Highly-efficient N-arylation of imidazole catalyzed by Cu(II) complexes with quaternary ammonium-functionalized 2-acetylpyridine acylhydrazone

Article abstract of DOI:10.1016/j.poly.2019.03.001

The reaction of (E)-N,N,N-trimethyl-2-oxo-2-(2-(1-(pyridin-2-yl)ethylidene)hydrazinyl)ethan-1-aminium-chloride (HLCl) with copper(II) perchlorate led to mononuclear [CuLCl]ClO₄ complex (1). The same reaction with excess of sodium azide gives di

Full text of DOI:10.1016/j.poly.2019.03.001

Accepted Manuscript
Highly-efficient N-arylation of imidazole catalyzed by Cu(II) complexes with
quaternary ammonium-functionalized 2-acetylpyridine acylhydrazone
Milica R. Milenković, Argyro T. Papastavrou, Dušanka Radanović, Andrej
Pevec, Zvonko Jagličić, Matija Zlatar, Maja Gruden, Georgios C.
Vougioukalakis, Iztok Turel, Katarina Anđelković, Božidar Čobeljić
PII:
S0277-5387(19)30166-4
https://doi.org/10.1016/j.poly.2019.03.001
POLY 13801
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
25 January 2019
28 February 2019
2 March 2019
Please cite this article as: M.R. Milenković, A.T. Papastavrou, D. Radanović, A. Pevec, Z. Jagličić, M. Zlatar, M.
Gruden, G.C. Vougioukalakis, I. Turel, K. Anđelković, B. Čobeljić, Highly-efficient N-arylation of imidazole
catalyzed by Cu(II) complexes with quaternary ammonium-functionalized 2-acetylpyridine acylhydrazone,
Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.03.001
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Highly-efficient N-arylation of imidazole catalyzed by Cu(II) complexes with quaternary
ammonium-functionalized 2-acetylpyridine acylhydrazone
Milica R. Milenkovića, Argyro T. Papastavroub, Dušanka Radanovićc, Andrej Pevecd, Zvonko
Jagličiće, Matija Zlatarc, Maja Grudena, Georgios C. Vougioukalakisb, Iztok Tureld, Katarina
Anđelkovića,* and Božidar Čobeljića,*
aUniversity of Belgrade, Faculty of Chemistry, Studentski trg 12-16, 11000 Belgrade, Serbia
bLaboratory of Organic Chemistry, Department of Chemistry, National and Kapodistrian
University of Athens, Panepistimiopolis, 15771 Athens, Greece
cInstitute of Chemistry, Technology and Metallurgy, University of Belgrade, Njegoševa 12, P.O.
Box 815, 11000 Belgrade, Serbia
dFaculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, 1000
Ljubljana, Slovenia
eInstitute of Mathematics, Physics and Mechanics & Faculty of Civil and Geodetic Engineering,
University of Ljubljana, Jadranska 19, Ljubljana, Slovenia
Abstract
The reaction of (E)-N,N,N-trimethyl-2-oxo-2-(2-(1-(pyridin-2-yl)ethylidene)hydrazinyl)ethan-1-
aminium-chloride (HLCl) with copper(II) perchlorate led to mononuclear [CuLCl]ClO4 complex
(1). The same reaction with excess of sodium azide gives dinuclear azido double end-on bridged
Cu(II) complex [Cu2L2(μ-1,1-N3)2](ClO4)2 (2). In both complexes hydrazone ligand is NNO
*Corresponding Authors:
Katarina Anđelković
E-mail: kka@chem.bg.ac.rs
ORCID: 0000-0003-1178-8326
Božidar Čobeljić
E-mail: bozidar@chem.bg.ac.rs
ORCID: 0000-0001-6335-0196
1

coordinated in monodeprotonated formally neutral zwitter-ionic form. Complexes were
characterized by elemental analysis, IR spectroscopy and single‐ crystal X‐ ray crystallography.
Variable‐ temperature magnetic susceptibility measurements for dinuclear Cu(II) complex
showed intra-dimer ferromagnetic coupling between Cu(II) ions (J = 7.4 cm–1). DFT-BS
calculations provided explanation for magnetic properties of dinuclear Cu(II) complex. Both
complexes were shown to highly efficiently catalyze the N-arylation of imidazole and
benzimidazole with electron-poor or electron-rich aryl iodides, under user-friendly and
sustainable conditions.
Keywords: Cu(II) complexes, hydrazones, DFT calculations, crystal structure, catalysis
1. Introduction
Polynuclear transition metal complexes with bridging ligands are a promising class of
compounds for the development of magnetic materials and understanding magneto-structural
correlations [1–6]. Pseudohalides (azide, cyanate, thiocyanate, etc.) have widely been used for
the synthesis of such systems, because of their ability to coordinate metal ions in different
modes, as monodentate or bridging ligands. The bridging modes strongly influence the magnetic
interactions between adjacent metal ions, ranging from antiferromagnetic to ferromagnetic
couplings of spins on paramagnetic centers. In general, the type and magnitude of the magnetic
interactions depend on the M···M separation, M–X bond length, M–X–M and X–M–X angles,
the dihedral angles between the planes containing the metal ions and the symmetry of the
bridging mode [7]. A homonuclear azido ligand can bind transition metal ions as monodentate or
bridging ligand, leading to the formation of mononuclear or polynuclear species, respectively. As
bridging bidentate ligand, the azide anion binds the metals via EO (end-on) or EE (end-to-end)
mode forming the following bridges: μ1,1-N3 (single EO), di-μ1,1-N3 (double EO), μ1,3-N3 (single
EE) and di-μ1,3-N3 (double end-to-end). The other coordination modes, including tridentate
bridges μ1,1,1-N3and μ1,1,3-N3 and rarer, tetradentate ones μ1,1,1,1-N3, μ1,1,3,3-N3andμ1,1,1,3,3,3-N3,
afford complexes with variable nuclearity, magnitude and type of exchange coupling
(antiferromagnetic or ferromagnetic) between the paramagnetic metallocenters [3].
2

From a synthetic point of view, it is not an easy task to predict the structure of metallo-
pseudohalide complexes obtained in the reaction of a transition metal salt, a pseudohalide ligand
and a blocking polydentate ligand. In the case of a particular pseudohalide, the nuclearity of
reaction product depends on metal ion, counter anion, organic (blocking) ligand, stoichiometric
ratio of the reactants, solvent, etc. Several NNO donor Schiff bases are used as blocking ligands
in polynuclearazide bridged complexes [4,8–11]. Acylhydrazone ligands are of particular
interest, since they exhibit keto-enol tautomerism and can coordinate metal ions in neutral or
deprotonated forms, increasing the diversity of their coordination compounds [7,8,10–14].
Girard’s reagents [Girard’s T (trimethylacetylhydrazide ammonium chloride), Girard’s D (N,N-
dimethylglycine hydrazide hydrochloride), and Girard’s P (pyridinioacetohydrazide chloride)]
are N-substituted glycine hydrazides [15], which readily reacts with carbonyl group forming
water soluble hydrazones. Girard’s T reagent hydrazones are quaternary ammonium salts, which
can coordinate metal ions either in their non-deprotonated, positively charged form, or in their
deprotonated, formally neutral, zwitter-ionic form. In recent years, one part of our research is
oriented towards the investigation of the structural and magnetic properties of pseudohalide
metal complexes with hydrazones of Girard's T reagent [15].
On a different note, the synthesis of imidazole-based compounds is very important due to
their numerous applications, amongst others as drugs, agrochemicals, and biomimetic catalysts
[16–20]. The N-arylimidazole scaffold, for example, is a moiety found in many biologically-
active natural products and pharmaceutically-related compounds with applications for diseases
such as congestive heart failure and myocardial fibrosis. These structures can be easily prepared
through N-substitution of imidazole [21,22]. One of the most promising strategy in forming the
corresponding C-N bond is the copper-catalyzed N-arylation of imidazoles with aryl halides,
(Scheme 1) [23] a transformation studied for the first time by Ullmann [24–30]. In this regard, a
number of Cu-based catalytic systems, employing salen or chelating Schiff base ligands, have
been found to catalyze these types of reactions in the past [31–35].
Scheme 1. Copper-catalyzed N-arylation of imidazoles with aryl halides.
3

Herein, we report on the synthesis, structural and magnetic characterization of mono (1)
and dinuclear (2) Cu(II) complexes with the condensation product of 2-acetylpyridine and
Girard's T reagent. The experimental studies on the magnetic properties of dinuclear azide
bridged Cu(II) complex have been accompanied by the density functional theory (DFT)
calculations. Also, we explored the catalytic activity of the complexes 1 and 2 in the N-arylation
of imidazole and benzimidazole under sustainable, user-friendly and low-cost conditions.
2. Experimental
2.1. Materials and methods
2-Acetylpyridine (≥99%) and Girard’s T reagent (99%) were obtained from Aldrich. IR
spectra were recorded on a Nicolet 6700 FT-IR spectrometer using the ATR technique in the
region 4000−400 cm−1 (s-strong, m-medium, w-weak). Elemental analyses (C, H, and N) were
performed by standard micro-methods using the ELEMENTARVario ELIII C.H.N.S.O analyzer.
Magnetic properties of a polygra in sample were investigated using a Quantum Design MPMS-
XL-5 SQUID magnetometer. Susceptibility has been measured between 2 K and 300 K in a
constant magnetic field of 1 kOe. NMR spectra were recorded with a Varian Mercury 200MHz
spectrometer. GC-MS spectra were recorded with a Shimandzu R GCMS-QP2010 Plus
Chromatograph Mass Spectrometer using a MEGAR (MEGA-5, F.T: 0.25μm, I.D.: 0.25mm, L:
30m, Tmax: 350 oC, Column ID# 11475) column.
2.1.1.
Synthesis
of
ligand
HLCl
(E)-N,N,N-trimethyl-2-oxo-2-(2-(1-(pyridin-2-
yl)ethylidene)hydrazinyl)ethan-1-aminium-chloride
The ligand HLCl was synthesized by the reaction of Girard’s T reagent (1.6764 g, 1.00
mmol) and 2-acetylpyridine (1.120 mL, 1.00 mmol) in methanol (50 mL). The reaction mixture
was acidified with 3–4 drops of 2M HCl and was refluxed for 2 h at 85 °C. IR: 3387 (w), 3127
(m), 3090 (m), 3049 (m), 3016 (m), 2950 (s), 1700 (vs), 1612 (w), 1549 (s), 1485 (m), 1400 (m),
1300 (w), 1253 (w), 1200 (s), 1153 (w), 1135 (m), 1095 (w), 1073 (m), 975 (w), 944 (w), 914
4

(m), 748 (w), 683 (w). Elemental analysis calcd for C12H19ClN4O: C 53.23 %, H 7.07 %, N
20.69 %, found: C 53.42 %, H 7.12 %, N 20.77 %.
2.1.2. Synthesis of mononuclear Cu(II) complex (1)
The mononuclear Cu(II) complex was synthesized by the reaction of Cu(ClO4)2∙6H2O
(111mg, 0.30 mmol) and ligand HLCl (70mg, 0.30 mmol) in methanol (20 mL). The solution
was refluxed for 4 h. After refrigeration of the reaction solution at –8 °C for two weeks, green
crystals suitable for X-ray analysis were formed. Yield: 102mg (79 %). IR (cm–1): 3096 (w),
3037 (w), 2958 (w), 1636 (w), 1603 (w), 1573 (w), 1525 (s), 1472 (m), 1447 (s), 1362 (w), 1399
(m), 1374 (w), 1339 (m), 1316 (w), 1263 (w), 1242 (w), 1152 (w), 1075 (vs), 966 (w), 930 (w),
912 (m), 815 (w), 785 (m), 754 (w), 682 (w), 647 (w), 622 (m), 568 (w). Elemental analysis
calcd for C12H18Cl2CuN4O5: C 33.31 %, H 4.19 %, N 12.95 %, found: C 33.27 %, H 4.22 %, N
12.78 %.
2.1.3. Synthesis of dinuclear Cu(II) complex (2)
Into a mixture of Cu(ClO4)2∙6H2O(111 mg, 0.30 mmol, dissolved in 5 mL of H2O) and
ligand HLCl (70 mg, 0.30 mmol, dissolved in 20 mL of methanol) excess of NaN3(52 mg, 0.90
mmol, dissolved in 5 mL of H2O) was added. The reaction mixture was refluxed for 4 h. After
refrigeration of the reaction solution at –8 °C for two weeks, dark green crystals suitable for X-
ray analysis were formed. Yield: 221mg (84 %). IR (cm–1): 3518 (m), 3348 (m), 2039 (vs/bs),
1628 (w), 1597 (w), 1568 (w), 1523 (m), 1468 (w), 1372 (w), 1340 (m), 1297 (m), 1146 (w),
1078 (w), 1027 (w), 974 (w), 910 (w), 779 (w), 684 (w). Elemental analysis calcd for
C24H36Cl2Cu2N14O10: C 32.81 %, H 4.13 %, N 22.32 %, found: C 32.67 %, H 4.18 %, N 22.48
%.
2.2. X-ray structure determinations
The molecular structures of complexes 1 and 2 were determined by single-crystal X-ray
diffraction. Crystallographic data and refinement details are given in Table 1. The X-ray
intensity data for 1 were collected at room temperature on a Nonius Kappa CCD diffractometer
5

equipped with graphite-monochromator utilizing MoKα radiation (λ = 0.71073 Å). Data
reduction and cell refinement was carried out using DENZO and SCALPACK [36]. Diffraction
data for 2 were collected at room temperature with an Agilent SuperNova dual source
diffractometer using an Atlas detector and equipped with mirror-monochromated MoKα
radiation (λ = 0.71073 Å). The data were processed by using CrysAlis PRO [37]. All the
structures were solved using SIR-92 [38] (2) and refined against F2 on all data by full-matrix
least-squares with SHELXL–2016 [39]. All non-hydrogen atoms were refined anisotropically.
All other hydrogen atoms were included in the model at geometrically calculated positions and
refined using a riding model. Crystallographic data for the structures reported in this paper have
been deposited with the CCDC 1886534 (for 1) and 1886535 (for 2).
Table 1
Crystal data and structure refinement details for 1 and 2.
1
2
formula
C12H18Cl2CuN4O5
432.74
C24H36Cl2Cu2N14O10
878.65
Fw (g mol–1)
crystal size (mm)
crystal color
crystal system
space group
a (Å)
0.150.050.05
green
0.400.300.20
green
monoclinic
P 21/c
monoclinic
C 2/c
9.9406(2)
9.5650(2)
18.8796(5)
94.7120(10)
1789.04(7)
4
16.4095(6)
13.6320(6)
17.1507(8)
108.145(5)
3645.7(3)
4
b (Å)
c (Å)
β (º)
V (Å3)
Z
calcd density (g cm-3)
1.607
1.601
F(000)
884
1800
6

no. of collected reflns
no. of independent reflns
Rint
7769
18585
5002
4065
0.0202
3215
0.0282
3826
no. of reflns observed
no. parameters
R[I> 2σ (I)]a
249
239
0.0432
0.1328
1.097
0.0345
0.0891
1.059
wR2(all data)b
Goof , Sc
maximum/minimum residual +0.54/–0.67
+0.30/–0.33
electron density (e Å–3)
2
2 2
aR = ∑||Fo| – |Fc||/∑|Fo|. bwR2 = {∑[w(Fo – Fc2)2]/∑[w(Fo ) ]}1/2.
cS = {∑[(Fo – Fc2)2]/(n/p}1/2 where n is the number of reflections and p is the total
2
number of parameters refined.
2.3. Computational details
The exchange coupling constant J of a dimer 2 was calculated within broken symmetry
DFT formalism [40–44] according to the Yamaguchi approach [45], from relativistic single-point
calculations on the experimentally determined X-ray structure:
(EBS  EHS )
J 
SH2S  SB2S
EHS and EBS are the energies of high-spin (triplet) and broken-symmetry states, respectively.
SH2S
SB2S
are their corresponding expectation values of the spin operator. All calculations
and
were performed with the ORCA program package (version 4.0.1.2) [46] using increased
integration grids (Grid4). Scalar relativistic effects were considered at the Zero-Order-Regular-
Approximation (ZORA) level [47]. BP86 functional [48–50] and ZORA-def2-TZVP [51,52]
basis set for all atoms have been used. The resolution of the identity (RI) approximation [53] in
the Split-RI-J variant with the scalar relativistically recontracted SARC/J [52,54,55] Coulomb
fitting sets has been used. Results are rationalized based on the Mulliken atomic spin populations
7

and spin-density map of the high-spin state. The analysis of the overlap of the non-orthogonal
corresponding orbitals [56] of the broken-symmetry solution is used as well.
2.4. Catalysis
General procedure for the catalytic reactions: In a flame-dried vessel, equipped with a
magnetic stirrer, under argon atmosphere, were added 0.3 mL of anhydrous acetonitrile, the
nucleophile (imidazole or benzimidazole – 0.75 mmol, 1.5 eq.), the electrophile (alkyl or aryl
iodide – 0.5 mmol, 1 eq.), the base (1 mmol, 2 eq.), and the copper catalyst (0.05 mmol – 10%
loading). The reaction vessel was heated to 80 °C and left under stirring for 24 hours. The
reaction mixture was then allowed to cool to room temperature, diluted with dichloromethane (5
mL) and filtered through celite. The celitepad was further washed with dichloromethane (2 × 5
mL). The combined organic phases were washed with water (2 × 5 mL) and brine (2 × 5mL).
The organic solvents were then removed in vacuo to yield the crude product, which was purified
by flash column chromatography on silica gel using a gradient mixture of ethyl acetate /
1
13
petroleum ether as eluent. The H and C NMR spectral data for all N-arylatedimidazoles and
benzimidazoles are in full agreement with those reported to literature [57–61].
3. Results and discussion
3.1. Synthesis
The
ligand
(HLCl),
(E)-N,N,N-trimethyl-2-oxo-2-(2-(1-(pyridin-2-
yl)ethylidene)hydrazinyl)ethan-1-aminium-chloride, was obtained from the condensation
reaction of 2-acetylpyridine and Girard’s T reagent (Scheme 2a). By the reaction of ligand HLCl
with Cu(ClO4)2·6H2O in a 1 : 1 molar ratio in methanol, mononuclear Cu(II) complex 1, with the
composition [CuLCl]ClO4, was obtained (Scheme 2b). Reaction of HLCl with Cu(ClO4)2·6H2O
and NaN3 in a1 : 1 : 3 molar ratios, in a mixture of methanol/water (2 : 1), gives dinuclear double
end-on azido bridged Cu(II) complex 2, with composition [Cu2L2(μ-1,1-N3)2](ClO4)2 (Scheme
2c).
8

Scheme 2. Synthesis of ligand HLCl (a) and complexes 1 (b) and 2 (c).
3.2. IR Spectroscopy
IR spectra of complexes 1 and 2 show that the hydrazone ligand is coordinated in its
deprotonated form. The vibration of deprotonated hydrazone moiety ν(−O–C=N) appears at 1636
cm−1 in the spectrum of 1 and at 1628 cm−1 in the spectrum of 2, compared with the band of the
carbonyl group in the free ligand at 1700 cm−1. Coordination of the azomethine nitrogen results
in a shift of ν(C=N) group, from 1612 cm−1 in the spectrum of HLCl to 1603 cm−1 in the
spectrum of complex 1 and 1597 cm−1 in the spectrum of complex 2. In the IR spectra of
complexes 1 and 2, vibrations of perchlorate anions are observed at 1075 cm−1 and 1078 cm−1,
respectively. The strong band at 2039 cm−1 in the spectrum of complex 2 corresponds to end-on
bonded azido ligands [62].
9

3.3. Crystal structures of complexes 1 and 2
Complex 1 crystallizes in the monoclinic centrosymmetric space group P21/c, with the

asymmetric unit (asu) comprising one complex cation [CuLCl]+ and ClO4 anion. The molecular
structure of the complex cation [CuLCl]+ with atom numbering scheme is presented in Fig. 1.
Selected bond lengths and bond angles are listed in Table 2. The complex cation features a four
coordinate Cu(II) center with the NNO donor set of tridentate zwitterionic ligand L and the Cl
ion supplementing the fourth coordination site. The coordination geometry around Cu(II) may be
described as a distorted square planar with 4 parameter [63] of 0.15 (
, where 
and  are the two largest angles around the central atom). The values of 4 can range from 1.00
for a perfect tetrahedral geometry to zero for a perfect square-planar geometry. Intermediate
structures, including trigonal pyramidal and seesaw, fall within the range of 0 to 1.00. The cis
bond angles (N1-Cu1-Cl1, 99.81(8); N2-Cu1-N1, 80.66(10); N2-Cu1-O1, 79.61(9) and O1-
Cu1-Cl1, 99.94(6)) show average deviation of nearly 10 from ideal (90). The trans bond angle
O1-Cu1-N1 is bent and the N2-Cu1-Cl1 is almost linear (O1-Cu1-N1, 160.25(10) and N2-Cu1-
Cl1, 178.19(8)). The tridentate NNO coordination of L to Cu(II) ion generates two five-
membered chelation rings (Cu-N-C-C-N and Cu-N-N-C-O) fused along Cu1-N2 bond. The
chelate rings are nearly coplanar, as indicated by the dihedral angle of 2.0. The N(CH3)3 group
from the side chain can occupy different positions to the rest of molecule by rotating around the
C8-C9 and C9-N4 bonds. The orientation of N(CH3)3 can be described by the dihedral angle N4-
C9-C8-N3 which amounts –137.7(3). The distance of N4 atom from the mean coordination
plane (Cu1, N1, N2, O1, Cl1) is 0.731(3) Å. The present orientation of the N(CH3)3 group is
supported by the intramolecular C11-H11AO1 hydrogen bond (Table S1 in the Supplementary
material).
In the environment of the Cu(II) ion two long contacts: Cu1O5A (2.73(1) Å) and Cu1Cl1i (i
stands for -x, -y, 1-z) (3.0003(9) Å), have been noticed. If these long CuOperchlorate and CuCl
contacts are viewed as bonds, the geometry around Cu(II) ion can be described as tetragonally
elongated octahedral with Cl-Cu-Operchlorate bond angle of 165.0(3). The Cu1 and Cu1i (i = -x, -y,
10

1-z) ions are bridged by Cl1 and Cl1i forming the centrosymmetric dimeric unit
[Cu2L2Cl2](ClO4)2 (Fig. 2) with CuCu separation of 3.5800(5) Å. The dimeric units are
reinforced by intermolecular C-HO hydrogen bonds involving (C11) methyl group and
pyridine carbon (C2) as H-bond donors and O2 from perchlorate anion as a double acceptor
(Table S1 and Fig. S1a in the Supplementary material). The perchlorate anions mediate in
joining the dimeric units of pseudo-octahedral geometry in two-dimensional layers parallel with
(100) lattice plane by means of intermolecular C-HO hydrogen bonds (Table S1 and Fig. S1a).
The adjacent layers are packed via CMe-HCl and CMe-HOperchlorate intermolecular hydrogen
bonds to form three-dimensional supramolecular structure (Table S1 and Fig. S1b in the
Supplementary material).
Table 2
Selected bond lengths (Å) and angles () for 1 and 2.
1
2
Cu1-N1
Cu1-N2
Cu1-O1
Cu1-Cl1
O1-C8
N3-C8
N2-C6
N2-N3
2.011(2)
1.926(2)
1.983(2)
2.2157(8)
1.286(4)
1.310(4)
1.281(4)
1.384(3)
Cu1-N1
Cu1-N2
Cu1-O1
Cu1-N5
O1-C8
N3-C8
N2-C6
N2-N3
2.0272(16)
1.9266(17)
1.9800(14)
1.9333(17)
1.277(2)
1.317(2)
1.284(2)
1.381(2)
N5-N6
N6-N7
1.189(2)
1.140(3)
N2-Cu1-O1
N2-Cu1-N1
O1-Cu1-N1
N2-Cu1-Cl1
O1-Cu1-Cl1
N1-Cu1-Cl1
79.61(9)
80.66(10)
160.25(10)
178.19(8)
99.94(6)
99.81(8)
N2-Cu1-N5
N2-Cu1-O1
N5-Cu1-O1
N2-Cu1-N1
N5-Cu1-N1
O1-Cu1-N1
N7-N6-N5
175.54(7)
79.91(6)
101.92(6)
79.85(7)
98.57(7)
159.34(7)
175.8(3)
11

Complex 2 crystallizes in the monoclinic space group C2/c, with the asymmetric unit


(asu) containing one Cu(II) center, zwitterionic ligand L, one azide N3 ligand and ClO4 anion.
The crystal structure displays a centrosymmetric dinuclear complex with the crystallographically
independent Cu(II) center coordinating to the three donor atoms (N1, N2 and O1) from the
deprotonated ligand L and two N atoms belonging to two bridging azido ligands (Fig. 3). The
coordination geometry of the crystallographically independent Cu1 center is distorted square
pyramidal, with the base formed by pyridyl (N1) and imine (N2) nitrogen atoms, an enolate
oxygen (O1) of L and one azide nitrogen atom (N5), while the axial position is occupied by a
nitrogen atom of another azide (N5ii, ii = 1.5-x, 1/2-y, 1-z). The coordination polyhedron around
Cu(II) may be described as an axially elongated square pyramid with an index of trigonality (5)
[64] of 0.27 [5 = ()/60, where  and  are the two largest angles around the central atom; 5
is 0 for regular square based pyramidal geometry and 1 for regular trigonal bipyramidal
geometry]. For more information about the bond distances and angles see Table 2. The structural
parameters correlating the geometry of related square pyramidal di(-1,1-azido) bridged Cu(II)
complexes with hydrazone-based ligands [7,10,12–14 and this work] are listed in Table 3. The
5 value calculated for the complex 2 lies within a range of values 0.22-0.29 obtained for
complexes 3-7 (Table 3). The azide anion bridges in an asymmetric (basal–apical) fashion so
that the Cu–Nazide bond lengths are significantly different, Cu1–N5 1.9333(17) Å and Cu1–N5ii is
2.590(2) Å. The basal–apical Cu–Nazide bond lengths observed in 2 are comparable to those
observed in complexes 3-7 [7,10,12–14]. The di-1,1-azide bridging nitrogen atoms are in a
planar Cu2N2 ring around the crystallographic inversion center, with the slightly bent azide
anions (N5–N6–N7, 175.8(3)). The Cu1–N5–Cu1ii bridging angle (96.10(7)) is similar to that
found in complex 7 [12], while the other complexes listed in Table 3 show somewhat narrower
bridging angles ranging from 89.9(4) to 94.77(12). The azide anions bridge the Cu(II) centers in
an end-on fashion leading to a Cu1Cu1ii separation of 3.3929(4.) Å [symmetry operation used
to generate equivalent atoms: ii = 1.5-x, 1/2-y, 1-z]. The complexes 3–7 [7,10,12–14] show
slightly shorter separation of the Cu(II) ions within Cu2N2 rings (3.1919(2)  3.2978(7) Å) with
respect to that observed in 2. In the analyzed complexes, the out-of plane deviation () of the
azide anions spans the range from 42.56(15) (for 5 [10]) to 52.27(15) (for 2). Similarly, as in
complex 1 the orientation of the N(CH3)3 group is supported by the intramolecular CMe-
12

HOenolate hydrogen bonds (Table S1 in the Supporting information). The dihedral angle N4-C9-
C8-N3 is 155.9(2) and the distance of N4 atom from the mean coordination plane (Cu1, N1, N2,
O1, N5) is 0.589(2) Å. In the environment of the Cu(II) ion a long non-bonding Cu1O5
contact of 2.801(2) Å was observed (Fig. 3). Taking into account this observation, the geometry
around Cu(II) ion could be described as tetragonally elongated octahedral with Nazide-Cu-
Operchlorate bond angle of 173.64(6). The dinuclear units of 2 are connected by means of CMe-
HNazide hydrogen bonds into 1D chains extending parallel with the [1 -1 0] direction (Table S1
and Fig. S2).
Table 3
Structural parameters correlating the geometry of square pyramidal di(-1,1-azido) bridged Cu(II)
complexes with hydrazone-based ligands.
b
Complex
Cu–Nazido(end-on) (Å)
Ref.
This
Cu–Nazido(end-on)–Cu ()
a()
CuCu (Å)
5
 (Å)c
96.10(7)
52.27(15)
3.3929(4)
1.9333(17)
2.590(2)
0.27
0.0064(3)
[Cu2L2(N3)2](ClO4)2 (2)
work
[Cu2(L1)2(N3)2]d(3)
[Cu2(L2)2(N3)2]e (4)
[Cu2(L3)2(N3)2]f (5)
[Cu2(L4)2(N3)2]g (6)
[Cu2(L5)2(N3)2]h(7)
89.9(4)
50.2(8)
3.205(6)
3.242(4)
3.1919(2)
3.272(1)
3.2978(7)
1.968(9)
2.534(10)
0.27
0.27
0.29
0.26
0.22
0.045(2)
[13]
92.31(7)
91.53(7)
94.77(12)
96.35(7)
49.97(15)
42.56(15)
43.2(2)
1.9454(19)
2.5164(19)
0.0488(3)
0.0486(3)
0.0410(4)
0.0887(3)
[14]
[10]
[7]
1.9401(2)
2.4832(2)
1.939(3)
2.478(3)
47.16(15)
1.9640(18)
2.4409(17)
[12]
a () is the out-of-plane deviation of the azide ion measured as the angle between Cu2N2 plane and the N-N bond.
bThe parameter 5 (5 = ()/60, where  and  are the two largest angles around the central atom) is an index of
the degree of trigonality, within the structural continuum between trigonal bipyramidal and square-based pyramidal
geometry.
c (Å) is the distance of metal ion from the mean basal plane of square pyramid toward the apical ligand.
dL1 = condensation product of benzhydrazide and 2-acetylpyridine.
eL2 = 2-benzoylpyridine-3-methoxybenzhydrazone.
fL3 = pyridine 2-carbaldehyde 4-hydroxy benzoyl hydrazone.
gL4= (E)-N'-(phenyl(pyridin-2-yl)methylene)isonicotinohydrazide.
hL5= methyl 2-pyridyl ketone semicarbazone.
13

Fig. 1. ORTEP presentation [65,66] of the complex cation [CuLCl] in [CuLCl]ClO4 (1).
Thermal ellipsoids are drawn at the 30% probability level.
Fig. 2. View of the dimeric unit of 1 of pseudo-octahedral geometry. Long non-bonding contacts
CuO and CuCl are represented as dashed orange lines. Symmetry code i stands for -x, -y,1-z.

ClO4 anion suffers from the positional disorder.
14

Fig. 3. ORTEP presentation of the dimeric unit of 2. Thermal ellipsoids are drawn at the 30%
probability level. Non-bonding contacts between Cu and O atoms are represented as dashed
lines. Symmetry codes: ii = 1.5-x, 1/2-y, 1-z.
3.4. Magnetic measurements
A measurement of the temperature dependence of susceptibility in a static magnetic field
of 1 kOe for 2 is shown in Fig. 4 as a product of the magnetic susceptibility and temperature
versus temperature. Instead of a horizontal line, which would be expected for a system of non-
interacting magnetic moments (Curie law), the curve strongly rises as the temperature gets lower.
In addition, the value for magnetic susceptibility measured at 300 K gives an effective magnetic
moment eff = 2.3 B per magnetic ion (as calculated from the Curie constant), which is
considerably greater than expected for Cu2+ ion (1.9 B, [67]). Both observations clearly imply
local ferromagnetic interactions between spins localized on an ion pair.
15

Fig. 4. Product of susceptibility  and temperature T as a function of temperature. A full green
line is a fit with function (1).
In order to estimate the magnitude of the intramolecular ferromagnetic coupling, the magnetic
data (the product *T) were fitted to the Bleaney-Bowers equation:

(1)
for two interacting copper(II) ions with the interaction Hamiltonian in the form H = –2 J S1S2
[68]. The least-squares fitting of the data with equation 1 leads to the interaction parameter J =
+7.4 cm–1. The theoretical curve is shown as a solid line in Fig. 4.
3.5. DFT calculations
To explain the origin of the ferromagnetic ground state of the dimer 2 broken-symmetry
DFT calculations were performed based on the X-ray data. Calculated J in [Cu2L2(N3)2]2+ at the
BP86/def2-TZVP level of theory is +5.36 cm–1. This is in excellent agreement with
experimentally determined Jexp = +7.4 cm–1. Taking into account perchlorate anions in
16

calculations, i.e. considering [Cu2L2(N3)2](ClO4)2 complex, gives even better agreement with
experiment, calculated J = +7.73 cm–1. Weak ferromagnetic coupling is observed irrespectively
of the level of theory employed (see Supplementary material). As explained in the discussion of
the crystal structure of 2, the geometry around Cu(II) ions can be considered either as elongated
2
2
square pyramidal or elongated octahedral. In both cases, the unpaired electron is in the local dx –y
2
orbital, while the local dz contains spin paired electrons. The spin density in the ground
ferromagnetic state is mainly located in the equatorial plane of the subunits, Fig. 5. There is an
extensive delocalization of the spin density towards directly coordinated equatorial donor atoms
(N1, N2, O1, N5) and toward terminal nitrogen atom of the azide group (N7), Fig. 5 and Table
4. All the atoms with considerable spin density have the same sign. The middle nitrogen atoms of
the bridging azide ligands (N6) have a very small negative spin population. This distribution of
the spin density indicates that spin delocalization is the main exchange mechanism. Weak
coupling is due to the nature of the bridge between monomeric units. Azido bridge is linked in
equatorial fashion to one Cu(II), but in axial to the second. Spin density is delocalized from one
Cu(II) to the bridging nitrogen atom (N5), but delocalization from the second Cu(II) is poor
because bridging nitrogen is in axial position with a longer Cu1ii-N5 distance. The situation is
equivalent for the second azide group. Consequently, the weak coupling is expected, as
observed. Weak ferromagnetic coupling can be understood based on the analysis of the
unrestricted corresponding orbitals [56] obtained from the broken-symmetry determinant, Fig. 6.
Magnetic orbital centered on the one Cu(II) is parallelly displaced to the orbital centered on the
other Cu(II). The equatorial-axial coordination of the bridging azide, and long axial Cu-N5
distance causes poor orbital overlap. Calculated overlap is 0.016 at the BP86/def2-TZVP level of
theory for [Cu2L2(N3)2](ClO4)2. Such a small overlap implies that the antiferromagnetic coupling
is quenched completely [69]. Thus, the ferromagnetic exchange interaction in 2 is favored.
Table 4
Mulliken spin densities for dimer 2 calculated at the BP86/ZORA-def2-TZVP level of theory.
[Cu2L2(N3)2]2+
[Cu2L2(N3)2](ClO4)2
Cu1
N1
0.489
0.509
0.087
0.119
0.099
N2
0.128
17

O1
N5
N6
N7
0.082
0.083
-0.021
0.104
0.074
0.086
-0.022
0.111
Fig. 5. Graphical representation of the spin density of the high-spin state of the dimer (2).
Isosurfaces were drawn at 0.003 e/Å3 with α-spin depicted by blue surfaces and β-spin depicted
by red surfaces.
Fig. 6. Unrestricted corresponding orbitals obtained from the broken-symmetry solution of the
dimer 2. α Spin-orbitals are depicted as red (positive)/yellow (negative) lobes; β spin-orbitals are
18

depicted as blue (positive)/grey (negative) lobes. Isosurfaces were drawn at 0.03 e/Å3. Their
spatial overlap is 0.016.
3.6. Catalysis
The reaction between iodobenzene and imidazole [70,71] was chosen as the benchmark
reaction for our optimization studies. As shown in Table 5, CuI and CuBr2 exhibit low reactivity,
towards the formation of the desired product, when Cs2CO3 was used as base in acetonitrile at
80oC. CuCl2 and CuCl provided better yields, affording 1-phenylimidazole in 24 and 37% yield,
respectively (entries 3 and 4, Table 5). On the other hand, 1 provided excellent isolated yields,
affording 1-phenylimidazole almost quantitatively under identical conditions (10% loading). At
8% catalyst loading (entry 7, Table 5) the yield of the reaction decreased to 84%.. A number of
other bases and solvents were also tested during the optimization of the reaction conditions
(Table 5). Triethylamine [72] was found to be incompatible with the reaction, while NaOH
afforded an 89% yield under the same conditions. Besides acetonitrile, DMF and THF were also
tested as solvents. In both cases, the reaction afforded the desired product, albeit in lower yield
than acetonitrile. Based on the optimization studies, the reaction affords excellent yields under
10% catalyst loading in the presence of two equivalents of cesium carbonate, in acetonitrile
when conducted at 80oC for 24h (entry 1 Table 6, and entry 1 Table 7).
With the optimized reaction conditions in hand, we explored the reactivity of both 1 and
2. A number of N-substituted imidazoles and benzimidazoles were successfully synthesized. As
shown in Tables 6 and 7, both catalysts show excellent reactivity in affording N-arylated
imidazoles with aryl groups bearing either electron-donating or electron-withdrawing moieties.
In addition, both catalysts exhibit similar reaction profiles when the scope of the reaction is
expanded to benzimidazole derivatives (entries 6, 7 Table 6, and 2, 7 Table 7). Para-iodophenol
was also utilized, but did not provide the desired product (entry 8, Table 6). On the other hand,
p-iodoaniline and m-iodopyridine provided the corresponding derivatives in very good yields
(entries 9-10, Table 6). Overall, both 1 and 2 are excellent catalysts in the synthesis of N-
arylatedimidazoles and benzimidazoles.
19

Table 5
N-arylation of imidazole – optimization conditions.
Catalyst
Reaction
yield (%)
-
Entry
Catalyst
-
Base
Solvent
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DMF
loading
-
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
NaOH
1
CuBr2
10%
10%
10%
10%
10%
8%
11
24
37
12
>99
84
89
0
2
CuCl2
3
CuCl
4
CuI
5
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
6
7
10%
10%
10%
10%
10%
8
Et3N
9
K2CO3
Cs2CO3
Cs2CO3
47
91
89
10
11
12
THF
Table 6
N-arylatedimidazoles obtained under 1 catalysis.
20

Isolated
yield (%)
Entry
1
Product
99%
99%
2
3
98%
99%
99%
4
5
99%
6
21

99%
7
8
No Reaction
78%
88%
9
10
Reaction conditions: aryl halide (0.5 mmol), imidazole (0.75 mmol),
Cs2CO3(1.0 mmol), complex 1 (0.05 mmol), CH3CN (0.3 mL), 80 °C, t=24h.
Table 7
N-arylated imidazoles obtained under 2 catalysis.
Isolated
yield (%)
Entry
1
Product
99%
22

99%
2
97%
99%
3
4
99%
99%
5
6
99%
7
Reaction conditions: aryl halide (0.5 mmol), imidazole (0.75 mmol),
Cs2CO3(1.0 mmol), complex 2 (0.05 mmol), CH3CN (0.3 mL), 80 °C, t=24h.
Although we did not study the mechanism of this transformation, two catalytic cycles
proposed in the literature account for the copper-catalyzed arylation of nucleophiles such as
imidazoles. According to the first proposed cycle, coordination of the nucleophile (imidazole)
takes place before the oxidative addition of the aryl halide, which is then followed by the
formation of the coupling product via reductive elimination. According to the second scenario,
23

oxidative addition of the aryl halide occurs in the first step, prior to the coordination of the
nucleophile [32,73].
4. Conclusion
In this paper, we presented a comprehensive discussion on the synthesis and structural
characterizations of mononuclear and dinuclear azido double end-on bridged Cu(II) complexes
with condensation product of 2-acetylpyridine and Girard’s T reagent. Variable‐ temperature
magnetic susceptibility studies for dinuclear Cu(II) complex showed intramolecular
ferromagnetic coupling between Cu(II) ions with interaction parameter J = + 7.4 cm–1. Broken-
symmetry DFT calculations based on the X-ray crystal structure of [Cu2L2(N3)2](ClO4)2 complex
were performed to explain the origin of the ferromagnetic ground state. Calculated J in
[Cu2L2(N3)2]2+ at the BP86/def2-TZVP level of theory is +5.36 cm-1. Taking into account
perchlorate anions in calculations obtained value for J is +7.73 cm-1. This is in excellent
agreement with experimentally determined Jexp=+7.4 cm-1.Weak ferromagnetic coupling is
observed irrespectively of the level of theory employed. The investigated complexes catalyze the
N-arylation of imidazole and benzimidazole with alkyl iodides, as well as electron-poor or
electron-rich aryl iodides, in a highly-efficient, user-friendly and sustainable protocol.
Acknowledgements
This work was supported by the Ministry of Education, Science and Technological
development of the Republic of Serbia (Grant OI 172055) and the Slovenian Research Agency
(P-0175 and P2-0348) which enabled the postdoctoral stay of Božidar Čobeljić at the University
of Ljubljana (labs of I. Turel). We thank the EN-FIST Centre of Excellence, Ljubljana, Slovenia,
for use of the SuperNova diffractometer. We also acknowledge the contribution of COST Action
CA15106 (C-H Activation in Organic Synthesis – CHAOS), as well as the Special Account for
Research Grants of the National and Kapodistrian University of Athens (Research Program
70/3/14872).
24

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Graphical abstract
The cover picture shows crystal structures of mono- and dinuclear Cu(II) complexes with the
condensation product of 2-acetylpyridine and Girard’s T reagent. Catalytic properties of Cu(II)
complexes in N-arylation of imidazole and benzimidazole with alkyl and aryl iodides were
investigated.
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