Structural and Magnetic Analysis of Retrosynthetically Designed Architectures Built from a Triply Bridged Heterometallic (CuL)2Co Node and Benzenedicarboxylates

Article abstract of DOI:10.1002/ejic.201500273

The flexible trinuclear metallatecton {(CuL)₂Co}²⁺ [H₂L = N,N′-bis(salicylidene)-1,3-propanediamine] has been used as a building block, and its coordinative interaction with ortho-, meta-, and para-benzenedicarboxylates (BDCs) has been investigated to evaluate the positional isomeric effect of these carboxylate ligands on the resulting compounds. Structural characterization reveals that o-H₂BDC produces a discrete trinuclear complex, namely [(CuL)₂Co(o-HBDC)₂]·H₂O (1). As expected from the retrosynthetic design, one-dimensional (1D) coordination polymers, i.e., [{(CuL)₂Co(m-BDC)}·CH₃OH]_∞ (2) and [{(CuL)₂Co(p-BDC)}·2CH₃OH]_∞ (3), are obtained with m-BDC and p-BDC linkers, respectively. In all three complexes, the triply bridged bimetallic trinuclear coordination clusters are linear and solely a syn-syn bridging mode is observed for the carboxylates. In 2, the nodes are connected sidewise and are inclined toward each other to be accommodated by the 120-angular m-BDC units, whereas in 3, the linear p-BDC linkers connect the parallel nodes diagonally. These differences in connectivity give rise to one-dimensional "great wall"-like chains for 2 and to quasi-linear coordination chains for 3. These distinct one-dimensional propagations induced by the different BDC isomers influence the three-dimensional columnar packing of 2 and 3. Magnetic susceptibility measurements and fitting of the results confirm that these three compounds are made by analogous magnetic building blocks, with very similar antiferromagnetic intramolecular magnetic exchange (with J in the -16.96 to -11.80 cm^-1 range, using the -2JS₁S₂ convention). The structural homogeneity of the magnetic fragment within this family of compounds has allowed magnetostructural correlations to be explored, which has not previously been attempted.

Full text of DOI:10.1002/ejic.201500273

FULL PAPER
DOI:10.1002/ejic.201500273
Structural and Magnetic Analysis of Retrosynthetically
Designed Architectures Built from a Triply Bridged
Heterometallic (CuL) Co Node and Benzenedicarboxylates
2
[
a]
[b]
[b,c]
Soumavo Ghosh, Guillem Aromí, Patrick Gamez,
Ashutosh Ghosh*[
and
a]
Keywords: Schiff bases / Heterometallic complexes / Bridging ligands / Coordination polymers / Magnetic properties
The flexible trinuclear metallatecton {(CuL)
2
Co}2+ [H
2
L =
accommodated by the 120°-angular m-BDC units, whereas
in 3, the linear p-BDC linkers connect the parallel nodes
diagonally. These differences in connectivity give rise to one-
dimensional “great wall”-like chains for 2 and to quasi-linear
coordination chains for 3. These distinct one-dimensional
propagations induced by the different BDC isomers influence
the three-dimensional columnar packing of 2 and 3. Mag-
netic susceptibility measurements and fitting of the results
confirm that these three compounds are made by analogous
magnetic building blocks, with very similar antiferromag-
netic intramolecular magnetic exchange (with J in the –16.96
N,NЈ-bis(salicylidene)-1,3-propanediamine] has been used
as a building block, and its coordinative interaction with or-
tho-, meta-, and para-benzenedicarboxylates (BDCs) has
been investigated to evaluate the positional isomeric effect
of these carboxylate ligands on the resulting compounds.
Structural characterization reveals that o-H
discrete trinuclear complex, namely [(CuL)
O (1). As expected from the retrosynthetic design, one-
dimensional (1D) coordination polymers, i.e., [{(CuL) Co(m-
BDC)}·CH OH] (2) and [{(CuL) Co(p-BDC)}·2CH OH] (3),
2
BDC produces a
2
Co(o-HBDC) ]·
2
H
2
2
3
ϱ
2
3
ϱ
–
1
1 2
are obtained with m-BDC and p-BDC linkers, respectively. In to –11.80 cm range, using the –2JS S convention). The
all three complexes, the triply bridged bimetallic trinuclear
coordination clusters are linear and solely a syn-syn bridging
mode is observed for the carboxylates. In 2, the nodes are
connected sidewise and are inclined toward each other to be
structural homogeneity of the magnetic fragment within this
family of compounds has allowed magnetostructural corre-
lations to be explored, which has not previously been at-
tempted.
often leads to random processes.[3] From the synthetic point
of view, the design of molecular building blocks that will
allow a higher rate of predictability is one of the crucial
Introduction
One of the most significant advances that has revolution-
ized synthetic chemistry in the second half of the 20th cen-
tury was the combinatorial assembly of small molecular
building blocks of complementary functionalities under
specific conditions to generate more intricate, finite, or infi-
nite architectures of different dimensions.[1] This applies
from conventional organic or inorganic chemistry to cur-
rent macromolecular (modules and connectors) or supra-
molecular chemistry (nodes and spacers).[ The predictabil-
ity of supramolecular architectures that are formed by using
organic modules based on a covalent frame is superior to
inorganic nodes involving noncovalent interactions, which
[4]
challenges of modern metallosupramolecular chemistry.
To confine the directionality of a metallic node, the use
of discrete complexes of various nuclearities as metallatec-
tons in combination with exodentate organic ligands (spa-
cers) has been the foremost strategy to obtain predictable
[5]
architectures. In such cases, the influence of a specific ste-
reochemical preference of a spacer used for a particular tec-
ton is undeniable. The use of spacers with monoatomic do-
nating sites (e.g., N-donor spacers) has been highly success-
2]
[6]
ful in this regard. On the other hand, spacers with poly-
atomic donating sites often show unpredictable results be-
cause they can exhibit more than one type of stereochemi-
cal orientation to their complementary tecton. Polycarbox-
ylate spacers are well known for inducing such unpredict-
[
a] Department of Chemistry, University College of Science,
University of Calcutta,
[
7]
ability (Scheme 1); such compounds even adjust their li-
gating properties through accommodation of the geometric
disposition of the carboxylate functionalities around an or-
ganic tecton (e.g., BDCs), which is often termed a posi-
9
2, A.P.C. Road, Kolkata 700009, India
E-mail: ghosh_59@yahoo.com
http://173.201.183.21/~cuchemistry/?page_id=439
b] Departament de Química Inorgànica, Universitat de Barcelona,
Martí i Franquès 1–11, 08028 Barcelona, Spain
c] Institució Catalana de Recerca i Estudis Avançats (ICREA),
Passeig Lluís Companys 23, 08010 Barcelona, Spain
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500273.
[
[
[8]
tional isomeric effect. However, the positional isomeric
effect should strictly be defined as the architectural changes
that arise solely from a particular geometrical disposition
Eur. J. Inorg. Chem. 2015, 3028–3037
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FULL PAPER
of the donating atoms, without changing the coordination
mode to a particular tecton. Such a crucial distinction has tained from distinct BDC isomers (Scheme 1) provide an
One-dimensional coordination polymers (CPs)[14] ob-
been largely overlooked for polycarboxylates.
exceptional platform from which to establish magnetic cor-
relation for heterometallic clusters, as long as the nodes
satisfy non-nil-spin ground state at low temperatures.[
8b,15]
Very recently, we studied discrete (ML) MЈ single-atom
2
bridged spin-coupled systems with axially bridged carb-
oxylates that showed interesting magnetic properties.[
Although one can envisage joining such trinuclear tectons
using polycarboxylates (Scheme 2), to our knowledge, no
studies involving such a strategy have been described.
13,16]
II
II
Heterometallic Cu /Co systems are expected to exhibit in-
tricate magnetic properties as a result of considerable or-
bital and spin-orbit coupling contributions to the effective
magnetic moment arising from the intrinsic orbital angular
II
momentum in the octahedral ground state of high-spin Co
4
[17,18]
ions [ T (F)].
A few structurally and magnetically
1g
characterized 1D/2D/3D CPs containing heterometallic di-
II
II
phenoxido-bridged Cu /Co nodes have been report-
[
11b,19]
ed,
and most of these are constructed with binuclear
2
Scheme 1. Representation of the different H BDC isomers and the vari-
ous coordination modes of monocarboxylates.
nodes derived from compartmental ligands with various
spacers.
Besides the spacer itself, the metallatecton can also play
In the present study, we report on the synthesis, charac-
a very important role in controlling the stereochemical pref- terization, crystal structures, and magnetic properties of
II
II
erences of the spacer around the available coordination sites three [(Cu L)
Co ] complexes [H
L = N,NЈ-bis(salicylid-
2
2
[
5a,9]
of a node.
been studied with mononuclear nodes; examples involving complex [(CuL)
polynuclear nodes are rare, most likely because of the dination polymers [{(CuL)
The positional isomeric effect has mainly ene)-1,3-propanediamine], namely the discrete trinuclear
[8]
Co(o-HBDC)
Co(m-BDC)}·CH
OH] (3). Complexes 2 and 3
similar dinuclear nodes with different polycarboxylato li- represent very rare cases of rationally designed connectivity
]·H
2 2
O (1), and two 1D coor-
2
OH] (2) and
2
3
ϱ
higher complexity of ligation around these.[ For instance, [{(CuL)
10]
Co(p-BDC)}·2CH
2
3
ϱ
[
11]
[12]
gand backbones and metal combinations may produce between triply bridged bimetallic trinuclear nodes in one-
[
20a]
entirely different architectures that would depend on the dimension,
symmetry, steric demand, conformational or coordinative assemblies of homometallic MOFs are common.
flexibility etc. of the metallatecton. For trinuclear The results show a significant modulation of the connectiv-
(ML) MЈ] clusters (H L = salen-type Schiff base), it has ity, architectures, and 3D packing; yet, the relative positions
recently been found that these species can adopt a symmet- within the linear Cu –Co –Cu triads are fixed, with sim-
rical linear conformation that only permits a syn–syn bridg- ilar Cu –O–Co angles. However, slight changes in angles
although such connections in serendipitous
[
20b,20c,20d]
[
2
2
II
II
II
II
II
ing mode; apparently, irrespective of the metal ions give rise to small variations in the magnitude of the antifer-
[
13]
(Scheme 2).
Therefore, such trinuclear nodes potentially romagnetic exchange interactions in these complexes, en-
represent ideal systems to investigate the positional isomeric abling a possible magnetostructural correlation to be
effect of benzene dicarboxylates. Such a study has not yet drawn.
been reported, although metallatectons derived from Rob-
son-type macrocycles, bicompartmental N O - and N,N,O-
2
4
[
5b] Results and Discussion
donor ligands have been tested previously in this regard.
Synthesis and Spectral Characterization
Recently, we successfully synthesized several discrete and
polymeric [(ML) MЈ]-type complexes (where, M represents
2
II
II
Cu and Ni , and MЈ are different s-, p-, d-, and f-block
[
21]
elements).
In these polymeric complexes, neutral or an-
ionic N-donor spacers that connect the (ML) MЈ tectons
2
[
21c,21d,21e]
through bis-/tris-monodentate modes were used.
The flexibility of the molecular building blocks plays a cru-
cial role in the formation of isomeric coordination poly-
mers. In our present endeavor, we have used carboxylato
spacers for the first time with this type of metallatectons
with the objective of quenching their flexibility and generat-
ing linear structures. To limit the flexibility factor of the
2 2
Scheme 2. [(ML) MЈ] clusters (H L = salen-type Schiff base).
Eur. J. Inorg. Chem. 2015, 3028–3037
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spacers, we have used different geometric isomers of the ri-
gid BDCs, to achieve structure predictability. All three com-
plexes were obtained as light-green crystals upon mixing
the required precursors in methanol at room temperature,
followed by slow evaporation of the solvent. The conforma-
tional adaptability and quenched flexibility of the linear tri-
II
II 2+
[13a]
nuclear [(Cu L) Co ]
coordination cluster
distin-
2
guishes polyatomic carboxylate functionalities at the axial
positions, and permits solely a syn–syn bridging mode of
ligation of the three isomeric BDCs. However, the singly
protonated phthalate produces a discrete trinuclear com-
plex, whereas isophthalate and terephthalate offer rare one-
dimensional chains of the same metallic core.
In addition to elemental analyses, all the complexes were
initially characterized by their IR spectra. The precursor
metalloligand [CuL] is neutral (i.e., it does not have a
counter anion), whereas all three complexes contain IR-
active carboxylate co-ligands. The carboxylato anion shows
characteristic bands for bidentate chelation in each com-
[
22]
pound.
In all complexes, a strong and sharp band as-
cribed to the azomethine ν(C=N) group of the Schiff base
Figure 1. Representation of the molecular structure of 1 together with
a partial atomic numbering scheme. Hydrogen atoms attached to carbon
and solvent molecules are omitted for clarity; Co (pink), Cu (light-
–
1
appears at 1620–1624 cm for [CuL], as observed ear-
lier.[
22c]
All three complexes and the free metalloligand have been green), O (red), N (blue), C (gray), H (orange); intramolecular hydrogen
bonding shown in dark-green.
characterized by UV/Vis spectroscopy in acetonitrile as well
as in the solid state. In solution, all compounds display very
similar UV/Vis spectra, with intense bands at approximately
3
2
65 (ligand-to-metal charge-transfer transition of [CuL]), bond lengths of the ligating atoms to the CoII center in
72, and 230 nm (intraligand n–π* and π–π* transitions of 1 are comparable with those reported for CoO elongated
6
[
(
CuL]), which are similar to those of the free metalloligand octahedra exhibiting weak Jahn–Teller distortion, a typical
see the Supporting Information, Figure S1, left). The solid- feature for high-spin Co ions. The cis angles with the axi-
II
state electronic spectra exhibit small shifts of the d–d transi- ally coordinated atoms [in the range of 88.5(2)–91.7(2)°] as
tion bands of the metalloligand, as a result of its complex- well as the trans angles [in the range of 179.3(2)–179.8(2)°]
II
ation with the Co center (see the Supporting Information, are close to the ideal values (90 and 180°, respectively), but
Figure S1, right). Hence, the corresponding band at 598 nm the cis angles in the equatorial plane [in the range of
for the free [CuL] moiety is observed at 590 (1), 615 (2), and 75.3(2)–105.1(2)°] deviate considerably, and are thus indica-
612 nm (3), whereas the charge-transfer bands are found at tive of a distorted octahedral geometry. The terminal Cu(1)
3
84 nm for 1, and at 380 nm for 2 and 3.
and Cu(2) atoms are pentacoordinate in a square-pyramidal
coordination geometry. The basal planes of each of the two
terminal copper atoms contain four donor atoms of the
Schiff base; namely, two imine N atoms [N(1), N(2) for
Cu(1) and N(3), N(4) for Cu(2)] with Cu–N bond lengths in
the range 1.963(7)–1.984(5) Å, and two phenoxido O atoms
Description of the Structures
Structure of 1
The X-ray crystal structure of 1 (Figure 1) consists of [O(1), O(2) for Cu(1) and O(5), O(6) for Cu(2)] with Cu–O
[
13a]
a linear
HBDC stands for singly deprotonated phthalic acid). Se- bridging carboxylato groups occupy the axial positions of
lected coordination bonds and angles are listed in Table 1. the square pyramids with Cu(1)–O(4) and Cu(2)–O(8) dis-
trinuclear unit, [(CuL) Co(o-HBDC) ]·H O (o- bond lengths in the range 1.945(5)–1.955(5) Å. The
2 2 2
The structure of complex 1 includes a hexacoordinate tances of 2.301(5) and 2.282(5) Å, respectively. The angles
central cobalt(II) atom in a CoO octahedral geometry to- between the axially and basally coordinated atoms of the
6
gether with two pentacoordinate square-pyramidal cop- square-pyramidal Cu centers vary within a narrow range
per(II) atoms from two [CuL] metalloligands. Two bridging of 90.3(3)–98.0(3)°. The so-called Addison parameter (τ5)
phenoxido oxygen atoms from each [CuL], i.e., O(1) and amounts to 0.132 and 0.116 for Cu(1) and Cu(2), respec-
O(2), and O(5) and O(6) form the equatorial plane of the tively, which confirms the slightly distorted square-pyrami-
CoO unit, with bond lengths of 2.059(4)–2.085(4) Å. The dal geometry for the copper(II) ions (τ is 0 for a perfect
6
5
axial positions are occupied by the oxygen atoms O(3) and square pyramid, whereas it has a value of 1 for a trigonal
[
23]
O(7) of the syn–syn bridging carboxylato ligands bipyramid). The r.m.s. deviations of the four basally co-
1κO:2κOЈ) belonging to two monoanionic phthalates, with ordinated donor atoms from the mean planes are 0.107 and
bond lengths of 2.144(5) and 2.146(5) Å, respectively. The 0.088 Å for Cu(1) and Cu(2), respectively, with the metal
(
Eur. J. Inorg. Chem. 2015, 3028–3037
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Table 1. Selected bond lengths [Å] and angles [°] for complexes 1–3.[a]
joined together through two monoatomic and one triatomic
bridges, in an edge-sharing manner. Previous reports state
1
2
3
that [(ML) MЈ]-type tectons usually play multiple roles in
2
Cu(1)–O(1)
Cu(1)–O(2)
Cu(1)–O(4)
Cu(1)–N(1)
Cu(1)–N(2)
Cu(2)–O(5)
Cu(2)–O(6)
Cu(2)–O(8)
Cu(2)–N(3)
Cu(2)–N(4)
Co(1)–O(1)
Co(1)–O(2)
Co(1)–O(3)
1.945(5)
1.955(5)
2.301(5)
1.982(8)
1.963(7)
1.951(4)
1.950(5)
2.282(5)
1.970(6)
1.984(5)
2.075(5)
2.085(4)
2.144(5)
2.066(5)
2.059(4)
2.146(5)
81.3(2)
96.4(2)
90.6(3)
163.9(3)
90.8(2)
171.9(3)
90.3(2)
90.3(3)
97.4(2)
97.5(3)
80.7(2)
94.8(2)
91.5(2)
165.0(3)
91.3(2)
172.0(2)
91.3(3)
90.9(2)
98.0(3)
96.0(3)
75.3(2)
91.1(2)
179.3(2)
105.1(2)
88.6(2)
89.9 (2)
104.1(2)
179.4(2)
90.0(2)
88.5(2)
89.6(2)
179.8(2)
75.5(2)
91.7(2)
90.5 (2)
–
1.959(6)
1.950(6)
2.197(7)
1.986(7)
1.993(9)
1.933(6)
1.971(6)
2.182(7)
1.998(9)
1.961(8)
2.138(6)
2.118(7)
2.059(6)
2.127(6)
2.088(5)
2.079(6)
81.6(3)
95.4(2)
90.6(3)
167.5(3)
93.9(3)
167.1(3)
89.9(3)
97.1(3)
94.3(3)
96.1(3)
81.2(2)
96.6(3)
90.8(3)
167.8(3)
96.5(3)
169.9(3)
90.8(3)
90.4(3)
93.4(3)
96.1(3)
73.8(2)
90.1(2)
180.00
106.3(2)
89.9(2)
88.8(2)
–
1.951(3)
1.970(4)
2.290(4)
1.988(5)
1.978(4)
–
–
–
the self-assembly because of their inherent flexibility in the
presence of co-ligands with monoatomic donating sites.[
However, carboxylates quench the flexibility of these tec-
tons, generating linear systems through their syn–syn bridg-
ing mode. Hence, the two carboxylato ligands bind to the
metallatecton in an anti fashion so that the coordination
cluster can be considered as “Z-shaped”. o-HBDC is a
“60°-angular-type” organic tecton that is expected to join
21]
–
–
2.079(4)
2.137(3)
2.090(4)
–
–
–
81.9(1)
92.1(1)
91.6(2)
166.4(2)
92.5(1)
164.8(2)
89.9(2)
101.5(2)
99.0(2)
93.7 (2)
–
–
–
–
–
–
–
–
–
–
[
(CuL) Co] metallatectons at both ends of the “Z” to pro-
2
duce a 1D chain, although, one end of the “60°-angular-
tecton” is not engaged in coordination, and instead remains
protonated (and participates in an intramolecular hydrogen
bond). Consequently, the complex is a discrete trinuclear
entity. All attempts to deprotonate the second carboxylic
acid group of o-HBDC to favor the formation of a 1D
chain led to the rupture the metallatectons, producing a
[Co(o-BDC)] MOF; presumably the chelate effect of the (o-
[
b]
Co(1)/Co(2) –O(5)
Co(1)/Co(2) –O(6)
[b]
[b]
Co(1)/Co(2) –O(7)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–O(4)
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(2)
O(2)–Cu(1)–O(4)
O(2)–Cu(1)–N(1)
O(2)–Cu(1)–N(2)
O(4)–Cu(1)–N(1)
O(4)–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
O(5)–Cu(2)–O(6)
O(5)–Cu(2)–O(8)
O(5)–Cu(2)–N(3)
O(5)–Cu(2)–N(4)
O(6)–Cu(2)–O(8)
O(6)–Cu(2)–N(3)
O(6)–Cu(2)–N(4)
O(8)–Cu(2)–N(3)
O(8)–Cu(2)–N(4)
N(3)–Cu(2)–N(4)
O(1)–Co(1)–O(2)
O(1)–Co(1)–O(3)
O(1)–Co(1)–O(5)/O(1a)/O(1b)
O(1)–Co(1)–O(6)/O(2a)/O(2b)
O(1)–Co(1)–O(7)/O(3a)/O(3b)
O(2)–Co(1)–O(3)
O(2)–Co(1)–O(5)
O(2)–Co(1)–O(6)
O(2)–Co(1)–O(7)/O(3a)/O(3b)
O(3)–Co(1)–O(5)
O(3)–Co(1)–O(6)
O(3)–Co(1)–O(7)
O(5)–Co(1)–O(6)
O(5)–Co(1)–O(7)
O(6)–Co(1)–O(7)
O(5)–Co(2)–O(6)
O(5)–Co(2)–O(7)
O(5)–Co(2)–O(5a)
O(5)–Co(2)–O(6a)
O(5)–Co(2)–O(7a)
O(6)–Co(2)–O(7)
O(6)–Co(2)–O(7a)
2–
BDC) anion at higher pH values [pK (o-H BDC) ϾϾ
a2
2
pK (o-H BDC)], together with a likely steric impediment,
a1
2
prevents the polymerization of the trinuclear cluster, giving
rise to its dissociation.
The crystal packing of 1 shows the presence of a disor-
dered water molecule in the interstitial spaces of the supra-
molecular assembly, with 50% occupancy at each site
nearer to the protonated carboxylic acid groups. Molecules
of 1 pack closely to generate a 1D supramolecular column
along the crystallographic c-axis by means of moderate
C(8)–H(8A)···O(12) interactions (symmetry code: 1 – x, –y,
–
1/2 + z), with H···O and C(H)···O contact distances of
2.69(1) and 3.47(1) Å, respectively, and ЄC–H···O of
38(1)° (see the Supporting Information, Figure S2).
75.1(1)
87.8(1)
180.00
104.9(1)
92.2(1)
90.7(1)
–
–
1
Structures of 2 and 3
The X-ray crystal structures of 2 and 3 consist of centro-
symmetric linear trinuclear coordination clusters (see Fig-
–
ures 2 and 3) of formulae [(CuL) Co(BDC)] (BDC stands
2
91.2(2)
–
–
–
–
–
–
74.1(2)
90.8(2)
180.00
105.9(2)
89.2(2)
89.1(2)
90.9(2)
89.3(1)
for doubly deprotonated benzenedicarboxylate and it repre-
–
–
–
–
–
–
–
–
–
–
–
–
–
sents isophthalate, i.e., m-BDC, for 2 and terephthalate, i.e.,
II
p-BDC for 3), where the Co ion sits at a center of sym-
metry. Selected coordination bonds and angles are listed in
Table 1.
–
–
–
–
–
–
[a] Symmetry codes: a: 2 – x, –y, 1 – z; b: –x, 2 – y, –z for complexes 2
and 3, respectively. [b] Co(1) for complexes 1 and 3 and Co(2) for com-
plex 2.
atoms located at 0.127(1) and 0.130(1) Å away from the
plane, towards O(4) and O(8) atoms, respectively.[
13a]
Figure 2. Representation of the molecular structure of 2 together with
a partial atomic numbering scheme. Hydrogen atoms and solvent mol-
The heterometallic coordination cluster in 1 is comprised
of one cobalt octahedron and two copper square pyramids ecules are omitted for clarity; color scheme as indicated in Figure 1.
Eur. J. Inorg. Chem. 2015, 3028–3037
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sists of half of the trinuclear unit. The [(CuL) Co] coordi-
2
nation cluster is equivalent to that in 2, with two pairs of
bridging phenoxido O atoms from two [CuL] metallo-
ligands [O(1) and O(2); O(1a) and O(2a)] in a planar ar-
rangement around the CoO octahedron, and two carboxyl-
6
ato O atoms [O(3) and O(3a)] at the axial positions. The
bond parameters around the Co coordination sphere are
similar to those found in 1 and 2, with a weak Jahn–Teller
distortion [Co(1)–O(2) : 2.137(3) Å; Co(1)–O : 2.079(4)
ax
eq
and 2.090(4) Å] of the octahedral geometry. The symmetri-
cally related Cu atoms from each metalloligand adopt a
five-coordinate square-pyramidal geometry, with an asym-
metric N O basal plane [O(1), O(2), N(1) and N(2)], and
2
2
an oxygen atom, i.e., O(4), from a μ1,3-bridging carboxylato
group at the axial position. The bond parameters of the Cu
centers are comparable with those of 1 and 2. The τ value
5
amounts to 0.027. The r.m.s. deviation from the mean plane
Figure 3. Molecular structure of 3 together with a partial atomic num- of the four basally coordinated donor atoms of the pentaco-
bering scheme. Hydrogen atoms and solvent molecules are omitted for
clarity; color scheme as indicated in Figure 1.
ordinate Cu(1) is 0.022 Å, with the metal atom being
0
.224(1) Å away from the plane, towards the atom O(4).
II
The archetype of [(ML) MЈ] clusters (M = Cu and MЈ
2
II
The asymmetric unit of {[(CuL) Co(m-BDC)] } (2) con- = Co ) in 2 and 3 remain unaltered as in 1; that is, dis-
2
n
tains two halves of (CuL) Co units connected by two carb- playing a “Z-shape” with very small differences in the
2
oxylate (1κO:2κOЈ) groups from one m-BDC acting as syn– phenoxido bridging angles (Table 2). The variations in the
syn bridging ligand (Figure 2). The structures of the trinu- structures are due to the different orientation of the carb-
clear units are comparable, with slight differences in bond oxylate group of the BDC linkers (Figure 4). For 2, the tri-
lengths and angles. They are formed by a hexacoordinate ply bridged trinuclear nodes are inclined toward each other
central cobalt(II) atom in a CoO octahedral geometry and to be accommodated by the 120°-angular dicarboxylate lin-
6
two pentacoordinate square-pyramidal copper(II) atoms kers, generating a 1D “great wall”-like coordination chain
[
24]
from two[CuL] metalloligands. For each Co atom, two pairs along the crystallographic c-axis (Figure 4, top). This as-
of bridging phenoxido O atoms from two metalloligands sembly represents a rare case of heterometallic coordination
[
25]
[
for Co(1); O(1), O(2), O(1a), O(2a), for Co(2); O(5), O(6), polymers involving isophthalic acid. For 3, the linear lin-
O(5a), O(6a)] are in a planar arrangement. Two additional kers keep the nodes parallel to each other to build up a 1D
O atoms, normal to this plane, from m-BDC linkers [for quasi-linear coordination chain along the crystallographic
[
26]
Co(1); O(3), O(3a) and for Co(2); O(4), O(4a)] complete the c-axis (Figure 4, bottom). In both cases, the coordination
CoO octahedron. The Co(1) octahedron is slightly com- modes of the complimentary building blocks are identical;
6
pressed [Co(1)–O(3) : 2.059(6) Å; Co(1)–O : 2.138(6) and the distinct chain propagations are due to the skeletal dis-
ax
eq
2.118(7) Å], whereas the Co(2) octahedron is slightly elon- position of the chemical functionality of the organic tec-
gated [Co(2)–O(5) : 2.127(6); Co(2)–O : 2.088(5) and tons.
ax
eq
2
.079(6) Å], thus indicating a weak Jahn–Teller distortion.
The distortions of the cis and trans angles of the Co octahe- Table 2. List of μ1,1 bridging angles for complexes 1–3.
dron are analogous to those of 1. The two symmetrically
related Cu atoms reside in a five-coordinate square-pyrami-
dal coordination space, as observed for 1, where the basal
μ
1,1 Bridging angle
1
2
3
Cu(1)–O(1)–Co(1)
Cu(1)–O(2)–Co(1)
98.3(2)
97.6(2)
98.0(2)
98.3(2)
97.6(3)
98.6(3)
97.2(3)
97.3(2)
99.6(1)
97.0(1)
–
–
[
a]
plane is formed by an asymmetric N O donor set from a Cu(2)–O(5)–Co(1)/Co(2)
2
2
Schiff-base ligand, the axial position being occupied by a Cu(2)–O(6)–Co(1)/Co(2)[
a]
μ1,3-bridging carboxylato group (Table 1). The Cu–N and [a] Co(1) for complex 1 and Co(2) for complex 2.
Cu–O bond lengths are comparable with those of 1. The τ5
values amount to 0.007 and 0.035, for Cu(1) and Cu(2),
For both 2 and 3, the adjacent chains run parallel to
respectively. The r.m.s. deviations of the four basally coordi- form a 2D grid structure through π-π stacking (4.8 and
nated donor atoms from the mean plane of the pentacoor- 3.8 Å for 2 and 3, respectively) and C–H···π interactions,
dinate Cu(1) and Cu(2) atoms in 2 are 0.009 and 0.023 Å, which are common features in solid-state structures of mol-
with the metal atoms being 0.178(1) and 0.140(1) Å away ecules containing aromatic moieties. These layers are fur-
from the plane, respectively, toward the atoms O(4) and ther stacked into a 3D framework through C–H···π and C–
O(8).
H···O interactions, with supramolecular voids containing
Compound {[(CuL) Co(p-BDC)] } (3) displays centers solvent molecules. In 2, the relative tilting of the tectons
2
n
of symmetry at the Co atom and at the ring centroid of the with a planar m-BDC creates a void that can accommodate
p-BDC linker (Figure 3). Hence, its asymmetric unit con- one methanol molecule. In the case of 3, the p-BDC is
Eur. J. Inorg. Chem. 2015, 3028–3037
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Figure 4. 1D packing diagrams (left panel) of 2 (top) and 3 (bottom), along the crystallographic c-axis, and their schematic representations (right
panel).
twisted, which leads to an increase in the void space that
It should be noted that a small number of symmetric
can accommodate two methanol molecules. The solid-state dinuclear metallatectons are known for which di- or tricarb-
packing of 2 and 3 are significantly different (Scheme 3). oxylates coordinate only in a bis- or tris-(syn–syn bidentate)
[
11a,30]
The 1D chains of 2 display a hexagonal columnar packing, mode.
In asymmetric heterodinuclear tectons, the co-
whereas closely packed tetragonal columns are observed for ordination modes of the dicarboxylates are usually unpre-
[27]
[11b,19b]
3.
dictable.
However, such unpredictability is greatly di-
minished by the use of symmetric linear trinuclear tectons
that only allow a bis(syn–syn bidantate) mode for the di-
[
31]
merization. These trinuclear tectons are derived from bi-
compartmental N O Schiff bases, which provide an ex-
2
4
tremely stable coordination core that blocks most of the
coordination sites of the central metal atom. Therefore, for
further networking, a coordination number of at least eight
is required for the central metal atom. In contrast, the use
of N O Schiff bases offers fewer blocking possibilities of
2
2
coordination sites of the central metal atom, at the expense
and 3 (tetragonal columnar packing: right), with 1D columns along the of a lower stability of the coordination core. As a result,
Scheme 3. 3D packing diagrams of 2 (hexagonal columnar packing: left)
crystallographic c-axis.
the robustness of such types of metal clusters is reduced,
making them very difficult to employ as nodes. Here, for
the first time, we succeeded in connecting this type of com-
plex with isomeric BDCs, ensuring that their donor func-
tionalities remain unchanged, which led to extremely rare
Polycarboxylate linkers can show diverse and irregular
connecting modes that usually lead to unpredictable and
often undesirable architectural changes, thus frustrating a
systematic control of structural features.[ In the present
study, the carboxylate functionalities interact with the metal
centers only through a syn–syn bridging mode, as antici-
pated, and, depending on the relative disposition of the
functionalities, they assemble in a discrete or 1D fashion,
with a different connectivity and packing. Evidently, the
9]
1D bimetallic chains of triply bridged trinuclear nodes.
Magnetic Properties
Complexes 1–3 add to the growing family of identified
II
II
systematic and rather predictable behavior exhibited by this heterometallic Cu /Co complexes. The study of their mag-
symmetric metallatecton illustrates its exceptional adapt- netic properties is complicated by the simultaneous pres-
ability to isomeric dicarboxylates with different shapes. ence of magnetic exchange together with the contribution
Such prediction and evaluation of positional isomeric effect of the orbital angular momentum of high-spin octahedral
II
of any organic tecton on bimetallic polynuclear nodes is Co . This can be done by a matrix diagonalization pro-
[
10a]
very rare.
The generation of heterometallic trinuclear cedure employing a magnetic Hamiltonian that considers
II
nodes with dicarboxylate entities had been accomplished the spin-orbit coupling within the Co ion, the exchange
previously through serendipitous assembly under solvother- between the spin angular momenta, and the Zeeman inter-
mal conditions, which do not allow the consequent archi- action of the various magnetic moments of the system with
tectures to be predicted.[20] Although, a few isolated reports the magnetic field (see below).
of connecting homometallic Cu O and Ni O tectons with
Magnetization measurements at variable temperature (2
2
2
2
2
BDCs that can be correlated as positional isomeric effect to 300 K) under a constant magnetic field (0.5 T) were col-
have been described.[
28,29]
lected on microcrystalline samples of 1–3; the results are
Eur. J. Inorg. Chem. 2015, 3028–3037
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presented in Figure 5 as χ T vs. T plots (χM is the molar ima in the χM vs. T plots. The model used also provides good
M
paramagnetic susceptibility). In all cases, the χ T product fittings of these plots (see the Supporting Information, Figure
M
is above the expected value for the spin only value expected S3).
II
II
for two Cu ions (S = 1/2) and one Co high spin center
The parameters obtained through this calculation are similar
(
3
2
S = 3/2), all magnetically dilute (3.51, 3.63, and to those of the previously studied analogue, also featuring single
3
–1
.71 cm Kmol for 1, 2, and 3, respectively; expected value syn,syn-carboxylate bridges between Cu and Co, in addition to
3
–1
.63 cm Kmol for all g = 2). The reason is the influence the two μ -phenoxide ligands. In a previous paper we unveiled
2
II
of the orbital angular momentum of Co , L = 1, on the an approximate correlation between the average [Cu–O–Co] an-
overall magnetic susceptibility of the system.
gle in bis-μ -phenoxide systems and the magnitude of the cou-
2
pling constant J. The significant dispersion of this correlation is
due to the large number of other variables that also have an
influence on this property. We here report three compounds
with the same molecular skeleton as that of the previously re-
[13a]
ported compound [(CuL) Co(OBz) ] (OBz = benzoate).
Pos-
2
2
sible magnetostructural correlations within this small family of
very similar analogues were thus investigated. Surprisingly, im-
portant distortions were encountered in the attempts to correlate
J with the average [Cu–O–Co] angle. The possibility for the
largest or the smallest angle within the bridged moiety to have
more weight than the other was then investigated. We found a
much better correlation by using the smallest angle for each
compound, which seems then to play a more important role
Figure 5. Plots of χ
T vs. T for complexes 1, 2, and 3. The solid lines than the other (see the Supporting Information, Figure S4).
M
are fits to the experimental data (see text for details).
The contribution of spin-orbit coupling prevents the defini-
tion of a total spin ground state for this molecule. However,
Co clusters with orbital angular momentum have shown slow
The program PHI[32] was employed to fit the experimen-
tal data by diagonalization of the Hamiltonian in Equation
II
relaxation of the magnetization (for example, see Zhang et
(1).
[34]
al. ). Therefore, AC magnetic susceptibility measurements
were performed for clusters 1, 2, and 3. No sign of an out-of-
phase component of the magnetization could be observed, even
for the largest frequency.
Ǟ
∧
∧
(σL∧1 + g Sˆ
B 1 1
) + μ + g
ˆ
ˆ ˆ
– 2J(S
ˆ ˆ
+ S
ˆ
ˆ
+ g
3 3
H
= λσL
1
S
1
1
S
2
1
S
3
2
S
2
S
)B
1)
(
∧
II
ˆ
In this Hamiltonian, L and S are isotropic orbital and spin
1
ˆ
ˆ
angular momenta of Co , respectively, whereas S and S are
2
3
the spin operators of the two copper ions in one molecule. Like-
Conclusions
wise, g , g , and g are isotropic gyromagnetic ratios of these
1
2
3
three metals, respectively, with terms J, λ, and σ corresponding,
respectively, to the Co···Cu exchange-coupling constant, the Co
By utilizing the tactical advantage of the particular coordina-
tive interaction and adaptability of the [(ML) MЈ] coordination
2
spin-orbit coupling constant, and a combined orbital reduction
cluster towards carboxylate functionalities, herein, we assess the
positional isomeric effect of three benzenedicarboxylates
Ǟparameter of this metal.[
33]
The terms μB and
B are the Bohr magneton and the external magnetic field, towards a trinuclear heterometallic metallatecton for the first
respectively. This model does not include any intermolecular in- time. As expected from a retrosynthetic view, the combinatorial
teractions because it is assumed from the structural data that self-assembly of complementary building blocks led to the for-
the coupling between clusters is negligible; thus the three com- mation of rare 1D-coordination polymers consisting of
II
II
pounds were treated in exactly the same manner.
[(Cu L) Co ] nodes and m-BDC and p-BDC linkers for 2 and
2
This fitting methodology was previously employed for an 3, respectively. On the other hand, o-BDC persists in its singly
II
II
II
[13a]
analogous series of [Cu -Co -Cu ] molecular clusters;
protonated form to constitute a discrete complex 1. Although 2
therefore, for the purpose of comparison, we used the same set and 3 have the same constitutional formulae, except for solvent
of fixed parameters employed before. Thus, reasonable values of content, the relative differences of the angularity of m-BDC and
2
.0 and 2.2 were employed for g and g = g , respectively, and p-BDC isomers change the propagation from “great wall”-like
1 2 3
–1
λ = 180 cm was employed for all the compounds. In addition, for 2 to quasi-linear for 3 along with 3D-supramolecular colum-
optimal fittings were obtained by including contributions by nar packing. Such changes in connectivity, propagation, and
II
small amounts of Co paramagnetic impurities (amounting to packing attributed only to the geometrical disposition of the
0.122, 0.18, and 0.18 molar fraction for 1, 2, and 3, respectively). chemical functionality in BDCs as the coordinating mode of
The best solutions were obtained for J and σ values for 1/2/3 of carboxylate remains unaltered in all three complexes. From the
–1
–
16.96/–11.80/–13.53 cm and –1.37/–1.54/–1.67, respectively; magnetic point of view, these structures are practically mono-
the resulting simulation curves are represented as solid lines in meric where the intramolecular magnetic exchange coupling
Figure 5. The moderate coupling, together with the effect of the constants J are found to be –16.96, –11.80, and –13.53 cm–1 for
paramagnetic impurities prevent the observation of clear max- complexes 1, 2, and 3, respectively. A possible magneto-struc-
Eur. J. Inorg. Chem. 2015, 3028–3037
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tural correlation suggests that the smaller oxido bridging angle Physical Measurements: Elemental analyses (C, H and N) were per-
formed with a Perkin–Elmer 2400 series II CHN analyzer. IR spectra
for each compound might play a more important role than the
larger angle for lowering the antiferromagnetic interaction.
–1
in KBr pellets (4000–500 cm ) were recorded with a Perkin–Elmer RXI
FTIR spectrophotometer. All solutions were prepared in spectroscopic
grade solvents. Electronic spectra were recorded in acetonitrile (800–
2
00 nm) in a 1 cm optical glass cuvette as well as in the solid state (800–
Experimental Section
300 nm) with a Hitachi U-3501 spectrophotometer, using the appropri-
Starting Materials: Salicylaldehyde and 1,3-propanediamine were ob-
tained from Spectrochem India, and were of reagent grade and used as
received. The reagents and solvents used were of commercially available
reagent quality, unless otherwise stated.
ate setup. The variable-temperature magnetic susceptibility data of crys-
talline samples of 1–3 were collected with a Quantum Design supercon-
ducting quantum interference device (SQUID) magnetometer at the
Serveis Cientificotècnics of the Universitat de Barcelona. Pascal’s con-
stants were used to estimate diamagnetic corrections to the molar para-
magnetic susceptibility, and a correction was applied for the sample
holder.
Caution! Perchlorate salts of metal complexes with organic ligands are
potentially explosive. Only a small amount of material should be pre-
pared and it should be handled with care.
Crystallographic Data Collection and Refinement: Suitable single crystals
of each of the three complexes were mounted with a Bruker-AXS
SMART APEX II diffractometer equipped with a graphite monochro-
Synthesis of the Schiff-Base Ligand H
2
L and the Metalloligand [CuL]:
The di-Schiff-base ligand H L was synthesized by following reported
2
[35]
methods. 1,3-Propanediamine (5 mmol, 0.42 mL) was mixed with sal-
icylaldehyde (10 mmol, 1.05 mL) in methanol (20 mL). The resulting
solution was heated to reflux for ca. 2 h, and subsequently cooled to
room temperature. The yellow methanolic solution was used directly for
α
mator and Mo-K (λ = 0.71073 Å) radiation. The crystals were posi-
tioned 60 mm from the CCD. 360 frames were measured with a count-
ing time of 10 s. The structures were solved by using the Patterson
method with SHELXS 97. Subsequent difference Fourier synthesis and
least-square refinement revealed the positions of the remaining non-
hydrogen atoms, which were refined with independent anisotropic dis-
placement parameters. However, for complex 1, the disordered oxygen
atom O(100) and O(200) was refined isotropically. Hydrogen atoms were
placed in idealized positions and their displacement parameters were
fixed to be 1.2 times larger than those of the attached non-hydrogen
atom. Successful convergence was indicated by a maximum shift/error
of 0.001 for the last cycle of the least-square refinement. The non-ideal
data obtained for complex 3 are due to the intrinsic nature of the crystal.
complex formation. To a methanolic solution (20 mL) of Cu(ClO
4 2
) ·
2
6H O (1.852 g, 5 mmol), a methanolic solution of H L (5 mmol, 20 mL)
2
and triethylamine (1.4 mL, 10 mmol) were added to prepare the respec-
tive precursor “metalloligand” [CuL].[
35]
{
[(CuL)
the precursor metalloligand [CuL] (0.359 g, 1 mmol), a solution of
Co(ClO ·6H O (0.183 g, 0.5 mmol) in methanol (2 mL) was added
and the mixture was stirred for 5 min. In another portion, phthalic acid
0.166 g, 1 mmol) was dissolved in methanol (3 mL), and triethylamine
138 μL, 1 mmol) was added. The phthalic acid solution was added
slowly to the mixture of metalloligand and Co(ClO with vigorous All calculations were carried out with SHELXS 97, SHELXL 97,
stirring for 1 h. The mixture was filtered and left undisturbed for slow
evaporation of the solvent. After ca. three days, crystals of X-ray quality collection and structure-refinement parameters and crystallographic data
2 2 2
Co(o-HBDC) ]·H O} (1): To a methanolic solution (20 mL) of
4
)
2
2
(
(
[
36]
Absorption corrections were carried out with the SADABS program.
[37]
[38]
4 2
)
PLATON 99,[ ORTEP-32, and WinGX system ver-1.64.[41] Data
39]
[40]
appeared at the bottom of the vessel. The crystalline compound was
collected, washed with cold methanol, air dried, and characterized by
elemental analysis, spectroscopic methods, and X-ray diffraction.
for the three complexes are given in Table 3.
CCDC-1052921 (for 1), -1052922 (for 2), and -1052923 (for 3) contain
the supplementary crystallographic data for this paper. These data can
2 4
Compound 1: Yield 0.345 g (63%). C50H44CoCu N O13 (1094.92): calcd.
C 54.85, H 4.05, N 5.12; found C 54.77, H 4.11, N 5.08. UV/Vis: λmax
Table 3. Parameters for data collection and structure refinement for
(
MeCN, absorbance) = 360, 270, 231 nm; λmax (solid, reflectance) = 590, complexes 1–3.
–
–
1
3
84 nm. IR (KBr): ν˜ s+as(COO ) = 1553, 1370, ν(C=N) = 1620 cm .
[(CuL) Co(m-BDC)]·CH OH} (2) and {[(CuL) Co(p-BDC)]·
CH OH} (3): To a solution of the precursor metalloligand [CuL]
0.036 g, 0.1 mmol) in methanol (20 mL), a solution of Co(ClO
O (0.018 g, 0.05 mmol, 5 mL of methanol) was added and the solu-
1
2
3
{
2
(
2
3
n
2
Formula
C
Cu
50
H
2
Co
44
N
4
O
13
43 40 4 9 44 44 4 10
C H N O C H N O
3
n
Cu
942.82
monoclinic monoclinic
2
Co
Cu
974.86
2
Co
4 2
) ·
Formula weight
Space group
Crystal system
a [Å]
b [Å]
c [Å]
1094.92
orthorhombic
Pna21
17.542(2)
10.803(1)
26.281(3)
90.0
4980(1)
4
6H
2
tion was stirred for 5 min. An aqueous solution (5 mL) of the disodium
salt of m-BDC or p-BDC (0.010 g of each, 0.05 mmol) was added drop-
wise with stirring, for 2 and 3, respectively. A green precipitate immedi-
ately appeared. The mixture was stirred for 2.5 h, and subsequently fil-
tered to remove any residue. The clear filtrate was allowed to stand
overnight at room temperature for slow evaporation of the solvent, and
crystals of X-ray quality appeared at the bottom of each vessel.
P2 /n
10.387(5)
P2 /c
9.360(5)
1
1
18.200(5)
21.465(5)
96.833(5)
4029(2)
4
20.213(5)
11.452(5)
113.741(5)
1983(1)
2
β [°]
V [Å ]
3
Z
D
–3
calc [gcm ]
1.458
1.243
2236
1.554
1.632
–
1
Compound 2: Yield 0.025 g (53%). C43
C 54.78, H 4.28, N 5.94; found C 54.87, H 4.18, N 5.86. UV/Vis: λmax
MeCN, absorbance) = 365, 272, 230 nm; λmax (solid, reflectance) = 615,
H
40CoCu
2
N
4
O
9
(942.82): calcd.
μ [mm ]
F(000)
1.515
1932
1.544
1002
R
int
0.0610
1.0–25.1
33157
0.0933
1.5–25.1
52647
7172
0.0397
2.0–24.8
8830
(
3
=
–
–1
θ range [°]
Total reflections
Unique reflections
80 nm. IR (KBr): ν˜ = νs+as(COO ) = 1602, 1547, 1376 cm , ν(C=N)
1622 cm .
–1
8756
3264
Compound 3: Yield 0.027 g (55%). C44
C 54.21, H 4.55, N 5.75; found C 54.30, H 4.46, N 5.84. UV/Vis: λmax
MeCN, absorbance) = 365, 271, 230 nm; λmax (solid, reflectance) = 612,
H
44CoCu
2
N
4
O
10 (974.86): calcd.
Data with IϾ2σ(I) 6610
5105
2384
R
1
on IϾ2σ(I)
2
[IϾ2σ(I)]
0.0517
0.1354
1.045
293
0.0807
0.2081
1.148
0.0439
0.1089
1.068
wR
GOF on F
(
2
–
–1
3
80 nm. IR (KBr): ν˜ = νs+as(COO ) = 1597, 1548, 1379 cm , ν(C=N)
Temperature [K]
293
293
–1
=
1624 cm .
Eur. J. Inorg. Chem. 2015, 3028–3037
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FULL PAPER
be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[11] a) M. Pascu, F. Lloret, N. Avarvari, M. Julve, M. Andruh, Inorg.
Chem. 2004, 43, 5189–5191; b) D. G. Branzea, A. Guerri, O. Fabelo,
C. Ruiz-Pérez, L.-M. Chamoreau, C. Sangregorio, A. Caneschi, M.
Andruh, Cryst. Growth Des. 2008, 8, 941–949.
Supporting Information (see footnote on the first page of this article):
Electronic absorption and reflectance spectra of the complexes and the
plot of χ vs. T and –J vs. Cu–O–Co angles of complexes 1–3.
M
[12] a) M. Fondo, A. M. García-Deibe, N. Ocampo, J. Sanmartín,
M. R. Bermejo, A. L. Llamas-Saiz, Dalton Trans. 2006, 4260–4270;
b) M. Fondo, A. M. García-Deibe, N. Ocampo, J. Sanmartín,
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The authors thank the Department of Science and Technology (DST),
New Delhi for financial support (grant number SR/S1/IC/0034/2012) as
well as for the DST-FIST-funded single-crystal X-ray diffractometer fa-
cility at the department of chemistry, University of Calcutta. S. G. is
thankful to the University Grants Commission (UGC), New Delhi for a
Senior Research Fellowship. P. G. acknowledges the ICREA (Institució
Catalana de Recercai Estudis Avançats) and the Ministerio de Econ-
omía y Competitividad (MINECO) of Spain (project numbers
CTQ2011-27929-C02-01 and CTQ2012-32247, grants to P. G. and
G. A., respectively). G. A. thanks the Generalitat de Catalunya for the
prize ICREA Academia 2008 and 2013, for excellence in research and
the European Research Council (ERC) for a starting grant (258060
FuncMolQIP).
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