[MoV12O30(mu 2-OH)10H2(NiII(H2O)3)4], a highly symmetrical epsilon-Keggin unit capped with four NiII centers: synthesis and magnetism

Organically Functionalized Mixed-Valent Polyoxo-30-molybdate Wheel and Neutral Tetramolybdenum(V) Oxo Cluster

The first organofunctionalized mixed-valent polyoxo-30-molybdate wheel, [Mo^VI₁₈Mo^V₁₂O₈₄{AsO₂(CH₃)₂}₁₈]^18- (Mo₃₀), was synthesized in aqueous, mildly acidic conditions, and upon further acidification, the neutral tetramolybdenum(V) oxo cluster [Mo^V₄O₈{AsO₂(CH₃)₂}₄] (Mo₄) was obtained. Single-crystal X-ray diffraction (XRD) revealed that Mo₃₀ comprises 18 Mo^VI and 12 Mo^V ions arranged in a cyclic fashion with alternating {Mo^V₂} and {Mo^VI₃} groups, which are capped by 18 dimethylarsinate ligands, resulting in a novel polyoxo-30-molybdate wheel with a central cavity of ∼1.5 nm. On the other hand, Mo₄ has a distorted-cubic structure, with the corners of the cube being occupied by alternating Mo^V ions and oxo ligands. The compounds were characterized in solution by ¹H and ¹³C NMR and UV-vis spectroscopy and in the solid state by X-ray photoelectron spectroscopy and powder XRD. Mo₃₀ represents a novel type of polyanionic cycle with manyfold possibilities regarding host-guest chemistry.

Assembly of ϵ-Keggin Polyoxometalate from Molecular Crystal to Zeolitic Octahedral Metal Oxide

Zeolitic octahedral metal oxides are inorganic crystalline microporous materials with adsorption and redox properties. New ϵ-Keggin nickel molybdate-based zeolitic octahedral metal oxides have been synthesized. ³¹ P NMR spectroscopy shows that reduction of Mo^VI -based molybdates forms an ϵ-Keggin polyoxometalate that immediately transfers to the solid phase. Investigation of the formation process indicates that a low Ni concentration, insoluble reducing agent, and long synthesis time are the critical factors for obtaining the zeolite octahedral metal oxides rather than the ϵ-Keggin polyoxometalate molecule. The synthesized zeolitic nickel molybdate with Na⁺ is used as the adsorbent, which effectively separates C₂ hydrocarbon mixtures.

Engineering Highly Reduced Molybdenum Polyoxometalates via the Incorporation of d and f Block Metal Ions

The assembly of nanoscale polyoxometalate (POM) clusters has been dominated by the highly reduced icosahedral {Mo₁₃₂} “browns” and the toroidal {Mo₁₅₄} “blues” which are 45 % and 18 % reduced, respectively. We hypothesised that there is space for a greater diversity of structures in this immediate reduction zone. Here we show it is possible to make highly reduced mix‐valence POMs by presenting new classes of polyoxomolybdates: [Mo^V₅₂Mo^VI₁₂H₂₆O₂₀₀]⁴²⁻ {Mo₆₄} and [Mo^V₄₀Mo^VI₃₀H₃₀O₂₁₅]²⁰⁻ {Mo₇₀}, 81 % and 57 % reduced, respectively. The {Mo₆₄} cluster archetype has a super‐cube structure and is composed of five different types of building blocks, each arranged in overlayed Archimedean or Platonic polyhedra. The {Mo₇₀} cluster comprises five tripodal {Mo^V₆} and five tetrahedral {Mo^V₂Mo^VI₂} building blocks alternatively linked to form a loop with a pentagonal star topology. We also show how the reaction yielding the {Mo₆₄} super‐cube can be used in the enrichment of lanthanides which exploit the differences in selectivity in the self‐assembly of the polyoxometalates.

Constructing superior Co-Mo HDS catalyst from a crystalline precursor separated from the impregnating solution

In this paper, a Co tetra-capped Keggin crystalline Co4Mo12 was separated from the impregnating solution through the self-assembly strategy. The crystalline Co4Mo12 was structurally characterized and used as a molecular...

Molecular magnetism in the multi-configurational self-consistent field method

We develop a structured theoretical framework used in our recent articles (2019 Eur. Phys. J. B 92 93 and 2020 Phys. Rev. B 101 094427) to characterize the unusual behavior of the magnetic spectrum, magnetization and magnetic susceptibility of the molecular magnet Ni₄Mo₁₂. The theoretical background is based on the molecular orbital theory in conjunction with the multi-configurational self-consistent field method and results in a post-Hartree-Fock scheme for constructing the corresponding energy spectrum. Furthermore, we construct a bilinear spin-like Hamiltonian involving discrete coupling parameters accounting for the relevant spectroscopic magnetic excitations, magnetization and magnetic susceptibility. The explicit expressions of the eigenenergies of the ensuing Hamiltonian are determined and the physical origin of broadening and splitting of experimentally observed peaks in the magnetic spectra is discussed. To demonstrate the efficiency of our method we compute the spectral properties of a spin-one magnetic dimer. The present approach may be applied to a variety of magnetic units based on transition metals and rare Earth elements.

Insights into Nature of Magnetization Plateaus of a Nickel Complex [Ni4(μ-CO3)2(aetpy)8](ClO4)4 from a Spin-1 Heisenberg Diamond Cluster

Magnetic and magnetocaloric properties of a spin-1 Heisenberg diamond cluster with two different coupling constants are investigated with the help of an exact diagonalization based on the Kambe’s method, which employs a local conservation of composite spins formed by spin-1 entities located in opposite corners of a diamond spin cluster. It is shown that the spin-1 Heisenberg diamond cluster exhibits several intriguing quantum ground states, which are manifested in low-temperature magnetization curves as intermediate plateaus at 1/4, 1/2, and 3/4 of the saturation magnetization. In addition, the spin-1 Heisenberg diamond cluster may also exhibit an enhanced magnetocaloric effect, which may be relevant for a low-temperature refrigeration achieved through the adiabatic demagnetization. It is evidenced that the spin-1 Heisenberg diamond cluster with the antiferromagnetic coupling constants J1/kB = 41.4 K and J2/kB = 9.2 K satisfactorily reproduces a low-temperature magnetization curve recorded for the tetranuclear nickel complex [Ni4(μ-CO3)2(aetpy)8](ClO4)4 (aetpy = 2-aminoethyl-pyridine) including a size and position of intermediate plateaus detected at 1/2 and 3/4 of the saturation magnetization. A microscopic nature of fractional magnetization plateaus observed experimentally is clarified and interpreted in terms of valence-bond crystal with either a single or double valence bond. It is suggested that this frustrated magnetic molecule can provide a prospective cryogenic coolant with the maximal isothermal entropy change −ΔSM=10.6 J·K−1·kg−1 in a temperature range below 2.3 K.

Modeling the Oxygen Vacancy at a Molecular Vanadium(III) Silica-Supported Catalyst

Here we report on the use of a silanol-decorated polyoxotungstate, [SbW₉O₃₃( ^tBuSiOH)₃]^3- (1), as a molecular support to describe the coordination of a vanadium atom at a single-site on silica surfaces. By reacting [V(Mes)₃·thf] (Mes = 2,4,6-trimethylphenyl) with 1 in tetrahydrofuran, the vanadium(III) derivative [SbW₉O₃₃( ^tBuSiO)₃V(thf)]^3- (2) was obtained. Compound 2 displays the paramagnetic behavior expected for a d²-V^III high spin complex (SQUID measurements) with a triplet electronic ground state (ca. 30 kcal·mol^-1 more stable than the singlet, from DFT calculations). Compound 2 proves to be a reliable model for reduced isolated-vanadium atom dispersed on silica surfaces [(≡Si-O)₃V^III(OH₂)], an intermediate that is often proposed in a Mars-van Krevelen type mechanism for partial oxidation of light alcohols. Oxidation of 2 under air produced the oxo-derivative [SbW₉O₃₃( ^tBuSiO)₃VO]^3- (3). In compound 2, the d²-electrons are localized in degenerated d(V) orbitals, whereas in the electronically analogous bireduced-[SbW₉O₃₃( tBuSiO)₃VO]^5-, 3·(2e), one electron is localized on d(V) orbital and the second one is delocalized on the polyoxotungstic framework, leading to a unique case of a bireduced heteropolyanion derivative with completely decoupled d¹-V(IV) and d¹-W(V). Our body of experimental results (EPR, magnetic measurements, spectroelectrochemical studies, Raman spectroscopy) and theoretical studies highlights (i) the role of the apical ligand coordination, i.e., thf (σ-donor) vs oxo (π-donor), in destabilizing or stabilizing the d(V) orbitals relative to the d(W) orbitals, and (ii) a geometrical distortion of the O₃VO entity that causes a splitting of the degenerated orbitals and the stabilization of one d(V) orbital in the bireduced compound 3·(2e).

One-step chemical vapor deposition synthesis and supercapacitor performance of nitrogen-doped porous carbon-carbon nanotube hybrids

Novel nitrogen-doped carbon hybrid materials consisting of multiwalled nanotubes and porous graphitic layers have been produced by chemical vapor deposition over magnesium-oxide-supported metal catalysts. CN _x nanotubes were grown on Co/Mo, Ni/Mo, or Fe/Mo alloy nanoparticles, and MgO grains served as a template for the porous carbon. The simultaneous formation of morphologically different carbon structures was due to the slow activation of catalysts for the nanotube growth in a carbon-containing gas environment. An analysis of the obtained products by means of transmission electron microscopy, thermogravimetry and X-ray photoelectron spectroscopy methods revealed that the catalyst's composition influences the nanotube/porous carbon ratio and concentration of incorporated nitrogen. The hybrid materials were tested as electrodes in a 1M H₂SO₄ electrolyte and the best performance was found for a nitrogen-enriched material produced using the Fe/Mo catalyst. From the electrochemical impedance spectroscopy data, it was concluded that the nitrogen doping reduces the resistance at the carbon surface/electrolyte interface and the nanotubes permeating the porous carbon provide fast charge transport in the cell.

Gas-Phase Fragmentation Pathways of Mixed Addenda Keggin Anions: PMo12-nW nO 40 3- (n = 0-12)

We report a collision-induced dissociation (CID) investigation of the mixed addenda polyoxometalate (POM) anions, PMo(12-n)W(n)O(40)(3-) (n = 0-12). The anions were generated in solution using a straightforward single-step synthesis approach and introduced into the gas phase by electrospray ionization (ESI). Distinct differences in fragmentation patterns were observed for the range of mixed addenda POMs examined in this study. CID of molybdenum-rich anions, PMo(12-n)W(n)O(40)(3-) (n = 0-2), generates an abundant doubly charged fragment containing seven metal atoms (M) and 22 oxygen atoms (M(7)O(22)(2-)) and its complementary singly charged PM(5)O(18)(-) ion. In comparison, the doubly charged Lindqvist anion, (M(6)O(19)(2-)) and its complementary singly charged PM(6)O(21)(-) ion are the dominant fragments of Keggin POMs containing more than two tungsten atoms, PMo(12-n)W(n)O(40)(3-) (n = 3-12). The observed transition in the dissociation pathways with an increase in the number of W atoms in the POM may be attributed to the higher barrier of tungsten-rich anions towards isomerization. We present evidence that the observed distribution of Mo and W atoms in the major M(6)O(19)(2-) and M(7)O(22)(2-) fragment ions is different from that predicted by a random distribution, indicating substantial segregation of the addenda metal atoms in the POMs. Charge reduction of the triply charged precursor anion resulting in formation of doubly charged anions is also observed. This is a dominant pathway for mixed POMs having a majority (8-11) of W atoms and a minor channel for other precursors indicating a close competition between fragmentation and charge loss pathways in CID of POM anions.

Four hybrid compounds based on a new type of molybdates and a flexible tripodal ligand: synthesis, structures, photochemical and electrochemical properties

Four hybrids based on a new type of molybdates have been prepared. Their electrochemical and photocatalytic activities were also investigated.

A novel POMos-based hybrid with penta-coordinated Mo in trigonal bipyramid: structure and an efficient precursor for hydrodesulfurization catalyst

The first POMos-based hybrid with penta-coordinated Mo in trigonal bipyramid for Mo–O clusters; an efficient precursor for hydrodesulfurization catalyst.

An α-Keggin polyoxometalate completely constructed from the late transition metal Co(II) as poly atom

We report a catalytic eight-cobalt-capped α-Keggin polyoxoazocobaltite with a highly symmetric structure completely constructed from the late transition-metal Co(II) as a poly atom, [Co(20)(OH)(24)(MMT)(12)(SO(4))](NO(3))(2)·6H(2)O (1) (MMT: 2-mercapto-5-methyl-1,3,4-thiadiazole). To the best of our knowledge, this is the first homometallic Co(II) magnetic cluster bearing Keggin structure. It can serve as a potential catalyst in the styrene polymerization.

[Co(II)4Mo(V)12O28(OH)12(H2O)12]·12H2O: facilitating single-crystal growth by deuteration

The structure of the neutral heterometal oxide cluster dodecaaqua-di-μ(3)-hydroxido-deca-μ(2)-hydroxido-octacosaoxidotetracobalt(II)dodecamolybdenum(V) dodecahydrate, [Mo(12)O(28)(μ(2)-OH)(10)(μ(3)-OH)(2){Co(H(2)O)(3)}(4)], is virtually identical to the previously reported Ni(II) analogue [Mo(12)O(28)(μ(2)-OH)(10)(μ(3)-OH)(2){Ni(II)(H(2)O)(3)}(4)] [Müller, Beugholt, Kögerler, Bögge, Budko & Luban (2000). Inorg. Chem. 39, 5176-5177], the first molecular magnet to exhibit signs of magnetostriction. The formation kinetics of the neutral cluster species, which is insoluble in water, can be significantly slowed by the use of deuterated reactants in order to grow single crystals of sufficient size for single-crystal X-ray diffraction studies using standard diffractometers. One half of the main cluster and six solvent water molecules constitute the asymmetric unit. The main cluster is located on a mirror plane.

Novel Doped Tungstovanadate and Molybdovanadate with Bi-antimony-Capped Keggin Structure

Three novel V-centred Keggin-based compounds containing cap-shaped Sb atoms: (HDMF)2[(VO4)W5.65V7.74Sb2.61O36.77]·6H2O (1), [Co(phen)3]2[(VO4)Mo9.58V2.42Sb2O36]·6H2O (2) and [Ni(phen)2(H2O)]2[(VO4)Mo8V4Sb2O36]·2H2O (3) (DMF = dimethylformamide; phen = 1,10-phenanthroline) have been synthesized and characterized. X-ray diffraction analysis revealed that compound 1 consists of an unusually one-dimensional 12-tungstovanadate Keggin-based polymer, in which the reduced tungstovanadate Keggin species are stabilized by introducing capping {VO} and {Sb} groups and then linked into chains via V(Sb)–O–V(Sb) interactions. Both 2 and 3 are built from secondary metal–ligand groups (Co-phen for 2, Ni-phen for 3) and 12-molybdovanadate doped Keggin-type cores with two antimony atoms capping two opposite sites. The anion in 3 represents a bi-capped and bi-supported Keggin-type structure. The electrochemical properties of 2- and 3-modified carbon paste electrodes (2-CPE and 3-CPE) are also studied.

A remarkable member of the polyoxometalates: the eight-nickel-capped alpha-keggin polyoxoazonickelate

The eight-nickel-capped polyoxoazonickelate, [Ni(20)(OH)(24)(MMT)(12)(SO(4))](NO(3))(2).6H(2)O (1; MMT = 2-mercapto-5-methyl-1,3,4-thiadiazole), has been synthesized, which has an alpha-Keggin structure with eight nickel caps. In this structure, the polyatom is the late transition metal Ni(II); the central heteroatom is S, and the organic terminal ligand becomes the primary part of the Keggin structure. This is a Keplerate-type cluster, which shows a central Ni(II)(12) cuboctahedron formed by the 12 Ni(II) centers of the classical alpha-Keggin core and a Ni(II)(8) hexahedron formed by the eight nickel caps.