Double end-on azido derivative of a tridentate (NNO) Schiff base dimeric copper(II) complex: synthesis, X-ray structure, magnetic property and catalytic effectiveness

Article abstract of DOI:10.1080/00958972.2020.1836360

A dimeric copper(II) complex, bis{(2-[1-(aminoethylimino)ethyl]-phenoxo}-di-μ_1,1-azido-dicopper(II), [Cu₂(L)₂(μ_2-1,1-N₃)₂] (1) [L = 2-[1-(aminoethylimino)ethyl]-phenoxo ion], has been isolated using a self-assembly reaction using a 1:1:1 molar ratio of Cu(NO₃)₂·3H₂O, HL and NaN₃ in methanol at room temperature and characterized through X-ray diffraction analysis and spectroscopic studies. X-ray structural analysis reveals that 1 consists of two distinct dinuclear molecular units, where each copper(II) center in the individual dinuclear unit adopts a distorted square pyramidal geometry with a CuN₄O chromophore ligated through a tridentate (NNO) Schiff base and two N atoms of two different bridging azides in μ_1,1-mode. Two Cu(II) centers are linked through double μ_2-1,1-N₃ bridges to form the dinuclear unit [Cu₂(L)₂(μ_2-1,1-N₃)₂]. In the crystalline state, the dinuclear units in 1 are associated through weak intermolecular N-H?O hydrogen bonds to afford a 2-D sheet structure viewed along the crystallographic a-axis. The small magnitude of the antiferromagnetic interaction (J = –0.45 cm^?1) is a result of the long Cu···Cu separation (3.205(2) ?). The catalytic efficacy of 1 was studied in a series of solvents for the epoxidation of alkenes using tert-butyl-hydroperoxide (TBHP) as an efficient oxidant under mild conditions.

Full text of DOI:10.1080/00958972.2020.1836360

Journal of Coordination Chemistry
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Double end-on azido derivative of a tridentate
(NNO) Schiff base dimeric copper(II) complex:
synthesis, X-ray structure, magnetic property and
catalytic effectiveness
Habibar Chowdhury , Rajesh Bera , Corrado Rizzoli & Chandan Adhikary
To cite this article: Habibar Chowdhury , Rajesh Bera , Corrado Rizzoli & Chandan Adhikary
(2020): Double end-on azido derivative of a tridentate (NNO) Schiff base dimeric copper(II)
complex: synthesis, X-ray structure, magnetic property and catalytic effectiveness, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2020.1836360
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1836360
Double end-on azido derivative of a tridentate (NNO)
Schiff base dimeric copper(II) complex: synthesis, X-ray
structure, magnetic property and catalytic effectiveness
Habibar Chowdhurya, Rajesh Berab, Corrado Rizzolic
and Chandan Adhikaryd
b
aDepartment of Chemistry, Kabi Nazrul College, Murarai, West Bengal, India; Department of
c
Chemistry, Dinabandhu Andrews College, Kolkata, West Bengal, India; Departimento di Scienze
d
ꢀ
Chimiche, della Vita e della Sostenibilita Ambientale, Parma, Italy; Department of Education, The
University of Burdwan, Burdwan, West Bengal, India
ABSTRACT
ARTICLE HISTORY
Received 30 May 2020
Accepted 3 September 2020
A dimeric copper(II) complex, bisf(2-[1-(aminoethylimino)ethyl]-
phenoxog-di-l1,1-azido-dicopper(II), [Cu2(L)2(l2-1,1-N3)2] (1) [L ¼ 2-
[1-(aminoethylimino)ethyl]-phenoxo ion], has been isolated using
KEYWORDS
a
self-assembly reaction using
a
1:1:1 molar ratio of
Copper(II) dimer; m2-1,1-N3
azide bridge; structure;
magnetic study; catalytic
effectiveness
Cu(NO3)2ꢀ3H2O, HL and NaN3 in methanol at room temperature
and characterized through X-ray diffraction analysis and spectro-
scopic studies. X-ray structural analysis reveals that 1 consists of
two distinct dinuclear molecular units, where each copper(II) cen-
ter in the individual dinuclear unit adopts a distorted square pyr-
amidal geometry with a CuN4O chromophore ligated through a
tridentate (NNO) Schiff base and two N atoms of two different
bridging azides in m1,1-mode. Two Cu(II) centers are linked
through double m2-1,1-N3 bridges to form the dinuclear unit
[Cu2(L)2(l2-1,1-N3)2]. In the crystalline state, the dinuclear units in 1
are associated through weak intermolecular N-Hꢀ ꢀ ꢀO hydrogen
bonds to afford a 2-D sheet structure viewed along the crystallo-
graphic a-axis. The small magnitude of the antiferromagnetic
interaction (J ¼ –0.45 cmꢁ1) is a result of the long CuꢀꢀꢀCu separ-
ation (3.205(2) Å). The catalytic efficacy of 1 was studied in a ser-
ies of solvents for the epoxidation of alkenes using tert-butyl-
hydroperoxide (TBHP) as an efficient oxidant under mild condi-
tions.
CONTACT Chandan Adhikary
cadhikary123@gmail.com
Department of Education, The University of Burdwan,
Golapbag, Burdwan 713104, West Bengal, India
Supplemental data for this article is available online at at 10.1080/00958972.2020.1836360
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
H. CHOWDHURY ET AL.
1. Introduction
Copper(II) complexes with different nuclearities and varied pseudohalide bridges
have spawn great interest because of their interesting structures, catalytic and mag-
netic properties [1–9], and also as models for the active sites of biomolecules [10].
Of them, homoatomic azide-bridged compounds [11] with m-1,1 (end-on, EO) and
m-1,3 (end-to-end, EE) bridging motifs show different molecular architectures and
magnetic interactions [9, 12–20]. The crystalline architectures of these compounds
are found to be interesting due to the variations of the nature of forces such as
weak intramolecular and/or intermolecular interactions. The magnetic super-exchange
pathways [9, 12–19] in these compounds depend on structural parameters, such as
Cu-N and Cu … .Cu distances, Cu–N–Cu angles and of course on the nature of the
magnetic orbitals. This indicates that the structural parameters have great influence
on the magnetic properties; thus a detailed knowledge of these parameters is essen-
tial for magneto-structural/-chemical correlations. Although the activity of Schiff base
copper(II) complexes toward several oxidation reactions both in homogeneous
[21–26] and heterogeneous conditions [27–31] are well documented, catalytic oxida-
tion of alkenes involving Schiff base copper azido complexes has been scarcely
reported [32–35]. Several oxidants such as molecular oxygen, 30% hydrogen perox-
ide and sodium hypochlorite are used for the epoxidation of alkenes. But industrial
processes still mainly rely on tert-butyl-hydroperoxide (tert-BuOOH) [36]. In industrial
epoxidation such as the Halcon–Arco and Sumitomo processes [37–40], alkyl-hydro-
peroxides are used on a large scale. The recycling of the co-product, e.g. tert-BuOH
has been realized in the Sumitomo process. In the present endeavor, we have
chosen a tridentate Schiff base (HL) (Scheme 1) to isolate one neutral dinuclear
copper(II) azido complex bisf(2-[1-(aminoethylimino)ethyl]-phenoxog-di-l1,1-azido-
dicopper(II), [Cu2(L)2(l2-1,1-N3)2] (1) [L ¼ 2-[1-(aminoethylimino)ethyl]-phenoxo ion.
Details of the synthesis, crystal structure, magnetic behavior and its catalytic

JOURNAL OF COORDINATION CHEMISTRY
3
OH
C
NH2
N
CH3
Scheme 1. Framework of Schiff base ligand (HL).
effectiveness toward epoxidation of alkenes using tert-BuOOH as an oxidant of this
compound are presented in the following sections.
2. Experimental
2.1. Materials
High purity 1-(2-hydroxyphenyl)ethanone (Aldrich, USA), ethane-1,2-diamine (Aldrich,
USA), sodium azide (E. Merck, India) and copper(II) nitrate trihydrate (Merck, India)
were purchased from their respective concerns and used without purification. All other
chemicals and solvents were of AR grade. The solvents used were distilled and dried
before use. The synthetic reactions and work-up were done in the open air.
Caution! Transition metal azido complexes are potentially explosive especially in the
presence of organic ligands. Although we have not encountered any problem during
our study, yet a small amount of materials should be prepared and it should be
handled with care.
2.2. Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were performed on a Perkin-
Elmer 240C elemental analyzer. The IR spectrum (KBr disc, 4000–200 cmꢁ1) was
recorded using a Nicolet Magna IR 750 Series II FTIR spectrometer. Ground state
absorption (in methanol) was measured with a Perkin Elmer LAMBDA EZ-301 spectro-
photometer. Variable-temperature (1.8–300 K) magnetic measurements were carried
out using a Quantum Design SQUID-based MPMSXL-5-type magnetometer. The palla-
dium rod sample (Materials Research Corporation, measured purity 99.9985%) was
used to calibrate the SQUID magnetometer where the superconducting magnet was
normally run from 0 to 5 T field strength. Measurements were made at a magnetic
field of 0.5 T. Corrections are based on subtracting the sample holder signal and con-
tribution vD estimated from the Pascal constants [41]. Magnetization measurements
were conducted at 2 K in the magnetic field from 0 to 5 T. X-band (9.5 GHz) EPR meas-
urements were done on polycrystalline samples using a Bruker ElexSys E 500 spec-
trometer. The measurements were performed at room and liquid nitrogen
temperature. The spectrometer was equipped with a NMR teslameter (ER 036TM) and
an X-band frequency counter (E 41 FC). Measurement parameters were as follows:
microwave power 10 mW, modulation amplitude 8 G, center field 3500 G, range 7000 G
for X-band.

4
H. CHOWDHURY ET AL.
2.3. Synthesis and analytical data for Schiff base (HL) and [Cu2(L)2
(l2-1,1-N3)2] (1)
2.3.1. Schiff base, 2-[1-(aminoethylimino)ethyl]-phenol (HL)
1-(2-Hydroxyphenyl)ethanone (0.136 g, 1.00 mmol) was refluxed (10 h) with ethane-1,2-
diamine (0.060 g, 1.00 mmol) in ethanol (20 mL). After 10 h, the reaction solution was
evaporated under reduced pressure to yield a gummy mass, which was dried and
stored in vacuo over CaCl2 for subsequent use (Supplementary Material Scheme S1).
Yield: 0.125 g (70%). Elemental Anal. Calcd for C10H14N2O (HL): C, 67.38; H, 7.92; N,
15.72%. Found: C, 68.10; H, 8.15; N, 14.80%. IR (KBr pellets, cmꢁ1): ꢀ(C ¼ N) 1640; ꢀ(N-
1
H) 3284. H NMR (300 MHz, CDCl3): (d 2.25 (m, 6H, CH3); 2.6 (t, 2H, CH2); 3.6 (t, 2H,
CH2); 7.2 (m, 3H, aromatic).
2.3.2. Bisf(2-[1-(aminoethylimino)methyl]-phenoxog-di-l1,1-azido-dicopper(II), [Cu2
(L)2(l2-1,1-N3)2] (1)
The tridentate Schiff base, HL (0.178 g, 1.00 mmol), in MeOH (20 mL) was added slowly
to a copper(II) nitrate trihydrate (0.242 g, 1.00 mmol) solution (20 mL) in the same solv-
ent producing an intense blue solution, which was stirred for 30 min. An aqueous solu-
tion (5 mL) of NaN3 (0.065 g, 1.00 mmol) was added dropwise to the resulting mixture
with continuous stirring, which gave a green solution. After filtration through a fine
glass-frit, the green solution filtrate was kept in air for slow evaporation. Black rect-
angular crystals of 1 were deposited within a week, which were separated by filtration
and dried in vacuo over silica gel indicator. Yield: 0.339 g (60%). Elemental Anal. Calcd
for C20H26N10O2Cu2 (1): C, 42.47; H, 4.63; N, 24.77%. Found: C, 42.84; H, 5.15; N,
25.08%. IR (KBr, cmꢁ1): tasy(N3) 2045, 2031; tbending(N3) 635. UV–vis [DMF; kmax/nm
(emax/dm3 molꢁ1 cmꢁ1]: 262, 292 (1.30 ꢂ 104), 382 (1.25 ꢂ 104), 609 (2.35 ꢂ 102). KM
(DMF, Xꢁ1 cm2 molꢁ1): 6.
2.4. X-ray data collection and structure refinement of [Cu2(L)2(l2-1,1-N3)2] (1)
A single crystal of 1 suitable for X-ray analysis was selected from those obtained by
slow evaporation of a methanol solution at room temperature. Diffraction data were
collected on a Bruker APEX IICCD diffractometer using graphite monochromated Mo-
Ka radiation (k ¼ 0.71073 Å) and were used to measure cell dimensions and diffraction
intensities. The x-h scan technique in the range to a maximum 2.7ꢃ < h < 25.5ꢃ has
been fixed for data collection. For the purpose of data reduction and cell refinement,
the program SAINT-Plus [42] was used. Absorption correction was applied using
SADABS [42]. The structure was solved by direct methods using SIR97 [43] and refined
with version 2018/3 of SHELXL [44] using least squares minimization. The amine H
atoms were located in a difference Fourier map and refined with Uiso(H) ¼ 1.2 Ueq(N).
The N-H bond distances were restrained to be equal using the SADI command with
an effective standard deviation of 0.05. Refinement of the C-bound H atoms was car-
ried out using a riding model, with C-H ¼ 0.96–0.97 Å and with Uiso(H) ¼ 1.2 Ueq(C) or
1.5 Ueq(C) for methyl H atoms. A large residual density peak (3.37 e Åꢁ3) in the final
difference Fourier map located at 1.62 Å from atom Cu3 should be considered as a
residual absorption artifact due to the poor quality of the crystal (Rint ¼ 10.4%). The

JOURNAL OF COORDINATION CHEMISTRY
5
Table 1. Crystallographic data and structure refinement parameters for 1.
Chemical formula
Formula mass
Crystal system
Space group
a (Å)
C20H26N10O2Cu2
565.59
Monoclinic
P21/c
16.866(11)
14.881(10)
14.879(10)
90
111.562(7)
90
b (Å)
c (Å)
a/ꢃ
b (ꢃ)
c (ꢃ)
V (Å3)
3473(4)
k (Å)
0.71073
1.623
qcalcd (mg cm–3
)
Z
6
T (K)
295(2)
1.877
1740
m (mm–1
F(000)
)
Crystal size (mm3)
h ranges (ꢃ)
Index ranges
0.16 ꢂ 0.11 ꢂ 0.06
1.30 to 25.25
–20 ꢄ h ꢄ 20
–17 ꢄ k ꢄ 17
–17 ꢄ l ꢄ 12
20,852
Reflections collected
Independent reflections
Tmax and Tmin
6274
0.896 and 0.769
6274/15/481
0.104
Data/restraints/parameters
R(int)
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
Largest peak and hole (eÅ–3
0.996
R ¼ 0.0815 and wR2 ¼ 0.2039
R ¼ 0.1540 and wR2 ¼ 0.2487
3.330 and ꢁ0.676
)
Weighting scheme: R ¼ RjjFoj ꢁ jFcjj/RjFoj, wR ¼ [Rw(Fo2 ꢁ Fc2)2/Rw(Fo2 )2]1/2, calcd w ¼ 1/[r2(Fo2) þ (0.1337P)2
þ
0.0000P] where P ¼ (Fo2 þ 2Fc2)/3.
final positional and thermal parameters are available as Supplementary Material. A
summary of the crystallographic data and structure determination parameters for 1 is
detailed in Table 1.
2.5. Catalytic reactions
The catalytic reactions were carried out in a glass batch reactor according to the fol-
lowing procedure. Substrate (10 mmol), solvent (8 mL) and catalyst (0.005 mmol) were
first mixed in a round-bottom flask. The mixture was then equilibrated to 65 ꢃC in an
oil bath. After addition of tert-BuOOH (20 mmol), the mixture was stirred continuously
for 24 h. The products of the oxidation reactions were collected at different time inter-
vals and were identified and quantified by a VARIAN CP-3800 gas chromatograph
equipped with an FID detector. A CP-Sil 8 CB capillary column was used for analysis of
the products.
3. Results and discussion
3.1. Synthesis and physical measurements
The pentacoordinate dinuclear copper(II) complex, bisf(2-[1-(aminoethylimino)methyl]-
phenoxog-di-l1,1-azido-dicopper(II), [Cu2(L)2(m2-1,1-N3)2] (1), was obtained in a one-pot

6
H. CHOWDHURY ET AL.
Scheme 2. Synthetic route of 1.
reaction of a 1:1:1 molar ratio of the copper(II) nitrate trihydrate, HL [HL ¼ 2-[1-(ami-
noethylimino)ethyl]-phenol] and NaN3 in MeOH solution at room temperature
(Scheme 2).
Complex 1 was characterized by microanalytical (C, H and N), spectroscopic and
other physico-chemical results. The microanalytical data are in good conformity with
the formulation. The air stable moisture insensitive compound is stable over long peri-
ods of time in powdery and crystalline states, and is soluble in MeOH, EtOH, MeCN,
DMF and DMSO, but is insoluble in water. In the IR spectrum, bands at 2045 and
2031 cmꢁ1 are assigned to the asymmetric vibration of the coordinated azide group
[tas N3 ] and the band at 635 cmꢁ1 is attributed to the bending vibration of the azide
–
group. Similar characteristic bands for azides have been observed in a number of
mononuclear and polynuclear azido-bridged complexes [45]. Complex 1 displays mul-
tiple absorption bands in the 200–1100 nm range. A weak low-intensity absorption
band at 609 nm is assignable to a d-d transition, consistent with the square pyramidal
(sp) geometry of the copper(II) centers. The absorption band at 382 nm may be attrib-
uted to the ligand to copper(II) charge transfer transition (LMCT) [46, 47]. Additionally,
two strong absorption bands at 262 and 292 nm may be assigned to a ligand-based
charge transfer transitions [48].
3.2. Structural description of [Cu2(L)2(l2-1,1-N3)2] (1)
A single crystal X-ray diffraction study of [Cu2(L)2(m2-1,1-N3)2] (1) was made to define
the coordination sphere. An ORTEP diagram and packing view of the 2-D sheet struc-
tures of 1 are shown in Figures 1 and 2, respectively. Selected bond distances and
angles relevant to the coordination spheres can be found in Table 2 and non-covalent
interaction parameters are given in Table 3. X-ray structural analysis reveals that 1 con-
sists of two distinct dinuclear molecular units as [Cu1Cu2(L)2(l2-1,1-N3)2] and
[Cu3 (L)2(l2-1,1-N3)2]. Each Cu(II) center in asymmetric unit adopts a distorted square
2
pyramidal geometry [s ¼ 0.283(Cu1)/0.227(Cu2)/0.198(Cu3)] with a CuN4O chromo-
phore [49] coordinated through two N atoms and one O atom [N1, N2,O1 in Cu1/N9,
N10,O2 in Cu2/N11, N12,O3 in Cu3] of a tridentate Schiff base (L) and two N atoms
[N3,N6 in both Cu1 and Cu2/N13,N13i in Cu3] of two different bridging azides in m1,1
-
mode (Figure 1) (symmetry code: (i) 1 – x, –y, 1 – z). The basal plane is ligated by N1,
N2, O1 and N3 in Cu1/N9, N10, O2 and N6 in Cu2/N11, N12, O3 and N13 in Cu3,
whereas the apical position is occupied by N6 in Cu1/N3 in Cu2/N13i in Cu3. A consid-
erable deviation from ideal square pyramidal geometry is seen, which is presumably

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 1. An ORTEP diagram of two distinct dinuclear molecular units in 1 as [Cu1Cu2(L)2(l2-1,1
-
N3)2] and [Cu3 (L)2(l2-1,1-N3)2] with displacement ellipsoids drawn at the 30% probability level.
2
Symmetry code: (i) 1 – x, –y, 1 – z.
due to the smaller bite angles produced by the ligand [N1-Cu1-N2 85.8(3)ꢃ in Cu1/N9-
Cu2-N10 86.5(3)ꢃ/N11-Cu3-N12 85.3(3)ꢃ]. The basal plane consists of four short bond
lengths [Cu1-N1: 1.991(7) Å, Cu1-N2: 1.957(7) Å, Cu1-O1: 1.873(6) Å, Cu1-N3: 2.021(8) Å
in Cu1/Cu2-N9:1.995(7) Å, Cu2-N10: 1.962(7) Å, Cu2-O2: 1.881(6) Å, Cu2-N6: 2.010(7) Å
in Cu2/Cu3-N11:1.994(9) Å, Cu3-N12: 1.996(7) Å, Cu3-O3: 1.897(6) Å, Cu3-N13: 1.999(9)
Å in Cu3] and the apical position is occupied by a longer bond [Cu1-N6: 2.414(7) Å in
Cu1/Cu2-N3: 2.448(8) Å in Cu2/Cu3-N13i: 2.457(9) Å in Cu3] (Table 2). The Cu1, Cu2 and
Cu3 copper(II) centers deviate 0.1370(12), 0.1221(12) and 0.1249(12) Å, respectively,
from the mean basal plane. In the [Cu1Cu2(L)2(l2-1,1-N3)2] unit, two Cu(II) centers (Cu1
and Cu2) are different in position linked and through double m2-1,1-N3 bridges to form
a dinuclear unit, whereas in [Cu3 (L)2(l2-1,1-N3)2] unit, two inversion related Cu(II) cen-
2
ters (Cu3) are connected via double m2-1,1-N3 bridges to construct the dinuclear unit
(Supplementary Material Figure S1). The Cu-Nazide-Cu bridging angles are similar [Cu1-
N3-Cu2: 91.1(3)ꢃ; Cu1-N6-Cu2: 92.4(3)ꢃ; Cu3-N13-Cu3i: 91.5(4)ꢃ]. The l2-1,1-N3 (end-on)
bridged azides are linear, which is reflected in their angles [N3-N4-N5: 176.8(12) Å; N6-
N7-N8: 179.2(10) Å; N13-N14-N15: 176.9(13) Å]. The Cu … Cu distance in the dinuclear
unit is 3.205(2) Å. In the crystalline state, the dinuclear units in 1 are associated
through weak intermolecular N-Hꢀ ꢀ ꢀO hydrogen bonds to afford a 2-D sheet structure

8
H. CHOWDHURY ET AL.
Figure 2. Packing diagram of 2-D supramolecular sheet structure in 1 through intermolecular N-
HꢀꢀꢀO hydrogen bonds viewed down the a axis. Hydrogens not involved in hydrogen bonding
(dashed lines) are omitted.
Table 2. Selected bond distances (Å) and angles (ꢃ) for 1.
Bond distances
Bond angles
Cu1
Cu2
Cu3
Cu-NA (NA ¼ N1/N9/N11)
Cu-NB (NB ¼ N2/N10/N12)
Cu-OC (OC ¼ O1/O2/O3)
Cu-ND (ND ¼ N3/N6/N13)
Cu-NE (NE ¼ N6/N3/N13i)
N3-N4/N6-N7/N13-N14
N4-N5/N7-N8/N14-N15
1.991(7)
1.957(7)
1.873(6)
2.021(8)
2.414(7)
1.170(11)
1.132(13)
1.995(7)
1.962(7)
1.881(6)
2.010(7)
2.449(8)
1.218(11)
1.169(11)
1.994(9)
1.996(7)
1.897(6)
1.999(9)
2.457(9)
1.153(12)
1.120(13)
NA-Cu-NB
NA-Cu-OC
NA-Cu-ND
NA-Cu-NE
NB-Cu-ND
NB-Cu-NE
NB-Cu-OC
ND-Cu-NE
ND-Cu-OC
NE-Cu-OC
Cu1-N3-Cu2
Cu1-N6-Cu2
Cu3-N13-Cu3i/ Cu3-N13i-Cu3i
N3-N4-N5
85.8(3)
176.7(3)
89.3(3)
88.5(3)
159.7(3)
110.9(3)
93.5(3)
88.6(3)
92.4(3)
88.7(3)
91.1(3)
92.4(3)
–
86.5(3)
175.3(3)
85.0(3)
90.1(3)
161.7(3)
109.7(3)
93.6(3)
87.9(3)
90.6(3)
91.3(3)
91.1(3)
92.4(3)
–
85.5(4)
178.4(4)
86.8(3)
94.0(4)
166.5(4)
104.8(3)
92.9(3)
88.6(4)
87.6(3)
93.5(3)
–
–
91.4(4)
–
176.8(12)
179.2(10)
–
176.8(12)
179.2(10)
–
N6-N7-N8
N13-N14-N15
176.9(13)
Symmetry code: (i) 1 – x, –y, 1 – z.

JOURNAL OF COORDINATION CHEMISTRY
9
ꢃ
Table 3. Hydrogen bond interaction parameters (Å, ) for 1.
D-HꢀꢀꢀA
D-H
HꢀꢀꢀA
DꢀꢀꢀA
D-HꢀꢀꢀA
Symmetry code
N1-H1NꢀꢀꢀO3
N1-H3NꢀꢀꢀO2
N9-H3NꢀꢀꢀO1
N9-H4NꢀꢀꢀO1
N11-H5NꢀꢀꢀO3
0.99(6)
0.97(6)
0.94(6)
0.95(6)
0.96(9)
2.12(6)
2.34(7)
2.26(7)
2.37(6)
2.42(10)
3.102(10)
3.157(10)
3.136(10)
3.303(10)
3.248(13)
171(5)
140(5)
155(5)
166(5)
144(7)
–
–
–
–x, –y, –z
1 – x, –y, 1 – z
Figure 3. X-band EPR spectrum of 1 at room temperature with gx ¼ 2.07; gy ¼ 2.08, gz ¼ 2.18
(gav ¼ 2.11).
viewed along the crystallographic a-axis (Figure 2). The weak N-HꢀꢀꢀO hydrogen bonds
between the O atom (O1/O2/O3) of the tridentate Schiff base (L) with H atoms (H1N/
H3N/H4N/H5N) of amines (-NH2) group (Table 3) are in operation to strengthen the
architecture of the dinuclear unit in 1.
3.3. EPR spectroscopy
The X-band EPR spectra of the magnetically concentrated polycrystalline samples were
recorded at room and liquid nitrogen temperatures. The EPR spectrum of 1 is depicted
in Figure 3. The principal g values have been determined by conventional methods of
EPR spectra [50]. At X-band, the EPR spectrum for 1 shows an axial symmetry with an
asymmetry in the perpendicular region with gx ¼ 2.07; gy ¼ 2.08; gz ¼ 2.18; gav
¼
2.11 (Figure 3). This approximately axial spectrum, in view of the significant anisotropy
between gx ꢅ gy and gz, indicates that copper(II) ion assumes either a square planar

10
H. CHOWDHURY ET AL.
Figure 4. Thermal dependence of (
of inverse magnetic susceptibility. The solid line is the calculated curve using Bleaney–
᭺
) vMT and (ꢆ) vM for 1. The inset shows thermal dependence
Bowers expression.
or a penta-coordinated geometry around it. The X-ray crystal structure shows the com-
plex indeed displays square-pyramidal geometry.
3.4. Magnetic study
Variable-temperature magnetic susceptibility measurements of 1 have been made on
a powdered monocrystalline sample in the temperature range 2–300 K under the
external magnetic field of 0.5 T. Figure 4 depicts the thermal dependence of vMT (vMT
vs. T) and vM (vM vs. T) for 1. The v values increase slowly with decrease of the tem-
perature and a rapid increase of the molar susceptibility values is observed in the low
temperature region without showing any peak. The product function vMT (and/or the
effective magnetic moment) increases slowly on cooling from room temperature down
to T ¼ 30 K: vMT (300 K) ¼ 0.67 cm3 molꢁ1 K (slightly lower than the spin-only value
expected for two isolated copper(II) ions with S ¼ 1= and g ¼ 2.00), v T (30 K) ¼
2
M
0.72 cm3 molꢁ1 K). Below 30 K, a rapid drop of the vMT product is registered until vMT
(2 K) ¼ 0.57 cm3 molꢁ1 K. This feature is characteristic of weak antiferromagnetic cou-
pling. The magnetic data were fitted to the modified Bleaney–Bowers expression for
two interacting copper(II) ions (S ¼ 1= ) (Eqs. (1) and (2)) with the Hamiltonian in the
2
^ ^
form H ¼ ꢁJS1S2 (Eq. (3)) [51]
À Á
J
Ng2l2B
3kT
3exp
kT
À Á
xd ¼
(1)
J
1 þ 3exp
kT

JOURNAL OF COORDINATION CHEMISTRY
11
Table 4. Main structural and magnetic parameters for asymmetric double end-on (m2-1,1-N3) azido
bridgedcomplexes.
J
Compound
Cu-N-Cu (ꢃ)
R (Å)
Cu-Cu (Å)
3.205(2)
s
(cm–1) Ref.
ꢇ
R (Å)
[Cu2(L)2(m2-1,1-N3)2]
91.1(3),
92.4(3),
91.5(4)
89.9
2.021(8),
2.010(7),
1.999(9)
1.95
2.414(7),
2.448(8),
2.457(9)
2.02
0.283,
0.227,
0.198
0.268
–
0.53 This
work
[Cu2(L)2(m1,1-N3)2]
[Cu2L2(N3)2]
3.205
3.166
0.75 55
24
86.9
–
2.55
55
[Cu2(L)2(N3)2][ClO4]2
Cu2(dmterpy)2(N3)]2(N3)2(NO3)(H2O)2
[CuL3(N3)]2
96.7
117.4
93.6
–
1.99
2.06
1.923
2.54
2.31
2.48
2.440
3.391
3.674
3.318
3.193
–
–
0.26
0.02
WF 55
2.9 55
2.9 55
–1.8 55
[CuL1(N3)]2
90.5
[Cu(dpt)(N3)]2(ClO4)2
101.0(2),
100.7(2)
2.022(4)
2.398(4)
3.314(1),
3.314(1)
0.28, 0.49 –2.9 55
ꢇ
R ¼ M-Nazido(basal); R ¼ M-Nazido(apical) Å; s ¼ trigonality index (s ¼ 0 infers a perfect square pyramid, s ¼ 1 a perfect
trigonal bipyramid).
vM
vM0
¼
(2)
(3)
ꢀ
ꢁ
2zJ0
1 ꢁ
vM
Nb2g2
ðEHSꢁEBSÞ
ðS1 þ S2ÞðS1 þ S2 þ 1Þ
J ¼ ꢁ
[in the weak coupling limit]
The small J value observed, comparable with those reported for similar complexes
described in the literature [52], may be explained by the fact that the magnetic orbi-
tals are unfavorably oriented to interact. The experimental data can be matched with
J ¼ ꢁ0.45 cmꢁ1, g ¼ 2.10, R ¼ R[(vT)exp – (vT)calc]2/R[(vT)exp]2 ¼ 2.53 ꢂ 10ꢁ6. For azido-
bridged binuclear copper(II) system, an unusual range of magnetic behavior is strongly
dependent on two main factors: the geometry of the dimers and the < Cu-N-N and
Cu-N3-Cu torsion angle in the bridging region. Consideration of the electronic states of
weakly interacting metal centers have revealed that the bridging ligand is most effect-
ive in supporting antiferromagnetic exchange interactions between the two metal ions
if the bridge has available molecular orbitals that can interact with the bonding com-
bination of the metal’s d-orbitals [53]. The antiferromagnetic contribution reduces
when the environment around metal atom changes from planar to tetrahedral geom-
etry. The coupling may be strongly antiferromagnetic when the azido bridge is in
basal–basal position, in which case two dx2ꢁy2 orbitals are involved in the exchange
mechanism. In the case of the examined complex, the Cu(II) ion assumes a square pyr-
amidal coordination environment and the azido bridge adopts a basal-apical asymmet-
ric disposition between copper ions involving dx2ꢁy2 and dz2 atomic orbitals. As a
result the interaction between Cu1 and Cu10 magnetic orbitals are expected to be
very weak, leading to a small exchange coupling parameter. According to literature
data [54], Cu(II) dimers with double end-on asymmetric azido bridges present anti-
ferromagnetic or ferromagnetic interaction depending not only on coordination geom-
etry, but also of several structural parameters such as dissymmetry or asymmetry of
the m-N3 bridge (the distortion on planarity described by out-of-plane displacement
(Table 4) [55] Cu–Cu distance, and the nature of the other ligating centers inclusive of
their fluxionality. Ferromagnetic ordering appears when the Cu-N-Cu angle is small
(due to spin polarization effects) and decreases with increasing Cu-N-Cu angle up to

12
H. CHOWDHURY ET AL.
Table 5. Homogeneous catalytic oxidation of olefins catalyzed by [Cu2(L)2(l2-1,1-N3)2] (1) in aceto-
nitrile media.
% yield of products
Substrate
Reaction time (hr)
24
Conversion (wt%)
82
Epoxide
49
Others
33a
24
24
78
68
56
38
22b
30c
Reaction conditions: alkenes (10 mmol); catalysts (0.005 mmol); tert-BuOOH (20 mmol); acetonitrile (10 mL); tempera-
ture 70 ꢃC.
aBenzaldehyde and benzoic acid.
bCyclooctane-1,2-diol,.
cCyclohex-2-en-1-ol and cyclohex-2-en-1-one.
antiferromagnetic ordering if the bridge is large (critical angle ꢅ104ꢃ) [55]. On the
other hand, the long Cu–Cu distance reduces the overlap between the MO’s bearing
unpaired electrons which will result in a reduction of the magnitude of JAF as evidence
of ferromagnetic contributions. Temperature-dependent measurements of magnetic
susceptibility reveal the occurrence of competitive exchange interactions with a net
very weak ferromagnetic character. However, the elaborate hydrogen-bonding network
is also able to transmit magnetic coupling [56]. Strong hydrogen bonds (Figure 2)
form the next magnetic pathway between the Cu ions of neighboring dimers in the
crystal network. The small magnitude of this antiferromagnetic interaction (J ¼
ꢁ0.45 cmꢁ1) is a result between the long CuꢀꢀꢀCu separation (3.205(2) Å). In order to
explain the weakness of intramolecular interaction in 1, two effects must be taken
into account: (i) dominant intermolecular antiferromagnetic interaction (J
¼
ꢁ0.45 cmꢁ1) and (ii) the large Cu-Cu distance (3.205(2) Å), the large Cu-Nazido(basal) dis-
tance (2.021(8) Å), smaller Cu-Nazido(apical) distance (2.414(7) Å), the large Cu-N-Cu angle
(92.4(3)ꢃ) and the large s parameter (0.283) (Table 4). The combined effect may be
attributable for the very weak nature of the magnetic character.
3.5. Catalytic activities
The catalytic activity of 1 in the epoxidation of various alkenes in homogeneous
medium is summarized in Table 5. The graphical representation of different alkene
conversion for 1 is shown in Figure 5. For 1, the conversion trend of alkenes follows
the order styrene > cyclooctene > cyclohexene, while the observed epoxide selectivity
is cyclooctene (72%) > styrene (60%) > cyclohexene (56%). The oxidation of styrene
gives styrene epoxide in 60% yield (selectivity 61%), and along with this a moderate
amount of benzaldehyde and benzoic acid are also detected. Cyclooctene epoxide is
produced in good yield, with high selectivity when cyclooctene reacts with tert-

JOURNAL OF COORDINATION CHEMISTRY
13
Figure 5. Reaction profile for the epoxidation of olefins with tert-BuOOH in the presence of 1.
BuOOH. In addition, a small amount of cyclooctane-1,2-diol was also formed. For cyclo-
hexene, epoxide selectivity is the least since its active allylic C-H site could also be
activated in the process of oxidation. Apart from cyclohexene oxide, cyclohex-2-en-1-
ol and cyclohex-2-en-1-one were also generated, owing to allylic C-H oxidation. We
have studied the epoxidation of alkenes over Schiff base copper(II) azido complexes
under homogeneous conditions in the recent past [35]. We have demonstrated that
Schiff base copper(II) azido complexes viz., [Cu(L1)(N3)],[Cu2(L2)2(m2-1,1-N3)2]
[Cu(L2)(N3)],[Cu(L3)(N3)] [where HL1 ¼ 1-(N-5-methoxy-ortho-hydroxyacetophenim ino)-
2,2-dimethyl-aminoethane], HL2 ¼ 1-(N-ortho-hydroxyacetophenimine)-2,2-diethyl-ami-
noethane and HL3 ¼ 1-(N-salicylideneimino)-2-(N,N-diethyl)-aminoethane] exhibit high
catalytic activity in the oxidation reactions of a variety of olefins with tert-BuOOH in
acetonitrile media. The results of the catalytic activity of these systems are compared
with 1 in Table 6. From Table 6, it is evident that in similar reaction conditions,
[Cu(L1)(N3)] is more efficient catalyst than the others. This might be due to the slight
variation in structural morphologies of the complexes. The decreased efficacy of 1 as
well as selectivity of the reaction may be due to the absence of an electron donating
group (-OMe) at the aryl ring of the Schiff base (Scheme 1). Rayati et al. reported that
copper(II) complexes of salen-type tetradentate Schiff-bases show improved catalytic
activity by introduction of p-electron-donating groups at the aromatic rings [26]. The
effect of various reaction media on epoxidation of cyclooctene catalyzed by 1 has also
been studied. A graphical representation of the relative efficacy of 1 as catalyst for the

14
H. CHOWDHURY ET AL.
Table 6. Comparison of catalytic efficacy of [Cu2(L)2(l2-1,1-N3)2] (1) with other reported copper(II)
azido catalysts for the epoxidation of alkenes with tert-BuOOH in acetonitrile media.
Conversion (%)
Complex
Styrene
Cyclooctene
Cyclohexene
References
[Cu(L1)(N3)]
98
96
90
82
95
90
85
78
82
84
70
68
[15]
[15]
[15]
[Cu2(L2)2(l2-1,1-N3)2] [Cu(L2)(N3)]
[Cu(L3)(N3)]
[Cu2(L)2(l2-1,1-N3)2]
This work
HL1 ¼ 1-(N-5-methoxy-ortho-hydroxyacetophenimino)-2,2-dimethyl-aminoethane], HL2 ¼ 1-(N-ortho-hydroxyaceto-
phenimine)-2,2-diethyl-aminoethane and HL3 ¼ 1-(N-salicylideneimino)-2-(N,N-diethyl)-aminoethane.
Figure 6. Reaction profile for the epoxidation of cyclooctene with tert-BuOOH in different solvents
over 1.
epoxidation of cyclooctene in different solvents has been given in Figure 6. From
Figure 6, it is evident that the best performance of the catalyst was observed in aceto-
nitrile medium. The catalytic efficiency follows the order acetonitri-
le > dichloromethane > methanol > chloroform. The facilitating factor of the
epoxidation may be attributed due to optimum polarity of acetonitrile to dissolve
both olefin and tert-BuOOH. A similar trend was found in our previous work [34, 35].
Bera et al. also reported the same trend in the study of epoxidation of cyclooctene
catalyzed by a single end-on azido-bridged Schiff base copper(II) complex [32]. We
studied the reactions of cyclooctene by varying the temperature from room tempera-
ture to 75 ꢃC in acetonitrile to determine the optimum temperature. It was found that

JOURNAL OF COORDINATION CHEMISTRY
15
Figure 7. Comparison of catalytic efficacy of 1 at different temperatures.
maximum conversion occurs at 70 ꢃC (Figure 7). The complex contains a five-coordin-
ate coordinatively unsaturated Cu-center, thus the complex has been used for Lewis
acid catalysis reactions under homogeneous conditions. The copper(II) binds the per-
oxo-group upon treatment with peroxides [41] to form the pre-catalyst containing
LxCu–OOH (where L ¼ ligand), which are capable of transferring the oxo-functionality
to the organic substrates to produce the oxidized products [42, 43]. We assume that a
similar kind of mechanism is operative in our case. X-ray crystal structure analysis
shows the presence of five-coordinate Cu(II) in [Cu2(L)2(l2-1,1-N3)2] (1). Thus, the coord-
ination environment around copper(II) is easily accessible for an external ligand. As a
result, tert-BuOOH gets enough space to bind copper in the intermediate stages of
the catalytic cycle.
4. Conclusion
One neutral diazido-bridged dinuclear copper(II) compound, bisf(2-[1-(aminoethylimi-
no)methyl]-phenoxog-di-l1,1-azido-dicopper(II) (1), containing a tridentate Schiff base
ligand has been isolated. Structural analysis reveals that each copper(II) center, with a
distorted square pyramidal geometry in 1, is linked to each other by asymmetric m2-
1,1-azido bridges in the square pyramid conformation. The structure of 1 is stabilized
by intermolecular weak N-HꢀꢀꢀO hydrogen bond interactions forming a 2-D sheet struc-
ture. Variable-temperature magnetic susceptibility measurement shows weak intermo-
lecular antiferromagnetic interactions among the dinuclear entities as a result between
the long CuꢀꢀꢀCu separation. The complex exhibited excellent catalytic activity in
homogeneous oxidation of alkenes. A better selectivity was found when acetonitrile
was used as a solvent. The catalytic efficacy of similar copper(II) azido complexes was
compared. It was found that the absence of an electron-donating group at the aryl
ring of the Schiff base decreases the efficiency of the catalyst.
Disclosure statement
No potential conflict of interest was reported by the authors.

16
H. CHOWDHURY ET AL.
Acknowledgement
Authors acknowledge Prof. Alina Bienko, Faculty of Chemistry, University of Wroclaw, 14 F.
Joliot-Curie, 50-383 Wroclaw, Poland for magnetic study.
Funding
The work is financially supported by the Department of Science and Technology, Government
of India, by a grant (SR/S1/IC-0013/2010) (to CA).
ORCID
Corrado Rizzoli
http://orcid.org/0000-0002-4841-6123
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