Crystal structure of bis{4-bromo-2-[(carbamimidamidoimino)methyl]phenolato-κ3 N,N′,O}cobalt(III) nitrate dimethylformamide monosolvate

In [Co(C8H8BrN4O)2]NO3·C3H7NO, the ligand molecules are deprotonated at the phenol O atom and octahedrally coordinate the CoIII atoms through the azomethine N and phenolate O atoms in a mer configuration. In the crystal lattice, the cations are arranged in layers in the ab plane divided by the nitrate anions and solvent molecules. All of the amine H atoms are involved in hydrogen bonding to nitrate, DMF or ligand O atoms or to one of the Br atoms, forming two-dimensional networks.

The title compound, [Co(C 8 H 8 BrN 4 O) 2 ]NO 3 ÁC 3 H 7 NO, is formed of discrete [CoL 2 ] + cations, nitrate anions and dimethylformamide (DMF) molecules of crystallization. The cation has no crystallographically imposed symmetry. The ligand molecules are deprotonated at the phenol O atom and octahedrally coordinate the Co III atoms through the azomethine N and phenolate O atoms in a mer configuration. The deprotonated ligand molecules adopt an almost planar conformation. In the crystal lattice, the cations are arranged in layers in the ab plane divided by the nitrate anions and solvent molecules. Nostacking is observed. All of the amine H atoms are involved in hydrogen bonding to nitrate, DMF or ligand O atoms or to one of the Br atoms, forming two-dimensional networks parallel to (100).

Chemical context
Aminoguanidine (AG) has been extensively studied as one of the most promising compounds for the treatment of diabetic complications (Thornalley, 2003). AG-based Schiff bases have attracted much research attention owing to experimental evidence that a pyridoxal-aminoguanidine Schiff base adduct exhibited advanced glycation inhibitory activity comparable to that of AG, while causing no decrease in the liver pyridoxal phosphate content of normal mice (Taguchi et al., 1998(Taguchi et al., , 1999. The study of the chelating properties of AG-based Schiff bases toward metal ions may help to understand the mechanism of action of drugs and possible benefits of chelation therapy in diabetes (Nagai et al., 2012).
Multinuclear Schiff base metal complexes, coupled systems in particular, are also of special interest in materials science. During the last few years, we have been exploring the chemistry of transition metal complexes of Schiff base ligands with the aim of preparing heterometallic polynuclear compounds with diverse potential advantages. In these studies, we continued to apply direct synthesis of coordination compounds, an approach that employs zero-valent metal (metal oxide) as a source of metal ions along with a salt of another metal (Vinogradova et al., 2001;Buvaylo et al., 2009;Semenaka et al., 2010;Nesterov et al., 2011). The metal powder is oxidized during the synthesis by dioxygen from the air. The main advantage of this approach is generation of building blocks in situ, in one reaction vessel, thus eliminating separate steps in building-block construction. Reactions of a metal powder and another metal salt in air with a solution containing a pre- ISSN 2056-9890 formed Schiff base ligand yielded a number of novel Cu/Cr and Co/Fe compounds (Nikitina et al., 2008;Chygorin et al., 2012).
The title compound was isolated in an attempt to prepare a heterometallic Co/Mn compound with the ligand, HLÁHNO 3 ( Fig. 1) that was synthesized from Schiff base formation of 5-bromosalicylaldehyde with AGÁHNO 3 . Mn powder and Co(NO 3 ) 2 Á6H 2 O were reacted with the Schiff base formed in situ in methanol/dimethylformamide (DMF) mixture in a 1:1:2 molar ratio. The isolated dark-red microcrystalline product was identified crystallographically to be the mononuclear Co III Schiff base complex CoL 2 NO 3 ÁDMF (I) which did not contain any manganese.

Structural commentary
The title compound [Co(C 8 H 8 BrN 4 O) 2 ]NO 3 ÁC 3 H 7 NO, (I), is formed of discrete [CoL 2 ] + cations, nitrate anions and DMF molecules of crystallization. The cation has no crystallographically imposed symmetry (Fig. 2). The ligand molecules are deprotonated at the phenol oxygen atom and coordinate to the Co III atom through four azomethine N and two phenol O atoms in such a way that the Co III atom is octahedrally surrounded by two anionic ligands in a mer configuration. The Co-N/O distances (Table 1) fall in the range 1.887 (2)-1.9135 (18) Å , the trans angles at the metal atom vary from 175.14 (9) to 177.14 (8) , the cis angles lie in the range 82.62 (9) to 94.35 (8) . The deprotonated ligand molecules adopt an almost planar conformation.
The coordination geometry around the metal atom has a close resemblance to that found in Co III complexes with a very similar ligand which results from the condensation between salicylaldehyde and AG hydrochloride: bis{2-[(guanidinoimino)methyl]phenolato-3 N,N 0 ,O]}cobalt(III) chloride hemihydrate (CSD refcode MEXGED; Buvaylo et al., 2013), and its solvatomorph trihydrate (CSD refcode GEMJOY; Chumakov et al., 2006). Co-N/O distances in MEXGED, which possesses two independent cations, vary from 1.8863 (8) to 1.9290 (8) Å , the trans angles at the metal atoms fall in the range 172.24 (4)-176.71 (4) , the cis angles are equal to 82.33 (4)-94.86 (4) . Obviously, the use of the 5-bromoderivative of salicylaldehyde in the present study does not change the coordination properties of the resulting Schiff base Scheme of HLÁHNO 3 .

Figure 2
The molecular structure of the title complex, showing the atomnumbering scheme. Non-H atoms are shown with displacement ellipsoids at the 50% probability level. ligand compared to that of parent salicylaldehyde-aminoguanidine Schiff base.

Supramolecular features
In the crystal lattice, the cations are arranged in layers in the ab plane divided by the nitrate anions and DMF molecules ( Fig. 3). Interactions between cations are weak, the closest CoÁ Á ÁCo intermolecular separation exceeds 5.76 Å . Nostacking is observed. All the amine hydrogen atoms are involved in hydrogen bonding to nitrate, DMF or ligand oxygen atoms or to one of the Br atoms, Br21, to form twodimensional networks parallel to (100) (Fig. 4). Hydrogenbonding geometrical details are listed in Table 2.

Database survey
Crystal structures of neither the ligand itself nor its metal complexes are found in the Cambridge Structure Database (Groom et al., 2016; CSD Version 5.37 plus one update). Eighteen reported structures of AG-based Schiff bases deposited in the Database incorporate various chloro, fluoro, hydroxy, methoxy, methylthio and nitro derivatives of benzaldehyde, pyridine and pyrimidine. These organic compounds exist as zwitterions as well as chloride, nitrate, acetate, dihydrogenphosphate and sulfate salts in the solid state.
Of 18 crystal structures of Schiff base metal complexes derived from AG, six are Fe, Cu and Zn compounds that contain a pyridoxal-aminoguanidine ligand. The latter has been of much interest due to its suggested superiority to AG in the treatment of diabetic complications. The remaining 12 compounds are mostly mononuclear Cu II complexes (four) and CuCl 4 2salts (four) with protonated Schiff base ligands as cations. Other mononuclear complexes and hybrid metal salts of AG-based Schiff base ligands comprise V, Co, and Ni, Cd structures, respectively. The Schiff base ligands derived from AG do not show any coordination variability in their metal complexes -the ligand tends to coordinate through two azomethine N atoms and phenoxy O atom from the ring if such one is present.

Figure 3
Crystal packing of (I) showing the layered arrangement of [CoL 2 ] + cations in the ab plane. H atoms are not shown.

Figure 4
Part of the crystal structure with intermolecular hydrogen bonds shown as blue dashed lines. CH hydrogen atoms have been omitted for clarity.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. 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, C-H = 0.98 Å , U iso (H) = 1.5U eq C for CH 3 ). NH hydrogen atoms were refined with bond lengths restrained to ideal values (N-H = 0.88 Å ). Anisotropic displacement parameters were employed for the non-hydrogen atoms.

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. NH hydrogen atoms were refined with bond distances restrained to ideal values. Two reflections which were considered to be masked by the beam stop were omitted from the refinement. Largest peak is 0.79 Angstroms from Br21. Largest trough is 0.64 Angstroms from Co1.