Solid and solution state structures of mono- and di-nuclear iron(III) complexes of related hexadentate and pentadentate aminopyridyl ligands

Article abstract of DOI:10.1039/b007799m

Two mononuclear iron(m) complexes formed with the related ligands N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine (tpen) and N-benzyl-N,N′,N′-tris(2-pyridylmethyl)ethylenediamine (bztpen) have been studied. X-Ray crystallography reveals for the complex

Full text of DOI:10.1039/b007799m

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Solid and solution state structures of mono- and di-nuclear iron(III)
complexes of related hexadentate and pentadentate aminopyridyl
ligands
Lars Duelund,*a Rita Hazell,b Christine J. McKenzie,c Lars Preuss Nielsenc and
Hans Toftlundc
a Institut für Physik, Medizinische Universität Lübeck, D-23538 Lübeck, Germany
b Department of Chemistry, Aarhus University, DK-8000 Aarhus, Denmark
c Department of Chemistry, University of Southern Denmark, Main Campus:
Odense University, DK-5230 Odense M, Denmark
Received 26th September 2000, Accepted 20th November 2000
First published as an Advance Article on the web 2nd January 2001
Two mononuclear iron() complexes formed with the related ligands N,N,NЈ,NЈ-tetrakis(2-pyridylmethyl)ethylene-
diamine (tpen) and N-benzyl-N,NЈ,NЈ-tris(2-pyridylmethyl)ethylenediamine (bztpen) have been studied. X-Ray
crystallography reveals for the complex [Fe(tpen)][ClO4]3 that tpen acts as a hexadentate ligand in the solid state.
In methanol or water containing solutions it was shown by EPR and UV-Vis spectroscopy that one pyridyl arm is
exchanged by a solvent molecule. In dmf solution only partial exchange of a pyridyl arm was observed. The complex
with the related pentadentate ligand bztpen [Fe(bztpen)Cl][ClO4]2 showed exchange of the coordinated chloride with
methanol and water, but not with dmf. In water containing solutions both complexes are slowly converted into the
dimeric µ-oxo complexes [(tpen)FeIII(Cl)OFeIII(Cl)(tpen)]2ϩ or [(bztpen)FeIII(Cl)OFeIII(Cl)(bztpen)]2ϩ. This reaction
is accelerated by addition of an excess of chloride.
We present here the crystal structure of [Fe(tpen)][ClO4]3
and a study of the formation of µ-oxo bridged dimers, in the
presence or absence of chloride. The produced oxo bridged
dimer was independently synthesized and characterised.
Introduction
The formation of iron() hydroperoxo complexes has
recently been reported starting from iron() complexes of the
aminopyridyl ligand N,N,NЈ,NЈ-tetrakis(2-pyridylmethyl)-
ethylenediamine (tpen)1 (Chart 1) and the related N-benzyl-
Experimental
Materials
Commercially available chemicals were used without prior
purification. Methanol and dmf were both Fluka puriss, water
content <0.01%. The ligand tpen was synthesized according to
a literature method.4
Physical measurements
UV-Visible absorption spectra were recorded on a Shimadzu
UV-3100 spectrophotometer, FAB mass spectra on a Kratos
MS50TC instrument, ES-MS spectra on a Finnigan TSQ 700
MAT triple quadrupole instrument equipped with a nano-
Chart 1
electrospray source, and NMR spectra on a 300 MHz Varian
N,NЈ,NЈ-tris(2-pyridylmethyl)ethylenediamine
(bztpen)2
Gemeni 2000 instrument. EPR spectra were recorded on a
Bruker EMX 113 spectrometer operating at X-band and
equipped with a continuous-flow liquid nitrogen cryostat.
Low spin spectra were simulated using the program Bruker
WINEPR Simfonia, version 1.25 and high spin spectra with
the Sim program.5 Elemental analyses were carried out by the
microanalytical laboratory of the H. C. Ørsted Institute,
Copenhagen.
(Chart 1). These are synthetic analogues of the active inter-
mediate in the reaction cycle of bleomycin (BLM) which has
been shown to be an iron() hydroperoxo complex.3 From the
EPR spectra of the iron() hydroperoxo species another low-
spin iron() complex appeared to be present but its structure
was not definitely assigned.2 Recently, the structures of several
chromium() complexes in which tpen acts as a hexa-, penta-
or tetra-dentate ligand have been published.4 These structures
of the more robust chromium() complexes, where it was
possible to trap the different complexes of tpen in the solid
state, might be relevant for the more labile iron() complexes
in solution. To clarify the solution structures we have therefore
undertaken a spectroscopic investigation of the iron() com-
plexes formed with the aminopyridyl ligands tpen and bztpen,
in different solvents.
Syntheses
N-Benzyl-N,NЈ,NЈ-tris(2-pyridylmethyl)ethylenediamine,
bztpen. A solution of N-benzylethylenediamine (2.0 g, 13.3
mmol) in CH2Cl2 (20 ml) was added to a solution of 2-(chloro-
methyl)pyridine hydrochloride (6.55 g, 40 mmol) in water
(20 ml). NaOH (3.2 g, 80 mmol) in water (10 ml) was added in
152
J. Chem. Soc., Dalton Trans., 2001, 152–156
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b007799m

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small portions over 5 days, keeping the reaction under an N2
atmosphere. The reaction mixture was extracted with methylene
chloride (3 × 25 ml). The combined organic phases were dried
over MgSO4 and evaporated to dryness to yield a white solid. It
was purified by extraction in a Soxhlet apparatus using low
1
boiling petroleum. Yield 3.2 g, 57%. H NMR (CDCl3): δ 8.48
(2H, m), 7.58 (3H, dt), 7.45 (3H, t), 7.19–7.32 (5H, m), 7.12
(3H, m), 3.77 (4H, s), 3.72 (2H, s), 3.59 (2H, s) and 2.74 (4H,
m). 13C NMR (CDCl3): δ 160.09, 159.65, 148.85, 148.70,
139.11, 136.22, 128.64, 128.09, 126.79, 122.64, 122.59, 121.75,
121.69, 60.71, 60.65, 60.51, 58.91, 52.15 and 51.83. FABMS:
m/z 424 (MHϩ, 76), 331 ([MH Ϫ C6H7N]ϩ, 15), 225 ([MH Ϫ
C12H13N3]ϩ, 94), 212 ([M Ϫ C14H15N2]ϩ, 100) and 211 ([M Ϫ
C13H14N3]ϩ, 95%). Calc. for C27H29N5 (%): C, 76.56; H, 6.90,
N, 16.53. Found (%): C, 76.46; H, 7.04; N, 16.65.
Fig. 1 An ORTEP11 drawing of the [Fe(tpen)]3ϩ cation. Ellipsoids at
30% probability and hydrogens omitted for clarity.
CAUTION! the following perchlorate compounds should be
handled as potential explosives. On one occasion [Fe(tpen)]-
[ClO4]3 exploded under handling.
The product was isolated in acceptable yields as an off-
white, crystalline solid, which is stable for months at room
temperature.
[Fe(tpen)][ClO4]3. Fe(ClO4)3ؒ9H2O (147.4 mg, 0.285 mmol)
in dry MeOH (10 ml) was added to a solution of tpen (60.6 mg,
0.143 mmol) and sodium acetate (24 mg, 0.293 mmol) in
dry methanol (15 ml). The mustard coloured precipitate,
which formed immediately, was dissolved by heating for 5 min.
The product precipitated as orange crystals. Yield: 87 mg,
78%. FABMS: m/z 579 ([Fe(tpen)(ClO4)]ϩ, 73%). Owing to
the explosive nature of the complex no microanalysis was
performed.
Synthesis of iron(III) complexes
[Fe(tpen)][ClO4]3 was isolated from the reaction of tpen with
Fe(ClO4)3ؒ9H2O (1:2) in the presence of two equivalents of
acetate. A mustard coloured impure precipitate was formed
initially, which redissolved by heating to give an orange solu-
tion from which the orange monomeric product could be
isolated. ES-MS spectra of acetone solutions of the mustard
coloured intermediate support formulations of the type [(tpen)-
Fe]2O(X)(Y)2ϩ (X = Y = Cl or ClO4 or X = Cl, Y = ClO4).
Chloride is probably introduced with the tpen or as an impurity
from the ES-MS experiment. Dominant peaks were identified
as the dimeric {(tpen)2Fe2OCl2}2ϩ (m/z 523), {(tpen)2Fe2O-
Cl(ClO4)}2ϩ (555) and {(tpen)2Fe2O(ClO4)2}2ϩ (587) and
monomeric [(tpen)Fe]2ϩ (m/z 240) and {(tpen)FeClO4}ϩ (579).
This indicates that the µ-oxo dimer may be an intermediate
for the formation of [Fe(tpen)][ClO4]3. No peaks containing
acetate could be identified in the ES-MS, and substitution
of acetate by triethylamine in the synthesis had no influence
on the yield and purity of the product. This shows that a base
is crucial for the formation of [Fe(tpen)]3ϩ, possibly to aid
formation of the above mentioned µ-oxo dimer.
[Fe(bztpen)Cl][ClO4]2ؒH2O. bztpen (100 mg, 0.236 mmol)
in abs. EtOH (5 ml) was added to a solution of FeCl3ؒ6H2O
(63.8 mg, 0.236 mmol) in abs. EtOH (3 ml). After stirring for
10 min, NaClO4ؒH2O (100 mg, 0.712 mmol) in abs. EtOH (3 ml)
was added. The resulting yellow precipitate was isolated by
filtration and washed with cold abs. EtOH (1 ml). Yield 131 mg,
76%. FABMS: m/z 613 ([Fe(bztpen)Cl(ClO4)]ϩ, 1) and 514
([Fe(bztpen)Cl]ϩ, 100%). Calc. for C27H31Cl3FeN5O9 (%): C,
44.32; H, 4.27; N, 9.57. Found (%): C, 44.32; H, 4.13; N, 9.40.
[(bztpen)Fe(Cl)OFe(Cl)(bztpen)][PF6]2ؒ3H2O. 2 M NaOH
(118 mg, 0.236 mmol) in dry MeOH (1 ml) was added to a
stirred solution of FeCl3 (38.2 mg, 0.236 mmol) in dry MeOH
(1 ml). bztpen (100 mg, 0.236 mmol) in dry MeOH (1 ml) was
added and the mixture stirred for 10 min. After addition of
NH4PF6 (50 mg, 0.307 mmol) the mustard coloured product
precipitated immediately. Yield 98 mg, 60%. FABMS: m/z 514
([Fe(bztpen)Cl]ϩ, 100%). Calc. for C27H32ClF6FeN5O2P (%):
C, 46.67; H, 4.64; N, 10.08. Found (%): C, 46.16; H, 4.30;
N, 10.04.
[Fe(bztpen)Cl][ClO4]2 was synthesized by a 1:1 reaction of
bztpen with anhydrous FeCl3 followed by precipitation with
NaClO4ؒH2O. No product could be obtained in the absence
of ClϪ ions from reaction of bztpen with Fe(ClO4)3ؒ9H2O in
various solvents.
Reaction of FeCl3 with bztpen in the presence of two
equivalents of NaOH, followed by addition of PF6Ϫ, resulted
in precipitation of the µ-oxo product, [(bztpen)Fe(Cl)-
OFe(Cl)(bztpen)][PF6]2. The metpen (N-methyl-N,NЈ,NЈ-
tris(2-pyridylmethyl)ethane) analogue, [(metpen)Fe(Cl)OFe-
(Cl)(metpen)]Cl[OH]ؒ7H2O has been isolated from a reaction
of [Et4N]2[Cl3FeOFeCl3] with metpen in acetone solution.8
However the preparation of [Et4N]2[Cl3FeOFeCl3]9 is quite
lengthy and extensive purification is necessary. Here we are
able to synthesize the oxo-dimer directly in a one-step reaction,
albeit in slightly lower yields.
The crystal structure of [Fe(tpen)][ClO4]3 shows that in the
solid state tpen acts as a hexadentate ligand. The complex
forms a distorted octahedron with the iron–nitrogen bond
distance typical of low-spin (LS) iron() complexes10 (Fig. 1
and Table 2). The major distortion from an ideal octahedron is
the angle N4–Fe–N5 which is large, 111.2Њ. The structure of the
iron() complex differs significantly from that of the iron()
complex of tpen, [Fe(tpen)][ClO4]2, which is a spin cross-over
compound.12 The low-spin iron() complex has slightly longer
bond distances than the LS iron() complex (Fe–N(pyridine)
X-Ray crystallography
Experimental and crystal data are collected in Table 1. The
crystal structure of [Fe(tpen)][ClO4]3 was solved by direct
methods (SIR 92).6 Two of the perchlorate groups are dis-
ordered, and the following constraints were applied in order
to get the best possible determination of the rest of the struc-
ture: all perchlorate groups are identical, regular tetrahedra;
two groups were each allowed two orientations with refined
occupancy, the most populated in both cases about 65%.
Atomic displacement parameters for each perchlorate group
were modelled by the TLS approximation.7
CCDC reference number 186/2274.
See http://www.rsc.org/suppdata/dt/b0/b007799m/ for crys-
tallographic files in .cif format.
Results and discussion
Ligand synthesis
Synthesis of the ligand bztpen is by nucleophilic substitution
of 2-(chloromethyl)pyridine with N-benzylethylenediamine.
J. Chem. Soc., Dalton Trans., 2001, 152–156
153

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1.99 and 1.97 Å respectively and Fe–N(amine) 2.03 and 1.99 Å)
and the angle N4–Fe–N5 is only 91Њ.12 In the corresponding
chromium() complex, [Cr(tpen)][ClO4]3, the angle N4–Fe–N5
is 116Њ. The difference in the angle N4–M–N5 could stem from
the ligand-field stabilisation energy (LFSE) (equal to Ϫ1.2 for
CrIII, Ϫ2.0 for FeIII and Ϫ2.4 for FeII.13
Table 1 Crystal and experimental data for [Fe(tpen)][ClO4]3
Solution species
EPR and UV-Vis spectroscopy show that [Fe(tpen)][ClO3]4
and [Fe(bztpen)Cl][ClO4]2 are labile towards solvent attack.
The reactions observed are summarised in Charts 3 and 4. In
the solid state at 100 K [Fe(tpen)][ClO3]4 shows a rhombic
EPR spectrum, typical of LS iron() centres,14 with g = 2.36,
2.15 and 1.92 (Table 3). In a frozen dmf solution at 100 K we
find two overlapping rhombic LS spectra. The minor signal,
with g = 2.38, 2.17, 1.93, is assigned to the six-coordinated
[Fe(tpen)]3ϩ species in which tpen acting as a hexadentate
ligand. The major signal with g = 2.34, 2.15, 1.93 is assigned
to a complex in which one pyridyl arm has been replaced by a
dmf molecule (the spectrum of [Fe(bztpen)(dmf)]3ϩ is similar,
see later). Exchange of a pyridyl ligand for a dmf molecule can
take place at two different sites. One site is trans to another
pyridyl group (N3 or N6 in Fig. 1) and the other is trans to a
secondary amine (N4 or N5 in Fig. 1). In the corresponding
chromium() complex, [Cr(tpen)][ClO4]3, an exchange of a
pyridyl arm for an hydroxide ion is observed at the position
trans to the secondary amine (see Chart 2).4 By analogy to this
complex we therefore suggest that the exchange at FeIII also
takes place at this position. The structures of complexes with
linear tetradentate ligands related to tpen furnish support
for this argument. For example those of cis-[Fe(LBzl2)Cl2]-
[PF6]10 (LBzl2 = N,NЈ-bis(benzyl)-N,NЈ-bis(2-pyridylmethyl)-
ethane-1,2-diamine) and cis-[Fe(bispicen)Cl2][ClO4] (bispicen =
Formula
Formula weight
Crystal system
Space group
a/Å
b/Å
c/Å
C26H23Cl3FeN4O12
778.78
Monoclinic
P21/n
9.816(2)
18.486(2)
18.156(2)
93.702(7)
3288(1)
4
295
0.71073
β/Њ
V/Å3
Z
T/K
Wavelength, λ(Mo-Kα)/Å
Diffractometer
µ/cmϪ1
Absorption correction
Reflections collected
Independent reflections
Rint
Reflections with I > 3σ(I)
R
wR
Huber four-circle diffractometer
0.771
None
6279
5793
0.029
4371
0.045
0.055
4371/392
Data/parameters
Table
2
Selected bond distances (in Å) and angles (in Њ) for
[Fe(tpen)][ClO4]3
Fe–N1
Fe–N2
Fe–N3
1.989(3)
1.986(3)
1.970(3)
Fe–N4
Fe–N5
Fe–N6
1.971(3)
1.973(3)
1.977(3)
N1–Fe–N2
N1–Fe–N3
N1–Fe–N4
N1–Fe–N5
N1–Fe–N6
N2–Fe–N3
N2–Fe–N4
N2–Fe–N5
87.2(1)
84.5(1)
81.2(1)
166.5(1)
97.5(1)
96.4(1)
167.7(1)
80.7(1)
N2–Fe–N6
N3–Fe–N4
N3–Fe–N5
N3–Fe–N6
N4–Fe–N5
N4–Fe–N6
N5–Fe–N6
84.5(1)
86.5(1)
90.7(1)
177.8(1)
111.2(1)
93.0(1)
87.6(1)
Chart 2
Table 3 EPR and UV-Vis data of the complexes
EPR Characteristic
Solvent, E/D (HS) or
g values (LS)
UV-Vis
Solvent and λmax/nm
Complex
Spin state
Reference
[Fe(bztpen)Cl][ClO4]2
[Fe(bztpen)Cl][ClO4]2
[Fe(bztpen)(dmf)]3ϩ
[Fe(bztpen)(OMe)]2ϩ
[Fe(bztpen)(OH)]2ϩ
[Fe(tpen)][ClO4]3
HS
HS
LS
LS
LS
LS
LS
LS
LS
LS
LS
HS
Solid
≈0.15
dmf
0.15
dmf
Not determined
This work
This work
dmf
300, 368
Not determined
2.35, 2.15, 1.93
MeOH–dmf
2.325; 2.140; 1.933
MeOH–dmf–water
2.39, 2.19, 1.91
Solid
2.36, 2.15, 1.92
dmf
2.385, 2.175, 1.930
dmf
2.34, 2,15, 1.93
MeOH–dmf
2.336, 2.140, 1.934
MeOH–dmf–water
2.39, 2.19, 1.91
MeOH
2.29, 2.12, 1.96
MeCN–dmf (2:1)
0.115
MeOH
358
Not determined
This work
This work
This work
This work
This work
This work
This work
17
Not determined
Not determined
Not determined
[Fe(tpen)]3ϩ
[Fe(tpen)(dmf)]3ϩ
[Fe(tpen)(OMe)]2ϩ
[Fe(tpen)(OH)]2ϩ
MeOH
358
Not determined
a
[Fe(N4Py)(OMe)][ClO4]2
[Fe(LBzl2)Cl2][PF6]b
MeOH
255, 360
MeCN
10
256, 300, 370
a N4Py = (bis-2-pyridylmethyl)bis(2-pyridylmethyl)amine. b LBzl2 = N,NЈ-bis(benzyl)-N,NЈ-bis(2-pyridylmethyl)ethane-1,2-diamine.
154
J. Chem. Soc., Dalton Trans., 2001, 152–156

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Chart 3
Chart 4
Fig. 3 UV-Vis spectra of (a) [Fe(bztpen)(dmf)]2ϩ in dmf and (b)
[Fe(bztpen)(OMe)]2ϩin MeOH at room temperature.
g equal to 8.16, 5.58, 4.3 and 3.67 at 100 K. The intensity of
the signal at g = 4.3 varies between samples and its position is
typical of an HS iron() complex with E/D = 1/3. This signal
has been seen for other HS polypyridyl complexes and attri-
buted to an impurity.10 The rest of the present spectrum can
be simulated using E/D = 0.15 and D = 0.3 cmϪ1. The complex
[Fe(bztpen)(dmf)]3ϩ is prepared by in situ mixing of equivalent
amounts of Fe(ClO4)3ؒ9H2O, and bztpen in dmf. It yields a
rhombic LS signal with g = 2.35, 2.15 and 1.93 at 100 K. This
in situ experiment supports assignment of the signal observed
in solution to the [Fe(bztpen)Cl]2ϩ ion and indicates that dmf is
not capable of replacing the coordinated chloride. The UV-Vis
spectrum of [Fe(bztpen)Cl][ClO4]2, in dmf solution at room
temperature, shows two absorption bands at 300 and 368 nm
(Fig. 3a) which may be attributed to ligand to metal charge
transfer (LMCT) transitions.16
In a MeOH–dmf (3:1) glass at 100 K both [Fe(tpen)][ClO4]3
and [Fe(bztpen)Cl][ClO4]2 show a new rhombic low-spin signal
with g = 2.32, 2.14 and 1.93 (Fig. 2c). The same signal is
also seen on in situ mixing of equivalent amounts of Fe-
(ClO4)3ؒ9H2O and bztpen in methanol. This leads us to the
conclusion that the one pyridyl arm (for tpen) or a chloride
(for bztpen) has been replaced by a methoxo ligand, forming
an LS species. The UV-Vis spectrum in methanol reveals a band
at 358 nm (Fig. 3) which is similar to the band observed for
[Fe(N4Py)(OMe)][ClO4]2 (N4Py = (bis-2-pyridylmethyl)bis(2-
pyridylmethylamine) in methanol.17 If water then is added to
the MeOH–dmf (3:1) solution at room temperature and the
spectrum subsequently measured at 100 K we observe a new
rhombic EPR signal with slightly different g values at 2.39, 2.18
and 1.91 (Fig. 2d). The intensity of this signal can be correlated
Fig. 2 EPR spectra of the bztpen complexes: (a) solid state [Fe-
(bztpen)Cl][ClO4]2, (b) [Fe(bztpen)Cl]2ϩ generated by in situ reaction of
Fe(ClO4)3ؒ9H2O, bztpen and tetrabutylammonium chloride, (c) [Fe-
(bztpen)(OMe)]2ϩ in MeOH–dmf (3:1), (d) simulation of (c), (e) after
addition of water to [Fe(bztpen)(OMe)]2ϩ in MeOH–dmf (3:1) and
(f) after addition of an excess of chloride to (e).
N,NЈ-bis(2-pyridylmethyl)-ethane-1,2-diamine)15 show that the
two halide ligands are found cis to each other (trans to the
secondary amines) and thus the pyridyl donors are trans to
each other. The alternate structures, halide trans and the pyridyl
donors cis or both the pyridyl donors and the halides cis,
have not been observed thus far.
The solid state EPR spectrum of [Fe(bztpen)Cl][ClO4]2 at
100 K consists of three very broad absorptions with effective
g values of ca. 8.0, 5.7 and 4.4 (Fig. 2a) indicative of a HS
iron() complex. The EPR spectra of HS d5 systems are usually
described by the spin Hamiltonian parameter D, which results
from axial zero field splitting, and E/D, describing the rhom-
bicity of the complex (0 ≤ E/D ≤ 1/3).14 In a frozen dmf solu-
tion, a nitromethane glass and by in situ mixing of equivalent
amounts of Fe(ClO4)3ؒ9H2O, bztpen and tetrabutylammonium
chloride in dmf at 100 K four lines are observed (Fig. 2b) at
J. Chem. Soc., Dalton Trans., 2001, 152–156
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with the amount of water added. We assign this signal to a
complex in which the methoxo ligand has been replaced by
a hydroxo ligand. Despite several attempts we were not able to
isolate any hydroxo or methoxo complexes of either tpen or
bztpen.
Conclusion
tpen acts as a hexadentate ligand toward iron() in the solid
state. This was shown by X-ray crystallography where a dis-
torted octahedra structure was observed. In solution we found
by EPR, UV-Vis and ES-MS that one pyridyl was exchanged
by a solvent molecule. In the case of dmf the exchange did not
go to completion, but in methanol and water it was total. We
found an analogous behaviour for the complex [Fe(bztpen)Cl)]-
[ClO4]2. Thus in dmf solution no exchange of chloride was
observed, whereas in methanolic and water containing solution
the chloride was exchanged with either a methoxo or a hydroxo
ligand, respectively.
Both the tpen and the bztpen complexes react further in
solution and form an oxo bridged dimer which we were able
to identify by comparison of the UV-Vis spectral charac-
teristics with those of the deliberately synthesized complex
[{Fe(bztpen)Cl}2O]2ϩ and of previous published µ-oxo dimers.
We have observed a difference in the dimerisation rates for the
monomers depending on the presence of chloride ion. Excess
of chloride makes the dimerisation process proceed much faster
and this can be explained by the fact that dimerisation of
HS iron() complexes is favoured and the excess of chloride
increases the concentration of the HS monomer.
Formation of oxo-bridged dimer complexes
The integrations of the EPR spectrum of [Fe(bztpen)(OMe)]2ϩ
in dry MeOH–dmf (3:1) solution kept at room temperature
remained unchanged after 48 h. In a MeOH–dmf (3:1) solu-
tion containing ca. 5% water the integrals changed significantly
with time. After 48 h the concentration of the methoxo complex
was reduced to approximately 15% of the starting concen-
tration. In the presence of both water and an excess of chloride
the signal was lost rapidly and after two days only 0.1% was left.
The UV-Vis spectra of aged solutions (48 h) containing both
water and an excess of chloride show an absorption band
at 320 nm and a stronger band at 374 nm, as found in the
UV-Vis spectra of the µ-oxo complexes [{(bztpen)FeCl}2O}]2ϩ
,
[{(metpen)FeCl}2O}]2ϩ 8 and [{(TPA)FeCl}2O]2ϩ (TPA =
tris(2-pyridylmethyl)amine)18 dimers. Furthermore ES-MS
experiments on the aged solution showed the presence of
[{(bztpen)FeCl}2O}]2ϩ (m/z 522). This indicates that the EPR
silent product formed is a µ-oxo complex. Analogous reactions
were observed with [Fe(tpen)][ClO4]3.
Acknowledgements
Although no detailed kinetic study was undertaken, our
results show that the formation of the µ-oxo dimer is
accelerated by the presence of chloride ions. This is under-
standable when considering the fact that an HS dimer is
stabilised through π donation from the O2Ϫ bridge and therefore
the dimerisation of HS iron() monomeric complexes will
proceed much faster than that of LS monomers. The addition
of chloride will shift the equilibrium towards HS species, which
will then form HS dimers. The reactions are summarised in
Chart 5.
Dr K. B. Jensen, University of Southern Denmark, Odense
Campus is thanked for assistance with the ES-MS experiment.
The Danish Natural Science research council is thanked for a
grant (C. J. M.).
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Chart 5
156
J. Chem. Soc., Dalton Trans., 2001, 152–156