Investigation of solid-state photochemical nitro–nitrito linkage isomerization: crystal structures of trans-bis(ethylenediamine)(isothiocyanato)nitritocobalt(III) salts: thiocyanate, chloride monohydrate, and perchlorate–thiocyanate(0.75/0.25)

The crystal structures of the title compounds have been studied to confirm that the solid-state photochemical nitro–nitrito linkage isomerization is restricted by the reaction cavity of the nitrite ion in the thiocyanate salt.


Chemical context
The nitrite ion is one of the well-known ligands that show linkage isomerism even in the solid state (Hatcher & Raithby, 2013). Adell (1971) prepared trans-[Co(en) 2 (NO 2 )(NCS)]X (en = ethylenediamine, X = a counter-anion and a solvent molecule if incorporated into the crystal structure) to show that irradiation by sunlight or visible light ( > 430 nm) alters the color of the crystals from orange to red for perchlorate and nitrate salts, indicating nitro-nitrito photochemical isomerization, but not for thiocyanate. These facts suggest that the photo-isomerization is interrupted by some steric condition in (I) where X = SCN À . Bö rtin (1976) determined the crystal structure of (I), but failed to find the steric obstacles to the reaction, and the puzzle has been left unsolved. Kubota & Ohba (1992) investigated the solid-state nitro-nitrito photochemical reaction of [Co(NH 3 ) 5 NO 2 ]Cl 2 to show that the shape of the reaction cavity in the nitro plane is of crucial importance. It is noted that not only the steric condition around the nitro group, but also the electronic effects of the co-existing ligands are important for the longer lifetime of the much less stable nitrito form (Miyoshi et al., 1983), the thiocyanate ligand at the trans position being favorable. When the powders were irradiated by a 150 W Xe lamp without filtering, the color changed immediately from yellow to orange for (II) and (III) but not for (I), in agreement with the observations of Adell (1971). In the present study, the structures of the three ISSN 2056-9890 title crystals were investigated to reveal the steric conditions that make (I) photo-inactive.

Structural commentary
The crystal structure of (I) has been redetermined in the present study with a more sophisticated treatment of the disorder of thiocyanate ions [R(F 2 ) = 0.048 for 2845 observed reflections] than that reported by Bö rtin (1976) [R(F) = 0.077 for 1970 reflections], and the s.u.'s of the bond lengths were reduced to less than half of the previous values. The molecular structures of (I)-(III) are shown in Figs. 1-3, respectively. The coordination geometry around the Co atoms is octahedral, and the Co-N(nitro) bond lengths are similar to one another, 1.905 (3) Å in (I), 1.912 (2) Å in (II) and 1.915 (4) and 1.916 (4) Å in (III). The conformations of the ethylenediammine ligands are gauche in (I) and (III), and envelope in (II). The short C17-C18 distance of 1.417 (8) Å in (I) may be an artifact of unresolved disorder over two orientations by the puckering of the chelate ring as mentioned by Bö rtin (1976). The combination of the two ethylenediamine chelate rings in each complex is and , and the Co(en) 2 moiety possesses approximate mirror symmetry. In (I), there are two independent thiocyanate counter-ions, which are disordered around twofold axes and are therefore half occupied. In (II), there is a chloride counter-ion and an ordered water molecule of crystallization. In (III), one of the two perchlorate ions (Cl4/O16-O19) lies on a center of symmetry, showing orientational disorder. Furthermore, an unexpected thiocyanate ion (S7/ C43/N32) exists on a center of symmetry, possessing two The molecular structure of (III), showing displacement ellipsoids at the 30% probability level. Only one of two possible orientations of the disordered thiocyanate (S7/C43/N32) and perchlorate (Cl4/O16-O19) ions is indicated for clarity.

Figure 1
The molecular structure of (I), showing displacement ellipsoids at the 30% probability level. Only one of two possible orientations of the disordered thiocyanate (N13/C20/S3 and N14/C21/S4) ions is indicated for clarity.

Figure 2
The molecular structure of (II), showing displacement ellipsoids at the 30% probability level.

Figure 4
The crystal structure of (I), projected along c. N-HÁ Á ÁO/N/S hydrogen bonds are shown as blue dashed lines. Both possible orientations of the disordered thiocyanate ions are indicated.

Figure 5
The crystal structure of (II), projected along a. Hydrogen bonds are shown as dashed lines in blue for O-HÁ Á ÁO/Cl and N-HÁ Á ÁO, and in red for N-HÁ Á ÁCl.
N(nitro) distance. Asymmetric intermolecular hydrogen-bond contacts are also observed in (III) (Fig. 9), and the reaction cavities show the vacancy at one of the two O atoms, O8 and O10 (Fig. 10). The R 2 2 (4) ring formed by the pair of nitro groups is observed not only in (III) but also in (I) and (II) (Fig. 11). These four-membered rings are essentially planar with the OÁ Á ÁH distances ranging from 2.33 to 2.49 Å . However, there are apparent differences in the geometry. That in (I) is a narrow rhomb with the interior angles at O6 and H10B being 33.3 and 146.7 , respectively, and inclined to the nitro plane by 79.2 (3) . The corresponding angles at O4 and H9A in (II) are 98.7 and 81.3 , and the dihedral angle with the nitro plane is 45.5 (2) . The shape of the ring in (III) is also nearly square with interior angles of 87.3-92.4 , and the dihedral angles with the nitro planes are 53.6 (2) and 53.8 (2) . Grenthe & Nordin (1979) reported the structures of trans-{Co(en) 2 (NO 2 )(NCS)]ÁX (X = ClO 4 À and I À ) obtained after solid-state thermal isomerization of the nitrito complexes (monoclinic P2 1 , Z = 2). The lattice constants did not correspond to the crystals grown from aqueous solutions of the nitro complexes. Except for Bö rtin (1976) (X = SCN À ) there is no other entry of the title nitrocobalt complex in the Cambridge Structural Database (CSD Version 5.39; Groom et al., 2016).

Figure 8
Comparison of the slices of the cavity around the nitro group within 0.1 Å from the plane in (I) and (II).

Figure 10
The slices of the cavity in (III) around the nitro groups within 0.1 Å from the planes. and then trans-[Co(en) 2 Cl(NO 2 )]NO 3 . The crystals of (I) were grown from a hot aqueous solution. Crystals of (I) were pulverized and dissolved in conc. HCl over a moderate heat, and impurities were removed by filtration. To the filtrate, some amount of ethanol was added. The solution was concentrated to precipitate the chloride (II), which was recrystallized with a small amount of water as solvent. To the saturated aqueous solution of (II), NaClO 4 powder was added to precipitate the perchlorate (III). Crystals of (III) were grown from an aqueous solution. The possibility of contamination of (III) by chloride ions was eliminated because no precipitation of AgCl occurred when AgNO 3 was added to an aqueous solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The H atoms bound to C and N were positioned geometrically. They were refined as riding, with N-H = 0.89 Å , C-H = 0.97 Å , and U iso (H) = 1.2U eq (C/ N). In (I), the non-coordinating thiocyanate ions S3/C20/N13 and S4/C21/N14 lie around the twofold axis with the molecular axes perpendicular and slightly inclined, respectively, showing orientational disorder. Their geometries were restrained with EADP or SIMU commands. Three reflections showing very poor agreement with I obs much smaller than I calc were omitted from the final refinement.
In (II), the H atoms of the water molecule were located from difference-density maps, and their coordinates were refined with the geometry restrained, and with U iso (H) = 1.5U eq (O). Eight reflections showing poor agreement were omitted from the final refinement, since their I obs were much smaller than I calc .

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

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.40 e Å −3 Δρ min = −1.15 e Å −3 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.