Synthesis, crystal structure and magnetic properties of new trinuclear Copper(II) complexes with Biphenol?Based dinucleating ligands

Article abstract of DOI:10.1016/j.molstruc.2020.128450

Treatment of (3,3′-bis-((2,6-dimethylphenyl)-imino)methyl)-(1,1′)-biphenyl-2,2′-diol) (H₂L²) and (3,3′-bis-((2-methoxyphenyl))imino)methyl)-(1,1′)-biphenyl-2,2′-diol) (H₂L³) with excess [Cu₂(OAc)₄(H₂O)₂] in the presence of triethylamine afforded new trinuclear complexes [Cu(3,3′-bis-((R)-iminomethyl)-(1,1′)-biphenyl-2,2′-dioxo)]₃ [R = 2,6-Me₂C₆H₃ (3e) and R = 2- OMeC₆H₃(4e)] respectively. The resulting complexes were characterized by elemental analysis, magnetic susceptibility, EPR, UV–Vis, IR, CV and Single crystal X-ray diffraction analysis. The magnetic susceptibility study of 3e and 4e was performed in the 5–300 K and revealed the existence of antiferromagnetic interaction in both complexes. The experimental data could be satisfactorily reproduced using an isotropic exchange model, H = -J (S₁S₂ + S₂S₃ + S₁S₃), yielding as best fit parameters: J = ?15 cm^?1, g = 1.99 for 3e and J = ?18 cm^?1, g = 1.99 for 4e. Both complexes have an angular C2/c-symmetric trinuclear core and each Cu(II) ion is in similar environments coordinated to two (O, N) donor sets as revealed from single-crystal X-ray diffraction studies. The coordination geometry can best be described as distorted square planar which could be judged using the τ₄ index, τ₄ = 360 – (α + β)/141°, where α and β are the two largest angles subtended by the ligand donor atoms in the four-coordinate complex. The average indexes for 3e and 4e were found to be 0.42 and 0.43 respectively, signifying a considerable distortion from an idealized square planar, D_4h geometry (τ₄ = 0.0) or idealized tetrahedral, T_d geometry (τ₄ = 1.0). The average Cu?Cu distance between closest copper(II) ions in the complex 3e is 4.48 ?. On the other hand, the single crystal X-ray analysis showed that each of the three Cu(II) centers in complex 4e is strongly tetracoordinated to two N and two O atoms & weakly to two O atoms of the methoxy groups of the ligand, N,N′,O,O′-3, 3′-bis ((2-methoxyphenyl)-iminomethyl)-[1, 1′-biphenyl]-2, 2′-diol (H₂L³). Generally, Cu?N & Cu?O bond lengths are (1.983–1.994) & (1.882–1.893) respectively, comparable to similar systems.

Full text of DOI:10.1016/j.molstruc.2020.128450

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Synthesis, crystal structure and magnetic properties of new trinuclear Copper(II)
complexes with Biphenol−Based dinucleating ligands
E. Adelew, K.G. Manoj
PII:
S0022-2860(20)30775-4
DOI:
https://doi.org/10.1016/j.molstruc.2020.128450
MOLSTR 128450
Reference:
To appear in: Journal of Molecular Structure
Received Date: 16 December 2019
Revised Date: 11 May 2020
Accepted Date: 13 May 2020
Please cite this article as: E. Adelew, K.G. Manoj, Synthesis, crystal structure and magnetic properties
of new trinuclear Copper(II) complexes with Biphenol−Based dinucleating ligands, Journal of Molecular
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First and corresponding Author, Adelew Estifanos Filkale, Ph.D,
ꢀ Designed the schemes
ꢀ Synthesized the compounds
ꢀ Wrote the manuscript
ꢀ Interpreted the results
Coauthor (2nd author) Manuj Kumar Gangwar, Ph.D
ꢀ Solved the crystal structures of the complexes
ꢀ participated in designing and proposing the schemes

Synthesis, Crystal Structure and Magnetic Properties of New Trinuclear
Copper(II) Complexes with Biphenol−Based Dinucleating Ligands
Adelew. E , Manoj. K. G.
Department of Chemistry, College of Computational & Natural sciences, Hawassa University,
P.O. Box, 05, Ethiopia
KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology,
P.O. Box, 4700, Thuwal, 23955-6900, Kingdom of Saudi Arabia.
A B S T R A C T
Treatment of (3,3′-bis-((2,6-dimethylphenyl)-imino)methyl)-(1,1')-biphenyl-2,2'-diol) (H2L2)
and (3,3′-bis-((2-methoxyphenyl))imino)methyl)-(1,1')-biphenyl-2,2'-diol ) (H2L3) with excess
[Cu2(OAc)4(H2O)2] in the presence of triethylamine afforded new trinuclear complexes [Cu(3,3′-
bis-((R)-iminomethyl)-(1,1')-biphenyl-2,2'-dioxo)]3 [R = 2,6-Me2C6H3 (3e) and R = 2-
OMeC6H3(4e)] respectively. The resulting complexes were characterized by elemental analysis,
magnetic susceptibility, EPR, UV-Vis, IR, CV and Single crystal X-ray diffraction analysis. The
magnetic susceptibility study of 3e and 4e was performed in the 5–300 K and revealed the
existence of antiferromagnetic interaction in both complexes. The experimental data could be
satisfactorily reproduced using an isotropic exchange model, H = -J (S1S2 + S2S3 + S1S3),
yielding as best fit parameters: J = -15 cm-1, g = 1.99 for 3e and J = -18 cm-1, g = 1.99 for 4e.
Both complexes have an angular C2/c-symmetric trinuclear core and each Cu(II) ion is in similar
environments coordinated to two (O, N) donor sets as revealed from single-crystal X-ray
diffraction studies. The coordination geometry can best be described as distorted square planar
which could be judged using the τ4 index, τ4 = 360 – (α + β)
/
141°, where α and β are the two
largest angles subtended by the ligand donor atoms in the four-coordinate complex. The average
indexes for 3e and 4e were found to be 0.42 and 0.43 respectively, signifying a considerable
distortion from an idealized square planar, D4h geometry (
τ
4=0.0) or idealized tetrahedral, Td
geometry ( 4=1.0). The average Cu•••Cu distance between closest copper(II) ions in the
τ
complex 3e is 4.48 Å. On the other hand, the single crystal X-ray analysis showed that each of
the three Cu(II) centers in complex 4e is strongly tetracoordinated to two N and two O atoms &
weakly to two O atoms of the methoxy groups of the ligand, N,N',O,O′-3, 3′-bis ((2-
methoxyphenyl)-iminomethyl)-[1, 1’-biphenyl]-2, 2′-diol (H2L3). Generally, Cu−N & Cu−O
bond lengths are (1.983 – 1.994) & (1.882 – 1.893) respectively, comparable to similar systems.
1

KEY WORDS: Biphenol, 2,6-dimethoxyaniline, 2,6-dimethylaniline, Dinucleating ligands
Trinuclear copper(II) complexes, Magnetic susceptibility
1. Introduction
Recent years have witnessed an explosion of interests in the research of polynuclear complexes,
not only because of their intriguing variety of architectures and multinuclear metals[1], but also
because of their fascinating extraordinary properties in the fields of catalysis, metalloenzyme and
magnetic materials[2–8]. In line with this, polynuclear copper complexes are studied for their
interesting magnetic and spectral properties which can serve as potential models of biological
systems in the characterization of the active sites of multicopper proteins [2,7,9,10] and
biological activities such as antitumor, antiviral, and anti-inflammatory[11,12]. Currently, there
are extensive studies on the synthesis and construction of the biphenol-based metal complexes,
which are derived from 2, 2’-dihydroxy-[1, 1’-biphenyl]-3, 3’-dicarbaldehyde with 2, 6-
dialkylanilines displaying interesting magnetic, electronic, and/or catalytic properties[13,14].
Among the various ligand modification reactions, salicylaldimine ligands derived from the
condensation of 2,2'-dihydroxy-[1,1'] biphenyl -3,3'-dicarbaldehyde and diamines such as 2,6-
diethylaniline, 2,6-dimethylaniline[3], and 2,6-diisopropylaniline represent an important series of
chelating agents that have been used to synthesize mono-, di-, or polynuclear transition metal
complexes. Copper(II) complexes, on the other hand, represent models of physical and chemical
behavior of biological copper systems that mimic copper metalloproteins such as
hemocyanin[15–17]. Polynuclear copper complexes with biphenol based ligands, in particular,
have gained considerable interest because of their relevance to the fungal enzyme galactose, with
recent progress in biomimetic model complexes which have been summarized in several
excellent reviews[8,18–20]. Complexes with two or more metal centers have shown also
promising efficiency in the field of catalysis mainly because of their cooperativity effect[5,21].
Each metal centers shown in Scheme 1, has tetradentate cavities coordinated to two N & O sites,
leaving the metal centers coordinatively unsaturated, and thus room for further coordination [22]
by exogenous ligands. In this paper we report the synthesis of biphenol-based N2O2 donor
ligands (H2L2) and (H2L3), formed by the facile condensation of 2, 2’-dihydroxy-[1, 1’-
biphenyl]-3, 3’-dicarbaldehyde (c) with 2-methoxy /2, 6-dimethylaniline and the corresponding
trinuclear copper(II) complexes (3e and 4e) (Scheme 1). All the ligands and complexes were
characterized by elemental analysis (EA) and spectroscopic techniques [21]. The variable
temperature magnetic behavior and single crystal X-ray diffraction (SCXRD) of the two
2

trinuclear complexes have also been described and the results have been compared with those of
similar complexes reported in the literature
2. Experimental
2.1. Materials
All starting materials were reagent grade and purchased from commercial sources. 2,6-
dimethylaniline and 2-methoxyaniline were purchased from Sigma Aldrich, Germany and used
without further purification. Solvents were purified and degassed by standard procedures[18].
2,2'-dihydroxy-[1,1'-biphenyl]-3,3'-dicarbaldehyde (c) was synthesized using literature
procedure[23].
2.2. Synthesis
2.2.1. Synthesis of 3,3′-bis[(2,6-dimethyl-phenylimino)-methyl]-[1,1'] biphenyl-2,2'-diol
(H2L2)
To a solution of 3,3′-diformyl-2,2’-dihydroxy-1,1′- biphenyl (c) (0.204 g, 0.843 mmol) in a
mixed medium of toluene (10 mL) and EtOH (10 mL), 2,6-dimethylaniline (0.306 g, 2.53 mmol)
was added dropwise in the presence of few drops of formic acid under magnetic stirring. The
reaction mixture was refluxed for 10 hours using a Dean stark trap apparatus in which the water
formed as a byproduct of the condensation reaction separated out. The volatiles were then
removed under high vacuum and the residue was washed with cold methanol three times and
1
dried under reduced pressure to give a yellow solid (H2L2). Yield: (0.289 g, 76 %). H NMR
(CDCl3, 400 MHz, 25 °C): δ 13.2 (s, 2H, 2OH), 8.39 (s, 2H, 2CHN), 7.61 (dd, 2H, 3JHH = 8 Hz,
3
4
4JHH = 2 Hz, 2C6H3), 7.32 (dd, 2H, JHH = 8 Hz, JHH = 2 Hz, 2C6H3), 7.05–7.02 (m, 6H,
2C6H3(CH3)2), 6.98 (t, 2H, 3JHH = 8 Hz, 2(2,6-C6H3(CH3)2)), 2.25 (s, 12H, 2(2,6-C6H3(CH3)2)).
Anal. Calcd.for C30H28N2O2 (448.56): C, 80.33; H, 6.29; N, 6.25. Found: C, 80.54; H, 5.81; N,
6.12 . Uv−Vis(nm): 236, 341.
2.2.2. Synthesis of 3,3'-bis(2-methoxyphenyl)imino)methyl)-[1,1'-biphenyl]-2,2'-diol (H2L3)
3

To a solution of 3, 3′-diformyl-2, 2’-dihydroxy-1, 1′- biphenyl (c) (0.204 g, 0.843 mmol) in a
mixed medium of toluene (10 mL) and EtOH (10 mL), 2-methoxyaniline (0.311 g, 2.53 mmol)
was added dropwise in the presence of few drops of formic acid under magnetic stirring. The
reaction mixture was refluxed for 10 hours using a Dean stark trap apparatus in which the water
formed as a by product of the condensation reaction separated out. The volatiles were then
removed under high vacuum and the residue was washed with cold methanol three times and
1
dried under reduced pressure to give a yellow brown solid (H2L3). Yield: 0.257 g (68 %). H
NMR (CDCl3, 400 MHz, 25 °C): δ 14.25 (s, 2H, 2OH), 8.51 (s, 2H, 2CHN), 7.61 (d, 2H, 3JHH
=
8 Hz, 2C6H3), 7.45 (d, 2H, 2C6H3), 7.25–7.40 (m, 2H, C6H3), 6.90–7.08 (m, 6H, C6H4), 6.8 (t,
2H, C6H4), 3.58 (s, 6H, 2(OCH3). )). Anal. Calcd.for C28H24N2O4 (452.50): C, 74.32; H, 5.35;
N, 6.19 %. Found: C, 74.45; H, 5.72; N, 6.04 %. Uv−Vis(nm): 249, 341.
2.2.3. Tris [3, 3′-bis [(2, 6-dimethylphenyl) imino) methyl]-[1, 1'] - biphenyl-2, 2’-dioxo copper
(II)] (3e)
A hot solution (50 °C) of [Cu2 (OAc)4(H2O)2] (0.334 g, 1.67 mmol)) in MeOH (10 mL) was
added dropwise while stirring to
a
hot suspension (60 °C) of (3,3′-bis[(2,6-
dimethylphenyl)imino)methyl]-[1,1']biphenyl-2,2'-diol (H2L2) (0.165 g, 0.368 mmol) in MeOH
(10 mL). The reaction mixture was then refluxed for 12 hours in the presence of few drops of
triethylamine, during which time the formation of precipitate was observed. The reaction was
cooled to room temprature and concentrated under reduced pressure. The residue was dissolved
in CH2Cl2 (40 mL), filtered over celite and the volatiles were removed under reduced pressure to
give a brownish powder. Single-crystals of compound 3e suitable for X-ray diffraction were
grown by slow evaporation of a solution of acetonitrile and dichloromethane (1:1) at room
temprature. Yield: 0.108 g (55 %). Anal. Calcd.for C90H78Cu3N6O6(1530.26): C, 70.64; H,
5.14; N, 5.49 %. Found: C, 70.45; H, 5.22; N, 5.23 %. IR (KBr): ν(cm-1): 3434 (w), 2960 (s),
2935 (s), 1601 (s), 1587 (s), 1545 (s), 1424 (s), 1169 (s), 747 (s), 519 (m). Uv−Vis(nm):230,
274, 310, 406.
2.2.4. Tris [3, 3’-bis ((2-methoxphenyl) imino) methyl)-[1, 1’-biphenyl]-2,2’-trioxo copper(II)]
(4e)
4

A hot solution (50 °C) of [Cu2 (OAc)4(H2O)2] (0.131 g, 0.654 mmol ) in MeOH (10 mL) was
added dropwise while stirring to a hot suspension (60 °C) of 3, 3’-bis ((2-methoxyphenyl) imino)
methyl)-[1, 1’-biphenyl]-2,2'-diol (0.164 g, 0.362 mmol) (H2L3) in MeOH (10 mL). The reaction
mixture was then refluxed for 12 hours in the presence of few drops of triethylamine, during which
time the formation of precipitate was observed. The reaction was cooled to room temprature and
concentrated under reduced pressure. The residue was dissolved in CH2Cl2 (40 mL), filtered over
celite and the volatiles were removed under reduced pressure to give dark green solid. Single-
crystals of compound 4e suitable for X-ray diffraction were grown by slow evaporation of a
solution of acetonitrile and dichloromethane (1:1) at room temprature. Yield: 0.103 g (52 %).
Anal. Calcd. for C84H66Cu3N6O12 (1542.09): C, 65.42; H, 4.31; N, 5.45 %. Found: C, 65.58; H,
4.69; N, 5.48 %. IR (KBr): ν(cm-1): 3437 (w), 2964 (s), 2931 (s), 1603 (s), 1588 (s), 1548 (s),
1424 (s), 1169 (s), 747 (s), 519 (m). Uv−Vis(nm):228, 275, 312, 405.
2.3. Methods and measurements
1H NMR and13C {1H} NMR spectra were recorded on Bruker 400 MHz and Bruker 500
MHz NMR spectrometers. Elemental Analysis was carried out on Thermo Finnigan FLASH EA
1112 SERIES (CHNS) Elemental Analyzer. Infrared spectra were recorded on a Perkin Elmer
Spectrum one FT-IR spectrometer. The electronic spectra were recorded in dichloromethane using
a Perkin Elmer Lamda 35 UV-Visible spectrophotometer. Mass spectrometry measurements were
performed on a Micromass Q-Tof spectrometer. Cyclic voltammetry was performed at room
temperature in dichloromethane under liquid nitrogen atmosphere, in a three-electrode cell
connected to a Schlenk line. The counter electrode was a platinum wire of 1 cm2 apparent surface
area. The reference was a saturated calomel electrode (SCE) separated from the solution by a
bridge (4 ml) filled with a 0.1 M. Variable-temperature magnetic susceptibility measurements
were performed on pulverized crystalline samples in the range 5 to 300 K with a Quantum Design
MPMS-5 SQUID magnetometer. Magnetic data were corrected for the diamagnetism of the
sample holder and diamagnetism of the sample using Pascal’s constants[24]. The EPR
measurements were made with a Varian model 109C E-line X-band spectrometer fitted with a
quartz Dewar for measurements at 77 K. The spectra were calibrated using tetracyanoethylene
(tcne) (g = 2.0037).
5

2.4. X-ray crystal structure determination
X-ray diffraction data were collected on a Bruker P4 diffractometer equipped with a SMART
CCD detector and crystal data collection and refinement parameters are summarized in Tables 3
and 4. Single-crystal X-ray data for 3e and 4e were collected at 100K and 150K respectively. The
structures were solved using direct methods and standard difference map techniques, and were
refined by full-matrix least-squares procedures on F2 with SHELXTL (Version)[2]. CCDC-
1400845 (for 3e) and CCDC- 1432509 (for 4e) contain the supplementary crystallographic data.
These data can be obtained free of charge from the Cambridge Crystallographic Data center via
www.ccdc.cam.ac.uk/data_request/cif.
3. Results and discussion
3.1. Synthesis of ligands 3, 3′-bis-((R)-iminomethyl)-(1,1')-biphenyl-2,2'-diol, where R = 2,6-
Me2C6H3 (H2L2), 2- OMeC6H4 (H2L3)
The tetradentate chelating ligands, H2Ln (n = 2, 3) used in this work were prepared in high
yields by the condensation reaction of 2, 2’-dihdroxy-[1, 1-biphenyl]-3, 3-dicarbaldehyde (c)
(Scheme 1) with three equivalents of 2,6-dialkylaniline in a mixed medium of toluene and EtOH
(1:1) at about 60 ◦C in the presence of catalytic amount of formic acid [25–27]. The condensation
reaction was confirmed by the complete disappearance of CHO proton 1HNMR resonance peak at
11.43 ppm and the appearance of–CHN−proton peak[28] at 8.40 ppm in the ligand H2L2 [6,29].
Information gained from HRMS, IR & UV-Vis[6] spectras indicate that both ligands possess four
coordination sites (two imino–N and two biphenolic–O atoms). When fully deprotonated, they
provide N2O2 tetradentate environments which could encapsulate two metal ions and bridge two
other similar ligands to form a trinuclear structure. Both H2L2 and H2L3 are soluble in organic
solvents such as chloroform, dichloromethane, ethanol, methanol or other low-polar organic
solvents.
3.2. Synthesis of trinuclear copper(II) complexes, [M (3,3′-bis-((R)-iminomethyl)-(1,1')-
biphenyl-2,2'-dioxo)]3, M =Cu(II), R = 2, 6-Me2C6H3 (3e) and M =Cu (II), R = R = 2-
OMe2C6H4 (4e)
6

The ligands were subsequently employed for the synthesis of corresponding trinuclear
complexes according to the synthetic pathways summarized in Scheme 1[17,30]. The
complexes were prepared by the reaction of methanolic solution of H2Ln with excess
[Cu2(OAc)4(H2O)2] in the presence of triethylamine[9,31,32]. Complexes 3e & 4e were isolated
using the appropriate workup as brownish and dark green colored solids with 55 and 52 % yields
respectively. The synthesized compounds are thermally stable, air and moisture insensitive on
storage under ordinary conditions. They exhibit good solubility in common non polar organic
solvents. The molecular identity and geometry of complexes 3e and 4e were elucidated by the
single crystal X-ray crystal structure determination. Good elemental analysis data comparable
with the calculated ones were obtained for the complexes. Solvent of crystallization was
detected in the single crystal X-ray structures of complexes. The solvents were used for the
crystal growth and believed to be incorporated in the voids or lattices, holding partly the three
dimensional crystal structures.
7

Scheme 1. Synthesis of ligand H2L2 and the corresponding trinuclear copper(II) complex 3e.
Scheme 2. Synthesis of ligand H2L3 and the corresponding trinuclear copper(II) complex 4e.
8

3.3. Spectroscopic characterizations
3.3.1. FTIR Spectra
In the FT-IR spectra of the ligands H2L2 and H2L3, strong absorption band at
ν
3440 cm-1
assignable to O-H stretching is observed[33,34]. The FT-IR spectra of complex 4e also show the
retention of this band in the said region due to the absorption of coordinated H2O[25,36].
However, this band is absent in the spectra of 3e which shows the absence of coordinated H2O
and complete deprotonation and coordination of OH to the Cu(II) core[27]. The azomethine
ν(C=N) absorption band and the biphenolic C−O stretching bands are observed at 1618 cm-1 and
1205 cm-1 in the spectra of H2L2 respectively [28,37]. Strong band at 1427 cm-1 is assigned to
ν(C=C) biphenyl stretching vibrations [16]. Up on complexation, the positions of C=N, C−O,
and C=C vibrations were shifted to lower frequencies 1602 , 1201, 1425 cm-1 respectively and
their intensities were changed[38]. This confirms the coordination takes place through the
azomethine nitrogen C=N and the deprotonated biphenol oxygen C−O leading to lower electron
density on the azomethine and oxygen of the ionized hydroxyl C−O of the biphenol ring[28].
The ν(C−O) shifts to lower frequencies because of the high electronegative oxygen of C−O
which exerts an electron withdrawing effect reducing the C−O bond electron density and
consequently weakening the force constant[28,39]. Similarly, the azomethine C=N stretching
and the biphenolic C−O stretching bands were observed at 1617 cm-1 and 1208 cm-1 in the
spectra of H2L3 respectively [28,37]. These bands were shifted to lower frequencies in the
corresponding complex 4e to 1603 cm-1 and 1174 cm-1 respectively (Table 1S, Supplementary
data).
3.3.2. Electronic spectra
The electronic spectra of ligands (H2L2 and H2L3) in CH2Cl2 showed strong absorption band
in the region 228–230 nm, attributable to π→ π* transition in −C=N− bond[34,40]. This band was
shifted to higher wave length 274–310 nm (red shift) up on complexation[33]. The band in the
region 274–275 nm is due to n → π* transition of the azomithine group[17]. This may involve the
transfer of lone pair electrons of azomethine-N to the lowest unoccupied orbitals (LUMO) of the
imine double bond. Upon complexation, it undergoes a blue shift in the complexes[32].
9

3e
4e
1.0
0.8
0.6
0.4
0.2
0.0
2
H L
2
200
250
300
350
400
450
500
Wave length/nm
Fig. 1. UV-visible spectra of the trinuclear copper complex, 3e, 4e and H2L2 in CH2Cl2.
This confirms the coordination of Cu(II) to the ligand through the azomethine nitrogen. In the
spectra of the trinuclear Cu(II) complexes, the position and intensity of the absorption band that is
characteristic of the ligand appeared to be modified with respect to those of the free ligands. The
bands observed in the region 310–312 nm could be assigned as the metal-to-ligand charge (MLCT)
transfer transition[36] which might be attributed to the metal d→ligand π* charge-transfer
transions (Table 2) (supporting info)[27]. The relatively intense broad bands in the regions 405–
406 nm are attributed to the CuII–O biphenolate ligand– to –metal charge transfer transitions[41].
These bands probably incorporate both d-d and charge transfer transitions[42]. The spectra of
complexes 3e and 4e show no apparent bands within the 900-500 nm range, although it is very
likely that the expected d-d transitions are masked by the broad and intense charge transfer
bands[6,41].
10

3.4. Cyclic voltammetry
Cyclic voltammograms of the trinuclear copper(II) complex 3e was measured in order to gain
insight about the tendency of the new trinuclear copper(II) complexes to undergo oxidation–
reduction. Experiments were carried out in CH2Cl2 (1 mM) solution of compound 3e with 0.1 M
(n-Bu4N)(ClO4) as the supporting electrolyte under liquid nitrogen atmosphere.
6.0
E
red
4.0
E
ox1
2.0
0.0
E
ox2
-2.0
-4.0
-6.0
-8.0
2
1
0
-1
-2
Potential (V)
Fig. 2. Cyclic voltammogram of 3e (1.0 mM) in CH2Cl2 with 0.1 M (n-Bu4N)(ClO4) obtained on
a glassy carbon electrode at a scan rate of 100mVs-1.
The anodic region (Figure 2) of 3e shows quasi reversible (Eox1) and (Eox2) oxidation processes at
anodic potential peak values: Eox1 = 0.164 V, Eox2 = 1.33 V respectively[43]. In the negative
direction, the cyclic voltammogram also shows quasireversible reduction signal at the cathodic
potential peak: Ered = -1.30 V. Based on comparison with the previous works , the oxidations
could be assigned to biphenolate-ligand centered processes ( BiphO22- → BiphO2• − → BiphO2
)
:
while the reduction (Ered) corresponds to the CuII/CuI) couple [14,28,44].
11

2-
Thus, the anodic peaks are assigned to the sequential one-electron oxidations, BiphO2
→
BiphO2• −(Eox1), BiphO2• −→ BiphO2
:
(Eox2), while the cathodic peak is assigned to reduction
involving (CuII/CuI)(Ered)[43]. The quasireversible redox waves suggest that each
oxidation/reduction is followed by a rearrangement, which is both electronic and structural.
The CuII ions are high spin while the CuI ions are low spin, thus, reduction of CuII may initially
produce a high-spin CuI species, which presumably undergoes a rapid spin change[7]. The
electrochemical properties of complex 4e is found to be analogous with that of complex 3e and
not presented here. Finally, it should be noted that comparison of our results with reported data
is delicate in case where different solvents have been employed, since it is obvious that the
potentials are markedly dependent on the solvent.
3.5. Magnetic properties
3.5.1. Magnetic Susceptibilities
The magnetic properties of the trimers 3e and 4e were measured using a SQUID
magnetometer over the temperature range 5–300 K at 100 Oe[1]. The temperature dependence
of χMT (χM being the magnetic susceptibility per Cu3 entity) for complexes 3e and 4e are shown
in Figures 4 and 5 respectively[42]. At room temperature, the χMT (χMT = ꢀeff2/8) value (0.958
cm3 K mol-1) of complex 3e is substantially lower than that expected for three uncoupled Cu(II)
ions (1.125 cm3 K mol-1, considering g = 2[1]). Plots of the magnetic susceptibility and
reciprocal magnetic susceptibility for 3e and 4e are given in Fig. 3 and 6 respectively[8,42]. The
Curie–Weiss behavior of 3e indicates a linear relation of the reciprocal magnetic susceptibility
curve at higher temperatures (100–300 K) with a Curie constant C = 0.236 cm3 mol-1 K and
Weiss-constant, θ = -8.26 K and a small anomaly is observed only at the lowest
temperatures[33]. The compound reveals a magnetic behavior which is characteristic of
antiferromagnetic spin coupling[34,45]. The χMT values decrease gradually with lowering the
temperature, from 0.958 cm3 mol-1 K at 300 K to 0.256 cm3 mol-1 K at 5.8 K [43]. Below 100 K
to 50 K the decrease observed on the χMT values is slightly less pronounced, tending to the
expected plateau, characteristic of an S = 1/2 system resulting from an antiferromagnetically
12

coupled trinuclear Cu(II) compound. Below 50 K the magnetic susceptibility shows a new rapid
decrease, which can only be attributable to antiferromagnetic intramolecular interactions [9,46].
Similarly, at room temperature, the χmT value of complex 4e is 0.987 cm3 K mol−1, when
lowering the temperature to 25 K, this value decreases to 0.61 cm3 K mol−1. The Curie–Weiss
behavior of 4e indicates a linear relation of the reciprocal magnetic susceptibility curve at higher
temperatures (100–300 K) with a Curie constant C = 0.182 cm3 mol-1 K and Weiss-constant,
θ = -5.05 K, which also suggests the presence of weak antiferromagnetic interactions among the
trimeric Cu(II) ions[47].
0.16
120
0.14
100
0.12
0.10
0.08
0.06
0.04
0.02
0.00
80
60
40
20
0
0
50
100
150
200
250
300
T (K)
Fig. 3. Temperature dependence of χm and1/χ plot for complex 3e at 100 Oe. The solid
line is the best exponential fit of the experimental data.
The χMT value decreases sharply and reaches to a minimum value of 0.45 cm3 K mol−1at 6 K
[48]. Furthermore, the effective magnetic moments of complexes 3e and 4e at room temperature
13

are 2.79 and 2.71 B.M. respectively[4], which is less than the spin only value of Cu(II), 3.87
B.M., calculated ([4S(S + 1)]1/2; S = 3(1/2)) for three uncoupled spin 1/2 centers [29,44,47,49].
1.0
0.8
0.6
0.4
0.2
0.0
0
50
100
150
200
250
300
T (K)
Fig. 4. Temperature dependence of χmT for complex 3e at 100 Oe. The solid line is the best
exponential fit of the experimental data
To quantify the magnetic interactions (J) in complexes 3e and 4e, it is shown that the three Cu(II)
ions of each [Cu3N2O2] system occupy vertices of an equilateral triangle as the three Cu(II)
centres are equivalent[12,46,50]. Each Cu(II) ions should interact with the two adjacent Cu(II)
ions similarly as the Cu(II)•••Cu(II) distances are equivalent[51]. The magnetic exchange
pathway for each pair involves the biphenyl-O and N- bridging group of the derived Schiff base
ligand. In order to investigate the magnetic behavior of these trinuclear complexes, an isotropic
Heisenberg–Dirac–Van Vleck (HDVV) Hamiltonian was used in the approach[34].
14

Ĥ = −J12S1S2 − J13S1S3− J23 S2S3
(1)
Since the three copper atoms of the [Cu3N2O2] unit define a quasi-equilateral triangle, the three
CuII ions can be considered equivalents, thus J12 = J13= J23 = JCuCu & Eq.(1) can be written in the
form Ĥ = − JCuCu (S1S2 − S1S3− S2S3).
The magnetic susceptibility deduced from the Hamiltonian is given in Eq. (2), where N, g, β and
k have their usual meanings.
χmT = ( Ng2β2͎/4KT)[1 + 5 exp(3J/2KT)]/(1+exp(3J/2KT)
(2)
The best fitting was obtained for 3e with an isotropic coupling constant JCuCu = −15 cm−1 and
gCuCu = 1.99, assuming a temperature-independent paramagnetic correction of χTIP = 2.4 × 10−3
cm3 K mol−1, Magnetic impurity (MI) = 0.18 % (Table 2). Similarly, the best agreement with
the experimental data was obtained for 4e with JCuCu = −18 cm−1 and gCuCu = 1.99, assuming a
temperature-independent paramagnetic correction of χTIP = 2.81 × 10−3 cm3 K mol −1, Magnetic
impurity (MI) = 0.16 % [14,47] (Table 2). Thus both complexes show weak antiferromagnetic
interactions (Table 3)[6,29]. Generally, the decrease in the magnitude of χMT with decreasing
system
temperature
for
these
types
of
complexes
could
result
fro
m several factors such as intramolecular M–M interactions between the Cu(II) centres, [Cu•••Cu
4.48Å (3e) and Cu•••Cu 4.54 Å (4e)], significant spin-orbit coupling inherent to the Cu(II) ions
as well as non-negligible intermolecular interactions (4.84 Å) even at room temperature
[6,10,14,47].
15

1.0
0.8
0.6
0.4
0.2
0.0
0
50
100
150
200
250
300
T (K)
Fig. 5. Temperature dependence of χmT for complex 4e at 100 Oe. The solid line is the best
exponential fit of the experimental data.
The antiferromagnetic coupling observed in the complexes can also be explained from the
magneto-structural correlations established for biphenol linkers[52]. These correlations
established that magnetic coupling strongly depend on the Cu−O, the Cu−Cu separation and to
small extent on the geometries around the coupling. In the present case, the exchange coupling
(J) can be explained in terms of the geometrical distortions that affect the copper coordination
spheres.
The core structures found in complexes 3e and 4e are reasonably similar with previously
reported[45] trinuclear Cu(II) complexes where three Cu(II) ions of each [Cu3N2O2] system
occupy alternate corners of a cube[53].
16

0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
100
80
60
40
20
0
0
50
100
150
200
250
300
T (K)
Fig. 6. Temperature dependence of χm and 1/χm for complex 4e at 100 Oe. The solid line is the
best exponential fit of the experimental data.
Therefore, each Cu(II) ion is connected to the adjacent Cu(II) ion through one [Cu3N2O2] group
and one biphenoxyl bridging group. The molecular structure also shows that the aromatic linkers
connect the Cu(II) centers in an equilateral fashion[54]. It is obvious that the single electron of
the Cu2+ in the dx2-y2 orbital always participates in a strong magnetic structural correlations of
complexes of such type show that larger Cu•••Cu angles favor larger antiferromagnetic
contributions[55].
In triangular complexes, where the arrangement of the magnetic centers corresponds to
equilateral triangles, the three cores show antiferromagnetic coupling which give rise to spin
frustration effects [34]. The spin frustration requires two conditions: (a) a highly symmetrical
triangle and (b) a degenerate ground state.[56] The existence of spin frustration for this kind of
complexes is most likely studied by means of EPR spectra at very low temperatures when only
17

the degenerate S = 1/2 states are populated[56]. One of the signatures of spin frustration is the
appearance of g values equal or close to 2.00[34].
3.5.2. Electron paramagnetic spectroscopy
The EPR spectra of complexes 3e and 4e was recorded as polycrystalline powder at 100K with
X-band microwave magnetic field. The spectrum of 4e at 100K shows a single absorption band
observed approximately at giso = 2.12., whereas the EPR spectra of 3e shows g value centered at
g
iso = 2.11 with no hyperfine splitting (A)[57], indicating antiferromagnetic interaction between
copper ions in both complexes[58,59]. The isotropic nature of the signal is probably due to the
exchange narrowing resulting from nearby Cu units in the crystal lattice[34].
Thus, EPR of both complexes at the lowest temperatures reveal only one unpaired electron on
the whole trimer molecule which can be related to a doublet (S = 1/2) species and to a higher
spin state generated by spin coupling within units becoming thermally populated at higher
temperatures[51]. They show unresolved single resonances peaks, due to exchange coupling
between the copper ions[8,34]. As can be noticed, the line width at 100 K (Fig. 7 & 8) is small,
but no anisotropy of g or hyperfine coupling with the Cu nuclei can be detected[61]. On the
other hand, a proper consideration of this interaction should in principle take into account three
Cu nuclei with different hyperfine couplings and the simultaneous presence of the two isotopes
63Cu and 65Cu, in comparable natural abundance [36]. Generally, the results indicate that the
ground state of both 3e and 4e is a doublet state[42]. The same explanation was suggested to
account for the EPR features of a similar trinuclear copper[62]. It is interesting to note that the g
factor components are similar to those measured recently at low temperature for a spin-frustrated
symmetric trinuclear copper complex, which clearly display the features of the quartet state at
high temperature [56].
18

600
400
200
0
giso = 2.11
-200
-400
2500
3000
3500
4000
Field(G)
Fig. 7. Polycrystalline X-band EPR spectra for complex 3e at 100 K.
800
600
400
giso = 2.12
200
0
-200
-400
-600
2000
3000
4000
Field(G)
Fig. 8. Polycrystalline X-band EPR spectra for complex 4e at 100 K.
19

Table 1
Magnetic data for 3e and 4e at 300 K.
S.No Metal complex
Obs.χM(cm3mol1)
1/χ(cm3mol)
χMT(cm3mol-1K)
ꢀs(B) ꢀs+l(B)
Obs.ꢀeff(ꢀB)
1
0.010
98.15
0.987
3.87
5.19
2.71
3e
2
0.011
90.9
0.958
3.87
5.19
2.79
4e
Table 2
List of magnetic fitting parameters in complexes 3e and 4e.
S.No. Metal complex
gcu(II)
1.99
1.99
JCuCu(cm-1) Temperature
Independent
Magnetic impurity (%)
Paramagnetism(10-3cm3mol)
1
2
-15
-18
2.40
0.18
0.16
3e
4e
2.81
20

3.6. Single Crystal X-ray diffraction
X-ray quality crystals were readily obtained for 3e and 4e through slow diffusion of
CH3CN into a CH2Cl2 solution of the complex
The single crystal X-ray diffraction data and experimental details for complexes 3e and 4e
are available in Table 3 and their structures along with selected bond lengths and angles are
given in Table 4.
Fig. 9. Molecular structure of 3e with thermal ellipsoids plotted at the 50 % probability level.
Description of the crystal structures of 3e and 4e
Each Cu(II) ion in complex 3e is coordinated through the lone pairs of biphenolate-O and the
imine-N atoms of the ligand H2L2[36]. The Cu(II) ion coordination polyhedron in 3e is
formed by the ligation of imine –N and biphenolate-O atoms[29]. The geometry around each
Cu(II) atom can be best described as distorted square planar where Cu(II) centers are in similar
21

environments coordinated to two (O, N) donor sets[52,63]. The distorted nature of the
geometry around each Cu(II) atoms can be judged from the 4 index,
4 = 360 – ( 141
are the two largest angles subtended by the ligand donor atoms in the four-
τ
τ
α
+
β)/
°
where
α and β
coordinate complex.
The average indexes for 3e and 4e were found to be 0.42 and 0.43 respectively, signifying a
considerable distortion from an idealized square planar, D4h geometry (
τ
4=0.0) or idealized
tetrahedral, Td geometry ( 4=1.0)[64]. Cu–O and Cu–N bond distances are in the respective
τ
ranges 1.870(7) –1.890(4) and 1.985(4)–1.975
[42,56]
Ǻ, which are consistent with literature values
Complexes 3e and 4e are isomorphous and crystallized in the monoclinic space group C2/c. All
Cu1, Cu2 and Cu3 in 4e display distorted square planar geometry, being surrounded by two
adjacent imine–N atoms and two –O atoms [7]. For each Cu(II) ions, the two oxygen atoms
come from the deprotonated biphenol oxygen atom –O and two nitrogen atoms from imine of the
ligand[34]. In addition to strong coordination in 4e with two (O, N) donor sets, each Cu(II) ion
is weakly coordinated with two O atoms from the two ortho methoxy group.
Both complexes 3e and 4e are trimeric and are centrosymmetric due to the presence of an
inversion center in the middle of each molecule[53]. In complex 4e, 2, 2-biphenol-based Schiff
base complex Cu1-O1-N1-N2 and Cu1i-O1i-N1i-N2i are formed by the two azomithine-N and
two biphenolate-O ligand H2L3 moieties and three cu(II) ions[51]. Crystallographically
equivalent atoms have equivalent bond lengths and bond angles. The structure of 4e consists of
a trinuclear unit containing a symmetrical Cu3N6O6 core with each Cu(II) ion located on the
corner of an equilateral triangle[3].
22

Fig. 10. Molecular structure of 4e with thermal ellipsoids plotted at the 50 % probability level.
The ORTEP drawing and atom-labeling scheme of complex 4e is shown in Fig. 10 and selected
geometrical parameters are given in Table 4. There is no direct covalent linkage between Cu(II)
ions and the trigonal arrangement of the biphenyl linkers at each Cu(II) ion leads these metals to
be separated by a distance of 4.804(1) Å[12]. Each Cu(II) ions is coordinated by two nitrogens
and alkoxy oxygen donor from the ligand [Cu(1)–N(1) 1.983(5) Å, Cu(1)–N(2) 2.013(5) Å,
Cu(1)–N(3) 1.976(4) Å and Cu(1)–O(1) (1.938(3) Å][34,52]. The geometry at each Cu (II) ion
may be considered as pseudo octahedral, being surrounded in the equatorial plane by two
nitrogens and two oxygens from different ligands [Cu(2)– N(4) 1.988(5) Å, Cu(2)–O(1) 1.957(3)
Å], and at axial positions by two oxygen atoms O(9) from methoxy group on the ligand which is
23

at significantly longer distance, 2.602(4) Å). Complexes ( 3 & 4)e represent the first family
which are characterized spectrometrically and structurally[51].
Table 3
X-ray crystallographic data for compounds 3e. 2(C7H8).H2 O and 4e. 2(CH2Cl2)
Data
3e. 2(C7 H8). H2 O
C104H96Cu3N6O7
Monoclinic
4e. 2(CH2Cl2)
C86H70Cl4Cu3N6O12
Monoclinic
Formula
Crystal system
Formula weight
Space group
1530.25
1542.0938
C
2/c
C2/c
a/
b/
c/
Ǻ
Ǻ
Ǻ
14.693(3)
15.287(3)
21.242(5)
90
25.215(60)
15.002(3)
20.902(5)
90
α/◦
β
/
◦
105.573(3)
90
109.399(3)
90
γ/
◦
V/
Ǻ
84.69(3)
4
74.58(3)
4
Z
temperature (K)
radiation ( , Å)
(calcd.), g cm-3
max, deg.
100
150
λ
0.71070
1.359
0.71070
1.525
ρ
θ
25.00
25.00
No. of data
7469
6573
No. of parameters
553
499
R1a
wR2b
0.0543
0.118
0.0808
0.2490
1.099
GOF
1.079
a R1=
bwR2 = {[
Σ
|
Fo
|
−
|
Fc|/
ΣFo
Σ
w
(
Fo−Fc)2]/[ Fo)2]}1/2
Σw(
24

Table 4
Selected bond lengths (
Ǻ
) and angles (
◦
) of complexes 3e and 4e
Complex
Complex
3e
4e
Cu(1)−O(2)
1.870(2)
1.887(2),
1.975(3),
1.975(3)
1.873(2),
1.873(2)
1.975(3)
1.975(3)
1.985(3)
1.985(3).
1.873(2),
1.873(2)
Cu(1)-O(1AA)
Cu(1)−O(0AA)
Cu(1)−N(2)
Cu(1)−N(1)
Cu(2)−O(3)
Cu(2)−O(3)¡
Cu(2)−N(3)
Cu(2)−N(3)¡
Cu(3)−O(3)
Cu(3)−O(3)¡
Cu(3)−N(3)
Cu(3)−N(3)¡
1.899(3)
1.910(3),
1.976(4),
1.984(4)
1.896(3)
1.896(3)
1.964(4)
1.964(4)
1.896(3)
1.896(3)
1.964(4)
1.964(4)
Cu(1)−O(1)
Cu(1)−N(2)
Cu(1)–N(1)
Cu(2)–O(4)
Cu(2)−O(3)
Cu(2)–N(3)
Cu(2)–N(4)
Cu(3)−N(5)
Cu(3)−N(6)
Cu(3)−O(5)
Cu(3)−O(6)
O(2)−Cu(1)−O(1)
O(2)−Cu(1)−N(2)
O(1)−Cu(1)−N(2)
O(2)−Cu(1)−N(1)
O(1)−Cu(1)−N(1)
N(2)−Cu(1)−N(1)
O(3)−Cu(2)−O(3i)
O(3)−Cu(2)−N(3)
O(3)−Cu(2)−N(3i)
O(3)−Cu(2)−N(3)
O(3i)−Cu(2)−N(3)
N(3)−Cu(2)−N(3i)
152.30(10),
92.90(10)
94.90(10)
94.71(10)
94.72(10)
143.38(11)
159.11(14)
91.79(10)
94.01(10),
94.01(10
O(1AA)−Cu(1)−O(0AA) 148.87(13)
O(1AA)−Cu(1)−N(2)
O(0AA)−Cu(1)−N(2)
O(1AA)−Cu(1)−N(1)
O(0AA)−Cu(1)−N(1)
N(2)−Cu(1)−N(1)
O(3)−Cu(2)−O(3)¡
O(3)−Cu(2)−N(3)
O(3)−Cu(2)−N(3)¡
O(3¡)−Cu(2)−N(3)
O(3)¡−Cu(2)−N(3)¡
N(3)−Cu(2)−N(3)¡
93.17(14)
94.17(14)
93.99(14)
92.61(14)
153.84(15)
140.08(18)
94.24(14)
94.64(14)
94.64(14)
94.24(14)
153.8(2)
91.79(10),
147.57(15).
25

4. Conclusion
In summary, we have synthesized the first example of trinuclear copper(II) complexes as revealed by
the spectrometric and spectroscopic techniques. The structure and coordination geometry of
complexes (3 and 4)e were deduced from the Single Crystal X-ray crystallographic studies. The
complexes show weak antiferromagnetic interaction. Evidence for intramolecular interaction was
obtained from the fitting of experimental magnetochemistry with the theoretical ones which give
coupling constant (J) values characteristics of antiferromagnetic interaction. The trimeric,
paramagnetic and redox active nature of the complexes probe good future magnetic (as single
molecule magnets), catalytic or antibiological applications. The study for catalytic applications of
the complexes is underway in our lab
.
Conflicts of interest
The authors declare no conflict of interest
Acknowledgments
The Authors would like to express their gratitude for the financial support from the Department of
Science and Technology (EMR/2014/000254), New Delhi, India and Ministry of Education,
Ethiopia. The authors gratefully acknowledge the central Facility, Department of Chemistry, IIT
Bombay, Mumbai, India, for the Single Crystal X-ray crystallographic characterization and
Professor P. Ghosh, Department of Chemistry, IIT Bombay for his technical support in this study.
Appendix A. Supplementary data
1H NMR, 13C{1H} NMR, IR, HRMS, elemental analysis data of 3,3
phenylimino)-methyl]-[1,1']biphenyl-2,2'-diol 3,3 bis[(2,4,6-trimethyl-phenylimino)-
(H2L1),
′-bis[(2,4,6-trimethyl-
′-
methyl]-[1,1']binaphthalenyl-2,2'-diol (H2L2) and the trinuclear copper (3e & 4e ) complexes ,CIF
file giving X-ray crystallographic data can be found in the journal webpage. This material is
available free of charge via journal webpage.
26

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