During the last 50 years a vast number of structural studies on Schiff bases derived from hydroxyaryl aldehydes and their coordination compounds have been undertaken. In this review we present a systematic and brief overview of the most important discoveries and achievements accomplished in this field. The occurrence of Schiff bases (and their complexes) derived from nine most commonly used hydroxyaryl aldehydes in the Cambridge Structural Database (CSD) has been investigated. Keto-enol tautomerism and intramolecular hydrogen bonding in salicylaldimines and naphthaldimines is discussed, with a comment of the most common errors in determining the correct tautomeric form. Also, the interrelationship of crystal packing and the substituents on the Schiff base is studied. Finally, we give a short overview of conformational differences between free Schiff bases and Schiff bases bonded to metal ions as well as some structural characteristics of Schiff base metal complexes.

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Schiff bases derived from hydroxyaryl aldehydes: molecular and crystal structure, tautomerism, quinoid effect, coordination compounds

The use of biophenols to develop bioantioxidants is devoted to sustainable development, reducing energy and environmental issues. In order to step closer to practical application, bioantioxidant effectiveness is as important as renewability. A novel intramolecular synergism strategy aiming at high-efficiency oxidative inhibition is proposed by combining biobased phenol and arylamine in one molecule. Mannich condensation of hydrogenated cardanol (HC) with arylamines and paraformaldehyde led to the formation of three phenol-arylamine antioxidants (BAs). BAs were characterized by HRMS, FT-IR, NMR, and thermogravimetic (TG) analysis. Free radical scavenging activity of BAs was assessed using a DPPH (1,1-diphenyl-2-picrylhydrazyl) method, and was found to be much higher than that of commercial antioxidants. BAs were evaluated as antioxidant and thermal-oxidative stabilizer in polyol in terms of pressure differential scanning calorimetry (PDSC), rotary bomb oxidation tests (RBOT), and TGA. In a comparison to co...

One-Pot Synthesis of Cardanol-Derived High-Efficiency Antioxidants Based on Intramolecular Synergism

The variable coordination behaviour of the mixed ligand 1-(2-hydroxyethyl)-3,5-dimethylpyrazole (L) towards Zn(II), Cd(II) and Hg(II) ions has been investigated. Three new complexes, [ZnCl2(L)] (1), [CdCl2(L)]2 (2), and [HgCl2(L)]n (3), with different structures and topologies have been fully characterized by analytical and spectroscopic techniques including heteronuclear 113Cd and 199Hg NMR analyses. Complexes 1–3 were also characterized by single crystal and powder X-ray diffraction techniques. The L ligand exhibits high versatility and provides different environments to the metal centre as a function of its diverse coordination mode. The weak π–π stacking interactions and intermolecular hydrogen bonds resulted in the two-dimensional network structures. All these results point out the structural diversity of the hybrid pyrazole ligand on its coordination. Finally, we determined the fluorescent properties of complexes 1–3. Surprisingly, L displays a high selectivity for Hg(II) ion. The findings indicate that the variable behaviour of the hydroxyl group substituent exerts an important effect on the spectroscopic properties. Moreover, the fluorescence of these complexes has been rationalized through theoretical calculations. The primary results suggest that the use of L is a promising strategy to synthesize novel materials with unique photoluminescent properties and further applications for the detection of heavy metals in real environmental samples.

Variable behaviour of flexible N,O-mixed pyrazole ligand towards Zn(II), Cd(II) and Hg(II) ions. Synthesis, crystal structure and fluorescent properties

Reaction of 2,6-bis(dialkylaminomethyl)-4-alkylphenols (L) with ZnCl2 in ethanol leads to zwitterionic Zn(II) complexes [ZnCl2L]. An X-ray diffraction study on the compound with L=2,6-bis(dimethylaminomethyl)-4- tert-butylphenol reveals the presence of a mononuclear zincate complex, in which the distorted tetrahedral coordination of the metal consists of two chlorine atoms and the ligand L acting as a bidentate chelating agent via the deprotonated phenol oxygen and one amino group. Migration of the phenolic hydrogen to the uncoordinated amino group in the other ortho substituent generates an ammonium group whose positive charge balances the negative charge of the zincate centre. This ammonium group forms an intramolecular N–H···O hydrogen bond with the coordinated phenoxide oxygen. Complex stability is enhanced by the reduction of electron density accompanying the formation of the phenoxide ‘bridge’ between the Zn atom and the ammonium proton. 1H and 13C NMR spectroscopies indicate that ligand dissociation does not occur and the complexes retain their solid-state structure in solution.

Mononuclear zinc(II) complexes of o-amino phenolate ligands. X-Ray structure of the zwitterionic [ZnCl2L] compound with 2,6-bis(dimethylaminomethyl)-4-tert-butylphenol

The interaction of copper(II) with medications, used for the regulation of arterial blood pressure, has been studied and discussed. The drugs include antihypotensives and antihypertensives of different types: β-blockers, vasodilators, ACE-inhibitors and diuretics. The paper summarizes the results obtained for the characterization of the complexation processes and determination of the structure and properties of the complexes formed.

Copper(II) complexes of blood pressure active drugs

Zinc(II) and copper(II) complexes of monoanionic chelating phenolato ligands containing an amine donor. Crystal and molecular structure of the (Et3NH)[ZnCl2L] and CuL2 compounds with L=2-[(diethylamino)methyl]-4-methylphenolate

Nouzha Habaddi

Papers

Zinc(II) and copper(II) complexes of monoanionic chelating phenolato ligands containing an amine donor. Crystal and molecular structure of the (Et3NH)[ZnCl2L] and CuL2 compounds with L=2-[(diethylamino)methyl]-4-methylphenolate