Crystal structure of bis(2-{[1,1-bis(hydroxymethyl)-2-oxidoethyl]iminomethyl}-6-methoxyphenolato)manganese(IV) 0.39-hydrate

Using a predesigned tripodal Schiff-base ligand, a monomeric octahedral MnIV complex has been synthesized and its structure determined at 100 K.

The title compound, [Mn(C 12 H 15 NO 5 ) 2 ]Á0.39H 2 O, is a 0.39 hydrate of the isostructural complex bis(2-{[1,1-bis(hydroxymethyl)-2-oxidoethyl]iminometh-yl}-6-methoxyphenolato)manganese(IV) that has previously been reported by Back, Oliveira, Canabarro & Iglesias [Z. Anorg. Allg. Chem. (2015), 641, 941-947], based on room-temperature data. The current structure that was determined at 100 K reveals a lengthening of the c cell parameter compared with the published one due to the incorporation of the partial occupancy water molecule. The title compound crystallizes in the tetragonal chiral space group P4 1 2 1 2; the neutral [Mn IV (C 12 H 15 NO 5 ) 2 ] molecule is situated on a crystallographic C 2 axis. The overall geometry about the central manganese ion is octahedral with an N 2 O 4 core; each ligand acts as a meridional ONO donor. The coordination environment of Mn IV at 100 K displays a difference in one of the two Mn-O bond lengths, compared with the room-temperature structure. In the crystal, the neutral molecules are stacked in a helical fashion along the c-axis direction.
Remarkably, the current structure that was determined at 100 K reveals shortening of the a cell parameter compared with the published one [8.0953 (2) (1)

Structural commentary
The title compound (1) crystallizes in the tetragonal chiral space group P4 1 2 1 2; the neutral [Mn IV (C 12 H 15 NO 5 ) 2 ] molecule is situated on a crystallographic C 2 axis, hence the asymmetric unit comprises one half of the metal complex and the O atom of a water molecule with occupancy 0.195 (15) (Fig. 1) (Kessissoglou et al., 1987;Pradeep et al., 2004). The MnO 4 equatorial fragment is approximately square planar, the maximum deviation from the mean plane being about 0.11 Å . The ranges of cis and trans angles at the metal atom are 84.14 (18)-98.44 (18) and 168.6 (3)-172.89 (18) , respectively ( Table 1). The Mn-N distance is longer than the average Mn-O distance by approximately 0.1 Å . This is significantly larger than the difference in covalent radii of N and O. Thus, the primary distortion of the MnN 2 O 4 octahedron is axial elongation along the MnN 2 axis.
The molecular structure of (1) closely resembles that of the Mn II complex of the same ligand, [Mn II (H 3 L) 2 ]Á2CH 3 OHÁ-0.5H 2 O (refcode ROMROB; Zhang et al., 2009) (Fig. 2). The latter crystallizes in the monoclinic space group P2 1 /n and has no crystallographically imposed symmetry. There is a marked increase in the ROMROB Mn II -O(H) bond length (mean 2.134 Å ) when (1) is compared to ROMROB which has two additional protons to compensate for the two additional electrons. In ROMROB, the Mn II -O(phenolate) and Mn II -N(imine) bonds are also elongated (mean lengths 2.011 and Table 1 Selected geometric parameters (Å , ).

Figure 1
The molecular structure of the title complex, showing the atomnumbering scheme. Non-H atoms are shown with displacement ellipsoids at the 50% probability level. Labelled atoms are related to unlabelled atoms by the symmetry operation y, x, Àz + 1.

Figure 2
Scheme showing the structure of the closely related ROMROB Mn II complex.
2.027 Å , respectively). (1) thus provides a rare structural example of variations in the metal coordination sphere to accommodate change in the metal oxidation state. The flexibility of the lattice, formed using the partly deprotonated H 4 L ligand, permits distortion of the structure in the solid state to allow for changes in the charge and spin state of the Mn atom without disrupting the integrity of the crystal structure.

Supramolecular features
In the crystal lattice, individual [Mn IV (H 2 L) 2 ] molecules are stacked in a helical fashion along the c axis, as shown in Fig. 3, with the minimum MnÁ Á ÁMn distances inside a column being 10.28 Å . Molecules that are translated by one unit cell in the aaxis direction [MnÁ Á ÁMn distance equals the a-axial length, 8.0953 (2) Å ] are intertwined by intermolecular hydrogen bonds between the hydroxyl groups and phenolic and methoxy oxygen atoms. There is also a possible hydrogen-bonding interaction between one hydroxyl group (O113) and the solvent water molecule (O1) considering the O113Á Á ÁO1 distance of 2.17 (2) but as the H atoms on O1 could not be located this contact could not be confirmed. Details of the hydrogen bonding are given in Table 2.  Kessissoglou et al., 1987;Chandra et al., 1990;Pradeep et al., 2004).

Synthesis and crystallization
2-Hydroxy-3-methoxy-benzaldehyde (0.30 g, 2 mmol) and tris(hydroxymethyl)aminomethane (0.24 g, 2 mmol), were added to methanol (20 ml) and stirred magnetically for 30 min. Next zinc powder (0.07 g, 1 mmol) and MnCl 2 Á4H 2 O (0.20 g, 1 mmol) were added to the yellow solution and the mixture was heated to 323 K under stirring until total dissolution of the zinc powder was observed (1 h). The resulting brown solution was filtered and allowed to stand at room temperature. Black microcrystals of the title compound were formed in several days. They were collected by filter-suction, washed with dry Pr i OH and finally dried in vacuo (yield: 43%). The IR spectrum of powdered (1) in the range 4000-400 cm À1 shows all the characteristic Schiff base vibration bands: (OH), (CH) and (C N) at 3400, 3000-2840, and 1602 cm À1 , respectively (see Supplementary data). A strong peak at 1618 cm À1 is due to the bending of the H 2 O molecule, providing evidence of the presence of water in (1). The major feature of the X-band solid-state EPR spectrum of (1) at 77 K is a strong and broad signal at g $4 and a weak but resolved response at g $2 (see Supplementary data). This corresponds to strong axial distortion with small zero-field splitting, 2D >> h (h 0.31 cm À1 at the X-band frequency) in agreement with structural findings. The 55 Mn hyperfine structure is not resolved.

Figure 3
Crystal packing of (1) showing the helical arrangement of Mn IV (H 2 L) 2 molecules in the c-axis direction. H atoms are not shown.
Associated hydrogen atoms were not located. The OH hydrogen atoms H112 and H113 were refined using a riding model with U iso (H) = 1.5U eq (O). All hydrogen atoms bound to carbon were included in calculated positions and refined using a riding model with isotropic displacement parameters based on those of the parent atom [C-H = 0.95 Å , U iso (H) = 1.2U eq (C) for CH and CH 2 , 1.5U eq (C) for CH 3 ].