Crystal structure of ammonium/potassium trans-bis(N-methyliminodiacetato-κ3 O,N,O′)chromate(III) from synchrotron data

In the title complex, the CrIII ion is coordinated to two methyliminodiacetate (mida) dianions acting as tridentate ligands through the N atom and two O atoms of each carboxylate group, in a distorted octahedral geometry. The partial ammonium cation is linked to two O atoms of carboxylate group from neighboring mida groups through N—H⋯O hydrogen-bonding interactions.


Chemical context
Methyliminodiacetate (abbreviated here as mida; C 5 H 7 NO 4 ) can coordinate to a central metal ion as a tridentate ligand through one N atom and two O atoms. The mida ligand differs from iminodiacetate (ida) in the substitution of the imino hydrogen with a methyl group. This change has significant consequences with respect to the configuration of the bischromate(III) complexes with these ligands. Two facial configurations in cis or trans mode relative to the two N atoms have been observed: for example K[Cr(ida) 2 ]Á3H 2 O (Mootz & Wunderlich, 1980) and Na[Cr(ida) 2 ]Á1.5H 2 O (Li et al., 2003) are cis-fac structures whereas Na[Cr(mida) 2 ] is a trans-fac structure (Suh et al., 1997). However, the trans meridional isomer of octahedrally coordinated chromium(III) with ida or mida ligands has not yet been identified. In order to confirm the bonding mode of the methyliminodiacetato ligand and its structural arrangement, we report herein on the crystal structure of the title salt, [(NH 4 ) 0.8 K 0.2 ][Cr(C 5 H 7 NO 4 ) 2 ], (I).

Structural commentary
Counter-ionic species play important roles in crystal packings and hydrogen-bonding patterns. The structure reported here is another example of a [Cr(mida) 2 ] À salt but with a different cation (Suh et al., 1996(Suh et al., , 1997. The structural analysis shows that the two tridentate mida dianions octahedrally coordinate to the Cr III metal atom through one N atom and two carboxylate O atoms in a facial configuration. The coordinating N atoms are mutually trans due to point group 1 for the entire anionic complex. The asymmetric unit of (I) comprises one half of the Cr III complex anion and one occupationally disordered ammonium/potassium cation (situated on a twofold rotation axis), respectively. An ellipsoid plot of title compound together with the atomic numbering is illustrated in Fig. 1.

Figure 2
A packing diagram of (I), viewed perpendicular to the ac plane. Dashed lines represent hydrogen-bonding interactions N-HÁ Á ÁO (cyan).
O bond length (C1-O2 and C3-O4) and smaller O-C-O bond angles of the carboxylate groups in the mida ligand of (I) compared to the ligand in Na[Cr(mida) 2 ] (Suh et al., 1997) may be attributed to the involvement of the two non-coordinating O atoms in hydrogen bonds with the N-H groups of the ammonium cation. The N-C and C-C distances in the mida moieties are close to those found in the free H 2 mida molecule (Shkol'nikova et al., 1986) and are equal to 1.479 (2)-1.494 (2) and 1.508 (3)-1.512 (2) Å , respectively.

Supramolecular features
The pattern of hydrogen bonding around the cation is different from the crystal packing network in the related sodium salt (Suh et al., 1996(Suh et al., , 1997. The cation is linked to four non-coordinating O atoms of carboxylate groups from four neighboring mida ligands through classical N-HÁ Á ÁO hydrogen bonds (Table 1). An array of these interactions generate a three-dimensional network of molecules whereby individual molecules are stacked along the b-axis direction ( Fig. 2).

Synthesis and crystallization
All chemicals were reagent-grade materials and were used without further purification. The starting material, K[Cr(mida) 2 ] was prepared by a method similar to that outlined previously (Wernicke et al., 1977;Uehara et al., 1970). The potassium salt (0.25 g) was dissolved in 15 mL of water at 343 K and added to 5 mL of water containing 0.50 g of NH 4 Cl. The resulting solution was filtered to remove any impurities and allowed to stand at room temperature for several days to give pale pink plate-like crystals of the mixed-occupancy ammonium/potassium salt, (I), suitable for X-ray diffraction analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms of the complex were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H distances of 0.97-0.98 Å and with U iso (H) values of 1.5 (methyl) and 1.2 times U eq (all others) of the parent atoms. The H atoms of the cation were located from difference Fourier maps and refined with DFIX and DANG restraints and fixed N-H distances of 0.855 (9) and 0.869 (9) Å , with U iso (H) values of 1.2U eq (N). The occupancy of mixed-occupied (NH 4 /K) first was refined and then fixed at a ratio of 0.8:0.2. The corresponding (NH 4 /K) sites was refined using EXYZ/EADP commands for the two atom types.     -bis(N-methyliminodiacetatoκ 3 O,N,O′)

Ammonium/potassium trans-bis(N-methyliminodiacetato-κ 3 O,N,O′)chromate(III)
Crystal data [(NH 4 )  Extinction correction: SHELXL2014 (Sheldrick, 2015b), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0118 (19) 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 )