# Synthesis, Spectroscopy, and Magnetic Characterization of Copper(II) and Cobalt(II) Complexes with 2-Amino-5-bromopyridine as Ligand

Authors: Raziyeh Arab Ahmadi; Farshideh Hasanvand; Giuseppe Bruno; Hadi Amiri Rudbari; Saeid Amani
Journal: ISRN Inorganic Chemistry
 (2013)

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## Abstract

The synthesis, spectroscopic, and magnetic characterization of two new copper(II) and cobalt(II) complexes are described. Both two compounds have the general formula [M(L)2(Cl)2], in which L= 2-amino-5-bromopyridine. These complexes were prepared in one-step synthesis and characterized by elemental analysis, FTIR, UV-Vis, and EPR spectroscopy. Moreover, the single crystal structure of complex (1) was studied by the X-ray diffraction method. This compound consists of mononuclear units consisting of two ligands linked to metal via the nitrogen of pyridine ring. The UV-Vis spectra of copper(II) and cobalt(II) complexes show three and five absorption bands, respectively, attributed to the d-d transition of the metal ion, ligand → metal charge transfer, and π→π∗ or n→π∗ transitions of the ligand. The FTIR spectra show MN2Cl2 vibrations at 500–300 cm−1. The complexes show room temperature magnetic moments of 1.78 BM and 4.12 BM for Cu(II) and Co(II), respectively. The X-band electron spin resonance (ESR) spectra of Cu(II) complex in DMF or DMSO frozen at liquid nitrogen temperature show the typical ΔMS=±1 transition.


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## Body

## 1. Introduction

The study of transition metal ion complexes by magnetic and optical techniques has furnished a considerable body of empirical data, much of which can be understood in terms of the phenomenological ligand field theory [1]. The major portion of this data is primarily concerned with complexes containing a single paramagnetic transition metal ion; relatively little information is available on dimeric and trimeric coordination complexes [2]. Since copper(II) ions are widely distributed in biological systems, a significant amount of research has centered on the search for relatively simple copper(II) complexes which may display some of the properties of the metalloproteins. One such system of potential importance is that formed by copper(II) halides with pyridine and substituted pyridines [3]. Small low-molecular-weight copper(II) coordination complexes with N-donor ligands have frequently been used to model the active site in copper proteins with the aim of obtaining insight into the correlation between structure and the spectroscopic and magnetic behavior [4–7]. Cobalt(II) complexes are important in biology mainly because of coenzyme B12 [8]. A report of insulin-like action of cobalt(II) chloride implies that such metal complexes may have similarities with vanadium compounds, which exhibit insulin-like effects [9–16]. Electronic properties of cobalt(II) compounds have received substantial attention [17–19] in part due to the fact that many of these complexes bind oxygen reversibly [20] and are model systems for vitamin B12r. Cobalt is a necessary trace element in mammals and has many uses in medicine, magnetic resonance imaging, and drug delivery [21]. Cobalt ethylenediamine complexes are potent antimicrobial agents [22]. Low-spin cobalt porphyrins [23, 24] are of interest since cobalt-substituted hemoglobin, “coboglobin,” binds oxygen in a cooperative fashion [25]. The growth of cobalt chemistry has necessitated the development of models to account for and predict the spectroscopic properties of cobalt(II) complexes. Cobalt(II) compounds have interesting magnetochemistry, as shown in a recent review on magnetic metal-organic frameworks [26]. Strong magnetic anisotropy of high-spin cobalt(II) is at the origin of the increasing interest in polynuclear compounds containing this metal ion [27]. Several examples of high-nuclear complexes with six-coordinate cobalt(II) behaving as single molecule magnets and single chain magnets have been reported [28–31]. We have been interested in copper(II) and cobalt(II) complexes with 2-amino-5-bromopyridine as ligand. Such complexes have potential for antibacterial activities, and we will focus on this subject in the future. The complexes were synthesized in a one-step synthesis and characterized by elemental analysis, Fourier transform infrared (FTIR), electronic spectra, room temperature magnetic moments, and a crystal structure.


## 2. Experimental

### 2.1. Chemicals

All of the chemicals and solvents were purchased from Merck Chemical Company and used as received without further purification.


### 2.2. Preparation of the Complexes

The coordination compounds were prepared according to the general procedure (Scheme1) [17].Scheme 1
Structure of ligand.Complex [Cu(2-amino-5-bromopyridine)
2
(Cl)
2
]; (1). One mmol of copper(II) chloride dehydrates and 2.1 mmol of 2-amino-5-bromopyridine (Scheme 1) were each dissolved in 25 mL of CH3OH. The Cu(II) salt solution was then added slowly to the ligand solution, thereby preventing any precipitation, and the solution was filtered to remove any solids. After the solution stood for five days, the product was separated. Yield about 85%.Elemental analysis for CuC10H10N4Br2Cl2; (1); found: C, 24.89; H, 2.21; N, 11.43; Cu, 13.68%. Cal.; C, 25.00; H, 2.10; N, 11.66; Cu, 13.26%.Complex [Co(2-amino-5-bromopyridine)
2
(Cl)
2
]; (2). One mmol of cobalt(II) chloride dehydrates and 2.1 mmol of 2-amino-5-bromopyridine each dissolved in 25 mL of CH3OH. The Co(II) salt solution was then added slowly to the ligand solution, thereby preventing any precipitation, and the solution was filtered to remove any solids. After the solution stood for five days, the product was separated. Yield about 79%.Elemental analysis for CoC10H10N4Br2Cl2; (2); found: C, 25.57; H, 2.17; N, 11.96; Co, 12.62%. Cal.; C, 25.24; H, 2.12; N, 11.77; Co, 12.38%.


## 2.1. Chemicals

All of the chemicals and solvents were purchased from Merck Chemical Company and used as received without further purification.


## 2.2. Preparation of the Complexes

The coordination compounds were prepared according to the general procedure (Scheme1) [17].Scheme 1
Structure of ligand.Complex [Cu(2-amino-5-bromopyridine)
2
(Cl)
2
]; (1). One mmol of copper(II) chloride dehydrates and 2.1 mmol of 2-amino-5-bromopyridine (Scheme 1) were each dissolved in 25 mL of CH3OH. The Cu(II) salt solution was then added slowly to the ligand solution, thereby preventing any precipitation, and the solution was filtered to remove any solids. After the solution stood for five days, the product was separated. Yield about 85%.Elemental analysis for CuC10H10N4Br2Cl2; (1); found: C, 24.89; H, 2.21; N, 11.43; Cu, 13.68%. Cal.; C, 25.00; H, 2.10; N, 11.66; Cu, 13.26%.Complex [Co(2-amino-5-bromopyridine)
2
(Cl)
2
]; (2). One mmol of cobalt(II) chloride dehydrates and 2.1 mmol of 2-amino-5-bromopyridine each dissolved in 25 mL of CH3OH. The Co(II) salt solution was then added slowly to the ligand solution, thereby preventing any precipitation, and the solution was filtered to remove any solids. After the solution stood for five days, the product was separated. Yield about 79%.Elemental analysis for CoC10H10N4Br2Cl2; (2); found: C, 25.57; H, 2.17; N, 11.96; Co, 12.62%. Cal.; C, 25.24; H, 2.12; N, 11.77; Co, 12.38%.


## 3. Physical Measurements

C, H, and N determination were undertaken using an Elementar Analysis System Gmb H Vario EL II. Copper and cobalt were determined on a Perkin-Elmer 2380 Atomic Absorption Spectrophotometer. Electronic spectra were recorded on a Perkin-Elmer Lambda 900 spectrophotometer using the diffuse reflectance technique, and MgO was used as a reference. FTIR spectra were obtained in the 4000–300 cm−1 range as KBr disks by using a Galaxy series FTIR 5000 spectrophotometer. The spectra were calibrated using the polystyrene bands at 3028, 1601, and 1208 cm−1. X-band electron paramagnetic resonance spectra were recorded of both powdered and frozen solutions of the complexes at both room temperature and at liquid nitrogen temperature in DMF or in DMSO on an IBM electron spin resonance spectrometer using DPPH (g=2.0036) as a standard. The room temperature magnetic moment of each complex was measured according to the Evans method. H NMR determination was carried out on a Bruker 300 MHz spectrometer.


## 4. Crystal Structure Determination and Refinement

The X-ray measurement of complex (1) was made on a Bruker-Nonius X8 ApexII diffractometer equipped with a CCD area detector by sung graphite-monochromated Mo kα radiation (λ = 0.71073 Å) generated from a sealed tube source. Data were collected and reduced by SAINT software in the Bruker package [32]. The structure was solved by direct methods [33] and then developed by least squares refinement on F2 [34, 35]. OLEX2 program for Windows was used to draw the structure [36]. All non-H atoms were placed in calculated positions and refined as isotopic with the “riding-model technique.” The complete conditions of the data collection and structure are given in Table 1.Table 1
Crystal data and refinement parameters for [Cu(2-amino-5-bromopyridine)2(Cl)2].
Empirical formula
C10H10Br2Cl2CuN4
Formula weight
480.48
T (K)
296(2)
Wavelength (Å)
0.71073
Crystal system
Monoclinic
Space group
P2 (1)/n
a (Å)
12.8921(4)
b (Å)
4.22090(10)
c (Å)
15.6291(4)
α (°)
90
β (°)
113.730(2)
γ (°)
90
Vol. (Å3)
778.57(4)
Z
2
Calculated density (Mg/m3)
2.050
Absorption coefficient (mm−1)
6.868
F
(
000
)
462
Crystal size (mm)
0.54 × 0.09 × 0.04
Theta range for data collection (°)
3.45 to 28.11
−17 ≤h ≤ 17
Limiting indices
−5 ≤k ≤ 5
−20 ≤l ≤ 20
Reflections collected/unique
23239/1902 [R(int) = 0.0388]
Completeness to theta
28.11,     99.7%
Refinement method
Full-matrix least squares onF2
Data/restraints/parameters
1902/0/88
Goodness-of-fit onF2
0.985
FinalR  indices [I > 2σ(I)]
R
1 = 0.0329, wR2  = 0.0943
R  indices (all data)
R
1 = 0.0451, wR2  = 0.1026
Largest diff. peak and hole (e·A−3)
1.210 and −0.866
W
=
1
/
[
δ
2
(
F
o
2
)
+
(
0.0613
P
)
2
+
0.7908
P
].
P
=
(
F
o
2
+
2
F
C
2
)
/
3.
S
=
∑
[
w
(
F
o
2
-
F
C
2
)
2
/
(
N
obs
-
N
param
)
]
1
/
2
.


## 5. Results and Discussion

### 5.1. Electronic Spectra

The spectroscopic data for both compounds (Scheme2) are presented in Table 2. The electronic spectra of the complexes were obtained from a solid sample using the diffuse reflectance technique. Copper(II) compound (1) shows a broadband at about 720 nm. This band represents the ligand field transition for the CuN2Cl2 chromophore [37, 38]. The second absorption bands at 270 nm are assigned to charge transfer from the nonbonding orbital of chloride ions to the vacant copper(II) d orbitals [37, 39]. The last absorption band observed at about 257 nm is associated with π→π* or n→π* transitions of the ligand [40]. On the basis of simplest model three spin-allowed bands are expected in tetrahedral cobalt(II) complexes, that is, A42(F)→T42(F), A42(F)→T41(F), and A42(F)→T41(P). Usually such complexes show two bands between 830 and 625 nm, which can be assigned to A42(F)→T42(F) and A42(F)→T41(F), respectively; A42(F)→T41(P) is usually observed as a well-defined shoulder at about 550 nm [41]. The UV-Vis spectra of complex (2) were obtained from solid sample using diffuse reflectance technique. The compound shows a broad shoulder with bands at 810, 640, and 535 nm is due to d→d transitions. Absorption at 290 is assigned to charge transfer from the nonbonding orbital of chloride ions to half-filled d orbitals of cobalt(II) [42]. Absorption at 259 nm is associated with π→π* or n→π* transitions of the ligand [43, 44].Table 2
Spectroscopic data for both complexes.
Complex
IR (Cu–N) cm−1
UV-Vis (nm)
ESR (powder)
ESR (solution)
μ (R.T)
IR (Cu–Cl) cm−1
IR (Co–N) cm−1
IR (Co–Cl) cm−1
(1)
459
720
2.15
g
|| = 2.32
334
270
g
⊥ = 2.07
1.78
257
A
|| = 160 × 10−4
A
N
⊥  = 14 × 10−4
(2)
434
810, 640
—
—
4.12
322
535, 290
259Scheme 2
Structure of complexes.


### 5.2. Infrared Spectra

The IR spectra of the free ligand and the complexes were obtained in the range of 4000–300 cm−1. All the bands present in the IR spectra of the free ligand were also observed in the spectra of the complexes. Based on data from earlier reports [45, 46], we assigned bands at 459 and 434 cm−1 for (1) and (2), respectively, to M-N vibrations. M-Cl vibrations for (1) and (2) are assigned to 334 and 322 cm−1, respectively [47, 48].


### 5.3. Magnetic Moment

The magnetic moment of both complexes were determined by the Evans method [49]. This method is based on the principle that the position of a given proton resonance (t-butyl alcohol) in the spectrum of a molecule is dependent on the bulk susceptibility of the medium in which the molecule is found. The shift of a proton resonance line of an inert substance due to the presence of paramagnetic ions is given by theoretical expression:
(1)Δvvo=(2π3)(χv-χv′).
In this equation Δv is the shift, vo is the applied field, χv is the volume susceptibility of the solution containing paramagnetic ions, and χv′ is the volume susceptibility of the reference solution. The values of 1.78 and 4.12 were found for complexes (1) and (2), respectively.


### 5.4. Electron Spin Resonance

The electron spin resonance (ESR) spectra of complex (1) in powder and in frozen solution form were recorded at X-band frequencies at room temperature and at liquid nitrogen temperature to aid in determining the ground-state configuration of copper(II) ions. In the solid state, the spectra shows only a weak and broad signal (gave.=2.15). Such behavior is also found in the literature [50]. The observed frozen solution (77 K) ESR spectra of the compound in DMSO or DMF shows two bands located at g||=2.32 and g⊥=2.07 corresponding to the ΔMS=±1 transition. This feature (Figure 1) is typical for copper(II) complexes. The g||>g⊥>2.0023 also exhibit large parallel hyperfine splitting and are associated with the dx2-y2 ground state [51]. In the spectrum of complex, the parallel region is clearly resolved, and all four transitions derived from the Cu (I = 3/2) hyperfine splitting can be directly observed. These hyperfine lines for complexes split the gI signal with an average spacing of 160 × 10−4 cm−1. In the perpendicular region, the spectrum is not well resolved, although some superhyperfine structure is observed. The lack of clear resolution in this spectral region is presumably due to the large number of overlapping ΔMI=0 and ΔMI>0 transitions and to the large intrinsic widths of the individual transitions. The structure observed in the perpendicular region of the spectra is due to nitrogen superhyperfine splitting of the ligands. The AN⊥ value of ≈14 × 10−4 cm−1 and the presence of three peaks for the nitrogen superhyperfine structure of the complex are in accordance with expectations for two N donors per copper(II) ions [52, 53].Figure 1
ESR spectra of Cu(II) complex in DMF at liquid nitrogen temperature.


## 5.1. Electronic Spectra

The spectroscopic data for both compounds (Scheme2) are presented in Table 2. The electronic spectra of the complexes were obtained from a solid sample using the diffuse reflectance technique. Copper(II) compound (1) shows a broadband at about 720 nm. This band represents the ligand field transition for the CuN2Cl2 chromophore [37, 38]. The second absorption bands at 270 nm are assigned to charge transfer from the nonbonding orbital of chloride ions to the vacant copper(II) d orbitals [37, 39]. The last absorption band observed at about 257 nm is associated with π→π* or n→π* transitions of the ligand [40]. On the basis of simplest model three spin-allowed bands are expected in tetrahedral cobalt(II) complexes, that is, A42(F)→T42(F), A42(F)→T41(F), and A42(F)→T41(P). Usually such complexes show two bands between 830 and 625 nm, which can be assigned to A42(F)→T42(F) and A42(F)→T41(F), respectively; A42(F)→T41(P) is usually observed as a well-defined shoulder at about 550 nm [41]. The UV-Vis spectra of complex (2) were obtained from solid sample using diffuse reflectance technique. The compound shows a broad shoulder with bands at 810, 640, and 535 nm is due to d→d transitions. Absorption at 290 is assigned to charge transfer from the nonbonding orbital of chloride ions to half-filled d orbitals of cobalt(II) [42]. Absorption at 259 nm is associated with π→π* or n→π* transitions of the ligand [43, 44].Table 2
Spectroscopic data for both complexes.
Complex
IR (Cu–N) cm−1
UV-Vis (nm)
ESR (powder)
ESR (solution)
μ (R.T)
IR (Cu–Cl) cm−1
IR (Co–N) cm−1
IR (Co–Cl) cm−1
(1)
459
720
2.15
g
|| = 2.32
334
270
g
⊥ = 2.07
1.78
257
A
|| = 160 × 10−4
A
N
⊥  = 14 × 10−4
(2)
434
810, 640
—
—
4.12
322
535, 290
259Scheme 2
Structure of complexes.


## 5.2. Infrared Spectra

The IR spectra of the free ligand and the complexes were obtained in the range of 4000–300 cm−1. All the bands present in the IR spectra of the free ligand were also observed in the spectra of the complexes. Based on data from earlier reports [45, 46], we assigned bands at 459 and 434 cm−1 for (1) and (2), respectively, to M-N vibrations. M-Cl vibrations for (1) and (2) are assigned to 334 and 322 cm−1, respectively [47, 48].


## 5.3. Magnetic Moment

The magnetic moment of both complexes were determined by the Evans method [49]. This method is based on the principle that the position of a given proton resonance (t-butyl alcohol) in the spectrum of a molecule is dependent on the bulk susceptibility of the medium in which the molecule is found. The shift of a proton resonance line of an inert substance due to the presence of paramagnetic ions is given by theoretical expression:
(1)Δvvo=(2π3)(χv-χv′).
In this equation Δv is the shift, vo is the applied field, χv is the volume susceptibility of the solution containing paramagnetic ions, and χv′ is the volume susceptibility of the reference solution. The values of 1.78 and 4.12 were found for complexes (1) and (2), respectively.


## 5.4. Electron Spin Resonance

The electron spin resonance (ESR) spectra of complex (1) in powder and in frozen solution form were recorded at X-band frequencies at room temperature and at liquid nitrogen temperature to aid in determining the ground-state configuration of copper(II) ions. In the solid state, the spectra shows only a weak and broad signal (gave.=2.15). Such behavior is also found in the literature [50]. The observed frozen solution (77 K) ESR spectra of the compound in DMSO or DMF shows two bands located at g||=2.32 and g⊥=2.07 corresponding to the ΔMS=±1 transition. This feature (Figure 1) is typical for copper(II) complexes. The g||>g⊥>2.0023 also exhibit large parallel hyperfine splitting and are associated with the dx2-y2 ground state [51]. In the spectrum of complex, the parallel region is clearly resolved, and all four transitions derived from the Cu (I = 3/2) hyperfine splitting can be directly observed. These hyperfine lines for complexes split the gI signal with an average spacing of 160 × 10−4 cm−1. In the perpendicular region, the spectrum is not well resolved, although some superhyperfine structure is observed. The lack of clear resolution in this spectral region is presumably due to the large number of overlapping ΔMI=0 and ΔMI>0 transitions and to the large intrinsic widths of the individual transitions. The structure observed in the perpendicular region of the spectra is due to nitrogen superhyperfine splitting of the ligands. The AN⊥ value of ≈14 × 10−4 cm−1 and the presence of three peaks for the nitrogen superhyperfine structure of the complex are in accordance with expectations for two N donors per copper(II) ions [52, 53].Figure 1
ESR spectra of Cu(II) complex in DMF at liquid nitrogen temperature.


## 6. Description of the Crystal Structure of Complex (1)

The solid-state structure of complex (1) determined by single-crystal X-ray diffraction, together with atom labeling is shown in Figure 2. Selected bond lengths and bond angles are presented in Table 3. Crystallographic data and structure.Table 3
Selected bond distances [Å] and angles [°].
Lenghts [Å]
Angles [°]
Br–C(1)
1.884(3)
C(5)–N(2)–C(4)
118.4(3)
Cl–Cu
2.2584(7)
C(5)–N(2)–C(4)
118.4(3)
N(2)–C(5)
1.348(4)
C(4)–N(2)–Cu
126.8(2)
N(2)–C(4)
1.350(4)
C(5)–C(1)–C(2)
119.6(3)
N(2)–Cu
2.039(2)
C(5)–C(1)–Br
119.1(3)
C(1)–C(5)
1.355(4)
C(2)–C(1)–Br
121.3(3)
C(1)–C(2)
1.389(5)
C(3)–C(2)–C(1)
118.3(3)
C(2)–C(3)
1.348(5)
C(3)–C(2)–H(2)
120.9
C(2)–H(2)
0.9300
C(1)–C(2)–H(2)
120.9
C(3)–C(4)
1.407(4)
C(2)–C(3)–C(4)
120.8(3)
C(3)–H(3)
0.9300
C(2)–C(3)–H(3)
119.6
C(4)–N(1)
1.334(4)
C(2)–C(3)–H(3)
119.6
C(5)–H(5)
0.9300
N(1)–C(4)–N(2)
119.2(3)
N(1)–H(1A)
0.8600
N(1)–C(4)–C(3)
120.7(3)
N(1)–H(1B)
0.8600
N(2)–C(4)–C(3)
120.1(3)
Cu–N(2)a
2.039(2)
N(2)–C(5)–C(1)
122.8(3)
Cu–Cl(a)
2.2584(7)
N(2)–C(5)–H(5)
118.6
C(1)–C(5)–H(5)
118.6
C(1)–C(5)–H(5)
118.6
C(4)–N(1)–H(1A)
120.0
C(4)–N(1)–H(1B)
120.0
H(1A)–N(1)–H(1B)
120.0
N(2)a–Cu–N(2)
180.00(14)
N(2)a–Cu–Cl(a)
88.66(7)
N(2) –Cu–Cl(a)
91.34(7)
N(2)a1–Cu–Cl
91.34(7)
N(2)–Cu–Cl
88.66(7)
Symmetry transformations used to generate equivalent atoms:
#1 −x, −y + 2, −z + 1.Figure 2
Crystal structure of complex [Cu(2-amino-5-bromopyridine)2(Cl)2].Refinement parameters are summarized in Table1. The structure of complex (1) consists of a Cu surrounded by two ligands and two chlorides. In this complex Cu(II) has a slightly distorted square planar geometry, with trans position of two halides. The structure is essentially a D2h symmetric, and the copper(II) ion is four coordinate, as expected with trans halides. The Cu–N distances are 2.039 Å and Cu–Cl 2.2584 Å. The angles N–Cu–N and Cl–Cu–Cl are 180.0 (°), while the N(2)a–Cu–Cl(a) and N(2)–Cu–Cl angles are 88.66 (°). The angles N(2)–Cu–Cl(a) and N(2)a–Cu–Cl are 91.34 (°). Four-coordinate copper(II) compounds with two different ligands are almost distorted square planar symmetry from a coupling between electronic and vibration wave factions that lower the ground-state energy [54].



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