Synthesis, Crystal Structure and Electrochemistry Properties of a Cobalt(II)Complex Based on Asymmetry Schiff Base Ligand①

FENG Xun SONG Hong-Ling YE Bo-Xin HUO Su-Zhen XIE Shi-Yu GUO Jin-Zhong (College of Chemistry nd Chemicl Engineering, Luoyng Norml University, Luoyng 471022, Chin)

b (College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China)

1 INTRODUCTION

More recently, the design, synthesis and characterization of transitional metal complexes with Schiff-base ligands play a relevant role in the coordination chemistry due to their importance as synthetic models for the functional materials[1-2].They show biological applications including antibacterial[3-4], antifungal[5-6]and antitumor activity[7].Diamino tetradentate Schiff bases and their complexes have been used as biological models to understand the structures of biomolecules and biological processes[8-10]. The interaction of metal complexes containing N2O2Schiff base ligands has been thoroughly considered[11,12]. The development of routes and strategies for the synthesis of complexes of 3d metals in moderate oxidation states is of great importance because these species provide substantial impetus for the development in several fields, including bioinorganic chemistry, magnetochemistry, materials chemistry and solid-state physics[9]. On the other hand, the cobalt ion is an essential component of many proteins and enzymes. Cobalt(II) complexes based on benzene/pyridyl multi-carboxylate could serve as models for the examination of biological and non-biological catalytic processes, which are found to play important roles in biological systems[9]. In recent years, the simple preparation of metal-salen based modified electrodes by oxidative electro-polymerization of metal complexes in poor coordinating solvents has prompted their applications in heterogeneous electro-catalysis[10], among which the cobalt atom is commonly seven-coordinated, linked to the nitrogen and oxygen atoms of the Schiff base, or is further ligated by the nitrogen, sulphur and oxygen atoms from the second ligand. However, single crystal of the cobalt(II) complex containing single asymmetry substituted salen Schiff type ligand has been less well investigated. In order to further study the influence of coordinating behavior by the electron withdrawing species and electronic properties in the selfassembly processes for Schiff base complexes, in this work, the synthesis and crystal structure of a new Co(II) complex containing asymmetry substituted salicylaldehyde Schiff base has been reported,together with its electrochemistry properties. The NO2-salen ligand presents a mono deprotonated fashion in the complex, and to the best of our knowledge, such coordination mode is seldom reported.

2 EXPERIMENTAL

2. 1 General

All chemicals and solvents purchased were of reagent grade and used without further purification.Elemental analyses for carbon, hydrogen and nitrogen were carried out on a Model 240 Perkin-Elmer elemental analyzer. The infrared spectrum was performed on an AvatarTM360 E.S.P. IR spectrometer in the 4000～400 cm-1region with KBr pellets. Electrochemical measurements were executed on RST3000 series electronic work station(Suzhou Risetech Instrument Co. Ltd China). The electrochemical cell used in cyclic-voltammetry was a closed standard three-electrode cell connected to a solution reservoir through a Teflon tube under nitrogen atmosphere. A platinum disk (1.5 mm diameter) was used as the working electrode, and a Hg/Hg2Cl2as the reference electrode equipped with a wire counter electrode. The ferrocene/ferrocinium(Fc/Fc+) redox couple served as the internal standard.

2. 2 Synthesis of the title compound

The mixture of Co(OAc)2·6H2O (1 mmol, 0.572 g)and NH4SCN (0.153 g, 2 mmol) was refluxed in anhydrous methanol (10 mL) for 50 min, decanted off, and filtered. To the resulting red solution were added 10 mL methanol solution of freshly distilled 5-nitrosalicylaldehyde and 10 mL methanol solution of ethylene-diamine at a molar ratio of 2:1 simultaneously, then they were condensed under stirring for 3 h. The resulting clear solution was diffused with diethyl ether vapor at room temperature for three weeks. The red block crystals were formed and collected by filtration and dried in air. Yield: 270 mg(56%, based on Co element). Elemental Anal. Calcd.(%) analysis for C18H20CoN6O6: C, 43.04; H, 4.61;N, 16.73. Found (%): C, 43.76; H, 4. 47; N, 16.57.IR (KBr disc, cm-1): 3622(s), 3435 (S), 3206 (m),1625(s), 1548 (m), 1311(s), 514 (m), 465 (m).

2. 3 X-ray structure determination

A single crystal of the title complex (0.33mm ×0.24mm × 0.17mm) was mounted on a Bruker SMART APEX II CCD diffractometer equipped with a graphite-monochromatized Mo Kα radiation(λ = 0.71073 ?) by using a φ/ω scan mode at room temperature in the range of 2.35≤θ≤25.25° with index ranges of -26≤h≤0, 0≤k≤31 and 0≤l≤42.A total of 4118 reflections were collected, of which 1892 were independent (Rint= 0.0137), and 769 with I ＞ 2σ(I) were observed and used for structure refinements. Corrections for Lp factors were applied and all non-hydrogen atoms were refined with anisotropic thermal parameters. The structure was solved by direct methods with SHELXS-97[11]. The hydrogen atoms were assigned with common isotropic displacement factors and included in the final refinement by use of geometrical restrains. A fullmatrix least-squares refinement on F2was carried out using SHELXL-97[12]. For the title complex, the disordered solvate molecules are difficult to identify,so the solvate molecules were accounted for by using the program PLATON/SQUEEZE (Spek, 2009)in order to remove the contributions of disordered solvent[23]. Elemental analyses and IR spectra were employed to further confirm the structure. The disordered solvents have been included in the chemical formula. PLATON estimates that one water molecule is located at the (six) Wyckoff 6b sites, two water molecules are at the (six) Wyckoff 6a sites,and two water molecules occupy the (eighteen)Wyckoff 18d sites, i.e., there are a total of 54 water molecules in the unit cell, so the formula unit is then C18H23CoN6O7.5. The final R = 0.0741, wR = 0.2410(w = 1/[σ2(Fo2) + (0.1168P2) + 7.2760P], where P =(Fo2+ 2Fc2)/3), S = 1.204, (Δρ)max= 1.212 and(Δρ)mix= –0.247 e/?3. The selected bond lengths and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (o) for the Title Complex

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of the title complex

The principal perspective view of the title complex with atomic labeling scheme is illustrated in Fig. 1.The thiocyanate anion does not present in the final product, unfortunately. The unit is composed of sixcoordinated cobalt ion, two NO2-salen ligands and 1.5 of free water molecules. The coordination polyhedron around the Co(II) ion can be visualized as a slightly distorted octahedral geometry with a novel CoO4N2mode. Among the donor set, the ligand(NO2-salen) affords four N and two O atoms to coordinate with the Co(II) ion. Single asymmetry substituted salen Schiff base ligand in this case has been less well investigated. Interestingly, the asymmetric coordination pattern of the ligand led to a crystal structure belonging to a centric space group.The bond distances of Co(1)–O(1) and Co(1)–O(4)are 1.887(5) and 1.896(5) ?, respectively, while the bond distances of Co(1)–N(5), Co(1)–N(2) and Co(1)–N(6) are found to be 1.899(6), 1.902(5) and 1.937(6) ?, respectively, giving the slightly distorted octahedral coordination geometry mainly due to the coordination of deprotonated NO donor Schiff base ligand[13]. The bond parameters are within the reasonable ranges for other six-coordinated Co(II) complexes with oxygen and nitrogen donating ligands[14].A slightly distorted octahedral geometry is also confirmed by the bond angles of O(1)–Co(1)– O(4),O(1)–Co(1)–N(5), O(1)–Co(1)–N(2) and O(4)–Co(1)–N(2), which are found to be 90.3(2), 87.4(2)93.9(2) and 88.1(2)o, respectively. Among the donor atoms, the equatorial plane of octahedron is occupied by one oxygen atom of the monodentate deprotonated hydroxyl group and three iminic nitrogen atoms from the diaminoethane moiety.However, another hydroxyl oxygen atom and an iminic nitrogen atom are located in axial position of the octahedron. Interestingly, just one iminic functional group of the diaminoethane molecule was deprotonated, giving rising to a mono asymmetry Schiff base in this case. The characteristic feature of the observed bridging mode is the use of single oxygen hetero-ring nitrogen atom to coordinate with the metal ion on one side, and two nitrogen atoms both bridging and chelating donated by the same tridentate ligand on the other two sides, respectively,with the O, O, N, N, N, N bonding moiety. Chelation of Co(II) ion leads to coplanarity of the two fragments, sharing a common central ion including nitro group, and makes a dihedral angle of 77.74°between the adjacent benzene rings. NO2- salen ligand coordinates to the Co(II) ion to form two six-membered chelate rings (Co(1)–C–C–C– N–O)and two five-membered rings (Co–N–C–C–N). This coordination fashion is compared to that of the reported calcium(II) complex with imidazole-4,5-dicarboxylate and water coligand[20]. In the complex,the two nitro groups are nearly coplanar with the benzene planes at the largest deviation of 0.3363(18)?, and the mean benzene ring is inclined at 4.2(6)and 4.0(2)° to the six-membered chelating ring each other, respectively. This coordination is essentially different from the analogous Schiff base compounds[15].

Fig. 1. Coordination environment of Co(II) ion in the symmetry unit of 1 viewed along the ac plane

Fig. 2 shows the alignment of a one-dimensional alternating chain viewed along the a b plane, in which the cobalt ions are bridged by NO2-salen ligand to maintain charge balance. The ligand bears one negative charge with nitro group each. It is noteworthy that there are obvious strong hydrogen bonds between the adjacent monomeric units. The hydrogen bonding parameters involving oxygen atoms belonging to coordinated water and oxygen atom from the carboxylic group (see Table 2 for details). Within chains, there is π-π stacking between neighboring benzene rings with centriod-centriod distance of 3.925 ? and the dihedral angle of 0°.

Fig. 2. Schematic illustration of the 1D chain structure through hydrogen bonds

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°) for Complex 1

3. 2 Infrared spectra

The infrared spectrum of the title complex displays a broad strong peak at 3435 cm-1, which is assigned to the coordinated water molecules. Lattice water molecules are indicated by the peaks appearing at ca. 3622 cm-1. It exhibits a strong band at 3206 cm-1, corresponding to ν(NH) stretching vibration of imino group from the ethane-diamine moiety[17]. This band shifts slightly to higher wave number side relative to the free ligand, indicating the coordination of metal ion due to the reduction of electron density in the amine link[18], as previously reported[19], which is consistent with the above structure analysis. The observation of strong band at 1625 cm-1was attributed to characteristic stretching vibration of C=N group. The new weak non-ligand bands at about 514 and 465 cm-1in the spectra of the complexes are assigned to the stretching frequencies of ν(M–O) and ν(M–N) bonds.

3. 3 Powder X-ray diffraction patterns

Form Fig. 3, we can find that the powder XRD patterns of compound 1 are well matched with the simulation ones based on single-crystal analysis,which indicates that the sample obtained is in a pure phase.

3. 4 Electronic chemical study

Fig. 3. Simulated and experimental PXRD patterns of the title complex

The cyclic voltammogram of the title complex was recorded at a sweep rate of 100 mV·s-1, as shown in Fig. 4. The cobalt(II) complex exhibits a one-electron reduction process in the cathodical potential in Epc= –314 mV and oxidation peak potential in Epa= +376 mV, with ipc= 0.057 mA and ipa= 0.024 mA. ΔE = |Epa– Epc| = 790 mV, ipa/ipc=0.87 and E1/2= 26 mV. The oxidation and reduction processes are irreversible in nature[20]. The complex showed an electrochemically reversible oxidation process in the anodic region corresponding to CoII/III couple. In the CoIII/II reduction region, the title complex shows only the Epcpeak, and Epawas not observed. This was possibly due to the loss of an ancillary ligand which itself resulted from the addition of an electron to the antibonding d2z orbital[21]. The behavior of the title complex is very similar to that of Co(III) complex [CoLCl3], where L=bis(pyrid-2-yl-methyl)-N-methyl-N-(pyrid-2-ylmethyl) ethylenediamine[22], for the lowest scan rate used is also comparable to that of the Fc/Fc+couple.The controlled potential electrolysis carried out shows that the peak corresponds to a one-electron transfer process, as given below:

This behavior is different from the case of[Co(saox)(bipy)2]Br[23], for which the cyclic voltammetry study in CH3CN showed the reversible redox couples Co(I)/Co(II) and Co(II)/Co(III), as well as the reversible oxidation of bipy or phen and the irreversible oxidation of the oxime ligands.

Fig. 4. Cyclic voltammogram diagram of 1 in DMF/0.1 M Bu4ClO4 at 100 mV s -1 scan rate

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