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Nitrogen–oxygen bond formation during oxidative reactions of copper(II)benzoate complexes having 3,5-dimethylpyrazole
Inorganic Chemistry Communications 9 (2006) 931–934 www.elsevier.com/locate/inoche
Nitrogen–oxygen bond formation during oxidative reactions of copper(II)benzoate complexes having 3,5-dimethylpyrazole Kaustavmoni Deka, Rupam J. Sarma, Jubaraj B. Baruah
*
Department of Chemistry, Indian Institute of Technology, Guwahati, Assam 781 039, India Received 20 May 2006; accepted 30 May 2006 Available online 10 June 2006
Abstract Dimeric and polymeric 3,5-dimethylpyrazole containing copper(II)benzoate complexes are prepared and structurally characterized. The complexes on oxidation led to formation of N–O–O by oxidation of N–H bond. 2006 Elsevier B.V. All rights reserved. Keywords: Copper(II)benzoate; 3,5-dimethylpyrazole; Co-ordination polymer; Oxidative reactions; Radical
Metal carboxylate complexes are important for designing advanced materials [1]. Among them copper(II) carboxylates are of special interest as they are easily obtained as polynuclear units having relevance to magnetic materials [2] and biology [3]. Due to different co-ordination modes of carboxylates [1b,4], it is essential to have control on the binding of carboxylate to a metal ion in specific manner in the presence of other ligand/s. Achieving such control will enable one to synthesise complexes having new structures and also would help in understanding reactive intermediates. It may be noted that in biological environment the metal complexes are formed in ambient condition from multiple-components. In addition to these many of the copper-enzymes binds with oxygen to cause oxidation reactions [3]. In this study we have prepared some structurally important copper(II) complexes from three reactants system of copper(II)acetate monohydrate, 3,5-dimethylpyrazole and benzoic acid and also isolated and characterized peroxo radical containing copper(II) complex from one pot reactions from reactions of four reactants. Mixing copper(II)acetate monohydrate with 3,5-dimethylpyrazole and benzoic acid selectively gives complex 1 (Scheme 1). The structure of complex 1 is already reported *
Corresponding author. Tel.: +91 361 2582301; fax: +91 361 2690762. E-mail address: [email protected] (J.B. Baruah).
1387-7003/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2006.05.035
[5]. This copper complex on treatment with sulphamic acid results in two interesting copper complexes 2 and 3 as illustrated in Scheme 1. Complex 2 has a regular paddle-wheel type of structure, in which the axial positions are occupied by two 3,5-dimethylpyrazole. In the crystal lattice the molecules of the complex are anchored to each other by weak C–H p interactions along a-crystallographic axis. The C– H p interaction occurs between the C–H of a methyl group of the 3,5-pyrazole and the aromatic ring of a benzoate group. The packing of the molecule along a-crystallographic axis is shown in Fig. 1. The Cu Cu distance in ˚ that correlates with the similar dimeric the dimer is 2.690 A copper complexes reported elsewhere [1b,4]. The compound 3 has a polymeric structure in which there is repeated benzoato bridged dimers held by intervening monomeric six co-ordinated copper(II)benzoate complexes having two 3,5-dimethylpyrazole ligands (Fig. 2). One of the oxygen atoms co-ordinating to the monomeric copper unit, bridges the dimer by occupying the axial coordination sites of adjacent dimeric copper unit. The dimeric units having paddle-wheel type of geometry has ˚ which is slightly higher than Cu Cu separation 2.667 A the Cu Cu distance observed for the dimeric cop˚ ). The Cu Cu disper(II)actetate dihydrate [6] (2.614 A tance between the copper atoms of the dimer and a ˚ . The monomeric units of the polymers monomer is 4.430 A
932
K. Deka et al. / Inorganic Chemistry Communications 9 (2006) 931–934 H2O2
Cu2 (OAc)4 2H2O in methanol
Cu2 (μ-OBz)4 (pz-O)2
pz + BzOH pz + BzOH
4
cis -Cu (pz)2 (OBz)2. trans -Cu(pz)2 (OBz)2 H2O 1
Sulphamic acid/ methanol
Cu2 (μ-OBz)4 (pz)2 2
+
(μ-OBz)4 Cu3 (OBz)2 (pz)2 3
where pz = 3,5-dimethylpyrazole, OAc = acetate anion, pz-O = OBz = benzoate anion
O O N N
Scheme 1.
have cis-disposition among the 3,5-dimethylpyrazole and the benzoate groups are also disposed to each other in cis-fashion. There is also N–H O interactions between the oxygen atoms of the benzoate groups with the N–H ˚ and ÆN– bonds of the 3,5-dimethylpyrazole (dd–a2.83 A H O 169.40). We have prepared a nitro-peroxide radical complex 4 from the reaction of copper(II)acetate monohydrate, benzoic acid, 3,5-dimethylpyrazole and hydrogen peroxide (Scheme 1). The N–O–O radical is formed in situ via oxidation of N–H bond of the 3,5-dimethylpyrazole. Compound 4 has interesting polymeric chain like structure [8] as shown in Fig. 3. The packing pattern of the compound revealed that the each N–O–O units from two independent units are close enough to interact among themselves. The separation distances are shown in Fig. 4. The geometry of four oxygen
Fig. 2. The crystal structure of 3 showing the alternate monomer and dimer in the polymeric structure.
atoms is such that it forms a rectangle with two sides hav˚ and 1.40 A ˚ along with diagonal distance of ing 1.35 A ˚ 1.48 A. These distances suggest that the molecules may have overlap either in diagonal manner or sidewise. The ˚, O–O bond distance in peroxide is approximately 1.48 A
Fig. 1. (a) Crystal structure of 2 (b) Packing of the complex 2 along a-crystallographic axis. (inset is the type of C–H p interactions responsible for stabilising the packing).
K. Deka et al. / Inorganic Chemistry Communications 9 (2006) 931–934
933
Fig. 3. Top: The crystal structure of 4; Bottom: The crystal packing arrangement along crystallographic a-axis.
Fig. 4. The arrangement of the -O–O in the lattice of 4.
and this shows that along the diagonal there can be peroxide type of linkage. However, the sidewise distances are slightly shorter than the peroxide type bond whereas it is ˚ ) species. This shows higher than the superoxide (1.28 A that although the interactions along the sides are there, the four oxygen atoms may be interacting each other in a co-operative manner through side wise and diagonal manner. Copper(II)nitroxide complexes are of special interest for their magnetic properties [2a,7]. It is already reported that monomeric [7a] and dimeric [2a] copper complexes derived from ligands having nitroxide radicals show ferromagnetic coupling. The reported literature on copper benzoate complexes having ligand possessing nitroxide radicals showed interaction of radical with the copper centers [2a]. But our system is unique as it posses dimeric copper units besides the N-peroxide radicals that are self-interacting. Complex 4 is of interest as it has a stable N–O–O radical that are coupled to each other in pairs and also shows interesting magnetic properties. At room temperature in the solid state the complex shows single esr signal from the copper(II) at 2.73 G and another single esr signal for the N–O–O radical at 1.88 G both with equal intensities. As the temperature is lowered the intensity of the signals
changes and the relative intensity signal at 2.73 G is enhanced with respect to the other signal. This suggests that the signal originating from the N–O–O disappears at lower temperature due to obvious reason of anti-ferromagnetic coupling arising from the type of assembly formation as depicted in Fig. 4. To complement this observation we have measured the variable temperature magnetic susceptibility of the complex and found that as the temperature is lowered the susceptibility increases till 60 K and below which there is a fall in the susceptibility (Fig. 5). Plot of log v vs 1/T shows ferromagnetic and anti-ferromagnetic regime for the complex. Although we do not have an adequate quantitative explanation on this issue but this observation makes the system unconventional. To add further support to the N–O–O formation we have obtained a trinuclear copper complex from one pot reaction between p-chloro benzoic acid, copper(II)acetate, 3,5-dimethylpyrazole and hydrogen peroxide. The crystal structure of the trimeric copper(II) complex 5 is shown in
Fig. 5. Normalized magnetic susceptibility of 4 vs temperature.
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K. Deka et al. / Inorganic Chemistry Communications 9 (2006) 931–934
Fig. 6. Crystal structure of 5.
Fig. 6. From the structure it is clear that a trimeric structure is formed by two bridging methanol and two benzoato group along with two oxo-bridges from the N–O–O unit. The compound has two terminal bidentate benzoate groups and has distorted octahedral geometry around all the three copper centers. Thus, we have synthesized and structurally characterized a few copper(II) complexes that are of structural interest and proper understanding of magnetic properties of these compounds will add a new dimension. We also demonstrated a one pot reaction for selective synthesis of complexes which other wise is difficult.
[2]
Acknowledgements The authors thank the department of Science and Technology, India for financial support and Dr. S. Ravi for help in the low temperature magnetic measurements. Appendix A. Supplementary data The CCDC deposit numbers for the CIF of the complexes 2, 3, 4 and 5 are 603939, 603940, 603941 and 606934. The experimental details for the synthesis of the complexes along with the table for crystallographic parameters are available as supplementary materials. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2006.05.035. References [1] (a) M. Eddaoudi, D. Moler, H. Li, B. Chen, T.M. Reineke, M. O’Keeffe, O.M. Yaghi, Acc. Chem. Res. 34 (2001) 319; (b) M. Casarin, C. Corvaja, C.D. Nicola, D. Falcomer, L. Franco, M. Monari, L. Pandolfo, C. Pettinari, F. Piccinelli, Inorg. Chem. 44 (2005) 6265; (c) M. Eddaoudi, H. Li, O.M. Yaghi, J. Am. Chem. Soc. 127 (2005) 7110;
[3]
[4]
[5] [6] [7]
[8]
(d) A.C. Sudik, A.R. Millward, N.W. Ockwig, A.P. Cote, J. Kim, O.M. Yaghi, J. Am. Chem. Soc. 122 (2000) 1391; (e) E. Tynan, P. Jensen, A.C. Lees, B. Moubaraki, K.S. Murray, P.E. Kruger, Crys. Eng. Comm. 7 (2005) 90; (f) Z. Wang, V.C. Kravtsov, M.J. Zaworotko, Angew Chem. Int. Ed. Eng. 44 (2005) 2877; (g) X-J. Luan, Y-Y. Wang, D-S. Li, P. Liu, H-H. Hu, Q-Z. Shi, S-M. Peng, Angew. Chem. Int. Ed. Eng. 44 (2005) 3864; (h) G. Cristou, Acc. Chem. Res. 22 (1989) 328. (a) R.E. Del Sesto, A.M. Arif, J.S. Miller, Inorg. Chem. 39 (2000) 4894; (b) O. Kahn, Chem. Phys. Lett. 265 (1997) 109; (c) M. Kato, Y. Muto, Coord. Chem. Rev. 92 (1988) 45; (d) S. Ferrer, F. Lioret, I. Beertmenu, G. Alzuet, J. Borras, S. GarciaGranda, M. Liu-Gonzales, J.G. Hassnoot, Inorg. Chem. 41 (2002) 5821; (e) O. Kahn, Y. Pei, Y. Journaux, in: D.W. Bruce, D. O’Hare (Eds.), Inorganic Materials, second ed., John Wiley & Sons, New York, 1996, pp. 66–119; (f) G. Christou, S.P. Perlepes, E. Libby, K. Folting, J.C. Huffman, R.J. Webb, D.N. Hendrickson, Inorg. Chem. 29 (1990) 3657, and references therein; (g) L. Porter, M. Dickman, R. Doedens, Inorg. Chem. 22 (1983) 1962; (h) L. Porter, R. Doedens, Inorg. Chem. 24 (1985) 1006. (a) R.H. Holm, P. Kennepohl, E.I. Solomon, Chem Rev. 96 (1996) 2239; (b) W. Kaim, J. Rall, Angew. Chem. Int. Ed. Eng. 35 (1996) 43; (c) M.A. Halcrow, Angew. Chem. Int. Ed. Eng. 40 (2001) 1433. M. Casarin, C. Corvaja, C. di Nicola, D. Falcomer, L. Franco, M. Monari, L. Pandolfo, C. Pettinari, F. Piccinelli, P. Taliatesta, Inorg. Chem. 43 (2004) 5865. K. Deka, M. Laskar, J.B. Baruah, Polyhedron in press. G.M. Brown, M. George, R. Chidambaram, Acta Crystal. Sec. 29B (1973) 2393. (a) D. Luneau, P. Rey, J. Laugier, P. Fries, A. Caneschi, D. Gatteschi, R. Sessoli, J. Am. Chem. Soc. 113 (1991) 1245; (b) K. Oshima, K. Okuda, M. Date, J. Phys. Soc. Jpn. 44 (1978) 757; (c) Y. Karaki, R. Masutomi, H. Ishimoto, T. Asano, Y. Ajiro, Physica B: Condensed matter 329-333 (2003) 1002. The oxygen attached to the nitrogen are disordered between two equivalent positions, thus, the structure is solved by assigning 50% occupancy to the O-atom of the N–O unit.
Nitrogen–oxygen bond formation during oxidative reactions of copper(II)benzoate complexes having 3,5-dimethylpyrazole Kaustavmoni Deka, Rupam J. Sarma, Jubaraj B. Baruah
*
Department of Chemistry, Indian Institute of Technology, Guwahati, Assam 781 039, India Received 20 May 2006; accepted 30 May 2006 Available online 10 June 2006
Abstract Dimeric and polymeric 3,5-dimethylpyrazole containing copper(II)benzoate complexes are prepared and structurally characterized. The complexes on oxidation led to formation of N–O–O by oxidation of N–H bond. 2006 Elsevier B.V. All rights reserved. Keywords: Copper(II)benzoate; 3,5-dimethylpyrazole; Co-ordination polymer; Oxidative reactions; Radical
Metal carboxylate complexes are important for designing advanced materials [1]. Among them copper(II) carboxylates are of special interest as they are easily obtained as polynuclear units having relevance to magnetic materials [2] and biology [3]. Due to different co-ordination modes of carboxylates [1b,4], it is essential to have control on the binding of carboxylate to a metal ion in specific manner in the presence of other ligand/s. Achieving such control will enable one to synthesise complexes having new structures and also would help in understanding reactive intermediates. It may be noted that in biological environment the metal complexes are formed in ambient condition from multiple-components. In addition to these many of the copper-enzymes binds with oxygen to cause oxidation reactions [3]. In this study we have prepared some structurally important copper(II) complexes from three reactants system of copper(II)acetate monohydrate, 3,5-dimethylpyrazole and benzoic acid and also isolated and characterized peroxo radical containing copper(II) complex from one pot reactions from reactions of four reactants. Mixing copper(II)acetate monohydrate with 3,5-dimethylpyrazole and benzoic acid selectively gives complex 1 (Scheme 1). The structure of complex 1 is already reported *
Corresponding author. Tel.: +91 361 2582301; fax: +91 361 2690762. E-mail address: [email protected] (J.B. Baruah).
1387-7003/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2006.05.035
[5]. This copper complex on treatment with sulphamic acid results in two interesting copper complexes 2 and 3 as illustrated in Scheme 1. Complex 2 has a regular paddle-wheel type of structure, in which the axial positions are occupied by two 3,5-dimethylpyrazole. In the crystal lattice the molecules of the complex are anchored to each other by weak C–H p interactions along a-crystallographic axis. The C– H p interaction occurs between the C–H of a methyl group of the 3,5-pyrazole and the aromatic ring of a benzoate group. The packing of the molecule along a-crystallographic axis is shown in Fig. 1. The Cu Cu distance in ˚ that correlates with the similar dimeric the dimer is 2.690 A copper complexes reported elsewhere [1b,4]. The compound 3 has a polymeric structure in which there is repeated benzoato bridged dimers held by intervening monomeric six co-ordinated copper(II)benzoate complexes having two 3,5-dimethylpyrazole ligands (Fig. 2). One of the oxygen atoms co-ordinating to the monomeric copper unit, bridges the dimer by occupying the axial coordination sites of adjacent dimeric copper unit. The dimeric units having paddle-wheel type of geometry has ˚ which is slightly higher than Cu Cu separation 2.667 A the Cu Cu distance observed for the dimeric cop˚ ). The Cu Cu disper(II)actetate dihydrate [6] (2.614 A tance between the copper atoms of the dimer and a ˚ . The monomeric units of the polymers monomer is 4.430 A
932
K. Deka et al. / Inorganic Chemistry Communications 9 (2006) 931–934 H2O2
Cu2 (OAc)4 2H2O in methanol
Cu2 (μ-OBz)4 (pz-O)2
pz + BzOH pz + BzOH
4
cis -Cu (pz)2 (OBz)2. trans -Cu(pz)2 (OBz)2 H2O 1
Sulphamic acid/ methanol
Cu2 (μ-OBz)4 (pz)2 2
+
(μ-OBz)4 Cu3 (OBz)2 (pz)2 3
where pz = 3,5-dimethylpyrazole, OAc = acetate anion, pz-O = OBz = benzoate anion
O O N N
Scheme 1.
have cis-disposition among the 3,5-dimethylpyrazole and the benzoate groups are also disposed to each other in cis-fashion. There is also N–H O interactions between the oxygen atoms of the benzoate groups with the N–H ˚ and ÆN– bonds of the 3,5-dimethylpyrazole (dd–a2.83 A H O 169.40). We have prepared a nitro-peroxide radical complex 4 from the reaction of copper(II)acetate monohydrate, benzoic acid, 3,5-dimethylpyrazole and hydrogen peroxide (Scheme 1). The N–O–O radical is formed in situ via oxidation of N–H bond of the 3,5-dimethylpyrazole. Compound 4 has interesting polymeric chain like structure [8] as shown in Fig. 3. The packing pattern of the compound revealed that the each N–O–O units from two independent units are close enough to interact among themselves. The separation distances are shown in Fig. 4. The geometry of four oxygen
Fig. 2. The crystal structure of 3 showing the alternate monomer and dimer in the polymeric structure.
atoms is such that it forms a rectangle with two sides hav˚ and 1.40 A ˚ along with diagonal distance of ing 1.35 A ˚ 1.48 A. These distances suggest that the molecules may have overlap either in diagonal manner or sidewise. The ˚, O–O bond distance in peroxide is approximately 1.48 A
Fig. 1. (a) Crystal structure of 2 (b) Packing of the complex 2 along a-crystallographic axis. (inset is the type of C–H p interactions responsible for stabilising the packing).
K. Deka et al. / Inorganic Chemistry Communications 9 (2006) 931–934
933
Fig. 3. Top: The crystal structure of 4; Bottom: The crystal packing arrangement along crystallographic a-axis.
Fig. 4. The arrangement of the -O–O in the lattice of 4.
and this shows that along the diagonal there can be peroxide type of linkage. However, the sidewise distances are slightly shorter than the peroxide type bond whereas it is ˚ ) species. This shows higher than the superoxide (1.28 A that although the interactions along the sides are there, the four oxygen atoms may be interacting each other in a co-operative manner through side wise and diagonal manner. Copper(II)nitroxide complexes are of special interest for their magnetic properties [2a,7]. It is already reported that monomeric [7a] and dimeric [2a] copper complexes derived from ligands having nitroxide radicals show ferromagnetic coupling. The reported literature on copper benzoate complexes having ligand possessing nitroxide radicals showed interaction of radical with the copper centers [2a]. But our system is unique as it posses dimeric copper units besides the N-peroxide radicals that are self-interacting. Complex 4 is of interest as it has a stable N–O–O radical that are coupled to each other in pairs and also shows interesting magnetic properties. At room temperature in the solid state the complex shows single esr signal from the copper(II) at 2.73 G and another single esr signal for the N–O–O radical at 1.88 G both with equal intensities. As the temperature is lowered the intensity of the signals
changes and the relative intensity signal at 2.73 G is enhanced with respect to the other signal. This suggests that the signal originating from the N–O–O disappears at lower temperature due to obvious reason of anti-ferromagnetic coupling arising from the type of assembly formation as depicted in Fig. 4. To complement this observation we have measured the variable temperature magnetic susceptibility of the complex and found that as the temperature is lowered the susceptibility increases till 60 K and below which there is a fall in the susceptibility (Fig. 5). Plot of log v vs 1/T shows ferromagnetic and anti-ferromagnetic regime for the complex. Although we do not have an adequate quantitative explanation on this issue but this observation makes the system unconventional. To add further support to the N–O–O formation we have obtained a trinuclear copper complex from one pot reaction between p-chloro benzoic acid, copper(II)acetate, 3,5-dimethylpyrazole and hydrogen peroxide. The crystal structure of the trimeric copper(II) complex 5 is shown in
Fig. 5. Normalized magnetic susceptibility of 4 vs temperature.
934
K. Deka et al. / Inorganic Chemistry Communications 9 (2006) 931–934
Fig. 6. Crystal structure of 5.
Fig. 6. From the structure it is clear that a trimeric structure is formed by two bridging methanol and two benzoato group along with two oxo-bridges from the N–O–O unit. The compound has two terminal bidentate benzoate groups and has distorted octahedral geometry around all the three copper centers. Thus, we have synthesized and structurally characterized a few copper(II) complexes that are of structural interest and proper understanding of magnetic properties of these compounds will add a new dimension. We also demonstrated a one pot reaction for selective synthesis of complexes which other wise is difficult.
[2]
Acknowledgements The authors thank the department of Science and Technology, India for financial support and Dr. S. Ravi for help in the low temperature magnetic measurements. Appendix A. Supplementary data The CCDC deposit numbers for the CIF of the complexes 2, 3, 4 and 5 are 603939, 603940, 603941 and 606934. The experimental details for the synthesis of the complexes along with the table for crystallographic parameters are available as supplementary materials. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2006.05.035. References [1] (a) M. Eddaoudi, D. Moler, H. Li, B. Chen, T.M. Reineke, M. O’Keeffe, O.M. Yaghi, Acc. Chem. Res. 34 (2001) 319; (b) M. Casarin, C. Corvaja, C.D. Nicola, D. Falcomer, L. Franco, M. Monari, L. Pandolfo, C. Pettinari, F. Piccinelli, Inorg. Chem. 44 (2005) 6265; (c) M. Eddaoudi, H. Li, O.M. Yaghi, J. Am. Chem. Soc. 127 (2005) 7110;
[3]
[4]
[5] [6] [7]
[8]
(d) A.C. Sudik, A.R. Millward, N.W. Ockwig, A.P. Cote, J. Kim, O.M. Yaghi, J. Am. Chem. Soc. 122 (2000) 1391; (e) E. Tynan, P. Jensen, A.C. Lees, B. Moubaraki, K.S. Murray, P.E. Kruger, Crys. Eng. Comm. 7 (2005) 90; (f) Z. Wang, V.C. Kravtsov, M.J. Zaworotko, Angew Chem. Int. Ed. Eng. 44 (2005) 2877; (g) X-J. Luan, Y-Y. Wang, D-S. Li, P. Liu, H-H. Hu, Q-Z. Shi, S-M. Peng, Angew. Chem. Int. Ed. Eng. 44 (2005) 3864; (h) G. Cristou, Acc. Chem. Res. 22 (1989) 328. (a) R.E. Del Sesto, A.M. Arif, J.S. Miller, Inorg. Chem. 39 (2000) 4894; (b) O. Kahn, Chem. Phys. Lett. 265 (1997) 109; (c) M. Kato, Y. Muto, Coord. Chem. Rev. 92 (1988) 45; (d) S. Ferrer, F. Lioret, I. Beertmenu, G. Alzuet, J. Borras, S. GarciaGranda, M. Liu-Gonzales, J.G. Hassnoot, Inorg. Chem. 41 (2002) 5821; (e) O. Kahn, Y. Pei, Y. Journaux, in: D.W. Bruce, D. O’Hare (Eds.), Inorganic Materials, second ed., John Wiley & Sons, New York, 1996, pp. 66–119; (f) G. Christou, S.P. Perlepes, E. Libby, K. Folting, J.C. Huffman, R.J. Webb, D.N. Hendrickson, Inorg. Chem. 29 (1990) 3657, and references therein; (g) L. Porter, M. Dickman, R. Doedens, Inorg. Chem. 22 (1983) 1962; (h) L. Porter, R. Doedens, Inorg. Chem. 24 (1985) 1006. (a) R.H. Holm, P. Kennepohl, E.I. Solomon, Chem Rev. 96 (1996) 2239; (b) W. Kaim, J. Rall, Angew. Chem. Int. Ed. Eng. 35 (1996) 43; (c) M.A. Halcrow, Angew. Chem. Int. Ed. Eng. 40 (2001) 1433. M. Casarin, C. Corvaja, C. di Nicola, D. Falcomer, L. Franco, M. Monari, L. Pandolfo, C. Pettinari, F. Piccinelli, P. Taliatesta, Inorg. Chem. 43 (2004) 5865. K. Deka, M. Laskar, J.B. Baruah, Polyhedron in press. G.M. Brown, M. George, R. Chidambaram, Acta Crystal. Sec. 29B (1973) 2393. (a) D. Luneau, P. Rey, J. Laugier, P. Fries, A. Caneschi, D. Gatteschi, R. Sessoli, J. Am. Chem. Soc. 113 (1991) 1245; (b) K. Oshima, K. Okuda, M. Date, J. Phys. Soc. Jpn. 44 (1978) 757; (c) Y. Karaki, R. Masutomi, H. Ishimoto, T. Asano, Y. Ajiro, Physica B: Condensed matter 329-333 (2003) 1002. The oxygen attached to the nitrogen are disordered between two equivalent positions, thus, the structure is solved by assigning 50% occupancy to the O-atom of the N–O unit.