Investigation of nitro–nitrito photoisomerization: crystal structures of trans-bis(acetylacetonato-O,O′)(pyridine/4-methylpyridine/3-hydroxypridine)nitrocobalt(III)

Study of the crystal structures of the title compounds reveals that the solid-state photochemical nitro–nitrito linkage isomerization is restricted by intermolecular C—H⋯O,O contacts in the 3-hydroxypyridine phase.


Chemical context
Solid-state reactions are restricted by the cage effect, which is helpful for stereo-selectivity, but it sometimes interrupts the reaction. The photochemical nitro-nitrito linkage isomerization in crystals was investigated for the salts of [Co(NH 3 ) 5 (NO 2 )] + , and indicated that insufficient free space around the nitro ligand prevents the isomerization from occurring (Boldyreva, 2001). For the salts of trans-[Co(en) 2 -(NO 2 )(NCS)] + , a certain geometry of the intermolecular N-HÁ Á ÁO hydrogen bonds restricts the photoisomerization (Ohba et al., 2018). In the present study, we investigated another type of nitrocobalt complex, trans-[Co(acac) 2 (NO 2 )-(X-py)], where acac stands for acetylacetonate ion, and X-py = pyridine (I) or pyridine derivative; 4-Me-py (II), 3-OH-py (III), and 3-Me-py (IV). The photoactivity of (I) in the solid state had been reported based on the infrared spectra while irradiated with a high-pressure mercury arc, a remarkable increase in absorption in the region 1000-1050 cm À1 being detected (Johnson & Martin, 1969). This is due to the symmetric N-O stretching mode of the nitrito form, and it corresponds to 1055 cm À1 for [Co(NH 3 ) 5 ONO]Cl 2 (Heyns & de Waal, 1989). The molecular structure of (I), showing displacement ellipsoids at the 30% probability level. Symmetry code: (i) x, Ày + 1, z.

Figure 2
The molecular structure of (II), showing displacement ellipsoids at the 30% probability level. Symmetry code: (i) x, Ày + 1, z. One of the two set of H-atom positions of the C18 methyl group is omitted for clarity.

Figure 3
The molecular structure of (III), showing displacement ellipsoids at the 30% probability level. Symmetry code: (i) x, Ày + 3 2 , z. The minor occupancy O5A/O5B atoms of the nitro group and one of two possible positions of the water molecule O7 are omitted for clarity.

Structural commentary
The molecular structures of (I)-(III) are shown in Figs. 1-3, respectively. In these crystals, the complex has crystallographic mirror symmetry, and the py/4-Me-py/3-OH-py ligands and the cobalt atom lie on a mirror plane. The nitro group also lies on the mirror plane in (I) and (II). However, in (III) the nitro group shows positional disorder, and the major component [O4-N8-O4 i , 67.2 (16)%] is oriented perpendicular to the mirror plane. The minor component [O5A-N8-O5B, 16.4 (8)%] and the water molecule (O7) are disordered near the mirror. The Co-N(nitro) bond distances are 1.923 (9) Å in (I), 1.949 (10) Å in (II) and 1.915 (3) Å in (III). In each case, a distorted trans-CoN 2 O 4 octahedral coordination polyhedron arises.

Supramolecular features
The crystal structures of (I)-(III) are shown in Figs. 4-6, respectively. In (I) and (II), the molecules are connected by C-HÁ Á ÁO hydrogen bonds (Tables 1-3), forming chains propagating along the a-axis direction. In (III), the complex molecules are connected via O-HÁ Á ÁO hydrogen bonds involving the water molecules, forming layers lying parallel to (010).
Slices of the reaction cavities around the nitro group near its plane in (I)-(IV) are compared in Fig. 7, where the radii of the neighboring atoms are assumed to be 1.0 Å greater than the corresponding van der Waals radii (Bondi, 1964)  The crystal structure of (I), projected along c. The C-HÁ Á ÁO hydrogen bonds are shown as blue dashed lines.

Figure 5
The crystal structure of (II), projected along c. The C-HÁ Á ÁO hydrogen bonds are shown as blue dashed lines.

Figure 8
The steric circumstance of the nitro group in (I). Only parts of the complex are shown for clarity. The C-HÁ Á ÁO hydrogen bonds are shown as blue dashed lines. The green dashed lines indicate other OÁ Á ÁH contacts shorter than 2.8 Å , O5Á Á ÁH15 iv =2.73 Å . Symmetry codes:

Figure 9
The steric circumstance of the nitro group in (II). Only parts of the complex are shown for clarity. The C-HÁ Á ÁO hydrogen bonds are shown as blue dashed lines. The green dashed lines indicate other OÁ Á ÁH contacts shorter than 2.8 Å . Symmetry codes:

Figure 10
The steric circumstance of the nitro group in (III
this ring will block the rotation of NO 2 À to become a nitrito form.

Synthesis and crystallization
The title compounds were prepared according to the method of Boucher & Bailar (1965) from Na[Co(acac) 2 (NO 2 ) 2 ] and the appropriate pyridine derivative. Dark-red plates of (I), dark-red prisms of (II) and dark-red needles of (III) were grown from acetonitrile, nitromethane and methanol solutions, respectively.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The H atoms bound to C were positioned geometrically, the methyl H atoms being introduced by an HFIX 137 command. They were refined as riding, with C-H = 0.93-0.96 Å , and U iso (H) = 1.2U eq (C) or 1.5U eq (C methyl ). (I): two reflections showing poor agreement with I obs much smaller than I calc were omitted from the final refinement. (II): one reflection showing poor agreement was omitted. The DELU instruction was applied to C15 and C18 to avoid the 10 s.u. of the Hirshfeld test difference. (III): six reflections showing poor agreement were omitted. The minor occupancy nitro atoms O5A and O5B were refined anisotropically with an ISOR instruction. The H atoms bound to O were positioned from difference density maps, and their positional parameters were refined with the geometry restrained and with U iso (H) = 1.5U eq (O). Compounds (I) and (II) were refined as inversion twins. For all structures, data collection: APEX3 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008) and CAVITY (Ohashi et al., 1981); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b) and publCIF (Westrip, 2010).

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.

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

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.

trans-Bis(acetylacetonato-κ 2 O,O′)(3-hydroxypyridine-κN)(nitro)cobalt(III) monohydrate (III)
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.