
Scan type
Scan range/Њ
θ Limits/Њ
ω–2θ
0.8 ϩ 0.345 tan θ
1–25
4
3
5
T/ЊC
Room temperature
Octants collected
No. data collected
No. unique data
No. unique data used for
refinement
Rint
Decay of standard reflections
R
RЈ*
Extinction parameter
Goodness of fit
No. variables
∆ρmin, ∆ρmax/e Å3
h Ϫ17 to 15, k 0–17, l 0–19
6120
6
7
N
N
2
1
5653
(C9-C14)
Ph
Ph
Ph
Ph
8
21
S
1684 [(Fo)2 > 3σ(Fo)2]
H
22
H
O
S
(C15-C20)
23
27
26
O
0.052
None
0.060
0.070
119
1.14
199
24
25
V
(iv)
Ϫ0.35, 0.34
* Weighting scheme of the form w = wЈ[1 Ϫ (||Fo| Ϫ |Fc||)/6σ(Fo)2]2 with
wЈ = 1/ΣΓΛΓTΓ(X) with coefficients 3.85, Ϫ0.902 and 2.74 for a Cheby-
N
N
shev series for which X = Fo/Fo(max)
.
Ph
Ph
Ph
Ph
SH
HS
out under argon. Addition of 1 equivalent of iron() chloride
to the L2Ϫ ligand in dmf yielded a deep purple solution. The
complex was first precipitated from dmf by anhydrous ether,
purified by dissolution in CH2Cl2 to eliminate sodium chlor-
ide formed during iron insertion, and finally isolated after
evaporation of the solvent under vacuum. Complex 1 (60%
yield) was characterized by elemental analysis and mass spec-
trometry. It is air stable in the solid state and in CH2Cl2 or dmf
solution.
H2L
Scheme 1 Synthesis of the proligand H2L: (i) 2,3-dihydropyran,
pyridinium toluene-p-sulfonate, CH2Cl2; (ii) LiBun, ether; (iii) thf,
P(NMe2)3O; (iv) AgNO3, MeOH–ethyl acetate, H2S, CH2Cl2
directly added without isolating complex 2, as follows: UV/VIS,
6.5 × 10Ϫ2 m 1 in dmf (3 cm3) Ϫ 0.1  NBu4OH (2 µl) in
methanol–toluene, 0.1  tfa (2 µl) in dmf; electrochemistry,
1 m 1 in dmf (3 cm3) Ϫ 1  NBu4OH (3 µl) in methanol, 0.1 
tfa (30 µl) in dmf; NMR, 5 m 1 in [2H7]dmf (0.5 cm3) Ϫ 1 
NBu4OH (2.5 µl) in methanol, 0.1  tfa (25 µl) in dmf.
Structure of complex 1
Single crystals of complex 1, suitable for X-ray crystallography,
were obtained by slow evaporation of a CH2Cl2 solution
under a stream of argon. The complex crystallizes in the
monoclinic space group P21/c. Fig. 1 shows its structure. The
iron is in a distorted square-pyramidal environment with the
two nitrogen atoms and the two sulfur atoms of the ligand
close to the mean equatorial plane, and the chlorine atom in
the axial position. The iron atom is approximately 0.48 Å out
Results
Synthesis of the tetradentate proligand H2L
The design of the proligand H2L was derived from that
reported for a tridentate NS2H2 compound previously syn-
thesized by Berg and Holm23 to model the active site and the
chemical reactivity of molybdenum oxotransferases. The syn-
thesis of the latter 2,6-bis(2,2-diphenyl-2-sulfanylethyl)pyridine
was based on the reaction of 2,6-bis(bromomethyl)pyridine
with the protected form III of diphenylmethanethiol I. Our
proligand has been prepared by reaction of the protected thiol
III with 6,6Ј-bis(bromomethyl)-2,2Ј-bipyridine II as depicted in
Scheme 1. Reaction of the carbanion IV resulting from the lithi-
ation of III at the gem-diphenylcarbon atom with II, afforded
the doubly protected dithiol V in 80% yield from II. Deprotec-
tion of V gave the proligand H2L in 92% yield. This compound
is unstable upon standing in air and has to be stored in its
protected form IV. This new tetradentate proligand has been
completely characterized by 1H NMR spectroscopy, mass spec-
trometry and elemental analysis (see Experimental section).
of the mean equatorial N2
S plane toward the axial Cl. The
2
two Fe᎐S bonds are equivalent with an average length of
2.221 Å while the two Fe᎐N bonds are inequivalent with
lengths of 2.076(8) and 2.027(8) Å. Finally the Fe᎐Cl dis-
tance [2.306(3) Å] is very long compared to other ones.
Selected interatomic bond lengths and angles are listed in
Table 2.
Electrochemistry of complex 1
The cyclic voltammogram of complex 1 in dmf in the presence
of ferrocene as internal standard exhibits two waves, with a
peak to peak separation of 100 mV, which can be described as
two one-electron quasi-reversible reduction steps (Fig. 2). The
III
₁
₁
first one at E Ϫ415 mV (E vs. SSCE) is attributed to the Fe –
₂
₂
Synthesis of complex 1, [FeL(Cl)]
II
₁
Fe reduction and the second one at E Ϫ1445 mV (vs. SSCE) to
₂
Synthesis of the corresponding iron complex has been carried
the FeII–FeI redox couple.
J. Chem. Soc., Dalton Trans., 1998, Pages 991–998
993

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Table 2 Selected bond distances (Å) and angles (Њ) in complex 1
Fe᎐S(1)
Fe᎐S(1Ј)
2.226(3)
2.217(3)
Fe᎐Cl
2.306(3)
Fe᎐N(1)
Fe᎐N(1Ј)
2.027(8)
2.076(8)
S(1)᎐C(8)
1.86(1)
1.32(1)
1.36(1)
1.41(2)
1.39(2)
1.40(2)
1.52(2)
1.52(2)
1.36(2)
1.29(2)
1.48(2)
1.36(2)
S(1Ј)᎐C(8Ј)
N(1)᎐C(6)
N(1Ј)᎐C(6Ј)
C(2)᎐C(2Ј)
C(4)᎐C(5)
C(6)᎐C(7)
C(8)᎐C(9)
C(9)᎐C(10)
C(10)᎐C(11)
C(12)᎐C(13)
C(15)᎐C(16)
C(16)᎐C(17)
1.83(1)
1.35(1)
1.32(1)
1.47(2)
1.37(2)
1.51(2)
1.54(2)
1.38(2)
1.41(2)
1.32(12)
1.36(2)
1.40(2)
C(17)᎐C(18)
C(19)᎐C(20)
C(3Ј)᎐C(4Ј)
C(5Ј)᎐C(6Ј)
C(7Ј)᎐C(8Ј)
C(8Ј)᎐C(15Ј)
C(9Ј)᎐C(14Ј)
C(11Ј)᎐C(12Ј)
C(13Ј)᎐C(14Ј)
C(15Ј)᎐C(20Ј)
C(17Ј)᎐C(18Ј)
C(19Ј)᎐C(20Ј)
1.32(2)
1.43(2)
1.39(2)
1.39(1)
1.58(1)
1.56(1)
1.40(2)
1.34(2)
1.40(2)
1.36(2)
1.32(2)
1.42(2)
C(18)᎐C(19)
C(2Ј)᎐C(3Ј)
C(4Ј)᎐C(5Ј)
C(6Ј)᎐C(7Ј)
C(8Ј)᎐C(9Ј)
C(9Ј)᎐C(10Ј)
C(10Ј)᎐C(11Ј)
C(12Ј)᎐C(13Ј)
C(15Ј)᎐C(16Ј)
C(16Ј)᎐C(17Ј)
C(18Ј)᎐C(19Ј)
1.41(2)
N(1)᎐C(2)
N(1Ј)᎐C(2Ј)
C(2)᎐C(3)
C(3)᎐C(4)
C(5)᎐C(6)
C(7)᎐C(8)
C(8)᎐C(15)
C(9)᎐C(14)
C(11)᎐C(12)
C(13)᎐C(14)
C(15)᎐C(20)
1.37(2)
1.40(2)
1.48(1)
1.51(1)
1.41(1)
1.38(2)
1.37(2)
1.37(2)
1.44(2)
1.33(2)
S(1)᎐Fe᎐S(1Ј)
S(1Ј)᎐Fe᎐Cl
S(1Ј)᎐Fe᎐N(1)
79.1(1)
104.0(1)
160.7(3)
S(1)᎐Fe᎐N(1Ј)
Cl᎐Fe᎐N(1Ј)
S(1)᎐Fe᎐Cl
141.2(3)
101.9(3)
116.7(1)
S(1)᎐Fe᎐N(1)
Cl᎐Fe᎐N(1)
S(1Ј)᎐Fe᎐N(1Ј)
91.7(3)
95.3(3)
96.5(2)
N(1)᎐Fe᎐N(1Ј)
80.0(3)
Fe᎐S(1)᎐C(8)
Fe᎐N(1)᎐C(2)
110.5(4)
114.6(7)
127.2(7)
119.5(9)
123.4(11)
119.4(11)
121.9(13)
121.7(11)
120.7(11)
109.1(8)
108.7(9)
111.7(9)
116.1(12)
Fe᎐S(1Ј)᎐C(8Ј)
Fe᎐N(1)᎐C(6)
Fe᎐N(1Ј)᎐C(2Ј)
109.4(3)
125.7(8)
113.1(7)
C(16)᎐C(17)᎐C(18)
C(18)᎐C(19)᎐C(20)
N(1Ј)᎐C(2Ј)᎐C(2)
C(2)᎐C(2Ј)᎐C(3Ј)
C(3Ј)᎐C(4Ј)᎐C(5Ј)
N(1Ј)᎐C(6Ј)᎐C(5Ј)
C(5Ј)᎐C(6Ј)᎐C(7Ј)
S(1Ј)᎐C(8Ј)᎐C(7Ј)
C(7Ј)᎐C(8Ј)᎐C(9Ј)
C(7Ј)᎐C(8Ј)᎐C(15Ј)
C(8Ј)᎐C(9Ј)᎐C(10Ј)
C(10Ј)᎐C(9Ј)᎐C(14Ј)
C(10Ј)᎐C(11Ј)᎐C(12Ј) 120.7(12)
C(12Ј)᎐C(13Ј)᎐C(14Ј) 118.1(13)
C(8Ј)᎐C(15Ј)᎐C(16Ј)
C(16Ј)᎐C(15Ј)᎐C(20Ј) 120.3(11)
C(16Ј)᎐C(17Ј)᎐C(18Ј) 120.5(13)
C(18Ј)᎐C(19Ј)᎐C(20Ј) 120.4(13)
120.2(13)
115.8(15)
114.1(10)
123.2(11)
118.0(12)
121.1(10)
119.1(10)
108.8(7)
106.8(8)
111.6(9)
119.6(10)
118.0(10)
C(17)᎐C(18)᎐C(19)
C(15)᎐C(20)᎐C(19)
N(1Ј)᎐C(2Ј)᎐C(3Ј)
C(2Ј)᎐C(3Ј)᎐C(4Ј)
C(4Ј)᎐C(5Ј)᎐C(6Ј)
N(1Ј)᎐C(6Ј)᎐C(7Ј)
C(6Ј)᎐C(7Ј)᎐C(8Ј)
S(1Ј)᎐C(8Ј)᎐C(9Ј)
S(1Ј)᎐C(8Ј)᎐C(15Ј)
C(9Ј)᎐C(8Ј)᎐C(15Ј)
C(8Ј)᎐C(9Ј)᎐C(14Ј)
C(9Ј)᎐C(10Ј)᎐C(11Ј)
C(11Ј)᎐C(12Ј)᎐C(13Ј) 122.4(14)
C(9Ј)᎐C(14Ј)᎐C(13Ј)
C(8Ј)᎐C(15Ј)᎐C(20Ј)
C(15Ј)᎐C(16Ј)᎐C(17Ј) 118.3(12)
C(17Ј)᎐C(18Ј)᎐C(19Ј) 121.1(13)
C(15Ј)᎐C(20Ј)᎐C(19Ј) 119.2(12)
121.5(15)
122.9(14)
122.7(11)
118.8(11)
119.9(11)
119.8(10)
111.5(9)
114.2(7)
102.8(7)
112.6(9)
122.3(10)
119.8(11)
Fe᎐N(1Ј)᎐C(6Ј)
C(2)᎐N(1)᎐C(6)
N(1)᎐C(2)᎐C(3)
C(3)᎐C(2)᎐C(2Ј)
C(3)᎐C(4)᎐C(5)
N(1)᎐C(6)᎐C(5)
C(5)᎐C(6)᎐C(7)
S(1)᎐C(8)᎐C(7)
C(7)᎐C(8)᎐C(9)
C(7)᎐C(8)᎐C(15)
C(8)᎐C(9)᎐C(10)
C(10)᎐C(9)᎐C(14) 121.1(13)
C(10)᎐C(11)᎐C(12) 121.6(20)
C(12)᎐C(13)᎐C(14) 116.3(19)
C8᎐C(15)᎐C(16)
C(16)᎐C(15)᎐C(20) 117.3(12)
C(2Ј)᎐N(1Ј)᎐C(6Ј) 119.4(9)
N(1)᎐C(2)᎐C(2Ј)
C(2)᎐C(3)᎐C(4)
C(4)᎐C(5)᎐C(6)
N(1)᎐C(6)᎐C(7)
C(6)᎐C(7)᎐C(8)
S(1)᎐C(8)᎐C(9)
S1᎐C(8)᎐C(15)
C(9)᎐C(8)᎐C(15)
C(8)᎐C(9)᎐C(14)
C(9)᎐C(10)᎐C(11) 117.1(16)
C(11)᎐C(12)᎐C(13) 125.3(22)
C(9)᎐C(14)᎐C(13) 118.8(14)
C(8)᎐C(15)᎐C(20) 117.1(11)
C(15)᎐C(16)᎐C(17) 122.2(12)
117.1(10)
115.6(12)
118.4(13)
118.1(10)
111.2(10)
112.4(8)
104.9(7)
110.0(9)
122.7(12)
121.0(12)
118.3(10)
121.3(10)
125.6(11)
Fig. 1 Crystal structure of complex 1 with atom labeling. For clarity the hydrogen atoms are omitted
UV/VIS spectroscopy
1H NMR spectroscopy
The electronic spectrum of complex 1 is characterized by
an absorption at 490 nm (ε = 7900 350 Ϫ1 cmϪ1) which
can be assigned to a sulfur-to-iron charge-transfer band
[Fig. 3(A)].
The 1H NMR spectrum of complex 1 in CD2Cl2 displays three
shifted resonances of equal intensity at δ 40 (2 H), -6 (2 H) and
Ϫ24 (2 H) [Fig. 4(A)]. The proton resonances of the ligand were
assigned from their chemical shifts, intensity ratios and 1H
994
J. Chem. Soc., Dalton Trans., 1998, Pages 991–998

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Fig. 2 Cyclic voltammogram of complex 1 (1 m in deaerated dmf,
0.1  NBu4BF4) at a glassy-carbon electrode. Potential sweep rate 50
mV sϪ1. The ferrocenium–ferrocene couple used as internal standard is
observed at ϩ450 mV (vs. SSCE)
Fig. 4 Proton NMR spectra in [2H7]dmf at 300 K of (A) 5 m
complex 1, (B) complex 2 obtained from (A) ϩ 1 equivalent NBu4OH
and (C) (B) ϩ 1 equivalent tfa
Magnetic susceptibility measurements
The variation of the magnetic susceptibility of complex 1 in the
solid state has been measured in the 2–300 K temperature
range. As shown in Fig. 5, above 10 K the effective magnetic
moment reaches a constant value of 3.8 µB, which is, within the
experimental error, the theoretical spin-only value expected for
three unpaired electrons S = ³ (3.87 µB). This behavior clearly
¯
²
Fig. 3 Electronic spectra at 298 K in dmf of (A) 65 µ complex 1, (B)
complex 2 prepared from (A) ϩ 1 equivalent NBu4OH and (C) (B) ϩ 1
equivalent tfa
shows that complex 1 is in a pure S = ³ spin state with no spin
¯
²
crossover and no participation of the higher spin state. Below
10 K the moment decreases to ca. 3.2 µB at 2 K, which presum-
ably results from a significant zero-field splitting leading to an
unequal population of the Kramer’s doublets of the S = ³ spin
longitudinal relaxation times (T1) which have been correlated to
the Fe ؒ ؒ ؒ H distances available from the crystal structure (Table
3). In this assignment the signals with the shortest T1 values (<7
ms) are associated with the protons closest to the iron (d < 4.3
Å). Complex 1 has almost the same symmetry as that of the
proligand with a pseudo-C2 axis. Nevertheless, the two protons
bound to the C(7) [or C(7Ј)] carbons are inequivalent with a
Fe ؒ ؒ ؒ H distance of 4.244 Å for H(71) and 2.973 Å for H(72).
The signal at δ Ϫ 6 corresponding to the shortest T1 relaxation
time (<1 ms) is attributed to the H(72) and H(7Ј2) protons
which are closest to the iron. Similarly, the resonances at
δ Ϫ 24.07 (T1 = 6.77) and 40.45 (T1 = 1.4 ms) are assigned to the
H(71) and H(7Ј1) protons bound to the C(7) and C(7Ј) carbons,
and to the protons of the phenyl rings H(14) and H(14Ј)
{d [Fe ؒ ؒ ؒ H(14)] = 3.599 Å}, respectively. Finally, the signals
between δ 0 and 10 are attributed to the protons of the pyridine
and phenyl moieties which are the farthest from the iron
[d(Fe ؒ ؒ ؒ H) > 5 Å)] and so have the largest T1 values (22–142.7
ms).
¯
²
state.
Mössbauer spectroscopy
Mössbauer spectra of complex 1 recorded at 293, 80 and 4.5 K
consist of a single asymmetric quadrupole-split doublet as
exemplified by the 80 K spectrum shown in Fig. 6. Although the
asymmetry decreases with decreasing temperature, the broader
low-energy absorption shows that the electronic system has not
yet reached the fast fluctuation limit at 4.5 K.27 The spectra
were least-squares fitted with Lorentzian lines and the resulting
isomer shift (δ) and quadrupole splitting parameters (∆EQ) are
listed in Table 4 together with Mössbauer parameters gathered
from the literature for square-pyramidal non-heme iron()
complexes. The magnitudes of δ and ∆EQ are comparable to
those observed for well characterized pure S = ³ spin systems28
¯
²
with an orbitally non-degenerate spin-quartet ground elec-
tronic state (Table 4 and refs. therein). This conclusion is further
J. Chem. Soc., Dalton Trans., 1998, Pages 991–998
995

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Table 3 Proton NMR data for complex 1
H(xy)
[H(xЈy)]
d[Fe ؒ ؒ ؒ H(xy)]
{d[Fe ؒ ؒ ؒ H(xЈy)]}/Å
δ (integration
ratio)
T1/ms
71
[7Ј1]
72
4.244
{4.329}
2.973
Ϫ24.07
(1 H)
Ϫ6
6.77
<1
[7Ј2]
14
[14Ј]
Other H
{3.148}
3.599
{3.200}
(1 H)
40.45
(1 H)
1.4
(Phenyl)
4.850–8.067
4.942–5.826



1.51–8.39
(10 H)
22–142.7
(Pyridine)
Fig. 7 X-Band EPR spectra of (A) complex 1 in the solid state at 293
K, (B) a CH2Cl2 frozen solution of 1 (5 m) at 4 K
Reversible formation of the ì-oxo complex
Fig. 5 Temperature dependence of the magnetic susceptibility and of
the effective magnetic moment for complex 1
Addition of tetrabutylammonium hydroxide to a CH2Cl2 solu-
tion of complex 1 leads to the precipitation of a new species 2
identified as a µ-oxo complex on the basis of its mass spectrum
and elemental analyses. Complex 2 is soluble in dmf and has
been further characterized in this solvent. It is EPR silent and
its electronic spectrum exhibits a charge-transfer band blue-
shifted from 490 to 465 nm [Fig. 3(B)]. The redox potential
of the FeIII–FeII couple determined by cyclic voltammetry is
shifted from Ϫ415 (for complex 1) to Ϫ740 mV (vs. SSCE).
Such a negative redox potential has been previously reported
III
₁
for the complex [Fe (salen)(OH)](E = Ϫ870 mV vs. SCE), the
₂
FeIII–FeII wave of which is also 520 mV cathodically shifted
relative to that of the [FeIII(salen)Cl] parent complex.32 The 1H
NMR spectrum of complex 2 in [2H7]dmf, shown in Fig. 4(B), is
characterized by two resonances at low field at δ ϩ15 and ϩ21.
When formed in situ upon addition of 1 equivalent of
tetrabutylammonium hydroxide to a dmf solution of complex
1, complex 2 reverted back to 1 after subsequent addition of
1 equivalent of trifluoroacetic acid [equation (1)]. All the
Fig. 6 Mössbauer spectrum of complex 1 recorded at 80 K in zero
field
NBu4OH, dmf
[FeL(Cl)]
[Fe2L2O]
(1)
CF3CO2H, dmf, ClϪ
supported by the magnetic susceptibility results in the previous
subsection.
1
2
characteristics of complex 1 were recovered as shown by
UV/VIS [Fig. 3(C)] and 1H NMR [Fig. 4(C)] spectroscopy and
cyclic voltammetry (data not shown).
EPR spectroscopy
The powder X-band EPR spectrum of complex 1 exhibits a
very weak absorption at g = 2 and a broad and unsymmetrical
signal at low field, the pattern of which is clearly temperature
dependent. While two maxima are located at g = 4.2 and 3.4 at
room temperature [Fig. 7(A)], a broad absorption centered at
g = 3.3 with a shoulder at g = 4.1 is observed at 100 K (data
not shown). At 4 K the EPR spectrum of the powder is
nearly identical to that of the CH2Cl2 frozen solution which
is not better resolved and exhibits a very broad resonance
with a maximum at g = 3.8 and a weak shoulder at g = 5.1
[Fig. 7(B)].
Discussion and Conclusion
This paper describes the synthesis, structure and spectroscopic
properties of a chloroiron() complex in a mixed nitrogen–
sulfur co-ordination environment with a S = ³ ground state.
¯
²
There are very few examples of such iron() complexes with
a S = ³ spin state. With the exception of some iron()
¯
²
porphyrins.29,33,34
o-bis(dialkyldithiocarbamato)halogeno-
iron()30,35,36 and some tetraaza macrocyclic iron() com-
996
J. Chem. Soc., Dalton Trans., 1998, Pages 991–998

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Table 4 Mössbauer parameters for S = ³ iron() complexes
¯
²
b,c
b,d
1
–
2Γ
Complex
δa,b
∆EQ
T/K
Donors
N2S2Cl
Ref.
1 [FeL(Cl)]
0.328(5)
2.543(9)
2.824(7)
2.806(5)
0.21(1)e
293
This work
0.153(5)f
0.212(2)e
0.158(1)f
0.195(5)e
0.153(3)f
0.452(2)
0.459(3)
80
N2S2Cl
N2S2Cl
This work
This work
4.5
[Fe{S2CN(CH3)2}2Cl]
[Fe{S2CN(C2H5)2}2Cl]
2.66g
1.2
77
1.2
1.2
77
1.2
77
77
77
300
77
298
4.2
298
4.2
S4Cl
S4Cl
S4Cl
S4Cl
S4Br
S4Br
S4I
30
29
30
30
29
30
29
29
28
28
28
31(a)
31(a)
31(b)
31(b)
0.492(8)
0.492(8)
2.70(1)
2.68(4)g
2.68(3)g
2.85(1)
2.88(4)g
2.89(1)
2.65(1)
2.55
[Fe{S2CN(i-C3H7)2}2Cl]
[Fe{S2CN(C2H5)2}2Br]
[Fe{S2CN(C2H5)2}2I]
0.509(8)
0.509(8)
0.13
[Fe{S2CN(C2H5)2}2(SCN)]
[Fe(Ph2[15]aneN4)(SPh)]
[Fe(Ph2[16]aneN4)(SPh)]
S5
N4S
N4S
N4S
N4Cl
N4Cl
N4O
N4O
0.26
1.93
2.28
3.68
[FeLЈ(Cl)]2Ϫ
0.14
0.25(3)
0.14
3.60(5)
4.29
4.19(4)
0.17
[FeLЉ(OH2)]Ϫ
0.14(2)
a Isomer shift (mm sϪ1). b Statistical standard deviations are given in parentheses. c Quadrupole splitting (mm sϪ1). d Half-width. e Half-width of the
low-energy line. f Half-width of the high-energy line. g No isomer shift data are available in ref 30; the quadrupole coupling constant 1/2e2qQ was
obtained from the best fit of the corresponding Zeeman spectrum.
O
O
2–
2–
O
O
MeO
MeO
NH
NH HN
HN
HN
N
N
N
N
N
N
N
N
O
Ph
Ph
Ph
Ph
NH
O
HN
NH
O
O
O
Ph2[15]aneN4
Ph2[16]aneN4
H4L′′
H4L′
plexes,28,31 iron() compounds are either low or high spin.
Moreover, all pure intermediate-spin non-heme iron() com-
plexes described so far have either a N4X (with X = Cl, S or
OH2) or a S4X (with X = Cl, I, Br, SCN, etc.) co-ordination
sphere. Thus 1 is the first example of S = ³ iron() complexes
plex 1 at 100 and 293 K further supports this interpretation.
Several factors including exchange broadening (or narrowing)
and spin–spin relaxation may contribute to the EPR linewidths;
however, the prevailing factor is usually the latter. In the case of
1, exchange broadening may be excluded on the basis of struc-
tural parameters and the temperature dependence of the EPR
spectra, although small, seems to indicate that spin–spin relax-
ation is mainly responsible for the observed linewidths.
¯
²
with a mixed nitrogen–sulfur donor set, N2S2Cl. Evidence for a
pure intermediate-spin state arises primarily from magnetic
susceptibility and Mössbauer data. The magnetic moment is
constant and equal to 3.8 µB over the 10–300 K range which is
very close to the 3.87 µB spin-only value expected for S = ³. The
Similarly to all other S = ³ iron() complexes, 1 exhibits a
¯
²
distorted square-pyramidal structure with a particularly long
axial bond. As a result of this long Fe᎐Cl axial bond, the
chloride ligand can easily be substituted by HOϪ upon addition
of tetrabutylammonium hydroxide. However the hydroxo
complex could be only observed at the trace level by EPR spec-
troscopy (data not shown), the final complex 2 being the µ-oxo
derivative.
¯
²
isomer shift and quadrupole splitting values for complex 1 and
their temperature dependence are similar to those observed
for well characterized S = ³ iron() spin systems (Table 4). Its
¯
²
powder and CH2Cl2 frozen-solution EPR spectra exhibit a
broad main resonance located near to g = 3.7, reminiscent of
the patterns observed for the S = ³ bis(dithiocarbamato)-
¯
²
iron() complexes and more particularly the thiocyanato
derivative.35 The EPR spectra of S = ³ iron() complexes
¯
²
Acknowledgements
experiencing a rhombically distorted cubic ligand field are
usually characterized by three resonances with effective g values
close to 2 for gz and 4 for gx and gy, the separation of which
is dependent upon the λ = E/D rhombicity factor. However
Chapps et al.35 have shown that the broad EPR pattern of
[Fe{S2CN(C2H5)2}2(SCN)] can be interpreted by a λ = E/D
rhombicity factor of 0.116 and linewidths of ca. 600 Oe. The
molecular structure of complex 1 clearly indicates a significant
rhombic distortion from the axial symmetry around the iron. It
seems thus reasonable to suggest that the broad main resonance
located at g = 3.7 arises from a combination of E/D ratio and
linewidths close to those evaluated by Chapps et al. for [Fe{S2-
CN(C2H5)2}2(SCN)]. Finally the presence of a weak resonance
close to g = 2 and a broad unsymmetrical resonance with two
maxima close to g = 4.2 and 3.4 in the powder spectra of com-
This work was supported by a grant from Ministère de la
Recherche et de la Technologie and Rhône-Poulenc (Pro-
gramme Bioavenir) and a fellowship to Marie-Aude Kopf
from Rhône-Poulenc. We thank Professor I. Morgenstern and
Dr. A. Bousseksou for helpful discussions on the EPR and
Mössbauer results.
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