An antiferromagnetic Mn(III) chain bridged by hydrogencyanamide: [Mn III(5-Brsalen)(μ1,3-NCNH)]n

Article abstract of DOI:10.1021/ic048547s

A one-dimensional Mn(III) chain, [Mn^III(5-Brsalen)(μ _1.3-NCNH)]_n, is constructed by a hydrogencyanamide bridge, and the chains are further connected to a 3-D network through hydrogen bonding. At low temperature, the compound shows weak ferromagnetism due to spin canting and an unusual spin reorientation induced by an applied field.

Full text of DOI:10.1021/ic048547s

Inorg. Chem. 2004, 43, 8221−8223
An Antiferromagnetic Mn(III) Chain Bridged by Hydrogencyanamide:
[MnIII(5-Brsalen)(
1,3-NCNH)]n
µ
Mei Yuan,† Song Gao,*,† Hao-Ling Sun,† and Gang Su‡
College of Chemistry and Molecular Engineering, State Key Laboratory of Rare Earth Materials
Chemistry and Applications, Peking UniVersity, Beijing 100871, P.R. China,
and College of Physical Sciences, Graduate School of the Chinese Academy of Sciences,
P.O. Box 3908, Beijing 100039, P.R. China
Received October 17, 2004
The design and synthesis of transition metal coordination
polymers bridged by small conjugated ligands, such as cyano,
azido, oxalato, and nitrido, are currently under intense
investigation in view of their structure diversity and in the
context of molecule-based magnets.1 As a potential nitrogen-
based ligand, cyanamide (NCNH2) and its basic forms
(NCNH- or NCN2-) have been used to prepare a number of
alkali-metal,2 alkaline-earth-metal,3 and rare-earth-metal4 salts
by different synthetic routes, but their transition metal
coordination chemistry and the ability to mediate the
magnetic coupling, which are important though, still remain
inactively explored. Among those reported structures of
cyanamide transition metal coordination complexes,5,6 most
have molecular cluster structures, and only three NCNH--
bridged coordination complexes, to our knowledge, have
been reported so far.6 Constructing magnetic coordination
polymers with cyanamide is still a great challenge. Since
NCNH- is isoelectronic with the azide anion, polymers
bridged by NCNH- should also exist with regard to those
well characterized polymer structures bridged by azide. On
the other hand, dicyanamide (dca, N(CN)2-) has been well
used as a functional ligand to synthesize coordination
polymers with interesting magnetic properties as well as
various topologies due to its versatile coordination modes.7
Moreover, among those different coordination modes of
bridging dca, the µ1,3-MsNtCsNsM pathway was found
to provide the strongest coupling which leads to the long-
range magnetic ordering. This observation further inspires
us to explore the cyanamide ligand as a bridging ligand to
obtain a new series of coordination polymers with interesting
magnetic properties. Here we report an unprecedented
hydrogencyanamide-bridged 1-D Mn(III) chain, [MnIII(5-
Brsalen)(µ1,3-NCNH)]n, 1. The magnetic study of 1 shows a
weak ferromagnetic ordering at low temperature due to spin
canting and an unusual spin reorientation induced by an
applied field. To the best of our knowledge, complex 1 is
the first hydrogencyamamide-bridged coordination polymer.
The simple reaction of NCNH2 with MnClO4‚6H2O,
5-Brsalen, and NaOH in a 1:1:1:2 molar ratio in methanol
solution yielded small deep-red crystals of complex 1.
Complex 1 features a NCNH-bridged Mn(III) chain.8 As
shown in Figure 1a, the Mn(III) ion adopts a distorted
octahedral geometry coordinated by the N2O2 donor atoms
of one 5-Brsalen ligand in the equatorial mode and the N
donor atoms of two NCNH- ions in the axial positions. Each
NCNH- ligand functions as a trans-µ1,3 bridge to link
monomeric [MnIII(5-Brsalen)]+ units into a 1-D zigzag chain
(Figure 1b), in comparison to µ1,3-N3-bridged chains.9 In the
equatorial plane, Mn(1), O(1), O(2), N(1), and N(2) are
* To whom correspondence should be addressed. E-mail: gaosong@
pku.edu.cn.
† Peking University.
‡ Graduate School of the Chinese Academy of Sciences.
(1) (a) Ferlay, S.; Mallah, T.; Ouahes, R.; Veillet, P.; Verdaguer, M. Nature
1995, 378, 701. (b) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K.
Science 1996, 272, 704. (c) Tamaki, H.; Zhong, Z. J.; Matsumoto,
N.; Kida, S.; Koikawa, K.; Achiwa, N.; Okawa, H. J. Am. Chem. Soc.
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and references therein.
(2) (a) Becker, M.; Jansen, M.; Lieb, A.; Milius, W.; Schnick, W. Z. Anorg.
Allg. Chem. 1998, 624, 113. (b) Becker, M.; Nuss, J.; Jansen, M. Z.
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Inorg. Chem. 2004, 43, 5884.
(5) (a) Becker, M.; Jansen, M. Z. Anorg. Allg. Chem. 2000, 626, 1639.
(b) Liu, X. H.; Mu¨ller, P.; Kroll, P.; Dronskowski, R. Inorg. Chem.
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Kuwata, S.; Ishii, Y. J. Am. Chem. Soc. 2002, 124, 6528.
(7) Batten, S. R.; Murray, K. S. Coord. Chem. ReV. 2003, 246, 103.
(8) Crystallographic data for 1 follow: monoclinic, space group P21/c,
deep red, a ) 13.3699(4) Å, b ) 11.7277(4) Å, c ) 11.5495(1) Å, â
) 105.978(2)°, V ) 1740.98(9) Å3, Z ) 4, Dcalcd ) 1.984 Mg/m3,
GOF ) 0.979, R1 ) 0.0326, wR2 ) 0.0670 (I > 2σ), R1 ) 0.0632,
wR2 ) 0.0733 (all data).
(9) (a) Kennedy, B. J.; Murray, K. S. Inorg. Chem. 1985, 24, 1552. (b)
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10.1021/ic048547s CCC: $27.50
Published on Web 11/18/2004
© 2004 American Chemical Society
Inorganic Chemistry, Vol. 43, No. 26, 2004 8221

COMMUNICATION
Figure 1. (a) Molecular structure of compound 1 (30% probability
ellipsoids). (b) View of the zigzag chain.
-1
nearly coplanar (rms deviation ) 0.012(9) Å). The bond
lengths of Mn(1)-O(1), Mn(1)-O(2), Mn(1)-N(1), and
Mn(1)-N(2) are 1.881(2), 1.880(1), 1.976(2), and 1.991(2)
Å, respectively, which are close to those in other Mn(III)-
salen complexes.9b As expected, the bond lengths in the axial
position of Mn(1)-N(3) and Mn(1)-N(4A) (2.245(2) and
2.407(3) Å, respectively) are elongated due to a Jahn-Taller
distortion at the high-spin d4 metal center,9 and also indicate
an asymmetric coordination of “NCNH” bridge. For com-
parison, in the previously reported dinuclear structure of [Cu2-
(C9H21N3)2(µ-NCNH)2]](ClO4)‚2H2O (C9H21N3 ) N,N′,N′′-
trimethyl-1,4,7-triazacyclononane) which also possesses an
end-to-end “NCNH” bridge, the NCNH bridges have close
Cu-N distances ranging from 1.962(5) to 2.017(6) Å.6b
In complex 1, the C(17)-N(3)H and C(17)-N(4) bond
distances at 1.294(4) and 1.171(4) Å, respectively, as well
as the N(4)-C(17)-N(3)H angle of 174.3(3)° emphasize the
importance of resonance structure (a) NtCsNH- much
more than resonance structure (b) N-dCdNH in the bonding
description of the NCNH- bridge.10 The intrachain distance
between the two MnIII centers bridged by NCNH- ion is
about 5.878 Å, which is longer than that in the complex [Mn-
(salen)N3].9b Surprisingly, the nearest interchain Mn-Mn
separation is as small as 5.386 Å owing to the rich hydrogen
bonding interactions (Figure S1).
Figure 2. (a) øm and øm vs T plots in an applied field of 2 kOe with a
theoretical fit for 1. Inset: øm vs T plots at different fields. (b) Field
dependence of magnetization at different temperatures. Inset: field depen-
dence of dM/dH.
3.015(5) cm3 mol-1 K and θ ) -16.1(1) K. The Curie
constant is close to 3.0 cm3 mol-1 K expected for one isolated
spin-only Mn(III) ion, while the negative θ value confirms
a dominant AF coupling between Mn(III) ions. Although this
is at very low temperatures where some anisotropy may be
significant, manganese(III) is expected to be Heisenberg-
like in its magnetic properties.9a The best fit of the magnetic
data above 5 K in terms of the Heisenberg 1-D chain using
the Fisher model11 with H ) -2JSMnSMn and intrachain
coupling J gives the following parameters: J ) -0.99(3)
cm-1, zJ′ ) -1.0(2) cm-1 (z ) 4), and g ) 2.004(2) with R
) 7.6 × 10-4 {R ) ∑[(ømT)obs - (ømT)calcd]2/∑(ømT)obs2}.
Fitting the same set of data using the model given by Weng
and modified by Hiller12 gives the following best fit
parameters: J ) -0.85(2) cm-1, zJ′ ) -1.3(1) cm-1 (z )
4), and g ) 2.0 (fixed) with R ) 7.6 × 10-4. These values
prove an obvious AF coupling between not only intrachain
Mn(III) ions but also interchain Mn(III) centers. As shown
in the inset of Figure 2a, øm versus T curves measured in a
field of 200-50 kOe show that the weak ferromagnetic phase
transition is greatly influenced by the increase of the applied
field; at 30 kOe, the characteristic of spin-canting nearly
disappears. Interestingly, at a higher field, 50 kOe, the shape
of the øm versus T plot is obviously different from those at
low fields, indicating another magnetic transition possibly
from a spin-canted AF phase to another AF state (Figure
S2). Furthermore, the maximum around 7.0-8.0 K suggests
an AF ordering in 1. This unusual filed-induced spin
The temperature dependence of the magnetic susceptibility
of 1 was measured from 2 to 300 K in an applied magnetic
field of 2 kOe (Figure 2a). On cooling, the øm value increases
slowly to a maximum at ca. 8.0 K, and then, after a small
downturn, it exhibits a relatively abrupt increase on further
cooling at about 4.0 K, which may indicate the presence of
an antiferromagnetic (AF) coupling and a weak ferromagnetic
phase transition due to spin canting at low temperature.
The data above 50 K obey the Curie-Weiss law with C )
(11) Wagner, G. R.; Friendberg, S. A. Phys. Lett. 1964, 9, 11.
(12) (a) Weng, C. Y. Ph.D. Thesis, Carnegie Institute of Technology, 1968.
(b) Hiller, W.; Strahle, J.; Datz, A.; Hanack, M.; Hatfield, W. E.; ter
Haar, L. W.; Guetlich, P. J. Am. Chem. Soc. 1984, 106, 329.
(10) Brader, M. L.; Ainscough, E. W.; Baker, E. N.; Brodie, A. M.; Ingham,
S. L. J. Chem. Soc., Dalton Trans. 1990, 2785.
8222 Inorganic Chemistry, Vol. 43, No. 26, 2004

COMMUNICATION
reorientation was further confirmed by the plots of M versus
H measured at different temperatures (Figure 2b). We found
that the curves of M versus H below ca. 5.0 K all exhibit an
inflection point as the applied field increases. The critical
field, at ca. 36 kOe, was determined by the dM/dH versus H
curve. Moreover, the magnetization only reaches a value of
ca. 0.84 Nâ mol-1 at 50 kOe, obviously below the theoretical
saturation value of 4 Nâ mol-1 (for Mn(III) g ) 2, S ) 2).
The divergence of the zero-field (ZFC) and field-cooled (FC)
M(T) data (Figure S3) below TC ) 2.5 K displays a long-
range ferromagnetic ordering at low field, while the peak at
ca. 7.5 K further confirms the presence of an AF ordering
in 1. The ac magnetic susceptibility data (Figure S4) also
ensure the appearance of magnetic ordering in 1. The critical
temperature Tc ) 2.8 K is determined by the peaks of both
øm′ versus T and øm′′ versus T plots, whereas another peak
at ca. 7.5 K in the øm′ versus T curve gives the critical
temperature of AF ordering, TN ) 7.5 K. Furthermore, a
hysteresis loop was detected in the ordered phase at 1.77 K
(Figure S5), characteristic of a weak ferromagnet, with a
coercive field of ca. 36 Oe and remnant magnetization of
ca. 0.007 Nâ mol-1.
In conclusion, a novel Mn(III) chain bridged by hydro-
gencyanamide was structurally and magnetically character-
ized for the first time in this paper. Although the magnetic
interaction through cyanamide in complex 1 is not as strong
as that through the azide bridge,9 this work provides a unique
example to construct magnetic coordination polymers through
a new type of small conjugated ligands. Recently, we have
obtained several other 1-D chain compounds bridged by
NCNH-, and the study of their magnetic properties is in
progress.
Acknowledgment. This work was supported by the
National Science Fund for Distinguished Young Scholars
(20125104) and NSFC 20221101, 20490210. The authors
thank Prof. X. X. Zhang for helpful discussion.
Supporting Information Available: Synthesis, IR, and more
structural diagrams and magnetic data (PDF). Crystal data (CIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC048547S
Inorganic Chemistry, Vol. 43, No. 26, 2004 8223