Bis(2,2′,2′′-nitrilotriacetamide-κ3 O,N,O′)nickel(II) dinitrate tetrahydrate

In the title compound, [Ni(C6H12N4O3)2](NO3)2·4H2O, the NiII cation is located on an inversion center and is N,O,O′-chelated by two nitrilotris(acetamide) molecules in a distorted octahedral geometry. The complex cations, nitrate anions and lattice water molecules are connected by O—H⋯O and N—H⋯O hydrogen bonds, forming a three-dimensional supramolecular structure.

In the title compound, [Ni(C 6 H 12 N 4 O 3 ) 2 ](NO 3 ) 2 Á4H 2 O, the Ni II cation is located on an inversion center and is N,O,O 0chelated by two nitrilotris(acetamide) molecules in a distorted octahedral geometry. The complex cations, nitrate anions and lattice water molecules are connected by O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds, forming a three-dimensional supramolecular structure.
Data collection: SMART (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT ; program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL. Coordination chemistry of nitrilotriacetic acid with metal ions is explored extensively owing to their flexible coordinating nature, but nitrilotriacetamide (H 3 NTA) is hardly studied (Niraj et al., 2012;Biswajit et al., 2009;Ben Amor et al., 1998). This is the first report of a bis(H 3 NTA)-nickel(II) structure in which only H 3 NTA acts as a tridentate ligand.
Complex I consists of a Ni(H 3 NTA) 2 cation, two nitrate anions and four solvent water molecules (Scheme). Ni(II) has an octahedral coordination environment which is centrosymmetric as Ni(II) occupies an inversion center. The Ni atom is coordinated in a planar geometry by the nitrilotriacetamide N and O atoms. Two trans axial sites of this coordination environment is occupied by O2 and its symmetry related O2′ oxygen atoms from ligands( Fig. 1). In the equatorial plane the Ni-N1 distance is 2.131 (2) Å and the Ni-O1 distance is 2.098 (2) Å. The axial Ni-O2 bond is appreciably shortented which is 2.036 (2) Å. A few more selected bond distances and bond angles are presented in Table 1. The molecules are stacked along the a axis and display N-H···O and O-H···O hydrogen-bonds interaction (Fig. 2).

Experimental
The synthesis of nitrilotriacetamide was carried out according to US patent 3799981 (Donald & George, 1974). The title compound was synthesized by adding solid Ni(NO 3 ) 2 .6H 2 O (291 mg, 1 mmol) to a solution of ligands (376 mg, 2 mmol) in ethanol/water (2:1, 20 ml), then the mixture was stirred for 2 h at room temperature. The solution was filtered and the filtrate was allowed to stand in air for 1 d, and blue crystals were formed at the bottom of the vessel on slow evaporation of the solvent at room temperature.Yield: 73%.

Refinement
Water H atoms were located in a difference Fourier map and the positional parameters were refined, U iso (H) = 1.5U eq (O).
Other H atoms were included in calculated positions with C-H = 0.93 or 0.97 and N-H = 0.86 Å, and refined using a riding-model with U iso (H) = 1.2U eq (C,N).

Computing details
Data collection: SMART (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).    The packed diagram for the title compound, viewed down the a axis with hydrogen bonds drawn as dashed lines. 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.