Computational Insights on the Mechanism of H₂ Activation at Ir₂S₂(PPh₃)₄: A Combination of Multiple Reaction Pathways Involving Facile H Migration Processes

The complex Ir₂S₂(PPh₃)₄ (<b>1</b>) is known to react with 1 and 2 equivalents of H₂ leading to [Ir­(H)­(PPh₃)₂]₂(μ-S)₂ (<b>2</b>) and Ir₂(μ-S)­(μ-SH)­(μ-H)­H₂(PPh₃)₄ (<b>4</b>), respectively (Linck, R. C.; Pafford, R. J.; Rauchfuss, T. B. J. Am. Chem. Soc. 2001, 123, 8856−8857). Herein, the results of a thorough computational (DFT) study of these formally homo- and heterolytic H₂ activation processes, respectively, are presented. These indicate that <b>2</b> is formed in a two-step process whereby the oxidative addition of H₂ at a single Ir^II center of <b>1</b> generates an intermediate (<b>A</b>) that rearranges into <b>2</b> by means of a facile H migration to the neighboring Ir center. Activation of the second equivalent of H₂ most likely occurs at the bridging sulfur ligands of <b>2</b> leading to a reaction intermediate (<b>3aa</b>) that features two (μ-SH) ligands. Intermediate <b>3aa</b> present two isomers that differ only on the stereochemistry of the (μ-SH) ligands, and both of them can further evolve into <b>4</b> via H migration from (μ-SH) to bridging (μ-H). Nevertheless, an alternative mechanism based on the initial isomerization of <b>2</b> into <b>A</b>, and followed by H₂ coordination and activation steps at a single Ir center cannot be completely ruled out. In general, the results herein show that the mechanisms for the activation of H₂ at <b>1</b> and <b>2</b> involve facile H migration processes, in agreement with the experimentally observed intermetallic hydride exchange in <b>2</b> and the exchange between Ir<i>H</i> and S<i>H</i> centers in <b>4</b>, which proceed with computed free energy barriers of ca. 19–23 kcal mol^–1

Computational Insights on the Geometrical Arrangements of Cu(II) with a Mixed-Donor N₃S₃ Macrobicyclic Ligand

The macrobicyclic mixed-donor N₃S₃ cage ligand AMME-N₃S₃sar (1-methyl-8-amino-3,13,16-trithia-6,10,19-triazabicyclo[6.6.6]­eicosane) can form complexes with Cu­(II) in which it acts as hexadentate (N₃S₃) or tetradentate (N₂S₂) donor. These two complexes are in equilibrium that is strongly influenced by the presence of halide ions (Br^– and Cl^–) and the nature of the solvent (DMSO, MeCN, and H₂O). In the absence of halides the hexadentate coordination mode of the ligand is preferred and the encapsulated complex (“Cu-in²⁺”) is formed. Addition of halide ions in organic solvents (DMSO or MeCN) leads to the tetradentate complex (“Cu-out⁺”) in a polyphasic kinetic process, but no Cu-out⁺ complex is formed when the reaction is performed in water. Here we applied density functional theory calculations to study the mechanism of this interconversion as well as to understand the changes in the reactivity associated with the presence of water. Calculations were performed at the B3LYP/(SDD,6-31G**) level, in combination with continuum (MeCN) or discrete-continuum (H₂O) solvent models. Our results show that formation of Cu-out⁺ in organic media is exergonic and involves sequential halide-catalyzed inversion of the configuration of a N-donor of the macrocycle, rapid halide coordination, and inversion of the configuration of a S-donor. In aqueous solution the solvent is found to have an effect on both the thermodynamics and the kinetics of the reaction. Thermodynamically, the process becomes endergonic mainly due to the preferential solvation of halide ions by water, while the kinetics is influenced by formation of a network of H-bonded water molecules that surrounds the complex

Mechanistic Elucidation of Zirconium-Catalyzed Direct Amidation

The mechanism of the zirconium-catalyzed condensation of carboxylic acids and amines for direct formation of amides was studied using kinetics, NMR spectroscopy, and DFT calculations. The reaction is found to be first order with respect to the catalyst and has a positive rate dependence on amine concentration. A negative rate dependence on carboxylic acid concentration is observed along with S-shaped kinetic profiles under certain conditions, which is consistent with the formation of reversible off-cycle species. Kinetic experiments using reaction progress kinetic analysis protocols demonstrate that inhibition of the catalyst by the amide product can be avoided using a high amine concentration. These insights led to the design of a reaction protocol with improved yields and a decrease in catalyst loading. NMR spectroscopy provides important details of the nature of the zirconium catalyst and serves as the starting point for a theoretical study of the catalytic cycle using DFT calculations. These studies indicate that a dinuclear zirconium species can catalyze the reaction with feasible energy barriers. The amine is proposed to perform a nucleophilic attack at a terminal η²-carboxylate ligand of the zirconium catalyst, followed by a C–O bond cleavage step, with an intermediate proton transfer from nitrogen to oxygen facilitated by an additional equivalent of amine. In addition, the DFT calculations reproduce experimentally observed effects on reaction rate, induced by electronically different substituents on the carboxylic acid

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

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

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

Bimetallic Complexes for Enhancing Catalyst Efficiency: Probing the Relationship between Activity and Intermetallic Distance

A series of new homoditopic ligands (<b>14</b>–<b>17</b>) containing two bis­(pyrazol-1-yl)­methane moieties connected to either flexible (1,6-bis­(bis­(pyrazol-1-yl)­methyl)­hexane, L_6C (<b>14</b>); 1,7-bis­(bis­(pyrazol-1-yl)­methyl)­heptane, L_7C (<b>15</b>)) or rigid scaffolds (4,5-bis­(bis­(pyrazol-1-yl)­methyl)-9,9-dimethylxanthene, L_Xan (<b>16</b>); 4,6-bis­(bis­(pyrazol-1-yl)­methyl)­dibenzofuran, L_Dib (<b>17</b>)) were synthesized. A series of bimetallic rhodium­(I) complexes [Rh₂(CO)₄(L_X)]­[BAr^F₄]₂ (X = Xan (<b>8</b>), Dib (<b>9</b>), Fc ((1,1′-bis­(bis­(pyrazol-1-yl)­methyl)­ferrocene) (<b>10</b>)), 6C (<b>11</b>), 7C (<b>12</b>)) and [Rh₂(COD)₂(L_X)]­[BAr^F₄]₂ (COD = 1,5-cyclooctadiene, X = 6C (<b>21</b>), 7C (<b>22</b>)) as well as the monometallic complexes [Rh­(CO)₂(L_Ph)]­[BAr^F₄] (<b>7</b>, L_Ph = α,α-bis­(pyrazol-1-yl)­toluene) and [Rh­(COD)­(L_Ph)]­[BAr^F₄] (<b>20</b>) were synthesized. The solid-state structures of <b>8</b>, <b>10</b>, <b>16</b>, <b>17</b>, and <b>21</b> were determined using single-crystal X-ray diffraction analysis. The catalytic activity of complexes <b>7</b>–<b>12</b> was established for the dihydroalkoxylation of the alkynediols 2-(5-hydroxypent-1-ynyl)­benzyl alcohol (<b>I</b>) and 2-(4-hydroxybut-1-ynyl)­benzyl alcohol (<b>II</b>). The rigid bimetallic scaffolds L_Xan and L_Dib were found to yield the most active catalysts, <b>8</b> and <b>9</b>, respectively, with <b>9</b> achieving a reaction rate 5–6 times faster than the monometallic complex <b>7</b> for the dihydroalkoxylation of <b>I</b>. Density functional theory calculations were used to examine the intermetallic Rh···Rh distances in <b>8</b> and <b>9</b>, and these were compared with those of three other related bimetallic catalysts reported previously. The calculations showed all these species to be very flexible at minimal energetic cost, both in terms of the Rh···Rh distance and in being able to access a range of different conformations. No clear correlation between Rh···Rh distance and catalytic activity was established here, which suggests that the observed experimental correlation between catalyst structure and activity may derive from the structures of key reaction intermediates

Photochemistry of Cp′Mn(CO)₂(NHC) (Cp′ = η⁵-C₅H₄Me) Species: Synthesis, Time-Resolved IR Spectroscopy, and DFT Calculations

UV irradiation of Cp′Mn­(CO)₃ (Cp′ = η⁵-C₅H₄Me) in the presence of the free N-heterocyclic carbenes IEt₂Me₂, IⁱPr₂Me₂, IMes, and IPr affords the NHC dicarbonyl complexes Cp′Mn­(CO)₂(NHC) (<b>1</b>–<b>4</b>). Time-resolved infrared spectroscopy in alkane solution reveals that <b>1</b>–<b>4</b> photodissociate CO to generate Cp′Mn­(CO)­(NHC) (<b>1-CO</b>, <b>2-CO</b>, <b>3-CO</b>, <b>4-CO</b>), which exhibit solvent-independent second-order rate constants (<i>k</i> _CO) for reaction with CO. These observations are consistent with <b>1-CO</b> to <b>4-CO</b> being stabilized by intramolecular agostic interactions with the NHCs rather than intermolecular alkane coordination. Density functional theory calculations provide support for this hypothesis and locate a series of agostic structures varying from δ-agostic (<b>1-CO</b>, <b>2-CO</b>), to ε-agostic (<b>3-CO</b>), to ϕ-agostic (<b>4-CO</b>). The atoms-in-molecules approach is used to characterize these species, along with the γ-agostic interaction seen in the CpMn­(CO)­(PPh₃) analogue (<b>5-CO</b>), and shows that these species are distinguished primarily by the magnitude of the electron density at the agostic ring critical point