Di­chloro­bis(DL-valine)­zinc(II)

In the title compound, [ZnCl₂(C₅H₁₁NO₂)₂], the Zn atom lies on the twofold axis within the unit cell, with two symmetrically related valine mol­ecules coordinating to it as ligands. The observed asymmetry of the carboxyl­ate group is most likely a result of the participation of only one of the carboxyl O atoms in the coordination environment. Zinc has a distorted tetrahedral environment with twofold-related Cl atoms and two carboxyl O atoms, one each from the twofold-related valine mol­ecules in the asymmetric unit.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](C-C) = 0.005 Å
• R factor = 0.053
• wR factor = 0.137
• Data-to-parameter ratio = 16.5

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

---

S1. Comment

Valine, an essential amino acid, is often found in the interior of protein molecules due to its hydrophobic nature and thus plays a key role in determining their three-dimensional structure. Structural data on simple valine–inorganic salt complexes seem to be very limited. Apart from the present study which reports the crystal structure of (I), a complex of DL-valine with ZnCl₂, namely dichlorobis(DL-valinato)zinc(II), the structure of DL-valine with CaCl₂ (Glowiak & Ciunik, 1978) remains the only other complex that has been reported so far. Zinc, which competes successfully with cadmium for protein-binding sites, also plays an important biological role in the form of a structural motif called zinc fingers. Interestingly, zinc finger motifs are characteristic of certain proteins that have an affinity to bind to DNA. Recently, the crystal structures of trichloro(sarcosinio)zinc(II) monohydrate (Krishnakumar et al., 2001) and dichlorobis(sarcosinato)zinc(II) (Subha Nandhini et al., 2001) have been reported.

Fig. 1 shows the molecular structure and the atom-numbering scheme adopted. Zn atom lies on the two fold axis within the unit cell with two symmetrically related valine molecules coordinating to it as ligands. Valine exists as a zwitterion as expected with C1—O1 and C1—O2 bond distances of 1.252 (3) and 1.233 (3) Å, respectively. This observed asymmetry of the carboxylate group is most likely a result of the participation of only one of the carboxyl O atoms (O1) in the coordination environment. The carboxylate group is quite planar and the amino nitrogen deviates from this plane by 0.486 (4) Å, leading to the twisting of the C—N bond out of the plane of the carboxyl group by 19.9 (3)°. The conformation of the valine molecule determined by the internal rotational angles ψ₂ [-19.4 (3)], χ₁₁ [-164.4 (3)° and χ₁₂ [70.5 (3)] agree well with the values observed for the monoclinic form of DL-valine (Mallikarjunan & Rao, 1969) and for the triclinic form of DL-valine (Dalhus & Gorbitz, 1996).

Zinc is known to have both tetrahedral and octahedral coordination in crystal structures (Cingi et al., 1972). In the present structure, zinc has a distorted tetrahedral environment with twofold related Cl atoms and two carboxyl O atoms, one each from the twofold-related valine molecules in the asymmetric unit. Similar metal–ligand coordination is observed in the crystal structures of glycine with ZnCl₂ (Hariharan et al., 1989) and L-proline with ZnCl₂ (Yukawa et al., 1985). However, in the complexes of ZnCl₂ with L-histidine (Foster et al., 1993) and sarcosine (Krishnakumar et al., 2001), the Zn atom exhibits a different coordination environment with one of the carboxyl O and three Cl atoms participating in it. Interestingly, in both these structures, the amino acid exists in an unusual cationic form.

Fig. 2 shows the packing of the molecules of (I) viewed down the b axis. The valine molecules coordinating to Zn through O1 form a linear chain characterized by a head-to-tail hydrogen bond between the centrosymmetrically related valine molecules and an N—H···Cl hydrogen bond. The linear chain, on either side, is flanked by the hydrophobic side chains of valine leading to alternating polar and non-polar regions along the a axis. This chain forms an infinite two-dimensional layered network parallel to the bc plane through head-to-tail hydrogen bonds between translationally related valine molecules along the b axis. O1 of the carboxyl group does not participate in the hydrogen bonding network and the strengths of the two head-to-tail hydrogen bonds involving O2 are nearly equal. Though the metal–ligand coordination observed in (I) is similar to those observed in some amino acid–ZnCl₂ complexes, the crystal structure itself does not bear any similarities with them.

S2. Experimental

Colourless single crystals of (I) were grown from a saturated aqueous solution containing DL-valine and ZnCl₂ in stoichiometric ratio.

S3. Refinement

The H atoms were placed at calculated positions and were allowed to ride on their respective parent atoms using SHELXL97 (Sheldrick, 1997) defaults.

Computing details

Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software; data reduction: CAD-4 Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 1999); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The structure of (I) with 50% probability displacement ellipsoids.
	Fig. 2. Packing of the molecules of (I) viewed down the b axis.

Dichlorobis(DL-valine)zinc(II)

Crystal data

[ZnCl2(C5H11NO2)2]	F(000) = 768
Mr = 370.57	Dx = 1.503 Mg m^−3 Dm = 1.52 Mg m^−3 Dm measured by flotation in a mixture of xylene and bromoform
Monoclinic, C2/c	Cu Kα radiation, λ = 1.54180 Å
a = 19.997 (2) Å	Cell parameters from 25 reflections
b = 6.2259 (10) Å	θ = 15–27°
c = 13.5028 (10) Å	µ = 5.19 mm^−1
β = 103.02 (2)°	T = 293 K
V = 1637.9 (3) Å^3	Plates, colorless
Z = 4	0.24 × 0.18 × 0.10 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	1394 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.048
Graphite monochromator	θmax = 67.9°, θmin = 4.5°
ω–2θ scans	h = −24→23
Absorption correction: ψ scan (North et al., 1968)	k = 0→7
Tmin = 0.40, Tmax = 0.60	l = −15→16
1526 measured reflections	2 standard reflections every 100 reflections
1435 independent reflections	intensity decay: <2%

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)] = 0.053	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.137	H-atom parameters constrained
S = 1.11	w = 1/[σ^2(Fo^2) + (0.1048P)^2 + 1.3333P] where P = (Fo^2 + 2Fc^2)/3
1435 reflections	(Δ/σ)max < 0.001
87 parameters	Δρmax = 1.06 e Å^−3
0 restraints	Δρmin = −0.89 e Å^−3

Special details

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	0.0000	0.23870 (6)	0.7500	0.0332 (3)
Cl1	0.08490 (3)	0.03328 (12)	0.83847 (5)	0.0481 (3)
N1	−0.06037 (12)	0.8226 (3)	0.89686 (18)	0.0377 (5)
H1A	−0.0770	0.9442	0.9163	0.057*
H1B	−0.0694	0.8170	0.8293	0.057*
H1C	−0.0152	0.8178	0.9213	0.057*
O1	−0.03602 (11)	0.4567 (3)	0.82769 (16)	0.0458 (5)
O2	−0.06796 (12)	0.2625 (3)	0.94793 (18)	0.0442 (6)
C2	−0.09324 (13)	0.6360 (4)	0.93640 (19)	0.0323 (6)
H2	−0.0814	0.6406	1.0109	0.039*
C3	−0.17161 (15)	0.6472 (5)	0.9000 (3)	0.0519 (8)
H3	−0.1856	0.7939	0.9122	0.062*
C1	−0.06352 (11)	0.4305 (4)	0.90147 (19)	0.0304 (5)
C4	−0.2063 (2)	0.4966 (8)	0.9619 (4)	0.0842 (14)
H4A	−0.2552	0.5059	0.9379	0.126*
H4B	−0.1939	0.5372	1.0322	0.126*
H4C	−0.1916	0.3519	0.9544	0.126*
C5	−0.19570 (19)	0.6031 (6)	0.7863 (3)	0.0724 (11)
H5A	−0.1735	0.7011	0.7491	0.109*
H5B	−0.2445	0.6222	0.7662	0.109*
H5C	−0.1842	0.4583	0.7722	0.109*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.0409 (4)	0.0207 (3)	0.0424 (4)	0.000	0.0188 (3)	0.000
Cl1	0.0471 (4)	0.0421 (5)	0.0536 (5)	0.0102 (3)	0.0083 (3)	−0.0017 (3)
N1	0.0418 (12)	0.0209 (10)	0.0499 (13)	−0.0003 (8)	0.0090 (10)	−0.0016 (9)
O1	0.0649 (13)	0.0294 (10)	0.0527 (12)	0.0067 (8)	0.0334 (10)	−0.0019 (8)
O2	0.0555 (13)	0.0218 (10)	0.0590 (14)	0.0052 (7)	0.0211 (11)	0.0057 (7)
C2	0.0408 (13)	0.0203 (11)	0.0391 (13)	0.0034 (9)	0.0160 (11)	−0.0005 (9)
C3	0.0369 (14)	0.0358 (15)	0.087 (2)	0.0070 (11)	0.0213 (14)	0.0019 (14)
C1	0.0302 (11)	0.0209 (11)	0.0407 (13)	0.0031 (9)	0.0092 (10)	−0.0019 (9)
C4	0.058 (2)	0.078 (3)	0.134 (4)	−0.0003 (19)	0.057 (2)	0.011 (3)
C5	0.0461 (17)	0.065 (2)	0.093 (3)	−0.0020 (16)	−0.0128 (17)	0.007 (2)

Geometric parameters (Å, º)

Zn1—O1	1.9500 (18)	C2—C3	1.535 (4)
Zn1—O1^i	1.9500 (18)	C2—H2	0.9800
Zn1—Cl1^i	2.2430 (8)	C3—C4	1.522 (5)
Zn1—Cl1	2.2430 (8)	C3—C5	1.526 (5)
N1—C2	1.492 (3)	C3—H3	0.9800
N1—H1A	0.8900	C4—H4A	0.9600
N1—H1B	0.8900	C4—H4B	0.9600
N1—H1C	0.8900	C4—H4C	0.9600
O1—C1	1.252 (3)	C5—H5A	0.9600
O2—C1	1.233 (3)	C5—H5B	0.9600
C2—C1	1.529 (3)	C5—H5C	0.9600
O1—Zn1—O1^i	91.77 (12)	C4—C3—C2	110.7 (3)
O1—Zn1—Cl1^i	110.88 (7)	C5—C3—C2	112.5 (3)
O1^i—Zn1—Cl1^i	115.94 (7)	C4—C3—H3	107.4
O1—Zn1—Cl1	115.94 (7)	C5—C3—H3	107.4
O1^i—Zn1—Cl1	110.88 (7)	C2—C3—H3	107.4
Cl1^i—Zn1—Cl1	110.47 (4)	O2—C1—O1	127.6 (2)
C2—N1—H1A	109.5	O2—C1—C2	118.4 (2)
C2—N1—H1B	109.5	O1—C1—C2	114.0 (2)
H1A—N1—H1B	109.5	C3—C4—H4A	109.5
C2—N1—H1C	109.5	C3—C4—H4B	109.5
H1A—N1—H1C	109.5	H4A—C4—H4B	109.5
H1B—N1—H1C	109.5	C3—C4—H4C	109.5
C1—O1—Zn1	128.24 (17)	H4A—C4—H4C	109.5
N1—C2—C1	107.9 (2)	H4B—C4—H4C	109.5
N1—C2—C3	110.5 (2)	C3—C5—H5A	109.5
C1—C2—C3	112.3 (2)	C3—C5—H5B	109.5
N1—C2—H2	108.7	H5A—C5—H5B	109.5
C1—C2—H2	108.7	C3—C5—H5C	109.5
C3—C2—H2	108.7	H5A—C5—H5C	109.5
C4—C3—C5	111.2 (3)	H5B—C5—H5C	109.5
O1^i—Zn1—O1—C1	−174.3 (3)	Zn1—O1—C1—O2	7.7 (4)
Cl1^i—Zn1—O1—C1	67.0 (2)	Zn1—O1—C1—C2	−173.36 (17)
Cl1—Zn1—O1—C1	−60.0 (2)	N1—C2—C1—O2	159.6 (2)
N1—C2—C3—C4	−164.4 (3)	C3—C2—C1—O2	−78.4 (3)
C1—C2—C3—C4	75.0 (4)	N1—C2—C1—O1	−19.4 (3)
N1—C2—C3—C5	70.5 (3)	C3—C2—C1—O1	102.6 (3)
C1—C2—C3—C5	−50.1 (3)

Symmetry code: (i) −x, y, −z+3/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···O2^ii	0.89	2.03	2.837 (3)	151
N1—H1B···Cl1^iii	0.89	2.59	3.370 (2)	146
N1—H1C···O2^iv	0.89	2.19	2.972 (3)	146

Symmetry codes: (ii) x, y+1, z; (iii) −x, y+1, −z+3/2; (iv) −x, −y+1, −z+2.