Newμ4-oxido-bridged copper benzoate quasi-tetrahedron and bis-μ3-hydroxido-bridged copper azide and copper thiocyanate stepped cubanes: Core conversion, structural diversity, and magnetic properties

Article abstract of DOI:10.1021/ic100356y

[Cu₂(μ₄-O)Cu₂] and [Cu ₂(μ₃-OH)₂Cu₂] geometrical arrangements are found in a new family of tetranuclear complexes: [Cu ₄(μ4-O)(μ-bip)₂(μ-O₂CPh) ₄]· 0.5CH₂Cl₂ (1 -0.5CH ₂Cl₂), [Cu₄(μ₃-OH) ₂(μ-bip)₂(N₃)₄] (2), and [Cu _4- (μ₃-OH)₂(μ-bip)₂(NCS) ₄(DMF)₂] (3·2DMF) [Hbip = 2,6- bis(benzyliminomethyl)-4-methylphenol; DMF = dimethylformamide]. These complexes have been characterized by X-ray crystallography, and their magnetic properties have been studied. Complex 1 reacts with azide and thiocyanate anions, leading to 2 and 3 with a change of the [Cu₄(μ₄-O)] core into [Cu₄(μ₃-OH)₂] units. These compounds are new examples of [Cu₄] complexes where Cu^II ions are connected by two types of water-derived ligands: oxide and hydroxide. Formation of these [Cu₄] complexes can be controlled by changing the bridging ligands, which allows an effective tuning of the self-assembly. The study of the magnetic properties reveals that these complexes exhibit strong intramolecular antiferromagnetic interactions to yield a S_T = O ground state. For the three complexes, the temperature dependence of the magnetic susceptibility was fitted using a model with two isolated S = 1/2 dimers based on the H = -2J{S_cu,1·S_cu,2} spin Hamiltonian with J/k _B = -289 K for 1; J/k_B = -464 and -405 K for 2 and 3, respectively (where J is the exchange constant through the oxido-phenoxido or hydroxido-phenoxido bridges, respectively).

Full text of DOI:10.1021/ic100356y

Inorg. Chem. 2010, 49, 6575–6585 6575
DOI: 10.1021/ic100356y
New μ4-Oxido-Bridged Copper Benzoate Quasi-Tetrahedron and
Bis-μ3-Hydroxido-Bridged Copper Azide and Copper Thiocyanate Stepped
Cubanes: Core Conversion, Structural Diversity, and Magnetic Properties
†
‡,)
§
Mrinal Sarkar, Rodolphe Clerac, Corine Mathoniere, Nigel G. R. Hearns,‡,) Valerio Bertolasi,^ and
ꢀ
ꢁ
Debashis Ray*,†
†
Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India, ‡CNRS, UPR 8641,
ꢀ
ꢀ
ꢀ
Centre de Recherche Paul Pascal (CRPP), Equipe “Materiaux Moleculaires Magnetiques”, 115 Avenue du
)
ꢀ
Dr. Albert Schweitzer, Pessac, F-33600, France, Universite de Bordeaux, UPR 8641, Pessac, F-33600, France,
§
ꢁ
ꢀ
ꢀ
Institut de Chimie de la Matiere Condensee de Bordeaux (ICMCB), CNRS, Universite de Bordeaux, 87 Avenue
du Dr. Albert Schweitzer, Pessac Cedex, F-33608, France, and ^Dipartimento di Chimica e Centro di
ꢁ
Strutturistica Diffrattometica, Universita di Ferrara, Via Borsari 46, 44100 Ferrara, Italy
Received February 22, 2010
[Cu2(μ4-O)Cu2] and [Cu2(μ3-OH)2Cu2] geometrical arrangements are found in a new family of tetranuclear
complexes: [Cu4(μ4-O)(μ-bip)2(μ-O2CPh)4] 0.5CH2Cl2 (1 0.5CH2Cl2), [Cu4(μ3-OH)2(μ-bip)2(N3)4] (2), and [Cu4-
3
3
(μ3-OH)2(μ-bip)2(NCS)4(DMF)2] (3 2DMF) [Hbip = 2,6-bis(benzyliminomethyl)-4-methylphenol; DMF = dimethyl-
3
formamide]. These complexes have been characterized by X-ray crystallography, and their magnetic properties have
been studied. Complex 1 reacts with azide and thiocyanate anions, leading to 2 and 3 with a change of the [Cu4(μ4-O)]
core into [Cu4(μ3-OH)2] units. These compounds are new examples of [Cu4] complexes where CuII ions are
connected by two types of water-derived ligands: oxide and hydroxide. Formation of these [Cu4] complexes can be
controlled by changing the bridging ligands, which allows an effective tuning of the self-assembly. The study of the
magnetic properties reveals that these complexes exhibit strong intramolecular antiferromagnetic interactions to yield
a ST = 0 ground state. For the three complexes, the temperature dependence of the magnetic susceptibility was
fitted using a model with two isolated S = 1/2 dimers based on the H = -2J{SCu,1
SCu,2} spin Hamiltonian with
J/kB = -289 K for 1; J/kB = -464 and -405 K for 2 and 3, respectively (where J is the exchange constant through the
oxido-phenoxido or hydroxido-phenoxido bridges, respectively).
3
Introduction
and cleavage,4 and oxidation of catechols2,5,6 and as building
units for the construction of tetra- and polynuclear com-
plexes with interesting magnetic properties.7-12 Following
this strategy, our research activity has focused on the design
of new families of molecular aggregates based on [Cu2]
building blocks. The tendency of the copper ions to receive
a fifth coordination group at the apical binding site and the
ability of the phenoxide ligands to bridge metal ions allow for
The synthetic coordination chemistry based on controlled
aggregation via self-assembly of small building units to form
polynuclear complexes is of contemporary interest. The role
of ligand design and fine-tuning of reaction conditions with
co-ligands are crucial to understand and control the aggrega-
tion process. Progresses in understanding the aggregation
reactions are likely to be developed by studying the self-
assembly processes of simple building units such as [Cu2]/
ligand units in the presence of other secondary nucleating
groups. [Cu2] complexes have often been used as models to
study the magnetic-exchange interactions,1-3 DNA binding
´
(5) Nanda, P. K.; Bertolasi, V.; Aromı, G.; Ray, D. Polyhedron 2009, 28,
987–993.
(6) Banu, K. S.; Chattopadhyay, T.; Banerjee, A.; Bhattacharya, S.;
Suresh, E.; Nethaji, M.; Zangrando, E.; Das, D. Inorg. Chem. 2008, 47,
7083–7093.
*To whom correspondence should be addressed. Fax: 91-3222-82252. Tel:
03222-283324. E-mail: dray@chem.iitkgp.ernet.in.
(1) Paital, A. R.; Wu, A.-Q.; Guo, G.-C.; Aromı
(7) Paital, A. R.; Mandal, D.; Huang, X.; Li, J.; Aromı, G.; Ray, D.
´
Dalton Trans. 2009, 1352–1362.
~
(8) Paital, A. R.; Bertolasi, V.; Aromı, G.; Ribas-Arino, J.; Ray, D.
´
~
´
, G.; Ribas-Arino, J.;
Ray, D. Inorg. Chem. 2007, 46, 2947–2949.
Dalton Trans. 2008, 861–864.
(9) Paital, A. R.; Hong, C. S.; Kim, H. C.; Ray, D. Eur. J. Inorg. Chem.
2007, 1644–1653.
(10) Nanda, P. K.; Aromı, G.; Ray, D. Chem. Commun. 2006, 3181–3183.
´
(11) Paital, A. R.; Nanda, P. K.; Das, S.; Aromı
(2) Banerjee, A.; Sarkar, S.; Chopra, D.; Colacio, E.; Rajak, K. K. Inorg.
Chem. 2008, 47, 4023–4031.
~
(3) Paital, A. R.; Mitra, T.; Ray, D.; Wong, W. T.; Ribas-Arino, J.;
Novoa, J. J.; Ribasa, J.; Aromı
´
, G. Chem. Commun. 2005, 5172–5174.
´
, G.; Ray, D. Inorg. Chem.
(4) Lahiri, D.; Bhowmick, T.; Pathak, B.; Shameema, O.; Patra, A. K.;
2006, 45, 505–507.
Ramakumar, S.; Chakravarty, A. R. Inorg. Chem. 2009, 48, 339–349.
(12) Nanda, P. K.; Aromı
´
, G.; Ray, D. Inorg. Chem. 2006, 45, 3143–3145.
r
2010 American Chemical Society
Published on Web 06/24/2010
pubs.acs.org/IC

6576 Inorganic Chemistry, Vol. 49, No. 14, 2010
Sarkar et al.
Scheme 1. Utilization of One μ4-A (O2-) and Two μ-B Bridges Provides a Tetrahedral [Cu4] Assembly, Whereas Participation of Two μ3-A (HO-),
Two μ-B, and Four Terminal C Units Provides a Stepped Cubane [Cu4] Assembly
Chart 1. Ligands
the possibility of associating dinuclear Cu(II) units into
discrete oligonuclear and polynuclear complexes. Depending
on the number and nature of secondary bridging or coordi-
nating groups, the assembly of two dinuclear units leads to
four types of [Cu4] aggregates: the tetrahedron, the cubane,
the fused defective dicubane, and the stepped cubane (Scheme 1).
Among these different geometries, tetrahedral and stepped
cubane structures appear much less frequently.13-16 We have
been engaged in studying [Cu4] complexes for some time
because of their interest in bioinorganic modeling, multi-
electron transfer, industrial and biological catalysis, and
methylphenol)23 with CuII metal ions in the presence of addi-
tional ligands such as PhCO2-, N3-, and NCS-. The use of
H3bemp (Chart 1, right, 2,6-bis[(2-hydroxyethylimino)methyl]-
4-methylphenol), a closely related ligand to Hbip, leads to
interesting systems, consisting of [Co4],24 [Ni6],25 and [Cu18]26
complexes and a large heterometallic [Mn6Cu10]27 species.
Herein, two classes of stable [Cu4]/bip- complexes are reported:
(i) [Cu4(μ4-O)(μ-bip)2(μ-O2CPh)4] (1), which contains a central
tetrahedral μ4-oxido coordinating group, and (ii) [Cu4(μ3-
OH)2(μ-bip)2(N3)4] (2) and [Cu4(μ3-OH)2(μ-bip)2(NCS)4-
(DMF)2] (3), obtained by converting 1 and its μ4-oxido group
into stepped cubane complexes with bis-OH bridges. These
complexes have been isolated and crystallographically charac-
terized, and their magnetic properties have been studied.
magnetostructural research.7-12,16 Water and water-derived
ligands such as hydroxido and oxido groups are most suitable
bridging ligands in the construction and/or stabilization
of [Cu2] moieties and more generally in coordination
chemistry.5,11,16,19-22 μ4-Oxidotetracopper(II) cores, [Cu4-
(μ4-O)], are important both in industrial catalysis of convert-
ing substituted phenols to quinones17 and in the biological
catalytic cycle of nitrous oxide reduction to dinitrogen.18
Recently, during oxidation of 2,3,6-trimethylphenol (TMP)
with O2 to trimethyl quinone (TMQ), a key intermediate of
vitamin E, in the presence of a catalytic amount of copper(II)
chloride in ionic liquids a hexa-μ-chlorido-tetrachlorido-μ4-
oxidocuprate (4-) species [Cu4(μ4-O)Cl10]4- was isolated as
an active species.17 In trying to isolate and study new forms of
such aggregates, we have been interested in exploring the
reactivity of the tetradentate/dinucleating phenolate Schiff
base ligand Hbip (Chart 1, left; 2,6-bis(benzyliminomethyl)-4-
Experimental Section
Materials. The chemicals used were obtained from the fol-
lowing sources: Copper acetate monohydrate, ammonium thio-
cyanate, sodium azide, and benzylamine from SRL (India) and
sodium benzoate from S.D. Fine Chemicals (India). All other
chemicals and solvents were reagent grade materials and were
used as received without further purification.
(13) Shakya, R.; Keyes, P. H.; Heeg, M. J.; Moussawel, A.; Heiney, P. A.;
Verani, C. N. Inorg. Chem. 2006, 45, 7587–7589.
(14) Kirillov, A. M.; Kopylovich, M. N.; Kirillova, M. V.; Haukka, M.;
Silva, M. F. C. G. da; Pombeiro, A. J. L. Angew. Chem., Int. Ed. 2005, 44,
4345–4349.
Syntheses. Hbip Ligand. The Hbip Schiff base is prepared
from the single-step condensation of 2,6-diformyl-5-methylphe-
nol (1.640 g, 10 mmol) and benzylamine (2.18 mL, 20 mmol) in
methanol (40 mL) under reflux for 1 h, as reported previously.23
[Cu4(μ4-O)(μ-bip)2(μ1,3-O2CPh)4] 0.5CH2Cl2 (1 0.5CH2Cl2).
(15) Isele, K.; Franz, P.; Ambrus, C.; Bernardinelli, G.; Decurtins, S.;
Williams, A. F. Inorg. Chem. 2005, 44, 3896–3906.
´
(16) Bera, M.; Wong, W. T.; Aromı, G.; Ribas, J.; Ray, D. Inorg. Chem.
2004, 43, 4787–4789.
(17) Sun, H.; Harms, K.; Sundermeyer, J. J. Am. Chem. Soc. 2004, 126,
9550–9551.
3
3
To the yellow MeOH solution (25 mL) of Hbip (0.342 g, 1.00 mmol)
is added Cu(OAc)2 H2O (0.400 g, 2.00 mmol) with stirring at
^
(18) Prudencio, M.; Pereira, A. S.; Tavares, P.; Besson, S.; Cabrito, I.;
Brown, K.; Samyn, B.; Devreese, B.; Beeumen, J. V.; Rusnak, F.; Fauque,
G.; Moura, J. J. G.; Tegoni, M.; Cambillau, C.; Moura, I. Biochemistry 2000,
39, 3899–3907.
3
ambient temperature in air. The brown solution formed initially
changes to green in about 5 min. The resulting green solution is
ꢀ
(19) Paul, S.; Clerac, R.; Hearns, N. G. R.; Ray, D. Cryst. Growth Des.
2009, 9, 4032–4040.
(20) Mukherjee, A.; Rudra, I.; Nethaji, M.; Ramasesha, S.; Chakravarty,
A. R. Inorg. Chem. 2003, 42, 463–468.
(21) Dhara, K.; Ratha, J.; Manassero, M.; Wang, X.-Y.; Gao, S.; Bane-
rjee, P. J. Inorg. Biochem. 2007, 101, 95–103.
(22) Pradeep, C. P.; Supriya, S.; Zacharias, P. S.; Das, S. K. Polyhedron
2006, 25, 3588–3592.
(23) Grzybowski, J. J.; Urbach, F. L. Inorg. Chem. 1980, 19, 2604–2608.
(24) Mandal, D.; Ray, D. Inorg. Chem. Commun. 2007, 10, 1202–1205.
~
(25) Mandal, D.; Bertolasi, V.; Ribas-Arino, J.; Aromı, G.; Ray, D. Inorg.
´
Chem. 2008, 47, 3465–3467.
(26) Shiga, T.; Maruyama, K.; Han, L. Q.; Oshio, H. Chem. Lett. 2005,
34, 1648–1649.
(27) Yamashita, S.; Shiga, T.; Kurashina, M.; Nihei, M.; Nojiri, H.; Sawa,
H.; Kakiuchi, T.; Oshio, H. Inorg. Chem. 2007, 46, 3810–3812.

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6577
stirred for ca. 10 min, and NaO2CPh (0.288 g, 2.0 mmol) is added.
The green suspensions formed initially change to green solution in
about 15 min, and after 1 h, a green precipitate appears. The green
solid is isolated by filtration, washed with cold methanol, and dried
under vacuum over P4O10. The green prismatic-shaped single crystals
suitable for X-ray analysis are obtained from a CH2Cl2-MeOH
solvent mixture after 5 days. Yield: 0.524 g, 73%. Anal. Calcd for
11.24. Found: C, 49.68; H, 4.75; N, 11.09. Selected FTIR bands
(KBr, cm-1): 3600 (br), 3435 (br), 2079 (vs), 1653 (vs), 1638 (vs),
1560 (s), 1450 (m), 1336 (m), 1090 (m), 829 (m), 758 (m), 713 (m).
Molar conductance, ΛM (DMF solution): 6 Ω-1 cm2 mol-1
.
UV-vis spectra [λmax, nm (ε, L mol-1 cm-1)] (DMF solution):
665 (521), 378 (6823), 353 (4136), 274 (13 840).
Physical Measurements. The elemental analyses (C, H, N) were
performed with a Perkin-Elmer model 240C elemental analyzer.
FTIR spectra were recorded on a Perkin-Elmer 883 spectrometer.
The solution electrical conductivity and electronic spectra were
obtained using a Unitech type U131C digital conductivity meter
with a solute concentration of about 10-3 M and a Shimadzu UV
3100 UV-vis-NIRspectrophotometer, respectively. The powder
X-ray diffraction (PXRD) patterns were obtained using a Phillips
PW 1710 diffractometer with Cu KR radiation. The magnetic
susceptibility measurements were obtained with the use of a
Quantum Design SQUID magnetometer MPMS-XL housed at
the Centre de Recherche Paul Pascal. This magnetometer works
between 1.8 and 300 K for dc applied fields ranging from -7 to
7 T. Measurements were performed on polycrystalline samples of
13.70, 29.33, and 12.16 mg for 1 0.5CH2Cl2, 2, and 3 2DMF,
C
74.5H63Cu4N4O11Cl (1479.97 g mol-1): C, 60.46; H, 4.29; N, 3.78.
Found: C, 60.38; H, 4.18; N, 3.58. Selected FTIR bands (KBr, cm-1
;
s = strong, vs = very strong, m = medium, br = broad): 1623 (s),
1609 (s), 1568 (s), 1455 (s), 1371 (s), 1234 (m), 760 (m), 718 (s), 679
(s), 623 (m), 494 (m). Molar conductance, ΛM (CH2Cl2 solution): 3
Ω-1 cm2 mol-1. UV-vis spectra [λmax, nm (ε, L mol-1 cm-1)]:
(CH2Cl2 solution) 680 (210), 389 (4895), 326 (3534), 253 (21695).
[CuII4(μ3-OH)2(μ-bip)2(N3)4] (2). Method A via Direct Route.
Cu(OAc)2 H2O (0.20 g, 1.00 mmol) dissolved in methanol (25 mL)
3
is added dropwise with stirring to a yellow MeOH solution (15 mL)
of Hbip (0.171 g, 0.50 mmol). The brown solution formed initially
changes to green after about 5 min of stirring. After ca. 10 min a
solution of NaN3 (0.130 g, 2.0 mmol) in MeOH (25 mL) is added
dropwise. The color of the reaction mixture changes to dark green,
and the reaction mixture is stirred further for about 30 min. The
obtained green precipitate is collected by filtration, washed with
cold methanol followed by water, and dried under vacuum over
P4O10. Single crystals suitable for X-ray analysis are obtained from
DMF after 7 days. Yield: 0.202 g, 71%. Anal. Calcd for C46H42N16-
O4Cu4 (1137.13 g mol-1): C, 48.58; H, 3.72; N, 19.70. Found: C,
48.50; H, 3.61; N, 19.44. Selected FTIR bands (KBr, cm-1): 3600
(br), 3448 (br) 2043 (vs), 1637 (vs), 1560 (vs), 1347 (s), 1092 (m), 829
(m), 755 (m), 711 (m). Molar conductance, ΛM (DMF solution):
6 Ω-1 cm2 mol-1. UV-vis spectra [λmax, nm (ε, L mol-1 cm-1)]:
(DMF solution): 655 (520), 378 (9480), 353 (4450), 274 (13 658).
Method B via Conversion of 1. A green solution of 1 (0.360 g,
0.25 mmol) in CH2Cl2 (25 mL) is stirred for about 5 min followed
by a dropwise addition of NaN3 (0.097 g, 1.50 mmol) dissolved in
MeOH (20 mL) at ambient temperature. During the reaction, there
is a noticeable color change from green to dark green followed by
separation of a green precipitate, which is easily collected by
filtration, washed with cold methanol, and dried under vacuum
over P4O10. Yield: 0.180 g, 63%. Anal. Calcd for C46H42N16O4Cu4
(1137.13 g mol-1): C, 48.58; H, 3.72; N, 19.70. Found: C, 48.45; H,
3.63; N, 19.54. Selected FTIR bands (KBr, cm-1): 3601 (br), 3448
(br) 2045 (vs), 1638 (vs), 1558 (vs), 1350 (s), 1094 (m), 832 (m), 758
(m), 707 (m). Molar conductance, ΛM (DMF solution): 8 Ω-1 cm2
mol-1. UV-vis spectra [λmax, nm (ε, L mol-1 cm-1)] (DMF
solution): 655 (525), 378 (9483), 353 (4452), 274 (13 655).
3
3
respectively. The magnetic data were corrected for the sample
holder and the diamagnetic contributions.
Crystal Data Collection and Refinement for 1 0.5CH2Cl2, 2,
3
and 3 2DMF. Crystal data of compound 1 0.5CH2Cl2 were
3
3
collected on a Nonius Kappa CCD diffractometer using gra-
phite-monochromated Mo KR radiation (λ = 0.7107 A) at low
temperature (120 K). Data sets were integrated with the Denzo-
SMN package28 and corrected for Lorentz-polarization and
absorption29 effects. The structure was solved by direct methods
(SIR97)30 and refined by full-matrix least-squares methods with
all non-hydrogen atoms anisotropic and hydrogens included on
calculated positions, riding on their carrier atoms. The phenyl
group C34/C39 was found disordered and was refined isotropi-
cally over two positions with occupancies of 0.6 and 0.4, respec-
tively. The molecule of solvent, CH2Cl2, was found disordered
around a center of symmetry and was refined over two positions
withoccupancy of0.5each. All calculations wereperformed using
SHELXL-9731 and PARST32 implemented in the WINGX sys-
tem of programs.33 The crystal data of the complex 2 were
collected on a Nonius CAD4 X-ray diffractometer using single
crystals with graphite-monochromated Mo KR radiation (λ =
0.7107 A) by the ω-scan method. The crystal data of the complex
3 2DMF were collected on a Bruker-APEX-2 CCD X-ray dif-
fractometer using single crystals with graphite-monochromated
3
ꢀ
Mo KR radiation (λ = 0.7107 A) by the hemisphere method.
Data were collected at 293 K. The coordinated DMF molecule is
disordered and was refined over two sites with occupancies of
0.6 and 0.4, respectively. The refinement was performed using
full-matrix least-squares with all non-hydrogen atoms refined
anisotropically, except those belonging to the disordered coordi-
nated DMF molecule, which were refined isotropically. The
hydrogens were included on calculated positions, riding on their
carrier atoms. The hydroxide O2-H hydrogen was found in the
difference Fourier maps and refined with an O-H fixed distance
of 0.90 A. The crystal parameters and other experimental details
of the data collections are summarized in Table 1.
Caution!! Azide complexes of metal ions involving organic ligands
are potentially explosive. Only small quantities of the complexes
should be prepared, and these should be handled with care.
[CuII4(μ3-OH)2(μ-bip)2(NCS)4(DMF)2] 2DMF (3 2DMF).
3
3
Method A via Direct Route. The green compound 3 is obtained
by a similar method to that described above for compound 2 by
using NH4SCN (0.152 g, 2.0 mmol) instead of NaN3. The reaction
in the presence of up to 6 mmol of NH4SCN yields complex 3
exclusively. Yield: 0.210 g, 70%. Green single crystals of 3 2DMF
3
suitable for X-ray analysis are obtained from DMF during 7 days.
Anal. Calcd for C62H72N12O8S4Cu4 (1495.77 g mol-1): C, 49.78;
H, 4.85; N, 11.24. Found: C, 49.70; H, 4.74; N, 11.02. Selected
FTIR bands (KBr, cm-1): 3600 (br), 3433 (br), 2081 (vs), 1655
(vs), 1636 (vs), 1562 (s), 1451 (m), 1333 (m), 1092 (m), 829 (m), 760
(m), 709 (m). Molar conductance, ΛM (DMF solution): 5 Ω-1 cm2
mol-1. UV-vis spectra [λmax, nm (ε, L mol-1 cm-1)] (DMF
solution): 665 (525), 378 (6820), 353 (4134), 274 (13 834).
Crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre
(28) Otwinowski, Z.; Minor, Z. In Methods Enzymol., Carter, C. W.; Sweet,
R. M., Eds.; Academic Press: London, 1977; Vol. 276, Part A, pp 307-326.
(29) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33–38.
(30) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G.; Polidori, G.; Spagna, R.
J. Appl. Crystallogr. 1999, 32, 115–119.
Method B via Conversion of 1. Compound 3 is also obtained
quantitatively following the conversion method used in the case
of 2 by replacing NaN3 by NH4SCN. The green precipitate is
collected by filtration, washed with cold methanol, and dried
under vacuum over P4O10. Yield: 0.180 g, 60%. Anal. Calcd for
C62H72N12O8S4Cu4 (1495.77 g mol-1): C, 49.78; H, 4.85; N,
(31) Sheldrick, G. M. SHELXL97, Program for Crystal Structure
€
€
Refinement; University of Gottingen: Gottingen, Germany, 1997.
(32) Nardelli, M. J. Appl. Crystallogr. 1995, 28, 659–660.
(33) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–838.

6578 Inorganic Chemistry, Vol. 49, No. 14, 2010
Sarkar et al.
Table 1. Crystallographic Data for 1 0.5CH2Cl2, 2, and 3 2DMF
3
3
1 0.5CH2Cl2
3
2
3 2DMF
3
formula
C74.5H63Cu4N4O11Cl
1479.97
P21/n
C46H42N16O4Cu4
1137.13
P21/n
C62H72N12O8S4Cu4
1495.77
P1
M g mol-1
space group
cryst syst
monoclinic
13.7694(2)
23.8283(3)
20.3394(3)
monoclinic
8.571(3)
triclinic
12.0338(6)
12.9150(7)
14.1274(8)
65.2500(10)
80.906(2)
87.583(2)
1968.18(18)
293
a/A
b/A
24.548(5)
12.048(3)
c/A
R/deg
β/deg
γ/deg
U/A3
T/K
97.0892(7)
109.52(7)
6622.37(16)
120
2389.3(15)
293
Z
4
1.484
1
1.581
2
1.385
Dc/g cm-3
F(000)
3036
13.73
1156
18.18
852
12.34
μ(Mo KR)/cm-1
measd reflns
unique reflns
Rint
63 571
11 643
4439
4145
26 923
8722
0.0851
9336
0.0647
2334
0.0327
5994
obsd reflns [I g 2σ(I)]
θmin-θmax/deg
hkl ranges
R(F2)a (obsd reflns)
wR(F2)a (all reflns)
no. variables
goodness of fit
ΔFmax; ΔFmin/e A-3
1.90-25.00
-16,16; -27,28; -24,24
0.0751
0.1937
844
1.98-24.97
-10,0; 0,29; -13,14
0.0606
0.1581
316
1.61-27.50
-14,15; -16,16; -18,18
0.0498
0.1511
458
1.159
1.46; -0.74
0.988
0.720; -0.769
1.011
1.057; -0.530
P
P
P
P
a R = (||Fo| - |Fc||)/ |Fo|. wR = [ w(|Fo| - |Fc|)2/ w(Fo)2]1/2. w = 0.75/(σ2(Fo) þ 0.0010Fo2).
as supplementary publications CCDC-716090, -637273, and
-744551. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (internat.) þ44-1223/336-033; e-mail: deposit@
ccdc.cam.ac.uk].
The elemental analysis and molar conductivity data are
consistent withtheformula[Cu4(μ4-O)(μ-bip)2(μ-O2CPh)4]
for 1. However no sign of formation of only phenoxido-
bridged Cu(II) dinuclear species (4, Scheme 2) was ob-
served, perhaps because of the better stability of 1, which
crystallizes with a central tetrahedral μ4-oxido group to
assemble a pair of [Cu2] units (it is worth noting that 4 was
also not obtained when a MeOH solution of Hbip and
Results and Discussion
Synthetic Considerations. The Schiff base 2,6-bis-
(benzyliminomethyl)-4-methylphenol (Hbip) has been
prepared (Scheme S1 in the Supporting Information)
following a literature procedure,23 and its reactions with
copper(II) salts have been systematically investigated
PhCO2Na was added to Cu(OAc)2 H2O in a 1:2 molar
3
ratio). Using a similar approach, two other reactions in
the presence of azide and thiocyanate anions have been
carried out. In these cases, the oxido bridge is converted
in a hydroxido bridge. Green complexes [Cu4(μ3-OH)2-
(μ-bip)2(N3)4] (2, Scheme 2) and [Cu4(μ3-OH)2(μ-bip)2-
(NCS)4(H2O)2] (3, Scheme 2) are directly synthesized in
∼70% yield from MeOH medium under aerobic condi-
tions at room temperature by stirring a reaction mix-
(Scheme 2). When the reaction of Cu(OAc)2 H2O was
3
carried out with Hbip and PhCO2Na in MeOH and in the
absence of base, [Cu4(μ4-O)(μ-bip)2(μ-O2CPh)4] (1,
Scheme 2) was obtained. In the absence of sodium
benzoate, [Cu4(μ4-O)(μ-bip)2(μ-OAc)4] was obtained as
a green powder from the reaction mixture. At room
ture of Cu(OAc)2 H2O, Hbip, and NaN3 or NH4SCN
3
in a 2:1:2 molar ratio for 1 h. The same reactions in
the presence of stoichiometric NEt3 or NaOH gave the
same products, confirming the formation of hydroxido
bridges independently of the type and nature of added
bases. The syntheses of 2 and 3 are summarized in
eqs 2 and 3.
temperature Cu(OAc)2 H2O, PhCO2Na, and Hbip in a
3
2:2:1 molar ratio have been mixed and stirred for 1 h to
lead to complex 1. The complex precipitates directly from
the reaction mixture as a green solid in ∼73% yield (note
that several Cu/Hbip ratios were explored, but the 2:2:1
molar ratio gives the best yield and clean samples).
The preparation of complex 1 is summarized in eq 1,
accounting for the formation of the oxido bridge from the
aqua molecule.
2Hbip þ 4CuðOAcÞ2 H2O þ 4NaN3 f ½Cu4ðμ3-OHÞ2
3
ðμ-bipÞ2ðN3Þ4ꢀ þ 4AcOH þ 4NaOAc þ 2H2O ð2Þ
2Hbip þ 4CuðOAcÞ2 H2O þ 4PhCO2Na
3
2Hbip þ 4CuðOAcÞ2 3 H2O þ 4NH4SCN
f ½Cu4ðμ4-OÞðμ-bipÞ2ðμ-O2CPhÞ4ꢀ þ 4CH3CO2Na
f ½Cu4ðμ3-OHÞ2ðμ-bipÞ2ðNCSÞ4ðH2OÞ2ꢀ þ 4AcOH
þ 4CH3CO2H þ 3H2O
ð1Þ
þ 4NH4OAc
ð3Þ

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6579
Scheme 2. Summary of the Possible Synthetic Routes That Lead to the Reported Complexes and Other Known Species for the Hbip Ligand
The elemental analysis and molar conductivity data
confirm the respective formula. Reactions with an ex-
cess of azide or thiocyanate anions did not lead to the
substitution of the hydroxido bridges in 2 and 3, in-
dicating the strong affinity and stability of the hydro-
xido bridges in these systems.
group that reacts irreversibly with water to produce HO-
anions trapped by 2 and 3 (eqs 4 and 5) in the same way as
the reaction of K2O(s) with H2O (K2O þ H2O f 2
KOH).34
½Cu4ðμ4-OÞðμ-bipÞ2ðμ-O2CPhÞ4ꢀ þ 4NaN3 þ H2O
f ½Cu4ðμ3-OHÞ2ðμ-bipÞ2ðN3Þ4ꢀ þ 4PhCO2Na ð4Þ
Conversion of 1 to Stepped Cubane [Cu4] Complexes.
Green complexes [Cu4(μ3-OH)2(μ-bip)2(N3)4] (2) and
[Cu4(μ3-OH)2(μ-bip)2(NCS)4(DMF)2] (3) can be obtained
from [Cu4(μ4-O)(μ-bip)2(μ-O2CPh)4] (1) on reaction
with sodium azide and ammonium thiocyanate salts.
The treatment of CH2Cl2-MeOH (1:1) solutions of 1
by four equivalents of NaN3 or NH4SCN readily leads to
stepped cubane complex 2 or 3 with color changes from
green to dark green followed by precipitation of green
solids. The precipitates were filtered, washed with MeOH,
dried, and dissolved in DMF for crystallization. Green
crystals of 2 and 3 grow slowly over several days and can
be isolated by filtration. Complexes 2 and 3 turn out to be
novel examples of tetranuclear CuII complexes in which
the core of complex 1 is successfully modified by (i)
replacing the benzoate bridges by N3- and SCN- anions
and (ii) converting (μ4-O) into (μ3-OH)2 groups. The
mechanism of the conversion of 1 to 2 and 3 summarized
in eqs 4 and 5 is clearly complicated. It should be men-
tioned that the encaged O2- anion in 1 is a strong basic
½Cu4ðμ4-OÞðμ-bipÞ2ðμ-O2CPhÞ4ꢀ þ 4ðNH4ÞSCN þ 3H2O
f ½Cu4ðμ3-OHÞ2ðμ-bipÞ2ðNCSÞ4ðH2OÞ2ꢀ þ 4PhCO2ðNH4Þ
ð5Þ
These two reactions also demonstrate the successful
replacement of the carboxylate groups with weak apical-
basal bridging modes by strong azide and thiocyanate
coordination groups on only the basal sites.
IR Spectroscopy. In the three [Cu4] complexes, the
characteristic bands of the bip- ligand appear clearly on
the FT-IR spectra. The ν~CdN stretching frequencies are
observed at 1623-1636 cm-l for complexes 1-3. For 1,
the asymmetric and the symmetric stretching vibrations
of the four bound carboxylate groups are detected at 1609 cm-l
(34) Myers, R. J. J. Chem. Educ. 1986, 63, 687–690.

6580 Inorganic Chemistry, Vol. 49, No. 14, 2010
Sarkar et al.
as ν~as(COO), while ν~s(COO) appeared at 1371 cm-1 with a
difference Δν~ = ν~as(COO) - ν~s(COO) = 238 cm-1. This Δν~
value is in accord with the presence of μ1,3-carboxylato
bridges between Cu(II) sites in 1 (it is worth noting that a
nonbridging carboxylato group would lead to a larger Δν~
separation of ca. 350 cm-1).35 Inthe IR spectra of 1, a sharp
absorption band at 623 cm-1 is detected. This band is
missing from the IR spectra of the free Hbip ligand, sodium
benzoate, and copper(II) acetate. This band is indeed
associated with ν~Cu-O frequencies originating from vibra-
tions in the [Cu4O] core in agreement with our recent report
on [Cu4(μ4-O)(bahped)2](ClO4)2,16 which exhibits a [Cu4O]
band at 566 cm-1. In the case of [Cu4Cl4(μ4-O)(OSR2)4]
(R = Et, nBu)36 and in copper 2,6-bis(morpholinomethyl)-
4-methylphenol complexes,37 this band is seen at 581-
Figure 1. FT-IR spectra of 1 0.5CH2Cl2, 2, and 3 2DMF between 1250
3
3
and 2200 cm-1
.
592 cm-1. As expected for complexes 2 and 3 2DMF, the
3
ν~COO and ν~Cu-O bands are missing and a broad band near
3600 cm-1 is observed due to the ν~OH stretching mode of
the HO- bridging groups.38 In addition, complex 2 shows a
very strong and sharp band at 2043 cm-1 for the
ν~as(N-N-N) asymmetric stretching vibration of the bound
-
terminal N3 groups, while the symmetric vibration
ν~s(N-N-N) appears as a weak band at 1347 cm-1.6 The
binding of thiocyanate anions in 3 2DMF is clearly seen by
3
the ν~C-N stretching mode, which appears as a single intense
band at 2081 cm-1 39
The ν~C-S stretching and the δNCS
.
bending frequencies in the 800-400 cm-1 region could not
be identified with certainty due to the presence of strong
absorptions of the organic ligand in this region. The
conversion of 1 to 2 or 3 in solution and in the solid state
via a mechanochemical route (vide infra) has been well
monitored using FT-IR spectroscopy and the characteristic
bands of complexes 1, 2, and 3 (Figure 1).
Figure 2. View of the molecular tetranuclear unit [Cu4(μ4-O)(μ-bip)2-
(μ-O2CPh)4] in 1 0.5CH2Cl2 with atom-numbering scheme. H atoms are
3
omitted for clarity. Color code: CuII brown, N blue, O red, C gray.
P21/n space group. The asymmetric unit contains one
tetranuclear complex and half of a dichloromethane
molecule. The tetranuclear complex consists of two de-
protonated bip- ligands, each of them delivering a set of
N2O donor atoms to the [Cu4] complex that assembles
around a central μ4-O group. Four benzoato groups
complete the coordination environments around each
CuII site (Figures 2 and 3). The O2- anion, O1, at the
center of the complex is surrounded by four copper(II)
ions organized in a distorted tetrahedral geometry with
Electronic Spectra. The three complexes in DMF solu-
tions show multiple bands in the 200-900 nm region. The
ligand-field spectra of the complexes show broad absorption
bands (λ), with maxima at 680 (ε = 210 L mol-1 cm-1), 655
(ε = 520 L mol-1 cm-1), and 665 nm (ε = 525 L mol-1
cm-1) for 1, 2, and 3, respectively. The intense absorptions
below 400 at 253 nm (ε = 21695 L mol-1 cm-1), 274 nm
(ε=13658Lmol-1 cm-1), and 274 nm (ε= 13834 L mol-1
cm-1) are dominated by the metal ion bound ligand-based
absorptions for 1 0.5CH2Cl2, 2, and3 2DMF, respectively.
the Cu Cu distances varying from 2.98 to 3.18 A
(Figure S1, it should be noted that these Cu Cu
3 3 3
3
3
3 3 3
Spectra of these complexes also record shoulders at 389 nm
(ε = 4895 L mol-1 cm-1), 378 nm (ε = 9480 L mol-1
cm-1), and 378 nm (ε = 6820 L mol-1 cm-1), respectively,
due to HO- f CuII and PhO- f CuII ligand-to-metal
charge transfer (LMCT) transitions.2
distances are longer than those in the known benzoato-
bridged copper paddlewheel dimers40). This O2- oxygen
anion thus provides a μ4-bridging connectivity to four
CuII. The coordination sphere of each CuII adopts a
distorted square-pyramid geometry due to the Jahn-Tel-
ler effect expected for 3d9 copper(II) ion. The copper
coordination sphere consists of two oxygen atoms from
the carboxylato groups, one phenoxido oxygen and ni-
trogen atoms from the bip- ligand, and one central O2-
anion (Figures S2 and S3). One of the carboxylato oxygen
atoms is coordinating at the apical position of the Cu(II)
sites with bond distances of 2.21-2.52 A, which are
significantly longer, by 0.2-0.3 A, than the other carbox-
ylato oxygen atoms coordinated in Cu(II) basal planes.
The Cu-Obz (bz = benzoate) bond lengths in the basal
plane (1.94-1.97 A) are longer than the Cu-(μ4-O)
(1.91-1.93 A) but close to the Cu-Oph (1.97-2.00 A)
Description of the Crystal Structures. [Cu4(μ4-O)(μ-
bip)2(μ-O2CPh)4] 0.5CH2Cl2 (1 0.5CH2Cl2). The molec-
ular structure of compound 1 is shown in Figure 2, and
3
3
important bond lengths and angles are given in Table 2.
Compound 1 0.5CH2Cl2 crystallizes in the monoclinic
3
(35) Nakamoto, K. In Infrared and Raman Spectra of Inorganic and
Coordination Compounds, part B; John Wiley & Sons: New York, 1997;
pp 59-62; 276-282.
(36) Guy, J. T., Jr.; Cooper, J. C.; Gilardi, R. D.; Flippen-Anderson,
J. L.; George, C. F., Jr. Inorg. Chem. 1988, 27, 635–638.
(37) Teipel, S.; Griesar, K.; Haase, W.; Krebs, B. Inorg. Chem. 1994, 33,
456–464.
_
(38) Motoda, K.-i.; Sakiyama, H.; Matsumoto, N.; Okawa, H.; Fentonb,
D. E. J. Chem. Soc., Dalton Trans. 1995, 3419–3425.
ꢀ
(39) Cortes, R.; Urtiaga, M. K.; Lezama, L.; Pizarro, J. L.; Arriortua,
M. I.; Rojo, T. Inorg. Chem. 1997, 36, 5016–5021.
(40) Takamizawa, S.; Miyake, R. Chem. Commun. 2009, 4076–4078.

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6581
Table 2. Selected Interatomic Distances (A) and Angles (deg) for 1 0.5 CH2Cl2
3
Distances
Cu1-O1
Cu1-O2
Cu1-O4
Cu1-O6
Cu1-N1
Cu2-O1
Cu2-O2
Cu2-O8
Cu2-O10
Cu2-N2
1.913(4)
1.992(4)
2.222(5)
1.974(4)
1.973(5)
1.919(4)
1.975(4)
2.380(6)
1.952(5)
1.977(5)
2.996(1)
3.189(1)
3.169(1)
Cu3-O1
Cu3-O3
Cu3-O5
Cu3-O7
Cu3-N3
Cu4-O1
Cu4-O3
Cu4-O9
Cu4-O11
Cu4-N4
Cu2...Cu3
1.932(4)
2.003(4)
1.945(4)
2.214(5)
2.000(5)
1.916(4)
1.976(4)
1.963(5)
2.522(6)
1.971(5)
3.309(1)
3.146(1)
2.986(1)
Cu1 Cu2
3 3 3
Figure 4. Viewofthe molecular tetranuclear unit[Cu4(μ3-OH)2(μ-bip)2-
(N3)4] in 2 with atom-numbering scheme. H atoms are omitted for clarity.
Color code: CuII brown, N blue, O red, C gray.
Cu1 Cu3
Cu1 Cu4
Cu2 Cu4
3 3 3
3 3 3
Cu3 Cu4
3 3 3
3 3 3
Angles
The Cu-O-C-O/O-C-O-Cu torsion angles (2.74ꢀ/
52.82ꢀ) indicate that copper atoms are bound by benzo-
ates in a syn-syn (ideal values 0ꢀ/0ꢀ) arrangement, having
out-of-plane binding for the second Cu atom. Within the
[Cu2(μ-bip)]3þ units, the bridging phenoxido oxygen
atom shows a basal-basal coordination mode to two
copper atoms. This mode of bridging allows the carboxy-
lato oxygens to exhibit a complementary basal-apical
coordination to CuII ions. This basal-apical and apical-
basal combination is responsible for the formation of an
interdimer cavity between [Cu2(μ-bip)]3þ units that nicely
traps the O2- ion in 1. The phenoxido oxygen bridge
squeezes the Cu-(μ4-O)-Cu (Cu1-O1-Cu2 and Cu3-
O1-Cu4) angles to 101.80ꢀ compared to the other angles,
at 112.04ꢀ (Cu1-O1-Cu3 and Cu2-O1-Cu4). Two
phenoxido-bridged [Cu2] fragments stay almost perpen-
dicular to each other, with a dihedral angle of 86.89ꢀ
between the two Cu2O2 planes. All these distances and
angles are in good agreement with previously reported
values.37,41-44 The four Cu atoms are placed approxi-
mately 0.13-0.19 A from their least-squares basal planes
toward the apical carboxylato oxygen atoms. The co-
ordination geometry about the metal ions is very close to
square pyramidal (Addison parameter τ varies from 0.004
to 0.210).45 Within the crystal packing no H-bonding
O1-Cu1-O2
O1-Cu1-O4
O1-Cu1-O6
O1-Cu1-N1
O2-Cu1-O4
O2-Cu1-O6
O2-Cu1-N1
O4-Cu1-O6
O4-Cu1-N1
O6-Cu1-N1
O1-Cu2-O2
O1-Cu2-O8
O1-Cu2-O10
O1-Cu2-N2
O2-Cu2-O8
O2-Cu2-O10
O2-Cu2-N2
O8-Cu2-O10
O8-Cu2-N2
O10-Cu2-N2
Cu1-O1-Cu2
Cu1-O1-Cu3
Cu1-O1-Cu4
Cu2-O1-Cu3
79.4(2)
98.4(2)
94.8(2)
166.7(2)
94.4(2)
167.1(2)
90.5(2)
97.9(2)
90.8(2)
93.3(2)
76.7(2)
94.5(2)
93.4(2)
169.4(2)
83.9(2)
162.6(2)
90.4(2)
112.7(2)
88.1(2)
95.1(2)
102.8(2)
112.0(2)
111.7(2)
118.5(2)
O1-Cu3-O3
O1-Cu3-O5
O1-Cu3-O7
O1-Cu3-N3
O3-Cu3-O5
O3-Cu3-O7
O3-Cu3-N3
O5-Cu3-O7
O5-Cu3-N3
O7-Cu3-N3
O1-Cu4-O3
O1-Cu4-O9
O1-Cu4-O11
O1-Cu4-N4
O3-Cu4-O9
O3-Cu4-O11
O3-Cu4-N4
O9-Cu4-O11
O9-Cu4-N4
O11-Cu4-N4
Cu2-O1-Cu4
Cu3-O1-Cu4
Cu1-O2-Cu2
Cu3-O3-Cu4
79.7(2)
96.2(2)
97.0(2)
160.8(2)
173.6(2)
90.6(2)
89.7(2)
94.8(2)
92.8(2)
99.1(2)
80.8(2)
94.5(2)
89.6(2)
169.3(2)
168.9(2)
82.0(2)
89.4(2)
108.1(2)
94.4(2)
93.3(2)
110.3(2)
101.8(2)
98.1(2)
97.3(2)
network or CH π interactions are present even in the
3 3 3
presence of solvate CH2Cl2 molecules within the lattice.
The crystal-packing diagram along the a axis is shown in
Figure S5 (see Supporting Information).
[Cu2(μ3-OH)(μ-bip)(N3)2]2 (2). The molecular structure
of [Cu2(μ3-OH)(μ-bip)(N3)2]2 is shown in Figure 4, and
selected bond lengths and angles are listed in Table 3. The
complex 2 crystallizes in the P21/n space group, and the
asymmetric unit contains one-half of the tetranuclear unit
(the second half of the tetramer is generated by the (-x,
-y, 1-z) symmetry operator). The [Cu4] complex results
from the assembly of two [Cu2(μ-OH)(μ-bip)(N3)2] frag-
ments through two hydroxido groups (O2 and O2*) that
Figure 3. View of the atom connectivity within the [Cu4] tetrahedral
core with the basal-apical carboxylate bridges in 1 0.5CH2Cl2. Each
€
(41) Mukherjee, S.; Weyhermuller, T.; Bothe, E.; Wieghardt, K.;
3
carboxylate bridge joins the basal position of one copper atom with the
apical position of another one. Color code: CuII brown, N blue, O red,
C black.
Chaudhuri, P. Eur. J. Inorg. Chem. 2003, 863–875.
(42) Chen, L.; Breeze, S.; Rousseau, R.; Wang, S.; Thompson, L. Inorg.
Chem. 1995, 34, 454–465.
(43) Breeze, S. R.; Wang, S.; Chen, L. J. Chem. Soc., Dalton Trans. 1996,
1341–1349.
(44) Reim, J.; Griesar, K.; Haase, W.; Krebs, B. J. Chem. Soc., Dalton
Trans. 1995, 2649–2656.
(45) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor,
G. V. J. Chem. Soc., Dalton Trans. 1984, 1349–1356.
bond lengths. A carboxylato bridge joins an apical
binding site of one copper atom with a basal posi-
tion of another copper atom (Figure S4). There are
four such bridges of this type in the [Cu4] molecule.

6582 Inorganic Chemistry, Vol. 49, No. 14, 2010
Sarkar et al.
Table 3. Selected Interatomic Distances (A) and Angles (deg) for 2
Distances
N(3)-Cu(1)
Cu(2)-N(6)
Cu(2)-O(2)
Cu(2)-O(1)
1.972(6)
1.933(7)
1.962(4)
1.963(4)
Cu(2)-N(2)
Cu(1)-O(2)
Cu(1)-N(1)
Cu(1)-O(1)
1.971(6)
1.934(5)
1.951(5)
1.987(4)
Angles
N(4)-N(3)-Cu(1)
N(6)-Cu(2)-O(2)
N(6)-Cu(2)-O(1)
O(2)-Cu(2)-O(1)
N(6)-Cu(2)-N(2)
O(2)-Cu(2)-N(2)
O(1)-Cu(2)-N(2)
O(2)-Cu(1)-N(1)
119.4(5)
95.3(2)
169.8(3)
76.82(18)
95.2(3)
169.2(2)
92.5(2)
165.6(2)
O(2)-Cu(1)-N(3)
N(1)-Cu(1)-N(3)
O(2)-Cu(1)-O(1)
N(1)-Cu(1)-O(1)
N(3)-Cu(1)-O(1)
Cu(2)-O(1)-Cu(1)
Cu(1)-O(2)-Cu(2)
N(7)-N(6)-Cu(2)
96.0(2)
96.5(2)
76.90(18)
92.47(19)
165.2(2)
101.88(18)
103.9(2)
121.5(7)
Figure 5. Viewofthe moleculartetranuclear unit[Cu4(μ3-OH)2(μ-bip)2-
(NCS)4(DMF)2] in 3 2DMF with atom-numbering scheme. H atoms are
3
omitted for clarity. Color code: CuII brown, N blue, O red, S yellow, C
gray.
bridge pentacoordinated pyramidal CuII ions (Cu2) in
the apical directions (Figures S6-S10). The coordination
sphere of Cu2 is very close to the perfect square-pyramidal
geometry (τ = 0.01) induced by the Jahn-Teller effect,
with significant elongation of the apical bonds by 0.4-
0.5 A (Cu2-O2*, 2.491 A). The in-plane Cu-Ohy (hy =
hydroxido) bond distances fall in the 1.926 to 1.958 A
range, which is comparable to those reported for other
μ-OH-bridged tetranuclear complexes.46,47 On the other
hand, Cu1 adopts a square-planar coordination sphere
consisting of one O atom from a bridging hydroxido
anion, one phenoxido O atom, one N atom from the
imine group of the bip- ligand, and one N atom from
an azido anion. In their square-pyramidal Cu2 and
square-planar Cu1 coordination sphere, the Cu ions
are displaced by 0.07 and 0.03 A, respectively, from
the O2N2 mean basal planes. The CdN and C-O dis-
tances of the dialdimine and phenoxide fragments are
consistent with the coordination of these functiona-
lities.48,49 The tetranuclear [Cu2(μ3-OH)(μ-bip)(N3)2]2
complex is centrosymmetric with a stepped cubane or
ladder-type geometry (Figure S7) that results from the
stacking of [Cu2O2] dinuclear cores (O are phenoxido and
hydroxido oxygen atoms). Within this phenoxido-
hydroxido-bridged [Cu2(μ-OH)(μ-bip)(N3)2] fragment,
Table 4. Selected Interatomic Distances (A) and Angles (deg) for 3 2DMF
3
Distances
Cu(1)-O(2)
Cu(1)-N(3)
Cu(1)-N(1)
Cu(1)-O(1)
Cu(1)-O(2)a
Cu(1)-Cu(2)
1.924(2)
1.949(3)
1.965(3)
1.978(2)
2.331(2)
3.0425(6)
Cu(2)-O(2)
Cu(2)-N(4)
Cu(2)-N(2)
Cu(2)-O(1)
Cu(2)-O(3)
O(2)-Cu(1)a
1.927(2)
1.940(3)
1.972(3)
1.975(2)
2.337(6)
2.331(2)
Angles
O(2)-Cu(1)-N(3)
O(2)-Cu(1)-N(1)
N(3)-Cu(1)-N(1)
O(2)-Cu(1)-O(1)
N(3)-Cu(1)-O(1)
N(1)-Cu(1)-O(1)
O(2)-Cu(1)-O(2)a
N(3)-Cu(1)-O(2)a
N(1)-Cu(1)-O(2)a
O(1)-Cu(1)-O(2)a
O(2)-Cu(2)-N(4)
94.82(12) O(2)-Cu(2)-N(2)
167.72(11) N(4)-Cu(2)-N2
96.65(13) O(2)-Cu(2)-O(1)
159.44(11)
97.08(13)
76.69(9)
170.31(12)
92.00(11)
103.14(16)
90.30(19)
94.32(16)
92.35(16)
169.5(4)
76.70(9)
N(4)-Cu(2)-O(1)
163.66(13) N(2)-Cu(2)-O(1)
91.11(11) O(2)-Cu(2)-O(3)
84.90(9)
N(4)-Cu(2)-O(3)
98.51(12) N(2)-Cu(2)-O(3)
97.69(11) O(1)-Cu(2)-O(3)
94.66(9)
C(24)-N(3)-Cu(1)
93.64(12) C(25)-N(4)-Cu(2)
169.0(4)
a = -x, -y, 2-z.
show a nonlinear monodentate binding mode on the
Cu(II) ions, having an average Cu-N-N angle of
∼120ꢀ and Cu-N separations of 1.93-1.97 A. These
Cu-N bond distances are short in comparison to the
basal-apical bridging azides (1.98 and 2.33 A).10 It is
interesting to note that the two outer copper ions (Cu1
the Cu ions are separated by 3.06 A. The Cu Cu
3 3 3
distance (3.31 A) is longer for the bis-hydroxido-bridged
Cu motif, which implies apical positions. As already
mentioned above, the O2 and O2* atoms of HO- anions
act as μ3 bridges forming two equally short bonds to Cu1
and Cu2 atoms, while the Cu-O bond to a third copper
(Cu2*) atom is longer by 0.53 A. Hydrogen atom posi-
tions as well as the temperature displacement parameters
on the hydroxido groups were refined without any re-
straints, indicating that the oxygen atoms O2 and O2* are
protonated. A monodentate azide anion is coordinated to
each copper site. The azido terminal groups are both
almost linear, with an N-N-N angle of ∼175ꢀ, but they
and Cu1*) show intermolecular Cu Naz contacts at a
3 3 3
distance of 2.79 A (Figures S11 and S12), while intramo-
lecular Cu Naz separations are around 2.87 A. These
3 3 3
intercomplex interactions are operative via a μ-1,1-brid-
ging coordination mode of the azide anion (with a
Cu-Naz-Cu angle of 98.99ꢀ, similar to the other azide-
bridged CuII complexes50) and lead to a one-dimensional
assembly of tetranuclear [Cu2(μ3-OH)(μ-bip)(N3)2]2
complexes.
[Cu4(μ3-OH)2(μ-bip)2(NCS)4(DMF)2] (3 2DMF). Com-
pound 3 crystallizes in triclinic space group P1 with
an inversion center at the midpoint of Cu2 Cu2*.
3
(46) Graham, B.; Hearn, M. T. W.; Junk, P. C.; Kepert, C. M.; Mabbs,
F. E.; Moubaraki, B.; Murray, K. S.; Spiccia, L. Inorg. Chem. 2001, 40,
1536–1543.
3 3 3
The molecular structure of 3 is shown in Figure 5, and the
relevant metric parameters are collected in Table 4. The
molecular structures of 2 and 3 are essentially the same
ꢀ
(47) Alzuet, G.; Real, J. A.; Borras, J.; Santiago-Garcıa, R.; Garcıa-
´ ´
Granda, S. Inorg. Chem. 2001, 40, 2420–2423.
(48) Mukhopadhyay, U.; Govindasamy, L.; Ravikumar, K.; Velmurugan,
D.; Ray, D. Inorg. Chem. Commun. 1998, 1, 152–154.
(49) Fondo, M.; Garcia-Deibe, A. M.; Bermejo, M. R.; Sanmartin, J.;
Liamas-Saiz, A. L. J. Chem. Soc., Dalton Trans. 2002, 4746–4750.
(50) Mondal, K. C.; Mukherjee, P. S. Inorg. Chem. 2008, 47, 4215–4225.

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6583
with the exception that all CuII metal ions are pentacoordi-
nated with square-pyramid geometry in 3 owing to the
Jahn-Teller effect, with significant elongation of the
apical bonds by 0.20-0.30 A. Compound 3 differs also
from 2 due to the presence of terminal NCS- groups
-
(instead of N3 anions in 2) and DMF molecules present
in the crystal packing and also on the apical position of
the Cu2 site. These differences induce only slight geome-
trical changes, while the overall structure of the complex
is preserved (Figure S13). The equatorial positions of
both copper sites, Cu1 and Cu2, are occupied by the imine
N1 (for Cu1) or N2 (for Cu2) and phenoxido O1 atoms
from the bip- ligand, a hydroxido O2 group, and N3
(for Cu1) or N4 (for Cu2) from NCS- anions. The
Nim-Cu-Nth (th = thiocyanate) and Nim-Cu-Oph bite
angles average 96.86ꢀ and 92.76ꢀ, respectively. The
Cu-Ohy basal distances fall in the 1.924-1.927 A range,
which is comparable to bond distances reported for
other μ-hydroxido-bridged tetranuclear complexes.46,47
The apical positions of the pentacoordinated Cu1 and
Cu2 metal ions are occupied by hydroxido O2* and
DMF oxygen, O3, atoms at Cu-O distances of 2.332 and
2.337 A, respectively. The Cu(II) metal ions are displaced by
0.14 and 0.16 A, respectively, from the O2N2 mean basal
planes. The Cu-Ohy apical distance (2.33 A) is significantly
shorter in 3 than in 2 (2.49 A). The phenoxido- and
Figure 6. Temperature dependence of χT product (χ being the molar
magnetic susceptibility defined as M/H) for 1 0.5CH2Cl2 (in black), 2 (in
3
red), and 3 2DMF (in blue) at 1000 Oe. The solid lines are the best fits
3
obtained using the isotropic Heisenberg model described in the text with
two isolated dinuclear S = 1/2 units.
substituted during the reaction by N3- and NCS- groups,
which coordinate in a monodentate fashion the Cu sites by
the N ends of these incoming groups. During this substitu-
tion the μ4-O central atom of 1 is converted to two HO-
bridges most probably from the attack of solvent water
molecules or moisture from the air (eq 4). The irreversible
conversions of 1 to 2 and 3 are due to a better coordina-
tion ability of pseudo-halide anions in monodentate
mode compared to the syn-syn basal-apical bridg-
hydroxido-bridged Cu atoms (Cu1 Cu2) are separated
3 3 3
-
ing mode of the benzoate groups. Thus these two N3
by 3.04 A with Cu-O-Cu angles of 104.38ꢀ and 100.65ꢀ,
and NCS- groups replace benzoates spontaneously.
Replacement of the four benzoates by the same numbers
whereas bis-hydroxido-bridged Cu atoms (Cu2 Cu2*)
show a longer Cu Cu distance of 3.15 A with a
3 3 3
3 3 3
of N3 or NCS- anions retains the electroneutrality
-
Cu-O-Cu angle of 95.08ꢀ. In 3, a thiocyanate anion is
coordinating each Cu site in a quasi-linear fashion with an
average Cu-Nth-C angle of ∼169.23ꢀ and Cu-Nth separa-
tions of 1.940-1.950 A. The thiocyanato groups are them-
selves almost linear, with an average N-C-S angle of
∼178ꢀ. Unlike compound 2, the presence of a DMF solvent
molecule in the apical position of the outer copper ions,
of the resulting complexes with concomitant oxido to
bis-hydroxido bridge conversion.
Water-Assisted Mechanochemical Core Conversions.
Intimate mixing and grinding of 1 with NaN3 or
NH4SCN in an agate mortar (as normally done for
preparing samples for IR spectroscopy) and thorough
washing with water (that removes sodium benzoate from
the products) led to the quantitative transformations to 2
and 3, as identified by FTIR measurement, solubility
differences, and determinations of unit cells of grown
single crystals from powder samples (the resulting IR
spectra and cell parameters are given in the Supporting
Information). This synthetic mechanochemical method is
an interesting alternative process to the traditional one
performed in solution. It allows a rapid test of possible
complex core conversions in a given metal-ligand sys-
tem. In the present case, this approach is useful to study
the stability of a [Cu4] stepped cubane compound with-
out the presence of multiple species in equilibrium in
traditional solution-based syntheses. The PXRD data
have been taken to identify the phase change from 1 to
2 or 3 core conversions. The PXRD patterns of the
three complexes are shown in Figure S17 (Supporting
Information). These patterns are consistent with those
of simulated ones obtained from the single-crystal X-ray
diffraction data.
Cu2, prevents any kind of short intermolecular Cu Sth
3 3 3
contacts between neighboring molecules (Figure S14).
Nevertheless, two intermolecular hydrogen bonds are ob-
served with O O distances of 2.745 A between oxygen
3 3 3
atoms of interstitial DMF molecules and the hydrogen
atoms of hydroxido groups (Figure S15).
Role of PhCO2- and N3- or NCS- Anions for Different
Molecular Assemblies. The well-established μ1,3 bridging
mode of PhCO2- groups helps the trapping of the oxido
group within the [Cu4O] tetrahedron in 1 by maintaining
the required Cu Cu separation to attract a μ4-O atom.
3 3 3
In the case of 2 and 3, a μ1,3 bridging mode is not possible
for either N3- or NCS-. As a result, the reaction of 1 with
N3 or NCS- substitutes all carboxylate groups and
-
converts the μ4-oxido core into a bis-hydroxido core with
the binding of the pseudo-halide ions in monodentate
fashions. The N3- or NCS- anions replace efficiently the
benzoate groups and cleave the tetrahedron [Cu4O] core
into two phenoxido-bridged [Cu2] fragments, which
ultimately reassemble around two hydroxido groups.
-
Binding of N3 and NCS- in [Cu4] Stepped Cubane
Magnetic Properties. The solid-state magnetic proper-
ties of 1 0.5CH2Cl2, 2, and 3 2DMF have been investi-
-
Complexes. The binding of N3 and NCS- anions to
3
3
the CuII centers of complex 1 in 1:1 fashion that yields
new neutral complexes 2 and 3 is reminiscent of the N2O
binding via N in CuII-based nitrous oxide reductase.
The bridging benzoate anions originally present in 1 are
gated by dc susceptibility measurements down to 1.8 K at
0.1 T (Figure 6). At room temperature, the χT products
are 0.59 cm3 K mol-1, 0.29 cm3 K mol-1, and 0.41 cm3
K mol-1 for 1, 2, and 3, respectively. The values of

6584 Inorganic Chemistry, Vol. 49, No. 14, 2010
Sarkar et al.
χT are far from the theoretical value of 1.5 cm3 K mol-1
(with g = 2) expected for four isolated paramagnetic CuII
ions (d9, S = 1/2), indicating strong and dominant anti-
ferromagnetic interactions between the CuII ions in the
three compounds. Upon cooling, both χT products conti-
nuously decrease to reach a value close to zero below 70,
120, and 100 K, for 1, 2, and 3, respectively (Figure 6), as
expected when intracomplex antiferromagnetic interac-
tions induce a singlet spin ground state.
Table 5. Parameters Deduced from the Analysis of the Magnetic Properties for 1,
2, and 3
g
J/kB (K)
F (%)
1
2
3
2.0(1)
2.2(1)
2.2(1)
-289(4)
-464(5)
-405(15)
2
0
2
seen for materials displaying a diamagnetic ground
state. Taking into account that the impurity possesses a
Curie paramagnetic behavior and the same molecular
weight as the major complex, the amount is about 2% for
1 and 3.
Generally speaking for the CuII ions, the magnetic
interactions mediated through their apical positions are
much weaker than those mediated by bridges coordinated
on the equatorial sites. Therefore, if one considers only the
magnetic pathways that involve equatorial bonds around
each CuII ion, the complexes 2 and 3 can be viewed as two
almost isolated Cu1-Cu2 dinuclear units with a double
phenoxido-hydroxido bridge (Figures 4 and 5). For 1,
the situation is slightly different due to the presence of the
central μ4-oxido bridge between Cu(II) ions. Neverthe-
less, the magnetic pathway through the double μ4-oxido/
phenoxido bridges is likely much more efficient than a
single μ4-oxido bridge between two Cu(II) metal ions. As
a consequence, the magnetic description of 1 is similar to 2
and 3 (vide supra) with two isolated Cu1-Cu2 and
Cu3-Cu4 dinuclear units with a double phenoxido-
oxido bridge (Figures 2 and 3). Moreover, considering
the similarity of the bridging modes in the two dinuclear
Cu1-Cu2 and Cu3-Cu4 units in 1 (Table 2), the inter-
action in the two spin dimers is expected to be similar.
Considering these magnetostructural remarks, the mag-
netic properties of the three compounds can be analyzed
in a first approximation with the same magnetic model
including two identical S = 1/2 spin dimers with only one
exchange parameter, noted J. The magnetic susceptibility
has been thus calculated from the following isotropic spin
Heisenberg Hamiltonian:
It is worth mentioning that for 1 the magnetic proper-
ties have also been analyzed with an alternative model
including two different intramolecular interactions, one
mediated by the double oxido/phenoxido bridges and the
other one mediated by single μ4-oxido bridges.54 Indeed
as soon as two interactions are considered in the model,
more than one set of parameters can reproduce well the
experimental data, suggesting an overparametrization in
the fitting procedure. Therefore the only relevant model
requires no more than one interaction parameter, as
shown in Figure 6, and thus it is not possible to evaluate
the magnetic interaction through the single μ4-oxido
bridge. The amplitude of the antiferromagnetic ex-
change parameter is relatively large for the three com-
pounds but stays in the range of magnitude observed
in similar compounds with same type of double oxido-
phenoxido bridges.54,55 These results confirm the efficiency
of this type of double oxido-phenoxido or hydroxido-
phenoxido bridge to mediate strong antiferromagnetic
couplings.
Conclusions
The coordination chemistry of the bip- phenolate-cen-
tered ligand in presence of Cu(II) metal ions has been
studied in this work and confirms the key role of ancillary
and nucleating groups in the synthesis of new [Cu4]
assemblies. The type of bridging co-ligands (O2- or HO-)
seems to dictate the final topologies for the resulting
complexes. Three tetranuclear copper complexes were
investigated by X-ray diffraction on single crystals and
magnetic susceptibility measurements. The dinucleating
ligand bip- facilitates the assembly of two CuII pairs in two
different topologies, e.g., tetrahedral and stepped cubanes,
leaving three coordinating sites open per metal for binding
exogenous ligands. The binding of ancillary ligands such
H ¼ - 2JfSCu,1 SCu,2
g
3
where Si are the spin operators for each center with S = 1/2.
The application of the van Vleck equation51,52 allows the
determination of the low-field analytical expression of the
magnetic susceptibility53 taking into account the presence of
residual paramagnetic contribution:
2Nμ2Bg2Cu
kB
1
Nμ2Bg2Cu
þ F
2kB
χT ¼ 2ð1 - FÞ
3 þ e- 2J=k T
B
as N3 and NCS- induces the conversion of the tetra-
-
hedral [Cu4(μ4-O)] core observed in 1 into stepped cubane
[Cu4(μ3-OH)2] units in 2 and 3. Our current work is dedi-
cated to the use of different external bridges such as sulfide,
peroxide, and bisulfide in this reaction system to induce
the formation of other types of homo- and heterometallic
complexes.
As shown in Figure 6, the above expression of the
magnetic susceptibility reproduces almost perfectly the
experimental data at 1000 Oe. The best sets of parameters
obtained are given in Table 5 for the three compounds.
The negative sign of the magnetic interactions implies
that these CuII dimer units and thus the tetranuclear
complexes possess an ST = 0 spin ground state, i.e.,
diamagnetic ground state. The F values indicate that a
very small amount of residual paramagnetic contribu-
tion is present in the measured samples of 1 and 3, as often
Acknowledgment. M.S. is thankful to the Council of
Scientific and Industrial Research, New Delhi, for the
(54) Hall, J. W.; Estes, W. E.; Estes, E. D.; Scaringe, R. P.; Hatfield, W. E.
Inorg. Chem. 1977, 16, 1572–1574.
(55) Adams, A.; Fenton, D. E.; Haque, S. R.; Heath, S.; Ohba, M.;
Okawa, H.; Spey, S. E. Dalton Trans. 2000, 1849–1856.
(56) Banerjee, A.; Sarkar, S.; Chopra, D.; Colacio, E.; Krishna, K. R.
Inorg. Chem. 2008, 47, 4023–4031.
(51) van Vleck, J. H. The Theory of Electric and Magnetic Susceptibility;
Oxford University Press, 1932.
(52) Kambe, K. J. Phys. Soc. Jpn. 1950, 5, 48–51.
(53) O’Connor, C. J. Prog. Inorg. Chem. 1982, 29, 203–283.

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6585
financial support. V.B. acknowledges Italian Ministry of
University and Scientific Research (MIUR, Rome). R.C.,
C.M., and N.H. thank MAGMANet (Grant NMP3-CT-
ꢀ
2005-515767), the Conseil Regional d’Aquitaine, GIS
Advanced Materials in Aquitaine (COMET Project), the
Science and Engineering Council (NSERC) of Canada
for a year of postdoctoral fellowship.
Supporting Information Available: X-ray crystallographic file
in CIF format, Schemes S1 to S2, Figures S1 to S17, and
synthesis and characterization of ligand Hbip. This material is
available free of charge via the Internet at http://pubs.acs.org.
ꢀ
Universite de Bordeaux, the CNRS, and the Natural