(2-Amino-7-methyl-4-oxidopteridine-6-carboxylato-κ3 O 4,N 5,O 6)aqua(ethane-1,2-diamine-κ2 N,N′)nickel(II) dihydrate

The NiII atom in the title complex, [Ni(C8H5N5O3)(C2H8N2)(H2O)]·2H2O, is six-coordinated in a distorted octahedral geometry by a tridentate 2-amino-7-methyl-4-oxidopteridine-6-carboxylate (pterin) ligand, a bidentate ancillary ethane-1,2-diamine (en) ligand and a water molecule. The pterin ligand forms two chelate rings. The en and pterin ligands are arranged nearly orthogonally [dihedral angle between the mean plane of the en molecule and the pterin ring = 77.1 (1)°]. N—H⋯O, O—H⋯N and O—H⋯O hydrogen bonds link the complex molecules and lattice water molecules into a three-dimensional network. π–π interactions are observed between the pyrazine and pyrimidine rings [centroid–centroid distance = 3.437 (2) Å].

Data collection: APEX2 (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molecular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: CRYSTALS. The importance of pterins in several classes of metalloenzymes has catalysed symbiotic developments of their coordination chemistry (Basu & Burgmayer, 2011;Burgmayer, 1998;Fitzpatrick, 2003;Fukuzumi & Kojima, 2008;Kaim et al., 1999). A SciFinder search reveals the existence of only one structurally characterized nickel(II)-pterin complex (Crispini et al., 2005), thereby highlighting the urgency of development in this direction. The present endeavour is concerned with the title complex, possessing both a tridentate pterin ligand and a σ-donor ligand like en. The sixcoordinated Ni II atom shows departure from a regular octahedral geometry with respect to both bond lengths and angles ( Fig. 1). The equatorial plane is formed by the two N atoms (N1, N2) of en, the pyrazine ring N atom (N3) of the pterin ligand and the aqua O atom (O6). The axial positions are occupied by the two pterin O atoms (O1 and O3), with the latter one forming the longest axial bond [2.327 (2) Å]. One important factor causing distortion from regular octahedral geometry is that this pterin ligand forms two five-membered chelate rings with small bite angles [76.31 (9) and 77.20 (10)°], instead of only one per pterin ligand for the earlier case (Crispini et al., 2005). A perusal of the charge balance of this complex indicates that this pterin ligand acts as a binegative tridentate ONO-donor. A near orthogonal disposition of the en ligand and pterin chelate ring is observed, which helps to minimize the steric repulsion. Of the three axes, least deviation from linearity is observed in the N3-Ni1-N2 direction [177.56 (11)°], where the highest electron density is concentrated [Ni1-N3 = 1.976 (2), Ni1-N2 = 2.065 (3) Å]. It represents the unique combination of a σ-donor atom N2 (en) and the N3 atom of the redox noninnocent pterin ligand from the opposite directions of the Ni II centre (d 8 ), with possible assistance from the π-donating phenolate and carboxylate O atoms (Kohzuma et al., 1988). Again, location of the pyrazine ring N atom (N3) in the equatorial plane is consistent with the earlier observations on related copper complexes (Odani et al., 1992).
In the crystal, the complex molecules and lattice water molecules are linked by intermolecular N-H···O, O-H···N and O-H···O hydrogen bonds (Table 1)  Experimental 2-Amino-4-hydroxy-7-methylpteridine-6-carboxylic acid sesquihydrate (C 8 H 7 N 5 O 3 .1.5H 2 O) was obtained by published procedure (Wittle et al., 1947). The title complex was prepared by the slow addition of an aqueous alkaline solution (NaOH: 44 mg, 1.1 mmol) of the pterin ligand (124 mg, 0.5 mmol) to a well stirred warm (323 K; paraffin oil bath) aqueous reaction mixture containing NiSO 4 .7H 2 O (140 mg, 0.5 mmol) and 1,2-ethanediamine (36 mg, 0.6 mmol) under subdued light; final volume was 35 ml. The pH value was adjusted to 9.2 and the stirring was continued for 3 h. Upon standing, the reaction medium deposited yellow-brown crystals after 2 days, which were suitable for single-crystal X-ray diffraction (yield: 30%). Analytically pure compound could be obtained by filtration, repeated washing with small quantities of water and drying in vacuo over silica gel. Analysis, calculated for C 10 H 19 N 7 NiO 6 : C 30.70, H 4.89, N 25.06%; found: C 30.51, H 5.11, N 24.55%.

Refinement
H atoms were all located in a difference map, but those attached to C atoms were repositioned geometrically. The H atoms were initially refined with soft restraints on bond lengths and angles to regularize their geometry (C-H = 0.93-0.98, N-H = 0.86-0.89, O-H = 0.82 Å) and with U iso (H) = 1.2-1.5U eq (parent atom), after which the positions were refined with riding constraints (Cooper et al., 2010).

Figure 1
The molecular structure of the title compound. Displacement ellipsoids are drawn at the 40% probability level. Lattice water molecules are omitted for clarity.  The crystal packing diagram of the title compound, viewed along the c axis. Dotted lines indicate hydrogen bonds.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (4)