Crystal structure of anhydrous poly[bis(μ2-sarcosinato-κ3 O,N:O′)copper(II)]

The copper(II) ion of the anhydrous form of bis(sarcosinato)copper(II) exhibits a [4 + 2] coordination sphere with four shorter equatorial bonds to the N and carboxylate O atoms of two sarcosinate anions, and two longer axial bonds to O atoms of neighboring complexes, leading to a sheet structure parallel to (001).

The title compound, [Cu(C 3 H 6 NO 2 ) 2 ] n , is a bis-complex of the anion of sarcosine (N-methylglycine). The asymmetric unit consists of a copper(II) ion, located on a center of inversion, and one molecule of the uninegative sarcosinate anion. The copper(II) ion exhibits a typical Jahn-Teller distorted [4 + 2] coordination geometry. The four shorter equatorial bonds are to the nitrogen and carboxylate O atoms of two sarcosinate anions, and the longer axial bonds are to carboxylate O atoms of neighboring complexes. The overall structure is made up from two chains formed by these longer axial Cu-O bonds, one extending parallel to [011] and the other parallel to [011]. Each onedimensional array is connected by the equatorial bridging moieties to the chains on either side, creating an extended two-dimensional framework parallel to (100). There is a single intermolecular hydrogen-bonding interaction within the sheets between the amino NH group and an O atom of an adjacent molecule.

Chemical context
The -amino acids are essential for life as they are the building blocks of all proteins and enzymes and a great deal is known about their structures and complexes. N-Methyl amino acids, such as sarcosine, are non-proteinogenic and hence differ from the proteinogenic amino acids used in living systems in that the amino N atom is achiral in the free molecule but chiral, R or S, when bound to a metal. Examples of complexes of sarcosine that exhibit chirality due to coordination of the amino N atom have been reported (Blount et al., 1967;Larsen et al., 1968;Prout et al., 1972). This is similar to the chirality that is observed on the binding of reduced tripodal Schiff base complexes of metals (Brewer et al., 2014;Al-Obaidi et al., 1996). In these cases, the binding of three achiral (due to rapid inversion) amine N atoms of the free ligand to the same metal resulted in the observation of a single enantiomeric pair (RRR and SSS) or a single enantiomer (RRR or SSS) if the molecule crystallized in one of the Sohncke space groups. In these cases, the binding of an organic ligand containing three achiral N atoms to a metal resulted in a preference for chirality correlation of the N atoms, RRR or SSS, resulting in homochiral complexes. Similarly, the reduced Schiff base complexes of the condensate of amino acids with salicylaldehyde have an energetic preference for the stereoisomer in which the chirality of the -C atom and the amine N atom are correlated (Koh et al., 1996).

ISSN 1600-5368
This preference for homochirality is not always observed: the copper complex of a Schiff base condensate of tyrosine is heterochiral as is the bis-adduct of cobalt(III) with histidine (Pradeep et al., 2006;Zie et al., 2007). The present complex was investigated to determine if there was a preference for homo-(RR or SS) versus heterochirality (RS) in a M(sarcosinato) 2 complex (M = divalent transition metal). Heterochirality, RS, was observed in this complex. Future work will focus on related complexes such as M 0 (sarcosinato) 3 (M 0 = trivalent transition metal) to determine if the presence of three chiral ligands bound to a single metal favors homochirality, which can serve as a method of enantiomeric separation.

Structural commentary
The title compound, [Cu(C 3 H 6 NO 2 ) 2 ] n , is a bis-complex of the sarcosinate anion with copper(II). The central metal cation is located on a center of inversion. It is six-coordinate and has a distorted octahedral [4 + 2] coordination sphere characteristic for Jahn-Teller systems. The four shorter equatorial bonds are to the amino N atom and carboxylate O atom of two sarcosinate anions (Fig. 1). The N and O atoms are trans to one another. The related [Cu(sarcosinato) 2 ]Á2H 2 O structure (Krishnakumar et al., 1994) is much simpler in that the two longer axial bonds are to water molecules so that there is no extended bonding to neighboring complexes. In both structures, the equatorial Cu-O and Cu-N bond lengths are very similar [Cu-O = 1.9758 (8) Å and Cu-N = 2.0046 (9) Å in the title compound, and 1.970 and 2.007 Å in the dihydrate], but the axial Cu-O distances are significantly different at 2.5451 (10) and 2.461 Å .

Supramolecular features
In the title compound, the individual coordination polyhedra are linked by longer axial Cu-O bonds into two chains, one extending parallel to [011] and the other parallel to [011]. The one-dimensional array is linked by equatorial bridging bonds to the chains on either side, creating an extended twodimensional framework (Fig. 2) parallel to (100). There is a single intermolecular hydrogen-bonding interaction within the sheets between the amino NH and an carboxylate O atom of an adjacent molecule (Table 1).

Database survey
The structure of the zwitterionic form of sarcosine has been reported by Rodrigues et al. (2005). The structure of the copper(II) and nickel(II) complexes of this same ligand have been reported as their dihydrates by Krishnakumar et al. (1994) and Guha (1973), respectively.

Figure 2
Packing diagram of the title compound viewed along the a axis. Hydrogen bonding is shown by dashed lines.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were positioned geometrically and refined using a riding model, with C-H distances of 0.93-0.99 Å , and with U iso (H) = 1.2U eq (C) or 1.5U eq (C) for methyl H atoms. The H attached to N was located in a difference Fourier map and refined using a riding model, with an N-H distance of 0.93 Å and U iso (H) = 1.2U eq (N).