Redox-noninnocence of the S,S′-coordinated ligands in bis(benzene-1,2-dithiolato)iron complexes

Article abstract of DOI:10.1021/ja040237l

The electronic structures of complexes of iron containing two S,S′-coordinated benzene-1,2-dithiolate, (L)^2-, or 3,5-di-tert-butyl-1,2-benzenedithiolate, (L^Bu)^2-, ligands have been elucidated in depth by electronic absorption, infrared, X-band EPR, and Moessbauer spectroscopies. It is conclusively shown that, in contrast to earlier reports, high-valent iron(IV) (d⁴, S = 1) is not accessible in this chemistry. Instead, the S,S′-coordinated radical monoanions (L^?)^1- and/or (L^Bu?)^1- prevail. Thus, five-coordinate [Fe(L)₂(PMe₃)] has an electronic structure which is best described as [Fe^III(L)(L ^?)(PMe₃)] where the observed triplet ground state of the molecule is attained via intramolecular, strong antiferromagnetic spin coupling between an intermediate spin ferric ion (S_Fe = 3/2) and a ligand radical (L^?)^1- (S_rad = 1/2). The following complexes containing only benzene-1,2-dithiolate(2-) ligands have been synthesized, and their electronic structures have been studied in detail: [NH(C₂H₅)₃]₂[Fe^II(L) ²] (1), [N(n-Bu)₄]₂[Fe^III ₂(L)₄] (2), [N(n-Bu)₄]₂[Fe ^III₂(L^Bu)₄] (3); [P(CH ₃)Ph₃][Fe^III(L)₂(t-Bu-py)] (4) where t-Bu-py is 4-tert-butylpyridine. Complexes containing an Fe ^III(L^?)-(L)- or Fe^III(L ^Bu)(L^Bu?)- moiety are [N(n-Bu)₄][Fe ^III₂(L^Bu)₃(L^Bu?)] (3^ox), [Fe^III(L)(L^?)(t-Bu-py)] (4 ^ox), [Fe^III(L^Bu)(L^Bu?)- (PMe₃)] (7), [Fe^III(L^Bu)(L^Bu?) (PMe₃)₂] (8), and [Fe^III(L^Bu)(L ^Bu?)(PPr₃)] (9), where Pr represents the n-propyl substituent. Complexes 2, 3^ox, 4, [Fe^III(L)(L ^?)(PMe₃)₂] (6), and 9 have been structurally characterized by X-ray crystallography.

Full text of DOI:10.1021/ja040237l

Published on Web 03/22/2005
Redox-Noninnocence of the S,S′-Coordinated Ligands in
Bis(benzene-1,2-dithiolato)iron Complexes
Kallol Ray, Eckhard Bill, Thomas Weyhermu¨ller, and Karl Wieghardt*
Contribution from the Max-Planck-Institute for Bioinorganic Chemistry, Stiftstrasse 34-36,
D-45470 Mu¨lheim an der Ruhr, Germany
Received October 14, 2004; E-mail: wieghardt@mpi-muelheim.mpg.de
Abstract: The electronic structures of complexes of iron containing two S,S′-coordinated benzene-1,2-
)
dithiolate, (L)2-, or 3,5-di-tert-butyl-1,2-benzenedithiolate, (LBu 2-, ligands have been elucidated in depth
by electronic absorption, infrared, X-band EPR, and Mo¨ssbauer spectroscopies. It is conclusively shown
that, in contrast to earlier reports, high-valent iron(IV) (d4, S ) 1) is not accessible in this chemistry. Instead,
the S,S′-coordinated radical monoanions (L•)1- and/or (LBu• 1- prevail. Thus, five-coordinate [Fe(L)2(PMe3)]
)
has an electronic structure which is best described as [FeIII(L)(L•)(PMe3)] where the observed triplet ground
state of the molecule is attained via intramolecular, strong antiferromagnetic spin coupling between an
intermediate spin ferric ion (SFe
) )
3/2) and a ligand radical (L•)1- (Srad 1/2). The following complexes
containing only benzene-1,2-dithiolate(2-) ligands have been synthesized, and their electronic structures
have been studied in detail: [NH(C2H5)3]2[FeII(L)2] (1), [N(n-Bu)4]2[FeIII2(L)4] (2), [N(n-Bu)4]2[FeIII2(LBu)4] (3);
[P(CH3)Ph3][FeIII(L)2(t-Bu-py)] (4) where t-Bu-py is 4-tert-butylpyridine. Complexes containing an FeIII(L•)-
(L)- or FeIII(LBu)(LBu•)- moiety are [N(n-Bu)4][FeIII2(LBu)3(LBu•)] (3ox), [FeIII(L)(L•)(t-Bu-py)] (4ox), [FeIII(LBu)(LBu•)-
(PMe3)] (7), [FeIII(LBu)(LBu•)(PMe3)2] (8), and [FeIII(LBu)(LBu•)(PPr3)] (9), where Pr represents the n-propyl
substituent. Complexes 2, 3ox, 4, [FeIII(L)(L•)(PMe3)2] (6), and 9 have been structurally characterized by
X-ray crystallography.
Interestingly, Holm et al.8 have shown that the dianion
Introduction
Coordination compounds of toluene-3,4-dithiolate, (LMe 2-
with iron ions have been known since 1963 when Gray et al.1-3
reported the synthesis of the dinuclear dianion [FeIII2(LMe)4]2-
[Fe2(LMe)4]2- can chemically be one-electron oxidized by iodine
)
,
1
yielding [N(n-Bu)4][Fe2(LMe)4] which possesses an S )
/
2
ground state. Its molecular and electronic structure has remained
elusive. The question of metal (FeIII/FeIV) or ligand ((LMe 2-
(L
Me• 1-) based mixed valency has not been addressed.
.
)
/
The crystal structure of [N(n-Bu)4]2[Fe2(LMe)4] was reported in
1986.4 Later, the corresponding complex containing the unsub-
stituted benzene-1,2-dithiolate, (L)2-, namely [NEt4]2[FeIII2(L)4],
was synthesized and structurally characterized.5,6 All early room-
temperature crystal structure determinations have large standard
deviations of the bond lengths which do not allow a detailed
description of the electronic structure of the dianions. Recently,
a better structure was reported for the 4,5-dicyanobenzene-1,2-
dithiolato analogue [FeIII2(LCN)4][N(n-Bu)4]2.7 Although, all
investigations agree that these dianions possess a diamagnetic
ground state, contradicting proposals for the intrinsic spin of
)
Monomeric, five-coordinate monoanions [FeIII(L)2X]1- where
X represents a neutral ligand such as pyridine2 or hydrazine9
3
have been reported, and in both cases an S ) /2 ground state
has been deduced from room-temperature magnetic susceptibil-
ity measurements. The structure of the former has not been
reported.
Monomeric ferrous complexes [FeII(L)2]2- and [FeII(L)-
(PMe3)3] have been synthesized and structurally character-
ized.10,11 We have recently shown12 that square planar [FeII(L)2]2-
possesses an S ) 1 ground state with a large zero-field splitting
of D ) +28 cm-1. [FeII(L)(PMe3)3] is five-coordinate and
diamagnetic (S ) 0), the iron ion is in a square-pyramidal S2P3
donor atom environment.
3
1
the ferric ions as SFe ) /2 (intermediate spin) or S ) /2 (low
spin) have been reported.8
(1) Gray, H. B.; Billig, E. J. Am. Chem. Soc. 1963, 85, 2019.
(2) Williams, R.; Billig, E.; Waters, J. H.; Gray, H. B. J. Am. Chem. Soc.
1966, 88, 43.
(3) Baker-Hawkes, M.-J.; Billig, E.; Gray, H. B. J. Am. Chem. Soc. 1966, 88,
4870.
(4) Sawyer, D. T.; Srivatsa, G. S.; Bodini, M. E.; Schaefer, W. P.; Wing, R.
M. J. Am. Chem. Soc. 1986, 108, 936.
In 1991, Sellmann et al.13 published an interesting paper
where the “stabilization of high-valent Fe(IV) centers” in the
(9) Sellmann, D.; Kreutzer, P.; Huttner, G.; Frank, A. Z. Naturforsch. 1978,
33b, 1341.
(10) Sellmann, D.; Kleine-Kleffmann, U.; Zapf, L.; Huttner, G.; Zsolnai, L. J.
Organomet. Chem. 1984, 263, 321.
(11) Sellmann, D.; Geck, M.; Moll, M. J. Am. Chem. Soc. 1991, 113, 5259.
(12) Ray, K.; Begum, A.; Weyhermu¨ller, T.; van Slageren, J.; Neese, F.;
Wieghardt, K. J. Am. Chem. Soc. 2005, 127, 4403.
(13) Sellmann, D.; Geck, M.; Knoch, F.; Ritter, G.; Dengler, J. J. Am. Chem.
Soc. 1991, 113, 3819.
(5) Wong, L.; Kang, B. J. Struct. Chem. 1987, 6, 94.
(6) Kang, B. S.; Weng, L. H.; Wu, D. X.; Wang, F.; Guo, Z.; Huang, L. R.;
Huang, Z. Y.; Liu, H. Q. Inorg. Chem. 1988, 27, 1129.
(7) Alves, H.; Simao, D.; Novais, H.; Santos, I. C.; Gimenez-Saiz, C.; Gama,
V.; Waerenborgh, J. C.; Henriques, R. T.; Almeida, M. Polyhedron 2003,
22, 2481.
(8) (a) Balch, A. L.; Holm, R. H. Chem. Commun. 1966, 552. (b) Balch, A.
L.; Dance, I. G.; Holm, R. H. J. Am. Chem. Soc. 1968, 90, 1139.
9
10.1021/ja040237l CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 5641-5654
5641

A R T I C L E S
Ray et al.
Scheme 1. Complexes
Scheme 2
to procedures described in the literature. The ligand 3,5-di-tert-butyl-
1,2-benzenedithiol, H2[LBu], has been synthesized as described in ref
16.
[N(n-Bu)4]2[Fe2(L)4] (2). To a solution of 1,2-benzenedithiol, H2[L]
(0.28 g; 2.0 mmol), in dry, degassed methanol (40 mL) were added
under strictly anaerobic conditions (Ar blanketing atmosphere) triethy-
lamine (0.4 mL; 4.0 mmol) and FeCl2‚4H2O (0.20 g; 1.0 mmol) and
[N(n-Bu)4](PF6) (0.78 g; 2.0 mmol). After being stirred at ambient
temperature for 15 min the solution was allowed to stand for 12 h at
20 °C during which time light yellow crystals of [N(n-Bu)4][FeII(L)2]
(1) precipitated and were collected by filtration and redissolved in an
acetone/toluene mixture (1:1 vol). Slow evaporation of the solvent in
the presence of air produced black cubes of 2 suitable for X-ray
crystallography. Yield: 0.15 g.
six-coordinate complex [FeIV(L)2(PMe3)2] and its five-coordinate
analogue [FeIV(L)2(PMe3)] was claimed. Both species were
shown to possess an S ) 1 ground state in agreement with a
low-spin d4 electron configuration of an FeIV ion. Zero-field
Mo¨ssbauer data were also reported which apparently supported
the above proposal. Both compounds are described as green-
[N(n-Bu)4]2[Fe2(LBu)4] (3). Sodium metal (0.069 g; 3.0 mmol) was
added to a solution of [N(n-Bu)4]Br (0.161 g; 0.5 mmol) and H2[LBu
]
(0.25 g; 1.0 mmol) in absolute ethanol (10 mL) under an argon
blanketing atmosphere. A solution of FeCl3 (0.081 g; 0.5 mmol) in
degassed ethanol (5 mL) was slowly added to the above solution at 20
°C under anaerobic conditions in the glovebox. The resultant dark red
solution was stirred for 30 min during which time dark black
microcrystalline 3 precipitated. Yield: 224 mg (56%). Anal. Calcd for
C88H152S8Fe2N2: C, 65.80; H, 9.54; N, 1.74. Found: C, 65.7; H, 9.3;
N, 1.8. Electrospray mass spectrum (CH2Cl2 solution) positive- and
negative-ion mode: m/z ) 560 {Fe(LBu)2}-; 242 {N(n-Bu)4}+.
[N(n-Bu)4][Fe2(LBu)4] (3ox). Sodium metal (0.069 g; 3.0 mmol) was
added to a solution of H2[LBu] (0.25 g; 1.0 mmol) in absolute ethanol
(10 mL) under an argon blanketing atmosphere. Solid FeCl3 (0.081 g;
0.5 mmol) was added to this solution. The resultant dark red solution
was exposed to air and stirred for 5 min. The solution was then filtered,
and to the filtrate was added a solution of [N(n-Bu)4]Br (0.161 g; 0.5
mmol) in ethanol (5 mL). Microcrystalline greenish black 3ox precipi-
tated from the solution. Yield: 136 mg (40%). Anal. Calcd for
C72H116S8Fe2N: C, 63.44; H, 8.52; N, 1.03. Found: C, 64.49; H, 8.20;
N, 1.20. Electrospray mass spectrum (CH2Cl2 solution) positive- and
negative-ion mode: m/z ) 560 {Fe(LBu)2}-; 242 {N(n-Bu)4}+.
[P(CH3)Ph3][Fe(L)2(t-Bu-py)] (4). Sodium metal (0.069 g; 3.0
mmol) was added to a solution of H2[L] (0.14 g; 1.0 mmol) and 4-tert-
butylpyridine (2 mL) in absolute ethanol (10 mL) under an argon
blanketing atmosphere. Solid FeCl3 (0.081 g; 0.5 mmol) was added to
this solution. A dark red powder of 4 precipitated upon addition of
[P(CH3)Ph3]Br (0.180 g; 0.5 mmol) to the resultant dark red solution.
Yield: 300 mg (80%). Anal. Calcd for C40H39S4FeNP: C, 64.16; H,
5.25; N, 1.87. Found: C, 64.19; H, 5.40; N, 1.91. Electrospray mass
spectrum (CH2Cl2 solution with a few drops of 4-tert-butylpyridine)
positive- and negative-ion mode: m/z ) 471.6 {Fe(L)2(4-tBuPy)}-;
277.3 {P(Me)Ph3}+. Single crystals suitable for X-ray crystallography
black crystalline materials with intense (∼104 M-1 cm-1
)
absorption maxima in the near-infrared (>600 nm). We felt that
this observation is not in accord with the proposed electronic
structure as Fe(IV) species.
We have recently shown14,15 that S,S′-coordinated benzene-
1,2-dithiolato(2-) ligands are readily oxidized by one electron
yielding an S,S′-coordinated benzene-1,2-dithiolato(1-) π radi-
cal anion, (L•)1- 14,15
In this case, the electronic structure of
.
the above complexes would have to be reformulated as [FeIII-
(L)(L•)(PMe3)2] and [FeIII(L)(L•)(PMe3)] where the S ) 1 ground
state would result from a very strong intramolecular antiferro-
magnetic coupling between an intermediate spin ferric ion (SFe
) 3/2) and a ligand π radical (Srad ) 1/2). We note that Sawyer
et al.4 have suggested that in [N(n-Bu)4]2[Fe2(LMe)4] the iron
ions may possess an +II oxidation state with S ) 2. In this
case, one antiferromagnetically coupled π radical anion per
[FeII(LMe)(LMe•)]1- unit must exist. No spectroscopic evidence
for this notion was given.
In view of these contradicting interpretations we have initiated
the following, in depth, synthetic and spectroscopic study. We
have synthesized and studied spectroscopically the complexes
shown in Scheme 1 by using the ligands shown in Scheme 2.
Experimental Section
Synthesis of Complexes. [NH(C2H5)3]2[FeII(L)2] (1),12 [Fe(L)2-
(PMe3)] (5),13 and [Fe(L)2(PMe3)2] (6)13 have been prepared according
(14) Ghosh, P.; Begum, A.; Herebian, D.; Bothe, E.; Hildenbrand, K.; Wey-
hermu¨ller, T.; Wieghardt, K. Angew. Chem., Int. Ed. 2003, 42, 563.
(15) Ray, K.; Weyhermu¨ller, T.; Goossens, A.; Craje´, M. W. J.; Wieghardt, K.
Inorg. Chem. 2003, 42, 4082.
(16) (a) Sellmann, D.; Freyberger, G.; Eberlein, R.; Bo¨hlen, E.; Huttner, G.;
Zsolnai, L. J. Organomet. Chem. 1987, 323, 21. (b) Sellmann, D.; Ka¨ppler,
O. Z. Naturforsch. 1987, 42b, 1291.
9
5642 J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005

Ligands in Bis(benzene-1,2-dithiolato)iron Complexes
A R T I C L E S
Table 1. Crystallographic Data for 2, 3ox, 4, 6, and 9
2
3ox
4
6
9
chem formula
fw
space grp
a, Å
b, Å
C56H88Fe2N2S8
1157.46
C72H116Fe2NS8
1363.84
P-1, No. 2
14.018(2)
17.260(3)
18.196(4)
73.91(2)
69.65(2)
82.85(2)
3964.1(12)
2
C40H39FeNPS4
748.78
C18H26FeP2S4
488.42
C37H61FePS4
720.92
P-1, No. 2
12.6561(6)
15.0960(8)
16.1084(8)
85.12(1)
88.24(1)
75.24(1)
2965.1(3)
2
Cc, No. 9
12.3310(3)
24.7174(5)
12.3106(4)
90
99.19(1)
90
3704.0(2)
4
P21212, No. 18
11.9489(4)
20.6634(8)
8.6716(4)
90
90
90
2141.1(2)
4
Pnma, No. 62
53.472(4)
7.0995(8)
10.6898(12)
90
90
90
4058.1(7)
4
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
T, K
100(2)
100(2)
100(2)
100(2)
100(2)
F calcd, g cm-3
refl. collected/2Θmax
unique reflns/I > 2σ(I)
no. of params/restr
µ(Mo KR), cm-1
R1a/ goodness of fit b
wR2 c(I > 2σ(I))
resid density, e Å-3
1.296
1.143
1.343
1.515
1.180
35992/66.4
22272/15817
613/0
8.08
0.0509/1.017
0.0931
8632/42.0
6656/4033
748/1
6.13
0.0956/1.107
0.1601
53762/62.0
11737/11069
428/2
7.05
0.0293/1.037
0.0599
46345/61.1
6549/6439
232/0
12.44
0.0172/1.046
0.0432
19492/47.0
3181/2307
256/0
12.88
0.0893/1.196
0.1494
+0.53/-0.71
+0.40/-0.36
+0.28/-0.28
+0.42/-0.32
+0.74/-0.46
2
2
2
a Observation criterion: I > 2σ(I). R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b GoF ) [Σ[w(Fo2 - Fc )2]/(n - p)]1/2
.
c wR2 ) [Σ[w(Fo2 - Fc )2]/Σ[w(Fo )2]]1/2 where w
) 1/σ2(Fo ) + (aP)2 + bP, P ) (Fo + 2Fc )/3.
2
2
2
were obtained by recrystallization of this material from a CH2Cl2/toluene
mixture (1:1 by volume).
mass spectrum (CH2Cl2 solution) positive-ion mode: m/z ) 720 {Fe-
(LBu)2(P(Pr)3)}+.
[Fe(L)2(t-Bu-py)] (4ox). To a solution of 4 (0.05 g; 0.07 mmol) in
CH3CN was added a few drops of 4-tert-butylpyridine. The resultant
solution was cooled to -30 °C, and solid tris(4-bromophenyl)amminium
hexachloroantimonate (0.06 g; 0.07 mmol) was added. The solution
was then frozen to liquid nitrogen temperature and used for Mo¨ssbauer
measurements.
[Fe(LBu)2(PMe3)] (7). A gentle stream of N2 was bubbled through
the boiling blue-green toluene solution (30 mL) of 8 (0.20 g; 0.28 mmol)
for 1 h. The solution was filtered while hot, whereupon microcrystalline
7 crystallizes out from solution. Yield: 106 mg (60%). Anal. Calcd
for C31H49S4FeP: C, 58.47; H, 7.76. Found: C, 58.31; H, 7.90.
Electrospray mass spectrum (CH2Cl2 solution) positive ion mode: m/z
) 636 {Fe(LBu)2(PMe3)}+.
Physical Measurements. The equipment used for IR, UV-vis-
NIR, X-band EPR, and Mo¨ssbauer spectroscopies has been described
in ref 15. The temperature-dependent magnetic susceptibilities of solid
samples of complexes were measured by using a SQUID magnetometer
(Quantum Design) at 1.0 T (2.0-300 K). Corrections for underlying
diamagnetism were made by using tabulated Pascal’s constants. Cyclic
voltammograms and coulometric experiments were performed with an
EG&G potentiostat/galvanostat in CH2Cl2 solutions (0.10 M [N(n-Bu)4]-
PF6) at a glassy carbon working electrode. Ferrocene was used as
internal standard; all redox potentials are given vs the ferrocenium/
ferrocene (Fc+/Fc) couple.
X-ray Crystallographic Data Collection and Refinement of the
Structures. Dark red single crystals of 4 and 9 and black crystals of 2,
3ox, and 6 were coated with perfluoropolyether, picked up with a glass
fiber, and mounted in the nitrogen cold stream of the diffractometer.
Intensity data were collected at 100 K using a Nonius Kappa-CCD
diffractometer equipped with a Mo-target rotating-anode X-ray source
and a graphite monochromator (Mo KR, λ ) 0.71073 Å). Final cell
constants were obtained from a least-squares fit of all measured
reflections. Crystal faces, except for compound 2, were determined and
the corresponding intensity data were corrected for absorption using
the Gaussian-type routine embedded in XPREP.17 The data set of 2
was left uncorrected. Crystallographic data of the compounds are listed
in Table 1. The Siemens ShelXTL17 software package was used for
solution and artwork of the structure, ShelXL9718 was used for the
refinement. The structures were readily solved by direct and Patterson
methods and subsequent difference Fourier techniques. All non-
hydrogen atoms were refined anisotropically and hydrogen atoms were
placed at calculated positions and refined as riding atoms with isotropic
displacement parameters.
[Fe(LBu)2(PMe3)](SbCl6) (7ox). To a CH2Cl2 solution (10 mL) of 7
(0.03 g; 0.05 mmol) at -30 °C was added 1 equiv of tris(4-
bromophenyl)aminium hexachloroantimonate (0.04 g; 0.05 mmol). The
solvent was removed at -30 °C by rotary evaporation. The blackish
residue of 7ox was cooled to liquid nitrogen temperature and used for
Mo¨ssbauer measurements. Complex 7ox proved to be very unstable at
room temperature, and further characterizations have not been possible
to date.
[Fe(LBu)2(PMe3)2] (8). A solution of NaOMe (0.107 g; 2 mmol)
and H2[LBu] (0.25 g; 1.0 mmol) in 10 mL of methanol was combined
with a solution of FeCl3 (0.081 g; 0.5 mmol) in 3 mL of methanol
under an argon blanketing atmosphere. Addition of PMe3 (0.2 mL, 2
mmol) and filtration afforded a red-violet solution. Bubbling oxygen
through this solution for 2 min yielded a dark green suspension. The
black-green microcrystals of 8 were collected by filtration, washed with
methanol, and dried. Yield: 292 mg (82%). Anal. Calcd for C34H58S4-
FeP2: C, 57.29; H, 8.20. Found: C, 57.34; H, 8.06. Electrospray mass
spectrum (CH2Cl2 solution) positive ion mode: m/z ) 636 {Fe(LBu)2-
(PMe3)}+.
[Fe(LBu)2(PPr3)] (9). A solution of NaOMe (0.107 g; 2 mmol) and
H2[LBu] (0.25 g; 1.0 mmol) in 10 mL of methanol was combined with
a solution of FeCl3 (0.081 g; 0.5 mmol) in 3 mL of methanol under
argon blanketing conditions. Addition of tris(n-propyl)phosphane (0.5
mL, 2 mmol) and filtration gave a red-violet solution. Bubbling oxygen
through this solution for 2 min yielded a dark green suspension. The
crude crystalline product obtained by filtration is recrystallized from a
1:1 mixture (volume) of CH2Cl2 and toluene to yield deep black
rectangular crystals of 9. Yield: 200 mg (56%). Anal. Calcd for
C37H61S4FeP: C, 61.64; H, 8.53. Found: C, 61.57; H, 8.32. Electrospray
Complex 9 was first solved and refined in the acentric space group
Pna21 (No.33) but the crystal appeared to be twinned by merohedry
giving rise to apparent disorder, mainly of the iron and the phosphorus
atom splitting on two positions. The structure was therefore finally
refined in centrosymmetric space group Pnma (No.62), now having a
mirror plane running through the basal S4-plane which “generates the
former split positions” (the iron center is off plane). The occupation
(17) ShelXTL V.5; Siemens Analytical X-ray Instruments, Inc.: Madison, WI,
1994.
(18) Sheldrick, G. M. ShelXL97; Universita¨t Go¨ttingen: Go¨ttingen, Germany,
1997.
9
J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005 5643

A R T I C L E S
Ray et al.
Table 2. Redox Potentials of Complexesa (V) vs Fc+/Fc (CH2Cl2;
0.10 M [N(n-Bu)4]PF6)
factor of the phosphane ligand (above and below the pseudoplane) was
set to 0.5 to account for the “disorder”.
complex
E1, V
E2, V
E3, V
E4, V
Results
2
3
4
-1.36 (r; 2e) -0.41 (r; 1e) -0.10 (r; 1e)
-1.45 (r; 2e) -0.63 (r; 1e) -0.35 (r; 1e) +0.40 (qr; 1e)
-0.175 (r; 1e)
Syntheses and Characterization of Complexes. As has been
established by Sellmann and co-workers,10,11 the reaction of
dilithium benzene-1,2-dithiolate(2-) with FeCl2‚4H2O (2:1)
under strictly anaerobic conditions in tetrahydrofuran yields
upon addition of [AsPh4]Cl, [NH(C2H5)3]Br, or [N(n-Bu)4]Br
the yellow crystalline salts [monocation]2[FeII(L)2] (1). The
square-planar dianion possesses an S ) 1 ground state and a
5b
6b
7
-1.42 (r; 1e) -0.68 (r; 1e) +0.28 (qr; 1e)
-1.45 (r; 1e) -0.68 (r; 1e) +0.30 (qr; 1e) -0.10 (irr Ep,ox
)
)
-1.50 (r; 1e) -0.84 (r; 1e)
-1.50 (r; 1e) -0.83 (r; 1e)
-1.42 (r; 1e) -0.79 (r; 1e)
0.085 (r; 1e)
0.085 (r; 1e) -0.13 (irr Ep,ox
0.17 (r; 1e)
8
9
a r ) reversible; qr ) quasireversible; irr ) irreversible. b Reference 13;
the redox potentials have been converted from NHE to the Fc+/Fc couple
by addition of -0.4 V.
large positive12 zero-field splitting parameter, D, of +28 cm-1
.
The molecular structure10,12 clearly shows the presence of two
closed-shell benzene-1,2-dithiolate dianions. The average C-S
bond lengths at 1.76 ( 0.01 Å are typical for S,S′-coordinated
benzene-1,2-dithiolates, and the C-C distances of two six-
membered, aromatic phenyl rings are equidistant at 1.398 (
0.006 Å as expected. The electronic structure has been verified
by density functional theoretical calculations12 which clearly
show the presence of an intermediate spin ferrous ion (S ) 1).
Thus, the Mo¨ssbauer parameters of the [FeII(L)2]2- ion11 are
considered to be a benchmark for such a square planar FeIIS4
core.
Oxidation of 1 in poorly coordinating solvents produces the
dimeric species [monocation]2[FeIII2(L)4] (2) (monocation )
[N(C2H5)4]+, [N(n-Bu)4]+).5,6 From a structural reinvestigation
of [N(n-Bu)4]2[FeIII2(L)4] in this work (see below) it is estab-
lished that the ligands are S,S′-coordinated, closed-shell dianions
(L)2- with the central iron ions being ferric. Thus, the oxidation
of 1 to 2 is metal centered and accompanied by dimerization.
The crystal structure of 2 does not support the notion that 2
Figure 1. CV of 3 in CH2Cl2 solution (80.10 M [N(n-Bu)4]PF6) at 20 °C
at a glassy carbon working electrode at scan rates 50, 100, 200, and 400
mV s-1
.
[Fe(L)2(PMe3)]- which has not been isolated to date. This
putative anion is readily oxidized in the presence of dioxygen
yielding neutral, five-coordinate [Fe(L)2(PMe3)] (5). The oxida-
tion has been proposed to be metal-centered yielding an [FeIV-
(L)2(PMe3)] species.13 Similarly, the reaction of Na2[L] with
FeCl3 and PMe3 (2:1:2) in methanol yields six-coordinate, black
[NMe4][FeIII(L)2(PMe3)2] with a doublet ground state (S ) 1/2).13
Its one-electron oxidation with dioxygen yields the neutral, six-
coordinate species [Fe(L)2(PMe3)2] (6)13 with a triplet ground
state (S ) 1). Complex 6 has also been described13 as high valent
iron(IV) species [FeIV(L)2(PMe3)2], whereas the corresponding
may possess an electronic structure such as [FeII (L•)2(L)2]2-
2
as has been suggested by Sawyer et al.4
By using disodium 3,5-di-tert-butyl-1,2-benzenedithiolate,
Na2[LBu], as a ligand it has been possible to isolate the analogous
crystalline black salt [N(n-Bu)4]2[FeIII2(LBu)4] (3). The reaction
of Na2[LBu] with FeIIICl3 (2:1) in ethanol under anaerobic
conditions also yields 3 upon addition of [N(n-Bu)4]Br. Interest-
ingly, the reaction of 3 dissolved in a CH2Cl2/CH3OH mixture
(1:1 vol) with a small amount of dioxygen yielded greenish-
black crystals of [N(n-Bu)4][Fe2(LBu)4] (3ox) suitable for X-ray
crystallography. It contains the dinuclear monoanion [Fe2(LBu)4]-.
Thus, the dianion [FeIII2(LBu)4]2- in 3 is oxidized by one
electron. A metal-centered oxidation of 3 would yield a mixed
valent species FeIIIFeIV but, of course, ligand oxidation must
also be considered to be a viable alternative affording
[FeIII2(LBu)3(LBu•)]-. As we will show below the latter is the
case for 3ox which is the analogue of Holm’s complex [N(n-
Bu)4][Fe2(LMe)4].8
1
monoanion (S ) /2) has been (correctly) assigned a ferric
oxidation state as in [FeIII(L)2(PMe3)2]- the [N(CH3)4]+ salt of
which has been structurally characterized.13
To gain more insight into the electronic structures of these
putative high valent complexes 5 and 6 we have synthesized
the corresponding analogues [Fe(LBu)2(PMe3)] (7) and [Fe(LBu)2-
(PMe3)2] (8) and [Fe(LBu)2(PPr3)] (9) where Pr is the n-propyl
substituent. To systematically decrease the errors of the crys-
tallographically determined bond lengths in 6 (at 298 K) we
have reinvestigated the structure at 100 K (see below).
The dimer [FeIII2(L)4]2- is known2,3 to react in coordinating
solvents such as pyridine with formation of the square-base
pyramidal monomer [FeIII(L)2(py)]- which possesses an inter-
mediate spin ground state (S ) 3/2).3,7 This species has not been
structurally characterized to date. Therefore, we have prepared
and isolated [P(CH3)Ph3][FeIII(L)2(t-Bu-py)] (4) and character-
ized it by X-ray crystallography (t-Bu-py is 4-tert-butylpyridine).
The presence of two closed-shell dianions (L)2- is clearly seen
Electro- and Spectroelectrochemistry. Cyclic voltammo-
grams (CVs) of 2-4, 7, and 8 have been recorded in CH2Cl2
solutions containing 0.10 M [N(n-Bu)4]PF6 as supporting
electrolyte by a glassy carbon working electrode and a Ag/
AgNO3 reference electrode. Ferrocene was used as internal
standard, and all potentials are referenced versus the ferroce-
nium/ferrocene couple (Fc+/Fc). The results are summarized
in Table 2.
3
which renders the iron ion ferric (SFe ) /2).
It is interesting that the axial ligand in 4 can be substituted
by a phosphine such as trimethylphosphine yielding the anion
Figure 1 shows the CV of 3 which displays one quasirevers-
ible and three reversible waves; the CV of 3ox is identical.
Coulometric experiments at appropriately fixed potentials show
9
5644 J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005

Ligands in Bis(benzene-1,2-dithiolato)iron Complexes
A R T I C L E S
Figure 3. Electronic spectra of 4 and 4ox in CH2Cl2 solution at 20 °C.
nm. We have analyzed the spectral changes in Figure 2 on the
assumption of the equilibrium shown in eq 3 and calculated
Figure 2. Electronic spectra of 3 in CH2Cl2 as a function of the temperature
(range +25 to -60 °C) (top) and spectral changes associated with the
coulometric one-electron oxidation of 3 to 3ox in CH2Cl2 at -25 °C (0.10
M [N(n-Bu)4]PF6).
the following thermodynamic data: ∆G300 ) -2.2 kJ mol-1
;
∆H ) +11 ( 1 kJ mol-1; ∆S300 ) 44 ( 3 J mol-1 K-1. Similar
values have been reported by Pierpont et al.20,21 for (catecho-
lato)cobalt(III) h (semiquinonato)cobalt(II) equilibria and in
ref 19 for the dinuclear [CoIII(LSIP)(LSISQ)]2. It is noted that this
valence tautomeric equilibrium of 3 is only observed in solution.
In the solid state, the spectrum of 3 does not change in the range
10-300 K. The solution spectrum of 2 also does not show this
temperature dependence.
that the first reversible wave at E1 ) -1.45 V corresponds
1/2
to a two-electron reduction of dinuclear 3 which may indicate
two successive one-electron-transfer steps as in eq 1 yielding
mononuclear [FeII(LBu)2]2-
.
[Fe2IIIL4
]
Bu 2- y+2ez 2[FeIIL2Bu]2-E1
(1)
1/2
-2e
Figure 2 (bottom) shows the spectral changes observed during
the electrochemical oxidation of 3 to 3ox in CH2Cl2 solution
(0.10 M [N(n-Bu)4PF6) at -25 °C. The appearance of intense
absorption bands >600 nm is a clear indication for the presence
The two waves at E2 ) -0.63 V and E3 ) -0.35 V
each correspond to a one-electron oxidation per dimeric dianion
in 3 as shown in eq 2.
1/2
1/2
of one S,S′-coordinated (LBu)2- ligand and one (L
in 3ox.
)
Bu• 1- radical
-1e
[Fe2IIIL4
]
Bu 2- y-1ez [Fe2L4
]
Bu 1- y z [Fe2L4
]
(2)
Bu 0
+1e
+1e
The CV of mononuclear 4 (not shown) displays a fully
reversible one-electron-transfer wave in CH2Cl2 solution (0.10
M [N(n-Bu)4]PF6) at 20 °C at E11/2 ) -0.175 V in the potential
range 0.0 to -0.8 V. Coulometric measurements establish that
4 is oxidized by one-electron generating a stable neutral complex
[Fe(L)2(t-Bu-py)] (4ox). Chemically 4 can be oxidized yielding
the same product by one equivalent of tris(4-bromophenyl)-
aminium hexachloroantimonate in CH3CN at -30 °C. Figure 3
displays the electronic spectra of 4 and chemically generated
4ox. The absence of intense maxima >600 nm in the spectrum
of 4 and their presence in that of 4ox (810 nm; ꢀ ) 0.6 × 104
M-1 cm-1) are taken as an indication that 4ox has an electronic
structure as in [FeIII(L)(L•)(t-Bu-py)]. Thus, the one-electron
oxidation of 4 is ligand centered.
The CVs of 5 and 6 have been reported previously.13 They
closely resemble those of 7 and 8 shown in Figure 4 (Table 2).
Sellmann et al.13 have assigned the first two reduction waves
at -0.68 V and -1.42 V as metal-centered Fe(IV/III) and Fe-
(III/II) couples, respectively. The irreversible peak 6 at -0.10
V was proposed to be due the irreversible oxidation of free
PMe3. The quasi-reversible oxidation peak at 0.30 V had been
proposed to be due to the metal centered oxidation of five-
coordinate 5 to an Fe(V) containing monocation.13 We disagree
with the above interpretations and propose the redox behavior
shown in Scheme 3.
Thus, one-electron oxidation of 3 yields reversibly the
monoanion 3ox which can again be reversibly oxidized yielding
a dinuclear neutral species. At this point it is not possible to
discern between metal- or ligand-centered processes but from
the following spectroscopic characterizations it is clear that the
monoanion is [FeIII2(LBu)3(LBu•)]- which may indicate that the
neutral species is [FeIII2(LBu)2(LBu•)2]0. The quasireversible one-
electron-transfer process at E4 ) +0.40 V has not been
1/2
investigated in detail because the generated cationic species is
not stable. The CV of 2 is very similar (Table 2) and the results
are to be interpreted analogously.
Figure 2 displays the electronic spectra of 3 in CH2Cl2
solution as a function of the temperature (25° to -60 °C).
Interestingly, there is a significant, reversible change of the
electronic spectrum in this temperature range. This has been
reported previously19 for the dimer [CoII (LSISQ)4] where
2
ISQ 1-
(LS
)
represents the π radical monoanion of 6-amino-2,4-
di-tert-butylthiophenolate(2-). In analogy to this species the
effect may be interpreted as valence tautomerism eq 3.
K
II Bu
[Fe2 (L )2(LBu•)2]2-
Bu 2- y
z
(3)
[Fe2IIIL4
]
>298 K
<213 K
The spectrum of 3 at 298 K in CH2Cl2 solution resembles
that of 4 (see below) which also displays a single intense LMCT
transition at ∼500 nm, whereas the spectrum at 213 K resembles
that of 3ox or 4ox all three of which exhibit IVCT bands >600
First of all, the equilibrium between 7 and 8 and uncoordi-
nated PMe3 has been experimentally proven to exist by
(20) Buchanan, R. M.; Pierpont, C. G. J. Am. Chem. Soc. 1980, 102, 4951.
(21) (a) Jung, O. S.; Pierpont, C. G. Inorg. Chem. 1994, 33, 2227. (b) Pierpont,
C. G.; Jung, O. S. Inorg. Chem. 1995, 34, 4281.
(19) Herebian, D.; Ghosh, P.; Chun, H.; Bothe, E.; Weyhermu¨ller, T.; Wieghardt,
K. Eur. J. Inorg. Chem. 2002, 1957.
9
J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005 5645

A R T I C L E S
Ray et al.
Figure 6. Electronic spectra of a mixture of 7 and 8, 7ox, and 8red in CH2-
Cl2 solution (0.10 M [N(n-Bu)4]PF6) at -25 °C.
Figure 4. CVs of 7 (bottom) and 8 (top) in CH2Cl2 solutions (0.10 M
[N(n-Bu)4]PF6) at a glassy carbon working electrode at 20 °C. Scan rates:
50, 100, and 200 mV s-1 for 7 and 50, 100, 200, and 400 mV s-1 for 8.
containing exclusively closed-shell benzene-1,2-dithiolate(2-)
ligands (i.e., no ligand radicals) exhibit no absorption maximum
>590 nm: this holds for 1-4 and 8red. It is very important to
note that all other complexes exhibit numerous intense (>103
M-1 cm-1) absorption maxima in the near-infrared between 600
and 1500 nm which cannot be assigned as d-d transitions of
an Fe(IV) species. Such bands have been identified as spin-
and dipole-allowed ligand-to-ligand charge-transfer bands of the
type
hν
(L)2-(L•)1- 98 (L•)1-(L)2-
They are markers for the presence of radical anions (L )
Bu• 1- or
Figure 5. Solid-state electronic spectra of 7 and 8.
(L•)1-. The energy dependence of this transition (see Table 3)
observed in five-coordinate 3ox, 4ox, 5, and 7 on the nature of
the fifth apical ligand X may be due to the fact that the central
iron ion is located slightly but to a varying degree-above the
S4-plane of the two dithiolate ligands.
Scheme 3
Infrared Spectra. In a recent publication,15 we had shown
that the presence of S,S′-coordinated radical monoanions,
(L )
Bu• 1-, is readily detected in the infrared spectrum where the
ν(CdS•) stretching frequency is observed at ∼1100 cm-1 as
an intense band. In contrast, in complexes containing only the
S,S′-coordinated closed-shell dianion (LBu)2- this band is not
detected. For example, the spectra of [N(n-Bu)4][CoIII(LBu)2]
or of the corresponding AuIII complex [N(n-Bu)2][AuIII(LBu)2]
do not show a ν(CdS•) band12 at 1100 cm-1. In contrast, in
[N(n-Bu)4][NiII(LBu)(LBu•)] an intense band at 1114 cm-1 has
been observed.12
dissolving 8 in CH3CN and recording the frozen solution
Mo¨ssbauer spectrum at 80 K (see below). Two quadrupole
doublets at an approximate ratio of 1:1 representing the spectra
of 7 and 8 have been detected. Thus, a solution 8 always
contains also 7 and free PMe3.
The solid-state electronic spectra of 7 and 8 shown in Figure
5 are very similar indeed. Both exhibit a broad band at ∼700
nm which is due to the presence of an intervalence charge-
The spectra of 1-4 do not exhibit a ν(CdS•) stretching mode
at 1100 cm-1 in the infrared. Therefore, we conclude that in
these compounds only closed-shell (LBu)2- dianions are present.
As shown in Figure 7 the species 3ox does display such a band
at 1107 cm-1 which is again absent in the spectrum of 3. This
is considered to be compelling evidence that the one-electron
oxidation of 3 to 3ox is ligand based. Similarly, the infrared
spectrum of 7 displays a ν(CdS•) band at 1111 cm-1 as does
the spectrum of 8 at 1117 cm-1. In contrast, the spectrum of
8red of which we had isolated the tetra-n-butylammonium salt
[N(n-Bu)4][FeIII(LBu)2(PMe3)2] lacks this band as is shown in
Figure 8. Therefore, it appears to be justified to assign an
electronic structure for 7 as [Fe(LBu)(LBu•)(PMe3)] and for 8 as
[FeIII(LBu)(LBu•)(PMe3)2].
transfer band (IVCT) between an (LBu)2- and an (LBu• 1- S,S′-
)
coordinated ligand as in [FeIII(LBu)(LBu•)(PMe3)] (7) and [FeIII-
(LBu)(LBu•)(PMe3)2] (8).
Figure 6 shows the spectra of electrochemically generated
(-25 °C) 7ox and 8red together with that of a mixture of 7 and
8 in CH2Cl2 solution. It is noteworthy that 8red, [FeIII(LBu)2-
(PMe3)2]-, does not show intense absorptions >600 nm which
indicates the absence of IVCT bands between (LBu• 1-
and
)
(LBu)2-. In contrast, the spectrum of 7°x displays two intense
maxima at 772 nm (ꢀ ) 1.4 × 104 M-1 cm-1) and 894 nm (0.4
× 104 M-1 cm-1) which are due to the presence of two (LBu• 1-
)
ligands as in [FeIII(LBu•)2(PMe3)]+.
The electronic spectra of all other complexes have been
recorded by using conditions given in Table 3. These spectra
are quite revealing. It is important to note that all complexes
Sellmann et al.13 had pointed out that in the infrared spectra
of 5 and 6 containing unsubstituted (L)2- ligands no ν(CdS•)
bands are observed. In fact, this observation had been used to
9
5646 J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005

Ligands in Bis(benzene-1,2-dithiolato)iron Complexes
A R T I C L E S
Table 3. Electronic Spectra of Complexes
1
complex
conditionsc
T,
°
C
λmax, nm (ꢀ )
, 104 M-1 cm-
1
3
a
b
20
20
315 (0.5), 389 (0.15)
365 (4.15), 590 (3.0), 680 (sh,0.42)
-60
-25
20
20
20
-25
-25
-25
363 (3.2), 480 (1.4), 530 (1.8), 590 (1.2), 630 (1.1)
367 (2.7), 495 (1.51), 590 (1.9), 645 (1.8), 860 (0.8)
391 (2.2), 485 (sh,1.5), 512 (1.7)
302 (1.6), 360 (1.45), 490 (0.8), 589 (0.9), 670 (0.6), 814 (0.7)
315 (sh,1.6), 360 (sh,0.74), 480 (0.51), 580 (sh,0.48), 645 (0.86), 730 (sh,0.52), 1210 (0.7), 1510( 0.8)
320 (2.9), 490 (1.01), 690 (2.0), 789 (0.7), 890 (0.2)
3ox a
4
b
c
c
d
b
b
b
4ox
5
7/8b
7ox a
8red a
310 (4.0), 460 (1.9), 692 (4.0), 772 (1.4), 894 (0.4)
360 (2.9), 480 (1.2), 510 (1.9), 570 (1.0)
a Electrochemically generated (0.10 M [N(n-Bu)4]PF6). b Equilibrium mixture. c Conditions: (a) tetrahydrofuran solution; (b) CH2Cl2 solution; (c) CH2Cl2
solution with a few drops of 4-tert-butylpyridine; (d) acetone solution (ref 13).
Figure 7. Infrared spectra (KBr disks) of solid 3 (top) and 3ox (bottom) in
the range 500-1500 cm-1
.
Figure 9. Temperature dependencies of the magnetic moments of solid
samples of 3 (top) and 3ox (bottom). The solid lines represent best fits by
using parameters given in the text.
reported:12 S ) 1; D ) +28 cm-1; giso ) 2.05; E/D ) 0.08.
Thus, 1 contains an intermediate spin ferrous ion.
Figure 9 (top) shows the temperature dependence of the
effective magnetic moment of 3 as a function of the temperature.
A monotonic decrease of µeff from 3.0 µB at 300 K to 0.4 µB at
2 K is indicative of an S ) 0 ground state and an intramolecular
antiferromagnetic coupling between two intermediate spin ferric
ions (SFe ) 3/2). The best fit of data was achieved with a coupling
Figure 8. Infrared spectra (KBr disks) of solid 7 (top), 8 (middle), and
[N(n-Bu)4][FeIII(LBu)2(PMe3)], 8red (bottom). The marked bands indicate
ν(CdS•) frequencies.
propose the presence of only (L)2- ligands and, consequently,
iron(IV) metal centers in 5 and 6. We have verified their
observation: 5 and 6 do not display ν(CdS•) bands with
appreciable intensity but they are nevertheless present in the
infrared spectra of 5 and 6 at 1084 cm-1, respectively, as very
weak bands. We note that in [N(CH3)4][FeIII(L)2(PMe3)2] this
band is not observed. From DFT calculations22 of the vibrational
spectra of such complexes which will be reported elsewhere
we are able to show that complexes containing 3,5-di-tert-butyl-
1,2-benzenedithiolate(1-) radicals do exhibit the ν(CdS•) band
with significant intensity whereas the same vibration is less
intense by at least a factor of ∼10 in the corresponding
complexes with unsubstituted (L•)1- ligands.
3
constant J of -84 cm-1 (H ) -2J S1‚S2; S1 ) S2 ) /2), a
fixed g ) 2.0, and a zero-field splitting parameter D1 ) D2 )
15 cm-1, E/D ) 0 (fixed) and a paramagnetic impurity with S
5
) /2 of 1.1%, and a temperature independent paramagnetism
øTIP of 150 × 10-6 emu. The magnetism of 2 (not shown) is
3
very similar: J ) -114 cm-1 (S1 ) S2 ) /2). D1 ) D2 ) 5
cm-1; E/D ) 0; g ) 2.0 (fixed); paramagnetic impurity with S
5
) /2 of 0.6%. It is noted that a reasonable fit of the data can
also be obtained assuming the presence of two low spin ferric
1
ions (S ) /2) but this is physically unlikely to be the case
because of the fact that monomeric 4 possesses an S ) 3/2 ground
state. It is also not in agreement with its Mo¨ssbauer data (see
below).
Magnetic Properties. All complexes have been investigated
by measuring their molar magnetic susceptibilities in the
temperature range 2-300 K and 1 T applied field by using a
SQUID magnetometer. The magnetic properties of 1 have been
The temperature dependence of the magnetic moments of 3ox
shown in Figure 9 (bottom) clearly establishes the S ) 1/2 ground
state of the monoanion (µeff (4.2 K) ) 1.6 µB). The increase of
(22) Neese, F. Unpublished work.
9
J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005 5647

A R T I C L E S
Ray et al.
Figure 10. Temperature dependence of the magnetic moment of 4. The
inset shows the magnetization data at 1, 4, and 7 T. For fit parameters see
text.
µ
eff with increasing temperature was modeled by using a three-
3
1
spin system S1 ) S3 ) /2 for the two ferric ions and S2 ) /2
for the ligand radical (L
)
Bu• 1-. The following parameters afford
the fit shown as solid line in Figure 9: J12 ) J23 ) -28 cm-1
;
J13 ) -98 cm-1; g1 ) g2 ) g3 ) 2.0 (fixed); D1 ) D3 ) 15
cm-1; E/D ) 0 (fixed), a paramagnetic impurity with S ) /2
5
of 1.4%, and a TIP of 500 × 10-6 emu. For the data <30 K a
small Weiss constant θ of -1.5 K improved the fit. We cannot
ascertain if this model is physically meaningful.
Figure 11. X-band EPR spectra. Top: spectrum of 3ox in CH2Cl2 at 10 K
(frequency 9.64 GHz; modulation 10.0 G; power 317 µW). Bottom:
spectrum of 4 in CH2Cl2 at 10 K (frequency: 9.64 GHz; modulation 10.0
G; power 317 µW). For simulation parameters see text.
The µeff vs T plot for 4 is shown in Figure 10 together with
variable-field, variable-temperature (VHVT) magnetization mea-
surements. The magnetic moment is constant at 3.8µB in the
range 20-290 K indicating the exclusive population of the spin
quartet ground state. Below 20 K µeff decreases due to
magnetization saturation by the applied field of 1 T and the
influence of zero-field splitting. The sign of D was determined
from VHVT measurements (inset Figure 10). With E/D fixed
at 0.32 (from EPR measurements; see below) the magnetization
behavior was satisfactorily fitted with D ) -2.3 cm-1 which
is in excellent agreement with the analysis of the applied field
Mo¨ssbauer data of 4.
For complexes 5 and 6 magnetic moments of 2.75µB at 295
K, respectively, have been determined in CDCl3 solution by
using Evan’s method; for both species an S ) 1 ground state
has been proposed.13 A more complete analysis for the corre-
sponding complexes 7 and 8 has been performed here where
magnetic moments in the range 2-300 K have been measured.
Both compounds exhibit temperature-independent magnetic
moments in the range 50-300 K of 2.83µB typical for S ) 1
ground states. Below 50 K, the temperature dependence was
satisfactorily modeled for 8 by a zero-field splitting parameter
D ) +13 cm-1 and a TIP of 375 × 10-6 emu. For 7, a similar
behavior was observed: D ) -27 cm-1, TIP ) 858 × 10-6
emu, and a small Weiss constant θ of 3 K.
prominent peak g ) 5-6 and a broad feature centered at g ∼
2. This is indicative of a spin-quartet with a large rhombic zero-
field splitting. The spectrum was fitted for S ) 3/2 by using an
axial ZFS parameter D ) 2.2 cm-1 and a rhombicity factor
E/D ) 0.32 which were obtained from the magnetization
measurements. Simulations showed that the experimental line
width cannot be reproduced with a single set of ZFS parameters.
Satisfactory fits were obtained with a Gaussian distribution of
E/D values, for which the half-width of the distribution is 0.11.
These fit parameters are quite similar to other intermediate spin
ferric complexes.23
Figure 12 exhibits the X-band EPR spectra at 10 K of 7ox
(top) and 8red (bottom) in frozen CH2Cl2 both of which possess
an S ) 1/2 ground state. Interestingly, both 7ox and 8red display
31P hyperfine splitting. Satisfactory simulations were obtained
by using the following parameters: for 7ox: gx ) 2.18, gy )
2.07, gz ) 2.03; S ) 1/2; Axx(31P) ) 1.44 × 10-4 cm-1, Ayy(31P)
) 28.7 × 10-4 cm-1, Azz(31P) ) 22.04 × 10-4 cm-1; 8red: gx
1
) 2.15, gy ) 2.05, gz ) 2.00; S ) /2; hyperfine coupling to
two phosphorus nuclei: Axx(31P) ) 6.0 × 10-4 cm-1, Ayy(31P)
) 33.3 × 10-4 cm-1, Azz(31P) ) 35.0 × 10-4 cm-1. Note that
Sellmann’s EPR spectrum of ([Fe(L)2(PMe3)2]-, S ) 1/2) which
is the equivalent of our 8red did not show 31P hyperfine
splitting.13
EPR Spectroscopy. From magnetic susceptibility data on a
solid sample of 3ox it had been established that this species
possesses an S ) 1/2 ground state.8 As shown in Figure 11 (top),
this is confirmed by X-band EPR spectroscopy. An axial signal
with gx ) gy ) 2.06, and gz ) 2.01 (giso ) 2.04) has been
observed at 10 K in frozen CH2Cl2 solution. This spectrum is
quite typical for an S-centered radical;15 it is not in accord with
a half-filled d orbital at one of the iron centers. A similar but
rhombic signal has been reported for [AuIII(LBu)(LBu•)] (gx )
2.0690, gy ) 2.0320; gz ) 1.911; giso ) 2.004).15
Crystal Structures. The crystal structures of 2, 3ox, 4, 6,
and 9 have been determined or redetermined (6). Crystal-
lographic details are given in Table 1. Table 4 summarizes
important bond distances. The structure of [N(n-Bu)4]2[FeIII
-
2
(L)4] (2) has been determined using a data set collected at 100
K. The room-temperature structure of the tetraethylammonium
3
Compound 4 possesses an S ) /2 ground state; its X-band
(23) (a) Hauser, C.; Bill, E.; Holm, R. H. Inorg. Chem. 2002, 41, 1615. (b)
Hans, M.; Buckel, W.; Bill, E. Eur. J. Biochem. 2000, 267, 7082.
EPR spectrum is shown in Figure 11 (bottom). It displays a
9
5648 J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005

Ligands in Bis(benzene-1,2-dithiolato)iron Complexes
A R T I C L E S
Table 4. Selected Bond Distances (Å)
Complex 2
Fe1-S11
Fe1-S12
Fe1-S1
2.2245(6)
2.2305(6)
2.2315(6)
2.2348(6)
2.4237(6)
2.2225(6)
2.2290(6)
2.2345(6)
2.2367(6)
2.4942(6)
S1-C1
1.764(2)
1.764(2)
1.750(2)
1.759(2)
1.763(2)
1.761(2)
1.758(2)
1.769(2)
S2-C2
S11-C11
S12-C12
S21-C21
S22-C22
S31-C31
S32-C32
Fe1-Fe1*
Fe2-Fe2*
Fe1-S2
Fe1-S1*
Fe2-S22
Fe2-S31
Fe2-S21
Fe2-S32
Fe2-S32*
avg C-C: 1.393 ( 0.01 Å
Complex 3ox
Fe1-S4
Fe1-S3
Fe1-S2
Fe1-S1
Fe1-S2*
Fe2-S8
Fe2-S7
Fe2-S6
Fe2-S5
Fe2-S6*
2.191(4)
2.197(4)
2.215(4)
2.216(4)
2.327(4)
2.184(4)
2.202(4)
2.209(4)
2.220(4)
2.338(4)
S1-C1
1.766(12)
1.778(12)
1.767(13)
1.720(13)
1.763(12)
1.771(14)
1.712(13)
1.730(11)
2.921(4)
S2-C2
S3-C21
S4-C22
S5-C41
S6-C42
S7-C61
S8-C62
Fe1-Fe1*
Fe2-Fe2*
2.851(4)
Complex 4
Fe1-N20
Fe1-S1
Fe1-S12
Fe1-S11
Fe1-S2
2.155(1)
S1-C1
1.755(2)
1.771(2)
1.765(2)
1.760(2)
Figure 12. X-band EPR spectra. Top: spectrum of electrochemically
generated 7ox in CH2Cl2 at 10 K (frequency 9.64 GHz; modulation 10 G;
power 323 µW). Bottom: spectrum of 8red in CH2Cl2 at 10 K (frequency
9.63; modulation 10 G; power 329 µW). Simulation parameters are given
in the text.
2.2264(4)
2.2336(5)
2.2354(4)
2.2526(5)
S2-C2
S11-C11
S12-C12
Complex 6
Fe-S11
Fe1-S2
Fe1-S1
Fe1-S12
Fe1-P20
2.1867(3)
2.1887(3)
2.2894(3)
2.2986(3)
2.3214(3)
S1-C1
1.750(1)
1.748(1)
1.747(1)
1.749(1)
salt has been reported previously.5,6 Complex 2 crystallizes in
the triclinic space group P1h with two crystallographically
independent dinuclear dianions [Fe2(L)4]2- in the unit cell; each
dianion is located on a crystallographic center of symmetry.
The two independent dianions adopt different conformations
in the solid state. Figure 13 (top) shows one of these; a schematic
overlay of the two conformers is shown below. Each five-
coordinate iron ion is in a square pyramidal FeS5 sulfur
environment. The two ligands in each Fe(L)2 unit can then either
adopt a chair or boat conformation. This is a packing effect
because only the [N(n-Bu)4]+ salt shows this effect. The average
C-S bond length of the nonbridging thiolates at 1.760 ( 0.006
Å is long and indicates the presence of closed-shell benzene-
1,2-dithiolates(2-) which in turn requires for the iron ions an
oxidation state of +III. Therefore, the electronic structure of 2
S2-C2
S11-C11
S12-C12
P20-Fe1-P30
159.79(1)
Fe1-P30
C11-C12
C12-C13
C13-C14
C14-C15
C15-C16
C16-C11
2.3228(3)
1.407(2)
1.402(2)
1.388(2)
1.395(2)
1.386(2)
1.399(2)
C1-C2
C1-C6
C2-C3
C3-C4
C4-C5
C5-C6
1.402(2)
1.405(2)
1.398(2)
1.386(2)
1.396(2)
1.385(2)
Complex 9
Fe1-S21
Fe1-S22
Fe1-S1
Fe1-S2
Fe1-P40
2.167(3)
2.190(2)
2.198(2)
2.205(3)
2.284(4)
C1-C1
1.768(8)
1.749(8)
1.751(8)
1.765(8)
S2-C2
S21-C21
S22-C22
is best described as [FeIII2(L)4]2-
.
For our purpose it is important to note that both thiolato ligands
have a somewhat shorter average C-S bond distance at 1.748
( 0.003 Å than is observed in complexes 2 and 4. This may be
interpreted as ligand oxidation; a dianion (L)2- and a monoan-
ionic radical (L•)1- may yield such average C-S distance. All
other features are the same as discussed previously by Sellmann
et al.13
The structure of the monoanion in crystals of 3ox is very
similar to that of the dianion in 2. Again, two crystallographi-
cally independent monoanions are present in the unit cell but
in the same conformation as shown in Figure 14. The quality
of this crystal structure determination is rather low and does
not permit the assignment of the ligand oxidation level(s) or of
the metal centers but the atom connectivity is established.
Crystals of 4 consist of well-separated mononuclear anions
[FeIII(L)2(t-Bu-py)]- and [PMePh3]+ cations (1:1 shown in
Figure 15 (top). The average C-S bond length of 1.763 ( 0.006
Å is long and indicates the presence of two S,S′-coordinated,
closed-shell ligands (L)2-. A neutral 4-tert-butylpyridine is
located in the apical position of a square pyramidal FeS4N-
polyhedron with a central ferric ion.
Figure 15 (bottom) shows the structure of the neutral complex
[FeIII(LBu)(LBu•)(PPr3)] in crystals of 9. In this five-coordinate
species the central iron ion is in a square-based pyramidal
environment of two S,S′-coordinated thiolato derivatives and
an axial phosphine ligand. The structure is very similar to that
reported for 5.13 The quality of the crystals was not excellent
and, therefore, the error of the bond distances is rather large
and the average C-S bond length at 1.758 + 0.02 Å does not
allow to discern between the presence of two (LBu)2- or one
The redetermination13 of the structure of 6 at 100 K has
indeed lowered the estimated standard deviations considerably.
Figure 15 (middle) shows the structure. The neutral complex
possesses a severely distorted octahedral polyhedron comprised
of two S,S′-coordinated benzene-1,2-dithiolate ligands and two
trimethyl phosphines in trans-position relative to each other.
(LBu)2- and one (LBu• 1-. The atom connectivity is established
)
beyond doubt. It is interesting that the room-temperature crystal
structure of neutral [Fe(L)2(PMe3)] displays a short average C-S
bond distance at 1.75 Å13 which may indicate the presence of
one (L)2- and one (L•)1- ligand. This is also bolstered by the
9
J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005 5649

A R T I C L E S
Ray et al.
Figure 13. Structure of one of the two crystallographycally independent
dianions [Fe2(L)4]2- in crystals of 2 (top) and schematic overlay of the two
conformers (bottom).
Figure 15. Structure of the monoanions [FeIII(L)2(t-Bu-py)]- in crystals
of 4 (top), and of the neutral complex [Fe(L)2(PMe3)2] in crystals of 6 and
of the neutral species in crystals of 9.
differences indicative of a localized difference in charge but
both do display frequencies at ∼1100 cm-1 in the infrared
indicating the presence of at least one (L )
Bu• 1- radical. We will
show in a subsequent paper that the square planar complexes
[M(LBu)(LBu•)]- (M ) NiII, PdII, PtII) also exhibit this band.
From density functional calculations it is deduced that the ligand
mixed valency between (LBu)2- and (LBu• 1-
is of delocalized
)
class III. Thus we propose that in complexes 7 and 8 a similar
behavior may be expected.
Mo1ssbauer Spectroscopy. Complexes 1-9 have been care-
fully investigated by applied-field (1-7 T) Mo¨ssbauer spec-
troscopy at 4.2 K and detailed spin Hamiltonian analyses have
been carried out. In addition, zero-field Mo¨ssbauer spectra of
polycrystalline or frozen solution samples were recorded at 80
K. The results are summarized in Tables 5 and 6.
The spectra of the square planar intermediate spin ferrous
species [NH(C2H5)3]2[FeII(L)2] (S ) 1) have been reported and
discussed previously.11,12 The following spin Hamiltonian
parameters were found at 4.2 K: S ) 1, zero-field splitting
Figure 14. Structure of one of the two crystallographically independent
monoanions [Fe2(LBu)4]- (3ox) and schematic overlay of both conformers.
observation that both phenyl rings display a weak quinoidal
distortion (which is not significant within the 3σ experimental
error limit). Thus, the electronic structures of 5 and 9 are
probably to be described as [FeIII(L)(L•)(PMe3)] and [Fe(LBu)-
(LBu•)(PPr3)], respectively.
We point out that the structures of 6 and 9 both display two
equivalent dithiolato ligands; they fail to show structural
9
5650 J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005

Ligands in Bis(benzene-1,2-dithiolato)iron Complexes
A R T I C L E S
Table 5. Zero-Field Mo¨ssbauer Data of Complexes
δ a
,
∆
EQ,b
Γ c
,
mm s-
1
1
1
d
complex
mm s-
mm s-
T, K
SFe
Ste
ref
1
0.44
0.45
0.31
0.36
0.28
0.33
0.29
0.12
0.17
0.29
0.11
0.25
0.15
0.24
0.09
1.20
1.21
2.95
2.87
2.70
3.03
3.02
3.03
1.54
1.67
3.18
2.60
1.46
1.56
3.19
0.28
80
4.2
77
80
80
80
80
1
1
11, 12
0.28
n.m.f
0.44
0.46
0.31
0.29
0.31
0.29
2
3
1.5
1.5
0
0
6
this work
3ox
4
1.5 0.5 this work
1.5 1.5 this work
1.5 1.0 this work
4ox
5
4.2 1.5 1.0 13
86
6
1.5 1.0 13
0.5 0.5 13
6red
7
0.38/0.32 92
0.28
0.36
0.29
0.28
0.30
80
80
80
80
80
1.5 1.0 this work
1.5 0.5 this work
1.5 1.0 this work
0.5 0.5 this work
1.5 1.0 this work
7ox
8
8red
9
a Isomer shift. b Quadrupole splitting. c Half width of signal. d Intrinsic
spin ground state of iron ion. e Spin ground state of molecule. f Not
measured.
D ) +28 cm-1, E/D ) 0.08; isomer shift δ ) 0.45 mm s-1
quadrupole splitting ∆EQ ) +1.21 cm-1, asymmetry parameter
η ) 0.44; nuclear hyperfine coupling constants Axx/gNâN
,
)
-4.15 T, Ayy/gNâN ) -5.58 T, and Azz/gNâN ) +0.18 T. We
will use these data as benchmark for an intermediate spin ferrous
ion.24
Figure 16. Applied field Mo¨ssbauer spectra of 4 in frozen CH3CN solution
at 4.2 K and 0.1, 1, 4, 7 T fields. For fit parameters see Table 6.
Similarly, the data for the five-coordinate anion [FeIII(L)2(t-
complexes where the intramolecular antiferromagnetic spin
coupling does not allow an independent direct determination
of the individual 3/2 spin states by magnetic measurements. We
also note that the reported Mo¨ssbauer parameters for [N(n-
Bu)4]2[FeIII2(dcbdt)4]8 (dcbdt2- ) 4,5-dicyanobenzene-1,2-dithi-
olate(2-)), namely δ ) 0.32 mm s-1; ∆EQ ) 3.01 mm s-1 at
75 K, are again identical to those mentioned above. In our view
Bu-py)]- in 4 may serve as benchmark values for an intermedi-
3
ate spin ferric ion (SFe ) /2);25-29 see Tables 5 and 6. The
Mo¨ssbauer spectra were recorded on frozen CH3CN solutions
of 4 in order to minimize possible intermolecular magnetic
interactions. Figure 16 displays the spectra and their simulations
at 0.1, 1, 4, and 7 T. These values are reassuringly similar to
those reported for other five-coordinate, intermediate spin ferric
complexes25-28 (Table 6). Hallmarks are a quite large quadru-
pole splitting of >3.0 mm s-1 as well as two major negative
components and a minor positive one of the A tensor. This
anisotropy is, in general, attributed to the spin dipolar term.
Since the latter is traceless, the contact contribution can be
obtained by averaging the principal components of A. The value
AFe/gNâN of -16.6 T compares also well with other five-
coordinate intermediate spin ferric complexes.25-29 The nature
of the donor atoms (sulfur or nitrogen) does not appear to play
a dominant role. These data give us confidence to assign an
intermediate spin ferric electronic structure in complexes by
applied-field Mo¨ssbauer spectroscopy.
1
this rules out an interpretation as low-spin ferric (SFe ) /2) as
was proposed by the authors.7
Up to this point, the oxidation states of the iron ions and
their spin ground states are unequivocally determined by
magnetic susceptibility measurements, X-ray crystallography,
and Mo¨ssbauer spectroscopy. They are in all cases fairly
straightforwardly determined because the thiolato ligands are
always redox-innocent, i.e., they are S,S′-coordinated closed-
shell dianions. This is not so for the one-electron oxidized form
1
of 3, namely the dinuclear monoanion with S ) /2 in crystals
of 3ox. Simple charge considerations immediately imply that in
case of a metal-centered one-electron oxidation a mixed valent
FeIII-FeIV monoanionic species with four benzene-1,2-dithi-
olato(2-) ligands is generated or, alternatively, if a ligand-
centered oxidation prevails, an [FeIII2(LBu)3(LBu•)]- species is
formed with two intermediate spin ferric ions and three (LBu)2-
For example, the diamagnetic dinuclear complexes 2 and 3
exhibit each a single quadrupole doublet with isomer shift and
quadrupole splitting parameters which are nearly identical to
those of 4 (Table 5). Therefore, we feel confident to assign local
ligands plus one radical (L )
Bu• 1-. The zero-field Mo¨ssbauer
3
spin states of /2 to each of two ferric ions in both dinuclear
spectrum of 3ox displays a single quadrupole doublet which rules
out the mixed valent FeIIIFeIV formulation. The isomer shift and
quadrupole splitting parameters for 3ox are in excellent agree-
ment with those of 2, 3, and 4 indicating the presence of two
intermediate spin ferric ions. Thus the oxidation of 3 to 3ox is
ligand-centered.
(24) (a) Barraclough, C. G.; Martin, R. L.; Mitra, S.; Sherwood, R. C. J. Chem.
Phys. 1970, 53, 1643. (b) Hudson, A.; Whitefield, H. J. Inorg. Chem. 1967,
6, 1120. (c) Moss, T. H.; Robinson, A. B. Inorg. Chem. 1968, 7, 1692.
(25) Niarchos, D.; Kostikas, A.; Simopoulos, A.; Coucouvanis, D.; Piltingsrud,
D.; Coffman, R. E. J. Chem. Phys. 1978, 69, 4411.
(26) Kostka, K. L.; Fox, B. G.; Hendrich, M. P.; Collins, T. J.; Rickard, C. E.;
Wright, L. J.; Mu¨nck, E. J. Am. Chem. Soc. 1993, 115, 6746.
Interestingly, one-electron oxidation of mononuclear 4 yield-
ing neutral 4ox is also not accompanied by significant changes
of the isomer shift or quadrupole splitting parameters (Table
5). Thus, a frozen CH3CN solution at 80 K of 4 to which 1
equivalent of the oxidant tris(4-bromophenyl)aminium hexachlo-
(27) Keutel, H.; Ka¨pplinger, I.; Ja¨ger, E.-G.; Grodzicki, M.; Schu¨nemann, V.;
Trautwein, A. X. Inorg. Chem. 1999, 38, 2320.
(28) Koch, W. O.; Schu¨nemann, V.; Gerdan, M.; Trautwein, A. X.; Kru¨ger,
H.-J. Chem. Eur. J. 1998, 4, 686.
(29) (a) Gupta, G. P.; Lang, G.; Scheidt, W. R.; Geiger, D. K.; Reed, C. A. J.
Chem. Phys. 1986, 85, 5212. (b) Gupta, G. P.; Lang, G.; Reed, C. A.;
Shelly, K.; Scheidt, W. R.; Geiger, D. K. J. Chem. Phys. 1987, 86, 5288.
9
J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005 5651

A R T I C L E S
Ray et al.
roantimonate had been added at -30 °C shows a single
quadrupole doublet: δ ) 0.29 mm s-1, ∆EQ ) 3.02 mm s-1
for 4ox. This indicates again a ligand-centered oxidation and
the electronic structure of 4ox is best described as [FeIII(LBu)-
(LBu•)(t-Bu-py)] S ) 1 with an intermediate spin ferric ion (SFe
) 3/2) coupled antiferromagnetically to a ligand radical (LBu• 1-
) .
The square-based pyramidal complex 513 is isoelectronic with
4ox; only the axial trimethylphosphine in 5 is substituted by a
4-tert-butylpyridine ligand in 4ox. It is therefore not surprising
that both compounds have similar zero-field Mo¨ssbauer param-
eters. The quadrupole splitting is 3.03 mm s-1 for 5 and 3.02
mm s-1 for 4ox at 80 K; the isomer shifts differ slightly by ∆δ
) 0.17 mm s-1 where δ is 0.29 mm s-1 for 4ox and 0.12 mm
s-1 for 5. Such a shift of δ has previously been shown to also
occur in isoelectronic [FeII(pyS4)X] complexes30 where X is
either a bridging thiolate (δ ) 0.44 mm s-1) or a trimethylphos-
phine (δ ) 0.34 mm s-1). The Mo¨ssbauer parameters for five-
coordinate 9 are also very similar to those of 5 and 4ox. Thus,
all of these species possess an intermediate spin central ferric
3
ion (S ) /2) and one (LBu)2- or (L)2- as well as one radical
monoanion (LBu• 1- or (L•)1- affording the observed S ) 1
)
Figure 17. Applied field Mo¨ssbauer spectra of 8 at 4.2 K and 1, 4, 7 T
fields. For fit parameters see Table 6.
ground states: [FeIII(L)(L•)(PMe3)] (5), [FeIII(L)(L•)(t-Bu-py)]
(4ox), and[FeIII(LBu)(LBu•)(PPr3)] (9). Complex 7 is also isoelec-
tronic and “isostructural” with 5 and the Mo¨ssbauer parameters
are again nearly identical (Tables 5 and 6). The electronic
structure of 7 is therefore [FeIII(LBu)(LBu•)(PMe3)] with a central
intermediate spin ferric ion coupled antiferromagnetically to a
be described as Fe(IV) species. The Mo¨ssbauer data for
Sellmann’s13 complex [NMe4][FeIII(L)2(PMe3)2]‚CH3OH (δ )
0.29 mm s-1; ∆EQ ) 1.67 mm s-1 at 92 K) are in excellent
agreement with our frozen solution data for 8red
.
ligand radical (LBu• 1-
affording the observed S ) 1 ground
)
Discussion
state. The zero-field Mo¨ssbauer spectrum of the frozen CH3-
1
CN solution of the one-electron oxidized form 7ox (S ) /2)
The most important goal of the present investigation is to
establish unequivocally the electronic structure of some five-
coordinate iron complexes containing two S,S′-coordinated
benzene-1,2-dithiolato type ligands and a fifth (axial) phosphine
ligand, [Fe(L)2(PR3)], in a square-base pyramidal FeS4P donor
atom set. The problem rests entirely on the ability to identify
spectroscopically a square planar [FeIII(L)(L•)]0 vs an [FeIV(L)2]0
moiety. In other words, the question is whether benzene-1,2-
dithiolates behave here as redox-noninnocent ligands with the
sulfur analogues of a catecholate or benzosemiquinonate radical
being present in a given coordination compound.
displays also a single quadrupole doublet (δ ) 0.25 mm s-1
;
∆EQ ) +2.60 mm s-1). These parameters again indicate the
presence of an intermediate spin ferric ion but now two ligand
1
radicals must be present yielding the observed S ) /2 ground
state: [FeIII(LBu•)2(PMe3)]+. The alternative description13 as FeV
species, namely [FeV(LBu)2(PMe3)]+, is not at all compatible
with the spectroscopic data presented here. In this case it is
again observed that one-electron oxidation of [FeIII(LBu)(LBu•)-
(PMe3)] (7) with S ) 1 ground-state yielding [FeIII(LBu•)2-
(PMe3)]+ with S ) 1/2 is a ligand-centered process; both species
contain a central intermediate spin ferric ion.
Many authors7,13 have attempted to discern between a
dithiolato(2-) and its monoanionic radical form in a given
complex by their crystallographically determined structural
parameters. The underlying assumption has always been that
the phenyl ring has either six equidistant C-C bonds in a
thiolate or displays a quinoidal distortion with two alternating
shorter CdC bonds and four longer ones in the radical form
and, in addition, the C-S bonds were assumed to shrink upon
oxidation of a dianionic thiolate to a monoanionic radical. This
behavior had been firmly established for the corresponding
complexes containing catecholates and semiquinonates.31 This
issin partsa misconception for the sulfur analogues. The
structures of the phenoxyl and the thiyl radical have been
calculated by theoretical computational methods.32,33 The results
clearly show that the expected structural changes in comparison
The applied field Mo¨ssbauer spectra of distorted octahedral
8 are shown in Figure 17; the results of the spin Hamiltonian
analysis are given in Table 6. The parameters are very similar
to those reported for all intermediate spin ferric complexes with
the exception of a smaller quadrupole splitting parameter of
only +1.46 mm s-1 as compared to an average value of 3.0
mm s-1 for the five-coordinate intermediate spin complexes 4,
7, and 7ox. Interestingly, Kru¨ger et al.28 have recently reported
a similar, octahedral ferric complex [FeIII(L-N4Me2)(S2C6H4)]-
3
(ClO4)‚0.5H2O (SFe ) /2; δ ) 0.38 mm s-1, ∆EQ ) 2.22 mm
s-1 at 200 K) which also exists in a low spin ferric form (SFe
)
1/2; δ ) 0.25 mm s-1, ∆EQ ) 2.22 mm s-1 at 200 K). The
latter Mo¨ssbauer parameters agree nicely with those measured
for 8red in Table 5. Thus, the electronic structures are best
3
descibed as intermediate spin ferric (SFe ) /2) in 8 and low
spin ferric in 8red
:
[FeIII(LBu•)(LBu)(PMe3)2] (S ) 1) and
(31) Pierpont, C. G.; Lange, C. W. Progr. Inorg. Chem. 1994, 33, 331.
(32) (a) Qin, Y.; Wheeler, R. A. J. Am. Chem. Soc. 1995, 117, 6083. (b) Himo,
F.; Gra¨slund, A.; Erikson, L. A. Biophys. J. 1997, 72, 1556. (c) Adamo,
R.; Subra, A.; Di Matteo, A.; Barone, V. J. Chem. Phys. 1998, 109, 10244.
(33) Tripathi, G. N. R.; Sun, Q.; Armstrong, D. A.; Chipman, D. M.; Schuler,
R. H. J. Phys. Chem. 1992, 96, 5344.
[FeIII(LBu)2(PMe3)2]- (S ) 1/2), respectively. Complex 8 cannot
(30) Li, M.; Bonnet, D.; Bill, E.; Neese, F.; Weyhermu¨ller, T.; Blum, N.;
Sellmann, D.; Wieghardt, K. Inorg. Chem. 2002, 41, 3444.
9
5652 J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005

Ligands in Bis(benzene-1,2-dithiolato)iron Complexes
A R T I C L E S
Table 6. Applied Field Mo¨ssbauer Parameters of Intermediate Spin Ferric Complexes at 4.2 K
e
δ
∆
EQ
A
xx/gNâN
(T)
A
yy/gNâN
(T)
A
zz/gNâN
(T)
A /gNâN
(T)
AFed/gNâN
(T)
D
(cm-
DFe
1
1
1
1
complex
St
(mm s-
)
(mm s-
)
)
E/D
(cm-
)
h
3ox
0.5
1.5
1.0
0.5
1.0
1.5
1.5
0.27
0.34
0.12
0.29
0.17
0.25
0.19
2.67
2.94
3.05
2.57
1.46
3.60
3.56
+2.5
-19.13
-32.43
-31.10
1.56
-22
-12.7
-25.1
-28.6
1.54
0.35
1.68
-38.22
5
-17.1
-16.61
-18.08
-20.72
-16.34
-11.8
-8.3
0.3
4a
-32.24
-22.16
-32.74
-12.35
-16.6
-16.61
-14.46f
-8.31g
-13.07f
-11.8
-3.80
-28.0
0.33
0.01
-3.80
-18h
0.02
0.74
0.45
0.10
0.15
0.0
7
7ox
8
14.35
-3.70
13
0.01
0.05
0.22
9.0h
-3.7
13
L′FeClb
LFeIc
-12.7
0.5
-8.3
f
g
a Frozen acetonitrile solution. b See ref 26. c See ref 27. d Intrinsic A component for SFe
)
3/2. e Intrinsic D value for SFe
)
3/2. AFe
)
4/5 A . AFe
)
h
2
2/5 A . DFe ) /3D.
Table 7. Compilation of Intrinsic Mo¨ssbauer Parameters for Five-Coordinate Complexes in Figure 18
b
1
1
complex
Sta
SFe
δ
c (mm s-
)
∆
EQd (mm s-
)
A(SFe
)
3/2)/gNâNe (T)
ref
3
3
[FeIII(L)2(t-Bu-py)]1-
[FeIII(LOISQ•)2I]
/
/
0.34
0.24
0.12
0.12
0.29
0.19
0.25
+2.94
+2.80
3.18
+3.05
+2.57
3.56
-19.1, -32.2, +1.54
-12.4, -20.3, +1.3
-7.4, -16.0, +0.6
-25.9, -17.7, +0.3
-12.44, -13.1, +0.68
-12.7, -12.7, +0.5
-22, -16.6, +5
this work
36
36
this work
this work
27
2
2
1
3
3
3
3
3
3
/
/
/
/
/
/
/
2
2
[FeIII(LSISQ)2I]
/
1
2
2
2
2
2
2
[FeIII(LBu)(LBu•)(PMe3)]0
[FeIII(LBu•)2(PMe3)]+
[FeIII(L)I]
1
1
/
2
3
/
2
3
[FeIII(L′)Cl]
/
3.60
26
2
a Ground state of molecule. b Intrinsic ground state of iron ion. c Isomer shift at 4.2 K relative to R-Fe at 298 K. d Quadrupole splitting at 4.2 K. e A(SFe
3
) /2): tensor components from spin projection techniques where appropriate.
with their closed-shell phenolate and thiophenolate forms are
only observed for the phenoxyl but not the thiyl. This observa-
tion is due to the fact that the spin density distribution of both
radicals differ. In a phenoxyl only 38% of the spin density
resides on the oxygen atom, 24% is found at each of the two
ortho carbon atoms and 32% is at the para carbon. In contrast,
in the thiyl radical 68% of the spin density resides at the sulfur
atom and only 32% is delocalized over the ortho and para
positions of the phenyl ring. Consequently, the six C-C
distances in a thiyl radical resemble more closely those of the
aromatic thiolate analogue but the C-S bond is slightly shorter
at 1.75 Å. Therefore, structural parameters may be of only
limited value for the identification of a benzene-1,2-dithiolate-
(1-) radical in a given complex. On the other hand, in the series
[NiII(LBu)2]2-/1-/0 the neutral species [NiII(LBu•)2]0 contains two
infrared spectra of 5 and 6 at 1084 cm-1, respectively, as very
weak bands. We note that in [N(CH3)4][FeIII(L)2(PMe3)2] this
band is not observed. From DFT calculations22 of the vibrational
spectra of such complexes which will be reported elsewhere
we are able to show that complexes containing 3,5-di-tert-butyl-
1,2-benzenedithiolate(1-) radicals do exhibit the ν(CdS•) band
with significant intensity whereas the same vibration is less
intense by at least a factor of ∼10 in the corresponding
complexes with unsubstituted (L•)1- ligands.
The second important spectroscopic marker for the presence
of an M(LBu)(LBu•) and/or M(L)(L•) moiety is the observation
of an intense (∼104 M-1 cm-1) intervalence transition M(L)-
(L•) T M(L•)(L) in the near-infrared. These intense bands with
intensities >0.5 × 104 M-1cm-1 cannot be assigned as
transitions from the π-orbital levels of the isolated radical
ligands. Complex [(bpy)PtII(LBu•)]+ displays such an intraligand
π- π* transition14 at 700 nm but with an absorption coefficient
of only 1.0 × 103 M-1cm-1. A similar ligand-to-ligand band is
observed for all square planar complexes [M(L•)2]n.35 Thus, this
band has been reported for [AuIII(LBu)(LBu•)]0 at 1452 nm (ꢀ )
(L
different from those of the dianion [NiII(LBu)2]2- 34
As pointed out by us previously15 in complexes containing
the radical anion (L
Bu• 1-, the ν(CdS•) stretching frequency is
)
Bu• 1- radicals and here the structural parameters are distinctly
.
)
clearly observed as a relatively intense band in the infrared at
1100 cm-1. This band is absent in the corresponding complexes
containing only (LBu)2- ligands. This is in accord with a
resonance Raman study of the phenylthiyl radical for which
the predominantly ν(CdS•) mode has been observed at 1073
cm-1 (ν7a Wilson mode).33 We propose that this band is a
significant spectroscopic marker for the presence of an S,S′-
2.7 × 104 M-1 cm-1 15
)
and for [NiII(LBu)(LBu•)]- at 860 nm (ꢀ
) 1.2 × 104 M-1 cm-1).34 These bands are absent in their one-
electron reduced forms [AuIII(LBu)2]- and [NiII(LBu)2]2-. In the
present series of iron complexes we noted that all complexes
containing exclusively two (LBu)2- or (L)2- ligands do not
display absorption maxima of intensities >500 M-1 cm-1 above
600 nm (complexes 1, 2, 3, 4, 6red, and 8red). Their respective
electronic structure should therefore be described as shown in
Scheme 1 with dianionic aromatic ligands. In contrast, com-
plexes 3ox (860 nm (0.8 × 104 M-1 cm-1)), 4ox (814 nm (0.7 ×
104 M-1 cm-1)), 5 (1210 nm (0.7 × 104), 1510 nm (0.8 × 104
M-1 cm-1)),13 7/8 (890 nm (0.2 × 104 M-1 cm-1)), display
this absorption and, therefore, are proposed to contain a [FeIII-
(LBu)(LBu•)] or a [FeIII(L)(L•)] moiety. The presence of [FeIV-
coordinated (LBu• 1-
radical. As noted above, the present
)
complexes 3ox, 7, and 8 display such a band.
Sellmann et al.13 had pointed out that in the infrared spectra
of 5 and 6 containing unsubstituted (L)2- ligands no ν(CdS•)
bands are observed. In fact, this observation had been used to
propose the presence of only (L)2- ligands and, consequently,
iron(IV) metal centers in 5 and 6. We have verified their
observation: 5 and 6 do not display ν(CdS•) bands with
appreciable intensity but they are nevertheless present in the
(35) (a) Herebian, D.; Bothe, E.; Neese, F.; Weyhermu¨ller, T.; Wieghardt, K.
J. Am. Chem. Soc. 2003, 125, 9116. (b) Herebian, D.; Wieghardt, K.; Neese,
F. J. Am. Chem. Soc. 2003, 125, 10997.
(34) Sellmann, D.; Binder, H.; Ha¨ussinger, D.; Heinemann, F. W.; Sutter, J.
Inorg. Chim. Acta 2000, 300-302, 829.
9
J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005 5653

A R T I C L E S
Ray et al.
(Tables 5,6). When the measured A-tensor components Aii of 7
1
(S ) 1), 7ox (S ) /2)), and 8 (S ) 1) in Table 6 are converted
3
to intrinsic values for SFe
)
/
by using spin-projection
2
3
techniques, one obtains the intrinsic A(SFe ) /2)/gNâN values
in Table 6. The values obtained agree nicely with those reported
for well-characterized five-coordinate intermediate-spin ferric
complexes with redox-innocent ligands (Table 7). In particular,
the similarity of the pronounced A anisotropies with the unusual
positive z component is significant. Furthermore, large quad-
rupole splittings at ∼3 mm s-1 and a positive electric field
gradient (EFG) component Vzz along the minor component of
the A-tensor were recognized as additional characteristic features
of intermediate-spin iron(III). We have previously reported
detailed analyses of the Mo¨ssbauer spectra of complexes [FeIII-
1-
(L•rad)2X] where (L•rad
)
represents an o-iminobenzosemi-
quinonate or its thio analogue (Figure 18 and Table 7).36 From
these spectroscopic data (Tables 6 and 7) it is evident that
complexes 5-9 contain an intermediate spin ferric ion and one
antiferromagnetically coupled ligand radical (L•)1- or (LBu• 1-
) .
It is not possible to interpret the Mo¨ssbauer data in Tables 5
and 6 consistently by assuming the presence of high valent iron-
(IV) ions.
Figure 18. Five-coordinate complexes containing none, one, and two π
radical ligands and a central intermediate spin ferric ion (SFe ) 3/2) yielding
Acknowledgment. We thank Dr. Ameerunisha Begum for
the synthesis and characterization of 2. We are grateful to the
Fonds der Chemischen Industrie for financial support, and K.R.
thanks the Max-Planck Society for a stipend.
3
1
St ) /2, St ) 1, and St ) /2 ground states, respectively.
(L)2] is ruled out. Complex 7ox is proposed to be [FeIII(LBu•)2-
(PMe3)]+ with two (L
)
Bu• 1- radical ligands (894 nm (0.4 × 104
Supporting Information Available: X-ray crystallographic
tables for complexes. This material is available free of charge
via the Internet at http://pubs.acs.org.
M-1 cm-1).
The third very valuable marker for a correct assignment of
the electronic structure of these complexes is zero- and applied-
field 57Fe Mo¨ssbauer spectroscopy. The spectra for mononuclear
JA040237L
3
4 (St ) /2) represent the benchmark for a pure intermediate
(36) Ghosh, P.; Bill, E.; Weyhermu¨ller, T.; Wieghardt, K. J. Am. Chem. Soc.
2003, 125, 3967.
spin ferric ion in a square base pyramidal ligand environment
9
5654 J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005