Long-distance ferromagnetic coupling through spin polarization in a linear heterotrinuclear iron(III)-copper(II)-iron(III) complex derived from 5-ferrocenyl-2-aminotropone

Article abstract of DOI:10.1039/b813944j

A novel hetrotrinuclear complex composed of two ferrocenium-ion moieties and copper complex of 2-aminotropones showed relatively strong intramolecular ferromagnetic coupling in the solid states owing to spin polarization mechanism. The Royal Society of Chemistry.

Full text of DOI:10.1039/b813944j

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Long-distance ferromagnetic coupling through spin polarization
in a linear heterotrinuclear iron(III)–copper(II)–iron(III) complex derived
from 5-ferrocenyl-2-aminotroponew
a
Yoshihiro Miyake,z Sayaka Watanabe,a Satoshi Aono,a Tohru Nishinaga,*a Akira Miyazaki,b
Toshiaki Enoki,b Hitoshi Miyasaka,ya Hiroyuki Otanic and Masahiko Iyoda*a
Received (in Cambridge, UK) 12th August 2008, Accepted 25th September 2008
First published as an Advance Article on the web 16th October 2008
DOI: 10.1039/b813944j
A novel hetrotrinuclear complex composed of two ferrocenium-ion
moieties and copper complex of 2-aminotropones showed rela-
tively strong intramolecular ferromagnetic coupling in the solid
states owing to spin polarization mechanism.
As such new p-spacers, we focused on 2-aminotropone.
2-Aminotropone is an amine analogue of tropolone. It was
first synthesized from the interest of non-benzenoid aromatics
as tropone derivatives.14 Owing to the contribution of
resonance structure of aromatic tropylium ion, basicity of
the carbonyl oxygen part are high and the oxygen atom and
deprotonated amide can effectively chelate metals.15 In addi-
tion, the chelating arms can conjugate with the p-backbone,
which might make the through-bond magnetic coupling by
spin-polarization mechanism possible. On the other hand, we
are interested in ferrocenium-ion as stable paramagnetic
source and have prepared several ferrocene derivatives directly
connected to p-conjugated systems.16 During the course of
that work, we found that the two-electron oxidized species of
copper(II) complex of ferrocenylaminotropone 1 showed intra-
molecular ferromagnetic coupling at low temperatures. Here
we report the synthesis and properties of 12+ together with the
results of theoretical calculations for discussion of the spin
polarization mechanism in the ferromagnetism.
The design and synthesis of molecular-based magnetic materials
have attracted increasing attention. The relatively new research
field, named molecular magnetism,1 was first pioneered with
organic p-electron systems2,3 and soon the objects were
expanded into transition metal complexes4 because of the more
stable and diverse magnetic states of transition metals. In the
research field, polynuclear complexes with intramolecular long-
distance magnetic coupling have actively been investigated.5
However, despite the significance of ferromagnetic coupling for
development of new magnetic materials, the examples of long-
distance ferromagnetic coupling within a molecule are still rare,
partly due to the lack of new spacers that enable the effective
magnetic communication between metal centers.
For long-distance ferromagnetic coupling, a spin polarization
mechanism6 with ‘‘non-disjoint’’ singly occupied molecular
orbitals (SOMOs)7 plays dominant role in organic p-electron
systems, while orthogonal magnetic orbitals have often been
utilized in metal complexes.8,9 In the application of spin polar-
ization mechanism to ferromagnetic metal complexes, the
spacers used so far have mostly been limited to the m-phenylene
linkage10 and related pyrimizine and 1,3,5-triazine ligand,11
even though the ferromagnetism of the early subjects in the
field, e.g., [Cp*2Fe]+[TCNE]ꢀ12 was explicated with a spin
polarization mechanism.13 In order to develop new ferro-
magnetic polynuclear complexes, new design of p-spacers under
the concept of spin polarization is important.
As shown in Scheme 1, the precursor ligand of 5-ferrocenyl-2-
(ethylamino)tropone (2a) was synthesized by Negishi coupling
between ferrocenylzinc chloride16 and 5-bromo-2-methoxy-
tropone17 followed by the substitution of methoxy group with
ethylamine in 80% overall yield. Then the copper(II) complex 1
was obtained quantitatively by the reaction with copper(II)
acetate in a mixed solvent of dichloromethane and methanol,
which could be recrystallized from THF and isopropyl ether. In
contrast, attempted synthesis of the tropolone analogue of 1 was
hampered because of the solubility problem. For the preparation
of 12+, chemical oxidation was difficult due to the limited
solubility of the product, and hence the electrochemꢀical oxidation
was conducted to give single crystals of 12+ 2PF6 when tetra-
n-butylammonium hexafluorophosphate and dichloroethane
were used as electrolyte and solvent, respectively.
a Department of Chemistry, Graduate School of Science and
Engineering, Tokyo Metropolitan University, Hachioji, Tokyo,
192-0397, Japan. E-mail: iyoda@tmu.ac.jp;
Fax: (+81)42-677-2525; Tel: (+81)42-677-2547
b Department of Chemistry, Graduate School of Science and
Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo,
152-8551, Japan. E-mail: enoki.t.aa@m.titech.ac.jp
c Faculty of Education and Human Sciences, Yokohama National
University, Hodogaya-ku, Yokohama, Kanagawa, 240-8501, Japan.
E-mail: otani@ed.ynu.ac.jp
w Electronic supplementary information (ESI) available: Synthesis,
UV-Vis-NIR spectra, ESR spectra, CV, SQUID. CCDC 691675 (1),
691674 (12+ 2PF6ꢀ), and 691676 (2b ClO4). For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
b813944j
z Present address: The University of Tokyo.
Scheme 1 Synthesis of 12+ 2PF6
.
ꢀ
y Present address: Tohoku University.
ꢁc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 6167–6169 | 6167

View Article Online
In the X-ray structure of 1 and 12+ 2PF6ꢀz (Fig. 1a), the
chelating arms show square-planar coordination around the
formal Cu(II) atoms with a conformation of Feꢂ ꢂ ꢂCuꢂ ꢂ ꢂFe
atoms being linearly arranged. From the comparison of these
molecular structures, no remarkable change in the bond
lengths and angles upon the two-electron oxidation is
observed except for a slight elongation of the Fe–Cp distances,
suggesting that the additional positive charge in 12+ is mostly
localized on the formal Fe(III) atoms of ferrocenium-ion
moieties. As a result, the intramolecular distances between
the Fe(III) (or II)ꢂ ꢂ ꢂCu(II) and Fe(III) (or II)ꢂ ꢂ ꢂFe(III) (or II)
atoms are almost identical for 1 (8.67 and 17.34 A) and 12+
(8.77 and 17.53 A). As concerns the packing structures, unlike
the packing of 1 in a herringbone manner, 12+ showed a
stacking structure (Fig. 1b), in which the nearest inter-
molecular Fe(III)ꢂ ꢂ ꢂCu(II) distance (5.17 A) is shorter than
the intramolecular Fe(III)ꢂ ꢂ ꢂCu(II) distance. Nevertheless, the
ferromagnetic coupling is obtained intramolecularly as
described below.
Fig. 2 Thermal dependence of wMT for 12+ 2PF6ꢀ; (J) experimental
data; (—) fitting curve.
solid states and in a frozen acetonitrile solution showed
identical quite broad signals ranging from 100 to
500 mT below 40 K, whereas 2b+ ClO4 only gave typical
ꢀ
ESR spectra of ferrocenium ion.18 Thus, the characteristic
ESR signal of 12+ at low temperatures is ascribed to the
magnetic interaction between the Cu(II) and Fe(III) atoms, and
the identical signal shape irrespective of the observation
conditions suggested the presence of intramolecular magnetic
coupling at low temperatures.
In electronic absorption spectra, a THF solution of 1
showed a very weak MLCT band at 730 nm (log e = 2.3) in
addition to the absorption band from the ferrocenylamino-
tropone moiety (lmax (log e); 409 nm (4.6)). In sharp contrast,
an acetonitrile solution of 12+ 2PF6 showed a very broad
ꢀ
The detailed magnetic properties wereꢀ examined with
SQUID. The magnetic behavior of 12+ 2PF6 as a wMT vs. T
plot is shown in Fig. 2. Upon cooling, the wMT values slightly
decrease until around 150 K due to the temperature independent
paramagnetism (TIP) of ferrocenium-ion moiety, then gradually
increase and reach a maximum up to 3.2 emu K molꢀ1 at 4.8 K,
and then rapidly decrease until 2 K. These results can be
explained that there are a stronger ferromagnetic interaction
and a weaker antiferromagnetic interaction between low spin
state (S = 1/2) of two Fe(III) atoms and Cu(II) atom (S = 1/2).
A good fit to the experimental data was obtained with
g = 2.69, J = +12.0 cmꢀ1, J0 = ꢀ0.05 cmꢀ1, and TIP =
absorption in the near-IR region (lmax (log e); 1230 nm (3.0)).
The characteristic broad band is apparently ascribed to the
strong p–d interaction between the p-spacer and the Fe(III)ꢂ
Cu(II) metals, because there is no such absorption band for 1
or 2b+ ClO4ꢀ8 (lmax (log e); 380 nm (4.2), 582 nm (3.5)). On
the other hand, the ESR spectrum of 1 in THF showed typical
signal pattern for Cu(II) complex with nitrogen ligands (g>
=
2.033 (aN = 1.04 mT), gJ = 2.127 (aCu = 7.89 mT)), whereas
the siꢀgnal from the Cu(II) part had almost disappeared for 12+
2PF6 in acetonitrile at room temperature, indicating the
presence of intramolecular exchange interactionꢀbetween the
Cu(II) and Fe(III) atoms. Furthermore, 12+ 2PF6 both in the
560 ꢃ 10ꢀ6 emu mol
.
ꢀ1 19 Taking the results of X-ray and ESR
analyses into consideration, we concluded that the intramole-
cular ferromagnetic and intermolecular antiferromagnetic
coupling take place despite of the longer Fe(III)–Cu(II) intra-
molecular distance than the intermolecular distance.
To support the conclusion, the electronic structures of
doublet and quartet states of 12+ were estimated by DFT
calculations using the X-ray structure.20 In these calculations,
the quartet state (spin densities, Fe(III) 1.1179, Cu(II) 0.6081)
was shown to lie below the doublet state (Fe(III) 1.1384, Cu(II)
ꢀ0.6089) by the energy gap of 97 cmꢀ1. Furthermore, when
the spin density distributions was depicted, a precise orbital
picture of the ferromagnetic coupling in the quartet state of
12+ was found to be apparent as shown in Fig. 3. In the
ferrocenium ion moiety, negative spin densities are induced in
the whole Cp ligand by the Fe(III) atom, as in the case of other
ferrocenium ions.13 On the other hand, the nitrogen and
oxygen atoms around the Cu(II) metal show a relatively large
positive spin distribution due to the contribution of resonance
structures between Cu(m)–N(or O)(mk) and Cu(mk)–N(or
O)(m). Since the calculated spin density on N (0.1578) is larger
than O (0.1140), the resonance between Cu and N atoms is
more important principally due to the substantial covalency of
the Cu–N bond. As a result, the sign alternation of spin
Fig. 1 X-ray structure of 12+ 2PF6 C2H4Cl2. (a) ORTEP drawing
ꢀ
(50% probability). (b) Packing structure.
ꢁc
This journal is The Royal Society of Chemistry 2008
6168 | Chem. Commun., 2008, 6167–6169

View Article Online
3 (a) H. Iwamura, Pure Appl. Chem., 1986, 58, 187; (b) H. Iwamura
and N. Koga, Acc. Chem. Res., 1993, 26, 346.
4 (a) H. Iwamura, K. Inoue and T. Hayamizu, Pure Appl. Chem.,
1996, 68, 243; (b) O. Kahn, Acc. Chem. Res., 2000, 33, 647.
5 (a) E. Pardo, J. Faus, M. Julve, F. Lloret, M. C. Munoz,
Y. J. Cano, X. Ottenwaelder, Y. Journaux, R. Carrasco,
G. Blay, I. Ferna
´
2003, 125, 10770; (b) E. Pardo, R. Carrasco, R. Ruiz-Garcı
ndez and R. Ruiz-Garcı
´
a, J. Am. Chem. Soc.,
´
a,
M. Julve, F. Lloret, M. C. Munoz, Y. Journaux, E. Ruiz and
J. Cano, J. Am. Chem. Soc., 2008, 130, 576, and references are cited
in the reports and supporting information.
6 H. M. McConnell, J. Chem. Phys., 1963, 39, 1910.
7 W. T. Borden, H. Iwamura and J. A. Berson, Acc. Chem. Res.,
1994, 27, 109.
Fig. 3 Calculated spin densities of the quartet state of 12+
.
density from the Cu(II) atom through the N, C2–C5, and
Cp ring to the Fe(III) atom is in accordance with the spin
polarization mechanism of the ferromagnetic coupling,
whereas there is a mismatch in the doublet state which
restrains Cu(mk)–N(or O)(m) delocalization, judging from
the lower spin density on N (ꢀ0.0693) and O (ꢀ0.0774).
In summary, we have revealed that the noꢀvel hetero-
trinuclear Fe(III)–Cu(II)–Fe(III) complex 12+ 2PF6 showed a
long-distance intramolecular ferromagnetic coupling at low
temperatures. From the DFT calculations, the spin polariza-
tion mechanism through the p-system of 2-aminotropone and
cyclopentadienyl ligand was clearly demonstrated for the
ferromagnetism in 12+. As a related example, a long-distance
(46 A) ferromagnetic coupling (J = +22 cmꢀ1) of a linear
trinuclear Cu(II)–Fe(III)–Cu(II) complex by the use of ortho-
gonal magnetic orbitals has once been reported.9 However, to
our knowledge, such a long-distance ferromagnetic coupling in
linear trinuclear complex including Cu(II) and Fe(III) centers as
reported in the present work has never been attained through
spin polarization mechanism. In this context, we have success-
fully added 2-aminotropone spacer as a potentially versatile
new ligand in molecular magnetism.
8 O. Kahn, Inorg. Chim. Acta, 1982, 62, 3.
9 For examples (a) C. A. Koch, C. A. Reed, G. Brewer, N. P. Rath,
W. R. Scheidt, G. P. Gupta and G. Lang, J. Am. Chem. Soc., 1989,
111, 7645; (b) G. P. Gupta, G. Lang, C. A. Koch, B. Wang,
W. R. Scheidt and C. A. Reed, Inorg. Chem., 1990, 29, 4234.
10 (a) H. Oshio and H. Ichida, J. Phys. Chem., 1995, 99, 3294;
(b) I. Fernandez, R. Ruiz, J. Faus, M. Julve, F. Lloret, J. Cano,
´
X. Ottenwaelder, Y. Journaux and C. Munoz, Angew. Chem., Int.
Ed., 2001, 40, 3039; (c) T. Glaser, M. Gerenkamp and R. Frchlich,
Angew. Chem., Int. Ed., 2002, 41, 3823; (d) T. Glaser,
M. Heidemeier, S. Grimme and E. Bill, Inorg. Chem., 2004, 43,
5192; (e) X. Ottenwaelder, J. Cano, Y. Journaux, E. Riviere,
C. Brennan, M. Nierlich and R. Ruiz-Garcıa, Angew. Chem., Int.
´
Ed., 2004, 43, 850; (f) T. Glaser, M. Heidemeier, J. B.
H. Strautmann, H. Bogge, A. Stammler, E. Krickemeyer,
¨
R. Huenerbein, S. Grimme, E. Bothe and E. Bill, Chem.–Eur. J.,
2007, 13, 9191.
11 (a) S.-i. Mitsubori, T. Ishida, T. Nogami and H. Iwamura, Chem.
Lett., 1994, 285; (b) T. Ishida, K. Nakayama, M. Nakagawa,
W. Sato and Y. Ishikawa, Synth. Met., 1997, 85, 1655;
(c) F. Lloret, G. De Munno, M. Julve, J. Cano, R. Ruiz and
A. Caneschi, Angew. Chem., Int. Ed., 1998, 37, 135; (d) T. Ishida,
S.-i. Mitsubori, T. Nogami, N. Takeda, M. Ishikawa and
H. Iwamura, Inorg. Chem., 2001, 40, 7059.
12 J. S. Miller, J. C. Calabrese, H. Rommelmann, S. Chittipeddi,
A. J. Epstein, J. H. Zhang and W. M. Reiff, J. Am. Chem. Soc.,
1987, 109, 769.
This work was supported by Grant-in-Aid for Scientific
Research on Priority Areas, Ministry of Education, Culture,
Sports, Science and Technology, Japan.
13 C. Kollmer and O. Kahn, Acc. Chem. Res., 1993, 26, 259.
14 P. L. Pauson, Chem. Rev., 1955, 55, 9.
15 T. Asao and Y. Kikuchi, Chem. Lett., 1972, 413.
16 (a) M. Iyoda, T. Okabe, T. Kondo, S. Sasaki, H. Matsuyama,
Y. Kuwatani and M. Katada, Mol. Cryst. Liq. Cryst., 1996, 286,
65; (b) M. Iyoda, T. Okabe, T. Kondo, S. Sasaki, H. Matsuyama,
Y. Kuwatani and M. Katada, Chem. Lett., 1997, 103; (c) M. Iyoda,
T. Kondo, T. Okabe, H. Matsuyama, S. Sasaki and Y. Kuwatani,
Chem. Lett., 1997, 35; (d) M. Iyoda, T. Okabe, M. Katada and
Y. Kuwatani, J. Organomet. Chem., 1998, 569, 225; (e) M. Iyoda,
T. Takano, N. Otani, K. Ugawa, M. Yoshida, H. Matsuyama and
Y. Kuwatani, Chem. Lett., 2001, 1310; (f) R. H. Herber, I. Nowik
and M. Iyoda, J. Organomet. Chem., 2002, 658, 210.
17 M. G. Banwell, J. N. Lambert, M. E. Reum and R. Onrust, Org.
Prep. Proced. Int., 1988, 20, 393.
18 R. Prins and F. J. Reinders, J. Am. Chem. Soc., 1969, 91,
4929.
19 Magnetic measurements were carried out on a SQUID magnetometer
on single crystals in the range from 2 K to 300 K. The susceptibility
curve was fitted using Heisenberg liner-type tri-spin model
(SFe1,SCu,SFe2) = (1/2, 1/2, 1/2) H = ꢀ2J(SFe1/SCu+SFe2/SCu).
20 Electronic structure calculations were performed with density
functional B3LYP method and the triple-z (TZV) basis set by
using Gaussian-03 program. The broken-symmetry approach was
implemented for the doublet state. The atomic spin densities were
obtained by Mulliken population analysis.
Notes and references
z Crystal data for 1. C38H36CuFe2N2O2, M = 727.93, monoclinic,
a = 20.359(5), b = 9.756(9), c = 8.232(4) A, b = 98.43(3)1, U =
1617.4(17) A3, T = 123 K, space group P21/c (no. 14), Z = 2, 3731
reflections measured. The final R1 and wR(F2) was 0.052 and 0.160
(I 4 2s(I)).
Crystal data for 12+ 2PF6ꢀ. C40H40Cl2CuF12Fe2N2O2P2, M = 1116.82,
triclinic, a = 10.197(10), b = 10.402(16), c = 10.513(10) A, a =
84.50(8), b = 68.16(7), g = 84.28(8)1, U = 1028(2) A3, T = 120 K,
space group P-1 (no. 2),
Z = 1, 3263 reflections measured.
The final R1 and wR(F2) was 0.063 and 0.146 (I 4 2s(I)).
8 Crystal data for 2b+ ClO4ꢀ. C21H23ClFeNO5, M = 460.70, mono-
clinic, a = 7.6219(6), b = 24.8096(18), c = 10.8126(8) A, b =
92.7620(10)1, U = 2042.2(3) A3, T = 293 K, space group P21/n (no.
14), Z = 4, 2941 reflections measured. The final R1 and wR(F2) was
0.0313 and 0.0836 (I 4 2s(I)).
1 (a) O. Kahn, Molecular Magnetism, VCH, New York, 1993;
(b) Molecular Magnetism, ed. K. Itoh and M. Kinoshita, Gordon
and Breach, Amsterdam (copublisher Kodansha Tokyo), 2000.
2 K. Itoh, Pure Appl. Chem., 1978, 50, 1251.
ꢁc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 6167–6169 | 6169