Diaquabis(1,3-propanediamine)nickel(II) squarate tetrahydrate

The asymmetric unit of the title compound, [Ni(C3H10N2)2(H2O)2](C4O4)·4H2O, contains one-half of the diaquabis(1,3-propanediamine)nickel(II) cation, one-half of the centrosymmetric squarate anion and two uncoordinated water molecules. In the cation, the NiII atom is located on a crystallographic inversion centre and has a slightly distorted octahedral coordination geometry. The six-membered chelate ring adopts a chair conformation. O—H⋯O hydrogen bonds link the cation and anion through the water molecule, while N—H⋯O hydrogen bonds link the cation and anion and cation and water molecules. In the crystal structure, intermolecular O—H⋯O and N—H⋯O hydrogen bonds link the molecules into a three-dimensional network structure.


Comment
The conformation of six-membered rings arranged by the bidentate coordination of pen (1, 3-propanediamine) to transition metals has long been of theoretical interest (Gollogly & Hawkins, 1972). Despite this interest, only a limited number of such complexes have been structurally described. Because of their ability to undergo solid-state phase transitions, some nickel(II) complexes of bis(N-substituted-pen) have been studied in recent times (Mukherjee et al., 1990;Pariya et al., 1995;Ghosh et al., 1997).
Squaric acid (H 2 C 4 O 4 ) has been of much interest because of its cyclic structure and possible aromaticity. Recently, considerable progress has been made in the crystal engineering of multidimensional arrays and networks containing metal ions as nodes. Squaric acid is a useful tool for constructing crystalline architectures, due to its rigid, planar four membered ring skeleton, and its proton donating and accepting capabilities for hydrogen bonding (Bertolasi et al., 2001;Reetz et al., 1994;Lam & Mak, 2000;Zaman et al., 2001;Mathew et al., 2002). In addition, squaric acid has been studied for potetial application in xerographic photoreceptors, organic solar cells and optical recording (Liebeskind et al., 1993;Seitz & Imming, 1992).
The asymmetric unit of the title compound contains one centrosymmetric cation, where Ni II is located on a crystallographic inversion centre, one centrosymmetric anion and two uncoordinated water molecules ( Fig.1 ), in which the bond lengths (Allen et al., 1987) and angles are within normal ranges. In cation, the Ni II is hexacoordinated by two O atoms of two water molecules in a trans order and by four N atoms of two pen ligands at the equatorial positions (Table 1). It is suggested that the trans geometry is preferred, when the amine ligand is more bulky. Thus, the coordination environment of Ni II is a slightly distorted octahedral. Intramolecular O-H···O hydrogen bonds (Table 2) link the cation and anion through the water molecule, while intramolecular N-H···O hydrogen bonds (Table 2) link the cation and anion and cation and water molecule.
In the crystal structure, intermolecular O-H···O and N-H···O hydrogen bonds (Table 2) link the molecules into chains ( Fig. 2), in which they may be effective in the stabilization of the structure.

Experimental
For the preparation of the title compound, a solution of squaric acid (0.57 g, 5 mmol) in water (25 ml) was neutralized with sodium hydroxide (0.40 g, 10 mmol) and added dropwise with stirring to a solution of Ni(CH 3 COO) 2 .4(H 2 O) (1.24 g, 5 mmol) in water (25 ml) at 323 K. The solution immediately became suspension and was stirred for 2 h. Then, 1,3-propandiamine (0.74 g, 10 mmol) in methanol (10 ml) was added dropwise to the obtained suspension. The clear solution was stirred for 2 h, and then cooled to room temperature. The crystals formed were filtered and washed with water (10 ml) and methanol (1:1), then dried in air. Anal. Calcd. : C 28.12,H 7.55,N 13.12%;Found C 28.06,H 7.61,N 13.18%. supplementary materials sup-2 Refinement Atoms H1E, H1F, H4A, H4B, H5A and H5B (for H 2 O) were located in difference syntheses and refined isotropically. The remaining H atoms were positioned geometrically with N-H = 0.90 Å (for NH 2 ) and C-H = 0.97 Å (for CH 2 ) and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C,N). Fig. 1. The molecular structure of the title molecule, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 40% probability [symmetry code: (i) 1 -x, 1 -y, 1 -z.

Diaquabis(1,3-propanediamine)nickel(II) squarate tetrahydrate
Crystal data [Ni(C 3  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 Rfactors(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.