Synthesis, Characterization, and Crystal Structure of [Co4(CH3CO2)2L4]2[BPh4]40.5H2O, Where HL = 4-(Salicylaldiminato)antipyrine
The title complex was isolated as a red solid from the reaction of 4-(salicylaldiminato)antipyrine, HL, and cobalt (II) acetate in ethanol. The complex has been characterized by elemental analysis, FTIR, UV-Vis, and X-ray single crystal diffraction. Two crystallographically different cationic units, A and B, of the title complex are found. Both units are essentially isostructural; nevertheless, small differences exist between them. Both units contain four cobalt atoms arranged at the corners of distorted cubane-like core alternatively with phenoxy oxygen of the Schiff base. In both cases, one cobalt binds to three coordinated sites from the corresponding tridentate Schiff base ligand, and the fourth one was bonded by the acetate oxygen, and the fifth and the sixth donor sites come from the phenolate oxygen of another Schiff base ligand.
Polynuclear as well as binuclear Co(II) complexes have attracted much attention due to their potential advantages than mononuclear complexes toward the preparation of molecular magnets and their application in data storage and memory devices . The discovery of single molecule magnetism (SMM) in high-spin Ni(II) molecular clusters, particularly in cubane-like tetranuclear Ni(II) complexes, revived the interest in such compounds in order to study the correlation between the magnetic anisotropy of the high-spin ground state and the magnetization at low temperatures . In comparison to mononuclear complexes, binuclear and polynuclear complexes could also provide more than one metal active center as Lewis acid in catalytic process, which is the interest of many researchers .
Crystal structures of complexes containing Co4O4 cubane-like core, as Co4O4(OAc)2(bpy)4(ClO4)2, Co4(CH3O)4(C5H7O2)4(C2H3O2), and Co4O4(C8H9N2O2) 7.5H2O , are well-known for several decades. Polynuclear metal complexes with tridentate ligands containing at least one hydroxyl group and oxygen as terminal coordinating atom have been reported and attracted much attention . These ligands often form polynuclear complexes with cubane or double cubane structure with missing one vertex each . Antipyrine and its derivatives are one of such compounds that act as tridentate ligands, and this type of ligands has been attractive to researchers, since they are used as antifever and pain reliving drugs .
Transition metal Schiff base complexes have been also used as antifungal and antibacterial reagents . It is shown that transition metal complexes containing 4-aminoantipyrine as a Schiff base have anticancer and antibacterial activity  and have an important effect on simulate enzymes . The type of the metal salt and the ligand plays an important role in tailoring the final product. Here, we report the first type of the Co(II) complex that contains two crystallographically different ions 1.a and 1.b, with tetranuclear cobalt atoms each. The Co(II) atoms reside in the corners of a cubane-like structure alternatively with oxygen atoms from the Schiff base ligands.
2.1. Material and Measurement
All reagent grade chemicals used in this work were obtained commercially from Aldrich or BDH and used without any further purification. All manipulations were carried out under atmospheric pressure. Elemental analysis (C, H, and N) was performed on a Vario El(III) elemental analyzer. FTIR spectra were recorded at room temperature with a Bruker IFS-25 OPUS/IR over the range from 400 to 4000 cm−1 with resolution of 4 cm−1. The electronic absorption spectrum was recorded over the range 200–800 nm using Cary 5000 UV-VISNR spectrophotometer.
2.1.1. Synthesis of (4-Salicylaldiminato)antipyrine-EtOH (HL·EtOH)
Yellow crystals of Schiff base, HL, were prepared using previously described method [11, 12]. A mixture of 4-aminoantipyrine (2.0 g, 9.85 mmol) in 20 mL ethanol and salicylaldehyde (1.22 g, 10 mmol) in 10 mL ethanol were refluxed together. Complete reaction with near quantitative conversion to the product required a period of 2.0 h. After cooling to room temperature, the yellow precipitate formed was collected by suction filtration and recrystallized from hot ethanol as deep yellow microcrystals (yield 2.81 g, 85%). The Schiff base was characterized by elemental analysis (Anal. Calc. for C20H22N3O3: C, 68.18; H, 6.25; N, 11.93; Found: C, 67.95; H, 6.25; N, 11.70).
2.1.2. Synthesis of Co4(CH3CO2)2L42BPh44·0.5H2O
A solution of HL (2.0 mmol, 0.6 g) in 15 ml ethanol and cobalt (II) acetate (2.0 mmol) in 15 mL ethanol were refluxed together; after 2 h, a red solid material was precipitated. The resulting solid was separated by filtration, washed with ethanol and dichloromethane, and air-dried. The product was dissolved in hot methanol and a suitable methanol solution of sodium tetraphenylborate was added and stirred for a while to give the title complex as a red solid. The final product was dissolved in acetone and filtered; on slow evaporation, red crystals were formed (yield 62%). The final product was characterized by elemental analysis, FTIR, UV-Vis, and single-crystal X-ray diffraction analysis (Anal. Calc. for C248H219B4Co8N24O24.5: C, 66.99; H, 4.93; N, 7.56. Found: C, 67.38; H, 5.4; N, 7.23).
2.1.3. Crystal Structure Determination and Refinement of the Title Complex
Suitable red single crystals of the complex were obtained by slow evaporation of an acetone solution at room temperature. X-ray single-crystal diffraction data were collected on an Oxford Gemini S diffractometer equipped with CCD detector at 295 K; MoKα radiation ( Å) was used and a multiscan correction for absorption was applied. The structure was solved by direct methods (SIR92)  and refined on by full matrix least-squares SHELXL97  and WinGX . All nonhydrogen atoms except were only partially occupied (s.o.f = 0.5) and their H atoms were not located. Positions of hydrogen atoms bonded to C were calculated geometrically and refined by a riding model. All aromatic C6-rings are constrained to be planar using FLAT instruction. Although there were indications that some rings are disordered, this behavior was not further explored due to very large number of atoms in the asymmetric unit. Data collection: CrysAlis PRO , cell refinement: CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SIR97 , program(s) used to refine structure: SHELXL97 , molecular graphics: ORTEP-3 for Windows , and Mercury , software used to prepare material for publication: publCIF , and PARST . Crystal Data. The details of the crystallographic data and structure refinement for the complex are shown in Table 1. Additional material available from the Cambridge Crystallographic Data Center comprises thermal parameters and remaining bond distances and angles (CCDC number 801603).
2.2. Crystal Data
See Table 1.
3. Results and Discussion
A novel cobalt complex Co4(CH3CO2)2L42BPh44·0.5H2O was synthesized by the reaction of cobalt (II) acetate with HL Schiff base in ethanolic solution. The IR spectrum of the complex exhibited strong absorptions at 1625 and 1562 cm−1 assignable to the carbonyl group of the pyrazolone ring, ν(C–O), and azomethine group (HC=N) of “L,” respectively. The first absorption band of the complex was shifted to the lower frequency for about 29 cm−1 relative to the free ligand (1654 cm−1) indicating that the ligand coordinates through the carbonyl oxygen of the pyrazolone ring. The second absorption was shifted to the lower frequency for about 29 cm−1 (free ligand at 1591 cm−1) suggesting the involvement of the nitrogen atom of the azomethine group in the coordination. The absorptions appears as shoulders at 1610 and 1457 cm−1 indicates the asymmetric and symmetric stretching frequencies of the acetate – groups. The separation (143 cm−1) between the two absorptions indicates that the acetate group acts as a bidentate bridging ligands . The ν(OH) absorption at 3446 cm−1 indicates the presence of water molecule. The tetraphenylborate showed a finger print absorption at 2927–3044 cm−1. The UV-Vis spectrum of the title complex was obtained in acetone solution. The spectrum shows a very intense band at 202 cm−1 due to transition characteristic for aromatic rings, and another intense band at 232 nm corresponding to the charge transfer band. The bands at 324 nm with a shoulder at 315 nm and 401 nm are assigned to the d-d transitions of the high-spin Co(II) ions in an octahedral geometry. There is a blue shift in this bands compared with the absorption spectra of some hexacoordinate Co(II) complexes with the peak maxima wavelength () at about 500–513 nm .
The asymmetric units of the title complex contain two tetranuclear cations, A and B, of the formula Co4(CH3CO2)2L42+ with a cubane-like Co4O4 core. In addition, four BPh4− ions and one water molecule with 0.5 occupancy are present. The cations are close to be isostructural, and they only slightly differ in corresponding bond distances and angles. Crystal structure of the title complex with atomic numbering scheme are presented as ORTEP (Figure 1), the stereoview of the cell contents of the title complex was shown in (Figure 2) and relevant bond distances were given in Table 2. From ORTEP presentation and molecular packing diagram (Figure 3), the ligand, “L,” acts as trichelate coordinating to the Co atom by two O and one N atoms. The oxygen of the phenolate group further bridges three Co(II) ions (μ3-O) in the cubane-like core. Both cationic units contain Co(II) residing in a pseudooctahedral environment with CoNO5 chromophore. Each complex cation can be regarded as a dimer of dimeric units composed by phenoxy bridged dicobalt (II) subunits. The basal-planes of Co(II) include two oxygen atoms from the phenolate groups of two “L” ligands and another oxygen atom from the pyrazolone ring; the fourth position was occupied by the azomethine nitrogen of “L.” Axial sites are occupied by two oxygen atoms, one from the acetate group and the other one from another phenolate group of “L.”
The acetate group acts as triatomic bidentate ligands bridging two Co atoms in their familiar syn-syn manner on opposite faces of the cube and the ligand “L” chelates the remaining four faces. The two faces bridged by acetate group in cationic unit A exhibit shorter Co≡Co separations of 2.996 and 3.022 Å, and more acute Co–O–Co angles ranging from 89.06 to 90.03° with Co–O–O–Co dihedral angles of 154.16 and 156.56° compared with those not bridged by acetate group, where Co≡Co separations are in the range of 3.24 to 3.31 Å, Co≡Co angles in the range of 97.26 to 102.28°, and Co–O–O–Co dihedral angles in the range of 177.06 to 178.99°. Cationic unit B is a little bit shorter in Co≡Co separations with more acute angles. These variations in bond distances and angles between faces of the cubane-like cores are responsible for their distortions. This distortion produces two types of Co–O–Co angles, those that are <90° at the top and bottom faces of the core and those that are >101° at the rest of the cubane faces. Therefore, one can expect that the Co(II) ions with small angles will lead to some magnetic properties comparable to the Ni4(OCH3)4(dbm)4(MeOH)4, dbm = dibenzoylmethane .
The average Co–OAc bond distances of the title complex of 2.02 Å are longer than that reported for Co4O4(dpah)4(CH3CO2)22V4O12·5H2O (1.98 Å) and shorter than that reported for Ni4(OCH3)4(OAc)2(TMB)4(BPh4)2·4CH2Cl2, TMP = 2,5-dimethyl-2,5-diisocyanohexane (2.05 Å) . The bond distances of Co–OAc in cationic unit B are a little bit longer than in cationic unit A by 0.02 Å. The difference in all bond distances of the acetate groups in both units ranges from 0.01 to 0.02 Å and is very close to that reported for Ni(II) complex . While the phenolate oxygen atom is ligating three cobalt atoms in a tetrahedral environment, the four cobalt atoms are located at four alternative corners of a distorted cubane-like core and the rest of the cubane vertices are occupied by four oxygen atoms from four L as μ3-O. Both units (Figure 1), as we mentioned before, differ slightly in corresponding bond distances and angles; even in the cubane itself the separations between cobalt atoms differ slightly, for example, the longest separation for cationic unit A of Co2≡Co4 (3.288 Å) and the shortest separation of Co3≡Co4 (2.996 Å), for cationic unit B, the longest separation of Co6≡Co8 (3.313 Å), and the shortest of Co7≡Co8 (2.979 Å). The shortest Co≡Co separation in the title complex is still longer than the longest one reported for Co4O4(dpah)4(OAc)22V4O12·5H2O, dpah = 2,2-dipyridylamine (2.848 Å) . The average Co≡Co separations in both units are very close to the separation in Co2L2(NCS)2(CH3OH)2 (3.1028 Å)  and in Co2L2Cl2(CH5OH)2 (3.085 Å) , where “L” is the same Schiff base in the first and 2-hydroxyisophtaldehyde oxime in the second case. The C–O bond distances in the phenolate groups of the ligands, which act as μ3-O bridges, range from 1.377(5) to 1.381(5) Å in cationic unit A and from 1.367(5) to 1.379(5) Å in cationic unit B. All of these distances are still longer than C–O distance of the phenolate group which acts as μ2-O in Co2L2(NCS)2(CH3OH)2 (1.330(2) Å) and even longer than the distance of the free Schiff base (1.345 Å) [11, 12].
Further analysis of the crystal structure revealed that this structure contains a half water molecule in the lattice. There is a short contact between O25 and H27E atom belonging to the acetate methyl group at 3.01(1)8 Å and another O24O25 contact of 2.90(2) Å indicating possible hydrogen bonds. The four counter ions BPh4− exhibit their normal geometry. The two A and B cationic units of the title complex are held together by an electrostatic attraction with BPh4− ions. Finally, similar complexes of Ni(II), Mn(II), mixed Co(II) and Ni(II), and mixed Co(II) and Cu(II) complexes were prepared and characterized by spectroscopic methods and their structures will be investigated in near future.
From all of these, we conclude that the novel Co(II) complex has two very large A and B cationic unit complexes with a cubane-like core; A and B units are almost isostructural and differ slightly in bond distance and angles. The Co(II) atoms reside in a pseudooctahedral environment and share the corners of a distorted cubane-like core equally with the oxygen of the phenolate group of the Schiff base ligands. The phenolate oxygen bridges three cobalt atoms, while the acetate group bridges two cobalt atoms. The distortion of the cationic unit complexes may lead to some magnetic interactions which is correlated with the small Co–O–Co angles of the top and bottom faces within the cubanes.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
CCDC no. 8011603 contains the supplementary crystallographic data for [Co4(CH3CO2)2 L4]2[BPh4]4. 0.5H2O, where HL= 4-(salicylaldiminato) antipyrine complex. The data can be obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/cif by e-mailing
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