Bis[tris(propane-1,3-diamine-κ2 N,N′)nickel(II)] diaquabis(propane-1,3-diamine-κ2 N,N′)nickel(II) hexabromide dihydrate

In the title compound, [Ni(C3H10N2)3]2[Ni(C3H10N2)2(H2O)2]Br6·2H2O, one Ni2+ cation, located on an inversion centre, is coordinated by four N atoms from two ligands and by two water O atoms. The other Ni2+ cation, located in a general position, is coordinated by six N atoms from three ligands. In both cases, the Ni2+ cation has an octahedral coordination environment. The overall structural cohesion is ensured by three types of hydrogen bonds, N—H⋯Br, O—H⋯Br and O—H⋯O, which connect the two types of complex cations, the bromide counter-anions and the lattice water molecules into a three-dimensional network.


Related literature
For the multiple coordination modes of amine derivatives as ligands to metal ions, see: Manzur et al. (2007); Ismayilov et al. (2007); Austria et al. (2007). For control of the aggregation of molecules or ions in the solid state in crystal engineering, see: Burrows (2004). For hydrogen bonding in bifunctional ligands, see: Simard et al. (1991); Zerkowski & Whitesides (1994).
Grateful thanks are expressed to Tarak Gargouri (Université de Sfax, Faculté des Sciences de Sfax) for his assistance with the single-crystal X-ray diffraction data collection.

Comment
Compounds having specific functional groups have received considerable attention due to their particular properties and applications. For example, derivatives of the amino acids have a biological activity and amine derivatives have potential ability to form metal-organic frameworks because of their multiple coordination modes as ligands to metal ions (Austria et al., 2007;Ismayilov et al., 2007;Manzur et al., 2007). The use of hydrogen bonds to control the aggregation of molecules or ions in the solid state is a key tool in crystal engineering (Burrows, 2004). Although such concepts were originally developed for organic systems, many studies have extended these ideas into the inorganic domain by using bifunctional ligands that are capable of simultaneously coordinating to a metal centre and presenting one or more hydrogen bonding (Simard et al., 1991;Zerkowski et al., 1994). In this context, we report here the chemical preparation and the crystal structure of a novel hybrid material using nickel as transition metal presenting the following formula [Ni(C 3 N 2 H 10 ) 2 (H 2 O) 2 ][Ni(C 3 N 2 H 10 ) 3 ] 2 Br 6 ·2H 2 O, (I). The asymmetric unit of I, represented in Fig. 1, contains two crystallographically independent nickel atoms. The first one occupies a general position and it is coordinated by three 1,3-diaminopropane molecules amine, which are bidentate ligands. The second type of nickel atom lies in a special position on inversion centre and it is coordinated by one molecule amine and one water molecule and their symmetric by the the inversion centre. Consequently, the nickel atoms, in this compound, adopt two different octahedral coordination.
The asymmetric unit of I conatins also two free water molecules and three bromine ions. As it can be seen in Fig. 2, the cohesion of the crystal structure is ensured by three types of hydrogen bonds, N-H···Br, O-H···Br and O-H···O, established between the different entities ginving rise to a three dimensional H-bonds network.

Experimental
The title compound is resulting from a chemical reaction between three reagents: 1,3-diaminopropane (C 3 H 10 N 2 ), hydrobromic acid (HBr) and nickel bromide (NiBr 2 ). The 1 mmol of NiBr 2 and 1 mmol of the diamine with excess of HBr were mixed in the DMF solvent. The obtained solution is kept at room temperature. After 4 days, purple platelets were formed.
The purity of the product was improved by a second recrystallization.

Refinement
The water H atoms were located in difference map and refined with O-H distance restraints of 0.85 (2)Å and H···H distance restraints of 1.35 (2)Å. The H atoms bonded to C and N atoms were positioned geometrically (with distances C -H = 0.97Å and N-H = 0.90Å) allowed to ride on their parent atoms, with U iso = 1.2U eq (C, N).   Projection of the structure of I along the a axis. Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.