Electronic structure of nitric oxide adducts of bis(diaryl-1,2-dithiolene) iron compounds: Four-membered electron-transfer series [Fe(NO)(L) 2]z (z = 1+, 0, 1-, 2-)

Article abstract of DOI:10.1021/ic061874a

Four members of the electron-transfer series [Fe(NO)(S₂C ₂R₂)₂]^z (z = 1+, 0, 1-, 2-) have been isolated as solid materials (R = p-tolyl): [1a](BF₄), [1a] ⁰, [Co(Cp)₂][

Full text of DOI:10.1021/ic061874a

Inorg. Chem. 2007, 46, 522−532
Electronic Structure of Nitric Oxide Adducts of
Bis(diaryl-1,2-dithiolene)iron Compounds: Four-Membered
Electron-Transfer Series [Fe(NO)(L)2]z (z
) 1+, 0, 1−, 2−)
Prasanta Ghosh,† Keira Stobie,‡ Eckhard Bill,† Eberhard Bothe,† Thomas Weyhermu
1
ller,†
Michael D. Ward,§ Jon A. McCleverty,‡ and Karl Wieghardt*,†
Max-Planck-Institut fu¨r Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mu¨lheim an der
Ruhr, Germany, School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, United Kingdom, and
Department of Chemistry, UniVersity of Sheffield, Sheffield S3 7HF, United Kingdom
Received September 29, 2006
Four members of the electron-transfer series [Fe(NO)(S2C2R2)2]z (z
materials (R
p-tolyl): [1a](BF4), [1a]0, [Co(Cp)2][1a], and [Co(Cp)2]2[1a]. In addition, complexes [2a]0 (R
diphenyl), [3a]0 (R
p-methoxyphenyl), [Et4N][4a] (R phenyl), and [PPh4][5a] (R ) −CN) have been synthesized
)
1+, 0, 1−, 2−) have been isolated as solid
)
)
4,4-
)
)
and the members of each of their electron-transfer series electrochemically generated in CH2Cl2 solution. All species
have been characterized electro- and magnetochemically. Their electronic, Mo¨ssbauer, and electron paramagnetic
resonance spectra as well as their infrared spectra have been recorded in order to elucidate the electronic structure
of each member of the electron-transfer series. It is shown that the monocationic, neutral, and monoanionic species
6
possess an
(S
generates a dianion with an
(S 1/2).
{
FeNO
}
(S
)
T
0) moiety where the redox chemistry is sulfur ligand-based, (L)2-(L•)1-: [Fe(NO)(L•)2]+
)
0), [Fe(NO)(L•)(L)]0
[Fe(NO)(L)(L•)]0 (S
)
1/2), [Fe(NO)(L)2]- (S
)
0). Further one-electron reduction
-
{
FeNO )
}
7 (S 1/2) unit and two fully reduced, diamagnetic dianions L2-: [Fe(NO)(L)2]2
)
Introduction
McCleverty and co-workers established in a series of
articles1-3 that five-coordinate NO adducts of bis(dithiolene)-
iron complexes exist as a maximally five-membered
electron-transfer series [Fe(NO)(dithiolene)2]z (z ) 1+, 0,
1-, 2-, 3-).
In contrast, the corresponding 1,2-benzenedithiolate series,
[Fe(NO)(bdt)2]z, consists only of two stable species, namely,
the mono- and dianion (z ) 1- or 2-) where salts of both
anions have been structurally characterized by X-ray crystal-
lography.4 None of the above bis(dithiolene)iron complexes
has been structurally characterized with the exception of
[Et4N][Fe(NO)(mnt)2]5 and [Et4N]2[Fe(NO)(mnt)2],6 where
the latter structure is of low quality (mnt2- ) maleonitrile-
1,2-dithiolate).
Despite the fact that for a number of neutral or anionic
species the Mo¨ssbauer,7,8 EPR, and UV-vis spectra have
been recorded and reported,1-3 it is surprising that the
electronic structures of these compounds have not yet been
studied in detail. The problem is that all three components
of these species can, in principle, be redox active: that is
* To whom correspondence should be addressed. E-mail: wieghardt@
mpi-muelheim.mpg.de.
† Max-Planck-Institut fu¨r Bioanorganische Chemie.
‡ University of Bristol.
§ University of Sheffield.
(1) Locke, J.; McCleverty, J. A.; Wharton, E. J.; Winscom, C. J. J. Chem.
Soc., Chem. Commun. 1966, 677.
(2) McCleverty, J. A.; Atherton, N. M.; Locke, J.; Wharton, E. J.;
Winscom, C. J. J. Am. Chem. Soc. 1967, 89, 6082.
(3) McCleverty, J. A.; Ratcliff, B. J. Chem. Soc. (A) 1970, 1627.
(4) Lee, C.-M.; Chen, C.-H.; Chen, H.-W.; Hsu, J.-L.; Lee, G.-H.; Liaw,
W.-F. Inorg. Chem. 2005, 44, 6670.
(5) Chatel, S.; Chauvin, A.-S.; Tuchagues, J.-P.; Leduc, P.; Bill, E.;
Chottard, J.-C.; Mansuy, D.; Artaud, J. Inorg. Chim. Acta 2002, 336,
19.
(6) Rae, A. I. Chem. Commun. 1967, 1245.
(7) Birchall, T.; Greenwood, N. N.; McCleverty, J. A. Nature 1967, 215,
625.
(8) Birchall, T.; Greenwood, N. N. J. Chem. Soc. (A) 1969, 286.
522 Inorganic Chemistry, Vol. 46, No. 2, 2007
10.1021/ic061874a CCC: $37.00
© 2007 American Chemical Society
Published on Web 12/24/2006

Adducts of Bis(diaryl-1,2-dithiolene)iron Compounds
Chart 1
Chart 2
(a) the NO ligand is the archetypal noninnocent ligand which
can be bound to an iron ion as NO+ (S ) 0), NO (S ) 1/2),
or NO- (S ) 0 or 1), (b) the central iron ion can possess a
d6 (SFe ) 0, SFe ) 1, SFe ) 2) or d5 (SFe ) 1/2, 3/2, 5/2)
electron configuration, and (c) the dithiolene ligands may
be coordinated as closed-shell dianion (SL ) 0) or as
π-radical monoanion (SL ) 1/2).
In order to avoid the assignment of a formal oxidation
state to the iron ion and a charge on the coordinated nitrosyl
ligand Enemark and Feltham introduced the {FeNO}n
notation where n corresponds to the familiar number of d
electrons on the metal when the nitrosyl ligand is formally
(for the sake of electron bookkeeping) considered to be bound
as NO+.9 In concert these three sources of redox activity
will determine the overall electronic structure and spin
ground state of each member of the electron-transfer series
[Fe(NO)(dithiolene)2]z complexes (z ) 1+/0/1-/2-).
plexes, and their mono- and dianions. This requires deter-
mination of n in the {FeNO}n moieties and the oxidation
level of the corresponding two dithiolene ligands. For this
purpose the νNO stretching frequencies, the Mo¨ssbauer
parameters of all complexes, and the EPR spectra of the
paramagnetic neutral and dianionic forms have been mea-
sured.
Experimental Section
Syntheses. The dinuclear complexes 1,10 4,11 and 512 and the
mononuclear nitrosyl complexes [3a], [Et4N][4a],2 and [Et4N] [5a]2
have been prepared according to literature procedures. We note
that neutral [1a] has also been prepared previously.3
In this study we used the ligands shown in Chart 1 which
can exist either as closed-shell dianions (Lx)2- or in their
one-electron oxidized forms as π radical monoanion (Lx•)1-.
The two forms are known to possess distinctly different C-S
and “olefinic” C-C bond lengths (see above). Therefore, it
should be possible to determine the oxidation level of the
dithiolene ligands in a given complex by high-quality X-ray
crystallography.
We prepared and isolated four new members of the nitrosyl
iron dithiolene electron-transfer series (Chart 2): [Fe(NO)-
(L1•)2]+ (S ) 0), [1a]+; [Fe(NO)(L1•)(L1)]0 T [Fe(NO)(L1)-
(L1•)]0 (S ) 1/2), [1a]0; [Co(Cp)2][Fe(NO)(L1)2] (S ) 0),
[1a]1-; [Co(Cp)2]2[Fe(NO)(L1)2] (S ) 1/2), [1a]2-. Com-
plexes [2a]0, [3a]0, [Et4N][4a], and [PPh4][5a] have been
synthesized according to literature procedures (see Experi-
mental Section).
[FeIII(L2•)(L2)]2 (2). To a suspension of 4,4′-diphenylbenzoin (4.0
g; 10 mmol) in 1,4-dioxan (80 mL) solid P4S10 (4.0 g) was added
and heated to reflux under argon for 2 h with stirring. The color of
the solution turned orange. It was cooled to room temperature and
filtered to remove excess P4S10. To the resulting orange solution
was added an aqueous solution (10 mL) of FeCl2‚4H2O (1.0 g; 5
mmol). After heating to reflux under argon for 2 h, the solution
turned dark green and black microcrystals separated out which were
filtered off and washed successively with methanol, water, and
acetonitrile and dried in air. Yield: 1.6 g (40% with respect to
iron). Anal. Calcd for C104H72S8Fe2: C, 73.91; H, 4.29; S, 15.18;
Fe, 6.61. Found: C, 73.9; H, 4.5; S, 14.9; Fe, 7.1. UV-vis (CH2-
Cl2): λmax(ꢀ) ) 356 (19 300), 625 nm (5582 L mol-1 cm-1).
[FeIII(L3•)(L3)]2 (3). A mixture of 4,4′-dimethoxybenzoin (4.00
g; 15 mmol) and P4S10 (4.00 g) in xylene (40 mL) was heated to
reflux for 2 h, during which time the yellow mixture became deep
It is the aim of this work to establish unequivocally the
electronic structures of the monocations, the neutral com-
(10) Patra, A. K.; Bill, E.; Weyhermu¨ller, T.; Stobie, K.; Bell, Z.; Ward,
M. D.; McCleverty, J. A.; Wieghardt, K. Inorg. Chem. 2006, 45, 6541.
(11) Schrauzer, G. N.; Mayweg, V. P. J. Am. Chem. Soc. 1965, 87, 1483.
(12) Schrauzer, G. N.; Mayweg, V. P.; Heinrich, W. Inorg. Chem. 1965,
4, 1615.
(9) (a) Enemark, J. H.; Feltham, R. D. Coord. Chem. ReV. 1974 13, 339.
(b) Feltham, R. D.; Enemark, J. H. Topics in Stereochemistry;
Geoffroy, G., Ed.; J. Wiley & Sons: New York, 1981; Vol. 12, p
155.
Inorganic Chemistry, Vol. 46, No. 2, 2007 523

Ghosh et al.
Table 1. Crystallographic Data for [1a], [2a]‚0.75 CH2Cl2, and [Co(Cp)2][1a]
[1a]
[2a]‚0.75CH2Cl2
[Co(Cp)2][1a]
formula
fw
C32H28FeNOS4
626.64
C52.75H37.50Cl1.50FeNOS4
938.60
C42H38CoFeNOS4
815.75
space group
a, Å
b, Å
c, Å
â, deg
V, Å
Pbcn, No. 60
17.0132(5)
21.5449(6)
16.2040(5)
90
C2/c , No. 15
18.7178(4)
23.4237(6)
21.2816(6)
95.555(6)
9286.9(4)
8
P21, No. 4
8.7605(3)
14.7549(5)
15.0400(6)
91.709(4)
1943.21(12)
2
5939.5(3)
8
Z
T, K
F
100(2)
1.402
100(2)
1.343
100(2)
1.394
calcd, g cm-3
reflns collected/2Θmax
unique reflns/I > 2σ(I)
no. of params/restraints
λ, Å /µ(KR), cm-1
44408/60.00
8646/7083
359/0
0.71073/8.16
0.0666/1.202
0.1236
81627/60.00
13473/12442
569/7
0.71073/6.30
0.0504/1.052
0.1374
32259/63.00
12751/12231
455/1
0.71073/10.48
0.0274/1.044
0.0631
R1a/goodness of fitb
wR2c (I > 2σ(I))
residual density, e Å-3
+0.80/-0.51
+1.94/-1.49
+0.39/-0.31
a Observation criterion: I > 2σ(I). R1 ) ∑||Fo| - |Fc||/∑|Fo|. b GooF ) [∑[w(Fo Fc )2]/(n - p)]1/2
.
c wR2 ) [∑[w(Fo Fc )2]/∑[w(Fo )2]]1/2, where
2 -
2
2 -
2
2
w ) 1/σ2(Fo ) + (aP)2 + bP, P ) (Fo + 2Fc )/3.
2
2
2
red in color. After cooling to room temperature, a solution of iron-
(II) sulfate (4.00 g; 14 mmol) dissolved in water/methanol (1:1 v/v,
30 mL) was added gradually. During additional heating at reflux
for 2 h, a black solution formed which was left overnight at 20 °C.
The black solid formed was collected by filtration, washed with
methanol and diethyl ether, and allowed to air dry.
[Fe(NO)(L1•)2](BF4) ([1a](BF4)). To a stirred suspension of
dinuclear [FeIII2(L1•)2(L1)2] (1) (200 mg; 0.16 mmol) in CH2Cl2 (40
mL) was added [NO](BF4) (38 mg; 0.33 mmol) under an Ar
blanketing atmosphere. Within 3 h of stirring at 20 °C the color of
the green solution turned brown. The solvent was removed by
evaporation under reduced pressure, and the resulting brown
material was washed with n-hexane (3×) and dried under Ar.
Yield: 145 mg (60%). ESI mass spectrum (CH2Cl2) positive-ion
mode: m/z ) 626.1, calcd for {Fe(NO)(L1)2}+ 626.6 and negative-
ion mode m/z ) 87.2 (BF4)-. Anal. Calcd for C32H28NOBF4S4Fe:
C, 53.87; H, 3.96; N, 1.96; S, 17.97; Fe, 7.83. Found: C, 53.7; H,
4.1; N, 1.9; S, 17.8; Fe, 7.7.
[Fe(NO)(L1•)(L1)] ([1a]0). Argon was bubbled through a vigor-
ously stirred suspension of 1 (200 mg; 0.16 mmol) in CH2Cl2 (40
mL) for 5 min. Then NO gas was bubbled through the solution for
80-90 s, after which time the flow of Ar was retained for 10 min.
The color of the solution turned dark brown. Anhydrous methanol
(10 mL) was added, and the reaction volume was reduced to one-
half by slow evaporation of the solvent(s) in an Ar stream. After
allowing this solution to stand for 3-4 days at 20 °C, black crystals
precipitated which were filtered off and dried in air. Yield: 160
mg (77%). Anal. Calcd for C32H28NOS4Fe: C, 61.33; H, 4.50; N,
2.23; S, 20.47; Fe, 8.91. Found: C, 61.6; H, 4.6; N, 2.2; S, 20.3;
Fe, 8.8.
atmosphere was added [Co(Cp)2] (61 mg; 0.32 mmol) with stirring
for 1.5 h. A microcrystalline light green solid separated out,
which was collected by filtration, dried, and stored under Ar.
Yield: 100 mg (62%). Anal. Calcd for C52H48NOS4FeCo2: C,
62.15; H, 4.81; N, 1.39; S, 12.76; Fe, 6.85. Found: C, 61.8; H,
5.2; N, 1.2; S, 12.5.
[Fe(NO)(L2•)(L2)] ([2a]0). Through a vigorously stirred suspen-
sion of dinuclear 2 (170 mg; 0.1 mmol) in CH2Cl2 (40 mL) under
Ar was bubbled NO gas for ∼90 s, after which time the Ar flow
through the solution was continued for 1 h. The color of the solution
changed to dark brown. After addition of dry methanol (10 mL)
the reaction volume was reduced by one-half by evaporation of
the solvent(s) in a stream of Ar. Within 2-3 days at 20 °C black
crystals separated out which were collected by filtration and air
dried. Yield: 150 mg (85%). ESI-mass spectrum (CH2Cl2):
positive-ion mode, m/z ) 874.7 and 844, calcd for the [Fe(NO)-
(L2)2]+ ion is 874.9 and the [Fe(L2)2]+ ion 844.9. Anal. Calcd for
C52H36NOS4Fe: C, 71.38; H, 4.15; N, 2.60; S, 14.66; Fe, 6.38.
Found: C, 70.8; H, 4.4; N, 2.4; S, 15.1; Fe, 5.9.
X-ray Crystallographic Data Collection and Refinement of
the Structures. Dark brown single crystals of [1a], [2a]‚0.75 CH2-
Cl2, and [Co(Cp)2][1a] were coated with perfluoropolyether, picked
up with nylon loops, and immediately mounted in the nitrogen cold
stream of the diffractometer to prevent loss of solvent. 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) was used. Final cell constants were obtained from least-
squares fits of all measured reflections. Intensity data of [1a] and
[2a] were corrected for absorption using intensities of redundant
reflections, but in the case of [Co(Cp)2][1a] crystal faces were
determined and a Gaussian absorption correction was performed.
The structures were readily solved by direct methods and
subsequent difference Fourier techniques. The Siemens ShelXTL13
software package was used for solution and artwork of the structure;
ShelXL9714 was used for refinement. All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were placed at calculated
positions and refined as riding atoms with isotropic displacement
parameters. Crystallographic data of the compounds are listed in
Table 1.
[Co(Cp)2][Fe(NO)(L1)2], ([Co(Cp2)][1a]). To a brown solution
of [1a]0 (100 mg; 0.16 mmol) in CH2Cl2 (30 mL) under an Ar
atmosphere was added [Co(Cp)2] (30 mg; 0.16 mmol) with stirring
for 2 h. To the resulting light brown solution was added methanol
(15 mL), which was allowed to slowly evaporate in a stream of
Ar. Within 3-4 days black crystals separated out which were
filtered off and dried under Ar. Yield: ∼100 mg (77%). ESI mass
spectrum (CH2Cl2 solution) negative-ion mode: m/z ) 596.3, calcd
for {Fe(NO)(L1)2-NO}- 596.6. Anal. Calcd for C42H38NOS4FeCo:
C, 61.84; H, 4.69; N, 1.72; S, 15.72; Fe, 6.85. Found: C, 61.3; H,
5.1; N, 1.6; S, 15.3; Fe, 7.0.
(13) ShelXTL, V.5; Siemens Analytical X-Ray Instruments Inc.: Madison,
WI, 1994.
[Co(Cp)2]2[Fe(NO)(L1)2], ([Co(Cp2)]2[1a]). To a brown solution
of [1a]0 (100 mg; 0.16 mmol) in CH2Cl2 (30 mL) under an Ar
(14) Sheldrick, G. M. ShelXL97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
524 Inorganic Chemistry, Vol. 46, No. 2, 2007

Adducts of Bis(diaryl-1,2-dithiolene)iron Compounds
Scheme 1
Table 2. ν(NO) Stretching Frequency of Complexes in the Infrared
complex
NO, cm-1 a (KBr disk)
ν
[1a](BF4)
[1a]
1833 (1760)
1800,1783 (1725)
1758 (1693)
1575,1530(<1500)
1800
[Co(Cp)2][1a]
[Co(Cp)2]2[1a]
[N(n-Bu)4][FeNO(bdt)2]
[N(n-Bu)4][Co(Cp)2][FeNO(bdt)2]
[2a]
1600
1791
[3a]
1764
[Et4N][4a]
1775
1814
1633
[PPh4][5a]b
[PPh4]2[5a]b
a Values in parentheses are for the corresponding 15N18O isotopomer.
b Reference 2.
electrolysis. After completion of electrolysis, samples of the
electrolyzed solutions were taken and rapidly frozen for EPR
analysis.
Infrared spectra of oxidized and reduced species were measured
by performing electrolysis (at -25 °C) in a temperature-controlled
optically transparent thin layer electrode (OTTLE; d ) 0.18 mm)
equipped with a platinum grid working electrode. The OTTLE was
mounted in an FT infrared spectrophotometer (Perkin-Elmer, system
2000) monitoring the infrared spectral changes directly during
electrolysis.
Two dichloromethane solvent molecules in compound [2a] were
found not to be fully occupied and to be disordered next to a
crystallographic 2-fold axis. Split positions with occupation factors
of 0.53 and 0.22 were refined to give a satisfactory disorder model.
Thermal displacement parameters and C-Cl and Cl-Cl distances
were restrained to be equal within errors using EADP and SADI
instructions of ShelXL97.
Physical Measurements. X-band EPR spectra were recorded
on a Bruker ELEXSYS E500 spectrometer equipped with a helium
flow cryostat (Oxford Instruments ESR 910) and Hewlett-Packard
frequency counter HP5253B. The spectra taken from frozen
solutions were simulated by using a powder routine for spin S )
1/2 with anisotropic g values and Gaussian line widths.
Results
Syntheses of Complexes. All mononuclear complexes of
the present study have been prepared according to literature
methods2,3 as shown in Scheme 1 using the dinuclear
complexes 1- 4 as starting materials. Dinuclear species 1
has recently been described in detail by us.10 By analogy
with our studies of complex 1, we suggest that the neutral
species 2-4 also possess terminal radical monoanions (L•)1-
and two bridging closed-shell dianions (L)2-, where (L)2-
and (L•)1- represent 1,2-diaryl-based ligands [S2C2Ar2]2-/1-
and its cis-1,2-dicyanoethene-1,2-dithiolate dianion (mnt)2-
or the (mnt•)1- radical monoanion. The central ferric iron
atoms possess an intrinsic intermediate spin state (S ) 3/2).
These two metal ions are intramolecularly antiferromagneti-
cally coupled, yielding an overall diamagnetic ground state
(St ) 0).10
Mo¨ssbauer data were recorded on an alternating constant-
acceleration spectrometer. The minimum experimental line width
was 0.24 mm s-1 (full width at half-height). The sample temperature
was maintained constant in an Oxford Instruments VARIOX
cryostat. Isomer shifts are quoted relative to iron metal at 300 K.
Magnetic susceptibilities of solid complexes were measured in
the temperature range 1.5-300 K using a SQUID susceptometer
(MPMS Quantum Design) with an external field of 1.0 T. Multiple-
field variable-temperature measurements were done at three dif-
ferent fields for which the magnetization was equidistantly sampled
on a 1/T temperature scale. Experimental data were corrected for
underlying diamagnetism by use of tabulated Pascal’s constants.
The susceptibility and magnetization data were simulated with our
own spin Hamiltonian routine for exchange-coupled systems.
Electrochemistry and Spectroelectrochemistry. In all experi-
ments potential control was achieved using an EG&G Potentiostat/
Galvanostat (model 273A) and M270 software, and solutions of
the complexes in CH2Cl2 containing 0.2 M [(n-Bu)4N]PF6 as
supporting electrolyte were employed throughout. Cyclic voltam-
mograms (CV) were recorded using a conventional three-electrode
arrangement consisting of a glassy carbon working electrode (2
mm diameter), an Ag/Ag+ (0.01 M AgNO3) reference electrode,
and a Pt wire counter electrode. Small amounts of ferrocene were
added as an internal standard after completion of an experiment,
and potentials are referenced versus the ferrocenium/ferrocene (Fc+/
Fc) couple.
As shown in Scheme 1 oxidation of 1 with two equivalents
of [NO]BF4 in CH2Cl2 yields the monocation [Fe(NO)-
(L1•)2]+, [1a]+, which has been isolated as a brown BF4
-
salt. The salt is diamagnetic (S ) 0) and displays an intense
νNO stretching frequency (Table 2) at 1833 cm-1 which is
typical for an {FeNO}6 moiety consisting of a low-spin
ferrous ion (d6, SFe ) 0) and an NO+ ligand coordinated
linearly to the iron ion15 which is bound to two radical
monoanions (L1•)1-: [FeII(NO+)(L1•)2]+ (S ) 0). This mono-
cation represents the most oxidized member of the electron-
transfer series (z ) 1+, 0, 1-, 2-).
Interestingly, reaction of 1 with NO gas in CH2Cl2 solution
produces black microcrystals of the neutral, paramagnetic
complex [Fe(NO)(L•)(L)]0 [1a] with S ) 1/2. In the solid
Controlled potential electrolysis at appropriate fixed potentials
was performed at -25 °C in a jacketed quartz cuvette with the
same type of reference electrode, a Pt mesh working electrode, and
a Pt brush counter electrode. The cuvette (optical path length 0.5
cm) was mounted directly in a spectrophotometer (Hewlett-Packard
HP 8453) and allowed recording of UV-vis spectra in situ during
(15) (a) Hauser, C.; Glaser, T.; Bill, E.; Weyhermu¨ller, T.; Wieghardt, K.
J. Am. Chem. Soc. 2000, 122, 4352. (b) Serres, R. C.; Grapperhaus,
C. A.; Bothe, E.; Bill, E.; Weyhermu¨ller, T.; Neese, F.; Wieghardt,
K. J. Am. Chem. Soc. 2004, 126, 5138.
Inorganic Chemistry, Vol. 46, No. 2, 2007 525

Ghosh et al.
Figure 1. Temperature dependence of the magnetic moments of solid
samples of [1a]0 (top) and [2a]0 (bottom). The solid lines represent best
fits using a model of two interacting monomers with S1 ) S2 ) 1/2, H )
-2JS1S2. Parameters are as follows: [1a] J ) -1.1 cm-1, g1 ) g2 ) 2.0
(fixed) and [2a] J ) -15 cm-1 and a Weiss constant θ of -10 K, g1 )
g2 ) 2.0 (fixed).
Figure 2. Structure of the two independent neutral molecules in crystals
of [1a]0.
state (KBr disk) it displays two νNO stretches at 1800 and
1783 cm-1, indicating again the presence of an {FeNO}6
moiety. Thus, one-electron reduction of [1a]+ is a ligand-
based process (L1•)1- + e f (L1)2-. The corresponding
neutral complex [2a] has also been isolated as a solid. Both
[1a] and [2a] are paramagnetic with an S ) 1/2 ground state.
Figure 1 exhibits the temperature-dependent magnetic
moments of solid samples of [1a] and [2a]. The solid lines
represent best fits which were obtained using a model where
the paramagnetic molecules are packed pairwise (see below)
with an antiferromagnetic spin exchange coupling between
two S ) 1/2 molecules: for [1a] a very small antiferromag-
netic coupling constant, J, of -1.1 cm-1 (H ) -2JS1S2;
S1 ) S2 ) 1/2; g ) 2.0 (fixed)) has been obtained, whereas
this coupling constant is -15 cm-1 for [2a] (an additional
Weiss constant of θ ) -10 K had to be included for a good
fit). We realize that this does not represent a physically
correct model for the magnetism of [2a], but it does show a
stronger antiferromagnetic coupling in [2a] than in [1a].
Reaction of neutral [1a] with one and two equivalents of
cobaltocene, [Co(Cp)2], in CH2Cl2 under strictly anaerobic
conditions produces the black salts [Co(Cp)2][Fe(NO)(L1)2]
([Co(Cp)2][1a]) and green [Co(Cp)2]2[Fe(NO)(L1)2] ([Co-
(Cp)2]2[1a]), respectively. Interestingly, the νNO stretching
frequencies of the mono- and dianion are observed at 1758
and 1575 cm-1, respectively. This large difference of 183
cm-1 is a clear indication that the {FeNO}6 moiety in the
monoanion [1a]1- is reduced to the {FeNO}7 unit in the
dianion [1a]2-.2,3 Note that νNO varies from 1833 cm-1 in
[1a]1+ to ∼1800 cm-1 in [1a]0 and to 1758 cm-1 in [1a]1-,
which represents ∼35 cm-1 per one-electron reduction step,
indicating that the {FeNO}6 moiety is retained throughout
Figure 3. Structure of the neutral molecule in crystals of [2a].
the three species. [Co(Cp)2]2[1a] is paramagnetic with an
S ) 1/2 ground state (see below).
Thus, it has been possible for the first time to isolate four
members of a single five-coordinate iron nitrosyl dithiolene
electron-transfer series [Fe(NO)(L1)2]z (z ) 1+, 0, 1-, 2-).
The following complexes have been synthesized according
to literature procedures: [Fe(NO)(L3•)(L3)] ([3a]0),3 [Et4N]-
[FeNO)(L4)2] ([Et4N][4a]),2 and [NEt4][Fe(NO)(mnt)2] ([NEt4]-
[5a]).2 The neutral complex [3a]0 is again paramagnetic (S
) 1/2), whereas complexes containing the monoanions [4a]1-
and [5a]1- are both diamagnetic. Their NO stretching
frequencies are given in Table 2. Notably, irrespective of
the charge (neutral or monoanion), the similarity of their νNO
stretching frequencies again indicates the presence of the
same {FeNO}6 moiety.
X-ray Structures. The structures of complexes [1a]0,
[2a]0‚0.75 CH2Cl2, and [Co(Cp)2][1a] have been determined
by X-ray crystallography at 100(2) K. Figures 2 and 3 display
the structures of the neutral molecules in crystals of [1a]
and [2a], respectively, whereas that of the monoanion and
526 Inorganic Chemistry, Vol. 46, No. 2, 2007

Adducts of Bis(diaryl-1,2-dithiolene)iron Compounds
Figure 4. Structure of the monocation [Co(Cp)2]+ and monoanion [1a]1-
in crystals of Co(Cp)2] [1a].
Table 3. Selected Bond Distances (Å)
[1a]a
Fe1-N21
1.636(3)
2.2118(7)
2.2039(7)
1.164(4)
1.641(3)
2.2090(8)
2.2172(8)
1.158(5)
S1-C1
S2-C2
C1-C2
1.703(3)
1.711(3)
1.387(4)
Fe1-S1
Fe1-S2
N21-O22
Fe2-N51
Fe2-S31
Fe2-S32
N51-O52
S31-C31
S32-C32
C31-C32
1.705(3)
1.708(3)
1.390(4)
[2a]
Fe1-N61
Fe1-S1
1.6430(2)
2.1902(5)
2.2068(5)
2.2099(5)
2.2549(5)
1.163(2)
S1-C1
1.714(2)
1.712(2)
1.397(3)
1.715(2)
1.718(2)
1.401(3)
S2-C2
Figure 5. Schematic packing diagram of two neutral molecules in [2a]
(top) and [1a] (bottom). The dotted lines indicate the shortest S‚‚‚S contacts
(see text).
Fe1-S2
C1-C2
Fe1-S31
Fe1-S32
N61-O62
S31-C31
S32-C32
C31-C32
planes roughly face each other, giving rise to four weak S‚
‚‚S interactions. These S‚‚‚S distances are for S2‚‚‚S32A and
S2A‚‚‚S32 at 3.728 Å and for S1‚‚‚S31 and S1A‚‚‚S31A at
3.705 Å. These interactions may provide a pathway for the
observed very weak antiferromagnetic spin exchange cou-
pling (J ) -1.1 cm-1) in solid [1a].
[Co(Cp)2][1a]
Fe1-N41
Fe1-S1
1.624(1)
N41-O42
1.176(2)
1.734(2)
1.743(2)
1.374(2)
1.752(2)
1.361(3)
2.2335(4)
2.2186(4)
2.2504(4)
2.2148(4)
1.755(2)
S1-C2
Fe1-S2
S2-C2
Fe1-S21
Fe1-S22
S22-C22
C1-C2
S21-C21
C21-C22
Interestingly, in solid [2a] the neutral molecules also pack
pairwise (Figure 5) but in a slightly different fashion. Only
one shorter S‚‚‚S distance at 3.619 Å is observed between
S32A and S32B of two different molecules. No other S‚‚‚S
interactions are present. This relatively shorter interaction
may account for the observed stronger intermolecular anti-
ferromagnetic coupling in [2a].
The geometrical features of the Fe(NO) moiety in [1a]
and [2a] are very similar to those reported for many five- or
six-coordinate nitrosyliron complexes containing an {FeNO}6
moiety.18 The {FeNO}6 unit is diamagnetic and has been
described from DFT calculations15b,16,19 as consisting of a
a Data for both crystallographically independent molecules are given.
cation in crystals of [Co(Cp)2][1a] are shown in Figure 4.
Table 3 summarizes important bond distances.
Complex [2a]0 crystallizes in an orthorhombic space group
containing two crystallographic independent molecules in the
unit cell. The conformations of the four p-tolyl groups differ
slightly in both molecules, but the geometrical features of
both Fe(NO)(S2C2)2 parts are within experimental error
identical.
Crystals of [1a]0 and [2a]0 consist of mononuclear, neutral
five-coordinate nitrosyliron species, [Fe(NO)(S2C2R2)2], where
R ) p-tolyl in [1a] and 4,4-diphenyl in [2a]. The nitrosyl
group is coordinated in the apical position of a square-based
pyramidal FeS4N coordination polyhedron; the two bidentate
dithiolate ligands occupy the four basal sites. The Fe-NO
groups are linear in both cases (180° for both conformers in
[1a] and 178.7(2)° in [2a]). The central iron ion lies ∼0.50
Å above the best plane of the four sulfur donor atoms.
As shown in Figure 5, two neutral molecules are packed
in the solid state in [1a] in such a way that the two basal S4
(16) Li, M.; Bonnet, D.; Bill, E.; Neese, F.; Weyhermu¨ller, T.; Blum, N.;
Sellmann, D.; Wieghardt, K. Inorg. Chem. 2002, 41, 3444.
(17) Sellmann, D.; Blum, N.; Heinemann, F.; Hess, B. A. Chem. Eur. J.
2000, 7, 1874.
(18) Lo´pez, J. P.; Heinemann, F. W.; Prakash, R.; Hess, B. A.; Horner,
O.; Jeandey, C.; Oddou, J.-L.; Latour, J.-M.; Grohmann, A. Chem.
Eur. J. 2002, 8, 5709.
(19) (a) Gonzales, M. C.; Scherlis, D. A.; Esiu´, G. L.; Olabe, J. A.; Estrin,
D. A. Inorg. Chem. 2001, 40, 4127. (b) Wanner, M.; Scheiring, T.;
Kaim, W.; Slep, L. D.; Baraldo, L. M.; Olabe, J. A.; Zalis, S.;
Baerends, E. J. Inorg. Chem. 2001, 40, 5704.
Inorganic Chemistry, Vol. 46, No. 2, 2007 527

Ghosh et al.
low-spin ferrous ion (SFe ) 0, d6) and an NO+ ligand. Thus,
the {FeNO}6 unit carries a 3+ charge which requires the
two dithiolene ligands to adopt two different oxidation
levels: one closed-shell dianion, [S2C2R2]2-, and a π-radical
monoanion, [S2C2R2]1-•. The average C-S distances in [1a]
and [2a] are short at 1.71 Å, and concomitantly, the average
“olefinic” C-C is long at 1.39 Å. These parameters are in
agreement with the presence of one radical and one closed-
shell ligand in [1a]0 and [2a]0, respectively.20,21 The bond
lengths are not in agreement with the presence of two closed-
shell dianons.21 In principle, the following three formal
electron distributions may be considered for the neutral
species [Fe(NO)(S2C2R2)]2]0 : [{FeNO}7(L•)2], [{FeNO}6-
(L•)(L)], and [{FeNO}5(L)2].
Figure 6. Cyclic voltammogram of [2a] in CH2Cl2 at 20 °C (0.20 M [N(n-
Bu)4]PF6) supporting electrolyte; glassy carbon working electrode; scan rates
50, 100, 200, and 400 mV s-1).
Table 4. Redox Potentials of Complexesa) Determined by Cyclic and
Square Wave Voltammetry at 20 °C
It is conceivable that the proposed chelating ligand mixed
valency in the {FeNO}6 case is of class III (delocalized),
giving thus rise to two equivalent ligands as observed in [1a]0
and [2a]0. Since an {FeNO}5 species has to the best of our
knowledge not been characterized to date, the crystallography
suggests an electronic structure for the neutral complexes
[1a]0 and [2a]0 as [FeII(NO+)(L•)(L)]0 T [FeII(NO+)(L)(L•)]0.
Reduction of [1a] produces the monoanion [Fe(NO)(L1)2]-,
the structure of which is shown in Figure 4 together with its
[Co(Cp)2]+ cation in crystals of [Co(Cp)2] [1a]. Overall the
structure of this anion is very similar to that of its neutral
counterpart [1a]. The geometric details of the Fe-NO moiety
in [1a] and [1a]- are within experimental error identical,
but the average C-S bond length at 1.75 Å and the av.
“olefinic” C-C bond at 1.36 Å in [1a]- have increased and
decreased, respectively, in comparison with the same bonds
in neutral [1a]. This is a clear indication that reduction of
[1a] involves only the dithiolene ligands in [1a]-. Thus, the
metrical details in [1a]- suggest an electronic structure as
in [FeII(NO+)(L1)2]-.
E1
,
E21/2, V
(0/1-)
E31/2, V
(1-/2-)
E41/2, V
(2-/3-)
1/2
complex
V/(1+/0)b
[1a]
[2a]
[3a]
[4a]
[5a]
0.17
0.22
0.17
0.26
n.o.
-0.44
-0.41
-0.46
-0.42
+0.65
-1.22
-1.17
-1.21
-1.21
-0.38
n.o.
n.o.
n.o.
n.o.
-1.83 (q.r.)
a Conditions: CH2Cl2 solution (0.20 M [N(n-Bu)4]PF6) at 20 °C, scan
rate 200 mV s-1, glassy carbon working electrode. b All redox potentials
are referenced vs the ferrocenium/ferrocene couple (Fc+/Fc); n.o. ) not
observed to -2.0 V vs Fc+/Fc; q.r. ) quasi reversible.
Figure 6 shows the cyclic voltammogram of [2a]; Table 4
summarizes the redox potentials.
In each case three reversible one-electron-transfer waves
are observed. From coulometric measurements at appropri-
ately fixed potentials it was established that the neutral
complexes [1a]-[4a] undergo a reversible one-electron
oxidation generating a monocation and two successive one-
electron reductions with successive formation of a mono-
and a dianion as shown in eq 1
Electro- and Spectroelectrochemistry. It was shown in
1967 by electrochemistry that the present nitrosyliron
complexes exist within a five-membered electron-transfer
chain,1-3 corresponding to [Fe(NO)(S2C2R2)2]z, where z can
be -3, -2, -1, 0, 1+. No single system permitted detection
and/or isolation of all five species. It was shown that the
(mnt)n- and perfluoromethyl ligands (S2C2R2; R ) CN or
CF3) stabilize the more reduced species, giving access to
the trianions but not to the monocations, whereas the diaryl-
based ligands, (L1-4), facilitate formation of the more highly
oxidized species, giving access to monocations but not
trianions.2,3
We studied complexes [1a]-[5a] by cyclic and square-
wave voltammetry in CH2Cl2 solutions containing 0.10 M
[N(n-Bu)4]PF6 as supporting electrolyte at 20 °C at a glassy
carbon working electrode. All potentials are referenced vs
the ferrocenium/ferrocene (Fc+/Fc) couple (internal standard).
-e
H
-e
H
-e
H
[1a]+
[1a]
[1a]1-
[1a]2-
(1)
+e
+e
+e
E1
E2
E3
1/2
1/2
1/2
For [Fe(NO)(mnt)2]- ([5a]-) it was shown that a di- and
trianion are accessible via two successive one-electron
reduction steps and a neutral species via a one-electron
oxidation, eq 2, but no monocation could be detected
[5a]0
[5a]
[5a]
[5a]
(2)
1-
2-
3-
-e
H
+e
-e
H
+e
-e
H
+e
Since all four diaryl-based species of each electron-transfer
series are stable in solution, their solution infrared spectra
have been recorded in the range 1500-1900 cm-1 where
the NO stretching band is generally observed. The 15N18O
isotopomers have also been prepared, and their IR solution
spectra have also been recorded. The results are shown in
Figure 7 for the series [1a]z (z ) 1+, 0, 1-, 2-).
Table 2 gives the NO stretching frequencies of all isolated
complexes (KBr disks). Interestingly, νNO decreases from
1838 cm-1 for the monocation [1a]+, to 1800 cm-1 in the
neutral complex [1a]0, and then to 1765 cm-1 in the
(20) Patra, A.; Bill, E.; Bothe, E.; Chlopek, K.; Neese, F.; Weyhermu¨ller,
T.; Stobie, K.; Ward, M. D.; McCleverty, J. A.; Wieghardt, K. Inorg.
Chem. 2006, 45, 7877.
(21) The av. C-S and C-C bond lengths in [AuIII(L)2]- are 1.77 and 1.36
Å, respectively, where (L)2- is 1,2-di(4-tert-butylphenyl)ethylene-1,2-
dithiolate(2-), see: Kokatam, S.; Ray, K.; Pap, J.; Bill, E.; Geiger,
W. E.; LeSuer, R. J.; Rieger, P. H.; Weyhermu¨ller, T.; Neese, F.;
Wieghardt, K. Inorg. Chem., in press.
528 Inorganic Chemistry, Vol. 46, No. 2, 2007

Adducts of Bis(diaryl-1,2-dithiolene)iron Compounds
Table 5. Electronic Spectra of Electrochemically Generated Complexes
in CH2Cl2 Solution (0.10 M [N(n-Bu)4]PF6) at 25 °C
complex
z
λ
max, nm (104 ꢀ, M-1 cm-1
)
[1a]z
2-
1-
0
1+
2-
1-
0
1+
2-
1-
0
1+
2-
1-
0
316(sh,3.0), 420(1.0), 710(0.12)
310(3.7), 410(sh,1.0), 678(0.12), 1560(0.6)
384(1.5), 490(0.7), 607(0.5), 855(1.1)
460(1.0), 523(1.0), 858(2.7)
421(sh,2.0), 500(sh,1.0), 709(0.17)
406(sh,1.8), 675(0.2), 1560(0.6)
402(1.7), 507(0.9), 630(0.5), 873(1.4)
376(sh,3.0), 486(1.0), 596(1.2), 903(3.0)
275(6.4)
[2a]z
[3a]z
[4a]z
[5a]z
383(sh), 1005(0.2)
386(sh), 671(sh), 926(0.6)
382(sh), 465(sh), 593(0.6), 954(1.7)
325(sh), 418(2.2)
309(6.6), 389(sh), 638(sh), 1510(0.7)
306(6.8), 375(sh), 490(sh), 590(sh), 822(1.8)
486(sh), 806(3.4)
383(1.2), 533(0.1)
319(2.2), 418(0.1)
435(sh), 1363(0.4)
295(2.6), 456(0.4), 752(0.2)
Figure 7. Observed infrared spectra changes during the electrochemical
oxidation of [1a]0 to [1a]+ and reductions of [1a] to [1a]1- and [1a]2- at
-25 °C in CH2Cl2 (0.10 M [N(n-Bu)4]PF6) (top) and its corresponding
15N18O-labeled isotopomers (bottom).
1+
3-
2-
1-
0
planar [MII(L•)2]0 (M ) Ni, Pd, Pt) complexes22 and
[FeII(CN)(L•)2];1-20 (L6•)1- representing the 1,2-di(4-tert-
butylbutylphenyl)-1,2-ethylenedithiolate(1-) radical and (L•)1-
any dithiolato radical anion. Since the latter complexes do
not contain a M-NO moiety, the presence of this transition
is an excellent marker for the FeII(L1•)2 unit. Thus, the
infrared and electronic spectra of the monocations indicate
that the electronic structure is probably best described as [FeII-
(NO+)(L•)2]1+.
The electronic spectra of the neutral species [1a]-[5a]
are also very similar: an intense absorption maximum in
the range 750-930 nm (ꢀ ≈ 104 M-1 cm-1) is again
observed, although it is noted that its intensity is ap-
proximately only one-half of that observed for the similar
CT band of the monocations. This transition is likely to be
an intervalence charge-transfer band (IVCT) of a planar
M(L)(L•) unit which has also been reported for all planar
[MII(L)(L•)]1- (M ) Ni, Pd, Pt) species.22 Thus, we propose
an electronic structure [FeII(NO+)(L•)(L)]0 T [FeII(NO+)-
(L)(L•)]0 (S ) 1/2) for the neutral complexes. The ligand
mixed valency may be of class III type (delocalized).
The monoanionic complexes display a weak charge-
transfer band at >1200 nm (ꢀ ≈ 5000 M-1 cm-1) but no
LLCT or IVCT band in the range 750-950 nm. Therefore,
it is concluded that reduction of the neutral species to the
corresponding monoanions is ligand-based involving reduc-
tion of the radical (L•)1- monoanion to the closed-shell
dianion (L)2-: [FeII(NO+)(L)2]1- (S ) 0).
Figure 8. Spectroelectrochemistry of [1a]0 showing the electronic spectra
of electrochemically generated [1a]+, [1a]1-, and [1a]2- and of [1a]0 in
CH2Cl2 solution (0.20 M [N(n-Bu)4]PF6) at -25 °C.
monoanion [1a]1-, i.e., a decrease of only ∼35 cm-1 per
one-electron reduction. In stark contrast, further reduction
of the monoanion [1a]1- to the corresponding dianion [1a]2-
induces a shift of ∼190 cm-1 (from 1765 cm-1 in [1a]1- to
1575 cm-1 in [1a]2-). It is noteworthy that the observed
bands at 1600 cm-1 in the monocation and at 1531 cm-1 in
the dianion do not involve the Fe-NO moiety since no shift
of the bands is observed in the corresponding 15N18O
isotopomers. Typically, νNO is shifted by ∼75 cm-1 to lower
energy in the 15N18O isotopomers. These results indicate that
a {Fe-NO}6 moiety prevails in the diamagnetic monocation,
paramagnetic neutral species, and monoanion. Only in the
dianion does an {FeNO}7 moiety appear to be present. In
the trianion [Fe(NO)(mnt)2]3- an {FeNO}8 moiety may be
present. However, we have no data at present to support this
notion, but in [Fe(NO)(cyclam-ac)]0 which contains an
{FeNO)}8 moiety νNO has been observed at 1271 cm-1.15b
Finally, reduction of the mono- to dianions involves the
process {FeNO}6 f {FeNO}7 which is NO based.16-19 This
implies that the coordinated cation (NO+) (S ) 0) is reduced
to a coordinated neutral (NO)• (S ) 1/2). This process is
accompanied by a dramatic decrease in the NO stretching
frequency (Table 2) but does not bring about a dramatic
change in the electronic spectrum with the exception that
the CT band in the near-infrared at ∼1200 nm is no longer
We also recorded the electronic spectra of [1a]0 and the
electrochemically generated species [1a]+, [1a]1-, and [1a]2-
in the range 250-2000 nm, and these are displayed in Figure
8. Table 5 summarizes the spectra of the other complexes.
It is remarkable that the spectra of the monocations,
[1a]+-[4a]+, are so very similar. In each case a very intense
absorption maximum at ∼850 nm (ꢀ ≈ 2.5 × 104 M-1 cm-1)
is observed which we assign to a ligand-to-ligand charge-
transfer band (LLCT) of a nearly planar (L•)FeII(L•) unit.
Such LLCT bands have been observed in the spectra of
(22) Ray, K.; Weyhermu¨ller, T.; Neese, F.; Wieghardt, K. Inorg. Chem.
2005, 44, 5345.
Inorganic Chemistry, Vol. 46, No. 2, 2007 529

Ghosh et al.
Table 6. X-Band EPR Spectra of Complexes in Frozen CH2Cl2
Solutions at 30 K
complex
gx
gy
gz
Axx, (14N)a Ayy, (14N)a Azz, (14N)a
[1a]0
[2a]0
[3a]0
[4a]0
[1a]2-
[2a]2-
2.0224 2.0127 1.999
2.0230 2.0130 2.000
2.019 2.010 1.997
2.020 2.010 1.997
2.0496 2.0297 2.0097
2.0512 2.0296 2.0102
n.o.
n.o.
n.o.
n.o.
0.0
n.o.
n.o.
n.o.
n.o.
14.4
14.2
15.4
n.o.
n.o.
n.o.
n.o.
15.6
14.7
15.8
0.0
13.9
[5a]2- b 2.060 2.028 2.005
a Nitrogen hyperfine coupling constants (×104 cm-1). b Reference 24;
n.o. ) not observed.
Figure 10. X-Band EPR spectrum of [2a]2- in frozen CH2Cl2 at 30 K.
Conditions: ν ) 9.43 GHz, power 1.0 × 10-5 mW; modulation 7.0 G. For
simulation parameters, see Table 6.
Table 7. Zero-Field Mo¨ssbauer Parameters of Complexes at 80 K
complex
δ, mm s-1 b
∆EQ, mm s-1 c
[1a](BF4)
[1a]
[Co(Cp)2][1a]
0.07
0.06
0.04
0.20
0.07
0.01
0.27
0.08
0.06
0.01
0.03
0.33
1.40
1.70
1.88
1.16
1.68
2.48
1.12
1.59
1.65
1.98d
1.70
0.79
[Co(Cp)2]2[1a]
[2a]
[(n-Bu)4N][FeNO(bdt)2]a
[Co(Cp)2][N(n-Bu)4]-[FeNO(bdt)2]a
[3a]
[4a]
[NEt4][4a]
[PPh4][5a]
[Et4N]2[5a]
Figure 9. X-Band EPR spectrum of [1a]0 in frozen CH2Cl2 at 20 K.
Conditions: ν ) 9.43 GHz; power 1.0 × 10-5 mW, modulation 3.0 G. g
values are given in Table 6.
present. We propose the following electronic structure for
the dianions: [FeII(NO•)(L)2]2- (S ) 1/2).
a (bdt)2- ) benzene-1,2-dithiolate(2-), ref 4. b Isomer shifts vs R-Fe at
298 K. c Quadrupole splitting. d Reference 8.
X-band EPR Spectroscopy. Two members, namely, the
neutral and dianionic complexes, of the present electron-
transfer series are paramagnetic, both possessing an S ) 1/2
ground state. The X-band EPR spectra have been recorded
in frozen CH2Cl2 solutions (0.10 M [N(n-Bu)4]PF6) at 30 K,
and the results are summarized in Table 6.
EPR spectra of the neutral complexes [1a]-[4a] are very
similar, consisting of a rhombic signal with small g anisot-
ropy (g ) 2.02-1.999), and 14N hyperfine splitting has not
been observed. The spectrum of [1a] is shown in Figure 9.
These spectra are typical of an S-centered radical which is
consistent with the presence of a π-radical monoanionic
ligand (L1-4•)1-. This simple EPR signal is representative
of a nearly planar (L•)FeII(L) T (L)FeII(L•) unit and has also
been observed in [AuIII(L)(L•)]0 T [AuIII(L•)(L)]0 com-
plexes.23 These spectra are in accord with an electronic
structure description of the neutral species as [Fe(NO)(L•)-
(L)]0 T [Fe(NO)(L)(L•)]0 where the diamagnetic {FeNO}6
unit consists of a low-spin Fe(II) (SFe ) 0; d6) and a
coordinated, diamagnetic NO+ ligand.
From DFT calculations16 it has been concluded that these
complexes should be described as low-spin Fe(II) coordinated
to a neutral NO• radical. The spectrum of [5a]2- has been
reported and interpreted previously.24
Mo1ssbauer Spectroscopy. Table 7 summarizes the zero-
field Mo¨ssbauer parameters for solid samples of complexes
at 80 K. In each case a single quadrupole doublet has been
observed.
It is significant that values for the isomer shift, δ, and
quadrupole splitting ∆EQ, for the monocations, the neutral
complexes, and the corresponding monoanions are all
observed in the narrow range δ ) 0.08-0.01 mm s-1 and
∆EQ ) 1.4-2.0 mm s-1. This is a clear indication that the
{FeNO}6 moiety is retained in the complexes irrespective
of their charge or dithiolene oxidation level. This conclusion
has also been reached by the infrared (Table 2) and UV-
vis spectral studies reported above.
In contrast, one-electron reduction of the monoanion
generating the corresponding dianion brings about a signifi-
cant change in the Mo¨ssbauer parameters. Now the isomer
shift is observed in the range δ ) 0.20-0.33 mm s-1 and
∆EQ ) 0.80-1.2 mm s-1. In agreement with previous
reports,8 we think that these complexes contain the {FeNO}7
moiety.
Interestingly, the EPR spectra of the dianions [1a]2-,
[2a]2-, and [5a]2- are quite different as shown in Figure 10
for [2a]2-. These spectra display a first-order hyperfine
interaction with the nitrogen nucleus (14N, I ) 1) of the NO
group. The spectra closely resemble those reported for other
five- and six-coordinated {FeNO}7 (S ) 1/2) species.15,16
We also measured the zero-field Mo¨ssbauer spectra of
structurally characterized7 [N(n-Bu)4][Fe(NO)(bdt)2] and [Co-
(23) Ray, K.; Weyhermu¨ller, T.; Goossens, A.; Craje´, M. W. J.; Wieghardt,
K. Inorg. Chem. 2003, 42, 4082.
(24) Teschmit, G.; Kirmse, R. Inorg. Chem. Commun. 1999, 2, 465.
530 Inorganic Chemistry, Vol. 46, No. 2, 2007

Adducts of Bis(diaryl-1,2-dithiolene)iron Compounds
Scheme 2
Table 8. Comparison of Zero-Field Mo¨ssbauer Parameters at 80 K of
Five- and Six-Coordinate {Fe(NO)}n (n ) 6-8) Complexes
×e7∆EQ×e7,
complex
δ, mm s-1 a
mm s-1 b
{FeNO}n, n ref
[Fe(NO)(cyclam-ac)]2+
[Fe(NO)(cyclam-ac)]1+
[Fe(NO)(cyclam-ac)]0
[Fe(NO)(pyS4)]+
0.01
0.26
0.41
0.04
0.33
0.04
0.31
0.05
0.21
1.76
0.74
1.69
1.63
0.40
1.84
0.84
1.68
0.97
6
7
8
6
7
6
7
6
7
15b
15b
15b
16
16
18
18
8a
8a
[Fe(NO)(pyS4)]0
[Fe(NO)(pyN4)]3+
[Fe(NO)(pyN4)]2+
[Et4N][Fe(NO)(mnt)2]
[PPh4]2[Fe(NO)(mnt)2]
a Isomer shift vs R-Fe at 298 K. b Quadrupole splitting, c 4.2 K.
(Cp)2][N(n-Bu)4][Fe(NO)(bdt)2], where again the data indi-
cate the presence of an {FeNO}6 and {FeNO}7 moiety,
respectively.
Discussion
In order to delineate the factors determining the electronic
structures of the various nitrosyl iron complexes [Fe(NO)-
(dithiolene)2]n (n ) 1+, 0, 1-, 2-, 3-), it is instructive to
compare their spectroscopic properties with those of other
five- and six-coordinate nitrosyl iron complexes containing
{FeNO}6-8 cores and redox-innocent, four- or five-coordinate
auxiliary ligands. Table 8 summarizes the Mo¨ssbauer pa-
rameters of six-coordinate species with an {FeNO}6 and/or
{FeNO}7 core. It is evident that the isomer shift, δ, increases
from ∼0.03 mm s-1 for {FeNO}6 to ∼0.26 mm s-1 for
{FeNO}7 complexes. Concomitantly, the quadrupole split-
ting, ∆EQ, decreases from ∼1.7 to ∼0.8 mm s-1. Recent
DFT calculations coupled to spectroscopic studies have
established15b,16,19 a consensual description of the electronic
structure of these complexes: the {FeNO}6 species consist
of a low-spin Fe(II) (SFe ) 0; d6) coordinated by a
diamagnetic NO+ cation, and the {FeNO}7 species with an
S ) 1/2 ground state are best described as low-spin Fe(II)
(SFe ) 0; d6) coordinated by a NO• radical (S ) 1/2).
Interestingly, the Mo¨ssbauer parameters do not vary greatly
with the coordination number (5 or 6) or the nature of the
donor atoms of the auxiliary ligand (4S + 1N or 5N).
It is therefore informative that the pair [Fe(NO)(bdt)2]1-/2-
(Table 7) and [Fe(NO)(mnt)2]1-/2- (Table 8) display the same
characteristic Mo¨ssbauer and EPR parameters as discussed
above. This is taken as evidence for the presence of {FeNO}6
and {FeNO}7 moieties, respectively, and closed-shell (bdt)2-
and (mnt)2- ligands in both cases. It is also revealing that
the electronic spectra of the dianions with an {FeNO}7 (S )
1/2) moiety do not display absorption maxima >500 nm with
molar extinction coefficients >5 × 103 M-1 cm-1. In
contrast, the monoanions display a relatively weak maximum
(∼4 × 103 M-1 cm-1) in the near-infrared (NIR) at 1304
nm7 for [Fe(NO)(bdt)2]1- and 1363 nm (4 × 103 M-1 cm-1)
for [Fe(NO)(mnt)2]1-. The exact nature of this transition,
which is also present in [1a]1-, [2a]1-, [3a]1-, and [4a]-
(Table 5), is at present unclear.
cm-1 in [Fe(NO)(bdt)2]1-/2- and from 1814 to 1633 cm-1
for [Fe(NO)(mnt)2]1-/2- (see ref 2)). This is again nicely in
accord with the notion that an NO+ ligand in a given
{FeNO}6 complex is reduced to an {FeNO}7 species with a
coordinated NO• molecule. Taken together, the spectroscopic
evidence clearly establishes that in all mono- and dianions
of this study ([1a]1-/2--[5a]1-/2-) the dithiolene ligands are
closed-shell dianions (Lx)2- (x ) 1-5).
Structurally, a few pairs of mono- and dianions have been
characterized by X-ray crystallography including [Fe(NO)-
(mnt)2]1-/2- 5,6 and [Fe(NO)(bdt)2]1-/2-.7 A linear Fe-NO
group is present in the {FeNO}6 species, as expected,
whereas in the {FeNO}7 complexes it is bent (150-170°).
Significantly, the C-S bond lengths in the mono- and
dianions are long and nearly identical which indicates the
presence of closed-shell dianions (mnt)2- and (bdt)2-, thereby
ruling out the presence of coordinated π-radical monoanions
(mnt•)1- or (bdt•)1-.
It is also a reliable spectroscopic feature of {FeNO}7 (S
) 1/2) complexes that their EPR spectra (including all
dianions of the present study as summarized in Table 6)
exhibit 14N hyperfine splitting. This has been discussed for
[Fe(NO)(mnt)2]2- by Kirmse and Teschmit24 and for
[Fe(NO)(S2CN(CH3)2)2]25 and [Fe(NO)(S2CNEt2)2].26 Note
that the latter two complexes contain two redox-innocent
A further significant spectroscopic observation is the fact
that on going from {FeNO}6 to {FeNO}7 species νNO
decreases by ∼200 cm-1 (for example, from 1800 to 1600
(25) Feltham, R. D.; Crain, H. Inorg. Chim. Acta 1980, 40, 37.
(26) Goodman, B. A.; Raynar, J. B.; Symons, M. C. R. J. Chem. Soc. (A)
1969, 2572.
Inorganic Chemistry, Vol. 46, No. 2, 2007 531

Ghosh et al.
dithiocarbamato(1-) ligands. A similar EPR spectrum has
been reported for [Fe(NO)(i-mnt)2]2-, where (i-mnt)2- is 1,1-
dicyanoethene-2,2-dithiolate(2-).27
In summary, all spectroscopic evidence and DFT calcula-
tions for the present mono- and dianions are in excellent
agreement with the electronic structures: (a) [FeII(NO+)(Ln)2]1-
(SFe ) 0; d6; St ) 0) containing two closed-shell dianions
(Ln)2- and (b) [FeII(NO•)(Ln)2]2- (SFe ) 0; d6; St ) 1/2)
containing again two dianions (Ln)2-.
species, namely, δ ) 0.11 and 0.08 mm s-1 and ∆EQ ) 2.55
and 2.55 mm s-1, respectively.
In addition, the electronic spectra of these two non-nitrosyl
species also display an intense (1.1 × 104 and 1.8 × 104
M-1 cm-1) absorption maximum at 723 and 687 nm,
respectively. From DFT calculations on [FeII(L•)2(CN)]- it
was shown that an electronic configuration containing low-
spin Fe(II) and two π-radical monoanions prevails. The above
intense absorption maximum is assigned as a ligand-to-ligand
z
CT band which is characteristic of all planar M(L•)2
As pointed out above it is quite remarkable that the
Mo¨ssbauer parameters for the monoanion, neutral complex,
and monocation are within a very narrow range identical,
indicating the presence of the same {FeNO}6 moiety (low-
spin Fe(II) with an NO+ ligand) irrespective of the overall
charge or spin state of the complex (the monocations and
monoanion possess an S ) 0 ground state, whereas the
neutral species is paramagnetic (S ) 1/2)). This result clearly
points to sulfur-ligand-based redox processes when oxidation
of the monoanion to the neutral and then to the corresponding
monocation occurs. The NO stretching frequency depends
somewhat on the overall charge of the complex, which is
normal: it is observed at the highest frequency in the
monocation and decreases by only ∼35 cm-1 per one-
electron reduction, a smaller decrease than frequently
observed, further indicating that reduction processes in this
series of complexes are not primarily associated with the
NO ligand.
Simple charge considerations require that the monoanionic
forms [1a]1--[5a]1- contain each two closed-shell dianionic
ligands (Lx)2-, but the neutral complexes [1a]0-[5a]0 must
have three negative charges per two dithiolene ligands (one
dianion (Lx)2- and one radical monoanion (L•)1-), and,
finally, the monocations [1a]1+-[4a]+ must contain two
negative charges per two dithiolene ligands (two π-radical
anions (Lx•)1-).
chromophores22 (M ) NiII, PdII, PtII, AuIII). This absorption
maximum is present in all of the monocations [1a]1+-[4a]1+
for which therefore an electronic structure as in [FeII(NO+)-
(L•)2]1+ is deduced from the above spectroscopic data.
Similarly, planar [M(L)(L•)]z T [M(L•)(L)]z chromophores
(M ) NiII, PdII, PtII, AuIII; z ) 1- or 0)22 have been shown
to display intervalence charge-transfer bands (IVCT) in the
NIR with approximately one-half of the intensity of the
LLCT band in the [M(L•)2]z species. The same holds true
for the neutral (nitrosyl)iron complexes [1a]0-[4a]0 (Table
5). Furthermore, the EPR spectra of the neutral S ) 1/2
species display rhombic signals with small g anisotropy and
no 14N hyperfine splitting. These spectra are characteristic
of sulfur-centered radicals. Therefore, from all spectroscopic
data we can safely deduce the following electronic structure
of these neutral complexes: [FeII(NO+)(L•)(L)]0 T [FeII-
(NO+)(L)(L•)]0.
Finally, a summary of all elucidated electronic structures
of the series [Fe(NO)(L)2]z (z ) 1+, 0, 1-, 2-) is given in
Scheme 2.
We note that the trianion [Fe(NO)(mnt)2]3- has been
deduced to exist from electrochemical experiments only.1-3
No spectroscopic data are available, and therefore, the
proposed structure in Scheme 2 is tentative. We also note
that the photoelectron spectra of [M(mnt)2]2-/1- species (M
) Fe, Co, Ni, Cu) have recently been reported.28
The {FeNO}6 moiety in the monocations, neutral com-
plexes, and monoanions is best described as containing a
low-spin Fe(II) (SFe ) 0; d6) coordinated by an NO+ ligand.
In a previous report20 we have shown that the monoanion
[FeII(L•)2(CN)]1- and the neutral complex [FeII(L•)2{P(OPh)3}]
also contain an FeII(L•)2 chromophore with a low-spin Fe-
(II) center where (L•)1- represents the π-radical monoanion
of di-(4-tert-butylphenyl)-1,2-ethylenedithiolate. The Mo¨ss-
bauer parameters of these two low-spin Fe(II) complexes
are remarkably similar to those of the present (nitrosyl)iron
Acknowledgment. We thank the Fonds der Chemischen
Industrie for financial support and the EPSRC (U.K.) for a
studentship (K.S.).
Supporting Information Available: Crystallographic details
on positional and displacement parameters; bond lengths and angles
for compounds [1a], [2a], and [Co(Cp)2][1a] in CIF format. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
IC061874A
(28) Waters, T.; Wang, X.-B.; Woo, H.-K.; Wang, L.-S. Inorg. Chem. 2006,
45, 5841.
(27) Mo¨ller, E.; Sieler, J.; Kirmse, R. Z. Naturforsch. 1997, 52b, 919.
532 Inorganic Chemistry, Vol. 46, No. 2, 2007