Synthesis, characterization and crystal structure of a mononuclear zinc(II) complex derived from 2-methoxy- 6-[(3-cyclohexylaminopropylimino)methyl]phenol

Colourless block-shaped crystals of Dichloro{2-methoxy-6-[(3-cyclohexylaminopropy-limino)methyl]phenolato}zinc(II) methanol, [Zn(C₁₇H₂₆N₂O₂)Cl₂].CH₃OH, have been obtained and characterized by elemental analysis, IR and X-ray single crystal determination. The complex crystallizes in the monoclinic space group P2₁/n with unit cell dimensions a=12.980(3) Å, b=13.139(3) Å, c=13.277(3) Å, β=107.00(3)°, V=2165.4(8) ų, Z=4, R ₁=0.0631 and wR2=0.1110. X-ray structure determination revealed that the complex consists of a [Zn(C₁₇H₂₆N₂O)Cl₂] moiety and a lattice MeOH molecule. In the crystal structure, molecules are linked through intermolecular N–H$\cdots$O, C–H$\cdots$Cl and O–H$\cdots$Cl hydrogen bonds, forming chains. It is the first complex derived from the Schiff base ligand 2-methoxy-6-[(3-cyclohexylaminopropylimino)methyl]phenol.

Introduction

Schiff base complexes play an important role in the development of coordination chemistry related to catalysis and enzymatic reactions, magnetism and molecular architectures.1 ^– 6 Zinc is an important metal in biology and acts as the active site of hydrolytic enzymes, where it is in a coordination environment of the hard donors nitrogen and oxygen.7 ^, 8 Zinc has been recognized as an important co-factor in biological molecules, either as a structural template in protein folding or as a Lewis acid catalyst that can readily adopt the coordination numbers 4, 5, or 6.9 Recently, we have reported a few complexes derived from the Schiff base ligand 2-[(3-cyclohexylaminopropylimino)methyl]phenol10 ^– 12 and its derivatives.13 ^, 14 However, to the best of our knowledge, no studies have been reported based on the Schiff base ligand 2-methoxy-6-[(3-cyclohexylaminopropylimino)methyl]phenol (HMCP; see Scheme 1) with metals. In this paper, we report the synthesis and structure of a mononuclear Schiff base zinc(II) complex derived from the ligand HMCP.

Scheme 1.

The Schiff base ligand HMCP.

Experimental

Materials and measurements

All chemicals (reagent grade) were commercially available and used without further purification. C, H, and N elemental analyses were performed on a Perkin-Elmer 240C elemental analyzer. The IR spectra were measured with a FT-IR 170-SX (Nicolet) spectrophotometer. The ¹HNMR spectra were recorded on Bruker AVANCE 500 MHz spectrometer with tetramethylsilane as the internal reference.

Synthesis of the ligand HMCP

To a MeOH solution (20 mL) of 3-methoxysalicylaldehyde (6.0 mmol, 913.2 mg) was added a MeOH solution (10 mL) of N-cyclohexyl-1,3-diaminopropane (6.0 mmol, 942.3 mg) with stirring. The mixture was stirred for 10 min at room temperature to give a great deal of yellow precipitation. The product was filtered, washed three times with MeOH, and dried in a vacuum desiccator containing anhydrous CaCl₂. Analysis calculated for C₁₇H₂₆N₂O₂: C, 70.31; H, 9.02; N, 9.65%; found: C, 70.17; H, 9.13; N, 9.72%. Selected IR data (KBr, cm⁻¹): 3554 (w), 3432 (w), 3067 (w), 2933 (m), 2859 (w), 1639 (vs), 1280 (s), 754 (s). ¹HNMR data (CDCl₃, ppm): δ=1.05 (q, 2H), 1.17 (m, 1H), 1.26 (m, 2H), 1.62 (m, 2H), 1.72 (m, 2H), 1.87 (m, 4H), 2.40 (q, 1H), 2.74 (m, 2H), 3.67 (t, 2H), 3.90 (s, 3H), 6.79 (t, 1H), 6.86 (d, 1H), 6.91 (d, 1H), 8.32 (s, 1H), 14.0 (e, 1H).

Synthesis of the complex [Zn(C ₁₇ H ₂₆ N ₂ O ₂ )Cl ₂ ].CH ₃ OH

To a MeOH solution (5 mL) of ZnCl₂ (0.1 mmol, 13.6 mg) was added a MeOH solution (10 mL) of HMCP (0.1 mmol, 15.2 mg), with stirring. The mixture was stirred for 10 min at room temperature and filtered. Upon keeping the filtrate in air for 5 days, colourless block-shaped crystals of the complex, suitable for X-ray single crystal structural determination, formed at the bottom of the vessel on slow evaporation of the solvent. The crystals were isolated, washed three times with cold MeOH and dried in a vacuum desiccator containing anhydrous CaCl₂. Analysis calculated for C₁₈H₃₀Cl₂N₂O₃Zn: C, 47.13; H, 6.59; N, 6.11%; found: C, 46.96; H, 6.67; N, 6.23%. Selected IR data (KBr, cm⁻¹): 3563 (m), 3447 (m), 3067 (w), 2933 (m), 2859 (w), 1631 (vs), 1468 (s), 1447 (s), 1299 (m), 1243 (s), 1219 (s), 1086 (w), 746 (m).

Crystal structure determination

X-ray diffraction intensities were collected using a Bruker SMART Apex CCD area detector equipped with graphite-monochromated Mo-Ka radiation (λ=0.71073 Å) at 298(2) K. Absorption correction was applied by SADABS program.15 The structure was solved by direct methods and refined on F ² by full-matrix least-squares methods using the SHELXTL version 5.1.16 All of the non-hydrogen atoms were refined anisotropically. H atoms were placed in calculated positions and constrained to ride on their parent atoms. The details of the crystallographic data are summarized in Table 1. Selected bond lengths and angles are summarized in Table 2. Hydrogen bonds are listed in Table 3.

Table 1. Crystal Data and Refinement Parameters for the Complex

Table 2. Selected Bond Distances (Å) and Angles (°) for the Compound

Table 3. Geometrical Parameters for Hydrogen Bonds

Results and discussion

To design novel structures of metal complexes, the ligands used in the synthesis are important. In this paper, we designed and synthesized a new Schiff base ligand, HMCP. The reason we use HMCP as the ligand is that it could adopts versatile coordination modes through the phenolate O atom, imine N atom and/or amine N atom. HMCP was prepared in an excellent yield (82.3%) in a MeOH solution. The compound is yellow crystallite, stable in air at room temperature. It is soluble in common polar organic solvents, such as DMSO, DMF, MeOH, EtOH, and MeCN, etc., but poorly soluble in water and Et₂O. The elemental analysis is in good agreement with the chemical formula proposed for the compound. Zinc(II) is a good candidate of tetrahedral coordination. The complex is also stable in air at room temperature, soluble in DMF, DMSO, MeOH, EtOH and MeCN, and poorly soluble in water and Et₂O.

Crystal structure description of the complex

Figure 1 gives a perspective view of the complex with the atomic labeling system. The complex is a mononuclear zinc(II) compound, which consists of a [Zn(C₁₇H₂₆N₂O₂)Cl₂] moiety and a lattice MeOH molecule. In the zinc(II) moiety, the Zn atom is four-coordinated by the phenolate O and imine N atom of the ligand MCP, and by two terminal Cl atoms, forming a tetrahedral coordination. The bond lengths related to the Zn atom are comparable to the values observed in other similar Schiff base zinc(II) complexes we reported recently.17 ^– 19 The bond angles subtended at the Zn atom are ranged from 94.87(14) to 115.25(11)°, indicating a distorted tetrahedral geometry. As expected, the N1–C8–C9–C10–N2–C11 moiety in the complex adopts zigzag form and the cyclohexyl group adopts chair conformation to minimize steric effects. The torsion angles N1–C8–C9–C10 and C9–C10–N2–C11 are −73.4(5)° and 4.7(5)°, respectively.

Fig. 1.

Molecular structure of the complex. Displacement ellipsoids are drawn at the 30 % probability level and H atoms are shown as small spheres of arbitrary radii; numbering according to Table 2.

In the crystal structure, the MeOH molecules are linked to the zinc complex moieties through intramolecular N–H$\cdots$O hydrogen bonds and intermolecular O–H$\cdots$Cl hydrogen bonds. The molecules are further linked through intermolecular N–H$\cdots$O and C–H···Clhydrogen bonds, forming chains as shown in Fig 2.

Fig. 2.

Molecular packing of the complex, viewed along the c axis. Intermolecular hydrogen bonds are shown as dashed lines.

Conclusions

The present study shows that the ligand HMCP coordinates to the Zn atom through the phenolate O and imine N atoms. The amine N atom of the ligand HMCP is protonated, and not coordinates to the Zn atom. The structure of the complex is stabilized by the intermolecular H-bonding interactions.