Synthesis, structural characterization and electrochemical studies of an ionic cobalt complex derived from a tridentate hydrazone Schiff base and azide ligands

Homo- and Hetero-Oligonuclear Complexes of Platinum Group Metals (PGM) Coordinated by Imine Schiff Base Ligands

Chemistry of Schiff base (SB) ligands began in 1864 due to the discovery made by Hugo Schiff (Schiff, H., Justus Liebigs Ann. der Chemie 1864, 131 (1), 118-119). However, there is still a vivid interest in coordination compounds based on imine ligands. The aim of this paper is to review the most recent concepts on construction of homo- and hetero-oligonuclear Schiff base coordination compounds narrowed down to the less frequently considered complexes of platinum group metals (PGM). The combination of SB and PGM in oligonuclear entities has several advantages over mononuclear or polynuclear species. Such complexes usually exhibit better electroluminescent, magnetic and/or catalytic properties than mononuclear ones due to intermetallic interactions and frequently have better solubility than polymers. Various construction strategies of oligodentate imine ligands for coordination of PGM are surveyed including simple imine ligands, non-innocent 1,2-diimines, chelating imine systems with additional N/O/S atoms, classic N2O2-compartmental Schiff bases and their modifications resulting in acyclic fused ligands, macrocycles such as calixsalens, metallohelical structures, nano-sized molecular wheels and hybrid materials incorporating mesoionic species. Co-crystallization and formation of metallophilic interactions to extend the mononuclear entities up to oligonuclear coordination species are also discussed.

One pot synthesis of two cobalt(iii) Schiff base complexes with chelating pyridyltetrazolate and exploration of their bio-relevant catalytic activities

Two new cobalt(iii) tetrazolato complexes [Co(L¹)(PTZ)(N₃)] (1) and [Co(L²)(PTZ)(N₃)] (2) {where H₂L¹ = 2((3-(methylamino)propylimino)methyl)-6-methoxyphenol, H₂L² = 2((3-(dimethylamino)propylimino)methyl)-6-ethoxyphenol and HPTZ = 5-(2-pyridyl)tetrazole}, have been synthesized via in situ 1,3-dipolar cycloaddition reaction of 2-cyanopyridine and sodium azide in the presence of cobalt(ii) nitrate hexahydrate and respective Schiff bases in the open atmosphere. The structures of both complexes have been confirmed by single crystal X-ray diffraction studies. Features of noncovalent interactions in the solid state of both complexes have been studied by means of DFT and MEP calculations and characterized using Bader's theory of “atoms in molecules” (AIM). These complexes act as biomimetic catalysts promoting the aerobic oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to the corresponding o-benzoquinone at room temperature. The reaction follows Michaelis–Menten enzymatic reaction kinetics with turnover numbers of ∼0.030 s⁻¹ in an acetonitrile–methanol (2 : 1) mixture. Both complexes are also reactive towards aerobic oxidation of o-aminophenol in acetonitrile–methanol (2 : 1) with turnover numbers ∼0.095 s⁻¹.

Two cobalt(iii) tetrazolato complexes have been synthesized and characterized. Noncovalent interactions have been analysed by DFT and MEP calculations and characterized using Bader's theory of AIM. Both complexes catalyze the aerial oxidation of 3,5-DTBC and OAPH.

Novel CoIII complexes containing fluorescent coumarin-N-acylhydrazone hybrid ligands: Synthesis, crystal structures, solution studies and DFT calculations

A series of new Co^III complexes of the type [Co(dien)(L1-L3)]ClO₄ (1-3), containing fluorescent coumarin-N-acylhydrazonate hybrid ligands, (E)-N'-(1-(7-oxido-2-oxo-2H-chromen-3-yl)ethylidene)-4-R-benzohydrazonate [where R=H (L1^2-), OCH₃ (L2^2-) or Cl (L3^2-)], were obtained and isolated in the low spin Co^III configuration. Single-crystal X-ray diffraction showed that the coumarin-N-acylhydrazones act as tridentate ligands in their deprotonated form (L^2-). The cation (+1) complexes contain a diethylenetriamine (dien) as auxiliary ligand and their structures were calculated by DFT studies which were also performed for the Co^II (S=1/2 and S=3/2) configurations. The LS Co^II (S=1/2) concentrated the spin density on the O-Co-O axis while the HS Co^II (S=3/2) exhibited a broad spin density distribution around the metallic center. Cyclic voltammetry studies showed that structural modifications made in the L^2- ligands caused a slight influence on the electronic density of the metal center, and the E_1/2 values for the Co^III/Co^II redox couple increased following the electronic effect of the R-substituent, in the order: 2 (R=OCH₃)<1 (R=H)<3 (R=Cl). The theoretical redox potentials (E°) of the process Co^III→Co^II were calculated for both Co^II spin states (S=1/2 and S=3/2) and a better correlation was found for Co^III→Co^II (S=1/2), compared with experimental values vs SHE (E^°_calc=-0.37, -0.36 and -0.32V vs E^°_exp.=-0.371, -0.406 and -0.358V, for 1-3 respectively). Complexes 1-3 exhibited a very intense absorption band around 470nm, assigned by DFT calculations as π-π* transitions from the delocalized coumarin-N-acylhydrazone system. 1-3 were very stable in MeOH for several days. Likewise, 1-3 were stable in phosphate buffer containing sodium ascorbate after 15h, which was attributed to the high chelate effect and σ-donor ability of the L^2- and dien ligands.

A selective naked-eye chemosensor derived from 2-methoxybenzylamine and 2,3-dihydroxybenzaldehyde - synthesis, spectral characterization and electrochemistry of its bis-bidentates Schiff bases metal complexes

A new colorimetric receptor HL, acting as a bidentate Schiff base ligand, has been synthesized by condensation of 2-methoxybenzylamine on 2,3-dihydroxybenzaldehyde in a methanolic solution. Interestingly, this chelating agent can selectively detect Cu²⁺, Co²⁺, Fe²⁺ and Fe³⁺ ions with a simple and an easy-to-make, well defined naked-eye visible color changes in two different solvents like acetonitrile and methanol. This bidentate ligand coordinates three metal ions of Co(II), Cu(II) and Fe(II) via nitrogen and oxygen atoms. The molecular structures of the synthesized compounds were elucidated by various physicochemical properties such as the elemental analysis, FT-IR, HNMR, UV-Vis and the Mass spectrometry. The resulting general formulae [M(L)₂·H₂O] (M(II)=Cu, Fe, Co) are proposed as mononuclear complexes. The solvatochromism properties of these compounds were studied with their absorption spectra using different solvents as methanol (MeOH), acetonitrile (AN), tetrahydrofuran (THF), dimethylformamid (DMF), dimethylsulfoxid (DMSO) and dichloromethane (DC). The Electrochemical behavior of copper complex was explored in DMF solutions by cyclic voltammetry (CV) with two working electrodes: glassy carbon (GC) and platinum electrode (Pt). This study reveals that copper complex shows successively two redox systems as Cu^III/II and Cu^II/I. The Fe^III/II and Co^II/I redox systems have also been studied in DMF and DMSO media.

A heterogenized chiral imino indanol complex of manganese as an efficient catalyst for aerobic epoxidation of olefins

A new composite was synthesized by grafting the (1R,2S)-1-(N-salicylideneamino)-2-indanol complex of Mn onto graphene oxide/Fe₃O₄@C. It catalysed the aerobic epoxidation of olefins with a selectivity of >98% and enantioselectivity of >99%.

Synthesis, structure, magnetic properties and EPR spectroscopy of a copper(ii) coordination polymer with a ditopic hydrazone ligand and acetate bridges

The synthesis, structure, EPR spectroscopy and magnetic properties of a 1D coordination polymer of Cu(ii) containing two kinds of acetate bridged dimers is reported.

Molecular oxygen reduction catalyzed by a highly oxidative resistant complex of cobalt–hydrazone at the liquid/liquid interface

Syntheses, structure and spectroscopic studies of a new cobalt complex with a hydrazine ligand. The first report on a highly oxidative resistant complex in the ORR electrocatalysis at the liquid–liquid interface.

A novel copper(ii) complex with an unsymmetrical tridentate-Schiff base: synthesis, crystal structure, electrochemical, morphological and electrocatalytic behaviors toward electroreduction of alkyl and aryl halides

This work describes the synthesis of a new unsymmetrical tetradentate copper(ii) Schiff base complex Cu(L)(Py)(ClO4) containing N₃O donor atoms.

Synthesis, Structural Characterization, and Electrochemical Studies of New Oxovanadium(V) Complexes Derived from 2-Furanoylhydrazon Derivatives

Five monooxovanadium(V) complexes [VO(L1)(OCH3)(OHCH3)](1), [VO(L2)(OCH3)(OHCH3)](2), [VO(L3)(OCH3)(OHCH3)](3), [VO(L4)(OCH3)(OHCH3)](4), and [VO(L5)(OCH3)(OHCH3)](5)were synthesized and characterized by IR, NMR UV-Vis, and single-crystal structure analysis [H2L1=(E)-N′-((2-hydroxynaphthalen-1-yl)methylene)furan-2-carbohydrazide,H2L2=(E)-N′-(2-hydroxybenzylidene)furan-2-carbohydrazide, H2L3(E)-N′-(5-bromo-2-hydroxybenzylidene)furan-2-carbohydrazide,H2L4=(E)-N′-(2-hydroxy-5-nitrobenzylidene)furan-2-carbohydrazide,H2L5=(E)-N′-(2-hydroxy-5-iodobenzylidene)furan-2-carbohydrazide]. In all1–3structures the vanadium atom has a distorted octahedral coordination with the three meridional donor atoms from the Schiff base dianion (L1–3)2−and one methoxylato group occupying the sites of the equatorial plane. The oxo group and one methanol molecule occupy the apical sites. In the complexes1,2, and3the conformation of 2-furanyl oxygen atom relative to the carbohydrazide oxygen atom iss-anti,s-anti/s-syn, ands-syn at 293 K, respectively. Cyclic voltammetric experiments of the solution species1–5in DMSO revealed a quasi-reversible behavior.