Synthesis, Characterization, and Structure of Mixed-Ligand Cobalt (II) Complex with N, O Donor Sites

Abstract

A mononuclear octahedral mixed-ligand cobalt (II) complex [Co(H₂L)(PhCOO)₂] (1) has been prepared by using H₂L (N,N′-dimethyl-N,N′-bi(2-hydroxy-3,5-di methyl benzyl)-ethylenediamine) as a facially coordinating tetradentate ligand with a N₂O₂ donor center along with sodium benzoate as an ancillary ligand. Complex 1 has been characterized by a single-crystal X-ray diffraction study, as well as by other spectroscopic tools. The complex crystallizes in the monoclinic space group C₂ with a = 31.73(3) Å, b = 7.868(3) Å, c = 19.131(15) Å, and β = 125.25(3)°. The single-crystal X-ray diffraction study shows that in the mononuclear cobalt (II) complex [Co(H₂L)(PhCOO)₂] (1), the metal center adopts an octahedral environment.

1. Introduction

Transition metal complexes with the tetradentate amine-bis(phenolate) ligand have been widely studied as catalysts [1,2,3,4,5,6,7], models of reaction centers of metalloenzymes [8,9], nonlinear optical materials [10], and biological models to understand the structures of biomolecules and biological processes [11]. Recently, research in transition metal complexes with bis(phenolate) ligands has been steadily increasing. A series of mono-, bi- and trinuclear cobalt (III) complexes with amine-bis(phenolate) ligands have been reported [12,13,14,15]. The catalytic activity of mononuclear cobalt (II/III) complexes with an amine-bis(phenolate) ligand for the coupling of CO₂ with propylene oxide to give cyclic propylene carbonate have been investigated [16]. At the same time, the catechol oxidase activity of mononuclear cobalt complexes with amine-bis(phenolate) ligands have been reported [17].

Inspired by the above knowledge, we report a mononuclear octahedral mixed-ligand cobalt (II) complex [Co(H₂L)(PhCOO)₂] (1). The molecular structure of the complex was established by a single-crystal X-ray diffraction study, as well as by other spectroscopic tools. The crystal structure reveals that the cobalt ion in this mononuclear complex is surrounded by a tetradentate ligand (H₂L) with a N₂O₂ donor site along with two benzoate ligands as ancillary ligands.

2. Results

2.1. General: Synthesis of 1

A sterically hindered tetradentate phenol-based N₂O₂ ligand (H₂L) has been used to synthesize a mononuclear mixed-ligand cobalt complex [Co(H₂L)(PhCOO)₂] (1). The tetradentate ligand N,N′-dimethyl-N,N′-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine (H₂L) was prepared according to a reported method [12,18]. The reactions are carried out in acetonitrile in the presence of sodium benzoate as the ancillary ligand and Co(ClO₄)₂.6H₂O as the metal ion precursor. The tetradentate ligand H₂L acts as a capping ligand in complex 1 because of its enormous flexibility, thus allowing benzoate ions to form a coordinate bond around the cobalt center in the mononuclear complex [19]. The compound is air stable in the solid state and also fairly stable in solution. The compound is also stable in solution for several days and no aerial oxidation takes place in solution in open air. The detailed strategy is summarized in Scheme 1. The structure of 1 was confirmed by a single-crystal X-ray diffraction study (Figure 1).

IR spectra of the complexes (1) display characteristic bands of the coordinated tetradentate ligand. IR spectra of the compound have a prominent band at the ca. 1183 cm⁻¹ region due to the ν(C-O/phenolate) stretching mode [20]. In addition, the carboxylato complex (1) displays a pair of strong bands at ca. the 1674 and 1453 cm⁻¹ regions due to symmetric and antisymmetric ν(COO⁻) vibrations, respectively. The difference in frequency (Δν) in all three cases is ca. 220 cm⁻¹, thus indicating a monodentate mode of carboxylate coordination [20].

2.2. X-ray Structure

The asymmetric unit of the crystal structure consists of two independent molecules with two units of solvent of crystallization, as shown in Figure 2. The central metal ion Co(II) is hexa-coordinated, forming an octahedral geometry, and is surrounded by two protonated phenolic O (Co1-O1, 2.163(4) Å; Co2-O3, 2.115(4) Å), two carboxylate O (Co1-O2, 2.059(5) Å; Co2-O4, 2.073(5) Å) and two N (Co1-N1, 2.213(6) Å; Co2-N2, 2.226(6) Å). The phenolic O are opposite to each other (O1-Co1-O1A, 170.47(15)°; O4-Co2-O4A, 170.90(16)°), whereas the tertiary amine N is opposite to the carboxylate O (N1-Co1-O2, 167.55(16)°, N1A-Co1-O2A, 167.55(16)°; N2-Co2-O5, 167.08(16)°, N2A-Co1-O5A, 167.08(16)°).

The phenolic proton participates in strong to moderately strong intra-molecular H-bonding (as shown in the packing structure, Figure 3). The H-bond parameters are given in

Table 1. Hydrogen-bonding geometries (Å and °) for 1.

Donor --- H....Acceptor	D - H	H...A	D...A	D - H...A	Type (Symmetry)
O1---H1O....O2	0.91(9)	2.48(8)	2.901(7)	109(5)	Intra
O1---H1O....O3	0.91(9)	1.60(9)	2.486(7)	163(8)	Intra
O4---H2O....O5	0.73(10)	2.42(9)	2.864(7)	121(7)	Intra
O4---H2O....O6	0.73(10)	1.77(10)	2.477(8)	163(8)	Intra
C17--- H17.... O4	0.93	2.58	3.422(9)	150.00	Inter (1 + x, 1 + y, z)

. In addition to intra-molecular H-bonds, the solvent, acetonitrile, also exhibits weak H-bonding to form a 1-D zig-zag chain along the b-axis (as shown in Figure 4). The contact distance between acetonitrile N and phenolic H is 2.70 Å with the bond angle of 147.85° (N4-H40-C40).

Phenolic O also participates in intermolecular H-bonding of moderate strength with the H of the adjacent phenyl ring.

2.3. Hirschfeld Surface Analysis

A Hirshfeld surface analysis was performed using Crystal Explorer [21]. The Hirshfeld surface was mapped using an iso-value of 0.5, with red contours indicating a contact less than the sum of the van der Waals radii of the respective elements, whereas blue and white contours indicate that the nearest external atom is at a distance greater than or equal to the sum of the van der Waals radii, respectively, from the atomic coordinate, as shown in Figure 2.

The Hirshfeld analysis of 1 also reveals an array of the H-interaction ranging from a strong H-bond that is intra-molecular (H1O....O3, 1.60(9) Å; H2O....O6, 1.77(10) Å) in nature to a moderate intermolecular H-bond involving the protonated phenolic oxygen atom, as shown in Figure 5. A 1-D zig-zag chain formed by the acetonitrile solvent is shown in Figure 6.

3. Materials and Methods

3.1. General

The tetradentate ligand N,N′-dimethyl-N,N′-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine (H₂L) was prepared according to a reported method [12,18]. Solvents were purified by taking suitable drying agents that were distilled under nitrogen prior to their use [22]. All reagents were purchased from E Merck, ( Bangalore, India). Chemicals were used without further purification.

UV–visible spectra of the complex were recorded on a Perkin-Elmer 950 UV/vis/NIR spectrophotometer (San Diego, CA, USA) and infrared spectra were taken on a Nicolet Magna 750 FT-IR spectrometer (San Diego, CA, USA), series II with samples prepared as KBr pellets.

3.2. Experimental: Synthesis of 1

The ligand H₂L (0.18 g, 0.5 mmol) was added to a stirred solution of cobalt (II) perchlorate hexahydrate (0.18 g, 0.5 mmol) in acetonitrile (25 mL). The stirring was continued for ca. 15 min and then solid sodium benzoate (0.14 g, 1 mmol) was portion-wise added to it. The resulting solution was refluxed for another 1 h. Then, the solution was filtered, and the filtrate was kept in open air for crystallization. A light-pink crystalline compound, along with diffraction-quality single crystals, were obtained by slow evaporation. It was collected by filtration and washed with cold diethyl ether (2 × 5 mL). Yield: 0.21 g (70%). Anal. Calcd for C₃₆H₄₀CoN₂O₆: C, 66.15; H, 6.12; N, 4.28. Found: C, 66.05; H, 5.98; N, 4.08%. IR (KBr disk, cm⁻¹): 3415, 1674, 1453, 1307, 1183, 751, 1076, 754 (See Supplementary Materials). UV–Vis [λ_max/nm (ε/M⁻¹ cm⁻¹)]: 312 (1090) (See Supplementary Materials).

The intensity data for a blue-block crystal, 0.12 × 0.14 × 0.20 mm, of the cobalt (II) complex (1) was collected at 296 K (23 °C) by mounting the crystal onto a quartz fiber on a Bruker D8 Quest diffractometer (Bruker, Karlsruhe, Germany) with an Incoatec microfocus source of Mo Kα radiation (λ = 0.71073 Å). APEX4 software [23] was used for the preliminary determination of the unit cell. Determination of integrated intensities and unit cell refinement was performed using SAINT [24]. The structure was solved with SHELXS [25] and subsequent structure refinements were performed with SHELX2019/1 [26]. Absorption corrections were performed by SADABS [27]. The crystal exhibits inversion twinning as confirmed by the Flack test [28]. Platon [29] was employed to obtain the twin law [−1 0 0 0 −1 0 0.69 0 1]. The final structure was refined with hkl5.

4. Conclusions

The sterically constrained tetradentate ligand H₂L has been used to synthesize mononuclear cobalt complexes in the presence of carboxylate as an ancillary ligand. The complex [Co(H₂L)(PhCOO)₂] (1) has been characterized by a single-crystal X-ray diffraction study. The geometry of the complex is octahedral in nature and it is a mixed-ligand complex.