Crystal structures of four dimeric manganese(II) bromide coordination complexes with various derivatives of pyridine N-oxide

The synthesis and crystal structures of four dimeric complexes composed of manganese(II) dibromide, a pyridine N-oxide and solvent molecules are reported. The pyridine N--oxide, 2-methylpyridine N-oxide, 3-methylpyridine N-oxide, and 4-methylpyridine N-oxide complexes all form similar structures with slight differences owing to the substituent group effects.


Chemical context
N-oxides have interesting binding modes that facilitate the growth of unique coordination structures. Their utility to facilitate organic oxotransfer reactions has been well documented over the years (see, for example, Eppenson, 2003). Many of these reactions are actually catalyzed by transitionmetal interactions with the N-oxide ligands (see, for example, Moustafa et al., 2014). Herein, we report four coordination dimers; however, many of these types of structures extend to the formation of coordination polymers. A recent report shows the utility of pyridine N-oxide to facilitate coordination polymer formation with both zinc(II) and manganese(II) metal ions with a single bifunctional ligand containing an acetate and N-oxide moiety (Ren et al., 2018). These have been reported by us (Lynch et al., 2018;Kang et al., 2017) and others (Sarma et al., 2008(Sarma et al., , 2009Sarma & Baruah, 2011). ISSN 2056-9890 Herein, we report the synthesis and solid-state structures of four pyridine N-oxide manganese(II) dimeric complexes, using pyridine N-oxide (PNO) and its mono-methyl-substituted forms, 2-methylpyridine N-oxide (2MePNO), 3-methylpyridine N-oxide (3MePNO), and 4-methylpyridine N-oxide (4MePNO). This was done to study the impact of substitution of the pyridine on the two-and three-dimensional solid-state structures, and to compare them to previous structures in which the bromide ions are replaced with chloride ions.

General structural details
The pyridine N-oxide complexes form dimers consisting of two Mn II atoms related by an inversion center; the dimer contains a six-coordinate metal center at each Mn II ion with four donor oxygen atoms and two bromides. The Mn1Á Á ÁMn1 0 dimer is bound trans by two 2 -1,1-PNO ligands, and the octahedral environment is completed by a water molecule of hydration or a solvent molecule, non-bridging PNO ligands,

Figure 3
A view of compound III, showing the atom labeling. Displacement ellipsoids are drawn at the 50% probability level. H atoms have been omitted for clarity. [Symmetry code: (i) Àx, Ày + 1, Àz]

Figure 1
A view of compound I, showing the atom labeling. Displacement ellipsoids are drawn at the 50% probability level. H atoms have been omitted for clarity. [Symmetry code: (i) Àx + 1, Ày + 1, Àz + 1] and bromide ions. The dimer is constructed from symmetryrelated atoms and molecules using a crystallographic inversion center of the space group (P1 and P2 1 /n). The molecular structures of compounds I, II, III and IV are given in Figs. 1, 2, 3 and 4, respectively.
Compound II is a dimer with two water molecules bound to each Mn II atom and to only one 2MePNO ligand. The structure has a second 2MePNO molecule not bound to an Mn atom. This unbound 2MePNO is hydrogen-bonded to the bound water molecules of two different dimers, O3Á Á ÁO2 = 2.731 (4) Å and O4Á Á ÁO2 ii = 2.721 (4) Å (Table 2). Neighboring dimers also form hydrogen bonds between bound water molecules and bromide ions, O3-H3BÁ Á ÁBr1 i with a distance of 2.44 (2) Å ( Fig. 6; see Table 2 for hydrogen-bond details and symmetry codes). Combined, these interactions form a hydrogen-bonded chain running parallel to the a axis. Neighboring chains are held together through offsetstacking between the non-bonded 2MePNO ligands (ring N2/ C7-C11), with an inter-centroid distance of the stacked aromatic rings of 3.516 (4) Å , so forming layers parallel to the ac plane (Fig. 6).
The packing in III is similar to that for compound I; however, the aromatic inter-centroid distance is longer than in the other two compounds, 4.545 (5) Å , with a significant centroid shift of 3.221 (9) Å preventing -stacking. Neighboring dimers are linked by O-HÁ Á ÁBr hydrogen-bonds forming chains parallel to the a axis. There are two observed interactions, O3-H3AÁ Á ÁBr1 i with a distance of 2.60 (2) Å and O3-H3BÁ Á ÁBr1 ii with a distance of 2.55 (2) Å ( Fig. 7; see Table 3 for hydrogen-bond details and symmetry codes).

Figure 5
Crystal packing diagram of compound I, viewed down the a axis. C-bound H atoms have been omitted for clarity. Hydrogen-bonding interactions are indicated by dashed lines (Table 1).   Table 4). The inter-centroid distance of the offset -stacked aromatic rings is 3.824 (5) Å between bridging 4MePNO molecules and non-bridging 4MePNO molecules. This results in the formation of chains running parallel to the b axis ( Fig. 8). There is no hydrogen-bonding observed between neighboring dimers in this structure.
Compound IV: Manganese(II) bromide tetrahydrate (0.302 g; 1.05 mmol) was dissolved in a minimal amount (20 ml) of methanol. Two molar equivalents of 4-methylpyridine N-oxide (4MPNO; 0.230 g, 2.11 mmol) were also dissolved in methanol. The solutions were mixed and stirred for 10 min and the solvent was allowed to evaporate to produce a powder (yield: 0.215 g, 44.1%). X-ray quality crystals were grown by recrystallizing a second time from Crystal packing diagram of compound III, viewed looking down the b axis. C-bound H atoms have been omitted for clarity. Hydrogen-bonding interactions are indicated by dashed lines (Table 3).  Compounds I and II have been reported analytically pure, whereas III and IV were not isolated analytically pure. The FT-IR spectra of the four N-oxide complexes all exhibit broad absorbances in the 3500-3100 cm À1 region characteristic of the (O-H) of the coordinated water or methanol molecules. In addition, the (N-O) stretching frequency that is due to the N-oxide pyridyl moiety is observed in the region between 1260 and 1195 cm À1 , as noted previously (Mautner et al., 2017).

Bis(µ-pyridine N-oxide)bis[aquadibromido(pyridine N-oxide)manganese(II)] (I)
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.

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

Bis(µ-2-methylpyridine N-oxide)bis[diaquadibromidomanganese(II)]-2-methylpyridine N-oxide (1/2) (II)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.72 e Å −3 Δρ min = −0.61 e Å −3 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.

Bis(µ-3-methylpyridine N-oxide)bis[aquadibromido(3-methylpyridine N-oxide)manganese(II)] (III)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 1.66 e Å −3 Δρ min = −0.84 e Å −3 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.

Bis(µ-4-methylpyridine N-oxide)bis[dibromidomethanol(4-methylpyridine N-oxide)manganese(II)] (IV)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.64 e Å −3 Δρ min = −0.74 e Å −3 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 )