Crystal structure of (2-amino-7-methyl-4-oxidopteridine-6-carboxylato-κ3 O 4,N 5,O 6)aqua(1,10-phenanthroline-κ2 N,N′)zinc trihydrate

In the title compound, [Zn(C8H5N5O3)(C12H8N2)(H2O)]·3H2O, a tridentate 2-amino-7-methyl-4-oxidopteridine-6-carboxylate ligand, a bidentate ancillary 1,10-phenanthroline (phen) ligand and a water molecule complete a distorted octahedral coordination geometry around the ZnII atom. The pterin ligand forms two chelate rings. The phen and pterin ring systems are nearly perpendicular [dihedral angle = 85.16 (5)°]. Classical N—H⋯O, O—H⋯N and O—H⋯O hydrogen bonds and weak C—H⋯O hydrogen bonds link the complex molecules and lattice water molecules into a three-dimensional network. π–π stacking contacts are observed as well, with centroid-to-centroid distances of 3.5679 (14), 3.7004 (14), 3.6641 (15), 3.6974 (13) and 3.3412 (12) Å.

The present effort is concerned with the title complex, possessing both a tridentate pterin ligand and a π-acceptor ancillary ligand like 1, 10-phenanthroline (phen). The six-coordinated Zn II atom exhibits 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 (N19, N26) of phen, the pyrazine ring N atom (N6) of the pterin ligand and the aqua O atom (O18). The axial positions are occupied by the two pterin O atoms (O2 and O16), with the latter one forming the longest axial bond [2.3724 (16) Å].
One important reason causing distortion from regular octahedral geometry is that this pterin ligand forms two fivemembered chelate rings with small bite angles [76.28 (7) and 74.66 (6)°], instead of only one per pterin ligand for the earlier case (Mitsumi et al., 1995). A consideration 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 phen ligand and pterin chelate ring is observed, which affords minimum steric repulsion. Of the three axes, least deviation from linearity is observed in the O18 The exocyclic bond length data of the pyrimidine ring, C15-O16 [1.257 (3) Å] and C13-N17 [1.335 (3) Å] merit attention. Participation by the pterin unit in the electron-shuffling process from the pyrazin ring N9 to the C15 carbonyl group is indicated, as suggested in the literature (Baisya & Roy, 2014;Beddoes et al., 1993;Kohzuma et al., 1988;Miyazaki et al., 2008;Russell et al., 1992). Formation of the Zn1-O16 bond assists this process.  (Table 1) into a three-dimensional network. The lattice water molecules play a decisive role in the crystal packing process (Fig. 2). Fig. 3 indicates π-π stacking interactions involving two parallel, inversionrelated pterin rings within the same unit cell and showing face-to-face distance of 3.6974 (13) and 3.3412 (12) Å. Besides this, the nearly parallel phen rings of adjacent molecules also display π-π stacking interactions with centroid-centroid distance of 3.5678 (14), 3.7004 (14) and 3.6641 (15) Å.

S2.1. Refinement
The H atoms were all located in a difference map, but those attached to carbon atoms were repositioned geometrically.
The H atoms were initially refined with soft restraints on the bond lengths and angles to regularise their geometry (C-H in the range 0.93-0.98 Å, N-H in the range 0.86-0.89 Å and O-H = 0.82 Å) and U iso (H) (in the range 1.2-1.5 times U eq of the parent atom), after which the positions were refined with riding constraints.

Figure 1
The molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are shown as spheres of arbitrary radius.  The crystal packing diagram of the title compound, viewed along the b axis. Dotted lines indicate hydrogen bonds.