Iron(III) complexes with the ligand N′,N′-bis[2-pyridyl-methyl]ethylenediamine (uns-penp) and its amide derivative N-acetyl-N′,N′-bis[(2-pyridyl)methyl]ethylenediamine (acetyl-uns-penp)

Article abstract of DOI:10.1002/ejic.200500902

Synthesis and structural characterization of the iron(III) complexes of the tripodal ligands N′,N′-bis[(2-pyridyl)methyl]-ethylenediamine (uns-penp), [Fe(uns-penp)Cl₂]ClO₄· CH₃CN, [{Fe(uns-penp)Cl}₂O](ClO_4<

Full text of DOI:10.1002/ejic.200500902

FULL PAPER
DOI: 10.1002/ejic.200500902
Iron(III) Complexes with the Ligand NЈ,NЈ-Bis[(2-pyridyl)-
methyl]ethylenediamine (uns-penp) and Its Amide Derivative N-Acetyl-NЈ,NЈ-
bis[(2-pyridyl)methyl]ethylenediamine (acetyl-uns-penp)
Jing-Yuan Xu,[a,b] Jörg Astner,[b] Olaf Walter,[c] Frank W. Heinemann,[d]
Siegfried Schindler,*[b] Michael Merkel,[e] and Bernt Krebs*[e]
Keywords: Iron / Tripodal ligands / Catechol dioxygenase / Amide hydrogen bonding / Carboxamide coordination
Synthesis and structural characterization of the iron(III) com-
plexes of the tripodal ligands NЈ,NЈ-bis[(2-pyridyl)methyl]-
tetrachlorocatecholate), and [{Fe(acetyl-uns-penp)(tcc)}2O]·
(C2H5)2O·CH3OH are reported. Catechol dioxygenase reac-
tivity of in situ prepared complex solutions was tested and
showed that all complexes reacted slower compared with the
iron tmpa system described in the literature.
ethylenediamine
(uns-penp),
[Fe(uns-penp)Cl2]ClO4·
CH3CN, [{Fe(uns-penp)Cl}2O](ClO4)2·2CH3CN and the
amide derivative N-Acetyl-NЈ,NЈ-bis[(2-pyridyl)methyl]eth-
ylenediamine (acetyl-uns-penp), [Fe2(acetyl-uns-penp)2O]-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
(ClO4)2·H2O, [Fe(acetyl-uns-penp)(tcc)Br]·(C2H5)2O (tcc
=
Introduction
Tripodal ligands such as tris[(2-pyridyl)methyl]amine
(tmpa, also abbreviated as tpa in the literature) as well as
derivatives, e.g. N4Py (Scheme 1), have been used success-
fully in experimental studies to model copper and iron en-
zymes.[1–5] Iron() complexes of the related ligand Rtpen (R
= Me, Bz etc.) were shown to react with an excess of hydro-
gen peroxide to form an end-on hydroperoxo complex that
can convert to a side-on peroxo complex at higher pH val-
ues.[6–10] More recently it was demonstrated that iron com-
plexes with the ligand Bztpen as well as N4Py can form an
iron() oxo species that is able to oxidise alkanes such as
cyclohexane.[11]
Scheme 1. The ligands discussed in the text.
Rtpen can be prepared in two different synthetic pro-
cedures. The first described previously starts from a mono-
substituted ethylendiamine that is reacted with picolylchlor-
ide.[6–8] The second takes advantage of the amine NЈ,NЈ-
bis[(2-pyridyl)methyl]ethylenediamine (uns-penp; Scheme 1),
a versatile tripodal ligand described previously,[12,13] as a
starting material. Interestingly, so far only an iron() com-
plex of uns-penp has been described in the literature that
has been investigated with regard to its spin crossover prop-
erties.[14] Furthermore, very recently an iron() complex of
a derivative of uns-penp was described.[15] However, so far
no iron() complexes have been described in the literature.
Therefore, we decided to study the appropriate iron()
complexes of this ligand as well as of its acetyl derivative
N-acetyl-NЈ,NЈ-bis[(2-pyridyl)methyl]ethylenediamine (ace-
tyl-uns-penp; Scheme 1), an amide that was obtained dur-
ing the synthesis of uns-penp.
[a] Pharmacy College, Tianjin Medical University,
300070 Tianjin, China
[b] Institut für Anorganische und Analytische Chemie der Justus-
Liebig-Universität Gießen,
Heinrich-Buff-Ring 58, 35392 Gießen, Germany
Fax: +49-641-9934149
E-mail: Siegfried.Schindler@chemie.uni-giessen.de
[c] Institut für Technische Chemie – Chemisch-Physikalische Ver-
fahren (ITC-CPV), Forschungszentrum Karlsruhe,
Postfach 3640, 76021 Karlsruhe, Germany
[d] Institut für Anorganische Chemie der Friedrich-Alexander-
Universität Erlangen-Nürnberg, Egerlandstraße 1, 91058 Er-
langen, Germany
[e] Institut für Anorganische und Analytische Chemie der
Westfälischen Wilhelms-Universität,
Results and Discussion
Only recently have different research groups started to
use uns-penp as a versatile ligand in different areas of coor-
Wilhelm-Klemm-Straße 8, 48149 Münster, Germany
Fax: +49-251-83-38366
E-mail: krebs@uni-muenster.de
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S. Schindler, B. Krebs et al.
FULL PAPER
dination chemistry and different synthetic procedures have Herein the amide is not deprotonated, however, the carbox-
been described.[12,13,15–19] However, most of the authors un- amido function of acetyl-uns-penp is no longer truly sp2
fortunately do not refer to the original synthesis of this li- hybridised and the nitrogen atom undergoes a weak interac-
gand by Mandel and co-workers[12] and in one case it is tion with the metal centre.[13] The copper complex of Pa-
presented a second time and described incorrectly as a new Py3H forms a dimer in which the oxygen atom of the amide
compound (ten years later in the same journal).[14] Some of is coordinated to the copper() ion.[32] In contrast, the
us have used this ligand previously in studies within the iron amide complexes of PaPy3H described by Mascharak
field of copper chemistry and have improved its prepara- and co-workers contained the deprotonated ligand
tion.[13] During the two-step synthesis of uns-penp its pro- form.[27,30,32,33] However, our own efforts to synthesise the
tected form, N-Acetyl-NЈ,NЈ-bis[(2-pyridyl)methyl]ethyl- mononuclear deprotonated iron() complex [Fe(acetyl-uns-
enediamine (acetyl-uns-penp), was easily obtained in pure penp)](ClO4)2 were only partially successful. Instead of the
form in high yields. Recrystallisation from petroleum ethers mononuclear species we obtained the dinuclear complex
afforded single crystals that were suitable for X-ray diffrac- [Fe2(acetyl-uns-penp)2O](ClO4)2·H2O (1). It is interesting to
tion studies. Acteyl-uns-penp crystallises with two mole- note at this point that the stoichiometrically identical com-
cules per unit cell. Those are dimerised by hydrogen bond- pound [Fe2(PaPy3H)2O](ClO4)2 (containing additional eth-
ing between the carboxamido group and one of the pyridine anol solvent molecules) derived through oxidation of the
groups. The molecular structure of one of these dimers is iron() complex is structurally completely different from
presented in Figure 1 (see Table 2 for a summary of the 1.[30] In this complex the oxo-bridge assembles two mono-
crystallographic data and refinement parameters). Besides nuclear amide complexes (with coordinated deprotonated
the hydrogen bonding, the crystal structure shows no ex- amide nitrogen atoms while the oxygen atoms are not coor-
traordinary features.
dinated) to form the dimer (as one would expect). In con-
trast and as discussed below, in 1 the ligand acetyl-uns-penp
additionally bridges the two iron() ions and involves the
amide nitrogen as well as the oxygen atom in the coordina-
tion.
Crystals of 1 were at first obtained from the reaction of
Fe(ClO4)3, acetyl-uns-penp and NaN3 in methanol followed
by recrystallisation of the precipitate in acetonitrile. In con-
trast to our expectations no azide complex was formed but
instead deprotonation of the amide occurred under these
conditions and because of the presence of water the oxo-
bridged dinuclear complex 1 was obtained. Taking this re-
sult into account it was furthermore possible to crystallise
1 successfully by replacing the NaN3 by Et3N or NaOH as
the deprotonating agent. The cation of 1, as depicted in
Figure 2, shows that the crystallographically equivalent
iron() centres are triply bridged by one oxo and two car-
boxamido groups from two acetyl-uns-penp ligands, respec-
tively, leading to an intramolecular Fe–Fe distance of
2.992 Å and an Fe–O–Fe angle of 113.1°. A summary of
the crystallographic data and refinement parameters for the
structures is presented in Table 2. Selected bond lengths and
angles for the iron() complexes are reported in Table 1.
It is interesting to note that in 1, the acetyl-uns-penp li-
gand shows an unusual pentadentate coordination mode
displaying deprotonated carboxamido NCO bridging
groups (η2, µ2) involving the delocalisation of π-bonding
[d(C–O) = 1.283, d(C–N) = 1.306 Å], which is in agreement
with the values of a related (η2, µ2) NCO-bridged iron com-
plex reported previously.[34] However, in contrast, structural
properties of 1 are quite different to a related iron() com-
plex with a bridging urea anion.[35] Compared to other di-
iron() complexes with three bridging ligands, of which at
least one is a µ-oxo unit, 1 exhibits some unique structural
features. The Fe–Fe separation is significantly shorter than
those of other µ-oxo-tribridged diiron() complexes (range
Figure 1. Thermal ellipsoid plot of an acetyl-uns-penp dimer (50%
probability).
Furthermore, iron complexation with carboxamide li-
gands has attracted the interest of inorganic chemists to
gain better understanding of metal–peptide bond coordina-
tion chemistry in life sciences[20] and to use such complexes
as model compounds for the anti-tumor drug bleomy-
cin[21,22] or nitrile hydratase.[23–25] Important in that regard
are the results reported by Mascharak and co-workers, who
have investigated in detail the structures and properties of
FeII/FeIII complexes of a number of carboxamide ligands,
several based on a pyridine-2-carboxamide framework, e.g.
N-{[bis(2-pyridylmethyl)amino]ethyl}pyridine-2-carbox-
amide (PaPy3H).[22,25–31] The ligand PaPy3H (Scheme 1) is
related to acetyl-uns-penp; instead of the methyl group in
acetyl-uns-penp it contains an additional coordinating
pyridyl unit.
[Fe2(acetyl-uns-penp)2O](ClO4)2·H2O (1)
Acetyl-uns-penp has so far only been used to synthesise 3.048–3.335 Å) and the Fe–O–Fe angle is among the small-
and characterise a copper() complex with this ligand. est of them all (range 113.8–134.7°).[36] Each iron() ion
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Iron() Complexes with NЈ,NЈ-Bis[(2-pyridyl)methyl]ethylenediamine Ligands
Table 1. Selected distances [Å] and angles [°] in 1–5.[a]
FULL PAPER
Atoms
1
Atoms
2
3
Atoms
4
Atoms
5
Fe(1)–O(1)
Fe(1)–O(2A)
Fe(1)–N(4)
Fe(1)–N(2)
Fe(1)–N(3)
Fe(1)–N(1)
O(1)–Fe(1A)
O(2)–Fe(1A)
Fe(1)···Fe(1A)
1.792(2)
2.013(2)
2.100(2)
2.157(2)
2.162(2)
2.245(2)
1.792(2)
2.013(2)
2.992(1)
Fe(1)–X
2.4813(9) 1.7929(5) Fe(1)–N(1)
2.127(2)
2.136(2)
2.172(2)
2.214(2)
2.2622(5)
2.3081(6)
Fe(1)–O(1)
Fe(1)–N(3)
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(4)
Fe(1)–Cl(1)
O(1)–Fe(1A)
Fe(1)···Fe(1A)
1.798(1)
2.141(2)
2.147(2)
2.213(2)
2.235(2)
2.3166(4)
1.798(1)
3.596(1)
Fe(1)–O(2)
Fe(1)–O(3)
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(3)
Fe(1)···Fe(1A)
Fe(1)···N(4)
O(2)···N(4)
1.961(3)
1.969(3)
2.266(4)
2.191(4)
2.159(4)
2.012(2)
2.008(2)
2.366(3)
2.170(3)
2.175(2)
3.586
Fe(1)–N(3)
Fe(1)–N(4)
Fe(1)–N(2)
Fe(1)–Cl(1)
Fe(1)–Cl(2)
3.692(3)
2.755(3)
O(1)–Fe(1)–O(2A) 98.35(5)
O(1)–Fe(1)–N(4) 103.62(6)
O(2A)–Fe(1)–N(4) 93.13(7)
O(1)–Fe(1)–N(2) 102.11(6)
O(2A)–Fe(1)–N(2) 85.80(6)
X–Fe(1)–O(2)
X–Fe(1)–O(3)
X–Fe(1)–N(1)
X–Fe(1)–N(2)
96.2(2)
99.8(2)
165.6(2)
93.5(2)
95.4(2)
82.9(2)
95.4(2)
168.1(2)
86.5(2)
90.1(2)
88.7(2)
162.3(2)
76.2(2)
76.8(2)
99.5(2)
98.25(6)
106.26(7) N(1)–Fe(1)–N(4)
161.78(7) N(3)–Fe(1)–N(4)
95.87(7)
92.24(7)
81.31(8)
95.28(9)
N(1)–Fe(1)–N(3)
153.61(6)
83.51(6)
90.55(6)
78.07(6)
75.55(6)
79.08(6)
O(1)–Fe(1)–N(3)
O(1)–Fe(1)–N(1)
N(3)–Fe(1)–N(1)
O(1)–Fe(1)–N(2)
N(3)–Fe(1)–N(2)
N(1)–Fe(1)–N(2)
O(1)–Fe(1)–N(4)
N(3)–Fe(1)–N(4)
N(1)–Fe(1)–N(4)
N(2)–Fe(1)–N(4)
O(1)–Fe(1)–Cl(1)
N(3)–Fe(1)–Cl(1)
N(1)–Fe(1)–Cl(1)
N(2)–Fe(1)–Cl(1)
N(4)–Fe(1)–Cl(1)
93.26(3)
90.95(3)
153.72(5)
93.08(3)
76.04(4)
77.84(4)
170.11(3)
88.86(4)
82.98(4)
78.04(4)
102.82(2)
100.45(3)
103.87(3)
163.93(3)
86.26(3)
N(1)–Fe(1)–N(2)
N(3)–Fe(1)–N(2)
N(4)–Fe(1)–N(2)
X–Fe(1)–N(3)
N(4)–Fe(1)–N(2)
O(1)–Fe(1)–N(3)
154.12(7)
96.96(5)
O(2)–Fe(1)–O(3)
O(2)–Fe(1)–N(1)
O(2)–Fe(1)–N(2)
O(2)–Fe(1)–N(3)
O(3)–Fe(1)–N(1)
O(3)–Fe(1)–N(2)
O(3)–Fe(1)–N(3)
N(1)–Fe(1)–N(2)
N(1)–Fe(1)–N(3)
N(2)–Fe(1)–N(3)
Fe(1A)–O(1)–Fe(1)
N(1)–Fe(1)–Cl(1) 105.57(4)
O(2A)–Fe(1)–N(3) 164.53(6)
160.74(9) N(3)–Fe(1)–Cl(1) 100.04(4)
N(4)–Fe(1)–N(3)
N(2)–Fe(1)–N(3)
O(1)–Fe(1)–N(1)
85.43(7)
88.81(6)
173.14(7)
96.38(9)
87.77(9)
82.20(9)
N(4)–Fe(1)–Cl(1) 89.42(4)
N(2)–Fe(1)–Cl(1) 167.56(5)
N(1)–Fe(1)–Cl(2) 89.33(5)
O(2A)–Fe(1)–N(1) 87.39(7)
161.50(9) N(3)–Fe(1)–Cl(2) 92.30(5)
N(4)–Fe(1)–N(1)
N(2)–Fe(1)–N(1)
N(3)–Fe(1)–N(1)
79.68(7)
74.44(7)
77.19(7)
74.16(9)
74.12(9)
96.10(9)
180
N(4)–Fe(1)–Cl(2) 169.15(4)
N(2)–Fe(1)–Cl(2) 91.49(4)
Cl(1)–Fe(1)–Cl(2) 100.38(2)
Fe(1A)–O(1)–Fe(1) 113.1(2)
Fe(1A)–O(1)–Fe(1) 180
[a] X = Br(1) in 2 or O(4) in 3 A: x, y, –z + 1/2 (for 1); –x, –y, –z (for 5).
amide groups facilitates the increase of bond lengths
around the FeIII ions (Table 1).
Model Complexes for Intradiol Catechol Dioxygenases
One reason for our efforts to obtain the mononuclear
iron() complex with the deprotonated acetyl-uns-penp as
ligand was our hope that this complex might be an excellent
functional model for intradiol catechol dioxygenases. These
mononuclear nonheme iron enzymes catalyse the oxidative
cleavage of catechol derivatives by insertion of both atoms
of dioxygen into the substrate−a key step in the degradation
of aromatic compounds.[4,37,38]
Figure 2. Thermal ellipsoid plot of the dinuclear complex in 1 (50%
probability; hydrogen atoms omitted for clarity).
Since Funabiki et al. reported the first functional models
for catechol dioxygenases, increased efforts have been made
adopts a distorted octahedron coordination geometry by a by bioinorganic chemists to mimic the structure and func-
N4O2 donor set, in which two pyridine nitrogen atoms (N2 tion of these enzymes.[4,37–42] Besides macrocyclic li-
and N3), one carboxamido nitrogen atom (N4) and a car- gands,[43–45] especially tetradentate tripodal ligands have
boxamide oxygen atom (O2A) from the second acetyl-uns- shown considerable abilities to regulate the properties of
penp ligand reside in the equatorial plane while oxo-bridged model complexes, indicating that dioxygenase activity
O(1) and the tertiary amino N(1) atoms occupy the axial strongly depends on the nature of the ligand. The most ef-
positions. The negative charge of the µ-oxo-bridge leads to fective biomimetic catalyst to date is the iron() tmpa (tpa)
a rather short Fe(1)–O(1) bond [1.792(2) Å] and likewise complex that was first reported by Que and co-workers.[4,46]
because of the trans effect, the opposing Fe(1)–N(1) bond Furthermore, several other systems with tripodal N4 donor
is weakened with a distance of 2.245(2) Å. All cis angles ligands showed considerable catechol dioxygenase ac-
around O(1) are larger than the ideal 90°, with values of tivity.[47,48] Complexes with enzyme-analogous N2O2 donor
98.35(5)° for O(1)–Fe(1)–O(2A), 102.11(6)° for O(1)–Fe(1)– sets represent good structural and spectroscopic model
N(2), 96.96(5)° for O(1)–Fe(1)–N(3) and 103.62(6)° for compounds, however, they are poor functional models so
O(1)–Fe(1)–N(4). Obviously, deprotonation of the carbox- far.[49–56] Our recent efforts to reach higher activities than
Eur. J. Inorg. Chem. 2006, 1601–1610
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S. Schindler, B. Krebs et al.
FULL PAPER
the iron() tmpa complex by increasing or decreasing the
chelate ring sizes in this system were unsuccessful.[57]
The substrate-binding process in the reaction cycle of the
catechol cleavage involves protonation of two ligands at the
active site, Tyr447 and a hydroxide.[58,59] These two proton
acceptors dissociate from the metal ion and thereby enable
the proton donor molecule, the catechol, to coordinate in
its dianionic form (see Scheme 2).
Figure 3. Thermal ellipsoid plot of the iron() complex in 2 (50%
probability; hydrogen atoms omitted for clarity).
The iron() core is ligated by three nitrogen atoms, two
catecholate oxygen atoms and one bromide atom. The long-
est bonds formed by iron and its donor atoms are found for
Fe(1)–Br(1) and Fe(1)–N(1) with 2.4813(9) and 2.266(4) Å,
respectively. This causes stretching of the coordination oc-
tahedron along the Br(1)–Fe(1)–N(1) axis. The cis angles
around the bromide ion are widened to an average angle of
96.2°. On the other hand, the formation of five-membered
chelate rings leads to small values for the N(1)–Fe(1)–N(2)
and N(1)–Fe(1)–N(3) angles of 76.2(2) and 76.8(2)°, respec-
tively.
Scheme 2. Proposed substrate-binding process in catechol-1,2-di-
oxygenases.
The special electronic properties of the leaving Tyr447
group have been studied extensively using spectroscopic
methods and seem to influence the reactivity of the enzyme
to a large extent.[60] In previous work some of us used a
tmpa-derived ligand [(6-bromo-2-pyridyl)methyl]bis[(2-pyr-
idyl)methyl]amine (brtpa) to mimic the weak bonding of
one of the donor groups, however, again it was observed
that catechol dioxygenase reactivity decreased compared to
[{Fe(acetyl-uns-penp)(tcc)}2O]·(C2H5)2O·CH3OH (3)
Interestingly, applying the same experimental conditions
the iron tmpa system, most likely because of steric hin- as for the synthesis of 2, though using a slightly larger
drance.[61]
amount of base, the dinuclear oxo-bridged complex
Replacing one pyridyl moiety of the tmpa ligand with a [{Fe(acetyl-uns-penp)(tcc)}2O]·(C2H5)2O·CH3OH (3) was
deprotonated carboxamide function we had hoped to fi- obtained (the syntheses of 2 and 3 could be reproduced ap-
nally increase reaction rates of the oxidation of catecholates plying these conditions). The molecular structure of the cat-
compared to the tmpa system. However, as discussed below ion of 3 is shown in Figure 4 (crystallographic data are pre-
we did not reach this goal and furthermore, in contrast to sented in Table 2 and Table 1).
the synthesis of complex 1, we did not succeed in the prepa-
ration of deprotonated amide complexes when using acetyl-
uns-penp, catecholates and base.
[Fe(acetyl-uns-penp)(tcc)Br]·(C2H5)2O (2)
When iron() bromide, acetyl-uns-penp, tetrachlorocate-
chol and triethylamine were mixed in acetone the complex
[Fe(acetyl-uns-penp)(tcc)Br]·(C2H5)2O (2) was obtained.
Slow diffusion of diethyl ether into the complex solutions
allowed the precipitation of single crystals that were suit-
able for X-ray diffraction studies. Four complex molecules
and four diethyl ether molecules form the unit cell of com-
pound 2. The diethyl ether molecules are attached to the
complex via hydrogen bonding to the noncoordinating car-
boxamide function of the ligand. The structure of 2 is de-
picted in Figure 3 (the solvate molecule has been omitted
Figure 4. Thermal ellipsoid plot of the dinuclear complex in 3 (50%
probability; hydrogen atoms omitted for clarity).
for clarity; crystallographic data are presented in Table 2
and Table 1).
The unit cell contains two complexes as well as four di-
ethyl ether and two disordered methanol molecules. Since
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Iron() Complexes with NЈ,NЈ-Bis[(2-pyridyl)methyl]ethylenediamine Ligands
FULL PAPER
the bridging oxygen atom O(4) is located on a symmetry nal base that was added to a mixture of iron salt, the ligand
centre, one half of the dinuclear complex is generated by and the substrate 3,5-dbc. At the beginning of the titration
inversion. The metal–metal distance is 3.586 Å and both (0.0–0.5 equiv. of piperidine; piperidine was used for com-
iron ions are surrounded by an N3O3 donor set. The nitro- parison with previous studies, however triethylamine works
gen donor atoms are provided by the ligand acetyl-uns- in the same way) an absorption band at 375 nm can be ob-
penp, whereas the oxygen atoms belong to the dianionic served that is assigned to an oxo-iron()-CT transition.
tetrachlorocatecholate ligand and the µ-oxo-bridge. The co-
We suggest that the main species in solution is a µ-oxo-
ordination sphere of the iron centre has a distorted octahe- bridged dinuclear compound without coordinated sub-
dral geometry. The negative charge of the µ-oxo-bridge strate. Such dinuclear complexes are thermodynamically
leads to a rather short Fe(1)–O(4) bond (1.793 Å) and be- favoured in the presence of water and many examples have
cause of a trans effect, the opposing Fe(1)–N(1) bond is been reported in the literature.[62–72] With regard to pre-
weakened. All cis angles around O(4) are larger than the vious results it is possible that the carboxamido group of
ideal 90°. Especially those to the catecholate oxygen atoms the ligand already undergoes a very weak interaction with
are widened, since this effect can be attributed to electro- the metal centre and is not truly sp2 hybridised, however is
static repulsion. The negative charge of O(2) is reduced by not yet deprotonated.[13] Furthermore, two weak transitions
the intramolecular hydrogen bond and therefore the corre- occur at 580 and 920 nm which are typical for catecholate-
sponding O(2)–Fe(1)–O(4) angle has a value of only iron()-CT transitions and indicate the presence of a low
98.25(6)° compared to 106.29(9)° for O(3)–Fe(1)–O(4).
concentration of the desired mononuclear substrate adduct.
The most striking feature of 3 is the intramolecular hy- Upon further addition of base (1.0–1.6 equiv.) these two ab-
drogen bond between the carboxamide nitrogen N(4) and sorption bands rise dramatically and so does the concentra-
O(2) of the catecholate. The donor acceptor distance has a tion of the mononuclear substrate adduct. In the last part
typical value of 2.755(3) Å and the distance between Fe(1) of the titration (1.8–4.0 equiv.) the bands at 580 and 920 nm
and N(4) is 3.692 Å. This is in good agreement with similar disappear and two new bands are seen at 430 and 690 nm
intramolecular hydrogen bonding reported recently.[61] In a that are also assigned to catecholate-iron()-CT transitions.
way this hydrogen bonding indicates a possible pathway for The shift to higher energies indicates a higher electron den-
the deprotonation of the catechol similar to the enzyme re- sity on the iron() core that makes the charge transfer from
action shown in Scheme 1.
the catecholate more difficult. In accordance to the crystal
structure of 3 we suggest that a dinuclear µ-oxo-bridged
substrate adduct is formed in which the large electron den-
sity of the oxo-group is partially transferred to the metal
ions and one ligand arm is detached. Finally, the amount
of base that is necessary to reach optimal reaction condi-
tions for the catechol cleavage was determined from a plot
of the absorption of the lower energy CT band vs. the
amount of base added (see Figure 6). The maximum of this
plot is located at 1.7 equiv..
Spectrophotometric Titrations
To gain a better understanding of the influence of the
base and furthermore to achieve optimised conditions for
catechol cleavage by molecular dioxygen it is necessary to
provide a high concentration of the mononuclear iron com-
plex with one coordinated catecholate dianion ([Fe(L)(3,5-
dbc)]+). To determine these ideal conditions for the catechol
cleavage experiments, spectrophotometric titrations were
performed (see Figure 5 and Exp. Sect.) as described pre-
viously for related systems.[61]
Figure 6. Absorbance vs. base equivalents plot ( λ = 980 nm).
Figure 5. Spectrophotometric titration of a solution of iron() per-
chlorate hydrate, acetyl-uns-penp and 3,5-dbc against piperidine;
solid lines: 0.0 and 0.5 equiv.; dashed lines: 1.0–1.6 equiv.; dotted
lines: 1.8–4.0 equiv..
Catechol 1,2-Dioxygenase Activity
An in situ prepared complex solution containing equi-
molar amounts of iron() perchlorate hydrate and acetyl-
This analysis allowed us to gain insight into the species uns-penp was treated with 1 equiv. of 3,5-dbc and 1.7 equiv.
distribution in solution depending on the amount of exter- of piperidine. The decrease in the lower energy LMCT band
Eur. J. Inorg. Chem. 2006, 1601–1610
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S. Schindler, B. Krebs et al.
FULL PAPER
was monitored by UV/Vis-spectroscopy. The reaction was thesis of the complexes such as for example the iron()
performed in air-saturated methanol, resulting in a more tmpa complex or the acetyl-uns-penp ligand system de-
than tenfold excess of dioxygen that ensures pseudo-first- scribed above. Addition of base can accelerate this reaction
order kinetics for the complete reaction. From the slope and therefore when base was added during the synthesis of
of the ln(absorbance) vs. time plot, the reaction rate was 4 the dinuclear oxo-bridged complex 5 was obtained in-
determined to be 0.05 –1 s–1, which is more than two or- stead. The thermal ellipsoid representation of the cation of
ders of magnitude lower than that reported for the iron- 5 is shown in Figure 8. The molecular structure is com-
tmpa system under similar reaction conditions.[46] Because posed of centrosymmetric dimeric cations with a linear Fe–
of this low reactivity no further experiments were per- O–Fe unit. Each iron centre is in a distorted octahedral
formed with this system.
environment ligated by the two pyridine nitrogen atoms, the
primary and tertiary amine nitrogen atoms, as well as the
oxygen atom which is bound to the second iron centre. The
Fe–O bond length of 1.798(1) Å is in keeping with the mean
values of 1.79(6) (with a range of 1.73–1.82 Å) for such
bond lengths in oxo-bridged iron() complexes.[75] The Fe–
Npy bonds of 2.141(2) Å and 2.147(2) Å are considerably
shorter than the Fe–Namine bonds [2.213(2) and 2.235(2) Å].
This is analogous to the structures of the respective (µ-oxo)
diiron() complexes of tmpa. The chloride ligands coordi-
nate trans to the tertiary amine nitrogen on each iron centre
and anti to each other relative to the Fe–O–Fe axis. The
Fe–Cl bond lengths of 2.3166(4) Å are slightly longer than
the values of 4 arising from steric hindrance in the dimer.
The Fe–Fe distance of 3.596 Å is typical for complexes with
singly bridged Fe–O–Fe cores, which are usually in the
range 3.4–3.6 Å, whereby the longer distances are associ-
ated with Fe–O–Fe angles that are linear or close to linear-
ity.
[Fe(uns-penp)Cl2]ClO4·CH3CN (4)
The ligand uns-penp was obtained in good yields accord-
ing to the procedures described in the literature. Mixing
uns-penp together with iron() salts in methanol afforded
a yellow material that could be recrystallised from acetoni-
trile by ether diffusion to yield crystals suitable for X-ray
structural analysis. The ORTEP representation of [Fe(uns-
penp)Cl2]+ is shown in Figure 7.
Figure 7. Thermal ellipsoid plot of the cation of 4 (50% prob-
ability; hydrogen atoms omitted for clarity).
The structure of the cation of 4 shows a distorted octahe-
dral geometry coordinated with four N atoms of the uns-
penp ligand and two chloride ions, as represented by the
trans ligand angles of 153.61(6)° for N(1)–Fe(1)–N(3),
167.56(5)° for N(2)–Fe(1)–Cl(1), and 169.15(4)° for N(4)–
Fe(1)–Cl(2). Moreover, the angle for Cl(1)–Fe(1)–Cl(2) is
100.38(2)°, which is significantly larger than 90°. As is often
the case with a tripodal ligand that forms five-membered
chelate rings, the coordination geometry is distorted toward
Figure 8. Thermal ellipsoid plot of the cation of 5 (50% prob-
ability; hydrogen atoms omitted for clarity).
the tertiary amino group (average Namine–Fe–Npy/amine
=
77.55°). The Fe–Npy bonds (av. 2.132 Å) are shorter than
the Fe–N(amine) bond (av. 2.193 Å), which are comparable
to the values of FeIII tmpa complexes.[46,73,74] Consequently
the Fe(1)–Cl(1) bond which is trans to the tertiary amino
group [2.2622(5) Å] is shorter than the Fe(1)–Cl(2) bond
trans to a primary amino nitrogen [2.3081(6) Å].
Catechol 1,2-Dioxygenase Activity of the Iron(III)-uns-penp
System
The iron()-uns-penp complex was investigated under
the same conditions used previously for the iron() tmpa
catecholate (3,5-dbc) system.[57] Stopped-flow kinetic in-
vestigations revealed again that the rate of the reaction of
the iron tmpa system is faster under these conditions, how-
[{Fe(uns-penp)Cl}2O](ClO4)2·2CH3CN (5)
It is well known that a general problem in iron() chem- ever, at least the iron-uns-penp complex was only slower
istry is the formation of oxo-bridged dimers during the syn- by a factor of 20. No further detailed kinetic studies were
1606
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Eur. J. Inorg. Chem. 2006, 1601–1610

Iron() Complexes with NЈ,NЈ-Bis[(2-pyridyl)methyl]ethylenediamine Ligands
FULL PAPER
1 could also be prepared analogously to the former synthesis except
that Et3N (101 mg, 1.0 mmol) or NaOH (40 mg, 1.0 mmol) were
used instead of NaN3.
performed on this system because of the fact that the rate
could not be increased and that no reactive intermediates
could be detected spectroscopically.
[Fe(acetyl-uns-penp)(tcc)Br]·(C2H5)2O (2): Anhydrous iron() bro-
mide (30 mg, 0.1 mmol) and acetyl-uns-penp (28 mg, 0.1 mmol)
were dissolved in acetone (7 mL). After addition of tetrachlorocate-
chol hydrate (27 mg, 0.1 mmol) and triethylamine (24 µL, 17 mg,
0.17 mmol) the reaction mixture was stirred for 10 min and filtered.
Vapor diffusion of diethyl ether into the complex solution yielded
single crystals of 2 (58 mg, 0.09 mmol, 90%), m.p. 181 °C (decom-
position). C22H20BrCl4FeN4O3 (without solvent, 666.0): calcd. C
39.7, H 3.0, N 8.4; found C 39.0, H 3.9, N 7.9.
Conclusion
Acetyl-uns-penp has not been used in coordination
chemistry so far despite its interesting ligand properties and
in contrast to related ligands (in which additional pyridine
or phenol donor groups are present) that have attracted a
lot of interest in amide chemistry recently.[18,27–33,76–78]
A
copper() complex was recently reported, where the carbox-
amido function of acetyl-uns-penp is no longer truly sp2
hybridised and the nitrogen atom undergoes a weak interac-
tion with the metal centre.[13] In our present work, we dem-
onstrated that acetyl-uns-penp is capable of influencing co-
ordination chemistry by building hydrogen bonds with hy-
drogen acceptors in the vicinity of the carboxamido func-
tion. The intramolecular hydrogen bond between one arm
of the tripodal ligand and a coordinated substrate molecule
in 3 suggests a pathway for the second substrate deproton-
ation step in the reaction cycle of intradiol cleaving catechol
dioxygenases according to Scheme 2. Taking into account
that the carboxamide can also undergo strong binding in-
teractions with a metal centre upon deprotonation, as dem-
onstrated in 1, acetyl-uns-penp is a very versatile ligand.
Furthermore, structural characterization of two iron()
complexes of the ligand uns-penp and its catechol dioxygen-
ase reactivity provided additional information on the chem-
istry of this interesting ligand.
[{Fe(acetyl-uns-penp)(tcc)}2O]·(C2H5)2O·CH3OH (3): The synthetic
procedure is identical to the preparation of 2 with the only differ-
ence being that a slightly larger amount of triethylamine (28 µL,
20 mg, 0.2 mmol) was used. Vapor diffusion of diethyl ether into
the complex solution yielded single crystals of 3 (37 mg, 0.03 mmol,
60%), m.p. 223 °C (decomposition). C44H40Cl8Fe2N8O7 (without
solvent, 1188.2): calcd. C 44.5, H 3.4, N 9.4; found C 43.8, H 3.7,
N 9.1.
[Fe(uns-penp)Cl2]·ClO4·CH3CN (4): To a solution of uns-penp
(219 mg, 0.9 mmol) in methanol (10 mL) was added a solution of
Fe(ClO4)3·xH2O (139 mg, 0.3 mmol) and FeCl3 (109 mg, 0.6 mmol)
in methanol (10 mL). The resulting brown solution was stirred for
1 h at room temperature during which time a greenish yellow solid
precipitated, which was then filtered. The precipitate was washed
with methanol and diethyl ether, and dried under vacuum. Yellow
prism crystals for crystallographic studies were obtained by vapor
diffusion of diethyl ether into the acetonitrile solution. Yield:
221 mg (ca. 50%). C16H21Cl3FeN5O4 (509.58): calcd. C 37.71, H
4.15, N 13.75; found C 37.59, H 4.13, N 13.68.
[{Fe(uns-penp)Cl}2O]·(ClO4)2·2CH3CN (5): To a methanol suspen-
sion (15 mL) of Fe(ClO4)3·xH2O (115 mg, 0.25 mmol), FeCl3
(41 mg, 0.25 mmol) and uns-penp ligand (122 mg, 0.5 mmol) was
added Et3N (51 mg, 0.5 mmol) in methanol (5 mL) whilst stirring.
After 1 h, the resulting greenish brown slurry was filtered and the
precipitate was washed with methanol and diethyl ether. Dark
brown cubic crystals suitable for X-ray diffraction analysis were
obtained by slow evaporation of an acetonitrile solution about
1 week. Yield: 200 mg (ca. 42%). C32H42Cl4Fe2N10O9 (964.26):
calcd. C 39.86, H 4.39, N 14.53; found C 39.62, H 4.24, N 14.41.
Experimental Section
Materials: All chemicals were obtained from commercial sources
and used without further purification.
Caution! The syntheses and procedures described below involve com-
pounds that contain perchlorate and azide ions, which can detonate
explosively and without warning. Although we have not encountered
any problems with the compounds used in this study, they should be
handled with extreme care.
X-ray Crystallographic Studies: Intensity data for 1 were collected
with a Siemens SMART CCD 1000 diffractometer using graphite
monochromated Mo-Kα radiation (λ = 0.71073 Å) by the ω-scan
technique. The collected reflections were corrected for absorption
effects.[79] The structure was solved by direct methods and refined
by least-squares techniques using the SHELX97 programme pack-
age.[80] Further data collection parameters are summarised in
Table 2.
Physical Measurements: UV/Vis spectroscopy was performed with
a Hewlett–Packard 8453 diode array spectrometer. Elemental
analyses were carried out with an Elementar Vario EL III analyzer
at the University of Münster.
Syntheses
Ligand Syntheses: The ligand acetyl-uns-penp as well as uns-penp
were prepared according to literature procedures.[13]
Intensity data for acetyl-uns-penp, 2 and 3 were collected with a
Bruker AXS SMART 6000 CCD diffractometer (Cu-Kα, λ =
1.54178 Å, Göbel mirror) using the ω-scan technique. The collected
reflections were corrected for absorption effects.[79] The structure
was solved by direct methods and refined by full-matrix least-
squares methods on F2.[80] Further data collection parameters are
summarised in Table 2.
[Fe2(acetyl-uns-penp)2O](ClO4)2·H2O (1): Iron() perchlorate
hexadrate (177 mg, 0.5 mmol) and acetyl-uns-penp (142 mg,
0.5 mmol) were combined in methanol (10 mL). After 10 min of
stirring, to the resulting brown solution NaN3 (49 mg, 0.75 mmol)
was added, immediately leading to a very dark red suspension. Af-
ter 2 h a brown precipitate was filtered off, washed with methanol
and diethyl ether, and dried under vacuum. Dark brown prism crys-
tals, air stable and suitable for X-ray diffraction analysis, were ob-
tained by slow evaporation of an acetonitrile solution after 1 week
(69 mg, 0.03 mmol, 30%). C16H20ClFeN4O6 (455.66): calcd. C 42.2,
H 4.4, N 12.3; found C 42.6, H 4.2, N 12.7.
Intensity data for 4 and 5 were collected at a temperature of 100 K
with a Bruker-Nonius KappaCCD diffractometer with graphite-
monochromated Mo-Kα radiation (λ = 0.71073 Å). Data were cor-
rected for Lorentz and polarization effects. Absorption effects were
corrected numerically[81] for 4 and by semi-empirical methods
Eur. J. Inorg. Chem. 2006, 1601–1610
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1607

S. Schindler, B. Krebs et al.
FULL PAPER
Table 2. Crystallographic data and experimental details.
Compound
Acetyl-uns-penp
1
2
3
4
5
Empirical formula
Mr
C16H20N4O
284.36
C32H40Cl2Fe2N8O12 C26H30BrCl4FeN4O4 C53H64Cl8Fe2N8O10 C16H21Cl3FeN5O4 C32H42Cl4Fe2N10O9
455.66
740.10
1368.42
509.58
964.26
Temperature [K]
140(2)
200(2)
100(2) K
150(2)
100(2)
100(2)
Radiation (λ [Å])
Crystal colour and shape
Cu-Kα, 1.54178
colourless, cuboid
Mo-Kα, 0.71073
brown, prism
Cu-Kα, 1.54178
red, plate
Cu-Kα, 1.54178
brown, cuboid
Mo-Kα, 0.71073
yellow, prism
Mo-Kα, 0.71073
brown, irregular
Crystal size [mm]
0.20 × 0.14 × 0.10 2.00 × 0.40 × 0.50 0.13 × 0.12 × 0.03
0.31 × 0.29 × 0.29 0.24 × 0.15 × 0.12 0.23 × 0.22 × 0.12
Crystal system
Space group
a [Å]
b [Å]
c [Å]
triclinic
P1 (No. 2)
monoclinic
C2/c (No. 15)
16.2248(17)
12.8536(13)
19.653(2)
monoclinic
P21/c (No. 14)
12.4448(5)
13.4022(6)
18.1585(8)
monoclinic
P21/n (No. 14)
10.4477(3)
20.9973(7)
13.9166(5)
monoclinic
P21/n (No 14)
11.241(1)
7.8366(6)
24.330(2)
triclinic
P1 (No 2)
¯
¯
9.3758(2)
9.6178(2)
10.2342(3)
82.807(2)
68.351(2)
62.401(2)
751.10(3)
2
8.4308(5)
11.2769(6)
11.8246(8)
71.583(4)
76.621(5)
80.422(4)
1032.4(2)
1
α [°]
β [°]
γ [°]
112.910(1)
92.758(3)
94.824(2)
91.443(7)
V [Å3]
Z
3775.2(7)
4
1.603
3025.1(2)
4
1.625
3042.1(2)
2
1.494
2142.6(3)
4
1.580
ρcalcd. [g·cm–3
]
1.257
0.652
1.551
1.024
µ [mm–1
]
0.983
9.127
7.570
1.111
F(000)
304
1880
1500
1412
1044
496
θ range [°]
Index ranges
4.66 to 71.30
–10 Յ h Յ 11
–10 Յ k Յ 11
–11 Յ l Յ 12
4369
2.09 to 28.28
–16 Յ h Յ 21
–17 Յ k Յ 16
–26 Յ l Յ 26
13663
3.56 to 71.35
–14 Յ h Յ 14
–15 Յ k Յ 16
–20 Յ l Յ 22
16266
3.82 to 71.44
–11 Յ h Յ 12
–22 Յ k Յ 24
–15 Յ l Յ 17
17174
3.14 to 27.88
–14 Յ h Յ 14
–10 Յ k Յ 10
–32 Յ l Յ 32
32406
3.30 to 27.87
–11 Յ h Յ 10
–14 Յ k Յ 14
–15 Յ l Յ 15
27493
Reflections collected
Unique reflections
Rint
2521
0.0285
4476
0.0263
5631
0.0766
5614
0.0650
5103
0.0792
4910
0.0272
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I Ͼ 2σ(I)]
2521/0/195
1.099
4476/0/266
1.040
5631/0/368
0.888
5614/0/380
0.954
5103/0/263
1.032
4910/0/260
1.065
R1 = 0.0490
wR2 = 0.1381
R1 = 0.0553
wR2 = 0.1443
0.210/–0.325
R1 = 0.0365
wR2 = 0.0954
R1 = 0.0428
wR2 = 0.1000
0.937/–0.644
R1 = 0.0492
wR2 = 0.0981
R1 = 0.0864
wR2 = 0.1082
1.458/–0.501
R1 = 0.0463
wR2 = 0.1065
R1 = 0.0683
wR2 = 0.1118
0.400/–0.692
R1 = 0.0330
wR2 = 0.0671
R1 = 0.0568
wR2 = 0.0721
0.401/–0.525
R1 = 0.0240
wR2 = 0.0580
R1 = 0.0323
wR2 = 0.0606
0.434/–0.442
R indices (all data)
Largest diff. peak/hole [e·Å–3
]
based on multiple scans[79] for 5. The structures were solved by
direct methods; full-matrix least-squares refinement was carried
out on F2 using SHELXTL NT 6.12.[82] All non-hydrogen atoms
were refined anisotropically. All hydrogen atoms were geometrically
positioned; their isotropic displacement parameters were tied to
those of their corresponding carrier atoms by a factor of 1.2 or 1.5.
the activity determinations. To avoid cleavage of the substrate, all
manipulations were carried out under argon. A sample of the 3,5-
H2dbc solution (0.1 mL) was added to the complex solution
(10 mL). The resulting mixture was titrated with piperidine and the
UV/Vis-spectra were monitored using a flow cell (1 cm).
CCDC-266956 (acetyl-uns-penp), -263645 (for 1), -266957 (for 2),
-266958 (for 3), -283894 (for 4), and -283895 (for 5) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments
We thank the Deutsche Forschungsgemeinschaft (SPP 1118 and
SFB 424) and the Fonds der Chemischen Industrie for financial
support. M. M. thanks the University of Münster and the Inter-
national Graduate College “Template Directed Chemical Synthe-
sis” for graduate fellowships.
Determination of the Catechol 1,2-Dioxygenase Activity: The cate-
chol cleaving activity of an in situ prepared complex solution was
tested using piperidine as an external base as described pre-
viously.[61] The amount of base needed to reach the highest reaction
rates was determined according to the spectrophotometric titration
method described below. To 2 mL of a 2·10–4  methanolic solution
of Fe(ClO4)3·H2O and the ligand was added a 2·10–2  (1 equiv.)
solution of 3,5-H2dbc (0.02 mL). The proper amount of base was
added to the reaction mixture from a 2·10–2  stock solution. To
limit errors, the oxidation of the complex was followed three times
by UV/Vis spectroscopy. It is important to note that during these
studies iron salts with noncoordinating ions (such as perchlorate or
triflate) were used and besides the catecholate ligand no additional
coordinating ions (such as bromide used in the crystallographic
studies) were present.
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Eur. J. Inorg. Chem. 2006, 1601–1610

Iron() Complexes with NЈ,NЈ-Bis[(2-pyridyl)methyl]ethylenediamine Ligands
FULL PAPER
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