Tetraaqua­tetrakis­{μ2-[1-(carb­oxylato­meth­yl)cyclo­hex­yl]methanaminium}bis(μ3-hydroxido)bis(nitrato-κ2 O,O′)tetrazinc(II)

Elise J C de Vries; Caryn Gamble; Ahmed Shaikjee

doi:10.1107/S1600536811011020

. 2011 Apr 13;67(Pt 5):m573–m574. doi: 10.1107/S1600536811011020

Tetraaquatetrakis{μ₂-[1-(carboxylatomethyl)cyclohexyl]methanaminium}bis(μ₃-hydroxido)bis(nitrato-κ² O,O′)tetrazinc(II)

Elise J C de Vries ^a,^*, Caryn Gamble ^a, Ahmed Shaikjee ^a

PMCID: PMC3089351 PMID: 21754301

Abstract

As the title gabapentin complex, [Zn₄(OH)₂(NO₃)₂(C₉H₁₇NO₂)₄(H₂O)₄](NO₃)₄ is located about a centre of inversion, the asymmetric unit contains two disordered nitrate ions and half a complex molecule. The two zinc ions have different coordination environments: one is slightly distorted octahedral and the other is trigonal–pyramidal. The conformation of the gabapentin molecule is defined by the formation of two intramolecular O—H⋯O hydrogen bonds. Furthermore, the ammonium H atoms are involved in numerous hydrogen bonds with the disordered nitrate anions.

Related literature

For related transition metal complexes with gabapentin, see: Braga et al. (2008 ▶). For structures with hexa- and tetra-coordinated zinc atoms, see: Clegg et al. (1991 ▶); Karmakar & Baruah (2008 ▶). For the structure of a gabapentin nitrate salt, see: de Vries et al. (2011 ▶).

Experimental

Crystal data

[Zn₄(OH)₂(NO₃)₂(C₉H₁₇NO₂)₄(H₂O)₄](NO₃)₄
M _r = 1424.64
Triclinic,
a = 10.0160 (2) Å
b = 11.3524 (2) Å
c = 14.2480 (2) Å
α = 88.740 (1)°
β = 74.021 (1)°
γ = 67.295 (1)°
V = 1430.28 (4) Å³
Z = 1
Mo Kα radiation
μ = 1.76 mm⁻¹
T = 173 K
0.51 × 0.30 × 0.22 mm

Data collection

Bruker APEXII CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2005 ▶) T _min = 0.468, T _max = 0.698
21157 measured reflections
6238 independent reflections
5401 reflections with I > 2σ(I)
R _int = 0.027

Refinement

R[F ² > 2σ(F ²)] = 0.047
wR(F ²) = 0.128
S = 1.05
6238 reflections
363 parameters
6 restraints
H-atom parameters constrained
Δρ_max = 1.54 e Å⁻³
Δρ_min = −1.42 e Å⁻³

Data collection: APEX2 (Bruker, 2005 ▶); cell refinement: SAINT-NT (Bruker, 2005 ▶); data reduction: SAINT-NT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008) ▶; program(s) used to refine structure: SHELXL97 (Sheldrick, 2008) ▶; molecular graphics: X-SEED (Barbour, 2001 ▶; Atwood & Barbour, 2003 ▶); software used to prepare material for publication: X-SEED.

Supplementary Material

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536811011020/ez2233sup1.cif

e-67-0m573-sup1.cif^{(42.6KB, cif)}

Structure factors: contains datablocks I. DOI: 10.1107/S1600536811011020/ez2233Isup2.hkl

e-67-0m573-Isup2.hkl^{(305.3KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Selected bond lengths (Å).

Zn1—O1	2.075 (2)
Zn1—O5ⁱ	2.075 (2)
Zn1—O5	2.077 (2)
Zn1—O6	2.099 (2)
Zn1—O7	2.109 (3)
Zn1—O3	2.127 (2)
Zn2—O4	1.933 (3)
Zn2—O5	1.954 (2)
Zn2—O2	1.979 (3)
Zn2—O2E	1.979 (7)
Zn2—O3B	2.122 (5)

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Symmetry code: (i) Inline graphic .

Table 2. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1C⋯O1	0.91	2.23	2.918 (4)	132
N1—H1D⋯O2Cⁱⁱ	0.91	2.18	3.003 (9)	149
N1—H1E⋯O1Bⁱⁱ	0.91	2.57	3.015 (6)	111
N1—H1E⋯O2Dⁱⁱⁱ	0.91	2.25	3.090 (8)	153
N1—H1E⋯O3Dⁱⁱⁱ	0.91	2.31	3.037 (8)	136
N2—H2C⋯O1D^iv	0.91	2.00	2.906 (8)	172
N2—H2D⋯O1Cⁱⁱ	0.91	2.02	2.884 (9)	158
N2—H2E⋯O3	0.91	2.01	2.748 (5)	138
O5—H5C⋯O1D	0.95	1.86	2.809 (8)	173
O6—H6C⋯O2Cⁱⁱ	0.98	1.97	2.944 (7)	170
O6—H6C⋯O2Cⁱ	0.98	2.01	2.819 (7)	138
O6—H6D⋯O3D^iv	0.95	2.58	3.010 (9)	108
O6—H6D⋯O3Bⁱ	0.95	2.00	2.927 (5)	163
O7—H7C⋯O3Cⁱ	0.95	1.78	2.735 (9)	178
O7—H7D⋯O2D	0.96	1.87	2.819 (8)	173

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Symmetry codes: (i) Inline graphic ; (ii) ; (iii) ; (iv) .

Acknowledgments

The authors thank the National Research Foundation of South Africa and the University of the Witwatersrand for financial support.

supplementary crystallographic information

Comment

The first transition metal complexes of gabapentin were published by Braga (Braga et al., 2008). Complexes were obtained by grinding together gabapentin and the inorganic salts ZnCl₂ and CuCl₂.2H₂O. In the resulting crystals of the zinc complex the Zn(II) cation is tetrahedrally coordinated to two chloride anions and two zwitterionic gabapentin molecules. In this study we investigated the effect of the nitrate counterion on the nature of the metal gabapentin complex formed.

The asymmetric unit contains [Zn₂(Gpn)₂(H₂O)₂(NO₃)OH]²⁺.(NO₃^-)₂ (complex (I)). This molecule is situated around the centre of inversion (0, 1/2, 1/2). The two zinc metal ions in the asymmetric unit have different coordination environments. The first zinc ion (Zn1) is in a slightly distorted octahedral environment, coordinated to two water oxygen atoms, two oxygen hydroxyl atoms (one generated by symmetry) and two gabapentin carboxylate oxygen atoms. The water and hydroxyl oxygen atoms are in both the axial and equatorial positions. The second zinc ion (Zn2) is in a trigonal pyramidal coordination environment and is coordinated to two gabapentin carboxylate oxygen atoms, one hydroxyl oxygen atom and an oxygen atom of a disordered nitrate molecule (found in the axial position). The two metal ions are therefore linked by the two hydroxyl groups and two gabapentins which act as a bridge (Fig. 1). As the asymmetric unit is located around a centre of inversion each hydroxyl group is bonded to three zinc metal ions, by symmetry. This type of hexa and tetra coordinated zinc has been observed before (Clegg et al., 1991; Karmakar & Baruah, 2008). Each gabapentin is in the chair conformation, with the ammonium group in the equatorial position. The conformation of gabapentin can be described by torsion angles which indicate that the two molecules are quite similar (Table 1). Each of the nitrate counterions was found to be disordered over two positions within the structure, with one of the nitrate counterions coordinating to a metal centre.

The crystal packing is determined by hydrogen bonds involving the nitrate anions, carboxylic acid and NH₃⁺ groups. The conformation of the gabapentin molecule is defined by the formation of two intramolecular hydrogen bonds between the ammonium and carboxylic acid oxygen atoms of gabapentin. This bond plays an important role in the determination of the structural properties of the metal complex and is also observed in Braga's Zn(II) complex (Braga et al., 2008). Furthermore, the two NH₃⁺ groups are involved in numerous hydrogen bonds, some of which are bifurcated, with the three disordered nitrate anions. The molecular conformation is further stabilized by four intermolecular C—H···O bonds which are formed between gabapentin and the nitrate anions with distances and angles in the range of 3.138–3.394 Å and 126–164 ° respectively.

Experimental

Gabapentin was purchased from Sigma-Aldrich. Complex (I) is obtained by reflux solution crystallization. The metal salt (0.371 g) and gabapentin (0.428 g) were combined in 1:2 stoichiometric proportions, then dissolved in 25 ml distilled water. The solution was refluxed at 60 degrees Celcius for an hour, then allowed to cool in a fridge at 12 degrees Celcius (until crystals formed).