Crystal structure of tetraaquabis(pyrimidin-1-ium-4,6-diolato-κO 4)manganese(II)

The crystal structure of the MnII complex of 4,6-dihydroxyprimidine (L), [MnL 2(H2O)4], shows that the ligand coordinates to the metal ion through one deprotonated hydroxy group from each of two ligands.


Chemical context
H-tautomeric forms of 4,6-dihydroxypyrimidine (DHP) are known to exist and are associated with low disproportionation energies (Katrusiak & Katrusiak, 2003). Although crystal structures have been reported where cobalt(II) and nickel(II) are coordinated by the 4,6-dihydroxypyrimidine ligand through a ring nitrogen atom (Huang et al., 2005;Wang et al., 2006), prior to this report no complexes with ligation through a phenolate oxygen atom have been reported even though this mode of coordination does occur in complexes of 3,6-dihydroxypyridizine (Shennara et al., 2015).

Structural commentary
Crystallographic analysis reveals that the title compound consists of a centrosymmetric mononuclear [Mn(C 4 H 3 N 2 O 2 ) 2 -(H 2 O) 4 ] complex in which the Mn II ion is in an O 6 environment that is close to octahedral. Two deprotonated 4,6-dihydroxypyrimidine ligands coordinate through the phenolate oxygen atom (O1) at axial positions, while four water molecules occupy the equatorial sites (Fig. 1). The bond lengths in the pyrimidine ligand are very similar to those found for the ISSN 2056-9890 Co and Ni complexes in which, however, ligation to the metal is through a nitrogen atom. For all three complexes, the structures indicate a zwitterionic form of the ligand resulting from transfer of a proton from the hydroxyl group to a ring nitrogen atom. Others have reported variability in the Htautomeric forms of 4,6-dihydroxypyrimidine associated with low disproportionation energies (Katrusiak & Katrusiak, 2003). The structure of the complex includes an intramolecular hydrogen bond between an aqua ligand (O2W) and the non-protonated N 3 ring atom (N2) ( Table 1).

Figure 2
Diagram showing how the molecules link up into chains through the formation of C(6)[R(6)R 2 2 (8)] hydrogen bonds. Atomic displacement parameters are drawn at the 50% probability level. Hydrogen bonds are shown as dashed lines.

Figure 3
Diagram showing one of the two mutually perpendicular chains linked through the formation of C(6)[R 2 3 (8)] hydrogen bonds. Atomic displacement parameters are drawn at the 50% probability level. Hydrogen bonds are shown as dashed lines.

Synthesis and crystallization
0.5 mM aqueous solutions of the ligand and anhydrous MnCl 2 , both purchased from Aldrich, were adjusted to pH 5.5 with NaOH/HCl and then mixed together in a 1:2 stoichiometry. The solutions were left to crystallize slowly at room temperature. Light-yellow crystals formed over two weeks. Room-temperature X-band EPR spectra of powdered crystals exhibited a single broad line centered at a g-value of near to 2.0 with a peak-to-peak line width of 660 G, the breadth of which indicates MnÁ Á ÁMn magnetic interactions, although not as strong as in the related maleic hydrazide (MH), Mn(MH) 2 (H 2 O) 4 , complex, for which a line width of 920 G was found (Shennara et al., 2015). EPR spectra of aqueous solutions of the title complex had g = 2.006 and A iso (Mn) = 95.2 G, similar to that of the Mn(MH) 2 complex

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were positioned geometrically and refined as riding: C-H = 0.95 Å with U iso (H) = 1.2U eq (C). N-H and O-H hydrogen atoms were refined isotropically without restrictions on the bond lengths. Four reflections which were obvious outliers were omitted from the refinement (132, 163, 100, 011). Acta Cryst. (2017). E73, 620-622 research communications Table 2 Experimental details.

Tetraaquabis(pyrimidin-1-ium-4,6-diolato-κO 4 )manganese(II)
Crystal data [Mn(C 4  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.