Na10Co4O10, an oligooxocobaltate(II, III) with unusual magnetic properties

Article abstract of DOI:10.1002/anie.200351506

The novel mixed-valent oligooxocobaltate can be synthesized through a direct reaction by the azide/nitrate route. The magnetic properties of the compound are characterized by the strongly distinct hierarchy in the magnetic exchange interactions within the complex anion (see picture).

Full text of DOI:10.1002/anie.200351506

Angewandte
Chemie
Mixed-Valent Oligooxocobaltates
metal oxide in appropriate ratios. By employing this techni-
que, we synthesized the new mixed valent, oligomeric
oxocobaltate Na10Co4O10.
Na10Co4O10, an Oligooxocobaltate(ii, iii) with
Unusual Magnetic Properties**
Thoroughly ground starting materials were reacted in a
specially designed vessel,[4b] according to Equation (1).
Mikhail Sofin, Hans-Ulrich Güdel, Roland Bircher,
Eva-Maria Peters, and Martin Jansen*
76 NaN3 þ 14NaNO 3 þ 12 Co3O4 ! 9 Na10Co4O10 þ 121 N2
ð1Þ
The as-obtained single-phase samples are sensitive to air,
and are black. They decompose at 6008C, leaving CoO and
NaCoO2 as the only solid residues.
The study of magnetic clusters of transition-metal ions is a
very active area of research. There are two independent
driving forces: one is based on the unusual behavior of so-
called spin-ladder compounds, such as KCuCl3 or Sr14Cu24O41,
which result from an interplay of strong and weak intra and
interdimer interactions, respectively.[1,2] The dominant anti-
ferromagnetic coupling within the dimers leads to a singlet
ground state and a triplet excited state. Singlet–triplet
excitations become delocalized, thus leading to dispersion,
by the presence of weak interdimer interactions. The appli-
cation of an external magnetic field leads to a crossing of the
Ms = ꢀ1 component of the triplet with the singlet state (Ms =
electron spin angular momentum), thus creating a quantum
critical point. The other driving force is based on the
observation, about 10 years ago, of slow magnetic relaxation
in molecular spin clusters.[3] These so-called single molecule
magnets (SMM) constitute a very interesting class of materi-
als. Besides the slow relaxation of the magnetization, they
exhibit quantum tunneling phenomena at low temperatures.
Typical SMM are composed of 3d transition-metal ions and
organic or inorganic bridging and coordinating ligands.
Alkalioxometallates of the transition elements can consist
of oligomeric complex oxoanions which contain magnetic
centers of diverse spin moments. Unlike molecular spin
clusters the oligomeric complexes in these compounds are not
magnetically shielded from each other, and we can expect an
interesting interplay of intra and intercluster interactions
leading to novel magnetic properties. Such compounds are
only accessible through conventional solid-state reactions, in
general, and this high-temperature route lacks sufficient
control for a directed synthesis of a targeted compound. In
particular, fixing mixed valence states of the transition
element components is an intricate task. As a rather efficient
approach to overcoming such difficulties, we have developed
the azide/nitrate route.[4] In this route, all starting materials
used are solid, and the composition of the product, including
the oxygen content, can be fixed by weighing out the
respective alkali azide, nitrate (or nitrite), and transition
X-ray single crystal structure analysis[5] reveals a novel
tetrameric oxoanion to be the prominent structural feature
(Figure 1). Two edge sharing tetrahedra form the central unit,
to which two CoO3 triangles are attached by sharing a vertex
with each one of the two tetrahedra. An inversion center is
10ꢀ
Figure 1. Crystal structure of the Co4O10 anion. Bond lengths (Š):
II
II
II
III
II
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Co O2 1.798, Co O3 1.958, Co O4 1.802, Co O1 1.927, Co O1a
II
II
II
III
ꢀ
ꢀ
1.951, Co O3 1.823, Co O5 1.777. Angles (8): Co -O3-Co : 1248,
CoIII-O1-CoIII: 928.
located at the middle of the edge common to the tetrahedra.
The threefold triangular planar coordination, which is the one
strongly preferred by Co2+ in alkali oxocobaltates (ii),[6]
justifies the assignment of the oxidation state of + 2 of
ꢀ
cobalt to this unit. This interpretation is backed by the Co O
bondlengths which, on average, agree well with data reported
in the literature.[6] As a consequence Co3+ is assigned to the
tetrahedral sites. This is rare, but not without precedent. We
are aware of five examples: RbCoO2,[7] b-KCoO2,[8]
[10]
Na5CoO4,[9] Li3Na2CoO4
and K5CoW12O40·20H2O[11] are
known to contain Co3+ in a tetrahedral coordination. Again,
the bond lengths observed comply with published data,[9,10]
and with the sums of the respective ionic radii.[12] Considering
ꢀ
the Co O separations in detail, one notices significant
variations, thus reflecting strong disturbances of the ideal
geometries of the coordination polyhedra (Figure 1). How-
ever, all deviations can be understood in terms of the different
functionalities of the oxygen atoms involved. As a rule, edges
shared by two polyhedra are contracted, while the centering
cations repel each other. Furthermore, the distances from
cobalt centers to bridging ligands are longer than those to
[*] Prof. Dr. M. Jansen, Dipl.-Chem. M. Sofin, E.-M. Peters
Max-Planck-Institut für Festkörperforschung
Heisenbergstrasse 1, 70569 Stuttgart (Germany)
Fax: (+49)711-689-1502
E-mail: m.jansen@fkf.mpg.de
Prof. Dr. H.-U. Güdel, Dipl.-Chem. R. Bircher
Department of Chemistry and Biochemistry
University of Bern
terminal ones. With these qualitative rules in mind, the
2+
distances Co3+ O1 (common edge) as well as Co O3
ꢀ
ꢀ
Freiestrasse 3, 3000 Bern 9 (Switzerland)
(bridging oxygen) being longest, and Co3+ O5, Co O2 and
2+
ꢀ
ꢀ
[**] We thank E. Brücher und Dr. R. K. Kremer for carrying out the
magnetic measurements
Co2+ O4(all terminal) being shortest, appear reasonable.
ꢀ
Angew. Chem. Int. Ed. 2003, 42, 3527 –3529
DOI: 10.1002/anie.200351506
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3527

Communications
Also, the O-Co3+-O angles varying from 88.48 to 118.88 are
consistent with this view. The complex anions are “isolated”
from each other by sodium ions which on their part have
irregular oxygen coordination numbers ranging from 4to 6
with bond lengths from 2.32 to 2.97 Š (Figure 2), which is
again in the expected range.
The magnetic properties of Na10Co4O10, represented as c
versus Tand cT versus T, are shown in Figure 3 (c = magnetic
susceptibility). The behavior is most unusual, with a very low
cT value of 5.4emuK per Co 4 unit at 330 K and a sharp
increase of c around 40 K. Below 35 K the zero-field cooled
and field-cooled data diverge, which indicates the onset of
magnetic order. We can clearly distinguish two different
temperature regimes reflecting different magnitudes of
exchange interactions. The smooth part of the curve obtained
between 330 K and 50 K is characteristic of dominant
antiferromagnetic (AF) interactions within the
cluster. The discontinuity at 38 K is attributed to
cooperative intercluster interactions. Both, the
increase and the divergence of c, point to a
magnetic ordering with a nonvanishing magnetic
moment.
¯
Figure 2. Crystal structure of Na10Co4O10. Viewed along 401. Co4O10-
clusters are given in cyan with oxygen atoms in red (for detailed struc-
ture of the oxoanion see Figure 1). Sodium atoms (gray balls) are
drawn only on the right hand part. The dashed line corresponds to a
possible interaction pathway between clusters (with CoII-CoII distances
d=4.1 Š) leading to cooperative effects below 38K.
We can identify and roughly quantify the
dominant exchange coupling by comparing the
present results with magnetic data obtained for
systems containing the constituents of our cluster
[13]
as isolated units. From the data on Na4CoO3
for Co2+ in triangular oxo coordination, we
deduce a cT value of 2 emuK per Co2+, which
is essentially independent on T between 300 K
and 100 K. Figure 4shows a comparison of cT
versus T values of the title compound and
Na6Co2O6.[14] The latter was prepared by a similar
route to Na10Co4O10, and the crystal structure
Figure 3. Magnetic susceptibility of Na10Co4O10 represented as c versus T and cT
reveals the presence of dimers of tetrahedrally
coordinated Co3+ ions sharing an edge, that is,
exactly the inner part of the tetramer. The cT
~
^
,
*
verus T. The applied field was 1 Tesla, and ZFC (&, ) and FC (
) refer to zero-
field cooled and field-cooled, respectively. The dotted and full lines correspond to
least-squares fits using equation (1) and the van Vleck equation. Dotted lines:
gCo2+ =gCo3+ set to 2.0, best fit 2J=ꢀ70 cmꢀ1. Full lines: Best fit with gCo2+ =1.71,
10ꢀ
values in Figure 4are given per Co 4O10
6ꢀ
ꢀ1
tetramer and Co2O6 dimer, respectively. The
gCo3+ =2.15, 2J=ꢀ8 1 cm .
two compounds have distinctly different mag-
netic properties. The dimer cT values are
significantly higher and essentially temperature
independent between 300 K and 100 K. We conclude
3+
that in our Co4 cluster, showing Co2+ Co and
ꢀ
Co3+ Co separation of 3.33 Š and 2.78 Š respec-
3+
ꢀ
tively, there are dominant antiferromagnetic inter-
actions between the terminal Co2+ ions and their
nearest Co3+ neighbors. These significantly reduce
cT from about 12 to 5.4emuK per tetramer at 330 K
and lead to the observed continuous decrease down
to about 50 K. We use the simplest possible model to
obtain an estimate of this coupling strength. The
magnetic moments of Co2+ and Co3+ are assumed
isotropic with S = 3/2 and S = 2 values, respectively
(S = spin angular momentum). The Co2+–Co3+ inter-
actions are expressed in terms of a Heisenberg
exchange Hamiltonian: [Eq. (2)]
*
Figure 4. Magnetic susceptibility of Na10Co4O10 (&) and Na6Co2O6 ( ) repre-
sented as cT per tetramer and dimer, respectively, versus T. The applied field
was 1 Tesla.
! !
! !
h ¼ ꢀ2 Jð S 1 S 2 þ S 3 S 4Þ
ð2Þ
3528
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Angewandte
Chemie
in which S1 = S4 = 3/2 and S2 = S3 = 2, and the Co3+–Co3+
interaction is neglected (J = spin coupling constant). A
least-squares fit of cT calculated on the basis of Equation (2)
and the van Vleck equation[15] with g = 2.0 (g = g-factor) for
all the ions to the data between 330 K and 100 K yields 2J =
ꢀ70 cmꢀ1 and the broken line in Figure 3. The full line
corresponds to a fit with the g values as additional fit
parameters: g1 = g4 = 1.71, g2 = g3 = 2.15 and 2J = ꢀ81 cmꢀ1.
Considering the very approximate character of our model the
fits are surprisingly good. Anisotropy, which is expected to be
important for both Co2+ and Co3+ in distorted triangular and
Td coordination, respectively, does not significantly affect the
magnetic properties above 100 K.
Cobalt oxide was prepared by heating Co(C2O4)·2H2O (Alfa,
Reagent Grade) in a stream of oxygen at 3758C for 24h. Precursors
(NaN3, NaNO3 and Co3O4) were mixed in the required ratio
according to Equation (1), ground in a ball mill, pressed in pellets
under 105 N, dried under vacuum (10ꢀ3 mbar) at 1508C for 12 h, and
placed under argon in a tightly closed steel container provided with a
silver inlay.[4b] In a flow of dry argon the following temperature
treatment was applied: 25!2608C (100 Khꢀ1); 260!3808C
(5 Khꢀ1); 380–5008C (20 Khꢀ1) followed by subsequent annealing
for 50 h at 5008C. The obtained black powder is very sensitive to air
and moisture and all following manipulation were performed in an
inert atmosphere of purified argon.
Single crystals were grown by subsequent annealing of the as-
prepared powder at 5008C for 2000 h in silver crucibles, which were
sealed in glass ampoules under dry argon.
The dominant antiferromagnetic Co2+–Co3+ interaction
with 2J = ꢀ81 cmꢀ1 is due to the superexchange through the
oxygen ion. The Co3+ lies approximately in the triangular
plane of the Co2+ coordination, and the Co2+-O-Co3+ angle is
1248. The planar Co2+ coordination forces two unpaired
electrons into the coordination plane. They will have signifi-
cant overlap with oxygen 2s and 2p orbitals. On the Co3+ side
all the t2 orbitals are singly occupied in tetrahedral coordina-
tion. As a result of the overlap with the oxygen orbitals some
spin density will be transferred also from Co3+ to oxygen. The
situation thus bears some resemblance with dihydroxo-
bridged Cu2+ dimers, in which the relevant orbital interactions
occur in the Cu2+-O-Cu2+ plane.[16] In these, the sign and the
magnitude of J is a function of the angle at the bridging
oxygen, and for an angle of 1248 it is strongly antiferromag-
netic. The interaction between the central Co3+ ions is much
weaker, and we attribute this mainly to the much smaller
Co3+-O-Co3+ angle of 928, which leads to a cancellation of
Magnetic measurements were performed on a SQUID-Magneto-
meter (MPMS 5.5, Quantum Design) between 2 and 330 K in a
magnetic field up to 5 T.
Received: March 27, 2003 [Z51506]
Keywords: cobalt · crystal structure · magnetic properties ·
.
mixed-valent compounds · oxo ligands
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[5] Crystal data: monoclinic, C2/c (No. 15), Z = 4, a = 14.9061(4),
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1 = 3.11, Single crystal X-ray structure determination: intensities
were measured at 293 K with graphite-monochromatized MoKa
radiation (l = 0.71069 Š) on a Bruker AXS Diffraktometer with
APEX Smart CCD; 3510 independent reflections (13458 total
measured) were analysed by direct method. The refinement by
full-matrix least squares gave final values R1(all) = 0.0294,
Rw(all) = 0.0689. Further details on the crystal structure inves-
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ferro and antiferromagnetic contributions to J within Co3+
ꢀ
3+
ꢀ
O/O Co plane. This view is supported by comparison with
Na6Co2O6 where for a virtually identical geometry there is no
manifestation of magnetic interactions down to about 100 K.
At 50 K in Na10Co4O10 we thus have two strongly AF
coupled Co2+–Co3+ dimers, each with a S = 1/2 ground state.
The coupling between these dimers is weak and cannot be
quantified because of the onset of the ferromagnetic order at
38 K. This cooperative effect results from intercluster inter-
actions. An inspection of the crystal structure (Figure 2)
reveals unusually short Co2+–Co2+ distances of d = 4.1 Š
along the b axis between neighboring clusters, which may act
as interaction pathways. From the small value of the ordered
moment M = 0.13 mB per Co4 at 10 K the order is more likely
canted antiferromagnetic rather than ferromagnetic.
In conclusion, the unusual new structure of the title
compound is accompanied by unusual magnetic properties.
We can clearly determine a hierarchy of exchange effects. A
strong antiferromagnetic intracluster exchange interaction
between Co2+ and Co3+ ions dominates the high T magnetic
behavior. Below 38 K cooperative effects become important.
As a consequence, the intracluster Co3+–Co3+ coupling and
the intercluster interactions cannot be separated.
[7] M. Jansen, R. Hoppe, Z. Anorg. Allg. Chem. 1975, 417, 31.
[8] C. Delmas, C. Fouassier, P. Hagenmuller, J. Solid State Chem.
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[10] J. Birx, R. Hoppe, Z. Anorg. Allg. Chem. 1991, 597, 19.
[11] L. C. Baker, T. P. McCutchean, J. Am. Chem. Soc. 1956, 78, 4503.
[12] R. D. Shannon, Acta Crystallogr. Sect. A 1976, 32, 751.
[13] A. Möller, Chem. Mater. 1998, 10, 3196.
[14] M. Sofin, E. M. Peters, M. Jansen, unpublished results.
[15] R. L. Carlin, Magnetochemistry, Springer, New York, 1986.
[16] O. Kahn, Molecular Magnetism, VCH Publishers, New York,
1993, p. 160.
Experimental Section
Starting materials for the preparation of Na10Co4O10 were NaN3
(Sigma, 99.5%), NaNO3 (Aldrich, 99%) and activated Co3O4.
Angew. Chem. Int. Ed. 2003, 42, 3527 –3529
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3529