Dichlorobis(DL-proline-κO)zinc(II)

The title compound, [ZnCl₂(C₅H₉NO₂)₂], crystallizes in the centrosymmetric space group C2/c with the Zn atom on a twofold axis. The two proline residues in any one complex thus have the same absolute configuration. Hydrogen bonding links the mol­ecules into linear chains, which run in the crystallographic b direction. The proline residues within any one chain also have an identical absolute configuration.

S1. Comment

Zn²⁺ ions play an essential role in the regulation and catalytic activity of biological systems (Fraústo da Silva & Williams, 1991). To obtain a deeper insight into the interaction of metal ions with amino acids, neutral salts can be studied. Neutral salts are formed by the interaction of neutral zwitterionic amino acids with metal salts, e.g. CaCl₂, SrCl₂, BaCl₂, and LiCl (Pfeiffer & Wittka, 1915). The crystal structures of neutral salts of Zn²⁺ with DL-penicillamine (Bell & Sheldrick, 1984), D-penicillamine (Bell & Sheldrick, 1984), L-proline (Yukawa et al., 1985), sarcosine (Subha Nandhini et al., 2001), L-t-leucine (Hoffmüller et al., 1999), glycine (Hariharan et al., 1989), DL-valine (Nandhini et al., 2001) and DL-alanine (Subha Nandhini et al., 2002) have already been reported. In all cases, the Zn²⁺ ions are tetrahedrally four-coordinated, with two halogenides and two O atoms of two negatively charged carboxylates as donors. The halogenides act as acceptors of hydrogen bonds, which are donated by the positively charged ammonium groups.

In the structure of dichlorobis(L-proline)zinc(II) (Yukawa et al., 1985), the Zn complex contains two independent L-proline residues. The five-membered ring of one of them has an envelope conformation on C4 and a C—N—C^α—C torsion angle of −0.8 (5)°, while the other has a twist conformation on the C3—C4 bond, with a C—N—C^α—C torsion angle of 11.0 (4)°. From quantum chemical calculations, it is known that the torsion angle of the carboxylate group is correlated with the ring puckering (Ramek et al., 1997). This correlation can also be found in the crystal structures of 83 proline complexes (133 residues) obtained from the Cambridge Structural Database (April 2002 release; Allen, 2002), as shown by a plot of the C—N—C^α—C ring torsion angles versus the N—C^α—C—O torsion angles (Fig. 1). A negative value for the C—N—C^α—C torsion angle results in a positive deviation from 0° for the N—C^α—C—O torsion angle, while a positive value results in a negative deviation. In dichlorobis(L-proline)zinc(II), the N—C^α—C—O torsion angles are −15.2 (5) and 20.9 (5)°, respectively, thus fitting badly into the correlation. The two independent proline residues also differ in the C—O—Zn angles, which are 122.4 (3) and 116.5 (3)°, respectively. For comparison with these data, we prepared the title racemic compound, dichlorobis(DL-proline)zinc(II), (I), and its structure is presented here. \sch

Compound (I) crystallizes in the centrosymmetric spacegroup C2/c. The Zn atom is located on a twofold axis in a distorted tetrahedral environment (Fig. 2). Both proline ligands of one complex consequently have the same absolute configuration, either both R or both S. Conformation analysis of the five-membered ring according to the method of Evans & Boeyens (1989) results in coefficients of 59 and 41% for the envelope and twist conformations, respectively, indicating an intermediate form. The C5—N1—C2—C3 torsion angle is 21.05 (14)° and the O1—C1—C2—N1 torsion angle is −9.81 (16)°, which is perfectly in line with the above-mentioned correlation between these two torsion angles. The C1—O1—Zn1 angle is 128.29 (9)°, which is significantly larger than the corresponding angles in the L-proline complex. As expected, the C1—O1 bond length of the coordinated O atom is significantly longer than C1—O2 of the non-coordinated O atom (Table 1).

Both H atoms of the positively charged ammonium group are involved in hydrogen bonding (Table 2). Atom H1A forms a bifurcated hydrogen bond, with atom O1 of the same proline residue and atom Cl1ⁱ as acceptors [symmetry code: (i) x, 1 + y, z]. The angle sum at H1A is 355 (2)°. Atom H1B forms an intermolecular hydrogen bond to atom O2ⁱ. Due to the hydrogen bonding, a linear chain is formed in the direction of the crystallographic b axis (Fig. 2). Because translation is the only symmetry operation in the generation of these chains, all proline residues within one chain have the same configuration. Of course, in the racemic centrosymmetric crystal, there is the same number of chains with R-proline as with S-proline residues.

Partial separation of chiral molecules in a racemic crystal has been reported before, for the structure of N-acetyl-DL-alanine methylester (Müller & Lutz, 2001), where the hydrogen-bonded chains consist of one R and two S molecules, or vice versa. A complete separation of the R and S forms would lead to a mixture of enantiopure crystals, as in the famous example of sodium ammonium tartrate (Pasteur, 1848). Indeed, we observed this separation in a crystallization experiment, where we obtained a crystalline powder of dichlorobis(L-proline)zinc(II) and dichlorobis(D-proline)zinc(II). The structure of the powder was analyzed by comparison of the measured powder pattern with that calculated from the single-crystal coordinates (Yukawa et al., 1985). This powder was later used as seed crystals for the crystallization of the racemic compound, (I). It seems that, in this case, the conditions of the crystallization experiment control which product is obtained, not the nature of the seed crystals.

S2. Experimental

DL-Proline (0.502 g; 99%, ACROS Organics) and ZnCl₂ (0.297 g; 98%, Fluka Chimica) were dissolved in a minimum amount of water. The solution was heated to 363 K and slowly cooled to room temperature, yielding a viscous oil. As seed crystals, a mixture of previously obtained dichlorobis(L-proline)zinc(II) and dichlorobis(D-proline)zinc(II) powder was added. After standing for 6 h, transparent colourless crystals of (I) of a suitable size for the diffraction experiment were obtained. The product can also be obtained by crystallization from aqueous ethanol.

S3. Refinement

H atoms on N atoms were refined freely with isotropic displacement parameters. All remaining H atoms were placed in geometrically idealized positions, with C—H distances in the range 0.99–1.00 Å, and constrained to ride on their parent atoms, with U_iso(H) = 1.2U_eq(C).

Computing details

Data collection: COLLECT (Nonius, 1999); cell refinement: DIRAX (Duisenberg, 1992); data reduction: EVAL14 (Duisenberg, 1998) and SORTAV (Blessing, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2002).

Figures

	Fig. 1. The correlation between the C—N—C^α—C and the N—C—C—O torsion angles in 83 crystal structures containing proline (133 observations). The two independent proline residues of dichlorobis(L-proline)zinc(II) (Yukawa et al., 1985) are marked with arrows.
	Fig. 2. The molecular structure of (I). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. A complex containing two R-proline residues is shown [symmetry code: (i) 1 − x, y, 3/2 − z].
	Fig. 3. Hydrogen-bond formation in the crystal structure of (I). The proline residues in the chain on the left-hand side have the R configuration, while those on the right-hand side have the S configuration [symmetry code: (i) x, 1 + y, z].

Dichlorobis(DL-proline-κO)zinc(II)

Crystal data

[Zn(C5H9NO2)2Cl2]	F(000) = 752
Mr = 366.53	Dx = 1.693 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 96 reflections
a = 18.6705 (8) Å	θ = 3.8–22.6°
b = 5.9427 (2) Å	µ = 2.09 mm^−1
c = 13.3961 (4) Å	T = 150 K
β = 104.637 (4)°	Needle, colourless
V = 1438.10 (9) Å^3	0.30 × 0.15 × 0.15 mm
Z = 4

Data collection

Nonius KappaCCD area-detector diffractometer	1648 independent reflections
Radiation source: rotating anode	1485 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.028
ϕ and ω scans	θmax = 27.5°, θmin = 2.3°
Absorption correction: multi-scan (SORTAV; Blessing, 1997)	h = −24→23
Tmin = 0.58, Tmax = 0.73	k = 0→7
12791 measured reflections	l = 0→17

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.018	Hydrogen site location: difference Fourier map
wR(F^2) = 0.048	H atoms treated by a mixture of independent and constrained refinement
S = 1.08	w = 1/[σ^2(Fo^2) + (0.0233P)^2 + 1.2477P] where P = (Fo^2 + 2Fc^2)/3
1648 reflections	(Δ/σ)max = 0.001
95 parameters	Δρmax = 0.37 e Å^−3
0 restraints	Δρmin = −0.25 e Å^−3

Special details

Experimental. 364 ϕ scans and 407 ω scans were measured at a constant detector distance of 40.0 mm with a rotation angle of 1° per frame. The exposure time was 15 s per frame with a generator setting of 60 kV, 50 mA.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Zn1	0.5000	0.04576 (4)	0.7500	0.01255 (8)
Cl1	0.568586 (19)	−0.17754 (6)	0.87247 (3)	0.01949 (9)
O1	0.44050 (5)	0.26580 (17)	0.80316 (8)	0.0194 (2)
O2	0.33580 (6)	0.07590 (16)	0.79946 (8)	0.0202 (2)
N1	0.39295 (7)	0.6528 (2)	0.85066 (9)	0.0143 (2)
H1A	0.4323 (10)	0.615 (3)	0.8303 (13)	0.022 (4)*
H1B	0.3740 (10)	0.779 (3)	0.8220 (14)	0.026 (5)*
C1	0.37325 (7)	0.2491 (2)	0.80777 (10)	0.0139 (3)
C2	0.33609 (7)	0.4691 (2)	0.82480 (10)	0.0129 (3)
H2	0.2979	0.5099	0.7604	0.015*
C3	0.29968 (8)	0.4624 (2)	0.91636 (11)	0.0178 (3)
H3A	0.2453	0.4794	0.8919	0.021*
H3B	0.3108	0.3186	0.9544	0.021*
C4	0.33353 (8)	0.6608 (3)	0.98500 (12)	0.0219 (3)
H4A	0.3037	0.7990	0.9652	0.026*
H4B	0.3374	0.6275	1.0586	0.026*
C5	0.40963 (8)	0.6856 (3)	0.96500 (11)	0.0202 (3)
H5A	0.4307	0.8367	0.9853	0.024*
H5B	0.4443	0.5695	1.0022	0.024*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.01256 (12)	0.00900 (12)	0.01765 (12)	0.000	0.00669 (8)	0.000
Cl1	0.02073 (18)	0.01919 (18)	0.01772 (17)	0.00258 (13)	0.00329 (14)	0.00354 (13)
O1	0.0146 (5)	0.0146 (5)	0.0318 (6)	−0.0003 (4)	0.0109 (4)	−0.0067 (4)
O2	0.0166 (5)	0.0110 (5)	0.0330 (6)	−0.0013 (4)	0.0063 (4)	−0.0022 (4)
N1	0.0150 (6)	0.0097 (6)	0.0201 (6)	0.0008 (5)	0.0081 (5)	−0.0001 (5)
C1	0.0140 (6)	0.0123 (6)	0.0149 (6)	0.0016 (5)	0.0029 (5)	−0.0005 (5)
C2	0.0120 (6)	0.0108 (6)	0.0162 (6)	−0.0005 (5)	0.0042 (5)	0.0005 (5)
C3	0.0162 (7)	0.0178 (7)	0.0221 (7)	0.0005 (6)	0.0097 (6)	0.0005 (6)
C4	0.0213 (7)	0.0250 (8)	0.0228 (7)	0.0011 (6)	0.0118 (6)	−0.0043 (6)
C5	0.0176 (7)	0.0227 (7)	0.0207 (7)	−0.0018 (6)	0.0056 (6)	−0.0062 (6)

Geometric parameters (Å, º)

Zn1—O1	1.9625 (10)	C2—H2	1.0000
Zn1—Cl1	2.2429 (4)	C3—C4	1.530 (2)
O1—C1	1.2763 (16)	C3—H3A	0.9900
O2—C1	1.2334 (17)	C3—H3B	0.9900
N1—C5	1.4967 (19)	C4—C5	1.518 (2)
N1—C2	1.5008 (18)	C4—H4A	0.9900
N1—H1A	0.876 (19)	C4—H4B	0.9900
N1—H1B	0.88 (2)	C5—H5A	0.9900
C1—C2	1.5243 (19)	C5—H5B	0.9900
C2—C3	1.5458 (19)
O1^i—Zn1—O1	96.43 (6)	C3—C2—H2	109.3
O1^i—Zn1—Cl1	112.88 (3)	C4—C3—C2	105.03 (11)
O1—Zn1—Cl1	113.56 (3)	C4—C3—H3A	110.7
Cl1—Zn1—Cl1^i	107.45 (2)	C2—C3—H3A	110.7
C1—O1—Zn1	128.29 (9)	C4—C3—H3B	110.7
C5—N1—C2	106.45 (11)	C2—C3—H3B	110.7
C5—N1—H1A	112.2 (12)	H3A—C3—H3B	108.8
C2—N1—H1A	109.9 (12)	C5—C4—C3	103.41 (12)
C5—N1—H1B	107.0 (12)	C5—C4—H4A	111.1
C2—N1—H1B	109.9 (12)	C3—C4—H4A	111.1
H1A—N1—H1B	111.2 (16)	C5—C4—H4B	111.1
O2—C1—O1	127.07 (13)	C3—C4—H4B	111.1
O2—C1—C2	117.40 (12)	H4A—C4—H4B	109.0
O1—C1—C2	115.53 (12)	N1—C5—C4	101.88 (12)
N1—C2—C1	109.65 (11)	N1—C5—H5A	111.4
N1—C2—C3	105.24 (11)	C4—C5—H5A	111.4
C1—C2—C3	113.87 (12)	N1—C5—H5B	111.4
N1—C2—H2	109.3	C4—C5—H5B	111.4
C1—C2—H2	109.3	H5A—C5—H5B	109.3
O1^i—Zn1—O1—C1	146.99 (14)	O1—C1—C2—N1	−9.81 (16)
Cl1—Zn1—O1—C1	−94.59 (12)	O2—C1—C2—C3	52.96 (17)
Cl1^i—Zn1—O1—C1	28.03 (13)	O1—C1—C2—C3	−127.40 (13)
Zn1—O1—C1—O2	15.9 (2)	N1—C2—C3—C4	5.27 (15)
Zn1—O1—C1—C2	−163.69 (9)	C1—C2—C3—C4	125.38 (13)
C5—N1—C2—C1	−101.81 (13)	C2—C3—C4—C5	−29.02 (15)
C5—N1—C2—C3	21.05 (14)	C2—N1—C5—C4	−39.19 (14)
O2—C1—C2—N1	170.55 (12)	C3—C4—C5—N1	41.68 (15)

Symmetry code: (i) −x+1, y, −z+3/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···Cl1^ii	0.876 (19)	2.755 (18)	3.3721 (13)	128.6 (14)
N1—H1A···O1	0.876 (19)	2.117 (18)	2.6014 (16)	114.2 (14)
N1—H1B···O2^ii	0.88 (2)	1.90 (2)	2.7498 (16)	163.8 (17)

Symmetry code: (ii) x, y+1, z.