## 5 Concluding Remarks  
Four novel iron(III) coordination compounds were derived from 3-amino-2-pyrazinecarboxylic acid (HL) under various synthetic conditions. They exhibit mononuclear (1 and 2), dinuclear (3), and 2D heterometallic polymeric (4) structures, on account of the various coordination modes of L\({}^{-}\). Single-crystal X-ray diffraction analysis revealed that 1 and 2 have different mononuclear structures, the latter with two different and crystallographically independent complex metal cations, whereas compound 3 has a methoxido-bridged dinuclear structure. Compound **4** features the first example of a two dimensional oxido- and aqua-bridged heterometallic Na(I)/Fe(III) polynuclear coordination polymer based on such a ligand. In all compounds, the Fe(III) centers adopted octahedral type coordination environments.  
Cyclic voltammetric studies, supported by controlled potential electrolyses, indicate that complexes 1-4 easily undergo reversible Fe(III) to Fe(II) reductions at a positive potential (0.08 V) vs. SCE, thus being easily reduceable (good oxidants), which is in line with their ability to act as efficient heterogeneous catalysts for the mild additive-free microwave-assisted peroxididative oxidation of cyclohexanol to cyclohexanone. Thus, the electrochemical studies appear as a useful tool to assign the ability of a metal complex to behave as an oxidant and, therefore, to predict its catalytic performance in oxidation catalysis. Moreover, the catalysts were easily recovered and reused, at least for five consecutive cycles, maintaining a high selectivity and activity.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/8/2692/s1](http://www.mdpi.com/2076-3417/10/8/2692/s1)]([http://www.mdpi.com/2076-3417/10/8/2692/s1](http://www.mdpi.com/2076-3417/10/8/2692/s1)), Figure S1: 2D fingerprint plots of 1-4, exhibiting different intermolecular interactions, Figure S2: FT-IR spectra of 3 before (in blue), after (in red) the microwave-assisted catalytic peroxididative oxidation of cyclohexanol, Figure S3: Cyclic voltammograms (current intensity in uA _vs._ potential in V), initiated by the anodic sweep, of compounds 1-4 in a 0.2 M [\"BuA][BF4]/DMSO solution, at a Pt disc working electrode. Internal standard used: [Fe(n\({}^{5}\)-C\({}_{5}\)H\({}_{5}\))2]\({}^{0/+}\) redox couple (E\({}_{1/2}\)\({}^{\infty}\) = 0.42 V vs. SCE), Table S1: Crystal data and structure refinement details for compounds 1-4, Table S2: Selected bond distances (A) and angles (\({}^{\circ}\)) for compounds 1-4, Table S3: Hydrogen bond geometry (A, \({}^{\circ}\)) in compounds 1-4, Table S4: MW-assisted oxidation of cyclohexanol to cyclohexanone with TBHP and catalyzed by 1 - 4.  
A.K.: Research planning, synthesis and characterization of complexes, Hirshfeld Surface Analysis, manuscript writing. L.M.D.R.S.M.: Catalytic studies. Y.Y.: Electrochemistry and Catalytic studies. M.F.C.G.d.S.: Single crystal X-ray diffraction analysis of compounds. A.J.L.P.: Manuscript reading and correcting. All authors have read and agreed to the published version of the manuscript.  
This research was funded by [Fundacao para a Ciencia e Tecnologia (FCT)] grant number [UIDB/00100/2020].  
[PERSON] expresses his gratitude to the FCT and Instituto Superior Tecnico for Scientific Employment contract (Contrato No: IST-ID/107/2018) under Decree-Law no. 57/2016, of August 29.  
There is no conflict of interest.