Synthesis and crystal structure of [Zn6Br4(C9H18NO)4(OH)4]·2C3H6O2

The complete molecule of the hexametallic title complex, namely, tetrabromidotetra-μ-hydroxido-hexakis[μ-2-methyl-3-(pyrrolidin-1-yl)propan-2-olato]hexazinc(II) acetone disolvate, is generated by a crystallographic centre of symmetry. Two of the unique zinc atoms adopt distorted ZnO2NBr tetrahedral coordination geometries and the other adopts a ZnO3N tetrahedral arrangement. Both unique alkoxide ligands are N,O-chelating and both hydroxide ions are μ2 bridging.


Chemical context
Zinc complexes have a wide range of applications. For example they can be found as catalysts in organic chemistry or in the human body in enzymes, such as oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases (Lipscomb & Strä ter, 1996). As a result of the filled d 10 shell of the Zn 2+ cation, zinc complexes can exhibit different coordination geometries, including tetrahedral, trigonal-bipyramidal and octahedral (Kimura et al., 1997). The tetrahedral coordination sphere is the most common because the ligands have the largest separation from each other (Holm et al., 1996).
Zinc alkoxides find applications in many fields. They are used in organic catalysis, for example in the amplification of an enantiomer through an autocatalytic cycle by building a tetrameric zinc alkoxide as an intermediate (Shibata et al., 1997;Soai et al., 1995). In addition, they are also used as catalysts in polymerization reactions, for example for the ringopening polymerization of lactides (Chen et al., 2006(Chen et al., , 2011. Moreover, zinc alkoxides are electronically favoured in comparison to the incorporation of hydroxide or water molecules (Bergquist & Parkin, 1999). Hence, zinc alkoxides are an important species in the human body for example for the liver alcohol dehydrogenase or the CO 2 transport through the circulatory system by carbonate anhydrase (Clegg et al., 1988;Siek et al., 2016). Liver alcohol dehydrogenase is an enzyme that catalyses the biological oxidation of alcohols to aldehydes and ketones (Bergquist et al., 2000). As part of this reaction, a tetrahedral zinc alkoxide complex is formed and after that, a formal hydride transfer occurs from the alkoxide to the oxidized form of NAD + (see Fig. 1). The entire process depicted in Fig. 1 involves the removal of a ketone from the zinc atom.
In the title compound, (I), an acetone molecule interacts with the complex through hydrogen bonding. It can therefore ISSN 2056-9890 be understood as an intermediate of the ketone removal during the dehydrogenation process shown in Fig. 1. The remaining interaction of the ketone with the zinc complex is interesting for a deeper understanding of the liver alcohol dehydrogenase cycle.

Structural commentary
Compound (I) was crystallized from a mixture of zinc bromide and an aminoalkoxide in an acetone/water/triethylamine mixture at 278 K. It crystallizes in the monoclinic crystal system in space group P2 1 /n together with one solvent molecule of acetone and the complete hexa-metallic molecule is generated by crystallographic inversion symmetry. The structure of (I) is shown in Fig. 2 and selected bond lengths and angles are given in Table 1.
The bond lengths between the zinc atom and the oxygen atom of the alkoxide ligand are 1.9593 (9) Å for Zn1-O1 and 1.9401 (9) Å for Zn2-O4. The bond length for Zn2 may be shorter because of the direct bonding of a bromide ion to Zn1. Bond lengths between a zinc atom and an alkoxide oxygen atom have been observed to be 1.936 (3) Å (Chen et al., 2014) and 1.971 (2) Å (Siek et al., 2016), thus the corresponding bonds in (I) lie between these limits. The bond lengths between the zinc atom and the bridging hydroxide O atom, Zn1-O2 and Zn2-O3, are 1.9165 (10) Å and 1.9147 (9) Å , respectively, which are elongated in comparison to a similar zinc-hydroxide bond in the literature, where the distance is 1.900 (2) Å (Siek et al., 2016)  The molecular structure of (I) with atom labelling and 50% displacement ellipsoids. Atoms with superscript a are generated by the symmetry operation 1 À x, 1 À y, 1 À z. Table 1 Selected geometric parameters (Å , ). In general, the bond angles in (I) are as expected (Table 1), apart from the O-Zn-N angles: these are significantly compressed from the ideal tetrahedral values with O1-Zn1-N1 = 88.54 (4) and O4-Zn2-N2 = 86.91 (4) , presumably because of the rigid structure of the aminoalkoxide and the higher steric demand of the tetrahedral nitrogen atom. This is supported by a similar compound in the literature with an O-Zn-N angle of 94.1 (1) (Chen et al., 2014). The N2-Zn2-O3 bond angle [112.11 (4) ] is slightly wider than the ideal tetrahedral angle, as is O2-Zn2-O4 [116.23 (4) ] but O2-Zn2-O3 is slightly compressed to 108.79 (4) . The angle of the O2 hydroxyl oxygen atom, Zn1 and the O1 atom of the alkoxide is 111.19 (4) , which is slightly expanded from the ideal tetrahedral angle. Finally, the N1-Zn1-Br1 bond angle is widened to 114.35 (3) , which is similar to a compound in literature, where the corresponding angle is 113.1 (1) (Chen et al., 2014).
The central structural features of (I) are two six-membered rings, which consist of zinc-oxygen bonds (Fig. 2). In the sixmembered rings two zinc atoms are bridged by one oxygen atom of the alkoxide and the other zinc centres are bridged by a hydroxide ion. Then, both six-membered rings are connected by two oxygen atoms of the alkoxide species, so the two parts are interconnected to each other and a central eightmembered ring is formed by the connection of the two sixmembered rings. The four nitrogen atoms of the piperidine rings coordinate to the zinc atoms of the six-membered ring. The coordination spheres of the other zinc atoms are completed by bromide ions. The chelating 2-methyl-1-(piperidine-1-yl)propan-2-olate anions lie at the edges of the complex, so they do not interact with the other anions.
One of the methyl groups of the acetone solvent molecule is disordered over two sets of sites with occupancies of 0.519 (6) and 0.481 (6). The disorder of just one methyl group of an acetone molecule has already been reported in the literature (Arias et al., 2013;Balogh-Hergovich et al., 1998).

Supramolecular features
In the extended structure of (I), the molecules are stacked along the a axis, as shown in Fig. 3. As noted already, an O-HÁ Á ÁO hydrogen bond links the O2-H2 hydroxide ion with the acetone solvent molecule ( Table 2). The graph-set motif of the O-HÁ Á ÁO hydrogen bonding is described by a discrete finite pattern [D(2)] and, because of the inversion symmetry of the complex, a second [D 2 2 (11)] pattern appears. The Hirshfeld surface analysis of (I) (CrystalExplorer17; Turner et al., 2017) highlights the hydrogen bonding between the main molecule and the acetone solvent molecule. The main molecule is shown (Fig. 4) with d norm in the range À0.5240 to +1.5598: the characteristic red spot adjacent to H2 indicates the hydrogen bond to O5. As a result of steric shielding, no intermolecular hydrogen bonding through the bridging O3 hydroxide group occurs.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The O-bound hydrogen atoms were located in difference-Fourier maps and refined independently. All C-bound hydrogen atoms were placed in geometrically calculated positions (C-H = 0.98-0.99 Å ) and refined as riding atoms with the constraint U iso (H) = 1.5U eq (Cmethyl) and 1.2U eq (C) for other H atoms.

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