Cobalt Complexes Research Articles

E-selective hydroboration of alkynes catalyzed by 3 N-donor Schiff-base cobalt(II) complex

Hydroboration is one of the landmark reactions of homogeneous catalysis. Despite the existence of many mature protocols, there is still a field for the search of new ones, especially utilizing active and selective base-metal catalysts. We report an effective and selective method for anti-Markovnikov hydroboration of alkynes using bench-stable cobalt(II) complex bearing a 3 N-donor Schiff-base ligand. The optimized procedure can be used both on aromatic and aliphatic alkynes leading to high yields of fully selective transformations within 90 min at room temperature.

Investigation of mono-nuclear cobalt(II) complexes with a tri-dentate quinoxalyl-hydrazone ligand for their potential in biological research and interaction with ct-DNA.

The condensing reaction of 2-hydroxy-1-naphthaldehyde with quinoxalyl-2-carbohydrazide resulted in synthesizing of a novel derivative of hydrazone quinoxalyl ligand (H2dpq). The bonding behavior between H2dpq and Co(II) ion was investigated in molar ratios of 1: 1 and 2: 1 to produce two different complexes, Codpq and Co(dpq)2, respectively. Their chemical structure was verified using several spectroscopic approaches. The characterization envolved micro-C, H, and N-elemental analyses, thermogravimetric investigations, as well as the examination of their magnetic and conductance properties. The inhibitory effects of H2dpq (the free ligand) and its Co(II)-chelates on the growing up of three specific bacterial series, three specific fungal series, and three famous human cancer cell lines were evaluated based on their structure features. The study referred to the pivotal function of Co(II) and its ratio in the two complexes. The antioxidant activity was determined for all current compounds within DPPH and SOD assays, reporting respectable antioxidant reactivity. The interaction of H2dpq, Codpq, and Co(dpq)2 with calf thymus DNA (ct-DNA) was assessed by analyzing through the alterations in the viscometric and spectrophotometric properties. Determination of binding constant (Kb, 13.12, 16.02, and 16.82×107mol-1dm3), Gibb's free energy (∆Gb≠, were -40.21, -46.13, and -46.67kJmol-1), and the chromism mode (hypo-character) were employed for H2dpq, Codpq, and Co(dpq)2 to assess the interactive modes, respectively. Rewardingly, the monometallic-chelates Co(II)-complexes displayed modified binding action more than that of H2dpq against ct-DNA.

Versatile applications of cobalt and copper complexes of biopolymeric Schiff base ligands derived from chitosan.

In the present study, biopolymeric Schiff base (SB) ligands were synthesized from chitosan and isatin. Consequently, their earth abundant transition metal complexes of cobalt and copper were synthesized. All compounds were extensively characterized using FTIR and UV spectroscopy, thermo-gravimetric (TG) analysis, X-ray powder diffraction (XRD) and FESEM (field emission scanning electron microscopy). The impetus of this study was to explore different applications of the same transition metal complexes, thereby converting them to broad spectrum antimicrobial drugs and to useful chemical catalysts. Thus, these cobalt and copper complexes were applied as anti-bacterial and anti-fungal agents and in reductive degradation of common pollutants. Specifically, chitosan, and all synthesized compounds were investigated against fungal species, namely, Candida and Aspergillus, and bacterial species, like Pseudomonas syringae, Escherichia coli, Staphylococcus aureus and Staphylococcus epidermis. All compounds demonstrated promising antimicrobial activity. Chitosan-modified compounds demonstrated a high antifungal activity against Candida albicans and possessed superior antibacterial action than free chitosan. Additionally, catalytic ability in reductive degradation of common dyes, methylene blue (MEB), Congo red (CR) and methyl orange (MO), was investigated. These dyes were conveniently degraded in presence of metal complexes, whereas neither chitosan nor the Schiff base ligand derived from it could carry out similar reaction.

Electrochemical Fabrication of Ni–Co Alloy over a Wide pH Range Using Sodium Citrate as a Complexing Agent

In this study, nickel–cobalt (Ni–Co) coatings were fabricated via electrodeposition using a 22 central composite factorial design with two central and two axial points, totaling ten experiments. The effects of pH and current density on the coatings’ chemical composition and properties were evaluated. Coatings were characterized by microstructure, morphology, magnetic properties, and corrosion resistance. The results showed that pH significantly influenced chemical composition, while current density had no notable effect. Acidic pH produced cobalt-rich coatings (43–81 at.%), with uniform morphology, higher saturation magnetization, and lower corrosion resistance. Maximum cobalt content (81 at.%) resulted in a mixed face-centered cubic (fcc) + hexagonal close-packed (hcp) phase. Alkaline pH yielded nickel-rich coatings (89–95 at.%), featuring nodular morphology, lower magnetization, higher corrosion resistance, and, exclusively, the fcc phase. The highest polarization resistance (66.1 kΩ) occurred at pH 8.83 and 60 mA/cm2, while resistance decreased with increasing cobalt content. The pH effect on deposition was linked to the formation of citrate complexes: ammonia and citrate complexes promoted nickel deposition under alkaline conditions, while stable cobalt complexes dominated in an acidic pH. These findings highlight the potential to tailor Ni–Co coatings for applications such as corrosion-resistant coatings (nickel-rich) or magnetic devices (cobalt-rich).

6,6′-{[Ethane-1,2-Diylbis(azaneylylidene)]bis(methaneylylidene)}bis[2-(4-Oxy(2,2,6,6-Tetramethylpiperidin-1-Oxyl)butyloxy)phenolato] Cobalt(II)

Salen-type complexes with transition metals and corresponding polymers attract great scientific interest due to their outstanding electrochemical properties and catalytic potential. Cobalt complexes of this kind are known as catalysts for a large variety of redox reactions, including oxygen reduction. Herein, we report the preparation of a modified cobalt Salen-type complex, modified by TEMPO pendants linked to the core by flexible butylenedioxy linkers. The resulting product was characterized by electrospray–high-resolution mass spectrometry and Fourier-transform infrared spectroscopy.

Hypoxia-activated dissociation of heteroleptic cobalt(III) complexes with functionalized 2,2'-bipyridines and a model anticancer drug esculetin.

A low oxygen level in solid tumors is behind the modern concept of selective chemotherapy by hypoxia-activated prodrugs, such as heteroleptic complexes of transition metals (cobalt(III), iron(III) or platinum(IV)) with bi- or tetradentate ligands and an anticancer drug molecule as a co-ligand. A series of new cobalt(III) complexes [Co(LR)2(esc)]ClO4 with esculetin (6,7-dihydroxycoumarin) and 2,2'-bipyridines (2,2'-bipy) functionalized by different substituents R were probed in the hypoxia-activated delivery of this model anticancer drug. Their combined study by cyclic voltammetry and in situ NMR spectroscopy allowed identifying linear correlations of the electrochemical reduction potentials and the rate of the hypoxia-activated dissociation of [Co(LR)2(esc)]ClO4 with the Hammett constants of the substituents in 2,2'-bipy ligands. The latter, therefore, should be decorated with the most electron-withdrawing groups (unless they preclude the formation of a heteroleptic complex) to promote the drug release and increase the anticancer activity towards, e.g., human epidermoid carcinoma A431.These correlations can be transferred to other types of bi- or tetradentate ligands, thereby paving the way towards the molecular design of cobalt complexes as prodrugs for hypoxia-activated anticancer drug delivery with high therapeutic efficiency.

SMM Behavior in Distorted Trigonal Bipyramidal and Tetrahedral Cobalt(II) Complexes Based on Tripodal Tetradentate Phenolic Amines

Four CoII complexes [Co(L1)(MeOH)]‧MeOH (1‧MeOH), [Co(L2)(MeOH)]‧MeOH (2‧MeOH), [Co(L3)(H2O)]‧MeOH (3‧MeOH) and [Co(L4)] (4), derived from tripodal tetradentate phenolic amine arms, H2L1‐4, were synthesized and structurally characterized. The complexes 1·MeOH, 2·MeOH and 3·MeOH displayed distorted trigonal bipyramidal geometry, whereas 4 exhibited distorted tetrahedral geometry, depending on the substituents at the phenolate rings and amine arm. The variation of the coordination geometries and interatomic parameters around the CoII center has an impact on the magnetic behavior of the compounds. The complexes show magnetic anisotropy (ZFS) of the MS = ± ½ and ± 3/2 sub‐levels, with D = 29.1(2), 22.7(1), 28.8(2) and 30.9(5) cm‐1 for 1‧MeOH, 2‧MeOH, 3‧MeOH and 4, respectively. The results obtained from ab initio CASSF calculations match well with the experimental data, revealing the origin of magnetic anisotropy. The dynamic ac magnetic investigation of the magnetic susceptibility revealed a slow magnetic relaxation behavior for 2·MeOH, 3·MeOH and 4. The field‐induced slow relaxation of the magnetization occurred through combination of Raman and Direct processes, depending on the variation in the coordination geometries imposed by the coordinated ligand and/or the interatomic parameters around the CoII center, which in turn have definite impact on the magnetic features of the compounds.

Low Temperature Emissive Cyclometalated Cobalt(III) Complexes.

A series of CoIII complexes [Co(RImP)2][PF6], with HMeImP = 1,1'-(1,3-phenylene)bis(3-methyl-1-imidazole-2-ylidene)) and R = Me, Et, iPr, nBu, is presented in this work. The influence of the strong donor ligand on the ground and excited-state photophysical properties was investigated in the context of different alkyl substituents at the imidazole nitrogen. X-ray diffraction revealed no significant alterations of the structures and all differences in the series emerge from the electronic structures. These were probed via cyclic voltammetry and UV-vis spectroscopy, detailing the influence of the different alkyl substituents on the ground-state properties. All complexes are emissive at 77 K from a 3MC state, which exhibits lifetimes in the range of 1-5 ns at room temperature, depending on the alkyl substituent. Therefore, it is clearly shown that even small differences in the electronic structure have a large impact on the details of the excited state landscape. The observed behavior was rationalized by a detailed DFT analysis, which shows that the minimum-energy crossing point to the ground-state is located only slightly above the MC energy: Consequently, nonradiative decay to the ground state at room temperature is enabled, while at 77 K this path is prohibited, leading to low-temperature 3MC emission.

2-Acetylpyridine thiosemicarbazones cause rapid substitution of amine and pyridyl ligands coordinated to cobalt(III)

The reaction of 2-acetylpyridine 4-phenyl-3-thiosemicarbazone, 2-acetylpyridine 4-methyl-3-thiosemicarbazone and 2-acetylpyridine 4-allyl-3-thiosemicarbazone with [(en)2Co(OSO2CF3)2]CF3SO3, (dien)Co(OSO2CF3)3 and [(picen)Co(OSO2CF3)2]CF3SO3 results in the rapid substitution of coordinated amine and pyridyl ligands to form cobalt(III) complexes coordinated by two deprotonated 2-acetylpyridine thiosemicarbazone ligands, each bound tridentate through the sulfur atom and two nitrogen atoms (N2S). While the mechanism of this reaction remains unclear, it is suspected that an electron transfer reaction occurs between the thiosemicarbazone and the metal complex resulting in reduction to cobalt(II). Rapid ligand loss from cobalt(II) followed by chelation of the tridentate ligands and subsequent re-oxidation to cobalt(III) results in formation of the final product. The resulting complexes were characterized by NMR. In addition, the solid-state structure (X-ray) of the methyl derivative confirms the coordination mode of the ligand.

A Pair of Hydrogen‐Bonded Cobalt(II) Complexes showing the Proton Conduction and Spin Crossover Property

Achieving high proton conductivity in spin‐crossover (SCO) compounds is promising for the development of magnetoelectric and spintronics devices. In this work we designed two spin‐crossover and proton‐conductive bifunctional Co(II) compounds, [Co(Pyrimidin‐terpy)2](BF4)2·2H2O (1·2H2O; Pyrimidine‐terpy = 4'‐(5‐pyrimidinyl)‐2,2':6',2"‐terpyridine) and [Co(Pyrimidin‐terpy)2](ClO4)2·2H2O (2·2H2O). Both compounds undergo the typical spin transitions and have a hydrogen−bonding network consisting of anions with solvent water molecules. At 353 K and under 95% relative humidity, the proton conductivity of 1·2H2O was 1.9 × 10–4 S cm–1 and that of 2·2H2O was 7.5 × 10–5 S cm–1. The activation energy analysis indicates that the proton conduction of 1·2H2O follows the Vehicle mechanism in the temperature range of 303−318 K, while the Grotthuss mechanism plays a dominant role in the higher temperature range of 323−353 K. Additionally, 2·2H2O also follows the Grotthuss mechanism in the temperature range of 338−353 K. This study provides new guidelines for the design of novel SCO molecular materials with proton conduction functionality.

Apoptotic cell death of stomach cancer lines (AGS) induced by Co-NTB complex through cellular organelles and DNA damage.

Given that stomach cancer is the fourth leading cause of cancer-related death, there is a need to develop new drugs. Among various methods, metal-based coordination compounds are considered as an efficient strategy against this type of cancer. Similarly, the benzimidazole moiety plays a crucial role in biology; thus, various benzimidazole-based compounds have been found to be active as potential anticancer drugs and are currently used in clinical trials. In this study, we explored the benzimidazole-based cobalt(ii) complex as an anticancer agent against AGS stomach cancer cell lines. Interestingly, the MTT assay of the Co-NTB complex shows a lower IC50 value of 4.25 μg mL-1 compared to cisplatin, which has an IC50 of 7.5 μg mL-1 against AGS cell lines. Light microscopy and Hoechst/propidium iodide dye staining clearly indicate that the complex damages DNA, leading to cell death through an apoptotic pathway. The apoptotic cell death pathway was further complemented by Lysotracker and Mitotracker staining, as well as transmission electron microscopy (TEM) imaging. Overall, the Co-NTB complex acts as an effective anticancer agent against AGS stomach cancer cell lines, with apoptotic cell death induced by targeting cellular organelles and DNA.

Cobalt-Catalyzed Enantioselective Reductive Coupling of Imines and Internal Alkynes.

Chiral allylamines are important structural components in natural products, pharmaceuticals, and chiral catalysts. Herein, we report a cobalt-catalyzed enantioselective reductive coupling of imines with internal alkynes to synthesize chiral allyl amines. The reaction is catalyzed by a cobalt complex derived from commercially available bisphosphine ligand utilizing zinc as the electron donor.The substrate scope is extensive. Symmetric and asymmetric alkyl and aryl alkynes have been successfully coupled with various imines derived from aryl and alkyl aldehydes. Tri- and tetra-substituted allyl amines were isolated in high yields exceeding 89%, with enantiomeric excess surpassing >99.9% and regioselectivities exceeding >20:1. These chiral allylamines can serve as versatile platformsfor subsequent transformations while preserving their stereochemical integrity. Extensive experimental and computational mechanistic studies were performed to elucidate the mechanism. These investigations have indicated that an in situ cobalt(I) catalyst enables the oxidative cyclization of alkynes and imines, and a spin crossover occurs during the enantio-determining step. Zinc plays a pivotal role in facilitating the transmetallation of the resulting azacobaltacycle. The observed enantioselectivity was interpreted by the stabilization of the transition state through higher stabilization interaction energy from high negative polarization, dispersion, and C-H•••π interactions.

Overcoming the Tradeoff Between Reaction Rate and Overpotential in Dinuclear Cobalt Complex Catalyzed Electrochemical Water Oxidation.

This study focuses on enhancing the water oxidation reaction (WOR) efficacy of dinuclear cobalt complex catalysts from both kinetic (turnover frequency, TOF) and thermodynamic (overpotential, η) perspectives. For this purpose, we synthesized six dinuclear cobalt complexes 1-6 comprising non-innocent ligands with different electronically active substituents (-OMe (1), -Me (2), -H (3), -F (4), -Cl (5), and -CN (6)). The electronic effects on the electrochemical WOR under neutral, acidic, and alkaline conditions were investigated experimentally and computationally. X-ray crystallography revealed that the valence state of the cobalt complexes was affected by electronic effects, affording different catalytic potentials, as evidenced by electrochemical measurements. Kinetic and spectroscopic studies confirmed that the synergistic effect of catalyst identity and reaction media changes the reaction mechanism of each catalyst. The WOR performance of 1-6 was tunable through ligand electronics and solvent effects, with the log(TOF)-η analysis highlighting an operational η as low as 40 mV. This study provides valuable insights into optimizing WOR catalysis through molecular design and environmental tuning.

Bifunctional catalytic and capacitive properties of CoC and CoC/Zn derived from cobalt complex pyrolysis

Bifunctional catalytic and capacitive properties of CoC and CoC/Zn derived from cobalt complex pyrolysis

Red and NIR light-triggered enhancement of anticancer and antibacterial activities of dinuclear Co(II)-catecholate complexes.

Photoactive complexes of bioessential 3d metals, activable within the phototherapeutic window (650-900 nm), have gained widespread interest due to their therapeutic potential. Herein, we report the synthesis, characterization, and light-enhanced anticancer and antibacterial properties of four new dinuclear Co(II) complexes: [Co(phen)(cat)]2 (Co-1), [Co(dppz)(cat)]2 (Co-2), [Co(phen)(esc)]2 (Co-3), and [Co(dppz)(esc)]2 (Co-4). In these complexes, phen (1,10-phenanthroline) and dppz (dipyrido[3,2-a:2',3'-c]phenazine) act as neutral N,N-donor ligands, while cat2- and esc2- serve as O,O-donor catecholate ligands derived from catechol (1,2-dihydroxybenzene) and esculetin (6,7-dihydroxy coumarin). Their high-spin paramagnetic nature and dimeric identity in solution were confirmed by magnetic susceptibility, UV-visible, emission, and mass spectral data. Co-1-Co-4 exhibited an absorption band within the 600-850 nm range, originating from a charge transfer transition. The electrically neutral complexes demonstrated sufficient solution stability both in the dark and under irradiated conditions. The dppz complexes Co-2 and Co-4 exhibited notable toxicity towards A549 lung carcinoma cells, with potency increasing significantly under brief (5 min) exposure to 660 nm (red) and 808 nm (NIR) laser light (IC50 ∼ 8.9 to 14.9 μM). Notably, their toxicity towards normal NIH-3T3 fibroblast cells was minimal. Cellular assays highlighted that the induced cell death followed an apoptotic pathway, primarily due to mitochondrial damage. Co-2 and Co-4 also demonstrated significant antibacterial potency against Gram-(+) S. aureus and Gram-(-) P. aeruginosa, with effectiveness significantly enhanced upon 808 nm laser irradiation (MIC ∼ 15-142 μM). The increase in the anticancer and antibacterial efficacies was attributed to the generation of cytotoxic singlet oxygen (1O2) species upon red/NIR light exposure. Notably, 808 nm NIR irradiation produced more pronounced effects compared to 660 nm. This study is the first to report on cobalt complexes exhibiting red and NIR light-triggered enhancement of antibacterial and anticancer activities, illuminating the path for the development of long-wavelength absorbing cobalt complexes with enhanced therapeutic efficacy.

Synthesis, spectroscopic analysis and theoretical study of novel cobalt(II) complexes: Solar radiations-driven photocatalytic degradation of organic pollutants in wastewater

Synthesis, spectroscopic analysis and theoretical study of novel cobalt(II) complexes: Solar radiations-driven photocatalytic degradation of organic pollutants in wastewater

Face-controlled chirality induction in octahedral thiacalixarene-based porous coordination cages.

Nanosized chiral octahedral M32 coordination cages were prepared via self-assembly of sulfonylcalix[4]arene tetranuclear M(II) clusters (M = Co or Ni) with enantiomerically enriched linkers based on tris(dipyrrinato)cobalt(III) complexes, appended with peripheral carboxylic groups. Two pairs of enantiomers of cages were obtained and unambiguously characterized from a structural point of view, using single crystal X-ray diffraction. Chiral-HPLC was used to evidence the enantiomers. In the solid state, the compounds present intrinsic and extrinsic porosity: the intrinsic porosity is linked with the size of the cages, which present an inner diameter of ca. 19 Å. The obtained solid-state supramolecular architectures demonstrated good performances as adsorbents for water and 2-butanol guest molecules.

Solvent-assisted control of the oxidation state in a dinuclear cobalt(II) complex with slow relaxation of the magnetization

Solvent-assisted control of the oxidation state in a dinuclear cobalt(II) complex with slow relaxation of the magnetization

Exploring acidity-dependent PCET pathways in imino-bipyridyl cobalt complexes.

The electrochemical proton reactivity of transition metal complexes has received intensive attention in catalyst research. The proton-coupled electron transfer (PCET) process, influenced by the coordination geometry, determines the catalytic reaction mechanisms. Additionally, the pKa value of a proton source, as an external factor, plays a crucial role in regulating the proton transfer step. Understanding the effects of variations in the pKa values of Brønsted acids on the PCET process is therefore essential. This study compares the PCET pathways of two high-spin cobalt (Co) complexes with contrasting exchange coupling interactions under acidic conditions with high and low pKa values. These findings reveal how proton reduction reactions in high-spin Co complexes are affected by the internal factor of the spin state, as well as an external factor related to the proton source. The corresponding reaction mechanisms are also proposed based on these observations.

Single-site cobalt complexes embedded into thiophene-ring doped carbon nitride aiming to promote photocatalytic hydrogen evolution

Single-site cobalt complexes embedded into thiophene-ring doped carbon nitride aiming to promote photocatalytic hydrogen evolution