Metal ion substitution and aldehyde exchange for CuII4 aggregates from two types of piperazine-based Schiff base ligands: Synthesis, X-ray structures, magnetic studies and theoretical validation

Article abstract of DOI:10.1016/j.ica.2020.119439

Two new tetranuclear Cu^II complexes [Cu₄(L2)₂(μ₂-OH)₂(H₂O)₂]·(ClO₄)₂·DMF·2H₂O (1) and [Cu₄(L3)₂(μ₂-OH)₂(H₂O)₂]·(ClO₄)₂·DMF·H₂O (2) have been synthesized containing piperazine based Schiff base ligands H₂L2 (asymmetric) and H₂L3 (symmetric). H₂L2 contains one o-vanillin and a salicylaldehyde unit while H₂L3 has two o-vanillin units. An interesting synthetic methodology involving transition metal ion induced aldehyde exchange reaction between two Schiff base ligands has been explored for the synthesis of H₂L2 and H₂L3. While the latter can be obtained by direct condensation of the respective amine and aldehyde, the former can only be synthesized by this procedure developed. Both 1 and 2 contain a piperazine ring bridging two Cu^II centers in chair conformation in the rare axial-equatorial mode of coordination. Two piperazine bridged Cu₂ moieties are further connected via two hydroxido and two phenoxido bridges. Of the two O_phenoxido–Cu bonds one is short (basal) and the other is long (apical) while the hydroxido O bridges the two Cu^II centers symmetrically in basal–basal manner giving rise to a unique [Cu₂O₂] moiety where, of the four bonds linking the metals, three are basal (short) and one axial (long). Magnetic studies reveal weak antiferromagnetic interactions across the two piperazine bridges with an average exchange constant value of ?4.007 cm^?1 and ?4.507 cm^?1 for 1 and 2 respectively. The antiferromagnetic exchanges across the phenoxido–hydroxido bridges were found to be stronger with the average value being ?43.036 cm^?1 for 1 and ?38.031 cm^?1 for 2. Computational studies further support the experimental findings and the spin density plots in the HS configuration reveal that the exchange across the piperazine bridge takes place via through space mechanism. The spin density analysis in the HS and BS configurations also reveal that the exchange interaction is propagated mainly through the hydroxido bridge. The unsymmetrical nature of H₂L2 leads to greater spin density at the hydroxido bridge in 1 leading to slightly stronger exchange interaction.

Full text of DOI:10.1016/j.ica.2020.119439

Journal Pre-proofs
Research paper
Metal Ion Substitution and Aldehyde Exchange for CuII 4 Aggregates from Two
Types of Piperazine–Based Schiff Base Ligands: Synthesis, X-ray Structures,
Magnetic Studies and Theoretical Validation
Dipmalya Basak, Debashis Ray
PII:
S0020-1693(19)31612-3
https://doi.org/10.1016/j.ica.2020.119439
ICA 119439
DOI:
Reference:
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Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
24 October 2019
31 December 2019
10 January 2020
Please cite this article as: D. Basak, D. Ray, Metal Ion Substitution and Aldehyde Exchange for CuII 4 Aggregates
from Two Types of Piperazine–Based Schiff Base Ligands: Synthesis, X-ray Structures, Magnetic Studies and
Theoretical Validation, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119439
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II
Metal Ion Substitution and Aldehyde Exchange for Cu Aggregates from
4
Two Types of Piperazine‒Based Schiff Base Ligands: Synthesis, X-ray
Structures, Magnetic Studies and Theoretical Validation
a
a
Dipmalya Basak, Debashis Ray*
aDepartment of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India
Abstract
II
Two new tetranuclear Cu complexes [Cu (L2) (µ -OH) (H O) ]·(ClO ) ·DMF·2H O (1) and
4
2
2
2
2
2
4 2
2
[
Cu (L3) (µ -OH) (H O) ]·(ClO ) ·DMF·H O (2) have been synthesized containing piperazine
4 2 2 2 2 2 4 2 2
based Schiff base ligands H L2 (asymmetric) and H L3 (symmetric). H L2 contains one
2
2
2
o‒vanillin and a salicylaldehyde unit while H L3 has two o‒vanillin units. An interesting
2
synthetic methodology involving transition metal ion induced aldehyde exchange reaction
between two Schiff base ligands has been explored for the synthesis of H L2 and H L3. While
2
2
the latter can be obtained by direct condensation of the respective amine and aldehyde, the
former can only be synthesized by this procedure developed. Both 1 and 2 contain a piperazine
II
ring bridging two Cu centers in chair conformation in the rare axial‒equatorial mode of
coordination. Two piperazine bridged Cu moieties are further connected via two hydroxido and
2
two phenoxido bridges. Of the two Ophenoxido‒Cu bonds one is short (basal) and the other is long
II
(
apical) while the hydroxido O bridges the two Cu centers symmetrically in basal‒basal manner
giving rise to a unique [Cu O ] moiety where, of the four bonds linking the metals, three are
2
2
basal (short) and one axial (long). Magnetic studies reveal weak antiferromagnetic interactions
-1
across the two piperazine bridges with an average exchange constant value of ‒4.007 cm and
4.507 cm-1 for 1 and 2 respectively. The antiferromagnetic exchanges across the
phenoxido‒hydroxido bridges were found to be stronger with the average value being ‒43.036
‒
-1
-1
cm for 1 and ‒38.031 cm for 2. Computational studies further support the experimental
findings and the spin density plots in the HS configuration reveal that the exchange across the
piperazine bridge takes place via through space mechanism. The spin density analysis in the HS
and BS configurations also reveal that the exchange interaction is propagated mainly through the

hydroxido bridge. The unsymmetrical nature of H L2 leads to greater spin density at the
2
hydroxido bridge in 1 leading to slightly stronger exchange interaction.
Keywords: Aldehyde exchange, Tetracopper coordination aggregate, Schiff base, Piperazine
bridge, Antiferromagnetic interaction, DFT
1
. Introduction
The synthesis of transition metal based coordination clusters has been attracting interest
for quite some time now due to their usefulness in bio-inorganic chemistry, catalysis,
magnetochemistry, etc. During synthesis of such polynuclear complexes it is desirable for the
ligand to possess multiple coordination sites in order to sequester several metal cations within the
same ligand backbone. In‒situ generated or externally added ancillary ligands further help in
connecting the initially formed fragments leading to higher nuclearity clusters. One approach to
introduce multiple coordination sites is to have spacer groups within the ligand backbone. Such
spacers can either be a single bridging atom like in the case of phenoxides or a rigid unit
destroying the flexibility of the ligand backbone like a piperazine ring in place of ethyl function.
Spacers can be of different kinds of different lengths which can be easily incorporated into the
ligand backbone. They can be rigid, semirigid and flexible. Spacer in multidentate ligands plays
crucial role in deciding the nuclearity and geometry of the molecule. The magnetic exchange
interactions between the paramagnetic metal centers mediated by these spacer units is of further
interest as it contributes to the overall magnetic property of the cluster. The phenoxido or
alkoxido bridge is known to propagate both antiferromagnetic and ferromagnetic exchange
interactions depending on the M‒O‒M bond angles [1‒5]. Earlier we have investigated the
exchange interaction mediated through a meta‒Phenylenediimine bridge (rigid spacer) between
II
Cu centers and found it to be ferromagnetic [6].
Substituted piperazine‒based ligands (semi‒rigid spacers) are of special interest due to
the many conformations they can adopt hence engaging the donor atoms in coordination bonds in
different directions [7‒9]. The piperazine moiety can have four distinct conformations Chair,
Boat, Twist‒boat and halfboat [10]. In the boat and twist‒boat forms, the piperazine unit chelates
one, or bridges, two metal ions to give the mono [10‒13] and dinuclear [14], [15] complexes,
respectively. In the chair conformation it usually bridges pairs of metal ions in a trans‒N,N′
configuration, either in the equatorial–equatorial (e,e) or the axial–axial (a,a) fashion (Scheme

1
, A and B) [16], [17]. The very rare cis‒N,N′ configuration with the piperazine moiety
coordinating in the axial‒equatorial (a,e) fashion (Scheme 1, C) was previously reported by us in
a [Cu ] complex [18].
4
Scheme 1. Three coordination modes in the chair conformation.
The chair conformation in particular due to its various kinetically accessible coordination
modes can give rise to systems with different nuclearity and dimensionality [19]. The magnetic
exchange interactions mediated through the piperazine ring in the chair conformation has been
II
previously investigated by our group in Cu complexes [16], [18], [19]. It is found that in the
II
axial‒axial mode the exchange interaction between two Cu ions is antiferromagnetic and is
mediated via through bond mechanism [16]. In the very rare axial‒equatorial mode the
interaction is again antiferromagnetic but is propagated by through space pathway via direct
overlap of N orbitals [18]. The exchange interaction in this case was averaged out together with
the interaction across the Cu‒Ohydroxido‒Cu bridge. In this work two new piperazine based ligand
systems, unsymmetrical H L2 (2-[(2-{4-[4-(2-hydroxyphenyl)but-3-en-1-yl]piperazin-1-
2
yl}ethyl)imino]methyl-6-methoxyphenol) and symmetrical H L3 (2-Hydroxo-3-methoxy-
2
benzylidene-[2-(4-{2-[(2-hydroxo-3-methoxy-benzylidene)-amino]-ethyl}-piperazin-1yl)-ethyl]-
amine) have been synthesized through an interesting synthetic strategy involving aldehyde
exchange between ligands H L4 (6,6'-(2,5,8,11-tetraazadodeca-1,11-diene-1,12-diyl)bis(2-
2
methoxyphenol)) and H L1 (2-Hydroxo-benzylidene-[2-(4-{2-[(2-hydroxo-benzylidene)-amino]-
2
II
II
ethyl}-piperazin-1yl)-ethyl]-amine) (Scheme 2) promoted by Co and Ni ions. Tetranuclear
Cu complexes 1 and 2 were obtained from H L2 and H L3 with the piperazine ring coordinating
4
2
2
in the rare axial‒equatorial mode of the chair conformation. Complexes 1 and 2 have very
similar structures only differing in the number of electron donating ‒OMe functions on the
ligand backbone and hence provide an opportunity to explore the electronic effects on the

magnetic exchange interactions propagated across the chair piperazine ring as well as the
Cu‒Ohydroxido‒Cu bridge. The introduction of ‒OMe functions lead to additional hydrogen
bonding interactions which opened up the Cu‒Ohydroxido‒Cu bond angle leading to separation of
the two exchange interactions as opposed to that previously reported for analogous compounds
[18].
Scheme 2. Plethora of ligands H L1, H L2, H L3 and H L4 exploited in this work.
2
2
2
2
2
. Experimental Section
2
.1. Reagents and starting materials
The chemicals used were obtained from the following sources: copper carbonate, nickel
carbonate and cobalt carbonate from SRL India; triethylamine perchloric acid and NaBH from
4
Merck, India; ortho-vanillin from Spetrochem, India; triethylenetetramine (trien),
salicylaldehyde and glyoxal from SD FineChem, India. All other chemicals and solvents used in
this work were reagent-grade materials and were used as received without further purification.
2
2
.2. Synthesis of Ligands
.2.1. 2-[4-(2-amino-ethyl)-piperazine-1-yl]-ethylamine
2
-[4-(2-amino-ethyl)-piperazine-1-yl]-ethylamine was synthesised according to a
reported procedure [20]. A solution of 40% glyoxal (0.725 g, 5mmol) in 5 ml ethanol was added
slowly to a mixture of triethylenetetramine (0.574 g, 5mmol) and NaBH (0.946 g, 25mmol) in
4
1
0 ml of ethanol at 0°C. After 2 hours of vigorous stirring the reaction was quenched by the

addition of 4N HCl solution. When evolution of hydrogen ceases the acidic mixture was
neutralized with 4N NaOH solution and the solvent was removed under vacuum. The residue
was extracted several times with chloroform and the combined extracts were dried over
anhydrous Na SO . Removal of solvent gave the amine in the form of an oily residue. It was
2
4
used without further purification.
.2.2. 2-Hydroxo-benzylidene-[2-(4-{2-[(2-hydroxo-benzylidene)-amino]-ethyl}-piperazin-1yl)-
2
ethyl]-amine (H L1)
2
Method A
To the MeOH solution of 2-[4-(2-amino-ethyl)-piperazine-1-yl]-ethylamine (0.344 g,
mmol), salicylaldehyde (0.484 g, 4mmol) was added under ice-cold condition and stirred for 3
2
h to give a yellow solid which was separated by filtration through G4 sintered bed and washed
thoroughly with ice cold MeOH. The isolated solid was next dried in vacuo over anhydrous
CaCl . The single crystal suitable for X-ray analysis were obtained from hot MeOH after 4 days
2

and found to be same as reported earlier [21]. Yield 1.92 g (∼87 % yield), mp 152 C. Selected
-1
IR peaks (KBr, cm , vs = very strong, br = broad, s = strong, m = medium): 2939(b), 2823(b),
1
1
641(vs), 1499(s), 1441(s), 1281(vs), 1159(s), 1017(s), 863(s), 764(s). H NMR (CDCl , ppm): δ
3
(phenolic OH) 13.43(s, 2H); 8.35 (s, 2H); 7.35-6.83 (m, 8H); 3.73(t, 4H); 2.70(t, 4H); 2.57 (s,
1
3
8
1
H). C NMR (CDCl , ppm): δ (C7,16) 165.52; (C1,22) 161.16; (C6,17) 132.13; (C3,5,18,20)
3
31.14; (C4,19) 118.44; (C2,21) 116.98; (C9,14) 58.59; (C8,15) 56.98; (C10,11,12,13) 53.33.
Method B
2
-[4-(2-amino-ethyl)-piperazine-1-yl]-ethylamine is present as an impurity in commercial
triene and was isolated in the form of 2-hydroxo-benzylidene-[2-(4-{2-[(2-hydroxo-
benzylidene)-amino]-ethyl}-piperazin-1yl)-ethyl]-amine [22]. 7.3 g of commercial trien was
dissolved in 20 ml of refluxing MeOH and treated with a solution of 11 g salicylaldehyde in 20
ml refluxing MeOH. The solution was brought to room temperature and stirred for 1 hour. The
compound was precipitated in the form of a bright yellow powder during this time. It was filtered
through a G4 sintered glass bed, washed with ice cold methanol and dried over anhydrous CaCl2.

Yield 1.670g (~8.79%), mp 152 C.
2
.2.3. 2-Hydroxo-3-methoxy-benzylidene-[2-(4-{2-[(2-hydroxo-3-methoxy-benzylidene)-amino]-
ethyl}-piperazin-1yl)-ethyl]-amine (H L3)
2

2
-[4-(2-amino-ethyl)-piperazine-1-yl]-ethylamine (0.344 g, 2mmol) was dissolved in 5
ml methanol and to it o-vanillin (0.608 g, 4mmol) in 5 ml MeOH was added. It was refluxed for
hours forming an orange coloured solution. Removal of solvent under vacuum gave a deep
3
orange semi-solid mass. It was redissolved in 20 ml MeOH and used without further purification
considering a quantitative yield.
2
2
.3. Synthesis of Complexes
.3.1. [Cu (L2) (µ -OH) (H O) ](ClO ) ·DMF·2H O (1)
4
2
2
2
2
2
4 2
2
To a yellowish brown solution of o-vanillin (0.076 g, 0.5mmol) and Cu(ClO ) ·6H O
4
2
2
(0.186 g, 0.50mmol) in 4 ml MeOH, trien (0.036 g, 0.25mmol) in 2 ml MeOH was added
dropwise over a period of 20 min time with constant stirring at room temperature. The deep
green solution was left to stir for 30 minutes (Solution I). H L1 (0.095 g, 0.25mmol) was
2
dissolved in 2 ml CHCl -MeOH (1:1) and treated with 2 ml MeOH solution of Co(ClO ) ·6H O
3
4 2
2
(
0.182 g, 0.5mmol) (Solution II). The resulting brown solution along with the brown precipitate
Solution II) was added to the stirring solution of initially prepared copper complex (Solution I)
(
followed by NEt (0.152 g, 1.5mmol) after 10 min, giving a moss green precipitate. The mixture
3
was stirred for 6 h and filtered through a G4 sintered glass crucible, washed with MeOH and
dried in vacuum over P O . The filtrate on standing for a day yielded further crop of precipitate.
4
10
Large green block shaped crystals for X-ray structure determination was grown by slow
evaporation of a DMF solution of the compound after 7 days. Overall yield of the reaction
product is ~ 60%. Anal.Calcd (%) for C H Cl Cu N O : C, 40.61; H, 5.08; N, 8.70. Found
4
9
73
2
4
9
21
-1
(
%): C, 40.58; H, 5.04; N, 8.65. Selected IR peaks: (KBr, cm , vs = very strong, br = broad, s =
strong, m = medium, w = weak): 3400 (br), 3273 (s), 3170 (s), 1576 (vs), 1476 (s), 1400 (m),
-1
2
-1
-1
-1
1
270 (m), 1236 (m).  : (MeCN solution) 230 Ω cm mol . UV-vis:  , nm (, L mol cm )]
M
max
(MeCN) = 566 (476), 371 (17200), 316 (22000), 273 (53400), 229 (87200).
2
.3.2. [Cu (L3) (µ -OH) (H O) ](ClO ) ·DMF·H O (2)
4 2 2 2 2 2 4 2 2
Method A
A procedure similar to that mentioned in Method A above for the preparation of 1 was
followed with Ni(ClO ) ·6H O in place of Co(ClO ) ·6H O. A solution of o-vanillin (0.076 g,
4
2
2
4 2
2
0
.5mmol) and Cu(ClO ) ·6H O (0.186 g, 0.5mmol) was prepared in 4 ml methanol at room
4 2 2
temperature. To this trien (0.036 g, 0.25mmol) in 2 ml MeOH was added dropwise over a period

of 20 min and the whole solution was further stirred for another period of 30 min (Solution III).
To a solution of H L1 (0.095 g, 0.25mmol) in 2 ml CHCl -MeOH (1:1), Ni(ClO ) ·6H O in 2 ml
2
3
4 2
2
methanol was added and stirred for 10 minutes (Solution IV). After stirring for 10 min the
reddish yellow solution (Solution IV) was added slowly to the stirred deep green solution
(Solution III) of the previously formed copper complex. The resulting mixture was further stirred
for 20 min and NEt (0.152 g, 1.5mmol) was added resulting in the slow separation of a bright
3
green precipitate. The reaction mixture was next stirred for 6 h and the formed deep green
precipitate was filtered through a G4 sintered glass crucible, washed with cold MeOH and dried
in air. The green filtrate gave further crop of green precipitate after standing for 2 days. Green
block shaped single crystals suitable for X-ray structure determination were obtained by slow
evaporation of a DMF solution of the complex after 6 days. Total yield of the product is ~ 55%.
Anal.Calcd (%) for C H Cl Cu N O : C, 41.08; H, 5.07; N, 8.45. Found (%): C, 41.10; H,
5
1
75
2
4
9
22
-1
5
.09; N, 8.47. Selected IR peaks: (KBr, cm , vs = very strong, br = broad, s = strong, m =
medium, w = weak): 3400 (br), 3273 (s), 3170 (s), 1576 (vs), 1476 (s), 1400 (m), 1270 (m), 1236
-1
2
-1
-1
-1
(
m).  : (MeCN solution) 255 Ω cm mol . UV-vis:  , nm (, Lmol cm )] (MeCN) = 573
M
max
(503), 375 (16100), 321 (18800), 277 (53300), 238 (86600).
Method B
A 5 ml yellow methanolic solution of 2-hydroxo-3-methoxy-benzylidene-[2-(4-{2-[(2-
hydroxo-benzylidene)-amino]-ethyl}-piperazin-1yl)-ethyl]-amine (H L3) (0.25mmol) was
2
treated with a solution of Cu(ClO ) ·6H O (0.186 g, 0.5mmol) in 5 ml of MeOH followed by
4
2
2
trien (0.101 g, 1.0mmol) producing a green precipitate. After stirring the mixture for 1 hour the
precipitate was filtered, washed with methanol and dried over suction. Yield ~ 45%. The
complex was crystallized by following the procedure described in Method A.
2
.4. Physical Measurements
Elemental analyses (C, H and N) were performed with a PerkinElmer model 240C
elemental analyzer. A Shimadzu UV 3100 UV/Vis/NIR spectrophotometer and a PerkinElmer
RX1 spectrometer were used to record the solution electronic absorption spectra and FTIR
spectra, respectively. A Bruker esquire 3000 plus mass spectrometer was employed to collect the
electrospray ionization (ESI) high resolution mass spectra of the two compounds.
2
.5. SQUID Measurements

Magnetic data of compounds 1 and 2 were recorded using a Quantum Design MPMS-
3
XL SQUID magnetometer. The polycrystalline samples were immobilized into PTFE capsules.
The data were acquired as a function of the temperature (2.0–300 K) at a constant magnetic field
of 0.1 T. The data were corrected for diamagnetic contributions from the sample holder and the
compounds.
2
.6. Computational Studies
Theoretical calculations were performed in the ORCA 4.0 computational package [23],
[24] on molecular fragments extracted from experimental X‒ray data. The B3LYP DFT
functional [25‒27] was used together with the polarized triple‒ζ quality basis set def2‒TZVP for
copper, zinc, nitrogen and oxygen atoms and the def2‒SVP basis set for carbon and hydrogen
atoms [28]. Exchange constants, J, were calculated following Ruiz’s approach [29], [30] by
comparing the energies of high‒spin (HS) and broken‒symmetry (BS) states. To speed up the
calculations the RI approximation together with AutoAux generation procedure [31] and the
chain‒of‒spheres (RIJCOSX) approximation [32], [33] were utilized as implemented in ORCA.
Increased integration grids (Gridx5 in the ORCA convention) and tight SCF convergence criteria
were used in all calculations. The calculated spin density was visualized with the Avogadro 1.2.0
program [34].
2
.7. Crystal Data Collection and Refinement
Single crystal X-ray diffraction data for 1–3 were collected on a Bruker SMART APEX-
II CCD X-ray diffractometer furnished with a graphite-monochromated Mo Kα ( = 0.71073 Å)
−
1
radiation by the ω scan (width of 0.3° frame ) method at 293 K with a scan rate of 4 s per frame.
SAINT and XPREP software [35] were used for Data processing and space group determination.
Direct method of SHELXS-2014 [36] were used to solve the structure and then refined with full-
matrix least squares using the SHELXL-(2014/7) [37] program package included into WINGX
system Version 2014.1 [38]. Data were corrected for Lorentz and polarization effects; an
empirical absorption correction was applied using the SADABS [39]. The locations of the
heaviest atoms (Cu) were determined easily. The O, N, Cl and C atoms and the H atoms of the
bridging hydroxido groups were subsequently determined from the difference Fourier maps.
These atoms are refined anisotropically. The other H atoms were incorporated at calculated
positions and refined with fixed geometry and riding thermal parameters with respect to their

carrier atoms. In 1 the disordered oxygen (O2) and carbon (C45) atoms of the ‒OMe group were
split over two positions as O2A (~81%), O2B (~19%) and C45A (~63%), C45B (~37%) and
refined anisotropically. The disordered water molecule (O4W) was split over two positions as
O4WA (~57%) and O4WB (~43%) and refined anisotropically. In disorder in the DMF molecule
in 2 was dealt with by splitting the carbon atoms C49, C50 and C51 over two positions each as
C49A (~64%), C49B (~36%); C50A (~66%), C50B (~34%) and C51A (~36%), C51B (~64%)
and refined anisotropically. In all of the above cases SIMU and ISOR restrictions were applied
on the split atoms. Crystallographic diagrams were presented using DIAMOND software [40]. A
summary of the crystal data and relevant refinement parameters is summarized in Table 1.
Crystallographic data (including structure factors) have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publications CCDC 1960913 and 1960920.
These data can also be obtained free of cost at www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre).
Table 1. Crystal data and structure refinement details for 1and 2.
parameters
Formula
1
2
C
49
H
73Cl
2
Cu
1449.27
Monoclinic
P 2 /n
Green
4
N
9
O
21
C
51
H
75Cl
2
Cu
1491.30
Monoclinic
P 2 /n
Green
4 9 22
N O
–
1
F.W. (g mol )
crystal system
space group
1
1
Crystal color
Crystal size / mm3
0.35×0.27×0.25
18.434(16)
15.399(13)
21.478(18)
90°
0.30×0.27×0.24
19.428(4)
15.988(4)
19.713(4)
90°
a / Å
b / Å
c / Å
α
β
96.91(3)°
90°
100.409(7) °
90°
γ
V / Å3
Z
6053(9)
4
6022(2)
4
D
c
/ g cm–3
1.587
1.645
μ / mm–1
F(000)
T / K
1.556
1.568
2980.0
3080.0
293(2)
293(2)

Total reflns
R(int)
54486
0.0824
72014
0.2402
Unique reflns
Observed reflns
Parameters
10231
10565
5931
5214
704
827
R
1
; wR
2
(I > 2σ(I))
0.0910, 0.3259
1.129
0.0873, 0.2794
1.019
GOF (F2)
−
3
Largest diff peak and hole (e Å )
1.876, -1.431
1960920
1
.591, -1.435
1960913
CCDC No.
3
. Results and Discussion
3
.1. Synthetic Methodology
Schiff base condensation of salicylaldehyde and o-vanillin with 2-[4-(2-amino-ethyl)-
piperazine-1-yl]-ethylamine in 2:1 molar ratio provided 2-hydroxo-benzylidene-[2-(4-{2-[(2-
hydroxo-benzylidene)-amino]-ethyl}-piperazin-1yl)-ethyl]-amine (H L1) and 2-Hydroxo-3-
2
methoxy-benzylidene-[2-(4-{2-[(2-hydroxo-3-methoxy-benzylidene)-amino]-ethyl}-piperazin-
1
yl)-ethyl]-amine (H L3) respectively in near quantitative yields from the reaction medium.
2
Generation of unsymmetrical H L2 and symmetrical H L3 in metal ions bound forms were
2
2
achieved from the inter-complex metal ion and aldehyde substitution reactions between metal ion
bound H L1 and H L4. The course of reactions proceeded through metal ion coordination driven
2
2
in-situ imine bond hydrolysis and aldehyde exchange promoted by Co(ClO ) ·6H O and
4
2
2
2
+
Ni(ClO ) ·6H O (Scheme 3). In solution the Cu (L4) fragment (A in Scheme 3) is labile for the
4
2
2
2
2
+
release of Cu ions in the reaction medium and is suitable for the metal ion substitution and
2
+
ligand exchange in the following stages. The in situ formed Co (L2) fragment (B in Scheme 3),
2
in a separate reaction, is also labile for both metal ion substitution and aldehyde exchange
2
+
forming 1. In a similar fashion another in situ formed labile Ni (L2) fragment (C in Scheme 3),
2
showed metal ion substitution by copper(II) and double aldehyde exchange to result 2.

Scheme 3. Formation of 1 and 2 through metal ion substitution and aldehyde exchange.
Both H L2 and thus 1 are thus formed through this elaborate synthetic methodology as
2
they could not be produced by direct condensation of the respective aldehyde and amine. Initial
reaction of o-vanillin with trien in 2:1 molar ratio and stoichiometric Cu(ClO ) ·6H O lead to the
4
2
2
generation of 6,6'-(2,5,8,11-tetraazadodeca-1,11-diene-1,12-diyl)bis(2-methoxyphenol) (H L4)
2
II
II
bound to Cu as precursor fragment A in solution which is suitable to supply Cu ions in
compounds 1 and 2. To this, addition of the solution of fragment B, generated from a mixture of
Co(ClO ) ·6H O and H L1 in 2:1 molar ratio followed by Et N gave moss green precipitate
4
2
2
2
3
which on crystallization from DMF solution resulted 1 as X-ray diffraction quality single crystals
in 60% yield (Eqs. (1), (2), (4)). In a similar procedure use of Ni(ClO ) ·6H O and H L1
4
2
2
2

provided 2 in 55% yield (Eqs. (1), (3), (5)). Complex 2 can be also be synthesized directly from
the reaction of Cu(ClO ) ·6H O and H L3 in the presence of Et N and crystallized as mentioned
4
2
2
2
3
before. The reaction proceeds through aldehyde exchange between the two Schiff base ligands
H L1 and H L4 and the nature of the M(ClO ) ·6H O salt determines whether the exchange is
2
2
4 2
2
partial (M = Co) or complete (M = Ni). In the process the transition metal cations are also
II
exchanged with the final product containing only Cu ions. We were unable to obtain any
II
II
heterometallic aggregates of type Cu Co or Cu Ni . The released Co and Ni ions are probably
2
2
2
2
sequestered by the new trien based ligands 2-(12-(2-hydroxyphenyl)-2,5,8,11-tetraazadodeca-
,11-dien-1-yl)-6-methoxyphenol (H L5) and 2,2'-(2,5,8,11-tetraazadodeca-1,11-diene-1,12-
1
2
diyl)diphenol (H L6) respectively formed through this aldehyde exchange process. The exact
2
mechanism of this exchange process is still under investigation. Interestingly use of only o-
vanillin instead of initial formation of H L4 or Cu(ClO ) ·6H O instead of either
2
4 2
2
Co(ClO ) ·6H O and Ni(ClO ) ·6H O lead to the formation of only 2 in negligible yields.
4
2
2
4 2
2
푀푒푂퐻 (푠표푙푣)
2
+
2
표 ― 푣푎푛푖푙푙푖푛 + 2퐶푢(퐶푙푂 ) ·6퐻 푂 + 푇푟푖푒푡ℎ푒푙푒푛푒푡푒푡푟푎푚푖푛푒
[퐶푢 (퐿4)(푠표푙푣) ]
4
+
2
2
2 4
―
+
8퐻 푂 + 2퐻 + 4퐶푙푂
(1)
2
4
푀푒푂퐻 (푠표푙푣)
2
+
+
―
퐻 퐿1 + 2퐶표(퐶푙푂 ) ·6퐻 푂
[퐶표 (퐿1)(푠표푙푣) ] + 6퐻 푂 + 2퐻 + 4퐶푙푂
4
(2)
(3)
2
4 2
2
2
6
2
푀푒푂퐻 (푠표푙푣)
2
+
+
―
퐻 퐿1 + 2푁푖(퐶푙푂 ) ·6퐻 푂
[푁푖 (퐿1)(푠표푙푣) ] + 6퐻 푂 + 2퐻 + 2퐶푙푂
2 6 2 4
2
4 2
2
2
+
2 +
+
―
2
[퐶푢 (퐿4)(푠표푙푣) ] + 2[퐶표 (퐿1)(푠표푙푣) ] +6Et N + 4H O + DMF + 4H +6ClO
2 4 2 6 3 2 4
MeOH then DMF
2
+
[
Cu (L2) (µ -OH) (H O) ](ClO ) ·DMF·2H O + 2Co(L5) + 2Co + 6(Et NH)
4 2 2 2 2 2 4 2 2
3
(
퐶푙푂4
)
(4)
2
+
2 +
+
―
2
[퐶푢 (퐿4)(푠표푙푣) ] + 2[푁푖 (퐿1)(푠표푙푣) ] +6Et N + 4H O + DMF + 4H +6ClO
2 4 2 6 3 2 4
MeOH then DMF
2
+
[
Cu (L3) (µ -OH) (H O) ](ClO ) ·DMF·H O + 2Ni(L6) + 2Ni + 6(Et NH)
4 2 2 2 2 2 4 2 2
3
(
퐶푙푂4
)
(5)
Both 1 and 2 consist of the dicationic units [Cu (L2) (OH) (H O) ]
II
2+
and
4
2
2
2
2
II
2+
[
Cu (L3) (OH) (H O) ] with the tetranuclear core being composed of two piperazine-briged
4
2
2
2
2
[
Cu O ] units wrapped around by two ligand anion strands in elongated pseudo-tetrahedron
2
2
geometry. Bothe the complexes precipitate from the reaction mixtures as a dark-green solid

products in the presence of cobalt(II) and nickel(II) ions. Elemental analysis, solution electrical
conductivity, and room-temperature magnetic susceptibility data are consistent with the
molecular formula of complex 1 and 2. The aim behind the synthesis of complexes having
similar structure with two types of ligand systems H L2 and H L3 is to explore the effects of
2
2
II
electron donating groups on the magnetic exchange interactions between the Cu centers
mediated by the chair piperazine bridge in equatorial-axial binding mode as well as the
hydroxido-phenoxido bridges.
3
.2. High resolution mass spectrometry analysis
2
‒
2‒
To further ascertain the formation of L2 and L3 in 1 and 2 and determine the pathway
of aggregation leading to the tetranuclear structures the HRMS(+ve) spectra of the complexes in
MeOH were scrutinized (Figures S2 and S3). The spectra of 1 (Figure S2) shows a base peak at
2
+
m/z = 303.0462 which can be attributed to the species [Cu (L2)(H O) ] (C H Cu N O ; calcd.
2
2
4
23 36
2
4
7
+
3
03.0583) while the peak at m/z = 411.2319 is due to the protonated ligand H L2-H
2
(
C H N O ; calcd. 411.2396). Similarly for 2 the base peak in the spectra (Figure S3) at m/z =
23 31 4 3
2
+
3
18.0643 arises from the species [Cu (L3)(H O) ] (C H Cu N O ; calcd. 318.0636) with the
2 2 4 24 38 2 4 8
+
protonated ligand H L3-H (C H N O ; calcd. 441.2502) exhibiting the peak at m/z =
2
24 33
4
4
2
‒
2‒
4
41.2519. This clearly shows that L2 and L3 are formed by the aldehyde exchange process. It
2
+
further elucidates the initial formation of dinuclear fragments like [Cu (L2)(H O) ] and
2
2
4
2
+
[
Cu (L3)(H O) ] which undergoes self-aggregation giving rise to the tetranuclear complexes.
2 2 4
3
3
.3. Description of crystal structures
.3.1. [Cu (L2) (µ -OH) (H O) ](ClO ) ·DMF·2H O
(1)
and
[Cu (L3) (µ -
4 2 2
4
2
2
2
2
2
4 2
2
OH) (H O) ](ClO ) ·DMF·H O (2)
2
2
2
4 2
2
Both 1 and 2 crystallize in monoclinic P 2 /n space group with Z = 4. Selected metric
1
parameters are represented in Table S1. The structure of the cationic parts of 1 and 2 are similar
except for the nature of the wrapping ligands with respect to the terminal hydroxy aldehyde
II
2+
donors isolating the tetranuclear cationic units
[Cu (L2) (OH) (H O) ]
and
4
2
2
2
2
II
2+
‒
[
Cu (L3) (OH) (H O) ] , respectively. The crystal lattices also accommodate two ClO
4 2 2 2 2 4
counter anions for charge compensation in both 1 and 2. Apart from this, two molecules of water
and a molecule of DMF are present in the crystal lattice of 1 while in 2 a molecule of water and a
molecule of DMF are present per formula unit. The molecular structures of 1 and 2 are shown in

Figure 1. Since both complexes have a similar structure, the structural description is presented
for 2 only citing the differences with 1.
Figure 1. Molecular structures of the cationic parts of 1 (left) and 2 (right). Hydrogen atoms and solvent
molecules are omitted for clarity. Color code: Grey, carbon; red, oxygen; blue, nitrogen; green, CuII
II
Each hexadentate N O donor ligand binds three Cu centers asymmetrically with the
2
4
bridging phenoxido O forming one shorter equatorial bond (Cu3···O3, 1.970(7) Å; Cu4···O6,
.974(7) Å) and a longer axial bond (Cu2···O6, 2.434(7) Å; Cu1···O3, 2.485(7) Å). Both
complexes 1 and 2 are very rare examples displaying a [Cu O ] moiety in which, of the four
1
2
2
bonds linking the metals, three are basal (short) and one axial (long). In both the two cases the
Cu ] cores of the complexes are composed of two 1,4-piperazine nitrogen bridged [Cu O ] units
[
4
2
2
enclosed by two ligand strands in the construction of tetranuclear double-helical structures where
II
the four Cu centers form elongated pseudo-tetrahedra. The NCuN and NCuO bites from
five- and six-membered chelates, record average angles of 84.16° and 91.97°, respectively.
Within [Cu O ] parts the average of the CuOCu angles for the hydroxido and phenoxido O
2
2
bridges are 89.98° and 110.34°, respectively for basalbasal and basalapical modes of
bridging. The terminal basal‒apical bridging mode of the phenoxido groups orient the CuII
bound square bases in perpendicular orientation allowing the helical binding of two ligand
II
anions. On the other hand the hydroxido O bridges the Cu centres symmetrically with the
CuOhydroxido distances ranging from 1.916 to 1.930 Å which are comparable to known µ-
hydroxido-bridged tetranuclear complexes [41], [42]. For the other basal bonds the Cu‒Npiperazine
distances (av. 2.115 Å) are longer than the Cu‒Nimine distances (av. 1.929 Å) primarily due to the
different states of hybridization of two types of nitrogen atoms. In 1 the bridging phenoxido O
belongs to the salicylaldehyde part of H L2 with the two o-vanillin units of the two ligand
2

molecules remaining at opposite ends mainly due to the hydrogen bonding interaction of the
II
‒
OMe group with the coordinated water molecules. The four Cu centers form an elongated
II
pseudo-tetrahedron. Interestingly, the apical position of one type of Cu , not bridged by
II
phenoxido O, is occupied by H O molecule. The coordination geometry around the Cu ions are
2
very close to square pyramid type with Cu1 (CShM = 5.511 for SPY) and Cu2 (CShM = 4.510
for SPY) having apical phenoxido bonds showing more distortion in geometry compared to Cu3
(
CShM = 1.753 for SPY) and Cu4 (CShM = 1.083 for SPY) having apical H O coordination
2
(Table S2).
Both the ligands H L2 and H L3 are expected to display a chair conformation of the
2
2
piperazine backbone with both N‒C bonds being equatorial in their free state. During formation
of the complexes the ligands undergo conformational isomerization with one N‒C bond deposed
equatorially while the other is axial. Unsymmetrical equatorial‒axial coordination of the
chair‒piperazine ring triggers unsymmetrical binding of the two tridentate halves of the ligands.
In 2 the average Cu···Cu distance along the ligand piperazine backbone is 5.67935 Å
(5.6989(10) Å, Cu1···Cu4 and 5.6598(9) Å, Cu2···Cu3), which is less compared to 6.908 Å for
chair-piperazine‒bridged Cu complex in trans‒bridging mode [16]. The intermetallic separation
2
across the hydroxido-phenoxido bridge were much shorter at 3.1613(4) Å (Cu1···Cu3) and
3
.1472(4) Å (Cu2···Cu4).
Figure 2. Core structure of 2 (left), the intermetallic separations within Cu core (right) and the
4
II
tetragonal disposition of the four Cu ions viewed along O6, O7, O8 and O3 (right upper corner). Carbon
and hydrogen atoms omitted for clarity. Color code: Red, oxygen; blue, nitrogen; green, CuII.

In 2 the coordinated water molecules show intramolecular hydrogen bonding interactions
with the non-bridging and terminal phenoxido oxygen (O1W···O1, 2.741(9) Å and O2W···O4,
2
2
.889(9) Å), and the ‒OMe group of the o-vanillin unit (O1W···O2, 2.971(11) Å and O2W···O5,
.975(11) Å). Intermolecular hydrogen bonding interactions were also operative between the
lattice water molecule and coordinated water molecule (O2W···O3W, 2.813(10) Å). The same
‒
lattice water molecule further connects to a ClO anion (O3W···O16, 2.986(14) Å) which in
4
turn is hydrogen bonded to the second coordinated water molecule of another Cu unit
4
(
(
(
O1W···O17, 2.861(16) Å) forming a one dimensional chain involving multiple Cu units
4
‒
Figure 3). A second ClO anion also remains hydrogen bonded to the lattice water molecule
4
O3W···O13, 2.943(10) Å). The bridging hydroxido oxygen atoms also trap a solvent DMF
molecule through hydrogen bonding interactions (O7···O19, 2.849(9) Å and O8···O19, 2.921(1)
Å).
Figure 3. Hydrogen bonding interactions in 2 showing 1D chain structure.
In case of 1 a second lattice water molecule is trapped in place of DMF molecule
(O7···O4WB, 2.774(4) Å and O8···O4WB, 2.741(7) Å) by the two bridging hydroxido oxygen
atoms. A similar one dimensional chain of Cu units is formed through intermolecular hydrogen
4
‒
bonding interactions through lattice water molecules and ClO counter anion as observed in 2
4
(Figure 4). A DMF molecule is further hydrogen bonded to the lattice water molecule
‒
(
O3W···O18, 2.803(2) Å) in place of a second ClO anion as found in 2.
4

Figure 4. Hydrogen bonding interactions in 1 showing 1D chain structure.
3
.4. Magnetic studies
The magnetic susceptibility data for 1 and 2 were collected in the temperature range
2
‒300 K at 0.1 T on polycrystalline samples. The data is presented as χ T vs. T plots in Figure 4
M
where χ is the molar magnetic susceptibility after subtraction of the diamagnetic contributions.
M
3
-1
3
-1
At 300 K the χ T values for 1 and 2 are 1.32 cm K mol and 1.34 cm K mol , respectively,
M
II
3
-1
which are close to that expected for four non‒interacting and isolated Cu ions (1.5 cm K mol
for g = 2.0). Upon cooling the value slowly decreases up to about 100 K and beyond which the
3
-1
3
-1
drop occurs more sharply reaching a minimum of 0.002 cm K mol and 0.001 cm K mol at 2
K for 1 and 2 respectively. This type of behavior indicates the presence of antiferromagnetic
II
exchange interactions between the Cu centers.
Figure 4. χ T vs. T plots for 1 and 2.
M

Figure 5. Spin‒coupling scheme used considered for the fitting.
In order to get a more quantitative picture of the various exchange interactions present in
the system, the spin‒coupling scheme depicted in Figure 5 was taken into consideration. The
Heisenberg spin Hamiltonian for superexchange according to this scheme is given below.
4
퐻 = ― 2퐽1
(
푆 푆
)
― 2퐽2
(
푆 푆
)
― 2퐽3
(
푆 푆
)
― 2퐽4
(
푆 푆
)
+ 푔휇 퐻 푆푖
(6)
1 2
2 3
3 4
4 1
퐵
∑
푖 = 1
The experimental data for 1 and 2 were fitted using the PHI program [43] making use of
the above Hamiltonian to extract the magnitude of the exchange interactions. The g value was
initially fixed at 2.0 and the J , J , J and J were allowed to vary freely. This procedure gave
1
2
3
4
multiple minima and definite values for the exchange constants could not be extracted. To reduce
the number of free parameters it was considered J = J and J = J taking the clue from the X-
1
3
2
4
ray structural parameters found for 1 and 2. This procedure also failed to give definite values for
the exchange parameters. Hence to get an idea about the starting values for the exchange
constants during the fitting process, DFT calculations were performed on the X‒ray crystal
structures of 1 and 2 using the broken symmetry approach (Table 2). Finally the fits for the
experimental data were carried out using DFT calculated values for the exchange constants. The
g value was allowed to vary freely and temperature‒independent paramagnetism (TIP) was also
considered. For 1 the values of the coupling constants, J , J , J and J , obtained were 43.024,
1
2
3
4
-1

4.044, 43.049 and 3.971 cm , while for 2 these were 38.038, 4.490, 38.025 and 4.525
cm . The g values were found to be 2.01 (1) and 2.00 (2) with the χ values being 0.12 × 10-6
-
1
TIP
3
-1
-6
3
-1
cm K mol (1) and 0.10 × 10 cm K mol (2). From the values of the coupling constants
arrived at through fitting of the experimental data, it is clear that J ≈ J and J ≈ J with average
1
3
2
4
-1
-1
-1
-1
values of 4.007 cm (1), 4.507 cm (2) and 43.036 cm (1), 38.031 cm (2) respectively.
This is in accordance with the similarity in structural parameters across the two pairs of
exchange pathways. The various J parameters for 1 and 2 are listed in Table 2.
Table 2. Experimental and theoretical exchange interaction parameters in 1 and 2.
-1
Theoretical (cm-1)
Complex
J
Experimental (cm )
1
J
1
, J
3
43.024, 43.049
43.331, 44.022

J
2
J
1
J
2
, J
4
, J
3
, J
4
4.044, 3.971
38.038, 38.025
4.490, 4.525
4.130, 4.525
38.211, 37.983
4.748, 4.957
2
There are two types of Cu···Cu links present in 1 and 2. One is through substituted
piperazine ring in chair conformation showing axial‒equatorial type coordination and the other
is formed by two µ‒O atoms from a basal‒basal hydroxido bridge and a basal‒apical phenoxido
bridge. Both the two types of linkages have been reported on very rare ocassions as means for
propagating magnetic exchange interactions [18], [19], [44‒46]. Across the
phenoxido‒hydroxido bridge the magnetic exchange should occur mainly through the µ‒HO‒
II
bridge as the overlap between the magnetic dx2-y2 orbitals of the Cu ions can only occur with the
atomic p-orbitals of the O atom connecting in the basal‒basal situation. The obtained coupling
-1
constant values are higher than that previously reported (16.9 cm ) for similar type of
compounds [18] Error! Bookmark not defined. mainly due to higher Cu‒Ohydroxido‒Cu bond angles
(
~109°). Interestingly, the antiferromagnetic exchange interaction within the [Cu O ] unit is
2 2
-1
-1
slightly stronger in 1 (43.036 cm ) compared to 2 (38.031 cm ) possibly due to the unequal
II
distribution of electron density on the two Cu centers in 1 as a result of the unsymmetrical
nature of H L2. In case of the piperazine bridge, both 1 and 2 are rare examples where the
2
II
chair‒piperazine coordinates two Cu ions in axial‒equatorial (a-e) mode and hence the
magnetic exchange interaction mediated through this unit has only previously been reported in
one instance [18]. It is suggested that in axial‒axial mode of coordination the favoured
mechanism of exchange is through the σ network of the ring [16] whereas the axial‒equatorial
mode would favour an interaction through space via direct N to N delocalization [18], [47]. The
-1
obtained values of exchange interaction (4.007 and 4.507 cm ) are much lower than that
mediated through the phenoxido‒hydroxido bridges as well as that taking place through a
-1
chair‒piperazine bridge in axial‒axial coordination mode (14.0 cm ) [16]. These findings are
in contrast to that previously reported for similar compounds where both the exchange
-1
interactions averaged out at 16.9 cm [18]. Thus in all probability interactions taking place by
through‒space pathway are weaker than those via through‒bond mechanism. Introduction of
‒
OMe functinality ortho to the phenoxido groups on the ligand backbone in the present study
lead to higher Cu‒O‒Cu angles as a result of additional hydrogen bonding interactions which

might have aided in the seggregation of the two exchange parameters. The similarity of the
coupling constants across the piperazine bridge in 1 and 2 indicate that difference in electron
II
density at the two interacting Cu centers have little effect on the magnitude of through‒space
exchange interactions.
3
.5. Computational studies
In order to obtain further insight into the mechanism of the exchange interactions taking
place in 1 and 2 and also to accounnt for the observed differences in the magnitude of coupling
constants in the two compounds DFT calculations were performed on the X-ray crystal structures
using B3LYP functional and def2‒TZVP basis set for Cu, N, O and def2‒SVP basis set for C, H.
The broken symmetry approach was utilized for calculating the values of the coupling constants.
For exchange iteractions (J and J ) across the piperazine bridges, Cu3 and Cu2 followed by Cu1
1
3
II
and Cu4 were replaced by diamagnetic Zn ions. Similarly in case of the interactions (J and J )
2
4
across the phenoxido‒hydroxido bridges an analogous approach of replacing Cu1 and Cu3 and
II
then Cu2 and Cu4 by Zn ions was undertaken. The theoretically calculated values of the
-1
-1
-1
-1
coupling constants are: J , J = 43.331 cm , 44.022 cm (1) and 38.211 cm , 37.983 cm
1
3
-1
-1
-1
-1
(
2) ; J , J = 4.130 cm , 4.525 cm (1) and 4.748 cm , 4.957 cm (2). The obtained values
2
4
are in excellent agreement with those derived from fitting of the experimental data (Table 2).
Analysis of the spin density distribution gives an idea about the mechanism of the excange
II
interactions in both the compounds. The atomic spin population values for the Cu centers and
the donor atoms of the ligands are listed in Tables 3 and 4. In 2 the Mulliken spin population
analysis for the high‒spin (HS) configuration shows that some spin is delocalised through the
hydroxido (ca. 0.215 e) and phenoxido (ca. 0.218 e) bridges as well as the piperazine N (ca.
0
.343 e). No spin delocalisation through the σ network of the piperazine ring is observed
confirming that the exchange interaction occurs via through‒space pathway. A considerable
II
amont of spin is supported by the Cu centers (ca. 2.430 e). It is to be noted that the spin density
on the individual N atoms of the piperazine bridge is much lower compared to the bridging O
atoms giving credance to the much weaker exchange interaction observed through the piperazine
bridge. The representation of the spin distributions in high-spin states of 1 and 2 are described
in Figure 6 while Figure 7 denotes the representation of spin density distributions relating to one

of the broken‒symmetry wave functions for each type of exchange interaction where α and β
spin states are denoted by positive (blue) and negative (red) signs, respectively.
Table 3. Mulliken atomic spin density values computed for the high spin configuration of 1 and
2
.
Atom Label
Cu1
Cu2
Cu3
Cu4
O1
Complex 1
0.606
0.603
0.617
0.606
0.139
0.107
0.143
0.116
0.111
0.112
0.115
0.076
0.089
0.113
0.112
0.075
0.090
0.111
Complex 2
0.602
0.609
0.609
0.610
0.128
0.109
0.138
0.109
0.107
0.108
0.117
0.079
0.090
0.117
0.111
0.078
0.095
0.111
O3
O4
O6
O7
O8
N1
N2
N3
N4
N5
N6
N7
N8
Table 4. The spin density values on selected atoms for 1 and 2 considering different coupling
interactions.
Atom Label
High spin
Complex 1
Cu1···Cu4
0.605
Low spin
Cu1
Cu4
N6
0.605
‒0.610
0.075
0.610
0.075
N7
0.092
‒0.092
Cu4···Cu2

Cu4
Cu2
O7
0.608
0.603
‒0.606
0.600
0.110
‒0.007
‒0.115
O6
0.116
Cu2···Cu3
0.603
Cu2
Cu3
N2
0.603
‒0.620
0.076
0.620
0.076
N3
0.091
‒0.090
Cu3···Cu1
0.617
Cu3
Cu1
O8
‒0.616
0.602
0.605
0.111
‒0.009
‒0.105
O3
0.107
Complex 2
Cu1···Cu4
0.601
Cu1
Cu4
N6
0.601
‒0.612
0.078
0.612
0.078
N7
0.097
‒0.097
Cu4···Cu2
0.608
Cu4
Cu2
O7
‒0.608
0.606
0.610
0.107
‒0.005
‒0.108
O6
0.110
Cu2···Cu3
0.608
Cu2
Cu3
N2
0.608
‒0.610
0.079
0.610
0.079
N3
0.092
‒0.092
Cu3···Cu1
0.608
Cu3
Cu1
O8
‒0.606
0.598
0.601
0.107
‒0.003
‒0.108
O3
0.110

–
3
Figure 6. Graphical representation of spin density (contour 0.002 e Å ) at the high spin configuration
of 1 (left) and 2 (right).
–
3
Figure 7. Graphical representation of spin density (contour 0.002 e Å ) at the low spin configuration of 1
(top) and 2 (bottom) with Cu2 and Cu3 (left) as well as Cu1 and Cu3 (right) being replaced by Zn.
II
When Cu2 and Cu3 are replaced by Zn ions, the broken‒symmetry spin population
values of +0.601 on Cu1 and 0.612 on Cu4 confirm them as the magnetic centers. The spin
delocalisation on N6 and N7 of the piperazine bridge amounted to +0.078 and 0.097
II
respectively. Again when Cu1 and Cu3 are replaced by Zn ions, the broken‒symmetry spin
population values of +0.606 on Cu4 and 0.608 on Cu2 confirm them as the magnetic centers
with spin delocalisation on O7 (hydroxido) and O6 (phenoxido) being of magnitudes 0.005 and

0.108 respectively. The much lower value of spin population on O7 as compared to O6

confirms that the exchange interaction takes place mainly through the hydroxido bridge. This is
also evident from the representation of spin population distribution relating to the
broken‒symmetry wavefunction (Figure 7) where it can be seen that both α and β spin states are
present on O7 while the sign of the spin on O6 corresponds to only that on Cu4. Similar
observations were made for 1 in high‒spin and broken‒symmetry configuration.
The observed difference in the coupling constants across the phenoxido‒hydroxido
bridge in 1 and 2 can be explained by comparing the spin density distributions in the high spin
state (Table 3). Since the exchange interaction is propagated mainly through the hydroxido
bridges the spin density at the O7 and O8 atoms were scrutinised. The Mulliken spin population
analysis shows the amount of spin delocalised over O7 and O8 to be 0.108 e and 0.107 e
respectively in 2. On the other hand the spin delocalisation over O7 and O8 is a bit higher in 1
amounting to 0.112 e and 0.111 e respectively. The greater spin density at the hydroxido O
atoms in 1 is suggestive of stronger exchange interactions compared to 2. This is due to the
unsymmetrical nature of ligand H L2 in 1 where only the phenoxido O coordinated to Cu1 and
2
Cu2 belongs to o-vanillin unit having an electron donating ‒OMe function whereas in case of
the symmetrical H L3 ligand in 2 all the four coordinated phenoxido O comes from o-vanillin
2
unit. The mismatch of electron donating ability at the two ends of the [Cu O ] moeity leads to
2
2
greater spin delocalisation over the hydroxido O atoms in 1.
4
. Concluding remarks
The synthetic effort reported in this work have demonstrated the relative capability of
II
II
II
three first transition series elements, Co , Ni and Cu in stabilizing the imine double bonds
within ligand frame. The solution state cross fragment reactions promoted the metal ion asssited
hydrolysis of the imine bonds and were responsible for the exchange of the hydrolyzable
II
2
2
aldehyde termini. The Cu bound L2 and L3 in tetranuclear forms as 1 and 2 were obtained
2
‒
2‒
via exchange of two and four aldehyde molecules respectively between L1 and L4 . In the
II
2
II
II
2‒
process the Cu ions tend to leave L4 and replace the Co and Ni ions (initially bound to L1 )
2

2
to form the final products. Both the L2 and L3 ligand anions showed unique conformational
isomerization at the piperazine coordination as a result of the interdimeric self-assembly process
with both 1 and 2 exhibiting a chair piperazine ring coordinating in the rare axial‒equatorial
mode. This unsymmetrical binding of the nitrogen donors of the chair piperazine ring essentially

prompts the unsymmetrical binding of the two tridentate halves of the ligands bearing similar or
II
dissimmilar aldehyde end functions. Two of the terminal phenolate groups bridges two Cu ions
in a equatorial‒axial manner which is not feasible for all axial coordination of the piperazine
ring. In both the two cases the experimental determined magnetic exchange interactions across
the chair‒piperazine and phenoxido‒hydroxido bridges were found to be antiferromagnetic in
-1
-1
nature. The interaction across the piperazine bridge (average J = ‒4.007 cm (1), ‒4.507 cm
2)) is weaker compared to that across the phenoxido‒hydroxido bridge (average J = ‒43.036
(
-1
-1
cm (1), ‒38.031 cm (2)) and that reported previously for axial‒axial mode of coordination.
DFT studies complement the experimental J values and further elucidate the mechanism of
exchange interaction across the piperazine bridge as through space via direct N‒to‒N
delocalization. It further reveals the build up of greater spin density at the bridging hydoxido O
2
‒
in 1 compared 2 due to the unsymmetrical nature of L2 which is responsible for a stronger
interaction across the phenoxido‒hydroxido bridge in the former.
Supporting Information
X-ray crystallographic data in CIF format, Tables S1–S3, Figures S1‒S3. CCDC 1960913 and
1
960920 contain the supplementary crystallographic data in CIF format for complexes 1 and 2.
Conflict of interests
The authors declare no competing financial interests.
Acknowledgements
DB is thankful to IIT Kharagpur for his research fellowship. We are also greateful to DST, New
Delhi for providing the Single Crystal X‒Ray Diffractometer facility in the Department of
Chemistry, IIT Kharagpur under the FIST program. The SQUID magnetometer facility at Central
Instrumentation Facility (CIF) at IISER Bhopal is highly acknowledged. DB is thankful to Ms.
Maya Khatun for her help with the ORCA 4.0 program.
Author Information
Corresponding Author
*E-mail: dray@chem.iitkgp.ac.in. Tel: (+91) 3222-283324. Fax: (+91) 3222-82252.
ORCID
Debashis Ray: 0000-0002-4174-6445

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