## 3 Results and Discussion
### Crystal Structure Analyses
Figure 2: (**A**) Ball-and-stick representation of the two crystallography independent complex units and counter-ions in **2** with partial atom numbering scheme. (**B**) Its hydrogen-bonded network (the NHE\({}_{3}\)\({}^{+}\) ions are represented in red and yellow space-fill model). The reaction of HL with FeCl\({}_{3}\) and sodium azide in solvothermal condition leads to the formation of the 2D heterometallic network 4 (Figure 4C). Its asymmetric unit contains two L\({}^{-}\) moieties, each one O- and N\({}_{\text{Pyrazine}}\)-bonded to a Fe\({}^{3+}\) cation, its octahedral type coordination sphere being completed with an N\({}_{\text{azide}}\) and half an O\({}_{\text{oxo}}\) anion; a Na\({}^{+}\) cation linked to the N\({}_{\text{pyrazine}}\) of one of the L\({}^{-}\) groups and a coordinated water molecule were also found (Figure 4A). Symmetry expansion reveals a hexa-coordination sodium center with the carboxylate groups of two L\({}^{-}\) acting in a chelating mode (Figure 4B). In this way, the 2D polymer is formed by sheets of iron and sodium cations connected by the organic moieties, the oxido anions, and the water molecules, respectively, bridging the Fe(III) and the Na(I) centers, thus connecting two infinite sheets. The Fe-O bond length of the \(\mu\)-oxido diiron(III) core is approximately 1.772 A, the Fe-O\({}_{\text{oxo}}\)-Fe bond angle is approximately 180\({}^{\circ}\), and the distance between the two symmetry-related Fe(III) centers is 3.544 A; these values are in the range of those observed for other iron(III) \(\mu\)-oxido-bridged complexes [38]. The distance between the two symmetry-related Na(I) centers is 3.963 A, and the Na-O\({}_{\text{water}}\)-Na bond angle is 113.3\({}^{\circ}\). This is the first example of an oxido- and aqua-bridged heterometallic Fe(III)-Na(I) coordination polymer based on the 3-amino-2-pyrazinecarboxylic acid proligand. The oxido-bridged diiron(III) core has structural resemblance with one of the intermediate structures of methane monooxygenase during the catalytic conversion of methane to methanol [38]. Extensive H-bond interactions can be found that play a role towards the extension to 3D supramolecular frameworks. A list of selected hydrogen bonding interactions is presented in Table S3 (Supplementary Materials). In this framework, intramolecular resonance-assisted N-H-\(\cdots\)O hydrogen bonding as well as various C-H-\(\cdots\)O interactions play a significant role in stabilizing the structure and extending it to the third dimension. Figure 3: (**A**) Ball-and-stick representation of **3** with partial atom numbering scheme. (**B**) Its 2D hydrogen-bonded network. Figure 4: (**A**) Ball-and-stick representation of the asymmetric unit of coordination polymer **4** with partial atom numbering scheme, (**B**) its crystal structure, and (**C**) two-dimensional depiction (arbitrary view). The 3-amino-2-pyrazinecarboxylate ligand in the compounds of this study exhibits three different coordination modes, shown in Scheme 2A. Mode I is the most common and is present in complexes 1, 2, and 3; L\({}^{-}\) acts as a N,O-bidentate chelating ligand and coordinates to one Fe(III) center. Modes II and III are only found in the coordination polymer **4**; the carboxylate group itself acts as a bridging chelate ligand, and in III, the pyrazine-N is also involved in coordination. Additionally, the 3-amino-2-pyrazinecarboxylate ligand is involved in various hydrogen bond interactions such as N-H\(\cdots\)-O, N-H\(\cdots\)-N, or intramolecular resonance-assisted hydrogen bonding (RAHB) contacts (Scheme 2B).