Sterically-controlled nuclearity in new copper(II) complexes with di-compartmental ligands: Formation of antiferromagnetically coupled angular trimer and mononuclear inclusion complex

Article abstract of DOI:10.1016/j.ica.2009.12.053

Two new copper(II) complexes, [Cu₃(L¹)₂(H₂O)₂](ClO₄)₂ (1) and [CuL²? (H₂O)] (2) have been derived from two di-compartmental Schiff base ligands H₂

Full text of DOI:10.1016/j.ica.2009.12.053

Inorganica Chimica Acta 363 (2010) 1395–1403
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Inorganica Chimica Acta
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Sterically-controlled nuclearity in new copper(II) complexes with
di-compartmental ligands: Formation of antiferromagnetically coupled
angular trimer and mononuclear inclusion complex
Santarupa Thakurta a, Corrado Rizzoli b, Ray J. Butcher c, Carlos J. Gómez-García d,
Eugenio Garribba e, Samiran Mitra a,
*
a Department of Chemistry, Jadavpur University, Kolkata 700 032, India
b Dipartimento di Chimica Generale ed Inorganica, Università di Parma, Viale, G.P. Usberti 17/A, I-43100 Parma, Italy
c Department of Chemistry, Howard University, 2400 Sixth Street, NW, Washington, DC 20059, USA
d Instituto de Ciencia Molecular (ICMol), Parque Científico, University of València, 46980 Paterna, Spain
e Department of Chemistry and Center for Biotechnology Development and Biodiversity Research, University of Sassari, Via Vienna 2, I-07100 Sassari, Italy
a r t i c l e i n f o
a b s t r a c t
Two new copper(II) complexes, [Cu3(L1)2(H2O)2](ClO4)2 (1) and [CuL2 ꢀ (H2O)] (2) have been derived
from two di-compartmental Schiff base ligands H2L1 and H2L2, respectively. Depending on slight modifi-
cation of the substituent group of the potentially N2O4 donor ligands, tri- and mononuclear structures are
obtained, which have been confirmed by single-crystal X-ray diffraction studies. Both complexes have
been characterized by elemental analysis, IR, UV–vis and EPR spectroscopy. Complex 1 consists of an
angular trinuclear array of copper ions, while complex 2 consists of a mononuclear copper center. Vari-
able temperature magnetic susceptibility measurements have been performed to investigate the mag-
netic behaviour of complex 1 and the result indicates a strong antiferromagnetic exchange interaction
Article history:
Received 7 September 2009
Received in revised form 14 December 2009
Accepted 28 December 2009
Available online 6 January 2010
Keywords:
Copper(II)
Compartmental Schiff bases
Angular trimer
Antiferromagnetic
Inclusion complex
(J = ꢁ120.1(2) cmꢁ1) between the adjacent copper(II) centers through two double
l2-phenoxo bridges.
Complex 2 is a mononuclear inclusion compound encapsulating one water molecule in the vacant exter-
nal compartment of the ligand through hydrogen-bonding interactions.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
species in literature have a linear geometry or a triangular Cu3(l3-
X) core (X = Cl, OH) [10–14], whereas in much lesser number of
complexes the array of CuII ions adopts a bent or angular configu-
ration [15–17]. Thus, among the 52 different copper(II) trimers
with double oxo bridges available in the CCDC database, only four
present a bent structure while the remaining 48 are linear.
The study of trinuclear copper(II) systems assumes a special po-
sition in coordination chemistry following the discovery of the tri-
nuclear active site in laccase [1], the ferromagnetically coupled CuII
triad in the subunit of methane mono-oxygenase [2] and the
angled trinuclear CuII unit of ascorbate oxidase [3]. Another impor-
tant aspect of the CuII trinuclear complexes is the possibilities for
intriguing magnetic interactions between the paramagnetic metal
ions which stimulate to develop better understanding of the funda-
mental science of exchange coupling [4–7]. Although the field of
exchange-coupled copper(II) complexes has been extensively stud-
ied with respect to various binuclear systems [8,9], the number of
reported and studied CuII trimers is comparatively scarce. One ap-
proach for the synthesis of magnetically relevant multinuclear
complexes is the introduction of suitable chelating ligands contain-
ing phenol and alcohol groups, which can act as bridging and/or
hydrogen-bonding functionalities. Most of the copper(II) trinuclear
In recent years, our research group reported some phenoxo-
bridged linear trinuclear copper(II) complexes of the Schiff base
N,N0-bis(2-hydroxyacetophenone)propylenediimine [18,19]. The
interesting results obtained for those complexes inspired us to
continue research on the dynamic properties of new trinuclear
clusters using various chelating ligands. In this context, the rich
chemistry of di-compartmental ligands, capable of holding metal
ions in close proximity, has drawn our attention. Particularly, a
number of discrete 3d–4f compounds have already been synthe-
sized from the bis(3-hydroxy or 3-methoxy/ethoxy) derivatives
of salen or salpn [20–24]. In these complexes, the internal N2O2
cavity of the Schiff base is adequate for complexation of the 3d ions
while the external O4 cavity is well suited for the 4f ions owing to
their larger size and oxophilic nature. In contrast, these hexaden-
tate Schiff bases may simultaneously accommodate 3d ions in both
compartments only in some limited instances when the respective
* Corresponding author. Tel.: +91 (033) 2414 6666x2779; fax: +91 (033) 2414
6414.
E-mail address: smitra_2002@yahoo.com (S. Mitra).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.12.053

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S. Thakurta et al. / Inorganica Chimica Acta 363 (2010) 1395–1403
dimensions match up with each other and therefore, very few
N2O2 + O4 homo- or heteronuclear 3d complexes are known to ex-
ist [25–28]. Furthermore, homometallic trinuclear structures are
really scanty [29–31], thereby leaving scope for new discoveries.
In addition, the compartmental nature of these ligands can also
be utilized to design specific molecular architectures which can
serve as suitable host molecules [32] with huge range of applica-
tions such as anion and solvent recognition, inclusion phenomena,
facilitated transport and catalysis. All these features of this family
of ligands encouraged our research group to explore their feasibil-
ity in the preparation of functional copper(II) materials presenting
useful magnetic and inclusion properties.
Caution! Perchlorate salts in presence of organic materials are
potentially explosive. They should be prepared in small amounts
and handled with extreme care.
2.2. Syntheses
2.2.1. Synthesis of the Schiff base ligands
N,N0-Bis(3-methoxysalicylidenimino)-1,3-diaminopropane (H2L1):
The ligand H2L1 was prepared by refluxing 3-methoxy-2-hydroxy-
benzaldehyde (20 mmol, 3.04 g) and 1,3-diaminoprapane
(10 mmol, 0.833 ml) in 50 ml of methanol at 65 °C for 1 h. Upon
cooling the solution, a deep-yellow crystalline solid was precipi-
tated which was collected as the ligand for the synthesis of the cor-
responding complex 1. Yield: 90%. Anal. Calc. for C19H22N2O4: C,
66.65; H, 6.48; N, 8.18. Found: C, 66.71; H, 6.42; N, 8.08%. FT-IR
In this work two di-compartmental Schiff bases, N,N0-
bis(3-methoxysalicylidenimino)-1,3-diaminopropane (H2L1) and
N,N0-bis(3-ethoxysalicylidenimino)-1,3-diaminopropane (H2L2)
(Scheme 1), were used to synthesize the new copper(II) complexes
[Cu3(L1)2(H2O)2](ClO4)2 (1) and [CuL2 ꢀ (H2O)] (2). The nuclearity
of the resulting complexes is modulated according to the steric de-
mand of the ligand substituent groups. Crystal structure of 1 shows
that adjacent copper centers are linked by double phenoxo bridges
furnishing a novel angular trinuclear structure. Magnetic studies
confirm the presence of a strong antiferromagnetic interaction
(J = ꢁ120.1(2) cmꢁ1) which can be suitably correlated with the
structural parameters. On the other hand, complex 2 can be
regarded as a ‘‘complex ligand” [33] owing to its potential to serve
as precursor for the synthesis of multinuclear homo- and
heterometallic complexes. Moreover, the water molecule in 2 is
encapsulated in the vacant O4 compartment through H-bonding
interactions, giving rise to the formation of a rare kind of inclusion
complex.
(KBr, cmꢁ1):
m(C@N) 1641. UV–vis (k, nm): 258, 398.
N,N0-Bis(3-ethoxysalicylidenimino)-1,3-diaminopropane (H2L2):
The Schiff base H2L2 was prepared following the same procedure
as adopted for H2L1 except that we used 3-ethoxy-2-hydroxybenz-
aldehyde (20 mmol, 3.32 g) as the carbonyl-containing compound.
The orange solid obtained as the ligand was dissolved in methanol
for the synthesis of the corresponding complex 2. Yield: 87%. Anal.
Calc. for C21H26N2O4: C, 68.09; H, 7.07; N, 7.56. Found: C, 68.01; H,
7.12; N, 7.51%. FT-IR (KBr, cmꢁ1):
260, 395.
m(C@N) 1640. UV–vis (k, nm):
2.2.2. Synthesis of the complexes
[Cu3(L1)2(H2O)2](ClO4)2 (1). Cu(ClO4)2ꢂ6H2O (1.11 g, 3 mmol) dis-
solved in 10 ml methanol was added to a 20 ml methanolic solu-
tion of the ligand H2L1 (0.684 g, 2 mmol) with constant stirring.
The solution was refluxed for 40 min at 65 °C and the resulting
deep-green filtrate was kept undisturbed. After 4 days, dark-green
prismatic single crystals suitable for X-ray diffraction appeared.
Yield: 72%. Anal. Calc. for C38H44Cl2Cu3N4O18: C, 41.25; H, 4.01;
N, 5.06. Found: C, 41.22; H, 4.05; N, 5.09%.
2. Experimental
2.1. Materials
All the chemicals and solvents employed for the synthesis were
of analytical grade. 1,3-Diaminopropane, 3-methoxy-2-hydroxy-
benzaldehyde and 3-ethoxy-2-hydroxybenzaldehyde were pur-
chased from Aldrich Co., USA. Copper(II) perchlorate hexahydrate
was prepared by the treatment of copper carbonate (E. Merck, In-
dia) with 80% perchloric acid (E. Merck, India) followed by slow
evaporation. It was then filtered through a fine glass-frit and pre-
served in CaCl2 desiccator.
[CuL2 ꢀ (H2O)] (2). Methanolic solution (10 ml) of Cu(ClO4)2ꢂ6H2O
(1.11 g, 3 mmol) was added dropwise to 20 ml of a methanolic
solution of H2L2 (0.741 g, 2 mmol) and the resulting solution was
refluxed for 40 min at hot condition. Dark-red plate-shaped
crystals were obtained from the filtrate after 3 days and they were
collected by filtration. Alternatively, complex 2 can also be synthe-
sized by taking the corresponding metal and ligand in 1:1 molar
CH3
O
CH2
O
CH3
OH
N
OH
OH
N
OH
N
N
CH3
O
O
CH3
CH2
H2L1
H2L2
Scheme 1. Compartmental Schiff base ligands used for synthesis.

S. Thakurta et al. / Inorganica Chimica Acta 363 (2010) 1395–1403
1397
C27 and C28 carbon atoms of one H2L1 ligand were found to be dis-
ordered over two positions (called position A and B), with site
occupancy factors of 0.6 and 0.4 respectively. Severe disorder af-
fected also the perchlorate anions and the water molecules and
the best fit for the disordered atoms was obtained by considering
the distribution of the atoms over two positions (A and B). The H
atoms of the disordered water molecules were positioned theoret-
ically and included in the structure-factor calculations with fixed
positional parameters. All other H atoms were placed at calculated
positions and refined using the riding-model approximation.
The intensity data collection of 2 was performed with an Oxford
Diffraction Gemini R-Ruby diffractometer using multi scan tech-
nique at 200 K. Data collection and unit cell refinement were per-
formed using CRYSALISPRO software [38]. The structure was solved by
direct method procedures with SHELXS and refined by full-matrix
least-squares methods based on F2 with SHELXL-97 [37]. The non-
hydrogen atoms were refined with anisotropic factors. All hydro-
gen atoms were positioned geometrically treated as riding on the
bound atom and those of the water molecules were located in dif-
ference Fourier maps. All calculations and graphical works for both
complexes have been done using PLATON [39] and WinGX [40]
programs. Selected crystallographic data and some important
structure refinement parameters of complexes 1 and 2 are summa-
rized in Table 1.
Table 1
Summary of crystal data and refinement details for 1 and 2.
1
2
Empirical formula
Formula weight
Crystal dimension (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C38H44Cl2Cu3N4O18
1106.32
0.18 ꢃ 0.07 ꢃ 0.04
orthorhombic
P212121
9.004(2)
18.462(4)
26.783(6)
90.00
4452.2(17)
4
296(2)
1.651
1.619
2260
1.3–25.0
35 676
7787
3592
0.0681
0.1097
0.93
0.099
C21H26CuN2O5
449.98
0.47 ꢃ 0.43 ꢃ 0.18
monoclinic
P21/n
12.4079(3)
10.2670(2)
16.4030(4)
98.961(2)
2064.10(8)
4
200(2)
1.448
1.093
940
4.70–32.51
18 939
6850
4321
0.0306
b (°)
V (Å3)
Z
Temperature (K)
Dcalc (g cmꢁ3
)
l
(mmꢁ1
)
F(0 0 0)
h (°)
Total data
Unique data
Observed data [I > 2
r(I)]
Ra
wR2b
0.0759
0.95
0.033
0.36
Goodness-of-fit on F2, S
Rint
D
D
qmax (e Åꢁ3
)
)
0.40
ꢁ0.31
qmin (e Åꢁ3
ꢁ0.31
a
R =
R
(|Fo ꢁ Fc|)/
R|Fo|.
2
2
1=2
wR2 ¼ f
R
½wðF2o ꢁ F2c Þ ꢄ=
R
½wðF2o Þ ꢄg
.
b
3. Results and discussion
proportions in 30 ml of a MeOH–H2O mixture (3:1 v/v). Yield: 66%.
Anal. Calc. for C21H26CuN2O5: C, 56.05; H, 5.82; N, 6.23. Found: C,
56.09; H, 5.78; N, 6.27%.
3.1. Discussion
As mentioned above, we had already used the N2O2 donor
ligand N,N0-bis(2-hydroxyacetophenone)propylenediimine for syn-
thesizing two centrosymmetric, linear trimeric complexes
[Cu3(L)2(CX3COO)2] (X = H and F) where the central copper atom
is distorted octahedral and the two terminal copper centers are
square pyramidal [19]. Useful magneto-structural correlations
were established which satisfactorily explained the plausible path-
ways of the antiferromagnetic exchange interactions shown by
those complexes. Thereafter, we are interested in the rational
designing of new trinuclear clusters in order to draw meaningful
interpretation of their magnetic results. Interestingly, in most of
the reported trinuclear complexes an auxiliary ligand, such as
syn–syn bridging carboxylate or nitrate group is present in order
to provide the required rigidity to the structure [41,42]. In this
work, we employed two compartmental ligands, H2L1 and H2L2
for the synthetic purpose because of their ability to span over adja-
cent metal centers by taking advantage of the respective methoxy
or ethoxy side-arms [43]. Here, H2L1 coordinates in a pentadentate
fashion with N2O2(phenolic)O(methoxy) donor set and utilizes its bridg-
ing and chelating properties simultaneously to furnish the trinu-
clear complex [Cu3(L1)2(H2O)2](ClO4)2 (1) where the desired
SqPy–Oct–SqPy topology of the triad is achieved without the sup-
port from any bridging co-ligand. Moreover, the phenoxo-bridged
central Cu3O4 core adopts a new kind of angular configuration
rather than the usual linear ones. Note that 3d homometallic di-
or multinuclear compounds derived from a hetero-compartmental
ligand system are not well-known because the ionic radius of the
3d metal usually fits in only one of the two cavities. In particular,
the ionic radius of the 3d metal, especially CuII, is often found to
be smaller with respect to the dimension of the outer O4 cavity.
Therefore complex 1 represents a significant contribution in
exploring the magnetic properties of 3d–3d homometallic com-
pounds in this ligand environment.
2.3. Physical measurements
The Fourier transform infrared spectra of the ligands and the
complexes were recorded on a Perkin Elmer Spectrum RX I FT-IR
spectrophotometer with KBr pressed pellets in the range 4000–
200 cmꢁ1. The electronic spectra were recorded on a Perkin Elmer
Lambda 40 (UV–vis) spectrometer using HPLC grade acetonitrile as
solvent in the range 800–200 nm. Elemental analyses were carried
out on a Perkin Elmer 2400 II Elemental Analyser. Electron para-
magnetic resonance (EPR) spectra of the complexes were recorded
on polycrystalline samples or in CH3CN and N,N-dimethylformam-
ide (DMF) solutions with an X-band Bruker EMX spectrometer. The
spectra were simulated with the computer program Bruker Win-
EPR SimFonia. The magnetic susceptibility measurements were
carried out in the temperature range 2–300 K with an applied mag-
netic field of 0.1 T on polycrystalline sample of complex
1
(mass = 44.64 mg) with a Quantum Design MPMS-XL-5 SQUID
magnetometer. The isothermal magnetization was performed on
the same sample at 2 K with magnetic fields up to 5 T. The suscep-
tibility data were corrected for the sample holders previously mea-
sured using the same conditions and for the diamagnetic
contributions of the salt as deduced by using Pascal’s constant ta-
bles (v
dia = ꢁ570.6 emu molꢁ1).
2.4. Crystallographic data collection and structure refinements
The intensity data of 1 were collected with a Bruker SMART
1000 CCD, using the
x-scan technique. Data collection and data
reduction were performed with the SMART and SAINT programs
[34]. Multi-scan absorption correction was carried out using the SA-
DABS program [35]. The structure was solved by direct methods by
using the SHELXS-97 program [36]. All non-hydrogen atoms were re-
fined with anisotropic displacement parameters by full-matrix
least-squares based on F2 using the SHELXL-97 program [37]. The
Interestingly, under similar reaction conditions the other ligand
H2L2 behaves as a simple N2O2(phenolic) donor tetradentate chelating
ligand yielding the mononuclear complex [CuL2 ꢀ (H2O)] (2). Thus

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S. Thakurta et al. / Inorganica Chimica Acta 363 (2010) 1395–1403
Fig. 1. ORTEP representation of complex 1. Displacement ellipsoids are drawn at the 40% probability level. Hydrogen atoms are omitted for clarity. Only the major
components of the disorder are shown.
it is quite evident that ligating properties have been influenced by
Table 2
the nature of substituents on the phenyl rings, and as a result,
these two potentially hexadentate ligands exhibit different coordi-
nation behaviours in the resulting complexes. The greater steric
hindrance of the ethoxy side-arm in H2L2 as compared with the
methoxy one in H2L1 prevents the binding of a second CuII ion in
the outer compartment, thereby restricting the extension of nucle-
arity of complex 2 contrary to complex 1. Nevertheless, the vacant
external O4 site of this mononuclear compound encapsulates one
water molecule by means of hydrogen-bonding interactions with
phenolic and ethoxy oxygen atoms of the ligand and thus complex
2 belongs to the category of host-guest or inclusion compounds.
The formation of an inclusion product in such ligand environment
is quite rare and so far only the mononuclear compounds of nick-
el(II) and oxovanadium(IV) are known to feature potential water
encapsulation [44–46]. In fact, there are only two structurally char-
acterized mononuclear copper(II) compounds of general formula
[CuL(H2O)] with this type of compartmental ligands: one of them
is derived from the ligand N,N0-ethylenebis(3-methoxysalicylaldi-
mine [47] and the other one, recently reported by our research
group is from H2L1 ligand (the one used for complex 1) [48]. In
these two last complexes there is a water molecule, but it is di-
rectly coordinated to the square pyramidal CuII center, showing
that the encapsulation of a water molecule in the vacant O4 cavity
as observed in the case of complex 2 is not a common phenomenon
in this type of metal compounds containing a vacant compartment.
Selected bond lengths and bond angles for 1.
Bond lengths (Å)
Cu2–O4
Cu2–O8
Cu2–O2
Cu2–O6
Cu2–O3
Cu2–O7
Cu1–O17A
2.233(7)
2.219(7)
2.204(7)
2.161(6)
1.889(6)
1.878(5)
2.457(14)
Cu1–O2
Cu1–O3
Cu1–N1
Cu3–O6
Cu3–O7
Cu3–N3
Cu3–O18A
1.917(6)
1.941(6)
1.930(7)
1.926(5)
1.930(6)
1.935(7)
2.367(14)
Bond angles (°)
O3–Cu2–O7
O2–Cu2–O4
O4–Cu2–O8
O6–Cu2–O7
O2–Cu2–O3
O7–Cu2–O4
O2–Cu2–O8
O2–Cu1–N2
N2–Cu1–N1
174.6(2)
149.8(3)
80.7(2)
73.5(2)
73.5(2)
99.4(3)
102.4(3)
167.9(4)
96.0(3)
O3–Cu1–N2
O2–Cu1–O3
O17A–Cu1–N1
O17A–Cu1–O2
O6–Cu3–N4
O6–Cu3–N3
N3–Cu3–N4
O6–Cu3–O7
O18A–Cu3–N3
O18A–Cu3–O6
91.7(3)
73.5(2)
118.5(4)
90.4(4)
167.9(3)
95.0(3)
96.9(4)
78.0(2)
112.2(4)
90.9(4)
a bis(l
2-phenoxo) bridged CuII–CuII motif. Another CuII center is
again bonded to this CuII–CuII motif via a similar bridging segment
and completes the trinuclear unit. An interesting feature remains
in the variation of the ligating modes of the o-vanillin moieties
present at the two fag ends of the completely symmetrical ligand
H2L1. In the potentially hexadentate ligand (L1)2ꢁ, one –OMe group
is bonded to the central Cu2 atom while its counterpart in the
other end of the same ligand remains uncoordinated. The same
behaviour is also observed for the other ligand. Thus, each H2L1 be-
haves in a pentadentate fashion utilizing the N2O2(phenoxo)O(methoxy)
donor set for coordinating the metals. The crystal structure of 1
presents two different coordination geometries: an octahedral
one for the six coordinated central Cu2 and two square pyramidal
for the five coordinated terminal Cu1 and Cu3.
3.2. Description of the crystal structures
3.2.1. [Cu3(L1)2(H2O)2](ClO4)2 (1)
A perspective ORTEP view with the atom labeling scheme of the
asymmetric unit of [Cu3(L1)2(H2O)2](ClO4)2 (1) is presented in
Fig. 1. Selected bond lengths and angles for complex 1 are given
in Table 2. The [Cu3(L1)2(H2O)2]2+ cation contains a trinuclear array
of CuII ions where two bridging phenoxo oxygen atoms of the
deprotonated Schiff base ligand (L1)2ꢁ connect each terminal cop-
per atom to the central copper atom. The overall +2 charge of the
unit is balanced by two perchlorate counter anions in the lattice.
The perchlorate anions and the water molecules are quite disor-
dered and only the major components of the disorder are shown
in Fig. 1. The Schiff base H2L1 acts as a N2O2 donor chelating ligand
with respect to the terminal CuII atom while both deprotonated
phenolic oxygens further bridge to the central Cu2 atom forming
The geometry around the central Cu2 can be best described as
distorted octahedral having an O2O2O2 donor set from the two li-
gands. The equatorial plane is constructed by four oxygen atoms,
among them two are from two deprotonated bridging phenoxo
oxygens [O2 and O6] while the other two are from two methoxy
oxygens [O4 and O8] coming from the two different ligands. The
two axial sites are occupied by the two remaining bridging phen-
oxo oxygens [O3 and O7]. The coordination geometry considerably

S. Thakurta et al. / Inorganica Chimica Acta 363 (2010) 1395–1403
1399
deviates from ideal octahedral since CuII ion being a d9 system
suffers from Jahn-Teller distortion. The Cu–Ophenoxo and Cu–
Omethoxy bond lengths in the equatorial plane comprised between
2.161(6) and 2.233(7) Å are much longer as compared to the axial
Cu–O distances ranging from 1.878(5) to 1.889(6) Å, giving a clear
indication of marked compression type of Jahn-Teller distortion
(Table 2). The only trans O–Cu–O bond angle close to 180° is the
axial one [O3–Cu2–O7 = 174.6(2)°], whereas the trans-angles in
the equatorial plane are much lower [149.8(3) to 150.4(3)°], as a
consequence of the distortion in the coordination sphere.
Both terminal copper atoms Cu1 and Cu3 in the salen-type cav-
ity are penta-coordinated bearing an almost identical environment
having only slight differences in the bonding parameters. Five-
coordination for CuII is very common and usually it may possess
either square pyramidal (SP) or trigonal bipyramidal (TBP) or any
of the distorted intermediated geometries. For a penta-coordinated
metal center, the distortion of structure from trigonal bipyramidal
(TBP) to square pyramidal (SP) can be evaluated by the Addison
distortion index,
the two largest coordination angles;
ideal TBP [49]. In complex 1 the calculated
s
, defined as
s
= |h ꢁ
U
|/60, where h and
= 0 for perfect SP and 1 for
values for Cu1 and
U are
s
s
Fig. 2. Packing diagram showing the extended H-bonded network of complex 1.
Hydrogen atoms not involved in H-bonding are omitted for clarity. Only the major
components of the disorder are shown.
Cu3 are 0.268 and 0.233 respectively, which clearly suggest dis-
torted square pyramidal geometry for the terminal copper atoms.
The basal plane of the square pyramid is formed by two imine
nitrogen atoms (N1 and N2 for Cu1, and N3 and N4 for Cu3) and
two bridging phenoxo oxygen atoms (O2 and O3 for Cu1, and O6
and O7 for Cu3). The apical position is occupied by an oxygen atom
from the coordinated water molecule (O17A and O18A in Cu1 and
Cu3, respectively). Both Cu1 and Cu3 are displaced from the mean
N2O2 plane of the Schiff base ligand by 0.17 and 0.18 Å respec-
tively, towards the apical oxygen. As expected, the Cu–Ophenoxo
equatorial bond distances around the terminal CuII ions
[1.917(6)–1.941(6) Å] are shorter than the apical Cu–Owater bond
distances [2.367(14)–2.457(14) Å]. The Cu–Nimine distances (Table
2) are also comparable to those observed in the similar systems
[29]. The average bite angles of the six-membered CuN2C3 and
CuONC3 chelate rings are 96.45° and 93.65°, respectively.
The most remarkable aspect of complex 1 is that the doubly
phenoxo-bridged trinuclear core adopts a unique angular arrange-
ment of the three copper centers (Fig. 1) which is evident from the
Cu1ꢂꢂꢂCu2ꢂꢂꢂCu3 intraatomic angle of 117.91°. Also, the interplanar
angle of 71.82° between the two mean Cu2O2 planes defined as
Cu2–O3–Cu1–O2 and O7–Cu3–O6–Cu2, is indicative of the signif-
icant twist in the trinuclear unit. In contrast, all the previous trinu-
clear complexes obtained by our research group exhibited linear
arrays of CuII ions in spite of containing analogous double phenoxo
bridges. Therefore, this angular disposition in 1 may result from
the steric strain imposed on the system due to the presence of both
coordinating and free –OMe side-arms of the ligands. Furthermore,
complex 1 is also original since none of the four bent trinuclear CuII
complexes with double oxo bridges found in the CCDC database
presents a CuII coordination environment similar to that of com-
plex 1. Thus, complex 1 is the first example where the central CuII
ion is octahedral and the two terminal ones are square pyramidal
(in the only known example with a central octahedral CuII ion,
the terminal ones present a square-planar coordination [50]). The
terminal Cu atoms are almost equidistant (ꢅ3.063 Å) from the cen-
tral atom, whereas the distance between the terminal atoms is
5.250 Å.
Table 3
Hydrogen-bonding parameters for 1.
D–Hꢂ ꢂ ꢂA
d(D–H) Å
d(Hꢂ ꢂ ꢂA) Å
d(Dꢂ ꢂ ꢂA) Å
\(DHA)°
O17A–H17Aꢂ ꢂ ꢂO14A
O17A–H17Aꢂ ꢂ ꢂO16A
C9–H9Aꢂ ꢂ ꢂO14A
0.90
0.90
0.97
2.39
2.57
2.48
3.27(2)
3.22(2)
3.376(15)
164
129
153
3.2.2. [CuL2 ꢀ (H2O)] (2)
A perspective view of the complex 2, together with the atom
labeling scheme, is shown in Fig. 3 and selected bond lengths
and angles are given in Table 4. In the complex, the Schiff base
H2L2 coordinates the metal ion via the imine nitrogen atoms and
the deprotonated phenolic oxygen atoms, while the ethoxy oxy-
gens remain uncoordinated. The four-coordinate copper center as-
sumes square-planar geometry where the equatorial plane is
constructed by two imine nitrogen atoms (N1A, N1B) and two phe-
nolic oxygen atoms (O1A, O1B). The copper(II) ion in the salen-type
cavity of (L2)2ꢁ is located almost exactly on the mean basal plane
(deviation of only 0.03 Å). The square plane is somewhat distorted
in nature as evidenced by the bond distances and angles (Table 4).
Thus, all the cis-angles fall within the range 84.73(4)°–94.52(5)°,
and the two trans-angles are in the range 166.16(5)°–169.52(5)°,
both being deviated from the respective ideal values of 90° and
180°. The Cu–Nimine and Cu–Ophenolic bond lengths are similar to
those of the literature values [48]. The six-membered chelate ring
CuC3N2 incorporating the trimethylene fragment from the starting
diamine exhibits a bite angle of 94.52°, while the average N–Cu–O
bite angle of the two six-membered rings is found to be 91.40°.
These values are consistent with those obtained for the six-mem-
bered chelate rings at the terminal copper centers of complex 1
and this in turn, establishes the similarity of the N2O2 chelating
mode of the two ligands, H2L1 and H2L2, despite of their different
bridging properties.
Several hydrogen bonds are observed in the crystal structure, as
illustrated in the packing diagram in Fig. 2. The perchlorate oxygen
atoms are engaged in O–HꢂꢂꢂO and C–HꢂꢂꢂO intermolecular hydro-
gen-bonding interactions with the coordinated waters and Schiff
base ligands respectively, giving rise to the formation of an ex-
tended network. Details of the hydrogen-bonding parameters are
listed in Table 3.
An important feature of this kind of mononuclear complexes
containing vacant ligand compartment is the potential to function
as host-guest compounds or inclusion compounds, where different
solvent molecules can be encapsulated, acting as guests, by form-
ing hydrogen bonds with the electron donor centers of the host
(ethoxy)
molecules. The external acyclic O2(phenolic)O2
compartment

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S. Thakurta et al. / Inorganica Chimica Acta 363 (2010) 1395–1403
Fig. 4. Representation of complex 2 in space filling model showing the encapsu-
lation of the water molecule in the outer O4 compartment of the ligand (colour
code: Cu = green, N = blue, O = red, Owater = dark red). (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
Fig. 3. ORTEP representation of complex 2. Displacement ellipsoids are drawn at
the 40% probability level. Dashed lines indicate hydrogen bonds.
Table 5
Hydrogen-bonding parameters for 2.
D–Hꢂ ꢂ ꢂA
d(D–H) Å
d(Hꢂ ꢂ ꢂA) Å d(Dꢂ ꢂ ꢂA) Å
\(DHA)°
Table 4
O(1W)–H(1W1)ꢂ ꢂ ꢂO(2A) 0.813(14) 2.131(15)
O(1W)–H(1W1)ꢂ ꢂ ꢂO(1A) 0.813(14) 2.55(2)
2.9082(16) 160(2)
3.1558(17) 132.4(19)
2.9534(18) 148(2)
3.0222(17) 142(2)
Selected bond lengths and bond angles for 2.
O(1W)–H(1W2)ꢂ ꢂ ꢂO(2B)
O(1W)–H(1W2)ꢂ ꢂ ꢂO(1B)
0.828(15) 2.216(17)
0.828(15) 2.327(19)
Bond lengths (Å)
Cu–O(1B)
Cu–O(1A)
Cu–N(1B)
Cu–N(1A)
1.9143(10)
1.9233(10)
1.9634(13)
1.9751(12)
hydrogen bonds responsible for such inclusion property of com-
plex 2 can be considered as fairly strong; those involving uncoor-
dinated ethoxy oxygens are slightly stronger. The details of
hydrogen-bonding parameters are given in Table 5. The phenome-
non of inclusion of water as shown by complex 2 is very subtle and
therefore, the examples of this kind of systems in similar ligand
environment are very scarce [44–46]. Till date the crystal structure
of only one copper(II)-mononuclear complex had been reported,
which exhibits encapsulation of water in the vacant O2O2 compart-
ment of N,N0-ethylenebis(3-ethoxysalicylaldimine) [51]. Structural
comparison reveals that complex 2 is very similar to this complex
as the bonding parameters of both of them are in nice agreement. It
is noteworthy that almost all such water inclusion compounds are
resulted by compartmental ligands of the type H2L–OEt, but not of
the type H2L–OMe, therefore it can be suggested that flexibility of
the ethoxy group is vital in shaping up a suitable cavity for better
accommodation of the guest water molecule.
Bond angles (°)
N(1B)–Cu–N(1A)
O(1A)–Cu–N(1A)
O(1A)–Cu–N(1B)
O(1B)–Cu–N(1A)
O(1B)–Cu–O(1A)
C(1A)–O(1A)–Cu
94.52(5)
91.10(5)
169.52(5)
166.16(5)
84.73(4)
124.10(10)
of the mononuclear unit of complex 2 behaves as a host site and
encapsulates one water molecule through hydrogen-bonding
interactions leading to the formation of an inclusion product. The
space filling model in Fig. 4 shows that the water molecule is well
embedded within the coordination pocket and is almost com-
pletely encapsulated by the ligand. In order to minimize the elec-
trostatic repulsion between the negative charge clouds of the
oxygen atoms, the water molecule is seated slightly above the
least-squares plane constructed by the exogenous O2O2 core
(Fig. 4). Each of the two hydrogen atoms of the guest water mole-
cule are engaged in bifurcated hydrogen-bonding interactions
(Fig. 3) with two oxygens of [L2]2ꢁ, one of which is a phenolic
and the second is an ethoxy oxygen. The presence of a few ring mo-
tifs can also be envisaged in the outer fragment as a direct outcome
of the hydrogen bonds which have a crucial role in stabilization of
the complex. Thus, two identical five-membered rings designated
by the graph set notation of R21 (5) are enclosed by these bifurcated
H-bonds and another six-membered ring involving the central CuII
ion is also formed which can be denoted by R22 (5). The
donorꢂ ꢂ ꢂacceptor contacts for the H2Oꢂ ꢂ ꢂphenolic and H2Oꢂ ꢂ ꢂeth-
oxy hydrogen bonds lie in the ranges 3.0222(17)–3.1558(17) Å
and 2.9082(16)–2.9534(18) Å, respectively, indicating that the
3.3. Infrared spectra
The solid state Fourier transform infrared spectra of the two
complexes are fully consistent with their single-crystal structures.
The strong m
C@N stretching vibrations at 1598 and 1602 cmꢁ1 for 1
and 2, respectively, are shifted considerably towards lower fre-
quencies compared to that of the free Schiff base ligands (ca.
1640 cmꢁ1) indicating the coordination of the imine nitrogen atom
to the metal center [52]. The phenolic C–O stretching is obtained at
1265 and 1272 cmꢁ1 for 1 and 2, respectively. Ligand coordination
to metal center is substantiated by prominent bands appearing at
nearly 455 and 375 cmꢁ1 which can be attributed to
and (Cu–O), respectively, in both complexes. The bands at
m(Cu–N),
m

S. Thakurta et al. / Inorganica Chimica Acta 363 (2010) 1395–1403
1401
2930 cmꢁ1 can be correlated to the alkyl C–H bond stretching of
alkoxy groups. The sharp, strong peak at 1087 cmꢁ1 for complex
1 is assigned to perchlorate stretching vibrations and the single
peak devoid of any splitting gives evidence of the presence of
non-coordinating perchlorate anion [53]. The IR spectra of the
complexes confirm the presence of water molecules. Complex 1
nal ones (Fig. 1), we have fit the magnetic properties to a simple
symmetrical S = 1/2 trimer model considering only one intratrimer
exchange constant, J (the Hamiltonian is written as H = ꢁJSiSi+1
)
[9]:
Ng2b2 1 þ e2x þ 10e3x
v
¼
;
where x ¼ J=kT:
4kT 1 þ e2x þ 2e3x
exhibits strong and broad band in the range 3325–3400 cmꢁ1
attributable to str(O–H) vibrations of the coordinated water mole-
,
m
In spite of its simplicity, this model reproduces very satisfactorily
cule [54]. Importantly, complex 2 exhibits two sharp bands at 3568
and 3518 cmꢁ1 due to the O–H stretching vibration of the water
molecule. Because of encapsulation, the motion of the water mol-
ecule is so restricted in 2 that the asymmetric and symmetrical
stretchings appear separately instead of a broad band, observed
commonly. The restricted motion of the water molecule is further
evidenced in the occurrence of one medium intensity band at
1541 cmꢁ1 due to the bending vibration mode of water [51].
the magnetic data of complex 1 in the whole temperature range
with g = 2.1000(6) and J = ꢁ120.1(2) cmꢁ1 (solid line in Fig. 5). As
expected, the magnetic coupling is antiferromagnetic and quite
strong. Nevertheless, this antiferromagnetic coupling is not able to
cancel the three S = 1/2 spins and, therefore, the ground spin state
is expected to be 1/2 (with only two of the three spins being can-
celled at low temperatures). The plateau at ca. 0.42 emu K molꢁ1 ob-
served at low temperatures confirms this result.
A further confirmation of the ground spin state at low temper-
atures comes from the isothermal magnetization at 2 K. This mea-
3.4. Electronic spectra
surement shows a saturation value of ca. 1.0
lB, which is the
In the electronic spectra of the complexes, the two strong
absorption bands in the region 350–385 nm are clearly charge
transfer in origin [55] and are attributed to the charge transfer
from the coordinated unsaturated ligand to the metal ion (LMCT).
Again, the intense high energy bands at about 245 and 380 nm may
expected value for a S = 1/2 with g = 2 (inset in Fig. 5). Additionally,
it is also supported by the fact that the isothermal magnetization
can be very well reproduced with a Brillouin function for a S = 1/2
with a g value of 2.0796(4) (solid line in the inset in Fig. 5).
The strong antiferromagnetic coupling observed in complex 1 is
not straightforward to understand since this complex presents two
different CuII coordination environments (Fig. 6). Nevertheless, a
more detailed inspection of the structure reveals that the complex
is almost symmetrical, which is manifested by the very similar
bonding parameters of the pair of double phenoxo bridges and also
by the fact that in both double phenoxo bridges, the oxygen atoms
connect equivalent positions of the metal ions. Thus, as can be seen
in Fig. 6, the central Cu2 is connected to each terminal CuII atom
(Cu1 and Cu3) through one short-short phenoxo bridge: (O3,
connecting Cu1 and Cu2 with Cu1–O3 = 1.940(6) Å and Cu2–O3 =
1.890(6) Å and O7, connecting Cu2 and Cu3 with Cu2–O7 =
1.878(5) and Cu3–O7 = 1.929(6) Å) and through one short-long
phenoxo bridge (O2, connecting Cu1 and Cu2 with Cu1-O2 =
1.917(6) Å and Cu2–O2 = 2.204(6) Å and O6, connecting Cu2 and
Cu3 with Cu3-O6 = 1.925(6) and Cu2–O6 = 2.161(6) Å). As ex-
pected, the short-short phenoxo bridges form larger Cu–O–Cu bond
angles (Cu1–O3–Cu2 = 106.4(3)° and Cu2–O7–Cu3 = 106.9(3)°)
whereas the short-long phenoxo bridges give rise to smaller
Cu–O–Cu bond angles (Cu1–O2–Cu2 = 96.0(3)° and Cu2–O6–
Cu3 = 96.8(3)°). Besides the similarities in the structural
parameters, the double phenoxo bridges also connect identical
be assigned to the intraligand
p ? p* and n ? p* transitions,
respectively. The broad low-intensity absorption band centered
at 605 nm for complex 1 is a typical d–d band for the copper(II)
ion [56]. For complex 2 this d–d band is obtained nearly at 645 nm.
3.5. Magnetic studies
The thermal variation of the molar magnetic susceptibility per
CuII trimer times the temperature (
vmT) for complex 1 shows at
room temperature a value of ca. 0.76 emu K molꢁ1 (Fig. 5). This va-
lue is significantly lower that the expected one for three non-inter-
acting CuII S = 1/2 ions (1.125 emu K molꢁ1 for g = 2.0). When
cooling down the sample, the
decrease to reach a plateau of ca. 0.42 emu K molꢁ1 at ca. 80 K. Be-
low this temperature mT remains constant down to 2 K. This
vmT product shows an almost linear
v
behaviour indicates that complex 1 presents a strong antiferro-
magnetic coupling, responsible of the decrease already observed
at room temperature. The
vmT value at low temperatures corre-
sponds to the expected one for an S = 1/2 ground spin state with
a g value of 2.101. Since the structure of complex 1 shows the pres-
ence of an angular CuII trimer with two almost identical double
phenoxo bridges connecting the central CuII ion to the two termi-
0.8
1.0
0.8
0.6
0.7
0.4
0.2
0.6
0.0
0.0
0.5
1.0
1.5
-1
2.0
2.5
H/T (T.K
)
0.5
0.4
0
50
100
150
200
250
300
T (K)
Fig. 6. Coordination environments in the CuII trimer of complex 1 showing the
bridging bond distances (Å) and angles (°) (colour code: Cu = green, N = blue,
O = pink). (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
Fig. 5. Thermal variation of the
vmT product for complex 1. Inset shows the
isothermal magnetization at 2 K. Solid lines represent the best fit to the models (see
text).

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S. Thakurta et al. / Inorganica Chimica Acta 363 (2010) 1395–1403
coordination positions of the CuII centers. The oxygen atoms O3 and
O7 can be considered as the axial positions of the central distorted
octahedron since Cu2 presents a contraction along this axis, as
clearly indicated by the bond distances around Cu2. We can see that
O2 and O6 occupy an equatorial position of Cu1 and Cu3, respec-
tively, and both of them are also equatorial with respect to the cen-
tral Cu2 atom (eq–eq). The other two bridging oxygens O3 and O7
connect an equatorial position of Cu1 and Cu3, respectively, but
they are bonded to the axial positions of central Cu2 atom (eq–
ax). Overall, the central copper (Cu2) is connected to either of the
terminal copper centers (Cu1 or Cu3) through double phenoxo
bridges of two distinct types; one of them is eq–eq while the other
represents eq–ax. It can be also noted that both terminal CuII atoms
are connected to the central Cu2 through their equatorial positions
(Fig. 6). Once established the equivalence between both bridges and
the nature of the bridging positions, we can now estimate the mag-
netic coupling expected through these double phenoxo bridges.
Since all the known CuII complexes connected through equatorial–
axial oxo bridges present weak antiferromagnetic couplings (with
J values in the range ꢁ4.3 to ꢁ0.25 cmꢁ1 [57], except in two cases
Fig. 7. X-band EPR spectra of polycrystalline complex 1 recorded at: (a) room
temperature and (b) 120 K. An asterisk indicated the position of the hyperfine
splitting with 63,65Cu nuclei.
where weak ferromagnetic couplings with J = 1.26 and 4.44 cmꢁ1
,
have been reported) [58,59], we can conclude that the magnetic
coupling through O3 and O7 must be negligible as compared with
the coupling through the eq–eq bridge (O2 and O6). The magneto-
structural correlations established for eq–eq double oxo bridges in
CuII complexes, show that the main parameters influencing the
magnetic coupling constants are the Cu–Cu distance and the Cu–
O–Cu bond angles [60–63]. These correlations suggest that for the
present complex the magnetic coupling constant should be ca.
ꢁ170 cmꢁ1 which is in quite good agreement with the experimental
value (ca. ꢁ120 cmꢁ1). This difference between the experimental
coupling constant and the estimated one may be attributed to the
quite significant deviations observed in the octahedral environment
of the central Cu2 atom that leads to a smaller overlap of the mag-
netic orbitals and, therefore, to a magnetic coupling weaker than
expected.
Fig. 8. X-band EPR anisotropic spectra of complex 1 recorded at 120 K in: (a) DMF
and (b) CH3CN.
In summary, we can conclude that the magnetic couplings be-
tween Cu2 and Cu1 as well as Cu2 and Cu3 must be very similar
(J) since they occur through similar phenoxo bridges and that this
coupling must be strong and antiferromagnetic, as confirmed by
the agreement between the model used (a symmetrical linear
S = 1/2 trimer) and the experimental data (Fig. 5).
and A|| = 187 ꢃ 10ꢁ4 cmꢁ1 in DMF and g|| = 2.236 and A|| = 191 ꢃ
10ꢁ4 cmꢁ1 in CH3CN), and match well with those reported for
square-planar CuII complexes formed by salicylaldehydeimine or
derivatives of salen (N,N0-bis(salicylidene)ethylenediimine)
[64–67].
The room temperature magnetic moment of the complex 2
which is characterized as a mononuclear ‘‘precursor” complex
The EPR spectra of a polycrystalline sample of complex 2 at 298
and 120 K resemble that of non-interacting monomeric CuII species
with well-resolved hyperfine features and g|| > g\ > ge. The spectral
was recorded for a polycrystalline sample at 30 °C. The
lue was found to be equal to 1.76 B, which is in excellent agree-
ment with the theoretical value (ca. 1.74 B) for one unpaired
leffective va-
l
2
2
features are characteristic of a square-planar geometry and a dx
ꢁy
l
ground state for the copper(II) center [68,69].
electron in magnetically diluted copper(II) complexes.
3.6. EPR spectra
4. Conclusion
The X-band spectra recorded on a polycrystalline sample of
complex 1 change significantly when passing from room tempera-
ture to 120 K (Fig. 7). Thus, at room temperature complex 1 shows
a weak signal with the typical hyperfine splitting due to the cou-
pling of the electron with the 63,65Cu nuclei (Fig. 7a). When the
temperature is decreased to 120 K this signal becomes isotropic
and appears centered at g = 2.083 (Fig. 7b), in good agreement with
A slight modification of the substituent groups of the two acy-
clic hetero-compartmental Schiff bases, H2L1 and H2L2 induces sig-
nificant variations in their coordinating properties, which in turn;
control the nuclearities of the resulting copper(II) complexes.
Although both ligands present very similar N2O4 donor sets, only
the ligand (L1)2ꢁ simultaneously utilizes its penta-coordination
mode and bridging capacities to furnish the expected trinuclear
complex 1. In contrast, the ligand (L2)2ꢁ remains as a tetradentate
chelating ligand to provide the mononuclear complex 2. The dou-
bly phenoxo-bridged trinuclear core in complex 1 adopts an unu-
sual angular configuration. Moreover, complex 1 has a special
importance from the magnetic viewpoint since 3d–3d homometal-
lic multinuclear complexes with such compartmental ligands are
very limited. Here, we have successfully drawn reasonable
magneto-structural correlations for the strong antiferromagnetic
the value obtained by the fitting of the variation of
vmT with the
temperature (see above). Also, this variation indicates the presence
of major changes in the magnetic signal of complex 1 as the tem-
perature is lowered, consistent with the SQUID measurements.
The spectrum of complex 1 in DMF or CH3CN is typical of a Cu2+
with an equatorial donor set (Ophenolic, Nimine, Nimine, Ophenolic) and
shows the characteristic super-hyperfine structure (Fig. 8). The
spectral parameters are very similar in both solvents (g|| = 2.245

S. Thakurta et al. / Inorganica Chimica Acta 363 (2010) 1395–1403
1403
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ing inclusion properties and encapsulates a water molecule in the
external cavity through hydrogen-bonding interactions. Overall,
here we have presented a comparative study on the formation,
structural aspects and properties of two very different complexes
(1 and 2) derived from two very similar ligands.
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CCDC 739722 and 739723 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
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