Crystal structure of bis(μ-4-nitrobenzoato-κ2 O:O′)bis[bis(4-methylpyridine-κN)(4-nitrobenzoato-κ2 O,O′)manganese(II)]

The crystal structure of a dinuclear tetracarboxylate complex of manganese(II) is reported wherein the MnII atoms are bridged by two carboxylate anions.


Chemical context
Polynuclear manganese complexes with carboxylate ligation have received great attention due to their potential applications in catalysis (Arafa et al., 2014), magnetism (Miyasaka et al., 2004) and their antitumor activity (Dey et al., 2015) as well as in other areas. The occurrence of Mn in a number of oxidation states (II-IV) under normal conditions and also the ability of carboxylato ligands to display a variety of coordination modes are the main reasons why Mn-carboxylates have received a lot of attention in the recent past. It has been reported that an Mn-based binuclear complex of composition [Mn 2 (bbppnol)(-O 2 CCH 3 ) 2 ] [bbppnol = N,N 0 -bis(2-hydroxybenzyl)N,N 0 -bis(2-methylpyridyl)-2-ol-1,3-propanediamine] with two bridging acetato ligands is active as a catalyst in the epoxidation of cyclohexene and cyclooctene (Castaman et al., 2009). A series of dimeric complexes with the general formula [Mn 2 (O 2 CCH 3 )L] {where L = 2,2 0 -[2-hydroxy-5-(pivalamidomethyl)-1,3-phenylene]bis(1H-benzo [d]imidazole-4-carboxylic acid), 2,2 0 -(5-benzyl-2-hydroxy-1,3-phenylene)bis (1H-benzo[d]imidazole-4-carboxylic acid) etc.} have been explored as catalysts for the water-oxidation reaction with a view to generating O 2 and H 2 (Arafa et al., 2014). Microwave-assisted alcohol oxidation with tert-butylhydroperoxide (TBHP) has been carried out (Sutradhar et al., 2014) using a Schiff base-containing Mn dimer. Manganese complexes are also recognized for their magnetic behaviour since coordination compounds of this metal often display large ground-state spin (S) values and the polynuclear manganese cluster [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ]Á2CH 3 -COOHÁ4H 2 O is considered to be the first single molecule magnet (SMM) (Uhrecký et al., 2013;Sessoli et al., 1993). Complexes of manganese are also considered to be important ISSN 2056-9890 in view of the occurrence of an Mn 4 Ca unit in the active site of Photosystem II that catalyses the water-splitting reaction to evolve oxygen in nature (Nocera, 2012).
Keeping in mind earlier results published from our laboratory (Chakrabarty et al., 2007) on the synthesis and catalytic properties of cobalt(III)-oxide pseudo-cubane units of the type [Co 4 O 4 (-O 2 CR) 4 L 4 ], where R is an alkyl or aryl group and L is a monodentate pyridyl ligand, and also due to their relevance as catalysts for the water-oxidation reaction (McCool et al., 2011), we explored whether analogous manganese complexes could also be synthesized. These efforts have led to the synthesis of the title complex, among others. Herein we report the synthesis, crystal structure and some salient properties of the dimeric manganese(II) compound [Mn 2 (-NBz) 2 ( 2 -NBz) 2 (4-Mepy) 4 ], I, which belongs to a structure type constituted of only a limited number of complexes (vide infra). Fig. 1 shows the molecular structure of the dimeric complex. The two Mn atoms are related by an inversion centre and are bridged by the carboxylate anions of two NBz ligands in a syn-syn fashion. Each Mn II atom is further coordinated by a carboxylato ligand in chelating mode. The four oxygen atoms -two from a pair of bridging NBz ligands and two from a chelating NBz ligand -are nearly coplanar with each of the central Mn atoms, forming an equatorial plane; the axial positions for both are occupied by two 4-methylpyridine ligands completing the distorted octahedral geometry around each Mn II atom. The bridging Mn-O(carboxyl) bond lengths ($2.1 Å ) are found to be shorter than the Mn-O(carboxyl) distances ($2.3 Å ) in the chelating ligands (Table 1). For the chelating NBz anions, the longer Mn-O distances can be attributed to the steric crowding imposed by the neighbouring bridging bis-monodentate NBz anions.
The highly distorted nature of the MnO 4 N 2 octahedron in the title species, which is probably due to the steric crowding of both the bridging and chelating NBz ligands surrounding the Mn II atom, is manifested by the O-Mn-O and O-Mn-N angles. While the former are in the range 56.95 (4)-150.77 (4) , the latter are in the range 88.02 (5)-94.59 (5) .
In the title compound, the carboxyl -COO and -NOO planes of the chelating NBz anion deviate slightly from the phenyl ring plane, forming dihedral angles of 2.6 (3) and 23.6 (4) , respectively. According to Kaduk (2000) and Kaduk & Golab (1999), completely planar phenyl carboxylates are associated with low conformational energy and any deviation from planarity leads to an increase in the energy of the system. However, this destabilization can be compensated for by efficient crystal packing in the solid state. An ORTEP-style view of the molecular structure of [Mn 2 (-NBz) 2 ( 2 -NBz) 2 (4-Mepy) 4 ] I with displacement ellipsoids drawn at the 50% probability level.

Supramolecular features
The crystal structure of I features several intramolecular as well as intermolecular C-HÁ Á ÁO interactions wherein the O atoms from -NO 2 and -CO 2 groups of the NBz ligand act as hydrogen acceptors (Table 2 and Fig. 2). While the DÁ Á ÁA separations for these weak contacts are in the range of 3.161 (2) to 3.369 (2) Å , the <C-HÁ Á ÁO angles are generally lower than 130 , except in one case where a hydrogen bond with a greater DÁ Á ÁA separation of 3.369 (2) Å forms has an angle of 172 . In addition, intermolecular C-HÁ Á Á interactions involving the pyridyl ring system of the 4-Mepy ligand link the complex molecules into chains along the a axis ( Fig. 3). Although each of the above non-covalent contacts is individually weak, the presence of many of these supramolecular contacts clearly result in extra stability of the species in the solid state. Indeed, the involvement of the -NO 2 and -CH 3 groups at the 4-positions of the phenyl ring of the NBz ligand and the pyridyl ring of the 4-Mepy ligand may explain why the isolation of complexes analogous to I has not been possible for some combinations of carboxylato and pyridyl ligands.

Database survey
A survey of the Cambridge Structural Database (Groom et al. 2016) shows that only a few dinuclear Mn complexes with both bridging and chelating carboxylate linkages are known. We have tabulated some of the available data for complexes of the Table 3 Table 2 Hydrogen-bond geometry (Å , ).

Figure 2
Packing diagram showing C-HÁ Á ÁO interactions (dashed lines) in the crystal structure of I.
[Mn 2 (-NBz) 2 ( 2 -NBz) 2 (4-Mepy) 4 ] a 4.1324 (4) 178.20 (1) (1) , respectively. The more pronounced asymmetry of bonding in the bridging carboxylato groups in I may be ascribed to steric factors and also to differences in molecular packing effects resulting from the presence of substituents on the aromatic rings of both types of ligand.

Synthesis and crystallization
A mixture of MnSO 4 ÁH 2 O (0.845 g, 5 mmol), NaNBz (1.89 g, 10 mmol) and 4-Mepy (1 ml, 10 mmol) was stirred mechanically in water (20 ml) at room temperature for 4 h. The yellow precipitate that appeared was washed thoroughly with water and then with methanol before being dried in a vacuum desiccator over fused CaCl 2 . Yield: 2.58 g (85% based on Mn). Light-yellow transparent crystals of I suitable for X-ray analysis were obtained in 2-3 days from a solution prepared by mixing 2 ml of a methanolic solution of NaNBz (1 mmol) with a solution (

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. Hydrogen atoms were positioned geometrically (aromatic C-H = 0.93 Å , methyl C-H = 0.96 Å ) and were included in the refinement in the ridingmodel approximation, with U iso (H) set at 1.2-1.5U eq (C).

Bis(µ-4-nitrobenzoato-κ 2 O:O′)bis[bis(4-methylpyridine-κN)(4-nitrobenzoato-κ 2 O,O′)manganese(II)]
Crystal data 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.