Tri- (M = CuII) and hexanuclear (M = NiII, CoII) heterometallic coordination compounds with ferrocene monocarboxylate ligands: Solid-state structures and thermogravimetric, electrochemical and magnetic properties

Article abstract of DOI:10.1016/j.poly.2017.09.021

The synthesis and characterization of the hexanuclear [M₂(κO-O₂CFc)₂(μ-O₂CFc)₂(μ-H₂O)(κ²N,N′-tmeda)₂] (M^II = Ni, 5; Co, 6; Fc = ferrocenyl, (η⁵-C₅H₄)(η⁵-C₅H₅)Fe; tmeda = N,N,N′,N′-tetramethylethylendiamine) and the trinuclear [Cu(κ²N,N′-tmeda)(κ²O,O′-O₂CFc)_{2 2}] (7) coordination compounds are described. Compounds 5–7 were prepared by the consecutive reaction of ferrocene carboxylic acid (FcCO₂H; 1) with [ⁿBu₄N]OH followed by treatment of in situ formed [ⁿBu₄N][FcCO₂] with the metal salts [M(tmeda)(NO₃)₂] (M = Ni, 2; Co, 3; Cu, 4). The structures of 5–7 in the solid state were determined by single crystal X-ray diffraction analysis. Isostructural 5 and 6 crystallise in the triclinic P1ˉ (5) and in the monoclinic space group P2₁/n (6). The two M^II(tmeda) entities of 5 and 6 with M^II = Ni, Co, respectively, are syn, syn-bridged by two FcCO₂^? functionalities and one μ-bridging water molecule. Additionally, two FcCO₂^? ligands are κO-coordinated to each M^II ion to form octahedral MN₂O₄ coordination setups. A related MN₂O₄ coordination setup is observed for 7 as well, whereby the Cu^II ion is coordinated by two O₂CFc and one tmeda ligand. Electrochemical investigations reveal that all individual Fc units of 5–7 are oxidized separately. Thermogravimetric analysis showed that 5 and 6 start to decompose at 110 and 125 °C and thus at significantly lower temperatures compared to 7 (200 °C). The mass residues obtained after decomposition are composed of Fe₂O₃, FeNi₃ and Fe_0.64Ni = Ni_0.36 (5), Fe and Co₃O₄ (6) and Cu₂O and CuFeO₂ (7), as determined by powder X-ray diffraction analysis (PXRD). Thermal susceptibility measurements of 5 and 6 determined a weak antiferromagnetic coupling in 5 and 6 with J = 1.1 K and J = 1.9 K, respectively.

Full text of DOI:10.1016/j.poly.2017.09.021

Accepted Manuscript
II
II
II
Tri- (M = Cu ) and hexanuclear (M = Ni , Co ) heterometallic coordination
compounds with ferrocene monocarboxylate ligands: Solid-state structures and
thermogravimetric, electrochemical and magnetic properties
Karoline Müller, Marcus Korb, Changhyun Koo, Rüdiger Klingeler, Dominique
Miesel, Alexander Hildebrandt, Tobias Rüffer, Heinrich Lang
PII:
S0277-5387(17)30598-3
https://doi.org/10.1016/j.poly.2017.09.021
POLY 12831
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
2 May 2017
14 September 2017
Please cite this article as: K. Müller, M. Korb, C. Koo, R. Klingeler, D. Miesel, A. Hildebrandt, T. Rüffer, H. Lang,
II
II
II
Tri- (M = Cu ) and hexanuclear (M = Ni , Co ) heterometallic coordination compounds with ferrocene
monocarboxylate ligands: Solid-state structures and thermogravimetric, electrochemical and magnetic properties,
Polyhedron (2017), doi: https://doi.org/10.1016/j.poly.2017.09.021
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II
II
II
Tri- (M = Cu ) and hexanuclear (M = Ni , Co ) heterometallic coordination compounds
with ferrocene monocarboxylate ligands: Solid-state structures and thermogravimetric,
electrochemical and magnetic properties
a
a
b
b,c
a
Karoline Müller, Marcus Korb, Changhyun Koo, Rüdiger Klingeler, Dominique Miesel,
a
a
a
Alexander Hildebrandt, Tobias Rüffer,* and Heinrich Lang
a
Technische Universität Chemnitz, Faculty of Natural Science, Institute of Chemistry,
Inorganic Chemistry, D-09107 Chemnitz, Germany
b
Kirchhoff Institute of Physics, Heidelberg University, D-69120, Heidelberg, Germany
Center for Advanced Materials (CAM), Heidelberg University, Heidelberg, Germany
c
ABSTRACT
The synthesis and characterization of the hexanuclear [M (O-O CFc) (µ-O CFc) (µ-
2
2
2
2
2
2
II
5
5
H O)( N,N’-tmeda) ] (M = Ni, 5; Co, 6; Fc = ferrocenyl, (η -C H )(η -C H )Fe; tmeda =
2
2
5
4
5
5
2

N,N,N’,N’-tetramethylethylendiamine) and the trinuclear [Cu( N,N’-tmeda)( O,O’-O CFc)
2
2
2]
(7) coordination compounds are described. Compounds 5 – 7 were prepared by the
n
consecutive reaction of ferrocene carboxylic acid (FcCO H; 1) with [ Bu N]OH followed by
2
4
n
treatment of in situ formed [ Bu N][FcCO ] with the metal salts [M(tmeda)(NO ) ] (M = Ni,
4
2
3 2
2; Co, 3; Cu, 4). The structures of 5 – 7 in the solid state were determined by single crystal X-
ray diffraction analysis. Isostructural 5 and 6 crystallise in the triclinic P
1
(5) and in the
II
II
monoclinic space group P2 /n (6). The two M (tmeda) entities of 5 and 6 with M = Ni, Co,
1
respectively, are syn,syn-bridged by two FcCO2 functionalities and one µ-bridging water
1

molecule. Additionally, two FcCO2 ligands are O-coordinated to each M ion to form
II
octahedral MN O coordination setups. A related MN O coordination setup is observed for 7
2
4
2
4
II
as well, whereby the Cu ion is coordinated by two O CFc and one tmeda ligand.
2
Electrochemical investigations reveal that all individual Fc units of 5 – 7 are oxidized
separately. Thermogravimetric analysis showed that 5 and 6 start to decompose at 110 and
125 °C and thus at significantly lower temperatures compared to 7 (200 °C). The mass
residues obtained after decomposition are composed of Fe O , FeNi and Fe0.64Ne0.36 (5), Fe
2
3
3
and Co O (6) and Cu O and CuFeO (7), as determined by powder X-ray diffraction analysis
3
4
2
2
(PXRD). Thermal susceptibility measurements of 5 and 6 determined a weak
antiferromagnetic coupling in 5 and 6 with J = 1.1 K and J = 1.9 K, respectively.
ARTICLE INFO
KEYWORDS
molecular structure, electrochemical studies, single crystal X-ray diffraction,
thermogravimetry, powder X-ray diffraction, magnetic susceptibility
CONTACT
Supplemental data for this article can be accessed at…
1
. Introduction
The discovery, that the electrical resistance of a magnetic device is tunable by modifying its
magnetic texture, thus of the giant magneto-resistance (GMR) effect [1–4], manifested the
beginning of the field of magneto-electronics or, in other words, of Spintronics. As
comprehensively reviewed by Sanvito [5], the use of organic molecules for the construction
2

of GMR devices gave rise to the new field Molecular Spintronics. Basically, “ … The general
idea behind spintronics is that of detecting the response of spins to an external stimulus …”
[
5], whereby the “organic molecule” could be even a single molecule magnet. Following this
idea we became interested in the possible interplay between complexes displaying both
intervalence charge/transfer properties and single molecule magnetic properties. Our group
reported frequently on the synthesis and electrochemical properties of novel charge-transfer
compounds, considering especially ferrocenyl-containing coordination compounds [6–10].
Previously, we have reported the electrochemical and magnetic properties of the isostructural
hexanuclear coordination compounds [M (O CFcCO ) (H O)(tmeda) ] (M = Ni, 8; M = Co,
2
2
2 2
2
2
9). We demonstrated, that the two ferrocenyl units of both 8 and 9 were oxidized at significant
different redox potentials and that 8 exhibits a small ferromagnetic (θ = 1.6 K) and 9 an
antiferromagnetic Weiss temperature (θ = 14.5 K) [11]. Furthermore, and in the search for
II
Ni -containing coordination compounds which decompose to pure metallic nickel we
2
recently reported on [Ni (O-O CH) (µ-O CH) (µ-H O)( N,N’-tmeda) ] (10) [12]. The
2
2
2
2
2
2
2
structure of 10 is closely related to the one of 8 and 9, and we were surprised that 10 exhibits
a weak intramolecular antiferromagnetic coupling (J = 7.8 K) together with an easy plane
magnetic anisotropy.
In order to narrow further to materials in which the spins response to an external stimuli we
aimed to replace the two O CFcCO ligands of 8 and 9 by four FcCO ligands as in [M (O-
2
2
2
2
2
II
O CFc) (µ-O CFc) (µ-H O)( N,N’-tmeda) ] (M = Ni, 5; Co, 6). Here we report on the
2
2
2
2
2
2
2

synthesis and solid state structures of 5 and 6 together with [Cu( N,N’-tmeda)( O,O’-
O CFc) ] (7). Furthermore, we describe the thermal and electrochemical properties of 5 – 7
2
2 2
and the results of susceptibility studies of 5 and 6.
3

2
.
Experimental Section
2
.1.
Materials and measurements.
All reactions were carried out under an atmosphere of argon unless otherwise stated. Starting
materials Fc(CO H) (1) and [M(tmeda)(NO ) ] (M = Ni (2), Co (3), Cu (4)) were prepared by
2
3 2
published procedures [11,13]. Pyridine was dried with KOH, distilled under argon atmosphere
and stored over molecular sieve 4 Å. Acetonitrile was purified by distillation from P O .
4
10
Diethyl ether and dichloromethane were dried using a MBraun MP SPS-800 system (double
column solvent filtration, working pressure 0.5 bar). The melting points were determined
using a Gallenkamp MFB 595 010 M melting point apparatus. The elemental analyses were
measured with a Thermo FlashEA 1112 Series instrument. IR spectra were measured using

1
KBr pellets in the range of 400 – 4000 cm with a Nicolet IR 200 spectrometer from Thermo
1
Electron Corporation. For each spectrum 16 scans were measured at a resolution of 2 cm .
High-resolution mass spectra were recorded with a Bruker Daltonik microTOF-QII
spectrometer. Thermal gravimetric measurements combined DSC measurements were
performed with a Mettler Toledo TGA/DSC1 1600 System equipped with a MX1 balance. X-
ray powder diffraction (PXRD) studies were performed with a STOE-STADI-P
diffractometer with CuKα1 = 1.540 Å in the range of 20 – 90 ° for 2θ. Electrochemical
measurements were performed on 1.0 mmol·L1 solutions of 5 – 7 in dichloromethane in a
dried, argon-pulled cell at 25 °C with a Radiometer Voltalab PGZ 100 electrochemical
n
1
workstation interfaced with a personal computer. [ Bu N][B(C F ) ] (0.1 mol·L ) was used
4
6 5 4
as supporting electrolyte.
Spectroelectrochemical UV-Vis/NIR measurements of 1.0 mmol L1 solution of 6 in
anhydrous acetonitrile containing 0.1 mol L1 of [ Bu N][B(C F ) ] as the supporting
n
4
6 5 4
electrolyte were performed in an OTTLE (= Optically Transparent Thin-Layer
Electrochemical) [14] cell with a Varian Cary 5000 spectrophotometer at 25 °C.
4

The static magnetic susceptibility χ(T) = M(T)/B of 5 and 6 was measured by means of a
Quantum Design MPMX XL-5 SQUID magnetometer. The measurements were done in the
temperature range of T = 2 - 300 K and at B = 5 T and 1 T, respectively. In order to account
for the contribution of the ligands, the temperature independent diamagnetic susceptibility of
the coordination compounds as calculated by means of Pascal’s constants has been subtracted
from the experimental data [15].
Crystallographic studies. All data were collected with an Oxford Gemini S diffractometer.
All structures were solved by direct methods using SHELXS-2013 and refined by full-matrix
least-square procedures on F² using SHELXL-2013 [16]. All non-hydrogen atoms were
refined anisotropically. All C-bonded hydrogen atoms were refined using a riding model. For
further details cf. Table S1 and additional explanation (SI).
2.2.
Synthesis of [Ni (O CFc) (H O)(tmeda) ] (5).
2 2 4 2 2
n
FcCO H (1) (150 mg, 0.652 mmol) was suspended in acetonitrile (20 mL) and [ Bu N]OH
2
4
(0.42 mL, 0.652 mmol, 40% in water) was added in a single portion at ambient temperature.
After stirring this solution for 20 min, a solution of [Ni(tmeda)(NO ) ] (2) (97 mg, 0.326
3
2
mmol) in acetonitrile (10 mL) was added dropwise. After stirring for 2 h the solution volume
was reduced to 5 mL and stored over night at 5 °C. The obtained precipitate was filtered off
and washed with cold acetonitrile (2 × 3 mL) and diethyl ether (2 × 3 mL). The solid was
recrystallized in a mixture of acetonitrile and diethyl ether (ratio 3:5, v/v) and stored at 5 °C.
After one week crystals suitable for crystallographic studies were obtained, filtered off,
washed with cold diethyl ether (2 × 3 mL) and were dried in vacuum. Yield: 0.143 mg (68 %
of 5 based on 1). Comment: Crystals selected under an optical microscope for crystallographic
characterisation were observed to become quickly brittle, which indicates loss of packing
5

solvent molecules. Note that crystallographic characterisation revealed a composition of
(5) (MeCN) (Et O)] (5’). The elemental analysis of 5 dried in vacuum confirmed the loss of
[
2
3
2
the packaging solvent molecules (Anal. Calcd. for C H N O Ni Fe (1285.96 g/mol, 5; %):
56
72
4
9
2
4
C, 52.30; H, 5.64; N, 4.36. Found: C, 51.98; H, 5.85; N, 4.34.), while material that was stored
on air produced an elemental analysis which refers to 5 with an incorporation of one molecule
of H O per molecule of 5. M.p.: 175 °C. Anal. Calcd. for C H N O Ni Fe (1303.97 g/mol,
2
56 74
4
10
2
4

1
5
3
1
7
·H O; %): C, 51.58; H, 5.72; N, 4.30. Found: C, 51.46; H, 5.80; N, 4.46. IR (KBr, cm ):
2
096 (w), 3018 (w), 2903–2840 (m), 2794 (w), 2055 (w, br), 1617 (s), 1534 (w, br), 1473 (s),
388 (s), 1359 (s), 1348 (m, sh), 1287 (w), 1187 (w), 1106 (w), 1024 (m), 957 (w), 802 (m),
+
+
73 (s). ESIMS: m/z = 403.0732 [M  Ni(tmeda)(O CFc) ] ; 1035.1187 [M – O CFc] . Fig.
2
3
2
S1 (SI) gives the IR spectra of 5 and 5∙H O and Figure S4 (SI) the ESI-MS spectrum of 5.
2
2
.3. Synthesis of [Co (O CFc) (H O)(tmeda) ] (6).
2 2 4 2 2
Compound 6 was prepared according to the procedure reported for 5. FcCO H (1) (150 mg,
2
n
0
.652 mmol) was reacted with [ Bu N]OH (0.42 mL, 0.652 mmol; 40% in water) and
4
[
Co(tmeda)(NO ) ] (3) (94 mg, 0.33 mmol) in acetonitrile. After stirring for 2 h at ambient
3 2
temperature the volume was reduced to 5 mL and stored over night at 5 °C. The formed
precipitate was filtered and washed with acetonitrile (2 × 3 mL) and diethyl ether (2 × 3 mL).
Crystals were obtained out of a mixture of acetonitrile and diethyl ether (ratio 3:5, v:v)
containing 6 after partial evaporation of diethyl ether. Yield: 0.125 mg (60 % for 1). M.p.: 195
°
C (decomp.). Anal. Calcd. for C H N O Co Fe (1286.44 g/mol; %): C, 52.28; H, 5.64; N,
56 72 4 9 2 4

1
4
–
.36. Found: C, 52.37; H, 5.74; N, 4.25. IR (KBr, cm ): 3090 (w), 3012 (w), 2974 (w), 2907
2839 (m), 2788 (w), 2048 (w, br), 1612 (s), 1532 (w, br), 1472 (s), 1388 (s), 1359 (m), 1347
(m), 1283 (w), 1158 (w), 1106 (w), 1015 (m), 951 (w), 799 (m), 767 (m), 560 (m). ESIMS:
6

+
+
m/z = 404.0660 [M  Co(tmeda)(O CFc) ] ; 1037.1144 [M – O CFc] . Fig. S1 (SI) gives the
2
3
2
IR spectrum of 6 and Fig. S5 (SI) gives the ESI-MS spectrum of 6.
2
.4. Synthesis of [(tmeda)Cu(O CFc) ] (7).
2 2
Compound 7 was prepared according to the procedure reported for 5. FcCO H (1) (150 mg,
2
n
0
.652 mmol) was reacted with [ Bu N]OH (0.42 mL, 0.652 mmol, 40% in water) and
4
[
Cu(tmeda)(NO ) ] (4) (99 mg, 0.326 mmol) in acetonitrile. After stirring the reaction mixture
3 2
for 1 h at ambient temperature, the precipitated solid was filtered and washed with diethyl
ether (3 × 10 mL). Single crystals were obtained by slow diffusion of diethyl ether into a
solution of dichloromethane containing 7 at ambient temperature. For purification the
crystallisation procedure had to be carried out twice. Yield: 0.176 mg (84 % for 1). M.p.: 230
°
C (decomp.). Anal. Calcd. for C H N O CuFe (637.82 g/mol; %): C, 52.73; H, 5.37; N,
28 34 2 4 2

1
4
.39. Found: C, 52.43; H, 5.44; N, 4.20. IR (KBr, cm ): 3109 – 3086 (w, m), 2979 – 2803
(
w, m), 1566 (νasym(CO ), vs), 1467 (ν (CO ), s), 1385 (ν (CO ), s), 1385 (ν (CO ), s),
2 asym 2 sym 2 sym 2
1
5
354 (m, sh), 1336 (s), 1289 (w), 1180 (m), 1105 (m), 1021 (m), 1003 (w), 957 (w), 797 (s),
+
+
+
12 (m). ESIMS: m/z = 408.0557 [M  O CFc] ; 637.0510 [M] ; 660.0408 [M + Na] . Fig.
2
S1 (SI) gives the IR spectrum of 7 and Fig. S6 (SI) gives the ESI-MS spectrum of 7.
7

3
.
Results and Discussion
3.1. Synthesis
The consecutive synthetic methodology to prepare 5 – 7 is shown in Scheme 1 and is
analogous to our previous report [11]. The reaction of ferrocene monocarboxylic acid
n
(
[
[
[
(
FcCO H, 1) with [ Bu N]OH in a 1:1 molar ratio in acetonitrile afforded in situ generated
2
4
n
Bu N][FcCO , which upon treatment with the appropriate amount of the metal salts
4
2
M(tmeda)(NO ) ] (M = Ni, 2; Co, 3; Cu, 4) produced either discrete hexanuclear M Fe
4
3
2
2
M (O CFc) (H O)(tmeda) ] (M = Ni, 5; Co, 6) or trinuclear CuFe [Cu(tmeda)(O CFc) (7)
2
2
4
2
2
2
2
2
Scheme 1).
Solids of 5 and 6 are soluble in most common organic solvents including diethyl ether and
acetonitrile, while 7 is insoluble in polar solvents. After appropriate work-up 5 – 7 were
isolated as yellow, orange or green solids, respectively (Experimental Section). Crystals of 5
were characterized as [(5) (MeCN) (Et O)] (5’) while crystals of 6 do not any contain packing
2
3
2
solvent.
8

Scheme 1. Synthesis of 5 – 7 from 1 and [M(tmeda)(NO ) ] (M = Ni, 2; M = Co, 3; M = Cu, 4).
3
2
3
.2.
IR spectroscopy
The IR spectra of 5 – 7 are shown in Fig. S1 (SI) and display similar spectral features for all
three coordination compounds. For example, all spectra exhibit four intense ν(CO ) vibrations
2
–
1
in the region of 1300 – 1600 cm due to the different binding motifs of the FcCO ligands.
2
Single crystal X-ray diffraction studies proved for isostructural 5 and 6 two OO CFc and
2
two µ-O CFc bonded ligands (Scheme 1, Fig. 1). Generally, it is possible to distinguish
2
between the different binding motifs of carboxylates by the intensities of the CO stretching
2
frequencies and the difference of the asymmetric (νasym) and symmetric (νsym) CO vibrations
2
(
ν, ν = νasym – ν ; ∆ν
(CO ) < ∆ν
(CO ) ≤ ∆ν (CO ) << ∆ν
(CO ))
ym
chelating
2
bridging
2
ionic
2
unidentate
2
[
17,18].
9

However, we recently reported for the IR spectroscopic characterization of 10, inclusive
accompanying DFT calculations [12], that these common assigning rules are not precisely
applicable. A related situation is observed for here reported 5 and 6 and thus it is not possible
to assign unambiguously the νasym(CO ) and ν (CO ) vibration modes. Another
2
sym
2
–
1
characteristic of both 5 and 6 is a very broad band at approximately 2000 cm , which
presents the OH vibration of the µ-bridging water ligand, as observed for 10 [12,19,20]. The
significant shift of the OH vibration of 5, 6 and 10 [12] to higher energies, when compared to
free water, is based on the intramolecular hydrogen bonding between the µ-bridging water
and the kO-bonded carboxylate ligands (Fig. 1) and is additionally verified by DFT
2
calculations [17,19,21–23]. For 7 the presence of two asymmetrically k O,O’-bonded O CFc
2
ligands was found (Scheme 1, Fig. 2). The respective ν(CO ) absorptions of 7 are, compared
2
to 5 and 6, somewhat shifted to smaller wavenumbers (1565, 1466 (νasym(CO )); 1385, 1336
2

1
1
(
νsym(CO )) cm ) (Experimental Section). The ∆ν (180 and 130 cm ) agree with the
2
observed binding motif of the O CFc ligands according to the literature [17,18].
2
3.3. The molecular structures of 5 – 7
The structures of 5 and 6: In case of 5’ the asymmetric unit of the centrosymmetric triclinic
unit cell comprises two crystallographically independent molecules of 5, denoted as 5A
(including Ni1 and Ni2) and 5B (including Ni3 and Ni4). Such a phenomena is observed for 6
as well, as in case of 6 the asymmetric unit of the centrosymmetric monoclinic unit cell in
P2 /n comprises two crystallographically independent molecules, denoted as 6A (including
1
Co1 and Co2) and 6B (including Co3 and Co4). Related structural features of 5A/5B and
6A/6B compare well with each other and therefore all further discussion refer to 5A and 6A
only. Thus, the molecular structures of 5A and 6A are presented in Fig. 1 and selected bond
lengths and angles are summarized in Table 1.
1
0

Fig. 1. ORTEP (25 % ellipsoid probability) of the molecular structures of 5A (left) and 6A (right). All C-bonded
hydrogen atoms were omitted for clarity. Of disordered Fc units of 6A only one atomic position is shown.
Intramolecular hydrogen bonds and iron to geometrical centroid distances of C H and C H units were indicated
5
4
5
5
by dotted lines.
II
2
II
2
Hexanuclear 5A and 6A (Fig. 1) belong to a family of Ni - [11,24–41] and Co -containing
[
11,19,24,31,41–64] coordination compounds possessing related bis(carboxylate)-1O;2O-
2
2
bis(µ-carboxylate-1:2k O,O’)-µ-aqua-1:2 O cores. In this family the organic groups R of the
carboxylate ligands O CR vary, as well as the nature of additional mono- or bidentate N-
2
donor ligands coordinated to the metal ions. Thus, each metal atom of 5A/6A is coordinated
by the µ-aqua ligand, by two O-donor atoms of two µ-FcCO ligands, by one O-donor atom
2
2
of a O-bonded FeCO entity and by a  N,N’-bonded tmeda unit, resulting in distorted
2
octahedral MO N setups. Accordingly, bond angles of trans-aligned donor atoms cover a
4
2
range from 169.8 to 178.7 ° for 5A, and 169.2 to 178.8 ° for 6A, while those of cis-aligned
donor atoms cover a range from 84.4 to 96.8 ° for 5A, and 82.1 to 98.8 ° for 6A (Table 1).
Bond lengths and angles of 5A and 6A compare well with related values described for already
crystallographically characterized analogous coordination compounds. To verify this, selected
structural features of coordination compounds of the type [M (O CFc) (H O)(tmeda) ] (M =
2
2
4
2
2
Ni, [11,24–41],[65] Co [11,19,24,31,41–64]) were searched with the Cambridge Structural
Database (CSD, version 5.37, 02/2016) and were compared with 5A and 6A as described in
the SI. In summary it can be stated, that isostructural 5A and 6A show no structural
1
1

peculiarities and compare well to already described analogous coordination compounds.
There is a final remark: Although 5A and 6A are isostructural, they are not isomorphic as
their crystallographic data (unit cell, space group) differ. In Fig. S8 (SI) an overlay of the
cores of both coordination compounds is displayed revealing minor deviation, although the
orientation of the terminal Fc substituents differs. That proves that the Fc substituents can
freely rotate in solution.
a)
Table 1. Selected bond lengths [Å] and angles [°] of 5A, 6A and 7.
Bond lengths
5
A
6A
5A
6A
7
M1–O1
M1–O3
M1–O7
M1–O9
M1–N1
M1–N2
C1–O1
C12–O3
C23–O5
C34–O7
2.033(4)
2.016(4)
2.094(4)
2.088(4)
2.158(5)
2.179(5)
1.253(6)
1.268(7)
1.277(7)
1.276(7)
1.644(3)
1.642(3)
1.637(2)
1.628(4)
2.045(5)
2.052(5)
2.063(18) M2–O5
2.125(5)
2.221(6)
2.257(6)
1.267(8)
1.249(8)
1.249(8)
M2–O2
M2–O4
2.026(4)
2.020(4)
2.064(4)
2.077(4)
2.161(5)
2.180(4)
1.251(7)
1.271(8)
1.264(7)
1.252(7)
1.643(3)
1.644(3)
1.641(3)
1.649(4)
2.040(5)
2.065(5)
2.099(5)
2.113(5)
2.222(6)
2.215(6)
1.242(8)
1.277(9)
1.252(8)
Cu1N1
Cu1N2
Cu1O1
Cu1O2
Cu1O3
Cu1O4
C1O1
2.034(3)
2.034(3)
2.550(2)
1.947(2)
1.953(2)
2.594(2)
1.245(4)
1.286(4)
1.285(4)
1.248(4)
1.647(2)
1.647(2)
1.648(2)
1.660(2)
M2–O9
M2–N3
M2–N4
C1–O2
C12–O4
C23–O6
C1O2
C12O3
1.256(18) C34–O8
1.256(19) C12O4
b)
b)
b)
b)
b)
c)
c)
c)
c)
Fe1–D1
Fe2–D3
Fe3–D5
Fe4–D7
1.651(4)
1.652(4)
1.639(4)
1.630(4)
Fe1–D2
Fe2–D4
Fe3–D6
Fe4–D8
1.657(4)
1.649(4)
1.666(4)
1.632(4)
Fe1D1
Fe1D2
Fe2D3
Fe2D4
b)
b)
b)
Bond angles
M1–O9–M2 116.49(15) 116.4(2)
O1–M1–O3 93.59(15) 95.4(2)
O1–M1–O7 177.48(16) 174.2(12) O2–M2–O5
N1Cu1N2 86.71(12)
O2Cu1O3 92.31(10)
O2Cu1N2 162.63(10)
O2–M2–O4
91.66(16)
88.61(16)
92.78(14)
93.77(16)
89.9(2)
87.6(2)
O1–M1–O9 92.67(15)
O3–M1–O9 92.74(15)
O3–M1–O7 86.37(15)
O7–M1–O9 89.85(15)
N1–M1–N2 84.40(19)
N1–M1–O1 87.05(16)
N1–M1–O3 86.04(18)
N1–M1–O7 90.43(17)
N1–M1–O9 178.73(17) 178.8(2)
N2–M1–O1 89.51(17) 89.7(2)
N2–M1–O3 169.78(18) 169.9(2)
N2–M1–O7 90.10(17)
N2–M1–O9 96.85(17)
91.90(18) O2–M2–O9
89.75(19) O4–M2–O9
93.62(19) O3Cu1N1 167.38(11)
93.10(18) O2Cu1N1 91.69(11)
87.5(18)
93.2(10)
82.3(2)
88.1(2)
89.1(2)
86.9(10)
O4–M2–O5
O5M2–O9 88.34(15)
177.86(16) 177.1(2)
O3Cu1N2 92.96(12)
88.53(18) O2C1O1
123.0(3)
O3C12O4 122.3(3)
N3–M2–N4
N3–M2–O2
N3–M2–O4
N3–M2–O5
N3–M2–O9
N4–M2–O2
N4–M2–O4
N4–M2–O5
N4–M2–O9
84.46(17)
86.30(16)
87.77(18)
90.13(18)
178.24(18) 176.9(2)
170.55(18) 169.2(2)
89.97(17)
89.43(17)
96.40(16)
82.1(2)
87.4(2)
82.1(2)
88.6(2)
c)
D1Fe1D2
D3Fe2D4
178.5(3)
176.5(3)
c)
92.8(2)
89.4(2)
96.7(2)
126.6(6)
125.9(6)
86.8(17)
98.8(2)
126.5(6)
126(3)
O1–C1–O2
O7–C34–O8 125.7(6)
127.1(5)
O3–C12–O4 128.0(6)
O5–C23–O6 123.8(6)
a) For 5A/6A M1 and M2 refer to Ni1 and Ni2/Co1 and Co2, respectively. Of disordered units only data of one
fragment are given. b) D1/D3/D5/D7 and D2/D4/D6/D8 denote geometrical centroids of C H and C H units,
5
4
5
5
respectively. c) D1/D3 and D2/D4 denote geometrical centroids of C H and C H units, respectively.
5
4
5
5
1
2

The structure of 7: The molecular structure of 7 is displayed in Fig. 2 and selected bond
2
lengths and angles are given in Table 1. The Cu atom of 7 is  N,N’-bonded by the tmeda and
2
by two  O,O’-bonded O CFc ligands (Fig. 2). The thus formed CuN O coordination unit
2
2
4
exhibits a strongly distorted octahedral geometry, which might be better described as a
distorted tetragonal-bipyramidal coordination geometry [66]. Related crystallographically
II
described Cu coordination compounds of the general composition [Cu(O CR) (N,N’)] (R =
2
2
organic group; N,N’ = bidentate ligand with C Csp3 bridges) [66–75] deposited with the
sp3
CSD were used in for a structural comparison with 7, as described in the SI. As a summary
thereof it can be stated that 7 does not show structural peculiarities and compares well with its
structural parameters to already described analogous coordination compounds.
Fig. 2. ORTEP diagram (50 % ellipsoid probability) of the molecular structure of 7. All hydrogen atoms were
omitted for clarity. Iron to geometrical centroid distances of C H and C H units were indicated by dotted lines.
5
4
5
5
3.4.
Thermogravimetric analysis
TG (= thermogravimetry) along with DSC (= differential scanning calorimetry) studies were
carried out to determine the decomposition behaviour of 5 – 7 in the temperature range
between 40 to 800 °C (Fig. S2, SI).
1
3

Hexanuclear 5 or 6 start to decompose at a significant lower temperatures (110 or 125 °C)
when compared to trinuclear 7 (190 °C). The residual masses were determined to 18.5 (5),
21.2 (6) and 24.5 % (7). PXRD analysis (= X-ray powder diffraction) confirmed the
formation of crystalline Fe O (maghemit-Q; ICDD C00-039-1346) for all three compounds
2
3
(
Fig. S3, SI). The formation of Fe O is attributed to traces of oxygen in the argon gas stream
2 3
as discussed for 10 [12]]. Additionally, the formation of Fe O under anaerobic conditions
2
3
goes coherent with previously reported studies by Yilmaz et al. [76]. Beyond that, PXRD
analysis reveal the formation of FeNi (alloy, ICDD C01-071-8324) and Fe0.64Ni0.36 (alloy,
3
ICDD C00-047-1405) for 5, Co O (ICDD C00-042-1467) and Fe (ICDD C00-006-0696) for
3
4
6
, and Cu O (cuprite; ICDD C00-005-0667) and CuFeO (delafossite; ICDD C00-039-0246)
2
2
for 7 (for more details see Fig. S3, SI).
3.5. Electrochemical properties
The redox properties of 5 – 7 have been investigated by cyclic voltammetry (CV) (25 °C) and
square wave voltammetry (= SWV) (25 °C) (Fig. 3). As supporting electrolyte a 0.1 M
n
solution of [ Bu N][B(C F ) ] [77–91] in anhydrous dichloromethane was used. All redox
4
6 5 4
+
5
potentials were referenced to the FcH/FcH redox couple (E°′ = 0 mV, FcH = Fe( -C H ) )
5
5 2
[
92]. The cyclic and square wave voltammograms are shown in Fig. 3 and data derived from
CV measurements are summarized in Table 2. For simulation of the SWV potentials three or
four Gaussian-shaped functions were applied to get fits well enough for an almost exact
overlay with the experimental voltammograms (Table 2).
Within the cyclic voltammetry measurements of 5 two redox processes for the four ferrocenyl
units were observed, whereas a third process is noticed as a shoulder. The difference between
the cathodic and anodic peak potential (ΔE = 132 mV) for the first event at E°′ = 38 mV
p
1
suggests two one electron processes in close potential proximity. In order to resolve the
1
4

ferrocenyl-based redox events deconvolution of the square wave voltammogram was
performed (Fig. 3). The area ratio of the obtained four functions is 1:1.19:1.01:1.04, cf. Fig. 3,
which corresponds to the subsequent oxidation of the ferrocenyls. Furthermore, the peak-to-
peak separation of two superimposed one-electron processes within the cyclic voltammogram
st
nd
was determined by the method of Richardson and Taube [93]. For the 1 and 2 redox
process of 5 ΔE°′ was identified to be approximately 95 mV. This small potential difference is
not resolved within standard cyclic voltammetry techniques.
For the ferrocenyl groups of 6 two redox events were observed within the CV. However, the
asymmetric shape of the event at 270 – 360 mV emphasises that most probably more than one
redox process takes place in a close potential range. The SWV for 6 shows four waves,
whereby only the first one appears as a single one-electron process, while the following
events were deconvoluted into three ferrocenyl-based partly superimposed oxidations (Fig. 3).
st
nd
Within the SWV an increased redox separation between the 1 and 2 redox process of 6
compared to compound 5 (ΔE°′ = 86 mV, 5; ΔE°′ = 209 mV, 6) was found, indicating
interactions between the Fc groups. In order to identify the origins of those interactions
spectroelectrical
measurements
have
been
carried
out.
The
UV-Vis/NIR
spectroelectrochemical study of 6 (1.0 mmol·L1 acetonitrile solution, 0.1 mol·L1 of
n
[
Bu N][B(C F ) ] as supporting electrolyte) was performed in an OTTLE (= Optically
4 6 5 4
Transparent Thin-Layer Electrochemical) cell [14]. During the measurements, oxidation of
+
4+
neutral 6 to mixed-valent [6] , and finally to the fully oxidized compounds [6] was
performed by a stepwise increase of the potentials (step width: 25, 50, 100 mV, Fig. S6, SI).
To prove the reversibility, 6 was reduced at 200mV after full oxidation and the afterwards
measured UV-Vis/NIR spectrum was identical to the one of the starting compound. During
+
the oxidation process (up to 1000 mV vs Ag /AgCl) no IVCT absorption (= inter-valence
charge-transfer) were observed in the NIR region, indication that the large redox separation is
caused mostly by electrostatic interactions between the Fc entities.
1
5

The two ferrocenyl groups in 7 oxidize separately (E°′ = 1 mV, E°′ = 171 mV). The
1
2
differences between the cathodic and anodic peak potential ΔE of 70 and 78 mV indicate the
p
reversible character of the ferrocenylbased oxidations. The redox separation ΔE°′ = 170 mV
might be influenced by electrostatic interactions between the ferrocenyl units. In addition, a
third process as a shoulder was observed at E°′ = 20 mV. Deconvolution of the SWV gave
3
rd
three redox processes of peak ratio 1:0.94:0.63. The 3 event is most likely attributed to the
II
oxidation of Cu which is in accordance with copper(II) carboxylates known to literature (–
0.6 to 0.6 V) [94–98].
ia
ia
ic
20 A
2 A
ic
5
1.19
1.01
1
1.04
6
7
0
.91
0.96
1
0.99
1
0.94
0.63
-
1.2 -0.8 -0.4 0.0 0.4 0.8 1.2
-0.4
-0.2
0.0
0.2
0.4
0.6
+
Potential in mV vs FcH/FcH
Potential in mV

1
n
Fig. 3. Left: Cyclic voltammograms of 5 – 7 (1 mmol·L dichloromethane solution, 25 °C, [ Bu N][B(C F ) ]
4
6 5 4

1
–1
(
0.1 mol·L ) as supporting electrolyte, scan rate = 100 mV s ). Right: Square wave voltammograms of 5 – 7
1
n
1
(
0.1 mol·L dichloromethane solution, 25 °C, [ Bu N][B(C F ) ] (0.1 mol·L ) as supporting electrolyte, scan
4 6 5 4
–1
rate = 2.5 mV·s ) (red line). Spectral deconvolution of the SWVs was accomplished using Gaussian-shaped
functions (dotted lines: individual electron transfer processes; dashed lines = sum of all electron transfer
+
processes). All potentials are referenced to the [FcH]/[FcH] redox couple.
1
6

Table 2. Cyclic and square-wave voltammetry data using three to four Gaussian-shaped functions for 5 – 7.
5
6
7
Cyclic voltammetry data a d)
g)
g)
E°′ , V (ΔE , V)
0.02 (0.13)
0.11 (0.10)
0 (0.07)
1
p
g)
f)
E°′ , V (ΔE , V)
0.19 (0.13)

0.17 (0.08)
2
p
e)
g)
e)
E°′ , V (ΔE , V)

0.20 
0.32
 
3
p
g)
ΔE°′ , V
0.15
0.17
1
Square-wave voltammetry
st
nd
rd
th
st
nd
rd
th
st
nd
rd
Process No.
ratio
1 
2
3
4
1
2
3 
4 
1 
2 
3 
1
1.19
1.01
0.18
0.10
1.04
0.28
0.14
1
0.99
0.10
0.14
0.91 0.96
0.20 0.29 0.02 0.16 0.27
0.11 0.13 0.12 0.11 0.17
1
0.94 0.63
E°′ /V
0.03 0.05
0.14 0.12
0.11
0.13
h)
FWHM /V
+
1
1
a) Potentials vs FcH/FcH , scan rate 100 mV·s at glassy-carbon electrode of 1.0 mmol·L solutions of 5 – 7 in
dry dichloromethane; 0.1 mol·L1 of [ Bu N][B(C F ) ] as supporting electrolyte at 25 °C. b) ΔE = difference
n
4
6
5 4
p
between anodic and cathodic peak potential (ΔE = E – E ). c) E°′ = formal potential (E°′ = E + E )/2). d) ΔE°′
1
p
pa
pc
pa
pc
=
difference between two redox processes (ΔE°′= E°′  E°′ ). e) E
values not possible to identify. Appears as a
pc/pa
2
1
shoulder. f) Epc value of second process not possible to identify, since second and third processes are superimposed.
g) Determined from potentials of SWV measurements. h) Full Width of Half Maximum.
3
.6.
Magnetic measurements
Fig. 4 shows the product of the static magnetic susceptibility, corrected by the diamagnetic
contribution of the ligands inferred from Pascal’s constants [15], and temperature, (χmol – χ )T
0
(
T), as well as the inverse susceptibility 1/(χmol – χ ) (T) of 5 and 6. For solids of 6, at high
0
temperatures, i.e. above T = 150 K, the data obey a Curie-Weiss-like behavior which is, e.g.,
shown by the fact that (χmol – χ )T exhibits a constant value of (χ – χ )T = 5.85 erg
0
mol
0
2
K/G /mol. As temperature decreases, (χ – χ )T smoothly decreases, followed by a more
mol
0
2
steep decrease below around 17 K. At T = 2 K, it amounts to 0.99 erg K/G /mol. The low
temperature decrease of (χmol – χ )T observed for 6 can be either interpreted as signature of
0
II
antiferromagnetic interaction between the Co moments or magnetic anisotropy of the metal
ions.
Fitting the linear region of the inverse susceptibility (Fig. 4a) at T > 150 K by means of the
Curie-Weiss law, i.e. χmol - χ = C/(T – θ), yields the Curie constant C = 6.13 ± 0.01 erg
0
2
K/G /mol and the Weiss temperature θ = –8.4 ± 0.4 K. The negative value of θ signals
II
II
predominant antiferromagnetic coupling between the Co moments. Assuming Co being in
1
7

the high-spin configuration S = 3/2 allows extracting the g-factor from the Curie constant, i.e.
ꢂꢃ
ꢀ
ꢁ
ꢈꢇꢉꢊ, which amounts to 2.55 ± 0.01.
ꢄꢅꢁꢅꢆꢇꢈ
Fig. 4. Product of static magnetic susceptibility and temperature, (χmol - χ )T (T), and inverse susceptibility,
0
1
/(χmol - χ ) (T) of (a) 6 and (b) 5. Black circles represent the experimental data. Blue lines show fits to the data
0
by means of the Curie-Weiss law, and red diamonds show fits according to Hamiltonian Eq. 1 (see the text).
In order to determine the exchange interaction between the metal ions as well as the magnetic
anisotropy, the data have been fitted using the julX simulation software package [99]
according to the following Hamiltonian:
ꢊ
ꢐꢑ ꢍ ꢊꢐꢔ
ꢎ
ꢎ
ꢗꢎ ꢎ
ꢖꢏꢍ ꢐꢔ
ꢗꢎ ꢎ
ꢖꢏꢍ
ꢊ
ꢋꢀꢌꢍ ꢏꢍ ꢐꢑ ꢍ
Eq. 1
ꢇ ꢊ
ꢇ ꢇꢒꢓ
ꢊ ꢊꢒꢓ
ꢕ ꢇ
ꢇ
ꢕ ꢊ
The first term is the exchange interaction term within the magnetic dimer with exchange
integral, J, the second and third terms are the axial anisotropy terms with anisotropy
II
constants, D. The fourth and fifth terms include the Zeeman energies for two individual Co
moments. As shown in Fig. 5a, the simulation results are well matched to the experimental

1
data with the parameters for 6 of J = 1.4 ± 0.02 cm , g = 2.53 ± 0.01, g = 2.56 ± 0.01, |D | =
1
2
1

1
1
6
3 ± 1 cm , and |D | = 58 ± 1 cm . The obtained g-values are well matched to the ones from
2
the Curie-Weiss analysis. While there is only a weak antiferromagnetic exchange of J  2 K
II
between the Co moments, a significant axial anisotropy is required to describe the data in
terms of the Hamiltonian, Eq. 1. It can be ascribed to spin-orbit coupling which results in zero
1
8

field splitting and related depopulation of the ±3/2 doublet with respect to the ±1/2 one.
II
Similar values are found in the literature for mononuclear high spin Co containing
coordination compounds in distorted octahedral coordination environments where, e.g., g =
-
1
1
2
.580, and |D| = 87.9 cm ,[100] or |D| = 60(3) cm , and g = 2.77(5), g = 3.04(5) [101]. The
z xy
II
slight discrepancy of g and D values between the two Co ions might be attributed to the
observation of two crystallographic independent molecules of 6 in the solid state, namely 6A
and 6B as described above.
Fig. 4b shows (χmol – χ )T (T) and 1/(χ – χ )T (T) of 5. The (χ – χ )T data exhibit constant
0
mol
0
mol
0
2
behavior with the amounts of 2.21 erg K/G /mol at high temperatures, i.e. in this case at T >
50 K. While the temperature scale is different from the finding for 6, the qualitative behaviour
2
is similar: T smoothly decreases upon cooling but drops below 20 K to 1.4 erg K/G /mol at 2
K. Fitting the data at T > 100 K by means of the Curie-Weiss law provides C = 2.247 ± 0.004
2
erg K/G /mol and θ = 2.2 ± 0.3 K. The Curie constant implies S = 1 and g = 2.12 ± 0.005
II
which is slightly smaller than the often found g-values of Ni ions, g = 2.15 – 2.20 [102,103],
II
but larger than g = 2.052 of Ni ions in binuclear coordination compound [Ni (μ-
2
O P(H)Ant) (bpy) ]Br [104]. The negative Weiss temperature again indicates predominant
2
2
4
2
antiferromagnetic exchange interactions which is contradicting to a previous observation of
weakly ferromagnetic behavior (θ = 1.6 K) observed for the similar species/compounds
[
Ni (O CFcCO ) (H O)(tmeda) ] (9) [11]. The julX-fits [99] with the Hamiltonian, Eq. 1,
2 2 2 2 2 2
yield a good description of the data, without including anisotropy terms, with the parameters J

1
=
0.7 cm , g = 2.119 ± 0.002, and g = 2.117 ± 0.002. The small value of J  1 K again
1
2
II
shows weak antiferromagnetic interaction between the two Ni moments. Note, that a
similarly good description of the susceptibility data is possible by including |D| up to 5 cm1

1
1
and varying J accordingly, i.e. in the limits 0.1 cm ≤ J ≤ 0.3 cm . This would account for
II
single ion anisotropy well established for Ni (cf., e.g., reference [103]). Typical values of
1
9

II
axial anisotropies of Ni ions in distorted octahedral environment obtained by high-frequency

1
1
1
ESR measurements range from D = 7.7 cm [105], 5.3 cm [106], 3.2 cm [107], 0.5

1
1
cm [108] to D = 1.875 cm [109] while in case of [Ni (O CH) (H O)(tmeda) ] (10) the
2
2
4
2
2
II
= 1.503 cm1 and DNi,2 = 1.83 cm1 [12],
magnetic anisotropy for the Ni ions is D
Ni,1
respectively.
Conclusion
Within this study, the synthesis of three discrete coordination compounds of type
2
2
2
[
(
M (tmeda) (μ -O CFc) (η -O CFc) (μ -H O)] (M = Ni, 5; Co, 6) and [Cu(tmeda)(O CFc) ]
2 2 2 2 2 2 2 2 2

7) by the reaction of O CFc with the metal salts [M(tmeda)(NO ) ] (M = Ni, 2; Co, 3; Cu, 4)
2
3 2
is reported. The structures of 5 – 7 in the solid state are presented, showing that 5 and 6 are
II

isostructural. In 5 and 6 two M ions are bridged by two μ-O CFc building blocks and one µ-
2
II
H O molecule. In contrast, in 7 the Cu ion is octahedrally surrounded by two asymmetric
2

chelate-bonded FcCO entities and one bidentate tmeda ligand. The thermal behaviour of 5 –
2
7
is discussed and decomposition afforded Fe O , FeNi and Fe0.64Ni0.36 for 5, Fe O , CoO and
2 3 3 2 3
Fe for 6 and Fe O , Cu O and CuFeO for 7 as evidenced by PXRD studies. Cyclic
2
3
2
2
voltammetry measurements for 5 – 7 were performed showing that the number of Fc
substituents corresponds to the number of redox events, however, in a close potential range.
Within the square wave voltammograms the appropriate ferrocenyl oxidations were separated
by using the method of deconvolution giving the expected four (5, 6) or three (7) one-electron
processes. Magnetic properties of 5 and 6 are characterized by weak antiferromagnetic
exchange interaction between the magnetic centers, i.e. J  1 K (5) and J  2 K (6). No
significant anisotropy is found for 5. In contrast, the magnetic properties of 6 may be

1
1
described by inferring uniaxial anisotropy of D = 63.3 cm and D = 58.1 cm for the two
1
2
II
Co moments which accounts for spin-orbit coupling and distorted ligand cage.
2
0

Acknowledgments
We are grateful to the Deutsche Forschungsgemeinschaft (FOR 1154 “Towards Molecular
Spintronics”) for generous financial support. We thank Prof. Dr. M. Mehring and Dipl.-Chem.
L. Mertens (TU Chemnitz) for PXRD measurements, Katrin Müller (TU Chemnitz) for
elemental analysis, Brigitte Kempe as well as Dr. Roy Buschbeck (TU Chemnitz) for ESI-MS
and Dipl.-Chem. Natalia Rüffer (TU Chemnitz) for TG measurements.
Appendix A. Supplementary data
CCDC 1547322 (5’), 1547331 (6), 1547330 (7) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via … , or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ UK; fax: (+44) 1223-
336-033; or e-mail: deposite@ccdc.cam.ac.uk.
Supplementary data associate with this article can be found, in the online version, at …
2
1

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The trinuclear complex [Cu(tmeda)(O CFc) ] (7) and the two hexanuclear complexes
2
2
[
M (O CFc) (µ-O CFc) (µ-H O)( tmeda) ] (M = Ni, 5; Co, 6) are shown to undergo
2 2 2 2 2 2 2
reversible redox events according to their number of ferrocenyl substituents. Susceptibility
measurements revealed for 5 and 6 a weak antiferromagnetism while 6 shows additionally an
uniaxial anisotropy.
2
7