supplementary materials

The primary motivation for pursuing coordination chemistry of pterins is the ubiquitous presence of this heterocyclic system in nature including a substantial number of metalloenzymes (Basu & Burgmayer, 2011; Burgmayer, 1998; Fitzpatrick, 2003; Fukuzumi & Kojima, 2008). Literature survey reveals the existence of only a few X-ray structurally characterized cobalt-pterin/pteridine/lumazine complexes as well as one containing an organocobalt moiety (Acuña-Cueva et al., 2003; Beddoes et al., 1997; Burgmayer & Stiefel, 1988; Funahashi et al., 1997). The concerned ligands usually act as bidentate O,N-donors and none of the above complexes possesses a typical π-acceptor ancillary ligand like 1,10-phenanthroline (phen). In this crystallographic study on the title cobalt(II) complex, possessing both a tridentate pterin ligand and a π-acidic ligand like phen, different aspects are considered, e.g. crystal, molecular and electronic structures.

In the title compound (Fig. 1), the stereochemistry around the Co^II atom is essentially distorted octahedral with two N atoms of phen, a pyrazine ring N atom (N3) of the pterin ligand and an aqua O atom forming the equatorial plane; two pterin O atoms (O1 and O3) define the longer axial positions, with the phenolate O3 forming the longest axial bond [2.270 (2) Å]. Extent of distortion of this coordination octahedron is much more pronounced as compared to that of the Co(II)-pteridine complexes reported earlier (Acuña-Cueva et al., 2003; Burgmayer & Stiefel, 1988; Funahashi et al., 1997). A major cause of this departure from regular geometry is that the pterin ligand forms two five-membered chelate rings having small bite angles [75.10 (10) and 76.26 (9)°], instead of only one per pteridine ligand for the earlier cases. Location of the short Co1—N3 bond [2.016 (3) Å] in the equatorial plane is consistent with the literature, which suggests a strong cobalt-pterin interaction (Odani et al., 1992). The pterin ligand is coordinated here as a binegative tridentate ONO donor, as evident from the charge balance of this complex. The phen and pterin rings are nearly perpendicular to each other for minimizing the steric repulsion. The Co1—N1 [2.079 (3) Å] and Co1—N2 [2.123 (3) Å] bond lengths are at par with that of the Co1—N3 bond [2.016 (3) Å] and indicate receipt of π-back donation to both phen and pterin rings from the Co(II) centre (d⁷) through dπ–pπ interactions. This process is further strengthened by the presence of π-donating phenolate and carboxylate O atoms around the metal centre (Kohzuma et al., 1988).

For rationalizing the near double bond nature of the O3—C18 [1.265 (4) Å] bond, a hypothesis of Joule (Beddoes et al., 1993; Russell et al., 1992) may be invoked, which suggests withdrawl of electron density from the pyrazine ring N6 by the pyrimidine ring C18-carbonyl group through mesomeric interaction. Formation of the O3—Co1 bond accentuates this electron withdrawal towards O3. The electron-rich N7—C17 [1.337 (4) Å] bond may also participate in this electron transfer. The pyrimidine ring is fairly planer and deviations of the C16/N5/C17 and C17/N4/C18 segments with respect to the N7—C17 multiple bonds are 2.6 and 0.7°, respectively.

In the crystal, intermolecular N—H···O, O—H···N and O—H···O hydrogen bonds (Table 1) link the complex molecules and lattice water molecules into a layer parallel to (001) (Fig. 2). The lattice water molecules are decisive for the crystal packing. Fig. 3 reveals π–π stacking interactions involving two parallel, inversion-related pterin rings within the same unit cell and showing face-to-face distance of 3.283 (4) and 3.366 (4) Å. Again the phen rings display two types of π–π stacking on either side of the unit cell. In one case, the adjacent phen rings are essentially parallel to each other with an average interplanar distance of 3.496 (4) Å; on the other side of the unit cell, the face-to-face seperations between parallel phen rings are 3.578 (4) and 3.629 (5) Å.