Pyridazine-based ligands and their coordinating ability towards first-row transition metals

Article abstract of DOI:10.1002/ejic.201000120

Tridentate pyridazine ligands with two different substituents at the 3- and 6-positions (pyridyl hydrazone at the 3-position and tolyl 3^tol or tert-butyl 3^tBu at the 6-position) were prepared in high yields from 3,6-disubstituted pyridazine hydrazines and pyridine aldehyde by Schiff base condensation. This provides a facile entry into a novel class of heterocyclic molecules exhibiting interesting coordinating abilities. They react: with first-row transition metal nitrates to yield complexes of the type [ML ₂](NO₃)₂ (L = 3^tol; M = Co 4, M = Ni 5, M = Zn 7) and [ML(NO₃)₂] (L = 3^tol, M = Cu 6; L = 3^tBu, M = Ni 8, M = Cu 9, M = Zn 10) as crystalline solids. With the exception of copper compound. 6 all complexes employing ligand 3 ^tol form complexes of the ML₂ type, whereas with ligand 3^tBu exclusively complexes of the ML type are formed, which demonstrates the influence of the substituent at the 6-position despite its being located far from the coordination site. This points to a pronounced electronic effect. Molecular structures of all complexes were determined by single-crystal X-ray diffraction analysis revealing six-coordinate metal centers in a N₆ coordination sphere consisting of two ligands 3 or in a N₃O₃ sphere with one ligand and two nitrates in the O,- and O,O′-coordination modes, respectively. Diamagnetic zinc complexes 7 and 10 were characterized in solution by ¹H NMR spectroscopy. All other complexes are paramagnetic (Evans' method). The redox behavior of all complexes was investigated by cyclic voltammetry, which revealed that the complexes possess the same stoichiometry in solution and in the solid state.

Full text of DOI:10.1002/ejic.201000120

FULL PAPER
DOI: 10.1002/ejic.201000120
Pyridazine-Based Ligands and Their Coordinating Ability towards First-Row
Transition Metals
Katrin R. Grünwald,[a] Gerald Saischek,[a] Manuel Volpe,[a] Ferdinand Belaj,[a] and
Nadia C. Mösch-Zanetti*[a]
Keywords: Nitrogen heterocycles / Schiff bases / Tridentate ligands / Chelates
Tridentate pyridazine ligands with two different substituents
at the 3- and 6-positions (pyridyl hydrazone at the 3-position
and tolyl 3tol or tert-butyl 3tBu at the 6-position) were pre-
pared in high yields from 3,6-disubstituted pyridazine hy-
drazines and pyridine aldehyde by Schiff base condensation.
This provides a facile entry into a novel class of heterocyclic
molecules exhibiting interesting coordinating abilities. They
react with first-row transition metal nitrates to yield com-
plexes of the type [ML2](NO3)2 (L = 3tol; M = Co 4, M = Ni 5,
M = Zn 7) and [ML(NO3)2] (L = 3tol, M = Cu 6; L = 3tBu, M =
Ni 8, M = Cu 9, M = Zn 10) as crystalline solids. With the
exception of copper compound 6 all complexes employing
ligand 3tol form complexes of the ML2 type, whereas with
ligand 3tBu exclusively complexes of the ML type are formed,
which demonstrates the influence of the substituent at the 6-
position despite its being located far from the coordination
site. This points to a pronounced electronic effect. Molecular
structures of all complexes were determined by single-crystal
X-ray diffraction analysis revealing six-coordinate metal cen-
ters in a N6 coordination sphere consisting of two ligands 3
or in a N3O3 sphere with one ligand and two nitrates in the
O,- and O,OЈ-coordination modes, respectively. Diamagnetic
zinc complexes 7 and 10 were characterized in solution by
1H NMR spectroscopy. All other complexes are paramagnetic
(Evans’ method). The redox behavior of all complexes was
investigated by cyclic voltammetry, which revealed that the
complexes possess the same stoichiometry in solution and in
the solid state.
atoms (3,6-positions) or secondary bridging ligands seem to
be a prerequisite to accommodate two or more metal atoms
in elaborate structures.[9–13] Thus, pyridazines without adja-
cent coordination sites act mostly as monodentate ligands
comparable to pyridine.[14,15]
Pyridazines with only one additional donor next to a ring
nitrogen atom, leading to bi- or tridentate ligands, have
been significantly less investigated in coordination chemis-
try, despite the unique electronic situation imposed by the
pyridazine ring. Here, we report on the preparation of
pyridazine-based ligands that contain additional donor sites
at the 3-position of the heterocycle, whereas the 6-position
is substituted by a noncoordinating aryl or alkyl group.
Introduction
Substituted pyridazine-based compounds are extensively
investigated with regard to their versatile applications.
Pyridazines interfere in regulation processes of various en-
zymes and find wide application in drug design.[1–3] Analo-
gously, agricultural science takes advantage of their high
biological activity, and pyridazines whose substitution pat-
terns have been modified display powerful fungicide proper-
ties.[4–6] Furthermore, simple pyridazines rate high as halo-
gen-free flame retardants.[7]
Pyridazines have also been used as ligands in coordina-
tion chemistry. The electron-deficient aromatic system con-
taining two nitrogen atoms leads to decreased electron-do-
nor abilities in comparison to pyridines, which is reflected
by significantly lower pKa values (2.24 for parent pyridazine
vs. 5.25 for pyridine).[8] The two vicinal nitrogen donor
atoms in the pyridazine heterocycle allow, in principle, the
coordination of two metal centers in close proximity. How-
ever, in nonmacrocyclic ligands, two additional donor sites
attached to the aromatic ring on both sides of the nitrogen
[a] Institut für Chemie, Bereich Anorganische Chemie,
Karl-Franzens-Universität Graz,
Schubertstrasse 1, 8010 Graz, Austria
Fax: +43-316-380-9835
Structurally, the ligands described here bear resemblance
to a pyridine-based ligand, which is widely known as
PAPHY (pyridine-2-aldehyde-2-pyridylhydrazone) but is
E-mail: nadia.moesch@uni-graz.at
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000120.
Eur. J. Inorg. Chem. 2010, 2297–2305
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K. R. Grünwald, G. Saischek, M. Volpe, F. Belaj, N. C. Mösch-Zanetti
FULL PAPER
expected to exhibit significantly different electronic features. 3tol and 3tBu in quantitative yields. The tolyl-substituted li-
PAPHY was first described in 1957[16] and subsequently ex- gand is obtained as a white crystalline solid and the tBu
tensively studied.[17–22] Throughout the literature two types derivative as a light yellow solid. They are characterized by
of complexes, namely the mono[23] and the bis complex[24] spectroscopic methods. Imine formation is indicated by the
can be found for this ligand type, depending on the donor absence of the NH2 signal (at ca. 4 ppm) and the appear-
ability of the counterion. The observed poor stabilities in ance of the imine resonance (singlet at δ = 7.83 ppm) in the
solution and the easy deprotonation of the hydrogen atom 1H NMR spectra. The resonances assignable to the other
at the hydrazone backbone represent the major drawbacks protons in the molecules are barely influenced by the substi-
in PAPHY complexes, as the pH value must be kept within tution. Both represent a completely novel type of ligand.
small ranges. Methylation of the nitrogen atom seemed to (NMR spectra are available as Supporting Information).
be a viable solution without changing the spectroscopic
properties of the complexes.[25] These methylated derivatives
Synthesis of the Complexes
recently experienced a renaissance as the key building
blocks in forming self-assembled morphologies. Complex
grid- and rack-type constructions[26–28] are induced by dy-
namic structural and conformational changes of the ligand
backbone as described by Lehn and co-workers.[29–31] Sim-
ilar research with pyridazine-based ligands was not pur-
sued, which is certainly due to the as yet elusive pyridazine
building blocks.
In this work, we describe a series of first-row transition
metal complexes with novel pyridazine ligands. Fine tuning
in the ligand backbone provides us with a simple tool to
direct complex stoichiometry. Molecular structures were de-
termined by X-ray diffraction, and electrochemical behavior
in solution was investigated by cyclic voltammetry.
The coordination ability of the synthesized ligands
towards some first-row transition metals was investigated.
Syntheses of the complexes were inspired by literature pro-
cedures.[37,38] Solutions of 3tol or 3tBu in ethanol were re-
fluxed, which was followed by the addition of M(NO3)2 (M
= Co, Ni, Cu, Zn) as a 0.1  solution in ethanol. Coordina-
tion was immediate, as a change of color was apparent. The
solutions were heated at reflux for an additional 20 min to
ensure completeness of the reaction. Most compounds were
obtained as air-stable crystalline solids upon cooling. Some
crystallized after the addition of diethyl ether to the ethanol
solution as indicated in the Experimental Section. For all
complexes, we were able to obtain crystals suitable for X-
ray diffraction analysis (vide infra), revealing the formation
of 1:1 or 1:2 complexes depending on the substituent at the
6-position of the pyridazine ring as shown in Scheme 2.
Results and Discussion
Synthesis of the Ligands
Two new pyridazine-based ligands were prepared starting
from 3-chloro-pyridazine according to Scheme 1. The latter
can be synthesized in a three-step procedure as described in
the literature.[32,33] Subsequent reaction with methyl hydraz-
ine in ethanol at refluxing temperature afforded 2tol and
2tBu in high isolated yields with high purities. NMR spec-
troscopy on crude samples shows the selective substitution
at the 1-position of methyl hydrazine. Schiff base condensa-
tion with 2-pyridine aldehyde in ethanol gave the ligands
Scheme 2. Formation of 1:1 and 1:2 complexes.
All complexes with ligand 3tBu form 1:1 complexes (8, 9,
and 10), whereas with 3tol both 1:1 (6) as well as 1:2 com-
plexes (4, 5, and 7) are formed. In complexes of the 1:1 type,
one η1- and one η2-nitrate ion complete the six-coordinate
coordination site. The ligand-to-metal ratio observed within
the complexes is independent of the molar ratio applied in
the reaction mixture. It seems unlikely that only the larger
steric demand of the tert-butyl group prevents coordination
of a second ligand, since it is located at the 6-position
pointing away form the metal site. Presumably, a combina-
tion of the steric bulk and the electron-releasing effect of
tBu, which leads to a higher electron donating capability,
prevents substitution of nitrate.
Scheme 1. Synthesis of ligands 3tol and 3tBu with two different sub-
stituents at the 6-position starting from the corresponding 3-chlo-
ropyridazine: (a) 1 equiv. methyl hydrazine in EtOH, reflux, 10 h;
(b) 1 equiv. 2-pyridine aldehyde in EtOH (3 drops of acetic acid),
reflux, 45 min.
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Coordination of Pyridazine-Based Ligands
All complexes are air-stable crystalline solids. Com- Experimental Section. Molecular views including atom-
pounds 4, 5, 6, and 7 are very poorly soluble in water and numbering schemes of complexes 5, 8 and 9 are given in
acetone, moderately soluble in acetonitrile and alcohols, Figures 1 and 2. Views of all other compounds can be
and fairly well soluble in dimethyl sulfoxide, but still not found in the Supporting Information (Figures S1–S4).
well enough to obtain a meaningful 13C NMR spectrum Crystallographic data and refinement parameters are given
even after measurement overnight. Complexes 8, 9, and 10 in Table 2, and selected bond lengths and angles of all com-
are just slightly more soluble in dmso, alcohols, acetonitrile, plexes are given in Tables 3 and 4. In the following para-
and acetone due to the tBu moiety; however, concentrations graphs, the two types of complexes (with ligand-to-metal
sufficient for measuring a 13C NMR spectrum could not be ratios of 2:1 and 1:1) are described.
obtained, either. All complexes are completely insoluble in
dichloromethane and ethers. For this reason, all spectro-
scopic and susceptibility data were obtained in dmso, this
being the only solvent capable of achieving sufficiently high
concentrations. Electrochemical measurements (vide infra),
however, could be carried out in dry acetonitrile.
The proton NMR spectra of the diamagnetic zinc com-
plexes 7 and 10 in [D6]dmso show in both cases one reso-
nance set for the ligands, confirming the equivalency of
both coordinated ligands in complex 7 and also confirming
the structures revealed by X-ray crystallography. The spec-
trum of 7 additionally contains resonances for ethanol,
which could not be removed by extensive drying under high
vacuum for 96 h. The chemical shifts of the coordinated
ligand resonances are only slightly different from those of
the free ligands, so NMR spectroscopy is not a suitable tool
to confirm coordination in solution for the diamagnetic
complexes. The paramagnetism of the cobalt (4), nickel (5
and 8), and copper (6 and 9) complexes prevented the deter-
mination of the structure by NMR spectroscopy. However,
their magnetism in solution could be determined by Evans’
method,[34] and the results are given in Table 1 along with
the theoretically expected number of unpaired electrons as-
suming the complexes to be octahedral high-spin species.
Figure 1. Molecular view of complex 5 (50% probability). Hydro-
gen atoms and solvent molecules have been omitted for clarity.
Table 1. Unpaired electrons determined by the Evans’ method.
µobsd. /BM Unpaired e– Unpaired e– (theor.)
[Co(3tol)2](NO3)2 (4)
[Ni(3tol)2](NO3)2 (5)
[Cu(3tol)(NO3)2] (6)
[Ni(3tBu)(NO3)2] (8)
[Cu(3tBu)(NO3)2] (9)
2.97
2.84
1.54
2.98
1.58
2.13
2.01
0.83
2.15
0.87
3
2
1
2
1
Figure 2. Molecular view of complex 8 and 9 (50% probability).
Hydrogen atoms have been omitted for clarity.
With the exception of the cobalt complex, the experimen-
tal and theoretical numbers agree fairly well. The low
number of unpaired electrons in the case of the cobalt com-
plex points to a significant contribution of the low-spin spe-
cies in solution and shows the ligand to be of medium
strong character.[8,39] Notwithstanding the paramagnetic
character of the reported complexes, no meaningful EPR
spectra (solution or solid state) could be acquired, the spec-
trum itself consisting generally of a single, broad, unre-
solved wave.
Molecular Structure of Complexes 4, 5, and 7 (Ligand/
Metal = 2:1)
Complexes 4, 5, and 7 are isostructural. They all contain
two equivalents of ligand 3tol, which coordinates in a triden-
tate fashion to form distorted octahedral cations. Both ni-
trate counterions are not coordinated to the metal center.
A representative molecular view of compound 5 including
the atom numbering scheme is shown in Figure 1. The dis-
tortion from ideal octahedral geometry is evident from, for
example, the values of the following angles: N2–M–N5
158.99(8)° (M = Co, 4), 155.12(12)° (M = Ni, 5), or
148.25(7)° (M = Zn, 7) or the angles N2–M–N24
Solid State Structures of the Complexes
The molecular structures of all complexes were deter- 173.46(12)° (M = Co, 4), 170.10(12)° (M = Ni, 5) or
mined by X-ray diffraction analysis. Suitable single crystals 164.19(10)° (M = Zn, 7). Metal-to-nitrogen bond lengths to
were obtained by crystallization procedures indicated in the the pyridine and the pyridazine N atoms are in the same
Eur. J. Inorg. Chem. 2010, 2297–2305
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K. R. Grünwald, G. Saischek, M. Volpe, F. Belaj, N. C. Mösch-Zanetti
Table 2. Crystallographic data and structure refinement for complexes 4–10.
FULL PAPER
4
5
6
7
8
9
10
Empirical formula
Mr /gmol–1
Crystal description
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
β /°
Co2C76N24H80O14 NiC38N12H40O7 CuC18N7H17O6 ZnC38N12H40O7 NiC15N7H19O6 CuC15N7H19O6 ZnC15N7H19O6
1671.50
needle, red
monoclinic
C2/c
835.53
490.94
842.19
block, yellow
monoclinic
C2/c
452.08
tablet, gold
monoclinic
P21/c
456.91
tablet, green
monoclinic
P21/c
458.74
plate, colorless
monoclinic
P21/c
column, red
monoclinic
P21/c
block, green
triclinic
¯
P1
15.7497(7)
24.4139(10)
20.6835(8)
90.00
104.506(2)
90.00
15.7949(10)
24.5127(15)
20.7312(11)
90.00
105.816(2)
90.00
11.635(3)
11.824(3)
15.185(4)
93.12(2)
102.81(2)
92.74(2)
2030.2(9)
4
15.6375(6)
24.9838(10)
20.6616(9)
90.00
104.079(2)
90.00
14.1108(5)
7.6035(3)
17.2073(6)
90.00
90.0980(10)
90.00
13.7491(4)
7.5464(2)
17.5964(5)
90.00
91.7710(10)
90.00
13.9955(6)
7.6897(3)
17.3050(7)
90.00
90.124(2)
90.00
γ /°
Volume /Å3
Z
7699.5(6)
4
7722.7(8)
8
7829.7(6)
8
1846.19(12)
4
1824.86(9)
4
1862.38(13)
4
T /K
100(2)
1.442
0.513
3480
100(2)
1.437
0.569
3488
95
1.606
1.129
1004
100(2)
1.429
0.693
3504
100(2)
1.626
1.102
936
100(2)
1.663
1.249
940
100(2)
1.636
1.370
944
Dc, /gcm–3
µ(Mo-Kα) /mm–1
F(000)
Reflections collected
Unique reflections
Reflections with IՆ2 σ (I)
R(int), R(σ)
Number of param./restraints
Final R1,[a] wR2[b] (IՆ2 σ)
R indices (all data)
GOF on F2
51924
7574
6300
0.0453, 0.0302
530/1
118725
17782
13549
8705
7956
6528
84110
7558
6593
35823
3855
3513
23761
4402
4034
42945
5832
4844
0.0443, 0.0381 0.0334, 0.0634 0.0389, 0.0231
1058/0 591/0 602/34
0.0607, 0.1423 0.0411, 0.0853 0.0385, 0.0815
0.0873, 0.1578 0.0551, 0.0918 0.0469, 0.0845
1.186
0.825, –0.617
0.0329, 0.0164 0.0292, 0.0204 0.0271, 0.0148
266/0 266/0 266/0
0.0216, 0.0524 0.0238, 0.0613 0.0217, 0.0570
0.0249, 0.0543 0.0272, 0.0632 0.0263, 0.0605
0.999
0.301, –0.279
0.0439, 0.1125
0.0560, 0.1203
1.058
1.034
0.354, –0.354
1.146
0.506, –0.339
1.035
0.468, –0.403
1.048
0.490, –0.274
Larg. diff. peak and hole /eÅ–3 0.866, –0.734
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = {Σ[w(Fo – Fc ) ] /Σ[w(Fo2)2]}1/2
.
2
2 2 2
Table 3. Selected bond lengths in Å and angles in ° of compounds
4, 5, and 7.
Table 4. Selected bond lengths in Å and angles in ° of compounds
6, 8, 9, and 10.
4 (M = Co)[a,b] 5 (M = Ni)[a]
7 (M = Zn)[a]
6 (M = Cu)[a] 8 (M = Ni)[a] 9 (M = Cu) 10 (M = Zn)
M–N2
M–N4
M–N5
M–N22
M–N24
M–N25
N2–M–N4
N2–M–N5
N2–M–N22
N2–M–N24
N2–M–N25
N4–M–N5
N4–M–N22
N4–M–N24
N4–M–N25
N5–M–N22
N5–M–N24
N5–M–N25
N22–M–N24
N22–M–N25
N24–M–N25
1.999(2)
1.900(2)
2.066(2)
1.999(2)a
1.900(2)a
2.066(2)a
79.76(8)
158.99(8)
92.31(11)
95.67(8)
93.86(8)
79.66(8)
95.67(8)
173.46(12)
105.18(8)
93.86(8)
105.18(8)
87.49(12)
79.76(8)
158.99(8)
79.66(8)
2.053(3)
2.004(3)
2.091(3)
2.055(3)
2.001(3)
2.085(3)
76.99(12)
155.12(12)
91.89(12)
95.85(12)
94.84(12)
78.44(12)
96.30(12)
170.10(12)
108.67(12)
94.63(12)
109.01(12)
89.32(12)
76.91(12)
155.00(12)
78.46(12)
2.135(2)
2.129 (2)
2.133 (2)
2.135(2)
2.129 (2)
2.133 (2)
73.17(7)
148.25(7)
91.51(10)
95.61(7)
97.55(7)
75.69(7)
95.61(7)
164.19(10)
116.14(7)
97.55(7)
116.14(7)
90.57(10)
73.17(7)
148.25(7)
75.69(7)
M–N2
M–N4
M–N5
M–O1
M–O2
M–O4
1.967(2)
1.960(2)
1.989(2)
1.965(2)
2.636(2)
2.239(2)
2.056(1)
2.008(1)
2.087 (1)
2.062(1)
2.154(1)
2.030(1)
76.95(4)
154.94(4)
101.31(4)
89.54(4)
86.77(4)
78.66(4)
167.46(4)
106.21(4)
91.29(4)
101.25(4)
91.82(4)
99.46(4)
61.25(4)
101.05(4)
160.80(4)
1.997 (1)
1.962(1)
2.019(1)
1.954(1)
2.678(1)
2.215(1)
78.30(5)
156.32(5)
102.60(4)
91.20(5)
84.82(4)
79.99(5)
169.75(5)
116.64(5)
96.98(5)
97.11(4)
90.14(5)
107.19(4)
53.30(5)
93.27(4)
144.61(5)
2.116(1)
2.138(1)
2.125(1)
2.023(1)
2.460(1)
2.036(1)
72.89(3)
146.19(4)
104.47(4)
86.11(3)
88.32(4)
75.35(3)
158.79(4)
102.12(3)
98.18(4)
100.87(3)
89.46(3)
107.53(4)
56.73(3)
102.80(4)
156.30(3)
N2–M–N4 79.19(10)
N2–M–N5 157.77(10)
N2–M–O1 97.97(9)
N2–M–O2 83.77(10)
N2–M–O4 92.80(9)
N4–M–N5 80.85(10)
N4–M–O1 174.11(9)
N4–M–O2 120.43(10)
N4–M–O4 97.30(9)
N5–M–O1 100.98(9)
N5–M–O2 98.09(10)
N5–M–O4 99.37(9)
O1–M–O2 53.87(10)
O1–M–O4 87.96(8)
O2–M–O4 140.43(10)
[a] Data of only one molecule of the two found in the asymmetric
unit are given. [b] Second ligand molecule generated by symmetry
operation (N2i, N4i and N5i are represented by N22, N24 and
N25).
[a] Data for only one molecule of the two found in the asymmetric
unit are given.
Crystal Structures of Complexes 6, 8, 9, and 10 (Ligand/
Metal = 1:1)
range [from 1.999(2) Å to 2.135(2) Å] and are similar to
those found in the literature for pyridine and pyridazine
All four complexes 6, 8–10 are isostructural. The central
metal bonds.[24,40–43] All M–N4 bond lengths are the short- metal atom is coordinated by the three nitrogen atoms of
est in all complexes, which was also observed for complexes one ligand molecule (3tol in the case of copper complex 6
of PAPHY and analogues.[28,44]
and 3tBu in 8–10) as well as by the oxygen atoms of one
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Coordination of Pyridazine-Based Ligands
O- and one slipped O,OЈ-coordinated nitrate ion forming Epa 1.022 V (3tBu)] are shifted to more positive potentials
distorted octahedral geometries. The M–O bond lengths in [Epa 1.595 V (4), Epa 1.314 V (5), Epa 1.672 V (8), Epa
the O,OЈ-coordinate NO3 ion differ significantly with the 1.641 V (6), Epa 1.877 V (9)]; thus, we expect easier ligand
oxygen occupying the apical position and increase as fol- reduction in the complexes (Table 5).
lows: 2.154(1) Å (8)ϽϽ2.460(1) Å (10) Ͻ 2.636(2) Å (6) Ͻ
2.678(1) Å (9). Thus, only the nickel compound 8 shows
octahedral geometry. Compounds 6, 9, and 10 are better
described as exhibiting square pyramidal geometries. Repre-
sentative molecular views of the two extremes, compounds
8 and 9, are shown in Figure 2. In the d9 CuII complexes 6
and 9, tetragonal distortion of the octahedron stabilizes the
system due to the Jahn–Teller effect.[45] The three N atoms
of the tridentate pyridazine ligands N2, N4, and N5 and
O1 of the O,OЈ-coordinate nitrate ions form distorted
square planes about the metal centers with torsion angles
of –1.14(5) (9), –3.41(4) (10), 3.32(4) (8) and –4.11(10)° (6).
A distorted pyramidal arrangement is manifested by N2–
M–O4 and N4–M–O4 angles of 92.80(9) and 97.30(9) (6),
86.77(4) and 91.29(4) (8) 84.82(4) and 96.98(5) (9), and
88.32(4) and 98.18(4)° (10). The metal–oxygen distances
range from 1.954(1) to 2.062(1) Å for O1 and from 2.030(1)
to 2.239(2) Å for O4, where Cu–O1 distances in the two
copper complexes (6 and 9) are slightly shorter than those
in the nickel (8) and zinc complexes (10). In contrast, the
Cu–O4 distances are significantly longer than Ni–O4 and
Zn–O4 distances consistent to the preferred square-planar
geometry typical for Jahn–Teller-distorted CuII ions.
Metal–nitrogen distances range in all complexes from
1.960(2) to 2.138(1) Å, increasing from the imino-N via the
pyridazine-N through to the pyridine–N bond. The coordi-
nated nitrate ions feature oxygen–metal–oxygen angles of
53.30 (9), 53.87 (6), 56.73 (10), and 61.25° (8), which are
similar to values reported in the literature.[46]
Figure 3. Cyclic voltammogram of complex 4 [Co(3tol)2](NO3)2 in
acetonitrile solution. Scan rate: 50 mVs–1.
Ligand exchange reactions employing metal halide or
pseudo-halide precursors are known to lead preferably to
complexes of the type metal/ligand = 1:1,[19–22] whereas em-
ploying precursors with noncoordinating counterions leads
to complexes of the type 2:1.[18,24–26] Nitrate precursors
seem to exhibit reactivity at the borderline, enabling both
stoichiometries. Thus, fine-tuning of electronic and steric
properties on the pyridazine ring results in two different
metal-to-ligand ratios.
Figure 4. Cyclic voltammograms of complexes 5 [Ni(3tol)2](NO3)2
and
50 mVs–1.
8
[Ni(3tBu)(NO3)2] in acetonitrile solution. Scan rate:
Electrochemical Properties of the Complexes
An additional reversible reduction couple MII Ǟ MI was
Redox properties of complexes 4–6, 8, and 9 were investi- observed for the nickel(II) complex 8 (E1/2 –0.901 V), which
gated by cyclic voltammetry in dry acetonitrile solutions. is more negative than the cobalt(II) complex 4 (E1/2
Cobalt complex 4 (Figure 3) exhibits two reversible redox –0.111 V) due to the electronic configuration of NiII
2
waves, which are assigned to the consecutive CoII Ǟ CoI (t2g6eg ). Nickel complex 5 employing ligand 3tol shows a
(E1/2 –0.111 V) and CoI Ǟ Co0 (E1/2 –1.008 V) redox pro- 2:1 stoichiometry and a N6 coordination environment
cesses. An additional pseudo-reversible reduction wave ap- around the metal center in the solid state in contrast to
peared at –1.719 V, possibly due to a coordinated ligand metal complex 8, which shows a N3O3 coordination sphere.
redox process. We observed similar reduction waves for the Thus, the electron density at the metal center is expected to
nickel complexes [E1/2 –1.260 V (5), E1/2 –1.387 V (8)], as be higher than that in 8, explaining the lower E1/2 value as
shown in Figure 4. The free ligands were redox inactive in electrochemical reduction is hindered. This suggests that in
the cathodic potential region. Upon coordination to the solution the same complex stoichiometries are present as in
metal center, the ligand oxidations [Epa 0.979 V (3tol) and the solid state.
Eur. J. Inorg. Chem. 2010, 2297–2305
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2301

K. R. Grünwald, G. Saischek, M. Volpe, F. Belaj, N. C. Mösch-Zanetti
FULL PAPER
Table 5. Electrochemical data for 4–6, 8, and 9 in acetonitrile solu- gands reminiscent of the methylated PAPHY ligand, but
tion at room temperature.[a]
exhibiting completely different electronic features. We dis-
covered two complex stoichiometries upon coordination to
various first-row transition metals depending on the substit-
uents at the 6-position of the heterocycle. We found bis
complexes with two ligands coordinated to the metal, which
is a prerequisite for the formation of the knots of grids,
nets, and cross-linked polymeric networks impressively
demonstrated by Lehn and co-workers.[29–31] Additionally,
we found stable mono complexes with only one ligand and
two nitrate ions, featuring easily accessible metal centers,
which are desirable for versatile catalytic applications. Here-
with, we have presented a powerful tool to direct the stoi-
chiometry of the formed complexes by simple variation of
the substituents. Furthermore, we proved by cyclic voltam-
metry that the complexes exhibit in solution the same stoi-
chiometry as in the solid state. The well-behaved character
of the compounds makes them suitable building blocks for
the design of novel materials.
Compound
Epc
/V
Epa
/V
∆Ep
/mV
E1/2
/V
Redox
process
[Co(3tol)2](NO3)2 (4)
–0.153 –0.069 84
–1.046 –0.970 76
–1.775 –1.662 113
1.595
–1.373 –1.146 227
1.314
–0.955 –0.846 109
–1.453 –1.321 132
1.672
–0.820 –0.604 216
1.641
–0.111 CoII Ǟ CoI
–1.008 CoI Ǟ Co
–1.719 L Ǟ L–[b]
L Ǟ L+[c]
[Ni(3tol)2](NO3)2 (5)
[Ni(3tBu)(NO3)2] (8)
–1.260 L Ǟ L+[b]
L Ǟ L+[c]
–0.901 NiII Ǟ NiI
–1.387 L Ǟ L–[b]
L Ǟ L+[c]
[Cu(3tol)(NO3)2] (6)
[Cu(3tBu)(NO3)2] (9)
–0.712 CuII Ǟ Cu
L Ǟ L+[c]
–0.808 –0.606 202
1.877
–0.707 CuII Ǟ Cu
L Ǟ L+[c]
3tol
0.979
1.022
L Ǟ L+
3tBu
L Ǟ L+
[a] Measured by CV in CH3CN at 50 mVs–1. E vs. Ag/AgNO3.
Conditions: Pt disc (working) and Ag/AgNO3 (reference) elec-
trodes, supporting electrolyte 0.05  TBAP solution in CH3CN in
an Ar atmosphere. [b] Ligand-centered reduction process upon co-
ordination to the metal, not observed in the CV of the parent li-
gand in the explored region of 0 to –2.5 V. [c] Ligand-centered oxi-
dation process, facilitated upon coordination to the metal.
Experimental Section
General Considerations: The chloropyridazines, 3-chloro-6-(4-meth-
ylphenyl)pyridazine (1tol
) and 3-chloro-6-(tert-butyl)pyridazine
Cyclic voltammograms of copper complexes 6 and 9, as
depicted in Figure 5, show both pseudo-reversible redox
couples with cathodic reductions at –0.820 V and –0.808 V,
respectively, as well as oxidative responses with narrow
widths and high peak currents at –0.604 V and –0.606 V,
which is typical for anodic stripping of copper metal.[47]
(1tBu), were prepared according to literature procedures.[32,33] All
other chemicals were purchased from commercial sources and used
without further purification. All NMR spectra were measured with
a Bruker AMX 360 spectrometer (360 MHz for 1H NMR) or a
Bruker Avance III spectrometer (300 MHz for 1H and 75 MHz for
1
13C NMR). The H NMR spectroscopic data are reported as s =
singlet, d = doublet, t = triplet, m = multiplet or unresolved, br =
broad signal, coupling constant(s) in Hz, shifts in ppm relative to
the solvent residual peak. All melting points were determined with
a Büchi Melting Point B-540 apparatus. Infrared spectra were mea-
sured with a Perkin–Elmer FTIR 1725X apparatus by using KBr
discs. Room-temperature magnetic susceptibilities, µobs in BM, and
the number of unpaired electrons (u. e.) were determined by the
Evans’ method[34] in [D6]dmso/3% benzene.
3-(1-Methylhydrazine)-6-(4-methylphenyl)pyridazine (2tol): A sus-
pension of 3-chloro-6-(4-methylphenyl)pyridazine (1tol) (8.20 g,
40 mmol) in ethanol (100 mL) was heated to reflux. Methylhydraz-
ine (10.7 mL, 200 mmol) was added to the resulting solution. The
reaction mixture was heated to reflux for an additional 10 h, after
which full conversion was evidenced by TLC analysis. After cooling
the solution to room temperature, a white precipitate was formed,
which was filtered off and washed with ice-cooled ethanol.
Recrystallization from hot ethanol afforded 6.6 g (77%) of pure
2
tol. M.p. 141 °C. 1H NMR (300 MHz, CDCl3, 298 K): δ = 2.41 (s,
3 H, CH3), 3.44 (s, 3 H, NCH3), 4.11 (br. s, 2 H, NH2), 7.42 (d, J
= 8.1 Hz, 2 H, ArH), 7.43 (d, J = 9.5 Hz, 1 H, ArH), 7.62 (d, J =
9.5 Hz, 1 H, ArH), 7.90 (d, J = 8.1 Hz, 2 H, ArH) ppm. 13C NMR
(300 MHz, CDCl3, 298 K): δ = 21.3 (CH3), 41.1 (N-CH3), 114.2,
125.0, 125.8, 129.5, 134.1, 138.6, 151.1, 160.8 (aromatic carbons)
ppm.
Figure 5. Cyclic voltammograms of complexes 6 [Cu(3tol)2](NO3)2
and
9
[Cu(3tBu)2](NO3)2 in acetonitrile solution. Scan rate:
50 mVs–1.
3-(1-Methylhydrazine)-6-(tert-butyl)pyridazine (2tBu): To a suspen-
sion of 3-chloro-6-(tert-butyl)pyridazine (3.50 g, 20.5 mmol) in dry
ethanol (50mL) was added methylhydrazine (5.4 mL, 102.6 mmol),
and the mixture was heated to reflux for two days. After full con-
version (evidenced by TLC analysis) all volatiles were removed un-
Conclusions
We have developed a new class of tridentate pyridazine
ligands. In contrast to known ligands containing this het-
erocycle, they feature donor substituents at only one posi-
tion adjacent to the two nitrogen atoms. This leads to li- der reduced pressure. The yellow residue was dissolved in dichloro-
2302 Eur. J. Inorg. Chem. 2010, 2297–2305
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Coordination of Pyridazine-Based Ligands
methane, and the resulting solution was washed with water. The
solvent was removed in vacuo to afford 2.8 g (77%) of pure 2tBu as
a light brown oil. 1H NMR (300 MHz, CDCl3, 300 K): δ = 1.36 [s,
9 H, C(CH3)3], 3.34 (s, 3 H, NCH3), 4.02 (br. s, 2 H, NH2), 7.27
(d, J = 9.5 Hz, 1 H, ArH), 7.32 (d, J = 9.5 Hz, 1 H, ArH) ppm.
13C NMR (300 MHz, CDCl3, 300 K): δ = 30.0 [C(CH3)3], 36.0
[C(CH3)3], 41.2 (NCH3), 114.3, 124.6, 160.7, 161.4 (aromatic car-
bons) ppm.
CoC30N10H38·H2O (434.6): calcd. C 53.54, H 4.49, N 20.81; found
C 53.31, H 4.29, N 20.61.
[Ni(3tol)2](NO3)2 (5): Following the general procedure by employing
3tol (0.1 g, 0.35 mmol) and Ni(NO3)2·6H2O (0.35 mmol, 3.5 mL of
a 0.1  ethanol solution), compound 5 was obtained as red col-
umns (124 mg, 85% based on the ligand) by crystallization at room
temperature for 2 d. IR (KBr): ν = 786 (m), 810 (m), 988 (m), 1034
˜
(m), 1130 (s), 1304 (s), 1338 (s), 1352 (s), 1452 (s), 1546 (m), 1588
(m), 1612 (m), 2350 (w), 2378 (w), 3028 (m), 3052 (m), 3436 (m,
br) cm–1. NiC36N12H34O6·CH3CH2OH·0.5H2O (844.5): C 54.15, H
4.42, N 21.05; found C 54.06, H 4.46, N 20.55.
3tol: An ethanol solution (60 mL) of 2tol (2.00 g, 9.3 mmol) and 2-
pyridine aldehyde (1.1 equiv., 1.10 g, 0.98 mL, 10.3 mmol) in the
presence of acetic acid (3 drops) as a catalyst were heated to reflux
for 45 min. After cooling to room temperature, the mixture was
poured on ice. The cloudy off-white precipitate was filtered off and
dried in vacuo. Recrystallization from hot ethanol yielded 2.58 g
(96%) of 3tol as a white crystalline solid. M.p. 187 °C. 1H NMR
(300 MHz, [D6]dmso, 300 K): δ = 2.37 (s, 3 H, CH3), 3.81 (s, 3 H,
NCH3), 7.33 (m, 3 H, ArH), 7.84 (t, J = 7.5 Hz, 1 H, ArH), 7.86
(s, 1 H, CH=N), 7.97 (d, J = 7.8 Hz, 2 H, ArH), 8.05 (m, 3 H,
ArH), 8.60 (br. s, 1 H, ArH) ppm. 13C NMR (300 MHz, [D6]dmso,
300 K): δ = 20.5 (CH3), 29.7 (NCH3), 115.3, 119.2, 122.6, 125.3,
125.6, 129.1, 133.1, 136.3, 136.9, 138.4, 149.0, 152.8, 154.0, 157.7
[Cu(3tol)(NO3)2] (6): Following the general procedure by employing
3tol (0.1 g, 0.35 mmol) and Cu(NO3)2·3H2O (0.35 mmol, 3.5 mL of
a 0.1  ethanol solution), compound 6 was obtained as green
blocks (149 mg, 87%) by crystallization by means of slow evapora-
tion of the solvent at room temperature. IR (KBr): ν = 776 (m),
˜
822 (m), 990 (m), 1014 (m), 1042 (m), 1130 (s), 1238 (m), 1290 (s),
1302 (s), 1384 (s), 1446 (s), 1484 (s), 1554 (m), 1592 (m), 1610 (m)
cm–1. CuC18N7H17O6·1.5H2O (517.9): C 41.74, H 3.89, N 18.93;
found C 41.43, H 3.36, N 19.02.
[Zn(3tol)2](NO3)2 (7): Following the general procedure by em-
ploying 3tol (0.1 g, 0.35 mmol) and Zn(NO3)2·4H2O (0.35 mmol,
3.5 mL of a 0.1  ethanol solution), compound 7 was obtained as
colorless needles (122 mg, 83% based on the ligand) by crystalli-
(aromatic and imino carbons) ppm. IR (KBr): ν = 776 (m), 822
˜
(s), 974 (m), 1102 (m), 1156 (m), 1176 (m), 1426 (s), 1460 (s), 1568
(m), 2355 (w), 2358 (w), 3434 (m, br) cm–1. C18H17N5 (303.4):
calcd. C 71.27, H 5.65, N 23.09; found C 70.61, H 5.32, N 23.00.
1
zation at room temperature overnight. H NMR (300 MHz, [D6]-
3tBu: An ethanol solution (60 mL) of 2tBu (2.00 g, 11.1 mmol) and
2-pyridine aldehyde (1.0 equiv., 1.19 g, 1.06 mL, 11.1 mmol) in the
presence of acetic acid (3 drops) as a catalyst were heated under
reflux for 45 min. After cooling the solution, all volatiles were re-
moved under reduced pressure, yielding 2.84 g (95%) of 3tBu as a
dmso, 300 K): δ = 1.05 (t, 1.5 H, CH3, EtOH), 2.38 (s, 3 H, CH3),
3.34 (dt, 1 H, CH2, EtOH), 3.84 (s, 3 H, NCH3), 4.35 (t, 0.5 H,
OH, EtOH), 7.34–7.37 (m, 3 H, ArH), 7.96–8.15 (m, 6 H, ArH),
8.58 (br. s, 1 H, ArH) ppm. No 13C NMR spectroscopic data are
available because of the poor solubility in [D ]dmso. IR (KBr): ν
˜
6
1
light brown oil. H NMR (300 MHz, [D6]dmso, 300 K): δ = 1.35
= 788 (m), 810 (s), 986 (s), 1030 (s), 1130 (s), 1302 (s), 1302 (s),
1338 (s, br), 1348 (s), 1450 (s), 1548 (s), 1590 (s), 2404 (m, br), 3052
(m, br), 3470 (m, br) cm–1 ppm. ZnC36N12H34O6·C2H5OH (842.2):
C 54.32, H 4.80, N 20.01; found C 53.95, H 4.55, N 20.01.
[s, 9 H, C(CH3)3], 3.75 (s, 3 H, NCH3), 7.32 (dd, J = 5.5, 6.7 Hz,
1 H, ArH), 7.69 (d, J = 9.5 Hz, 1 H, ArH), 7.83 (t, J = 7.8 Hz, 1
H, ArH), 7.85 (s, 1 H, N=CH), 7.96 (d, J = 9.5 Hz, 1 H, ArH),
8.02 (d, J = 7.8 Hz, 1 H, ArH), 8.58 (d, J = 4.8 Hz, 1 H, ArH)
ppm. 13C NMR (300 MHz, [D6]dmso, 300 K): δ = 29.7 [C(CH3)3
+C(CH3)3], 36.0 (NCH3), 115.3, 119.1, 122.9, 125.4, 136.3, 136.4,
149.1, 154.3, 157.4, 163.5 (aromatic and imino carbons) ppm. IR
[Ni(3tBu)(NO3)2] (8): Following the general procedure by employing
3
tBu (0.1 g, 0.37 mmol) and Ni(NO3)2·6H2O (0.37 mmol, 3.7 mL of
a 0.1  ethanol solution), compound 8 was obtained as golden tab-
lets (57 mg, 34%) by crystallization by means of diffusion of di-
isopropyl ether into the solution of the complex in acetonitrile. IR
(KBr): ν = 776 (m), 842 (m), 972 (s), 1136 (s), 1284 (s), 1428 (s),
˜
1464 (s), 1480 (s), 1574 (s), 1714 (m), 2342 (m), 2362 (m), 2960 (s),
3422 (m, br) cm–1. C15H19N5 (269.3): C 66.89, H 7.11, N 26.00;
found C 66.87, H 7.03, N 25.39.
(KBr): ν = 792 (m), 834 (m), 992 (m), 1018 (m), 1090 (m), 1158
˜
(m), 1240 (m), 1268 (s), 1298 (s), 1310 (s), 1356 (m), 1386 (m) tBu,
1446 (s), 1504 (s), 2324 (w), 2346 (w), 2944 (m, br), 3434 (m, br)
cm–1. NiC15N7H19O6·CH3CN (493.1): C 41.41, H 4.50, N 22.72;
found C 41.55, H 4.27, N 22.47.
Synthesis of the Complexes
General Procedure: To a boiling solution of the ligand (100 mg) in
ethanol (40 mL) was added the metal salt [Co(NO3)2·6H2O,
Ni(NO3)2·6H2O, Cu(NO3)2·3H2O or Zn(NO3)2·4H2O] (1 equiv.,
0.1  solution in ethanol). A color change was immediately appar-
ent. The reaction mixtures were heated to reflux for additional
20 min followed by cooling to room temperature. The products
crystallized after several days. The crystals obtained were isolated
by filtration and dried in vacuo. For all compounds single crystals
suitable for X-ray diffraction analysis were obtained. The specific
crystallization methods are described below.
[Cu(3tBu)(NO3)2] (9): Following the general procedure by employing
3
tBu (0.1 g, 0.37 mmol) and Cu(NO3)2·3H2O (0.37 mmol, 3.7 mL of
a 0.1  ethanol solution), compound 9 was obtained as green tab-
lets (104 mg, 62%) by crystallization by means of diffusion of di-
isopropyl ether into the ethanol solution of the complex. IR (KBr):
ν = 798 (m), 836 (m), 994 (m), 1010 (m), 1094 (m), 1156 (m), 1242
˜
(m), 1284 (s), 1302 (s), 1358 (m), 1386 (s) tBu, 1486 (s), 1550 (m),
1590 (m), 1590 (m), 1600 (m), 1616 (m), 2332 (w), 2344 (w), 2362
(w), 2970 (m, br), 3434 (s, br) cm–1. CuC15N7H19O6·H2O (474.9):
C 37.94, H 4.46, N 20.64; found C 37.52, H 4.13, N 20.48.
[Co(3tol)2](NO3)2 (4): Following the general procedure by em-
ploying 3tol (0.1 g, 0.35 mmol) and Co(NO3)2·6H2O (0.35 mmol,
3.5 mL of a 0.1  ethanol solution), compound 4 was obtained as
red needles (138 mg, 47% based on the ligand) by crystallization
by means of slow evaporation of the solvent at room temperature.
[Zn(3tBu)(NO3)2] (10): Following the general procedure by em-
ploying 3tBu (0.1 g, 0.37 mmol) and Zn(NO3)2·4H2O (0.37 mmol,
3.7 mL of a 0.1  ethanol solution), compound 10 was obtained as
1
colorless plates (64 mg, 38%) by crystallization in acetonitrile. H
IR (KBr): ν = 786 (s), 810 (s), 986 (s), 1032 (s), 1128 (s), 1160 (s), NMR (300 MHz, [D6]dmso, 300 K): δ = 1.34 [s, br 9 H, C(CH3)3],
˜
1304 (s), 1350 (s, br), 1430 (s), 1452 (s), 1544 (s), 1588 (s), 1606 (s),
3.78 (s, 3 H, NCH3), 7.67–8.54 (m, 6 H, ArH) ppm. No 13C NMR
spectroscopic data are available because of the poor solubility in
2398 (w), 2406 (w), 3042 (s, br), 3436 (m, br) cm–1.
Eur. J. Inorg. Chem. 2010, 2297–2305
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2303

K. R. Grünwald, G. Saischek, M. Volpe, F. Belaj, N. C. Mösch-Zanetti
FULL PAPER
[4]
[5]
[6]
C. Lamberth, S. V. Wendeborn, S. Trah, R. Dumeunier, PCT
Int. Appl. WO2008089934 2008.
S. Trah, C. Lamberth, S. Wendeborn, V. PCT Int. Appl.
[D ]dmso. IR (KBr): ν = 794 (m), 836 (m), 990 (m), 1014 (m), 1086
˜
6
(m), 1158 (s), 1240 (m), 1284 (s), 1300 (s), 1356 (m), 1386 (m), 1490
(s), 1550 (m), 1598 (m), 1622 (m), 2322 (w), 2332 (w), 2342 (w),
2360 (w), 2966 (m, br), 3458 (s, br) cm–1. ZnC15N7H19O6·
1.5CH3CN (520.3): C 41.55, H 4.55, N 22.88; found C 41.51, H
4.41, N 22.10.
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X-ray Crystallographic Studies: Intensity data sets for compounds
6 and 7 were collected by using a Stoe four circle diffractometer
and for compounds 4, 5, and 8–10 by using a Bruker Smart Apex
2 diffractometer, both equipped with a graphite monochromator
(Mo-Kα radiation, λ = 0.71073 Å) and the latter with a CCD detec-
tor. All compounds were measured at 100 K, except for 6, which
was measured at 95 K. The structures were solved by direct meth-
ods (SHELXS-97)[35] and refined by full-matrix least-squares tech-
niques against F2 (SHELXL-97).[35] The asymmetric units of com-
plexes 4, 5, 6, and 7 contain two metal complex molecules and
those of 4 and 7 contain additionally one molecule of ethanol, and
that of 5 two molecules of ethanol. In complex 4 the C1S–C2S
bond length of the ethanol solvent molecule in the crystal was re-
strained to 1.50(2) Å. For compound 7 one anion and the ethanol
solvent molecule are disordered over two sites, which were refined
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
–
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Received: February 2, 2010
Published Online: April 16, 2010
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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