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New Mn(II) complexes with benzoxazole-based ligands: Synthesis, structure and their electrochemical behavior
Accepted Manuscript New Mn(II) complexes with benzoxazole-based ligands: synthesis, structure and their electrochemical behavior Dawid Marcinkowski, Marta A. Fik, Teresa Łuczak, Maciej Kubicki, Violetta Patroniak PII: DOI: Reference:
S0277-5387(17)30767-2 https://doi.org/10.1016/j.poly.2017.11.039 POLY 12941
To appear in:
Polyhedron
Received Date: Accepted Date:
27 October 2017 26 November 2017
Please cite this article as: D. Marcinkowski, M.A. Fik, T. Łuczak, M. Kubicki, V. Patroniak, New Mn(II) complexes with benzoxazole-based ligands: synthesis, structure and their electrochemical behavior, Polyhedron (2017), doi: https://doi.org/10.1016/j.poly.2017.11.039
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New Mn(II) complexes with benzoxazole-based ligands: synthesis, structure and their electrochemical behavior Dawid Marcinkowski, Marta A. Fik, Teresa Łuczak, Maciej Kubicki, Violetta Patroniak* Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61614 Poznań, Poland *Fax: +48 618291508; Tel: +48 618291677; E-mail: [email protected] Graphical abstract Abstract Six complexes of various Mn(II) salts: MnBr2·4H2O (1), Mn(NO3)2·4H2O (2), MnCl2·4H2O (3) with 2-(1-methyl-2-(pyridin-2-ylmethylene)hydrazinyl)benzo[d]oxazole LA (complexes 1A – 3A) and 2-(2-((1H-imidazol-4-yl)methylene)-1-methylhydrazinyl)benzo[d]oxazole HLB (complexes 1B – 3B) have been synthesized. All complexes were obtained in 1:1 stoichiometry. Their structures have been established through analytical and spectroscopic (ESI-MS, IR, 1H NMR and microanalyses) methods as well as by X-ray structure determinations. In order to gain some insights into the electron-richness of studied systems, the electrochemical properties of ligands LA and HLB and complexes 1A and 1B were investigated by cyclic voltammetry. Complexes 1A and 1B were chosen due to isostructurality what allows to the direct comparison of their behavior. The impact of pyridine and imidazole substituents on electrochemical behavior of synthesized systems was evaluated and showed that the presence of NH moiety in imidazole ring of HLB does not feasible the adsorption on the bare gold electrode. 1. Introduction The ongoing interest in well-defined stable systems exhibiting some distinct properties has led researchers to extensively explore the chemistry of Schiff-bases. They are common enzymatic intermediates where an amine reversibly reacts in vivo with an aldehyde or ketone of cofactor of substrate, e.g. the common enzyme cofactor PLP (pyridoxal 5’-phosphate) forms an intermediate imine with lysine residue and is transferred to the substrate [1]. Moreover, they participate in photosynthesis and oxygen transportation in living organisms [2]. The versatility of imines and their ability to coordinate metal ions has allowed one to create a great variety of potentially applicative materials. Many complexes of Schiff-base ligands with d-electron metal ions have been investigated as models for active sites of enzymes [3], antifungal [4] and antibacterial [5,6] as well as anticancer [7,8] drugs. Their biocompatibility enables biologists to develop new DNA-cleavage and repair systems [9-11]. They also provide some valuable magnetic properties which are useful in fabrication of novel electronic nano-devices [12,13]. They are already used as catalysts in multiple organic and metalorganic reaction such as, for instance, ring-opening olefin epoxidation [14] or alcohol oxidation [15] as well as in photocatalytic organic matter degradation processes [16]. Some d10-electron metal ions (as Zn2+, Cd2+, Ag+) coordinated by Schiff base type ligands display intense luminescence and exhibit high quantum yields thus may be utilized in design of new OLED’s (Organic Light Emitting Diodes) or as components in WOLED’s (White Organic Light Emitting Diodes) construction [17,18]. The coordination chemistry of Mn(II) has been extensively explored in recent years mainly due to its essential role in biological systems. For instance, Manganese ions act as cofactors of numerous enzymes, such as oxidoreductases, transferases, ligases or lyases [19]. Manganese is also present in eukaryotic mitochondria in the form of superoxide dismutase (Mn-SOD), which is responsible for deactivation of superoxide O2 formed from the one1
electron reduction of oxygen [20]. At the same time, the role of Manganese in plant cells cannot be neglected, since it is present in the oxygen-evolving complex (OVC) which is a part of photosystem II [21]. The latter one continuously encourages researchers to evaluate the redox activeness of artificial, bio-mimetic manganese complexes in H2O splitting processes because it could be used for solar energy conversion [22,23]. The aim of our work presented here was to obtain and characterize new Mn(II) redox active compounds. Therefore, we have investigated structural properties of six new complexes with N3-donor Schiff base ligands containing benzoxazole moieties and evaluated electrochemical properties of two of them (Fig. 1.) of general formula [MnLBr2(CH3OH)]. The impact of pyridine and imidazole substituents on electrochemical behavior of synthesized systems was evaluated. 2. Experimental 2.1. Materials and methods The metal salts and organic compounds as well as NaOH, H2SO4 and H2O2, LiClO4 were supplied from Aldrich and POCH. Tetrabutyloammonium perchlorate (TBAP) and dimethylformamide (DMF) were purchased from Merck. All chemicals mentioned above were of analytical grade quality and were used as obtained without further purification. NMR spectra were run on a Varian Gemini 300 MHz spectrometer and were calibrated against the residual protonated solvent signals (DMSO-d6, δ 2.50) which are given in parts per milion. ESI mass spectra for MeOH solutions ~ 10-4 M were measured using a Waters Micromass ZQ spectrometer. Microanalyses were obtained using a Vario EL III CHN element analyzer. IR spectra were recorded with a Bruker FTIR IFS 66/s spectrometer and peak positions are reported in cm-1. 2.2. Synthesis of ligands LA and HLB Ligand LA and intermediate A were synthesized according to previously described procedure [24]. Ligand HLB was obtained within two subsequent steps starting from commercially available 2-chlorobenzoxazole according to the Fig. 1. Formation of products thus formed was established by 1H NMR, 13C NMR, FT-IR spectroscopy and ESI-MS spectrometry.
Fig. 1. Perspective of synthetic protocol of ligands LA and HLB. 2.2.1. Synthesis of ligand HLB To the colorless solution of A (120.1 mg, 666 mol) in EtOHabs (4 ml) an equimolar amount of 4-(2H-imidazole)carboxyaldehyde (64.0 mg, 666mol) was added. The mixture was allowed to react for 4 h under inert atmosphere in reflux and for another 20 h at ambient temperature. The white crystalline solid precipitated overnight and was filtered under reduced pressure. Yield: 151.6 mg, 82.3%. 1H NMR (DMSO-d6, 300 MHz): δ(ppm) – 12.51 (sbroad, 1H), 7.98 (s, 1H), 7.76 (s, 1H), 7.53 (d, 1H, J = 7.78 Hz), 7.49 (s, 1H), 7.43 (d, 1H, J = 7.78 Hz), 7.23 (t, 1H, J = 7.57 Hz), 7.12 (t, 1H, J = 7.78 Hz), 3.74 (s, 3H). 13C NMR (DMSO-d6, 300 MHz): δ(ppm) – 168.36, 153.23, 151.62, 149.52, 138.91, 136.93, 131.73, 126.00, 123.82, 122.49, 121.40, 119.99, 119.43, 32.73. IR (KBr, cm-1): ν(N-H)imidazole 3116; ν(C-H)arom 3061, 3027; νas(C-H)alif 2969; νs(C-H)alif 2953; ν(C=C) 1639, 1579; ν(C=N) 1458, 1446, δ(CH3) 1296, ν(C-O) 1245, 1156, γ(C-H)arom 992, 926, 909, 853, 808, 747, 626. ESI-MS(+) m/z (%):
2
242 (100) [H2L]+; (-) m/z (%): 240 (100) [L]-. Anal. calc. for (C12H11N5O) (240.25): C, 59.99; H, 4.62; N, 29.16; found: C, 62.99; H, 4.29; N, 30.91%. 2.3. Synthesis of complexes 1A – 3A and 1B – 3B Complexes were prepared in the same manner by treating equimolar amounts of the ligands LA (30.5 mg, 126 µM) and HLB (31.8 mg, 126 µM) in MeOH (10 ml) with MnBr2·4H2O (36.1 mg, 126 µM) (1), Mn(NO3)2·4H2O (31.6 mg, 126 µM) (2) and MnCl2·4H2O (24.9 mg, 126 µM) (3) in 5 ml of MeOH. Pale yellow solutions formed instantly and the reaction mixtures were stirred for 24 h at room temperature. After evaporation of solvents under reduced pressure, the residues were dissolved in minimum volume of MeOH and precipitated by excess of Et2O. Orange solids were filtered via suction filtration and dried in the vacuum. 2.3.1. Complex 1A [Mn(LA)Br2(CH3OH)] Yield: 39.2 mg, 65.0%. Crystals suitable for X-ray analysis were obtained via slow diffusion methods in MeOH/iPr2O system. IR (KBr, cm-1): νbroad(O-H)methanol 3447; ν(C-H)arom 3025; νas(C-H)alif 2971; νs(C-H)alif 2955; ν(C=C) 1641, 1635, 1583; ν(C=N) 1463, 1410, 1367, δ(CH3) 1291, ν(C-O) 1256, 1249, 1150, γ(C-H)arom 1087, 1038, 1011, 943, 933, 898, 819, 784. ESI-MS(+) m/z (%): 153 (30) [MnLA]2+, 253 (100) [HLA]+, 280 (100) [Mn(LA)2]2+, 387 (40) [MnLABr]+; (-) m/z (%): 294 (100) [MnBr3]-. Anal. calc. for [Mn(C14H12N4O)Br2(CH3OH)] (499.08): C, 36.01; H, 3.24; N, 11.23; found: C, 35.57; H, 3.45; N, 11.04%. 2.3.2. Complex 2A [Mn(LA)(O2-NO)(O-NO2)(CH3OH)] Yield: 34.7 mg, 70.5%. Crystal suitable for X-ray analysis was obtained via slow diffusion methods in MeOH/tBuOMe system. IR (KBr, cm-1):νbroad(O-H)methanol 3375; ν(C-H)arom 3084, 3023; νas(C-H)alif 2968; νs(C-H)alif 2938; ν(C=C) 1636, 1620, 1581; ν(C=N) 1456, 1404, 1365, δ(CH3) 1289, ν(C-O) 1250, 1151, 1097, γ(C-H)arom 1037, 1006, 941, 898, 817, 740. ESIMS(+) m/z (%): 253 (40) [HLA]+, 275 (20) [NaLA]+, 369 (100) [MnLANO3]+, 621 (10) [Mn(LA)2NO3]+; (-) m/z (%): 241 (100) [Mn(NO3)3]-. Anal. calc. for [Mn(C14H12N4O)(NO3)2(CH3OH)] (463.28): C, 38.88; H, 3.49; N, 18.14; found: C, 37.52; H, 3.45; N, 17.25%. 2.3.3. Complex 3A [Mn(LA)Cl2] Yield: 28.9 mg, 73.4%. Crystals suitable for X-ray analysis were obtained via slow diffusion methods in MeOH/iPr2O system. IR (KBr, cm-1):νbroad(O-H)methanol 3424; ν(C-H)arom 3096, 3052, 3022; νas(C-H)alif 2958; νs(C-H)alif 2920; ν(C=C) 1633, 1620, 1580; ν(C=N) 1464, 1406, 1368, δ(CH3) 1289, ν(C-O) 1254, 1143, 1100, γ(C-H)arom 1036, 1011, 944, 933, 898, 817, 760. ESI-MS(+) m/z (%): 253 (10) [HLA]+, 275 (50) [NaLA]+, 342 (100) [MnLACl]+; (-) m/z (%): 160 (100) [MnCl3]-. Anal. calc. for [Mn(C14H12N4O)Cl2] (378.12): C, 44.47; H, 3.21; N, 11.23; found: C, 44.77; H, 3.45; N, 11.70%. 2.3.4. Complex 1B [Mn(HLB)Br2(CH3OH)] Yield: 38.6 mg, 59.8%. Crystal suitable for X-ray analysis was obtained via slow diffusion methods in MeOH/Et2O system. IR (KBr, cm-1): νbroad(O-H)methanol 3414; ν(NH)imidazole 3165; ν(C-H)arom; 3017; νas(C-H)alif 2943; νs(C-H)alif 2889; ν(C=C) 1638, 1578, 1461, 1452; ν(C=N) 1408; δ(CH3) 1292; ν(C-O) 1292, 1153, γ(C-H)arom 1044, 999, 946, 849, 803, 755.ESI-MS(+) m/z (%): 242 (100) [H2LB]+, 295 (60) [MnLB]+, 375 (100) [Mn(HLB)Br]+, 536 (100) [MnLBHLB]+; (-) m/z (%): 294 (100) [MnBr3]-. Anal.calc. for
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[Mn(C12H11N5O)Br2(CH3OH)] (488.06): C, 32.00; H, 3.01; N, 14.35; found: C, 30.89; H, 2.96; N, 14.29%. 2.3.5. Complex 2B [Mn(HLB)(O2-NO)(CH3OH)2] Yield: 32.2 mg, 66.5%. Crystal suitable for X-ray analysis was obtained via slow diffusion methods in MeOH/tBuOMe system. IR (KBr, cm-1): ν(N-H)imidazole 3248; ν(C-H)arom 3028; νas(C-H)alif 2948; νs(C-H)alif 2895; ν(C=C) 1636, 1621, 1584; ν(C=N) 1433, 1402, 1320, δ(CH3) 1289, ν(C-O) 1255, 1156, γ(C-H)arom 1087, 1033, 1000, 942, 931, 892, 848, 754. ESIMS(+) m/z (%): 148 (30) [MnLB]2+, 242 (40) [HLB]+, 358 (80) [Mn(HLB)NO3]+, 536 (40) [MnLBHLB]+; (-) m/z (%): 241 (100) [Mn(NO3)3]-. Anal. calc. for [Mn(C12H11N5O)(NO3)(CH3OH)2] (484.30): C, 35.72; H, 3.96; N, 17.36; found: C, 36.28; H, 3.51; N, 16.97%. 2.3.6. Complex 3B [Mn(HLB)Cl2(CH3OH)] Yield: 33.9 mg, 74.7%. Crystal suitable for X-ray analysis was obtained via slow diffusion methods in MeOH/iPr2O system. IR (KBr, cm-1): ν(N-H)imidazole 3142; ν(C-H)arom 3061; νas(CH)alif 2979; νs(C-H)alif 2856; ν(C=C) 1635, 1618, 1581; ν(C=N) 1462, 1433, 1410, 1337, δ(CH3) 1289, 1278, ν(C-O) 1255, 1204, 1148, 1100, γ(C-H)arom 1076, 1039, 1000, 946, 890, 847, 760. ESI-MS(+) m/z (%): 242 (100) [HLB]+, 268 (60) [Mn(LB)2]2+, 331 (80) [Mn(HLB)Cl]+, 536 (40) [MnLBHLB]+; (-) m/z (%): 159 (100) [MnCl3]-. Anal. calc. for [Mn(C12H11N5O)Cl2(CH3OH)] (399.14): C, 39.12; H, 3.80; N, 17.55; found: C, 38.43; H, 3.99; N, 18.01%. 2.4. X-Ray diffraction Diffraction data were collected by the ω-scan technique on Agilent Technologies fourcircle diffractometers: 1A, 1B, 2B, 3B at 100(1) K, 2A at 120(1) K on Xcalibur with Eos CCD detector and graphite-monochromated MoKα radiation (λ=0.71069 Å), 3A at 130(1) K on SuperNova with Atlas CCD detector, equipped with Nova microfocus CuKα radiation source (λ = 1.54178 Å). The data were corrected for Lorentz-polarization as well as for absorption effects [25]. Precise unit-cell parameters were determined by a least-squares fit of 1855 (1A), 581 (1B), 4986 (2A), 3503 (2B), 4408 (3A), and 9157 (3B) reflections of the highest intensity, chosen from the whole experiment. The structures were solved with SHELXT [26] and refined with the full-matrix least-squares procedure on F2 by SHELXL2013 [26]. All non-hydrogen atoms were refined anisotropically, hydrogen atoms were placed in idealized positions and refined as ‘riding model’ with isotropic displacement parameters set at 1.2 (1.5 for methyl groups) times Ueq of appropriate carrier atoms. Table 1 lists the relevant experimental data and refinement details. Crystallographic data (excluding structure factors) for the structural analysis has been deposited with the Cambridge Crystallographic Data Centre, Nos. CCDC - 1057746 (1A), CCDC - 1057747 (1B), CCDC - 1559507 (2A), CCDC - 1559508 (2B), CCDC - 1559509 (3A), and CCDC 1559510 (3B). Copies of this information may be obtained free of charge from: The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK. Fax: +44(1223)336-033, e-mail: [email protected], or www: www.ccdc.cam.ac.uk.
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Table 1. Crystal data, data collection and structure refinement Compound Formula Formula weight Crystal system Space group a(Å) b(Å) c(Å) α(º) (º) γ(º) V(Å3) Z Dx(g cm-3) F(000) (mm-1) range (0) Reflections: collected unique (Rint) with I>2σ(I) R(F) [I>2σ(I)] wR(F2) [I>2σ(I)] R(F) [all data] wR(F2) [all data] Goodness of fit max/min (e Å3
1A
2A
3A
1B
2B
3B
C15H16Br2MnN4O2 2 499.08 triclinic P-1 6.9932(11) 9.3269(15) 14.4133(17) 79.227(12) 76.925(12) 70.070(15) 854.7(2) 2 1.94 490 5.46 2.90 – 28.62
C15H16MnN6O8 463.28 monoclinic P21/n 10.0212(8) 15.2609(8) 12.6793(12) 90 107.677(8) 90 1847.5(3) 4 1.67 948 0.78 3.10 – 28.37
C14H12Cl2MnN4O 378.12 triclinic P-1 7.2384(5) 7.4065(5) 14.8532(10) 75.822(5) 75.956(6) 79.007(5) 741.64(9) 2 1.69 382 10.61 3.14 – 67.46
C13H15Br2MnN5O2 488.06 triclinic P-1 7.0451(8) 9.2663(8) 14.0438(13) 80.066(8) 77.326(8) 68.213(9) 826.37(15) 2 1.96 478 5.65 2.99 – 28.24
C14H19MnN6O6·NO3 484.30 monoclinic P21/n 10.1252(2) 13.1365(4) 14.8069(4) 90 96.550(2) 90 1956.61(9) 4 1.64 996 0.74 3.40 – 28.15
C13H15Cl2MnN5O2 399.14 monoclinic P21/c 9.0718(2) 6.94904(13) 25.6177(5) 90 93.070(2) 90 1612.63(6) 4 1.64 812 1.17 2.82 – 27.08
6485 3568 (0.048) 2682 0.063 0.156 0.086 0.176 1.03 2.72/-1.65
16837 4094 (0.053) 3022 0.043 0.113 0.061 0.118 1.02 0.91/-0.44
4667 2658 (0.032) 2578 0.050 0.136 0.052 0.136 1.07 0.84/-0.89
5612 3364 (0.017) 3079 0.023 0.052 0.026 0.054 1.03 0.59/-0.69
7797 4047 (0.014) 3663 0.026 0.065 0.030 0.067 1.04 0.27/-0.27
17052 3312 (0.022) 3189 0.022 0.053 0.023 0.053 1.12 0.32/-0.21
)
5
2.5. Cyclic voltammetry The electrochemical measurements were carried out in a conventional threecompartment cell separated by glass frits using an Autolab potentiostat/galvanostat analyzer (AUTOLAB PGSTAT 302N, Eco Chemie, B. V., Utrecht, The Netherlands). As a supporting electrolyte the solution of 0.1 M TBAP in DMF was used. The electrochemical activation of the working bare gold electrode was carried out in the 0.1 M NaOH water solution. The solutions were prepared prior to use. All solutions under investigation were performed at room temperature and purged with high-purity argon before measurements. The working electrode was the bare gold electrode of 0.3 cm2 geometric area. A gold sheet was used as an auxiliary electrode. As the reference electrode the saturated calomel electrode (SCE) was used. The gold (purity 99.999%) electrodes and the SCE electrode were purchased from the Polish State Mint and EuroSensor, Poland, respectively. In order to activate the electrode, the bare gold electrode was polished with aluminium slurries of successively decreasing final grades (down to 0.05 m, Buehler) on polishing cloths (Buehler). Then it was rinsed carefully with acetone and water purified in a Millipore Milli-Q system (resistivity ≥ 18 MΩcm-1), respectively. As a preliminary step to each experiment, the working electrode was electrochemically activated in the 0.1 M NaOH solution by cycling the electrode potential in the potential range from E = -1.2 vs. SCE to E = 0.6 V vs. SCE (v = 0.1 V s-1) eg between the onset of hydrogen and oxygen evolution, until a stable voltammogram (j - E curve) was obtained. This procedure avoids structural changes on the gold surface. Furthermore, the roughness factor of the activated electrode remains constant [27]. C At the end of each series of experiments in non-aqueous solutions, the gold electrode was rinsed with acetone and water and the j - E curves were recorded again in a freshly prepared 0.1 M NaOH electrolyte solution. The shapes of j – E curves in each series of experiments were the same as those taken before the measurements in non-aqueous solutions. This means that the surface of the working electrode did not change during the experiment. Before subsequent use of working electrode in new electrochemical measurements, gold electrode was washed in the Piranha solution, rinsed with water and dried. Next the procedure of polishing and activation of the bare electrode was repeated. 3. Results and discussion 3.1. Synthesis of ligands L and their complexes Reaction of divalent Manganese(II) salts with two Schiff base type ligands containing N3-donor binding pockets with pyridine (LA) or imidazole (HLB) moiety in MeOH resulted in formation of complexes in 1:1 stoichiometry. In general, central ion is coordinated by single ligand molecule and the coordination sphere is complemented with counterions (3A) or counterions and solvent molecules (1A, 2A, 1B-3B). Crystals suitable for X-Ray analysis were obtained by double diffusion crystallization approach. FT-IR spectra correspond well with obtained X-Ray structures. One may observe stretching bands arising from N-H present in imidazole moiety in HLB and complexes 1B-3B at 3116 and 3165 cm-1, respectively. Slight shift may be due to the coordination. Spectrometric studies show that monomeric species persist in solution however harsh conditions in ESI-MS measurements led to partial decomposition of complexes during analysis thus peaks arising from ligands or MnX3- (X- = counterion) may be observed. 3.2. Crystal data Figures 2-6 show perspective views of the complexes as observed in their crystal structures.
6
The crystal structures can be divided into three categories, depending on their composition: (1) MnLX2(CH3OH) (1A, 1B, 3B), (2) MnL1X2 (3A), and (3) two complexes with nitro group, one neutral MnL1(NO3)2(CH3OH) (2A) and one cationic [Mn(HL2)(NO3)(CH3OH)2]+·NO3- (2B). In all cases the conformations of ligand molecules (defined by the values of dihedral angles between the planar fragments, cf. Table 2) are similar, i.e. ligands are generally planar, the central linkers C-C=N-N-C are also to good approximations planar. The above-mentioned category determines the coordination number and geometry of the complexes. In the three complexes from category 1, Manganese ion is six-coordinated (by three nitrogen atoms from ligand molecule, two halide ions and one oxygen atom from methanol molecule), in a distorted octahedral manner. The distortion is caused by the nature of the ligand L – the angle N3-Mn-N15 cannot be much closer to 180° than it is observed in the crystal (ca. 140°). Category 2 complex is five-coordinated, by three nitrogen atoms from the ligand molecule (N3, N10 and N15 – the same coordination mode of the ligand is present in all the complexes) and two halogens. The coordination is close to tetragonal pyramid (N3X base almost planar, second X atom at the apex, and Manganese ion displaced from the basal plane towards the apex by 0.5469(12)Å in 3A. Finally, two nitro-complexes (2A and 2B) are 7-coordinated, and it may be stressed that in cases when nitro groups coordinate via two oxygen atoms, both Mn-O distances are almost equal. The coordination polyhedron of these complexes is pentagonal bipyramid, with five atoms (three N from ligand molecule and two oxygen atoms from one nitro group) to a good approximation coplanar (maximal deviation ca. 0.15Å), and two oxygen atoms- from second nitro group and methanol molecule in 2A, from two methanol molecules in 2B – at the apexes, almost equally distanced from the basal plane (2.175 and -2.119Å in 2A, 2.181 and 2.198Å in 2B. In the nitro complexes the hydrogen bonds between methanol OH groups and NH (2B) and nitro groups are the main packing creation forces. Also, the intermolecular interactions and crystal packing are to some extent determined by the group to which a given complex belongs. In complex 3A the only specific intermolecular interaction which to some extent can be deemed responsible for the crystal packing is π···π stacking between rings of neighbouring molecules. In the crystal structures intermolecular hydrogen bonds of a type O-H···X and NH···X connect complex molecules into three-dimensional structures (cf. Tab. 3). Besides, relatively short π···π stacking interactions (centroid-to-centroid distances around 3.65Å, interplanar separations as short as 3.25Å) can be found between the molecules related by the consecutive centers of symmetry, thus producing infinite stacks of pyridine or imidazole rings. The complexes 1B and 3B are an example of one-dimensional isostructurality; although the unit cell parameters are quite similar, the packing along only one direction (a) is almost identical.
7
Fig. 2. Perspective view of the complex 1A; ellipsoids are drawn at the 50% probability level, hydrogen atoms are represented by spheres of arbitrary radii.
Fig 3. Perspective view of the complex 3A; ellipsoids are drawn at the 50% probability level, hydrogen atoms are represented by spheres of arbitrary radii.
Fig. 4. Perspective views of the complexes 1B (left) and 3B (right); ellipsoids are drawn at the 50% probability level, hydrogen atoms are represented by spheres of arbitrary radii.
Fig. 5. Perspective view of the complex 2A; ellipsoids are drawn at the 50% probability level, hydrogen atoms are represented by spheres of arbitrary radii.
8
Fig. 6. Perspective view of the complex 2B; ellipsoids are drawn at the 50% probability level, hydrogen atoms are represented by spheres of arbitrary radii. Table 2 Relevant geometrical parameters [A, °] with s.u.’s in parantheses. A, B, C denote the mean planes of benzoxazole ring system, CNNCC linker and imidazole ring, respectively. 1A 2A 3A 1B 2B 3B Mn-N3A 2.227(6) 2.2560(18) 2.252(3) 2.2546(19) 2.3163(12) 2.2429(12) Mn-N12A 2.316(6) 2.3384(19) 2.281(3) 2.332(2) 2.3558(12) 2.3259(12) Mn-N15A 2.301(6) 2.247(3) 2.2773(19) 2.2533(12) 2.2739(12) Mn-Br 2.5290(13) 2.3418(19) 2.5473(5) 2.6687(13) 2.7119(5) Mn-Cl 2.3549(9) 2.4056(4) 2.3691(9) 2.5449(4) Mn2.257(5) 2.1802(18) 2.2376(17) 2.1692(11) 2.2496(12) O(MeOH) 2.2136(11) Mn2.1889(19) 2.3247(11) O(nitrate) 2.2958(18) 2.3558(12) 2.3257(18) angles (3)
172.38(14) 172.26(13) 139.4(2)
163.53(7) 153.75(6) 151.29(6)
150.31(7) 136.75(11) 106.66(3)
171.60(5) 171.57(5) 140.34(7)
176.04(4) 152.57(4) 151.75(4)
172.72(3) 172.01(3) 140.28(5)
A/B B/C A/C
4.9(5)
7.3(3) 4.1(3) 8.78(14)
6.93(19) 8.73(17) 11.55(11)
4.13(17) 4.13(18) 6.80(10)
4.37(13) 4.20(14) 0.52(8)
5.18(8) 3.88(9) 6,65(6)
Table 3 Hydrogen bond data (A, °).
D
H
A
D-H
H···A
D···A
O1D
H1D
Br1Ci
D-H···A
0.95
2.30
3.224(5)164
1B 0.79(3) 0.88 2A
2.45(3) 2.51
3.2306(19) 3.364(2)
174(3) 164
2.02(3)
2.764(3)
174(4)
1.98(2) 1.97(2)
2.7473(15) 2.7372(16)
173(2) 171(2)
1A
O1D N17A
H1D H17A
Br1Cii Br1Ciii
O1D
H1D
O2Biv
O1C O1D
H1C H1D
O3E O2Ev
0.75(3) 2B 0.77(2) 0.77(2) 3B
9
O1D N17A
H1D H17A
Cl1Bvi Cl1Cvii
0.77(2) 0.88
2.35(3) 2.35
3.1208(13) 3.1503(13)
174(2) 151
Symmetry codes: i-1+x,y,z; ii-1+x,y,z; iii 2-x,-y,-z; iv -1/2+x,3/2-y,-1/2+z; v 1/2+x,3/2-y,1/2+z; vi x,-1+y,z; vii 2-x,-1/2+y,3/2-z. 3.3. Electrochemical properties In order to gain some insights into the electron-richness of studied systems, the electrochemical properties of ligands LA and HLB, as well as complexes 1A and 1B were investigated by cyclic voltammetry. The measurements were performed in deaerated DMF containing 0.1 M of TBAClO4 as the supporting electrolyte solution (SES). Complexes 1A and 1B were chosen due to their isostructurality what allows to the direct comparison of their behavior. The electrochemical behavior of the polycrystalline gold electrode in the presence of both the supporting electrolyte solution alone, studied ligands and their complexes are presented in Figs. 8 and 9. Fig. 7 presents the cyclic voltammograms of the bare polycrystalline gold electrode obtained in SES, TBAClO4 (curve x) and LiClO4 (curve y), respectively. Considering the j - E curve obtained in the first SES and going from a negative to positive potential limit there are two peaks visible on the anodic branch of CV at E = -0.775 V vs SCE and at E = 1.6 V vs SCE (which begins to grow already from 1.3 V vs. SCE), respectively. At the cathodic part of SES j - E curve three peaks appeared at E = 0.75 V vs SCE, E = -0.75 and E = -1.05 vs SCE respectively. There is no doubt that the anodic peak at E = 1,6 V vs SCE and the cathodic peak at E = 0.75 V vs SCE are assigned to the gold oxide formation at the electrode surface and to its reduction in cathodic scan, respectively. It is characteristic for gold electrodes both in aqueous and in non-aqueous solutions [28,29]. To explain the source of anodic peak at E = -0.775 V vs SCE and cathodic peaks E = -0.75 and E = -1.05 vs SCE on the j - E curve obtained in 0.1 M TBAClO4, the cyclic voltammogram in 0.1 M LiClO4 in DMF was recorded (curve y at Fig. 7). These two supporting electrolyte solutions differ only in cations. From literature it is known that ClO4 – anions both do not adsorb and do not undergo any faradic reactions on the polycrystalline gold electrode surface [30,31]. As can be seen, the only difference between the curve x recorded in 0.1 M TBAClO4 and the curve y obtained in 0.1 M LiClO4 presented on Fig. 7 is such that the latter j - E curve lacks the peak at E = 0.775 V vs SCE. So, it may be stated that the latter peak is related to electroactivity of the organic cation TBA+. While, as it was mentioned above, since ClO4 – anions do not take part in electrochemical reaction on gold, the two following peaks in the cathodic scan at E = -0.75 and E = -1.05 vs SCE which appeared both at x and y j - E curves are assigned to the redox processes related to DMF molecules. It was observed that the latter effect is intensified when ligands or, especially, complexes are added into the supporting electrolyte solution. As depicted in Fig. 8A both considered ligands show one reduction peak at 0.775 V vs SCE without any anodic counterpart, which may be attributed to the formation of the radical cation of heterocyclic moiety [32,33] (for comparison, the j - E curve of the bare gold electrode in the supporting electrolyte solution is added as the inset in Fig. 8A). One may envisage the benzoxazole part of ligands being involved in this redox process, since peaks positions are alike for both ligands and no distinct changes may be observed due to the presence of imidazole or pyridine moiety. This peak cannot be related to the gold oxide layer reduction, otherwise one should expect much higher current response. The increase of current observed in the potential range of the gold oxide layer is related to adsorption of the studied ligands on the electrode/solution interface. One can underline that, in comparison to both ligands, the cyclic voltammograms of complexes 1A and 1B exhibit some ligand-independent peaks (Fig. 8B as well as Figs. 9). During the anodic potential scan both complexes show oxidative responses at 0.75 V vs SCE, which are assigned to the Mn(II)Mn(III) conversion 10
[34,35]. An additional peak occurring at 0.04 V vs SCE in the cathodic part of the j-E curve is attributed to some electron interactions in the benzoxazole rings induced by complexed manganese ions. Similarly to the ligands an increase of the intensity of the peak observed at 1.25 V vs SCE in case of both complexes may be generated due to their adsorption and, additionally, adsorption of bromides, which are present in the coordination sphere of metal ions and preset in the electric double layer region [36]. The latter processes are intensified by gradual increase of concentration of ligands and their complexes (Figs. 10). It should be added that the peak visible at -0.775 V vs SCE which is assigned to the organic cation TBA+ fades-out in case of both, ligands and Mn(II) complexes. It may be caused by electrostatic interaction of TBA+ with oxygen atoms present in benzoxazole ring, thus gradual decay of this peak is observed due to the addition of analyzed compounds. It is worth pointing out that the CV curves presented in this manuscript are fully repeatable after the first potential scan for each concentrations of ligand or complex used. Therefore, one can conclude that in the presented studies, the adsorption of the ligand/complex residues does not occur at gold, so the gold electrode surface is not passivated during the measurements. Thus, it is a good template for evaluating the electrochemical properties of the prepared ligands and their metal complexes.
Fig. 7. Cyclic voltammograms of the polycrystalline gold electrode recorded in: 0.1 M TBAClO4 in DMF solution (curve x) and in 0.1 M LiClO4 in DMF solution (curve y); dE/dv = 0.1 Vs-1.
Fig. 8. A) Cyclic voltammograms of polycrystalline gold electrode recorded in: 0.1 M TBAP solution (inset) and in presence of ligands LA and HLB in 0.1 M TBAP in DMF solution and B) complexes 1A and 1B in 0.1 M TBAP in DMF solution; dE/dv = 0.1 Vs-1.
11
Fig. 9. Cyclic voltammograms of polycrystalline gold electrode recorded in 0.1 M TBAP solution in DMF; Comparison of complexes and their parent ligands: A) ligand LA and complex 1A; B) ligand HLB and complex 1B; dE/dv = 0.1 Vs-1.
Fig. 10. Cyclic voltammogram of polycrystalline gold electrode in 0.1 M TBAP solution in DMF containing increasing amounts (10-5 ÷ 2·10-3 M) of A) ligand LA, B) complex 1A, C) ligand HLB and D) complex 1B. Grey curves – supporting electrolyte solution. dE/dv = 0.1 Vs-1. 4. Conclusion As a continuation of our previous work concerning various metal complexes and their properties we report six newly synthesized Manganese(II) complexes with Schiff base type ligands: an earlier reported ligand LA and a new ligand HLB. X-Ray studies revealed isostructurality between complexes 1A and 1B and formation of dimers supported by OH···Br bonds in solid state. Additionally, dimers of complex 1B are confined due to N-H···Br interactions. This is manifested in the density of crystals equal to 1.94 and 1.96 g/cm3 of complexes 1 and 2, respectively. Cyclic voltammetry studies show only irreversible oxidation Mn(II)Mn(III) at 0.75 V vs SCE and several peaks attributed to
12
electron processes in ligands or DMF. Our investigations evidence that additional NH unit in complex 2 do not facilitate the adsorption of compounds on a bare gold electrode. Further studies on synthesis and characterization of Manganese(II) species with tridentate N-heterocyclic scaffolding are underway. Our next attempt will be to focus on the synthetic modification of ligands presented herein by incorporating additional coordinating moieties and investigation on electrochemical character of their Manganese complexes. 5. Acknowledgements This research was carried out as a part of the Polish National Science Center project (Grant No. 2016/21/B/ST5/00175). 6. References [1] S. Dajnowicz, J.M. Parks, X. Hu, K. Gesler, A.Y. Kovalevsky, T.C. Mueser, J. Biol. Chem. 292 (2017) 5970-5980. [2] M.M. Najafpour, M.Z. Ghobadi, B. Haghighi, T. Tomo, J.-R. Shen, S.I. Allakhverdiev, Biochim. Biophys. Acta 1847 (2015) 294–306. [3] B. Xu, W. Jiang, Y. Wang, Z. Xiang, F. Liu, Y. Wu, Colloid. Surface. A 456(2014) 222– 230. [4] P. Mondal, R. Kumar, R. Gogoi, Bioorg. Chem. 70 (2017) 153-162. [5] H. Zafar, A. Kareem, A. Sherwani, O. Mohammad, M.A. Ansari, H.M. Khan, T.A. Khan, J. of Photochem. Photobiol. B 142 (2015) 8–19. [6] A. Rauf, A. Shah, A.A. Khan, A.H. Shah, R. Abbasi, I.Z. Qureshi, S. Ali, Spectrochim. Acta A. 176 (2017) 155-167. [7] S.A. Al-Harbi, M.S. Bashandy, H.M. Al-Saidi, A.A.A. Emara, T.A.A. Mousa, Spectrochim. Acta A 145 (2015) 425–439. [8] Y. Gou, Y. Zhang, J. Qi, Z. Zhou, F. Yang, H. Liang, J. Inorg. Biochem. 144 (2015) 47– 55. [9] K.J. Davis, C. Richardson, J.L. Beck, B.M. Knowles, A. Guédin, J.-L. Mergny, A.C. Willis, S.F. Ralph, Dalton Trans. 44 (2015) 3136-3150. [10] M. Das, R. Nasani, M. Saha, S.M Mobin, S. Mukhopadhyay, Dalton Trans. 44 (2015) 2299-2310. [11] M. Shabbir, I. Ahmad, H. Ismail, S. Ahmed, V. McKee, Z. Akhter, Polyhedron 133, 270278. [12] J. Cisterna, V. Dorcet, C. Manzur, I. Ledoux-Rak, J.-R. Hamon, D. Carrillo, Inorg. Chim. Acta 430 (2015) 82–90. [13] Z.-G. Gu, C.-Y. Pang, D. Qiu, J. Zhang , J.-L. Huang, L.-F. Qin, A.-Q. Sun, Z. Li, Inorg. Chem. Commun. 35 (2013) 164–168. [14] D.-F. Liu, L.-Q. Zhu, J. Wu, L.-Y. Wu, X.-Q. Lü, RSC Adv. 5 (2015) 3854-3859. [15] A.-R. Judy-Azar, S. Mohebbi, J. Mol. Cat.A 397 (2015) 158–165. [16] M.A. Fik, A.E. Odachowska, M. Kubicki, J. Karpińska, V. Patroniak, Eur. J. Inorg. Chem. (2016) 5530-5538. [17] S. Roy, K. Harms, S. Chattopadhyay, Polyhedron 91 (2015) 10–17. [18] F. Dumur, L. Beouch, M.-A. Tehfe, E. Contal, M. Lepeltier, G. Wantz, B. Graff, F. Goubard ,C.R. Mayer, J. Lalevée, D. Gigmes, Thin Solid Films 564 (2014) 351–360. [19] D.P. Kessissoglou, Bioinorg. Chem.NATO ASI Series 459 (1995) 299-320. [20] M. Maiti, D. Sadhukhan, S. Thakurta, E. Zangrando, G. Pilet, A. Bauzá, A. Frontera, B. Dede, S. Mitra, Polyhedron 75 (2014) 40–49. [21] C. Tommos, G.T. Babcock, Biochem. Biophys. Acta 1458 (2000) 199-219. [22] M. Kato, J.Z. Zhang, N. Paul, E. Reisner, Chem. Soc. Rev. 43 (2014) 6485-6497.
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[23] Y. Xu, A. Fischer, L. Duan, L. Tong, E. Gabrielsson, B. Åkermark, L. Sun, Angew. Chem., Int. Ed., 49 (2010) 8934–8937. [24] M.A. Fik, M. Loffler, M. Weselski, M. Kubicki, M.J. Korabik, V. Patroniak, Polyhedron, 102 (2015) 609-614. [25] Agilent Technologies, CrysAlis PRO (Version 1.171.33.36d), Agilent Technologies Ltd, 2011. [26] G. M. Sheldrick, Acta Crystallogr. (2008) A64, 112-122. [27] T. Łuczak, Collect. Czech. Chem. Commun. 70 (2005) 2027 – 2037. [28] R.C. Newman, G.T. Burstein, J. Electroanal. Chem. (1981) 129 343-348. [29] A. Gorczyński, D. Pakulski, M. Szymańska, M. Kubicki, K. Bułat, T. Łuczak, V. Patroniak, Talanta (2016) 149 347-335. [30] C.N. van Huong, J. Electroanal. Chem. (1986) 235-246. [31] T. Łuczak, M. Bełtowska-Brzezinska, M. Bron, R. Holze, Vib. Spectroscopy (1997) 15 17-25. [32] I. Kuźniarska-Biernacka, O. Rodrigues, M.A. Carvalho, P. Parpot, K. Biernacki, A.L. Magalhaes, A.M. Fonseca, I.C. Neves, Eur. J. Inorg. Chem.(2013) 2768-2776. [33] A.M. Foneseca, M. Belsley, E.M. Gomes, M.C.R. Castro, M.M.M. Raposo, Eur. J. Inorg. Chem.(2010) 2998-3004. [34] M.L. Dianu, A. Kriza, A.M. Musuc, J. Therm. Anal. Calorim. 112 (2013) 585-593. [35] M.S. Jana, A.K. Pramanik, D. Sarkar, S. Biswas, T.K. Mondal, Polyhedron (2014) 6673. [36] B. B. Damaskin, O.A. Petrii, V.V Batrakov, “Adsorption organischer Verbindungen an Elektroden”, Akademie-Verlag, Berlin 1975.
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Electrochemical activity of Mn(II) complexes was evaluated by cyclic voltammetry. Studies show only irreversible oxidation Mn(II)Mn(III) at 0.75 V vs SCE and several peaks attributed to electron processes in ligands or DMF. Presence of NH moiety in imidazole ring of HLB does not feasible the adsorption on the bare gold electrode.
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S0277-5387(17)30767-2 https://doi.org/10.1016/j.poly.2017.11.039 POLY 12941
To appear in:
Polyhedron
Received Date: Accepted Date:
27 October 2017 26 November 2017
Please cite this article as: D. Marcinkowski, M.A. Fik, T. Łuczak, M. Kubicki, V. Patroniak, New Mn(II) complexes with benzoxazole-based ligands: synthesis, structure and their electrochemical behavior, Polyhedron (2017), doi: https://doi.org/10.1016/j.poly.2017.11.039
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New Mn(II) complexes with benzoxazole-based ligands: synthesis, structure and their electrochemical behavior Dawid Marcinkowski, Marta A. Fik, Teresa Łuczak, Maciej Kubicki, Violetta Patroniak* Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61614 Poznań, Poland *Fax: +48 618291508; Tel: +48 618291677; E-mail: [email protected] Graphical abstract Abstract Six complexes of various Mn(II) salts: MnBr2·4H2O (1), Mn(NO3)2·4H2O (2), MnCl2·4H2O (3) with 2-(1-methyl-2-(pyridin-2-ylmethylene)hydrazinyl)benzo[d]oxazole LA (complexes 1A – 3A) and 2-(2-((1H-imidazol-4-yl)methylene)-1-methylhydrazinyl)benzo[d]oxazole HLB (complexes 1B – 3B) have been synthesized. All complexes were obtained in 1:1 stoichiometry. Their structures have been established through analytical and spectroscopic (ESI-MS, IR, 1H NMR and microanalyses) methods as well as by X-ray structure determinations. In order to gain some insights into the electron-richness of studied systems, the electrochemical properties of ligands LA and HLB and complexes 1A and 1B were investigated by cyclic voltammetry. Complexes 1A and 1B were chosen due to isostructurality what allows to the direct comparison of their behavior. The impact of pyridine and imidazole substituents on electrochemical behavior of synthesized systems was evaluated and showed that the presence of NH moiety in imidazole ring of HLB does not feasible the adsorption on the bare gold electrode. 1. Introduction The ongoing interest in well-defined stable systems exhibiting some distinct properties has led researchers to extensively explore the chemistry of Schiff-bases. They are common enzymatic intermediates where an amine reversibly reacts in vivo with an aldehyde or ketone of cofactor of substrate, e.g. the common enzyme cofactor PLP (pyridoxal 5’-phosphate) forms an intermediate imine with lysine residue and is transferred to the substrate [1]. Moreover, they participate in photosynthesis and oxygen transportation in living organisms [2]. The versatility of imines and their ability to coordinate metal ions has allowed one to create a great variety of potentially applicative materials. Many complexes of Schiff-base ligands with d-electron metal ions have been investigated as models for active sites of enzymes [3], antifungal [4] and antibacterial [5,6] as well as anticancer [7,8] drugs. Their biocompatibility enables biologists to develop new DNA-cleavage and repair systems [9-11]. They also provide some valuable magnetic properties which are useful in fabrication of novel electronic nano-devices [12,13]. They are already used as catalysts in multiple organic and metalorganic reaction such as, for instance, ring-opening olefin epoxidation [14] or alcohol oxidation [15] as well as in photocatalytic organic matter degradation processes [16]. Some d10-electron metal ions (as Zn2+, Cd2+, Ag+) coordinated by Schiff base type ligands display intense luminescence and exhibit high quantum yields thus may be utilized in design of new OLED’s (Organic Light Emitting Diodes) or as components in WOLED’s (White Organic Light Emitting Diodes) construction [17,18]. The coordination chemistry of Mn(II) has been extensively explored in recent years mainly due to its essential role in biological systems. For instance, Manganese ions act as cofactors of numerous enzymes, such as oxidoreductases, transferases, ligases or lyases [19]. Manganese is also present in eukaryotic mitochondria in the form of superoxide dismutase (Mn-SOD), which is responsible for deactivation of superoxide O2 formed from the one1
electron reduction of oxygen [20]. At the same time, the role of Manganese in plant cells cannot be neglected, since it is present in the oxygen-evolving complex (OVC) which is a part of photosystem II [21]. The latter one continuously encourages researchers to evaluate the redox activeness of artificial, bio-mimetic manganese complexes in H2O splitting processes because it could be used for solar energy conversion [22,23]. The aim of our work presented here was to obtain and characterize new Mn(II) redox active compounds. Therefore, we have investigated structural properties of six new complexes with N3-donor Schiff base ligands containing benzoxazole moieties and evaluated electrochemical properties of two of them (Fig. 1.) of general formula [MnLBr2(CH3OH)]. The impact of pyridine and imidazole substituents on electrochemical behavior of synthesized systems was evaluated. 2. Experimental 2.1. Materials and methods The metal salts and organic compounds as well as NaOH, H2SO4 and H2O2, LiClO4 were supplied from Aldrich and POCH. Tetrabutyloammonium perchlorate (TBAP) and dimethylformamide (DMF) were purchased from Merck. All chemicals mentioned above were of analytical grade quality and were used as obtained without further purification. NMR spectra were run on a Varian Gemini 300 MHz spectrometer and were calibrated against the residual protonated solvent signals (DMSO-d6, δ 2.50) which are given in parts per milion. ESI mass spectra for MeOH solutions ~ 10-4 M were measured using a Waters Micromass ZQ spectrometer. Microanalyses were obtained using a Vario EL III CHN element analyzer. IR spectra were recorded with a Bruker FTIR IFS 66/s spectrometer and peak positions are reported in cm-1. 2.2. Synthesis of ligands LA and HLB Ligand LA and intermediate A were synthesized according to previously described procedure [24]. Ligand HLB was obtained within two subsequent steps starting from commercially available 2-chlorobenzoxazole according to the Fig. 1. Formation of products thus formed was established by 1H NMR, 13C NMR, FT-IR spectroscopy and ESI-MS spectrometry.
Fig. 1. Perspective of synthetic protocol of ligands LA and HLB. 2.2.1. Synthesis of ligand HLB To the colorless solution of A (120.1 mg, 666 mol) in EtOHabs (4 ml) an equimolar amount of 4-(2H-imidazole)carboxyaldehyde (64.0 mg, 666mol) was added. The mixture was allowed to react for 4 h under inert atmosphere in reflux and for another 20 h at ambient temperature. The white crystalline solid precipitated overnight and was filtered under reduced pressure. Yield: 151.6 mg, 82.3%. 1H NMR (DMSO-d6, 300 MHz): δ(ppm) – 12.51 (sbroad, 1H), 7.98 (s, 1H), 7.76 (s, 1H), 7.53 (d, 1H, J = 7.78 Hz), 7.49 (s, 1H), 7.43 (d, 1H, J = 7.78 Hz), 7.23 (t, 1H, J = 7.57 Hz), 7.12 (t, 1H, J = 7.78 Hz), 3.74 (s, 3H). 13C NMR (DMSO-d6, 300 MHz): δ(ppm) – 168.36, 153.23, 151.62, 149.52, 138.91, 136.93, 131.73, 126.00, 123.82, 122.49, 121.40, 119.99, 119.43, 32.73. IR (KBr, cm-1): ν(N-H)imidazole 3116; ν(C-H)arom 3061, 3027; νas(C-H)alif 2969; νs(C-H)alif 2953; ν(C=C) 1639, 1579; ν(C=N) 1458, 1446, δ(CH3) 1296, ν(C-O) 1245, 1156, γ(C-H)arom 992, 926, 909, 853, 808, 747, 626. ESI-MS(+) m/z (%):
2
242 (100) [H2L]+; (-) m/z (%): 240 (100) [L]-. Anal. calc. for (C12H11N5O) (240.25): C, 59.99; H, 4.62; N, 29.16; found: C, 62.99; H, 4.29; N, 30.91%. 2.3. Synthesis of complexes 1A – 3A and 1B – 3B Complexes were prepared in the same manner by treating equimolar amounts of the ligands LA (30.5 mg, 126 µM) and HLB (31.8 mg, 126 µM) in MeOH (10 ml) with MnBr2·4H2O (36.1 mg, 126 µM) (1), Mn(NO3)2·4H2O (31.6 mg, 126 µM) (2) and MnCl2·4H2O (24.9 mg, 126 µM) (3) in 5 ml of MeOH. Pale yellow solutions formed instantly and the reaction mixtures were stirred for 24 h at room temperature. After evaporation of solvents under reduced pressure, the residues were dissolved in minimum volume of MeOH and precipitated by excess of Et2O. Orange solids were filtered via suction filtration and dried in the vacuum. 2.3.1. Complex 1A [Mn(LA)Br2(CH3OH)] Yield: 39.2 mg, 65.0%. Crystals suitable for X-ray analysis were obtained via slow diffusion methods in MeOH/iPr2O system. IR (KBr, cm-1): νbroad(O-H)methanol 3447; ν(C-H)arom 3025; νas(C-H)alif 2971; νs(C-H)alif 2955; ν(C=C) 1641, 1635, 1583; ν(C=N) 1463, 1410, 1367, δ(CH3) 1291, ν(C-O) 1256, 1249, 1150, γ(C-H)arom 1087, 1038, 1011, 943, 933, 898, 819, 784. ESI-MS(+) m/z (%): 153 (30) [MnLA]2+, 253 (100) [HLA]+, 280 (100) [Mn(LA)2]2+, 387 (40) [MnLABr]+; (-) m/z (%): 294 (100) [MnBr3]-. Anal. calc. for [Mn(C14H12N4O)Br2(CH3OH)] (499.08): C, 36.01; H, 3.24; N, 11.23; found: C, 35.57; H, 3.45; N, 11.04%. 2.3.2. Complex 2A [Mn(LA)(O2-NO)(O-NO2)(CH3OH)] Yield: 34.7 mg, 70.5%. Crystal suitable for X-ray analysis was obtained via slow diffusion methods in MeOH/tBuOMe system. IR (KBr, cm-1):νbroad(O-H)methanol 3375; ν(C-H)arom 3084, 3023; νas(C-H)alif 2968; νs(C-H)alif 2938; ν(C=C) 1636, 1620, 1581; ν(C=N) 1456, 1404, 1365, δ(CH3) 1289, ν(C-O) 1250, 1151, 1097, γ(C-H)arom 1037, 1006, 941, 898, 817, 740. ESIMS(+) m/z (%): 253 (40) [HLA]+, 275 (20) [NaLA]+, 369 (100) [MnLANO3]+, 621 (10) [Mn(LA)2NO3]+; (-) m/z (%): 241 (100) [Mn(NO3)3]-. Anal. calc. for [Mn(C14H12N4O)(NO3)2(CH3OH)] (463.28): C, 38.88; H, 3.49; N, 18.14; found: C, 37.52; H, 3.45; N, 17.25%. 2.3.3. Complex 3A [Mn(LA)Cl2] Yield: 28.9 mg, 73.4%. Crystals suitable for X-ray analysis were obtained via slow diffusion methods in MeOH/iPr2O system. IR (KBr, cm-1):νbroad(O-H)methanol 3424; ν(C-H)arom 3096, 3052, 3022; νas(C-H)alif 2958; νs(C-H)alif 2920; ν(C=C) 1633, 1620, 1580; ν(C=N) 1464, 1406, 1368, δ(CH3) 1289, ν(C-O) 1254, 1143, 1100, γ(C-H)arom 1036, 1011, 944, 933, 898, 817, 760. ESI-MS(+) m/z (%): 253 (10) [HLA]+, 275 (50) [NaLA]+, 342 (100) [MnLACl]+; (-) m/z (%): 160 (100) [MnCl3]-. Anal. calc. for [Mn(C14H12N4O)Cl2] (378.12): C, 44.47; H, 3.21; N, 11.23; found: C, 44.77; H, 3.45; N, 11.70%. 2.3.4. Complex 1B [Mn(HLB)Br2(CH3OH)] Yield: 38.6 mg, 59.8%. Crystal suitable for X-ray analysis was obtained via slow diffusion methods in MeOH/Et2O system. IR (KBr, cm-1): νbroad(O-H)methanol 3414; ν(NH)imidazole 3165; ν(C-H)arom; 3017; νas(C-H)alif 2943; νs(C-H)alif 2889; ν(C=C) 1638, 1578, 1461, 1452; ν(C=N) 1408; δ(CH3) 1292; ν(C-O) 1292, 1153, γ(C-H)arom 1044, 999, 946, 849, 803, 755.ESI-MS(+) m/z (%): 242 (100) [H2LB]+, 295 (60) [MnLB]+, 375 (100) [Mn(HLB)Br]+, 536 (100) [MnLBHLB]+; (-) m/z (%): 294 (100) [MnBr3]-. Anal.calc. for
3
[Mn(C12H11N5O)Br2(CH3OH)] (488.06): C, 32.00; H, 3.01; N, 14.35; found: C, 30.89; H, 2.96; N, 14.29%. 2.3.5. Complex 2B [Mn(HLB)(O2-NO)(CH3OH)2] Yield: 32.2 mg, 66.5%. Crystal suitable for X-ray analysis was obtained via slow diffusion methods in MeOH/tBuOMe system. IR (KBr, cm-1): ν(N-H)imidazole 3248; ν(C-H)arom 3028; νas(C-H)alif 2948; νs(C-H)alif 2895; ν(C=C) 1636, 1621, 1584; ν(C=N) 1433, 1402, 1320, δ(CH3) 1289, ν(C-O) 1255, 1156, γ(C-H)arom 1087, 1033, 1000, 942, 931, 892, 848, 754. ESIMS(+) m/z (%): 148 (30) [MnLB]2+, 242 (40) [HLB]+, 358 (80) [Mn(HLB)NO3]+, 536 (40) [MnLBHLB]+; (-) m/z (%): 241 (100) [Mn(NO3)3]-. Anal. calc. for [Mn(C12H11N5O)(NO3)(CH3OH)2] (484.30): C, 35.72; H, 3.96; N, 17.36; found: C, 36.28; H, 3.51; N, 16.97%. 2.3.6. Complex 3B [Mn(HLB)Cl2(CH3OH)] Yield: 33.9 mg, 74.7%. Crystal suitable for X-ray analysis was obtained via slow diffusion methods in MeOH/iPr2O system. IR (KBr, cm-1): ν(N-H)imidazole 3142; ν(C-H)arom 3061; νas(CH)alif 2979; νs(C-H)alif 2856; ν(C=C) 1635, 1618, 1581; ν(C=N) 1462, 1433, 1410, 1337, δ(CH3) 1289, 1278, ν(C-O) 1255, 1204, 1148, 1100, γ(C-H)arom 1076, 1039, 1000, 946, 890, 847, 760. ESI-MS(+) m/z (%): 242 (100) [HLB]+, 268 (60) [Mn(LB)2]2+, 331 (80) [Mn(HLB)Cl]+, 536 (40) [MnLBHLB]+; (-) m/z (%): 159 (100) [MnCl3]-. Anal. calc. for [Mn(C12H11N5O)Cl2(CH3OH)] (399.14): C, 39.12; H, 3.80; N, 17.55; found: C, 38.43; H, 3.99; N, 18.01%. 2.4. X-Ray diffraction Diffraction data were collected by the ω-scan technique on Agilent Technologies fourcircle diffractometers: 1A, 1B, 2B, 3B at 100(1) K, 2A at 120(1) K on Xcalibur with Eos CCD detector and graphite-monochromated MoKα radiation (λ=0.71069 Å), 3A at 130(1) K on SuperNova with Atlas CCD detector, equipped with Nova microfocus CuKα radiation source (λ = 1.54178 Å). The data were corrected for Lorentz-polarization as well as for absorption effects [25]. Precise unit-cell parameters were determined by a least-squares fit of 1855 (1A), 581 (1B), 4986 (2A), 3503 (2B), 4408 (3A), and 9157 (3B) reflections of the highest intensity, chosen from the whole experiment. The structures were solved with SHELXT [26] and refined with the full-matrix least-squares procedure on F2 by SHELXL2013 [26]. All non-hydrogen atoms were refined anisotropically, hydrogen atoms were placed in idealized positions and refined as ‘riding model’ with isotropic displacement parameters set at 1.2 (1.5 for methyl groups) times Ueq of appropriate carrier atoms. Table 1 lists the relevant experimental data and refinement details. Crystallographic data (excluding structure factors) for the structural analysis has been deposited with the Cambridge Crystallographic Data Centre, Nos. CCDC - 1057746 (1A), CCDC - 1057747 (1B), CCDC - 1559507 (2A), CCDC - 1559508 (2B), CCDC - 1559509 (3A), and CCDC 1559510 (3B). Copies of this information may be obtained free of charge from: The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK. Fax: +44(1223)336-033, e-mail: [email protected], or www: www.ccdc.cam.ac.uk.
4
Table 1. Crystal data, data collection and structure refinement Compound Formula Formula weight Crystal system Space group a(Å) b(Å) c(Å) α(º) (º) γ(º) V(Å3) Z Dx(g cm-3) F(000) (mm-1) range (0) Reflections: collected unique (Rint) with I>2σ(I) R(F) [I>2σ(I)] wR(F2) [I>2σ(I)] R(F) [all data] wR(F2) [all data] Goodness of fit max/min (e Å3
1A
2A
3A
1B
2B
3B
C15H16Br2MnN4O2 2 499.08 triclinic P-1 6.9932(11) 9.3269(15) 14.4133(17) 79.227(12) 76.925(12) 70.070(15) 854.7(2) 2 1.94 490 5.46 2.90 – 28.62
C15H16MnN6O8 463.28 monoclinic P21/n 10.0212(8) 15.2609(8) 12.6793(12) 90 107.677(8) 90 1847.5(3) 4 1.67 948 0.78 3.10 – 28.37
C14H12Cl2MnN4O 378.12 triclinic P-1 7.2384(5) 7.4065(5) 14.8532(10) 75.822(5) 75.956(6) 79.007(5) 741.64(9) 2 1.69 382 10.61 3.14 – 67.46
C13H15Br2MnN5O2 488.06 triclinic P-1 7.0451(8) 9.2663(8) 14.0438(13) 80.066(8) 77.326(8) 68.213(9) 826.37(15) 2 1.96 478 5.65 2.99 – 28.24
C14H19MnN6O6·NO3 484.30 monoclinic P21/n 10.1252(2) 13.1365(4) 14.8069(4) 90 96.550(2) 90 1956.61(9) 4 1.64 996 0.74 3.40 – 28.15
C13H15Cl2MnN5O2 399.14 monoclinic P21/c 9.0718(2) 6.94904(13) 25.6177(5) 90 93.070(2) 90 1612.63(6) 4 1.64 812 1.17 2.82 – 27.08
6485 3568 (0.048) 2682 0.063 0.156 0.086 0.176 1.03 2.72/-1.65
16837 4094 (0.053) 3022 0.043 0.113 0.061 0.118 1.02 0.91/-0.44
4667 2658 (0.032) 2578 0.050 0.136 0.052 0.136 1.07 0.84/-0.89
5612 3364 (0.017) 3079 0.023 0.052 0.026 0.054 1.03 0.59/-0.69
7797 4047 (0.014) 3663 0.026 0.065 0.030 0.067 1.04 0.27/-0.27
17052 3312 (0.022) 3189 0.022 0.053 0.023 0.053 1.12 0.32/-0.21
)
5
2.5. Cyclic voltammetry The electrochemical measurements were carried out in a conventional threecompartment cell separated by glass frits using an Autolab potentiostat/galvanostat analyzer (AUTOLAB PGSTAT 302N, Eco Chemie, B. V., Utrecht, The Netherlands). As a supporting electrolyte the solution of 0.1 M TBAP in DMF was used. The electrochemical activation of the working bare gold electrode was carried out in the 0.1 M NaOH water solution. The solutions were prepared prior to use. All solutions under investigation were performed at room temperature and purged with high-purity argon before measurements. The working electrode was the bare gold electrode of 0.3 cm2 geometric area. A gold sheet was used as an auxiliary electrode. As the reference electrode the saturated calomel electrode (SCE) was used. The gold (purity 99.999%) electrodes and the SCE electrode were purchased from the Polish State Mint and EuroSensor, Poland, respectively. In order to activate the electrode, the bare gold electrode was polished with aluminium slurries of successively decreasing final grades (down to 0.05 m, Buehler) on polishing cloths (Buehler). Then it was rinsed carefully with acetone and water purified in a Millipore Milli-Q system (resistivity ≥ 18 MΩcm-1), respectively. As a preliminary step to each experiment, the working electrode was electrochemically activated in the 0.1 M NaOH solution by cycling the electrode potential in the potential range from E = -1.2 vs. SCE to E = 0.6 V vs. SCE (v = 0.1 V s-1) eg between the onset of hydrogen and oxygen evolution, until a stable voltammogram (j - E curve) was obtained. This procedure avoids structural changes on the gold surface. Furthermore, the roughness factor of the activated electrode remains constant [27]. C At the end of each series of experiments in non-aqueous solutions, the gold electrode was rinsed with acetone and water and the j - E curves were recorded again in a freshly prepared 0.1 M NaOH electrolyte solution. The shapes of j – E curves in each series of experiments were the same as those taken before the measurements in non-aqueous solutions. This means that the surface of the working electrode did not change during the experiment. Before subsequent use of working electrode in new electrochemical measurements, gold electrode was washed in the Piranha solution, rinsed with water and dried. Next the procedure of polishing and activation of the bare electrode was repeated. 3. Results and discussion 3.1. Synthesis of ligands L and their complexes Reaction of divalent Manganese(II) salts with two Schiff base type ligands containing N3-donor binding pockets with pyridine (LA) or imidazole (HLB) moiety in MeOH resulted in formation of complexes in 1:1 stoichiometry. In general, central ion is coordinated by single ligand molecule and the coordination sphere is complemented with counterions (3A) or counterions and solvent molecules (1A, 2A, 1B-3B). Crystals suitable for X-Ray analysis were obtained by double diffusion crystallization approach. FT-IR spectra correspond well with obtained X-Ray structures. One may observe stretching bands arising from N-H present in imidazole moiety in HLB and complexes 1B-3B at 3116 and 3165 cm-1, respectively. Slight shift may be due to the coordination. Spectrometric studies show that monomeric species persist in solution however harsh conditions in ESI-MS measurements led to partial decomposition of complexes during analysis thus peaks arising from ligands or MnX3- (X- = counterion) may be observed. 3.2. Crystal data Figures 2-6 show perspective views of the complexes as observed in their crystal structures.
6
The crystal structures can be divided into three categories, depending on their composition: (1) MnLX2(CH3OH) (1A, 1B, 3B), (2) MnL1X2 (3A), and (3) two complexes with nitro group, one neutral MnL1(NO3)2(CH3OH) (2A) and one cationic [Mn(HL2)(NO3)(CH3OH)2]+·NO3- (2B). In all cases the conformations of ligand molecules (defined by the values of dihedral angles between the planar fragments, cf. Table 2) are similar, i.e. ligands are generally planar, the central linkers C-C=N-N-C are also to good approximations planar. The above-mentioned category determines the coordination number and geometry of the complexes. In the three complexes from category 1, Manganese ion is six-coordinated (by three nitrogen atoms from ligand molecule, two halide ions and one oxygen atom from methanol molecule), in a distorted octahedral manner. The distortion is caused by the nature of the ligand L – the angle N3-Mn-N15 cannot be much closer to 180° than it is observed in the crystal (ca. 140°). Category 2 complex is five-coordinated, by three nitrogen atoms from the ligand molecule (N3, N10 and N15 – the same coordination mode of the ligand is present in all the complexes) and two halogens. The coordination is close to tetragonal pyramid (N3X base almost planar, second X atom at the apex, and Manganese ion displaced from the basal plane towards the apex by 0.5469(12)Å in 3A. Finally, two nitro-complexes (2A and 2B) are 7-coordinated, and it may be stressed that in cases when nitro groups coordinate via two oxygen atoms, both Mn-O distances are almost equal. The coordination polyhedron of these complexes is pentagonal bipyramid, with five atoms (three N from ligand molecule and two oxygen atoms from one nitro group) to a good approximation coplanar (maximal deviation ca. 0.15Å), and two oxygen atoms- from second nitro group and methanol molecule in 2A, from two methanol molecules in 2B – at the apexes, almost equally distanced from the basal plane (2.175 and -2.119Å in 2A, 2.181 and 2.198Å in 2B. In the nitro complexes the hydrogen bonds between methanol OH groups and NH (2B) and nitro groups are the main packing creation forces. Also, the intermolecular interactions and crystal packing are to some extent determined by the group to which a given complex belongs. In complex 3A the only specific intermolecular interaction which to some extent can be deemed responsible for the crystal packing is π···π stacking between rings of neighbouring molecules. In the crystal structures intermolecular hydrogen bonds of a type O-H···X and NH···X connect complex molecules into three-dimensional structures (cf. Tab. 3). Besides, relatively short π···π stacking interactions (centroid-to-centroid distances around 3.65Å, interplanar separations as short as 3.25Å) can be found between the molecules related by the consecutive centers of symmetry, thus producing infinite stacks of pyridine or imidazole rings. The complexes 1B and 3B are an example of one-dimensional isostructurality; although the unit cell parameters are quite similar, the packing along only one direction (a) is almost identical.
7
Fig. 2. Perspective view of the complex 1A; ellipsoids are drawn at the 50% probability level, hydrogen atoms are represented by spheres of arbitrary radii.
Fig 3. Perspective view of the complex 3A; ellipsoids are drawn at the 50% probability level, hydrogen atoms are represented by spheres of arbitrary radii.
Fig. 4. Perspective views of the complexes 1B (left) and 3B (right); ellipsoids are drawn at the 50% probability level, hydrogen atoms are represented by spheres of arbitrary radii.
Fig. 5. Perspective view of the complex 2A; ellipsoids are drawn at the 50% probability level, hydrogen atoms are represented by spheres of arbitrary radii.
8
Fig. 6. Perspective view of the complex 2B; ellipsoids are drawn at the 50% probability level, hydrogen atoms are represented by spheres of arbitrary radii. Table 2 Relevant geometrical parameters [A, °] with s.u.’s in parantheses. A, B, C denote the mean planes of benzoxazole ring system, CNNCC linker and imidazole ring, respectively. 1A 2A 3A 1B 2B 3B Mn-N3A 2.227(6) 2.2560(18) 2.252(3) 2.2546(19) 2.3163(12) 2.2429(12) Mn-N12A 2.316(6) 2.3384(19) 2.281(3) 2.332(2) 2.3558(12) 2.3259(12) Mn-N15A 2.301(6) 2.247(3) 2.2773(19) 2.2533(12) 2.2739(12) Mn-Br 2.5290(13) 2.3418(19) 2.5473(5) 2.6687(13) 2.7119(5) Mn-Cl 2.3549(9) 2.4056(4) 2.3691(9) 2.5449(4) Mn2.257(5) 2.1802(18) 2.2376(17) 2.1692(11) 2.2496(12) O(MeOH) 2.2136(11) Mn2.1889(19) 2.3247(11) O(nitrate) 2.2958(18) 2.3558(12) 2.3257(18) angles (3)
172.38(14) 172.26(13) 139.4(2)
163.53(7) 153.75(6) 151.29(6)
150.31(7) 136.75(11) 106.66(3)
171.60(5) 171.57(5) 140.34(7)
176.04(4) 152.57(4) 151.75(4)
172.72(3) 172.01(3) 140.28(5)
A/B B/C A/C
4.9(5)
7.3(3) 4.1(3) 8.78(14)
6.93(19) 8.73(17) 11.55(11)
4.13(17) 4.13(18) 6.80(10)
4.37(13) 4.20(14) 0.52(8)
5.18(8) 3.88(9) 6,65(6)
Table 3 Hydrogen bond data (A, °).
D
H
A
D-H
H···A
D···A
O1D
H1D
Br1Ci
D-H···A
0.95
2.30
3.224(5)164
1B 0.79(3) 0.88 2A
2.45(3) 2.51
3.2306(19) 3.364(2)
174(3) 164
2.02(3)
2.764(3)
174(4)
1.98(2) 1.97(2)
2.7473(15) 2.7372(16)
173(2) 171(2)
1A
O1D N17A
H1D H17A
Br1Cii Br1Ciii
O1D
H1D
O2Biv
O1C O1D
H1C H1D
O3E O2Ev
0.75(3) 2B 0.77(2) 0.77(2) 3B
9
O1D N17A
H1D H17A
Cl1Bvi Cl1Cvii
0.77(2) 0.88
2.35(3) 2.35
3.1208(13) 3.1503(13)
174(2) 151
Symmetry codes: i-1+x,y,z; ii-1+x,y,z; iii 2-x,-y,-z; iv -1/2+x,3/2-y,-1/2+z; v 1/2+x,3/2-y,1/2+z; vi x,-1+y,z; vii 2-x,-1/2+y,3/2-z. 3.3. Electrochemical properties In order to gain some insights into the electron-richness of studied systems, the electrochemical properties of ligands LA and HLB, as well as complexes 1A and 1B were investigated by cyclic voltammetry. The measurements were performed in deaerated DMF containing 0.1 M of TBAClO4 as the supporting electrolyte solution (SES). Complexes 1A and 1B were chosen due to their isostructurality what allows to the direct comparison of their behavior. The electrochemical behavior of the polycrystalline gold electrode in the presence of both the supporting electrolyte solution alone, studied ligands and their complexes are presented in Figs. 8 and 9. Fig. 7 presents the cyclic voltammograms of the bare polycrystalline gold electrode obtained in SES, TBAClO4 (curve x) and LiClO4 (curve y), respectively. Considering the j - E curve obtained in the first SES and going from a negative to positive potential limit there are two peaks visible on the anodic branch of CV at E = -0.775 V vs SCE and at E = 1.6 V vs SCE (which begins to grow already from 1.3 V vs. SCE), respectively. At the cathodic part of SES j - E curve three peaks appeared at E = 0.75 V vs SCE, E = -0.75 and E = -1.05 vs SCE respectively. There is no doubt that the anodic peak at E = 1,6 V vs SCE and the cathodic peak at E = 0.75 V vs SCE are assigned to the gold oxide formation at the electrode surface and to its reduction in cathodic scan, respectively. It is characteristic for gold electrodes both in aqueous and in non-aqueous solutions [28,29]. To explain the source of anodic peak at E = -0.775 V vs SCE and cathodic peaks E = -0.75 and E = -1.05 vs SCE on the j - E curve obtained in 0.1 M TBAClO4, the cyclic voltammogram in 0.1 M LiClO4 in DMF was recorded (curve y at Fig. 7). These two supporting electrolyte solutions differ only in cations. From literature it is known that ClO4 – anions both do not adsorb and do not undergo any faradic reactions on the polycrystalline gold electrode surface [30,31]. As can be seen, the only difference between the curve x recorded in 0.1 M TBAClO4 and the curve y obtained in 0.1 M LiClO4 presented on Fig. 7 is such that the latter j - E curve lacks the peak at E = 0.775 V vs SCE. So, it may be stated that the latter peak is related to electroactivity of the organic cation TBA+. While, as it was mentioned above, since ClO4 – anions do not take part in electrochemical reaction on gold, the two following peaks in the cathodic scan at E = -0.75 and E = -1.05 vs SCE which appeared both at x and y j - E curves are assigned to the redox processes related to DMF molecules. It was observed that the latter effect is intensified when ligands or, especially, complexes are added into the supporting electrolyte solution. As depicted in Fig. 8A both considered ligands show one reduction peak at 0.775 V vs SCE without any anodic counterpart, which may be attributed to the formation of the radical cation of heterocyclic moiety [32,33] (for comparison, the j - E curve of the bare gold electrode in the supporting electrolyte solution is added as the inset in Fig. 8A). One may envisage the benzoxazole part of ligands being involved in this redox process, since peaks positions are alike for both ligands and no distinct changes may be observed due to the presence of imidazole or pyridine moiety. This peak cannot be related to the gold oxide layer reduction, otherwise one should expect much higher current response. The increase of current observed in the potential range of the gold oxide layer is related to adsorption of the studied ligands on the electrode/solution interface. One can underline that, in comparison to both ligands, the cyclic voltammograms of complexes 1A and 1B exhibit some ligand-independent peaks (Fig. 8B as well as Figs. 9). During the anodic potential scan both complexes show oxidative responses at 0.75 V vs SCE, which are assigned to the Mn(II)Mn(III) conversion 10
[34,35]. An additional peak occurring at 0.04 V vs SCE in the cathodic part of the j-E curve is attributed to some electron interactions in the benzoxazole rings induced by complexed manganese ions. Similarly to the ligands an increase of the intensity of the peak observed at 1.25 V vs SCE in case of both complexes may be generated due to their adsorption and, additionally, adsorption of bromides, which are present in the coordination sphere of metal ions and preset in the electric double layer region [36]. The latter processes are intensified by gradual increase of concentration of ligands and their complexes (Figs. 10). It should be added that the peak visible at -0.775 V vs SCE which is assigned to the organic cation TBA+ fades-out in case of both, ligands and Mn(II) complexes. It may be caused by electrostatic interaction of TBA+ with oxygen atoms present in benzoxazole ring, thus gradual decay of this peak is observed due to the addition of analyzed compounds. It is worth pointing out that the CV curves presented in this manuscript are fully repeatable after the first potential scan for each concentrations of ligand or complex used. Therefore, one can conclude that in the presented studies, the adsorption of the ligand/complex residues does not occur at gold, so the gold electrode surface is not passivated during the measurements. Thus, it is a good template for evaluating the electrochemical properties of the prepared ligands and their metal complexes.
Fig. 7. Cyclic voltammograms of the polycrystalline gold electrode recorded in: 0.1 M TBAClO4 in DMF solution (curve x) and in 0.1 M LiClO4 in DMF solution (curve y); dE/dv = 0.1 Vs-1.
Fig. 8. A) Cyclic voltammograms of polycrystalline gold electrode recorded in: 0.1 M TBAP solution (inset) and in presence of ligands LA and HLB in 0.1 M TBAP in DMF solution and B) complexes 1A and 1B in 0.1 M TBAP in DMF solution; dE/dv = 0.1 Vs-1.
11
Fig. 9. Cyclic voltammograms of polycrystalline gold electrode recorded in 0.1 M TBAP solution in DMF; Comparison of complexes and their parent ligands: A) ligand LA and complex 1A; B) ligand HLB and complex 1B; dE/dv = 0.1 Vs-1.
Fig. 10. Cyclic voltammogram of polycrystalline gold electrode in 0.1 M TBAP solution in DMF containing increasing amounts (10-5 ÷ 2·10-3 M) of A) ligand LA, B) complex 1A, C) ligand HLB and D) complex 1B. Grey curves – supporting electrolyte solution. dE/dv = 0.1 Vs-1. 4. Conclusion As a continuation of our previous work concerning various metal complexes and their properties we report six newly synthesized Manganese(II) complexes with Schiff base type ligands: an earlier reported ligand LA and a new ligand HLB. X-Ray studies revealed isostructurality between complexes 1A and 1B and formation of dimers supported by OH···Br bonds in solid state. Additionally, dimers of complex 1B are confined due to N-H···Br interactions. This is manifested in the density of crystals equal to 1.94 and 1.96 g/cm3 of complexes 1 and 2, respectively. Cyclic voltammetry studies show only irreversible oxidation Mn(II)Mn(III) at 0.75 V vs SCE and several peaks attributed to
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electron processes in ligands or DMF. Our investigations evidence that additional NH unit in complex 2 do not facilitate the adsorption of compounds on a bare gold electrode. Further studies on synthesis and characterization of Manganese(II) species with tridentate N-heterocyclic scaffolding are underway. Our next attempt will be to focus on the synthetic modification of ligands presented herein by incorporating additional coordinating moieties and investigation on electrochemical character of their Manganese complexes. 5. Acknowledgements This research was carried out as a part of the Polish National Science Center project (Grant No. 2016/21/B/ST5/00175). 6. References [1] S. Dajnowicz, J.M. Parks, X. Hu, K. Gesler, A.Y. Kovalevsky, T.C. Mueser, J. Biol. Chem. 292 (2017) 5970-5980. [2] M.M. Najafpour, M.Z. Ghobadi, B. Haghighi, T. Tomo, J.-R. Shen, S.I. Allakhverdiev, Biochim. Biophys. Acta 1847 (2015) 294–306. [3] B. Xu, W. Jiang, Y. Wang, Z. Xiang, F. Liu, Y. Wu, Colloid. Surface. A 456(2014) 222– 230. [4] P. Mondal, R. Kumar, R. Gogoi, Bioorg. Chem. 70 (2017) 153-162. [5] H. Zafar, A. Kareem, A. Sherwani, O. Mohammad, M.A. Ansari, H.M. Khan, T.A. Khan, J. of Photochem. Photobiol. B 142 (2015) 8–19. [6] A. Rauf, A. Shah, A.A. Khan, A.H. Shah, R. Abbasi, I.Z. Qureshi, S. Ali, Spectrochim. Acta A. 176 (2017) 155-167. [7] S.A. Al-Harbi, M.S. Bashandy, H.M. Al-Saidi, A.A.A. Emara, T.A.A. Mousa, Spectrochim. Acta A 145 (2015) 425–439. [8] Y. Gou, Y. Zhang, J. Qi, Z. Zhou, F. Yang, H. Liang, J. Inorg. Biochem. 144 (2015) 47– 55. [9] K.J. Davis, C. Richardson, J.L. Beck, B.M. Knowles, A. Guédin, J.-L. Mergny, A.C. Willis, S.F. Ralph, Dalton Trans. 44 (2015) 3136-3150. [10] M. Das, R. Nasani, M. Saha, S.M Mobin, S. Mukhopadhyay, Dalton Trans. 44 (2015) 2299-2310. [11] M. Shabbir, I. Ahmad, H. Ismail, S. Ahmed, V. McKee, Z. Akhter, Polyhedron 133, 270278. [12] J. Cisterna, V. Dorcet, C. Manzur, I. Ledoux-Rak, J.-R. Hamon, D. Carrillo, Inorg. Chim. Acta 430 (2015) 82–90. [13] Z.-G. Gu, C.-Y. Pang, D. Qiu, J. Zhang , J.-L. Huang, L.-F. Qin, A.-Q. Sun, Z. Li, Inorg. Chem. Commun. 35 (2013) 164–168. [14] D.-F. Liu, L.-Q. Zhu, J. Wu, L.-Y. Wu, X.-Q. Lü, RSC Adv. 5 (2015) 3854-3859. [15] A.-R. Judy-Azar, S. Mohebbi, J. Mol. Cat.A 397 (2015) 158–165. [16] M.A. Fik, A.E. Odachowska, M. Kubicki, J. Karpińska, V. Patroniak, Eur. J. Inorg. Chem. (2016) 5530-5538. [17] S. Roy, K. Harms, S. Chattopadhyay, Polyhedron 91 (2015) 10–17. [18] F. Dumur, L. Beouch, M.-A. Tehfe, E. Contal, M. Lepeltier, G. Wantz, B. Graff, F. Goubard ,C.R. Mayer, J. Lalevée, D. Gigmes, Thin Solid Films 564 (2014) 351–360. [19] D.P. Kessissoglou, Bioinorg. Chem.NATO ASI Series 459 (1995) 299-320. [20] M. Maiti, D. Sadhukhan, S. Thakurta, E. Zangrando, G. Pilet, A. Bauzá, A. Frontera, B. Dede, S. Mitra, Polyhedron 75 (2014) 40–49. [21] C. Tommos, G.T. Babcock, Biochem. Biophys. Acta 1458 (2000) 199-219. [22] M. Kato, J.Z. Zhang, N. Paul, E. Reisner, Chem. Soc. Rev. 43 (2014) 6485-6497.
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Electrochemical activity of Mn(II) complexes was evaluated by cyclic voltammetry. Studies show only irreversible oxidation Mn(II)Mn(III) at 0.75 V vs SCE and several peaks attributed to electron processes in ligands or DMF. Presence of NH moiety in imidazole ring of HLB does not feasible the adsorption on the bare gold electrode.
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