Crystal structure of (2-{[(8-aminonaphthalen-1-yl)imino]methyl}-4,6-di-tert-butylphenolato-κ3 N,N′,O)bromidonickel(II)

The coordination environment of the NiII atom is slightly distorted square planar, whereas the appearance of the whole molecule is twisted.


Chemical context
There has been an emergent interest in the design and synthesis of non-symmetrical iminoaryl bis(salen)-based ligands because of their facile synthesis and tunable properties. As a result, their nickel complexes have been used in a variety of applications and properties, including metal-organic frameworks (Crane & MacLachlan, 2012), catalysis for styrene polymerization (Ding et al., 2017), unique redox behavior (Rotthaus et al., 2006;Kochem et al., 2013), and non-linear optics (Cisterna et al., 2015;Trujillo et al., 2010). One of the synthetic methods utilizes the half-unit Schiff base as a precursor for the preparation of non-symmetrical iminoaryl bis(salen) ligands. Surprisingly, ligands are mostly limited to phenyl derivatives as the backbone. Some metal complexes bearing non-symmetrical iminonaphthyl bis(salen) ligands have been reported in the literature (Villaverde et al., 2011;Boghaei & Mohebi, 2002;Sundaravadivel et al., 2013Sundaravadivel et al., , 2014, but their crystal structures were not determined. As part of our work on the synthesis of nickel complexes bearing nonsymmetrical iminoaryl bis(salen)-based ligands, we report here the crystal structure of (2-{[(8-aminonaphthalen-1-yl)imino]methyl}-4,6-di-tert-butylphenolato-3 N,N 0 ,O)bromidonickel(II), (I).

Structural commentary
The molecular structure of the title compound, (I), is given in Fig. 1, with selected bond lengths and angles collated in Table 1. The structure confirms the nickel cation to be fourcoordinate and bound by two N atoms (imine N1 and amine N2), the phenolic O atom (O1), and the Br atom (Br1). The amino nitrogen atom (N2H 2 ) is neutral, with both hydrogen atoms well-defined in difference electron density maps. The O1-C1 bond length of 1.312 (4) Å indicates a phenolate resonance form for the ligand. The Schiff base double N1 C7 bond is within the range expected for a metal-coordinating Schiff base-imine fragment.
The coordination environment around the Ni II cation can be best described as slightly distorted square-planar, with an r.m.s deviation from planarity for the NiN 2 OBr fragment of 0.0943 Å . Interestingly, the Ni1-N1, Ni1-N2, and Ni1-O1 bond lengths are slightly longer than those observed in the phenyl backbone counterpart of (I), [Ni(NNO)OAc] (II) (NNO = 2-{[(2-aminophenyl)imino]methyl}-4,6-di-tert-butylphenolate; Ding et al., 2017), which could be attributed to the increased steric bulk of the naphthyl backbone in (I). In line with this increased steric demand are the value for the angle N2-Ni1- O1 [170.15 (11) ], and that of the torsion angle C6-C7-N1-C16 [163.1 (3) ], which are significantly larger than those observed for (II) (176 and 178 , respectively). The steric profile of the aryl backbone appears to play an important role in altering both bond lengths and angles around the coordination center.
The increased steric demand in (I) does not substantially affect the bond lengths and angles of the individual ligand fragments. Both the naphtyl as well as the iminomethyl phenolate fragments are essentially planar, with r.m.s deviations from planarity of only 0.062 and 0.072 Å , respectively (the least-squares planes include the N and O atoms attached to the fragments). They do, however, yield to the steric strain by substantially rotating out of the plane of the NiN 2 OBr plane, and with respect to each other, giving the molecule as a whole a twisted appearance. The dihedral angle of the naphthalene-1,8-diamine unit with the central NiN 2 OBr plane is 38.92 (7) , the equivalent angle of the iminomethyl phenolate substitutent is 37.22 (8) . The interplanar angle between the two organic fragments is 50.33 (5) . This contrasts starkly with (II). The less sterically strained counterpart of (I) is essentially planar, with interplanar angles of the NiN 2 O 2 fragment with the phenylene di-amine of only 5.91 and 7.39 [note that there are two independent molecules in the structure of (II)], and of only 7.08 and 3.58 towards the iminomethyl phenolate fragments.

Supramolecular features
The crystal-packing of (I) is steered by a number of medium strength and weak intermolecular interactions. Most prominent is an intermolecular N-HÁ Á ÁBr hydrogen bond, Table 2, which connects individual molecules into dimers. The hydrogen bond involves H2B of the amine group. The other amine H atom, H2A, does not form a hydrogen bond. Instead, it interacts with the electron cloud of the phenolate ring, with two close N-HÁ Á ÁC() contacts (

Figure 1
The molecular structure of the title compound showing atom labels, with displacement ellipsoids at the 50% probability level.

Database survey
The most recent version of the Cambridge Structural Database (Version 5.39, updated November 2017; Groom et al., 2016) has no entries related to iminonaphthyl mono(salen) supported metal complexes. However, a closely related compound, a nickel(II) complex bearing an iminophenyl mono(salen) ligand, has been reported as its acetate complex, and has been compared to the title compound in the Structural commentary. A broader exploration showed eight entries corresponding to iminophenyl mono (

Synthesis and crystallization
Starting materials were commercially available and were used without further purification. Ligand synthesis: 3,5-di-tertbutyl-2-hydrobenzaldehyde (1.00 g, 4.27 mmol) dissolved in ethanol (20 ml) was added to 1,8-diaminonaphthalene (1.36 g, 8.53 mmol) in ethanol (20 ml) in a 100 ml round-bottom flask. The reaction mixture was refluxed for 24 h. Volatiles were removed under reduced pressure, and the residue was crystallized at 253 K to yield light-purple crystals (1.17 g, 73% Synthesis of the title compound: To a stirred solution of (E)-2-{[(8-aminonaphthalen-1-yl)imino]methyl}-4,6-di-tertbutylphenol (80 mg, 0.21 mmol) in THF (3 mL) at ambient temperature under an N 2 atmosphere was added a suspension of potassium tert-butoxide (26 mg, 0.24 mmol) in THF (2 mL) for 2 h. Solid NiBr 2 (DME) (69 mg, 0.22 mmol) was added, and the resulting slurry was stirred for 18 h at ambient temperature. Volatiles were removed under reduced pressure, and the residue was extracted with toluene and filtered through Celite. The filtrate was dried in vacuo to yield a dark-red solid (21 mg, 95%). Crystals suitable for X-ray diffraction were grown from a concentrated solution in Et 2 O at ambient temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms attached to carbon atoms were positioned geometrically and constrained to ride on their parent atoms, with C-H bond lengths of 0.95 Å for alkene and aromatic moieties, and 0.98 Å for aliphatic CH 3 moieties, respectively. Methyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density. Amine H atom positions were refined with N-H distances restrained to 0.88 (2) Å . U iso (H) values were set to a multiple of U eq (C/N) with 1.5 for CH 3 , and 1.2 for C-H and N-H units, respectively. Reflections (0 0 2), (1 0 2) and (0 1 3) were obstructed by the beam stop and were omitted from the refinement.

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 −0.0553 (