Manganese(II) chloride complexes with pyridine N-oxide (PNO) derivatives and their solid-state structures

The synthesis and structures of three manganese(II) pyridine N-oxide complexes are presented.


Chemical context
The utility of aromatic N-oxides 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 transition metal interactions with the N-oxide ligands (see, for example, Moustafa et al., 2014). Furthermore, N-oxide metal interactions have recently attracted much interest in a variety of other areas, including metal organic frameworks (MOFs) (Hu et al., 2014). These MOFs synthesized using N-oxide derivatives take advantage of the multiple binding modes of the sp 3 O atom and the ease of modification of the organic backbone of the N-oxide. The utility of the MOFs has been examined in areas such as catalysis (Liu et al., 2014) and sensors (Hu et al., 2014). The constructs extend to the supramolecular study of coordination polymers that have been found in this type of complex ISSN 2056-9890 because of their incredible versatility as ligands (Sarma & Baruah, 2011).
In this context, we report the synthesis and solid-state structures of three pyridine N-oxide manganese(II) complexes. Notably, we used the ligands pyridine N-oxide, 2-methylpyridine N-oxide, and 3-methylpyridine N-oxide to study the impact of substitution of the pyridine on the two-and threedimensional solid-state structures. The pyridine N-oxide (PNO) and 2-methylpyridine N-oxide (2MePNO) complexes form coordination polymers with subtle differences. The 3-methylpyridine N-oxide (3MePNO), however, forms a dimeric complex.

Structural commentary
Complex I exhibits the repeating motif of [MnCl 2 -(PNO)(H 2 O)] n and crystallizes in the triclinic space group P1, containing two formula units per unit cell (Fig. 1). The coordination sphere around each Mn II atom is a distorted octahedron, with the equatorial atoms being two bridging chlorides alternating with two bridging pyridine N-oxide (PNO) molecules (Fig. 2). In the equatorial plane, the bridging chlorides and the bridging pyridine N-oxides are cis to one another. The axial positions are a terminal chloride and a water molecule. The Mn1-O1 bond length is 2.177 (3) Å , whereas the Mn1-O1 vii bond length is slightly longer at 2.182 (3) Å for the bridging PNO [symmetry code (vii) Àx + 1, Ày + 1, Àz + 1]. The bridging chlorides are found to have Mn-Cl2 distances of 2.5240 (19) and 2.532 (19) Å , respectively. Axially, the water is located 2.250 (3) Å from the Mn II cation and the terminal chloride is at 2.479 (2) Å . The bond angles around the equator are severely compressed at the two bridging N-oxides, with the O1-Mn1-O1 i angle observed at 72.03 (10) . The remaining three angles are found to all be similar at 95.58 (7) (Cl2-Mn1-Cl2 i ), 96.80 (8) (O1-Mn1-Cl2), and 94.69 (9) (O1 vii -Mn1-Cl2 vii ). Axially, the bond angle from the water through manganese(II) and the terminal chloride (O2-Mn1-Cl1) is nearly linear at 177.36 (8) .

Figure 2
Crystal packing diagram of compound I, viewed along the b axis. H atoms have been omitted for clarity.
The formation of the polymeric structure in I and II versus the dimer in III is likely due to the steric influence of the methyl group in the 3-position in 3MePNO and the core constituents. One can define the Mn 2 'N-oxide diamond core' in each of the structures as follows: I is alternating Mn 2 Cl 2 and Mn 2 O 2 (oxygen bridges via PNO) cores, II is Mn 2 ClO (oxygen bridge via 2MePNO) and III Mn 2 O 2 (oxygen bridges via 3MePNO). In I, the unsubstituted pyridine N-oxide group does not generate as much steric strain, allowing for polymer formation. In II, the core is formed to permit alternating up and down pyridine N-oxides with the 2-methyl substituents also facing in opposite directions. This limits the steric interactions and the N-oxide slightly tilts out of the polymeric core line to allow the methyl group to effect less steric interactions. In III, the methyl group appears to inhibit polymer formation due to the position of this bulky substituent. Subsequently, when the polymer is not formed, an extra water molecule is required to fill the sixth coordination site on the metal cation occupied by a bridging atom in I and II.

Supramolecular features
The packing of I forms a coordination polymer of alternating bis-bridges of two chlorides and two pyridine N-oxides in the a-axis direction (Fig. 2). The aromatic rings stack at 6.860 (7) Å , outside of -stacking distance due to the alternating chloride and pyridine N-oxide bridges. The single water molecule is locked into weak hydrogen-bonding interactions in two different modes. One hydrogen-bond interaction (H2A) is located down the bridge to the terminal chloride (Cl1), on the adjacent Mn II atom, and the O2-H2AÁ Á ÁCl1 i distance is 2.53 (2) Å . The other hydrogen-bond interaction (H2B) is across to the next polymeric chain with Cl1; the O2-H2BÁ Á ÁCl1 ii distance is 2.52 (3) Å (see Table 1 for hydrogenbond details and symmetry codes).

Figure 4
Crystal packing diagram of compound II, viewed along the b axis. H atoms have been omitted for clarity. and a single chloride in each bridge. Similar to I, the hydrogenbonding interactions are to a terminal chloride (Cl2) on the adjacent Mn II atom. There are two observed interactions, viz. O2-H2AÁ Á ÁCl2 iii with a distance of 2.49 Å and O2-H2BÁ Á ÁCl2 iv with a distance of 2.26 Å (see Table 2 for hydrogen-bond details and symmetry codes). The H2AÁ Á Á Cl2 interaction is in the coordination polymer and the H2BÁ Á ÁCl2 interaction is across the polymeric chains. Similar to I, the aromatic rings stack too far apart to be interacting in the a direction, at a distance of 6.862 (11) Å . As noted above, compound III does not form a coordination polymer but is observed in the solid state as a dimer with two water molecules for each Mn II atom (versus one aqua equivalent in I and II) (Fig. 5). The aromatic inter-centroid distance is longer than in the other two molecules, at 7.902 (7) Å . In compound III, a single water molecule hydrogen bonds from the equatorial plane of one dimer to an axial chloride on another dimer. Conversely, the axial water hydrogen bonds to an equatorial chloride on a different dimer. These interactions are found to be O2-H2BÁ Á ÁCl1 v [distance 2.38 (2) Å ] and O3-H3AÁ Á ÁCl2 vi [distance 2.28 (2) Å ] (see Table 3 for hydrogen-bond details and symmetry codes).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. All carbon-bound H atoms were positioned geometrically and refined as riding, with C-H = 0.95 or 0.98 Å and U iso (H) = 1.2U eq (C) or U iso (H) = 1.5U eq (C) for C(H) and CH 3 groups, respectively. In order to ensure chemically meaningful O-H distances for the bound water molecules in compound I, the H2A-O2 and H2B-O2 distances were restrained to a target value of 0.84 (2) Å (using a DFIX command in SHELXL2017; Sheldrick, 2015b). In compound II, water H atoms were refined as riding, with the O-H distance constrained to 0.892 Å and U iso (H) = 1.5U eq (O) using an AFIX 7 command, and in compound III, H2A-O2, H2B-O2, H3A-O3, and H3B-O3 were restrained using DFIX as for compound I. A rotating-group model was applied for the methyl groups. Structure refinement of II exhibits inversion twinning. Several crystals were tried and the centrosymmetric space group Pnma was tested. In all cases, there was a significant reduction in the R value for the inversion twinning P2 1 2 1 2 1 solution.

(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.

oxide)] (II)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.95 e Å −3 Δρ min = −0.73 e Å −3 Absolute structure: Refined as an inversion twin. Absolute structure parameter: 0.44 (8) 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. Refinement. Refined as a 2-component inversion twin.

Bis(µ-3-methylpyridine N-oxide)bis[diaquadichloridomanganese(II)] (III)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.56 e Å −3 Δρ min = −0.41 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.