Bis{bis(azido-κN)bis[bis(pyridin-2-yl-κN)amine]cobalt(III)} sulfate dihydrate

The crystal structure of bis{bis(azido-κN)bis[bis(pyridin-2-yl-κN)amine]cobalt(III)} sulfate dihydrate is comprised of discrete [Co(dpa)2(N3)2]+ cations, SO4 2− anions and solvent water molecules in a 2:1:2 ratio; extensive hydrogen-bonding interactions link the species into a three-dimensional supramolecular framework.


Chemical context
In recent years, molecular magnetism has attracted great attention due to the interest in designing new molecular materials with interesting magnetic properties and potential applications (Kahn, 1993;Miller & Gatteschi, 2011). Connecting paramagnetic ions by use of bridging polynitrile or pseudohalide ligands is an important strategy in the design of such materials (Setifi et al., 2002(Setifi et al., , 2013(Setifi et al., , 2014Benmansour et al., 2008Benmansour et al., , 2009Yuste et al., 2009). As a short bridging ligand and efficient superexchange mediator, the pseudohalide azide ion has proven to be very versatile and diverse in both coordination chemistry and magnetism. It can link metal ions in -1,1 (end-on, EO), -1,3 (end-to-end, EE) and -1,1,1 coordination modes among others, and effectively mediate either ferromagnetic or antiferromagnetic coupling. Many azide-bridged systems with different dimensionality and topology have been synthesized by using various auxiliary ligands, and a great diversity of magnetic behaviors have been demonstrated (Ribas et al., 1999;Gao et al., 2004;Liu et al., 2007;Mautner et al., 2010). In view of the possible roles of the versatile azido ligand, we have been interested in using it in combination with other chelating or bridging neutral coligands to explore their structural and electronic characteristics in the field of molecular materials exhibiting interesting magnetic exchange coupling. During the course of attempts to prepare such complexes with di-2-pyridylamine, we isolated the title compound, whose structure is described herein. ISSN 2056-9890

Structural commentary
The structure of the title compound is composed of discrete [Co(dpa) 2 (N 3 ) 2 ] + cations, SO 4 2À anions, and solvent water molecules in a 2:1:2 ratio (Fig. 1). The sulfate anion is located on a twofold rotational axis, and all other atoms lie on general positions. The central Co III ion has an approximately octahedral coordination geometry formed by four N-donors from the pyridyl rings of two chelating bidentate dpa ligands and two N-donors from the terminal azide anions with the cisoid angles ranging from 84.97 (8) to 94.15 (8) and transoid angles ranging from 174.02 (8) to 176.36 (8) (Table 1). While the bite angles of the dpa ligands are both less than 90 the smallest cisoid angle observed is for N4-Co1-N10 (Table 1), and the pyridyl ring containing N4 and azide anion containing N10 are involved in a weak C-HÁ Á ÁN interaction (C11-H11Á Á ÁN10) ( Table 2). The two pyridyl rings of each chelating dpa ligand coordinate to the metal in a cis-disposition, and the azide anions are also coordinating cis to each other.
Three conformations are known for dpa, cis-cis, cis-trans, or trans-trans (Fig. 2); cis and trans refer to the relation of the pyridyl nitrogen atoms to the amine nitrogen (Gornitzka & Stalke, 1998). Several bonding modes are possible involving just the pyridyl nitrogen atoms (Fig. 3). Only bonding modes I-II and IV-VI are observed for dpa with transition metals, but additional bonding modes are possible for anionic dpa Conformations of dpa. Cis and trans refer to the relation of the pyridyl N atoms to the amine N atom.

Figure 3
Possible coordination modes of dpa involving only pyridyl N atoms. Only modes I-II and IV-VI are observed with transition metals. Table 3 Deviations of atoms from the least-squares planes and angle between planes (Å , ).
Note: (*) an atom that was not used to define the plane.

Figure 4
The sulfate anions, highlighted in space-filling mode, are sandwiched between two symmetry related layers of complex cations and water molecules.

Database survey
Free dpa crystallizes as one of several polymorphs, but only in the cis-trans conformation with an intramolecular C-HÁ Á ÁN hydrogen bond between the two pyridyl rings [CSD refcodes: DPYRAM (Johnson & Jacobson, 1973); DPYRAM01 (Pyrka & Pinkerton, 1992); DPYRAM03 and DPYRAM04 (Schö del et al., 1996)]. Theoretical calculations by Wu et al. (2013) give the cis-trans conformation at 2.5 and 8.0. kcal mol À1 more stable than the cis-cis and trans-trans conformations, respectively, and the authors suggest that the instability of free dpa in the trans-trans conformation is due to repulsive interactions between of the pyridyl nitrogen lone pairs. However, when dpa coordinates to a transition metal, the trans-trans conformation is preferred.   Scatter plot of py cent -N a -py cent angles versus N py -M-N py bite angles for all transition metal complexes reported to the CSD with dpa in coordination mode VI. Blue dots represent all complexes with dpa coordinating in bonding mode VI to a transition metal. Red dots represent compounds where the metal has a coordination environment similar to the title compound: two dpa in bonding mode VI and two terminal azide anions. Black dots represent the title compound.

Figure 8
Scatter plot of the folding angles about N a -M versus N py -M-N py bite angles for all transition metal complexes reported to the CSD with dpa in coordination mode VI. Blue dots represent all complexes with dpa coordinating in bonding mode VI to a transition metal. Red dots represent compounds where the metal has a coordination environment similar to the title compound: two dpa in bonding mode VI and two terminal azide anions. Black dots represent the title compound.
trans conformation and acts as a chelating ligand in bonding mode VI.
As mentioned in the Structural commentary, dpa is a flexible ligand and adopts a wide range of Py cent -N a -Py cent and N py -M-N py bite angles in transition metal complexes. A comparison of these angles in the title compound to those observed in all structures reported to the CSD involving dpa coordinating to a transition metal in bonding mode VI reveals no simple trend (Brogden & Berry, 2016) (Fig. 7). Comparison of the folding angle about N a -M versus the N py -M-N py bite angle (Fig. 8) as well as the folding angle about N a -M versus the mean N py -M distance (Fig. 9) in the title compound to those observed in all structures reported to the CSD involving dpa coordinating to a transition metal in bonding mode VI also supports the flexible nature of dpa as a chelating ligand; however, no simple trend between the folding angle and the bite angle or the folding angle and the mean N py -M distance is indicated.
A more narrow search for structures involving at least one terminal azide anion and one dpa ligand in bonding mode VI within the coordination sphere of a transition metal cation returned 30 hits for 25 unique structures. Of the 25 structures, 23 involve M II cations; there is one report for Co III [CSD refcode: HUFNUR (Du et al., 2001)] and another report for Pt IV [CSD refcode: YATYOJ (Ha, 2012)]. Five structures are reported where the metal cation has a coordination sphere similar to that of the title compound. In each case, an approximately octahedral coordination geometry is formed by four N-donors from the pyridyl rings of two dpa ligands and two N-donors from terminal azide anions. In  (Rahaman et al., 2005)], neutral complexes are observed. In each case, the azide anions coordinate to the metal cation in a cis-fashion, and hydrogen bonding, face-to-facestacking, and edge-to-face C-HÁ Á Á interactions result in a threedimensional supramolecular framework. In [Cu(dpa) 2 (N 3 ) 2 ]Á-2H 2 O, the azide anions coordinate to the Cu II ion weakly in a trans-fashion, resulting in a tetragonally elongated octahedral coordination sphere for the Cu II ion, and hydrogen bonding and face-to-facestacking interactions result in twodimensional supramolecular sheets that lie parallel to the bcplane [CSD refcode: XUYWIX (Du et al., 2003)].
[Co(dpa) 2 (N 3 ) 2 ]ClO 4 is most closely related to the title complex in that the Co III ions are coordinated by two chelating dpa ligands and two azide anions in a cis-fashion to form [Co(dpa) 2 (N 3 ) 2 ] + complex cations [CSD refcode: HUFNUR (Du et al., 2001)]. The structure is stabilized by strong N-HÁ Á ÁO interactions between the complex cation and perchlorate anions. Consideration of additional weak C-HÁ Á ÁN interactions between the cations (which were not discussed by the authors) results in supramolecular ribbons that run parallel to the c axis.

Synthesis and crystallization
The title compound was synthesized hydrothermally under autogenous pressure from a mixture of cobalt(II) sulfate heptahydrate (28 mg, 0.1 mmol), di-2-pyridylamine (17 mg, 0.1 mmol) and sodium azide NaN 3 (13 mg, 0.2 mmol) in Scatter plot of the folding angles about N a -M versus mean M-N py bond length for all transition metal complexes reported to the CSD with dpa in coordination mode VI. Blue dots represent all complexes with dpa coordinating in bonding mode VI to a transition metal. Red dots represent compounds where the metal has a coordination environment similar to the title compound: two dpa in bonding mode VI and two terminal azide anions. Black dots represent the title compound. Computer programs: APEX2 and SAINT (Bruker, 2009), SHELXS97 (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015) and X-SEED (Barbour, 2001).
water-methanol (4:1 v/v, 20 ml). The mixture was sealed in a Teflon-lined autoclave and heated at 423 K for two days and cooled to room temperature at 10 K h À1 . The crystals were obtained in ca 20% yield based on cobalt. CAUTION! Although not encountered in our experiments, azido compounds of metal ions are potentially explosive. Only a small amount of the materials should be prepared, and it should be handled with care.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. All aromatic H atoms were positioned geometrically and refined using a riding model with C-H = 0.93 Å and U iso (H) = 1.2U eq (C). The N-H and O-H-atoms were located in difference Fourier maps and then refined as riding on the carrying nitrogen or oxygen atom with U iso (H) = 1.2U eq (N) or U iso (H) = 1.5U eq (O). Two reflections considered to be affected by beam stop interference, 0 0 2 and 2 0 0, were omitted from the refinement.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.