The crystal structure of bis[(E)-4-bromo-2-({[2-(pyridin-2-yl)ethyl]imino}methyl)phenol]nickel(II) bis[(E)-4-bromo-2-({[2-(pyridin-2-yl)ethyl]imino}methyl)phenolato]nickel(II) bis(perchlorate) methanol monosolvate, a structure containing strong inter-species hydrogen bonds

The title compound, [Ni(C14H12BrN2O)2][Ni(C13H12BrN2O)2](ClO4)2 2(MeOH) consists of two mononuclear ([Ni(HL)2]2+ and [NiL 2]) complexes linked by strong hydrogen bonding [O⋯O separations of only 2.430 (5) Å], which is the shortest reported to date for such species.


Chemical context
Metal-Schiff base complexes have been of interest for a variety of reactions, in particular catalytic reactions (Egekenze et al., 2017a(Egekenze et al., ,b, 2018a. The metalloenzyme urease contains Ni II at its active site. Ureases can be found in a variety of species and efficiently accelerate by several orders of magnitude the rate of hydrolysis of urea into CO 2 and NH 3 (Mobley, 2001). It has been of great interest to catalyze a variety of reactions to mimic the catalytic efficiency of metalloenzymes. The crystal structures of related Ni II -Schiff base complexes have been reported (Ayikoé et al., 2011;Butcher et al., 2009;Elmali et al., 2000;Kobayashi et al., 2017;Kuchtanin et al., 2016;Okeke et al., 2017;Duran et al., 1989). Similar complexes have been studied in relation to catalytic redox reactions, catechol oxidase activity, and alkaline phosphatase reactivity (Ö zalp-Yaman et al., 2005;Sanyal et al., 2016;Bhardwaj & Singh, 2014). In view of this interest and in a continuation of our previous research listed above, the title Ni II -Schiff base complex has been synthesized to be used as a catalyst for the hydrolysis of phosphate esters.

Supramolecular features
The main point of interest in this structure is the presence of very strong inter-species hydrogen bonding between the phenol and phenolate moieties as mentioned above. In addition, the perchlorate anions link the complexes and methanol solvate molecules through both C-HÁ Á ÁO and O-HÁ Á ÁO interactions (Table 1). These, along with C-HÁ Á ÁBr interactions (Table 1), link all the species into a complex threedimensional array as shown in Fig. 4.

Figure 3
Diagram of both the cation and neutral complex linked by strong hydrogen bonding (shown as dashed lines). For the cation, only the major component of the disordered group is shown. Atomic displacement parameters are at the 30% probability level.

Synthesis and crystallization
2-(2-Pyridyl)ethylamine (0.1613 g, 1.320 mmol) was added to a reaction flask and dissolved in 50 ml of methanol. 5-Bromosalicylaldehyde (0.2654 g, 1.320 mmol) was added to the solution. The mixture was refluxed for 5 h. The nickel(II) complex was prepared by reacting the ligand in 50 ml of methanol with Ni(ClO 4 ) 2 Á6H 2 O (0.7242 g, 1.980 mmol) with no added base. The mixture was stirred at room temperature overnight. The product was crystallized by slow diffusion in methanol for two weeks giving green crystals.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. For the neutral NiL 2 , each 2-ethylaminepyridine arm is disordered over two equivalent conformation with occupancies of 0.750 (8)

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. Refinement. Refined as a two-component twin.