Cobalt Center Research Articles

Tartrate bridging ligands in divalent Co(II)/Co(III) dinuclear anion

A cobalt(II/III) ionic compound with bridging tartaric acid (H4L) anion (CH6N3)3[Co2II/III(L)2(H2O)2]·4H2O 1 was synthesized. The crystallites were isolated from highly alkaline aqueous solution in the presence of guanidinium cations (CH6N3)+ being then incorporated as counter ions.A determined structure reveals the dinuclear coordination anion with two cobalt centers, each within its distinctive coordination octahedron CoO6 sphere showing different Co−O bond lengths. The metal center with shorter coordination bonds is assigned as Co(III), while the other with Co(II), all in agreement with overall net charge equilibrium. This involves three guanidinium cations, two tartrate(4-) anions, the latter within the coordination anion, as well as with bond valence sums for both cobalt centers. Both tartrates (L) are triply bonded on the Co(III) center, thus enabling CoIIIO6 coordination bonds. On the other hand, only two coordination bonds are enabled by each tartrate towards Co(II), while the remaining two in CoIIO6 are completed by two terminal water molecules. The electronic spectrum shows clearly the d-d bands at 400 and 665 nm for Co(III), while at 520 nm for Co(II), for each CoO6, respectively. The temperature dependent magnetic susceptibility shows the paramagnetic behavior of the compound, due to the d7 Co(II) ions being in a high spin state, while the d6 low spin Co(III) ions are diamagnetic.

Catalytic Behavior of Cobalt Complexes Bearing Pyridine-Oxime Ligands in Isoprene Polymerization.

Several cobalt(II) complexes Co1-Co3 bearing pyridine-oxime ligands (L1 = pyridine-2-aldoxime for Co1; L2 = 6-methylpyridine-2-aldoxime for Co2; L3 = phenyl-2-pyridylketoxime for Co3) and picolinaldehyde O-methyl oxime (L4)-supported Co4 were synthesized and well characterized by FT-IR, mass spectrum and elemental analysis. The single-crystal X-ray diffraction of complex Co2 reveals that the cobalt center of CoCl2 is coordinated with two 6-methylpyridine-2-aldoxime ligands binding with Npyridine and Noxime atoms, which feature a distorted octahedral structure. These Co complexes Co1-Co4 displayed extremely high activity toward isoprene polymerization upon activation with small amount of AlClEt2 in toluene, giving polyisoprene with high activity up to 16.3 × 105 (mol of Co)-1(h)-1. And, the generated polyisoprene displayed high molecular weights and narrow molecular distribution with a cis-1,4-enriched selectivity. The type of cobalt complexes, cocatalyst and reaction temperature all have effects on the polymerization activity but not on the microstructure of polymer.

Cation Concavities Induced d-Band Electronic Modulation on Co/FeOx Nanostructure to Activate Molecular and Interfacial Oxygen for CO Oxidation.

Cobalt-based catalysts have been identified for effective CO oxidation, but their activity is limited by molecular O2 and interfacial oxygen passivation at low temperatures. Optimization of the d-band structure of the cobalt center is an effective method to enhance the dissociation of oxygen species. Here, we developed a novel Co/FeOx catalyst based on selective cationic deposition to anchor Co cations at the defect site of FeOx, which exhibited superior intrinsic low-temperature activity (100%, 115 °C) compared to that of Pt/Co3O4 (100%, 140 °C) and La/Co2O3 (100%, 150 °C). In contrast to catalysts with oxygen defects, the cationic Fe defect in Co/FeOx showed an exceptional ability to accept electrons from the Co 3d orbital, resulting in significant electron delocalization at the Co sites. The Co/FeOx catalyst exhibited a remarkable turnover frequency of 178.6 per Co site per second, which is 2.3 times higher than that of most previously reported Co-based catalysts. The d-band center is shifted upward by electron redistribution effects, which promotes the breaking of the antibonding orbital *π of the O═O bond. In addition, the controllable regulation of the Fe-Ov-Co oxygen defect sites enlarges the Fe-O bond from 1.97 to 2.02 Å to activate the lattice oxygen. Moreover, compared to CoxFe3-xO4, Co/FeOx has a lower energy barrier for CO oxidation, which significantly accelerates the rate-determining step, *COO formation. This study demonstrates the feasibility of modulating the d-band structure to enhance O2 molecular and interfacial lattice oxygen activation.

A new mixed-ligand mononuclear Co(II) complex derived from 4,4,4-trifluoro-1-phenylbutane-1,3-dione and 1,10-phenanthroline: Synthesis, structural elucidation and quantum-computational studies

In this study, a novel mixed-ligand mononuclear Co(II) complex, denoted as {[Co(TFPBD)2(PHE)]DMF}, was synthesized and characterized using single crystal X-ray diffraction analysis. The central cobalt atom coordinated with two 4,4,4-trifluoro-1-phenylbutane-1,3-dione ligands (TFPBD1 and TFPBD2) and a 1,10-phenanthrene (PHE) ligand, adopting a slightly distorted octahedral geometry. Notably, the non-planar characteristics of the ligands led to the manifestation of non-covalent interactions within the molecular structure, further enhanced by the involvement of the lattice DMF solvent. The crystal lattice arrangement in the {[Co(TFPBD)2(PHE)]DMF} complex was a result of diverse non-covalent interactions, including hydrogen bonds, C-F…π, and Π…π stacking interactions, influenced the crystal's stability. Additionally, the study delved into the unique lattice arrangement consisted of pairwise R12(5) ring motifs along with one-dimensional molecular arrays formed along the crystallographic axes. Furthermore, intermolecular C-F…π Cg…Cg interactions resulting in two-dimensional linear molecular arrays along the a- and b-axes. Quantum computational studies using density functional theory (DFT) were employed to examine the complex's electronic structure and stability and reactivity, highlighting electron-rich and electron-deficient regions. Hirshfeld surface analysis (HSA) and Non-covalent interaction analysis confirmed the significance of intermolecular interactions where 2D fingerprint plots emphasized the prominence of CH…F interactions within the crystal structure. These findings contribute significantly to our understanding of mixed-ligand metal complexes and their potential applications in various scientific domains.

Effective interactions and phase behavior of protein solutions in the presence of hexamine cobalt(III) chloride

It is well established that deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) exhibit a reentrant condensation (RC) phase behavior in the presence of the trivalent hexamine cobalt(III) cations (Hac) which can be important for their packing and folding. A similar behavior can be observed for negatively charged globular proteins in the presence of trivalent metal cations, such as Y3+ or La3+. This phase behavior is mainly driven by charge inversion upon an increasing salt concentration for a fixed protein concentration (cp). However, as Hac exhibits structural differences compared to other multivalent metal cations, with six ammonia ligands (NH3) covalently bonded to the central cobalt atom, it is not clear that Hac can induce a similar phase behavior for proteins. In this work, we systematically investigate whether negatively charged globular proteins β-lactoglobulin (BLG), bovine serum albumin (BSA), human serum albumin (HSA) and ovalbumin (OVA) feature Hac-induced RC. Effective protein–protein interactions were investigated by small-angle X-ray scattering. The reduced second virial coefficient (B2/B2HS) was obtained as a function of salt concentration. The virial coefficient analysis performed confirms the reentrant interaction (RI) behavior for BLG without actually inducing RC, given the insufficient strengths of the interactions for the latter to occur. In contrast, the strength of attraction for BSA, HSA and OVA are too weak to show RC. Model free analysis of the inverse intensity 1/Iq→0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1/I\\left( {q \ o 0} \\right)$$\\end{document} also supports this finding. Looking at different q-range by employing static (SLS) and dynamic light scattering experiments, the presence of RI behavior can be confirmed. The results are further discussed in view of metal cation binding sites in nucleic acids (DNA and RNA), where Hac induced RC phase behavior.Graphical abstract

The Co-N bond cleavage in the adenosyncobalamin cofactor in advance to glutamate mutase and methylmalonyl-CoA mutase processes

The in vivo experiments show that the adenosylcobalamin cofactor in glutamate mutase and methylmalonyl-CoA mutase processes lose its dimethylbenzimidazole axial ligand before starting the enzymatic processes. Complete active space self-consistent field geometry optimization of the vitamin B12 active forms plus substrates joint models have been performed. These joint models include the adenosylcobalamin cofactor, the carboxyl negative ion model of the studied processes’ active substrates, and the histidine molecule. Partial electronic density is transferred from the highest occupied substrate molecular orbitals to the lowest unoccupied antibonding molecular orbitals, which consist of corrin ring and dimethylbenzimidazole ligand common molecular orbitals during the multi-configurational self-consistent field molecular orbital mixing process. As a result, the Co-N axial bond is permanently elongated during the complete active space self-consistent field geometry optimization until its complete rupture and until the removal of the dimethylbenzimidazole ligand from the central cobalt atom and the corrin ring is complete. The Co-N bond cleavage in the adenosylcobalamin cofactors in the studied processes is running as no energy barrier process under the influence of their active substrates and histidine molecule.

Sterically attenuated electronic communication in cobalt complexes of meridional isoquinoline-derived ligands for applications in electrocatalysis.

The ability to synthetically tune the ligand frameworks of redox-active molecules is of critical importance to the economy of solar fuels because manipulating their redox properties can afford control over the operating potentials of sustained electrocatalytic or photoelectrocatalytic processes. The electronic and steric properties of 2,2':6',2″-terpyridine (Terpy) ligand frameworks can be tuned by functional group substitution on ligand backbones, and these correlate strongly to their Hammett parameters. The synthesis of a new series of tridentate meridional ligands of 2,4,6-trisubstituted pyridines that engineers the ability to finely tune the redox potentials of cobalt complexes to more positive potentials than that of their Terpy analogs is achieved by aryl-functionalizing at the four-position and by including isoquinoline at the two- and six-positions of pyridine (Aryl-DiQ). Their cobalt complex syntheses, their electronic properties, and their catalytic activity for carbon dioxide (CO2) reduction are reported and compared to their Terpy analogs. The cobalt derivatives generally experience a positive shift in their redox features relative to the Terpy-based analogs, covering a complementary potential range. Although those evaluated fail to produce any quantifiable products for the reduction of CO2 and suffer from long-term instability, these results suggest possible alternate strategies for stabilizing these compounds during catalysis. We speculate that lower equilibrium association constants to the cobalt center are intrinsic to these ligands, which originate from a steric interaction between protons on the pyridine and isoquinoline moieties. Nevertheless, the new Aryl-DiQ ligand framework has been engineered to selectively tune homoleptic cobalt complexes' redox potentials.

Cobalt-59 NMR relaxation times of symmetric tris(γ-substituted-acetylacetonato)cobalt(III) complexes in benzene

Cobalt-59 NMR relaxation times, T1(59Co), and chemical shifts, δ(59Co), of symmetric tris(γ-substituted-acetylacetonato)cobalt(III) complexes, [Co(acac-X)3], which have chloride, bromide, nitro, and phenyl groups at the methine carbon atoms in the chelate ligands, have been measured in benzene at four temperatures between 284 and 319 K. In view of the viscosity dependence of 1/T1(59Co), the 59Co relaxation is caused by the reorientation of the static electric field gradient at the nucleus in every complex. Assuming that the rotational volume is replaced by the van der Waals volume, the nuclear quadrupolar coupling products, {(1 + ηq2/3)(eQq/h)2}1/2, were obtained for the series of [Co(acac-X)3] complexes in benzene. The negative correlation between {(1 + ηq2/3)(eQq/h)2}1/2 and δ(59Co) was observed for the four complexes [Co(acac)3], [Co(acac-Cl)3], [Co(acac-Br)3], and [Co(acac-NO2)3] except for [Co(acac-Ph)3]. This discrepancy strongly suggests that the rotational volume of the propeller-like [Co(acac-Ph)3] complex with the protruding phenyl groups cannot be simply treated as the van der Waals volume, different from the other almost spherical complexes. Actually, the estimation of the rotational volumes from the distances between the central cobalt and g-substituents gave much better correlation between {(1 + ηq2/3)(eQq/h)2}1/2 and δ(59Co) for all the complexes including [Co(acac-Ph)3].

Synthesis, Infrared Spectroscopic Characterization and Antimicrobial Studies of Co(III) Dimethylglyoximato Complexes with Aniline and Aniline-derived Axial Ligands

Complexes with cobalt(III) have been used as models in the study of vitamin B12 chemistry. In this study, cobalt(III) complexes with dimethylglyoxime as equatorial ligand and aniline or substituted aniline as well as thiocyanate ion as axial ligands have been synthesized and characterized based on melting point, conductivity, solubility and infrared (IR) spectral studies. The complexes are formulated as [Co(Hdmg)2(B)(SCN)] where Hdmg is dimethylglyoximato, B = aniline (A), 4-bromoaline (4BrA), 2- nitroaniline (2NA), 4-nitroaniline (4NA) or 4-anisidine (4PA). The complexes were formed in low yields (33 – 37 %) except [Co(Hdmg)2(A)(SCN)] which was obtained in significant yield (65 %). The percentage yield of the synthesized cobaloximes follows the order A > 4NA > 4PA > 4BrA = 2NA. They are of various colours, soluble or slightly soluble in the solvents used and have melting points in the range (77 – 120 0C). The data from IR spectra indicated that the coordination compounds coordinated via the nitrogen of the ligands. Further supportive fact of the coordination of the ligands to the metal ions was deduced from the emergence of new bands due to v(M-N) in the metal complexes. In the proposed octahedral coordination sphere of the central cobalt atom, four coordination sites in the equatorial positions are occupied by nitrogen atoms of two dimethylglyoximate monoanions, and the other two sites are occupied by atoms of monodentate neutral ligands and thiocyanate anion. The measured molar conductance showed that the complexes are non-electrolytes, except [Co(Hdmg)2(4BrA)(SCN)] which has conductivity of 149 Scm2 mol-1, hence electrolytic in nature. The in vitro antibacterial potency of the complexes were assayed against some strains of bacteria such as MRSA (Methicillin Resistance Staphylococcus) Staphylococcus aureus, Bacillus subtilis, Streptococcus pneumoniae, and the Gram negative bacteria, Salmonella typhi, Klebsiella pneumoniae, Escherichia coli, and Pseudomonas aeruginosa. The results of the antimicrobial assay uncovered that the complexes have more potency than their free ligands but less than that of the standard drug (Gentamicin)

Synthesis and reactivity of cobalt dinitrogen complex supported by nonsymmetrical pincer ligand

A nonsymmetrical PNN pincer ligand [6-(tBu2PNH)C5H4N-2-(3-Mes)C3H2N2] and its corresponding cobalt-N2 complex were synthesized and characterized. By the stoichiometric reaction of the PNN ligand lithium salt with CoCl2, the complex 3, (PNN)CoCl, was obtained. Then, reduction of 3 with NaBHEt3 under a dinitrogen atmosphere yielded complex 5, (PNN)Co(I)(η1-N2). Single-crystal X-ray analysis, IR spectrum, and DFT calculations revealed that the dinitrogen in 5 was only weakly reduced by the cobalt center. The reactions of 5 with carbon monoxide and 2,6-dimethylphenyl isocyanide gave carbonyl and isocyanide complexes 6 and 7 with the release of N2, respectively. Furthermore, these cobalt complexes, especially complex 5, demonstrated the capacity to convert dinitrogen to N(TMS)3 with moderate efficiency.

Spin state-tailored tetrahedral and octahedral cobalt centers on millimetric Co-Al oxide catalysts as dual sites for synergistic peroxymonosulfate activation

Rationally developing millimetric metal catalysts with tailored electron function is crucial for water decontamination, but challenges remain due to insufficient understanding on their electronic structure engineering and the underlying mechanism. Here, through constructing bonded Co-O-Al unit sites on the millimetric γ-Al2O3 spheres, we report a feasible and scalable strategy of modulating the 3d electron configurations and spin states of supported tetrahedral and octahedral cobalt cations just by adjusting the radius of support curvature. The tetrahedral and octahedral cobalt centers with tailored electron spin states showed a synergism in peroxymonosulfate activation to generate SO4• and reactive oxygen (O*), in which the latter was converted into 1O2 and O2•. The electronic structure-oriented spin catalysis affected the formation of reactive species and the pathway of pollutant degradation. This work will stimulate the design and mass production of macroscopic spin catalysts with superior activity, reusability and stability for catalytic water purification.

Ligand-Mediated Regulation of the Chemical/Thermal Stability and Catalytic Performance of Isostructural Cobalt(II) Coordination Polymers.

Regulating the chemical/thermal stability and catalytic activity of coordination polymers (CPs) to achieve high catalytic performance is topical and challenging. The CPs are competent in promoting oxidative cross-coupling, yet they have not received substantial attention. Here, the ligand effect of the secondary ligand of CPs for oxidative cross-coupling reactions was investigated. Specifically, four new isostructural CPs [Co(Fbtx)1.5(4-R-1,2-BDC)]n (denoted as Co-CP-R, Fbtx = 1,4-bis(1,2,4-triazole-1-ylmethyl)-2,3,5,6-tetrafluorobenzene, 4-R-1,2-BDC = 4-R-1,2-benzenedicarboxylate, R = F, Cl, Br, CF3) were prepared. It was found that in the reactions of oxidative amination of benzoxazoles with secondary amines and the oxidative coupling of styrenes with benzaldehydes, both the chemical and thermal stabilities of the four Co-CPs with the R group followed the trend of -CF3 > -Br > -Cl > -F. Density functional theory (DFT) calculations suggested that the difference in reactivity may be ascribed to the effect of substituent groups on the electron transition energy of the cobalt(II) center of these Co-CPs. These findings highlight the secondary ligand effect in regulating the stability and catalytic performance of coordination networks.

Atomic high-spin cobalt(II) center for highly selective electrochemical CO reduction to CH3OH

In this work, via engineering the conformation of cobalt active center in cobalt phthalocyanine molecular catalyst, the catalytic efficiency of electrochemical carbon monoxide reduction to methanol can be dramatically tuned. Based on a collection of experimental investigations and density functional theory calculations, it reveals that the electron rearrangement of the Co 3d orbitals of cobalt phthalocyanine from the low-spin state (S = 1/2) to the high-spin state (S = 3/2), induced by molecular conformation change, is responsible for the greatly enhanced CO reduction reaction performance. Operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy measurements disclose accelerated hydrogenation of CORR intermediates, and kinetic isotope effect validates expedited proton-feeding rate over cobalt phthalocyanine with high-spin state. Further natural population analysis and density functional theory calculations demonstrate that the high spin Co2+ can enhance the electron backdonation via the dxz/dyz−2π* bond and weaken the C-O bonding in *CO, promoting hydrogenation of CORR intermediates.

Structural, physicochemical characterization and antimicrobial activities of a new tris(8-quinolinolato-κ2N,O)cobalt(III) ethanol solvate [Co(C9H6NO)3].(C2H6O)

We report herein the synthesis and physicochemical characterization of a new 8-hydroxyquinoline cobalt(III) complex ethanol solvate with formula [Co(C9H6NO)3].(C2H6O). This compound was been prepared by slow evaporation at room temperature and characterized by single crystal X-ray diffraction. It crystallizes in the monoclinic system, spaces group P21/n. The central cobalt ion has a slightly distorted square bipyramidal environment, coordinated by three chelating 8-hydroxyquinoline molecules. Structural cohesion is primarily established by π-π interactions between the neighboring rings of quinoline groups and intermolecular hydrogen bonds O-H…O connecting the cobalt complex entities and uncoordinated ethanol molecules.The title compound has been characterized by the infrared (IR) and the UV–Visible (UV–Vis) spectra. Intermolecular interactions in the crystal structure were quantified by Hirshfeld surface analysis. The band gap of the complex was calculated using solid-state reflectance spectra, and the results show that the complex acts as a semiconductor. Moreover, the new compound exhibits high antibacterial and antifungal activities, as detected by the well agar diffusion against various bacterial and fungal clinical strains. The superior inhibitory effect was observed against Klebsiella pneumoniae, Candida albicans and Candida glabrata with zones inhibition of about 17.5, 17.5 and 17 mm, respectively.

One-pot cocrystallization of mononuclear and 1D cobalt(II) complexes based on flexible triclopyr and 2,2′-bipyridine coligands: Structural analyses, conformation comparison, non-covalent interactions and magnetic properties

A pair of cobalt(II) complexes with formulas of [Co(3,5,6-tcpa)(2,2′-bpy)Cl] (1A) and [Co(3,5,6-tcpa)2(2,2′-bpy)]n (1B) have been cocrystallized by the solvothermal reaction of cobalt chloride hexahydrate with triclopyr (systematic name 2-((3,5,6-trichloropyridin-2-yl) oxy)acetic acid, abbreviation 3,5,6-Htcpa) and 2,2′-bipyridine (2,2′-bpy). Single-crystal X-ray diffraction analysis indicated that both 1A and 1B crystallized in triclinic system, space group P1¯ (no. 2). Compound 1A is mononuclear and 3,5,6-tcpa anion in it is a bidentate chelate ligand. In contrast, 1B is a 1D double-stranded chain structure in which 3,5,6-tcpa are bidentate bridging-μ2 linkers. The non-covalent interactions of Cl···Cl halogen bonds, C–H···O/Cl hydrogen bonds as well as π···π stacking interactions play an important part in their crystal packing, which have been confirmed by the careful Hirshfeld surface analyses. After deeply structural analyses, it was proposed that flexible conformation alteration of 3,5,6-tcpa might be in charge of the cocrystallization phenomenon of 1A and 1B. In addition, the magnetic characterizations suggested antiferromagnetic coupling between neighbouring cobalt(II) centers in 1B.

Organo-Functionalized Lacunary Double Cubane Type Oxometallates: Synthesis, Structure, and Properties of [(MIICl)2(VIVO)2{((HOCH2CH2)(H)N(CH2CH2O))(HN(CH2CH2O)2)}2] (M = Co, Zn).

Organofunctionalized tetranuclear clusters [(MIICl)2(VIVO)2{((HOCH2CH2)(H)N(CH2CH2O))(HN(CH2CH2O)2)}2] (1, M = Co; 2, M = Zn) containing an unprecedented oxometallacyclic {M2V2Cl2N4O8} (M = Co, Zn) framework are prepared by solvothermal reactions. The new oxo-alkoxide compounds are fully characterized by spectroscopic methods, magnetic susceptibility measurement, DFT and ab-initio computational methods, and complete single crystal X-ray diffraction structure analysis. The isostructural clusters are formed of edge-sharing octahedral {VO5N} and trigonal bipyramidal {MO3NCl} units. Diethanolamine ligates the bimetallic lacunary double cubane core of 1 and 2 in an unusual two-mode fashion, unobserved previously. In the crystalline state, the clusters of 1 and 2 are joined by hydrogen-bonds to form a three-dimensional network structure. Magnetic susceptibility data indicate weakly antiferromagnetic interactions between the vanadium centers [Jiso(VIV-VIV) = -5.4(1); -3.9(2) cm-1], and inequivalent antiferromagnetic interactions between the cobalt and vanadium centers [Jiso (VIV-CoII) = -12.6 and -7.5 cm-1] contained in 1.

Engineering Coordination Environment of Cobalt Center in Molecular Catalysts for Improved Photocatalytic CO2 Reduction

Comprehensive SummaryThe creation of effective and inexpensive catalysts is essential for photocatalytic CO2 reduction. Homogeneous molecular catalysts, possessing definite crystal structures, are desirable to study the relationship between catalytic performance and coordination microenvironment around catalytic center. In this report, we elaborately developed three Co(II)‐based molecular catalysts with different coordination microenvironments for CO2 reduction, named [CoN3O]ClO4, [CoN4]ClO4, and [CoN3S]ClO4, respectively. The optimal [CoN3O]ClO4 photocatalyst has a maximum TON of 5652 in photocatalytic reduced CO2 reduction, which is 1.28 and 1.65 times greater than that of [CoN4]ClO4 and [CoN3S]ClO4, respectively. The high electronegativity of oxygen in L1 (N,N‐bis(2‐pyridylmethyl)‐N‐(2‐hydroxybenzyl)amine) provides the Co(II) catalytic centers with low reduction potentials and a more stable *COOH intermediate, which facilitates the CO2‐to‐CO conversion and accounts for the high photocatalytic activity of [CoN3O]ClO4. This work provides researchers new insights in development of catalysts for photocatalytic CO2 reduction.

Cationic Cobalt(II) Bisphosphine Hydroformylation Catalysis: In Situ Spectroscopic and Reaction Studies.

[HCo(CO)x(bisphosphine)](BF4), x = 1-3, is a highly active hydroformylation catalyst system, especially for internal branched alkenes. In situ infrared spectroscopy (IR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance studies support the proposed catalyst formulation. IR studies reveal the formation of a dicationic Co(I) paramagnetic CO-bridged dimer, [Co2(μ-CO)2(CO)(bisphosphine)2]2+, at lower temperatures formed from the reaction of two catalyst complexes via the elimination of H2. DFT studies indicate a dimer structure with square-pyramidal and tetrahedral cobalt centers. This monomer-dimer equilibrium is analogous to that seen for HCo(CO)4, reacting to eliminate H2 and form Co2(CO)8. EPR studies on the catalyst show a high-spin (S = 3/2) Co(II) complex. Reaction studies are presented that support the cationic Co(II) bisphosphine catalyst as the catalyst species present in this system and minimize the possible role of neutral Co(I) species, HCo(CO)4 or HCo(CO)3(phosphine), as catalysts.

Size and Electronic Effects on the Performance of (Corrolato)cobalt-Modified Electrodes for Oxygen Reduction Reaction Catalysis.

Considering the worldwide efforts for designing catalysts that are not based on platinum group metals while still reserving the many advantages thereof, this study focused on the many variables that dictate the performance of cathodes used for fuel cells, regarding the efficient and selective reduction of oxygen to water. This was done by investigating two kinds of porous carbon electrodes, modified by molecular cobalt(III) complexes chelated by corroles that differ very much in size and electron-withdrawing capability. Examination of the electronic effect uncovered shifts in the CoII/CoIII redox potentials and also large differences in the affinity of the cobalt center to external ligands. Spontaneous absorption of the catalysts was found to depend on the size of the corrole's substituents (C6F5 ≫ CF3 ≫ H) and the metal's axial ligands (PPh3 versus pyridine), as well as on the porosity of the carbon electrodes (BP2000 > Vulcan). The better-performing cobalt-based catalysts were almost as active and selective as 20% platinum on Vulcan in terms of the onset potential and the only 2-10% undesirable formation of hydrogen peroxide. Durability was also addressed by using the best-performing modified cathode in a proper anion-exchange membrane fuel cell setup, revealing very little voltage change during 12 h of operation.

Correlating Metal Redox Potentials to Co(III)K(I) Catalyst Performances in Carbon Dioxide and Propene Oxide Ring Opening Copolymerization

AbstractCarbon dioxide copolymerization is a front‐runner CO2 utilization strategy but its viability depends on improving the catalysis. So far, catalyst structure‐performance correlations have not been straightforward, limiting the ability to predict how to improve both catalytic activity and selectivity. Here, a simple measure of a catalyst ground‐state parameter, metal reduction potential, directly correlates with both polymerization activity and selectivity. It is applied to compare performances of 6 new heterodinuclear Co(III)K(I) catalysts for propene oxide (PO)/CO2 ring opening copolymerization (ROCOP) producing poly(propene carbonate) (PPC). The best catalyst shows an excellent turnover frequency of 389 h−1 and high PPC selectivity of >99 % (50 °C, 20 bar, 0.025 mol% catalyst). As demonstration of its utility, neither DFT calculations nor ligand Hammett parameter analyses are viable predictors. It is proposed that the cobalt redox potential informs upon the active site electron density with a more electron rich cobalt centre showing better performances. The method may be widely applicable and is recommended to guide future catalyst discovery for other (co)polymerizations and carbon dioxide utilizations.