Reactivity of Lewis Basic Platinum Complexes Towards Fluoroboranes

We herein report detailed investigations into the interaction of Lewis acidic fluoroboranes, for example BF₂Pf (Pf=perfluorophenyl) and BF₂Ar^F (Ar^F=3,5‐bis(trifluoromethyl)phenyl), with Lewis basic platinum complexes such as [Pt(PEt₃)₃] and [Pt(PCy₃)₂] (Cy=cyclohexyl). Two presumed Lewis adducts could be identified in solution and corresponding secondary products of these Lewis adducts were characterized in the solid state. Furthermore, the concept of frustrated Lewis pairs (FLP) was applied to the activation of ethene in the system [Pt(BPf₃)(CH₂CH₂)(dcpp)] (dcpp=1,3‐bis(dicyclohexylphosphino)propane; Pf=perfluorophenyl). Finally, DFT calculations were performed to determine the interaction between the platinum‐centered Lewis bases and the boron‐centered Lewis acids. Additionally, several possible mechanisms for the oxidative addition of the boranes BF₃, BCl_3, and BF₂Ar^F to the model complex [Pt(PMe₃)₂] are presented.     

Conformational studies on heteroleptic trifluoromethyl‐substituted phenylboranes

Organoboranes carrying electron‐withdrawing substituents are commonly used as Lewis acidic catalysts or cocatalysts in a variety of organic processes. These Lewis acids also became popular through their application in `frustrated Lewis pairs', i.e. combinations of Lewis acids and bases that are unable to fully neutralize each other due to steric or electronic effects. We have determined the crystal and molecular structures of four heteroleptic arylboranes carrying 2‐(trifluoromethyl)phenyl, 2,6‐bis(trifluoromethyl)phenyl, 3,5‐bis(trifluoromethyl)phenyl or mesityl substituents. [3,5‐Bis(trifluoromethyl)phenyl]bis[2‐(trifluoromethyl)phenyl]borane, C₂₂H₁₁BF₁₂, (I), crystallizes with two molecules in the asymmetric unit which show very similar geometric parameters. In one of the two molecules, both trifluoromethyl groups of the 3,5‐bis(trifluoromethyl)phenyl substituent are disordered over two positions. In [3,5‐bis(trifluoromethyl)phenyl]bis[2,6‐bis(trifluoromethyl)phenyl]borane, C₂₄H₉BF₁₈, (II), only one of the two meta‐trifluoromethyl groups is disordered. In [2,6‐bis(trifluoromethyl)phenyl]bis[3,5‐bis(trifluoromethyl)phenyl]borane, C₂₄H₉BF₁₈, (III), both meta‐trifluoromethyl groups of only one 3,5‐bis(trifluoromethyl)phenyl ring are disordered. [3,5‐Bis(trifluoromethyl)phenyl]dimesitylborane, C₂₆H₂₅BF₆, (IV), carries only one meta‐trifluoromethyl‐substituted phenyl ring, with one of the two trifluoromethyl groups disordered over two positions. In addition to compounds (I)–(IV), the structure of bis[2,6‐bis(trifluoromethyl)phenyl]fluoroborane, C₁₆H₆BF₁₃, (V), is presented. None of the ortho‐trifluoromethyl groups is disordered in any of the five compounds. In all the structures, the boron centre is in a trigonal planar coordination. Nevertheless, the bond angles around this atom vary according to the bulkiness and mutual repulsion of the substituents of the phenyl rings. Also, the ortho‐trifluoromethyl‐substituted phenyl rings usually show longer B—C bonds and tend to be tilted out of the BC₃ plane by a higher degree than the phenyl rings carrying ortho H atoms. A comparison with related structures corroborates the conclusions regarding the geometric parameters of the boron centre drawn from the five structures in this paper. On the other hand, CF₃ groups in meta positions do not seem to have a marked effect on the geometry involving the boron centre. Furthermore, it has been observed for the structures reported here and those reported previously that for CF₃ groups in ortho positions of the aromatic ring, disorder of the F atoms is less probable than for CF₃ groups in meta or para positions of the ring.     

Poly‐Schiff bases. V. Synthesis and characterization of novel soluble fluorine‐containing polyether azomethines

A new dialdehyde monomer, 4,4′‐(hexafluoroisopropylidine) bis(p‐phenoxy) benzaldehyde, was prepared; it led to a number of novel poly‐Schiff bases in reactions with different diamines, such as 4,4′‐diaminidiphenyl ether, 4,4′‐(isopropylidine) bis(p‐phenoxy) dianiline, 4,4′‐(hexafluoroisopropylidine) bis(p‐phenoxy) dianiline, and benzidine. The polymers were characterized with viscosity measurements, nitrogen analyses, and IR and ¹H NMR spectroscopy. These poly‐Schiff bases showed good thermal stability up to 491 °C for 10% weight loss in thermogravimetric analysis under air and high glass‐transition temperatures up to 215 °C in differential scanning calorimetry. These polymers were soluble in a wide range of organic solvents, such as CHCl₃, dimethylformamide (DMF), dimethyl sulfoxide, and 1‐methyl‐2‐pyrrolidon (NMP), and were insoluble in toluene and acetone. Thin films of these polymers cast from DMF exhibited tensile strengths up to 38 MPa. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 383–388, 2001     

Design,synthesis, and evaluation of unsymmetrical difluoro‐boron complexes with imidazoline as potential fungicides

A series of unsymmetrical difluoro‐boron(BF₂) complexes with pyridine and imidazoline were synthesized by reaction of new chelating ligands (arylmethyl‐imidazolidinylidene)‐pyridin‐2‐yl‐amine with boron trifluoride diethyl etherate. All the ligands and BF₂ complexes were structurally characterized by IR, HRMS, ¹H, ¹³C,¹¹B, and ¹⁹F NMR, indicating the bidentate complexation of imidazoline nitrogen and the pyridine nitrogen to the boron center. Evaluation of agricultural bioactivities showed that some of the BF₂ complexes exhibited moderate fungicidal activities, and most of the BF₂ complexes exhibited higher activities than the none‐BF₂ complexed substrates. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 20:418–424, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20567     

Synthesis,characterization and biocidal studies of ruthenium(II) carbonyl complexes containing tetradentate Schiff bases

Stable ruthenium(II) complexes of Schiff bases have been prepared by reacting [RuHCl(CO)(PPh₃)₂(B)] (B = PPh₃, pyridine or piperidine) with bis(o-vanillin)ethylenediimine (valen), bis(o-vanillin)propylene-diimine (valpn), bis(o-vanillin)tetramethylenediimine (valtn), bis(o-vanillin)o-phenylenediimine (valphn), bis(salicylaldehyde)tetramethylenediimine (saltn) and bis(salicylaldehyde)o-phenylenediimine (salphn). These complexes have been characterised by elemental analyses, i.r., electronic, ¹H- and ³¹P{¹H}-n.m.r. spectral studies. In all the above reactions, the Schiff bases replace two molecules of Ph₃P, a hydride and a halide ion from the starting complexes, indicating that the Ru–N bonds present in the complexes containing heterocyclic nitrogen bases are stronger than the Ru–P bond to Ph₃P. The new complexes of the general formula [Ru(CO)(B)(L)] (B = PPh₃, py or pip; L = tetradentate Schiff bases) have been assigned an octahedral structure. Some of the Schiff bases and the new complexes have been tested against the pathogenic fungus Fusarium sp.     

Design,Synthesis and Antitumor Activity of Asymmetric Bis(s‐triazole Schiff‐base)s Bearing Functionalized Side‐Chain

1‐Amino‐2‐pyrid‐3‐yl‐5‐(2‐benzoylethylthio)‐s‐triazole ( 1 ) was condensed with 1‐amino‐3‐mercapto‐5‐ [(un)substituted phenyl]‐s‐triazoles and subsequently substituted with chloroacetic acid to afford bis‐s‐triazole sulfanylacetic acid mono‐Schiff bases ( 3a – 3e ), which were condensed with 9‐formylanthracene to produce asymmetric bis(s‐triazole Schiff base) sulfanylacetic acids ( 4a – 4e ). The structures of new synthesized compounds were characterized by elemental analysis and spectral data, and their in vitro antitumor activity against L1210, CHO and HL60 cell lines was evaluted via the respective IC₅₀ values by methylthiazole trazolium (MTT) assay.     

Room‐Temperature Phosphorescence of Crystalline Metal‐Free Organoboron Complex

The photoluminescence (PL) properties of a metal‐free organoboron complex, bis(4‐iodobenzoyl)methanatoboron difluoride ( 1BF₂ ), were elucidated. At room temperature, 1BF₂ emits blue fluorescence (FL) in nBuCl upon photoexcitation. In contrast, crystals of 1BF₂ emit green PL comprised of FL and phosphorescence (PH). The room‐temperature PH of crystalline 1BF₂ is a consequence of 1) suppression of thermal deactivation of the S₁ and T₁ excited states and 2) enhancement of intersystem crossing (ISC) from the S₁ to T₂ or T₁. The results of X‐ray crystallographic and theoretical studies supported the proposal that the former (1) is a result of intermolecular interactions caused by π‐stacking in the rigid crystal packing structure of 1BF₂ . The latter (2) is an effect of not only the heavy‐atom effect of iodine, but also the continuous π‐stacking alignment of 1BF₂ molecules in crystals, which leads to a forbidden S₁→S₀ transition and a small energy gap between the S₁ and T₂ or T₁.     

The Synthesis and Characterization of s‐Triazine‐Cored Tripodal Structure and Its Salen/Salophen‐Bridged Fe/Cr(III) Capped Complexes

A novel Schiff base compound was synthesized, and its complexation properties with Fe(III) and Cr(III) were investigated. Tripodal ligand was synthesized by the reaction of s‐triazine and 4‐hydroxybenzaldehyde. Then a Schiff base involving 8‐hydroxyquinoline was synthesized by the reaction of 5‐aminomethyl‐8‐hydroxyquinoline ( QN ) and 2,4,6‐tris(p‐formylphenoxy)‐1,3,5‐triazine ( TRIPOD ) in methanol/chloroform media. The obtained Schiff base ( QN-TRIPOD ) was then reacted with four trinuclear Fe(III) and Cr(III) complexes including tetradentate Schiff bases N ,N ′‐bis(salicylidene)ethylenediamine (salenH₂)/bis(salicylidene)‐o‐phenylenediamine (SalophenH₂). The synthesized ligand and complexes were characterized by means of elemental analysis carrying out ¹H NMR, FTIR spectroscopy, thermal analyses, and magnetic susceptibility measurements. Finally, metal ratios of the prepared complexes were determined by using atomic adsorption spectrometry.     

Titanium (IV) complexes of some tetra‐dentate symmetrical bis‐Schiff bases of 1,6‐hexanediamine: Synthesis,characterization, and in silico prediction of potential inhibitor against coronavirus (SARS‐CoV‐2)

Symmetrical bis‐Schiff bases (LH ₂) have been synthesized by the condensation of 1,6‐hexanediamine (hn) and carbonyl or dicarbonyl. One of the synthesized Schiff bases has been subjected to the molecular docking for the prediction of their potentiality against coronavirus (SARS‐CoV‐2). Molecular docking revealed that tested Schiff base possessed high binding affinity with the receptor protein of SARS CoV‐2 compared with hydroxychloroquine (HCQ). The ADMET analysis showed that ligand is non‐carcinogenic and less toxic than standard HCQ. Schiff bases acting as dibasic tetra‐dentate ligands formed titanium (IV) complexes of the type [TiL(H₂O)₂Cl₂] or [TiL(H₂O)₂]Cl₂ being coordinated through ONNO donor atoms. Ligands and complexes were characterized by the elemental analysis and physicochemical and spectroscopic data including FTIR, ¹H NMR, mass spectra, UV‐Visible spectra, molar conductance, and magnetic measurement. Optimized structures obtained from quantum chemical calculations supported the formation of complexes. Antibacterial, antifungal, and anti‐oxidant activity assessments have been studied for synthesized ligands and complexes.     

1,ω‐Bis(4‐amino‐1,2,4‐triazole‐5(1H)‐thion‐3‐ylsulfanyl)alkanes: Versatile precursors for novel bis(S‐triazolo[3,4‐b][1,3,4]thiadiazines) as well as novel bis(macrocyclic schiff bases)

Two synthetic routes were attempted for the synthesis of the novel bis(5,6‐dihydro‐S‐triazolo[3,4‐b]thiadiazines) 12a,b and 14 . In the first route the bis(aminotriazoles) 4a,b were reacted with the appropriate α‐haloketones or α‐haloesters to give the corresponding bis(S‐triazolo[3,4‐b]thiadiazines) 11a‐d followed by reduction with NaBH₄. In the second route, the bis(Schiff bases) 13d were reacted with the appropriate α‐haloesters in refluxing DMF containing TEA to give the target compound 14 . Cyclocondensation of 4a,b with the appropriate bis(carbonyl) ethers 15a,b in refluxing acetic acid under high dilution conditions afforded the corresponding macrocyclic Schiff bases 16a‐c . The latter underwent alkylation with the appropriate halo compounds to give the corresponding alkylated derivatives 17a‐d .     

Synthesis of novel boron‐containing epoxy–novolac resins and properties of cured products

Epoxy–novolac resins were synthesized by modifying a commercial novolac resin with epichlorohydrin. These epoxy–novolac resins were characterized and further modified with different contents of bis(benzo‐1,3,2‐dioxa‐borolanyl)oxide or bis(4,4,5,5‐tetramethyl‐1,3,2‐dioxa‐borolanyl)oxide. The boron‐containing epoxy–novolac resins were autocatalytically crosslinked or crosslinked with BF₃MEA and their thermal stability and flame retardancy were determined by thermogravimetric analysis and limiting oxygen index (LOI) values. These LOI values for the bis(benzo‐1,3,2‐dioxa‐borolanyl)oxide derivatives were higher than the boron‐free novolac resins, which shows the benefit of the presence of boron. To test the role of boron in the enhancement of flammability, scanning electronic microscopy and energy‐dispersive X‐ray spectroscopy images were made. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6332–6344, 2006     

Highly efficient,catalyst‐free,one‐pot,pseudo five‐component synthesis of novel pyrazoline‐containing Schiff bases,metal complexes formation and computational studies via DFT method

In this study, a green and high yielding synthesis of novel pyrazoline‐containing Schiff bases via a one‐pot pseudo five‐component condensation reaction under catalyst‐free conditions in EtOH at room temperature is described. Initially, the reaction of 1,1‐bis(methylthio)‐2‐nitroethylene (BMTNE), with NH₂NH₂.H₂O is used for situ preparation of 1,1‐dihydrazino‐2‐nitroethylene (DHNE). Then, wide varieties of aldehydes are added to the mixture to afford the desired products good to excellent yields. This synthetic route is presented with several unique merits such as simplicity in operation and workup, readily presented starting materials, and high functional group tolerance. Additionally, the stoichiometric complex formation of these Schiff bases as ligand (L) with various metal ions [Mn(OAc)₂, Cu(OAc)₂ and Zn(OAc)₂] in general molecular formula [ML] is examined via Job's method. Subsequently, theoretical analysis of the product is accomplished using density functional theory (DFT) calculations. According to the frontier molecular orbital (FMO) analysis, it is found that the aryl group linked to the imine bond contributes as an electron donor in the ligand–metal complex.     

Accelerated Silane Water‐Crosslinking Kinetics of Ethylene–Propylene Copolymer by Boron Trifluoride Complexes

The hydrolysis and overall water‐crosslinking kinetics of vinyltrimethoxysilane‐grafted ethylene–propylene copolymer (EPR‐g‐VTMS) containing boron trifluoride monoethylamine complex (BF₃–MEA), boron trifluoride methanol complex (BF₃–MeOH), or di‐n‐butyltin(II) dilaurate (DBTL) as a silane‐crosslinking catalyst was followed by ATR‐FT‐IR and measuring the gel fraction, respectively. Kinetics results by ATR‐FT‐IR technique showed that using BF₃–MEA provided much lower activation energy for the hydrolysis reactions than that of DBTL; however, BF₃–MeOH had little catalytic effect on both the hydrolysis and overall water‐crosslink reaction of EPR‐g‐VTMS. We propose that the actual catalytic effect of ethylamine molecule dissociated from BF₃–MEA results in the displacement of Si? OCH₃ groups via an S_N2‐Si mechanism.

     

Polymer Supported Calix[4]arene Schiff Bases: A Novel Chelating Resin for Hg2+ and Dichromate Anions

This article describes the synthesis and characterization of two new calix[4]arene Schiff bases and their polymeric resins. The extraction properties of these “proton switchable extractants” with alkali, transition, post transition metal cations and for dichromate anions are reported. The two new calix[4]arene based Schiff bases (5 and 6) have been synthesized from 5,17‐diformyl‐25,27‐dipropoxy‐26,28‐dihydroxycalix[4]arene (4) by treatment with 3‐amino‐methylpyridine and 1,8‐diaminooctane in two separate reaction flasks following the same procedure. Compounds 5 and 6 have been appended to a polymeric resin by treatment with Merrifield resin through a nucleophilic substitution reaction. The receptor compounds (3 and 5–8) do not extract alkali metal cations, but show some selectivity toward transition metal cations, and a particularly high selectivity to Hg²⁺ and Pb²⁺. The protonated forms of all of the calixarene‐based receptors are good extractants for transferring Cr₂O₇ ^2?/HCr₂O₇ ^? anions from an aqueous into a dichloromethane layer.     

Synthesis of a New Series of N‐Mannich Bases and Polyhydroxy Mannich Bases of Pharmaceutical Interest Related to Isatin and Its Schiff Bases

The reaction of isatin 1 with benzaldehyde and a sec‐amine or the appropriate aldimine afforded the N‐Mannich bases 2 – 3 and the bis‐base 4 . Treatment of 1 with glutaric dialdehyde and morpholine gave the bis‐base 5 . Mannich reaction of the Schiff bases 6a – f derived from 1 , led to the new Mannich bases and bis‐bases 7 – 9 . The use of N‐methyl‐D‐glucamine as the amine component in the Mannich reaction with 6b – f led to the polyhydroxy Mannich bases 11 – 13 .     

Some divalent metal(II) complexes of novel potentially tetradentate Schiff base N,N′‐bis(2‐carboxyphenylimine)‐2,5‐thiophenedicarboxaldhyde: Synthesis,spectroscopic characterization and bioactivities

A novel tetradentate dianionic Schiff base ligand, N ,N ′‐bis(2‐carboxyphenylimine)‐2,5‐thiophenedicarboxaldhyde (H₂L) and some first row d‐transition metal chelates (Co(II), Cu(II), Ni(II) and Zn(II)) were synthesized and characterized using various physicochemical and spectroscopic methods. The spectroscopic data suggested that the parent Schiff base ligand coordinates through both deprotonated carboxylic oxygen and imine nitrogen atoms. The free Schiff base and its metal chelates were screened for their antimicrobial activities for various pathogenic bacteria and fungi using the agar well diffusion method. The antibacterial and antifungal activities of all the newly synthesized compounds are significant compared to the standard drugs ciprofloxacin and nystatin. The antioxidant activities of the compounds were determined by reduction of 1,1‐diphenyl‐2‐picrylhydrazyl and compared with that of vitamin C as a standard. DNA binding ability of the novel Schiff base and its complexes was investigated using absorption spectroscopy, fluorescence spectroscopy, viscosity measurements and thermal denaturation. The obtained results clearly demonstrate that the binding affinity with calf thymus DNA follows the order: Cu(II) complex > Ni(II) complex > Zn(II) complex > Co(II) complex >H₂L. Furthermore, the DNA cleavage activity of the newly synthesized ligand and its metal complexes was investigated using supercoiled plasmid DNA (pUC18) gel electrophoresis.     

Synthesis of metallosupramolecular polymers incorporating cage silsesquioxanes in the main chains

We first time prepared main‐chain type cage octasilsesquioxane (T₈) containing coordination polymers based on transition metal complexes with a bis‐Schiff base T₈ ligand ( bis‐SA‐T ₈ ), which was prepared from para‐bis(3‐aminopropyl)hexaisobutyl‐substituted T₈ cage ( bis‐T ₈ ) with salicylaldehyde. A mono‐Schiff base T₈ ligand ( SA‐T ₈ ) was prepared from 3‐aminopropylheptaisobutyl‐substituted T₈ cage ( mono‐T ₈ ) with salicylaldehyde. Coordination of bis‐SA‐T ₈ with Cu²⁺ and Zn²⁺ in a solution as well as the film state was confirmed by the d–d transition appearing as broad shoulder on the longer wavelength at around 600 nm and the fluorescence at around 440–470 nm, respectively. The resulting metallosupramolecular polymers gave optically transparent films in the visible region by a casting method. However, opaque green and whitish films were obtained in the case of the Cu²⁺ and Zn²⁺ complexes with SA‐T ₈ , respectively. The thermal stabilities of the metallosupramolecular polymers were higher than those of the corresponding metal complexes of SA‐T ₈ . © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2260–2266     

Wrapping an Organic Reducing Reagent in a Cationic Boron Complex and Its Use in the Synthesis of Polyhalide Monoanionic Networks

The reaction between BF₃ ? OEt₂ and one of two guanidines, 1,8‐bis(tetramethylguanidinyl)naphthalene (btmgn) and 1,2,4,5‐tetrakis(tetramethylguanidinyl)naphthalene (ttmgn), yields the salts [(btmgn)(BF₂)]BF₄ and [(ttmgn)(BF₂)₂](BF₄)₂. NMR spectroscopic data show that the boron atoms in the cation and anion exchange in the case of [(ttmgn)(BF₂)₂](BF₄)₂, but not in the case of [(btmgn)(BF₂)]BF₄. The rate constant for this exchange was estimated to be 4 s^?1 at 80 °C for solutions in CH₃CN. These salts were subsequently used for the reduction of dihalides Br₂ or I₂ to give polyhalide salts. We report the synthesis and first complete characterization (including structural analysis) of salts that contain pentabromide monoanions. In these salts, the Br₅^? anions interact to give dimeric units or polymeric chains. Our results are compared to previous quantum chemical calculations on the gas‐phase structure of the Br₅^? anion. The possible pathways that lead to the polyhalides are evaluated. In the case of [(ttmgn)(BF₂)₂](BF₄)₂, reduction is accompanied by ttmgn oxidation, whereas in the case of [(btmgn)(BF₂)]BF₄ reduction is initiated by aromatic substitution.     

Experimental and theoretical studies of the products of addition–elimination reactions between benzil dihydrazone and three isomeric chlorobenzaldehydes

A series of mono‐ and di‐Schiff bases formed between benzil dihydrazone {BDH; systematic name: (1Z)‐[(2E)‐2‐hydrazinylidene‐1,2‐diphenylethylidene]hydrazine} and three isomeric chlorobenzaldehydes were designed and synthesized to be used as model compounds to help to explain the reaction mechanisms for the formation of Schiff bases. These compounds are 1‐(2‐chlorobenzylidene)‐2‐{2‐[2‐(2‐chlorobenzylidene)hydrazin‐1‐ylidene]‐1,2‐diphenylethylidene}hydrazine (BDHOCB), and the 3‐chloro (BDHMCB) and 4‐chloro (BDHPCB) analogues, all having the formula C₂₈H₂₀Cl₂N₄. Surprisingly, only di‐Schiff bases were obtained; our attempts to push the reaction in favour of the mono‐Schiff bases all failed. Density functional theory (DFT) calculations were performed to explain the trend in the experimental results. In the case of the systems studied, the type of Schiff base produced exhibits a clear dependence on the HOMO–LUMO energy gaps (ΔE_HOMO–LUMO), i.e. the product is mainly governed by its stability. The compounds were characterized by single‐crystal X‐ray diffractometry, elemental analysis, melting point, ¹H NMR and ¹³C NMR spectroscopy. The structural features of the three new Schiff bases are similar. For instance, they have the same chemical formula, all the molecules have a symmetrical double helix structure, with each Ph—C=N—N=C—Ph arm exhibiting an anti conformation, and their supramolecular interactions include intermolecular π–π and weak C—H...π stacking interactions. The crystal systems are different, however, viz. triclinic (space group P) for BDHPCB, monoclinic (space group P2₁/n) for BDHOCB and orthorhombic (space group Pnna) for BDHMCB.     

Phosphorus‐based Schiff bases and their complexes as nontoxic antioxidants: Structure–activity relationship and mechanism of action

Phosphorus‐based Schiff base were synthesized by treating bis{3‐[2‐(4‐amino‐1.5‐dimethyl‐2‐phenyl‐pyrazol‐3‐ylideneamino)ethyl]‐indol‐1‐ylmethyl}‐phosphinic acid with paraformaldehyde and characterized as a novel antioxidant. Its corresponding complexes [(VO)₂L(SO₄)₂], [Ni₂LCl₄], [Co₂LCl₄], [Cu₂LCl₄], [Zn₂LCl₄], [Cd₂LCl₄], [Hg₂LCl₄], [Pd₂LCl₄], and [PtLCl]Cl₂ were analyzed by Fourier transform‐infrared, (¹H and ¹³C) nuclear magnetic resonance, and mass and UV–Vis spectroscopy. Experimental data showed that the ligand coordinated with the metal ions via donor atoms such as nitrogen to form an octahedral arrangement of the Schiff base around the central transition‐metal atom. The nature of these complexes was identified using the molar ratio and Job's methods, with the results agreeing with a metal‐to‐ligand (M:L) molar ratio of 2:1, expect for Pt, whose M:L was 1:1. Thermodynamic activation parameters such as ?E*, ?H*, ?S*, ?G*, and K were determined from the thermogravimetric analysis curve using the Coats–Redfern method. The antioxidant activities of the prepared compounds were assessed by using 1.1‐diphenyl‐2‐picrylhydrazyl as the free radical, and the results show that the complex Schiff bases were found to possess potent antioxidant activity. The structure–activity relationship of the ligand and its complexes indicates that the presence of electron‐donating moieties, such as Co(II) and Ni(II), in the chemical structure increases the antioxidant activity, whereas the Pt(IV) and Pd(II) complexes diminished the antioxidant activity, indicating the superior activity of the hydroxyl radical (OH^·) over the superoxide radical.