Nitrosyliron complexes of macrocyclic [N42-] and open-chain [N2O22-] chelate ligands: Influence of the equatorial ligand on the NO binding mode

Article abstract of DOI:10.1016/S0020-1693(02)01114-3

Several new nitrosyliron complexes FeL-NO (L^2-: macrocyclic [N₄] coordinated or open-chain [N₂O₂] coordinated Schiff base ligand) were synthesised and compared with respect to their chemical behaviour (reactivity with dioxygen; redox potentials), their T-dependent magnetic susceptibility and their molecule structure. These properties reflect clearly the electronic influence of the equatorial chelate ligand (if present, also of an additional axial ligand) on the electron density at the central atom and, as a consequence, on the NO-binding mode. The macrocyclic complexes with the strongest equatorial donor ligands give pentacoordinated low spin (S=1/2) derivatives, best to interpret as [Fe^IIIL⁺-NO^-] - similar as the halides or pseudo-halides FeL-X. These complexes show no tendency to add additional axial ligands and are insensitive to air in non-coordinating solvents. In presence of a donor ligand D (pyridine, MeOH) the reaction with dioxygen results in an octahedral nitro derivative FeL(NO₂)×D that can also be prepared by reaction of the iron(III) complex [FeL×2D]⁺ with sodium nitrite. This could be proved especially for the most electron-rich complex Fe1-NO which can also be oxidised electrochemically in MeCN to give [Fe1-NO]⁺ and [Fe1-NO]²⁺ in two reversible steps. The [N₂O₂] coordinated complexes with weaker donor ligands in the equatorial plane show a more differentiated behaviour depending on electronic effects of the peripheral ligand substituents. Octahedral adducts FeL-NO×D could be isolated from several of such complexes under anaerobic conditions. Their reaction with dioxygen gives the μ-oxo derivative (FeL)₂O as the main product. Decreasing electron density at the central atom results in a trend towards decreasing transition temperature between a low-temperature S=1/2 and a high-temperature S=3/2 state. The atom distances within the first coordination sphere of the complex Fe5-NO×MeOH with the most electron withdrawing substitution of the equatorial ligand are significantly longer than those of the other complexes and show a surprising similarity to those of octahedral high-spin di-adducts FeL×2D of the iron(II) complexes. This fact suggests an interpretation as [Fe^IIL-NO^·×MeOH] with a coupling of the iron(II) S=4/2 state with the unpaired electron of the NO radical. The electronic influence of the equatorial ligands is also reflected in a significant correlation between the angle Fe-N-O (varying from 140 to 167°) and the NO stretching frequencies (varying from 1629 to 1812 cm^-1).

Full text of DOI:10.1016/S0020-1693(02)01114-3

Inorganica Chimica Acta 337 (2002) 247Á265
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www.elsevier.com/locate/ica
Nitrosyliron complexes of macrocyclic [N42ꢀ]
and open-chain [N2O22ꢀ] chelate ligands:
influence of the equatorial ligand on the NO binding mode
B. Weber, H. Gorls, M. Rudolph, E.-G. Jager *
¨ ¨
Institute of Inorganic and Analytical Chemistry, Friedrich-Schiller-Universitat Jena, August-Bebel-Str. 2, D-07743 Jena, Germany
¨
Received 8 March 2002; accepted 7 June 2002
Dedicated to Karl Wieghardt on the occasion of his 60th birthday
Abstract
Several new nitrosyliron complexes FeLꢀ
/
NO (L2ꢀ: macrocyclic [N4] coordinated or open-chain [N2O2] coordinated Schiff base
ligand) were synthesised and compared with respect to their chemical behaviour (reactivity with dioxygen; redox potentials), their T-
dependent magnetic susceptibility and their molecule structure. These properties reflect clearly the electronic influence of the
equatorial chelate ligand (if present, also of an additional axial ligand) on the electron density at the central atom and, as a
consequence, on the NO-binding mode. The macrocyclic complexes with the strongest equatorial donor ligands give
pentacoordinated low spin (Sꢁ
FeLꢀX. These complexes show no tendency to add additional axial ligands and are insensitive to air in non-coordinating solvents. In
presence of a donor ligand D (pyridine, MeOH) the reaction with dioxygen results in an octahedral nitro derivative FeL(NO2)ꢃ
that can also be prepared by reaction of the iron(III) complex [FeLꢃ
2D]ꢂ with sodium nitrite. This could be proved especially for
the most electron-rich complex Fe1ꢀNO which can also be oxidised electrochemically in MeCN to give [Fe1ꢀ
NO]ꢂ and [Fe1ꢀ
NO]2ꢂ in two reversible steps. The [N2O2] coordinated complexes with weaker donor ligands in the equatorial plane show a more
differentiated behaviour depending on electronic effects of the peripheral ligand substituents. Octahedral adducts FeLꢀNOꢃ
/
1/2) derivatives, best to interpret as [FeIII
L
ꢂ ꢀ
/
NOꢀ]*
/
similar as the halides or pseudo-halides
/
/D
/
/
/
/
/
/D
could be isolated from several of such complexes under anaerobic conditions. Their reaction with dioxygen gives the m-oxo
derivative (FeL)2O as the main product. Decreasing electron density at the central atom results in a trend towards decreasing
transition temperature between a low-temperature Sꢁ
coordination sphere of the complex Fe5ꢀNOꢃMeOH with the most electron withdrawing substitution of the equatorial ligand are
significantly longer than those of the other complexes and show a surprising similarity to those of octahedral high-spin di-adducts
FeLꢃ MeOH] with a coupling of the iron(II)
2D of the iron(II) complexes. This fact suggests an interpretation as [FeIILꢀ
NO+ ꢃ
Sꢁ4/2 state with the unpaired electron of the NO radical. The electronic influence of the equatorial ligands is also reflected in a
significant correlation between the angle FeꢀNꢀO (varying from 140 to 1678) and the NO stretching frequencies (varying from 1629
to 1812 cmꢀ1).
# 2002 Elsevier Science B.V. All rights reserved.
/
1/2 and a high-temperature Sꢁ
/
3/2 state. The atom distances within the first
/
/
/
/
/
/
/
/
Keywords: Iron; Nitrosyl complexes; Macrocycles; Cyclovoltammetry; Magnetic moment; Molecule structures
1. Introduction
There has been a renewed interest in the chemistry of
nitric oxide (NO) during the last few years because of its
role in biological systems. NO is a biological messenger
in a wide range of physiological processes, it is essential
* Corresponding author
E-mail address: ernst-gottfried.jaeger@uni-jena.de (E.-G. Jager).
¨
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 1 1 4 - 3

248
B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á265
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in host defence and immunological reactions and
responsible for the colour and stability of cured meats
[1,2]. Nitric oxide is synthesised in vivo by the hem
binding conditions of the NO to the iron. This is an
important question because in biological systems the
equatorial ligand together with the protein controls the
electronic (and geometric) features of the active site and
the reactivity of the iron towards the binding and
activation of small molecules, their precursors, and their
metabolites.
enzyme nitric oxide synthase by stepwise oxidation of L-
arginine to NO and -citrulline [1,2] and it is also an
L
intermediate in the reduction of nitrite to ammonia by
the hem enzyme nitrite reductase [3].
The groundwork relating to current studies was laid
about three decades ago when NO has been used as EPR
active probe for studying the reaction of hem proteins
and related derivatives with small molecules [4,5]. The
reaction of nitric oxide with ferrous and ferric porphy-
rins in the presence or absence of additional axial
ligands and the formed nitrosyl complexes have been
investigated in detail using X-ray absorption, EPR,
In this paper, we present the synthesis and character-
isation of ‘{FeNO}7’ nitrosyliron complexes with dia-
nionic tetradentate chelate ligands derived either from
macrocyclic [N4] coordinated bis(3-imino-enamines)
(type I) or from open-chain [N2O2] coordinated bis(3-
oxo-enamines) (type II). The advantage of these ligands
is their broad variability. Former investigations of
copper [11,12b], nickel [12b,13,14], cobalt [12b,13,15]
and iron complexes [12,13,16] type I and II showed the
principal possibility to control such properties as the
affinity of vacant axial coordination sites towards
additional ligands, the reduction and oxidation poten-
tials, the binding and activation of dioxygen [15] or
carbon dioxide [17], the magnetic behaviour and special
structural features by use of electronic (and in part
steric) effects of the bridges R, R? and the peripheral
substituents R1 and R2 (with a few exceptions, R3 is H in
our complexes). The formula of the base iron(II)
complexes and their abbreviations used in this paper
are given in Scheme 1. The aim of our studies is to gain
information on the NO binding mode in the complexes
Mossbauer, IR and UVÁVis spectroscopy [6]. In addi-
/
¨
tion NO-adducts of several non-porphyrin iron chelate
complexes as tetramethylcyclam, phthalocyanine and
quadridentate Schiff bases ligands, have been prepared
and spectroscopically characterised. Some of them
exhibit Sꢁ
intermediate spin (Sꢁ
dence of the MꢀNꢀO geometry of the number of
/
3/2 ground states or low spin (Sꢁ
/
1/2) l
/
/3/2) transitions [7]. The depen-
/
/
electrons determined by the electron-counting formalism
of Enemark and Feltham [8] has been discussed in
various cases [6,7,9]. Selected structural, IR- and mag-
netic data of several known nitrosyliron complexes are
listed in Table 1.
Despite the enormous number of known nitrosyliron
complexes with porphyrins and related anionic equator-
ial ligands, up to now there have been surprisingly few
systematic investigations on relationships between the
electronic properties of the equatorial ligand and the
FeLꢀ
as UVÁ
axial ligands and stability on air, the redox behaviour,
the spin state and the molecule structure*with those of
the corresponding base complexes FeL. Prior to our
/
NO by comparison of their typical properties-such
/Vis and IR spectra, reactivity with additional
/
Table 1
Several known nitrosyliron complexes
a
dMꢀN(NO) (A) dMꢀL (A) ÞMNO (8) dNꢀO (A) nNO (cmꢀ1
)
S
Reference
˚
˚
˚
Complex
‘{FeNO}7’
[Fe(TPP)NO]
b
1.717
1.722
1.737
1.716
1.783
1.80
149.2
144.4
1.122
1.167
1.137
1.17
1670
1666
1/2
[6]
[6]
[Fe(OEP)NO] (monoclinic)
[Fe(TMC)NO](BF4)2
[Fe(Me4TAA)NO]
177.5
144.1
1840
1636
1/2l3/2 [7b]
1/2
3/2
1/2
[7c]
[Fe(Salen)NO] (1)
[Fe(Salen)NO] (2)
144, 150
122, 132
142.1
1.11
1.15
1710
1630
[7d,10]
[7d,10]
[6]
[Fe(TPP)NO(1MeIm)]
[Fe(TpivPP)NO(NO2)]ꢀ (average of 3 values)
1.743
1.802
2.180
2.075
1.121
1.155
1625
1616, 1668
138.1
[6]
b
‘{FeNO}6’
[Fe(OEP)NO]ꢂ
1.644
1.652
1.6465
176.9
174.4
177.28
1.112
1.150
1.135
1862
1937
1921
0
0
[6]
[6]
[6]
[Fe(TPP)NO(H2O)]ꢂ
[Fe(OEP)NO(1MeIm)]ꢂ
2.001
1.9889
a
Abbreviations used in this paper: TPP, tetraphenylporphyrin, OEP, octaethylporphyrin, TMC, tetramethyl-tetraaza-cyclotetradecan, Me4TAA,
tetramethyl-dibenzotetraaza-[14]annulen, Salen, bis-salicylaldehyde-ethylenediimin, TpivPP, tetra(pivaloylphenyl)-porphyrin; Him, imidazole,
MeOH, methanol, Py, pyridine, DMF, dimethyformamide.
b
Designation after Enemark and Feltham [8].

B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á
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249
Magnetic measurements of powered samples were
performed on a Quantum-Design-MPMSR-5s-SQUID-
Magnetometer in a temperature range from 1.7 to 300
K. All measurements were carried out at two field
strength (0.2 and 0.5 T). No significant field dependence
was observed. Diamagnetic corrections were made by
using the molar susceptibilities of the analogous dia-
magnetic nickel(II) complexes without NO. They are in
satisfactory agreement with the values calculated with
tabulated Pascal constants (as a rule, suitable values can
be estimated as xdia
:
/
0.6ꢃ
10ꢀ6 Mcomplex).
/
Mass spectroscopic investigations were performed
using a SSQ 710 or a MAT95XL mass spectrometer
and direct electron beam ionisation as stimulation.
2.2. Syntheses
2.2.1. Iron(II) complexes
The iron(II)complexes Fe1 [12a], Fe2 [12b], Fe3 [20],
Fe4ꢃ
/
2Py, Fe5ꢃ
/
3Py, Fe6ꢃ
/
2MeOH, Fe6a, Fe7aꢃ
/
2Py [16a] and the ligands H21 [21] H27a, [14a] and
H27b [14a,21] have been prepared as described in the
literature. All preparations were done using Schlenk
techniques and argon as inert gas [19].
Fe7aꢃ
bis(iminomethylidyne)bis(3-oxo-3-phenylpropa-
nato)}(2-)-N,N?,O3,O3?]iron(II)):ꢃ
2 methanol: 1.7 g
(9.9 mmol) iron(II)acetate and 4.24 g (8.3 mmol) H27a
were dissolved in 50 ml methanol and heated to reflux
for 1 h. After cooling down, the black precipitate was
filtered off and dried in vacuo. Yield: 4.3 g (91%). Anal.
/
2MeOH. ([E,E]-[{diethyl 2,2?-1,2-phenylene-
/
Scheme 1. Abbreviations of iron(II) complexes used as starting
material for nitrosyl derivatives.
work the complex Fe6ꢀ
/
NO was synthesised by Numata
et al. [18] and later investigated magnetically by Konig
¨
et al. [7e]. The NO derivative of the iron complex type I
Calc. for C30H26N2O6Feꢃ
N, 4.44; Fe, 9.86. Found: C, 57.04; H, 4.53; N, 4.33; Fe,
9.73%. IR (Nujol): nCÄO
1691 cmꢀ1. MS (DEI): Base
peak: 105 m/z (fragment; 100%); 566 m/z (Mꢂ, 50%).
Fe7bꢃ2Py ([E,E]-[{diethyl-2,2?-1,2-ethylenebis(imi-
nomethylidyne)bis(3-oxo-3-phenyl-propanato)}(2-)-
N,N?,O3,O3?]-iron(II))ꢃ
2pyridine: 2.0 g (11.4 mmol)
/2CH3OH: C, 60.96; H, 5.44;
with Rꢁ
/
R?ꢁ
/
1,2-phenylene, R1ꢁ
(FeMe4TAA) was earlier synthesised and structurally
/
R3ꢁ
/
Me and R2ꢁ
H
/
ꢁ
/
characterised by Floriani et al. [7c].
/
/
2. Experimental
iron(II)acetate, 4.1 g (8.8 mmol) H27b and 2.32 g (34.2
mmol) sodiumethanolat were dissolved in 30 ml metha-
nol and 30 ml pyridine and heated to reflux for 1 h. The
solution was left in the freezer over night to yield bright
to dark read crystals, which were collected, washed with
a little amount of methanol and dried in vacuo. Yield:
2.1. General procedures and instrumentation
The solvents methanol, toluene and pyridine were
purified as described in the literature [18].
Infrared measurements were performed in the range
2.84 g (63%). Anal. Calc. for C26H26N2O6Feꢃ
C, 63.91; H, 5.36; N, 8.28. Found: C, 62.73; H, 5.29; N,
8.15%. IR (Nujol): nCÄO
1671 cmꢀ1. MS (DEI): Base
peak: 79 m/z (Pyridine; 100%); 518 m/z (Mꢂ, 30%); 473
m/z (Mꢂ ꢀ
OCH2CH3, 10%).
/2C5H5N:
of 4000Á
/
650 cmꢀ1 with a PerkinÁ
Elmer System 2000
/
FT IR-Spectrometer as Nujol mulls at room tempera-
ture (r.t.). The NO-stretching frequency was obtained by
comparison of the infrared spectra of the NO- adduct
with the IR spectra of the corresponding iron(II)
ꢁ
/
/
complex and iron(III) halide complex. In most casesÁ
with the exception of Fe7bꢀNOÁthe position of the NO-
stretching frequency could be determined clearly.
Electronic spectra were measured with a Varian Cary
5 spectrophotometer at r.t. using methanol, toluene and
pyridine as solvents.
/
2.2.2. Nitrosyl derivatives
/
/
2.2.2.1. General procedure. Nitric oxide (formed and
purified according to Dickinson et al. [22]) was bubbled
through a solution (Fe1, Fe6, Fe6a) or suspension (Fe2,
Fe3, Fe4, Fe5, Fe7a, Fe7b) of the corresponding iron(II)

250
B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á265
/
complex. The formed product was filtered off and dried
in vacuo.
N3,N3?,O1,O1?]-nitrosyliron)ꢃ
Fe5ꢃ3Py were suspended in 50 ml MeOH. Nitric oxide
/
MeOH: 0.47 g (0.7 mmol)
/
Fe1ꢀ
/
NO ((6,13-bis(ethoxycarbonyl)-7,12-dimethyl-
was bubbled through the suspension till a clear solution
was obtained. The solution was left in the freezer to
yield black crystals. Yield: 0.13 g (39%). Anal. Calc for
C19H20N5O6Fe: C, 48.53; H, 4.29; N, 14.89. Found: C,
1,4,8,11-tetraazacyclotetradeca-5,7,12,14-tetraena-
to(2-))nitrosyliron(II)): 1.09 g (6.2 mmol) iron(II)acetate
and 2.17 g (5.9 mmol) ligand H21 were dissolved in 50
ml MeOH and heated to reflux for 1 h. After cooling
down nitric oxide was bubbled through the solution.
The colour of the solution changed from red to green
and the product precipitates immediately. The precipi-
tate was collected and dried in vacuo. The mother
solution was allowed to stand over night to produce
single crystals of Fe1ꢀ
lography. Yield: 1.85
C18H26N5O5Fe: C, 48.25; H, 5.85; N, 15.64; Fe, 12.46.
Found: C, 45.21; H, 5.40; N, 13.84; Fe, 12.28%. IR
47.72; H, 4.11; N, 14.35%. IR (Nujol) nNÄO
cmꢀ1. MS (DEI): Base peak: 30 m/z (NOꢂ, 100%); 408
m/z (Mꢂ ꢀ
NO, 70%).
Fe5ꢀNOꢃMeOH(2): The same procedure carried
out at ꢀ78 8C (dry ice CO2Áethanol) yielded black
ꢁ1744
/
/
/
/
/
/
crystals which rearranged quickly into the above pro-
duct at r.t. The identity of these crystals was proved by
X-ray analysis, but it was not possible to measure the
characteristic IR using standard methods.
/
NO suitable for X-ray crystal-
g
(69%). Anal. Calc. for
Fe5ꢀ
no)bis(2-cyano-2-propenoato)](2-)-N3,N3?,O1,O1?]-nitro-
syliron): 0.81 g (1.3 mmol) Fe5ꢃ3Py were suspended in
/
NO ((Z,Z)-[diethyl 3,3?-(1,2-phenylenediimi-
(Nujol) nNO
z (Mꢂ ꢀNO, 100%); 30 m/z (NOꢂ, 15%), 448 m/z (Mꢂ,
10%).
Fe2ꢀ
ꢁ
/
1629 cmꢀ1. MS (DEI): Base peak: 418 m/
/
/
50 ml MeOH. Nitric oxide was bubbled through the
suspension for one and a half hour. The microcrystalline
product was filtered off and dried in vacuo. Yield: 0.49 g
(85%). Anal. Calc. for C18H16N5O5Fe: C, 49.33; H, 3.68;
N, 15.98. Found: C, 48.09; H, 3.75; N, 15.13%. IR
/
NO
(6,13-bis(ethoxycarbonyl)-7,12-dimethyl-
benzo-[b]-1,4,8,11-tetraazacyclotetradeca-5,7,12,14-tet-
raenato(2-)nitrosyliron(II)): 0.19 g (0.4 mmol) Fe2 were
suspended in 20 ml MeOH and nitric oxide was bubbled
through the suspension for half an hour. The crystalline
product was filtered off and dried in vacuo. To obtain
single crystals suitable for X-ray crystallography some
of the product was slowly crystallised from a pyridineÁ
methanol solution. Yield: 0.16 g (75%). Anal. Calc. for
C22H26N5O5Fe: C, 53.24; H, 5.28; N, 14.11. Found: C,
(Nujol) nNÄO
ꢁ
/
1812 cmꢀ1. MS (DEI): Base peak: 30 m/
NO, 5%).
z (NOꢂ, 100%); 408 m/z (Mꢂ ꢀ
/
Fe6ꢀ
/
NO
ne)bis(2,4-pentanedionato)(2-)-N,N?,O2,O2?]-nitrosy-
liron): Nitric oxide was bubbled through a solution of
([3,3?]-[1,2-phenylenebis(iminomethylidy-
/
1.1 g (2.88 mmol) Fe6ꢃ2MeOH in 100 ml methanol.
/
52.15; H, 5.02; N, 13,96%. IR (Nujol) nNÄO
cmꢀ1. MS (DEI): Base peak: 30 m/z (NOꢂ, 100%); 466
m/z (Mꢂ ꢀNO, 80%); 496 m/z (Mꢂ, 5%).
Fe3ꢀNO (6,13-bis(ethoxycarbonyl)-7,12-dimethyl-di-
ꢁ1637
/
The precipitate was filtered off and dried in vacuo.
Yield: 0.94 g (79%). Anal. Calc. for C18H18N3O5Fe: C,
52.45; H, 4.40; N, 10.20. Found: C, 52.36; H, 4.30; N,
/
/
10.10%. IR (Nujol) nNÄO
peak: 30 m/z (NOꢂ, 100%); 412 m/z (Mꢂ, 1%); 382 m/
z (Mꢂ ꢀ
NO, 15%).
NO
ꢁ
/
1790 cmꢀ1. MS (DEI): Base
benzo-[b,i]-1,4,8,11-tetraazacyclotetradeca-5,7,12,14-tet-
raenato(2-)nitrosyliron(II)): 0.23 g (0.5 mmol) Fe3 were
suspended in 30 ml MeOH and nitric oxide was bubbled
through the solution for half an hour. The crystalline
product was filtered off and dried in vacuo. Yield: 0.22 g
(80%). Anal. Calc. for C26H26N5O5Fe: C, 57.36; H, 4.81;
N, 12.87. Found: C, 54.32; H, 4.31; N, 10.72%. IR
/
Fe6aꢀ
/
(Diethyl-2,2?-[1,2-phenylene-bis(imino-
methylidyne)bis(3-oxo-butanoato)(2-)-N,N?,O3,O3?]-ni-
trosyliron): Nitric oxide was bubbled through a solution
of 0.1 g (0.2 mmol) Fe6aꢃ2MeOH in 15 ml methanol.
/
ꢁ
1675 cmꢀ1. MS (DEI): Base peak: 514
/
The precipitate was filtered off and dried in vacuo.
Yield: 0.08 g (85%). Anal. Calc. for C20H22N3O7Fe: C,
50.86; H, 4.69; N, 8.90. Found: C, 52.73; H, 5.01; N,
(Nujol) nNÄO
m/z (Mꢂ ꢀNO, 100%); 30 m/z (NOꢂ, 80%), 544 m/z
(Mꢂ, 5%).
Fe4ꢀNOꢃ
phenylenebis ( iminomethylidyne ) } bis - ( propanodiato )
(2-)N,N?,O4, O4?]-nitrosyliron)ꢃ
MeOH: 0.4 g (0.8
mmol) Fe4ꢃ2Py were suspended in 20 ml MeOH.
/
8.11%. IR (Nujol) nNÄO
peak: 442 m/z (Mꢂ ꢀNO, 100%); 30 m/z (NOꢂ, 100%).
Fe7aꢀNO ([E,E]-[{diethyl-2,2?-1,2-phenylene-bis(imi-
ꢁ
1776 cmꢀ1, MS (DEI): Base
/
/
/MeOH
((E,E)-[tetraethyl
2,2?-{1,2-
/
/
/
nomethylidyne)bis(3-oxo-3-phenylpropanoato)}(2-)-
N,N?,O3,O3?]-nitrosyliron): Nitric oxide was bubbled
/
Nitric oxide was bubbled through the suspension till a
clear solution was obtained. The solution was left in the
freezer to yield dark brown crystals. Yield: 0.26 g (63%).
Anal. Calc for C23H30N3O10Fe: C, 48.94; H, 5.36; N,
7.45. Found: C, 49.10; H, 5.43; N, 7.21%. IR (Nujol)
through a suspension of 0.75 g (1.3 mmol) Fe7aꢃ
/
2MeOH in 60 ml MeOH for 1 h. The black fine
crystalline product was filtered off and dried in vacuo.
Yield: 0.63 g (84%) Anal. Calc. for C30H26N3O7Fe: C,
60.41; H, 4.39; N, 7.05. Found: C, 59.78; H, 4.43; N,
nNÄO
(NOꢂ, 100%); 502 m/z (Mꢂ ꢀ
Fe5ꢀNOꢃMeOH(1) ((Z,Z)-[diethyl
phenylenediimino)bis(2-cyano-2-propenoato)](2-)-
ꢁ
/
1716 cmꢀ1. MS (DEI): Base peak: 30 m/z
NO, 5%).
6.96%. IR (Nujol) nNÄO
peak: 566 m/z (Mꢂ ꢀ
NO, 100%); 30 m/z (NO, 80%);
521 m/z (Mꢂ ꢀ
NOꢀOCH2CH3, 20%).
ꢁ
/
1778 cmꢀ1. MS (DEI): Basis
/
/
/
/
3,3?-(1,2-
/
/

B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á
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251
Fe7bꢀ
/
NO ([E,E]-[{diethyl 2,2?-1,2-ethylenebis(imino-
The structures were solved by direct methods (SHELXS
[25]) and refined by full-matrix least-squares techniques
methylidyne)bis(3-oxo-3-phenylpropanoato)}(2-)-
N,N?,O3,O3?]-nitrosyliron): Nitric oxide was bubbled
2
against Fo
(
SHELXL-97 [26]). The quality of the data of
through a suspension of 1.7 g (2.5 mmol) Fe7bꢃ
/
2Py in
compound Fe6ꢀ/NO is too bad. We will only be
30 ml methanol until a clear solution was obtained. The
solution was left in the freezer over night to yield a fine
crystalline black product that was filtered off and dried
in vacuo. Yield: 0.69 g (52%). Anal. Calc. for
C26H26N3O7Fe: C, 56.95; H, 4.78; N, 7.66. Found: C,
56.75; H, 4.67; N, 7.34%. IR (Nujol) nNÄO, nCÄO: 1700,
1696, 1659, 1655 cmꢀ1. MS (DEI): Base peak: 30 m/z
publishing the conformation of the molecule and the
crystallographic data. We will not deposit the data in the
Cambridge Crystallographic Data Centre. For the
compound Fe5ꢀ
were located by difference Fourier synthesis and refined
isotropically. For the compound Fe4ꢀNOꢃMeOH
/
NOꢃ/MeOH(1) all hydrogen atoms
/
/
only the hydrogen atom of the hydroxy-group of the
methanol was located by difference Fourier synthesis
and refined isotropically. The other hydrogen atoms
were included at calculated positions with fixed thermal
parameters. All non-hydrogen atoms were refined
anisotropically [26]. XP (SIEMENS Analytical X-ray
Instruments, Inc.) was used for structure representa-
tions.
(NOꢂ, 100%); 518 m/z (Mꢂ ꢀ
/NO, 60%).
2.3. Electrochemistry
Cyclic and square wave voltammetric measurements
have been conducted in 3-electrode technique using an
home-built computer controlled instrument based on
the PCI-MIO-16E-1 data acquisition board (National
Instruments). The experiments were performed either in
methylenechloride or in acetonitrile containing 0.5M
tetra-n-butylammonium-perchlorate under a blanket of
solvent-saturated argon. The complex concentration
was about 0.015 m in all experiments. The ohmic
resistance, which had to be compensated for, was
determined by measuring the impedance of the system
at potentials where the Faradayic current was negligibly
small. Background correction was accomplished by
subtracting the current curves of the blank electrolyte
(containing the same concentration of supporting elec-
trolyte) from the experimental CVs. The reference
electrode was Ag/AgCl in acetonitrile containing 0.25
M tetra-n-butylammonium chloride (abbreviated ‘Ag’).
The potential of this reference system was calibrated by
2.4.2. Crystal data for Fe1ꢀ
/
NO
C18H26FeN5O5, Mrꢁ
size 0.22ꢃ0.20ꢃ
aꢁ10.0030(5), bꢁ
103.017(3), bꢁ109.871(3),
1019.45(8) A , Tꢁ 90 8C, Zꢁ
cmꢀ3, m (Mo Ka)ꢁ7.8 cmꢀ1, F(000)ꢁ
reflections in h(ꢀ12/10), k(ꢀ12/13), l(ꢀ12/13), mea-
sured in the range 3.245U 527.358, completeness
Umax 97.2%, 4488 independent reflections, Rint
/
448.29 g molꢀ1, brown prism,
¯
0.12 mm, triclinic, space group P1;
/
/
/
˚
/
/
10.3890(5), cꢁ
gꢁ
2, rcalc
/
10.8640(5) A; aꢁ
94.539(3)8; Vꢁ
1.460 g
470, 6788
/
/
/
/
3
˚
/
ꢀ
/
/
ꢁ
/
/
/
/
/
/
/
/
ꢁ
/
ꢁ
/
0.031, 3588 reflections with Foꢀ
/
4s(Fo), 272 para-
1
restraints, Robs
2
0.054, wRobs
meters,
1
Robs
0
0.076, wRobs
ꢁ
/
ꢁ0.120,
/
2
ꢁ
/
ꢁ
/
0.131, GOOFꢁ
/
1.011, largest dif-
ꢀ3
˚
0.489 e A
ference peak and hole: 0.601/ꢀ
/
.
measuring the potential of the ferroceneÁ
couple at the end of each experiment. The latter was
found to be at 0.85590.002 V in methylenechloride and
0.78590.001 V in acetonitrile, respectively. Calibra-
tion using the complex Ni1-the potentials of which
versus SCE (EHꢁ0.241 V) are nearly independent from
the solvents MeCN, DMF, and pyridine [16f]*leads to
a potential of EH(‘Ag’)ꢀ0.163 V in MeCN and ꢀ0.237
/
ferricinium
2.4.3. Crystal data for Fe2ꢀ
C22H26FeN5O5, Mrꢁ
496.33 g molꢀ1, brown or
green prism, size 0.12ꢃ0.10ꢃ0.09 mm, monoclinic,
space group P21/n, aꢁ7.6622(2), bꢁ13.8944(5), cꢁ
95.048(2)8, Vꢁ2164.33(11) A , Tꢁ
4, rcalc 7.43
1.523 g cmꢀ3, m (Mo Ka)ꢁ
cmꢀ1, F(000)ꢁ
1036, 8555 reflections in h(ꢀ9/9), k(ꢀ
18/16), l(ꢀ26/26), measured in the range 2.935U 5
27.478, completeness Umax 98.2%, 4852 independent
reflections, Rint 0.026, 3741 reflections with Foꢀ
4s(Fo), 298 parameters, 0 restraints, Robs
/
NO
/
/
/
/
ꢂ
/
/
/
/
/
3
˚
˚
20.4088(5) A; bꢁ
/
/
/
/
ꢀ
/
90 8C, Zꢁ
/
ꢁ
/
/
/
/
/
/
/
/
/
/
/
V in methylenechloride for this electrode. The working
electrode was either an hanging mercury drop (mHg-
ꢁ
/
ꢁ
/
/
1
dropꢁ
/
3.95Á/4 mg) produced by the CGME instrument
ꢁ
/
0.045,
0.130, GOOFꢁ
1.038, largest difference peak and hole: 0.544/ꢀ0.510 e
2
or a 1.5 mm Pt disk electrode (both from Bioanalytical
Systems, Inc., West Lafayette, USA).
wRobs
ꢁ
/
0.119, Ra1llꢁ
/
0.064, wRa2llꢁ
/
/
/
ꢀ3
˚
A
.
2.4. X-ray crystallography
2.4.4. Crystal data for Fe3ꢀ
C26H26FeN5O5, Mrꢁ
544.37 gmolꢀ1, brown prism,
size 0.19ꢃ0.17ꢃ0.12 mm, orthorhombic, space group
Pbcn, aꢁ38.8930(10), bꢁ
Tꢁ 90 8C, Zꢁ8, rcalc
Ka)ꢁ
6.73 cmꢀ1, F(000)ꢁ
h(ꢀ50/50), k(ꢀ9/9), l(ꢀ19/19), measured in the range
/
NO
2.4.1. Crystal structure determination
/
The intensity data for the compounds were collected
on a Nonius kCCD diffractometer, using graphite-
monochromated Mo Ka radiation. Data were corrected
for Lorentz and polarisation effects, but not for
absorption [23,24].
/
/
˚
15.2520(6) A;
/
/
8.1527(3), cꢁ
1.495 g cmꢀ3, m (Mo
2264, 9424 reflections in
/
/
ꢀ
/
/
ꢁ
/
/
/
/
/
/

252
B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á265
/
2.885
independent reflections, Rint
with Foꢀ4s(Fo), 334 parameters, 0 restraints, Robs
0.069, wRobs
/
U 5
/
27.518, completeness Umax
ꢁ
/
95.4%, 5303
F(000)ꢁ
/
852, 6433 reflections in h(ꢀ
/
6/6), k(ꢀ
U 527.328,
98.1%, 3869 independent reflec-
/
18/20),
ꢁ/0.095, 3180 reflections
l(ꢀ25/25), measured in the range 3.295
/
/
/
1
/
ꢁ
/
completeness Umax
tions.
ꢁ
/
2
ꢁ
/
0.119, Ra1llꢁ
/
0.138, wRa2llꢁ
/0.138,
GOOFꢁ
/
1.007, largest difference peak and hole:
ꢀ3
˚
0.392 e A
0.447/ꢀ
/
.
3. Results and discussion
2.4.5. Crystal data for Fe4ꢀ
C23H30FeN3O10, Mrꢁ
564.34 g molꢀ1, brown prism,
size 0.20ꢃ0.18ꢃ0.12 mm, monoclinic, space group
P21/n, aꢁ8.1399(2), bꢁ
bꢁ96.018(3)8, Vꢁ2562.69(17) A , Tꢁ
4, rcalc
1.460 g cmꢀ3, m (Mo Ka)ꢁ
F(000)ꢁ1176, 9520 reflections in h(ꢀ8/8), k(ꢀ
l(ꢀ35/35), measured in the range 3.475U 5
completeness Umax
tions, Rint 0.142, 2348 reflections with Foꢀ
338 parameters, 0 restraints, Robs
0.158, Ra1llꢁ0.210, wRa2llꢁ
0.200, GOOFꢁ
gest difference peak and hole: 0.389/ꢀ
/
NOꢃ
/
MeOH
/
3.1. Syntheses, general characterisation and reactivity
/
/
˚
/
/
11.5547(6), cꢁ
/
27.398(1) A;
90 8C, Zꢁ
6.49 cmꢀ1
The iron(II) complexes Fe1ÁFe7 (with exception of
/
3
˚
/
/
/
ꢀ
/
/
Fe7b) and several of their iron(III) derivatives have been
described earlier [12,13,16]. Their properties reflect the
ꢁ
/
/
,
/
/
/
15/14),
27.508,
strong effect of the equatorial ligand and*
of additional axial ligands.
The macrocyclic complexes Fe1Á
strongest equatorial ligands exist mainly either as penta
(4ꢂ1) coordinated species with intermediate spin
ground state (FeII: Sꢁ1; FeIII: Sꢁ
3/2) or as octahedral
derivatives with low spin ground state (Sꢁ0, 1/2)
/
if present*
/
/
/
/
ꢁ
/
89.7%, 5282 independent reflec-
/Fe3 having the
ꢁ
/
/
4s(Fo),
1
2
ꢁ/0.079, wRobs
ꢁ
/
/
/
/
/
0.997, lar-
/
/
ꢀ3
˚
/0.501 e A
.
/
[12,13,16]. Their first oxidation potentials are always
metal centred (FeII/III) and can serve as a measure of the
electron density at the central atom. They are strongly
dependent on the macrocyclic ligand, the solvent and on
2.4.6. Crystal data for Fe5ꢀ
C19H20FeN5O6, Mrꢁ
470.25 g molꢀ1, dark-brown
prism, size 0.28ꢃ0.22ꢃ0.20 mm, monoclinic, space
group P21/n, aꢁ7.4269(8), bꢁ14.625(2), cꢁ19.927(2)
90 8C, Zꢁ
7.42 cmꢀ1
9/9), k(ꢀ18/0),
25/0), measured in the range 2.475U 527.458,
completeness Umax 99.7%, 4922 independent reflec-
tions, Rint 0.038, 2795 reflections with Foꢀ4s(Fo),
360 parameters, 0 restraints, Robs
0.097, Ra1llꢁ0.131, wRa2llꢁ
0.122, GOOFꢁ
gest difference peak and hole: 0.405/ꢀ
/
NOꢃ
/
MeOH(1)
/
/
/
/
/
/
the type of additional axial ligands. For Fe1 (with R2ꢁ
/
3
˚
˚
A; bꢁ
/
91.790(8)8, Vꢁ
1.444 g cmꢀ3, m (Mo Ka)ꢁ
972, 5056 reflections in h(ꢀ
/
2163.4(4) A , Tꢁ
/
ꢀ
/
/
COMe instead of COOEt) in MeCN a spread over more
than 0.7 V has been observed [16e] (for better compar-
ability all the following selected values are estimated for
4, rcalc
F(000)ꢁ
l(ꢀ
ꢁ
/
/
,
/
/
/
/
/
/
NHE by EH(FeII/III)ꢁ
/
Eref(FeII/III)ꢂ
0.163 V in MeCN and EH(SCE)ꢁ
/
EH(ref) using
ꢁ
/
EH(‘Ag’)ꢁ
/
ꢀ
/
/0.241
ꢁ
/
/
V, tNC: tosylmethylisocyanide; chNC: cyclohexyliso-
cyanide):
1
2
ꢁ/0.041, wRobs
ꢁ
/
/
/
/
0.979, lar-
ꢀ3
˚
/0.345 e A
.
Axial ligand
EH(FeII/III1)/V
tNC
0.17
MeCN
0.05
chNC
0.02
2.4.7. Crystal data for Fe5ꢀ
/
NOꢃMeOH(2)
/
›
C19H20FeN5O6, Mrꢁ
size 0.12ꢃ0.10ꢃ
aꢁ8.574(1), bꢁ
86.804(7), bꢁ76.623(6), gꢁ
90 8C, Zꢁ2, rcalc
7.63 cmꢀ1, F(000)ꢁ
480, 7324 reflections in h(ꢀ
11/10), k(ꢀ11/11), l(ꢀ19/18), measured in the range
3.285U 527.758, completeness Umax 93.7%, 4658
independent reflections, Rint 0.084, 3153 reflections
with Foꢀ4s(Fo), 280 parameters, 0 restraints, Robs
0.167, wRobs
/
470.25 g molꢀ1, brown prism,
¯
4CNPy
Py
N-MeIm
CN
/
/
0.09 mm, triclinic, space group P
/
1;
ꢀ
/
0.02
ꢀ0.16
/
ꢀ
/
0.39
ꢀ
/
0.56
˚
/
/
8.955(1), cꢁ
64.604(5)8; Vꢁ
1.475 g cmꢀ3, m (Mo
/
/
15.638(1) A; aꢁ
/
/
/
/
1053.9(2)
Depending on the macrocyclic ligand the potentials
increase under identical conditions with decreasing
electron density in the order
3
˚
A , Tꢁ
Ka)ꢁ
/
ꢀ
/
/
ꢁ
/
/
/
/
/
/
/
ꢁ
/
Electron density
EH(MeCN)/V [12b,16e]
EH(Py)/V [16e]
Fe1ꢀ
0.05
0.16 0.02
/
Fe2ꢀ
/
Fe3
0.69
0.15
ꢁ
/
0.34
1
/
ꢁ
/
ꢀ
/
2
ꢁ
/
0.204, Ra1llꢁ
/
0.226, wRa2llꢁ
0.226,
/
GOOFꢁ
/
1.214, largest difference peak and hole:
ꢀ3
(As expected, the trans Áaxial binding of the donor
/
˚
0.558 e A
0.641/ꢀ
/
.
pyridine becomes stronger with increasing Lewis acidity
of the central atom and levels the electron density. This
results in a lower gradation of the potentials.)
2.4.8. Crystal data for Fe6ꢀ
C18H18FeN3O5, Mrꢁ
412.20 g molꢀ1, brown prism,
size 0.22ꢃ0.18ꢃ0.10 mm, monoclinic, space group
P21/n, aꢁ5.4089(4), bꢁ
bꢁ93.995(4)8, Vꢁ1756.1(2) A , Tꢁ
rcalc 1.559
cmꢀ3
(Mo Ka)ꢁ
/
NO
/
The complexes Fe4Á
negative) oxygen donors in the (weaker) open-chain
ligand give mainly high spin derivatives (FeII: Sꢁ
4/2,
FeIII: Sꢁ
5/2) in a square pyramidal pentacoordinated
(e.g. FeII6aꢃHIm [13,27], FeIIILꢃ
halide [13,16a]) or
/
Fe7 with the two (more electro-
/
/
˚
/
/
16.108(1), cꢁ
/
20.205(1) A;
90 8C, Zꢁ4,
8.95 cmꢀ1
/
3
˚
/
/
/
ꢀ
/
/
/
ꢁ
/
g
,
m
/
,
/
/

B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á
/265
253
octahedral coordination sphere [13]. Exceptions were
found with strong axial ligands such as imidazole (the
reaction with NO, depending on the concentration of an
potential axial ligand, resembles the equilibrium reac-
tion of the corresponding cobalt(II) complex with
dioxygen in presence of different concentrations of
pyridine [13,15a], e.g.
low spin Sꢁ
transition into the high spin Sꢁ
[13,27,28]). The adduct FeII6aꢃ
2Py is high spin at r.t.
but changes to low spin below 180 K [13,27]. The
oxidation potentials in pyridine are*as expected*
/
0 derivative Fe6aꢃ/2HIm undergoes a
/4/2 state above 328 K
/
CoII4ꢂPy?PyCoII4(Sꢁ1=2)
(1)
(2)
(3)
/
/
more positive than those of the macrocycles. They
indicate clearly the increasing electron-withdrawing
influence of the peripheral substituents on the electron
II
III
›
š
PyCo 4ꢂO2 ?PyCo 4 O2 (Sꢁ1=2)
III
›
II
š
PyCo 4 O2 ꢂPy?Co 4(Py)2(Sꢁ3=2)ꢂO2
density in the order (EHꢁ
/
ESCEꢂ0.24 V):
/
A slower decomposition, accelerated by coordinating
solvents or potential axial bases, was observed also for
Fe7bꢀ
/
Fe6ꢀ
/
Fe6aꢀ
0.42
0.25
/
Fe7aꢀ
0.44
0.38
/
Fe4ꢀ
0.46
0.27
/
Fe5
0.6
0.41
EH(Py)/V
EH(DMF)/V
:/0.34 0.38
ꢀ
/
Fe7bꢀ
/
NO.
:/0.27 0.25
The nitrosyl derivatives of Fe6a,b are better soluble
and require a much longer time for crystallisation. This
may be the reason for difficulties in isolation of pure
(DMF seems to be a stronger donor than pyridine in
this case and lowers the potentials. Additionally, its high
tendency to form H-bridges with the ligand periphery
results in a more complicated dependence [16a].)
The differences of the oxidation potentials are man-
ifested in the air sensitivity of the iron(II) complexes:
products. Fe6aꢀ
/
NO always contains small impurities of
the m-oxo derivative (Fe6a)2O, and we could not isolate
Fe6b in satisfactory purity.
The macrocycles*especially Fe1-are extremely sensitive
to oxygen, independent of the axial ligands. The open-
chain complexes vary in their air sensitivity from stable
/
Table 2
Selected IR data of the iron complexes with different axial ligands
Complex
nNO (cmꢀ1
)
nCO or nCN (cmꢀ1
)
as solid over years (Fe5ꢃ
/
3Py) or for a few days (Fe4ꢃ
/
2Py) to highly air sensitive (ethylene bridged open-chain
complexes).
This different sensitivity to air is also reflected in the
reaction with nitric oxide. The macrocyclic complexes
react in solution (e.g. methanol) very fast with NO
undergoing a colour change from dark red or violet to
Fe1
Fe1ꢀCl
Fe1ꢀNO
Fe2
1671
1680
1629
1637
1675
1678
1676
Fe2ꢀCl
Fe2ꢀNO
Fe3
1685
1676
1686
Fe3ꢀCl
1697, 1683
1700, 1687
1669
dark green or brownÁgreen. The nitrosyl derivatives
/
Fe3ꢀNO
Fe4ꢃ2MeOH
Fe4ꢃ2Pyridin
Fe4ꢀCl
Fe4ꢀNOꢃMeOH
Fe5
(fine crystalline black solids) can be stored as solids at
air for a few days without decomposition. The open-
chain complexes Fe6 and Fe7a,b behave similarly. The
a
1687 (1697
1693
1698
)
complex Fe7bÁNO is the only ethylene bridged open-
/
1716
a
2220 (CN)
2201; 2184 (CN)
2207 (CN)
chain iron nitrosyl complex where a defined, pure
product could be obtained.
The reaction of Fe4 and Fe5 requires a higher partial
pressure of NO for complete transformation. As crystal-
line solids the open-chain nitrosyl complexes can be
exposed to air for a few min, but longer exposition
Fe5ꢃ3Pyridin
(Fe5)2O
Fe5ꢀNO
Fe5ꢀNO?MeOH(1)
Fe6
1776 b/1812
1744
2200, 2204 (CN)
2204, 2152 (CN)
c
a
1655
Fe6ꢃ2MeOH
Fe6ꢀCl
Fe6ꢀNO
Fe6a
1619
1662
results in decomposition. The complexes Fe4ꢀ
/
NOꢃ
/
1790 (s), 1717 (w) 1638
1715
MeOH and Fe5ꢀNOꢃMeOH must be stored at
/
/
a
temperatures below r.t. (freezer) to prevent a loss of
methanol. The formation of the solvent free derivatives
can be followed by IR spectroscopy (shift of the NO-
stretching frequency to higher frequencies). These ni-
trosyl complexes decompose under argon in pure
solvents (without excess of NO) to give the iron(II)
complex and free nitric oxide. The elimination of NO
(indicated in form of gas bubbles) is accelerated by
addition of pyridine to a solution of the complexes in
methanol or toluene. The iron(II) complexes crystallise
as their adducts with pyridine, the starting materials for
the synthesis of the nitrosyl derivatives. This reversible
Fe6aꢀNO
Fe7aꢃ2Py
Fe7aꢀNO
Fe7bꢃ2Py
Fe7bꢀNO
1776
1704
a
1671 (1685
1677
1671 (1605, 1598. . ..)
)
1778
d
1700, 1696, 1659, 1655?
These earlier in KBr measured values [16a] are about 10 cmꢀ1
higher than those performed in nujol mull with the new instrumenta-
tion.
a
b
This band shows time-dependent increasing intensity and is
apparently to assign to an impurity of a more stable octahedral isomer
with intermolecular CN-coordination.
c
R.t. modification.
Doubtless assignment to the nNO difficult.
d

254
B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á265
/
š
›
All the fine crystalline black precipitates are char-
acterised by a new IR band which is with exception of
could therefore be interpreted as d5 [FeIIIL NO ]
species*at least those of the macrocycles. Their
/
Fe7bꢀ
/
NO well separated from the CO or CN stretching
frequency of R2 (Table 2). The n(NO) stretching
frequencies of the derivatives without additional axial
n(NO) frequencies are within the range of 1520Á1720
/
cmꢀ1 which is considered for anionic NO in its
›
complexes, but for NO as three electron donor (bound
š
ligands increase from 1629 cmꢀ1 for Fe1ꢀ
/
NO to 1812
NO and show a significant correlation
with the increasing oxidation potentials of the iron(II)
complexes. This suggests*with respect to the stretching
frequency n(NO+)ꢁ
1906.5 cmꢀ1 for the 2P state of the
free NO radicalÁan increasing population of the anti-
as NO ) a similar broad range (1950Á
/
1600 cmꢀ1) is
cmꢀ1 for Fe5ꢀ
/
discussed, too [31]. The formulation FeIII and NO was
also elucidated by Solomon et al. [30] on the base of
detailed experimental and theoretical studies for other
nitrosyliron complexes.
›
/
/
/
The formation of the NO derivatives from the iron(II)
complexes is accompanied by a typical change of the Vis
absorption spectrum. This is displayed in Fig. 1 for Fe1.
The NO derivative is characterised by a new broad band
at about 615 nm. It resembles the typical visible CT-
absorption of pentacoordinated iron(III) derivatives
bonding p* orbital [29] with increasing electron density
at the central atom. As expected, an additional trans
donor ligand (MeOH in the adducts of Fe4ꢀ
MeOH and Fe5ꢀ
transfer to the NO ligand and lowers the NO frequency.
The NO derivatives of our complexes which belong to
the {FeNO}7 type in the EnemarkÁ
Feltham notation [8]
/
NOꢃ
/
/
NOꢃMeOH) increases the charge
/
Fe1ꢀ
/
X (X: halide, N3ꢀ, NCSꢀ) [12c,13], but the
/
intensity is much lower, and its wavelength is hardly
Fig. 1. Vis-Spectra of Fe-1 and their derivatives (a) [FeII1] (1) and [Fe1ꢀ
pyridine (1, 3; cꢁ0.0022 M) inert (1, 2) and after exposure to air (3, 4; the band at 751 nm increases strongly, and the shoulder at 403 nm decreases
after longer interaction with air). (c) [FeIII1ꢀ
(2) followed by addition of NaNO2 (3). (d) [FeIII1ꢀ
solution of [FeIII1(NO2)2]ꢀ in MeOH (3Á
6) and saturated with NaNO2 (7).
/
NO] (2) in MeOH (inert). (b) [Fe1ꢀ
/
NO] in toluene (2ꢁ
/
4; cꢁ0.0011 M) and
/
/
/
NO2ꢃ
/
MeOH] (2) and [FeIII1(NO2)2]ꢀ (3) formed from [Fe1ꢀ
/
NO] in MeOH (1) by oxidation with air
ꢂ
/
Cl] in CH2Cl2 (1); [FeIII1solv
]
from [FeIII1ꢀ
/
Cl] in MeOH (2); ‘titration’ with an equimolar
/

B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á
/265
255
Table 3
UVÁVis data of Fe1ꢀNO
a
/
Solvent
c (mol lꢀ1
)
l1 (nm)
Log (o1/(l molꢀ1 cmꢀ1))
l2 (nm)
Log (o2/(l molꢀ1 cmꢀ1))
Methanol
Toluene
Pyridine
7.4ꢃ10ꢀ4
1.1ꢃ10ꢀ3
2.2ꢃ10ꢀ3
401
403
405
3.72
3.64
3.58
616
614
614
2.88
2.78
2.76
a
All spectra were measured at r.t. under argon with dꢁ0.1 cm.
Table 4
Potentials and kinetic data for the electrochemical reduction of selected nitrosyliron complexes in methylenechlorideꢂ0.5 m Bu4NClO4
a
Complex (type)
E‘Ag’8 (V)
ks (cm sꢀ1
)
D (cm2 sꢀ1
)
Fe1ꢀNO (I)
Fe2ꢀNO (I)
Fe3ꢀNO (I)
Fe7bꢀNO (II)
ꢀ0.775(ꢀ1.01)
ꢀ0.660(ꢀ0.90)
ꢀ0.520(ꢀ0.76)
ꢀ0.410(ꢀ0.65)
0.3
0.8
1.0
0.3
8ꢃ10ꢀ6
8ꢃ10ꢀ6
7ꢃ10ꢀ6
5ꢃ10ꢀ6
b
Epc (V)
Epa (V)
c,d
c
Fe6ꢀNO (II)
Fe6aꢀNO (II)
Fe5ꢀNO (II)
ꢀ0.345(ꢀ0.58)
ꢀ0.345(ꢀ0.58)
ꢀ0.133..ꢀ0.128 (ꢀ0.37)
ꢀ0.375
ꢀ0.375
ꢀ0.158. . .ꢀ0.155
ꢀ0.314...ꢀ0.317
ꢀ0.313
ꢀ0.107...ꢀ0.101
c
c
MeCNꢂ0.25 m nBu4NCl. In parentheses: EHꢁE‘Ag’ ꢀ0.237 V (see Section 2.3).
a
Hanging mercury drop; vs. AgÁ
/
AgClÁ
/
b
c
Kinetic parameters not determined.
With increasing scan-rate.
Weak satellite at lower potential, caused probably by partial isomerisation; not observed in MeCN.
d
dependent on the solvent (Table 3). This independence
of the spectrum under anaerobe conditions indicates
that the pentacoordinated complex does not bind an
additional axial ligand (MeOH, pyridine). Indeed, our
attempts to obtain any adducts of the macrocyclic
over a broad range of nitrite concentration is in
agreement with the observation, that nitro derivatives
of our macrocyclic iron(III) complexes were often
isolated with an trans Á
Fe2ꢀNO2ꢃH2O [13,32].
The complexes Fe4ꢀ
MeOH, and Fe5ꢀNO are air sensitive even in non-
/
axially bound O-donor, e.g.
/
/
complexes Fe1ꢀ
/
NO to Fe3ꢀ
/
NO were not successful.
/
NOꢃ
/
MeOH, Fe5ꢀ
/
NOꢃ
/
Recrystallisation from MeOHÁ
/Py yields the pure sol-
/
vent free starting material. In contrast, some of the
complexes with open-chain ligands were isolated from
coordinating solvents. The aerobic decomposition re-
sults in the formation of the corresponding m-oxo-
species as the main product.
methanol as their monoadducts FeLꢀ
As solids and as solutions in non-coordinating
solvents (toluene) Fe1ꢀNO, Fe2ꢀNO and Fe3ꢀNO are
/
NOꢃ/MeOH.
/
/
/
stable on air for some time without decomposition. In
coordinating solvents (pyridine; MeOH) the reaction
with dioxygen is indicated by a characteristic change of
Table 5
Oxidation potentials of macrocyclic nitrosyliron complexes measured
by cyclovoltammetry (CV) and square wave voltammetry (SWV)
the spectrum (displayed for Fe1ꢀ
1(b), curve 3). A detailed spectral comparison of the
products formed by oxidation of Fe1ꢀNO in pyridine or
MeOH, followed by addition of sodium nitrite (Fig.
1(c)), with those formed from FeIII1ꢀI or FeIII1ꢀ
Cl in
/NO in pyridine in Fig.
d
Complex
DEp (mV) E‘Ag’ (V) (CV) E‘Ag’ (V) (SWV) Efc (V)
/
a
a
a
b
c
Fe1ꢀNO
Fe2ꢀNO
Fe3ꢀNO
Fe1ꢀNO
68
72
61
66
76
0.975(0.74)
0.815(0.58)
0.840(0.60)
0.89 (0.73)
1.16 (1.00)
0.977. . .0.983
0.808. . .0.813
0.834. . .0.841
0.12
c
c
ꢀ0.04
ꢀ0.02
0.11
/
/
MeOH (Fig. 1(d)) or pyridine by titration with sodium
nitrite, reveal without any doubt the following sequence
for the observed reactions:
0.38
a
In CH2Cl2ꢂ0.5 m nBu4NClO4; Pt disc electrode; vs. AgÁ
/
AgClÁ
/
MeCNꢂ0.25 m nBu4NCl. In parentheses: EHꢁE‘Ag’ ꢀ0.237 V.
Fe1ꢀNOꢂMeOHꢂ1=2O2 [Fe1ꢀNO2 ꢃMeOH
Fe1ꢀNO2 ꢃMeOHꢂNO2
(4)
b
In MeCNꢂ0.5 m nBu4NClO4; Pt disc electrode; vs. AgÁ
/
AgClÁ
/
›
MeCNꢂ0.25 m nBu4NCl. In parentheses: EHꢁE‘Ag’ ꢀ0.163 V.
Shift with increasing frequency (six steps from 25 to 800 Hz).
c
›
[[Fe1ꢀ(NO2)2] ꢂMeOH
(5)
d
fcꢁferrocenÁ
/
ferricinium; E‘Ag’ (fcÁ
fcꢂ)ꢁ0.857 V in methyl-
/
enechloride, 0.785 V in MeCN.
The stability of the intermediate Fe1ꢀ
/
NO2ꢃMeOH
/

256
B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á265
/
Fig. 3. Voltammetric oxidation of selected complexes I and II (a)
Oxidation of [Fe3ꢀNO] in CH2Cl2 (0.5 M nBu4NClO4, Pt-disk
electrode) at a scan rate of 1 V sꢀ1. (b) Cyclic voltammetric oxidation
of [Fe1ꢀNO] (left) and [Fe2ꢀNO] (right) in CH2Cl2 (0.5 M nBu4N-
ClO4, Pt-disk electrode) at a scan rate of 1 V sꢀ1. (c) Cyclic
voltammetric oxidation of [Fe1ꢀNO] in acetonitrile (0.5 M nBu4N-
ClO4, Pt-disk electrode) at a scan rate of 1 V sꢀ1. (d) Cyclic
voltammetric oxidation of [Fe1ꢀNO] in CH2Cl2 (0.5 M nBu4NClO4,
Pt-disk electrode) at a scan rate of 1 V sꢀ1
Fig. 2. Cyclic voltammetric reduction of selected complexes I and II
(a) Comparison of experimental (solid line) and simulated (open
/
circles) CVs for the reduction of [Fe1ꢀ
nBu4NClO4, mercury drop electrode) at scan rates 2, 5, 10, 20, 40,
67, 83, 125 and 200 V sꢀ1, respectively. (b) Reduction of [Fe3ꢀ
NO]
2NO] (right) in CH2Cl2 (0.5 M n-
/
NO] in CH2Cl2 (0.5 M
/
/
/
/
(left), [Fe2ꢀ
/
NO] (middle) and [Fe1ꢀ
/
Bu4NClO4, mercury drop electrode) at a scan rates of 40 V sꢀ1. (c)
Comparison of experimental (solid line) and simulated (open circles)
/
.
CVs for the reduction of [Fe7bꢀNO] in CH2Cl2 (0.5 M nBu4NClO4,
/
mercury drop electrode) at scan rates 5, 10, 20, 40, 62, 83 and 125 V
sꢀ1, respectively. (d) First and second cycle of the cyclic voltammetric
methylenechloride, for Fe1ꢀ
results are listed in Tables 4 and 5. Characteristic
examples of voltammograms are displayed in Figs. 2Á4.
/
NO also in MeCN. The
reduction of [Fe7bꢀ
/
NO] in CH2Cl2 (0.5 M nBu4NClO4, mercury drop
.
electrode) at a scan rates of 1 V sꢀ1
/
From Fe5ꢀ
/
NOꢃMeOH two isomers were isolated
/
depending on the temperature of preparation (cf.
Section 2.4.6 and Section 2.4.7). Both show significant
differences in stability and molecule structure. The low-
temperature isomer Fe5ꢀ
/
NOꢃ
/
MeOH(2) converts
quickly into the stable isomer Fe5ꢀ
/
NOꢃMeOH(1) at
/
r.t. Therefore, we were not able to measure the
characteristic spectroscopic, electrochemical and mag-
netic data for the low temperature form. Fortunately it
was possible, however, to solve the crystal and molecule
structure of both isomers by X-ray analysis (cf. Section
3.4).
Fig. 4. Square wave voltammograms of the oxidation of [Fe3ꢀNO] in
/
CH2Cl2 (0.5 M nBu4NClO4, Pt-disk electrode) at frequencies of 25, 50,
100, 200, 400 and 800 sꢀ1 (order of increasing peak currents) using a
square wave amplitude of 25 mV and potential steps of 5 mV. The SW
voltammograms show a small frequency-dependent potential shift of 6
mV indicating a slow follow-up reaction of the oxidised species, which
cannot be detected in the CVs depicted in Fig. 3(a).
3.2. Electrochemistry
The redox behaviour of the new nitrosyliron com-
plexes was investigated using cyclovoltammetry (and, in
part, frequency dependent square wave voltammetry) in
Fig. 5. Temperature dependence of the effectie magnetic moment of selected nitrosyl deriaties of iron complexes with macrocyclic Fe1NO and open-
chain ligands type II. All measurements were carried out at two magnetic field strengths, e.g. 0.2 and 0.5 T. The deiations are within the circles in the
cures as displayed for Fe5NO and Fe7b NO.

B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á
/265
257
Fig. 5

258
B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á265
/
All nitrosyl complexes show a (quasi)reversible re-
duction (Table 4). In case of the macrocycles Fe1ꢀNO
to Fe3ꢀNO and the aliphatically bridged open-chain
complex Fe7bꢀNO the kinetic parameter could be
determined by simulation of the CVs using the Digi-
SimTM algorithm [33,34] (cf. Table 4 and the examples
of Fig. 2(a), (c)). Remarkable are the relatively high rate
constants ks for the heterogeneous electron transfer
reaction. They are in the same magnitude as those
observed for planar nickel complexes of type I [17] and
II [35]. The values of the macrocyclic complexes increase
with increasing extension of the p-electron system when
aliphatic bridges are successively replaced by aromatic
bridges.
5, Fig. 3(a),(b)). The potentials measured by cyclovol-
tammetry and by square wave voltammetry (Fig. 4) are
in good agreement, although the latter show a small but
significant shift depending on the frequency. The
gradation of the equilibrium potentials in dependence
on the equatorial macrocycle is different from those
found for the oxidation of the base iron(II) complexes
and for the reduction of their nitrosyl derivatives. The
/
/
/
complex Fe1ꢀ
macrocycle shows the most positive potential (0.97 V vs.
‘Ag’), whereas Fe2ꢀNO and Fe3ꢀNO have lower and
/NO with the strongest electron-donating
/
/
scarcely different values (0.82 and 0.84 V, respectively).
The base iron(II) complexes Fe1, Fe2 and Fe3, show a
second oxidation [FeL]1ꢂ/2ꢂ in methylenechloride at
The NO derivatives with open-chain ligands are less
stable in solution without an excess of free NO. The
lower affinity towards NO becomes particulary visible if
the complex is reduced in a CV experiment. As shown in
Fig. 2(d), the second cycle at the low scan rate of 1 V sꢀ1
exhibits a decimated signal of the nitrosyliron redox
couple and a new signal of the NO- free FeII/III redox
E‘Ag’
ꢁ1.46, 1.30 and 1.27 V, respectively, which had
/
been classified as belonging to FeIII/IV for Fe1 and to an
oxidation of the aromatically bridged anionic macro-
cycle for Fe2 and Fe3 [12b]. The similar gradation of the
oxidation potentials of the NO derivatives suggests an
analogous interpretation, whereby the (cis-) influence of
›
the strongly donating anionic NO ligand makes the
(equatorial ligand-) oxidation easier and lowers the
potential.
A quantity worth mentioning is the difference be-
tween the oxidation and the reduction potentials of the
nitrosyl derivatives on the one hand (1.75, 1.48, and 1.33
couple. Fe5ꢀ
/
NO and Fe6ꢀ
/
NO show a small but
significant drift of the peak potentials in dependence
on the scan rate (Table 4). Additionally, in some cases
(e.g. Fe6aꢀ
/
NO) the CVs indicate small impurities of the
m-oxo iron(III) complex that is formed by oxidative
decomposition of the nitrosyl derivatives. The CVs of
V for Fe1ꢀ
/
NO to Fe3ꢀ
/
NO) and between the second and
the first oxidation potentials of the base iron(II)
Fe6ꢀ
/
NO in methylenechloride (but not in MeCN) show
a weaker satellite at lower potentials that is possibly
caused by partial isomerisation of the complex in
solution. (A similar peculiarity in solution had been
observed in the EPR spectra of the pure cobalt(II)
complex with the same ligand [36], although the crystal
structure of Co6 indicates only identical molecules in
solid state.) No evaluable CVs were obtained with the
complexes on the other hand (1.17, 0.75, and 0.71 V
Fe3). It is a measure of the width of the
for Fe1Á
/
thermodynamic stability of the middle oxidation state*
/
in our imagination iron(III). This is in good agreement
with the expectation in an electrostatic view: increasing
electron-donating effect of the whole ligand sphere
results in an increasing stabilisation of the iron(III)
state.
The nitrosyl derivative Fe1ꢀ
equatorial donor ligand gives a second reversible oxida-
tion (Fig. 3(c)), but only in MeCN (E‘Ag’ 1.16 V) and
complex Fe4ꢀ/NO.
The values of the equilibrium reduction potentials of
the nitrosyl derivatives show the same trend in depen-
dence on the equatorial ligand (cf. Table 4, Fig. 2(b)) as
the FeII/III couples of the base iron(II) complexes (cf.
Section 3.1), but on a much lower (more negative) level.
The potentials are significantly (non-linear) correlated
with the n(NO) stretching frequency. This behaviour
suggests all in all the interpretation of the one electron
reduction of the nitrosyl derivatives as a mainly metal
centred FeII/III couple. The low level of the potentials
could be explained with the presence of anionic NOꢀ as
a very strong axial donor ligand. The potential becomes
more negative with increasing electron donating effect of
the equatorial and the NO ligand (i.e. increasing
electron density on the iron), and the n(NO) decreases
because the transfer of electron density from the
antibonding NO p* orbital to the central atom becomes
more difficult.
/
NO with the strongest
ꢁ
/
not in methylenechloride (Fig. 3(d)). There is no
counterpart of this in the redox chemistry of the base
complex Fe1. Therefore, it should be assigned to a
(MeCN-assisted?) mainly NO centred oxidation result-
ing in a [FeIV1ꢀ
/
NO+]2ꢂ species. All in all, this complex
with the aliphatically bridged equatorial [N42ꢀ] coordi-
nated macrocycle is characterised by the following series
of four reversible redox couples in the EnemarkÁ
Feltham notation:
/
8
7
6
š
›
f[Fe1ꢀNO] g ?fFe1ꢀNOg ?f[Fe1ꢀNO] g
5
?f[Fe1ꢀNO]2š
g
The nitrosyl derivatives with equatorial open-chain
ligands Fe4ꢀNO to Fe7ꢀNO give no reversible oxida-
tion.
The macrocycles give also a reversible couple [FeLꢀ
/
/
/
NO]0/[FeLꢀNO]ꢂ for the one electron oxidation (Table
/

B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á
/265
259
With respect to its reduction and oxidation potentials
as well as their difference, the complex Fe1ꢀNO is
surprisingly closely related to the {FeNO}7 species
trans-[(cyclam)Fe(NO)Cl](ClO4) (cyclamꢁ1,4,8,11-tet-
Table 6
˚
Selected bond lengths (A) and angles (8) for macrocyclic nitrosyliron
complexes type I
/
/
Complex:
bond/Þ
Fe1ꢀNO Fe2ꢀNO Fe3ꢀNO FeMe4-
TAAꢀNO
a
raazacyclotetradecane) that was synthesised and studied
in detail (together with similar isostructural nitrosyl
complexes) by Wieghardt, et al. [9]. This complex shows
in MeCN reversible couples for the oxidation at 0.14 V
Bond lengths
FeꢀN1
FeꢀN2
FeꢀN3
FeꢀN4
FeꢀN(NO)
Feꢀpl(N4)eq
NꢀO
1.928(2) 1.909(2) 1.914(3) 1.948
1.895(2) 1.915(2) 1.935(3) 1.931
1.920(2) 1.942(2) 1.975(3) 1.945
1.954(2) 1.936(2) 1.945(3) 1.951
1.732(2) 1.712(2) 1.716(3) 1.716
and the reduction at ꢀ
/
1.30 V (vs. fcÁ
fcꢂ). Based on
/
variable-temperature Mossbauer spectroscopy in ap-
¨
plied field, this complex with an Sꢁ
was proposed to contain an intermediate spin (Sꢁ
iron(III) ion, antiferromagnetically coupled with an
/1/2 ground state
b
0.376
1.185(3) 1.164(3) 1.171(4) 1.165
0.353
0.442
0.386
/3/2)
Bond angles
FeꢀNꢀO
N1ꢀFeꢀN2
N3ꢀFeꢀN4
N2ꢀFeꢀN3
N4ꢀFeꢀN1
›
NO anion in its triplet (Sꢁ
/1) state. This inter-
140.3(2) 145.1(2) 152.0(3) 144.0
83.46(10) 83.60(9) 82.19(12) 97.4
86.71(11) 86.11(10) 83.53(12) 83.1
91.51(11) 91.12(9) 91.74(13) 93.5
89.70(10) 91.57(9) 90.63(12) 92.4
pretation*and for Fe1ꢀNO also the description of
/
/
the oxidation product as an iron(IV) complex-is in full
agreement with our imagination of the NO binding
mode in the macrocyclic complexes Fe1ꢀ
/
NO to Fe3ꢀ
/
c
N1ꢀFeꢀNꢀO
Pl(N4)eq pl(FeNO)
ꢂ133.2
90.6
5.6
ꢀ15.9
90.2
4.8
ꢂ121.7
91.4
4.0
ꢂ117.8
90.2
4.9
›
NO. The product of the reduction, {[FeLꢀ
/
NO] }8, is in
our case, however, rather an iron(II) species with an
d
Á/
e
Tilt angle
›
NO monoanion than an iron(III) complex with an
a
NO2› dianion, as proposed for the above mentioned
cyclam complex by Wieghardt et al. Attempts to isolate
and characterise the reduction and oxidation products
of our macrocyclic nitrosyl complexes are in progress.
Data from CSD database v. 5.22 (Oct. 01) Code GEFNIO, cf. [7c].
Displacement of the iron from the equatorial [N4] plane.
Torsion angle.
b
c
d
Dihedral angle between the equatorial [N4] plane and the [FeNO]
plane.
e
Tilt angle between the FeꢀN(NO) bond and the axis vertical to the
[N4] plane.
3.3. Magnetochemistry
The magnetic susceptibilities of all new nitrosyliron
complexes were measured from 1.7 to 300 K. In no case
was a strict linear relationship between xꢀ1 and T
below). The Sꢁ
complex Fe7bꢀNO, but the magnetic moment increases
significantly above :250 K*indicating a slow transfer
into a higher (Sꢁ3/2?) spin state. This behaviour
reflects the middle position of this aliphatically bridged
complex between the macrocycles and the other (ar-
omatically bridged) complexes with open-chain ligands
(cf. the oxidation potential of the base iron(II) com-
plexes, the reduction potential and the decomposition in
solution of the nitrosyl derivatives etc.).
/
1/2 state is also dominant in the
/
/
/
according to the CurieÁWeiss law observed. Apart from
/
/
spin crossover processes within the investigated tem-
perature range, there are in some cases (especially for
Fe7aꢀ
/
NO, Fe7bꢀ
/
NO, in much weaker extent also for
Fe1ꢀNO to Fe3ꢀ
/
/
NO) deviations at low temperature (B
/
10 K) which correspond with a decrease of the magnetic
moment below the value for one unpaired electron. The
reason for this*
pling of a {FeNO}7 complex or spin-orbit coupling in a
NO+ radical bound to low-spin FeII*
is not clear. For
/intermolecular antiferromagnetic cou-
The pentacoordinated complex Fe5ꢀ
/
NO with the
most electron-withdrawing periphery of the equatorial
ligand represents the other extreme: The magnetism
/
better comparison of the ground state, the more
informative magnetic moments meff in dependence on
the temperature are depicted for selected examples in
Fig. 5.
corresponds with an Sꢁ
down to about 30 K. An explanation for this should at
least consider either a high spin iron(III) (Sꢁ5/2),
antiferromagnetically coupled with the triplet state of
/3/2 ground state which is stable
/
All complexes with macrocyclic ligands have an Sꢁ
2 ground state over the whole temperature range-as it is
displayed for Fe1ꢀNO in Fig. 5. This is in agreement
with the above mentioned interpretation as an inter-
mediate spin (Sꢁ3/2) iron(III), antiferromagnetically
coupled with a coordinated NO anion in its triplet
(Sꢁ1) state. This assumption is strongly supported by
the fact, that all pentacoordinated iron(III) derivatives
of our macrocycles, FeLꢀX (Xꢁhalide, pseudo halide),
have also an Sꢁ3/2 ground state*and a nearly
identical geometry of the first coordination sphere (see
/1/
›
an NO
anion, or a high spin iron(II) (Sꢁ
/2),
antiferromagnetically coupled with the doublet (Sꢁ
/
1/
/
2) state of a neutral NO+ radical. Both possibilities
would agree with the fact, that the base iron(II) complex
/
›
Fe5*
its iron(III) derivatives are always high spin complexes
(Sꢁ4/2, 5/2). The first possibility was derived from
/
even with rather strong axial ligands*as well as
/
/
/
combined physical measurements and spin polarised
SCF-Xa calculations by Solomon et al. [30] for Wie-
/
/
/
/
ghardt’s
Sꢁ
/
3/2
nitrosyl
complex
Fe(Me3-

260
B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á265
/
Table 7
Selected bond lengths (A) and -angles (8) for nitrosyliron complexes type II with open-chain [N2O22ꢀ] ligands
˚
c
Complex:
Fe4ꢀNOꢃMeOH Fe5ꢀNOꢃMeOH(1) b Fe5ꢀNOꢃMeOH(2) Fe6ꢀNO FeSalen-
NO(1)
FeSalen-
a
NO(2)
a
Bond lengths
FeꢀN1
1.905(4)
1.915(4)
1.969(4)
1.977(4)
2.252(4)
1.721(5)
0.175
2.015(2)
2.035(2)
2.014(2)
1.998(2)
2.296(3)
1.761(3)
0.237
2.0728(16)
2.0862(16)
2.0601(14)
2.0363(15)
2.2009(16)
1.7796(18)
0.177
2.012
1.966
1.948
1.956
2.07(1)
2.08(1)
1.923(9)
1.892(9)
1.97(2)
1.98(3)
1.93(2)
1.87(2)
FeꢀN2
FeꢀO1
FeꢀO2
FeꢀO(MeOH)
FeꢀN(NO)
Feꢀpl(N2O2)eq
NꢀO
1.701
0.504
1.174
1.78(2)
0.465
1.11(4)
1.793
0.360
1.15
d
1.169(6)
1.137(4)
1.107(3)
Bond angles
FeꢀNꢀO
N1ꢀFeꢀN2
O1ꢀFeꢀO2
N2ꢀFeꢀO1
O2ꢀFeꢀN1
N1ꢀFeꢀNꢀO
142.1(4)
83.97(19)
149.5(3)
79.94(9)
96.71(10)
89.77(9)
90.41(10)
ꢂ14.7
149.7(2)
78.69(6)
104.51(6)
87.41(6)
87.72(6)
ꢀ34.0
93.1
167
144;150
76.8
95.1
122;132
79.5
88.9
80.5
90.5
87.6
86.5
133.6
89.0
2.8
89.70(16)
92.32(17)
92.24(18)
87.3
88.1
90.9
92.7
e
ꢂ23.5
90.3
4.4
ꢀ34.6
87.3
2.9
ꢀ27.2
90.8
5.3
f
Pleq
Á
/
pl(FeNO)
90.7
4.5
g
Tilt angle
3.8
a
Data from CSD database v. 5.22 (Oct. 01) Code NSALFE and NSALFE01, cf. [10].
Generated at r.t.
b
c
d
e
f
Generated at ꢀ68 8C.
Displacement of the iron from the equatorial [N2O2] plane.
Torsion angle.
Dihedral angle between the equatorial [N2O2] plane and the axial [FeNO] plane.
Tilt angle between the FeꢀN(NO) bond and the axis vertical to the [N2O2] plane.
g
below), it seems that both possibilities are realised in the
both isomers Fe5ꢀNOꢃMeOH(1) and Fe5ꢀNOꢃ
MeOH(2).
The other complexes (including Fe5ꢀ
wherein the additional trans ligand increases the overall
ligand field strength of Fe5ꢀNO) show the typical
conversion between a low temperature Sꢁ1/2 and a
high temperature Sꢁ3/2 state, that has been described
earlier by Konig, Larkworthy et al. for Fe6ꢀNO [7e,7f].
In respect to this, our 1,2-phenylene bridged complexes
of the type FeIIꢀNO are closely related to N,N?-
ethylen-bis(salicylideneiminato)nitrosyliron, Fe(salen)ꢀ
NO, which can be classified as belonging to the general
type II with an annelated aromatic ring for R1ꢂR2. The
latter was first described by Earnshaw, King and
Larkworthy [38] to give an Sꢁ3/2[Sꢁ1/2 transition
at decreasing temperature*a phenomenon that was
/
/
/
/
/
NOꢃMeOH(1)
/
/
/
/
Fig. 6. Unified atom numbering scheme used in Tables 6Á8 for the
equatorial donor atoms. The numbers may deviate from those in Fig.
7.
/
/
¨
/
/
TACN)(NO)(N3)2] (TACNꢁ/triazacyclononane) [37]
and for the nitrosyl derivative of FeEDTA. For Fe5ꢀ
/
/
NO the second possibility would be supported by the
high oxidation potential of the base complex Fe5, the
low reactivity of this iron(II) complex and the corre-
sponding cobalt(II) complex Co5 with dioxygen (which
is in striking contrast to the behaviour of all the other
iron(II) and cobalt(II) complexes studied in our labora-
/
/
/
/
subsequently intensively studied by different experimen-
tal and theoretical methods [7d,7e,10]. In contrast to
some of our type II complexes with Rꢁ1,2-phenylene,
the corresponding nitrosyliron complexes of aromati-
cally bridged salicylaldimines have in general a T-
/
tory [13,15a]), and by the low stability of Fe5ꢀ
/
NO in
solution (especially the relatively fast elimination of free
NO in absence of an excess of NO and in presence of
potential axial ligands). As the X-ray studies show (see
independent Sꢁ3/2 ground state [38].
/
Fig. 7. Molecule structures of selected nitrosyliron complexes with macrocyclic N42 coordinated and open-chain N2O22 coordinated Schiff base
ligands. All structures are displayed in approximately the same projection with NO as apex and the N1 of Fig. 6 in front left. FeMe4TAA is the
complex Type I with RR1,2-phenylene, R1R3Me and R2H data taken from the CSD database 5.22, October 01, GEFNIO.

B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á
/265
261
Fig. 7

262
B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á265
/
It is interesting to note that some of the base iron(II)
complexes of type II show similar spin crossover
phenomena*depending on the number and strength
of axial ligands. For instance, Fe6a gives a pentacoordi-
nated mono adduct with imidazole, Fe6aꢃHIm, that is
in the high spin (Sꢁ2) state, independent of the
temperature. The corresponding diadduct, Fe6aꢃ
2HIm, is a low-spin (Sꢁ0) complex up to 325 K, where
ꢀ175 8C) [7b]. The atom numbering in the Tables
corresponds with the one given in Fig. 6. Fig. 7 shows
the molecule structures (including the one of
/
/
FeMe4TAAꢀ
/
NO), all approximately in the same projec-
/
tion of the basic chelate complex (the atom numbering
in the pictures is that of the original files and may differ
from that in the Tables).
/
/
/
In Table 8, some*
/
still unpublished [42,44]Ástruc-
/
a very sharp and strictly reversible conversion into the
high spin isomer takes place [13,27,28]. The octahedral
tural data for several iron(II/III) complexes are listed
which are derived from the same or very similar
equatorial ligands as the nitrosyl derivatives described
in this paper. Additional data are published for sixteen
iron complexes of type I and one iron(II) complex type
II in [12d,16c,16d,43]. If the atomic distances and angles
around the iron would be influenced by the spin state,
the oxidation step and the coordination number, the
comparison with such base complexes should be helpful
to elucidate the binding mode of NO in the nitrosyl
derivatives.
adduct Fe6aꢃ
ligand than imidazole) undergoes a spin change between
Sꢁ0 and Sꢁ2 at about 180 K [13]. Conversion
between low spin (Sꢁ1/2) and high spin (Sꢁ5/2) states
/
2Py with two pyridine (a weaker axial
/
/
/
/
has also been observed for many iron(III) complexes
with salen derivatives as equatorial chelate ligand [39].
Particular consideration ought to be paid to the spin
conversion of the nitrosyliron complex Fe5ꢀ
/
NOꢃ
/
MeOH(1). It takes place in two badly separated but
significant steps (Fig. 5). Such behaviour (spin transition
The central atom in the macrocyclic nitrosyl deriva-
tives has an approximately square-pyramidal coordina-
tion and is located above the plane of the four nitrogen
‘type d’ in the notation of Gutlich et al. [40]) is well
¨
established and intensively studied e.g. for [Fe(2-
picolylamin)3]Cl2ꢃ
/
EtOH, and seems to be caused by
atoms. The average Feꢀ
/
Neq distance within the macro-
˚
NO (1.942 A) and
˚
a cooperative 1:1 ordering of low-spin and high spin
molecules [40]. In our case, an intermolecular interaction
through H-bridges between the MeOH ligand and the
peripheral CN substituents could be responsible for such
behaviour.
cyclic framework is for Fe3ꢀ
/
FeMe4TAAꢀ
Fe1ꢀNO (1.924 A) and Fe2ꢀ
differences are within the standard deviation of the
/
NO (1.944 A) slightly longer than for
˚
˚
NO (1.925 A), but the
/
/
˚
average 1.934(0.020) A for all sixteen individual bonds
in all four complexes. The nitrosyl group points in all
four structures towards the six-membered chelate ring of
3.4. X-ray Investigations
the macrocycle as shown in Fig. 7. The Feꢀ
is nearly vertical to the N4 plane and the Feꢀ
approximately straight up to this plane. The macrocycle
in Fe3ꢀNO has the typical saddle shaped distortion of
the most dibenzo-tetraaza[14]annulenes [41] but on the
non-methylated side of the ligand in a lower extent as in
FeMe4TAA.
/
Nꢀ
/
O plane
Suitable crystals for X-ray structural investigations
/
N bond is
were obtained from all solvent free macrocycles Fe1ꢀ
NO, Fe2ꢀNO and Fe3ꢀNO, the likewise solvent free
Fe6ꢀNO, as well as from the methanol monoadducts
Fe4ꢀNOꢃ NOꢃMeOH*the latter
/
/
/
/
/
/
/
MeOH and Fe5ꢀ
/
/
/
in both preparations, the stable and the metastable
isomer (1) and (2). Unfortunately, the attempts to
In the case of the macrocyclic base complexes I, the
isolate good crystals from the solvent free Fe5ꢀ
and from the derivatives with R1ꢁ
Ph, Fe7a,bꢀNO,
failed. All measurements were carried out at ꢀ90 8C.
Under these conditions, the macrocycles are in the pure
Sꢁ1/2 state whereas Fe6ꢀNO and Fe5ꢀNOꢃ
MeOH(1) should remain in the Sꢁ3/2 state*possibly
with a beginning conversion into the Sꢁ1/2 state.
Unfortunately, the magnetic properties of the meta-
stable isomer Fe5ꢀNOꢃMeOH(2) are unknown. In
case of Fe4ꢀNOꢃMeOH the phase change into the
Sꢁ1/2 state should be nearly complete.
/
NO
average atom distances Feꢀ
/
Neq depend hardly on the
oxidation step, the spin state and the coordination
/
/
/
number of the iron. The average amounts 1.904(0.012)
˚
A for 21 derivatives of Fe1, Fe2 and Fe3 involving three
planar or 4ꢂ1 coordinated (Sꢁ1; FeꢀNeq(avg.)ꢁ
1.902 A) and two octahedral (Sꢁ0; FeꢀNeq(avg.)ꢁ
1.917 A) iron(II) complexes, two square-pyramidal
pentacoordinated iron(III) halide derivatives (Sꢁ3/2;
1.914 A) and fourteen octahedral ad-
ducts in the iron(III) state (Sꢁ Neq(avg.)ꢁ
/
/
/
/
/
/
/
/
˚
˚
/
/
/
/
/
/
/
˚
/
/
Feꢀ
/
Neq(avg.)ꢁ
/
/
/
/
1/2; Feꢀ
/
/
˚
/
1.901(0.011) A) with neutral, anionic, and mixed axial
ligands. The only exception is the m-oxo-iron(III)
Selected atom distances and angles within the first
coordination sphere are listed in Table 6 (for the
macrocycles) and Table 7 (for the open-chain com-
plexes). For comparison, some data from literature are
complex [(Fe3)2O(H2O)]2 with an average Feꢀ
/
Neq-
distance of 2.033 A. No conclusions about the binding
˚
mode of the NO in the nitrosyl derivatives can be drawn
included: the macrocyclic complex FeMe4TAAꢀ
and both phases of the T-dependent spin conversion of
FeSalenꢀNO (1) (taken at r.t.) and (2) (taken at
/
NO [7c]
by comparison of these Feꢀ
/
Neq distances. However,
considering additionally the displacement of the central
˚
atom from the [N4] plane (0.38; 0.35; 0.44 and 0.39 A for
/

B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á
/265
263
Fe1ꢀ
/
NO, Fe2ꢀ
/
NO, Fe3ꢀ
/
NO and FeMe4TAAꢀ
/
NO,
the more electron-withdrawing substituted complex
Fe5ꢀNOꢃMeOH(1) (Sꢁ3/2; average FeꢀLeqꢁ2.016
A). In the metastable isomer of the latter complex, Fe5ꢀ
NOꢃMeOH(2) (the magnetism of which is unknown),
the FeꢀLeq distances are more lengthened up to a
surprisingly big average of Feꢀ
be understand as a trend to lower transfer of electron
density from iron to NO. Thus, a formulation as [FeIILꢀ
NO+] with antiferromagnetically coupled high-spin
iron(II) (Sꢁ2) and neutral NO (Sꢁ1/2) becomes
respectively), the only satisfactory relation is found to
˚
/
/
/
/
/
˚
the Sꢁ
/
3/2 halides Fe1ꢀ
/
Cl (0.405 A) and Feꢀ
/I (0.343
/
˚
A). Although this is not a strict proof, there is also no
/
contradiction against the assumption that an anionic
›
/
˚
Leqꢁ2.064 A. This can
NO is bound in its Sꢁ
/
1 state and antiferromagneti-
3/2)
/
/
cally coupled to iron(III) in its intermediate (Sꢁ
state.
The nitrosyliron complexes with open-chain ligands,
Fe4ꢀNO to Fe6ꢀNO, show a more diverse behaviour
(Table 7)*in the same way as their base iron(II/III)
complexes (Table 8). According to their different spin
states, the complex Fe4ꢀNOꢃMeOH (dominant Sꢁ
1/2) has significantly lower FeꢀLeq distances (average
/
/
/
/
/
/
/
more relevant. This interpretation is in agreement with
the loose binding of nitric oxide, that is eliminated
(reversibly) from these complexes in solution in absence
of an excess of NO.
/
/
/
/
˚
1.942 A; nearly the same as for the macrocycles) than
Table 8
˚
Bond lengths (A) and -angles (8) within the first coordination sphere of iron(II/III) complexes type I and II with different coordination number CN
and spin state S (unpublished data [42,44])
Macrocyclic complexes type I
a
a
b
Complex
CN
S
FeꢀN1,2
FeꢀN3,4
averageFeLeq
FeꢀLax
Feꢀpleq
FeII1
4ꢂ1
4ꢂ1
1
1.877 1.879 1.907 1.908 1.893
1.879 1.906 1.915 1.906 1.902
1.912 1.836 1.864 2.036 1.911
1.917 1.904 1.925 1.932 1.920
1.909 1.912 1.920 1.948 1.922
1.893 1.887 1.909 1.918 1.902
1.910 1.902 1.931 1.930 1.918
1.896 1.892 1.923 1.926 1.909
1.908 1.849 1.901 1.912 1.893
3/2 1.895 1.904 1.925 1.927 1.913
2.702
2.329
0.077
0.110
FeII1ꢃMeOH
1
FeII2
4
6
5
6
6
6
6
6
5
1
c
FeII2(Py)2
0
2.018 2.037
2.262
0.017
0.405
0.015
FeIII1ꢀCl
3/2
1/2
1/2
1/2
1/2
[FeIII2(Py)2]
2.022 2.040
2.002 2.004
1.947 1.954
š
[FeIII(CN)2]
›
[FeIII1(NCS)2]
0.003
0.032
0.018
0.589
›
[FeIII3(NO2)2]
1.962 1.995
1.963 2.055
1.777
›
d
[FeIII2(NO2)(OH2)]
{[(FeIII3)2OꢃH2O]}2
1/2?
/
/
e
5/2? 3/2? 2.025 2.020 2.046 2.042 2.033
Complexes type II with Schiff base-like open-chain ligands
a
a
b
Complex
CN
S
FeꢀN1,2
FeꢀO1,2
Average FeꢀLeq
FeꢀLax
Feꢀpleq
Angle OeqFeOeq
FeII6a(HIm)
5
6
6
6
6
6
6
6
6
6
5
5
6
6
5
2
0
0
2
2
2
2
2
2
2
2.074 2.097 1.995 1.999 2.041
1.903 1.895 1.943 1.960 1.925
1.923 1.918 1.962 1.955 1.940
2.062 2.053 1.990 2.017 2.031
2.102 2.110 2.031 2.035 2.070
2.115
0.379
0.005
0.016
0.027
0.006
101.5
88.2
92.4
f
FeII6a(Him)2
2.014 2.017
2.023 2.025
2.256 2.195
2.223 2.219
2.271
FeII6a(Py)2 (180 K)
FeII6a(Py)2 (290 K)
FeII6a(MeOH)2
FeII6b(Py)2
106.3
110.7
114.4
111.1
116.4
112.1
103.3
90.9
g
2.105
2.097
2.124
2.042
2.023
2.077
2.074
2.060
2.101
FeII6b(MeOH)2
FeII7b(Py)2
2.267
2.230
FeII4(Py)2
2.109 2.104 2.049 2.043 2.076
2.067 2.056 2.026 2.010 2.040
2.045 2.046 1.925 1.921 1.984
2.060 2.062 1.934 1.925 1.995
2.063 2.068 1.951 1.960 2.011
2.050 2.083 1.977 1.958 2.017
2.075 2.072 1.961 1.964 2.018
2.256 2.233
2.246 2.249
2.205
0.053
0.007
0.573
0.560
0.198
0.191
FeII4(MeOH)2
FeIII6aꢀCl
5/2
5/2
5/2
5/2
?
FeIII6aꢀBr
2.351
92.0
FeIII6aꢀCl(MeOH)
FeIII6ꢀCl(MeOH)
2.273 2.197
2.285 2.148
1.775 1.779
104.6
105.8
93.3
[(FeIII6a)2O]
0.587 0.591
h
a
Designation according to Fig. 6.
b
c
d
e
f
Displacement of the iron form the main plane of the equatorial donor atoms.
Mixed crystals with the free ligand.
Average of 1:1 disordered molecules.
Average of distances for four iron atoms.
Sharp and strongly reversible spin crossover (Sꢁ0=Sꢁ2) with small hysteresis at 328 K [28].
g
Cooperative magnetic behaviour with spontaneous magnetisation B10 K, caused by strong intermolecular H-bridges between R2 and MeOH:
B.R. Muller, G. Leibeling, E.-G. Jager, Mol. Cryst. Liq. Cryst. 334 (1999) 389.
¨
¨

264
B. Weber et al. / Inorganica Chimica Acta 337 (2002) 247Á265
/
ments in combination with SCCXa-DFT calculations.
This will be the topic of a subsequent paper.
4. Supplementary Material
Further details of the crystal structure investigations
are available on request from the director of the
Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1 EZ, UK (fax: ꢂ44-1223-
/
336033; e-mail:deposit@ccdc.cam.ac.uk; www: http://
www.ccdc.cam.ac.uk), on quoting the depository num-
ber CCSD-180321 (Fe1ꢀ
NO), CCSD-180322 (Fe3ꢀ
NOꢃMeOH), CCSD-181607, (Fe5ꢀ
NOꢃMeOH(2)), the names
/
NO), CCSD-180325 (Fe2ꢀ
NO), CCSD-180324 (Fe4ꢀ
NOꢃMeOH(1))
/
/
/
/
/
/
and CCSD-180323 (Fe5ꢀ
/
/
Fig. 8. Correlation between the NO stretching frequencies and the
of the authors, and the journal citation.
angles Feꢀ
tetradentate equatorial chelate ligands. The octahedral derivatives
Fe5ꢀNOꢃMeOH and Fe4ꢀNOꢃMeOH (italic) are not included
into the calculated line. (The equation of the linear fit is ‘angle (8)ꢁ
0.165nNO/cmꢀ1
128’ with Rꢁ0.987).
/
NꢀO in pentacoordinated nitrosyliron complexes with
/
/
/
/
/
/
Acknowledgements
ꢀ/
/
This work was supported by the Deutsche For-
schungsgemeinschaft (Sonderforschungsbereich 436
‘Metallvermittelte Reaktionen nach dem Vorbild der
Natur’). B.W. acknowledges a grant from the Fonds der
Chemischen Industrie. The authors would like to thank
A very indicative structural feature for the spin state
of octahedral iron(II) complexes with tetradentate open-
chain Schiff base ligands is the ‘bite’, that means the Oꢀ
/
FeꢀO angle in the equatorial plane. It is about 908 for
/
low spin iron(II) (cf. FeII6a(HIm)2 and the low-tem-
perature form of FeII6a(Py)2 in Table 8) and increases
Dipl.-Ing. Regina Gollmick for the UVÁVis and Dr.
/
Wolfgang Poppitz for the MS measurements.
up to about 1108 in the octahedral high spin (Sꢁ
derivatives. Indeed, this ‘bite’ increases from 89.7 to
104.58 going from Fe4ꢀNOꢃMeOH over Fe5ꢀNOꢃ
MeOH(1) to Fe5ꢀNOꢃMeOH(2).
/2)
/
/
/
/
References
/
/
With respect to the binding mode of the NO ligand it
[1] J. Lancaster (Ed.), Nitric Oxide: Principles and Actions, Aca-
demic Press, San Diego, 1996.
is also interesting to note, that there exists a highly
[2] P.L. Feldman, O.W. Griffith, D.J. Stuehr, Chem Eng. News 20
(1993) 26.
significant correlation between the angle Feꢀ
/
Nꢀ
/
O and
the stretching frequency nNÄO of the NO group. This is
displayed in Fig. 8 for our pentacoordinated nitrosyl
derivatives, completed by some literature data for other
[3] O. Einsle, A. Messerschmidt, P. Stach, G.P. Bourenkov, H.D.
Bartunik, R. Huber, P.M.H. Kroneck, Nature 400 (1999) 476.
[4] B.B. Wayland, L.W. Olson, J. Chem. Soc., Chem. Commun.
(1973) 897.
pentacoordinated nitrosyliron complexes of the {Feꢀ
/
NO}7 type. (Our octahedral derivatives are also de-
picted, but nut included in the correlation). The increase
of the angle (from about 140 to nearly 1808), accom-
panied by an increase of the force constant of the NO
bond is expected for a decreasing population of the
antibonding p* orbital, i.e. for a decreasing transfer of
electron density from iron(II) to NO. This indicates the
strong effect of the equatorial ‘controlling’ ligands on
the NO binding mode and confirms our assumption that
the macrocyclic derivatives are best to describe as
[5] (a) W.R. Scheidt, M.E. Frisse, J. Am. Chem. Soc. 97 (1975) 17;
(b) W.R. Scheidt, P.C. Piciulo, J. Am. Chem. Soc. 98 (1976) 1913.
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R. Guilard, (Eds.) The Porphyrin Handbook, Biochemistry and
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New York, 2000, pp. 219Á339;
/
(b) W.R. Scheidt, M.K. Ellison, Acc. Chem. Res. 32 (1999) 350
(and references therein).
[7] (a) C. Ercolani, C. Neri, J. Chem. Soc. (A) (1967) 1715Á1718;
/
(b) K.D. Hodges, R.G. Wollmann, S.L. Kessel, D.N. Hendrick-
son, D.G. Van Derveer, E.K. Barefield, J. Am. Chem. Soc. 101
(1979) 906;
(c) P. Berno, C. Floriani, A. Chiesi-Villa, C. Guastini, J. Chem.
Soc., Dalton Trans. (1988) 1409;
š
›
[FeIIIL
ligands with electron-withdrawing periphery the formu-
NO+] becomes more probability.
ꢀ
/
NO ] whereas in the case of open-chain
(d) F.V. Wells, S.W. McLann, S.L. Kessel, D.N. Hendrickson,
R.D. Feltham, Inorg. Chem. 21 (1982) 2306;
(e) E. Konig, G. Ritter, J. Dengler, L.F. Larkworthy, Inorg.
¨
lation [FeIILꢀ
/
We hope to gain deeper insight into the NO binding
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