Crystal structure of a tetranuclear CuII complex with an O,N,N′-donor Schiff base ligand: hexa-μ2-acetato-bis(2-{[(2,2,6,6-tetramethylpiperidin-4-yl)imino]methyl}phenolato-κ3 O,N,N′)tetracopper(II)

In the title compound, the symmetry-unique terminal CuII ion is O,N,N′-coordinated by a 2-{[(2,2,6,6-tetramethylpiperidin-4-yl)imino]methyl}phenolate ligand and an O atom from an acetate group in a slightly distorted tetrahedral coordination environment. The symmetry-unique central CuII ion is coordinated by a different O atom of the same acetate group and by four bridging acetate ligands, which connect the asymmetric unit into a dimeric complex and form a slightly distorted square-pyramidal coordination environment.


Chemical context
The chemistry of metal complexes with Schiff base ligands and their applications has attracted considerable attention, mainly due to their preparative accessibility, structural variability, magnetic properties and biological properties (Karahan et al., 2015). The design of suitable building blocks and the utilization of coordinate bonds and non-covalent interactions to generate self-assemblies of various dimensions having aesthetic beauty and properties for possible use as functional materials are the major objectives in supramolecular chemistry and crystal engineering (Sasmal et al., 2011). Within this context, we report herein the crystal structure of the title complex.

Structural commentary
The molecular structure of the title complex is shown in Fig. 1. The complex lies across a twofold rotation axis. The asymmetric unit contains two independent Cu II ions, Cu1 and Cu2. Cu1 is coordinated by atoms O1, N1 and N2 of a 2-{[(2,2,6,6- ISSN 2056-9890 tetramethylpiperidin-4-yl)imino]methyl}phenolate ligand and by atom O2 from an acetate group in a slightly distorted square-planar coordination environment. Cu2 is coordinated by atom O3 of the same acetate group mentioned above and by four bridging acetate ligands, which connect the asymmetric unit into a dimeric complex. Cu2 is in a distorted square-pyramidal coordination environment. The CuÁ Á ÁCu distance is 2.6225 (9) Å . The piperidine rings are in boat conformations. Within the complex, there are two symmetryequivalent intramolecular N-HÁ Á ÁO hydrogen bonds (Table 1).

Supramolecular features
In the crystal, weak C-HÁ Á ÁO hydrogen bonds link the complex molecules, forming a three-dimensional network (see Table 1 and Figs. 2 and 3).

Database survey
A search of the Cambridge Structural Database (Version 5.37, update 1; Groom & Allen, 2014) for compounds containing the same Schiff base ligand as the title compound found only one hit, namely bis[N-(2,2,6,6-tetramethylpiperidin-4-yl)salicylaldiminato]copper(II) (Golovina et al., 1975). In this compound, the ligand acts as only an N,O donor with the -N-H group remaining non-coordinating, unlike in the title compound. However, the precision of the determined geometric parameters is not sufficient to make a meaningful comparison with the title compound. Although, in a closely related compound, namely, hexakis( 2 -acetato)bis[1-(5bromosalicylaldimino)-3-(2-methylpiperidino)propane]tetracopper (Chiari et al., 1993), the Cu-O and Cu-N distances for each coordination center are in agreement. A comprehensive study of the compound tetrakis( 2 -acetato)bis(acetic acid)dicopper(II), which is the basic core of the title compound, has been carried out by Vives et al. (2003).

Figure 2
Part of the crystal structure, viewed along the b axis, with hydrogen bonds shown as dashed lines. Only H atoms involved in hydrogen bonding are shown.

Figure 3
Part of the crystal structure, viewed along the c axis, with hydrogen bonds shown as dashed lines. Only H atoms involved in hydrogen bonding are shown.

Figure 1
The molecular structure of the title compound with 50% probability ellipsoids. For clarity, H atoms bonded to C atoms are not shown. The unlabeled part of the molecule is related by the symmetry code (Àx + 1, y, Àz + 1 2 ).

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.