Synthesis and Crystal Structure of the First Example of a Tris-Chelated Co ( II ) Complex Based on Oxamide Dioxime Ligand

A new cobalt(II) complex, [Co(H2oxado)3]C2O4·H2oxado·2H2O (H2oxado = oxamide dioxime), has been synthesized in aqueous solution and characterized by elemental analysis and single crystal X-ray structure determination. The complex crystallizes in the triclinic space group P-1, with the parameters a = 9.46(4), b = 11.84(5), c = 12. 81(5) Å, α = 104.94(6), β = 99.29(5), γ = 106.73(5), V = 1284(9) Å3, Z = 2. The central cobalt(II) cation is pseudo-octahedrally coordinated by six imino N atoms of the neutral oxamide dioxime ligand. In the solid state, each of the following bricks, namely the cationic complexes, the oxalate dianions as well as the oxamide dioxime crystallization molecules, pile up parallel to the a axis. The bulk structure is consolidated by an extended three-dimensional network of hydrogen bridgings—that link the ionic partners, oxamide dioxime and water molecules to one another—and by coulombic interactions.

Along the line of this research program, an attempt to prepare the elusive tris(oxamide dioxime)cobalt(III) tris(oxalato the structure of which is reported herein.The present finding turns out to be quite interesting, in as much as it provides, to the best of our knowledge, the first example of a complex salt containing the dipositive [Co(H 2 oxado) 3 ] 2+ cation, as highlighted in Scheme 1.

Synthesis of [Co(H2oxado)3]C2O4•H2oxado•2H2O
The title compound was obtained as follows: dark green crystals of K 3 [Co(C 2 O 4 ) 3 ]•3H 2 O (0.5 g, 1 mmol) were dissolved in H 2 O (50 mL) acidified with a drop of HNO 3 .To the filtered solution were added with stirring at 30˚C successive small portions of oxamide dioxime (0.36 g, 3 mmol), followed by portions of finely powdered Co(NO 3 ) 2 •6H 2 O (0.3 g, 1 mmol).The reaction mixture was stirred magnetically for 1 h and allowed to decant over 2 h.The reddish-pink precipitate of CoC 2 O 4 was carefully filtered off, and the solution left to evaporate slowly in a hood at room temperature.After two weeks, prismatic reddish crystals suitable for single crystal X-ray studies were harvested.Anal.Calcd.for C 10 H 28 CoN 16 O 14 : C, 18.33; H, 4.30; N, 34.20%.Found: C, 18.32; H, 4.30; N, 34.18%.

Crystal Structure Determination and Refinement
A suitable single crystal of the title compound with dimensions 0.5 × 0.12 × 0.10 mm was mounted on a glass Scheme 1.Chemical diagram of the title compound.
HO fiber and fixed to the goniometer head.Data collection [13] was carried out on a Bruker-Nonius X8 kappa APEX II CCD area-detector diffractometer using graphite-monochromatic radiation λ(Mo Kα) = 0.71073 Å at 296(2) K. Data reduction was performed using SAINT [13], and absorption corrections were carried out by multi-scan method by SADABS [14].The structure was solved by direct methods and refined against F 2 by full-matrix least-squares techniques with SHELXTL [15].All non-hydrogen atoms were refined with anisotropic displacement parameters.The hydrogen atoms were included from calculated positions and refined riding on their respective parent atoms with isotropic displacement parameters.Crystal data and structure refinement details for the title compound are summarized in Table 1, and selected bond lengths and angles in Table 2.

Results and Discussion
Single-crystal X-ray structural analysis reveals that the title compound crystallizes in the triclinic space group   Completeness to θ = 25.00,% 97.9  Finally, it should be noted that the title compound, in spite of the fact that it contains an unfamiliar [Co(H 2 oxado) 3 ] 2+ ion, happens to be a serendipitous isolation.Therefore, a systematic procedure designed to fabricate this compound is to be applied in our forthcoming works.

Conclusion
We have isolated from aqueous solution the salt [Co(H 2 oxado) 3 ]C 2 O 4 •H 2 oxado•2H 2 O containing the unprecedented tris-chelated Co II complex cation, [Co(H 2 oxado) 3 ] 2+ .In this cation, as well as in the familiar [Co(H 2 oxado) 3 ] 3+ ion, the central cobalt atom is in the same pseudo-octahedral coordination geometry of the six imino N atoms of H 2 oxado.In the crystal, the [Co(H 2 oxado) 3 ] 2+ ions are hydrogen-bonded into centrosymmetric dimers, thus generating zig-zag chains running parallel to [001].It is interesting to note that the dipositive complex cation [Co(H 2 oxado) 3 ] 2+ appears as resulting from the reduction of the tripositive complex cation [Co(H 2 oxado) 3 ] 3+ , a process reminiscent of catalytic or biological reactions involving transfer of electrons.

Co
H 2 oxado•2H 2 O.As shown in Figure1, the molecular structure is composed of one [Co(H 2 oxado) 3 ] 2+ complex cation, one oxalate (2-) anion, one oxamide dioxime molecule and two solvent water molecules.The point of prime interest in this report is the assessment of the existence of the pseudo-octahedral complex cation, tris(oxamide dioxime)cobalt(II), [Co(H 2 oxado) 3 ] 2+ , and this, as far as we know, is raised here for the first time.The oxamide dioxime acting as a crystallization molecule has been described previously[16].Selected bond lengths and angles of the title compound are listed in Table2, and compared with the corresponding values (in brackets) for the reported [Co(H 2 oxado) 3 ] 3+ cation[6]-[8].Data derived from this tripostive cation reflect nicely the C3 symmetry of the complex entity, whereas those derived from the dipositive complex cation, [Co(H 2 oxado) 3 ] 2+ , deviate noticeably from this symmetry.However, the pseudo-octahedral coordination of the central cobalt and the chiral nature of the complex cation are maintained.Whereas the O-N bond lengths within the [Co(H 2 oxado) 3 ] 2+ entities of the present structure range from 1.290(3) to 1.490(2) Å, its average value (1.383 Å) is slightly shorter than O-N bonds in the familiar [Co(H 2 oxado) 3 ] 3+ cation whose common value is 1.391(6) Å (Table 2).The projection of the structure along [100] highlights the [Co(H 2 oxado) 3 ] 2+ ions disposed in centrosymmetric dimers leading to a zig-zag chain running along c axis (Figure 2).The threedimensional crystal packing of the title compound is reinforced by extended O-H•••O, O-H•••N and N-H•••O bridges which interlink the complex cations, [Co(H 2 oxado) 3 ] 2+ , oxalate anions, oxamide dioxime molecules and water molecules.

Figure 2 .
Figure 2. Projection of the achiral unit cell of the title compound along [100], showing centro-symmetric dimerization of Co II complex cations and interconnection of these dimers, C 2 O 4 2− dianions, H 2 oxado and H 2 O molecules via hydrogen bridges (dotted lines).

Table 1 .
Crystal data and structure refinement details for the title compound.