The electronic structures of an isostructural series of octahedral nitrosyliron complexes {Fe-NO}6,7,8 elucidated by Mossbauer spectroscopy

Article abstract of DOI:10.1021/ja994161i

From the reaction of cis-[(cyclam)Fe(III)(Cl)₂]Cl (cyclam = 1,4,8,11- tetraazacyclotetradecane) with hydroxylamine in water the octahedral nitrosyliron complexes trans-[(cyclam)Fe(NO)Cl](ClO₄) (1) and cis- [(cyclam)Fe(NO)I]I (2) have been isolated as crystalline solids. EPR spectroscopy and variable-temperature susceptibility measurements established that 1 possesses an S = 1/2 and 2 an S = 3/2 ground state; both species are of the {Fe-NO}⁷ type. Electrochemically, 1 can be reversibly one-electron oxidized yielding trans-[(cyclam)Fe(NO)Cl]²⁺, an {Fe-NO}⁶ species, and one-electron reduced yielding trans-[(cyclam)Fe(NO)Cl]⁰, an {Fe-NO}⁸ species. These complexes have been characterized in CH₃CN solutions by UV- vis and EPR spectroscopy; both possess a singlet ground state. All of these nitrosyliron complexes, including [LFe(NO)(N₃)₂] (S = 3/2; L = 1,4,7- trimethyl-1,4,7-triazacyclononane) and [L'Fe(NO)(ONO)(NO₂)](ClO₄) (S = 0; L' = 1,4,7-triazacyclononane), have been studied by variable-temperature Mossbauer spectroscopy both in zero and applied fields. The oxidation of 1 is best described as metal-centered yielding a complex with an Fe(IV) (S = 1) coupled antiferromagnetically to an NO^- (S = 1), whereas its reduction is ligand-centered and yields a species with a low-spin ferric ion (S = 1/2 ) antiferromagnetically coupled to an NO^2- (S = 1/2 ). In agreement with Solomon et al. (J. Am. Chem. Soc. 1995, 117, 715) both {Fe-NO}⁷ (S = 3/2) species in this work are described as high-spin ferric (S = 5/2) antiferromagnetically coupled to an NO^- (S = 1). Complex 1 is proposed to contain an intermediate spin ferric ion (S = 3/2) antiferromagnetically coupled to NO^- (S = 1). The alternative descriptions as low-spin ferric antiferromagnetically coupled to NO^- (S = 1) or low-spin ferric with an NO^- (S = 0) ligand are ruled out by the applied field Mossbauer spectra.

Full text of DOI:10.1021/ja994161i

4352
J. Am. Chem. Soc. 2000, 122, 4352-4365
The Electronic Structures of an Isostructural Series of Octahedral
Nitrosyliron Complexes {Fe-NO}6,7,8 Elucidated by Mo¨ssbauer
Spectroscopy
Christina Hauser, Thorsten Glaser, Eckhard Bill,* Thomas Weyhermu1ller, and
Karl Wieghardt*
Contribution from the Max-Planck-Institut fu¨r Strahlenchemie, Stiftstrasse 34-36,
D-45470 Mu¨lheim a.d. Ruhr, Germany
ReceiVed NoVember 29, 1999
Abstract: From the reaction of cis-[(cyclam)FeIII(Cl)2]Cl (cyclam ) 1,4,8,11-tetraazacyclotetradecane) with
hydroxylamine in water the octahedral nitrosyliron complexes trans-[(cyclam)Fe(NO)Cl](ClO4) (1) and cis-
[(cyclam)Fe(NO)I]I (2) have been isolated as crystalline solids. EPR spectroscopy and variable-temperature
susceptibility measurements established that 1 possesses an S ) 1/2 and 2 an S ) 3/2 ground state; both species
are of the {Fe-NO}7 type. Electrochemically, 1 can be reversibly one-electron oxidized yielding trans-[(cyclam)-
Fe(NO)Cl]2+, an {Fe-NO}6 species, and one-electron reduced yielding trans-[(cyclam)Fe(NO)Cl]0, an {Fe-
NO}8 species. These complexes have been characterized in CH3CN solutions by UV-vis and EPR spectroscopy;
3
both possess a singlet ground state. All of these nitrosyliron complexes, including [LFe(NO)(N3)2] (S ) /2;
L ) 1,4,7-trimethyl-1,4,7-triazacyclononane) and [L′Fe(NO)(ONO)(NO2)](ClO4) (S ) 0; L′ ) 1,4,7-
triazacyclononane), have been studied by variable-temperature Mo¨ssbauer spectroscopy both in zero and applied
fields. The oxidation of 1 is best described as metal-centered yielding a complex with an FeIV (S ) 1) coupled
antiferromagnetically to an NO- (S ) 1), whereas its reduction is ligand-centered and yields a species with a
low-spin ferric ion (S ) 1/2) antiferromagnetically coupled to an NO2- (S ) 1/2). In agreement with Solomon
3
et al. (J. Am. Chem. Soc. 1995, 117, 715) both {Fe-NO}7 (S ) /2) species in this work are described as
high-spin ferric (S ) 5/2) antiferromagnetically coupled to an NO- (S ) 1). Complex 1 is proposed to contain
3
an intermediate spin ferric ion (S ) /2) antiferromagnetically coupled to NO- (S ) 1). The alternative
descriptions as low-spin ferric antiferromagnetically coupled to NO- (S ) 1) or low-spin ferric with an NO-
(S ) 0) ligand are ruled out by the applied field Mo¨ssbauer spectra.
Introduction
represents the dianion of ethylenediaminetetraacetic acid and
L is 1,4,7-trimethyl-1,4,7-triazacyclononane. Both species are
Non-heme ferrous centers in a number of metalloproteins are
known to react reversibly with NO1-7 with formation of
paramagnetic nitrosyliron centers which according to the
Enemark and Feltham notation8 are of the {Fe-NO}7 type.
3
of the type {Fe-NO}7 (S ) /2) and the latter complex has
been structurally characterized11 and spectroscopically studied
by using X-ray absorption,12a,b resonance Raman, magnetic
circular dichroism, electron paramagnetic resonance,12a and, in
part, Mo¨ssbauer spectroscopies as well as SQUID magnetic
susceptibility.12a In addition, SCF-XR-SW12a and, very recently,
GGA3 density functional calculations have been reported. Table
1 lists some spectroscopic properties of these model complexes
and metalloproteins.
It is therefore rather discomforting that despite these enormous
spectroscopic efforts and calculations using the most advanced
computational programs available the conclusions with regard
to the best description of the electronic structure of this important
class of {Fe-NO}7 (S ) 3/2) complexes remains to be somewhat
controversial.3,12 While Solomon et al.12 have presented compel-
ling spectroscopic evidence (with the exception of Mo¨ssbauer
These non-heme iron enzyme nitrosyl derivatives invariably
3
possess an EPR-active S )
/ ground state. In inorganic
2
chemistry9-11 two complexes have become archetype models,
namely [Fe(EDTA)(NO)]1 and [LFe(NO)(N3)2],11 where EDTA
(1) Arciero, D. M.; Lipscomb, J. D.; Huynh, B. H.; Kent, T. A.; Mu¨nck,
E. J. Biol. Chem. 1983, 258, 14981.
(2) Nocek, J. M.; Kurtz, D. M., Jr.; Sage, J. T.; Xia, Y.-M.; Debrunner,
P.; Shiemke, A. K.; Sanders-Loehr, T. M. Biochemistry 1988, 27, 1014.
(3) Rodriguez, J. H.; Xia, Y.-M.; Debrunner, P. J. Am. Chem. Soc. 1999,
121, 7846.
(4) Bill, E.; Bernhardt, F.-H.; Trautwein, A. X.; Winkler, H. Eur. J.
Biochem. 1985, 147, 177.
(5) Haskin, C. J.; Ravi, N.; Lynch, J. B.; Mu¨nck, E.; Que, L., Jr.
Biochemistry 1995, 34, 11090.
(6) Chen, V. J.; Orville, A. M.; Harpel, M. R.; Frolik, C. A.; Sureros,
K.; Mu¨nck, E.; Lipscomb, J. D. J. Biol. Chem. 1989, 264, 21677.
(7) Orville, A. M.; Chen, V. J.; Kriauciunas, A.; Harpel, M. R.; Fox, B.
G.; Mu¨nck, E.; Lipscomb, J. D. Biochemistry 1992, 31, 4602.
(8) Enemark, J. H.; Feltham, R. D. Coord. Chem. ReV. 1974, 13, 339.
(b) Feltham, R. D.; Enemark, J. H. Topic in Inorganic and Organometallic
Stereochemistry; Geoffroy, G. L., Ed.; Topics in Stereochemistry, Vol. 12;
Allinger, N. L., Eliel, E. L., series Eds.;; Wiley: New York, 1981, 155. (c)
Westcott, B. L.; Enemark, J. H. In Inorganic Electronic Structure and
Spectroscopy; Solomon, E. I., Lever, A. B. P., Eds.; Wiley: New York,
1999; Vol. II, p 403.
(9) Wells, F. V.; McCann, S. W.; Wickman, H. H.; Kessel, S. L.;
Hendrickson, D. N.; Feltham, R. D. Inorg. Chem. 1982, 21, 2306.
(10) Feig, A.; Bantista, M. T.; Lippard, S. J. Inorg. Chem. 1996, 35,
6892.
(11) Pohl, K.; Wieghardt, K.; Nuber, B.; Weiss, J. J. Chem. Soc., Dalton
Trans. 1987, 187.
(12) Brown, C. A.; Pavlosky, M. A.; Westre, T. E.; Zhang, Y.; Hedman,
B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 1995, 117, 715. (b)
Westre, T. E.; Di Cicco, A.; Filipponi, A.; Natoli, C. R.; Hedman, B.;
Solomon, E. I.; Hodgson, K. O. J. Am. Chem. Soc. 1994, 116, 6757.
10.1021/ja994161i CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/22/2000

Octahedral Nitrosyliron Complexes {Fe-NO}6,7,8
Table 1. Selected Spectral Parameters for {Fe-NO}7 (S ) /2) Systems
CNe) δ, mm s-1 a ∆EQ, mm s-1 b
J. Am. Chem. Soc., Vol. 122, No. 18, 2000 4353
3
R(Fe-NO), degc
ν(NO), cm-1 d
ref
proteins
protocatechuate 4,5-dioxygenase/substrate
deoxyhemerythrin
deoxyhemerythrin F-NO
putidamonooxin/substrate
ribonucleotide reductase
isopenicillin N synthase
isopenicillin N synthase-ACV
model complexes
[Fe(5-Cl-salen)(NO)]
[LFe(NO)(N3)2]
[Fe(EDTA)(NO)]
[Fe2(NO)2(Et-HPTB)(O2CPh)](BF4)2
cis-[(cyclam)Fe(NO)I]I (2)
6
6
0.66
0.68
0.77
0.68
0.70
0.75
0.65
-1.67
+0.61
1.03f)
-1.4
-1.7
-1.0
-1.2
1
2,3
3
4
5
6,7
7
6
6
6
5
6
5
6
6
0.654
0.62
0.66
0.67
0.64
0.575f)
-1.28
-1.67
1.44f)
147(298 K), 127(175 K)
9
155.6
156(5)
167
1690,1712
1776
1785
this work
1
10
this work
-1.78
1726
a Isomer shift at 80 K (or below). b Quadrupole splitting. c Bond angle from X-ray or EXAFS. d NO stretching frequency. e CN ) coordination
number. f Sign not determined
1
Table 2. Selected Complexes of the {Fe-NO}7 (S ) /2) Type and EPR Spectra
A(14NNO), G
complex
g1
g2
g3
A1
A2
A3
ref
14e
HbNO
MbNO
2.082
2.0728
2.102
2.080.
2.02
2.043
2.046
2.039
2.015
2.085
2.0254
2.0068
2.064
2.040
1.99
2.039
2.039
2.035
1.9909
1.9850
2.010
2.003
1.97
2.031
2.028
2.025
1.988
2.01
27.1
18.6
17.2
19.3
14d
13a
13a
13b
13c
13c,d
13e,f
13g
51
[(tpp)Fe(NO)]
12.6
17.3
21.7
24.0
12.2
14.9
15.5
[(tpp)Fe(NO)(pip)]
[(tim)Fe(NO)(CH3CN)]
[(i-mnt)Fe(NO)]2-
[(Me2dtc)2Fe(NO)]
[(E2dtc)2Fe(NO)]
16.0
12.6
13.4
14.5
12.2
12.1
[(das)2Fe(NO)Br]+
[(TpivPP)Fe(NO)(NO)2]-
spin crossover systems
[(tmc)Fe(NO)]2+
2.032
24
16.5
16
S ) 3/2 H S ) 1/2
21
[(Salen)Fe(NO)]
[(Salophen)Fe(NO)]
9, 13h
13i
1
spectroscopy) and calculations which favor a bonding model
been reported to possess an S ) /2 ground state.14 Five- and
six-coordinate species of this type have been structurally
characterized but little advanced Mo¨ssbauer spectroscopic data
are available to date. A few cases of {Fe-NO}7 species which
exhibit high-spin (S ) 3/2) H low-spin (S ) 1/2) equilibria have
also been reported and structurally characterized.
5
which invokes high-spin FeIII (S ) /2) antiferromagnetically
3
coupled to NO- (S ) 1) yielding the observed S ) /2 ground
state, Rodriguez et al.3 have pointed out that the observed isomer
shift in the Mo¨ssbauer spectra of {Fe-NO}7 complexes is
intermediate between that of octahedral ferric and ferrous
species. Their density functional theoretical calculations has
allowed them to trace these unusual isomer shifts to “strong
valence electron delocalization within the {Fe-NO}7 unit,
whereby some electrons are almost equally shared by metal (i.e.
dxz and dyz) and NO π* orbitals.”
Our strategy to gain a better understanding of the electronic
structure of such species has been to design and synthesize a
series of “isostructural” complexes and study each member by
a combination of spectroscopic methods including a detailed
variable-temperature Mo¨ssbauer investigation in zero and ap-
plied fields.
It has been stated repeatedly in the literature that Mo¨ssbauer
parameters and, specifically, the isomer shift do not readily allow
the assignment of formal oxidation states in nitrosyliron
complexes.8,10 In no instance has a complete series of isostruc-
tural octahedral complexes containing the three {Fe-NO}6,7,8
moieties been systematically investigated by this spectroscopic
method.
Similarly, {Fe-NO}7 complexes with an S ) 1/2 ground state
have been reported in the literature (Table 2).13 The nitrosyl
derivatives of myo- and hemoglobin (MbNO, HbNO) have also
In a recent paper15 we have shown that the isomer shift of a
series of iron complexes of the Werner type decreases linearly
with increasing formal oxidation state: trans-[(cyclam)FeII-
(N3)2]0 (S ) 0), trans-[(cyclam)FeIII(N3)2]+ (S ) 1/2), [(cyclam)-
(N3)FeIII)NdFeIV(cyclam)(N3)]2+ (S ) 1/2 and 3/2 isomers), and
[(cyclam)FeV(N)(N3)]+ (S ) /2) where cyclam is the macro-
3
cyclic amine 1,4,8,11-tetraazacyclotetradecane. Note that in this
series the dn electron configuration of the iron ion varies
systematically by one-electron steps from t6 to t3 (in Oh
2g
2g
symmetry) and all complexes contain an octahedral FeN6
coordination polyhedron. In the latter two species the strong
(13) Wayland, B. B.; Olson, L. W. J. Am. Chem. Soc. 1974, 96, 6037.
(b) Chen, Y.; Sweetland, A.; Shepherd, R. E. Inorg. Chim. Acta 1997, 260,
163. (c) Mo¨ller, E.; Sieler, J.; Kirmse, R. Z. Naturforsch. 1997, 52b, 919.
(d) Feltham, R. D.; Crain, H. Inorg. Chim. Acta 1980, 40, 37. (e) Doodman,
B. A.; Raynor, J. B.; Symons, M. C. R. J. Chem. Soc. 1969, 2572. (f) Guzy,
C. M.; Raynor, J. B.; Symons, M. C. R. J. Chem. Soc. A 1969, 2987. (g)
Enemark, J. H.; Feltham, R. D.; Huie, B. T.; Johnson, P. L.; Bizot Swedo,
K. J. Am. Chem. Soc. 1977, 99, 3285. (h) Haller, K. J.; Johnson, P. L.;
Feltham, R. D.; Enemark, J. H.; Ferraro, J. R.; Basile, L. J. Inorg. Chim.
Acta 1979, 33, 119. (i) Leeuwenkamp, O. R.; Plug, C. M.; Bult, A.
Polyhedron 1987, 6, 295.
(14) Lang, G.; Marshall, W. Proc. Phys. Soc. 1966, 87, 3. (b) Oosterhuis,
W. T.; Lang, G. J. Chem. Phys. 1969, 50, 4381. (c) Spartalian, K.; Lang,
G.; Yonetani, T. Biochim. Biophys. Acta 1976, 428, 281. (d) Dickinson, L.
C.; Chien, C. W. J. Am. Chem. Soc. 1971, 93, 5036. (e) Chien, J. C. J.
Chem. Phys. 1969, 51, 4220. (f) Saucier, K. M.; Freeman, G.; Mills, J. S.
Science 1962, 137, 752.
(15) Meyer, K.; Bill, E.; Mienert, B.; Weyhermu¨ller, T.; Wieghardt, K.
J. Am. Chem. Soc. 1999, 121, 4859.

4354 J. Am. Chem. Soc., Vol. 122, No. 18, 2000
Hauser et al.
Table 3. Crystallographic Data for trans-[(cyclam)Fe(NO)Cl]ClO4
(1) and [L′Fe(NO)(NO2)2](ClO4)
nitrido π-donor ligand is present whereas in the former two this
is not the case. Clearly, the Mo¨ssbauer isomer shift is an
excellent marker for the given dn electron configuration at the
iron ion.
In this work we report a related series of octahedral
nitrosyliron complexes, namely trans-[(cyclam)Fe(NO)Cl]-
(ClO4) [an {Fe-NO}7 (S ) 1/2) species], cis-[(cyclam)Fe(NO)I]I
[an {Fe-NO}7 (S ) 3/2) species], trans-[(cyclam)Fe(NO)Cl]2+
[an {Fe-NO}6 (S ) 0) species], and trans-[(cyclam)Fe(NO)I]0
[an {Fe-NO}8 (S ) 0) species]. We have spectroscopically
(UV-vis, EPR, Mo¨ssbauer) characterized their electronic
structures. The results show that Mo¨ssbauer spectroscopy is a
valuable tool for the correct assignment of oxidation levels.
1
[L′Fe(NO)(NO2)2](ClO4)
chem formula
FW
space group
a, Å
b, Å
c, Å
C10H24Cl2FeN5O5
421.09
Pbca
13.010(2)
10.853(1)
23.692(4)
90o
3345.3(8)
8
C6H15ClFeN6O9
406.54
Pn
13.450(2)
8.1261(8)
13.478(2)
109.43(2)
1389.2(3)
4
â, deg
V, Å3
Z
T, K
100(2)
1.672
100(2)
1.944
Siemens SMART
0.71073
13.41
8915
5814
439/47
F
calc, g cm-3
diffractometer used Siemens SMART
λ(Mo KR), Å
0.71073
12.52
21031
3528
Experimental Section
µ, cm-1
The ligand 1,4,8,11-tetraazacyclotetradecane (cyclam)16 and com-
plexes cis-[(cyclam)Fe(Cl2)]Cl17 as well as [LFe(NO)(N3)2]11 have been
prepared as decribed in the literature.
no. of data
no. of unique data
no. of parameters/
restraints
244/0
cis-[(cyclam)56/57Fe(Cl)2]Cl. To prepare 57Fe isotopically enriched
complexes the starting material cis-[(cyclam)56/57Fe(Cl2)]Cl was syn-
thesized as follows. 57Fe (34.4 mg) and 56Fe (59.9 mg) foil was dissolved
in concentrated hydrochloric acid in the presence of air by heating to
reflux. The clear solution was dried in vacuo and the 56/57FeCl3 residue
was dissolved in dry methanol (10 mL). Addition of the ligand cyclam
(0.46 g; 2.44 mmol) and heating to reflux for 1 h produced a dark
yellow solution that was cooled to 20 °C. Dropwise addition of
30% HCl initiated the precipitation of brown microcrystalline cis-
[(cyclam)56/57Fe(Cl)2]Cl (35% 57Fe), which was filtered off. Further
addition of HCl to the solution produced colorless crystals of
[H2cyclam]Cl2 which were removed by filtration. Further addition of
HCl yielded another crop of crystalline cis-[(cyclam)56/57Fe(Cl2)]Cl.
Yield: 0.55 g (90%).
R1a; wR2 (all data)b 0.054; 0.124
0.046; 0.1207
a R1 ) ∑(|Fo| - |Fc|)/∑|Fo| observation criterion: I > 2σ(I). b wR2
2
2
) [∑[w(Fo2 - Fc2)2]/∑[w(Fo )2]1/2 where w ) 1/σ2(Fo ) + (aP)2 + bP,
2
P ) (Fo + 2Fc2)/3.
data were collected by hemisphere runs taking frames at 0.30° in ω.
The data sets were corrected for Lorentz and polarization effects and
for 1 a semi-empirical absorption correction was carried out using the
program SADABS.18 Crystallographic data are listed in Table 3. The
Siemens ShelXTL19 software package was used for solution, refinement
and artwork of the structures. All non-hydrogen atoms were refined
with anisotropic displacement parameters. The hydrogen atoms were
placed at calculated positions and refined as riding atoms.
The oxygen atoms of the perchlorate anion in 1 were found to be
disordered over two sites. The two sets of atoms were refined with
occupancy factors of 0.67 and 0.33, respectively. A weak hydrogen
bridge between the cation and the perchlorate anions in 1 gives rise to
a slight disorder of the iron complex. This results in abnormally large
thermal parameters for all atoms not lying on the Cl-Fe-NNO axis of
the complex.
trans-[(cyclam)Fe(NO)(Cl)](ClO4) (1). To a solution of cis-[(cyclam)-
Fe(Cl)2]Cl (0.20 g; 0.55 mmol) in water (20 mL) was added solid
[NH3OH]Cl (1.15 g; 16.5 mmol) whereupon the starting complex cis-
[(cyclam)FeCl)2]Cl started to precipitate again. To this yellow suspen-
sion was added dropwise 1.0 M aqueous NaOH with stirring at ambient
temperature until at pH 6-7 a deep blue clear solution was obtained.
The color of this solution changed within 5 min to brown-red. Addition
of NaClO4 (2.0 g) initiated the precipitation of brown microcrystals of
1. Yield: 0.08 g (36%). Slow recrystallization of the crude material
from acetonitrile solution produced single crystals of 1 suitable for X-ray
crystallography. Anal. Calcd for C10H24N5O5FeCl2: C, 28.50; H, 5.74;
N, 16.63. Found: C, 28.3; H, 5.7; N, 16.3.
One of the two crystallographically independent cations in [L′Fe-
(NO)(ONO)(NO2)](ClO4) displays a disorder of the carbon atoms of
the 1,4,7-triazacyclononane backbone. This disorder was successfully
modeled by a split atom model for these C atoms with occupancy factors
of 0.65 and 0.35, respectively.
cis-[(cyclam)Fe(NO)(I)]I (2). To a suspension of cis-[(cyclam)Fe-
(Cl)2]Cl (0.20 g; 0.55 mmol) and [NH3OH]Cl (1.15 g) in water (10
mL) was added NaI (3.0 g; 20 mmol). Immediately after the addition
of NaI an aqueous solution of 1.0 M NaOH was added dropwise with
stirring (pH 6-7) until from the reddish reaction mixture dark green
microcrystals precipitated which were rapidly filtered off, washed with
cold ethanol and diethyl ether, and air-dried. Yield: 0.20 g (67%). Anal.
Calcd for C10H24N5OI2Fe: C, 22.24; H, 4.48; N, 12.97. Found: C, 22.1;
H, 4.3; N, 12.8.
[L′Fe(NO)(ONO)(NO2)](ClO4). A solution of L′FeCl3 (0.8 g), where
L′ represents 1,4,7-triazacyclononane, and NaNO2 (7.5 g) in 0.10 M
aqueous hydrochloric acid (120 mL) was stirred at ambient temperature
for 1.5 h. To the filtered brown solution was added NaClO4‚H2O (2.5
g). Upon storage at 0 °C for 12 h a brown crystalline precipitate formed
which was collected by filtration. Yield: 0.46 g. Anal. Calcd for C6H15-
ClFeN6O9: C, 17.73; H, 3.72; N, 20.67. Found: C, 17.5; H, 3.7; N,
20.7.
Physical Measurements. The equipment used for IR, UV-vis,
Mo¨ssbauer, and EPR spectroscopy has been described in ref 15. Low-
temperature EPR, magnetic Mo¨ssbauer spectra, and magnetization
measurements were analyzed on the basis of a spin-Hamiltonian
description of the electronic ground-state spin multiplet as is described
in detail also in ref 15.
Results
Preparation and Characterization of Complexes. To obtain
a deeper understanding of the electronic structures of a series
of octahedral complexes containing the {Fe-NO}6,7,8 structural
motif as electronically unperturbed by the co-ligands as possible,
we used the redox-innocent macrocycles 1,4,7-trimethyl-1,4,7-
triazacyclononane (L) in the previously synthesized and struc-
11
turally characterized complex [LFe(NO)(N3)2] ({Fe-NO}7,
S ) 3/2) and triazacyclononane (L′) in [L′Fe(NO)(ONO)-
(NO2)]ClO411 ({Fe-NO}6, S ) 0), and in this work, 1,4,8,11-
tetraazacyclotetradecane (cyclam).
X-ray Crystallographic Data Collection and Refinement. A
brown-violet single crystal of 1 and a brown crystal of [L′Fe(NO)-
(ONO)(NO2)](ClO4) were sealed in a glass capillary and mounted on
a Siemens SMART CCD-detector diffractometer equipped with a
cryogenic nitrogen cold stream at 100 K, respectively. Graphite
monochromated Mo KR radiation (λ ) 0.71073 Å) was used. Intensity
From the reaction of cis-[(cyclam)Fe(Cl2)2]Cl, a high-spin
ferric precursor,17 with hydroxylamine in water at 20 °C we
(18) SADABS, Sheldrick, G. M., Universita¨t Go¨ttingen, 1994.
(19) ShelXTL V. 5.0; Siemens Analytical X-ray Instruments, Inc.
Maddison, WI.
(16) Barefield, K. E.; Wagner, F. Inorg. Synth. 1976, 16, 220.
(17) Chan, P.-K.; Poon, C.-K. J. Chem. Soc., Dalton Trans. 1976, 858.

Octahedral Nitrosyliron Complexes {Fe-NO}6,7,8
J. Am. Chem. Soc., Vol. 122, No. 18, 2000 4355
Table 4. Selected Bond Distances (Å) and Angles (deg) of 1 and
[L′Fe(NO)(ONO)(NO2)](ClO4)
Complex 1
Fe-N1
Fe-N2
Fe-N3
Fe-N4
2.005(3)
2.007(3)
2.010(3)
1.996(3)
Fe-N5
1.820(4)
2.456(1)
1.006(4)
144.0(4)
Fe-Cl
N5-O1
Fe-N5-O1
Complex [L′(NO)(ONO)(NO2)](ClO4)
Fe1-N1
1.644(4)
1.914(3)
1.969(4)
1.993(4)
1.996(4)
125.1(3)
116.3(4)
122.4(4)
Fe1-N6
1.999(4)
1.163(5)
1.216(6)
1.322(5)
1.216(6)
1.282(5)
171.2(4)
120.9(4)
116.6(4)
Fe1-O2
N1-O1
Fe1-N3
N2-O3
Fe1-N4
N2-O2
Fe1-N5
N3-O4
Fe1-O2-N2
O2-N2-O3
O4-N3-O5
N3-O5
Fe1-N1-O1
Fe1-N3-O4
Fe1-N3-O5
Figure 1. Perspective view of the monocation in crystals of 1 (top)
and of one crystallographically independent cation in crystals of
[L′Fe(NO)(ONO)(NO2)](ClO4) (bottom). Small circles represent N-H
protons while those of the methylene groups are not shown.
obtained upon addition of NaClO4 brown microcrystals of trans-
[(cyclam)Fe(NO)(Cl)](ClO4) (1). Using similar reaction condi-
tions but adding NaI instead of NaClO4 and performing the
precipitation and workup more rapidly we obtained green
microcrystals of cis-[(cyclam)Fe(NO)I]I (2).
Figure 2. Temperature dependence of magnetic moments, µeff/µB, of
solid samples of 1 and 2. The solid lines represent best fits using the
parameters given in the text.
ion is displaced by only 0.086 Å from the best plane defined
by the four amine nitrogens which deviate by an average of
only 0.004 Å from this plane. The sixth coordination site in 1
is occupied by a loosely bound chloride ion with an Fe-Cl
bond length of 2.456(1) Å.
The cis and trans configuration of the coordinated ligand
cyclam in 2 and 1, respectively, was clearly established by their
infrared spectra.20 For 1 a single out-of-plane wagging mode,
γ(N-H) at 888 cm-1, and a single rocking mode, δ(CH2) at
808 cm-1, have been observed indicative of the trans config-
uration. In contrast, for 2 two frequencies are observed for each
The electronic ground states of 1 and 2 have been established
from variable-temperature and variable-field susceptibility
measurements by using a SQUID magnetometer. Figure 2 shows
the temperature dependence of the effective magnetic moments
of 1 and 2 in an external magnetic field of 1 T and a best fit of
the data. Both species are paramagnetic. The fit shown in Figure
2 for 1 has been obtained by using the following parameters:
a fixed g ) 2.012 which was determined from EPR measure-
ments (see below) and a temperature-independent paramagnet-
ism, øTIP, of 483 × 10-6 cm3 mol-1. Thus 1 possesses an S )
1/2 ground state of {Fe-NO}7. In contrast, complex 2 exhibits
of these modes: γ(N-H) 862, 852, and δ(CH2) 805, 793 cm-1
.
The ν(NO) stretching frequency has been detected at 1611 cm-1
for 1 and 1726 cm-1 for 2.
Complex 2 must be rapidly precipitated from solution. We
have found that these preparations invariably are contaminated
5
with ∼10% cis-[(cyclam)FeIII(I)2]I (S ) /2). It is not possible
to purify 2 by fractional recrystallization because 2 isomerizes
fairly rapidly even at lower temperatures within a few seconds
yielding a trans-configurated species.
The crystal structure of 1 has been determined by single-
crystal X-ray crystallography at 100(2) K. Figure 1 shows the
structure of the monocation in crystals of 1 and Table 4 gives
selected bond distances and angles. The cyclam ligand is bound
in its strain-free trans-III or two-up/two-down configuration to
the central iron ion (R, S, S, R configuration at the chiral
nitrogen atoms). The average Fe-Namine bond distance at 2.03
( 0.01 Å is in excellent agreement with simple low-spin ferric
complexes of the type trans-[(cyclam)FeIII(X)2]+. For example,
3
an S ) /2 ground state. The fit of the data in Figure 2 was
obtained by including 12% of a paramagnetic impurity of cis-
[(cyclam)FeIII(I)2]I (S ) 5/2) as was determined from Mo¨ssbauer
spectroscopy (see below). The marked decrease of the magnetic
moment of 2 at temperatures <50 K is due to a large zero-field
splitting of 2 which was modeled with D3/2 ) +12.6 cm-1 in
agreement with Mo¨ssbauer spectroscopic simulations for an S
3
) /2 ground state. In conjunction with the simulations of the
-
for X ) N3 an average Fe-Namine distance of 2.00 ( 0.01 Å
magnetic Mo¨ssbauer spectra we remeasured also the magnetic
susceptibility of the S ) 3/2 complex [LFe(NO)(N3)2]11 by using
variable fields (1, 4, 7 T). In contrast to the very large value
D3/2 ) 20 cm-1 reported previously,12a our data are best
described with a ZFS of D3/2 ) +13.6 cm-1 (Figure S1,
Supporting Information).
has recently been determined.15 The coordinated nitrosyl forms
a bent Fe-NO moiety with a bond angle at 144.0(4)°. The Fe-
NNO bond at 1.820(4) Å indicates considerable double bond
character. The iron ion is bound in the in-plane mode; the Fe
(20) Poon, C.-K. Inorg. Chim. Acta 1970, 5, 322.

4356 J. Am. Chem. Soc., Vol. 122, No. 18, 2000
Hauser et al.
Structurally characterized octahedral {Fe-NO}6 complexes
are extremely rare. [(tmc)Fe(NO)(OH)](ClO4) (tmc ) tetra-
methylcyclam) represents an early example which has not been
spectroscopically characterized due to its somewhat problematic
synthetic accessibility.21 In 1986 we had described the synthesis
of [L′Fe(NO)(NO2)2](ClO4)‚2H2O but at that time did not obtain
single crystals.11 From its infrared spectrum we concluded that
this species contains a nitrosyl(dinitro)iron core: ν(NdO) 1890;
νas(NO2) 1430; and νs(NO2) 1310 cm-1. In an attempt to grow
single crystals we have now obtained an isomer of this complex
containing the nitrosyl(nitrito)nitroiron core, namely, [L′Fe(NO)-
(ONO)(NO2)](ClO4). In the infrared two ν(NO) stretching
frequencies at 1907 and 1885 cm-1 are observed; the O-
coordinated nitrito ligand gives rise to two ν(NdO) stretching
frequencies at 1477 and 1464 cm-1, and the N-coordinated nitro
group displays N-O stretching frequencies νas(NO2) at 1439,
1419 and νs(NO2) at 1315, 1306 cm-1. [L′Fe(NO)(ONO)(NO2)]-
(ClO4) is diamagnetic indicating an S ) 0 ground state.
The crystal structure determination reveals that two crystal-
lographically independent cations [L′Fe(NO)(ONO)(NO2)]+ are
present in the crystals. The cation possesses two sources of
chirality. The iron ion is a center of chirality giving rise to ∆,
Λ configurations, and the three five-membered Fe-N-C-C-N
chelate rings of the 1,4,7-triazacyclononane ligand adopt either
a (λλλ) or a (δδδ) conformation. Thus the diastereomers ∆(λλλ)
and ∆(δδδ) and their respective enantiomers (Λ(δδδ), Λ(λλλ)
are present in equal amounts in the crystals. The latter are
interrelated by a crystallographic gliding mirror plane. Figure
1 (bottom) shows the structure of one of these cations [L′Fe-
(NO)(ONO)(NO2)]+ containing a nearly linear {Fe-NO}6
moiety, an O- and an N-coordinated nitrito and a nitro group,
respectively. Table 4 gives selected bond distances and angles.
The most salient feature of this structure are the short Fe-Namine
distances at an average 1.996 Å which are slightly shorter than
those in 1, an {Fe-NO}7 (S ) 1/2) species with av. Fe-Namine
) 2.004 Å) and much shorter than those in [LFe(NO)(N3)2],
an {Fe-NO}7 (S ) 3/2) species with av. 2.252 Å. For
comparison, in the low-spin ferrous and ferric species [L′2Fe]2+/3+
the average Fe-Namine distance is found at 2.03 and 1.99 Å,22
respectively, whereas this distance is >2.10 Å in all octahedral
high-spin ferric and >2.20 Å in high-spin ferrous species. These
observations lend strong support to the notion that [LFe(NO)-
(N3)2] (S ) 3/2) contains a high-spin ferric ion whereas in 1 the
eg metal orbitals (in Oh symmetry) must, at least partly, be
unoccupied as in a low or intermediate spin ferric ion. On
extrapolation of this argument we conclude that a low-spin FeIV
ion (S ) 1) may be present in [L′Fe(NO)(ONO)(NO2)]+. The
diamagnetic ground state is then attained by intramolecular
antiferromagnetic coupling of this FeIV ion to an NO- (S ) 1)
ligand.
Figure 3. Cyclic (top) and square-wave (middle) voltammograms of
1 in acetonitrile at 298 K (0.10 M [(n-Bu)4N]PF6 supporting electrolyte,
glassy carbon working electrode, reference electrode Ag/AgNO3). The
potentials are referenced vs the ferrocenium/ferrocene (Fc+/Fc) couple.
Bottom: Cyclic voltammograms of the oxidation and reduction wave,
respectively, at scan rates 2.0, 1.0, 0.50, 0.20, 0.10, and 0.05 V s-1
respectively.
,
in three different oxidation levels, namely a dicationic {Fe-
NO}6, a monocationic {Fe-NO}7, and a neutral {Fe-NO}8
species. From the coulometric measurements it was established
that the oxidized {Fe-NO}6 species is stable at ambient
temperature for at least 2 h whereas the reduced {Fe-NO}8
form is only stable at -30 °C. Hence the oxidized and reduced
species were readily investigated in solution by UV-vis, EPR,
and Mo¨ssbauer spectroscopy.
Figures 4 and 5 show the results of the spectroelectrochemical
oxidation of 1 at 20 °C and of the corresponding reduction at
-30 °C. The electronic spectrum of 1 in CH3CN displays two
Electro- and Spectroelectrochemistry. Figure 3 displays the
cyclic and square-wave voltammograms of 1 in acetonitrile (0.10
M [(n-Bu)4N]PF6) at 20 °C. In the following all redox potentials
are given vs the ferrocenium/ferrocene couple (Fc+/Fc). Clearly,
two reversible one-electron-transfer waves are detected where
the process at E1/2 ) 0.14 V corresponds to a one-electron
oxidation generating trans-[(cyclam)Fe(NO)Cl]2+ and that at
-1.30 V is due to a reversible one-electron reduction affording
trans-[(cyclam)Fe(NO)Cl]0 as was established from coulometric
measurements at appropriately fixed potentials. Thus 1 exists
absorption maxima in the visible at 398 (ꢀ ) 300 M-1 cm-1
)
and 560 nm (ꢀ ) 145 M-1 cm-1). Upon one-electron oxidation
these maxima disappear and a new band at 362 nm (ꢀ ) 500
M-1 cm-1) is generated.
Conversely, upon one-electron reduction of 1 the position of
the two absorption maxima of 1 remains unchanged at 398 and
560 nm only their intensity increases to ∼500 and ∼190 M-1
cm-1, respectively. This is a remarkable result: the spectra of
1 and trans-[(cyclam)Fe(NO)Cl]2+ are completely different in
nature whereas the spectra of 1 and trans-[(cyclam)Fe(NO)-
Cl]0 are nearly the same. If one assumes that due to the overall
low intensity of all maxima in the visible these transitions are
(21) Hodges, K. D.; Wollmann, R. G.; Kessel, S. L.; Hendrickson, D.
N.; Van Derveer, D. G.; Barefield, E. K. J. Am. Chem. Soc. 1979, 101,
906.
(22) Boeyens, J. C. A.; Forbes, A. G. S.; Hancock, R. D.; Wieghardt,
K. Inorg. Chem. 1985, 24, 2926-2931.

Octahedral Nitrosyliron Complexes {Fe-NO}6,7,8
J. Am. Chem. Soc., Vol. 122, No. 18, 2000 4357
Figure 6. Q-band EPR spectrum of 1 in a CH3CN/CH3OH (2:1)
mixture at 50 K. Conditions: ν ) 33.923 GHz; power 0.1 mW,
modulation amplitude 2 mT. Simulation parameters are given in the
text. Gaussian lines with angular- and mI-dependent widths were used
with parameters W ) (0.2, 0.2, 6.) mT, C1 ) (0.36, 0.37, -1.) mT/
GHz, and C2 ) (20.7, 5.4, 6.9) mT.28
Figure 4. Spectroelectrochemical changes observed during the one-
electron oxidation of 1 in CH3CN at 298 K. The arrows indicate the
appearance or disappearance of peaks during the oxidation.
) 0) the ν(NO) mode has been observed at 1890 cm-1 (tmc )
tetramethylcyclam).21 Scheidt et al.23 report a ν(NO) at 1937
cm-1 for six-coordinate [Fe(TPP)(NO)(H2O)](ClO4), also an
octahedral {Fe-NO}6 species.23 For [L′Fe(NO)(ONO)(NO2)]-
(ClO4) two ν(NO) bands at 1907 and 1885 cm-1 corresponding
to the two crystallographically independent cations have been
observed.
On the other hand, the well-characterized diamagnetic {Fe-
NO}8 (S ) 0) species containing a linear Fe-NO moiety in,
for example, [L4Fe(NO)]+ (L ) P(OMe)3, P(OEt)3),24 [Fe(NO)-
(np3)] (np3 ) tris(2-diphenylphosphinoethyl)amine),25 and other
five-coordinate organometallic phosphine complexes26,27 dis-
plays ν(NO) bands in the region ∼1730 ( 20 cm-1. If our
reduced trans-[(cyclam)Fe(NO)Cl]0 would be of this type we
would have detected its ν(NO) band using the above spectro-
electrochemical conditions. Thus these experiments point to a
different electronic structure of the reduced form of 1.
EPR Spectroscopy. The X-band EPR spectrum of 1 in a
frozen solution of acetonitrile at 10 K displays a broad (13.5
mT) isotropic signal at giso ) 2.012. It appears that intermo-
lecular dipolar spin couplings broaden the spectra and reduce
the spectral resolution, even in diluted samples (≈50 µM).
However, at higher frequency (Q-band) the higher resonance
fields apparently decouple the molecular spins so that a
reasonably well resolved EPR spectrum of 1 was obtained. The
spectrum is shown in Figure 6. The simulation was achieved
by involving first-order hyperfine interaction to one nitrogen
(of the NO ligand) (14N, I ) 1) with the parameters gx ) 2.0520,
gy ) 2.0145, gz ) 1.9698 and A(14N) ) (112, 49.9, 59.1) ×
10-4 cm-1 (117, 53, 64 G) with angular- and mI-dependent
Gaussian line shapes.28 This spectrum is remarkably similar to
those reported for many five-coordinate {Fe-NO}7 species with
Figure 5. Spectroelectrochemical changes observed during the one-
electron reduction of 1 in CH3CN at -30 °C. The arrows indicate the
increase in absorbance during reduction.
predominantly of d-d character, one arrives at the conclusion
that the {Fe-NO}7 and {Fe-NO}8 species have a rather similar
dn configuration implying that the added electron resides in a
π*(NO) ligand orbital. In contrast, {Fe-NO}6 and {Fe-NO}7
must then have different dn electron configurations. In other
words, the one-electron oxidation of 1 may be considered to be
metal-centered and the one-electron reduction to be ligand-
centered (NO).
We have also performed a coulometric one-electron oxidation
and a one-electron reduction of 1 dissolved in dimethyl sulfoxide
(dmso) at ambient temperature in a thin-layer electrochemical
cell and recorded the infrared spectra in the range 1500-2000
cm-1. For the parent complex 1 the ν(NO) stretching frequency
is observed at 1620 cm-1 which upon oxidation disappears and
a new band at 1902 cm-1 is observed which we assign to the
{Fe-NO}6 species trans-[(cyclam)Fe(NO)Cl]2+. Similarly, upon
one-electron reduction of 1 at -20 °C the 1620 cm-1 mode
disappears but we have not been able to identify a new ν(NO)
stretching frequency for the generated {Fe-NO}8 species in
our experimental spectral window. Under our experimental setup
using dmso as solvent this implies that this ν(NO) frequency
must be below 1500 cm-1. When the fully oxidized or reduced
species were rereduced or reoxidized, respectively, the ν(NO)
mode of 1 at 1620 cm-1 was fully regenerated indicating the
reversibility of the process. It is noted that for [(tmc)Fe(NO)-
(OH)](ClO4)2‚CH3CN which represents the first structurally
characterized example for an octahedral {Fe-NO}6 species (S
1
an S ) /2 ground state containing porphinato and dithiocarb-
amato ligands (Table 2). Interestingly, it also resembles closely
spectra recorded for HbNO and MbNO.14d,e The large anisotropy
of the line width is characteristic for {Fe-NO}7 species
(23) Scheidt, W. R.; Lee, Y. J.; Hatano, K. J. Am. Chem. Soc. 1984,
106, 3191.
(24) Albertin, G.; Bordignon E. Inorg. Chem. 1984, 23, 3822.
(25) Di Vaira, M.; Ghilardi, C. A.; Sacconi, L. Inorg. Chem. 1976, 15,
1555.
(26) Johnson, B. F. G.; Segal, J. A. J. Chem. Soc., Dalton Trans. 1972,
1268.
(27) Hoffman, P. R.; Miller, J. S.; Ungermann, C. B.; Caulton, K. J. J.
Am. Chem. Soc. 1973, 95, 7902.

4358 J. Am. Chem. Soc., Vol. 122, No. 18, 2000
Hauser et al.
to trans-[(cyclam)Fe(NO)I]+ with an S ) 1/2 ground state. This
signal becomes the dominant signal; after three warm-ups only
this signal is observed. The resonance at g ∼ 4 has completely
vanished.
The signal at g ∼ 4 is assigned to the S ) 3/2 ground state of
2 which is quite similar to that reported for [LFe(NO)(N3)].
eff
The axial spectrum was simulated with effective g values g
) 3.99 and g|eff ) 1.98, which are characteristic of the |ms )
(1/2 Kramers doublets in a S ) /2 manifold with large zero-
3
field splitting (2D3/2 . µB) and axial symmetry E/D3/2 ≈ 0. A
plot of the EPR intensity of 2, I × T versus T, is consistent
with the magnetic susceptibility result D3/2 ) +12.6 cm-1. Since
for axial zero-field symmetry the |ms ) (3/2 Kramers doublet
is EPR-silent at X-band, the intensity data are not very sensitive
for the numerical value of D3/2 but they clearly show that the
resonance doublet is a magnetic ground state and, hence, that
D3/2 is positive. For [LFe(NO)(N3)2] the zero-field splitting is
very similar, D3/2 ) +13.6 cm-1
.
Mo1ssbauer Spectroscopy. (a) Isomer Shifts and Quadru-
pole Splittings. Zero-field Mo¨ssbauer spectra on solid samples
or frozen solutions of complexes were measured in the tem-
perature range 2-295 K. All complexes show practically
symmetric quadrupole doublets in the whole temperature range.
The absence of magnetic hyperfine splittings also in the
paramagnetic {Fe-NO}7 compounds (in the solid state) shows
fast electronic spin relaxation with respect to the nuclear
precession rates. In this regime the internal fields at the nucleii
average to zero without externally applied fields. The obtained
zero-field quadrupole spectra were simulated with Lorentzian
doublets; the results are summarized in Table 5.
The spectra of [LFe(NO)(N3)2] and 2 show very similar
isomer shifts of 0.62 and 0.64 mm s-1 at 4.2 K (Figure S2,
Supporting Information). Isomer shifts in the range 0.63-0.78
mm s-1 have been observed for all octahedral {Fe-NO}7 S )
3/2 complexes including the respective sites in metalloproteins
such as the nitrosyl derivative of deoxyhemerythrin2,3 (δ ) 0.68
mm s-1) (see Table 1).
Also in line with previous data on octahedral {Fe-NO}7 (S
) 3/2) species is the observed temperature-independence of the
quadrupole splitting, ∆EQ, of [LFe(NO)(N3)2] and 2 (4.2-295
K). In contrast, the isomer shift, δ, of the two complexes
increases slightly by 0.11 mm s-1 with decreasing temperature
in the range 295-4.2 K, which is probably due to the second-
order Doppler effect. For the {Fe-NO}7 unit in nitrosyl
deoxyhemerythrine the same trend of the same magnitude is
observed.3 This is strong evidence for the notion that the
electronic structure is dominated by the common {Fe-NO}7
moiety.
The zero-field Mo¨ssbauer spectrum of 1 at 80 K also consists
of a symmetrical doublet with an isomer shift of 0.27 mm s-1
and a quadrupole splitting, |∆EQ|, of 1.26 mm s-1 (Figure 8,
middle). In the temperature range 4.2-80 K both isomer shift
and quadrupole splitting are temperature-independent within
experimental error.
Figure 7. X-band EPR spectra of 2 in CH3OH at 3.0 K as a function
of warm-up and freeze-quench experiments showing the isomerization
2 f 1. Conditions: ν ) 9.645 GHz; power 2.0 × 10-4 mW, modulation
amplitude 11.4 G. Simulation parameters are given in the text.
containing four equatorial nitrogen donors. In general, smaller
line widths are observed in the direction of the Fe-NO vector
as compared to the plane of four ligand nitrogen donors. In 1
additional coupling to protons of the secondary amine ligand
cyclam can cause a further increase in line width.
3
The S ) /2 ground state of the {Fe-NO}7 complex 2 has
been investigated by X-band EPR spectroscopy both in the solid
state and frozen methanol solution in the temperature range 3
to 50 K.
Due to fast isomerization of 2 f trans-[(cyclam)Fe(NO)-
(I)]I the solution EPR spectra of 2 have been obtained only by
using a special freeze-quench technique. The carefully degassed
solvent methanol in an EPR tube was frozen and a few crystals
of 2 were added to the frozen solution under an Ar blanketing
atmosphere. Upon a very brief warm-up of this mixture in the
EPR spectrometer, until the methanol had liquified, these crystals
sank slowly to the bottom of the cavity with dissolution of a
very small amount of 2. Then the solution was immediately
frozen again and the spectrum was recorded. The result of such
an experiment is shown in Figure 7 where this liquify-freeze
process was repeated after 1, 2, and 6 min. The first recorded
spectrum displays a strong signal at g ∼ 4 and a weaker isotropic
signal at g ) 2.01. Quantitation of these signals reveals that
the latter accounts for 24% of the total intensity; it is assigned
It is now an important observation that the isomer shift of
the cis and trans-“isomers”, 2 and 1, containing an {Fe-NO}7
(S ) 3/2) and an {Fe-NO}7 (S ) 1/2) moiety, respectively, differ
by 0.37 mm s-1. For the corresponding high-spin and low-spin
3
1
forms of five-coordinate [(salen)Fe(NO)] (S ) /2 H S ) /2)
this difference is smaller at ∼0.16 mm s-1.9 It is of significance
that this difference in isomer shifts at 80 K is 0.56 mm s-1 for
the high- and low-spin ferrous isomers cis-[(cyclam)FeII(N3)2]
(S ) 2) and trans-[(cyclam)FeII(N3)2] (S ) 0) and 0.17 mm s-1
for their high- and low-spin ferric analogues cis-[(cyclam)FeIII-
(28) The line widths for the field-swept spectrum are determined
according to σ(field) ) sqrt(∑i)x,y,zσi2li2), where li denotes the direction
cosines and σi the angular dependent line width σi2 ) Wi + C1,i ‚ν + ∑mIC2,i
‚
mI2 where Wi is the intrinsic residual line width, ν is the microwave
,
frequency, and mI ) -1, 0, 1 is the magnetic quantum number of the 14N
nucleus (I ) 1). For further details see for instance: Pilbrow, J. R. Transition
Ion Electron Paramagnetic Resonance; Clarendon Press: Oxford, 1990.

Octahedral Nitrosyliron Complexes {Fe-NO}6,7,8
J. Am. Chem. Soc., Vol. 122, No. 18, 2000 4359
Table 5. Spin Hamiltonian and Hyperfine Parameters of Complexes at 4.2 K
trans-[(cyclam)-
Fe(NO)Cl]+ (1)
trans-[(cyclam)-
trans-[(cyclam)-
cis-[(cyclam)-
Fe(NO)I]+I (2)
[L′Fe(NO)(NO2)2]+-
[LFe(NO)(N3)2]
Fe(NO)Cl]2+
Fe(NO)Cl]0
(ClO4)
3
1
3
St
D
/
/
0
0
/
0
2
2
2
3/2, cm-1
+13.6
0
+12.6
<0.01
3.99, 1.98
-20.0
-24.8
-23.4
-14.3
-17.7
-16.7
-16.2
0.64
E/D3/2
geff
4.0,2.0
-21.3
-20.8
-25.4
-15.2
-14.9
-18.2
-16.1
0.62
2.052, 2.015, 1.970
-23.4
-13.7
11.9
Axeff/gNâN,Ta)
Ayeff/gNâN,T
Azeff/gNâN,T
A
A
A
A
Fe(III),x/gNâN,Tb
Fe(III),y/gNâN,Tb
Fe(III),z/gNâN,Tb
-14.0d
-8.2d
+7.1d
b,c
iso/gNâN
-5.0d
δ, mm s-1
∆EQ, mm s-1
η
0.27
0.04
+2.05
0
0.27
+0.77
0.5
0.03
+1.37
0.1
-1.28
0.2
0.42
+1.26
-1.78
0.4
0.39
<0.1
Γ, mm s-1
0.33
0.27
0.42
0.3
3
1
a Hyperfine coupling tensor in the system of the total molecular spins St ) /2 and /2, respectively. b Hyperfine coupling tensor with respect to
c
1
3
local iron spin. Aiso ) /3tr(AhFe(III)). d Values with respect to a local iron spin of S ) /2 (intermediate spin).
s-1 observed for the complexes trans-[(cyclam)FeII(N3)2] (S )
1
0) and low-spin trans-[(cyclam)FeIII(N3)2]+ (S ) /2).15 Thus
one-electron oxidation of the {Fe-NO}7 (S ) 1/2) core in 1 to
{Fe-NO}6 (S ) 0) represents a metal-centered one-electron
oxidation.
The small isomer shift of trans-[(cyclam)Fe(NO)Cl]2+, an
{Fe-NO}6 system, deserves a comment. We are aware of only
one other report where even smaller isomer shifts of +0.04 and
-0.02 mm s-1 at 80 K have been reported for two five-
coordinate, structurally characterized {Fe-NO}6 (S ) 0)
species, namely [Fe(HL)NO](NO3) and [Fe(n-C3H7)2Q(NO)],
where H3L is 2,4-pentanediane-bis(S-methylisothiosemicarb-
azone) and (n-C3H7)2Q is the dianion nitromalonedialdehyde-
bis(S-propylisothiosemicarbazonate).29 The authors discuss the
electronic structure of these compounds as intermediate between
FeIV(NO)1- (FeIV S ) 1 and NO- S ) 1) and FeIII(NO) (FeIII
1
1
low spin S ) /2 and coordinated neutral NO S ) /2).
Figure 8 (bottom) shows the zero-field Mo¨ssbauer spectrum
of an ∼70% reduced sample of 1 which contains ∼30% of the
starting material, the Mo¨ssbauer spectrum of which is known.
Two quadrupole doublets in the ratio 68:32 are observed. The
most salient feature of this spectrum is the fact that the isomer
shift at 80 K of trans-[(cyclam)Fe(NO)Cl]0 at 0.27 mm s-1 is
within experimental error identical with that of 1. Only the
quadrupole splitting parameters differ significantly; |∆EQ| at
80 K is 1.26 mm s-1 for 1 but only 0.77 mm s-1 for the reduced
form which contains an {Fe-NO}8 moiety. We consider this
result remarkable because it proves that the additional electron
in the reduced complex does not change the s-electron density
at the iron nucleus and must, therefore, occupy a π* orbital of
the nitrosyl ligand which is not involved in nitrogen-to-iron
bonding. Thus in contrast to the above metal-centered one-
electron oxidation of 1 the corresponding reduction is ligand-
centered.
Figure 8. Zero-field Mo¨ssbauer spectra at 80 K of solid 1 (middle):
the one-electron oxidized form of 1 (top) in frozen CH3CN (0.10 M
[(n-Bu)4N]PF6) and of the one-electron reduced (∼70%) form of 1
(bottom) where the minor spectrum (‚‚‚) is that of 1.
(N3)2]+ (S ) 5/2) and trans-[(cyclam)FeIII(N3)2]+ (S ) 1/2).15 It
is remarkable that the observed ∆δ of 0.37 mm s-1 for the pair
of complexes 2 and 1 is exactly between the corresponding
differences for similar ferric and ferrous complexes. The
magnetic properties given below, however, show that it is not
the valence state of iron that is ambiguous in this case but rather
the spin states are unusual. The iron can be consistently
described as Fe(III) for 1 and 2.
To measure the Mo¨ssbauer spectra of the one-electron
oxidized and one-electron reduced forms of 1 we have used
35% 57Fe isotopically enriched 1 and electrochemically gener-
ated these forms in acetonitrile solution (0.10 M [(n-Bu)4N]-
PF6).
Figure 8 (top) shows the zero-field spectrum of trans-
[(cyclam)Fe(NO)Cl]2+ at 80 K. A single quadrupole doublet
with an isomer shift of 0.04 mm s-1 and a quadrupole splitting
of 2.05 mm s-1 is observed. Thus one-electron oxidation of 1
decreases the isomer shift by 0.23 mm s-1. It is again instructive
to compare this shift with the corresponding shift of 0.26 mm
3
Oxidation and reduction derivatives of the S ) /2 system
could not be studied directly by using the cis-cyclam complex
2 because of its instability in solution. However, the oxidized
{Fe-NO}6 complex [L′Fe(NO)(ONO)(NO2)]+ (S ) 0) was
prepared as described above. The zero-field Mo¨ssbauer spectrum
of the solid material (Figure S2, Supporting Information) shows
a pure, well-resolved quadrupole doublet. The isomer shift is
found at 0.02 mm s-1 (0.03 mm s-1 at 4.2 K) which is 0.6 mm
(29) Gerbeleu, N. V.; Arion, V. B.; Simonov, Yu. A.; Zavodnik, V. E.;
Stavrov, S. S.; Turta, K. I.; Gradinaru, D. I.; Birca, M. S.; Pasynskii, A.
A.; Ellert, O. Inorg. Chim. Acta 1992, 202, 173.

4360 J. Am. Chem. Soc., Vol. 122, No. 18, 2000
Hauser et al.
Figure 9. Applied-field Mo¨ssbauer spectra of solid [LFe(NO)(N3)2]
at 4.2 K. Simulation parameters are given in Table 5.
s-1 below the value found for the {Fe-NO}7 species [LFe(NO)-
(N3)2]. The quadrupole splitting is 1.37 mm s-1 (temperature-
independent). A magnetically perturbed measurement performed
at 4.2 K (see below) reveals a change of sign of Vzz of the
electric field gradient on going from [L′Fe(NO)(ONO)(NO2)]+
where it is positive to [LFe(NO)(N3)2] where it is negative.
For the purpose of an educated comparison we have also
recorded the zero-field Mo¨ssbauer spectra at 80 K of an
octahedral ferrous species, [FeII(P(OC2H5)3)5Br]+ (S ) 0), and
its five-coordinate, trigonal bipyramidal nitrosyl iron complex
[Fe(NO)(P(OC2H5)3)4]BPh4 (S ) 0) of the {Fe-NO}8 type
which probably contains a linear {Fe-NO}8 moiety25 and a
ν(NO) stretching frequency at 1734 cm-1. The first of these
complexes displays a symmetrical quadrupole doublet at 0.15
mm s-1 and a quadrupole splitting of 0.76 mm s-1, whereas
for the nitrosyl complex these parameters are -0.02 and 1.85
mm s-1 (Figure S3, Supporting Information). The shift of ∼0.2
mm s-1 of the isomer shift on going from a low-spin ferrous
species to the corresponding nitrosyl iron {Fe-NO}8 species
indicates to us that the iron ion is more reduced in the latter
than in the former. Thus the electronic structure of [Fe(NO)-
(P(OC2H5)3)4]BPh4 can be described as Fe0(NO+) (which we
Figure 10. Applied-field Mo¨ssbauer spectra of solid 2 at 4.2 K.
Simulation parameters are given in Table 5.
to the large zero-field splitting (ZFS) (2D3/2/k > 30 K). Since
fast spin relaxation prevails in the spectra of [LFe(NO)(N3)2]
and 2, it is the thermal average BS of the spin expectation
T
value which determines the internal field BBint ) Aheff BS . In the
T
fast relaxation regime the internal field depends on the applied
field because BS varies for variant Zeeman splitting of the
T
ground-state Kramers doublet. This behavior, however, is
modified by the mixing of magnetic sublevels due to competing
ZFS and Zeeman interaction which, hence, yields a measure of
the ZFS. On the basis of these considerations the magnetic
Mo¨ssbauer spectra of [LFe(NO)(N3)2] and 2 are readily con-
sistent with the large ZFS parameters D3/2 determined from the
magnetic susceptibility data. In particular, the positive sign of
D3/2 is unambiguous from the magnetic anisotropy observed in
the spectra.
According to the axial symmetry of the ZFS interaction as
determined by the EPR spectra of both compounds (E/D ) 0),
3
the ground-state Kramers doublet | /2, +1/2 shows an “easy-
1
1
prefer) or FeI(NO*) (FeI S ) /2, NO- S ) /2). A description
as FeII(NO-) (FeII S ) 0, NO- S ) 0) is not appropriate. Note
that this difference of isomer shifts on going from the intermedi-
plane” of magnetization. This means for the molecules in a
powder sample that the spin expectation values and the resulting
internal fields BBint are much stronger for applied fields in the
molecular x/y-direction (of the ZF-interaction) than for those
close to the z-direction. Therefore, the x- and y-components of
the A-tensor are better determined by the simulations than the
z-component. The values of Axeff and Ayeff are found to be very
close to each other for both compounds, whereas the fits of the
asymmetry of the hyperfine lines and the field dependence of
the magnetic splittings could be improved if Azeff was allowed
to take larger values than Axeff/Ayeff. In any case, the signs of
3
ate spin of the Fe(III) ion (S ) /2) in [trans-(cyclam)2{FeIII-
1
(µ-N)FeIV}(N3)2]2+ (St ) /2)15 to trans-[(cyclam)Fe(NO)Cl]+
1
(S ) /2) is only 0.07 mm s-1, which indicates to us that the
iron ions in both complexes have the same oxidation state and
similar dn electron configuration, namely intermediate spin ferric.
(b) Applied Field Mo1ssbauer Spectra. Application of 1.5-7
T magnetic fields at liquid helium temperatures resolves the
magnetic hyperfine splitting of solid 2 and [LFe(NO)(N3)2] as
is shown in Figures 9 and 10. The spectra were readily simulated
for an S ) 3/2 spin system in the limit of fast relaxation. These
Spin-Hamiltonian simulations yield hyperfine coupling tensors,
the sign and magnitude of the quadrupole splitting, and the
isomer shift with the other parameters (g, D, E/D) set to the
values determined from EPR and magnetic susceptibility
measurements. Details of the Spin-Hamiltonian analyses are
described in ref 15; the parameters are given in Table 5.
In strong applied field (7 T) at 4.2 K both complexes show
an asymmetric magnetic six-line pattern with overall splittings
of 6.2 mm s-1 for [LFe(NO)(N3)2] and 7.5 mm s-1 for 2. The
corresponding internal fields originate from induced spin
expectation values of the electronic spin ground-state Kramers
3
the Aheff components are negative for both S ) /2 complexes.
For magnetic Mo¨ssbauer spectra with strong internal fields
and dominating nuclear Zeeman interaction the quadrupole
interaction is determined in first order only by the components
of the electric field gradients (efg) tensor that are oriented along
the internal field. Since the internal fields for [LFe(NO)(N3)2]
are predominantly in the x/y-plane only the Vxx and Vyy
components of the efg-tensor are effective for the appearance
of the spectra. Since these components have the opposite sign
than the main component Vzz and the apparent quadrupole shifts
in the magnetic hyperfine pattern are positive, Vzz is in fact
negatiVe which is consistent with other examples of {Fe-NO}7
(S ) 3/2) systems (Table 1). For both complexes the asymmetry
parameter of the efg is found to be small (η ≈ 0.2-0.4). For
3
doublet | /2, (1/2 which is exclusively populated at 4.2 K due

Octahedral Nitrosyliron Complexes {Fe-NO}6,7,8
J. Am. Chem. Soc., Vol. 122, No. 18, 2000 4361
[LFe(NO)(N3)2] we could not detect significant orientational
differences of the principal axis systems (pas) of the efg and
ZF interaction, except for a minor rotation of Vz′z′ off from the
z-axis of the ZFS. Systematic searches yield an Euler-rotation
(â,y′) of the efg with â ) 18°. For 2 the efg rotations appear to
be more complex. We found sizable Euler angles R ) 51°, â
) 21°, γ ) 59° for the three rotations with respect to the axes
z, y′, z′′. However, most significant is again â ) 21° which
means again that the main component Vz′z′ is oriented away from
the z-axis by about 20°, similar as for [LFe(NO)(N3)2]. We
mention that rotations around the z-axis are less important
because of the high, almost axial symmetry of the ZFS and,
similarly, the A-tensor.
The negative A-tensor components of [LFe(NO)(N3)2] and 2
support the suggested spin-coupling model for the {Fe-NO}7
3
(S ) /2) unit which is based on the assignments of high-spin
FeIII (S ) 5/2) and NO- (S ) 1). In this model the ground state
3
S ) /2 of the molecule results from strong antiferromagnetic
coupling between the two local spin systems. Since then the
spin of FeIII is parallel to the system spin, the Aeff-tensor in the
coupled system has the same (negative) sign as the intrinsic
tensor AFe(III). (This is at least true for the isotropic part Aiso.)
From an application of the Wigner-Eckart theorem one deduces
the relation of intrinsic and coupled A-tensor components:
AFe(III) ) +5/7Aeff. The resulting intrinsic values of AhFe(III) are
given in Table 5. The isotropic parts Aiso/gNâN ) -16 T for
both complexes are lower than that of typical “ionic” FeIII
moieties (-20 to -22 T)30 but are quite consistent with that
found for FeIII in iron-sulfur clusters (≈ -16.5 T)31 which also
experience significantly covalent bonds.
Figure 11. Variable-temperature, variable-field Mo¨ssbauer spectra of
1, trans-[(cyclam)Fe(NO)Cl]+ (35% 57Fe enriched), in CH3CN (0.10
M [(n-Bu)4N]PF6). Simulation parameters are given in Table 5.
The anisotropy of the A-tensor of [LFe(NO)(N3)2] was
recently explained by covalent delocalization of some of the
d-orbitals.3 Anisotropic covalency of d-orbitals is also the reason
for the large quadrupole splittings found in some cases of FeIII
compounds, which in the “ionic” limit would show no or only
small splittings due to the symmetry of the S-state ion. For
instance, similar large values such as those of [LFe(NO)(N3)2]
and 2 or even larger ones are reported for various compounds
with the (µ-oxo)diiron(III) unit.32
The magnetic Mo¨ssbauer spectrum of 1 measured at 150 K
with 7 T applied field as shown in Figure 11 (top) unambigu-
ously reveals a positive sign of the efg main-component Vzz,
and a vanishing asymmetry parameter η ) 0 (error range +0.1).
At liquid helium temperatures the applied-field spectra recorded
at 1, 3.5, 5, and 7 T show only rather broad lines and a relatively
weak overall splitting, in accordance with the low total spin
state. In addition, the weak magnetic splitting indicates also a
small isotropic part of the hyperfine coupling tensor. The
asymmetry of the spectra shown in Figure 11, in conjunction
with a large quadrupole splitting, is typical of substantial
anisotropy of the hyperfine tensor. Spin-Hamiltonian simulations
which were performed with S ) 1/2, g values taken from EPR,
and the g matrix elements all set positive, yield for 1 a coupling
tensor Aeff/gNâN ) (-23.4, -13.7, +11.9) T.
by a small angle â ) 6°. Interestingly, this angle resembles the
tilting angle θ ) 7.6° that was found between the normal of
the cyclam(nitrogen) plane and the axis through Fe and the
average of N and O atom positions.
The isotropic part of the Aeff tensor obtained for 1 is clearly
1
negative, /3tr(A)/gNâN ) -8.4 T. This observation rules out
the description of 1 as an antiferromagnetically coupled low-
spin ferric ion (S )1/2) and an NO- (S ) 1) ligand, since in
this case the apparent Aeff tensor in the coupled spin system
would exhibit a positive isotropic part for a negative intrinsic
value of iron. Even if this argument might be obscured by
undefined signs of the g values, which can be a problem for
low-spin FeIII (the Mo¨ssbauer spectra are sensitive only for the
product gi‚Ai),33 the magnetic spectra of 1 are not consistent
with the interpretation of low-spin FeIII at all for the following
reasons: (i) The efg does not match: The small anisotropy of
the g values obtained from the EPR spectra would clearly mean
for a low-spin FeIII interpretation that the electron-hole in the
(t2g)5 configuration has to reside in an energetically well isolated
d-orbital which is virtually not mixed by spin-orbit coupling
(∑gi2 ) 12.15). For instance, in the crystal-field model
developed by Griffith34 and Taylor,35 the best description of 1
would be obtained with large axial and rhombic orbital splittings
of ∆/λ ) 96 and V/λ ) (46, which corresponds to a situation
of a practically pure configuration (dxy, dxz)4(dyz)1 (or, equiva-
lently, (dxy, dyz)4(dxz)1). The resulting charge anisotropy of the
valence electrons caused by the electron-hole in a single
“pancake”-shaped t2g orbital would lead to a strong negatiVe
valence contribution to the main component of the efg30 (either
in the x- or the y-direction), which is in contradiction to the
When different orientations of efg and Ah1/2 tensor principal
axes systems were allowed, the fits could be significantly
improved only by an Euler rotation of the efg around the y-axis
(30) Srivastava, J. K.; Bhargara, S. C.; Iyengar, P. K.; Thosar, B. V. In
AdVances in Mo¨ssbauer Spectroscopy; Thosar, B. V., Iyengar, P. K., Eds.;
Elsevier: New York, 1983; pp 1-121.
(31) Mu¨nck, E. Methods Enzymol. 1978, 54, 346. (b) Huynh, B. H.; Kent,
T. A. In AdVances in Mo¨ssbauer Spectroscopy; Thosar, B. V., Iyengar, P.
K., Eds.; Elsevier: New York, 1983; pp 490-560.
(33) Huynh, B. H.; Emptage, M. H.; Mu¨nck, E. Biochim. Biophys. Acta
1978, 534, 295-306.
(34) Griffith, J. S. Proc. R. Soc. London, A 1956, 235, 23-36.
(35) Taylor, C. P. S. Biochim. Biophys. Acta 1977, 491, 137.
(32) Rodriguez, J. H.; Xia, Y.-M.; Debrunner, P. G.; Chaudhuri, P.;
Wieghardt, K. J. Am. Chem. Soc. 1996, 118, 7542-7550 and references
therein.

4362 J. Am. Chem. Soc., Vol. 122, No. 18, 2000
Hauser et al.
2
2
experimental observation. Even if covalency and lattice con-
tributions significantly influence the actual values of the efg
components, it is difficult to envisage a reversal of the sign of
Vz′z′, particulary in this case of a strong axially symmetric efg.
(ii) The symmetry of the magnetic hyperfine coupling tensor
Aeff as can be derived from the t2g-hole description of 1 is also
not consistent with the experiment: The orbital and spin-dipolar
contributions to A in this model33,36 yield the largest, negative
component of A in the direction of the efg main component,
whereas in the simulations of the spectra the Aeff component
along the efg z-direction was found to be the weakest (and
positive).
to the assumption that the antibonding dx -y orbital is too high
2
in energy above the dz and the dπ orbitals to be populated.
The resulting ground-state configuration is basically (dxy)2(dxz)1-
1
2
(dyz)1(dz ) .
High-field Mo¨ssbauer measurements (7 T, 4.2 K) of the {Fe-
NO}8 and the {Fe-NO}6 species derived from 1 by electro-
chemical reduction and oxidation, respectively, reveal an S )
0 ground state for both complexes, namely trans-[(cyclam)Fe-
(NO)(Cl)]0 and trans-[(cyclam)Fe(NO)(Cl)]2+. Also, for [L′Fe-
(NO)(ONO)(NO2)]+, an {Fe-NO}6 species, a diamagnetic
ground state has been established. We assume that the S ) 0
ground state of these complexes owes its origin to strong
intramolecular spin couplings (see below). The magnetic Mo¨ss-
bauer spectra (Figure S4) were simulated by using the isomer
shifts and the electric quadrupole splittings from the respective
zero-field spectra. All species, namely the {Fe-NO}6 and the
{Fe-NO}8 derivatives of 1 and [L′Fe(NO)(ONO)(NO2)]+,
possess a positive sign of the quadrupole splitting and high axial
symmetry of the efg with η ≈ 0. The low isomer shift and large,
positive quadrupole splitting with η ≈ 0 of the two present {Fe-
NO}6 species are consistent with those observed for Werner-
type octahedral low-spin FeIV complexes.15 The axial efg can
be rationalized in a ligand field model for the spin-orbit
interaction of distorted octahedral (t2g)4 complexes. In this model
the valence contribution to the efg originates from two electron
holes in the t2g subshell which is positive for a (dxy)2(dxz)1(dyz)1
ground-state configuration of FeIV.44-47 Thus the electronic
structures of trans-[(cyclam)Fe(NO)Cl]2+ and [L′Fe(NO)(ONO)-
(NO2)]+, both of which are of the {Fe-NO}6 (S ) 0) type, can
be described as low-spin FeIV (S ) 1) coupled antiferromag-
netically to an NO- (S ) 1) ligand.
These arguments led us to consider the alternative description
of 1, not as a low-spin FeIII complex but as an antiferromag-
netically coupled system of intermediate-spin ferric ion (S )
3/2) and an NO- (S ) 1). Since in this picture iron has the major
eff
local spin, the sign of the isotropic part Aiso with respect to
1
the system spin St ) /2 is the same as that of the (negative)
intrinsic value. The value of the isotropic part turns out to be
rather weak in this model (1/3tr(AFe(III),S)3/2) ) -5 T), when
the experimental value for the system spin is converted
according to the spin-projection formula AFe(III),S)3/2 ) +3/5 Aeff.
However, this does not contradict the basic assumption of
intermediate spin FeIII, because it is clear that spin-dipolar
contributions and covalent orbital delocalizations cannot be
neglected for a nitrosyl complex.
Iron(III) intermediate spin states were first reported for five-
coordinate iron porphyrinates with square-pyramidal geometry;37
but other examples with salen and dithiocarbamate and other
ligands are also described in detail.38-40 The large pos-
itive quadrupole splitting (≈+3 mm s-1, η ≈ 0) was recognized
as a characteristic feature of intermediate-spin ferric por-
phyrinates.41 Detailed analyses of EPR and Mo¨ssbauer spectra
showed the presence of a highly anisotropic hyperfine tensor
with large, negative components in x/y-directions and weak,
positive z-components. Both features, the positive, axial quad-
rupole interaction and the anisotropic magnetic hyperfine tensor,
are very similar to what we found for 1. In the theoretical model
developed by Maltempo42 the “spin-Hamiltonian” parameters43
are derived by mixing of close lying 4A2 and 6A1 orbital singlet
states. Square-planar geometry was taken as the starting point
for the description of the spin-quartet ground state, according
Discussion
Rodriguez et al.3 have recently reported a detailed Mo¨ssbauer
study and density functional theory calculation on the nitrosyl
derivatives of deoxyhemerythrin which contains a high-spin
ferrous site with isomer shift and quadrupole splitting at 1.21
and +2.66 mm s-1, respectively, and a nitrosyl iron site {Fe-
NO}7 (S ) 3/2) with 0.68 and +0.61 mm s-1, respectively. While
the isomer shift for the former was considered to be normal
(typical) for an octahedral ferrous ion in an oxygen/nitrogen
donor environment, the latter value has been considered
“unusual” or not typical (in fact enhanced) with respect to a
normal high-spin ferric ion or, conversely, reduced with respect
to a high-spin ferrous ion. They concluded that an isomer shift
of 0.68 mm s-1 is intermediate between high-spin ferrous and
ferric and does not allow an oxidation state assignment.
In contrast to this interpretation we would like to point out
that the isomer shift is only considered to be unusual because
there are but few high-spin ferric complexes reported to date
with such large isomer shift. For example, Lippard et al.48 have
structurally characterized a (µ-1,2-peroxo)diiron(III) complex
(36) Oosterhuis, W. T.; Lang, G. Phys. ReV. 1969, 178, 439-456. (b)
Lang, G.; Marshall, W. Proc. Phys. Soc. 1966, 87, 3-34. (c) Safo, M. K.;
Gupta, G. P.; Walker, F. A.; Scheidt, W. R. J. Am. Chem. Soc. 1991, 113,
5497-5510. (d) Walker, F. A.; Huynh, B. H.; Scheidt, W. R.; Osvath, S.
R. J. Am. Chem. Soc. 1986, 108, 5288-5297.
(37) Debrunner, P. G. Mo¨ssbauer Spectroscopy of Iron Porphyrins;
Debrunner, P. G., Ed.; VCH: Weinheim, 1989; Vol. III, pp 137-234. (b)
Scheidt, W. R.; Osvath, S. R.; Lee, Y. J.; Reed, C. A.; Shaevitz, B.; Gupta,
G. P. Inorg. Chem. 1989, 28, 1591-1595.
(38) Niarchos, D.; Kostikas, A.; Simopoulos, A.; Coucouvanis, D.;
Piltingsrud, D.; Coffman, R. E. J. Chem. Phys. 1978, 69, 4411-4418. (b)
Ganguli, P.; Marathe, V. R.; Mitra, S. Inorg. Chem. 1975, 14, 970-973.
(c) Ganguli, P.; Hasselbach, K. M. Z. Naturforsch. 1979, 34a, 1500-1506.
(d) Wells, F. V.; McCann, S. W.; Wickman, H. H.; Kessel, S. L.;
Hendrickson, D. N.; Feltham, R. D. Inorg. Chem. 1982, 21, 2306-2311.
(e) Kuang, X.-Y.; Morgenstern, I.; Rodriguez, M.-C. Phys. ReV. B 1993,
48, 6676-6679. (f) Kuang, X.-Y.; Morgenstern-Badarau, I. Phys. Status
Solidi B 1995, 191, 395-400.
(39) Kostka, K. L.; Fox, B. G.; Hendrich, M. P.; Collins, T. J.; Rickard,
C. E. F.; Wright, L. J.; Mu¨nck, E. J. Am. Chem. Soc. 1993, 115, 6746.
(40) Kentel, H.; Ka¨pplinger, I.; Ja¨ger, E.-G.; Grodzicki, M.; Schu¨nemann,
V.; Trautwein, A. X. Inorg. Chem. 1999, 38, 2320. (b) Koch, S.; Holm, R.
H.; Frankel, R. B. J. Am. Chem. Soc. 1975, 97, 6714. (c) Gupta, G.; Lang,
G.; Reed, C. A.; Shelly, K.; Scheidt, W. R. J. Chem. Phys. 1987, 86, 5288.
(41) Dolphin, D. H.; Sams, J. R.; Tsin, T. B. Inorg. Chem. 1977, 16,
711.
(44) Moss, T.; Ehrenberg, A.; Bearden, A. J. Biochemistry 1969, 8,
4159-4162. (b) Schulz, C. E.; Rutter, R.; Sage, J. T.; Debrunner, P. G.;
Hager, L. P. Biochemistry 1984, 23, 4743-4754. (c) Mandon, D.; Weiss,
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1992, 31, 4404-4409 and references therein.
(45) (a) Jayaraj, K.; Gold, A.; Austin, R. N.; Ball, L. M.; Terner, J.;
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Schu¨nemann, V.; Trautwein, A. X. Inorg. Chem. 1997, 36, 4555-4566.
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9080 and references therein.
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Octahedral Nitrosyliron Complexes {Fe-NO}6,7,8
J. Am. Chem. Soc., Vol. 122, No. 18, 2000 4363
3
(N3)]+ (S ) /2)15 which are labeled d, e, and h in Figure 12.
The oxidation state +IV is attained in two isomers of [(cyclam)-
(N3)FeIIIdNdFeIV(N3) (cyclam)]2+ with either an St ) 3/2 or St
1
) /2 ground state. Both give rise to two quadrupole doublets
3
1
(FeIII (S ) /2 or /2) and FeIV (S ) 1)) for each complex; the
isomer shift of both FeIV (S ) 1) ions in the two isomers is
nearly identical (labels f and g in Figure 12). Again the isomer
shift increases linearly with decreasing oxidation number.15
Interestingly, the change of isomer shift per one-electron change
of dn configuration is significantly smaller (only 0.19 mm s-1
)
than that for the above high-spin complexes. Note that all low-
spin complexes considered here have an octahedral FeN6
coordination polyhedron; three contain the strong π donor ligand
nitride which is bound to high-valent iron in a quite covalent
fashion50 but the other two do not.
Closer inspection of the two straight lines in Figure 12 reveals
an interesting facet of this analysis. From the plot we can
immediately see that a high-spin-low-spin crossover affects
the isomer shift in a different fashion for each oxidation state
(see the inset in Figure 12). A very large positive ∆δ (high-
low spin) of ∼0.6 mm s-1 is observed for ferrous complexes;
for ferric complexes this difference is still positive but smaller
at +0.2 mm s-1; on going to FeIV this difference becomes
negatiVe, -0.2 mm s-1. Thus high-spin FeIV complexes have a
smaller isomer shift than their low-spin counterparts, and for
FeIII and FeII high- and low-spin species it is the other way
around.
Up to this point the above analysis should not be controver-
sial. Obviously, the conclusions drawn rely heavily on the
structural (and electronic) similarity of complexes chosen to
generate a “series”. We do not claim that these correlations hold
for all iron complexes, e.g. [Fe(CN)6]3-/4- complexes do not
fit into the picture. Nevertheless, the concept has heuristic value
as we attempt to show now.
Figure 12. Plot of the formal oxidation state of the iron ions in
complexes vs the isomer shift, δ: cis-[(cyclam)FeII(N3)2] (a); cis-
[(cyclam)FeIII(N3)2]+ (b); [Et4N][FeIVCl(η4-MAC*)] (c); trans-
[(cyclam)FeII(N3)2] (d); trans-[(cyclam)FeIII(N3)2]+ (e); [(cyclam)(N3)-
FeIIIdNdFeIV(N3)(cyclam)]2+ S ) 3/2 (g); [(cyclam)(N3)FeIIIdNdFeIV-
1
(N3)(cyclam)]2+ S ) /2 (f); trans-[(cyclam)FeV(N)(N3)]+ (h); trans-
[(cyclam)Fe(NO)Cl]2+ (q); trans-[(cyclam)Fe(NO)Cl]+ (r); trans-
[(cyclam)Fe(NO)Cl]0 (s); cis-[(cyclam)Fe(NO)I]I (t), [LFe(NO)(N3)2]
(u), and [LFe(NO)(ONO)(NO2)]+ (v). The inset shows the isomer shift
difference, ∆δ, between high and low spin complexes in a similar ligand
environment at a given common oxidation state (FeIV, FeIII, or FeII).
that contains two octahedral high-spin ferric ions (N3O3Fe) and
exhibits in frozen toluene solution at 4.2 K a Mo¨ssbauer
spectrum consisting of a symmetrical doublet with isomer shift
and quadrupole splitting of δ ) 0.66 mm s-1 and ∆EQ ) 1.40
mm s-1 (Γ ) 0.15 mm s-1). This isomer shift corresponds
exactly to the value reported for the peroxodiferric intermediate
of methane monooxygenase.49 Since the oxidation level of the
n-
O2 bridge is unequivocally peroxide (O22-), the assignment
{Fe-NO}7 (S ) 3/2). The complex [LFe(NO)(N3)2] has
played a role model for nitrosyl iron complexes of the type {Fe-
of a +III oxidation state for both iron ions is also unequivocal.
The Fe-Operoxo bond is relatively short and displays covalent
character. It resembles in this respect the situation in nitrosyliron
or nitridoiron complexes.
3
NO}7 (S ) /2). In-depth spectroscopic investigations12 and
calculations3,12 have led to the conclusion that it can be best
described as a high-spin ferric ion coupled to a NO- (S ) 1)
yielding the observed S ) 3/2 ground state. At 4.2 K this complex
has an isomer shift of +0.61 mm s-1. Adding this data point to
the plot in Figure 12 (labeled t) immediately shows that the
isomer shift falls very close to the linear correlation of a, b,
and c if an oxidation state of +III is assumed. In principle, the
measured isomer shift could also be achieved by a low-spin
ferrous ion (S ) 0) using correlation d, e, f, g, and h but this
necessitates the description of the {Fe-NO}7 (S ) 3/2) species
In the following we attempt to correlate in a more general
fashion Mo¨ssbauer isomer shifts with formal oxidation states
of a given iron ion or, more to the point, with the dn electron
configuration of the central iron ion in a series of structurally
very similar, preferably octahedral, complexes. In Figure 12 we
have plotted the isomer shifts (at 4.2 K) of three high-spin
configured complexes, namely cis-[(cyclam)FeII(N3)2] (S ) 2),15
cis-[(cyclam)FeIII(N3)2]+ (S ) 5/2),15 and [Et4N][FeIVCl(η4-
MAC*)] (S ) 2)39 which we label a, b, and c, respectively.
H4[η4-MAC*] is the tetradentate ligand 1,3,8,11-tetraaza-13,13-
diethyl-2,2,5,5,7,7,10,10-octamethyl-3,6,9,12,14-pentaoxocy-
clotetradecane which is coordinated to an FeIV ion very much
like cyclam except that the four nitrogen donor atoms are amides
rather than amines in cyclam. The monoanionic complex is five-
coordinate (FeN4Cl). These three complexes are for all our
purposes considered to be “isostructural”. Their isomer shifts
increase linearly with decreasing dn configuration d6, d5, d4. In
other words, a constant shift of the isomer shift of 0.57 mm s-1
per one-electron change is observed for this series.
as FeII (S ) 0) coordinated to an NO- radical which must then
3
possess an S )
/ configuration. This is of course not
2
acceptable. Thus Mo¨ssbauer and Fe K-edge energy X-ray
absorption spectroscopy (XAS) are fully in accord with a
formalism where a high-spin ferric ion is antiferromagnetically
coupled to an NO- (S ) 1) ligand. Exactly the same arguments
hold for cis-[(cyclam)Fe(NO)I]I (data point u in Figure 12) and
for all {Fe-NO}7 (S ) 3/2) species including the nitrosyl
derivatives of non-heme proteins listed in Table 1.
1
{Fe-NO}7 (S ) /2). For complex 1 the isomer shift at 4.2
K has been determined to be 0.27 mm s-1. The trans-cyclam
ligand in octahedral ferric or ferrous complexes enforces in
general a low-spin configuration. Recently a few cases have
been reported where this trans configuration also stabilizes an
In a second series we correlate, as above, known low-spin
iron complexes:15 trans-[(cyclam)FeII(N3)2] (S ) 0), trans-
1
[(cyclam)FeIII(N3)2]+ (S ) /2),15 and trans-[(cyclam)FeV(N)-
(49) Liu, K. E.; Valentine, A. M.; Qiu, D.; Edmonson, D. E.; Appelman,
E. H.; Spiro, T. G.; Lippard, S. J. J. Am. Chem. Soc. 1995, 117, 4997. (b)
Liu, K. E.; Valentine, A. M.; Wang, D.; Huynh, B. H.; Edmonson, D. E.;
Salifoglou, A.; Lippard, S. J. J. Am. Chem. Soc. 1995, 117, 10174.
(50) Ju¨stel, T.; Mu¨ller, M.; Weyhermu¨ller, T.; Kressl, C.; Bill, E.;
Hildebrandt, P.; Lengen, M.; Grodzicki, M.; Trautwein, A. X.; Nuber, B.;
Wieghardt, K. Chem. Eur. J. 1999, 5, 793.

4364 J. Am. Chem. Soc., Vol. 122, No. 18, 2000
Hauser et al.
3
intermediate spin state (S ) /2) at a ferric ion.39,40 Using the
{Fe-NO}8 (S ) 0). All diamagnetic complexes of this
electronic structure reported to date are five-coordinate (trigonal
bipyramidal) and possess a linear Fe-NO moiety. The co-
ligands are invariably strong π-acceptors such as phosphines
and phospites.24-27 Their ν(NO) stretching frequencies are
usually observed in the range 1730 ( 20 cm-1. This class of
complexes represents the least well characterized with regard
to their electronic structure. Most authors imply a description
involving a low-spin ferrous ion (S ) 0) coordinated to an NO-
(S ) 0!). As we have shown for [Fe(NO)(P(OC2H5)3)4]BPh4,24
this {Fe-NO}8 (S ) 0) species possesses a very small isomer
shift of -0.02 mm s-1 at 80 K. We have argued above that
such species are probably best described as Fe0(NO+) species
because their “innocent” low-spin ferrous counterparts display
a larger isomer shift at 0.15 mm s-1 for [FeII(P(OC2H5)3)5Br]+
(S ) 0).
two correlations in Figure 12 one could expect an isomer shift
of 0.64 - 0.2 (difference h.s.-l.s.) ) ∼0.4 mm s-1 for 1, if we
assume a high-spin ferric ion adopting a low-spin electron
configuration (S ) /2). Clearly, data point r is off the line
defined by d, e, f, g, and h, but not far. As pointed out above,
1
1
the description of 1 as low-spin ferric (S ) /2) antiferromag-
netically coupled to NO- (S ) 1 or 0) is not compatible with
the applied-field Mo¨ssbauer spectra, but the description with
3
an intermediate spin ferric ion (S ) /2) antiferromagnetically
coupled to an NO- (S ) 1) ligand is.
Pure intermediate spin, five-coordinated complexes containing
an FeIIIN4X coordination polyhedron have been reported to have
isomer shifts of, for example, 0.1840a and 0.14 mm s-1 39
, which
is somewhat smaller than the isomer shift of low-spin ferric
complexes at ∼0.3 mm s-1. We have shown15 that the dinuclear
complex [trans-(cyclam)2{FeIII(µ-N)FeIV}(N3)2]2+ (S ) 1/2)
consists of an octahedral FeIV ion (S ) 1) antiferromagnetically
coupled to an octahedral intermediate spin ferric ion (S ) 3/2).
Its Mo¨ssbauer spectrum at 80 K displays two quadrupole
doublets with isomer shifts at 0.20 mm s-1 for the ferric and at
0.11 for the FeIV ion. The difference of isomer shifts on going
from 2 to 1 can then be calculated as 0.64 - 0.2 (difference
h.s.-l.s.) - 0.1 (difference l.s.-i.s.) ) 0.3 mm s-1, which nicely
corresponds to the observed value for 1 of 0.27 mm s-1. This
estimate is further supported by the distinctly more negative
Clearly, the electronic structure of reduced 1, namely trans-
[(cyclam)Fe(NO)Cl]0 (S ) 0), must be very different. We do
not observe a ν(NO) band at ∼1730 cm-1 and its isomer shift
has been determined to be 0.27 mm s-1 which is exactly the
same value as that for 1. Consequently, we propose an electronic
structure invoking a low-spin ferric ion (S ) 1/2) antiferromag-
1
netically coupled to the dianionic radical NO2- (S ) /2). We
expect the Fe-NO unit to be strongly bent (R(FeNO) ∼ 110-
130°) and the N-O distance approaching that of a single bond
with ν(NO) < 1500 cm-1. More experimental spectroscopic data
(Mo¨ssbauer and EXAFS) are called for on this interesting class
of compounds; the above interpretation is admittedly speculative
and rests entirely on the validity of the correlations in Figure
12.
isomer shift of a genuinely low-spin ferric {FeNO}7 (S ) /2)
1
complex, namely the recently published compound [Fe(NO)-
(TC-515)].51 The iron in this system which is five-coordinate
but otherwise has rather similar ligands as 1 shows an isomer
shift of only 0.06 mm s-1. Thus Mo¨ssbauer spectroscopy gives
a reasonable indication of the nature of the electronic structure
{Fe-NO}6 (S ) 0). Octahedral diamagnetic {Fe-NO}6
complexes are structurally well characterized, the earliest
examples being cis-[Fe(NO)(S2CN(C2H5)2)2(NO2)]58 which
exhibits a ν(NO) band at 1835 cm-1 and [Fe(ttp)(NO)(H2O)]-
ClO423 with a ν(NO) band at 1937 cm-1 (see also ref 21). The
oxidized form of 1, namely trans-[(cyclam)Fe(NO)Cl]2+ (S )
0), and [L′Fe(NO)(ONO)(NO2)]+ fit well into this series. Their
small isomer shifts clearly indicate to us that the one-electron
oxidation of 1 is metal-centered in nature. These isomer shifts
are in perfect agreement with other octahedral FeIV (S ) 1)
species such as the FeIV ions in [(cyclam)(N3)FeIIIdNdFeIV-
3
of 1 as intermediate spin ferric (S ) /2) antiferromagnetically
coupled to NO- (S ) 1).
Recently, the Mo¨ssbauer spectra of two distinctly different
crystalline forms of [Fe(TpivPP)(NO)(NO2)]-, a six-coordinate
porphinato complex of {Fe-NO}7 (S ) /2) type, have been
1
reported52 which display distinctly different isomer shifts at 0.28
mm s-1 (4.2 K) for form 1 and 0.35 mm s-1 (4.2 K) for form
2. Applied-field Mo¨ssbauer spectra for these two forms have
not been reported. It is tempting to assign the following
electronic structures using the correlation in Figure 12: form
3
1
(cyclam)(N3)]2+ (S ) /2 or /2) for which an isomer shift of
0.11 mm s-1 has been determined.15 On the basis of Figure 12
we can assign the following electronic structures to the {Fe-
NO}6 species: a low-spin FeIV (S ) 1) ion antiferromagnetically
coupled to an NO- (S ) 1) ligand. This has also been suggested
by Gerbeleu et al.29 for five-coordinate [Fe(HL)(NO)](NO3) with
an isomer shift of +0.04 mm s-1 at 80 K.
3
1, intermediate-spin ferric (S ) /2) coupled antiferromagneti-
cally to NO- (S ) 1), and form 2, low-spin ferric (S ) /2)
1
coupled to NO- (S ) 1). The authors have pointed out the
similarity of the spectra of these model complexes to those
measured for NO-bound hemes, e.g. of d1 heme in diheme
cytochrome cd1 nitrite reductase53 and siroheme in the hemo-
protein subunit of E. coli sulfite reductase.54
Conclusion
It appears that this model is also supported by the spin-density
distribution on the Fe-NO fragment which was determined for
In this study we have shown that Mo¨ssbauer spectroscopy is
a very useful spectroscopic tool for the elucidation of the actual
electronic structure of a series of nitrosyliron complexes of the
type {Fe-NO}6,7,8. In particular, the isomer shift, δ, is a very
sensitive parameter for the assignment of oxidation states of
the iron ions (and, indirectly, of the nitrosyl ligand). We have
shown that a consistent interpretation can be achieved by using
an ionic bonding model for these complexes with defined metal
oxidation states: FeIII (low- and intermediate-spin electron
1
nitrosylated hemes (S ) /2). Whereas the most sophisticated
DFT calculations55 still do not yield a very illustrative picture
in this respect, the experimental data derived from EPR
measurements56,57 indicate that nearly 30% of the spin-density
resides on the NO ligand in these complexes.
(51) Franz, K. J.; Lippard, S. J. J. Am. Chem. Soc. 1999, 121, 10504.
(52) Nasri, H.; Ellison, M. K.; Chen, S.; Huynh, B. H.; Scheidt, W. R.
J. Am. Chem. Soc. 1997, 119, 6274.
(53) Liu, M.-C.; Huynh, B. H.; Payne, W. J.; Peck, H. D., Jr.; Der
Vartanian, D. V.; Le Gall, J. Eur. J. Biochem. 1987, 169, 253.
(54) Christner, J. A.; Mu¨nck, E.; Janick, P. A.; Siegel, L. M. J. Biol.
Chem. 1983, 258, 11147.
(55) Rovira, C.; Kunc, K.; Hutter, J.; Ballone, P.; Parinello, M. J. Phys.
Chem. A 1997, 101, 8914.
(56) Hori, H.; Sarto, J.; Yonetani, T. J. Biol. Chem. 1981, 256, 7849-
1
3
configuration (S ) /2 and /2), respectively, antiferromagneti-
(57) Tyryshkin, A. M.; Dikanov, S. A.; Reijerse, E. J.; Burgard, C.;
Hu¨ttermann, J. J. Am. Chem. Soc. 1999, 121, 3396-3406. (b) Kappe, R.;
Hu¨ttermann, J. In AdVanced EPR Applications in Biology and Biochemistry;
Hoff, A. J., Ed.; Elsevier: Amsterdam, 1989.
7855.
(58) Ileperuma, O. A.; Feltham, R. D. Inorg. Chem. 1977, 16, 1876.

Octahedral Nitrosyliron Complexes {Fe-NO}6,7,8
J. Am. Chem. Soc., Vol. 122, No. 18, 2000 4365
5
cally coupled to an NO- (S ) 1) or high-spin FeIII (S ) /2)
antiferromagnetically coupled to an NO- (S ) 1)) and FeIV (S
) 1) antiferromagnetically coupled to NO- (S ) 1). For trans-
[(cyclam)Fe(NO)Cl]0, an {Fe-NO}8 (S ) 0) species, we arrive
at the conclusion that a low-spin ferric ion (S ) 1/2) is
Supporting Information Available: Tables of crystal-
lographic and structure refinement data, atom coordinates, bond
lengths and angles, anisotropic thermal parameters, and calcu-
lated positional parameters of H atoms for complexes 1 and
[L′Fe(NO)(ONO)(NO2)](ClO4) (PDF) and an X-ray crystal-
lographic file in CIF format. Figures S1, S2, S3, and S4
displaying variable-temperature, variable-field magnetization
curves of [LFe(NO)(N3)2]; variable-temperature zero-field Mo¨ss-
bauer spectra of [LFe(NO)(N3)2], 2, [L′Fe(NO)(ONO)(NO2)]-
(ClO4), [FeII(P(OC2H5)3)5Br](ClO4), and [Fe(NO)(P(OC2H5)3)4]-
BPh4, and applied field Mo¨ssbauer spectra at 4.2 K of trans-
[(cyclam)(NO)(Cl)]2+ and [L′Fe(NO)(ONO)(NO2)](ClO4) (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
antiferromagnetically coupled to an NO2- (S ) /2) ion. Our
1
results are in excellent agreement with those of Solomon et al.
for all {Fe-NO}7 (S ) /2) species where the high-spin ferric
3
5
(S ) /2) electron configuration has been independently estab-
lished by Fe K-edge XAS, magnetic circular dichroism,
resonance Raman, EPR, and Mo¨ssbauer spectroscopy.7,12
Acknowledgment. We thank the Fonds der Chemischen
Industrie for financial support. B. Mienert is thanked for
measuring many Mo¨ssbauer spectra. We are grateful to Dr.
Karsten Meyer for many helpful suggestions concerning the
syntheses.
JA994161I