A promising new Schiff base-like ligand for the synthesis of octahedral iron(II) Spin crossover complexes

Article abstract of DOI:10.1515/znb-2010-0315

Abstract A new tetradentate Schiff base-like ligand H₂L (1) ((E,E)-dimethyl-2,2'-[1,2-phenylenebis(amino-methylidyne)]bis[3-oxobutanoate]) was synthesised and structurally characterised. Its reaction with iron(II) acetate leads to the formation of the octahedral N₂O ₄-coordinated complex [FeL(MeOH)₂] (2). The complex is a pure high-spin (HS) compound as is evident from magnetic measurements and X-ray crystallography.

Full text of DOI:10.1515/znb-2010-0315

A Promising New Schiff Base-like Ligand for the Synthesis
of Octahedral Iron(II) Spin Crossover Complexes
Wolfgang Bauer, Tanja Ossiander, and Birgit Weber
Center for Integrated Protein Science Munich, Department Chemie und Biochemie,
Ludwig-Maximilians-Universita¨t Mu¨nchen, Butenandtstraße 5 – 13 (Haus F), 81377 Mu¨nchen,
Germany
Reprint requests to PD Dr. Birgit Weber. Fax: (+49) 89-2180-77407.
E-mail: bwmch@cup.uni-muenchen.de
Z. Naturforsch. 2010, 65b, 323 – 328; received November 16, 2009
Dedicated to Professor Rolf W. Saalfrank on the occasion of his 70th birthday
A new tetradentate Schiff base-like ligand H2L (1) ((E,E)-dimethyl-2,2ꢀ-[1,2-phenylenebis(amino-
methylidyne)]bis[3-oxobutanoate]) was synthesised and structurally characterised. Its reaction with
iron(II) acetate leads to the formation of the octahedral N2O4-coordinated complex [FeL(MeOH)2]
(2). The complex is a pure high-spin (HS) compound as is evident from magnetic measurements and
X-ray crystallography.
Key words: Schiff Base-like Ligand, Iron Complexes, Magnetic Properties
Introduction
first example of this type of complexes, where we were
able to analyse the X-ray structure of both, the free lig-
and and its iron complex.
The bistability of spin transition complexes (spin
crossover, SCO) is one of the most promising possi-
bilities for new electronic devices in molecular mem-
ories and switches as it may be controlled by different
physical perturbations such as temperature, pressure
or light [1, 2]. Of the possible types of spin transition
Results and Discussion
Synthesis
Scheme 1 gives the general synthetic route to the
(gradual, abrupt, with hysteresis, step wise, incom- tetradentate Schiff base-like ligand H2L (1) and its oc-
plete), much of the interest is focused on the bistability tahedral iron(II) complex [FeL(MeOH)2] (2). The syn-
in highly cooperative systems (hysteresis or memory thesis of the ligand is an adaptation of Claisen [4], who
effect) as such compounds can exist in two different first described the preparation of oxymethylene deriva-
electronic states, depending on the history of the sys- tives and their reaction with amines in the late 19th cen-
tem. With regard to this we recently characterised an tury, and of Ja¨ger and Wolf [5], who first described the
iron(II) spin crossover complex with a 70 K wide ther- preparation of this type of ligand in the 1960s. The syn-
mal hysteresis loop around room temperature based on thesis of H2L is carried out in two independent steps.
a 2D network of hydrogen bonds between the complex At first the methoxymethylene derivative 1a is directly
molecules [3]. In order to more deeply understand the synthesised by oxymethylation of methyl acetoacetate
role of hydrogen bonds for cooperative effects in spin using trimethyl orthoformate in acetic anhydride. Con-
crossover systems we designed a new ligand with two trary to literature it is essential to use trimethyl ortho-
ethyl groups substituted by methyl groups – a compar- formate instead of triethyl orthoformate, in order to
atively small modification that can, however, signifi- prevent the substitution of the methoxy ester group by
cantly influence the magnetic properties. In Scheme 1 an ethoxy ester group. The second step is the condensa-
the general procedure for the synthesis of the new lig- tion of 1a with 1,2-ortho-phenylenediamine (stoichio-
and and its iron(II) complexes is given. In this paper we metric ratio 2 : 1) in methanol as solvent, which gives 1
present the synthesis and characterisation of the free (H2L) as a yellow precipitate. The synthesis of the oc-
ligand H2L and its octahedral iron(II) complex with tahedral complex 2 ([FeL(MeOH)2]), where L2−, the
two methanol molecules as axial ligands. This is the deprotonated form of H2L, acts as an equatorial ligand
c
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W. Bauer et al. · Octahedral Iron(II) Spin Crossover Complexes
Table 1. Crystal structure data for 1 – 2.
1
2
Formula
Mr
C18H20N2O6
360.36
C20H26N2O8Fe
478.28
Crystal size, mm3
Crystal system
Space group
0.35 × 0.25 × 0.20 0.36 × 0.20 × 0.15
orthorhombic
Pbca
monoclinic
P21
0.71069
173(2)
8.9590(6)
7.4730(5)
15.840(3)
92.96(1)
1059.1(2)
2
˚
λ, A
0.71073
200(2)
T, K
˚
a, A
15.0360(2)
8.60200(10)
27.3832(4)
90
3541.73(8)
8
˚
b, A
˚
c, A
β, deg
3
˚
V, A
Z
Dcalcd, g cm−3
µ(MoKα ), mm−1
θ, deg
1.35
0.1
1.50
0.8
3.3 – 25.0
1520
−17 ≤ h ≤ +17
−10 ≤ k ≤ +10
−32 ≤ l ≤ +32
18940
4.3 – 26.4
500
−11 ≤ h ≤ +9
−6 ≤ k ≤ +9
−19 ≤ l ≤ +18
4371
3094 / 0.0247
286
0.0392 / 0.0603
−0.009(15)
0.942
F(000), e
hkl range
Refl. measured
Refl. unique / Rint
Param. refined
3124 / 0.0297
239
Scheme 1. General route for the synthesis of the Schiff base-
like ligand 1 (H2L) and its octahedral iron(II) complex 2
([FeL(MeOH)2]).
R(F) / wR(F2)a (all refl.) 0.0476 / 0.1199
x(Flack)
–
GoF (F2)b
1.072
0.28 / −0.27
−3
˚
∆ρfin (max / min), e A
0.22 / −0.29
a
2
R1 = ΣꢁFo| − |Fcꢁ/Σ|Fo|, wR2 = [Σw(Fo − Fc2)2/Σw(Fo2)2]1/2
,
w = [σ2(Fo2)+(AP)2+BP]−1, where P = (Max(Fo2, 0)+2Fc2)/3 and A
and B are constants adjusted by the program; b GoF = S = [Σw(Fo
−
2
Fc2)2/(nobs −nparam)]1/2, where nobs is the number of data and nparam
the number of refined parameters.
˚
Table 2. Selected bond lengths (A) and angles (deg) of com-
plex 2 with estimated standard deviations in parentheses.
Fe1–O1
Fe1–O3
Fe1–N1
2.0113(17)
2.264(2)
2.075(2)
Fe1–O2
Fe1–O4
Fe1–N2
1.9904(19)
2.189(2)
2.0810(19)
O1–Fe1–O2
105.62(8)
O3–Fe1–O4
174.77(8)
X-Ray structure analysis
Crystals suitable for X-ray analysis of the free equa-
torial ligand 1 and its iron(II) complex 2 were obtained
by slow crystallisation out of the mother liquor. The
crystallographic data are summarised in Table 1. Fig. 1
displays the asymmetric units of 1 and 2, respectively.
Selected bond lengths and angles are summarised in
Table 2. Compound 1 crystallises with the orthorhom-
bic space group Pbca. The unit cell contains 8 formula
units. The molecular structure of 1 is non-planar, and
Fig. 1. ORTEP drawing of the asymmetric units of ligand 1
(top) and complex 2 (bottom) with selected bond lengths. For
clarity reasons the hydrogen atoms have been omitted for 2.
Displacement ellipsoids are shown at the 50 % probability
level.
and the methanol molecules as axial ligands, is carried thus the N2O2 coordination sites are not in a common
out in a one-pot reaction by treatment of 1 with a slight plane. This is indicated by the two dihedral angles,
excess of anhydrous iron(II) acetate in methanol.
which are defined by atoms C3–N1–C4–C5 and C10–
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W. Bauer et al. · Octahedral Iron(II) Spin Crossover Complexes
325
Fig. 2. 1H-NMR spectrum of H2L with the sig-
nal assignment given at the left.
˚
Table 3. Bond lengths (A) and angles (deg) of inter-
molecular hydrogen bonds of complex 2 with d(D···A)
˚
< R(D) + R(A) + 0.50, d(H···A) < R(H) + R(A) − 0.12 A,
D–H···A > 100.0◦.
O3–H31···O5a
O4–H41···O5b
D–H
0.84
0.84
H···A
D···A
D–H···A
1.93
2.763(3)
175
1.92
2.758(3)
173
a Symmetry code: 2−x, −1/2+y, −z; b symmetry code: 2−x,1/2+
y, −z.
Scheme 2. Possible tautomeric structures of 1, keto-enamine
(left) and enol-imine (right). The equilibrium is shifted to the
keto-enamine form.
lengths and angles around the iron atom are within the
range reported for octahedral HS iron(II) complexes
of the same ligand type with two axial ligands [6, 7].
N2–C9–C8 and display the torsion of the conjugated
aminomethylidene chains, which build up the chelate
ring, relative to the phenylene ring. Their values are
7.6(3)◦ and 34.3(2)◦. The average bond lengths within
˚
˚
The average values are 2.08 A (Fe–Neq), 2.00 A (Fe–
˚
Oeq) and 2.23 A (Fe–Oax). A characteristic tool for the
determination of the spin state of this type of iron(II)
complexes is the Oeq–Fe–Oeq angle, which changes
from about 110◦ in the HS state to about 90◦ in the LS
state [6, 7]. With 105.6◦ compound 2 is clearly in the
HS state. The average values of the bond lengths within
˚
the conjugatedπ-system of the chelate ring [1.34 A (C-
˚
˚
˚
N), 1.38 A (C=C), 1.46 A (C–C) and 1.24 A (C=O)]
are in line with literature values for similar single and
double bonds. This leads to the result that the equi-
librium between the two possible tautomeric structures
enol-imine and keto-enamine (see Scheme 2) is shifted
to the keto-enamine form, in contrast to classic Schiff
base ligands (e. g. salen). The 1H-NMR spectrum, pic-
tured in Fig. 2, in which a CH–NH coupling constant
in the range of 12 Hz is observed, supports this find-
ing. Between the carbonyl oxygen atoms O1 and O2
of the acetyl groups and the amino groups N1–H1 and
N2–H2 intramolecular hydrogen bonds are formed.
Compound 2 crystallises in the monoclinic space
group P21, with 2 formula units per cell. The bond
˚
the conjugated π-system of the chelate ring [1.30 A
˚
˚
˚
(C–N), 1.43 A (C–C), 1.42 A (C–C) and 1.26 A (C–O)]
reveal that the negative charge of the deprotonated
form of the Schiff base-like ligand L2− is delocalised
over the six-membered chelate ring. For the discussion
of cooperative interactions or spontaneous magnetic
ordering, intermolecular interactions such as hydrogen
bonding or π-stacking are thought to play a central role
as such contacts are necessary for long-range ordering
effects. In complex 2 two different intermolecular hy-
drogen bonds can be found, which are listed in Table 3,
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326
W. Bauer et al. · Octahedral Iron(II) Spin Crossover Complexes
Fig. 3. Crystal structure of the hydrogen-bonded molecules
of complex 2 projected along [1 0 0].
leading to an infinite one-dimensional chain with the
base vector [0 1 0], as can be seen in Fig. 3. The hy-
droxyl oxygen atoms of both axial methanol ligands
act as hydrogen bond donors. In both cases, an ester
carbonyl oxygen atom O5 of the respective neighbour-
ing complex is the acceptor.
Magnetic susceptibility data
The magnetic susceptibility of the iron(II) complex
2 was measured in the temperature range from 300 to
−1
Fig. 4. Plot of the product χM T and of χM vs. T for com-
−1
2 K. The plots of χM T and χM versus T (χM being
plex 2 between 5 and 300 K (top) and the fits according to the
the molar susceptibility) are given in Fig. 4. Over the
whole temperature range investigated, 2 remains in the
paramagnetic high-spin state, with typical χM T values,
considering four unpaired electrons (S = 2). The χM T
product slightly decreases from a value of 3.34 cm3
Kmol−1 at 300 K to 3.29 cm3 Kmol−1 at 110 K, and
then again rises to 3.34 cm3 Kmol−1 at 20 K. The de-
crease of χM T below 20 K is due to zero field split-
ting. The susceptibility data above 20 K can be fitted
with the Curie-Weiss law (χM = C/(T − θ)) with the
Curie-Weiss law, χM = C/(T − θ), with the parameters θ =
−0.65 and C = 3.33 cm3 K mol−1 for complex 2. Bottom:
(zero) field-cooled magnetisation ((Z)FCM) plots measured
under a weak magnetic field of H < 30 G.
pected from results of the magnetic susceptibility mea-
surement only very weak indications for long-range
magnetic ordering can be found. The FCM measure-
ment exhibits no abrupt increase in the magnetisation
(typical for spontaneous magnetic ordering), but only
a small slope in the magnetisation curve at about 10 K.
This is in line with the ZFCM measurement. However,
the spontaneous magnetic ordering is significantly less
pronounced compared to other iron(II) complexes of
this type with methanol molecules as axial ligands and
a Schiff base-like equatorial ligand [8].
parameters θ = −0.65 K and C = 3.33 cm3 K−1 mol−1
.
The Curie constant C of the complex is in a region
expected for iron(II) complexes in the high-spin state,
and the negative Weiss constant θ together with the
temperature-dependent decrease of the χM T product
above 110 K are indications of weak antiferromagnetic
interactions between the spin centres. The increase of
the magnetic moment below 110 K is probably due to
spin canting as observed previously for similar com-
Conclusion
Compound 1 is the first example of this ligand type
plexes of this ligand system [8]. A zero field-cooled where the determination of the X-ray structure was
(ZFCM) and a field-cooled magnetisation (FCM) mea- possible. This enabled us to provide a further proof
surement with H = 30 G was performed for compound for the suggested keto-enol-tautomeric structure for
2, and the result is shown at the bottom of Fig. 4. As ex- this type of Schiff base-like ligands. Upon coordina-
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W. Bauer et al. · Octahedral Iron(II) Spin Crossover Complexes
327
tion to a metal centre the two amino nitrogens are de- [FeL(MeOH) ] (2)
2
protonated, and the negative charge is delocalised over
2.95 g anhydrous iron(II) acetate (17.0 mmol) and 3.60 g
H2L (10.0 mmol) were dissolved in 100 mL methanol, and
the mixture was heated to reflux for one hour. After two days
a black fine-crystalline precipitate had formed which was
filtered off, washed two times with 5 mL of methanol and
dried in vacuo. Crystals suitable for X-ray analysis formed
in the mother liquor within further two days. Yield: 4.26 g
(71 %). – C20FeH26N2O8 (478.3): calcd. C 50.23, H 5.48,
N 5.86; found C 49.80, H 4.43, N 5.85. – MS ((+)-DEI),
70 eV): m/z (%) = 414 (100) [FeL1]+. – IR (KBr): ν = 3398
(O–H), 1702 (OC=O), 1572 (C=O), 1435, 1417, 1388, 1269,
the six-membered chelate ring. The magnetic proper-
ties of 2 demonstrate how small changes in the ligand
structure influence the magnetic properties of the re-
lated complexes. While for other methanol diadducts
of this complex type a spontaneous magnetisation due
to canted antiferromagnetism is observed, for com-
plex 2 the observed effects in the FCM/ZFCM mea-
surements are too small to be seriously discussed any
further.
1200, 1087 cm −1
.
Experimental Section
The synthesis of the iron(II) complex was carried out
under argon using Schlenk tube techniques. The solvent
methanol was purified as described in the literature [9] and
distilled under argon. The synthesis of iron(II) acetate is de-
scribed in the literature [10].
Magnetic measurements
Magnetic susceptibility data were collected using a Quan-
tum Design MPMSR-2 SQUID magnetometer under an ap-
plied field of 0.5 T over the temperature range 2 – 300 K. The
samples of 2 were placed in gelatine capsules held within a
plastic straw. The data were corrected for the diamagnetic
magnetisation of the ligands, using tabulated Pascal’s con-
stants, and of the sample holder.
(E)-Methyl 2-(methoxymethylidyne)-3-oxobutanoate (1a)
A mixture of 58.1 g methyl acetoacetate (0.5 mol), 53.1 g
trimethyl orthoformate (0.5 mol) and 102.1 g acetic anhy-
dride (1.0 mol) was heated to reflux for 75 min, while the
colour of the reaction mixture turned to dark red. At first, un-
der normal pressure the low-boiling side products were dis-
tilled off. At second, by fractionated vacuum distillation 1a
was isolated as a slightly yellow oil at about 90 ◦C (1.7 mbar)
and used immediately for further synthesis. Yield: 39.5 g
(50 %). – 1H NMR (270 MHz, CDCl3): δ = 2.30 (s, 3 H,
COMe), 3.72 (s, 3 H, OMe), 3.95 (s, 3 H, CO2Me), 7.52 (s,
1 H, CH).
X-Ray structure determination
The intensity data of 1 were collected on a Nonius Kappa
CCD diffractometer using graphite-monochromatised MoKα
˚
radiation (λ = 0.71073 A). The intensity data of 2 were col-
lected on an Oxford XCalibur diffractometer likewise using
graphite-monochromatised MoKα radiation. The data were
corrected for Lorentz and polarisation effects. The structures
were solved by Direct Methods (SIR 97) [11] and refined by
2
full-matrix least-squares techniques against F0 (SHELXL-
97) [12]. The hydrogen atoms were included at calculated po-
sitions with fixed displacement parameters. ORTEP-III [13]
was used for the representation of the molecular structures,
SCHAKAL-99 [14] to illustrate the crystal structures.The
crystallographic data are summarised in Table 1.
CCDC 753713 (1) and 753714 (2) contain the supple-
mentary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data request/
cif.
H L (1)
2
To
a solution of 4.56 g ortho-phenylenediamine
(42.4 mmol) in 60 mL methanol was added drop-wise a
solution of 14.7 g 1a (93.4 mmol) in 20 mL of methanol.
The reaction mixture was heated to reflux for 30 min. After
cooling to r. t. light-yellow crystals formed which were re-
crystallised from methanol. Crystals suitable for X-ray anal-
◦
ysis formed in the mother liquor at 4 C within one day.
Yield: 14.0 g (92 %). – M. p. 132 – 133 ◦C. – C18FeH20N2O6
(360.36): calcd. C 59.99, H 5.59, N 7.77; found C 60.07,
H 5.51, N 7.73. – 1H NMR (270 MHz, CDCl3): δ = 2.54
Acknowledgements
This work has been supported financially by the Deutsche
(s, 6 H, COMe), 3.75 (s, 6 H, CO2Me), 7.23 – 7.30 (m, 4 H, Forschungsgemeinschaft (SPP 1137), the Fonds der Chemi-
CH-arom), 8.36 (d, J = 12.0 Hz, 2 H, CH), 12.95 (d, J = schen Industrie, the Center for Integrated Protein Science
12.0 Hz, 2 H, NH). – MS ((+)-DEI), 70 eV): m/z (%) = Munich (CIPSM), and the University of Munich. The authors
360.2 (17) [M]+, 285.1 (84) [C15H13N2O4]+, 253.1 (100) thank M. Reichvilser and P. Mayer for the acquisition of the
3
3
[C14H9N2O4]+, 232.1 (83) [C12H12N2O3]+.
crystallographic data.
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328
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