Bis{μ-2-methoxy-6-[(methylimino)methyl]phenolato}bis({2-methoxy-6-[(methylimino)methyl]phenolato}copper(II))

The title compound, [Cu2(C9H10NO2)4], is built of discrete centrosymmetric dimers. The CuII atoms are each five coordinated by two deprotonated Schiff base ligands that are bonded differently to the metal atoms. Of the two phenolate O atoms, one is coordinated to one CuII atom, whereas another bridges the two metal atoms. The basal plane of the square pyramid around CuII atoms is formed by the imino N and phenolate O atoms of the bidentate and the monodentate/bidentate Schiff base ligands. The bridging phenolate oxygen occupies the apical position of the coordination sphere with a considerably longer Cu—O bond length. In the crystal, the dimeric molecules pack relative to each other in such a way that the Cu2O2 planes of adjacent dimers are orthogonal.

The title compound, [Cu 2 (C 9 H 10 NO 2 ) 4 ], is built of discrete centrosymmetric dimers. The Cu II atoms are each five coordinated by two deprotonated Schiff base ligands that are bonded differently to the metal atoms. Of the two phenolate O atoms, one is coordinated to one Cu II atom, whereas another bridges the two metal atoms. The basal plane of the square pyramid around Cu II atoms is formed by the imino N and phenolate O atoms of the bidentate and the monodentate/bidentate Schiff base ligands. The bridging phenolate oxygen occupies the apical position of the coordination sphere with a considerably longer Cu-O bond length. In the crystal, the dimeric molecules pack relative to each other in such a way that the Cu 2 O 2 planes of adjacent dimers are orthogonal.

Comment
The Schiff base ligand 2-methoxy-6-iminomethylphenol (HL) (Chatziefthimiou et al. 2006)  The Schiff base, a bright yellow crystalline solid, is usually obtained by the standard method of condensation of the substituted salicylaldehyde with aqueous solution of methylamine in methanol (Meally et al., 2010). In the present work, we used a mixture of 2-hydroxy-3-methoxy-benzaldehyde and methylamine hydrochloride to react with copper powder and transition metal salt, NiCl 2 . 6H 2 O, in an attempt to prepare a heterometallic complex with HL ligand. Details of the used synthetic approach as well as its applications were given by Chygorin et al. (2012a) and Chygorin et al. (2012b).
However, the monometallic [Cu 2 L 4 ] 1 was isolated instead. As there is no evidence of the influence of NiCl 2 . 6H 2 O on the formation of 1 it can be presumed that the given copper complex may be synthesized starting from metallic copper or copper salt as well. To the best of our knowledge no copper complexes of HL have been structurally characterized.
The molecular structure of 1 consists of discrete centrosymmetric dimers [Cu 2 L 4 ] (Fig. 1). The copper atoms are five coordinated each by two deprotonated Schiff base ligands that are bonded differently to the metal centres. Of the two phenolate oxygen atoms, O21 is coordinated to one copper atom, whereas O11 bridges the two metal centres. The basal plane of the square pyramid around copper atoms is formed by the coordination of the imino nitrogen, N27, and phenolate oxygen, O21, atoms of the bidentate Schiff base ligand and N17 and O11 donor atoms of the tridentate L with Cu-O/N distances in the range 1.9044 (7)-2.0032 (8) Å (Table 1). The bridging phenolate oxygen O11{-x + 1,-y + 1,-z + 1} occupies the apical position of the coordination sphere with the bond distance of 2.4329 (8) Å. The elongation of the apical contact is typical for copper (II) complexes. The trans angles at the metal atom are equal to 169.53 (3) and 175.12 (3)°, the cis angles vary from 79.17 (3) to 105.57 (3)°. The deviation of the copper(II) ion from the basal plane is 0.13 Å. The bridge angle Cu1-O11-Cu1{-x + 1,-y + 1,-z + 1} involving the phenolate oxygen is 100.8 (2)°, the separation between the metal centres is about 3.37 Å.
In the crystal lattice, the dimeric molecules pack relative to each other in such a way that Cu 2 O 2 planes of the adjacent dimers are orthogonal (Fig. 2

Experimental
2-Hydroxy-3-methoxy-benzaldehyde (0.30 g, 2 mmol), CH 3 NH 2 . HCl (0.14 g, 2 mmol), NEt 3 (0.3 ml, 2 mmol) were added to 20 ml of methanol and stirred magnetically for 30 min. After that copper powder (0.06 g, 1 mmol) and NiCl 2 . 6H 2 O (0.23 g, 1 mmol) were added to the yellow solution and the mixture was heated to 323 K under stirring for an hour. The resulting green solution was filtered and allowed to stand at room temperature. Dark-green rhombic plates of the title compound were formed next day. They were collected by filter-suction, washed with dry Pr i OH and finally dried in vacuo (yield: 32%).

Refinement
Hydrogen atoms were placed at idealized positions (C-H = 0.95 Å, U iso H = 1.2U eq C for CH, 0.98 Å, 1.5U eq C for CH 3 ) and refined as part of riding models.

Figure 1
Molecular structure of the complex with the numbering scheme (the non-hydrogen atoms shown as 30% thermal ellipsoids). Symmetry code: (i) -x + 1, -y + 1, -z + 1.  Packing diagram viewed down the c axis (CH and CH 3 hydrogen atoms were omitted for clarity).

Bis{µ-2-methoxy-6-[(methylimino)methyl]phenolato}bis({2methoxy-6-[(methylimino)methyl]phenolato}copper(II))
Crystal data  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.004 Δρ max = 0.64 e Å −3 Δρ min = −0.37 e Å −3 Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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.