A two-dimensional homospin Cu(II) ferrimagnet featuring S-shaped hexanuclear secondary building blocks

Article abstract of DOI:10.1039/c4dt00375f

The decomposition of 2-(1-hydroxy-2-methylpropan-2-yl-imino)-1,2- diphenylethanone in the presence of copper acetate promoted the formation of a two-dimensional homospin Cu(ii) ferrimagnet featuring S-shaped hexanuclear secondary building blocks and a new pentanodal topology. the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00375f

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A two-dimensional homospin Cu(II) ferrimagnet
featuring S-shaped hexanuclear secondary
building blocks†
Cite this: Dalton Trans., 2014, 43,
8154
Received 1st March 2014,
Accepted 6th March 2014
Zilu Chen,* Long Liu, Yifei Wang, Huahong Zou, Zhong Zhang and Fupei Liang*
DOI: 10.1039/c4dt00375f
www.rsc.org/dalton
The decomposition of 2-(1-hydroxy-2-methylpropan-2-yl-imino)- ment for these spin carriers, ensuring the noncancellation of
1,2-diphenylethanone in the presence of copper acetate promoted the same kind of spins.9 In spite of the limited achievement, it
the formation of a two-dimensional homospin Cu(II) ferrimagnet is still of great difficulty to tune the reaction conditions to
featuring S-shaped hexanuclear secondary building blocks and a meet the requirements mentioned above for the preparation of
new pentanodal topology.
the targeted ferrimagnets. Taking the challenge, we managed
to explore the synthetic conditions for the formation of the
aimed homospin ferrimagnet by a combination of all above-
mentioned methods. Carboxylate and alkoxide groups can
both mediate magnetic coupling effectively and are both
capable of promoting AF and F interactions by adopting appro-
priate bridging modes and tuning the M–O–M bridging
angles, respectively. Furthermore, they are both facile to form
a triangular lattice (odd-sided polyhedra) via bridging three or
more metal ions simultaneously. Thus we managed to use
carboxylate and alkoxide ligands to synergistically tune the
magnetic topology and magnetic interactions between metal
ions with the specific reaction conditions provided by a syner-
gistic decomposition reaction of a Schiff base.
Ferrimagnets are drawing increased research interest from
both fundamental and device-related perspectives for their
permanent magnetization due to the noncancellation of the
antiferromagnetic coupled spins.1 Conventional ferrimagnetic
systems are usually achieved by different spin carriers which
can be either two kinds of metal ions with different spins2 or
the same metal with different oxidation states,3 as well as a
metal ion with an organic radical4 or just one organic com-
ponent bearing two or more kinds of organic radicals with
different spin-multiplicities.5 In contrast, the homometallic
homospin ferrimagnetic systems are uncommon and still
remain a synthetic challenge for their prerequisites of the
particular spatial arrangement and the thus resulting non-
compensation of individual spin moments which are difficult
to achieve.6–9 Undertaking the task, a few researchers suc-
ceeded in obtaining homospin ferrimagnets. From the mag-
netic topological point of view, magnetic lattices containing
odd-sided polyhedra such as triangles may achieve the non-
compensation in antiferromagnetically coupled spin moments,
thereby showing ferrimagnetism.8,10 Based on a consideration
of magnetic interaction, the introduction of ferromagnetic inter-
action and the specific alternation of antiferromagnetic (AF)
and ferromagnetic (F) interactions are regarded as one of the
important strategies for achieving these systems.7 In the syn-
thetic strategy, mixed ligands were used to induce different
coordination environments and a special topological arrange-
We report here
a ferrimagnetic complex [Cu3(NH2C-
(CH3)2CH2O)-(CH3COO)5]n (1) separated from the reaction of
Cu(CH3COO)2 with a Schiff base of 2-(1-hydroxy-2-methyl-
propan-2-ylimino)-1,2-diphenylethanone, the formation of which
was driven by the decomposition of the Schiff base. There are
three crystallographically independent Cu(II) ions, five acetato
groups and one 2-amino-2-methyl-1-propanolato ligand in the
asymmetric unit of 1 (Fig. 1a). The three Cu(II) ions present
square pyramidal geometries completed by five oxygen atoms
from five acetato groups for Cu1 and Cu2, and by four oxygen
atoms and one nitrogen atom from two acetato groups and
two 2-amino-2-methyl-1-propanolato anions for Cu3. Cu1 and
Cu2 are connected by four syn–syn bridging acetato groups to
form a paddle-wheel [Cu2(CH3COO)4] unit with a Cu⋯Cu
distance of 2.5992(6) Å. Cu3 and Cu3B are bridged by two
2-amino-2-methyl-1-propanolato anions in chelating-bridging
modes to build a [Cu2(H2NC(Me2)CH2O)2]2+ unit with a
Cu⋯Cu distance of 2.9119(9) Å. Two [Cu2(CH3COO)4] units are
bridged by one [Cu2(H2NC(Me2)CH2O)2]2+ unit via two acetato
O atoms to construct an S-shaped hexanuclear {[Cu2(CH3COO)4]-
School of Chemistry and Pharmaceutical Sciences, Guangxi Normal University,
Guilin 541004, P. R. China. E-mail: zlchen@mailbox.gxnu.edu.cn,
fliangoffice@yahoo.com; Fax: (+86)773-2120958
†Electronic supplementary information (ESI) available: Experimental details,
and additional tables and figures for 1. CCDC 976195. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c4dt00375f
[Cu2(H2NC(Me2)CH2O)2][Cu2(CH3COO)4](CH3COO)2}
unit.
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Fig. 1 (a) Molecular structure of 1 with selected atoms labelled. Sym-
metry codes: (A) −x + 2, y − 1/2, −z + 1/2; (B) −x + 2, −y + 1, −z + 1; (C)
−x + 2, y + 1/2, −z + 1/2. (b) Two-dimensional sheet of 1.
Fig. 3 (a) Plots of χM and χMT vs. T for 1. (b) Plots of χM and χMT (inset)
vs. T measured between 2 and 20 K at the indicated applied fields. (c)
Temperature-dependent ac magnetic susceptibilities of
1 under
Each hexanuclear unit is further connected to another four different frequencies (100, 300, 499 and 997 Hz) with a zero DC field
and an oscillating field of 2.5 Oe. (d) Field dependence of the magneti-
zation below 50.26 kOe for 1. (Inset: the whole field dependence of the
magnetization between −50.26 and 50.26 kOe emphasizing the
absence of a hysteresis effect.) Solid lines are guides for the eyes.
ones in a head-to-side style, constructing a two-dimensional
sheet. Alternatively, [Cu2(CH3COO)4] units are bridged along
the b-axis by acetato groups in anti–anti bridging modes to
form one-dimensional chains, which are further bridged by
[Cu2(H2NC(Me2)CH2O)2]2+ units along the c-axis to form the
two-dimensional sheet (Fig. 1b and S1†). It shows a new penta-
nodal 3-connected (3·162)(3·4·5)2(4·162)2 topology (Fig. S2†).
The five acetato groups in the asymmetric unit of 1 present
three kinds of bridging modes (Fig. 2a–c). Three carboxylato
(Fig. 3b) shows that the maximum in χMT at 7.9 K became less
groups in the [Cu2(CH3COO)4] unit behave as μ2-bridges
(Fig. 2a: syn–syn), and the other one in the [Cu2(CH3COO)4]
unit adopts a μ3-bridging mode (Fig. 2b: syn–syn : anti). The
actions between the adjacent Cu(II) centers.11 A further study
of χMT vs. T in the low temperature range at different fields
prominent upon increasing the field. This transition at 7.9 K
could indicate the onset of long-range magnetic ordering.12
To complete the characterization of the magnetic ordering
and define more precisely the critical ordering temperature,
the real χ′ and the imaginary χ″ components of ac suscepti-
fifth acetato group in 1 acts as a different μ3-bridging mode
(Fig. 2c: anti–anti : syn). The 2-amino-2-methyl-1-propanolato
ligand in 1 behaves also as a μ2-bridging mode (Fig. 2d: chelat-
bility (Fig. 3c) were measured with a frequency of 100, 300, 499
ing-bridging).
and 997 Hz and an amplitude of the driving field of 2.5 Oe. It
reveals in the real part of the susceptibility a broad and asym-
The magnetic behavior of 1 (solid state) observed by a
SQUID magnetometer at a DC field of 1000 Oe is shown in
metric peak with a frequency-independent maximum of χ′ at
Fig. 3a as the temperature dependent χM and χMT plot per
8.8 K and a plateau around 4.4 K showing a tiny frequency
dependence. The imaginary part of the ac susceptibility
hexanuclear building unit. Upon cooling, χM increases
smoothly first, and then increases rapidly below about 22 K
reaching a maximum of 0.75 cm3 mol−1 at 2.5 K. Decreasing
becomes non-zero below 9.3 K, which defines the critical
temperature. In comparison to the real χ′ components, the
imaginary χ″ components present first a shoulder-peak at
8.4 K and then an apparent peak at 4 K which shows some fre-
quency dependency. This indicates the presence of long-range
the temperature, the value of χMT decreases steadily from
1.98 cm3 K mol−1 at 300 K to a round minimum of 0.65 cm3 K
mol−1 at about 40 K. As the temperature is lowered further,
χMT increases rapidly and reaches a maximum of 4.33 cm3 K
magnetic ordering.13 The emergence of two peaks on both the
real χ′ and the imaginary χ″ components of ac susceptibility
was also seen in a few heteronuclear ferrimagnetic com-
pounds,14 but seldom for homonuclear ferrimagnetic com-
pounds. Field-dependent magnetization (M) measurements
(0 to 50.26 kOe) at 2.0 K were carried out as presented in
Fig. 3d. As the applied field increases, a rapid increase below
about 0.5 kOe followed by a much slower increase led to a
magnetization of 0.76Nβ at 50.26 kOe, which is much lower
than that expected for a total alignment of the moments and
suggests a magnetic ordering. No magnetic hysteresis is
observed at 2 K for this compound.
mol−1 at 7.9 K, and finally decreases rapidly reaching 1.45 cm3
K mol−1 at 1.96 K. All of these are characteristic of a ferri-
magnetic behaviour with dominant antiferromagnetic inter-
Fig. 2 Bridging modes of acetate and 2-amino-2-methyl-1-propano-
lato ligands in 1.
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As demonstrated in the structural discussion, compound ing unit to another four ones, resulting in a 2D magnetic
1 presents a 2D structure constructed from S-shaped hexa- order.
nuclear units. The noncompensation of the Cu(II) spin carriers
and the thus resulting long range magnetic order can be inter-
preted based on their special arrangement as shown in Fig. 4.
Cu1 and Cu2 are linked by four carboxylato groups in syn–syn-
Conclusions
bridging modes to form a paddle-wheel motif, which favours In summary, we demonstrate here a rare homospin Cu(II) ferri-
strong antiferrromagnetic coupling between the two spin car- magnet featuring S-shaped hexanuclear secondary building
riers.15 Magneto-structural correlation studies for hydroxide/ blocks and a new pentanodal 3-connected 2D topology. The
alkoxide bridged dimers revealed an angular dependence decomposition of 2-(1-hydroxy-2-methyl-propan-2-ylimino)-1,2-
which marks a crossover from antiferromagnetic to ferro- diphenylethanone made a significant contribution to the for-
magnetic coupling at a Cu–O–Cu bridging angle of about mation of the title compound, which provides a strategy for
97.6°.16,17 However, there exist both cases between Cu2 and preparing ferrimagnets by tuning the structural parameters via
Cu3 ions (Cu2–O2–Cu3: 96.28(10)° and Cu2–O9–Cu3: 103.12(10)°). the control of specific reaction environments.
As stated in documents, the antiferromagnetic coupling in this
case usually dominates over the ferromagnetic coupling.17 As a
result, Cu2 and Cu3 are presumably antiferromagnetically
coupled. Cu3 and Cu3B are double bridged by alcoholic
Acknowledgements
oxygen atoms in an equatorial–equatorial Cu–O–Cu The authors acknowledge the financial support by the National
exchange pathway with the Cu3–O11–Cu3B bridging angles of Natural Foundation of China (grant no. 21261004 and
97.40(11)°, a τ angle of 41.3° and the Cu3–O11 and Cu3–O11B 21271050), Guangxi Natural Science Foundation of China
bond lengths of 1.933(2) and 1.943(2) Å, which is facile for (grant no. 2013GXNSFGA019008 and 2013GXNSFAA019039),
ferromagnetic interaction.18 Cu1 and Cu3 are bridged by one and Program for New Century Excellent Talents in University
syn–anti carboxylato group in an equatorial–apical and nonpla- (NCET-10-0095).
nar Cu–O–C–O–Cu exchange pathway mediating a weak ferro-
magnetic interaction,19 which also supports the above-
mentioned magnetic couplings between Cu1 and Cu2, as well
as Cu2 and Cu3. Cu1 in the hexanuclear secondary construct-
Notes and references
ing unit is further linked to Cu2A and Cu3A by one carboxylato
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syn–anti mode for nonplanar Cu1–O–C–O–Cu3A via an apical–
equatorial exchange pathway), which facilitates antiferro-
magnetic and ferromagnetic couplings for Cu1 and Cu2A, and
Cu1 and Cu3A, respectively.19 This kind of magnetic inter-
action leads to the linkage of the hexanuclear secondary build-
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