Complexes of the 14th group elements with tridentate redox-active ligand

New methods of synthesis of the 14th group metal (Si, Sn, Pb) complexes containing a redox-active ligand, 3,5-di-tert-butyl-1,2-quinone-1-(2-hydroxy-3,5-di-tert-butylphenyl)imine, in various redox states were developed. New compounds were studied by the ESR spectroscopy in solutions. The redox properties of the paramagnetic tin compounds are characterized by the cyclic voltammetry. The solvent and temperature effect on the ESR spectra parameters of paramagnetic complexes of Sn(IV)and Pb(IV) was revealed.     

Functionalized derivatives of sterically hinderedo-quinones and pyrocatechols. 3,6-di-tert-butyl-4-dicyanometyl-1,2-benzoquinone and its isomers

Base-catalyzed interaction of 3,6-di-tert-butyl-1,2-benzoquinone with malononitrile mainly occurs as 1,4-addition to give 3,6-di-tert-butyl-4-dicyanomethylpyrocatechol. Its oxidation leads to 3,6-di-tert-butyl-4-dicyanomethyl-1,2-benzoquinone, which converts into 3,6-di-tert-butyl-2-hydroxy-α,α-dicyano-1,4-quinomethane in solution and in the solid state. The latter rearranges into isomeric 3,6-di-tert-butyl-5-dicyanomethylenecyclohex-3-ene-1,2-dione. Reverse conversion occurs under the action of amines. Semiquinone complexes of dicyanomethylquinone were studied in solutions by ESR.     

New polydentate ligands based on sterically hindered o-benzoquinones (pyrocatechols) containing the 1,4-diazadiene group

New bifunctional ligands containing the 1,4-diazadiene and 1,2-benzoquinone groups were synthesized by the reactions of 4-amino-3,6-di-tert-butylpyrocatechol with glyoxal and its derivatives. Their o-semiquinone complexes with transition and nontransition metals were studied in solution by the ESR method.     

3,6-Di-tert-butyl-o-benzosemiquinone complexes of copper(1) with bidentate bis(diphenylphosphine) ligands. Synthesis, structures, and properties

New mononuclear 3,6-di-tert-butyl-o-benzosemiquinone complexes of copper(1) with bis(diphenylphosphine) ligands were synthesized: (DBSQ)Cu(dppe) (1) (DBSQ=3,6-di-tert-butyl-o-benzosemiquinone and dppe=1,2-bis(diphenylphosphino)ethane), (DBSQ)Cu(dppp) (2) (dppp=1,3-bis(diphenylphosphino)propane), (DBSQ)Cu(dppn) (3) (dppn=2,2′-bis(diphenylphosphino)-1,1′-binaphthyl), and (DBSQ)Cu(dppfc) (4) (dppfc=1,1′-bis(diphenylphosphino)ferrocene). The compositions and structures of complexes1–4 were characterized by elemental analysis and electronic absorption, IR, and ESR spectroscopy. The molecular structures of complexes3 and4 were established by X-ray diffraction analysis. The reactions of elimination and replacement of neutral ligands in the coordination sphere of the complexes were studied by ESR spectroscopy. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2333–2340, November, 1998.     

Dinuclear chelates of acyclic and cyclic tridentate Schiff bases derived from sterically hindered o-aminophenols. A new type of reactivity of tridentate ligands under electrosynthesis conditions

The chemical and electrochemical synthesis of cobalt(II), nickel(II), and copper(II) complexes based on sterically hindered tridentate Schiff bases obtained by the coupling of 2-amino-4,6-di-tert-butylphenol with salicylaldehyde derivatives (H₂L, H₂L’) was performed. The resulting dinuclear complexes were characterized by elemental analysis, IR spectroscopy, and magnetochemical measurements in the 300–2 K temperature range. The structures of the dinuclear nickel(II) and copper(II) complexes with the composition Ni₂L₂·2AcOH·2MeOH and Cu₂L₂, respectively, were established by X-ray diffraction. The copper chelates are characterized by the presence of antiferromagnetic exchange interactions. The mononuclear copper(II) complexes (DMSO)bis[2-(5,7-di-tert-butyl-4-hydroxybenzoxazol-2-yl)phenolato]copper(II) and (DMF)₂bis[2-(5,7-di-tert-butyl-4-hydroxybenzoxazol-2-yl)phenolato]copper(II) were isolated upon electrochemical dissolution of copper(0) and were structurally characterized.     

Functionalization of sterically hindered catechol and <Emphasis Type="Italic">o</Emphasis>-benzoquinone with 2,2,6,6-tetramethylpiperidine 1-oxyl

Sterically hindered 4,6-di-tert-butyl-3-formylcatechol and 3,6-di-tert-butyl-o-benzoquinone react with 4-amino-2,2,6,6-tetramethylpiperidine 1-oxyl to give new chelate ligands of the o-quinone type bearing a 2,2,6,6-tetramethylpiperidine 1-oxyl neutral radical moiety. Structures of the synthesized compounds were established by ESR spectroscopy, IR spectroscopy, mass spectrometry, and X-ray diffraction analysis.     

Synthesis, structure, and antioxidant activity of hybrid N-substituted salicylic acid amides

Sterically hindered phenols, 3-tert-butyl-N-(3,5-di-tert-butyl-4-hydroxyphenyl)-5-ethyl-2-hydroxybenzamide and N-[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propyl]-2-hydroxybenzamide, were synthesized as potential antioxidants, and the formation of hydrogen bonds by their molecules was shown by UV and IR spectroscopy.     

Synthesis and coordination properties of the zinc complex of dimeric porphyrin in reactions with imidazole, 2-methylimidazole, and the pyridine in benzene

Dimeric porphyrin(2,6-bis[15′-(3″,5″-di-tert-butylphenol)-3′,7′,13′,17′-tetramethyl-2′,8′,12′,18-tetraethylporphin-5′-yl]-4-tert-buthylphenol) and its binuclear zinc complex were obtained from 4,4′-dimethyl-3,3′-diethyldipyrrolylmethane, 2,6-diformyl-4-tert-butylphenol and 3,5-di-tert-buthylbenzaldehyde. Coordina-tion properties of dimeric zincporphyrin in the intermolecular reaction with nitrogen-containing bases (imidazole, 2-methylimidazole, and pyridine) in benzene were studied. Geometry and electronic structure of the zincporphyrin and its molecular complexes were calculated by a quantum-chemical method. Energy characteristics of the intermolecular reaction of the dimeric zincporphyrin with bases were determined. The calculated energies of the central metal atom interaction with the nitrogen atom of an extra-ligand agree well with the stability of the Zn-porphyrin molecular complexes. The influence of the deformation distortions of the porphyrin ligand on the strength of the metal-extra-ligand σ-bond was established.     

The oxidation of 2-alkoxy-3,6-di-tert-butylphenols. The reversible dimerization of 2-alkoxy-3,6-di-tert-butylphenoxy radicals

The oxidation of 2-alkoxy-3,6-di-tert-butylphenols has been studied. It was found that 2-RO-3,6-di-tert-butylphenoxy radicals (R = Et, Pr, Ph) undergo dimerization by C-O coupling. The X-ray structure of 2-ethoxy-3,6-di-tert-butylphenoxy dimer has been determined. Radical dimers dissociate reversibly in solution to give two phenoxy radicals at 200-350 K. By using EPR spectroscopy the equilibrium concentration of the phenoxy radicals, equilibrium constants and ΔH and ΔS of dissociation have been determined.     

Synthesis and characterization of an unsymmetric salicylaldimine ligand derived from 1-(2-Aminoethyl) piperazine and investigation of its analytical properties for the extraction and preconcentratoion of some divalent cations

The synthetic, structural, spectroscopic and analytical properties of steric hindered Schiff-base ligand [N-(3,5-di-tert-butylsalicylaldimine)-1-(2-Aminoethyl) piperazine (HL)] and its mononuclear Cu(II), Co(II) and Ni(II) complexes are described. The new unsymmetric steric hindered Schiff base ligand containing a donor set of NONO was prepared by the reaction of 1-(2-Aminoethyl) piperazine with 3,5-di-tert-butylsalicylaldehyde. Certain metal complexes of this ligand were synthesized by treating an ethanolic solution of the ligand with an equimolar amount of metal salts. The ligand and its metal complexes were characterized by FT-IR, UV-Vis, ¹H NMR, elemental analysis, molar conductivity and magnetic susceptibility techniques. The reaction of this ligand in a 1:2 mole ratio with metal acetate afforded mononuclear metal complexes. The molar conductivity (??_M) values of the metal complexes of Ni(II), Co(II) and Cu(II) were in the range of 6.4 to 9.8 ??^?1 cm² mol^?1 at room temperature. Preconcentration and separation of Cu²⁺ from aqueous solution using N-(3,5-di-tert-butylsalicylaldimine)-1-(2-Aminoethyl) piperazine (HL) as a new extractant were studied. The extraction experiments were carried out at various pHs. While Cu²⁺ showed the highest extractability and selectivity at pH 7.0, extractions of Co²⁺ and Ni²⁺ were unsuccessful due to precipitate formation.     

Synthesis and ESR study of Co and Ni hydrazides and hydroxamates containing 2,6-di-tert-butylphenol moiety in the ligands

A series of Co and Ni hydrazides and hydroxamates containing the 2,6-di-tert-butylphenol moiety as a potential free radical precursor in the ligand environment of the complexes were synthesized. ESR spectroscopy was used to study the dependence of the stability of metal complexes incorporating a paramagnetic center in the ligand on the type of the coordination environment of the metal and the nature of the ligand. The complexes, in which a saturated aliphatic bridging group separates the 2,6-di-tert-butylphenol moiety and the coordination environment of the metal, can undergo oxidation to produce stable paramagnetic species with an unpaired electron in the ligand environment. The interaction of the free radical center in the ligand with the coordination environment of the metal is practically absent (excepting the weak spin-spin interaction). Metal complexes incorporating a hydrazine moiety conjugated with the 2,6-di-tert-butylphenol substituent do not produce detectable stable paramagnetic forms with a free radical in the ligand.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1794–1797, September, 1995.     

Template Assembling of the Pentadentate Redox-Active Ligand in the Coordination Sphere of Tin(IV)

Heptacoordinated tin complexes with pentadentate redox-active ligands containing the diiminopyridine fragment combined with two sterically hindered phenolate coordination centers, LSn-Cl₂ and L'SnCl₂ (L and L' are dianions of deprotonated 2,6-bis[2,4-di-tert-butyl-6-(methylidenylamino) phenol]pyridine and 2,6-bis[2,4-di-tert-butyl-6-(ethylidenylamino)phenol]pyridine, respectively), are synthesized. The molecular and electronic structures of the synthesized compounds were studied by X-ray diffraction analysis (for complex I, CIF file CCDC no. 1557838), a set of spectral methods, and quantum-chemical calculations. The redox properties of the obtained complexes are characterized by cyclic voltammetry.     

Homo-and heterometal complexes with sterically eclipsed ortho-nitrosophenol

Novel homo-and heterometal complexes based on 4,6-di-tert-butyl-2-nitrosophenol (HL) were synthesized, and their physicochemical properties were studied. The data of the ¹H NMR, electronic spectroscopy, magnetochemical measurements, and the atomic absorption study suggest that the nitroso group of a ligand can be bidentate in polynuclear coordination compounds. The experimental data were obtained for the first time on self-assembling of both homo-and heterometal trinuclear complexes with predicted properties from the initial metal chelates of ortho-nitrosophenols.     

Synthesis and redox properties of novel ferrocenes with redox active 2,6-di-tert-butylphenol fragments: The first example of 2,6-di-tert-butylphenoxyl radicals in ferrocene system

Novel Schiff bases of ferrocenecarboxaldehyde bearing 2,6-di-tert-butyphenol fragments N-(3,5-di-tert-butyl-4-hydroxyphenyl)iminomethylferrocene (1) and N-(3,5-di-tert-butyl-4-hydroxybenzyl)iminomethylferrocene (2) have been synthesized and characterized. The oxidation of the compounds 1 and 2 by PbO₂ in solution leads to the formation of stable phenoxyl radicals 1′ and 2′ studied by EPR spectroscopy. The redox properties of ferrocenes 1 and 2 were studied using cyclic voltammetry.     

Effect of two isomeric tetrapyrazolyl ligands on the catalytic oxidation of 3,5-di-tert-butylcatechol

Copper(II) salts were combined with a tetrapyrazolyl ligand ({N,N,N’,N’-tetrakis-[(3,5-dimethylpyrazol-1-yl)methyl)-1,4-phenylenediamine}L1 or {N,N,N’,N’-tetrakis-[(1,5-dimethylpyrazol-3-yl)methyl)-1,4-phenylenediamine}L2) and assessed as oxidation catalysts. The corresponding dioxygen complexes were generated in situ by mixing the copper salt and the pyrazolyl donor ligand in air. The oxidation of 3,5-di-tert-butylcatechol (DTBC), which affords 3,5-di-tert-butylquinone (DTBQ), was studied. The reaction rate was found to depend essentially on the nature of the junction linking two pyrazolyl neighbours.     

UV/VIS Spectral Studies of Catecholase Activity of Binuclear Copper(II) Complexes [Cu2REP(μ-X)(Y)2]

By means of ultraviolet and visible spectroscopy we have studied the catecholase activity of two binuclear copper(II) complexes of general formula [Cu₂REP(u-X)(Y)₂] (1, X=OH and Y=ClO₄; 2, X=Y=CI), REP =deprotonated 2, 6-bis(1′-(4′-(2″-pyridyl)-2′-thiabutyl))-4-methylphenol. Both complexes promote catalytic autoxidation of 3, 5-di-tert-butylcatechol (3, 5-DTBC) to 3, 5-di-tert-butylquinone (3, 5-DTBQ) in methanol, but not in acetonitrile.     

Coordination compounds based on 4,6-di-tert-butyl-2-nitrosophenol

New metal mono-, bis-, and tris(chelates) based on 4,6-di-tert-butyl-2-nitrosophenol were synthesized and their physicochemical properties were studied. Conclusions concerning the structures of the complexes and the influence of the nature of the complexing metal on the electronic structure of the coordinated ligand were drawn from X-ray diffraction, ¹H NMR, magnetochemical, and electronic spectroscopy data.     

Syntheses and structures of the cobalt, nickel, and zinc complexes with 1,4-diaza-1,3-butadiene ligands

Several cobalt(II), nickel(II), and zinc(II) complexes with a series of ligands of the 1,4-diaza-1,3-butadiene type bearing aryl (2,6-di-iso-propylphenyl, mesityl) and alkyl (tert-butyl, iso-propyl) substituents at the nitrogen atoms are synthesized. The obtained complexes are characterized by X-ray structure analysis, IR spectroscopy, and elemental analysis.     

Two-dimensional correlation NMR study of the structure of by-product in the reaction of 2-methylquinoline with 3,5-di-<Emphasis Type="Italic">tert</Emphasis>-butyl-1,2-benzoquinone

The reaction of 2-methylquinoline with 3,5-di-tert-butyl-1,2-benzoquinone afforded a mixture of 5,7-di-tert-butyl-3-hydroxy-2-(quinolin-2-yl)cyclohepta-2,4,6-trien-1-one and previously unknown 10-tert-butylindolo[1,2-a]quinoline-8,11-dione. The structure of the latter was determined by two-dimensional heteronuclear correlation NMR spectroscopy.     

New lead(II) catecholate and o-semiquinone complexes

Lead(II) catecholate complexes were prepared by reduction of 3,6-di-tert-butyl-o-benzoquinone and its derivatives with lead metal in THF. The molecular structure of the (CatPb)₄·(PbO)₂·6C₃H₆O complex (Cat is the dianion of 3,6-di-tert-butylcatechol), which was synthesized by hydrolysis of lead 3,6-di-tert-butylcatecholate in acetone, was established by X-ray diffraction. A series of lead(II) o-semiquinone complexes, which were prepared by the addition of the phenoxyl radical to lead catecholates or by oxidation of the latter with mercury(II), copper(II), or silver(I) halides, were studied by the ESR method. Lead(II) mono-o-semiquinolate complexes undergo symmetrization to form stable bis-o-semiquinolates, which were isolated and characterized in individual state. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1103–1111, July, 2006.