Crystal structure of trans-bis(diethanolamine-κ3 O,N,O′)manganese(II) bis(3-aminobenzoate)

The title salt, [Mn(C4H11NO2)2](C7H6NO2)2, contains a centrosymmetric cation with the Mn2+ ion coordinated octahedrally by two tridentate diethanolamine (DEA) ligands. The cations are connected to the anions through O—H⋯O and N—H⋯O hydrogen bonds into a three-dimensional network structure.


Chemical context
In contrast to the two other isomers of aminobenzoic acid, viz. p-aminobenzoic acid (or vitamin B 10 ) and o-aminobenzoic acid (or antranylic acid), m-aminobenzoic acid (3-aminobenzoic acid or MABA) is not biologically active. Nevertheless, we are studying this substance within the context of mixed-ligand coordination complex formation including benzoic acid isomers and ethanolamines (Ashurov et al., 2015). As a result of the presence of two spatially separated electrondonor functional groups in the MABA molecule, the reported metal complexes of this ligand are mostly coordination polymers. Polymerization may take place involving both COOH and NH 2 functional groups (Wang et al., 2004;Flemig et al., 2008;Tan et al., 2006;Wei et al., 2006;Shen & Lush, 2010;Wang et al., 2006;), or only one of them: COOH (Kozioł et al., 1992;Murugavel & Banerjee, 2003;Flemig et al., 2008;Tsaryuk et al., 2014) or, more infrequently, NH 2 (Wang et al., 2004).
In discrete monoligand complexes, the MABA molecules coordinate to metal ions only bidentately through the oxygen atoms of the carboxylic group (Ozhafarov et al., 1981) while in mixed-ligand complexes, the carboxylic group can feature mono- (Sundberg et al., 1998;) or bidentate (Palanisami et al., 2013) coordination modes. Coordination through the nitrogen atom is observed only in an Ag complex with participation of the co-ligand p-toluenesulfonate (Smith et al., 1998).
The disposition of MABA molecules as non-coordinating counter-ions (in their benzoate form) is characteristic for mixed-ligand Mn (Fang & Nie, 2011) or Cd complexes (Gao et al., 2011) with 4,4-bipyridine as co-ligand whereas the simultaneous presence of coordinating and non-coordinating MABA species was reported for an Mn complex with 1,10phenanthroline as an additional ligand (Zhang, 2006). Diethanolamine (DEA) ligands can coordinate to metal ions in a mono- (Petrović et al., 2006), bi- (Yilmaz et al., 2000) or tridenentate (Buvaylo et al., 2009) mode if two ligand molecules are situated around the central atom. However, a combination of these modes, for example, in a bi-and tridentate fashion, is also possible (Bertrand et al., 1979).
A search in the Cambridge Structural Database (CSD; Groom & Allen, 2014) revealed that crystal structures have been reported for complexes of MABA and DEA with many metal ions, including zinc, copper, nickel, manganese, cadmium, cobalt, etc. However, no mixed-ligand metal complex including MABA and DEA is documented in the CSD. In order to prepare such compounds, we carried out a synthesis in a solution containing an Mn salt, MABA and DEA. Instead of the desired complex, the title salt, [Mn(C 4 H 11 NO 2 ) 2 ](C 7 H 6 NO 2 ) 2 , consisting of discrete [Mn(DEA) 2 ] 2+ cations and MABA À anions was obtained.

Structural commentary
The asymmetric unit consists of one DEA ligand, one MABA À anion and one Mn 2+ -ion, the latter being located on an inversion centre (Fig. 1). Coordination of the DEA ligand to the metal ion takes place in a tridentate O,N,O 0 mode. The Mn-ligand bond lengths cover a range from 2.065 (2) to 2.096 (2) Å with an angular range of 81.79 (10) to 98.21 (10) , leading to a slightly distorted MnN 2 O 4 octahedron. Since the DEA ligands are in their neutral form, a charged component in the outer sphere is required for charge compensation.
Hence, two MABA À anions in the benzoate form are present per complex ion. The carboxylate group of the anionic molecule is tilted by 14.4 (4) relative to the aromatic ring.

Supramolecular features
The MABA À anion is connected to the complex ion by a pair of rather strong O-HÁ Á ÁO hydrogen bonds involving the DEA hydroxy groups [2.562 (3) and 2.611 (3) Å ; Table 1], which give rise to the formation of a supramolecular motif with graph-set notation R 2 2 (8). The resulting supramolecular cationic:anionic 1:2 units are associated to other such units by relatively weak N-HÁ Á ÁO hydrogen bonds [2.965 (4) and 3.008 (4) Å ; Table 1] involving the secondary amine function of the DEA ligand and one of the H atoms of the MABA À amino group; notably, the second H atom (H1B) of the amino group remains without an acceptor. These four hydrogen bonds associate the different moieties into a three-dimensional network (Fig. 2).

Synthesis and crystallization
To an aqueous solution (5 ml) of MnCl 2 Á4H 2 O (0.098 g, 0.5 mmol) was slowly added an ethanolic solution (5 ml) containing DEA (96 ml) and MABA (0.137 g, 1 mmol) under constant stirring. A light-pink crystalline product was obtained at room temperature by solvent evaporation after 20 days.

Figure 1
The structures of the molecular moieties in the title salt. Displacement ellipsoids are drawn at the 50% probability level and the asymmetric unit is identified by the numbering of its atoms. Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
The crystal packing in the title structure. Hydrogen bonds are shown as dashed lines.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The positions of the O-and Nbound hydrogen atoms were located from difference Fourier maps. Whereas O-bound hydrogen atoms were refined freely, N-bound H atoms were refined with soft distance restraints of 0.98 Å for the secondary amine function and of 0.95 Å for the primary amine function. The C-bound hydrogen atoms were placed in calculated positions and refined as riding atoms with C-H = 0.93 and 0.97 Å for aromatic and methylene hydrogen atoms, respectively, and with U iso (H) = 1.2U eq (C).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq