A new iron(II) complex with strongly saddle shaped Schiff base like ligand

Article abstract of DOI:10.1002/zaac.201300176

The synthesis of two new Schiff base like ligands containing a 1, 8-diaminonaphthalene unit in order to improve π-π interactions between the molecules is described. The corresponding iron(II), copper(II), and nickel(II) complexes were synthesized and characterized using NMR spectroscopy, magnetic measurements, and for the iron(II) complex X-ray structure analysis. The crystal structure shows a strongly saddle shaped ligand. In contrast to related complexes with a phenylene unit that prefer an octahedral coordination sphere, this complex crystallizes pentacoordinate. The keto-group of a neighboring complex serves as axial ligand resulting in the formation of infinite chains. Magnetic susceptibility measurements reveal nearly ideal curie behavior for the copper(II) and the iron(II) complex. Copyright

Full text of DOI:10.1002/zaac.201300176

ARTICLE
DOI: 10.1002/zaac.201300176
A New Iron(II) Complex with Strongly Saddle Shaped Schiff Base like
Ligand
Stephan Schlamp,[a] Peter Thoma,[a] Tobias Bauer,[a] Rhett Kempe,[a] and
Birgit Weber*[a]
Dedicated to Professor Heinrich Nöth on the Occasion of His 85th Birthday
Keywords: Iron; Schiff base ligand; X-ray structure; Copper; Nickel
Abstract. The synthesis of two new Schiff base like ligands containing
a 1,8-diaminonaphthalene unit in order to improve π–π interactions
between the molecules is described. The corresponding iron(II), cop-
strongly saddle shaped ligand. In contrast to related complexes with a
phenylene unit that prefer an octahedral coordination sphere, this com-
plex crystallizes pentacoordinate. The keto-group of a neighboring
per(II), and nickel(II) complexes were synthesized and characterized complex serves as axial ligand resulting in the formation of infinite
using NMR spectroscopy, magnetic measurements, and for the iron(II) chains. Magnetic susceptibility measurements reveal nearly ideal curie
complex X-ray structure analysis. The crystal structure shows a
behavior for the copper(II) and the iron(II) complex.
ligand family was synthesized, where the phenylene unit is
replaced by the extended π system of a naphthalene unit.
Introduction
Magnetic bistability is a very interesting phenomenon that
can occur in transition metal complexes with d4–d7 electron
configuration of the central atom, the so-called spin crossover
(SCO) or spin transition (ST).[1,2] Through variation of exter-
nal conditions like temperature, pressure, or by light irradia-
tion, the system can be switched between two electronic states.
This involves physical changes like color shift, or a change in
the magnetism for example from the paramagnetic high-spin
(HS) to the diamagnetic low-spin (LS) state in iron(II) com-
plexes.[1,2] This makes these compounds a matter of particular
interest concerning applications like storage of information, or
as optical sensors for example for cold channel control units.[3]
For some of those applications a cooperative ST with hystere-
sis is needed. This involves the forwarding of the information
of the spin state change from one molecule to the other within
the crystal. Intermolecular interactions like van der Waals in-
teractions, π–π interactions,[4] or hydrogen bonds[5] are respon-
sible for such effects.[6] This leads to a constant search for new
ligands, where those interactions are optimized. In order to
improve the cooperative interactions, particularly the π–π in-
teractions of Schiff-base like ligands used in our group, a new
Results and Discussion
Synthesis of the Ligands and their Complexes
In Scheme 1 the general synthesis of the ligands and their
complexes is given. 1,8-Diaminonaphthalene and the desired
keto-enol ether[7,8] were converted in ethanol yielding the new
ligands H2L1 and H2L2 that were characterized by elemental
analysis, mass spectrometry, IR, and NMR spectroscopy. In
1
Figure S1 (Supporting Information), the H NMR spectrum of
H2L1 is given together with the signal assignment.
Scheme 1. General synthesis of the ligands H2L1 and H2L2 and the
corresponding metal complexes [Fe(L1)], [Cu(L2)], and [Ni(L2)].
The ligands were converted with the corresponding metal(II)
acetate, to give the desired complexes. For H2L2 the corre-
sponding nickel(II) ([Ni(L2)]) and copper(II) complex
([Cu(L2)]·1.5EtOH) were obtained in good yield. In contrast
to this it was not possible to isolate the related iron(II) complex
using similar conditions. This is different to the synthesis of
analogue iron(II) complexes of Jäger-type Schiff base like li-
gands, that are normally obtained in good yields. Similar diffi-
* Prof. Dr. B. Weber
Fax: +49-921-552157
E-Mail: weber@uni-bayreuth.de
[a] Inorganic Chemistry II
Universität Bayreuth
Universitätsstr. 30
95440 Bayreuth, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201300176 or from the au-
thor.
Z. Anorg. Allg. Chem. 2013, 639, (10), 1763–1767
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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B. Weber et al.
ARTICLE
culties for the synthesis of the iron(II) complex occurred for chelate six-rings are with an average of 2.01 Å in the expected
H2L1. However, a mixture of the free ligand and the iron com-
plex was obtained. The black and colorless crystals could be
easily separated enabling the characterization of the iron com-
plex [Fe(L1)]·2MeOH ([Fe(L1)]) by single-crystal X-ray
structure analysis. All three complexes were characterized by
elemental analysis, IR spectroscopy, and magnetic measure-
ments.
region for iron(II) complexes of this ligand type in the high-
spin state.[11] The Fe–N bonds have an average value of 2.08 Å
in agreement with previously published structures of similar
complexes. The bond length to the axial oxygen atom is with
an average of 2.05 Å relatively short compared to other penta-
coordinate iron(II) complexes of this ligand type.[11] The
O–Fe–O angles are with an average of 95.7° smaller than
found for comparable pentacoordinate complexes of such
Jäger-type ligands in the high spin state, that all show
O–Fe–O angles around 100°.[11] This is due to the two aro-
matic rings, where the two methylidyne groups are in the 1,8-
position. Because of this the N–Fe–N angle is slightly enlarged
in comparison to complexes with only one phenylene ring (ca.
85° instead of 80°). With 2.79 Å, the N–N distance is about
0.15 Å larger than in the comparable complexes with only one
ring (about 2.65 Å).[11] So the “N,N-bite” of the ligand is big-
ger on the N,N-diaminonaphthalene side.
Due to the penta-coordination, the central iron atom is lo-
cated above the equatorial ligand with an out of plane (oop)
distance of the iron atom to the N2O2 plane of 0.458 Å for
Fe1 and 0.455 Å for Fe2, respectively. The increased distance
between the two nitrogen ligands induces strain into the ligand
and it is strongly bent in a saddle shaped way to still allow the
coordination of the central iron atom. The two N,O-chelate
rings with a delocalized π system (O1C3C2C1N1 and
O2C7C8C9N2) are bent in an angle of 40.04° and 41.46° in
Fe1, and 41.80° and 45.54° (O6C25C26C28N4,
O5C39C40C41N3) in Fe2, with regard to the N2O2 plane. The
parts that are connected to a neighboring iron atom with the
oxygen atoms of their keto group are slightly stronger bent
than the parts that do not. Thus the molecule packing further
supports the pronounced saddle shape of the ligand. A further
supporting factor for the deviation of the ligand from a planar
structure are restraining steric interactions between the protons
of C1/C17 and C9/C11 (see Figure 1), if a planar structure of
the ligand is assumed. Similar restraining interactions are made
responsible for the saddle shaped structure of analogue macro-
cyclic complexes with two phenylene bridges.[15] The naphth-
alene is arranged in a 26.07° angle to the N2O2 plane at Fe1
and a 24.52° angle at Fe2. The distortion of the equatorial
Schiff base like ligand is much more pronounced than for re-
lated open-chain and macrocyclic ligands of this general type.
Figure 2 displays the molecular packing of [Fe(L1)] along
[1 0 0]. The complex molecules are arranged in infinite chains
that run along [0 1 0], where the iron atom Fe1 is coordinated
by the oxygen atom O4 of a neighboring Fe1 complex
molecule, and the iron atom Fe2 is coordinated to the oxygen
atom O8 of a neighboring Fe2 complex molecule.
Discussion of the X-ray Structure
Black block-like crystals of [Fe(L1)] suitable for X-ray
structure analysis were obtained thus the molecular setup of
the new complex could be investigated. The crystal structure
is displayed in Figure 1, selected bond lengths and angles
within the first coordination sphere are given in Table 1. The
crystallographic data are summarized in Table 3. The complex
crystallizes in the monoclinic space group P21 and the asym-
metric unit consists of two complex molecules and three meth-
anol molecules (solvent). In contrast to previously published
iron(II) complexes of this ligand type the methanol does not
coordinate at the central iron atom.[9–14] Thus not the expected
octahedral coordination sphere with an equatorial N2O2 coordi-
nating chelate ligand and two axial ligands is observed but a
pentacoordinate central iron atom. The fifth ligand in axial po-
sition is the oxygen atom of the keto group of a neighboring
complex. Regarding the inner [N2O3] coordination sphere
around the two crystallographic different central iron atoms
(Fe1 and Fe2), the bond lengths and angles are in the same
order of magnitude. Bond lengths to the oxygen atoms of the
Figure 1. Ortep drawing of the molecule structure of [Fe(L1)]. Hydro-
gen atoms and solvent molecules are omitted for clarity. Ellipsoids are
drawn at the 50% probability level.
Table 1. Selected bond lengths /Å, angles /° and distances /Å of 1.
Fe–Neq
Fe–Oeq
Fe–Oax
O–Fe–O
N–Fe–N
oop
Fe1
Fe2
2.069(2), 2.096(2) 2.010(2), 2.011(2) 2.068(2)a)
2.076(2), 2.088(2) 2.023(2), 1.995(2) 2.040(2)b)
95.75(8)
95.62(8)
84.10(9)
83.87(9)
0.458
0.455
Symmetry codes a): –x, ½+y, –z; b) 1–x, ½+y, 1–z.
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Z. Anorg. Allg. Chem. 2013, 1763–1767

Iron(II) Complex with Saddle Shaped Schiff Base like Ligand
ligand field strength), or because even in pure pyridine only
penta-coordination is obtained, cannot be answered based on
the experiments done. The steric strain of the system is also
reflected in the low yields for the iron complex. It just precipi-
tated as a side product, next to the ligand. Due to the high
affinity of nickel(II) and copper(II) to a square planar or
square-pyramidal coordination arrangement, the synthesis of
those complexes succeeded with a significantly higher yield.
Due to the larger N,N-bite of the ligand, it will be interesting
to test the complex formation with larger transition metal ions
like 4d or 5d elements or lanthanides.
Magnetism
Magnetic susceptibility data of [Fe(L1)] and [Cu(L2)] were
recorded on cooling and warming over the temperature range
300–10 K. Both complexes show nearly ideal Curie behavior.
In Figure 3 the results for both complexes are given. At room
temperature, the value of χMT of 4.03 cm3·K·mol–1 for
[Fe(L1)] clearly indicates that the complex resides in the HS
state, with a slightly increased magnetic susceptibility that can
be explained with an orbital momentum contribution. The cor-
responding room temperature value for [Cu(L2)] is with
Figure 2. Packing of the molecules of [Fe(L1)] in the crystal projected
along [1 0 0]. Hydrogen bonds are drawn in dashed lines.
The oxygen atom O7 of a Fe2 complex molecule is con-
nected by a hydrogen bond to the oxygen atom O10 of a meth-
anol molecule, which forms itself also a hydrogen bond to the
oxygen atom O11 of a neighboring methanol molecule (see
Table 2). In the Fe1 complex molecules, the non-coordinating
oxygen of a keto group (O3) forms a hydrogen bond to the
oxygen atom O9 of the third methanol molecule. So the both
chains are embedding the methanol molecules in their middle,
as can be seen in Figure 2.
Table 2. Selected distances /Å and angles /° of the hydrogen bonds in
the crystal packing of [Fe(L1)].
D–H···A
D–H
H···A
D···A
D–H···A
O9–H9A···O3
0.84
0.84
1.94
1.94
1.90
2.772(4)
2.778(4)
2.713(5)
171
175
163
O10–H10···O7a)
O11–H11A···O10 0.84
Symmetry code a): x, 1+y, –1+z.
In comparable complexes with just one aromatic phenylene
ring in the ligand the iron is always axially coordinated with
two methanol molecules.[9–14] For this new iron complex it is
obviously more convenient to crystallize pentacoordinate in-
stead of hexacoordinate. This can be attributed to steric effects.
Due to the pronounced saddle shape of the equatorial ligand
the sixth coordination side is blocked and the introduction of
a sixth ligand is difficult. Attempts were made to introduce
stronger ligands to see if an octahedral coordination sphere
could be obtained. Thus the complex was dissolved in pyridine
to try to precipitate an octahedral product, but with no success.
The pyridine solution was cooled down with liquid nitrogen to
see if a spin transition takes place. With such an approach it
was possible to demonstrate a spin transition in solution for
imidazole or pseudo-halide complexes although no octahedral
complexes were obtained in the solid state.[16] With the new
ligand presented herein no indications for a spin transition
Figure 3. (A) χMT vs. T plot of [Cu(L2)] displayed in the temperature
range 300–2 K (filled cycles) and Curie plot of [Cu(L2)] (open cy-
cles). (B) χMT vs. T plot of [Fe(L1)] displayed in the temperature
range 300–10 K (filled cycles) and Curie-plot of [Fe(L1)] (open cy-
were observed. If this is due to the equatorial ligand (too weak cles).
Z. Anorg. Allg. Chem. 2013, 1763–1767
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B. Weber et al.
ARTICLE
0.43 cm3·K·mol–1 in a typical range for copper(II) complexes
of this ligand type (theoretical value: 0.37 cm3·K·mol–1 for one
unpaired electron). The corresponding nickel(II) complex is
tometer under an applied field of 0.05 T over the temperature range
10–300 K in the sweep mode. The sample was placed in a gelatine
capsule held within plastic straws. The data were corrected for the
diamagnetic magnetization of the ligand, which was estimated using
tabulated Pascal’s constants and of the sample holder.
1
diamagnetic according to H NMR spectroscopy. In Figure S2
1
(Supporting Information), the H NMR spectra of the copper
and the nickel complex are given together with the spectrum
Crystal Structure Determination: The intensity data of [Fe(L1)]
of the free ligand H2L2. In the case of [Ni(L2)] the NMR
were collected with a STOE-IPDS II using graphite-monochromated
spectrum is very similar to that of the free ligand. Due to the Mo-Kα radiation. The data were corrected for Lorentz and polarization
effects. The structure was solved by Direct Methods (Sir 97)[18] and
coordination of the nickel the resonances are slightly shifted
(coordination induced shift) and a slight broadening is ob-
2
refined by full-matrix least-square techniques against F0 (SHELXL-
97).[19] The hydrogen atoms were included at calculated positions with
served. In contrast to this a strong shift of the NMR signals is
fixed displacement parameters. All non-hydrogen atoms were refined
observed for the paramagnetic copper complex together with
anisotropically. ORTEP-III[20] was used for the structure representa-
a significant broadening of the lines. The diamagnetic behavior
tion, Schakal-99[21] for the representation of the molecule packing.
of the nickel complex is not unexpected if a square-planar co-
ordination sphere is assumed and in agreement with the results
for other nickel complexes of closely related ligands.[8]
Crystal structure parameters are listed in Table 3.
Table 3. Crystal structure parameters for [Fe(L1)].
In the case of [Cu(L2)], the magnetic susceptibility does not
change significantly upon cooling. For [Fe(L1)] the χMT value
[Fe(L1)]
at 50 K is about 3.7 cm3·K·mol–1. Below 50 K a significant
drop of the magnetic moment is observed that could be due to
zero field splitting, probably in combination with weak antifer-
romagnetic interactions within the 1D chains. A fit of the mag-
netic data with an analytical expression for infinite chains of
equally spaced local spins of S = 2[17] resulted in a coupling
constant J of –0.83(1) cm–1 [g = 2.32(1)] that is almost negligi-
ble. 1/χ was plotted against T (Curie plot) (Figure 3, open cy-
cles) showing a linear dependence (Curie behavior). A linear
fit of the data resulted in a Weiss constant θ of –6.33 K and a
Curie constant C of 4.09 K in agreement with high-spin
iron(II) atoms, weakly antiferromagnetic coupled.
Sum formula
Formula weight
Temperature /K
Crystal size /mm
Crystal system
Space group
Radiation /Å
a /Å
b /Å
c /Å
α /°
β /°
2(C22H20FeN2O4), 3(CH4O)
960.63
133
0.16ϫ0.18ϫ0.48
monoclinic
P21
0.71069 (Mo-Kα)
11.5940(4)
13.3460(4)
14.6640(5)
90
92.313(3)
90
γ /°
V /Å3
2267.16(13)
2
1.407
0.705
Z
ρcalcd. /g·cm–3
μ /mm–1
Conclusions
F(000)
θ range /°
1004
1.4–25.6
A new ligand system with an increased aromatic π system
and its iron(II), copper(II), and nickel(II) complexes were syn-
thesized. The results show, that the ligand exhibits steric strain
what precludes the formation of octahedral iron(II) complexes,
a precondition for the observation of spin crossover activity. It
is more favorable for the complex to crystallize pentacoordi-
nate due to the strongly saddle shaped equatorial ligand. The
steric strain is also reflected in the low yields of the iron com-
plex, whereas for the copper(II) and nickel(II) high yields are
obtained. Magnetic measurements show weak antiferromag-
netic interactions between the central iron atoms. The nickel
complex is diamagnetic indicating a square-planar coordina-
tion sphere, whereas for the copper complex a typical behavior
for a complex with one unpaired electron and no significant
magnetic interactions is observed. For future investigations it
will be interesting to introduce other central metal atoms. Due
to the larger bite of the ligand compared to other Schiff base
like ligands of this family, larger metal ions could be interest-
ing.
hkl
–14 to 14; –16 to 16; –17 to 17
30938
8540
Measured reflections
Independent reflections
Rint
R
wR2
S
Δρmax / Δρmin
0.046
0.0351
0.0909
1.06
–0.47 / 0.32
Crystallographic data (excluding structure factors) for the structure in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
number CCDC-931181 [Fe(L1)] (Fax: +44-1223-336-033; E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk)
Synthesis of H2L1: 1,8-Diaminonaphthalene (2 g, 12.65 mmol) and
ethoxymethyleneacetylacetone[7] (4.34 g, 27.81 mmol, 2.2 equiv.) were
dissolved in ethanol (200 mL) and heated to reflux for 2 h. After stor-
age at –28 °C, a fine crystalline white powder precipitated that was
collected and recrystallized from ethanol (100 mL). The product, a
pinkish white powder was filtered and dried on air. Yield: 0.4 g (8%).
C22H22N2O4 (378.42 g·mol–1): calcd. C 69.83, H 5.86, N 7.40%;
Experimental Section
found: C 69.88, H 5.76, N 6.86%. MS (DEI+): m/z (%) = 378 (75)
Magnetic Measurements: Magnetic susceptibility data of [Fe(L1)] [M]+. H NMR (CDCl3, 399.8 MHz, 296 K): δ = 2.4 (s, 6 H, CH3),
1
were collected with a Quantum Design MPMSXL5 SQUID magne-
2.45 (s, 6 H, CH3), 7.30 (d, 2 H, 7.4 Hz, HArom), 7.55 (t, 2 H, 7.6 Hz,
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Iron(II) Complex with Saddle Shaped Schiff Base like Ligand
HArom), 7.82 (d, 2 H, 7.5 Hz, HArom), 8.17 (d, 2 H, 12.0 Hz, C=H),
13.0 (d, 2 H, 12.0 Hz, NH) ppm. IR: ν˜ = 1611 vs. (CO), 1389 vs,
1243 vs, 982 s, 926 m, 764 s, 753 s cm–1.
Acknowledgements
This work has been supported financially by the Deutsche Forschungs-
gemeinschaft (SFB 840, TP A10) and the University of Bayreuth.
H2L2 ({2,2-[1,2-Phenylenebis(iminomethylidyne)] bis[1-phenylbu-
tane-1,3-dione](2–)-N,NЈ,O3,O3Ј}): 1,8-Diaminonaphthalene (2.54 g,
16 mmol) was dissolved in methanol (10 mL) and ethoxymethylene-
1-phenylbutane-1,3-dione[8] (7.7 g, 35.28 mmol, 2.2 equiv.), dissolved
in warm methanol (20 mL), was added. The dark red solution was
heated to reflux for 90 min. After cooling to room temperature, a yel-
low solid precipitated that was collected and washed with methanol.
Yield: 1.66 g (21%). C32H26N2O4·MeOH (534.60 g·mol–1): calcd.
C 74.14, H 5.66, N 5.24%; found: C 74.30, H 5.70, N 5.28%. MS
(DEI+): m/z (%) = 502 (93) [M]+, 105 (100). 1H NMR (CDCl3,
399.8 MHz, 296 K): δ = 2.42 (s, 6 H, CH3), 7.12 (d, 2 H, 7.5 Hz,
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[Cu(L2)]: H2L2 (0.1 g, 0.2 mmol) and copper(II) acetate monohydrate
(0.04 g, 0.22 mmol, 1.1 equiv.) were dissolved in ethanol (10 mL) and
the green solution was heated to reflux for 90 min. After cooling to
room temperature, the green fine crystalline precipitate was filtered,
washed with methanol, and dried. Yield: 0.11 g (98%).
CuC32H24N2O4·1.5MeOH (633.12 g·mol–1): calcd. C 66.39, H 5.25,
N 4.42%; found: C 66.00, H 4.67, N 4.70%. IR: ν˜ = 1630 m, 1583 w,
1552 vs, 1386 vs, 1263 s, 991 m, 972 m, 888 s, 733 s, 698 s, 666 s
cm–1.
[Ni(L2)]: H2L2 (0.10 g, 0.2 mmol) and nickel(II) acetate tetrahydrate
(0.05 g, 0.22 mmol, 1.1 equiv.) were dissolved in ethanol (10 mL) and
the brownish black solution was heated to reflux for 75 min. After
cooling to room temperature, the black fine crystalline precipitate was
filtered, washed with methanol, and dried. Yield: 0.09 g (81%).
NiC32H24N2O4 (559.24 g·mol–1): calcd. C 68.73, H 4.33, N 5.01%;
found: C 68.62, H 4.48, N 5.09%. IR: ν˜ = 1630 m, 1577 w, 1552 vs,
1385 vs, 1265 vs, 999 m, 891 s, 715 m, 696 s, 668 s cm–1.
[Fe(L1)]: Under argon, H2L1 (0.36 g, 0.95 mmol) and iron(II) acet-
ate[22] (0.28 g, 1.61 mmol, 1.7 equiv.) were heated to reflux in meth-
anol (30 mL) for 80 min. After cooling, fine crystalline ligand precipi-
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tated again. After 15 d, also black block-like crystals were formed out [20] a) L. Farrugia, J. Appl. Crystallogr. 1997, 30, 565; b) C. K. John-
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of the dark red solution. The overlaying solution and the ligand were
decanted, the product washed thrice with 3 mL methanol and dried in
vacuo. Yield: approx. 0.05 g (5%). C47H52Fe2N4O11 (960.63 g·mol–1):
calcd. C 58.76, H 5.46, N 5.83%; found: C 58.68, H 5.26, N 6.32%.
IR: ν˜ = 1552 vs. (CO), 1383 vs, 1247 vs, 983 s, 939 s, 762 vs. cm–1.
2011, 637, 102–107.
Supporting Information (see footnote on the first page of this article):
1H NMR spectra of the ligands H2L1 and H2L2 and the complexes
[Ni(L2)] and [Cu(L2)].
Received: March 27, 2013
Published Online: July 2, 2013
Z. Anorg. Allg. Chem. 2013, 1763–1767
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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