Bis(μ₂-2,2′-((2-(hydroxy)propane-1,3-diyl)bis((nitrilo)eth-1-yl-1-ylidene))diphenolato)-dicobalt(III)

Abstract

A new cobalt(III) complex with a pentadentate Schiff base was synthesized using a reaction of 2,2′-{(2-hydroxypropane-1,3-diyl)bis(nitriloeth-1-yl-1-ylidene)}diphenol (H₃L) and cobalt(II) acetate in a methanolic solution. This synthesis resulted in the isolation of dinuclear compound [Co^III₂L₂] (1), which was characterized using elemental analyses and XRF, FTIR, and TG/DSC techniques. The molecular structure of the complex was confirmed using single-crystal X-ray diffraction. The structure of 1 consists of a centrosymmetric dimer in which two crystallographically equivalent cobalt(III) ions are bridged by two alkoxido oxygen atoms. In addition, each metal center is coordinated by two Schiff bases.

1. Introduction

Polydentate ligands are extensively synthesized and studied in coordination chemistry. The use of such a compound makes it possible to obtain a wide range of complexes with different compositions and structures and interesting physicochemical properties. 2,2′-{(2-hydroxypropane-1,3-diyl)bis(nitriloeth-1-yl-1-ylidene)}diphenol (H₃L) is polydentate ligand that has five donor atoms. It contains three oxygen atoms (two phenolic groups and one alcohol group) and two nitrogen atoms (azomethine groups) which can participate in the coordination of metal ions. The literature confirms that there is information about the mono- (vanadium), di- (iron and cobalt), tera- (copper, nickel, and cobalt), and hexanuclear (copper and nickel) metal complexes of the mentioned Schiff base [1,2,3,4,5,6,7,8,9].

The coordination chemistry of cobalt complexes with Schiff bases has been the subject of extensive research for many years. Focusing on the complexes of cobalt(III) with Schiff bases, the great interest in them is because such compounds have a diverse range of applications as they possess interesting catalytic, magnetic, photochemical, and biological properties [3,10,11,12,13,14,15].

Herein, as a continuation of our work on polydentate Schiff-base complexes, we report the synthesis, crystal structure, and thermal stability of a dinuclear solvent-free double μ₂-alkoxido-bridged Co(III) compound ([Co₂L₂]). The dinuclear cobalt(III) complex was also prepared by Mitra et al. [3], but the coordination environment around the metal ions is slightly different. The previously described Co(III) complex contains a coordinated water molecule in its structure, and only one alkoxide group acts as a bridge between the two metal ions. In addition, one Schiff-base molecule is involved in the coordination of one central atom (Scheme 1). In the complex described in this paper, each cobalt(III) ion is coordinated by both Schiff-base ligands.

2. Results and Discussion

The synthesis of the cobalt(III) complex was performed by a reaction between the Co^II ions and the ligand at a 1:1 stoichiometric ratio in a hot methanol solution, as indicated in Scheme 2. The obtained compound is solvent-free, which was confirmed by the results of the elemental and thermal analyses and XRF and FT-IT spectroscopies. In addition, X-ray analyses of a single crystal allowed verifying the research results obtained by carrying out other measurements.

2.1. FT-IR Spectroscopy and Thermal Analysis

The FT-IR spectrum of the complex was recorded within the range of 4000–530 cm⁻¹ (Figure S1 in Supplementary Materials) and compared with that of the free ligand. It should be noted that there are no bands at 3600–3100 cm⁻¹, which indicates that all hydroxyl groups were deprotonated and the complex is solvent-free. The free ligand spectrum shows the bending δ(O-H) vibrations of hydroxyl groups, i.e., phenolic (1346 cm⁻¹) and alcohol (1277 cm⁻¹). These vibrations are not recorded in the spectrum of 1, confirming that all OH groups have been deprotonated and are involved in the coordination of metal centers. In addition, the ν(C-O_phenol) stretching vibration, which appeared as a strong band at 1268 cm⁻¹ in the H₃L spectrum, is observed at 1264 cm⁻¹ in the complex and exhibited lower intensity [4,9]. The metal ions are also bound by imine nitrogen atoms. The intense band of C=N stretching vibrations, which was recorded at 1607 cm⁻¹ in the Schiff-base spectrum [4], is slightly shifted towards higher frequencies in the [Co₂L₂] spectrum (1610 cm⁻¹) and exhibited lower intensity. This shows the participation of the nitrogen atoms of azomethine groups in the coordination of cobalt(III) ions.

The thermal behavior of the complex was studied in an oxidizing atmosphere. As observed in Figure 1, the compound is characterized by good thermal stability. Its decomposition begins above 250 °C and proceeds in one stage. In the beginning, the defragmentation of the ligand is most likely followed by its combustion, which is evidenced by a large exothermic effect on the DSC curve. The final product is formed at a temperature of about 410 °C and is Co₃O₄ (found and calculated total mass losses are 78.79 and 79.00%).

2.2. Description of the Molecular Structure

The compound crystallizes in centrosymmetric monoclinic space group P2₁/c. The asymmetric unit of the cobalt(III) complex together with the atomic numbering scheme is shown in Figure 2a. It comprises one Co(III) ion and one Schiff-base ligand. However, the structure of the compound consists of a centrosymmetric dimer [Co₂L₂] (Figure 2b) in which the two six-coordinated cobalt(III) ions are bridged by two deprotonated alkoxido oxygen atoms of the pentadentate ligand. The geometry around the cobalt(III) ion is a distorted octahedral (Figure S2 in Supplementary Materials). One fully deprotonated Schiff-base ligand (L³⁻) coordinates one Co(III) center via one nitrogen atom (N(1)) and two oxygen atoms (phenoxido O(1) and alkoxido O(2)). The three remaining sites of the cobalt(III) ion are coordinated by one azomethine nitrogen atom N(2), phenoxido oxygen atoms O(3), and a bridging μ₂-alkoxido atom (O(2)) of the second L³⁻ anion. The similar chelating modes of the Schiff-base ligands lead to the formation of a symmetric dimer with the Co₂O₂ core at its center. This way of coordinating metal ions causes the ligand to be bent. The dihedral angle between the mean planes defined by the phenyl rings (C(1)–C(6) and C(14)–C(19)) is 42.17(14)°. As observed in

Table 1. Selected geometric parameters for [Co₂L₂].

Bond Lengths (Å)
Co(1)-;O(1)	1.894(2)	Co(1)-;O(3)i	1.896(2)
Co(1)-;O(2)	1.955(2)	Co(1)-;O(2)i	1.943(2)
Co(1)-;N(1)	1.896(2)	Co(1)-;N(2)i	1.908(2)
Co(1)-;Co(1)i	2.930(1)
Angles (°)
O(1)-;Co(1)-;N(1)	88.30(9)	O(3)i-;Co(1)-;O(2)i	171.63(8)
O(1)-;Co(1)-;O(3)i	90.34(8)	N(2)i-;Co(1)-;O(2)i	84.78(9)
N(1)-;Co(1)-;O(3)i	90.59(9)	O(1)-;Co(1)-;O(2)	171.64(8)
O(1)-;Co(1)-;N(2)i	88.65(9)	N(1)-;Co(1)-;O(2)	84.16(9)
N(1)-;Co(1)-;N(2)i	176.86(10)	O(3)i-;Co(1)-;O(2)	93.32(8)
O(3)i-;Co(1)-;N(2)i	88.70(9)	N(2)i-;Co(1)-;O(2)	98.94(9)
O(1)-;Co(1)-;O(2)i	94.74(8)	O(2)i-;Co(1)-;O(2)	82.54(9)
N(1)-;Co(1)-;O(2)i	96.20(9)

Symmetry transformations used to generate equivalent atoms: (i) −x + 1, −y + 1, −z + 1.

, Co-O_alkoxido bond distances are comparable and amount to 1.955(2) Å for Co(1)-;O(2) and 1.943(2) Å for Co(1)-;O(2)ⁱ. The bridging Co(1)–O(2)–Co(1)ⁱ angle is 82.54(9)° with a Co(1)–Co(1)ⁱ separation of 2.930(1) Å. In the complex described by Mitra et al. [3], the asymmetric unit consists of two crystallographically independent Co^III ions. The different modes of Schiff-base ligands and various coordination environments of the metal center result in the formation of an asymmetric dimer. Co(1) and Co(2) ions are bridged only by one deprotonated alkoxido oxygen atom, which causes the Co(1)–O_alkoxido–Co(2) angle to become larger (135.5(3)°), and the distance between metal ions is greater (3.572(2) Å) [3]. In the structure of the title compound, there are no strong hydrogen bonds (Table S1 in Supplementary Materials). The dimeric units are linked by weak C−H⋯O bonds into a supramolecular layer (Figure S3 in Supplementary Materials).

3. Materials and Methods

3.1. Materials

3-diamino-2-propanol, 2′-hydroxyacetophenon, Co(CH₃COO)₂·4H₂O, and methanol were purchased from commercial vendors and used as received without further purification. The Schiff base (2,2′-{(2-hydroxypropane-1,3-diyl)bis(nitriloeth-1-yl-1-ylidene)}diphenol, H₃L) was synthesized from 1,3-diamino-2-propanol with 2′-hydroxyacetophenon in hot methanol (30 mL) according to the procedure given in the literature [4].

3.2. Synthetic Procedures

Co(CH₃COO)∙4H₂O (0.500 mmol, 0.124 g) was dissolved in 15 mL of methanol at room temperature. Next, the acetate solution was added dropwise to a hot solution of H₃L (0.500 mmol, 0.172 g) in 20 mL of methanol. The initial yellow solution of the Schiff base immediately turned dark brown. After a few minutes, a small amount of green solid (0.030 g) precipitated out of the solution. The mixture was stirred for 30 min before being filtered. The filtrate was kept at room temperature. After a few days of slow evaporation, dark green crystals suitable for X-ray diffraction were collected. Yield of 65%, Anal. (%) for C₃₈H₃₈Co₂N₄O₆ (M_W: 764.58). Calcd: C, 59.69; H, 5.01; N, 7.33; Co, 15.42. Found: C, 59.43; H, 5.03; N, 7.13; Co, 15.24. FTIR bands (ν/cm⁻¹): 3044 (w), 3016 (w); 2939 (w), 2883 (w), 2843 (w), 1610 (s), 1593 (s), 1540 (m), 1470 (m), 1436 (vs), 1375 (w), 1320 (s); 1264 (m), 1237 (s, br); 1195 (w), 1154 (w), 1043 (m), 1005 (m), 961 (s), 925 (w), 862 (s), 846 (m), 823 (m), 814 (m), 771 (m), 757 (vs), 739 (vs), 642 (w), 632 (m), 622 (m), 560 (m), 579 (m), 564 (m), 534 (w).

3.3. Methods

The Fourier transform infrared spectrum (4000–400 cm⁻¹) of the complex was recorded on a Nicolet 6700 FTIR (ATR technique) spectrometer with a resolution of 4 cm⁻¹ for 16 scans. C, H, N microanalyses were performed using a CHN/CHNS EuroEA3000 analyser (EuroVector), and cobalt content was determined by XRF using an Axios mAX spectrophotometer (PANalytical). The thermal stability of the complex was determined using the TG-DSC method. For this purpose, the sample (7 mg) was heated in an open ceramic crucible on a Setaram Setsys 16/18 derivatograph within the range of 30–800 °C at a heating rate of 10 °C min⁻¹ in flowing air (v = 0.75 dm³ h⁻¹).

3.4. Crystallographic Details

Data collection for the complex was performed using an Oxford Diffraction Xcalibur CCD diffractometer. The measurement was carried out using a graphite monochromated MoK_α (λ = 0.71073 Å) radiation. Data collection and reduction were performed by using the CrysAlis program [16]. Information about crystal data structure and refinement is provided in

Table 2. Details of data collection and structure refinement parameters for 1.

CCDC	2267146
Temperature K	100(2)
Crystal system	monoclinic
Space group	P21/c
a (Å)	12.0356(10)
b (Å)	9.7908(7)
c (Å)	13.5890(13)
β (°)	107.086(9)
Volume (Å3)	1530.6(2)
Z	2
Calculated density (g cm−3)	1.659
μ (mm−1)	1.144
Absorption correction	multi-scan
F(000)	792
Crystal size (mm)	0.400 × 0.400 × 0.350
θ range (°)	2.605-;26.370
Reflections collected/unique	10,259/3114
Rint	0.0486
Data/restraints/parameters	3114/0/228
GooF on F2	1.073
Final R indices [I > 2σ(I)]	R1 = 0.0424, wR2 = 0.1015
R indices (all data)	R1 = 0.0543, wR2 = 0.1098
Largest diff. peak/hole, e Å−3	0.878/−0.569

. WinGX software [17] was used to solve (direct method, SHELXS-2018) and refine structure (full-matrix least squares against F², SHELXL-2018/3 [18]). All non-hydrogen atoms were found from difference Fourier maps and refined anisotropically, while H atoms were positioned geometrically and refined using a riding model (C—H = 0.95–0.99 Å, and with U_iso(H) = 1.2 (1.5 for methyl groups) times Ueq(C). ORTEP3 [17] and Mercury [19] programs were used to visualize the structure of the compound. Geometrical calculations were carried out in the PLATON program [20].

4. Conclusions

The reaction between cobalt(II) acetate and 2,2′-{(2-hydroxypropane-1,3-diyl)bis(nitriloeth-1-yl-1-ylidene)}diphenol results in the formation of a new dinuclear di(μ-alkoxido)-bridged complex. During syntheses, Co^II was oxidized to Co^III. The coordination of metal ions occurs via both Schiff bases.