**4. Conclusions**

Three Fe(III) compounds (**1**-**3**) derived from *N*--acetylpyrazine-2-carbohydrazide were successfully applied for the peroxidative oxidation of cyclohexane under di fferent energy stimuli (microwave irradiation, ultrasound, and conventional heating). In order to improve the catalytic performance, the e ffects of di fferent reaction parameters were studied, namely the reaction time, catalyst amount, substrate:oxidant ratio, and presence of additives. Compounds **1**-**3** catalyzed the oxidation of cyclohexane via a radical mechanism, yielding cyclohexanol and cyclohexanone as the main products; the former with a high selectivity upon the reduction of the primary product cyclohexylperoxide (up to 95%). Complexes **1** and **3** exhibited similar catalytic activities, with 14–15% yields of cyclohexanol and cyclohexanone under 3 h of microwave irradiation at 50 ◦C, whereas complex **2** reached a total product yield of 10.4%.

Apart from being undertaken under microwaves, the peroxidative oxidation of cyclohexane was also performed using other types of energy inputs, i.e., conventional heating and ultrasound irradiation. The catalysts responded di fferently to the various energy stimuli, with the best performance of **1** being observed under microwave radiation, whereas complex **2** showed the maximum activity under conventional heating and compound **3** under ultrasounds. However, in all cases, the highest selectivity to cyclohexanol was verified when the reaction was assisted by microwave radiation.

In the case of **1,** a significant increase of the total yield was observed from 15 to 38% in the presence of an additive (HNO3), although such an influence was not found for **2** and **3**. Therefore, the combined use of nitric acid and the mononuclear cationic Fe(III) compound **1** plays a crucial role in accelerating the catalytic oxidation process.

This study can help to foster the fruitful use of the environmentally acceptable oxidant aqueous TBHP and the application of microwave heating or ultrasounds to promote catalysis, which have significant environmental implications.

**Supplementary Materials:** CCDC number 2019464 contains the supplementary crystallographic data for **1**. This can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)-1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.

**Author Contributions:** Conceptualization, M.S. and E.C.B.A.A.; Methodology, M.S. and E.C.B.A.A.; Software, M.S., M.d.F.C.G.d.S. and T.R.B.; Validation, M.S., M.d.F.C.G.d.S. and E.C.B.A.A.; Formal Analysis, M.S. and T.R.B.; Investigation, M.S. and T.R.B; Resources, M.S. and A.J.L.P.; Data Curation, M.S., M.d.F.C.G.d.S. and T.R.B.; Writing-Original Draft Preparation, M.S., T.R.B. and E.C.B.A.A.; Writing-Review & Editing, M.d.F.C.G.d.S. and A.J.L.P.; Visualization, M.S.; Supervision, M.S. and E.C.B.A.A.; Project Administration, A.J.L.P.; Funding Acquisition, A.J.L.P and E.C.B.A.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work has been supported by the Fundação para a Ciência e Tecnologia (FCT) 2020-2023 multiannual funding to Centro de Química Estrutural (project UIDB/00100/2020).

**Acknowledgments:** The authors are grateful to the Fundação para a Ciência e Tecnologia (FCT) project PTDC/QUI-QIN/29778/2017 for financial support. M.S. acknowledges the FCT and IST for a working contract "DL/57/2017" (Contract no. IST-ID/102/2018).

**Conflicts of Interest:** The authors declare no conflict of interest.
