Tris­[(R)-lact­amide-κ²O,O′]­zinc(II) tetra­bromo­zincate

The title compound, tris­[(R)-2-hydroxy­propan­amide-κ²O,O′]­zinc(II) tetra­bromo­zincate, [Zn(C₃H₇NO₂)₃][ZnBr₄], contains one monomeric six-coordinate zinc complex cation and one tetrahedral [ZnBr₄]²⁻ anion. Both Zn^II atoms lie on threefold axes. Coordination in the cation occurs via the amide and hydroxy O atoms [Zn—O = 2.074 (5) and 2.073 (6) Å] and has a distorted octahedral geometry, with cis-O—Zn—O angles in the range 76.2 (2)–109.2 (2)°. In the crystal structure, the cations and anions are linked by N—H...Br and O—H...O hydrogen bonds, generating a three-dimensional network.

S1. Comment

Metal-containing proteins have been studied in relation to severe diseases. Alzheimer's and mad cow diseases imply metal–protein interactions. Metalloproteases are implied in cancer dispersion and angiotensin converting enzyme (ACE) in blood pressure control. For theses reasons, amide–metal complexes have attracted much interest. Following our work on metal–amide crystalline complexes (Bekaert et al., 2002a; Bekaert et al., 2002b; Bekaert et al., 2003), we now report a new zinc complex< (I), with 2-hydroxypropanamide (lactamide).

Compound (I) (Fig. 1) contains one monomeric six-coordinate zinc complex cation, [Zn(C₃H₇NO₂)₃]²⁺, and one tetrahedral [ZnBr₄]²⁻ anion. In both the cation and the anion, the Zn atoms lie on threefold axes in space group P6₃. In the cation, the Zn atom is surrounded by three symmetry-related R-lactamide ligands, coordinated in a bidentate fashion via amide atom O1 and hydroxy atom O2, and their symmetry equivalents. The Zn coordination may be described as distorted octahedral. The O1—Zn1—O1* angles [* is either at (1 − x + y, 2 − x,z) or (2 − y,1 + x-y,z)] are 91.6 (2)°, while the O2—Zn1—O2* angles are 88.1 (2)°. The Zn1(O1)₃ and Zn1(O2)₃ moieties are rotated about the threefold axis, some 31° from a fully eclipsed conformation, and the three symmetry-related trans O—Zn—O angles are 156.0 (2)°. The two Zn—O distances (Table 1) are not significantly different and are in agreement with those reported in the literature. Among the rare crystal structures of the metal complexes with lactamide or its derivatives, described in the literature, two coordination modes are possible, viz. N,O-coordination in [copper(II)-(N-pyridin-2-ylmethyl)chlorolactamide] monohydrate? (Tounsi et al., 2004) and O,O-coordination for the lactamide moiety in molybdenum(VI)oxodiperoxo N,N-dimethyllactamide (Winter et al., 1980); this latter structure is similar to that of (I), with the same geometry for the lactamide entity.

In the crystal structure the individual cation complexes are linked into chains running parallel to the c axis by inter-ion N—H···O hydrogen bonds (Table 2 and Fig. 2). Such chains are cross-linked by the use of the O2 hydroxy group forming inter-cation O—H···O hydrogen bonds with amide atom O1. The fact that both Zn1 and Zn2 lie on threefold axes in space group P6₃ then leads to a three-dimensional hydrogen-bonded network.

S2. Experimental

Compound (I) was prepared by mixing a solution of R-lactamide (0.272 g, 3 mmol) in hot acetic acid (5 ml, 353 K) and a solution of ZnBr₂ (0.496 g, 2 mmol) in boiling acetic acid (10 ml). Upon slow cooling, crystals of (I) were recovered.

S3. Refinement

C– and N-bound H atoms were placed in calculated positions, with C—H distances of 0.96 or 0.98 Å, N—H distances of 0.86 Å and U_iso(H) values of 1.2U_eq(C,N), and were refined in the riding-model approximation. Coordinates for hydroxy atom H2 were located in a difference map and then allowed for as riding, with a U_iso(H) value of 1.2U_eq(O). The absolute configuration of (I) was known from the synthesis route and the Flack value [0.06 (6)] is entirely in agreement with the known configuration.

Computing details

Data collection: CAD-4 EXPRESS (Enraf–Nonius, 1994); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

Fig. 1. A view of the title compound, showing the atomic numbering. Displacement ellipsoids are shown at the 50% probability level for non-H atoms. The additional letters `a', `b', `c' and `d' in the atom labels denote atoms at equivalent positions (1 − y,1 + x-y,z), (-x + y,1 − x,z), (2 − y,1 + x-y,z) and (1 − x + y,2 − x,z), respectively.

Fig. 2. A view showing part of the N—H···O and O—H···O hydrogen-bond network in the title compound. Displacement ellipsoids are shown at the 30% probability level for non-H atoms. The atoms in the Zn2—Br2 bond lie along the z direction, with coordinates (1/3, 2/3, z). The additional letters in the atom labels denote atoms at the following equivalent positions: a (1 − y,1 + x-y,z), b (-x + y,1 − x,z), e = f (1 − x,2 − y,-1/2 + z), g = h (x,y,-1 + z), k (1 + x-y,x,-1/2 + z), l (1 + x-y,x,1/2 + z).

Tris[(R)-2-hydroxypropanamide-κ²O,O']zinc(II) tetrabromozincate

Crystal data

[Zn(C3H7NO2)3][ZnBr4]	Dx = 2.161 Mg m^−3 Dm = 2.14 (2) Mg m^−3 Dm measured by flotation (CCl4 / C2H4Br2)
Mr = 717.67	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63	Cell parameters from 25 reflections
Hall symbol: P 6c	θ = 7.5–13.6°
a = 11.337 (5) Å	µ = 9.45 mm^−1
c = 9.910 (5) Å	T = 293 K
V = 1103.1 (9) Å^3	Parallelepiped, colourless
Z = 2	0.30 × 0.20 × 0.15 mm
F(000) = 688

Data collection

Enraf–Nonius CAD-4 diffractometer	818 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.130
Graphite monochromator	θmax = 30.0°, θmin = 3.6°
ω – 2θ scans	h = −15→15
Absorption correction: empirical (using intensity measurements) multi-scan (SADABS; Sheldrick, 1996; Blessing, 1995)	k = −15→15
Tmin = 0.131, Tmax = 0.284	l = 0→13
6629 measured reflections	3 standard reflections every 60 min
1109 independent reflections	intensity decay: 1%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.044	H-atom parameters constrained
wR(F^2) = 0.088	w = 1/[σ^2(Fo^2) + (0.0315P)^2 + 0.1638P] where P = (Fo^2 + 2Fc^2)/3
S = 1.02	(Δ/σ)max < 0.001
1109 reflections	Δρmax = 0.66 e Å^−3
75 parameters	Δρmin = −0.70 e Å^−3
1 restraint	Absolute structure: Flack (1983), No. of Friedel pairs?
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.06 (6)

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	1.0000	1.0000	0.00000 (15)	0.0286 (3)
O1	0.8802 (5)	1.0503 (5)	−0.1175 (6)	0.0312 (12)
O2	0.8371 (5)	0.9595 (7)	0.1247 (7)	0.0415 (15)
H2	0.8269	0.9405	0.2135	0.050*
N1	0.6730 (7)	1.0320 (8)	−0.1394 (9)	0.049 (2)
H1A	0.6850	1.0493	−0.2243	0.059*
H1B	0.5983	1.0168	−0.1016	0.059*
C1	0.7684 (8)	1.0288 (8)	−0.0673 (9)	0.0305 (18)
C2	0.7424 (8)	1.0003 (9)	0.0816 (8)	0.0298 (18)
H20	0.6491	0.9257	0.0959	0.036*
C3	0.7623 (11)	1.1273 (10)	0.1519 (11)	0.055 (3)
H3A	0.7497	1.1109	0.2473	0.083*
H3B	0.6971	1.1506	0.1184	0.083*
H3C	0.8527	1.2010	0.1348	0.083*
Br1	0.36350 (9)	0.88266 (8)	0.03123 (12)	0.0423 (2)
Br2	0.3333	0.6667	0.35162 (19)	0.0587 (5)
Zn2	0.3333	0.6667	0.11173 (19)	0.0380 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.0318 (4)	0.0318 (4)	0.0222 (7)	0.0159 (2)	0.000	0.000
O1	0.036 (3)	0.037 (3)	0.022 (2)	0.020 (3)	0.006 (2)	0.006 (2)
O2	0.035 (3)	0.069 (4)	0.026 (3)	0.030 (3)	0.004 (2)	0.016 (3)
N1	0.052 (5)	0.082 (6)	0.036 (4)	0.050 (5)	0.006 (3)	0.010 (4)
C1	0.029 (4)	0.027 (4)	0.038 (5)	0.017 (4)	0.002 (4)	0.004 (3)
C2	0.026 (4)	0.040 (5)	0.018 (3)	0.012 (4)	0.006 (3)	0.002 (3)
C3	0.071 (7)	0.066 (7)	0.042 (6)	0.044 (6)	0.000 (5)	−0.009 (5)
Br1	0.0423 (5)	0.0392 (5)	0.0466 (5)	0.0214 (4)	0.0062 (4)	0.0055 (4)
Br2	0.0689 (8)	0.0689 (8)	0.0383 (9)	0.0345 (4)	0.000	0.000
Zn2	0.0354 (6)	0.0354 (6)	0.0430 (10)	0.0177 (3)	0.000	0.000

Geometric parameters (Å, º)

Zn1—O2^i	2.073 (6)	N1—H1B	0.86
Zn1—O2	2.073 (6)	C1—C2	1.507 (11)
Zn1—O2^ii	2.073 (6)	C2—C3	1.511 (12)
Zn1—O1^ii	2.074 (5)	C2—H20	0.98
Zn1—O1	2.074 (5)	C3—H3A	0.96
Zn1—O1^i	2.074 (5)	C3—H3B	0.96
O1—C1	1.266 (9)	C3—H3C	0.96
O2—C2	1.431 (9)	Br1—Zn2	2.4315 (15)
O2—H2	0.90	Br2—Zn2	2.377 (3)
N1—C1	1.312 (10)	Zn2—Br1^iii	2.4315 (15)
N1—H1A	0.86	Zn2—Br1^iv	2.4315 (15)
O2^i—Zn1—O2	88.1 (2)	O1—C1—N1	122.7 (7)
O2^i—Zn1—O2^ii	88.1 (2)	O1—C1—C2	119.7 (7)
O2—Zn1—O2^ii	88.1 (2)	N1—C1—C2	117.6 (7)
O2^i—Zn1—O1^ii	156.0 (2)	O2—C2—C1	105.3 (6)
O2—Zn1—O1^ii	109.2 (2)	O2—C2—C3	113.1 (7)
O2^ii—Zn1—O1^ii	76.2 (2)	C1—C2—C3	109.1 (7)
O2^i—Zn1—O1	109.2 (2)	O2—C2—H20	109.7
O2—Zn1—O1	76.2 (2)	C1—C2—H20	109.7
O2^ii—Zn1—O1	156.0 (2)	C3—C2—H20	109.7
O1^ii—Zn1—O1	91.6 (2)	C2—C3—H3A	109.5
O2^i—Zn1—O1^i	76.2 (2)	C2—C3—H3B	109.5
O2—Zn1—O1^i	156.0 (2)	H3A—C3—H3B	109.5
O2^ii—Zn1—O1^i	109.2 (2)	C2—C3—H3C	109.5
O1^ii—Zn1—O1^i	91.6 (2)	H3A—C3—H3C	109.5
O1—Zn1—O1^i	91.6 (2)	H3B—C3—H3C	109.5
C1—O1—Zn1	117.1 (5)	Br2—Zn2—Br1	109.15 (6)
C2—O2—Zn1	118.7 (4)	Br2—Zn2—Br1^iii	109.15 (6)
C2—O2—H2	110.0	Br1—Zn2—Br1^iii	109.79 (6)
Zn1—O2—H2	129.1	Br2—Zn2—Br1^iv	109.15 (6)
C1—N1—H1A	120.0	Br1—Zn2—Br1^iv	109.79 (6)
C1—N1—H1B	120.0	Br1^iii—Zn2—Br1^iv	109.79 (6)
H1A—N1—H1B	120.0
O2^i—Zn1—O1—C1	−86.4 (6)	O1^i—Zn1—O2—C2	53.0 (9)
O2—Zn1—O1—C1	−3.3 (6)	Zn1—O1—C1—N1	−166.9 (7)
O2^ii—Zn1—O1—C1	47.5 (9)	Zn1—O1—C1—C2	14.2 (9)
O1^ii—Zn1—O1—C1	106.0 (7)	Zn1—O2—C2—C1	15.9 (8)
O1^i—Zn1—O1—C1	−162.4 (6)	Zn1—O2—C2—C3	−103.2 (7)
O2^i—Zn1—O2—C2	101.9 (7)	O1—C1—C2—O2	−19.2 (10)
O2^ii—Zn1—O2—C2	−169.9 (6)	N1—C1—C2—O2	161.8 (7)
O1^ii—Zn1—O2—C2	−95.1 (6)	O1—C1—C2—C3	102.4 (9)
O1—Zn1—O2—C2	−8.3 (6)	N1—C1—C2—C3	−76.5 (10)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2···O1^v	0.90	1.82	2.696 (8)	166
N1—H1A···Br1^vi	0.86	2.68	3.487 (8)	157
N1—H1B···Br1	0.86	2.66	3.478 (8)	158

Symmetry codes: (v) x−y+1, x, z+1/2; (vi) −x+1, −y+2, z−1/2.