Kinetic and DFT Studies on the Mechanism of C–S Bond Formation by Alkyne Addition to the [Mo₃S₄(H₂O)₉]⁴⁺ Cluster

Reaction of [Mo₃(μ₃-S)­(μ-S)₃] clusters with alkynes usually leads to formation of two C–S bonds between the alkyne and two of the bridging sulfides. The resulting compounds contain a bridging alkenedithiolate ligand, and the metal centers appear to play a passive role despite reactions at those sites being well illustrated for this kind of cluster. A detailed study including kinetic measurements and DFT calculations has been carried out to understand the mechanism of reaction of the [Mo₃(μ₃-S)­(μ-S)₃(H₂O)₉]⁴⁺ (<b>1</b>) cluster with two different alkynes, 2-butyne-1,4-diol and acetylenedicarboxylic acid. Stopped-flow experiments indicate that the reaction involves the appearance in a single kinetic step of a band at 855 or 875 nm, depending on the alkyne used, a position typical of clusters with two C–S bonds. The effects of the concentrations of the reagents, the acidity, and the reaction medium on the rate of reaction have been analyzed. DFT and TD-DFT calculations provide information on the nature of the product formed, its electronic spectrum and the energy profile for the reaction. The structure of the transition state indicates that the alkyne approaches the cluster in a lateral way and both C–S bonds are formed simultaneously

Kinetic Analysis and Mechanism of the Hydrolytic Degradation of Squaramides and Squaramic Acids

The hydrolytic degradation of squaramides and squaramic acids, the product of partial hydrolysis of squaramides, has been evaluated by UV spectroscopy at 37 °C in the pH range 3–10. Under these conditions, the compounds are kinetically stable over long time periods (>100 days). At pH >10, the hydrolysis of the squaramate anions shows first-order dependence on both squaramate and OH^–. At the same temperature and [OH^–], the hydrolysis of squaramides usually displays biphasic spectral changes (A → B → C kinetic model) with formation of squaramates as detectable reaction intermediates. The measured rates for the first step (<i>k</i>₁ ≈ 10^–4 M^–1 s^–1) are 2–3 orders of magnitude faster than those for the second step (<i>k</i>₂ ≈ 10^–6 M^–1 s^–1). Experiments at different temperatures provide activation parameters with values of Δ<i>H</i>^⧧ ≈ 9–18 kcal mol^–1 and Δ<i>S</i>^⧧ ≈ −5 to −30 cal K^–1 mol^–1. DFT calculations show that the mechanism for the alkaline hydrolysis of squaramic acids is quite similar to that of amides

Influence of the Ligand Alkyl Chain Length on the Solubility, Aqueous Speciation, and Kinetics of Substitution Reactions of Water-Soluble M₃S₄ (M = Mo, W) Clusters Bearing Hydroxyalkyl Diphosphines

Water-soluble [M₃S₄X₃(dhbupe)₃]⁺ diphosphino complexes (dhbupe = 1,2-bis­(bis­(hydroxybutyl)­phosphino)­ethane), <b>1</b>⁺ (M = Mo, X = Cl) and <b>2</b>⁺ (M = W; X = Br), have been synthesized by extending the procedure used for the preparation of their hydroxypropyl analogues by reaction of the M₃S₄(PPh₃)₃X₄(solvent)_<i>x</i> molecular clusters with the corresponding 1,2-bis­(bishydroxyalkyl)­diphosphine. The solid state structure of the [M₃S₄X₃(dhbupe)₃]⁺ cation possesses a <i>C</i>₃ symmetry with a cuboidal M₃S₄ unit, and the outer positions are occupied by one halogen and two phosphorus atoms of the diphosphine ligand. At a basic pH, the halide ligands are substituted by hydroxo groups to afford the corresponding [Mo₃S₄(OH)₃(dhbupe)₃]⁺ (<b>1</b>_<b>OH</b>⁺) and [W₃S₄(OH)₃(dhbupe)₃]⁺ (<b>2</b>_<b>OH</b>⁺) complexes. This behavior is similar to that found in 1,2-bis­(bis­(hydroxymethyl)­phosphino)­ethane (dhmpe) complexes and differs from that observed for 1,2-bis­(bis­(hydroxypropyl)­phosphino)­ethane (dhprpe) derivatives. In the latter case, an alkylhydroxo group of the functionalized diphosphine replaces the chlorine ligands to afford Mo₃S₄ complexes in which the deprotonated dhprpe acts in a tridentate fashion. Detailed studies based on stopped-flow, ³¹P­{¹H} NMR, and electrospray ionization mass spectrometry techniques have been carried out in order to understand the solution behavior and kinetics of interconversion between the different species formed in solution: <b>1</b> and <b>1</b>_<b>OH</b>⁺ or <b>2</b> and <b>2</b>_<b>OH</b>⁺. On the basis of the kinetic results, a mechanism with two parallel reaction pathways involving water and OH^– attacks is proposed for the formal substitution of halides by hydroxo ligands. On the other hand, reaction of the hydroxo clusters with HX acids occurs with protonation of the OH^– ligands followed by substitution of coordinated water by X^–

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

Synthesis and Structure of Trinuclear W₃S₄ Clusters Bearing Aminophosphine Ligands and Their Reactivity toward Halides and Pseudohalides

The aminophosphine ligand (2-aminoethyl)­diphenylphosphine (edpp) has been coordinated to the W₃(μ-S)­(μ-S)₃ cluster unit to afford trimetallic complex [W₃S₄Br₃(edpp)₃]⁺ (<b>1⁺</b>) in a one-step synthesis process with high yields. Related [W₃S₄X₃(edpp)₃]⁺ clusters (X = F^–, Cl^–, NCS^–; <b>2⁺</b>–<b>4⁺</b>) have been isolated by treating <b>1⁺</b> with the corresponding halide or pseudohalide salt. The structure of complexes <b>1⁺</b> to <b>4⁺</b> contains an incomplete W₃S₄ cubane-type cluster unit, and only one of the possible isomers is formed: the one with the phosphorus atoms trans to the capping sulfur and the amino groups trans to the bridging sulphurs. The remaining coordination position on each metal is occupied by X. Detailed studies using stopped-flow, ³¹P­{¹H} NMR, and ESI-MS have been carried out in order to understand the solution behavior and the kinetics of interconversion among species <b>1⁺</b>, <b>2⁺</b>, <b>3⁺</b>, and <b>4⁺</b> in solution. Density functional theory (DFT) calculations have been also carried out on the reactions of cluster <b>1⁺</b> with the different anions. The whole set of experimental and theoretical data indicate that the actual mechanism of substitutions in these clusters is strongly dependent on the nature of the leaving and entering anions. The interaction between an entering F^– and the amino group coordinated to the adjacent metal have also been found to be especially relevant to the kinetics of these reactions

Equilibrium, Kinetic, and Computational Studies on the Formation of Cu²⁺ and Zn²⁺ Complexes with an Indazole-Containing Azamacrocyclic Scorpiand: Evidence for Metal-Induced Tautomerism

Cu²⁺ and Zn²⁺ coordination chemistry of a new member of the family of scorpiand-like macrocyclic ligands derived from tris­(2-amino­ethyl)­amine (<i>tren</i>) is reported. The new ligand (<b>L1</b>) contains in its pendant arm not only the amine group derived from <i>tren</i> but also a 6-indazole ring. Potentiometric studies allow the determination of four protonation constants. UV–vis and fluorescence data support that the last protonation step occurs on the indazole group. Equilibrium measurements in the presence of Cu²⁺ and Zn²⁺ reveal the formation of stable [M<b>L1</b>]²⁺, [MH<b>L1</b>]³⁺, and [M<b>L1</b>(OH)]⁺ complexes. Kinetic studies on the acid-promoted decomposition of the metal complexes were carried out using both absorbance and fluorescence detection. For Zn²⁺, both types of detection led to the same results. The experiments suggest that [Zn<b>L1</b>]²⁺ protonates upon addition of an acid excess to form [ZnH<b>L1</b>]³⁺ within the mixing time of the stopped-flow instrument, which then decomposes with a first-order dependence on the acid concentration. The kinetic behavior is more complex in the case of Cu²⁺. Both [Cu<b>L1</b>]²⁺ and [CuH<b>L1</b>]³⁺ show similar absorption spectra and convert within the mixing time to a new intermediate species with a band at 750 nm, the process being reverted by addition of base. The intermediate then decomposes with a second-order dependence on the acid concentration. However, kinetic experiments with fluorescence detection showed the existence of an additional faster step. With the help of DFT calculations, an interpretation is proposed in which protonation of [Cu<b>L1</b>]²⁺ to form [CuH<b>L1</b>]³⁺ would involve dissociation of the <i>tren</i>-based NH group in the pendant arm and coordination of a 2<i>H</i>-indazole group. Further protonation would lead to dissociation of coordinated indazole, which then will convert to the more stable 1<i>H</i> tautomer in a process signaled by fluorescence changes that would not be affecting to the d–d spectrum of the complex

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

Water-Soluble Mo₃S₄ Clusters Bearing Hydroxypropyl Diphosphine Ligands: Synthesis, Crystal Structure, Aqueous Speciation, and Kinetics of Substitution Reactions

The [Mo₃S₄Cl₃(dhprpe)₃]⁺ (<b>1</b>⁺) cluster cation has been prepared by reaction between Mo₃S₄Cl₄(PPh₃)₃ (solvent)₂ and the water-soluble 1,2-bis­(bis­(hydroxypropyl)­phosphino)­ethane (dhprpe, L) ligand. The crystal structure of [<b>1</b>]₂[Mo₆Cl₁₄] has been determined by X-ray diffraction methods and shows the typical incomplete cuboidal structure with a capping and three bridging sulfides. The octahedral coordination around each metal center is completed with a chlorine and two phosphorus atoms of the diphosphine ligand. Depending on the pH, the hydroxo group of the functionalized diphosphine can substitute the chloride ligands and coordinate to the cluster core to give new clusters with tridentate deprotonated dhprpe ligands of formula [Mo₃S₄(dhprpe-H)₃]⁺ (<b>2</b>⁺). A detailed study based on stopped-flow, ³¹P­{¹H} NMR, and electrospray ionization mass spectrometry techniques has been carried out to understand the behavior of acid–base equilibria and the kinetics of interconversion between the <b>1</b>⁺ and the <b>2</b>⁺ forms. Both conversion of <b>1</b>⁺ to <b>2</b>⁺ and its reverse process occur in a single kinetic step, so that reactions proceed at the three metal centers with statistically controlled kinetics. The values of the rate constants under different conditions are used to discuss on the mechanisms of opening and closing of the chelate rings with coordination or dissociation of chloride