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′)copper(II) trihydrate

In a hydrated copper(II) complex, 2-amino-7-methyl-4-oxidopteridine-6-carboxylate and 1,10-phenanthroline ligands chelate the CuII cation while a water molecule further coordinates to the CuII cation to complete the elongated distorted octahedral coordination geometry.


Chemical context
The ubiquitous presence of pterins in nature including several classes of metalloenzymes, has catalysed developments of their coordination chemistry (Basu & Burgmayer, 2011;Burgmayer, 1998;Dix & Benkovic, 1988;Erlandsen et al., 2000;Fitzpatrick, 2003). Literature survey reveals the paucity of structurally characterized Cu II complexes involving tridentate pterin coordination (Kohzuma et al., 1989). The present work is concerned with the title complex, possessing both a tridentate pterin ligand and a -acidic ligand like phen.

Structural commentary
The hexacoordinated Cu II atom is located in an axially elongated distorted octahedron ( Fig. 1 and Table 1). The equatorial plane is formed by the two N atoms of phen, the pyrazine ring N atom of the pterin ligand and the aqua O atom. The axial positions are occupied by the two pterin O ISSN 1600-5368 atoms, with the former one forming the longest axial bond [2.384 (3) Å ]. Apart from the characteristic Jahn-Teller effect, another reason for distortion from a regular octahedral geometry is that the pterin ligand forms two five-membered chelate rings with small bite angles [76.47 (10) and 74.66 (11) ]. Consideration of the charge balance of this complex indicates that this pterin ligand acts as a binegative tridentate O,N,O 0 -donor. A near orthogonal disposition of the phen ligand and pterin chelate ring helps to minimize the steric repulsion. Of the three axes, the least deviation from linearity is observed in the O4-Cu1-N2 direction [174.45 (13) ]. Location of the pyrazine ring N atom (N6) in the equatorial plane is in agreement with earlier observations on related copper and cobalt complexes (Baisya et al., 2013;Odani et al., 1992); the Cu1-N6 bond length [1.999 (3) Å ] is the shortest one in this case.
The multiple bond character of the O1-C13 bond [1.237 (4) Å ] may be elucidated in the light of Joule's hypothesis (Beddoes et al., 1993;Russell et al., 1992), suggesting electron-density withdrawal from the pyrazine ring N5 by the pyrimidine ring C13 carbonyl group through mesomeric interaction. Formation of the O1-Cu1 bond assists this electron flow towards atom O1, with possible participation of the electron-rich N7-C14 [1.327 (5) Å ] bond in this process.

Supramolecular features
In the crystal, intermolecular N-HÁ Á ÁÁO, O-HÁ Á ÁÁN and O-HÁ Á ÁO hydrogen bonds (Table 2) link the complex molecules and lattice water molecules into a layer parallel to (001) (Fig. 2). Intermolecular weak C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á interactions are also observed in the crystal. In addition,stacking between nearly parallel pterin ring systems of adjacent molecules occurs in the crystal structure, the centroid-centroid distance being 3.352 (2) Å (Fig. 3). Again, the nearly parallel phen rings of adjacent molecules also displaystacking interactions with centroids distances of 3.546 (3), 3.706 (3) and 3.744 (3) Å . These intermolecular interactions link the molecules into a three-dimensional supramolecular architecture.

Database survey
The crystal structures of copper(II) complexes chelated by the pterin-6-carboxylate anion have been reported by Kohzuma et The molecular structure of the title compound. Displacement ellipsoids are drawn at the 30% probability level.  Hydrogen-bond geometry (Å , ).

Figure 2
The crystal packing diagram of the title compound, viewed along the a axis. Hydrogen bonds (dotted lines) assist the formation of a layer structure parallel to (001).
al. (1989) and Funahashi et al. (1999). In both complexes, the Cu II atom has the elongated distorted octahedral coordination geometry.

Synthesis and crystallization
2-Amino-4-hydroxy-7-methylpteridine-6-carboxylic acid sesquihydrate (C 8 H 7 N 5 O 3 Á1.5H 2 O) was obtained by a published procedure (Wittle et al., 1947). The title complex could be obtained by two different methods; the crystals obtained by method B have been used for the present structural study. The X-ray structural data of the crystals synthesized by method A, are available from the Cambridge Crystallographic Data Center (CCDC deposition No. 985054).
Method A. The title complex was synthesized by bubbling oxygen into an aqueous reaction mixture (50 ml) containing Cu(NO 3 ) 2 Á3H 2 O (30 mg, 0.125 mmol), 1,10-phenanthroline monohydrate (25 mg, 0.125 mmol) and pterin (31 mg, 0.125 mmol) dissolved in NaOH (11 mg, 0.275 mmol) for 60 h at 310-312 K under subdued light; additional NaOH solution was added for adjusting the initial pH at 10.5. Within a short while the initial bright-green solution turned hazy blue due to the presence of a fine white precipitate which gradually disappeared substantially. The final blue solution was slightly hazy. Upon storage under aerobic conditions for one week the clear blue solution yielded green crystals, suitable for X-ray structure determination.
Method B. Using NaBH 4 reduction in equimolar proportion of the original complex (obtained by Method A) and subsequent aerial reoxidation of the reduced complex to the present crystals merits attention due to the involvement of intricate redox chemistry. The NaBH 4 treatment (Beddoes et al., 1993;Russell et al., 1992) leads to the formation of a dark-brown compound in solution, which could be isolated in the solid state and characterized (microanalysis, ESIMS, 2DNMR, etc.,) to be Na 2 [Cu 2 I (L 0 ) 2 (phen)(H 2 O) 4 ]Á2H 2 O, where L 0 is the 7,8dihydro form of the present pterin ligand anion (C 8 H 5 N 5 O 3 ) (Burgmayer, 1998); it is able to convert bromobenzene into 4bromophenol in the presence of oxygen (Baisya & Roy, unpublished results). However, in the absence of any substrate (e.g. bromobenzene; Dix & Benkovic, 1988), aerial oxidation reconverts the reduced compound to the title complex (Method B).
Although the title compound could be obtained by two alternative methods, the present structural data obtained using the crystals from Method B, represent better accuracy [R = 0.057 and wR(F 2 )= 0.135] as compared to the other one [R = 0.113 and wR(F 2 ) = 0.279].
Cyclic voltammetry data of this complex indicate an E 0 value of À0.68 V; now using an E 0 value of À0.80 V for NaBH 4 in neutral medium (Chatenet et al., 2006;Celikkan et al., 2007), an E cell value (E cell = E 1 À E 2 ; Segel, 1976) of 0.12 V is obtained for the Cu II ! Cu I reduction in the title complex; it is within the range of E cell value (0.033 V) for the Fe IIItetrahydrobiopterin reduction in phenylalanine hydroxylase (Hagedoorn et al., 2001;Gorren et al., 2001). The dark-brown reduced complex (as above) shows an E 0 value of À0.67 V (cyclic voltammetry); using an E 0 value of 0.70 V for the 350 Baisya and Roy [Cu (C 8 A molecular packing diagram highlightingstacking interactions between neighbouring phen-phen and pterin-pterin rings. Computer programs: APEX2 and SAINT (Bruker, 2007), SHELXS97 (Sheldrick, 2008), CRYSTALS (Betteridge et al., 2003) and CAMERON (Watkin et al., 1996).
O 2 /H 2 O 2 couple, an E cell value of 1.37 V is obtained, indicating facile aerial oxidation. Now using an E 0 value of 0.19 V for the chelated pterin ligand (oxidized/aromatic; Eberlein et al., 1984), synchronization of its reduction or oxidation with the above redox process may be rationalized. Actually, for pterincontaining metalloenzymes the redox processes at the metal centres could be linked to the changes in the pterin ring oxidation level (Burgmayer, 1998;Erlandsen et al., 2000).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3

(2-Amino-7-methyl-4-oxidopteridine-6-carboxylato-κ 3 O 4 ,N 5 ,O 6 )aqua(1,10-phenanthroline-κ 2 N,N′)copper(II)
trihydrate Crystal data Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. 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 > 2sigma(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.