Crystal structure and Hirshfeld surface analysis of bis(μ-4-tert-butoxy-4-oxobut-2-en-2-olato)bis[(4-tert-butoxy-4-oxobut-2-en-2-olato)ethanolzinc(II)]

The crystal structure of bis(μ-4-tert-butoxy-4-oxobut-2-en-2-olato)bis[(4-tert-butoxy-4-oxobut-2-en-2-olato)ethanolzinc(II)] is reported and discussed.

The molecular and crystal structure of the title binuclear Zn 2+ complex, [Zn 2 (C 8 H 13 O 3 ) 4 (C 2 H 5 OH) 2 ], with enolated anionic tert-butylacetoacetate and ethanol was analysed. The coordination polyhedra of the Zn atoms are distorted octahedra formed by six oxygen atoms that belong to three ligand molecules and a coordinated ethanol molecule. In the crystal phase, alternating layers can be distinguished parallel to the ac plane. A Hirshfeld surface analysis showed that there are no strong intermolecular interactions in the structure. The most significant contributions to the overall crystal packing are from HÁ Á ÁH intermolecular contacts.
Our research group is developing coordination compounds soluble in non-polar organic solvents, including metal complexes of acetoacetic acid esters (Koval et al., 2009), which can potentially be used as environmentally friendly additives to industrial products. Previously, we reported the structure of a trimeric zinc complex synthesized in a rather complicated way using diethyl zinc (Shtokvish et al., 2014). After that, we developed a much simpler and relatively more efficient method for the synthesis of cobalt and nickel ketoesterates (Shtokvish et al., 2015, Shtokvysh et al., 2018. The use of this method for the synthesis of Zn complexes made it possible to obtain dimeric complexes with cyclohexylacetoacetate (Shtokvysh et al., 2020) and tert-butylacetoacetate. In the present work, we report the synthesis and structural analysis of the new complex [Zn 2 (C 8 H 10 O 3 ) 4 (C 2 H 5 OH) 2 ].

Structural commentary
The title compound, systematic name bis(-4-tert-butoxy-4oxobut-2-en-2-olato)bis[(4-tert-butoxy-4-oxobut-2-en-2-olato)ethanolzinc(II)], is a binuclear complex that resides on a special position with the unit cell's central inversion centre being close to the refined zinc(II) atom and directly in between this and the symmetry-generated zinc atom [symmetry code: (i) Àx + 1, Ày + 1, Àz + 1] (Fig. 1). The coordination polyhedron of the Zn centre is a distorted octahedron formed by six oxygen atoms. One bidentate acetylacetonate type ligand (O1, O2) binds only to one zinc centre. Its oxygen atoms occupy an axial (O1) and an equatorial position (O2). The second bidentate ligand (O4, O5) binds the zinc centre only equatorially, while O4 also binds the symmetry-generated second zinc atom of the binuclear complex. This also means that the symmetry-generated O4 i atom occupies the fourth equatorial position. The octahedral coordination sphere is completed by axially coordinated ethanol (O7). The bonds of zinc atoms with the enol atom of the bridging ligand are not equivalent. The Zn1-O4 bond length in the chelate is shorter than the Zn1-O4 i bond length with the symmetry-generated bridging ligand [2.076 (2) and 2.141 (3) Å , respectively;  Table 1). The connection between the nuclei of the complex is additionally stabilized by two intramolecular hydrogen bonds between the hydrogen atoms of the hydroxyl groups of ethanol and the enol oxygen atoms of the terminal ligands belonging to another nucleus (Table 2).

Supramolecular features
There are no short intermolecular contacts between neighbouring molecules in the crystal phase. However, visually we can distinguish alternating layers parallel to the ac plane ( Fig. 2a) Symmetry code: (i) Àx þ 1; Ày þ 1; Àz þ 1.

Figure 2
(a) Crystal packing of the title compound and (b) differently-coloured layers in the same projection.

Figure 1
The molecular structure of the title compound, showing 30% probability displacement ellipsoids. H atoms and the minor occupancy disordered component have been omitted for clarity. Unlabelled atoms are related by the symmetry operation 1 À x, 1 À y, 1 À z.
respect to each other and mirrored with respect to the molecules of the neighbouring layer ( Fig. 2b).

Hirshfeld surface analysis and finger print plots
A Hirshfeld surface analysis was performed and the associated two-dimensional fingerprint plots were generated using Crystal Explorer 21.5 software (Spackman et al., 2021), with a standard resolution of the three-dimensional d norm surfaces plotted over a fixed colour scale of 0.0290 (white) to 1.706 (blue) a.u. (Fig. 3). Usually contacts shorter than the sums of van der Waals radii are shown in red, those longer in blue, and those approximately equal as white spots. There are no red spots on the d norm surface. This indicates that there are no strong intermolecular interactions in the structure. The overall two-dimensional fingerprint plot, and those decomposed into various interactions are given Fig. 4. The most significant contributions to the overall crystal packing are from HÁ Á ÁH (89.2%) proximities, which are located mostly in the middle region of the fingerprint plot. There is also a small contribution from HÁ Á ÁO/OÁ Á ÁH (6.5%) and HÁ Á ÁC/ CÁ Á ÁH (4.3%) intermolecular 'contacts'.

Synthesis and crystallization
The title compound was synthesized in accordance with the methodology reported earlier (Shtokvish et al., 2015). ZnCl 2 (0.1 g, 7 mmol) was dissolved in 2 ml of ethanol (azeotrope with water, 95.6% alcohol). Then liquid tert-butyl acetoacetate was added to the solution (0.244 ml, 14 mmol). The components were then mixed. The test tube with the reaction mixture was placed in a container together with a vessel containing triethylamine (0.4 ml, 28 mmol). The container was sealed and left in the refrigerator for 1-2 days at a temperature of 281 K. The structural study was performed for a crystal taken directly and immediately from the reaction mixture, since this compound is prone to degradeation. The crystals were filtered on a P2 (P100) fritted glass filter (to separate thin powders of by-products and degradeation products), then washed several times with ethanol and dried in air for no more than 1 h. The yield is 0.078 g, which is 25.3% of the theoretical value. The obtained crystals can be stored at 261 K and below.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were placed in calculated positions [C-H = 0.93 Å (0.96 Å for C-methyl)] and refined as riding with U iso (H) = 1.2U eq (C) or 1.5U eq (C-methyl).
The C atoms of the coordinated ethanol molecule are disordered over two positions with an occupancy of 50%. Restraints were applied to the bond lengths in the disordered The overall two-dimensional fingerprint plot and those delineated into specified interactions.

Bis(µ-4-tert-butoxy-4-oxobut-2-en-2-olato)bis[(4-tert-butoxy-4-oxobut-2-en-2-olato)ethanolzinc(II)]
Crystal data 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (