An adaptable heterometallic trinuclear coordination cluster in the synthesis of tailored one-dimensional architecture: Structural characterization, magnetic analysis and theoretical calculations

Article abstract of DOI:10.1016/j.poly.2015.10.014

A new trinuclear complex [(CuL^α-Me)₂Co(bnz)₂] (1) has been synthesized by using a metalloligand [CuL^α-Me] (H₂L^α-Me = N,N′-bis(α-methylsalicylidene)-1,3-propanediamine, bnz = benzoate) with

Full text of DOI:10.1016/j.poly.2015.10.014

Polyhedron 102 (2015) 366–374
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
An adaptable heterometallic trinuclear coordination cluster in the
synthesis of tailored one-dimensional architecture: Structural
characterization, magnetic analysis and theoretical calculations
⇑
Soumavo Ghosh, Sanjib Giri, Ashutosh Ghosh
Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India
a r t i c l e i n f o
a b s t r a c t
a
a-
A new trinuclear complex [(CuL -Me)2Co(bnz)2] (1) has been synthesized by using a metalloligand [CuL
Article history:
a
Me] (H2L -Me = N,N0-bis(
a
-methylsalicylidene)-1,3-propanediamine, bnz = benzoate) with trans-coordi-
nated syn–syn bridging benzoate group (1
Received 13 July 2015
Accepted 8 October 2015
Available online 22 October 2015
j
O:2
j
O0). The syn–syn coordinative selectivity of carboxylate
towards this trinuclear unit leads exclusively to the formation of linear coordination cluster. Such coor-
dinative adaptability is exploited for supramolecular assembly using a dicarboxylate linker, terephthalate
(tph, 1,4-benzenedicarboxylate) which yielded a tailored one-dimensional quasi-linear coordination
Keywords:
Crystal engineering
Heterometallic
polymer [(CuL -Me)2Co(tph)]nꢀ2nH2O (2) having the linear trinuclear node. Isothermal magnetization
a
measurement at 2 K suggests that both 1 and 2 posses S = 1/2 ground spin state indicating the presence
of antiferromagnetic coupling at low temperature. The variable-temperature magnetic susceptibility
measurements also reveal that both compounds are antiferromagnetically coupled with exchange cou-
pling constants (J) of ꢁ17.3 and ꢁ9.2 cmꢁ1 for 1 and 2, respectively. The nature and magnitude of
exchange interactions are further corroborated by density functional calculations.
Ó 2015 Elsevier Ltd. All rights reserved.
Building block
Magnetic exchange
Density functional theory
1. Introduction
from 2 + 1 condensation of a 2-phonolic carbonyl and a primary
diamine (Scheme 1) [4]. Previous reports indicate that, when the
The chemistry of self organization process through non-cova-
lent interactions has evolved to the idea of chemical programming
i.e., systems capable of spontaneously generating precise
supramolecular architectures by combinatorial self-assembly from
reliable supramolecular synthons, thus behaving as programmed
systems [1]. The program is molecular, where the information is
stored in the covalent structural framework of the individual com-
ponents and its operation is supramolecular, making use of recog-
nition algorithms based on specific interaction patterns [2]. When
the structural instructions are sufficiently strong, the system is
narrowed down to singularity. This strategy is widely adopted in
synthetic design of coordination polymers where mononuclear or
carbonyl is an aldehyde (salicyldehyde), the trinuclear complex
[(CuL)2M]n+ can desirably be transformed to linear or bent tecton
to form discrete or infinite architectures with judicial choice of col-
igands [4d,5]. On the other hand, use of 2-hydroxyacetophenone
a
invariably yields discrete bent complexes [(CuL -Me)2M]n+ that
act very inertly in the attempt of joining them to form coordination
polymer [6]. The preference for discrete bent trinuclear species
may arise as the
a-methyl group modulates (less ꢁR effect of
keto-carbonyl group) the property of phenoxido donor atoms in a
way that the interactions between two terminal metalloligands
are sufficiently strong through axial site of the Cu centers which
could not be perturbed by N-donor coligands. However, poly-
atomic O-donor ligands e.g. carboxylates are known to bind
strongly with this type clusters to form discrete linear complexes
[5a,7]. Therefore, a mono-carboxylate is expected to subdue the
Cu–O axial interactions from phenoxido atoms and to form a
discrete trinuclear complex whereas a di-carboxylate should join
polynuclear
(homo-/heterometallic)
complexes
containing
replaceable or available coordination sites (regarded as metallate-
cton) are combined with exodentate organic/inorganic ligands
(anionic or neutral) as complementary tecton [3]. Recently, we
had been intrigued by the robustness and adaptability of a family
of bimetallic trinuclear Cu2M (s, p, d and f metal ions) complexes
derived from N2O2 donor Salen type di-Schiff base ligands obtained
the linear cluster to generate
a
one dimensional (1D)
coordination polymer [8]. Such endeavor with polycarboxylates
has been pursued frequently to generate various oxido-bridged
1D-heterodinuclear systems while there is only one report that
deals with heterometallic coordination polymer comprised of
⇑
Corresponding author. Fax: +91 33 2351 9755.
E-mail address: ghosh_59@yahoo.com (A. Ghosh).
http://dx.doi.org/10.1016/j.poly.2015.10.014
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

S. Ghosh et al. / Polyhedron 102 (2015) 366–374
367
N
O
N
O
N
O
N
O
Cu
Cu
a
Scheme 1. Metalloligands [CuL] and [CuL -Me].
oxido-bridged 3d–3d0 trinuclear node [9]. Moreover, theoretical
DFT based evaluation of the magnetic exchange couplings between
such heterometallic spin centers of 3d-block ions through single
atom bridges or a long spacer (if any) are still considered to be very
rare [10].
crystals of X-ray quality appeared at the bottom of the vessel of
each solution.
Compound 1: Yield 0.031 g (59%) Anal. Calc. for C52H50N4O8Cu2-
Co (1044.99): C 59.77, H 4.82, N 5.36. Found: C 59.67, H 4.91, N
5.28%. IR (KBr):
= 1603 cmꢁ1
Compound 2: Yield 0.028 g, (56%) Anal. Calc. for C46H48N4O10
Cu2Co (1002.92): C 55.09, H 4.82, N 5.59. Found: C 55.18, H 4.72,
N 5.51%. IR (KBr): (C@N)
s+as(COOꢁ) = 1579, 1539, 1368 cmꢁ1
= 1600 cmꢁ1
m , m(C@N)
s+as(COOꢁ) = 1578, 1566, 1400 cmꢁ1
Herein we report synthesis, crystal structure and magnetic
properties of two new heterometallic coordination complexes of
.
-
a
a
formulae [(CuL -Me)2Co(bnz)2] (1) and [(CuL -Me)2Co(tph)]nꢀ2nH2O
(2) [H2L -Me = N,N0-bis(
a
-methylsalicylidene)-1,3-propanediamine;
m
, m
a
bnzꢁ and tph2ꢁ are benzoate and terephthalate, respectively]. As
is expected, the monocarboxylate (benzoate) and dicarboxylate
(terephthalate) groups resulted in the linear coordination cluster
and one-dimensional quasi-linear coordination polymer, respec-
.
2.4. Physical measurements
tively, through syn–syn bridging mode (1jO:2j
O0). The variable-
temperature magnetic susceptibility measurement reveals that
both compounds show very similar magnetic properties with anti-
ferromagnetic exchange coupling (J) of ꢁ17.3 and ꢁ9.2 cmꢁ1 for 1
and 2, respectively. A magneto-structural correlation along with
density functional theory based calculations and spin density plot
successfully validate the magnetic behavior of these complexes.
Elemental analyses (C, H and N) were carried out using a Perkin-
Elmer 2400 series II CHN analyzer. IR spectra (4000–500 cmꢁ1
)
were recorded by a Perkin-Elmer RXI FT-IR spectrophotometer in
KBr pellets. All solutions were prepared in spectroscopic grade
methanol. Variable-temperature magnetic-susceptibility data
were collected on crystalline samples of 1 and 2 with a Quantum
Design SQUID VSM magnetometer housed at the Center for
Research in Nanoscience and Nanotechnology (CRNN) of the
University of Calcutta. Pascal’s constants were used to quantify
diamagnetic corrections to the molar paramagnetic susceptibility,
and a correction was applied for the sample holder.
2. Experimental
2.1. Starting materials
Reagent grade 2-hydroxyacetophenone and 1,3-propanedi-
amine was obtained from Spectrochem, India and used as received.
The sodium salts were prepared by reacting the corresponding car-
boxylic acid (10 mmol) with Na2CO3 (10 mmol) in water (100 ml).
The solid product is collected through filtration and recrystallized
from hot water. Other reagents and solvents used were of commer-
cially available reagent quality, unless otherwise stated.
Caution! Perchlorate salts of metal complexes with organic
ligands are potentially explosive. Though not encountered
throughout the experiment, only a small amount of material
should be prepared and it should be handled with care.
2.5. Crystallographic data collection and refinement
Suitable single crystals of each of the two complexes were
mounted on a Bruker-AXS SMART APEX II diffractometer ready
with a graphite monochromator and Mo Ka (k = 0.71073 Å) radia-
tion. The crystals were placed at 60 mm from the CCD. 360 frames
were measured with a counting time of 5 s. The structures were
solved using Patterson method by using the SHELXS 97. Subsequent
difference Fourier synthesis and least-square refinement revealed
the positions of the remaining non-hydrogen atoms that were
refined with independent anisotropic displacement parameters.
However for complex 2, the disordered oxygen atom O(11) of
water molecule was refined isotropically in three positions. Hydro-
gen atoms were placed in idealized positions and their displace-
ment parameters were fixed to be 1.2 times larger than those of
the attached non-hydrogen atom except the interstitial solvent
water molecules in 2 in which H atoms could not be located in
the Fourier map. Successful convergence was indicated by the
maximum shift/error of 0.001 for the last cycle of the least squares
refinement. The non-ideal data obtained for complex 2 are due to
the intrinsic nature of the crystal which on removal from mother
liquor readily releases lattice solvent and disintegrate during data
collection. The data set presented here is the best collected.
Absorption corrections were carried out using the SADABS program
[12]. All calculations were carried out using SHELXS 97 [13], SHELXL
97 [14], PLATON 99 [15], ORTEP-32 [16] and WINGX system ver-1.64
[17]. Data collection with selected structure refinement parame-
ters and selected bond parameters for both the complexes are
given in Table 1 and Table S1, ESI respectively.
a
-Me
2.2. Synthesis of the Schiff base ligand H2L
and the metalloligand
a
-Me
[CuL
]
a
-Me
a-
The di-Schiff base ligand H2L
and the metalloligand [CuL
Me] were prepared by the reported method described earlier [11].
a
a
2.3. Synthesis of [(CuL -Me)2Co(bnz)2] (1) and [(CuL -Me)2Co
(tph)]nꢀ2nH2O (2)
a
-Me
To a solution (10 mL) of the precursor metalloligand [CuL
]
(0.038 g, 0.1 mmol) in methanol, a solution of Co(ClO4)2ꢀ6H2O
(0.018 g, 0.05 mmol, 5 mL methanol) was added and stirred for five
minutes. To it an aqueous solution (5 mL) of sodium salt of bnz
(benzoate) and tph (terephthalate) (0.014 g 0.1 mmol and
0.010 g, 0.05 mmol) for 1 and 2 respectively was slowly added drop
wise with stirring. A green precipitate appeared in case of 2. The
mixtures were stirred for 2.5 h and then filtered. The clear filtrate
was allowed to stand overnight at room temperature, and single

368
S. Ghosh et al. / Polyhedron 102 (2015) 366–374
Table 1
benzoate produces discrete trinuclear linear coordination cluster
whereas terephthalate yields a rare one-dimensional chain com-
prising of the same bimetallic trinuclear core.
Crystal data and structure refinement of complexes 1 and 2.
Compound
1
2
Besides elemental analyses, both compounds 1 and 2 were ini-
tially characterized by IR spectra (Fig. S1, ESI). The precursor met-
Formula
C
52H50N4O8Cu2Co
C46H48N4O10Cu2Co
1002.92
Formula weight
Space group
Crystal system
a (Å)
b (Å)
c (Å)
1044.99
monoclinic
P21/n
11.346(5)
10.808(5)
19.288(5)
90
105.944(5)
90
2274.3(16)
2
1.526
1.349
1078
0.0318
1.9–25.6
15480
4213
a
alloligand [CuL -Me] is neutral and does not have any counter
triclinic
ꢀ
P1
anion, whereas both the complexes include IR active carboxylato
coligands. The carboxylato anion shows its characteristic bands
for bidentate chelation for each compound [5]. In both complexes,
11.644(5)
11.787(5)
18.098(7)
91.579(9)
90.987(8)
113.674(8)
2272.9(16)
2
1.459
1.349
1026
0.0776
1.1–25.0
10874
7254
a strong and sharp band due to the azomethine m(C@N) group of
a
(°)
a
the Schiff base appears at 1600–1603 cmꢁ1 for [CuL -Me] as
b (°)
c
(°)
observed before [5].
V (Å3)
Z
Dcalc (g cmꢁ3)
3.2. Structure descriptions
l
(mmꢁ1
)
a
Both complexes 1 and 2 consist of linear trinuclear (CuIIL -Me)2-
F(000)
Rint
h (°)
Total reflections
Unique reflections
II
a
a
Co units [5a] of formulae [(CuL -Me)2Co(bnz)2] and [(CuL -Me)2Co
(tph)]nꢀ2nH2O, respectively where CoII resides at the center of
inversion. The asymmetric unit of complex
2
possesses
crystallographically two different halves of (CuL -Me)2Co units
a
Data with I > 2
r
(I)
3461
3052
a
R1 on I > 2
r
r
(I)
0.0296
0.0720
1.037
0.0936
0.2358
1.002
with slightly different bond parameters, which are connected by
b
wR2 (I > 2
GOFc on F2
T (K)
(I))
two carboxylate (1jO:2j
O0) groups from one tph linker. The
corresponding molecular structures are depicted in Figs. 1 and 2,
and selected bond lengths and angles are listed in Table S1, ESI.
The trinuclear units in both structures contain a hexacoordinate
central cobalt atom in a CoO6 octahedral geometry together with
two pentacoordinate square-pyramidal copper atoms. The four
296
296
a
b
c
R1
wR2 = [
GOF = [
=
R
||Fo| ꢁ |Fc||/
R|Fo|.
R
w(F2o ꢁ F2c)2/
R .
w(F2o)2]1/2
R
[w(F2o ꢁ F2c)2/(Nobs ꢁ Nparams)]1/2
.
l
2-bridging phenoxido oxygen atoms around CoII from two
(CuL -Me) metalloligands are in a planar arrangement. These are
O(1) and O(2) and their symmetry related ones (O(1a) and O(2a))
for Co(1) in complex 1. For complex 2, these atoms are O(1), O(2)
and O(5), O(6) and their symmetry related ones (O(1a), O(2a)
and O(5a), O(6a)) for Co(1) and Co(2) respectively. The Co–O dis-
tances for 1 are 2.093(2) and 2.208(2) Å whereas those for 2 ranges
2.115(8)–2.184(9) Å. These are comparable to the reported high
spin octahedral CoII–O bond distances [5a]. A pair of O atoms
a
2.6. Computational methodology
To calculate the coupling constant (J) for complexes 1 and 2, we
have performed DFT calculation and the energies of high spin (Ehs)
and broken symmetry (Ebs) states. The hybrid B3LYP functional
[18–20] and def2-TZVP [21] basis set has been applied in all calcu-
lations as implemented in the ORCA package [22]. We have incor-
porated zeroth-order regular approximation (ZORA) to describe
scalar relativistic effects along with tight SCF convergence criteria
(Grid4) [23]. To speed up the calculations, we have used RI approx-
imation by considering auxiliary def2-TZVP/J coulomb fitting basis
sets [24]. Finally, the J values have been obtained from the Eq. (1)
as proposed by Ruiz et al. [25].
from two syn–syn bridging (1jO:2j
O0) carboxylato donors
coordinate trans- to the central Co atom to complete the
octahedral coordination sphere of CoII. These atoms for complex
1; O(3) and O(3a) are distances at 2.023(2) Å and for complex 2;
O(3), O(3a) and O(7), O(7a) (for Co(1) and Co(2) respectively) are
distances at 2.009(8) and 2.035(8) Å respectively. In each of the
Co centers for both complexes, the cis angles involving the
carboxylato oxygen atoms [in the ranges of 87.7(1)°–92.3(1)° for
1 and 88.6(3)°–91.4(3)° for 2] are close to the ideal values (90°)
but the cis angles in the plane of phenoxido O atoms [range
between 73.8(1)°–106.2(1)° for 1 and 76.2(3)°–103.8(3)° for 2]
deviate considerably indicating a distorted octahedral geometry.
The symmetrically related Cu atoms from each metalloligand of
Ebs ꢁ Ehs
2J ¼
ð1Þ
2S1S2 þ S1
where S1 P S2
3. Result and discussions
3.1. Syntheses and spectroscopic characterizations of complexes 1
and 2
a
these centrosymmetric linear (CuIIL -Me)2CoII units take up a five
coordinated square pyramidal geometry in both complexes. Each
of these square pyramids is comprised of a N2O2 basal plane
belonging to two phenoxido O atoms and two imine N atoms
(O(1), O(2), N(1) and N(2) for Cu(1) in 1; O(1), O(2), N(1), N(2)
and O(5), O(6), N(3), N(4) for Cu(1) and Cu(2) respectively in 2)
Previously, we have synthesized quite a few trinuclear com-
a
plexes using [CuL] and [CuL -Me] as metalloligands [H2L = N,N0-
bis(salicylidene)-1,3-propanediamine] (Scheme S1, ESI) [4–6]. Sev-
eral trinuclear complexes derived from [CuL] were connected suc-
cessfully by N-donor linker to produce oligo-/polymeric
complexes. In contrast, every attempt of joining the metallatecton
a
-Me
from doubly deprotonated H2L
ligand. The axial positions are
occupied by coordinated oxygen atoms (O(4) for Cu(1) in 1; O(4)
and O(8) for Cu(1) and Cu(2) respectively in 2) of the syn–syn
bridging carboxylato groups. The Cu–O and Cu–N bond lengths at
the N2O2 basal planes in 1 are 1.932(2), 1.955(2) and 1.990(2),
1.996(2) Å, respectively and the axial Cu–O bond distance is
2.257(2) Å. Whereas the Cu–O and Cu–N bond lengths at the
N2O2 basal planes in 2 are ranges between 1.943(8)–1.973(9) Å
and 1.949(16)–1.999(13) Å, respectively and axial Cu–O bond
distances are 2.199(11) and 2.221(10) Å. The r.m.s. deviation of
the four basal atoms from the mean plane for complex 1 is 0.016
a
derived from [CuL -Me] metalloligands with the same linkers failed
and only discrete trinuclear complexes were formed [6,26]. In the
present study, we chose O-donor ligands benzoate and terephtha-
late that can potentially coordinate to the metal centers in a stron-
ger way (one coordination bond for N-donor linker vs. two
coordination bonds for O-donor carboxylate linker). Both the com-
plexes were obtained as dark green crystals upon mixing the
required precursors in methanol at room temperature and subse-
quent slow evaporation of the solvent (Scheme 2). As expected,

S. Ghosh et al. / Polyhedron 102 (2015) 366–374
369
Co
O
Cu
O
2+
N
N
O
O
N
Cu
O
O
Co
Cu
N
O
O
Trinuclear Bimetallic
Metallatecton
N
N
O
O
N
Cu
O
O
Co
Cu
N
ONa
O
2
Methanol
O
ONa
M
O
O
O
e
th
a
n
o
l
ONa
O
O
O
O
N
N
O
O
N
N
O
O
N
Cu
Cu
O
O
Co
Cu
O
O
Co
Cu
N
N
N
O
O
O
O
O
Complex 1
Complex 2
O
Cu
Co
Scheme 2. Syntheses of complexes 1 and 2.
(4) Å with the metal atom 0.180(1) Å from this plane toward the
axially coordinated O(4) atom of Cu(1). The same deviations for
Cu(1) and Cu(2) centers in complex 2 are 0.054(23) and 0.026
(23) Å with the metal atoms 0.233(2) and 0.191(2) Å from the
plane toward the axially coordinated O(4) and O(8) atoms, respec-
tively. The Addison parameters of Cu(1) atom in complex 1 as well
as of Cu(1) and Cu(2) atoms in complex 2 are 0.013, 0.103 and
0.055 respectively indicating negligible amount of distortion
toward trigonal bipyramid geometry [27].
The structural characterizations of complexes 1 and 2 reveal
a
that the basic structural unit, (CuIIL -Me)2CoII metallatecton has
an intriguing ability to distinguish a carboxylato functionality only
at its trans-positions in a particular coordinating (syn–syn bridging)
mode that makes it useful for programmed supramolecular system
(Fig. 3) [5a,b]. It arises due to formation of a typical linear Cu2Co
coordination cluster comprising of a central Co-octahedron and
two terminal Cu-square pyramids in an edge-sharing manner
where the apical coordination sites of two Cu centers are
mutually trans and aligned with the axial sites of Co atom to
accommodate two carboxylato groups. Such coordinative
selectivity of the trinuclear unit towards carboxylate
(coordinative adaptability) would not allow any coordination
mismatch. As a result, it leads to the formation of a discrete
trinuclear complex, 1 with benzoate (a mono-carboxylate donor)
whereas with terephthalate (a di-carboxylate: ditopic linker) the
trinuclear metallatectons are connected to form a heterometallic
one-dimensional quasi-linear chain 2. The results indicate that
Fig. 1. Molecular structure of complex 1 along with atomic numbering scheme
(H atoms are omitted for clarity; color scheme Co pink, Cu green, O red, N blue,
C white; symmetry operation a = 2 ꢁ x, ꢁy, 2 ꢁ z). (Color online.)
a
the fidelity of the (CuIIL -Me)2CoII metallatecton in forming
precisely designed architectures with carboxylato based

370
S. Ghosh et al. / Polyhedron 102 (2015) 366–374
Fig. 2. Molecular structure of complex 2 with atomic numbering scheme (H atoms and solvent molecules are omitted for clarity; color scheme unchanged; symmetry
operation a = 3 ꢁ x, 3 ꢁ y, 1 ꢁ z and 2 ꢁ x, 2 ꢁ y, 2 ꢁ z for Co(1) and Co(2) respectively). (Color online.)
Fig. 3. Supramolecular growth from discrete linear complex 1 to 1D quasi-linear chain of 2 with identical coordination cluster substituting benzoate by terephthalate.
functionalities is good enough to use them as reliable
supramolecular synthon for crystal engineering and molecular
tectonics.
In case of 1, C–Hꢀ ꢀ ꢀ
p (phenyl) interaction is established between
the hydrogen atoms H(3) of a phenyl ring of one molecule and the
phenyl ring of the Schiff base of another molecule and vice versa,
forming a one-dimensional (1D) supramolecular column parallel
to crystallographic b-axis at C(3)–H(3)ꢀ ꢀ ꢀCg(1) and C(3)ꢀ ꢀ ꢀCg(1)
(Cg = centroid of the phenyl ring) distances of 2.61 and 3.47(1) Å,
respectively with \C(3)–H(3)ꢀ ꢀ ꢀCg(1) angles of 155° (symmetry
code: x, ꢁ1 + y, z). These supramolecular columns are closely
It is to be noted that a large number of bimetallic Cu(II)–M tec-
ton (M = Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Hg(II), Na(I), K(I), Ln(III))
consisting of Schiff base ligands derived from the reaction of ami-
nes with aldehydes have been used to prepare coordination poly-
mers [4d,6c,9,26,28]. Such example with Schiff bases derived
from ketone is extremely rare. Complex 2 represents only the
second coordination polymer where the Schiff base moiety of the
bimetallic tecton is made by 2 + 1 condensation of a ketone
(2-hydroxyacetophenone) and 1,3-propanediamine [29]. More-
over, the previously used bimetallic tectons are mostly dinuclear,
derived from bicompartmental Schiff base ligands. Compared to
them fewer bimetallic trinuclear tectons had been joined with
N-donor linkers (dicyanamide, cyanometallates, 4,40-bipridine
etc.) to form coordination polymers [4d,6c,9a,26,28c,28d,29,30].
There are some examples where very similar trinuclear units have
been joined with dicarboxylates but the used carbonyls are
aldehyde [9d,28e,31]. Complex 2 is the first example where a
keto-carbonyl derived bimetallic trinuclear tecton has been
connected with a dicarboxylate to form a rationally designed 1D-
coordination polymer.
ꢀ
packed to form a two dimensional layer parallel to 1 0 1 plane by
another C–Hꢀ ꢀ ꢀ
p (phenyl) interaction through H(15) at C(15)–H
(15)ꢀ ꢀ ꢀCg(2) and C(15)ꢀ ꢀ ꢀCg(2) with the distances of 2.97 and 3.82
(1) Å, respectively and \C(15)–H(15)ꢀ ꢀ ꢀCg(2) angles of 154° (sym-
metry code: 5/2 ꢁx, 1/2 + y, 5/2 ꢁ z). These layers are stacked to
form three dimensional architecture by C–Hꢀ ꢀ ꢀ
p (side chain)
interaction through H(9A) of the methylene chain of the Schiff
base. The C(9)–H(9A)ꢀ ꢀ ꢀCg(2) and C(9)ꢀ ꢀ ꢀCg(2) distances are 2.99
and 3.91(1) Å, respectively with \C(9)–H(9A)ꢀ ꢀ ꢀCg(2) angle of
157° (symmetry code: 3 ꢁ x, ꢁy, 2 ꢁ z). These stacking is further
reinforced by
through C(8)–H(8B)ꢀ ꢀ ꢀO(4) of the
a
non-classical C–Hꢀ ꢀ ꢀO interaction occurring
a-methyl group of the Schiff base
and carboxylato oxygen at H(8B)ꢀ ꢀ ꢀO(4) and C(8)ꢀ ꢀ ꢀO(4) distances
of 2.67 and 3.51(1) Å, respectively with \C(8)–H(8B)ꢀ ꢀ ꢀO(4) angles
of 146° (symmetry code: 3 ꢁ x, ꢁy, 2 ꢁ z).
In case of 2, the 1D-quasilinear chains being parallel to each
other (no supramolecular entanglement) mutually interact to form
a two dimensional supramolecular sheet like architecture through
a C–Hꢀ ꢀ ꢀO interaction parallel to 011 plane. It takes place through
3.3. Description of supramolecular interactions
The packing of the molecules in both complexes 1 and 2 is
a
-methyl group of the Schiff base with the carboxylate functions of
controlled by C–Hꢀ ꢀ ꢀ
types of C–Hꢀ ꢀ ꢀ interactions, namely, C–Hꢀ ꢀ ꢀ
C–Hꢀ ꢀ ꢀ (phenyl), between the neighboring molecules of 1 and 2
p
and C–Hꢀ ꢀ ꢀO interactions. There are two
terephthalate linkers at C(8)–H(8B)ꢀ ꢀ ꢀO(8) and C(8)ꢀ ꢀ ꢀO(8) dis-
tances of 2.71 and 3.61(3) Å, respectively with \C(8)–H(8B)ꢀ ꢀ ꢀ
O(8) angles of 157° (symmetry code: ꢁ1 + x, ꢁ1 + y, z). These
sheets further interact to form 3D-supramoleclar network by
p
p
(side chain) and
p
to generate a three-dimensional (3D) supramolecular architecture
(Fig. 4).

S. Ghosh et al. / Polyhedron 102 (2015) 366–374
371
Fig. 4. 3D supramolecular packing (right panel) of trinuclear complexes 1 (upper panel) and 2 (lower panel) formed by stacking of 2D layers (highlighted as red boxes) which
are assembled (left panel) from interaction between 1D columns/chains (highlighted as gray shading). See text for details. (Colour online.)
C–Hꢀ ꢀ ꢀ
p
(side chain) interaction through H(31A) of the
a
-methyl
at low temperature (a small bump in compound 1, ascribed to
the occurrence of a small portion of a paramagnetic impurity).
The decrease in the observed vMT may be account for two possible
reasons: firstly, it may be an outcome of the first-order spin–orbit
coupling present in Co(II) complexes; secondly, it may be because
of an antiferromagnetic coupling between the central Co(II) ion
and the terminal Cu(II) ones per Cu2Co unit. Since the spin–orbit
group of the Schiff base at C(31)–H(31A)ꢀ ꢀ ꢀCg(1) and C(31)ꢀ ꢀ ꢀ
Cg(1) distances of 2.99 and 3.81(2) Å, respectively with \C(9)–H
(9A)ꢀ ꢀ ꢀCg(2) angles of 144° (symmetry code: 1 ꢁ x, 2 ꢁ y, 2 ꢁ z).
The introduction of the tph in 2 is likely to form structural voids
that are occupied by guest water molecules which are not observed
in 1.
coupling in isolated Co(II) complexes normally led to
vMT values
at 2 K in the range of 1.0–2.0 cm3 K molꢁ1 which is well above
the observed value of ca. 0.1 cm3 K molꢁ1 on an average for both
compounds [33]. This indicates that both compounds present an
antiferromagnetic coupling between the central Co(II) and the ter-
minal Cu(II) ions.
3.4. Magnetic properties
Powdered polycrystalline samples of 1 and 2 were used for the
molar paramagnetic susceptibility (vM) measurements in the tem-
perature (T) range of 2–300 K under a constant magnetic field of
0.05 T. The compounds exhibited very similar magnetic properties
as expected from their analogous bond parameters (Fig. 5). The
resultant of the molar magnetic susceptibility times the tempera-
The presence of antiferromagnetic couplings in both com-
pounds are further confirmed from the temperature dependence
of the molar susceptibility vM that shows maxima at ca. 53 and
31 K for 1 and 2, respectively, which are slightly masked by the
paramagnetic contribution (Curie tail) of the S = 1/2 ground state
at low temperatures. This contribution of the S = 1/2 ground state
is clearly seen in the isothermal magnetization measurements of
1 and 2 at 2 K that show saturation values almost of 1 lB (0.93
and 1.04 lB for 1 and 2 respectively) which is the projected value
for a S = 1/2 spin ground state with g = 2 (Figs. S3 and S4, ESI).
Both compounds possess a centrosymmetric linear trinuclear
unit with a central Co(II) ion connected to two terminal Cu(II) ions
through a double phenoxido bridge and single carboxylato bridged.
As there is a negligible difference present between two Cu2Co tri-
mers in one asymmetric unit of compound 2 they can be consid-
ered as equivalent units like 1. We have considered a model that
ascribed for a ground state of 4T1 for the octahedral Co(II) ion with
S = 3/2 spin state with angular momentum L = 1. The Hamiltonian
ture (v
MT) shows the values of ca. 3.47 and 3.55 cm3 K molꢁ1 at
room temperature for compounds 1 and 2 respectively which are
within the usual range expected for two non-interacting Cu(II) ions
and a Co(II) ion i.e. per Cu2Co trimer. The values of ca. 2.67 and
2.75 cm3 K molꢁ1 for the Co(II) contribution of compounds 1 and
2, respectively are found by subtracting the expected room tem-
perature contribution for two Cu(II) ions (ca. 0.80 cm3 K molꢁ1
,
gCu(II) = 2.00) for each trimeric units (within the normal range of
2.5–3.1 cm3 K molꢁ1 observed in isolated high-spin octahedral Co
(II) complexes, gCo(II) = 2.1–7.0 [32]. These values are way above
the expected ones for an S = 3/2 spin ground state (1.875 cm3 -
K molꢁ1) that signifies the presence of an orbital contribution aris-
ing from the 4T1 ground state of high-spin octahedral Co(II)
complexes. The
vMT values exhibit a constant decrease by lowering
the temperature to reach a smooth plateau of ca. 0.1 cm3 K molꢁ1

372
S. Ghosh et al. / Polyhedron 102 (2015) 366–374
Fig. 5.
vMT vs. T plots for complexes 1 (left) and 2 (right) at an applied dc field of 0.05 T.
for this trinuclear system comprises of three part: (a) an isotropic
exchange interaction between Cu(II) and Co(II) real spins (S) that
based on the Lines model [6c,34], (b) a spin–orbit interaction
term in the Co(II) ion, and (c) a term considering the effect of
axial distortions [35]:
the carboxylato functionalities are bound to the square pyramidal
Cu(II) ions through the long axial positions, their interaction with
the dx2
magnetic orbitals of the Cu(II) ions and, thus, their con-
ꢁy2
tribution to the magnetic coupling is expected to be negligible
[41]. Thus in the present system, Cu–O–Co bridges are only active
for magnetic couplings which is mainly controlled by the Cu–O–Co
bond angle and bond distances. Correlations show that for such
cases the coupling is antiferromagnetic (even average bond angle
as low as 91.5°) and increases (in absolute value) as the Cu–O–Co
angle increases [5a].
In compounds 1 and 2 the Cu–O–Co bond angles are 97.0(1)°
and 100.2(1)° (in 1) and 96.2(3)°, 95.3(3)°, 97.3(3)°, and 96.5(4)°
(in 2). We can conclude that for the angles present in compounds
1 and 2 the Cu–Co coupling is expected to be antiferromagnetic
and moderate, in agreement with the experimental results. Fur-
thermore, since the average Cu–O–Co bond angles are significantly
larger in compound 1 (98.6(1)°) than in 2 (96.3(8)°), compound 2 is
expected to show a smaller antiferromagnetic coupling which is in
agreement with the observed J values.
The antiferromagnetic interaction observed in compounds 1
and 2 are of the same order as those of the only two reported dis-
crete linear triply bridged Cu2Co complexes with similar double
oxido bridges [5a]. The present study thus allows investigating
any strong magnetic exchange taking place between the
connected units. However such complexes behaved similar as
discrete magnetic unit while linking them with such a long
spacer as terephthalate due to very small exchange taking place
through linker compared to that through phenoxido bridges. In
order to increase the magnetic communication between two
consecutive trinuclear units the linker better be much shorter
albeit, one cannot establish any comparison.
^
H ¼ ꢁ2JðSCu1SCo þ SCu2SCoÞ þ A
r
kSCoL þ DðL2 þ LðL þ 1ÞÞ
z
In this Hamiltonian J is the exchange coupling constant between
Co and Cu, is the orbital reduction factor and k is the spin–orbit
r
coupling constant. The A factor, described in the perspective of T
and P term isomorphism that allows differentiation between the
matrix elements of the orbital angular momentum operator, calcu-
lated with the employment of the P term basis from those calcu-
lated with the wave functions of the 4T1 term [36]. The Zeeman
interaction is supposed to be isotropic and can be demonstrated as:
^
H ¼ bðgeSCo þ A
rLÞH þ bgCuðSCu1 þ SCu2ÞH
where the first part illustrates the interaction of an octahedral Co(II)
ion with an external magnetic field together with both spin and
orbital Zeeman contributions (ge is the electronic g factor) and the
second term only tells about the spin Zeeman contribution of the
Cu(II) ions.
Since, the problem does not allow for an analytical solution for
fitting the magnetic properties to the wave equation from the
resulting Hamiltonian [37]; therefore, to simulate the magnetic
properties in these systems, it is necessary to employ matrix diag-
onalization procedures [38] and the program PHI [39] was
employed for the diagonalization of the matrix arising from this
Hamiltonian and to obtain the parameters that best fit the experi-
mental data. A very reasonable fit of the
vMT product is obtained
for both compounds with the parameters displayed in (solid lines
in Fig. 5). The data could only be fitted satisfactorily if the contri-
bution of a small amount of a Cu(II) paramagnetic impurity (unre-
acted metalloligand) was considered (amounting to molar
fractions of 1.5% and 2.5% in 1 and 2, respectively). The best solu-
tions obtained for J, for compounds 1 and 2 were, ꢁ17.3 and
ꢁ9.2 cmꢁ1 respectively. Very weak coupling shall take place across
the terephthalate linkers which is negligible compared to phenox-
ido bridges thus cannot be quantified. The solutions were acquired
3.5. DFT calculations
To obtain further insight into the magneto-structural phenom-
ena along with exchange mechanism, we adopted DFT calculations
using broken symmetry approach and calculated J values theoret-
ically. The DFT calculation gives the following values of J = ꢁ19.01
and ꢁ8.75 cmꢁ1 for complexes 1 and 2, which agree well with the
for g, k, and
r values of (in the 1/2 format) 2.02/2.08,
Table 2
ꢁ180/ꢁ179 cmꢁ1, and ꢁ1.25/ꢁ1.10, respectively, and the conse-
Calculated and experimentally obtained coupling constant (J) parameters of com-
plexes 1 and 2.
quent simulations are represented as solid lines in Fig. 5.
The antiferromagnetic coupling observed in these two com-
pounds can be explained from their structures. They both present
double 1,1-phenoxido and single 1,3-carboxylato bridges connect-
ing the central Co(II) ion with the two terminal Cu(II) ones which
are known to be active as magnetic couplers [40]. However, as
Complex Average Cu–O–Co
Coupling constant (J)
Coupling constant
bond angles (°)
Ruiz method (cmꢁ1
)
(J) Exp (cmꢁ1
)
1
2
98.6(1)
96.3(8)
ꢁ19.01
ꢁ8.75
ꢁ17.26
ꢁ9.2

S. Ghosh et al. / Polyhedron 102 (2015) 366–374
373
Fig. 6. Spin density plot corresponds to the low spin state (S = 1/2) for 1 (left) and 2 (right). Positive and negative values are represented as white and blue surfaces with
isosurface cutoff value 0.004 e/Å3. (Colour online.)
experimentally obtained values of J = ꢁ17.26 and ꢁ9.2 cmꢁ1
,
antiferromagnetically coupled with exchange coupling (J) of ꢁ17.3
and ꢁ9.2 cmꢁ1 for 1 and 2 respectively. The difference in values of
J can be correlated with phenoxido bridging angles of such com-
pounds. The nature and magnitude of exchange interactions were
further supported from density functional calculations. These dis-
crete and polymeric systems with structural similarities give an
excellent platform for comparative study of magnetic properties. It
suggests that the phenoxido bridging atoms act as the significant
spin carriers in these systems whereas possible contribution of mag-
respectively (Table 2). The spin density plots (Fig. 6) show that
the dx2
orbitals of the Cu(II) centers combined with the hybrid
ꢁy2
p-orbitals of the bridging phenoxy ligands and Co(II) d-orbital
are playing main role in the hetero exchange coupling.
It is clear from the Mulliken atomic spin distributions (Table S2,
ESI) that both of the complexes at high spin state support direct
spin delocalization mechanism in which Co atoms carry ꢂ80% of
net spin and the remaining part is delocalized over coordinating
and bridging atoms of the ligands. The spin delocalization is strong
as it is found that ꢂ20% of the spins for the unpaired electrons on
the Cu(II) and Co(II) centers are delocalized onto the ligand atoms.
It is also seen that the phenoxo-bridged oxygen are carrying a
moderate amount of (0.10–0.11 au) positive spin density. This indi-
cates a weak to moderate antiferromagnetic exchange through
direct spin delocalization mechanism. It is worth to note that with
increase in positive spin density at bridging phenoxo oxygen
causes more delocalization which in turn is responsible for stron-
netic exchange through
l-1,3-carboxylate bridges or along tereph-
thalate is negligible. This DFT-based theoretical perspective of
magnetic interaction pathways of the heterometallic nodes is pre-
sented in this study which has rarely been evaluated previously.
Acknowledgements
A.G. thank the Department of Science and Technology (DST),
New Delhi, India, for financial support (SR/S1/IC/0034/2012) as
well as for the DST-FIST funded single crystal X-ray diffractometer
facility at the department of Chemistry, University of Calcutta. We
also thank CRNN, University of Calcutta, Kolkata, India for mag-
netic measurement. S. Ghosh and S. Giri are thankful to the Univer-
sity Grants Commission (UGC), New Delhi, for the Senior Research
Fellowship and Dr. D. S. Kothari Postdoctoral Fellowship
respectively.
ger antiferromagnetic interaction [42]. Considering l-1,3-carboxy-
late bridges, it is seen that noticeable spin densities are delocalized
on the carboxylate oxygen connecting to cobalt center; however,
spin densities on carboxylate oxygen connecting to copper center
is negligible. The central carbon atoms of carboxylates group also
show a very small spin density in both of the cases implying a very
weak antiferromagnetic interaction through the carboxylate linker.
4. Conclusions
Appendix A. Supplementary data
Herein, we synthesized two bimetallic CuI2ICoII complexes,
derived from a N2O2-donor salen-type Schiff base metalloligand
The electronic supplementary information file contains the IR
spectra, the synthetic scheme for trinuclear tecton, the plots for
molar paramagnetic susceptibility (vM) measurements in the tem-
a
[CuL -Me], from keto precursor that has been very rarely used to syn-
thesize coordination polymer. This metalloligand usually produces
discrete bent trinuclear complexes even in the presence of bridging
anions. However, utilizing the coordinative selectivity of the flexible
perature (T) range of 2–300 K and field dependant molar magneti-
zation plots at 2 K (Figs. S1–S3) as well as tables for bond
parameters and Mulliken atomic spin densities of the complexes
(Tables S1 and S2). CCDC 1055823 (for 1) and 1055824 (for 2) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge via http://www.ccdc.cam.ac.
uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary
a
trinuclear [(CuL -Me)2Co]2+ coordination cluster towards carboxylate
donors, we succeed to steer its geometry into linear form as is found
in the discrete complex with benzoate (1). Our attempt in joining
these linear nodes by dicarboxylate to generate 1D-polymeric archi-
tecture is successful with terephthalate yielding complex 2. Mag-
netic susceptibility measurement reveals that both compounds are

374
S. Ghosh et al. / Polyhedron 102 (2015) 366–374
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