Structures of substituted pyridine N-oxide with manganese(II) acetate

The synthesis and structures of three coordination polymers composed of manganese(II) acetate and pyridine N-oxide complexes are reported. The pyridine N-oxide, 2-methylpyridine N-oxide, and 4-methylpyridine N-oxide complexes form different networks owing to the substituent group effects.

. Compounds (I) and (II) both have three unique Mn atoms; in both compounds two of them sit on a crystallographic inversion center while the third is on a general position. In compound (III), the single unique Mn atom sits on a general position. Pseudo-octahedral six-coordinate manganese(II) centers are found in all compounds. All of the compounds form chains of Mn atoms bridged by acetate ions and the oxygen atom of the N-oxide in pyridine N-oxide (PNO), 2-methylpyridine N-oxide (2MePNO), or 4-methylpyridine N-oxide (4MePNO). Compound (I) and (II) both exhibit a bound water of solvation. In (I), the water hydrogen bonds to a nearby acetate whereas in (II) the water molecule forms bridging hydrogen bonds between two neighboring acetates. In compound (III) a water molecule of solvation is found in the lattice, not bound to the metal ion but hydrogen bonding to a bridging acetate.

Chemical context
N-Oxides and acetates both have interesting binding modes that facilitate the growth of unique coordination structures. The structures take advantage of the versatility of the acetate ions and the hybridization and dipole at the oxygen atom on the N-oxide. The structures extend to the formation of coordination polymers that have been reported previously (Sarma et al., 2008(Sarma et al., , 2009Sarma & Baruah, 2011). 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). In a previous paper in this series, we examined the initial utility of aromatic N-oxide ligands to form polymeric structures with manganese(II) chloride (Kang et al., 2017). Complexes have also been used previously as metal centers for catalytic transformations (Liu et al., 2014).
In this contribution, we report the synthesis and solid-state structures of three manganese(II) complexes with the versatile mono-or bidentate bridging ligands acetate and three derivatives of pyridine N-oxide . In this study, each of the ligands pyridine N-oxide, 2-methyl and 4-methyl pyridine N-oxide has an impact on the structures of manganese(II) acetate complexes. All three complexes form coordination polymers with the N-oxide bridging in a 2 -1,1 mode and varying acetate ligation. The study was conducted to investigate the utility of both acetate and substituted pyridine Noxide to facilitate the growth of unique coordination polymers. A view of compound I, showing the atom labeling. Displacement ellipsoids are drawn at the 50% probability level, H atoms not involved in hydrogen bonding have been omitted for clarity. [Symmetry codes: (i) Àx + 1, -y + 1, Àz + 1; (ii) x + 1 2 , y À 1 2 , z; (iii) Àx + 3 2 , Ày + 1 2 , Àz + 1.]

Figure 3
A view of compound III, showing the atom labeling. Displacement ellipsoids are drawn at the 50% probability level, H atoms not involved in hydrogen bonding have been omitted for clarity. [Symmetry code: (i) Àx + 1 2 , y À 1 2 , Àz + 3 2 .]   (Fig. 4). In this orientation, the pyridine rings also stack; however, they are not stacked because of the separation distance caused by the methyl group of an acetate ligand in between each aromatic group. Interpolymeric chain hydrogen bonding is observed from the water molecule (O10) on Mn2 to an oxygen atom (O6) on an Mn2bound acetate ligand (Table 1). The structure contains a sixcoordinate metal center at each Mn II atom with all six donor atoms being oxygen. Mn1 sits on an inversion center and is bound trans by two 2 -1,1-PNOs (to Mn2), trans by two 2 -1,3-acetates (to Mn2), and trans by two 3 -1,3,3-acetates (to both Mn2 and Mn3). Mn2 is also six-coordinate with a 2 -1,1-PNO (from Mn1), a 2 -1,3-acetate (from Mn1), and a 3 -1,3,3acetate (from Mn1 and Mn3). Further, the octahedral environment is completed by a water of hydration, a 2 -1,1-acetate (to Mn3), and a 2 -1,3-acetate (to Mn3). Mn3 also sits on an inversion cente, showing an octahedral enviroment where all the six coordinated oxygen atoms belong to acetate ligands. The coordination sphere comprises two 3 -1,3,3-acetates (uniquely bound to Mn2 and Mn1), two 2 -1,1-acetates (to Mn2) and two 2 -1,3-acetates (to Mn2). The 2-methylpyridine N-oxide (2MePNO) complex, compound II, is similar to I in that it is a repeating tetrameric coordination polymer. The polymer crystallizes in the triclinic system, space group P1. The manganese atoms align as an Mn3, Mn2, Mn1, Mn2 chain similar to I with Mn1 and Mn3 sitting on inversion centers. Examining the molecule across the BC vertex, as in I, the Mn3 and Mn1 sit in a line whereas the Mn2 atoms all sit along a different line with respect to this orientation. The atom-to-atom connectivity in the Mn3, Mn2, Mn1, Mn2 repeating unit can best be described as zigzag (Fig. 5). In this orientation, the 2-methylpyridine ring planes are twisted by 85.31 (2) with respect to the Mn1/O2/Mn2 plane with all the methyl groups pointing in two symmetryrelated directions. As observed in I, interpolymeric chain hydrogen bonding is observed from the water molecule (O10) on Mn2 to an oxygen atom on the adjacent Mn2 on the next Crystal packing diagram of compound I, viewed along the b axis. Displacement ellipsoids are drawn at the 50% probability level, H atoms not involved in hydrogen bonding have been omitted for clarity, hydrogen bonds are rendered in blue. Table 1 Hydrogen-bond geometry (Å , ) for I. Symmetry codes: (i) Àx þ 3 2 ; Ày þ 1 2 ; Àz þ 1; (ii) Àx þ 3 2 ; Ày þ 3 2 ; Àz þ 1.

Figure 5
Crystal packing diagram of compound II, viewed along the a axis. H atoms not involved in hydrogen bonding have been omitted for clarity, polymer. However, symmetry dictates that the hydrogen bonding is to oxygen atoms (O5 and O8) on two acetates bound to Mn2 ( Table 2). The structure can be formulated with the same empirical stoichiometry as I, [Mn 4 (2MePNO) 2 (OAc) 7 (H 2 O) 4 ] n . Compound II has one important variation from the PNO derivative outlined above. There is no evidence of the 2 -1,1-acetate bridge found above. While the singular 3 -1,3,3-acetate bridge is retained between Mn2 and Mn3, the 2 -1,1 has been replaced by a 2 -1,3 acetate bridge. This is likely because of the steric demands of the 2-methyl substituent. The 4-methylpyridine N-oxide (4MePNO) complex, compound III, is a repeating coordination polymer with one unique Mn II ion that crystallizes in the monoclinic system, space group P2 1 /n. In the coordination polymer, the manganese atoms are aligned along the b-axis direction. The structure can be formulated as [Mn(4MePNO) 2 (OAc) 4 (H 2 O)] n . The six-coordinate metal center is bridged by two oxygen atoms from 2 -1,1 4MePNO and four 2 -1,3 acetate bridges. The 4MePNO complex molecules alternate above and below the line formed by the manganese atoms. Unlike I and II, compound III only forms intramolecular hydrogen bonding in the polymeric chain (Table 3, Fig. 6). The water observed in the lattice forms a hydrogen bond at 2.21 (2) Å with the O2 atom belonging to one of the acetate 2 -1,3 acetate bridges.

Supramolecular features
The packing of I forms a polymeric structure bisecting the a axis and b axis. Because of the complexity of the structure, many of the details were outlined above. The structure is not linear but forms a zigzag chain in which the bridging acetates and N-oxide ligands connect the Mn ions. There is no evidence for stacking but interpolymeric chain hydrogen bonding is present.
Compound II forms a similar polymeric structure to I, with the chain bisecting the unit cell at ( 1 2 , 0, 1) and ( 1 2 , 1, 0). The chain sets up in a similar fashion as I; however, 2 -1,3 and 3 -1,3,3 are the bridges observed while the 2 -1,1 bridge noted in I is absent.
Compound III forms a polymeric chain which is observed in the b-axis direction. Each manganese(II) atom is bridged by a single 4MePNO and two 2 -1,3 acetate ions. The 4MePNO bridging ligands are alternating up and down in the a-axis direction. There is no evidence for stacking due to the long distance found in the structure with the aromatic rings at separations of 7.334 (6) Å .

Synthesis and crystallization
The manganese(II) coordination polymers were all synthesized by a similar method. 0.245 g (1.00 mmol) manganese(II) acetate tetrahydrate (MnAc 2 Á4H 2 O, FW 245 g mol À1 ) was dissolved in a minimal amount (20 mL) of methanol. 2 molar equivalents of the appropriate N-oxide (0.191 g pyridine N-oxide, PNO; 0.220 g 2-methylpyrdine N-oxide, 2MePNO; 0.220 g 4-methylpyridine N-oxide, 4MePNO) were similarly dissolved in 10 mL of methanol. The N-oxide alcoholic solution was added in one portion to the Mn II one. The combined reaction mixture was stirred for 30 minutes, filtered and the filtrate was allowed to evaporate by slow diffusion. X-ray quality crystals were obtained by precipitation from the mother liquor. A final wash with a minimal amount of methanol was performed to assist with removal of excess Noxide.

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 the compounds, the oxygen-to-hydrogen distances were restrained to a target value of 0.84 (2) Å (using a DFIX command in SHELXL2017; Sheldrick, 2015b) and U iso (H) = 1.5U eq (O).  (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

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.

catena-Poly[[manganese(II)-di-µ 2 -acetato-µ-(4-methylpyridine N-oxide)] monohydrate] (III)
Crystal data  (9) 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.