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Article

Structural and Electrochemical Studies of Cobalt(II) and Nickel(II) Coordination Polymers with 6-Oxonicotinate and 4,4′-Bipyridine

by
Ivana Škugor Rončević
1,
Nives Vladislavić
1,
Nabanita Chatterjee
2,
Vesna Sokol
3,
Clive L. Oliver
2,* and
Boris-Marko Kukovec
3,*
1
Department of General and Inorganic Chemistry, Faculty of Chemistry and Technology, University of Split, Ruđera Boškovića 35, 21000 Split, Croatia
2
Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
3
Department of Physical Chemistry, Faculty of Chemistry and Technology, University of Split, Ruđera Boškovića 35, 21000 Split, Croatia
*
Authors to whom correspondence should be addressed.
Chemosensors 2021, 9(12), 352; https://doi.org/10.3390/chemosensors9120352
Submission received: 15 November 2021 / Revised: 2 December 2021 / Accepted: 3 December 2021 / Published: 10 December 2021
(This article belongs to the Section Analytical Methods, Instrumentation and Miniaturization)
Browse Figures
Versions Notes

Abstract

:
The 6-oxonicotinate (6-Onic) salts of a one-dimensional cationic cobalt(II) or nickel(II) coordination polymers with 4,4′-bipyridine (4,4′-bpy), namely {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1) and {[Ni(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (2), were prepared hydrothermally by reactions of cobalt(II) nitrate hexahydrate or nickel(II) nitrate hexahydrate, respectively, 6-hydroxynicotinic acid and 4,4′-bipyridine in a mixture of ethanol and water. In the hydrogen-bonded frameworks of 1 and 2, the one-dimensional polymeric chains of {[M(4,4′-bpy)(H2O)4]2+}n (M = Co, Ni), the 6-oxonicotinate anions and the lattice water molecules were assembled via strong intermolecular O–H···O and N–H···O hydrogen bonds and π–π interactions, leading to the formation of the representative hydrogen-bond ring motifs: trimeric R23(10) motif, the centrosymmetric tetrameric R24(8) and R24(12) motifs and the pentameric R45(12) motif. The isostructural coordination polymers 1 and 2 exhibited a different electrochemical behavior, as observed by cyclic voltammetry, which can be attributed to the nature of the metal ions (cobalt(II) vs. nickel(II)).

1. Introduction

The design of coordination polymers has become an important field of crystal engineering due to their various functional properties and many possible applications, e.g., in catalysis, gas and energy storage, gas separation, magnetism, luminescence, molecular sensing, biomedical imaging [1,2,3,4,5,6,7,8]. The coordination preferences of the particular metal ions and the respective geometric and electronic properties of the ligands have been extensively studied in the past, but are still not understood completely. Furthermore, the supramolecular assembling of coordination polymers employing weak interactions (hydrogen bonds, halogen bonds, π–π interactions) is also less predictable, but gains more and more attention, in the hope to shed light on the control of these weak interactions in the formation of coordination polymers with the desired frameworks and properties.
By properly selecting experimental parameters such as a choice of solvents, starting metal salts, additional ligands, temperature, hydrothermal conditions, pH value of the reaction mixture etc., it is possible to optimize the design of the desired coordination polymers, and thus control the outcome of the crystallization experiments [9,10,11,12]. However, the preparation and crystallization of coordination polymers still remain very challenging.
The electrochemical properties of coordination polymers have been neglected in the past, but are attracting more interest as of recently. Cyclic voltammetry is the most frequently used electrochemical technique in the characterization of coordination polymers, whether providing information on redox activity [13,14] of the respective metal ions and ligands, or is simply used to elucidate the electrochemical sensor properties [15,16] of coordination polymers, e.g., as an electrode surface modifier because of their potential to undergo reversible single-electron processes which enable them to sense many analytes, e.g., anions [17,18], cations [19,20] or organic compounds [21]. The electrochemical sensing ability of coordination polymers is generally affected by the type of the corresponding metal ions and functional organic ligands. For instance, cobalt(II) coordination polymers are considered to be promising candidates for electrochemical sensing because of their low cost, high accessibility and great electrocatalytic performance [22]. On the other hand, organic ligands in coordination polymers contribute significantly to their electrochemical sensing potential, also enhancing their electron conduction rate [23].
We have prepared two structurally different 1D cobalt(II) coordination polymers with bromine and chlorine halosubstituents: {[Co(2-Clnic)2(4,4′-bpy)(H2O)2]·4H2O}n and {[Co(4,4′-bpy)(H2O)4] [Co(5-Brnic)4(H2O)2]·4,4′-bpy·2H2O}n. Using DFT calculations, the influence of the type and position of halosubstituents in the pyridine ring of nicotinate ligands was deciphered. Their position results in different structural types of coordination polymers. These structural differences should be ascribed solely to the different positions of the introduced halogen atoms in the pyridine ring of the ligands and not to the types of the halogen atoms [24]. Similar nickel(II) coordination polymers were prepared in the same way, with fluorine and chlorine halosubstituents: {[Ni(6-Fnic)2(4,4′-bpy)(H2O)2]·3H2O}n [25] and {[Ni(4,4′-bpy)(H2O)4](6-Clnic)2·4H2O}n [26]. Although being prepared under the same experimental conditions, two different types of 1D coordination polymers were obtained. The fluorine coordination polymer resembles the analogous chlorine cobalt(II) coordination polymer, while chlorine nickel(II) polymer consists of a complex cation {[Ni(4,4′-bpy)(H2O)4]2+}n and non-coordinated 6-chloronicotinate anions. Since all experimental parameters remained unchanged, these structural differences in the nickel(II) coordination polymers should be probably attributed to the type of halosubstituents (fluorine and chlorine) in the nicotinate ligands used [25,26].
Our current motivation was to further explore whether the introduction of substituents, other than the halide atoms, e.g., hydroxyl, in the specific position (-6) in the aromatic ring of the nicotinate ligands (6-hydroxynicotinate (6-OHnic) as opposed to 6-fluoronicotinate and 6-chloronicotinate) has any effect on the coordination environments of the metal ions and, consecutively, on the structural types of the obtained 1D coordination polymers. In order to ensure this, we again used well-established cobalt(II) and nickel(II) ions (with similar ionic radii, but different electronic configurations) and 4,4′-bipyridine linker to enable the formation of the analogous 1D polymers. Since the halosubstituents in the nicotinate ligands are now replaced by hydroxyl substituent, which have far greater potential in hydrogen-bonding than halosubstituents, we were particularly interested to check the differences in the formed hydrogen-bonded frameworks, with a special attention on the hydrogen-bond motifs within, which might arise from an enhanced participation of hydroxyl substituent in hydrogen bonding.
In this report, we prepared two isostructural 1D cobalt(II) and nickel coordination polymers with 4,4′-bipyridine (4,4′-bpy) and 6-oxonicotinate (6-Onic), namely {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1) and {[Ni(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (2). Voltammetric studies on these two polymers were carried out to explore their different electrochemical properties and to determine the origin of these electrochemical differences in this isostructural system.

2. Materials and Methods

2.1. Materials and Physical Measurements

The chemicals and materials were commercially available, of reagent purity and used as received without further purification.
A Carlo Erba microanalyzer at the Microanalytical Laboratory of the University of Vienna, Austria, was used to perform CHNO elemental analyses.
IR spectra (KBr pellets) were collected on a Shimadzu IRAffinity–1 FT-IR spectrometer in a range of 4000–400 cm−1.
TGA/DTA curves were recorded on a Shimadzu DTG-60H analyzer (from room temperature (RT) to 800 °C) in a stream of synthetic air and by applying a heating rate of 10 °C min−1.

2.2. Preparation of Coordination Polymers 1 and 2

6-Hydroxynicotinic acid (0.050 g, 0.358 mmol) was dissolved in 4 mL of distilled water, 4,4′-bipyridine (0.028 g, 0.179 mmol) was dissolved in 2 mL of ethanol and cobalt(II) (for 1) or nickel(II) (for 2) nitrate hexahydrate (0.052 g, 0.179 mmol) was dissolved in 2 mL of distilled water. The solutions of the two ligands were first mixed together under stirring. The resulting solutions was then slowly added to the cobalt(II) (for 1) or nickel(II) nitrate (for 2) solutions under stirring. The pH of the final solutions was adjusted to 7 by adding a sodium hydroxide solution dropwise. The reaction mixtures were heated in the Teflon lined autoclave for 3 days at 130 °C and then left to cool slowly to the room temperature over 1 day.
The pink precipitate of 1 and the green precipitate of 2 were obtained, collected by filtration, washed with water and dried in a desiccator over CaCl2. Yield of 1: 0.0376 g (35%). Yield of 2: 0.0231 g (21%).
The clear solutions, remained after filtration, were left to evaporate slowly at room temperature until pink crystals of 1 and green crystals of 2, suitable for X-ray diffraction measurements, were obtained. It took approximately two days for the crystals of 1 and seven days for the crystals of 2.
(1): 0.0283 g (26%). Anal. calc. for C22H28CoN4O12 (Mr = 599.41): C, 44.08; H, 4.71; N, 9.35; O, 32.03%. Found: C, 43.91; H, 4.73; N, 9.32; O, 33.34%. IR (KBr, cm−1): 3549–3415 (m) [ν(O–H)], 3234 (w), 2367 (w), 2028 (w), 1643 (s) and 1618 (s) [νas(COO−)], 1562 (m), 1530 (m), 1473 (w), 1422 (m), 1382 (s) [νs(COO−)], 1217 (m), 1132 (w), 1067 (w), 1030 (w), 986 (w), 941 (w), 802 (m), 733 (w), 637 (m), 497 (w), 469 (w), 401 (w) (Figure S1 in the Supplementary Materials).
(2): 0.0273 g (25%). Anal. calc. for C22H28NiN4O12 (Mr = 599.19): C, 44.07; H, 4.71; N, 9.33; O, 32.04%. Found: C, 43.81; H, 4.69; N, 9.33; O, 31.34%. IR (KBr, cm−1): 3413 (m) [ν(O–H)], 2035 (w), 1870 (w), 1643 (s) and 1618 (s) [νas(COO−)], 1562 (m), 1532 (m), 1476 (w), 1420 (m), 1382 (s) [νs(COO−)], 1220 (m), 1134 (w), 1067 (w), 1031 (w), 986 (w), 943 (w), 802 (m), 733 (w), 636 (m), 528 (w), 500 (w), 470 (w) (Figure S2 in the Supplementary Materials).
The CHNO analyses and IR spectroscopy were used to confirm that the precipitates and the crystals are the same compounds, in case of both 1 or 2.

2.3. Electrochemical Measurements

Preparation of MWCNT suspension (γ = 0.5 mg mL−1; V = 10 mL): 5 mg of MWCNT was added to 10 mL of N,N-dimethylformamide (DMF), and the mixture was treated in an ultrasonic bath for 10 h to obtain a homogeneous suspension.
Preparation of MWCNT|1(2) suspension (γ = 0.5 mg mL−1; V = 10 mL): 5 mg of MWCNT and 5 mg of coordination polymers 1 or 2 was added to 10 mL of N,N-dimethylformamide (DMF), and the mixture was treated in an ultrasonic bath for 10 h to obtain a homogeneous suspension.
The working electrode was glassy carbon (GC) electrode (electrode diameter 5 mm), modified with a suspension of multi-walled carbon nanotubes (MWCNT) and prepared polymers 1 and 2. Prior modification, GC electrode was polished with alumina powder (1 and 0.05 μm) and rinsed with distilled water. A homogeneous suspension (15 µL) was transferred dropwise to the GC surface and dried in an oven for 30 min at a temperature of 50 °C.
All electrochemical experiments were performed in a standard three-electrode cell (Pt-counter electrode and Ag|AgCl, with 3.0 mol dm−3 KCl, as a reference electrode). Cyclic voltammograms were carried out using a potentiostat (Autolab PGSTAT 302N), connected to a PC and GPES 4.9 Software (Eco Chemie). EIS measurements were carried out using a Solartron SI 1287 electrochemical interface and a Solartron SI 1255 frequency response analyzer controlled by a personal computer.
Cyclic voltammograms were recorded in a broader range from −1.0 to 1.2 V vs. Ag/AgCl, with a scan rate of 50 mV s−1 at MWCNT electrode, MWCNT|1 electrode and MWCNT|2 electrode in 0.1 mol L−1 KNO3 solution, pH 7.0 (Figure S3 in the Supplementary Materials). EIS measurements were performed at the open circuit potential (EOCP) over the frequency range 100 kHz to 10 mHz and 5 mV amplitude. Nyquist plots were fitted using the Randles modified equivalent circuit, deduced by using complex non-linear least squares (Figure S4 in the Supplementary Materials).

2.4. X-ray Crystallographic Analysis

The suitable single crystals of 1 and 2 were selected and mounted in Paratone-N oil onto cryoloops. The Bruker D8 Venture diffractometer producing MoKα (λ = 0.71073 Å) radiation at 50 kV and 1.4 mA, with the crystal cooled with an Oxford Cryostream 800 (100(2) K), was used for data collection. The cell refinement and data reduction were performed using SAINT [27]. The X-ray diffraction data have been corrected for Lorentz-polarization factor and scaled for absorption effects by multi-scan using SADABS [28]. The crystal structures were solved by SHELXT [29] and refined by SHELXL-2018/3 [30]. The refinement procedure was obtained by full-matrix least-squares methods based on F2 values against all reflections. The C1 and C2 atoms in polymers 1 and 2 were disordered and refined in two positions, resulting with site occupancy factors of 0.502(8) for 1 and 0.508(8) for 2. These values exhibit virtually 50:50 ratio of the disorder components of those atoms (all figures show only one component of the disordered atoms, C1A and C2A). The C–N and C–C distances around the disordered atoms were refined freely. The figures were made with MERCURY (Version 2020.2.0) [31]. The crystallographic data for 1 and 2 are shown in Table 1.

3. Results and Discussion

3.1. Syntheses

The coordination polymers 1 and 2 were prepared by reactions of cobalt(II) nitrate hexahydrate or nickel(II) nitrate hexahydrate, respectively, 6-hydroxynicotinic acid and 4,4′-bipyridine in the mixture of ethanol and water under hydrothermal conditions (Scheme 1). The said compounds can be prepared exclusively under hydrothermal conditions, as the syntheses attempts at normal pressure and room temperature lead to the crystallization of the ligands from the reaction mixtures.

3.2. Crystal Structures

The metal ions (cobalt(II) in 1 and nickel(II) in 2), nitrogen (N1 and N2) and carbon (C3 and C6) atoms are situated on a two-fold axis. Consequently, the asymmetric units of both 1 and 2 contain a half of a metal ion, two coordinated water molecules, a 6-oxonicotinate anion, a half of a 4,4′-bipyridine ligand and a lattice water molecule. The molecular structures of 1 and 2 contain a one-dimensional polymeric {[Co(4,4′-bpy)(H2O)4]2+}n cation in 1 and a {[Ni(4,4′-bpy)(H2O)4]2+}n cation in 2, respectively. Per repeating polymeric unit of 1 and 2, there are also two lattice water molecules and two 6-oxonicotinate anions in their keto tautomeric form (with the protonated pyridine nitrogen atoms) (Figure 1a,b).
Compounds 1 and 2 are isostructural, as can be seen from the overlay of the respective molecules of 1 and 2 (Figure 1c). Moreover, both compounds 1 and 2 crystallize in the same space group (C2/c) and with the similar unit cell parameters (Table 1). The metal(II) ions in the respective polymeric {[M(4,4′-bpy)(H2O)4]2+}n cations (M = Co, Ni) in 1 and 2 are octahedrally coordinated by four water molecule O atoms (O1, O2, O1i, and O2i) (symmetry code (i): −x+1, y, −z+3/2) and by two 4,4′-bipyridine N atoms (N1 and N2) in the trans position (N1–M–N2 = 180°) (Figure 1a,b; Table S1 in the Supplementary Materials).
The infinite one-dimensional polymeric chains of {[M(4,4′-bpy)(H2O)4]2+}n extend along the [0 1 0] direction and are composed of the metal(II) ions connected with bridging 4,4′-bipyridine ligands (Figure 2a). As can be seen from the angles for the trans (175.8(2)° and 175.1(2)° in 1 and 176.1(2)° and 174.6(2)° in 2) and cis (87.55(8)°–92.45(8)° in 1 and 87.29(9)°–92.71(9)° in 2) pairs of the ligating atoms, the octahedral coordination environments around the metal(II) ions in 1 and 2 are only slightly distorted (Table S1 in the Supplementary Materials). The M–O and M–N bond lengths in 1 and 2 can be compared to the analogous bond lengths in the already known structures containing {[Co(4,4′-bpy)(H2O)4]2+}n [32] or {[Ni(4,4′-bpy)(H2O)4]2+}n cations [32,33,34,35,36,37,38,39,40].
There are strong intermolecular O–H···O and N–H···O hydrogen bonds and weak C–H···O hydrogen bonds, observed in the structures of 1 and 2 (Table 2). The polymeric chains of {[M(4,4′-bpy)(H2O)4]2+}n, the 6-oxonicotinate anions and the lattice water molecules are connected by strong hydrogen bonds into hydrogen-bonded frameworks (Figure 2b). There are two types of π–π interactions in the respective structures:
π–π interactions between 4,4′-bipyridine and 6-oxonicotinate pyridine rings [a Cg1···Cg2(–x, –y, 1–z) distance = 3.871(2) Å (in 1) and 3.874(2) Å (in 2); dihedral angle between the planes = 11.9(2)° (in 1) and 12.2(2)° (in 2); slippage = 1.368 Å (in 1) and 1.355 Å (in 2); Cg1 is the centroid of the 4,4′-bipyridine ring N2/C4/C5/C6/C5i/C4i and Cg2 is the centroid of the 6-oxonicotinate pyridine ring N3/C7–C11], and π–π interactions between symmetry-related 6-oxonicotinate pyridine rings N3/C7–C11 [a Cg2···Cg2(1/2–x, 1/2–y, 1–z) distance = 3.613(3) Å (in 1) and 3.573(3) Å (in 2); dihedral angle between the planes = 0.0(2)° in 1 and 2 and a slippage = 1.206 Å (in 1) and 1.098 Å (in 2)].
The polymeric chains of {[M(4,4′-bpy)(H2O)4]2+}n are hydrogen-bonded solely to 6-oxonicotinate anions and lattice water molecules. However, besides being hydrogen-bonded to other 6-oxonicotinate anions, lattice water molecules and polymeric {[M(4,4′-bpy)(H2O)4]2+}n cations, the 6-oxonicotinate anions are additionally assembled together by π–π interactions between symmetry-related 6-oxonicotinate pyridine rings (Cg2···Cg2) within the hydrogen-bonded framework of 1 and 2.
The most distinguished hydrogen-bonded ring motifs within the frameworks of 1 and 2 are the trimeric R23(10) motif, the centrosymmetric tetrameric R24(8) and R24(12) motifs and the pentameric R45(12) motif (Figure 3).
The trimeric R23(10) motif is formed between two 6-oxonicotinate anions (one of them linked via an oxo atom and a pyridinium N atom, the other via carboxylate O atom) and a polymeric cation (linked via two coordinated water molecules). The tetrameric R24(8) motif is formed between two lattice water molecules and two 6-oxonicotinate ions (both linked via a single carboxylate O atom). In addition, a tetrameric R24(12) motif is formed between two polymeric cations (both linked via two coordinated water molecules) and two 6-oxonicotinate anions (both linked via a single oxo atom). The pentameric R45(12) motif is formed between two polymeric cations (both linked via a single coordinated water molecule), two 6-oxonicotinate anions (one of them linked via two carboxylate O atoms and the other via oxo atom) and a single lattice water molecule.

3.3. Thermal Analysis

The TGA/DTA curves of compounds 1 and 2 are almost identical, as it would be expected for isostructural compounds (Figures S5 and S6 in the Supplementary Materials). Both compounds are thermally stable up to 100 °C, followed by a release of four coordinated and two lattice water molecules in a single step with an endothermic peak at 137.2 °C in 1 and 143.3 °C in 2 (observed mass losses 18.4% in 1 and 18.6% in 2, calculated 18.0% in 1 and 2). Both coordinated and lattice water molecules are released at the same temperature most probably due to the existence of thermally stable hydrogen-bonded frameworks of 1 and 2 (as revealed by their crystal structures), in which these water molecules participate extensively. Therefore, their simultaneous release upon the collapse of hydrogen-bonded frameworks of 1 and 2 is expected. The thermal decomposition of 1 and 2 continues through two consecutive, not well-resolved steps; first one representing an endothermic process (peaks at 342.7 °C in 1 and 350.2 °C in 2) and the second representing an exothermic process (peaks at 403.5 °C in 1 and 394.9 °C in 2). These endothermic (observed mass losses 41.9% in 1 and 34.7% in 2) and exothermic (observed mass losses 27.5% in 1 and 36.0% in 2) steps correspond to the complete decomposition of 6-oxonicotinate and 4,4′-bipyridine ligands. The observed residues (12.2% in 1 and 10.7% in 2) at 800 °C correspond to CoO in the case of 1 and to NiO in the case of 2. The last decomposition steps in 1 and 2 are exothermic, which is in accordance with the formation of thermodynamically stable metal oxides (CoO and NiO) upon total decomposition of 1 and 2, respectively.

3.4. Electrochemical Characterization

The electrochemical properties of the modified glassy carbon (GC) electrodes were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using a solution of 1 mmol L−1 [Fe(CN)6]3−/4− in 0.1 mol L−1 KNO3. Figure 4 shows CVs of 1 mmol L−1 [Fe(CN)6]3−/4− and 0.1 mol L−1 KNO3 at multi-walled carbon nanotubes (MWCNT) electrode, MWCNT|1 electrode and MWCNT|2 electrode, recorded in a potential range from −0.2 to 0.6 V (vs. Ag/AgCl) with a scan rate of 50 mV s−1. Potential ranges depend on the nature of electrode material and is established after recording in a wider potential range. Those limits agreed roughly with the potential range characteristic for Fe(III)/Fe(II) redox couple.
As observed in Figure 4, the MWCNT|2 electrode exhibits two redox waves, one quasi-reversible (ΔEp is much larger than theoretical value [41]) and one reversible. The quasi-reversible peak corresponds to the Fe(III)/Fe(II) couple, Epa1 = 0.462 V, Epc1 = −0.086 V, and the average formal potential E1/2 is 0.188 V. The peak-to-peak separation between the anodic and cathodic peaks (ΔEp) is 0.548 V. The ratio of cathodic peak current to anodic peak current (ipc1/ipa1) is −20.826 (ipc1 = −45.40 μA, ipa1 = 2.18 μA), and the ratio of anodic peak current to cathodic peak current (ipa1/ipc1) is −0.048. The second reduction peak at ca. 0.406 V vs. Ag/AgCl corresponds to the reduction of 2. The anodic peak corresponds to Fe(III)/Fe(II) couple obtained at MWCNT|1 and is significantly lower than those obtained at MWCNT. In all cases, the broadened and little defined peaks are obtained with the higher oxidation potential (compound 1, Ep,a = 467 mV, and compound 2, Ep,a = 462 mV) with respect to the MWCNT (Ep,a = 0.269 mV), revealing an electroinhibitory effect of the 1 and 2 on the Fe(III)/Fe(II) redox reaction. The large peak-to-peak separation in the corresponding cyclic voltammograms (ΔE 548 mV vs. ΔE 146 mV of the MWCNT) in comparison with the MWCNT electrode can also be attributed to the significant electroinhibitory effect of the 1 and 2 on the Fe(III)/Fe(II) redox reaction.
Cyclic voltammograms suggest very similar behavior of both 1 and 2 vs. Fe(III)/Fe(II) redox reaction, while in the case of polymer 2 there is an additional redox pair corresponding to a redox reaction of polymer 2. The observed differences can be related to the reaction between 2 and the Fe(III)/Fe(II) system and can be attributed to the different electronic configurations of cobalt(II) and nickel(II) ions, leading to a different reactivity of 1 and 2 towards the Fe(III)/Fe(II) system.

4. Conclusions

The introduction of a hydroxyl substituent (instead of a chlorine atom) at position -6 in the pyridine ring of nicotinate ligands did not affect either coordination environments of the metal ions (cobalt(II), nickel(II)) or structural type of the obtained 1D coordination polymers 1 and 2, if compared to the structurally analogous nickel(II) coordination polymer with 6-chloronicotinate and 4,4′-bipyridine ligands, {[Ni(4,4′-bpy)(H2O)4](6-Clnic)2·4H2O}n. The only difference is in the number of lattice water molecules [26]. Indeed, the hydroxyl substituent participates more extensively in the hydrogen bonding than it was the case with chlorine substituent, leading to the different hydrogen-bond motifs in the hydrogen-bonded frameworks of 1 and 2, as compared to {[Ni(4,4′-bpy)(H2O)4](6-Clnic)2·4H2O}n (only R24(8) motif is the same). However, other motifs are related, differing only in the ring size (e.g., R45(12) and R24(12) in 1 and 2 comparing to R45(16) and R24(8) in {[Ni(4,4′-bpy)(H2O)4](6-Clnic)2·4H2O}n, respectively) [26].
The electrochemical studies of coordination polymers 1 and 2 showed that the reduction is metal-centered, and only polymer 2 underwent a reduction in the region of potentials from −0.2 to 0.6 V vs. Ag|AgCl, while polymer 1 did not exhibit any redox activity. This different electrochemical behavior of the isostructural 1 and 2 can be attributed to the nature of metal ions (cobalt(II) vs. nickel(II)). Both metal ions have different electronic configurations, leading to a different reactivity of 1 and 2 towards the Fe(III)/Fe(II) system.

Supplementary Materials

The following are available online at www.mdpi.com/article/10.3390/chemosensors9120352/s1, Table S1. Selected bond lengths (Å) and angles (°) for {[Co(4,4’-bpy)(H2O)4](6-Onic)2·2H2O}n (1) and {[Ni(4,4’-bpy)(H2O)4](6-Onic)2·2H2O}n (2), Figure S1. IR spectrum of {[Co(4,4’-bpy)(H2O)4](6-Onic)2·2H2O}n (1), Figure S2. IR spectrum of {[Ni(4,4’-bpy)(H2O)4](6-Onic)2·2H2O}n (2), Figure S3. Cyclic voltammograms recorded in a broader range at MWCNT electrode, MWCNT|1 electrode and MWCNT|2 electrode in 0.1 mol L−1 KNO3 solution (pH 7.0), Figure S4. Electrochemical impedance spectra of the 1 mmol L−1 [Fe(CN)6]3−/4− redox probe in 0.1 mol L−1 KNO3 (pH 7.0) recorded at Eocp for MWCNT electrode, MWCNT|1 electrode and MWCNT|2 electrode, Figure S5. TGA/DTA curve of {[Co(4,4’-bpy)(H2O)4](6-Onic)2·2H2O}n (1), Figure S6. TGA/DTA curve of {[Ni(4,4’-bpy)(H2O)4](6-Onic)2·2H2O}n (2). Deposition numbers 2122274 (for 1) and 2122275 (for 2) contain the supplementary crystallographic data for this paper.

Author Contributions

Conceptualization, C.L.O. and B.-M.K.; methodology, all authors; valida tion, all authors; formal analysis, I.Š.R., C.L.O. and B.-M.K.; investigation, all authors; resources, I.Š.R., N.V., V.S., C.L.O. and B.-M.K.; data curation C.L.O. and B.-M.K.; writing—original draft preparation, I.Š.R. and B.-M.K.; writing—review and editing, all authors; visualization, I.Š.R. and B.-M.K.; project administration, B.-M.K.; funding acquisition, I.Š.R., N.V., V.S., C.L.O. and B.-M.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research and the APC were funded by the Faculty of Chemistry and Technology, University of Split institutional funding. C.L.O. also thanks the National Research Foundation of South Africa for funding (grant no: 120849).

Data Availability Statement

These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service at www.ccdc.cam.ac.uk/structures.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

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Chemosensors 09 00352 sch001 550
Scheme 1. Preparation of {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1) and {[Ni(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (2) under hydrothermal conditions.
Scheme 1. Preparation of {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1) and {[Ni(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (2) under hydrothermal conditions.
Chemosensors 09 00352 sch001
Chemosensors 09 00352 g001 550
Figure 1. ORTEP-style plots of {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1) (a) and {[Ni(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (2) (b), with the atomic numbering schemes (symmetry code (i): −x+1, y, −z+3/2). The thermal ellipsoids are drawn at the 50% probability level at 100(2) K and hydrogen atoms are shown as spheres of arbitrary radii; the components of the disordered C1 and C2 atoms in 1 and 2 (C1B and C2B) are not shown. The overlay (RMS value of 0.0447 Å) (c) of {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1) (red) and {[Ni(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (2) (blue). The Co, Ni, N, and O atoms were chosen for the overlay.
Figure 1. ORTEP-style plots of {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1) (a) and {[Ni(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (2) (b), with the atomic numbering schemes (symmetry code (i): −x+1, y, −z+3/2). The thermal ellipsoids are drawn at the 50% probability level at 100(2) K and hydrogen atoms are shown as spheres of arbitrary radii; the components of the disordered C1 and C2 atoms in 1 and 2 (C1B and C2B) are not shown. The overlay (RMS value of 0.0447 Å) (c) of {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1) (red) and {[Ni(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (2) (blue). The Co, Ni, N, and O atoms were chosen for the overlay.
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Chemosensors 09 00352 g002 550
Figure 2. The infinite one-dimensional polymeric chain of {[Co(4,4′-bpy)(H2O)4]2+}n cation in 1, extending along the [0 1 0] direction (a). A fragment of the infinite hydrogen-bonded framework of {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1), viewed down the [0 1 0] direction (b), with the {[Co(4,4′-bpy)(H2O)4]2+}n cations (represented as monomeric molecules in this projection), 6-oxonicotinate anions and lattice water molecules being connected via strong intermolecular O–H···O and N–H···O hydrogen bonds (represented by the dotted lines).
Figure 2. The infinite one-dimensional polymeric chain of {[Co(4,4′-bpy)(H2O)4]2+}n cation in 1, extending along the [0 1 0] direction (a). A fragment of the infinite hydrogen-bonded framework of {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1), viewed down the [0 1 0] direction (b), with the {[Co(4,4′-bpy)(H2O)4]2+}n cations (represented as monomeric molecules in this projection), 6-oxonicotinate anions and lattice water molecules being connected via strong intermolecular O–H···O and N–H···O hydrogen bonds (represented by the dotted lines).
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Chemosensors 09 00352 g003 550
Figure 3. The most distinguished hydrogen-bonded ring motifs (shown by dotted lines) found within the hydrogen-bonded framework of {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1), e.g., the trimeric R23(10) motif, the centrosymmetric tetrameric R24(8) and R24(12) motifs and the pentameric R45(12) motif.
Figure 3. The most distinguished hydrogen-bonded ring motifs (shown by dotted lines) found within the hydrogen-bonded framework of {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1), e.g., the trimeric R23(10) motif, the centrosymmetric tetrameric R24(8) and R24(12) motifs and the pentameric R45(12) motif.
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Chemosensors 09 00352 g004 550
Figure 4. Cyclic voltammograms of the 1 mmol L−1 [Fe(CN)6]3−/4− redox probe in 0.1 mol L−1 KNO3 (pH 7.0) at MWCNT electrode, MWCNT|1 electrode and MWCNT|2 electrode recorded with a scan rate of 50 mV s−1.
Figure 4. Cyclic voltammograms of the 1 mmol L−1 [Fe(CN)6]3−/4− redox probe in 0.1 mol L−1 KNO3 (pH 7.0) at MWCNT electrode, MWCNT|1 electrode and MWCNT|2 electrode recorded with a scan rate of 50 mV s−1.
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Table
Table 1. The crystallographic data for {[Co(4,4′-bpy)(H2O)4](6-OHnic)2·2H2O}n (1) and {[Ni(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (2).
Table 1. The crystallographic data for {[Co(4,4′-bpy)(H2O)4](6-OHnic)2·2H2O}n (1) and {[Ni(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (2).
Compound12
Formula{[C10H16CoN2O4](C6H4NO3)2·2H2O}n{[C10H16NiN2O4](C6H4NO3)2·2H2O}n
Mr599.41599.19
Crystal system, space groupmonoclinic, C2/c
(No. 15)		monoclinic, C2/c
(No. 15)
a (Å)19.0114(13)18.9891(9)
b (Å)11.4133(7)11.3111(5)
c (Å)12.0374(7)12.0639(6)
β (°)99.333(2)99.169(2)
V (Å3)2577.3(3)2558.1(2)
Z44
Dcalc (g cm−3)1.5451.556
μ (mm−1)0.7370.830
R [I ≥ 2σ(I)]0.04800.0473
wR [all data]0.12600.1228
Table
Table 2. The hydrogen bond geometry for {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1) and {[Ni(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (2).
Table 2. The hydrogen bond geometry for {[Co(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (1) and {[Ni(4,4′-bpy)(H2O)4](6-Onic)2·2H2O}n (2).
D–H···Ad(D–H)/Åd(H···A)/Åd(D···A)/Å∠(D–H···A)/°Symmetry Code on A
1
N3–H31···O30.88(1)1.88(2)2.732(4)163(4)−x + 1/2, y + 1/2, −z + 3/2
O1–H11···O50.84(1)1.85(1)2.680(4)172(4)x, −y + 1, z + 1/2
O1–H12···O30.84(1)1.87(1)2.699(4)169(4)−x + 1/2, y + 1/2, −z + 3/2
O2–H21···O60.84(1)1.87(1)2.704(4)170(5)x, y, z
O2–H22···O50.84(1)1.90(2)2.729(4)167(5)x, y, z
O6–H61···O40.84(1)1.99(1)2.823(4)175(5)−x + 1/2, −y + 1/2, −z + 1
O6–H62···O40.84(1)1.92(1)2.755(4)175(4)x, y + 1, z
C1B–H1B···O10.952.452.989(9)116−x + 1, y, −z + 3/2
C7–H7···O40.952.603.287(5)130−x + 1/2, y + 1/2, −z + 3/2
2
N3–H31···O30.88(1)1.88(2)2.721(4)161(4)−x + 1/2, y + 1/2, −z + 3/2
O1–H11···O50.84(1)1.86(2)2.687(4)166(5)x, −y + 1, z + 1/2
O1–H12···O30.84(1)1.86(1)2.696(4)171(4)−x + 1/2, y + 1/2, −z + 3/2
O2–H21···O60.84(1)1.87(1)2.702(4)172(5)x, y, z
O2–H22···O50.84(1)1.90(2)2.724(4)165(5)x, y, z
O6–H61···O40.84(1)1.99(1)2.823(4)175(4)−x + 1/2, −y + 1/2, −z + 1
O6–H62···O40.84(1)1.91(1)2.748(4)174(5)x, y + 1, z
C1B–H1B···O10.952.402.923(9)115−x + 1, y, −z + 3/2
C7–H7···O40.952.573.270(5)131−x + 1/2, y + 1/2, −z + 3/2
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Škugor Rončević, I.; Vladislavić, N.; Chatterjee, N.; Sokol, V.; Oliver, C.L.; Kukovec, B.-M. Structural and Electrochemical Studies of Cobalt(II) and Nickel(II) Coordination Polymers with 6-Oxonicotinate and 4,4′-Bipyridine. Chemosensors 2021, 9, 352. https://doi.org/10.3390/chemosensors9120352

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Škugor Rončević I, Vladislavić N, Chatterjee N, Sokol V, Oliver CL, Kukovec B-M. Structural and Electrochemical Studies of Cobalt(II) and Nickel(II) Coordination Polymers with 6-Oxonicotinate and 4,4′-Bipyridine. Chemosensors. 2021; 9(12):352. https://doi.org/10.3390/chemosensors9120352

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Škugor Rončević, Ivana, Nives Vladislavić, Nabanita Chatterjee, Vesna Sokol, Clive L. Oliver, and Boris-Marko Kukovec. 2021. "Structural and Electrochemical Studies of Cobalt(II) and Nickel(II) Coordination Polymers with 6-Oxonicotinate and 4,4′-Bipyridine" Chemosensors 9, no. 12: 352. https://doi.org/10.3390/chemosensors9120352

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