**3. Experimental**

### *3.1. General Materials and Procedures*

The syntheses were performed in air. Reagents and solvents were obtained from commercial sources and used as received, without further purification or drying. Fe(NO3)3.9H2O was used as the source of metal ion for the synthesis of **1**. Anhydrous FeCl2was used for the syntheses of **2** and **3**.

C, H, and N elemental analyses were carried out by the Microanalytical Service of the Instituto Superior Técnico. Infrared spectra (4000–400 cm<sup>−</sup>1) were recorded on a Bruker Vertex 70 instrument (Bruker Corporation, Ettlingen, Germany) in KBr pellets; wavenumbers are given in cm<sup>−</sup>1. Mass spectra were run in a Varian 500-MS LC Ion Trap Mass Spectrometer (Agilent technologies, Amstelveen, The Netherlands) equipped with an electrospray (ESI) ion source. The drying gas and flow rate were optimized (for electrospray ionization) according to the particular sample, with a 35 p.s.i. nebulizer pressure. Scanning was performed from *m*/*z* 100 to 1200 in methanol solution. The compounds were observed in the positive mode (capillary voltage = 80–105 V).

### *3.2. Synthesis of the Pro-Ligand H2L*

*N*--acetylpyrazine-2-carbohydrazide (H2L), the pro-ligand, (Scheme 1), was synthesized as described in the literature [45], by the acetylation of pyrazine-2-carbohydrazide.

Yield: 86.0%. Anal. calc. for (C7H8N4O2): C, 46.67; H, 4.48; N, 31.10; found: C, 46.62; H, 4.43; N, 31.06%. IR (KBr pellet, cm<sup>−</sup>1): 3324 ν(NH), 3216 ν(NH), 1698 <sup>ν</sup>(C=O), 1670 <sup>ν</sup>(C=O). 1H NMR (DMSO-*d*6, δ): 9.18-8.86 (m, 3H, C4H3N2), 8.74 (s, 2H, NH), 1.91 (s, 3H, CH3).

### *3.3. Syntheses of Fe(III) Complexes of N*-*-acetylpyrazine-2-carbohydrazide*
