Structure of copper(II) complexes grown from ionic liquids – 1-ethyl-3-methylimidazolium acetate or chloride

Crystals of four new copper(II) complexes containing Cu2(AcO)4 paddle-wheel units and 1-ethyl-3-methylimidazolium cations have been grown from ionic liquid–water mixtures and characterized by X-ray analysis. Two of the synthesized complexes are one-dimensional coordination polymers with anionic chains and counter-ions between them.


Chemical context
Ionic liquids (ILs) with melting point below 373 K were discovered in 1888 (Gabriel & Weiner, 1888), but have been specific laboratory substances for a long time. However, over the past two decades ionic liquids have been of increased interest for researchers owing to the awareness of their unique properties, such as low dielectric permeability, low movability, wide range of liquid states, high ionic density, high ionic conductivity, good solubility for many substances, very low volatility among others (Buszewski et al., 2006;Hallett & Welton, 2011). It is important that the properties of ionic liquids can be varied not only by structural design, but also by mixing with other substances, especially with water (Kohno & Ohno, 2012). The use of ILs as unique solvents for the replacement of traditional solvents and the synthesis of new substances from ionic liquids are the goals of many investigations. The application of ILs has already allowed the synthesis of new polyoxometallates, transition metal clusters, main-group element clusters and nanomaterials; the most important catalytic organic syntheses have also been performed in ionic liquids under mild conditions (Sasaki et al.,

Structural commentary
Compound 1 consists of two 1-ethyl-3-methylimidazolium cations and a binuclear complex anion [Cu 2 (AcO) 4 Cl 2 ] 2À in which two copper(II) atoms are bonded through four bridging acetate ions. Two chloride ions are situated in the axial positions of both metal atoms, forming the axis of a paddle-wheel structure with the copper(II) ions ( Fig. 1).
Compound 2 is a polymer; in the main chain chloride ions and the two copper(II) ions, connected by four acetate ions, alternate with each other (Fig. 2). Disordered 1-ethyl-3methylimidazolium cations and water molecules are present in the regions between the polyanionic chains. The interatomic CuÁ Á ÁCu distances in the clusters decrease (Table 1) with the transition from the binuclear compound 1 to the polymer 2.
Compound 3 is also a polymer, but differs from 2 in the bridging ligand between clusters and the absence of water molecules (Fig. 3). It is evident that the replacement of the chloride ion by acetate leads to a significant increase in the copper-copper distances between neighboring cluster units. However, the interatomic metal-metal distances in the clusters are practically unchanged (Table 1).

Figure 2
Compound 2 with displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) 2 À x, 1 À y, Àz; (ii) 2 À x, 1 À y, 1 À z; (iii) x, y, À1 + z; (iv) 1 À x, 1 À y, 1 À z.] Table 1 Metal-metal distances (Å ) in complexes 1-4. and comprises two copper(II) ions and six acetate ions, four of which act as bridges between metal atoms. The other cluster is not charged and differs from the first by the non-bridging ligands (in this case they are water molecules). Furthermore, compound 4 contains 1-ethyl-3-methylimidazolium ions and water molecules. The metal-metal distances in the clusters in 4 are somewhat shorter than in the polymeric compounds 2 and 3 (Table 1).

Supramolecular features
In the crystal of 1, weak interactions are found between the [Cu 2 (AcO) 4 Cl 2 ] 2À anion and the surrounding six 1-ethyl-3methylimidazolium cations, namely C1-H1Á Á ÁO2, C2-H2Á Á ÁO5 and C3-H3Á Á ÁO3 contacts (see Table 2 for details). The last contact is relatively short and probably the strongest of them. Two different orientations of the paddle-wheels units form herringbone motif (Fig. 5).
Polymeric chains in 2 propagate along the c-axis direction (Fig. 6). The water molecule forms hydrogen bonds with oxygen atoms of the acetate residues of two neighbouring clusters in one chain (see Table 3). Those interactions decrease the Cu-Cl-Cu angle from 180 to 169.5 on the side of water molecule and distort the linearity of the polymeric chains.

Figure 5
The packing of compound 1, viewed along the a and b axes.

Figure 8
The packing of compound 4, viewed along the b axis. mean value of other structures deposited in the CSD. It should be mentioned that in 1 the CuÁ Á ÁCu distance is very close to the maximum distance shown in Fig. 9. This long CuÁ Á ÁCu distance can be explained by the strong interaction between the copper(II) atoms and the chloride ions.

Synthesis of 2:
A mixture of 1-ethyl-3-methylimidazolium chloride (0.60 g, 4.1 mmol), copper(II) acetate hydrate (0.40 g, 2 mmol) and water (0.60 g, 33 mmol) was stirred in a closed vial at 343 K for 20 h. After several weeks, a green precipitate had formed from the solution. This precipitate consisted of crystals of compounds 1 and 2 with 1 predominant (and hence the yield of 2 was not determined).

Figure 9
Histogram of the distribution of CuÁ Á ÁCu distances in the Cu 2 (AcO) 4 fragment based on a fragment search in the CSD.

Synthesis of 4:
A mixture of 1-ethyl-3-methylimidazolium acetate (1.0 g, 5.9 mmol), copper(II) acetate hydrate (0.078 g, 0.39 mmol) and copper(II) chloride dihydrate (0.133 g, 0.78 mmol) was stirred in a closed vial at 323 K for 30 h. After several weeks, blue crystals were formed from the solution. The yield was not determined because the precipitate additionally contained small green crystals of complex 1. In the absence of copper(II) chloride, compound 3 was grown from the solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. In 2, the Emim cations and water molecules are disordered over two positions with an occupancy ratio of 0.513 (12):0.487 (12) and were refined with constraints and restraints. In 4, the water molecules refined using restraints. Water H atoms were located in difference-Fourier maps and refined using constraints with U iso (H) = 1.2U eq (O). C-bound H atoms were positioned geometrically and refined using a riding model with C-H = 0.95 (aromatic), 0.98 (methyl or 0.99 Å (methylene bridges) with U iso (H) = 1.2U eq (C) or 1.5U eq (Cmethyl).

Bis(1-ethyl-3-methylimidazolium) tetra-µ-acetato-bis[chloridocuprate(II)] (1)
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.

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