Synthesis, acid-base and complexing properties with Cu(II), Co(II) and Zn(II) in aqueous solution of a novel 1H-benzimidazol-2-ylmethyl diethyl phosphate ligand: Comparison with other 2-substituted benzimidazole ligands

Article abstract of DOI:10.1016/j.poly.2013.01.006

1H-Benzimidazol-2-ylmethyl diethyl phosphate (2-BimOpe) was synthesized via condensation of phosphorochloridic acid diethyl ester with 2- benzimidazolemethanol in the presence of triethylamine. This phosphate derivative of benzimidazole was chosen because of its expected biological properties, indicated up to now for many other related compounds. The coordinating properties of 2-BimOpe towards Cu(II), Co(II) and Zn(II) in aqueous solution have been studied through a combined application of potentiometric and spectroscopic methods (electronic absorption for Cu(II) and Co(II) as well as EPR spectroscopy in the case of Cu(II)). It was shown that 2-BimOpe coordinates Cu(II) yielding CuL and CuL₂ species (the CuL₂ complex being confirmed only by EPR). The same measurements have been made with benzimidazole (Bim) and 2-(hydroxymethyl)benzimidazole (2-CH₂OHBim), the latter ligand capable of bidentate {N,O} coordination. Moreover, the potentiometric results for the novel diethyl phosphate derivative of 1H-benzimidazol-2-ylmethanol, 2-BimOpe, have been compared with the literature data reported recently for (1H-benzimidazol-2-yl-methyl)phosphonate (Bimp ^2-). The difference between complexation abilities of Bim and 2-BimOpe comes evidently from a bulky effect of the estrificated phosphoric groups preventing the benzimidazole fragment to be involved in the consecutive complexation steps. On the other hand the N(3) nitrogen is the most acidic one (pK_a = 4.04) in 2-BimOpe among the compounds considered. The release of the N(3)H⁺ proton occurs at a relatively low pH for 2-BimOpe, most probably due to the known electron withdrawing effect of the phosphate group.

Full text of DOI:10.1016/j.poly.2013.01.006

Polyhedron 53 (2013) 20–25
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, acid–base and complexing properties with Cu(II), Co(II) and Zn(II)
in aqueous solution of a novel 1H-benzimidazol-2-ylmethyl diethyl phosphate
ligand: Comparison with other 2-substituted benzimidazole ligands
a
b
c
Aleksander Kufelnicki a, , Magdalena Wozniczka , Urszula Kalinowska-Lis , Julia Jezierska ,
⇑
´
Justyn Ochocki b
a
´
´
´
Department of Physical and Biocoordination Chemistry, Faculty of Pharmacy, Medical University of Łódz, 1 Muszynski Str., 90-151 Łódz, Poland
b
´
´
´
Department of Bioinorganic Chemistry, Faculty of Pharmacy, Medical University of Łódz, 1 Muszynski Str., 90-151 Łódz, Poland
c Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie Str., 50-383 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
1H-Benzimidazol-2-ylmethyl diethyl phosphate (2-BimOpe) was synthesized via condensation of phosp-
horochloridic acid diethyl ester with 2-benzimidazolemethanol in the presence of triethylamine. This
phosphate derivative of benzimidazole was chosen because of its expected biological properties, indi-
cated up to now for many other related compounds. The coordinating properties of 2-BimOpe towards
Cu(II), Co(II) and Zn(II) in aqueous solution have been studied through a combined application of poten-
tiometric and spectroscopic methods (electronic absorption for Cu(II) and Co(II) as well as EPR spectros-
copy in the case of Cu(II)). It was shown that 2-BimOpe coordinates Cu(II) yielding CuL and CuL2 species
(the CuL2 complex being confirmed only by EPR). The same measurements have been made with benz-
imidazole (Bim) and 2-(hydroxymethyl)benzimidazole (2-CH2OHBim), the latter ligand capable of biden-
tate {N,O} coordination. Moreover, the potentiometric results for the novel diethyl phosphate derivative
of 1H-benzimidazol-2-ylmethanol, 2-BimOpe, have been compared with the literature data reported
recently for (1H-benzimidazol-2-yl-methyl)phosphonate (Bimp2À). The difference between complexation
abilities of Bim and 2-BimOpe comes evidently from a bulky effect of the estrificated phosphoric groups
preventing the benzimidazole fragment to be involved in the consecutive complexation steps. On the
other hand the N(3) nitrogen is the most acidic one (pKa = 4.04) in 2-BimOpe among the compounds con-
sidered. The release of the N(3)H+ proton occurs at a relatively low pH for 2-BimOpe, most probably due
to the known electron withdrawing effect of the phosphate group.
Received 13 September 2012
Accepted 5 January 2013
Available online 26 January 2013
Keywords:
2-Substituted benzimidazoles
Divalent metals
Bioinorganic chemistry
pH potentiometry
UV–Vis spectroscopy
EPR spectroscopy
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
ceived significant attention as therapeutic or potentially therapeu-
tic agents [9,10]. Moreover, also many phosphate derivatives of
It is known that some Cu(II) and also Zn(II) complexes with 2-
substituted benzimidazole ligands indicate biological and pharma-
cological activity [1,2]. Similarly, such properties involve, among
others, the cytotoxic [3,4] and antiviral [4] activity of Pt(II) com-
plexes. On the other hand, the benzimidazole moiety alone, also
addressed as 1,3-dideazapurine, is structurally related to purine
bases. The benzimidazole residue is also a constituent (axial ligand
bound through N3 nitrogen) of vitamin B12 and other cobalamines.
Then, it is not surprising that the biological properties of benzimid-
azole and related compounds have been and are still studied inten-
sively because of their antiarrythmic, antibacterial, antiviral,
antitumor properties and application in treatment of acid-peptic
diseases [5–8]. Recently, the phosphonate derivatives have re-
benzimidazole, in particular nucleotide analogs, show various bio-
logical properties [11].
The title ligand (2-BimOpe) is a diethyl ester of 1H-ben-
zimidazol-2-phosphonic acid described up to now only as a ligand
in
a trans-bis(1H-benzimidazol-2-ylmethyl-j
N3 diethyl phos-
phate)dichloropalladium(II) monohydrate [12] complex and also
investigated as involved in solid chloride Cu(II), Co(II) and Zn(II)
complexes well soluble in ethanol [13]. In the copper complex
recrystallized from ethanol the Cu atoms together with the chlo-
ride ions form a characteristic closed cage-like structure with a
water molecule inside. As shown by X-ray structure analysis the
Co and Zn complexes are isostructural. Their central metal adopts
tetrahedral arrangement with two chloride ions and two endocy-
clic nitrogen atoms in the coordination sphere. The Cu(II) complex
proved to be a cytotoxic agent against the A549 lung cancer cell
lines and a human colorectal adenocarcinoma cell line (HT29)
⇑
Corresponding author. Tel.: +48 42 6779204; fax: +48 42 678 83 98.
E-mail address: aleksander.kufelnicki@umed.lodz.pl (A. Kufelnicki).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.01.006

A. Kufelnicki et al. / Polyhedron 53 (2013) 20–25
21
Scheme 1. Structures of the ligands.
[14]. The second active complex was Zn(II) which, in spite of the
necessity of being used in high doses, remained virtually neutral
for normal human peripheral blood lymphocytes.
(300 MHz, DMSO), d (ppm): 1.22 (t, 6H, 2 Â OCH2CH3); 4.05 (dq,
3
4H, 2 Â OCH2CH3); 5.18 (d, 2H, (bi)CH2OP, JHP = 8.1 Hz); 7.21 (m,
2H, (bi)H(5)H(6)); 7.58 (m, 2H, (bi)H(4)H(7); 12.69 (s, 1H, NH).
The results presented now show the coordination behavior of
the synthesized 2-BimOpe in aqueous solution towards three diva-
lent cations (Cu2+, Co2+ and Zn2+), which favor rather N than O do-
nors [15]. A comparison is done with benzimidazole (Bim), a ligand
coordinating only via the N3 nitrogen and also with two other li-
gands, 2-(hydroxymethyl)benzimidazole (2-CH2OHBim) and (1H-
benzimidazol-2-yl-methyl)phosphonate (Bimp2À), which may
even participate in chelatation owing to additional O donors in
the 2-methyl substituents [10]. To extend the comparison to other
metal ions we report here also the attempts of studies on Co(II)
complexes with 2-CH2OHBim.
31P NMR (121 MHz, DMSO) d (ppm): À0.375. IR (KBr)
mmax
(cmÀ1): (N–H, C–H stretching) 3397(s), 3186(s), 3062(s), 2986(s),
2916(s), (C@N, C@C) 1625(w), 1592(vw), (–CH2 scissors) 1458(s),
1442(s), (P@O) 1264(vs br), (P–O–C) 1077(s), 1032(vs br),
984(m); (vs: very strong; s: strong; m: medium; w: weak; vw: very
weak; br: broad).
2.2. Potentiometric measurements
The stability constants of proton and metal complexes were
determined with a Molspin automatic titration kit equipped with
a combined microelectrode Russell CMAWL/4/5/S7. All the experi-
ments were carried out at constant temperature of 25.0 0.1 °C.
The water soluble ligands were dissolved in 10 ml flasks, filled
up with HNO3 in excess, KNO3 and in the titrations in presence
of the metal also with an appropriate metal nitrate. Then the ionic
strength of the final solution was 0.1 (KNO3). The protonation and
formation constants were determined by pH-metric titrations of
4.0 cm3 samples. Alkali (0.1 M NaOH carbonate-free, Malinckrodt
Baker B.V.) was added from a 0.500 cm3 calibrated micro syringe.
The measurement cell was daily calibrated with 0.1 M NaOH in
the Àlog[H+] scale by titration of aqueous 0.005 M HNO3 (contain-
ing KNO3 up to I = 0.1), according to the method of Irving et al. [16].
Purified argon was bubbled through the titration vessel to ensure
absence of oxygen and carbon dioxide. Overall concentration
formation constants: bmlh = [MmLlHh]/[M]m[L]l[H]h were calculated
from at least three titration files by SUPERQUAD [17] and then
HYPERQUAD 2008 [18]. Graphical simulation of speciation diagrams
on the basis of calculated constants was carried out by HySS
2009 [19]. The total concentration of the ligand in each sample
ranged within 3.0–5.0 Â 10À3 M. The metal–ligand interaction
was studied at ligand-to-metal ratios 4:1, 6:1 and 8:1. Cu(NO3)2
p.a. of Fluka, Co(NO3)2 p.a. of POCh Gliwice and Zn(NO3)2 p.a. of
Fluka were used – the standard solutions were titrated with
disodium salt of EDTA in the presence of murexide.
The structures of the new ligand (2-BimOpe) and these studied
as references (Bim, 2-CH2OHBim as well as Bimp2À) are shown in
Scheme 1.
2. Experimental
2.1. Materials and physical measurements
Starting materials and solvents for synthesis were obtained
commercially and used as received. The silica gel 60 (63–200 mesh,
Merck) was used for column chromatography. Analytical thin-layer
chromatography was performed using Merck 60 F254 silica gel
(precoated sheets, 0.2 mm thick). Infrared spectrum was recorded
in the range 4000–400 cmÀ1 on a Bruker IFS66 spectrophotometer
using KBr pellets. NMR (1H, 31P) spectra were collected on a Mer-
cury 300 MHz spectrometer using DMSO as a solvent.
2.1.1. Synthesis of 1H-benzimidazol-2-ylmethyl diethyl phosphate (2-
BimOpe)
2-Benzimidazolemethanol (0.01 mol, 1.48 g) and triethylamine
(0.01 mol, 1.01 g) were dissolved in anhydrous N,N-dimethylform-
amide (20 ml) at room temperature. Then diethyl chlorophosphate
(0.01 mol, 1.73 g) was added. The reaction mixture was stirred for
1 h at room temperature. The precipitated N(C2H5)3ÁHCl was
filtrated and discarded. The solvent (DMF) was evaporated from
the solution under reduced pressure by repeated evaporation with
toluene. The oily residue was purified by column chromatography
(silica gel, acetone) giving a pale yellow oil product: RF = 0.54;
2.3. Spectroscopic measurements
Electronic spectra under argon were recorded on a Cary 50 Bio
spectrophotometer, slit width 1.5 nm, equipped with a fiber-optic
device. Thanks to this device it was possible to investigate the
C
12H17N2O4P (M = 284.2 g/mol); (1.59 g, 56% yield). 1H NMR

22
A. Kufelnicki et al. / Polyhedron 53 (2013) 20–25
equilibrium systems spectrophotometrically, simoultaneously
with the pH measurements controlled by another Molspin titrator.
The fiber-optic probe of 5 mm (which corresponds to path length
1 cm) was dipped directly into the titration vessel (initial volume
25.0 cm3, total metal concentration 8.3 Â 10À4 up to 7.5 Â 10À3
M
owing to optimum absorbance measurements, ligand:metal ratio
6:1, 8:1, 10:1, temperature 25.0 0.1 °C). In the experiments with-
out the metal the total concentration of L amounted to 1.5–
2.0 Â 10À4 M. Because of the highly disturbing absorption of the ni-
trate ion at ca 300 nm, all the UV experiments were carried out
mainly in perchlorate medium (electrolyte NaClO4 instead of
KNO3). The ionic strength in aqueous phase was 0.5, i.e. higher than
in the solely potentiometric measurements, by that permitting to
avoid undesired dilution of samples during titration with 0.1 M
NaOH. The used alkali (0.5 M NaOH carbonate-free, Malinckrodt
Baker B.V.) was added from a 0.500 cm3 calibrated micro syringe.
After each base addition the pH and EMF were controlled by the
Molspin titrator with a combined InLab Semi-Micro (METTLER TO-
LEDO) electrode. A time delay was given to equilibrate the system.
After each point or set of points a pause was made and the spec-
trum was recorded at a slow scan (300 nm/min). After the collec-
tion of each curve, new aliquots of base were added and the
procedure repeated. It was therefore possible to collect a spectrum
at each titration point or at each chosen pH. The molar absorbances
of species have been calculated after deconvolution by HypSpec
(part of HYPERQUAD 2008 suite, Protonic Software). The HypSpec pro-
gram resolves a linear equation system based on Lambert–Beer’s
law, using the spectrophotometric data and known (or estimated)
equilibrium constants, yielding the initial molar absorbances of
individual absorbing species. Then, optionally the program can be
used to refine the estimated equilibrium constants from spectro-
photometric data.
Caution! Perchlorate salts are potentially explosive and were han-
dled only in small quantities with care.
The EPR spectra were measured using a Bruker Elexsys E500
spectrometer equipped with NMR teslameter (ER 036TM) and fre-
quency counter (E 41 FC) at X-band. The simulations of the exper-
imental spectra were performed using computer program WINEPR
Simfonia, version 1.26 beta and the program written by Dr. Andrew
Ozarowski from NHMFL, University of Florida, with resonance field
calculated by diagonalization of energy matrix. The spectra were
measured with a modulation frequency 100 kHz, modulation
amplitude of 7 gauss and microwave power of 10 mW. The ligand
(L) and Cu2+ concentrations were CL = 5 Â 10À3 and CCu2+
=
6.25 Â 10À4 M, respectively. To avoid the aggregation of Cu(II)
complexes in water to the studied solutions 10% (v/v) of ethyl
glycol was added.
3. Results and discussion
3.1. Acid–base properties
Fig. 1. Titrations of the H+/Metal ion/2-BimOpe systems under argon. The negative
values of base equivalent, a, correspond to HNO3 needed to protonate the ligand.
All the ligands accept a proton at N(3) of the benzimidazole res-
idue. According to Fig. 1 the N(1)H cannot be considered as proton
release and coordination site – the end-point in the ligand curve
occurs just after adding one equivalent of base needed to neutral-
ize the mineral acid used for protonation. If N(1)H is also a proton
release site the number of base equivalents needed to reach the
end-point would be relatively higher. Moreover, as it was shown
in the literature for related compounds containing the benzimid-
azole moiety, the N(1)–H site is deprotonated at very high pH 13
[9].
Metal ions: (a) Cu(II), (b) Co(II), (c) Zn(II). Curves:
–
ligand-to-metal 4:1,
– ligand-to-metal
CL = 3.0 Â 10À3 M; – ligand-to-metal 6:1, CL = 5.0 Â 10À3 M;
8:1, CL = 5.0 Â 10À3 M;
– ligand in absence of the metal, CL = 3.0 Â 10À3 M.
electron withdrawing effect of the phosphate group [20,21]. The
solubility of 2-BimOpe at neutral and basic pH is also decreased.
Contrariwise, the acidity of N(3)H+ of Bim is only slightly lowered
in comparison with two ligands, 2-CH2OHBim and Bimp2À, able to
release additional proton and possessing another negative charged
group; both these latter ligands are sufficiently soluble in the alka-
line medium. Moreover, the acidity constants reported in Table 1
are in reasonable agreement with those of the references cited,
As it follows from Table 1, the N(3) nitrogen is the most acidic
(pKa = 4.04) in 2-BimOpe. The release of the accepted N(3)H+ pro-
ton occurs at a relatively low pH, most probably due to the known

A. Kufelnicki et al. / Polyhedron 53 (2013) 20–25
23
Table 1
Negative logarithms of dissociation constants at 25.0 0.1 °C, I = 0.1 (KNO3) and UV spectral data of the ligands used. Standard deviations in parentheses refer to random errors
only. The values are given for the particular sites.
Ligands
N(3)H+
pKa
OH
pKa
–
pKa
Refs.
kmax
(
emax
)
kmax(emax)
2-BimOpe
4.04(1)
–
269(7.9 Â 103)
275(7.6 Â 103)
–
Bim
5.70(2)
5.55(6)
5.37(2)a
–
5.58
5.64
5.54
5.68
[22]
[22]
[23]
[24]
240(3.9 Â 103)
267(6.2 Â 103)
274(6.7 Â 103)
–
2-CH2OHBim
11.84(6)
7.41(2)a,b
5.40;11.55
5.48;11.48
[25]
[29]
236(4.1 Â 103)
268(8.5 Â 103)
275(8.4 Â 103)
244(5.8 Â 103)
273(6.7 Â 103)
279(8.4 Â 103)
Bimp2À
[9]
a
Data found only in [9].
b
Concerning the –P(O)2À(OH) group.
since the differences do not exceed 0.2–0.3 log unit. The analysis of
UV absorption spectra via their HypSpec deconvolution on the ba-
sis of the equilibrium model accepted potentiometrically (Figs. S1–
S3) made possible to determine molar absorbances of the particu-
lar ionic species of the ligands – Table 1.
of aqua molecules deprotonation. The results for the Zn(II)–2-Bim-
Ope system, based only on potentiometric data owing to the d10
configuration of the metal, agree with the results described above
for Cu(II) and Co(II) but for two of the three other ligands (Bim and
Bimp2À) a comparison could be made only with Co(II) (Table 2).
In contrast with Cu(II)–2-BimOpe, the potentiometric results re-
veal Cu(II) coordination by at least three Bim ligands (Table 2)
through N(3), although some literature data indicate that even
fourth Bim is bound to Cu(II) [28]. This is possible because a steric
hindrance is not produced by this ligand, opposite to 2-BimOpe
providing a bulky diethyl phosphate group. Also for the Cu(II)–2-
CH2OHBim system, potentiometric results suggest that more than
one N(3) nitrogen donor are involved in the formed complex. Fur-
thermore, the presence of 2-hydroxymethyl OÀ donor permits to-
gether with N(3) a {N,O} chelatation mode, evidenced in high
stability constants (Table 2) by more than six orders of magnitude
higher than for solely N(3) coordination.
3.2. Complexing behavior
Potentiometric data for 2-BimOpe, although carried out at ex-
cess of the ligand (Fig. 1), indicate the formation of L:M = 1:1 spe-
cies with Cu(II), Co(II) and Zn(II) – Table 2. The complexes exist
until pH 7–8 depending on metal ion, whereas in neutral and alka-
line solutions the hydrolytic products of the particular cations be-
come the predominant species (example for Cu(II) in Fig. 2).
According to Fig. 2, the contribution of metal complex attains max-
imum relative to the total metal concentration at pH > 4, where
more than 50% of the ligand occurs in deprotonated N(3)H+ form.
However, the disturbances caused by poorly soluble hydrolytic
products at higher pH make impossible to confirm the formation
of L:M = 2:1 species, likely owing to the possible next coordination
step. Further alkalization leads to appearance and subsequent
increasing contribution of the species formed by ML with products
It seemed to be interesting to compare the potentiometric data
of the Cu(II) complexes with the ligands not containing the coordi-
nating oxygen donors beside N(3), with those observed for the
complexes
with
(1H-benzimidazol-2-yl-methyl)phosphonate
(Bimp2À), additionally studied as reference. The first stepwise
Table 2
Logarithms of cumulative stability constants bmlh = [MmLlHh]/[M]m[L]l[H]h and UV spectral data [kmax max) for Cu2+ and Co2+] of the complexes with 2-BimOpe and with the other
(e
related ligands at 25.0 0.1 °C, I = 0.1 (KNO3). Standard deviations in parentheses refer to random errors only.
M2+
2-BimOpe
3.20(3)
Bim
2-CH2OHBim
Present data
Bimp2Àa
Present data
Refs.
Refs.
Cu2+
CuL
3.17(6);
6.15(7);
8.94(8)
3.44(10);
6.29(3);
8.70(6);
10.87(9)
8.89(4)
16.80(7)
9.03;
16.81 [29]
4.21(8);
7.68(5)
CuL2
CuL3
CuL4
[kmax
[28a]
[23]
[23]
(
e
max)]
ꢀ440(25)sh
312(241)c
719(58)d
2.74(8)
321(530)c
717(85)d
d
747(30)
Co2+
Zn2+
CoL
3.17(2)
2.98
–
–
2.31(8);
4.92(5)
b
[kmax
ZnL
(e
max)]
ꢀ440(25)sh
747(30)d
3.15(2)
510(9)d
3.00(4)
b
–
–
–
2.45
–
62.1
5.79(4)
a
b
c
Data found only in [10]. For Bimp2À the two stability constants are stepwise constants, they correspond to the N(3)H and phosphonate protons, respectively.
Measurement disturbance caused by hydrolytic products.
CT transition.
d–d transition.
d

24
A. Kufelnicki et al. / Polyhedron 53 (2013) 20–25
stability constant of Bimp2À corresponds to coordination via N(3)
and is of one order of magnitude higher than the constants for
2-BimOpe and Bim. Apart from the chelate effect the role of
negatively charged residue, four bonds away, in facilitating M2+
binding at N(3) was shown in Ref. [10].
The wavelengths at maximum and molar absorbances, calcu-
lated by deconvolution, are shown in Table 1 (for the ligands)
and in Table 2 (for the 2-BimOpe amd Bim complexes with Cu(II)
and Co(II) as well as for the Cu(II) complex with 2-CH2OHBim).
As it follows, the lower energetic bands species are blue shifted
2+
for both the metals relative to the corresponding Cu(H2O)6 and
2+
Co(H2O)6 aqua ions [26]. Noteworthy is the presence of a weak
shoulder at ꢀ440 nm besides the d–d transition of 747 nm usually
expected for typical hexacoordinated Cu(II) complexes of distorted
octahedral symmetry (Fig. S5). The blue shifts of the asymmetric
d–d bands in the visible region, observed for Cu(II) and Co(II) with
2-BimOpe occurred in the pH ranges up to 4.5 for copper(II) and 5.3
for cobalt(II), where the formation of ML reaches maximum
(Figs. S4 and S5). Such a result may indicate a slight additional
symmetry lowering in relation to the Cu(II) complexes with octa-
hedral geometry distorted by the Jahn–Teller effect, described later
in context of Bim and 2-CH2OHBim.
It may be concluded that the electronic absorption spectra
could confirm only the first step of complexation both for Cu(II)–
Bim and Cu(II)–2-CH2OHBim (Figs. S6 and S7) as alkalization above
pH 3.75 or pH 5.0, respectively, brought about a disturbance
caused by light scattering arising from the hydrolytic products. A
comparison of the spectrophotometric features resulting from
titrations and then deconvolution performed by HypSpec (Table 2)
shows that the CuL complexes with L = Bim or 2-CH2OHBim are
very similar as regarding the coordination modes. Within the visi-
ble region, the electronic spectra display absorption at kmax ca
720 nm, whereas in UV range they are characterized by absorption
at kmax ca 310–320 nm. This suggests that the remaining coordina-
tion sites in Cu(II) centre are occupied by H2O molecules resulting
in six-coordinated species. Then, the higher energy bands may be
assigned to p–
p⁄ ligand-based LMCT transitions and the lower en-
ergy bands to d–d transitions in hexacoordinated Cu(II) complexes
with a distorted octahedral geometry [5,26,27].
The spectrophotometric titrations of the systems with Co(II)
ions were limited to a very narrow pH range due to the concomi-
tant hydrolysis. A scarce blue shift could be noticed only for
Co(II)–Bim within the pH range 4.4–5.5 (Fig. S8) by that indicating
only the initial stage of complexation. For Co(II)–2-CH3OHBim the
Fig. 2. Species distribution curves as a function of pH for the complexes formed in
the Cu(II) ion/2-BimOpe systems at 8:1 ligand-to-metal ratio. CL = 5.0 Â 10À3 M.
Fig. 3. (a) Frozen solution EPR spectra (single differentiated absorption) of Cu(II) complexes with Bim or CH2OHBim or 2-BimOpe ligands at pH slightly higher than 6, together
with simulated spectra (sim1–sim3, respectively) calculated using the parameters given in the Table 3; (b) high-field regions of perpendicular orientation of copper(II) z axis
to magnetic field direction after next (double) differentiation together with their simulated counterparts (see text).

A. Kufelnicki et al. / Polyhedron 53 (2013) 20–25
25
Table 3
(pKa = 4.04) in 2-BimOpe may be assigned to the known electron
withdrawing effect of the phosphate group. Hence, this benzimid-
azole related ligand may be useful in such biological applications
where the metal ion substitutes H+ ion of the protonated ligand at
relatively low pH. It is known, for instance, that the acid–base
chemistry of benzimidazole containing proton pump inhibitor is
specifically responsible for the acid-catalyzed conversion into the
active inhibitors [8].
EPR parameters of frozen solutions of Cu(II)–L systems, where L = Bim or 2-
CH2OHBim or 2-BimOpe.
L
pH
g||
g\
A|| (10À4 cmÀ1
)
A (14N) [G]
Bim
2-CH2OHBim
2-BimOpe
6.5
7.0
5.5–6.5
2.256
2.312
2.312
2.054
2.062
2.063
192
152
169
14
13
13
spectrophotometric titrations were disturbed by hydrolytic prod-
ucts already at low pH.
Acknowledgements
The EPR spectra of the complexes formed in the systems con-
taining Cu(II) ions and ligands: L = Bim or L = 2-CH2OHBim or
L = 2-BimOpe at pH about 6.5 are shown in Fig. 3. The spectra cor-
respond to g|| ꢁ g\ > 2.0023 relation, typical for axial symmetry of
This work was partially supported by the Polish Ministry of Sci-
ence and Higher Education (Grant No. N405 303236) and by the
Medical University of Łódz´ (Statute Fund No. 503/3-016-02/503-
01 – J. Ochocki and Statute Fund No. 503/3-014-02/503-01 – A.
Kufelnicki).
tetragonal geometry of Cu(II) complexes, with dx2
orbital of un-
Ày2
paired electron being very sensitive to the ligands in xy plane.
Although 2-BimOpe with the estrificated phosphonic group pro-
vides only imidazole nitrogen donor, similarly to that in Bim, the
EPR spectra of the formed complexes exhibit higher g|| and lower
A|| for the complex with 2-BimOPe ligand (Table 3). It is associated
with a smaller amount of the ligands involved in Cu(II) coordina-
tion sphere due to bulky effect of estrificated phosphonic groups.
The EPR parameters of the complexes formed with 2-CH2OHBim
may be assigned to Cu(II) coordination through two NO chelating
ligands supported by the hyperfine splitting pattern assigned to
interaction with two nitrogen donors. A second derivative of the
resonance absorptions at the magnetic field region close to g\,
exhibited in Fig. 3b, for both the experimental and simulated spec-
tra shows a distinctly resolved splitting due to the hyperfine inter-
action between Cu(II) unpaired electron and the ligands nitrogen
nuclei with I(14N) = 1. The best agreement between the hyperfine
splitting pattern in experimental and theoretical spectrum was
achieved for simulation assuming four nitrogen nuclei for the spec-
trum of Cu(II)–Bim system and two nitrogen nuclei for the com-
plexes with L = 2-CH2OHBim and 2-BimOpe. Weak and
overlapped EPR spectra of the studied Cu(II) systems at pH about
4.5 show the equilibrium between the Cu(II) ions surrounded by
water molecules and the first formed complex with parameters
g|| = 2.360, g\ = 2.070 and A|| = 145 Â 10À4 cmÀ1, suggesting lower
number of nitrogen donors in Cu(II) coordination sphere.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2013.01.006.
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