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

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.

Base-Free Catalytic Hydrogen Production from Formic Acid Mediated by a Cubane-Type Mo3S4 Cluster Hydride

Formic acid (FA) dehydrogenation is an attractive process in the implementation of a hydrogen economy. To make this process greener and less costly, the interest nowadays is moving toward non-noble metal catalysts and additive-free protocols. Efficient protocols using earth abundant first row transition metals, mostly iron, have been developed, but other metals, such as molybdenum, remain practically unexplored. Herein, we present the transformation of FA to form H₂ and CO₂ through a cluster catalysis mechanism mediated by a cuboidal [Mo₃S₄H₃(dmpe)₃]⁺ hydride cluster in the absence of base or any other additive. Our catalyst has proved to be more active and selective than the other molybdenum compounds reported to date for this purpose. Kinetic studies, reaction monitoring, and isolation of the [Mo₃S₄(OCHO)₃(dmpe)₃]⁺ formate reaction intermediate, in combination with DFT calculations, have allowed us to formulate an unambiguous mechanism of FA dehydrogenation. Kinetic studies indicate that the reaction at temperatures up to 60 °C ends at the triformate complex and occurs in a single kinetic step, which can be interpreted in terms of statistical kinetics at the three metal centers. The process starts with the formation of a dihydrogen-bonded species with Mo-H···HOOCH bonds, detected by NMR techniques, followed by hydrogen release and formate coordination. Whereas this process is favored at temperatures up to 60 °C, the subsequent β-hydride elimination that allows for the CO₂ release and closes the catalytic cycle is only completed at higher temperatures. The cycle also operates starting from the [Mo₃S₄(OCHO)₃(dmpe)₃]⁺ formate intermediate, again with preservation of the cluster integrity, which adds our proposal to the list of the infrequent cluster catalysis reaction mechanisms.

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.