ESR and pH-potentiometric study of the mixed-ligand complex formation in the copper(II)-4-fluorosalicylic acid-N,N-diethylnicotinamide system: Structure and spectral properties of [Cu(4-fluorosalicylate)2(N,N- diethylnicotinamide)2(H2O)2] complex

Article abstract of DOI:10.1016/j.poly.2011.06.030

EPR simulation method together with pH-potentiometry combined with UV-Vis spectrophotometry were used for the study of the ternary system 4-fuorosalicylic acid (HA)-N,N-diethylnicotinamide (B)-copper(II) in aqueous solution. The N,N-diethylnicotinamide ligand is a weak donor, its mixed-ligand complexes with 4-fluorosalicylate anions are more favoured. The number of coordinated N,N-diethylnicotinamide molecules increases with decreasing temperature: up to four ones were detected in the coordination sphere of copper(II) in frozen solutions. The formation of [CuH_-1AB₂] and [CuH _-1A] was detected by all methods at neutral pH. At lower pH values, [CuA₂B₂] and [CuB] become dominant, and this fact is in good agreement with [CuA₂B₂(H₂O)₂] crystals obtained from similar solutions. The structural unit of the [CuA ₂B₂(H₂O)₂] complex consists of a copper(II) ion, which is monodentately coordinated by a pair of 4-fluorosalicylate anions and by a pair of N,N-diethylnicotinamide in trans positions in the basal plane, and by two water molecules in the axial positions of a tetragonal bipyramid.

Full text of DOI:10.1016/j.poly.2011.06.030

Polyhedron 30 (2011) 2421–2429
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
ESR and pH-potentiometric study of the mixed–ligand complex formation in the
copper(II)–4-fluorosalicylic acid–N,N-diethylnicotinamide system: Structure and
spectral properties of [Cu(4-fluorosalicylate)2(N,N-diethylnicotinamide)2(H2O)2]
complex
Terézia Szabó-Plánka a, Ján Moncol d, Eszter Tóth c, Béla Gyurcsik c, Nóra Veronika Nagy b, Zuzana Vasková d,
Antal Rockenbauer b, Dušan Valigura d,
⇑
a Department of Physical Chemistry and Materials Sciences, University of Szeged, Aradi vértanúk tere 1, H-6720 Szeged, Hungary
b Chemical Research Center, Institute of Structural Chemistry, Hungarian Academy of Sciences, Pusztaszeri út 59-67, H-1025 Budapest, Hungary
c Department of Inorganic and Analytical Chemistry, University of Szeged, Dóm tér 7, Szeged H-6720, Hungary
d Department of Inorganic Chemistry, Slovak Technical University, Radlinskeho 9, 812 37 Bratislava, Slovakia
a r t i c l e i n f o
a b s t r a c t
Article history:
EPR simulation method together with pH-potentiometry combined with UV–Vis spectrophotometry
were used for the study of the ternary system 4-fuorosalicylic acid (HA)–N,N-diethylnicotinamide (B)–
copper(II) in aqueous solution. The N,N-diethylnicotinamide ligand is a weak donor, its mixed–ligand
complexes with 4-fluorosalicylate anions are more favoured. The number of coordinated N,N-diethylnic-
otinamide molecules increases with decreasing temperature: up to four ones were detected in the coor-
dination sphere of copper(II) in frozen solutions. The formation of [CuHꢀ1AB2] and [CuHꢀ1A] was
detected by all methods at neutral pH. At lower pH values, [CuA2B2] and [CuB] become dominant, and
this fact is in good agreement with [CuA2B2(H2O)2] crystals obtained from similar solutions. The struc-
tural unit of the [CuA2B2(H2O)2] complex consists of a copper(II) ion, which is monodentately coordinated
by a pair of 4-fluorosalicylate anions and by a pair of N,N-diethylnicotinamide in trans positions in the
basal plane, and by two water molecules in the axial positions of a tetragonal bipyramid.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 21 April 2011
Accepted 29 June 2011
Available online 12 July 2011
Keywords:
Copper(II) complexes
4-Fluorosalicylic acid
N,N-Diethylnicotinamide
Speciation
Electron spin resonance
pH-potentiometry
Crystal structure
1. Introduction
stoichiometry and bridging mode of den bonding has been found
in the polymeric [Cu2(RCOO)4(den)]n complexes built up of dimeric
The great attention of many research groups dealing with the
role played by therapeutically active substances in organisms has
been in the latest decades focused to the metal complex formation
[1–8]. The most mentioned biological characteristics of the N,N-
diethylnicotinamide (abbreviated as den) is its ‘‘breathing stimu-
lant’’ activity [9] and it still is in use nowadays [10]. Its possible
interactions with different biometals in living systems and proven
biological activity of some complexes [11–15] are the reasons for
the substantial interest of bioinorganic chemists in these com-
plexes. This could be documented by about seventy metal com-
plexes with known structure, and the copper(II) complexes with
den ligands are more than half of all ones. The den ligand is in com-
plexes usually bonded monodentately via the pyridine nitrogen
donor atom, and rarely a bridging N,O mode could be found too.
There are three different Cu(II):den stoichiometries observed in
the group of carboxylatocopper(II) complexes. In few cases, 2:1
Cu2(RCOO)4 molecules and den ligands [16,17]. Another rare
Cu(II):den stoichiometry is 1:1, that could be found in paddle–
wheel complexes [11,12,18,19] of the type [Cu2(RCOO)4(den)2]
and in polymeric [Cu(RCOO)2(den)(H2O)] complexes that were just
recently published [20,21]. The most frequent 1:2 stoichiometry
with unidentate den ligands can be found in monomeric com-
plexes of general formula Cu(RCOO)2(den)2(H2O)x, where x = 0
[22,23], x = 2 [17,24–29] or x = 4 [30]. Within the structural study
of the group [Cu(x-Me(O)sal)2(den)2(H2O)2] (where x-Me(O)-
sal = x-methyl- or x-methoxy-salicylate, x = 3, 4 or 5), the confor-
mational polymorphism with its consequence in supramolecular
isomerism has been found [29] for the pair of [Cu(3-Mesal)2
(den)2(H2O)2] complexes.
Salicylic acid itself [31] and its derivatives [32–39] have been
studied for their therapeutic performance that have led to the thor-
ough investigation of their properties in the presence of copper(II)
because of its ability to act in a synergistic manner with the sali-
cylic acid derivatives [40–44], the mechanism of which is not yet
fully understood. Our recent attention focused on the various
⇑
Corresponding author.
E-mail address: dusan.valigura@stuba.sk (D. Valigura).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.06.030

2422
T. Szabó-Plánka et al. / Polyhedron 30 (2011) 2421–2429
corresponding formation constants (bCu
lated using the PSEQUAD computer program [51].
ꢃ bpqrs) were calcu-
isomers of fluorosalicylic acids and their complexes in solid state
and in solutions [45–48] is in the present paper oriented toward
the system containing copper(II) ions in the presence of 4-fluoro-
salicylic (4F-sal) acid and N,N-diethylnicotinamide.
pHqArBs
The protonation constant of the B ligand was determined from 3
independent titrations (80–90 data points per titration), the pro-
tonation constant of A ligand was previously determined [48].
The complex formation constants of the copper(II)–B binary sys-
tem and of the copper(II)–HA–B ternary system were evaluated
from 4 to 6 independent titrations (50–90 data points per titra-
tion). The metal-to-ligand ratios varied between 1:1 and 1:100 in
the copper(II)–B system; 1:2:2 and 1:2:4 in the copper(II)–HA–B
2. Experimental
2.1. Materials
The ligands 4-fluorosalicylic acid and N,N-diethylnicotinamide
were of analytical grade, and were purchased from Aldrich Chem-
ical Co. They are symbolized by HA and B in their neutral forms,
respectively, when giving the composition of various complexes.
Doubly deionized water and freshly distilled methanol were used
as solvents. Copper(II) perchlorate (Fluka) solutions were stan-
dardized complexometrically. The pH-metric titrations were per-
formed with NaOH (Aldrich) standard solution. To ensure acidic
starting conditions, HClO4 (Aldrich) was added to the solutions be-
fore titration. Other reagents were of analytical grade, supplied by
Aldrich or Sigma, and were used as received.
system, and the ligand concentrations between 8.9 ꢄ 10ꢀ4
–
1.8 ꢄ 10ꢀ3 M for HA and 4.5 ꢄ 10ꢀ4–1.0 ꢄ 10ꢀ1 M for B.
2.4. UV–Vis spectroscopic measurements
UV–Vis absorption spectra in aqueous solutions were recorded
by means of Ocean Optics PC2000 plug in fiber optic spectropho-
tometer, in the wavelength interval from 350 to 800 nm, 1 cm opti-
cal pathlength, and by a Hewlett Packard 8452 diode array
spectrophotometer in a quartz cell of 4 cm optical pathlength.
2.5. X-ray crystallography
2.2. Preparation of [Cu(4-fluorosalicylate)2(N,N-
diethylnicotinamide)2(H2O)2] complex
Data collection and cell refinement were carried out using a
axis diffractometers Bruker Kappa APEXII CCD [52] at 150 K with
graphite monochromated Mo K radiation. The diffraction intensi-
j-
The title compound was prepared by mixing an aqueous solu-
tion of copper(II) acetate (0.32 mmol) with an aqueous solution
(about 80 mL) containing mixture of 4-fluorosalicylic acid
(0.64 mmol) with N,N-diethylnicotinamide (0.64 mmol). The reac-
tion mixture was stirred until the reaction finished and the color of
the product remained unchanged. The light blue microcrystalline
product which precipitated was filtered off and mother liquor
was left to crystallize at ambient temperature. The blue crystals
suitable for X-ray structure determination were separated after
several days and dried at ambient temperature.
a
ties were corrected for Lorentz and polarization factors. The struc-
ture was solved by direct methods using SIR-97 [53] and refined by
the full-matrix least-squares procedure with SHELXL-97 [54]. The
multi-scan absorption correction was applied the program SADABS
[55]. Geometrical analyses were performed with SHELXL-97. The
structures were drawn with XP in SHELXTL [54] and PLATON [56]. The
crystal data, conditions of data collection and refinement are re-
ported in Table 1.
2.6. ESR measurements
2.3. pH-metric measurements
The total (analytical) copper(II) concentration, TCu was 1 mM in
all cases. For the Cu(II)–HA system, the total ligand concentration
TA = 2 mM was applied, while for the Cu(II)–B system, TB was 2, 5
or 25 mM. For the Cu(II)–HA–B system, TA was 2 mM and TB was
The protonation and coordination equilibria were investigated
by potentiometric titrations in aqueous solution (I = 0.1 M NaClO4,
and T = 298.0 0.1 K) under argon atmosphere, using an automatic
titration set including a PC controlled Dosimat 665 (Metrohm)
autoburette and an Orion 710A precision digital pH-meter. The
Metrohm semimicro combined pH glass electrode (125 mm) was
calibrated [49] via the modified Nernst Eq. (1):
Table 1
Crystallographic data for 1 complex.
1
JOH ꢁ Kw
E ¼ E0 þ K ꢁ log½Hþꢂ þ JH ꢁ ½Hþꢂ þ
ð1Þ
Code
Chemical formula
Mr
RS36ml1
34H40CuF2N4O10
766.24
½Hþꢂ
C
where JH and JOH are fitting parameters in acidic and alkaline media
for the correction of experimental errors, mainly due to the liquid
junction and to the alkaline and acidic errors of the glass electrode;
Kw = 10ꢀ13.75 M2 is the autoprotolysis constant of water [50]. The
parameters were calculated by the non-linear least squares method.
The complex formation was characterized by the following general
equilibrium process (2):
ꢀ
Cell setting, space group
triclinic, P1
150(2)
T (K)
a (Å)
b (Å)
c (Å)
7.5360(5)
8.2930(4)
14.4060(12)
85.312(6)
81.244(7)
76.513(6)
864.39(10)
1
a
(°)
b (°)
c
(°)
V (Å3)
pCu þ qH þ rA þ sB b ꢀ r s CupHqArBs
½CupHqArBsꢂ
Cu
H
q
A
B
Z
p
Radiation type
Mo Ka
0.706
l
(mmꢀ1
)
bCu
¼
ð2Þ
Crystal size (mm)
Diffractometer
Absorption correction
Tmin, Tmax
0.497 ꢄ 0.216 ꢄ 0.176
Bruker Kappa APEXII CCD
SADABS
0.719, 0.883
1.084
0.0360, 0.0781
3548/0/235
0.186, ꢀ0.164
pHqArBs
p
q
r
s
½Cuꢂ ½Hꢂ ½Aꢂ ½Bꢂ
where Cu denotes the metal ion, while A and B the non-protonated
ligands (A = 4-fluorosalicylate anion and B = N,N-diethylnicotina-
mide molecule). Here and in the figures the charges are omitted
for simplicity, but they can easily be calculated taking into account
the compositions and charges of the fully protonated ligands. The
S
R1[F2 > 2
r
(F2)], wR2(F2)
Data/restrains/parameters
D
qmax
,
D
qmin (e Åꢀ3
)

T. Szabó-Plánka et al. / Polyhedron 30 (2011) 2421–2429
2423
2, 5 or 25 mM. The pH was adjusted with NaOH (0.2 M) to a value
between 4.60 and 4.67, to an accuracy of 0.01 pH unit, measured
with a Radiometer PHN 240 pH-meter equipped with a Metrohm
LL combined microelectrode, which was calibrated with IUPAC
Standard Buffers (Radiometer). The ESR spectra were recorded at
298 K using an X-band Bruker EleXsys E500 instrument. Before
the measurements, the signal of the capillary tube filled with dis-
tilled water was recorded as background.
conditions for their pH-metric detection. In order to promote com-
plexation, we varied the metal-to-ligand ratio from 1:1 to 1:100.
The buffering effect of the large quantity of the ligand at high li-
gand excess, however, prevented to detect significant pH effect
attributed to the complex formation. Finally, in the acidic pH range
the complex [CuB]2+ could only be detected by pH-metry (Table 2).
On the other hand, the UV–Vis absorption maxima of the solutions
with increasing ligand excess continuously shifted from ꢅ800 nm
(Cu:B = 1:2) to ꢅ706 nm (Cu:B = 1:100) at pH ꢅ4.5. This clearly
shows that parent complexes with more ligand molecules were
formed in the solutions, too. Similarly, stepwise formation of the
parent complexes was detected by the ESR measurements under
high ligand excess (see Section 3.3). The formation of these, how-
ever, occurred in the pH range where B was in non-protonated
form, therefore, it did not cause a significant pH effect, and hence,
the stability constant for these species could not be determined
accurately by pH-potentiometry.
The low stability of these complexes was not able to prevent the
hydrolytic processes of copper(II) around pH 6.5. It was also ob-
served that the deprotonation step(s) – resulting in the formation
of Cu(OH)2 precipitate and/or [CuHꢀ1B]+ and CuHꢀ2B complexes
– were shifted to higher pH (by about 0.5 unit) at 100-fold ligand
excess, indicating that more ligand molecules around the metal
ion can hinder the hydrolytic processes.
Samples of 0.100 cm3 volume were taken from the solutions at
298 K, 0.010 cm3 methanol was added to each, and after mixing
they were frozen in liquid nitrogen. Then the ESR spectra were re-
corded at 77 K with the same spectrometer as above.
2.7. Evaluation of ESR spectra
Analysis of the isotropic spectra recorded at 298 K was pre-
ceded by the elimination of the background signal. Then the spec-
tra were evaluated by the EPR program [57], which allows to take in
consideration one to four component curves. The ESR spectra of the
various species were described by the parameters g0, the copper
hyperfine coupling constant A0, the nitrogen superhyperfine cou-
pling constant aN0 and the relaxation parameters a, b and c relat-
ing to the line widths of the copper hyperfine multiplet as
WM
¼
a
þ bMI þ
c
M2 (MI is the magnetic quantum number of cop-
I
I
The ternary complex formation in aqueous solution was inves-
tigated by pH-potentiometric titrations carried out under similar
conditions than the crystallization experiments (see later), in
1:2:2 or 1:2:4 copper(II):HA:B systems. The stability constants of
the species yielding the best fit with the experimental titration
curves are collected in Table 2. The species distribution diagram
of the latter system (Fig. 1) shows that the ternary complexes dom-
inate over the binary ones in the whole pH range. The first com-
plexes were [CuB]2+ and [CuA2B2] with concentration maxima at
pH 4.0 and 4.3, respectively. The latter is most probably the species
that crystallized from the solution (see Section 3.2). Under these
conditions, a substantial amount of the aqua complex is still pres-
ent, but significantly less than in the Cu:HA 1:2 or Cu:B 1:4
systems, indicating the enhanced formation of the ternary com-
plexes. The shift of the absorption maximum (kmax ꢅ 750 nm) com-
pared to the binary systems (kmax ꢅ 765 nm for Cu:HA 1:2, and
kmax ꢅ 798 nm for Cu:B 1:4) suggests an increased ligand field
around the metal ion, in accordance with the coordination of two
carboxylate oxygens of molecules A, and two pyridyl nitrogens
from ligands B, similar to the coordination mode in the crystals.
The next deprotonation step results in a chelate-type complex
by the coordination of the phenolate and carboxylate oxygens of
one of the 4-fluorosalicylate ligands. The development of the band
at ꢅ390 nm in the UV–Vis absorption spectra corresponding to the
phenolate–copper(II) charge transfer (CT) [45], and the ESR spectra
also support this. Simultaneously, the rearrangement of the coordi-
nation sphere occurs: one of the B or the second A ligand must be
per nuclei). Since a natural mixture of copper isotopes was used,
the spectra were calculated as the sum of the curves of molecules
containing isotope 63Cu or 65Cu weighted by their natural abun-
dances. The hyperfine coupling constants and the relaxation
parameters given in the Tables refer to the 63Cu isotope. The cou-
pling constants and the relaxation parameters are given in gauss
(G) units throughout the paper 1 G = 10ꢀ4 T.
The quality of fit for the jth spectrum was characterized by the
noise-corrected regression parameter Rj computed from the aver-
age square deviation between the respective experimental and cal-
culated curves. The noise was deduced from the quadratic error of
the fit to obtain Rj = 1 for perfect fit.
For the anisotropic ESR spectra, the EPR program allows the
description of the experimental spectra as the superposition of
one to three component curves. The spectral fits (characterized by
the noise-corrected regression parameter Rj, see above) achieved
with the assumption of either axial or rhombic g-, hyperfine, super-
hyperfine, and quadrupole coupling tensors were compared to gain
information about the symmetry of the coordination polyhedron in
various species. Anisotropy of the relaxation parameters a, b and c
was also taken into consideration. In some cases, a rhombic zero-
field splitting was also considered. Calculated spectra were com-
posed of the curves of complexes containing isotope 63Cu or 65Cu
as for the isotropic spectra.
3. Results and discussion
3.1. Solution speciation in the copper(II)–HA–B system
Table 2
The stability constants of the species included in the characterization of the
copper(II)–A–B system (4-fluorosalicylic acid = HA and N,N-diethylnicotinamide = B).
Under the conditions described in the experimental part, 4-flu-
orosalicylic acid (HA) undergoes one deprotonation process up to
pH 11, which is related to the carboxylic group with a pKa of
2.76 [48]. Similarly, only one protonation process of the N,N-dieth-
ylnicotinamide (B) was observed. The proton complex was defined
as [HL]+, its pKa = 3.34(1) is very close to that determined for nico-
tinamide (pKa = 3.3 [58]; pKa = 3.6 0.2 [59]; pKa = 3.42; [60];
pKa = 3.42 [61]).
Since there were no literature data under the present conditions
for the copper(II)–B system, we first investigated the binary com-
plexes. The low pKa of B, as expected, allows for the formation of
parent complexes with low stability, and therefore, rather poor
Species
log b
2.76a
HA
[HB]+
3.34(1)
CuHꢀ1A
ꢀ2.59a
[CuHꢀ2A2]2ꢀ
[Cu2Hꢀ3A2]ꢀ
[CuB]2+
ꢀ7.70a
ꢀ10.37a
2.47(4)
CuA2B2
11.91(4)
3.79(1)
ꢀ7.55(2)
CuHꢀ1AB2
[CuHꢀ2AB]ꢀ
a
From Ref. [48]. The uncertainty of the constants is shown in parentheses.

2424
T. Szabó-Plánka et al. / Polyhedron 30 (2011) 2421–2429
Table 3
100
80
60
40
20
0
Selected geometric parameters (Å, °).
CuH-2 AB
Cu(II)
1
CuH AB2
-1
Cu1–N1
Cu1–O1
Cu1–O1W
2.006(2)
1.984(2)
2.422(2)
%Cu(II)
O1–Cu1–N1
O1–Cu1–O1W
N1–Cu–O1W
90.07(7)
92.65(7)
92.01(7)
CuA2B2
CuB
CuH A
-1
CuH-2 A2
O1Wꢁ ꢁ ꢁO2
2.723(3)
1.92
160
2.789(3)
1.97
165
2.536(3)
1.79
146
H1Wꢁ ꢁ ꢁO2
Cu2H-3A2
O1W–H1W–O2
O1Wꢁ ꢁ ꢁO4ii
H2Wꢁ ꢁ ꢁO4ii
O1W–H2W–O4ii
O3ꢁ ꢁ ꢁO2
2
3
4
5
6
7
8
9
10
11
pH
Fig. 1. The species distribution diagram for the Cu:A:B = 1:2:4 system calculated on
the basis of the stability constants depicted in Table 1. The ternary species are
represented by solid lines while other species with dashed lines.
H3Oꢁ ꢁ ꢁO2
O3–H3Oꢁ ꢁ ꢁO2
c
Cu(II) = 5.0 ꢄ 10ꢀ4 M; I = 0.1 M, NaClO4; T = 298 K.
Symmetry codes: (i) ꢀx + 1, ꢀy + 1, ꢀz + 1; (ii) ꢀx + 2, ꢀy, ꢀz + 1.
pulled out from the equatorial coordination. It is worth to note that
the deprotonation in the ternary system occurred at lower pH than
in the Cu:A binary system, and so the overlapping equilibria make
the assignment of the macroscopic processes to the individual
deprotonation steps ambiguous. The intensity of the above-men-
tioned UV-band suggests somewhat higher amount of the deproto-
nated complexes than the macroscopic species distribution does.
The spectral data suggest that the composition of this complex is
[CuHꢀ1AB2] rather than [Cu(Hꢀ1A)AB]ꢀ (see later). The formation
of neutral ternary complexes ([CuA2B2] and [CuHꢀ1AB2]) may en-
hance the passage of B across the hydrophobic cell membranes
by passive diffusion, while the passage for the charged molecules
(like HB+ in acidic media) is expected to be slow in the absence
of a carrier [62].
At pH ꢅ7.0, a new deprotonation process started that could be
best fitted with the composition of [CuHꢀ2AB]ꢀ, similarly to the
ternary system copper(II)–3-pyridylmethanol–5-fluorosalicylic
acid [63]. In this complex, one of the B ligands is probably replaced
by a deprotonated water molecule, resulting in the {COOꢀ, pheno-
late-Oꢀ, pyridyl-N, OHꢀ} coordination around the copper(II) ion.
The coordination modes for two of the mixed–ligand complexes
could be characterized in more detail: [CuA2B2] was obtained also
in the crystalline state, and it was investigated by X-ray diffraction
and UV–Vis and IR spectroscopy, while ESR spectroscopic data of-
fered information on the structure of [CuHꢀ1AB2] in solution. Fur-
thermore, the ESR spectroscopic studies revealed some features of
the copper(II)–B binary equilibrium system, too.
3.2. Structure and spectral properties of [CuA2B2(H2O)2]
The principal structural features of [CuA2B2(H2O)2] (1) are illus-
trated in Fig. 2. Selected bond lengths and angles as well as struc-
tural parameters of hydrogen bonds of 1 are shown in Table 3. The
coordination environment of the copper atom is elongated tetrag-
onal bipyramidal. The tetragonal plane is built up by a pair of uni-
dentate Aꢀ anions using carboxylate oxygen atoms [Cu1–
O1 = 1.984(2) Å] and by a pair of neutral B molecules using pyri-
dine ring nitrogen atoms [Cu1–N1 = 2.006(2) Å] in trans positions.
The two axial positions of tetragonal bipyramid are occupied by
two coordinated water molecules [Cu1–O1W = 2.422(2) Å], thus T
parameter [64] is 0.82. Intramolecular hydrogen bonds involving
an axial coordinated water molecule and uncoordinated carboxyl-
ate oxygen atoms [O1W–H1Wꢁ ꢁ ꢁO2, with O1Wꢁ ꢁ ꢁO2 distance
2.723(3) Å], stabilize the molecular structure of 1. The hydroxyl
groups of Aꢀ anions are connected into intramolecular hydrogen
bonds [O3–H3Oꢁ ꢁ ꢁO2, with O3ꢁ ꢁ ꢁO2 distance 2.536(3) Å] in six-
membered intramolecular rings S(6) [65], where the acceptors of
hydrogen bonds are also uncoordinated carboxylate oxygen atoms
of Aꢀ anions. Similar coordination environments around the cop-
per atoms have been observed in the crystal structures of some
methyl- and methoxysalicylatocopper(II) complexes [29,30] as
well as other [Cu(RCOO)2(den)2(H2O)2] complexes [17,20,24–28].
The orientation of the pyridine rings in relation to the equatorial
planes of the coordination polyhedra, and the value of the dihedral
Fig. 2. Perspective view of 1 complex, with the atom numbering scheme. Thermal ellipsoids are drawn at the 30% probability level.

T. Szabó-Plánka et al. / Polyhedron 30 (2011) 2421–2429
2425
Fig. 3. Supramolecular chain from molecules of 1, connecting through O–Hꢁ ꢁ ꢁO hydrogen bonds.
atoms of B ligands of adjacent complexes molecules through
R22(16) rings [65]. The R22(16) rings [65] form supramolecular
chains in direction [1 1 0] (Fig. 3). The supramolecular chains are
a
ꢀ
b
c
d
connected through
pyridine aromatic rings of B ligands of two neighboring complex
p–p stacking interactions [66] between two
[ꢀx + 1, ꢀy, ꢀz + 1]. The plane–plane and centroid–centroid dis-
tances of
respectively.
In the IR spectrum of the complex, the bands assigned to
as(COOꢀ) and s(COOꢀ) are at 1610 and 1369 cmꢀ1, respectively.
The value of for
s = 241 cmꢀ1) is greater than
p–p stacking interactions are 3.38 and 3.47 Å,
m
m
e
D
m
(D
m
=
mas
ꢀ
m
Dm
the ionic form of the 4-fluorosalicylate salt (158 cmꢀ1), and corre-
sponds to an unidentate mode of coordination [67]. The band at
3494 cmꢀ1 corresponds to the OH vibration and confirms the pres-
ence of water molecules in the compound.
f
g
The solid state electronic spectrum of the complex exhibits a
broad asymmetrical absorption band attributed to d d transi-
tions with maximum positioned at about 631 nm, typical of tetrag-
onal–bipyramidal copper(II) [68].
h
3.3. ESR spectra in liquid solution at 298 K
The isotropic ESR spectra recorded at pH ꢅ4.6 are shown in
Fig. 4. The curves of the Cu(II)–B system at a 2- or 5-fold excess
of ligand are very similar to the spectrum of the CuCl2 solution
{containing the aqua complex [Cu(aqua)]2+}. Evaluating with one
component spectrum, the parameters slightly differed from those
of the aqua complex, however, if two or three components were
considered, the ESR parameters became uncertain. Most probably,
the major species is the aqua complex in these solutions, too, and
the minor species also have broad, unresolved spectra with similar
g and A that makes the decomposition of experimental curves
uncertain. At 25-fold excess of B (Fig. 4d), two component spectra
could be identified. Comparing their parameters (Table 4) to the
data of the 3-pyridylmethanol complexes [47], the major one can
be assigned to [CuB]2+, while the minor one to [CuB2]2+. In contrast
to the Cu(II)–3-pyridylmethanol system, the complexes [CuB3]2+
and [CuB4]2+ could not be shown at all, also indicating lower com-
plexing ability of B, which can be explained by much lower basicity
of its pyridyl nitrogen.
Fig. 4. Isotropic ESR spectra in aqueous solution at 298 K; (a) CuCl2 solution, pH
3.8; (b)–(e) binary systems at TCu = 1 mM: (b) TB = 2 mM, pH 4.66; (c) TB = 5 mM, pH
4.67; (d) TB = 25 mM, pH 4.65; (e) TA = 2 mM, pH 4.60; (f)–(h) ternary systems at
TCu = 1 mM and TA = 2 mM: (f) TB = 2 mM, pH 4.67; (c) TB = 5 mM, pH 4.66; (d)
TB = 25 mM, pH 4.67.
angle [38.2°] between the pyridine ring plane and the equatorial
plane of the coordination polyhedron N1–O1–Cu–O1i–Ni of com-
plex 1 are very similar to those observed in the crystal structures
of polymorph-II of [Cu(3-Mesal)2(den)2(H2O)2] (3-Mesal = 3-meth-
ylsalicylate anion) [29] and complex [Cu(4-MeOsal)2(den)2(H2O)2]
(4-MeOsal = 4-methoxylsalicylate anion) [29]. All complexes show
similar orientations of carboxamide groups, thus the C9–C10–C14–
O4 torsion angles have similar values: 64.3° for 1, 52.0° for [Cu
(3-Mesal)2(den)2(H2O)2] [29], and 63.3° for [Cu(4-MeOsal)2
(den)2(H2O)2] [29].
The molecules of 1 are joined by intermolecular hydrogen
bonds [O1W–H2Wꢁ ꢁ ꢁO4ii [Symmetry code: (ii) ꢀx + 2, ꢀy, ꢀz + 1),
with O1Wꢁ ꢁ ꢁO4ii distance of 2.789(3) Å], between the hydrogen
atoms of the coordinated water molecules and the amide oxygen
In the Cu(II)–HA system at pH 4.61 (Fig. 4e), the formation of
two minor species{[Cu(aqua)]2+ and [CuA]+} and a major complex
{[CuH-1A]} can be expected [48]. Accordingly, the experimental
curve was decomposed into three component spectra. The param-

2426
T. Szabó-Plánka et al. / Polyhedron 30 (2011) 2421–2429
a
Rj=0.996918
[CuH-1A]
50 %
b
c
d
[CuH-1AB2]
40 %
e
Fig. 5. Experimental curve (black line) taken in aqueous solution at 298 K, at
TCu = 1 mM, TA = 2 mM and TB = 5 mM analytical concentrations, together with the
curve calculated as a four-component spectrum (gray line). Its major components
are shown below (gray line).
f
g
h
eters for the major one (Table 4) are in satisfactory agreement with
the data for [CuHꢀ1A], obtained by the two-dimensional evaluation
method in 50 v/v% methanol/water solvent mixture [48].
For the Cu(II)–HA–B system, the spectra (Fig. 4f and g) are rem-
iniscent of the former one, however, a fourth, new component is
also necessary for the good spectral fit (Fig. 5). The weight of this
component curve (i.e. the concentration of the corresponding com-
plex in the solution) increases in parallel with the increasing
amount of B, while the contribution of [CuHꢀ1A] decreases (Table
4). At high excess of B, the new species becomes predominant (Ta-
ble 4 and Fig. 4h). In accordance with the pH potentiometric find-
ings, probably it is the mixed–ligand complex [CuHꢀ1AB2].
Compared to [CuHꢀ1A], its lower g0 and higher A0 reflect on a
stronger ligand field, which can be attributed to the equatorial
coordination of the two B ligands through their pyridyl nitrogens.
The line-broadening (Fig. 5) can be explained by a faster ligand ex-
change allowed by the monodentate coordination of molecules B.
A similar phenomenon occurs in the Cu(II)–3-pyridylmethanol sys-
tem [47], too.
Fig. 6. Anisotropic ESR spectra in frozen 9 v/v% methanol/water at 77 K; (a) CuCl2
solution, pH 3.8; (b)–(e) binary systems at TCu = 1 mM: (b) TB = 2 mM, pH 4.66; (c)
TB = 5 mM, pH 4.67; (d) TB = 25 mM, pH 4.65; (e) TA = 2 mM, pH 4.60; (f)–(h) ternary
systems at TCu = 1 and TA = 2 mM: (f) TB = 2 mM, pH 4.67; (c) TB = 5 mM, pH 4.66; (d)
TB = 25 mM, pH 4.67.
The concentration data in Table 4 suggest that the binary com-
plex [CuHꢀ1A] is more favoured in the ternary system than
Table 4
Isotropic ESR parametersa for the various complexes formed in the binary and ternary systems in aqueous solution at 298 K.
TCu (mM)
TA (mM)
TB (mM)
pH
g0
A0 (G)
a
(G)
b (G)
c
(G)
Species
W (%)
1
1
0
0
0
25
3.80
4.65
2.191
2.177
30.7
42.0
51.0
f
50.0
48.2
36.0
f
ꢀ2.1
0.0
1.7
16.9
f
[Cu(aqua)]2+
[CuB]2+
100
81
19
11
3
86
9
3
ꢀ1.4
2.150
ꢀ14.8
[CuB2]2+
1
1
2
2
0
2
4.61
4.67
fb
f
f
[Cu(aqua)]2+
f
f
f
f
[CuA]+
2.159
f
f
54.6
f
f
30.7
f
f
ꢀ7.3
0.6
f
f
[CuHꢀ1A]
[Cu(aqua)]2+
[CuA]+
f
f
2.160
2.151
f
55.1
60.6
f
29.9
38.9
f
ꢀ9.7
ꢀ1.1
f
1.8
ꢀ0.4
f
[CuHꢀ1A]
[CuHꢀ1AB2]
[Cu(aqua)]2+
[CuA]+
[CuHꢀ1A]
[CuHꢀ1AB2]
[Cu(aqua)]2+
[CuHꢀ1AB2]
58
30
8
1
1
2
2
5
4.66
4.67
f
f
f
f
f
2
2.160
2.151
f
55.0
61.7
f
29.8
40.7
f
ꢀ9.5
ꢀ4.0
f
ꢀ2.8
1.9
0.0
f
50
40
2
25
2.153
59.7
44.8
1.1
98
a
The estimated error for g0 is 0.001, while for A0 and the relaxation parameters it falls between 0.1 and 1.0 G.
‘f’ means that the respective parameter was fixed in the course of parameter fitting; for [Cu(aqua)]2+, the values obtained from the analysis of the spectrum of the CuCl2
b
solution, while for [CuA]+, literature data from Ref. [47] were used.

T. Szabó-Plánka et al. / Polyhedron 30 (2011) 2421–2429
2427
Table 5
Anisotropic ESR parametersa for the various complexes formed in the binary and ternary systems in 9 v/v% methanol/water solution at 77 K.
b
TCu (mM) TA (mM) TB (mM) pH
gxx
gyy
gzz
g0
Axx (G) Ayy (G) Azz (G) A0 (G)b aN\ (G)c D (G) E (G) Complex
W (%)
1
1
0
0
0
2
3.80 2.081 2.081 2.424 2.195
4.66 2.070 2.070 2.371 2.170
2.063 2.063 2.326 2.151
2.0
1.1
1.8
1.3
2.0
1.1
1.8
1.3
14.3
5.1
111.9
126.0
143.0
141.1
166.5
167.7
109.4
130.0
38.6
42.7
48.9
47.9
65.0
59.3
37.5
50.9
[Cu(aqua)]2+ 100
1.1
2.0
0.0
7.6
14.7
[CuB]2+
42
58
55
45
100
43
[CuB2]2+
1
0
5
4.67 2.055 2.055 2.319 2.143
[CuB2]2+
2.064 2.064 2.294 2.141 14.3
4.65 2.057 2.057 2.280 2.131
4.01 2.081 2.081 2.423 2.195
2.066 2.066 2.365 2.166 11.3
2.290 2.081 2.436 2.269
4.67 2.061 2.061 2.314 2.145 15.6
2.230 2.059 2.369 2.219
4.66 2.060 2.060 2.300 2.140 13.3
2.270 2.065 2.371 2.235
4.67 2.054 2.054 2.273 2.127
[CuB3]2+
1
1
0
2
25
0
5.1
1.6
[CuB4]2+
1.6
11.3
[Cu(aqua)]2+
[CuHꢀ1A]
Dimer 1
40
17
537
517
525
ꢀ1
1
1
1
2
2
2
2
5
15.6
13.3
6.0
147.9
159.6
169.6
59.7
62.1
60.5
13.1
12.4
14.4
[CuHꢀ1AB2]
Dimer 2
60
40
ꢀ17
ꢀ17
[CuHꢀ1AB2]
Dimer 2
70
30
100
25
6.0
[CuB4]2+
a
The estimated error for g is 0.001, while for A it falls between 0.1 and 1.0 G in the case of monomeric complexes. For the dimeric species, g is much less reliable, its error is
likely to be in the order of magnitude of 0.01.
b
Average of the principal values of the corresponding tensor.
The nitrogen superhyperfine coupling tensor was assumed to be axial; aNxx = aNyy are symbolized by aN\; aNzz is not given in the Table, since the broad parallel lines did
c
not allow to obtain a reliable value for it.
[CuHꢀ1AB2], in contrast to pH-potentiometric findings. This can be
explained, on the one hand, by the fact that pH-potentiometry
seems to overestimate the mixed–ligand complex formation, com-
pared also to UV–Vis spectral data (see Section 3.1). On the other
hand, the decomposition of individual isotropic ESR spectra yields
less reliable relative concentrations than the simultaneous analysis
of series of spectra (elaborated for binary systems), and can over-
estimate the contribution of well-resolved spectra – [CuHꢀ1A] –
at the expense of curves with broader lines as for [CuHꢀ1AB2].
same solutions at 298 K (Tables 4 and 5). It should be noted that,
compared to [CuHꢀ1A], the coordination of the two B ligands in-
creases ligand field strength in solid phase, too, which is reflected
in the significant decrease in g and increase in A for [CuHꢀ1AB2]
(Table 5).
A further evidence for the favoured ligation of B at low temper-
ature is that in frozen solution this ligand, present in high excess,
replaces even the chelating ‘‘Hꢀ1A’’: at TB = 25 mM, the spectra of
the Cu(II)–B and Cu(II)–HA–B systems are closely similar regarding
both their g and A values (Table 5) and superhyperfine patterns
(Fig. 6). In the mixed ligand complex [CuHꢀ1AB2], a 2 N splitting
is expected, however, the resolved superhyperfine structure in
the perpendicular region cannot be described by the 2 N model
(Fig. 7). We could describe the positions of most superhyperfine
lines by the 3 N model, similar to the 4 N model, however, only
the latter model could describe satisfactorily also the intensity dis-
tribution of the lines (Fig. 6), suggesting the equatorial coordina-
tion of four ligands B.
In the systems Cu(II)–HA and Cu(II)–HA–B, in frozen solution
there is a spectral component with considerable zero-field splitting
and very broad lines (Figs. 8 and 9 and Table 5), which suggests
oligomerization processes. The nature of these oligomers is diffi-
cult to characterize. In the Cu(II)–4-fluorosalicylic acid system, a
small amount of [Cu2Hꢀ2A2] was shown in liquid solution, where
two monomeric units are bridged by the mutual equatorial ligation
of the phenolate oxygens to the neighboring metal ions [47]. Prob-
ably, the above spectral component (Fig. 8) can be assigned to this
species. For the mixed ligand system, the ratio of the spectral
3.4. ESR spectra in frozen solution at 77 K
The anisotropic spectra are shown in Fig. 6. Their well-resolved
character allowed a reliable spectrum-decomposition, offering fur-
ther information on the coordination modes. It should be noted
that for the Cu(II)–HA binary system, low temperature promoted
also the formation of [CuHꢀ2A2]2ꢀ, resulting a multiply superim-
posed spectrum at pH 4.67, therefore, we choose the spectrum of
a more acidic solution where an unique spectrum decomposition
could be carried out. The anisotropic ESR parameters and the rela-
tive concentrations for the various species are summarized in Table
5. It can be concluded from the concentration data that freezing in-
duced considerable changes in speciation: (a) it promoted the
coordination of B to the metal ion both in the Cu(II)–B and
Cu(II)–HA–B systems, and (b) led to the formation of oligomeric
species. The former fact is not surprising: Szabó-Plánka et al. have
shown [47] that – besides the fact that glass-forming compounds
can lower the equilibrium freezing point – a significant undercool-
ing occurs upon fast freezing of the samples, too, so the equilibria
‘‘freeze’’ at considerably lower temperature than the freezing point
of dilute aqueous solutions (ꢅ273 K). If the formation enthalpies of
various species differ from each other significantly, their formation
constants alter to different extent upon freezing, in terms of the
van’t Hoff equation, as it was observed with the Cu(II) – 3-pyridyl-
methanol system, too.
4N
3N
For the Cu(II)–B system, though in liquid solution only [CuB]2+
and [CuB2]2+ could be shown, in frozen solution we could identify
also [CuB3]2+ and [CuB4]2+, moreover, the latter was predominant
at 25-fold excess of B. Compared to 3-pyridylmethanol, the lower
basicity of the donor N of this ligand is manifested in weaker
Cu–N bonds, and this leads to higher gk and lower Ak for the B
(Table 5) than for the corresponding 3-pyridylmethanol complexes
[47].
2N
Fig. 7. The experimental curve (black line) taken in 9 v/v% methanol/water at 77 K,
at TCu = 1 mM, TA = 2 mM and TB = 25 mM analytical concentrations, pH 4.65,
together with the spectra calculated with a 4, 3 or 2 N superhyperfine splitting
model (gray line).
For the Cu(II)–HA–B system, the concentration of the mixed–li-
gand complex is significantly higher in the frozen state than in the

2428
T. Szabó-Plánka et al. / Polyhedron 30 (2011) 2421–2429
there is not enough experimental evidence for or against the struc-
tures of dimeric species.
Rj=0.997473
Finally, it should be noted that the differences between the
parameters of the same complex, obtained by the decomposition
of different anisotropic spectra, slightly exceed the experimental
errors in some cases (Table 5). The most probable reason for this
is that, in most cases, the anisotropic spectra were described as
the superpositions of two component curves, though different
minor complexes might also be present in the solutions in small
concentration. A minimum concentration of 10–20% is necessary
for the consideration of a species in the decomposition of an aniso-
tropic spectrum. If a complex of lower concentration is included in
the model, the uncertainties of all fitted parameters may increase
considerably. If, in turn, the corresponding minor species is omit-
ted, the parameters assigned to the major species are slightly mod-
ified by the contribution of the neglected species, or, in other
words, they are, in fact, average values.
[Cu(aqua)]2+
43 %
[CuH-1A]
40 %
Dimer 1
17 %
4. Conclusions
The ligand N,N-diethylnicotinamide is a weak complexing
agent, in accordance with the low basicity of its pyridyl nitrogen.
At low temperature, its complexation is much more favoured than
at 298 K. When both N,N-diethylnicotinamide and 4-fluorosalicylic
acid are present, the formation of mixed–ligand complexes is
favoured. In acidic solution, the monodentate ligation of two 4-flu-
orosalicylate anions and the coordination of two N,N-diethylnicoti-
namide molecules occur, and in the crystalline state two water
molecules occupy the axial sites of the elongated square-bipyrami-
dal complex, which take part in extensive H-bond network. In
moderately acidic solution, where the deprotonated 4-fluorosali-
cylic acid is chelated to copper(II) by its carboxylate and phenolate
oxygens, also the simultaneous ligation of two N,N-diethylnicoti-
namide molecules occurs. This mixed–ligand complex is a major
species at a moderate, while predominant at a 25-fold excess of
N,N-diethylnicotinamide. At low temperature, four molecules of
the latter ligand are coordinated to copper(II), if it is present in
high excess, replacing 4-fluorosalicylic acid. Dimers with a consid-
erable zero-field splitting in their spectra are also present in frozen
solution; their amount decreases with increasing excess of N,N-
diethylnicotinamide.
Fig. 8. Top: experimental curve (black line) taken in
9 v/v% methanol/water
solution at 77 K, at TCu = 1 mM and TA = 2 mM analytical concentrations, pH 4.01,
together with the curve calculated as a three-component spectrum (gray line). The
component spectra are shown by gray lines (bottom).
Rj=0.997085
[CuH-1AB2]
60 %
Acknowledgments
The work was performed in the framework of Hungarian–Slo-
vak Intergovernmental S&T Cooperation (Project SK-25/2006). Fur-
thermore, the financial support of the Hungarian Scientific
Research Fund OTKA (Grants K72781, F67581 and NI61786) and
the Slovak Academy of Sciences (Grants VEGA 1/2452/05 and
APVT-20-005504) are gratefully acknowledged.
Dimer 2
40 %
Appendix A. Supplementary data
CCDC 821897 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Fig. 9. Top: experimental curve (black line) taken in
9 v/v% methanol/water
solution at 77 K, at TCu = 1 mM, TA = 2 mM and TB = 2 mM analytical concentrations,
pH 4.67, together with the curve calculated as a two-component spectrum (gray
line). The component spectra are shown by gray lines (bottom).
component with zero-field splitting is even higher, so its presence
cannot be explained by the dimerization of the minor [CuHꢀ1A]
alone (if the latter is formed at all under these conditions). Pheno-
late bridges may connect the monomers of the mixed–ligand com-
plex, too, or carboxylate bridges can be formed between the
monomeric units of [CuA2B2]. These changes would require a con-
siderable rearrangement in the coordination sphere. However,
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