Hexaaquamanganese(II) bis{[N-(3-methoxy-2-oxidobenzylidene)glycylglycinato]copper(II)} hexahydrate

The ligand N-(2-hydroxy-3-methoxybenzylidene)glycylglycine (H3 L), a Schiff base derived from glycylglycine and 3-methoxysalicylaldehyde, was used in the synthesis of a new organic–inorganic coordination complex, [Mn(H2O)6][Cu(C12H11N2O5)]2·6H2O. The MnII atom is located on an inversion center and is coordinated to six water molecules in a slightly distorted octahedral geometry. The CuII atom is chelated by the tetradentate Schiff base ligand in a distorted CuN2O2 square-planar coordination. In the crystal structure, the complex [Mn(H2O)6]2+ cations and the [CuL]− anions are arranged in columns parallel to the a axis and are held together by O—H⋯O hydrogen bonding. Additional hydrogen bonds of the same type further link the columns into a three-dimensional network.


Related literature
Transition metal complexes of salicylaldehyde-peptide-and salicylaldehyde-amino-acid-derived Schiff bases are suitable non-enzymatic models for pyridoxal amino acid systems, which are of considerable importance as key intermediates in metabolic reactions, see: Bkouche-Waksman et al. (1988); Wetmore et al. (2001); Zabinski & Toney (2001). For the preparation, structural characterization, spectroscopic and magnetic studies of Schiff base complexes derived from salicylaldehyde and amino acids, see: Ganguly et al. (2008) and references cited therein. For Schiff bases derived from simple peptides, see: Zou et al. (2003).

Comment
Transition metal complexes of salicylaldehyde-peptides and salicylaldehyde-amino acid Schiff-bases are non-enzymatic models for pyridoxal-amino acid systems, which are of considerable importance as key intermediates in many metabolic reactions of amino acids catalyzed by enzymes (Zabinski et al., 2001;Wetmore et al., 2001;Bkouche-Waksman et al.,1988). Considerable effort has been devoted to the preparation, structural characterization, appropriate spectroscopic and magnetic studies of Schiff-base complexes derived from salicylaldehyde and amino acids and reduced salicylidene amino acids (Ganguly et al., 2008), but little attention has been devoted to Schiff bases derived from simple peptides (Zou et al., 2003). The asymmetric unit of structure (I) consist of one-half of a [Mn(H 2 O) 6 ] 2+ cation (completed by crystallographic inversion symmetry), one [CuL]anion and three water molecules ( Fig. 1 and Table 1). The coordination environment of the Cu II atoms is approximately square-planar. The Schiff-base ligand is deprotonated, thus acting as a triple negatively charged tetradentate ONNO chelate. It coordinates to the Cu II atom via one phenolic oxygen atom (O2), one deprotonated amide nitrogen atom (N2), one imino nitrogen atom (N1) and one carboxylate oxygen atom (O4). The two Cu-N bond distances  Table 1).

Experimental
The Schiff base was prepared through the condensation of glycylglycine and 3-methoxy-salicylaldehyde. Glycylglycine (10 mmol) was dissolved and refluxed in absolute methanol (40 ml) containing LiOH . H 2 O (10 mmol). After cooling to room temperature, a solution of 3-methoxy-salicylaldehyde (10 mmol) in absolute methanol was added slowly under stirring for 10 min. Then Cu(NO 3 ) 2 (10 mmol) was added to the HLLi solution and the resulting solution was adjusted to pH = 9-11 by 1.0 mol/L NaOH solution. After stirring at room temperature for 30 min, the volume was reduced to ca. 5 ml in vacuo.
supplementary materials sup-2 Anhydrous ethanol was added to precipitate the product, which then was recrystallized in methanol solution. Na [CuL] . 2H 2 O (2 mmol) was dissolved in 10 ml water. Then MnCl 2 . 4H 2 O (1 mmol) was added to the solution under stirring. The resulting crude product was precipitated. It was recrystallized in hot water 363 K and filtered. The filtrate was allowed to evaporate slowly at room temperature. After several days red to violet crystals suitable for X-ray diffraction were obtained.

Refinement
The water H atoms in the complex were located in a difference Fourier map and were refined with a distance restraint of O-H = 0.85 Å and U iso (H) = 1.5U eq (O). All other H atoms were positioned geometrically and were constrained as riding atoms, with C-H distances of 0.93-0.97 Å and U iso (H) set to 1.2 or 1.5U eq (C) of the parent atom.

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.
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 Rfactors(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.