Investigation of nitro–nitrito photoisomerization: crystal structures of trans-{2,2′-[ethane-1,2-diylbis(nitrilomethylidyne)]diphenolato}(pyridine/4-methylpyridine)nitrocobalt(III)

The crystal structures of the title compounds have been studied to clarify the characteristics of the NO2 − ligand as a C—H⋯O hydrogen-bond acceptor, in relation to the solid-state photochemical linkage isomerization.

H 14 N 2 O 2 )(NO 2 )(C 6 H 7 N)], (II), have been investigated to reveal that the intermolecular C Me -HÁ Á ÁO(nitro) contacts in (II) are unfeasible for the nitro-nitrito photochemical linkage isomerization process. In (I), there are two independent complexes showing similar conformations, and the central five-membered chelate ring of the tetradentate salen ligand adopts the same absolute configuration. This is the result of pseudospontaneous resolution, since the configuration of the five-membered chelate ring may frequently be reversed in solution. In the crystals of (I) and (II), the molecules are linked into three-dimensional networks by C-HÁ Á ÁO hydrogen bonds.
achieve the N,O-bidentate transition state toward the nitrito form, in accord with the observed photo-activity of (I). In (II), the cavity of O2-N6-O3 has a tail, which is connected to that of the symmetry-related one, as seen in Fig. 7. These nitro groups are connected via C Me -HÁ Á ÁO hydrogen bonds to form an R 4 4 (12) ring, there being a narrow void around the center of the ring. The photo-stability of (II) suggests that the  The crystal structure of (I), projected along a. The C-HÁ Á ÁO hydrogen bonds are shown as blue dashed lines.

Figure 4
The crystal structure of (II), projected along c. The C-HÁ Á ÁO hydrogen bonds are shown as blue dashed lines.

Figure 5
Comparison of the slices of the cavity around the nitro group within 0.1 Å from the plane in (I) and (II). rotation of the NO 2 À group in its plane will be blocked by the C-HÁ Á ÁO hydrogen bonds. The steric condition of O7-N15-O8 in (I) is similar to that in (II), suggesting that the photoreaction in (I) mainly occurs at the Co1 complex site.

Synthesis and crystallization
Cobalt(II) acetate tetrahydrate, sodium nitrite, H 2 salen, and pyridine/4-methylpyridine (molar ratio 1:1:1:1) were reacted in methanol. Air was bubbled through the solution at 328 K for 1 h to precipitate the title compound. Brown needles of (I) and (II) were grown from a dimethyl sulfoxide solution and an N, N 0 -dimethylformamide solution, respectively, by diffusion of diethyl ether vapour.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. 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 = 0.93-0.97 Å , and U iso (H) = 1.2U eq (C) or 1.5U eq (C Me ). (I): Since the c axis is longer than 40 Å , the overlapping of reflections was avoided in the intensity measurement by a longer sample-to-detector distance than the usual. (II): Six reflections showing poor agreement were omitted from the final refinement.

κN)cobalt(III) (II)
Crystal data [Co(C 16  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.