Communication pubs.acs.org/crystal
Chiral Crystal Structure of a P212121 Kryptoracemate Iron(II) Complex with an Unsymmetric Azine Ligand and the Observation of Chiral Single Crystal Circular Dichroism Yukinari Sunatsuki,*,† Kunihiro Fujita,† Hisashi Maruyama,† Takayoshi Suzuki,† Hiroyuki Ishida,† Masaaki Kojima,† and Robert Glaser*,‡ †
Department of Chemistry, Faculty of Science, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
‡
S Supporting Information *
ABSTRACT: A chiral complex, [Fe(HL)2](PF6)2 (1), where HL denotes 2-pyridylmethylidenehydrazono-4-(2methylimidazolyl)methane, was prepared. X-ray structure analysis revealed that it crystallizes as a kryptoracemate of sesquihydrate chiral crystals in the orthorhombic noncentrosymmetric space group P212121 (Z = 8, Z′ = 2). Two diastereomeric cationic complexes with opposite configuration reside within the asymmetric unit. KBr pellets prepared using selected single crystals showed enantiomorphous circular dichroism patterns.
C
glide relationship quality in a kryptoracemate P21 (Z = 4, Z′ = 2) chiral crystal whose packing arrangement mimicked that of achiral P21/c (Z = 4, Z′ = 1). Herein, we describe the crystal structure of an iron(II) complex in a P212121 kryptoracemate emulating a higher order Pbca space filling. We also present the first report of chiroptical measurements (solid-state circular dichroism) performed on kryptoracemate single crystals. A C2-symmetry chiral iron(II) complex, [FeII(HL)2](PF6)2 (1), where HL denotes 2-pyridylmethylidenehydrazono-4-(2methylimidazolyl)methane, was prepared by the reaction of FeCl2·4H2O, 2-pyridinecarboxaldehyde, 2-methyl-4-formylimidazole, and hydrazine monohydrate, in a 1:2:2:2 molar ratio in methanol, followed by the addition of an aqueous solution of NH4PF6 (2 equiv). Complex 1 was isolated as dark blue crystals of the sesquihydrate (1·1.5H2O). The reaction of three components, 2-pyridinecarboxaldehyde, 2-methyl-4-formylimidazole, and hydrazine in a 1:1:1 molar ratio may form three kinds of azine ligands; two of them (A, B) have symmetrical halves, and the third (C = HL) has nonsymmetrical halves (see Scheme S1, Supporting Information). The statistical formation ratio of A:B:C is equal to 1:1:2. The reaction of a mixture of the ligands with iron(II) can form six kinds of complexes, [FeA2]2+, [FeAB]2+, [FeAC]2+, [FeB2]2+, [FeBC]2+, and [FeC2]2+ when the ligands function as tridentate ligands. The formation ratio will be [FeA2]2+: [FeAB]2+: [FeAC]2+: [FeB2]2+: [FeBC]2+: [FeC2]2+ = 1:2:4:1:4:4. Thus, the theoretical yield of 1 will be 25%. The actual yield (79%) of 1·1.5H2O was much higher
hirality is an important topic in chemistry, pharmacy, biochemistry, and materials science.1 When racemates aggregate and condense, in >90% of the cases they crystallize as achiral crystals (racemic compounds or racemic modif ications) in which the two enantiomers are present in equal quantities and in well-defined positions within the crystal lattice.2 Spontaneous resolution produces a conglomerate of chiral crystals in approximately 10% of the time. Since the asymmetric unit usually contains one molecule (Z′ = 1), the chiral space group then generates only one enantiomer per crystal. Kryptoracemates (hidden racemates) are produced in a crystallization of a racemic mixture that yields a conglomerate of chiral crystals containing two molecules of opposite handedness in the asymmetric unit (i.e., diastereomers +X and −Y reside within one crystal, whereas −X and +Y occupy the other enantiomorphic crystal).3 These type of crystals have also been referred to as false conglomerates.4 Brock and Fábián3 reported that only 0.1% of the entries in the Cambridge Structural Database (CSD) are kryptoracemates. Of this very small amount, about 60% of them exhibited a pseudosymmetric relationship between the diastereomers.3 The spatial arrangement of oppositely handed diastereomers in kryptoracemates often emulates the space filling in higher order achiral crystals. The Söhncke space group5 crystals exhibit only symmetry operations of the First Kind (i.e., operations that preserve the handedness of the exchanged molecules). When the kryptoracemate chiral subgroup space filling emulates that of a higher order achiral supergroup, diastereomers of inverted handedness are related by pseudosymmetry relationships of the Second Kind having varying degrees of fidelity. Steinberg and Glaser6 have reported algorithms to measure pseudoinversion and pseudo© 2014 American Chemical Society
Received: May 2, 2014 Revised: July 1, 2014 Published: July 10, 2014 3692
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Figure 1. ORTEP X-ray molecular structures of the diastereomeric cations of [Fe(HL)2](PF6)2·1.5H2O (1·1.5H2O) with the selected atom numbering scheme showing thermal ellipsoids drawn at 50% probability. Hydrogen atoms were omitted for clarity. Black color-coded non-hydrogen atoms are carbons. (a) The Fe1 complex cation with the C configuration. (b) The Fe2 complex cation with the A configuration. The absolute configurations, C and A, are defined according to the IUPAC nomenclature.
Under these conditions, the two cationic complexes (1a2+ and 1b2+) become diastereomeric in the asymmetric unit of these kryptoracemate crystals (Table S2, Supporting Information). However, differences in their diastereomeric geometries are so small that they resemble enantiomers to the naked eye. All of the Fe−N coordinate bond distances (1.950(3)− 1.995(3) Å, Table S2) are typical for low-spin FeII10 in accordance with magnetic susceptibility measurements on a microcrystalline sample. The N−Fe−N angles for the sixmembered chelate rings (91.18(10)−91.93(11)°) are approximately right angles and are significantly larger than those for the five-membered chelate rings (80.69(11)−81.77(10)°). The crystal structure is shown in Figures 3 and S1. The imidazole hydrogen atom, N5−H10 of the complex cation involving Fe1, is linked to the azine N18 atom of the neighboring cation involving Fe2 by a hydrogen bond [N5··· N18 = 2.905(4) Å]. The imidazole hydrogen atom, N15−H32 of the complex cation involving Fe2, is also linked to the azine N3 atom of the neighboring cation involving Fe1 by a hydrogen bond [N15···N3 = 2.981(4) Å]. Thus, a 1D helical chain structure running along the a-axis is constructed by alternating N5···N18 and N15···N3 hydrogen bonds, as shown in Figure 3a. In addition, imidazole hydrogen atom, N10−H21, is hydrogen bonded to the water O1 atom [N10···O1 = 2.868(5) Å]. The distances and angles of hydrogen bonds are listed in Table S3, Supporting Information. Within the pseudoinversion asymmetric unit, there are two diastereotopic edge-to-face CH−π interactions (5.007 and 5.037 Å centroidto-centroid) and one face-to-face π−π stacking arrangement (3.635 Å centroid-to-centroid). Weak intrachain π−π stacking interactions are observed between a pyridine ring and an imidazole ring of the neighboring crystallographically related cation (red dotted arrows in Figure 3b). Stacking distances for cations involving Fe1 and Fe2 are 3.579(5) and 3.481(5) Å, respectively. Additionally, the helical chains are connected to each other by the N20−H43···O2 and O2−H48···N8 hydrogen bonds [N20···O2 = 2.776(4) Å, O2···N8 = 2.902(4) Å] and by π−π stacking and CH−π interactions to form a 3D network structure as shown in Figures S1 and S2. The data are listed in Tables S3 and S4. Emulation of higher order achiral Pbca supergroup packing was evident in the lower order P212121 chiral subgroup space filling. This mimicry of Pbca achiral packing is based upon pseudoinversion and pseudoglide relationships between
than that suggested from simple statistical formation (25%). The result can be explained by the relatively low solubility of 1· 1.5H2O and by the labile nature of the CN bonds.7 The crystal structure of 1·1.5H2O8 was determined by X-ray single crystal diffraction at 193 K. Compound 1 crystallizes as a kryptoracemate of sesquihydrate chiral crystals in the orthorhombic noncentrosymmetric space group P212121 (No. 19) with Z = 8, Z′ = 2, and the crystallographic data are listed in Table S1, Supporting Information. The asymmetric unit consists of two oppositely handed [Fe(HL)2]2+ diastereomeric cations (1a2+ and 1b2+), four PF6− anions, and three water molecules. Selected externally diastereotopic bond lengths and angles for the diastereomers are listed in Table S2, Supporting Information. ORTEP drawings of the cations of 1·1.5H2O are shown in Figure 1. Each FeII ion binds two HL ligands and has a pseudo-octahedral coordination geometry. Each ligand in the Z−E configuration serves as a tridentate ligand and coordinates meridionally to the metal ion with one imidazole, one pyridine, and one azine nitrogen atom, and the other azine nitrogen atom and the imidazole NH group remain uncoordinated. There are two possible coordination modes when HL coordinates meridionally to a metal ion as a tridentate ligand (Figure 2). In mode (i), the chelate involving the imidazol
Figure 2. Two possible coordination modes, i and ii, by tridentate coordination of the HL ligand to a metal ion.
moiety forms a six-membered ring, while that involving the pyridine moiety a five-membered ring. The situation is opposite in mode (ii). In the structure of 1, all ligands take the coordination mode (i). Solutions of the cationic complex consist of a pair of enantiomers, and their configurations are defined as C (clockwise) and A (anticlockwise) according to the IUPAC nomenclature (Figure 1).9 In the solid-state, the enantiomeric solution-state C2-symmetry complexes 1 are desymmetrized down to C1-symmetry since the C and A molecules occupy general positions of symmetry upon entering the P212121 lattice. 3693
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Figure 3. Helical one-dimensional arrangement of [Fe(HL)2](PF6)2·1.5H2O (1·1.5H2O) running along the a-axis. Color code: purple: Fe; blue: N; gray: C; light green: H; red: O. (a) Projection along the a-axis. A helical structure is constructed by alternating N5−H10···N18 and N15−H32···N3 hydrogen bonds. (b) Projection along the c-axis. Weak intrachain π−π stacking interactions (red dotted arrows) are observed between a pyridine ring and an imidazole ring of the neighboring cations.
the noncentrosymmetric lattice with space group P212121 resulted in complete loss of their ideal superimposable isometric relationship. Instead, within the confines of the lattice they became a pair of oppositely handed anisometric isomers (1a2+ and 1b2+) having identical constitutions (i.e., diastereomers). The realization that one crystal contained (+)-1a2+ and (−)-1b2+ while (−)-1a2+ and (+)-1b2+ resided in the other was the impetus for us to measure solid-state CD spectra11 from single crystals chosen from the conglomerate, because the UV−vis absorption spectrum of complex 1 in methanol showed intense charge-transfer (CT) transition bands in the visible region (Figure S3, Supporting Information). The solid-state CD spectra of KBr pellets prepared using two selected chiral single crystals showed explicit and enantiomorphous CD patterns (Figure 4), which were confirmed to the real CD of
oppositely handed diastereomers within the chiral crystal. The accuracy (fidelity or quality) of a nonsuperimposable pseudoenantiomeric relationship between diastereomers can be calculated using the rmS(σ) and rmS(i) algorithms of Steinberg and Glaser.6 For space group pseudosymmetry based upon translation, i.e., pure translation, glide-reflection (translation and reflection via a glide plane) and screw-rotation (translation and rotation via a screw axis), the rmS(translation) degree of discrepancy for translation is calculated. This is the root-mean-square difference between actual versus ideal 1/2unit cell translations. The rmS(glide) index is the sum of rmS(trans) + rmS(σ) components.6 These parameters enable the determination of quality ranking for a series of lower order chiral crystals that emulate higher order achiral space filling. Using the algorithms of Steinberg and Glaser,6 the accuracy of the pseudoinversion relationship in the kryptoracemate crystal structure of 1·1.5H2O was calculated to be rmS(i) = 0.07(1) Å. This low value corresponds to small (but not negligible) deviations from the ideal symmetries, which may or may not be visibly perceivable. Three mutually perpendicular pseudoglide-reflection planes are evident in the crystal. Two are n-pseudoglides (i.e., translations are on two axes), and the third is a c-pseudoglide. It is noted that in Pbca all the translations are parallel to only one of the three axes. N-Pseudoglide 1 cuts the a-axis, c-pseudoglide 2 cuts the b-axis, and n-pseudoglide 3 cuts the c-axis. The pseudoreflection glide-component rmS(σ) values were calculated as 0.04(3) Å, 0.07(6) Å, and 0.07(6) Å, respectively. Like rmS(i), these low values correspond to very small (but not negligible) deviations from the ideal symmetries. However, all the pseudotranslation glide-component relationships showed structurally signif icant divergences from the ideal. RmS(y-trans) and rmS(z-trans) for plane 1 were respectively 2.1(2) and 7.3(2) Å, rmS(x-trans) and rmS(z-trans) for plane 2 were repectively 0.5(2) and 5.7(2) Å, while rmS(x-trans) and rmS(y-trans) for plane 3 were respectively 1.5(1) and 5.8(1) Å. As a result, the pseudoinversion and pseudoreflection glidecomponents were all found to be fairly accurate, but the translation components of the pseudoglides were all very significantly distorted from the ideal. Our X-ray diffraction study revealed that a racemic mixture of 1·1.5H2O crystallized as a conglomerate of enantiomorphous kryptoracemate chiral crystals. Residence of the enantiomers in
Figure 4. Enantiomorphous solid-state CD spectra of single crystals of 1·1.5H2O dispersed in KBr pellets.
the sample (not the artifacts) by rotation and turnover of the pellets giving the identical CD spectra. Furthermore, a KBr pellet prepared similarly using a racemic compound (achiral crystal) of monohydrate, [FeII(HL)2](PF6)2·H2O,12 did not show any CD signal in this region. The observation of CD signals in the kryptoracemate chiral crystal (1·1.5H2O) may be attributed to the helical arrangement of the complex cations. Although some examples of complex-based kryptoracemates have been reported,13 the solid state CD has not been measured. As expected, CD was not detected in the spectrum 3694
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(6) Steinberg, A.; Ergaz, E.; Toscano, R. A.; Glaser, R. Cryst. Growth Des. 2011, 11, 1262. (7) Hogg, L.; Leigh, D. A.; Lusby, P. J.; Morelli, A.; Parsons, S.; Wong, J. K. Y. Angew. Chem., Int. Ed. 2004, 43, 1217. (8) Crystal data for 1: C22H25F12FeN10O1.5P2, Mr = 799.28; orthorhombic, P212121; a = 9.2876(4) Å, b = 25.1207(9) Å, c = 25.9555(9) Å, α = β = γ = 90°; V = 6055.7(4) Å3; Z = 8 (Z′ = 2); Dcalc = 1.753 g cm−3; T = 193 K; reflections collected/unique = 59482/ 13860, Rint = 0.0356, Friedel pairs = 6209; R1 = 0.0443 [I > 2.00σ(I)], R = 0.0475, wR2 = 0.1237 (all data), GOF = 1.040, Flack parameter = 0.025(12). CCDC: 978562. (9) Connelly, N. G., Damhus, T., Hartshorn, R. M., Hutton, A. T., Eds.; Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005; Royal Society of Chemistry: Cambridge, England, 2005. (10) Beattie, J. K. Adv. Inorg. Chem. 1988, 32, 1. (11) (a) Castiglioni, E.; Biscarini, P.; Abbate, S. Chirality 2009, 21, E28. (b) Kuroda, R.; Honma, T. Chirality 2000, 12, 269. (12) The crystals were obtained by recrystallization from nitromethane/diethyl ether. Crystal data: blue, platelet, C22H24F12FeN10OP2, Mr = 790.30, T = 193 K, monoclinic, P21/n, a = 12.516(1), b = 14.482(1), c = 19.611(1) Å. β = 96.704(6)°. V = 3530.4(4) Å3, R1 [I > 2σ(I)] = 0.063, wR2 = 0.197 (all data). The details are reported elsewhere. (13) (a) Bernal, I.; Somoza, F.; Banh, V. J. Coord. Chem. 1997, 42, 1. (b) Bouamaied, I.; Constable, E. C.; Housecroft, C. E.; Neuburger, M.; Zampese, J. A. Dalton Trans. 2012, 41, 10276. (c) Cai, J.; Myrczek, J.; Chun, H.; Bernal, I. J. Chem. Soc., Dalton Trans. 1998, 4155. (d) Cai, J.; Hu, Z.-P.; Yao, J.-H.; Ji, L.-N. Inorg. Chem. Commun. 2001, 4, 478. (e) Zasurskaya, L. A.; Polyakova, I. N.; Rybakov, V. B.; Polynova, T. N.; Poznyak, A. L.; Sergienko, V. S. Cryst. Rep. 2006, 51, 448.
measured in solutions of the dissolved solid kryptoracemate crystals. In summary, we prepared a chiral complex of [Fe(HL)2](PF6)2 (1), and X-ray structural analysis revealed that it underwent crystallization to afford sesquihydrate kryptoracemate chiral crystals (1·1.5H2O). Two cationic complexes with opposite configuration are resident in the asymmetric unit, and their bonding parameters are not identical. The CD observed in the solid state is ascribed probably to the solid-state helical arrangement of the molecules formed by hydrogen bonds. To the best of our knowledge, this is the first report of a chiroptical measurement (solid-state CD) performed on kryptoracemate single crystals.
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ASSOCIATED CONTENT
* Supporting Information S
Experimental section, tables of X-ray crystallographic data, bond lengths (Å) and angles (deg), data and figures related to hydrogen bonds and face-to-face π−π stacking and edge-to-face C−H−π aromatic−aromatic interactions, scheme for formation of three possible ligands, UV−vis electronic absorption spectrum, and an X-ray crystallographic file in CIF format for 1·1.5H2O. This material is available free of charge via the Internet at http://pubs.acs.org. The crystallographic information file is also available free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.
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AUTHOR INFORMATION
Corresponding Authors
*(Y.S.) E-mail:
[email protected]. Fax: +81-86-2517833. *(R.G.) E-mail:
[email protected]. Fax: +972-8-6900084. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research No. 23550078 (to Y.S.) and a Grant-in-Aid for Challenging Exploratory Research No. 23655052 (to T.S.) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan, and the Ben-Gurion University of the Negev Fund for Scientific Interactions (to R.G.).
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REFERENCES
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