Within the current challenges in medicinal chemistry, the development of new and better therapeutic agents effective against infectious diseases produced by bacteria, fungi, viruses, and parasites stands out. With chemotherapy as one of the main strategies against these diseases focusing on the administration of organic and inorganic drugs, the latter is generally based on the synergistic effect produced by the formation of metal complexes with biologically active organic compounds. In this sense, Schiff bases (SBs) represent and ideal ligand scaffold since they have demonstrated a broad spectrum of antitumor, antiviral, antimicrobial, and anti-inflammatory activities, among others. In addition, SBs are synthesized in an easy manner from one-step condensation reactions, being thus suitable for facile structural modifications, having the imine group as a coordination point found in most of their metal complexes, and promoting chelation when other donor atoms are three, four, or five bonds apart. However, despite the wide variety of metal complexes found in the literature using this type of ligands, only a handful of them include on their structures tridentate SBs ligands and their biological evaluation has been explored. Hence, this review summarizes the most important antimicrobial activity results reported this far for pincer-type complexes (main group and d-block) derived from SBs tridentate ligands.

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Reactions of 1,4-bis[(S) methylbenzyl]-1,4-diazabutadiene (1), 1,4-bis[(R) 1 0 -chlorobutan-2 0 -yl]-1,4-diazabutadiene (2), 1,4-bis[(S,S) 1 0 -chloro-1 0 -phenyl-propan-2 0 -yl]-1,4-diazabutadiene (3), and 1,4-di-tert-butyl-1,4diazabutadiene (4) with aluminum and indium halides were performed. Reaction of 1,4-diazabutadienes 1-4 with two equivalents of aluminum halides in toluene afforded ionic aluminum coordination compounds (5-10) which in THF solution are transformed into 1,3-dialkyl-2-methaneiminealkyl imidazolium heterocycles (11-14) by condensation of two 1,4-diazabutadienes and elimination of amine hydrochloride. Equimolar reactions of 1-4 with InCl 3 in dry acetonitrile at À78 1C afforded the neutral and diamagnetic InCl 3 coordination compounds (15-18), which are stable in the solid state under dry conditions, but in THF solution are slowly transformed into the corresponding 1,3-dialkyl-imidazolium heterocycles 19-22. The X-ray diffraction analyses of compounds 2, 3, 10, 11, 14 and 17 are described.Quantum mechanical calculations were performed in order to find the minimum energy conformers of 1,4-diazabutadienes, 1,3-dialkyl-2-methaneiminealkyl imidazolium and 1,3-dialkyl-imidazolium derivatives as well as the aluminum and indium coordination compounds.Scheme 1 Neutral, anion and dianionic oxidation states of 1,4diazabutadienes.

The Diels-Alder cycloaddition reaction of oxazole with ethylene is facilitated by addition of an alkyl group or Brønsted or Lewis acids to the oxazole nitrogen atom. The efficacy consists of stabilizing the transition state, lowering the activation barrier and the HOMO(dienophile)-LUMO(diene) gap, and increasing the reaction exothermicity.

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