research papers Acta Crystallographica Section B
Structural Science, Crystal Engineering and Materials
Enantiopure and racemic alanine as bridging ligands in Ca and Mn chain polymers
ISSN 2052-5206
Kevin Lamberts, Andreas Mo ¨ller and Ulli Englert* Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, Aachen 52074, Germany
Correspondence e-mail: [email protected]
Under accelerated and controlled evaporation, chain polymers crystallize from aqueous solutions of CaII and MnII halides with enantiopure l-alanine or racemic dl-alanine. In all ten solids thus obtained zwitterionic amino acid ligands bridge neighbouring cations. The exclusively O-donor-based coordination sphere around the metal cations is completed by aqua ligands; the halides remain uncoordinated and act as counter-anions for the cationic strands. Despite the differences in ionic radii and electronic structure between the main group and the transition metal cation, their derivatives with lalanine share a common structure type. In contrast, the solids derived from dl-alanine differ and adopt structures depending on the metal cation and the halide. Homochiral chains of either chirality or heterochiral chains with different arrangements of crystallographic inversion centres along the polymer strands are encountered. On average, the six-coordinated CaII cations, devoid of any ligand field effect, show more pronounced deviation from idealized octahedral geometry than the d-block cation MnII.
Received 1 August 2014 Accepted 26 September 2014
1. Introduction
# 2014 International Union of Crystallography
Acta Cryst. (2014). B70, 989–998
Aspects of structural systematics are often addressed in the context of inorganic compounds, e.g. in mineralogy. Such considerations are significantly less popular in the realm of molecular crystals or coordination polymers. Group–subgroup relationships (Ba¨rnighausen, 1980; Mu¨ller, 2013) are more readily perceived for structures of higher symmetry based on more rigid constituents than for conformationally soft molecules mostly crystallizing in low-symmetry space groups. However, although it might be challenging to establish structural relationships within a series of coordination polymers, such an effort may well be rewarding and lead to new insights. Our group has systematically studied halide-bridged chain polymers with pyridine ligands (Englert, 2010) and identified a simple structure type as stable at low (Hu & Englert, 2005) or high temperature only (Hu & Englert, 2006). We have investigated the structural chemistry of the reaction products of manganese halides with racemic and enantiopure proline and highlighted the role of chirality for the solid-state structures of the products (Lamberts & Englert, 2012). Additionally we have shown that a solid-state property such as the temperature for a certain phase transition may be tuned by isomorphous replacement (Lamberts et al., 2011), similar to the situation in classical inorganic salts. In this contribution we address the structural chemistry of racemic and enantiopure alanine derivatives of metal halides, both for the alkaline earth cation CaII and the transition metal ion MnII. Fig. 1 summarizes these combinations and puts them into the context of two earlier contributions: Mrozek et al. (1999) (BEFYUG) reported the doi:10.1107/S2052520614021398
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Figure 1 Overview of the reactions performed and the naming scheme of the resulting structures and structure types. The derivative from CaCl2 and lalanine is hitherto unknown. Structures [a] (Mrozek et al., 1999) and [b] (Ciunik & Głowiak, 1980) have been reported earlier but [a] could not be reproduced.
reaction product of l-alanine with MnCl2 ; we have not been able to obtain a crystalline solid according to their experimental information. The reaction product from MnBr2 and dl-alanine (9) has been studied by Ciunik & Głowiak (1980) (BRALMN) before. We can fully confirm the earlier report and decided to collect intensity data at low temperature for a better comparison. Only one Ca complex of alanine has been described to date; in this solid, the amino acid is deprotonated (Fox et al., 2007) (TEWYOK). Our crystal structures (1)–(10) formally represent adducts of the zwitterionic amino acid alanine to metal dihalides and reflect the interplay between the variables chirality, halide anion and metal cation. A comprehensive overview of amino acid structures can be found in a recently published book (Fleck & Petrosyan, 2014).
The best results were attained from reactions with a 1:2 metal halide-to-amino acid ratio despite the different target compositions. Crystallization in 1:1 stoichiometry was not observed but yielded an oil that later turned into a glass-like solid. The racemic products containing dl-alanine always crystallized faster than their chiral analogues from l-alanine. Our attempt to reproduce the compound from manganese chloride and l-alanine resulted in an oily residue which even after 10 months did not crystallize. Additionally, syntheses in the same stoichiometries were repeated in a mortar without solvent. All products except the manganese-containing compounds (3) and (4) were obtained both from 1:2 and 1:3 stoichiometry. Similar to the crystallization from solution, compounds containing dl-alanine showed higher crystallinity and could even be obtained from a 1:1 metal halide-to-alanine ratio. With increasing iodine content the products became more paste-like due to hygroscopicity. Additional experiments involved the concomitant complexation of two metal halides with the amino acid. Metal halides were mixed at 3:1, 1:1 and 1:3 ratio while an overall metal halide to alanine stoichiometry of 1:2 was maintained. These experiments did not result in any new structure types: On the one hand, combinations of different metal halides within the same structure type [either (I) or (IV)] resulted in solid solutions isomorphous with the constituents. These experiments were usually done by grinding; however, a single crystal with intermediate composition between (3) and (4) was grown from a solution containing MnBr2, MnI2 and l-alanine. Attempts at mixing different structure types always reproduced the structure of only one constituent in crystalline form.
2. Experimental 2.1. Chemicals and reagents
The following chemicals were used without further purification: CaCl2 Fluka > 99%, CaBr2xH2O Merck > 80%, CaI2 Sigma–Aldrich 99.95%, MnCl22H2O Merck 99%, MnBr2xH2O Alfa Aesar 98%, MnI2 Alfa Aesar 98%, lalanine Evonik 99.8%, dl-alanine Merck > 99%. 2.2. Syntheses
Compounds (1)–(10) were crystallized from aqueous solutions in 1:2 and 1:3 stoichiometries of manganese and calcium halides with l- and dl-alanine. MX2xH2 O (0.4 mmol; M = Mn, Ca; X = Cl, Br, I) and 0.4, 0.8 or 1.2 mmol of either lalanine or dl-alanine were dissolved in 2 ml of H2O. Because of the high solubility of the products no common purification technique, e.g. washing, has been applied. Colourless to yellow needles suitable for single-crystal diffraction were obtained by solvent evaporation. A homebuilt crystallization chamber (see supporting information1 for further details) maintained a flow of dry air under controlled temperature and thus achieved crystallization from water within 1–3 d rather than weeks. 1 Supporting information for this paper is available from the IUCr electronic archives (Reference: GP5078).
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2.3. Powder diffraction
Confirmation of the phase purity of the products was achieved by powder X-ray diffraction. Representative powder patterns for each type are given as supporting information. Experiments were performed at the Institute of Inorganic Chemistry, RWTH Aachen, using a Stoe Stadi P diffract˚, ometer with Guinier geometry (Cu K1, = 1.54059 A Johannson germanium monochromator, Stoe image plate detector IP-PSD, 0.005 step width in 2).
2.4. Single-crystal diffraction experiments
Suitable single crystals were mounted on glass fibers. Intensity data were collected with a Bruker SMART APEX CCD detector equipped with an Incoatec microsource ˚ , multilayer optics). (Mo K radiation, = 0.71073 A Temperature control was achieved using an Oxford Cryostream 700 at 100 K. Collected data were integrated with SAINT+ (Bruker, 2009) and multi-scan absorption corrections were applied with SADABS (Bruker, 2004). For the nonmerohedrally twinned crystal (10) data were processed with CELL NOW (Sheldrick, 2008b) instead and corrected for absorption with TWINABS (Sheldrick, 2008c). Acta Cryst. (2014). B70, 989–998
research papers Table 1 Experimental details. Absolute structure parameter was calculated according to Parsons & Flack (2004).
Crystal data Chemical formula Mr Crystal system, space group Temperature (K) ˚) a, b, c (A ( ) ˚ 3) V (A Z Radiation type (mm1) Crystal size (mm) Data collection Diffractometer Absorption correction Tmin, Tmax No. of measured, independent and observed [I > 2(I)] reflections Rint ˚ 1) (sin /)max (A Refinement R[F 2 > 2(F 2)], wR(F 2), S No. of reflections No. of parameters No. of restraints H-atom treatment ˚ 3) max, min (e A Absolute structure parameter
(1)
(2)
(3)
(4)
C9H23Br2CaN3O7 485.20 Monoclinic, P21 100 4.8971 (9), 15.812 (3), 11.773 (2) 93.746 (3) 909.7 (3) 2 Mo K 4.77 0.30 0.06 0.03
C9H23I2CaN3O7 579.18 Monoclinic, P21 100 4.9164 (8), 16.416 (3), 12.039 (2) 92.435 (3) 970.8 (3) 2 Mo K 3.53 0.20 0.04 0.03
C9H23Br2MnN3O7 500.06 Monoclinic, P21 100 4.7809 (4), 15.7146 (13), 11.6246 (10) 93.581 (1) 871.65 (13) 2 Mo K 5.38 0.19 0.15 0.07
C9H23I2MnN3O7 594.04 Monoclinic, P21 100 4.7932 (16), 16.261 (5), 11.894 (4) 92.255 (6) 926.3 (5) 2 Mo K 4.08 0.12 0.04 0.04
Bruker APEX CCD Multi-scan SADABS 0.455, 0.745 10 950, 3731, 3445
Bruker APEX CCD Multi-scan SADABS 0.554, 0.745 11 583, 3955, 3580
Bruker APEX CCD Multi-scan SADABS 0.486, 0.746 12 613, 4600, 4269
Bruker APEX CCD Multi-scan SADABS 0.588, 0.745 10 911, 3762, 3371
0.074 0.627
0.063 0.625
0.046 0.681
0.069 0.626
0.042, 0.099, 1.02 3731 235 38 H atoms treated by a mixture of independent and constrained refinement 0.67, 0.68 0.007 (13)
0.037, 0.065, 1.04 3955 225 38 H atoms treated by a mixture of independent and constrained refinement 0.72, 0.62 0.04 (3)
0.030, 0.059, 1.00 4600 234 12 H atoms treated by a mixture of independent and constrained refinement 0.46, 0.35 0.028 (7)
0.045, 0.099, 1.01 3762 235 30 H atoms treated by a mixture of independent and constrained refinement 1.59, 1.04 0.03 (4)
Computer programs: SHELXL2013 (Sheldrick, 2008a), SADABS (Bruker, 2004).
2.5. Refinement
Structures were solved by Patterson methods (SHELXS97 (Sheldrick, 2008a) and were refined on F 2 as implemented in SHELXL13 (Sheldrick, 2008a). Non-H atoms were assigned anisotropic displacement parameters. Atoms C8 and C9 in (2) were only refined isotropically and C7, C8 and C9 in structure (4) were treated with isotropicity restraints. In all structures H atoms connected to C were placed in idealized positions and included as riding with Uiso(H) = 1.2Ueq(C) for CH and Uiso(H) = 1.5Ueq(C) for methyl groups. H atoms of the amino groups and water molecules were found in Fourier maps and restrained to have similar distances to the respective hetero atom. H3W of (7) was placed in an idealized position pointing towards I1 ðx; y 1; zÞ for a reasonable hydrogen-bond network. For (8) and (10) the H atoms of the coordinated water were restrained to subtend a minimum distance of ˚. 1.3 A
3. Results and discussion Crystal data and refinement results are given in Tables 1–3. In total 10 structures are reported, all of which are coordination Acta Cryst. (2014). B70, 989–998
polymers with similar architecture. One-dimensional cationic chains are formed by bridging the metal centres with the carboxylato moieties of the zwitterionic alanine ligands. All structures contain aqua ligands; the halide anions are uncoordinated and reside between the polymer strands; their closest neighbors are H atoms, either associated with positively charged amino groups or with aqua oxygen. Several trends can be observed and the structures have been assigned to four different types with respect to structural motifs. An overview is given in Fig. 2. The structure of (5) will be discussed separately because of its chelating alanine ligands.
3.1. Isomorphous structures from L-alanine [type (I); (1)–(4)]
Isomorphous structures of type (I) were found for the reaction of l-alanine with CaBr2 (1), CaI2 (2), MnBr2 (3) and MnI2 (4). Reaction of the chlorides with l-alanine did only yield glass-like solids; a structure from MnCl2 has been reported in the literature (Mrozek et al., 1999). This solid is polymeric with bridging alanine ligands. However, chloride occurs both as anions and ligands in bridging and terminal positions. Crystal data and refinement results of type (I) Kevin Lamberts et al.
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research papers structures are given in Table 1. This compound is not isomorphous to type (I) and could not be reproduced by us. Structures of type (I) crystallize in the chiral monoclinic space group P21 with an asymmetric unit of M(l-alanine)3(H2O)X2 (Fig. 3). The metal cation is coordinated in distorted octahedral geometry by six O atoms. Four coordination sites are contributed by two bridging 2-O,O0 -l-alanine ligands. The coordination sphere is completed by a terminal lalanine and a water ligand in a cis position to each other. M O distances are compiled in Table 4. The resulting polymeric chains extend along the shortest axis (a) which is also the shortest metal-to-metal distance. Both bridging carboxylato moieties coordinate in syn–anti and anti–syn conformations. An eclipsed arrangement is found for the amino groups towards the anti-carboxylato O atoms. The terminal l-alanine coordinates with an almost linear M O—C angle. The resulting double-bridged cationic chains are stabilized by intra-chain hydrogen bonds and by N— H X and O—H X interactions with the halide anions. Each l-alanine ligand forms an N—H O hydrogen bond towards its symmetry-equivalent neighbour along the chain. The acceptors are the syn coordinating and the non-coordinating O atoms. Both H atoms of the water molecule also
engage in intra-chain hydrogen bonds, leaving two carboxylato-O without any hydrogen bond interaction (O1 and O5) and the terminal carboxylato-O engaged in two hydrogen bonds (O6; Fig. 4). The remaining six NH donors each form a hydrogen bond towards the two halides and concatenate two polymeric chains in the c direction in the form of a helical arrangement between symmetry equivalents of N2, N3, X1 and X2 (see Fig. 5a). N1 completes a trigonal bonding of both halides extending the connectivity between the chains along b; an overall threedimensional framework results. A full list of hydrogen bonds together with the relevant symmetry operations can be found as supporting information. 3.2. {[Ca(l-DL-Ala)2(H2O)2]Cl2(H2O)2}11 (5)
While no solid product could be obtained from l-alanine and both chlorides, racemic alanine in combination with calcium cloride yields a crystalline solid that stands out against all other reported structures. Its coordination mode and coordination number show an alternative option in the architecture of coordination polymers from amino acids and metal halides that was previously found for dl-valine (VALCAC) and dl-2-aminobutyric acid (AMBCAC; Glowiak & Ciunik, 1978). Structure (5) crystallizes in the orthorhombic space group Pbcn with an asymmetric unit of Ca(alanine)(H2O)Cl. The calcium cation occupies Wyckoff position 4c and is eightfold coordinated. No higher local symmetry is perceived than the crystallographic site symmetry 2 (Fig. 6). The alanine ligand is simultaneously chelating and bridging in 2 - 1 : 2 coordination. Hence the coordination sphere is built by two chelating symmetry-equivalent alanine molecules ðx þ 1; y; z þ 12Þ, two symmetry-equivalent water molecules and two bridging carboxylato O atoms of adjacent chain fragments. Ca O distances show a rather long contact of the bridging oxygen ˚ ]. This contact, towards the chelated calcium [2.716 (2) A however, is significantly shorter than the distance from the anti-coordinated carboxylate towards the neighbouring metal ˚ ). Disregarding this in the other reported structures (3.3–3.6 A contact, the carboxylate bridges in a syn–anti conformation. The cationic racemic chain extends along the shortest unitcell axis c with a Ca Ca distance of 4.0479 (12), which is also the shortest distance between metal cations found in all structures. All hydrogen-bond donors find a suitable acceptor but none of them forms a bond within the chain. In total, the coordination polymer is linked to a three-dimensional network. 3.3. {[Ca(l-DL-Ala)2(DL-Ala)(H2O)]Br2}11 [type (II); (6)]
Figure 2 Schematic representation of the core features of the obtained structures and assignment to structure types. Water molecules and counterions are omitted for clarity. Red and blue stick models represent l-alanine and dalanine ligands, respectively. Structures [a] (Mrozek et al., 1999) and [b] (Ciunik & Głowiak, 1980) have been reported previously.
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The reaction of calcium bromide with dl-alanine yields a product (6) [type (II)] with a very close relationship to structure (1) and type (I) structures in general. The structure crystallizes in the polar orthorhombic space group Pc21 n2 with an asymmetric unit of Ca(alanine)3(H2O)Br2 which is exactly 2 This is a non-standard setting of Pna21 (33) transformed by ðabcÞ ¼ ð010==001== 100Þða0 b0 c0 Þ for better comparison to type I structures.
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research papers Table 2 Experimental details.
Crystal data Chemical formula Mr Crystal system, space group Temperature (K) ˚) a, b, c (A ( ) ˚ 3) V (A Z Radiation type (mm1) Crystal size (mm) Data collection Diffractometer Absorption correction Tmin, Tmax No. of measured, independent and observed [I > 2(I)] reflections Rint ˚ 1) (sin /)max (A Refinement R[F 2 > 2(F 2)], wR(F 2), S No. of reflections No. of parameters No. of restraints H-atom treatment ˚ 3) max, min (e A Absolute structure parameter
(5)
(6)
(7)
C6H20Cl2CaN2O7 343.22 Orthorhombic, Pbcn 100 23.347 (7), 8.702 (3), 7.391 (2) 90 1501.5 (8) 4 Mo K 0.80 0.20 0.08 0.04
C9H23Br2CaN3O7 485.20 Orthorhombic, Pc21n 100 4.8727 (5), 16.0701 (17), 23.129 (2) 90 1811.1 (3) 4 Mo K 4.79 0.50 0.09 0.07
C15H45I4Ca2N5O15 1123.32 Monoclinic, P21/c 100 22.842 (4), 8.3609 (13), 18.807 (3) 97.734 (3) 3559.1 (10) 4 Mo K 3.85 0.18 0.06 0.05
Bruker APEX CCD Multi-scan SADABS 0.571, 0.745 16 504, 1545, 1164
Bruker APEX CCD Multi-scan SADABS 0.437, 0.745 20 251, 3739, 3450
Bruker APEX CCD Multi-scan SADABS 0.570, 0.755 41 724, 7339, 5475
0.129 0.626
0.060 0.628
0.103 0.627
0.037, 0.083, 1.06 1545 102 6 H atoms treated by a mixture of independent and constrained refinement 0.34, 0.31 –
0.028, 0.062, 1.03 3739 234 38 H atoms treated by a mixture of independent and constrained refinement 1.06, 0.30 0.004 (6)
0.046, 0.101, 1.05 7339 420 15 H atoms treated by a mixture of independent and constrained refinement 2.00, 1.46 –
Computer programs: SHELXL2013 (Sheldrick, 2013a), SADABS (Bruker, 2004).
the same as in structure (1). The structures hence formally share the same supergroup Pm21 n. Not only the composition but also the connectivity are the same as in its enantiopure analogue (see Table 4). Structure (6) is built by doublebridged homochiral, cationic chains extending along the shortest axis a; the same coordination sphere is found for the Ca cation and within the chains the same hydrogen bonds as in type (I) can be found (hence Figs. 3 and 4 also apply for this structure type). A necessary difference to type (I) structures, however, is the packing of the homochiral chains because both l- and dalanine build the crystalline solid. In chain direction (a) and along the polar axis (b) the crystal symmetry retains the handedness of the molecules; neighbouring chains along b correspond to the same chirality. Only in the c-direction do the glide planes mirror the structure so that l-chains are connected to d-chains (Fig. 7). Hence the hydrogen bonds between ionic residues (N—H Br) differ from those in the enantiopure type. The aforementioned helical arrangement changes to a rhombic connectivity (Fig. 5).
an isomorphous replacement of bromide with iodide was not possible. While product (7) is still a coordination polymer with bridging alanine molecules, incorporated water and noncoordinating halides (iodide), its composition (metal to alanine ratio) and connectivity is remarkably different from structure type (I) and (II). The asymmetric unit consists of two
3.4. {[Ca2(l-DL-Ala)5(H2O)2]I4(H2O)2}11 [type (III); (7)]
Figure 3
In contrast to the four isomorphous structures from enantiopure alanine, the reaction of CaI2 with racemic alanine yields a structurally independent solid in space group P21 =c:
Section of chain polymer (1). Atoms of the asymmetric unit are shown as displacement ellipsoids at 80% probability, adjacent atoms as capped sticks. The local coordination in all other compounds in structure type (I) and (II) is very similar.
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research papers Table 3 Experimental details for (7)–(10).
Crystal data Chemical formula Mr Crystal system, space group Temperature (K) ˚) a, b, c (A ( ) ˚ 3) V (A Z Radiation type (mm1) Crystal size (mm) Data collection Diffractometer Absorption correction Tmin, Tmax No. of measured, independent and observed [I > 2(I)] reflections Rint ˚ 1) (sin /)max (A Refinement R[F 2 > 2(F 2)], wR(F 2), S No. of reflections No. of parameters No. of restraints H-atom treatment ˚ 3) max, min (e A
(8)
(9)
(10)
C6H22Cl2MnN2O8 376.09 Monoclinic, P21 =c 100 4.7294 (9), 19.101 (4), 9.0705 (17) 103.395 (3) 797.1 (3) 2 Mo K 1.19 0.40 0.07 0.07
C6H22Br2MnN2O8 465.01 Monoclinic, P21 =c 100 4.7592 (6), 19.571 (2), 9.1998 (11) 102.450 (2) 836.72 (18) 2 Mo K 5.59 0.30 0.07 0.07
C6H22I2MnN2O8 558.99 Monoclinic, P21 =c 100 4.794 (3), 20.114 (11), 9.482 (5) 100.047 (10) 900.3 (9) 2 Mo K 4.19 0.23 0.04 0.02
Bruker APEX CCD Multi-scan SADABS 0.565, 0.745 9455, 1637, 1419
Bruker APEX CCD Multi-scan SADABS 0.507, 0.746 12 568, 2500, 2207
Bruker APEX CCD Multi-scan SADABS 0.327, 0.745 3312, 3312, 2231
0.069 0.628
0.043 0.720
0.129 0.625
0.030, 0.074, 1.09 1637 110 5 H atoms treated by a mixture of independent and constrained refinement 0.41, 0.28
0.029, 0.062, 1.04 2500 110 5 H atoms treated by a mixture of independent and constrained refinement 0.83, 0.43
0.063, 0.143, 1.03 3312 111 6 H atoms treated by a mixture of independent and constrained refinement 1.07, 1.76
Computer programs: SHELXL2013 (Sheldrick, 2013a), SADABS (Bruker, 2004).
Table 4 ˚ ) in type (I) structures and in (6). M O distances (in A
i
M1 O1 M1 O2 M1 O3 M1 O4i M1 O5 M1 O1W Average
(1)
(2)
(3)
(4)
(6)
2.303 (5) 2.353 (5) 2.338 (5) 2.396 (6) 2.252 (6) 2.372 (6) 2.336 (5)
2.298 (7) 2.366 (7) 2.327 (7) 2.388 (7) 2.253 (7) 2.380 (8) 2.335 (7)
2.194 (3) 2.172 (3) 2.209 (3) 2.240 (4) 2.096 (4) 2.188 (3) 2.199 (3)
2.183 (9) 2.199 (10) 2.202 (10) 2.249 (9) 2.077 (10) 2.202 (11) 2.185 (10)
2.296 (4) 2.368 (3) 2.330 (4) 2.395 (4) 2.245 (4) 2.379 (5) 2.336 (4)
connects to Ca2 with three 2 -O,O0 -alanine molecules of the same chirality (e.g. all l-alanine) in a syn–anti conformation. Ca2 is then bridged to the next chain segment (Ca1
Symmetry operation: (i) x þ 1; y; z.
independent Ca positions, five bridging alanine ligands, three coordinated aqua ligands as well as two solvent water molecules and the four iodide anions (Fig. 8). All atoms are located in a general position. On the one hand, the coordination sphere of Ca2 is very similar to that in the aforementioned structures: distorted octahedral coordination by five carboxylato O atoms and one water molecule. On the other hand, Ca1 is surrounded by five carboxylato O atoms and two water molecules, resulting in a distorted pentagonal bipyramidal coordination (CN 7). The chains extend along the crystallographic c-axis. Two independent Ca atoms linked to a coordination polymer also allow for two different bridging modes. Ca1
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Figure 4 Intra-chain hydrogen bonds in (1); symmetry operation: (i) x þ 1; y; z. Similar interactions occur in all other compounds in structure types (I) and (II). Acta Cryst. (2014). B70, 989–998
research papers remaining hydrogen-bond donors find acceptors in such a modelled hydrogen-bond network. Overall a three-dimensional network is formed by crosslinking the polymeric chains directly in b and via hydrogen bonds towards the iodide channels in the a direction.
3.5. Isomorphous structures from DL-alanine [type (IV), (8)–(10)]
The three isomorphous compounds of the type (IV) were obtained from reactions of MnCl2 (8), MnBr2 (9) and MnI2 (10) with Figure 5 racemic alanine. Structure (9) is Comparison of inter-chain hydrogen bonds in (1) [(a) type (I)] and (6) [(b) type (II)]. A helical already known from the literature arrangement is found for structure type (I) whereas rhombi between the chains form in structure type (Ciunik & Głowiak, 1980), but no (II). Symmetry operations: (ii) x þ 1; y 12 ; z; (iii) x þ 1; y 12 ; z þ 1; (iv) x; y 12 ; z; (v) x 32 ; y 12 ; z þ 12. instruction on the synthesis is available and the data collection had been performed at room temperature. Crystal data and x; 12 y; 12 þ z) by only two 2-O,O0 -alanine ligands. Partirefinement results are given in Table 3. cularly, this segment is built by both alanine enantiomers. The The asymmetric unit of type (IV) structures consists of a Mn next chain segment is a symmetry equivalent of the triple atom occupying Wyckoff position 2b in space group P21 =c bridge but in reverse chirality. Each alanine molecule bridges a coordinated by one alanine and an aqua ligand. One cocryssyn–anti conformation, whereas the amino group is staggered tallized water molecule and the respective halide are located towards the anti coordinating oxygen. between the cationic chains (Fig. 10). One hydrogen bond is formed within the polymeric chain Because of the 2-O,O0 -bridging nature of the alanine from each amino group towards the next chain segment. Four ligand, the coordination of the manganese centre is octahedral connect to syn coordinating carboxylato O atoms; N2 forms with axial water ligands and equatorial carboxylato coordithe hydrogen bond towards the water ligand O4W (Fig. 9). All nation. The carboxylato group is in a syn–anti conformation and staggered to the amino funcionality. Overall a centrosymmetric polymeric chain extends along b (shortest Mn Mn distance).
Figure 7 Figure 6 Section of chain polymer (5). Atoms of the asymmetric unit are shown as displacement ellipsoids at 80% probability, adjacent atoms as capped sticks. Acta Cryst. (2014). B70, 989–998
Packing of homochiral chains in the bc-plane of (1) (a) and (6) (b). Neighbouring chains along b are related by the 21 axis. Red represents lchains, blue d-chains. Switching the centre and border colours indicates chains of reverse direction. Kevin Lamberts et al.
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research papers Table 5 Distortion (h 2i) of the octahedrally coordinated metal centres. Distortion = 111 h 2i (grd2) h 2i (grd2)
P12
i¼1
ði 90Þ2 .
(1) 102.2 (7) 87.6
(2) 100.2 (8) 18.3
(3) 75.4 (9) 18.1
(4) 75.8 (10) 19.0
(6) 100.8
Our solids (1)–(10), obtained from alanine ligands, and calcium and manganese halides, show the sensitivity and flexibility of their structures towards small changes during their self-assembly. The discussion of structural trends may be divided into influences exerted by the metal cation or the halide, but also with respect to the usage of enantiopure or racemic alanine. While the transition metal cation MnII features dominant dorbital ligand-field influence and hence forms defined geometries (mostly octahedral), the coordination sphere of the larger alkaline earth cation CaII is more flexible and may easily accomodate a higher number of ligands; coordination numbers 7 and 8 are found in similar frequencies in the literature (Lamberts et al., 2014). However, in the series
Figure 8 Asymmetric unit of structure (7) [type (III)] as a displacement ellipsoid plot at 80% probability.
Hydrogen bonds occur in a similar fashion to the other structure types: One hydrogen bond is formed by the amino group towards the syn-carboxylato-O of the next chain fragment and the other two NH groups connect to the halides. The two independent water molecules also interact by hydrogen bonds and eventually connect directly coordinated water with the anionic halides completing the three-dimensional linkup of structure type (IV).
Figure 9 Hydrogen bonds in structure (7) within the chain.
4. Conclusion The CSD database (Allen, 2002) documents a total of 2311 structures3 in which any amino acid is O-coordinated to at least one metal cation. In 127 of these structures, at least one zwitterionic amino acid acts as a bridge between cations in a one-dimensional polymer in which no metal–halide bonds occur. 51 of these chain polymers contain the particularly popular, simple and achiral amino acid glycine. In addition to the structures described in this article, only four additional chain polymers based on alanine have been reported (BOPYOV, Fleck et al., 2008; BRALMN, Ciunik & Głowiak, 1980; MAKJAL, Li et al., 1997; SOYDOZ, Glowiak et al., 1991). 3 This and the following numbers refer to error-free structures without disorder for which three-dimensional coordinates are available.
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Figure 10 Section of chain polymer (10). Atoms of the asymmetric unit are shown as displacement ellipsoids at 80% probability, adjacent atoms as capped sticks. The local coordination in all other compounds in structure type (IV) is very similar. Acta Cryst. (2014). B70, 989–998
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Figure 11 Relation of the enantiopure solids (red) to three different types of combining both enantiomers to a racemic crystal.
reported here only two of six independent Ca positions exhibit those higher coordination numbers. This may be caused by the favourable structural motif found in structure types (I), (II) and (III): bridging and terminal alanine molecules with a stabilizing hydrogen-bond backbone. The exceptional structure (5) does not form hydrogen bonds directly within the chain. The difference between the naturally octahedral manganese and the more flexible calcium coordination is well reflected in their angular distortion (Robinson et al., 1971; Ertl et al., 2002; Table 5). The isomorphous coordination in structure types (I) and (II) is a well suited example: the meansquare angular deviation in the calcium structures is about 25 grd2 larger than in the manganese compounds. On the one hand, formal substitution of halides often leads to isomorphous replacement. On the other hand, differences with respect to radii, electronegativity and Pearson hardness are not neglegible. It is therefore not obvious whether halide variation will retain or change a structure type, and indeed both scenarios have been encountered among the compounds reported here: Solids within the structure types (I) and (IV) are isomorphous, whereas individually different structures (5), (6) and (7) are formed from the calcium halides with dlalanine. We have reported earlier (Lamberts & Englert, 2012) a comparable co-existence of an isomorphous series for racemic and distinctly different individual structures for enantiopure proline derivatives of MnX2 (X = Cl, Br, I). We will shortly address the role of chirality. Both Ca and Mn derivatives of enantiopure alanine adopt the same structure type (I). The racemic compounds are linked to this structure by three different concepts, but all follow the more popular alternative (Jacques et al., 1994) and crystallize in space groups containing either enantiomorph; no case of spontaneous resolution is encountered. The relationships between the enantiopure structure type (I) and the racemic compounds are graphically summarized in Fig. 11: the individual chains in type (II) are homochiral and very similar to those in type (I); however, strands of either chirality combine to a racemic but polar solid in crystal class m2m. Compound (III) is built from heterochiral chains which consist of short homochiral segments related by inversion. In both structure types (II) and Acta Cryst. (2014). B70, 989–998
(III) all atoms are in a general position; in contrast, structure type (IV) consists of heterochiral chains with a centre of inversion in every metal cation. We recall the fact documented in Table 5: sixfold coordination for MnII matches the ideal octahedral symmetry more closely than for CaII, and an inversion centre is well compatible with this higher symmetry. The more pronounced distortion for CaII derivatives apart, compounds (1)–(5) [all crystallizing in structure type (I)] and compound (6) [type (II)] share pseudo-octahedral coordination, tolerating the variation in the cation and in the halide as a counter-anion. Table 4 shows that the average M—O distances in this series reflect the differences in ionic radii: According to ˚ larger the compilation of Shannon (1976), CaII is ca 0.17 A II than Mn . In our comparison, we find that the difference in average Mn—O bond distances between compounds (1) (derived from CaCl2) and (3) (derived from MnCl2) and between the corresponding bromides (2) and (4) amounts to ˚ . Our future work will address the comparison between 0.15 A monocationic main group (e.g. NaI) and transition elements (e.g. AgI). The authors thank Tim Riebe for his experimental help. Evonik Industries is acknowledged for providing l-alanine.
References Allen, F. H. (2002). Acta Cryst. B58, 380–388. Ba¨rnighausen, H. (1980). MATCH Commun. Math. Chem. 9, 139– 175. Bruker (2004). SADABS, Version 2004/1. Bruker AXS Inc., Madison, Wisconsin, USA. Bruker (2009). SAINT+, Version 7.68. Bruker AXS Inc., Madison, Wisconsin, USA. Ciunik, Z. & Głowiak, T. (1980). Inorg. Chim. Acta, 44, L249–L250. Englert, U. (2010). Coord. Chem. Rev. 254, 537–554. Ertl, A., Hughes, J. M., Pertlik, F., Foit, F. F., Wright, S. E., Brandstatter, F. & Marler, B. (2002). Can. Mineral. 40, 153–162. Fleck, M., Lengauer, C., Bohaty, L. & Tillmanns, E. (2008). Acta Chim. Slov. 55, 880–888. Fleck, M. & Petrosyan, A. M. (2014). Salts of Amino Acids, Crystallization, Structure and Properties. Berlin: Springer. Fox, S., Bu¨sching, I., Barklage, W. & Strasdeit, H. (2007). Inorg. Chem. 46, 818–824. Glowiak, T. & Ciunik, Z. (1978). Bull. Acad. Pol. Sci. Ser. Sci. Chim. 26, 43–51. Glowiak, T., Legendziewicz, J., Dao, C. N. & Huskowska, E. (1991). J. Less Common Met. 168, 237–248. Hu, C. & Englert, U. (2005). Angew. Chem. Int. Ed. 44, 2281–2283. Hu, C. & Englert, U. (2006). Angew. Chem. Int. Ed. 45, 3457–3459. Jacques, J., Collet, A. & Wilen, S. H. (1994). Enantiomers, Racemates and Resolutions. Malabar, Florida: Krieger Publishing Company. Lamberts, K. & Englert, U. (2012). Acta Cryst. B68, 610–618. Lamberts, K., Kalf, I., Ramadan, A., Mu¨ller, P., Dronskowski, R. & Englert, U. (2011). Polymers, 3, 1151–1161. Lamberts, K., Porsche, S., Hentschel, B., Kuhlen, T. & Englert, U. (2014). CrystEngComm, 16, 3305–3311. Li, J., Liping, Z. & Jin, T. (1997). J. Chin. Rare Earth Soc. 15, 97. Mrozek, R., Rzaczyn´ska, Z., Sikorska-Iwan, M., Jaroniec, M. & Głowiak, T. (1999). Polyhedron, 18, 2321–2326. Mu¨ller, U. (2013). Symmetry Relationships between Crystal Structures. Oxford University Press. Parsons, S. & Flack, H. (2004). Acta Cryst. A60, s61. Kevin Lamberts et al.
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research papers Robinson, K., Gibbs, G. V. & Ribbe, P. H. (1971). Science, 172, 567– 570. Shannon, R. D. (1976). Acta Cryst. A32, 751–767. Sheldrick, G. M. (2008a). Acta Cryst. A64, 112–122.
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Sheldrick, G. M. (2008b). CELL NOW, Version 2008/2. Bruker-AXS Inc, Madison, Wisconsin, USA. Sheldrick, G. M. (2008c). TWINABS, Version 2008/4. University of Go¨ttingen, Germany.
Acta Cryst. (2014). B70, 989–998
Structural Science, Crystal Engineering and Materials
Enantiopure and racemic alanine as bridging ligands in Ca and Mn chain polymers
ISSN 2052-5206
Kevin Lamberts, Andreas Mo ¨ller and Ulli Englert* Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, Aachen 52074, Germany
Correspondence e-mail: [email protected]
Under accelerated and controlled evaporation, chain polymers crystallize from aqueous solutions of CaII and MnII halides with enantiopure l-alanine or racemic dl-alanine. In all ten solids thus obtained zwitterionic amino acid ligands bridge neighbouring cations. The exclusively O-donor-based coordination sphere around the metal cations is completed by aqua ligands; the halides remain uncoordinated and act as counter-anions for the cationic strands. Despite the differences in ionic radii and electronic structure between the main group and the transition metal cation, their derivatives with lalanine share a common structure type. In contrast, the solids derived from dl-alanine differ and adopt structures depending on the metal cation and the halide. Homochiral chains of either chirality or heterochiral chains with different arrangements of crystallographic inversion centres along the polymer strands are encountered. On average, the six-coordinated CaII cations, devoid of any ligand field effect, show more pronounced deviation from idealized octahedral geometry than the d-block cation MnII.
Received 1 August 2014 Accepted 26 September 2014
1. Introduction
# 2014 International Union of Crystallography
Acta Cryst. (2014). B70, 989–998
Aspects of structural systematics are often addressed in the context of inorganic compounds, e.g. in mineralogy. Such considerations are significantly less popular in the realm of molecular crystals or coordination polymers. Group–subgroup relationships (Ba¨rnighausen, 1980; Mu¨ller, 2013) are more readily perceived for structures of higher symmetry based on more rigid constituents than for conformationally soft molecules mostly crystallizing in low-symmetry space groups. However, although it might be challenging to establish structural relationships within a series of coordination polymers, such an effort may well be rewarding and lead to new insights. Our group has systematically studied halide-bridged chain polymers with pyridine ligands (Englert, 2010) and identified a simple structure type as stable at low (Hu & Englert, 2005) or high temperature only (Hu & Englert, 2006). We have investigated the structural chemistry of the reaction products of manganese halides with racemic and enantiopure proline and highlighted the role of chirality for the solid-state structures of the products (Lamberts & Englert, 2012). Additionally we have shown that a solid-state property such as the temperature for a certain phase transition may be tuned by isomorphous replacement (Lamberts et al., 2011), similar to the situation in classical inorganic salts. In this contribution we address the structural chemistry of racemic and enantiopure alanine derivatives of metal halides, both for the alkaline earth cation CaII and the transition metal ion MnII. Fig. 1 summarizes these combinations and puts them into the context of two earlier contributions: Mrozek et al. (1999) (BEFYUG) reported the doi:10.1107/S2052520614021398
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Figure 1 Overview of the reactions performed and the naming scheme of the resulting structures and structure types. The derivative from CaCl2 and lalanine is hitherto unknown. Structures [a] (Mrozek et al., 1999) and [b] (Ciunik & Głowiak, 1980) have been reported earlier but [a] could not be reproduced.
reaction product of l-alanine with MnCl2 ; we have not been able to obtain a crystalline solid according to their experimental information. The reaction product from MnBr2 and dl-alanine (9) has been studied by Ciunik & Głowiak (1980) (BRALMN) before. We can fully confirm the earlier report and decided to collect intensity data at low temperature for a better comparison. Only one Ca complex of alanine has been described to date; in this solid, the amino acid is deprotonated (Fox et al., 2007) (TEWYOK). Our crystal structures (1)–(10) formally represent adducts of the zwitterionic amino acid alanine to metal dihalides and reflect the interplay between the variables chirality, halide anion and metal cation. A comprehensive overview of amino acid structures can be found in a recently published book (Fleck & Petrosyan, 2014).
The best results were attained from reactions with a 1:2 metal halide-to-amino acid ratio despite the different target compositions. Crystallization in 1:1 stoichiometry was not observed but yielded an oil that later turned into a glass-like solid. The racemic products containing dl-alanine always crystallized faster than their chiral analogues from l-alanine. Our attempt to reproduce the compound from manganese chloride and l-alanine resulted in an oily residue which even after 10 months did not crystallize. Additionally, syntheses in the same stoichiometries were repeated in a mortar without solvent. All products except the manganese-containing compounds (3) and (4) were obtained both from 1:2 and 1:3 stoichiometry. Similar to the crystallization from solution, compounds containing dl-alanine showed higher crystallinity and could even be obtained from a 1:1 metal halide-to-alanine ratio. With increasing iodine content the products became more paste-like due to hygroscopicity. Additional experiments involved the concomitant complexation of two metal halides with the amino acid. Metal halides were mixed at 3:1, 1:1 and 1:3 ratio while an overall metal halide to alanine stoichiometry of 1:2 was maintained. These experiments did not result in any new structure types: On the one hand, combinations of different metal halides within the same structure type [either (I) or (IV)] resulted in solid solutions isomorphous with the constituents. These experiments were usually done by grinding; however, a single crystal with intermediate composition between (3) and (4) was grown from a solution containing MnBr2, MnI2 and l-alanine. Attempts at mixing different structure types always reproduced the structure of only one constituent in crystalline form.
2. Experimental 2.1. Chemicals and reagents
The following chemicals were used without further purification: CaCl2 Fluka > 99%, CaBr2xH2O Merck > 80%, CaI2 Sigma–Aldrich 99.95%, MnCl22H2O Merck 99%, MnBr2xH2O Alfa Aesar 98%, MnI2 Alfa Aesar 98%, lalanine Evonik 99.8%, dl-alanine Merck > 99%. 2.2. Syntheses
Compounds (1)–(10) were crystallized from aqueous solutions in 1:2 and 1:3 stoichiometries of manganese and calcium halides with l- and dl-alanine. MX2xH2 O (0.4 mmol; M = Mn, Ca; X = Cl, Br, I) and 0.4, 0.8 or 1.2 mmol of either lalanine or dl-alanine were dissolved in 2 ml of H2O. Because of the high solubility of the products no common purification technique, e.g. washing, has been applied. Colourless to yellow needles suitable for single-crystal diffraction were obtained by solvent evaporation. A homebuilt crystallization chamber (see supporting information1 for further details) maintained a flow of dry air under controlled temperature and thus achieved crystallization from water within 1–3 d rather than weeks. 1 Supporting information for this paper is available from the IUCr electronic archives (Reference: GP5078).
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2.3. Powder diffraction
Confirmation of the phase purity of the products was achieved by powder X-ray diffraction. Representative powder patterns for each type are given as supporting information. Experiments were performed at the Institute of Inorganic Chemistry, RWTH Aachen, using a Stoe Stadi P diffract˚, ometer with Guinier geometry (Cu K1, = 1.54059 A Johannson germanium monochromator, Stoe image plate detector IP-PSD, 0.005 step width in 2).
2.4. Single-crystal diffraction experiments
Suitable single crystals were mounted on glass fibers. Intensity data were collected with a Bruker SMART APEX CCD detector equipped with an Incoatec microsource ˚ , multilayer optics). (Mo K radiation, = 0.71073 A Temperature control was achieved using an Oxford Cryostream 700 at 100 K. Collected data were integrated with SAINT+ (Bruker, 2009) and multi-scan absorption corrections were applied with SADABS (Bruker, 2004). For the nonmerohedrally twinned crystal (10) data were processed with CELL NOW (Sheldrick, 2008b) instead and corrected for absorption with TWINABS (Sheldrick, 2008c). Acta Cryst. (2014). B70, 989–998
research papers Table 1 Experimental details. Absolute structure parameter was calculated according to Parsons & Flack (2004).
Crystal data Chemical formula Mr Crystal system, space group Temperature (K) ˚) a, b, c (A ( ) ˚ 3) V (A Z Radiation type (mm1) Crystal size (mm) Data collection Diffractometer Absorption correction Tmin, Tmax No. of measured, independent and observed [I > 2(I)] reflections Rint ˚ 1) (sin /)max (A Refinement R[F 2 > 2(F 2)], wR(F 2), S No. of reflections No. of parameters No. of restraints H-atom treatment ˚ 3) max, min (e A Absolute structure parameter
(1)
(2)
(3)
(4)
C9H23Br2CaN3O7 485.20 Monoclinic, P21 100 4.8971 (9), 15.812 (3), 11.773 (2) 93.746 (3) 909.7 (3) 2 Mo K 4.77 0.30 0.06 0.03
C9H23I2CaN3O7 579.18 Monoclinic, P21 100 4.9164 (8), 16.416 (3), 12.039 (2) 92.435 (3) 970.8 (3) 2 Mo K 3.53 0.20 0.04 0.03
C9H23Br2MnN3O7 500.06 Monoclinic, P21 100 4.7809 (4), 15.7146 (13), 11.6246 (10) 93.581 (1) 871.65 (13) 2 Mo K 5.38 0.19 0.15 0.07
C9H23I2MnN3O7 594.04 Monoclinic, P21 100 4.7932 (16), 16.261 (5), 11.894 (4) 92.255 (6) 926.3 (5) 2 Mo K 4.08 0.12 0.04 0.04
Bruker APEX CCD Multi-scan SADABS 0.455, 0.745 10 950, 3731, 3445
Bruker APEX CCD Multi-scan SADABS 0.554, 0.745 11 583, 3955, 3580
Bruker APEX CCD Multi-scan SADABS 0.486, 0.746 12 613, 4600, 4269
Bruker APEX CCD Multi-scan SADABS 0.588, 0.745 10 911, 3762, 3371
0.074 0.627
0.063 0.625
0.046 0.681
0.069 0.626
0.042, 0.099, 1.02 3731 235 38 H atoms treated by a mixture of independent and constrained refinement 0.67, 0.68 0.007 (13)
0.037, 0.065, 1.04 3955 225 38 H atoms treated by a mixture of independent and constrained refinement 0.72, 0.62 0.04 (3)
0.030, 0.059, 1.00 4600 234 12 H atoms treated by a mixture of independent and constrained refinement 0.46, 0.35 0.028 (7)
0.045, 0.099, 1.01 3762 235 30 H atoms treated by a mixture of independent and constrained refinement 1.59, 1.04 0.03 (4)
Computer programs: SHELXL2013 (Sheldrick, 2008a), SADABS (Bruker, 2004).
2.5. Refinement
Structures were solved by Patterson methods (SHELXS97 (Sheldrick, 2008a) and were refined on F 2 as implemented in SHELXL13 (Sheldrick, 2008a). Non-H atoms were assigned anisotropic displacement parameters. Atoms C8 and C9 in (2) were only refined isotropically and C7, C8 and C9 in structure (4) were treated with isotropicity restraints. In all structures H atoms connected to C were placed in idealized positions and included as riding with Uiso(H) = 1.2Ueq(C) for CH and Uiso(H) = 1.5Ueq(C) for methyl groups. H atoms of the amino groups and water molecules were found in Fourier maps and restrained to have similar distances to the respective hetero atom. H3W of (7) was placed in an idealized position pointing towards I1 ðx; y 1; zÞ for a reasonable hydrogen-bond network. For (8) and (10) the H atoms of the coordinated water were restrained to subtend a minimum distance of ˚. 1.3 A
3. Results and discussion Crystal data and refinement results are given in Tables 1–3. In total 10 structures are reported, all of which are coordination Acta Cryst. (2014). B70, 989–998
polymers with similar architecture. One-dimensional cationic chains are formed by bridging the metal centres with the carboxylato moieties of the zwitterionic alanine ligands. All structures contain aqua ligands; the halide anions are uncoordinated and reside between the polymer strands; their closest neighbors are H atoms, either associated with positively charged amino groups or with aqua oxygen. Several trends can be observed and the structures have been assigned to four different types with respect to structural motifs. An overview is given in Fig. 2. The structure of (5) will be discussed separately because of its chelating alanine ligands.
3.1. Isomorphous structures from L-alanine [type (I); (1)–(4)]
Isomorphous structures of type (I) were found for the reaction of l-alanine with CaBr2 (1), CaI2 (2), MnBr2 (3) and MnI2 (4). Reaction of the chlorides with l-alanine did only yield glass-like solids; a structure from MnCl2 has been reported in the literature (Mrozek et al., 1999). This solid is polymeric with bridging alanine ligands. However, chloride occurs both as anions and ligands in bridging and terminal positions. Crystal data and refinement results of type (I) Kevin Lamberts et al.
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research papers structures are given in Table 1. This compound is not isomorphous to type (I) and could not be reproduced by us. Structures of type (I) crystallize in the chiral monoclinic space group P21 with an asymmetric unit of M(l-alanine)3(H2O)X2 (Fig. 3). The metal cation is coordinated in distorted octahedral geometry by six O atoms. Four coordination sites are contributed by two bridging 2-O,O0 -l-alanine ligands. The coordination sphere is completed by a terminal lalanine and a water ligand in a cis position to each other. M O distances are compiled in Table 4. The resulting polymeric chains extend along the shortest axis (a) which is also the shortest metal-to-metal distance. Both bridging carboxylato moieties coordinate in syn–anti and anti–syn conformations. An eclipsed arrangement is found for the amino groups towards the anti-carboxylato O atoms. The terminal l-alanine coordinates with an almost linear M O—C angle. The resulting double-bridged cationic chains are stabilized by intra-chain hydrogen bonds and by N— H X and O—H X interactions with the halide anions. Each l-alanine ligand forms an N—H O hydrogen bond towards its symmetry-equivalent neighbour along the chain. The acceptors are the syn coordinating and the non-coordinating O atoms. Both H atoms of the water molecule also
engage in intra-chain hydrogen bonds, leaving two carboxylato-O without any hydrogen bond interaction (O1 and O5) and the terminal carboxylato-O engaged in two hydrogen bonds (O6; Fig. 4). The remaining six NH donors each form a hydrogen bond towards the two halides and concatenate two polymeric chains in the c direction in the form of a helical arrangement between symmetry equivalents of N2, N3, X1 and X2 (see Fig. 5a). N1 completes a trigonal bonding of both halides extending the connectivity between the chains along b; an overall threedimensional framework results. A full list of hydrogen bonds together with the relevant symmetry operations can be found as supporting information. 3.2. {[Ca(l-DL-Ala)2(H2O)2]Cl2(H2O)2}11 (5)
While no solid product could be obtained from l-alanine and both chlorides, racemic alanine in combination with calcium cloride yields a crystalline solid that stands out against all other reported structures. Its coordination mode and coordination number show an alternative option in the architecture of coordination polymers from amino acids and metal halides that was previously found for dl-valine (VALCAC) and dl-2-aminobutyric acid (AMBCAC; Glowiak & Ciunik, 1978). Structure (5) crystallizes in the orthorhombic space group Pbcn with an asymmetric unit of Ca(alanine)(H2O)Cl. The calcium cation occupies Wyckoff position 4c and is eightfold coordinated. No higher local symmetry is perceived than the crystallographic site symmetry 2 (Fig. 6). The alanine ligand is simultaneously chelating and bridging in 2 - 1 : 2 coordination. Hence the coordination sphere is built by two chelating symmetry-equivalent alanine molecules ðx þ 1; y; z þ 12Þ, two symmetry-equivalent water molecules and two bridging carboxylato O atoms of adjacent chain fragments. Ca O distances show a rather long contact of the bridging oxygen ˚ ]. This contact, towards the chelated calcium [2.716 (2) A however, is significantly shorter than the distance from the anti-coordinated carboxylate towards the neighbouring metal ˚ ). Disregarding this in the other reported structures (3.3–3.6 A contact, the carboxylate bridges in a syn–anti conformation. The cationic racemic chain extends along the shortest unitcell axis c with a Ca Ca distance of 4.0479 (12), which is also the shortest distance between metal cations found in all structures. All hydrogen-bond donors find a suitable acceptor but none of them forms a bond within the chain. In total, the coordination polymer is linked to a three-dimensional network. 3.3. {[Ca(l-DL-Ala)2(DL-Ala)(H2O)]Br2}11 [type (II); (6)]
Figure 2 Schematic representation of the core features of the obtained structures and assignment to structure types. Water molecules and counterions are omitted for clarity. Red and blue stick models represent l-alanine and dalanine ligands, respectively. Structures [a] (Mrozek et al., 1999) and [b] (Ciunik & Głowiak, 1980) have been reported previously.
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The reaction of calcium bromide with dl-alanine yields a product (6) [type (II)] with a very close relationship to structure (1) and type (I) structures in general. The structure crystallizes in the polar orthorhombic space group Pc21 n2 with an asymmetric unit of Ca(alanine)3(H2O)Br2 which is exactly 2 This is a non-standard setting of Pna21 (33) transformed by ðabcÞ ¼ ð010==001== 100Þða0 b0 c0 Þ for better comparison to type I structures.
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research papers Table 2 Experimental details.
Crystal data Chemical formula Mr Crystal system, space group Temperature (K) ˚) a, b, c (A ( ) ˚ 3) V (A Z Radiation type (mm1) Crystal size (mm) Data collection Diffractometer Absorption correction Tmin, Tmax No. of measured, independent and observed [I > 2(I)] reflections Rint ˚ 1) (sin /)max (A Refinement R[F 2 > 2(F 2)], wR(F 2), S No. of reflections No. of parameters No. of restraints H-atom treatment ˚ 3) max, min (e A Absolute structure parameter
(5)
(6)
(7)
C6H20Cl2CaN2O7 343.22 Orthorhombic, Pbcn 100 23.347 (7), 8.702 (3), 7.391 (2) 90 1501.5 (8) 4 Mo K 0.80 0.20 0.08 0.04
C9H23Br2CaN3O7 485.20 Orthorhombic, Pc21n 100 4.8727 (5), 16.0701 (17), 23.129 (2) 90 1811.1 (3) 4 Mo K 4.79 0.50 0.09 0.07
C15H45I4Ca2N5O15 1123.32 Monoclinic, P21/c 100 22.842 (4), 8.3609 (13), 18.807 (3) 97.734 (3) 3559.1 (10) 4 Mo K 3.85 0.18 0.06 0.05
Bruker APEX CCD Multi-scan SADABS 0.571, 0.745 16 504, 1545, 1164
Bruker APEX CCD Multi-scan SADABS 0.437, 0.745 20 251, 3739, 3450
Bruker APEX CCD Multi-scan SADABS 0.570, 0.755 41 724, 7339, 5475
0.129 0.626
0.060 0.628
0.103 0.627
0.037, 0.083, 1.06 1545 102 6 H atoms treated by a mixture of independent and constrained refinement 0.34, 0.31 –
0.028, 0.062, 1.03 3739 234 38 H atoms treated by a mixture of independent and constrained refinement 1.06, 0.30 0.004 (6)
0.046, 0.101, 1.05 7339 420 15 H atoms treated by a mixture of independent and constrained refinement 2.00, 1.46 –
Computer programs: SHELXL2013 (Sheldrick, 2013a), SADABS (Bruker, 2004).
the same as in structure (1). The structures hence formally share the same supergroup Pm21 n. Not only the composition but also the connectivity are the same as in its enantiopure analogue (see Table 4). Structure (6) is built by doublebridged homochiral, cationic chains extending along the shortest axis a; the same coordination sphere is found for the Ca cation and within the chains the same hydrogen bonds as in type (I) can be found (hence Figs. 3 and 4 also apply for this structure type). A necessary difference to type (I) structures, however, is the packing of the homochiral chains because both l- and dalanine build the crystalline solid. In chain direction (a) and along the polar axis (b) the crystal symmetry retains the handedness of the molecules; neighbouring chains along b correspond to the same chirality. Only in the c-direction do the glide planes mirror the structure so that l-chains are connected to d-chains (Fig. 7). Hence the hydrogen bonds between ionic residues (N—H Br) differ from those in the enantiopure type. The aforementioned helical arrangement changes to a rhombic connectivity (Fig. 5).
an isomorphous replacement of bromide with iodide was not possible. While product (7) is still a coordination polymer with bridging alanine molecules, incorporated water and noncoordinating halides (iodide), its composition (metal to alanine ratio) and connectivity is remarkably different from structure type (I) and (II). The asymmetric unit consists of two
3.4. {[Ca2(l-DL-Ala)5(H2O)2]I4(H2O)2}11 [type (III); (7)]
Figure 3
In contrast to the four isomorphous structures from enantiopure alanine, the reaction of CaI2 with racemic alanine yields a structurally independent solid in space group P21 =c:
Section of chain polymer (1). Atoms of the asymmetric unit are shown as displacement ellipsoids at 80% probability, adjacent atoms as capped sticks. The local coordination in all other compounds in structure type (I) and (II) is very similar.
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research papers Table 3 Experimental details for (7)–(10).
Crystal data Chemical formula Mr Crystal system, space group Temperature (K) ˚) a, b, c (A ( ) ˚ 3) V (A Z Radiation type (mm1) Crystal size (mm) Data collection Diffractometer Absorption correction Tmin, Tmax No. of measured, independent and observed [I > 2(I)] reflections Rint ˚ 1) (sin /)max (A Refinement R[F 2 > 2(F 2)], wR(F 2), S No. of reflections No. of parameters No. of restraints H-atom treatment ˚ 3) max, min (e A
(8)
(9)
(10)
C6H22Cl2MnN2O8 376.09 Monoclinic, P21 =c 100 4.7294 (9), 19.101 (4), 9.0705 (17) 103.395 (3) 797.1 (3) 2 Mo K 1.19 0.40 0.07 0.07
C6H22Br2MnN2O8 465.01 Monoclinic, P21 =c 100 4.7592 (6), 19.571 (2), 9.1998 (11) 102.450 (2) 836.72 (18) 2 Mo K 5.59 0.30 0.07 0.07
C6H22I2MnN2O8 558.99 Monoclinic, P21 =c 100 4.794 (3), 20.114 (11), 9.482 (5) 100.047 (10) 900.3 (9) 2 Mo K 4.19 0.23 0.04 0.02
Bruker APEX CCD Multi-scan SADABS 0.565, 0.745 9455, 1637, 1419
Bruker APEX CCD Multi-scan SADABS 0.507, 0.746 12 568, 2500, 2207
Bruker APEX CCD Multi-scan SADABS 0.327, 0.745 3312, 3312, 2231
0.069 0.628
0.043 0.720
0.129 0.625
0.030, 0.074, 1.09 1637 110 5 H atoms treated by a mixture of independent and constrained refinement 0.41, 0.28
0.029, 0.062, 1.04 2500 110 5 H atoms treated by a mixture of independent and constrained refinement 0.83, 0.43
0.063, 0.143, 1.03 3312 111 6 H atoms treated by a mixture of independent and constrained refinement 1.07, 1.76
Computer programs: SHELXL2013 (Sheldrick, 2013a), SADABS (Bruker, 2004).
Table 4 ˚ ) in type (I) structures and in (6). M O distances (in A
i
M1 O1 M1 O2 M1 O3 M1 O4i M1 O5 M1 O1W Average
(1)
(2)
(3)
(4)
(6)
2.303 (5) 2.353 (5) 2.338 (5) 2.396 (6) 2.252 (6) 2.372 (6) 2.336 (5)
2.298 (7) 2.366 (7) 2.327 (7) 2.388 (7) 2.253 (7) 2.380 (8) 2.335 (7)
2.194 (3) 2.172 (3) 2.209 (3) 2.240 (4) 2.096 (4) 2.188 (3) 2.199 (3)
2.183 (9) 2.199 (10) 2.202 (10) 2.249 (9) 2.077 (10) 2.202 (11) 2.185 (10)
2.296 (4) 2.368 (3) 2.330 (4) 2.395 (4) 2.245 (4) 2.379 (5) 2.336 (4)
connects to Ca2 with three 2 -O,O0 -alanine molecules of the same chirality (e.g. all l-alanine) in a syn–anti conformation. Ca2 is then bridged to the next chain segment (Ca1
Symmetry operation: (i) x þ 1; y; z.
independent Ca positions, five bridging alanine ligands, three coordinated aqua ligands as well as two solvent water molecules and the four iodide anions (Fig. 8). All atoms are located in a general position. On the one hand, the coordination sphere of Ca2 is very similar to that in the aforementioned structures: distorted octahedral coordination by five carboxylato O atoms and one water molecule. On the other hand, Ca1 is surrounded by five carboxylato O atoms and two water molecules, resulting in a distorted pentagonal bipyramidal coordination (CN 7). The chains extend along the crystallographic c-axis. Two independent Ca atoms linked to a coordination polymer also allow for two different bridging modes. Ca1
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Figure 4 Intra-chain hydrogen bonds in (1); symmetry operation: (i) x þ 1; y; z. Similar interactions occur in all other compounds in structure types (I) and (II). Acta Cryst. (2014). B70, 989–998
research papers remaining hydrogen-bond donors find acceptors in such a modelled hydrogen-bond network. Overall a three-dimensional network is formed by crosslinking the polymeric chains directly in b and via hydrogen bonds towards the iodide channels in the a direction.
3.5. Isomorphous structures from DL-alanine [type (IV), (8)–(10)]
The three isomorphous compounds of the type (IV) were obtained from reactions of MnCl2 (8), MnBr2 (9) and MnI2 (10) with Figure 5 racemic alanine. Structure (9) is Comparison of inter-chain hydrogen bonds in (1) [(a) type (I)] and (6) [(b) type (II)]. A helical already known from the literature arrangement is found for structure type (I) whereas rhombi between the chains form in structure type (Ciunik & Głowiak, 1980), but no (II). Symmetry operations: (ii) x þ 1; y 12 ; z; (iii) x þ 1; y 12 ; z þ 1; (iv) x; y 12 ; z; (v) x 32 ; y 12 ; z þ 12. instruction on the synthesis is available and the data collection had been performed at room temperature. Crystal data and x; 12 y; 12 þ z) by only two 2-O,O0 -alanine ligands. Partirefinement results are given in Table 3. cularly, this segment is built by both alanine enantiomers. The The asymmetric unit of type (IV) structures consists of a Mn next chain segment is a symmetry equivalent of the triple atom occupying Wyckoff position 2b in space group P21 =c bridge but in reverse chirality. Each alanine molecule bridges a coordinated by one alanine and an aqua ligand. One cocryssyn–anti conformation, whereas the amino group is staggered tallized water molecule and the respective halide are located towards the anti coordinating oxygen. between the cationic chains (Fig. 10). One hydrogen bond is formed within the polymeric chain Because of the 2-O,O0 -bridging nature of the alanine from each amino group towards the next chain segment. Four ligand, the coordination of the manganese centre is octahedral connect to syn coordinating carboxylato O atoms; N2 forms with axial water ligands and equatorial carboxylato coordithe hydrogen bond towards the water ligand O4W (Fig. 9). All nation. The carboxylato group is in a syn–anti conformation and staggered to the amino funcionality. Overall a centrosymmetric polymeric chain extends along b (shortest Mn Mn distance).
Figure 7 Figure 6 Section of chain polymer (5). Atoms of the asymmetric unit are shown as displacement ellipsoids at 80% probability, adjacent atoms as capped sticks. Acta Cryst. (2014). B70, 989–998
Packing of homochiral chains in the bc-plane of (1) (a) and (6) (b). Neighbouring chains along b are related by the 21 axis. Red represents lchains, blue d-chains. Switching the centre and border colours indicates chains of reverse direction. Kevin Lamberts et al.
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research papers Table 5 Distortion (h 2i) of the octahedrally coordinated metal centres. Distortion = 111 h 2i (grd2) h 2i (grd2)
P12
i¼1
ði 90Þ2 .
(1) 102.2 (7) 87.6
(2) 100.2 (8) 18.3
(3) 75.4 (9) 18.1
(4) 75.8 (10) 19.0
(6) 100.8
Our solids (1)–(10), obtained from alanine ligands, and calcium and manganese halides, show the sensitivity and flexibility of their structures towards small changes during their self-assembly. The discussion of structural trends may be divided into influences exerted by the metal cation or the halide, but also with respect to the usage of enantiopure or racemic alanine. While the transition metal cation MnII features dominant dorbital ligand-field influence and hence forms defined geometries (mostly octahedral), the coordination sphere of the larger alkaline earth cation CaII is more flexible and may easily accomodate a higher number of ligands; coordination numbers 7 and 8 are found in similar frequencies in the literature (Lamberts et al., 2014). However, in the series
Figure 8 Asymmetric unit of structure (7) [type (III)] as a displacement ellipsoid plot at 80% probability.
Hydrogen bonds occur in a similar fashion to the other structure types: One hydrogen bond is formed by the amino group towards the syn-carboxylato-O of the next chain fragment and the other two NH groups connect to the halides. The two independent water molecules also interact by hydrogen bonds and eventually connect directly coordinated water with the anionic halides completing the three-dimensional linkup of structure type (IV).
Figure 9 Hydrogen bonds in structure (7) within the chain.
4. Conclusion The CSD database (Allen, 2002) documents a total of 2311 structures3 in which any amino acid is O-coordinated to at least one metal cation. In 127 of these structures, at least one zwitterionic amino acid acts as a bridge between cations in a one-dimensional polymer in which no metal–halide bonds occur. 51 of these chain polymers contain the particularly popular, simple and achiral amino acid glycine. In addition to the structures described in this article, only four additional chain polymers based on alanine have been reported (BOPYOV, Fleck et al., 2008; BRALMN, Ciunik & Głowiak, 1980; MAKJAL, Li et al., 1997; SOYDOZ, Glowiak et al., 1991). 3 This and the following numbers refer to error-free structures without disorder for which three-dimensional coordinates are available.
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Figure 10 Section of chain polymer (10). Atoms of the asymmetric unit are shown as displacement ellipsoids at 80% probability, adjacent atoms as capped sticks. The local coordination in all other compounds in structure type (IV) is very similar. Acta Cryst. (2014). B70, 989–998
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Figure 11 Relation of the enantiopure solids (red) to three different types of combining both enantiomers to a racemic crystal.
reported here only two of six independent Ca positions exhibit those higher coordination numbers. This may be caused by the favourable structural motif found in structure types (I), (II) and (III): bridging and terminal alanine molecules with a stabilizing hydrogen-bond backbone. The exceptional structure (5) does not form hydrogen bonds directly within the chain. The difference between the naturally octahedral manganese and the more flexible calcium coordination is well reflected in their angular distortion (Robinson et al., 1971; Ertl et al., 2002; Table 5). The isomorphous coordination in structure types (I) and (II) is a well suited example: the meansquare angular deviation in the calcium structures is about 25 grd2 larger than in the manganese compounds. On the one hand, formal substitution of halides often leads to isomorphous replacement. On the other hand, differences with respect to radii, electronegativity and Pearson hardness are not neglegible. It is therefore not obvious whether halide variation will retain or change a structure type, and indeed both scenarios have been encountered among the compounds reported here: Solids within the structure types (I) and (IV) are isomorphous, whereas individually different structures (5), (6) and (7) are formed from the calcium halides with dlalanine. We have reported earlier (Lamberts & Englert, 2012) a comparable co-existence of an isomorphous series for racemic and distinctly different individual structures for enantiopure proline derivatives of MnX2 (X = Cl, Br, I). We will shortly address the role of chirality. Both Ca and Mn derivatives of enantiopure alanine adopt the same structure type (I). The racemic compounds are linked to this structure by three different concepts, but all follow the more popular alternative (Jacques et al., 1994) and crystallize in space groups containing either enantiomorph; no case of spontaneous resolution is encountered. The relationships between the enantiopure structure type (I) and the racemic compounds are graphically summarized in Fig. 11: the individual chains in type (II) are homochiral and very similar to those in type (I); however, strands of either chirality combine to a racemic but polar solid in crystal class m2m. Compound (III) is built from heterochiral chains which consist of short homochiral segments related by inversion. In both structure types (II) and Acta Cryst. (2014). B70, 989–998
(III) all atoms are in a general position; in contrast, structure type (IV) consists of heterochiral chains with a centre of inversion in every metal cation. We recall the fact documented in Table 5: sixfold coordination for MnII matches the ideal octahedral symmetry more closely than for CaII, and an inversion centre is well compatible with this higher symmetry. The more pronounced distortion for CaII derivatives apart, compounds (1)–(5) [all crystallizing in structure type (I)] and compound (6) [type (II)] share pseudo-octahedral coordination, tolerating the variation in the cation and in the halide as a counter-anion. Table 4 shows that the average M—O distances in this series reflect the differences in ionic radii: According to ˚ larger the compilation of Shannon (1976), CaII is ca 0.17 A II than Mn . In our comparison, we find that the difference in average Mn—O bond distances between compounds (1) (derived from CaCl2) and (3) (derived from MnCl2) and between the corresponding bromides (2) and (4) amounts to ˚ . Our future work will address the comparison between 0.15 A monocationic main group (e.g. NaI) and transition elements (e.g. AgI). The authors thank Tim Riebe for his experimental help. Evonik Industries is acknowledged for providing l-alanine.
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Sheldrick, G. M. (2008b). CELL NOW, Version 2008/2. Bruker-AXS Inc, Madison, Wisconsin, USA. Sheldrick, G. M. (2008c). TWINABS, Version 2008/4. University of Go¨ttingen, Germany.
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