N,N′,N′′ versus N,N′,O imine-containing coordination motifs: ligand-directed synthesis of mononuclear and binuclear CuII compounds

It is shown that tridentate imine ligands can control the nuclearity of copper(II) complexes based on the donor atoms present in the ligand. While the N,N′,N′′-donating imine ligand led to a mononuclear compound, the N,N′,O-donating imine ligand produced a binuclear metal complex.


Chemical context
Copper(II) complexes with imine ligands have attracted much attention in the past few decades due to a variety of possible applications, including catalysis [aerobic oxidation of alcohols (Nairn et al., 2006;Alaji et al., 2014), olefin epoxidation (Das et al., 1997) and ring-opening reactions (John et al., 2007)], and also in medicinal chemistry for both antibacterial (Ali et al., 2015) and antitumour applications (Creaven et al., 2010;Pervez et al., 2016).
Nonmacrocyclic binuclear copper compounds are of interest because they can serve as models for metalloproteins and metalloenzymes, as well as representing interesting subjects for studying molecular magnetism. Strong magnetic exchange is present in the two copper(II) sites of haemocyanin (Chen & Solomon, 2004), which represents a challenge that must be considered when synthetic models are developed. One ISSN 2056-9890 strategy, introduced by Robson (1970), makes use of symmetrical imino ligands containing a phenolate bridge to keep the Cu II atoms close in space. Imines represent an interesting class of ligands because they can be easily synthesized and finetuned to the desired application by introducing extra donor atoms or groups with the desired steric properties into the side chains. A limited number of binuclear copper(II) compounds containing substituted 2-iminomethylphenole ligands have been reported in the literature (Gao et al., 2011;Tang et al., 2008). This kind of structure, where the polydentate ligand has fewer donor atoms than the coordination number of the metal centre, is of interest for the design of more flexible binuclear model compounds.
We describe here the crystal structures of mononuclear (1) and binuclear (2) copper(II) complexes with tridentate iminecontaining ligands obtained by a one-pot synthetic method. The nuclearity of the complexes was shown to be directed by the different donor atoms present in the imine ligand.

Structural commentary
The mononuclear compound 1 has the central Cu II cation in a square-pyramidal coordination environment (Fig. 1a). The Cu II cation is displaced from the least-squares plane defined by the four coordinating atoms of the square base (N1, N2, N3 and Cl2) by 0.334 Å . The bond lengths to these atoms are: Cu-N1 = 2.060 (2), Cu-N2 = 1.978 (2), Cu-N3 = 2.058 (2) and Cu-Cl2 = 2.2639 (8) Å ; the Cu-Cl bond length to the apical Cl1 atom that completes the first coordination sphere is considerably longer, at 2.5013 (8) Å . In order to assess the coordination geometry of copper(II) more quantitatively, the 5 index as defined by Addison et al. (1984) can be used. A perfect square-pyramidal coordination geometry is defined by 5 = 0.0, while it is 1.0 for a perfect trigonal-bipyramidal coordination geometry. For compound 1, 5 is 0.059, indicating an almost perfect square-pyramidal coordination geometry.

Supramolecular features
The presence of a water molecule in the crystal structure of the mononuclear compound 1 leads to the formation of a hydrogen-bonded chain along [101] involving the apical ligand Cl1 ( Fig. 1b and Table 1). In addition, a short contact between the C-H group of the imine group and the apical Cl1 ligand is observed (C5-H5Á Á ÁCl1, Table 1). Finally, a similar C-HÁ Á ÁCl interaction between an aromatic H atom of the pyridine ring and the Cl2 ligand of the square base likewise contributes to the packing in the solid state (C9-H9Á Á ÁCl2, Table 1). Besides these hydrogen bonds, an offsetstacking is observed between adjacent pyridine rings [centroid-tocentroid distance of 3.5709 (18) Å ; symmetry code: Àx, Ày, Àz].
In terms of intermolecular contacts, a single set of hydrogen bonds is present in the crystal structure of 2, established between the non-substituted terminal amine group of N,Ndimethylethylenediamine and the apical chloride ligand Cl1 ( Fig. 2b and Table 2). Similar to compound 1, a nonclassical hydrogen bond between an aromatic H atom of the phenolic ring and the Cl2 ligand also contributes to the intermolecular network (C8-H8Á Á ÁCl2, Table 2). Differing from the structure of 1, a C-HÁ Á Á interaction is observed for compound 2, with a C12-H12Á Á Ácentroid(phenyl) distance of 3.393 (2) Å (symmetry code: Àx + 1, Ày + 1, Àz + 1). The partly occupied water molecule participates in a hydrogen bond with the 2bridging Cl2 ligand (Table 2).

Figure 3
Structures used as queries for the search of the CSD. Analogues of both the mononuclear and binuclear Cu II compounds were searched for. NM represents any non-metal, dashed lines represent any bond type and D9 represents the CuÁ Á ÁCu distance.

Figure 4
Polyhedral representation of the coordination spheres of Cu II in 1 and 2, compared with analogous compounds previously reported in the literature. Square-pyramidal coordination spheres (typical and distorted) are represented in blue and the octahedral coordination sphere in red.

Table 3
Averages of selected bond lengths (as represented in Fig. 3) obtained by searching the CSD for compounds analogous to 1 and 2.  (10) phenolate ligand were considered as analogues of 2. A total of 12 hits were found as analogues of 1, while 11 hits were found for analogues of 2, including both mono-and bis(iminomethyl)phenolate ligands. Averages of selected bond lengths (see representations in Fig. 3) were obtained using ConQuest (Version 1.19) and the statistical analysis module in Mercury (Version 3.9) (Macrae et al., 2008). The averaged values are collated in Table 3 and are in good agreement with the bond lengths in the structures of 1 and 2.
The closest relation to 1 is associated with the nonhydrated analogue (CCDC entry TAWMEK; Yuan & Zhang, 2005), which has the Cu II cation in a more distorted square-pyramidal coordination geometry than 1, with the following bond lengths: Cu-N1 = 2.275 (2), Cu-N2 = 2.104 (2) and Cu-N3 = 2.236 (2) Å , and almost identical Cu-Cl1 = 2.2573 (5) and Cu-Cl2 = 2.22561 (6) Å distances. The Cu II cation is displaced from the mean plane defined by the four coordinating atoms of the square base by 0.622 Å . While for 1 5 = 0.0593, for the structure of TAWMEK 5 = 0.302. The differences in the coordination environment of copper(II) probably arise as a consequence of the presence of the hydrogenbonded network established between the chloride ligands and the water molecules in the crystal structure of 1. The coordination spheres around the Cu II cations in 1 and TAWMEK are compared in Fig. 4.
Regarding the binuclear compound 2, the search returned only two examples of binuclear Cu II complexes containing a single 2 -(monoiminomethyl)phenolate ligand [VAMJIE (Gao et al., 2011) and UFATEB (Tang et al., 2008)]. The two structures have one Cu II cation in a square-pyramidal environment, comprising the tridentate imine ligand, and one octahedrally surrounded Cu II site, bridged by the phenolate and a chloride ligand. Structure 2, on the other hand, comprises a binuclear copper(II) complex with a single 2 -(monoiminomethyl)phenolate ligand that has two Cu II coordination sites in square-pyramidal environments. The coordination spheres around the two Cu II cations in 2 and UFATEB are compared in Fig. 4.

Synthesis and crystallization
Copper(II) chloride dihydrate was purchased from Vetec (Brazil). N,N-dimethylethylenediamine, pyridine-2-carboxaldehyde and salicylaldehyde were purchased from Sigma-Aldrich and used without further purification.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 4. H atoms were placed in calculated positions, with C-H = 0.99 (CH 2 ) or 0.95 Å (CH), with U iso (H) = 1.2U eq (C), and C-H = 0.98 Å (CH 3 ) and U iso (H) = 1.5U eq (C). For structure 1, the H atoms of the water molecule were refined with an O-H distance restraint of 0.82 (1) Å and a HÁ Á ÁH separation of 1.29 (2) Å , and with U iso (H) = 1.5U eq (O). For structure 2, the H atoms of the amine functionality (H3A and H3B) were refined freely. The occupancy of the partly occupied water solvent molecule was refined to a value of 0.11 (1); for this molecule, H atoms were not located and they were not considered in the final model.

Dichlorido[N,N-dimethyl-N′-(pyridin-2-ylmethylidene)ethane-1,2-diamine]copper(II) monohydrate (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 Cu1 0.25867 (6) 0.11867 (2) 0.18700 (5)   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.32 e Å −3 Δρ min = −0.28 e Å −3 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.