Geometrical Study of a New Cobalt(III) Complex, fac-[Co(HEAC)₂ ]ClO₄ , Containing Eight Chiral Centers with Mixed Donor (NN’O) Ligand

Introduction

The synthesis of β-amino alcohols is of current interest as they are important sub-units commonly used in the synthesis of biologically active natural and synthetic products, pharmaceuticals, synthetic amino acids and pesticides¹⁻² and as cross-linking reagents in the synthesis of precursor systems for superconductors³.

In the past few years we have studied complexation of multi N-donor ligands⁴⁻⁷. In continuation of our previous studies, in this work the preparation of cobalt(III) complex, [Co(HEAC)₂]ClO₄∙2.5H₂O  (1), with 2-(2-(2-hydroxyethylamino)ethylamino)cyclohexanol (HEAC, scheme 1) was described. This complex was characterized by elemental analysis, FT-IR and UV-Vis spectroscopy, cyclic voltammetry and X-ray crystallography.

Scheme 1. The structure of HEAC. Click here to View scheme

Material and Methods

General methods

All starting chemicals and solvents were reagent or analytical grade and used as received. The HEAC was prepared accordingly to the literature⁸. A conventional three–electrode system voltammetry was employed, with a PGSTAT101 and glassy carbon working electrode. The infrared spectrum of a KBr pellet was recorded in the range 400–4000 cm⁻¹ using a FT-IR 8400-Shimadzu spectrometer. The carbon, hydrogen and nitrogen contents were determined in a Thermo Finnigan Flash Elemental Analyzer 1112 EA. Melting point was determined using a Barnsted Electrothermal 9200 electrically heated apparatus. The electronic spectrum was recorded in H₂O using a Shimadzu model 2550 UV-Vis spectrophotometer (190–900 nm).

Synthesis of [Co(L)₂]ClO₄∙2.5H₂O, 1.

A solution of HEAC (2 mmol, 0.40 g) in methanol (25 mL) was mixed with CoCl₂∙6H₂O (1 mmol, 0.24 g), to produce a purple solution. O₂ was bubbled through the solution for 12 h, then the solution was left at room temperature for 2 d. Then it was diluted to l.5 L with water, and sorbed onto a column of Sephadex-SP C-25 cation exchange resin. Elution of a fast-moving major purple band was achieved with 0.2 M NaClO₄.Crystals suitable for X-ray diffraction were grown from a dilute water solution. Yield (0.51 g) 84 %; m.p.: 148 °C. Anal. Calcd for C₂₀H₄₇ClCoN₄O_10.5(%): C, 39.64; H, 7.82; N, 9.25. Found: C, 40.11; H, 7.77; N, 9.42. IR (KBr, cm⁻¹): 3267 s (ν OH), 3069 m (ν NH), 2928 s (ν CH), 2827 m (ν CH₂), 1445 m (δ_as CH₂), 1350 m (δ_s CH₂), 1230 w (ν CO), 1121 s (ν₁ ClO₄), 1116 s (ν CN), 1025 (ν₄ ClO₄), 988 (ν₂ ClO₄), 815 (ν₃ ClO₄) and 626 (ν₅ ClO₄) cm⁻¹. UV-Vis (H₂O, λ_max (nm)/ε): 229/6727 (n→π^*), 401/99 (¹A₁g→¹T₂g), 553/81 (¹A₁g→¹T₁g). Cyclic voltammetry (glassy carbon, 100 mV s⁻¹ scan rate): E_½ (Co^III/Co^II) −0.32 V, E_½ (Co^II/Co⁰) −1.19 V vs. Ag/AgCl.

Crystal structure determination and refinement

The data collection for 1 was carried with a Bruker APEX-II CCD diffractometer, using graphite-monochromatized MoKα (λ = 0.71073 Å) radiation at 296 K. The data were integrated with SAINT⁹ and corrected for Lorentz and polarization effects. Absorption was corrected for using SADABS¹⁰. The structure was solved by Patterson methods, implemented in SHELXS-97¹¹. Refinement by full-matrix least-squares methods based on F² values against all reflections has been performed by SHELXL-97¹¹, including anisotropic displacement parameters for all non-H atoms. The position of hydrogen atoms belonging to the carbon atoms Csp² were geometrically optimized applying the riding model [Csp²–H, 0.93 Å; U_iso(H) = 1.2 U_eq(C)]. Calculations concerning the molecular geometry, the verification of space group and the analysis of hydrogen bonds were performed with PLATON¹². Table 1 contains crystallographic data and details of the data collection and structure refinement. Selected bond lengths (Å) and angles (°) are listed in table 2.

Table 1. Crystal data and structure refinement for 1.

Empirical formula	C20H47ClCoN4O10.5
Formula weight (g mol−1)	606.00
Temperature (K)	293
Crystal system	Triclinic
Space group	12P1″>
Unit cell dimensions (Å, °)
a	9.5363(6)
b	9.7054(6)
c	17.6096(8)
α	85.119(4)
β	87.865(4)
γ	63.567(6)
Volume (Å3), Z	1454.15(14), 2
Calculated density (g cm−3)	1.384
Absorption coefficient (mm−1)	0.74
F(000)	646
Crystal size (mm3)	0.22 × 0.16 × 0.04
θ range for data collection (°)	2.7–29.1
h, k, l ranges	−12:12, −12:9, −22:23
Reflections collected	11969
Independent reflections	6512
Rint	0.053
Data / restraints / parameters	6512 / 99 / 380
Goodness-of-fit on F2	0.90
Final R indices [I > 2σ(I)]	R1 = 0.079, wR2 = 0.1989
R indices (all data)	R1 = 0.1848, wR2 = 0.229
Largest diff. peak and hole (e.Å−3)	0.83 and −0.40

 

Table 2. Selected bond length (Å) and angles (°) for 1 with estimated standard deviations in parentheses.

Distances		Angles
Co1A–O1A	1.897(3)	O1A–Co1A–N1A	93.2(2)
Co1A–N1A	1.959(4)	O1A–Co1A–N2A	96.0(2)
Co1A–N2A	1.979(5)	N2A–Co1A–N1A	85.8(2)
Cl1–O1	1.359(9)	O1–Cl1–O3	108.3(6)
Cl1–O2	1.383(6)	O3–Cl1–O2	107.1(5)
Cl1–O3	1.39(1)	O2–Cl1–O4	108.0(5)
Cl1–O4	1.419(6)	O4–Cl1–O1	109.1(5)

Results and Discussion

The reactions between methanolic solution of HEAC and cobalt(II) chloride in presence of O₂ provided purple crystals of 1. It is air-stable and soluble in DMSO and H₂O.

The ν (N–H) in the IR spectrum of 1 shifted 28 cm⁻¹ to lower energy respect to the free ligand, indicating coordination through the nitrogen atoms. The characteristic frequencies corresponding to the perchlorate group¹³ appear at about 1121 (ν₁), 1025 (ν₄), 988 (ν₂), 815 (ν₃) and 626 (ν₅) cm⁻¹.

UV–Vis spectrum of 1 in aqueous solution exhibited a broad absorption with the λ_max at 229 nm for n→π^* transition, 401 and 553 nm for d→d transitions of the cobalt(III) complex.

Cyclic voltammetry data for the cobalt(III) complex in the range of −0.32 to −1.19 V and 0.10 M TBAP-DMSO solution are listed in the experimental section. The cyclic voltammogram of 1 showed two reversible oxidation and reduction waves, corresponding to the Co^III/Co^II and Co^II/Co⁰ couple.

Description of the crystal structure

The crystal structure of 1 was determined by X-ray single-crystal diffraction. The molecular graphics were drawn with ORTEP-III¹⁴ and Diamond¹⁵.

Figure 1: The ORTEP-III diagram of the molecular structure of 1. The ellipsoids are drawn at the 50 % probability level. The C−H bonds were omitted for clarity. Click here to View figure

In the crystal structure of 1 (figure 1), the cobalt atom has a distorted octahedral coordination environment. Four sites are occupied by nitrogen atoms with the Co–N bond lengths in the range of 1.959(4)–1.979(5) Å. The two other sites are occupied by two oxygen atoms with the Co–O bond lengths 1.897(3) Å. The hydroxyl group of the cyclohexyl ring is coordinated to the metal ion while the other is clearly not. The complex 1 has one center of inversion which is placed on the cobalt atom and C_i symmetry (figure 2).

Figure 2. Showing the Ci symmetry of 1. All hedrogen atoms were omitted for clarity. Each CoN4O2 unit is shown as an octahedron. Click here to View figure

 

The cobalt(III) ion has d⁶ configuration in low spin status and no Jahn-Teller effects. The axial Co−N bond lengths are similar to equatorial ones, proving this issue. Of particular interest from coordination chemistry perspective asymmetrical HEAC ligand, is that several diastereomers are possible in complexation. There are six possible arrangement of the pair of ligands around the octahedral metal ion exist (scheme 2) for octahedral coordination of two HEAC ligand as tridentate chelate. Among these possible diastereomers, the all-trans–fac form was observed in 1.

Scheme 2. Six possible diastereomers for coordination of the two HEAC around the octahedral metal ion. Click here to View scheme

 

Supplementary material

CCDC 748469 for [Co(L)₂]ClO₄∙2.5H₂O (1) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

Acknowledgement

We are grateful to Payame Noor University of I. R. Iran for financial support.

References

1. Ager D. J., Prakash I. and Schaad D. R., Chem. Rev., 96, 835 (1996).
2. Corey E. J. and Zhang F.-Y., Angew. Chem., Int. Ed., 38, 1931 (1999).
3.  Wang S., Trepanier S. J., Zheng J. C., Pang Z. and Wagner M. J., Inorg. Chem., 31, 2118 (1992).
4. Hakimi M., Mardani Z., Moeini K., Minoura M. and Raissi H., Z. Naturforsch., 66b, 1122 (2011).
5. Hakimi M., Mardani Z., Moeini K., Mohr F., Schuh E. and Vahedi H., Z. Naturforsch., 67b, 452 (2012).
6. Hakimi M., Maeder M. and Lawrance G. A., J. Coord. Chem., 64, 105 (2011).
7. Hakimi M., Kukovec B.-M., Normohammadzadeh Z., Schuh E. and Mohr F., J. Chem. Crystallogr., 42, 180 (2012).
8. Hakimi M., Mardani Z., Moeini K. and Fernandes M. A., J. Coord. Chem., 65, 2221 (2012).
9. SAINT (version 7.12), Bruker Analytical X-ray Instruments Inc., Madison, Wisconsin, USA (2004).
10. Sheldrick G. M., SADABS, Program for Empirical Absorption Correction of Area Detector Data, University of Gottingen, Gottingen, Germany (1996).
11. Sheldrick G. M., Acta Crystallogr., A46, 467 (1990).
12. Spek A. L., J. Appl. Crystallogr., D65, 148 (2009).
13. Hakimi M., Moeini K., Mardani Z., Fernandes M. A., Mohr F. and Schuh E., J. Coord. Chem. 65, 1232 (2012).
14. Farrugia L. J., J. Appl. Crystallogr., 30, 565 (1997).
15. Bergerhoff G., Berndt M. and Brandenburg K., J. Res. Natl. Inst. Stand. Technol., 101, 221 (1996).

---

This work is licensed under a Creative Commons Attribution 4.0 International License.