Tris(3-nitropentane-2,4-dionato-κ2 O,O′)cobalt(III)

The structure of the title compound, [Co(C5H6NO4)3], consists of a CoIII ion octahedrally coordinated by three bidentate 3-nitropentane-2,4-dionate ligands. The complex was prepared via the nitration of tris(2,4-pentanedionato-κ2 O,O′)cobalt(III) with a solution of copper(II) nitrate in glacial acetic acid. The central C atom and the nitro group of one 3-nitropentane-2,4-dionate ligand are disordered over two positions with an occupancy ratio of 0.848 (4):0.152 (4). A second nitro group is also disordered over two orientations with an occupancy ratio of 0.892 (7):0.108 (7). Two of the ligand methyl groups form C—H⋯O interactions with two different nitro groups to form chains running along the c axis. Additional C—H⋯O interactions are found between ligand methyl groups and the cobalt-bound O atoms, also resulting in the formation of chains along the c axis.

The structure of the title compound, [Co(C 5 H 6 NO 4 ) 3 ], consists of a Co III ion octahedrally coordinated by three bidentate 3nitropentane-2,4-dionate ligands. The complex was prepared via the nitration of tris(2,4-pentanedionato-2 O,O 0 )cobalt(III) with a solution of copper(II) nitrate in glacial acetic acid. The central C atom and the nitro group of one 3-nitropentane-2,4dionate ligand are disordered over two positions with an occupancy ratio of 0.848 (4):0.152 (4). A second nitro group is also disordered over two orientations with an occupancy ratio of 0.892 (7):0.108 (7). Two of the ligand methyl groups form C-HÁ Á ÁO interactions with two different nitro groups to form chains running along the c axis. Additional C-HÁ Á ÁO interactions are found between ligand methyl groups and the cobalt-bound O atoms, also resulting in the formation of chains along the c axis.

Comment
The electrophilic substitution chemistry of the 2,4-pentanedionato (acetylacetonate, or acac) ligand has been studied for many years (Collman, et al., 1962;Collman, et al., 1963;Collman, 1965;Schirado, et al., 1971), but relatively few of these derivatives have been studied crystallographically, especially for the tri-substituted complexes. The nitro derivative of the cobalt complex is readily prepared and its synthesis and characterization have been described as part of several educational laboratory activities (James, 1974;Shalhoub, 1980). The average cobalt-oxygen bond length in the title compound is 1.869 (4) Å, slightly shorter than the average cobaltoxygen bond length observed for the [Co(acac) 3 ] complex determined at a similar temperature (von Chrzanowski, et al., 2007). All three nitro groups are twisted with respect to their 2,4-pentanedionato ligands (Fig. 1), forming angles of 49.3 (1), 59.3 (2), and 50.3 (2) degrees for the major components and 67.2 (2) and 51.6 (8) degrees for the minor disorder components. These are similar to the angle of 50.7 degrees observed for the mono-nitro cobalt complex (Appleton et al., 1992). The disorder in the positioning of one chelate ring has been observed previously (as large thermal parameters) for analogous complexes of cobalt and manganese (Appleton et al., 1992).
Analysis of packing (Fig. 2) and close contacts shows two different types of C-H···O interactions ( Table 1). The first type, shown in Figure 3(a) and 3(b), forms between methyl group hydrogen atoms and the nitro group on an adjacent molecule. The second type of C-H···O, shown in Figure 3(c) and 3(d), forms between methyl group hydrogen atoms and the cobalt-bound oxygen atom on an adjacent molecule. This second type of interaction is commonly seen in 2,4-pentanedionato complexes (von Chrzanowski et al., 2007). These hydrogen-bonding interactions result in the formation of four different types of C(6) chains (Bernstein et al., 1995), shown in Figure 4(a) through 4(d). In all four cases, the primary direction of the chain is along the c axis.

Experimental
The complex was prepared according to the procedure of Collman et al. (1963). Approximately 5.37 g (0.023 mol) of finely ground copper(II) nitrate trihydrate was mixed with 100 ml (1.06 mol) of acetic anhydride. Cobalt(III) acetylacetonate (2.5 g, 0.0070 mol) was added to the mixture and stirred with cooling for approximately two hours. A combination of water (300 ml), ice (300 g), and sodium acetate (7.5 g, 0.055 mol) was then added and the mixture was stirred for an additional two hours. The dark-green precipitate was vacuum filtered and washed with water and cold ethanol. The crude product was recrystallized from boiling chloroform and hot ethanol. The final product consisted of large, dark green crystals that were obtained in an overall yield of 62% (2.14 g).

Refinement
All hydrogen atoms were located in the difference map and refined with the atom positions constrained to an ideal tetrahedron with C-H distances of 0.98 Å. A riding model was used for all hydrogen atoms with U iso (H) = 1.5 times U iso (C).
One of the 3-nitropentane-2,4-dionato ligands was modeled as disordered over two positions for four atoms, C13/C13A, N3/N3A, O11/O11A, and O12/O12A and refined to give an occupancy ratio of 0.848 (4):0.152 (4). Carbon-carbon distances between similar atoms in the disordered ligand were restrained to be similar within a standard deviation of 0.02 Å. The nitro groups and their respective carbon atoms (C13/N3/O11/O12, C13A/N3A/O11A/O12A) were restrained to lie in a common plane, as were atoms C12, C13A, C14 and N3A. The anisotropic displacement parameters for the atom pairs N3/N3A and C13/C13A were constrained to be the same. The nitro group on a second ligand (N2, O9, O10) was modeled as a disordered group over two orientations and refined to give an occupancy ratio of 0.892 (7):0.108 (7).
Anisotropic displacement parameters were also restrained to be similar (with a standard deviation of 0.01 Å 2 ) for all atoms within the disordered nitro groups.

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