Cooperative Interactions in the Second Coordination Sphere of Pyridazine/Pyridine Containing Polyazaheterocyclic Iron(II) Complexes Favor Protonation

Article abstract of DOI:10.1002/ejic.201800533

The new pyridazine containing iron complexes, [N,N,N′,N′-tetrakis(3-pyridazylmethyl)propylenediamine]iron(II)(PF₆)₂ (1) and [N,N′-bis(2-pyridazylmethyl)-N,N′-bis(2-pyridylmethyl)propylenediamine]iron(II) (PF₆)₂ (2) were synthesized and their reactivity towards protonation was compared to that of the analogous tetrapyridine complex [N,N,N′,N′-tetrakis(2-pyridylmethyl)propylenediamine]iron(II)(PF₆)₂ (3). The solution and solid-state structures were confirmed by NMR and X-ray crystallographic studies. For 1–3, the ligands bind in a hexadentate fashion giving similar octahedral structures with an N₆ coordination environment. Across the series, the increasing number of pyridazines has only modest effects on the spectroscopic and electrochemical properties of the metal. Nevertheless, their reactivity towards protonation is drastically different. While 2 and 3 decompose in the presence of strong acids, 1 is able to be stably protonated as a result of cooperative second sphere interactions.

Full text of DOI:10.1002/ejic.201800533

A Journal of
Accepted Article
Title: Cooperative interactions in the second coordination sphere
of pyridazine/pyridine containing polyazaheterocyclic iron(II)
complexes increase their stability towards protonation.
Authors: Ahmad Hammoud, Jean-Boris Nshimyumuremyi, Jérémie
Bourotte, Fabio Lucaccioni, Koen Robeyns, Marinela M Dîrtu,
Yann Garcia, and Michael Singleton
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800533
Link to VoR: http://dx.doi.org/10.1002/ejic.201800533

10.1002/ejic.201800533
European Journal of Inorganic Chemistry
FULL PAPER
Cooperative interactions in the second coordination sphere of
pyridazine/pyridine containing polyazaheterocyclic iron(II)
complexes favor protonation.
Ahmad Hammoud, Jean-Boris Nshimyumuremyi, Jérémie Bourotte, Fabio Lucaccioni, Koen Robeyns,
Marinela M. Dîrtu, Yann Garcia,* Michael L. Singleton*
Abstract:The new pyridazine containing iron complexes, (N,N,N’,N’-
tetrakis(3-pyridazylmethyl)propylenediamine)iron(II)(PF6)2 (1) and (N,N’-
bis(2-pyridazylmethyl)-N,N’-bis(2-pyridylmethyl)propylenediamine)iron(II)
(PF6)2 (2) were synthesized and their reactivity towards protonation was
compared to the analogous tetrapyridine complex (N,N,N’,N’-tetrakis(2-
pyridylmethyl)propylenediamine)iron(II)(PF6)2 (3). The solution and solid
state structures were confirmedby NMR and X-ray crystallographic studies.
For 1-3, the ligandsbindina hexadentate fashion giving similar octahedral
structures with an N6 coordination environment. Across the series, the
increasing number of pyridazines has only modest effects on the
spectroscopic and electrochemical properties of the metal.
Nevertheless, their reactivity tow ards protonation is drastically
different. While 2 and 3 decompose in the presence of strong acids, 1
is able to be stably protonated as a result of cooperative second
sphere interactions.
notably in higher valent metals, pyridazines typically coordinate in
a monodentate fashion through a single nitrogen.21-30 This is likely
due to the initial coordination w ith the first nitrogen decreasing the
basicity of remaining site for interaction w ith an additional metal.
Despite this potential for deactivation, examples of metal bound
pyridazines have been reported w here the basicity of the
additional endocyclic nitrogen is potentially important for reactivity.
As an example, Yang and co-w orkers10 recently described a
number of iron based catalysts for w ater oxidation w here pyridine
groups w here substituted for pyridazines in tetradentate ligands.
They proposed that the additional nitrogen could act as a proton
relay during the deprotonation steps in the catalytic cycle and
show ed that it could retain some basicity. Nevertheless, the new
complexes show ed similar reactivity for w ater oxidation as
compared to the analogous pyridine complexes. Interestingly, for
unbound pyridazines, their role in proton transfer during catalysis
has been show n to be more significant despite the basicities of
these systems being reported as similar.5-6 As few examples of
iron bound pyridazines have been reported, the synthesis of this
type of complex and the study of their reactivity could lead to a
better understanding of the factors that might alter their ability to
participate in proton transfer steps and extend their potential for
catalytic applications.
Introduction
First row transition metal complexes containing pyridazine like
moieties have attracted interest for applications in medicine,
materials development, and catalysis.1-10 In these systems, the
pyridazine ring provides a number of important effects for tuning
the properties and reactivity of the metal complex. Notably, the
presence of the second nitrogen gives additional reactive sites in
the second coordination sphere for interaction w ith other
molecules or substrates.
Relative to the analogous pyridine ligands, pyridazines have
distinctly different electronic properties that can alter the
properties of the metal site. The second nitrogen decreases its σ-
donating ability and also makes it
a
better π-acceptor.11
Additionally, for coordination, pyridazine can act as a bridging
ligand, potentially leading to the formation of more complex
multimetallic structures.12-20 How ever, for first row transition
metals, this generally requires that the pyridazine is part of a
larger ligand scaffold or macrocycle functionalized w ith additional
pendant donor sites. In the absence of this cooperation and
Figure 1. Series of polypyridyl/pyridazyl Fe (II) complexes studied.
To this end, w e designed a series of pyridazine containing iron
complexes using analogs of the ligand N,N,N’,N’-tetrakis(2-
pyridylmethyl)-1,3-propylenediamine (tptn),31-34 Figure 1, w here
four (1) or tw o (2), of the pyridine groups have been replaced by
pyridazine and compared these to the complex of the unmodif ied
tetrapyridine ligand (3). Relative to iron complexes of the related
A. Hammoud, J.-B. Nshimyumuremyi, J. Bourotte, F. Lucaccioni, K.
Robeyns, M. M. Dîrtu, Y. Garcia, M. L. Singleton
Institute of Condensed Mater and Nanosciences, Molecules, Solids
and Reactivity (IMCN/MOST)
Université catholique de Louvain
N,N,N’,N’-tetrakis(2-pyridylmethyl)-1,2-ethylenediamine
(tpen)
Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium
E-mail: m.singleton@uclouvain.be, yann.garcia@uclouvain.be
https://uclouvain.be/en/research-institutes/imcn/most/michael-
singleton.html; http://ygarcia.homestead.com/index.html
ligand, w hich have been w idely studied for both their interesting
spin-crossover behavior and as mimics
of superoxide
dismutase,31-32,35-46 the Fe(tptn) complexes have received
significantly less attention. This is in part due to their high stability,
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201800533
European Journal of Inorganic Chemistry
FULL PAPER
both tow ards oxidation as w ellas to ligand substitution by solvent,
and their stable low -spin ground state.31,47 While this stability limits
the applications of the Fe(tptn) complexes, it does provide an
ideal scaffold for studying the effects of the pyridine/pyridazine
substitutions and the reactivity of the second sphere nitrogens.
Herein w e describe the full characterization of 1, 2, and 3 by NMR,
UV-Vis, and Mӧssbauer spectroscopies, as w ell as, by cyclic
voltammetry and X-ray crystallography. Despite having similar
structural and electronic properties, studies on the protonation
behavior of the complexes show that cooperativity betw een
multiple nitrogens in the second coordination sphere lead to a
dramatic change in the acidic stability and reactivity of 1 versus
either 2 or 3.
can occur more cleanly w hich allow s for easier isolation of the
ligands.
The complexes 1, 2, and 3 w ere synthesized by addition of one
equivalent of FeCl2 to an aqueous solution of the respective ligand.
An immediate color change from light yellow to dark red is
observed upon metal addition. The complexes can then be
precipitated from solution as red-orange pow ders by addition of
KPF6. All three are air-stable over the course of several days both
as solids and in solution. They are highly soluble in MeCN, but
only moderately soluble in alcohols or w ater at room temperature.
1H NMR spectra of 1, 2, and 3, Figure 2, show relatively sharp
signals over a limited spectral range. This is consistent w ith the
diamagnetic nature of low spin iron(II) in an octahedral geometry,
a geometry that w ould be expected if the ligand binds in a
hexadentate fashion, and matches w ell w ith the magnetic
behavior previously reported for the perchlorate salt of 3.32
For all three complexes, octahedral coordination should result
in axial chirality. In the NMR, this should lead to the splitting of the
resonances for both the benzylic methylene and the -CH2 units of
the propylene linker into diastereotopic pairs. For 1 and 3, this is
observed by the presence of four doublets, integrating for two
each, betw een 4.0-5.0 ppm for the benzylic -CH2 units. The
number of signals fits w ith the fact that the homogenous ligand
sets (all pyridazine or all pyridine) for these complexes should
result in a 2-fold symmetry axis that makes both of the equatorial
aromatic rings equivalent and both of the axial rings equivalent.
The presence of this symmetry element also supports the
hexadentate binding by the ligand and the octahedral geometry of
complexes 1 and 3.
Results and Discussion
Synthesis of ligands and complexes
The pyridazine containing polyazaheterocyclic ligands w ere
synthesized in a manner similar to reported procedures for the
tetrapyridine
pyridylmethyl)propylenediamine
ligand,
N,N,N’,N’-tetrakis(2-
(L3).47-48 Alkylation of 1,3-
propylenediamine or N,N’-bis(2-pyridylmethyl)propylene diamine
w ith 3-(chloromethyl)pyridazine, Scheme 1, affords ligands L1 &
L2 in moderate yield, 50-70 %.
Scheme 1. Synthesis of complexes 1 and 2: (a) 3-(chloromethyl)pyridazine,
KBr, K2CO3, MeCN, Δ; (b) FeCl2, KPF6, H2O; (c) 3-(chloromethyl)pyridazine
hydrogen chloride, K2CO3, MeCN
Figure 2. Comparison of the 1H NMR spectra in CD3CN of complexes 1 (a), 2
(b), and 3 (c) in the regions between 3.5-5 ppm and 7-9.25 ppm.
For
these
reactions,
the
synthesis
of
the
3-
(chloromethyl)pyridazine w as initially accomplished through the
chlorination of 3-methylpyridazine using trichloroisocyanuric
acid.49 How ever, other methods offered better results. Notably,
the chlorination of pyridazin-3-ylmethanol,50 w hich is easily
synthesized from furfurylalcohol (see SI) using a minor variation
of established procedures,51-52 provides a more scalable and
consistent route to the stable 3-(chloromethyl)pyridazine
hydrochloride salt. Using the material obtained follow ing this
approach, the incorporation of the pyridazyl unit into L1 and L2
The situation is more complicated for 2. Each aliphatic nitrogen
has one pyridine and one pyridazine, making multiple
conformations possible. Using the positions of the pyridazines to
describe these and assuming octahedral geometry, the complex
could adopt a conformation w here both pyridazines are axial, both
equatorial, or one axial-one equatorial. The tw o former
conformations should both exhibit C2 symmetry similar to
complexes 1 and 3, w hile the latter w ould lack this symmetry and
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10.1002/ejic.201800533
European Journal of Inorganic Chemistry
FULL PAPER
a greater number of signals w ould be expected. The presence of
eight doublets for the benzylic protons and the doubling of the
number of aromatic signals supports the axial-equatorial
conformation as the major species for this complex. A second
species (10% by integration) can also be observed in the spectra
for 2. This has been assigned as one of the higher symmetry
conformers due to the presence of only tw o aromatic signals at
8.3 and 8.9 ppm for the C-H groups adjacent to the endocyclic
nitrogens and four doublets betw een 4.0 - 5.0 ppm for the benzylic
CH2 groups. Nevertheless, despite our best efforts, this minor
compound could not be separated from the major species and so
this assignment remains tentative.
Interestingly, despite being similar in the unbound ligands, for 1,
2, and 3 the chemical shift for the resonances for the 6-position
protons (C-H adjacent to the endocyclic nitrogens in the
pyridazine or pyridine units) vary significantly from complex to
complex. In 1, the tw o signals for this position in the pyridazines
are observed just below 9.0 ppm w ith only 0.06 ppm difference in
the shifts. In the corresponding tetrapyridine complex, one of the
signals is observed at 8.7 ppm, w hile the second appears
significantly upfield shifted at 7.9 ppm. A similar upfield shift is
observed for one of the pyridine 6-position resonances in 2, (it
appears at ~7.7 ppm) w hile the resonances for the 6-position in
the remaining three heterocycles appear betw een 8.8 and 9.2
ppm. We postulate that the observed shift is due to the positioning
of the axial pyridine 6-position C-H directly above one of the
equatorial rings, thus leading to shielding of this proton due to the
aromatic ring current effect. In the case of the axial pyridazines,
this C-H w ould be positioned further aw ay from the equatorial
rings (equivalent to the 5-position on the pyridine rings) and be
affected less by the π-system of the equatorial aromatic group.
Thus for complex 2, the large observed shift for only one of the
pyridine 6-position protons could additionally be expected for the
axial/equatorial conformation described above.
appearance of fissures on the crystal surface w ithin moments
after crystal manipulation. Transfer to a viscous oil slow ed down
this process and enabled mounting and flash cooling of the crystal.
Given the limited crystal quality, a resolution cut-off of 1Å was
imposed during integration and refinement. The structure was
determined by Molecular Replacement53 in P65, as direct methods
failed due to the initially undetected tw inning around the reciprocal
b axis, combined w ith tw inning by inversion. Possibly the reported
ClO4- analogue of 3 suffered from the same issues and although
no cell parameters w ere given, w ith the postulated unit cell
volume of ca. 7000 Å3 and the hexagonal system a similar
desolvated structure might be hypothesized, likew ise the
theorized space group P6522/P6122 might be the higher metric
symmetry often encountered w ith tw inning.32
Refinement converged w ithout distance and angle restraints for
the Fe complex 3, similar restraints w ere used for the disordered
PF6 anions. Both w ere found to be disordered, one show ed tw o-
fold disorder in an almost 50/50 ratio. The other w as refined over
3 parts w ith a 50/30/20 ratio. Refined tw in fractions (4 domains)
indicate a 48% contribution of the main domain w ith a 40%
contribution of the inverted rotation around the reciprocal b axis.
The remaining tw o domains have a combined contribution of
about 10%. Diffuse solvent contribution to the diffraction
intensities w as treated by the squeeze procedure in Platon.54
Crystallographic and refinement details are tabulated in the
supporting information.
The crystal structures of 1, 2, and 3, Figure 3, each show a
single iron center bound in a hexadentate fashion by their
respective ligands. The metal centers exhibit
a distorted
octahedral geometry w ith N6 coordination environment
a
consisting of the tw o aliphatic amines of the propylene diamine
linker and four endocyclic nitrogens provided by the pendant
pyridine or pyridazine units. Relevant structural metrics are given
in Table 1. To compare the overall geometry of the three
complexes the RMS deviations betw een each pair w ere
calculated, revealing that all three complexes are highly similar
w ith calculated values of 0.12 Å2 betw een 1 and 2 and 0.21 Å2
betw een 1 and 3 (and betw een 2 and 3).
The reported Fe complexes show pseudo C2-symmetry, w ith
the 2-fold axis passing though the equatorial plane (See Figure 3).
In 2, the mixed pyridine/pyridazine complex, the 2-fold symmetry
is broken by a cis-equatorial configuration of the pyridine and
pyridazine moieties, but otherw ise maintains the global
appearance throughout the three complexes. The Fe-Npropylene
distances in the complexes range betw een 1.89-2.04 Å, w hich is
on the short end of the distance range found in the CSD for Fe –
tertiary amine distances (< 2.215 Å > ). Direct comparison
betw een the Fe-pyridine or Fe-pyridazine bond distances based
on complexes 1 and 3 is complicated due to the disparity in values
observed for the four pyridine groups in 3. Indeed, w hile the Fe-
Nheterocycle distances in 1 only range betw een 1.945-1.959 Ÿ in 3
the Fe-pyridine distances display a much larger variation, 1.907-
2.005 Å. How ever, in the mixed pyridine/pyridazine containing 2,
the Fe-Npyridazine bonds appear to be moderately shorter (1.951
and 1.944 Å) than the Fe-Npyridine distances (1.961 and 1.985 Å).
Nevertheless, for all of the rings, these values remain w ell w ithin
the range of the analogous tpen or polypyridyl Fe complexes and
Solid state structures
Further support for the structures of each complex w ere
obtained from X-ray diffraction studies. Single crystals of X-ray
quality for 1 and 2 w ere obtained by a three layer approach.
Hexane w as first layered onto solutions of the complexes in
MeCN, follow ed by addition of toluene on top of the hexane. After
diffusion of the toluene into the MeCN crystals w ere obtained. For
complex 3, crystals w ere grow n by slow diffusion of an MeCN
solution of 3 into CHCl3. Complexes 1 and 2, crystalize in the
monoclinic space groups P21/c and P21/n respectively.
1
crystalizes w ith tw o molecules of MeCN per complex (Z=4) in the
unit cell and 2 crystallizes as the monosolvated MeCN species.
While the perchlorate salt of 3 has been reported,32 attempts at
obtaining an X-ray structure of this complex have been previously
unsuccessful and only limited crystallographic details have been
reported. In the case of the PF6 salt reported herein, structural
refinement w as also complicated due to the instability of the
crystals outside the crystallization solution. The final structure
show s very large cavities ca. 40% of the total cell volume, present
as open column running along the c-axis. It is believed that the
volatile MeCN emerges almost instantly from the crystal w hen
removed from the mother liquid, this is further evidenced by the
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10.1002/ejic.201800533
European Journal of Inorganic Chemistry
FULL PAPER
are similar to values reported for Fe-N distances in other low -spin
Fe(II) complexes.31-47
2.935, 3.066 and 3.233 Å from each other for 1, 2, and 3
respectively. In 1, this separation betw een the unbound nitrogens
Table 1. Comparison of selected structural metrics for complexes 1, 2,
and 3.
Distance (Å)
1
2
3
Fe-N2
Fe-N31
Fe-N24
Fe-N9
Fe-N17
2.038(1)
1.945(1)
1.946(1)
1.959(1)
2.013(2)
1.948(1)
2.044(2)
1.961(2)
1.951(2)
1.944(2)
2.030(2)
1.985(2)
1.890(15)
1.907(13)
1.926(15)
1.984(8)
1.991(13)
2.005(19)
Fe-N16
Angle (º)
N31-Fe-N24
N31-Fe-N16
N31-Fe-N2
N31-Fe-N17
N31-Fe-N9
N24-Fe-N9
N24-Fe-N16
N24-Fe-N17
N16-Fe-N9
N16-Fe-N2
N9-Fe-N17
N9-Fe-N2
N17-Fe-N2
90.70(6)
96.18(6)
92.12(6)
84.18(6)
175.68(6)
93.35(6)
100.70(6)
81.22(6)
84.55(6)
82.98(6)
94.96(6)
83.74(6)
95.08(6)
88.92(6)
94.55(6)
93.25(6)
84.41(6)
177.18(6)
93.85(6)
101.67(7)
81.29(7)
85.42(6)
83.10(6)
95.47(6)
83.94(6)
93.98(6)
87.5(6)
99.6(6)
92.8(6)
82.9(6)
174.3(5)
93.7(5)
105.0(7)
79.4(6)
85.4(5)
85.5(8)
91.9(5)
85.1(5)
90.1(7)
Dihedral Angle (°)a
Equatorial Aromatic Rings
Axial Aromatic Rings
34.2
79.0
28.3
89.8
41.8
81.6
[a] Defined as the angle between the planes of the equatorial or axial
aromatic rings
in the pyridazine rings could potentially allow them to act in a
bidentate or cooperative fashion. How ever, the dihedral angle for
these rings is significantly deviated from the coplanar
conformation ideal for this purpose. We observe across the series
of complexes that this angle (measured as the angle betw een the
planes of equatorial aromatic rings) increases from 28.3 º in 2 to
34.2 º in 1 and ~41 º in 3. The larger angle in 3 likely results from
the minimization of the steric interaction betw een the 6-position
C-H groups, but may additionally be influenced by a possible C-
H--F interaction betw een the PF6- counter ion and the 6-position
C-H of one of the equatorial pyridines based on the observed
short F-C distance of 1.997 Å.
Figure 3. Thermal ellipsoid plots (50%) of 1 (a), 2 (b), and 3 (c). Numbering for
UV-vis spectra and Mӧssbauer Analysis
the three complexes corresponds to the numbering in Table 1. Hydrogen atoms
and PF6 counter ions have been omitted for clarity.
-
The UV-vis spectra for complexes 1-3, recorded in MeCN, are
show n in Figure 4. Wavelength and molar absorptivity data are
given in Table 2. The spectra of 3 is in good agreement w ith the
previously reported UV-vis studies of its perchlorate salt.33 Below
300 nm the spectra of all three complexes are similar, show ing
tw o absorptions, one betw een 205-209 nm and another betw een
242-252 nm. Based on the low w avelengths and high molar
absorptivity values, these are assigned to π-π*transitions in the
ligands.55 While these remain largely similar across the series, the
nature of the heterocycles on the ligand has a stronger influence
on the absorptions at higher w avelengths.
In fact, the only major differences from the tpen derivatives are
observed in the angles around the iron center. The propylene
linker used in 1-3 results in a greater bite angle for the aliphatic
nitrogens to iron (~90 – 95 º) as compared to derivatives w ith the
ethylene linker (<86 º). This relaxed angle also leads to a ~10 º
decrease in the bite angle for the equatorial heterocyclic ligands
to iron. As a result, the distance betw een these aromatic units also
decreases, placing the ortho N or C of the equatorial rings at
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10.1002/ejic.201800533
European Journal of Inorganic Chemistry
FULL PAPER
Despite being w eaker σ-donors than pyridine, pyridazines are
typically better π-acceptors. As a result, their incorporation into
the ligands has a significant impact on the observed MLCT bands.
The tetrapyridine complex, 3, show s an absorption maxima at 421
nm w hich has been previously assigned as an MLCT band.33 This
absorption is bathochromically shifted in the tetrapyridazine
containing 1, appearing at 449 nm, and has a molar absorptivity
significantly higher than that observed for 3. Given the analogous
nature of complexes 1 and 3, this absorption has also been
assigned as an MLCT band. For 2, tw o different absorption
maxima are observed, one at 384 nm and the other at 458 nm,
w ith similar molar absorptivities as 3. These bands overlap w ell
w ith the MLCT absorptions from 1 and 3, and are assigned as, for
384 nm, the MLCT from iron to pyridine and, for 458 nm, from iron
to pyridazine. No other absorptions w ere observed above 600 nm.
Figure 4. Comparison of the UV-v s spectra in CD3CN of complexes 1 (a), 2 (b),
and 3 (c). Concentration = 0.2 mM.
Figure 5. 57Fe Mössbauer spectra for 1 (a), 2 (b), and 3 (c) recorded at 77 K.
Mӧssbauer spectra of complexes 1-3, Figure 5, w ith natural
abundance 57Fe w ere collected at 77 K. The Mӧssbauer
parameters are given in Table 2. All three complexes can be fitted
to a single quadrupole doublet w ith parameters consistent w ith
low -spin iron(II), as expected by the solution NMR studies and the
magnitude of molar absorption observed in the UV. The isomer
shift, δ, is slightly low er for 1 w ith δ = 0.27 mm/s compared to the
one of 2 and 3, w hile the quadrupole splitting, ΔEQ, is identical for
1 and 2, compared to 3. This low er quadrupole splitting for 3
points to a low er distortion degree of the iron(II) coordination
sphere because of the absence of any electric field gradient
contribution to ΔEQ in the low -spin state.
Table 2. 57Fe Mössbauer parameters for complexes 1-3
Complex
1
2
3
T [K]
77
77
77
δ [mm/s]
ΔEQ [mm/s]
Γ/2 [mm/s]
Relative Area [%]
Sites
0.27(2)
0.30(2)
0.16(1)
100
0.33(1)
0.29(1)
0.15(1)
100
0.33(1)
0.22(2)
0.20(1)
100
LS
LS
LS
δ: isomer shift (with respect to α-Fe at r.t.); ΔEQ: quadrupole splitting;
Γ/2: half width at half maximum; LS : low spin
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10.1002/ejic.201800533
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Cyclic Voltammetry
The presence of the pyridazine units also has a moderate effect
on the electrochemical properties of the complexes. The
potentials for the oxidations and reductions (vs FcH/FcH+) are
show n in Table 3. The voltammograms of 1-3, Figure S1, each
show a reversible oxidation event, Figure 6, and one or more
irreversible reductions. For 1, the reductions are observed at -
2.25 V and -1.94 V, w hile complexes 2 and 3 each only show a
single event, at -1.99 V and -2.23 V respectively w ithin the
electrochemical w indow .33
A single reversible oxidation event is also observed for 1-3 and
is assigned to the FeII/III couple.33 Relative to complex 3, w hich has
an oxidation at 0.44 V, the oxidation in 1 and 2 occur at more
positive potentials (0.56 V and 0.52 V respectively). Given the
similar structures of 1-3, the positive shift observed for both the
oxidation and reduction potentials in 1 and 2 should thus only be
due to the increased number of pyridazines. This shift does not
appear to be linearly correlated w ith the number of pyridazines as
the exchange of tw o pyridines for pyridazines in 2 results in a
change of ~80 mV but the additional substitution of tw o pyridines
(2 to 1) only gives a 40 mV increase in the oxidation potential. Still,
the overall change is consistent w ith the metal being less electron
rich as the number of pyridazines increases.
Protonation Behavior
To study the protonation behavior of Fe(II) bound pyridazines,
w e initially performed a UV-vis titration of 1-3 w ith trifluoroacetic
acid. We w ere surprised to observe that the second sphere
nitrogens in these complexes are relatively inert. In fact, 1 shows
little to no change by Uv-Vis even upon addition of 100
equivalents of trifluoroacetic acid. Relative to the literature
examples w here bound counter-ions likely limit the electrostatic
destabilization of the protonated species,10 it is proposed that in
1, the positive charge of the complex additionally disfavors the
protonation w ith trifluoroacetic acid. This effect should be able to
be countered by using a stronger acid. Indeed, 1 could be
protonated w hen triflic acid w as used for the titrations.
Figure 7. Uv -vis spectra of the titration of 1 (0.2 mM) with 0 - 2 equivalents of
TfOH in acetonitrile.
Addition of one equivalent of triflic acid to 1 in MeCN leads to a
noticeable color change from red to dark purple. In the UV-vis
spectra, Figure 7, the intensity of the MLCT band at 449 nm
decreases w ith added triflic acid, w hile tw o new bands appear at
415 and 564 nm. This large change for the MLCT bands could be
expected if protonation occurs at the pyridazine nitrogens .
Isosbestic points, observed at 421 and 515 nm, are consistent
w ith conversion of 1 into a single new species. This new species
Figure 6. Cyclic voltammograms showing the reversible oxidation for
complexes 1, 2, and 3. [1] = [2] = [3] =0.001 M in MeCN with 100 mM TBABF4.
All values are reported versus FcH/FcH+.
Table 3. Electrochemical data for complexes 1, 2, and 3.
appears to be moderately stable to additional acid and
a
Potential vs FcH/FcH+ (V)
significant decrease in the intensity of the new bands
corresponding to ligand dissociation is only observed after
Complex
Ered
Ered2
Eox 1/2
0.56
0.52
0.44
1
addition of multiple equivalents of the acid. By H NMR, Figure 8,
as the amount of acid is increased, the signals corresponding to
the initial product begin to broaden and shift but remain a single
set of signals. This w ould fit w ith protonation w here the proton
transfer equilibrium betw een the complexes is fast on the NMR
timescale. With additional acid, the broad resonances quickly
sharpen and a new set of signals appear. Based on the number
of signals and the narrow peak w idths observed, the new species
1
2
3
-1.94
-1.99
-2.23
-2.25
Solvent = MeCN; [Fe] = 1mM; [TBABF4] = 100mM; Scan rate = 100 mV/s
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10.1002/ejic.201800533
European Journal of Inorganic Chemistry
FULL PAPER
is structurally similar to 1 and retains the diamagnetic character of
the parent complex. Notably, the presence of four doublets
corresponding to the benzylic -CH2 groups is good evidence that
the C2 symmetry of 1 is still present in the new species and that
dissociation of one of the pyridazine rings has not occurred. The
largest shifts observed occur in the aromatic region w here it
appears that the resonances corresponding to one pair of the
pyridazines shifts dow nfield by more than 0.5 ppm, w hile only
minor shifts occur for the resonances corresponding to the other
pair. Importantly, these spectral changes are fully reversible. The
new species can be converted back into 1 by addition of
triethylamine, (See SI, Figure S2), further supporting the fact that
the changes to the complex are due to protonation. Based on the
above observations, it is proposed that this protonation occurs at
an equatorial pyridazine and is likely stabilized through interaction
w ith the second nitrogen of the neighboring equatorial pyridazine,
Scheme 2A. Further support for protonation in this position and
the need for the additional interaction to stabilize the protonated
species comes from comparison to the reactivity of the tw o other
complexes.
broad signals indicating fast exchange are observed and that the
second set of signals corresponding to the dissociated ligands
begin to appear even w ith sub-stoichiometric amounts of the acid.
While this reactivity is unsurprising for 3 as no additional basic
sites are present in the ligand, if protonation in 1 w as solely due
to the presence of the additional nitrogens, the pyridazine rings in
2 should have allow ed protonation of the complex prior to
degradation. We postulate that the dissimilarity in reactivity of 1
compared to 2 is due to the difference in the second coordination
sphere resulting from the axial-equatorial conformation of the
pyridazine rings in 2. Looking at the potential protonation sites in
these molecules, Scheme 2, there are tw o possibilities for the
protonation of the pyridazines. Either an equatorial pyridazine or
an axial pyridazine can be protonated. Based on the presence of
the C-H groups in similar positions in 3, protonation of the
pyridazine group in either position should not be prohibited due to
steric factors. If the protonation w ere to occur in the axial position,
both complexes 1 and 2 should show similar reactivity tow ards
protonation since the resulting structural motif w ould be identical
i.e.
a protonated pyridazine oriented above an equatorial
pyridazine ring, Scheme 2C&D. By contrast, equatorial pyridazine
protonation w ould lead to the proton being in tw o different local
environments in 1 versus 2, Scheme 2A&B. In 1 the proton w ould
be oriented tow ards the nearby nitrogen of the neighboring
pyridazine. The distance betw een the tw o second sphere
endocyclic nitrogens (2.9 Å) and the identical pKa of these sites
should allow for moderate strength hydrogen bond formation. As
such the tw o nitrogens can act in a cooperative fashion to stabilize
the proton. In 2, no such interaction is possible. Protonation of the
equatorial pyridazine w ould orient the proton tow ards the 6-
position C-H of the neighboring equatorial pyridine w hich w ould
not provide any stabilizing interaction. The lack of any such
interaction combined w ith the w eak basicity of the pyridazine
nitrogens is likely w hat prevents 2 from being stably protonated.
These results highlight the potential of multiple cooperative
interactions in the second coordination sphere of metal
complexes for controlling and modifying reactivity.
Figure 8. 1H NMR spectra of the titration of (a) 1 (2 mM) with (b) 0.2; (c) 0.4; (d)
0.6; (e) 0.8 and (f) 1.0 equivalents of TfOH in CD3CN.
In contrast to 1, neither complex 2 nor 3 show evidence of a
stable protonated species. Rather, even w ith less than one
equivalent of acid, ligand dissociation occurs as evidenced by
both UV-vis and 1H NMR studies, Figures S3-S6. Upon addition
of triflic acid to complexes 2 and 3, a gradual loss of color is
observed. In the UV-vis spectra, Figures S2 and S3, this coincides
w ith a decrease and eventual disappearance of the MLCT bands,
w hile the higher energy bands assigned to the π-π*transitions of
1
the ligand show only minor differences. In the H NMR spectra of
both complexes, Figures S4 and S5, the addition of one
equivalent of acid leads to complete loss of the diastereotopicity
observed for the benzylic signals. For 3, these become a single
signal at 4.25 ppm, w hile for 2, tw o different resonances for the
benzylic positions are observed at 4.6 and 4.7 ppm consistent
w ith the presence of the tw o types of pendent heterocycles in the
ligand. These changes w ould be expected upon dissociation of
the ligand. It is also important to note that for these complexes no
Scheme 2. Comparison between potential protonation sites in 1 versus 2 with
potential cooperative interactions (shown in red) that may stabilize protonation.
Conclusions
A series of polyazaheterocycle containing mononuclear iron
complexes, including tw o novel pyridazine containing complexes,
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10.1002/ejic.201800533
European Journal of Inorganic Chemistry
FULL PAPER
w ere synthesized and characterized by NMR, UV-vis and
Mossbauer spectroscopies, as w ell as by X-ray diffraction and
electrochemical studies. Both solution and solid state studies
indicate that all three compounds have similar octahedral
structures. Across the series, the increasing number of
pyridazines has only modest effects on the spectroscopic and
electrochemical properties of the metal. Nevertheless, despite the
similarities across the complexes, their reactivity tow ards
protonation is drastically different. While addition of a strong acid
Louvain. Mössbauer spectra were recorded in transmission geometry with
a
conventional Mössbauer spectrometer equipped with a 57Co(Rh)
radioactive source operating at room temperature. The samples were
sealed in aluminum foil andmounted on a nitrogen Oxford bath cryostat.
The spectra were fitted to the sum of Lorentzians by a least-squares
refinement usingor Recoil 1.05Mössbauer AnalysisSoftware.57 All isomer
shifts refer to α-Fe at room temperature. Absence of any oxideimpurities
was checked by high velocity transmission measurements (vmax = 12
mm/s). Electrochemical data were recorded on a Metroohm Autolab
potentiostat. The experimentswere carried out in a three-electrode glass
cell with a glassy carbon working electrode (d = 3mm), a platinum wire
counter electrode, and a Ag/AgNO3(0.01 M in MeCN with 0.1 M Bu4NBF4)
reference electrode. Voltammogramswere recorded in MeCN with 0.1 M
tetrabutylammonium tetrafluoroborate. All potentials are reported as
versus the Fc/Fc+ couple.
to
both
the
tetrapyridine
containing
3
and
the
bispyridazine/bispyridine containing 2 leads only to degradation
of the complex due to ligand dissociation, the tetrapyridazine
containing 1 is able to be stably protonated. Based on the
spectroscopic studies, the protonation of
1 occurs in the
equatorial position and is likely stabilized by cooperative
interactions betw een the second coordination sphere nitrogens.
Given the interest that pyridazines have attracted as pendant
bases or proton relays in catalysis5,6,10 the above observations
may offer some insight into the design of new systems. While the
pyridazines can, in some cases, retain moderate basicity even
upon binding to a metal center, our results demonstrate that this
is certainly not alw ays the case. The basicity of these systems, as
w ellas their stability in acidic media, has a strong dependence on
the specific structure and charge of the corresponding complex.
This has the consequence of potentially limiting the range of
applicability of metal bound pyridazines, or similar groups w ith
exogenous basic sites, for catalytic purposes. Nevertheless, the
current study also show s that by taking into account other groups
in the second coordination sphere and designing new systems
w ith additional cooperative interactions, both the stability of the
metal complex and the basicity of the groups may still be able to
be fine-tuned to allow the additional nitrogens to have some
reactivity. For complexes w here protonation is more favored, this
could significantly augment the ability of these groups to
participate in proton transfer steps during catalysis.
X-ray crystallography: All diffraction data were recorded on a MAR345
image plate using MoKα radiation generated by a Rigaku UltraX 18S
rotating anode (XenocsFox3d mirrors). Prior to data collection the crystals
were flash frozen to 150K. Data integration andreduction wasperformed
by CrysAlisPR,58 and the implementedabsorption correction wasapplied.
The structures were solved by SHELXT59 and refined against F2 by
SHELXL-2014/7.60 All non-hydrogen bonds were refined anisotropically,
hydrogen atoms were added in calculated positions and refined in riding
mode, with temperature factors1.2 timeshigher thantheir parent atoms.
N,N,N’,N’-tetrakis(3-pyridazylmethyl)-1,3-propylenediamine (L1)
A
solution of 3-(chloromethyl)pyridazine (1 eq., 1.75 g, 13.60 mmol) in
CH3CN (20 mL) wasadded to a solutionof 1.,3-diaminopropane(0.17eq.,
194 µL, 2.33 mmol), K2CO3 (2 eq., 3.22g, 23.33 mmol) and KBr (1.5 eq.,
2.08 g, 17.49 mmol) inCH3CN (100 mL). Thereactionmixture washeated
at 40°C with an oil bath. Theconversionof the reaction wasmonitored by
1H NMR using dmso-d6 as solvent. Once the completeconversion of the
1,3-diaminopropane was observed (after ~ 7 days), the reaction was
cooled to room temperature and filtered on a celite padand the solidwas
washed twice with acetonitrile (2*100 mL). The solvent was evaporated
under reduce pressure. The crudeproduct waspurifiedover basic alumina
using 97% DCM/ 3% methanol asthe eluent. Theligand wasprecipitated
by slow addition of cyclohexaneto itssolutionin DCM to give0.55g of L1
(53%). 1H NMR (300 MHz, CDCl3) δ 9.07 (dd, J = 4.9, 1.8 Hz, 4H), 7.56
(dd, J = 8.4, 1.8 Hz, 4H), 7.45 (dd, J = 8.4, 4.9 Hz, 4H), 3.95 (s, 8H), 2.58
(t, J = 7.1 Hz, 4H), 1.81 (p, J = 7.4 Hz, 2H). 13C NMR (75 MHz, CDCl3) δ
161.22, 150.74, 127.08, 126.87, 58.03,52.21, 24.86.
Experimental Section
General Procedures. Unless otherwise stated all solvents and reagents
were used without further purification. When required, dry solvents were
obtained directly prior to use by distillation from CaH2 (CHCl3, toluene,
DCM) or from sodium/benzophenone (THF, Et2O). Anhydrousacetonitrile
was
purchased
from Sigma or
Alfa Aesar.
N,N’-bis(2-
pyridylmethyl)propylenediamine56
and N,N,N’,N’-tetrakis(2-
pyridylmethyl)propylenediamine48 (L3) were synthesized according to
reported procedures. NMR spectra were recorded on a Bruker 300MHz
UltrashieldTM. 1H and 13C NMR chemical shiftsare reported in ppm andare
calibrated against residual 1H and 13C solvent signals of CDCl3, CD3CN,
or DMSO. Couplingconstantsare reported inhertz. The followingnotation
is used for the 1H NMR spectral splittingpatterns: singlet(s), broad singlet
(bs), doublet (d), triplet (t), multiplet (m). UV-visible experiments were
performed on a VWR UV-1600PC Scanning Spectrophotometer equipped
with Deuterium/Tungsten HalogenLamp. HR MassSpectra were recorded
by the Mass Spectrometry service of the ASM platform of the Institute of
Condensed Matter and Nanosciences at the Université catholique de
N,N’-bis(3-pyridazylmethyl)-N,N’-bis(2-
pyridylmethyl)propylenediamine
(L2) 3-(Chloro-methyl)pyradizine
hydrochloride (3.1 g, 0.0189 mole) was added to a mixture of N,N-bis(2-
pyridylmethyl)propylene diamine (2.5 g, 0.009 mole) and K2CO3 (7.4 g,
0.054 mole) in dry acetonitrile (150 mL) at room temperature. The mixture
was heated to reflux andallowed to stir for 24 h under argon. The reaction
was then cooled down to room temperature, and the solidwasfiltered off
and the solvent removed by rotary evaporation. The crude product was
purified over basic alumina using (97% DCM/ 3% methanol) to yield 2.64
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10.1002/ejic.201800533
European Journal of Inorganic Chemistry
FULL PAPER
g (66 %) of L2. 1H NMR (300 MHz, Chloroform-d) δ 9.01 (dd, J = 4.9, 1.8
Hz, 2H), 8.48 (ddd, J = 4.9, 1.8, 0.9Hz, 2H), 7.63 – 7.53(m, 4H), 7.37(dd,
J = 8.4, 4.9 Hz, 2H), 7.30 (dt, J = 7.8, 1.1 Hz, 2H), 7.11 (ddd, J = 7.5, 4.9,
1.2 Hz, 2H), 3.91 (s, 4H), 3.73 (s, 4H), 2.59 – 2.47 (m, 4H), 1.85– 1.70(m,
2H). 13C NMR (75 MHz, CDCl3) δ 162.03, 159.19, 150.54, 149.22, 136.53,
126.73, 126.68, 123.07, 122.19, 60.41,58.23, 52.42, 24.86.
Theoretical: 247.09135; Experimental: 247.09111. UV-vis (MeCN) max
(nm) [ (M-1,cm-1)] 205 [14996], 249 [10793], 384[5712], 458 [8601]. Anal.
Calc’d. for C25H26F12FeN8P2 (784.30 g.mol-1): C, 38.19; H, 3.59; N, 14.24.
Found: C, 37.89; H, 3.87; N, 13.98%.
(N,N,N’,N’-tetrakis(2-pyridylmethyl)propylenediamine)iron(II)
hexafluorophosphate (3) To a solutionof L3(0.50g, 1.1 mmol) in distilled
water (5.0 ml), was added FeCl2 (0.144 g, 1.1mmol) resulting in an
immediate color change from yellow to red. The solution was allowed to
stir at 80 ºC for 20 minutesafter which timesolidKPF6 (0.420 g, 2.3 mmol)
was added and the mixture wasstirred for an additional 10 minutesat 80
ºC. The flaskwas then allowed to cool to room temperature, during which
time a red solid precipitated from the solution. Thesolid wascollected by
filtration to give complex 3 (0.73 g, 82%) asa red-orange microcrystalline
solid. The metal complex was crystalized by layering acetonitrile onto a
chloroform solutionof 3. 1H NMR (300 MHz, CD3CN) δ 8.66 (d, J = 5.3 Hz,
2H), 7.92 (d, J = 5.3 Hz, 4H), 7.88 (t, J = 7.6 Hz, 4H), 7.71-7.65 (m, 4H),
7.34 (t, J = 6.4 Hz, 2H), 7.25-7.20 (m, 4H), 4.79 (d, J = 20.3 Hz, 2H), 4.48
(d, J = 15.2 Hz, 2H), 4.12 (d, J = 15.8 Hz, 1H), 4.12 (d J = 19.3 Hz, 1H),
3.30 (m, 2H), 2.47-2.25 (m, 4H). 13C NMR (300 MHz, CD3CN) δ 166.07,
163.01, 157.00, 154.68, 138.34, 137.93, 125.96, 124.24, 121.39, 70.98,
64.41, 54.10, 22.29. HRMS for C27H30N6Fe2+ (-2PF6) Theoretical:
246.09603; Experimental: 246.09586. UV-vis (MeCN) max (nm) [ (M-
1,cm-1)] 205 [12865], 252 [10953], 386 (shoulder) [6350], 421 [7756]. Anal.
Calc’d. for C27H30F12FeN6P2 (784.34 g.mol-1): C, 41.35; H, 3.86; N, 10.71.
Found: C, 41.36; H, 4.19; N, 10.45%.
(N,N,N’,N’-tetrakis(2-pyridazylmethyl)propylenediamine)iron(II)
hexafluorophosphate (1) To a solutionof L1(0.10g, 0.2 mmol) in distilled
water (1.0 ml), was added FeCl2 (0.029 g, 0.2 mmol) resulting in an
immediatecolor change from yellow to darkred. The solutionwasallowed
to stir at 80 ºC for 20 minutes after which time solid KPF6 (0.083 g, 0.5
mmol) wasadded and themixture wasstirred for an additional 10 minutes
at 80 ºC. The flask was then allowed to cool to room temperature, during
which time a red solid precipitated from the solution. The solid was
collected by filtration to give complex 1 (0.11 g, 62%) as
a red
microcrystalline solid. Single crystals suitable for X-ray analysis were
obtained by the slow-diffusion method by first layering a solution of 1 in
acetonitrile with hexane, followed by addition of toluene on top of the
hexane layer. 1H NMR (300 MHz, CD3CN) δ 8.93 (d, J = 4.7 Hz, 2H), 8.86
(d, J = 4.4, 2H Hz), 7.68 (d, J = 8.2 Hz, 2H), 7.61 (dd, J = 8.0, 4.7 Hz, 2H),
7.55 (dd, J = 8.5, 4.9 Hz, 2 H), 7.44 (d, J = 8.2 Hz, 2H), 4.89 (d, J = 19.7
Hz, 2H), 4.48 (d, J = 15.8 Hz, 2H), 4.30 (d, J = 19.7 Hz, 2H), 4.26 (d, J =
15.8 Hz, 2H), 3.32 (m, 2H), 2.58 – 2.35 (m, 4H) . 13C NMR (75 MHz,
CD3CN) δ 170.55, 168.73, 165.76, 153.92, 151.93,127.33, 126.91,126.39,
125.54, 69.42, 63.46, 54.83, 22.22. HRMS for C23H26N10Fe2+ (-2PF6)
Theoretical: 248.08655; Experimental: 248.08635. UV-vis (MeCN) max
(nm) [ (M-1,cm-1)] : 209 [15506], 242 [11795], 449 [13884]. Anal. Calc’d.
for C23H26F12FeN10P2 (788.29g.mol-1): C, 35.04; H, 3.32; N, 17.76. Found:
C, 34.6; H, 3.53; N, 17.37%.
Acknowledgements
(N,N’-bis(2-pyridazylmethyl)-N,N’-bis(2-
pyridylmethyl)propylenediamine)iron(II) hexafluorophosphate (2) To
a solution of L2 (0.20 g, 0.5 mmol) in distilled water (3.0 ml), was added
FeCl2 (0.058 g, 0.5mmol) resulting in an immediate color change from
This w orkw as supported by the Université Catholique de Louvain
Budget d’installation de Jeune Académique and Fonds Speciaux
de Recherche FSR15 pour A. H. We also acknow ledge support
from the Fonds de la Recherche Scientifique-FNRS (PDR
T.0102.15) and chargé de recherches fellow hip to M. M. D.
(16511866). We also thank the COST actions CM1305 and
CA15128.
yellow to darkred. The solution wasallowedto stir at 80 ºC for 20 minutes
after which time solid KPF6 (0.167 g, 1.0 mmol) wasadded and themixture
was stirred for an additional 10 minutes at 80 ºC. The flask was then
allowed to cool to room temperature, during which time a red solid
precipitated from the solution. The solid wascollectedby filtrationto give
complex 2 (0.27 g, 76%) as a red microcrystalline solid. Single crystals
suitable for X-ray analysiswere obtainedby the slow-diffusionmethod by
first layering a solutionof 2 in acetonitrile with hexane, followed by addition
of toluene on top of the hexane layer. 1H NMR (300 MHz, CD3CN) δ 9.07
(d, 1H, J = 5.3 Hz), 8.97 (dd, J = 4.1, 2.3 Hz, 1H), 8.76(dd, J = 5.3, 1.8 Hz,
1H), 7.86 (t, J = 7.6 Hz, 1H), 7.74 (t, J = 7.6 Hz, 1H), 7.64-7.56 (m, 4H),
7.46 (dd, J = 8.2, 4.7 Hz, 1H), 7.38-7.24 (m, 3H), 7.18 (t, J = 7.0 Hz, 1H),
4.86 (d, J = 20.5 Hz, 1H), 4.79 (d, J = 20.5 Hz, 1H), 4.39 (d, J = 15.8 Hz,
1H), 4.35 (d, J = 15.8 Hz, 1H), 4.20 (d, J = 18.2 Hz, 1H), 4.14 (d, J = 18.8
Hz, 1H), 4.10 (d, J = 15.8 Hz, 1H), 4.03 (d, J = 15.8 Hz, 1H), 3.30-3.15 (m,
2H), 2.62-2.52 (m, 1H), 2.37-2.34 (m, 3H); 13C NMR (75 MHz, CD3CN) δ
170.14, 166.72, 165.06,161.77, 156.95, 154.34, 153.37, 152.82, 138.48,
138.18, 128.06, 126.89, 126.41, 125.37, 124.65, 123.62, 121.40, 70.52,
69.37, 65.04, 63.03, 54.42, 22.23. HRMS for C25H28N8Fe2+ (-2PF6)
Keywords:iron, pyridazines, second coordination sphere,
protonation, acid stability
CCDC
1824892-1824894
contain
the
supplementary
crystallographic data for this paper. The data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
w ww.ccdc.cam.ac.uk/structures.
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European Journal of Inorganic Chemistry
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This article is protected by copyright. All rights reserved.

10.1002/ejic.201800533
European Journal of Inorganic Chemistry
FULL PAPER
Entry for the Table of Contents
FULL PAPER
The synthesis and characterization of
Stabilizing Protonated Complexes
X = CH
+ TfOH
Decomposes
a
series
of
pyridazine/pyridine
iron(II)
Ahmad Hammoud, Jean-Boris
Nshimyumuremyi, Jérémie Bourotte,
Fabio Lucaccioni, Koen Robeyns,
Marinela M. Dîrtu, Yann Garcia,*
Michael L. Singleton*
containing
octahedral
complexes and their reactivity tow ards
protonation is described. The presence
of additional basic sites in the second
coordination sphere greatly enhances
the acidic stability of the complexes but
only w hen the pyridazine groups are
positioned in a w ay that allow s them to
act in a cooperative manner.
Page No. – Page No.
+ TfOH
X = N
Cooperative interactions in the
second coordination sphere of
pyridazine/pyridine containing
polyazaheterocyclic iron(II)
complexes increase their stability
towards protonation.
Stable
Protonation
This article is protected by copyright. All rights reserved.