Crystal structure of hexakis(μ2-4-tert-butoxy-4-oxobut-2-en-2-olato)trizinc

The structure of a centrosymmetric trinuclear zinc(II) complex with the formula [Zn{ZnL 3}2], where L is 4-tert-butoxy-4-oxobut-2-en-2-olate, is presented.


Chemical context
-Dicarbonyl complexes of zinc are used to obtain ZnO films by metal-organic chemical vapour deposition (MOCVD) processes (Matthews et al., 2006) and in catalysis of organic reactions (Mimoun, 2001). There are only a few reports related to the complexes of -ketoesters with zinc and bis-(ethyl acetoacetate)zinc(II) was described as a thermal stabilizer for polyvinyl halide resins (Backus & Wood, 1969). Our research group has been developing coordination compounds soluble in non-polar organic solvents, including metal complexes of acetoacetic acid esters (Koval et al., 2008;Koval, Rusanov et al., 2009), which can potentially be used as environmentally friendly additives for industrial products.

Structural commentary
The crystal structure of the zinc complex synthesized in our group with the formula [Zn{ZnL 3 } 2 ], where L is a deprotonated tert-butyl acetoacetate ligand, is presented here (Fig. 1). In the applied labelling scheme, symmetric independence of the three ligands is reflected in the suffixes A, B and C, whereas the atom numbers demonstrate the complete identity of their chemical structures and mode of coordination. The molecules of the title complex are trinuclear with all three zinc(II) atoms arranged in a linear fashion. The molecule is centrosymmetric with atom Zn1 located on an inversion centre; however, its non-crystallographic symmetry is higher as this molecule approximates C 3i symmetry. All Zn II cations are in a distorted octahedral environment formed by six O atoms. Both of the symmetry-equivalent terminal Zn2 atoms are chelated through the carbonyl O2 atoms of the ester groups and the enolate O1 atoms of the aceto groups of the tert-butyl acetoacetate ligands A, B and C. The six-membered chelate rings are virtually planar with r.m.s. deviations of 0.0257, 0.0221 and 0.0378 Å , respectively. The range of Zn2-O1 bond lengths is 2.0947 (12) (Dö hring et al., 1997). Very similar complexes of Mg II , but with crystallographic C 3i symmetry, have been reported with ethyl acetoacetate (Petrov et al., 1992) and with adamantan-1-yl acetoacetate . A common feature of these complexes is that the metal bonds to the carbonyl groups are shorter then those to the bridging enolate groups, whereas in mononuclear complexes an opposite trend has been found (Barclay & Cooper, 1965;Hall et al., 1966;Fawcett et al., 1997;Koval, Rusanov et al., 2009). Thus, there is enough evidence to suggest that ketoesters always form {M[ML 3 ] 2 } complexes with bridging enolate oxygen atoms with divalent metals with coordination number 6 when there are no other ligands able to coordinate to the central atom.

Supramolecular features
There are no short intermolecular contacts between neighbouring molecules in the crystal. The molecules are closely packed into (101) layers (Fig. 2). The molecules within the layers are arranged so that their tert-butyl ends are directed towards the central parts of neighbouring molecules (Fig. 3).

Synthesis and crystallization
To a solution of tert-butyl acetoacetate (0.01 mol) in 100 ml of toluene was added dropwise 5 ml of a 1 M solution of Zn(C 2 H 5 ) 2 (0.005 mol) in hexane. The procedure was carried out under an argon atmosphere at 233 K with vigorous stirring. The stirring under the argon atmosphere was stopped when the cooling bath (cyclohexanone with solid CO 2 ) 484 Shtokvish et al. The molecular structure of the title compound, showing 50% probability displacement ellipsoids. H atoms have been omitted for clarity. Unlabelled atoms are related by the symmetry operation (Àx, 1 À y, Àz).

Figure 2
The crystal packing of the title compound, in a projection along the b axis. H atoms have been omitted for clarity. reached room temperature. Next day, the reaction mixture was evaporated and a mobile yellowish liquid was obtained. After one day, a small amount of solid hydrolysis products precipitated from the liquid. The liquid was filtered off and hexane was added. A considerable amount of precipitate was obtained. The precipitate was filtered off and washed with toluene. Crystals suitable for X-ray diffraction analysis were obtained by very slow evaporation of the solvent from the filtrate at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were placed in geometrically idealized positions and constrained to ride on C atoms, with C-H bonds for the vinyl and methyl groups of 0.95 and 0.98 Å , respectively, with U iso (H vinyl ) = 1.2U eq (C) and U iso (H methyl ) = 1.5U eq (C). The methyl groups were allowed to rotate freely about the C-C bonds.

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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.