An unusual magnetic response in a π-stacked 66-dia net structure of [4 + 2] copper(II) cubane

Article abstract of DOI:10.1039/c5ra07548c

A phenoxo bridged antiferromagnetic copper(ii) cubane features a π-stacked 6⁶-dia net framework and creates long range ferromagnetic ordering, as evidenced from a coercivity maximum (～2000 Oe) at 20 K with very unusual saturation magnetization.

Full text of DOI:10.1039/c5ra07548c

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: S. Chattopadhyay,
M. Das, B. K. shaw, S. K. saha, B. N. Ghosh and K. Rissanen, RSC Adv., 2015, DOI: 10.1039/C5RA07548C.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Page 1 of 12
RSC Advances
View Article Online
DOI: 10.1039/C5RA07548C
Unusual magnetic response in πꢀstacked 66ꢀdia net structure
of [4+2] copper(II) cubane
Mithun Das,‡,a Bikash Kumar Shaw,‡,b Biswa Nath Ghosh,c Kari Rissanen,c
Shyamal Kumar Saha*,b and Shouvik Chattopadhyay*,a
Abstract
A phenoxo bridged antiferromagnetic copper(II) cubane, features a πꢀstacked 66ꢀdia net
framework, which creates long range ferromagnetic ordering, as evidenced from a coercivity
maximum (~2000 Oe) at 20K with very unusual saturation magnetization.
Keywords: copper(II); [4+2] cubane; πꢀstacked 66ꢀdia net; long range ferromagnetic ordering;
coercivity
Highꢀnuclearity transition metal complexes have attracted much attention because of their
unusual physical properties1 and relevance to biological function in metalloproteins.2 Among
them, oxygen bridged tetrameric copper(II) clusters having Cu4O4 cubane cores have been
extensively studied in order to provide clear insight into the relationship between their structural
features and the strength of the magnetic exchange interaction between the copper(II). It is now
well established that the exchange coupling of phenoxo bridged polynuclear copper(II)
complexes depends on various structural features such as the coordination geometry of the
copper(II), Cu−O(R)−Cu angles, Cu−O bond lengths, Cu···Cu distances etc.3 The copper(II)
cubanes may have six equivalent Cu···Cu distances, two short and four long Cu···Cu distances
or four short and two long Cu···Cu distances.4 Although, there are several reports on the
correlations between the structural parameters and magnetic properties for oxygen bridged
copper(II) cubanes,5 the consequence of long range ferromagnetic ordering is not explored. The
long range ferromagnetic ordering could be achieved by strong interꢀcubane coupling and several

RSC Advances
Page 2 of 12
View Article Online
DOI: 10.1039/C5RA07548C
strategies could be developed to increase the interꢀcubane coupling, such as the creation of
hydrogen bonding and π···π stacking among neighbouring cubanes. The importance of hydrogen
bonding in propagating the magnetic interactions among the metal centers of neighboring
molecules was reported in literature.6 The supramolecular Hꢀbonding interaction intervenes the
ferromagnetic superꢀexchange coupling in the dinuclear assemblies. These works shows that
Water molecules, carboxylate bridges and even the nitrate ion mediate the exchange coupling
between the metal centers. 18ꢀcrownꢀ6 also participates in the exchange processes through Hꢀ
bonding interactions. Even the magnetic interaction between metal centers connected through
aromatic moieties via strong π···π stacking was shown to depend upon the stacking angle and
interaction between the spin densities of the stacked layers.7 However, best to our knowledge,
there is no report regarding the long range ferromagnetic ordering in copper(II) cubanes
connected through strong π···π stacking.
We have used a semicarbazone Schiff base, HL, to prepare a phenoxo bridged copper(II)
cubane, [Cu4(L)4](ClO4)4·4H2O (Scheme S1, ESI). The complex features supramolecular 66ꢀdia
net framework and cyclic perchlorate–water octameric clusters in its solid state structure.
Variable temperature magnetic susceptibility measurements showed that the copper(II) spins in
the cubane core are antiferromagnetically coupled and the uncompensated spins of the adjacent
cubane cores are ferromagnetically coupled through π···π stacking.
All starting materials were commercially available, reagent grade, and used as purchased
from SigmaꢀAldrich without further purification. Caution!!! Although no problems were
encountered in this work, perchlorate salts containing organic ligands are potentially explosive.
Only a small amount of the material should be prepared and they should be handled with care.
A methanol solution (20 ml) of semicarbazide hydrochloride (1 mmol, 120 mg) and 3ꢀ
methoxysalicylaldehyde (1 mmol, 150 mg) were refluxed for ca. 1 h to produce the ligand 3ꢀ
methoxysalicylaldehydesemicarbazone (HL). The ligand was not isolated. A methanol solution
(10 ml) of copper(II) perchlorate hexahydrate (1 mmol, 370 mg) was added to it with constant
stirring. The stirring was continued for additional 2 h. The resulting green solution was allowed
to stand at room temperature. Single crystals, suitable for Xꢀray diffraction, started to separate
from the resulting solution and was collected by filtration after ca. 2 days.

Page 3 of 12
RSC Advances
View Article Online
DOI: 10.1039/C5RA07548C
The complex crystallizes in tetragonal space group I41/a. The asymmetric unit consists of
a cationic copper(II) complex, [Cu(L)]+, one perchlorate anion and a water of crystallization
(Figure 1A). Such four cationic units assembled together to form ꢁ3ꢀphenoxo bridged cubane
(Figure 1B). Within the cubane unit, Cu(1)···Cu(1)†, Cu(1)···Cu(1)◊, Cu(1)†···Cu(1)* and
Cu(1)◊···Cu(1)* distances are significantly shorter (3.281 Å) compared to Cu(1)···Cu(1)* and
*
◊
Cu(1)†···Cu(1)◊ distances (3.410 Å) († = ꢀ1/4+x,3/4ꢀy, 7/4ꢀz; = 1ꢀx,1/2ꢀy,z and = 3/4ꢀ
x,1/4+y,7/4ꢀz). Thus the complex may be classified as a cubane with two short and four long
Cu···Cu distances.4 Selected bond angles (°) around copper(II) is shown in Table S1 (ESI). Each
copper(II) in the cubane resides in identical environment. Cu(1) is coordinated by two oxygen
atoms, O(1) and O(3), and one nitreogen atom, N(1), of the deprotonated ligand (L)ꢀ and two
oxygen atoms, O(1)†, O(2)† of a symmetry related (L)ꢀ. The sixth coordination site is occupied by
an oxygen atom O(1)* from another symmetry related (L)ꢀ. There are six Cu2O2 faces in the
cubane, of which four Cu2O2 faces have two different Cu–O–Cu bridging angles {88.3° and
112.8° (β)} with three short (two 1.988 Å (d1) and one 1.951 Å) and one long (d2 ~2.672 Å) Cu–
O bond lengths (Figure S1, ESI). The other two Cu2O2 faces have similar Cu–O–Cu bridging
angles {93.7°(α)} with two short (~1.951 Å) and two long (d2 ~2.672 Å) Cu–O bond lengths.
The out of plane shift of the carbon atoms linked to bridged oxygen atoms makes the angle (τ)
around 49.5°. The Cu–N bond distance (1.938 Å) is found to be shorter than the Cu–O bond
distances (1.951–2.672 Å).
̑
A cyclic perchlorateꢀwater octameric ® (16) ring has been produced by hydrogen
̑
bonding interactions among the lattice water molecules and perchlorate ions (Figure S2, ESI).
This perchlorateꢀwater octamer is hydrogen bonded with the cubane structure to form a three
dimensional architecture (Figure S3, ESI). The details of hydrogen bonding interactions are
given in ESI. The phenyl ring R24 [C(1)–C(2)–C(3)–C(4)–C(5)–C(6)] present within [Cu(L)]+
unit shows π···π stacking with symmetry related (3/2ꢀx,1/2ꢀy,3/2ꢀz) phenyl ring R24 (Table S2,
ESI) and each cubane behaves like a tetrahedron vertex of a self assembled 66ꢀdia net structure
(Figure 2).8
The temperature dependence of the magnetic susceptibility (χ) (ZFCꢀFC) for the complex
measured in the temperature range between 2 to 300 K is shown in Figure 3A. The ZFCꢀFC
curves indicate a higher ordering temperature at around 300K. From the least squares analysis

RSC Advances
Page 4 of 12
View Article Online
DOI: 10.1039/C5RA07548C
(Figure S4), a strong antiferromagnetic exchange interaction is found between the bridged
copper(II) centers with J = –40.8 cm–1 and a relatively weaker one with J′ = –4.9 cm–1 between
the unbridged copper(II) centers respectively. (Details are given in ESI)
The most interesting feature in the magnetic properties of this intracluster
antiferromagnetic structure is the observation of complicated hysteresis loops which show
marked changes under application of thermal energy as shown in Figure S5 (ESI). Unlike other
ferromagnetic materials, here, the hysteresis loops become wider as the temperature rises from
2K to 20K. Above 20K, a gradual decrease in coercive field is observed with a minimum of 160
Oe at 300K, which is in close agreement with ZFCꢀFC behavior. The variation of coercivities
with temperature is summarized in Table S3 (ESI). At lower temperatures up to 10 K,
magnetization becomes paramagnetic at higher field region due to unsaturation. The observation
of highest coercivity at 20 K with clear saturation in magnetization is very unusual in this
antiferromagnetic cubane structure (Figure 3B). This behavior clearly reveals a certain phase
transition operated at lower temperatures.
To confirm the presence of magnetic phase, ac magnetic susceptibility measurements
were carried out as a function of temperature. The measurements were performed at an applied
dc field of 5 Oe and ac field of 3 Oe oscillating in the frequency range from 1.1–21.1 Hz. The
relaxation peak, signifying the existence of magnetic phase is appeared both in real (χ') and
imaginary (χ'') parts of susceptibility (Figure 3C,D).9
The unusual magnetic property could be explained with the help of supramolecular π···π
stacking which vary with thermal energy. In our previous work, we have reported the
enhancement of ferromagnetic ordering under the application of thermal energy.10 In case of
ferromagnetism that originated due to interaction between the S = ½ spins of metal centers via
π···π stacking increases as a result of increase in overlapping area of the aromatic πꢀstacked
moieties due to the strain generated in the crystal lattice resulting better ferromagnetic ordering
at high temperature. The perfect overlapping of the aromatic πꢀstacked moieties results long
range ferromagnetic interaction through the antiferromagnetic cubane cores. To explain the
observed unusual ferromagnetic hysteresis loops we have considered the model schematically
shown in Figure 4.

Page 5 of 12
RSC Advances
View Article Online
DOI: 10.1039/C5RA07548C
In this case, the copper(II) spins in the cubane core are antiferromagnetically coupled
while the uncompensated spins of the adjacent cubane cores are ferromagnetically coupled
through π···π stacking between the ligands. Basically, the magnitude of the uncompensated spin
moment in the cubane core is smaller at 2K as the antiferromagnetic singlet state predominates at
lower temperatures. However, at higher temperatures, the magnitude of the uncompensated spin
moment increases in the antiferromagnetic core due to splintering of the coupled spins at higher
temperature. At the same time, with increase in temperature overlapping between the πꢀstacked
moieties increases upto 20K. This creates a path for the uncompensated spins to be coupled
ferromagnetically in a long range manner, showing maximum ferromagnetic ordering at 20K.
After 20K, the ferromagnetic ordering gradually decreases with a minimum coercivity of 160 Oe
at temperature 300K, similar to a high temperature ferromagnet. Another point to be noted is the
appearance of asymmetry in the hysteresis loop near the zero field region and the unsaturation in
magnetization at high field region particularly for lower temperatures. This is correlated with the
rise in magnetization below 20 K in the susceptibility data. The inverse of susceptibility (Figure
S6) does not show linearity at low temperature which indicates that it does not follow Curie
paramagnetism at all. This also implies that the rise in magnetization at low temperature is not
due to paramagnetic impurities rather the uncompensated spin moment arises in the exchange
process into the cubane core resulting the anomaly. The asymmetry (sharp fall of magnetization
near zero field) arises due to antiferromagnetic coupling between the copper(II) spins in the
cubane core. The exchange bias originated due to sequence of alternate ferromagnetic and
antiferromagnetic exchange interactions causes the asymmetry in the coercivity as observed the
highest value at 30 K.
In conclusion,
a
phenoxo bridged antiferromagnetic copper(II) cubane,
[Cu4(L)4](ClO4)4ꢂ4H2O forms 66ꢀdia net supraꢀmolecular framework via π···π stacking which, in
turn, is responsible for a long range ferromagnetic ordering of the uncompensated spins of the
adjacent cubane cores. The coercivity unusually increases with temperature. The origin of
asymmetry in the hysteresis curve is the appearance of alternate antiferromagnetic and
ferromagnetic interactions in the complex.

RSC Advances
Page 6 of 12
View Article Online
DOI: 10.1039/C5RA07548C
Acknowledgements
S.C. acknowledges DST, India under FAST Track Scheme (Order No. SR/FT/CSꢀ
118/2010, dated 15/02/2012). S.K.S. acknowledges DST, India (project No. SR/NM/NSꢀ
1089/2011) for providing facilities of magnetic measurements.
Notes and references
a Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata 700032, India.
Eꢀmail: shouvik.chem@gmail.com
b
Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata
700032, India. Eꢀmail: cnssks@iacs.res.in
c
Department of Chemistry, Nanoscience Center, University of Jyväskylä, P.O. Box 35, 40014
Jyväskylä, Finland.
‡ These authors contributed equally to this work.
Electronic supplementary information (ESI) available: Experimental details, Crystal data
collection and refinement details, IR and electronic spectra, Photophysical and powder XRD
study, Hydrogen bonding interactions and magnetic property.
Yield: 270 mg, 69%, based on copper(II). Anal. Calc. for C36H48Cu4N12O32Cl4 (1556.82): C,
27.77; H, 3.11; N, 10.80. Found: C, 27.9; H, 3.4; N, 10.6%. IR (cmꢀ1): 3426 (OH), 3381, 3298
(NH2); 1642 (C=N), 1088 (Cl–O). UV–Vis, λmax (nm) [εmax(L molꢀ1 cmꢀ1)] (acetonitrile): 627
(229), 498 (534), 375 (7131).
Crystal data: formula = C36H48Cu4N12O32Cl4, formula weight = 1556.82, crystal size (mm) =
0.14 x 0.19 x 0.29, temperature (K) = 173, crystal system = Tetragonal, space group = I41/a, a =
19.0353(6) Å, b = 19.0353(6) Å, c = 15.4705(7) Å, dcalc (gcm−3) = 1.845, ꢁ (mm−1) = 1.795,
F(000) = 3152, total reflections = 5120, unique reflections = 2604, observed data [I > 2σ(I)] =
2049, R(int) = 0.019, R1, wR2 (all data) = 0.0681, 0.1446, R1, wR2 [I > 2σ(I)] = 0.0502, 0.1274.

Page 7 of 12
RSC Advances
View Article Online
DOI: 10.1039/C5RA07548C
1. (a) Z.ꢀH. Wei, C.ꢀY. Ni, H.ꢀX. Li, Z.ꢀG. Ren, Z.ꢀR. Sun and J.ꢀP. Lang, Chem. Commun.,
2013, 49, 4836ꢀ4838; (b) G. Novitchi, W. Wernsdorfer, L. F. Chibotaru, J.ꢀP. Costes, C. E.
Anson and A. K. Powell, Angew. Chem. Int. Ed., 2009, 48, 1614–1619; (c) R. Saha, B. Joarder,
A. S. Roy, S. M. Islam and S. Kumar, Chem. Eur. J., 2013, 19, 16607–16614.
2. (a) R. Than, A. A. Feldmann and B. Krebs, Coord. Chem. Rev., 1999, 182, 211ꢀ241; (b) Jr. L.
Que and W. B. Tolman, Angew. Chem. Int. Ed., 2002, 41, 1114ꢀ1137.
3. (a) J.ꢀP. Costes, C. Duhayon and L. Vendier, Inorg. Chem., 2014, 53, 2181−2187; (b) C. J.
Calzado, Chem. Eur. J., 2013, 19, 1254–1261.
4. (a) E. Ruiz, A. RodriguezꢀFortea, P. Alemany and S. Alvarez, Polyhedron, 2001, 20,
1323−1327; (b) E. Ruiz, P. Alemany, S. Alvarez and J. Cano, J. Am. Chem. Soc., 1997, 119,
1297−1303.
5. (a) D. Maity, A. D. Jana, M. Debnath, N. G. R. Hearns, M.ꢀH. Sie, H. M. Lee, R. Clérac and
M. Ali, Eur. J. Inorg. Chem., 2010, 3484–3490; (b) J. Tercero, E. Ruiz, S. Alvarez, A.
RodriguezꢀFortea and P. Alemany, J. Mater. Chem. 2006, 16, 2729−2735.
6. (a) M. Zeng, W. Zhang, X. Sun and X. Chen, Angew. Chem., Int. Ed., 2005, 44, 3079–3082;
(b) H. Miyasaka, H. Okawa, A. Miyazaki and T. Enoki, Inorg. Chem., 1998, 37, 4878ꢀ4883. (c)
F. Yoe, M. FloresꢀAlamo, F. Morales, R. Escudero, H. CortesꢀHernandez, M. Castro and N.
BarbaꢀBehrens, Inorg. Chim. Acta, 2014, 423, 36–45. (d) A. Drljaca, D. C. R. Hockless, B.
Moubaraki, K. S. Murray and L. Spiccia, Inorg. Chem., 1997, 36, 1988ꢀ1989; (e) R.P. Doyle,
M.Julve, F. Lloret, M. Nieuwenhuyzen and P. E. Kruger, Dalton Trans., 2006, 2081–2088. (f) J.
S. Costa, N. A. G. Bandeira, B. Le Guennic, V. Robert, P. Gamez, G. Chastanet, L. OrtizꢀFrade,
and L. Gasque, Inorg. Chem. 2011, 50, 5696–5705.
7. (a) K. Yoshizawa and R. Hoffmann, J. Am. Chem. Soc., 1995, 117, 6921ꢀ6926; (b) L. Li, K.
Lin, C. Ho, C. Sun and H. Yang, Chem. Commun., 2006, 1286–1288; (c) Y. Zheng, M. Tong, W.
Xue, W. Zhang, X. Chen, F. Grandjean and G. Long, Angew. Chem., 2007, 119, 6188–6192; (d)
V. Lageta, C. Hornic, P. Rabu, M. Drillon and R. Ziessel, Coord. Chem. Rev., 1998, 178–180,
1533–1553.

RSC Advances
Page 8 of 12
View Article Online
DOI: 10.1039/C5RA07548C
8. (a) M. Scudder and I.Dance, CrystEngComm, 2001, 12, 1ꢀ4; (b) Z.ꢀL. Wang, W.ꢀH. Fanga and
G.ꢀY. Yang, Chem. Commun., 2010, 46, 8216–8218.
9. N. Motokawa, H. Miyasaka, M. Yamashita and K. R. Dunbar, Angew. Chem. Int. Ed., 2008,
47, 7760 –7763.
10. S. Giri and S. K. Saha, J. Phys. Chem. Lett., 2011, 2, 1567–1572.

Page 9 of 12
RSC Advances
View Article Online
DOI: 10.1039/C5RA07548C
Figure 1: Perspective view of (A) [CuL]+ unit and (B) Cu4O4 core of the complex. Selected
bond lengths: Cu(1)ꢀO(1) = 1.951(4), Cu(1)ꢀO(3) = 1.956(4), Cu(1)ꢀO(1)* = 2.672(4), Cu(1)ꢀ
O(1)† = 1.988(3), Cu(1)ꢀO(2)† = 2.244(4), Cu(1)ꢀN(1) = 1.938(5).

RSC Advances
Page 10 of 12
View Article Online
DOI: 10.1039/C5RA07548C
Figure 2: (A) π···π stacking between cubane units and (B) πꢀstacked 66ꢀdia net structure of the
complex.

Page 11 of 12
RSC Advances
View Article Online
DOI: 10.1039/C5RA07548C
Figure 3: (A) Change in dc susceptibility (χ) values with temperature for the zero field cooling
and field cooling processes measured at 100 Oe. (B) Variation of magnetization vs. magnetic
field at 20K. Inꢀphase (χ') (C) and outꢀofꢀphase (χ'') (D) AC susceptibilities at 5 Oe dc field and 3
Oe ac oscillating field.

RSC Advances
Page 12 of 12
View Article Online
DOI: 10.1039/C5RA07548C
Figure 4: Schematic representation of antiferromagnetic exchange coupling between bridged
copper atoms in the intracubane core (above) and intercubane long range ferromagnetic ordering
mediated through π···π stacking (below)