Crystal structures and spectroscopic properties of copper(II)–dipicolinate complexes with benzimidazole ligands

Article abstract of DOI:10.1007/s11243-018-0236-2

The syntheses, crystal structures and spectroscopic properties of three Cu(II)–dipicolinate complexes with benzimidazole ligands, namely [Cu(bzim)(dipic)(MeOH)] (1), [Cu₂(2-Etbzim)₂(dipic)₂]_n·0.5nH₂O (2) and [Cu₂(2-ⁱPrbzim)₂(dipic)₂]_n (3), where dipic = dipicolinate, bzim = 1-H-benzimidazole, 2-Etbzim = 2-ethyl-1-H-benzimidazole and 2-iPrbzim = 2-isopropyl-1-H-benzimidazole, are reported. Crystal structure studies revealed different coordination modes of the dipicolinate ligands; tridentate chelating for monomeric complex 1, and both tridentate chelating and bridging for similar polymeric complexes 2 and 3. Polymers 2 and 3 both contain two units, in which the Cu(II) central atoms Cu1 and Cu2 have different coordination polyhedra. The first unit {Cu(dipic)₂} with Cu1 is connected to the second via two bidentate carboxylate groups of an μ₃-bridging dipicolinate. In the second unit, Cu2 is coordinated by two imidazole nitrogen atoms from 2-ethyl-1-H-benzimidazole (2) or 2-isopropyl-1-H-benzimidazole (3) ligands. Complex 2 is of higher symmetry and has a localized Cu(II) atom Cu2 in a special position on the twofold axis. EPR spectra of all three Cu(II) complexes, which were measured at both room temperature and 98?K, indicate distorted tetragonal coordination spheres for all the Cu(II) atoms. The g-factor relation (g_//> g_┴ > 2.0023) is consistent with a dx2-y2 ground electronic state in each case.

Full text of DOI:10.1007/s11243-018-0236-2

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0236-2
Crystal structures and spectroscopic properties
of copper(II)–dipicolinate complexes with benzimidazole ligands
1
1
2
1
Petra Masárová · Vladimír Kuchtanin · Milan Mazúr · Ján Moncol
Received: 17 January 2018 / Accepted: 12 April 2018
©
Springer International Publishing AG, part of Springer Nature 2018
Abstract
The syntheses, crystal structures and spectroscopic properties of three Cu(II)–dipicolinate complexes with benzimidazole
i
ligands, namely [Cu(bzim)(dipic)(MeOH)] (1), [Cu (2-Etbzim) (dipic) ] ·0.5nH O (2) and [Cu (2- Prbzim) (dipic) ] (3),
2
2
2 n
2
2
2
2 n
where dipic=dipicolinate, bzim=1-H-benzimidazole, 2-Etbzim=2-ethyl-1-H-benzimidazole and 2-iPrbzim=2-isopro-
pyl-1-H-benzimidazole, are reported. Crystal structure studies revealed diꢀerent coordination modes of the dipicolinate
ligands; tridentate chelating for monomeric complex 1, and both tridentate chelating and bridging for similar polymeric
complexes 2 and 3. Polymers 2 and 3 both contain two units, in which the Cu(II) central atoms Cu1 and Cu2 have diꢀerent
coordination polyhedra. The ꢁrst unit {Cu(dipic) } with Cu1 is connected to the second via two bidentate carboxylate groups
2
of an μ -bridging dipicolinate. In the second unit, Cu2 is coordinated by two imidazole nitrogen atoms from 2-ethyl-1-H-
3
benzimidazole (2) or 2-isopropyl-1-H-benzimidazole (3) ligands. Complex 2 is of higher symmetry and has a localized
Cu(II) atom Cu2 in a special position on the twofold axis. EPR spectra of all three Cu(II) complexes, which were measured
at both room temperature and 98 K, indicate distorted tetragonal coordination spheres for all the Cu(II) atoms. The g-factor
relation (g >g >2.0023) is consistent with a d
2
2
ground electronic state in each case.
/
/
┴
x −y
Introduction
of the solution, choice of solvent, van der Waals forces and
others [4, 5]. The selection of organic ligands depends on
factors such as steric and electronic eꢀects, ꢂexibility and
length of the ligands and position of their functional groups
[6].
The design and construction of self-assembled supramo-
lecular architectures and metal–organic frameworks with
desired topologies and interesting properties such as cata-
lytic activity, electrical conductivity, magnetic properties,
and pharmacological activity [1, 2] constitute a major area of
supramolecular chemistry. The many factors which have an
inꢂuence on the self-assembly of supramolecular structures
and metal–organic frameworks include the choice of metal,
organic ligands that are capable of intermolecular interac-
tions such as π–π stacking [3] and hydrogen bonding, pH
Benzimidazole and its derivatives are useful N-donor
ligands for the synthesis of supramolecular architectures,
due to their large conjugated π systems and capacity to act
as hydrogen bond donors [6]. Furthermore, imidazole and
its derivatives are constituents of metalloenzymes [7, 8].
Dipicolinate ligand (dipic=anion derived from pyridine-
2,6-dicarboxylic acid) can coordinate with metal ions in a
variety of ways. It is a versatile, multidentate ligand with
one nitrogen and two oxygen donor atoms and can exhibit
a large number of binding modes varying from bidentate or
tridentate chelating to bridging via one or more carboxylate
oxygen atoms, which leads to many structural variations
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-018-0236-2) contains
supplementary material, which is available to authorized users.
*
Ján Moncol
[
9, 10]. Moreover, dipicolinate ligand can behave both as
jan.moncol@stuba.sk
an acceptor and a donor of hydrogen bonds. These proper-
ties make dipicolinates valuable ligands in the synthesis of
supramolecular coordination compounds [11]. In addition,
dipicolinate compounds may exhibit insulin-like properties
[12] and superoxide dismutase (SOD) activity [13], they
can also activate or inactivate some metalloenzymes, inhibit
1
Department of Inorganic Chemistry, Faculty of Chemical
and Food Technology, Slovak University of Technology,
8
12 37 Bratislava, Slovak Republic
2
Department of Physical Chemistry, Faculty of Chemical
and Food Technology, Slovak University of Technology,
8
12 37 Bratislava, Slovak Republic
Vol.:(0
1
123456789)
3

Transition Metal Chemistry
1
13
electron transport, and function as electron carriers in DNA
cleavage [14]. Due to their low toxicity and amphophilic
nature, they can serve as models for pharmacologically
active compounds [15].
The H NMR and C NMR spectra of the proligands are
given in the supplementary materials (see Supplementary
Materials Figs. S1–S6).
−
1
IR (ATR, cm ) for 1-H-benzimidazole: 3113(w),
3061(w), 3002(w), 2939(w), 2860(w), 2789(w), 1771(m),
1620(m), 1588(m), 1477(m), 1456(s), 1406(s), 1364(s),
1300(s), 1271(s), 1244(s), 1202(m), 1132(m), 1003(m),
958(m), 887(m), 767(m), 743(s), 634(m), 626(m), 617(m),
578(w), 417(s).
Crystal structures of some dipicolinate complexes with a
variety of benzimidazole ligands, similar to the complexes
reported in the present work, have been described [16–20].
In this work, we present the synthesis, crystal struc-
tures and spectral properties (IR, UV/Vis and EPR) of
three new Cu(II)–dipicolinate complexes with vari-
ous benzimidazole ligands, namely [Cu(bzim)(dipic)
−1
IR (ATR, cm ) for 2-ethyl-1-H-benzimidazole: 3053(w),
2974(w), 1622(m), 1589(w), 1541(m), 1456(s), 1439(s),
1425(s), 1408(s), 1379(m), 1324(m), 1270(s), 1070(m),
1043(m), 966(m), 881(m), 793(m), 738(vs), 617(m),
476(m), 432(m).
(
[
MeOH)] (1), [Cu (2-Etbzim) (dipic) ] ·0.5nH O (2) and
2
2
2 n
2
i
Cu (2- Prbzim) (dipic) ] (3). The crystal structures of all
2
2
2 n
three complexes are discussed in detail.
−
1
IR (ATR, cm ) for 2-isopropyl-1-H-benzimidazole:
049(w), 2968(w), 1622(m), 1591(w), 1537(m), 1456(m),
411(s), 1389(m), 1360(m), 1321(m), 1273(s), 1228(m),
3
1
Experimental
1213(m), 1093(m), 1016(m), 995(m), 930(w), 868(w),
7
68(m), 739(vs), 619(m), 449(m), 420(m).
All the benzimidazole proligands were synthesized as
described in our previous paper [21], as follows. A mixture
of o-phenylenediamine (0.1 mol) and 0.1 mol of formic acid
The synthesis of the complexes [Cu(bzim)(dipic)
(MeOH)] (1), [Cu (2-Etbzim) (dipic) ] ·0.5nH O (2) and
2
2
2 n
2
i
[Cu (2- Prbzim) (dipic) ] (3) was carried out in the follow-
2
2
2 n
3
(
for 1-H-benzimidazole), propionic acid (for 2-ethyl-1-H-
ing manner. To a water (40 cm , complex 2) or a methanol
3
benzimidazole) or isobutyric acid (for 2-isopropyl-1-H-ben-
(40 cm , complexes 1 and 3) solution of copper(II) acetate
3
zimidazole) was added to 4 M hydrochloric acid (50 cm ).
monohydrate (0.15 g, 0.75 mmol) was added 1-H-benzimi-
dazole (for 1, 0.09 g, 0.75 mmol), 2-ethyl-1-H-benzimida-
zole (for 2, 0.22 g, 1.5 mmol) or 2-isopropyl-1-H-benzimi-
dazole (for 3, 0.24 g, 1.5 mmol). The mixture was stirred
for 30 min at ambient temperature to obtain a homogeneous
solution, then further treated with dipicolinic acid (0.125 g,
0.75 mmol) and heated under reꢂux. The resulting blue or
violet-blue solutions were cooled to ambient temperature,
ꢁltered, and left to crystallize. Single crystals of complexes
1–3 suitable for X-ray diꢀraction were formed within a few
days.
The resulting mixture was heated for 4 h under reꢂux. The
solution was cooled and the pH was adjusted to 8 with 26%
ammonia solution. The solid was collected by ꢁltration, dis-
solved in 1 M hydrochloric acid, and treated with charcoal.
The mixture was ꢁltered and the pH was adjusted to 8 with
ammonia solution, and the resulting precipitate was collected
by ꢁltration. The ꢁltre cake was washed with cold water to
obtain 1-H-benzimidazole in 75% yield, 2-ethyl-1-H-benzi-
midazole in 85% yield and 2-isopropyl-1-H-benzimidazole
in 85% yield, (all white solids).
1
H NMR (300 MHz, DMSO-d , δ ppm) for 1-H-benzi-
6
−
1
midazole: 8.21 (s, 1H), 7.64 (dd, J=6.0, 3.2 Hz, 2H), 7.23
Complex 1 IR (ATR, cm ): 3440(w), 3371(w), 3163(w),
3091(w), 2810(w), 2642(w), 1651(s), 1628(s), 1615(s),
1595(s), 1496(m), 1470(w), 1430(m), 1365(s), 1346(s),
1307(m), 1279(m), 1254(m), 1178(m), 1146(w), 1079(m),
1007(w), 984(w), 912(m), 876(m), 848(w), 824(w), 765(s),
750(s), 737(s), 679(s), 635(w), 624(w), 589(w), 548(w),
437(s), 426(s).
(
dd, J=6.0, 3.2 Hz, 2H).
1
3
C NMR (75 MHz, DMSO-d , δ ppm) for 1-H-benzimi-
6
dazole: 142.4, 138.9, 122.3, 115.9.
1
H NMR (300 MHz, DMSO-d , δ ppm) for 2-ethyl-1-H-
6
benzimidazole: 7.50 (dd, J = 5.9, 3.2 Hz, 2H), 7.14 (dd,
J = 6.0, 3.2 Hz, 2H), 2.88 (q, J = 7.6 Hz, 2H), 1.38 (t,
J=7.6 Hz, 3H).
Found (calc.): C 47.6 (47.6), N 10.9 (11.1), H 3.7 (3.5).
UV/Vis (nm): 202, 249, 273, 738.
1
3
C NMR (75 MHz, DMSO-d , δ ppm) for 2-ethyl-1-H-
6
benzimidazole: 156.8, 139.6, 121.6, 115.0, 22.6, 12.7.
1
−1
H NMR (300 MHz, DMSO-d , δ ppm) for 2-isopro-
Complex 2 IR (ATR, cm ): 3182(w), 2978(w), 2654(w),
6
pyl-1-H-benzimidazole: 7.51 (dd, J=6.0, 3.2 Hz, 2H), 7.15
1630(s), 1614(s), 1579(s), 1489(w), 1454(s), 1427(m),
1371(s), 1280(s), 1182(m), 1080(s), 1051(w), 1005(w),
916(w), 862(w), 819(w), 782(m), 762(w), 734(s), 683(m),
633(w), 436(m).
(
(
dd, J=6.0, 3.2 Hz, 2H), 3.21 (hept, J=6.9 Hz, 1H), 1.41
d, J=6.9 Hz, 6H).
1
3
C NMR (75 MHz, DMSO-d , δ ppm) for 2-isopro-
6
pyl-1-H-benzimidazole: 160.4, 139.3, 121.7, 115.1, 29.0,
Found (calc.): C 50.6 (50.1), N 11.4 (11.0), H 3.5 (3.7).
UV/Vis (nm): 215, 246, 279, 547, 755.
2
1.9.
1
3

Transition Metal Chemistry
−
1
Complex 3 IR (ATR, cm ): 3180(w), 3082(w), 2976(w),
X‑ray crystallography
2
1
1
6
879(w), 2775(w), 2654(w), 1618(s), 1585(s), 1452(m),
426(w), 1359(s), 1277(s), 1181(m), 1080(m), 1031(w),
003(w), 912(w), 862(w), 818(m), 785(m), 762(m), 730(s),
83(m), 434(m).
The data collections for complexes 1–3 at 100 K were made
with a Stoe StadiVari diꢀractometer using a Pilatus3R 300 K
HPAD detector and microfocus source Xenocs GeniX3D
HF with CuKα radiation. The structures were solved
using SHELXT [22] or SUPERFLIP [23] and reꢁned by
full-matrix least-squares procedures with SHELXL (ver-
sion 2017/1) [24]. The absorption corrections were made
by multi-scan methods using the Stoe LANA programme
[25]. Geometrical analyses were performed with SHELXL.
The structures were drawn using the OLEX2 package [26].
Crystal data, conditions of data collection and reꢁnement
are reported in Table 1. The crystal structure of complex 2
showed disorder of the dipicolinate and 2-ethyl-1-H-ben-
zimidazole ligands. The whole 2-ethyl-1-H-benzimidazole
ligand was disordered in two positions [N3–N4/C12–C20]
and [N3A–N4A/C12A–C20A] with occupancy factors
0.671(4) (green line) and 0.329(4) (violet line) (see Supple-
mentary Materials Fig. S11). One of two dipicolinate ligands
[N1/O1–O4/C1–C7] was disordered around the twofold axis
(the two orientations of the dipicolinate ligand are drawn
using green and violet lines, see Supplementary Materials
Fig. S11). Crystallographic data (excluding structure factors)
for the structures reported in this paper have been depos-
ited within the Cambridge Crystallographic Data Centre as
supplementary publication nos. 1586345–1586347. Copies
Found (calc.): C 52.2 (52.5), N 10.0 (10.8), H 4.0 (3.9).
UV/Vis (nm): 205, 248, 279, 560, 749.
The UV/Vis spectra of complexes 1–3 are shown
in the Supplementary Materials Fig. S7. Complex
1
has a molar absorptivity coefficient at λ = 738 nm
−
1
−1
(
ε7 = 86.5 L mol cm ) (see Supplementary Materials
38
Fig. S8).
Room temperature (RT) values of magnetic moments
μeꢀ for complexes 2 and 3 were obtained as 2.48 μ for 2,
B
indicative of dimeric behaviour in a magnetic ꢁeld and
μ
= 1.76 μ for complex 3, which corresponds to the
eꢀ
B
spin-only value for S = ½ and one unpaired electron in the
9
3
2
d system (μ = 1.73 μ ). Decrease of μ values below
eꢀ
B
eꢀ
0 K is caused by zero-ꢁeld splitting (ZFS), but could also
indicate antiferromagnetic exchange interactions between
the Cu(II) atoms for 2 and 3 (see Supplementary Mate-
rials Figs. S9, S10). The presence of antiferromagnetic
exchange coupling between the copper(II) atoms conꢁrms
signiꢁcant deviation from linear behaviour of the function
χ = f(1/T) (see Supplementary Materials Figs. S9, S10).
Table 1 Crystallographic data
1
2
3
for complexes 1–3
Chemical formula
Mr
Crystal system
Space group
T (K)
C H CuN O
378.82
Triclinic
P1̄
C H Cu N O
758.67
Monoclinic
C2/c
100(1)
C H Cu N O
8
777.72
Monoclinic
Cc
100(1)
1
5
13
3
5
32 27
2
6
8.5
34 30
2
6
100(1)
a (Å)
b (Å)
c (Å)
6.4426(3)
10.7774(6)
11.0318(5)
76.570(4)
88.770(4)
83.840(4)
740.73(6)
2
10.4082(4)
17.8362(5)
17.3477(7)
90
94.777(3)
90
10.3374(3)
18.0741(4)
17.9011(5)
90
92.401(2)
90
(
°)
β (°)
γ (°)
V (Å )
3
3209.3(2)
4
3341.69(15)
4
Z
λ (Cu–Kα) (Å)
1.54186
2.394
1.54186
2.176
1.54186
2.093
μ (mm−
1
)
Crystal size (mm)
0.40×0.05×0.05
1.698
0.12×0.06×0.05
1.570
0.16×0.09×0.05
1.546
ρcalc (g cm−
3
)
S
0.995
1.059
1.159
R1 [I>2σ(I)]
0.0567
0.1509
1.32, −1.29
1,586,345
0.0457
0.1279
0.84, −0.95
1,586,346
0.0644
0.1821
0.51, −0.88
1,586,347
wR [all data]
2
∆
〉max, ∆〉min/e (Å−3
)
CCDC
1
3

Transition Metal Chemistry
of the data can be obtained free of charge on application
to the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
[
fax: (internat.) +44 1223336033; e-mail:deposit@ccdc.
cam.ac.uk].
Physical measurements
All purchased chemicals and solvents were of analytical
grade (Aldrich, Sigma) and used as received. Elemental
analyses of carbon, hydrogen and nitrogen were carried out
with an automated analyser (Vario, Micro Cube) using a
CHNSO FlashEA™ 1112 Automatic Elemental Analyser.
The electronic spectra were recorded as Nujol mulls and in
−
3
methanol solution (10 M) on a Specord 250 plus, Analyti-
cal Jena spectrometer in the range of 200–800 nm at room
temperature. IR spectra were measured by ATR technique
Fig. 1 Molecular structure of complex 1
−
1
(
4000–400 cm ) on a Magna FTIR 750, Nicolet spectro-
Table 2 Selected geometric parameters (Å, °) for 1
photometer at room temperature. All the magnetic measure-
ments were performed on a SQUID magnetometer (Quantum
Design, model MPMS SQUID XL-7). The diamagnetic cor-
rections of the molar magnetic susceptibilities were applied
using Pascal’s constants. Magnetic data were corrected for
background values and then processed into χT product func-
tion. EPR spectra of the polycrystalline Cu(II) complexes
Cu1–O3
Cu1–O1
Cu1–N2
O3–Cu1–O5
O1–Cu1–O5
N2–Cu1–O5
N1–Cu1–O3
N1–Cu1–O1
2.023(2)
2.018(2)
1.941(3)
92.17(9)
95.02(9)
98.91(11)
95.60(10)
80.74(10)
Cu1–O5
Cu1–N1
2.271(2)
1.907(3)
O1–Cu1–O3
N2–Cu1–O3
N2–Cu1–O1
N1–Cu1–O5
N1–Cu1–N2
160.54(10)
95.38(10)
101.31(10)
95.45(10)
165.23(12)
1
–3 were recorded on an X-band (≈ 9.4 GHz) EMX EPR
spectrometer (Bruker, Germany) at both RT and 98 K. A
special procedure was used for thin-walled quartz EPR tubes
packing and precise positioning within the microwave cav-
ity, as described previously [27, 28]. The spin Hamiltonian
parameter values were obtained from the experimental EPR
spectra by WinEPR (Bruker, [29]) and then further reꢁned
Cu1–N2 distance of 1.941(3) Å (Table 2)]. The apical posi-
tion is occupied by the oxygen atom of a methanol ligand
[Cu1–O5 distance of 2.271(2) Å (Table 2)]. The crystal
1
by computer simulation using SimFonia (Bruker, [30]). H
structure of a very similar complex [Cu(bzim)(dipic)(H O)]
2
1
3
NMR and C NMR spectra were recorded with a Varian
with an axial water ligand was published by Dong et al. [20].
The molecules of complex 1 are connected by N–H···O
hydrogen bonds between the imidazole NH groups (N3) and
uncoordinated carboxylate oxygen atoms (O4) [N3–H3···O4
with an N3···O4 distance of 2.760(3) Å (see Supplemen-
tary Materials Table S1)]. There are also O–H···O hydro-
gen bonds between the methanol oxygen atoms (O5) and
uncoordinated carboxylate oxygen atoms (O2) [O5–H5···O2
with an O5···O2 distance of 2.766(4) Å (see Supplementary
Materials Table S1)]. In this way, the complex molecules
are assembled into 1D supramolecular band chains (Fig. 2).
T h e p r i n c i p a l s t r u c t u r a l fe a t u r e s o f
[Cu (2-Etbzim) (dipic) ] ·0. 5nH O (2) and
1
13
INOVA-300 spectrometer ( H, 300 MHz, and C, 75 MHz)
in DMSO-d at 65 °C.
6
Results and discussion
Crystal structures
The crystal structure of [Cu(bzim)(dipic)(MeOH)] (1) is
illustrated in Fig. 1. Selected bond lengths and angles are
tabulated in Table 2. The complex crystallizes in the tri-
clinic space group P−1. The copper(II) atom in [Cu(bzim)
2
2
2
n
2
i
(
dipic)(MeOH)] is ꢁve-coordinated with a square pyrami-
dal geometry. The molecular structure of [Cu(bzim)(dipic)
MeOH)] consists of a Cu(II) atom in a basal plane formed
[Cu (2- Prbzim) (dipic) ] (3) are illustrated in Figs. 3
2
2
2 n
and 4, respectively. Complexes 2 and 3 crystallize in the
monoclinic space groups C2/c and Cc, respectively. Both
compounds form similar 1D coordination chains, with two
diꢀerent coordination polyhedra around the copper(II) atoms
(Figs. 3, 4). The crystal structure of centrosymmetric com-
plex 2 contains 1D coordination chains and also uncoordi-
nated water molecules (Fig. 3). The 1D coordination chains
(
by the pyridine nitrogen atom and two carboxylate oxygen
atoms of a tridentate chelating pyridine-2,6-dicarboxylate
anion, and an imidazole nitrogen atom [Cu1–N1 distance
of 1.907(3) Å (Table 2), Cu1–O1 distance of 2.018(2) Å
(
Table 2), Cu1–O3 distance of 2.023(2) Å (Table 2)], and
1
3

Transition Metal Chemistry
Fig. 2 1D Supramolecular chain
structure of complex 1. The
hydrogen atoms are omitted for
clarity
Fig. 3 Molecular structure of
complex 2
are in both cases formed by copper(II) atoms (Cu2) coordi-
nated by two imidazole nitrogen atoms from 2-ethyl-1-H-
benzimidazole (2) or 2-isopropyl-1-H-benzimidazole (3)
bidentate carboxylate groups (Figs. 3, 4). The copper(II)
atoms (Cu1) are in both cases six-coordinated by two tri-
dentate dipicolinate ligands, through two pyridine nitrogen
atoms (N1, N2) [Cu1–N bond distances are in the range
of 1.874(2)–1.995(7) Å (Tables 3, 4)] and four carboxylate
ligands, and bridged by {Cu(dipic) } units containing
2
Cu1 atoms, which are bonded to the Cu2 atoms by two
1
3

Transition Metal Chemistry
Fig. 4 Molecular structure of
complex 3
oxygen atoms [O1, O4 and 2 × O5 for 2 (Fig. 3), and O1,
O3, O5 and O7 for 3 (Fig. 4). The Cu1–O bond lengths are
in the range of 2.025(6)–2.381(6) Å (Tables 3, 4)]. One of
the dipicolinate ligands in both complexes acts as a triden-
distance of 2.634(2) Å)] or weakly bonded [O1 and O3 for 3
(Cu2–O distances of 2.892(6) and 2.935(6) Å, respectively]
to Cu2. The one disordered dipicolinate ligand around the
special position of complex 2 is occasioned by the localiza-
tion of atom Cu2 on the twofold axis of the crystal structure.
The coordination polymers 2 and 3 are connected through
N–H···O hydrogen bonds between the imidazole nitrogen
and carboxylate oxygen atoms [N···O distances are in the
range of 2.728(15)–2.911(15) Å (see Supplementary Materi-
als Table S1)] into 2D supramolecular frameworks (Figs. 5,
6). The crystal structure of complex 2 also includes O–H···O
hydrogen bonds between solvent water molecules (O1W)
and carboxylate oxygen (O1 or O4) or neighbouring solvent
water molecules (O1W) [O···O distances are in the range
2.78(4)–3.06(3) Å (see Supplementary Materials Table S1)].
tate chelating terminal ligand, and the second is μ -bridging
3
(
tridentate to Cu1, and bidentate to two Cu2 atoms). The
Cu2 atoms in both cases are coordinated by two pairs of
carboxylate oxygen atoms and two imidazole nitrogen atoms
of 2-ethyl-1-H-benzimidazole ligands (complex 2, Fig. 3) or
2
-isopropyl-1-H-benzimidazole ligands (complex 3, Fig. 4)
in an elongated tetragonal bipyramid.
In complex 2, atom Cu2 is localized in a special position
on the twofold axis. The Cu2 atoms of both complexes 2
and 3 are bonded in a trans square planar arrangement to
the imidazole nitrogen atoms of two disordered 2-ethyl-1-H-
benzimidazole ligands (2 × N3 for main part, Fig. 3, or
2
× N3A for minor part) or two 2-isopropyl-1-H-benzimi-
IR, UV/Vis and EPR spectroscopy
dazole ligands (N3 and N5, Fig. 4) [Cu2–N distances are
in the range 1.964(4)–1.977(8) (Tables 3, 4)], plus one
carboxylate oxygen atom from two bridging dipicolinate
The IR spectra of complexes 1–3 exhibit some similar
features. Vibrational bands observed at 3163, 3182 and
−
1
ligands of {Cu(dipic) } units (2 × O6 for 2 (Fig. 3) or O2
3180 cm for 1, 2 and 3, respectively, are attributed to the
N–H stretching vibrations of the benzimidazole ligands.
The spectrum of complex 1 also shows stretching vibra-
tions assigned to the O–H groups at 3440 and at 3371 cm−
2
and O4 for 3 (Fig. 4)) [Cu2–O distances are in the range
1
.964(6)–1.977(3) (Tables 3, 4)]. The remaining two car-
1
boxylate oxygen atoms are strongly [2 x O5 for 2 (Cu2–O5
1
3

Transition Metal Chemistry
Table 3 Selected geometric parameters (Å, °) for 2
Table 4 Selected geometric parameters (Å, °) for 3
Cu1–O5
Cu1–O1
Cu1–N1
Cu2–O5
Cu2–O6
Cu2–N3
Cu2–N3A
O5–Cu1–O5
2.346(2)
2.046(4)
1.874(2)
2.634(2)
1.977(3)
1.972(3)
1.964(4)
150.97(11)
75.49(5)
100.9(10)
88.6(12)
110.09(19)
174.01(15)
76.8(10)
91.0(13)
124.31(8)
55.69(8)
180.0
Cu1–O5i
Cu1–O4
Cu1–N2
2.346(2)
2.046(4)
1.988(3)
2.634(2)
1.977(3)
1.972(3)
1.964(4)
75.49(5)
98.9(11)
95.8(13)
159.4(7)
98.91(19)
82.9(11)
94.5(12)
180.0
124.31(8)
55.69(8)
93.4(4)
86.6(4)
88.6(3)
91.4(3)
180.0
88.3(7)
87.6(7)
Cu1–O5
Cu1–O1
Cu1–N1
Cu2–O1
Cu2–O3
Cu2–N3
O1–Cu1–O3
O3–Cu1–O5
O3–Cu1–O7
N1–Cu1–O1
N1–Cu1–O5
N2–Cu1–O1
N2–Cu1–O5
N1–Cu1–N2
O1–Cu2–O2
2.027(6)
2.381(6)
1.995(7)
2.892(6)
2.935(6)
1.977(8)
150.4(2)
88.5(2)
95.3(2)
74.9(2)
102.1(3)
108.3(3)
80.3(3)
176.0(3)
50.9(2)
119.6(2)
49.8(2)
91.9(3)
88.7(3)
91.7(3)
86.6(2)
Cu1–O3
Cu1–O7
Cu1–N2
Cu2–O2
Cu2–O4
Cu2–N5
O1–Cu1–O5
O1–Cu1–O7
O5–Cu1–O7
N1–Cu1–O3
N1–Cu1–O7
N2–Cu1–O3
N2–Cu1–O7
O1–Cu2–O3
O2–Cu2–O3
O2–Cu2–O4
N3–Cu2–O2
N3–Cu2–O4
N3–Cu2–O1
N5–Cu2–O1
N3–Cu2–N5
2.368(6)
2.025(6)
1.923(7)
1.965(6)
1.964(6)
1.969(7)
94.4(2)
91.7(2)
160.4(2)
75.6(3)
97.5(3)
101.3(3)
80.2(3)
169.2(2)
139.7(2)
170.5(3)
90.2(3)
89.2(3)
89.8(3)
91.6(2)
177.9(3)
ii
Cu2–O5
Cu2–O6
Cu2–N3
ii
ii
iii
iii
ii
Cu2–N3A
i
N2–Cu1–O5
N2–Cu1–O1
O1–Cu1–O5
O1–Cu1–O4
N1–Cu1–O5i
N1–Cu1–O1
O4–Cu1–O5
N2–Cu1–O5i
N2–Cu1–O4
O1–Cu1–O5i
N1–Cu1–O5
N1–Cu1–N2
N1–Cu1–O4
iii
iii
iii
i
ii
O4–Cu1–O5
O5–Cu2–O5
ii
ii
iii
O5 –Cu2–O6
O5–Cu2–O6
O1–Cu2–O4
ii
ii
ii
iii
iii
O5–Cu2–O6
O6–Cu2–O6
O5 –Cu2–O6
O3 –Cu2–O4
N5–Cu2–O2
ii
ii
iii
N3 –Cu2–O5
ii
iii
N3–Cu2–O5
93.4(4)
86.6(4)
88.6(3)
91.4(3)
91.7(7)
92.4(7)
180.0
N3–Cu2–O5
N5–Cu2–O4
N3–Cu2–O3
N5–Cu2–O3
ii
ii
ii
iii
N3 –Cu2–O5
N3 –Cu2–O6
ii
iii
N3–Cu2–O6
N3–Cu2–O6
N3 –Cu2–O6
ii
ii
N3–Cu2–N3
Symmetry code: (iii) x+1/2, −y+3/2, z+1/2
ii
N3A–Cu2–O5
N3A–Cu2–O6
N3A–Cu2–N3A
N3A –Cu2–O5
ii
N3A –Cu2–O6
ii
groups. In this case, the corresponding Δν values for com-
−
1
Symmetry codes: (i) −x+1, y, −z+3/2; (ii) −x+1/2, −y+1/2,
z+1
plexes 2 and 3 are also in the range of 125–176 cm .
The solid-state electronic spectra show four (complex 1)
or ꢁve (complexes 2 and 3) maxima. All three complexes
exhibit absorption bands in the ranges of 202–215, 246–249
and 273–279 nm in the ultraviolet region, which can be
assigned to the π→π* and n→π* intraligand charge transfer
transitions of dipicolinate and benzimidazole. In the visible
region, one absorption band was observed for the mono-
meric compound 1, whereas polymeric compounds 2 and
3 with elongated tetragonal bipyramidal geometries around
Cu2 both gave two absorption bands. Broad bands centred
at 738–755 and 547–560 nm (for 2 and 3) correspond to the
d–d transitions of the Cu(II) atoms, with {CuN O O′} or
−
from the methanol ligand. The aromatic and aliphatic
υ(C–H) stretching vibrations are observed at 3091–3082
−
1
and at 2978–2810 cm , respectively [31].
Very intense vibrational bands around 1630–1579
−
1
and 1371–1346 cm , respectively, for complexes 1–3
should correspond to the asymmetric ν (COO) and sym-
as
metric ν (COO) stretching vibrations of the carboxy-
s
late groups, but their correct identiꢁcation is ambigu-
ous due to overlapping and interfering bands, as well as
involvement of the carboxylate groups in the O–H···O
hydrogen bonding networks. Moreover, in the case of
polymeric compounds 2 and 3 several diꢀerent vibra-
tional bands for the carboxylate groups are expected, as
a consequence of their diverse coordination modes. The
values of Δν = ν (COO) − ν (COO) vary in the range of
2
2
{CuN O } chromophores, respectively. The observation of
2
4
two absorption bands for the d–d transitions of Cu(II) for
complexes 2 and 3 is the result of Jahn–Teller distortion,
which causes splitting of the bands in the UV/Vis spectrum
due to reduction in symmetry (O to D ) [33].
as
s
h
4h
−
1
2
08–282 cm indicating according to Deacon et al. [32]
The experimental and computer simulated EPR spectra
of complexes 1–3, which were recorded at (a) room tem-
perature and (b) 98 K, are shown in Fig. 7. Unresolved
hyperfine splitting is observed in all the EPR spectra. In
Table 5, the g-factor values, which were evaluated from
the experimental EPR spectra and then further refined
by computer simulation, are summarized. At both tem-
peratures, the line shapes of the EPR spectra for each
unidentate binding of the carboxylate groups. This is con-
sistent with the single crystal X-ray structure of 1, but
complexes 2 and 3 also revealed chelating and bridging
coordination modes of some carboxylate groups. There-
−
1
fore, for complexes 2 and 3 peaks at 1454 and 1452 cm ,
respectively, have been tentatively ascribed to the sym-
metric ν (COO) stretching vibrations of the carboxylate
s
1
3

Transition Metal Chemistry
Fig. 5 2D supramolecular framework structure of complex 2. The hydrogen atoms are omitted for clarity
complex are very similar, and likewise the corresponding
g-factor values are (within experimental errors) identical.
The EPR spectra of complexes 1 and 2 exhibit axial sym-
metry features. However, orthorhombic EPR spectra were
obtained in the case of complex 3. The axially symmetric
EPR spectra of complexes 1 and 2 show the usual relation
Conclusion
Cu(II)–dipicolinate complexes with diꢀerent benzimi-
dazoles as N-donor ligands, namely [Cu(bzim)(dipic)
(MeOH)] (1), [Cu (2-Etbzim) (dipic) ] ·0.5nH O (2) and
2
2
2 n
2
i
[Cu (2- Prbzim) (dipic) ] (3), were synthesized and struc-
2
2
2 n
(
g >g >2.0023), which is consistent with a d
2
2
ground
turally characterized by single crystal X-ray diꢀraction
analysis. Complex 1 is a monomer of square pyramidal
geometry (4+1) with a methanol ligand in the apical posi-
tion, while complexes 2 and 3 are polymers with similar
crystal structures. The dipicolinate in 2 and 3 acts as both
/
/
┴
x −y
electronic state.
The average g-factor, g [34] and geometric parameter,
av
G [35] were calculated for each axially symmetric EPR
spectrum, as given in Table 5. In the case of complex 1,
the relation G > 4 indicates negligible exchange interac-
tion between the Cu(II) centres. However, for complex 2,
the relation G<4 indicates the presence of some exchange
coupling between the Cu(II) atoms [35, 36]. The obtained
g-values are in a good accordance with those reported for
similar Cu(II) complexes in the literature [35–37].
tridentate chelating terminal, and a μ -bridging ligand.
3
Both polymeric complexes consist of two units with diꢀer-
ent coordination polyhedra around Cu(II). The copper(II)
atoms, Cu1 from the ꢁrst unit {Cu(dipic) } and Cu2 from
2
the second unit, made up of Cu2 atom coordinated by two
1
3

Transition Metal Chemistry
Fig. 6 2D supramolecular framework structure of complex 3. The hydrogen atoms are omitted for clarity
Fig. 7 The experimental EPR
spectra of polycrystalline
(
a)
(b)
1
1
copper(II) complexes 1–3 meas-
ured at a room temperature, b
9
8 K (black line), together with
calculated EPR spectra (red
line). The spectral parameters
for the best ꢁt are given in
Table 5. (Color ꢁgure online)
2
2
3
3
2
000
2500
3000
3500
4000
4500
2000
2500
3000
3500
4000
4500
B [10-4 T]
B [10-4 T]
1
3

Transition Metal Chemistry
Table 5 The EPR parameters of copper(II) complexes 1–3
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N (1991) J Chem Soc, Dalton Trans 1:3193–3201
Copper(II)
complex
EPR parameters
8
.
Subramanian PS, Suresh E, Shukla RS (2005) Inorg Chim Acta
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58:2651–2660
g⊥ (± 0.002)
g| (± 0.002)
gav
G
9
.
Mistri S, Zangrando E, Manna SC (2013) Inorg Chim Acta
4
05:331–338
1
2
2.063
2.088
2.274
2.224
2.134
2.133
gav
4.33
2.55
–
1
1
0. Das B, Baruah J-B (2012) Polyhedron 31:361–367
1. van Albada GA, Ghazzali M, Al-Farhan K, Bouwman E, Reedijk
J (2013) Polyhedron 52:1059–1064
g
g
g
1
2
3
(
± 0.002)
(± 0.002)
2.132
(± 0.002)
2.208
1
2. Yang L, Crans DC, Miller SM, la Cour A, Anderson OP, Kaszyn-
ski PM, Godzala ME, Austin LTD, Willsky GR (2002) Inorg
Chem 41:4859–4871
3
2.061
2.134
–
The g-factor values obtained for each complex measured at RT and at
8 K are (within experimental error) identical
g =(2 g⊥+g|)/3, g =(g +g +g )/3, G=(g|−2)/(g⊥−2)
13. Siddiqi ZA, Khalid M, Kumar S, Shahid M, Noor S (2010) Eur J
Med Chem 45:264–269
9
1
4. Tabatabaee M, Bordbar M, Ghassemzadeh M, Tahriri M, Tah-
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av
av
1
2
3
7
0:364–371
1
5. Gonzalez-Baró AC, Castellano EE, Piro OE, Parajón-Costa BS
imidazole nitrogen atoms from 2-ethyl-1-H-benzimidazole
(
2005) Polyhedron 24:49–55
(
2) or 2-isopropyl-1-H-benzimidazole (3) ligands, are con-
16. Kong F, Yu Z (2011) Acta Crystallogr E67:m783
17. Liu SH, Li YZ, Meng QJ (2005) Acta Crystallogr
E61:m1183–m1184
nected to each other via two pairs of carboxylate oxygen
atoms from μ -bridging dipicolinate. The EPR spectra of
3
1
8. Li YH, Li FF, Liu XH, Zhao LY (2012) Acta Crystallogr
E68:m739
the complexes indicate distorted tetragonal coordination
spheres for the Cu(II) atoms, with a d
2
x −y
2
ground elec-
19. Feng R, Yang Y, Lu Z, Lu F-Y, Nie F-M (2015) Z Kristallogr
230:201–202
tronic state. This work contributes to our knowledge of
Cu(II)–dipicolinates containing benzimidazoles, their
crystal structures and spectroscopic properties.
2
0. Dong GY, Fan LH, Yang LX, Khan IU (2010) Acta Crystallogr
E66:m532
2
1. Puchoňová M, Kuchtanin V, Mazur M, Valigura D (2016) Acta
Chim Slovaca 9:23–27
Acknowledgements The authors would like to thank Dr. Michal Šoral
22. Sheldrick GM (2015) Acta Crystallogr A 71:3–8
23. Palatinus L, Chapuis G (2007) J Appl Crystallogr 40:786–790
24. Sheldrick GM (2015) Acta Crystallogr C 71:3–8
25. Koziskova J, Hahn F, Richter J, Kozisek J (2016) Acta Chim Slo-
vaca 9:136–140
1
13
for recording H NMR and C NMR spectra of organic proligands
and Dr. Ivan Šalitroš for carrying out magnetic measurements. This
work was supported by the Slovak Research and Development Agency
under the Contact Nos. APVV-15-0053, APVV-16-0039 and APVV-
1
4-0078, the Scientiꢁc Grant Agency of the Slovak Republic (Projects
26. Dolomanov OV, Bourhis LJ, Gildea RJ, Howard JAK, Puschmann
H (2009) J Appl Crystallogr 42:339–341
VEGA 1/0686/17, VEGA 1/0026/18 and 1/0639/18) and by the STU
Grant scheme for Support of Young Researchers (1687). This article
was created with the support of the MŠVVaŠ of the Slovak Republic
within the Research and Development Operation Programme for the
project “University Science Park of STU Bratislava” (ITMS Project
No. 26240220084) cofounded by the European Regional Development
Fund.
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333:253–265
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Bruker Analytic GmbH
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3
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0. Weber RT (1995) WINEPR SimFonia. EPR Division, Bruker Instr
Inc., Billerica
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