Cuboidal Cluster Research Articles

Cuboidal molybdenum sulfur clusters as a platform to construct novel catalysts for propane dehydrogenation

Growing demand for short chain olefins such as propylene for plastic production drives the development of high performing catalysts for propane dehydrogenation (PDH). Here, a confinement strategy originating from metal-sulfur clusters was employed to fabricate defective MoPtSx phase via anchoring by coordinatively unsaturated Al sites present on alumina, followed by reduction under H2 flow at high temperature. This catalyst with a low Pt content exhibits great ability for C–H activation with high intrinsic activity and facile propylene desorption in PDH reaction. More importantly, the catalyst shows excellent stability during long-term tests with both stable conversion and selectivity. Through combined X-ray absorption spectroscopy, electron microscopy and electron paramagnetic resonance measurements it could be affirmed that interaction between isolated and electron-enriched Pt sites as well as their proximity to adjacent sulfur vacancies is vital for the first and second C–H bond activation, while suppressing C–C cleavage which otherwise lead to coking.

Selective Dehydrogenation of Formic Acid Catalyzed by Air‐Stable Cuboidal PN Molybdenum Sulfide Clusters

AbstractFormic acid is considered as a promising hydrogen storage material in the context of a green hydrogen economy. In this work, we present a series of aminophosphino and imidazolylamino Mo3S4 cuboidal clusters which are active and selective for formic acid dehydrogenation (FAD). Best results are obtained with the new [Mo3S4Cl3(ediprp)3](BPh4) (4(BPh4)) (ediprp=(2‐(diisopropylphosphino)ethylamine)) cluster, which is prepared through a simple ligand exchange process from the Mo3S4Cl4(PPh3)3(H2O)2 precursor. Under the conditions investigated, complex 4+ showed significantly improved performance (TOF=4048 h−1 and 3743 h−1 at 120 °C in propylene carbonate using N,N‐dimethyloctylamine as base after 10 min and 15 min, respectively) compared to the other reported molybdenum compounds. Mechanistic investigations based on stoichiometric and catalytic experiments show that cluster 4+ reacts with formic acid in the presence of a base to form formate substituted species [Mo3S4Cl3‐x(OCOH)x(ediprp)3]+ (x=1–3) from which the catalytic cycle starts. Subsequently, formate decarboxylation of the partially substituted [Mo3S4Cl3‐x(OCOH)x(ediprp)3]+ (x=1, 2, 3) catalyst through a β‐hydride transfer to the metal generates the trinuclear Mo3S4 cluster hydride. Dehydrogenation takes place through protonation by HCOOH to form Mo−H⋅⋅⋅HCOOH dihydrogen adducts, with regeneration of the Mo3S4 formate cluster. This proposal has been validated by DFT calculations.

A new application of salamo-type compounds: As a photocatalyst of designed self-assembling Cu(II) dimer and tetranuclear Co(II) cuboidal cluster

The design and synthesis of structurally rich chemical materials for detection, analysis, and purification is challenging. In this regard, two functional 3d metal(II) complexes bearing the half-salamo-type compound HL1 (2-[O-(1-ethyloxyamide)]oxime-3‑methoxy-5‑bromo-phenol) have been synthesized, namely: [Cu2(L2)2]·2CH3CN (1) and [Co4(L2)4(CH3CH2OH)4] (2). Complexes 1 and 2 were fully characterized by single-crystal X-ray diffraction, elemental analysis, and FT-IR and UV–Vis spectroscopy. The crystal structure analysis revealed that complex 1 is a centrosymmetric oxo-bridged Cu(II) dimer formed by connecting two metalloligand [Cu(L2)] units. The coordination self-assembly in complex 2 was triggered to obtain a unique tetranuclear Co(II) cuboidal cluster of S4 symmetry based on a [Co(L2)] metalloligand unit. The experimental results showed that HL1 spontaneously catalyzes the formation of new compound H2L2 during the formation of complexes 1 and 2. The photocatalytic degradation properties of dimer 1 and cluster 2 were investigated for several dye pollutants: methyl orange (MO), methylene blue (MB), and Rhodamine B (RhB) under H2O2 initiation. Additionally, DFT calculations were performed to further understand the structure-property relationship between the electron characteristics and the photocatalytic degradation behaviors.

Facile and dynamic cleavage of every iron–sulfide bond in cuboidal iron–sulfur clusters

Nature employs weak-field metalloclusters to support a wide range of biological processes. The most ubiquitous metalloclusters are the cuboidal Fe-S clusters, which are comprised of Fe sites with locally high-spin electronic configurations. Such configurations enhance rates of ligand exchange and imbue the clusters with a degree of structural plasticity that is increasingly thought to be functionally relevant. Here, we examine this phenomenon using isotope tracing experiments. Specifically, we demonstrate that synthetic [Fe4S4] and [MoFe3S4] clusters exchange their Fe atoms with Fe2+ ions dissolved in solution, a process that involves the reversible cleavage and reformation of every Fe-S bond in the cluster core. This exchange is facile-in most cases occurring at room temperature on the timescale of minutes-and documented over a range of cluster core oxidation states and terminal ligation patterns. In addition to suggesting a highly dynamic picture of cluster structure, these results provide a method for isotopically labeling pre-formed clusters with spin-active nuclei, such as 57Fe. Such a protocol is demonstrated for the radical S-adenosyl-l-methionine enzyme, RlmN.

Spin-Gated Selectivity of the Water Oxidation Reaction Mediated by Free Pentameric CaxMn5-xO5+ Clusters.

We report on the first preparation of isolated ligand-free CaMn4O5+ gas-phase clusters, as well as other pentameric CaxMn5-xO5+ (x = 0-4) clusters with varying Ca contents, which serve as molecular models of the natural CaMn4O5 inorganic cluster in photosystem II. Ion trap reactivity studies with D2O and H218O reveal a pronounced cluster composition-dependent ability to mediate the oxidation of water to hydrogen peroxide. First-principles density functional theory simulations elucidate the mechanism of water oxidation, proceeding via formation of a terminal oxyl radical followed by oxyl/hydroxy (O/OH) coupling. The critical coupling reaction step entails a single electron transfer from the oxyl radical to the accommodating cluster core with a concurrent O/OH coupling forming an adsorbed OOH intermediate group. The spin-conserving electron transfer step takes place when the spin of the transferred electron is aligned with the spins of the d-electrons of the Mn atoms in the cuboidal high-spin cluster isomer. The d-electrons provide a ferromagnetically ordered environment that facilitates the spin-gated selective electron transfer process, resulting in parallel-spin-exchange stabilization and a lowered transition state barrier for the coupling reaction involving the frontier orbitals of the oxyl and hydroxy reactant intermediates.

A cuboidal [Cu4(SO4)4] structure supported by β-picoline ligands.

The solid-state structure of the cobalt-β-picoline-sulfate complex tetra-μ3-sulfato-tetra-kis-[bis-(3-methyl-pyridine)-cobalt(II)], [Co4(SO4)4(C6H7N)8], is reported. The tetra-meric cobalt cluster contains a cuboidal core comprised of four cobalt(II) cations and four sulfate anions at alternate cube vertices. The cobalt corners are each capped with two β-picoline ligands. The sulfate anions adopt a rare [3.2110] bridging motif, and the cuboidal cluster is unprecedented in coordination chemistry.

A Synthetic Approach to Trinuclear Cluster with [Co3(μ3‐O)(μ‐OH)] Core: A Comparative Study on Cluster having [Co3(μ3‐O)(μ‐OH)] and [Co4(μ3‐O)4] Core

AbstractA synthetic approach to synthesize the cationic trinuclear cluster with [Co3(μ3‐O)(μ‐OH)] core has been reported. A comparative study with the closely relevant polynuclear cluster containing [Co4(μ3‐O)4] active core has also been carried out along with a plausible pathway for these two clusters. The minor spectroscopic and electrochemical changes observed for the cores are quite significant to distinguish them. The rigidity of the core can be well characterized by the solvent interaction. The involvement of the hydroxo group in the electrochemical process may help these clusters to be used for catalytic processes, although these [Co3(μ3‐O)(μ‐OH)] cationic clusters are thermally less stable than [Co4(μ3‐O)4] cuboidal clusters.

A Cuboidal Cu4S4 Cluster Supported by Bulky Iminothiolate Ligands: Synthesis, Solid-State Structure, and Solution Study

Abstract A new tetracopper(I) complex containing a cuboidal Cu4S4 core, [Cu4(dap)4], where dap− is dibenzodiazepinethiolate(1–), has been synthesized and structurally characterized by single-crystal X-ray diffraction analysis. The monoanionic iminothiolate ligand dap− renders the cluster framework exceptionally stable to retain the tetranuclear structure in solution, which allows us to examine for the first time the solution properties of this class of clusters, including UV-vis and NMR spectroscopy and cyclic voltammetry.

An [Fe4S4]3+-Alkyl Cluster Stabilized by an Expanded Scorpionate Ligand.

Alkyl-ligated iron-sulfur clusters in the [Fe4S4]3+ charge state have been proposed as short-lived intermediates in a number of enzymatic reactions. To better understand the properties of these intermediates, we have prepared and characterized the first synthetic [Fe4S4]3+-alkyl cluster. Isolation of this highly reactive species was made possible by the development of an expanded scorpionate ligand suited to the encapsulation of cuboidal clusters. Like the proposed enzymatic intermediates, this synthetic [Fe4S4]3+-alkyl cluster adopts an S = 1/2 ground state with giso > 2. Mössbauer spectroscopic studies reveal that the alkylated Fe has an unusually low isomer shift, which reflects the highly covalent Fe-C bond and the localization of Fe3+ at the alkylated site in the solid state. Paramagnetic 1H NMR studies establish that this valence localization persists in solution at physiologically relevant temperatures, an effect that has not been observed for [Fe4S4]3+ clusters outside of a protein. These findings establish the unusual electronic-structure effects imparted by the strong-field alkyl ligand and lay the foundation for understanding the electronic structures of [Fe4S4]3+-alkyl intermediates in biology.

Assembly of eight spherical magnets into a dotriacontapole configuration

The magnetic field of a cuboidal cluster of eight magnetic spheres is measured. It decays with the inverse seventh power of the distance. This corresponds formally to a hitherto unheard-of multipole, namely a dotriacontapole. This strong decay is explained on the basis of dipole-dipole interaction and the symmetry of the ensuing ground state of the cuboidal cluster. A method to build such dotriacontapoles is provided.

Novel mixed-metal cubane-type {Mo3NiS4} and {Mo3PdS4} clusters coordinated with 2,2′-bipyridine type ligands

New cuboidal cluster complexes of general formula [Mo3S4(M′X)Cl3L3]0/+ (M′ = Ni0, Pd0; L = dnbpy – 4,4′-dinonyl-2,2′-bipyridine or dbbpy – 4,4′-di-tert-butyl-2,2′-bipyridine; X = tu, Cl) were synthesized by reactions of trinuclear precursors [Mo3S4Cl3L3]+ with Ni(COD)2 (COD = 1,5-cyclooctadiene) or Pd2(dba)3·CHCl3 (dba = dibenzylideneacetone) in the presence of thiourea (tu) in good yields. The complexes were characterized by elemental analysis, IR, 1H NMR and ESI-MS techniques. Crystal structures of [Mo3S4(Nitu)Cl3(dbbpy)3]Cl·tu (1a), [Mo3S4(Nitu)Cl3(dbbpy)3]Cl (1b) and [Mo3S4(Pdtu)Cl3(dbbpy)3]Cl (2) were determined by X-ray analysis. The DFT calculations of the electronic structure of [Mo3S4(Pd′X)Cl3(bpy)3]0/+ (X = Cl or tu) were carried out.

Direct Numerical Simulation of Fluid Flow and Mass Transfer in Particle Clusters.

In this paper, an efficient ghost-cell based immersed boundary method is applied to perform direct numerical simulation (DNS) of mass transfer problems in particle clusters. To be specific, a nine-sphere cuboid cluster and a random-generated spherical cluster consisting of 100 spheres are studied. In both cases, the cluster is composed of active catalysts and inert particles, and the mutual influence of particles on their mass transfer performance is studied. To simulate active catalysts the Dirichlet boundary condition is imposed at the external surface of spheres, while the zero-flux Neumann boundary condition is applied for inert particles. Through our studies, clustering is found to have negative influence on the mass transfer performance, which can be then improved by dilution with inert particles and higher Reynolds numbers. The distribution of active/inert particles may lead to large variations of the cluster mass transfer performance, and individual particle deep inside the cluster may possess a high Sherwood number.

Evaluating the component contribution to nonlinear optical performances using stable [Ni4O4] cuboidal clusters as models.

Five stable clusters sharing the cuboidal [Ni4O4] skeleton are subjected to third-order nonlinear optical (NLO) property measurements. Preliminary results suggest that the NLO property is largely defined by the cluster core skeleton and the directly coordinated atoms, with limited contribution from the heavy atoms peripherally attached to the aromatic ligands.

Rectangle and [2]catenane from cluster modular construction.

Reaction of [Et4N][Tp*WS3] (1) with [Cu(MeCN)4]PF6, CsCl, isonicotinic acid and CuCN, and treatment of [Et4N][Tp*WS3(CuCl)3] (2)/[Et4N][{Tp*WS3Cu3Cl}2(μ-Cl)2(μ4-Cl)] (3) with AgOTf and bpp (Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate; bpp = 1,3-di(4-pyridyl)propane) give rise to [Et4N]2[{Tp*WS3Cu3(CN)0.5}2(μ-Cl)2(μ4-Cl)]2(PF6)2 (4) and [(Tp*WS3Cu3)2(μ3-Cl)2(bpp)3]2(OTf)4 (5), respectively. Compounds 4 and 5 feature cluster-based rectangle and [2]catenane architecture, and both exhibit enhanced third-order nonlinear optical responses relative to those of 1.

The first heterocubane cluster with a [W3GaS4] core

The paramagnetic cuboidal clusters [Mo3(GaCl)(μ3-S)4(dppe)3Cl3] and [W3(GaBr)0.81(μ3-S)4(dppe)3Br3] were synthesized by treatment of the corresponding triangular clusters [M3S4(dppe)3X3]X with [Ga(η5-C5Me5)]6.

Tetrahedral and Cuboidal Clusters in Complexes of Uranyl and Alkali or Alkaline-Earth Metal Ions with rac- and (1R,2R)-trans-1,2-Cyclohexanedicarboxylate

Uranyl nitrate was treated with racemic or enantiopure (1R,2R) forms of trans-1,2-cyclohexanedicarboxylic acid (H2chdc and R-H2chdc, respectively) in the presence of additional cations, mostly alkali or alkaline-earth metal cations, under solvohydrothermal conditions to generate a series of one homo- and seven heterometallic complexes which all contain the pseudotetrahedral [(UO2)4((R-)chdc)6]4– cluster previously found in sodium(I)-, silver(I)-, and lead(II)-containing derivatives. These clusters are for the first time obtained as isolated species in [NH4]4[(UO2)4(chdc)6] (1), in which the ammonium cations are held by hydrogen bonds close to the faces of the tetrahedron. In both compounds [(UO2)4K4(R-chdc)6(H2O)6] (2) and [(UO2)4Ba2(R-chdc)6(H2O)8] (8), the uranyl tetrahedra are assembled into three-dimensional frameworks by bridging potassium(I) or barium(II) cations, these being bound to carboxylate groups from different clusters. A closer association of uranyl tetrahedra and countercations is found wi...

Synthesis, molecular structures and EPR spectra of the paramagnetic cuboidal clusters with Mo3S4Ga cores

The paramagnetic cuboidal clusters [Mo3(GaBr)(μ3-S)4(diphos)3Br3] (diphos = dppe, dmpe) were synthesized by the reduction of triangular clusters [Mo3S4(diphos)3Br3]Br with elemental Ga.

Kinetics Aspects of the Reversible Assembly of Copper in Heterometallic Mo3CuS4 Clusters with 4,4'-Di-tert-butyl-2,2'-bipyridine.

Treatment of the triangular [Mo3S4Cl3(dbbpy)3]Cl cluster ([1]Cl) with CuCl produces a novel tetrametallic cuboidal cluster [Mo3(CuCl)S4Cl3(dbbpy)3][CuCl2] ([2][CuCl2]), whose crystal structure was determined by X-ray diffraction (dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine). This species, which contains two distinct types of Cu(I), is the first example of a diimine-functionalized heterometallic M3M'S4 cluster. Kinetics studies on both the formation of the cubane from the parent trinuclear cluster and its dissociation after treatment with halides, supported by NMR, electrospray ionization mass spectrometry, cyclic voltammetry, and density functional theory calculations, are provided. On the one hand, the results indicate that addition of Cu(I) to [1]+ is so fast that its kinetics can be monitored only by cryo-stopped flow at -85 °C. On the other hand, the release of the CuCl unit in [2]+ is also a fast process, which is unexpectedly assisted by the CuCl2- counteranion in a process triggered by halide (X-) anions. The whole set of results provide a detailed picture of the assembly-disassembly processes in this kind of cluster. Interconversion between trinuclear M3S4 clusters and their heterometallic M3M'S4 derivatives can be a fast process occurring readily under the conditions employed during reactivity and catalytic studies, so their occurrence is a possibility that must be taken into account in future studies.

CT interactions of the cuboidal cluster Fe4S4. Photoredox decomposition of (C5H5)4Fe4S4

The cluster complex (C5H5)4Fe4S4 formally contains Fe(III) and sulfide as constituents. In agreement with this assignment, the long-wavelength absorptions of the cluster are attributed to S2− to Fe(III) LMCT transitions. In solutions of acetonitrile, LMCT excitation of the cluster complex leads to a photoredox decomposition yielding Fe(II) and elemental sulfur. Upon a one-electron oxidation of (C5H5)4Fe4S4, the cation [(C5H5)4Fe4S4]+ is obtained, which is much more stable to water, air and light than the neutral parent cluster. Upon addition of [M(CN)6]4− with MRu and Os to aqueous solutions of this cation, dark blue and violet colors, respectively, immediately develop, which are attributed to outer-sphere CT absorptions of the ion pairs [(C5H5)4Fe4S4]+/[M(CN)6]4−.

Photosynthetic water oxidation: insights from manganese model chemistry.

Catalysts for light-driven water oxidation are a critical component for development of solar fuels technology. The multielectron redox chemistry required for this process has been successfully deployed on a global scale in natural photosynthesis by green plants and cyanobacteria using photosystem II (PSII). PSII employs a conserved, cuboidal Mn4CaOX cluster called the O2-evolving complex (OEC) that offers inspiration for artificial O2-evolution catalysts. In this Account, we describe our work on manganese model chemistry relevant to PSII, particularly the functional model [Mn(III/IV)2(terpy)2(μ-O)2(OH2)2](NO3)3 complex (terpy = 2,2';6',2″-terpyridine), a mixed-valent di-μ-oxo Mn dimer with two terminal aqua ligands. In the presence of oxo-donor oxidants such as HSO5(-), this complex evolves O2 by two pathways, one of which incorporates solvent water in an O-O bond-forming reaction. Deactivation pathways of this catalyst include comproportionation to form an inactive Mn(IV)Mn(IV) dimer and also degradation to MnO2, a consequence of ligand loss when the oxidation state of the complex is reduced to labile Mn(II) upon release of O2. The catalyst's versatility has been shown by its continued catalytic activity after direct binding to the semiconductor titanium dioxide. In addition, after binding to the surface of TiO2 via a chromophoric linker, the catalyst can be oxidized by a photoinduced electron-transfer mechanism, mimicking the natural PSII process. Model oxomanganese complexes have also aided in interpreting biophysical and computational studies on PSII. In particular, the μ-oxo exchange rates of the Mn-terpy dimer have been instrumental in establishing that the time scale for μ-oxo exchange of high-valent oxomanganese complexes with terminal water ligands is slower than O2 evolution in the natural photosynthetic system. Furthermore, computational studies on the Mn-terpy dimer and the OEC point to similar Mn(IV)-oxyl intermediates in the O-O bond-forming mechanism. Comparison between the OEC and the Mn-terpy dimer indicates that challenges remain in the development of synthetic Mn water-oxidation catalysts. These include redox leveling to couple multielectron reactions with one-electron steps, avoiding labile Mn(II) species during the catalytic cycle, and protecting the catalyst active site from undesired side reactions. As the first example of a functional manganese O2-evolution catalyst, the Mn-terpy dimer exemplifies the interrelatedness of biomimetic chemistry with biophysical studies. The design of functional model complexes enriches the study of the natural photosynthetic system, while biology continues to provide inspiration for artificial photosynthetic technologies to meet global energy demand.