Investigation of nitro–nitrito photoisomerization: crystal structure of trans-chloridonitro(1,4,8,11-tetraazacyclotetradecane-κ4 N,N′,N′′,N′′′)cobalt(III) chloride

The crystal structure of the title compound has been studied to show that the macrocyclic cyclam ligand is very suitable as the co-ligand for the nitro–nitrito photo-isomerization of the CoIII complex.

Investigation of nitro-nitrito photoisomerization: crystal structure of trans-chloridonitro (1,4,8,11tetraazacyclotetradecane-j 4  The reaction cavity of the nitro group in the crystal of the title compound, [CoCl(NO 2 )(C 10 H 24 N 4 )]Cl, (I), was investigated to confirm that it offers sufficient free space for linkage isomerization to occur in accordance with the observed photochemical reactivity. The complex cation has crystallographic 2/m symmetry and the nitro and chloro ligands at the trans positions are statistically disordered. The complete cyclam ligand is generated by symmetry from a quarter of the molecule. In the crystal of (I), the complex cations and Cl À ions are linked into a three-dimensional network by N-HÁ Á ÁCl(counter-ion) hydrogen bonds.
When a KBr disk of (I) was irradiated for 30 min with a Xe lamp, the IR spectrum showed an apparent change involving an increase in intensity of the absorption peak of ca 1000 cm À1 (see the Figure in the supporting information), which corresponds to the symmetric N-O stretching mode of the nitrito form (Eslami et al., 2014). The IR spectrum of the irradiated complex was almost unchanged on standing at room ISSN 2056-9890 temperature for 2 h, indicating the long life-time of the nitrito form as in (II), and reverted to that before irradiation by heating at 55 C for 45 min. The crystal structure of (I) was determined to establish the dimensions of the reaction cavity and steric circumstance of the nitro group, and to compare them with those in (II).

Structural commentary
The molecular structure of (I) is shown in Fig. 1. The coordination geometry around the Co atom is a distorted octahedron with the N5 (nitro) and Cl2 atoms at the trans positions. The macrocyclic ligand cyclam adopts the trans-III conformation of Tobe's classification (Bosnich et al., 1965). The metal atom lies at site symmetry 2/m, and the atoms N5, Cl2 and C9 (the central C atom in the six-membered chelate ring) also lie on the mirror plane. There is a twofold axis running through the Co1 atom and midpoint of the C7-C7 iii bond in the five-membered chelate ring of cyclam, indicating that the positions of the Cl2 and nitro N5 atoms are exchanged. Similar orientational disorder of the chloridonitrocobalt complexes is observed for trans-[Co(en) 2 Cl-(NO 2 )]ClO 4 (Ohba & Eishima, 2000a) and the NO 3 salt (Ohba & Eishima, 2000b).

Supramolecular features
The crystal structure of (I) is shown in Fig. 2. The complex cations and chloride ions are connected by N- Figure 1 The molecular structure of (I), showing displacement ellipsoids at the 30% probability level. A crystallographic twofold axis runs through atom Co1 and the midpoint of the C7-C7 iii bond. Only one of two possible orientations of the nitro and chloride ions is shown for clarity. Symmetry codes: (i) Àx + 1, Ày, Àz + 1; (ii) x, y, Àz + 1; (iii) Àx + 1, Ày, z.

Figure 2
The crystal structure of (I), projected along b. The N-HÁ Á ÁCl hydrogen bonds are shown as red dashed lines. Only one of two possible orientations of the complex cation is shown for clarity.
HÁ Á ÁCl(counter-ion) hydrogen bonds, forming a threedimensional network. In (II), there are two independent nitro ligands at the trans positions, and the O atoms of each nitro group show two possible positions (occupation factors 65 and 35%; Ohba, et al., 2001). In the following discussion, the minor O(nitro) atoms will be neglected in (II). Slices of the reaction cavities around the NO 2 À group near its plane in (I) and (II) are compared in Fig. 3, where the radii of neighboring atoms are assumed to be 1.0 Å greater than the corresponding van der Waals radii (Bondi, 1964), except for Co, its radius being set to 1.90 Å . The shape of the cavity in the nitro plane is mainly defined by the N/C-HÁ Á ÁO(nitro) contacts which are shown in Figs. 4 and 5. Since the radius of the neighboring H atoms is assumed to be 2.20 Å , the cavity around the nitro O atoms is narrow in the intra-and intermolecular hydrogenbond directions. In (I), the cavity has sufficient free space to both side of the nitro O atoms for rotation to become the nitrito form, as suggested by the observed photoreactivity. In (II), the cavities of the nitro groups have space at one of the O atoms for conversion to the mono-and di-nitrito forms. The bifurcated N-HÁ Á ÁO,O hydrogen bonds form an R 2 2 (4) ring ( Comparison of the slices of the cavity around the nitro group within 0.1 Å from the plane in (I) and (II), where the minor O atoms (occupancy 35%) of each nitro ligand are omitted for clarity in (II). Symmetry codes for (I): (i) Àx + 1, Ày, Àz + 1; (ii) x, y, Àz + 1; (iii) Àx + 1, Ày, z.

Synthesis and crystallization
trans-[Co(cyclam)Cl 2 ]Cl was prepared by a literature method (Nakahara & Shibata, 1977) from cobalt(II) chloride hexahydrate and cyclam, and converted to trans-[Co(cyclam)Cl(NH 3 )]Cl 2 ÁH 2 O according to the method of Lee & Poon (1973). Then, trans-[Co(cyclam)Cl(NH 3 )]Cl 2 ÁH 2 O (1.0 mmol) was dissolved in 11 ml of 1% NH 3 aqueous solution and neutralized with diluted HCl. To the solution sodium nitrite (8.0 mmol) and 1 ml of 1 M HCl were added, and the reaction mixture was stirred for 3 h at room temperature, and concentrated to precipitate the title compound, (I). Orangered plate-like crystals of (I) were grown from a dimethyl sulfoxide solution by diffusion of diethyl ether vapour.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The electron densities of the nitro N5 and Cl2 atoms overlap with each other because of the orientational disorder of the complex cation. An EADP command was used for atoms N5 and Cl2, and the Co1-N5 bond distance was restrained to be 1.960 Å (s.u. = 0.001 Å ) to obtain a reasonable geometry for the nitro group. The H atoms bound to C and N were positioned geometrically. They were refined as riding, with C-H/N-H = 0.97-0.98 Å , and U iso (H) = 1.2U eq (C/N). One reflection showing poor agreement was omitted from the final refinement.  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).
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )