Dramatic modifications of magnetic properties through dehydration-rehydration processes of the molecular magnetic sponges CoCu(obbz)(H2O)4·2H2O and CoCu(obze)(H2O)4·2H2O, with obbz = N,N'-bis(2-carboxyphenyl)oxamido and obze = N-(2-carboxyphenyl)-N'-(carboxymethyl)oxamido

Article abstract of DOI:10.1021/ic970719g

The two compounds CoCu(obbz)(H₂O)₄·2H₂O (I) and CoCu(obze)(H₂O)₄·2H₂O (II) have been synthesized, and their crystal structures have been determined at room temperature. Both compounds crystallize in the monoclinic system, space group P2₁n. The lattice parameters are a = 7.8190(6) A?, b = 12.534(2) A?, c = 20.2770(10) A?, β = 97.306(6)°, and Z = 4 for CoCu(obbz)(H₂O)₄·2H₂O and a = 17.966(5) A?, b = 6.907(1) A?, c = 13.678(3) A?, β= 97.69(1)°, and Z = 4 for CoCu(obze)(H₂O)₄·2H₂O. For both compounds, the structure consists of isolated Co²⁺Cu²⁺ pairs (in which the metal ions are bridged by an oxamide group) and noncoordinated water molecules. A thermogravimetric analysis has been carried out. Three water molecules are removed at a temperature T₂ (96 °C for I and 120 °C for II), and five water molecules are removed at a temperature T₃ (205 °C for I and 190 °C for II). When cooling down, the two compounds reabsorb water slowly and then return to the initial state. The dehydration-rehydration process is totally reversible, provided that the temperature does not exceed 275 °C. The magnetic properties of the starting materials, as well as the materials containing three water molecules and then one, have been investigated, both in the dc and ac modes. The two starting materials, CoCu(obbz)(H₂O)₄·2H₂O and CoCu(obze)(H₂O)₄·2H₂O, possess a nonmagnetic ground state. The compounds CoCu(obbz)(H₂O)₃ and CoCu(obze)(H₂O)₃ behave as one-dimensional ferrimagnets without long-range magnetic ordering down to 2 K. The compounds CoCu(obbz)(H₂O) and CoCu(obze)(H₂O), finally, also behave as ferrimagnets, but with a long-range magnetic ordering occurring at 25 and 20 K, respectively. Upon rehydration, the compounds return to the initial state with a nonmagnetic ground state. Infrared and Raman vibrational spectroscopies have been used to obtain further insights into the stuctural modifications accompanying the removal and the uptake of water molecules.

Full text of DOI:10.1021/ic970719g

6374
Inorg. Chem. 1997, 36, 6374-6381
Dramatic Modifications of Magnetic Properties through Dehydration-Rehydration
Processes of the Molecular Magnetic Sponges CoCu(obbz)(H2O)4‚2H2O and
CoCu(obze)(H2O)4‚2H2O, with obbz ) N,N′-Bis(2-carboxyphenyl)oxamido and obze )
N-(2-Carboxyphenyl)-N′-(carboxymethyl)oxamido
Joulia Larionova,† Suvarna A. Chavan,†,‡ J. V. Yakhmi,‡ Anne Gulbrandsen Frøystein,§
Jorunn Sletten,§ Claude Sourisseau,| and Olivier Kahn*,†
Laboratoire des Sciences Mole´culaires, Institut de Chimie de la Matie`re Condense´e de Bordeaux,
UPR CNRS No. 9048, 33608 Pessac, France, Chemistry Division, Bhabha Atomic Research Center,
400 085 Mumbai, India, Department of Chemistry, University of Bergen, 5007 Bergen, Norway,
and Laboratoire de Spectroscopie Mole´culaire et Cristalline, URA CNRS No. 124,
Universite´ de Bordeaux 1, 33405 Talence, France
ReceiVed June 12, 1997X
The two compounds CoCu(obbz)(H2O)4‚2H2O (I) and CoCu(obze)(H2O)4‚2H2O (II) have been synthesized, and
their crystal structures have been determined at room temperature. Both compounds crystallize in the monoclinic
system, space group P21/n. The lattice parameters are a ) 7.8190(6) Å, b ) 12.534(2) Å, c ) 20.2770(10) Å,
â ) 97.306(6)°, and Z ) 4 for CoCu(obbz)(H2O)4‚2H2O and a ) 17.966(5) Å, b ) 6.907(1) Å, c ) 13.678(3)
Å, â ) 97.69(1)°, and Z ) 4 for CoCu(obze)(H2O)4‚2H2O. For both compounds, the structure consists of isolated
Co2+Cu2+ pairs (in which the metal ions are bridged by an oxamide group) and noncoordinated water molecules.
A thermogravimetric analysis has been carried out. Three water molecules are removed at a temperature T2 (96
°C for I and 120 °C for II), and five water molecules are removed at a temperature T3 (205 °C for I and 190 °C
for II). When cooling down, the two compounds reabsorb water slowly and then return to the initial state. The
dehydration-rehydration process is totally reversible, provided that the temperature does not exceed 275 °C.
The magnetic properties of the starting materials, as well as the materials containing three water molecules and
then one, have been investigated, both in the dc and ac modes. The two starting materials, CoCu(obbz)(H2O)4‚2H2O
and CoCu(obze)(H2O)4‚2H2O, possess a nonmagnetic ground state. The compounds CoCu(obbz)(H2O)3 and CoCu-
(obze)(H2O)3 behave as one-dimensional ferrimagnets without long-range magnetic ordering down to 2 K. The
compounds CoCu(obbz)(H2O) and CoCu(obze)(H2O), finally, also behave as ferrimagnets, but with a long-range
magnetic ordering occurring at 25 and 20 K, respectively. Upon rehydration, the compounds return to the initial
state with a nonmagnetic ground state. Infrared and Raman vibrational spectroscopies have been used to obtain
further insights into the stuctural modifications accompanying the removal and the uptake of water molecules.
Introduction
contain two kinds of spin carriers, either two different metals
ions11-20 or a metal ion and an organic radical.21-28 In at least
two cases, three spin carriers are involved.29-31
A few decades ago the distinction between solid state
chemistry looking toward materials sciences and molecular
chemistry looking toward reactivity, homogeneous catalysis, or
life sciences was well pronounced. For some time, a molecular
material science has existed. Among its achievements one can
mention molecular conductors and superconductors, liquid
crystals, molecular systems active in nonlinear optics, and more
recently molecule-based magnets. The first molecular com-
pounds exhibiting a spontaneous magnetization below a certain
critical temperature were reported in 1967.1,2 It was however
only during the 1980s that the field of molecule-based magnets
has developed rapidly, and today a large variety of such
compounds are known.3-10 In many cases, these compounds
(4) Kahn, O.; Pei, Y.; Verdaguer, M.; Renard, J. P.; Sletten, J. J. Am.
Chem. Soc. 1988, 110, 782.
(5) Miller, J. S.; Epstein, A. J. Angew. Chem., Int. Ed. Engl. 1994, 33,
385.
(6) Kahn, O. Molecular Magnetism; VCH: New York 1993.
(7) Kahn, O. AdV. Inorg. Chem. 1995, 43, 179.
(8) Nakazawa, Y.; Tamura, M.; Shirakawa, N.; Shiomi, D.; Takahashi,
M.; Kinoshita, M.; Ishikawa, M. Phys. ReV. 1992, B46, 8906.
(9) Chiarelli, R.; Nowak, M. A.; Rassat, A.; Tholence, J. L. Nature 1993,
363, 147.
(10) Gatteschi, D. AdV. Mater. 1994, 6, 635.
(11) Nakatani, K.; Carriat, J. Y.; Journaux, Y.; Kahn, O.; Lioret, F.; J.
Renard, J. P.; Pei, Y.; Sletten, J.; Verdaguer, M. J. Am. Chem. Soc.
1989, 111, 5739.
(12) Stumpf, H. O.; Pei, Y.; Kahn, O.; Sletten, J.; Renard, J. P. J. Am.
Chem. Soc. 1993, 115, 6738.
(13) Tamaki, H.; Zhong, Z. J.; Matsumoto, N.; Kida, S.; Koikawa, M.;
Achiwa, N.; Hashimoto, Y.; Okawa, H. J. Am. Chem. Soc. 1992, 114,
6974.
(14) Okawa, H.; Mitsumi, M.; Ohba, M.; Kodera, M.; Matsumoto, N. Bull.
Chem. Soc. Jpn. 1994, 67, 2139.
(15) Decurtins, S.; Schmalle, H. W.; Oswald, H. R.; Linden, A.; Ensling,
J.; Gu¨tlich, P.; Hauser, A. Inorg. Chim. Acta 1994, 216, 65.
(16) Decurtins, S.; Schmalle, H. W.; Schneuwly, P.; Ensling, J.; Gu¨tlich,
P. J. Am. Chem. Soc. 1994, 116, 9521.
† Institut de Chimie de la Matie`re Condense´e de Bordeaux.
‡ Bhabha Atomic Research Center.
§ University of Bergen.
| Universite´ de Bordeaux 1.
X Abstract published in AdVance ACS Abstracts, December 1, 1997.
(1) Wickman, H. H.; Trozollo, A. M.; Williams, H. J..; Hull, G. W.;
Merritt, F. R. Phys. ReV. 1967, 155, 563.
(2) deFotis, G. C.; Palacio, F.; O’Connor, C. J.; Bhatia, S. N.; Carlin, R.
L. J. Am. Chem. Soc. 1977, 99, 8314.
(3) Miller, J. S.; Calabrese, J. C.; Rommelman, H.; Chittipedi, S. R.; Zang,
J. H.; Reiff, W. M.; Epstein, A. J. J. Am. Chem. Soc. 1987, 109, 769.
(17) Turner, S.; Kahn, O.; Rabardel, L. J. Am. Chem. Soc. 1996, 118, 6428.
S0020-1669(97)00719-2 CCC: $14.00 © 1997 American Chemical Society

Magnetic Properties of Molecular Magnetic Sponges
Inorganic Chemistry, Vol. 36, No. 27, 1997 6375
Table 1. Crystallographic Data for CoCu(obbz)(H2O)4‚2H2O (I)
and CoCu(obze)(H2O)4‚2H2O (II)
One of the questions the people working in the field of
molecular materials are faced with may be expressed as
follows: The synthetic procedures leading to molecular materials
are obviously totally different from those leading to more
classical solid state materials. However, once these materials
are obtained, are there still some differences according to
whether they arise from molecular or solid state chemistry? In
the field of magnetic materials, it has already been emphasized
that the main originality of the molecular compounds arises from
the fact that they are usually weakly colored while the classical
magnets are opaque. Several groups including ours are presently
investigating the optical and photophysical properties of mo-
lecular magnetic materials.32,33 One of the important issues
along this line would be to detect a synergy between optical
(or photophysical) and magnetic properties.
Here, we are concerned by another specificity of molecular
materials, namely the softness of their crystal lattices as
compared to ionic or metallic lattices. We will show that it is
possible to modify dramatically and reversibly the magnetic
properties of molecular compounds through mild dehydration-
rehydration processes. More precisely, this paper describes first
the crystal structures and the magnetic properties of the
bimetallic compounds CoCu(obbz)(H2O)4‚2H2O (I) and CoCu-
(obze)(H2O)4‚2H2O (II), where obbz stands for N,N′-bis-
(carboxymethyl)oxamido and obze for N-(2-carboxymethyl)-
N′-(carboxymethyl)oxamido, and then the reversible trans-
formation between paramagnetic and ferromagnetic states as
the compounds are successively dehydrated and rehydrated.
To save space, we will detail the results obtained with the
CoCu(obbz) compounds and will mention much more briefly
those obtained with the CoCu(obze) compounds, pointing out
the few differences between the two systems. A preliminary
report of a long-range ordering in compound I after dehydration
has already appeared.34
I
II
empirical formula
fw
C16H20CoCuN2O12
554.81
C11H18CoCuN2O12
492.75
temp/K
λ/Å
293(1)
293(1)
0.71073
P21/n (No. 14)
7.8190(6)
12.534(2)
20.2770(10)
97.306(6)
1971.1(4)
4
1.870
1.992
0.034
0.070
0.71073
P21/n (No. 14)
17.966(5)
6.907(1)
13.678(3)
97.69(1)
1682(1)
4
1.94
2.319
0.059
0.058
space group
a/Å
b/Å
c/Å
â/deg
V/Å3
Z
F(calc)/g cm-3
µ/mm-1
R1 [I > 2σ(I)]a
wR [I > 2σ(I)]a
2
a For compound I: R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR ) {∑[w(Fo
-
2
2
Fc2)2]/∑[w(Fo )2]}1/2; w ) 1/σ2(Fo ) + (0.0342P)2 + 0.7008P, where P
) [0.3333 × (maximum of 0 or Fo ) + 0.6667Fc2. For compound II:
2
R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR ) {∑[w(|Fo| - |Fc|)2]/∑[w|Fo| ]}1/2; w
2
) 1/σ2(Fo), where σ(F) ) σ(I) (ILp)-1/2 and σ(I) ) [σc2 + (0.02F2)2]1/2
.
dianions [Cu(obbz)]2- and [Cu(obze)]2- are schematized as follows:
Experimental Section
Syntheses. The organic molecules H4obbz and H4obze as well as
the copper(II) precursors Na2[Cu(obbz)]‚4H2O and Na2[Cu(obze)]‚2H2O
were synthesized as already described.35,36 The structures of the two
CoCu(obbz)(H2O)4‚2H2O (I) was synthesized as follows. A 20 mL
portion of an aqueous solution containing 5 × 10-4 mol (0.145 g) of
Co(NO3)2‚6H2O was slowly added to 200 mL of a freshly filtered
aqueous solution containing 5 × 10-4 mol (0.22 g) of Na2[Cu(obbz)]‚-
4H2O. A pale green-gray precipitate appeared. It was filtered off,
washed with water, and dried in a desiccator containing silica gel. Anal.
Calc for C16H20N2O12CoCu (I): C, 34.64; H, 3.63; N, 5.05; Co, 10.62;
Cu, 11.45. Found: C, 35.02; H, 3.59; N, 4.85; Co, 10.42; Cu, 11.29.
CoCu(obbz)(H2O)3 and CoCu(obbz)(H2O) were obtained by heating
under vacuum the starting material to 96 and 205 °C, respectively. Well-
shaped single crystals were obtained by slow diffusion in an H-shaped
tube of two 10-3 molar aqueous solutions containing Na2[Cu(obbz)]‚-
4H2O and Co(NO3)2‚6H2O, respectively. CoCu(obze)(H2O)4‚2H2O (II)
was prepared in a similar way, using Na2[Cu(obze)]‚2H2O as a
precursor. Anal. Calc for C11H18N2O12CoCu (II): C, 26.81; H, 3.68;
N, 5.69; Co, 11.96; Cu, 12.90. Found: C, 26.99; H, 3.76; N, 5.62;
Co, 12.02; Cu, 12.81.
X-ray Structure Determination and Refinement. Diffraction data
were collected at 293 K with an Enraf-Nonius CAD-4 diffractometer,
using graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å).
Crystal parameters and refinement results are compiled in Table 1. Cell
dimensions were determined on the basis of the setting angles of 25
reflections in the 2θ range 29-42 and 19-35° for I and II, respectively.
A total of 3828 (I) and 2970 (II) reflections within 2θ ) 50° were
recorded using the ω/2θ scan technique. Three reference reflections
monitored throughout the data collection showed a moderate decay of
3% on average for I and no significant decay for II. The data were
corrected for Lorentz and polarization effects and for linear decay.
(18) Mallah, T.; Thiebaut, S.; Verdaguer, M.; Veillet, P. Science 1993, 262,
1554.
(19) Entley, W. R.; Girolami, G. S. Science 1995, 268, 397.
(20) Ferlay, S.; Mallah, T.; Ouahe`s, R.; Veillet, P.; Verdaguer, M. Nature
1995, 378, 701.
(21) Miller, J. S.; Calabrese, J. C.; Dixon, D. A.; Epstein, A. J.; Bigelow,
R. W.; Zhang, J. H.; Reiff, W. M. J. Am. Chem. Soc. 1987, 109, 769.
(22) Broderick, W. E.; Thompson, J. A.; Day, E. P.; Hoffman, B. M. Science
1990, 249, 410.
(23) Yee, G. T.; Manriquez, J. M.; Dixon, D. A.; McLean, R. S.; Groski,
D. M.; Flippen, R. B.; Narayan, K. S.; Epstein, A. J.; Miller, J. S.
AdV. Mater. 1991, 3, 309.
(24) Broderick, W. E.; Hoffman, B. M. J. Am. Chem. Soc. 1991, 113, 6334.
(25) Eichhorn, D. M.; Skee, D. C.; Broderick, W. E.; Hoffman, B. M. Inorg.
Chem. 1993, 32, 491.
(26) Miller, J. S.; Calabrese, J. C.; McLean, R. S.; Epstein, A. J. AdV. Mater.
1992, 4, 498.
(27) Caneschi, A.; Gatteschi, D.; Sessoli, R.; Rey, P. Acc. Chem. Res. 1989,
22, 392.
(28) Inoue´, K.; Iwamura, H. J. Am. Chem. Soc. 1994, 116, 3173.
(29) Stumpf, H. O.; Ouahab, L.; Pei, Y.; Grandjean, D.; Kahn, O. Science
1993, 261, 447.
(30) Stumpf, H. O.; Ouahab, L.; Pei, Y.; Bergerat, P.; Kahn, O. J. Am.
Chem. Soc. 1994, 116, 3866.
(31) Stumpf, H. O.; Pei, Y.; Michaut, C.; Kahn, O.; Renard, J. P.; Ouahab,
L. Chem. Mater. 1994, 6, 657.
(32) Day, P. Science 1993, 261, 431.
(33) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 272,
704.
(34) Chavan, S. A.; Yakhmi, J. V.; Gopalakrishnan, I. K. Mol. Cryst. Liq.
Cryst. 1995, 274, 11.
(35) Pei, Y.; Kahn, O.; Nakatani, K.; Codjovi, E.; Mathonie`re, C.; Sletten,
J. J. Am. Chem. Soc. 1991, 113, 6558.
(36) Nakatani, K.; Carriat, J. Y.; Journaux, Y.; Kahn, O.; Lloret, F.; Renard,
J. P.; Pei, Y.; Sletten, J.; Verdaguer, M. J. Am. Chem. Soc. 1989,
111, 5739.

6376 Inorganic Chemistry, Vol. 36, No. 27, 1997
Larionova et al.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
CoCu(obbz)(H2O)4‚2H2O (I)
CoCu(obze)(H2O)4 ‚2H2O (II)
Bond Lengths
Bond Lengths
Cu-O(3)
Cu-O(4)
Cu-N(1)
Cu-N(2)
Co-O(10)
Co-O(1)
Co-O(2)
Co-O(7)
Co-O(9)
Co-O(8)
O(1)-C(1)
1.905(2)
1.909(2)
1.934(3)
1.935(3)
2.053(3)
2.065(2)
2.065(2)
2.107(3)
2.118(3)
2.121(4)
1.250(4)
O(2)-C(2)
O(3)-C(5)
O(4)-C(12)
O(5)-C(5)
O(6)-C(12)
N(1)-C(1)
N(1)-C(3)
N(2)-C(2)
N(2)-C(10)
C(1)-C(2)
1.250(4)
1.283(4)
1.266(4)
1.250(4)
1.259(4)
1.318(4)
1.416(4)
1.325(4)
1.423(4)
1.543(5)
Cu-O(3)
Cu-O(4)
Cu-N(1)
Cu-N(2)
Co-O(1)
Co-O(2)
Co-O(7)
Co-O(8)
Co-O(9)
Co-O(10)
O(1)-C(1)
1.976(6)
O(2)-C(2)
O(3)-C(4)
O(4)-C(7)
O(5)-C(4)
O(6)-C(7)
N(1)-C(1)
N(1)-C(3)
N(2)-C(2)
N(2)-C(5)
C(1)-C(2)
1.24(1)
1.28(1)
1.31(1)
1.25(1)
1.25(1)
1.27(1)
1.45(1)
1.36(1)
1.42(1)
1.54(1)
1.869(6)
1.893(7)
1.930(7)
2.106(6)
2.060(6)
2.130(8)
2.103(8)
2.084(7)
2.103(6)
1.25(1)
Bond Angles
Bond Angles
O(3)-Cu-O(4)
O(3)-Cu-N(1)
O(4)-Cu-N(1)
O(3)-Cu-N(2)
O(4)-Cu-N(2)
N(1)-Cu-N(2)
O(10)-Co-O(1)
O(10)-Co-O(2)
O(1)-Co-O(2)
O(10)-Co-O(7)
O(1)-Co-O(7)
85.21(10)
93.61(10)
176.82(11)
173.70(12)
93.09(11)
88.40(11)
93.63(12)
170.51(13)
78.09(9)
O(2)-Co-O(7)
O(10)-Co-O(9)
O(1)-Co-O(9)
O(2)-Co-O(9)
O(7)-Co-O(9)
O(10)-Co-O(8)
O(1)-Co-O(8)
O(2)-Co-O(8)
O(7)-Co-O(8)
O(9)-Co-O(8)
90.39(13)
89.39(13)
175.97(12)
99.11(11)
88.79(14)
89.3(2)
91.68(12)
95.5(2)
172.48(14)
85.69(13)
O(3)-Cu-O(4)
O(3)-Cu-N(1)
O(3)-Cu-N(2)
O(4)-Cu-N(1)
O(4)-Cu-N(2)
N(1)-Cu-N(2)
O(1)-Co-O(2)
O(1)-Co-O(7)
O(1)-Co-O(8)
O(1)-Co-O(9)
O(1)-Co-O(10)
95.1(3)
82.9(3)
167.5(3)
177.2(4)
97.2(3)
84.8(3)
79.4(2)
88.6(3)
93.9(3)
168.2(3)
99.4(3)
O(2)-Co-O(7)
O(2)-Co-O(8)
O(2)-Co-O(9)
O(2)-Co-O(10)
O(7)-Co-O(8)
O(7)-Co-O(9)
O(7)-Co-O(10)
O(8)-Co-O(9)
O(8)-Co-O(10)
O(9)-Co-O(10)
95.2(3)
88.4(3)
90.0(3)
176.0(4)
176.0(3)
87.4(3)
88.6(3)
90.8(3)
87.8(3)
91.6(3)
85.6(2)
94.10(12)
Absorption correction for I was done on the basis of ψ-scan measure-
ments using seven reflections,37 and that for compound II, by the
Gaussian integration method.
Structure I was solved by direct methods; for compound II, the
coordinates of the isomorphous MnCu compound were used as a starting
point.36 Full-matrix least-squares refinements were carried out, for I
based on F2 and for II based on F. Non-hydrogen atoms were
anisotropically refined. Hydrogen atoms bonded to carbon were
included at idealized calculated positions. Hydrogen atoms bonded to
oxygen atoms were located in difference Fourier maps and were
isotropically refined. In the case of II, hydrogen atoms belonging to
the noncoordinated water molecules as well as to two of the coordinated
water molecules could not be unambiguously located. An extinction
parameter was included in the final refinement cycles. In the case of
I, the refinement, based on F2 and including all reflections, converged
at R1 ) 0.060, wR2 ) 0.081. The conventional R1 factor including
the 2544 reflections with I > 2σ(I) is 0.034. For II, the refinement,
based on F and including 1757 reflections with I > 2σ(I), converged
at R1 ) 0.059, Rw ) 0.058.
For structure I, the data reduction was done with the XCAD
program38 and all other calculations were performed with the SHELXS-
86, SHELXL-93, and XPL programs.39 For structure II, all calculations
were carried out using the Enraf-Nonius Structure Determination
Programs.40 Selected bond lengths are listed in Table 2, and selected
bond angles, in Table 3.
Thermogravimetric Measurements. These were carried out with
a Setaram TAGDSC apparatus, in the range 20-250 °C under a
nitrogen atmosphere.
Figure 1. The molecular unit CoCu(obbz)(H2O)4 in compound I.
Thermal ellipsoids are plotted at the 70% probability level.
were recorded from polycrystalline samples dispersed in Nujol or
Fluorolube oil on a Digilab (Bio-Rad, DA3) interferometer with a
resolution of 2 cm-1, using CaF2 or CsI flat optical windows. The
Raman spectra (2000-50 cm-1) were recorded in the backscattering
geometry, using an optical multichannel instrument (OMARS 89 from
DILOR) in conjunction with a confocal microscope (100× objective)
and a liquid-nitrogen-cooled CCD detector (EG and G, PARC model
1530C). The power of the incident laser radiation (514.5 nm) was
minimized (5-10 mW) in order to avoid any thermal or photochemical
effect, and the laser beam was focused on polycrystalline grains through
the wall of sealed Pyrex tubes. The wavenumber measurements ((1
cm-1) were calibrated using plasma lines.
Magnetic Measurements. These were performed with a Quantum
Design MPMS-5S SQUID magnetometer, working in both the dc and
the ac modes, in the temperature range 2-300 K, and in the 0-50
kOe field range. The temperature dependences of the ac magnetic
susceptibility were recorded at a field alternation frequency of 125 Hz
and a drive amplitude of 1 Oe. The chemical compositions of all
compounds under magnetic investigation were checked.
Infrared and Raman Spectra. All samples were handled in a dry-
Description of the Structures
air glovebag atmosphere. The mull infrared spectra (4000-400 cm-1
)
CoCu(obbz)(H2O)4‚2H2O (I). The structure is isomorphous
with that of NiCu(obbz)(H2O)4‚2H2O already described.35 It
consists of neutral CoCu(obbz)(H2O)4 units (see Figure 1) and
noncoordinated water molecules. The Co atom has a somewhat
distorted octahedral environment with two oxygen atoms of the
oxamido group and four water molecules in the coordination
sphere. All Co-O bond lengths are somewhat longer than the
corresponding Ni-O bond lengths in the isomorphous com-
pound. It is interesting to note that Co-O(10) is significantly
(37) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.
Sect. A 1968, 24, 351; Kopfman, G.; Huber, R. Acta Crystallogr.,
Sect. A 1968, 24, 348.
(38) Harms, K. XCAD. Philipps-Universita¨t Marburg: Marburg, Germany.
(39) Sheldrick, G. M. Acta Crystallogr. Sect. A 1990, 46, 467. Sheldrick,
G. M. SHELXL-93; University of Go¨ttingen: Go¨ttingen, Germany,
1993. Sheldrick, G. M. SHELXTL-PLUS, Version 4.21; Siemens
Analytical X-Ray Instruments, Inc.: Madison, WI, 1990.
(40) Frez, B. A. The SDP-User’s Guide (SDPVAX V.3); Enraf-Nonius:
Delft, The Netherlands, 1985.

Magnetic Properties of Molecular Magnetic Sponges
Inorganic Chemistry, Vol. 36, No. 27, 1997 6377
Figure 3. The molecular unit CoCu(obze)(H2O)4 in compound II.
Thermal ellipsoids are plotted at the 70% probability level.
Figure 2. Hydrogen bonding involving the coordinated water mol-
ecules in compound I. The reference molecule is in position x, y, z.
3
Symmetry operations: a ) -1/2 + x, /2 - y, -1/2 + z; b ) 1 - x, 1
- y, 1 - z; c ) 1 - x, 2 - y, 1 - z.
shorter than the other Co-O(water) bonds; this feature was also
evident in the Ni compound. The Cu environment may be
described as square planar with a slight tetrahedral distortion,
the axial positions being screened by carbon atoms in neighbor-
ing molecules related by inversion centers with Cu---C(7)(1 -
x, 2 - y, 1 - z) ) 3.416(4) Å and Cu---C(15)(1 - x, 1 - y, 1
- z) ) 3.236(4) Å. The dihedral angles between the bridging
oxamido group and the Cu and Co equatorial planes are 7.89
and 8.55°, respectively.
Figure 4. Overlay of temperature and percentage weight loss versus
time, obtained by a thermogravimetric study of CoCu(obbz)(H2O)4‚-
2H2O (I).
The Co---Cu distance within the binuclear unit is 5.295(1)
Å. The shortest intermolecular distance between metal atoms
occurs between molecules related by the n-glide, Co---Cu(-1/2
3
1
+ x, /2 - y, - /2 + z) being 5.109(1) Å. Across inversion
centers there are Cu---Cu distances of 6.203(1) Å [Cu---Cu(1
- x, 1 - y, 1 - z)] and 6.695(1) Å [Cu---Cu(1 - x, 2 - y, 1
- z)], and between molecules related by screw axes Co---Co-
The Co---Cu distance within the binuclear unit is 5.319(2)
Å. The shortest Cu---Cu intermolecular distances occur between
molecules related by inversion centers, Cu---Cu(1 - x, 1 - y,
2 - z) ) 4.612(2) Å alternating with Cu---Cu(1 - x, - y, 2 -
z) ) 5.931(2) Å along the b axis. The shortest Co---Cu
intermolecular distance is found between molecules related by
1
1
(1/2 - x, /2 + y, /2 - z) is 6.417(1) Å.
The molecules are connected through an extensive network
of hydrogen bonds (see Figure 2). In particular, one may notice
that molecules related by the n-glide (shortest intermolecular
Co---Cu) are laced into chains through three hydrogen bonds
involving coordinated water in one molecule and carboxylate
groups of the next molecule: O(7)-H(72)---O(5)a, O(9)-
H(92)---O(3)a, and O(10)-H(102)---O(4)a (where a signifies
1
1
the n-glide, Cu---Co(1/2 + x, /2 - y, /2 + z) ) 5.838 Å.
Furthermore, pairs of centrosymmetrically related Co atoms are
separated by 5.448(2) Å [Co---Co(1 - x, -y, 1 - z)], the two
coordination spheres being linked through hydrogen bonds
between O(7) and O(10). The O(6) atom is hydrogen bonded
to the oxygens of the axially coordinated water molecules O(7)
and O(8) in molecular units one unit cell apart along the b axis.
The Co---O(6)(1 - x, -y, 2 - z) and Co---O(6)(1 - x, 1 - y,
2 - z) distances are only 3.951 and 3.987 Å.
3
1
symmetry operation -1/2 + x, /2 - y, - /2 + z). Axially
coordinated water molecules also form hydrogen bonds to
carboxylate groups in molecules related by centers of sym-
metry: O(7)-H(71)---O(6)b and O(8)-H(82)---O(5)c (where
b and c denote symmetry operations 1 - x, 1 - y, 1 - z and
1 - x, 2 - y, 1 - z, respectively).
For both compounds I and II, the X-ray powder patterns are
strongly modified upon dehydration, but the compounds remain
crystalline. Upon rehydration, the patterns return to those of
the starting compounds, whose crystal structures are described
above.
CoCu(obze)(H2O)4‚2H2O (II). The structure is isomorphous
with that of MnCu(obze)(H2O)4‚2H2O.36 It consists of CoCu-
(obze)(H2O)4 units and noncoordinated water molecules (see
Figure 3). The Co environment is very similar to that found
for CoCu(obbz)(H2O)4. The Cu atom is coordinated to two
oxamido nitrogen atoms and two carboxylato oxygen atoms in
a slightly distorted square planar arrangement. The axial
positions of Cu are screened by the proximity of the phenyl
rings of centrosymmetrically related molecules with Cu---C(6)-
(1 - x, 1 - y, 2 - z) and Cu---C(10)(1 - x, 1 - y, 2 - z)
equal to 3.31 Å. The dihedral angles between the bridging
oxamido group and the Cu and Co equatorial planes are 6.9
and 6.0°, respectively.
Thermogravimetric Analyses
CoCu(obbz)(H2O)4‚2H2O (I). Figure 4 shows the overlay
of time versus temperature and time versus percentage change
in weight for a sample of CoCu(obbz)(H2O)4‚2H2O. Initially,
the temperature was increased at the rate of 30 °C/h, giving a
region of rapid weight loss between 21 °C (point T1) and 96
°C (point T2). The percentage weight loss at T2 corresponds
to three molecules of water per CoCu unit. As the temperature
was increased further, a second inflection point (T3) was

6378 Inorganic Chemistry, Vol. 36, No. 27, 1997
Larionova et al.
Figure 5. øMT versus T plot for CoCu(obbz)(H2O)4‚2H2O (Ia).
Figure 6. øMT versus T plot for CoCu(obbz)(H2O)3 (Ib). The insert
emphasizes the minimum in the øMT versus T plot.
observed around 200 °C, corresponding to a total loss of five
molecules of water. Increasing again the temperature up to ca.
275 °C resulted in a very progressive loss of half a water
molecule, before the compound decomposed irreversibly. If
the heating was stopped at T3, the sample began to reabsorb
water slowly. The degree of hydration after 24 h depended on
the humidity of atmosphere. If the sample was kept at room
temperature in the close vicinity of a beaker of water, the
rehydration was complete, and we obtained a material whose
chemical composition, structure, and physical properties (vide
infra) were strictly identical to those of the starting material.
The dehydration-rehydration process could be repeated as many
times as desired without any detectable fatigability of the
material, provided that the temperature never exceeded 275 °C.
CoCu(obze)(H2O)4‚2H2O (II). The thermogravimetric be-
havior of CoCu(obze)(H2O)4‚2H2O is rather similar to that of
the obbz derivative. The most noticeable differences are the
following: (i) the T2 and T3 temperatures are found at 120 and
190 °C, respectively; (ii) the rehydration process is much faster,
so that the magnetic studies of the dehydrated compounds were
performed with sample holders sealed under vacuum after
heating.
Figure 7. øMT versus T plot for CoCu(obbz)(H2O) (Ic). The insert
emphasizes the minimum in the øMT versus T plot.
Cu2+ Kramers doublet gives rise to a singlet nonmagnetic
ground state and a zero-field-split pseudotriplet magnetic excited
state for the pair. One can also say that the ground Kramers
doublet of Co2+ may be characterized by an S′Co ) 1/2 effective
local spin.6 The antiferromagnetic interaction between S′Co and
Magnetic Properties
1
SCu ) /2 leads to an S ) 0 ground pair state.
The CoCu(obbz) System. The magnetic studies were carried
out on four compounds, namely the starting material (Ia), the
material heated to 96 °C (Ib), the material heated to 205 °C
(Ic), and finally the material fully rehydrated after thermal
treatment (Id). Let us present successively the magnetic
properties of each of these materials.
(b) CoCu(obbz)(H2O)3 (Ib). The øMT versus T curve for
compound Ib, shown in Figure 6, is already deeply modified
with regard to that of compound Ia; øMT is equal to 2.70 emu
K mol-1 at 300 K, decreases very smoothly as T decreases,
reaches a minimum value at 74 K, with øMT ) 2.12 emu K
mol-1, then increases as T is lowered further, and reaches a
value of 12 emu K mol-1 at 2 K. The minimum in the øMT
versus T curve is characteristic of ferrimagnetic behavior.6
Through extended Co2+-Cu2+ antiferromagnetic interactions
the Co2+ magnetic moments tend to align along the field
direction and the Cu2+ magnetic moments along the opposite
direction. The compound, however, shows no long-range
magnetic ordering down to 2 K; the out-of-phase ac magnetic
susceptibility is zero in the whole 2-300 K temperature range.
(c) CoCu(obbz)(H2O) (Ic). The magnetic properties for this
compound reveal not only a ferrimagnetic behavior but also a
long-range magnetic ordering at Tc ) 25 K. The øMT versus T
curve is shown in Figure 7. øMT is equal to 2.60 emu K mol-1
at room temperature, decreases as T is lowered, presents a
minimum at 130 K, with øMT ) 2.22 emu K mol-1, and then
increases very abruptly as T is lowered further. A very high
maximum of øMT is observed around 25 K, with øMT ) 300
emu K mol-1 (with an applied field of 102 Oe). The decrease
of øMT below 25 K is due to saturation effects.
(a) CoCu(obbz)(H2O)4‚2H2O (Ia). The temperature (T)
dependence of the dc molar magnetic susceptibility (øM) is
shown in Figure 5 in the form of the øMT versus T plot; øMT is
equal to 2.89 emu K mol-1 at room temperature, decreases
continuously as T is lowered, and reaches a value very close to
zero at 2 K. This behavior is quite characteristic of an
antiferromagnetically coupled Co2+Cu2+ pair, with a nonmag-
netic ground state. A very similar curve was obtained and
quantitatively interpreted for CoCu(obp)(H2O)3‚H2O with obp
) N,N′-(2-carboxyethyl)oxamido.41 We will not repeat here
the theoretical treatment and will limit ourselves to recall why
the ground state is nonmagnetic at the first order.6,41,42 The
4T1 ground state of Co2+ in cubic symmetry is split into three
spin-quartet states in the rhombic symmetry observed for the
compound. Each of these spin-quartet states is split further into
two Kramers doublets. The antiferromagnetic interaction
between the Co2+ Kramers doublet of lowest energy and the
(41) Gulbrandsen, A.; Sletten, J.; Nakatani, K.; Pei, Y.; Kahn, O. Inorg.
Chim. Acta 1993, 212, 271.
(42) Kahn, O.; Tola, P.; Coudanne, H. Chem. Phys. 1979, 42, 355.
The onset of a long-range magnetic transition is confirmed
by the temperature dependences of the in-phase, ø′, and out-

Magnetic Properties of Molecular Magnetic Sponges
Inorganic Chemistry, Vol. 36, No. 27, 1997 6379
water molecules and a development of the polymerization
process, yielding a ferrimagnetic material, Ic, which exhibits a
long-range magnetic ordering at Tc ) 25 K, and a rather strong
coercivity below Tc. What is quite remarkable is that the
dehydration process is perfectly reversible. In a humid atmo-
sphere, the molecule-based magnet Ic slowly reabsorbs water
and transforms back into compound Ia with a nonmagnetic
ground state.
Let us now try to go further, and to figure out the transforma-
tions leading to compounds Ib and Ic.
The first three water molecules which are removed are
certainly the two noncoordinated water molecules along with
one of the molecules linked to the cobalt atom, so that the
formula of the compound can be written as CoCu(obbz)(H2O)3.
A very similar compound of formula CoCu(obp)(H2O)3, with
obp ) N,N′-bis(2-carboxyethyl)oxamido, does exist.40 This
compound has a chain structure. The cobalt atom is surrounded
by six oxygen atoms, two of them coming from the oxamido
bridge, three from water molecules, and the sixth one from the
carboxylato group of a neighboring unit. In CoCu(obp)(H2O)3,
the Co2+-Cu2+ interaction through the carboxylato bridge was
found to be too weak to give rise to the ferrimagnetic regime
with a minimum in the øMT versus T curve. However, it has
been shown that the magnitude of this interaction may dramati-
cally depend on the configuration around this carboxylato
bridge.43-45
Figure 8. Temperature dependence of the in-phase (b) and out-of-
phase (O) ac molar magnetic susceptibilities for CoCu(obbz)(H2O) (Ic).
The situation is more complicated as far as compound Ic is
concerned. This compound has only one water molecule in the
cobalt coordination sphere. This could suggest that the Co2+
ion is in a tetrahedral environment. Such a hypothesis, however,
may be ruled out. Indeed, compound Ic is a rather hard magnet.
Therefore, the Co2+ ion in Ic is a magnetically anisotropic spin
carrier. This would not be the case if this ion was in a
tetrahedral environment with a 4A2 ground state, without first-
order orbital momentum. We must admit that the Co2+ ion in
Ic is in an octahedral environment, as it was in Ia and Ib. This
is possible if the two water molecules which are removed when
passing from Ib to Ic are replaced by two carboxylato oxygen
atoms belonging to neighboring chains, resulting in a two- or a
three-dimensional network. Let us note here that the electronic
absorption spectrum usually allows one to distinguish between
octahedral and tetrahedral Co2+ chromophores. In the present
case, the spectrum in the visible range is dominated by the d-d
bands of the Cu2+ chromophore and the bands arising from Co2+
cannot be assigned.
Let us now examine whether the infrared and Raman
spectrocopy data are in line with the propositions above. We
first look at the infrared data in the 1300-1800 cm-1 range,
corresponding to the νCOO vibrations.46 The spectra of Ia, Ib,
and Ic are compared in Figure 10. The spectrum of Ia shows
several intense and broad bands in the 1550-1620 cm-1 range
which can be assigned to the antisymmetric νCOO vibrations of
the monodentate carboxylato groups. For Ib, the relative
intensities of these bands decrease and new bands appear in
the 1600-1620 cm-1 range. Those probably correspond to
antisymmetric νCOO vibrations of bridging carboxylato groups
(Cu-O-C-O-Co). Another difference between the spectra
of Ia and Ib is the appearance of new bands in the 1660-1690
Figure 9. Magnetic hysteresis loop at 5 K for CoCu(obbz)(H2O) (Ic).
of-phse, ø′′, ac molar magnetic susceptibilities displayed in
Figure 8; both ø′ and ø′′ present a peak, at 27 and 25 K,
respectively. The temperature of the maximum of ø′′ does not
depend on the frequency between 1 and 103 Hz. This
compound, Ic, may be considered as a rather hard magnet. As
a matter of fact, the field dependence of the magnetization at 5
K reveals a well-shaped hysteresis loop, with a coercive field
of 3.0 × 103 Oe (see Figure 9).
(d) CoCu(obbz)(H2O)4‚2H2O (Id). This compound, ob-
tained after thermal treatment up to 205 °C and complete
rehydration, behaves magnetically exactly as the starting
compound Ia, with øMT tending to zero as T approaches absolute
zero.
CoCu(obze) System. Very similar results were obtained
concerning the magnetic properties of the CoCu(obze) com-
pounds. CuCo(obze)(H2O)4‚2H2O possesses a nonmagnetic
ground state; CuCo(obze)(H2O)3 behaves as a ferrimagnet
without long-range ordering down to 2 K; CuCo(obze)(H2O)
also behaves as a ferrimagnet but exhibits a long-range magnetic
ordering at Tc ) 20 K.
Discussion
In this section, we will focus on the CoCu(obbz) system. Let
us start the discussion by summing up what is already clear
from the results described above. The reaction of [Cu(obbz)]2-
with Co2+ yields the binuclear species of formula CoCu(obbz)-
(H2O)4‚2H2O with four water molecules in the cobalt coordina-
tion sphere and two additional noncoordinated water molecules.
The ground state of the Co2+Cu2+ pair is nonmagnetic at the
first order. When this compound, Ia, is heated, three water
molecules are removed; this removal is accompanied by a sort
of polymerization process yielding a low-dimensional com-
pound, Ib, with a ferrimagnetic behavior. Heating this com-
pound further up to 200 °C results in the removal of two more
(43) Pei, Y.; Kahn, O.; Sletten, J.; Renard, J. P.; Georges, R.; Gianduzzo,
J. C.; Curely, J.; Xu, Q. Inorg. Chem. 1988, 27, 47.
(44) Nakatani, K.; Sletten, J.; Halut-Desporte, S.; Jeannin, S.; Jeannin, Y.
Inorg. Chem. 1991, 30, 164.
(45) Kahn, O.; Pei, Y.; Nakatani, K.; Journaux, Y. New. J. Chim. 1992,
16, 269.
(46) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 4th ed.; Wiley: New York, 1986.

6380 Inorganic Chemistry, Vol. 36, No. 27, 1997
Larionova et al.
Figure 11. Comparison of the Raman spectra of compounds Ia, Ib,
and Ic in the 1100-1700 cm-1 wavenumber range.
Figure 10. Comparison of the infrared spectra of compounds Ia, Ib,
and Ic in the 1300-1800 cm-1 wavenumber range.
would create Co-O bonds involving Co2+ ions of neighboring
chains. The presence of infrared bands around 1680 cm-1
suggests that the coordination would be as
cm-1 range. Their higher energy suggests that they arise from
very asymmetrical carboxylato groups, with the same oxygen
atom linked to both Co and Cu atoms. The relative intensities
of these two groups of bands increase further upon passing from
Ib to Ic, which might be related to the formation of two Co-O
bonds involving both bridging and strongly asymmetrical
carboxylato groups at the expense of two Co-H2O bonds.
The Raman spectra in the same wavenumber range are
compared in Figure 11. The spectrum of Ia is dominated by
the signal at 1415 cm-1 assignable to the symmetric νCOO
vibrations of monodentate carboxylato groups. Upon dehydra-
tion, this band is shifted toward higher energy and appears at
1428 cm-1 for Ic. This wavenumber agrees with what is
expected for the symmetric νCOO vibration of bridging carboxy-
lato groups. One can also notice that a weak signal around
1415 cm-1 is still present in the spectrum, as a low-energy
shoulder of the signal at 1428 cm-1. Let us finally mention
that the perfect reversibility of the dehydration-rehydration
process was checked again from the Raman spectra by accom-
modating a tiny air leak in the sealed tube containing Ic. The
shift of the νCOO mode from 1428 down to 1415 cm-1 was
observed, along with the variation of the half-bandwidth from
If it was so, the Co2+ ion in Ic would be surrounded by six
oxygen atoms, one arising from a water molecule, two from
the oxamido group, one from a carboxylato group with an atom
of the type O(5) or O(6) (see Figure 1), and two from
carboxylato groups with atoms of the type O(3) and O(4),
schematized below as follows:
10 to ca. 22 cm-1
.
To conclude this section, let us summarize what precedes.
It is quite reasonable to suggest that compound Ib has a chain
structure reminiscent of that found for CoCu(obp)(H2O)3 and
behaves as a one-dimensional ferrimagnet without long-range
ordering. In Ic, the chains would not be isolated from each
other anymore but would associate to afford a network of higher
dimensionality. The monodentate carboxylato groups of a chain
Such a description, however, does not explain why the bands
around 1680 cm-1 are already visible in the infrared spectrum
of Ib. Perhaps, at 96 °C, compound Ic begins to form, or more
likely the structure of Ib is more disordered than suggested

Magnetic Properties of Molecular Magnetic Sponges
Inorganic Chemistry, Vol. 36, No. 27, 1997 6381
above, with some Co2+ ions possessing oxygen atoms of the
type O(3) or O(4) in their coordination spheres.
At this stage, we wish to mention that this hydration-
dehydration process accompanied by a paramagnetic-ferro-
magnetic transformation has already been observed, in particular
for the system (RNH3)2CrX4-(RNH3)2[CrX4(H2O)2].47
is due to the softness of these crystal lattices. The Co-O bonds
can be broken and created without destroying the essence of
the molecular architecture. Dehydration increases the magnetic
dimensionality from zero to 2 or 3; rehydration decreases this
dimensionality.
Sometimes, we are asked to describe the properties of
molecular magnetic materials as compared to classical solid-
state magnetic materials. The phenomena reported in this paper
certainly represent striking properties of molecule-based matter.
Conclusion
The two compounds, I and II, investigated in this paper may
be considered as molecular magnetic sponges. They can
reversibly release and absorb five water molecules, a process
which is accompanied by a dramatic change of magnetic
properties. When the sponges are hydrated, or wet, they exhibit
a nonmagnetic ground state; øMT tends to zero as T approaches
absolute zero. When the sponges are dehydrated, or dry, they
exhibit a spontaneous magnetization below critical temperatures
of 25 and 20 K, respectively. The reversibility of the process
seems to us to be the most remarkable finding of this work. It
Acknowledgment. This work was carried out under Project
No. 1308-G funded by the Indo-French Center for the Promotion
of Advanced Research (IFCPAR). The authors thank R.
Cavagnat for his technical assistance in Raman measurements.
Supporting Information Available: Tables of X-ray experimental
details, atomic coordinates, anisotropic thermal parameters for the non-
hydrogen atoms, bond lengths and angles, and hydrogen bonds for
compounds I and II (18 pages). Ordering information is given on any
current masthead page.
(47) We thank the reviewer who brought this interesting system to our
attention.
IC970719G