Cobalt Clusters Research Articles

Slow magnetic relaxation in mixed-valence coordination polymer, containing Co(III) cluster and Co(II) nodes

Single-molecule magnets in cobalt clusters is much less well known than it is in manganese(II)/(III)/(IV) clusters. To explore new types of cobalt clusters and their potential applications, a new mixed valence coordination polymer ([Co(III)3Co(II)2(mba)6(Hdtba) (H2O)4]n, H2dtba=2,2’-dithiodibenzoic acid, H2mba=2-mercaptobenzoic acid) (1) is constructed by employing cobalt chloride hexahydrate and H2mba as starting reactants under hydrothermal conditions. Single-crystal XRD studies indicate that 1 consists of a trinuclear cobalt(III)-sulfur cluster moiety and two discrete cobalt(II) nodes, which result in the formation of a covalent three-dimensional complex network. Similarly, the cobalt(III) cluster moiety acts as a building unit in this infinite coordination polymer 1 instead of the discrete and tiara-like clusters previously reported in the literature. Magnetic studies show its slow magnetic relaxation in the absence of an applied dc field, which is characteristic behavior of single-ion magnets, caused by the individual Co(II) ions.

Observation of the Reaction Intermediates of Methanol Dehydrogenation by Cationic Vanadium Clusters

AbstractA mass spectrometric study of the reactions of vanadium cationic clusters with methanol in a low‐pressure collision cell is reported. For comparison, the reaction of methanol with cobalt cationic clusters was studied. For vanadium, the main reaction products are fully dehydrogenated species, and partial dehydrogenation and non‐dehydrogenation species are observed as minors, for which the relative intensities increase with cluster size and also at low cluster source temperature cooled by liquid nitrogen; no dehydrogenation products were observed for cobalt clusters. Quantum chemical calculations explored the reaction pathways and revealed that the fully dehydrogenation products of the reaction between Vn+ and methanol are Vn(C)(O)+, in which C and O are separated owing to the high oxophilicity of vanadium. The partial dehydrogenation and non‐dehydrogenation species were verified to be reaction intermediates along the reaction pathway, and their most probable structures were proposed.

Observation of the Reaction Intermediates of Methanol Dehydrogenation by Cationic Vanadium Clusters.

A mass spectrometric study of the reactions of vanadium cationic clusters with methanol in a low-pressure collision cell is reported. For comparison, the reaction of methanol with cobalt cationic clusters was studied. For vanadium, the main reaction products are fully dehydrogenated species, and partial dehydrogenation and non-dehydrogenation species are observed as minors, for which the relative intensities increase with cluster size and also at low cluster source temperature cooled by liquid nitrogen; no dehydrogenation products were observed for cobalt clusters. Quantum chemical calculations explored the reaction pathways and revealed that the fully dehydrogenation products of the reaction between Vn + and methanol are Vn (C)(O)+ , in which C and O are separated owing to the high oxophilicity of vanadium. The partial dehydrogenation and non-dehydrogenation species were verified to be reaction intermediates along the reaction pathway, and their most probable structures were proposed.

Infrared action spectroscopy of nitrous oxide on cationic gold and cobalt clusters.

Understanding the catalytic decomposition of nitrous oxide on finely divided transition metals is an important environmental issue. In this study, we present the results of a combined infrared action spectroscopy and quantum chemical investigation of molecular N2O binding to isolated Aun+ (n ≤ 7) and Con+ (n ≤ 5) clusters. Infrared multiple-photon dissociation spectra have been recorded in the regions of both the N[double bond, length as m-dash]O (1000-1400 cm-1) and N[double bond, length as m-dash]N (2100-2450 cm-1) stretching modes of nitrous oxide. In the case of Aun+ clusters only the ground electronic state plays a role, while the involvement of energetically low-lying excited states in binding to the Con+ clusters cannot be ruled out. There is a clear preference for N-binding to clusters of both metals but some O-bound isomers are observed in the case of smaller Con(N2O)+ clusters.

Decomposition of nitrous oxide in hydrated cobalt(I) clusters: a theoretical insight into the mechanistic roles of ligand-binding modes.

Hydrated cobalt(i) cluster ions, [Co(H2O)n]+, can decompose the inert nitrous oxide molecule, N2O. Density functional theory suggests that N2O can anchor to Co+ of [Co(N2O)(H2O)n]+ through either O end-on (η1-OL) or N end-on (η1-NL) coordinate mode. The latter is thermodynamically more favorable resulting from a subtle π backdonation from Co+ to N2O. N2O decomposition involves two major processes: (1) redox reaction and (2) N-O bond dissociation. The initial activation of N2O through an electron transfer from Co+ to N2O yields anionic N2O-, which binds to the metal center of [Co2+(N2O-)(H2O)n] also through either O end-on (η1-O) or N end-on (η1-N) mode and is stabilized by water molecules through hydrogen bonding. From η1-O, subsequent N-O bond dissociation to liberate N2, producing [CoO(H2O)n]+, is straightforward via a mechanism that is commonplace for typical metal-catalyzed N2O decompositions. Unexpectedly, the N-O bond dissociation directly from η1-N is also possible and eliminates both N2 and OH, explaining the formation of [CoOH(H2O)n]+ as observed in a previous experimental study. Interestingly, formation of [CoO(H2O)n]+ is kinetically controlled by the initial redox process between Co+ and the O-bound N2O, the activation barriers of which in large water clusters (n ≥ 14) are higher than that of the unexpected N-O bond dissociation from the N-bound structure forming [CoOH(H2O)n]+. This theoretical discovery implies that in the present of water molecules, the metal-catalyzed N2O decomposition starting from an O-bound metal complex is not mandatory.

Rylene- and diaza-rylene-derived cobalt clusters for solid-state pyrolysis towards undoped and N-doped carbon nanoparticles

A set of four tetraalkynyl rylene and diaza-rylene (L) templated clusters of the type [LCo8(CO)24] was synthesized and fully characterized. Their solid state pyrolysis led to carbon nanoparticles of different shapes depending on the C : N and C : Co ratio.

Two new uncommon 3D cobalt-based metal organic frameworks: Temperature induced syntheses and enhanced photocatalytic properties against aromatic dyes

The reaction of 5-fluorine-3-(3,5-dicarboxylphenxoy)benzoic acid (H3L) with cobalt(II) salt in presence of 1,1'-(1,4-butanediyl)bis (imidazole) (bib) linker yielded two new metal–organic frameworks (MOFs) with compositions [Co4(μ3-OH)2(H2O)2(L)2 (bib)1.5(CH3CN)·3H2O·CH3CN] (1) and [Co8(μ3-OH)4(L)4 (bib)4·8CH3CN] (2). The single crystal X-ray diffraction revealed that 1 exhibited an unusual 3D 3,8-connected framework with 3,8T25 topology based on [Co4(μ3-OH)2] tetranuclear cobalt clusters as secondary building units (SBU) with a (43) (412·54·64·88) Schläfli symbol. In 2, by substituting the CH3CN with neutral N-ligand, a 3D 3,8-connected MOF with 3,10T9 topology and a point symbol of {418.624.83}.{43}2 has been obtained. This represents a highly connected uninodal network topology presently known for flexible carboxylate-based MOF systems. Both the MOFs have been used as photocatalysts for photodegradation of a model organic dye methyl violet (MV). The photocatalytic experiments demonstrated that both 1 and 2 displayed efficient photocatalytic performances to degrade MV under UV irradiation. The plausible mechanism through which both the MOFs displayed photocatalytic properties have been proposed by using band gap calculations.

Investigation into the adsorption of methylene blue on the surface of a "core–shell" type catalyst for the Fenton system

The catalytic oxidation of organic substances, in particular by the Fenton method, is one of the effective methods of wastewater treatment. The oxidation efficiency is affected by the adsorption properties of the catalyst when using heterogeneous catalysts in the Fenton process. In this work, the adsorption kinetics of methylene blue dye from aqueous solutions of different concentrations on the surface of the synthesized nanostructured magnetosensitive catalyst CoFe2O4/SiO2/CoMnO2 was studied by spectrophotometric analysis and the kinetic parameters of the process were determined. The CoMnO2-based "core–shell" catalyst has a magnetically sensitive cobalt ferrite core coated with a protective layer of porous SiO2, on the surface of which clusters of cobalt and manganese oxides are placed as catalytic centers. The process of adsorption of methylene blue from an aqueous solution can be considered as a quasi-chemical reaction of displacement of solvent molecules from the adsorption layer by dye molecules. The equilibrium state is described by the Langmuir equation, in which the effective adsorption constant is the ratio of the equilibrium constants of the Langmuir interactions of the dye and the solvent, respectively, with the active center of the adsorbent. The limiting adsorption of methylene blue and the adsorption constant characterizing the affinity of the dye to the catalyst surface were determined. The specific area of the catalyst was calculated under the condition of a known landing plane of the dye molecule. The influence of the adsorption properties of the catalyst on the efficiency of the Fenton process was established.

A Three‐Dimensional Pentanuclear Co(II) Coordination Polymer: Structural Topology, Hirshfeld Surface Analysis and Magnetic Properties

AbstractA 3D pentanuclear coordination polymer of Co(II); {[Co5(L2‐)4(HL‐)2(bpmp)2 (H2O)2].(H2O)5}n (CP1) was synthesized by the self‐assembly of semi‐flexible 5‐benzylamino‐isophthalic acid (H2L) and N‐donor auxiliary ligand 1,4‐bis(4‐pyridinylmethyl)piperazine (bpmp) using Co(NO3)2 ⋅ 6H2O under hydrothermal condition. The CP1 has been structurally characterized by elemental analysis, TGA, Powder‐XRD and IR spectroscopy which was further corroborated by single crystal X‐ray studies. Structurally, it is a 3D pentanuclear cobalt cluster secondary building units (SBUs) which are formed by carboxylates oxygen of H2L and nitrogen of bpmp. The topological analysis revealed that CP1 possesses a 2‐nodal 5,6‐connected three‐dimensional (3D) coordination framework with pcu net. Variable temperature magnetic measurements demonstrate that CP1 shows antiferromagnetic behavior.

Preadsorption Effect of Carbon Monoxide on Reactivity of Cobalt Cluster Cations toward Hydrogen.

We report gas-phase reactions of free Con(CO)m+ (n = 3-11, m = 0-2) with H2, expecting a catalytic reaction of coadsorbed CO and H2 on Con+. Preadsorption of CO molecules is found to promote H2 adsorption, in particular, on Con(CO)+ (n = 5, 8-10). Density functional theory (DFT) calculations reveal that the reactivity is governed by the molecular-orbital energy of Con+, which is tuned by preadsorbed CO molecules. Collision-induced-dissociation experiments performed on ConCOH2+ (n = 8-10) imply that at least some of the CO and H2 molecules are bound together on Con+.

Fabrication of a new 2D Co(II)-organic framework tuned by semi-flexible dicarboxylate and 1,4-bis(4-pyridinylmethyl)piperazine ligands: Topology, DFT/UB3LYP calculations, Hirshfeld surface analysis and magnetic studies

A new 2D-coordination polymer, {[Co3(L−2)2(HL−)2(bpmp)](H2O)2}n (CP1) was synthesized by using a dicarboxylate ligand, H2L (5–benzylamino–isophthalic acid) and a nitrogen donor ligand 1,4-bis(4-pyridinylmethyl)piperazine (bpmp) with Co(NO3)2•6H2O under hydrothermal conditions via careful tuning the basicity of reaction conditions. Single crystal X-ray studies revealed that CP1 exist as a linear trinuclear cobalt cluster. DFT/UB3LYP/6-31G(d,p) calculations have been performed to study the ground state molecular geometry, IR spectrum, magnetic behavior by electron spin densities distribution as well as to explore various structure based molecular properties. The topological analysis of CP1 results an underlying sql net with a point symbol of {44.62}.Variable temperature magnetic susceptibility measurements of CP1 were carried out over the temperature range 2–300 K which indicates the existence of paramagnetic behavior.

Decoration of photocatalytic TiO2 particles by cobalt clusters

Decoration of photocatalytic TiO2 particles by cobalt clusters

Nanopore Confinement of Electrocatalysts Optimizing Triple Transport for an Ultrahigh-Power-Density Zinc-Air Fuel Cell with Robust Stability.

Metal-air fuel cells with high energy density, eco-friendliness, and low cost bring significantly high security to future power systems. However, the impending challenges of low power density and high-current-density stability limit their widespread applications. In this study, an ultrahigh-power-density Zn-air fuel cell with robust stability is highlighted. Benefiting from the water-resistance effect of the confined nanopores, the highly active cobalt cluster electrocatalysts reside in specific nanopores and possess stable triple-phase reaction areas, leading to the synergistic optimization of electron conduction, oxygen gas diffusion, and ion transport for electrocatalysis. As a result, the as-established Zn-air fuel cell shows the best stability under high-current-density discharging (>90 h at 100mA cm-2 ) and superior power density (peak power density: >300mW cm-2 , specific power: 500 Wgcat -1 ) compared to most reported non-noble-metal electrocatalysts. The findings will provide new insights in the rational design of electrocatalysts for advanced metal-air fuel cell systems.

Rational one-step synthesis of cobalt clusters embedded-graphitic carbon nitrides for the efficient photocatalytic CO2 reduction under ambient conditions

Metal clusters catalyst embedded in semiconductors is the key factor for an efficient photocatalyst toward photocatalytic CO2 reduction. However, developing a simple method to synthesize such photocatalyst with these features has remained challenging. Herein, a rational one-step synthesis of cobalt clusters embedded in g-C3N4 layers is developed for an enhanced photocatalytic CO2 reduction. The tiny cobalt clusters are embedded into the g-C3N4 structure and stabilized through the interaction with NH2-bpy moieties. Besides, the ligands of cobalt complex can copolymerize with urea during the synthesis owing to the presence of amino groups, thus providing a direct electron transfer between carbon nitride and cobalt clusters, which act as the light harvester and active reduction site, respectively. Consequently, a volcano relationship between the CO generation rate and the cobalt clusters incorporated into the carbon nitride matrix was found. Under optimized conditions, representative C3N4-0.5CoOx exhibits the very high CO generation rate of 2.6 μmol.h−1. Its apparent quantum yield of 3.2% at 420 nm and 1.0% at 460 nm, which is one of the highest values reported so far. The finding offers new opportunities for designing metal clusters catalysts toward photocatalytic CO2 reduction based on the judicious use of the metal complex.

Synthesis and X-ray crystallography of nonanuclear cobalt cluster and mononuclear cobalt complex: Effect of phosphine on the formation of cobalt complexes

Nonanuclear complex [Co{Co4(µ4-O)(1-methylimidazole-2-thiolate)6}2](BF4)2 and mononuclear complex [Co(bis(1-methyl-imidazol-2-yl)disulfide)2](BF4)2 were synthesized using almost the same method and characterized by X-ray diffraction study. Although almost the same method was used during synthesis, the presence and absence of phosphine produced the nonanuclear complex and mononuclear complex, respectively. In addition, the mononuclear complex has bis(1-methyl-imidazol-2-yl)disulfide that can be obtained from the oxidation of 1-methylimidazole-2-thiolate ligand. Therefore, phosphine ligand could prevent the oxidation of 1-methylimidazole-2-thiolate ligand.

A novel penta-nuclear cobalt cluster exhibiting slow magnetic relaxation behavior

A novel CoII3CoIII2 cluster, namely, [CoII3CoIII2 (HL)6](1), has been successfully obtained by using a multidentate organic ligand via solvothermal method, and further characterized by crystallography, elemental analysis (EA), infrared spectra (IR), powder X-ray diffraction (PXRD) and magnetic study. Structural analysis shows that two different coordination environments of CoII and CoIII ions were existed in the penta-nuclear cobalt cluster. The magnetic measurement reveals that 1 exhibits slow magnetic relaxation behavior under a zero direct current (dc) field.

Co13O8-metalloxocubes: a new class of perovskite-like neutral clusters with cubic aromaticity.

Exploring stable clusters to understand structural evolution from atoms to macroscopic matter and to construct new materials is interesting yet challenging in chemistry. Utilizing our newly developed deep-ultraviolet laser ionization mass spectrometry technique, here we observe the reactions of neutral cobalt clusters with oxygen and find a very stable cluster species of Co13O8 that dominates the mass distribution in the presence of a large flow rate of oxygen gas. The results of global-minimum structural search reveal a unique cubic structure and distinctive stability of the neutral Co13O8 cluster that forms a new class of metal oxides that we named as ‘metalloxocubes’. Thermodynamics and kinetics calculations illustrate the structural evolution from icosahedral Co13 to the metalloxocube Co13O8 with decreased energy, enhanced stability and aromaticity. This class of neutral oxygen-passivated metal clusters may be an ideal candidate for genetic materials because of the cubic nature of the building blocks and the stability due to cubic aromaticity.

Size-dependent velocity distributions and temperatures of metal clusters in a helium carrier gas

Combining a laser vaporization cluster source, a velocity scan system, and a time-of-flight mass spectrometer, we measured velocity distributions of few-atom cobalt clusters (${\text{Co}}_{n}, n=6--60$) suspended in a helium carrier gas and expanded into vacuum. The velocity distributions provide information about the cluster size dependence of the translational temperature, flow velocity, and velocity slip with respect to the helium carrier gas. The system of clusters in a carrier gas is found to violate the equipartition theorem. Although the clusters in the expansion do not thermalize with the helium gas, they do experience significant, size-dependent internal cooling. While expanding into vacuum, the clusters collide, at least a couple of hundred times, superelastically with the carrier gas, thereby transferring internal vibrational energy into self-acceleration and increasing the flow velocity of the gas as a whole. It is also demonstrated that the proposed velocity distribution measurements can be used to test whether a source produces thermalized clusters.

Reactivity of Cobalt Clusters Con±/0 with Ammonia: Co3+ Cluster Catalysis for NH3 Dehydrogenation.

A customized reflection time-of-flight (Re-TOF) mass spectrometer combined with a 177 nm deep-ultraviolet laser has enabled us to observe well-resolved cobalt clusters Con±/0 and perform a comprehensive study of their reactivity with ammonia (NH3). The anions Con- are found to be inert, the neutrals allow the adsorption of multiple NH3 molecules, while the cationic Con+ clusters readily react with NH3 giving rise to dehydrogenation. However, incidental dehydrogenation of NH3 on Con+ is only observed for n ≥ 3. The dramatic charge- and size-dependent reactivities of Con±/0 clusters with NH3 are studied by the density functional theory (DFT)-calculation results of energetics, density of states, orbital interactions, and reaction dynamics. We illustrate the dehydrogenation from two NH3 molecules, where a significantly reduced transition-state energy barrier is found pertaining to the dimolecular co-catalysis effect. The reactivity of Co3+ with NH3 is illustrated showing effective catalysis for N-H dissociation to produce hydrogen applicable for designing ammonia fuel cells.

A multifunctional pseudo-polyrotaxane coordination polymer based on the trinuclear cluster [Co3(COO−)4(OH−)2]: Synthesis, structure and properties

A trinuclear cobalt-based multifunctional coordination polymer with a dual loop pseudo-rotaxane motif, namely [Co3(DPFE‧H+)2(sdba2−)4(OH−)2](DPFE‧2H+)(CH3CN) (1), has been synthesized by a mixed ligand strategy under hydrothermal conditions (DPFE = 2,7-di(pyridin-4-yl)-9H-fluorene, H2sdba = 4,4′-sulfonyldibenzoic acid). Compound 1 has 1D + zero-dimensional (0D) → 1D armed pseudo-polyrotaxane character, in which the 1D dual ring chain [Co3(COO−)4(OH−)2(sdba2−)4 is threaded by doubly protonated free DPFE ligands. The trinuclear cobalt cluster and π-electron rich system involving the ligands jointly endow compound 1 with magnetism, photocatalytic and electrochemical properties. Magnetic property measurements of 1 indicate that there exists weak antiferromagnetic coupling in the trinuclear [Co3(COO−)4(OH−)2] unit, the experiments of photocatalytic degradation of dyes demonstrated that 1 has good catalytic activity for the degradation of methylene blue (MB) and the electrochemical results show that compound 1 exhibits a specific capacitance of 52.25 F g−1 at a current density of 1 A g−1 and retains 93.75% specific capacitance after 960 cycles.