New phenoxido-bridged quasi-tetrahedral and rhomboidal [Cu4] compounds bearing μ4-oxido or μ1,1-azido ligands: Synthesis, chemical reactivity, and magnetic studies

Article abstract of DOI:10.1021/ic102226j

[Cu₂(μ₄-O)Cu₂] and [Cu ₂(μ_1,1-N₃)₄Cu₂] geometrical arrangements are found in a new family of tetranuclear copper(II) complexes: [Cu₄(μ₄-O)(μ-cip)

Full text of DOI:10.1021/ic102226j

ARTICLE
pubs.acs.org/IC
New Phenoxido-Bridged Quasi-Tetrahedral and Rhomboidal [Cu4]
Compounds Bearing μ4-Oxido or μ1,1-Azido Ligands:
Synthesis, Chemical Reactivity, and Magnetic Studies
Mrinal Sarkar,† Rodolphe Clꢀerac,*,‡,§ Corine Mathoniꢁere,^ 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 “Matꢀeriaux Molꢀeculaires Magnꢀetiques”,
115 avenue du Dr. Albert Schweitzer, Pessac F-33600, France
§Universitꢀe de Bordeaux, UPR 8641, Pessac F-33600, France
^CNRS, Universitꢀe de Bordeaux, ICMCB, 87 avenue du Dr. A. Schweitzer, Pessac F-33608, France
Dipartimento di Chimica e Centro di Strutturistica Diffrattometica, Universitꢁa di Ferrara, Via Borsari 46, 44100 Ferrara, Italy
S Supporting Information
b
ABSTRACT: [Cu2(μ4-O)Cu2] and [Cu2(μ1,1-N3)4Cu2] geo-
metrical arrangements are found in a new family of tetranuclear
copper(II) complexes: [Cu4(μ4-O)(μ-cip)2Cl4] (1), [Cu4(μ4-O)-
(μ-cip)2(μ1,3-O2CPh)4] 2CH3OH (2 2CH3OH), and
3
3
[Cu4(μ1,1-N3)4(μ-cip)2(N3)2] DMF (3 DMF) [Hcip = 2,6-
3
3
bis(cyclohexyliminomethylene)-4-methylphenol; CH3OH =
methanol; DMF = dimethylformamide]. These complexes have
been characterized by X-ray crystallography, and their magnetic properties have been studied. 1 and 2 form quasi-tetrahedral
[Cu4(μ4-O)] complexes, and 3 is the first example of a rhomboidal [Cu4(μ1,1-N3)] compound. Formation of the [Cu4] compounds
is achieved via ligand-exchange reactions. The relative binding strength of the three ancillary ligands as N3ꢀ > PhCO2ꢀ > Clꢀ has
been demonstrated from the core-conversion and peripheral ligand-exchange reactions. For the three complexes, the magnetic
susceptibility measurements in the range of 1.8ꢀ300 K have been performed and modeled using two isolated S = 1/2 dimers based
on the spin Hamiltonian H = ꢀ2J{SCu,1 Cu,2} with J/kB = ꢀ513, ꢀ340, and ꢀ315 K for 1ꢀ3, respectively (where J is the exchange
S
3
constant through the oxidoꢀphenoxido and azidoꢀphenoxido bridges, respectively).
’ INTRODUCTION
the apical position and the ability of the phenoxido ligands to
bridge metal ions make the association of dinuclear [Cu2]
moieties into discrete oligo- and polynuclear complexes possible.
The assembly of two dinuclear [Cu2] units depends on the
number and nature of secondary bridging or coordinating groups
present in various reactions in such a way that four types of [Cu4]
aggregates can be obtained with cubane, tetrahedron, stepped-
cubane, rhomboid, and double-cubane geometry. Out of these
various aggregation possibilities, cubane, stepped-cubane, and
double-cubane assemblies were not formed in the present work
(Scheme 1). Among the ancillary ligands used, azido and
carboxylato groups have been employed because of their versatile
bridging modes (Scheme 2). The carboxylato group can adopt
numerous bridging conformations like the synꢀsyn, synꢀanti,
antiꢀanti, and μ3-1,1,3-modes, and depending upon the two
binding positions (basal or apical) around CuII ions, these
bridging modes can mediate a large variety of exchange interac-
tions. Quite generally, synꢀsyn and synꢀanti configurations favor
During the last 2 decades, the synthesis and characterization of
polynuclear transition-metal aggregates have received consider-
able interest because of their importance for examples as mag-
netic materials, bioinorganic model compounds, and catalysts.1ꢀ3
The self-assembly of small building units with coligands can be
used to synthesize polynuclear compounds with controlled
aggregation. The strong and highly directional metalꢀligand
interactions can effectively be used in lieu of several weak
interactions (like hydrogen-bonding and πꢀπ interactions) to
direct the formation of polynuclear coordination compounds.
The role of the ligand geometry and the tuning of the reaction
conditions with transition-metal ions are crucial to identify and
direct the aggregation process. Phenoxido-bridged [Cu2] frag-
ments have quite often been studied as chemical models for
magnetic exchange interactions4ꢀ6 and as building units for the
construction of tetra- and polynuclear complexes exhibiting
interesting magnetic properties.7ꢀ12 Recently, our research work
has been devoted to the preparation of a new family of [Cu4]
aggregates based on the phenoxido-bridged [Cu2] units. The
tendency of the CuII ions to accept a fifth coordinating group in
Received: November 12, 2010
Published: April 04, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
ARTICLE
Scheme 1. Different Tetranuclear Copper Assemblies Depending on the Precursors and Ancillary Ligands
Scheme 2. Different Bridging Modes of Azido and Carbox-
Scheme 3. Hcip and Related H3bemp Ligands
ylato Anions
cyclohexylamine from SRL (India) and sodium benzoate from SD Fine
Chemicals (India). All other chemicals and solvents were reagent-grade
materials and were used as received without further purification.
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.
Syntheses. Hcip Ligand. The Hcip Schiff-base ligand was prepared
from the single-step condensation of 2,6-diformyl-4-methylphenol
(1.640 g, 10 mmol) and cyclohexylamine (2.29 mL, 20 mmol) in
methanol (40 mL) under reflux for 1 h as reported previously.17
assemblies with antiferromagnetic interactions, while antiꢀanti
mode induces medium-to-weak antiferromagnetic or ferromag-
netic interactions.13ꢀ15 As a result of extensive research over the
last 2 decades, the superexchange mechanisms through various
bridging modes of azide are now well established. For example,
symmetric μ-1,3-copper(II) azide complexes are strongly anti-
ferromagnetic, whereas copper(II) complexes with double-sym-
metric μ-1,1 azide bridges are strongly ferromagnetic, provided that
the CuꢀNazideꢀCu angle is less than 108ꢀ. Usually asymmetric μ-
1,3 azido bridges lead to weak antiferromagnetic coupling, whereas
asymmetric μ-1,1 azide bridges propagate weak-to-moderately
strong ferromagnetic or antiferromagnetic interactions.16 In trying
to prepare and study new types of [Cu4] aggregates, we have been
interested in exploring the reactivity of the tetradentate phenoxide
Schiff-base ligand 2,6-(cyclohexyliminomethylene)-4-methylphe-
[Cu4(μ4-O)(μ-cip)2Cl4] (1). Cu(NO3)2 3H2O (0.483 g, 2.00 mmol)
3
dissolved in methanol (25 mL) was added dropwise with stirring to a
yellow CH3OH solution (15 mL) of Hcip (0.326 g, 1.00 mmol). The
brown solution, formed initially, changes to green in about 5 min. After
about 10 min of stirring, solid KCl (0.298 g, 4.0 mmol) was added to the
reaction. The green solution was further stirred for 1 h, and a green
precipitate appeared. The obtained green precipitate was collected by
filtration, washed with cold methanol followed by water, and dried under
vacuum over P4O10. Green single crystals suitable for X-ray analysis were
obtained from a CH2Cl2ꢀCH3OH solvent mixture after 5 days. Yield:
0.398 g, 75%. Anal. Calcd for C42H58N4O3Cl4Cu4 (1062.94 g molꢀ1):
C, 47.45; H, 5.49; N, 5.27. Found: C, 47.12; H, 5.32; N, 5.01. Selected
FTIR bands (KBr, cmꢀ1; s = strong, m = medium, and br = broad): 3447
(br), 1633 (s), 1559 (s), 1458 (m), 1348 (m), 1038 (m), 622 (m), 564
(m), 502 (m). Molar conductance, ΛM (CH2Cl2 solution): 3.0 Ωꢀ1 cm2
molꢀ1. UVꢀvis spectra [λmax, nm (ε, L molꢀ1 cmꢀ1)] (CH2Cl2
solution): 728 (688), 373 (5879), 256 (21 632).
nol, abbreviated Hcip (Scheme 3),17 with Cu(NO3)2 3H2O in
3
the presence of additional ligands like Clꢀ, PhCO2ꢀ, and N3ꢀ. It is
worth noting that the use of one closely related ligand of Hcip,
named H3bemp (Scheme 3, right, 2,6-bis[(2-hydro-
xyethylimino)methyl]-4-methylphenol), led to interesting systems,
consisting of [Co4],18 [Ni6],19 and [Cu18]20 complexes and a
heterobimetallic [Mn6Cu10]21 aggregate. Herein, three new [Cu4]
complexes based on the cipꢀ anionic ligand are reported: 1 and
2 2CH3OH, both containing a central tetrahedral μ4-oxido anion
3
to stabilize tetrahedral complexes and a bridged bis-N3 rhomboid
compound, 3 DMF. These complexes have been isolated and
3
[Cu4(μ4-O)(μ-cip)2(μ1,3-O2CPh)4] 2CH3OH (2 2CH3OH)
3
3
crystallographically characterized, and their magnetic properties
have been studied.
Method A via a Direct Route. Compound 2 was prepared by the
same method as that used for 1, only replacing KCl with PhCO2Na.
Green single crystals suitable for X-ray analysis were obtained from a
CH2Cl2ꢀCH3OH solvent mixture after 7 days. Yield: 0.534 g, 76%.
Anal. Calcd for C72H86N4O13Cu4 (1469.67 g molꢀ1): C, 58.84; H, 5.89;
N, 3.81. Found: C, 58.61; H, 5.78; N, 3.67. Selected FTIR bands
(KBr, cmꢀ1; vs = very strong, s = strong, m = medium, and br = broad):
’ EXPERIMENTAL SECTION
Materials. The chemicals used were obtained from the following
sources: copper nitrate trihydrate, potassium chloride, sodium azide, and
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Inorganic Chemistry
ARTICLE
3430 (br), 1624 (s), 1606 (s), 1569 (s), 1460 (m), 1372 (vs), 1063 (m),
717 (s), 678 (m), 625 (m), 503 (m). Molar conductance, ΛM (CH2Cl2
solution): 3.0 Ωꢀ1 cm2 molꢀ1. UVꢀvis spectra [λmax, nm (ε, L
molꢀ1cmꢀ1)] (CH2Cl2 solution): 681 (241), 378 (5033), 353
(3682), 248 (21 613).
diffractometer with Cu KR radiation. The magnetic susceptibility
measurements were obtained with the use of a Quantum Design
MPMS-XL magnetometer housed at the Centre de Recherche Paul
Pascal. This magnetometer works between 1.8 and 300 K for direct-
current (dc) applied fields ranging from ꢀ7 to 7 T. Measurements were
performed on polycrystalline samples of 14.34, 10.26, and 30.82 mg for
1, 2 2CH3OH, and 3 DMF, respectively. The magnetic data were
Method B via Anion Exchange of 1. The green suspension of 1
(0.265 g, 0.25 mmol) in CH3OH (25 mL) was stirred for about 5 min
before the addition of PhCO2Na (0.144 g, 1.0 mmol) at ambient
temperature. During the reaction, noticeable changes were observed
from a green suspension to a green solution followed by the appearance
of a green precipitate, which was collected by filtration, washed with cold
methanol, and dried under vacuum over P4O10. Yield: 0.224 g, 64%.
Anal. Calcd for C72H86N4O13Cu4 (1469.67 g molꢀ1): C, 58.84; H, 5.89;
N, 3.81. Found: C, 58.46; H, 5.73; N, 3.64. Selected FTIR bands
(KBr, cmꢀ1; s = strong, m = medium, and br = broad): 3447 (br), 1624
(s), 1605 (s), 1570 (s), 1460 (m), 1371 (vs), 1063 (m), 717 (s), 678
(m), 625 (m), 504 (m). Molar conductance, ΛM (CH2Cl2 solution): 3.0
Ωꢀ1 cm2 molꢀ1. UVꢀvis spectra [λmax, nm (ε, L molꢀ1 cmꢀ1)]
(CH2Cl2 solution): 681 (211), 378 (4406), 353 (3223), 248 (18 922).
[Cu4(μ1,1-N3)4(μ-cip)2(N3)2] DMF (3 DMF)
3
3
corrected for the sample holder and the diamagnetic contributions.
Crystal Data Collection and Refinement for 1, 2 2CH3OH, and
3
3 DMF. Single-crystal data of complexes 1, 2 2CH3OH, and 3 DMF
3
3
3
were collected on a Bruker APEX-2 CCD X-ray diffractometer using
graphite-monochromated Mo KR radiation (λ = 0.7107 Å). Data
were collected at 293 K. The refinement was performed anisotropi-
cally using full-matrix least squares with all non-H atoms. The H
atoms were included on calculated positions, riding on their carrier
atoms. The hydroxy H atom O2ꢀH was found in the difference
Fourier maps and refined with a OꢀH fixed distance of 0.90 Å. The
crystal parameters and other experimental details of data collection
are summarized in Table 1.
3
3
’ RESULTS AND DISCUSSION
Method A via a Direct Route. Compound 3 was prepared by the
same method as that used for 1, only replacing KCl with NaN3. The
obtained brown precipitate was collected by filtration, washed with cold
methanol followed by water, and dried under vacuum over P4O10.
Brown single crystals suitable for X-ray analysis were obtained from
DMF after 5 days. Yield: 0.416 g, 72%. Anal. Calcd for C45H65N23O3Cu4
(1230.35 g molꢀ1): C, 43.93; H, 5.32; N, 26.18. Found: C, 43.89; H,
5.01; N, 25.86. Selected FTIR bands (KBr, cmꢀ1, vs = very strong,
s = strong, m = medium, and w = weak): 2929 (vs), 2854 (vs), 2084 (vs),
2045 (vs), 1638 (vs), 1563 (vs), 1342 (s), 1041 (m), 829 (m), 765 (w).
Molar conductance, ΛM (DMF solution): 6 Ωꢀ1 cm2 molꢀ1. UVꢀvis
spectra [λmax, nm (ε, L molꢀ1 cmꢀ1)] (DMF solution): 695 (567), 380
(15 950), 301 (9540), 273 (19 240).
Synthetic Considerations. The Schiff-base ligand 2,6-bis-
(cyclohexyliminomethylene)-4-methylphenol (Hcip) was pre-
pared (Scheme S1 in the Supporting Information) following a
literature procedure,17 and its reactions with copper(II) salts
have been systematically investigated, as summarized in
Scheme 4. When the reaction of Cu(NO3)2 3H2O was carried
3
out with Hcip in CH3OH in the presence of KCl and in the
absence of base, 1 was obtained (Scheme 4). Several Cu/Hcip
ratios were explored, and we report here only the optimized ones
that gave clean and characterizable products in high yield. At
room temperature, complex 1 is easily isolated by stirring a
Method B from 1. The green solution of 1 (0.265 g, 0.25 mmol) in
CH2Cl2 (25 mL) was stirred for about 5 min followed by a dropwise
addition of NaN3 (0.097 g, 1.50 mmol) dissolved in CH3OH (20 mL) at
ambient temperature. During the reaction, the solution changed color
from green to brown before the appearance of a brown precipitate, which
was collected by filtration, washed with cold methanol, and dried under
vacuum over P4O10. Yield: 0.196 g, 68%. Anal. Calcd for C45H65N23O3-
Cu4 (1230.35 g molꢀ1): C, 43.93; H, 5.32; N, 26.18. Found: C, 43.12; H,
4.97; N, 25.64. Selected FTIR bands (KBr, cmꢀ1, vs = very strong,
s = strong, m = medium, and w = weak): 2929 (vs), 2854 (vs), 2082 (vs),
2055 (vs), 1639 (vs), 1564 (vs), 1342 (s), 1042 (m), 829 (m), 766 (w).
Molar conductance, ΛM (DMF solution): 6 Ωꢀ1 cm2 molꢀ1. UVꢀvis
spectra [λmax, nm (ε, L molꢀ1cmꢀ1)] (DMF solution): 695 (583), 380
(16 400), 301 (9809), 273 (19 783).
methanolic solution of Cu(NO3)2 3H2O, KCl, and Hcip in a
3
2:2:1 molar ratio for 1 h. The complex precipitates directly from
the reaction mixture as a green solid in ∼75% yield. The
preparation of 1 is summarized by eq 1, accounting for the
formation of the oxido bridge from a water molecule. It is worth
mentioning that the use of CuCl2 2H2O instead of Cu-
3
(NO3)2 3H2O leads also directly to the synthesis of 1.
3
2Hcip þ 4CuðNO3Þ2 3 3H2O þ 4KC1 f
½Cu4ðμ4-OÞðμ-cipÞ2Cl4ꢁ þ 4KNO3 þ 4HNO3 þ 11H2O
ð1Þ
The elemental analysis and molar conductivity data are con-
sistent with the formula [Cu4(μ4-O)(μ-cip)2Cl4] for 1. How-
ever, no sign of the formation of phenoxido-bridged [Cu2]
dinuclear species was observed because of the better stability of
1, which crystallizes with a central μ4-oxido group to assemble a
pair of [Cu2] units (vide infra). This synthetic procedure was
further explored with NaO2CPh and NaN3 successively in place
of KCl. Green crystals of 2 and brown crystals of 3 (Scheme 4)
were directly synthesized in ∼70% yield in CH3OH by stirring
Method C from 2. Compound 3 can also be prepared from 2 using
method B but replacing complex 1 by 2. Yield: 0.190 g, 66%. Anal. Calcd
for C45H65N23O3Cu4 (1230.35 g molꢀ1): C, 43.93; H, 5.32; N, 26.18.
Found: C, 43.02; H, 5.11; N, 25.76. Selected FTIR bands (KBr, cmꢀ1
,
vs = very strong, s = strong, m = medium, w = weak): 2928 (vs), 2854 (vs),
2082 (vs), 2055 (vs), 1639 (vs), 1563 (vs), 1342 (s), 1042 (m), 829 (m),
765 (w). Molar conductance, ΛM (DMF solution): 6 Ωꢀ1 cm2 molꢀ1
.
UVꢀvis spectra [λmax, nm (ε, L molꢀ1 cmꢀ1)] (DMF solution): 695
(533), 380 (14 993), 301 (8968), 273 (18 086).
reaction solutions of Cu(NO3)2 3H2O, Hcip, and PhCO2Na or
Physical Measurements. Elemental analyses (C, H, and N) were
performed with a Perkin-Elmer model 240C elemental analyzer. Fourier
transform infrared (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 10ꢀ3 M and a Shimadzu UV 3100
UVꢀvisꢀnear-IR spectrophotometer, respectively. The powder X-ray
diffraction (PXRD) patterns were obtained using a Philips PW 1710
3
NaN3 in a 2:1:2 or 2:1:3 molar ratio for 1 h under aerobic
conditions at room temperature. For 2, the same reactions in the
presence of a stoichiometric addition of either NEt3 or NaOH
gave the same product, confirming the formation of hydroxido
bridges independently of the type and nature of the base. The
direct syntheses of 2 and 3 from Hcip are summarized in eqs 2
and 3. It is interesting to note that 2 can also be obtained from the
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Inorganic Chemistry
ARTICLE
Table 1. Crystallographic Data for 1, 2 2CH3OH, and 3 DMF
3
3
compound
formula
1
2 2CH3OH
3 DMF
3
3
C42H58N4O3Cl4Cu4
C72H86N4O13Cu4
C45H65N23O3Cu4
1230.35
C2/c
M
1062.94
1469.67
space group
cryst syst
a/Å
I41/a
C2/c
tetragonal
monoclinic
monoclinic
17.6314(10)
16.5650(10)
19.2469(11)
90.0
12.6302(5)
15.0640(19)
b/Å
12.6302(5)
35.820(5)
c/Å
28.8439(12)
13.1134(17)
R/deg
90.0
90.0
β/deg
90.0
103.036(4)
103.887(2)
90.0
γ/deg
90.0
90.0
V/Å3
4601.2(3)
6893.5(15)
5457.0(5)
293
T/K
293
293
Z
4
4
4
Dc/g cmꢀ3
1.534
1.408
1.498
F(000)
2184
3032
2544
μ(Mo KR)/cmꢀ1
measd reflns
unique reflns
Rint
20.97
12.82
16.00
28 248
49 833
37 353
4129
9743
6493
0.0325
0.0470
0.0503
obsd reflns [I g 2σ(I)]
θminꢀθmax/deg
h; k; l ranges
R(F2) (obsd reflns)
wR(F2) (all reflns)
no. of variables
GOF
2702
6754
4523
1.76ꢀ32.88
1.14ꢀ29.74
1.71ꢀ28.00
ꢀ15, þ17; ꢀ16, þ19; ꢀ42, þ44
ꢀ20, þ20; ꢀ49, þ41; ꢀ18, þ17
ꢀ23, þ23; ꢀ21, þ21; ꢀ24, þ23
0.0488
0.0589
0.0430
0.2282
0.1805
0.1186
131
424
337
0.844
1.078
1.007
ΔFmax; ΔFmin (e Åꢀ3
)
1.028; ꢀ0.943
2.531; ꢀ0.566
0.559; ꢀ0.370
Cu(O2CPh)2 H2O precursor.
Ligand-Exchange Reactivity Studies on Complex 1. As
summarized by eq 4, 2 can be easily obtained by reacting 1 with
sodium benzoate in methanol. Treatment of a green suspension
of 1 in CH3OH by sodium benzoate shifts the equilibrium in
favor of 2 with noticeable dissolution of the suspension to a clear
green solution followed by precipitation of 2 within 5 min. The
precipitate was filtered, washed with CH3OH, dried, and dis-
solved in CH2Cl2 for crystallization and layered with CH3OH.
Green crystals of 2 grew slowly over several days and were
isolated by filtration.
3
2Hcip þ 4CuðNO3Þ2 3H2O þ 4PhCO2Na f
3
½Cu4ðμ4-OÞðμ-cipÞ2ðμ-O2CPhÞ4ꢁ þ 4HNO3 þ 4NaNO3 þ 11H2O
ð2Þ
2Hcip þ 4CuðNO3Þ2 3H2O þ 6NaN3 f
3
½Cu4ðμ-N3Þ4ðμ-cipÞ2ðN3Þ2ꢁ þ 2HNO3 þ 6NaNO3 þ 12H2O
ð3Þ
½Cu4ðμ4-OÞðμ-cipÞ2Cl4ꢁ þ 4PhCO2Na f
The elemental analysis and molar conductivity data confirm
the respective formula of the two compounds. The nature of the
final complex is greatly influenced by the presence of the ligands,
solvent, and sequence of the addition of the reactants. Weakly
coordinating anions such as Clꢀ and PhCO2ꢀ in both nonbrid-
ging and bridging modes support the μ4-O-bridged cores in 1 and
2, respectively. As for 1, the formation of dinuclear phenoxido-
bridged [Cu2] species was not observed, suggesting a better
stability of the μ4-O- and μ-N3-bridged [Cu4] complexes. As
illustrated by the synthesis of 1ꢀ3, reactions between the Hcip
ligand and copper(II) salts lead to different self-assemblies
depending on the nature of the additional ligands. Therefore,
the formation of these three [Cu4] complexes can be directꢀly
½Cu4ðμ4-OÞðμ-cipÞ2ðμ-O2CPhÞ4ꢁ þ 4NaCl
ð4Þ
The mechanism of the conversion of 1 to 2 is relatively
straightforward. During this reaction, terminal chlorides are
exchanged by bridging bidentate benzoates in the equatorial
and apical sites of the four CuII centers without affecting the
central μ4-oxido core structure. Concomitantly, the four CuII
ions switch from a square-planar to a square-pyramidal geometry.
Transformation of the [Cu4(μ4-O)] Tetrahedral Species to a
[Cu4(μ-N3)4] Rhomboidal Complex. The strength and nature ꢀof
the binding ability of the three ancillary ligands Clꢀ, PhCO2
,
and N3ꢀ together with the plasticity of the copper coordination
geometries can be discussed considering the three compounds
and their interconversions. The replacement of both oxido and
chlorido groups or both oxido and benzoato groups in 1 and 2,
respectively, by azido ligands leads to transformation of the
controlled by choosing the right ancillary ligands, N3ꢀ, PhCO2
,
and Clꢀ, and their relative binding strength can be studied from
ligand-exchange reactions.
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Inorganic Chemistry
ARTICLE
Scheme 4. Schematic Representation of the Different [Cu4] Assemblies Obtained in This Work
[Cu4(μ4-O)] tetrahedral cores into a new [Cu4] assembly. At
ambient temperature and in air, the treatment of 1 or 2 by 6 equiv
of sodium azide in methanol or CH2Cl2ꢀCH3OH (1:1) favors
the formation of 3 with a characteristic change of color from
green to brown followed by precipitation of a brown solid within
10 min. The precipitate was filtered, washed with CH3OH, dried,
and dissolved in DMF for crystallization. Brown crystals of 3
formed slowly over several days and were isolated by filtration.
During conversions of 1 to 3 and 2 to 3, all of the terminal ligands
are exchanged by three different types of azide anions: two types
of μ1,1-N3ꢀ and terminal N3ꢀ ligands. The mechanism of such
conversions is clearly complicated because it involves the follow-
ing: (i) in 1, abstraction of peripheral chlorido anions by azido
groups together with substitution of the μ4-oxido center by two
μ1,1-N3ꢀ bridges, leading to a change of the coordination sphere
for two CuII sites from square-planar to square-pyramidal
geometry; (ii) in 2, the central μ4-oxido bridge and the benzoato
groups are exchanged in a similar fashion. Abstraction of central
μ4-oxido ions by μ1,1-N3ꢀ bridges triggers the structural change
from a tetrahedral [Cu4(μ4-O)] form to a rhomboidal [Cu4(μ-
N3)4] complex. Considering the quantitative and spontaneous
conversions of 1 to 2 (eq 4), 1 to 3 (eq 5), and 2 to 3 (eq 6), the
relative binding strength of the three ligands can be established
without ambiguity: N3ꢀ > PhCO2ꢀ > Clꢀ.
½Cu4ðμ4-OÞðμ-cipÞ2Cl4ꢁ þ 6NaN3 þ H2O f
½Cu4ðμ-N3Þ4ðμ-cipÞ2ðN3Þ2ꢁ þ 2NaOH þ 4NaCl
ð5Þ
½Cu4ðμ4-OÞðμ-cipÞ2ðμ-O2CPhÞ4ꢁ þ 6NaN3 þ H2O f
½Cu4ðμ-N3Þ4ðμ-cipÞ2ðN3Þ2ꢁ þ 2NaOH þ 4PhCO2Na ð6Þ
Ligand-exchange and core-conversion reactions have been iden-
tified by FTIR measurement, solubility differences, and the
checking of unit cell parameters and crystal systems of grown
single crystals from powder samples (the resulting IR spectra are
given in the Supporting Information). PXRD data have been
taken to identify the phase change from 1 to 2 or 3 and 2 to 3
conversions. The PXRD patterns of the three complexes are
shown in Figure S1 in the Supporting Information. These
patterns are consistent with those of the simulated ones obtained
from the single-crystal X-ray diffraction data.
3926
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Table 2. Selected Interatomic Distances (Å) and Angles
(deg) for Complex 1a
Distances
2.2437(10)
Cl1ꢀCu1
O2ꢀCu1*1
O2ꢀCu1*2
O2ꢀCu1*3
O2ꢀCu1
Cu1ꢀO1
1.9670(18)
1.968(2)
1.9084(3)
1.9084(3)
1.9084(3)
1.9084(3)
Cu1ꢀN1
Cu1ꢀCu1*1
O1ꢀCu1*1
2.9947(7)
1.9670(18)
Angles
O2ꢀCu1ꢀO1
O2ꢀCu1ꢀN1
O1ꢀCu1ꢀN1
Cu1ꢀO1ꢀCu1*1
78.74(6)
O2ꢀCu1ꢀCl1
O1ꢀCu1ꢀCl1
N1ꢀCu1ꢀCl1
89.00(3)
169.99(8)
91.99(10)
99.15(12)
158.20(6)
100.98(8)
103.37(2)
Cu1ꢀO2ꢀCu1*1
1
Figure 1. FTIR spectra of 1, 2 2CH3OH, and 3 DMF between 1330
3
3
a *1, ꢀx, 1/2 ꢀ y, z; *2, 1/4 ꢀ y, 1/4 þ x, 1/4 ꢀ z; *3, ꢀ /4 þ y, 1/4 ꢀ x, 1/4
and 2150 cmꢀ1
.
ꢀ z.
FTIR Spectra. The three [Cu4] complexes show characteristic
bands of the CuII-bound cipꢀ ligands. νCdN stretching modes are
h
observed at 1624ꢀ1639 cmꢀl for 1ꢀ3. For 2, the asymmetric
and symmetric stretching vibrations of the four bound carbox-
ylate groups are detected at 1605 cmꢀl as νas(COO), while νs(COO)
h
h
appeared at 1371 cmꢀ1. The difference, Δν = 234 cmꢀ1, is in
h
accordance with the presence of μ1,3-bridging carboxylates in 2.
From the above Δν value, we can conclude that the benzoate
h
anions act as bidentate anions bridging two metal ion sites. For
nonbridging carboxylato coordination, this Δν separation is
h
usually larger (ca. 350 cmꢀ1).22 In the spectra of 1 and 2, two
medium absorption bands at 622 and 625 cmꢀ1 were detected,
respectively. These bands are missing in the spectra of the free
ligand, KCl, sodium benzoate, or copper(II) nitrate. These bands
are associated with νCuꢀO vibrations originating from the
h
[Cu4O] core. Recently, we have reported this [Cu4O] band at
566 cmꢀ1 for [Cu4(μ4-O)(bahped)2](ClO4)2,23 while in the
case of [Cu4Cl4(μ4-O)(OSR2)4] (R = Et, nBu)24 and copper 2,6-
bis(morpholinomethyl)-4-methylphenol complexes,25 the
νCuꢀO vibrations are observed at 581ꢀ592 cmꢀ1. In addition,
h
3 exhibits a strong band at ∼2082 cmꢀ1, which splits into three
components, and another strong band at 2055 cmꢀ1. These
bands can be easily assigned to the asymmetric stretching mode
of the two different types of azide ligands. Finally, the band at
1342 cmꢀ1 corresponds to the νsym stretching vibration of the
h
Figure 2. Molecular structure of [Cu4(μ4-O)(μ-cip)2Cl4] in 1 with an
atom numbering scheme with a [Cu4] tetrahedron arrangement.
N3ꢀ groups.26
The conversion of 1 to 2 or 3 and 2 to 3 in solution has been
well monitored using FTIR spectroscopy and the characteristic
bands of complexes 1ꢀ3 (Figure 1).
Description of Structures. [Cu4(μ4-O)(μ-cip)2Cl4] (1). Com-
pound 1 crystallizes in the tetragonal centrosymmetric I41/a space
group with four molecules in the unit cell without any interstitial
solvent molecule. The molecular structure of 1 is shown in
Figure1, and selected bond lengths and angles are given in Table2.
1 is composed ofneutral moleculesconsistingoftwodeprotonated
cipꢀ ligands, each providing a N2O set of donor atoms to the
[Cu4] core that is assembled around a central μ4-O group. Four
chloride anions complete the square-planar coordination spheres
around the CuII metal ions (Figure 2). Four equivalent copper
atoms are located at the corners of an almost regular tetrahedron
Electronic UVꢀVis Spectra. The three complexes in CH2Cl2
solutions show multiple bands in the 200ꢀ900 nm region. Broad
absorption bands (λ), with maxima at 728 nm (ε = 688 L
molꢀ1 cmꢀ1), 681 nm (ε = 241 L molꢀ1 cmꢀ1), and 695 nm
(ε = 567 L molꢀ1 cmꢀ1) for 1ꢀ3, respectively, are induced by
the cip ligand. The intense absorptions below 400 nm at 256 nm
(ε = 21 632 L molꢀ1 cmꢀ1), 248 nm (ε = 21 613 L molꢀ1 cmꢀ1),
and 273 nm (ε = 19 240 L molꢀ1 cmꢀ1) are dominated by metal-
to-ligand charge-transfer transitions for 1, 2 2CH3OH, and
3
3 DMF, respectively. Spectra of these complexes also show
3
shoulders at 373 nm (ε = 5879 L molꢀ1 cmꢀ1), 378 nm
(ε = 5053 L molꢀ1 cmꢀ1), and 380 nm (ε = 15 950
L molꢀ1 cmꢀ1), respectively, due to HOꢀ f CuII and PhOꢀ
f CuII ligand-to-metal charge-transfer transitions.27
centered on a O2ꢀ anion, O2, and with Cu Cu edges varying
3 3 3
from 2.99 to 3.18 Å. The previously reported tetranuclear copper-
(II) complexes possessing the [Cu4(μ4-O)]6þ tetrahedral core are
known to have phenoxido and other supports for the oxido core.28
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Figure 3. Molecular structure of [Cu4(μ4-O)(μ-cip)2)(μ1,3-O2CPh)4] in 2 2CH3OH with an atom numbering scheme. H atoms are omitted for
3
clarity.
Table 3. Selected Interatomic Distances (Å) and Angles
The square-planar coordination sphere of the Cu1 site is occupied
by one imine N1 atom and one O1 atom of a deprotonated cipꢀ
ligand, one Cl1 atom, and the μ4-O2 center (Figure S2 in the
Supporting Information). Only two of the six edges of the [Cu4]
tetrahedron are supported by phenoxido bridges (O11), while the
other edges remain unsupported (Figure S3 in the Supporting
Information). Apart from the phenoxido O1 and the central O2
bridges, no other additional supports are needed to ensure stability
to the tetrahedron core. The phenoxido-bridged [Cu2] fragments
(deg) for Complex 2 2CH3OHa
3
Distances
O7ꢀCu2*
O7ꢀCu2
O7ꢀCu1
O7ꢀCu1*
O1ꢀCu1
O1ꢀCu1*
O4ꢀCu2
O4ꢀCu2*
O6ꢀCu1*
1.9150(19)
1.9150(19)
1.9179(19)
1.9179(19)
1.975(2)
O2ꢀCu1
O3ꢀCu2*
Cu1ꢀN1
Cu1ꢀO6*
Cu1ꢀCu1*
Cu2ꢀO5
Cu2ꢀN2
Cu2ꢀO3*
Cu2ꢀCu2*
1.952(2)
2.343(3)
2.000(3)
2.325(3)
3.0105(8)
1.952(2)
1.991(3)
2.343(3)
2.9929(8)
present a Cu Cu distance of 2.995 Å, which is much shorter
3 3 3
1.975(2)
than the unbridged edges with long Cu Cu distances of around
3 3 3
∼3.18 Å (av.). It is worth mentioning for comparison that we
reported earlier a tetranuclear copper(II) complex, [Cu4(μ4-O)-
(bahped)2](ClO4)2, containing only a μ4-oxido bridge (i.e., with-
out any carboxylate or phenoxido bridge along the six tetrahedral
edges) to assemble the tetrahedron core.23 In 1, the CuII site
adopts in square-planar (spl) geometry without any kind of apical
interactions to the Clꢀ groups. Around the distorted square plane
of the CuII site, trans positions are occupied by N1 and the central
O2 with a N1ꢀCu1ꢀO2 angle of 169.99ꢀ and the phenoxide O1
and Cl1 atoms with a O1ꢀCu1ꢀCl1 angle of 158.20ꢀ (Figure S2
in the Supporting Information). The Cu1ꢀCl1 bond length in the
basal plane (2.244 Å) is longer than that of Cu1ꢀμ4-O2 (1.908 Å)
and the Cu1ꢀO1ph (1.967 Å) and CuꢀN1im (1.968 Å) bond
lengths. The Cu1ꢀO1phꢀCu1 angle spans about 99.1ꢀ. The
phenoxido oxygen bridge squeezes the Cuꢀμ4-OꢀCu
(Cu1ꢀO2ꢀCu1*1 and Cu1*2ꢀO2ꢀCu1*3) angles to 103.4ꢀ
compared to the other angles at 112.6ꢀ (Cu1ꢀO2ꢀCu1*3 and
Cu1*1ꢀO2ꢀCu1*2). All of these distances and angles are in good
agreement with the literature values.29 Two phenoxido-bridged
[Cu2] fragments stay exactly perpendicular to each other with a
dihedral angle of 90ꢀ between the two Cu2O2 planes. All of the
cyclo-hexyl rings adopt a chair conformationwiththe equatorialN1
atom of the cip- ligands after condensing as imine bonds. In crystal
packing, no hydrogen-bonding network has been identified. The
1.965(2)
1.965(2)
2.325(3)
Angles
O7ꢀCu1ꢀO2
O7ꢀCu1ꢀO1
92.77(10)
78.63(10)
168.93(10)
162.08(9)
95.31(11)
91.12(10)
98.83(6)
O7ꢀCu2ꢀO5
O7ꢀCu2ꢀO4
O5ꢀCu2ꢀO4
O7ꢀCu2ꢀN2
O5ꢀCu2ꢀN2
O4ꢀCu2ꢀN2
O7ꢀCu2ꢀO3*
O5ꢀCu2ꢀO3*
O4ꢀCu2ꢀO3*
N2ꢀCu2ꢀO3*
Cu1ꢀO7ꢀCu1*
Cu2ꢀO7ꢀCu2*
93.68(11)
79.01(10)
172.27(11)
162.06(9)
95.31(11)
91.06(11)
96.48(7)
O2ꢀCu1ꢀO1
O7ꢀCu1ꢀN1
O2ꢀCu1ꢀN1
O1ꢀCu1ꢀN1
O7ꢀCu1ꢀO6*
O2ꢀCu1ꢀO6*
O1ꢀCu1ꢀO6*
N1ꢀCu1ꢀO6*
Cu1ꢀO1ꢀCu1*
Cu2ꢀO4ꢀCu2*
a * = 1 ꢀ x, y, 1/2 ꢀ z.
98.72(11)
89.57(7)
98.27(11)
85.19(8)
95.72(10)
99.31(14)
99.21(14)
97.54(10)
103.19(14)
102.80(14)
crystal-packing diagram along the b axis is shown in Figure S4 in
the Supporting Information.
[Cu4(μ-cip)2(μ4-O)(μ1,3-O2CPh)4] 2CH3OH
The molecular structure of complex 2 is given in Figure 3, and
(2 2CH3OH).
3
3
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Inorganic Chemistry
ARTICLE
(6.63/36.57ꢀ) indicate that the benzoate groups bridge the CuII
metal ions in a synꢀsyn mode (ideal values 0ꢀ/0ꢀ) with a slightly
out-of-plane binding for the second Cu atom. Within the [Cu2(μ-
cip)]3þ units, the phenoxido O atom is bridging CuII ions in only
equatorial positions. The CuꢀOphꢀCu angles span about 99.2ꢀ,
squeezing the Cuꢀμ4-OꢀCu (Cu1ꢀO7ꢀCu1* and Cu2ꢀO7ꢀ
Cu2*) angles to 103.09ꢀ in comparison to the Cu1ꢀO7ꢀCu2*
and Cu2ꢀO7ꢀCu1* angles of 112.86ꢀ, which are in good agree-
ment with the literature values.31,32 The dihedral angle between the
two Cu2O2 planes of the phenoxido-bridged [Cu2] fragments is
89.83ꢀ, indicating a distortion similar to that observed for 1. The
Addison τ parameter for the CuII centers falls within the 0.11ꢀ0.17
range, highlighting only a slight tbp (trigonal-bipyramid) distortion
from ideal spy geometry.33 The CuII ions in phenoxido-bridged
[Cu2] fragments are separated by about 3.00 Å, which is shorter
than the 3.189 Å Cu Cu distance in benzoato-bridged moieties.
3 3 3
The longest Cu Cu distances, ∼3.194 Å, are found for the CuII
3 3 3
ions unbridged along the tetrahedral edge. O atoms of the interstitial
CH3OH molecules are involved in intermolecular hydrogen-bond-
ing interactions with two benzoato O atoms (Figure S9 in the
Supporting Information). The crystal packing along the a axis is
shown in Figure S10 (see the Supporting Information).
Figure 4. Partial molecular structure of 2 2CH3OH highlighting the
polynuclear coordination core.
3
[Cu4(μ1,1-N3)4(μ-cip)2(N3)2] DMF (3 DMF). The molecular
3
3
structure of [Cu2(μ1,1-N3)2(μ-cip)(N3)]2 is shown in Figure 5,
and selected bond lengths and angles are listed in Table 4. 3
crystallizes in the C2/c space group, and the asymmetric unit
contains only half of the tetranuclear complex, with the second
half being generated by the (ꢀx, y, 0.5 ꢀ z)* symmetry operator.
The neutral centrosymmetrical [Cu4] complex 3 is composed of
two [Cu2(μ1,1-N3)2(μ-cip)(N3)] fragments that are assembled
by two μ1,1-azido bridges (N9 and N9*) between Cu2 and Cu1*
and between Cu2* and Cu1 in an equatorialꢀaxial binding mode
(Figure 5). In the formed [Cu4] rhomboidal complex, the two
μ1,1-N3ꢀ (N3 and N3*) anions that bridge Cu1 and Cu2 in their
basal planes have obviously replaced the central μ4-O group
present in 1 and 2. The resulting azido- and phenoxido-bridged
[Cu4] rhomboidal core structure is not known in the literature.
This structure is different from the double cubane structure
because the azido bridges (N3 and N3*) within the core of the
rhomboidal structure are only of the bridging type. In the
[Cu2(μ1,1-N3)2(μ-cip)(N3)] moieties, the deprotonated cipꢀ
ligand provides a N2O set of donor atoms to the two CuII sites
important bond lengths and angles for 2 are listed in Table 3.
2 2CH3OH crystallizes in the monoclinic C2/c space group with
3
a crystallographic inversion center located on the O7 atom,
implying that the asymmetric unit contains half of a tetranuclear
molecule and one CH3OH molecule. 2 is composed of neutral
complexes consisting of two deprotonated cipꢀ ligands, each
providing N2O donor atoms to the [Cu4] core that is assembled
by a central μ4-O7 group. Four benzoate groups complete the
coordination sphere of the CuII centers (Figures 3 and 4). The
four CuII centers arrange around the central μ4-O2ꢀ anion in a
distorted Td geometry, with Cu Cu distances varying from
3 3 3
2.99 to 3.19 Å, which are significantly longer than those in the
known benzoate-bridged copper paddlewheel dimers (Figure S5
in the Supporting Information).30 The CuII coordination spheres
display a distorted square-based pyramid (spy) geometry con-
sisting of two O atoms of bridging carboxylate anions, one
phenoxido O atom, one central O2ꢀ anion, and one N atom of
the cipꢀ imine group (Figure S6 in the Supporting Information).
The square base is occupied in a trans fashion by the N atom and
the central O2ꢀ anion with NꢀCuꢀO angles of 162.06ꢀ162.08ꢀ
and by a phenoxido atom and one carboxylato O atom with
OꢀCuꢀO angles of 168.93ꢀ172.24ꢀ. The spy geometry is
completed by the second carboxylato O atom bound to the
apical CuII positions, with bond distances of 2.32ꢀ2.34 Å that are
longer by 0.3ꢀ0.4 Å than those found in the basal plane:
CuꢀObz (bz = benzoate) ≈ 1.95 Å; Cuꢀμ4-O ≈ 1.91 Å;
CuꢀOph ≈ 1.96ꢀ1.97 Å; CuꢀNim ≈ 1.99ꢀ2.00 Å. The Cu
atoms are shifted by about 0.15ꢀ0.20 Å from their least-squares
basal planes toward the apical carboxylato O atoms. The four
benzoato groups are bridging CuII metal ions along two edges of
the [Cu4O] tetrahedron, while the phenoxido groups connect
two other edges (Figures 3 and S7 in the Supporting In-
formation). Thus, only two edges remain unsupported, leaving
the [Cu4O] tetrahedron less open than in 1. All of the benzoato
groups coordinate uniformly in the apicalꢀequatorial positions
of the CuII sites (Figures 3 and S8 in the Supporting In-
formation). The CuꢀOꢀCꢀO/OꢀCꢀOꢀCu torsion angles
(Cu Cu distance of 3.121 Å) that are sharing the bridging
3 3 3
phenoxido O1 atom. While Cu2 adopts a square-planar (sp)
geometry, Cu1 exhibits a distorted square-based pyramid (spy)
coordination sphere. The square plane of Cu2 is occupied by two
N atoms (N9 and N3) of the bridging azide anions, N2 and O1
atoms from respectively the imine and phenoxido groups of the
cipꢀ ligand (Figure S11 in the Supporting Information). It is
worth mentioning that Cu2 is displaced by 0.075 Å from the
mean N9ꢀN3ꢀO1ꢀN2 plane and that the free apical position
of square-planar Cu2 atom shows interaction with formyl O
atoms of disordered DMF molecules at 2.518 Å. The square base
of Cu1 is occupied by N1 and O1 atoms from respectively the
imine and phenoxido groups of the cipꢀ ligand and two N atoms
(N6 from a terminal azide ligand and N3) of azide anions. Cu1
displaced by 0.236 Å from the N6ꢀN3ꢀO1ꢀN1 mean basal
plane. The distortion of the spy geometry around Cu1 is larger
than that observed in 2 with an Addison parameter of 0.35. In the
basal plane of both Cu sites, the imine N1 or N2 atom is
coordinating to CuII in the trans position of the bridging azido
N3 atom with NꢀCuꢀN3 angles of 167.35 and 168.45ꢀ.
3929
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ARTICLE
Figure 5. Molecular structure of [Cu4(μ-N3)4(μ-cip)2(N3)2] in 3 DMF with an atom numbering scheme. H atoms are omitted for clarity.
3
arrangement, the four Cu atoms are coplanar with Cu1 Cu2 =
Table 4. Selected Interatomic Distances (Å) and Angles
3 3 3
(deg) for Complex 3 DMFa
3.121 Å and Cu1 Cu2* = 3.512 Å (Figure S12 in the
3 3 3
3
Supporting Information) and with short Cu2 Cu2* = 3.659
3 3 3
Å and long Cu1 Cu1* = 5.546 Å diagonals (Figure S13 in the
Supporting Information).
3 3 3
Distances
Cu1ꢀN6
Cu1ꢀN1
Cu1ꢀN3
Cu1ꢀO1
Cu1ꢀN9*
1.943(3)
1.953(2)
1.994(2)
2.0147(19)
2.429(3)
Cu2ꢀN2
Cu2ꢀN3
Cu2ꢀO1
Cu2ꢀN9
1.968(2)
1.978(3)
1.9809(19)
1.966(3)
Magnetic Properties. The solid-state magnetic properties of
1, 2 2CH3OH, and 3 DMF have been investigated using dc
3
3
susceptibility measurements in the 1.8ꢀ300 K temperature
range under a magnetic field of 1000 Oe (Figure 5). At 300 K,
the χT products are 0.22, 0.49, and 0.68 cm3 K molꢀ1 for 1ꢀ3,
respectively. These χT values are far from the theoretical value of
1.5 cm3 K molꢀ1 expected for four isolated paramagnetic Cu2þ
ions (d9, S = 1/2) with g = 2, indicating that strong and dominant
antiferromagnetic interactions exist between the Cu2þ ions in
these complexes. Upon cooling, the χT products continuously
decrease to reach a value close to zero below 120, 80, and 70 K for
1ꢀ3, respectively.
Angles
N6ꢀCu1ꢀN1
N6ꢀCu1ꢀN3
101.85(12)
90.80(12)
167.35(11)
145.88(14)
92.89(9)
N3ꢀCu1ꢀN9*
O1ꢀCu1ꢀN9*
N9ꢀCu2ꢀN2
N9ꢀCu2ꢀN3
N2ꢀCu2ꢀN3
N9ꢀCu2ꢀO1
N2ꢀCu2ꢀO1
N3ꢀCu2ꢀO1
Cu1ꢀN3ꢀCu2
85.27(11)
90.03(9)
N1ꢀCu1ꢀN3
96.10(11)
95.13(11)
168.45(11)
165.02(10)
92.31(9)
N6ꢀCu1ꢀO1
N1ꢀCu1ꢀO1
Generally speaking, for the CuII ions, the magnetic interac-
tions mediated through their axial positions are much weaker
than those mediated by their equatorial sites. Therefore, if one
considers only the magnetic pathways that involve equatorial
bonds around each CuII ion, 3 can be viewed as two almost
isolated Cu1ꢀCu2 dinuclear units with a double phenoxi-
doꢀazido bridge (Figure 5). For 1 and 2, the situation is slightly
different because of the presence of the central μ4-oxido bridge
between CuII ions. Nevertheless, the magnetic pathway through
the double μ4-oxidoꢀphenoxido bridges is likely much more
efficient than a single μ4-oxido bridge between two CuII metal
ions. As a consequence, the magnetic description of 1 and 2 with
two isolated dinuclear units with a double phenoxidoꢀoxido
bridge (Figures 2 and 3) is similar to that of 3 (vide supra). For 1
like for 3, only one spin dimer has to be considered. For 2,
considering the similarity of the bridging modes in the two
N3ꢀCu1ꢀO1
76.19(9)
N6ꢀCu1ꢀN9*
N1ꢀCu1ꢀN9*
Cu1ꢀO1ꢀCu2
Cu1ꢀN9*ꢀCu2*
a * = ꢀx, 1 ꢀ y, 1 ꢀ z.
120.65(15)
88.41(10)
102.73(9)
105.61(11)
77.34(9)
103.61(11)
Similarly, the phenoxido O1 and monodentate azide N6 or
bridging azide N9 atoms are trans to each other with O1ꢀCuꢀN
angles of 145.88 and 165.02ꢀ. The CuꢀNaz (az = azide) bond
lengths involving a CuII basal plane (av. 1.98 Å) are shorter than
the Cu1ꢀN9az* bond distance (2.429 Å) in the axial direction
due to the JahnꢀTeller effect and longer than the Cu1ꢀN6az one
at 1.943 Å in the case of the monodentate azide anion (Figure S11
in the Supporting Information). Within the rhomboidal [Cu4]
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dinuclear [Cu2] units (Table 3), the interaction in the two spin
dimers is expected to be similar. Considering these magnetos-
tructural remarks, the magnetic properties of the three com-
pounds 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, denoted as J. The magnetic
susceptibility has thus been calculated from the following iso-
tropic spin HeisenbergꢀHamiltonian:
the tetranuclear complexes possess an ST = 0 spin ground state, i.
e., a diamagnetic ground state. The F values indicate that a very
small amount of residual paramagnetic contribution is present in
the measured samples of 1 and 3, as it is seen for materials
displaying a diamagnetic ground state. Taking into account that
the impurity possesses a Curie S = 1/2 paramagnetic behavior
and the same molecular weight as the major complex, the amount
is about 3 and 2% for 2 and 3, respectively.
It is worth mentioning that the magnetic properties of these
complexes have also been analyzed with an alternative model
including two different intramolecular interactions. For 1 and 2,
one could consider one interaction mediated by the double
oxidoꢀphenoxido bridges and the another one mediated by a
single μ4-oxido bridge.37 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 overparame-
trization of the fitting procedures. Therefore, the only relevant
model requires no more than one interaction parameter, as shown
in Figure 7, and thus it is not possible to evaluate the magnetic
interaction through the single μ4-oxido bridge in 1 and 2 or the
single azido bridges in 3. The amplitude of the antiferromagnetic
exchange parameter is relatively large for the three compounds
but stays in the range of magnitude observed in similar com-
pounds with the same type of double oxidoꢀphenoxido or
azidoꢀphenoxido bridges.38ꢀ40 It is well known that alkoxi-
doꢀphenoxido-bridged Cu2 complexes show antiferromagnetic
interaction when the CuꢀOphenoxido/alkoxidoꢀCu angle is larger
than 97.6ꢀ, and the antiferromagnetic interaction increases with
increasing angle.41 Thus, in the present case, in all three com-
plexes having CuꢀOphenoxidoꢀCu and CuꢀOalkoxidoꢀCu angles
close to 99ꢀ and 103ꢀ, respectively, the coupling through the
alkoxidoꢀphenoxido group should be antiferromagnetic. On the
other hand, the ferromagnetic interaction for μ1,1-N3 bridges
decreases with an increase in the CuꢀNazidoꢀCu angle from 85ꢀ
and can exhibit antiferromagnetic behavior for a CuꢀNazidoꢀCu
angle g104ꢀ.42 In complex 3, the CuꢀNazidoꢀCu and
CuꢀOphenoxidoꢀCu angles (102.73ꢀ and 103.61ꢀ) are close to
the cutoff value of 104ꢀ for ferromagnetic character, and very
weak ferromagnetic coupling may be expected through the
bridging azido group. Experimentally, this weak ferromagnetic
interaction is indeed dominated by the strong antiferromagnetic
coupling mediated by the phenoxido bridge.
H ¼ ꢀ 2JfSCu,1 SCu,2
g
ð7Þ
3
1
where Si are the spin operators for each center with S = /2.
Application of the van Vleck equation34,35 allows the determina-
tion of the low-field analytical expression of the magnetic
susceptibility36 taking into account the presence of a residual
extrinsic paramagnetic contribution:
2
2
2
2
2NμB gCu
1
NμB gCu
χT ¼ 2ð1 ꢀ FÞ
þ F
ð8Þ
3 þ eꢀ2J=k T
B
kB
4kB
As shown in Figure 6, the above expression of the magnetic
susceptibility reproduces almost perfectly the experimental χT vs
T 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 interaction implies that these CuII dimer units and thus
Figure 6. Temperature dependence of the χT product (with χ being the
molar magnetic susceptibility defined as M/H) for 1, 2 2CH3OH, and
3
3 DMF. The solid lines are the best fit obtained using the isotropic
3
dinuclear S = 1/2 Heisenberg model described in the text.
Concluding Remarks. The coordination chemistry of the
cipꢀ ligand has been studied and confirms the key role played by
the ancillary ligands in the synthesis of three new tetranuclear
copper(II) complexes. Chloride, benzoate, and azide ligands
have been used to control the molecular topology of the final
[Cu4] products. The tridentate cipꢀ ligand facilitates the trap-
ping of two CuII pairs, leaving available coordinating sites for
the binding of oxido, benzoato, and azido anions, which help
Table 5. Parameters Deduced from Analysis of the Magnetic
Properties for 1, 2 2CH3OH, and 3 DMF
3
3
g
J/kB (K)
F (%)
1
2
3
2.2(1)
ꢀ513.(15)
ꢀ340.(10)
ꢀ315.(5)
0
3
2
2.03(10)
2.3(1)
Figure 7. Magnetic cores of 1 and 2.
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Inorganic Chemistry
ARTICLE
the assembly of two CuII pairs in tetrahedral and rhomboidal
molecular topologies. Spontaneous conversions and ancillary
ligand-exchangeꢀreactions allꢀowed us to study the binding
efficiency of N3 and PhCO2 over Clꢀ, suggesting a rational
synthetic approach to the synthesis of tetranuclear complexes.
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We are currently working on other ancillary bridges such as O2ꢀ
,
O22ꢀ, S2ꢀ, HSꢀ, etc., in this reaction system to induce the
formation of new types of homo- and heterometallic assembled
cage complexes.
’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic data in
b
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CIF format, Scheme S1, Figures S1ꢀS13, and synthesis and
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charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
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2005, 34, 1648–1649.
Corresponding Author
* E-mail: clerac@crpp-bordeaux.cnrs.fr (R.C.), dray@chem.
iitkgp.ernet.in (D.R.). Tel: (þ33) 5 56 84 56 50 (R.C.), (þ91)
3222-283324 (D.R.). Fax: (þ33) 5 56 84 56 00 (R.C.), (þ91)
3222-82252 (D.R.).
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