Spontaneous Resolution Induced by a Chiral Ni ... - Wiley Online Library

Article DOI: 10.1002/bkcs.10157

S. Sarkar et al.

BULLETIN OF THE KOREAN CHEMICAL SOCIETY

Spontaneous Resolution Induced by a Chiral Ni(II) Complex with an Achiral Tripodal Ligand# Shuranjan Sarkar,† Dohyun Moon,‡ Seog K. Kim,§ Myoung Soo Lah,¶ and Hong-In Lee†,* †

Department of Chemistry, Kyungpook National University, Daegu 702-701, Republic of Korea. *E-mail: [email protected] ‡ Pohang Accelerator Laboratory, Pohang 790-784, Republic of Korea § Department of Chemistry, Yeungnam University, Gyeongsan 712-749, Republic of Korea ¶ Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea Received October 27, 2014, Accepted November 6, 2014, Published online February 18, 2015 A chiral nickel(II) complex, [Ni(II)H3L](ClO4)2 (1), with an achiral ligand H3L (=tris{2-(4-imidazolyl)methyliminoethyl}amine) was synthesized by in situ reaction between nickel(II) perchlorate hexahydrate and a condensation mixture of 4-imidazolecarboxaldehyde and tris(2-aminoethyl)amine. Single crystal X-ray analysis revealed that the H3L ligand hexadentately binds to Ni(II) ion through three Schiff-base imine N atoms and three imidazole N atoms with distorted octahedral geometry. Both single-crystal X-ray diffraction and circular dichroism investigations found that the crystal of complex 1 was an enantiopure conglomerate. The hydrogen-bond network of NimidazoleH OClO4 HNimidazole induced spontaneous resolution to form the conglomerate. The capped tripodshaped [Ni(II)H3L]2+ complex ions are hydrogen-bonded in a tail-to-tail mode and array in an up-and-down manner repeatedly to honeycomb an extended two-dimensional homochiral network with trigonal voids. Keywords: Ni(II) complex, Spontaneous resolution, Conglomerate

Introduction When a racemate (or racemic mixture) crystallizes, one of three different types of crystal can be formed: conglomerate (or racemic conglomerate), racemic compound (or true racemate), and pseudoracemate (or racemic solid solution). In contrast to a racemic compound and a pseudoracemate where an enantiomer pair coexists in a crystal, a conglomerate consists of one of the enantiomer pair in its crystal.1 Because homochiral interactions are in most cases weaker than heterochiral interactions, less than 10% of crystallized racemates have been found to belong to the conglomerates.2,3 The process to form a conglomerate is called spontaneous resolution. It is hard to predict whether spontaneous resolution will occur or not from the information of a molecular structure. Therefore, designing a chiral molecule for the purpose of inducing spontaneous resolution has been such a challenging task. One of the ways to induce spontaneous resolution makes use of the self-complementary building units which can be extended to multi-dimensional network via a coordination bond or a hydrogen bond.4 Potentially-hexadentate tripodal ligands containing imidazoles have been used for such purposes.4–11 We have reported an Mn(II) complex with a tripodal ligand, tris{2-(4-imidazolyl)methyliminoethyl}amine (H3L), in which spontaneous resolution derived two different kinds

# This paper is dedicated to Professor Kwan Kim on the occasion of his honorable retirement. Bull. Korean Chem. Soc. 2015, Vol. 36, 838–842

of two-dimensional (2D) networks through hydrogen bonds.12 In this study, we further extend the previous work to synthesize and structurally characterize a homochiral assembly of [Ni(II)H3L]2+ to understand the characteristics of spontaneous resolution found in this kind of ligands. Experimental Materials. Following chemicals were purchased from commercial sources and were used without further purification: tris(2-aminoethyl)amine, 4-imidazolecarboxaldehyde, and nickel(II) perchlorate hexahydrate from Sigma-Aldrich Chemical. All solvents were purified before use according to the standard procedures. Caution!. Perchlorate salts of metal complexes with organic ligands are potentially explosive. Only small quantities of material should be prepared, and the samples should be handled with care. Preparation of Tris{2-(4-imidazolyl)methyliminoethyl} amine (H3L). The H3L ligand was prepared by using 1:3 condensation between tris(2-aminoethyl)amine and 4-imidazolecarboxaldehyde in methanol as in the previously published literatures.12,13 The ligand was identified by FTIR recorded from KBr pellet and 1H NMR recorded in CD3OD at 25 C, showing identical features to the previously reported data. Synthesis of [Ni(II)H3L](ClO4)2 (1). 4-Imidazolecarboxaldehyde (0.59 g, 6 mmol) and tris(2-aminoethyl)amine

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Spontaneous Resolution Induced by a Chiral Ni(II) Complex

(0.31 ml, 2 mmol) were refluxed in 25 mL of methanol for 2 h to give a yellow solution. After cooling the solution, Ni(ClO4)26H2O (0.73 g, 2 mmol) dissolved in 10 mL dry methanol was added drop by drop and refluxed 2 h. After 1 h, a sky pink crystalline powder (1.0 g) was collected by filtration. The powder was again dissolved in dry methanol and taken in a test tube with diethyl ether. Suitable crystals for X-ray analysis were obtained from the solution after one day. Although the crystallographic data (see below) contain additional hydrogens owing to the disorder of ClO4− units, elemental analysis and fast atom bombardment (FAB) mass spectra (MS) measurements confirmed the composition of complex 1 is [Ni(II)H3L](ClO4)2. Elemental analysis calcd. for C18H24Cl2NiN10O8: C, 33.88%; H, 3.79%; N, 21.95%. Found: C 33.84%, H 3.90%, and N 21.65%. FAB MS analysis (+ ion mode): m/z, ion, base ; 436.8, [NiH2L]1+, 100%; 536.7, {[NiH3L](ClO4)}+, 25%. Crystallographic Data Collection and Refinement of the Structure. A crystal of 1 was mounted on glass fiber. The diffraction data were collected at 173 K using a Bruker SMART CCD Detector single crystal X-ray diffractometer with a graphite monochromate Mo-Kα (λ = 0.71073 Å). The SMART and SAINT software package was used for data collection and integration.14 SADABS was used for absorption correction.15 The crystal structure of complex 1 was solved by the direct method and refined by full-matrix least-squares calculation with the SHELXTL software package.16 All non-hydrogen atoms were refined anisotropically. The hydrogen atoms attached to N or C atoms were assigned isotropic displacement coefficients U(H) = 1.2U (N or C), and their coordinates were allowed to ride on their respective atoms. A summary of the crystal and some crystallography data is given in Table 1. Details of the crystallographic information can be obtained from the CCDC CIF depository request (CCDC-1030790). Physical Measurements. Fourier transform infrared (FT-IR) spectra of samples were recorded with a Bruker IFS 66v FT-IR spectrophotometer in the region of 400–4000 cm−1 using KBr pellets. The elemental analysis was performed using a PerkinElmer 2400 CHN analyzer. Nuclear magnetic resonance (NMR) spectra were collected on a Bruker AVANCE digital 400. Circular dichroism (CD) spectra were obtained using Jasco J-810. UV/Vis electronic spectra were recorded with a Scinco UV S-2100 spectrophotometer. High-resolution FAB MS were recorded using a Jeol JMS-700 mass spectrometer at the Daegu center of KBSI, Korea.


DMSO. Crystals were obtained by the slow diffusion of diethyl ether to the methanol solution of the complex. Crystal Structure of [Ni(II)H3L]2+. Figure 1 depicts the ORTEP drawing of the Ni(II) center in complex 1. H3L ligand binds to Ni(II) ion through three Schiff-base imine N atoms and three imidazole N atoms with distorted octahedral geometry. The crystal structure of [Ni(II)H3L]2+ shows C3 symmetry about its Ni(II)–Ntertiary amine axis. The overall structure is similar to those of the complexes with the same ligand.4,6,7,9,10,12,13,17 In some complexes containing this kind of tripodal ligand, a pseudo-coordination between metal ion and apical N atom has been found. The distance from the Ni(II) ion to the apical tertiary amine N atom of the ligand is 3.311(3) Å. This distance is longer than the distance of Table 1. Crystal data and structure refinement for complex 1. Empirical formula Formula weight Temperature, K Wavelength, Å Crystal system Space group a, Å b, Å c, Å α, β, γ, Volume, Å3 Z Independent reflections Absorption correction Goodness-of-fit on F2 Final R indices [I > 2σ(I)] R indices (all data) Largest diff. peak and hole, eÅ−3 R1 =

C18H33Cl2NiN10O6.50 623.15 173(2) 0.71073 Hexagonal R32 12.0758(9) 12.0758(9) 34.759(5) 90 90 120 4389.7(8) 6 3251 [R(int) = 0.0226] Semi-empirical from equvalents 1.176 R1 = 0.0593, wR2 = 0.1703 R1 = 0.0655, wR2 = 0.1797 0.871 and −0.382

hX i1=2 X X 2 X w F02 −Fc2 = wF04 . jjF0 j − jFc jj= jF0 j, wR2 =

Results and Discussion Synthesis of [Ni(II)H3L](ClO4)2 (1). Solution of tris(2-aminoethyl)amine and 4-imidazolecarboxaldehyde were refluxed in dry methanol to yield the yellow Schiff-base product, H3L. The purified ligand in methanol exhibited λmax at 290 nm of UV absorbance spectra. In situ reaction of nickel perchlorate hexahydrate without further separation of the ligand in dry methanol solution formed sky pink solid complex (Scheme 1). The complex is soluble in CH3OH, DMF, and Bull. Korean Chem. Soc. 2015, Vol. 36, 838–842

Scheme 1.


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2.824(3) Å found in [Mn(II)H3L]2+ with pseudocoordination,12 indicating no pseudo-coordination is present in [Ni(II)H3L]2+. Table 2 lists selected bond lengths and bond angles with their standard deviations in parentheses for [Ni(II) H3L]2+. Ni N bond lengths are found to be 2.073(3) Å for Ni–Nimidazole and 2.123(3) Å for Ni–Nimine. The bite and trans angles of Nimidazole–Ni–Nimine are 79.03(12) and 170.24 (13) , respectively. These lengths and angles are comparable to d(Ni–N) = 2.076(3)–2.188(5) Å, ∠bite(N–Ni–N) = 78.5(3)– 79.4(2) , and ∠trans(N–Ni–N) = 169.07(6)–174.2(2) found in [Ni(II)H3LMe]2+ and [Ni(II)H3LPh]2+ where methyl and phenyl groups are substituted in imidazole rings of the H3L ligand, respectively.11,18 In other complexes containing H3L ligand, metal N bond lenngths of d(Fe–N) = 1.941(2)– 1.990 Å for the low-spin [Fe(III)H3L]3+, d(Fe–N) = 2.060 (3)–2.118(3) Å for the high-spin [Fe(III)H3L]3+, d(Ru–N) = 2.151(7)–2.232(8) Å for [Ru(III)H3L]3+, and d(Mn–N) = 2.255(3)–2.286(2) Å for the high spin [Mn(II)H3L]2+ have been reported.6,7,10,12 The trend of metal N bond lengths are well fit to the Shannon's ionic radii of hexacoordinate Fe(III), Mn(II), and Ru(III): 0.55 Å for low-spin Fe(III), 0.645 Å for high-spin Fe(III), and 0.68 Å for Ru(III), 0.83 Å for high-spin Mn(II).19 The Ni N bond distances observed in [Ni(II)H3L]2+ (this work), [Ni(II)H3LMe]2+,18 and [Ni(II) H3LPh]2+ 11 are within the range of the lengths found in the high-spin [Fe(III)H3L]3+ and [Ru(III)H3L]3+. Considering the Shannon's ionic radii of hexacoordinate Fe(III), Ru(III), and low-spin Ni(II) whose ionic radius is 0.69 Å,19 the crystal structure of [Ni(II)H3L]2+ suggests that Ni(II) is in low-spin state. However, magnetic property measurements on [Ni(II) H3LMe]2+ showed high-spin Ni(II) in [Ni(II)H3LMe]2+.18 Therefore, a high-spin Ni(II) is also expected in [Ni(II) H3L]2+. Complete assignment requires a magnetic property measurement. Spontaneous Resolution Induced by Self-Organization. H3L, an achiral tripodal ligand, can arrange around Ni(II) with screw type to generate a chiral complex with either a Δ or Λ absolute configuration. Figure 1 shows that the analyzed crystal has the Λ absolute configuration of [Ni(II)H3L]2+. Flack parameter for the crystal (CCDC-1030790) is zero, suggesting that the crystal is a pure enantiomer. The crystallinity is confirmed by the CD spectra obtained from two different crystals of complex 1. (Figure 2) Each shows a positive and a negative peak at 325 and 330 nm, respectively.


As mentioned above, a conglomerate is formed by spontaneous resolution which induced by self-organization of each enantiomer. Figure 3 shows such the organization of Ni(II) centers. Each [Ni(II)H3L]2+ center is connected to three [Ni(II) H3L]2+ centers residing in a nearby layer by three hydrogen bonds between NimidazoleH OClO4 HNimidazole, forming a puckered 2D network. Because of the disorder of ClO4−, the hydrogen bond distances are not completely determined but those are estimated to be 2.2 Å. The presence of the hydrogen bonds are also confirmed by the relatively short NimidazoleNimidazole distance of 4.80 Å, which is much shorter than the sum (8.5 Å) of van der Waals radii of two N, two O, and two H atoms.20 The puckered 2D networks are wedged each other in along the caxis to constitute overall stack. Above intermolecular hydrogen bonds of either homochiral (ΔΔΔ or ΛΛΛ ) or heterochiral (ΔΛΔ ) arrangement of enantiomers construct 2D honeycomb networks. The networks can be classified into two groups by void shapes, trigonal, or hexagonal voids.4,12 Figure 4 shows (a) five [Ni(II) H3L]2+ centers and (b) their extended 2D network. The capped

Figure 1. ORTEP diagram of [Ni(II)H3L]2+ of complex 1 with 33% probability ellipsoids and atom numbering scheme. Hydrogen atoms have been omitted for clarity. Symmetry transformations used to generate equivalent atoms: #1−y + 1, x−y + 1, z; #2−x + y, −x + 1, z.

Table 2. Selected bond lengths and angles for [Ni(II)H3L]2+. Bond lengths (Å) Ni1 N2 Bond angles ( ) N2 Ni1 N3 N2 Ni1 N3b N2 Ni1 N3b a b

2.123(3) 79.03(12) 92.61(12) 170.24(13)

Ni1 N3 N3 Ni1 N3a N2 Ni1 N2a

2.075(3) 92.38(12) 96.58(11)

−y + 1, x − y + 1, z. −x + y, −x + 1, z.

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Figure 2. CD spectra of two different crystals of complex 1. The crystals were separately selected to be dissolved in methanol for CD measurements. Inset is the UV/Vis absorption spectrum of complex.

Figure 3. Schematic diagram displaying the arrangement and hydrogen-bond network of complex 1. Dotted lines represent the hydrogen bonds of NimidazoleH OClO4 HNimidazole. Os represents the oxygen atoms of ClO4− ions. Each [Ni(II)H3L]2+ center is connected to three [Ni(II)H3L]2+ centers. The diagram omits the third hydrogenbond connection of each Ni(II) center for clarity.

Figure 4. (a) Five [Ni(II)H3L]2+ centers showing the hydrogen bonds (dotted lines) of NimidazoleH OClO4 HNimidazole in tail-to-tail mode and (b) their extended 2D arrangement. Hydrogen atoms not involved in the hydrogen bonds are omitted for clarity. Arrows indicate the axis directions of the crystal unit cell. Bull. Korean Chem. Soc. 2015, Vol. 36, 838–842



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tripod-shaped [Ni(II)H3L]2+ centers are hydrogen-bonded in a tail-to-tail mode and array in an up-and-down manner repeatedly to honeycomb an extended 2D puckered network with trigonal voids. Similar homochiral 2D layer structures with trigonal voids have been reported for the crystals of [Fe(II) H3LMe][Fe(II)LMe](NO3),5 [Fe(II)H3L][Fe(III)L](NO3)2,7 [Co (III)H1.5 L]Cl1.54H2O,4 and [Mn(II)H1.5 L]0.5+ centers of [Mn(II)H1.5 L]2[Mn(II)H3L]2(ClO4)53H2O.12 In those crystals, the chiral centers are connected via imidazole–imidazolate (NimidazoleH Nimidazolate) hydrogen-bond network. In the crystals of [Fe(III)H3L]0 and [Fe(II)H3L] (BF4)23H2O, the same homochiral 2D networks with trigonal voids are constructed by O atom-mediated hydrogen bondings.9,10 Although it is not mature yet, surveying the previous works and this work on the complexes containing H3L type ligands tends to suggest that fine-tuning of trigonal or hexagonal voids in the crystals with honeycomb networks depends on the distances between two hydrogen-bond connected imidazole N atoms. Conclusion In situ reaction of nickel(II) perchlorate hexahydrate with the condensation mixture of 4-imidazolecarboxaldehyde and tris (2-aminoethyl)amine in dry methanol formed sky pink solid of complex 1, [Ni(II)H3L](ClO4)2, where H3L is tris{2-(4-imidazolyl)methyliminoethyl}amine. X-ray crystallographic study revealed that the H3L ligand hexadentately binds to Ni(II) ion through three Schiff-base imine N atoms and three imidazole N atoms with distorted octahedral geometry. Crystallographic and CD investigations on the compounds indicated the crystal is a enantiopure conglomerate. The capped tripod-shaped [Ni (II)H3L]2+ complex ions are hydrogen-bonded in a tail-to-tail mode and array in an up-and-down manner repeatedly to honeycomb an extended 2D puckered network with trigonal voids. Acknowledgment. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (NRF-2010-0024929). Supporting Information. Details of the crystallographic information of complex 1 can be obtained from the CCDC CIF depository request (CCDC-1030790).

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BULLETIN OF THE KOREAN CHEMICAL SOCIETY References

1. J. J. Jacques, A. Collet, S. H. Wilen, Enantiomers, Racemates and Resolution, John Wiley & Sons, New York, NY, 1981. 2. J. Jacques, M. Leclercq, M.-J. Brienne, Tetrahedron 1981, 37, 1727. 3. C. P. Brock, W. B. Schweizer, J. D. Dunitz, J. Am. Chem. Soc. 1991, 113, 9811. 4. I. Katsuki, Y. Motoda, Y. Sunatsuki, N. Matsumoto, T. Nakashima, M. Kojima, J. Am. Chem. Soc. 2002, 124, 629. 5. Y. Ikuta, M. Ooidemizu, Y. Yamahata, M. Yamada, S. Osa, N. Matsumoto, S. Iijima, Y. Sunatsuki, M. Kojima, F. Dahan, J. P. Tuchagues, Inorg. Chem. 2003, 42, 7001. 6. T. Yamaguchi, K. Harada, Y. Sunatsuki, M. Kojima, K. Nakajima, N. Matsumoto, Eur. J. Inorg. Chem. 2006, 3236. 7. Y. Sunatsuki, Y. Ikuta, N. Matsumoto, H. Ohta, M. Kojima, S. Iijima, S. Hayami, Y. Maeda, S. Kaizaki, F. Dahan, J. P. Tuchagues, Angew. Chem. Int. Ed. 2003, 42, 1614. 8. S. Nagasato, I. Katsuki, Y. Motoda, Y. Sunatsuki, N. Matsumoto, M. Kojima, Inorg. Chem. 2001, 40, 2534. 9. F. Lambert, C. Policar, S. Durot, M. Cesario, L. Yuwei, H. Korri-Youssoufi, B. Keita, L. Nadjo, Inorg. Chem. 2004, 43, 4178. 10. Y. Sunatsuki, H. Ohta, M. Kojima, Y. Ikuta, Y. Goto, N. Matsumoto, S. Iijima, H. Akashi, S. Kaizaki, F. Dahan, J. P. Tuchagues, Inorg. Chem. 2004, 43, 4154. 11. I. Katsuki, N. Matsumoto, M. Kojima, Inorg. Chem. 2000, 39, 3350. 12. S. Sarkar, D. Moon, M. S. Lah, H.-I. Lee, Bull. Korean Chem. Soc. 2010, 31, 3173. 13. H. S. He, K. R. Rodgers, A. M. Arif, J. Inorg. Biochem. 2004, 98, 667. 14. SMART, SAINT, Area Detector Software Package and SAX Area Detector Integration Program, Bruker Analytical X-ray, Madison, WI, 1997. 15. SADABS, Area Detector Absorption Correction Program, Bruker Analytical X-ray, Madison, WI, 1997. 16. G. M. Sheldrick, Acta Crystallogr. A 2008, A64, 112. 17. C. Brewer, G. Brewer, R. J. Butcher, E. E. Carpenter, L. Cuenca, B. C. Noll, W. R. Scheidt, C. Viragh, P. Y. Zavalij, D. Zielaski, Dalton Trans. 2006, 1009. 18. G. Brewer, C. Brewer, R. J. Butcher, E. E. Carpenter, Inorg. Chim. Acta 2006, 359, 1263. 19. R. D. Shannon, Acta Crystallogr. A 1976, A32, 751. 20. A. Bondi, J. Phys. Chem. 1964, 68, 441.



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