Coordination environment of new Co(II), Ni(II) and Cu(II) complexes with 4-bromophenoxyacetic acid: Structural, spectroscopic and theoretical studies

Article abstract of DOI:10.1016/j.poly.2017.05.005

We report the synthesis of new Co(II), Ni(II) and Cu(II) complexes with 4-bromophenoxyacetic acid. We characterize these compounds structurally using various laboratory (XRF, IR, UV–vis) and synchrotron (EXAFS, XANES) spectroscopic methods, magnetic measurements and DFT calculations. We found that in the powder form of complexes Co(II) and Ni(II) cations are coordinated by two monodentate carboxylate ligands and the first coordination sphere is completed by water molecules that form an octahedron. The coordination polyhedron of Cu(II) exhibits a distorted tetragonal–pyramidal geometry with two ligands coordinating in bidentate and monodentate fashion via carboxylate O atoms and two water molecules. The X-ray single crystal structure analysis performed for Cu(II) complex after its recrystallization from N,N-dimethylformamide (DMF) solution revealed the formation of dinuclear complex. In crystals, the carboxylate O atoms occupy the bases of two tetragonal pyramids formed around two Cu(II) ions with the DMF molecules in the apical positions.

Full text of DOI:10.1016/j.poly.2017.05.005

Accepted Manuscript
Coordination environment of new Co(II), Ni(II) and Cu(II) complexes with 4-
bromophenoxyacetic acid: Structural, spectroscopic and theoretical studies
Aleksandra Drzewiecka-Antonik, Wiesława Ferenc, Paweł Rejmak, Anna
Wolska, Marcin T. Klepka, Beata Cristóvão, Barbara Mirosław, Jan Sarzyński,
Dariusz Osypiuk
PII:
S0277-5387(17)30333-9
DOI:
http://dx.doi.org/10.1016/j.poly.2017.05.005
POLY 12624
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
22 March 2017
3 May 2017
3 May 2017
Please cite this article as: A. Drzewiecka-Antonik, W. Ferenc, P. Rejmak, A. Wolska, M.T. Klepka, B. Cristóvão,
B. Mirosław, J. Sarzyński, D. Osypiuk, Coordination environment of new Co(II), Ni(II) and Cu(II) complexes with
4-bromophenoxyacetic acid: Structural, spectroscopic and theoretical studies, Polyhedron (2017), doi: http://
dx.doi.org/10.1016/j.poly.2017.05.005
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Coordination environment of new Co(II), Ni(II) and Cu(II)
complexes with 4-bromophenoxyacetic acid: Structural,
spectroscopic and theoretical studies
Aleksandra Drzewiecka – Antonika, Wiesława Ferencb, Paweł Rejmaka, Anna Wolskaa,
Marcin T. Klepkaa, Beata Cristóvãob, Barbara Mirosławb, Jan Sarzyńskic, Dariusz Osypiukb
aInstitute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland,
bFaculty of Chemistry, Maria Curie-Skłodowska University, 20-031 Lublin, Poland,
cInstitute of Physics, Maria Curie-Skłodowska University, 20-031 Lublin, Poland.
Abstract:
We report the synthesis of new Co(II), Ni(II) and Cu(II) complexes with 4-
bromophenoxyacetic acid. We characterize these compounds structurally using various
laboratory (XRF, IR, UV-VIS) and synchrotron (EXAFS, XANES) spectroscopic methods,
magnetic measurements and DFT calculations. We found that in the powder form of
complexes Co(II) and Ni(II) cations are coordinated by two monodentate carboxylate ligands
and the first coordination sphere is completed by water molecules that form an octahedron.
The coordination polyhedron of Cu(II) exhibits a distorted tetragonal-pyramidal geometry
with two ligands coordinating in bidentate and monodentate fashion via carboxylate O atoms
and two water molecules.
The X-ray single crystal structure analysis performed for Cu(II) complex after its
recrystallization from N,N–dimethylformamide (DMF) solution revealed the formation of
dinuclear complex. In crystals, the carboxylate O atoms occupy the bases of two tetragonal
pyramids formed around two Cu(II) ions with the DMF molecules in the apical positions.
1

1. Introduction
The phenoxyacetic acids are very active weed-killer substances. They are widely used in
herbicide and pesticide formulations [1]. Such herbicides are absorbed by the overground
parts of plants (especially by the leaves) easily penetrating into the cell bunches and move to
the roots toxically influencing all their parts. Therefore, they are applied for the destructions
of weeds with rich root systems and are very often used in the protection of wheat, rye, barley
and oat (which they leave relatively unaffected). In weak-acid and neutral environments the
phenoxyacetic acids react with bivalent metal ions and form complexes of limited solubility
in water [2]. The carboxylic group present in the phenoxyalkanoic acid molecules may
interact as monodentate [3,4] or bidentate ligands [4-6] with metal ions affecting their
biological activity. Due to the wide applications of phenoxyacetic acid derivatives in
agriculture it becomes an important environmental and health issue. Therefore, it is important
to deepen the knowledge on the structure and physico-chemical properties of their transition
metal salts.
In our previous studies we presented the binding mode of 2,4-dichlorophenoxyacetic acid
(widely used herbicide called 2,4-D) anion to the Co(II), Ni(II) and Cu(II) cations [7]. Such
complexes may be formed between metal ions present in soil and herbicides used for plant
spraying [2]. It was found that in all cases the carboxylate group of 2,4-D anion coordinates to
the metal cation in a monodentate fashion and the coordination sphere is completed by water
molecules to form octahedron or square.
Our present studies are focused on little studied 4-bromophenoxyacetic acid ligand and its
interactions with selected bivalent cations. Detailed literature survey showed that not too
much attention had been devoted to the p-bromophenoxyacetic acid (p-BPAA) and its
1
derivatives. The electronic, Raman, infrared, mass and H NMR spectra for p-BPAA were
published [8-10]. However, the search of the Cambridge Structural Database [11] for solid
phases of ‘pure’ acid and its metal complexes did not show any crystal structure. Further
search for information in the REAXYS database [12] showed the synthesis only for sodium,
potassium and silver salts of p-bromophenoxyacetate [13,14].
Herein, we report the synthesis of new Co(II), Ni(II) and Cu(II) complexes with 4-
bromophenoxyacetic acid (1-Co, 2-Ni, 3-Cu). The aim of the study was to propose the
molecular structure of investigated complexes in their powder form. We found difficulties in
crystallizing these compounds in neutral solvent (that does not enter the metal coordination
sphere). Only the crystal structure of complex 3-Cu (after its dissolution and crystallization
from N,N-dimethylformamide) was determined. Unfortunately, the DMF solvent molecule
2

modified the complex structure and a new compound was obtained, labeled as 4-Cu. Based
on our experience with various kinds of transition metal complexes with O-donor ligands [15-
19] we determined geometry of complex molecules in amorphous solids using a combination
of several laboratory techniques, X-ray absorption spectroscopy methods and molecular
modelling. The stoichiometry of studied complexes was determined by elemental analysis, X-
ray fluorescence spectroscopy (XRF) and thermogravimetry methods. Infrared (IR)
spectroscopy was used for identifications of possible coordinating atoms from p-BPAA anion.
The geometry around Co(II), Ni(II) and Cu(II) cations was described by UV-VIS
spectroscopy as well as the results of magnetic susceptibility measurements. The extended X-
ray absorption fine structure (EXAFS) analysis delivered information about the type and
quantity of atoms around the metal cation as well as about the average distance between
absorbing atom and the neighboring atoms. The molecular structure of p-BPAA was
optimized by density functional theory (DFT) modelling and used to build structural models
of studied complexes. Finally, the detailed of metal to carboxylate ligand (p-BPAA) binding
was refined using DFT calculations and the X-ray absorption near edge structure (XANES)
technique.
2. Materials and methods
The content of carbon and hydrogen in complexes was determined by elemental analysis
using CHN 2400 Perkin-Elmer analyzer. The amounts of metal and bromine were established
by X-ray fluorescence XRF method using spectrophotometer with energy dispersion ED
XRF-1510 (Canberra – Packard).
The thermal stability and decomposition processes of complexes were studied in air using a
Setsys 16/18 (Setaram) with TG, DTG and DSC techniques. The experiments were carried out
under air flow in the temperature range of 293 – 1173 K at a heating rate of 5 K·min-1. The
initial masses of complex samples changed from 4.34 mg to 5.44 mg. The compounds were
heated in Al2O3 crucibles.
The ATR-FTIR spectra of complexes and parent acid were recorded over the range of 4000–
400 cm-1 on a Thermo Scientific Nicolet iS5 spectrometer.
The electronic reflectance spectra of free ligand and complexes were collected on
SHIMADZU UV-VIS Spectrophotometer UV-2600Plus with Integrating Sphere in the range
of 185–1400 nm.
3

2.1. Magnetic studies
Magnetic susceptibilities of complexes were investigated at 77 – 303 K using Gouy method
and for Ni(II) and Cu(II) complexes additionally also at 1.8 – 300 K with Quantum Design
SQUID-VSM magnetometer. The SQUID-VSM magnetometer was calibrated with the
palladium rod sample. In the case of Gouy’s method the mass changes were obtained from
Cahn RM-2 electrobalance. The calibrate employed was Hg[Co(SCN)4] for which the
magnetic susceptibility was assumed to be 1,644 · 10-5cm3 g-1. The measurements were made
at a magnetic field strength of 0.99 T. Correction for diamagnetism of the constituent atoms
was calculated by the use of Pascal’s constants [20,21]. The effective magnetic moment
values were calculated from the equation: µeff = 2.83 (χm·T)1/2, where: µeff - effective magnetic
moment, χm – magnetic susceptibility per molecule and T - absolute temperature.
2.2. X-ray absorption spectroscopy
The K-edges of Co, Ni and Cu for complexes were measured at the XAFS beamline at Elettra
synchrotron (Trieste, Italy). Double crystal Si 111 monochromator was calibrated using Cu
foil. The XAFS spectra were recorded in the transmission mode. For the EXAFS analysis, the
Athena and Artemis programs included in the IFEFFIT package were applied [22]. The k2
weighted χ(k) data were Fourier transformed in the k range from 2.3 to 11 Å-1 for 1-Co, 2.4 to
12 Å-1 for 2-Ni and 2.4 to 11.7 Å-1 for 3-Cu. The fitting was performed in the R range from 1
to around 2.8 Å (without phase correction) for all complexes. The amplitude reduction factor
(S02) was estimated based on reference oxides and later evaluated during fitting procedure.
Theoretical ab-initio XANES calculations were carried out using FEFF 9.6 code [23]. The
XANES, Self-Consistent Field and Full Multiple Scattering cards were used. The Hedin-
Lundqvist potential was chosen. The geometry of the models was adjusted by a LigandRot
program [24].
2.3. Computational part
The density functional theory (DFT) studies were performed using Turbomole ver. 6.5
computational package [25]. Perdew-Burke-Ernzerhof (PBE) gradient exchange–correlation
functional [26], def2-TZVP Gaussian basis set [27] and D3 Grimme correction for dispersion
interaction [28] were applied.
2.4. X-ray diffraction
The X-ray diffraction patterns of compounds (1-Co, 2-Ni, 3-Cu) and the products of their
decomposition process were taken on a HZG-4 (Carl-Zeiss, Jena) diffractometer using Ni
filtered CuKα radiation. The measurements were made within the range of 2θ = 4-80º by
4

means of Bragg-Brentano method. For the interpretation of diffractograms the Dicvol 06
programme was used [29].
The crystal structure of Cu(II) complex after its dissolution and crystallization from N,N-
dimethylformamide was determined. Since the DMF solvent molecule modified the complex
structure, the new compound was obtained labeled as 4-Cu. The X-ray diffraction intensities
for the single crystal of Cu(II) complex were collected at 120(2) K on SuperNova X-ray
diffractometer equipped with Atlas S2 CCD detector using the mirror-monochromatized
CuKα radiation (λ = 1.54184 Å). The structure was solved by direct methods using the
SHELXS-97 program and refined by the full-matrix least-squares method on F2 using the
SHELXL-97 program [30]. Crystal data and experimental parameters can be found in the
Supplementary Material. The experimental details and final atomic parameters for 4-Cu have
been deposited with the Cambridge Crystallographic Data Centre as a supplementary material
(CCDC ID: 1537060).
Table 1
The unit cell parameters for polycrystalline complexes.
V (Å3)
536.12
2190.07
475.21
Complex
1-Co
Crystal system
triclinic
a (Å)
4.92
b (Å)
5.80
4.97
6.98
c (Å)
19.11
23.11
13.64
α (°)
81.88
90.00
83.20
β (°)
89.27
117.20
87.24
γ (°)
83.25
90.00
76.68
monoclinic
triclinic
21.43
5.16
2-Ni
3-Cu
2.5. Synthesis
All chemicals and solvents used for the synthesis were of commercially available reagent
grade and were applied without further purification.
4-Bromophenoxyacetate of ammonium (pH ~ 5) of 0.1 mol/L concentration was prepared by
the addition of 15.42cm3 of NH3 (aq) solution (25% pure, Polish Chemical Reagents in
Gliwice, Poland) to 23.1g of 4-bromophenoxyacetic acid (p-BPAA) in water solution (99%
pure, Aldrich Chemical Company).
The complexes of Co(II), Ni(II) and Cu(II) with p-BPAA were prepared by adding the
equivalent quantities of 0.1 mol L-1 ammonium salt of p-BPAA (pH ~ 5) to a warm solutions
of respective chlorides (1.5g): CoCl2·6H2O, NiCl2·6H2O and CuCl2·2H2O (analytically pure,
Polish Chemical Reagents in Gliwice, Poland) and crystallizing at 293 K. The volumes of
0.1M ammonium 4-bromophenoxyacetate added to suitable chloride solutions of Co(II),
Ni(II) and Cu(II) were equal to: 126, 126 and 175 cm3, respectively. For the reach of
5

equilibrium state the solids were constant stirring for 1 h. Next they were filtered off, washed
with warm water to remove ammonium and chloride ions and dried at 303 K to a constant
mass.
Co(p-BPAA)2·4H2O (1-Co)
Yield: 42% as a pink solid. IR: 3476 (νOH), 3371 (νOH), 1578 (νasCOO¯ ), 1488 (νCAr−CAr),
1335 (νsCOO¯ ), 1282 (ωCH2), 1229 (νCAr−Oether), 1061 (vCAr−Br) cm-1. Anal. Calc. for
Co(C8H5O3Br)2·4H2O: C, 32.49; H, 3.38; Br, 27.07; Co, 9.98 % Found: C, 32.43; H, 3.27; Br,
27.0; Co, 9.52 %.
Ni(p-BPAA)2·4H2O (2-Ni)
Yield: 53% as a green solid. IR: 3449 (νOH), 3362 (νOH), 1576 (νasCOO¯ ), 1488 (νCAr−CAr),
1333 (νsCOO¯ ), 1282 (ωCH2), 1228 (νCAr−Oether), 1061 (vCAr−Br) cm-1. Anal. Calc. for
Ni(C8H5O3Br)2·4H2O: C, 32.64; H, 3.18; Br, 27.82; Ni, 10.26 % Found: C, 32.86; H, 3.20;
Br, 27.90; Ni, 10.20 %.
Cu(p-BPAA)2 ·2H2O (3-Cu)
Yield: 89% as a blue solid. IR: 3226 (νOH), 3152 (νOH), 1579 (νasCOO¯ ), 1488 (νCAr−CAr),
1345 (νsCOO¯ ), 1281 (ωCH2), 1224 (νCAr−Oether), 1061 (vCAr−Br) cm-1. Anal. Calc. for
Cu(C8H5O3Br)2·2H2O: C, 34.32; H, 2.86; Br, 28.60; Cu, 11.35 % Found: C, 34.17; H, 2.75;
Br, 28.40; Cu, 10.98 %.
The green crystals of 4-Cu, suitable for an X-ray single crystal structure analysis, were
obtained by a slow evaporation of the solvent from an DMF solution at room temperature.
3. Results
3.1. Thermal analysis
The thermogravimetric studies of analyzed complexes were carried out in air in the range of
293 – 1173 K (Table 2, Fig. 1). TG, DTG and DSC curves were recorded using the DSC/TG
technique. Heating to 1173 K the complexes decompose in three steps. They are stable up to
303 K. Next in the range of 303 – 412 K they dehydrate in one step with endothermic peaks
(DSC curves) losing from one to four molecules of water and form anhydrous compounds.
The energetic effects accompanying the dehydration process were also determined. The
enthalpy values, ∆H, change from 43.12 kJ·mol-1 to 26.63 kJ·mol-1 per one molecule of water.
These values show that in these compounds the water molecules are bounded with different
6

strengths depending on their positions in the complex coordination spheres and on the kind of
central ion. On further heating the anhydrous complexes of 1-Cu, 2-Ni and 3-Cu form
mixture of metal oxybromides and their oxides (513 – 655 K) being the intermediate products.
In the third stage of complex decomposition the metal oxides are formed. The final product
obtained as CoO, NiO and CuO were verified by comparing their FTIR spectra and powder
diffractograms with those of pure oxides.
Table 2
Temperature ranges of thermal stability in air at 293 – 1173 K and enthalpy values of dehydration
process for analysed complexes.
Mass losses
(%)
Mass residue
(%)
Complex
∆T1
Mass loss (%)
n
∆T2
∆H1H2O
(K)
H2O
(K)
calcd. found
calcd. found calcd. found (kJ/mol)
329-378 12.18 11.75
303-393 12.19 13.02
4
4
2
530-613 85.08 85.29 14.95 14.71
513-655 85.11 84.61 14.89 15.39
523-623 63.50 61.20 14.21 13.73
42.85
26.63
43.12
1-Co
2-Ni
3-Cu
362-412 6.43
6.58
∆T1 - temperature range of dehydration process, ∆T2 - temperature range of anhydrous complex
degradation, n – number of water molecules lost in one stage, ∆H1H2O - enthalpy value for 1 water
molecule of dehydration process
Fig. 1. TG, DTG and DSC curves for 3-Cu.
7

3.2. Infrared spectra
The IR spectroscopy was used to monitor the complexation reactions and to identify atoms of
carboxylate ligand coordinating to the respective cations (Fig. 2).
For parent carboxylic ligand, p-BPAA, the broad band with several maxima at 3100–2600 cm-
1
is assigned to the O–H and C–H stretching vibration. For its metal complexes (1-Co, 2-Ni
and 3-Cu) this broad band is extended to 3500 cm-1 which confirms the presence of water
molecules as it was suggested by the elemental and thermal analyses.
The –COOH group in p-BPAA molecule yields absorption band corresponding to a carboxyl
C=O stretching vibration at 1670 cm-1. After deprotonation, ν(C=O) shifts to lower and higher
energy and two strong bands, due to stretching vibrations of carboxylate group, are observed
(Fig. 2). The bands with maxima at 1578, 1576, 1579 cm-1 and at 1335, 1333, 1345 cm-1 are
assigned to asymmetric and symmetric stretching motion of 1-Co, 2-Ni and 3-Cu,
respectively.
Fig. 2. ATR-FTIR spectra in the range 1800–1000 cm-1 of: p-BPAA parent ligand (black solid line); 1-
Co (pink dashed line); 2-Ni (green dash dotted line) and 3-Cu (blue dashed dot dotted line).
8

The very strong band at 1227 (for p-BPAA), 1229 (1-Co), 1228 (2-Ni) and 1224 cm-1 (3-Cu)
is assigned to the C–Oether stretching vibration. As the position of these bands is nearly
identical in ligand and its complexes, one can exclude the possibility of binding to transition
metal ion via ether O atom. The same situation is observed for the band corresponding to the
C–Br stretching vibration. Its position in complex spectra (1061 cm-1) is practically the same
as that in parent ligand spectrum (1068 cm-1), which suggests that Br atom does not
coordinate to the metal center.
The fingerprint regions have been also compared. Several bands present in the spectrum of p-
BPAA are observed in different positions than those in IR spectra of its metal complexes.
3.3. UV-VIS spectra
The electronic spectra of complexes are presented in Fig. 3 and their assignment was used to
describe the geometry around metal cation and the oxidation state of cobalt, nickel and
copper. Recorded spectra of 1-Co, 2-Ni and 3-Cu are characteristic for complexes of studied
metals with oxidation state of +2 with weak ligand fields and they are very similar to those for
Co(II) and Ni(II) complexes with herbicide 2,4-dichlorophenoxyacetic acid [7]. The UV-VIS
spectrum for 1-Co and 2-Ni is typical for d-d transitions of Co(II) and Ni(II) in octahedral
environment. 1-Co complex gives two bands in these region: one at 515 nm assigned to the
4
4
4
4T1(F) → T1(P) and T1 → A2 transitions, and the second one around 1250 nm
corresponding to the 4T1 → 4T2 excitation. Three bands at 390, 680 and 1130 nm are observed
at the UV-VIS spectrum for 1-Ni complex. These bands arise most likely due to the
3
3
3
3
transitions from the A2 term to the T2(F), T1 and T2(P) terms, respectively. Regarding 3-
Cu, the broad feature is seen between 600 and 1000 nm, with two apparent peaks around 705
and 840 nm. The appearance of multiple Cu d-d transitions at such long waves implies that Cu
coordination may be rather of low symmetry (i.e. lower than tetragonal, square planar or
tetrahedral ones).
9

Fig. 3. UV-VIS spectra in the 1350–300 nm range of Co(II) complex (1-Co, pink dashed line), Ni(II)
complex (2-Ni, green dash dotted line) and Cu(II) complex (3-Cu, blue dotted line).
3.4. Magnetic measurements
To study the coordination spheres of transition metal ions the magnetic susceptibility of
compounds was measured over the range of 76 – 303 K using Gouy’s method. For Ni(II) and
Cu(II) complexes the measurements with SQUID were performed in wider temperature range
of 1.8 – 300 K. The results are collected in Table 3 and Figs 4, 5.
The complexes were found to obey the Curie-Weiss law. The effective magnetic moments in
all cases are close to the spin only values for free divalent ions. This implies that in all cases
the complexes are mononuclear ones. Deviations from spin only values of magnetic moments
may be ascribed to spin-orbit coupling (being the strongest for cobalt), interactions between
ligand and d metal orbitals, as well as due to the weak interactions between distant magnetic
centers. The measured magnetic susceptibilities for 1-Co and 2-Ni compounds are typical for
the mononuclear high spin complexes with octahedral environment of transition metal ions.
The more detailed description of magnetic properties of studied complexes can be found in SI.
10

Table 3
The comparison of spin only value of magnetic moments (µ) for studied transition metal ions with the
experimental values found for their complexes in literature and measured in this work.
Complex Ion
Spin
Spin only
Μ
Exp. µ1
(298 K)
4.3-5.2
2.8-3.5
1.7-2.2
Gouy method
(76-303 K)
4.56-4.59
SQUID
(2-300 K)
-
multiplicity
Co2+
4
3
2
3.88
1-Co
2-Ni
3-Cu
Ni2+
2.83
2.59-2.70
2.53-2.99
0.58-1.72
Cu2+
1.73
2.30-2.18
1 Ref. 31 (Kettle).
-1
Fig. 4. Relationship of χm and χmT versus T for 2-Ni.
Fig. 5. Dependence of χm and χm-1 values versus T for 3-Cu
11

3.5. XAFS studies: EXAFS
The analysis of EXAFS data provides information about the local surrounding within Co(II),
Ni(II) and Cu(II) coordination sphere of studied complexes. It gives information on the type
and number of coordinating atoms (N), their average distances (R) to the absorbing atom and
relative structural disorder (σ2). In all cases EXAFS studies of the powdered samples excluded
the formation of multinuclear complexes. Analysis revealed that for 1-Co and 2-Ni complexes
there are six O atoms at the distance above 2 Å in the first coordination sphere and two C
atoms around 3 Å (Fig. 6, Table 4). In case of 3-Cu complex four O atoms with the Cu–O
distance slightly below 2 Å were observed. Further analysis showed that one C atom is
located around 2.5 Å while another C atom is detected around 3 Å from the Cu central atom.
Fig. 6. Moduli of the Fourier transformed experimental EXAFS oscillations for complexes together
with the fits. The insets show filtered EXAFS oscillation together with the best fit.
12

Table 4
The mean distance (R) from central metal (M) cation to the next atom from EXAFS and DFT studies;
the number of atoms (N), EXAFS Debye-Waller factor (σ2) evaluated during EXAFS fitting
procedure. R-factor describes quality of the fit.
EXAFS
DFT
Complex M–O
R [Å]
N
σ2 [Å2]
R-factor
0.009
R [Å]
Co–O
1-Co
2.08(1)
3.00(1)
2.05(1)
2.98(1)
1.95(1)
2.45(1)
2.80(1)
3.03(1)
6
2
6
2
4
1
1
1
0.007(1)
0.007(1)
0.007(1)
0.007(1)
0.003(1)
0.004(7)
0.003(1)
0.002(9)
2.14/2.151
2.91/2.92
2.10/2.101
2.89/2.90
2.09
Co–C
Ni–O
2-Ni
0.005
0.013
Ni–C
Cu–O
Cu–C
3-Cu
Fixed2
Cu-O
fixed
Cu–C
fixed
1Centrosymmetirc/noncentrosymmetric model. 2The distances fixed to the EXAFS values.
3.6. DFT calculations
The chemical composition of complexes: Co(p-BPAA)2·4H2O (1-Co), Ni(p-BPAA)2·4H2O
(2-Ni), Cu(p-BPAA)2·2H2O (3-Cu) and discussed above spectroscopic data (notably EXAFS)
were used to construct the initial structural models subjected to DFT energy minimization
procedure.
For 1-Co and 2-Ni the coordination sphere should consist of six O atoms, which can be
realized either by (i) four water molecules and two carboxylate atoms bound in monodentate
way, (ii) two water molecules and two bidentate carboxylate ligands or (iii) two water
molecules and two ligand chelating metal (M) via one carboxylate and one ether O atoms. In
all these cases mean M–O distances in the optimized structures were found to be about 2.1 Å,
in good agreement with EXAFS data, see Table 4. However, only for monodentate
carboxylate coordination M–C distances matches well EXAFS distances (about 3 Å), as for
the bidentate coordination these distances drop below 2.5 Å. The models with metal ion being
chelated by carboxylate and ether O atoms can be disregarded, as these would have four C
atoms in the second coordination sphere with Cu–C distance about 2.9 Å and besides no shift
in C–Oether frequency was observed in FTIR.
13

Regarding the most likely models with monodentate carboxylate binding, for each metal ion
two types of such models were considered, centrosymmetric (Ci) and noncentrosymmetric
(C1) one, see Table 5. In both types of models mean M–O and M–C distances are virtually the
same. For both 1-Co and 2-Ni Ci models were found to be 10 kJ/mol more stable than the
corresponding C1 ones. This is however the energy difference of order of intermolecular
interaction energies, thus somewhat less stable C1 models cannot be ruled out as present in
solid phase. Note that even in C1 model the first coordination sphere MO6 is approximately
octahedral, thus these models do not contradict UV-Vis data.
Table 5
The coordination sphere of Co(II), Ni(II) and Cu(II) cations in studied complexes.
Monodentate carboxylate binding
Ci point group
C1 point group
1-Co
2-Ni
Mono- / bidentate carboxylate binding
3-Cu
14

For 3-Cu complex the best EXAFS fit points towards four O atoms in the first coordination
sphere of Cu(II) and somewhat mixed second coordination sphere, consisting of two C and
one O atoms unevenly spaced from metal ion. The EXAFS Cu–C distances imply that one
carboxylate ligand is coordinated in monodentate and the other in bidentate manner. The
optimization of the models with such mixed coordination converged to structures with
monodentate coordination only, therefore the optimization with Cu–C and the longest Cu–O
distance fixed to the EXAFS value (the latter O atom represented as belonging to water
molecule) was performed. Such conditional energy minimum, possibly stabilized in solid
phase by intermolecular interactions, indeed corresponds to the complex with mixed bi-
/mono-dentate carboxylate binding. The coordination sphere of optimized Cu(II) complex
with mixed carboxylate binding is presented in the Table 5.
3.7. XAFS studies: XANES
The DFT optimized gas phase models were used as a starting point in the XANES
calculations. Then, the LigandRot program [24] was employed to adjust ligand position
around the central atom. The ligands were assumed to be rigid and were rotated around the
M–O bonds using procedure described elsewhere [7].
The best agreement between the experimental and theoretical XANES spectra for 1-Co and 2-
Ni was obtained for the model postulating the noncentrosymmetric (C1) octahedral geometry
around the complex (Fig. 7). The polyhedron around cations for 1-Co and 2-Ni complexes are
formed by two monodentate carboxylate group of p-BPAA anion and four water molecules.
The XANES calculations for 3-Cu complex (Fig. 7) confirmed that the polyhedron around
cation is formed by one monodentate and one bidentate carboxylate group of p-BPAA anion
and two water molecules.
15

1.8
1.2
0.6
0.0
1.5
1.0
0.5
0.0
1-Co experiment
calculations
2-Ni experiment
calculations
7700
7750
7800
7850
8350
8400
8450
8500
Energy (eV)
Energy (eV)
1.2
0.6
0.0
3-Cu experiment
calculations
9000
9050
9100
9150
Energy (eV)
Fig. 7. The comparison of experimental and theoretical XANES spectra for studied complexes with
the metal-ligand binding geometry of the best structural model of 1-Co, 2-Ni and 3-Cu from XANES
studies.
3.8. X-ray single crystal structure analysis
The crystal structure was determined by the X-ray technique only for complex 4-Cu (after
dissolution of compound 3-Cu and recrystallization from N,N-dimethylformamide). It turned
out that the solvent molecules modified the complex structure and the formation of a new
compound was observed. The new complex was labelled as 4-Cu. The asymmetric units of 4-
Cu (Fig. 8) is composed of one metal cation, one DMF molecule and two deprotonated p-
bromophenoxyacetate anions. The coordination unit is located at the centre of symmetry,
therefore the binuclear species are formed. The p-BPAA anions exhibit a bidentate bridging
binding mode. The carboxylic O atoms occupy the bases of two tetragonal pyramids formed
around two Cu(II) ions with the DMF molecules in the apical positions. This fourfold bridge
forces metal cores to be very close to each other. The observed short intermetallic Cu–Cu
16

distance of 2.631(1)Å (below the sum of van der Waals radii 2.80 Å) indicates a possible
strong closed-shell metal-metal interaction [32,33].
In the 4-Cu crystal the coordination units stack in columns along the a crystallographic axis
(Fig. 9). There are weak intermolecular C–H…O and C–H…Br hydrogen bonds and C–H…π
contacts (SI).
Fig. 8. Molecular structure of 4-Cu and atom labelling scheme. The labels are presented only for the
asymmetric unit. Displacement ellipsoids were drawn at 50 % probability level.
Fig. 9. Crystal packing of 4-Cu with the dinuclear coordination units stacked in columns along the a
crystallographic axis.
17

4. Discussion
Elemental analysis, XRF and thermogravimetric measurements allowed to establish the
stoichiometry of the studied compounds. On the basis of these data the following formulas
were proposed: Co(p-BPAA)2·4H2O for 1-Co, Ni(p-BPAA)2·4H2O for 2-Ni and Cu(p-
BPAA)2·2H2O for 3-Cu complexes, respectively. The thermal decompositions of analyzed
metal (M) complexes in air proceed in three steps in the following way: M(p-BPAA)2·nH2O
M(p-BPAA)2
mixture of appropriate metal oxides and M2OBr2
MO,
→
→
→
where: M(II)=Co, Ni and Cu; n = 2 for 3-Cu ; 4 for 1-Co and 2-Ni. In the range of 303 – 412
K, the complexes dehydrate in one step with endothermic peaks losing all molecules of water
and form anhydrous compounds. On further heating the anhydrous complexes form mixture
of respective metal oxides with oxybromides (513 – 655 K) being the intermediate products
which in the third stage of complex destruction form the oxides of respective metals.
The broad band at 3500–3200 cm-1 in the ATR-IR spectra of complexes 1-Co, 2-Ni and 3-Cu
agrees with the presence of water molecules in compounds as it was suggested by the
elemental and thermal analyses. The –COOH group in p-BPAA yields very strong absorption
band corresponding to a carboxyl C=O stretching vibration at 1670 cm-1. After deprotonation,
the ʋ(C=O) band shifts to lower and higher energy, and two strong bands, due to asymmetric
and symmetric stretching vibrations of carboxylate anion, are observed for all complexes.
Since O ether and Br atoms of p-BPAA are inactive in IR spectra of studied compounds, this
proves that metal–ligand binding occurs only through O carboxylate atoms.
UV-Vis and magnetic measurements indicated mononuclear complexes with the metal ions at
+2 oxidation state. These results are in good agreement with those of EXAFS studies as no
other metal atom was found in the proximity of the central one. Moreover, the EXAFS
analysis indicated the presence of six O atoms for 1-Co and 2-Ni with the M–O distances 2.08
and 2.05 Å respectively. Two C atoms around 3 Å from Co(II) and Ni(II) were identified
which indicates that two carboxylate groups coordinate to the metal cations in monodentate
fashion. For 3-Cu four O atoms were found at the distances shorter than 2 Å and one O atom
around 2.8 Å from absorption center. The Cu–C distances are around 2.5 and 3 Å
respectively, which suggests that the one ligand coordinate in bidentate and the second one in
monodentate fashion.
All these spectroscopic data allowed to build a set of structural models for studied complexes.
These models were verified by DFT calculations and XANES analysis. For Co(II) and Ni(II)
cations the analogous structures were proposed with two monodentate carboxylate group and
four water molecules within the first coordination sphere. Both cations exhibits octahedral
18

geometry very similar to that presented in Co(II) and Ni(II) complexes with herbicide 2,4-
dichlophenoxyacetic acid [7].
These results agree with the crystal structure reported in Cambridge Structural Database for
Co(II) and Ni(II) complexes with water and phenoxyacetic acid derivatives containing only
one carboxylic and one ether groups. The mononuclear complexes with Ni(II) octahedral
coordination consisting of two carboxylate and four water O atoms were reported [34,35]. For
Co(II) ion the former types of mononuclear complexes were found [36,37], along with the
coordination polymers with octahedral Co(II) sites bridged with carboxylate O atoms [38,39].
In the case of 3-Cu, the metal cation is penta-coordinated having a distorted tetragonal
pyramidal polyhedron with one monodentate and one bidentate carboxylate groups.
Among the crystal structures resolved for hydrated Cu(II) complexes with simple
phenoxyacetic acid derivatives the most common are those with Cu(II) being chelated by two
ligand molecules via carboxylate and ether O atoms, with tetragonal coordination completed
by two apical water molecules [37,39]. This type of coordination is inconsistent with our
EXAFS (the number of C atoms in the second coordination shell) and FTIR (no red shift in C-
Oether stretching frequency) data. However, other types of coordination compounds were also
reported, like (i) mononuclear complexes with Cu(II) ion coordinated by carboxylate O atoms
in the monodentate as well as bidentate fashions [40-43], (ii) binuclear complexes with
carboxylate bridges [44] and (iii) the latter complexes stacked, forming coordination polymer
[45]. Clearly, Cu(II) ion and phenoxyacetic acid ligand pair is very flexible system, able to
form rich variety of coordination compounds.
The crystals of Cu(II) complex were obtained after its dissolutions in DMF solvent. The X-ray
single crystal structure analysis revealed the formation of binuclear complex in which the
solvent molecules built the tops of [Cu2O10] center and carboxylate bridging group of p-
BPAA occupy the bases of two tetragonal pyramids formed around two Cu(II) ions.
CONCLUSIONS
The new complexes with p-bromophenoxyacetic acid (p-BPAA) have been prepared from
ammonium salt of acid and metal chlorides. The synthesized compounds form hydrates with
formula: Co(p-BPAA)2·4H2O, Ni(p-BPAA)2·4H2O and Cu(p-BPAA)2·2H2O. We succeeded
to determine the metal-ligand geometry of complexes in the powder form, as obtained from
the direct synthesis. This was achieved by application beyond several laboratory methods (IR,
19

UV-VIS, thermal analysis, magnetic studies) also the X-ray absorption spectroscopy (EXAFS
and XANES) as well as DFT calculations. We found that complexes, in their powder form,
are mononuclear. Two monodentate carboxylate groups and four water molecules form
octahedron around Co(II) and Ni(II) cations, whereas Cu(II) ion is surrounded by three O
atoms from mono- and bidentate carboxylate groups and two water molecules. The X-ray
diffraction on the single crystal of Cu(II) complex, after its recrystallization from N,N–
dimethylformamide solution, revealed the formation of new dinuclear complex. The
carboxylate O atoms occupy the bases of two tetragonal pyramids formed around two Cu(II)
ions with the DMF molecules in the apical positions. This shows that recrystallization from
solution is not always merely purification process and the formation of new complex may
occur in course. Therefore techniques, such as X-ray absorption spectroscopy, which are able
to provide structural information even for poorly crystalline or amorphous samples are
invaluable tools in coordination chemistry.
Acknowledgment
This research was supported in part by PL-Grid Infrastructure.
This work was partially supported by the EAgLE international project (FP7-REGPOT-2013-
1, Project No. 316014) and the international project co-financed by Polish Ministry of Science
and Higher Education, Grant Agreement 2819/7.PR/2013/2.
The research was carried out with the equipment purchased thanks to the financial support of
the European Regional Development Fund in the framework of the Operational Program
Development of Eastern Poland 2007-2013 (Contract No. POPW.01.03.00-06-009/11-00,
Equipping the laboratories of the Faculties of Biology and Biotechnology, Mathematics,
Physics and Informatics, and Chemistry for studies of biologically active substances and
environmental samples.
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Graphical abstract
We report the synthesis of Co(II), Ni(II) and Cu(II) complexes with p-bromophenoxyacetic
acid (p-BPAA). For powder form of compounds, we find that mononuclear complexes are
formed with two carboxylate ligands coordinating to the metal cation. In single crystal
structure of Cu(II) complex, the bridging carboxylate groups and DMF molecules stabilize
dinuclear Cu2O10 center.
23