Spin-Crossing in the (<i>Z</i>)‑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

Cuboidal Mo₃S₄ Clusters as a Platform for Exploring Catalysis: A Three-Center Sulfur Mechanism for Alkyne Semihydrogenation

We report a trinuclear Mo₃S₄ diamino cluster that promotes the semihydrogenation of alkynes. Based on experimental and computational results, we propose an unprecedented mechanism in which only the three bridging sulfurs of the cluster act as the active site for this transformation. In the first step, two of these μ-S ligands react with the alkyne to form a dithiolene adduct; this process is formally analogous to the olefin adsorption on MoS₂ surfaces. Then, H₂ activation occurs in an unprecedented way that involves the third μ-S center, in cooperation with one of the dithiolene carbon atoms. Notably, this step does not imply any direct interaction between H₂ and the metal centers, and directly results in the formation of an intermediate featuring one (μ-S)–H and one C–H bond. Finally, such half-hydrogenated intermediate can either undergo a reductive elimination step that results in the <i>Z</i>-alkene product, or evolve into an isomerized analogue whose subsequent reductive elimination generates the <i>E</i>-alkene product. Interestingly, the substituents on the alkynes have a major impact on the relative barriers of these two processes, with the semihydrogenation of dimethyl acetylenedicarboxylate (dmad) resulting in the stereoselective formation of dimethyl maleate, whereas that of diphenylacetylene (dpa) leads to mixtures of <i>Z</i>- and <i>E</i>-stilbene. The results herein could have significant implications on the understanding of the catalytic properties of MoS₂-based materials

Cuboidal Mo₃S₄ Clusters as a Platform for Exploring Catalysis: A Three-Center Sulfur Mechanism for Alkyne Semihydrogenation

We report a trinuclear Mo₃S₄ diamino cluster that promotes the semihydrogenation of alkynes. Based on experimental and computational results, we propose an unprecedented mechanism in which only the three bridging sulfurs of the cluster act as the active site for this transformation. In the first step, two of these μ-S ligands react with the alkyne to form a dithiolene adduct; this process is formally analogous to the olefin adsorption on MoS₂ surfaces. Then, H₂ activation occurs in an unprecedented way that involves the third μ-S center, in cooperation with one of the dithiolene carbon atoms. Notably, this step does not imply any direct interaction between H₂ and the metal centers, and directly results in the formation of an intermediate featuring one (μ-S)–H and one C–H bond. Finally, such half-hydrogenated intermediate can either undergo a reductive elimination step that results in the <i>Z</i>-alkene product, or evolve into an isomerized analogue whose subsequent reductive elimination generates the <i>E</i>-alkene product. Interestingly, the substituents on the alkynes have a major impact on the relative barriers of these two processes, with the semihydrogenation of dimethyl acetylenedicarboxylate (dmad) resulting in the stereoselective formation of dimethyl maleate, whereas that of diphenylacetylene (dpa) leads to mixtures of <i>Z</i>- and <i>E</i>-stilbene. The results herein could have significant implications on the understanding of the catalytic properties of MoS₂-based materials

Kinetics Aspects of the Reversible Assembly of Copper in Heterometallic Mo₃CuS₄ Clusters with 4,4′-Di-<i>tert</i>-butyl-2,2′-bipyridine

Treatment of the triangular [Mo₃S₄Cl₃(dbbpy)₃]Cl cluster ([<b>1</b>]­Cl) with CuCl produces a novel tetrametallic cuboidal cluster [Mo₃(CuCl)­S₄Cl₃(dbbpy)₃]­[CuCl₂] ([<b>2</b>]­[CuCl₂]), whose crystal structure was determined by X-ray diffraction (dbbpy = 4,4′-di-<i>tert</i>-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 [<b>1</b>]⁺ 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 [<b>2</b>]⁺ 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

Kinetics Aspects of the Reversible Assembly of Copper in Heterometallic Mo₃CuS₄ Clusters with 4,4′-Di-<i>tert</i>-butyl-2,2′-bipyridine

Treatment of the triangular [Mo₃S₄Cl₃(dbbpy)₃]Cl cluster ([<b>1</b>]­Cl) with CuCl produces a novel tetrametallic cuboidal cluster [Mo₃(CuCl)­S₄Cl₃(dbbpy)₃]­[CuCl₂] ([<b>2</b>]­[CuCl₂]), whose crystal structure was determined by X-ray diffraction (dbbpy = 4,4′-di-<i>tert</i>-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 [<b>1</b>]⁺ 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 [<b>2</b>]⁺ 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