Crystal structure of bis(azido-κN)bis(quinolin-8-amine-κ2 N,N′)iron(II)

Exploring the role of azido anions led to the structure of this heteroleptic Fe complex with two azide ions and two quinolin-8-amine ligands.


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 centers by use of bridging polynitrile or pseudohalide ligands is an important strategy to design such materials (Setifi et al., 2002Gaamoune et al., 2010;Miyazaki et al., 2003;Benmansour et al., 2008Benmansour et al., , 2009Yuste et al., 2009;Setifi et al., 2013Setifi et al., , 2014Addala et al., 2015). As a short bridging ligand and efficient superexchange mediator, the pseudohalide azide ion has proved 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), -1,1,1 and other modes, and effectively mediate either ferromagnetic or antiferromagnetic coupling. Many azide-bridged systems with different dimensionality and topologies have been synthesized by using various auxiliary ligands, and a great diversity of magnetic behavior has 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 quinolin-8-amine, we isolated the title compound, whose structure is described herein.

Structural commentary
The title compound shows an octahedral coordination around the Fe II atom. The Fe complex is a neutral and discrete molecule and the two coordinating N 3 À anions occupy adjacent sites, classifying the title compound as a cis-complex. Fig. 1 shows the molecular structure.
The octahedral positions are occupied by six nitrogen atoms where the quinoline aromatic nitrogen atoms are found in the trans positions. All six Fe-N bond lengths are essentially uniform [2.104 (3)-2.284 (3) Å ] and typical for high-spin iron(II) compounds (Table 1). The Fe-NH 2 bond lengths are somewhat longer ($0.10 Å ) than the other Fe-N bonds. As a result of the quinolin-8-amine bite angle of about 75 the octahedral geometry is slightly distorted, allowing better separation of the negative charges on the azide ligands.

Supramolecular features
Looking down the a axis ( Fig. 2) one can notice alternating layers (stacked along the b-axis direction) of hydrophilic and aromatic regions. This layering can also be seen at the level of the complex itself, where the aromatic quinoline moieties are located above and below the hydrophilic plane formed by the NH 2 and N 3 À groups. These latter are engaged in hydrogen bonds expanding along the ac plane (Table 2). Both H atoms of the NH 2 group involving N1 form hydrogen bonds with the terminal nitrogen atoms of two neighboring (symmetryrelated) azide ligands. The other NH 2 group has one of its hydrogen atoms (N3-N3A) involved in a similar interaction, and the other hydrogen (N3-N3B) shows a very weak interaction with the coordinating end of a neighboring azide ion. The aromatic rings on the other hand show parallel displaced -stacking between pairs of quinoline (Q) moieties, the distance between the two quinoline planes is 3.38 Å (measured as the distance between the centroid of Q1 and the plane through Q2), or 3.35 Å , when interchanging Q1 and Q2. Some of the hydrogen bonds (Table 2) are rather long and the stabilization of the crystal packing comes from the combined effect of the hydrogen-bonding interactions, which direct the orientation of the neighboring complexes and the additional stacking interactions that hold the complexes in place.

Figure 1
The molecular structure of the title compound, showing the atomlabelling scheme. Displacement ellipsoids are drawn at the 50% probability level.

Database survey
A search in the Cambridge Structural Database (Version 5.37, Feb 2016 with two updates; Groom et al., 2016) reveals that only nine Fe II complexes with quinolin-8-amine groups have been reported. None of these complexes involve azide groups, neither coordinating nor as a free anion. There is one known Cd complex that contains 8-aminoquinoline and bound azide; rather than forming discrete entities, the Cd complex is polymeric, expanding into chains where the azides act as bridging ligands [refcodes WIJWES (Paira et al., 2007) and WIJWES01 (Xu et al., 2008)] in the EO mode. Considering the azides and their coordination modes, the predominant N 3 À binding mode is as monodentate (2210 entries), among the bridging modes the 2 modes either 1,1 EO (1652 entries) or 1,3 EE (931 entries) are most favored. The other EO modes 3 (159 entries) or 4 (11 entries) are far less frequent. Similar observations are made for the more complex end-to-end bridging modes: 3 -1,1,3 (131), 4 -1,1,3,3 (13), 4 -1,1,1,3 (11), 5 -1,1,1,3,3 (1). For completeness, the occurrence of N 3 À as a free anion is not so common, as only 92 entries were identified in the CSD database.

Synthesis and crystallization
The title compound was synthesized hydrothermally under autogenous pressure from a mixture of iron(II) sulfate heptahydrate (28 mg, 0.1 mmol), quinolin-8-amine (15 mg, 0.1 mmol) and sodium azide NaN 3 (13 mg, 0.2 mmol) in water-methanol (4:1 v/v, 20 ml). The mixture was sealed in a Teflon-lined autoclave and heated at 453 K for two days and cooled to room temperature at 10 K h À1 . The crystals were obtained in ca 20% yield based on iron and proved to consist of a mononuclear heteroleptic Fe complex rather than the expected polymeric architecture with bridging azides.
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 3. All H atoms were placed in geometrically idealized positions and constrained to ride on

Bis(azido-κN)bis(quinolin-8-amine-κ 2 N,N′)iron(II)
Crystal data 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.