Sulfur-Centered Mechanism in Catalytic Methanolysis of Hydrosilanes Mediated by Air-Stable Mo3S4 Clusters

Methanolysis of hydrosilanes is catalyzed by incomplete cubane-type Mo3(μ3-S)(μ-S)3 clusters functionalized with diamino and imidazolyl amino ligands under mild conditions. Silane activation mediated by the air-stable [Mo3(μ3-S)(μ-S)3Cl3(ImNH2)3]Cl (ImNH2 = (1-methyl-1H-imidazol-2-yl)methanamine) ([3]Cl) cluster salt has been elucidated through a comprehensive experimental and theoretical study. Our results support the operation of a sulfur-centered mechanism without direct participation of the metals in clear contrast with all previously reported mechanisms catalyzed by transition metal complexes. The reaction proceeds in two steps, with the first one being the rate-determining step. The process starts with the hydrosilane Si-H bond activation, which occurs at one of the bridging sulfur atoms of the Mo3(μ3-S)(μ-S)3 cluster unit. This step takes place through a concerted and asynchronous transition state with the participation of one methanol molecule to yield the silyl ether product and a bis(hydrosulfido) intermediate. Analysis of this transition state reveals that its imaginary frequency is basically associated with the silane hydride transfer and the formation of the Si-O bond in agreement with the observed KIE results. The second step consists in the hydrogen release from the bis(hydrosulfido) intermediate, from which the cluster catalyst is recovered. The same mechanism operates for the diamino [Mo3S4Cl3(en)3]Cl (en = ethylenediamine) ([1]Cl) and [Mo3S4Cl3(dmen)3]+ (dmen = N,N'-dimethylethylenediamine) ([2]Cl) cluster salts. The calculated free energy barriers for those cluster catalysts agree with the observed catalytic activities, giving further support to our mechanistic proposal.

Unprecedented Mo3S4 cluster-catalyzed radical C-C cross-coupling reactions of aryl alkynes and acrylates

A new method for the generation of benzyl radicals from terminal aromatic alkynes has been developed, which allows the direct cross coupling with acrylate derivatives. Our additive-free protocol employs air-stable diamino Mo3S4 cubane-type cluster catalysts in the presence of hydrogen. A sulfur-centered cluster catalysis mechanism for benzyl radical formation is proposed based on catalytic and stoichiometric experiments. The process starts with the cluster hydrogen activation to form a bis(hydrosulfido) [Mo3(μ3-S)(μ-S)(μ-SH)2Cl3(dmen)3]+ intermediate. The reaction of various aromatic terminal alkynes containing different functionalities with a series of acrylates affords the corresponding Giese-type radical addition products.

Spin-Crossing in the (Z)-Selective Alkyne Semihydrogenation Mechanism Catalyzed by Mo3S4 Clusters: A Density Functional Theory Exploration

Semihydrogenation of internal alkynes catalyzed by the air-stable imidazolyl amino [Mo3S4Cl3(ImNH2)3]+ cluster selectively affords the (Z)-alkene under soft conditions in excellent yields. Experimental results suggest a sulfur-based mechanism with the formation of a dithiolene adduct through interaction of the alkyne with the bridging sulfur atoms. However, computational studies indicate that this mechanism is unable to explain the experimental outcome: mild reaction conditions, excellent selectivity toward the (Z)-isomer, and complete deuteration of the vinylic positions in the presence of CD3OD and CH3OD. An alternative mechanism that explains the experimental results is proposed. The reaction begins with the hydrogenation of two of the Mo3(μ3-S)(μ-S)3 bridging sulfurs to yield a bis(hydrosulfide) intermediate that performs two sequential hydrogen atom transfers (HAT) from the S-H groups to the alkyne. The first HAT occurs with a spin change from singlet to triplet. After the second HAT, the singlet state is recovered. Although the dithiolene adduct is more stable than the hydrosulfide species, the large energy required for the subsequent H2 addition makes the system evolve via the second alternative pathway to selectively render the (Z)-alkene with a lower overall activation barrier.

Molecularly Engineering Defective Basal Planes in Molybdenum Sulfide for the Direct Synthesis of Benzimidazoles by Reductive Coupling of Dinitroarenes with Aldehydes

Developing more sustainable catalytic processes for preparing N-heterocyclic compounds in a less costly, compact, and greener manner from cheap and readily available reagents is highly desirable in modern synthetic chemistry. Herein, we report a straightforward synthesis of benzimidazoles by reductive coupling of o-dinitroarenes with aldehydes in the presence of molecular hydrogen. An innovative molecular cluster-based synthetic strategy that employs Mo₃S₄ complexes as precursors have been used to engineer a sulfur-deficient molybdenum disulfide (MoS₂)-type material displaying structural defects on both the naturally occurring edge positions and along the typically inactive basal planes. By applying this catalyst, a broad range of functionalized 2-substituted benzimidazoles, including bioactive compounds, can be selectively synthesized by such a direct hydrogenative coupling protocol even in the presence of hydrogenation-sensitive functional groups, such as double and triple carbon-carbon bonds, nitrile and ester groups, and halogens as well as diverse types of heteroarenes.

CO-free, aqueous mediated, instant and selective reduction of nitrobenzene via robustly stable chalcogen stabilised iron carbonyl clusters (Fe3E2(CO)9, E = S, Se, Te)

Highly stable and thermally robust iron chalcogenide carbonyl clusters Fe₃E₂(CO)₉ (E = S, Se or Te) have been explored for the reduction of nitrobenzene. A 15 min thermal heating of an aqueous solution of nitrobenzene and hydrazine hydrate in the catalytic presence of Fe₃E₂(CO)₉ (E = S, Se or Te) clusters yield average to excellent aniline transformations. Among the S, Se and Te based iron chalcogenised carbonyl clusters, the diselenide cluster was found to be most efficient and produce almost 90% yield of the desired amino product, the disulfide cluster was also found to be significantly active, produce the 85% yield of amino product, while the ditelluride cluster was not found to be active and produced only 49% yield of the desired product. The catalyst can be reused up to three catalytic cycles and it needs to be dried in an oven for one hour prior to reuse for the best results. The developed method is inexpensive, environmentally benign, does not require any precious metal or a high pressure of toxic CO gas and exclusively brings the selective reduction of the nitro group under feasible and inert free conditions.

In this study, a strongly feasible method for the reduction of nitrobenzene has been developed through highly stable and thermally robust iron chalcogenide carbonyl clusters Fe₃E₂(CO)₉ (E = S, Se or Te).

Biomass-derived carbon-supported Ni catalyst: an effective heterogeneous non-noble metal catalyst for the hydrogenation of nitro compounds

The biomass-derived carbon material supported Ni catalysts (Ni/C) demonstrated a high catalytic activity for the hydrogenation of nitro compounds into primary amines at room temperature.

Interaction of Hydrophobic Tungsten Cluster Complexes with a Phospholipid Bilayer

Nanoconstructions composed of lipid vesicles and inorganic units (nanoparticles, metal complexes) arouse much interest across materials science and nanotechnology as hybrid materials combining useful functionalities from both parts. Ideally, these units are to be embedded into the bilayer to keep the biophysical performance of lipid vesicles having inorganic moieties screened from the environment. This can be achieved by doping a lipid bilayer with cluster complexes of transition metals. In this work, we report the preparation of nanoparticles from trinuclear W₃S₄ cluster complexes and egg phosphatidylcholine. A systematic study of their properties was performed by the differential scanning calorimetry, NMR spectroscopy, dynamic light scattering, and transmission electron microscopy. Phospholipids and clusters have been found to spontaneously self-assemble into novel cluster-lipid hybrid materials. The behavior of clusters in the hydrophobic lipid environment is determined by the structure of the ligands and cluster-to-lipid ratio. Intact cluster complexes bearing compact hydrophobic ligands are embedded into the hydrophobic midplane of a lipid bilayer, whereas cluster complexes bearing larger ligands drive the aggregation of lipids and cluster complexes. Considering these differences, it could be possible to obtain different self-assembled associates such as cluster-doped liposomes or lipid-covered crystals. These cluster-lipid hybrids can be a platform for the design of new materials for nanotechnology.

On the catalytic transfer hydrogenation of nitroarenes by a cubane-type Mo3S4 cluster hydride: disentangling the nature of the reaction mechanism

Cubane-type Mo₃S₄ cluster hydrides decorated with phosphine ligands are active catalysts for the transfer hydrogenation of nitroarenes to aniline derivatives in the presence of formic acid (HCOOH) and triethylamine (Et₃N). The process is highly selective and most of the cluster species involved in the catalytic cycle have been identified through reaction monitoring. Formation of a dihydrogen cluster intermediate has also been postulated based on previous kinetic and theoretical studies. However, the different steps involved in the transfer hydrogenation from the cluster to the nitroarene to finally produce aniline remain unclear. Herein, we report an in-depth computational investigation into this mechanism. Et₃N reduces the activation barrier associated with the formation of Mo-HHOOCH dihydrogen species. The global catalytic process is highly exergonic and occurs in three consecutive steps with nitrosobenzene and N-phenylhydroxylamine as reaction intermediates. Our computational findings explain how hydrogen is transferred from these Mo-HHOOCH dihydrogen adducts to nitrobenzene with the concomitant formation of nitrosobenzene and the formate substituted cluster. Then, a β-hydride elimination reaction accompanied by CO₂ release regenerates the cluster hydride. Two additional steps are needed for hydrogen transfer from the dihydrogen cluster to nitrosobenzene and N-phenylhydroxylamine to finally produce aniline. Our results show that the three metal centres in the Mo₃S₄ unit act independently, so the cluster can exist in up to ten different forms that are capable of opening a wide range of reaction paths. This behaviour reveals the outstanding catalytic possibilities of this kind of cluster complexes, which work as highly efficient catalytic machines.

Hydrogenation of nitroarenes to anilines in a flow reactor using polystyrene supported rhodium in a catalyst-cartridge (Cart-Rh@PS)

The present methodology described the chemo-selective hydrogenation of various nitroarenes in a flow reactor under polystyrene supported rhodium in a catalyst-cartridge (Cart-Rh@PS) as a heterogeneous nano-catalyst.

Co-Ag alloy protected by nitrogen doped carbon as highly efficient and chemoselective catalysts for the hydrogenation of halogenated nitrobenzenes

The design of lower-cost alternative heterogeneous catalysts for the hydrogenation of halogenated nitrobenzenes using green method to synthesize the corresponding anilines is highly desirable. In this study, Ag was incorporated into the Co-MOFs during the growing process (Co-Ag(n)-MOFs), and then followed the carbothermal reduction process without any additional procedures, we synthesized a series of Co-Ag(n)@NCs. The self-supported catalysts exhibited excellent and stable catalytic performances for the chemoselective hydrogenation of halogenated nitrobenzenes without obvious dehalogenation. The Co-Ag bimetallic alloy nanoparticles were well-dispersed and protected from aggregation and leaching by the porous nitrogen doped carbon. Besides, either hydrazine hydrate (N₂H₄·H₂O, generating byproducts N₂ and H₂O) or H₂ could be used as green reducing agent with excellent selectivity towards synthesizing the corresponding anilines. And when the Co/Ag content ratio was approximate 1:1, the Co-Ag(1:1)@NC showed the best catalytic performance. Moreover, the Co-Ag(1:1)@NC could be efficiently recovered by using an external magnetic force and reused without obvious decrease of catalytic activity. Thus, such highly efficient, inexpensive, stable and magnetically recyclable catalysts could show great potentials in practical applications for many important reactions.

Investigation of electronic structure of tri- and tetranuclear molybdenum clusters by X-ray photoelectron and emission spectroscopies and quantum chemical methods

Charge state studies of compounds [Mo₃S₄(tu)₈(H₂O)]Cl₄·4H₂O (1), [Mo₃S₄Cl₃(dbbpy)₃]Cl·5H₂O (2), [Mo₃S₄(CuCl)Cl₃(dbbpy)₃][CuCl₂] (3), containing {Mo₃S₄}⁴⁺ and {Mo₃CuS₄}⁵⁺ cluster cores bearing terminal thiourea (tu) or 4,4'-di-tert-butyl-2,2'-bipyridine (dbbpy) ligands, have been performed by X-ray photoelectron and X-ray emission spectroscopies combined with quantum chemical calculations. The best agreement between theory and experiments has been obtained using the B3LYP method. According to the experimental and calculated data, the Mo atoms are in the oxidation state 4+ for all compounds. The energies and shapes of the Cu2p lines indicate formal oxidation states of Cu as 1+. The coordination of Сu(I) to the cluster {Mo₃S₄} in 3 does not lead to significant changes in the charge state of the molybdenum atoms and the {Mo₃S₄} unit can be considered as a tridentate metallothia crown ether.

Hemilability of phosphine-thioether ligands coordinated to trinuclear Mo3S4 cluster and its effect on hydrogenation catalysis

Phosphine-thioether ligands were coordinated to the Mo₃S₄ cluster to afford [Mo₃S₄Cl₃(PS)₃]⁺ complexes. Their catalytic activity in nitrobenzene reduction reflects the different hemilabile behaviours of PS1, PS2 and PS3.

Tetrametallic Thorium Compounds with Th4E4 (E = S, Se) Cubane Cores

Tetrametallic thorium compounds with a Th₄E₄ core (E = S, Se) having a distorted cubane structure can be prepared by ligand-based reductions of elemental E with thorium chalcogenolates, prepared by in situ oxidation of Th metal with a 3:1 mixture of PhEEPh and F₅C₆EEC₆F₅. Four compounds, (py)₈Th₄S₄(μ₂-SPh)₄(SC₆F₅)₄, (py)₈Th₄S₄(μ₂-SPh)₄(SeC₆F₅)₄, (py)₈Th₄Se₄(μ₂-SePh)₄(SeC₆F₅)₄, and (py)₈Th₄Se₄(μ₂-SePh)₄(SC₆F₅)₄, were isolated and characterized by NMR spectroscopy and X-ray diffraction. These compounds clearly demonstrate the chemical impact of ring fluorination, with the less-nucleophilic EC₆F₅ ligands occupying the terminal binding sites and the EPh ligands bridging two metal centers. For this series of compounds, crystal packing and intermolecular π···π and H-bonding interactions result in a consistent motif and crystallization in a body-centered tetragonal unit cell. Solution-state ⁷⁷Se NMR spectroscopy reveals that the solid-state structures are maintained in pyridine.

Efficient and Selective N-Methylation of Nitroarenes under Mild Reaction Conditions

Herein, we report a straightforward protocol for the preparation of N,N-dimethylated amines from readily available nitro starting materials using formic acid as a renewable C₁ source and silanes as reducing agents. This tandem process is efficiently accomplished in the presence of a cubane-type Mo₃ PtS₄ catalyst. For the preparation of the novel [Mo₃ Pt(PPh₃ )S₄ Cl₃ (dmen)₃ ]⁺ (3⁺ ) (dmen: N,N'-dimethylethylenediamine) compound we have followed a [3+1] building block strategy starting from the trinuclear [Mo₃ S₄ Cl₃ (dmen)₃ ]⁺ (1⁺ ) and Pt(PPh₃ )₄ (2) complexes. The heterobimetallic 3⁺ cation preserves the main structural features of its 1⁺ cluster precursor. Interestingly, this catalytic protocol operates at room temperature with high chemoselectivity when the 3⁺ catalyst co-exists with its trinuclear 1⁺ precursor. N-heterocyclic arenes, double bonds, ketones, cyanides and ester functional groups are well retained after N-methylation of the corresponding functionalized nitroarenes. In addition, benzylic-type as well as aliphatic nitro compounds can also be methylated following this protocol.