Cobalt Metal Center Research Articles

Atomic Cobalt Metal Centers with Asymmetric N/B‐Coordination for Promoting Oxygen Reduction Reaction

Atomic Cobalt Metal Centers with Asymmetric N/B‐Coordination for Promoting Oxygen Reduction Reaction

Investigating the Effect of Trace Levels of Manganese Ions During Solvothermal Synthesis of Massey University Framework-16 on CO2 Uptake Capacity.

The effects of impurities on reaction precursors for metal-organic framework (MOF) synthesis have not been studied in extensive detail. The impact of these impurities can be an important factor while considering scale-up of these materials. In this work, we study the apparently positive impact of the presence of manganese ions for the synthesis of a Co-based MOF, Massey University Framework-16 (MUF-16). The presence of a trace amount of manganese in the reaction mixture led to consistently high CO2 uptake across multiple batches. Characterization including X-ray diffraction, scanning electron microscopy, Fourier transform infrared-attenuated total reflectance, ultraviolet-visible spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure spectroscopy led us to hypothesize that the differences in CO2 adsorption among materials with differing synthesis routes arise from variations in the local environment around the cobalt metal center. Aided by density functional theory calculations, we speculate that manganese ions get inserted into the structure during crystallization and act as catalysts for ligand substitution, improving the possibility for octahedral coordination of cobalt with the ligand, thus leading to Co-based pristine structures with higher CO2 uptakes.

Antibacterial, antibiofilm, and antioxidant activities of two novel metal-organic frameworks (MOFs) based on 4,6-diamino-2-pyrimidinethiol with Zn and Co metal ions as coordination polymers.

In the present era, the increase in free radical species (FRs) and multidrug-resistant (MDR) bacteria represents a major worldwide concern for public health. Biofilm development and the overuse and misuse of antibiotics could lead to the adaptation of bacteria to antimicrobial agents. Consequently, finding novel multifunctional species with antibacterial, antioxidant, and antibiofilm properties has become crucial in the fight against challenging bacterial infections and chronic inflammatory conditions. Metal-organic frameworks (MOFs) with zinc and cobalt metal centers are widely utilized in biological and environmental remediation owing to their versatility. In this study, multifunctional Zn-MOFs and Co-MOFs were successfully synthesized with zinc and cobalt as metal centers and 4,6-diamino-2-pyrimidinethiol as an organic linker using a hydrothermal technique. Numerous characterization techniques were used to fully examine the MOF structure, functionality, chemical makeup, crystalline structure, surface appearance, thermal behavior, and magnetic characteristics; the techniques included XPS, PXRD, FTIR, FESEM, EDX, UV-visible, BET, BJH, TGA/DTG, DSC, and magnetic susceptibility measurement. The antioxidant, antibacterial, and antibiofilm activities of the MOFs were examined, and they demonstrated potent activity in each of these aspects. The proposed mechanisms of antibacterial activity suggest that bacterial cell death results from multiple toxic effects, including electrostatic interaction and lipid peroxidation, when MOFs are attached to bacteria, leading to the formation of reactive oxygen species (ROSs). Zn-MOFs exhibit high antibacterial and antibiofilm efficacy owing to their large surface-to-volume ratio and porous nature, while Co-MOFs exhibit high antioxidant capacity owing to their redox properties.

Crystalline-amorphous Co/Co3O4 derived from a low-temperature etching of ZIF-67 for oxygen-invovled catalytic reactions

Development of a simple method for synthesizing transition metal-based bifunctional catalysts is of great importance in the context of oxygen-involved electrochemical energy conversion and storage devices. In this effort, electrochemical catalysts consisting of amorphous cobalt and cobalt oxides on graphene oxide (GO) were fabricated using a solvothermal process followed by etching ZIF-67 with oxygen-rich functional groups on GO (≈ 29 at. %) in a weak reduction atmosphere at 400 ℃. During the calcination process forming the ZIF-67@GO-based materials, exposed cobalt metal centers are created by partial vaporization and oxidation of ZIF-67. Also, metal ions are reduced to form Co and partially combine with oxygen from GO to generate amorphous Co3O4. Moreover, the porous framework structure of ZIF is retained under the mild calcination temperatures used. Benefiting from their porous framework and heteroatoms-doped composition, the ZIF-67@GO-based materials have low overpotentials of 319 mV at 10 mA cm−2 and high catalytic activities toward the oxygen evolution and oxygen reduction reactions in an alkaline electrolyte. In addition, the liquid zinc-air battery assembled with the ZIF-67@GO composite produced at a calcination temperature of 400 ℃ displays a high performance. Overall, the results demonstrate that the ZIF-67@GO composites have the potential to be high-performing catalysts in energy conversion devices. The readily controllable and high-yielding calcination method in this study is expected to serve as a model for new strategies to synthesize low-cost and efficient bifunctional electrocatalysts.

Screening and Experimental Validation for Selection of Open Metal Sites Metal-Organic Framework (M-CPO-27, M = Co, Mg, Ni and Zn) to Capture CO2

The release of CO2 into the atmosphere has become a primary issue nowadays. Recently, researchers found Metal-Organic Frameworks M-CPO-27 (M = Mg, Co, Ni, and Zn) to be revolutionary for CO2 adsorption due to the presence of open metal sites enhancing CO2 binding and leading to higher capacity. This study aims to select the best metal center for CPO-27 with the high performance of CO2 adsorption by screening metal centers using simulation as a preliminary selection method. Then, the different metal centers were synthesized using the solvothermal process for validation. The synthesis of MOFs is confirmed through PXRD and FTIR analysis. Subsequently, by using simulation and experimental methods, it is discovered that Ni-CPO-27 gives the best performance compared with magnesium, zinc, and cobalt metal centers. The CO2 adsorption capacity of synthesized Ni-CPO-27 is 5.6 mmol/g, which is almost 20% higher than other MOFs. In conclusion, the prospective outcome of changing the metal from Mg-CPO-27 to Ni-CPO-27 would be advantageous in this investigation owing to its excellent performance in capturing CO2.

Synthesis, Spectral, Redox, and Sensing Studies of β-Dicyanovinyl-Appended Corroles and Their Metal Complexes.

A new family of β-dicyanovinyl (DCV)-appended corroles represented as MTPC(MN) (where M = 3H, Cu, Ag, and Co(PPh3) and MN = malononitrile and TPC = 5,10,15-triphenylcorrole) were synthesized starting from the free base mono β-formyl corrole, H3TPC(CHO), and characterized along with their respective MTPC(CHO) and MTPC complexes as to their spectroscopic and electrochemical properties in nonaqueous media. Comparisons between the two series of corroles demonstrate a pronounced substituent effect of the β-DCV group on the physicochemical properties making the MTPC(MN) derivatives substantially easier to reduce and more difficult to oxidize than the formyl or unsubstituted corroles. In addition, the colorimetric and spectral detection of 11 different anions (X) in the form of tetrabutylammonium salts (TBAX, X = PF6-, OAc-, H2PO4-, CN-, HSO4-, NO3-, ClO4-, F-, Cl-, Br-, and I-) were also investigated in nonaqueous media. Of the investigated anions, only CN- was found to induce changes in the UV-vis and 1H NMR spectra of the β-DCV metallocorroles. This data revealed that CuTPC(MN) and AgTPC(MN) act as chemodosimeters for selective cyanide ion detection via a nucleophilic attack at the vinylic carbon of the DCV substituent, while (PPh3)CoTPC(MN) acts as a chemosensor for cyanide ion sensing via axial coordination to the cobalt metal center. A low-limit detection of cyanide ions was observed at 1.69 ppm for CuTPC(MN) and 1.17 ppm for AgTPC(MN) in toluene.

Alumina-supported cobalt catalysts in the hydrogenation of CO2 at atmospheric pressure

20 wt.% cobalt catalysts supported on pure and 5 wt.% silica-containing alumina have been prepared and characterized by X-Ray Diffraction, IR and DR-UV-vis-NIR spectroscopies and Field-Emission Scanning Electron Microscopy (FE-SEM). The presence of a cobalt-containing surface spinel phase Co3-xAlxO4 and, for the silica-containing sample, of a segregated Co3O4 phase is evident. These catalysts have been tested in CO2 hydrogenation at atmospheric pressure in steady state conditions in the temperature range 523–773 K. Both catalysts are active in CO2 hydrogenation to methane (methanation) and to CO (reverse Water Gas Shift, rWGS). CO2 hydrogenation activity is higher on freshly pre-reduced silica-free Co/Al2O3, while selectivity to methane is slightly higher on the silica-containing sample. Spent catalysts contain clustered or amorphous cobalt metal centers as active sites for methanation. The silica-containing catalyst shows slow deactivation in CO2 hydrogenation upon 13 h experiments, with quite stable or even slightly increasing rWGS activity and decreasing CH4 selectivity. This confirms previous data suggesting that, over cobalt catalysts, sites for methanation are metal centers prone to deactivation by carbon deposition. However, in contrast with what happens with unsupported and silica-supported cobalt catalysts, where deactivation is very fast, over these alumina-based catalysts carbon deposition and deactivation occur much more slowly. Sites for rWGS are unreduced cobalt centers which do not undergo such a deactivation phenomenon.

2D/3D covalent organic frameworks based on cobalt corroles for CO binding

In the present work, the synthesis, characterization, and gas adsorption properties of new 2D covalent organic framework (COF) (2D-COF-Cor) and 3D COF (3D-COF-Cor) based on corrole macrocycles are reported. The two COFs have been synthesized by Schiff base condensation from readily available C3-symmetric aldehyde or Td-symmetric aldehyde as platforms, and diamine-functionalized free-base corrole as building block linker, to access 2D and 3D polymers, respectively. Cobalt-metalated COFs were also synthesized using a post-metalation procedure to give 2D-COF-CorCo and 3D-COF-CorCo as porous materials. The design of the 2D and 3D structures of the materials are reported, as well as the relationship between their structure and their performances for carbon monoxide (CO) adsorption. Spectroscopy analyses such as 1H NMR, Fourier-Transform InfraRed (FTIR), powder X-ray diffraction, microscopic analyses, and sorption measurements were used to fully characterize the structure and the porosity of the COF materials. Their properties for capture and sensing of CO were also studied with the analysis of their isotherms using a multisite Langmuir isotherm model and the Ideal Adsorbed Solution Theory (IAST) theory. The affinity, capacity and selectivity of these materials for CO sorption were calculated. Compared to 3D-COF-CorCo, 2D-COF-CorCo reveals the highest adsorption capacity of 32.2 cm3/g for CO (298 K, 1 atm) with a high selectivity over N2, O2, CO2, up to 50, 130, 5090, 3170, respectively. In addition, FTIR analysis gave clear evidence of the involved solid–gas interactions, and the reversibility of CO binding on the cobalt metal center of the corrole within the materials. These results point out an appealing way of using cobalt corrole-based COF as efficient chemosensors to detect trace amounts of CO.

Molecular Switch Cobalt Redox Shuttle with a Tunable Hexadentate Ligand.

Strong-field hexadentate ligands were synthesized and coordinated to cobalt metal centers to result in three new low-spin to low-spin Co(III/II) redox couples. The ligand backbone has been modified with dimethyl amine groups to result in redox potential tuning of the Co(III/II) redox couples from -200 to -430 mV versus Fc+/0. The redox couples surprisingly undergo a reversible molecular switch rearrangement from five-coordinate Co(II) to six-coordinate Co(III) despite the ligands being hexadentate. The complexes exhibit modestly faster electron self-exchange rate constants of 2.2-4.2 M-1 s-1 compared to the high-spin to low-spin redox couple [Co(bpy)3]3+/2+ at 0.27 M-1 s-1, which is attributed to the change in spin state being somewhat offset by this coordination switching behavior. The complexes were utilized as redox shuttles in dye-sensitized solar cells with the near-IR AP25 + D35 dye system and exhibited improved photocurrents over the [Co(bpy)3]3+/2+ redox shuttle (19.8 vs 18.0 mA/cm2). Future directions point toward pairing the low-spin to low-spin Co(II/III) tunable series to dyes with significantly more negative highest occupied molecular orbital potentials that absorb into the near-IR where outer sphere redox shuttles have failed to produce efficient dye regeneration.

Efficient Reductive Defluorination of Branched PFOS by Metal-Porphyrin Complexes.

Vitamin B12 (VB12) has been reported to degrade PFOS in the presence of TiIII citrate at 70 °C. Porphyrin-based catalysts have emerged as VB12 analogues and have been successfully used in various fields of research due to their interesting structural and electronic properties. However, there is inadequate information on the use of these porphyrin-based metal complexes in the defluorination of PFOS. We have therefore explored a series of porphyrin-based metal complexes for the degradation of PFOS. CoII-5,10,15,20-tetraphenyl-21H,23H-porphyrin (CoII-TPP), CoII-5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphyrin (CoII-M-TPP), and CoIII-M-TPP exhibited efficient reductive defluorination of the branched PFOS. Within 5-8 h, these compounds achieved the same level of PFOS defluorination as VB12 achieved in 7-10 days. For branched isomers, the specific removal rate of the CoII-TPP-TiIII citrate system is 64-105 times higher than that for VB12-TiIII citrate. Moreover, the CoII-TPP-TiIII citrate system displayed efficient (51%) defluorination for the branched PFOS (br-PFOS) in 1 day even at room temperature (25 °C). The effects of the iron and cobalt metal centers, reaction pH, and several reductants (NaBH4, nanosized zerovalent zinc (nZn0), and TiIII citrate) were systematically investigated. Based on the analysis of the products and previously published reports, a new possible defluorination pathway of branched PFOS is also proposed.

A Polysulfone/Cobalt Metal–Organic Framework Nanocomposite Membrane with Enhanced Water Permeability and Fouling Resistance

Ultrafiltration membranes are widely used in water and wastewater applications. The two most important membrane characteristics that determine the cost-effectiveness of an ultrafiltration membrane process are membrane permeability and fouling resistance. Metal–organic frameworks (MOFs) have been intensively investigated as highly selective sorbents and superior (photo) catalysts. Their potential as membrane modifiers has also received attention recently. In this study, a non-functionalized, water-stable, nanocrystalline mixed ligand octahedral MOF containing carboxylate and amine groups with a cobalt metal center (MOF-Co) was incorporated into polysulfone (PSF) ultrafiltration (UF) membranes at a very low nominal concentration (2 and 4 wt %) using the conventional phase inversion method. The resultant PSF/MOF-Co_4% membrane exhibited water permeability up to 360% higher than of the control PSF membrane without sacrificing the selectivity of the membrane, which had not been previously achieved by an unmodified MOF. In addition, the PSF/MOF-Co_4% membrane showed strong resistance to fouling by natural organic matter (NOM), with 87 and 83% reduction in reversible and irreversible NOM fouling, respectively, compared to the control PSF membrane. This improvement was attributed to the increases in membrane porosity and surface hydrophilicity resulting from the high hydrophilicity of the MOF-Co. The capability of increasing membrane water permeability and fouling resistance without compromising membrane selectivity makes the MOF-Co and potentially other hydrophilic MOFs excellent candidates as membrane additives.

Proof of concept for CUK family metal-organic frameworks as environmentally-friendly adsorbents for benzene vapor

The utility of metal-organic frameworks (MOFs) such as the CUK family (CUK – Cambridge University–KRICT) has been explored intensively for adsorption/separation of airborne volatile organic compounds (VOCs). In this article, three M-CUK analogs (M = Mg, Co, or Ni) were synthesized hydrothermally under similar conditions to assess the effects of their isostructural properties and metal centers on adsorption of benzene vapor (0.05–1 Pa). A list of performance metrics (e.g., breakthrough volume (BTV) and partition coefficient (PC)) were used to assess the role of the metal type (in M-CUK-1s) in the adsorption of VOCs. Specifically, Co-CUK-1 (average pore size of 8.98 nm) showed 2–3 times greater performance (e.g., in terms of 10% BTV (2012 L atm g−1) and PC (6 mol kg−1 Pa−1)) over other analogs when exposed up to 0.05 Pa benzene vapor. The superiority of mesoporous Co-CUK-1 (e.g., enhanced adsorption diffusion mechanism through favorable metal-π and π- π interactions) can be attributed to the presence of cobalt metal centers (e.g., in reference to Mg- or Ni-CUK-1).

A Hybrid Metal-Organic Framework-Reduced Graphene Oxide Nanomaterial for Selective Removal of Chromate from Water in an Electrochemical Process.

Hexavalent chromium Cr(VI) is a highly toxic groundwater contaminant. In this study, we demonstrate a selective electrochemical process tailored for removal of Cr(VI) using a hybrid MOF@rGO nanomaterial synthesized by in situ growth of a nanocrystalline, mixed ligand octahedral metal-organic framework with cobalt metal centers, [Co2(btec)(bipy)(DMF)2]n (Co-MOF), on the surface of reduced graphene oxide (rGO). The rGO provides the electric conductivity necessary for an electrode, while the Co-MOF endows highly selective adsorption sites for CrO42-. When used as an anode in the treatment cycles, the MOF@rGO electrode exhibits strong selectivity for adsorption of CrO42- over competing anions including Cl-, SO42-, and As(III) and achieves charge efficiency (CE) >100% due to the strong physisorption of CrO42- by Co-MOF; both electro- and physisorption capacities are regenerated with the reversal of the applied voltage, when highly toxic Cr(VI) is reduced to less toxic reduced Cr species and subsequently released into brine. This approach allows easy regeneration of the nonconducting Co-MOF without any chemical addition while simultaneously transforming Cr(VI), inspiring a novel electrochemical method for highly selective degradation of toxic contaminants using tailor-designed electrodes with high affinity adsorbents.

Development of a "Turn-on" Fluorescent Probe-Based Sensing System for Hydrogen Sulfide in Liquid and Gas Phase.

A “turn-on” fluorescence sensing system based on a BODIPY-cobaloxime complex for the detection of H2S in liquid and gas phase was developed. To that aim, two cobaloxime complexes bearing an axial pyridyl-BODIPY ligand were initially evaluated as sensitive fluorescent HS− indicators in aqueous solution. The sensing mechanism involves the selective substitution of the BODIPY ligand by the HS− anion at the cobalt center, which is accompanied by a strong fluorescence enhancement. The selection of a complex with an ideal stability and reactivity profile toward HS− relied on the optimal interaction between the cobalt metal-center and two different pyridyl BODIPY ligands. Loading the best performing BODIPY-cobaloxime complex onto a polymeric hydrogel membrane allowed us to study the selectivity of the probe for HS− against different anions and cysteine. Successful detection of H2S by the fluorescent “light-up” membrane was not only accomplished for surface water but could also be demonstrated for relevant H2S concentrations in gas phase.

New Heteroleptic Cobalt Precursors for Deposition of Cobalt-Based Thin Films.

A new series of heteroleptic complexes of cobalt were synthesized using aminoalkoxide and β-diketonate ligands. The complexes, [Co(dmamp)(acac)]2 (3), [Co(dmamp)(tfac)]2 (4), [Co(dmamp)(hfac)]2 (5), [Co(dmamp)(tmhd)]2 (6), and [Co(dmamb)(tmhd)]2 (7), were prepared by two-step substitution reactions and studied using Fourier transform infrared spectroscopy, mass spectrometry, elemental analysis, and thermogravimetric analysis (TGA). Complexes 3–7 displayed dimeric molecular structures for all of the complexes with cobalt metal centers interconnected by μ2-O bonding by the alkoxy oxygen atom. TGA and a thermal study of the complexes displayed high volatilities and stabilities for complexes 6 and 7, with sublimation temperatures of 120 °C/0.5 Torr and 130 °C/0.5 Torr, respectively.

Perfluoroalkyl Cobalt(III) Fluoride and Bis(perfluoroalkyl) Complexes: Catalytic Fluorination and Selective Difluorocarbene Formation.

Four perfluoroalkyl cobalt(III) fluoride complexes have been synthesized and characterized by elemental analysis, multinuclear NMR spectroscopy, X-ray crystallography, and powder X-ray diffraction. The remarkable cobalt fluoride (19)F NMR chemical shifts (-716 to -759 ppm) were studied computationally, and the contributing paramagnetic and diamagnetic factors were extracted. Additionally, the complexes were shown to be active in the catalytic fluorination of p-toluoyl chloride. Furthermore, two examples of cobalt(III) bis(perfluoroalkyl)complexes were synthesized and their reactivity studied. Interestingly, abstraction of a fluoride ion from these complexes led to selective formation of cobalt difluorocarbene complexes derived from the trifluoromethyl ligand. These electrophilic difluorocarbenes were shown to undergo insertion into the remaining perfluoroalkyl fragment, demonstrating the elongation of a perfluoroalkyl chain arising from a difluorocarbene insertion on a cobalt metal center. The reactions of both the fluoride and bis(perfluoroalkyl) complexes provide insight into the potential catalytic applications of these model systems to form small fluorinated molecules as well as fluoropolymers.

Novel cathode catalyst for DMFC: Study of the density of states of oxygen adsorption using density functional theory

Platinum (Pt) is the most common cathode catalyst in Direct Methanol Fuel Cells (DMFC). However, Pt involved both in the oxidation reaction of methanol and the reduction of oxygen on the cathode side, which limits the performance of DMFCs. Therefore, this study investigates cobalt phthalocyanine (CoPc) as a potential DMFC cathode catalyst capable of inhibiting the reactivity of methanol oxidation. This study investigates the reaction mechanism and adsorption energy of oxygen molecules on Cobalt Phthalocyanine (CoPc) using the Density Functional Theory (DFT). The basis sets of semi-core pseudopotential (DSPP) and effective core potential are used to compare the geometric optimization of the Cobalt Phthalocyanine structure. The adsorption strength of molecular oxygen on the Cobalt metal center of Phthalocyanine is investigated using Partial Density of States and compared with that of Iron Phthalocyanine (FePc). Finally, the results show that the adsorption energy of carbon monoxide and methanol on the Cobalt and Iron metal centers of the Phthalocyanine complex yield low energy adsorption on CoPc. It was observed that the adsorption energy when two metal sites of CoPc and supported CoPc on tungsten atom are involved yields high energy adsorption and bond length of molecular oxygen that would lead to oxygen bond dissociation. Therefore, this study concludes that CoPc has the potential to replace Pt due to its high tolerance to methanol and carbon monoxide oxidation.

Trans-Dichloridotetrakis[(dimethylphosphoryl)methanaminium-κO]cobalt(II) tetrachloridocobaltate(II)

The asymmetric unit of the title structure, [CoCl2(C3H11NOP)4][CoCl4]2, consists of one half of the trans-dichlorido­tetra­kis­[(di­methyl­phosphor­yl)methanaminium]cobalt(II) tetra­cation lying on an inversion center and one tetra­chloridocobaltate(II) dianion on a general position. Four O-coordinated cationic (di­methyl­phosphor­yl)methanaminium (dpmaH+) ligands occupy the equatorial coordination sites, whereas the chloride ligands occupy axial positions of the roughly o­cta­hedral coordination polyhedron of the cobalt metal center. Intra­molecular hydrogen bonds between the aminium groups and the O atom of the phosphoryl groups and additional hydrogen bonds between the aminium groups and the chloride ligands are present. Furthermore, four of the six H atoms not involved in intra­molecular bonding of each cobalt(II) tetra­cation form weak hydrogen bonds to four adjacent tetra­chloridocobaltate(II) counter-anions. By these inter­molecular hydrogen bonds, one-dimensional polymeric strands are formed along the b-axis direction. The hydrogen bonding is analyzed using the graph-set method and the structural similarity with dpmaHCl is discussed.

Syntheses and crystal structures of mono-, di- and trinuclear cobalt complexes of a salen type ligand

Three cobalt complexes containing the salen type ligand, bis(salicylidene)-meso-1,2-diphenylethylenediaminato (mdpSal 2−), are reported. The complexes differ in nuclearity and include the mononuclear, Co(mdpSal) ( 1), which contains a Co(II) metal center bound to one mdpSal −2 ligand frame in a square planar geometry. The second complex is the dinuclear [Co(mdpSal)Cl] 2 ( 2) in which both cobalt ions have been oxidized to the +3 oxidation state. The overall geometry of complex 2 is an edge-sharing bioctahedron with the coordination sphere around each cobalt metal center consisting of one mdpSal −2 ligand and one Cl − ion. The shared edge between the Co(III) ions contains two bridging phenolate groups, one from each ligand frame. Complex 3 is a linear, mixed valence, trinuclear species, [Co(mdpSal)(OAc)(μ-OAc)] 2Co, with the oxidation states of the metal centers assigned as Co(III)–Co(II)–Co(III). The terminal Co(III) centers are equivalent with the central Co(II) lying on the inversion center of the molecule. Each cobalt ion in 3 adopts an octahedral geometry with the terminal Co(III) ions being bound to one mdpSal 2− ligand each. All phenolate groups bridge to the central Co(II). The coordination sphere about each metal center in the trinuclear complex is completed by four acetate groups, two of which bind in a μ-fashion bridging from the terminal Co(III) metal centers to the central Co(II). The complexes have been characterized by X-ray crystallography as well as UV–Vis and IR spectroscopy.

Spectroscopic characterization of cobalt-containing solid catalysts

The nature of the cobalt centers in oxidized and reduced CoOx/Al2O3 catalysts and in oxidized Co-H-mordenite (Co-H-MOR) have been characterized by UV-Vis spectroscopy and by IR spectroscopy. The UV-Vis spectroscopy of CoOx/Al2O3 catalysts is dominated by the features of divalent cobalt in tetrahedral coordination. These sites can be reduced by hydrogen to cobalt metal centers that have been characterized by IR spectroscopy of adsorbed carbon monoxide. The surface of CoOx/Al2O3 when it is in contact with CO/H2 mixtures and works as the catalyst for hydrocarbon synthesis has also been characterized by IR. Surface formate species were detected that could act as reaction intermediates. The active sites of Co-H-MOR are also constituted by tetrahedrally coordinated Co2+ that act as medium strong Lewis sites. The presence of Co species seems to cause a slight increase of the average Brønsted acidity of the residual hydroxy groups in Co-H-MOR samples.